AN EXAFS STUDY OF Bi DOPING IN CHALCOGENIDE GLASSES

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AN EXAFS STUDY OF Bi DOPING IN CHALCOGENIDE GLASSES

S. Elliott, A. Steel

To cite this version:

S. Elliott, A. Steel. AN EXAFS STUDY OF Bi DOPING IN CHALCOGENIDE GLASSES. Journal

de Physique Colloques, 1986, 47 (C8), pp.C8-399-C8-402. �10.1051/jphyscol:1986879�. �jpa-00226201�

AN EXAFS STUDY OF Bi DOPING IN CHALCOGENIDE GLASSES

S.R. ELLIOTT and A.T. STEEL"

Department of Physical Chemistry, University of Cambridge, Lensfield Road, GB-Cambridge CB2 I E P , Great-Britain

' A . T . Steel, Daresbury Laboratory, Daresbury,

GB-Warrington WA4 4AD, Great-Britain

A b s t r a c t

The r o l e o f Bi i n causing a t r a n s i t i o n from p-type t o n-type e l e c t r i c a l behaviour i n Ge chalcogenide glasses has been s t u d i e d by EXAFS. I t i s found t h a t the c o o r d i n a t i o n number o f the B i i s 3, independent o f B i content, b u t t h a t the Debye-Waller f a c t o r increases by a f a c t o r o f two a t the p-n t r a n s i t i o n .

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

Chalcogenide glasses are normally very r e s i s t a n t t o e l e c t r i c a l doping.

However, B i has been found (1) t o a c t as a dopant i n Ge chalcogenide glasses, changing t h e dominant c a r r i e r type from holes t o e l e c t r o n s a t a c o n c e n t r a t i o n o f

=9%; the d.c. e l e c t r i c a l c o n d u c t i v i t y changes concomitantly by about s i x orders o f magnitude.

The doping mechanism i s n o t w e l l understood : several mechanisms have been suggested, most o f which a s c r i b e t h e doping e f f e c t t o a disturbance o f t h e e q u i l i b r i u m between t h e charged d e f e c t centres D+, D- ( 2 ) present i n t h e glass, (which otherwise pin- t h e Fermi l e v e l ) by the i n c o r p o r a t i o n o f t h e B i i n a charged s t a t e . Thus mechanisms i n v o l v i n g p o s i t i v e l y charged B i i n t h r e e - f o l d ( 6 ) and f o u r - f o l d (4,5) c o o r d i n a t i o n and n e g a t i v e l y charged B i i n t w o - f o l d (6,7) and s i x - f o l d c o o r d i n a t i o n (4,'8) have v a r i o u s l y been proposed. I n a d d i t i o n , a mechanism i n v o l v i n g p e r c o l a t i v e t r a n s p o r t between m i c r o c r y s t a l l i n e B i C 3 (C=S,Se) i n c l u s i o n s i n a glassy Ge-chalcogenide m a t r i x has a l s o been suggeste$ ( 9 ) . I t i s t h e r e f o r e apparent t h a t a study of the l o c a l s t r u c t u r a l environment about t h e Bi, e.g. by EXAFS, should p e r m i t t h e v a r i o u s competing mechanisms f o r t h e doping e f f e c t t o be discriminated.

2. Experimental

B i Ge S ( 3 < x < 10) glasses were made by m e l t quenching; EXAFS samplesX ~ @ e ~ ~ ~ ~ e p a r e d by f i n e l y powdering t h e g l ass and p r e s s i n g between s e l l o t a p e . Transmission EXAFS measurements were taken a t room temperature beyond t h e B i L -edge on t h e w i g g l e r l i n e a t t h e Daresbury synchrotron r a d i a t i o n source. he ^{data were analysed by} f i t t i n g i n r e c i p r o c a l space t h e back-transformed, F o u r i e r - f i l t e r e d and background-subtracted data using standard Daresbury r o u t i n e s based on t h e curved-wave approximation f o r t h e EXAFS amplitude and using c a l c u l a t e d p h a s e - s h i f t s f o r the elements i n question.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1986879

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3. R e s u l t s

The p r i n c i p a l r e s u l t o f our study can be seen i n Fig. 1. I t i s seen t h a t on i n c r e a s i n g t h e B i c o n c e n t r a t i o n from 3 t o 10 a t . % t h e EXAFS amplitude, and concomitantly the magnitude o f t h e F o u r i e r transform, decreases by Z 40%. A c u r v e - f i t t i n g a n a l y s i s o f the data i n d i c a t e s t h a t t h i s d i f f e r e n c e i s due t o an increase by a f a c t o r o f two i n t h e Debye-Waller (DW) f a c t o r r a t h e r than t o a change i n t h e c o o r d i n a t i o n number about the Bi, which remains unchanged a t 3 (S atoms).

Fig.1 EXAFS spectra taken above

glasses (x=3, 10): a) k3-weighted

t h e B i LIII-edge f o r B i Ge S x 20 80-x EXAFS amp1 i tude, b) F o u r i e r transform.

A c o n f i r m a t i o n o f t h i s f i n d i n g i s shown i n f i g . 2 where the r a t i o o f the EXAFS amplitudes (taken a t extrema i n the spectra) f o r 3 and 10 a t . % B i samples p l o t t e d l o g a r i t h m i c a l l y versus k 2 i s shown. I n t h e case o f a s i n g l e b a c k - s c a t t e r i n g s h e l l and i n t h e plane-wave approximation, a d i f f e r e n c e i n c o o r d i n a t i o n numbers would be s i g n i f i e d by a l i n e o f zero slope, and a d i f f e r e n c e i n D W f a c t o r s by a s t r a i g h t l i n e having a f i n i t e slope whose g r a d i e n t i s equal t o t w i c e the d i f f e r e n c e i n D W f a c t o r s (10). The data i n f i g . 2 c l e a r l y i n d i c a t e t h a t t h e l a t t e r case i s predominantly t r u e here, and the d i f f e r e n c e i n D W f a c t o r s (0.006A2) obtained from a least-squares f i t t o the data ( e x c l u d i n g t h e f i r s t t h r e e data p o i n t s a t low k, where t h e p l ane-wave approximation i s n o t v a l i d ) agrees very c l o s e l y w i t h t h a t obtained from curve f i t t i n g (see Table). The non-zero i n t e r c e p t i n f i g . 2 i m p l i e s a d i f f e r e n c e i n c o o r d i n a t i o n numbers o f l e s s than 8%, c o n s i s t e n t w i t h t h e

u n c e r t a i n t y associated w i t h t h e f i t s t o x i k ) (see Table). F i n a l l y , a comparison o f t h e EXAFS spectra f o r glassy Bil,,Ge,oS,o and c r y s t a l l i n e Bi2S, ( f i g . 3) c l e a r l y demonstrates t h a t t h e r e i s no s t r u c t u r a l s i m i l a r i t y between these two m a t e r i a l s .

Table 1

S t r u c t u r a l parameters obtained from f i t t o EXAFS o f BixGe20S80-x glasses

x

R,(A) A,( it2)

N,,R, and A, are 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, bond l e n g t h and t w i c e t h e

Debye-Wal l e r f a c t o r 2a2, r e s p e c t i v e l y .

(x=3, 10 a t . %) p l o t t e d l o g a r i t h m i c a l l y versus k2.

f o r glassy BiloGe20S70 and c r y s t a l l i n e Bi,S3.

4. Discussion

We have shown t h a t a pronounced l o c a l s t r u c t u r a l change accompanies/causes the p-n doping t r a n s i t i o n occasioned by t h e i n c o r p o r a t i o n o f Bi i n Ge chalcogenide glasses. Our r e s u l t s r u l e o u t several o f the contending proposed mechanisms f o r the doping e f f e c t . I n p a r t i c u l a r , our f i n d i n g t h a t t h e c o o r d i n a t i o n number o f t h e B i remains a t 3 (S atoms) f o r a71 B i concentrations r u l e s o u t those models which invoke coordinations lower ( r e f s 6,7) o r higher ( r e f s 4-8) than t h i s value.

Furthermore, t h e d i s s i m i l a r i t y between t h e l o c a l s t r u c t u r e s o f glassy B i ,oGe,,oS,o and c r y s t a l l i n e Bi2S i n d i c a t e s t h a t the model o f p e r c o l a t i v e t r a n s p o r t i n an inhomogeneous m a t e r ~ a j c o n t a i n i n g B i 2S3 (micro) c r y s t a l l i t e s (9) i s l i k e l y t o be i n a p p r o p r i a t e . Based on t h e present EXAFS r e s u l t s , t h e mechanism f o r doping by B i which we propose i s as f o l l o w s ( E l l i o t t and Steel, t o be published). A t low B i concentrations ( x t 9 a t . %), the B i atoms occupy t h r e e - f o l d coordinated s i t e s ,

Fig.4 EXAFS data taken above the Sb K-edge f o r SbxGe20S80-x (x=4,10).

:a) k3-wei ghted EXAFS ampl i tude, b) F o u r i e r transform.

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bonded t o 3 S atoms by covalent linkages, i n accordance w i t h the usual valence employing only p - e l e c t r o n s i n the bonding. Above a c e r t a i n c r i t i c a l concentration, some o f the Bi-S bonds acquire a p a r t i a l l y i o n i c character, a circumstance favoured by t h e e l e c t r o p o s i t i v i t y o f the B i . We suppose t h a t , a t l e a s t a t some s i t e s , a p a r t i a l l y i o n i c Bi6+-s6- bond forms; such bonds are markedly s h o r t e r than bonds having more c o v a l e n t c h a r a c t e r i n c r y s t a l l i n e B i s u l p h o s a l t s (111, and we assume t h a t t h i s bond-length d i f f e r e n c e i s t h e cause o f t h e increase i n t h e s t a t i c D W f a c t o r w i t h i n c r e a s i n g B i content. The formation o f p o s i t i v e l y charged B i atoms ( 3 ) upsets t h e e q u i l i b r i u m between D+ and D- charged d e f e c t centres by the law o f mass a c t i o n (12); as a r e s u l t , the Fermi l e v e l r i s e s towards the conduction band and the m a t e r i a l becomes n-type. The observation (13) o f a decrease i n the value o f t h e d.c. c o n d u c t i v i t y a c t i v a t i o n energy t o 2/3 o f i t s value on passing through the D-n t r a n s i t i o n can a l s o be understood on t h i s b a s i s ( E l 1 i o t t and Steel, t o be pub1 i shed).

F i n a l l y , we remark t h a t we b e l i e v e t h a t Bi i s unique i n i t s doping behaviour because o f t h e ease w i t h which p a r t i a l l y i o n i c bonds may form, w i t h t h e consequent e f f e c t on t h e charged d e f e c t e q u i l i b r i u m . For instance, t h e i n c o r p o r a t i o n o f Sb i n t o Ge chalcogenide glasses does n o t cause e l e c t r i c a l doping (4); the EXAFS spectra taken on S ~ X G ~ ~ O S ~ O - ~ glasses beyond the Sb K-edge a r e very s i m i l a r f o r a l l Sb contents (see f i g . 4 1 , i n d i c a t i n g t h a t t h e Sb i s always i n c o r p o r a t e d i n t o a l l o y i n g , c o v a l e n t s i t e s , r a t h e r than doping, charged s i t e s ( E l l i o t t and Steel, t o be published).

5. Conclusions

EXAFS experiments g i v i n g i n f o r m a t i o n on t h e l o c a l s t r u c t u r a l environment o f B i i n c o r p o r a t e d i n t o Ge chalcogenide glasses have enabled a mechanism f o r t h e e l e c t r i c a l doping e f f e c t o f B i t o be elucidated.

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