Growth pressure dependence of selective area metalorganic vapor phase epitaxy on planar patterned substrates

,. . . . . . . .

ELSEVIER

CRYSTAL GIROW'rH

Journal of Crystal Growth 156 (1995)59-66

Growth pressure dependence of selective area metalorganic vapor phase epitaxy on planar patterned substrates Takuya Fujii *, Mitsuru Ekawa, Susumu Yamazaki Fujitsu Laboratories Ltd., 10-1 Morinosato-Wakamiya, Atsugi 243-01, Japan

Received 15 November 1994; manuscript receivedin final form 24 May 1995

Abstract

We investigated the growth mechanism of selective area metalorganic vapor phase epitaxy (MOVPE) on planar mask-patterned substrates by analyzing the growth pressure dependence of the in-plane growth rate distribution using an equation we reported previously IT. Fujii, M. Ekawa and S. Yamazaki, J. Crystal Growth 146 (1995) 475]. Experiments showed that the lateral diffusion constants defined at the epilayer and mask surfaces are inversely proportional to growth pressure. This dependence is evidence of negligible surface migration both on the epilayer and mask surfaces. The growth rate enhancement produced by mask patterning increased and saturated as growth pressure increased. Our theory predicts, however, that enhancement is proportional to growth pressure when the growing probability on the mask surface is exactly zero. This contradiction suggests that chemical reaction at the mask surface reduces the vapor phase concentration of group III source materials. This assumption successfully quantitatively predicted the experimental results.

I. Introduction

Selective area metalorganic vapor phase epitaxy (MOVPE) of compound semiconductors modulates the in-plane distribution of thickness and composition of epilayers [1-10]. Aoki et al. [2] reported that mask patterning improves the growth rate of epilayers near the mask edge, and that the improvement increases as the mask region increases. These results indicate that source materials for epitaxial growth diffuse laterally from the mask region to the epilayer region. Analyzing growth rate modulation lets us deter-

* Corresponding author.

mine some material constants, such as the diffusion constant and lifetime of surface source materials. Selective area growth is a useful experimental way to investigate the M O V P E growth mechanism. Lateral diffusion in M O V P E generally consists of vapor phase diffusion and surface migration. To discuss the behavior of surface source materials, we have to divide lateral diffusion into vapor phase diffusion and surface migration. Previous studies [3-5] have grown epilayers on selectively mask-patterned substrates with deep trenches on the epilayer region to investigate surface migration effects. Their experiments showed that the growth rate distribution around deep trenches is independent of the trenches. This suggests that surface migration plays a minor role, at least on the epilayer surface. Gibbon and

0022-0248/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSDI 0022-0248(95)00256-1

60

T. Fujii et al. / Journal of Crystal Growth 156 (1995) 59-66

Thrush et al. [3,4] proposed a quantitative theory that excludes surface migration both on the epilayer and mask surfaces. They numerically solved a three-dimensional Laplace equation for the vapor phase concentration of source materials using diffusion length defined at the epilayer surface as a parameter and evaluated their experimental results. However, no clear quantitative evidence of negligible surface migration on the mask surface has been reported. The dominant effect in lateral diffusion must be discussed using a theory that includes effects of vapor phase diffusion and surface migration. Sasaki et al. [5] showed experimentally that the distribution of growth rate enhancement near the mask edge closely fits an exponential curve. This suggests that the vapor phase concentration of source materials near the substrate surface is governed by a two-dimensional diffusion equation. The concentration of source materials migrating on the surface is also expected to be governed by a two-dimensional diffusion equation. This analogy predicts that vapor phase effects, such as vapor phase diffusion and readsorption of desorbed source materials, can be included in the two-dimensional surface equation. If the behavior of selective area growth is expressed as a two-dimensional equation, it is easy to investigate the effects of vapor phase diffusion and surface migration. In a previous study [1], we developed a two-dimensional equation that describes the behavior of selective area MOVPE on planar patterned substrates. To analyze selective area MOVPE, we divided the MOVPE system into four regions: the fluid layer, the stagnant layer, the surface layer, and the growing solid, where the vapor phase concentration of source materials at the top of the stagnant layer is assumed to be kept constant by the fluid layer. Our theory is based on the law of conservation of source materials in the stagnant layer, at the solid surface, and at the interface between the vapor phase and the solid surface. Analyzing the model equations, we found that the vapor phase concentration near the substrate surface can be expressed as a two-dimensional diffusion equation. From this, we developed a simple two-dimensional equation for se-

lective area MOVPE on planar patterned substrates that includes effects of vapor phase diffusion and surface migration. This equation revealed that three parameters of surface source materials, renormalized by the vapor phase effects, dominate the growth rate distribution: diffusion length on the epilayer, diffusion length on the mask, and the lifetime ratio for the mask and epilayer to desorb and solidify. Diffusion lengths dominate the shape of growth rate distribution. The lifetime ratio dominates the maximum growth rate enhancement. If surface migration has a significant effect, these parameters are independent. If vapor phase diffusion dominates, the lifetime ratio is determined by the ratio of the diffusion lengths, and the number of independent parameters is reduced from three to two. In this study, we investigated the growth pressure dependence of selective area MOVPE [6] using our equation. Experimental investigation of the growth pressure dependence directly divides effects of vapor phase diffusion and surface migration because we know the growth pressure dependence of vapor phase parameters, such as that the vapor phase diffusion constant is inversely proportional to growth pressure. We can investigate the dependence of selective area MOVPE on the vapor phase parameters independently of surface parameters by measuring the effects of growth pressure.

2. Theoretical prediction of pressure dependence In the previous study [1], we revealed that the distribution of surface concentration of source materials, P(x,y), which includes effects of vapor phase diffusion and surface migration, can be described by a two-dimensional differential equation. This dimensional reduction from three to two results from the fact that the in-plane distribution of vapor phase concentration of source materials near the surface determines the distribution of source supply from the vapor phase to the surface and that the law of conservation of source materials determines the relationship between the in-plane vapor phase and surface distributions of source materials. In general, the

T. Fujii et al./Journal of Crystal Growth 156 (1995) 59-66

two-dimensional differential equation for P(x,y) consists of a power expansion of the in-plane Laplacian. However, the Laplacian of higherorder than the square in the equation has to be treated as zero when the behavior of vapor phase source materials in the stagnant layer is governed by the Laplace equation. We developed an equation that describes the in-plane distribution of MOVPE growth rate, R(x,y), on planar patterned substrates. R(x,y) is proportional to P(x,y):

R(x,y) =GP(x,y).

(1)

G is the solidifying probability per unit time of surface source materials. P(x,y) is governed by the two-dimensional diffusion equation: P( x, y) - P0 6v:P(x,y)

=

Po = ?Jo"

,

(2)

(3)

P0 is P ( x , y ) for uniform surface systems. The tilde ( ~ ) indicates a quantity that includes the effects of vapor phase diffusion and surface migration. /) is the lateral diffusion constant of surface source materials. ? is the lifetime for surface source materials to desorb and solidify. J0 is the uniform source flux from the vapor phase to the surface: ovap _ _ ~)top

J0 = Lstag~'~

'

(4)

1 ? = (1 + 3')G + 3'K'

(5)

6 = (1 + 3')Dsurf + 3'D lat,

(6)

where Dlat = 1( K + G)(Lstag) 2.

(7)

D *ap is the vapor phase diffusion constant. L sty* is the stagnant layer thickness. QtOp is the vapor phase concentration at the top of the stagnant layer, which is assumed to be kept constant by the fluid layer. K is the desorbing probability per unit time of surface source materials./) incorporates the diffusion constant of surface migration, D ~urr, and the vapor phase lateral diffusion constant, D )~t. 3' is a non-dimensional vapor phase

61

parameter that corresponds to the ratio of the mean free path of vapor phase source materials and

zstag:

T

OZ L s t a g v therm "

6

D yap (8)

is the thermal velocity of vapor phase source materials. 6 is the degree of freedom of thermal velocity direction, a is the non-dimensional adsorbing probability of vapor phase source materials to the surface, a and V therm determine the relationship between P0 and the vapor phase concentration near the surface of uniform surface systems, Q0: z~ ~1 a lrvI therm ~0 = (K+G)Po. (9) V therm

Eq. (9) gives the sticking coefficient of source materials from the vapor phase to the growing solid, 0:

0 = G/co,

(10)

where

co = ( K + G ) / a ,

(11)

and co is the oscillation frequency of sticking molecules. 3' is a special parameter in MOVPE systems. The source materials diffuse in the vapor phase colliding with other molecules. A collision reflects one half of the diffusing source materials. The reciprocal of 3' gives the number of reflectors in the stagnant layer. Reflectors reduce the desorbing probability of surface source materials and the vapor phase lateral diffusion constant. When we control growth pressure, Pg, independently of L stag, 3' becomes a unique parameter which depends on Pg. We can produce such growth conditions by keeping the total inlet gas flow to the reactor constant, because the Reynolds' number in the reactor is independent of Pg. When L stag is independent of Pg, D vap determines the growth pressure dependence of 3': G 7-

0 K+G

Pg'

(12)

where Dvap 6 Lstagvth .... -

(13)

T. Fujii et al. /Journal of CrystalGrowth156 (1995)59-66

62

The coefficient of the reciprocal of Pg in Eq. (12) gives the critical pressure of MOVPE. When growth pressure is less than the critical pressure, growth behaves like chemical beam epitaxy (CBE) [11]. The mean free path of vapor phase source materials reaches L stag at this critical pressure. Eqs. (5) and (6) are derived from the law of conservation of the source flux from the vapor phase to the surface and the law of conservation of P(x,y): d

ovap--7--a(x,y,z ) OZ

= jsup(x,y) - K P ( x , y ) , Iz=0 (14)

jsup(x,y) = (K + a ) e ( x , y ) - D~""V~/'(x,y). (15) Q(x,y) is the vapor phase concentration of source materials in the stagnant layer. J~"P(x,y) is the source supply from the vapor phase to the surface. In the previous study [1], we defined J"~P(x,y) in terms of Q(x,y,O): jsup(x,y ) =1-~o~Vtherm Q( x,y,z ) l~=o.

1 therm a(x,y,z)L=~v,,~. -~aV

(17)

This new formula leads to revised equations for ? and /5: ?

1 G + 7K'

(19)

(1 + y) in Eqs. (5) and (6) are substituted with 1. Substituting Eq. (12) into Eqs. (18) and (19) gives the growth pressure dependence of t" and /): a+

0 K+a

g0]

R 0 = 1 + 0 g + G Pg

(22)

Jo"

? and /} can be experimentally determined by the distribution of R(x,y). Eq. (2) leads to

P( x,y ) = e( x,y ) /e,~,

(23)

(L)2 =/}?,

(24)

?M = ?M/? E,

(25)

V2fiE(x,y) -

PE(x,y)

1

(LE) 2

(LE)2,

fiM( x,y ) v2pM( X'y )

--

"[M

2

(L ~)

(26)

2"

(L M)

(27)

E and M indicate quantities of the epilayer and mask. fi(x,y) is P(x,y) normalized by PoE. L is the diffusion length, i~ is the lifetime ratio of the mask and epilayer. The continuous conditions of fi(x,y) and the flux of fi(x,y) at the interface between the epilayer and mask regions are: p E ( x , y ) linterface= pM(x,y)linterface,

(28)

0

rE--° On PE(x'Y) ~°te.ace= OM~pM(x'y) ~.tcr~aoo (29)

(18)

/~ = D surf + "yDlat.

i=

[ 1

(16)

This definition is, however, an approximation for 3' much less than 1. For low pressure selective area growth, where 3' is not much smaller than 1, we have to define JS~P(x,y) at a distance of the mean free path from the surface:

Js°°(x,y)

The desorbing probability and vapor phase lateral diffusion constant of source materials are inversely proportional to Pg. Inserting Eq. (20) into Eq. (1) gives the growth rate for maskless growth, R0:

'

where n is the direction normal to the interface. L E, L M, and iEM uniquely determine the distribution of fiE(x,y), which equals the distribution of the growth rate normalized by maskless growth, R(x,y). We can therefore determine L E, LM, and iEM by analyzing the distribution of R(x,y).

(20) 3. Experiments

/~ =D~,~f + OG 1 (Lstag) 2 /,go

P,

(21)

We grew lnP layers on planar mask-patterned InP (001) substrates by low-pressure MOVPE.

T. Fujii et al. /Journal of Crystal Growth 156 (1995) 59-66

[iiii

~/~

63

Table 1 Parameters determined by growth rate distribution

,n.,oo,, 20 to 200 pm Fg~~ -~-" ; ~:~ ~ '~,~!~]

Pg (Torr)

R o (nm/s)

/7,E (~m)

/~ra (o.m)

10 25 50 100 180

0.31 0.30 0.28 0.28 0.28

150 100 70 50 40

300 210 190 130 100

?~ 3.2 4.1 6.0 7.7 8.6

X

300 pm L ~ ~ - 1 2 0 0 pm ,1800 pm

-'

where x 0 is the position of the mask edge. /M and ?E M were determined by iteration using Eqs. (26)-(29). Table 1 summarizes the growth pressure dependence of R0, L E, /M, and ?E M.

[

Fig. 1. Configuration of the mask pattern.

4. Results and discussions Source materials were trimethylindium (TMIn) and phosphine (PH3). The carrier gas was hydrogen. The gas mixture was introduced into a vertical reactor from a gas nozzle 4 cm from the substrate. The gas nozzle was 6 cm in diameter. Growth temperature was 630°C, total flux was 6 slm, and the V / I l l ratio was 100. The TMIn source flux was 0.4 sccm. We varied the growth pressure from 10 to 180 Torr. The growing time was 6 min. We obtained 0.1 I~m thick epilayers for maskless growth and observed no deposition on the mask surface under all growth conditions. We formed SiO 2 dielectric masks by chemical vapor deposition (CVD). We periodically arranged tetragonal masks on planar substrates, as shown in Fig. 1. The periodic epilayer area consisted of a striped region and an open region. Horizontal (x-direction) and vertical (y-direction) periodicities were 1800 I~m and 300 I~m. Horizontal widths of the striped and open regions were 600 Ixm and 1200 Ixm. We varied the stripe width from 20 I~m to 200 Ixm. We measured the distribution of the epilayer thickness by Dektak. The experimental error in the thickness was about 2%. Ro and L E were determined by Eq. (26). W h e n L E is much less than the open region width, R(x,y) in the open region is R(x,y)=(R(xo,y

(x-x0 t )-Ro)ex p -~ff ] + R o ,

(30)

To determine the growth pressure dependence of epilayer parameters, ? E and /~E, we measured the distribution of epilayer thickness in the open region, as shown in Fig. 2. Solid circles and triangles show experimental distributions for Pg of 100 Torr and 10 Torr. Solid curves show the corresponding theoretical distributions from Eq. (30). R 0 was independent of _P~, as shown in Fig. 3. From Eq. (20), we can use i ~ as a unit of time, which is independent of Pg: 1 KE /,go (31) 0 E K E + G E p << 1, g

f E= 1/G E.

(32)

300 • 100 torr

• 10 torr

-- Theoretical

g "8

t[--1 rL---)

.

20 pm 0

.

0

.

.

.

.

.

.

.

.

i

.

.

.

.

.

.

.

.

.

100

200

Distance from mask edge ~m)

Fig. 2. Epilayer thickness distribution in the open region normal to the mask edge.

64

T. Fujii et al. /Journal of Crystal Growth 156 (1995) 59-66 15o

3

r

,

I

'

'

r

'

'

'

,

r

.

,

A

E C

1

t-

KE

<< 1

0 E K E + G E Po

O

09

;

• lOO torr • I0 torr

( ~ ' l "~

4)

P~

~,

•

e-

10o

2

G-

D. 0 w

-o m

5o

1

o

e-

j

z

_o

T

e.

80pm

I-

0 0

.

.

.

.

.

.

.

.

.

,

100

.

.

.

.

.

.

.

.

.

,

,

l~ :1300pm ,

J

200

200

,

:,

,

i

400

,

,

,

i

600

,

,

,

t

,

800

Position in stripe direction (/Jm)

Growth pressure (torr) Fig. 3. Growth pressure dependence of growth rate far from the mask region.

Fig. 5. Growthrate distributionnormalizedby masklessgrowth in the stripe direction.

This leads to the following well-known formula for R0:

We concluded that /)E is inversely proportional to Pg. This dependence indicates negligible surface migration on the epilayer in our growth pressure range. Substituting the growth pressure dependence in Fig. 4 into Eq. (21) gives

Dvap

R 0 = -~-i-ffa t°p.

(33)

L E decreased as Pg increased. Fig. 4 shows the growth pressure dependence of /5 E with a time unit of the reciprocal of G E, which is ([E)2. We determined the error bars assuming a 2% error in R 0. The best fit solution from Eq. (21) gives a (DSU~f)E of 3 X 10 -6 cm 2 G E, which gives a surface migration length of 17 txm. The amount of (DS"a) E is, however, within the error allowance.

o

10 5

........

,

........

,

......

P~ hztag/2 - 7 x 10- 3 cm 2 tort 0E

p

~-

j

--

10 4

o

¢:

o "~ ~::

10 3

........

, 10

.

.

.

.

.

.

.

.

~

.......

10 2

10 3

Growth pressure (torr) Fig. 4. Growth pressure dependence of lateral diffusion stant defined on the epilayer surface.

c o n

-

ego t s t a g ) 2 = 0E ( 7 X 10 - 3 c m 2 T o r r .

(34)

To determine the growth pressure dependence of mask parameters, ~'M and /9 M, we measured the normalized growth rate distribution in the stripe direction, R(x,0). Fig. 5 compares experimental and theoretical results. The stripe width was 60 Ixm. Solid circles and solid triangles show experimental distributions for Pg of 100 Torr and 10 Torr. Solid curves show corresponding theoretical distributions from Eqs. (26)-(29). Theoretical curves quantitatively predict the distribution of R(x,0). Fig. 6 shows the dependence of R(0,0) on the stripe width. Solid circles and solid triangles show ,~(0,0) for Pg of 100 Torr and 10 Torr. Theoretical curves also predict the dependence of R(0,0) on the stripe width, where we used the parameters from Fig. 5. Fig. 7 shows the growth pressure dependence o f / 5 M with a time unit of the reciprocal of G E, which is (LM)2/[EM. The solid line shows the growth pressure dependence of D E. /) M is also inversely proportional to Pg. From Figs. 4 and 7, we can conclude that vapor phase diffusion domi-

T. Fujiiet al. /Journal of Crystal Growth 156 (1995)59-66 8

. . . . . . . . .

,

. . . . . . . . .

,

i

10

. . . . . . . . .

65

.........

, ........

• 1O0 tort • 10 ton" - - Theoretical

i

.lg W

t~

E

GM

e-

)

2

o 4)

f

E

20 to 200pro

Z

..I 0

o

1oo

200

0

300

.

.

.

.

nates lateral diffusion in both the epilayer and mask regions for our growth pressure range. We found no great difference between /5 M and /) E. Their ratio is 1.0 + 0.4.

(35)

Eqs. (21) and (35) indicate that to in Eq. (10) is independent of surface parameters: GE// o E

:

(36)

GM / o M.

In the previous study [1], we obtained a diffusion constant ratio of 0.4 at 76 Torr in a different MOVPE system. The contradiction between our

10 5

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

,

.

.

.

.

.

E Q t~

c o

,

i

.

.

.

.

.

.

.

.

.

200

Fig. 8. Growth pressure dependence of the lifetime of source materials at the mask surface.

previous results and Eq. (35) may be because of the error in measuring R 0. The growth pressure dependence in Figs. 4 and 7 is direct evidence that vapor phase diffusion dominates lateral diffusion of source materials in selective area MOVPE. Fig. 8 shows the growth pressure dependence of ?M with a time unit of the reciprocal of G E, which is ?M. ?M increases and saturates as Pg increases. Substituting Eq. (36) into Eq. (20) gives i M with a time unit of the reciprocal of GE:

[

~'M= GM -~-

KM

1

,:]1

+ K M + GM oE pg j

G M / G E = 0.1,

DM : 6 E

E

,

.

(37)

The ratio of solidifying probabilities, GE/GM, dominates the saturated ?M. The solid curve in Fig. 8 is the theoretical prediction from Eq. (37) using

.

0

N

.

Growth pressure (torr)

Fig. 6. Stripe width dependence of the growth rate normalized by maskless growth at the center of the striped region.

=

.

100

Strlpe wldth (pm)

/)M/DE

.

K M

pO

10 4

K M + G M 0E e.o o

(38) = 3 Tore

(39)

If we assume that K M is much larger than G M, Eq. (39) gives

o

pO/oE= 3 Tore

(40)

10 3 10

Growth

10 2

10 3

pressure (torr)

Fig. 7. Growth pressure dependence of lateral diffusion constant defined on the mask surface.

The mean free path of vapor phase source materials reaches L stab at this critical pressure. 'When growth pressure is less than this critical pressure, the presence of the mask has no effect on the

T. Fujiiet al. /Journal of Clystal Growth 156 (1995) 59-66

66

growth rate [6,11]. Inserting Eq. (40) into Eq. (34) gives Z stag = 5 )< 10-2cm.

(41)

Our theory quantitatively explained the growth pressure dependence of selective area M O V P E with a finite G M. The meaning of the finite G M is discussed below. When surface migration can be neglected, the maximum growth rate, i~a, is determined by the ratio of the vapor phase concentration near the surface for the uniform surface system, Q0, between the mask and epilayer regions:

iMz = QoM/ Q~.

(42)

If G M is zero, i ~ is proportional to Pg, as presented in Eq. (37). = 0)

= 0 Pg/Pg."

(43)

Pg and pg0 in Eq. (43) correspond to the growth pressure dependencies of Q0M and Q0E respectively. This dependence, however, contradicts the experiments in Fig. 8. The finite G M represents Qc~ < QtOp.

(44)

We think that the finite G M corresponds to a chemical reaction at the mask surface which produces non-reactive species, such as a chemical reaction between T M I n colliding with the mask surface and PH 3 terminating the mask surface. There may be another chemical reaction model which explains the finite G M. Direct investigation needs in-situ experiments, such as mass spectroscopy, Which d e t e r m i n e species near the mask surface. Further investigation will clarify the details of how GM is formed.

5. Summary We investigated the growth pressure dependence of selective area M O V P E . We grew InP layers on InP (001) mask-patterned substrates at 630°C from 10 to 180 Torr. We experimentally determined the growth pressure dependence of the lateral diffusion constants and lifetimes of surface source materials on the epilayer and mask. The M O V P E growth rate for maskless growth

was independent of growth pressure. This indicates that we can neglect the desorbing probability of source materials at the epilayer surface. The lateral diffusion constants defined at the epilayer and mask surfaces were inversely proportional to growth pressure. This is direct evidence that vapor phase diffusion dominates selective area MOVPE. Maximum growth rate enhancement increased and saturated as growth pressure increased. Our theory predicts, however, that the amount is proportional to growth pressure when the growing probability on the mask surface is exactly zero. This contradiction suggests that chemical reaction at the mask surface reduces the vapor phase concentration of group III source materials. Using this assumption, our theory with three independent parameters: POg/oE, L stag, and G M / G E, quantitatively predicted the results of experiments, pO/oE and L stag determine the growth pressure dependence of the lateral diffusion constant. G M / G E gives the saturated maxim u m growth rate enhancement. When growth pressure is less than pgO/oE, growth behaves like CBE.

References [1] T. Fujii, M. Ekawa and S. Yamazaki, J. Crystal Growth 146 (1995) 475. [2] M. Aoki, H. Sato, M. Suzuki, M. Takahashi, K. Uomi and A. Takai, Electron. Lett. 27 (1991) 2137. [3] M. Gibbon, J.P. Stagg, C.G. Cureton, E.J. Thrush, C.J. Jones, R.E. Mallard, R.E. Pritchard, N. Collis and A. Chew, Semicond. Sci. Technol. 8 (1993) 998. [4] E.J. Thrush, J.P. Stagg, M.A. Gibbon, R.E. Mallard, B. Hamilton, J.M. Jowett and E.M. Allen, Mater. Sei. Eng. B 21 (1993) 130. [5] T. Sasaki, M. Kitamura and I. Mito, IPRM (1993) 44. [6] E. Colas, C. Caneau, M. Frei, E.M. Clausen, Jr., W.E. Quinn and M.S. Kim, Appl. Phys. Lett. 59 (1991) 2019. [7] Y.D. Galeuchet, P. Roentgen and V. Graf, J. Appl. Phys. 68 (1990) 560~ [8] J. Finders, J. Geurts, A. Kohl, M. Weyers, B. Opitz, O. Kayser and P. Balk, J. Crystal Growth 107 (1991) 151. [9] T. Itagaki, T. Kimura, K. Goto, Y. Mihashi, S. Takamiya and S. Mitsui, J. Crystal Growth 145 (1994) 256. [10] F. Scholz, D. Ottenw5lder, M. Eckel, M. Wild, G. Frankowsky, T. Wacker and A. Hangleiter, J. Crystal Growth 145 (1994) 242. [I 1] W.T. Tsang, L. Yang, M.C. Wu and Y.K. Chan, Electron. Lett. 27 (1991) 3.