LOW-TEMPERATURE GROWTH CONDITIONS AND PROPERTIES OF AlGa (As) Sb ON GaSb SUBSTRATE BY LPE

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LOW-TEMPERATURE GROWTH CONDITIONS AND PROPERTIES OF AlGa (As) Sb ON GaSb

SUBSTRATE BY LPE

S. Fujita, N. Hamaguchi, Y. Takeda, A. Sasaki

To cite this version:

S. Fujita, N. Hamaguchi, Y. Takeda, A. Sasaki. LOW-TEMPERATURE GROWTH CONDITIONS

AND PROPERTIES OF AlGa (As) Sb ON GaSb SUBSTRATE BY LPE. Journal de Physique Col-

loques, 1982, 43 (C5), pp.C5-29-C5-38. �10.1051/jphyscol:1982505�. �jpa-00222224�

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JOURNAL DE PHYSIQUE

CoZZoque C5, suppZ6ment au n012, Tome 43, d6cembre 1982 page C5-29

LOW-TEMPERATURE GROWTH CONDITIONS AND PROPERTIES OF A ~ G ~ ( A S )

Sb O N

GaSb SUBSTRATE BY LPE

S.

F u j i t a ,

N.

Hamaguchi,

Y.

Takeda and

A.

Sasaki

Department o f EZectricaZ Engineering, Kyoto University, Kyoto 606, Japau

Resume.- La croissance par epitaxie en phase liquide des a l l i a g e s semiconducteurs AlGaSb e t AlGaAsSb sur l e s u b s t r a t GaSb a St& etudiee en fonction de l a temperature de croissance (Tc) . A une temperature de croissance inferieure

&

450°C, l a morpho- logie e t l a v i t e s s e de croissance des couches obtenues par e p i t a x i e sont fortement influencees par l e processus de preearation du s u b s t r a t . Les couches t e r n a i r e s ob- tenues par epitaxie A1 Ga Sb avec x=0,84 sont obtenues avec succPs sans instabi- l i t 6 d ' i n t e r f a c e entrexle&- ouches e t l e s u b s t r a t en diminuant l a temperature de croissance

1

400°C.

A

une temperature de croissance de 540°C, l e plus grand taux de composition en A1 pour une couche t e r n a i r e sans instabilitfi d ' i n t e r f a c e e s t -0,64 a l o r s que, pour une couche quaternaire, i l e s t -0,78. Les couches quaternaires A1 Ga As Sbl-y

1

i n t e r f a c e d r o i t peuvent &re obtenues de maniere sOre pour ung3@mp0e~%urg dc croissance de 540°C. La concentration en porteurs e s t reduite d'une maniere signi ficativelgn ab i s s a n t l a temperature de croissance

:

p=1017cm-3

1

TC=54O0C a l o r r

que

p=8x10

?I

TC=40O0C. La photoluminescence des couches ternai res obtenues par @pi taxi e met en 6vi dence quatre bandes d' emi ssion di s t i nctes 1 4 , 2 K probablement dues

1

l a recombinaison radiative d'un exciton l i b r e , d'un ex- citon l i e

&

l'accepteur neutre e t d'une paire

D-A lice 1 2

niveaux accepteurs d i f f e r e n t s .

Abstract. - Liquid-phase epitaxial growth of AlGaSb and AlGaAsSb alloy semiconductors on GaSb substrates has been investigated a s a function of growth temperature(

TG ).

A t a growth temperature lower than 450°C, the morphology and the growth r a t e of the epilayer a r e greatly affected by the substrate preparation processes. AlxGal-xSb ternary epilayer with x=0.84 i s successfully grown without i n t e r f a c e i n s t a b i l i t y between the epilayer and the substrate by lowering the growth temperature t o 400°C.

A t a

TG

of 540°C, the l a r g e s t Al-composition r a t e f o r a ternary layer without the i n s t a b i l i t y i s ~ 0 . 6 4 , whereas i t f o r a quaternary layer extends t o ~ 0 . 7 8 . A10.78 Ga0.22As~Sbl-~ layer with s t r a i g h t i n t e r f a c e can be r e l i a b l y grown a t T~=540"C.

Carrier concentration i s s i g n i f i c a n t l y reduced by decreasing TG: p=l 017cm-3 a t

TG=

540°C, whereas p=8x1 o1 5cm-3 a t T~=400"C. Photo1 uminescence of the ternary 1 ayers exhibits four d i s t i n c t emission bands a t 4.2K which a r e presumably a r i s e d from radi- a t i v e recombination of f r e e exciton, exciton bound t o neutral acceptor and

D-A

pair re1 ated t o two d i f f e r e n t acceptor 1 eve1 s .

1 . Introduction. - Much attention has been given t o AlGa(As)Sb ternary o r quaternary

a l l o y semiconductor as an a t t r a c t i v e candidate material f o r optoelectronic device

application in a 1-vm wavelength range because of i t s wide variation of bandgap

energy by adjusting a l l o y composition r a t e together with i t s close o r exact l a t t i c e

matching with GaSb. Investigations in l a s e r diodes and photodetectors u t i l i z i n g an

AlGa(As)Sb/GaSb heterostructure have been carried out by manyauthor$l-121 as an

a1 t e r n a t i v e t o an InGaAsP/InP system. A1 though A1 Ga(As)Sb a1 loy semiconductors

have intensively grown by a

1

iquid-phase epitaxial (LPE) technique, problems s t i l l

remain with the growth of device-quality s i n g l e crystal and also there s t i l l remain

unresolved in e l e c t r i c a l and optical properties of these epilayersC131. In order

t o make the best use of high potential of AlGa(As)Sb/GaSb system a s a material f o r

1 ong wave1 ength optoel ectronic devices, i t i s of great importance t o investigate

further the growth conditions f o r high-quality epilayers imperative f o r a high device

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1982505

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performance.

In t h i s paper, we describe experimental r e s u l t s of LPE-grown AlGaSb o r AlGaAsSb on a GaSb substrate. The relationship between the growth temperature and the epi- layer properties i s investigated by changing the temperature over a wide range from 600 t o 400°C. In p a r t i c u l a r , we demonstrate t h a t the i n t e r f a c e i n s t a b i l i t y between the epilayer and the substrate can be e f f e c t i v e l y suppressed by lowering the growth temperature even i n the epilayer with a high A1 composition r a t e . Arsenic addition to ternary layer releases the interface i n s t a b i l i t y : a t the same growth temperature, the 1argestAl compositicn r a t e becomes higher i n AlGaAsSb quaternary epilayer than in AlGaSb ternary epilayers which can be grokn without the i n s t a b i l i t y . Further, i t i s described t h a t c a r r i e r concentration i s s i g n i f i c a n t l y reduced by decreasing the growth temperature. Finally, i t i s also reported t h a t two d i s t i n c t acceptor levels a r e found t o e x i s t i n the epilayers by photoluminescence measurements.

2. Experimental procedures.- The epitaxial layers of the ternary or the quaternary a l l o y semiconductors a r e grown from Ga-rich solution on (111)-B oriented GaSb sub- s t r a t e s using a s l i d i n g boat technique in a Pd-diffused hydrogen stream i n horizontal furnace. The s l i d i n g boat used in t h i s work i s composed by a double-bin type c r u s i - ble which enables us t o supply fresh and almost thermal equilibrium melt t o a

sub-

strate[13]. The GaSb substrate

( MCP

Electronics

LTD. )

a r e both of n-type

(

Te aoped, n-6x1017 t o lx1018cm-3

)

and p-type

(

Zn doped, p=3x1018cm-3

)

. These sub- s t r a t e s a r e etched in a solution of HF:HN03:CH3COOH=1:19:30

( F

solution

)

o r

B r 2

in methanol and subsequently rinsed in deionized water o r methanol, prior to loading to the furnace. The preparation procedures f o r t h e substrate a r e important f o r growing uniform and smooth epilayers. The s u b s t r a t e orientation must be accurate within k0.2" t o obtain an as-grown mirror-like epilayer. The best r e s u l t s are well reproduced with thorough r i n s e by pure methanol a f t e r a final etching of the sub- s t r a t e in the F solution. The raw materials f o r the melt a r e 7N-Ga, 5N-A1, 6N-Sb, and a GaAs polycrystal. The dew point a t the downstream of the growth tube i s lower than -76°C during the growth runs. The heating cycles f o r the growth a r e schemati- c a l l y i l l u s t a t e s i n Fig. 1. The cycle

A

i s f o r the melt baking of Ga and Sb, where the temperature i s kept a t 70G°C f o r 3 t o

24

hrs. After the baking, A1 and GaAs a r e adaed t o the me1 t and the substrate i s loaded to the boat

(

cycle

B ):

For a high temperature growth

( TG >

500°C

) ,

the growth solution i s heated to a tempera- t u r e s l i g h t l y above the growth temperature and maintained a t t h i s temperature f o r 30 min t o e q u i l i b r a t e the me1 t , and then the growth i s s t a r t e d

(

Fig. 1 ( a )

) .

For a low temperature growth

(

TG

<

500°C

)

, a heat cleaning cycle

C

f o r the substrate a t 550°C f o r 1 hr has t o be added before the growth in order t o obtain smooth and uniform a l l o y epilayers

(

Fig. (b)

). A

cooling r a t e of 0.2 t o 0.3OC/min i s used and the thickness of the epilayers i s

about 0.5 t o 15um depending on the grow-

^W^a₃

t h i n t e r v a l . 2

a

W a

3. Results and discussions.

I w +

3.1 Relationship between epilayer

W u a

growth and substrate preparation

z

process.

^a³

In

LPE

technique, good "wetting"

^u.

between a melt and a substrate i s needed for uniform and smooth morphology toge- ther with thickness control of a grown layer. The "wetting" is greatly depend-

ent on a substrate surface condition .just

^TIME

-

7 0 0 ~ ~ 3-6hrs HEAT CYCLE FOR LPE

70$C 3-24hrs (b) A

-

prior t o the growth. We investigate the of sub- temperature growth ( a ) and low tempera- Fig. 1. Temperature p r o f i l e s of high s t r a t e preparation condition and growth

temperature on the "wetting" o r the t u r e growth ( b ) . growth r a t e of AlGaSb ternary a l l o y epi-

layer.

A s shown i n Table I , the apparent

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growth r a t e f o r A10.3gGa0.61Sb

grown a t T~=500"C e x h i b i t e d r e - ^{Table I} Relationship between substrate preparation processes

markable dependence on t h e GaSb and A10.39Ga0.61Sb growth characteristics.

s u b s t r a t e p r e p a r a t i o n process. Thickness (m) of Surface The growth r a t e i s seen t o be r e -

:b7~:iLp,,

the epilayer a t

TG = 500°C morphology

duced w i t h decreasing t h e r e s i s - t i v i t y o f t h e water used as a

f i n a l r i n s e s o l u t i o n f o r t h e sub- ^{(a) Pure}_methanol ^9.3 mirror-1 i k e

s t r a t e . A t T ~ = 4 0 0 and 450°C,

Al(j.3yGa0.61Sb was n o t grown a t ( b ) Water 7.5 m i r r o r - l i k e (p>lY(R.cm)

a l l unless t h e h e a t c y c l e C men-

t i o n e d i n t h e previous s e c t i o n (c) Uater

( ~ ? . l R - c m ) 2.8 rough surface

was introduced p r i o r t o the

growth regardless o f t h e process (d) water

subjected, t o t h e substrate, where- ( P % B K ~ . c ~ ) ^0%2.9 no growth o r rough surface

as no s i g n i f i c a n t d i f f e r e n c e i n t h e growth r a t e f o r A10.3gGa0.61 Sb was observed a t a h i g h e r TG

than 550°C w i t h v a r i o u s kinds o f t h e f i n a l r i n s e f o r GaSb.

Judging from the above experimental observation, t h e s u r f a c e o f t h e GaSb sub- s t r a t e would be presumably covered by some undesirable t h i n l a y e r o f which thickness i s dependent on t h e q u a l i t y o f t h e water, which gives g r e a t influence on t h e "wet- t i n g " o r t h e apparent growth r a t e o f the e p i l a y e r . Although i t i s s t i l l unrevealed whether t h e t h i n s u r f a c e l a y e r covering t h e s u b s t r a t e s u r f a c e i s formed b y an o x i d e o r o t h e r m a t e r i a l s , t h e f a c t t h a t t h e growth r a t e a t a h i g h TG i s l e s s s e n s i t i v e t o t h e f i n a l r i n s e s o l u t i o n s f o r GaSb used i n t h i s experiment suggests t h a t t h e t h i n surface l a y e r can e a s i l y be removed by thermal e t c h i n g a t a h i g h temperature j u s t b e f o r e t h e m e l t comes i n t o c o n t a c t w i t h t h e substrate, which leads t o a good "wet- t i n g ' ' o f t h e m e l t w i t h a substrate. However, t h e surface morphology o f t h e e p i l a y e r was u n i f o r m and smooth when u s i n g pure methanol o r water w i t h p > 15N2.cm as a f i n a l r i n s e s o l u t i o n f o r t h e substrate. Thus, t h e thickness c o n t r o l o f the e p i l a y e r w i t h e x c e l l e n t morphology becomes e a s i e r i n t h e LPE growth a t a lower temperature.

3.2 Low-temperature growth o f AlGaSb and AlGaAsSb

The low-temperature growth o f AlGa(As)Sb has been found t o be very e f f e c t i v e t o reduce t h e c o n c e n t r a t i o n o f background i m p u r i t i e s o r defectsC9, 13, 143 and f u r t h e r t o suppress t h e i n t e r f a c e i n s t a b i l i t y between t h e a l l o y e p i l a y e r and t h e GaSb sub- s t r a t e [ l 3 ] . C a l c u l a t i o n s and experiments f o r t h e phase diagram o f the t e r n a r y a1 l o y semiconductors have been c a r r i e d o u t by several authorsCl5-191. However, t h e r e a r e o n l y a few r e p o r t s [13, 14, 171 on experimental s t u d i e s f o r low-temperature LPE grow-

~ 6 0 . 1 3 ~ I ~ I lB~1*~ ~ ~ I

g . Al,Gq,Sb AI,Ga,-x Sb

z ^-

z 0.6

-

5

2 0 -

I- 3

g

^10-

+

c

A1 ATOM FRACTION I N LIQUID 0 0 1 1 , 1 1 1 I 1 1 1 1 1

0.001 0 01 0.1

Al ATOM FRACTION IN LIQUID Fig. 2. S o l i d u s isotherms i n AlGaSb a t

TG=400 and 540°C. S o l i d curves a r e t h e Fig. 3. A1 d i s t r i b u t i o n c o e f f i c i e n t c a l c u l a t e d r e s u l t s a f t e r Cheng e t a1 .[I71 o f AlGaSb a t T ~ = 4 0 0 and 540°C.

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l ' l ' l ' l ' 5

.- C Al, Gal, Sb

E

0.5

5

0 0.2 0.4 0.6 0.8 1.0

A1 COMPOSITION RATE IN SOLID

0 . 0 5 0

400 500 600

GROWTH TEMPERATURE ('C ) Fig. 4. R e l a t i o n s h i p between A1 GaSb F i g . 5. Growth temperature dependence growth r a t e and A1 composition r a t e on t h e growth r a t e o f AlGaSb.

i n s o l i d .

t h o f these t e r n a r y and quaternary a l l o y semiconductors. I n o r d e r t o o b t a i n high- q u a l i t y heteroepi t a x i a l l a y e r s o f these m a t e r i a l s , the LPE growth was performed a t TG'S be1 ow 540°C.

Experimentally determined s o l i d u s isotherms i n AlGaSb system grown a t 400 and 540°C a r e shown i n F i g . 2. These experimental r e s u l t s a r e i n good agreement a t a low

xA1

r e g i o n w i t h t h e c a l c u l a t e d r e s u l t s based on a simple s o l u t i o n model r e p o r t e d by Cheng e t a1 .[17]. F i g u r e 3 shows the r e l a t i o n s h i p between A1 a t o m - f r a c t i o n i n s o l i d u s uhase and i n l i ~ u i d u s ohase, i.e., A1 d i s t r i b u t i o n c o e f f i c i e n t . I t i s seen t h a t A1 d i s t r i b u t i o n c o e f f i c i e n t ,

xil/xA1 1

i n A1 GaSb

a t T~=400"C decreases from 50 t o 15 w i t h i n c r e a s i n g

xA,

from 0.001 t o 0.55. The r a i s i n g o f TG t o 540°C reduces

~2~ /xil

by approximately one ha1 f w i t h i n

xA1

employed i n t h i s The growth r a t e decreases w i t h i n c r e a s i n g b o t h TG's, as shown i n F i g . 4.

I n p a r t i c u l a r , a submicron AlGaSb e p i l a y e r can be easi 1 y reproduced by decreasing TG t o 400°C.

F i g u r e 5 i n d i c a t e s t h e growth temperature dependence on t h e growth r a t e o f AlxGal-xSb w i t h x=0.38

-

0.40.

I t i s w e l l known t h a t c r y s t a l i n s t a b i l i t y i s o f t e n observed a t t h e i n t e r f a c e i n a h e t e r o e p i t a x i a l growth o f AlGaSb ob GaSb[13, 17, 20, 211. We observed t h a t t h e low-temperature growth o f AlGaSb i s

verv e f f e c t i v e t o suuuress t h e i n t e r f a c e i n s t a b i l i t v (a) a n d - t h a t t h e As a d d i t i o n t o t h e melt, i . e . , the

t e r n a r y e p i l a y e r growth leads t o u n i f o r m s t r a i g h t i n t e r f a c e . The A1 composition r a t e x i n AlxGal -xSb beyond which the i n t e r f a c e becomes nonuniform was extended from 0.47 t o U.84 bv l o w e r i n a TG from 600 t o 400°C. F i g u r e 6 shows a - t y p i c a l e x a i p l e o f

A1~.84Gao.l6Sb l a y e r surface and cleaved c r o s s sec-

-

t i o n where TG i s 400°C. No i n t e r f a c e i n s t a b i l i t y i s observed. Re1 a t i o n s h i p between t h e i n t e r f a c e i n s t a b i l i t y and t h e growth temperature as w e l l as t h e As a d d i t i o n i s shown i n F i g . 7 where one can c l e a r l y see t h a t t h e i n s t a b i l i t y i s a b l e t o be suppress b y

l o w e r i n g TG o r by t h e As a d d i t i o n even i n t h e hetero- (b)

e p i t a x i a l growth o f these a l l o y s w i t h a h i g h A1 com- F i g . 6. Surface (a) and p o s i t i o n r a t e . The avoidance o f i n s t a b i l i t y i s ess- cleaved cross s e c t i o n ( b ) o f e n t i a l l y important i n wide-gap h e t e r o j u n c t i o n device A10.84Ga0.16Sb grown a t TG=

a p p l i c a t i o n s . I n general, i n t e r f a c e i n s t a b i l i t y i s 4000C. I

caused by a c o n s t i t u t i o n a l supercooling phenomenon[22]. 1 0um

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F i g . 8. Cleaved cross s e c t i o n As Sb w i t h y=

Of A10.4Ga0.6 y 1-y

0.007 grown a t 540°C on,GaSb.,

--

2 > , - 20vm

cl,

Assume t h a t t h e i n s t a b i l i t y i n t h e present experiments i s due- t o t h e supercooling phenomenon.

I n o r d e r t o avoid t h e supercool- i n g , a c r i t e r i o n must be f o l l o w e d t h a t r e q u i r e d GL/V be g r e a t e r than a c e r t a i n value[22], where GL i s t h e 'temperature g r a d i e n t i n t h e l i q u i d u s

a t t h e i n t e r f a c e and v t h e growth v e l o c i t y . I f GL i s assumed t o be c o n s t a n t i n each growth r u n i n Fig.7, a value o f GL/V would be increase w i t h reducinq Tc o r w i t h t h e As addi- Fig. 7. R e l a t i o n s h i p between i n t e r f a c e t i o n since ttie growth r a t e i s s i g n i - i n s t a b i l i t y and growth temperature. f i c a n t l y decreased by 1 owering TG o r

by t h e As a d d i t i o n , which agrees w i t h t h e requirement f o r suppression of c o n s t i t u t i o n a l supercooling. A t t h e same TG, on t h e o t h e r hand, t h e growth r a t e decreases w i t h i n c r e a s i n g A1 composition r a t e as shown i n F i g . 4. Howeverythe i n - s t a b i l i t y i s easy t o occur i n t h e growth a t higher A1 composition r a t e s . Therefore, i t cannot be explained o n l y i n terms o f t h e growth r a t e . Since t h e c r i t i c a l value o f GL/V depends on t h e l i q u i d u s slope, t h e l i q u i d concentration, t h e d i s t r i b u t i o n c o e f f i c i e n t and t h e d i f f u s i o n constant a t the i n t e r f a c e o f each component, i t i s f u r - t h e r r e q u i r e d t o take these f a c t o r s i n t o account f o r t h e i n t e r p r e t a t i o n of t h e i n s t a - b i l i t y phenomenon.

The As composition r a t e i s q u i t e l i m i t e d i n t h e AlGaAsSb quaternary a l l o y system, which r e s u l t s i n d i f f i c u l t y i n growing AlGaAsSb l a t t i c e - m a t c h e d w i t h GaSb. This would be due t o low s o l u b i l i t y o f As i n t o the l i q u i d u s phase[2] o r due t o a m i s c i b i - l i t y gap i n a Ga-As-Sb systemC23, 24). Therefore, AlGaAsSb w i t h a h i g h As-composi- t i o n lattice-matched w i t h GaSb cannot be grown a t a l o w growth temperatureC9, 191.

However, even i n t h e quaternary e p i l a y e r 1 a t t i c e - m i smatched w i t h GaSb, we were a b l e t o r e p r o d u c i b l y grow t h e excel 1 e n t morphological e p i l a y e r w i t h a h i g h A1 composition r a t e , which i s f r e e from t h e i n t e r f a c e i n s t a b i l i t y as shown i n F i g . 8.

F i g u r e 9 shows t h e TG dependence o f t h e As composition r a t e i n s o l i d u s phase of AlGaAsSb. A t T~=400'C, t h e maximum As composition r a t e y i n A10.4Ga0.6AsySbl-y i s estimated around 0.0045. A10.3gGa0.61AsySbl- w i t h y-0.025%0.03 l a t t i c e - m a t c h e d w i t h A10.07Ga0.93Sb was reproduced by r a i s i n g

fG

up t o 540°C as shown i n F i g . 10.

However, t h e increase i n As atom f r a c t i o n i n l i q u i d u s phase leads t o i r r e g u l a r v a r i a - t i o n i n t h e l a t t i c e constant o f t h e quaternary l a y e r s , which i s probably caused by the m i s c i b i l i t y gap o f t h i s quaternary system[2, 241. Since t h e growth r a t e o f t h e quaternary e p i l a y e r i s s i g n i f i c a n t l y decreased w i t h t h e As composition r a t e i n l i q u i - dus phase as i n d i c a t e d i n F i g . 11, we can s u c c e s s f u l l y reproduce t h e submicron ( ~ 0 . 5 pm ) quaternary l a y e r s , which i s very u s e f u l f o r double h e t e r o j u n c t i o n a p p l i c a t i o n s .

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GROWTH TEMPERATURE(°C )

Fig. 9. Growth temperature dependence of Fig. 10. Relationship between As atom the As composition r a t e in solidus phase f r a c t i o n in liquidus phase

xis

^and

in A1 0.4Ga0.6As Sbl y quaternary epilayers. l a t t i c e constant of A1 xGal -xAsySbl -y The r e s u l t s by gotoiugi e t a1 .[19], and quaternary epi 1 ayers

.

Law e t a1 .[21 are a l s o indicated f o r com-

parison.

-

.- C ¹

-

_I A!, Ga .sAs,Sq-,: _I _J

-

_W

_- -

3.3 Electrical and 1 uminescent properties of A1 GaSb.

In order t o investigate the e l e c t r i c a l properties of epilayers, the Hall measurements have been carried out by a p-n junction separation technique because of a semi- insulating GaSb substrate unavailable.

The undoped AlGaSb epilayers show p- type conduction regardless of TG's employed

a t present work. However, hole concentra- 0.01 ^I ^I

tions a r e s i g n i f i c a n t l y reduced by about 0 1 .o 2.0 (XIO-3)

two orders in magnitude by lowering TG from As ATOM FRACTION IN LIQUID

600 t o 400 O C as shown in Fig. 12.

A10.4Ga0.6Sb epilayer a t T~=400'C i s seen t o Fig. 11. Dependence of growth r a t e

have a typical value of p=8x101 5cm-3 a t room _~ _~ _i _~ _~ _~ _i _~ _~ _~ _i ₆ _~ _~ _$ _s _~ _~ _; _{ _t _~ _~ _~ _~ _~ _~ _~ _~ _~ _n temperature. Temperature dependence of hole i n l i q u i d u s

phase XAs.

concentration of the epilayers a t T~=400 and 536OC i s shown in Fig. 13. These r e s u l t s

could allow us t o evaluate acceptor l e v e l s , the acceptor and the compensating donor concentrations by computer analyses. Although attempts t o estimate these material parameters of the epilayers by the computer analyses have been made, i t i s found to d i f f i c u l t t o evaluate the reasonable values of the parameters because of remarkable influence on the Hall measurements a t a high temperature region of leakage current of p-n junction used e l e c t r i c a l separation between the epilayers and t h e substrates.

A rough estimation from the data a t a low temperature region gives a value of 25 t o 30 meV f o r an acceptor level.

In undoped p-type GaSb, a model of doubly-ionizable acceptor whose origin i s connected with an i n t r i n s i c Sb vacancy has been reportedC25-J. I t has been indicated, on the other hand, t h a t by reducing

TG

undoped AlGaSb o r GaSb epilayers exhibit

n-

type[9 14, 261 and f u r t h e r residual donor concentration decreases d r a s t i c a l l y up t o 2-4xl0f5cm-3 f o r A10.2Gao.aSb and up t o 2xl013cm-3 f o r GaSb[14, 261. The origin of p-type conduction in our epilayers i s s t i l l unknown whether i t i s due t o i n t r i n s i c defects related t o a Sb vacancy[25] o r to e x t r i n s i c impurities such a s Si[30].

Further experiments f o r obtaining high-purity epilayers a r e needed to elucidate the origin.

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I I I I

R.T. I " " I ' ~ " 1 " " I

GROWTH TEMPERATURECC)

Fig . 13. Temperature dependence Fig. 12 Growth temperature dependence of hole concentration of p-A1 GaSb of hole concentration of p-AlGaSb l a e r a t T~=400 and 540°C. Rapid in- Solid square i s a f t e r Gautier e t al.f32j. crease in hole concentration i s

due t o leakaae current from

p-n

junction a t high temperatures.

Photoluminescence

( PL )

measurements were made f o r A10.07Ga0.93Sb ternary epi-

layers grown a t TG's from 550 t o 400°C t o obtain information on impurity or defect l e v e l s involved. A high-pressure

Hg

lamp with a f i l t e r o r argon ion l a s e r

(

1W

)

f o r excitation source and a cooled PbS photoconductive c e l l f o r a detector were used f o r the measurements.

Fig. 14 shows

PL

spectra measured a t 77 and 4.2K in an epilayer grown a t TG=450

OC.

A t 4.2K four emission bands labelled a s I x , 11, Ia , and Ic can be observed a t 0.873, 0.870, 0.844, and 0.815eV, respectively, while one can see two d i s t i n c t emis- sion bands of 10 and I$ located a t 0.870 and 0.842eV a t 77K, respectively.

The 10 emission band a t 77K varies in peak photon energy depending on the s l i g h t difference in the A1 composition r a t e among the epilayers grown a t d i f f e r e n t TG's, a s shown in Fig. 15. However, the peak

energy difference between 10 and I$ re- mains almost unchanged among the epi-

layers: the value i s about 27meV. Fur-

₈₆₇₄

.

^TG-^450.C

t h e r , the i n t e n s i t y of I$ w i t h respect t o t h a t of 10 tends t o decrease a t low- e r TG's which lead t o decrease i n t h e acceptor concentration.

From these r e s u l t s , the 10 emis- sion band may be arised from d i r e c t t r a n s i t i o n from electron i n the conduc- tion band t o hole i n the valence band, deducing from data of temperature dep- endence of bandgap f o r GaSb together with A1 composition dependence of t h a t f o r the ternary a1 loy systemC28-301.

And i n addition,

I$

can be ascribed t o

r a d i a t i v e recombination due t o a tran-

^1.40 1.44 1 . 4 1.52

s i t i o n of f r e e electron i n t o an accep-

WAVELENGTH ( pm)

t o r level EA located a t about 27meV

above the valence band. This value Fig. 14.

PL

spectra a t 77 and 4.2K

o f

an i s interpreted with the acceptor level A1 0.07Ga0. 93Sb epilayer grown a t 450°C.

deduced from the e l e c t r i c a l measure-

ments.

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PHOTON ENERGY (eV )

0.900 0.850 0.800

- !

-

>

t V)

z

r

f

U W

z W

S:

W

I

e 3

P

1.40 1.50

WAVELENGTH ( pm )

Table I 1 Sunnary of emission peak energies and proposed mechanisms for PL spectra of Alpl-,Sb with ~ 2 0 . 0 7 .

A 7K 77K

Emission

eE:k

band ( e V Y Mechanism band e g k ( e ~ ? Mechanism Ix 0.873 Free exciton IO 0.870 d i r e c t band to

band 1, 0.870 Exciton bound to 1; 0.842 Free to shallbw

neutral acceptor acceptor

I, 0.844 0-A pair(donor- shallow acceptor)

I b 0.815 0-A pair(donor-

deep acceptor?)

A t 4.2K, t h e emission i n t e n s i t y o f I 1 band increases strong1 y w i t h i n c r e a s i n g t h e e x c i t a t i o n i n t e n s i t y b u t no displacement o f t h e emission peak energy i s observed, and i n a d d i t i o n , t h e emis- s i o n i n t e n s i t y of I 1 i s s t r o n g l y dependent on TG:

t h e I 1 i n t e n s i t y of an e p i l a y e r grown a t T~=400'C i s r e l a t i v e l v s t r o n s e r than t h a t o f an e ~ i l a v e r Fig. 15. PL spectra a t 77K a t TG=%O'C whose c a r r i e r c o n c e n t r a t i o n i s h i g h e r by about one order i n magnitude than t h a t grown a t

Of A1xGal-xSb at various T~=400'C. Further, t h e I 1 i n t e n s i t y i s d r a s t i c a l -

temperatures

.

l y reduced a t above 4.2K. These observations

s t r o n g l y suggest t h a t t h e I 1 band i s due t o r a d i - a t i v e recombination from e x c i t o n bound t o n e u t r a l acceptor, c o n s i d e r i n g t h a t t h e e p i l a y e r s a r e p-type. S i m i l a r c a r r i e r c o n c e n t r a t i o n dependence on emission i n t e n s i t y o f bound e x c i t o n complex t o our r e s u l t s has been observed i n GaAs[31]. The r e l a t i v e l y broad emission band o f I 1 would be ascribed t o f l u c t u a t i o n o f energy s t a t e due t o perturbed d i s o r d e r arrangement o f t h e c o n s t i t u e n t group-I11 atoms i n t h e l a t t i c e .

The b i n d i n g energy of f r e e e x c i t o n , Eex f o r A10.07Ga0.93Sb can be estimated t o be Eex-3meV by e x t r a p o l a t i n g p h y s i c a l constants f o r each b i n a r y compound[23]. I f t h e h i g h e s t energy emission band Ix l o c a t e d a t 0.873eV i s assumed t o be due t o f r e e e x c i t o n emission, t h e band gap energy EG a t 4.2K f o r t h e a l l o y e p i l a y e r i s evaluated t o be 0.876eV which i s s m a l l e r by about 15meV than t h a t deduced from PL data r e p o r t e d by A l l e g r e e t a1 .[28]. This r e l a t i v e l y l a r g e discrepancy i n t h e energy gap i s n o t c l e a r , b u t i t would be caused by t h e p o s s i b i l i t y t h a t t h e A1 composition r a t e d e t e r - mined by an X-ray d i f f r a c t i o n technique s l i g h t l y d i f f e r s from the r a t e o f the a c t u a l e p i l a y e r . Using EA, Eex, and EG thus estimated, we can c a l c u l a t e t h e b i n d i n g energy o f 11. From an e f f e c t i v e mass arguments developed by Hopfield[27], t h e photon energy e m i t t e d by r a d i a t i v e recombination o f e x c i t o n bound n e u t r a l acceptor, E(AO,X) i s g i v e n by a f o l l o w i n g e q u a t i o n : E(A0,X) = E G - E e x - 0 . 0 6 E ~

where t h e e l e c t r o n mass, me and t h e h o l e mass, mh f o r A10.07Ga0.93Sb i s taken as 0.0476mo and 0.479mo[23] (mo i s f r e e e l e c t r o n mass), r e s p e c t i v e l y . Then, the c a l - c u l a t e d value f o r E(A0,X) i s 0.871eV which i s i n good agreement w i t h t h e peak energy of 11.

As f o r I,observed a t 4.2K, we can assign i t as t h e D-A p a i r emission because the emission peak p o s i t i o n i s s l i g h t l y d i s p l a c e d t o h i g h e r energy as t h e e x c i t a t i o n i n t e n s i t y increases. Although t h e emission i n t e n s i t i e s o f I b among t h e samples i s too weak t o c h a r a c t e r i z e the emission mechanism, i t i s presumably a r i s e d from t h e D-A p a i r emission where t h e r e l a t e d acceptor i s deep one w i t h an a c t i v a t i o n energy of around 50 t o 60meV. Table I 1 shows the summary o f t h e peak energies and t h e

proposed mechanisms o f each l i n e observed by t h e PL measurements.

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4. Conclusions.

-

The experimental r e s u l t s o b t a i n e d i n t h i s work can be summarized i n t h e f o l l w i n g :

( i ) The morphology and t h e growth r a t e o f AlGa(As)Sb e p i l a y e r s a r e g r e a t l y a f f e c t - ed by t h e GaSb s u b s t r a t e p r e p a r a t i o n processes when growing a t a low tempera- t u r e o f 400 o r 450°C. A heat c l e a n i n g process a t 500 o r 550°C j u s t p r i o r t o t h e low temperature growth gives t h e e x c e l l e n t e p i l a y e r s .

( i i ) I n t e r f a c e i n s t a b i l i t y can be avoided by l o w e r i n g TG. A t T~=540"C, t h e l a r g e s t A1 -composi t i o n r a t e f o r a t e r n a r y l a y e r w i t h o u t t h e i n s t a b i l i t y i s around 0.64, whereas i t f o r a quaternary l a y e r extends t o about 0.78.

( i i i ) Lowering the growth temperature c o n t r i b u t e s s i g n i f i c a n t l y t o r e d u c t i o n i n

c a r r i e r concentration: p-l 018cm-3 a t T~=600'C whereas p-8xl015cm-3 a t T ~ = 4 0 0 ~ C . ( i v ) PL spectra o f Alr).07Gao.g3Sb show f o u r emission bands a t 4.2K. Ix a t 0.873eV,

I 1 a t 0.870eVY Ia a t 0.844eVY and I b a t 0.815eV a r e presumably a r i s e d from f r e e e x c i ton, e x c i t o n bound t o n e u t r a l acceptor, donor-shallow acceptor p a i r , and donor-deep acceptor p a i r , r e s p e c t i v e l y . A t 77K, band-to-band and f r e e - t o - bound recombination a r e dominant i n t h e t e r n a r y e p i l a y e r .

Experiments w i l l be f u r t h e r continued t o o b t a i n t h e h i g h - q u a l i t y e p i l a y e r s impe- r a t i v e f o r o p t o e l e c t r o n i c device a p p l i c a t i o n s , which a l s o enables us t o e l u c i d a t e b u l k and surface p r o p e r t i e s o f AlGa(As)Sb a l l o y semiconductors

Acknowledgments.

-

The authors would l i k e t o thank A. Ohishi and E. Sogawa f o r t h e i r h e l p on growth experiments and measurements.

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