Optically detected cyclotron resonance in Ge, Si and ZnSe

PHYSICA

Physica B 184 (1993) 141-148 North-Holland

Optically detected cyclotron resonance in Ge, Si and ZnSe T. O h y a m a , T. T o m a r u a n d E . O t s u k a Department of Physics, College of General Education, Osaka University, Japan We have investigated optically detected cyclotron resonance (ODCR) and ordinary cyclotron resonance (CR) under the same excitation condition, in Ge, Si and ZnSe, which include both high-purity and doped materials. We have discussed various new features about the impact dissociation process for excitons, bound excitons, bound-multiple-exciton-complex and electron-hole droplets, which are the origin of ODCR. In addition, some information related to hot-electron distribution induced by CR are obtained through the ODCR spectra.

1. Introduction

2. Samples and experimental procedures

T h e cyclotron resonance experiment is one of the most powerful and direct tools to obtain information about the band structure and about the transport behaviour of semiconductors. A n o t h e r most direct and perhaps simplest m e t h o d for probing the band structure and the dynamical behaviour of semiconductors is to measure the photo-luminescence spectrum. By combining the above two methods and studying the changes in photo-luminescence spectra induced by the cyclotron resonance, one can discover new aspects of dynamical behaviour of nonequilibrium electronic systems and learn much about the interaction between hot carriers and localized or condensed carriers. This report presents very recent results obtained by the above-mentioned method n a m e d 'Optically Detected Cyclotron Resonance' ( O D C R ) in germanium, silicon and zincselenide.

In this experiment we have employed four g e r m a n i u m , two silicon and three zinc-selenide samples. A typical sample size is 3.5 x 3.5 x 1.5 m m 3. Figure 1 shows the block diagram of our measuring system for O D C R . The microwave setup is a nonresonant reflection-type waveguide system working at 35 G H z and the sample is placed in the b o t t o m of the wave-guide. As an exciting light source an Ar + laser for Ge and Si or an H e - C d laser for ZnSe is used. The exciting light is guided to the sample face by a silica rod, which is also used for guiding photoluminescence from the sample face to a spectrometer. Details of the fiber-system are schematically shown in the inset of fig. 1. One end of the optical fibers was bundled into a form of concentric circles, with an inner diameter of 1.5 m m and an outer d i a m e t e r of 3.0 mm. The other end of the bundle was f o r m e d into a rectangle of 5.3 x 1 m m 2 for the purpose of fitting to the slit of the spectrometer. To prevent direct incidence of undesirable light to the spectrometer, a piece of suitable color glass filter was employed. Detectors for O D C R m e a s u r e m e n t s were G e - P I N photodiode (North Coast EO-817) for Ge and Si, and a

Correspondence to: T. Ohyama, Department of Physics, College of General Education, Osaka University, Toyonaka, Osaka 560, Japan.

0921-4526/93/$06,00 © 1993- Elsevier Science Publishers B.V. All rights reserved

142

T. Ohyama et al. / Optically detected cyclotron resonance in Ge, Si and ZnSe to Spectrometer

=~

Laser Beam

~

~FECLA)

pure-Ge

3.2K

mtnescence

o I':L

i

/

/

\

slit

U ~

Silica Rod

z

to Sample

715

Chopper

~ ' t ~

Spect ro-

-Oetector

~

Ge-PIN

/-',.-J

700

PHOTON ENERGY ( m e V ) Fig. 2. Luminescence spectrum for high-purity Ge. The tracing is the measured spectra at 4.2 K and 1.5 K.

Filter

O p t l e o l ~

705

710

meter

Silica Rod

Ref.

system for a high-purity Ge sample. No luminescence line from the electron-hole droplet (EHD) is seen at 4.2 K and only the free-exciton (FE) line is observed. In the same excitation condition, on the other hand, only an EHD line is observed at 1.5 K. After adjusting the spectrometer to the free-exciton or EHD line on the photoluminescence spectra, we observe ODCR signals by sweeping an external magnetic field. Figures 3 and 4(a) show typical ODCR signals obtained by monitoring the FE line at 4.2 K and the EHD line at 1.5 K, respectively. Besides the electron resonance peaks, light and heavy hole resonance peaks are observed in both figures.

Fig. 1. Block diagram for ODCR measurements. The inset shows a specially designed optical fiber system.

photomultiplier (Hamamatsu R980) for ZnSe. The ODCR signal was caught by a lock-in amplifier with a light modulation technique.

ODCR FE 35GHz 4.2K

F,~

BI/[O01~

pure-Ge

microwaves off

3. Experimental results and discussions

o

3.1. High-purity germanium Figure 2 shows typical photoluminescence spectra taken by the use of the above-mentioned

o~,

,~

o!~

oI,

o~

o.~

MAGNETIC FIELD ( T )

Fig. 3. Typical ODCR signal using free-exciton FE (LA) of the photoluminescence spectrum.

T. Ohyama et al. / Optically detected cyclotron resonance in Ge, Si and Z n S e d. n e x

dt

Z

!

t i

0.1

0.2

c

~.. ,

0.3

MAGNETIC

~

/-5dB

,.

,

0.4

05

FIELD

( T )

"~

CR

•.

(b)

nex = g % x ( n / g r e -- 1).

g/ex = grex(1 - " r e x A I / 2 r ~ B T ) .

~.-lldB 0

i

0.1

012

,.

03

,

,

0.4

0.5

0.6

MAGNETIC FIELD ( T ) Fig. 4. (a) Typical ODCR signal using the EHD line of the photoluminescence spectrum. (b) Ordinary cyclotron resonance absorption under the same condition as (a). Figure 4(b) shows ordinary cyclotron resonance absorption in the same experimental condition. In the first step, we will discuss our experimental results on the O D C R signals for the FE system. A t 4.2 K, only free excitons are observed in the photoluminescence measurement. We thus find that only free excitons, free electrons and free holes exist in this system at 4.2 K. We can expect that the electron density is equal to the hole density under intrinsic light excitation. In such a situation, the rate equations which govern the kinetic relation in this system are given by dn

n

d----t = g - B T n 2 + A v n e× + A I n n ~x -

-re

(2)

(3)

In case the experimental condition is appropriate, expanding an original equation derived from eqs. (1) and (2) in a series of A~ and taking terms to the first order in A I [1], we have

o

o

nex

0.6

Z

and

A T n ~ x -- A I n n e x

H e r e n is the electron density, n~x the exciton density, By, A T and A~ are thermal capture, thermal dissociation and impact ionization coefficients for excitons, respectively, r e and rex are the electron and exciton lifetimes, g represents a generation rate of free carriers by intrinsic light excitation. In a steady state condition, i.e. when d n / d t = d n ~ x / d t = 0, we can easily solve eqs. (1) and (2) as follows:

(a)

•

BTn2-

%x

ODCR EHD 35GHz 1.5K BI135 ° from C0013

z

-

143

(1)

(4)

Though % and %x are practically constant, the impact ionization coefficient A~ is an increasing function of the kinetic energy of the free carriers and B T is a decreasing function of the same variable. The exciton density thus decreases with increasing kinetic energy of free carriers by cyclotron resonance, and the O D C R signal for the F E line shows downward peaks as illustrated in fig. 3. Next we will touch upon the O D C R signals concerned with E H D . In addition to electron and fundamental-hole cyclotron resonance, the third, fourth and fifth harmonics of the heavyhole resonance are observed. It is found that the signal intensity of the hole resonance compared to that of the electron resonances increases with a rise in microwave power. The reason is as follows: energy separations between Landau levels in the conduction band are equal in Ge, so that cyclotron resonance of electrons always occurs sharply for high-purity Ge. The energy separations in the valence band, on the other hand, are not equal, but gradually become so with increase in the Landau quantum n u m b e r n. Holes are thus acceleratively heated up with a rise in microwave power. Then resonance lines

144

T. Ohyama et al. / Optically detected cyclotron resonance in Ge, Si and ZnSe

which are spread by inhomogeneous broadening converge, so that the resonance line becomes sharp at higher microwave power.

o~

3.2. Doped germanium

Cn ~ L~ 0

(a)

"', ~',,

ODCR

o

Figures 5(a) and 5(b) show typical photoluminescence spectra for arsenic-doped Ge ( G e : A s ) . Both bound-exciton (BE) and E H D lines are observed. Figures 6(a) and 6(b) show O D C R signals making use of the BE or E H D line in the photoluminescence spectrum. It is worth noting that O D C R signals using the E H D line decrease at all resonances, while the O D C R signal using the BE line decreases at electron resonance and increases at hole resonances. We now focus our attention to the difference between electron and hole resonances. Figure 7 shows O D C R signals using the BE line in P-, Asor Ga-doped Ge. It is found that peaks directed upward and those directed downward are inverted between n-type samples ( G e : P and G e : A s ) and p-type sample ( G e : G a ) . These phenomena are explained as follows: both B E and E H D are generally broken by the

B//£1113 Ge:As

",

',,

35GHz

~ N ¢.;

1.5K

"~. 0,, E "-.

"=

o -....

==

~

z;d

(b)

z ;d ~J Z ~9

0.1

0.2

0.3

0.4

0.5

0.6

MAGNETIC FIELD ( T ) Fig. 6. O D C R signals using BE (NP) or E H D (LA) lines under (a) weak and (b) strong excitation conditions which correspond to those of fig. 5(a) and 5(b), respectively.

(a) G e: As 1.5K e~

excitation

1/100 x5

Z

(b)

z

8E(NP)

EHD(LA)

m z

z

EHD(TA) [ ~ _ _ j ~ f k BE(LA)

.1

xl ~

iiI,

740

I i ~t

730

i 1 i i-i

i

720

710

PHOTON ENERGY (

, ,'~-, i

meV

700 )

Fig, 5. Photoluminescence spectra of Ge : As under (a) weak and (b) strong excitation conditions. Here T A and LA in parentheses imply emission lines assisted with TA and LA phonons, respectively, while NP implies a phononless line.

impact of hot'carriers. In fact, E H D are broken easily, and at the same time a large amount of F E are released. FE, however, are not long-lived at 1.5 K - one can indeed see no FE peak in fig. 5(b). It means that FE are immediately captured by neutral impurities in the vicinity of FE to form BE. Accordingly the number of BE is determined by competition between increasing and decreasing factors. Thus in the case of increasing factors surpassing decreasing ones, the O D C R signals for the BE line show peaks directed upward. It is concluded that excitons bound to As or P impurities are destroyed much more easily by the impact of electrons than by that of holes. Increases on both sides of the electron resonance at 0 . 2 4 T and a dent on the heavy-hole resonance shown in fig. 6 are also due to this competitive effect. Since the efficiency to break E H D is expected

145

T. Ohyama et al. / Optically detected cyclotron resonance in Ge, Si and ZnSe

~1

tn

"6

c

estimation of the impact-dissociation cross section, we expect that the interaction between the same kind of charged particles is larger than that between different ones on account of the 'exchange interaction'. Considering constituents of d o n o r - b o u n d excitons, one can expect, based on the above insight, that donor-bound excitons interact rather m o r e strongly with impinging electrons than with holes. Conversely acceptorbound excitons interact rather m o r e strongly with holes than with electrons. As a result, a d o n o r - b o u n d exciton possesses a larger impactdissociation cross section for electrons than for holes.

OBCR BE 35GHz 1.5K B/I [1113

0

rsj

Z r~

z

z

3.3. High-purity silicon and d o p e d silicon

Q

0.1

0.2

0.3

0.4

0.5

0.6

MAGNETIC FIELD ( T ) Fig. 7. ODCR signals using BE (NP) lines on photoluminescence spectra of Ge : P, Ge:As and Ge:Ga under strong excitation conditions. to be the same in n- and p-type samples, there should be little difference in supply of BE. Thus experimental facts suggest that d o n o r - B E are b r o k e n up much m o r e easily by the impact of electrons than by that of holes, conversely acc e p t o r - B E are broken up much m o r e easily by holes than by electrons. In other words, we can say that d o n o r - B E have a larger impact-dissociation cross section for impinging electrons than for holes, while acceptor-BE have a larger cross section for the holes than for the electrons. A strongly noticeable characteristic is that the impurity-type dependence of the impact-dissociation cross section of bound excitons has the same tendency as that of the carrier-scattering cross section [2]. T h e a b o v e - m e n t i o n e d experimental findings are understood as follows. The donor-bound exciton consists of two electrons, one hole and the positively charged donor impurity. The acceptorb o u n d exciton, on the other hand, consists of two holes, one electron and a negatively charged acceptor impurity. As a guiding principle for the

In photoluminescence spectra of high-purity Si, luminescence lines arising from FE, E H D and bound-multiple-exciton-complexes ( B M E C ) are observed. B M E C and E H D are all broken on impact by hot carriers and then FE are released. Accordingly, only the O D C R signals for F E are direct upward. In the photoluminescence spectra of borond o p e d Si shown in fig. 8, weak FE lines and

Si:B 4.2K Bt(TO)

Slit

z

B3(TO)

I

1110

I

1100

I

I

1090

I

I

1080

I

/

1070

I

1060

P H O T O N E N E R G Y ( m eV )

Fig. 8. Photoluminescence spectrum in B-doped Si showing an extended series of bound-multiple-exciton-complexs (BMEC) lines and weak free-exciton lines.

T. Ohyama et al. / Optically detected cyclotron resonance in Ge, Si and ZnSe

146

some strong B M E C lines are observed. Labels, B 1 , B 2 , B 3 . . . . , express radiative recombination lines of single, double, t r i p l e , . . , excitons bound to boronic impurities (BMEC). Figures 9(a) and 9(b) show changes in luminescence intensity due to electron and hole cyclotron resonance, respectively, where the magnetic field is adjusted to resonance position. Experimental results demon-

(a) FE(TO)

Si:B

4.2K 35GHz B/111113 0.33T Electron CR

•

II BI (TO) FE(LO) }1 °,i

o BI(LO) B4(TO)

1,4

Slit B3(TO) B :TO) I

1110

I

I

I

I

I

I

1100 1090 1080

I

I

I O70

1060

(b)

Si:B 4.2K 35GHz BflE111] 0.19T Light Hole CI:

FE(TO) ,~. BI(TO)

•~

x2 ~¢i-G

o

1"4

1

+

Slit

B3{TO) B2(TO) t

1110

I

I

I

I

1100 to90

PHOTON

I

1080

ENERGY

I

I

t

1070

lo6o

( meV )

Fig. 9. Typical O D C R signals o f S i : B obtained with microwave modulation technique. Change in photoluminescence

spectrum caused by electron-CR (a) and light-hole-CR (b), respectively. An upward direction expresses an increase of luminescence intensity and a downward direction expresses a decrease.

strate that by impact of hot carriers against BE or B M E C , free excitons are released one by one. For example, B 3 breaks up into B 2 and FE through one process. Next B 2 breaks up into B 1 and FE. A remarkable difference between 8(a) and 8(b) is seen in the signal intensity of the B 1 peak. The consequence is that the efficiency with which hot electrons break BE or B M E C is different from that with which hot holes do. The experimental results indicate that hot holes break excitons bound to a B impurity more efficiently than hot electrons• 3.4. Zinc selenide

Though ZnSe is expected to be one of the most promising I I - V I compound semiconductors for efficient blue-light-emitting devices, there is as yet no well-established technique to make such devices. This is because characterization of the electronic properties of ZnSe has always suffered from the fact that even an undoped ZnSe crystal contains many native defects and residual impurities which act as donors. Crystal-growth methods of ZnSe have been variously attempted, e.g., vapor-phase epitaxy ( V P E ) , liquid-phase epitaxy ( L P E ) and chemical vapor deposition (CVD). The VPE-grown crystal is generally of high quality. However, it is frequently twinned, bringing some trouble in fabricating devices. The twin-boundary, on the other hand, is a physically noticeable and appealing system, because it functions as a natural potential well for electrons. Some physical properties of a two-dimensional (2D) electron system constructed in this potential well was studied by cyclotron resonance experiments [3]. Figure 10 shows photoluminescence spectra of the three samples. The assignment of most lines on the spectra was done by previous workers [4]. The I1° line is due to radiative recombination of excitons bound to deep acceptors arising from Zn vacancies Vz,. Radiative recombinations of excitons bound to various neutral donors are labeled 12. Radiative recombination lines labeled 13 a r e controversial and assignment of the S line has not yet been done. Figure 11 shows an ordinary cyclotron reso-

T. Ohyama et al. / Optically detected cyclotron resonance in Ge, Si and ZnSe

PHOTON ENERGY(eV) 2.80 2.75 I

I

I

I

I

I

I

I

i,d

2.70 I

I

I

4.2 K

S

I2

]l

12

I?-LO

z

II

z p4

Sample A

/ t .J ~

S-LO

Sample B

,,,,I

Sample C I

440

I

I

444

448

I

f

I

I

452

I

456

460

WAVELENGTH (nm) Fig. 10. Photoluminescence samples.

spectra

for

various

ZnSe

Sample A

M

~

4.2K

z

£

ODCR (FE)

-15dB

,4

0

0.1 '

0.2 '

MAGNETIC

0i 3

0.4

F I E L D (T)

Fig. 11. O D C R traces obtained from F E lines for sample A and the ordinary cyclotron resonance trace. A downward deviation m e a n s a decrease in luminescence intensity. Values indicated with decibel are relative'microwave powers.

147

nance trace and O D C R traces obtained by monitoring the free-exciton ( F E ) line for sample A, which is not twinned. The relative microwave p o w e r is indicated on the right. The peak position of the ordinary cyclotron resonance varies to the high-field side by about 1% as the microwave p o w e r increases to the maximum. Each O D C R trace, roughly speaking, shows a decrease in luminescence intensity with increasing microwave power, which is due to the decrease in the free-exciton-formation rate caused by increasing electron kinetic energy. In addition, there is a fact that the peak position strongly depends on microwave power and the m a x i m u m shift to a higher magnetic field reaches to 4% of the resonance field for ordinary cyclotron resonance. Considering the strong dependence of this peak position on microwave power, we expect that it is caused by a resonant formation of free excitons. Free carriers cannot emit an L O - p h o n o n at capture processes in such a case as e b < htOLO, where e b and hWLo are the binding energy of a b o u n d state, i.e. free excitons and bound excitons, and the L O - p h o n o n energy, respectively. T h e situation is changed however when the condition E + E'b > h 0 , J L O is satisfied by increasing the electron kinetic energy E. Then the free carrier can emit one L O - p h o n o n , and therefore the capture rate suddenly grows• This process corresponds to a resonant formation of free excitons, namely, it occurs at E = hO.)LO - - e b = 12 meV with h W L o = 32 meV and e b = 20 meV for ZnSe [5]. In addition, the reason why the p e a k position of O D C R is displaced to a higher magnetic field from the resonance field of ordinary cyclotron resonance is explained by a polaron effect. A n ordinary cyclotron resonance line is the sum of the absorption by all phonondressed conduction electrons, i.e. by all polarons, while the peak of an O D C R trace for high microwave power is caused only by the polarons which are heated up to more than 12 meV through cyclotron resonance absorption. Figure 12 shows a change in the photoluminescence spectrum induced by cyclotron resonance for sample C with twin boundaries. H e r e the I 2 lines as well as the S line, which is expected to be related with shallow impurities,

148

T. Ohyama et al. / Optically detected cyclotron resonance in Ge, Si and ZnSe

2.80 I

I

PHOTON ENERGY (eV) 2.75 I

I

I

I

I

l~

I

I

2.70 I

ODCR 0.14 T 4.2K

I

8:40"

Sample C ,6

lid- LO

S

~, f _ _ . . ~

I~-2LO

s

12

440

I

I

444

I

~48

I

I

452

I

I

456

4. Summary

I

I

460

WAVELENGTH (nm) Fig. 12. ODCR spectrum obtained with the microwave modulation method under the cyclotron resonance condition for sample C.

Various new features of dynamical properties in G e , Si and ZnSe have been observed by O D C R . The impact ionization of excitons by free carriers is the essential feature for O D C R in a free carrier-exciton coexisting system including no E H D . Concerning the system including E H D , it would obviously be expected that the impact ionization of E H D by free carriers is essential for ODCR. We have m a d e clear the kinetic relation a m o n g FE, B E and B M E C in Si. Furthermore, it is found that the polaron as well as the resonant formation effects for FE is essential for understanding the O D C R signals in ZnSe.

Acknowledgements and its replicas are directed downward, while the 11d line and its replicas related to deep impurities are directed upward. To understand this p h e n o m e n a , we discuss the formation process of excitons bound to deep acceptors VZn. If the following process is dominant for the formation of deep bound excitons, experimental observations are well explained. Electrons are captured after holes are captured by neutral acceptors: A ° + h + + e - - - - > A + + e - - - - > ( A ° × ) . The electron-capture rate by A + centers with one L O - p h o n o n emission increases with electron velocity, since the chance of electrons encountering A ÷ centers grows as the electron velocity increases owing to the cyclotron resonance. The photoluminescence lines 12 and S, on the other hand, which are related to shallow impurities, decrease on account of impact dissociation by electrons with sufficient energy.

We are greatly indebted to M. Isshiki and K. Igaki for their sample offer. This work is partially supported by Grant-in-Aid for Scientific Research on Priority Areas from the Ministry of Education, Science and Culture, Japan.

References [1] T. Tomaru, T. Ohyama and E. Otsuka, J. Phys. Soc. Jpn. 58 (1989) 3718. [2] E. Otsuka, K. Murase and J. Iseki, J. Phys. Soc. Jpn. 21 (1966) 1104. [3] T. Ohyama, K. Sakakibara, E. Otsuka, M. Isshiki and K. Igaki, Phys. Rev. B 37 (1988) 6153. [4] M. Isshiki, T. Kyotani, K. Masumoto, W. Uchida and S. Suto, Phys. Rev. B 36 (1987) 2568. [5] G.E. Hite, D.T.F. Marple, M. Aven and B. Segall, Phys. Rev. 156 (1967) 850.