GaAs METAL ORGANICS VAPOUR PHASE EPITAXY : RESIDUAL CARBON

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GaAs METAL ORGANICS VAPOUR PHASE EPITAXY : RESIDUAL CARBON

B. El Jani, M. Leroux, J. Grenet, P. Gibart

To cite this version:

B. El Jani, M. Leroux, J. Grenet, P. Gibart. GaAs METAL ORGANICS VAPOUR PHASE EPI- TAXY : RESIDUAL CARBON. Journal de Physique Colloques, 1982, 43 (C5), pp.C5-303-C5-310.

�10.1051/jphyscol:1982535�. �jpa-00222255�

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JOURNAL DE PHYSIQUE

Colloque C5, supplément au n°12, Tome 43, décembre 1982 page C5-303

GaAs METAL ORGANICS VAPOUR PHASE EPITAXY : RESIDUAL CARBON B. e l J a n i , M. L e r o u x , J . C . Grenet and P . G i b a r t

Laboratoire de Physique du Solide—Energie Solaire, CNRS, B.P. 1, Sophia- kntipolis, F-06560 VaTbonne, France

RESUME

Les couches de GaAs élaborées â partir de trimëthylgallium (TMG) et d'Arsi- ne (AsH,) contiennent du carbone résiduel qui s'incorpore essentiellement sous forme d'accepteur. Les spectres de photoluminescence présentent en dehors des faits habituels un pic à 1.477 eV, qui est d'autant plus intense que le rapport As/Ga.dans la phase vapeur est élevé. Un recuit sous mélange H2 + AsH3 augmente l'intensité de ce pic. Des couches de GaAs ont été élaborées avec un excès de CH^. Ceci donne des couches très compensées. Le pic à 1.477 eV existe dans tous ces échantillons. La cinétique de croissance est profondément modifiée par 1"excès de méthane.

ABSTRACT

GaAs grown from trimethylgallium (TMG) and arsine (AsH3) countaing residual carbonC. It is mostly Incorporated essentialy as acceptor. Photo!uminescence spec- tra exhibit besides the usual features a peak at 1.477 eV the higher the grea- ter the ratio As/Ga. Annealing under H2 + AsH3 enhances the intensity of this peak. GaAs layers were grown with excess CH-. This gives highly compensated layers; the 1.477 eV peak appears in all these samples. The kinetics of growth

is drastically changed by the excess CH4.

1 - INTRODUCTION

MO-VPE had become a widely used method for growing GaAs epitaxial layers with electronic quality equalling that obtained in the Ga - AsCl, - H2 method.

IE o

Residual carbon impurities < 10 cm are always found in MO-VPE GaAs from TMG and AsH,. This residual carbon gives an acceptor peak in photoluminescence (PL) spectra. Its density was measured by nuclear activation.

In 1975, Seki (1) using TEG instead of TMG obtained high mobility GaAs. His GaAs was almost f m e o f C. He explained his results assuming that no Ga-C-Ga bond could be formed during the pyrolysisr process-; (Ihthe case of TMA1, Al-C-Al bridges do exist and Al-C, may be obtained. To avoid the formation of carbide in growing AlSb.tr^mobulaluminium was used as aluminium source (2). It has been argued too (3) that contaminants may be responsable for the carbon pollution).

Recently Bhat (4) grew high purity GaAs from TMG of TEG sources. He got high mobility samples ^ 1 05 cni2/V.§ec in GaAs grown from TEG. PL spectra did not show any carbon acceptor peak.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1982535

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C5-304 JOURNAL DE PHYSIQUE

Experiments r e p o r t e d here deal w i t h some aspect of C contamination. Emphasis was p u t on d e t a i l e d PL s p e c t r a o n samples grown under d i f f e r e n t c o n d i t i o n s . Heat treatment under ASH

+

H was done on s p e c i f i c samples i n order t o change t h e n a t i v e d e f e c t s equi?ibru8, thus t h e i n c o r p o r a t i o n o f carbon e i t h e r i n As o r Ga g t e .Somelayerswheregrown w i t h excess CH i n t h e gas phase. These p r e l i m i n a r y

r e s u l t s snow t h a t , t h e e x t r a CH4 as w e l l %s 12; a r i s i n g from t h e thermal decbm- p 6 f f t i o . n o f TE!G, i s t o some extend 'incorporate$ i n t h e growing GaAs Tayers.

2

-

EXPERIMENTAL

2.1.1

-

GaAs l a y e r s from TMG and AsH3

We checked

-

as most t h e authors i n v o l v e d i s t h i s f i e l d d i d

-

^{t h a t}^va ^{t h e}

speed o f d e p o s i t i o n i s independant o f T t h e d e p o s i t i o n temperature (between 600 and 750aC), o f PAsH and i s proport?onnal t o PTMG, The nature of t h e r e s i d u a l i m p u r i t i e s dep2nls on t h e r a t i o As/Ga i n t h e vapour phase. Depending on As/Ga p o r n t y p e l a y e r s a r e obtained.

2.1.2

-

Photoluminescence spectra

F i g . 1 shows on a l o g s c a l e 4.2K PL spectra of t h r e e d i f f e r e n t samples grown a t 680°C, w i t h As/Ga = 15, 37, 74 r e s p e c t i v e l y . Besides t h e usual peaks, i . e . e x c i t o n i c f e a t u r e s near t h e band gap ; (Coy e) peak a t 1.493 eV (6), LO rep1 ic a 36 meV below, t h e s p e c t r a show f o r h i g h As/Ga values a shoulder about 1.477eV. This peak i s g r e a t e r t h e higher t h e As/Ga r a t i o .

This peak i s u s u a l l y assigned t o Ge acceptor. However SIMS a n a l y s i s does n o t show any t r a c e o f Ge. Furthermore t h e i n c o r p o r a t i o n of Ge acceptors GeA;

(a pASi1) should decrease w i t h i n c r e a s i n g PAsH

.

This i s n o t t h e case. There i s y e t no #orma1 assignment f o r t h i s peak. 3

2.1.3

-

Annealing

Annealing under HZ

+

AsH3 (lo-' at.) d u r i n g one hour a t 680°C was c a r r i e d o u t on samples grown w i t h As/Ga = 37 and 74. An increase o f VGa r e s u l t s from t h i s h e a t treatment ; t h i s a1 lows t h e r e d i s t r i b u t i o n o f CAs towards Ga s i t e and p o s s i b l y t h e f o r m a t i o n o f

c i s - c i a

complexes. I n h i g h l y doped S i

-

GaAs, h e a t treatment under p d r t i a l pressure o f AsH3 r e s u l t s i n r e d i s t r i b u t i o n of S i among As and Ga s i t e s and i n t h e f o r m a t i o n o f SiA5

siG;

complexes ( 7 ) .

As 2

For a t y p i c a l sample grown a t 680°C, w i t h ⁼37,

u300

= 3500 cm /V.sec, ND

-

NA = 2 . 1 0 ' ~ cmJ (K = 0.8). A f t e r annealing,ND

-

NA = 2.6.10'~ c m - 3 ~ k e r e a ~

u300

= 1660 cm 2 /V .sec. The conversion o f CAs i n t o CG, and t h e formation o f some k i n d of complexe l i k e

c i s - ^{c i a}

i s c o n s i s t e n t w i t h these data.

On t h e PL s p e c t r a of annealed samples, t h e h e i g h t o f t h e 1.477eV peak i n - creases by several orders o f magnitude (Fig.2).

2.2

-

MO-VPE o f GaAs w i t h excess o f CH, i n the gas phase

An excess o f CH4 i n t h e i n p u t gases Hz, TMG, AsH3 r e s u l t s i n decreasing t h e speed o f d e p o s i t i o n V V e x p e r i m e n t a l l y v e r i f i e s t h e law V a PCH - O e 3 a t a

9 ' 9

9

g i v e n temperature (680°C) ( F i g . 3). Furthermore a t constant AslGa,

fg

i s tempe- r a t u r e dependent.

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For T > 630°C, V i s t h e r m a l l y a c t i v a t e d ( F i g . 4) 9

V a exp

-

€,/KT w i t h Ea = 26 Kcal.

9

I n standard c o n d i t i o n s T = 680°C, 4s/Ga = 1 5 a t y p i c a l sample shows t h e f o l - l o w i n g p r o p e r t i e s

w i t h an excess

These r e s u l t s show t h a t C i s i n c o r p o r a t e d b o t h as donor and acceptor. ( I t i s r o u g h l y v e r i f i e d o v e r f i v e r u n s thatN,,.

P

- 3 , N A a pCHi2).

CH4

The PL spectra o f these samples a r e shown on Fig. 5. Th$ main peaks a r e present as usual (Coy e) overlapping (CO, Do) ; (A0, X), (D

,

x ) . However t h e (Ae,FJ peak, corresponding probably t o CAs i n h i g h e r than i n standard samples grown w i t h o u t CH4.This peak i s t o be r e l a t e d t o C t o same f o r a t o be

hypothezied.

2.3

-

Other r e s u l t s

GaAs grown from TEG do n o t present carbon contamination (1, 4 ) . The 1.493 (CO, e) acceptor peak does n o t e x i s t i n PL spectra. On t h e o t h e r hand Ga.4~

p:"epdred from TMG and t r i m s t h y l a r s e n i c (TMAs) ( i n s t e a d of ASH ) shows poor m o b i l i t i e s

-

3500 em /Vsec. GaAs grown from TMG and T M A ~ i s formed by t h e simultan28tls p y r o l y s i s o f TMG and TMAs. This b r i n g s CH4 molecules i n t h e gas phase and f i n a l l y s i m i l a r f e a t u r e s than GaAs grown w i t h excess eHa a r e obser- ved, i.e, h i g h l y compensated samples, broad acceptor peaks i n PL spectra ( C O ? e) and probably o t h e r acceptor i m p u r i t i e s , , t h e speed o f d e p o s i t i o n decreases w i t h i n c r e a s i n g As/Ga ( o r w i t h i n c r e a s i n g p a r t i a l p a r t i a l pressure o f CH4 i n t h e gas phase).

It must be emphasized t h a t i n these both examples, no add compounds a r e formed.

3

-

_PRIMARY ANALYSIS OF THE DATA 3.1

-

PL peak a t 1.477eV

I t was a l r e a d y shown t h a t t h i s peak i s n o t due t o Ge acceptor and t h i s was discussed r e c e n t l y by Hess e t a1 t o o (8). A p l a u s i b l e explanation-of a l l t h e observed f e a t u r e s i s t h e e x m n c e o f some complexe, p o s s i b l y CAs

-

^{CGa. The}

heat treatment under AsH3 causes o u t d i f f u s i o n o f VGa then r e s u l t s i n t h e r e d i s - t r i b u t i o n o f C among As and Ga s i t e , and increases t h e CAs

-

CGa d e n s i t y (observed by t h e enhancement o f t h e 1.477eV peak)". The s l i g h t difference between c a l c u l a t e d and measured energy can be a t t r i b u t e d t o t h e Stokes s h i f t . Prelimina- r y r e s u l t s on t h e temperature behaviour o f t h i s luminescence peak do n o t seem t o i n d i c a t e an e l e c t r o n t o acceptor t y p e peak. F u r t h e r experiments i s anyhow s t i l l needed.

*

The d i s t r i b u t i o n o f C on t h e two s u b l a t t i c e s v a r i e s w i t h t h e c o n c e n t r a t i o n of C.

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JOURNAL DE PHYSIQUE

3.2

-

K i n e t i c s d a t a

It i s assumed i n standard MO-VPE experiments t h a t t h e speed o f d e p o s i t i o n i s l i m i t e d by mass t r a n s f e r t t h r o u g h t a boundary l a y e r , The speed i s d i , r e c t l y p r o - p o r t i o n n a l t o PTMG. The a d d i t i o n o f e x t r a CH4 changes t h e n a t u r e o f k i n e t i c s towards a surface 1 im i t e d process (Fig. 4).

Schlyer ( 9 ) showed t h a t TMG and AsH3 form. an add compound Ga (CH3)3

-

^AsH3

-

Around 200

-

250°C t h i s add compound looses CH4 molecules t o g i v e Ga(CH3)3-x complexes. The a d d i t i o n o f e x t r a CH4 changes t h e e q u i l i b r u m o f f o r m a t i o n of t h i s complexe, and t h e k i n e t i c s o f i t s . decomposition i n t o GaAs.

4

-

FURTHER DEVELOPMENTS

-

Suggestions

A d i r e c t p r o o f o f t h e i n c o r p o r a t i o n o f carbon i n GaAs from CH4 molecules (and .. f r o o t 3 e r carbon source o r some Ga

-

C

-

Ga b r i d g e ) can be brought by

EPR u s i n g

'kill.

13 C e x q y i t s an h y p e r f i n e f i e l d whereas

'*c

does not, EPR i s a good t o o l be recognize C i n GaAs.

An i n s i t u method t o i n v e s t i g a t e the gas phase i s o f g r e a t impor- t a n c e t o d e f i n e t h e temperature p r o f i l e and t h e composition o f t h e gas phase. For instance. Raman spectroscopy, more p r e c i s e l y CARS (coherent a n t i s t o k e Raman spectroscopy) A l a s e r beam, i s scanned along a normal t o t h e s u t x t r a t e and t h e Raman s i g n a l i s analyzed. P r e l i m i nary experiments i n closed tubes show t h a t TMG and ASH e x h i b i t we1 1 d e f i n e d Raman spectra. Add complexes 1 ik e t h e one described b$ Schlyer Ga(CH3)3-x Am3-, a r e expected t o be i d e n t i f i e d .

5

-

CONCLUSION

A b e t t e r understanding o f t h e chemistry and aerothermochemistry i n MO-VPE i s u r g e n t l y needed t o improve t h e q u a l i t y o f t h e l a y e r s . A non d e s t r i c t i v e , and non i n t r u s i v e method l i k e i n s i t u Raman spectroscopy should prove be very help- f u l l .

ACKNOWLEGMENTS

The authors wish t o express t h e i r thanks t o M. GUITTARD f o r h e l p f u l 1 experimental support and R. DRUILHE f o r some unpublished r e s u l t s ,

This work was supported by DAII under c o n t r a c t 80-35-009.

REFERENCES

1

-

^Y. SEKI, K. TANNO, K. IiDA and E. ICHIKI, J. Electrochem. Soc. (1975) 1108

2

-

A. TROMSON-CARLI, P. GIBART and C. BERNARD, J. Cryst. Growth

--

55 (1981) 125 3

-

S. ITO, T. SHINOHARA and Y. SEKI, J. Electrochem. Soc.

120

(1973) 1419 4

-

^R. BHAT, P. O'CONNOR, H. TEMKIN, R. DINGLE and V. G. KERAMIDAS, I n s t . Phys.

Conf. Ser No 63 (1982) 101

5

-

B. EL JANI, These 3@me cycle, Paris V I I , 1982

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6

-

The 1.494eV peak i n GaAs a t 4.2K was unambigously assigned t o (CQ, e) a f t e r carbon i m p l a n t a t i o n i n S.1 GaAs : G.B. S t r i n g f e l l o w , W. KOSCHEL, F. BRIONES, J. GLADSTONE and G. PATTERSON, Appl. Phys. L e t t .

2

(1981) 581

7

-

W .G. SPITZER and W. ALLRED, 3. Appl

.

^Phys.

29

(1968) 4999

8

-

K.L. HESS, P.D. DAPKUS, H.M. MANASEVIT, T.S. LOW, B.J. SKROMME and G.E. STILLMAN ( t o be published)

9

-

D.J. SCHLYER and M.A. RING, J. Electrochem. Soc.

122

(1977) 569

FIGURE 1

Photoluminescence spectra a t 4.2 K (Log Scale) of t h r e e OM-VPE samples grown a t 680°C w i t h

( a )

As/Ga = 15, (b) 37 and ( c ) 74 r e s p e c t i v e l y

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I L L I 1 6 I L B 1 50 1 5 2 V

1.4779 14922

1.L555

-1

1.4435

I I I I I I

8 600 8 500 8 LOO 83M) 8 2W 8 100

A [ A )

FIGURE 2

Photolumines~ence s p e c t r a a t 4.2 K (Log Scale) on samples grown a t 680°C, annealed one hour under ArH3

(lo-'

atmt Hz

( a ) As ⁼74 ( b ) As/Ga = 37

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FIGURE 3

Speed of d e p o s i t i o n of W-VPE GaAs grown w i t h excess Me ( A s I G a = 15, TD = 680°C) a s a f u n c t i o n o f the p a r t i a l Pressure of excess CH4

Speed o f deposition of MO-VPE GaAs grown w i t h excess CH4 as a f u n c t i o n o f I / T

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FIGURE 5

Photoluminescence spectra a t 4.2 K (Log Scale) o f MO-VPE GaAs samples grown a t 680°C, As/Ga = 15 w i t h excess CH4

( a ) pCH = 7.5.10 -4 a t n

(b)

PCH = atm

4 4