Case studies of metal-III–V compound interactions

i

Applied Surface Science 70/71 (1993) 488-495 North-Holland

applied surface science

Case studies of metal-III-V compound interactions S.S. L a u Department of Electrical and Computer Engineering, University of California at San Diego, La Jolla, CA 92093-0407, USA Received 28 September 1992; accepted for publication 11 October 1992

The interactions between metal and III-V compound semiconductors have been under increasingly more intensive investigation since the early 1980's. Over the years rather significant progress in understanding the interactions has been made by various laboratories. In this discussion, we emphasize solid state reactions that are pertinent to the formation of non-spiking contacts to nand p-type A1GaAs and GaAs, as well as those that could induce compositional disordering in III-V superlattice structures. Examples are given to illustrate the diverse application of simple reactions to microelectronics and optoelectronics.

1. Introduction M e t a l c o n t a c t s to s e m i c o n d u c t o r s a r e t r a d i tionally u s e d as S c h o t t k y b a r r i e r s o r as o h m i c contacts. U n i f o r m a n d r e l i a b l e c o n t a c t s o n Si rely on u n i f o r m i n t e r a c t i o n s b e t w e e n t h e m e t a l overl a y e r a n d t h e u n d e r l y i n g Si s u b s t r a t e to f o r m t h e r m a l l y s t a b l e silicides. T h e b a r r i e r h e i g h t o f a given silicide d i c t a t e s t h e c o n t a c t f u n c t i o n a l use. It a p p e a r s t h a t u n i f o r m i n t e r a c t i o n is t h e m o s t i m p o r t a n t k e y in successful Si c o n t a c t s as rep o r t e d in n u m e r o u s r e s e a r c h a n d review p a p e r s [1,2]. I n G a A s a n d o t h e r I I I - V c o m p o u n d contact t e c h n o l o g y , it is o f t e n t h e lack o f u n i f o r m i t y a n d stability in t h e m e t a l / s e m i c o n d u c t o r i n t e r f a cial r e g i o n t h a t c a u s e s difficulties in m a k i n g eit h e r s t a b l e S c h o t t k y o r o h m i c contacts. I n this r e p o r t , e x a m p l e s will b e given to show how uniform interactions between metal and III-V comp o u n d s c a n l e a d to s t a b l e contacts. I n a d d i t i o n to m a k i n g contacts, m e t a l - I I I - V c o m p o u n d s i n t e r a c t i o n s a r e s o m e t i m e s useful in p e r f o r m i n g o t h e r t e c h n o l o g i c a l l y i m p o r t a n t f u n c t i o n s such as c o m -

p o s i t i o n a l d i s o r d e r i n g , p h o t o e l a s t i c effects a n d epitaxial liftoff.

2. Thermal and chemical stable Schottky barriers on GaAs and other I I I - V compounds T h e h i g h - t e m p e r a t u r e stability o f Schottky b a r r i e r s o n G a A s d e p e n d s on t h e inactivity bet w e e n t h e m e t a l a n d t h e u n d e r l y i n g substrate. Since single e l e m e n t a l m e t a l c o n t a c t s on G a A s a n d o t h e r I I I - V c o m p o u n d s a r e c h e m i c a l l y unstable, t h e r m a l l y stable Schottky b a r r i e r s a r e n o t e x p e c t e d f r o m t h e s e systems. A l t e r n a t i v e l y , s o m e o f t h e m e t a l silicides, n o t a b l y T a x S i a n d WxSi, a r e c h e m i c a l l y stable o n G a A s [3,4] a n d a r e m o r e likely to f o r m t h e r m a l l y stable Schottky contacts. Since we e m p h a s i z e cases w h e r e i n t e r a c t i o n s a r e of i n t e r e s t h e r e , t h e issue o f n o n - i n t e r a c t i o n s will n o t b e discussed f u r t h e r in this r e p o r t . T h e r e is a n o t h e r a p p r o a c h to f o r m stable contacts o n I I I - V c o m p o u n d s . T h a t is to grow epitaxial a n d t h e r m o d y n a m i c a l l y s t a b l e m e t a l transi-

0169-4332/93/$06.00 © 1993 - Elsevier Science Publishers B.V. All rights reserved

S.S. Lau

(a)

(b)

fM,( MxAB

/

489

Case studies of metal-lll-V compound interactions

I I I - V s e m i c o n d u c t o r heterostructures have m a n y interesting features and properties; the subject has b e e n reviewed by Sands et al. recently [5], interested readers are referred to their paper.

(c)

AB:Mr_I

3. The solid-phase regrowth concept

AB

Fig. 1. Schematic diagram illustrating the reaction sequence during solid-phase regrowth by reaction-driven decomposition of an interfiaediate ternary phase. (a) Unreacted films of M (e.g., Pd or Ni) and M' (e.g., Si, Ge, or In) on a compound semiconductor substrate (e.g., GaAs or InP). (b) Intermediate stage during which the MM~/M and M / M x A B interfaces are moving toward each other. (c) Final configuration after reaction to form MM'r. As a result, AB, perhaps doped or alloyed with M', regrows epitaxially on the AB substrate. Note that the crystalline quality of the epitaxial layer will depend on where the regrown AB is nucleated. For example, if AB nucleates at the MM'r/MxAB interface and if the MxAB layer is polycrystalline, the AB layer will be polycrystalline. However, if AB nucleates at the interface with the substrate, of if the MxAB phase is highly oriented, the regrown AB layer will be nominally monocrystalline, (after ref. [6]). tion gallides structure ( T M - H I ) and the rareearth monopnicitides with NaCI structure ( R E - V ) on the I I I - V c o m p o u n d s by m e a n s o f molecular b e a m epitaxy. These stable and epitaxial m e t a l /

Si

T h e solid-phase epitaxial regrowth o f I I I - V c o m p o u n d semiconductors was first d e m o n strated by Sand, Marshall and W a n g [6] in 1988. T h e regrowth of d o p e d or lightly alloyed comp o u n d s e m i c o n d u c t o r layers by reaction-driven decomposition o f intermediate t e r n a r y phases can be described as follows: Films of the two elements M and M ' are deposited on a c o m p o u n d s e m i c o n d u c t o r substrate AB. T h e film adjacent to the substrate consists of a metal M that reacts at low t e m p e r a t u r e s to f o r m a ternary phase M x A B : xM + AB ~ MxAB.

(1)

T h e second film consists of an e l e m e n t M ' that is chosen such that it reacts with M x A B to form a c o m p o u n d M M y that is sufficiently stable so as to drive the reaction x y M ' + M x A B ~ A B + xMM'y

to the right. Because o f the proximity o f the

Si o ~0 ¢~

NiSi

E e-

c .g

Ni

,e-

u~ Ni-Si NixGaAs

'

voids

..t

m

u~ GaAS o~

I,/_ ,.., J NiAs ppts " - " ~ - - - ~ T J F - a t regrowth ~ interface ~lGaAs

~ AlAs

~jAIAs

c GaAs

I AlAs ia) as deposited

(2)

(b) 250°C, 30rnin

(c)250°C,30min+350°C,30min

Fig. 2. Schematic diagram summarizing TEM results, (after ref. [6]).

S.S. Lau / Case studies of m e t a l - l l l - V compound interactions

490

~200oc. 250oc

~100°C

.Si Pd

Si Pd Pda GaAs

GaAs (a)

..~

~200oC. 250oc

Si

Si

Pd 2 S i " Pd Pcl 4 GaAs

Pd2Si _.~

> 300oc Si (Pd) Pd 2Si

~d4GaAs(SI)

. rl+-G_a.A.s.{~ 1)

GaAs

GaAs

GaAs

GaAs

(b)

(c)

(d)

(e)

Fig. 3. Regrowth mechanism to form n +-GaAs surface layer using the Si/Pd contact as an example.

crystalline AB substrate, the epitaxial regrowth of AB is likely. Furthermore, the regrown AB may be doped or alloyed with M'. This regrowth mechanism is illustrated schematically in fig. 1. The only metals M that are known to react with GaAs and InP at low temperatures ( < 200°C) to form MxAB ternary phases are Ni and Pd. In fact, it has been demonstrated that the reaction xM + GaAs --->MxGaAs (M = Ni or Pd)

To demonstrate the regrowth mechanism, Sands et al used the S i / N i / G a A s system. The reactions are as follows:

Ge

Pd

Pd ..~

GaAs (a)

(4)

xSi + NixGaAs ~ NiSi + GaAs (regrown).

(5)

Fig. 2 shows the schematic diagram summarizing the results obtained from TEM investigations.

(3)

occurs even at temperatures below 80°C. The choice of M' depends on the requirement that reaction (2) be driven to the right and on the desired electronic properties of the epitaxial layer. In the absence of thermodynamic data on PdxGaAs and NixGaAs, one cannot quantitatively predict the sign and magnitude of the freeenergy change during reaction (2) for various elements M'. Nevertheless, the known thermal stabilities of the silicides and germanides suggest that Si and Ge would be likely candidates for M', especially in cases where a heavily doped n+-AB regrown layer is desired. Recent results on S i / P d / n - G a A s ohmic contacts provide experimental evidence for this regrowth mechanism.

Go

xNi + GaAs --->NixGaAs,

4. Non-spiking ohmic contacts to GaAs and AIGaAs based on the solid-phase regrowth concept

The Si/Pd and the G e / P d systems were the first systems used to fabricate non-spiking and low-resistance ohmic contacts on n-GaAs based on the solid phase regrowth (SPR) approach [7]. Fig. 3 shows the schematic regrowth mechanism of the S i / P d / n - G a A s system. It is interesting to point out that ohmic behavior was observed only when the intermediate phase Pd4GaAs disappears, thus forming the regrown GaAs layer. The formation of PdaGaAs also seems to be self-limiting in thickness. Fig. 4 shows the SPR reactions

Ge

PdGe

Ge

PdGe

Pd4 GaAs

Pd Pd 4 GaAs

GaAs

GaAs

GaAs

(b)

(c)

(d)

• C-_~-~,s

PdGe epi-Ge F+-G_a_A~.LG_eL

GaAs (e)

Fig. 4. Reaction steps of the G e / P d / G a A s system based on the regrowth mechanism.

S.S. Lau / Case studies of metal-III-V compound interactions

of the G e / P d / G a A s system. Both systems yield low contact resistivities (_< 1 x 10 - 6 ~-~"cmZ). Electron microscopic examination indicate planar and non-spiking interfaces. The obvious difference between the S i / P d and the G e / P d system is that in the G e / P d system, the top Ge layer (initially amorphous in structure) was transport across the PdGe layer to form an epitaxial Ge layer (see fig. 4e), whereas the Si layer (also initially amorphous in structure) stays at the sample surface (fig. 3e). This observation shows that a heterojunction formed between Ge (or Si) and GaAs was not required to make low-resistance ohmic contact. From electrical measurements as a function of temperature (fig. 5), we conclude that the ohmic behavior is due to tunneling. It is, therefore, possible to infer that a heavily doped n + surface regrown layer ( ~ 30 .~) was formed in both the S i / P d and G e / P d system after the regrowth. The doping mechanism was believed to be due to a site selection mechanism where the intermediate compound PdxGaAs is As rich [8], thus enabling the occupancy of either Ge or Si in the excess Ga vacant sites. Although the G e / P d and the S i / P d system yield planar and low-resistance contacts; due to the amphoteric nature of the dopants, these contacts may not be appropriate for applications that require long-term reliability or thermal stability at elevated temperatures. An alternative and more thermally robust method to form an ohmic contact to n-GaAs is to introduce an InGaAs interfacial layer between GaAs and the contacting metal. This interfacial layer can be grown by molecular beam epitaxy [9,10] or liquid-phase epitaxy [11,12], but most conveniently formed by the SPR process [13]. An intense effort has gone into the development of a hybrid P d / I n / P d ( G e ) contact [14]. Fig. 6 shows the solid-phase reactions of the contact. Upon evaporation of a Pd layer on the substrate or during the rise of in temperature of the furnace, a layer of Pd4GaAs is formed at the metallization/GaAs interface:

491

( ~ 100°C, at the P d / I n interface). At higher temperatures (>_ 550°C) PdIn 3 + PdnGaAs ~ PdIn + InxGaa_xAs

with x --- 0.4. For annealing below ~ 550°C, we conclude that x is about zero in eq. (8). Ohmic behavior is observed only if Ge ( ~ 10-20 .~) was in the contact structure as shown in fig. 6. Due to the hybrid nature of this contact, the processing window for ohmic formation extends from ~ 250 to 675°C (fig. 7). For p-type contacts, the G e / P d or the S i / P d systems are not suitable. The Si/Ni(Mg) system [15] was developed based o n the SPR process. The reactions are similar to these shown in eqs. (4) and (5), except a thin layer of Mg is imbedded in the Ni layer to provide p-type dopant in the regrown GaAs layer. Planar contacts with resistivities less than 1 x 106 ~ . cm 2 can be fabricated T (K) 10-2

.3(30,

100

50

(6)

( ~ 100°C, at the P d / G a A s interface), and 3In + P d ~ Pdln 3

(7)

40

AI/V/Ge/Pd/GaAs

30 B

10-"

>-

10-~

s_

I

10~ H

o.

UJ

104

u"

09

10-~

5

1'0 ' 1'5 ' 2'0 ' 2'5 ' 3'0 ,

4Pd + GaAs ~ Pd4GaAs

(8)

xlO (K

_,

'

)

Fig. 5. Dependence of specific contact resistivity of A l / V / G e / P d / n - G a A s system on reciprocal temperature. The doping concentration of sample B increases from 4.3 X 1015 to 4X 10 TM cm -3 for sample U.

S.S. Lau / Case studies of m e t a l - I l l - V compound interactions

492

High Temp

Pdln In Ga As :Ge x 1-x

(>550 °C) n-GaAs

x -0.4 Pd

L~ 25oA

Pd

]

~iiiiiiii~;

(-lO0°C)

lo4oA 25oA

Pdln 3 Pd 4GaAs(Ge) • n-GaAs

Low Temp

Pdln II[IIIII[IIIW[llllllHfllLIIIl[lllllllll i 4 _ . GaAs:Ge

(< 550°C) n-GaAs

I

Fig. 6. Schematic diagram showing the solid-phase reaction of the Pd/In/Pd(Ge) contact. Upon evaporation or during the rise in temperature of the furnace, a layer of Pd4GaAs is formed at the metallization/GaAs interface. At temperatures above 550°C, a layer of InxGa l_~As (x -- 0.4) doped with Ge is formed at the interface. At temperatures below 550°C, a regrown layer of GaAs doped with Ge is formed at the interface. o n p - G a A s d o p e d to > 1.5 × 1018 cm -3. It should be p o i n t e d out that while the formation of P d 4 G a A s is self-limiting in thickness, N i ~ G a A s apparently is not. T h e thickness of Ni~ G a A s

10 .2.

10"~

~L

•

' Pd/ln/Pd(Ge)

- - -A- -

Pd/ln/Pd

..... [] ....

Ge/Pd

A

and, therefore, the regrown G a A s layer are limited by the initial Ni thickness. If the N i x G a A s is thicker than 5000 to 6000 .A the stresses in the film may cause cracks in the substrate. F r o m the discussion and examples given above, it is clear that the S P R process is a significant development in fabricating non-spiking contacts to p- and n-type G a A s and A I G a A s [16,17]; and this type of contact, especially the G e / P d contact, is increasingly being a c c e p t e d for technology applications.

10 "~

~a

/x i tI

10 .5 /)3

~ ' .

....

5. Compositional disordering by solid phase regrowth

'?

/"

10 "~

10

200

0 .

.

300

.

.

.

/

400

-

500

600

"

700

Annealing Temperature(°C)

Fig. 7. Contact resistivity versus annealing temperatures for the Ge/Pd, Pd/In/Pd, and Pd/In/Pd(Ge) contact. The annealing time is 30 min for annealing temperatures between 250 and 375°C, 30 s for annealing temperatures between 400 and 500°C, and 5-10 s, for annealing temperatures between 550 and 675°C. An ohmic formation temperature range of about 400°C is observed for the Pd/ln/Pd(Ge) contact.

T h e intermixing of elements in structures of I I I - V c o m p o u n d q u a n t u m wells and superlattices has b e e n investigated extensively in recent years [18]. T h e layered structure of q u a n t u m wells is generally thermally stable; there is little or no intermixing b e t w e e n individual layers at t e m p e r a tures as high as 1000°C in some cases if p r o p e r precautions are taken to prevent vaporization. In 1981, researchers at the University of Illinois r e p o r t e d that the disordering of an A I A s - G a A s superlattice could be greatly e n h a n c e d by diffus-

S.S. Lau / Case studies of metal-III-V compound interactions

493

AIGaAs

GaAs

°-- I I (&)

(b)

~ , S SupetlatUces,

NI depo~tion.

(c) Annealing at 250~C to 350°C, NIAIGaAs forms, pedodic structure remains in the quatenaw phase.

(d)

(e)

Si deposition.

Annealing a~ 400°C to 650°C, uniform layer mgrows.

Fig. 8. Schematic showing the sequence ot events ot the solid-phase reactions leading to compositional disordering (a) initial superlattice on GaAs, (b) deposition of Ni, (c) annealing at 250-350°C to form NixAIGaAs , (d) deposition of Si, (e) annealing at 400-650°C to decompose NixAIGaAS , resulting in a NiSi layer and a mixed AlGaAs regrown layer.

o~ W, O4

W

Ca)

Cb3

Cc3

Fig. 9. Cross-sectnonal TEM micrographs of a AIGaAs/GaAs. sample before and after processing. (a) The as-grown 16-period AIGaAs/GaAs superlattice with a top GaAs layer about 150 A in thickness, (b) after Ni deposition and annealed at 250°C for 60 min, a quaternary phase of NixA1GaAs is formed with the periodic structure still visible, and only 4.5 periods of the superlattice are left unconsumed by the Ni reaction, and (c) after Si deposition and annealed at 600°C for 2 min, a compositionally disordered A1GaAs regrown layer, ~ 1840 ,~ in thickness, is formed.

494

S.S. Lau / Case studies of m e t a l - l l l - V compound interactions

ing a p-type dopant, Zn, into the structure at low temperatures (_< 600°C). Since this discovery, impurity disordering in the temperature range of 650-900°C has been reported in many III-V systems using shallow dopants. Compositional disordering can be induced by several methods: (1) impurity-induced disordering where the impurities are introduced into the structure either by diffusion or by ion implantation followed by a high-temperature annealing step, (ii) disordering induced by ion mixing where ions are implanted into the structure held at moderately elevated temperatures, and (iii) defect-induced disordering where a stripe pattern of SiO 2 is deposited on the surface of the sample (usually structures of AIGaAs/GaAs superlattice) followed by high-temperature annealing (> 850°C). All these technique require ion implantation or high-temperature annealing or both. We found that the concept of SPR can be applied to compositional disordering of III-V superlattice structures [19]. The process is shown schematically in fig. 8 using the Si/Ni system on AlGaAs/GaAs superlattices. The idea is to form an intermediate compound between Ni and the AlGaAs/GaAs superlattice, which subsequently decomposes to form a homogeneous AlGaAs alloy with the average composition of the superlattice. From TEM observations (fig. 9), it was found that the intermixing of Al and Ga atoms occurs when the hexagonal NixAlGaAs phase decomposes to form the cubic AIGaAs regrown layer. The SPR approach to cause disordering only requires thin film deposition and low temperature short duration annealing (i.e. 650°C for 30 s). Recently we have made low-loss AlGaAs/GaAs waveguides using the SPR method. These waveguides are planar and have propagation losses as low as 1.6 dB/cm. We expect the SPR method to be useful in planarization and other applications for photonic device fabrication.

6. Photoelastic effect by thin film reactions

Lateral confinement of carriers and photons in superlattice and quantum well structures is generally recognized to be an important feature in a

number of electric and optoelectric devices. Quite a few techniques have been invented to induce lateral confinement in these structures with varying degrees of success. One of the simplest but less commonly used technique is to utilize the photoelastic effect for optical mode guiding. This technique to form waveguides in III-V compound semiconductors was rather actively investigated in the mid 1970's [20,21] and in 1979 Kirkby et al. published a rather comprehensive paper on the complex stress state in the semiconductor structure due to various stripe geometries and their effects on the dielectric constant of the material [22]. Generally speaking, waveguides in a semiconductor can be formed by changing the dielectric constant laterally in local regions, these lateral changes of dielectric content (are therefore the refractive index and the bandgap) are conveniently induced by depositing stripes of the film with the proper stress on the semiconductor. In principle, the photoelastic effect is the simpliest way to cause local bandgap changes as compared to other techniques such as impurity induced compositional disordering in superlattice structures. While the concept is simple to understand, the control of the stress state in the stripes appears to be rather difficult in practice. It is commonly recognized that the stress state in SiO 2 and Si3H 4 layers is deposition system dependent. The stress state in most metal thin films is usually tensile, the magnitude and sometimes the sign of the stress are strongly dependent on deposition methods and parameters. The stress state of thin deposited layers may vary from run to run, not to mention the variability from one laboratory to another. It is, perhaps, for this reason that the photoelastic effect has not been commonly used in inducing lateral confinement in recent years. In order to utilize the photoelastic effect to induce local bandgap change, the stress in the stripes must be predictable and precisely controlled. The simplest way to accomplish this is to take away all the variables in film deposition and depend only on the volume change in the structure due to interfacial reactions between the stripe and the substrate. For example, Ni films deposited by evaporation on GaAs is usually under tension, however, the sign and the magnitude

S.S. Lau / Case studies of metal-llI-V compound interactions

may change from one deposition to the other. Since the volume change of a specific reaction is generally known from crystallography, annealing the sample to induce a reaction between the Ni layer and the GaAs substrate to Ni 3 GaAs would yield a known volume change. This change of volume should be the same from run to run as long as the thickness of the Ni layer is well controlled, which is easily achieved in thin film deposition. In reference to the above mentioned reaction: 3Ni + GaAs ~ Ni 3 GaAs. 1,~ 1.4,A 2A

(9)

This interfacial reaction should lead to a stripe of NiaGaAs under compression. However, since the reaction takes place inside the substrate, the stress state in the substrate may be much more complex than that of just a stripe deposit on the surface of the substrate. The control and the understanding of the stress state due to interracial reactions are the key issues in utilizing the photoelastic effect for lateral confinement purposes. We have recently succeeded in making relatively low-loss waveguides in the AIGaAs/GaAs system using reaction (9) to form Ni3GaAs stresser stripes. There are other interesting applications utilizing metal I I I - V interactions, they will not be discussed due to space limitation here.

7. Conclusion The discussion and examples given above tend to suggest that while the field of metal-semiconductor interactions is maturing, there can be many interesting and exciting new aspects of this field that may merit further research efforts.

Acknowledgements S.S. Lau acknowledges many of his colleagues in teaching him and in collaborative efforts with him over the years. He is also grateful to N.S.F. and D.A.R.P.A. for their interests and sponsorship in this field of research.

495

References [1] S.P. Muraka, Silicides for VLSI Applications (Academic Press, New York, 1983). [2] M.-A. Nicolet and S.S. Lau, Formation and Characterization of Transition-Metal Silicides, in: VLSI Electronics: Microstructure Science, Vol. 6, Eds. N.G. Einspruch and G. B. Larrabee (Academic Press, New York, 1983). [3] S.S. Lau, W.X. Chen, E.D. Marshall, C.S. Pai, W.F. Tseng and T.F. Kuech, Appl. Phys. Lett. 47 (1985) 1298. [4] T. Yokoyama, T. Ohnishi and T. Misugi, in: Layered Structures and Interface Kinetics, Ed. S. Furukawa (KTK, Tokyo, 1985). [5] T. Sands, C.J. Palmstrcm, J.P. Harbison, V.G. Keramidas, N. Tabatabaie, T.L. Cheeks, R. Ramesh and Y. Silberberg, Mater. Sci. Rep. 5 (1990) 99. [6] T. Sands, E.D. Marshall and L.C. Wang, J. Mater. Res. 3 (1988) 914. [7] L.C. Wang, F. Fang, E.D. Marshall and S.S. Lau, Contact Metallization for GaAs - A report of the Development of a Non-Alloyed ohmic Contact Scheme, Defect and Diffusion Forum, Vol. 59, Eds. D. Gupta, A.D. Romig and M.A. Dayananda, in: Diffusion in High Technology Materials 1988 (Trans. Tech. Publications, 1988) pp. 111-128. [8] J.C. Lin, K.C. Hsieh, K.J. Schulz and Y.A. Chang, J. Mater. Res. 3 (1988) 148. [9] J.M., J.F. Freeouf, G.D. Pettit, T. Jackson and P. Kirchner, J. Vac. Sci. Technol. 19 (1981) 626. [10] S.L. Wright, R.F. Marks, S. Tiwari, T.N. Jackson and H. Baratte, Appl. Phys. Lett. 49 (1986) 1545. [11] A.A. Lakhani, J. Appl. Phys. 56 (1984) 1888. [12] J. Ding, J. Washburn, T. Sands and V.G. Keramidas, Appl. Phys. Lett. 49 (1986) 818. [13] L.C. Wang, X.Z. Wang, S.S. Lau, T. Sands, W.K. Chan and T.F. Kuech, Appl. Phys. Lett. 56 (1990) 2129. [14] L.C. Wang, X.Z. Wang, S.N. Hsu, S.S. Lau, P.S.D. Lin, T. Sands, S.A. Schwarz, D.L. Plumton and T.F. Kuech, J. Appl. Phys. 69 (1991) 4364. [15] C.C. Han, X.Z. Wang, S.S. Lau, R.M. Potemski, M.A. Tischler and T.F. Kuech, J. Appl. Phys. 69 (1991) 3124. [16] E.D. Marshall, L.S. Yu, S.S. Lau, T.F. Kuech and K.L. Kavanagh, Appl. Phys. Lett. 54 (1989) 721. [17] L.C. Wang, S.S. Lau, E.K. Hsieh and J.R. Velebir, Appl. Phys. Lett. 54 (1989) 2677. [18] D. Deppe and N. Holonyak, Jr., J. Appl. Phys. 64 (1988) R93. [19] X. Wei, C.C. Han, S.A. Pappert, S.H. Hsu, Z.F. Guan, P.K.L. Yu and S.S. Lau, Appl. Phys. Lett. 58 (1991) 625. [20] Y. Yamamoto, T. Kamiya and H. Yanai, Appl. Opt. 14 (1975) 322. [21] J.C. Campbell, F.A. Blum, D.W. Shaw and K.L. Lawley, Appl. Phys. Lett. 27 (1975) 202. [22] P.A. Kirkby, P.R.. Selway and L.D. Westbrook, Appl. Phys. 50 (1979) 4567.