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SURFACE AND INTERFACIAL RECOMBINATION IN SEMICONDUCTORS
Chapter 3 SURf"ACE AND INTERFACIAL RECOMBINATION IN SEMICONDUCTORS Annamraju Kasi Viswanath Center for Materials for Electronics Technology, Ministry of Information Technology, Pune 411 008, India Contents 1.
Introduction
2.
Shockley-Read-Hall Recombination
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.
Auger Recombination
4.
218
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
220
III-V Semiconductors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
221
4.1.
G a A s Epilayers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
221
4.2.
Novel Techniques to Characterize Surface R e c o m b i n a t i o n in G a A s
. . . . . . . . . . . . . . .
231
4.3.
M e t a l - G a A s Interface
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
233
4.4.
F e r r o m a g n e t - G a A s Interface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
234
4.5.
Heterostructures and Q u a n t u m Wells of G a A s
235
6.
. . . . . . . . . . . . . . . . . . . . . . . . . .
4.6.
G a A s Q u a n t u m Wires
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
238
4.7.
InP . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
242
4.8.
Other I I I - V Binary S e m i c o n d u c t o r s . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
243
4.9.
I I I - V Ternary S e m i c o n d u c t o r s . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
244
4.10. I I I - V Quarternary S e m i c o n d u c t o r s 5.
218
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
250
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
251
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
251
I I - V I Semiconductors 5.1.
CdS
5.2.
CdSe
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
254
5.3.
CdSSe . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
256
5.4.
ZnS
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
258
5.5.
ZnSe . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
258
5.6.
ZnCdSe
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
258
5.7.
CdTe
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
259
5.8.
HgCdTe . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
259
5.9.
CdZnTe
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
259
5.10. H g Z n T e
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
260
5.11. PbSe . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
260
Group IV S e m i c o n d u c t o r s . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
260
6.1.
Si Single Crystals and Wafers
260
6.2.
Si Solar Cells . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
261
6.3.
A m o r p h o u s Si . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
262
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
6.4.
Si-based Q u a n t u m Wells and Superlattices
. . . . . . . . . . . . . . . . . . . . . . . . . . . .
264
6.5.
Porous Si
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
264
6.6.
Si/SiO 2 Interfaces . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
265
6.7.
Ge . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
7.
Group IV Binary S e m i c o n d u c t o r s
8.
Conclusions
7.1.
SiC
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
269 269
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
269
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
270
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
271
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
271
Handbook of Surfaces and Interfaces of Materials, edited by H.S. N a l w a Volume 1: Surface and Interface Phenomena C o p y r i g h t 9 2001 by A c a d e m i c Press All rights o f reproduction in any form reserved.
217
218
KASI VISWANATH
1. INTRODUCTION Recombination of carriers at semiconductor surfaces and interfaces is important not only from the basic physics point of view, but also because it has many implications in almost all semiconductor technological devices, such as light-emitting diodes (LEDs), semiconductor lasers, photodetectors, solar cells, heterojunction bipolar transistors (HBTs), metal-oxide semiconductor field-effect transistors (MOSFETs), and so forth [ 1-19]. When electrons in the conduction band and holes in the valence band recombine they give rise to radiative recombination. If a semiconductor surface has defects such as vacancies, it creates dangling bonds. The vacancies have energies that lie in the middle of the bandgap. These energy levels are called trap states, which also participate in the recombination with electrons and holes. These recombinations are called nonradiative recombinations and have very detrimental effects on the performance of the semiconductor device as well as on the device lifetime itself. The lifetime of the injected carriers in the semiconductor is a structure-sensitive property of the material. This happens because the recombination process takes place through the medium of imperfections in the semiconductor. For a semiconductor that has many surface states, the Fermi level is said to be "pinned" at almost the midgap of the semiconductor. Fermi-level pinning and its removal in semiconductor surfaces and interfaces has been a long-standing problem in semiconductor technology, and has been studied by a number of workers for more than two decades. A semiconductor surface or interface that has many dangling bonds can be cleaned by chemical treatment, a process known as surface passivation. Unpinning of the Fermi level was shown to be possible by a variety of passivation techniques. The improvement in the quality of the surface can be evaluated by continuous wave (cw) and time-resolved photoluminescence (PL) spectroscopies. In cw photoluminescence the PL intensities before and after passivation provide evidence of the improvement in the quality of surface, whereas in timeresolved spectroscopy this is done by the carrier lifetimes. With the advances in highly powerful femtosecond laser technology and detection systems such as ultrahigh-speed streak cameras, it is quite possible today to measure the surfacerecombination lifetimes very accurately. Also, the quality of semiconductor surfaces and interfaces can be judged very precisely using these ultrafast techniques. The reader is referred to the excellent review article by Aspnes [ 12] to understand the initial efforts made on the study of surface recombination in semiconductors. This chapter reviews the present status of understanding of surface- and interface-related recombination in semiconductors including epitaxial layers, bulk single crystals, quantum-confined systems such as quantum wells, quantum wires, and quantum dots, semiconductor devices such as light-emitting diodes, lasers, metal-oxide field-effect transistors (MOSFETs), solar cells, and so forth. The following experimental techniques were considered: cw photoluminescence (PL); cathodoluminescence (CL); electroluminescence (EL); time-resolved
spectroscopic techniques in nano-, pico- and femtosecond timescales; luminescence decay to measure radiative and nonradiative recombination lifetimes; surface-recombination velocities; femtosecond transient absorption; pump-probe techniques; degenerate four-wave mixing, and novel techniques to measure surface-recombination lifetimes such as photoacoustic methods, near-field scanning optical microscopy (NSOM), microwave modulated photoluminescence (MMPL). An in-depth coverage of surface-passivation techniques that have been employed so far in the literature for a number of semiconductors has been given. Also discussed are radiative and nonradiative recombination processes in surface-level quantum dots, for example, InAs quantum dots on GaAs surface, GaAs surface quantum wells that have applications in emerging microelectronic engineering. It is hoped that this review will be a ready reference for those who are already experts in the field of semiconductor surfaces and interfaces. For those who are not at all familiar with the topic of surface states in semiconductors, the present review will serve as a starting point. In order to meet these two objectives, attempts were made to give as many references as possible along with illustrative figures under each heading.
2. SHOCKLEY-READ-HALL RECOMBINATION The surface recombination in semiconductors that is mediated by surface traps was discussed originally by Shockley and Read [9] and Hall [10], and is popularly known as ShockleyRead-Hall (SRH) recombination. Figure 1 illustrates how the electrons and holes recombine through the traps. To discuss the phenomenological picture we will follow the generally accepted Stevenson-Keys model [11, 12]. In this model the dependence of recombination on surface Fermi-level and bulk doping was considered. From this picture, one can understand how to reduce surface recombination by controlling the material properties. Figure 1 illustrates a trap that may exist in either of two states differing by one electronic unit of charge, that is, it can be either negative or neutral. Similar treatment may be applied to other possibilities, for example, neutral or positive. If the trap is neutral it can capture an electron from the conduction band. The energy loss of the electron is then converted into
t uE t J w
o E
Ev (a)
(bj
911,.
q,
(c)
(d)
Fig. 1. The basic processes involved in recombination by trapping: (a) electron capture; (b) electron emission; (c) hole capture; (d) hole emission. Reprinted with permission from W. Shockley and W. T. Read, Phys. Rev. 87, 835 (1952). Copyright 1952 by The American Physical Society.
SURFACE AND INTERFACIAL RECOMBINATION IN SEMICONDUCTORS heat or light or both depending upon the nature of the trapping process, ft can also capture an electron from the valence band represemed by part (d) of the figure. In this case it acquires a negative charge and leaves a hole in the valence band. If the trap i~ negatively charged it can emit an electron that goes to the c{:Jnduction band (Fig. l b) or capture a hole from the valence l:,and (Fig. l c). Shockley and Read [9], in their analysis of t:~e statistics of recombination of holes and electrons involving,: the trap, have assumed that the readjustment time for the t:apped electron for other physical processes such as relaxati,a~ from the excited state to the ground state is negligible conq',ared to the time required on the average for the trap to emit t l~e electron or to capture a hole. At r,),:,m temperature the electrons in the conduction band and hol,z ~in the valence band of the semiconductor move with an avera~;:;e speed v T = x / 3 k T / m * , which is around 10 7 cm]s. Here, v r is the thermal velocity and m* is the effective mass of the ca:Tier. The instantaneous current density Ji due to the thermal ~:aotion of the ith carrier is given by
219
%+An
|
Ec
0 0 % 000%%%~ o~ .
.
.
.
.
.
.
.
.
.
Cn, e
EF
ET, NT
--
G,R
ep, cp
EV
|
(9
|
PO+ ,~p Fig. 2. Schematic representation of the principal channels by which electrons and holes equilibrate. Band-to-band generation and recombination are denoted by G and R. The corresponding processes involving traps are emission and capture e n, c, and ep, Cp of electrons and holes, respectively. Reprinted with permission from D. E. Aspnes, Surface Sci. 132, 406 (1983). Copyright 1983 by Elsevier Science.
At equilibrium Ji -
q i V T i 6 ( r -- ri)
//
where qi is the charge of the carrier, and r/is its position. At equilibrium the current density for electrons or holes can be written as J = ~
q i V T i t ~ ( r -- ri) ---- 0
This happens because the number of carriers going in one direction is equal to the number of carriers going in the opposite direction. However, the situation will be different if we consider the recombination at the semiconductor surface or interface, as the balance will not be maintained. There will be a net flow of carriers t~ the surface or interface that can be represented as a current oF the form J = n q v s. The important parameter here is velocity t's that indicates the lost carriers in the recombination process. The sarface-recombination velocity S is defined as the number (:,f carriers recombining on the surface per unit area per unit time per unit volume of excess bulk carriers at the botmdary between the quasi-neutral and space-charge regions [:2]. Figure 2 shows the various principal channels by which electrons and holes equilibrate in a photoexcited semiconductor that has trap levels; n is the number of electrons in the conduction band and p is the number of holes in the valence band; G is the generation rate and R is the band-to-band recombination rate per unit volume; R should be proportional to n and also p. It is also proportional to the mean thermal velocity v- ~ . Therefore, R can be written as R-
npcrv
where tr is a proportionality constant and has the dimension of area; tr is called the recombination cross section.
~
H 0
P - - Po G -
R o - - noPoCrv
This means that the volume generation rate is equal to the recombination rate at equilibrium. The net rate of change of nonequilibrium values of n and p due to intrinsic band-to-band processes in the absence of external generation is dn/dt
-
dp/dt
-
G-
R
= noPotrV-
npcrv
-- ov(noP o - np) -- -crv(np-
noPo)
=
where n~ - - n o P o
or ni-
nx/h~op%
Here, n/ is the intrinsic carrier concentration. After the semiconductor is excited by a pulse of light, electron-hole pairs are created. Let n = n0+An p=
p0+Ap
where An and Ap represent nonequilibrium carrier distributions. For charge neutrality An -- Ap dn/dt
= dp/dt
-
d/dt
An
- - _ O r V [ ( n 0 .qt_p 0 ) A H _]_ A n 2]
220
KASI VISWANATH
If An << (n o + P0), then An 2 is a very small quantity and hence can be neglected:
d/dt
=
+ Po)] a , .
The solution to this equation is a simple exponential with a time constant T, where '7 -- '7"o -- 1/ ( o v ( n o .+ P o ) )
Now we will consider the situation for a semiconductor that has defects. Let: Nr = defect-level density at temperature T; E r = energy of the trap; c n = electron capture rate; en = electron emission rate; and o-, = capture cross section. Because the recombination processes involved obey the FermiDirac statistics, we introduce the symbol f to represent the probability that a quantum state will be occupied; f is a function of energy level Er of the quantum state and of the Fermi level EF, and can be written as f = 1/(1 + e x p [ ( E / - E r ) / k T ] ) The electron capture rate G is proportional to n, G, ~r, and the volume density of unoccupied trap sites N r ( 1 - f ) . The electron emission rate e n is proportional to the volume density of occupied sites N r f . At equilibrium, the electron emission and electron capture rates are equal, that is, e~ = GUnder the assumption that nondegenerate statistics is applicable, the net rate of capture of electrons by the traps can be shown to be R r -- ~r, v n N r / ( 1 + exp[(Ef - E r ) / k T l )
x ({n-- n r e x p [ ( E f - E r ) / k T ] } ) where n r is the density of electrons that would be in the conduction band, and if the equilibrium Fermi level E F were located at the trap energy E r. One can arrive at a similar expression for holes. When there is quasi-equilibrium condition Rn Rp = R, =
R -- NT'OrnO'pVnVp(n p -- n ~ ) / [O'nV n (n + n r ) + OpVp(p + p r ) ] This is the well-known Shockley-Read-Hall expression, which provides an additional channel for recombination with an exponential decay with a shorter time constant. The expression for surface-recombination velocity can be written as [ 11 ] S = R/An
-- N s 9 O'nO'plJnVp(n0 "-t-PO)
/[O'nVn("s +
+ %V,,(Ps +
where N s is surface or areal density of traps, and n s and Ps are the surface carrier densities; S will be maximum if both
the surface Fermi level and trap energies lie at the midgap. Therefore, S can be reduced by shifting the defect energy levels out of the forbidden gap; S can also be reduced by pinning the surface Fermi level at or near the band edges.
3. AUGER RECOMBINATION In the previous section we have seen that the recombination of excess charge carriers via deep impurity levels in semiconductors occurs by a two-step process where electrons and holes are successively captured into the deep level. The Shockley-Read-Hall model accounts for the two-step nature of the recombination and in general gives a good description of the basic recombination kinetics. However, so far, the physical mechanism of the capture process is not understood. Several studies have attempted to solve this problem [20-31]. There are basically four models that discuss the recombination at deep impurities [22, 23]. The first model considers the capture by multiphonon emission. Here, the transition between two states is due to their vibronic coupling and the excess energy is dissipated in the form of localized phonons. The reader is referred to the review on this topic by Morgan [24]. Morgan has considered a system consisting of a point defects that is capable of binding an electron in a deep state within the bandgap. Because of the interactions between the bound electron and the surrounding atoms of the lattice, there will be coupling between electronic states and the normal modes of the lattice that have an appreciable amplitude near the defect. It was shown that the required transitions are induced by coupling of the vibronic states to the surrounding medium through the electron-phonon and phonon-phonon interaction terms, and that this additional lattice phonon scattering enters as the important part of the theory of nonradiative capture. This model is very similar to the electronic relaxation in chemical systems discussed by Jortner, Rice, and Hochstrasser [32], Freed [33], and by Avouris, Gelbart, and E1-Sayed [34], and Engleman and Jortner [35]. Lax [25] has proposed the cascade capture mechanisms. Here, the carrier, which is going to be captured, loses its energy by passing through a series of closely spaced levels, emitting one phonon during each step. Landsberg et al. [26] and Haug [27] have considered classical Auger capture of free carriers by the impurity. In this process, two independent free carriers meet at the impurity site. Then their electron-electron interaction induces a transistion of one of the carriers into a deeply bound state, while the excess energy is transferred to the other carrier, which is highly excited, into its respective band. Hangleiter and coworkers [28-31] have proposed an excitonic Auger process. The e - e - h Auger recombination is shown in Figure 3. An electron recombines with a hole in a band-to-band recombination and another electron absorbs this recombination energy and moves to an excited conductionband state. The transition is mediated by Coulomb repulsions between the electrons. The excitonic Auger process is shown in Figure 4. When a free exciton meets the impurity,
SURFACE AND INTERFACIAL R E C O M B I N A T I O N IN S E M I C O N D U C T O R S
,2'
I
BQnd
I, 9 2 C o n d u c t i o n
1~
Valence Bond
Fig. 3. e ~-h Auger recombination. Reprinted with permission from D. B. Lak et al., Phys. Rev. Lett. 61, 1229 (1988). Copyright 1988 by The AmericaJl )hysical Society.
/ /;x
ca
I
i
I
I
Er
, , ~ a
~
, 4
cs ET
I
(o) electron coptur~
(b) hole capture
Fig. 4. E:{citonic Auger capture processes into deep impurity level E r. One particle (m: of the free exciton (FE) is captured, transferring its excess energy to the oth,:r one. Reprinted with permission from A. Hangleiter, Phys. Rev. B 37, 259,= (1988). Copyright 1988 by The American Physical Society.
the eleciron in the exciton is captured into the deep level, whereas the hole absorbs the excess energy and is highly excited into the valence band. Similarly, a hole can be captured into the impurity and the electron in the free exciton is excited into the conduction band. For a complete recombination of an electron-hole pair, two excitonic Auger processes are required. The situation is very similar to the ShockleyRead-Hall recombination.
4. Ill-X" SEMICONDUCTORS
4.1. Gats
Epilayers
From th.~ luminescence measurements on GaAs it was found that the minority-carrier lifetime ~'mc, the internal quantum efficiencies r/i and the radiative recombination co-efficient B were setlsitive to surface recombination [36, 37], recombination at substrate-epilayer interface [38], and self-absorption of recombination radiation [39, 40]. Because of the large surfacerecombination velocity [36] and also because of the large interface recombination at the substrate-epilayer interface, the initial measurements of luminescence decay times were dominated by the interface effects, and hence the measured values were not characteristic of the bulk material. Nelson et al. [41, 42] found that the interfacial recombination velocity was
221
dependent on the doping level in the GaAs/GaA1As structure. Henry and Logan [43] have reported a very high value of 4 x 105 cm/s for the surface-recombination velocity in GaAs free surface. Nelson and Sobers [44] have investigated the double heterostructure (DH) of the type GaA1As-GaAsGaA1As and shown that the surface and interfacial recombination were very much reduced. Luminescence decay times observed were 1.2 ns for heavily doped samples and were up to 1.3 s for lightly doped samples. Comparison of timedecay data was made for the GaAs samples with and without cladding layers. It was found that the confinement provided by the cladding layers reduced the surface and interface recombination. Experimental values of radiative recombination coefficient B were compared with the first-principles calculations of Stern [45, 46] and Casey and Stern [47] and excellent agreement was observed for high doping levels. For doping levels below P0 = 1 x 1017 cm -3, the experimental B values were significantly larger than the calculated values, which was possibly due to Coulomb interactions. Jastrzebski et al. [48] have reported the surfacerecombination velocity evaluated by scanning electron microscopy (SEM). This method is based on the relationship between the effective diffusion length of the minority carriers, the penetration depth of the electron beam, and the surfacerecombination velocity. Ryam and Eberhardt [49] have shown earlier that with decreasing accelerating voltage, that is, with decreasing penetration depth of the electron beam, the effective diffusion length of the minority carriers Leff decreases due to the increasing influence of the surface recombination. In n-type GaAs the surface recombination velocity was found to be 2.5 x 106 cm]s and a saturation value of 3 x 106 cm]s was observed if the carrier concentration exceeded 1018 cm -3 [48]. The same group has reported the surface-recombination velocity of GaAs p-n junction [50]. They found that the S value changes along the surface of GaAs. Surface impurities or surface-compositional inhomogeneities were thought to be responsible for the spatial variation of S.
4.1.1. Sulfur Passivation To prepare GaAs surfaces of superior electronic quality it is necessary to remove the native oxides from the surface, and it is also essential to satisfy the dangling bonds on the surface. The surfaces were passivated by aqueous sulfide treatment [51-93]. Sandroff et al. [51] were the first to report a very simple chemical treatment of semiconductor surfaces with NazS. 9H20, and observed dramatic enhancement in the gain of a GaAs/GaA1As heterostructure bipolar transistor (HBT). The structure of HBT, which was grown by metal-organic chemical vapor deposition (MOCVD) is shown in Figure 5. At low collector currents the current gain of the device increased 60-fold. The chemical reaction mechanism responsible for passivation was described by a two-step process. The native oxide and elemental arsenic were etched away, thus exposing a pristine surface, and then the sulfur gets bonded strongly. It is known that sulfur forms many binary stable compounds with both Ga and As; GaS and A s z S 3 are layered semiconductors.
222
KASI VISWANATH
0.2 pm
n*" I0
GoAs emitter AIxGal-xAs
3 /~m
_o
le cm'3
N~', 5 x IOI7r
0.15/am
bose" GoAs
p,,.,i x iOI8 cm "3
0.5/.tin
collector" G o A s
n , ~ 5 x l O I T c m "3
Yablonovitch et al. [52] have discovered that ( N H 4 ) 2 S and Li2S also have excellent passivating properties, in addition to Na2S. 9H20. The surface-recombination velocity at the interface between the sulfide and GaAs was found to be very close to that of the nearly ideal A1GaAs/GaAs interface. Figure 6 shows the time-resolved spectra of different samples of GaAs. The initial nonexponential behavior of the double heterostructure decay curve is due to the influence of bulk Auger and radiative recombination. By using the double heterostructure decay curve as a reference, the effect of bulk recombination was subtracted. It can be seen in Figure 6 that Na2S. 9H20 treated GaAs and the ideal GaAs/GaA1As structure have almost identical decay behavior indicating the improvement in the quality of surface treatment. The dependence of surface-recombination velocity on carrier density is shown in Figure 7 [52]; S is highest at the highest carrier densities and is lowest for the lowest carrier
I
i
I
1 1
1 2
t 3
I.I
E
o 9- 1017 0 o. >., I-.,.,.
r
10~=
lO~S
,< 0 1014
0
1 o
(n
10
TM
101r
CARRIER DENSITY (per cm =)
Fig. 5. Schematic representation of the A1GaAs/GaAs HBT device. Reprinted with permission from C. J. Sandroff et al., Appl. Phys. Lett. 51, 33 (1987). Copyright 1987 by The American Institute of Physics.
z u,I cI er ILl
u 1::
r
n ~ GoAs substrate
......
101=
el" Q
TM
i
-5
,
10
I
4
TIME after laser pulse (ps) Fig. 6. Decay of carrier density in a GaAs double heterostructure excited by delta function optical injection at t - - - 0 : (a) A1GaAs/GaAs; (b) Na2S.9H20/GaAs; (c) photochemical treatment; (d) clean GaAs surface exposed to air. Reprinted with permission from E. Yablonovitch et al., Appl. Phys. Lett. 51, 439 (1987). Copyright 1987 by The American Institute of Physics.
Fig. 7. Variation of S with optically injected carrier density for Na2S-9H20/GaAs. At high density the bands tend to flatten as the surface charge is screened out. At low density, minority carriers are repelled from the surface, thus reducing S. The solid line is a theoretical curve for N - 2 x 10 ]1 charges/cm 2, and SO = 9000 cm/s. Reprinted with permission from E. Yablonovitch et al., Appl. Phys. Lett. 51,439 (1987). Copyright 1987 by The American Institute of Physics.
densities. These results were explained by considering the effects of surface-fixed charge, such as band bending, near the interface. The chemical reaction mechanism for the low S at the NazS. 9HzO/GaAs interface was explained by considering a two-step process: GaAs surface is first etched giving a very clean surface to which sulfur can form strong covalent bonds. The clean surface is obtained by the joint action of sulfide ions and hydroxide ions in the aqueous solutions. It was also noted that the GaAs surfaces treated with pure NH4OH, KOH also gave surface-recombination velocities similar to those observed with sulfide treatment. Surface oxides in the form of A s 2 0 3 and Ga203 along with elemental arsenic are responsible for high concentrations of midgap defects associated with the GaAs surface. Surface oxides are dissolved in alkaline solutions and sulfide ions react with elemental arsenic. This interpretation was very similar that given by Nelson et al. [94] for the action of KOH/KzSe on GaAs. Skromme et al. [55] have observed that the surfacerecombination related notch features in the free and bound exciton emission spectra of NazS. 9HzO-treated GaAs at low temperature were eliminated as shown in Figures 8 and 9. They concluded that the residual band bending under illumination is less than 0.15 V. Wilmsen et al. [59] investigated the effects of N2, O 2 and H20 on GaAs passivated by sulfides. They observed that both water vapor and oxygen must be present in order to obtain a large PL signal. Lee et al. [62] passivated n-GaAs with P2Ss/NH4OH and a fivefold increase in PL intensities was recorded. Besser and Helms [73] compared the quality of GaAs surfaces treated with sodium sulfide and ammonium sulfide. From the PL measurements it was concluded that ammonium sulfide is better than sodium sulfide. This is in ageement with the report by Liu et al. [58]. Sodium is known to be extremely mobile in ionic forms in silicondevice technology. The presence of sodium ions in SiO 2 films was a major problem in the realization of practical insulated gate field-effect transistors. Their reduction has given rise to
223
S U R F A C E A N D I N T E R F A C I A L R E C O M B I N A T I O N IN S E M I C O N D U C T O R S
-
1.slso -I
ENERGY (eV) 1.s~2s 1.s~oo . . . . I . . . . . . . I
( 0 " . X)
VPE
ENERGY (eV) 1 .SiSO " MBE
GaAs
T=
T = 1 .SK PL = 700 (O " . X)/ 9 , h)
mWlcm
1.5125 -
,
-
,
i
2
X e x c = 5 1 4 5 ,~
..
_
I
l.sloo ! = .
.
.
.
.
.
.
.
~ . x)
GaAs 1,8K
PL
700 mW/cm
~exc =
5145
2
A CfA ", X)
I I ) -
~_ z
= Lu
CARE SURFACE FE "
_z
I (O
GeIA ". X= FFilt (a. xl
iJll/
) fO" h
Na2S APPLIED I
8170
8180
8190
8200
WAVELENGTH
8210
8220
(A)
Fig. 8. N,:,rmalized excitonic luminescence spectra (intensity in arbitrary units) of a~:~n-type VPE GaAs layer (n = 4 x 1014 cm -3, 77 K mobility -9200 cm2P~' s) before and after Na2S.9H20 surface passivation. The position of the grot ad state (D~ peak is indicated by the tic mark at 8187 A. Two excited stales are also marked at shorter wavelengths. Incident laser intensity (PL)alld wavelength ()texc) are indicated. Reprinted with permission from B. S. Skro:ame et al., Appl. Phys. Letr 51, 2022 (1987). Copyright 1987 by The Ameri,'an Institute of Physics.
very high-quality silicon, very large-scale integrated (VLSI) circuits. I'herefore, large quantities of sodium are not advisable for device processing. From these arguments ammonium sulfide was thought to be a better alternative to sodium sulfide. Shikata imd Hayashi [77] have used PL to characterize various kin,~ls of processes and insulators for overpassivation of (NH4)2S <.-treated GaAs. The influence of annealing on overpassivati,)n was investigated for the fabrication of devices. The overpassivation films deposited by electron-cyclotron resonance ,:hemical-vapor deposition (ECR-CVD) maintain the effect of sulfur treatment. The other overpassivations deposited by sput:ering, pyrolytic CVD and plasma-enhanced CVD degrade the effects of sulfur treatment. The deposited films were SiN and SiC 2. Annealing between 350-500 ~ has given better results for ECR-CVD SiN. By contrast, for SiC2 a gradual degradation with increasing temperature was observed. The luminescence of free and bound excitons was observed even for bulk-grown GaAs at 4.2 K, which indicated reduction of surface-recombination centers. Lunt et al. at Cal Tech. [78] used sulfides and thiols to passivate GaAs surfaces. They reported the surface electrical and chemical properties of (100)-oriented GaAs that has been exposed to a variety of sulfur, nitrogen and oxygen donors.
1,. . . . . 8170
8180
t., 8190
,
,I 8200
,
I 8210
, 8220
WAVELE.~IGTH f A )
Fig. 9. PL spectra for a p-type MBE GaAs sample (p -- 4 x 10 ]5 cm-3), before and after Na2S.9H20 surface passivation. Reprinted with permission from B. S. Skromme et al., Appl. Phys. Letr 51, 2022 (1987). Copyright 1987 by The American Institute of Physics.
Many of these treatments have given improved GaAs surface recombination properties, demonstrating that a general class of chemical reactivity can be exploited to obtain information on the GaAs surface-state properties. Steady-state photoluminescence and time-resolved spectroscopy have been employed. Organic thiols, with general formula R - S H where R---CH3CH2SH o r - C 6 H 4 C 1 , which were dissolved in nonaqueous solvents, were found to be good candidates for surface passivation. An increase in PL intensities was observed with the chemical treatment of GaAs by thiols. X-ray photoelectron spectroscopy (XPS) studies have shown that thiols were bound to the GaAs surface. However, elemental As was not removed and A s z S 3 overlayers were not found on the GaAs surface. It was argued that complete removal of As atoms and the formation of monolayers of A s z S 3 is not necessary to effect the reduction in the recombination at the surface. Figure 10 shows the time-resolved spectra of GaAs under different treatment conditions recorded by Lunt et al. [78]. The experiments were done under high-level injection conditions. In this situation bands are almost flattened and the effects due to band bending are minimum. The changes in the PL signal will be due to variation in the density and capture cross sections of the surface traps. In the low-level injection case, both the changes in the surface-recombination velocity S and the equilibrium surface Fermi-level affect the PL intensity.
224
KASI VISWANATH
~.
to3
i
a)
N
!| 100
time (ns) Fig. 10. Time-resolved photoluminescence decay curves of 2.8-/xm-thick epilayer GaAs samples taken at 880 nm. The excitation source was a Nd:YAG pumped dye laser providing light at 685 nm: (a) A10.3Gao.vAscapped GaAs sample, as-grown; (b) Alo.3Ga0.7As cap etched off with 0.05% Br2 in methanol and GaAs sample immersed in 1.0-M Na2S.9H20 (aq) for 30 min; (c) etched GaAs sample immersed in 1.0-M 4-Cl-thiophenol in C C I 4 for 30 min; (d) etched GaAs sample immersed in 1.0-M NaOCH3 in methanol for 30 min; (e) GaAs sample only exposed to etch A. Reprinted with permission from S. R. Lunt et al., J. Appl. Phys. 70, 7449 (1991). Copyright 1991 by The American Institute of Physics.
For GaAs with A1GaAs cap layer S was found to be 500 cm/s, whereas for the GaAs surface that was uncapped and exposed to air, very fast PL decay was observed with a best-fit S value of 2 x 105 cm/s. These two extreme cases were used as benchmarks to evaluate the effect of various surface treatments. The GaAs surface, which was treated with NazS. 9H20, produced an interface with a lower surface-recombination velocity than that obtained from the unpassivated, air-exposed GaAs surface. Best fits of the PL decays have given S -- 7 • 104 cm/s for this interface. Analysis of PL decays after exposure to 4-Cl-thiophenol also yielded an S value of 7 • 10 4 cm/s. On the other hand, the GaAs surface that was treated with 1.0 M NaOCH 3 in CH 3OH gave an S value of 2 • 105 m/s, which was the same as that observed for the air-exposed GaAs surface. From these data it was argued that increases in the steadystate PL signal do not necessarily correlate with decreases in surface-recombination rates under high injection conditions. The steady-state PL measurements [78] indicated a correlation between the electron-donating ability of a chemical and the improvement in PL. Increases in the intensity of photoluminescence with methoxide, phenoxide, ethylenediamine, and hydroxide have indicated that other strong electron donors besides sulfides can be used to passivate the surface-recombination centers in GaAs. Charge alone was not sufficient to improve the PL because it was found that several ionic species did not produce any decrease in the observed surface-recombination rate. Time-resolved measurements have shown [78] that the surface treatment decreases the cross section or density of the surface traps. Other reagents, such as sodium methoxides, have given increases in the steady-state PL intensities, but did not affect the GaAs surface carrier-trapping rates. Sodium methoxide treatment did not change the high-injection level
PL lifetime, but the intensity of steady-state PL has shown an increase. This was interpreted as being caused by changes in the surface Fermi-level position. Hung et al. [79] have grown CaF 2 and BaF 2 films on GaAs surfaces that were chemically cleaned by (NH4)2S x. These materials have applications in epitaxial insulator layers or buffer layers for heteroepitaxy. Wang et al. [80] have passivated GaAs with P2Ss/(NH4)2S solutions. A number of techniques such as photoluminescence, X-ray photoelectron spectroscopy (XPS), and Auger electron spectroscopy (AES), scanning-electron microscopy, were used. It was noted that the addition of P2S 5 to (NH4)2S has given better GaAs surfaces. When P2Ss-containing solutions were used, P was detected on the surface but it was bound only to either S or O, never to Ga or As. When the PL intensity decreases, XPS detected an increase of O on the surface, and for longer times found Ga-O bonding, but As-O bonding was not detected. Wang et al. [80] have also studied the differences between passivation by evaporation of elemental sulfur or absorption of hydrogen sulfide and passivation by sulfide solutions. Both evaporation and absorption have resulted in the same saturation of S on GaAs at room temperature. These S layers were not as resistant as the S layers obtained by sulfide-solution treatment. With the help of XPS results, it was argued that the degradation in the PL intensities upon exposure to air is due to the generation of electrically active defects that are created by the reaction of oxygen at structural point defects in the sulfur passivation layer. Continued long-term degradation is thought to be due to the continued reactions at the surface. Several methodologies were also developed in order to have a uniformly passivated surface layer. Oh et al. [82] have shown that (NH4)2S x treatment improves the surface quality of GaAs by observing a highly resolved photoluminescence spectra as shown in Figure 11. Note that in the untreated GaAs sample several peaks due to many defects can be seen. The peak at 1.5148 eV might be due to excitons bound to antisite defect complexes [95]. The (d, X) peak is due to Ga vacancies resulting from the interaction between A s 4 and Ga or their related complexes acting as acceptors [96]. Kunzel and Ploog [97] reported that this (d, X) peak appears mainly for As-stabilized (2 • 4) surfaces grown by molecular beam epitaxy using A s 4 and Ga molecules. In the sulfur-treated PL spectrum the lines due to all these defect complexes are greatly reduced in intensity. Liu and Kauffman [84] have investigated the excitation power dependence on the photoluminescence of bare and Na2S-treated n-GaAs. It was shown that for both samples there was an increase in PL quantum yield with increasing excitation power density. In the chemically treated semiconductor there will be a reduction in the number density of surface impurities responsible for midgap levels, which, in turn, results in reduced band bending and/or a reduction in the effective surface-recombination velocity. Photoexcitation alone can give rise to these two effects in GaAs, with a reduction in band bending resulting from charge separation in the depletion layer following photoexcitation and a reduction in
SURFACE AND INTERFACIAL RECOMBINATION IN SEMICONDUCTORS
]
(D~,X) ---1 T = 10K
]1~
I
I
!
1.45
(MIS) structures, as the organics have insulating atoms made up of alkyl chains. A new sulfur-c0ntaining organic solution of CH3CSNH 2 was shown to effectively passivate the GaAs surface [86]. Liu and Kuech [87] have utilized nearfield scanning optical microscopy (NSOM) along with atomic force microscopy (AFM) to study the spatial variations in photoluminescence and their relation to topographic features in untreated and (NH4)zS-treated GaAs samples that were grown by metal-organic vapor-phase epitaxy. Anderson et al. [88] have achieved the growth of ferromagnetic Fe overlayers on sulfur-passivated (001) GaAs surfaces. Geisz et al. [90] have utilized photoreflectance spectroscopy to show that GaAs surfaces that were photowashed degraded very easily, whereas sulfide-treated surfaces were very stable as shown in Figures 13(a) and 13(b).
--i.5148eV
. (d,X) ----]
1,~18
1.51
225
1.54
Energy (eV)
Fig. 11. I)L spectra of (a) as-grown and (b) sulfur-treated GaAs measured at 10 K. lieprinted with permission from Y. T. Oh et al., J. Appl. Phys. 76, 1959 (1(.'9 t). Copyright 1994 by The American Institute of Physics. surface- :rap density resulting from midgap state filling that is short co~npared with the actual recombination event [98-100]. Figure 1 2 shows the influence of excitation power on the measured enhancement of PL following the surface treatment of GaAs. [1: is very important to note that PL enhancement varies dramatically with excitation power. Asai et al. [85] have electrodeposited organic molecules containi~lg reactive sulfur as a passivating layer on GaAs. This pr(~cess has two advantages. First, it passivates the GaAs surface without any surface damage. Second, it can be used as dielectric in the metal-insulator-semiconductor 20 f. . . . . . . . . . .
,
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Fig. 12. Relative PL efficiency curves for bare and Na2S-treated GaAs: Filled circles represent bare GaAs and open circles are for the treated sample. The PL enhancement is also shown. Note the different scale for enhancement and efficiency. Reprinted with permission from C. S. Liu and J. E Kauffman, Appl. Phvs Lett. 66, 3504 (1995). Copyright 1995 by The American Institute of Physics.
1.3
_0
i
1.4
i
1.5
1.6
Energy (eV)
(b) Fig. 13. (a) Effects of photowashing on the PR spectra from bulk n-GaAs. (b) Effects of sulfide passivation on the PR spectra from bulk n-GaAs Reprinted with permission from J. E Geisg, S. A. Safvi, and T. E Kuech, J. Electrochem. Soc. 144, 732 (1997). Copyright 1997 by The Electrochemical Society, Inc.
226
KASI VISWANATH
MacInnes et al. [101], Jenkins et al. [102], and Tabib-Azar et al. [103] have employed chemical vapor-deposition techniques to form a very stable cubic-phase GaS-passivating film on GaAs. A GaAs-based metal-insulator-semiconductor fieldeffect transistor (MISFET), in which cubic GaS was used as the insulating material, was fabricated (Fig. 14a). Such a GaS/GaAs MISFET showed an on-to-off resistance ratio comparable to that of a commercial device. An enhancement in PL intensity was also shown. Cao et al. [92, 93] have studied the electronic structure of these systems by Auger spectroscopy. The C-V curves of the GaS/GaAs MIS diode are shown in Figure 14b. 4.1.2. Electronic Structure of Sulfur-Passivated
GaAs Surfaces The electronic structure of sulfur-passivated GaAs surfaces has been studied by a number of workers. Surface-characterization techniques such as photoelectron spectroscopy and Auger spectroscopy have been employed. Sugahara et al. [104, 106, 108], Scimeca et al. [ 105, 107], Maeyama et al. [ 109], Oshima Gate (Au) -5 p.m_ m Cubic GaS
Drain (Au) 100 I~m
Source (Au) 100 p.m
n..type. GaAs active layer .
I
p.m
. ]~t
p.rn ) (
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Semi-insulating , GaAs substrat6
~ I
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AI/GaS/GaAs
u. 120
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(b) Fig. 14. (a) Schematic diagram of the GaS-GaAs field-effect transistor (FET): not to scale. Reprinted with permission from R R Jenkins et al., Science 263, 1751 (1994). Copyright 1994 by the American Association for Advancement of Science. (b) High-frequency (1 MHz) C-V curves of the GaS/GaAs MIS diode at room temperature. Reprinted with permission from X. A. Cao et al., J. Appl. Phys. 86, 6940 (1999). Copyright 1999 by The American Institute of Physics.
et al. [110], and Sugiyama et al. [111-113] have made indepth studies by using synchrotron photoemission analysis, and discussed the surface-passivation mechanisms. They have concluded that Ga-S, As-S and S-S bonds exist on the sulfurtreated GaAs surface. It was also observed that Ga-S bonds become dominant after annealing at 360 ~ The initial upward band bending of the As-treated samples suggested that there is a reduction of Asca anti-site defects. Annealing has caused the shift of the surface Fermi level by 0.3 eV towards the conduction-band minimum. After annealing the band bending has decreased; Ga-S bonding formation was thought to be the most important reason for the improved electronic quality of sulfur-treated samples. Changes in the chemical bonding as a function of temperature were also studied [105]. The surface structure of S chemisorbed GaAs was the same as (NH4)zS-treated GaAs [106]. The relative stability of sulfur on (NHa)zS-treated GaAs was in the order ( l l l ) B > (100) > ( I l l ) A , based on the Ga coordination number [ 107]. Direct correlation between PL degradation and the Ga oxide formation resulting in drastic upward band bending was observed [110]. From the X-ray standing-wave measurements it was shown that oxidized sulfur atoms were about 22% of the surface sulfur atoms and were randomly distributed, whereas the unoxidized sulfur atoms in a Ga-S bonding state remain at the first-layer As sites although they are disordered [ 111 ]. Sato and Ikoma [114], Sato et al. [115], and Sakata et al. [116] have measured X-ray photoelectron spectra of sulfurtreated GaAs that did not show the existence of Ga and As oxides. They have concluded, after a number of systematic studies, that the most robust surface passivation of GaAs is PzS5/NH4OH treatment plus cleaning in deionized water and thermal annealing. Moriarty et al. [ 117, 118] have used scanning tunneling microscopy (STM) and synchrotron photoemission techniques and have shown that sulfur substitutes for As in atomic layers below the surface, which will increase the surface band bending from that of the clean GaAs surface. Bessolov et al. [89, 119 120, 122-126], and Lebedev and Aono [121] have shown that the surface barrier height and depletion-layer width decrease with the decrease of the dielectric constant of the passivating medium as shown in Figure 15. The decrease of the surface barrier in n-GaAs means that the surface Fermi level shifts toward the conduction band after treatment with sulfur. Annealing at 360 ~ or higher results in the change of the surface Fermi level from 1.2 to 0.85 eV above the valence-band maximum as depicted in Figure 16. The changes in the surface Fermi-level position were also correlated with the PL intensity changes for various treatments of GaAs. There are several contradictory reports [74, 127-132] on Fermi-level pinning and band bending in surface-treated GaAs. Besser and Helms [127] have concluded that unpinning of the surface Fermi level does not occur in the surface-treated GaAs although there is a reduction in the surface recombination. They have obtained a model that accounts for this effect
SURFACE AND INTERFACIAL RECOMBINATION IN SEMICONDUCTORS 5.0 0.8
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IIc" Fig. 15. I)ependence of the surface barrier heights (a) and the depletionlayer width~ (b) for GaAs treated in different solutions of ammonium sulfide on the reciprocal effective dielectric constant of the solution used. Reprinted with permission from V. N. Bessolov, M. V. Lebedev, and D. R. T. Zahn, J. Appl. Phys. 82, 2640 (1997). Copyright 1997 by The American Institute of Physics.
based or,, the changes in band bending after surface treatment. D(>wnward band bending near a surface gives rise to a field that tends to repel electrons that are coming from the bulk. Be,:ause the recombination centers are assumed to be at the surface, nonradiative recombination can be effectively decreased by inhibiting the carriers from arriving at the surface. Hence, the PL intensity can increase without a reduction in the density of surface-recombination centers and without unpinning the surface Fermi level. Spindt et al. [128], and Spindt and Spicer [129], obtained a model for reduced surface recombination without band flattening. The electronic states thought to be responsible were midgap donors compensated by acceptors that have energies near the valence-band maximum. Double donors and double acceptors were also considered. In the double-donor case the reduction in the surface-recombination velocity was shown to be greater by a factor than the reduction in the surfacestate density. The As and Ga anti site defects were considered for the donor and acceptor states. This model is shown in
Fig. 16. Binding energy relative to the valence-band maximum (a) and intensity (b) of feature B (squares) as well as C and D (circles) of the valence-band spectra of p-GaAs(?00) treated in the solution of ammonium sulfide in isopropanol as a function of annealing temperature. The intensity of features is referred to the intensity of bulk feature with binding energy 6.7 eV. Results are depicted by dots with dashed lines given as a guide to the eye. Reprinted with permission from M. V. Lebedev and M. Aono, J. Appl. Phys. 87, 289 (2000). Copyright 2000 by The American Institute of Physics.
Figure 17 and is consistent with the advanced unified defect model of Lindau et al. [15] and Chye et al. [16] for the surface and interfacial recombination in semiconductors. Hasegawa et al. [130] made observations that are very similar to those made by Besser and Helms [127] independently and at the same time. They also concluded that unpinning of the Fermi level does not take place in surface-treated GaAs. Yablonovitch et al. [74] have contradicted the results and interpretation of Besser and Helms [ 127] and Hasegawa [ 130]. They have obtained a model for band bending and Fermi-level pinning for sulfur or chemically treated GaAs (see Fig. 18). Fermi-level pinning implies band bending, but band bending does not necessarily imply pinning. Band bending is caused by the total surface charge, which is the sum of the surface-trap charge due to defect states in the gap, and the surface-fixed charge due to defect states in the gap and surface-fixed charge arising as a result of ionic charges on the surface. When only the surface-fixed charge is present the Fermi level can move freely. However, if the density of the gap states is very high, then the Fermi level is pinned. Even low surface-trap charges can cause the band bending. Under the various conditions of optical excitation by a laser the changes in the quasi-Fermi level and band bending are shown in Figure 18.
228
KASI VISWANATH
CBM
"r
Energy from CBM
1.4eV
Donor
dark
--0.65eV
Acceptor ~ l ~l.leV L
,
.
.
.
.
.
.
.
'
'
'
,
(b)
-
,, Iy,... EFn
(a)
'"l
CBM
1.4eV
weak light
EFp --
....
Energy from CBM ~0.65eV
Double Donor ~ Double Acceptor { Z
(c)
EFn
~0.9eV --1.1eV
t"
M
strong light
EFp_I: -(d)
(b) Fig. 17. Energy diagrams showing midgap donor states compensated by acceptors at the surface: (a) the simplest case of a single donor compensated by a single acceptor; (b) the single-level states are replaced by double donors and acceptors. The conduction-band minimum (CBM), valence-band maximum (VBM), and bandgap (1.4 eV) are also indicated. Reprinted with permission from C. J. Spindt and W. E. Spicer, Appl. Phys. Lett. 55, 1653 (1989). Copyright 1989 by The American Institute of Physics.
4.1.3. Selenium Passivation
There are many reports [64, 135-149] on Se-passivated GaAs. For example, Mauk et al. [64] have passivated GaAs p-n junction solar cells by KzSe aqueous solution and observed a reduction in the surface-recombination velocity from 5 x 10 6 ClTI/S for the untreated sample to 103 cm/s for the treated sample. Improvement in open-circuit voltage and reduction in saturation current also indicated reduced surface recombination. Sandroff et al. [134] measured PL of a Se-treated sample that was 400 times enhanced compared to that recorded for a similarly grown GaAs surface. Berrigan et al. [137], Scimeca et al. [138, 139, 141, 142], and Maeda et al. [140] reported the chemical bonding and atomic structure of Se-treated GaAs. They studied in detail synchrotron-radiation photoelectron spectroscopy of Se-treated (111) A, (100) and (111) B GaAs. If the deposition is made at relatively high temperature, As on the surface is replaced by Se and band bending is reduced considerably. Deposition of a thin A1 layer on a Se/GaAs surface has yielded a better quality surface [139]; GaAs samples on which ZnSe epilayers were grown have shown very good PL properties [143, 144]. Kuruvilla et al. [145, 146] observed the formation of the As2Se 3 phase in their X-ray photoelectron spectra of Se/GaAs.
.....
EFn very
strong light
~V
EFp
Fig. 18. (a) Band bending in the dark for the Na2S. 9H20 case. The t- represent the surface traps largely responsible for dark bank bending. For (NH4)2S the Fermi level stays above t- (b) Low injection conditions appropriate to this experiment. The electron Fermi level moves freely, but the hole Fermi level is temporarily "pinned." The bands begin to flatten. (c) Strong injection, both Fermi levels move freely, and traps t- become empty, but some surfacefixed charge Q/remains. (d) Near degenerate injection, all band bending is screened out. Reprinted with permission from E. Yablonovitch et al., Appl. Phys. Lett. 54, 555 (1989). Copyright 1989 by The American Institute of Physics.
Sun et al. [147, 148] conducted very elegant experiments by using high-resolution core-level photoemission spectroscopy excited with a synchrotron-radiation source to clarify the nature of bonding in Se-treated GaAs. They have identified arsenic-based sulfides and selenides on the topmost surface and gallium-based selenides adjacent to the bulk GaAs substrate. Thermal annealing was done at different temperatures and the shift of the surface Fermi level was monitored. This procedure allowed the authors to identify the chemical entities that cause band bending. Figure 19 shows the shift in the surface Fermi level as a function of annealing temperature. It was demonstrated that GazSe 3 bonds are responsible for the unpinning of the surface Fermi level by removing midgap states.
4.1.4. Phosphorous Passivation Improvement in the electronic quality of GaAs surfaces was achieved by phosphorous passivation [150-158]. Olego et al. [150] reported the unpinning of the Fermi level and lowering
SURFACE AND INTERFACIAL RECOMBINATION IN SEMICONDUCTORS
CBM 1.4
!
!
....
i
'
-
'l
1
i
1.2 1.0 0.8 0.6 0.4 RT
0.2
V f3M 0.0
T ';
1;0 2(~0" 3(~)0"'4;0 500' 6;0 Annealing Temperature(~
Fig. 19. P~isition of the surface Fermi level relative to the band edges as function c f Lhe annealing temperature of a highly n-doped GaAs(100). The shift of th,~ ~urface Fermi level from the unpinned position near CBM toward midgap is :lue to the progressive desorption of the passivating overlayer. Reprinted v~ith permission from J. Sun et al., J. Appl. Phys. 85, 969 (1999). Copyright ~ )99 by The American Institute of Physics.
of the stHface barrier from 0.7 to 0.18 eV in n-GaAs on which an amorphous phosphorous overlayer was deposited. Raman and ph()loluminescence spectroscopies were used for these studies. A model was proposed that considers the P atoms in the interface in the place of As vacancies that pin the Fermi level at midgap in n-GaAs. Viktorovitch et al. [151] carried out the thermal treatment of GaAs in PH 3 that causes the As/P exchange. This gives rise to a thin layer of GaP on the GaAs surface and the formation of arsenic oxide on the GaAs surface, thus preventing the formation of arsenic oxide on the GaAs surface. Wada and Wada [153] directly deposited a thin layer of InP on the GaAs surface. Davidson et al. [155] employed a frequency-doubled argon ion laser emitting 257 nm light to conduct the photolysis of P(CH3) 3 and phosphidation of the GaAs surface. Harrison et al. [156] have done a very simple heat treatment of GaAs in the presence of tertiary butyl-phosphine vapor and obtained an excellent PL spectrum. A very thin GaP layer was formed on the GaAs surface. Enhancement in PL intensity was thought to be due to reduced surface recombination. Glemb()cki et al. [157] used P2S5 to passivate GaAs surfaces exposed to CI2/Ar plasma. The Fermi level of p-GaAs shifts from neltr the valence band to midgap. The treatment with P2S5 wa:, found to remove the damage caused by plasma etching. Excess As was evidenced for the etched sampled at the GaAs/oxide interface from Auger spectroscopy; passivation by P2S5 removes the excess arsenic.
4.1.5. Arsenic and Antimony Passivation Mada et al. [81] suppressed the degradation of the (NH4)2Streated GaAs by coating the surface with an evaporated A s z S 3 film. The interfaces were also examined by studying the characteristics of a metal-insulator-semiconductor (MIS) structure, in addition to the PL measurements. In a later study [83] n-GaAs was passivated by directly depositing A s z S 3 film on the etched surface. It was also shown that the surface Fermi
229
level moves to 0.25 eV below the conduction band minimum. Passlack et al. [159] reported very low interface recombination velocity of S << 1000 cm/s for As cap layers of GaAs stored in ultrahigh vacuum. Wang et al. [160] applied a two-step process to passivate deep-level defects in GaAs grown on Si. The first step is the H plasma passivation followed by annealing in AsH 3. The p-n junction properties of GaAs on Si were found to be greatly improved. Passivation of GaAs by antimony overlayers was reported by Zinck et al. [ 161 ]. Maeda et al. [ 162, 164] and Sugiyama et al. [163] examined the surface reconstruction and changes in the chemical bonding in the Sb/GaAs (001) surface that was prepared by depositing Sb on As-terminated GaAs (001) at room temperature and annealing it up to Sb desorption temperature.
4.1.6. Passivation by Nitridation One of the best methods to passivate GaAs is by nitridation [165-172]. In this method a thin layer of GaN is coated on GaAs. The main advantage in group III nitrides is that they are chemically very stable. In order to form the passivating nitrides, the native oxide is first removed from the surface, and then a nitride film is grown. Vogt and Kohl have performed nitridation with hydrazine at low temperature around 400 ~ Higher PL intensity in the bandgap luminescence was observed. Park et al. [166] nitrided GaAs by irradiation with electron-cyclotron resonance nitrogen plasma at various substrate temperatures from room temperature to 600 ~ Nitridation by using helicon-wave excited N 2 plasma was achieved by Okamoto et al. [167], Kasahara et al. [168], Hara et al. [ 169, 171], Wada et al. [ 170], and Tanemura et al. [ 172]; Ga-N bonds were identified in the film grown on the GaAs surface.
4.1.7. C1 Passivation Lu et al. [173] obtained an ordered and air-stable GaAs by chemical etching and passivation by dilute HCL solution. X-ray photoelectron spectroscopy gave evidence for Ga-C1 bonds along the surface normal. The C1 termination has removed surface bandgap states. An enhancement in the nearband PL was observed. Surface photovoltage measurements indicated reduction in midgap states.
4.1.8. Ru Passivation Nelson et al. [174] were the first to observe reduction in surface-recombination velocity in GaAs after chemisorption of Ru ions. Time-resolved luminescence experiments were performed on the untreated and Ru-treated GaAs.
4.1.9. Ga203 Passivation Hwang et al. [175] used molecular beams of gallium oxide to deposit an oxide film on GaAs. From photoreflectance and photoluminescence experiments the interfacial state density was found to be very small; Ga203-GaAs is a very
230
KASI VISWANATH
good material to realize metal-oxide semiconductor fieldeffect transistors (MOSFETs), which will be better than Sibased MOSFETs, as they can offer higher speed and low power consumption. Callegari et al. [176] employed the rf plasma-cleaning technique to remove the midgap states at the Ga oxide/GaAs interface; MOS capacitors were also fabricated that have excellent capacitance-voltage characteristics. Ping and Ruda [ 177] performed ab initio molecular orbital calculations to understand the mechanism of Ga203 passivation of GaAs surfaces.
4.1.10. Hydrogen Passivation Introduction of hydrogen in semiconductors is a very wellknown technique in semiconductor technology because it improves the electrical and optical properties of any III-V compound. Native oxides on GaAs substrate surfaces are generally removed before epitaxial growth by heating the substarte around 500 ~ In this process many surface contaminants such as carbon remain without evaporation. The surface contaminants form a carrier-depletion layer between the substrate and epitaxial layer interface. The carrier-depletion layer affects the optical and electronic properties. Hydrogen can passivate many shallow donors and acceptors. Also, it will effectively passivate the dangling bonds on the surface. Hydrogen passivation in GaAs has been reported by a number of researchers [178-193]. Carbon and nitrogen impurities in the GaAs surface were removed by a H 2 plasma in the experiments conducted by Friedel and Gourrier [178]. Surface photovoltage measurements confirmed the reduction in band bending. Stillman's group could passivate C acceptors in both molecular beam epitaxy (MBE) and metal-organic chemical vapor deposition (MOCVD) GaAs samples by hydrogenation. In sutu chemical etching of GaAs substrates with a combination of HC1 gas and hydrogen prior to MBE growth effectively cleaned the substrate surface [181]. A novel photochemical vapor-deposition system for hydrogen passivation was described by Chen et al. [182]; PL spectroscopy was utilized to evaluate the passivated surfaces. This technique has no electron or ion bombardment, therefore the sample will not be damaged during the processing. Swaminathan et al. [183] reported the photoluminescence measurements of hydrogenated GaAs grown on an InP substrate. This heteroepitaxial structure has great importance as it makes it feasible for the development of optoelectronic integration of GaAs devices with InP-based photonic devices on the same wafer. However, the major problem in this material system is a large lattice mismatch of about 3.7% between GaAs and InP, which gives rise to high density of interface defects, such as dislocations. To solve the problems of lattice mismatch, several techniques have been reported: For example, growth on misoriented substrates, usage of buffer layers such as GaAs/ GaA1As multiple quantum wells, and GaAs/InGaAs strained layer superlattice prior to GaAs growth. The authors have found that the hydrogenated sample has better PL properties.
Gottrcho et al. [184] developed a realtime PL monitoring method to study the hydrogen-plasma treatment of GaAs, and were able to find the best conditions for plasma treatment. It is very important to note that the surface treatment by plasma not only passivates the defects, but it can also damage the surface. Therefore, a trade-off between the plasma pressure and exposure time is very essential. Ineffective plasma treatment at room temperature can be due to low pressure. Excess arsenic on the surface is removed by plasma and midgap states are removed. Enhancement in PL intensity was attributed to removal of midgap energy levels. A remote Ar discharge was used to excite and dissociate hydrogen for in sutu cleaning of the GaAs surface [185]. Carbon and oxygen on the surface were removed as evidenced in Auger spectroscopy. Swaminathan et al. [ 186] passivated Si 6-doped GaAs grown by molecular beam epitaxy by employing a low-frequency hydrogen plasma at 250 ~ for 30 min. It was demonstrated that ~-doped material can be passivated by atomic hydrogen in exactly the same way as a uniformly doped semiconductor. Low-temperature photoluminescence and capacitance-voltage measurements confirmed the effects of passivation. Two hydrogenation schemes, that is, the rf glow-discharge system and the photochemical vapor system, were used to passivate GaAs heteroepitaxially grown on InP [187]. The copper-arsenic vacancy complex was eliminated. The damage due to rf glow-discharge was removed by the photochemical deposition method. The photolumnescence peaks at 1.503 and 1.462 eV have intensities stronger than those in homoepitaxial GaAs epilayers. These samples were grown by molecular beam epitaxy. An ECR hydrogen-plasma treatment was utilized by the AT&T group to remove the native oxides, followed by annealing at 500 ~ to prepare GaAs substrates for MBE overgrowth [189]. Oxygen was completely removed and carbon impurities were greatly reduced in concentration. A very good structural order at the epilayer/substrate interface was obtained. A Ga-stabilized reconstructed surface was achieved. The temperature at which annealing was done was shown to be important. Low-temperature annealed samples were found to have many defects, for example, dislocations at the epilayer/substrate interface. The same research group in a later publication [190] conducted the C12 chemical etching as a follow-up process after the ECR hydrogen-plasma treatment. This additional procedure has reduced the C, Si, and O surface contaminants from the GaAs substrate that was similar to a very clean GaAs epitaxial layer. Wang et al. [ 191, 192] and Soga et al. [ 193] have passivated GaAs solar cells grown on Si by using a phosphine-added plasma treatment. The samples were grown by MOCVD. Narrowing of PL peaks (Fig. 20) was observed after hydrogenation, which was thought to be due to passivation of localized states. The minority carrier lifetime of PH3/Hz-treated sample was higher than that of the sample treated only with H 2 as shown in Figure 21. The surface-treated solar cells have
231
SURFACE AND INTERFACIAL RECOMBINATION IN SEMICONDUCTORS
A
B
C D
shown very good performance characteristics. Conversion efficiency 77 of 18.3% (Fig. 22), and increased open-circuit voltage due to reduction of the p-n junction dark current by defect passivation were reported. For an excellent review on hydrogen in semiconductors the reader is referred to the article by Pearton et al. [194].
.
4.2K
, t + _~~ (700 ~ ....2].
3.93 meV
hydrogenated annealed
L4.19
5
Hsu and Lau [195] described the effects of polyimide passivation of MOCVD-grown GaAs. The samples were spin coated with polyimide acid; PL was used to evaluate the samples. Rao et al. [196], Manorama et al. [197], and Bhide et al. [198] employed polythiophene and polyphenylene sulfide as passivating materials.
ahnYdn;aogll~(nl(at~:>O+C)__jr ~ / ~
_>, _c
4.1.11. Polymer Passivation
moV
,
hydr~
~
__
,.
4.1.12. LB Film Passivation as-grown ___L
%
8(:)0
L
_ "__
|
|
_*
820
,
880
860
840
Wavelength [nm] Fig. 20. The 4.2 K PL spectra of GaAs on Si for various plasma-treatment conditions. Reprinted with permission from T. Soga et al., J. Appl. Phys. 87, 2285 (2000). Copyright 2000 by The American Institute of Physics.
Interface trap density in GaAs was reduced by coating a thin Langmuir-Blodgett (LB) film that was also used as an insulator in an MIS structure [199]. The LB films have very important electrical, chemical and optical properties and their incorporation in GaAs devices will open up many possibilities for novel devices.
4.1.13. Quantum Wells of Si as Passivating Barriers Metal-insulator-semiconductor (MIS) structures of GaAs with quantum wells of Si as passivating medium were reported [200] that have shown excellent electrical properties. 4.2. Novel Techniques to Characterize Surface Recombination in GaAs ~ *
(c)
X:2.74 ns
Riech et al. [201] have developed a photoacoustic (PA) technique to measure surface-recombination velocity. The main advantage of their method is that it is noninvasive. The PA signal can be interpreted in terms of recombination events.
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o c
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15
20
25
Time (ns) Fig. 21. Time-resolved PL decay curves of GaAs epilayers on Si measured at room temperature: (a) as-grown; (b) H 2 plasma passivated, (c) PH 3 (PH3/H2 -- 10%) plasma passivated, and (d) PH 3 plasma passivated followed by annealing in H 2 ambient at 450 ~ The solid lines represents fitted results. Reprinted with permission from G. Wang et al., Appl. Phys. Lett. 76, 730 (2000). Copyright 2000 by The American Institute of Physics.
I15 : 0
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-
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232
KASI VISWANATH
When the semiconductor absorbs light energy electron-hole pairs are created that will thermalize in a very short time, that is, of the order of a few picoseconds. The excess energy equal to ( h v - E g ) where hv is the incident photon energy, and Eg is the bandgap energy, is given to the lattice. After thermalization, some of the carriers diffuse towards the bulk for about a distance equal to their diffusion length L in time z (where z is their lifetime), and recombine in the bulk. The other fraction diffuses towards the surface and participates in surface recombination in a nonradiative process. The first process of bulk recombination is determined by the minority carrier diffusion coefficient D and the second channel of nonradiative recombination is determined by the surface recombination velocity. Only non-radiative recombinations contribute to the PA signal. Figure 23 shows the experimental arrangement for the PA measurements. Electron-hole pairs are created by employing an Ar ion laser, and the sample is attached to a microphone. The laser light was mechanically chopped by a chopper, and a lock-in-amplifier was used as the detection system. Figure 24 shows the surface-recombination velocity for different carrier concentrations in Ge-doped GaAs. Paulson et al. [202] and Safvi et al. [203] have designed very novel photoreflectance (PR) and photoluminescence (PL) near-field scanning optical microscopy (NSOM) techniques. Photoreflectance is an electromodulation spectroscopic method. A laser is used to create electron-hole pairs in the semiconductor by using appropriate energy of the laser light, and a second tunable laser is used as the probe. The reflected light of his probe laser is monitored. Franz Keldysh oscillations (FKOs) are seen in the A R / R spectra due to electric-field modulation of the reflectivity of the sample when the wavelength of the probe is scanned (Fig. 25). Figure 26 shows the main physical processes involved in the NSOM-based PL technique. The photoexcited carriers can diffuse as well as drift in all the directions from the point of excitation. There will be an electric field at the surface of the material due to surface states that causes band bending at the surface. The minority carriers will drift towards the surface by this electric field.
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CarrierConcentration(cm-3) Fig. 24. Correlation between surface-recombination velocity and doping concentration. Reprinted with permission from L. Riech et al., Phys. Stat. Sol. (a) 169, 275 (1998). Copyright 1998 by Wiley Interscience.
The photogenerated carriers will recombine nonradiatively at the surface with a finite recombination velocity. Therefore, the surface states, Fermi level, surface electric field in the depletion region, and the variation in the surface-recombination velocity can be studied by the NSOM-PL method. DeLong et al. [204, 205] and Inglefield [206-209] at the University of Utah, developed a new and elegant microwave-
9 . Sapphire . Laser l Tttamum . 1 ~ ""
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Fig. 25. In the photoreflectance NSOM system the pump and probe beams are combined in a laser-to-single-mode-fiber coupler. The light is brought to the surface through a single-mode fiber, and reflected light is collected by a lens. The PR signal is detected at a photodiode. A computer controls the tuning of the titanium sapphire laser to the wavelengths of interest and controls data acquisition. Typical light intensities are shown. Reprinted with permission from C. Paulson et al., Mat. Res. Soc. Sym. Proc. 588, 13 (2000). Copyright 2000 by Material Research Society, U.S.A.
SURFACE AND INTERFACIAL RECOMBINATION IN SEMICONDUCTORS
EF E,, Lateral PL
Diffusion
FWHM
1~ 'in-depth' PL
E
Photo-
Surface +1 ~
Stat,~.s
PL
I E ~
A
~_ Depth
generated carriers
Tip
Drift
Fig. 26. S :hematic of the main physical processes that are in operation during NSOM based photoluminescence measurements. The lateral diffusion of photogenex+Lted carriers can affect the lateral resolution achievable in these measurenLeJlts. Surface recombination, bulk carrier lifetime, and internal drift fields all s +rve to restrict this lateral diffusion and hence improve the resolution o1 :he luminescence measurements. Reprinted with permission from S. A. Safvi J. Liu, and T. E Kuech, J. Appl. Phys. 82, 5352 (1997). Copyright 1997 by The American Institute of Physics.
modulated photoluminescence (MMPL) method to characterize semiconductor surfaces. A schematic diagram of the MMPL technique is shown in Figure 27. The sample was placed irL a microwave cavity, and the photoluminescence was then measured as usual after applying microwave power. The MMPL experiment has many common features with other per:urbation techniques such as optically detected magnetic res<)nance (ODMR), optically detected cyclotron resonance (ODCR), and thermally modulated photoluminescence (TMPL). In ODMR, changes in spin-dependent recombination
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processes are observed as changes in PL intensity or polarization. In this case the spin states of a center taking part in the recombination process is changed by application of a pulsed microwave plus magnetic fields. The ODCR technique is similar except that microwave coupling is via cyclotron resonance of free carriers rather than changes in spin states. The TMPL is based on changes in luminescence spectra induced by pulsed changes in the sample temperature. The PL spectrum is affected by the application of the microwave electric fields. The free carriers are accelerated to higher kinetic energy than that corresponding to lattice temperature. The microwave-accelerated carriers will equilibrate with the lattice, which will heat the sample. In this case the MMPL spectrum is the same as that of a sample that is heated. This can be observed as the long transients in the signals that result from insufficient cooling of the sample. There is a second effect due to accelerated carriers, namely, the capture cross section of luminescence process depends on the energy of carriers. Figure 28 shows the usual PL and MMPL of GaAs structures. When microwaves are applied the nonradiative lifetime of minority carriers becomes short compared to the radiative lifetime, and the luminescence efficiency decreases rapidly. From the differences in the MMPL spectra of GaAs exposed vs double hetrostructure it was inferred that the surfacerecombination velocity for exposed surface was higher compared to that in the double hetrostructure. Figure 29 shows the surface-recombination velocity for different microwave powers for the GaAs sample. Wang and Neugroschel [210] described a novel method of measuring surface-recombination velocity that is based on recording photoluminescence decay as a function of self-absorption under different excitation conditions. Surfacerecombination velocity in MBE-grown p-type GaAs was discussed by a method that combines PL and lifetime measurements [211]. Toba and Tajima [212], and Noji et al. [213] reported room-temperature photoluminescence mapping and spatially resolved PL to characterize surface and interface states in GaAs epitaxial wafers. 4.3. M e t a l - G a A s Interface
Microwave Modulated PL Apparatus
!~ t
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.
.
.
.
.
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Bardeen [214] proposed that electronic states at a metalsemiconductor interface "pin" the metal Fermi energy and account for the Schottky-barrier phenomena. In this model the Schottky barrier for an n-type semiconductor interface is the difference between the conduction band minimum at the surface and the energy of the pinning state. Later, Chye et al. [16] and Spicer et al. [17-19] proposed the advanced unified-defect model that explains the metal-semiconductor as well as oxide-semiconductor interface electronic structure. According to these researchers, the pinning states are associated with semiconductor alone because they have energies virtually independent of the metal in the metal-semiconductor contact. It was also proposed that the native defects are near or at the semiconductor surface rather than being intrinsic
234
KASI VISWANATH
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9
4 mW R=/~.71
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1.54
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1.38 (b)
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Fig. 29. Surface-recombination velocity in GaAs as a function of ratio of PL efficiencies (microwaves on/microwaves off) at several microwave powers. Data points are experimental values for a single sample at three microwave powers, and solid lines are theoretical fittings. Reprinted with permission from C. E. Inglefield et al., unpublished.
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0.94
Ratio o f PL Efficiencies qon/Vloff
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Fig. 28. The PL and MMPL from the nominally identical GaAs layers in two different structures (structures also shown in figure). The scales are the same for the MMPL and PL spectra shown on each plot. The MMPL from the GaAs with the exposed surface (a) is entirely negative while that from the buried layer (b) has a significant positive component. This indicates a larger surface-recombination velocity in the sample shown in (a). Reprinted with permission from C. E. Inglefield et al., J. Vac. Sci. Technol. B 15, 1201 (1997). Copyright 1997 by The American Vacuum Society.
or metal-induced surface states. Allen and Dow [215] have shown theoretically that surface antisite defects that have energies in the bandgap of the semiconductor are responsible for the pinning of the Fermi level and cause the barrier formation. Very good metal-semiconductor interfaces are important in order to realize several devices such as metal-semiconductor field-effect transistors (MESFETS), etc. Several groups have achieved good-quality metal-semiconductor interfaces by passivating schemes such as sulfide, selenide, hydrogen plasma, hydrogen fluoride, phosphide treatments, inclusion of rare earths to GaAs, and inclusion of Si interface layers, and so forth [68, 114-116, 154, 216-235]. Sulfide treatments using (NH4)2S [216, 217] and (NH4)2S x [218, 220] were initially attempted by two groups led by Fan et al. [218-220] and Carpenter et al. [217]. Surface states were removed that pin the Fermi level. The Schottky-barrier height (SBH) was
then shown to be dependent on the metal-work function. Lee et al. [222] observed improved characteristics of GaAs MESFETS after (NH4)2S x treatment due to the enhanced Schottky-barrier height. Chang et al. [228, 232, 233] and Lin et al. [230] added Pr/Pr203 to Ga melt for LPE-grown GaAs and reported increased SBH and reduced leakage currents. Chen et al. [226] measured the photoluminescence of Schottky diodes and concluded that anodic (NH4)2S x treatment improves the stability of passivation. Nie and Nannichi [236-238] and Nie alone [239] studied the effect of hydrogen on a Pd/GaAs Schottky diode. Christianen et al. [240, 241] reported ultrafast phenomena in a Au-GaAs Schottky diode. Subpicosecond photoluminescence correlation was used to measure luminescence decay for different reverse-bias voltages and excitation powers (Figure 30). With increasing reverse-bias voltage the decay time reduces from several nanoseconds to a few picoseconds as shown in Figure 31. When the reverse-bias voltage is small, the depletion width of the Schotky barrier is estimated to be thinner (0.1 mm) than the GaAs active-layer thickness (0.3 mm). Then the decay time will be almost the same as the spontaneous carrier lifetime because carriers in the fiat band region determine photoluminescence decay. When the reversebias voltage increases, the depletion width increases and the carrier sweep out becomes faster. This results in small decay times, and the decay times were found to increase with excitation power. 4.4. F e r r o m a g n e t - G a A s
Interface
The hybrid ferromagnetic-semiconductor structures have attracted the interest of many scientists and many new device concepts have been proposed, such as the spin-polarized fieldeffect transistor (Fig. 32) [242], the spin-valve transistor [243], the spin-filter/aligner [244], the spin-polarized light-emitting
235
S U R F A C E A N D I N T E R F A C I A L R E C O M B I N A T I O N IN S E M I C O N D U C T O R S Exc. Power: 0.8 ,,m W . . . . 0,everse blas:" 0.0 V
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l
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Fig. 32. Proposed spin-polarized field effect transistor (FET); DEG represents 2D electron gas. Reprinted with permission from S. Datta and B. Das, Appl. Phys. Lett. 56, 665 (1990). Copyright 1990 by The American Institute of Physics. 0
10
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Delay Time (ps) Reverse Bias: 2.0 V
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40
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Delay Time (ps) Fig. 30. ,~orrelated photoluminescence (PL) intensity as function of time delay belween the pulses for different reverse bias voltages (a) and excitation powers (b',. The solid curves correspond to the best fits to the data. Some of the traces are vertically shifted and multiplied by the given numbers for clarity. Reprinted with permission from P. C. M. Christianen et al., J. Appl. Phys. 80, f!831 (1996), Copyright 1996 by The American Institute of Physics. 104
9
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diodes [245], and the ferromagnetic-semiconductor injector [246]. All these device concepts are based on injecting spinpolarized electrons into the semiconductor. Prinz [247] has reviewed the advantages in using III-V compounds in utilizing spin-electronic devices. Exactly as in any semiconductor device, surface states hamper the fabrication of such devices. Fermi-level pinning has deleterious effects on the carrier lifetime and spin relaxation at the metal-semiconductor interface [248]. Jonker et al. [249] at the Naval Research Laboratory, USA, studied the carrier lifetimes in a ferromagnetic-semiconductor hybrid structure for the first time. They have shown that lifetimes are enhanced in Fe/GaAs relative to uncoated or sulfur-passivated surfaces. It was demonstrated that epitaxial Fe film provides a ferromagnetic contact and suppresses midgap state formation. Hence, Fermi-level pinning does not occur in this system. Formation of the Fe-As bond at the interface, which prevents the formation of As dangling bonds, was the proposed mechanism. The carrier lifetimes and Schottkybarrier heights were estimated from the photoreflectance spectra shown in Figure 33. Manago et al. [250] have reported improvements in the Schottky diodes of sulfur-passivated MnSb/GaAs. 4.5. Heterostructures and Quantum Wells of GaAs
Johnston [251] was the first to report the photoluminescence of a GaAs-GaA1As hetrostructure in which two broad dark regions (known as large dark spots or LDS) extended nearly 100~ on either side of the surface defects. Johnston and Logan [252] have assigned them to regions of nonradiative recombination due to interface states. Later, Henry and Logan [253] interpreted LDS as being caused by long-range transport of photogenerated carriers to the defect site where they recombine nonradiatively. The source of nonradiative recombination giving rise of LDS was attributed to surface recombination in the active layer. GaAs has applications not only in making light-emitting diodes (LEDs), but also in solar cells. The GaAs solar cells have several advantages compared to Si solar cells. For example, they have higher theoretical conversion efficiency of light to electricity (24%), better radiation resistance, and higher temperature of operation. The long-standing
236
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samples have given open-circuit voltage of 0.96 V and conversion efficiency of almost 18% [256]. Luminescence up-conversion was observed at the GaAsGaInP 2 interface, which was attributed to cold Auger processes and the presence of metastable states in GaInP 2 (Fig. 34) [257]. Rosenwaks et al. [258, 259] achieved a very low surface-recombination velocity of 200 cm/s in a p-GaAs epilayer on which a 30-50 A thin epilayer of GaInP 2 was deposited. Reduction of the interface recombination velocity was observed in GaAs coated with a Ga203 layer in addition to an increase in photoluminescence intensity [260]. Low threshold currents observed in vertical cavity semiconductor lasers that incorporate wet oxidation of AlAs for current confinement were explained by time-resolved studies [261]. In these experiments interface recombination velocities before and after oxidation were estimated that gave clues to understanding the mechanism. Thermal oxidation and sublimation of a semi-insulating GaAs substrate has improved the mobility and sheet concentration of two-dimensional electron gas in selectively doped GaAs/n-A1GaAs heterostructures with very thin GaAs buffer layers [262]. The main origin of the carrier depletion was thought to be the carbon concentration in the initial growth surface. An interface luminescence band was reported in the PL studies of GaAs/GaA1As heterostructure [263]. Similar experiments have identified luminescence due to vacancy complexes at the interface [264]. Sandroff et al. [51] were the first to report the enhancement of the current gain of a GaAs/GaA1As heterojunction bipolar transistor (HBT) by a factor of 60 after the sulfide treatment. Later, there were several studies made to clean the semiconductor surfaces and devices. Improvements in the dc characteristics of a GaAs/GaA1As transistor that was chemically treated with S2C12 solution has been reported [265]. Similar results were obtained by ( N H 4 ) 2 S x t r e a t m e n t of a HBT [266]. Nottenburg et al. [267] reported later on the reduction of surface-recombination velocity by sulfide treatment, which resulted in ideal transport characteristics of a HBT device. By using thin n+-doped layers at the surface and substrate interface, Smith et al. [268] observed increased radiative efficiencies and lifetimes of photoexcited carriers in GaAs epitax-
Fig. 33. The 300 K PR spectra for (a) native oxide reference sample, F = 58 kV/cm; (b) same sample following sulfur passivation, F = 61 kV/cm; (c) A1/GaAs-(2 x 4) sample, F = 55 kV/cm, and (d) Fe/GaAs-(2 x 4) sample, F = 60 kV/cm. The thickness of the undoped spacer is 1070/k for (a)-(c), and 1500 A for (d). Carrier lifetimes were extracted from these spectra. Reprinted with permission from B. T. Jonker et al., Phys. Rev. Lett. 79, 4886 (1997). Copyright 1997 by The American Physical Society.
]
IE I loser problem in using GaAs is the high surface-recombination velocity. Therefore, in order to have good collection efficiency, an A1GaAs heterojunction was grown epitaxially on GaAs as a surface-passivating layer. As an alternative approach In0.sGa0.sP layers were grown on GaAs that has reduced the recombination velocity in GaAs by several orders of magnitude [254, 255]. Preliminary studies on solar cells of the same
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Fig. 34. Sketch of the cold Auger process at the GaAs/GalnP 2 interface, carrier cooling and trapping, and PL from the GalnP 2 with its composite structure. Reprinted with permission from E A. J. M. Driessen, Appl. Phys. Lett. 67, 2813 (1995). Copyright 1995 by The American Physical Society.
SURFACE AND INTERFACIAL RECOMBINATION IN SEMICONDUCTORS ial layers grown by MOCVD. Prior to this report there were several studies carried out to improve the radiative recombination in GaAs by using A1GaAs as the surface-passivating window [44, 47, 269-274] in which carriers in GaAs are confined by the l:~rger-bandgap A1GaAs and thus are prevented from reaching the crystal surface. However, controlling the nonradiative ctmters is not very easy as demonstrated by the fact that all lile reports gave different values for lifetimes, which extend b'l several orders of magnitude. An ~n s i t u HC1 gas-etching process has enabled a research group al: Mitsubishi to reduce the interface recombination velocity n the GaAs/GaA1As heterointerface [275]. Similarly, hydrog,~J ation has improved the quality of the interface in a GaA,,jI3aA1As resonant tunneling structure [276]. RecombinatioJl models based on ambipolar diffusion equation were discuss e,:[ for GaAs/GaA1As and GaAs/GaInP heterostructures [277]. Capacitance-voltage simulation profiles were used t~ estimate the interface recombination velocities in GaAs/AGaAs structures that were correlated with optical time-re~,:,lved spectroscopy measurements [278]. Rosel~waks et al. [279, 280] reported for the first time the infl.~ence of electric fields on carrier dynamics in GaAs/G:~InP heterostructure interfaces studied both theoretically alld experimentally. Many groups have estimated the surface-recombination velocities under a high injection limit. In this ,:',ase the field in the space-charge region (SCR) is screened by the high density of photogenerated carriers. However, this; approach is not very rigorous and there are many drawbacks. For example, the electric field in the space-charge region is not completely screened even with high injection conditions, and the screening decreases with time as excess carrier concentration changes. When the sample is impurity doped it is essential to use very high injection levels. The analysis in tllis situation is very difficult because of other effects such as !)and filling, bandgap renormalization, carrier degeneracy, Auger processes, and so forth. Also, when high laser powers :~re used the samples can become damaged. Whei~ the injection level is low the time-resolved PL (TRPLt data are analyzed considering only minority-carrier transport with the following assumptions" (1) [,uminescence emitted from the space-charge region c~m be neglected. (2) The electric field in SCR is constant with time. (3) ]'he electric field created by spatial separation of excess carriers can be neglected. (4) An effective recombination velocity Sere is assumed to take care of the effects due to the fields. Figure 35 shows the energy-band diagram of a typical heterostructure used in the analysis by Rosenwaks et al. [279, 280]. They have made the self-consistent calculation of the effect of electrical fields on time-resolved PL (TRPL) of a semiconductor. They have also conducted the femtosecond and picosecond experiments and found good agreement with theory. Under high initial-injection conditions the spacecharge region electric field is screened by the photogenerated carriers in less than 0.2 ps. At lower injection levels the
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TRPL spectra were effected by the electric field. The PL was quenched in the first few picoseconds due to the electron-hole spatial separation. As the applied bias increases the depletionregion width increases and the effects on TRPL spectra is larger. Under low-injection levels the shape of the PL decay was quite different. This was because the surface band bending was reduced only by a factor of about 0.6 to 0.7 times its original dark value. In this case the decay of the luminescence was governed by a large Seff, which increases the PL decay rate. Very efficient radiative recombination, with low interfacerecombination velocity was reported in GaAs grown on a Si substrate. This was achieved by using a step-graded Ge/GeSi buffer [281]. Reich et al. [282] described the photoacoustic measurements on a GaAs/GaA1As heterostructure from which surface-recombination velocity was estimated. The values were compared with the time-resolved results. Several groups have discussed the interface-recombination velocity in GaAs quantum wells [269, 274, 283-315]. Dawson and Woodbridge [269] have reported a very low interfacerecombination velocity of 60 cm/s for GaAs in a double heterostructure. In order to explain the dependence of exciton temperature on quantum-well width in GaAs/GaA1As singlequantum wells, the effect of interface-recombination velocity was considered [283]. Sheldon et al. [284] measured exciton lifetimes in GaAs quantum wells with different well widths and with and without A1 in the GaAs active layer [284]; GaAs active layers that contain A1 have shown very poor bulk lifetimes, which was attributed to Shockley-Read-Hall recombination. Nelson and Sobers [285] have determined the interfacial recombination velocity of 450 cm/s for the GaAs/GaA1As interface in a double heterostructure. They have also considered the self-absorption of luminescence for the active layer thickness d > 1 /xm. From the luminescence efficiency measurements a very low interface recombination velocity of 53 cm/s was
238
KASI VISWANATH
reported by Hoofl et al. [286] for the metal-organic vaporphase epitaxially (MOVPE) grown GaAs/GaA1As double heterostructures. Molenkamp and Van't Blik [274] were able to reduce the interface recombination velocity to as low as 18 cm/s in a double heterostructure of GaAs. In their study the influence of modulation doping of the active layer was also examined. Yokoyama et al. [287] and Iwata et al. [288] were successful in decreasing the lasing threshold current and interfacial recombination in GaAs/GaA1As single-quantum well lasers by employing superlattice buffer layers. The usefulness of incorporating the superlattice buffer layers in reducing the nonradiative recombination was demonstrated by conducting experiments on two quantum-well laser structures, one with and the second without a buffer superlattice. Wolford et al. [289, 292] and Gilliland et al. [290, 291] at IBM research division have examined a number of "surface-free" quantum wells with a variety of barriers. Using the reactive ion-etching method, quantum dots with diameters 200/zm-60 nm were fabricated from GaAs/GaA1As single-quantum wells by Clausen et al. [293]. When the diameter of the quantum dot was smaller than a particular value, luminescence could not be observed, which was thought to be due to dominant nonradiative processes at the surface of the quantum dot. It was shown that the standard diffusion model of surface recombination fails in such a situation. The luminescence results were understood in terms of damage layer thickness ~. The value of sc was shown to determine the smallest size of the quantum dot that will give luminescence. Ahrenkiel et al. [294] and Olson et al. [295] observed an ultralow interface-recombination velocity of 1.5 cm/s for the GaAs/GalnP double heterostructure. Very long photoluminescence lifetimes of the order of 14/zs have been estimated. The decay times varied with temperature as T L59, which was very characteristic of radiative recombination not limited by bulk or surface nonradiative-recombination processes. Harris et al. [296] also studied the same system by picosecond population modulation spectroscopy. Surface and interface recombination have been reported in a number of other systems based on GaAs, for example, coupled quantum wells [297], self-electrooptic devices (SEED) [298], double-quantum wells [299], mesa structures [300], surface-quantum wells [301], surface-free quantum wells [302], 6-doped quantum wells [303], n-type quantum wells [304], interface traps [305, 306], and so forth. Zhao et al. [307] studied the recombination processes due to 2-dimensional (2D) carriers at the interface of a GaAs/GaA1As quantum-confined system. They observed two luminescence bands called H bands; the HB 1 band is due to recombination of 2D electrons at the interface with holes in the valence band, whereas the HB2 band is due to the recombination of 2D electrons with acceptor bound holes. Figure 36 is the illustration of heterostructure potential and recombination mechanisms. Mochizuki et al. [308] examined the recombination at single crystalline/polycrystalline GaAs interfaces by timeresolved spectroscopy. Low-resistance polycrystalline GaAs is
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important in the fabrication of heterojunction bipolar transistors. The effect of hydrogen passivation in GaAs quantum wells has been reported by a number of researchers [309-314]. Buyanova et al. [312] at Linkoping University, Sweden, measured the PL of hydrogen-passivated Be-doped GaAs/GaA1As quantum wells (Fig. 37). It was observed that if hydrogen passivation is done for a longer time it will degrade the sharpness of interfaces, which was thought to be due to hydrogenenhanced intermixing. Wang et al. [314] achieved enhanced spontaneous emission in H 2 plasma-passivated vertical cavity surface-emitting laser (VCSEL) grown on Si substrate. An increase in minority carrier lifetime was noted as shown in Figure 38. The results were explained as being due to passivation of nonradiative defects by hydrogen. An increase in intensity of the cavitymode emission peak was also recorded. Lipsanen et al. [315] in Finland used a novel method of passivating GaAs/GaA1As near surface quantum wells. They deposited a thin layer of InP on the surface of the quantumwell barrier. Passivated samples with the barrier thickness of <5 nm have shown improved PL intensities as seen in Figure 39. 4.6. G a A s Q u a n t u m Wires
Many research groups around the world have been working on the development of ultralarge-scale integration (ULSI) of semiconductor devices; ULSI is one more step above VLSI (very large-scale integration) technology and represents the ultimate sophistication in the device technology of semiconductors. Here, the size of the semiconductor is in the nanometer (nm) range. When the size of the semiconductor microstructure is of the order of the deBroglie
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S U R F A C E A N D I N T E R F A C I A L R E C O M B I N A T I O N IN S E M I C O N D U C T O R S 9 "'"
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wavelength of electrons, quantum-confinement effects occur. Therefore., in order to understand the operation and optimization of these devices, it is essential to understand the basic physical phenomena in these microstructures. Such nanometer-sized devices may be considered as experimental quantum-mechanical objects. In the previous section we have reviewed the case of quantum well i of GaAs. By reducing the dimensions of the semiconductor it is possible to make one- and zero-dimensional
0.0
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.~'~ 10.3
Fig. 37. qhe PL spectra of A10.3Ga07As/GaAs QWs with different Be doping concentrati :,ns before (solid lines) and after (dashed lines) hydrogen passivation, respe~:tively. The hydrogen passivation was performed during 1 h. All spectra o)]~tain FE and BE lines, with the relative intensity of Be-related BE emissk,a being increased with increasing doping. At higher doping concentrations (>1018 cm-3), an increasing contribution from the free-to bound recombination involving Be acceptors is observed in the PL spectra about 18 MeV ~t~t.[ow the FE line Reprinted with permission from I. A. Buyanova et al., Ap?,,. Phys. Lett. 68, 1365 (1996). Copyright 1996 by The American Institute (,f Physics.
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Fig. 39. Integrated PL intensity of passivated (open circles) and unpassivated QWs after a few hours from the growth (open squares) and after about five days (filled squares) as a function of top barrier thickness d. The passivated samples showed no degradation after five days. Passivation was done by depositing InP monolayer on the barrier of QW. Reprinted with permission from H. Lipsanen et al., Appl. Phys. Lett. 68, 2216 (1996). Copyright 1996 by The American Institute of Physics.
electron systems. A one-dimensional electron system is a quantum wire in which electron motion is confined in two directions. In a zero-dimensional system such as a quantum dot, the confinement of electron motion occurs in all three directions. In very low-dimensional quantum-confined structures, where electron scattering is minimum, there are reports of new electron-transport phenomena such as quantum interference effects [316, 317], point-contact conductance quantization [318, 319], and ballistic electron transport [320]. Sakaki [321] has predicted that in a single-channel quantum wire, the electron scattering would be reduced and electron mobility would be extremely high. Datta et al. [316, 322] proposed a quantum-interference transistor that was based on the electrostatic Aharnov-Bohm effect. The important application of quantum-wire and quantumdot structures to semiconductor lasers has been examined by Arakawa et al. [323]. The conventional bulk active layer is replaced by an array of quantum wires or quantum dots. The most important property of the quantum-confined system is the density-of-states (DOS) function. As the confinement increases, the DOS becomes sharper. This sharpening is directly related to the optical gain spectrum. For the k-conserving transitions, the optical gain is proportional to the effective density-of-states function of the electronic system and the Fermi occupancy factor giving the strength of inversion. Therefore, quantum-wire structures with narrow gain spectra are the best candidates for low-threshold current semiconductor lasers. One main problem with these systems is that the surface-to-volume ratio is very high and hence surfacerecombination processes become very important. Viswanath et al. [324-327] reported the surfacerecombination lifetimes and surface-recombination velocities for GaAs quantum whiskers that were grown by MOCVD at Hitachi. Figure 40 shows the SEM photograph of GaAs
240
KASI VISWANATH Fiber Pulse compressor
Locked Nd : YAG laser ] t
Cryostat
SHG lI
Sync.pumped dye laser
Monochromator
Streak Camera
Fig. 42. Block diagram of subpicosecond laser system. Reprinted with permission from A. Kasi Viswanath et al., Appl. Phys. Commun. 13, 55 (1994). Copyright 1994 by Marcel Dekker, Inc.
Fig. 40. The SEM photograph of GaAs quantum whiskers on a semiinsulating GaAs substrate. Reprinted with permission from A. Kasi Viswanath, K. Hiruma, and T. Katsuyama, Superlattices and Microstructures 14, 105 (1993). Copyright 1993 by Academic Press.
whiskers on a semi-insulating GaAs substrate. Free-standing quantum whiskers were grown without ion etching or ionbeam lithography. Therefore, these quantum whiskers were of very high quality because they do not have dislocations and impurities due to semiconductor processing. Exciton recombination was observed in the photoluminescence spectra. Appearance of free excitons was a manifestation of the optical quality of the samples. Temperature dependence of the main transition was found to be similar to the variation of bandgap of GaAs (Fig. 41), which confirmed that main transition was exciton related. Figure 42 shows a block diagram of the femtosecond timeresolved system used by our group at Hitachi Central Research Laboratory, in Japan. The samples were excited by using a Spectra Physics femtosecond laser system. The Nd: YAG laser
gave about 100-ps pulses at 1.06/zm. The output power of the YAG laser was kept at 10 W. These pulses were compressed using a fiber-optic pulse-compression technique to about 5 ps. A potassium titanyl phosphate (KTP) crystal was used for second harmonic generation of the output light from fiber. The power of the green light was typically 1.0 W, and a dye laser was synchronously pumped by the green light. The dye beam can be tuned by using a birefringence filter. The dye laser pulses were 500 fs in pulsewidth. The dye laser beam was guided on to the sample using the usual beam optics. The sample was kept at an angle of 45 ~ to the incident beam. The detector was kept perpendicular to the incident beam. The dye laser power could be changed by using neutral density filters. The pulse-repetition rate of the Nd : YAG laser was 82 MHz. The sample was mounted on a cold finger of an Air Products Helitran liquid helium cryostat. The temperature could be maintained at any desired value between 6 and 300 K. The photoluminescence was dispersed by a monochromator and detected by a streak camera. Recombination takes place both radiafively and nonradiatively. The PL decay can be written as l / - r = l/7"r -+- 1/'mr
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where ~'br is the bulk nonradiative lifetime, 2S/L is the surface recombination lifetime, and S is the surface recombination velocity. Here, the factor 2 appears because of the top and bottom surfaces of the sample. The photoluminescence decay curves for different wire diameters are shown in Figure 43. It can be seen that the luminescence decay can be split into two parts--an initial fast component and then a relatively slow component. The
241
SURFACE AND INTERFACIAL RECOMBINATION IN S E M I C O N D U C T O R S 500
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t
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TIME (ps) Fig. 43. Time-resolved photoluminescence decay for 100-, 70-, and 50-nm quantum wires at 7 K. Reprinted with permission from A. Kasi Viswanath et al., Proc. Intern. Conf. Solid State Devices and Materials, 1992, p. 296.
Viswanath et al. [326] have also reported the temperature dependence of the bulk part of carrier lifetime for a 70-nm quantum wire that is shown in Figure 45. An increase in lifetime with T showed that the recombination is mostly radiative. This means that most of the nonradiative recombination takes place only on the surface of the quantum wire. Surface passivation of quantum wires was done by sulfur treatment [327]. A blue shift in the PL (Fig. 46) and an increase in the surface-recombination lifetime (Fig. 47) were observed. This was consistent with the reduction of surfacerecombination centers at the surface after sulfur treatment. After the surface treatment the depletion layer was removed and hence the wire diameter effectively increases. This is
2000
fast con]tl)onent was interpreted as the surface-recombination part and '~:he slow component as the bulk-recombination part. Figure 4-,~ shows the surface-recombination velocities as a function ~)f wire diameter. The surface-recombination velocity was found to increase as the diameter of the wire decreased. The surface-recombination velocity is defined as the number of carriers recombining on the surface per unit area per unit time. Therefore, it was concluded that as the wire diameter decreased, the density of recombination centers at the surface increased. The physical interpretation was given as follows. The spatial profile of the electronic wavefunction is greatly dependent on the size of the quantum whisker and hence on the size of the quantum-confinement effect. For higher quantum confinement the electronic wavefunction leaks out of the quantum well and hence increases the number of surface-recombination centers. These observations were consistent with the theoretical predictions of Duggan et al. [328].
~q
1000
,I,
JI,
I
_l
50
100
150
200
,,
,
250
TEMPERATURE (K) Fig. 45. Temperature dependence of the bulk part of carrier lifetime for a 70-nm quantum wire. Reprinted with permission from A. Kasi Viswanath et al., Appl. Phys. Commun. 13, 55 (1994). Copyright 1994 by Marcel Dekker, Inc.
242
KASI VISWANATH Cross-sectional View
rown EZ
Depletion Layer Surface Passiv~atio
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Z EZ ,J
9
I
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L
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1.52 Energy Diagram
P H O T O N E N E R G Y (eV) Fig. 46. Photoluminescence spectra of GaAs quantum wires at 77 K before and after surface treatment. Peak photon energy is marked with a vertical line. Reprinted with permission from A. Kasi Viswanath et al., Microwave and Optical Technol. Lett. 7, 94 (1994). Copyright 1994 by Wiley Interscience.
shown schematically in Figure 48. Mayer et al. [329] reported the surface recombination in dry-etched GaAs quantum wires. 4.7. InP Ahrenkiel [330] has written an excellent review on the properties of InE There are many applications of InP that include semiconductor lasers, light-emitting diodes, integrated electooptics, avalanche photodiodes based on Gal_xInxAs]_yPy and Ga]_~InxAs. It also finds use in microwave and high-speed digital circuits because of its high saturation-drift velocity and high mobility. Most importantly, InP is a very good candidate for space-based solar cells because they have very high radiation resistance and high efficiency.
As-Grown r~
Surface Passivated
s
0
500
.....
i
1000
9
1500
a
2000
TIME ( ps )
Fig. 47. The PL decay at 77 K for 200-nm quantum wire showing the effect of surface treatment. Reprinted with permission from A. Kasi Viswanath et al., Microwave and Optical Technol. Lett. 7, 94 (1994). Copyright 1994 by Wiley Interscience.
i Fig. 48. Schematic cross-sectional view of the profile of the depletion layer and its shrinkage after the surface treatment. Energy diagram corresponds to a change in the ground-state quantum level of electrons with surface treatment. Reprinted with permission from A. Kasi Viswanath et al., Microwave and Optical Technol. Lett. 7, 94 (1994). Copyright 1994 by Wiley Interscience.
The first reports on surface-recombination velocity (SRV) in InP were based on steady-state PL measurements [331, 332]. The PL intensities in InP were compared with GaAs and guesses made as to the SRV values with the assumption that PL intensity is directly related to surface recombination. A low SRV for n-type InP and an SRV comparable to that of GaAs for p-InP were assigned. Ahrenkiel et al. [333] have determined a very long lifetime for InP epitaxial layers from time-resolved measurements. Hoffman et al. [334] measured SRV for n- and p-type InP as 1 x 104 and 2 x 105 cm/s, respectively. Bothra et al. [335] found that SRV in InP was dependent on doping, which was attributed to surface Fermi-level pinning. Rosenwaks et al. [336-338, 340-342] and Lin and Rosenwaks [339] have published numerous papers on surface recombination in InP. They have reported [336] similar values of SRV for both n-type and p-type InP that contradict previous findings. The lower PL intensity observed in p-type samples was interpreted as due to shorter bulk lifetime and large surface band-bending effects. The SRV of metal/InP interfaces was found to depend on the metal deposited on InP by thermal evaporation [337]. This was thought to be due to metal-induced defects in the bandgap. In n-type InP the nonradiative lifetime was found to be very long (~'nr = 320 ns) and the bulk recombination was mostly radiative. By contrast, ~'nr of p-InP was very small (<33 ns), which was interpreted as being due to the high concentration of deep traps. That is, in p-InP nonradiative bulk recombination was very
SURFACE AND INTERFACIAL RECOMBINATION IN SEMICONDUCTORS dominanl [338]. Liu and Rosenwaks [339] have concluded that the Shockley-Read-Hall lifetime was very long (> 10/xs) and radiative recombination was very dominant in undoped InE In p.-InE nonradiative recombination and trapping dominate evew~ in low-doped samples and the effective lifetimes were much shorter than in n-InE These results were consistent with their earlier findings. Rosenwaks et al. [341, 342] also studied the InP-liquid interfaces. They found that the surfacerecombination velocity increases with redox potential of the metal in the metal ion solution as shown in Figure 49. If the redox potential is more positive, then it will be reduced easily on th,:; crystal surface. These adsorbed metals or surface compoun:ts induce surface states, mostly by forming lattice defects. Krawczyk et al. [343] at France Telecom have developed a a;~vel quantitative mapping of surface-recombination velocity in InP [343]. From surface photovoltage spectra of strained IaP on GaAs the interface-recombination velocity was extracted [344]. Very high interface-recombination velocity due to spatially indirect transition was observed in quantum wells of A10.55In0.45As/InP [345]. In order to reduce the surface and interfacial recombination several passivation techniques were tried, for example: Passivation by sulfide/sulfur [346-3581; phosphates [359, 360]; arsenic [361-363]; antimony [364]; nitridation [365]; hydrogen plasma [366-368]; insertion of silicon quantum wells [369-372]; acids and bases [373]; silicon nitride overlayer [374]; and UV irradiation [375]. Cohen et al. [376] observed electron-hole recombination in surface quantum wells of InE Radiative recombination involving surface states of InP was also reported [377]. Asakawa et al. [378] have written a comprehensive review on chlorine-based dry etching of III-V compounds and, in particular, InP. They have also discussed the surface recombination in InE
4.8. Other III-V Binary Semiconductors 4.8.1. GaSb
Although GaSb has many optoelectronic applications, devices of GaSb could not be realized because of the large number of 2
l'
,'
10 s
'l
9
i
|'
9
E
l
9
n-lnP
i
-
l
-
i
Cu*+/Cu o 2 10 4
:>
rr" or)
9
Ag+/Ag~
~.. o
J
]~ Cr+a/Cr+= = 10 a ]; I
I
.
i
surface defects. Dutta et al. [379] have passivated GaSb surfaces by sulfur. They observed improvements in PL intensities in the passivated materials. 4.8.2. GaN
Viswanath [380] has recently reviewed the optical properties of GaN and related materials and their applications in photonics. At present, GaN is the most important semiconductor material [380-382]. Many photonic and electronic devices such as semiconductor lasers, light-emitting diodes, highelectron mobility transistors, photodetectors, and so forth, were realized. Blue-emitting LEDs and lasers are commercialized. Nonlinear optical devices based on room-temperature excitons in GaN were also proposed by Viswanath et al. [383]. Very large exciton binding energy was shown to be responsible for the observation of room-temperature lasing in GaN-based systems by Viswanath et al. [384]. However, the performance of GaN devices should be still improved. There are very few reports on the surface-related problems in GaN and related materials. Pearton et al. [385] observed enhancement in the luminescence of Er-doped A1N, a semiconductor that emits at 1.54/xm, which is suitable for optical communications. Better luminescence properties were attributed to passivation of defects in A1N by hydrogen. Arsenic passivation of cubic GaN was shown to modify its surface reconstruction [386]. Surface recombination in homojunction photodiodes was removed by using a heterojunction GaN/A1GaN in a photodiode [387]. Sun et al. [388] studied the HC1- and KOH-treated p-GaN that reduced the metal contact resistivity. This was interpreted as being caused by the movement of the surface Fermi level. Diluted KOH solutions were used to obtain thin and smooth oxide films on n-GaN samples that have the potential for the realization of metal-oxide semiconductor field-effect transistors (MOSFETs) [389]. This is a better approach than using SiO 2 on GaN that will give rise to high interface trap density. Huh et al. [390] have reported the surface passivation of n-GaN by alcohol-based sulfide solutions, which enabled them to observe very good luminescent properties of the treated samples. This result was thought to be due to the removal of surface defects. X-ray photoelctron spectroscopy of (NH4)2S xtreated Mg-doped GaN has shown that native oxides were removed from the GaN surface by the sulfide treatment [391]. The formation of Ga-S bonds and the occupation of nitrogenrelated vacancies by sulfur were proved to give a stable GaN surface. Behn et al. utilized photoreflectance spectroscopy to understand the movement of the surface Fermi level in a number of GaN samples grown by MBE and MOCVD on sapphire and SiC substrates [392]. 4.8.3. GaP
Zn++lZn ~ 9
243
.
.
.
.
l
.
=
-0.8-0.8-o.4-ci~ o'.o 0'.2 o;4 0.6 0.8 Redox Potential ( V ) Fig. 49. Surface-recombination velocity (SRV) of n-InP as a function of the redox potential of the four-metal ion solution. Reprinted with permission from Y. Rosenwaks et al., J. Phys. Chem. 97, 10421 (1993). Copyright 1993 by the American Chemical Society.
Lee et al. [393] studied the electronic structure of As-etched GaP and (NH4)2Sx-treated GaP [393]. They have shown that Ga vacancies Vca and/or Vca-related complexes are created on clean surfaces by the adsorption of oxygen atoms. Asakawa and coworkers were able to passivate the impurity related defects in GaP by hydrogen [394].
244
KASI VISWANATH
4.8.4. InAs
._. <
6OO
Katayama et al. [395] reported the surface structure of sulfur-treated InAs. They conducted synchrotron-radiation photoemission-spectroscopy studies on (NH4)2S x passivated samples. The As-treated surface of InAs has both S-In and S-As bonds. When the samples were heated to 380 ~ S-As bonds disappeared. Luminescence due to surface-quantum wells of InAs grown on InP was reported [396]. Because radiative recombination within the InAs layer can be distinguished from PL arising from both bulk and surface defects, it was suggested that this system is a good monitor to study the stability of InAs/InP and InAs/air interfaces. Kane et al. [397] studied surface and bulk recombination in InAs/A1As0.~6Sb0.s4 light-emitting diodes that emit at 3.54 /zm. They have shown that the quantum efficiency of these LEDs was limited by interface recombination at the InAs/A1As0.~6Sb0.84 heterojunction and reabsorption in the active layer (Fig. 50). Farad [398] has recently reported the luminescence from the near-surface InAs/GaAs quantum dots. State-filling spectroscopy of these samples is shown in Figure 51. The spectra from five well-resolved shells (s, p, d, f , g) were observed. This was attributed to QD ensemble with good homogeneity [399]. Charge transfer between the surface states and QD states was also noted.
--t
500
oc
400
._.o
3O0
- '
.'. '....
v
.~. 9
As InSb is a narrow-gap semiconductor, it can form alloys with InAs to give bandgaps in the IR range and hence it has many applications in fabricating optoeloctronic devices in the infrared. These devices include detectors for the 10-/xm range based on InSb nipi doping superlattices or InAsxSb~_x/InSb strained-layer superlattices. Wagner et al. [400] reported the surface Fermi-level pinning in epitaxial InSb.
Data
*~
,.~176176
.~
4..d
Q. 200 :3 0 100 a LU J 0
;
,
0
2
(a) ~, <
,
600,
>,
o
10
12
r] =0~14, LoS/D=0.053
~" soo t-
,
8
Thickness (pm)
. . . . . q =0.17, LoS/D=0.086
. - - . r I =0.2. LoS/O=0.12 400
~
~
~-
9
0ata
.m
._.o
300
9 ~176176 . "~176
9
(3. 200 -1 o 100 D LU --J 0
!!
-'*"
11
I1
..~x=.-:
1 I
i't.~ I
L
..... ..
2
0
(b) 4.8.5. InSb
q=l, LoS/D=0.42 ...... q =0.17, LoS/O=0.1 _ _ . . q =0.075, LoS/D=O
'l
~ .
I
L
I
I
&
4
6
8
10
Thickness (pro)
Fig. 50. Front emission efficiency of a series of InAs/A1As0.~6Sb0.84 LEDs with a p-type doping of 5 x 1017 in their active region as a function of active layer thickness. The solid lines are a fit to a model of the LED efficiency that includes bulk and interface recombination and reabsorption and uses as variable parameters the bulk efficiency r/ and the dimensionless parameter LDS/D, where L D is the diffusion length, S is the surface-recombination velocity, and D is the diffusion constant; (a) illustrates the range of parameters that can be used to explain a single data point; (b) shows the best fit and error range of fits to all data points in the figure. Reprinted with permission from M. J. Kane et al., Appl. Phys. Lett. 76, 943 (2000). Copyright 2000 by The American Institute of Physics.
4.9. III-V Ternary Semiconductors 4.9.1. InGaAs Tai et al. [401] at AT&T Bell Laboratories have performed picosecond timescale pump-probe experiments on InGaAs/InP quantum wells grown by chemical-beam epitaxy. The InGaAs/InP quantum-well mesa structures are important in high-speed optoelectronics [402] and all-optical switching [403]. For optical switching based on carrier-induced nonlinearities near the bandgap, Lee et al. [404] have shown that surface recombination was a very good method for speeding up device response without degrading bulk optical properties. For the details of the pump-probe technique the reader is referred to the article by Ippen and Shank [405]. Figure 52 shows the pump-probe spectra of the quantum well. The surface-recombination velocity in the smaller mesa structure was reported as 1.02 x 1 0 4 cm]s. Maile et al. [406] reported the wire-width dependence of the quantum efficiency in dry-etched InGaAs/InP quantum wires. The results were explained by considering both surface
~.
rTK
850
900
950
1000
1050
1100
1150
Wavelength (nm) Fig. 5 l. State-fillingspectroscopy of near-surface InAs/GaAs QDs covered with the indicated cap thickness. The PL spectra are excited with a few kW/cm 2 and vertical offsets have been used for clarity. The QD shells are labeled with the atomic notation: s, p, d, etc. Reprinted with permission from S. Farad, Appl. Phys. Lett. 76, 2707 (2000). Copyright 2000 by The American Institute of Physics.
245
S U R F A C E A N D I N T E R F A C I A L R E C O M B I N A T I O N IN S E M I C O N D U C T O R S
=I
6
/
v
.
.-
-.
--
--..-..-..
'i_. t . - 500
J
__/----
-.-.
v , -
j
1
I O
I
1 3OO
PROBE DELAY (p s)
Fig. 52. 17he AT/T signal vs probe delay for InGaAs/InP quantum wells. The dasb e:[ curve is obtained for a mesa with a diameter of 35/zm. The solid curve is f(: a mesa with a diameter of 35/~m. Zero is at the bottom for both curves. ~.el)rinted with permission from K. Tai et al. Appl. Phys. Lett. 53, 302 (1988). (.h,pyright 1988 by The American Institute of Phyics.
recombiE~ation and the inactive dead layer. Surface recombination v, as found to be high at higher temperatures whereas for wi~e,,; of smaller diameter the dead layer determines the quantu~I~ efficiency. Nanll:,u et al. [407] studied the relationship between the carrier l:fetime refr and the width of the dry-etched region R for lnGaAs/GaAs quantum wires. The calculated curve is shown ill Figure 53. Analysis was done by considering the carrier-d:tffusion length D r. When the R value is large enough, as shown by region (a), the carrier decay is determined by the intrinsic lifetime Ti,t. When the R value is small enough as shown i1~.region (c), the carrier decay is limited by the surface recombination. In this region Teff is proportional to R / S r . In the middle region (b) the carrier decay is accelerated with
log q:,tf/~k
(c)
(b)
(a)
' / c a r d e r decayis . . . . . l limitedby : ca'Tierdecay is : ca.,'ncr qecay,Is ] surface : limitedby : . oc!crml,nr oy recombination : diffusion :, mmnssc ttlcUmc >:< >.., log "ti.,I.................. !............... .:'--i.........xr
T=40 K ..,.~.~_~..-,.~--
"T2D
~
~z
.~
C:
=.,,.,=l
i ..... ;r . . . . ::" . . . . :
...................
decreasing R because of surface recombination. Diffusion also has an influence on the carrier decay in this region. In another report the surface recombination was found to limit the electron-hole plasma density attainable in dry etched InGaAs/InP quantum wires [408]. Band-filling and bandgap renormalization due to many-body effects were also observed at higher excitation densities. Nonradiative recombination due to surface states was found to decrease the intensity with decreasing lateral size in the photoluminescence of dry etched quantum wires and quantum dots of InGaAs/InP structures studied by Hubner et al. [409]. In a later report, the research team led by Forchel concluded that surface recombination was not dominated by diffusive carrier transport to the wire surfaces in deep-etched InGaAs/InP quantum wires in contrast to several previous reports [410]. Wire-width dependence of the carrier lifetime is shown in Figure 54. In a very interesting and important paper, Juodawlkis and Ralph [411] discussed the mechanism for photoinduced absorption in a semiconductor containing a large number of electron traps in the bandgap. When a semiconductor is photoexcited there will be competition between phase-space filling and bandgap renormalization (BGR). These two phenomena, which arise due to transient many-body effects, are very important as they determine the nonlinear optical response of the semiconductor. The relative dominance of phase-space filling and bandgap renormalization depends on the densityof-states (DOS) function and the time-dependent electron and hole distributions. Figure 55 shows the band-filling and bandgap renormalization processes in a semiconductor containing many electron traps in the bandgap. Figure 55b shows the two competing effects--a decrease in absorption due to band-filling and an increase in absorption due to bandgap renormalization. When there are a large number of electron traps in the bandgap the electrons in the conduction band are removed, leaving photoexcited holes that give rise to hole-induced
Faj
10~
E 0
.-I
/~
"Ceff" ~
calculation:
;'
L s = 5 nm , " r s = 1 4 0 ps ,
/
experiment
m
."
:
.
]
101
>
log@)
logLn ---log (2.41 ~ t )
,
1
9
'
i
I ! ,
I
!
i
t ~
10 2
Wire Width L~ [nm]
log R
Fig. 53. Calculated reff vs R curve where Telr is the carrier lifetime and R is the width of the dry-etched region. Reprinted with permission from Y. Nambu et al., Appl. Phys. Lett. 65, 481 (1994). Copyright 1994 by The American Institute of Physics.
Fig. 54. Wire-width dependence of the carrier lifetime in deep-etched In0.53Ga0.47As/InP quantum wires. Circles represent experimental data, and the solid line indicates calculated lifetimes assuming that the surface recombination is controlled by the probability of finding electrons within 5 nm of the wire surfaces. Reprinted with permission from E Kieseling et al., Phys. Rev. B 51, 13809 (1995). Copyright 1995 by The American Physical Society.
246
KASI VISWANATH t<0 Ot
:
0< t< t r
Or, I
t2
<
lZr< t< ~t
Gt:
Or, >
1.0
(]lt !
'"'
'
\
0.5
)=t
N m t--
E
'-
it
w
-
,,
l
,
, 14,Onrn
-
'
(a)]
\
.....
1530 nm
|
~
-- .... ,555nm
t
0.0
O
Z -0.5
k"kt
~
E"E~
k: -20
1.0
(a)
(b)
20 ,;o .... go Probe delay [ps]
o -
w
-
|
-
i
-
u"
' -
io ,
Ioo -
'
(c)
Fig. 55. Energy-mometum (E-k) diagrams showing band-filling and bandgap renormalization (BGR) processes during photoexcitation and recovery in a semiconductor containing a large density of electron traps: (a) t < 0, before photoexcitation, (b) 0 < t < TT, initial absorption increase (a < a~) and (c) z r < t < z R, after appreciable electron trapping (a < a~); ~-~ and z R are the electron-trapping and trapped-electron/hole recombination times, respectively. Reprinted with permission from P. W. Juodawlkis and S. E. Ralph, Appl. Phys. Lett. 76, 1722 (2000). Copyright 2000 by The American Institute of Physics.
bandgap renormalization. Therefore, once a large number of electrons are removed from the conduction band, many states become available for occupation. This gives rise to increased absorption as shown in Figure 55c. Juodawlkis and Ralph [411] have also done very elegant pump-probe differential transmission experiments with femtosecond lasers. Figure 56 shows experimental results and simulated curves. Shockley-Read-Hall theory was considered in simulation of theoretical curves. In Figure 56b please note that the A T / T is negative when A is 1555 nm. This corresponds to photoinduced absorption, which agrees well with the theoretical predictions. Boroditsky et al. [412] have reported surface-recombination studies in a number of III-V materials. Photoluminescence due to interface defects in strained InGaAs/GaAs single-quantum wells was indentified by Joyce et al. [413]. Juang et al. [414] could distinguish the surface and bulk luminescence peaks in InGaAs epilayers based on the excitation intensity-dependence studies as shown in Figure 57. Figure 58 shows the surface structure and InGaAs layers studied. A decrease in interface recombination has been observed in InGaAs/InAs heterostructures when the active layer thickness was increased [415]. Sulfur passivation of n-InGaAs Schottky diodes [416], InGaAs/InP metal-semiconductor-metal photodetectors [417], InGaAs/A1GaAs single quantum-well lasers [418], and InGaAs/InP heterojunction bipolar transistors [419, 420] were shown to have reduced the surface/interface recombination. Surface passivations by InP [421], hydrofluoric acid [422, 423], hydrogen [424-428], and hydroxides [429] have also given better optical quality surfaces in InGaAs materials and devices. Yablonovitch et al. [429] have achieved very highquality In0.53Ga0.a7As "naked" quantum wells by etching with NaOH. Band-to-band recombination spectra are shown in Figure 59. These quantum wells are grown on InP substrate. As the InGaAs becomes thinner and thinner, the carriers
0.5
e=
0.0
t_.
z -0.5 -20
0
2'0 ' 4() 6'0 Probe delay [ps]
"' 8'0
100
Fig. 56. Normalized differential transmission AT~T) of annealed (600 ~ for 30 s) LTG-InGaAs/InA1As MQWs with Be doping of 5 • 1017 cm -3, revealing wavelength-dependent photoinduced absorption: (a) simulated Shockley-Read-Hall (SRH) recovery response, and (b) measured pump/probe response. In (a), the rate-equation time constants and photoexcitation density (An -- Ap -- 1018 cm -3) are fixed so that the variation of the AT/T recovery with wavelength results solely from changes in the impact of hole-induced bandgap renormalization (BGR). Reprinted with permission from P. W. Juodawlkis and S. E. Ralph, Appl. Phys. Lett. 76, 1722 (2000). Copyright 2000 by The American Institute of Physics.
have more and more of their probability amplitude in InR In the extreme case the electrons become unbound because the electron-potential well is very shallow in this heterojunction. In the usual case of the quantum well that has a barrier on both sides, the electrons are in the bound states. In the naked quantum well, which is very shallow and has a barrier on only one side, the electrons are in unbound states. These structures are very useful in the fabrication of lateral quantum-confinement devices. 4.9.2. InGaN As a very important semiconductor material, InGaN has a bandgap that can be varied from 1.95 to 3.4 eV depending on the In content. It is the active layer for the blue and green LEDs and semiconductors lasers. Many of these devices are already commercially available. Major breakthroughs in this field have been made by Nakamura and coworkers [381 ]. They have achieved cw and pulsed lasing in InGaN quantum wells that emit in almost any region of the visible spectrum. Most of the optical studies have been directed to understanding the localization phenomena of excitons in the InGaN system, which is well known for its compositional inhomogeneity and the associated potential fluctuations [430-435]. Viswanath et al. [435] conducted an in-depth study of radiative and nonradiative recombination in InGaN multiple quantum wells. Nonradiative recombination arises due to interface
247
S U R F A C E A N D I N T E R F A C I A L R E C O M B I N A T I O N IN S E M I C O N D U C T O R S
In 0 53 Ga 0.47 As
22K
A lit ~' c
,
' i'
As-grown
''
l
T
,
9
, bulk
Ino.s~Gao.,~s
~;
eh-surface
,
~;: :
95
80
sec.
sec.
;r
50 30 20 sec. sec.sec,
10
begin
/ ~
sec. I
/
\
/I
I
A eh-bulk
:i
1280 W/cm 2 --~
1.0
1.1
'~.2
1.3
-
114
WAVELENGTH Q
It
X30
/ ~ ~ .
80 W/cm 2
X300
M,_ 20 W/cm2 I
_ z
I
1450
t
1600 Wavelength (nm)
" (.6
1.7
Fig. 59. Band-to-band room-temperature photoluminescence from a 50-A thick NaOH-coated "naked" quantum well as it is gradually thinned down by slow etching. The spectrum marked "begin" is the starting point. The quantum-shifted spectra induced by etch thinning for a given number of seconds are shown. The spectrum at 95 s is distorted because of a silicon window in the optical train. Reprinted with permission from E. Yablonovitch, H. M. Cox, and T. J. Gmitter, Appl. Phys. Lett. 52, 1002 (1988). Copyright 1988 by The American Institute of Physics.
320 W/cm 2
i
1~5 (IJm)
1700
Fig. 57. Photoluminescence spectra of LPE-grown In0.53Gao.47As layers used in the experiments under various excitation power densities at 22 K. Surface band to band (eh-surface), bulk band to band (eh-hulk), and conduction band to zinc accepters (eZn) are shown. Reprinted with permission from C. Juang et al., J. Appl. Phys. 72, 684 (1992). Copyright 1992 by The American Institute of Physics.
defects in the quantum-well structure. It is very essential to understartd the effects of the interface defects on the photoluminescence efficiency and its temperature dependence. Figure 60 shows the cw PL of an InGaN multiple quantum well with GaN barrier. From the appearance of the barrier emission it was concluded that the interfaces were of high quality with minimum defects. Figure 61 shows the experimental setup for picosecond time-resolved experiments [436]. This setup was constructed by our research group at the National Creative Research Initiative Center for Ultrafast Optics Control, Korea Research Institute of Standards
and Science. A coherent mode-locked Ar-ion laser that had about 2-ps pulsewidth at a 3.8-MHz repetition rate pumps the coherent double-jet, cavity-dumped dye laser. A synchronous pumping configuration was used in which the cavity lengths of both lasers should exactly match. The dye laser beam was frequency-doubled using a/3-barium borate nonlinear optical crystal. The UV light thus generated was used to excite the sample. For some experiments the Spectra Physics Tsunami Titanium Sapphire laser was used. Nonlinear optical crystals were utilized for harmonic generation. The PL signal was dispersed by a McPherson monochromator and detected by a Hamamatsu photomultiplier tube. The time dependence of the decay was measured by time-correlated single-photon counting. The setup has a time resolution of 10 ps after deconvolution. The sample was mounted on the cold finger of a liquid helium cryostat.
InGaN Multiple Quantum Well 10 K
v
InGaAssurface
disorder layer
+-,~--.i-..: L i...~...i.-.i., 4..,..
00 r r
barner
.--I
Q. . . . . .
;
:
:
',
:
i
:
,
:
; :
I 2.6
Fig. 58. Surface structure of In0.53Ga0.47As layers exposed t o C F 4 - - ~ - O 2 plasmas. Disorder layer and point-defect regions are indicated by horizontal and vertical dotted lines, respectively. Reprinted with permission from C. Juang et al., J. Appl. Phys. 72, 684 (1992). Copyright 1992 by The American Institute of Physics.
,
I 2.8
j
I 3.0
~
!
~
3.2
I 3.4
~
I 3.6
, 3.8
photon energy (eV) Fig. 60. The PL spectrum at 10 K for InGaN/GaN multiple quantum wells. The emission from the GaN barrier is observed very clearly. Reprinted with permission from A. K. Viswanath et al., unpublished.
248
KASI VISWANATH
I .o0 -LoC 0
20
I
InGaN Multiple Quantum Well
15
(/) vE
E
10
>
.m
I
,
"13 i_ E O Z
,
5
nun 9
I
COUNT, I
i
I
I
i cou. , I
I lco.Pu, , I
9
9
9
9
9
0
I
i,, 0
I
50
.
I
100
,
I
150
,
I
200
.
I
250
9
300
Temperature (K)
Fig. 61. Schematic diagram of picosecond photon counting system constructed by our group; CD denotes cavity dumper. Reprinted with permission from A. K. Viswanath, J. I. Lee, and D. Kim, unpublished.
Fig. 63. Nonradiative lifetime as a function of temperature for InGaN quantum-well emission. Reprinted with permission from A. K. Viswanath et al., unpublished.
4.9.3. InGaP Figure 62 shows the quantum efficiency of the InGaN quantum well as a function of temperature. As can be seen in the figure, quantum efficiency decreases with the increase in temperature. This was attributed to the increase in interf acial recombination with temperature. In order to understand this problem Viswanath et al. [435] evaluated the temperature dependence of nonradiative lifetime (~'nr), which is shown in Figure 63. By a very careful study of carrier lifetimes and quantum efficiency as a function of temperature, 'Tnr was evaluated. It was also proposed by Viswanath et al. [435] that the chemical passivation methods should be performed, which will reduce the interfacial recombination in these systems.
1.0
9
InGaN Multiple Quantum Well 9
"~ o c
emission
0.8
(!) .m
0.6
E
0.4
o
quantum well
E m
O
0.2 9
9
9 9
0.0 ,
0
I
50
,
i
100
,
I
150
,
I
~
200
9
I
250
9
9
,
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300
,
350
Temperature (K) Fig. 62. Quantumefficiency at different temperatures for InGaN/GaNmultiple quantum wells. Reprinted with permission from A. K. Viswanath,J. I. Lee, and D. Kim, unpublished.
Pearton et al. [437] reported the surface recombination velocities in InGaP p-n junctions. The ternary compound InGaP has applications in strained quantum-well lasers, and heterojunction bipolar transistors. The InGaP/GaAs heterojunction is less prone to surface oxidation than the A1GaAs/GaAs system and hence it is a better candidate to make quantum-well lasers. In these lasers the optical damage of the laser facets was expected to be minimum. The valence-band discontinuity between GaAs and InGaP is higher than in the case of GaAs/A1GaAs and thus it is more suitable for the fabrication of high electron-mobility transistors and heterojunction bipolar transistors. Pearton et al. [437] have also examined the effects of (NH4)2S x t r e a t m e n t on surface-recombination velocity. Mullenborn et al. [438] performed photoluminescence power-dependence measurements on InGaP/GaAs heterojunction interfaces and estimated the lifetimes that were correlated with the dislocation density at the interface (Fig. 64). The Shockley-Read-Hall recombination was considered and diffusion model calculations were also done. The decrease in lifetime was attributed to an increase in dislocation density at the interface. The band diagram used for diffusion model calculations is shown in Figure 65. Lee et al. [439] studied the effects of various chemical treatments on the performance of Schottky diodes of InGaP (Fig. 66). When samples were annealed above 500 ~ a drastic degradation of barrier height was observed. This was thought to be due to interdiffusion and penetration of metals into the semiconductor. Gorbylev et al. [440] passivated InGaP by hydrogen, which enabled them to obtain better quality surfaces. Photoluminescence measurements on InGaP/GaAs heterostructures have
249
S U R F A C E A N D I N T E R F A C I A L R E C O M B I N A T I O N IN S E M I C O N D U C T O R S
t~ t"l
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~ 1 , ,,,1
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4.,, tD
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,
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Dislocation Density/Length (era-a) Fig. 64. ]~ifetime as derived from PL power dependencies and diffusion model c~d:ulations as a function of dislocation density per dislocation length from HI~ J~:RD experiments. GaInP/GaAs interface data (O) fitted by an inverse square-r,)(,t function (line). Reprinted with permission from M. Mullenborn et al., J ,lppl. Phys. 75, 2418 (1994). Copyright 1994 by The American Institute c:' Physics. GolnP
GoK~
ep,~,r l uu~~ Ioyer
Lossf
shown that the hydrogen passivates the nonradiative recombination defect centers [441]. X-ray photoelectron spectroscopy experiments conducted by Tsai and Lee on the ( N H 4 ) 2 S xpassivated InGaP surface have shown that surface oxide was removed [442]. Evidence was also given for the bonding between sulfur atoms and In and Ga atoms on the surface of sulfur-treated InGaR It was found that sulfur atoms occupy the phosphorous vacancies instead of bonding with phosphorous atoms. Lee et al. [443], Stringfellow et al. [444], Shurtleff et al. [445], Jun et al. [446], Fetzer et al. [447], and Lee et al. [448] developed a novel method of surface reconstruction by adding surfactants during the process of crystal growth of InGaP by metal-organic vapor-phase epitaxy. Surfactant is a substance that accumulates at the surface and alters the surface properties of the material. The surfactant can modify the bonding at the surface, which results in changes in the surface energy. The Fermi-level position in the bandgap can also change by surfactants. Dopants such as Sb, As, and Te modify the surfaces. Figure 67 shows the PL spectrum of InGaP material that has both disordered and ordered regions. The bandgap energy was changed by adding Sb during the growth process. This is a very good method of producing quantum wells and heterostructures. This novel technique may be coined "bandgap engineering by surface reconstruction." 4.9.4. InA1As
N---- 1la~
=~
Fig. 65. Band diagram of the simplified heterojunction for the diffusion model cal,;ulations; B is the dislocated region with increased recombination. Reprinted with permission from M. Mullenborn et al., J. Appl. Phys. 75, 2418 (1994). C~)pyright 1994 by The American Institute of Physics. 1o .2
I
"
I
'
Benyattou et al. [449, 450] identified a PL peak in InP/InA1As/InP interfaces, which they ascribed to interface recombination. In this type I interface the lineup of the two semiconductor bandgaps has a staggered shape. The band bending at the interface depends on the doping type and concentration of the two semiconductors involved. In this case
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t..
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e4
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.6 meV 0.2
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1800
1850
1900 1950 Energy (rneV)
2000
2050
Bias V o l t a g e (V)
Fig. 66. Typical forward current-voltage characteristics of the GaAs and InGaP Schottky diodes for various chemical surface treatments. Reprinted with permission from C. T. Lee et al., Solid State Electron. 41, 1 (1997). Copyright 1997 by Elsevier Science.
Fig. 67. The 20 K PL spectrum for a GalnP disorder-on-order heterostructure grown by modulating the TESb flow during growth. Reprinted with permission from J. K. Shurtleff, et al., Appl. Phys. Lett. 75, 1914 (1999). Copyright 1999 by The American Institute of Physics.
250
KASI VISWANATH
quantum wells have triangular shapes, one for the electrons in the semiconductor having the lowest conduction band edge and one in the other material for holes. In this situation, when charges are injected, electrons and holes are separated at the interface. Therefore electron-hole recombination occurs through the interface. Hwang et al. [451], Chou et al. [452], and Chang et al. [453] utilized the photoreflectance technique to study surface states in InA1As; the InA1As material has many useful applications in high-speed electronics.
The most effective pass!vat!on was demonstrated by using 0.1-/xm thick A10.iaGa0.86As layer at the interfacesl A 16-fold increase in the effective lifetime has been observed in this approach. In another report, improved surface properties were observed in Al-rich AlxGa~_xAs multilayer structures [457]. Pass!vat!on by sulfide [458] and hydrogen [459-461] has enabled different groups to obtain good-quality surfaces and interfaces of A1GaAs. 4.9.7. A1GaN
4.9.5. InAsP
Only one report has been made on the surface-recombination velocity in A1GaN and that was by Boroditsky et al. [412]. They have given a value of S - 3 • 104 cm/s from the photoluminescence quantum-efficiency measurements.
Pikal et al. [454] studied the temperature dependence of recombination coefficients in InAsP/InP quantum-well semiconductor lasers. These lasers emit at 1.3 /xm and this long wavelength light is very useful for optical communications. Recombination coefficients were obtained from carrierlifetime measurements. In their analysis, the carrier population in the barrier was taken into account, and the results are shown in Figure 68. The frequently observed temperature insensitivity of the Auger coefficient in the InAsP/InP quantum-well laser was thought to be due to carriers spilling out of the quantum well at higher temperatures.
4.9.8. GaAsP
4.9. 6. A1GaAs
4.10. III-V Quarternary Semiconductors
Henry et al. [455] have shown that the 2kT current in A1GaAs p-n junction was due to the surface recombination. Measurements of surface-recombination velocities in AlxGal_xAs/AlyGa]_yAs interfaces was reported by Timmons et al. [456] based on time-resolved photoluminescence experiments. Different pass!vat!on methods were attempted.
4.10.1. InGaAIP
~-,
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.
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!
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.
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--
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Heterostructures of In05(Gal_xAlx)0. 5 P/In05Ga0.sP lattice matched to GaAs are the semiconductor laser materials that emit in the red. Boroditsky et al. [412] have measured surfacerecombination velocity in the InGaA1P/InA1P quantum well. The structure is shown in Figure 69; S was estimated as 105 cm/s from the luminescence efficiency experiments; PL signals were also recorded after thinning down the cap layer. Initially, the intensity increased and when the cap layer is completely removed the PL intensity suddenly decreased because of the dominant nonradiative recombination at the exposed surfaces. This is shown in Figure 70. Hydrogen-pass!vat!on effects in InGaA1P/InGaP quantum wells were reported by Gorbylev et al. [440].
- - ' - - - Bulk BEFF
8
v
'
The reduction in interface recombination with increasing phosphorous content was observed in GaAs]_xP x epitaxial layers that were grown by MOVPE [462]. Time-resolved spectroscopy was used to estimate the recombination velocities.
"
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=
-=
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. =
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40
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Temperature (*C) Fig. 68. Temperature dependence of the extracted recombination coefficients of the deep well InAsP/InP laser using both the bulk and QW analysis. Reprinted with permission from J. M. Pikal et al., Appl. Phys. Lett. 76, 2659 (2000). Copyright 2000 by The American Institute of Physics.
The InGaAsP/InP quantum wells have applications in the realization of several optoelectronic devicesmedge or surfaceemitting long-wavelength semiconductor lasers, light-emitting diodes, photodiodes, and solar cells. These materials have emission in the range 1.1 to 1.6 /xm. This spectral range is very much suitable for fiber-optic communication systems because optical fibers have the lowest attenuation and dispersion in this range. The InGaAsP is also considered as a very good candidate for metal-insulator semiconductor field-effect transistors (MISFETs). Dow and Allen [463] have theoretically shown that the self-reproducing dangling bond degrades the shelf life of the InGaAsP-based lasers. The schematic of this mechanism is shown in Figure 71.
SURFACE AND INTERFACIAL RECOMBINATION IN SEMICONDUCTORS E
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251
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(b) Fig. 69. (~l) The InGaA1P sample consists of 0.75-/xm thick In0.5(Gao,2,kl0.0a)0.sP (A = 630 nm) active region doped at the n = 10 ~7 cm -3 le,~,e sandwiched between n-type InA1P cladding layers grown on absorbing (3aAs substrate; (b) when the top InA1P cladding layer is etched away, the nonradiative surface recombination on the exposed surface of the active region becomes the dominant recombination process. Reprinted with perm}~sion from M. Boroditsky et al., J. Appl. Phys. 87, 3497 (2000). Copyright 2000 by The American Institute of Physics.
Mozer et al. [464] examined the degradation of InGaAsP/InP lasers due to nonradiative and interface defectrelated recombinations. Rajesh et al. [465] have done the sulfur passivation of the InGaAsP (100) grown on lattice-matched InP substrate and studied its surface chemistry, as well as the relative movements of the surface Fermi level. Etch Depth (nm) 0
100
200
300
400
500
600
0.6 0
0.5
0
= 0.4 ,-; < 0.3 .a . 0.2
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,
9
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150 200 Etch Time (s)
lOO
-
,
-
250
r
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Fig. 70. Dependence of PL signal from the InGaA1P sample on the etch depth. The experimental data are shown with open circles. The signal increases as the cap layer becomes thinner, and drops when the active layer becomes exposed. The solid line represents the fit obtained from solving the diffusion equation. Reprinted with permission from M. Boroditsky et al., J. Appl. Phys. 87, 3497 (2000). Copyright 2000 by The American Institute of Physics.
Fig. 71. Schematic illustration of how the self-reproducing dangling bond mechanism of laser degradation might occur. Transport (single arrows) from a moderately shallow level to a deep nonradiative "dangling bond" center might occur, after which the capture and nonradiative recombination events (jagged arrow) eventually lead to the breaking of another bond (double arrow), producing a daughter vacancylike dangling bond. Reprinted with permission from J. D. Dow and R. E. Allen, Appl. Phys. Lett. 41,672 (1982). Copyright 1982 by The American Institute of Physics.
4.10.3. AlGaAsSb Quartenary compounds of AlxGal_xASySb~_y are used as cladding layers in the fabrication of GaSb-based doubleheterostructure semiconductor lasers that emit at 2-/xm wavelength. Polyakov et al. [466] have reported the effect of ( N H 4 ) 2 S t r e a t m e n t on the surface properties of these materials.
5. II-VI SEMICONDUCTORS 5.1. CdS Huppert et al. [467] and Rosenwaks et al. [468-470] studied the effect of metal reactivity on the surface-recombination velocity of CdS and CdSe, which were deposited with various metals. The surface-recombination velocity (SRV) decreased if the metal is very reactive, such as A1, Ti and Zn. The SRV increased if the metal is unreactive, such as Cu, Au, and so forth. The results were understood considering metal-induced recombination centers at the semiconductor interface. Many papers have been published on the surface states in semiconductor quantum dots doped in glasses. These materials have large optical nonlinearities [471-474] which were observed for the first time by Jain and Lind [471]. Because of the large nonlinear optical properties they find applications in several optical and optoelectronic devices such as saturable absorbers that are useful in the generation of pico- and femtosecond laser pulses [475, 476], optical phase conjugation [471], pulse shaping [477], optical waveguides [478, 479], ultrafast photonic bistable devices [480, 481], and harmonic generation [482]. Higher nonlinearity of these materials is due to the quantum confinement in three dimensions. Most of the optical results in these materials were explained by various workers considering shallow and deep surface traps.
252
KASI VISWANATH 0 0.4
!00 ,,
200
300
400
500
600
700
800
900
I000
0.3
O.2 o 0.! 0.0-
m m
. II 9
9 ' " ' " " 9 9 ,1 I
9
. . . . . . .
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9
Fluency per pulse, mJ/cm2 Fig. 72. Transmission changes measured at the 700-nm wavelength vs irradiation of the layer by 347-nm pump beam. Each point corresponds to 100 shot accumulation. Reprinted with permission from S. Juodkazis et al., Opt. Commun. 148, 242 (1998). Copyright 1998 by Elsevier Science.
Juodkazis et al. [483] studied photodarkening in CdS quantum dots doped in (Si0.2Yi0.8)O 2 sol-gel waveguides. Photodarkening is an optical phenomenon that is nothing but the photoinduced decrease of transmission. To understand the photodarkening (shown in Fig. 72) picosecond pump-probe experiments were done. Transmission-bleaching experimental results are shown in Figure 73. Very fast decay was observed when the laser power was 4 mJ/cm 2, which produced photodarkening in these samples and attributed to deep traps. Vanagas et al. [484] reported laser-induced grating in CdS-doped sol-gel glass and suggested a three-level model that includes surface- and defect-related trap levels to explain the observed biexponential decay (Fig. 74). Many groups have reported the double-exponential decays in their optical measurements [485-490]. Jursenas et al. [491, 492, 494, 495] and Zukauskas and Jursenas [493] measured carrier lifetimes in CdS microcrystallites doped in glasses by picosecond time-resolved spectroscopy [491-495]. The results were interpreted by considering surface recombination of nonequilibrium electronhole plasma. The crystallite size was varied from 1000 to 5 4 / k . The decrease in carrier lifetime with decrease in quantum dot size was observed as shown in Figure 75. This was thought to be due to increase in the surface-to-volume ratio, which enhances the surface recombination. As the quantumdot size decreases, the density of surface states that participate in nonradiative capture increases [496]. Surface states arise due to dangling bonds [497] and self-trapped states arise due to lattice deformation at the interface [498]. The ultrafast capture is due to multiphonon emission [499]. Chestnoy et al. [500] examined the multiphonon processes in nanocrystals, but they did not study the participation of surface states in these processes. Zukauskas and Jursenas [493], and Jursenas et al. [494, 495] reported the surface recombination due to multiphonon emission. Burda and E1-Sayed [501] and Logunov et al. [502] made very elegant studies on surface states in CdS semiconductor
Ii 0
'".... n m, .lm.., 250
500
750
!000
o ll 1250
1500
1750
2000
Probe delay, ps Fig. 73. Transmission-bleaching change dependence on the probe beam delay (probe beam wavelength: 700 nm). The different curves are registered at different fluences of pump beam (wavelength: 347 nm: 400 short irradiation): (a) 0.5 mJ/cm2 per pulse: (b) 0.8 mJ/cm2; (c) 1.5 mJ/cm2; (d) 0.8 mJ/cm2 after measurements at 1.5 mJ/cm2; (e) 4 mJ/cm 2. Dotted lines are the best fit according to the decay by linear and Auger recombination, dashed and solid lines are the fit by linear and bimolecular recombination. Reprinted with permission from S. Juodkazis et al., Opt. Commun. 148, 242 (1998). Copyright 1988 by Elsevier Science.
nanoparticles by using very high-resolution femtosecond transient absorption experiments. Figure 76a shows the absorption and emission spectra for passivated and unpassivated CdS nanoparticles, whereas Figure 76b is the femtosecond transient absorption for the same samples. The unpassivated samples show the signals due to deep trap states. After passivation the trap-related peaks do not appear. Passivation by O H - and ZnS was known to increase the quantum efficiency of CdS quantum dots [503-505].
I
-~
//Z
n3 Tz t
Fig. 74. Schemeof a three-level system T21,T22 , T23 are the relaxation times associated with the different transitions; n2, and n3 are the populations of level 2 and 3, respectively, and AE is the energy difference between level 2 and 3. Reprinted with permission from E. Vanagas et al., J. Appl. Phys. 81, 3586 (1997). Copyright 1997 by The American Institute of Physics.
SURFACE AND INTERFACIAL RECOMBINATION IN SEMICONDUCTORS L
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~=25oA
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0.12,
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0.04
~ 10~
13o
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0
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,
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550
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OELA Y T;~E Cps) Fig. 75. *l'emporal evolution of the carrier effective temperature for fi = 250 A (A) and of the spectrally integrated luminescence intensity (B) of the CdS micro,:rystallites of various sizes. The intensity scale is arbitrarily shifted for each dependence. Reprinted with permission from S. Jursenas et al., Solid State Com,~un. 87, 577 (1993). Copyright 1993 by Elsevier Science.
By u,,~ing picosecond time-resolved fluorescence and femtosecond transient absorption experiments, Logunov et al. [502] examined the effects of electron acceptors adsorbed on the surface of CdS on the surface recombination. The electron transfer to viologen acceptors was shown to be very efficient and competes with surface-trapping and electron-hole recombination processes. Therefore bandgap emission and deep-trap emissiov, were quenched. These results were consistent with the report by Klimov et al. [506]. Murpl~y and Ellis [507, 512], and Zhang and Ellis [508] along w th Meyer et al. [509, 511], Lisensky et al. [510] and Murphy et al. [513] have done extensive work on the reduction of ,,urface-recombination velocity by treating CdS and CdSe surfaces with Lewis acids and Lewis bases. Very good correlation was observed between the surface-recombination rate and the electron-donating ability of the adsorbate. It was demonstrated that the chemical passivation of defect sites was much less important than the donation or withdrawl of electron density into the depletion region of the semiconductor. A similar approach was used to passivate the quantum dots of II-VI semiconductors [504, 514, 515]. Surface recombination was also reduced by treatments with (CzHs)sN, (CH3)3N,
Fig. 76. The ground-state absorption and steady-state emission of two different CdS NP samples in water. The unpassivated sample shows strong deeptrap emission, which is almost completely--but not perfectly--quenched in the case of the surface-passivated sample. The passivated sample emits mainly at the band-edge energy and weakly at a wavelength between the band-edge and deep-trap-emission wavelength. This indicates that surface passivation eliminates mainly the deep trapping sites; b) Femtosecond transient absorption spectra of CdS NPs 50 ps after laser excitation at 400 nm with 30 mJ/cm 2 to achieve high exciton density. The unpassivated CdS NPs show a much more intense absorption, which also takes place at higher energy than the absorption of the surface-passivated NPs. The decrease in transient absorption caused by the surface passivation suggests that the observed high-density absorption originates from surface-trapped charge carriers. Reprinted with permission from C. Burda and M. A. E1-Sayed, Pure Appl. Chem. 72, 165 (2000). Copyright 2000 by International Union of Pure and Applied Chemistry.
1,4-diazabicyclo [2.2.2] octane, and alkanethiols [516]. Lewis [517] at Cal Tech. made a thorough study of the surface and interface recombination in several semiconductor/liquid interfaces by time-resolved spectroscopy. These systems have very important applications in the development of photoelectrochemical cells. Optically detected magnetic resonance (ODMR) measurements were made on CdS nanoparticles doped in phosphate glass [518], and trapping sites at the surface and core were identified. The sites at the surface were found to have weak e-h exchange interaction and anisotropic broadening due to variable e-h distances around the circumference of the nanoparticle. By contrast, the sites inside the core have shown very strong e-h exchange interaction. These materials
254
KASI VISWANATH
have applications as laser materials [519]. Zhang et al. [520] have reported interfacial electron-hole recombination in aqueous CdS colloids that are useful in electrooptics, photocatalysis, and solar-energy conversion [521-525]. 5.2. C d S e Burda et al. [526] reported the femtosecond pump-probe spectroscopy of CdSe colloidal nanoparticles. Transient absorption spectra for 4-nm diameter quantum dots for different delay times are shown in Figure 77. Changes in transient absorption were attributed to the state-filling effect and surface trapping of the exciton. Both shallow and deep traps were identified. These results were consistent with the earlier reports by Bawendi et al. [485] and Hunsche et al. [527]. In a new report, E1-Sayed's group has demonstrated electron-shuttling of the excited electron from the conduction to the valence band of CdSe nanoparticles through naphthaquinone on the surface of the nanoparticle [526b]. Figure 78 shows the time dependence of bleach and transient absorption. It was observed that the decay of the bleach and the transient absorption have the same lifetimes. It was concluded that the electron was shuttling from the conduction band to the valence band. Mechanisms for bleach recovery in CdSe and CdS with adsorbed electron acceptors on their surfaces were also studied (Fig. 79). Surface and interface recombination in CdSe quantum dots was passivated by coating a thin layer of CdS or ZnS
[528-532]. Murray et al. [533] developed the passivating methods for II-VI semiconductor quantum dots by chemical treatments. Passivation of surface states was done by ligating with various organic capping groups. Mattoussi et al. [534] demonstrated a very novel method of surface passivation of CdSe quantum dots by dispersing them in a block copolymer. The block copolymer is [(norbornenylmethoxy carbonyl) biphenyl-yl-tert-butylphenyl oxadiazole]ls0 [norbornene-z-ylCH20 (CH2)sP (oct)2110 abbreviated as [NBPBD]150 [NBP]~ 0. This polymer acts as a passivating layer and also as an electron-transporting layer in the light-emitting diode (LED) structure. Figure 80 shows the electroluminescence spectra for the LED device. Enhancement in the luminescence due to passivation by the block copolymer was achieved. Figure 81 shows the energy-level diagram for the device. When voltage is applied, the holes tunnel into the PPV layer where they are transported to the heterojunction. At the aluminum cathode, electrons are injected into the PBD matrix or the dots. The barrier to injection of electrons into PBD is around 1.7 eV. Electron injection into CdSe bare nanocrystals or ZnS overcoated nanocrystals is energetically favorable. Recombination due to alloy formation at the interface in CdSe/ZnSe quantum well has been reported [535]. Bessler-Podorowski et al. [536] measured the time-resolved photoluminescence of CdSe single crystals immersed in various aqueous solvents and estimated the surface-recombination
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700
750
Wavelength ~. /nm Fig. 77. Transient absorption spectra of CdSe NPs with an average diameter of 4 nm, pumped with 400-nm femtosecond-laser pulses with a laser power of 4 mJ/cm2. The delay times of the spectra are 200 fs up to 110 ps. The measured absorption changes can be explained by the state-filling effect and surface trapping of the exciton. The inset shows the corresponding kinetic traces of the bleach decay observed at 480 and 570 nm. The short lifetime (~'~) of the 570-nm transient reflects fast trapping of the charge carriers at the NP surface to the shallow trap states. The longer lifetimes of the bleach decay are assigned to the charge carrier trapping by deeper traps. Reprinted with permission from C. Burda et al., J. Phys. Chem. B 103, 10775 (1999). Copyright 1999 by The American Chemical Society.
255
SURFACE AND INTERFACIAL R E C O M B I N A T I O N IN S E M I C O N D U C T O R S
'
'
'
'
I
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= 530 nm
. . . . . . . . . . . . . .
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.
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.
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.
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~
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: ool
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,
,
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.
.
.
.
l
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450
t
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i
i
i
l
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~
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~"i 4 / +...1
-0.15
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+, = 3 o o ~
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_
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, 0
.
.
. . . . . . . 2 4 6 Time l pe . I . . .
.
600
, 8
10 J
.
650
700
WavelengthX / n m Fig. 78. i:ime dependence of the bleach spectrum (480-590 nm) of the CdSe NP and the transient absorption spectrum of the surface-adsorbed naphthaquinone (NQ) radi,:al anion (600-680 nm) in the CdSe-NQ combined system in colloidal solution (fs laser excitation at 530 nm). The fact that the decay of the bleach (bottom o!: the inset) and the transient absorption of the NQ radical anion (top of the inset) have the same lifetimes strongly suggests the electron-shuttling mechanisr for the excited electron from the conduction to the valence band of the NP through the naphthaquinone on the surface of the NE Reprinted with permissio]~ from C. Burda, T. C. Green, S. Link and M. A. E1-Sayed, J. Phys. Chem. B 103, 1783 (1999). Copyright 1999 by The American Chemical Society. C d S e ) ~ , r = 530 nm
C d S Z,,,,r = 400 nm I o.o-t
0.0.
1 ~, = 560nm
-0.2-
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!
!
|
t
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-
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bleach = 480 nm
9
+
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;
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9
._! 414 I Oo o ~ ~ 1 7 6 1 7 6 1 7 6 1 7 6 1 7 6 1 7 6 1 7 6~1177661177661177661 7 6 1 7 6 1 7 6 1 7 6
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o -0.4.
-
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%. =20Ors
o
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~o
30
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0 "
"c = 200 fs
m
"
10
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.
,
.
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9 CdSM~ %,,== 7 ps
;~ =300 fs ~ = Z 8 5 ps
2D
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.
,
,
,.ot
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.
,
,.
=
,
absorption Xt=` = 6 1 0 n m
0.8
0.8-
__0.6
"!
,~ 0.4
0.60.40.2-
O2
%= : ~ I s %~=3.15 ~ 0.0
0.0 .
0
,
I 1
,
i 2
,
i 3
J o
/
/ % , = 2 0 0 - 300 fs 9~ > lOOps | 1
,
I 2
.
1 3
~r~/~ Fig. 79. (;omparison of the decay kinetics of the CdSe and CdSe-quinone system (left) with CdS and CdS-MV 2+ (right) suggests that different mechanisms are responsible for the bleach recovery in the respective semiconductor NPs. In CdS, charge carrier trapping leads to the bleach recovery. In the absence of MV 2+ (circles), electron trapping is rate limiting, while after addition of MV 2+ (squares), the hole trapping becomes the rate-limiting process. The forward electron transfer of MV 2+ take place within 200-300 fs, and this creates a long-lived charge-separated state (bottom). After the addition of electron acceptors on the CdSe surface, however, fast electron-hole pair recombination accelerates the bleach recovery to about 3 ps as the electron is shuttled back to the CdSe NP valence band by the quinone. The decay of the radical anion absorption (bottom) matches the bleach recovery dynamics. Reprinted with permission from C. Burda, '['. C. Green, S. Link and M. A. E1-Sayed, J. Phys. Chem. B 103, 1783 (1999). Copyright 1999 by The American Chemical Society.
256
KASI VISWANATH Vacuum
200
(a) Bare AI
[ ~": t ~
V=13V V=12V [i - - . - V = l l V i ...... V = l O V
cathode
160 E
"TI
.
.
.
.
.
:: Z
.
Composite
. ~ 120 PPV HTL ITO a~ode
C
E
hv
--1 ILl X ,
o
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Sub'strate
8O
,-u
hv
--::%~ L
,
I
n
,
2.1eV
I 4eV
4r
I
AI
[
(b) ZnS overcoated
[ I~ - -
.., 3o
[ [ ......
t-" "-I
V=13V ; V=12V V=l lV V=lOV
.d L20
tr C
1o
ZnS
Fig. 81. Schematic band diagram for the bilayer devices with composite nanocrystals (bare and ZnS overcoated) embedded in a [NBPBD]~50 [NBP]I0 block copolymer, built on top of a PPV MTL. An additional barrier is involved in the device operation with ZnS overcoating. The energy levels for PPV are taken from the literature, whereas those for the nanocrystals are derived from the bulk values corrected to account for the confinement of the exciton (mostly electron) and the absorption data. The energy levels for ZnS are extrapolated from the bulk values. Reprinted with permission from H. Mattoussi et al., J. Appl. Phys. 86, 4390 (1999). Copyright 1999 by The American Institute of Physics.
///. \,,\
0
_.. ~ ---.~. "..
"" . ~ ...,~,~. .
0 ~- ,,. 400
ZnS
|
d
uo
PPV
i
40
~ . ~
V
~-._~
~
450
500
550
600
650
700
k (nm)
Fig. 80. The EL spectra of devices using block copolymer/quantum dot hybrid films containing 60% (vol) at increasing applied voltage: (a) bare dots and (b) ZnS overcoated dots. The relative intensities of the two films at the same applied voltages indicate almost an order of magnitude higher EL from the bare dots. The inset shows the configuration of the devices used in this study. Reprinted with permission from H. Mattoussi et al., J. Appl. Phys. 86, 4390 (1999). Copyright 1999 by The American Institute of Physics.
velocities. They have found that the surface-recombination velocity was greatly dependent on the ionic-solution composition and concentration. Comparison was also made with their earlier report on CdS. By correlating with Auger spectroscopy and atomic absorption spectroscopy results on the same samples, it was concluded that the ions were chemisorbed on the semiconductor surface. Solutions of 13 , Br 2, Ag +, Hg 2+, Se zwere used in these experiments. The solubility product of the solution was thought to be one of the important parameters that determines the chemisorption and hence the recombination velocity. Ellis et al. [537], M e y e r [538], Neu [539], and M o o r e et al. [540] demonstrated the applicability of CdSe and CdS surfaces in chemical sensing. Figure 82 shows the energy-level diagram that is useful for chemical sensor applications. Adsorption of foreign atoms on the surface effects both the depletion width and surface-recombination velocity. Band bending near the surface depends on the adsorbed atoms or molecules as shown in Figure 83. Figure 84 shows the changes in PL intensity when N H 3 and CH3BBr 2 were adsorbed on CdSe surface. The aforementioned authors have also shown that CdSe sur-
faces can be used to sense the o x y g e n as shown in Figure 85. Very important application of sensing various gases is in the M O C V D reactor. A simple design for PL-based sensing by using an optical fiber is shown in Figure 86.
5.3. C d S S e Seo et al. [541] reported the time-resolved differential transmittance of C d S S e - d o p e d color-glass filters. They identified shallow and deep traps in their samples. Semiconductorglass interface states were studied by optical experiments [542-544]. The interface states arise due to surface dangling bonds and impurities [545, 546]. The excited electrons can
a)
b)
c.
CB
j
Ev
VB
VB
Fig. 82. Idealized diagram relevant to chemical sensing of energy as a function of distance from the surface of an n-type semiconductor: (a) Semiconductor-gas interface (vertical line) before establishment of thermal equilibrium in the dark. Shown are the valence band (VB) and conduction band (CB) edges of the solid (separated by the bandgap energy), the Fermi level EF, and a hypothetical distribution of surface states; (b) establishment of thermal equilibrium in the dark. Electrons in the conduction band come from the bulk to fill the surface states up to the Fermi level (shaded portion of the hypothetical surface-state distribution). This charge separation establishes an electric field in the near-surface region of the solid, represented by the bending of the band edges. Reprinted with permission from A. B. Ellis et al., J. Chem. Edu. 74, 680 (1997). Copyright 1997 by the Division of Chemical Education, Inc., American Chemical Society.
257
S U R F A C E A N D I N T E R F A C I A L R E C O M B I N A T I O N IN S E M I C O N D U C T O R S "'*
"
"
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.
!
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.
Nitrogen
Oxygen:
~
OO
0.11 atm
4.J I
C
.J
0.Z0 arm
O.
1.00 ~--
.
Fig. 83. ,i'.hemical sensing based on modulation of the depletion width and PL intensJ~y by surface adduct formation. The center figure represents the band betiding present in the reference ambient, corresponding to a nonemissive zon,~ or dead layer of thickness D 0. Flanking this picture to the left and right ~lre the thicker and thinner dead layers resulting from adsorption of Lewis ac ic:~ and bases, respectively, from the ambient. Reprinted with permission froH A. B. Ellis et al., J. Chem. Edu. 74, 680 (1997). Copyright 1997 by the Di~ ision of Chemical Education, Inc., American Chemical Society.
.
.
.
.
I 0
rain.
-~
L . . . . . .
1
.
.
.
.
.
l
4., m r @ (b) CH3BBr 2
. =C, , --I
a.
/
! 10 mtn.
.
.
.
.
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. . . . . .
1
.
.
.
.
Time
(a)
{"
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3
(b)
-
{
.o-, ~
0.4 i
be deeply trapped into these point defects and will recombine radiatively. Long lifetimes observed in photoluminescence ~ ere attributed to these deep levels [500, 545, 546]. Seo et Etl. [541] have shown that electron trapping at the microcr},stal-glass interfaces occurs within less than 1 ps after photoex~:itation. From the analysis of photoinduced absorp-
arm
1 K = 1 3 •
=
0
=
=
.2
9
=
.4
,A
.
.
.6
.
.
~
P(02), atm
Itm
-1
.
1
0
20
I0
-1
-1
P ( 0 2 ) , arm
Fig. 85. Upper panel : changes in the PL intensity of a Co(3-MeO-salen)coated n-CdSe sample as a function of oxygen partial pressure; the reference level is nitrogen. Lower panels: left, fractional surface coverage 0 vs dioxygen partial pressure; right, double-reciprocal plot of the same data; the linearity of the plot (correlation coefficient of 99) represents a good fit to the Langmuir adsorption isotherm model, and the reciprocal of the slope yields an equilibrium constant K of 13 -t- 1 atm -~. Reprinted with permission from D. E. Moore, G. C. Lisensky, and A. B. Ellis, J. Am. Chem. Soc. 116, 9487 (1994). Copyright 1994 by The American Chemical Society.
tion, the following conclusions were made. Shallow traps give rise to short-lived photoinduced absorption with a lifetime of 60 ps, and the deep trap levels give rise to long-lived photoinduced absorption with a lifetime longer than 3.2 ns. These results were consistent with earlier published work [490, 547, 549]. Ivanda et al. [550] have measured photoluminescence and Raman spectra of deep traps in a number of SCHOTT glasses--GG400, GG420, GG435, GG455, GG475, GG495, OG515, OG530, OG550, OG570, OG590, and RG695. Here, the numbers indicate the cutoff wavelengths of different glasses. The filter glasses consist of CdSxSel_ x nanocrystal-
Time
Fig. 84. l a) Changes in PL intensity of n-CdSe resulting from alternating exposur~ 1,~ N 2 (initial response) and decreasing partial pressures of ammonia (ranging ~!'om 0.1 atm for the first trio of exposures to 0.001 atm at the end of t!]~', sequence). (Adapted from Reference [538]); (b) Changes in PL intensity ~:f n-CdSe resulting from alternating exposure to vacuum (initial response) itnd increasing pressures of CH3BBr 2 (ranging from = 3 x 10-4 atm for the l ir~t exposure to = 4 x 10 -2 atm, the last exposure). The maximum change in dead-layer thickness with exposure to the borane is 350 /k. In both experiments, PL was monitored at the PL band maximum, = 720 nm. Reprinted with permission from D. R. Neu, J. A. Olson, and A. B. Ellis, J. Phys. Chem. 97, 5713 (1993). Copyright 1993 by The American Chemical Society.
h~ --~
--~ h~ /
Semico
Fig. 86. A simple design for PL-based sensing by using a bifurcated optical fiber. Reprinted with permission from A. B. Ellis et al., J. Chem. Edu. 74, 680 (1997). Copyright 1997 by the Division of Chemical Education, Inc., American Chemical Society.
258
KASI VISWANATH
lites with diameters of a few nanometers doped in a glass matrix. Nemec and Maly [551] have published a very good paper on the temperature dependence of trap-related luminescence dynamics in CdSxSel_ x nanocrystals in glass. They attributed the luminescence due to recombination of an electron in a shallow trap with a hole in the deep trap. Carrier tunneling between localized sites was also suggested. There have been many studies on the localized carriers in glasses by nonlinear optical experiments [552-557]. 5.4. ZnS
Chen et al. [558] have identified surface states in their PL and thermoluminescence spectra of Zns quantum dots in colloidal state. 5.5. ZnSe
Jonker et al. [559] have shown that a Se overlayer on ZnSe passivates the surface. Pong et al. [560] have obtained good-quality ZnSe epilayers by hydrogenation that removed many surface defects. Wang et al. [561] reported surfacerecombination velocity of S= 5.8 x 105 cm/s for ZnSe crystals based on femtosecond time-resolved one- and two-photon photoluminescence, von Freymann et al. [562] have correlated the photoluminescence in ZnSe/ZnMgSSe quantum wells with the atomic force microscopy images, and bowtie-shaped defects on the surface were spectroscopically imaged. 5.6. ZnCdSe
The II-VI quantum-well structures are very much suitable for the realization of blue, green semiconductor lasers and light-emitting diodes [563-568]. In these materials roomtemperature excitons were observed. Because of their large binding energy excitons exist up to room temperature. Haase et al. [563] were the first to achieve an electrically pumped Zn0.8Cd0.2Se single-quantum well laser with ZnSe waveguides. The cladding layers were composed of ZnSe0.97S0.03, which was lattice matched to GaAs substrate. The II-V semiconductor lasers were also made by Grillo et al. [565]. Their laser structure also was based on the ZnCdSe quantum well with ZnSSe waveguide layers and quarternary ZnMgSSe cladding layers. These lasers were operated at room temperature under pulsed lasing conditions. However, the operating lifetime of these lasers was only one hour. Sony Corporation could make cw room-temperature quantum-well blue emitting lasers using MgZnSeS cladding layers, which had an operating lifetime of only one second. The major problem with these lasers was the nonradiative recombination due to surface and interface defects [569-571]. The degradation mechanism in these lasers has been studied by Hua et al. [570] and Guha et al. [569, 571 ]. Hovinen et al. [572] have performed very novel experiments to simulate the electrical degradation by optically injecting electron-hole pairs to induce dark defects directly in the ZnCdSe quantum well. Dark defects are the nonradiative
regions in the quantum well of the separate-confinement heterostructure (SCH). Correlation between optical experiments and transmission electron microscopy (TEM) was also shown. The stacking-fault pairs originate at or near the II-VI/III-V interface and extend through the quantum-well layer with a V-shaped cross section. In the degraded area, each stacking fault was associated with a dislocation network. By examining the dependence of degradation rate on optical injection, it was demonstrated that the fraction of excess carriers that undergo nonradiative recombination processes at point-defect locations was of great importance in determining the degradation rate. The mechanism for degradation was given as follows: The energy released in nonradiative recombination processes at defects is transferred to the lattice by multiphonon process. This nonthermal heating provides a local power supply that enhances dislocation dynamics. Spiegel et al. [573] reported the surface-recombination velocity in ZnCdSe/ZnSe and ZnSe/ZnSeS quantum wires. They studied the recombination dynamics by picosecond time-resolved photoluminescence spectroscopy. In the case of ZnSe/ZnSeS quantum wires the lifetime reduced from 130 to 10 ps as the size changed from two-dimensional reference to wires of 60 nm wide. The surface-recombination velocity for this system was 5 • 105 cm/s. By contrast, the lifetimes of ZnCdSe/ZnSe quantum wires did not show any dependence on the wire width as seen in Figure 87. The surface-recombination velocity in this case was 2 x 104 cm/s. It was concluded that the surface-recombination velocity in ZnCdSe/ZnSe quantum wires was reduced by two orders of magnitude compared to ZnSe/ZnSeS quantum wires. Exciton localization at semiconductor interfaces was thought to be the reason for the reduction in the surface-recombination velocity in ZnCdSe/ZnSe quantum wells. Reactive ion etching of ZnCdSe/ZnSe quantum wells was shown to have created nonradiative recombination centers by a study conducted by Sparing et al. [574, 575]. Later, the same group reported the origin for the degradation in the
10z
L-J
T=2K A Cdo.],~Zno.enSe/ZnSe ZnSe//ZnSeo.aeSo 14 101 ,
1(i I
,
~
I
i
i
Li[
.
,
,
I
,
10 2
, ,,I
i
I
10 3
wire width Lx [nrn]
Fig. 87. Wire-width dependence at T = 2 K of the exciton lifetime in deepetched ZnSe/Zn(Se, S) wires (stars) compared to a theoretical fit (solid line). Compared the T = 2 K data of (Zn, Cd)Se/ZnSe quantum wires are shown (triangles). Reprinted with permission from R. Spiegel et al., Phys. Rev. B 53, R4233 (1996). Copyright 1996 by The American Physical Society.
S U R F A C E A N D I N T E R F A C I A L R E C O M B I N A T I O N IN S E M I C O N D U C T O R S
1.0
~ _ ~ .
Data ......... Fit A. Unetched
0.8
or
"~ =.. r "-'
n,
24op~
B. Etched 3 rnin @ 75V
0.6.
Decay:. 240ps
_
\
"~.
c. E,=.~ a . , . o 2oov
0.4
0.2
"~""
~"*~%
9
A
0.0 0
Delay Time
(ps)
Fig. 88. {'lot of transient reflectivity data for an undamaged and two damaged Zn,,:,Cd0.25Se/ZnSe SQW samples with 50-nm cap layers9 The decay rate for th,:. 75 V samples and the undamaged sample are very similar9 There is a dram~tic decrease in the carrier lifetime for the 200-V sample (70 ps) indicating :lonradiative recombination within the QW. Reprinted with permission from L. M. Sparing et al., J. Appl. Phys. 87, 3063 (2000). Copyright 2000 by "Ihe American Institute of Physics.
same qtantum well by a combined study of photoluminescence and time-resolved reflectivity [576]. Figure 88 shows the transient reflectivity spectra for Zn0.vsCd0.z5Se/ZnSe singlequantum wells under different conditions. It was shown that etching .~tthigher voltages damages the surface and increases the surf~.ce or nonradiative recombination. Dislocations at the surface/interface were thought to be the origin of nonradiative recombi~mtion. Faschinger and Nurnberger [577] realized ZnCdSe quantum-well lasers grown on InP substrates that have a very h)iig lifetime. In these structures n-ZnMgCdSe and p-ZnMgSeTe were used as waveguide layers. Dark defects were no1: found in these diodes. The addition of Te was supposed to improve the performance of the laser. 5.7. Cd'l?e In the ,,;emiconductor industry, CdTe is a very important semiconductor because it has many applications. High-quality CdTe crystals are useful as substrates to grow epitaxial HgCdq-e, which is a well-known infrared detector material; CdTe is ~tlso useful in making X-ray and y-ray detectors, optical anct acousto-optic modultors, infrared windows, and so forth. ()ae major problem associated with CdTe is the high density ~f surface states. Several groups have tried to passivate CdTe surfaces by chemical treatments [578-583]. Sobiesierski et al. [578] have used the following chemicals: Bromine/methanol (Br/Me); (Br/Me) + KOH; (Br/Me) + hydrazine; (Br/Me) + Na2S204; ( B r / M e ) -,: K z C r 2 0 7 ; oxidizing etch followed by reducing etch. Amritharaj and Dhar [579] used the following etchants: Br2/ CH3OH; KOH/CH3OH; NazSzO4/NaOH, KzCrzOv/HNO3. Cohen et al. [582] used organic molecules to achieve high electronic-quality surfaces. These molecules were chemisorbed on the semiconductor surface as monolayers; (NH4)2S = treatment also has given good photoluminescence
259
of p-CdTe [583]; additionally, H 2 plasma [584] and atomic hydrogen [585] were found to be good passivating agents. Dharmadasa et al. [586] studied the effect of surface treatment on the Schottky barrier at metal/n-CdTe. Chou et al. [587] and Chou and Rohatgi [588] discussed the interfacial recombination in CdTe/CdS heterojunction solar cells. Illing et al. [589] reported the surface recombination at the sidewalls of CdTe/Cd~_xMgxTequantum wires for wire diameters ranging from 60 nm to 5 /xm that were fabricated by electron-beam lithography and wet-chemical etching. Picosecond time-resolved experiments were done on wires of various diameters. Kronik et al. [590] discussed the possibility of using surface photovoltage spectroscopy to study the surface states in CdTe. Cohen et al. [591] reported an unusually low surface-recombination velocity of 200 cm/s for n-type CdTe single crystals from their analysis of time-resolved photoluminescence experiments. Delgadillo et al. [592], Vargas and Miranda [593] and Bernal-Alvarada et al. [594] used the photoacoustic technique to estimate the surface-recombination velocity in CdTe [592-594]. 5.8. HdCdTe
The Hg~_xCdxTe alloy is a very important material because it has applications in infrared detectors. The HdCdTe is a narrow bandgap material, and a small density of defects or impurities can easily form a band that will become a band tail. Relatively low defect concentrations can modify the absorption edge in HdCdTe. Hydrogen passivation [595] and anodic oxidation [596] has been reported in the literature. Epitaxially grown CdTe passivating layers have given very good-quality HgCdTe photodiodes [597]; ZnS/CdTe passivation was reported by Bahir et al. [598]. The effect of hydrogenation on ZnS passivating films was examined by White et al. [599]. Borodovskii et al. [600] performed magnetoresistance measurements and estimated the surface-recombination velocity in n-CdxHgl_xTe epitaxial layers. Su and Lin [601] reported a very low surfacerecombination velocity of 300 cm/s for Hg0.sCd0.zTe surfaces. They also observed that multiple passivating layers were more effective than a single passivating layer. They have used CdTe and ZnS for the surface passivation. Voitsektnovskii et al. [602] reported the carrier lifetimes in narrow bandgap Hg]_xCdxTe epilayers that were grown by molecular beam epitaxy [602]. Excitation was done by a pulsed laser at various wavelengths. It was shown that in p-type layers the lifetime was determined by Shockley-ReadHall recombination, whereas in n-type layers Auger recombination was dominant with some minor contributions coming from other mechanisms such as surface recombination. 5.9. CdZnTe Because its lattice constant can be made to match that of the mercury cadmium telluride or HgZnTe, Cdl_xZnxTe is a very good substrate material for HdCdTe. Chen et al. [603] have passivated the deep-level defects in CdZnTe by hydrogen.
260
KASI VISWANATH
Improvement in the photoluminescence was noted. Free exciton as well as the higher orders of free exciton transitions were observed for the first time. Exciton binding energy was also estimated. Kim et al. [604, 605] used bromine solution and hydrogen for passivation, and Chen et al. [606] improved the surface quality of Cd0.9Zn0.1Te crystals by bromine treatment, which has shown very good photoluminescence properties.
5.10. HgZnTe Oh et al. [607] measured the minority carrier lifetime in n-type HgZnTe by photoconductive method. A correlation was made between the surface-passivation technique and carrier lifetime. Shockley-Read-Hall recombination as well as Auger recombination were found to contribute to the carrier decay. Surfacerecombination velocities were also estimated.
5.11. PbSe Lead selenide is a narrow bandgap semiconductor. This has applications in the fabrication of infrared semiconductor lasers and also infrared photodetectors. Because it is a low bandgap material, surface purity is very important exactly the same as in the case of CdTe. Meinke et al. [608] achieved passivation of these materials by anodic oxidation.
6. GROUP IV SEMICONDUCTORS 6.1. Si Single Crystals and Wafers Silicon is the most important of all the semiconductors because the semiconductor industry at present is mostly dependent on silicon technology. The density of integrated circuits has been ever increasing in the last twenty years and very large-scale integration (VLSI) technology has been realized. In the near future we will have ultralarge-scale integration (ULSI) technology. In this case the size of the semiconductor will have quantum limits. When the semiconductor device is very small surface recombination becomes extremely important. Therefore, the Si surface must be very clean and free from any contamination. Linnros [609, 610] has reported the carrier lifetimes and surface-recombination velocities in silicon wafers. An optical pump-probe method was used and free-carrier absorption transients were analyzed. Schmidt and Aberle [611] described an easy method of surface passivation and monitoring of surface-recombination velocities for p- and n-type silicon wafers. Keskitalo et al. [612] measured the Shockley-ReadHall lifetime in p-type silicon. They have also examined the effect of temperature and injection level on the lifetimes. A novel photoluminescence surface-state spectroscopy for the measurement of surface-recombination velocity in silicon wafers was attempted by Saitoh et al. [613]. A multicolor method for the determination of surface and bulk recombination in Si was discussed by Ostendorf and Endros [614]. In another paper, surface-recombination velocity in Si crystals was estimated by photoluminescence time-resolved spectroscopy [615]. Eichinger and Rommel [616] used a two-color
technique to identify metals on a Si surface. Two diode lasers with different absorption depths in silicon were used to estimate the surface-recombination velocities. The electrolytic metal analysis tool (ELYMAT) technique makes it possible to identify trace metals and oxides on the semiconductor surface. The surface-photovoltage technique was reported by Brubaker et al. [617] who measured surface-recombination lifetimes in silicon, and the recombination centers were identified as interfacial bonding defects and iron contamination. A pump-probe optical method was used by an Italian group to study bulk and surface recombination lifetimes [618]. The dependence of S on excitation light intensity was theoretically shown [619]. It was also shown that shifting the surface Fermi level towards band edges reduces the S value. Photoconductive decay in Si solar cell wafers was numerically simulated by Wang and Ciszek [620]. Bi-surface photoconductive decay measurements were useful in the evaluation of S [621]. Schieck and Kunst [622] conducted microwave photoconductivity experiments that enabled them to find the injection-level dependent surface/interface-recombination velocity. Yablonovitch et al. [623] reported a very low surface-recombination velocity in silicon. A very large number of papers have been published on the passivation of silicon dangling bonds by a variety of treatments such as hydrogen [624-628], hydrogen fluoride [629-636], ammonium fluoride [637-645], silicon nitride films [646-655], SiO 2 films [656, 657], halogens [658-660], silane/helium [661], cyanide [662], sulfur [663], C60 monolayer [664, 665], copper [666], contact bonding [667], rare earth oxides [668], low-temperature oxidation prior to UV radiation [669], and laser annealing [670]. The effect of metallic impurities on surface-recombination velocity and carrier lifetimes has been reported by a number of research groups [671-679]. Tajima and Ibuka [680] and Tajima et al. [681-683] have shown the sensitivity of photoluminescence to the microdefects and interfacial quality of silicon-on-insulator (SOI) wafers [680-683]. The SOI structure is important for the development of low-power and high-speed integrated circuits. Also, SOI layers are used in the fabrication of complementary metal-oxide-semicondcutor (CMOS) devices. Ultrathin silicon-on-insulator layers were grown by the forementioned authors and they observed the luminescence due to electron-hole liquid (EHL) in these wafers. The phase diagram of electrons and holes in Si is shown in Figure 89, which was given originally by Shah et al. [684]. As exciton density increases, the excitons dissociate into plasma, which is described by Mott transition [685]. If the sample temperature is lowered, then the excitons condense to form electron-hole liquid. As can be seen in Figure 89 the coexistence of free excitons, electron-hole plasma and electron-hole liquid depends upon the sample temperature and excition density. The luminescence observed in SIMOX wafers was attributed to electron-hole liquid as shown in Figure 90. Interfacial microdefects were thought to be responsible for the luminescence.
SURFACE AND INTERFACIAL RECOMBINATION IN SEMICONDUCTORS
Fig. 89. ]'hasediagram of electrons and holes in Si: FE, EHP and EHL, are analogou..; :~ gas, plasma, and liquid phase, respectively. In the hatched region EHL coexists with FE or EHE This phase diagram was originally reported by Shah et at. i~eprintedwith permission from J. Shah, M. Combescot, and A. H. Dayem, t't~vs. Rev. Lett. 38, 1497 (1977). Copyright 1977 by The American Physical S,:ciety. Bedrc,~sian and de la Rubia [686] have observed smoothing of surfaces of Si(100), which were irradiated with 5-keV He ions at 130 K and then annealed at 160 K. Smoothing of Si surfaces ~,as interpreted as resulting from surface recombination of p~fint defects. The point defects were supposed to have been ger~erated when the semiconductor was irradiated below the surl'a.ze, which will migrate to the semiconductor surface at 110 K Smoothing was also found upon irradiation with Ar ions at 1 l0 K.
6.2. Si Solar Cells In a solar cell solar energy is converted into electricity. The developn~ent of solar cell materials and devices has been dominated by the following: Green at the University of New
Fig. 90. Two types of excitation intensity dependence of condensate emission from $IMOX wafers at 16 K under UV light excitation. The EHL emission appea~s at low excitation intensity in the first type (a), while evolution of EHP with ~xcitation intensity is recognized in the second type (b). Reprinted with permission from M. Tajima, S. Ibuka, and M. Warashina, Mat. Sci. Forum. 255-263, 1731 (1997). Copyright 1997 by Trans Tech Publications, Switzerland.
261
South Wales in Australia, Rohatgi at Georgia Tech, Ahrenkiel at NREL, Colarado, and Aberle and Hezel at the Institute for Solar Energy Forschung (ISFH) in Germany. Green has written several reviews and books that are very useful for beginners as well as experts [687-692]. Hezel's review covers the metal-insulator-semiconductor (MIS) solar cells [693]. A book on solar cells has been written by Aberle [694], currently at the University of New South Wales in Australia. In this section we will discuss only the latest developments in surface passivation and its effect on increasing the efficiency of solar cell. The most important factor of a solar cell is its conversion efficiency. The efficiency of a solar cell is limited by the surface and interfacial recombination in the semiconductor. Therefore, logically the efforts to improve the performance of the solar cell devices have been directed to reducing the surface-state density. Several groups around the world have reported on the surface recombination and surface passivation in silicon solar cells [695-715]. Aberle et al. [696] developed the passivated-emitter rear locally diffused (PERL) silicon solar cell shown in Figure 91. This solar cell has given 24% efficiency. In this structure, a S i O 2 layer was used on both sides of the p-silicon, which passivates front and rear surfaces. Inverted pyramids were designed to reduce reflection losses and increase the light trapping. Robinson et al. [695] have also done simulation of Shockley-Read-Hall recombination and its effect on I-V characteristics of the silicon solar cell [695]. Figure 92 shows the schematic of solar cell structure used for their theoretical work. Surface-recombination velocities below 10 cm/s were reported by Stephens et al. [714] by utilizing a thermally grown SiO 2 layer. By incorporating an A1 cap layer the surface-recombination velocity of 20-50 cm/s was observed at an injection level of 1014 c m -3 [715]. Silicon nitride was later found to be a very good passivating layer and record low surface-recombination velocities of 4 and 20 cm/s were observed for 1.5 and 0.7 ~ cm p-silicon wafers, respectively [649]. Dependence of S on the carrier concentration for a ptype Si wafer is shown in Figure 93. Bifacial silicon solar cells were later developed in which surface recombination was
Fig. 91. Passivatedemitter, rear locally diffused cell (PERL cell). Reprinted with permission from A. G. Aberle et al., Prog. Photov. 1, 133 (1993). Copyright 1993 by John Wiley & Sons.
262
KASI V I S W A N A T H
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reduced in both surfaces [650]. The schematic of this cell is shown in Figure 94. In a bifacial silicon solar cell sunlight that was incident on both front and rear surfaces will be converted into electricity. Therefore, the conversion efficiency will be high. In these structures silicon nitride layers were used as surface-passivating layers. A simplified energy-band diagram of a Si-SiN interface is shown in Figure 95. Karazhanov [716] has discussed the degradation mechanism in silicon solar cells. Carrier lifetimes and short-circuit current were shown to decrease with increasing illumination intensity.
Silicon nitride AI rear contact Fig. 94. Schematic representation of a bifacial p-n junction silicon solar cell developed at ISFH, Germany. Both surfaces of this cell are passivated by RPECVD silicon nitride. Reprint with permission from T. Lauinger et al., J. Vac. Sci. Technol. A 16, 530 (1998). Copyright 1998 by The American Vacuum Society.
hydrogenated-fluorinated amorphous silicon [717-720]. When a semiconductor is excited optically the generated excess carriers interact with phonons and dissipate their excess energy by three channelsmhot-carrier thermalization to the band edges, capture by midgap states, and nonradiative recombination. In crystalline semiconductors band-filling effects give rise to photoinduced bleaching. In amorphous semiconductors the relaxation of k-vector conservation enhances the absorption
6.3. A m o r p h o u s Si Insulator charges
Amorphous Si has important applications in many semiconductor devices such as solar cells. There are some differences between crystalline Si and amorphous Si in the optically induced processes. Ackley et al. [717], Vardeny and Tanc [718], and Vardeny et al. [719, 720] studied the picosecond photoinduced absorption due to the midgap states in nonhydrogenated, hydrogenated, fluorinated, and
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(< 2 nm) Fig. 95. Simplified energy-band diagram of the Si-SiN x interface (band bending not shown). Note that the positive insulator charge Q/consists of two components, namely a constant contribution from the Pox-like defects in the interfacial SiNxOy film and an operating condition dependent contribution from the charged K+ centers within a ~20-nm-wide section of the SiN x film. Reprinted with permission from A. G. Aberle, "Crystalline Silicon Solar Cells," Center for Photovoltaic Engineering, University of New South Wales, Sydney, Australia, 1999.
263
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cross se~:tion of excited carriers, which leads to photoinduced absorpti~Jn. Figures 96 and 97 show the photoinduced absorption of a-Si prepared under different conditions and at different temperat~res, respectively. The photoinduced absorption in a-Si was interpreted as due to midgap states9 Staebter and Wronski [721] were the first to observe the degradation in photoconductivity in a-Si after it was exposed to light ~or a long time. This effect has deleterious effects on solar cells made with a-Si. Light-induced defects in a-Si were als(~ found in electron-spin resonance [722], photoluminescence [723], transient photoconductivity [724], and transient grating spectroscopy [725]. Strait and Tauc [726] have performed photoinduced transient absorption experiments on a-Si:H, which was illuminated for different time periods. The spectra recorded at 300 and 150 K are shown in Figures 98 and 99. ~:espectively. The illumination of the semiconductor produces defects that act as deep traps. From the analysis of
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264
KASI VISWANATH
the results it was concluded that the defects were Si dangling bonds. Many groups have reported on the surface and interface states in hydrogenated a-Si [727-729]. Guha et al. [727] interpreted the light-induced changes in a-Si as due to recombination rather than carrier trapping. Jackson et al. [728] arrived at the silicon dangling bond density as 1012/cm 2 by comparing the defect absorption with dangling-bond spin densities. Lower interface state densities were observed in the a-Si layers that were treated with chemicals or oxygen-containing plasmas [730]. Tsuo et al. [731] fabricated p-i-n solar cells of a-Si and hydrogen plasma treatment has improved the efficiency of the solar cells. Atomic hydrogen from plasmas was found to be very effective in passivating the defects in a-Si [732]. In these studies Sill bonds were observed in the IR (infrared) spectra after hydrogen treatment. Jang and Lim [733] achieved improved performance of the solar cell in which hydrogentreated boron-doped hydrogenated amorphous silicon carbide (a-SiC:H) film was used as a p-type passivating layer. In this solar cell a p-i-n-type structure was used, which consists of amorphous silicon. Georgiev et al. [734] performed the numerical modeling on the effect of surface recombination in a-Si solar cells. Experimental and simulated results on the effect of defects in a-SiGe solar cells were reported by Zimmer et al. [735].
6.4. Si-based Quantum Wells and Superlattices Quantum wells and superlattices of Si/SiGe have possible applications in fabricating room-temperature light-emitting diodes and high-speed devices. Gnutzmann and Clausecker [736] were the first to propose the concept of zone folding in semiconductors. They have shown that the minimum of the conduction band of an indirect bandgap semiconductor can be folded back to the F point by creating artificial periodicity in the structure. After this proposal, several groups attempted to make quasi-direct bandgap structures based on Si. Arbert-Engels et al. [737] have grown SimGen superlattices by moleculer beam epitaxy. They passivated the superlattices with hydrogen, which enabled them to observe improved photoluminescence properties. St. Amour et al. [738] have shown that passivation can increase the photoluminescence intensity in Si~_xGex/Si heterostructure by an order of magnitude as shown in Figure 100. Usami et al. [739] have attributed the luminescence in Si~_xGe x heterostructure to interface located excitons. A similar explanation was given by Turton and Jaros [740] for the luminescence recorded for Si-Ge superlattices. Galeckas et al. [741] reported a surface-recombination velocity of S = 1.5 • 10 4 cm/s for a SiGe/Si strained-layer superlattice. These workers have done picosecond scale pump-probe transient reflection and dynamic gratings experiments.
6.5. Porous Si When single-crystal silicon was anodized by electrochemical method, porous silicon (PSI) was obtained [742, 743]. Photoluminescence in porous silicon was demonstrated by Can-
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265
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optical absorption coefficient of porous silicon by considering phonon-,~ssisted transitions in the indirect bandgap Si. Moyer et al. [773] reported time-resolved spectroscopy of porous silicon. Experiments were done for different excitation wavelengths and excitation intensities. Figure 101 shows the time-resolved data for three different excitation wavelengths. If the radiative recombination is due to quantum-confined states, then we should expect a decrease in radiative lifetime as the excitation energy increases. Because this was not observed and also because the lifetimes were very long, the recombination was thought to be due to surface states. Stalmans et al. proposed that porous silicon can be used as a very good passivating layer in silicon solar cells. Enhanced PL efficiencies in porous silicon have been observed by various groups after surface passivation was done by different methods--hydrogen [775-779], HF [780, 781], annealing and rapid oxidation [782], wet thermal oxidation [783], thermal treatment in NH 3 [784], acids, methanol, iodine [785], and copper ions in solution [786]. Pavesi et al. [787] have fabricated light-emitting diodes based on porous silicon as the active layer. They attempted different configurations to examine the effects of interface states on the efficiency. They found that the performance of the n-Si/porous silicon LED structure was better than the metal/porous silicon structure. This was explained by considering the band diagrams of these structures, which are shown in Figure 102. In the metal-PS device, many interface states will be present that will pin the Fermi level at the midgap. The interface of metal/PS acts as an ohmic contact and electrons are injected into midgap levels. These electrons have to gain energy to move into the conduction band before radiative recombination with holes in the valence band. On the other hand, n-Si/PS acts as a standard nip heterojunction. In this case the electrons are injected directly into the conduction band of porous silicon and then they recombine radiatively with holes. There are no midgap levels in the
Fig. 102. Schematic band structure diagram of a metal/PS LED device (top panel) and of the test device (bottom panel). Here CB and VB refer to the conduction and valence effective bands, which roughly approximate the lowest empty and the highest filled quantum confined states of the various quantum dots which form porous silicon. Reprinted with permission from L. Pavesi et al., J. Appl. Phys. 86, 6474 (1999). Copyright 1999 by The American Institute of Physics.
n-Si/porous silicon as it is a better interface. In fact, this model was first discussed by Ben-Chorin et al. [788].
6.6. Si/SiO2 Interfaces The Si/SiO 2 interface is the most important material system in semiconductor technology because it is used in a number of devices, for example, metal-oxide semiconductor field-effect transistors (MOSFETs), high-efficiency solar cells, complementary metal-oxide semiconductor (CMOS) devices, and in the modern integrated circuit technology. There are several reviews and edited volumes that discuss the importance of the high-quality Si/SiO2 interface required to make reliable semiconductor devices [789-795]. Yablonovitch et al. [796] studied the electron-hole recombination at the Si-SiO 2 interface by optical spectroscopy. The surface-recombination current was measured as a function of the surface density of electrons and holes. It was noted that the surface states that have electron capture cross section 100 times greater than the hole capture cross section dominate the interface recombination. The Shockley-ReadHall recombination model was considered to analyze the data. At higher carrier densities bandgap renormalization was observed. Surface-recombination velocities of electrons and holes were found to be S, = 330 cm/s and S p - - 1080 cm/s, respectively. Chen et al. [797, 798] deposited very thin SiO 2 layers on Si wafers by plasma-enhanced chemical-vapor deposition (PECVD). After rapid thermal annealing (RTA), very high
266
KASI V I S W A N A T H
cartier lifetimes >5 ms and very low surface-recombination velocity <2 cm/s were observed in these samples. Interface recombination lifetimes were measured by the photoconductive decay method and the experiments were done for different thicknesses (L) of the sample. Figure 103 shows the plot of I/teef as a function of 1/L from which the value S = 2 cm/s was estimated. The bulk recombination lifetime ~'b was found to be 23 ms. Surface-recombination velocity S is a function of Si/SiO 2 interface-state density, positive-charge density in the oxide, carrier injection level, and capture cross section for electrons and holes [799, 800]. The low S value observed by Chen et al. [797, 798] could be due to either low interface state density or high density of positive charge in the PECVD oxide or a combination of both. If the positive-charge density is larger than the interface-state density, then the energy band bending will be downward. This will increase the difference in concentrations of electrons and holes, which leads to reduced interface recombination. Figure 104 shows the dependence of surface-recombination velocity on positive charge in the oxide. Very high oxide charge was not good for the integrated circuits as it effects the threshold and breakdown voltage, but the low surface-recombination velocity was thought to be excellent for the high-efficiency silicon solar cells; F 2 treatment was used by a Japanese team to passivate the interface states in Si/SiO 2 [801]. Injection-level dependence of surface-recombination velocity in Si/SiO2 interface was examined by Aberle et ah [799, 802, 805] and Glunz et al. [803, 804 808]; Seff was found to decrease with increasing injection level for highly doped p-type silicon. However, Seef decreased for both low-doped p-type Si and n-type Si as shown in Figure 105 [804]. Aberle et al. [805] have reported the effect of oxide parameters on the characteristics of Si/SiO 2 interface. Ghannam and Mertens [809] and Girisch et al. [810, 811] have also thoroughly studied the interface-recombination parameters in Si/SiO 2, which was optically illuminated.
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Many groups have shown that in a metal-oxidesemicondcutor (MOS) structure interface states are created either due to hot-electron transport through the oxide film or electron-hole recombination at the surface [812-822]. Buchanan [823] has measured interface-state density as a function of electron injection in Si/SiO2 interfaces. This is shown in Figure 106. The Si dangling bond will be created at the Si/SiO 2 interface [792] due to hot electron stress [824] or irradiation [825]. Poindexter et al. [792] have called these dangling Si bonds Pb centers. Several workers have reported on the nature and mechanism of creation of the silicon dangling bonds in Si/SiO 2 interfaces [824-837]. It was proved that hydrogen is responsible for the degradation of metal-oxidesemiconductor (MOS) structures [829-831]. Under electrical stress, the hydrogen, which was present in the device, will be released by hot carriers. The following reaction was proposed [828, 830]: PbH + H --+ Pb -k-H2
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267
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where PbH is the hydrogen-passivated silicon dangling bond. Several scientists have agreed on the hydrogen-release model given in the preceding based on various experiments [831-836]. Cartier and Stathis [837] have shown that the silicon dangling bonds were passivated during hot electron stress, and the creation of unidentified defects was observed. This degradation was found to be very similar to that observed due to hydrogen plasma. Based on this, the authors have suggested that the hydrogen-release model was correct. It was also proposed that the silicon dangling bond was not the dominant interface defect. Devine et al. [838] have performed the degradation studies by hot-electron injection in metal-oxide-semiconductor transistors. The devices were annealed in H 2 or D. The results have indicated that D 2 was better than H 2 in improving the lifetime of the device. These findings were in agreement with the report by Lyding et al. [839]. In other reports, the microstructure [840] and mechanism for the generation of the interface-state precursor [841] were discussed. In the interfacial region of Si and SiO 2 there will be various kinds of silicon dangling bonds and bond distortions. This gives rise to a very broad distribution of interface states with different energy states. This was confirmed by a variety of experiments such as capacitance-voltage (C-V) and surfacephotovoltage measurements [842-846]. Femtosecond lasers have been used to study the interfacialrelated recombinations and carrier-trapping processes in Si/SiO 2 [847-861]. Lupke et al. [847], Bloch et al. [848], Mihaychuk et al. [849], and Shamir et al. [851, 860] have published a series of very interesting papers in which this approach was used. They have developed novel methods based on second-harmonic generation due to electric fields generated at the interfaces, which enabled them to understand charge trapping and detrapping phenomena. In addition, Shamir et al. [860] studied the charge trapping and
detrapping in Si/SiO 2 interfaces by using femtosecond laserinduced multiphoton-photoemission (MPPE) phenomena. This is shown schematically in Figure 107. Very intense laser radiation with photon energy 1.55 eV transfers electrons from silicon via three-photon or four-photon photoemission. The electric field generated can be monitored by electric fieldinduced second harmonic generation (EFISH) or multiphoton photoemission (MPPE). As shown in Figure 108 the photoemission current signal did not come to its original value, which indicated that some of the traps have a very long lifetime. The samples used are (a): 15 Sn : 1.5 nm oxide grown at 85 K on a low-doping n-Si substrate; and (b) CLn : the oxide of 15 Sn that was removed by heating at 1400 K. Wang et al. [861] observed a new and unusual enhancement of electric field at the Si/SiO 2 interface after the laser was switched off. The interface electric fields were monitored by optical second-harmonic generation. This work was part of their major program to understand fundamental physics dealing with carrier dynamics and recombination processes at semiconductor interfaces. Figure 109 shows the enhancement of second harmonic generation after the laser was switched off. The results were explained by considering multiphoton hole injection into the oxide. The electron and holes were also thought to have distinct trapping and detrapping behaviors. The model proposed was given in Figure 110. Wu et al. [862] have found that the interface-state density increased with temperature in the Si-SiO 2 system. Leakage currents observed in metal-oxide-semiconductor (MOS) devices were thought to be due to trap-assisted tunneling and recombination in oxide traps [863]. In a series of elegant papers, Stesmans [864-867] reported the passivation of different silicon dangling bonds in Si/SiO 2 by hydrogen. He has utilized electron spin resonance spectroscopy to understand the electronic structure of the defects before and after passivation by hydrogen. Mishima et al. [868] performed spin-dependent ..... 4P/PE
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268
KASI VISWANATH
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Fig. 110. Schematic diagrams of a three-photon electron and a four-photon hole transfer process from silicon into an ultrathin oxide. For simplicity only the flat-band condition is shown in the top diagram. Reprinted with permission from W. Wang et al., Phys. Rev. Lett. 81, 4224 (1998). Copyright 1998 by The American Physical Society.
recombination spectroscopy on Si/SiO2 and identified two types of silicon dangling bonds, called as Pbl and Pb0 as shown in Figure 111. There were many earlier reports on spindependent recombination involving interface defects in the Si/SiO 2 system [869-875]. Mishima's spin-dependent recomb i n a t o n s p e c t r o s c o p y e x p e r i m e n t s h a v e c o n f i r m e d the exist e n c e of energy levels of Pbl centers in the silicon bandgap. This result contradicted the reports by Stesmans and Afans'ev
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Fig. 109. Time-dependent SHG signal from a 40-,~ thermal oxide grown on a p-type Si(001) substrate, where A~ represents the beam-on saturated SHG signal and A2 represents the dark field enhanced SHG signal. The inset shows the TDSHG signal from the same wafer after the oxide has been etched down to 10/~ thickness. Reprinted with permission from W. Wang et al., Phys. Rev. Lett. 81, 4224 (1998). Copyright 1998 by The American Physical Society.
Fig. 111. Spin dependent recombination spectra illustrating the Pbo and Pb~ spectra for both 28Si (nuclear spin-zero) and 29Si (Nuclear spin-I/2) sites. The scan was taken at a gate voltage of -0.45 V. Although the two variant line shapes are not resolvable in the center line, both are clearly present in the 29Si hyperfine (hf) satellite peaks. Reprinted with permission from T. D. Mishima, P. M. Lenahan, and W. Weber, Appl. Phys. Lett. 76, 3771 (2000). Copyright 2000 by The American Institute of Physics.
SURFACE AND INTERFACIAL RECOMBINATION IN SEMICONDUCTORS [876, 877]. However, it was in agreement with the proposal given by Gerardi et al. [878]. Law et al. [879] have identified interstitial-related recombination in Si/SiO 2 interface and thus have clarified many controversies in the literature about this topic. A model for surface-recombination velocity of silicon interstitials in Si/SiO 2 interfaces was discussed by Tsamis and Tsoukalas [880] and Tsoukalas and Tsamis [881]. Interface-state generation was found to be dependent on temperature [882].
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The early work on surface recombination in Ge has been reviewed by Aspnes [12] and Fitzgerald and Grove [13]. Norton's group has reported on the efficient passivation of Ge surface by sulfur [883]. From Auger spectroscopy they ha~e concluded that S atoms were bonded to the Ge atoms. Timoshenko et al. [884] have developed an in sutu photoluminescence technique to characterize passivated surfaces of Ge.
269
substrate
Fig. 112. Schematic illustration of the vertical structure of visible a-SiC:Hbased p-i-n thin film light-emitting diode (TFLED). Intrinsic 50-mm-thick a-SiC:H film with bandgap of 3.0 eV was used as the important luminescent active layer. The p-/~c-Si:H and n-/zc-Si:H layers were used as hole and electron injectors into the luminescent active i layer, respectively. Intrinsic a-SiC:H layer of 1.5 nm with a wide bandgap of 3.3 eV was used to increase the efficiency of hole injection into the luminescently active i layer. Reprinted with permission from J. W. Lee and K. S. Lim, J. Appl. Phys. 81, 2432 (1997). Copyright 1997 by The American Institute of Physics.
7. GR()UP IV BINARY SEMICONDUCTORS 7.1. SiC
One of the best candidates for high-temperature and high-power electronics is SiC, which is a wide bandgap semiconductor. It also has high thermal conductivity, high electron-saturation velocity, very high breakdown voltage, and excellent physical stability. These properties make it suitable for several applications. Depending on the stacking order, SiC exists with either zinc-blende (cubic) or wurtzite (hexagonal) crystal structure. Because of the large bandgap SiC is very much suitable for light-emitting diodes in the visible region. Recent efforts have been centered around the development of thin-film lightemiting diodes (TFLEDs) with the idea of using them for flat panel displays. For this purpose hydrogenated amorphous silicon carbide (a-SiC:H) was utilized with p-i-n structure [885-8871]. Lee and Lim of South Korea [888, 889] have performed hydrogen passivation and observed improved characteristics of the LED. Figure 112 shows the schematic of an LED structure based on SiC. The diode was fabricated on a tin oxide (SnOz)-coated glass substrate by a photochemical vapordeposition system; p- and n-type hydrogenated Si layers were used as hole and electron injectors into the active layer. The amorphous SiC was the active luminescent layer. After hydrogen passivation, improved electroluminescence was observed as shown in Figure 113. After hydrogen treatment the luminescence spectrum has shifted to higher energy and also narrowed down considerably. This was explained by the energy-level diagram shown in Figure 114. After passivation midgap levels decrease considerably and the band tails become sharper. Passivation also improved the brightness of the LED as shown in Figure 115. Surface-recombination velocity in SiC epilayers was reported by Grivickas et al. [890]. Wahab et al. [891] have
observed improved electrical performance of SiC Schottky diodes after hydrogen annealing. Afans'ev et al. [892, 893] and Afans'ev and Stesmans [894] studied the origin of interface defects in the SiC/SiO 2 system; SiC is very much suitable for the fabrication of metal-oxide field-effect transistors (MOSFETs) because the insulating SiO 2 layer can easily be grown on SiC by thermal methods [895-898]. Afans'ev et al. [892, 893] and Afans'ev and Stesmans [894] identified interface traps in SiO2/SiC by conducting a number of studies, such as low-temperature electrical measurements and photon-stimulated electron-tunneling experiments. It was
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270
KASI VISWANATH
CO
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E?
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suggested that surface imperfections at the interface were responsible for the degradation of the SiC-based devices. Xu et al. [899] and Lai et al. [900] have observed improved interface properties of the SiO2/SiC material system after nitridation.
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8. CONCLUSIONS Surface and interfacial recombination studies in semiconductors were reviewed in this chapter. If a semiconductor surface has defects such as vacancies, it creates dangling bonds. These vancancies have energies in the bandgap of the semiconductor. Foreign impurity atoms also have midgap energies. These trap states participate in nonradiative recombination with electrons and holes. The nonradiative recombinations have detrimental effects on the performance of the semiconductor device and shelf life of the device. Fermi-level pinning due to surface states was discussed. Shockley-Read-Hall recombination in the framework of Stevenson-Keys formalism was reviewed. This gives the theoretical basis to understand the surface and interface recombination in semiconductors. Unpinning of the Fermi level by a number of surfacepassivation techniques were described. The improvement in the quality of surfaces can be evaluated by photoluminescence techniques. With the tremendous advances in femtosecond laser technology and detection systems such as streak cameras, it is now possible to evaluate the quality of the semiconductor surface very accurately. Measurements of surfacerecombination lifetimes and surface-recombination velocities in a number of systems have been elaborated. In the discussion part some importance has been given to very novel techniques such as near-field scanning optical microscopy (NSOM), microwave-modulated photoluminescence (MMPL), photoacoustic methods, electric field-induced second-harmonic generation (EFISHG), multiphoton photoemission (MMPE), and so forth. Also discussed were pump-probe spectroscopy and wave-mixing spectroscopy. Using these techniques surfaceelectron recombination and electron shuttling in picosecond timescales could be observed. There are tremendous opportunities now to understand the electronic structure of semiconductor surfaces by advanced laser techniques that was not possible two decades ago. These research efforts will enable us to understand the basic physics of semiconductor surfaces thus enabling the design of better materials. Growth of GalnP is one such example. By controlling the order and disorder on the surface, the required bandgap could be achieved. This very novel area can be called bandgap engineering by surface reconstruction. In the future it should be possible to make a totally new class of superlattices and quantum wells by this method. This chapter has attempted to summarize the work done in the last 25 years on surface-related recombinations in semiconductors. Most of the work has been done on GaAs and Si. Also discussed was surface and interface recombination in a number of devices, such as light-emitting diodes, semiconductor lasers, metal-oxide semiconductor field-effect transistors (MOSFETs), metal-insulator-semiconductor (MIS) devices, solar cells, and so forth. Improvement in the performance characteristics of various devices fabricated with a number of semiconductors has been achieved by different passivation techniques. In conclusion, surface and interface recombination is a very important problem in semiconductor technology. With
S U R F A C E A N D I N T E R F A C I A L R E C O M B I N A T I O N IN S E M I C O N D U C T O R S
the advanced laser technology that is available today the electronic structure of surface states has been undertsood to a great extent. Surface-passivation methods were useful in controlling the surface Fermi level. In the future it should be possible to achieve e~tremely pure semiconductor surfaces that are necessary for ultralarge-scale integration (ULSI) technology.
ACKNOWLEDGMENTS The original work of the author discussed in this chapter was done in Japan and South Korea. Surface-recombination studies in GaAs quantum wires have been done at the Advanced Microelectronics Center of Hitachi Central Research Laboratory, Tokyo, Japan. I would like to thank Dr. Shorijo Asai, the then Deputy General Manager, Hitachi, and currently Managing Director of Hitachi, for the invitation to come to Japan. It is a great pleasure to thank Dr. Toshio Katsuyama, the group leader, for the GaAs quantum wire project, and Dr. Kensuke Ogawa, senior researcher at Hitachi, who is currently at Femtosecond Technology Research Association at Tsukuba, for their wonderful collaboration. The author wishes to acknowledge the help received from them in using the femtosecond laser facility and for introducing him to the field of optical phenomena in semiconductors. He would also like to thank Dr. Toshikazu Shimada, Manager of the Advanced Microelectronics Center at Hitachi and General Manager of Hitachi Cambridge Laboratory, Cambridge, England for his constant encouragement. Thanks are due also to Dr. M. Yazawa and Dr. K. Hiruma for fabrication of free-standing GaAs quantum wires, Dr. M. Miyao, Department Manager and Dr. Nakamura, General Manager of Hitachi Central Research Laboratory for their support, and to Prof. Ryoichi Ito, Professor of Applied Physics, University of Tokyo, who is currently at Meiji University, for many stimulating discussions on semiconductor optical physics. This work was financed by the R & D Association of Future Electron Devices, as part of Basic Technology for Future Industries supported by NEDO (New Energy and Industrial Development Organization). The studies on nonradiative recombination due to interface defects in InGaN multiple quantum wells were done in the National Creative Research Initiative Center for Ultrafast Optics Control, Korea Research Institute of Standards and Science (KRISS), Taejon, South Korea. The author would like to thank the brain pool program of South Korea for the invitation to Korea, where he worked for three years. It was indeed a great opportunity to have collaborated with Dr. Dongho Kim who provided one of the best laser laboratories in the world. Thanks are due to Dr. Jooin Lee for his cooperation on the picosecond recombination dynamics studies in GaN materials, Dr. Sunkyu Yu, Dr. Yoonju Shin, Dr. S.C. Jeoung, and Dr. Nam Woong Song for their help in the laser laboratory. It is a great pleasure to thank Prof. Howard H. Patterson, University of Maine, Orono, Maine, U.S.A. with whom I have worked for several years. He was the first to introduce me to
271
the field of optical phenomena in the solid state, lasers, laser spectroscopy, and photonics. I also thank my wife Mrs. Annapurna Viswanath and son Mr. Srinivas for their understanding and support during the course of experimental investigations when I was working in Japan and South Korea as well as during the write up of this review.
REFERENCES 1. A. S. Grove, "Physics and Technology of Semiconductor Devices," Wiley, New York, 1967. 2. A. Heller, Acc. Chem. Res. 154, 154 (1981). 3. E. Ellis and T. S. Moss, Soild State Electron. 13, 1 (1970). 4. H. Kressel, in "Semiconductors and Semimetals," Vol. 16, R. K. Willardson and A. C. Beer, eds., Academic Press, New York, 1981, p. 1. 5. S. K. Gandhi, "Semiconductor Power Devices," Wiley-Interscience, New York, 1977. 6. B.J. Baliga, "Modern Power Devices," Wiley-Interscience, New York, 1987. 7. W. Shockley, "Electrons and Holes in Semicondcutors," Van Nostrand, New York, 1959. 8. A. Many, Y. Goldstein, and N. B. Grover, "Semiconductor Surfaces," North Holland, Amsterdam, 1965. 9. W. Shockley and W. T. Read, Phys. Rev. 87, 835 (1952). 10. R. N. Hall, Phys. Rev. 87, 387 (1952). 11. D. T. Stevenson and R. J. Keyes, Physics 20, 1041 (1954). 12. D. E. Aspnes, Surface Sci. 132, 406 (1983). 13. D. J. Fitzgerald and A. S. Grove, Surface Sci. 9, 347 (1968). 14. S. M. Sze, "Physics of Semiconductor Devices," John Wiley, New York, 1981. 15. I. Lindau, P. W. Chye, C. M. Garner, P. Pianetta, C. Y. Su, and W. E. Spicer, J. Vac. Sci. Technol. 15, 1332 (1978). 16. P. W. Chye, I. Lindau, P. Pianetta, C. M. Garner, C. Y. Su, and W. E. Spicer, Phys. Rev. B 18, 5545 (1978). 17. W. E. Spicer. P. W. Chye, P. R. Skeath, C. Y. Su, and I. Lindau, J. Vac. Sci. Technol. 16, 1422 (1979). 18. W. E. Spicer, I. Lindau, P. R. Skeath, C. Y. Su, and P. W. Chye, Phys. Rev. Lett. 44, 420 (1980). 19. W. E. Spicer, I. Lindau, P. Skeath, and C. Y. Su, J. Vac. Sci. Technol. 17, 1019 (1980). 20. N. E Mott, Solid State Electron. 21, 1275 (1978). 21. H. J. Queisser, Solid State Electron. 21, 1495 (1978). 22. A. M. Stroneham, Rep. Prog. Phys. 44, 1251 (1981). 23. P. T. Landsberg, Phys. Status Solidi 41,457 (1970). 24. T. N. Morgan, Phys. Rev. B 28, 7141 (1983). 25. M. Lax, Phys. Rev. 119, 1502 (1960). 26. E T. Landsberg, C. Phys-Roberts, and P. Lal, Proc. Phys. Soc. 84, 915 (1964). 27. A. Haug, Phys. Status Solid B 97, 481 (1982). 28. A. Hangleiter, Phys. Rev. B 35, 9149 (1987). 29. A. Hangleiter, Phys. Rev. B 37, 2594 (1988). 30. D. B. Laks, G. E Neumark, A. Hangleiter, and S. T. Pantelides, Phys. Rev. Lett. 61, 1229 (1988). 31. A. Hangleiter and R. Hacker, Phys. Rev. Lett. 65, 215 (1990). 32. J. Jortner, S. A. Rice, and R. M. Hochstrasser, Adv. Photochem. 7, 149 (1969). 33. K. E Freed, in "Energy Dependence of Electronic Relaxation Processes in Polyatomic Molecules," Vol. 15 of "Topics in Applied Physics," E K. Fong, ed., Springer, New York, 1976, p. 23. 34. E Avouris, W. M. Gelbart, and M. A. E1-Sayed, Chem. Rev. 77, 793 (1977).
272
KASI VISWANATH
35. R. Engleman and J. Jortner, Mol. Phys. 18, 145 (1970). 36. D. L. Keune, H. Holonyak, Jr., R. D. Burnham, D. R. Scifres, H. R. Zwicker, J. W. Burd, M. G. Crawford, D. L. Dickus, and M. J. Fox, J. Applied Phys. 42, 2048 (1971); E D. Dapkus, N. Holonyak, Jr., R. D. Burnham, D. L. Keune, J. W. Burd, K. L. Lawley, and R. E. Wallin, J. Appl. Phys. 41, 4194 (1971). 37. C. J. Hwang, J. Appl. Phys. 42, 4408 (1971). 38. C. Van Opdrop, C. Werkhoven, and A. T. Vink, Appl. Phys. Lett. 30, 40 (1977). 39. W. E Dumke, Phys. Rev. 105, 139 (1957). 40. P. Asbeck, J. Appl. Phys. 48, 820 (1977). 41. R. J. Nelson and R. G. Sobers, Appl. Phys. Lett. 32, 761 (1978). 42. R. J. Nelson, J. Vac.Sci. Technol. 15, 1475 (1978). 43. C. J. Henry and R. A. Logan, J. Appl. Phy. 48, 3962 (1977). 44. R. J. Nelson and R. G. Sobers, J. Appl. Phys. 49, 6103 (1978). 45. E Stem, IEEE J. Quantum Electron. QE-9, 290 (1973). 46. E Stem, J. Appl. Phys. 47, 5382 (1976). 47. H. C. Casey, Jr. and E Stem, J. Appl. Phys. 47, 631 (1976). 48. L. Jastrzebski, J. Lagowski, and H. C. Gatos, Appl. Phys. Lett. 27, 537 (1975). 49. R. D. Ryan and J. E. Eberhardt, Solid State Electron. 15, 865 (1972). 50. L. Jastrzebski, H. C. Gatos, and L. Jagowski, J. Appl. Phys. 48, 1730 (1977). 51. C. J. Sandroff, R. N. Nottenburg, J. C. Bischoff, and R. Bhat, Appl. Phys. Lett. 51, 33 (1987). 52. E. Yablonovitch, C. J. Sandroff, R. Bhat, and T. Gmitter, Appl. Phys. Lett. 51,439 (1987). 53. L. A. Farrow, C. J. Sandroff, and M. C. Tamargo, Appl. Phys. Lett. 51, 1931 (1987). 54. R. N. Nottenburg, C. J. Sandroff, O. H. Humphrey, T. H. Hollenbeck, and R. Bhat, Appl. Phys. Lett. 52, 1707 (1988). 55. B. J. Skromme, C. J. Sandroff, E. Yablonovitch, and T. G. Gmitter, Appl. Phys. Lett. 51, 2022 (1988). 56. R. S. Besser and C. R. Helms, Appl. Phys. Lett. 52, 2157 (1988). 57. M. S. Carpenter, M. R. Melloch, M. S. Lundstrom, and S. E Tobin, Appl. Phys. Lett. 52, 2157 (1988). 58. D. Liu, T. Zhang, R. A. LaRue, J. S. Harris, Jr., and T. W. Sigmon, Appl. Phys. Lett. 53, 1059 (1988). 59. C. W. Wilmsen, E D. Kirschner, and J. M. Woodall, J. Appl. Phys. 64, 3287 (1988). 60. R. Iyer, R. R. Chang, and D. L. Lile, Appl. Phys. Lett. 53, 134 (1988). 61. D. K. Gaskill, N. Bottka, and R. S. Sillmon, J. Vac. Sci. Technol. B 6, 1497 (1988). 62. H. H. Lee, R. J. Racicot, and S. H. Lee, Appl. Phys. Lett. 54, 724 (1989). 63. H. Hasegawa, T. Saitoh, S. Konishi, H. Ishii, and H. Ohno, Jpn. J. Appl. Phys. 27, L 2177 (1988). 64. M. G. Mauk, S. Xu, D. J. Arent, R. E Mertens, and G. Borghs, Appl. Phys. Lett. 54, 213 (1989). 65. J. E Fan, H. Oigawa, and Y. Nannichi, Jpn. J. Appl. Phys. 28, L 1331 (1988). 66. C. J. Sandroff, M. S. Hegde, L. A. Farrow, C. C. Chang, and J. P. Harbison, Appl. Phys. Lett. 54, 362 (1989). 67. B. A. Cowens, Z. Dradas, W. N. Oelgass, M. S. Carpenter, and M. R. Melloch, Appl. Phys. Lett. 54, 365 (1989). 68. Y. Nannichi, J. Fan, H. Oigawa, and A. Koma, Jpn. J. Appl. Phys. 27, L 2367 (1988). 69. T. Tiedje, E C. Wong, K. A. R. Mitchell, W. Eberhardt, Z. Fu, and D. Sondericker, Solid State Commun. 70, 355 (1989). 70. C. J. Spindt, D. Liu, K. Miyano, E L. Meissner, T. T. Chiang, T. Kendelewicz, I. Lindau, and W. E. Spicer, Appl. Phys. Lett. 55, 861 (1989). 71. C. M. Wilmsen, E D. Kirchner, J. M. Baker, D. T. McInturff, G. D. Pettit, and J. M. Woodall, J. Vac. Sci. Technol. B 6, 1180 (1988). 72. C. J. Sandroff, M. S. Hegde, and C. C. Chang, J. Vac. Sci. Technol. B 7, 841 (1989). 73. R. S. Besser and C. R. Helms, J. Appl. Phys. 65, 4306 (1989).
74. E. Yablonovitch, B. J. Skromme, R. Bhat, J. P. Harbison, and T. J. Gmitter, Appl. Phys. Lett. 54, 555 (1989). 75. D. Biswas, E R. Berger, and E Bhattacharya, J. Appl. Phys. 65, 2571 (1989). 76. C. J. Sandroff, E S. Turco-Sandroff, L. T. Florez, and J. E Harbison, J. Appl. Phys. 70, 3632 (1991). 77. S. Shikata and H. Hayashi, J. Appl. Phys. 70, 3721 (1991). 78. S. R. Lunt, G. N. Ryba, E G. Santangelo, and N. S. Lewis, J. Appl. Phys. 70, 7449 (1991). 79. L. S. Hung, G. H. Braunstein, and L. A. Bosworth, Appl. Phys. Lett. 60, 201 (1992). 80. Y. Wang, Y. Darici, and E H. Holloway, J. Appl. Phys. 71, 2746 (1992). 81. Y. Mada, K. Wada, and Y. Wada, Appl. Phys. Lett. 61, 2993 (1992). 82. Y. T. Oh, S. C. Byun, B. R. Lee, T. W. Kang, C. Y. Hong, S. B. Park, H. K. Lee, and T. W. Kim, J. Appl. Phys. 76, 1959 (1994). 83. Y. Mada and K. Wada, Appl. Phys. Lett. 66, 733 (1995). 84. C. S. Liu and J. E Kauffman, Appl. Phys. Lett. 66, 3504 (1995). 85. K. Asai, T. Miyashita, K. Ishigure, and S. Fukatsu, J. Appl. Phys. 77, 1582 (1995). 86. E. D. Lu, E E Zhang, S. H. Xu, X. J. Yu, E S. Xu, Z. E Han, E Q. Xu, and X. Y. Zhang, Appl. Phys. Lett. 69, 2282 (1996). 87. J. Liu and T. E Kuech, Appl. Phys. Lett. 69, 662 (1996). 88. G.W. Anderson, M. C. Hanf, E R. Norton, M. Kowalewski, K. Myrtle, and N. Heinrich, J. Appl. Phys. 79, 4954 (1996). 89. V. N. Bessolov, M. V. Lebedev, and D. R. T. Zahn, J. Appl. Phys. 82, 2640 (1997). 90. J. E Geisz, S. A. Safvi, and T. E Kuech, J. Electrochem. Soc. 144, 732 (1997). 91. K. Remashan and K. N. Bhat, IEEE Electron. Dev. Lett. 19, 466 (1998). 92. X. Cao, H. Hu, X. Ding, Z. Yuan, Y. Dong, X. Chen, B. Lai, and X. Hou, J. Vac. Sci. Technol. B 16, 2656 (1998). 93. X. A. Cao, H. T. Hu, Y. Dong, X. M. Ding, and X. Y. Hou, J. Appl. Phys. 86, 6940 (1999). 94. R. J. Nelson, J. S. Williams, H. J. Leamy, B. I. Miller, H. C. Casey, Jr., B. A. Parkinson, and A. Heller, Appl. Phys. Lett. 38, 76 (1980). 95. E Briones and D. M. Collins, J. Electron. Mater 11,847 (1982). 96. W. I. Wang, R. E Marks, and L. Vina, J. Appl. Phys. 59, 3 (1986). 97. H. Kunzel and K. Ploog, Appl. Phys. Lett. 37, 4 (1980). 98. J. E Kauffman and G. L. Richmond, J. Appl. Phys. 73, 1912 (1993). 99. J. E Kauffman, B. A. Balko, and G. L. Richmond, J. Phys. Chem. 96, 6371 (1992); 96, 6374 (1992). 100. T. Sawada, K. Numata, S. Tohdoh, T. Saitoh, and H. Hasegawa, Jpn. J. Appl. Phys. 32, 511 (1993). 101. A. N. MacInnes, M. B. Power, and A. R. Barron, Appl. Phys. Lett. 62, 713 (1993). 102. E E Jenkins, A. N. MacInnes, M. Tabib-Azar, and A. R. Barron, Science 263, 1751 (1994). 103. M. Tabib-Azar, S. Kang, A. N. MacInnes, M. B. Power, A. R. Barron, E E Jenkins, and A. E Hepp, Appl. Phys. Lett. 63, 625 (1993). 104. H. Sugahara, M. Oshima, H. Oigawa, H. Shigekawa, and Y. Nannichi, J. appl. Phys. 69, 4349 (1991). 105. T. Scimeca, Y. Muramatsu, M. Oshima, H. Oigawa, and Y. Nannichi, Phys. Rev. B 44, 12927 (1991). 106. H. Sugahara, M. Oshima, R. Klauser, H. Oigawa, and Y. Nannichi, Surf. Sci. 242, 335 (1991). 107. T. Scimeca, Y. Muramatsu, M. Oshima, H. Oigawa, and Y. Nannichi, appl. Surf. Sci. 60/61,256 (1992). 108. H. Sugahara, M. Oshima, H. Oigawa, and Y. Nannichi, Thin Solid Films 220, 212 (1992). 109. S. Maeyama, M. Sugiyama, M. Oshima, H. Sugahara, H. Oigawa, and Y. Nannichi, Appl. Surf. Sci. 60/61,513 (1992). 110. M. Oshima, T. Scimeca, Y. Watanabe, H. Oigawa, and Y. Nannichi, Jpn. J. Appl. Phys. 32, 518 (1993). 111. M. Sugiyama, S. Maeyama, and M. Oshima, Phys Rev. Lett. 71,2611 (1993).
SURFACE A N D I N T E R F A C I A L R E C O M B I N A T I O N IN S E M I C O N D U C T O R S
112. M. Sugiyama, S. Maeyama, and M. Oshima, Phys. Rev. B 50, 4905 (1994). 113. M. Sugiyama, S. Maeyama, and M. Oshima, Phys. Rev. B 48, 11037 (1993). 114. K. Sato and H. Ikoma, Jpn. J. Appl. Phys. 32, 921 (1993). 115. K. Sato, M. Sakata, and H. Ikoma, Jpn. J. Appl. Phys. 32, 3354 (1993). 116. M. Sakata, M. Hayakawa, N. Nakano, and H. Ikoma, Jpn. J. Appl. Phys. 34, 3447 (1995). 117. P. Moriarty, B. Murphy, L. Roberts, A. A. Cafolla, G. Hughes, L. Koenders, P. Bailey, and D. A. Woolf, Appl. Phys. Lett. 67, 383 (1995). 118. P. Moriarty, B. Murphy, L. Roberts, A. A. Cafolla, G. Hughes, L. Koenders, and P. Bailey, Phys. Rev. B 50, 14237 (1994). 119. V. N. Bessolov, E. V. Konenkova, and M. V. Lebedev, Mat. Sci. & Engg. B 44, 376 (1997). 120. V. N. Bessolov, M. V. Lebedev, A. E Ivankov, W. Bauhofer, and D. R. T. Zahn, Appl. Surf. Sci. 133, 17 (1998). 121. M. V. Lebedev, and M. Aono, J. Appl. Phys. 87, 289 (2000). 122. V. N. Bessolov, E. V. Konenkova, M. V. Lebedev, and D. R. T. Zahn. Physics of the Solid State 41,793 (1999). 123. V. N. Bessolov, E. V. Konenkova, and M. V. Lebedev, J. Vac. Sci. Technol. B 14, 2761 (1996). 124. V.N. Bessolov, E. V. Konenkova, and M. V. Lebedev, Tech. Phys. Lett. 22, 749 (1996). 125. V. N. Bessolov, M. V. Lebedev, N. M. Binh, M. Friedrich, and D. R. T. Zahn, Semicond. Sci. Technol. 13, 611 (1998). 126. V. N. Bessolov, E. V. Konenkova, M. V. Lebedev, and D. R. T. Zahn, Semiconductors 31, 1164 (1997). 127. R. S. Besser and C. R. Helms, Appl. Phys. Lett. 52, 1707 (1988). 128. C. J. Spindt, R. S. Besser, R. Cao, K. Miyano, C. R. Helms, and W. E. Spicer, Appl. Phys. Lett. 54, 1148 (1989). 129. C. J. Spindt and W. E. Spicer, Appl. Phys. Lett. 55, 1653 (1989). 130. H. Hasegawa, H. Ishii, T. Sawada, T. Saitoh, S. Konishi, Y. Liu, and H. Ohno, J. Vac. Sci. Technol. B 6, 1184 (1988). 131. V. L. Berkovits, V. N. Bessolov, T. N. L'vova, E. B. Novikov, V. I. Safarov, R. V. Khasieva, and B. V. Trarenkov, J. Appl. Phys. 70, 3707 (1991). 132. W. R. Miller and G. E. Stillman, J. Appl. Phys. 70, 7635 (1991). 133. Y. Hirota, E Maeda, Y. Watanabe, and T. Ogino, J. Appl. Phys. 82, 1661 (1997). 134. C.J. Sandroff, M. S. Hegde, L. A. Farrow, R. Bhat, J. E Harbison, and C. C. Chang, J. Appl. Phys. 67, 586 (1990). 135. E S. Turco, C. J. Sandroff, D. M. Hwang, T. S. Ravi, and M. C. Tamargo, J. Appl. Phys. 68, 1038 (1990). 136. S. A. Chambers and V. S. Sundaram, J. Vac. Sci. Technol. B 9, 2256 (1991). 137. R. Berrigan, Y. Watanabe, T. Scimeca, and M. Oshima, Jpn. J. Appl. Phys. 31, 3523 (1992). 138. T. Scimeca, Y. Watanbe, R. Berrigan, and M. Oshima, Phys. Rev. B 46, 10201 (1992). 139. T. Scimeca, K. Prabhakaran, Y. Watnabe, E Maeda, and M. Oshima, Appl. Phys. Lett. 63, 1807 (1993). 140. E Maeda, Y. Watanabe, T. Scimeca, and M. Oshima, Phys. Rev. B 48, 4956 (1993). 141. T. Scimeca, Y. Watanabe, R. Maeda, R. Berrigan, and M. Oshima, Appl. Phys. Lett. 62, 1667 (1993). 142. T. Scimeca, Y. Watnabe, E Maeda, R. Berrigan, and M. Oshima, J. Vac. Sci. Technol. B 12, 3090 (1994). 143. D. J. Olego, Appl. Phys. Lett. 51, 1422 (1987). 144. S. K. Gandhi, S. Tyagi, and R. Venkatasubramanian, Appl. Phys. Lett. 53, 1308 (1988). 145. B.A. Kuruvilla, A. Datta, G. S. Shekhawat, A. K. Sharma, P. D. Vyas, R. P. Gupta, and S. K. Kulakarni, J. Appl. Phys. 80, 6274 (1996). 146. B. A. Kuruvilla, S. V. Ghaisas, A. Datta, S. Banerjee, and S. K. Kulkarni, J. Appl. Phys. 73, 4384 (1993). 147. J. Sun, D. J. Seo, W. L. O'Brien, E J. Himpsel, and T. E Kuech, Mat. Res. Soc. Sym. Proc. 484, 589 (1998).
273
148. J. Sun, D. J. Seo, W. L. O'Brien, E J. Himpsel, A. B. Ellis, and T. E Kuech, J. Appl. Phys. 85, 969 (1999). 149. J. Arokiaraj, K. Ohtsuka, T. Soga, T. Jimbo, and M. Umeno, Inst. Phys. Conf. Ser. 162, 799 (1999); Jpn. J. appl. Phys. 38, 6587 (1999). 150. D. J. Olego, R. Schachter, and J. A. Baumann, Appl. Phys. Lett. 45, 1127 (1984). 151. P. Viktorovitch, M. Gendry, S. K. Krawczyk, E Krafft, E Abraham, A. Bekkaoui, and Y. Monteil, Appl. Phys. Lett. 58, 2387 (1991). 152. J. A. Dagata, W. Tseng, J. Bennett, J. Schneir, and H. H. Harary, Appl. Phys. Lett. 59, 3288 (1991). 153. Y. Wada and K. Wada, Appl. Phys. Lett. 63, 379 (1993). 154. M. Sakata and H. Ikoma, Jpn. J. Appl. Phys. 33, 3813 (1994). 155. J. L. Davidson, P. John, E G. Roberts, M. G. Jubber, and J. I. B. Wilson, Appl. Phys. Lett. 65, 1397 (1994). 156. D. A. Harrison, R. Ares, S. E Watkins, M. L. W. Thewalt, C. R. Bolognesi, D. J. S. Beckett, and A. J. S. Thorpe, Appl. Phys. Lett. 70, 3275 (1997). 157. O. J. Glembocki, J. A. Tuchman, J. A. Dagata, K. K. Ko, S. W. Pang, and C. E. Stutz, Appl. Phys. Lett. 73, 114 (1998). 158. R. Beaudry, S. E Watkins, X. Xu, and P. Yeo, J. Appl. Phys. 87, 7838 (2000). 159. M. Passlack, R. Droopad, Z. Yu, C. Overgaard, B. Bowers, and J. Abrokawah, Appl. Phys. Lett. 72, 3163 (1998). 160. G. Wang, K. Ohtsuka, T. Soga, T. Jimbo, and M. Umeno, Inst. Phys. Conf. Ser. 162, 597 (1999). 161. J. J. Zinck, E. J. Tarsa, B. Brar, and J. S. Speck, J. Appl. Phys. 82, 6067 (1997). 162. E Maeda, Y. Watanabe, and M. Oshima, Phys. Rev. B 48, 14733 (1993). 163. M. Sugiyama, S. Maeyama, E Maeda, and M. Oshima, Phys. Rev. B 52, 2678 (1995). 164. E Maeda, Y. Watanabe, and M. Oshima, J. Cryst. Growth 150, 1164 (1995). 165. K. W. Vogt and P. A. Kohl, J. Appl. Phys. 74, 6448 (1993). 166. Y. J. Park, E. K. Kim, I. K. Han, S. K. Min, E O'Keefe, H. Mutoh, H. Munekata, and H. Kukimoto, Appl. Surf. Sci. 117/118. 551 (1997). 167. N. Okamoto, E Kasahara, and H. Ikoma, Jpn. J. AppI. Phys. 38, L 424 (1999). 168. F. Kasahara, K. Kanazawa, N. Okamoto, and H. Ikoma, Jpn. J. Appl. Phys. 38, 6597 (1999). 169. A. Hara, R. Nakamura, and H. Ikoma, J. Vac. Sci. Technol. 16, 183 (1998). 170. S. Wada, K. Kanazawa, N. Okamoto, and H. Ikoma, J. Vac. Sci. Technol. 17, 1516 (1999). 171. A. Hara, E Kasahara, S. Wada, and H. Ikoma, J. Appl. Phys. 85, 3234 (1999). 172. H. Tanemura, K. Kanazawa, and H. Ikoma, Jpn. J. Appl. Phys. 39, 1629 (2000). 173. Z. H. Lu, E Chatenoud, M. M. Dion, M. J. Graham, H. E. Ruda, I. Koutzarov, Q. Liu, C. E. J. Mitchell, I. G. Hill, and A. B. MeLean, Appl. Phys Lett. 67, 670 (1995). 174. R. J. Nelson, J. S. Williams, H. J. Leamy, B. Miller, H. C. Casey, Jr., B. A. Parkinson, and A. Heller, Appl. Phys. Lett. 36, 76 (1980). 175. J. S. Hwang, Y. C. Wang, W. Y. Chou, S. L. Tyan, M. Hong, J. P. Mannaerts, and J. Kwo, J. Appl. Phys. 83, 2857 (1998). 176. A. Callegari, P. D. Hoh, D. A. Buchanan, and D. Lacey, Appl. Phys. Lett. 54, 332 (1989). 177. J. G. Ping and H. E. Ruda, J. Appl. Phys. 83, 5880 (1998). 178. E Friedel and S. Gourrier, Appl. Phys. Lett. 42, 509 (1983). 179. N. Pan, S. S. Bose, N. H. Kim, G. E. Stillman, E Chambers, G. Devane, C. R. Ito, and M. Feng, Appl. Phys. Lett. 51,596 (1987). 180. E E McCluskey, L. Pfeiffer, K. W. West, J. Lopata, M. L. Schnoes, T. D. Harris, S. J. Pearton, W. C. Dautremont-Smith, Appl. Phys. Lett. 54, 1769 (1989). 181. J. Saito and K. Kondo, J. Appl. Phys. 67, 6274 (1990). 182. Y. E Chen, C. S. Tsai, and Y. Chang, Appl. Phys. Lett. 57, 70 (1990). 183. V. Swaminathan, U. K. Chakrabarti, W. S. Hobson, R. Caruso, J. Lopata, S. J. Pearton, and H. S. Luftman, J. Appl. Phys. 68, 902 (1990).
274
KASI VISWANATH
184. R. A. Gottrcho, B. L. Preppernau, S. J. Pearton, A. B. Emerson, and K. P. Giapis, J. Appl. Phys. 68, 440 (1990). 185. S. V. Hattangady, R. A. Rudder, M. J. Mantni, G. G. Fountain, J. B. Posthill, and R. J. Markunas, J. Appl. Phys. 68, 1233 (1990). 186. V. Swaminathan, M. T. Asom, G. Livescu, M. Geva, F. A. Stevie, S. J. Pearton, and J. Lopata, Appl. Phys. Lett. 57, 2928 (1990). 187. Y. E Chen, W. S. Chen, S. H. Huang, and E Y. Juang, J. Appl. Phys. 69, 3360 (1991). 188. A. W. R. Leitch, Th. Prescha, and J. Weber, Phys. Rev. B 45, 14400 (1992). 189. K. D. Choquette, M. Hong, H. S. Luftman, S. N. G. Chu, J. P. Mannaerts, R. C. Wetzel, and R. S. Freund, J. Appl. Phys. 73, 2035 (1993). 190. M. Hong, R. S. Freund, K. D. Choquette, H. S. Luffman, J. P. Mannaerts, and R. C. Wetzel, Appl. Phys. Lett. 62, 2658 (1993). 191. G. Wang, K. Ohtsuka, T. Soga, T. Jimbo, and M. Umeno, Jpn. J. Appl. Phys. 38, 3504 (1999). 192. G. Wang, T Ogawa, M. Umeno, T. Soga, and T. Jimbo, Appl. Phys. Lett. 76, 730 (2000). 193. T. Soga, T. Jimbo, G. Wang, K. Ohtsuka, and M. Umeno, J. Appl. Phys. 87, 2285 (2000). 194. S. J. Pearton, J. W. Corbett, and T. S. Shi, Appl. Phys. A 43, 153 (1987). 195. J. K. Hsu and K. M. Lau, J. Appl. Phys. 63, 962 (1988). 196. V. J. Rao, V. Manorama, and S. V. Bhoraskar, Appl. Phys. Lett. 54, 1799 (1989). 197. V. Manorama, S. V. Bhoraskar, V. J. Rao, and S. T. Kshirsagar, Appl. Phys. Lett. 55, 1641 (1989). 198. R. S. Bhide, S. V. Bhoraskar, and V. J. Rao, J. Appl. Phys. 72, 1464 (1992). 199. M. Tabib-Azar, A. S. Dewa, and W. H. Ko, Appl. Phys. Lett. 52, 206 (1988). 200. Z. H. Lu and J. M. Baribeau, Appl. Phys. Lett. 70, 1989 (1997). 201. I. Riech, E. Marin, E Diaz, J. J. Alvarado-Gil, J. G. Medoza-Alvarez, H. Vergas, A. Cruz-Orea, M. Vargas, and J. Bernal-Alvarado, Phys. Stat. Sol. (a) 169, 275 (1998). 202. C. Paulson, B. Hawkins, J. Sun, A. B. Ellis, L. Mccaughan, and T. E Kuech, Mat. Res. Soc. Sym. Proc. 588, 13 (2000). 203. S. A. Safvi, J. Liu, and T. E Kuech, J. Appl. Phys. 82, 5352 (1997). 204. M. C. DeLong, I. Viohl, W. D. Ohlsen, E C. Taylor, E E Dabkowski, K. Meehan, J. E. Williams, and M. Hopkinson, unpublished results. 205. M. C. Delong, W. D. Ohlsen, I. Viohl, X. Yin, E C. Taylor, D. Sengupta, G. E. Stillman, J. M. Olson, and W. A. Harrison, Phys. Rev. B 48, 5157 (1993). 206. C.E. Inglefield, M. C. DeLong, E C. Taylor, and W. A. Harrison, Phys. Rev. B 56, 12434 (1997). 207. C. E. Inglefield, M. C. DeLong, E C. Taylor, and W. A. Harrison, J. Vac. Sci. Technol. B 16, 2328 (1998). 208. C. E. Inglefield, M. C. DeLong, E C. Taylor, J. E Geisz, and J. M. Olson, J. Vac. Sci. Technol. B 15, 1201 (1997). 209. C. E. Inglefield, M. C. DeLong, E C. Taylor, and W. A. Harrison, unpublished. 210. C. H. Wang and A. Neugroschel, J. Appl. Phys. 74, 3257 (1993). 211. H. Ito and T. Ishibashi, Jpn. J. Appl. Phys. 33, 88 (1994). 212. R. Toba and M. Tajima, J. Cryst. Growth 103, 28 (1990). 213. M. Noji, M. Kiyama, and M. Tajima, Jpn. J. Appl. Phys. 39, 2585 (2000). 214. J. Bardeen, Phys. Rev. 71,717 (1947). 215. R. E. Allen and J. D. Dow, Phys. Rev. B 25, 1423 (1982). 216. J. Fan, H. Oigawa, and Y. Nannichi, Jpn. J. Appl. Phys. 27, 1331 (1988). 217. M. S. Carpenter, M. R. Melloch, and T. E. Dungan, Appl. Phys. Lett. 53, 66 (1988). 218. J. Fan, H. Oigawa, and Y. Nannichi, Jpn. J. Appl. Phys. 27, 2125 (1988). 219. J. Fan, Y. Kurata, and Y. Nannichi, Jpn. J. Appl. Phys. 28, 2255 (1989).
220. H. Oigawa. J. Fan, Y. Nannichi, H. Sugahara, and M. Oshima, Jpn. J. Appl. Phys. 30, 322 (1990). 221. K. C. Hwang and S. S. Li, J. Appl. Phys. 67, 2162 (1990). 222. J. L. Lee, D. Kim, S. J. Maeng, H. H. Park, J. Y. Kang, and Y. T. Lee, J. Appl. Phys. 73, 3539 (1993). 223. H. Sugahara, M. Oshima, H-Oriwa, and Y. Nannichi, J. Vac. Sci. Technol. A 11, 52 (1993). 224. J. E. Samaras and R. B. Darling, J. Appl. Phys. 72, 168 (1992). 225. M. Sugiyama, S. Maeyama, T. Scimeca, M. Oshima, H. Oigawa, and Y. Nannichi, Appl. Phys. Lett. 63, 2540 (1993). 226. Z. Chen, W. Kim, A. Salvador, S. N. Mohammad, O. Aktas, and H. Morkoc, J. Appl. Phys. 78, 3920 (1995). 227. H. Xu, X. Belkouch, C. Aktik, and W. Rasmussen, Appl. Phys. Lett. 66, 2125 (1995). 228. L. B. Chang, H. T. Wang, and L. C. Yang, Jpn. J. Appl. Phys. 36, 3429 (1997). 229. G. Wang, G. Y. Zhao, T. Soga, T. Jimbo, and M. Umeno. Jpn. J. Appl. Phys. 37, L 1280 (1998). 230. J. Lin, M. J. Hwu, and L. B. Chang, Jpn, J. Appl. Phys. 37, L 1437 (1998). 231. M. J. Jeng, H. T. Wang, L. B. Chang, Y. C. Cheng, and S. T. Chou, J. Appl. Phys. 86, 6261 (1999). 232. L. B. Chang, Y. C. Cheng, and H. T. Wang, Jpn. J. Appl. Phys. 37, 811 (1998). 233. L. B. Chang, H. T. Wang, Y. C. Cheng, T. W. Shong, and E. K. Lin, Microelectronics Journal 30, 521 (1999). 234. G. Wang, T. Ogawa, T. Soga, T. Egawa, T. Jimbo, and M. Umeno, Appl. Surf. Sci. 159, 191 (2000). 235. J. Ivanco, H. Kobayashi, J. Almedia, and G. Margaritondo, J. Appl. Phys. 87, 795 (2000). 236. H. Y. Nie and Y. Nannichi, Jpn. J. Appl. Phys. 30, 906 (1991). 237 237. H. Y. Nie and Y. Nannichi, Jpn. J. Appl. Phys. 32, L 890 (1993). 238. H. Y. Nie and Y. Nannichi, J. Appl. Phys. 76, 4205 (1994). 239. H. Y. Nie, J. Appl. Phys. 87, 4327 (2000). 240. E C. M. Christianen, E J. Van Hall, H. J. A. Bluyssen, and J. H. Wolter, Semicond. Sci. Technol. 9, 707 (1994). 241. E C. M. Christianen, E J. van Hall, H. J. A. Bluyssen, M. R. Leys, L. Drost, and J. H. Wolter, J. Appl. Phys. 80, 6831 (1996). 242. S. Datta and B. Das, Appl. Phys. Lett. 56, 665 (1990). 243. D. J. Monsma, R. Vlutters, and J. C. Lodder, Science 281,407 (1998). 244. M. Oestreich, J. Hubner, D. Hagele, E J. Klar, W. Heimbrodt, W. W. Ruble, D. E. Ashenford, and B. Lunn, Appl. Phys. Lett. 74, 1251 (1999). 245. R. Flederling, M. Deim, G. Reuscher, W. Ossau, G. Schmidt, A. Waag, and L. W. Molenkamp, Nature 402, 787 (1999). 246. Y. Ohno, D. K. Young, B. Beschoten, E Matukura, H. Ohno, and D. D. Awshalom, Nature 402, 790 (1999). 247. G. A. Prinz, Science 250, 1092 (1990). 248. M.W.J. Prins, H. van Kempen, H. van LeuKen, R. A. de Groot, W van Roy, and J. De Boeck, J. Phys. Condens. Matter 7, 9447 (1995). 249. B. T. Jonker, O. J. Glembocki, R. T. Holm, and R. J. Wagner, Phys. Rev. Lett. 79, 4886 (1997). 250. T. Manago, S. Miyanishi, H. Akinaga, W. van Roy, R. E B. Roelfsema, T. Sato, E. Tamura, and S. Yuasa, J. Appl. Phys. 88, 2043 (2000). 251. W. D. Johnston, Jr., Appl. Phys. Lett. 24, 494 (1974). 252. W. D. Johnston, Jr., and R. A. Logau, Appl. Phys. Lett. 28, 140 (1976). 253. C. H. Henry and R. A. Logan, Appl. Phys. Lett. 31,203 (1977). 254. M. Ettenberg, C. J. Nuese, and G. H. Olsen, J. Appl. Phys. 48, 2543 (1977). 255. R. U. Martinelli and G. H. Olsen, J. Appl. Phys. 47, 1332 (1976). 256. G. H. Olsen, M. Ettemberg, and R. V. D'Aiello, Appl. Phys. Lett. 33, 606 (1978). 257. E A. J. M. Driessen, Appl. Phys. Lett. 67, 2813 (1995). 258. Y. Rosenwaks, B. R. Thacker, K. Bertness, and A. J. Nozik, J. Phys. Chem. 99, 7871 (1995). 259. Y. Rosenwaks, B. R. Thacker, and A. J. Nozik, Appl. Surf. Sci. 106, 396 (1996).
S U R F A C E A N D I N T E R F A C I A L R E C O M B I N A T I O N IN S E M I C O N D U C T O R S
260. M. P~tsslack, M. Hong, E. E Schubert, J. R. Kwo, J. E Mannaerts, S. N. G. Chu, N. Moriya, and E A. Thiel, Appl. Phys. Lett. 66, 625 (19951. 261. J. A. Kash, B. Pezeshki, E Agahi, and N. A. Bojarczuk, Appl. Phys. Lett. 67, 2022 (1995). 262. J. Saito, K. Nanbu, T. Ishikawa, and T. Kondo, J. Appl. Phys. 63, 404 (1988,. 263. V. N. Bessolov, V. V. Evrtropov, M. V. Lebedev, and V. V. Rossin, Phys. gev. B 51, 16801 (1995). 264. K. Aktmoto, K. Tamamura, J. Ogawa, Y. Mori, and C. Kozima, J. Appl. Phys. Ii3, 460 (1988). 265. X. Ca:,, X. Hou, X. Y. Chen, Z. Li, R. Su, X. Ding, and X. Wang, Appl. t'hys. Lett. 70, 747 (1997). 266. S. Shikata, H. Okada, and H. Hayashi, J. Appl. Phys. 69, 2717 (1991). 267. R. N. Nottenburg, C. J. Sandroff, D. A. Humphrey, T. H. Hollenbeck, and R. Bhat, Appl. Phys. Lett. 52, 218 (1988). 268. L.M. Smith, D. J. Wolford, R. Venkatasubramanian, and S. K. Gandhi, Appl. Phys. Lett. 57, 1572 (1990). 269. E Dawson and K. Woodbridge, Appl. Phys. Len. 45, 1227 (1984). 270. J. E. Fouquet and R. D. Burnham, IEEE J. Quantum Electron. QE-22, 1799 (1986). 271. H. Matsueda and K. Hara, Appl. Phys. Lett. 55, 362 (1989). 272. A. Hariz, E D. Dapkus, H. C. Lee, W. E Menu, and S. E DenBaars. Appl. Phys. Lett. 54, 635 (1989). 273. E. Yablonovitch, R. Bhat, and J. P. Harbison, Appl. Phys. Lett. 50, 1197 (1987). 274. L. W. Molenkamp and H. F. J. Van't Blik, J. Appl. Phys. 64, 4253 (1988). 275. H. Kizuki, M. Miyashita, Y. Kajikawa, and Y. Mihashi, Jpn. J. Appl. Phys. 36, 6290 (1997). 276. D. Yang, J. W. Garland, P. M. Raccah, C. Coluzza, P. Frankl, M. Capizzi, E Chambers, and G. Devane, Appl. Phys. Lett. 57, 2829 (1990). 277. M. MuUenborn and N. M. Haegel, J. Appl. Phys. 74, 5748 (1993). 278. J. S. Rimmer, B. Hamitton, P. Dawson, M. Missous, and A. R. Peaker, J. Appl. Phys. 73, 5032 (1993). 279. Y. Rosenwaks, B. R. Thacker, R. K. Ahrenkiel, A. J. Nozik, and I. Ya~,neh, Phys. Rev. B 50, 1746 (1994). 280. Y. Rosenwaks, A. J. Nozik, and I. Yavneh, Appl. Phys. Lett. 75, 4255 (1994). 281. R. M. Sieg, J. A. Carlin, S. A. Ringel, M. T. Currie, S. M. Ting, T. A. Langdo, G. Taraschi, E. A. Fitzgerald, and B. M. Keyes, Appl. Phys. Lett. 73. 3111 (1998). 282. I. Reich, E Diaz, T. Prutskij, J. Mendoza, H. Vargas, and E. Marin, J. Appl. Phys. 86, 6222 (1999). 283. C. Colvard, D. Bimberg, K. Alavi, C. Maierhofer, and N. Nouri, Phys. Rev. B 39, 3419 (1989). 284. E Sheldon, B. M. Keyes, R. K. Ahrenkiel, and S. E. Asher, J. Vac. Sci. Technol A 11, 1011 (1993). 285. R. J. Nelson and R. G. Sobers, Appl. Phys. Lett. 32, 761 (1978). 286. G. W.'t Hooft, M. R. Lays, and E Roozeboom, Jpn. J. Appl. Phys. 24, L 761 (1985). 287. H. Yokoyama, H. Iwata, M. Sugimoto, K. Onabe, and R. Lang, J. Appl. Phys. 63, 4755 (1988). 288. H. Iwata, H. Yokoyama, M. Sugimoto, N. Hamao, and K. Onabe, Appl. Phys. Lett. 54, 2427 (1989). 289. D.J. Wolford, G. D. Gilliland, T. E Kuech, L. M. Smith, J. Martinsen, J. A. Bradley, C. E Tsang, R. Venkatasubramanian, S. K. Gandhi, and H. P. Hjalmarson, J. Vac. Sci. Technol. B 9, 2369 (1991). 290. G. D. Gilliland, D. J. Wolford, T. E Kuech, and J. A. Bradley, Appl. Phys. Lett. 59, 216 (1991). 291. G. D. Gilliland, D. J. Wolford, T. F. Kuech, J. A. Bradley, and H. E Hjalmarson, J. Appl. Phys. 73, 8386 (1993). 292. D. J. Wolford, G. D. Gilliland, T. E Kuech, J. E Klem, H. E Hjalmarson, J. A. Bradley, C. F. Tsang, and J. Martinsen, Appl. Phys. Lett. 64, 1416 (1994).
275
293. E. M. Clausen, Jr., H. G. Craighead, J. M. Worlock, J. E Harbison, L. M. Schiavone, L. Florez, and B. van der Gaag, Appl. Phys. Lett. 55, 1427 (1989). 294. R. K. Ahrenkiel, J. M. Olson, D. J. Dunlavy, B. M. Keyes, and A. E. Kibbler, J. Vac. Sci. Technol. A 8, 3002 (1990). 295. J. M. Olson, R. K. Ahrenkiel, D. J. Dunlavy, B. Keyes, and A. E. Kibbler, Appl. Phys. Lett. 55, 1208 (1989). 296. J. H. Harris, S. Sugai, and A. V. Nurmikko, Appl. Phys. Lett. 40, 885 (1982). 297. M. Krahl, D. Bimberg, R. K. Bauer, D. E. Mars, and J. N. Miller, J. Appl. Phys. 67, 434 (1990). 298. V. Swaminathan, J. M. Freund, L. M. E Chirovsky, T. D. Harris, N. A. Kuebler, and L. A. D'Asaro, J. Appl. Phys. 68, 4116 (1990). 299. H. Hillmer, A. Forchel, T. Kuhn, G. Mahler, and H. E Meier, Phys. Rev. B 43, 13992 (1991). 300. N. Hamao, M. Sugimoto, and H. Yokoyama, Appl. Phys. Lett. 59, 1488 (1991). 301. J. M. Mison, K. Elcess, E Houzay, J. Y. Marzin, J. M. Gerard, E Barthe, and M. Bensoussan, Phys. Rev. B 41, 12945 (1990). 302. J. S. Rimmer, I. D. Hawkins, and B. Hamilton, J. Appl. Phys. 70, 5404 (1991). 303. D. Richards, J. Wagner, A. Fischer, and K. Ploog, Appl. Phys. Lett. 61, 2685 (1992). 304. G. D. Gilliland, D. J. Wolford, T. E Kuech, and J. A. Bradley, Phys. Rev. B 43, 14251 (1991). 305. Y.J. Ding, J. V. D. Veliadis, and J. B. Khurgin, J. Appl. Phys. 75, 1727 (1994). 306. K. Xie, C. R. Wie, J. A. Varriano, and G. W. Wicks, Appl. Phys. Lett. 60, 428 (1992). 307. Q. X. Zhao, J. E Bergman, E O. Holtz, B. Monemar, K. Ensslin, M. Sundaram, J. L. Merz, and A. C. Gossard, Superlattices and Microstructuress 9, 161 (1991). 308. K. Mochizuki, J. Kasai, and T. Tanoue, Appl. Phys. Len. 66, 2092 (1995). 309. Y. L. Chang, I. H. Tan, Y. H. Zhang, J. Merz, E. Hu, A. Frova, and V. Emiliani, Appl. Phys. Lett. 62, 2697 (1993). 310. J.M. Zavada, E Voillot, N. Lauret, R. G. Wilson, and B. Theys, J. Appl. Phys. 73, 8489 (1993). 311. J. R. Botha and A. W. R. Leitch, Appl. Phys. Lett. 63, 2534 (1993). 312. I.A. Buyanova, A. C. Ferreira, E O. Holtz, B. Monemar, K. Campman, J. L. Merz, and A. C. Gossard. Appl. Phys. Lett. 68, 1365 (1996). 313. S. S. Shi., E. L. Hu, J. E Zhang, Y. I. Chang, E Parikh, and U. Mishra, Appl. Phys. Lett. 70, 1293 (1997). 314. G. Wang, T. Soga, T. Egawa, T. Jimbo, and M. Umeno, Electronics Lett. 36, 1462 (2000). 315. H. Lipsanen, M. Sopanen, M. Taskinen, and J. Tulkki, Appl. Phys. Len. 68, 2216 (1996). 316. S. Datta, M. R. Melloch, S. Bandyophadhyay, and R. Norens, Phys. Rev. Lett. 55, 2344 (1985). 317. K. Ishibashi, Y. Takagaki, K. Gamo, S. Namba, S. Ishida, K. Murase, Y. Aoyagi, and M. Kawabes, Solid State Commun. 64, 573 (1987). 318. B.J. van Wees, H. van Houten, C. W. J. Beenakker, J. G. Williamson, L. E Kouwenhoven, D. van der Marel, and C. T. Foxon, Phys. Rev. Lett. 29, 848 (1988). 319. D. A. Wharam, T. J. Thornton, R. Newbury, M. Pepper, H. Ahmed, J. E. E Frost, D. G. Hasko, D. C. Peacock, D. A. Ritchie, and G. A. C. Jones, J. Phys. C 21, L209 (1988). 320. G. Timp, H. U. Baranger, E de Vevgar, J. E. Cunningham, R. E. Howard, R. Behringer, and E M. Mankiewich, Phys. Rev. Lett. 60, 2081 (1988). 321. H. Sakaki, Jpn. J. Appl. Phys. 19, L 735 (1980). 322. S. Datta, M. R. Melloch, S. Bandyopadhyay, and M. S. Lundstrom, Appl. Phys. Lett. 48, 487 (1986). 323. A. Arakawa, K. Vahala, and A. Yariv, Appl. Phys. Len. 45,950 (1984). 324. A. Kasi Viswanath, K. Hiruma, K. Ogawa, and T. Katsuyama, "Proc. Intern. Conf. Solid State Devices and Materials," 1992, p. 296.
276
KASI VISWANATH
325. A. Kasi Viswanath, K. Hiruma, and T. Katsuyama, Superlattices and Microstructures 14, 105 (1993). 326. A. Kasi Viswanath, K. Hiruma, K. Ogawa, and T. Katsuyama, Appl. Phys. Commun. 13, 55 (1994). 327. A. Kasi Viswanath, K. Hiruma, M. Yazawa, K. Ogawa, and T. Katsuyama, Microwave and Optical Technol. Lett. 7, 94 (1994). 328. G. Duggan, H. I. Ralph, and R. J. Elliot, Solid State Commun. 56, 17 (1985). 329. G. Mayer, B. E. Maile, R. Germann, A. Forchel, P. Gambrow, and H. P. Meier, Appl. Phys. Lett. 56, 2016 (1990). 330. R. K. Ahrenkiel, "Properties of InP," INSPEC, London, 1991, p. 77. 331. H. C. Carey, Jr. and E. Buehler, Appl. Phys. Lett. 30, 247 (1977). 332. J. M. Moison, M. van Rombay, and M. Bensoussan, Appl. Phys. Lett. 48, 1362 (1986). 333. R. K. Ahrenkiel, D. J. Dunlavy, and T. Hanak, J. Appl. Phys. 64, 1916 (1988). 334. C. A. Hoffman, H. J. Gerritsen, and A. V. Nurmikko, J. Appl. Phys. 51, 1603 (1978). 335. S. Bothra, S. Tyagi, S. K. Gandhi, and J. M. Borrego, Solid State Electron 34, 47 (1991). 336. Y. Rosenwaks, Y. Shapira, and D. Huppert, Phys. Rev. B 44, 13097 (1991). 337. Y. Rosenwaks, Y. Shapira, and D. Huppert, Appl. Phys. Lett. 57, 2552 (1990). 338. Y. Rosenwaks, Y. Shapira, and D. Huppert, Phys. Rev. B 45, 9108 (1992). 339. A. Liu and Y. Rosenwaks. J. Appl. Phys. 86, 430 (1999). 340. Y. Rosenwaks, B. R. Thacker, A. J. Nozik, R. J. Ellingson, K. C. Burr, and C. L. Tang, in "Ultrafast Phenomena IX," P. E Barbara, W. H. Knox, G. A. Mourou, and A. H. Zewail, eds., "Springer Series in Chemical Physics," Vol. 60, Springer-Verlag, Berlin, 1994, p. 409. 341. Y. Rosenwaks, B. R. Thacker, A. J. Nozik, R. J. Ellingson, K. C. Burr, and C. L. Tang, J. Phys. Chem. 98, 2739 (1994). 342. Y. Rosenwaks, B. R. Thacker, A. J. Nozik, Y. Shapira, and D. Huppert, J. Phys. Chem. 97, 10421 (1993). 343. S. Krawczyk, M. Bejar, A. Khoukh, R. Blanchet, B. Sermage, D. Cui, and D. Pavlidis, Jpn. J. Appl. Phys. 38, 992 (1999). 344. W. Zhuang, C. Chen, D. Teng, J. Yu, and Y. Li, J. Vac. Sci. Technol. A 9, 983 (1991). 345. J. Lammasniemi, K. Tappura, and K. Smekalin, J. Appl. Phys. 77, 4801 (1995). 346. R. Iyer, R. R. Chang, and D. L. Lile, Appl. Phys. Lett. 53, 134 (1988). 347. R. Leonelli, C. S. Sundararaman, and J. E Currie, Appl. Phys. Lett. 57, 2678 (1990). 348. H. Oigawa, J. E Fan, Y. Nannichi, H. Sugahara, and M. Oshima, Jpn. J. Appl. Phys. 30, L 322 (1991). 349. R. Iyer, and D. L. Lile, Appl. Phys. Lett. 59, 437 (1991). 350. Z. H. Lu, M. J. Graham, X. H. Feng, and B. X. Yang, Appl. Phys. Lett. 60, 2773 (1992). 351. F. Maeda, Y. Watanabe, and M. Oshima, Appl. Phys. Lett. 62, 297 (1993). 352. D. Gallet and G. Hollinger, Appl. Phys. Lett. 62, 982 (1993). 353. K. Vaccaro, H. M. Dauplaise, A. Davis, S. M. Spaziani, and J. P. Lorenzo, Appl. Phys. Lett. 67, 527 (1995). 354. I. K. Han, D. H. Woo, H. J. Kim, E. K. Kim, J. I. Lee, S. H. Kim, K. N. Kang, H. Lim, and H. L. Park, J. Appl. Phys. 80, 4052 (1996). 355. M. Shimomura, K. Naka, N. Sanada, Y. Suziki, Y. Fukuda, and P. J. Moiler, J. Appl. Phys. 79, 4193 (1996). 356. I. K. Han, E. K. Kim, J. I. Lee, S. H. Kim, K. N. Kang, Y. Kim, H. Lim, and H. L. Park, J. Appl. Phys. 81, 6986 (1997). 357. S. Morikata, T. Motegi, and H. Ikoma, Jpn, J. Appl. Phys. 38, L 1512 (1999). 358. L. He, H. Dauplaise, A. Davis, E. Martin, S. Spaziani, K. Vaccaro, W. Waters, and J. P. Lorenzo, Jpn. J. Appl. Phys. 38, 1119 (1999). 359. G. Hollinger, D. Gallet, M. Gendry, M. B. Besland, and J. Joseph, Appl. Phys. Lett. 59, 1617 (1991).
360. Y. Robach, M. E Besland, J. Joseph, G. Hollinger, E Viktorovitch, E Ferret, M. Pitaval, A. Falcou, and G. Post, J. Appl. Phys. 71, 2981 (1992). 361. J. M. Moison, M. Van Rompay, and M. Bensoussan, Appl. Phys. Lett. 48, 1362 (1986). 362. J. Chave, A. Choujaa, C. Santinelli, R. Blanchet, and E Viktorovitch, J. Appl. Phys. 61,257 (1987). 363. Y. Mada and K. Wada, J. Appl. Phys. 83, 2025 (1998). 364. H. Nobusawa and H. Ikoma, Jpn. J. Appl. Phys. 32, 3713 (1993). 365. H. Hayashi, I. Hatanaka, S. Sato, and H. Ikoma, Jpn. J. Appl. Phys. 36, 4235 (1997). 366. T. Sugino, H. Ninomiya, T. Yamada, J. Shirafuji, and K. Matsuda, Appl. Phys. Lett. 60, 1226 (1992). 367. G. Bruno, M. Losurdo, E Capezzuto, V. Capozzi, T. Trovato, G. Perna, and G. E Lorusso, Appl. Phys. Lett. 69, 685 (1996). 368. S. Balasubramanian, K. S. R. Koteswara Rao, N. Bala Subramaniam, and V. Kumar, J. Appl. Phys. 77, 5398 (1995). 369. H. Takahashi, T. Hashizume, and H. Hasegawa, Appl. Surf. Sci. 123/124, 615 (1998). 370. H. Takahashi, T. Hashizume, and H. Hasegawa, Jpn. J. Appl. Phys. 38, 1128 (1999). 371. H. Takahashi, T. Yoshida, M. Mutoh, T. Sakai, and H. Hasegawa, Solid State Electron. 43, 1561 (1999). 372. H. Hasegawa, Jpn. J. Appl. Phys. 38, 1098 (1999). 373. S. K. Krawczyk and G. Hollinger, Appl. Phys. Lett. 45, 870 (1984). 374. A. Kapila and V. Malhotra, Appl. Phys. Lett. 62, 1009 (1993). 375. R. Driad, Z. Hong, S. Laframboise, D. Scansen, W. R. McKinnon, and S. E McAlister, Jpn. J. Appl. Phys. 38, 1124 (1999). 376. R.M. Cohen, M. Kitamura, and Z. M. Fang, Appl. Phys. Lett. 50, 1675 (1987). 377. S. D. Lester, T. S. Kim, and B. G. Stretman, J. Appl. Phys. 62, 2950 (1987). 378. K. Asakawa, T. Yoshikawa, S. Kohmoto, Y. Nambu, and Y. Sugimoto, Jpn, J. Appl. Phys. 37, 373 (1998). 379. E S. Dutta, K. S. Sangunni, H. L. Bhat, and V. Kumar, Appl. Phys. Lett. 65, 1695 (1994). 380. A. Kasi Viswanath, in "Handbook of Advanced Electronic and Photonic Materials and Devices," in "Semiconductors," Vol. 1, H. S. Nalwa, ed., Academic Press, 2000. 381. S. Nakamura, in "Group III Nitride Semiconductor Compounds," B. Gil, ed., Clarendon Press, Oxford, 1998. 382. S. Nakamura, and G. Fasol, "The Blue Laser Diode," Springer Verlag, Berlin, 1997. 383. A. K. Viswanath, J. I. Lee, S. Yu, D. Kim, Y. Choi, and C. H. Hong, J. Appl. Phys. 84, 3848 (1998). 384. A. K. Viswanath, J. I. Lee, D. Kim, C. R. Lee, and J. Y. Leem. Phys. Rev. B 58, 16333 (1998). 385. S. J. Pearton, C. R. Abernathy, J. D. Mackenzie, U. Hommerich, X. Wu, R. G. Wilson, R. N. Schwartz, J. M. Zavada, and E Ren, Appl. Phys. Lett. 71, 1807 (1997). 386. H. Okumura, H. Hamaguchi, G. Feuillet, Y. Ishida, and S. Yoshida, Appl. Phys. Lett. 72, 3056 (1998). 387. W. Wang, T. Nohova, S. Krishnankutty, R. Torreano, S. Mc Pherson, and H. Marsh, Appl. Phys. Lett. 73, 1086 (1998). 388. J. Sun, K. A. Rickert, J. M. Redwing, A. B. Ellis, E J. Himpsel, and T. E Kuech, Appl. Phys. Lett. 76, 415 (2000). 389. T. Rotter, D. Mistele, J. Stemmer, E Fedler, H. Aderhold, J. Graul, V. Schwegler, C. Kirchner, M. Kamp, and M. Heuken, Appl. Phys. Lett. 76, 3923 (2000). 390. C. Huh, S. W. Kim, H. S. Kim, I. H. Lee, and S. J. Park, J. Appl. Phys. 87, 4591 (2000). 391. Y. J. Lin, C. D. Tsai, Y. T. Lyu, and C. T. Lee, Appl. Phys. Lett. 77, 687 (2000). 392. U. Behn, A. Thamm, O. Brandt, and H. T. Grahn, J. Appl. Phys. 87, 4315 (2000). 393. J. L. Lee, L. Wei, S. Tanigawa, H. Oigawa, and Y. Nannichi, J. Appl. Phys. 70, 2877 (1991).
S U R F A C E A N D I N T E R F A C I A L R E C O M B I N A T I O N IN S E M I C O N D U C T O R S
394. M. Mizuta, Y. Mochizuki, N. Takadoh, and K. Asakawa, J. Appl. Phys. 66, 891 (1989). 395. M. Katayama, M. Aono, H. Oigawa, Y. Nannichi, H. Sugahara, and M. Oshima, Jpn. J. Appl. Phys. 30, L 786 (1991). 396. Z. Sobiesierski, S. A. Clark, R. H. Williams, A. Tabata, T. Benyattou, G. Guillot, M. Gendry, G. Hollinger, and P. Viktorovitch, Appl. Phys. Lett. 58, 1863, (1991). 397. M. J. Kane, G. Braithwaite, M. T. Emeny, D. Lee, T. Martin, and D. R. Wright, Appl. Phys. Lett. 76, 943 (2000). 398. S. Farad, Appl. Phys. Lett. 76, 2707 (2000). 399. S. Farad, R. Leon, D. Leonard, J. L. Merz, and E M. Petroff, Phys. Re~'. B 52, 5752 (1995). 400. J. Wagner, A. L. Alvarez, J. Schmitz, J. D. Ralston, and P. Koidl, Appl. Phys. Lett. 63, 349 (1993). 401. K. Tai, T. R. Hayes, S. L. McCall, and W. T. Tsang, Appl. Phys. Lett. 53, 302 (1988). 402. T. P. Pearsall, Ed., "GaInAsP Alloy Semiconductors," Wiley, New York, 1982. 403. K. Tai, J. L. Jewell, W. T. Tsang, H. Temkin, M. Panish, and Y. Twu, Appl. Phys. Lett. 50, 795 (1987). 404. Y. H. Lee, H. M. Gibbs, J. L. Jewell, J. E Duffy, T. Venkatesan, A. C. Gossard, W. Wiegmann, and J. H. English, Appl. Phys. Len. 49, 486 (1986). 405. E. E Ippen and C. V. Shank, in "Ultrashort Light Pulses," S. L. Shapiro, ed., Springer, New York, 1977. 406. B.E. Maile, A. Forchel, R. Germann, and D. Grutzmacher, Appl. Phys. Lett. 54, 1552 (1989). 407. Y. Nambu, H. Yokoyama, T. Yoshikawa, Y. Sugimoto, and K. Asakawa, Appl. Phys. Lett. 65, 481 (1994). 408. S. Q. Gu, X. Liu, M. Covington, E. Reuter, H. Chang, R. Panepucci, I. Adesida, S. G. Bishop, C. Caneau, and R. Bhat, J. Appl. Phys. 75, 8071 (1994). 409. B. Hubner, B. Jacobs, Ch. Greus, R. Zengerle, and A. Forchel, J. Vac. Sci. Technol. B 12, 3658 (1994). 410. E Kieseling, W. Braun, P. Ils, M. Michel, A. Forchel, I. Gyuro, M. Klenk, and E. Zielinski, Phys. Rev. B 51, 13809 (1995). 411. P. W. Juodawlkis and S. E. Ralph, Appl. Phys. Lett. 76, 1722 (2000). 412. M. Boroditsky, I. Gontijo, M. Jackson, R. Vrijen, E. Yablonovitch, T. Krauss, C. C. Cheng, A. Scherer, R. Bhat, and M. Krames, J. Appl. Phys. 87, 3497 (2000). 413. M. J. Joyce, M. Gal, and J. Tann, J. Appl. Phys. 65, 1377 (1989). 414. C. Juang, J. E Hsu, I. S. Yen, and H. S. Shiau, J. Appl. Phys. 72, 684 (1992). 415. M. K. Parry and A. Krier, Thin Solid Films 259, 163 (1995). 416. K. C. Hwang, S. S. Li, C. Park, and T. J. Anderson, J. Appl. Phys. 67, 6571 (I 990). 417. U. Schade, St. Kollakowski, E. H. Bottchev, and D. Bimberg, Appl. Phys. l.ett. 64, 1389 (1994). 418. V. N. Bessolov, M. V. Lebedev, Y. M. Shernyakov, B. V. Tsarenkov, MateJ: Sci. Engg. B 44, 380 (1997). 419. R. Dria,t, Z. H. Lu, S. Charbonneau, W. R. McKinnon, S. Laframboise, P. J. l'~,ole, and S. E McAlister, Appl. Phys. Len. 73, 665 (1998). 420. R. Driad, Z. H. Lu, W. R. McKinnon, S. Laframboise, S. P. McAlister, P. J. Pool, S. Raymond, and S. Charbonneau, Mat. Res. Soc. Symp. Proc. 573, 227 (1999). 421. H. Lipsanen, M. Sopanen, J. Ahopelto, J. Sandmann, and J. Feldmann, Jpn. J. Appl. Phys. 38, 1133 (1999). 422. R. Driad, Z. H. Lu, S. Laframboise, D. Scansen, W. R. McKinnon, and S. P. McAlister, Jpn. J. Appl. Phys. 38, 1124 (1999). 423. R. Driad, W. R. McKinnon, Z. H. Lu, S. P. McAlister, E J. Poole, and S. Charbonneau, J. Vac. Sci. Technol. A 18, 697 (2000). 424. M. Gal, A. Tavendale, M. J. Johnson, and B. F. Usher, J. Appl. Phys. 66, 968 (1989). 425. M. Capizzi, C. Coluzza, V. Emiliani, E Frankl, A. Frova, E Sarto, A. A. Bonapasta, Z. Sobiesierski, and R. N. Sacks, J. Appl. Phys. 72, 1454 (1992).
277
426. S. M. Lord, G. Roos, J. S. Harris, Jr., and N. M. Johnson, J. Appl. Phys. 73, 740 (1993). 427. Y. L. Chang, I. H. Tan, E. Hu, J. Merz, V. Emiliani, and A. Frova, J. Appl. Phys. 75, 3040 (1994). 428. Y. L. Chang, I. H. Tan, C. Reaves, J. Merz, E. Hu, A. Frova, V. Emiliani, and B. Bonnani, Appl. Phys. Lett. 64, 2658 (1994). 429. E. Yablonovitch, H. M. Cox, and T. J. Gmitter, Appl. Phys. Lett. 52, 1002 (1988). 430. Y. Narukawa, Y. Kawakami, S. Fujita, S. Fujita, and S. Nakamura, Phys. Rev. B 55, R 1938 (1997). 431. A. Satake, Y. Masumoto, T. Miyajima, T. Asatsuma, E Nakamura, and M. Ikeda, Phys. Rev. B 57, R 2041 (1998). 432. Y. Narakawa, Y. Kawakami, S. Fujita, and S. Nakamura, Phys. Rev. B 59, 10283 (1999). 433. Y. K. Song, M. Kuball, A. V. Nurmikko, G. E. Bulman, K. Doverspike, S. T. Sheppard, T. W. Weeks, M. Leonard, H. S. Kong, H. Dieringer, and J. Edmond, Appl. Phys. Lett. 72, 1418 (1998). 434. S. Bidnyk, T. J. Schmidt, Y. H. Cho, G. H. Gainer, J. J. Song, S. Keller, U. K. Mishra, and S. P. DenBaars, Appl. Phys. Len. 72, 1623 (1998). 435. A. K. Viswanath, J. I. Lee, D. Kim, and H. G. Lee, unpublished. 436. A. K. Viswanath, J. I. Lee, and D. Kim, unpublished. 437. S. J. Pearton, E Ren, W. S. Hobson, C. R. Abernathy, R. L. Masaitis, and U. K. Chakrabarti, Appl. Phys. Lett. 63, 3610 (1993). 438. M. Mullenborn, K. Matney, M. S. Goorsky, N. M. Haegel, and S. M. Vernon, J. Appl. Phys. 75, 2418 (1994). 439. C. T. Lee, H. P. Shiao, N. T. Yeh, C. D. Tsai, Y. T. Lyu, and Y. K. Tu, Solid State Electron. 41, 1 (1997). 440. V. A. Gorbylev, A. A. Chelniy, A. Y. Polyakov, S. J. Pearton, N. B. Smirnov, R. G. Wilson, A. G. Milnes, A. A. Cnekalin, A. V. Govorkov, B. M. Leiferov, O. M. Borodina, and A. A. Balmashnov, J. Appl. Phys. 76, 7390 (1994). 441. J. C. Fan, J. C. Wang, and Y. F. Chen, Appl. Phys. Lett. 74, 1463 (1999). 442. C. D. Tsai and C. T. Lee, J. Appl. Phys. 87, 4230 (2000); C. D. Tsai, H. P. Shiao, C. T. Lee, and Y. K. Tu, IEEE Photonics Technology Len. 9, 660 (1997). 443. S.H. Lee, C. Y. Fetzer, G. B. Stringfellow, D. H. Lee, and T. Y. Seong, J. Appl. Phys. 85, 3590 (1999). 444. G. B. Stringfellow, R. T. Lee, C. M. Fetzer, J. K. Shurtleff, Y. Hsu, S. W. Jun, S. Lee, and T. Y. Seong, J. Electron. Mater. 29, 134 (2000). 445. J. K. Shurtleff, R. T. Lee, C. M. Fetzer, and G. B. Stringfellow, Appl. Phys. Lett. 75, 1914 (1999). 446. S. W. Jun, C. M. Fetzer, R. T. Lee, J. K. Shurtleff, and G. B. Stringefellow, Appl. Phys. Lett. 76, 2716 (2000). 447. C. M. Fetzer, R. T. Lee, J. K. Shurtleff, G. B. Stringfellow, S. M. Lee, and T. Y. Seong, Appl. Phys. Lett. 76, 1440 (2000). 448. R.T. Lee, J. K. Shurtleff, C. M. Fetzer, G. B. Stringfellow, S. Lee, and T. Y. Seong, J. Appl. Phys. 87, 3730 (2000). 449. T. Benyattou, M. A. Garcia, S. Moueger, A. Tabata, M. Sacilotti, E Abraham, Y. Monteil, and R. Landers, Appl. Surf. Sci. 63, 197 (1993). 450. M. Sacilotti, P. Abraham, M. Pitaval, M. Ambri, T. Benyattou, A. Tabata, M. A. G. Perez, P. Motisuke, R. Landers, J. Morais, N. Le Corre, and S. Loualiche, Can. J. Phys. 74, 202 (1996). 451. J. S. Hwang, W. Y. Chou, S. L. Tyan, H. H. Lin, and T. L. Lee, Appl. Phys. Lett. 67, 2350 (1995). 452. W. Y. Chou, G. S. Chang, W. C. Hwang, and J. S. Hwang, J. Appl. Phys. 83, 3690 (1998). 453. G. S. Chang, W. C. Hwang, Y. C. Wang, Z. E Yang, and J. S. Hwang, J. Appl. Phys. 86, 1765 (1999). 454. J. M. Pikal, C. S. Menoni, E Thiagarajan, G. Y. Robinson, and H. Temkin, Appl. Phys. Lett. 76, 2659 (2000). 455. C. H. Henry, R. A. Logan, and E R. Merritt, J. Appl. Phys. 49, 3530 (1978). 456. M. L. Timmons, T. S. Colpitts, R. Venkatasubramanian, B. M. Keyes, D. J. Dunlavy, and R. K. Ahrenkiel, Appl. Phys. Lett. 56, 1850 (1990).
278
KASI VISWANATH
457. H. Gebretsadik, K. Zhang, K. Kamath, and E Bhattacharya, Appl. Phys. Lett. 71, 3865 (1997). 458. V. L. Berkovits, V. M. Lantratov, T. V. L'vova, G. A. Shakiashvili, V. P. Ulin, and D. Paget, Appl. Phys. Lett. 63, 970 (1993). 459. S. Gotoh and H. Horikawa, Appl. Phys. Lett. 69, 641 (1996). 460. L. Pavesi, E Marteli, D. Martin, and E K. Reinhart, Appl. Phys. Lett. 54, 1522 (1989). 461. G. Wang, T. Ogawa, K. Ohtsuka, G. Y. Zhao, T. Soga, T. Jimbo, and M. Umeno, Jpn. J. Appl. Phys. 38, L 796 (1999). 462. R. A. J. Thommer, A. van Geelen, S. M. Olsthoorn, G. J. Bauhuis, M. van Schalkwijk, and L. J. Giling, "Proc. Int. Sym. GaAs and Related Compounds," Frieburg, Germany, 1993. 463. J. D. Dow and R. E. Allen, Appl. Phys. Lett. 41,672 (1982). 464. A. Mozer, K. M. Romanek, W. Schmidt, M. H. Pilkuhn, and E. Schlosser, Appl. Phys. Lett. 41,964 (1982). 465. K. Rajesh, L. J. Huang, W. M. Lau, R. Bruce, S. Ingrey, and D. Landheer, J. Appl. Phys. 81, 3304 (1997). 466. A. Y. Polyakov, M. Stam, A. G. Milnes, A. E. Bochkarev, and S. J. Pearton, J. Appl. Phys. 71, 4411 (1992). 467. D. Huppert, M. Evenor, and Y. Shapira, J. Vac. Sci. Technol. A 2, 532 (1984). 468. Y. Rosenwaks, L. Burstein, Y. Shapira, and D. Huppert, Appl. Phys. Lett. 57, 458 (1990). 469. Y. Rosenwaks, L. Burstein, Y. Shapira, and D. Huppert, J. Phys. Chem. 94, 6842 (1990). 470. Y. Rosenwaks, Y. Shapira, and D. Huppert, Vaccum 41, 1009 (1990). 471. R. K. Jain and R. C. Lind, J. Opt. Soc. Am. 73, 647 (1983). 472. S. S. Yao, C. Karaguleff, A. Gabel, R. Fortenberry, C. T. Seaton, and G. I. Stegeman, Appl. Phys. Lett. 46, 801 (1985). 473. G. R. Olbright and N. Peyghambarian, Appl. Phys. Lett. 48, 1184 (1986). 474. E Roussignol, D. Ricard, J. Lukasik, and C. Flytzanis, J. Opt. Soc. Am. B 4, 5 (1987). 475. G. Bret and E Gires, Appl. Phys. Lett. 4, 175 (1964). 476. N. Sarukura, Y. Ishida, T. Yanagawa, and H. Nakano, Appl. Phys. Lett. 57, 229 (1990). 477. B. Danielzik, K. Nattermann, and D. vonder Linde, Appl. Phys. B 38, 31 (1985). 478. T. J. Cullen, C. N. Ironside, C. T. Seaton, and G. I. Stegeman, Appl. Phys. Lett. 49, 1403 (1986). 479. B. J. Ainslie, H. P. Girdlestone, and D. Cotter, Electron. Lett. 23, 405 (1987). 480. J. Yumoto, S. Fukushima, and K. Kudobera, Opt. Lett. 12, 832 (1987). 481. N. Finlayson, W. C. Banyai, E. M. Wright, C. T. Seaton, G. I. Stegeman, T. J. Cullen, and C. N. Ironside, Appl. Phys. Lett. 53, 1144 (1988). 482. N. M. Lawandy and, R. L. MacDonald, J. Opt. Soc. Am. B 8, 1307 (1991). 483. S. Juodkazis, E. Bernstein, J. C. Plenet, C. Bovier, J. Dumas, J. Mugnier, and J. V. Vaitkus, Opt. Commun. 148, 242 (1998). 484. E. Vanagas, J. Moniatte, P. Riblet, B. Honerlage, S. Juodkazis, E Paille, J. C. Plenet, J. G. Dumas, M. Petrauskas, and J. Vailtkus, J. Appl. Phys. 81, 3586 (1997). 485. M. G. Bawendi, W. L. Wilson, L. Rothberg, P. J. Carroll, T. M. Jedju, M. L. Steigerwald, and L. E. Brus, Phys. Rev. Lett. 65, 1623 (1990). 486. K. Misawa, H. Yao, T. Hayashi, and T. Kobayashi, Chem. Phys. Lett. 183, 113 (1991). 487. H. Shinojima, J. Yumoto, and N. Uesugi, Appl. Phys. Lett. 60, 298 (1992). 488. T. Inokuma, T. Arai, and M. Ishikawa, Phys. Rev. B 42, 11093 (1990). 489. J. J. Shiang, S. H. Risbud, and A. P. Alivisatos, J. Chem. Phys. 98, 8432 (1993). 490. N. Peyghambarian, B. Fluegel, D. Hulin, A. Migus, M. Joffre, A. Antonetti, S. W. Koch, and M. Lindberg, IEEE J. Quantum Electron. QE-25, 2516 (1989). 491. S. Jursenas, M. Strumskis, A. Zukauskas, and A. I. Ekimov, Solid State Commun. 87, 577 (1993).
492. S. Jursenas, V. Stepankevicius, M. Strumskis, and A. Zukauskas, Semiconductor Sci. Technol. 10, 302 (1995). 493. A. Zukauskas and S. Jursenas, Appl. Phys. Lett. 67, 1095 (1995). 494. S. Jursenas, G. Kurilcik, M. Strumskis, and A. Zukauskas, Appl. Phys. Lett. 71, 2502 (1997). 495. S. Jursenas, G. Tamulaitis, G. Kurilcik, and A. Zukauskas, Appl. Phys. Lett. 72, 241 (1998). 496. P. Nemec, E Trojanek, and P. Maly, Phys. Rev. B 52, 8605 (1995). 497. J. P. Zheng, L. Shi, E S. Choa, P. L. Liu, and H. S. Kwok, Appl. Phys. Lett. 53, 643 (1988). 498. G. Allan, C. Delerue, and M. Lannoo, Phys. Rev. Lett. 76, 2961 (1996). 499. C. H. Henry and D. V. Lang, Phys. Rev. B 15, 989 (1977). 500. N. Chestnoy, T. D. Haris, R. Hull, and L. E. Brus, J. Phys. Chem. 90, 3393 (1986). 501. C. Burda and M. A. E1-Sayed, Pure Appl. Chem. 72, 165 (2000). 502. S. Logunov, T. Green, S. Marguet, and M. A. E1-Sayed, J. Phys. Chem. 102, 5652 (1998). 503. L. Spanhel, M. Haase, H. Weller, and A. Henglein, J. Am. Chem. Soc. 109, 5649 (1987). 504. M. L. Steigerwald and L. E. Brus, Acc. Chem. Res. 23, 183 (1990). 505. H. Weller, Adv. Mater 5, 88 (1993). 506. V. Klimov, P. Haring Bolivar, and H. Kurz, Phys. Rev. B 53, 1463 (1996); V. I. Klimov, P. Haring Bolivar, H. Kurz, and V. Karavinskii, Superlattices and Microstrucures 20, 395 (1996). 507. C. J. Murphy and A. B. Ellis, J. Phys. Chem. 94, 3082 (1990). 508. J. Z. Zhang and A. B. Ellis, J. Phys. Chem. 96, 2700 (1992). 509. G. J. Meyer, G. C. Lisensky, and A. B. Ellis, J. Am. Chem.Soc. 110, 4914 (1988). 510. G. C. Lisensky, R. L. Penn, C. J. Murphy, and A. B. Ellis, Science 248, 840 (1990). 511. G. J. Meyer, L. K. Leung, J. C. Yu, G. C. Lisensky, and A. B. Ellis, J. Am. Chem. Soc. 111, 5146 (1989). 512. C. J. Murphy and A. B. Ellis, Polyhedron 9, 1913 (1990). 513. C. J. Murphy, G. C. Lisensky, L. K. Leung, G. R. Kowach, and A. B. Ellis, J. Am. Chem. Soc. 112, 8344 (1990). 514. A. Henglein, Chem. Rev. 89, 1816 (1989). 515. M. L. Steigerwald and L. E. Brus, Annu. Rev. Mater Sci. 19, 471 (1989). 516. T. Dannhauser, M. O. Neil, K. Johansson, D. Whitten, and G. McLendon, J. Phys. Chem. 90, 6074 (1986). 517. N. S. Lewis, J. Phys. Chem. 102, 4843 (1998). 518. E. Lifshitz, I. D. Litvin, H. Porteanu, and A. A. Lipovskii, Chem. Phys. Lett. 295, 249 (1998). 519. A. Ekimov, J. Lumin. 70, 1 (1996). 520. J. Z. Zhang, R. H. O'Neil, and T. W. Roberti, Appl. Phys. Lett. 64, 1989 (1994). 521. M. Gratzel, "Heterogeneous Photochemical Electron Transfer," CRC, Boca Raton, FL, 1989. 522. M. A. Fox and M. T. Dulay, Chem. Rev. 93, 341 (1993). 523. P. V. Kamat, Chem. Rev. 93, 267 (1993). 524. N. Serpone and E. Pelizzetti, "Photocatalysis, Fundamentals and Applications," Wiley, New York, 1989, p 123. 525. J. R. Norris, Jr. and D. Meisel, Eds., "Photochemical Energy Conversion," Elsevier, New York, 1989. 526. (a) C. Burda, S. Link, T. C. Green, and M. A. E1-Sayed, J. Phys. Chem. 103, 10775 (1999). 526. (b) C. Burda, T. C. Green, S. Link and M. A. E1-Sayed, J. Phys. Chem. 103, 1783 (1999). 527. S. Hunsche, T. Dekorsy, V. Klimov, and H. Kurz, Appl. Phys. B 62, 3 (1996). 528. M. A. Hines and P. Guyot-Sionnest, J. Phys. Chem. 100, 468 (1996). 529. B. O. Dabbousi, J. Rodrigez-Viejo, E V. Mikulec, J. R. Heine, H. Mattoussi, R. Ober, and M. G. Bawendi, J. Phys. Chem. 101, 9463 (1997). 530. X. Peng, M. C. Schlamp, A. V. Kadavanich, U. Banin, and A. P. Alivisatos, J. Am. Chem. Soc. 119, 7019 (1997).
S U R F A C E A N D I N T E R F A C I A L R E C O M B I N A T I O N IN S E M I C O N D U C T O R S 531. D. t!. Fogg, L. H. Radzilowski, R. Blanski, R. R. Schrock, and E. L. Tho has, Micromolecules 30, 417 (1997). 532. D. L',. Fogg, L. H. Radzilowski, B. O. Dabbousi, R. R. Schrock, E. L. Tho~nas, and M. G. Bawendi, Macromolecules 30, 8433 (1997). 533. C. B Murray, D. J. Norris, and M. G. Bawendi, J. Am Chem. Soc. 115, 87()~, (1993). 534. H. lX~attoussi, L. H. Radzilowski, B. O. Dabbousi, D. E. Fogg, R. R. Schrock, E. L. Thomas, M. E Rubner, and M. G. Bawendi, J. Appl. PhyLa. 86, 4390 (1999). 535. Z. Zhu, H. Yoshihara, K. Takebayashi, and T. Yao, Appl. Phys. Lett. 63, 1678 (1993). 536. E B,:~ssler-Podorowski, D. Huppert, Y. Rosenwaks, and Y. Shapira, J. Pkvs. Chem. 95, 4370 (1991). 537. A. B. Ellis, R. J. Brainard, K. D. Kepler, D. E. Moore, E. J. Winder, T. F. Kuech, and G. C. Lisensky, J. Chem. Edu. 74, 680 (1997). 538. G. J. Meyer, G. C. Lisensky, and A. B. Ellis, J. Am. Chem. Soc. 110, 4914(1988). 539. D. R. Neu, J. A. Olson, and A. B. Ellis, J. Phys. Chem. 97, 5713 (19q3). 540. D. E. Moore, G. C. Lisensky, and A. B. Ellis, J. Am. Chem. Soc. 116, 9487 (1994). 541. J. C. Seo, D. Kim, and H. J. Kong, Appl. Phys. A 64, 445 (1997). 542. N. ]~. Borrelli, D. W. Hall, H. J. Holland, and D. W. Smith, J. Appl. Phys. 61, 5399 (1987). 543. A. [. Ekimov, I. A. Kudryavtsev, M. G. Ivanov, and A. L. Efros, J. Lumin. 46, 83 (1990). 544. E Hache, M. C. Klein, D. Ricard, and C. Flytzanis, J. Opt. Soc. Am. B 8, 1802 (1991). 545. J. Warnock and D. D. Awschalom, Phys. Rev. B 32, 5529 (1985). 546. J. P. Zheng, L. Shi, E S. Choa, P. L. Liu, and H. S. Kwok, Appl. Phys. Lett. 53, 645 (1988). 547. Y.Z. Hu, S. W. Koch, M. Lindberg, N. Peyghambariun, E. L. Pollock, and F. E Abraham, Phys. Rev. Lett. 64, 1805 (1990). 548. Y.Z. Hu, M. Lindberg, and S. W. Koch, Phys. Rev. B 42, 1713 (1990). 549. P. V. Kamat, T. W. Ebbesen, N. M. Dimitrijevic, and A. J. Nozik, Chem. Phys. Lett. 157, 384 (1989). 550. M. Ivanda, T. Bischof, G. Lermann, A. Matemy, and W. Kiefer, J. Appl. Phys. 82, 3116 (1997). 551. E Nemec and P. Maly, J. Appl. Phys. 87, 3342 (2000). 552. H. Ma. A. S. Gomez, and C. N. de Arajo, J. Opt. Soc. Am. B 9, 2230 (1992) 553. G. Be;~die, E. Sauvain, A. S. L. Gomez, and N. M. Lawandy, Phys. Rev. B 51, 2180 (1995). 554. P. Maly, E Trojanek, and A. Svoboda, J. Opt. Soc. Am. B 10, 1890 (1993). 555. U. V~bggon, M. Muller, I. Ruckmann, J. Kolenda, and M. Petrauska, Phys. Status Solid B 160, K 79 (1990). 556. M. C. Schane-Klein, L. Piveteau, M. Ghanassi, and D. Ricard, Appl. Phys. llett. 67, 579 (1995). 557. J. T. Rcmillard, H. Wang, M. D. Webb, and D. G. Steel, J. Opt. Soc. Am. B "7, 899 (1990). 558. W. Chen, Z. Wang, Z. Lin, and L. Lin, J. Appl. Phys. 82, 3111 (1997). 559. B. T. Jonker, J. J. Krebs, and G. A. Prinz, J. Appl. Phys. 63, 5885 (1988). 560. C. Pong, N. M. Johnson, R. A. Street, J. Walker, R. Feigelson, and R. C. De Mattei, Appl. Phys. Lett. 61, 3026 (1992). 561. H. Wang, K. S. Wong, B. A. Forman, Z. Y. Yang, and G. K. L. Wong, J. Appl. Phys. 83, 4773 (1998). 562. G.von Freymann, D. Luerben, C. Rabenstein, M. Mikolaiczyk, H. Richter, H. Kalt, Th. Schimmel, M. Wegener, K. Okhawa, and D. Hommel, Appl. Phys. Lett. 76, 203 (2000). 563. M. A. Haase, J. Qui, J. M. Depuydt, and H. Cheng, Appl. Phys. Lett. 59, 1272 (1991). 564. J. M. Gaines, R. R. Drenten, K. W. Haberem, T. Marshall, E Mensz, and J. Petruzzello, Appl. Phys. Lett. 62, 2462 (1993). 565. D. C. Grillo, Y. Fan, J. Han, L. He, R. L. Gunshor, A. Salokatve, H. Hagerott, H. Jeon, A. V. Nurmikko, G. C. Hua, and N. Otsuka, Appl. Phys. Lett. 63, 2723 (1993).
279
566. S. Itoh, H. Okuyama, S. Matsumoto, N. Nakayama, T. Ohata, T. Miyajima, A. Ishibashi, and K. Akimoto, Electron. Lett. 29, 766 (1993). 567. N. Nakayama, S Itoh, H. Okuyama, M. Ozawa, T. Ohata, K. Nakano, M. Ikeda, A. Ishibashi, and Y. Mori, Electron. Lett. 29, 2194 (1993). 568. H. Morkoc, S. Strite, G. B. Gao, M. E. Lin, B. Sverdlov, and M. Burns, J. Appl. Phys. 76, 1363 (1994). 569. S. Guha, J. M. Depuydt, M. A. Haase, J. Qiu, and H. Cheng, Appl. Phys. Lett. 63, 3107 (1993). 570. G. C. Hua, N. Otsuka, D. C. Grillo, Y. Fan, J. Han, M. D. Ringle, R. L. Gunshor, M. Hovinen, and A. V. Nurmikko, Appl. Phys. Lett. 65, 1331 (1994). 571. S. Guha, H. Cheng, M. A. Haase, J. M. Depuydt, J. Qiu, B. J. Wu, and G. E. Hofler, Appl. Phys. Lett. 65, 801 (1994). 572. M. Hovinen, J. Ding, A. V. Nurmikko, G. C. Hua, D. C. Grillor, L. He, J. Han, and R. L. Gunshor, Appl. Phys. Lett. 66, 2013 (1995). 573. R. Spiegel, G. Bacher, K. Herz, M. Illing, T. Kummell, A. Forchel, B. Jobst, D. Hommel, G. Landwehr, J. Sollner, and M. Heuken, Phys. Rev. B 53, R 4233 (1996). 574. L. M. Sparing, P. D. Wang, S. H. Xin, S. W. Short, S. S. Shi, J. K. Furdyna, J. L. Merz, and G. L. Snider, J. Vac. Sci. Technol. B 14, 3654 (1996). 575. L. M. Sparing, E D. Wang, A. M. Mintairov, S. Lee, U. Bindley, C. H. Chen, S. S. Shi, J. K. Furdyna, J. L. Merz, and G. L. Snider, J. Vac. Sci. Technol. B 15, 2652 (1997). 576. L. M. Sparing, A. M. Mintairov, J. H. Hodak, I. B. Martini, G. V. Hartland, U. Bindley, S. Lee, J. K. Furdyna, J. L. Merz, and G. L. Snider, J. Appl. Phys. 87, 3063 (2000). 577. W. Faschinger and J. Nurnberger, Appl. Phys. Lett. 77, 187 (2000). 578. Z. Sobiesierski, I. M. Dhermadasa, and R. H. Willaims, Appl. Phys. Lett. 53, 2623 (1988). 579. E M. Amritharaj and N. K. Dhar, J. Appl. Phys. 67, 3107 (1990). 580. J. Garcia-Garcia, J. Gonzalez-Hernandez, J. G. Mendoza-Alvarez, E. L. Cruz, and G. Contreras-Puente, J. Appl. Phys. 67, 3810 (1990). 581. Y. H. Kim, S. Y. An, J. Y. Lee, I. J. Kim, K. N. Oh, S. U. Kim, M. J. Park, and T. S. Lee, J. Appl. Phys. 85, 7370 (1999). 582. R. Cohen, S. Bastide, D. Cahen, J. Libman, A. Shanzer, and Y. Rosenwaks, Optical Materials 9, 394 (1998).; R. Cohen, L. Kronik, A. Shanzer, D. Cahen, A. Liu, Y. Rosenwaks, J. K. Lorenz, and A. B. Ellis, J. Am. Chem. Soc. 121, 10545 (1999). 583. C. K. Kang, Sh. U. Yuldashev, J. H. Leem, Y. S. Ryu, J. K. Hyun, H. S. Jung, H. I. Lee, Y. D. Woo, and T. W. Kim, J. Appl. Phys. 88, 2013 (2000). 584. A.J. Nelson, S. E Frigo, and R. A. Rosenberg, J. Appl. Phys. 75, 1632 (1994). 585. S. Gurumurthy, H. L. Bhat, B. Sundersheshu, R. K. Bagai, and V. Kumar, Appl. Phys. Lett. 68, 2424 (1996). 586. I. M. Dharmadasa, J. M. Thornton, and R. H. Willaims, Appl. Phys. Lett. 54, 137 (1989). 587. H. C. Chou, A. Rohatgi, N. M. Jokerst, S. Kamra, S. R. Stock, S. L. Lowrie, R. K. Ahrenkiel, and D. H. Levi, Mater. Chem. Phys. 43, 178 (1996). 588. H. C. Chou and A. Rohatgi, J. Electron. Mater. 23, 31 (1994). 589. M. Illing, G. Bacher, A. Forchel, T. Litz, A Waag, and G. Landwehr, Appl. Phys. Lett. 66, 1815 (1995). 590. L. Kronik, L. Burstein, Y. Shapira, and M. Oron, Appl. Phys. Lett. 63, 60 (1993). 591. R. Cohen, V. Lyahovitskaya, E. Poles, A. Liu, and Y. Rosenwaks, Appl. Phys. Lett. 73, 1400 (1998). 592. I. Delgadillo, M. Vargas, A. Cruz-Orea, J. J. Alvarado-Gil, R. Baquero, E Sanchez-Senencio, and H. Vergas, Appl. Phys. B 64, 97 (1997). 593. H. Vargas and L. C. M. Miranda, Phys. Rep. 161, 45 (1988). 594. J. Bernal-Alvarada, M. Vargas, J. J. Alvarado-Gil, I. Delgadillor, A. Cruz-Orea, H. Vargas, M. Tufino-Velazquez, M. L. Albor-Aguilera, and M. A. Gonzalez-Trujillo, J. App. Phys. 83, 3807 (1998). 595. Y. E Chen and W. S. Chen, Appl. Phys. Lett. 59, 703 (1991).
280
KASI VISWANATH
596. W. M. Lau, J. W. He, and E R. Norton, Appl. Phys. Lett. 54, 772 (1989). 597. O. E Agnihotri, C. A. Musca, and L. Faraone, Semicond. Sci. Technol. 13, 839 (1998). 598. G. Bahir, V. Ariel, V. Garber, and D. Rosenfeld, Appl. Phys. Lett. 65, 2725 (1994). 599. J. K. White, C. A. Musca, H. C. Lee, and L. Faraone, Appl. Phys. Lett. 76, 2448 (2000). 600. P. A. Borodovskii, A. E Buldygin, and V. S. Varavin, Semiconductors 32, 963 (1998). 601. Y. K. Su and C. T. Lin, Intern. J. High Speed Electronics and Systems 8, 703 (1997). 602. A.V. Voitsektnovskii, Yu. A. Denisov, A. E Kokhanenko, V. S. Varavin, S. A. Dvoretskii, N. N. Mikhailov, Yu.G. Sidorov, and M. V. Yakushev, Russian Physics Journal 40, 915 (1997). 603. Y. E Chen, C. S. Tsai, Y. H. Chang, Y. M. Chang, T. K. Chen, and Y. M. Pang, Appl. Phys. Lett. 58, 493 (1991). 604. M.D. Kim, T. W. Kang, G. H. Kim, M. S. Han, H. D. Cho, J. M. Kim, Y. T. Jeoung, and T. W. Kim, J. Appl. Phys. 73, 4074 (1993). 605. M. D. Kim, T. W. Kang, J. M. Kim, H. K. Kim, Y. T. Jeoung, and T. W. Kim, J. Appl. Phys. 73, 4077 (1993). 606. H. Chen, J. Tong, Z. Hu, D. T. Shi, G. H. Wu, K. T. Chen, M. A. George, W. E. Collins, R. B. James, C. M. Stahle, and L. M. Bartlett, J. appl. Phys. 80, 3509 (1996). 607. K. N. Oh, K. H. Kim, J. K. Hong, Y. C. Chung, S. U. Kim, and M. J. Park, J. Cryst. Growth 184-185, 1247 (1998). 608. H. Meincke, D. G. Ebling, J. Heinze, M. Tacke, and H. Bottner, J. Electrochem. Soc. 145, 2806 (1998). 609. J. Linnros, J. Appl. Phys. 84, 275 (1998). 610. J. Linnros, J. Appl. Phys. 84, 284 (1998). 611. J. Schmidt and A. G. Aberle, Prog. Photovolt. 6, 259 (1998). 612. N. Keskitalo, E Jonsson, K. Nordgren, H. Bleichner, and E. Nordlander, J. Appl. Phys. 83, 4206 (1998). 613. T. Saitoh, Y. Nisimoto, and H. Hasegawa, Solar Energy Materials and Solar Cells 34, 161 (1994). 614. H. C. Ostendorf and A. L. Endros, Appl. Phys. Lett. 71, 3275 (1997). 615. K. Tholmann, M. Yamaguchi, and A. Yahata, Scanning Microscopy Supplement 9, 125 (1995). 616. E Eichinger and M. Rommel, "Proc. Sym. Cryst. Defects and Contamination: Their Impact and Control in Device Manufacturing," Electrochem. Soc., Pennington, NJ, p. 363. 617. M. Brubaker, E Roman, J. Staffa, and J. Ruzyllo, Electrochem. Solid State Lett. 1, 130 (1998). 618. A. Cutolo, S. Daliento, A. Sanseverino, G. E Vitale, and L. Zeni, Solid State Electron. 42, 1035 (1998). 619. B. Adamowicz and H. Hasegawa, Jpn. J. Appl. Phys. 37, 1631 (1998). 620. T. H. Wang and T. E Ciszek, "Proc. Twenty-Sixth IEEE Photovoltaic Specialists Conference," New York, 1997, p. 55. 621. Y. Ogita, Y. Uematsu, and H. Daio, "Sym. Science and Technology of Semicondcutor Surface Preparation," Material Research Society, Pittsburgh, PA, 1997, p. 329. 622. R. Schieck and M. Kunst, Solid State Electron. 41, 1755 (1997). 623. E. Yablonovitch, D. L. Allara, C. C. Chang, T. Gmitter, and T. B. Bright, Phys. Rev. Lett. 57, 249 (1986). 624. J. Cho, T. E Schneider, J. Vanderweide, H. Jeon, and R. J. Nemanich, Appl. Phys. Lett. 59, 1995 (1991). 625. I. A. Veloarisoa, M. Stavola, D. M. Kozuch, R. E. Peale, and G. D. Watkins, Appl. Phys. Lett. 59, 2121 (1991). 626. M. Ishii, K. Nakashima, I. Tajima, and M. Yamamoto, Appl. Phys. Lett. 58, 1378 (1991). 627. J. I. Hanoka, C. H. Seager, D. J. Sharp, and J. K. G. Panitz, Appl. Phys. Lett. 42, 618 (1983). 628. W. M. Chen, I. A. Buyanova, W. X. Ni, G. V. Hansson, and B. Monemar, Appl. Phys. Lett. 70, 369 (1997). 629. K. Ljungberg, A. Soderbarg, and U. Jansson, Appl. Phys. Lett. 67, 650 (1995).
630. M. Niwano, Y. Kimura, and N. Miyamoto, Appl. Phys. Lett. 65, 1692 (1994). 631. M. Egawa and H. Ikoma, Jpn. J. Appl. Phys. 33, 943 (1994). 632. T. Konoshi, T. Yao, M. Tajima, H. Oshima, H. Ito, and T. Hattori, Jpn. J. Appl. Phys. 31, L 1216 (1992). 633. L. Li, H. Bender, T. Trenkler, P. W. Mertens, M. Meuris, W. Vandervorst, and M. M. Heyns, J. Appl. Phys. 77, 1323 (1995). 634. M. Niwano, Y. Takeda, Y. Ishibashi, K. Kurita, and N. Miyamoto, J. appl. Phys. 71, 5646 (1992). 635. T. Takahagi, I. Nagai, A. Ishitani, H. Kuroda, and Y. Nagasawa, J. Appl. Phys. 64, 3516 (1988). 636. K. Utani, T. Suziki, and S. Adachi, J. Appl. Phys. 73, 3467 (1993). 637. Th. Dittrich, V. Yu. Timoshenko, and J. Rappich, Appl. Phys. Lett. 72, 1635 (1998). 638. H. M'saad, J. Michel, A. Reddy, and L. C. Kimmerling, J. Electrochem. Soc. 142, 2833 (1995). 639. S. Adachi, T. Arai, and K. Kobayashi, J. Appl. Phys. 80, 5422 (1996). 640. G. J. Kluth and R. Maboudian, J. Appl. Phys. 80, 5408 (1996). 641. U. Neuwald, A. Feltz, U. Memmert, and R. J. Behm, J. Appl. Phys. 78, 4131 (1995). 642. M. R. Houston and R. Maboudian, J. Appl. Phys. 78, 3801 (1995). 643. M. Niwano, Y. Takeda, and N. Miyamoto, J. Appl. Phys. 72, 2488 (1992). 644. H. J. Lewerenz and G. Schlichthorl, J. Appl. Phys. 75, 3544 (1994). 645. S. Rauscher, Th. Dittrich, M. Aggour, J. Rappich, H. Flietner, and H. J. Lewerenz, Appl. Phys. Lett. 66, 3018 (1995). 646. J. Schmidt, E M. Schuurmans, W. C. Sinke, S. W. Glunz, and A. G. Aberle, Appl. Phys. Lett. 71,252 (1997). 647. A. G. Aberle, T. Lauinger, J. Schmidt, and R. Hezel, Appl. Phys. Lett. 66, 2828 (1995). 648. A. G. Aberle, Recent Res. Devel. Vacuum Sci. Tech. 1, 177 (1999). 649. T. Lauinger, J. Schmidt, A. G. Aberle, and R. Hezel, Appl. Phys. Lett. 68, 1232 (1996). 650. T. Lauinger, J. Moschner, A. G. Aberle, and R. Hezel, J. Vac. Sci. Technol. A 16, 530 (1998). 651. A. G. Aberle and R. Hezel, Prog. Photovoltaics 5, 29 (1997). 652. B. Bitnar, R. Glathaar, S. Keller, J. Kugler, M. Spiegel, P. Fath, G. Willeke, E. Bucher, E Duerinckx, J. Szlufcik, J. Nijs, R. Mertens, H. Nussbaumer, and E Ferrazza, "Proc. 14 European Photovoltaic Solar Energy Conference," Barcelona, Spain, 1997, p. 1491. 653. J. Kolnik, J. Ivanco, and M. Ozvold, Phys. Stat. Sol. (a) 130, 245 (1992); J. Ivanco, I. Thurzo, and E. Pincik, Appl. Phys. Lett. 65, 2594 (1994). 654. Y. Saito, N. Lio, Y. Kameshima, R. Takeda, and H. Kuwano, J. Appl. Phys. 63, 1117 (1988). 655. J. M. Shannon and B. A. Morgan, J. Appl. Phys. 86, 1548 (1999). 656. H. E. Elgamel, A. Barnett, A. Rohatgi, Z. Chen, C. Vinckier, J. Nijs, and R. Mertens, J. Appl. Phys. 78, 3457 (1995). 657. S. Okamoto, Sharp Technical Journal 70, 24 (1998). 658. H. M'saad, J. Michel, J. J. Lappe, and L. C. Kimmerling, J. Electronic Mater. 23, 487 (1994). 659. T. Aoyama, T. Yamazaki, and T. Ito, Appl. Phys. Lett. 59, 2576 (1991). 660. I. Bello, W. H. Chang, and W. M. Lau, J. Appl. Phys. 75, 3092 (1994). 661. H. C. Neitzert, N. Layadi, E R. Cabarrocas, and R. Vanderhaghen, Appl. Phys. Lett. 65, 1260 (1994). 662. H. Kobayashi, S. Tachibana, K. Yamanaka, Y. Nakato, and K. Yoneda, J. Appl. Phys. 81, 7630 (1997). 663. E Moriarty, L. Koenders, and G. Hughes, Phys. Rev. B 47, 15950 (1993). 664. A. W. Dunn, E Moriarty, M. D. Upward, and E H. Beton, Appl. Phys. Lett. 69, 506 (1996). 665. E Moriarty, M. D. Upward, A. W. Dunn, Y. R. Ma, E H. Beton, and D. Teehan, Phys. Rev. B 57, 362 (1998). 666. J. G. Lee and S. R. Morrision, J. Appl. Phys. 64, 6679 (1988). 667. E Grey and K. Hermansson, Appl. Phys. Lett. 71, 3400 (1997). 668. A.I. Petrov and V. A. Rozhkov, Technical Physics Lett. 24, 254 (1998). 669. W. E Lee and Y. L. Khong, J. Appl. Phys. 87, 7845 (2000).
S U R F A C E A N D I N T E R F A C I A L R E C O M B I N A T I O N IN S E M I C O N D U C T O R S 670. R. 1~ Wood, R. D. Westbrook, and G. E. Jellison, Jr., Appl. Phys. Lett. 50, 107 (1987). 671. G..;. Norga, M. Platero, K. A. Black, A. J. Reddy, J. Michel, and L. ('. Kimmerling, J. Electrochem. Soc. 145, 2602 (1998). 672. B. Nesterenko, O. Stadnik, A. Cricenti, and E Perfetti, Nuovo Cimento 20D, 1025 (1998). 673. E Tardif, J. E Joly, A. Courteaux, U. Straube, A. Danel, and G. Kamarinos, "Proc. Symp. Crystalline Defects and Contamination: Their Impact and Control in Device Manufacturing," Electrochem. Sot., Pennington, New Jersey, 1997, p. 385. 674. A. Buczkowski, E Kirscht, and H. Koya, "Proc. Symp. Crystalline Defects and Contamination: Their Impact and Control in Device Manufacl aring," Electrochem. Soc., Pennington, New Jersey, 1997, p. 376. 675. M. 1.,. Polignano, C. Bresolin, G. Pavia, V. Soncini, F. Zanderigo, G. Q~aeirolo, and M. di Dio, Mater. Sci. Engg. B 53, 300 (1998). 676. H. P',lrk, C. R. Helms, and D. Ko, J. Electrochem. Soc. 145, 1724 (1995). 677. D. ~. Ramappa and W. B. Henley, Appl. Phys. Lett. 72, 2298 (1998). 678. G. J. Norga, M. R. Black, K. A. Black, H. M'saad, J. Michel, and L. C. Kimurling, "Proc. Intern. Conf. on Defects in Semiconductors," Sendai, Japan, 1995. 679. G. J. Norga, K. A. Black, H. M'saad, J. Michel, and L. C. Kimurling, Mater. Sci. Technol. 11, 90 (1995). 680. M. Yajima and S. Ibuka, J. Appl. Phys. 84, 2224 (1998). 681. M. Tajima, S. Ibuka, and M. Warashina, Mat. Sci. Forum 258-263, 1731 (1997). 682. M. Tajima, S. Ibuka, H. Aga, and T. Abe, Appl. Phys. Lett. 70, 231 (1997). 683. M. Tajima, A. Ogura, T. Karasawa, and A. Mizoguchi, Jpn. J. Appl. Phys. 37, L 1199 (1998). 684. J. Shah, M. Combescot, and A. H. Dayem, Phys. Rev. Lett. 38, 1497 (1977). 685. N.F. Mott, "Metal-Insulator Transitions," Taylor & Francis, New York, 1990. 686. E J. Bedrossian and T. D. de la Rubia, J. Vac. Sci. Technol. A 16, 1043 (1998). 687. M. A. Green, "Solar Cells," University of New South Wales, Sydney, Australia, 1982. 688. M. A. Green, "High Efficiency Silicon Solar Cells," Trans Tech, Switzerland, 1987. 689. M. A. Green, Prog. Photov. 2, 87 (1994). 690. M. A. Green, Semicond. Sci. Technol. 8, 1 (1993). 691. M. A. Green, Prog. Photov. 2, 73 (1994). 692. M. A. Green, "Silicon Solar Cells: Advanced Principles and Practice, Centre for Photovoltaic Devices and Systems," University of New South Wales, Sydney, Australia, 1995. 693. R. Hezel, Progr. Photov. 5, 109 (1997). 694. A.G. Aberle, "Crystalline Silicon Solar Cells," Centre for Photovoltaic Engineering, University of New South Wales, Sydney, Australia, 1999. 695. S.J. Robinson, A. G. Aberle, and M. A. Green, J. Appl. Phys. 76, 7920 (1994) 696. A. G. Aberle, S. J. Robinson, A. Wang, J. Zhao, S. R. Wenham, and M. A. Green, Prog. Photov. 1, 133 (1993). 697. P. E Altermatt, G. Heiser, X. Dai, J. Jurgens, A. Aberle, S. J. Robinson, T. Young, S. R. Wenham, and M. A. Green, J. Appl. Phys. 80, 3574 (1996). 698. S. Narasimha, G. Crotty, A. Rohatgi, and D. L. Meier, Solid State Electron. 9, 1631 (1998). 699. Y. Bai, J. E. Phillips, and A. M. Barnett, IEEE Trans. Electron Devices 45, 1784 (1998). 700. S. Narasimha and A. Rohatgi, IEEE Trans. Electron Devices 45, 1776 (1998). 701. E Doshi and A. Rohatgi, IEEE Trans. Electron Devices 45, 1710 (1998). 702. D. D. Smith and A. M. Barnett, "Proc. 26th IEEE Photovoltaic Specialists Conference," IEEE, New York, 1997, p. 767.
281
703. E Doshi, J. Moschner, J. Jeorj, A. Rohatgi, R. Singh, and S. Narayanan, "Proc. 26th IEEE Photovoltaic Specialists Conference," IEEE, New York, 1997, p. 87. 704. M. Stocks, A. Blakers, and A. Cuevas, "Proc. 26th IEEE Photovoltaic Specialists Conference," IEEE, New York, 1997, p. 67. 705. S. Narasimha and A. Rohatgi, "Proc. 26th IEEE Photovoltaic Specialists Conference," IEEE, New York, 1997, p. 63. 706. A. Metz, R. Meyer, B. Kuhlmann, M. Grauvogl, and R. Hezel, "Proc. 26th IEEE Photovoltaic Specialists Conference," IEEE, New York, 1997, p. 31. 707. A. U. Ebong and S. H. Lee, Solid State Electron. 41, 1745 (1997). 708. C. Hebling, S. W. Glunz, C. Schetter, J. Knobloch, and A. Raeuber, Solar Energy Materials and Solar Cells 48, 335 (1997). 709. Y. H. Cho, A. U. Ebong, E. C. Cho, D. S. Kim, and S. H. Lee, Solar Energy Materials and Solar Cells 48, 173 (1997). 710. J. Salami, A. Shibta, D. Meier, E. Kochka, S. Yamanaka, P. Davis, J. Easoz, T. Abe, and K. Kinoshita, Solar Energy Materials and Solar Cells 48, 159 (1997). 711. L. Cai, A. Rohatgi, S. Han, G. May, and M. Zou, J. Appl. Phys. 83, 5885 (1998). 712. S. Narasimha and A. Rohatgi, Appl. Phys. Lett. 72, 1872 (1998). 713. A. Rohatgi, E Doshi, J. Moschner, T. Lauinger, A. G. Aberle, and D. S. Ruby, IEEE Trans. Electron Devices 47, 987 (2000). 714. A. W. Stephens, A. G. Aberle, and M. A. Green, J. Appl. Phys. 76, 363 (1993). 715. A. G. Aberle, P. E Altermatt, G. Heiser, S. J. Robinson, A. Wang, J. Zhao, U. Krumbein, and M. A. Green, J. Appl. Phys. 77, 3491 (1995). 716. S. Zh. Karazhanov, J. Appl. Phys. 76, 2689 (2000). 717. D. E. Ackley, J. Tauc, and W. Paul, Phys. Rev. Lett. 43, 715 (1979). 718. Z. Vardeny and J. Tauc, Phys. Rev. Lett. 46, 1223 (1981). 719. Z. Vardeny, J. Strait, D. Pfost, J. Tauc, and B. Abeles, Phys. Rev. Lett. 48, 1132 (1982). 720. Z. Vardeny, J. Strait, and J. Tauc, Appl. Phys. Lett. 42, 580 (1983). 721. D. L. Staebler and C. R. Wronski, Appl. Phys. Lett. 31, 292 (1977); J. Appl. Phys. 51, 3262 (1980). 722. H. Dersch, J. Stuke, and J. Beichler, Appl. Phys. Lett. 38, 456 (1981). 723. J. I. Pankove and E. I. Berkeyheiser, Appl. Phys. Lett. 37, 705 (1980). 724. R. A. Street, Appl. Phys. Lett. 42, 507 (1983). 725. S. Komuro, Y. Aoyagi, Y. Segawa, S. Namba, A. Mosuyama, H. Okamoto, and Y. Hamakawa, Appl. Phys. Lett. 42, 79 (1983). 726. J. Strait and J. Tauc, Appl. Phys. Lett. 47, 589 (1985). 727. S. Guha, J. Yang, W. Czubatyj, S. J. Hudgens, and M. Hack, Appl. Phys. Lett. 42, 588 (1983). 728. W. B. Jackson, D. K. Biegelsen, R. J. Namanich, and J. C. Knights, Appl. Phys. Lett. 42, 105 (1983). 729. A. V. Gelator, J. D. Cohen, and J. P. Harbison, Appl. Phys. Lett. 49, 722 (1986). 730. R. C. Frye, J. J. Kumler, and C. C. Wong, Appl. Phys. Lett. 50, 101 (1987). 731. Y. S. Tsuo, Y. Xu, R. S. Crandall, H. S. Ullal, and K. Emery, "Proc. Conf. Amorphous Silicon Technology," Mater. Res. Soc., Pittsburgh, PA, 1989, p. 471. 732. S. A. McQuaid, S. Holgado, J. Garrido, J. Martinez, J. Piqueras, R. C. Newman, and J. H. Tucker, J. Appl. Phys. 81, 7612 (1997). 733. J. H. Jang and K. S. Lim, Appl. Phys. Lett. 71, 1846 (1997). 734. S. S. Georgiev, N. Nedev, D. Sueva, and A. Toneva, J. Phys: Condens. Matt. 10, 5515 (1998). 735. J. Zimmer, H. Stiebig, J. Folsch, E Finger, Th.Eickhoff, and H. Wagner, "Proc. Symp. Amorphous and Microcrystalline Silicon Technology," Mater. Res. Soc., Pittsburgh, PA, 1997, p. 735. 736. U. Gnutzmann and K. Clausecker, Appl. Phys. 3, 9 (1974). 737. V. Arbet-Engels, M. A. Kallel, and K. L. Wang, Appl. Phys. Lett. 59, 1705 (1991). 738. A. St. Amour, J. C. Sturm, Y. Lacroix, and M. L. W. Thewalt, Appl. Phys. Lett. 65, 3344 (1994). 739. N. Usami, Y. Shiraki, and S. Fukatsu, Appl. Phys. Lett. 68, 2340 (1996).
282
KASI VISWANATH
740. R. J. Turton and M. Jaros, Appl. Phys. Lett. 69, 2891 (1996). 741. A. Galeckas, S. Juodkazis, E. Vanagas, V. Netiksis, M. Petrauskas, A. Bitz, J. L. Staehli, and M. Willander, Appl. Phys. 83, 4756 (1998). 742. A. Uhlir, Bell Syst. Tech. J. 35, 333 (1956). 743. D. R. Turner, J. Electrochem. Soc. 105, 402 (1958). 744. L. T. Canham, Appl. Phys. Lett. 50, 1046 (1990). 745. V. Lehmann and U. Gosele, Appl. Phys. Lett. 58, 856 (1991). 746. A. Halimaoui, C. Oules, G. Bomchil, A. Bsiesy, E Gaspard, R. Herino, M. Ligeon, and E Muller, Appl. Phys. Lett. 59, 304 (1991). 747. A. Cullis and L. T. Canham, Nature 353, 335 (1991). 748. A. Bsiesy, J. C. Vial, E Gaspard, R. Herino, M. Ligeon, E Muller, R. Romestain, A. Wasiela, A. Halimaoui, and G. Bomchcil, Surf. Sci. 254, 195 (1991). 749. R. E Vasquez, R. W. Fathauer, T. George, A. Kesendzov, and T. L. Lin, Appl. Phys. Lett. 60, 1004 (1992). 750. C. Tsai, K. H. Li, J. Sarathy, S. Shih, J. C. Campbell, B. K. Hance, and J. M. White, Appl. Phys. Lett. 59, 2814 (1992). 751. C. Tsai, K. H. Li, D. S. Kinosky, R. Z. Qian, T. C. Hsu, J. T. Irby, S. K. Banerjee, A. E Tasch, and J. C. Campbell, Appl. Phys. Lett. 60, 1700 (1992). 752. S. M. Prokes, O. J. Glembocki, V. M. Bermudez, R. Kaplan, L. E. Friedersdorf, and P. C. Searson, Phys. Rev. B 45, 13788 (1992). 753. M. B. Robinson, A. C. Dillon, D. R. Haynes, and S. M. George, Appl. Phys. Lett. 61, 1414 (1992). 754. S. M. Prokes, Appl. Phys. Lett. 62, 3244 (1993). 755. S. M. Prokes, W. E. Carlos, and O. J. Glembocki, Phys. Rev. B 50, 17093 (1994). 756. J. L. Gole and D. A. Dixon, J. Phys. Chem. B 101, 8098 (1997). 757. M. S. Brandt, H. D. Fuchs, M. Stutzmann, J. Weber, and M. Cardona, Solid State Commun. 81,307 (1992). 758. E Koch, V. Petrova, T. Muschik, A. Nikolov, and V. Gavrilenko, Mater. Res. Soc. Symp. Proc. 283, 197 (1993). 759. G. G. Qin and Y. Q. Jia, Solid State Commun. 86, 559 (1993). 760. Y. Kanemitsu, H. Uto, and Y. Masumoto, Phys. Rev. B 48, 2827 (1993). 761. S. Sawada, N. Hamada, and N. Ookubo, Phys. Rev. B 49, 5236 (1994). 762. H. Koyama, T. Ozaki, and N. Koshida, Phys. Rev. B 52, R 11561 (1995). 763. Q. K. Andersen and E. Veje, Phys. Rev. B 53, 15643 (1996). 764. L. Jia, S. E Wong, I. H. Wilson, S. K. Hark, S. L. Zhang, Z. E Liu, and S. M. Cai, Appl. Phys. Lett. 71, 1391 (1997). 765. G. G. Qin, J. Lin, J. Q. Duan, and G. Q. Yao, Appl. Phys. Lett. 69, 1689 (1996). 766. G. G. Qin, X. S. Liu, S. Y. Ma, J. Lin, G. Q. Yao, X. Y. Lin, and K. X. Lin, Phys. Rev. B 55, 12876 (1997). 767. D. Kovalev, G. Polisski, M. Ben-Chorin, and E Koch, J. Appl. Phys. 80, 5978 (1996). 768. D. T. Jiang, I. Coulthard, T. K. Sham, J. W. Lorimer, S. P. Frigo, X. H. Feng, and R. A. Rosenberg, J. Appl. Phys. 74, 6335 (1993). 769. D. Xu, G. Guo, L. Gui, Y. Tang, B. R. Zhang, and G. G. Qin, J. Appl. Phys. 86, 2066 (1999). 770. T. Dittrich, V. Yu. Timoshenko, and J. Rappich, Appl. Phys. Lett. 72, 1635 (1998). 771. Z. Hajnal and E Deak, J. Non-Cryst. Sol. 227, 1053 (1998). 772. S. M. Prokes, "Proc. Int. Sym. Pits and Pores: Formation, Properties, and Significance for Advanced Luminescent Materials," Electrochem. Soc., Pennington, NJ, 1997, p. 378. 773. E J. Moyer, A. Pridmore, T. Martin, J. Schmidt, T. Hasche, L. Eng, and J. L. Gole, Appl. Phys. Lett. 76, 2683 (2000). 774. L. Stalmans, J. Poortmans, H. Bender, M. Caymax, K. Said, E. Vazsonyi, J. Nijs, and R. Mertens, Prog. Photovolt. 6, 233 (1998). 775. R. Kumar, Y. Kitoh, and K. Hara, Appl. Phys. Lett. 63, 3032 (1993). 776. U. Gruning, S. C. Gujrathi, S. Poulin, Y. Diawara, and A. Yelon, J. Appl. Phys. 75, 8075 (1994). 777. J. Salonen and E. Laine, J. Appl. Phys. 80, 5984 (1996). 778. S. E Withrow, C. W. White, A. Meldrum, J. D. Budai, D. M. Hembree, Jr., and J. C. Barbour, J. Appl. Phys. 86, 396 (1999).
779. K. S. Min, K. V. Shcheglov, C. M. Yang, H. A. Atwater, M. L. Brongersma, and A. Polman, Appl. Phys. Lett. 69, 2033 (1996). 780. Th. Dittrich and V. Yu. Timoshenko, J. Appl. Phys. 75, 5436 (1994). 781. H. Koyama, T. Nakagawa, T. Ozaki, and N. Koshida, Appl. Phys. Lett. 65, 1656 (1994). 782. S. Gardelis and B. Hamilton, J. Appl. Phys. 76, 5327 (1994). 783. H. Chen, X. Hou, G. Li, E Zhang, M. Yu, and X. Wang, J. Appl. Phys. 79, 3282 (1996). 784. G. Li, X. Hou, S. Yuan, H. Chen, E Zhang, H. Fan, and X. Wang, J. Appl. Phys. 80, 5967 (1996). 785. A. A. Seraphin, S. T. Ngiam, and K. D. Kolenbrander, J. Appl. Phys. 80, 6429 (1996). 786. N. Rigakis, J. Hilliard, L. A. Hassan, J. M. Hetrick, D. Andsager, and M. H. Nayfeh, J. Appl. Phys. 81,440 (1997). 787. L. Pavesi, R. Chierchia, P. Bellutti, A. Lui, E Fuso, M. Labardi, L. Pardi, E Sbrana, M. Allegrini, S. Trusso, C. Vasi, P. J. Ventura, L. C. Costa, M. C. Carmo, and O. Bisi. J. Appl. Phys. 86, 6474 (1999). 788. M. Ben-Chorin, E Moiler, and E Koch, Phys. Rev. B 49, 2981 (1994). 789. E. H. Nicollian and J. R. Brews, "MOS Physics and Technology," Wiley, New York, 1982. 790. G. Lucovsky, S. Pantelides, and E Galeener, Eds., "The Physics of MOS Interfaces," Pergamon, New York, 1980. 791. Semicond. Sci. Technol. 4, 961 (1989). This special issue discusses defect physics of Si/SiO 2. 792. E. Poindexter, P. Caplan, B. Deal, and R. Razouk, J. Appl. Phys. 52, 879 (1981). 793. R. Helms and E. Poindexter, Rep. Prog. Phys. 57, 791 (1994). 794. C.R. Helms and B. E. Deal, Eds., "The Physics and Chemistry of SiO 2 and The SiO2 Interface," Plenum, New York, 1988. 795. E. Poindexter and P. Kaplan, Prog. Surf. Sci. 14, 211 (1983). 796. E. Yablonovitch, R. M. Swanson, W. D. Eades, and B. R. Weinberger, Appl. Phys. Lett. 48, 245 (1986). 797. Z. Chen, S. K. Pang, K. Yasutake, and A. Rohatgi, J. Appl. Phys. 74, 2856 (1993). 798. Z. Chen, K. Yasutake, A. Doolittle, and A. Rohatgi, Appl. Phys. Lett. 63, 2117 (1993). 799. A. G. Aberle, S. Glunz, and W. Warta, J. Appl. Phys. 71, 4422 (1992). 800. R. B. M. Girisch, R. P. Mertens, and R. E De Keersmaecker, IEEE Trans. Electron Devices ED-35, 203 (1988). 801. K. Inoue, M. Nakamura, M. Okuyama, and Y. Hamakawa, Appl. Phys. Lett. 55, 2402 (1989). 802. A. G. Aberle, S. Glunz, and W. Warta, Solar Energy Materials and Solar Cells 29, 175 (1993). 803. S. W. Glunz, A. B. Sproul, S. Sterk, and W. Warta, "Proc. 12th European Photovoltaic Solar Energy Conf.," Amsterdam, 1994, p. 492. 804. S. Glunz, A. B. Sproul, W. Warta, and W. Wettling, J. Appl. Phys. 75, 1611 (1994). 805. A. G. Aberle, S. W. Glunz, A. W. Stephens, and M. A. Green, Prog. Photov. 2, 265 (1994). 806. A. W. Stephens, A. G. Aberle, and M. A. Green, J. Appl. Phys. 76, 363 (1994). 807. S.J. Robinson, S. R. Wenham, P. P. Altermatt, A. G. Aberle, G. Heiser, and M. A. Green, J. Appl. Phys. 78, 4740 (1995). 808. S. W. Glunz, D. Biro, S. Rein, and W. Warta, J. Appl. Phys. 86, 683 (1999). 809. M. Y. Ghannam and R. P. Mertens, IEEE Electron Dev. Lett. 10, 242 (1989). 810. R. Girisch, R. P. Mertens, and R. van Overstraeten, "Proc. 16th IEEE Photovoltic Specialists Conf," 1982, p. 1231. 811. R. Girisch, R. P. Mertens, and R. E De Keersmaecher, Appl. Surf. Sci. 30, 127 (1987). 812. S. K. Lai, Appl. Phys. Lett. 39, 58 (1981). 813. S. Pang, S. A. Lyon, and W. C. Johnson, Appl. Phys. Lett. 40, 709 (1982). 814. S. K. Lai, J. Appl. Phys. 54, 2540 (1983). 815. M. M. Heyns, R. E DeKeersmaecker, and M. W. Hillen, Appl. Phys. Lett. 44, 202 (1984).
S U R F A C E A N D I N T E R F A C I A L R E C O M B I N A T I O N IN S E M I C O N D U C T O R S
816. Y. Nissen-Cohen, J. Shappir, and D. Frohman-Bentchkowsky, J. Appl. Phy~'. 60, 2024 (1986). 817. D. 3 DiMaria, Appl. Phys. Lett. 51,655 (1987). 818. M ~slam, J. Appl. Phys. 62, 159 (1987). 819. Y. Nissan-Cohen and T. Gorczyca, Electron. Dev. Lett. EDL-9, 287 (1')~;8). 820. S. J Wang, J. M. Sung, and S. A. Lyon, Appl. Phys. Lett. 52, 1431 821. 822. 823. 824. 825. 826. 827. 828. 829. 830. 831. 832. 833. 834. 835. 836. 837. 838. 839. 840. 841. 842. 843. 844.
845. 846. 847. 848. 849. 850. 851. 852.
853.
854.
855. 856.
D. J. DiMaria and J. Stasiak, J. Appl. Phys. 65, 2342 (1989). D. A. Buchanan and D. J. DiMaria, J. App1. Phys. 67, 7439 (1990). D. A. Buchanan, Appl. Phys. Lett. 65, 1257 (1994). W. 1.. Warren and E M. Lenahan, Appl. Phys. Lett. 49, 1296 (1986). P. ~'d Lenahan and E V. Dressendorfer, J. AppI. Phys. 55, 3495 (1984). A. Slesmans, Phys. Rev. Lett. 70, 1723 (1993). J. 1~ Stathis and D. J. DiMarria, Appl. Phys. Lett. 61, 2887 (1992). K. L. Brower and S. M. Meyers, Appl. Phys. Lett. 57, 162 (1990). D. '~ilillaume, A. Mir, R. Bouchakour, and M. Jordain, J. Appl. Phys. 73, 777 (1993). D. L. Griscom, J. Electron. Mater. 21,763 (1972). E. C.!rtier and D. J. DiMarria, Microelectron. Eng. 22, 207 (1993). E. C~,crtier, J. H. Stathis, and D. A. Buchanan, Appl. Phys. Lett. 63, 1510 (1993). E. t~artier, D. A. Buchanau, and G. J. Dunn, Appl. Phys. Lett. 64, 901 (19'),1). J. I-L Stathis and E. Cartier, Phys. Rev. Lett. 72, 2745 (1994). E. Cartier, D. A. Buchanau, J. H. Stathis, and D. J. DiMarria, J. NonCrypt. Solids 187, 244 (1995). E. (;artier and J. H. Stathis, Microelectron. Eng. 28, 3 (1995). E. (;artier and J. H. Stathis, Appl. Phys. Lett. 69, 103 (1996). R. A. B. Devine, J. L. Autran, W. L. Warren, K. L. Vanheusdan, and J. C. Rostaing, Appl. Phys. Lett. 70, 2999 (1997). J. W. Lyding, K. Hess, and I. C. Kizilyalli, Appl. Phys. Lett. 68, 2526 (1996). Y. B. Park and S. W. Rhee, J. Appl. Phys. 86, 1346 (1999). J.F. Zhang, H. K. Sii, R. Degraeve, and G. Groeseneken, J. Appl. Phys. 87, 2967 (2000). W. Fussel, M. Schmidt, H. Angermann, G. Mende, and H. Flietner, Nuel. lnstrum. Methods Phys. Res. A 377, 177 (1996). M.J. l~Jren, J. H. Stathis, and E. Cartier, J. Appl. Phys. 80, 3915 (1996). N. M. Johnson, D. K. Biegelsen, M. D. Moyer, S. T. Chang, E. H. Poindexter, and P. J. Caplan, Appl. Phys. Lett. 43, 563 (1983). H. Flietner, Mater Sci. Forum 185-188, 73 (1995). J. Albohn, W. Fussel, N. D. Sinh, K. Kliefoth, and W. Fuhs, J. Appl. Phys. 88, 842 (2000). G. Lupke, D. J. Bottomley, and H. M. van Driel, Phys. Rev. B 47, 10389 (1993). J. Bloch, J. G. Mihaychuk, and H. M. van Driel, Phys. Rev. Lett. 77, 920 (1996). J. G. Mihaychuk, J. Bloch, Y. Liu, and H. M. van Driel, Opt. Lett. 20, 2063 (1995). J. E Me Gilp, Prog. Surf. Sci. 49, 1 (1995). N. Shamir, J. G. Mihaychuk, and H. M. van Driel, J. Vac. Sci. Technol. A 15, 2081 (1997). O. A. Aktsipetrov, A. A. Fedyanin, E. D. Mishina, A. A. Nikulin, A. N. Rubtsov, C. W. van Hasselt, M. A. C. Devillers, and T. Rasing, Phys. Rev. Lett. 78, 46 (1997). J. I. Dadap, X. E Hu, N. M. Russel, J. G. Ekerdt, J. K. Lowell, and M. C. Downer, IEEE J. Sel. Top. Quantum Electron. 1, 1145 (1996). U. Emmerichs, C. Meyer, H. J. Bakker, H. Kurz, C. H. Bjorkman, C. E. Shearon, Jr., Y. Ma, T. Yasuda, Z. Jing, G. Lucovsky, and J. L. Whitten, Phys. Rev. B 50, 5506 (1994). C. Meyer, G. Lupke, U. Emmerichs, E Wolter, H. Kurz, C. H. Bjoorkman, and G. Lucovsky, Phys. Rev. Lett. 74, 3001 (1995). C. W. van Hasselt, M. A. C. Devillers, T. Rasing, and O. A. Aktsipetrov, J. Opt. Soc. Am. B 12, 33 (1995).
283
857. J. I. Dadap, X. E Hu, M. H. Anderson, M. C. Downer, J. K. Lowell, and O. A. Aktsipetrov, Phys. Rev. B 53, R 7607 (1996). 858. E Godefroy, W. de Jong, C. W. van Hasselt, M. A. C. Devillers, and T. Rasing, Appl. Phys. Lett. 68, 1981 (1996). 859. A. Nahata, T. F. Heinz, and J. A. Misewich, Appl. Phys. Lett. 69, 746 (1996). 860. N. Shamir, J. G. Mihaychuk, and H. M. van Driel, J. Appl. Phys. 88, 896 (2000). 861. W. Wang, G. Lupke, M. Di Ventra, S. T. Pantelides, J. M. Gilligan, N. H. Tolk, I. C. Kiziyalli, E K. Roy, G. Margaritondo, and G. Lucovsky, Phys. Rev. Lett. 81, 4224 (1993). 862. J. K. Wu, S. A. Lyon, and W. C. Johnson, Appl. Phys. Lett. 42, 585 (1983). 863. D. Lelmini, A. S. Spinelli, and A. L. Lacaita, Appl. Phys. Lett. 76, 1719 (2000). 864. A. Stesmans, Appl. Phys. Lett. 68, 2076 (1976). 865. A. Stesmans, Appl. Phys. Lett. 68, 2723 (1996). 866. A. Stesmans, Phys. Rev. B 61, 8393 (2000). 867. A. Stesmans, J. Appl. Phys. 88, 489 (2000). 868. T. D. Mishima, E M. Lenahan, and W. Weber, Appl. Phys. Lett. 76, 3771 (2000). 869. J. W. Gabrys, E M. Lenahan, and W. Weber, Microelectron. Eng. 22, 273 (1993). 870. R.L. Vranch, B. Henderson, and M. Pepper, Appl. Phys. Lett. 52, 1161 (1988). 871. M. A. Jupina and E M. Lenahan, IEEE Trans. Nucl. Sci. 36, 1800 (1989). 872. P. M. Lenahan and M. A. Jupina, Colloids Surface 45, 191 (1990). 873. M. A. Jupina and E M. Lenahan, IEEE Trans. Nucl. Sci. 37, 1650 (1990). 874. J.T. Krick, E M. Lenahan, and G. J. Dunn, Appl. Phys. Lett. 59, 3437 (1991). 875. C. A. Billman, E M. Lenahan, and W. Weber, Microelectron. Eng. 36, 271 (1997). 876. A. Stesmans and V. V. Afans'ev, J. Phys.: Condens. Matter 10, L19 (1998). 877. A. Stesmans and V. V. Afans'ev, Phys. Rev. B 57, 10030 (1998). 878. G.J. Gerardi, E. H. Poindexter, P. J. Caplan, and N. H. Johnson, Appl. Phys. Lett. 49, 348 (1986). 879. M. E. Law, Y. M. Haddara, and K. S. Jones, J. Appl. Phys. 84, 3555 (1998). 880. C. Tsamis and D. Tsoukalas, J. Appl. Phys. 84, 6650 (1998). 881. D. Tsoukalas and C. Tsamis, IEEE Electron Dev. Lett. 18, 90 (1997). 882. A. E1-Hdiy, Appl. Phys. Lett. 63, 3338 (1993). 883. G.W. Anderson, M. C. Hanf, E R. Norton, Z. H. Lu, and M. J. Graham, Appl. Phys. Lett. 66, 1123 (1995). 884. V. Yu. Timoshenko, J. Rappich, and Th. Dittrich, Appl. Surf. Sci. 123-124, 111 (1998). 885. D. Kruangam, D. Deguchi, T. Toyama, H. Okamoto, and Y. Hamakawa, IEEE Trans. Electron Devices 35, 957 (1988). 886. S. M. Paasche, T. Toyama, H. Okamoto, and Y. Hamakawa, IEEE Trans. Electron Devices 36, 2895 (1989). 887. T. S. Jen, J. W. Pan, N. E Shin, W. C. Tsay, J. W. Hong, and C. Y. Chang, Jpn. J. Appl. Phys. 33, 827 (1994). 888. J. W. Lee and K. S. Lim, Appl. Phys. Lett. 68, 1031 (1996). 889. J. W. Lee and K. S. Lim, J. Appl. Phys. 81, 2432 (1997). 890. V. Grivickas, J. Linnros, and A. Galeckas, Mater. Sci. Forum 264-268, 529 (1998). 891. Q. Wahab, E. B. Macak, J. Zhang, L. D. Madsen, and E. Janzen, "Proc. European SiC Conference," Erlangen, Germany, 2000. 892. V. V. Afans'ev, A. Stesmans, M. Bassler, G. Pensl, and M. J. Schulz, Appl. Phys. Lett. 76, 336 (2000). 893. V. V. Afans'ev, A. Stesmans, M. Bassler, G. Pensl, M. J. Schulz, and C. I. Harris, J. Appl. Phys. 85, 8292 (1999). 894. V. V. Afans'ev and A. Stesmans, Mater Sci. Eng. B 71,309 (2000).
284
KASI V I S W A N A T H
895. M. Ruff, H. Mitlehner, and R. Helbig, IEEE Trans. Electron Devices 41, 1040 (1994). 896. M. R. Melloch and J. A. Cooper, Jr., MRS Bull. 22, 42 (1997). 897. J. N. Shenoy, G. L. Chindalore, M. R. Melloch, J. A. Cooper, Jr., J. W. Palmour, and K. G. Irvine, J. Electron. Mater 24, 303 (1995).
898. J. A. Cooper, Jr., Phys. Status Solid A 162, 305 (1997). 899. J. E Xu, E T. Lai, C. L. Chan, and Y. C. Cheng, Appl. Phys. Lett. 76, 372 (2000). 900. E T. Lai, S. Chakraborty, C. L. Chan, and Y. C. Cheng, Appl. Phys. Lett. 76, 3744 (2000).
Introduction
2.
Shockley-Read-Hall Recombination
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.
Auger Recombination
4.
218
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
220
III-V Semiconductors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
221
4.1.
G a A s Epilayers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
221
4.2.
Novel Techniques to Characterize Surface R e c o m b i n a t i o n in G a A s
. . . . . . . . . . . . . . .
231
4.3.
M e t a l - G a A s Interface
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
233
4.4.
F e r r o m a g n e t - G a A s Interface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
234
4.5.
Heterostructures and Q u a n t u m Wells of G a A s
235
6.
. . . . . . . . . . . . . . . . . . . . . . . . . .
4.6.
G a A s Q u a n t u m Wires
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
238
4.7.
InP . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
242
4.8.
Other I I I - V Binary S e m i c o n d u c t o r s . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
243
4.9.
I I I - V Ternary S e m i c o n d u c t o r s . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
244
4.10. I I I - V Quarternary S e m i c o n d u c t o r s 5.
218
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
250
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
251
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
251
I I - V I Semiconductors 5.1.
CdS
5.2.
CdSe
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
254
5.3.
CdSSe . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
256
5.4.
ZnS
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
258
5.5.
ZnSe . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
258
5.6.
ZnCdSe
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
258
5.7.
CdTe
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
259
5.8.
HgCdTe . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
259
5.9.
CdZnTe
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
259
5.10. H g Z n T e
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
260
5.11. PbSe . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
260
Group IV S e m i c o n d u c t o r s . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
260
6.1.
Si Single Crystals and Wafers
260
6.2.
Si Solar Cells . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
261
6.3.
A m o r p h o u s Si . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
262
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
6.4.
Si-based Q u a n t u m Wells and Superlattices
. . . . . . . . . . . . . . . . . . . . . . . . . . . .
264
6.5.
Porous Si
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
264
6.6.
Si/SiO 2 Interfaces . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
265
6.7.
Ge . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
7.
Group IV Binary S e m i c o n d u c t o r s
8.
Conclusions
7.1.
SiC
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
269 269
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
269
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
270
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
271
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
271
Handbook of Surfaces and Interfaces of Materials, edited by H.S. N a l w a Volume 1: Surface and Interface Phenomena C o p y r i g h t 9 2001 by A c a d e m i c Press All rights o f reproduction in any form reserved.
217
218
KASI VISWANATH
1. INTRODUCTION Recombination of carriers at semiconductor surfaces and interfaces is important not only from the basic physics point of view, but also because it has many implications in almost all semiconductor technological devices, such as light-emitting diodes (LEDs), semiconductor lasers, photodetectors, solar cells, heterojunction bipolar transistors (HBTs), metal-oxide semiconductor field-effect transistors (MOSFETs), and so forth [ 1-19]. When electrons in the conduction band and holes in the valence band recombine they give rise to radiative recombination. If a semiconductor surface has defects such as vacancies, it creates dangling bonds. The vacancies have energies that lie in the middle of the bandgap. These energy levels are called trap states, which also participate in the recombination with electrons and holes. These recombinations are called nonradiative recombinations and have very detrimental effects on the performance of the semiconductor device as well as on the device lifetime itself. The lifetime of the injected carriers in the semiconductor is a structure-sensitive property of the material. This happens because the recombination process takes place through the medium of imperfections in the semiconductor. For a semiconductor that has many surface states, the Fermi level is said to be "pinned" at almost the midgap of the semiconductor. Fermi-level pinning and its removal in semiconductor surfaces and interfaces has been a long-standing problem in semiconductor technology, and has been studied by a number of workers for more than two decades. A semiconductor surface or interface that has many dangling bonds can be cleaned by chemical treatment, a process known as surface passivation. Unpinning of the Fermi level was shown to be possible by a variety of passivation techniques. The improvement in the quality of the surface can be evaluated by continuous wave (cw) and time-resolved photoluminescence (PL) spectroscopies. In cw photoluminescence the PL intensities before and after passivation provide evidence of the improvement in the quality of surface, whereas in timeresolved spectroscopy this is done by the carrier lifetimes. With the advances in highly powerful femtosecond laser technology and detection systems such as ultrahigh-speed streak cameras, it is quite possible today to measure the surfacerecombination lifetimes very accurately. Also, the quality of semiconductor surfaces and interfaces can be judged very precisely using these ultrafast techniques. The reader is referred to the excellent review article by Aspnes [ 12] to understand the initial efforts made on the study of surface recombination in semiconductors. This chapter reviews the present status of understanding of surface- and interface-related recombination in semiconductors including epitaxial layers, bulk single crystals, quantum-confined systems such as quantum wells, quantum wires, and quantum dots, semiconductor devices such as light-emitting diodes, lasers, metal-oxide field-effect transistors (MOSFETs), solar cells, and so forth. The following experimental techniques were considered: cw photoluminescence (PL); cathodoluminescence (CL); electroluminescence (EL); time-resolved
spectroscopic techniques in nano-, pico- and femtosecond timescales; luminescence decay to measure radiative and nonradiative recombination lifetimes; surface-recombination velocities; femtosecond transient absorption; pump-probe techniques; degenerate four-wave mixing, and novel techniques to measure surface-recombination lifetimes such as photoacoustic methods, near-field scanning optical microscopy (NSOM), microwave modulated photoluminescence (MMPL). An in-depth coverage of surface-passivation techniques that have been employed so far in the literature for a number of semiconductors has been given. Also discussed are radiative and nonradiative recombination processes in surface-level quantum dots, for example, InAs quantum dots on GaAs surface, GaAs surface quantum wells that have applications in emerging microelectronic engineering. It is hoped that this review will be a ready reference for those who are already experts in the field of semiconductor surfaces and interfaces. For those who are not at all familiar with the topic of surface states in semiconductors, the present review will serve as a starting point. In order to meet these two objectives, attempts were made to give as many references as possible along with illustrative figures under each heading.
2. SHOCKLEY-READ-HALL RECOMBINATION The surface recombination in semiconductors that is mediated by surface traps was discussed originally by Shockley and Read [9] and Hall [10], and is popularly known as ShockleyRead-Hall (SRH) recombination. Figure 1 illustrates how the electrons and holes recombine through the traps. To discuss the phenomenological picture we will follow the generally accepted Stevenson-Keys model [11, 12]. In this model the dependence of recombination on surface Fermi-level and bulk doping was considered. From this picture, one can understand how to reduce surface recombination by controlling the material properties. Figure 1 illustrates a trap that may exist in either of two states differing by one electronic unit of charge, that is, it can be either negative or neutral. Similar treatment may be applied to other possibilities, for example, neutral or positive. If the trap is neutral it can capture an electron from the conduction band. The energy loss of the electron is then converted into
t uE t J w
o E
Ev (a)
(bj
911,.
q,
(c)
(d)
Fig. 1. The basic processes involved in recombination by trapping: (a) electron capture; (b) electron emission; (c) hole capture; (d) hole emission. Reprinted with permission from W. Shockley and W. T. Read, Phys. Rev. 87, 835 (1952). Copyright 1952 by The American Physical Society.
SURFACE AND INTERFACIAL RECOMBINATION IN SEMICONDUCTORS heat or light or both depending upon the nature of the trapping process, ft can also capture an electron from the valence band represemed by part (d) of the figure. In this case it acquires a negative charge and leaves a hole in the valence band. If the trap i~ negatively charged it can emit an electron that goes to the c{:Jnduction band (Fig. l b) or capture a hole from the valence l:,and (Fig. l c). Shockley and Read [9], in their analysis of t:~e statistics of recombination of holes and electrons involving,: the trap, have assumed that the readjustment time for the t:apped electron for other physical processes such as relaxati,a~ from the excited state to the ground state is negligible conq',ared to the time required on the average for the trap to emit t l~e electron or to capture a hole. At r,),:,m temperature the electrons in the conduction band and hol,z ~in the valence band of the semiconductor move with an avera~;:;e speed v T = x / 3 k T / m * , which is around 10 7 cm]s. Here, v r is the thermal velocity and m* is the effective mass of the ca:Tier. The instantaneous current density Ji due to the thermal ~:aotion of the ith carrier is given by
219
%+An
|
Ec
0 0 % 000%%%~ o~ .
.
.
.
.
.
.
.
.
.
Cn, e
EF
ET, NT
--
G,R
ep, cp
EV
|
(9
|
PO+ ,~p Fig. 2. Schematic representation of the principal channels by which electrons and holes equilibrate. Band-to-band generation and recombination are denoted by G and R. The corresponding processes involving traps are emission and capture e n, c, and ep, Cp of electrons and holes, respectively. Reprinted with permission from D. E. Aspnes, Surface Sci. 132, 406 (1983). Copyright 1983 by Elsevier Science.
At equilibrium Ji -
q i V T i 6 ( r -- ri)
//
where qi is the charge of the carrier, and r/is its position. At equilibrium the current density for electrons or holes can be written as J = ~
q i V T i t ~ ( r -- ri) ---- 0
This happens because the number of carriers going in one direction is equal to the number of carriers going in the opposite direction. However, the situation will be different if we consider the recombination at the semiconductor surface or interface, as the balance will not be maintained. There will be a net flow of carriers t~ the surface or interface that can be represented as a current oF the form J = n q v s. The important parameter here is velocity t's that indicates the lost carriers in the recombination process. The sarface-recombination velocity S is defined as the number (:,f carriers recombining on the surface per unit area per unit time per unit volume of excess bulk carriers at the botmdary between the quasi-neutral and space-charge regions [:2]. Figure 2 shows the various principal channels by which electrons and holes equilibrate in a photoexcited semiconductor that has trap levels; n is the number of electrons in the conduction band and p is the number of holes in the valence band; G is the generation rate and R is the band-to-band recombination rate per unit volume; R should be proportional to n and also p. It is also proportional to the mean thermal velocity v- ~ . Therefore, R can be written as R-
npcrv
where tr is a proportionality constant and has the dimension of area; tr is called the recombination cross section.
~
H 0
P - - Po G -
R o - - noPoCrv
This means that the volume generation rate is equal to the recombination rate at equilibrium. The net rate of change of nonequilibrium values of n and p due to intrinsic band-to-band processes in the absence of external generation is dn/dt
-
dp/dt
-
G-
R
= noPotrV-
npcrv
-- ov(noP o - np) -- -crv(np-
noPo)
=
where n~ - - n o P o
or ni-
nx/h~op%
Here, n/ is the intrinsic carrier concentration. After the semiconductor is excited by a pulse of light, electron-hole pairs are created. Let n = n0+An p=
p0+Ap
where An and Ap represent nonequilibrium carrier distributions. For charge neutrality An -- Ap dn/dt
= dp/dt
-
d/dt
An
- - _ O r V [ ( n 0 .qt_p 0 ) A H _]_ A n 2]
220
KASI VISWANATH
If An << (n o + P0), then An 2 is a very small quantity and hence can be neglected:
d/dt
=
+ Po)] a , .
The solution to this equation is a simple exponential with a time constant T, where '7 -- '7"o -- 1/ ( o v ( n o .+ P o ) )
Now we will consider the situation for a semiconductor that has defects. Let: Nr = defect-level density at temperature T; E r = energy of the trap; c n = electron capture rate; en = electron emission rate; and o-, = capture cross section. Because the recombination processes involved obey the FermiDirac statistics, we introduce the symbol f to represent the probability that a quantum state will be occupied; f is a function of energy level Er of the quantum state and of the Fermi level EF, and can be written as f = 1/(1 + e x p [ ( E / - E r ) / k T ] ) The electron capture rate G is proportional to n, G, ~r, and the volume density of unoccupied trap sites N r ( 1 - f ) . The electron emission rate e n is proportional to the volume density of occupied sites N r f . At equilibrium, the electron emission and electron capture rates are equal, that is, e~ = GUnder the assumption that nondegenerate statistics is applicable, the net rate of capture of electrons by the traps can be shown to be R r -- ~r, v n N r / ( 1 + exp[(Ef - E r ) / k T l )
x ({n-- n r e x p [ ( E f - E r ) / k T ] } ) where n r is the density of electrons that would be in the conduction band, and if the equilibrium Fermi level E F were located at the trap energy E r. One can arrive at a similar expression for holes. When there is quasi-equilibrium condition Rn Rp = R, =
R -- NT'OrnO'pVnVp(n p -- n ~ ) / [O'nV n (n + n r ) + OpVp(p + p r ) ] This is the well-known Shockley-Read-Hall expression, which provides an additional channel for recombination with an exponential decay with a shorter time constant. The expression for surface-recombination velocity can be written as [ 11 ] S = R/An
-- N s 9 O'nO'plJnVp(n0 "-t-PO)
/[O'nVn("s +
+ %V,,(Ps +
where N s is surface or areal density of traps, and n s and Ps are the surface carrier densities; S will be maximum if both
the surface Fermi level and trap energies lie at the midgap. Therefore, S can be reduced by shifting the defect energy levels out of the forbidden gap; S can also be reduced by pinning the surface Fermi level at or near the band edges.
3. AUGER RECOMBINATION In the previous section we have seen that the recombination of excess charge carriers via deep impurity levels in semiconductors occurs by a two-step process where electrons and holes are successively captured into the deep level. The Shockley-Read-Hall model accounts for the two-step nature of the recombination and in general gives a good description of the basic recombination kinetics. However, so far, the physical mechanism of the capture process is not understood. Several studies have attempted to solve this problem [20-31]. There are basically four models that discuss the recombination at deep impurities [22, 23]. The first model considers the capture by multiphonon emission. Here, the transition between two states is due to their vibronic coupling and the excess energy is dissipated in the form of localized phonons. The reader is referred to the review on this topic by Morgan [24]. Morgan has considered a system consisting of a point defects that is capable of binding an electron in a deep state within the bandgap. Because of the interactions between the bound electron and the surrounding atoms of the lattice, there will be coupling between electronic states and the normal modes of the lattice that have an appreciable amplitude near the defect. It was shown that the required transitions are induced by coupling of the vibronic states to the surrounding medium through the electron-phonon and phonon-phonon interaction terms, and that this additional lattice phonon scattering enters as the important part of the theory of nonradiative capture. This model is very similar to the electronic relaxation in chemical systems discussed by Jortner, Rice, and Hochstrasser [32], Freed [33], and by Avouris, Gelbart, and E1-Sayed [34], and Engleman and Jortner [35]. Lax [25] has proposed the cascade capture mechanisms. Here, the carrier, which is going to be captured, loses its energy by passing through a series of closely spaced levels, emitting one phonon during each step. Landsberg et al. [26] and Haug [27] have considered classical Auger capture of free carriers by the impurity. In this process, two independent free carriers meet at the impurity site. Then their electron-electron interaction induces a transistion of one of the carriers into a deeply bound state, while the excess energy is transferred to the other carrier, which is highly excited, into its respective band. Hangleiter and coworkers [28-31] have proposed an excitonic Auger process. The e - e - h Auger recombination is shown in Figure 3. An electron recombines with a hole in a band-to-band recombination and another electron absorbs this recombination energy and moves to an excited conductionband state. The transition is mediated by Coulomb repulsions between the electrons. The excitonic Auger process is shown in Figure 4. When a free exciton meets the impurity,
SURFACE AND INTERFACIAL R E C O M B I N A T I O N IN S E M I C O N D U C T O R S
,2'
I
BQnd
I, 9 2 C o n d u c t i o n
1~
Valence Bond
Fig. 3. e ~-h Auger recombination. Reprinted with permission from D. B. Lak et al., Phys. Rev. Lett. 61, 1229 (1988). Copyright 1988 by The AmericaJl )hysical Society.
/ /;x
ca
I
i
I
I
Er
, , ~ a
~
, 4
cs ET
I
(o) electron coptur~
(b) hole capture
Fig. 4. E:{citonic Auger capture processes into deep impurity level E r. One particle (m: of the free exciton (FE) is captured, transferring its excess energy to the oth,:r one. Reprinted with permission from A. Hangleiter, Phys. Rev. B 37, 259,= (1988). Copyright 1988 by The American Physical Society.
the eleciron in the exciton is captured into the deep level, whereas the hole absorbs the excess energy and is highly excited into the valence band. Similarly, a hole can be captured into the impurity and the electron in the free exciton is excited into the conduction band. For a complete recombination of an electron-hole pair, two excitonic Auger processes are required. The situation is very similar to the ShockleyRead-Hall recombination.
4. Ill-X" SEMICONDUCTORS
4.1. Gats
Epilayers
From th.~ luminescence measurements on GaAs it was found that the minority-carrier lifetime ~'mc, the internal quantum efficiencies r/i and the radiative recombination co-efficient B were setlsitive to surface recombination [36, 37], recombination at substrate-epilayer interface [38], and self-absorption of recombination radiation [39, 40]. Because of the large surfacerecombination velocity [36] and also because of the large interface recombination at the substrate-epilayer interface, the initial measurements of luminescence decay times were dominated by the interface effects, and hence the measured values were not characteristic of the bulk material. Nelson et al. [41, 42] found that the interfacial recombination velocity was
221
dependent on the doping level in the GaAs/GaA1As structure. Henry and Logan [43] have reported a very high value of 4 x 105 cm/s for the surface-recombination velocity in GaAs free surface. Nelson and Sobers [44] have investigated the double heterostructure (DH) of the type GaA1As-GaAsGaA1As and shown that the surface and interfacial recombination were very much reduced. Luminescence decay times observed were 1.2 ns for heavily doped samples and were up to 1.3 s for lightly doped samples. Comparison of timedecay data was made for the GaAs samples with and without cladding layers. It was found that the confinement provided by the cladding layers reduced the surface and interface recombination. Experimental values of radiative recombination coefficient B were compared with the first-principles calculations of Stern [45, 46] and Casey and Stern [47] and excellent agreement was observed for high doping levels. For doping levels below P0 = 1 x 1017 cm -3, the experimental B values were significantly larger than the calculated values, which was possibly due to Coulomb interactions. Jastrzebski et al. [48] have reported the surfacerecombination velocity evaluated by scanning electron microscopy (SEM). This method is based on the relationship between the effective diffusion length of the minority carriers, the penetration depth of the electron beam, and the surfacerecombination velocity. Ryam and Eberhardt [49] have shown earlier that with decreasing accelerating voltage, that is, with decreasing penetration depth of the electron beam, the effective diffusion length of the minority carriers Leff decreases due to the increasing influence of the surface recombination. In n-type GaAs the surface recombination velocity was found to be 2.5 x 106 cm]s and a saturation value of 3 x 106 cm]s was observed if the carrier concentration exceeded 1018 cm -3 [48]. The same group has reported the surface-recombination velocity of GaAs p-n junction [50]. They found that the S value changes along the surface of GaAs. Surface impurities or surface-compositional inhomogeneities were thought to be responsible for the spatial variation of S.
4.1.1. Sulfur Passivation To prepare GaAs surfaces of superior electronic quality it is necessary to remove the native oxides from the surface, and it is also essential to satisfy the dangling bonds on the surface. The surfaces were passivated by aqueous sulfide treatment [51-93]. Sandroff et al. [51] were the first to report a very simple chemical treatment of semiconductor surfaces with NazS. 9H20, and observed dramatic enhancement in the gain of a GaAs/GaA1As heterostructure bipolar transistor (HBT). The structure of HBT, which was grown by metal-organic chemical vapor deposition (MOCVD) is shown in Figure 5. At low collector currents the current gain of the device increased 60-fold. The chemical reaction mechanism responsible for passivation was described by a two-step process. The native oxide and elemental arsenic were etched away, thus exposing a pristine surface, and then the sulfur gets bonded strongly. It is known that sulfur forms many binary stable compounds with both Ga and As; GaS and A s z S 3 are layered semiconductors.
222
KASI VISWANATH
0.2 pm
n*" I0
GoAs emitter AIxGal-xAs
3 /~m
_o
le cm'3
N~', 5 x IOI7r
0.15/am
bose" GoAs
p,,.,i x iOI8 cm "3
0.5/.tin
collector" G o A s
n , ~ 5 x l O I T c m "3
Yablonovitch et al. [52] have discovered that ( N H 4 ) 2 S and Li2S also have excellent passivating properties, in addition to Na2S. 9H20. The surface-recombination velocity at the interface between the sulfide and GaAs was found to be very close to that of the nearly ideal A1GaAs/GaAs interface. Figure 6 shows the time-resolved spectra of different samples of GaAs. The initial nonexponential behavior of the double heterostructure decay curve is due to the influence of bulk Auger and radiative recombination. By using the double heterostructure decay curve as a reference, the effect of bulk recombination was subtracted. It can be seen in Figure 6 that Na2S. 9H20 treated GaAs and the ideal GaAs/GaA1As structure have almost identical decay behavior indicating the improvement in the quality of surface treatment. The dependence of surface-recombination velocity on carrier density is shown in Figure 7 [52]; S is highest at the highest carrier densities and is lowest for the lowest carrier
I
i
I
1 1
1 2
t 3
I.I
E
o 9- 1017 0 o. >., I-.,.,.
r
10~=
lO~S
,< 0 1014
0
1 o
(n
10
TM
101r
CARRIER DENSITY (per cm =)
Fig. 5. Schematic representation of the A1GaAs/GaAs HBT device. Reprinted with permission from C. J. Sandroff et al., Appl. Phys. Lett. 51, 33 (1987). Copyright 1987 by The American Institute of Physics.
z u,I cI er ILl
u 1::
r
n ~ GoAs substrate
......
101=
el" Q
TM
i
-5
,
10
I
4
TIME after laser pulse (ps) Fig. 6. Decay of carrier density in a GaAs double heterostructure excited by delta function optical injection at t - - - 0 : (a) A1GaAs/GaAs; (b) Na2S.9H20/GaAs; (c) photochemical treatment; (d) clean GaAs surface exposed to air. Reprinted with permission from E. Yablonovitch et al., Appl. Phys. Lett. 51, 439 (1987). Copyright 1987 by The American Institute of Physics.
Fig. 7. Variation of S with optically injected carrier density for Na2S-9H20/GaAs. At high density the bands tend to flatten as the surface charge is screened out. At low density, minority carriers are repelled from the surface, thus reducing S. The solid line is a theoretical curve for N - 2 x 10 ]1 charges/cm 2, and SO = 9000 cm/s. Reprinted with permission from E. Yablonovitch et al., Appl. Phys. Lett. 51,439 (1987). Copyright 1987 by The American Institute of Physics.
densities. These results were explained by considering the effects of surface-fixed charge, such as band bending, near the interface. The chemical reaction mechanism for the low S at the NazS. 9HzO/GaAs interface was explained by considering a two-step process: GaAs surface is first etched giving a very clean surface to which sulfur can form strong covalent bonds. The clean surface is obtained by the joint action of sulfide ions and hydroxide ions in the aqueous solutions. It was also noted that the GaAs surfaces treated with pure NH4OH, KOH also gave surface-recombination velocities similar to those observed with sulfide treatment. Surface oxides in the form of A s 2 0 3 and Ga203 along with elemental arsenic are responsible for high concentrations of midgap defects associated with the GaAs surface. Surface oxides are dissolved in alkaline solutions and sulfide ions react with elemental arsenic. This interpretation was very similar that given by Nelson et al. [94] for the action of KOH/KzSe on GaAs. Skromme et al. [55] have observed that the surfacerecombination related notch features in the free and bound exciton emission spectra of NazS. 9HzO-treated GaAs at low temperature were eliminated as shown in Figures 8 and 9. They concluded that the residual band bending under illumination is less than 0.15 V. Wilmsen et al. [59] investigated the effects of N2, O 2 and H20 on GaAs passivated by sulfides. They observed that both water vapor and oxygen must be present in order to obtain a large PL signal. Lee et al. [62] passivated n-GaAs with P2Ss/NH4OH and a fivefold increase in PL intensities was recorded. Besser and Helms [73] compared the quality of GaAs surfaces treated with sodium sulfide and ammonium sulfide. From the PL measurements it was concluded that ammonium sulfide is better than sodium sulfide. This is in ageement with the report by Liu et al. [58]. Sodium is known to be extremely mobile in ionic forms in silicondevice technology. The presence of sodium ions in SiO 2 films was a major problem in the realization of practical insulated gate field-effect transistors. Their reduction has given rise to
223
S U R F A C E A N D I N T E R F A C I A L R E C O M B I N A T I O N IN S E M I C O N D U C T O R S
-
1.slso -I
ENERGY (eV) 1.s~2s 1.s~oo . . . . I . . . . . . . I
( 0 " . X)
VPE
ENERGY (eV) 1 .SiSO " MBE
GaAs
T=
T = 1 .SK PL = 700 (O " . X)/ 9 , h)
mWlcm
1.5125 -
,
-
,
i
2
X e x c = 5 1 4 5 ,~
..
_
I
l.sloo ! = .
.
.
.
.
.
.
.
~ . x)
GaAs 1,8K
PL
700 mW/cm
~exc =
5145
2
A CfA ", X)
I I ) -
~_ z
= Lu
CARE SURFACE FE "
_z
I (O
GeIA ". X= FFilt (a. xl
iJll/
) fO" h
Na2S APPLIED I
8170
8180
8190
8200
WAVELENGTH
8210
8220
(A)
Fig. 8. N,:,rmalized excitonic luminescence spectra (intensity in arbitrary units) of a~:~n-type VPE GaAs layer (n = 4 x 1014 cm -3, 77 K mobility -9200 cm2P~' s) before and after Na2S.9H20 surface passivation. The position of the grot ad state (D~ peak is indicated by the tic mark at 8187 A. Two excited stales are also marked at shorter wavelengths. Incident laser intensity (PL)alld wavelength ()texc) are indicated. Reprinted with permission from B. S. Skro:ame et al., Appl. Phys. Letr 51, 2022 (1987). Copyright 1987 by The Ameri,'an Institute of Physics.
very high-quality silicon, very large-scale integrated (VLSI) circuits. I'herefore, large quantities of sodium are not advisable for device processing. From these arguments ammonium sulfide was thought to be a better alternative to sodium sulfide. Shikata imd Hayashi [77] have used PL to characterize various kin,~ls of processes and insulators for overpassivation of (NH4)2S <.-treated GaAs. The influence of annealing on overpassivati,)n was investigated for the fabrication of devices. The overpassivation films deposited by electron-cyclotron resonance ,:hemical-vapor deposition (ECR-CVD) maintain the effect of sulfur treatment. The other overpassivations deposited by sput:ering, pyrolytic CVD and plasma-enhanced CVD degrade the effects of sulfur treatment. The deposited films were SiN and SiC 2. Annealing between 350-500 ~ has given better results for ECR-CVD SiN. By contrast, for SiC2 a gradual degradation with increasing temperature was observed. The luminescence of free and bound excitons was observed even for bulk-grown GaAs at 4.2 K, which indicated reduction of surface-recombination centers. Lunt et al. at Cal Tech. [78] used sulfides and thiols to passivate GaAs surfaces. They reported the surface electrical and chemical properties of (100)-oriented GaAs that has been exposed to a variety of sulfur, nitrogen and oxygen donors.
1,. . . . . 8170
8180
t., 8190
,
,I 8200
,
I 8210
, 8220
WAVELE.~IGTH f A )
Fig. 9. PL spectra for a p-type MBE GaAs sample (p -- 4 x 10 ]5 cm-3), before and after Na2S.9H20 surface passivation. Reprinted with permission from B. S. Skromme et al., Appl. Phys. Letr 51, 2022 (1987). Copyright 1987 by The American Institute of Physics.
Many of these treatments have given improved GaAs surface recombination properties, demonstrating that a general class of chemical reactivity can be exploited to obtain information on the GaAs surface-state properties. Steady-state photoluminescence and time-resolved spectroscopy have been employed. Organic thiols, with general formula R - S H where R---CH3CH2SH o r - C 6 H 4 C 1 , which were dissolved in nonaqueous solvents, were found to be good candidates for surface passivation. An increase in PL intensities was observed with the chemical treatment of GaAs by thiols. X-ray photoelectron spectroscopy (XPS) studies have shown that thiols were bound to the GaAs surface. However, elemental As was not removed and A s z S 3 overlayers were not found on the GaAs surface. It was argued that complete removal of As atoms and the formation of monolayers of A s z S 3 is not necessary to effect the reduction in the recombination at the surface. Figure 10 shows the time-resolved spectra of GaAs under different treatment conditions recorded by Lunt et al. [78]. The experiments were done under high-level injection conditions. In this situation bands are almost flattened and the effects due to band bending are minimum. The changes in the PL signal will be due to variation in the density and capture cross sections of the surface traps. In the low-level injection case, both the changes in the surface-recombination velocity S and the equilibrium surface Fermi-level affect the PL intensity.
224
KASI VISWANATH
~.
to3
i
a)
N
!| 100
time (ns) Fig. 10. Time-resolved photoluminescence decay curves of 2.8-/xm-thick epilayer GaAs samples taken at 880 nm. The excitation source was a Nd:YAG pumped dye laser providing light at 685 nm: (a) A10.3Gao.vAscapped GaAs sample, as-grown; (b) Alo.3Ga0.7As cap etched off with 0.05% Br2 in methanol and GaAs sample immersed in 1.0-M Na2S.9H20 (aq) for 30 min; (c) etched GaAs sample immersed in 1.0-M 4-Cl-thiophenol in C C I 4 for 30 min; (d) etched GaAs sample immersed in 1.0-M NaOCH3 in methanol for 30 min; (e) GaAs sample only exposed to etch A. Reprinted with permission from S. R. Lunt et al., J. Appl. Phys. 70, 7449 (1991). Copyright 1991 by The American Institute of Physics.
For GaAs with A1GaAs cap layer S was found to be 500 cm/s, whereas for the GaAs surface that was uncapped and exposed to air, very fast PL decay was observed with a best-fit S value of 2 x 105 cm/s. These two extreme cases were used as benchmarks to evaluate the effect of various surface treatments. The GaAs surface, which was treated with NazS. 9H20, produced an interface with a lower surface-recombination velocity than that obtained from the unpassivated, air-exposed GaAs surface. Best fits of the PL decays have given S -- 7 • 104 cm/s for this interface. Analysis of PL decays after exposure to 4-Cl-thiophenol also yielded an S value of 7 • 10 4 cm/s. On the other hand, the GaAs surface that was treated with 1.0 M NaOCH 3 in CH 3OH gave an S value of 2 • 105 m/s, which was the same as that observed for the air-exposed GaAs surface. From these data it was argued that increases in the steadystate PL signal do not necessarily correlate with decreases in surface-recombination rates under high injection conditions. The steady-state PL measurements [78] indicated a correlation between the electron-donating ability of a chemical and the improvement in PL. Increases in the intensity of photoluminescence with methoxide, phenoxide, ethylenediamine, and hydroxide have indicated that other strong electron donors besides sulfides can be used to passivate the surface-recombination centers in GaAs. Charge alone was not sufficient to improve the PL because it was found that several ionic species did not produce any decrease in the observed surface-recombination rate. Time-resolved measurements have shown [78] that the surface treatment decreases the cross section or density of the surface traps. Other reagents, such as sodium methoxides, have given increases in the steady-state PL intensities, but did not affect the GaAs surface carrier-trapping rates. Sodium methoxide treatment did not change the high-injection level
PL lifetime, but the intensity of steady-state PL has shown an increase. This was interpreted as being caused by changes in the surface Fermi-level position. Hung et al. [79] have grown CaF 2 and BaF 2 films on GaAs surfaces that were chemically cleaned by (NH4)2S x. These materials have applications in epitaxial insulator layers or buffer layers for heteroepitaxy. Wang et al. [80] have passivated GaAs with P2Ss/(NH4)2S solutions. A number of techniques such as photoluminescence, X-ray photoelectron spectroscopy (XPS), and Auger electron spectroscopy (AES), scanning-electron microscopy, were used. It was noted that the addition of P2S 5 to (NH4)2S has given better GaAs surfaces. When P2Ss-containing solutions were used, P was detected on the surface but it was bound only to either S or O, never to Ga or As. When the PL intensity decreases, XPS detected an increase of O on the surface, and for longer times found Ga-O bonding, but As-O bonding was not detected. Wang et al. [80] have also studied the differences between passivation by evaporation of elemental sulfur or absorption of hydrogen sulfide and passivation by sulfide solutions. Both evaporation and absorption have resulted in the same saturation of S on GaAs at room temperature. These S layers were not as resistant as the S layers obtained by sulfide-solution treatment. With the help of XPS results, it was argued that the degradation in the PL intensities upon exposure to air is due to the generation of electrically active defects that are created by the reaction of oxygen at structural point defects in the sulfur passivation layer. Continued long-term degradation is thought to be due to the continued reactions at the surface. Several methodologies were also developed in order to have a uniformly passivated surface layer. Oh et al. [82] have shown that (NH4)2S x treatment improves the surface quality of GaAs by observing a highly resolved photoluminescence spectra as shown in Figure 11. Note that in the untreated GaAs sample several peaks due to many defects can be seen. The peak at 1.5148 eV might be due to excitons bound to antisite defect complexes [95]. The (d, X) peak is due to Ga vacancies resulting from the interaction between A s 4 and Ga or their related complexes acting as acceptors [96]. Kunzel and Ploog [97] reported that this (d, X) peak appears mainly for As-stabilized (2 • 4) surfaces grown by molecular beam epitaxy using A s 4 and Ga molecules. In the sulfur-treated PL spectrum the lines due to all these defect complexes are greatly reduced in intensity. Liu and Kauffman [84] have investigated the excitation power dependence on the photoluminescence of bare and Na2S-treated n-GaAs. It was shown that for both samples there was an increase in PL quantum yield with increasing excitation power density. In the chemically treated semiconductor there will be a reduction in the number density of surface impurities responsible for midgap levels, which, in turn, results in reduced band bending and/or a reduction in the effective surface-recombination velocity. Photoexcitation alone can give rise to these two effects in GaAs, with a reduction in band bending resulting from charge separation in the depletion layer following photoexcitation and a reduction in
SURFACE AND INTERFACIAL RECOMBINATION IN SEMICONDUCTORS
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(MIS) structures, as the organics have insulating atoms made up of alkyl chains. A new sulfur-c0ntaining organic solution of CH3CSNH 2 was shown to effectively passivate the GaAs surface [86]. Liu and Kuech [87] have utilized nearfield scanning optical microscopy (NSOM) along with atomic force microscopy (AFM) to study the spatial variations in photoluminescence and their relation to topographic features in untreated and (NH4)zS-treated GaAs samples that were grown by metal-organic vapor-phase epitaxy. Anderson et al. [88] have achieved the growth of ferromagnetic Fe overlayers on sulfur-passivated (001) GaAs surfaces. Geisz et al. [90] have utilized photoreflectance spectroscopy to show that GaAs surfaces that were photowashed degraded very easily, whereas sulfide-treated surfaces were very stable as shown in Figures 13(a) and 13(b).
--i.5148eV
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225
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Energy (eV)
Fig. 11. I)L spectra of (a) as-grown and (b) sulfur-treated GaAs measured at 10 K. lieprinted with permission from Y. T. Oh et al., J. Appl. Phys. 76, 1959 (1(.'9 t). Copyright 1994 by The American Institute of Physics. surface- :rap density resulting from midgap state filling that is short co~npared with the actual recombination event [98-100]. Figure 1 2 shows the influence of excitation power on the measured enhancement of PL following the surface treatment of GaAs. [1: is very important to note that PL enhancement varies dramatically with excitation power. Asai et al. [85] have electrodeposited organic molecules containi~lg reactive sulfur as a passivating layer on GaAs. This pr(~cess has two advantages. First, it passivates the GaAs surface without any surface damage. Second, it can be used as dielectric in the metal-insulator-semiconductor 20 f. . . . . . . . . . .
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(b) Fig. 13. (a) Effects of photowashing on the PR spectra from bulk n-GaAs. (b) Effects of sulfide passivation on the PR spectra from bulk n-GaAs Reprinted with permission from J. E Geisg, S. A. Safvi, and T. E Kuech, J. Electrochem. Soc. 144, 732 (1997). Copyright 1997 by The Electrochemical Society, Inc.
226
KASI VISWANATH
MacInnes et al. [101], Jenkins et al. [102], and Tabib-Azar et al. [103] have employed chemical vapor-deposition techniques to form a very stable cubic-phase GaS-passivating film on GaAs. A GaAs-based metal-insulator-semiconductor fieldeffect transistor (MISFET), in which cubic GaS was used as the insulating material, was fabricated (Fig. 14a). Such a GaS/GaAs MISFET showed an on-to-off resistance ratio comparable to that of a commercial device. An enhancement in PL intensity was also shown. Cao et al. [92, 93] have studied the electronic structure of these systems by Auger spectroscopy. The C-V curves of the GaS/GaAs MIS diode are shown in Figure 14b. 4.1.2. Electronic Structure of Sulfur-Passivated
GaAs Surfaces The electronic structure of sulfur-passivated GaAs surfaces has been studied by a number of workers. Surface-characterization techniques such as photoelectron spectroscopy and Auger spectroscopy have been employed. Sugahara et al. [104, 106, 108], Scimeca et al. [ 105, 107], Maeyama et al. [ 109], Oshima Gate (Au) -5 p.m_ m Cubic GaS
Drain (Au) 100 I~m
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(b) Fig. 14. (a) Schematic diagram of the GaS-GaAs field-effect transistor (FET): not to scale. Reprinted with permission from R R Jenkins et al., Science 263, 1751 (1994). Copyright 1994 by the American Association for Advancement of Science. (b) High-frequency (1 MHz) C-V curves of the GaS/GaAs MIS diode at room temperature. Reprinted with permission from X. A. Cao et al., J. Appl. Phys. 86, 6940 (1999). Copyright 1999 by The American Institute of Physics.
et al. [110], and Sugiyama et al. [111-113] have made indepth studies by using synchrotron photoemission analysis, and discussed the surface-passivation mechanisms. They have concluded that Ga-S, As-S and S-S bonds exist on the sulfurtreated GaAs surface. It was also observed that Ga-S bonds become dominant after annealing at 360 ~ The initial upward band bending of the As-treated samples suggested that there is a reduction of Asca anti-site defects. Annealing has caused the shift of the surface Fermi level by 0.3 eV towards the conduction-band minimum. After annealing the band bending has decreased; Ga-S bonding formation was thought to be the most important reason for the improved electronic quality of sulfur-treated samples. Changes in the chemical bonding as a function of temperature were also studied [105]. The surface structure of S chemisorbed GaAs was the same as (NH4)zS-treated GaAs [106]. The relative stability of sulfur on (NHa)zS-treated GaAs was in the order ( l l l ) B > (100) > ( I l l ) A , based on the Ga coordination number [ 107]. Direct correlation between PL degradation and the Ga oxide formation resulting in drastic upward band bending was observed [110]. From the X-ray standing-wave measurements it was shown that oxidized sulfur atoms were about 22% of the surface sulfur atoms and were randomly distributed, whereas the unoxidized sulfur atoms in a Ga-S bonding state remain at the first-layer As sites although they are disordered [ 111 ]. Sato and Ikoma [114], Sato et al. [115], and Sakata et al. [116] have measured X-ray photoelectron spectra of sulfurtreated GaAs that did not show the existence of Ga and As oxides. They have concluded, after a number of systematic studies, that the most robust surface passivation of GaAs is PzS5/NH4OH treatment plus cleaning in deionized water and thermal annealing. Moriarty et al. [ 117, 118] have used scanning tunneling microscopy (STM) and synchrotron photoemission techniques and have shown that sulfur substitutes for As in atomic layers below the surface, which will increase the surface band bending from that of the clean GaAs surface. Bessolov et al. [89, 119 120, 122-126], and Lebedev and Aono [121] have shown that the surface barrier height and depletion-layer width decrease with the decrease of the dielectric constant of the passivating medium as shown in Figure 15. The decrease of the surface barrier in n-GaAs means that the surface Fermi level shifts toward the conduction band after treatment with sulfur. Annealing at 360 ~ or higher results in the change of the surface Fermi level from 1.2 to 0.85 eV above the valence-band maximum as depicted in Figure 16. The changes in the surface Fermi-level position were also correlated with the PL intensity changes for various treatments of GaAs. There are several contradictory reports [74, 127-132] on Fermi-level pinning and band bending in surface-treated GaAs. Besser and Helms [127] have concluded that unpinning of the surface Fermi level does not occur in the surface-treated GaAs although there is a reduction in the surface recombination. They have obtained a model that accounts for this effect
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IIc" Fig. 15. I)ependence of the surface barrier heights (a) and the depletionlayer width~ (b) for GaAs treated in different solutions of ammonium sulfide on the reciprocal effective dielectric constant of the solution used. Reprinted with permission from V. N. Bessolov, M. V. Lebedev, and D. R. T. Zahn, J. Appl. Phys. 82, 2640 (1997). Copyright 1997 by The American Institute of Physics.
based or,, the changes in band bending after surface treatment. D(>wnward band bending near a surface gives rise to a field that tends to repel electrons that are coming from the bulk. Be,:ause the recombination centers are assumed to be at the surface, nonradiative recombination can be effectively decreased by inhibiting the carriers from arriving at the surface. Hence, the PL intensity can increase without a reduction in the density of surface-recombination centers and without unpinning the surface Fermi level. Spindt et al. [128], and Spindt and Spicer [129], obtained a model for reduced surface recombination without band flattening. The electronic states thought to be responsible were midgap donors compensated by acceptors that have energies near the valence-band maximum. Double donors and double acceptors were also considered. In the double-donor case the reduction in the surface-recombination velocity was shown to be greater by a factor than the reduction in the surfacestate density. The As and Ga anti site defects were considered for the donor and acceptor states. This model is shown in
Fig. 16. Binding energy relative to the valence-band maximum (a) and intensity (b) of feature B (squares) as well as C and D (circles) of the valence-band spectra of p-GaAs(?00) treated in the solution of ammonium sulfide in isopropanol as a function of annealing temperature. The intensity of features is referred to the intensity of bulk feature with binding energy 6.7 eV. Results are depicted by dots with dashed lines given as a guide to the eye. Reprinted with permission from M. V. Lebedev and M. Aono, J. Appl. Phys. 87, 289 (2000). Copyright 2000 by The American Institute of Physics.
Figure 17 and is consistent with the advanced unified defect model of Lindau et al. [15] and Chye et al. [16] for the surface and interfacial recombination in semiconductors. Hasegawa et al. [130] made observations that are very similar to those made by Besser and Helms [127] independently and at the same time. They also concluded that unpinning of the Fermi level does not take place in surface-treated GaAs. Yablonovitch et al. [74] have contradicted the results and interpretation of Besser and Helms [ 127] and Hasegawa [ 130]. They have obtained a model for band bending and Fermi-level pinning for sulfur or chemically treated GaAs (see Fig. 18). Fermi-level pinning implies band bending, but band bending does not necessarily imply pinning. Band bending is caused by the total surface charge, which is the sum of the surface-trap charge due to defect states in the gap, and the surface-fixed charge due to defect states in the gap and surface-fixed charge arising as a result of ionic charges on the surface. When only the surface-fixed charge is present the Fermi level can move freely. However, if the density of the gap states is very high, then the Fermi level is pinned. Even low surface-trap charges can cause the band bending. Under the various conditions of optical excitation by a laser the changes in the quasi-Fermi level and band bending are shown in Figure 18.
228
KASI VISWANATH
CBM
"r
Energy from CBM
1.4eV
Donor
dark
--0.65eV
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Double Donor ~ Double Acceptor { Z
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~0.9eV --1.1eV
t"
M
strong light
EFp_I: -(d)
(b) Fig. 17. Energy diagrams showing midgap donor states compensated by acceptors at the surface: (a) the simplest case of a single donor compensated by a single acceptor; (b) the single-level states are replaced by double donors and acceptors. The conduction-band minimum (CBM), valence-band maximum (VBM), and bandgap (1.4 eV) are also indicated. Reprinted with permission from C. J. Spindt and W. E. Spicer, Appl. Phys. Lett. 55, 1653 (1989). Copyright 1989 by The American Institute of Physics.
4.1.3. Selenium Passivation
There are many reports [64, 135-149] on Se-passivated GaAs. For example, Mauk et al. [64] have passivated GaAs p-n junction solar cells by KzSe aqueous solution and observed a reduction in the surface-recombination velocity from 5 x 10 6 ClTI/S for the untreated sample to 103 cm/s for the treated sample. Improvement in open-circuit voltage and reduction in saturation current also indicated reduced surface recombination. Sandroff et al. [134] measured PL of a Se-treated sample that was 400 times enhanced compared to that recorded for a similarly grown GaAs surface. Berrigan et al. [137], Scimeca et al. [138, 139, 141, 142], and Maeda et al. [140] reported the chemical bonding and atomic structure of Se-treated GaAs. They studied in detail synchrotron-radiation photoelectron spectroscopy of Se-treated (111) A, (100) and (111) B GaAs. If the deposition is made at relatively high temperature, As on the surface is replaced by Se and band bending is reduced considerably. Deposition of a thin A1 layer on a Se/GaAs surface has yielded a better quality surface [139]; GaAs samples on which ZnSe epilayers were grown have shown very good PL properties [143, 144]. Kuruvilla et al. [145, 146] observed the formation of the As2Se 3 phase in their X-ray photoelectron spectra of Se/GaAs.
.....
EFn very
strong light
~V
EFp
Fig. 18. (a) Band bending in the dark for the Na2S. 9H20 case. The t- represent the surface traps largely responsible for dark bank bending. For (NH4)2S the Fermi level stays above t- (b) Low injection conditions appropriate to this experiment. The electron Fermi level moves freely, but the hole Fermi level is temporarily "pinned." The bands begin to flatten. (c) Strong injection, both Fermi levels move freely, and traps t- become empty, but some surfacefixed charge Q/remains. (d) Near degenerate injection, all band bending is screened out. Reprinted with permission from E. Yablonovitch et al., Appl. Phys. Lett. 54, 555 (1989). Copyright 1989 by The American Institute of Physics.
Sun et al. [147, 148] conducted very elegant experiments by using high-resolution core-level photoemission spectroscopy excited with a synchrotron-radiation source to clarify the nature of bonding in Se-treated GaAs. They have identified arsenic-based sulfides and selenides on the topmost surface and gallium-based selenides adjacent to the bulk GaAs substrate. Thermal annealing was done at different temperatures and the shift of the surface Fermi level was monitored. This procedure allowed the authors to identify the chemical entities that cause band bending. Figure 19 shows the shift in the surface Fermi level as a function of annealing temperature. It was demonstrated that GazSe 3 bonds are responsible for the unpinning of the surface Fermi level by removing midgap states.
4.1.4. Phosphorous Passivation Improvement in the electronic quality of GaAs surfaces was achieved by phosphorous passivation [150-158]. Olego et al. [150] reported the unpinning of the Fermi level and lowering
SURFACE AND INTERFACIAL RECOMBINATION IN SEMICONDUCTORS
CBM 1.4
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0.2
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1;0 2(~0" 3(~)0"'4;0 500' 6;0 Annealing Temperature(~
Fig. 19. P~isition of the surface Fermi level relative to the band edges as function c f Lhe annealing temperature of a highly n-doped GaAs(100). The shift of th,~ ~urface Fermi level from the unpinned position near CBM toward midgap is :lue to the progressive desorption of the passivating overlayer. Reprinted v~ith permission from J. Sun et al., J. Appl. Phys. 85, 969 (1999). Copyright ~ )99 by The American Institute of Physics.
of the stHface barrier from 0.7 to 0.18 eV in n-GaAs on which an amorphous phosphorous overlayer was deposited. Raman and ph()loluminescence spectroscopies were used for these studies. A model was proposed that considers the P atoms in the interface in the place of As vacancies that pin the Fermi level at midgap in n-GaAs. Viktorovitch et al. [151] carried out the thermal treatment of GaAs in PH 3 that causes the As/P exchange. This gives rise to a thin layer of GaP on the GaAs surface and the formation of arsenic oxide on the GaAs surface, thus preventing the formation of arsenic oxide on the GaAs surface. Wada and Wada [153] directly deposited a thin layer of InP on the GaAs surface. Davidson et al. [155] employed a frequency-doubled argon ion laser emitting 257 nm light to conduct the photolysis of P(CH3) 3 and phosphidation of the GaAs surface. Harrison et al. [156] have done a very simple heat treatment of GaAs in the presence of tertiary butyl-phosphine vapor and obtained an excellent PL spectrum. A very thin GaP layer was formed on the GaAs surface. Enhancement in PL intensity was thought to be due to reduced surface recombination. Glemb()cki et al. [157] used P2S5 to passivate GaAs surfaces exposed to CI2/Ar plasma. The Fermi level of p-GaAs shifts from neltr the valence band to midgap. The treatment with P2S5 wa:, found to remove the damage caused by plasma etching. Excess As was evidenced for the etched sampled at the GaAs/oxide interface from Auger spectroscopy; passivation by P2S5 removes the excess arsenic.
4.1.5. Arsenic and Antimony Passivation Mada et al. [81] suppressed the degradation of the (NH4)2Streated GaAs by coating the surface with an evaporated A s z S 3 film. The interfaces were also examined by studying the characteristics of a metal-insulator-semiconductor (MIS) structure, in addition to the PL measurements. In a later study [83] n-GaAs was passivated by directly depositing A s z S 3 film on the etched surface. It was also shown that the surface Fermi
229
level moves to 0.25 eV below the conduction band minimum. Passlack et al. [159] reported very low interface recombination velocity of S << 1000 cm/s for As cap layers of GaAs stored in ultrahigh vacuum. Wang et al. [160] applied a two-step process to passivate deep-level defects in GaAs grown on Si. The first step is the H plasma passivation followed by annealing in AsH 3. The p-n junction properties of GaAs on Si were found to be greatly improved. Passivation of GaAs by antimony overlayers was reported by Zinck et al. [ 161 ]. Maeda et al. [ 162, 164] and Sugiyama et al. [163] examined the surface reconstruction and changes in the chemical bonding in the Sb/GaAs (001) surface that was prepared by depositing Sb on As-terminated GaAs (001) at room temperature and annealing it up to Sb desorption temperature.
4.1.6. Passivation by Nitridation One of the best methods to passivate GaAs is by nitridation [165-172]. In this method a thin layer of GaN is coated on GaAs. The main advantage in group III nitrides is that they are chemically very stable. In order to form the passivating nitrides, the native oxide is first removed from the surface, and then a nitride film is grown. Vogt and Kohl have performed nitridation with hydrazine at low temperature around 400 ~ Higher PL intensity in the bandgap luminescence was observed. Park et al. [166] nitrided GaAs by irradiation with electron-cyclotron resonance nitrogen plasma at various substrate temperatures from room temperature to 600 ~ Nitridation by using helicon-wave excited N 2 plasma was achieved by Okamoto et al. [167], Kasahara et al. [168], Hara et al. [ 169, 171], Wada et al. [ 170], and Tanemura et al. [ 172]; Ga-N bonds were identified in the film grown on the GaAs surface.
4.1.7. C1 Passivation Lu et al. [173] obtained an ordered and air-stable GaAs by chemical etching and passivation by dilute HCL solution. X-ray photoelectron spectroscopy gave evidence for Ga-C1 bonds along the surface normal. The C1 termination has removed surface bandgap states. An enhancement in the nearband PL was observed. Surface photovoltage measurements indicated reduction in midgap states.
4.1.8. Ru Passivation Nelson et al. [174] were the first to observe reduction in surface-recombination velocity in GaAs after chemisorption of Ru ions. Time-resolved luminescence experiments were performed on the untreated and Ru-treated GaAs.
4.1.9. Ga203 Passivation Hwang et al. [175] used molecular beams of gallium oxide to deposit an oxide film on GaAs. From photoreflectance and photoluminescence experiments the interfacial state density was found to be very small; Ga203-GaAs is a very
230
KASI VISWANATH
good material to realize metal-oxide semiconductor fieldeffect transistors (MOSFETs), which will be better than Sibased MOSFETs, as they can offer higher speed and low power consumption. Callegari et al. [176] employed the rf plasma-cleaning technique to remove the midgap states at the Ga oxide/GaAs interface; MOS capacitors were also fabricated that have excellent capacitance-voltage characteristics. Ping and Ruda [ 177] performed ab initio molecular orbital calculations to understand the mechanism of Ga203 passivation of GaAs surfaces.
4.1.10. Hydrogen Passivation Introduction of hydrogen in semiconductors is a very wellknown technique in semiconductor technology because it improves the electrical and optical properties of any III-V compound. Native oxides on GaAs substrate surfaces are generally removed before epitaxial growth by heating the substarte around 500 ~ In this process many surface contaminants such as carbon remain without evaporation. The surface contaminants form a carrier-depletion layer between the substrate and epitaxial layer interface. The carrier-depletion layer affects the optical and electronic properties. Hydrogen can passivate many shallow donors and acceptors. Also, it will effectively passivate the dangling bonds on the surface. Hydrogen passivation in GaAs has been reported by a number of researchers [178-193]. Carbon and nitrogen impurities in the GaAs surface were removed by a H 2 plasma in the experiments conducted by Friedel and Gourrier [178]. Surface photovoltage measurements confirmed the reduction in band bending. Stillman's group could passivate C acceptors in both molecular beam epitaxy (MBE) and metal-organic chemical vapor deposition (MOCVD) GaAs samples by hydrogenation. In sutu chemical etching of GaAs substrates with a combination of HC1 gas and hydrogen prior to MBE growth effectively cleaned the substrate surface [181]. A novel photochemical vapor-deposition system for hydrogen passivation was described by Chen et al. [182]; PL spectroscopy was utilized to evaluate the passivated surfaces. This technique has no electron or ion bombardment, therefore the sample will not be damaged during the processing. Swaminathan et al. [183] reported the photoluminescence measurements of hydrogenated GaAs grown on an InP substrate. This heteroepitaxial structure has great importance as it makes it feasible for the development of optoelectronic integration of GaAs devices with InP-based photonic devices on the same wafer. However, the major problem in this material system is a large lattice mismatch of about 3.7% between GaAs and InP, which gives rise to high density of interface defects, such as dislocations. To solve the problems of lattice mismatch, several techniques have been reported: For example, growth on misoriented substrates, usage of buffer layers such as GaAs/ GaA1As multiple quantum wells, and GaAs/InGaAs strained layer superlattice prior to GaAs growth. The authors have found that the hydrogenated sample has better PL properties.
Gottrcho et al. [184] developed a realtime PL monitoring method to study the hydrogen-plasma treatment of GaAs, and were able to find the best conditions for plasma treatment. It is very important to note that the surface treatment by plasma not only passivates the defects, but it can also damage the surface. Therefore, a trade-off between the plasma pressure and exposure time is very essential. Ineffective plasma treatment at room temperature can be due to low pressure. Excess arsenic on the surface is removed by plasma and midgap states are removed. Enhancement in PL intensity was attributed to removal of midgap energy levels. A remote Ar discharge was used to excite and dissociate hydrogen for in sutu cleaning of the GaAs surface [185]. Carbon and oxygen on the surface were removed as evidenced in Auger spectroscopy. Swaminathan et al. [ 186] passivated Si 6-doped GaAs grown by molecular beam epitaxy by employing a low-frequency hydrogen plasma at 250 ~ for 30 min. It was demonstrated that ~-doped material can be passivated by atomic hydrogen in exactly the same way as a uniformly doped semiconductor. Low-temperature photoluminescence and capacitance-voltage measurements confirmed the effects of passivation. Two hydrogenation schemes, that is, the rf glow-discharge system and the photochemical vapor system, were used to passivate GaAs heteroepitaxially grown on InP [187]. The copper-arsenic vacancy complex was eliminated. The damage due to rf glow-discharge was removed by the photochemical deposition method. The photolumnescence peaks at 1.503 and 1.462 eV have intensities stronger than those in homoepitaxial GaAs epilayers. These samples were grown by molecular beam epitaxy. An ECR hydrogen-plasma treatment was utilized by the AT&T group to remove the native oxides, followed by annealing at 500 ~ to prepare GaAs substrates for MBE overgrowth [189]. Oxygen was completely removed and carbon impurities were greatly reduced in concentration. A very good structural order at the epilayer/substrate interface was obtained. A Ga-stabilized reconstructed surface was achieved. The temperature at which annealing was done was shown to be important. Low-temperature annealed samples were found to have many defects, for example, dislocations at the epilayer/substrate interface. The same research group in a later publication [190] conducted the C12 chemical etching as a follow-up process after the ECR hydrogen-plasma treatment. This additional procedure has reduced the C, Si, and O surface contaminants from the GaAs substrate that was similar to a very clean GaAs epitaxial layer. Wang et al. [ 191, 192] and Soga et al. [ 193] have passivated GaAs solar cells grown on Si by using a phosphine-added plasma treatment. The samples were grown by MOCVD. Narrowing of PL peaks (Fig. 20) was observed after hydrogenation, which was thought to be due to passivation of localized states. The minority carrier lifetime of PH3/Hz-treated sample was higher than that of the sample treated only with H 2 as shown in Figure 21. The surface-treated solar cells have
231
SURFACE AND INTERFACIAL RECOMBINATION IN SEMICONDUCTORS
A
B
C D
shown very good performance characteristics. Conversion efficiency 77 of 18.3% (Fig. 22), and increased open-circuit voltage due to reduction of the p-n junction dark current by defect passivation were reported. For an excellent review on hydrogen in semiconductors the reader is referred to the article by Pearton et al. [194].
.
4.2K
, t + _~~ (700 ~ ....2].
3.93 meV
hydrogenated annealed
L4.19
5
Hsu and Lau [195] described the effects of polyimide passivation of MOCVD-grown GaAs. The samples were spin coated with polyimide acid; PL was used to evaluate the samples. Rao et al. [196], Manorama et al. [197], and Bhide et al. [198] employed polythiophene and polyphenylene sulfide as passivating materials.
ahnYdn;aogll~(nl(at~:>O+C)__jr ~ / ~
_>, _c
4.1.11. Polymer Passivation
moV
,
hydr~
~
__
,.
4.1.12. LB Film Passivation as-grown ___L
%
8(:)0
L
_ "__
|
|
_*
820
,
880
860
840
Wavelength [nm] Fig. 20. The 4.2 K PL spectra of GaAs on Si for various plasma-treatment conditions. Reprinted with permission from T. Soga et al., J. Appl. Phys. 87, 2285 (2000). Copyright 2000 by The American Institute of Physics.
Interface trap density in GaAs was reduced by coating a thin Langmuir-Blodgett (LB) film that was also used as an insulator in an MIS structure [199]. The LB films have very important electrical, chemical and optical properties and their incorporation in GaAs devices will open up many possibilities for novel devices.
4.1.13. Quantum Wells of Si as Passivating Barriers Metal-insulator-semiconductor (MIS) structures of GaAs with quantum wells of Si as passivating medium were reported [200] that have shown excellent electrical properties. 4.2. Novel Techniques to Characterize Surface Recombination in GaAs ~ *
(c)
X:2.74 ns
Riech et al. [201] have developed a photoacoustic (PA) technique to measure surface-recombination velocity. The main advantage of their method is that it is noninvasive. The PA signal can be interpreted in terms of recombination events.
'~
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."
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r
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0.8
o"
o c
Y
"~ 0.6
,.... q...
E 0.4
I,,,
r.
0
5
I0
15
20
25
Time (ns) Fig. 21. Time-resolved PL decay curves of GaAs epilayers on Si measured at room temperature: (a) as-grown; (b) H 2 plasma passivated, (c) PH 3 (PH3/H2 -- 10%) plasma passivated, and (d) PH 3 plasma passivated followed by annealing in H 2 ambient at 450 ~ The solid lines represents fitted results. Reprinted with permission from G. Wang et al., Appl. Phys. Lett. 76, 730 (2000). Copyright 2000 by The American Institute of Physics.
I15 : 0
~'/
0.2
~,,
0.0 400
-
t
......
hydrogenated
_ ]
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hydrogenated
t
600
+ annea~ (,5o ~
+ annexed (65O "C)
1
800
1000
Wavelength [nm] Fig. 22. Quantum efficiencies of GaAs solar cells on Si substrate with various plasma-treatment conditions. Reprinted with permission from T. Soga et al., J. Appl. Phys. 87, 2285 (2000). Copyright 2000 by The American Institute of Physics.
232
KASI VISWANATH
When the semiconductor absorbs light energy electron-hole pairs are created that will thermalize in a very short time, that is, of the order of a few picoseconds. The excess energy equal to ( h v - E g ) where hv is the incident photon energy, and Eg is the bandgap energy, is given to the lattice. After thermalization, some of the carriers diffuse towards the bulk for about a distance equal to their diffusion length L in time z (where z is their lifetime), and recombine in the bulk. The other fraction diffuses towards the surface and participates in surface recombination in a nonradiative process. The first process of bulk recombination is determined by the minority carrier diffusion coefficient D and the second channel of nonradiative recombination is determined by the surface recombination velocity. Only non-radiative recombinations contribute to the PA signal. Figure 23 shows the experimental arrangement for the PA measurements. Electron-hole pairs are created by employing an Ar ion laser, and the sample is attached to a microphone. The laser light was mechanically chopped by a chopper, and a lock-in-amplifier was used as the detection system. Figure 24 shows the surface-recombination velocity for different carrier concentrations in Ge-doped GaAs. Paulson et al. [202] and Safvi et al. [203] have designed very novel photoreflectance (PR) and photoluminescence (PL) near-field scanning optical microscopy (NSOM) techniques. Photoreflectance is an electromodulation spectroscopic method. A laser is used to create electron-hole pairs in the semiconductor by using appropriate energy of the laser light, and a second tunable laser is used as the probe. The reflected light of his probe laser is monitored. Franz Keldysh oscillations (FKOs) are seen in the A R / R spectra due to electric-field modulation of the reflectivity of the sample when the wavelength of the probe is scanned (Fig. 25). Figure 26 shows the main physical processes involved in the NSOM-based PL technique. The photoexcited carriers can diffuse as well as drift in all the directions from the point of excitation. There will be an electric field at the surface of the material due to surface states that causes band bending at the surface. The minority carriers will drift towards the surface by this electric field.
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CarrierConcentration(cm-3) Fig. 24. Correlation between surface-recombination velocity and doping concentration. Reprinted with permission from L. Riech et al., Phys. Stat. Sol. (a) 169, 275 (1998). Copyright 1998 by Wiley Interscience.
The photogenerated carriers will recombine nonradiatively at the surface with a finite recombination velocity. Therefore, the surface states, Fermi level, surface electric field in the depletion region, and the variation in the surface-recombination velocity can be studied by the NSOM-PL method. DeLong et al. [204, 205] and Inglefield [206-209] at the University of Utah, developed a new and elegant microwave-
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Fig. 25. In the photoreflectance NSOM system the pump and probe beams are combined in a laser-to-single-mode-fiber coupler. The light is brought to the surface through a single-mode fiber, and reflected light is collected by a lens. The PR signal is detected at a photodiode. A computer controls the tuning of the titanium sapphire laser to the wavelengths of interest and controls data acquisition. Typical light intensities are shown. Reprinted with permission from C. Paulson et al., Mat. Res. Soc. Sym. Proc. 588, 13 (2000). Copyright 2000 by Material Research Society, U.S.A.
SURFACE AND INTERFACIAL RECOMBINATION IN SEMICONDUCTORS
EF E,, Lateral PL
Diffusion
FWHM
1~ 'in-depth' PL
E
Photo-
Surface +1 ~
Stat,~.s
PL
I E ~
A
~_ Depth
generated carriers
Tip
Drift
Fig. 26. S :hematic of the main physical processes that are in operation during NSOM based photoluminescence measurements. The lateral diffusion of photogenex+Lted carriers can affect the lateral resolution achievable in these measurenLeJlts. Surface recombination, bulk carrier lifetime, and internal drift fields all s +rve to restrict this lateral diffusion and hence improve the resolution o1 :he luminescence measurements. Reprinted with permission from S. A. Safvi J. Liu, and T. E Kuech, J. Appl. Phys. 82, 5352 (1997). Copyright 1997 by The American Institute of Physics.
modulated photoluminescence (MMPL) method to characterize semiconductor surfaces. A schematic diagram of the MMPL technique is shown in Figure 27. The sample was placed irL a microwave cavity, and the photoluminescence was then measured as usual after applying microwave power. The MMPL experiment has many common features with other per:urbation techniques such as optically detected magnetic res<)nance (ODMR), optically detected cyclotron resonance (ODCR), and thermally modulated photoluminescence (TMPL). In ODMR, changes in spin-dependent recombination
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233
processes are observed as changes in PL intensity or polarization. In this case the spin states of a center taking part in the recombination process is changed by application of a pulsed microwave plus magnetic fields. The ODCR technique is similar except that microwave coupling is via cyclotron resonance of free carriers rather than changes in spin states. The TMPL is based on changes in luminescence spectra induced by pulsed changes in the sample temperature. The PL spectrum is affected by the application of the microwave electric fields. The free carriers are accelerated to higher kinetic energy than that corresponding to lattice temperature. The microwave-accelerated carriers will equilibrate with the lattice, which will heat the sample. In this case the MMPL spectrum is the same as that of a sample that is heated. This can be observed as the long transients in the signals that result from insufficient cooling of the sample. There is a second effect due to accelerated carriers, namely, the capture cross section of luminescence process depends on the energy of carriers. Figure 28 shows the usual PL and MMPL of GaAs structures. When microwaves are applied the nonradiative lifetime of minority carriers becomes short compared to the radiative lifetime, and the luminescence efficiency decreases rapidly. From the differences in the MMPL spectra of GaAs exposed vs double hetrostructure it was inferred that the surfacerecombination velocity for exposed surface was higher compared to that in the double hetrostructure. Figure 29 shows the surface-recombination velocity for different microwave powers for the GaAs sample. Wang and Neugroschel [210] described a novel method of measuring surface-recombination velocity that is based on recording photoluminescence decay as a function of self-absorption under different excitation conditions. Surfacerecombination velocity in MBE-grown p-type GaAs was discussed by a method that combines PL and lifetime measurements [211]. Toba and Tajima [212], and Noji et al. [213] reported room-temperature photoluminescence mapping and spatially resolved PL to characterize surface and interface states in GaAs epitaxial wafers. 4.3. M e t a l - G a A s Interface
Microwave Modulated PL Apparatus
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0aut.a~
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.
.
.
.
.
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Computer Fig. 27. Schematic diagram of MMPL apparatus. Reprinted with permission from M. C. Delong et al., unpublished results.
Bardeen [214] proposed that electronic states at a metalsemiconductor interface "pin" the metal Fermi energy and account for the Schottky-barrier phenomena. In this model the Schottky barrier for an n-type semiconductor interface is the difference between the conduction band minimum at the surface and the energy of the pinning state. Later, Chye et al. [16] and Spicer et al. [17-19] proposed the advanced unified-defect model that explains the metal-semiconductor as well as oxide-semiconductor interface electronic structure. According to these researchers, the pinning states are associated with semiconductor alone because they have energies virtually independent of the metal in the metal-semiconductor contact. It was also proposed that the native defects are near or at the semiconductor surface rather than being intrinsic
234
KASI VISWANATH
It)
2 pm GaA=
9
4 mW R=/~.71
,o'
mW
1 ~P'~wO'92
ill
0.1 pm GalnP
r
u o
c:
c
lo s
>
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c
c:
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tO
/
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10
4
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R./R~O.SS
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IZ U
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-
II If
c:
103
1o2 i
i
i
i
i
i
i
1.40
1.42
1.44
1.46
1.48
1.50
1.52
(a)
1.54
0.1
/[
,
PL
r C: c: .c::
......
1.38 (b)
'
i
i
i
1.40
1.42
1.44
0,96
0.98
1.00
Fig. 29. Surface-recombination velocity in GaAs as a function of ratio of PL efficiencies (microwaves on/microwaves off) at several microwave powers. Data points are experimental values for a single sample at three microwave powers, and solid lines are theoretical fittings. Reprinted with permission from C. E. Inglefield et al., unpublished.
pm GalnP
r J~ (,.) t,.. o>.. (/) c,,..,.,, (-
0.94
Ratio o f PL Efficiencies qon/Vloff
Emission Energy (eV)
,.,,.?. v0~
0.92
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i
l
i
i
1.46
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Emission Energy (eV)
Fig. 28. The PL and MMPL from the nominally identical GaAs layers in two different structures (structures also shown in figure). The scales are the same for the MMPL and PL spectra shown on each plot. The MMPL from the GaAs with the exposed surface (a) is entirely negative while that from the buried layer (b) has a significant positive component. This indicates a larger surface-recombination velocity in the sample shown in (a). Reprinted with permission from C. E. Inglefield et al., J. Vac. Sci. Technol. B 15, 1201 (1997). Copyright 1997 by The American Vacuum Society.
or metal-induced surface states. Allen and Dow [215] have shown theoretically that surface antisite defects that have energies in the bandgap of the semiconductor are responsible for the pinning of the Fermi level and cause the barrier formation. Very good metal-semiconductor interfaces are important in order to realize several devices such as metal-semiconductor field-effect transistors (MESFETS), etc. Several groups have achieved good-quality metal-semiconductor interfaces by passivating schemes such as sulfide, selenide, hydrogen plasma, hydrogen fluoride, phosphide treatments, inclusion of rare earths to GaAs, and inclusion of Si interface layers, and so forth [68, 114-116, 154, 216-235]. Sulfide treatments using (NH4)2S [216, 217] and (NH4)2S x [218, 220] were initially attempted by two groups led by Fan et al. [218-220] and Carpenter et al. [217]. Surface states were removed that pin the Fermi level. The Schottky-barrier height (SBH) was
then shown to be dependent on the metal-work function. Lee et al. [222] observed improved characteristics of GaAs MESFETS after (NH4)2S x treatment due to the enhanced Schottky-barrier height. Chang et al. [228, 232, 233] and Lin et al. [230] added Pr/Pr203 to Ga melt for LPE-grown GaAs and reported increased SBH and reduced leakage currents. Chen et al. [226] measured the photoluminescence of Schottky diodes and concluded that anodic (NH4)2S x treatment improves the stability of passivation. Nie and Nannichi [236-238] and Nie alone [239] studied the effect of hydrogen on a Pd/GaAs Schottky diode. Christianen et al. [240, 241] reported ultrafast phenomena in a Au-GaAs Schottky diode. Subpicosecond photoluminescence correlation was used to measure luminescence decay for different reverse-bias voltages and excitation powers (Figure 30). With increasing reverse-bias voltage the decay time reduces from several nanoseconds to a few picoseconds as shown in Figure 31. When the reverse-bias voltage is small, the depletion width of the Schotky barrier is estimated to be thinner (0.1 mm) than the GaAs active-layer thickness (0.3 mm). Then the decay time will be almost the same as the spontaneous carrier lifetime because carriers in the fiat band region determine photoluminescence decay. When the reversebias voltage increases, the depletion width increases and the carrier sweep out becomes faster. This results in small decay times, and the decay times were found to increase with excitation power. 4.4. F e r r o m a g n e t - G a A s
Interface
The hybrid ferromagnetic-semiconductor structures have attracted the interest of many scientists and many new device concepts have been proposed, such as the spin-polarized fieldeffect transistor (Fig. 32) [242], the spin-valve transistor [243], the spin-filter/aligner [244], the spin-polarized light-emitting
235
S U R F A C E A N D I N T E R F A C I A L R E C O M B I N A T I O N IN S E M I C O N D U C T O R S Exc. Power: 0.8 ,,m W . . . . 0,everse blas:" 0.0 V
....
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T
r~
Fe
O
4
V//,////////////////////A In AlAs
l
,/
Fe
O
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.J
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10
20
30
40
50
Delay Time (ps) Reverse Bias: 2.0 V
Exc. power: 2.00 m~,
.6 I_..
,,d
x5
0.75
,.d o
x 250
.0.32
(b) 0
20
10
30
40
50
Delay Time (ps) Fig. 30. ,~orrelated photoluminescence (PL) intensity as function of time delay belween the pulses for different reverse bias voltages (a) and excitation powers (b',. The solid curves correspond to the best fits to the data. Some of the traces are vertically shifted and multiplied by the given numbers for clarity. Reprinted with permission from P. C. M. Christianen et al., J. Appl. Phys. 80, f!831 (1996), Copyright 1996 by The American Institute of Physics. 104
9
,
9
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,,
v
'
','
i
Excitation power: o 0.10 mW
10 3
~10
9 0.75 mW
z O
~n O
10a
O O O 9
10 ~ -1
L
0
-
t
9
t
9
l
3 Reverse bias (V) 1
2
=
|
4
Fig. 31. Short correlated photoluminescence (PL) decay times as functions of reverse ~ias voltage for input powers 0.1 mW (open triangles) and 0.75 mW (filled triangles). The values are obtained by fitting the experimental correlation curves. Reprinted with permission from E C. M. Christianen, et al., J. Appl. Phys. 80, 6831 (1996), Copyright 1996 by The American Institute of Physics.
diodes [245], and the ferromagnetic-semiconductor injector [246]. All these device concepts are based on injecting spinpolarized electrons into the semiconductor. Prinz [247] has reviewed the advantages in using III-V compounds in utilizing spin-electronic devices. Exactly as in any semiconductor device, surface states hamper the fabrication of such devices. Fermi-level pinning has deleterious effects on the carrier lifetime and spin relaxation at the metal-semiconductor interface [248]. Jonker et al. [249] at the Naval Research Laboratory, USA, studied the carrier lifetimes in a ferromagnetic-semiconductor hybrid structure for the first time. They have shown that lifetimes are enhanced in Fe/GaAs relative to uncoated or sulfur-passivated surfaces. It was demonstrated that epitaxial Fe film provides a ferromagnetic contact and suppresses midgap state formation. Hence, Fermi-level pinning does not occur in this system. Formation of the Fe-As bond at the interface, which prevents the formation of As dangling bonds, was the proposed mechanism. The carrier lifetimes and Schottkybarrier heights were estimated from the photoreflectance spectra shown in Figure 33. Manago et al. [250] have reported improvements in the Schottky diodes of sulfur-passivated MnSb/GaAs. 4.5. Heterostructures and Quantum Wells of GaAs
Johnston [251] was the first to report the photoluminescence of a GaAs-GaA1As hetrostructure in which two broad dark regions (known as large dark spots or LDS) extended nearly 100~ on either side of the surface defects. Johnston and Logan [252] have assigned them to regions of nonradiative recombination due to interface states. Later, Henry and Logan [253] interpreted LDS as being caused by long-range transport of photogenerated carriers to the defect site where they recombine nonradiatively. The source of nonradiative recombination giving rise of LDS was attributed to surface recombination in the active layer. GaAs has applications not only in making light-emitting diodes (LEDs), but also in solar cells. The GaAs solar cells have several advantages compared to Si solar cells. For example, they have higher theoretical conversion efficiency of light to electricity (24%), better radiation resistance, and higher temperature of operation. The long-standing
236
KASI V I S W A N A T H ""
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,d)Fe/?aAs:2x,| 1.8
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2
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Energy (eV)
samples have given open-circuit voltage of 0.96 V and conversion efficiency of almost 18% [256]. Luminescence up-conversion was observed at the GaAsGaInP 2 interface, which was attributed to cold Auger processes and the presence of metastable states in GaInP 2 (Fig. 34) [257]. Rosenwaks et al. [258, 259] achieved a very low surface-recombination velocity of 200 cm/s in a p-GaAs epilayer on which a 30-50 A thin epilayer of GaInP 2 was deposited. Reduction of the interface recombination velocity was observed in GaAs coated with a Ga203 layer in addition to an increase in photoluminescence intensity [260]. Low threshold currents observed in vertical cavity semiconductor lasers that incorporate wet oxidation of AlAs for current confinement were explained by time-resolved studies [261]. In these experiments interface recombination velocities before and after oxidation were estimated that gave clues to understanding the mechanism. Thermal oxidation and sublimation of a semi-insulating GaAs substrate has improved the mobility and sheet concentration of two-dimensional electron gas in selectively doped GaAs/n-A1GaAs heterostructures with very thin GaAs buffer layers [262]. The main origin of the carrier depletion was thought to be the carbon concentration in the initial growth surface. An interface luminescence band was reported in the PL studies of GaAs/GaA1As heterostructure [263]. Similar experiments have identified luminescence due to vacancy complexes at the interface [264]. Sandroff et al. [51] were the first to report the enhancement of the current gain of a GaAs/GaA1As heterojunction bipolar transistor (HBT) by a factor of 60 after the sulfide treatment. Later, there were several studies made to clean the semiconductor surfaces and devices. Improvements in the dc characteristics of a GaAs/GaA1As transistor that was chemically treated with S2C12 solution has been reported [265]. Similar results were obtained by ( N H 4 ) 2 S x t r e a t m e n t of a HBT [266]. Nottenburg et al. [267] reported later on the reduction of surface-recombination velocity by sulfide treatment, which resulted in ideal transport characteristics of a HBT device. By using thin n+-doped layers at the surface and substrate interface, Smith et al. [268] observed increased radiative efficiencies and lifetimes of photoexcited carriers in GaAs epitax-
Fig. 33. The 300 K PR spectra for (a) native oxide reference sample, F = 58 kV/cm; (b) same sample following sulfur passivation, F = 61 kV/cm; (c) A1/GaAs-(2 x 4) sample, F = 55 kV/cm, and (d) Fe/GaAs-(2 x 4) sample, F = 60 kV/cm. The thickness of the undoped spacer is 1070/k for (a)-(c), and 1500 A for (d). Carrier lifetimes were extracted from these spectra. Reprinted with permission from B. T. Jonker et al., Phys. Rev. Lett. 79, 4886 (1997). Copyright 1997 by The American Physical Society.
]
IE I loser problem in using GaAs is the high surface-recombination velocity. Therefore, in order to have good collection efficiency, an A1GaAs heterojunction was grown epitaxially on GaAs as a surface-passivating layer. As an alternative approach In0.sGa0.sP layers were grown on GaAs that has reduced the recombination velocity in GaAs by several orders of magnitude [254, 255]. Preliminary studies on solar cells of the same
! I,,
'.,.
I
PL
I I
Fig. 34. Sketch of the cold Auger process at the GaAs/GalnP 2 interface, carrier cooling and trapping, and PL from the GalnP 2 with its composite structure. Reprinted with permission from E A. J. M. Driessen, Appl. Phys. Lett. 67, 2813 (1995). Copyright 1995 by The American Physical Society.
SURFACE AND INTERFACIAL RECOMBINATION IN SEMICONDUCTORS ial layers grown by MOCVD. Prior to this report there were several studies carried out to improve the radiative recombination in GaAs by using A1GaAs as the surface-passivating window [44, 47, 269-274] in which carriers in GaAs are confined by the l:~rger-bandgap A1GaAs and thus are prevented from reaching the crystal surface. However, controlling the nonradiative ctmters is not very easy as demonstrated by the fact that all lile reports gave different values for lifetimes, which extend b'l several orders of magnitude. An ~n s i t u HC1 gas-etching process has enabled a research group al: Mitsubishi to reduce the interface recombination velocity n the GaAs/GaA1As heterointerface [275]. Similarly, hydrog,~J ation has improved the quality of the interface in a GaA,,jI3aA1As resonant tunneling structure [276]. RecombinatioJl models based on ambipolar diffusion equation were discuss e,:[ for GaAs/GaA1As and GaAs/GaInP heterostructures [277]. Capacitance-voltage simulation profiles were used t~ estimate the interface recombination velocities in GaAs/AGaAs structures that were correlated with optical time-re~,:,lved spectroscopy measurements [278]. Rosel~waks et al. [279, 280] reported for the first time the infl.~ence of electric fields on carrier dynamics in GaAs/G:~InP heterostructure interfaces studied both theoretically alld experimentally. Many groups have estimated the surface-recombination velocities under a high injection limit. In this ,:',ase the field in the space-charge region (SCR) is screened by the high density of photogenerated carriers. However, this; approach is not very rigorous and there are many drawbacks. For example, the electric field in the space-charge region is not completely screened even with high injection conditions, and the screening decreases with time as excess carrier concentration changes. When the sample is impurity doped it is essential to use very high injection levels. The analysis in tllis situation is very difficult because of other effects such as !)and filling, bandgap renormalization, carrier degeneracy, Auger processes, and so forth. Also, when high laser powers :~re used the samples can become damaged. Whei~ the injection level is low the time-resolved PL (TRPLt data are analyzed considering only minority-carrier transport with the following assumptions" (1) [,uminescence emitted from the space-charge region c~m be neglected. (2) The electric field in SCR is constant with time. (3) ]'he electric field created by spatial separation of excess carriers can be neglected. (4) An effective recombination velocity Sere is assumed to take care of the effects due to the fields. Figure 35 shows the energy-band diagram of a typical heterostructure used in the analysis by Rosenwaks et al. [279, 280]. They have made the self-consistent calculation of the effect of electrical fields on time-resolved PL (TRPL) of a semiconductor. They have also conducted the femtosecond and picosecond experiments and found good agreement with theory. Under high initial-injection conditions the spacecharge region electric field is screened by the photogenerated carriers in less than 0.2 ps. At lower injection levels the
Snbt ~ Ec
9r
e-
237 e"
"r
AIGaAs
\~P
~l
EV
Sbpt~ O h + I I I I I
I i
x.=d
I
In+
I X=W
x=O
Fig. 35. Energy-band diagram of a typical heterostructure used. Reprinted with permission from Y. Rosenwaks et al., Phys. Rev. B 50, 1746 (1994). Copyright 1994 by The American Physical Society.
TRPL spectra were effected by the electric field. The PL was quenched in the first few picoseconds due to the electron-hole spatial separation. As the applied bias increases the depletionregion width increases and the effects on TRPL spectra is larger. Under low-injection levels the shape of the PL decay was quite different. This was because the surface band bending was reduced only by a factor of about 0.6 to 0.7 times its original dark value. In this case the decay of the luminescence was governed by a large Seff, which increases the PL decay rate. Very efficient radiative recombination, with low interfacerecombination velocity was reported in GaAs grown on a Si substrate. This was achieved by using a step-graded Ge/GeSi buffer [281]. Reich et al. [282] described the photoacoustic measurements on a GaAs/GaA1As heterostructure from which surface-recombination velocity was estimated. The values were compared with the time-resolved results. Several groups have discussed the interface-recombination velocity in GaAs quantum wells [269, 274, 283-315]. Dawson and Woodbridge [269] have reported a very low interfacerecombination velocity of 60 cm/s for GaAs in a double heterostructure. In order to explain the dependence of exciton temperature on quantum-well width in GaAs/GaA1As singlequantum wells, the effect of interface-recombination velocity was considered [283]. Sheldon et al. [284] measured exciton lifetimes in GaAs quantum wells with different well widths and with and without A1 in the GaAs active layer [284]; GaAs active layers that contain A1 have shown very poor bulk lifetimes, which was attributed to Shockley-Read-Hall recombination. Nelson and Sobers [285] have determined the interfacial recombination velocity of 450 cm/s for the GaAs/GaA1As interface in a double heterostructure. They have also considered the self-absorption of luminescence for the active layer thickness d > 1 /xm. From the luminescence efficiency measurements a very low interface recombination velocity of 53 cm/s was
238
KASI VISWANATH
reported by Hoofl et al. [286] for the metal-organic vaporphase epitaxially (MOVPE) grown GaAs/GaA1As double heterostructures. Molenkamp and Van't Blik [274] were able to reduce the interface recombination velocity to as low as 18 cm/s in a double heterostructure of GaAs. In their study the influence of modulation doping of the active layer was also examined. Yokoyama et al. [287] and Iwata et al. [288] were successful in decreasing the lasing threshold current and interfacial recombination in GaAs/GaA1As single-quantum well lasers by employing superlattice buffer layers. The usefulness of incorporating the superlattice buffer layers in reducing the nonradiative recombination was demonstrated by conducting experiments on two quantum-well laser structures, one with and the second without a buffer superlattice. Wolford et al. [289, 292] and Gilliland et al. [290, 291] at IBM research division have examined a number of "surface-free" quantum wells with a variety of barriers. Using the reactive ion-etching method, quantum dots with diameters 200/zm-60 nm were fabricated from GaAs/GaA1As single-quantum wells by Clausen et al. [293]. When the diameter of the quantum dot was smaller than a particular value, luminescence could not be observed, which was thought to be due to dominant nonradiative processes at the surface of the quantum dot. It was shown that the standard diffusion model of surface recombination fails in such a situation. The luminescence results were understood in terms of damage layer thickness ~. The value of sc was shown to determine the smallest size of the quantum dot that will give luminescence. Ahrenkiel et al. [294] and Olson et al. [295] observed an ultralow interface-recombination velocity of 1.5 cm/s for the GaAs/GalnP double heterostructure. Very long photoluminescence lifetimes of the order of 14/zs have been estimated. The decay times varied with temperature as T L59, which was very characteristic of radiative recombination not limited by bulk or surface nonradiative-recombination processes. Harris et al. [296] also studied the same system by picosecond population modulation spectroscopy. Surface and interface recombination have been reported in a number of other systems based on GaAs, for example, coupled quantum wells [297], self-electrooptic devices (SEED) [298], double-quantum wells [299], mesa structures [300], surface-quantum wells [301], surface-free quantum wells [302], 6-doped quantum wells [303], n-type quantum wells [304], interface traps [305, 306], and so forth. Zhao et al. [307] studied the recombination processes due to 2-dimensional (2D) carriers at the interface of a GaAs/GaA1As quantum-confined system. They observed two luminescence bands called H bands; the HB 1 band is due to recombination of 2D electrons at the interface with holes in the valence band, whereas the HB2 band is due to the recombination of 2D electrons with acceptor bound holes. Figure 36 is the illustration of heterostructure potential and recombination mechanisms. Mochizuki et al. [308] examined the recombination at single crystalline/polycrystalline GaAs interfaces by timeresolved spectroscopy. Low-resistance polycrystalline GaAs is
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~
! 200
i
S00
I
A
6oo~ I/~HB1 HB2 ~ _ ~ 9~ l
AIGaAs
GaAs
NA =,10is cm.3 EA =25 meV "%
SL
Fig. 36. An illustration of a heterostructure potential and the recombination mechanism giving rise to the H-bands: A 2D electron recombines with a hole in the GaAs valence band (HB 1) or a hole bound to a shallow acceptor (HB2). Reprinted with permission from Q. X. Zhao et al., Superlattices and Microstructures 9, 161 (1991). Copyright 1991 by Academic Press.
important in the fabrication of heterojunction bipolar transistors. The effect of hydrogen passivation in GaAs quantum wells has been reported by a number of researchers [309-314]. Buyanova et al. [312] at Linkoping University, Sweden, measured the PL of hydrogen-passivated Be-doped GaAs/GaA1As quantum wells (Fig. 37). It was observed that if hydrogen passivation is done for a longer time it will degrade the sharpness of interfaces, which was thought to be due to hydrogenenhanced intermixing. Wang et al. [314] achieved enhanced spontaneous emission in H 2 plasma-passivated vertical cavity surface-emitting laser (VCSEL) grown on Si substrate. An increase in minority carrier lifetime was noted as shown in Figure 38. The results were explained as being due to passivation of nonradiative defects by hydrogen. An increase in intensity of the cavitymode emission peak was also recorded. Lipsanen et al. [315] in Finland used a novel method of passivating GaAs/GaA1As near surface quantum wells. They deposited a thin layer of InP on the surface of the quantumwell barrier. Passivated samples with the barrier thickness of <5 nm have shown improved PL intensities as seen in Figure 39. 4.6. G a A s Q u a n t u m Wires
Many research groups around the world have been working on the development of ultralarge-scale integration (ULSI) of semiconductor devices; ULSI is one more step above VLSI (very large-scale integration) technology and represents the ultimate sophistication in the device technology of semiconductors. Here, the size of the semiconductor is in the nanometer (nm) range. When the size of the semiconductor microstructure is of the order of the deBroglie
239
S U R F A C E A N D I N T E R F A C I A L R E C O M B I N A T I O N IN S E M I C O N D U C T O R S 9 "'"
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wavelength of electrons, quantum-confinement effects occur. Therefore., in order to understand the operation and optimization of these devices, it is essential to understand the basic physical phenomena in these microstructures. Such nanometer-sized devices may be considered as experimental quantum-mechanical objects. In the previous section we have reviewed the case of quantum well i of GaAs. By reducing the dimensions of the semiconductor it is possible to make one- and zero-dimensional
0.0
T=12 K
.~'~ 10.3
Fig. 37. qhe PL spectra of A10.3Ga07As/GaAs QWs with different Be doping concentrati :,ns before (solid lines) and after (dashed lines) hydrogen passivation, respe~:tively. The hydrogen passivation was performed during 1 h. All spectra o)]~tain FE and BE lines, with the relative intensity of Be-related BE emissk,a being increased with increasing doping. At higher doping concentrations (>1018 cm-3), an increasing contribution from the free-to bound recombination involving Be acceptors is observed in the PL spectra about 18 MeV ~t~t.[ow the FE line Reprinted with permission from I. A. Buyanova et al., Ap?,,. Phys. Lett. 68, 1365 (1996). Copyright 1996 by The American Institute (,f Physics.
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10.2
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Time (ns) Fig. 38. The PL decay curves of vertical cavity surface-emitting laser (VCSEL) structures on Si measured at the cavity mode at room temperature: (i) as-grown sample; (ii) H plasma-passivated sample. Reprinted with permission from G. Wang et al., Electronics Lett. 36, 1462 (2000). Copyright 2000 by Institute of Electrical Engineers, U.K.
Fig. 39. Integrated PL intensity of passivated (open circles) and unpassivated QWs after a few hours from the growth (open squares) and after about five days (filled squares) as a function of top barrier thickness d. The passivated samples showed no degradation after five days. Passivation was done by depositing InP monolayer on the barrier of QW. Reprinted with permission from H. Lipsanen et al., Appl. Phys. Lett. 68, 2216 (1996). Copyright 1996 by The American Institute of Physics.
electron systems. A one-dimensional electron system is a quantum wire in which electron motion is confined in two directions. In a zero-dimensional system such as a quantum dot, the confinement of electron motion occurs in all three directions. In very low-dimensional quantum-confined structures, where electron scattering is minimum, there are reports of new electron-transport phenomena such as quantum interference effects [316, 317], point-contact conductance quantization [318, 319], and ballistic electron transport [320]. Sakaki [321] has predicted that in a single-channel quantum wire, the electron scattering would be reduced and electron mobility would be extremely high. Datta et al. [316, 322] proposed a quantum-interference transistor that was based on the electrostatic Aharnov-Bohm effect. The important application of quantum-wire and quantumdot structures to semiconductor lasers has been examined by Arakawa et al. [323]. The conventional bulk active layer is replaced by an array of quantum wires or quantum dots. The most important property of the quantum-confined system is the density-of-states (DOS) function. As the confinement increases, the DOS becomes sharper. This sharpening is directly related to the optical gain spectrum. For the k-conserving transitions, the optical gain is proportional to the effective density-of-states function of the electronic system and the Fermi occupancy factor giving the strength of inversion. Therefore, quantum-wire structures with narrow gain spectra are the best candidates for low-threshold current semiconductor lasers. One main problem with these systems is that the surface-to-volume ratio is very high and hence surfacerecombination processes become very important. Viswanath et al. [324-327] reported the surfacerecombination lifetimes and surface-recombination velocities for GaAs quantum whiskers that were grown by MOCVD at Hitachi. Figure 40 shows the SEM photograph of GaAs
240
KASI VISWANATH Fiber Pulse compressor
Locked Nd : YAG laser ] t
Cryostat
SHG lI
Sync.pumped dye laser
Monochromator
Streak Camera
Fig. 42. Block diagram of subpicosecond laser system. Reprinted with permission from A. Kasi Viswanath et al., Appl. Phys. Commun. 13, 55 (1994). Copyright 1994 by Marcel Dekker, Inc.
Fig. 40. The SEM photograph of GaAs quantum whiskers on a semiinsulating GaAs substrate. Reprinted with permission from A. Kasi Viswanath, K. Hiruma, and T. Katsuyama, Superlattices and Microstructures 14, 105 (1993). Copyright 1993 by Academic Press.
whiskers on a semi-insulating GaAs substrate. Free-standing quantum whiskers were grown without ion etching or ionbeam lithography. Therefore, these quantum whiskers were of very high quality because they do not have dislocations and impurities due to semiconductor processing. Exciton recombination was observed in the photoluminescence spectra. Appearance of free excitons was a manifestation of the optical quality of the samples. Temperature dependence of the main transition was found to be similar to the variation of bandgap of GaAs (Fig. 41), which confirmed that main transition was exciton related. Figure 42 shows a block diagram of the femtosecond timeresolved system used by our group at Hitachi Central Research Laboratory, in Japan. The samples were excited by using a Spectra Physics femtosecond laser system. The Nd: YAG laser
gave about 100-ps pulses at 1.06/zm. The output power of the YAG laser was kept at 10 W. These pulses were compressed using a fiber-optic pulse-compression technique to about 5 ps. A potassium titanyl phosphate (KTP) crystal was used for second harmonic generation of the output light from fiber. The power of the green light was typically 1.0 W, and a dye laser was synchronously pumped by the green light. The dye beam can be tuned by using a birefringence filter. The dye laser pulses were 500 fs in pulsewidth. The dye laser beam was guided on to the sample using the usual beam optics. The sample was kept at an angle of 45 ~ to the incident beam. The detector was kept perpendicular to the incident beam. The dye laser power could be changed by using neutral density filters. The pulse-repetition rate of the Nd : YAG laser was 82 MHz. The sample was mounted on a cold finger of an Air Products Helitran liquid helium cryostat. The temperature could be maintained at any desired value between 6 and 300 K. The photoluminescence was dispersed by a monochromator and detected by a streak camera. Recombination takes place both radiafively and nonradiatively. The PL decay can be written as l / - r = l/7"r -+- 1/'mr
1.52
ii
> 1.51 -
I I
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v
1.50 -
I
e-
uJ 1.49 -
I I
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~ 1.48-
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I I
~. 1 . 4 7 1.46 0
I
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I
50
100
150
200
Temperature (K) Fig. 41. Temperature dependence of photoluminescence peak energy for a 70-nm quantum wire. Reprinted with permission from A. Kasi Viswanath, K. Hiruma, and T. Katsuyama, Superlattices and Microstructures 14, 105 (1993). Copyright 1993 by Academic Press.
where ~'br is the bulk nonradiative lifetime, 2S/L is the surface recombination lifetime, and S is the surface recombination velocity. Here, the factor 2 appears because of the top and bottom surfaces of the sample. The photoluminescence decay curves for different wire diameters are shown in Figure 43. It can be seen that the luminescence decay can be split into two parts--an initial fast component and then a relatively slow component. The
241
SURFACE AND INTERFACIAL RECOMBINATION IN S E M I C O N D U C T O R S 500
t
100 nm Quantum Wire
7K
@ 400
z @ 300
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t
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70 nm Quantum Wire
[Z [Z
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I
50 nm Quantum Wire
t
t
,IL
500
1000
1500
2000
TIME (ps) Fig. 43. Time-resolved photoluminescence decay for 100-, 70-, and 50-nm quantum wires at 7 K. Reprinted with permission from A. Kasi Viswanath et al., Proc. Intern. Conf. Solid State Devices and Materials, 1992, p. 296.
Viswanath et al. [326] have also reported the temperature dependence of the bulk part of carrier lifetime for a 70-nm quantum wire that is shown in Figure 45. An increase in lifetime with T showed that the recombination is mostly radiative. This means that most of the nonradiative recombination takes place only on the surface of the quantum wire. Surface passivation of quantum wires was done by sulfur treatment [327]. A blue shift in the PL (Fig. 46) and an increase in the surface-recombination lifetime (Fig. 47) were observed. This was consistent with the reduction of surfacerecombination centers at the surface after sulfur treatment. After the surface treatment the depletion layer was removed and hence the wire diameter effectively increases. This is
2000
fast con]tl)onent was interpreted as the surface-recombination part and '~:he slow component as the bulk-recombination part. Figure 4-,~ shows the surface-recombination velocities as a function ~)f wire diameter. The surface-recombination velocity was found to increase as the diameter of the wire decreased. The surface-recombination velocity is defined as the number of carriers recombining on the surface per unit area per unit time. Therefore, it was concluded that as the wire diameter decreased, the density of recombination centers at the surface increased. The physical interpretation was given as follows. The spatial profile of the electronic wavefunction is greatly dependent on the size of the quantum whisker and hence on the size of the quantum-confinement effect. For higher quantum confinement the electronic wavefunction leaks out of the quantum well and hence increases the number of surface-recombination centers. These observations were consistent with the theoretical predictions of Duggan et al. [328].
~q
1000
,I,
JI,
I
_l
50
100
150
200
,,
,
250
TEMPERATURE (K) Fig. 45. Temperature dependence of the bulk part of carrier lifetime for a 70-nm quantum wire. Reprinted with permission from A. Kasi Viswanath et al., Appl. Phys. Commun. 13, 55 (1994). Copyright 1994 by Marcel Dekker, Inc.
242
KASI VISWANATH Cross-sectional View
rown EZ
Depletion Layer Surface Passiv~atio
< I
i
I
I
EC/}
ce Passivated
Z EZ ,J
9
I
1.50
L
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1.52 Energy Diagram
P H O T O N E N E R G Y (eV) Fig. 46. Photoluminescence spectra of GaAs quantum wires at 77 K before and after surface treatment. Peak photon energy is marked with a vertical line. Reprinted with permission from A. Kasi Viswanath et al., Microwave and Optical Technol. Lett. 7, 94 (1994). Copyright 1994 by Wiley Interscience.
shown schematically in Figure 48. Mayer et al. [329] reported the surface recombination in dry-etched GaAs quantum wires. 4.7. InP Ahrenkiel [330] has written an excellent review on the properties of InE There are many applications of InP that include semiconductor lasers, light-emitting diodes, integrated electooptics, avalanche photodiodes based on Gal_xInxAs]_yPy and Ga]_~InxAs. It also finds use in microwave and high-speed digital circuits because of its high saturation-drift velocity and high mobility. Most importantly, InP is a very good candidate for space-based solar cells because they have very high radiation resistance and high efficiency.
As-Grown r~
Surface Passivated
s
0
500
.....
i
1000
9
1500
a
2000
TIME ( ps )
Fig. 47. The PL decay at 77 K for 200-nm quantum wire showing the effect of surface treatment. Reprinted with permission from A. Kasi Viswanath et al., Microwave and Optical Technol. Lett. 7, 94 (1994). Copyright 1994 by Wiley Interscience.
i Fig. 48. Schematic cross-sectional view of the profile of the depletion layer and its shrinkage after the surface treatment. Energy diagram corresponds to a change in the ground-state quantum level of electrons with surface treatment. Reprinted with permission from A. Kasi Viswanath et al., Microwave and Optical Technol. Lett. 7, 94 (1994). Copyright 1994 by Wiley Interscience.
The first reports on surface-recombination velocity (SRV) in InP were based on steady-state PL measurements [331, 332]. The PL intensities in InP were compared with GaAs and guesses made as to the SRV values with the assumption that PL intensity is directly related to surface recombination. A low SRV for n-type InP and an SRV comparable to that of GaAs for p-InP were assigned. Ahrenkiel et al. [333] have determined a very long lifetime for InP epitaxial layers from time-resolved measurements. Hoffman et al. [334] measured SRV for n- and p-type InP as 1 x 104 and 2 x 105 cm/s, respectively. Bothra et al. [335] found that SRV in InP was dependent on doping, which was attributed to surface Fermi-level pinning. Rosenwaks et al. [336-338, 340-342] and Lin and Rosenwaks [339] have published numerous papers on surface recombination in InP. They have reported [336] similar values of SRV for both n-type and p-type InP that contradict previous findings. The lower PL intensity observed in p-type samples was interpreted as due to shorter bulk lifetime and large surface band-bending effects. The SRV of metal/InP interfaces was found to depend on the metal deposited on InP by thermal evaporation [337]. This was thought to be due to metal-induced defects in the bandgap. In n-type InP the nonradiative lifetime was found to be very long (~'nr = 320 ns) and the bulk recombination was mostly radiative. By contrast, ~'nr of p-InP was very small (<33 ns), which was interpreted as being due to the high concentration of deep traps. That is, in p-InP nonradiative bulk recombination was very
SURFACE AND INTERFACIAL RECOMBINATION IN SEMICONDUCTORS dominanl [338]. Liu and Rosenwaks [339] have concluded that the Shockley-Read-Hall lifetime was very long (> 10/xs) and radiative recombination was very dominant in undoped InE In p.-InE nonradiative recombination and trapping dominate evew~ in low-doped samples and the effective lifetimes were much shorter than in n-InE These results were consistent with their earlier findings. Rosenwaks et al. [341, 342] also studied the InP-liquid interfaces. They found that the surfacerecombination velocity increases with redox potential of the metal in the metal ion solution as shown in Figure 49. If the redox potential is more positive, then it will be reduced easily on th,:; crystal surface. These adsorbed metals or surface compoun:ts induce surface states, mostly by forming lattice defects. Krawczyk et al. [343] at France Telecom have developed a a;~vel quantitative mapping of surface-recombination velocity in InP [343]. From surface photovoltage spectra of strained IaP on GaAs the interface-recombination velocity was extracted [344]. Very high interface-recombination velocity due to spatially indirect transition was observed in quantum wells of A10.55In0.45As/InP [345]. In order to reduce the surface and interfacial recombination several passivation techniques were tried, for example: Passivation by sulfide/sulfur [346-3581; phosphates [359, 360]; arsenic [361-363]; antimony [364]; nitridation [365]; hydrogen plasma [366-368]; insertion of silicon quantum wells [369-372]; acids and bases [373]; silicon nitride overlayer [374]; and UV irradiation [375]. Cohen et al. [376] observed electron-hole recombination in surface quantum wells of InE Radiative recombination involving surface states of InP was also reported [377]. Asakawa et al. [378] have written a comprehensive review on chlorine-based dry etching of III-V compounds and, in particular, InP. They have also discussed the surface recombination in InE
4.8. Other III-V Binary Semiconductors 4.8.1. GaSb
Although GaSb has many optoelectronic applications, devices of GaSb could not be realized because of the large number of 2
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i
surface defects. Dutta et al. [379] have passivated GaSb surfaces by sulfur. They observed improvements in PL intensities in the passivated materials. 4.8.2. GaN
Viswanath [380] has recently reviewed the optical properties of GaN and related materials and their applications in photonics. At present, GaN is the most important semiconductor material [380-382]. Many photonic and electronic devices such as semiconductor lasers, light-emitting diodes, highelectron mobility transistors, photodetectors, and so forth, were realized. Blue-emitting LEDs and lasers are commercialized. Nonlinear optical devices based on room-temperature excitons in GaN were also proposed by Viswanath et al. [383]. Very large exciton binding energy was shown to be responsible for the observation of room-temperature lasing in GaN-based systems by Viswanath et al. [384]. However, the performance of GaN devices should be still improved. There are very few reports on the surface-related problems in GaN and related materials. Pearton et al. [385] observed enhancement in the luminescence of Er-doped A1N, a semiconductor that emits at 1.54/xm, which is suitable for optical communications. Better luminescence properties were attributed to passivation of defects in A1N by hydrogen. Arsenic passivation of cubic GaN was shown to modify its surface reconstruction [386]. Surface recombination in homojunction photodiodes was removed by using a heterojunction GaN/A1GaN in a photodiode [387]. Sun et al. [388] studied the HC1- and KOH-treated p-GaN that reduced the metal contact resistivity. This was interpreted as being caused by the movement of the surface Fermi level. Diluted KOH solutions were used to obtain thin and smooth oxide films on n-GaN samples that have the potential for the realization of metal-oxide semiconductor field-effect transistors (MOSFETs) [389]. This is a better approach than using SiO 2 on GaN that will give rise to high interface trap density. Huh et al. [390] have reported the surface passivation of n-GaN by alcohol-based sulfide solutions, which enabled them to observe very good luminescent properties of the treated samples. This result was thought to be due to the removal of surface defects. X-ray photoelctron spectroscopy of (NH4)2S xtreated Mg-doped GaN has shown that native oxides were removed from the GaN surface by the sulfide treatment [391]. The formation of Ga-S bonds and the occupation of nitrogenrelated vacancies by sulfur were proved to give a stable GaN surface. Behn et al. utilized photoreflectance spectroscopy to understand the movement of the surface Fermi level in a number of GaN samples grown by MBE and MOCVD on sapphire and SiC substrates [392]. 4.8.3. GaP
Zn++lZn ~ 9
243
.
.
.
.
l
.
=
-0.8-0.8-o.4-ci~ o'.o 0'.2 o;4 0.6 0.8 Redox Potential ( V ) Fig. 49. Surface-recombination velocity (SRV) of n-InP as a function of the redox potential of the four-metal ion solution. Reprinted with permission from Y. Rosenwaks et al., J. Phys. Chem. 97, 10421 (1993). Copyright 1993 by the American Chemical Society.
Lee et al. [393] studied the electronic structure of As-etched GaP and (NH4)2Sx-treated GaP [393]. They have shown that Ga vacancies Vca and/or Vca-related complexes are created on clean surfaces by the adsorption of oxygen atoms. Asakawa and coworkers were able to passivate the impurity related defects in GaP by hydrogen [394].
244
KASI VISWANATH
4.8.4. InAs
._. <
6OO
Katayama et al. [395] reported the surface structure of sulfur-treated InAs. They conducted synchrotron-radiation photoemission-spectroscopy studies on (NH4)2S x passivated samples. The As-treated surface of InAs has both S-In and S-As bonds. When the samples were heated to 380 ~ S-As bonds disappeared. Luminescence due to surface-quantum wells of InAs grown on InP was reported [396]. Because radiative recombination within the InAs layer can be distinguished from PL arising from both bulk and surface defects, it was suggested that this system is a good monitor to study the stability of InAs/InP and InAs/air interfaces. Kane et al. [397] studied surface and bulk recombination in InAs/A1As0.~6Sb0.s4 light-emitting diodes that emit at 3.54 /zm. They have shown that the quantum efficiency of these LEDs was limited by interface recombination at the InAs/A1As0.~6Sb0.84 heterojunction and reabsorption in the active layer (Fig. 50). Farad [398] has recently reported the luminescence from the near-surface InAs/GaAs quantum dots. State-filling spectroscopy of these samples is shown in Figure 51. The spectra from five well-resolved shells (s, p, d, f , g) were observed. This was attributed to QD ensemble with good homogeneity [399]. Charge transfer between the surface states and QD states was also noted.
--t
500
oc
400
._.o
3O0
- '
.'. '....
v
.~. 9
As InSb is a narrow-gap semiconductor, it can form alloys with InAs to give bandgaps in the IR range and hence it has many applications in fabricating optoeloctronic devices in the infrared. These devices include detectors for the 10-/xm range based on InSb nipi doping superlattices or InAsxSb~_x/InSb strained-layer superlattices. Wagner et al. [400] reported the surface Fermi-level pinning in epitaxial InSb.
Data
*~
,.~176176
.~
4..d
Q. 200 :3 0 100 a LU J 0
;
,
0
2
(a) ~, <
,
600,
>,
o
10
12
r] =0~14, LoS/D=0.053
~" soo t-
,
8
Thickness (pm)
. . . . . q =0.17, LoS/D=0.086
. - - . r I =0.2. LoS/O=0.12 400
~
~
~-
9
0ata
.m
._.o
300
9 ~176176 . "~176
9
(3. 200 -1 o 100 D LU --J 0
!!
-'*"
11
I1
..~x=.-:
1 I
i't.~ I
L
..... ..
2
0
(b) 4.8.5. InSb
q=l, LoS/D=0.42 ...... q =0.17, LoS/O=0.1 _ _ . . q =0.075, LoS/D=O
'l
~ .
I
L
I
I
&
4
6
8
10
Thickness (pro)
Fig. 50. Front emission efficiency of a series of InAs/A1As0.~6Sb0.84 LEDs with a p-type doping of 5 x 1017 in their active region as a function of active layer thickness. The solid lines are a fit to a model of the LED efficiency that includes bulk and interface recombination and reabsorption and uses as variable parameters the bulk efficiency r/ and the dimensionless parameter LDS/D, where L D is the diffusion length, S is the surface-recombination velocity, and D is the diffusion constant; (a) illustrates the range of parameters that can be used to explain a single data point; (b) shows the best fit and error range of fits to all data points in the figure. Reprinted with permission from M. J. Kane et al., Appl. Phys. Lett. 76, 943 (2000). Copyright 2000 by The American Institute of Physics.
4.9. III-V Ternary Semiconductors 4.9.1. InGaAs Tai et al. [401] at AT&T Bell Laboratories have performed picosecond timescale pump-probe experiments on InGaAs/InP quantum wells grown by chemical-beam epitaxy. The InGaAs/InP quantum-well mesa structures are important in high-speed optoelectronics [402] and all-optical switching [403]. For optical switching based on carrier-induced nonlinearities near the bandgap, Lee et al. [404] have shown that surface recombination was a very good method for speeding up device response without degrading bulk optical properties. For the details of the pump-probe technique the reader is referred to the article by Ippen and Shank [405]. Figure 52 shows the pump-probe spectra of the quantum well. The surface-recombination velocity in the smaller mesa structure was reported as 1.02 x 1 0 4 cm]s. Maile et al. [406] reported the wire-width dependence of the quantum efficiency in dry-etched InGaAs/InP quantum wires. The results were explained by considering both surface
~.
rTK
850
900
950
1000
1050
1100
1150
Wavelength (nm) Fig. 5 l. State-fillingspectroscopy of near-surface InAs/GaAs QDs covered with the indicated cap thickness. The PL spectra are excited with a few kW/cm 2 and vertical offsets have been used for clarity. The QD shells are labeled with the atomic notation: s, p, d, etc. Reprinted with permission from S. Farad, Appl. Phys. Lett. 76, 2707 (2000). Copyright 2000 by The American Institute of Physics.
245
S U R F A C E A N D I N T E R F A C I A L R E C O M B I N A T I O N IN S E M I C O N D U C T O R S
=I
6
/
v
.
.-
-.
--
--..-..-..
'i_. t . - 500
J
__/----
-.-.
v , -
j
1
I O
I
1 3OO
PROBE DELAY (p s)
Fig. 52. 17he AT/T signal vs probe delay for InGaAs/InP quantum wells. The dasb e:[ curve is obtained for a mesa with a diameter of 35/zm. The solid curve is f(: a mesa with a diameter of 35/~m. Zero is at the bottom for both curves. ~.el)rinted with permission from K. Tai et al. Appl. Phys. Lett. 53, 302 (1988). (.h,pyright 1988 by The American Institute of Phyics.
recombiE~ation and the inactive dead layer. Surface recombination v, as found to be high at higher temperatures whereas for wi~e,,; of smaller diameter the dead layer determines the quantu~I~ efficiency. Nanll:,u et al. [407] studied the relationship between the carrier l:fetime refr and the width of the dry-etched region R for lnGaAs/GaAs quantum wires. The calculated curve is shown ill Figure 53. Analysis was done by considering the carrier-d:tffusion length D r. When the R value is large enough, as shown by region (a), the carrier decay is determined by the intrinsic lifetime Ti,t. When the R value is small enough as shown i1~.region (c), the carrier decay is limited by the surface recombination. In this region Teff is proportional to R / S r . In the middle region (b) the carrier decay is accelerated with
log q:,tf/~k
(c)
(b)
(a)
' / c a r d e r decayis . . . . . l limitedby : ca'Tierdecay is : ca.,'ncr qecay,Is ] surface : limitedby : . oc!crml,nr oy recombination : diffusion :, mmnssc ttlcUmc >:< >.., log "ti.,I.................. !............... .:'--i.........xr
T=40 K ..,.~.~_~..-,.~--
"T2D
~
~z
.~
C:
=.,,.,=l
i ..... ;r . . . . ::" . . . . :
...................
decreasing R because of surface recombination. Diffusion also has an influence on the carrier decay in this region. In another report the surface recombination was found to limit the electron-hole plasma density attainable in dry etched InGaAs/InP quantum wires [408]. Band-filling and bandgap renormalization due to many-body effects were also observed at higher excitation densities. Nonradiative recombination due to surface states was found to decrease the intensity with decreasing lateral size in the photoluminescence of dry etched quantum wires and quantum dots of InGaAs/InP structures studied by Hubner et al. [409]. In a later report, the research team led by Forchel concluded that surface recombination was not dominated by diffusive carrier transport to the wire surfaces in deep-etched InGaAs/InP quantum wires in contrast to several previous reports [410]. Wire-width dependence of the carrier lifetime is shown in Figure 54. In a very interesting and important paper, Juodawlkis and Ralph [411] discussed the mechanism for photoinduced absorption in a semiconductor containing a large number of electron traps in the bandgap. When a semiconductor is photoexcited there will be competition between phase-space filling and bandgap renormalization (BGR). These two phenomena, which arise due to transient many-body effects, are very important as they determine the nonlinear optical response of the semiconductor. The relative dominance of phase-space filling and bandgap renormalization depends on the densityof-states (DOS) function and the time-dependent electron and hole distributions. Figure 55 shows the band-filling and bandgap renormalization processes in a semiconductor containing many electron traps in the bandgap. Figure 55b shows the two competing effects--a decrease in absorption due to band-filling and an increase in absorption due to bandgap renormalization. When there are a large number of electron traps in the bandgap the electrons in the conduction band are removed, leaving photoexcited holes that give rise to hole-induced
Faj
10~
E 0
.-I
/~
"Ceff" ~
calculation:
;'
L s = 5 nm , " r s = 1 4 0 ps ,
/
experiment
m
."
:
.
]
101
>
log@)
logLn ---log (2.41 ~ t )
,
1
9
'
i
I ! ,
I
!
i
t ~
10 2
Wire Width L~ [nm]
log R
Fig. 53. Calculated reff vs R curve where Telr is the carrier lifetime and R is the width of the dry-etched region. Reprinted with permission from Y. Nambu et al., Appl. Phys. Lett. 65, 481 (1994). Copyright 1994 by The American Institute of Physics.
Fig. 54. Wire-width dependence of the carrier lifetime in deep-etched In0.53Ga0.47As/InP quantum wires. Circles represent experimental data, and the solid line indicates calculated lifetimes assuming that the surface recombination is controlled by the probability of finding electrons within 5 nm of the wire surfaces. Reprinted with permission from E Kieseling et al., Phys. Rev. B 51, 13809 (1995). Copyright 1995 by The American Physical Society.
246
KASI VISWANATH t<0 Ot
:
0< t< t r
Or, I
t2
<
lZr< t< ~t
Gt:
Or, >
1.0
(]lt !
'"'
'
\
0.5
)=t
N m t--
E
'-
it
w
-
,,
l
,
, 14,Onrn
-
'
(a)]
\
.....
1530 nm
|
~
-- .... ,555nm
t
0.0
O
Z -0.5
k"kt
~
E"E~
k: -20
1.0
(a)
(b)
20 ,;o .... go Probe delay [ps]
o -
w
-
|
-
i
-
u"
' -
io ,
Ioo -
'
(c)
Fig. 55. Energy-mometum (E-k) diagrams showing band-filling and bandgap renormalization (BGR) processes during photoexcitation and recovery in a semiconductor containing a large density of electron traps: (a) t < 0, before photoexcitation, (b) 0 < t < TT, initial absorption increase (a < a~) and (c) z r < t < z R, after appreciable electron trapping (a < a~); ~-~ and z R are the electron-trapping and trapped-electron/hole recombination times, respectively. Reprinted with permission from P. W. Juodawlkis and S. E. Ralph, Appl. Phys. Lett. 76, 1722 (2000). Copyright 2000 by The American Institute of Physics.
bandgap renormalization. Therefore, once a large number of electrons are removed from the conduction band, many states become available for occupation. This gives rise to increased absorption as shown in Figure 55c. Juodawlkis and Ralph [411] have also done very elegant pump-probe differential transmission experiments with femtosecond lasers. Figure 56 shows experimental results and simulated curves. Shockley-Read-Hall theory was considered in simulation of theoretical curves. In Figure 56b please note that the A T / T is negative when A is 1555 nm. This corresponds to photoinduced absorption, which agrees well with the theoretical predictions. Boroditsky et al. [412] have reported surface-recombination studies in a number of III-V materials. Photoluminescence due to interface defects in strained InGaAs/GaAs single-quantum wells was indentified by Joyce et al. [413]. Juang et al. [414] could distinguish the surface and bulk luminescence peaks in InGaAs epilayers based on the excitation intensity-dependence studies as shown in Figure 57. Figure 58 shows the surface structure and InGaAs layers studied. A decrease in interface recombination has been observed in InGaAs/InAs heterostructures when the active layer thickness was increased [415]. Sulfur passivation of n-InGaAs Schottky diodes [416], InGaAs/InP metal-semiconductor-metal photodetectors [417], InGaAs/A1GaAs single quantum-well lasers [418], and InGaAs/InP heterojunction bipolar transistors [419, 420] were shown to have reduced the surface/interface recombination. Surface passivations by InP [421], hydrofluoric acid [422, 423], hydrogen [424-428], and hydroxides [429] have also given better optical quality surfaces in InGaAs materials and devices. Yablonovitch et al. [429] have achieved very highquality In0.53Ga0.a7As "naked" quantum wells by etching with NaOH. Band-to-band recombination spectra are shown in Figure 59. These quantum wells are grown on InP substrate. As the InGaAs becomes thinner and thinner, the carriers
0.5
e=
0.0
t_.
z -0.5 -20
0
2'0 ' 4() 6'0 Probe delay [ps]
"' 8'0
100
Fig. 56. Normalized differential transmission AT~T) of annealed (600 ~ for 30 s) LTG-InGaAs/InA1As MQWs with Be doping of 5 • 1017 cm -3, revealing wavelength-dependent photoinduced absorption: (a) simulated Shockley-Read-Hall (SRH) recovery response, and (b) measured pump/probe response. In (a), the rate-equation time constants and photoexcitation density (An -- Ap -- 1018 cm -3) are fixed so that the variation of the AT/T recovery with wavelength results solely from changes in the impact of hole-induced bandgap renormalization (BGR). Reprinted with permission from P. W. Juodawlkis and S. E. Ralph, Appl. Phys. Lett. 76, 1722 (2000). Copyright 2000 by The American Institute of Physics.
have more and more of their probability amplitude in InR In the extreme case the electrons become unbound because the electron-potential well is very shallow in this heterojunction. In the usual case of the quantum well that has a barrier on both sides, the electrons are in the bound states. In the naked quantum well, which is very shallow and has a barrier on only one side, the electrons are in unbound states. These structures are very useful in the fabrication of lateral quantum-confinement devices. 4.9.2. InGaN As a very important semiconductor material, InGaN has a bandgap that can be varied from 1.95 to 3.4 eV depending on the In content. It is the active layer for the blue and green LEDs and semiconductors lasers. Many of these devices are already commercially available. Major breakthroughs in this field have been made by Nakamura and coworkers [381 ]. They have achieved cw and pulsed lasing in InGaN quantum wells that emit in almost any region of the visible spectrum. Most of the optical studies have been directed to understanding the localization phenomena of excitons in the InGaN system, which is well known for its compositional inhomogeneity and the associated potential fluctuations [430-435]. Viswanath et al. [435] conducted an in-depth study of radiative and nonradiative recombination in InGaN multiple quantum wells. Nonradiative recombination arises due to interface
247
S U R F A C E A N D I N T E R F A C I A L R E C O M B I N A T I O N IN S E M I C O N D U C T O R S
In 0 53 Ga 0.47 As
22K
A lit ~' c
,
' i'
As-grown
''
l
T
,
9
, bulk
Ino.s~Gao.,~s
~;
eh-surface
,
~;: :
95
80
sec.
sec.
;r
50 30 20 sec. sec.sec,
10
begin
/ ~
sec. I
/
\
/I
I
A eh-bulk
:i
1280 W/cm 2 --~
1.0
1.1
'~.2
1.3
-
114
WAVELENGTH Q
It
X30
/ ~ ~ .
80 W/cm 2
X300
M,_ 20 W/cm2 I
_ z
I
1450
t
1600 Wavelength (nm)
" (.6
1.7
Fig. 59. Band-to-band room-temperature photoluminescence from a 50-A thick NaOH-coated "naked" quantum well as it is gradually thinned down by slow etching. The spectrum marked "begin" is the starting point. The quantum-shifted spectra induced by etch thinning for a given number of seconds are shown. The spectrum at 95 s is distorted because of a silicon window in the optical train. Reprinted with permission from E. Yablonovitch, H. M. Cox, and T. J. Gmitter, Appl. Phys. Lett. 52, 1002 (1988). Copyright 1988 by The American Institute of Physics.
320 W/cm 2
i
1~5 (IJm)
1700
Fig. 57. Photoluminescence spectra of LPE-grown In0.53Gao.47As layers used in the experiments under various excitation power densities at 22 K. Surface band to band (eh-surface), bulk band to band (eh-hulk), and conduction band to zinc accepters (eZn) are shown. Reprinted with permission from C. Juang et al., J. Appl. Phys. 72, 684 (1992). Copyright 1992 by The American Institute of Physics.
defects in the quantum-well structure. It is very essential to understartd the effects of the interface defects on the photoluminescence efficiency and its temperature dependence. Figure 60 shows the cw PL of an InGaN multiple quantum well with GaN barrier. From the appearance of the barrier emission it was concluded that the interfaces were of high quality with minimum defects. Figure 61 shows the experimental setup for picosecond time-resolved experiments [436]. This setup was constructed by our research group at the National Creative Research Initiative Center for Ultrafast Optics Control, Korea Research Institute of Standards
and Science. A coherent mode-locked Ar-ion laser that had about 2-ps pulsewidth at a 3.8-MHz repetition rate pumps the coherent double-jet, cavity-dumped dye laser. A synchronous pumping configuration was used in which the cavity lengths of both lasers should exactly match. The dye laser beam was frequency-doubled using a/3-barium borate nonlinear optical crystal. The UV light thus generated was used to excite the sample. For some experiments the Spectra Physics Tsunami Titanium Sapphire laser was used. Nonlinear optical crystals were utilized for harmonic generation. The PL signal was dispersed by a McPherson monochromator and detected by a Hamamatsu photomultiplier tube. The time dependence of the decay was measured by time-correlated single-photon counting. The setup has a time resolution of 10 ps after deconvolution. The sample was mounted on the cold finger of a liquid helium cryostat.
InGaN Multiple Quantum Well 10 K
v
InGaAssurface
disorder layer
+-,~--.i-..: L i...~...i.-.i., 4..,..
00 r r
barner
.--I
Q. . . . . .
;
:
:
',
:
i
:
,
:
; :
I 2.6
Fig. 58. Surface structure of In0.53Ga0.47As layers exposed t o C F 4 - - ~ - O 2 plasmas. Disorder layer and point-defect regions are indicated by horizontal and vertical dotted lines, respectively. Reprinted with permission from C. Juang et al., J. Appl. Phys. 72, 684 (1992). Copyright 1992 by The American Institute of Physics.
,
I 2.8
j
I 3.0
~
!
~
3.2
I 3.4
~
I 3.6
, 3.8
photon energy (eV) Fig. 60. The PL spectrum at 10 K for InGaN/GaN multiple quantum wells. The emission from the GaN barrier is observed very clearly. Reprinted with permission from A. K. Viswanath et al., unpublished.
248
KASI VISWANATH
I .o0 -LoC 0
20
I
InGaN Multiple Quantum Well
15
(/) vE
E
10
>
.m
I
,
"13 i_ E O Z
,
5
nun 9
I
COUNT, I
i
I
I
i cou. , I
I lco.Pu, , I
9
9
9
9
9
0
I
i,, 0
I
50
.
I
100
,
I
150
,
I
200
.
I
250
9
300
Temperature (K)
Fig. 61. Schematic diagram of picosecond photon counting system constructed by our group; CD denotes cavity dumper. Reprinted with permission from A. K. Viswanath, J. I. Lee, and D. Kim, unpublished.
Fig. 63. Nonradiative lifetime as a function of temperature for InGaN quantum-well emission. Reprinted with permission from A. K. Viswanath et al., unpublished.
4.9.3. InGaP Figure 62 shows the quantum efficiency of the InGaN quantum well as a function of temperature. As can be seen in the figure, quantum efficiency decreases with the increase in temperature. This was attributed to the increase in interf acial recombination with temperature. In order to understand this problem Viswanath et al. [435] evaluated the temperature dependence of nonradiative lifetime (~'nr), which is shown in Figure 63. By a very careful study of carrier lifetimes and quantum efficiency as a function of temperature, 'Tnr was evaluated. It was also proposed by Viswanath et al. [435] that the chemical passivation methods should be performed, which will reduce the interfacial recombination in these systems.
1.0
9
InGaN Multiple Quantum Well 9
"~ o c
emission
0.8
(!) .m
0.6
E
0.4
o
quantum well
E m
O
0.2 9
9
9 9
0.0 ,
0
I
50
,
i
100
,
I
150
,
I
~
200
9
I
250
9
9
,
I
300
,
350
Temperature (K) Fig. 62. Quantumefficiency at different temperatures for InGaN/GaNmultiple quantum wells. Reprinted with permission from A. K. Viswanath,J. I. Lee, and D. Kim, unpublished.
Pearton et al. [437] reported the surface recombination velocities in InGaP p-n junctions. The ternary compound InGaP has applications in strained quantum-well lasers, and heterojunction bipolar transistors. The InGaP/GaAs heterojunction is less prone to surface oxidation than the A1GaAs/GaAs system and hence it is a better candidate to make quantum-well lasers. In these lasers the optical damage of the laser facets was expected to be minimum. The valence-band discontinuity between GaAs and InGaP is higher than in the case of GaAs/A1GaAs and thus it is more suitable for the fabrication of high electron-mobility transistors and heterojunction bipolar transistors. Pearton et al. [437] have also examined the effects of (NH4)2S x t r e a t m e n t on surface-recombination velocity. Mullenborn et al. [438] performed photoluminescence power-dependence measurements on InGaP/GaAs heterojunction interfaces and estimated the lifetimes that were correlated with the dislocation density at the interface (Fig. 64). The Shockley-Read-Hall recombination was considered and diffusion model calculations were also done. The decrease in lifetime was attributed to an increase in dislocation density at the interface. The band diagram used for diffusion model calculations is shown in Figure 65. Lee et al. [439] studied the effects of various chemical treatments on the performance of Schottky diodes of InGaP (Fig. 66). When samples were annealed above 500 ~ a drastic degradation of barrier height was observed. This was thought to be due to interdiffusion and penetration of metals into the semiconductor. Gorbylev et al. [440] passivated InGaP by hydrogen, which enabled them to obtain better quality surfaces. Photoluminescence measurements on InGaP/GaAs heterostructures have
249
S U R F A C E A N D I N T E R F A C I A L R E C O M B I N A T I O N IN S E M I C O N D U C T O R S
t~ t"l
2
~ 1 , ,,,1
"''.< "''.<
v 1-0
101"
tr r tJ t~
"--,~0.5
e..-
c= tD
E
GalnP/GaAs
4.,, tD
" ~ " ~ ' ~ D
Interface
--A !
i t , ill
1 011
1
,
2
$
1 ! i lILt
1 012
1
l
2
8
, 1 i ill
l
1 018
1
1
2
S
l
i
I I I
1 014
Dislocation Density/Length (era-a) Fig. 64. ]~ifetime as derived from PL power dependencies and diffusion model c~d:ulations as a function of dislocation density per dislocation length from HI~ J~:RD experiments. GaInP/GaAs interface data (O) fitted by an inverse square-r,)(,t function (line). Reprinted with permission from M. Mullenborn et al., J ,lppl. Phys. 75, 2418 (1994). Copyright 1994 by The American Institute c:' Physics. GolnP
GoK~
ep,~,r l uu~~ Ioyer
Lossf
shown that the hydrogen passivates the nonradiative recombination defect centers [441]. X-ray photoelectron spectroscopy experiments conducted by Tsai and Lee on the ( N H 4 ) 2 S xpassivated InGaP surface have shown that surface oxide was removed [442]. Evidence was also given for the bonding between sulfur atoms and In and Ga atoms on the surface of sulfur-treated InGaR It was found that sulfur atoms occupy the phosphorous vacancies instead of bonding with phosphorous atoms. Lee et al. [443], Stringfellow et al. [444], Shurtleff et al. [445], Jun et al. [446], Fetzer et al. [447], and Lee et al. [448] developed a novel method of surface reconstruction by adding surfactants during the process of crystal growth of InGaP by metal-organic vapor-phase epitaxy. Surfactant is a substance that accumulates at the surface and alters the surface properties of the material. The surfactant can modify the bonding at the surface, which results in changes in the surface energy. The Fermi-level position in the bandgap can also change by surfactants. Dopants such as Sb, As, and Te modify the surfaces. Figure 67 shows the PL spectrum of InGaP material that has both disordered and ordered regions. The bandgap energy was changed by adding Sb during the growth process. This is a very good method of producing quantum wells and heterostructures. This novel technique may be coined "bandgap engineering by surface reconstruction." 4.9.4. InA1As
N---- 1la~
=~
Fig. 65. Band diagram of the simplified heterojunction for the diffusion model cal,;ulations; B is the dislocated region with increased recombination. Reprinted with permission from M. Mullenborn et al., J. Appl. Phys. 75, 2418 (1994). C~)pyright 1994 by The American Institute of Physics. 1o .2
I
"
I
'
Benyattou et al. [449, 450] identified a PL peak in InP/InA1As/InP interfaces, which they ascribed to interface recombination. In this type I interface the lineup of the two semiconductor bandgaps has a staggered shape. The band bending at the interface depends on the doping type and concentration of the two semiconductors involved. In this case
0..i.1
1.2
lO ~
135.1 rneV
,4---
,I
1(1.4
lO -s lO-e
~
10 .7
<.5. c--
10-a
>, 0.6
10-~
m
1974.7 rneV
t..
= (..3
0.8
.4/
10-1o
~o4 - - = - - InGaP " HCI+20H20
10-11
e4
10-12 10-13 10 -14
i
).o
---e--InGaP 9 NH4OH+10H20 4-InGaP 9 BOE - - v - - InGaP "NONE , GaAs 9 NH4OH+10H20
012
I.
O4
=..
I
0.6
,
I
,
0.8
.6 meV 0.2
0 1750
1800
1850
1900 1950 Energy (rneV)
2000
2050
Bias V o l t a g e (V)
Fig. 66. Typical forward current-voltage characteristics of the GaAs and InGaP Schottky diodes for various chemical surface treatments. Reprinted with permission from C. T. Lee et al., Solid State Electron. 41, 1 (1997). Copyright 1997 by Elsevier Science.
Fig. 67. The 20 K PL spectrum for a GalnP disorder-on-order heterostructure grown by modulating the TESb flow during growth. Reprinted with permission from J. K. Shurtleff, et al., Appl. Phys. Lett. 75, 1914 (1999). Copyright 1999 by The American Institute of Physics.
250
KASI VISWANATH
quantum wells have triangular shapes, one for the electrons in the semiconductor having the lowest conduction band edge and one in the other material for holes. In this situation, when charges are injected, electrons and holes are separated at the interface. Therefore electron-hole recombination occurs through the interface. Hwang et al. [451], Chou et al. [452], and Chang et al. [453] utilized the photoreflectance technique to study surface states in InA1As; the InA1As material has many useful applications in high-speed electronics.
The most effective pass!vat!on was demonstrated by using 0.1-/xm thick A10.iaGa0.86As layer at the interfacesl A 16-fold increase in the effective lifetime has been observed in this approach. In another report, improved surface properties were observed in Al-rich AlxGa~_xAs multilayer structures [457]. Pass!vat!on by sulfide [458] and hydrogen [459-461] has enabled different groups to obtain good-quality surfaces and interfaces of A1GaAs. 4.9.7. A1GaN
4.9.5. InAsP
Only one report has been made on the surface-recombination velocity in A1GaN and that was by Boroditsky et al. [412]. They have given a value of S - 3 • 104 cm/s from the photoluminescence quantum-efficiency measurements.
Pikal et al. [454] studied the temperature dependence of recombination coefficients in InAsP/InP quantum-well semiconductor lasers. These lasers emit at 1.3 /xm and this long wavelength light is very useful for optical communications. Recombination coefficients were obtained from carrierlifetime measurements. In their analysis, the carrier population in the barrier was taken into account, and the results are shown in Figure 68. The frequently observed temperature insensitivity of the Auger coefficient in the InAsP/InP quantum-well laser was thought to be due to carriers spilling out of the quantum well at higher temperatures.
4.9.8. GaAsP
4.9. 6. A1GaAs
4.10. III-V Quarternary Semiconductors
Henry et al. [455] have shown that the 2kT current in A1GaAs p-n junction was due to the surface recombination. Measurements of surface-recombination velocities in AlxGal_xAs/AlyGa]_yAs interfaces was reported by Timmons et al. [456] based on time-resolved photoluminescence experiments. Different pass!vat!on methods were attempted.
4.10.1. InGaAIP
~-,
3.8 !o. :
.
.....
!
9
.
x ]--=--< 3.21--o--
.
!
.
'.
!
',
!
9
!
9
!
.
Bulk AEF~
-'~'o
QW
"
Aw
1
L !
9
i
-~- lo oE
E 0 ._.
i
9
i
~
a
Bw
"O-
a
.
~O . . . .
~
-
'
-
a
a
-
-
-O
4.10.2. InGaAsP
--
~
i
i
9
i
"'
i
'
i
10
8
m,~_..Bulk - - o-- Q W
CE~ C,~
6
.o .....-0"
4
X
2 ~J.
,,s
.
....o
9
O
""
"
4 i
=.9. m
,'
--o--QW O . . . .
X
m
i
Heterostructures of In05(Gal_xAlx)0. 5 P/In05Ga0.sP lattice matched to GaAs are the semiconductor laser materials that emit in the red. Boroditsky et al. [412] have measured surfacerecombination velocity in the InGaA1P/InA1P quantum well. The structure is shown in Figure 69; S was estimated as 105 cm/s from the luminescence efficiency experiments; PL signals were also recorded after thinning down the cap layer. Initially, the intensity increased and when the cap layer is completely removed the PL intensity suddenly decreased because of the dominant nonradiative recombination at the exposed surfaces. This is shown in Figure 70. Hydrogen-pass!vat!on effects in InGaA1P/InGaP quantum wells were reported by Gorbylev et al. [440].
- - ' - - - Bulk BEFF
8
v
'
The reduction in interface recombination with increasing phosphorous content was observed in GaAs]_xP x epitaxial layers that were grown by MOVPE [462]. Time-resolved spectroscopy was used to estimate the recombination velocities.
"
"
"
. .,o- - " 0 . . . .
-.0-
=
-=
"
"
. =
;0' 2'0'
3'0
--=
-=--------. i
40
s'o
.
6'0
Temperature (*C) Fig. 68. Temperature dependence of the extracted recombination coefficients of the deep well InAsP/InP laser using both the bulk and QW analysis. Reprinted with permission from J. M. Pikal et al., Appl. Phys. Lett. 76, 2659 (2000). Copyright 2000 by The American Institute of Physics.
The InGaAsP/InP quantum wells have applications in the realization of several optoelectronic devicesmedge or surfaceemitting long-wavelength semiconductor lasers, light-emitting diodes, photodiodes, and solar cells. These materials have emission in the range 1.1 to 1.6 /xm. This spectral range is very much suitable for fiber-optic communication systems because optical fibers have the lowest attenuation and dispersion in this range. The InGaAsP is also considered as a very good candidate for metal-insulator semiconductor field-effect transistors (MISFETs). Dow and Allen [463] have theoretically shown that the self-reproducing dangling bond degrades the shelf life of the InGaAsP-based lasers. The schematic of this mechanism is shown in Figure 71.
SURFACE AND INTERFACIAL RECOMBINATION IN SEMICONDUCTORS E
0.2. m
InAIP Cap
!__z
,,,,,
InGaAIP (n--lO"cm ')
251
0.75 lain
CO~NDUCTION BAND
InAIP Cladding
llam
,
EXCI TON
250 ~tm
GaAs Substrate
(a)
,
0ANGLING BONO ~ ~ ~ ' ~
Exposed, s u r f a c ~ F InG aA IP
.75
,
,
VALENCE BANO
InAIP Cladding
t .
.
.
.
.
1 ~m
.
250 ~m
GaAs Substrate
(b) Fig. 69. (~l) The InGaA1P sample consists of 0.75-/xm thick In0.5(Gao,2,kl0.0a)0.sP (A = 630 nm) active region doped at the n = 10 ~7 cm -3 le,~,e sandwiched between n-type InA1P cladding layers grown on absorbing (3aAs substrate; (b) when the top InA1P cladding layer is etched away, the nonradiative surface recombination on the exposed surface of the active region becomes the dominant recombination process. Reprinted with perm}~sion from M. Boroditsky et al., J. Appl. Phys. 87, 3497 (2000). Copyright 2000 by The American Institute of Physics.
Mozer et al. [464] examined the degradation of InGaAsP/InP lasers due to nonradiative and interface defectrelated recombinations. Rajesh et al. [465] have done the sulfur passivation of the InGaAsP (100) grown on lattice-matched InP substrate and studied its surface chemistry, as well as the relative movements of the surface Fermi level. Etch Depth (nm) 0
100
200
300
400
500
600
0.6 0
0.5
0
= 0.4 ,-; < 0.3 .a . 0.2
0
0
0.1
0.0
,
9
0
50
9
,
.v
150 200 Etch Time (s)
lOO
-
,
-
250
r
3OO
Fig. 70. Dependence of PL signal from the InGaA1P sample on the etch depth. The experimental data are shown with open circles. The signal increases as the cap layer becomes thinner, and drops when the active layer becomes exposed. The solid line represents the fit obtained from solving the diffusion equation. Reprinted with permission from M. Boroditsky et al., J. Appl. Phys. 87, 3497 (2000). Copyright 2000 by The American Institute of Physics.
Fig. 71. Schematic illustration of how the self-reproducing dangling bond mechanism of laser degradation might occur. Transport (single arrows) from a moderately shallow level to a deep nonradiative "dangling bond" center might occur, after which the capture and nonradiative recombination events (jagged arrow) eventually lead to the breaking of another bond (double arrow), producing a daughter vacancylike dangling bond. Reprinted with permission from J. D. Dow and R. E. Allen, Appl. Phys. Lett. 41,672 (1982). Copyright 1982 by The American Institute of Physics.
4.10.3. AlGaAsSb Quartenary compounds of AlxGal_xASySb~_y are used as cladding layers in the fabrication of GaSb-based doubleheterostructure semiconductor lasers that emit at 2-/xm wavelength. Polyakov et al. [466] have reported the effect of ( N H 4 ) 2 S t r e a t m e n t on the surface properties of these materials.
5. II-VI SEMICONDUCTORS 5.1. CdS Huppert et al. [467] and Rosenwaks et al. [468-470] studied the effect of metal reactivity on the surface-recombination velocity of CdS and CdSe, which were deposited with various metals. The surface-recombination velocity (SRV) decreased if the metal is very reactive, such as A1, Ti and Zn. The SRV increased if the metal is unreactive, such as Cu, Au, and so forth. The results were understood considering metal-induced recombination centers at the semiconductor interface. Many papers have been published on the surface states in semiconductor quantum dots doped in glasses. These materials have large optical nonlinearities [471-474] which were observed for the first time by Jain and Lind [471]. Because of the large nonlinear optical properties they find applications in several optical and optoelectronic devices such as saturable absorbers that are useful in the generation of pico- and femtosecond laser pulses [475, 476], optical phase conjugation [471], pulse shaping [477], optical waveguides [478, 479], ultrafast photonic bistable devices [480, 481], and harmonic generation [482]. Higher nonlinearity of these materials is due to the quantum confinement in three dimensions. Most of the optical results in these materials were explained by various workers considering shallow and deep surface traps.
252
KASI VISWANATH 0 0.4
!00 ,,
200
300
400
500
600
700
800
900
I000
0.3
O.2 o 0.! 0.0-
m m
. II 9
9 ' " ' " " 9 9 ,1 I
9
. . . . . . .
'! 10
9
Fluency per pulse, mJ/cm2 Fig. 72. Transmission changes measured at the 700-nm wavelength vs irradiation of the layer by 347-nm pump beam. Each point corresponds to 100 shot accumulation. Reprinted with permission from S. Juodkazis et al., Opt. Commun. 148, 242 (1998). Copyright 1998 by Elsevier Science.
Juodkazis et al. [483] studied photodarkening in CdS quantum dots doped in (Si0.2Yi0.8)O 2 sol-gel waveguides. Photodarkening is an optical phenomenon that is nothing but the photoinduced decrease of transmission. To understand the photodarkening (shown in Fig. 72) picosecond pump-probe experiments were done. Transmission-bleaching experimental results are shown in Figure 73. Very fast decay was observed when the laser power was 4 mJ/cm 2, which produced photodarkening in these samples and attributed to deep traps. Vanagas et al. [484] reported laser-induced grating in CdS-doped sol-gel glass and suggested a three-level model that includes surface- and defect-related trap levels to explain the observed biexponential decay (Fig. 74). Many groups have reported the double-exponential decays in their optical measurements [485-490]. Jursenas et al. [491, 492, 494, 495] and Zukauskas and Jursenas [493] measured carrier lifetimes in CdS microcrystallites doped in glasses by picosecond time-resolved spectroscopy [491-495]. The results were interpreted by considering surface recombination of nonequilibrium electronhole plasma. The crystallite size was varied from 1000 to 5 4 / k . The decrease in carrier lifetime with decrease in quantum dot size was observed as shown in Figure 75. This was thought to be due to increase in the surface-to-volume ratio, which enhances the surface recombination. As the quantumdot size decreases, the density of surface states that participate in nonradiative capture increases [496]. Surface states arise due to dangling bonds [497] and self-trapped states arise due to lattice deformation at the interface [498]. The ultrafast capture is due to multiphonon emission [499]. Chestnoy et al. [500] examined the multiphonon processes in nanocrystals, but they did not study the participation of surface states in these processes. Zukauskas and Jursenas [493], and Jursenas et al. [494, 495] reported the surface recombination due to multiphonon emission. Burda and E1-Sayed [501] and Logunov et al. [502] made very elegant studies on surface states in CdS semiconductor
Ii 0
'".... n m, .lm.., 250
500
750
!000
o ll 1250
1500
1750
2000
Probe delay, ps Fig. 73. Transmission-bleaching change dependence on the probe beam delay (probe beam wavelength: 700 nm). The different curves are registered at different fluences of pump beam (wavelength: 347 nm: 400 short irradiation): (a) 0.5 mJ/cm2 per pulse: (b) 0.8 mJ/cm2; (c) 1.5 mJ/cm2; (d) 0.8 mJ/cm2 after measurements at 1.5 mJ/cm2; (e) 4 mJ/cm 2. Dotted lines are the best fit according to the decay by linear and Auger recombination, dashed and solid lines are the fit by linear and bimolecular recombination. Reprinted with permission from S. Juodkazis et al., Opt. Commun. 148, 242 (1998). Copyright 1988 by Elsevier Science.
nanoparticles by using very high-resolution femtosecond transient absorption experiments. Figure 76a shows the absorption and emission spectra for passivated and unpassivated CdS nanoparticles, whereas Figure 76b is the femtosecond transient absorption for the same samples. The unpassivated samples show the signals due to deep trap states. After passivation the trap-related peaks do not appear. Passivation by O H - and ZnS was known to increase the quantum efficiency of CdS quantum dots [503-505].
I
-~
//Z
n3 Tz t
Fig. 74. Schemeof a three-level system T21,T22 , T23 are the relaxation times associated with the different transitions; n2, and n3 are the populations of level 2 and 3, respectively, and AE is the energy difference between level 2 and 3. Reprinted with permission from E. Vanagas et al., J. Appl. Phys. 81, 3586 (1997). Copyright 1997 by The American Institute of Physics.
SURFACE AND INTERFACIAL RECOMBINATION IN SEMICONDUCTORS L
70O
............
a
P~siwted CdS NP$
A
1
1I
~=25oA
~oo
3
O
rj
9OO
I r'
f
t'
!
,-
i
O u'J
i
1
rm ,
X'tt i
10 s
4\\ 43 o,,,,,
i ~ ~
BULK
i
'Q....,,..
.d
~
L
1000
400
i,,
I
~ i
. . . . . . -._.
I
500
i
m.
"'-.
I
600
e L
~"
I
700
Wavelength
~.,
rll m,
~2
k
253
800
2 / nm
0.16
10 a
b
unpassivated CdS N P s
0.12,
250 0.08.
0.04
~ 10~
13o
y .o, 1
0
200
,
B] t,,
|
400
t
I
600
0.00
550
. l
BOO
~;o
"
6;0
"
"
,;o
W a v e l e n g t h / nm
OELA Y T;~E Cps) Fig. 75. *l'emporal evolution of the carrier effective temperature for fi = 250 A (A) and of the spectrally integrated luminescence intensity (B) of the CdS micro,:rystallites of various sizes. The intensity scale is arbitrarily shifted for each dependence. Reprinted with permission from S. Jursenas et al., Solid State Com,~un. 87, 577 (1993). Copyright 1993 by Elsevier Science.
By u,,~ing picosecond time-resolved fluorescence and femtosecond transient absorption experiments, Logunov et al. [502] examined the effects of electron acceptors adsorbed on the surface of CdS on the surface recombination. The electron transfer to viologen acceptors was shown to be very efficient and competes with surface-trapping and electron-hole recombination processes. Therefore bandgap emission and deep-trap emissiov, were quenched. These results were consistent with the report by Klimov et al. [506]. Murpl~y and Ellis [507, 512], and Zhang and Ellis [508] along w th Meyer et al. [509, 511], Lisensky et al. [510] and Murphy et al. [513] have done extensive work on the reduction of ,,urface-recombination velocity by treating CdS and CdSe surfaces with Lewis acids and Lewis bases. Very good correlation was observed between the surface-recombination rate and the electron-donating ability of the adsorbate. It was demonstrated that the chemical passivation of defect sites was much less important than the donation or withdrawl of electron density into the depletion region of the semiconductor. A similar approach was used to passivate the quantum dots of II-VI semiconductors [504, 514, 515]. Surface recombination was also reduced by treatments with (CzHs)sN, (CH3)3N,
Fig. 76. The ground-state absorption and steady-state emission of two different CdS NP samples in water. The unpassivated sample shows strong deeptrap emission, which is almost completely--but not perfectly--quenched in the case of the surface-passivated sample. The passivated sample emits mainly at the band-edge energy and weakly at a wavelength between the band-edge and deep-trap-emission wavelength. This indicates that surface passivation eliminates mainly the deep trapping sites; b) Femtosecond transient absorption spectra of CdS NPs 50 ps after laser excitation at 400 nm with 30 mJ/cm 2 to achieve high exciton density. The unpassivated CdS NPs show a much more intense absorption, which also takes place at higher energy than the absorption of the surface-passivated NPs. The decrease in transient absorption caused by the surface passivation suggests that the observed high-density absorption originates from surface-trapped charge carriers. Reprinted with permission from C. Burda and M. A. E1-Sayed, Pure Appl. Chem. 72, 165 (2000). Copyright 2000 by International Union of Pure and Applied Chemistry.
1,4-diazabicyclo [2.2.2] octane, and alkanethiols [516]. Lewis [517] at Cal Tech. made a thorough study of the surface and interface recombination in several semiconductor/liquid interfaces by time-resolved spectroscopy. These systems have very important applications in the development of photoelectrochemical cells. Optically detected magnetic resonance (ODMR) measurements were made on CdS nanoparticles doped in phosphate glass [518], and trapping sites at the surface and core were identified. The sites at the surface were found to have weak e-h exchange interaction and anisotropic broadening due to variable e-h distances around the circumference of the nanoparticle. By contrast, the sites inside the core have shown very strong e-h exchange interaction. These materials
254
KASI VISWANATH
have applications as laser materials [519]. Zhang et al. [520] have reported interfacial electron-hole recombination in aqueous CdS colloids that are useful in electrooptics, photocatalysis, and solar-energy conversion [521-525]. 5.2. C d S e Burda et al. [526] reported the femtosecond pump-probe spectroscopy of CdSe colloidal nanoparticles. Transient absorption spectra for 4-nm diameter quantum dots for different delay times are shown in Figure 77. Changes in transient absorption were attributed to the state-filling effect and surface trapping of the exciton. Both shallow and deep traps were identified. These results were consistent with the earlier reports by Bawendi et al. [485] and Hunsche et al. [527]. In a new report, E1-Sayed's group has demonstrated electron-shuttling of the excited electron from the conduction to the valence band of CdSe nanoparticles through naphthaquinone on the surface of the nanoparticle [526b]. Figure 78 shows the time dependence of bleach and transient absorption. It was observed that the decay of the bleach and the transient absorption have the same lifetimes. It was concluded that the electron was shuttling from the conduction band to the valence band. Mechanisms for bleach recovery in CdSe and CdS with adsorbed electron acceptors on their surfaces were also studied (Fig. 79). Surface and interface recombination in CdSe quantum dots was passivated by coating a thin layer of CdS or ZnS
[528-532]. Murray et al. [533] developed the passivating methods for II-VI semiconductor quantum dots by chemical treatments. Passivation of surface states was done by ligating with various organic capping groups. Mattoussi et al. [534] demonstrated a very novel method of surface passivation of CdSe quantum dots by dispersing them in a block copolymer. The block copolymer is [(norbornenylmethoxy carbonyl) biphenyl-yl-tert-butylphenyl oxadiazole]ls0 [norbornene-z-ylCH20 (CH2)sP (oct)2110 abbreviated as [NBPBD]150 [NBP]~ 0. This polymer acts as a passivating layer and also as an electron-transporting layer in the light-emitting diode (LED) structure. Figure 80 shows the electroluminescence spectra for the LED device. Enhancement in the luminescence due to passivation by the block copolymer was achieved. Figure 81 shows the energy-level diagram for the device. When voltage is applied, the holes tunnel into the PPV layer where they are transported to the heterojunction. At the aluminum cathode, electrons are injected into the PBD matrix or the dots. The barrier to injection of electrons into PBD is around 1.7 eV. Electron injection into CdSe bare nanocrystals or ZnS overcoated nanocrystals is energetically favorable. Recombination due to alloy formation at the interface in CdSe/ZnSe quantum well has been reported [535]. Bessler-Podorowski et al. [536] measured the time-resolved photoluminescence of CdSe single crystals immersed in various aqueous solvents and estimated the surface-recombination
0.05
0.00 -0.05
-0.10
$. at 4 8 0 nm: x
-0.05
x2 > 3 0 p s
=2.5
j~
-0.1o i,~
-0.15
/
-0.15
-0.20
-0.20 ;
obs. at 5 7 0 nm: z~ = 5 . 8 p s
. /
"t2 = 70ps
-0.25, J
-0.25 (1Se-lSh)
"--~
~
o
20
40
6o
time I ps
-0.30
so
~oo I
450
500
550
600
650
700
750
Wavelength ~. /nm Fig. 77. Transient absorption spectra of CdSe NPs with an average diameter of 4 nm, pumped with 400-nm femtosecond-laser pulses with a laser power of 4 mJ/cm2. The delay times of the spectra are 200 fs up to 110 ps. The measured absorption changes can be explained by the state-filling effect and surface trapping of the exciton. The inset shows the corresponding kinetic traces of the bleach decay observed at 480 and 570 nm. The short lifetime (~'~) of the 570-nm transient reflects fast trapping of the charge carriers at the NP surface to the shallow trap states. The longer lifetimes of the bleach decay are assigned to the charge carrier trapping by deeper traps. Reprinted with permission from C. Burda et al., J. Phys. Chem. B 103, 10775 (1999). Copyright 1999 by The American Chemical Society.
255
SURFACE AND INTERFACIAL R E C O M B I N A T I O N IN S E M I C O N D U C T O R S
'
'
'
'
I
'
'
'
'
I
'
0.05
= 530 nm
. . . . . . . . . . . . . .
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0.00
~/~]wl",'
r ",,1~-,
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.
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,,,,,~,
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.
,4
+
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_
,8
.
10 , ,,
.
ps
q:
1.5 ps
-0.05
I
0.5 ps
-0.10
~
! i
o,1
tk 5
: ool
*< +"-
,J
1
.
,
,
l
.
.
.
.
l
500
450
t
x.=, =S6onm:
i
i
i
l
550
,.. = 200 fs
~
-l.Ol -2 ,
.""~"-~,,,-,,~,~
~"i 4 / +...1
-0.15
, _ = 2oo~
+, = 3 o o ~
"
_
t: = 2.85 ps
, 0
.
.
. . . . . . . 2 4 6 Time l pe . I . . .
.
600
, 8
10 J
.
650
700
WavelengthX / n m Fig. 78. i:ime dependence of the bleach spectrum (480-590 nm) of the CdSe NP and the transient absorption spectrum of the surface-adsorbed naphthaquinone (NQ) radi,:al anion (600-680 nm) in the CdSe-NQ combined system in colloidal solution (fs laser excitation at 530 nm). The fact that the decay of the bleach (bottom o!: the inset) and the transient absorption of the NQ radical anion (top of the inset) have the same lifetimes strongly suggests the electron-shuttling mechanisr for the excited electron from the conduction to the valence band of the NP through the naphthaquinone on the surface of the NE Reprinted with permissio]~ from C. Burda, T. C. Green, S. Link and M. A. E1-Sayed, J. Phys. Chem. B 103, 1783 (1999). Copyright 1999 by The American Chemical Society. C d S e ) ~ , r = 530 nm
C d S Z,,,,r = 400 nm I o.o-t
0.0.
1 ~, = 560nm
-0.2-
.•"
!
!
|
t
412
~
-
1
1
bleach = 480 nm
9
+
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;
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9
._! 414 I Oo o ~ ~ 1 7 6 1 7 6 1 7 6 1 7 6 1 7 6 1 7 6 1 7 6~1177661177661177661 7 6 1 7 6 1 7 6 1 7 6
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o -0.4.
-
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~,,,,o o
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%=2.5m ~=40~
~ ~
o
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-1.0
%. =20Ors
o
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~o
30
~
0 "
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m
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10
tirne l ps ,
.
,
.
CdS %= = 30 ps
9 CdSM~ %,,== 7 ps
;~ =300 fs ~ = Z 8 5 ps
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.
,
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.
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absorption Xt=` = 6 1 0 n m
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0.0 .
0
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i 3
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,
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.
1 3
~r~/~ Fig. 79. (;omparison of the decay kinetics of the CdSe and CdSe-quinone system (left) with CdS and CdS-MV 2+ (right) suggests that different mechanisms are responsible for the bleach recovery in the respective semiconductor NPs. In CdS, charge carrier trapping leads to the bleach recovery. In the absence of MV 2+ (circles), electron trapping is rate limiting, while after addition of MV 2+ (squares), the hole trapping becomes the rate-limiting process. The forward electron transfer of MV 2+ take place within 200-300 fs, and this creates a long-lived charge-separated state (bottom). After the addition of electron acceptors on the CdSe surface, however, fast electron-hole pair recombination accelerates the bleach recovery to about 3 ps as the electron is shuttled back to the CdSe NP valence band by the quinone. The decay of the radical anion absorption (bottom) matches the bleach recovery dynamics. Reprinted with permission from C. Burda, '['. C. Green, S. Link and M. A. E1-Sayed, J. Phys. Chem. B 103, 1783 (1999). Copyright 1999 by The American Chemical Society.
256
KASI VISWANATH Vacuum
200
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[ I~ - -
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V=13V ; V=12V V=l lV V=lOV
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Fig. 81. Schematic band diagram for the bilayer devices with composite nanocrystals (bare and ZnS overcoated) embedded in a [NBPBD]~50 [NBP]I0 block copolymer, built on top of a PPV MTL. An additional barrier is involved in the device operation with ZnS overcoating. The energy levels for PPV are taken from the literature, whereas those for the nanocrystals are derived from the bulk values corrected to account for the confinement of the exciton (mostly electron) and the absorption data. The energy levels for ZnS are extrapolated from the bulk values. Reprinted with permission from H. Mattoussi et al., J. Appl. Phys. 86, 4390 (1999). Copyright 1999 by The American Institute of Physics.
///. \,,\
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Fig. 80. The EL spectra of devices using block copolymer/quantum dot hybrid films containing 60% (vol) at increasing applied voltage: (a) bare dots and (b) ZnS overcoated dots. The relative intensities of the two films at the same applied voltages indicate almost an order of magnitude higher EL from the bare dots. The inset shows the configuration of the devices used in this study. Reprinted with permission from H. Mattoussi et al., J. Appl. Phys. 86, 4390 (1999). Copyright 1999 by The American Institute of Physics.
velocities. They have found that the surface-recombination velocity was greatly dependent on the ionic-solution composition and concentration. Comparison was also made with their earlier report on CdS. By correlating with Auger spectroscopy and atomic absorption spectroscopy results on the same samples, it was concluded that the ions were chemisorbed on the semiconductor surface. Solutions of 13 , Br 2, Ag +, Hg 2+, Se zwere used in these experiments. The solubility product of the solution was thought to be one of the important parameters that determines the chemisorption and hence the recombination velocity. Ellis et al. [537], M e y e r [538], Neu [539], and M o o r e et al. [540] demonstrated the applicability of CdSe and CdS surfaces in chemical sensing. Figure 82 shows the energy-level diagram that is useful for chemical sensor applications. Adsorption of foreign atoms on the surface effects both the depletion width and surface-recombination velocity. Band bending near the surface depends on the adsorbed atoms or molecules as shown in Figure 83. Figure 84 shows the changes in PL intensity when N H 3 and CH3BBr 2 were adsorbed on CdSe surface. The aforementioned authors have also shown that CdSe sur-
faces can be used to sense the o x y g e n as shown in Figure 85. Very important application of sensing various gases is in the M O C V D reactor. A simple design for PL-based sensing by using an optical fiber is shown in Figure 86.
5.3. C d S S e Seo et al. [541] reported the time-resolved differential transmittance of C d S S e - d o p e d color-glass filters. They identified shallow and deep traps in their samples. Semiconductorglass interface states were studied by optical experiments [542-544]. The interface states arise due to surface dangling bonds and impurities [545, 546]. The excited electrons can
a)
b)
c.
CB
j
Ev
VB
VB
Fig. 82. Idealized diagram relevant to chemical sensing of energy as a function of distance from the surface of an n-type semiconductor: (a) Semiconductor-gas interface (vertical line) before establishment of thermal equilibrium in the dark. Shown are the valence band (VB) and conduction band (CB) edges of the solid (separated by the bandgap energy), the Fermi level EF, and a hypothetical distribution of surface states; (b) establishment of thermal equilibrium in the dark. Electrons in the conduction band come from the bulk to fill the surface states up to the Fermi level (shaded portion of the hypothetical surface-state distribution). This charge separation establishes an electric field in the near-surface region of the solid, represented by the bending of the band edges. Reprinted with permission from A. B. Ellis et al., J. Chem. Edu. 74, 680 (1997). Copyright 1997 by the Division of Chemical Education, Inc., American Chemical Society.
257
S U R F A C E A N D I N T E R F A C I A L R E C O M B I N A T I O N IN S E M I C O N D U C T O R S "'*
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~
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Fig. 83. ,i'.hemical sensing based on modulation of the depletion width and PL intensJ~y by surface adduct formation. The center figure represents the band betiding present in the reference ambient, corresponding to a nonemissive zon,~ or dead layer of thickness D 0. Flanking this picture to the left and right ~lre the thicker and thinner dead layers resulting from adsorption of Lewis ac ic:~ and bases, respectively, from the ambient. Reprinted with permission froH A. B. Ellis et al., J. Chem. Edu. 74, 680 (1997). Copyright 1997 by the Di~ ision of Chemical Education, Inc., American Chemical Society.
.
.
.
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.
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.
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be deeply trapped into these point defects and will recombine radiatively. Long lifetimes observed in photoluminescence ~ ere attributed to these deep levels [500, 545, 546]. Seo et Etl. [541] have shown that electron trapping at the microcr},stal-glass interfaces occurs within less than 1 ps after photoex~:itation. From the analysis of photoinduced absorp-
arm
1 K = 1 3 •
=
0
=
=
.2
9
=
.4
,A
.
.
.6
.
.
~
P(02), atm
Itm
-1
.
1
0
20
I0
-1
-1
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Fig. 85. Upper panel : changes in the PL intensity of a Co(3-MeO-salen)coated n-CdSe sample as a function of oxygen partial pressure; the reference level is nitrogen. Lower panels: left, fractional surface coverage 0 vs dioxygen partial pressure; right, double-reciprocal plot of the same data; the linearity of the plot (correlation coefficient of 99) represents a good fit to the Langmuir adsorption isotherm model, and the reciprocal of the slope yields an equilibrium constant K of 13 -t- 1 atm -~. Reprinted with permission from D. E. Moore, G. C. Lisensky, and A. B. Ellis, J. Am. Chem. Soc. 116, 9487 (1994). Copyright 1994 by The American Chemical Society.
tion, the following conclusions were made. Shallow traps give rise to short-lived photoinduced absorption with a lifetime of 60 ps, and the deep trap levels give rise to long-lived photoinduced absorption with a lifetime longer than 3.2 ns. These results were consistent with earlier published work [490, 547, 549]. Ivanda et al. [550] have measured photoluminescence and Raman spectra of deep traps in a number of SCHOTT glasses--GG400, GG420, GG435, GG455, GG475, GG495, OG515, OG530, OG550, OG570, OG590, and RG695. Here, the numbers indicate the cutoff wavelengths of different glasses. The filter glasses consist of CdSxSel_ x nanocrystal-
Time
Fig. 84. l a) Changes in PL intensity of n-CdSe resulting from alternating exposur~ 1,~ N 2 (initial response) and decreasing partial pressures of ammonia (ranging ~!'om 0.1 atm for the first trio of exposures to 0.001 atm at the end of t!]~', sequence). (Adapted from Reference [538]); (b) Changes in PL intensity ~:f n-CdSe resulting from alternating exposure to vacuum (initial response) itnd increasing pressures of CH3BBr 2 (ranging from = 3 x 10-4 atm for the l ir~t exposure to = 4 x 10 -2 atm, the last exposure). The maximum change in dead-layer thickness with exposure to the borane is 350 /k. In both experiments, PL was monitored at the PL band maximum, = 720 nm. Reprinted with permission from D. R. Neu, J. A. Olson, and A. B. Ellis, J. Phys. Chem. 97, 5713 (1993). Copyright 1993 by The American Chemical Society.
h~ --~
--~ h~ /
Semico
Fig. 86. A simple design for PL-based sensing by using a bifurcated optical fiber. Reprinted with permission from A. B. Ellis et al., J. Chem. Edu. 74, 680 (1997). Copyright 1997 by the Division of Chemical Education, Inc., American Chemical Society.
258
KASI VISWANATH
lites with diameters of a few nanometers doped in a glass matrix. Nemec and Maly [551] have published a very good paper on the temperature dependence of trap-related luminescence dynamics in CdSxSel_ x nanocrystals in glass. They attributed the luminescence due to recombination of an electron in a shallow trap with a hole in the deep trap. Carrier tunneling between localized sites was also suggested. There have been many studies on the localized carriers in glasses by nonlinear optical experiments [552-557]. 5.4. ZnS
Chen et al. [558] have identified surface states in their PL and thermoluminescence spectra of Zns quantum dots in colloidal state. 5.5. ZnSe
Jonker et al. [559] have shown that a Se overlayer on ZnSe passivates the surface. Pong et al. [560] have obtained good-quality ZnSe epilayers by hydrogenation that removed many surface defects. Wang et al. [561] reported surfacerecombination velocity of S= 5.8 x 105 cm/s for ZnSe crystals based on femtosecond time-resolved one- and two-photon photoluminescence, von Freymann et al. [562] have correlated the photoluminescence in ZnSe/ZnMgSSe quantum wells with the atomic force microscopy images, and bowtie-shaped defects on the surface were spectroscopically imaged. 5.6. ZnCdSe
The II-VI quantum-well structures are very much suitable for the realization of blue, green semiconductor lasers and light-emitting diodes [563-568]. In these materials roomtemperature excitons were observed. Because of their large binding energy excitons exist up to room temperature. Haase et al. [563] were the first to achieve an electrically pumped Zn0.8Cd0.2Se single-quantum well laser with ZnSe waveguides. The cladding layers were composed of ZnSe0.97S0.03, which was lattice matched to GaAs substrate. The II-V semiconductor lasers were also made by Grillo et al. [565]. Their laser structure also was based on the ZnCdSe quantum well with ZnSSe waveguide layers and quarternary ZnMgSSe cladding layers. These lasers were operated at room temperature under pulsed lasing conditions. However, the operating lifetime of these lasers was only one hour. Sony Corporation could make cw room-temperature quantum-well blue emitting lasers using MgZnSeS cladding layers, which had an operating lifetime of only one second. The major problem with these lasers was the nonradiative recombination due to surface and interface defects [569-571]. The degradation mechanism in these lasers has been studied by Hua et al. [570] and Guha et al. [569, 571 ]. Hovinen et al. [572] have performed very novel experiments to simulate the electrical degradation by optically injecting electron-hole pairs to induce dark defects directly in the ZnCdSe quantum well. Dark defects are the nonradiative
regions in the quantum well of the separate-confinement heterostructure (SCH). Correlation between optical experiments and transmission electron microscopy (TEM) was also shown. The stacking-fault pairs originate at or near the II-VI/III-V interface and extend through the quantum-well layer with a V-shaped cross section. In the degraded area, each stacking fault was associated with a dislocation network. By examining the dependence of degradation rate on optical injection, it was demonstrated that the fraction of excess carriers that undergo nonradiative recombination processes at point-defect locations was of great importance in determining the degradation rate. The mechanism for degradation was given as follows: The energy released in nonradiative recombination processes at defects is transferred to the lattice by multiphonon process. This nonthermal heating provides a local power supply that enhances dislocation dynamics. Spiegel et al. [573] reported the surface-recombination velocity in ZnCdSe/ZnSe and ZnSe/ZnSeS quantum wires. They studied the recombination dynamics by picosecond time-resolved photoluminescence spectroscopy. In the case of ZnSe/ZnSeS quantum wires the lifetime reduced from 130 to 10 ps as the size changed from two-dimensional reference to wires of 60 nm wide. The surface-recombination velocity for this system was 5 • 105 cm/s. By contrast, the lifetimes of ZnCdSe/ZnSe quantum wires did not show any dependence on the wire width as seen in Figure 87. The surface-recombination velocity in this case was 2 x 104 cm/s. It was concluded that the surface-recombination velocity in ZnCdSe/ZnSe quantum wires was reduced by two orders of magnitude compared to ZnSe/ZnSeS quantum wires. Exciton localization at semiconductor interfaces was thought to be the reason for the reduction in the surface-recombination velocity in ZnCdSe/ZnSe quantum wells. Reactive ion etching of ZnCdSe/ZnSe quantum wells was shown to have created nonradiative recombination centers by a study conducted by Sparing et al. [574, 575]. Later, the same group reported the origin for the degradation in the
10z
L-J
T=2K A Cdo.],~Zno.enSe/ZnSe ZnSe//ZnSeo.aeSo 14 101 ,
1(i I
,
~
I
i
i
Li[
.
,
,
I
,
10 2
, ,,I
i
I
10 3
wire width Lx [nrn]
Fig. 87. Wire-width dependence at T = 2 K of the exciton lifetime in deepetched ZnSe/Zn(Se, S) wires (stars) compared to a theoretical fit (solid line). Compared the T = 2 K data of (Zn, Cd)Se/ZnSe quantum wires are shown (triangles). Reprinted with permission from R. Spiegel et al., Phys. Rev. B 53, R4233 (1996). Copyright 1996 by The American Physical Society.
S U R F A C E A N D I N T E R F A C I A L R E C O M B I N A T I O N IN S E M I C O N D U C T O R S
1.0
~ _ ~ .
Data ......... Fit A. Unetched
0.8
or
"~ =.. r "-'
n,
24op~
B. Etched 3 rnin @ 75V
0.6.
Decay:. 240ps
_
\
"~.
c. E,=.~ a . , . o 2oov
0.4
0.2
"~""
~"*~%
9
A
0.0 0
Delay Time
(ps)
Fig. 88. {'lot of transient reflectivity data for an undamaged and two damaged Zn,,:,Cd0.25Se/ZnSe SQW samples with 50-nm cap layers9 The decay rate for th,:. 75 V samples and the undamaged sample are very similar9 There is a dram~tic decrease in the carrier lifetime for the 200-V sample (70 ps) indicating :lonradiative recombination within the QW. Reprinted with permission from L. M. Sparing et al., J. Appl. Phys. 87, 3063 (2000). Copyright 2000 by "Ihe American Institute of Physics.
same qtantum well by a combined study of photoluminescence and time-resolved reflectivity [576]. Figure 88 shows the transient reflectivity spectra for Zn0.vsCd0.z5Se/ZnSe singlequantum wells under different conditions. It was shown that etching .~tthigher voltages damages the surface and increases the surf~.ce or nonradiative recombination. Dislocations at the surface/interface were thought to be the origin of nonradiative recombi~mtion. Faschinger and Nurnberger [577] realized ZnCdSe quantum-well lasers grown on InP substrates that have a very h)iig lifetime. In these structures n-ZnMgCdSe and p-ZnMgSeTe were used as waveguide layers. Dark defects were no1: found in these diodes. The addition of Te was supposed to improve the performance of the laser. 5.7. Cd'l?e In the ,,;emiconductor industry, CdTe is a very important semiconductor because it has many applications. High-quality CdTe crystals are useful as substrates to grow epitaxial HgCdq-e, which is a well-known infrared detector material; CdTe is ~tlso useful in making X-ray and y-ray detectors, optical anct acousto-optic modultors, infrared windows, and so forth. ()ae major problem associated with CdTe is the high density ~f surface states. Several groups have tried to passivate CdTe surfaces by chemical treatments [578-583]. Sobiesierski et al. [578] have used the following chemicals: Bromine/methanol (Br/Me); (Br/Me) + KOH; (Br/Me) + hydrazine; (Br/Me) + Na2S204; ( B r / M e ) -,: K z C r 2 0 7 ; oxidizing etch followed by reducing etch. Amritharaj and Dhar [579] used the following etchants: Br2/ CH3OH; KOH/CH3OH; NazSzO4/NaOH, KzCrzOv/HNO3. Cohen et al. [582] used organic molecules to achieve high electronic-quality surfaces. These molecules were chemisorbed on the semiconductor surface as monolayers; (NH4)2S = treatment also has given good photoluminescence
259
of p-CdTe [583]; additionally, H 2 plasma [584] and atomic hydrogen [585] were found to be good passivating agents. Dharmadasa et al. [586] studied the effect of surface treatment on the Schottky barrier at metal/n-CdTe. Chou et al. [587] and Chou and Rohatgi [588] discussed the interfacial recombination in CdTe/CdS heterojunction solar cells. Illing et al. [589] reported the surface recombination at the sidewalls of CdTe/Cd~_xMgxTequantum wires for wire diameters ranging from 60 nm to 5 /xm that were fabricated by electron-beam lithography and wet-chemical etching. Picosecond time-resolved experiments were done on wires of various diameters. Kronik et al. [590] discussed the possibility of using surface photovoltage spectroscopy to study the surface states in CdTe. Cohen et al. [591] reported an unusually low surface-recombination velocity of 200 cm/s for n-type CdTe single crystals from their analysis of time-resolved photoluminescence experiments. Delgadillo et al. [592], Vargas and Miranda [593] and Bernal-Alvarada et al. [594] used the photoacoustic technique to estimate the surface-recombination velocity in CdTe [592-594]. 5.8. HdCdTe
The Hg~_xCdxTe alloy is a very important material because it has applications in infrared detectors. The HdCdTe is a narrow bandgap material, and a small density of defects or impurities can easily form a band that will become a band tail. Relatively low defect concentrations can modify the absorption edge in HdCdTe. Hydrogen passivation [595] and anodic oxidation [596] has been reported in the literature. Epitaxially grown CdTe passivating layers have given very good-quality HgCdTe photodiodes [597]; ZnS/CdTe passivation was reported by Bahir et al. [598]. The effect of hydrogenation on ZnS passivating films was examined by White et al. [599]. Borodovskii et al. [600] performed magnetoresistance measurements and estimated the surface-recombination velocity in n-CdxHgl_xTe epitaxial layers. Su and Lin [601] reported a very low surfacerecombination velocity of 300 cm/s for Hg0.sCd0.zTe surfaces. They also observed that multiple passivating layers were more effective than a single passivating layer. They have used CdTe and ZnS for the surface passivation. Voitsektnovskii et al. [602] reported the carrier lifetimes in narrow bandgap Hg]_xCdxTe epilayers that were grown by molecular beam epitaxy [602]. Excitation was done by a pulsed laser at various wavelengths. It was shown that in p-type layers the lifetime was determined by Shockley-ReadHall recombination, whereas in n-type layers Auger recombination was dominant with some minor contributions coming from other mechanisms such as surface recombination. 5.9. CdZnTe Because its lattice constant can be made to match that of the mercury cadmium telluride or HgZnTe, Cdl_xZnxTe is a very good substrate material for HdCdTe. Chen et al. [603] have passivated the deep-level defects in CdZnTe by hydrogen.
260
KASI VISWANATH
Improvement in the photoluminescence was noted. Free exciton as well as the higher orders of free exciton transitions were observed for the first time. Exciton binding energy was also estimated. Kim et al. [604, 605] used bromine solution and hydrogen for passivation, and Chen et al. [606] improved the surface quality of Cd0.9Zn0.1Te crystals by bromine treatment, which has shown very good photoluminescence properties.
5.10. HgZnTe Oh et al. [607] measured the minority carrier lifetime in n-type HgZnTe by photoconductive method. A correlation was made between the surface-passivation technique and carrier lifetime. Shockley-Read-Hall recombination as well as Auger recombination were found to contribute to the carrier decay. Surfacerecombination velocities were also estimated.
5.11. PbSe Lead selenide is a narrow bandgap semiconductor. This has applications in the fabrication of infrared semiconductor lasers and also infrared photodetectors. Because it is a low bandgap material, surface purity is very important exactly the same as in the case of CdTe. Meinke et al. [608] achieved passivation of these materials by anodic oxidation.
6. GROUP IV SEMICONDUCTORS 6.1. Si Single Crystals and Wafers Silicon is the most important of all the semiconductors because the semiconductor industry at present is mostly dependent on silicon technology. The density of integrated circuits has been ever increasing in the last twenty years and very large-scale integration (VLSI) technology has been realized. In the near future we will have ultralarge-scale integration (ULSI) technology. In this case the size of the semiconductor will have quantum limits. When the semiconductor device is very small surface recombination becomes extremely important. Therefore, the Si surface must be very clean and free from any contamination. Linnros [609, 610] has reported the carrier lifetimes and surface-recombination velocities in silicon wafers. An optical pump-probe method was used and free-carrier absorption transients were analyzed. Schmidt and Aberle [611] described an easy method of surface passivation and monitoring of surface-recombination velocities for p- and n-type silicon wafers. Keskitalo et al. [612] measured the Shockley-ReadHall lifetime in p-type silicon. They have also examined the effect of temperature and injection level on the lifetimes. A novel photoluminescence surface-state spectroscopy for the measurement of surface-recombination velocity in silicon wafers was attempted by Saitoh et al. [613]. A multicolor method for the determination of surface and bulk recombination in Si was discussed by Ostendorf and Endros [614]. In another paper, surface-recombination velocity in Si crystals was estimated by photoluminescence time-resolved spectroscopy [615]. Eichinger and Rommel [616] used a two-color
technique to identify metals on a Si surface. Two diode lasers with different absorption depths in silicon were used to estimate the surface-recombination velocities. The electrolytic metal analysis tool (ELYMAT) technique makes it possible to identify trace metals and oxides on the semiconductor surface. The surface-photovoltage technique was reported by Brubaker et al. [617] who measured surface-recombination lifetimes in silicon, and the recombination centers were identified as interfacial bonding defects and iron contamination. A pump-probe optical method was used by an Italian group to study bulk and surface recombination lifetimes [618]. The dependence of S on excitation light intensity was theoretically shown [619]. It was also shown that shifting the surface Fermi level towards band edges reduces the S value. Photoconductive decay in Si solar cell wafers was numerically simulated by Wang and Ciszek [620]. Bi-surface photoconductive decay measurements were useful in the evaluation of S [621]. Schieck and Kunst [622] conducted microwave photoconductivity experiments that enabled them to find the injection-level dependent surface/interface-recombination velocity. Yablonovitch et al. [623] reported a very low surface-recombination velocity in silicon. A very large number of papers have been published on the passivation of silicon dangling bonds by a variety of treatments such as hydrogen [624-628], hydrogen fluoride [629-636], ammonium fluoride [637-645], silicon nitride films [646-655], SiO 2 films [656, 657], halogens [658-660], silane/helium [661], cyanide [662], sulfur [663], C60 monolayer [664, 665], copper [666], contact bonding [667], rare earth oxides [668], low-temperature oxidation prior to UV radiation [669], and laser annealing [670]. The effect of metallic impurities on surface-recombination velocity and carrier lifetimes has been reported by a number of research groups [671-679]. Tajima and Ibuka [680] and Tajima et al. [681-683] have shown the sensitivity of photoluminescence to the microdefects and interfacial quality of silicon-on-insulator (SOI) wafers [680-683]. The SOI structure is important for the development of low-power and high-speed integrated circuits. Also, SOI layers are used in the fabrication of complementary metal-oxide-semicondcutor (CMOS) devices. Ultrathin silicon-on-insulator layers were grown by the forementioned authors and they observed the luminescence due to electron-hole liquid (EHL) in these wafers. The phase diagram of electrons and holes in Si is shown in Figure 89, which was given originally by Shah et al. [684]. As exciton density increases, the excitons dissociate into plasma, which is described by Mott transition [685]. If the sample temperature is lowered, then the excitons condense to form electron-hole liquid. As can be seen in Figure 89 the coexistence of free excitons, electron-hole plasma and electron-hole liquid depends upon the sample temperature and excition density. The luminescence observed in SIMOX wafers was attributed to electron-hole liquid as shown in Figure 90. Interfacial microdefects were thought to be responsible for the luminescence.
SURFACE AND INTERFACIAL RECOMBINATION IN SEMICONDUCTORS
Fig. 89. ]'hasediagram of electrons and holes in Si: FE, EHP and EHL, are analogou..; :~ gas, plasma, and liquid phase, respectively. In the hatched region EHL coexists with FE or EHE This phase diagram was originally reported by Shah et at. i~eprintedwith permission from J. Shah, M. Combescot, and A. H. Dayem, t't~vs. Rev. Lett. 38, 1497 (1977). Copyright 1977 by The American Physical S,:ciety. Bedrc,~sian and de la Rubia [686] have observed smoothing of surfaces of Si(100), which were irradiated with 5-keV He ions at 130 K and then annealed at 160 K. Smoothing of Si surfaces ~,as interpreted as resulting from surface recombination of p~fint defects. The point defects were supposed to have been ger~erated when the semiconductor was irradiated below the surl'a.ze, which will migrate to the semiconductor surface at 110 K Smoothing was also found upon irradiation with Ar ions at 1 l0 K.
6.2. Si Solar Cells In a solar cell solar energy is converted into electricity. The developn~ent of solar cell materials and devices has been dominated by the following: Green at the University of New
Fig. 90. Two types of excitation intensity dependence of condensate emission from $IMOX wafers at 16 K under UV light excitation. The EHL emission appea~s at low excitation intensity in the first type (a), while evolution of EHP with ~xcitation intensity is recognized in the second type (b). Reprinted with permission from M. Tajima, S. Ibuka, and M. Warashina, Mat. Sci. Forum. 255-263, 1731 (1997). Copyright 1997 by Trans Tech Publications, Switzerland.
261
South Wales in Australia, Rohatgi at Georgia Tech, Ahrenkiel at NREL, Colarado, and Aberle and Hezel at the Institute for Solar Energy Forschung (ISFH) in Germany. Green has written several reviews and books that are very useful for beginners as well as experts [687-692]. Hezel's review covers the metal-insulator-semiconductor (MIS) solar cells [693]. A book on solar cells has been written by Aberle [694], currently at the University of New South Wales in Australia. In this section we will discuss only the latest developments in surface passivation and its effect on increasing the efficiency of solar cell. The most important factor of a solar cell is its conversion efficiency. The efficiency of a solar cell is limited by the surface and interfacial recombination in the semiconductor. Therefore, logically the efforts to improve the performance of the solar cell devices have been directed to reducing the surface-state density. Several groups around the world have reported on the surface recombination and surface passivation in silicon solar cells [695-715]. Aberle et al. [696] developed the passivated-emitter rear locally diffused (PERL) silicon solar cell shown in Figure 91. This solar cell has given 24% efficiency. In this structure, a S i O 2 layer was used on both sides of the p-silicon, which passivates front and rear surfaces. Inverted pyramids were designed to reduce reflection losses and increase the light trapping. Robinson et al. [695] have also done simulation of Shockley-Read-Hall recombination and its effect on I-V characteristics of the silicon solar cell [695]. Figure 92 shows the schematic of solar cell structure used for their theoretical work. Surface-recombination velocities below 10 cm/s were reported by Stephens et al. [714] by utilizing a thermally grown SiO 2 layer. By incorporating an A1 cap layer the surface-recombination velocity of 20-50 cm/s was observed at an injection level of 1014 c m -3 [715]. Silicon nitride was later found to be a very good passivating layer and record low surface-recombination velocities of 4 and 20 cm/s were observed for 1.5 and 0.7 ~ cm p-silicon wafers, respectively [649]. Dependence of S on the carrier concentration for a ptype Si wafer is shown in Figure 93. Bifacial silicon solar cells were later developed in which surface recombination was
Fig. 91. Passivatedemitter, rear locally diffused cell (PERL cell). Reprinted with permission from A. G. Aberle et al., Prog. Photov. 1, 133 (1993). Copyright 1993 by John Wiley & Sons.
262
KASI V I S W A N A T H
front metal contact X~
f~
. . . . . . . . .
Ti-Pd-Ag front contact
.,,///anti-reflection coating
Silicon nitride
front surface
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rear metal contact'
Fig. 92. Schematic diagram of the basic solar-cell structure used in the computer simulations. Defects are introduced into this structure at particular locations to investigate their impact on the I-V curves. Reprinted from S. J. Robinson, A. G. Aberle, and M. A. Green, J. Appl. Phys. 76, 7920 (1994). Copyright 1994 by The American Institute of Physics.
reduced in both surfaces [650]. The schematic of this cell is shown in Figure 94. In a bifacial silicon solar cell sunlight that was incident on both front and rear surfaces will be converted into electricity. Therefore, the conversion efficiency will be high. In these structures silicon nitride layers were used as surface-passivating layers. A simplified energy-band diagram of a Si-SiN interface is shown in Figure 95. Karazhanov [716] has discussed the degradation mechanism in silicon solar cells. Carrier lifetimes and short-circuit current were shown to decrease with increasing illumination intensity.
Silicon nitride AI rear contact Fig. 94. Schematic representation of a bifacial p-n junction silicon solar cell developed at ISFH, Germany. Both surfaces of this cell are passivated by RPECVD silicon nitride. Reprint with permission from T. Lauinger et al., J. Vac. Sci. Technol. A 16, 530 (1998). Copyright 1998 by The American Vacuum Society.
hydrogenated-fluorinated amorphous silicon [717-720]. When a semiconductor is excited optically the generated excess carriers interact with phonons and dissipate their excess energy by three channelsmhot-carrier thermalization to the band edges, capture by midgap states, and nonradiative recombination. In crystalline semiconductors band-filling effects give rise to photoinduced bleaching. In amorphous semiconductors the relaxation of k-vector conservation enhances the absorption
6.3. A m o r p h o u s Si Insulator charges
Amorphous Si has important applications in many semiconductor devices such as solar cells. There are some differences between crystalline Si and amorphous Si in the optically induced processes. Ackley et al. [717], Vardeny and Tanc [718], and Vardeny et al. [719, 720] studied the picosecond photoinduced absorption due to the midgap states in nonhydrogenated, hydrogenated, fluorinated, and
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SiN~
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(< 2 nm) Fig. 95. Simplified energy-band diagram of the Si-SiN x interface (band bending not shown). Note that the positive insulator charge Q/consists of two components, namely a constant contribution from the Pox-like defects in the interfacial SiNxOy film and an operating condition dependent contribution from the charged K+ centers within a ~20-nm-wide section of the SiN x film. Reprinted with permission from A. G. Aberle, "Crystalline Silicon Solar Cells," Center for Photovoltaic Engineering, University of New South Wales, Sydney, Australia, 1999.
263
S U R F A C E A N D I N T E R F A C I A L R E C O M B I N A T I O N IN S E M I C O N D U C T O R S o- si (EV)
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cross se~:tion of excited carriers, which leads to photoinduced absorpti~Jn. Figures 96 and 97 show the photoinduced absorption of a-Si prepared under different conditions and at different temperat~res, respectively. The photoinduced absorption in a-Si was interpreted as due to midgap states9 Staebter and Wronski [721] were the first to observe the degradation in photoconductivity in a-Si after it was exposed to light ~or a long time. This effect has deleterious effects on solar cells made with a-Si. Light-induced defects in a-Si were als(~ found in electron-spin resonance [722], photoluminescence [723], transient photoconductivity [724], and transient grating spectroscopy [725]. Strait and Tauc [726] have performed photoinduced transient absorption experiments on a-Si:H, which was illuminated for different time periods. The spectra recorded at 300 and 150 K are shown in Figures 98 and 99. ~:espectively. The illumination of the semiconductor produces defects that act as deep traps. From the analysis of
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264
KASI VISWANATH
the results it was concluded that the defects were Si dangling bonds. Many groups have reported on the surface and interface states in hydrogenated a-Si [727-729]. Guha et al. [727] interpreted the light-induced changes in a-Si as due to recombination rather than carrier trapping. Jackson et al. [728] arrived at the silicon dangling bond density as 1012/cm 2 by comparing the defect absorption with dangling-bond spin densities. Lower interface state densities were observed in the a-Si layers that were treated with chemicals or oxygen-containing plasmas [730]. Tsuo et al. [731] fabricated p-i-n solar cells of a-Si and hydrogen plasma treatment has improved the efficiency of the solar cells. Atomic hydrogen from plasmas was found to be very effective in passivating the defects in a-Si [732]. In these studies Sill bonds were observed in the IR (infrared) spectra after hydrogen treatment. Jang and Lim [733] achieved improved performance of the solar cell in which hydrogentreated boron-doped hydrogenated amorphous silicon carbide (a-SiC:H) film was used as a p-type passivating layer. In this solar cell a p-i-n-type structure was used, which consists of amorphous silicon. Georgiev et al. [734] performed the numerical modeling on the effect of surface recombination in a-Si solar cells. Experimental and simulated results on the effect of defects in a-SiGe solar cells were reported by Zimmer et al. [735].
6.4. Si-based Quantum Wells and Superlattices Quantum wells and superlattices of Si/SiGe have possible applications in fabricating room-temperature light-emitting diodes and high-speed devices. Gnutzmann and Clausecker [736] were the first to propose the concept of zone folding in semiconductors. They have shown that the minimum of the conduction band of an indirect bandgap semiconductor can be folded back to the F point by creating artificial periodicity in the structure. After this proposal, several groups attempted to make quasi-direct bandgap structures based on Si. Arbert-Engels et al. [737] have grown SimGen superlattices by moleculer beam epitaxy. They passivated the superlattices with hydrogen, which enabled them to observe improved photoluminescence properties. St. Amour et al. [738] have shown that passivation can increase the photoluminescence intensity in Si~_xGex/Si heterostructure by an order of magnitude as shown in Figure 100. Usami et al. [739] have attributed the luminescence in Si~_xGe x heterostructure to interface located excitons. A similar explanation was given by Turton and Jaros [740] for the luminescence recorded for Si-Ge superlattices. Galeckas et al. [741] reported a surface-recombination velocity of S = 1.5 • 10 4 cm/s for a SiGe/Si strained-layer superlattice. These workers have done picosecond scale pump-probe transient reflection and dynamic gratings experiments.
6.5. Porous Si When single-crystal silicon was anodized by electrochemical method, porous silicon (PSI) was obtained [742, 743]. Photoluminescence in porous silicon was demonstrated by Can-
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265
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optical absorption coefficient of porous silicon by considering phonon-,~ssisted transitions in the indirect bandgap Si. Moyer et al. [773] reported time-resolved spectroscopy of porous silicon. Experiments were done for different excitation wavelengths and excitation intensities. Figure 101 shows the time-resolved data for three different excitation wavelengths. If the radiative recombination is due to quantum-confined states, then we should expect a decrease in radiative lifetime as the excitation energy increases. Because this was not observed and also because the lifetimes were very long, the recombination was thought to be due to surface states. Stalmans et al. proposed that porous silicon can be used as a very good passivating layer in silicon solar cells. Enhanced PL efficiencies in porous silicon have been observed by various groups after surface passivation was done by different methods--hydrogen [775-779], HF [780, 781], annealing and rapid oxidation [782], wet thermal oxidation [783], thermal treatment in NH 3 [784], acids, methanol, iodine [785], and copper ions in solution [786]. Pavesi et al. [787] have fabricated light-emitting diodes based on porous silicon as the active layer. They attempted different configurations to examine the effects of interface states on the efficiency. They found that the performance of the n-Si/porous silicon LED structure was better than the metal/porous silicon structure. This was explained by considering the band diagrams of these structures, which are shown in Figure 102. In the metal-PS device, many interface states will be present that will pin the Fermi level at the midgap. The interface of metal/PS acts as an ohmic contact and electrons are injected into midgap levels. These electrons have to gain energy to move into the conduction band before radiative recombination with holes in the valence band. On the other hand, n-Si/PS acts as a standard nip heterojunction. In this case the electrons are injected directly into the conduction band of porous silicon and then they recombine radiatively with holes. There are no midgap levels in the
Fig. 102. Schematic band structure diagram of a metal/PS LED device (top panel) and of the test device (bottom panel). Here CB and VB refer to the conduction and valence effective bands, which roughly approximate the lowest empty and the highest filled quantum confined states of the various quantum dots which form porous silicon. Reprinted with permission from L. Pavesi et al., J. Appl. Phys. 86, 6474 (1999). Copyright 1999 by The American Institute of Physics.
n-Si/porous silicon as it is a better interface. In fact, this model was first discussed by Ben-Chorin et al. [788].
6.6. Si/SiO2 Interfaces The Si/SiO 2 interface is the most important material system in semiconductor technology because it is used in a number of devices, for example, metal-oxide semiconductor field-effect transistors (MOSFETs), high-efficiency solar cells, complementary metal-oxide semiconductor (CMOS) devices, and in the modern integrated circuit technology. There are several reviews and edited volumes that discuss the importance of the high-quality Si/SiO2 interface required to make reliable semiconductor devices [789-795]. Yablonovitch et al. [796] studied the electron-hole recombination at the Si-SiO 2 interface by optical spectroscopy. The surface-recombination current was measured as a function of the surface density of electrons and holes. It was noted that the surface states that have electron capture cross section 100 times greater than the hole capture cross section dominate the interface recombination. The Shockley-ReadHall recombination model was considered to analyze the data. At higher carrier densities bandgap renormalization was observed. Surface-recombination velocities of electrons and holes were found to be S, = 330 cm/s and S p - - 1080 cm/s, respectively. Chen et al. [797, 798] deposited very thin SiO 2 layers on Si wafers by plasma-enhanced chemical-vapor deposition (PECVD). After rapid thermal annealing (RTA), very high
266
KASI V I S W A N A T H
cartier lifetimes >5 ms and very low surface-recombination velocity <2 cm/s were observed in these samples. Interface recombination lifetimes were measured by the photoconductive decay method and the experiments were done for different thicknesses (L) of the sample. Figure 103 shows the plot of I/teef as a function of 1/L from which the value S = 2 cm/s was estimated. The bulk recombination lifetime ~'b was found to be 23 ms. Surface-recombination velocity S is a function of Si/SiO 2 interface-state density, positive-charge density in the oxide, carrier injection level, and capture cross section for electrons and holes [799, 800]. The low S value observed by Chen et al. [797, 798] could be due to either low interface state density or high density of positive charge in the PECVD oxide or a combination of both. If the positive-charge density is larger than the interface-state density, then the energy band bending will be downward. This will increase the difference in concentrations of electrons and holes, which leads to reduced interface recombination. Figure 104 shows the dependence of surface-recombination velocity on positive charge in the oxide. Very high oxide charge was not good for the integrated circuits as it effects the threshold and breakdown voltage, but the low surface-recombination velocity was thought to be excellent for the high-efficiency silicon solar cells; F 2 treatment was used by a Japanese team to passivate the interface states in Si/SiO 2 [801]. Injection-level dependence of surface-recombination velocity in Si/SiO2 interface was examined by Aberle et ah [799, 802, 805] and Glunz et al. [803, 804 808]; Seff was found to decrease with increasing injection level for highly doped p-type silicon. However, Seef decreased for both low-doped p-type Si and n-type Si as shown in Figure 105 [804]. Aberle et al. [805] have reported the effect of oxide parameters on the characteristics of Si/SiO 2 interface. Ghannam and Mertens [809] and Girisch et al. [810, 811] have also thoroughly studied the interface-recombination parameters in Si/SiO 2, which was optically illuminated.
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Many groups have shown that in a metal-oxidesemicondcutor (MOS) structure interface states are created either due to hot-electron transport through the oxide film or electron-hole recombination at the surface [812-822]. Buchanan [823] has measured interface-state density as a function of electron injection in Si/SiO2 interfaces. This is shown in Figure 106. The Si dangling bond will be created at the Si/SiO 2 interface [792] due to hot electron stress [824] or irradiation [825]. Poindexter et al. [792] have called these dangling Si bonds Pb centers. Several workers have reported on the nature and mechanism of creation of the silicon dangling bonds in Si/SiO 2 interfaces [824-837]. It was proved that hydrogen is responsible for the degradation of metal-oxidesemiconductor (MOS) structures [829-831]. Under electrical stress, the hydrogen, which was present in the device, will be released by hot carriers. The following reaction was proposed [828, 830]: PbH + H --+ Pb -k-H2
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where PbH is the hydrogen-passivated silicon dangling bond. Several scientists have agreed on the hydrogen-release model given in the preceding based on various experiments [831-836]. Cartier and Stathis [837] have shown that the silicon dangling bonds were passivated during hot electron stress, and the creation of unidentified defects was observed. This degradation was found to be very similar to that observed due to hydrogen plasma. Based on this, the authors have suggested that the hydrogen-release model was correct. It was also proposed that the silicon dangling bond was not the dominant interface defect. Devine et al. [838] have performed the degradation studies by hot-electron injection in metal-oxide-semiconductor transistors. The devices were annealed in H 2 or D. The results have indicated that D 2 was better than H 2 in improving the lifetime of the device. These findings were in agreement with the report by Lyding et al. [839]. In other reports, the microstructure [840] and mechanism for the generation of the interface-state precursor [841] were discussed. In the interfacial region of Si and SiO 2 there will be various kinds of silicon dangling bonds and bond distortions. This gives rise to a very broad distribution of interface states with different energy states. This was confirmed by a variety of experiments such as capacitance-voltage (C-V) and surfacephotovoltage measurements [842-846]. Femtosecond lasers have been used to study the interfacialrelated recombinations and carrier-trapping processes in Si/SiO 2 [847-861]. Lupke et al. [847], Bloch et al. [848], Mihaychuk et al. [849], and Shamir et al. [851, 860] have published a series of very interesting papers in which this approach was used. They have developed novel methods based on second-harmonic generation due to electric fields generated at the interfaces, which enabled them to understand charge trapping and detrapping phenomena. In addition, Shamir et al. [860] studied the charge trapping and
detrapping in Si/SiO 2 interfaces by using femtosecond laserinduced multiphoton-photoemission (MPPE) phenomena. This is shown schematically in Figure 107. Very intense laser radiation with photon energy 1.55 eV transfers electrons from silicon via three-photon or four-photon photoemission. The electric field generated can be monitored by electric fieldinduced second harmonic generation (EFISH) or multiphoton photoemission (MPPE). As shown in Figure 108 the photoemission current signal did not come to its original value, which indicated that some of the traps have a very long lifetime. The samples used are (a): 15 Sn : 1.5 nm oxide grown at 85 K on a low-doping n-Si substrate; and (b) CLn : the oxide of 15 Sn that was removed by heating at 1400 K. Wang et al. [861] observed a new and unusual enhancement of electric field at the Si/SiO 2 interface after the laser was switched off. The interface electric fields were monitored by optical second-harmonic generation. This work was part of their major program to understand fundamental physics dealing with carrier dynamics and recombination processes at semiconductor interfaces. Figure 109 shows the enhancement of second harmonic generation after the laser was switched off. The results were explained by considering multiphoton hole injection into the oxide. The electron and holes were also thought to have distinct trapping and detrapping behaviors. The model proposed was given in Figure 110. Wu et al. [862] have found that the interface-state density increased with temperature in the Si-SiO 2 system. Leakage currents observed in metal-oxide-semiconductor (MOS) devices were thought to be due to trap-assisted tunneling and recombination in oxide traps [863]. In a series of elegant papers, Stesmans [864-867] reported the passivation of different silicon dangling bonds in Si/SiO 2 by hydrogen. He has utilized electron spin resonance spectroscopy to understand the electronic structure of the defects before and after passivation by hydrogen. Mishima et al. [868] performed spin-dependent ..... 4P/PE
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Fig. 107. Schematic diagram of the multiphoton photoemission (MPPE) processes. Reprinted with permission from N. Shamir, J. G. Mihaychuk, and H. M. van Driel, J. Appl. Phys. 88, 896 (2000). Copyright 2000 by The American Institute of Physics.
268
KASI VISWANATH
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.
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Fig. 108. Photoemission current (PEC) measurement sequences before, during, and following exposures to various oxygen pressures (shifted in time, for convenience) for; (a) 155 Sn:A-beam off, B-gas admission. C-beam on, D-gas pumping; and (b) CLn, where the dashed area gives the range of direct filling saturation for different spots on this sample. Reprinted with permission from N. Shamir, J. G. Mihaychuk, and H. M. van Driel, J. Appl. Phys. 88, 896 (2000). Copyright 2000 by The American Institute of Physics.
Fig. 110. Schematic diagrams of a three-photon electron and a four-photon hole transfer process from silicon into an ultrathin oxide. For simplicity only the flat-band condition is shown in the top diagram. Reprinted with permission from W. Wang et al., Phys. Rev. Lett. 81, 4224 (1998). Copyright 1998 by The American Physical Society.
recombination spectroscopy on Si/SiO2 and identified two types of silicon dangling bonds, called as Pbl and Pb0 as shown in Figure 111. There were many earlier reports on spindependent recombination involving interface defects in the Si/SiO 2 system [869-875]. Mishima's spin-dependent recomb i n a t o n s p e c t r o s c o p y e x p e r i m e n t s h a v e c o n f i r m e d the exist e n c e of energy levels of Pbl centers in the silicon bandgap. This result contradicted the reports by Stesmans and Afans'ev
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Fig. 109. Time-dependent SHG signal from a 40-,~ thermal oxide grown on a p-type Si(001) substrate, where A~ represents the beam-on saturated SHG signal and A2 represents the dark field enhanced SHG signal. The inset shows the TDSHG signal from the same wafer after the oxide has been etched down to 10/~ thickness. Reprinted with permission from W. Wang et al., Phys. Rev. Lett. 81, 4224 (1998). Copyright 1998 by The American Physical Society.
Fig. 111. Spin dependent recombination spectra illustrating the Pbo and Pb~ spectra for both 28Si (nuclear spin-zero) and 29Si (Nuclear spin-I/2) sites. The scan was taken at a gate voltage of -0.45 V. Although the two variant line shapes are not resolvable in the center line, both are clearly present in the 29Si hyperfine (hf) satellite peaks. Reprinted with permission from T. D. Mishima, P. M. Lenahan, and W. Weber, Appl. Phys. Lett. 76, 3771 (2000). Copyright 2000 by The American Institute of Physics.
SURFACE AND INTERFACIAL RECOMBINATION IN SEMICONDUCTORS [876, 877]. However, it was in agreement with the proposal given by Gerardi et al. [878]. Law et al. [879] have identified interstitial-related recombination in Si/SiO 2 interface and thus have clarified many controversies in the literature about this topic. A model for surface-recombination velocity of silicon interstitials in Si/SiO 2 interfaces was discussed by Tsamis and Tsoukalas [880] and Tsoukalas and Tsamis [881]. Interface-state generation was found to be dependent on temperature [882].
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The early work on surface recombination in Ge has been reviewed by Aspnes [12] and Fitzgerald and Grove [13]. Norton's group has reported on the efficient passivation of Ge surface by sulfur [883]. From Auger spectroscopy they ha~e concluded that S atoms were bonded to the Ge atoms. Timoshenko et al. [884] have developed an in sutu photoluminescence technique to characterize passivated surfaces of Ge.
269
substrate
Fig. 112. Schematic illustration of the vertical structure of visible a-SiC:Hbased p-i-n thin film light-emitting diode (TFLED). Intrinsic 50-mm-thick a-SiC:H film with bandgap of 3.0 eV was used as the important luminescent active layer. The p-/~c-Si:H and n-/zc-Si:H layers were used as hole and electron injectors into the luminescent active i layer, respectively. Intrinsic a-SiC:H layer of 1.5 nm with a wide bandgap of 3.3 eV was used to increase the efficiency of hole injection into the luminescently active i layer. Reprinted with permission from J. W. Lee and K. S. Lim, J. Appl. Phys. 81, 2432 (1997). Copyright 1997 by The American Institute of Physics.
7. GR()UP IV BINARY SEMICONDUCTORS 7.1. SiC
One of the best candidates for high-temperature and high-power electronics is SiC, which is a wide bandgap semiconductor. It also has high thermal conductivity, high electron-saturation velocity, very high breakdown voltage, and excellent physical stability. These properties make it suitable for several applications. Depending on the stacking order, SiC exists with either zinc-blende (cubic) or wurtzite (hexagonal) crystal structure. Because of the large bandgap SiC is very much suitable for light-emitting diodes in the visible region. Recent efforts have been centered around the development of thin-film lightemiting diodes (TFLEDs) with the idea of using them for flat panel displays. For this purpose hydrogenated amorphous silicon carbide (a-SiC:H) was utilized with p-i-n structure [885-8871]. Lee and Lim of South Korea [888, 889] have performed hydrogen passivation and observed improved characteristics of the LED. Figure 112 shows the schematic of an LED structure based on SiC. The diode was fabricated on a tin oxide (SnOz)-coated glass substrate by a photochemical vapordeposition system; p- and n-type hydrogenated Si layers were used as hole and electron injectors into the active layer. The amorphous SiC was the active luminescent layer. After hydrogen passivation, improved electroluminescence was observed as shown in Figure 113. After hydrogen treatment the luminescence spectrum has shifted to higher energy and also narrowed down considerably. This was explained by the energy-level diagram shown in Figure 114. After passivation midgap levels decrease considerably and the band tails become sharper. Passivation also improved the brightness of the LED as shown in Figure 115. Surface-recombination velocity in SiC epilayers was reported by Grivickas et al. [890]. Wahab et al. [891] have
observed improved electrical performance of SiC Schottky diodes after hydrogen annealing. Afans'ev et al. [892, 893] and Afans'ev and Stesmans [894] studied the origin of interface defects in the SiC/SiO 2 system; SiC is very much suitable for the fabrication of metal-oxide field-effect transistors (MOSFETs) because the insulating SiO 2 layer can easily be grown on SiC by thermal methods [895-898]. Afans'ev et al. [892, 893] and Afans'ev and Stesmans [894] identified interface traps in SiO2/SiC by conducting a number of studies, such as low-temperature electrical measurements and photon-stimulated electron-tunneling experiments. It was
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270
KASI VISWANATH
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suggested that surface imperfections at the interface were responsible for the degradation of the SiC-based devices. Xu et al. [899] and Lai et al. [900] have observed improved interface properties of the SiO2/SiC material system after nitridation.
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8. CONCLUSIONS Surface and interfacial recombination studies in semiconductors were reviewed in this chapter. If a semiconductor surface has defects such as vacancies, it creates dangling bonds. These vancancies have energies in the bandgap of the semiconductor. Foreign impurity atoms also have midgap energies. These trap states participate in nonradiative recombination with electrons and holes. The nonradiative recombinations have detrimental effects on the performance of the semiconductor device and shelf life of the device. Fermi-level pinning due to surface states was discussed. Shockley-Read-Hall recombination in the framework of Stevenson-Keys formalism was reviewed. This gives the theoretical basis to understand the surface and interface recombination in semiconductors. Unpinning of the Fermi level by a number of surfacepassivation techniques were described. The improvement in the quality of surfaces can be evaluated by photoluminescence techniques. With the tremendous advances in femtosecond laser technology and detection systems such as streak cameras, it is now possible to evaluate the quality of the semiconductor surface very accurately. Measurements of surfacerecombination lifetimes and surface-recombination velocities in a number of systems have been elaborated. In the discussion part some importance has been given to very novel techniques such as near-field scanning optical microscopy (NSOM), microwave-modulated photoluminescence (MMPL), photoacoustic methods, electric field-induced second-harmonic generation (EFISHG), multiphoton photoemission (MMPE), and so forth. Also discussed were pump-probe spectroscopy and wave-mixing spectroscopy. Using these techniques surfaceelectron recombination and electron shuttling in picosecond timescales could be observed. There are tremendous opportunities now to understand the electronic structure of semiconductor surfaces by advanced laser techniques that was not possible two decades ago. These research efforts will enable us to understand the basic physics of semiconductor surfaces thus enabling the design of better materials. Growth of GalnP is one such example. By controlling the order and disorder on the surface, the required bandgap could be achieved. This very novel area can be called bandgap engineering by surface reconstruction. In the future it should be possible to make a totally new class of superlattices and quantum wells by this method. This chapter has attempted to summarize the work done in the last 25 years on surface-related recombinations in semiconductors. Most of the work has been done on GaAs and Si. Also discussed was surface and interface recombination in a number of devices, such as light-emitting diodes, semiconductor lasers, metal-oxide semiconductor field-effect transistors (MOSFETs), metal-insulator-semiconductor (MIS) devices, solar cells, and so forth. Improvement in the performance characteristics of various devices fabricated with a number of semiconductors has been achieved by different passivation techniques. In conclusion, surface and interface recombination is a very important problem in semiconductor technology. With
S U R F A C E A N D I N T E R F A C I A L R E C O M B I N A T I O N IN S E M I C O N D U C T O R S
the advanced laser technology that is available today the electronic structure of surface states has been undertsood to a great extent. Surface-passivation methods were useful in controlling the surface Fermi level. In the future it should be possible to achieve e~tremely pure semiconductor surfaces that are necessary for ultralarge-scale integration (ULSI) technology.
ACKNOWLEDGMENTS The original work of the author discussed in this chapter was done in Japan and South Korea. Surface-recombination studies in GaAs quantum wires have been done at the Advanced Microelectronics Center of Hitachi Central Research Laboratory, Tokyo, Japan. I would like to thank Dr. Shorijo Asai, the then Deputy General Manager, Hitachi, and currently Managing Director of Hitachi, for the invitation to come to Japan. It is a great pleasure to thank Dr. Toshio Katsuyama, the group leader, for the GaAs quantum wire project, and Dr. Kensuke Ogawa, senior researcher at Hitachi, who is currently at Femtosecond Technology Research Association at Tsukuba, for their wonderful collaboration. The author wishes to acknowledge the help received from them in using the femtosecond laser facility and for introducing him to the field of optical phenomena in semiconductors. He would also like to thank Dr. Toshikazu Shimada, Manager of the Advanced Microelectronics Center at Hitachi and General Manager of Hitachi Cambridge Laboratory, Cambridge, England for his constant encouragement. Thanks are due also to Dr. M. Yazawa and Dr. K. Hiruma for fabrication of free-standing GaAs quantum wires, Dr. M. Miyao, Department Manager and Dr. Nakamura, General Manager of Hitachi Central Research Laboratory for their support, and to Prof. Ryoichi Ito, Professor of Applied Physics, University of Tokyo, who is currently at Meiji University, for many stimulating discussions on semiconductor optical physics. This work was financed by the R & D Association of Future Electron Devices, as part of Basic Technology for Future Industries supported by NEDO (New Energy and Industrial Development Organization). The studies on nonradiative recombination due to interface defects in InGaN multiple quantum wells were done in the National Creative Research Initiative Center for Ultrafast Optics Control, Korea Research Institute of Standards and Science (KRISS), Taejon, South Korea. The author would like to thank the brain pool program of South Korea for the invitation to Korea, where he worked for three years. It was indeed a great opportunity to have collaborated with Dr. Dongho Kim who provided one of the best laser laboratories in the world. Thanks are due to Dr. Jooin Lee for his cooperation on the picosecond recombination dynamics studies in GaN materials, Dr. Sunkyu Yu, Dr. Yoonju Shin, Dr. S.C. Jeoung, and Dr. Nam Woong Song for their help in the laser laboratory. It is a great pleasure to thank Prof. Howard H. Patterson, University of Maine, Orono, Maine, U.S.A. with whom I have worked for several years. He was the first to introduce me to
271
the field of optical phenomena in the solid state, lasers, laser spectroscopy, and photonics. I also thank my wife Mrs. Annapurna Viswanath and son Mr. Srinivas for their understanding and support during the course of experimental investigations when I was working in Japan and South Korea as well as during the write up of this review.
REFERENCES 1. A. S. Grove, "Physics and Technology of Semiconductor Devices," Wiley, New York, 1967. 2. A. Heller, Acc. Chem. Res. 154, 154 (1981). 3. E. Ellis and T. S. Moss, Soild State Electron. 13, 1 (1970). 4. H. Kressel, in "Semiconductors and Semimetals," Vol. 16, R. K. Willardson and A. C. Beer, eds., Academic Press, New York, 1981, p. 1. 5. S. K. Gandhi, "Semiconductor Power Devices," Wiley-Interscience, New York, 1977. 6. B.J. Baliga, "Modern Power Devices," Wiley-Interscience, New York, 1987. 7. W. Shockley, "Electrons and Holes in Semicondcutors," Van Nostrand, New York, 1959. 8. A. Many, Y. Goldstein, and N. B. Grover, "Semiconductor Surfaces," North Holland, Amsterdam, 1965. 9. W. Shockley and W. T. Read, Phys. Rev. 87, 835 (1952). 10. R. N. Hall, Phys. Rev. 87, 387 (1952). 11. D. T. Stevenson and R. J. Keyes, Physics 20, 1041 (1954). 12. D. E. Aspnes, Surface Sci. 132, 406 (1983). 13. D. J. Fitzgerald and A. S. Grove, Surface Sci. 9, 347 (1968). 14. S. M. Sze, "Physics of Semiconductor Devices," John Wiley, New York, 1981. 15. I. Lindau, P. W. Chye, C. M. Garner, P. Pianetta, C. Y. Su, and W. E. Spicer, J. Vac. Sci. Technol. 15, 1332 (1978). 16. P. W. Chye, I. Lindau, P. Pianetta, C. M. Garner, C. Y. Su, and W. E. Spicer, Phys. Rev. B 18, 5545 (1978). 17. W. E. Spicer. P. W. Chye, P. R. Skeath, C. Y. Su, and I. Lindau, J. Vac. Sci. Technol. 16, 1422 (1979). 18. W. E. Spicer, I. Lindau, P. R. Skeath, C. Y. Su, and P. W. Chye, Phys. Rev. Lett. 44, 420 (1980). 19. W. E. Spicer, I. Lindau, P. Skeath, and C. Y. Su, J. Vac. Sci. Technol. 17, 1019 (1980). 20. N. E Mott, Solid State Electron. 21, 1275 (1978). 21. H. J. Queisser, Solid State Electron. 21, 1495 (1978). 22. A. M. Stroneham, Rep. Prog. Phys. 44, 1251 (1981). 23. P. T. Landsberg, Phys. Status Solidi 41,457 (1970). 24. T. N. Morgan, Phys. Rev. B 28, 7141 (1983). 25. M. Lax, Phys. Rev. 119, 1502 (1960). 26. E T. Landsberg, C. Phys-Roberts, and P. Lal, Proc. Phys. Soc. 84, 915 (1964). 27. A. Haug, Phys. Status Solid B 97, 481 (1982). 28. A. Hangleiter, Phys. Rev. B 35, 9149 (1987). 29. A. Hangleiter, Phys. Rev. B 37, 2594 (1988). 30. D. B. Laks, G. E Neumark, A. Hangleiter, and S. T. Pantelides, Phys. Rev. Lett. 61, 1229 (1988). 31. A. Hangleiter and R. Hacker, Phys. Rev. Lett. 65, 215 (1990). 32. J. Jortner, S. A. Rice, and R. M. Hochstrasser, Adv. Photochem. 7, 149 (1969). 33. K. E Freed, in "Energy Dependence of Electronic Relaxation Processes in Polyatomic Molecules," Vol. 15 of "Topics in Applied Physics," E K. Fong, ed., Springer, New York, 1976, p. 23. 34. E Avouris, W. M. Gelbart, and M. A. E1-Sayed, Chem. Rev. 77, 793 (1977).
272
KASI VISWANATH
35. R. Engleman and J. Jortner, Mol. Phys. 18, 145 (1970). 36. D. L. Keune, H. Holonyak, Jr., R. D. Burnham, D. R. Scifres, H. R. Zwicker, J. W. Burd, M. G. Crawford, D. L. Dickus, and M. J. Fox, J. Applied Phys. 42, 2048 (1971); E D. Dapkus, N. Holonyak, Jr., R. D. Burnham, D. L. Keune, J. W. Burd, K. L. Lawley, and R. E. Wallin, J. Appl. Phys. 41, 4194 (1971). 37. C. J. Hwang, J. Appl. Phys. 42, 4408 (1971). 38. C. Van Opdrop, C. Werkhoven, and A. T. Vink, Appl. Phys. Lett. 30, 40 (1977). 39. W. E Dumke, Phys. Rev. 105, 139 (1957). 40. P. Asbeck, J. Appl. Phys. 48, 820 (1977). 41. R. J. Nelson and R. G. Sobers, Appl. Phys. Lett. 32, 761 (1978). 42. R. J. Nelson, J. Vac.Sci. Technol. 15, 1475 (1978). 43. C. J. Henry and R. A. Logan, J. Appl. Phy. 48, 3962 (1977). 44. R. J. Nelson and R. G. Sobers, J. Appl. Phys. 49, 6103 (1978). 45. E Stem, IEEE J. Quantum Electron. QE-9, 290 (1973). 46. E Stem, J. Appl. Phys. 47, 5382 (1976). 47. H. C. Casey, Jr. and E Stem, J. Appl. Phys. 47, 631 (1976). 48. L. Jastrzebski, J. Lagowski, and H. C. Gatos, Appl. Phys. Lett. 27, 537 (1975). 49. R. D. Ryan and J. E. Eberhardt, Solid State Electron. 15, 865 (1972). 50. L. Jastrzebski, H. C. Gatos, and L. Jagowski, J. Appl. Phys. 48, 1730 (1977). 51. C. J. Sandroff, R. N. Nottenburg, J. C. Bischoff, and R. Bhat, Appl. Phys. Lett. 51, 33 (1987). 52. E. Yablonovitch, C. J. Sandroff, R. Bhat, and T. Gmitter, Appl. Phys. Lett. 51,439 (1987). 53. L. A. Farrow, C. J. Sandroff, and M. C. Tamargo, Appl. Phys. Lett. 51, 1931 (1987). 54. R. N. Nottenburg, C. J. Sandroff, O. H. Humphrey, T. H. Hollenbeck, and R. Bhat, Appl. Phys. Lett. 52, 1707 (1988). 55. B. J. Skromme, C. J. Sandroff, E. Yablonovitch, and T. G. Gmitter, Appl. Phys. Lett. 51, 2022 (1988). 56. R. S. Besser and C. R. Helms, Appl. Phys. Lett. 52, 2157 (1988). 57. M. S. Carpenter, M. R. Melloch, M. S. Lundstrom, and S. E Tobin, Appl. Phys. Lett. 52, 2157 (1988). 58. D. Liu, T. Zhang, R. A. LaRue, J. S. Harris, Jr., and T. W. Sigmon, Appl. Phys. Lett. 53, 1059 (1988). 59. C. W. Wilmsen, E D. Kirschner, and J. M. Woodall, J. Appl. Phys. 64, 3287 (1988). 60. R. Iyer, R. R. Chang, and D. L. Lile, Appl. Phys. Lett. 53, 134 (1988). 61. D. K. Gaskill, N. Bottka, and R. S. Sillmon, J. Vac. Sci. Technol. B 6, 1497 (1988). 62. H. H. Lee, R. J. Racicot, and S. H. Lee, Appl. Phys. Lett. 54, 724 (1989). 63. H. Hasegawa, T. Saitoh, S. Konishi, H. Ishii, and H. Ohno, Jpn. J. Appl. Phys. 27, L 2177 (1988). 64. M. G. Mauk, S. Xu, D. J. Arent, R. E Mertens, and G. Borghs, Appl. Phys. Lett. 54, 213 (1989). 65. J. E Fan, H. Oigawa, and Y. Nannichi, Jpn. J. Appl. Phys. 28, L 1331 (1988). 66. C. J. Sandroff, M. S. Hegde, L. A. Farrow, C. C. Chang, and J. P. Harbison, Appl. Phys. Lett. 54, 362 (1989). 67. B. A. Cowens, Z. Dradas, W. N. Oelgass, M. S. Carpenter, and M. R. Melloch, Appl. Phys. Lett. 54, 365 (1989). 68. Y. Nannichi, J. Fan, H. Oigawa, and A. Koma, Jpn. J. Appl. Phys. 27, L 2367 (1988). 69. T. Tiedje, E C. Wong, K. A. R. Mitchell, W. Eberhardt, Z. Fu, and D. Sondericker, Solid State Commun. 70, 355 (1989). 70. C. J. Spindt, D. Liu, K. Miyano, E L. Meissner, T. T. Chiang, T. Kendelewicz, I. Lindau, and W. E. Spicer, Appl. Phys. Lett. 55, 861 (1989). 71. C. M. Wilmsen, E D. Kirchner, J. M. Baker, D. T. McInturff, G. D. Pettit, and J. M. Woodall, J. Vac. Sci. Technol. B 6, 1180 (1988). 72. C. J. Sandroff, M. S. Hegde, and C. C. Chang, J. Vac. Sci. Technol. B 7, 841 (1989). 73. R. S. Besser and C. R. Helms, J. Appl. Phys. 65, 4306 (1989).
74. E. Yablonovitch, B. J. Skromme, R. Bhat, J. P. Harbison, and T. J. Gmitter, Appl. Phys. Lett. 54, 555 (1989). 75. D. Biswas, E R. Berger, and E Bhattacharya, J. Appl. Phys. 65, 2571 (1989). 76. C. J. Sandroff, E S. Turco-Sandroff, L. T. Florez, and J. E Harbison, J. Appl. Phys. 70, 3632 (1991). 77. S. Shikata and H. Hayashi, J. Appl. Phys. 70, 3721 (1991). 78. S. R. Lunt, G. N. Ryba, E G. Santangelo, and N. S. Lewis, J. Appl. Phys. 70, 7449 (1991). 79. L. S. Hung, G. H. Braunstein, and L. A. Bosworth, Appl. Phys. Lett. 60, 201 (1992). 80. Y. Wang, Y. Darici, and E H. Holloway, J. Appl. Phys. 71, 2746 (1992). 81. Y. Mada, K. Wada, and Y. Wada, Appl. Phys. Lett. 61, 2993 (1992). 82. Y. T. Oh, S. C. Byun, B. R. Lee, T. W. Kang, C. Y. Hong, S. B. Park, H. K. Lee, and T. W. Kim, J. Appl. Phys. 76, 1959 (1994). 83. Y. Mada and K. Wada, Appl. Phys. Lett. 66, 733 (1995). 84. C. S. Liu and J. E Kauffman, Appl. Phys. Lett. 66, 3504 (1995). 85. K. Asai, T. Miyashita, K. Ishigure, and S. Fukatsu, J. Appl. Phys. 77, 1582 (1995). 86. E. D. Lu, E E Zhang, S. H. Xu, X. J. Yu, E S. Xu, Z. E Han, E Q. Xu, and X. Y. Zhang, Appl. Phys. Lett. 69, 2282 (1996). 87. J. Liu and T. E Kuech, Appl. Phys. Lett. 69, 662 (1996). 88. G.W. Anderson, M. C. Hanf, E R. Norton, M. Kowalewski, K. Myrtle, and N. Heinrich, J. Appl. Phys. 79, 4954 (1996). 89. V. N. Bessolov, M. V. Lebedev, and D. R. T. Zahn, J. Appl. Phys. 82, 2640 (1997). 90. J. E Geisz, S. A. Safvi, and T. E Kuech, J. Electrochem. Soc. 144, 732 (1997). 91. K. Remashan and K. N. Bhat, IEEE Electron. Dev. Lett. 19, 466 (1998). 92. X. Cao, H. Hu, X. Ding, Z. Yuan, Y. Dong, X. Chen, B. Lai, and X. Hou, J. Vac. Sci. Technol. B 16, 2656 (1998). 93. X. A. Cao, H. T. Hu, Y. Dong, X. M. Ding, and X. Y. Hou, J. Appl. Phys. 86, 6940 (1999). 94. R. J. Nelson, J. S. Williams, H. J. Leamy, B. I. Miller, H. C. Casey, Jr., B. A. Parkinson, and A. Heller, Appl. Phys. Lett. 38, 76 (1980). 95. E Briones and D. M. Collins, J. Electron. Mater 11,847 (1982). 96. W. I. Wang, R. E Marks, and L. Vina, J. Appl. Phys. 59, 3 (1986). 97. H. Kunzel and K. Ploog, Appl. Phys. Lett. 37, 4 (1980). 98. J. E Kauffman and G. L. Richmond, J. Appl. Phys. 73, 1912 (1993). 99. J. E Kauffman, B. A. Balko, and G. L. Richmond, J. Phys. Chem. 96, 6371 (1992); 96, 6374 (1992). 100. T. Sawada, K. Numata, S. Tohdoh, T. Saitoh, and H. Hasegawa, Jpn. J. Appl. Phys. 32, 511 (1993). 101. A. N. MacInnes, M. B. Power, and A. R. Barron, Appl. Phys. Lett. 62, 713 (1993). 102. E E Jenkins, A. N. MacInnes, M. Tabib-Azar, and A. R. Barron, Science 263, 1751 (1994). 103. M. Tabib-Azar, S. Kang, A. N. MacInnes, M. B. Power, A. R. Barron, E E Jenkins, and A. E Hepp, Appl. Phys. Lett. 63, 625 (1993). 104. H. Sugahara, M. Oshima, H. Oigawa, H. Shigekawa, and Y. Nannichi, J. appl. Phys. 69, 4349 (1991). 105. T. Scimeca, Y. Muramatsu, M. Oshima, H. Oigawa, and Y. Nannichi, Phys. Rev. B 44, 12927 (1991). 106. H. Sugahara, M. Oshima, R. Klauser, H. Oigawa, and Y. Nannichi, Surf. Sci. 242, 335 (1991). 107. T. Scimeca, Y. Muramatsu, M. Oshima, H. Oigawa, and Y. Nannichi, appl. Surf. Sci. 60/61,256 (1992). 108. H. Sugahara, M. Oshima, H. Oigawa, and Y. Nannichi, Thin Solid Films 220, 212 (1992). 109. S. Maeyama, M. Sugiyama, M. Oshima, H. Sugahara, H. Oigawa, and Y. Nannichi, Appl. Surf. Sci. 60/61,513 (1992). 110. M. Oshima, T. Scimeca, Y. Watanabe, H. Oigawa, and Y. Nannichi, Jpn. J. Appl. Phys. 32, 518 (1993). 111. M. Sugiyama, S. Maeyama, and M. Oshima, Phys Rev. Lett. 71,2611 (1993).
SURFACE A N D I N T E R F A C I A L R E C O M B I N A T I O N IN S E M I C O N D U C T O R S
112. M. Sugiyama, S. Maeyama, and M. Oshima, Phys. Rev. B 50, 4905 (1994). 113. M. Sugiyama, S. Maeyama, and M. Oshima, Phys. Rev. B 48, 11037 (1993). 114. K. Sato and H. Ikoma, Jpn. J. Appl. Phys. 32, 921 (1993). 115. K. Sato, M. Sakata, and H. Ikoma, Jpn. J. Appl. Phys. 32, 3354 (1993). 116. M. Sakata, M. Hayakawa, N. Nakano, and H. Ikoma, Jpn. J. Appl. Phys. 34, 3447 (1995). 117. P. Moriarty, B. Murphy, L. Roberts, A. A. Cafolla, G. Hughes, L. Koenders, P. Bailey, and D. A. Woolf, Appl. Phys. Lett. 67, 383 (1995). 118. P. Moriarty, B. Murphy, L. Roberts, A. A. Cafolla, G. Hughes, L. Koenders, and P. Bailey, Phys. Rev. B 50, 14237 (1994). 119. V. N. Bessolov, E. V. Konenkova, and M. V. Lebedev, Mat. Sci. & Engg. B 44, 376 (1997). 120. V. N. Bessolov, M. V. Lebedev, A. E Ivankov, W. Bauhofer, and D. R. T. Zahn, Appl. Surf. Sci. 133, 17 (1998). 121. M. V. Lebedev, and M. Aono, J. Appl. Phys. 87, 289 (2000). 122. V. N. Bessolov, E. V. Konenkova, M. V. Lebedev, and D. R. T. Zahn. Physics of the Solid State 41,793 (1999). 123. V. N. Bessolov, E. V. Konenkova, and M. V. Lebedev, J. Vac. Sci. Technol. B 14, 2761 (1996). 124. V.N. Bessolov, E. V. Konenkova, and M. V. Lebedev, Tech. Phys. Lett. 22, 749 (1996). 125. V. N. Bessolov, M. V. Lebedev, N. M. Binh, M. Friedrich, and D. R. T. Zahn, Semicond. Sci. Technol. 13, 611 (1998). 126. V. N. Bessolov, E. V. Konenkova, M. V. Lebedev, and D. R. T. Zahn, Semiconductors 31, 1164 (1997). 127. R. S. Besser and C. R. Helms, Appl. Phys. Lett. 52, 1707 (1988). 128. C. J. Spindt, R. S. Besser, R. Cao, K. Miyano, C. R. Helms, and W. E. Spicer, Appl. Phys. Lett. 54, 1148 (1989). 129. C. J. Spindt and W. E. Spicer, Appl. Phys. Lett. 55, 1653 (1989). 130. H. Hasegawa, H. Ishii, T. Sawada, T. Saitoh, S. Konishi, Y. Liu, and H. Ohno, J. Vac. Sci. Technol. B 6, 1184 (1988). 131. V. L. Berkovits, V. N. Bessolov, T. N. L'vova, E. B. Novikov, V. I. Safarov, R. V. Khasieva, and B. V. Trarenkov, J. Appl. Phys. 70, 3707 (1991). 132. W. R. Miller and G. E. Stillman, J. Appl. Phys. 70, 7635 (1991). 133. Y. Hirota, E Maeda, Y. Watanabe, and T. Ogino, J. Appl. Phys. 82, 1661 (1997). 134. C.J. Sandroff, M. S. Hegde, L. A. Farrow, R. Bhat, J. E Harbison, and C. C. Chang, J. Appl. Phys. 67, 586 (1990). 135. E S. Turco, C. J. Sandroff, D. M. Hwang, T. S. Ravi, and M. C. Tamargo, J. Appl. Phys. 68, 1038 (1990). 136. S. A. Chambers and V. S. Sundaram, J. Vac. Sci. Technol. B 9, 2256 (1991). 137. R. Berrigan, Y. Watanabe, T. Scimeca, and M. Oshima, Jpn. J. Appl. Phys. 31, 3523 (1992). 138. T. Scimeca, Y. Watanbe, R. Berrigan, and M. Oshima, Phys. Rev. B 46, 10201 (1992). 139. T. Scimeca, K. Prabhakaran, Y. Watnabe, E Maeda, and M. Oshima, Appl. Phys. Lett. 63, 1807 (1993). 140. E Maeda, Y. Watanabe, T. Scimeca, and M. Oshima, Phys. Rev. B 48, 4956 (1993). 141. T. Scimeca, Y. Watanabe, R. Maeda, R. Berrigan, and M. Oshima, Appl. Phys. Lett. 62, 1667 (1993). 142. T. Scimeca, Y. Watnabe, E Maeda, R. Berrigan, and M. Oshima, J. Vac. Sci. Technol. B 12, 3090 (1994). 143. D. J. Olego, Appl. Phys. Lett. 51, 1422 (1987). 144. S. K. Gandhi, S. Tyagi, and R. Venkatasubramanian, Appl. Phys. Lett. 53, 1308 (1988). 145. B.A. Kuruvilla, A. Datta, G. S. Shekhawat, A. K. Sharma, P. D. Vyas, R. P. Gupta, and S. K. Kulakarni, J. Appl. Phys. 80, 6274 (1996). 146. B. A. Kuruvilla, S. V. Ghaisas, A. Datta, S. Banerjee, and S. K. Kulkarni, J. Appl. Phys. 73, 4384 (1993). 147. J. Sun, D. J. Seo, W. L. O'Brien, E J. Himpsel, and T. E Kuech, Mat. Res. Soc. Sym. Proc. 484, 589 (1998).
273
148. J. Sun, D. J. Seo, W. L. O'Brien, E J. Himpsel, A. B. Ellis, and T. E Kuech, J. Appl. Phys. 85, 969 (1999). 149. J. Arokiaraj, K. Ohtsuka, T. Soga, T. Jimbo, and M. Umeno, Inst. Phys. Conf. Ser. 162, 799 (1999); Jpn. J. appl. Phys. 38, 6587 (1999). 150. D. J. Olego, R. Schachter, and J. A. Baumann, Appl. Phys. Lett. 45, 1127 (1984). 151. P. Viktorovitch, M. Gendry, S. K. Krawczyk, E Krafft, E Abraham, A. Bekkaoui, and Y. Monteil, Appl. Phys. Lett. 58, 2387 (1991). 152. J. A. Dagata, W. Tseng, J. Bennett, J. Schneir, and H. H. Harary, Appl. Phys. Lett. 59, 3288 (1991). 153. Y. Wada and K. Wada, Appl. Phys. Lett. 63, 379 (1993). 154. M. Sakata and H. Ikoma, Jpn. J. Appl. Phys. 33, 3813 (1994). 155. J. L. Davidson, P. John, E G. Roberts, M. G. Jubber, and J. I. B. Wilson, Appl. Phys. Lett. 65, 1397 (1994). 156. D. A. Harrison, R. Ares, S. E Watkins, M. L. W. Thewalt, C. R. Bolognesi, D. J. S. Beckett, and A. J. S. Thorpe, Appl. Phys. Lett. 70, 3275 (1997). 157. O. J. Glembocki, J. A. Tuchman, J. A. Dagata, K. K. Ko, S. W. Pang, and C. E. Stutz, Appl. Phys. Lett. 73, 114 (1998). 158. R. Beaudry, S. E Watkins, X. Xu, and P. Yeo, J. Appl. Phys. 87, 7838 (2000). 159. M. Passlack, R. Droopad, Z. Yu, C. Overgaard, B. Bowers, and J. Abrokawah, Appl. Phys. Lett. 72, 3163 (1998). 160. G. Wang, K. Ohtsuka, T. Soga, T. Jimbo, and M. Umeno, Inst. Phys. Conf. Ser. 162, 597 (1999). 161. J. J. Zinck, E. J. Tarsa, B. Brar, and J. S. Speck, J. Appl. Phys. 82, 6067 (1997). 162. E Maeda, Y. Watanabe, and M. Oshima, Phys. Rev. B 48, 14733 (1993). 163. M. Sugiyama, S. Maeyama, E Maeda, and M. Oshima, Phys. Rev. B 52, 2678 (1995). 164. E Maeda, Y. Watanabe, and M. Oshima, J. Cryst. Growth 150, 1164 (1995). 165. K. W. Vogt and P. A. Kohl, J. Appl. Phys. 74, 6448 (1993). 166. Y. J. Park, E. K. Kim, I. K. Han, S. K. Min, E O'Keefe, H. Mutoh, H. Munekata, and H. Kukimoto, Appl. Surf. Sci. 117/118. 551 (1997). 167. N. Okamoto, E Kasahara, and H. Ikoma, Jpn. J. AppI. Phys. 38, L 424 (1999). 168. F. Kasahara, K. Kanazawa, N. Okamoto, and H. Ikoma, Jpn. J. Appl. Phys. 38, 6597 (1999). 169. A. Hara, R. Nakamura, and H. Ikoma, J. Vac. Sci. Technol. 16, 183 (1998). 170. S. Wada, K. Kanazawa, N. Okamoto, and H. Ikoma, J. Vac. Sci. Technol. 17, 1516 (1999). 171. A. Hara, E Kasahara, S. Wada, and H. Ikoma, J. Appl. Phys. 85, 3234 (1999). 172. H. Tanemura, K. Kanazawa, and H. Ikoma, Jpn. J. Appl. Phys. 39, 1629 (2000). 173. Z. H. Lu, E Chatenoud, M. M. Dion, M. J. Graham, H. E. Ruda, I. Koutzarov, Q. Liu, C. E. J. Mitchell, I. G. Hill, and A. B. MeLean, Appl. Phys Lett. 67, 670 (1995). 174. R. J. Nelson, J. S. Williams, H. J. Leamy, B. Miller, H. C. Casey, Jr., B. A. Parkinson, and A. Heller, Appl. Phys. Lett. 36, 76 (1980). 175. J. S. Hwang, Y. C. Wang, W. Y. Chou, S. L. Tyan, M. Hong, J. P. Mannaerts, and J. Kwo, J. Appl. Phys. 83, 2857 (1998). 176. A. Callegari, P. D. Hoh, D. A. Buchanan, and D. Lacey, Appl. Phys. Lett. 54, 332 (1989). 177. J. G. Ping and H. E. Ruda, J. Appl. Phys. 83, 5880 (1998). 178. E Friedel and S. Gourrier, Appl. Phys. Lett. 42, 509 (1983). 179. N. Pan, S. S. Bose, N. H. Kim, G. E. Stillman, E Chambers, G. Devane, C. R. Ito, and M. Feng, Appl. Phys. Lett. 51,596 (1987). 180. E E McCluskey, L. Pfeiffer, K. W. West, J. Lopata, M. L. Schnoes, T. D. Harris, S. J. Pearton, W. C. Dautremont-Smith, Appl. Phys. Lett. 54, 1769 (1989). 181. J. Saito and K. Kondo, J. Appl. Phys. 67, 6274 (1990). 182. Y. E Chen, C. S. Tsai, and Y. Chang, Appl. Phys. Lett. 57, 70 (1990). 183. V. Swaminathan, U. K. Chakrabarti, W. S. Hobson, R. Caruso, J. Lopata, S. J. Pearton, and H. S. Luftman, J. Appl. Phys. 68, 902 (1990).
274
KASI VISWANATH
184. R. A. Gottrcho, B. L. Preppernau, S. J. Pearton, A. B. Emerson, and K. P. Giapis, J. Appl. Phys. 68, 440 (1990). 185. S. V. Hattangady, R. A. Rudder, M. J. Mantni, G. G. Fountain, J. B. Posthill, and R. J. Markunas, J. Appl. Phys. 68, 1233 (1990). 186. V. Swaminathan, M. T. Asom, G. Livescu, M. Geva, F. A. Stevie, S. J. Pearton, and J. Lopata, Appl. Phys. Lett. 57, 2928 (1990). 187. Y. E Chen, W. S. Chen, S. H. Huang, and E Y. Juang, J. Appl. Phys. 69, 3360 (1991). 188. A. W. R. Leitch, Th. Prescha, and J. Weber, Phys. Rev. B 45, 14400 (1992). 189. K. D. Choquette, M. Hong, H. S. Luftman, S. N. G. Chu, J. P. Mannaerts, R. C. Wetzel, and R. S. Freund, J. Appl. Phys. 73, 2035 (1993). 190. M. Hong, R. S. Freund, K. D. Choquette, H. S. Luffman, J. P. Mannaerts, and R. C. Wetzel, Appl. Phys. Lett. 62, 2658 (1993). 191. G. Wang, K. Ohtsuka, T. Soga, T. Jimbo, and M. Umeno, Jpn. J. Appl. Phys. 38, 3504 (1999). 192. G. Wang, T Ogawa, M. Umeno, T. Soga, and T. Jimbo, Appl. Phys. Lett. 76, 730 (2000). 193. T. Soga, T. Jimbo, G. Wang, K. Ohtsuka, and M. Umeno, J. Appl. Phys. 87, 2285 (2000). 194. S. J. Pearton, J. W. Corbett, and T. S. Shi, Appl. Phys. A 43, 153 (1987). 195. J. K. Hsu and K. M. Lau, J. Appl. Phys. 63, 962 (1988). 196. V. J. Rao, V. Manorama, and S. V. Bhoraskar, Appl. Phys. Lett. 54, 1799 (1989). 197. V. Manorama, S. V. Bhoraskar, V. J. Rao, and S. T. Kshirsagar, Appl. Phys. Lett. 55, 1641 (1989). 198. R. S. Bhide, S. V. Bhoraskar, and V. J. Rao, J. Appl. Phys. 72, 1464 (1992). 199. M. Tabib-Azar, A. S. Dewa, and W. H. Ko, Appl. Phys. Lett. 52, 206 (1988). 200. Z. H. Lu and J. M. Baribeau, Appl. Phys. Lett. 70, 1989 (1997). 201. I. Riech, E. Marin, E Diaz, J. J. Alvarado-Gil, J. G. Medoza-Alvarez, H. Vergas, A. Cruz-Orea, M. Vargas, and J. Bernal-Alvarado, Phys. Stat. Sol. (a) 169, 275 (1998). 202. C. Paulson, B. Hawkins, J. Sun, A. B. Ellis, L. Mccaughan, and T. E Kuech, Mat. Res. Soc. Sym. Proc. 588, 13 (2000). 203. S. A. Safvi, J. Liu, and T. E Kuech, J. Appl. Phys. 82, 5352 (1997). 204. M. C. DeLong, I. Viohl, W. D. Ohlsen, E C. Taylor, E E Dabkowski, K. Meehan, J. E. Williams, and M. Hopkinson, unpublished results. 205. M. C. Delong, W. D. Ohlsen, I. Viohl, X. Yin, E C. Taylor, D. Sengupta, G. E. Stillman, J. M. Olson, and W. A. Harrison, Phys. Rev. B 48, 5157 (1993). 206. C.E. Inglefield, M. C. DeLong, E C. Taylor, and W. A. Harrison, Phys. Rev. B 56, 12434 (1997). 207. C. E. Inglefield, M. C. DeLong, E C. Taylor, and W. A. Harrison, J. Vac. Sci. Technol. B 16, 2328 (1998). 208. C. E. Inglefield, M. C. DeLong, E C. Taylor, J. E Geisz, and J. M. Olson, J. Vac. Sci. Technol. B 15, 1201 (1997). 209. C. E. Inglefield, M. C. DeLong, E C. Taylor, and W. A. Harrison, unpublished. 210. C. H. Wang and A. Neugroschel, J. Appl. Phys. 74, 3257 (1993). 211. H. Ito and T. Ishibashi, Jpn. J. Appl. Phys. 33, 88 (1994). 212. R. Toba and M. Tajima, J. Cryst. Growth 103, 28 (1990). 213. M. Noji, M. Kiyama, and M. Tajima, Jpn. J. Appl. Phys. 39, 2585 (2000). 214. J. Bardeen, Phys. Rev. 71,717 (1947). 215. R. E. Allen and J. D. Dow, Phys. Rev. B 25, 1423 (1982). 216. J. Fan, H. Oigawa, and Y. Nannichi, Jpn. J. Appl. Phys. 27, 1331 (1988). 217. M. S. Carpenter, M. R. Melloch, and T. E. Dungan, Appl. Phys. Lett. 53, 66 (1988). 218. J. Fan, H. Oigawa, and Y. Nannichi, Jpn. J. Appl. Phys. 27, 2125 (1988). 219. J. Fan, Y. Kurata, and Y. Nannichi, Jpn. J. Appl. Phys. 28, 2255 (1989).
220. H. Oigawa. J. Fan, Y. Nannichi, H. Sugahara, and M. Oshima, Jpn. J. Appl. Phys. 30, 322 (1990). 221. K. C. Hwang and S. S. Li, J. Appl. Phys. 67, 2162 (1990). 222. J. L. Lee, D. Kim, S. J. Maeng, H. H. Park, J. Y. Kang, and Y. T. Lee, J. Appl. Phys. 73, 3539 (1993). 223. H. Sugahara, M. Oshima, H-Oriwa, and Y. Nannichi, J. Vac. Sci. Technol. A 11, 52 (1993). 224. J. E. Samaras and R. B. Darling, J. Appl. Phys. 72, 168 (1992). 225. M. Sugiyama, S. Maeyama, T. Scimeca, M. Oshima, H. Oigawa, and Y. Nannichi, Appl. Phys. Lett. 63, 2540 (1993). 226. Z. Chen, W. Kim, A. Salvador, S. N. Mohammad, O. Aktas, and H. Morkoc, J. Appl. Phys. 78, 3920 (1995). 227. H. Xu, X. Belkouch, C. Aktik, and W. Rasmussen, Appl. Phys. Lett. 66, 2125 (1995). 228. L. B. Chang, H. T. Wang, and L. C. Yang, Jpn. J. Appl. Phys. 36, 3429 (1997). 229. G. Wang, G. Y. Zhao, T. Soga, T. Jimbo, and M. Umeno. Jpn. J. Appl. Phys. 37, L 1280 (1998). 230. J. Lin, M. J. Hwu, and L. B. Chang, Jpn, J. Appl. Phys. 37, L 1437 (1998). 231. M. J. Jeng, H. T. Wang, L. B. Chang, Y. C. Cheng, and S. T. Chou, J. Appl. Phys. 86, 6261 (1999). 232. L. B. Chang, Y. C. Cheng, and H. T. Wang, Jpn. J. Appl. Phys. 37, 811 (1998). 233. L. B. Chang, H. T. Wang, Y. C. Cheng, T. W. Shong, and E. K. Lin, Microelectronics Journal 30, 521 (1999). 234. G. Wang, T. Ogawa, T. Soga, T. Egawa, T. Jimbo, and M. Umeno, Appl. Surf. Sci. 159, 191 (2000). 235. J. Ivanco, H. Kobayashi, J. Almedia, and G. Margaritondo, J. Appl. Phys. 87, 795 (2000). 236. H. Y. Nie and Y. Nannichi, Jpn. J. Appl. Phys. 30, 906 (1991). 237 237. H. Y. Nie and Y. Nannichi, Jpn. J. Appl. Phys. 32, L 890 (1993). 238. H. Y. Nie and Y. Nannichi, J. Appl. Phys. 76, 4205 (1994). 239. H. Y. Nie, J. Appl. Phys. 87, 4327 (2000). 240. E C. M. Christianen, E J. Van Hall, H. J. A. Bluyssen, and J. H. Wolter, Semicond. Sci. Technol. 9, 707 (1994). 241. E C. M. Christianen, E J. van Hall, H. J. A. Bluyssen, M. R. Leys, L. Drost, and J. H. Wolter, J. Appl. Phys. 80, 6831 (1996). 242. S. Datta and B. Das, Appl. Phys. Lett. 56, 665 (1990). 243. D. J. Monsma, R. Vlutters, and J. C. Lodder, Science 281,407 (1998). 244. M. Oestreich, J. Hubner, D. Hagele, E J. Klar, W. Heimbrodt, W. W. Ruble, D. E. Ashenford, and B. Lunn, Appl. Phys. Lett. 74, 1251 (1999). 245. R. Flederling, M. Deim, G. Reuscher, W. Ossau, G. Schmidt, A. Waag, and L. W. Molenkamp, Nature 402, 787 (1999). 246. Y. Ohno, D. K. Young, B. Beschoten, E Matukura, H. Ohno, and D. D. Awshalom, Nature 402, 790 (1999). 247. G. A. Prinz, Science 250, 1092 (1990). 248. M.W.J. Prins, H. van Kempen, H. van LeuKen, R. A. de Groot, W van Roy, and J. De Boeck, J. Phys. Condens. Matter 7, 9447 (1995). 249. B. T. Jonker, O. J. Glembocki, R. T. Holm, and R. J. Wagner, Phys. Rev. Lett. 79, 4886 (1997). 250. T. Manago, S. Miyanishi, H. Akinaga, W. van Roy, R. E B. Roelfsema, T. Sato, E. Tamura, and S. Yuasa, J. Appl. Phys. 88, 2043 (2000). 251. W. D. Johnston, Jr., Appl. Phys. Lett. 24, 494 (1974). 252. W. D. Johnston, Jr., and R. A. Logau, Appl. Phys. Lett. 28, 140 (1976). 253. C. H. Henry and R. A. Logan, Appl. Phys. Lett. 31,203 (1977). 254. M. Ettenberg, C. J. Nuese, and G. H. Olsen, J. Appl. Phys. 48, 2543 (1977). 255. R. U. Martinelli and G. H. Olsen, J. Appl. Phys. 47, 1332 (1976). 256. G. H. Olsen, M. Ettemberg, and R. V. D'Aiello, Appl. Phys. Lett. 33, 606 (1978). 257. E A. J. M. Driessen, Appl. Phys. Lett. 67, 2813 (1995). 258. Y. Rosenwaks, B. R. Thacker, K. Bertness, and A. J. Nozik, J. Phys. Chem. 99, 7871 (1995). 259. Y. Rosenwaks, B. R. Thacker, and A. J. Nozik, Appl. Surf. Sci. 106, 396 (1996).
S U R F A C E A N D I N T E R F A C I A L R E C O M B I N A T I O N IN S E M I C O N D U C T O R S
260. M. P~tsslack, M. Hong, E. E Schubert, J. R. Kwo, J. E Mannaerts, S. N. G. Chu, N. Moriya, and E A. Thiel, Appl. Phys. Lett. 66, 625 (19951. 261. J. A. Kash, B. Pezeshki, E Agahi, and N. A. Bojarczuk, Appl. Phys. Lett. 67, 2022 (1995). 262. J. Saito, K. Nanbu, T. Ishikawa, and T. Kondo, J. Appl. Phys. 63, 404 (1988,. 263. V. N. Bessolov, V. V. Evrtropov, M. V. Lebedev, and V. V. Rossin, Phys. gev. B 51, 16801 (1995). 264. K. Aktmoto, K. Tamamura, J. Ogawa, Y. Mori, and C. Kozima, J. Appl. Phys. Ii3, 460 (1988). 265. X. Ca:,, X. Hou, X. Y. Chen, Z. Li, R. Su, X. Ding, and X. Wang, Appl. t'hys. Lett. 70, 747 (1997). 266. S. Shikata, H. Okada, and H. Hayashi, J. Appl. Phys. 69, 2717 (1991). 267. R. N. Nottenburg, C. J. Sandroff, D. A. Humphrey, T. H. Hollenbeck, and R. Bhat, Appl. Phys. Lett. 52, 218 (1988). 268. L.M. Smith, D. J. Wolford, R. Venkatasubramanian, and S. K. Gandhi, Appl. Phys. Lett. 57, 1572 (1990). 269. E Dawson and K. Woodbridge, Appl. Phys. Len. 45, 1227 (1984). 270. J. E. Fouquet and R. D. Burnham, IEEE J. Quantum Electron. QE-22, 1799 (1986). 271. H. Matsueda and K. Hara, Appl. Phys. Lett. 55, 362 (1989). 272. A. Hariz, E D. Dapkus, H. C. Lee, W. E Menu, and S. E DenBaars. Appl. Phys. Lett. 54, 635 (1989). 273. E. Yablonovitch, R. Bhat, and J. P. Harbison, Appl. Phys. Lett. 50, 1197 (1987). 274. L. W. Molenkamp and H. F. J. Van't Blik, J. Appl. Phys. 64, 4253 (1988). 275. H. Kizuki, M. Miyashita, Y. Kajikawa, and Y. Mihashi, Jpn. J. Appl. Phys. 36, 6290 (1997). 276. D. Yang, J. W. Garland, P. M. Raccah, C. Coluzza, P. Frankl, M. Capizzi, E Chambers, and G. Devane, Appl. Phys. Lett. 57, 2829 (1990). 277. M. MuUenborn and N. M. Haegel, J. Appl. Phys. 74, 5748 (1993). 278. J. S. Rimmer, B. Hamitton, P. Dawson, M. Missous, and A. R. Peaker, J. Appl. Phys. 73, 5032 (1993). 279. Y. Rosenwaks, B. R. Thacker, R. K. Ahrenkiel, A. J. Nozik, and I. Ya~,neh, Phys. Rev. B 50, 1746 (1994). 280. Y. Rosenwaks, A. J. Nozik, and I. Yavneh, Appl. Phys. Lett. 75, 4255 (1994). 281. R. M. Sieg, J. A. Carlin, S. A. Ringel, M. T. Currie, S. M. Ting, T. A. Langdo, G. Taraschi, E. A. Fitzgerald, and B. M. Keyes, Appl. Phys. Lett. 73. 3111 (1998). 282. I. Reich, E Diaz, T. Prutskij, J. Mendoza, H. Vargas, and E. Marin, J. Appl. Phys. 86, 6222 (1999). 283. C. Colvard, D. Bimberg, K. Alavi, C. Maierhofer, and N. Nouri, Phys. Rev. B 39, 3419 (1989). 284. E Sheldon, B. M. Keyes, R. K. Ahrenkiel, and S. E. Asher, J. Vac. Sci. Technol A 11, 1011 (1993). 285. R. J. Nelson and R. G. Sobers, Appl. Phys. Lett. 32, 761 (1978). 286. G. W.'t Hooft, M. R. Lays, and E Roozeboom, Jpn. J. Appl. Phys. 24, L 761 (1985). 287. H. Yokoyama, H. Iwata, M. Sugimoto, K. Onabe, and R. Lang, J. Appl. Phys. 63, 4755 (1988). 288. H. Iwata, H. Yokoyama, M. Sugimoto, N. Hamao, and K. Onabe, Appl. Phys. Lett. 54, 2427 (1989). 289. D.J. Wolford, G. D. Gilliland, T. E Kuech, L. M. Smith, J. Martinsen, J. A. Bradley, C. E Tsang, R. Venkatasubramanian, S. K. Gandhi, and H. P. Hjalmarson, J. Vac. Sci. Technol. B 9, 2369 (1991). 290. G. D. Gilliland, D. J. Wolford, T. E Kuech, and J. A. Bradley, Appl. Phys. Lett. 59, 216 (1991). 291. G. D. Gilliland, D. J. Wolford, T. F. Kuech, J. A. Bradley, and H. E Hjalmarson, J. Appl. Phys. 73, 8386 (1993). 292. D. J. Wolford, G. D. Gilliland, T. E Kuech, J. E Klem, H. E Hjalmarson, J. A. Bradley, C. F. Tsang, and J. Martinsen, Appl. Phys. Lett. 64, 1416 (1994).
275
293. E. M. Clausen, Jr., H. G. Craighead, J. M. Worlock, J. E Harbison, L. M. Schiavone, L. Florez, and B. van der Gaag, Appl. Phys. Lett. 55, 1427 (1989). 294. R. K. Ahrenkiel, J. M. Olson, D. J. Dunlavy, B. M. Keyes, and A. E. Kibbler, J. Vac. Sci. Technol. A 8, 3002 (1990). 295. J. M. Olson, R. K. Ahrenkiel, D. J. Dunlavy, B. Keyes, and A. E. Kibbler, Appl. Phys. Lett. 55, 1208 (1989). 296. J. H. Harris, S. Sugai, and A. V. Nurmikko, Appl. Phys. Lett. 40, 885 (1982). 297. M. Krahl, D. Bimberg, R. K. Bauer, D. E. Mars, and J. N. Miller, J. Appl. Phys. 67, 434 (1990). 298. V. Swaminathan, J. M. Freund, L. M. E Chirovsky, T. D. Harris, N. A. Kuebler, and L. A. D'Asaro, J. Appl. Phys. 68, 4116 (1990). 299. H. Hillmer, A. Forchel, T. Kuhn, G. Mahler, and H. E Meier, Phys. Rev. B 43, 13992 (1991). 300. N. Hamao, M. Sugimoto, and H. Yokoyama, Appl. Phys. Lett. 59, 1488 (1991). 301. J. M. Mison, K. Elcess, E Houzay, J. Y. Marzin, J. M. Gerard, E Barthe, and M. Bensoussan, Phys. Rev. B 41, 12945 (1990). 302. J. S. Rimmer, I. D. Hawkins, and B. Hamilton, J. Appl. Phys. 70, 5404 (1991). 303. D. Richards, J. Wagner, A. Fischer, and K. Ploog, Appl. Phys. Lett. 61, 2685 (1992). 304. G. D. Gilliland, D. J. Wolford, T. E Kuech, and J. A. Bradley, Phys. Rev. B 43, 14251 (1991). 305. Y.J. Ding, J. V. D. Veliadis, and J. B. Khurgin, J. Appl. Phys. 75, 1727 (1994). 306. K. Xie, C. R. Wie, J. A. Varriano, and G. W. Wicks, Appl. Phys. Lett. 60, 428 (1992). 307. Q. X. Zhao, J. E Bergman, E O. Holtz, B. Monemar, K. Ensslin, M. Sundaram, J. L. Merz, and A. C. Gossard, Superlattices and Microstructuress 9, 161 (1991). 308. K. Mochizuki, J. Kasai, and T. Tanoue, Appl. Phys. Len. 66, 2092 (1995). 309. Y. L. Chang, I. H. Tan, Y. H. Zhang, J. Merz, E. Hu, A. Frova, and V. Emiliani, Appl. Phys. Lett. 62, 2697 (1993). 310. J.M. Zavada, E Voillot, N. Lauret, R. G. Wilson, and B. Theys, J. Appl. Phys. 73, 8489 (1993). 311. J. R. Botha and A. W. R. Leitch, Appl. Phys. Lett. 63, 2534 (1993). 312. I.A. Buyanova, A. C. Ferreira, E O. Holtz, B. Monemar, K. Campman, J. L. Merz, and A. C. Gossard. Appl. Phys. Lett. 68, 1365 (1996). 313. S. S. Shi., E. L. Hu, J. E Zhang, Y. I. Chang, E Parikh, and U. Mishra, Appl. Phys. Lett. 70, 1293 (1997). 314. G. Wang, T. Soga, T. Egawa, T. Jimbo, and M. Umeno, Electronics Lett. 36, 1462 (2000). 315. H. Lipsanen, M. Sopanen, M. Taskinen, and J. Tulkki, Appl. Phys. Len. 68, 2216 (1996). 316. S. Datta, M. R. Melloch, S. Bandyophadhyay, and R. Norens, Phys. Rev. Lett. 55, 2344 (1985). 317. K. Ishibashi, Y. Takagaki, K. Gamo, S. Namba, S. Ishida, K. Murase, Y. Aoyagi, and M. Kawabes, Solid State Commun. 64, 573 (1987). 318. B.J. van Wees, H. van Houten, C. W. J. Beenakker, J. G. Williamson, L. E Kouwenhoven, D. van der Marel, and C. T. Foxon, Phys. Rev. Lett. 29, 848 (1988). 319. D. A. Wharam, T. J. Thornton, R. Newbury, M. Pepper, H. Ahmed, J. E. E Frost, D. G. Hasko, D. C. Peacock, D. A. Ritchie, and G. A. C. Jones, J. Phys. C 21, L209 (1988). 320. G. Timp, H. U. Baranger, E de Vevgar, J. E. Cunningham, R. E. Howard, R. Behringer, and E M. Mankiewich, Phys. Rev. Lett. 60, 2081 (1988). 321. H. Sakaki, Jpn. J. Appl. Phys. 19, L 735 (1980). 322. S. Datta, M. R. Melloch, S. Bandyopadhyay, and M. S. Lundstrom, Appl. Phys. Lett. 48, 487 (1986). 323. A. Arakawa, K. Vahala, and A. Yariv, Appl. Phys. Len. 45,950 (1984). 324. A. Kasi Viswanath, K. Hiruma, K. Ogawa, and T. Katsuyama, "Proc. Intern. Conf. Solid State Devices and Materials," 1992, p. 296.
276
KASI VISWANATH
325. A. Kasi Viswanath, K. Hiruma, and T. Katsuyama, Superlattices and Microstructures 14, 105 (1993). 326. A. Kasi Viswanath, K. Hiruma, K. Ogawa, and T. Katsuyama, Appl. Phys. Commun. 13, 55 (1994). 327. A. Kasi Viswanath, K. Hiruma, M. Yazawa, K. Ogawa, and T. Katsuyama, Microwave and Optical Technol. Lett. 7, 94 (1994). 328. G. Duggan, H. I. Ralph, and R. J. Elliot, Solid State Commun. 56, 17 (1985). 329. G. Mayer, B. E. Maile, R. Germann, A. Forchel, P. Gambrow, and H. P. Meier, Appl. Phys. Lett. 56, 2016 (1990). 330. R. K. Ahrenkiel, "Properties of InP," INSPEC, London, 1991, p. 77. 331. H. C. Carey, Jr. and E. Buehler, Appl. Phys. Lett. 30, 247 (1977). 332. J. M. Moison, M. van Rombay, and M. Bensoussan, Appl. Phys. Lett. 48, 1362 (1986). 333. R. K. Ahrenkiel, D. J. Dunlavy, and T. Hanak, J. Appl. Phys. 64, 1916 (1988). 334. C. A. Hoffman, H. J. Gerritsen, and A. V. Nurmikko, J. Appl. Phys. 51, 1603 (1978). 335. S. Bothra, S. Tyagi, S. K. Gandhi, and J. M. Borrego, Solid State Electron 34, 47 (1991). 336. Y. Rosenwaks, Y. Shapira, and D. Huppert, Phys. Rev. B 44, 13097 (1991). 337. Y. Rosenwaks, Y. Shapira, and D. Huppert, Appl. Phys. Lett. 57, 2552 (1990). 338. Y. Rosenwaks, Y. Shapira, and D. Huppert, Phys. Rev. B 45, 9108 (1992). 339. A. Liu and Y. Rosenwaks. J. Appl. Phys. 86, 430 (1999). 340. Y. Rosenwaks, B. R. Thacker, A. J. Nozik, R. J. Ellingson, K. C. Burr, and C. L. Tang, in "Ultrafast Phenomena IX," P. E Barbara, W. H. Knox, G. A. Mourou, and A. H. Zewail, eds., "Springer Series in Chemical Physics," Vol. 60, Springer-Verlag, Berlin, 1994, p. 409. 341. Y. Rosenwaks, B. R. Thacker, A. J. Nozik, R. J. Ellingson, K. C. Burr, and C. L. Tang, J. Phys. Chem. 98, 2739 (1994). 342. Y. Rosenwaks, B. R. Thacker, A. J. Nozik, Y. Shapira, and D. Huppert, J. Phys. Chem. 97, 10421 (1993). 343. S. Krawczyk, M. Bejar, A. Khoukh, R. Blanchet, B. Sermage, D. Cui, and D. Pavlidis, Jpn. J. Appl. Phys. 38, 992 (1999). 344. W. Zhuang, C. Chen, D. Teng, J. Yu, and Y. Li, J. Vac. Sci. Technol. A 9, 983 (1991). 345. J. Lammasniemi, K. Tappura, and K. Smekalin, J. Appl. Phys. 77, 4801 (1995). 346. R. Iyer, R. R. Chang, and D. L. Lile, Appl. Phys. Lett. 53, 134 (1988). 347. R. Leonelli, C. S. Sundararaman, and J. E Currie, Appl. Phys. Lett. 57, 2678 (1990). 348. H. Oigawa, J. E Fan, Y. Nannichi, H. Sugahara, and M. Oshima, Jpn. J. Appl. Phys. 30, L 322 (1991). 349. R. Iyer, and D. L. Lile, Appl. Phys. Lett. 59, 437 (1991). 350. Z. H. Lu, M. J. Graham, X. H. Feng, and B. X. Yang, Appl. Phys. Lett. 60, 2773 (1992). 351. F. Maeda, Y. Watanabe, and M. Oshima, Appl. Phys. Lett. 62, 297 (1993). 352. D. Gallet and G. Hollinger, Appl. Phys. Lett. 62, 982 (1993). 353. K. Vaccaro, H. M. Dauplaise, A. Davis, S. M. Spaziani, and J. P. Lorenzo, Appl. Phys. Lett. 67, 527 (1995). 354. I. K. Han, D. H. Woo, H. J. Kim, E. K. Kim, J. I. Lee, S. H. Kim, K. N. Kang, H. Lim, and H. L. Park, J. Appl. Phys. 80, 4052 (1996). 355. M. Shimomura, K. Naka, N. Sanada, Y. Suziki, Y. Fukuda, and P. J. Moiler, J. Appl. Phys. 79, 4193 (1996). 356. I. K. Han, E. K. Kim, J. I. Lee, S. H. Kim, K. N. Kang, Y. Kim, H. Lim, and H. L. Park, J. Appl. Phys. 81, 6986 (1997). 357. S. Morikata, T. Motegi, and H. Ikoma, Jpn, J. Appl. Phys. 38, L 1512 (1999). 358. L. He, H. Dauplaise, A. Davis, E. Martin, S. Spaziani, K. Vaccaro, W. Waters, and J. P. Lorenzo, Jpn. J. Appl. Phys. 38, 1119 (1999). 359. G. Hollinger, D. Gallet, M. Gendry, M. B. Besland, and J. Joseph, Appl. Phys. Lett. 59, 1617 (1991).
360. Y. Robach, M. E Besland, J. Joseph, G. Hollinger, E Viktorovitch, E Ferret, M. Pitaval, A. Falcou, and G. Post, J. Appl. Phys. 71, 2981 (1992). 361. J. M. Moison, M. Van Rompay, and M. Bensoussan, Appl. Phys. Lett. 48, 1362 (1986). 362. J. Chave, A. Choujaa, C. Santinelli, R. Blanchet, and E Viktorovitch, J. Appl. Phys. 61,257 (1987). 363. Y. Mada and K. Wada, J. Appl. Phys. 83, 2025 (1998). 364. H. Nobusawa and H. Ikoma, Jpn. J. Appl. Phys. 32, 3713 (1993). 365. H. Hayashi, I. Hatanaka, S. Sato, and H. Ikoma, Jpn. J. Appl. Phys. 36, 4235 (1997). 366. T. Sugino, H. Ninomiya, T. Yamada, J. Shirafuji, and K. Matsuda, Appl. Phys. Lett. 60, 1226 (1992). 367. G. Bruno, M. Losurdo, E Capezzuto, V. Capozzi, T. Trovato, G. Perna, and G. E Lorusso, Appl. Phys. Lett. 69, 685 (1996). 368. S. Balasubramanian, K. S. R. Koteswara Rao, N. Bala Subramaniam, and V. Kumar, J. Appl. Phys. 77, 5398 (1995). 369. H. Takahashi, T. Hashizume, and H. Hasegawa, Appl. Surf. Sci. 123/124, 615 (1998). 370. H. Takahashi, T. Hashizume, and H. Hasegawa, Jpn. J. Appl. Phys. 38, 1128 (1999). 371. H. Takahashi, T. Yoshida, M. Mutoh, T. Sakai, and H. Hasegawa, Solid State Electron. 43, 1561 (1999). 372. H. Hasegawa, Jpn. J. Appl. Phys. 38, 1098 (1999). 373. S. K. Krawczyk and G. Hollinger, Appl. Phys. Lett. 45, 870 (1984). 374. A. Kapila and V. Malhotra, Appl. Phys. Lett. 62, 1009 (1993). 375. R. Driad, Z. Hong, S. Laframboise, D. Scansen, W. R. McKinnon, and S. E McAlister, Jpn. J. Appl. Phys. 38, 1124 (1999). 376. R.M. Cohen, M. Kitamura, and Z. M. Fang, Appl. Phys. Lett. 50, 1675 (1987). 377. S. D. Lester, T. S. Kim, and B. G. Stretman, J. Appl. Phys. 62, 2950 (1987). 378. K. Asakawa, T. Yoshikawa, S. Kohmoto, Y. Nambu, and Y. Sugimoto, Jpn, J. Appl. Phys. 37, 373 (1998). 379. E S. Dutta, K. S. Sangunni, H. L. Bhat, and V. Kumar, Appl. Phys. Lett. 65, 1695 (1994). 380. A. Kasi Viswanath, in "Handbook of Advanced Electronic and Photonic Materials and Devices," in "Semiconductors," Vol. 1, H. S. Nalwa, ed., Academic Press, 2000. 381. S. Nakamura, in "Group III Nitride Semiconductor Compounds," B. Gil, ed., Clarendon Press, Oxford, 1998. 382. S. Nakamura, and G. Fasol, "The Blue Laser Diode," Springer Verlag, Berlin, 1997. 383. A. K. Viswanath, J. I. Lee, S. Yu, D. Kim, Y. Choi, and C. H. Hong, J. Appl. Phys. 84, 3848 (1998). 384. A. K. Viswanath, J. I. Lee, D. Kim, C. R. Lee, and J. Y. Leem. Phys. Rev. B 58, 16333 (1998). 385. S. J. Pearton, C. R. Abernathy, J. D. Mackenzie, U. Hommerich, X. Wu, R. G. Wilson, R. N. Schwartz, J. M. Zavada, and E Ren, Appl. Phys. Lett. 71, 1807 (1997). 386. H. Okumura, H. Hamaguchi, G. Feuillet, Y. Ishida, and S. Yoshida, Appl. Phys. Lett. 72, 3056 (1998). 387. W. Wang, T. Nohova, S. Krishnankutty, R. Torreano, S. Mc Pherson, and H. Marsh, Appl. Phys. Lett. 73, 1086 (1998). 388. J. Sun, K. A. Rickert, J. M. Redwing, A. B. Ellis, E J. Himpsel, and T. E Kuech, Appl. Phys. Lett. 76, 415 (2000). 389. T. Rotter, D. Mistele, J. Stemmer, E Fedler, H. Aderhold, J. Graul, V. Schwegler, C. Kirchner, M. Kamp, and M. Heuken, Appl. Phys. Lett. 76, 3923 (2000). 390. C. Huh, S. W. Kim, H. S. Kim, I. H. Lee, and S. J. Park, J. Appl. Phys. 87, 4591 (2000). 391. Y. J. Lin, C. D. Tsai, Y. T. Lyu, and C. T. Lee, Appl. Phys. Lett. 77, 687 (2000). 392. U. Behn, A. Thamm, O. Brandt, and H. T. Grahn, J. Appl. Phys. 87, 4315 (2000). 393. J. L. Lee, L. Wei, S. Tanigawa, H. Oigawa, and Y. Nannichi, J. Appl. Phys. 70, 2877 (1991).
S U R F A C E A N D I N T E R F A C I A L R E C O M B I N A T I O N IN S E M I C O N D U C T O R S
394. M. Mizuta, Y. Mochizuki, N. Takadoh, and K. Asakawa, J. Appl. Phys. 66, 891 (1989). 395. M. Katayama, M. Aono, H. Oigawa, Y. Nannichi, H. Sugahara, and M. Oshima, Jpn. J. Appl. Phys. 30, L 786 (1991). 396. Z. Sobiesierski, S. A. Clark, R. H. Williams, A. Tabata, T. Benyattou, G. Guillot, M. Gendry, G. Hollinger, and P. Viktorovitch, Appl. Phys. Lett. 58, 1863, (1991). 397. M. J. Kane, G. Braithwaite, M. T. Emeny, D. Lee, T. Martin, and D. R. Wright, Appl. Phys. Lett. 76, 943 (2000). 398. S. Farad, Appl. Phys. Lett. 76, 2707 (2000). 399. S. Farad, R. Leon, D. Leonard, J. L. Merz, and E M. Petroff, Phys. Re~'. B 52, 5752 (1995). 400. J. Wagner, A. L. Alvarez, J. Schmitz, J. D. Ralston, and P. Koidl, Appl. Phys. Lett. 63, 349 (1993). 401. K. Tai, T. R. Hayes, S. L. McCall, and W. T. Tsang, Appl. Phys. Lett. 53, 302 (1988). 402. T. P. Pearsall, Ed., "GaInAsP Alloy Semiconductors," Wiley, New York, 1982. 403. K. Tai, J. L. Jewell, W. T. Tsang, H. Temkin, M. Panish, and Y. Twu, Appl. Phys. Lett. 50, 795 (1987). 404. Y. H. Lee, H. M. Gibbs, J. L. Jewell, J. E Duffy, T. Venkatesan, A. C. Gossard, W. Wiegmann, and J. H. English, Appl. Phys. Len. 49, 486 (1986). 405. E. E Ippen and C. V. Shank, in "Ultrashort Light Pulses," S. L. Shapiro, ed., Springer, New York, 1977. 406. B.E. Maile, A. Forchel, R. Germann, and D. Grutzmacher, Appl. Phys. Lett. 54, 1552 (1989). 407. Y. Nambu, H. Yokoyama, T. Yoshikawa, Y. Sugimoto, and K. Asakawa, Appl. Phys. Lett. 65, 481 (1994). 408. S. Q. Gu, X. Liu, M. Covington, E. Reuter, H. Chang, R. Panepucci, I. Adesida, S. G. Bishop, C. Caneau, and R. Bhat, J. Appl. Phys. 75, 8071 (1994). 409. B. Hubner, B. Jacobs, Ch. Greus, R. Zengerle, and A. Forchel, J. Vac. Sci. Technol. B 12, 3658 (1994). 410. E Kieseling, W. Braun, P. Ils, M. Michel, A. Forchel, I. Gyuro, M. Klenk, and E. Zielinski, Phys. Rev. B 51, 13809 (1995). 411. P. W. Juodawlkis and S. E. Ralph, Appl. Phys. Lett. 76, 1722 (2000). 412. M. Boroditsky, I. Gontijo, M. Jackson, R. Vrijen, E. Yablonovitch, T. Krauss, C. C. Cheng, A. Scherer, R. Bhat, and M. Krames, J. Appl. Phys. 87, 3497 (2000). 413. M. J. Joyce, M. Gal, and J. Tann, J. Appl. Phys. 65, 1377 (1989). 414. C. Juang, J. E Hsu, I. S. Yen, and H. S. Shiau, J. Appl. Phys. 72, 684 (1992). 415. M. K. Parry and A. Krier, Thin Solid Films 259, 163 (1995). 416. K. C. Hwang, S. S. Li, C. Park, and T. J. Anderson, J. Appl. Phys. 67, 6571 (I 990). 417. U. Schade, St. Kollakowski, E. H. Bottchev, and D. Bimberg, Appl. Phys. l.ett. 64, 1389 (1994). 418. V. N. Bessolov, M. V. Lebedev, Y. M. Shernyakov, B. V. Tsarenkov, MateJ: Sci. Engg. B 44, 380 (1997). 419. R. Dria,t, Z. H. Lu, S. Charbonneau, W. R. McKinnon, S. Laframboise, P. J. l'~,ole, and S. E McAlister, Appl. Phys. Len. 73, 665 (1998). 420. R. Driad, Z. H. Lu, W. R. McKinnon, S. Laframboise, S. P. McAlister, P. J. Pool, S. Raymond, and S. Charbonneau, Mat. Res. Soc. Symp. Proc. 573, 227 (1999). 421. H. Lipsanen, M. Sopanen, J. Ahopelto, J. Sandmann, and J. Feldmann, Jpn. J. Appl. Phys. 38, 1133 (1999). 422. R. Driad, Z. H. Lu, S. Laframboise, D. Scansen, W. R. McKinnon, and S. P. McAlister, Jpn. J. Appl. Phys. 38, 1124 (1999). 423. R. Driad, W. R. McKinnon, Z. H. Lu, S. P. McAlister, E J. Poole, and S. Charbonneau, J. Vac. Sci. Technol. A 18, 697 (2000). 424. M. Gal, A. Tavendale, M. J. Johnson, and B. F. Usher, J. Appl. Phys. 66, 968 (1989). 425. M. Capizzi, C. Coluzza, V. Emiliani, E Frankl, A. Frova, E Sarto, A. A. Bonapasta, Z. Sobiesierski, and R. N. Sacks, J. Appl. Phys. 72, 1454 (1992).
277
426. S. M. Lord, G. Roos, J. S. Harris, Jr., and N. M. Johnson, J. Appl. Phys. 73, 740 (1993). 427. Y. L. Chang, I. H. Tan, E. Hu, J. Merz, V. Emiliani, and A. Frova, J. Appl. Phys. 75, 3040 (1994). 428. Y. L. Chang, I. H. Tan, C. Reaves, J. Merz, E. Hu, A. Frova, V. Emiliani, and B. Bonnani, Appl. Phys. Lett. 64, 2658 (1994). 429. E. Yablonovitch, H. M. Cox, and T. J. Gmitter, Appl. Phys. Lett. 52, 1002 (1988). 430. Y. Narukawa, Y. Kawakami, S. Fujita, S. Fujita, and S. Nakamura, Phys. Rev. B 55, R 1938 (1997). 431. A. Satake, Y. Masumoto, T. Miyajima, T. Asatsuma, E Nakamura, and M. Ikeda, Phys. Rev. B 57, R 2041 (1998). 432. Y. Narakawa, Y. Kawakami, S. Fujita, and S. Nakamura, Phys. Rev. B 59, 10283 (1999). 433. Y. K. Song, M. Kuball, A. V. Nurmikko, G. E. Bulman, K. Doverspike, S. T. Sheppard, T. W. Weeks, M. Leonard, H. S. Kong, H. Dieringer, and J. Edmond, Appl. Phys. Lett. 72, 1418 (1998). 434. S. Bidnyk, T. J. Schmidt, Y. H. Cho, G. H. Gainer, J. J. Song, S. Keller, U. K. Mishra, and S. P. DenBaars, Appl. Phys. Len. 72, 1623 (1998). 435. A. K. Viswanath, J. I. Lee, D. Kim, and H. G. Lee, unpublished. 436. A. K. Viswanath, J. I. Lee, and D. Kim, unpublished. 437. S. J. Pearton, E Ren, W. S. Hobson, C. R. Abernathy, R. L. Masaitis, and U. K. Chakrabarti, Appl. Phys. Lett. 63, 3610 (1993). 438. M. Mullenborn, K. Matney, M. S. Goorsky, N. M. Haegel, and S. M. Vernon, J. Appl. Phys. 75, 2418 (1994). 439. C. T. Lee, H. P. Shiao, N. T. Yeh, C. D. Tsai, Y. T. Lyu, and Y. K. Tu, Solid State Electron. 41, 1 (1997). 440. V. A. Gorbylev, A. A. Chelniy, A. Y. Polyakov, S. J. Pearton, N. B. Smirnov, R. G. Wilson, A. G. Milnes, A. A. Cnekalin, A. V. Govorkov, B. M. Leiferov, O. M. Borodina, and A. A. Balmashnov, J. Appl. Phys. 76, 7390 (1994). 441. J. C. Fan, J. C. Wang, and Y. F. Chen, Appl. Phys. Lett. 74, 1463 (1999). 442. C. D. Tsai and C. T. Lee, J. Appl. Phys. 87, 4230 (2000); C. D. Tsai, H. P. Shiao, C. T. Lee, and Y. K. Tu, IEEE Photonics Technology Len. 9, 660 (1997). 443. S.H. Lee, C. Y. Fetzer, G. B. Stringfellow, D. H. Lee, and T. Y. Seong, J. Appl. Phys. 85, 3590 (1999). 444. G. B. Stringfellow, R. T. Lee, C. M. Fetzer, J. K. Shurtleff, Y. Hsu, S. W. Jun, S. Lee, and T. Y. Seong, J. Electron. Mater. 29, 134 (2000). 445. J. K. Shurtleff, R. T. Lee, C. M. Fetzer, and G. B. Stringfellow, Appl. Phys. Lett. 75, 1914 (1999). 446. S. W. Jun, C. M. Fetzer, R. T. Lee, J. K. Shurtleff, and G. B. Stringefellow, Appl. Phys. Lett. 76, 2716 (2000). 447. C. M. Fetzer, R. T. Lee, J. K. Shurtleff, G. B. Stringfellow, S. M. Lee, and T. Y. Seong, Appl. Phys. Lett. 76, 1440 (2000). 448. R.T. Lee, J. K. Shurtleff, C. M. Fetzer, G. B. Stringfellow, S. Lee, and T. Y. Seong, J. Appl. Phys. 87, 3730 (2000). 449. T. Benyattou, M. A. Garcia, S. Moueger, A. Tabata, M. Sacilotti, E Abraham, Y. Monteil, and R. Landers, Appl. Surf. Sci. 63, 197 (1993). 450. M. Sacilotti, P. Abraham, M. Pitaval, M. Ambri, T. Benyattou, A. Tabata, M. A. G. Perez, P. Motisuke, R. Landers, J. Morais, N. Le Corre, and S. Loualiche, Can. J. Phys. 74, 202 (1996). 451. J. S. Hwang, W. Y. Chou, S. L. Tyan, H. H. Lin, and T. L. Lee, Appl. Phys. Lett. 67, 2350 (1995). 452. W. Y. Chou, G. S. Chang, W. C. Hwang, and J. S. Hwang, J. Appl. Phys. 83, 3690 (1998). 453. G. S. Chang, W. C. Hwang, Y. C. Wang, Z. E Yang, and J. S. Hwang, J. Appl. Phys. 86, 1765 (1999). 454. J. M. Pikal, C. S. Menoni, E Thiagarajan, G. Y. Robinson, and H. Temkin, Appl. Phys. Lett. 76, 2659 (2000). 455. C. H. Henry, R. A. Logan, and E R. Merritt, J. Appl. Phys. 49, 3530 (1978). 456. M. L. Timmons, T. S. Colpitts, R. Venkatasubramanian, B. M. Keyes, D. J. Dunlavy, and R. K. Ahrenkiel, Appl. Phys. Lett. 56, 1850 (1990).
278
KASI VISWANATH
457. H. Gebretsadik, K. Zhang, K. Kamath, and E Bhattacharya, Appl. Phys. Lett. 71, 3865 (1997). 458. V. L. Berkovits, V. M. Lantratov, T. V. L'vova, G. A. Shakiashvili, V. P. Ulin, and D. Paget, Appl. Phys. Lett. 63, 970 (1993). 459. S. Gotoh and H. Horikawa, Appl. Phys. Lett. 69, 641 (1996). 460. L. Pavesi, E Marteli, D. Martin, and E K. Reinhart, Appl. Phys. Lett. 54, 1522 (1989). 461. G. Wang, T. Ogawa, K. Ohtsuka, G. Y. Zhao, T. Soga, T. Jimbo, and M. Umeno, Jpn. J. Appl. Phys. 38, L 796 (1999). 462. R. A. J. Thommer, A. van Geelen, S. M. Olsthoorn, G. J. Bauhuis, M. van Schalkwijk, and L. J. Giling, "Proc. Int. Sym. GaAs and Related Compounds," Frieburg, Germany, 1993. 463. J. D. Dow and R. E. Allen, Appl. Phys. Lett. 41,672 (1982). 464. A. Mozer, K. M. Romanek, W. Schmidt, M. H. Pilkuhn, and E. Schlosser, Appl. Phys. Lett. 41,964 (1982). 465. K. Rajesh, L. J. Huang, W. M. Lau, R. Bruce, S. Ingrey, and D. Landheer, J. Appl. Phys. 81, 3304 (1997). 466. A. Y. Polyakov, M. Stam, A. G. Milnes, A. E. Bochkarev, and S. J. Pearton, J. Appl. Phys. 71, 4411 (1992). 467. D. Huppert, M. Evenor, and Y. Shapira, J. Vac. Sci. Technol. A 2, 532 (1984). 468. Y. Rosenwaks, L. Burstein, Y. Shapira, and D. Huppert, Appl. Phys. Lett. 57, 458 (1990). 469. Y. Rosenwaks, L. Burstein, Y. Shapira, and D. Huppert, J. Phys. Chem. 94, 6842 (1990). 470. Y. Rosenwaks, Y. Shapira, and D. Huppert, Vaccum 41, 1009 (1990). 471. R. K. Jain and R. C. Lind, J. Opt. Soc. Am. 73, 647 (1983). 472. S. S. Yao, C. Karaguleff, A. Gabel, R. Fortenberry, C. T. Seaton, and G. I. Stegeman, Appl. Phys. Lett. 46, 801 (1985). 473. G. R. Olbright and N. Peyghambarian, Appl. Phys. Lett. 48, 1184 (1986). 474. E Roussignol, D. Ricard, J. Lukasik, and C. Flytzanis, J. Opt. Soc. Am. B 4, 5 (1987). 475. G. Bret and E Gires, Appl. Phys. Lett. 4, 175 (1964). 476. N. Sarukura, Y. Ishida, T. Yanagawa, and H. Nakano, Appl. Phys. Lett. 57, 229 (1990). 477. B. Danielzik, K. Nattermann, and D. vonder Linde, Appl. Phys. B 38, 31 (1985). 478. T. J. Cullen, C. N. Ironside, C. T. Seaton, and G. I. Stegeman, Appl. Phys. Lett. 49, 1403 (1986). 479. B. J. Ainslie, H. P. Girdlestone, and D. Cotter, Electron. Lett. 23, 405 (1987). 480. J. Yumoto, S. Fukushima, and K. Kudobera, Opt. Lett. 12, 832 (1987). 481. N. Finlayson, W. C. Banyai, E. M. Wright, C. T. Seaton, G. I. Stegeman, T. J. Cullen, and C. N. Ironside, Appl. Phys. Lett. 53, 1144 (1988). 482. N. M. Lawandy and, R. L. MacDonald, J. Opt. Soc. Am. B 8, 1307 (1991). 483. S. Juodkazis, E. Bernstein, J. C. Plenet, C. Bovier, J. Dumas, J. Mugnier, and J. V. Vaitkus, Opt. Commun. 148, 242 (1998). 484. E. Vanagas, J. Moniatte, P. Riblet, B. Honerlage, S. Juodkazis, E Paille, J. C. Plenet, J. G. Dumas, M. Petrauskas, and J. Vailtkus, J. Appl. Phys. 81, 3586 (1997). 485. M. G. Bawendi, W. L. Wilson, L. Rothberg, P. J. Carroll, T. M. Jedju, M. L. Steigerwald, and L. E. Brus, Phys. Rev. Lett. 65, 1623 (1990). 486. K. Misawa, H. Yao, T. Hayashi, and T. Kobayashi, Chem. Phys. Lett. 183, 113 (1991). 487. H. Shinojima, J. Yumoto, and N. Uesugi, Appl. Phys. Lett. 60, 298 (1992). 488. T. Inokuma, T. Arai, and M. Ishikawa, Phys. Rev. B 42, 11093 (1990). 489. J. J. Shiang, S. H. Risbud, and A. P. Alivisatos, J. Chem. Phys. 98, 8432 (1993). 490. N. Peyghambarian, B. Fluegel, D. Hulin, A. Migus, M. Joffre, A. Antonetti, S. W. Koch, and M. Lindberg, IEEE J. Quantum Electron. QE-25, 2516 (1989). 491. S. Jursenas, M. Strumskis, A. Zukauskas, and A. I. Ekimov, Solid State Commun. 87, 577 (1993).
492. S. Jursenas, V. Stepankevicius, M. Strumskis, and A. Zukauskas, Semiconductor Sci. Technol. 10, 302 (1995). 493. A. Zukauskas and S. Jursenas, Appl. Phys. Lett. 67, 1095 (1995). 494. S. Jursenas, G. Kurilcik, M. Strumskis, and A. Zukauskas, Appl. Phys. Lett. 71, 2502 (1997). 495. S. Jursenas, G. Tamulaitis, G. Kurilcik, and A. Zukauskas, Appl. Phys. Lett. 72, 241 (1998). 496. P. Nemec, E Trojanek, and P. Maly, Phys. Rev. B 52, 8605 (1995). 497. J. P. Zheng, L. Shi, E S. Choa, P. L. Liu, and H. S. Kwok, Appl. Phys. Lett. 53, 643 (1988). 498. G. Allan, C. Delerue, and M. Lannoo, Phys. Rev. Lett. 76, 2961 (1996). 499. C. H. Henry and D. V. Lang, Phys. Rev. B 15, 989 (1977). 500. N. Chestnoy, T. D. Haris, R. Hull, and L. E. Brus, J. Phys. Chem. 90, 3393 (1986). 501. C. Burda and M. A. E1-Sayed, Pure Appl. Chem. 72, 165 (2000). 502. S. Logunov, T. Green, S. Marguet, and M. A. E1-Sayed, J. Phys. Chem. 102, 5652 (1998). 503. L. Spanhel, M. Haase, H. Weller, and A. Henglein, J. Am. Chem. Soc. 109, 5649 (1987). 504. M. L. Steigerwald and L. E. Brus, Acc. Chem. Res. 23, 183 (1990). 505. H. Weller, Adv. Mater 5, 88 (1993). 506. V. Klimov, P. Haring Bolivar, and H. Kurz, Phys. Rev. B 53, 1463 (1996); V. I. Klimov, P. Haring Bolivar, H. Kurz, and V. Karavinskii, Superlattices and Microstrucures 20, 395 (1996). 507. C. J. Murphy and A. B. Ellis, J. Phys. Chem. 94, 3082 (1990). 508. J. Z. Zhang and A. B. Ellis, J. Phys. Chem. 96, 2700 (1992). 509. G. J. Meyer, G. C. Lisensky, and A. B. Ellis, J. Am. Chem.Soc. 110, 4914 (1988). 510. G. C. Lisensky, R. L. Penn, C. J. Murphy, and A. B. Ellis, Science 248, 840 (1990). 511. G. J. Meyer, L. K. Leung, J. C. Yu, G. C. Lisensky, and A. B. Ellis, J. Am. Chem. Soc. 111, 5146 (1989). 512. C. J. Murphy and A. B. Ellis, Polyhedron 9, 1913 (1990). 513. C. J. Murphy, G. C. Lisensky, L. K. Leung, G. R. Kowach, and A. B. Ellis, J. Am. Chem. Soc. 112, 8344 (1990). 514. A. Henglein, Chem. Rev. 89, 1816 (1989). 515. M. L. Steigerwald and L. E. Brus, Annu. Rev. Mater Sci. 19, 471 (1989). 516. T. Dannhauser, M. O. Neil, K. Johansson, D. Whitten, and G. McLendon, J. Phys. Chem. 90, 6074 (1986). 517. N. S. Lewis, J. Phys. Chem. 102, 4843 (1998). 518. E. Lifshitz, I. D. Litvin, H. Porteanu, and A. A. Lipovskii, Chem. Phys. Lett. 295, 249 (1998). 519. A. Ekimov, J. Lumin. 70, 1 (1996). 520. J. Z. Zhang, R. H. O'Neil, and T. W. Roberti, Appl. Phys. Lett. 64, 1989 (1994). 521. M. Gratzel, "Heterogeneous Photochemical Electron Transfer," CRC, Boca Raton, FL, 1989. 522. M. A. Fox and M. T. Dulay, Chem. Rev. 93, 341 (1993). 523. P. V. Kamat, Chem. Rev. 93, 267 (1993). 524. N. Serpone and E. Pelizzetti, "Photocatalysis, Fundamentals and Applications," Wiley, New York, 1989, p 123. 525. J. R. Norris, Jr. and D. Meisel, Eds., "Photochemical Energy Conversion," Elsevier, New York, 1989. 526. (a) C. Burda, S. Link, T. C. Green, and M. A. E1-Sayed, J. Phys. Chem. 103, 10775 (1999). 526. (b) C. Burda, T. C. Green, S. Link and M. A. E1-Sayed, J. Phys. Chem. 103, 1783 (1999). 527. S. Hunsche, T. Dekorsy, V. Klimov, and H. Kurz, Appl. Phys. B 62, 3 (1996). 528. M. A. Hines and P. Guyot-Sionnest, J. Phys. Chem. 100, 468 (1996). 529. B. O. Dabbousi, J. Rodrigez-Viejo, E V. Mikulec, J. R. Heine, H. Mattoussi, R. Ober, and M. G. Bawendi, J. Phys. Chem. 101, 9463 (1997). 530. X. Peng, M. C. Schlamp, A. V. Kadavanich, U. Banin, and A. P. Alivisatos, J. Am. Chem. Soc. 119, 7019 (1997).
S U R F A C E A N D I N T E R F A C I A L R E C O M B I N A T I O N IN S E M I C O N D U C T O R S 531. D. t!. Fogg, L. H. Radzilowski, R. Blanski, R. R. Schrock, and E. L. Tho has, Micromolecules 30, 417 (1997). 532. D. L',. Fogg, L. H. Radzilowski, B. O. Dabbousi, R. R. Schrock, E. L. Tho~nas, and M. G. Bawendi, Macromolecules 30, 8433 (1997). 533. C. B Murray, D. J. Norris, and M. G. Bawendi, J. Am Chem. Soc. 115, 87()~, (1993). 534. H. lX~attoussi, L. H. Radzilowski, B. O. Dabbousi, D. E. Fogg, R. R. Schrock, E. L. Thomas, M. E Rubner, and M. G. Bawendi, J. Appl. PhyLa. 86, 4390 (1999). 535. Z. Zhu, H. Yoshihara, K. Takebayashi, and T. Yao, Appl. Phys. Lett. 63, 1678 (1993). 536. E B,:~ssler-Podorowski, D. Huppert, Y. Rosenwaks, and Y. Shapira, J. Pkvs. Chem. 95, 4370 (1991). 537. A. B. Ellis, R. J. Brainard, K. D. Kepler, D. E. Moore, E. J. Winder, T. F. Kuech, and G. C. Lisensky, J. Chem. Edu. 74, 680 (1997). 538. G. J. Meyer, G. C. Lisensky, and A. B. Ellis, J. Am. Chem. Soc. 110, 4914(1988). 539. D. R. Neu, J. A. Olson, and A. B. Ellis, J. Phys. Chem. 97, 5713 (19q3). 540. D. E. Moore, G. C. Lisensky, and A. B. Ellis, J. Am. Chem. Soc. 116, 9487 (1994). 541. J. C. Seo, D. Kim, and H. J. Kong, Appl. Phys. A 64, 445 (1997). 542. N. ]~. Borrelli, D. W. Hall, H. J. Holland, and D. W. Smith, J. Appl. Phys. 61, 5399 (1987). 543. A. [. Ekimov, I. A. Kudryavtsev, M. G. Ivanov, and A. L. Efros, J. Lumin. 46, 83 (1990). 544. E Hache, M. C. Klein, D. Ricard, and C. Flytzanis, J. Opt. Soc. Am. B 8, 1802 (1991). 545. J. Warnock and D. D. Awschalom, Phys. Rev. B 32, 5529 (1985). 546. J. P. Zheng, L. Shi, E S. Choa, P. L. Liu, and H. S. Kwok, Appl. Phys. Lett. 53, 645 (1988). 547. Y.Z. Hu, S. W. Koch, M. Lindberg, N. Peyghambariun, E. L. Pollock, and F. E Abraham, Phys. Rev. Lett. 64, 1805 (1990). 548. Y.Z. Hu, M. Lindberg, and S. W. Koch, Phys. Rev. B 42, 1713 (1990). 549. P. V. Kamat, T. W. Ebbesen, N. M. Dimitrijevic, and A. J. Nozik, Chem. Phys. Lett. 157, 384 (1989). 550. M. Ivanda, T. Bischof, G. Lermann, A. Matemy, and W. Kiefer, J. Appl. Phys. 82, 3116 (1997). 551. E Nemec and P. Maly, J. Appl. Phys. 87, 3342 (2000). 552. H. Ma. A. S. Gomez, and C. N. de Arajo, J. Opt. Soc. Am. B 9, 2230 (1992) 553. G. Be;~die, E. Sauvain, A. S. L. Gomez, and N. M. Lawandy, Phys. Rev. B 51, 2180 (1995). 554. P. Maly, E Trojanek, and A. Svoboda, J. Opt. Soc. Am. B 10, 1890 (1993). 555. U. V~bggon, M. Muller, I. Ruckmann, J. Kolenda, and M. Petrauska, Phys. Status Solid B 160, K 79 (1990). 556. M. C. Schane-Klein, L. Piveteau, M. Ghanassi, and D. Ricard, Appl. Phys. llett. 67, 579 (1995). 557. J. T. Rcmillard, H. Wang, M. D. Webb, and D. G. Steel, J. Opt. Soc. Am. B "7, 899 (1990). 558. W. Chen, Z. Wang, Z. Lin, and L. Lin, J. Appl. Phys. 82, 3111 (1997). 559. B. T. Jonker, J. J. Krebs, and G. A. Prinz, J. Appl. Phys. 63, 5885 (1988). 560. C. Pong, N. M. Johnson, R. A. Street, J. Walker, R. Feigelson, and R. C. De Mattei, Appl. Phys. Lett. 61, 3026 (1992). 561. H. Wang, K. S. Wong, B. A. Forman, Z. Y. Yang, and G. K. L. Wong, J. Appl. Phys. 83, 4773 (1998). 562. G.von Freymann, D. Luerben, C. Rabenstein, M. Mikolaiczyk, H. Richter, H. Kalt, Th. Schimmel, M. Wegener, K. Okhawa, and D. Hommel, Appl. Phys. Lett. 76, 203 (2000). 563. M. A. Haase, J. Qui, J. M. Depuydt, and H. Cheng, Appl. Phys. Lett. 59, 1272 (1991). 564. J. M. Gaines, R. R. Drenten, K. W. Haberem, T. Marshall, E Mensz, and J. Petruzzello, Appl. Phys. Lett. 62, 2462 (1993). 565. D. C. Grillo, Y. Fan, J. Han, L. He, R. L. Gunshor, A. Salokatve, H. Hagerott, H. Jeon, A. V. Nurmikko, G. C. Hua, and N. Otsuka, Appl. Phys. Lett. 63, 2723 (1993).
279
566. S. Itoh, H. Okuyama, S. Matsumoto, N. Nakayama, T. Ohata, T. Miyajima, A. Ishibashi, and K. Akimoto, Electron. Lett. 29, 766 (1993). 567. N. Nakayama, S Itoh, H. Okuyama, M. Ozawa, T. Ohata, K. Nakano, M. Ikeda, A. Ishibashi, and Y. Mori, Electron. Lett. 29, 2194 (1993). 568. H. Morkoc, S. Strite, G. B. Gao, M. E. Lin, B. Sverdlov, and M. Burns, J. Appl. Phys. 76, 1363 (1994). 569. S. Guha, J. M. Depuydt, M. A. Haase, J. Qiu, and H. Cheng, Appl. Phys. Lett. 63, 3107 (1993). 570. G. C. Hua, N. Otsuka, D. C. Grillo, Y. Fan, J. Han, M. D. Ringle, R. L. Gunshor, M. Hovinen, and A. V. Nurmikko, Appl. Phys. Lett. 65, 1331 (1994). 571. S. Guha, H. Cheng, M. A. Haase, J. M. Depuydt, J. Qiu, B. J. Wu, and G. E. Hofler, Appl. Phys. Lett. 65, 801 (1994). 572. M. Hovinen, J. Ding, A. V. Nurmikko, G. C. Hua, D. C. Grillor, L. He, J. Han, and R. L. Gunshor, Appl. Phys. Lett. 66, 2013 (1995). 573. R. Spiegel, G. Bacher, K. Herz, M. Illing, T. Kummell, A. Forchel, B. Jobst, D. Hommel, G. Landwehr, J. Sollner, and M. Heuken, Phys. Rev. B 53, R 4233 (1996). 574. L. M. Sparing, P. D. Wang, S. H. Xin, S. W. Short, S. S. Shi, J. K. Furdyna, J. L. Merz, and G. L. Snider, J. Vac. Sci. Technol. B 14, 3654 (1996). 575. L. M. Sparing, E D. Wang, A. M. Mintairov, S. Lee, U. Bindley, C. H. Chen, S. S. Shi, J. K. Furdyna, J. L. Merz, and G. L. Snider, J. Vac. Sci. Technol. B 15, 2652 (1997). 576. L. M. Sparing, A. M. Mintairov, J. H. Hodak, I. B. Martini, G. V. Hartland, U. Bindley, S. Lee, J. K. Furdyna, J. L. Merz, and G. L. Snider, J. Appl. Phys. 87, 3063 (2000). 577. W. Faschinger and J. Nurnberger, Appl. Phys. Lett. 77, 187 (2000). 578. Z. Sobiesierski, I. M. Dhermadasa, and R. H. Willaims, Appl. Phys. Lett. 53, 2623 (1988). 579. E M. Amritharaj and N. K. Dhar, J. Appl. Phys. 67, 3107 (1990). 580. J. Garcia-Garcia, J. Gonzalez-Hernandez, J. G. Mendoza-Alvarez, E. L. Cruz, and G. Contreras-Puente, J. Appl. Phys. 67, 3810 (1990). 581. Y. H. Kim, S. Y. An, J. Y. Lee, I. J. Kim, K. N. Oh, S. U. Kim, M. J. Park, and T. S. Lee, J. Appl. Phys. 85, 7370 (1999). 582. R. Cohen, S. Bastide, D. Cahen, J. Libman, A. Shanzer, and Y. Rosenwaks, Optical Materials 9, 394 (1998).; R. Cohen, L. Kronik, A. Shanzer, D. Cahen, A. Liu, Y. Rosenwaks, J. K. Lorenz, and A. B. Ellis, J. Am. Chem. Soc. 121, 10545 (1999). 583. C. K. Kang, Sh. U. Yuldashev, J. H. Leem, Y. S. Ryu, J. K. Hyun, H. S. Jung, H. I. Lee, Y. D. Woo, and T. W. Kim, J. Appl. Phys. 88, 2013 (2000). 584. A.J. Nelson, S. E Frigo, and R. A. Rosenberg, J. Appl. Phys. 75, 1632 (1994). 585. S. Gurumurthy, H. L. Bhat, B. Sundersheshu, R. K. Bagai, and V. Kumar, Appl. Phys. Lett. 68, 2424 (1996). 586. I. M. Dharmadasa, J. M. Thornton, and R. H. Willaims, Appl. Phys. Lett. 54, 137 (1989). 587. H. C. Chou, A. Rohatgi, N. M. Jokerst, S. Kamra, S. R. Stock, S. L. Lowrie, R. K. Ahrenkiel, and D. H. Levi, Mater. Chem. Phys. 43, 178 (1996). 588. H. C. Chou and A. Rohatgi, J. Electron. Mater. 23, 31 (1994). 589. M. Illing, G. Bacher, A. Forchel, T. Litz, A Waag, and G. Landwehr, Appl. Phys. Lett. 66, 1815 (1995). 590. L. Kronik, L. Burstein, Y. Shapira, and M. Oron, Appl. Phys. Lett. 63, 60 (1993). 591. R. Cohen, V. Lyahovitskaya, E. Poles, A. Liu, and Y. Rosenwaks, Appl. Phys. Lett. 73, 1400 (1998). 592. I. Delgadillo, M. Vargas, A. Cruz-Orea, J. J. Alvarado-Gil, R. Baquero, E Sanchez-Senencio, and H. Vergas, Appl. Phys. B 64, 97 (1997). 593. H. Vargas and L. C. M. Miranda, Phys. Rep. 161, 45 (1988). 594. J. Bernal-Alvarada, M. Vargas, J. J. Alvarado-Gil, I. Delgadillor, A. Cruz-Orea, H. Vargas, M. Tufino-Velazquez, M. L. Albor-Aguilera, and M. A. Gonzalez-Trujillo, J. App. Phys. 83, 3807 (1998). 595. Y. E Chen and W. S. Chen, Appl. Phys. Lett. 59, 703 (1991).
280
KASI VISWANATH
596. W. M. Lau, J. W. He, and E R. Norton, Appl. Phys. Lett. 54, 772 (1989). 597. O. E Agnihotri, C. A. Musca, and L. Faraone, Semicond. Sci. Technol. 13, 839 (1998). 598. G. Bahir, V. Ariel, V. Garber, and D. Rosenfeld, Appl. Phys. Lett. 65, 2725 (1994). 599. J. K. White, C. A. Musca, H. C. Lee, and L. Faraone, Appl. Phys. Lett. 76, 2448 (2000). 600. P. A. Borodovskii, A. E Buldygin, and V. S. Varavin, Semiconductors 32, 963 (1998). 601. Y. K. Su and C. T. Lin, Intern. J. High Speed Electronics and Systems 8, 703 (1997). 602. A.V. Voitsektnovskii, Yu. A. Denisov, A. E Kokhanenko, V. S. Varavin, S. A. Dvoretskii, N. N. Mikhailov, Yu.G. Sidorov, and M. V. Yakushev, Russian Physics Journal 40, 915 (1997). 603. Y. E Chen, C. S. Tsai, Y. H. Chang, Y. M. Chang, T. K. Chen, and Y. M. Pang, Appl. Phys. Lett. 58, 493 (1991). 604. M.D. Kim, T. W. Kang, G. H. Kim, M. S. Han, H. D. Cho, J. M. Kim, Y. T. Jeoung, and T. W. Kim, J. Appl. Phys. 73, 4074 (1993). 605. M. D. Kim, T. W. Kang, J. M. Kim, H. K. Kim, Y. T. Jeoung, and T. W. Kim, J. Appl. Phys. 73, 4077 (1993). 606. H. Chen, J. Tong, Z. Hu, D. T. Shi, G. H. Wu, K. T. Chen, M. A. George, W. E. Collins, R. B. James, C. M. Stahle, and L. M. Bartlett, J. appl. Phys. 80, 3509 (1996). 607. K. N. Oh, K. H. Kim, J. K. Hong, Y. C. Chung, S. U. Kim, and M. J. Park, J. Cryst. Growth 184-185, 1247 (1998). 608. H. Meincke, D. G. Ebling, J. Heinze, M. Tacke, and H. Bottner, J. Electrochem. Soc. 145, 2806 (1998). 609. J. Linnros, J. Appl. Phys. 84, 275 (1998). 610. J. Linnros, J. Appl. Phys. 84, 284 (1998). 611. J. Schmidt and A. G. Aberle, Prog. Photovolt. 6, 259 (1998). 612. N. Keskitalo, E Jonsson, K. Nordgren, H. Bleichner, and E. Nordlander, J. Appl. Phys. 83, 4206 (1998). 613. T. Saitoh, Y. Nisimoto, and H. Hasegawa, Solar Energy Materials and Solar Cells 34, 161 (1994). 614. H. C. Ostendorf and A. L. Endros, Appl. Phys. Lett. 71, 3275 (1997). 615. K. Tholmann, M. Yamaguchi, and A. Yahata, Scanning Microscopy Supplement 9, 125 (1995). 616. E Eichinger and M. Rommel, "Proc. Sym. Cryst. Defects and Contamination: Their Impact and Control in Device Manufacturing," Electrochem. Soc., Pennington, NJ, p. 363. 617. M. Brubaker, E Roman, J. Staffa, and J. Ruzyllo, Electrochem. Solid State Lett. 1, 130 (1998). 618. A. Cutolo, S. Daliento, A. Sanseverino, G. E Vitale, and L. Zeni, Solid State Electron. 42, 1035 (1998). 619. B. Adamowicz and H. Hasegawa, Jpn. J. Appl. Phys. 37, 1631 (1998). 620. T. H. Wang and T. E Ciszek, "Proc. Twenty-Sixth IEEE Photovoltaic Specialists Conference," New York, 1997, p. 55. 621. Y. Ogita, Y. Uematsu, and H. Daio, "Sym. Science and Technology of Semicondcutor Surface Preparation," Material Research Society, Pittsburgh, PA, 1997, p. 329. 622. R. Schieck and M. Kunst, Solid State Electron. 41, 1755 (1997). 623. E. Yablonovitch, D. L. Allara, C. C. Chang, T. Gmitter, and T. B. Bright, Phys. Rev. Lett. 57, 249 (1986). 624. J. Cho, T. E Schneider, J. Vanderweide, H. Jeon, and R. J. Nemanich, Appl. Phys. Lett. 59, 1995 (1991). 625. I. A. Veloarisoa, M. Stavola, D. M. Kozuch, R. E. Peale, and G. D. Watkins, Appl. Phys. Lett. 59, 2121 (1991). 626. M. Ishii, K. Nakashima, I. Tajima, and M. Yamamoto, Appl. Phys. Lett. 58, 1378 (1991). 627. J. I. Hanoka, C. H. Seager, D. J. Sharp, and J. K. G. Panitz, Appl. Phys. Lett. 42, 618 (1983). 628. W. M. Chen, I. A. Buyanova, W. X. Ni, G. V. Hansson, and B. Monemar, Appl. Phys. Lett. 70, 369 (1997). 629. K. Ljungberg, A. Soderbarg, and U. Jansson, Appl. Phys. Lett. 67, 650 (1995).
630. M. Niwano, Y. Kimura, and N. Miyamoto, Appl. Phys. Lett. 65, 1692 (1994). 631. M. Egawa and H. Ikoma, Jpn. J. Appl. Phys. 33, 943 (1994). 632. T. Konoshi, T. Yao, M. Tajima, H. Oshima, H. Ito, and T. Hattori, Jpn. J. Appl. Phys. 31, L 1216 (1992). 633. L. Li, H. Bender, T. Trenkler, P. W. Mertens, M. Meuris, W. Vandervorst, and M. M. Heyns, J. Appl. Phys. 77, 1323 (1995). 634. M. Niwano, Y. Takeda, Y. Ishibashi, K. Kurita, and N. Miyamoto, J. appl. Phys. 71, 5646 (1992). 635. T. Takahagi, I. Nagai, A. Ishitani, H. Kuroda, and Y. Nagasawa, J. Appl. Phys. 64, 3516 (1988). 636. K. Utani, T. Suziki, and S. Adachi, J. Appl. Phys. 73, 3467 (1993). 637. Th. Dittrich, V. Yu. Timoshenko, and J. Rappich, Appl. Phys. Lett. 72, 1635 (1998). 638. H. M'saad, J. Michel, A. Reddy, and L. C. Kimmerling, J. Electrochem. Soc. 142, 2833 (1995). 639. S. Adachi, T. Arai, and K. Kobayashi, J. Appl. Phys. 80, 5422 (1996). 640. G. J. Kluth and R. Maboudian, J. Appl. Phys. 80, 5408 (1996). 641. U. Neuwald, A. Feltz, U. Memmert, and R. J. Behm, J. Appl. Phys. 78, 4131 (1995). 642. M. R. Houston and R. Maboudian, J. Appl. Phys. 78, 3801 (1995). 643. M. Niwano, Y. Takeda, and N. Miyamoto, J. Appl. Phys. 72, 2488 (1992). 644. H. J. Lewerenz and G. Schlichthorl, J. Appl. Phys. 75, 3544 (1994). 645. S. Rauscher, Th. Dittrich, M. Aggour, J. Rappich, H. Flietner, and H. J. Lewerenz, Appl. Phys. Lett. 66, 3018 (1995). 646. J. Schmidt, E M. Schuurmans, W. C. Sinke, S. W. Glunz, and A. G. Aberle, Appl. Phys. Lett. 71,252 (1997). 647. A. G. Aberle, T. Lauinger, J. Schmidt, and R. Hezel, Appl. Phys. Lett. 66, 2828 (1995). 648. A. G. Aberle, Recent Res. Devel. Vacuum Sci. Tech. 1, 177 (1999). 649. T. Lauinger, J. Schmidt, A. G. Aberle, and R. Hezel, Appl. Phys. Lett. 68, 1232 (1996). 650. T. Lauinger, J. Moschner, A. G. Aberle, and R. Hezel, J. Vac. Sci. Technol. A 16, 530 (1998). 651. A. G. Aberle and R. Hezel, Prog. Photovoltaics 5, 29 (1997). 652. B. Bitnar, R. Glathaar, S. Keller, J. Kugler, M. Spiegel, P. Fath, G. Willeke, E. Bucher, E Duerinckx, J. Szlufcik, J. Nijs, R. Mertens, H. Nussbaumer, and E Ferrazza, "Proc. 14 European Photovoltaic Solar Energy Conference," Barcelona, Spain, 1997, p. 1491. 653. J. Kolnik, J. Ivanco, and M. Ozvold, Phys. Stat. Sol. (a) 130, 245 (1992); J. Ivanco, I. Thurzo, and E. Pincik, Appl. Phys. Lett. 65, 2594 (1994). 654. Y. Saito, N. Lio, Y. Kameshima, R. Takeda, and H. Kuwano, J. Appl. Phys. 63, 1117 (1988). 655. J. M. Shannon and B. A. Morgan, J. Appl. Phys. 86, 1548 (1999). 656. H. E. Elgamel, A. Barnett, A. Rohatgi, Z. Chen, C. Vinckier, J. Nijs, and R. Mertens, J. Appl. Phys. 78, 3457 (1995). 657. S. Okamoto, Sharp Technical Journal 70, 24 (1998). 658. H. M'saad, J. Michel, J. J. Lappe, and L. C. Kimmerling, J. Electronic Mater. 23, 487 (1994). 659. T. Aoyama, T. Yamazaki, and T. Ito, Appl. Phys. Lett. 59, 2576 (1991). 660. I. Bello, W. H. Chang, and W. M. Lau, J. Appl. Phys. 75, 3092 (1994). 661. H. C. Neitzert, N. Layadi, E R. Cabarrocas, and R. Vanderhaghen, Appl. Phys. Lett. 65, 1260 (1994). 662. H. Kobayashi, S. Tachibana, K. Yamanaka, Y. Nakato, and K. Yoneda, J. Appl. Phys. 81, 7630 (1997). 663. E Moriarty, L. Koenders, and G. Hughes, Phys. Rev. B 47, 15950 (1993). 664. A. W. Dunn, E Moriarty, M. D. Upward, and E H. Beton, Appl. Phys. Lett. 69, 506 (1996). 665. E Moriarty, M. D. Upward, A. W. Dunn, Y. R. Ma, E H. Beton, and D. Teehan, Phys. Rev. B 57, 362 (1998). 666. J. G. Lee and S. R. Morrision, J. Appl. Phys. 64, 6679 (1988). 667. E Grey and K. Hermansson, Appl. Phys. Lett. 71, 3400 (1997). 668. A.I. Petrov and V. A. Rozhkov, Technical Physics Lett. 24, 254 (1998). 669. W. E Lee and Y. L. Khong, J. Appl. Phys. 87, 7845 (2000).
S U R F A C E A N D I N T E R F A C I A L R E C O M B I N A T I O N IN S E M I C O N D U C T O R S 670. R. 1~ Wood, R. D. Westbrook, and G. E. Jellison, Jr., Appl. Phys. Lett. 50, 107 (1987). 671. G..;. Norga, M. Platero, K. A. Black, A. J. Reddy, J. Michel, and L. ('. Kimmerling, J. Electrochem. Soc. 145, 2602 (1998). 672. B. Nesterenko, O. Stadnik, A. Cricenti, and E Perfetti, Nuovo Cimento 20D, 1025 (1998). 673. E Tardif, J. E Joly, A. Courteaux, U. Straube, A. Danel, and G. Kamarinos, "Proc. Symp. Crystalline Defects and Contamination: Their Impact and Control in Device Manufacturing," Electrochem. Sot., Pennington, New Jersey, 1997, p. 385. 674. A. Buczkowski, E Kirscht, and H. Koya, "Proc. Symp. Crystalline Defects and Contamination: Their Impact and Control in Device Manufacl aring," Electrochem. Soc., Pennington, New Jersey, 1997, p. 376. 675. M. 1.,. Polignano, C. Bresolin, G. Pavia, V. Soncini, F. Zanderigo, G. Q~aeirolo, and M. di Dio, Mater. Sci. Engg. B 53, 300 (1998). 676. H. P',lrk, C. R. Helms, and D. Ko, J. Electrochem. Soc. 145, 1724 (1995). 677. D. ~. Ramappa and W. B. Henley, Appl. Phys. Lett. 72, 2298 (1998). 678. G. J. Norga, M. R. Black, K. A. Black, H. M'saad, J. Michel, and L. C. Kimurling, "Proc. Intern. Conf. on Defects in Semiconductors," Sendai, Japan, 1995. 679. G. J. Norga, K. A. Black, H. M'saad, J. Michel, and L. C. Kimurling, Mater. Sci. Technol. 11, 90 (1995). 680. M. Yajima and S. Ibuka, J. Appl. Phys. 84, 2224 (1998). 681. M. Tajima, S. Ibuka, and M. Warashina, Mat. Sci. Forum 258-263, 1731 (1997). 682. M. Tajima, S. Ibuka, H. Aga, and T. Abe, Appl. Phys. Lett. 70, 231 (1997). 683. M. Tajima, A. Ogura, T. Karasawa, and A. Mizoguchi, Jpn. J. Appl. Phys. 37, L 1199 (1998). 684. J. Shah, M. Combescot, and A. H. Dayem, Phys. Rev. Lett. 38, 1497 (1977). 685. N.F. Mott, "Metal-Insulator Transitions," Taylor & Francis, New York, 1990. 686. E J. Bedrossian and T. D. de la Rubia, J. Vac. Sci. Technol. A 16, 1043 (1998). 687. M. A. Green, "Solar Cells," University of New South Wales, Sydney, Australia, 1982. 688. M. A. Green, "High Efficiency Silicon Solar Cells," Trans Tech, Switzerland, 1987. 689. M. A. Green, Prog. Photov. 2, 87 (1994). 690. M. A. Green, Semicond. Sci. Technol. 8, 1 (1993). 691. M. A. Green, Prog. Photov. 2, 73 (1994). 692. M. A. Green, "Silicon Solar Cells: Advanced Principles and Practice, Centre for Photovoltaic Devices and Systems," University of New South Wales, Sydney, Australia, 1995. 693. R. Hezel, Progr. Photov. 5, 109 (1997). 694. A.G. Aberle, "Crystalline Silicon Solar Cells," Centre for Photovoltaic Engineering, University of New South Wales, Sydney, Australia, 1999. 695. S.J. Robinson, A. G. Aberle, and M. A. Green, J. Appl. Phys. 76, 7920 (1994) 696. A. G. Aberle, S. J. Robinson, A. Wang, J. Zhao, S. R. Wenham, and M. A. Green, Prog. Photov. 1, 133 (1993). 697. P. E Altermatt, G. Heiser, X. Dai, J. Jurgens, A. Aberle, S. J. Robinson, T. Young, S. R. Wenham, and M. A. Green, J. Appl. Phys. 80, 3574 (1996). 698. S. Narasimha, G. Crotty, A. Rohatgi, and D. L. Meier, Solid State Electron. 9, 1631 (1998). 699. Y. Bai, J. E. Phillips, and A. M. Barnett, IEEE Trans. Electron Devices 45, 1784 (1998). 700. S. Narasimha and A. Rohatgi, IEEE Trans. Electron Devices 45, 1776 (1998). 701. E Doshi and A. Rohatgi, IEEE Trans. Electron Devices 45, 1710 (1998). 702. D. D. Smith and A. M. Barnett, "Proc. 26th IEEE Photovoltaic Specialists Conference," IEEE, New York, 1997, p. 767.
281
703. E Doshi, J. Moschner, J. Jeorj, A. Rohatgi, R. Singh, and S. Narayanan, "Proc. 26th IEEE Photovoltaic Specialists Conference," IEEE, New York, 1997, p. 87. 704. M. Stocks, A. Blakers, and A. Cuevas, "Proc. 26th IEEE Photovoltaic Specialists Conference," IEEE, New York, 1997, p. 67. 705. S. Narasimha and A. Rohatgi, "Proc. 26th IEEE Photovoltaic Specialists Conference," IEEE, New York, 1997, p. 63. 706. A. Metz, R. Meyer, B. Kuhlmann, M. Grauvogl, and R. Hezel, "Proc. 26th IEEE Photovoltaic Specialists Conference," IEEE, New York, 1997, p. 31. 707. A. U. Ebong and S. H. Lee, Solid State Electron. 41, 1745 (1997). 708. C. Hebling, S. W. Glunz, C. Schetter, J. Knobloch, and A. Raeuber, Solar Energy Materials and Solar Cells 48, 335 (1997). 709. Y. H. Cho, A. U. Ebong, E. C. Cho, D. S. Kim, and S. H. Lee, Solar Energy Materials and Solar Cells 48, 173 (1997). 710. J. Salami, A. Shibta, D. Meier, E. Kochka, S. Yamanaka, P. Davis, J. Easoz, T. Abe, and K. Kinoshita, Solar Energy Materials and Solar Cells 48, 159 (1997). 711. L. Cai, A. Rohatgi, S. Han, G. May, and M. Zou, J. Appl. Phys. 83, 5885 (1998). 712. S. Narasimha and A. Rohatgi, Appl. Phys. Lett. 72, 1872 (1998). 713. A. Rohatgi, E Doshi, J. Moschner, T. Lauinger, A. G. Aberle, and D. S. Ruby, IEEE Trans. Electron Devices 47, 987 (2000). 714. A. W. Stephens, A. G. Aberle, and M. A. Green, J. Appl. Phys. 76, 363 (1993). 715. A. G. Aberle, P. E Altermatt, G. Heiser, S. J. Robinson, A. Wang, J. Zhao, U. Krumbein, and M. A. Green, J. Appl. Phys. 77, 3491 (1995). 716. S. Zh. Karazhanov, J. Appl. Phys. 76, 2689 (2000). 717. D. E. Ackley, J. Tauc, and W. Paul, Phys. Rev. Lett. 43, 715 (1979). 718. Z. Vardeny and J. Tauc, Phys. Rev. Lett. 46, 1223 (1981). 719. Z. Vardeny, J. Strait, D. Pfost, J. Tauc, and B. Abeles, Phys. Rev. Lett. 48, 1132 (1982). 720. Z. Vardeny, J. Strait, and J. Tauc, Appl. Phys. Lett. 42, 580 (1983). 721. D. L. Staebler and C. R. Wronski, Appl. Phys. Lett. 31, 292 (1977); J. Appl. Phys. 51, 3262 (1980). 722. H. Dersch, J. Stuke, and J. Beichler, Appl. Phys. Lett. 38, 456 (1981). 723. J. I. Pankove and E. I. Berkeyheiser, Appl. Phys. Lett. 37, 705 (1980). 724. R. A. Street, Appl. Phys. Lett. 42, 507 (1983). 725. S. Komuro, Y. Aoyagi, Y. Segawa, S. Namba, A. Mosuyama, H. Okamoto, and Y. Hamakawa, Appl. Phys. Lett. 42, 79 (1983). 726. J. Strait and J. Tauc, Appl. Phys. Lett. 47, 589 (1985). 727. S. Guha, J. Yang, W. Czubatyj, S. J. Hudgens, and M. Hack, Appl. Phys. Lett. 42, 588 (1983). 728. W. B. Jackson, D. K. Biegelsen, R. J. Namanich, and J. C. Knights, Appl. Phys. Lett. 42, 105 (1983). 729. A. V. Gelator, J. D. Cohen, and J. P. Harbison, Appl. Phys. Lett. 49, 722 (1986). 730. R. C. Frye, J. J. Kumler, and C. C. Wong, Appl. Phys. Lett. 50, 101 (1987). 731. Y. S. Tsuo, Y. Xu, R. S. Crandall, H. S. Ullal, and K. Emery, "Proc. Conf. Amorphous Silicon Technology," Mater. Res. Soc., Pittsburgh, PA, 1989, p. 471. 732. S. A. McQuaid, S. Holgado, J. Garrido, J. Martinez, J. Piqueras, R. C. Newman, and J. H. Tucker, J. Appl. Phys. 81, 7612 (1997). 733. J. H. Jang and K. S. Lim, Appl. Phys. Lett. 71, 1846 (1997). 734. S. S. Georgiev, N. Nedev, D. Sueva, and A. Toneva, J. Phys: Condens. Matt. 10, 5515 (1998). 735. J. Zimmer, H. Stiebig, J. Folsch, E Finger, Th.Eickhoff, and H. Wagner, "Proc. Symp. Amorphous and Microcrystalline Silicon Technology," Mater. Res. Soc., Pittsburgh, PA, 1997, p. 735. 736. U. Gnutzmann and K. Clausecker, Appl. Phys. 3, 9 (1974). 737. V. Arbet-Engels, M. A. Kallel, and K. L. Wang, Appl. Phys. Lett. 59, 1705 (1991). 738. A. St. Amour, J. C. Sturm, Y. Lacroix, and M. L. W. Thewalt, Appl. Phys. Lett. 65, 3344 (1994). 739. N. Usami, Y. Shiraki, and S. Fukatsu, Appl. Phys. Lett. 68, 2340 (1996).
282
KASI VISWANATH
740. R. J. Turton and M. Jaros, Appl. Phys. Lett. 69, 2891 (1996). 741. A. Galeckas, S. Juodkazis, E. Vanagas, V. Netiksis, M. Petrauskas, A. Bitz, J. L. Staehli, and M. Willander, Appl. Phys. 83, 4756 (1998). 742. A. Uhlir, Bell Syst. Tech. J. 35, 333 (1956). 743. D. R. Turner, J. Electrochem. Soc. 105, 402 (1958). 744. L. T. Canham, Appl. Phys. Lett. 50, 1046 (1990). 745. V. Lehmann and U. Gosele, Appl. Phys. Lett. 58, 856 (1991). 746. A. Halimaoui, C. Oules, G. Bomchil, A. Bsiesy, E Gaspard, R. Herino, M. Ligeon, and E Muller, Appl. Phys. Lett. 59, 304 (1991). 747. A. Cullis and L. T. Canham, Nature 353, 335 (1991). 748. A. Bsiesy, J. C. Vial, E Gaspard, R. Herino, M. Ligeon, E Muller, R. Romestain, A. Wasiela, A. Halimaoui, and G. Bomchcil, Surf. Sci. 254, 195 (1991). 749. R. E Vasquez, R. W. Fathauer, T. George, A. Kesendzov, and T. L. Lin, Appl. Phys. Lett. 60, 1004 (1992). 750. C. Tsai, K. H. Li, J. Sarathy, S. Shih, J. C. Campbell, B. K. Hance, and J. M. White, Appl. Phys. Lett. 59, 2814 (1992). 751. C. Tsai, K. H. Li, D. S. Kinosky, R. Z. Qian, T. C. Hsu, J. T. Irby, S. K. Banerjee, A. E Tasch, and J. C. Campbell, Appl. Phys. Lett. 60, 1700 (1992). 752. S. M. Prokes, O. J. Glembocki, V. M. Bermudez, R. Kaplan, L. E. Friedersdorf, and P. C. Searson, Phys. Rev. B 45, 13788 (1992). 753. M. B. Robinson, A. C. Dillon, D. R. Haynes, and S. M. George, Appl. Phys. Lett. 61, 1414 (1992). 754. S. M. Prokes, Appl. Phys. Lett. 62, 3244 (1993). 755. S. M. Prokes, W. E. Carlos, and O. J. Glembocki, Phys. Rev. B 50, 17093 (1994). 756. J. L. Gole and D. A. Dixon, J. Phys. Chem. B 101, 8098 (1997). 757. M. S. Brandt, H. D. Fuchs, M. Stutzmann, J. Weber, and M. Cardona, Solid State Commun. 81,307 (1992). 758. E Koch, V. Petrova, T. Muschik, A. Nikolov, and V. Gavrilenko, Mater. Res. Soc. Symp. Proc. 283, 197 (1993). 759. G. G. Qin and Y. Q. Jia, Solid State Commun. 86, 559 (1993). 760. Y. Kanemitsu, H. Uto, and Y. Masumoto, Phys. Rev. B 48, 2827 (1993). 761. S. Sawada, N. Hamada, and N. Ookubo, Phys. Rev. B 49, 5236 (1994). 762. H. Koyama, T. Ozaki, and N. Koshida, Phys. Rev. B 52, R 11561 (1995). 763. Q. K. Andersen and E. Veje, Phys. Rev. B 53, 15643 (1996). 764. L. Jia, S. E Wong, I. H. Wilson, S. K. Hark, S. L. Zhang, Z. E Liu, and S. M. Cai, Appl. Phys. Lett. 71, 1391 (1997). 765. G. G. Qin, J. Lin, J. Q. Duan, and G. Q. Yao, Appl. Phys. Lett. 69, 1689 (1996). 766. G. G. Qin, X. S. Liu, S. Y. Ma, J. Lin, G. Q. Yao, X. Y. Lin, and K. X. Lin, Phys. Rev. B 55, 12876 (1997). 767. D. Kovalev, G. Polisski, M. Ben-Chorin, and E Koch, J. Appl. Phys. 80, 5978 (1996). 768. D. T. Jiang, I. Coulthard, T. K. Sham, J. W. Lorimer, S. P. Frigo, X. H. Feng, and R. A. Rosenberg, J. Appl. Phys. 74, 6335 (1993). 769. D. Xu, G. Guo, L. Gui, Y. Tang, B. R. Zhang, and G. G. Qin, J. Appl. Phys. 86, 2066 (1999). 770. T. Dittrich, V. Yu. Timoshenko, and J. Rappich, Appl. Phys. Lett. 72, 1635 (1998). 771. Z. Hajnal and E Deak, J. Non-Cryst. Sol. 227, 1053 (1998). 772. S. M. Prokes, "Proc. Int. Sym. Pits and Pores: Formation, Properties, and Significance for Advanced Luminescent Materials," Electrochem. Soc., Pennington, NJ, 1997, p. 378. 773. E J. Moyer, A. Pridmore, T. Martin, J. Schmidt, T. Hasche, L. Eng, and J. L. Gole, Appl. Phys. Lett. 76, 2683 (2000). 774. L. Stalmans, J. Poortmans, H. Bender, M. Caymax, K. Said, E. Vazsonyi, J. Nijs, and R. Mertens, Prog. Photovolt. 6, 233 (1998). 775. R. Kumar, Y. Kitoh, and K. Hara, Appl. Phys. Lett. 63, 3032 (1993). 776. U. Gruning, S. C. Gujrathi, S. Poulin, Y. Diawara, and A. Yelon, J. Appl. Phys. 75, 8075 (1994). 777. J. Salonen and E. Laine, J. Appl. Phys. 80, 5984 (1996). 778. S. E Withrow, C. W. White, A. Meldrum, J. D. Budai, D. M. Hembree, Jr., and J. C. Barbour, J. Appl. Phys. 86, 396 (1999).
779. K. S. Min, K. V. Shcheglov, C. M. Yang, H. A. Atwater, M. L. Brongersma, and A. Polman, Appl. Phys. Lett. 69, 2033 (1996). 780. Th. Dittrich and V. Yu. Timoshenko, J. Appl. Phys. 75, 5436 (1994). 781. H. Koyama, T. Nakagawa, T. Ozaki, and N. Koshida, Appl. Phys. Lett. 65, 1656 (1994). 782. S. Gardelis and B. Hamilton, J. Appl. Phys. 76, 5327 (1994). 783. H. Chen, X. Hou, G. Li, E Zhang, M. Yu, and X. Wang, J. Appl. Phys. 79, 3282 (1996). 784. G. Li, X. Hou, S. Yuan, H. Chen, E Zhang, H. Fan, and X. Wang, J. Appl. Phys. 80, 5967 (1996). 785. A. A. Seraphin, S. T. Ngiam, and K. D. Kolenbrander, J. Appl. Phys. 80, 6429 (1996). 786. N. Rigakis, J. Hilliard, L. A. Hassan, J. M. Hetrick, D. Andsager, and M. H. Nayfeh, J. Appl. Phys. 81,440 (1997). 787. L. Pavesi, R. Chierchia, P. Bellutti, A. Lui, E Fuso, M. Labardi, L. Pardi, E Sbrana, M. Allegrini, S. Trusso, C. Vasi, P. J. Ventura, L. C. Costa, M. C. Carmo, and O. Bisi. J. Appl. Phys. 86, 6474 (1999). 788. M. Ben-Chorin, E Moiler, and E Koch, Phys. Rev. B 49, 2981 (1994). 789. E. H. Nicollian and J. R. Brews, "MOS Physics and Technology," Wiley, New York, 1982. 790. G. Lucovsky, S. Pantelides, and E Galeener, Eds., "The Physics of MOS Interfaces," Pergamon, New York, 1980. 791. Semicond. Sci. Technol. 4, 961 (1989). This special issue discusses defect physics of Si/SiO 2. 792. E. Poindexter, P. Caplan, B. Deal, and R. Razouk, J. Appl. Phys. 52, 879 (1981). 793. R. Helms and E. Poindexter, Rep. Prog. Phys. 57, 791 (1994). 794. C.R. Helms and B. E. Deal, Eds., "The Physics and Chemistry of SiO 2 and The SiO2 Interface," Plenum, New York, 1988. 795. E. Poindexter and P. Kaplan, Prog. Surf. Sci. 14, 211 (1983). 796. E. Yablonovitch, R. M. Swanson, W. D. Eades, and B. R. Weinberger, Appl. Phys. Lett. 48, 245 (1986). 797. Z. Chen, S. K. Pang, K. Yasutake, and A. Rohatgi, J. Appl. Phys. 74, 2856 (1993). 798. Z. Chen, K. Yasutake, A. Doolittle, and A. Rohatgi, Appl. Phys. Lett. 63, 2117 (1993). 799. A. G. Aberle, S. Glunz, and W. Warta, J. Appl. Phys. 71, 4422 (1992). 800. R. B. M. Girisch, R. P. Mertens, and R. E De Keersmaecker, IEEE Trans. Electron Devices ED-35, 203 (1988). 801. K. Inoue, M. Nakamura, M. Okuyama, and Y. Hamakawa, Appl. Phys. Lett. 55, 2402 (1989). 802. A. G. Aberle, S. Glunz, and W. Warta, Solar Energy Materials and Solar Cells 29, 175 (1993). 803. S. W. Glunz, A. B. Sproul, S. Sterk, and W. Warta, "Proc. 12th European Photovoltaic Solar Energy Conf.," Amsterdam, 1994, p. 492. 804. S. Glunz, A. B. Sproul, W. Warta, and W. Wettling, J. Appl. Phys. 75, 1611 (1994). 805. A. G. Aberle, S. W. Glunz, A. W. Stephens, and M. A. Green, Prog. Photov. 2, 265 (1994). 806. A. W. Stephens, A. G. Aberle, and M. A. Green, J. Appl. Phys. 76, 363 (1994). 807. S.J. Robinson, S. R. Wenham, P. P. Altermatt, A. G. Aberle, G. Heiser, and M. A. Green, J. Appl. Phys. 78, 4740 (1995). 808. S. W. Glunz, D. Biro, S. Rein, and W. Warta, J. Appl. Phys. 86, 683 (1999). 809. M. Y. Ghannam and R. P. Mertens, IEEE Electron Dev. Lett. 10, 242 (1989). 810. R. Girisch, R. P. Mertens, and R. van Overstraeten, "Proc. 16th IEEE Photovoltic Specialists Conf," 1982, p. 1231. 811. R. Girisch, R. P. Mertens, and R. E De Keersmaecher, Appl. Surf. Sci. 30, 127 (1987). 812. S. K. Lai, Appl. Phys. Lett. 39, 58 (1981). 813. S. Pang, S. A. Lyon, and W. C. Johnson, Appl. Phys. Lett. 40, 709 (1982). 814. S. K. Lai, J. Appl. Phys. 54, 2540 (1983). 815. M. M. Heyns, R. E DeKeersmaecker, and M. W. Hillen, Appl. Phys. Lett. 44, 202 (1984).
S U R F A C E A N D I N T E R F A C I A L R E C O M B I N A T I O N IN S E M I C O N D U C T O R S
816. Y. Nissen-Cohen, J. Shappir, and D. Frohman-Bentchkowsky, J. Appl. Phy~'. 60, 2024 (1986). 817. D. 3 DiMaria, Appl. Phys. Lett. 51,655 (1987). 818. M ~slam, J. Appl. Phys. 62, 159 (1987). 819. Y. Nissan-Cohen and T. Gorczyca, Electron. Dev. Lett. EDL-9, 287 (1')~;8). 820. S. J Wang, J. M. Sung, and S. A. Lyon, Appl. Phys. Lett. 52, 1431 821. 822. 823. 824. 825. 826. 827. 828. 829. 830. 831. 832. 833. 834. 835. 836. 837. 838. 839. 840. 841. 842. 843. 844.
845. 846. 847. 848. 849. 850. 851. 852.
853.
854.
855. 856.
D. J. DiMaria and J. Stasiak, J. Appl. Phys. 65, 2342 (1989). D. A. Buchanan and D. J. DiMaria, J. App1. Phys. 67, 7439 (1990). D. A. Buchanan, Appl. Phys. Lett. 65, 1257 (1994). W. 1.. Warren and E M. Lenahan, Appl. Phys. Lett. 49, 1296 (1986). P. ~'d Lenahan and E V. Dressendorfer, J. AppI. Phys. 55, 3495 (1984). A. Slesmans, Phys. Rev. Lett. 70, 1723 (1993). J. 1~ Stathis and D. J. DiMarria, Appl. Phys. Lett. 61, 2887 (1992). K. L. Brower and S. M. Meyers, Appl. Phys. Lett. 57, 162 (1990). D. '~ilillaume, A. Mir, R. Bouchakour, and M. Jordain, J. Appl. Phys. 73, 777 (1993). D. L. Griscom, J. Electron. Mater. 21,763 (1972). E. C.!rtier and D. J. DiMarria, Microelectron. Eng. 22, 207 (1993). E. C~,crtier, J. H. Stathis, and D. A. Buchanan, Appl. Phys. Lett. 63, 1510 (1993). E. t~artier, D. A. Buchanau, and G. J. Dunn, Appl. Phys. Lett. 64, 901 (19'),1). J. I-L Stathis and E. Cartier, Phys. Rev. Lett. 72, 2745 (1994). E. Cartier, D. A. Buchanau, J. H. Stathis, and D. J. DiMarria, J. NonCrypt. Solids 187, 244 (1995). E. (;artier and J. H. Stathis, Microelectron. Eng. 28, 3 (1995). E. (;artier and J. H. Stathis, Appl. Phys. Lett. 69, 103 (1996). R. A. B. Devine, J. L. Autran, W. L. Warren, K. L. Vanheusdan, and J. C. Rostaing, Appl. Phys. Lett. 70, 2999 (1997). J. W. Lyding, K. Hess, and I. C. Kizilyalli, Appl. Phys. Lett. 68, 2526 (1996). Y. B. Park and S. W. Rhee, J. Appl. Phys. 86, 1346 (1999). J.F. Zhang, H. K. Sii, R. Degraeve, and G. Groeseneken, J. Appl. Phys. 87, 2967 (2000). W. Fussel, M. Schmidt, H. Angermann, G. Mende, and H. Flietner, Nuel. lnstrum. Methods Phys. Res. A 377, 177 (1996). M.J. l~Jren, J. H. Stathis, and E. Cartier, J. Appl. Phys. 80, 3915 (1996). N. M. Johnson, D. K. Biegelsen, M. D. Moyer, S. T. Chang, E. H. Poindexter, and P. J. Caplan, Appl. Phys. Lett. 43, 563 (1983). H. Flietner, Mater Sci. Forum 185-188, 73 (1995). J. Albohn, W. Fussel, N. D. Sinh, K. Kliefoth, and W. Fuhs, J. Appl. Phys. 88, 842 (2000). G. Lupke, D. J. Bottomley, and H. M. van Driel, Phys. Rev. B 47, 10389 (1993). J. Bloch, J. G. Mihaychuk, and H. M. van Driel, Phys. Rev. Lett. 77, 920 (1996). J. G. Mihaychuk, J. Bloch, Y. Liu, and H. M. van Driel, Opt. Lett. 20, 2063 (1995). J. E Me Gilp, Prog. Surf. Sci. 49, 1 (1995). N. Shamir, J. G. Mihaychuk, and H. M. van Driel, J. Vac. Sci. Technol. A 15, 2081 (1997). O. A. Aktsipetrov, A. A. Fedyanin, E. D. Mishina, A. A. Nikulin, A. N. Rubtsov, C. W. van Hasselt, M. A. C. Devillers, and T. Rasing, Phys. Rev. Lett. 78, 46 (1997). J. I. Dadap, X. E Hu, N. M. Russel, J. G. Ekerdt, J. K. Lowell, and M. C. Downer, IEEE J. Sel. Top. Quantum Electron. 1, 1145 (1996). U. Emmerichs, C. Meyer, H. J. Bakker, H. Kurz, C. H. Bjorkman, C. E. Shearon, Jr., Y. Ma, T. Yasuda, Z. Jing, G. Lucovsky, and J. L. Whitten, Phys. Rev. B 50, 5506 (1994). C. Meyer, G. Lupke, U. Emmerichs, E Wolter, H. Kurz, C. H. Bjoorkman, and G. Lucovsky, Phys. Rev. Lett. 74, 3001 (1995). C. W. van Hasselt, M. A. C. Devillers, T. Rasing, and O. A. Aktsipetrov, J. Opt. Soc. Am. B 12, 33 (1995).
283
857. J. I. Dadap, X. E Hu, M. H. Anderson, M. C. Downer, J. K. Lowell, and O. A. Aktsipetrov, Phys. Rev. B 53, R 7607 (1996). 858. E Godefroy, W. de Jong, C. W. van Hasselt, M. A. C. Devillers, and T. Rasing, Appl. Phys. Lett. 68, 1981 (1996). 859. A. Nahata, T. F. Heinz, and J. A. Misewich, Appl. Phys. Lett. 69, 746 (1996). 860. N. Shamir, J. G. Mihaychuk, and H. M. van Driel, J. Appl. Phys. 88, 896 (2000). 861. W. Wang, G. Lupke, M. Di Ventra, S. T. Pantelides, J. M. Gilligan, N. H. Tolk, I. C. Kiziyalli, E K. Roy, G. Margaritondo, and G. Lucovsky, Phys. Rev. Lett. 81, 4224 (1993). 862. J. K. Wu, S. A. Lyon, and W. C. Johnson, Appl. Phys. Lett. 42, 585 (1983). 863. D. Lelmini, A. S. Spinelli, and A. L. Lacaita, Appl. Phys. Lett. 76, 1719 (2000). 864. A. Stesmans, Appl. Phys. Lett. 68, 2076 (1976). 865. A. Stesmans, Appl. Phys. Lett. 68, 2723 (1996). 866. A. Stesmans, Phys. Rev. B 61, 8393 (2000). 867. A. Stesmans, J. Appl. Phys. 88, 489 (2000). 868. T. D. Mishima, E M. Lenahan, and W. Weber, Appl. Phys. Lett. 76, 3771 (2000). 869. J. W. Gabrys, E M. Lenahan, and W. Weber, Microelectron. Eng. 22, 273 (1993). 870. R.L. Vranch, B. Henderson, and M. Pepper, Appl. Phys. Lett. 52, 1161 (1988). 871. M. A. Jupina and E M. Lenahan, IEEE Trans. Nucl. Sci. 36, 1800 (1989). 872. P. M. Lenahan and M. A. Jupina, Colloids Surface 45, 191 (1990). 873. M. A. Jupina and E M. Lenahan, IEEE Trans. Nucl. Sci. 37, 1650 (1990). 874. J.T. Krick, E M. Lenahan, and G. J. Dunn, Appl. Phys. Lett. 59, 3437 (1991). 875. C. A. Billman, E M. Lenahan, and W. Weber, Microelectron. Eng. 36, 271 (1997). 876. A. Stesmans and V. V. Afans'ev, J. Phys.: Condens. Matter 10, L19 (1998). 877. A. Stesmans and V. V. Afans'ev, Phys. Rev. B 57, 10030 (1998). 878. G.J. Gerardi, E. H. Poindexter, P. J. Caplan, and N. H. Johnson, Appl. Phys. Lett. 49, 348 (1986). 879. M. E. Law, Y. M. Haddara, and K. S. Jones, J. Appl. Phys. 84, 3555 (1998). 880. C. Tsamis and D. Tsoukalas, J. Appl. Phys. 84, 6650 (1998). 881. D. Tsoukalas and C. Tsamis, IEEE Electron Dev. Lett. 18, 90 (1997). 882. A. E1-Hdiy, Appl. Phys. Lett. 63, 3338 (1993). 883. G.W. Anderson, M. C. Hanf, E R. Norton, Z. H. Lu, and M. J. Graham, Appl. Phys. Lett. 66, 1123 (1995). 884. V. Yu. Timoshenko, J. Rappich, and Th. Dittrich, Appl. Surf. Sci. 123-124, 111 (1998). 885. D. Kruangam, D. Deguchi, T. Toyama, H. Okamoto, and Y. Hamakawa, IEEE Trans. Electron Devices 35, 957 (1988). 886. S. M. Paasche, T. Toyama, H. Okamoto, and Y. Hamakawa, IEEE Trans. Electron Devices 36, 2895 (1989). 887. T. S. Jen, J. W. Pan, N. E Shin, W. C. Tsay, J. W. Hong, and C. Y. Chang, Jpn. J. Appl. Phys. 33, 827 (1994). 888. J. W. Lee and K. S. Lim, Appl. Phys. Lett. 68, 1031 (1996). 889. J. W. Lee and K. S. Lim, J. Appl. Phys. 81, 2432 (1997). 890. V. Grivickas, J. Linnros, and A. Galeckas, Mater. Sci. Forum 264-268, 529 (1998). 891. Q. Wahab, E. B. Macak, J. Zhang, L. D. Madsen, and E. Janzen, "Proc. European SiC Conference," Erlangen, Germany, 2000. 892. V. V. Afans'ev, A. Stesmans, M. Bassler, G. Pensl, and M. J. Schulz, Appl. Phys. Lett. 76, 336 (2000). 893. V. V. Afans'ev, A. Stesmans, M. Bassler, G. Pensl, M. J. Schulz, and C. I. Harris, J. Appl. Phys. 85, 8292 (1999). 894. V. V. Afans'ev and A. Stesmans, Mater Sci. Eng. B 71,309 (2000).
284
KASI V I S W A N A T H
895. M. Ruff, H. Mitlehner, and R. Helbig, IEEE Trans. Electron Devices 41, 1040 (1994). 896. M. R. Melloch and J. A. Cooper, Jr., MRS Bull. 22, 42 (1997). 897. J. N. Shenoy, G. L. Chindalore, M. R. Melloch, J. A. Cooper, Jr., J. W. Palmour, and K. G. Irvine, J. Electron. Mater 24, 303 (1995).
898. J. A. Cooper, Jr., Phys. Status Solid A 162, 305 (1997). 899. J. E Xu, E T. Lai, C. L. Chan, and Y. C. Cheng, Appl. Phys. Lett. 76, 372 (2000). 900. E T. Lai, S. Chakraborty, C. L. Chan, and Y. C. Cheng, Appl. Phys. Lett. 76, 3744 (2000).