THE STRUCTURAL ENVIRONMENT OF Te DOPANTS IN GaAs USING EXAFS IN FLUORESCENCE MODE

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THE STRUCTURAL ENVIRONMENT OF Te DOPANTS IN GaAs USING EXAFS IN

FLUORESCENCE MODE

G. Greaves, P. Halfpenny, G. Lamble, K. Roberts

To cite this version:

G. Greaves, P. Halfpenny, G. Lamble, K. Roberts. THE STRUCTURAL ENVIRONMENT OF Te

DOPANTS IN GaAs USING EXAFS IN FLUORESCENCE MODE. Journal de Physique Colloques,

1986, 47 (C8), pp.C8-901-C8-904. �10.1051/jphyscol:19868175�. �jpa-00226080�

JOURNAL D E PHYSIQUE

Colloque C8, supplbment au n o 12, Tome 47, dbcembre 1986

THE STRUCTURAL ENVIRONMENT OF T e DOPANTS IN GaAs USING EXAFS IN FLUORESCENCE MODE

G.N. GREAVES, P.J. HALFPENNY*, G.M. LAMBLE*" and K.J. ROBERTS"

SERC Daresbury Laboratory, GB-Warrington W A 4 4AD, Great-Britain ' ~ e p a r t m e n t of Engineering Materials, University of Southampton, GB-Southampton SO9 5NH, Great-Britain

t *

Department of Pure and Applied Chemistry, University of Strathclyde, GB-Glasgow G1 IXL, Great-Britain

A b s t r a c t

The l o c a l c c r o r d i n a t i o n of 'grown-in' Te in GaAs h a s been determined by FLEXAFS measurements. A t c a r r i e r c o n c e n t r a t i o n s of 8

1018 an-3 c o n s i d e r a b l e d i l a t i o n of t h e f i r s t c c r o r d i n a t i o n s p h e r e around t h e dopant t a k e s p l a c e , w i t h a p p r e c i a b l e c o n t r a c t i o n of t h e second c o - o r d i n a t i o n sphere. The f i r s t , second and t h i r d n e a r e s t neighbour d i s t a n c e s were found t o b e 2.58 (t 0.02) a, ^{3.52 (f 0.05)} a ^and

4.58 (i 0.1) a, r e s p e c t i v e l y . These a r e d i s c u s s e d i n t e r m s of v a r i o u s models which have been proposed f o r group V I dopants i n GaAs.

I n t r o d u c t i o n

The s t r u c t u r a l and e l e c t r o n i c e f f e c t s of dopant i m p u r i t i e s i n t e t r a h e d r a l semiconductors can, i n g e n e r a l , be p r e d i c t e d according to a simple m i s - f i t model a s d e s c r i b e d by Vegard's Law. However, in t h e c a s e of GaAs t h e r e a r e a number of dopants (e.g. Sn,Te ,Se) which d i l a t e t h e l a t t i c e to a much g r e a t e r e x t e n t . Te doping has r e c e i v e d s p e c i f i c a t t e n t i o n and s e v e r a l s t u d i e s have shown t h a t t h i s d i l a t i o n is c o n s i d e r a b l e a t c o n c e n t r a t i o n s of g r e a t e r than l 0 l a an-3 [I I . It has t h u s provided a u s e f u l model system f o r s t u d y i n g dopant environments in 1 1 1 - V semiconductors.

Experiment and Analysis

S i n g l e c r y s t a l wafers of GaAs were o b t a i n e d which were about 2.5 an i n diameter and contained 'grown-in' impurity c o n c e n t r a t i o n s of Te, t h e measurements r e p o r t e d h e r e r e l a t e to a c a r r i e r c o n c e n t r a t i o n of 8 x 1018 R e f l e c t i o n X-ray t o p o r graphs r e v e a l e d t h e wafer to b e of good c r y s t a l p e r f e c t i o n , w i t h no l a r g e - s c a l e mosaicity. FLEXAFS s p e c t r a were recorded a t t h e Te K-edge u s i n g t h e Wiggler s t a t i o n 9.2 on t h e SRS a t SERC Daresbury Laboratory. The wafer samples were i n c l i n e d a t an a n g l e of approximately 10 degrees t o t h e i n c i d e n t monochromatic beam, and r o t a t e d i n t h e i r p l a n e to average o u t any l a u e d i f f r a c t i o n e f f e c t s i n t h e recorded spectrum.

S e v e r a l scans were averaged to achieve adequate s i q n a l / n o i s e . To a s s i s t i n d a t a a n a l y s i s s t a n d a r d s p e c t r a were a l s o recorded. These were measured in t r a n s m i s s i o n mode using powdered samples of GaAs ( f o r t h e Ga and As K-edges) and elemental Te

( f o r t h e Te K-edge). Fig. 1 shows t h e k3 weighted s p e c t r a f o r As and Ga in GaAs, t o g e t h e r w i t h t h e p h a s e - s h i f t c o r r e c t e d F o u r i e r transforms. Three s h e l l s of t h e zinc-blende s t r u c t u r e can b e r e a d i l y i d e n t i f i e d and t h e e q u i v a l e n c e of t h e As and Ga s i t e s i s w e l l reproduced. The EXAFS of Te and i t s atomic d i s t r i b u t i o n are shown in

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19868175

C8-902 JOURNAL DE PHYSIQUE

f i g . 2; e l e m e n t a l Te h a s a c h a i n s t r u c t u r e and t h e i n t r a and i n t e r chain

c o r r e l a t i o n s can b e c l e a r l y seen. Data s e t s were background s u b t r a c t e d , F o u r i e r transformed and l e a s t - s q u a r e s r e f i n e d with t h e a i d of t h e Daresbury EXCURVE program

[21. The d o t t e d c u r v e s i n f i g . 2 show t h e f i t to experiment using t h e curved wave t h e o r y [31. They have been o m i t t e d from f i g . 1 f o r c l a r i t y but t h e d i s t a n c e s , FWHM widths and co-ordination numbers f o r

and Ga a r e included i n Table 1. Fig. 3 shows t h e weighted EXAFS spectrum and F o u r i e r t r a n s f o r m s f o r Te i m p u r i t i e s i n GaAs.

The d o t t e d l i n e i s t h e l e a s t - s q u a r e s f i t to experiment and t h e s t r u c t u r a l parameters f o r t h e f P r s t t h r e e s h e l l s a r e a l s o included i n Table 1.

Fig. 1. k 3 weighted Ga

and

K-edge EXAFS and F o u r i e r transforms f o r GaAs .

k( A-'1 R ( A )

Fig. 2. k3 weighted K-edge EXAFS and F o u r i e r t r a n s f o r m s o f elemental Te. The d o t t e d l i n e s a r e t h e 'curved wave' t h e o r y f i t s .

Fig. 3 demonstrates t h a t t h e dopant environment of Te i s q u i t e d i s t i n c t f r m t h e matrix environment o f As and Ga. Note t h a t t h e f i r s t s h e l l i s d i l a t e d and t h e

second s h e l l i s c o n t r a c t e d f o r Te compared to A s ( o r Gal. In p a r t i c u l a r t h e f i r s t

s h e l l r a d i u s of 2.58 8, is 0.13 8, longer than t h e As-Ga bond l e n g t h , i n d i c a t i n g con-

s i d e r a b l e r e l a x a t i o n to accommodate t h e l a r g e r anion. The second s h e l l r a d i u s of

3.52 8, i s 0.47 8, s h o r t e r than t h e e q u i v a l e n t s h e l l in. t h e zinc-blende s t r u c t u r e ,

i n d i c a t i n g t h e p o s s i b i l i t y of a vacancy i n t h i s s h e l l . The t h i r d s h e l l a l s o a p p e a r s

t o ' b e s l i g h t l y c o n t r a c t e d , a l t h o u g h t h i s change is o u t s i d e t h e p r e c i s i o n of a n a l y s i s

Fig. 3. k 3 weighted Te K-edge EXAFS and F o u r i e r transform of 8 x lo1* , T e doped &As. Dotted l i n e s g i v e ' curved wave' t h e o r y f i t s . Parameters a r e included i n Table 1.

Table 1. I n t e r a t o m i c d i s t a n c e s , s h e l l widths and c o - o r d i n a t i o n numbers from t h e a n a l y s i s of s p e c t r a shown in f i g s . 1 t o 3.

S h e l l D i s t a n c e ( 8 ) W H M ( 8 ) Co-ordination Number

Pure Doped Pure Doped Pure Doped

Models

( a ) Te i n t h e A s s i t e with Ga v a c a n c i e s in t h e f i r s t s h e l l . T h i s i s a f a i r l y common model f o r t h e Te i n c o r p o r a t i o n and has been proposed by Iiuang [41 and m l l i n e t d l . [51 (amongst o t h e r s ) . This would b e expected to reduce t h e f i r s t s h e l l c o - o r d i n a t i o n number and i n t r o d u c e some d i s o r d e r . T h i s i s n o t observed.

( b ) Te on an A s s i t e with G a i n t e r s t i t i a l s i n t h e f i r s t s h e l l . This was proposed by Mullin e t a l . [ 5 ] and could c e r t a i n l y e x p l a i n t h e d i l a t i o n observed. However, it would a l s o l e a d to an i n c r e a s e i n t h e f i r s t s h e l l c o - o r d i n a t i o n number.

( c ) A Te/& i n t e r s t i t i a l complex a l i g n e d a l o n g [I001 p o s i t i o n e d on a Ga s i t e and a s s o c i a t e d with a Ga vacancy. Such a d e f e c t was proposed by Fewster 161. T h i s d e f e c t would s p l i t t h e f i r s t c o - o r d i n a t i o n s h e l l and provide some s h o r t e r Te-Ga and Te-As c o n t a c t s , which a r e c l e a r l y n o t seen ( f i g . 3 ) .

( d ) Te on an A s site a s s o c i a t e d with A s v a c a n c i e s i n t h e second s h e l l . This has n o t y e t been suggested f o r Te b u t a r e c e n t s t r u c t u r a l s t u d y by S e t t e e t a l . I71 has proposed such a model f o r S-doped GaAs a t a l e v e l of between 2 x 1019 and

2 x 1020 atoms In t h e case of Te i n GaAs with this model we m u l d expect a

l a r g e expansion of t h e f i r s t s h e l l , t o g e t h e r w i t h a r e l a x a t i o n o f t h e second s h e l l .

The second s h e l l cc-ordination number should a l s o b e reduced.

JOURNAL DE PHYSIQUE

Conclusions

Model ( d ) p r o v i d e s t h e c l o s e s t agreement w i t h t h e environment of Te we have measured ( s e e Table 1 ) . Unlike t h e case of S, where t h e f i r s t s h e l l d i s o r d e r was

found t o b e g r e a t e r t h a n t h e second o r t h i r d s h e l l s , p o i n t i n g to a m i x t u r e of s i t e s , we f i n d t h e widths of a l l t h r e e s h e l l s a r e a s narrow ( o r s l i g h t l y narrower) than f o r Ga o r

and conclude t h a t Te EXAFS a t a c a r r i e r c o n c e n t r a t i o n of 8

lo1*

r e l a t e s t o a s i n g l e s u b s t i t u t i o n a l s i t e . Acknowledgements

We g r a t e f u l l y acknowledge D r . J. B. Mullin (RSRE Malvern) f o r t h e p r o v i s i o n of t h e Te-doped GaAs and SERC f o r funding this p r o j e c t . The a s s i s t a n c e of

Docherty, on computational a s p e c t s , i s much a p p r e c i a t e d .

References

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F. Beleznay, G. Ferenczi and J. Giber (Springer-Verlag, B e r l i n 1980

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[2] S.

Gurman, N. Binsted and I. Ross, J. Phys. C 17, 143 (1984).

[3] P.A. Lee and J.B. Pendry, Phys. Rev. e, 2795 (1975).

[4] C.

Hwang, J. Appl. Phys. 40, 4584 (1969).

[5] J.B. Mullin, B.W. Straughen, C.M.H. D r i s c o l l and A.F.W. Willoughby, J. Appl.

Phys. 47, 2584 (1976).

161 P.F. Fewster, J. Phys. Chem. S o l . 42, 883 (1981).

[7] F. S e t t e , S.J. Pearton, J.M. Poate, J.E. Rowe and J. S t & . . , Phys. Rev. L e t t . 56,

2637 (1986).