EXAFS STUDIES OF PHOTOSTRUCTURAL CHANGES IN CHALCOGENIDE GLASSES

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EXAFS STUDIES OF PHOTOSTRUCTURAL CHANGES IN CHALCOGENIDE GLASSES

S. Elliott, G. Greaves, A. Lowe, L. Gladden, J. Rennie

To cite this version:

S. Elliott, G. Greaves, A. Lowe, L. Gladden, J. Rennie. EXAFS STUDIES OF PHOTOSTRUC-

TURAL CHANGES IN CHALCOGENIDE GLASSES. Journal de Physique Colloques, 1986, 47 (C8),

pp.C8-363-C8-368. �10.1051/jphyscol:1986871�. �jpa-00226193�

JOURNAL DE PHYSIQUE

Colloque C8, supplkment au n o 12,

47, dkcembre 1986

EXAFS STUDIES OF PHOTOSTRUCTURAL CHANGES IN CHALCOGENIDE GLASSES

S.R. ELLIOTT, G.N. GREAVES*, A.J. LOWE('), L.F. GLADDEN

and

Department of Physical Chemistry, University of Cambridge, Lensfield Road, GB-Cambridge CB2 IEP, Great-Britain

" ~ a r e s b u r y Laboratory, GB-Warrington W A 4 4AD, Great-Britain

A b s t r a c t

EXAFS measurements have been performed on a number o f chalcogenide glasses e x h i b i t i n g b o t h r e v e r s i b l e and i r r e v e r s i b l e p h o t o s t r u c t u r a l e f f e c t s . Changes i n the l o c a l s t r u c t u r e induced by t h e absorption o f bandgap l i g h t have been observed and these are discussed i n t h e l i g h t o f c u r r e n t models f o r t h e e f f e c t .

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

Chalcogenide glasses, a l l o y s o f S and Se w i t h As,Ge etc., e x h i b i t a wide v a r i e t y o f photo-induced e f f e c t s (see r e f s . 1, 2 f o r a review). Changes can v a r i o u s l y be induced i n s t r u c t u r a l , o p t i c a l , mechanical and chemical p r o p e r t i e s . I n a d d i t i o n , the changes can be e i t h e r r e v e r s i b l e ( i .e. annealed o u t by h e a t i n g the ill uminated sample t o t h e g l a s s - t r a ~ e m p e r a t u r e , T o r i r r e v e r s i b l e depending on t h e p r e p a r a t i o n method and thermal h i s t o r y o f 9 t h e sample. Thus me1 t-quenched b u l k g l asses o r annealed t h i n f i 1 ms general l y e x h i b i t r e v e r s i b l e e f f e c t s , whereas as-deposited t h i n f i l m s can e x h i b i t i n a d d i t i o n i r r e v e r s i b l e e f f e c t s . However, c e r t a i n photo-induced phenomena, such as metal p h o t o - d i s s o l u t i o n are, by t h e i r very nature, i r r e v e r s i b l e .

I n many cases, the fundamental mechanisms r e s p o n s i b l e f o r these photo-induced phenomena remain u n c e r t a i n . I t i s our view, however, t h a t i n most cases a photo-induced change i n t h e s t r u c t u r e i s t h e cause of the o t h e r photo-induced phenomena, and so an understanding o f the p h o t o - s t r u c t u r a l (PSI changes which take p l a c e can shed considerable l i g h t on t h e n a t u r e o f o t h e r photo-induced e f f e c t s . F o r t h i s reason we have undertaken a s e r i e s o f EXAFS experiments i n order t o probe any photo-induced changes i n the local s t r u c t u r e o f t h e glasses which may occur.

2. Experimental

Bulk glassy As2S3, GeSe, and GeS,, synthesised from t h e elements and prepared by melt-quenching, were used i n powdered form f o r s t u d i e s o f the r e v e r s i b l e PS e f f e c t . Samples f o r EXAFS measurements were made e i t h e r by d i s p e r s i n g the powder i n a b i n d e r and forming a s e l f - s u p p o r t i n g tape (As$ 1, o r by spreading upon re1 1 otape (GeSe2,GeS2) ; ill umi n a t i o n and subsequent annea? i ng of t h e As8S3 tape was performed i n s i t u , and i n the case o f GeSe, and GeS, i n d i v i d u a l p o r t i o n s o f the powdered samples were separately i l l u m i n a t e d and then annealed a t Tg. Samples f o r s t u d i e s o f t h e i r r e v e r s i b l e PS e f f e c t s were prepared by vacuum evaporation onto A1 o r mylar substrates, e i t h e r deposited a t normal incidence (As S,, and GeSe f o r Ag p h o t o - d i s s o l u t i o n ) o r a t a v a r i e t y o f angles o f incidence (~ete,, GeS,). ~ X A F S measurements beyond t h e K-edges o f the elements i n question were taken both i n conventional transmission mode and a t g l a n c i n g angle f o r measurements a t t h e Ag K-edge a t t h e Daresbury synchrotron r a d i a t i o n source u s i n g b o t h channel-cut and double-crystal monochromators. Measurements on t h e r e v e r s i b l e PS e f f e c t f o r glassy As2S3 were taken a t 80K; a l l others were taken a t room temperature.

("present address : Department of Physics, University of Hull, GB-Hull HU6 7FX. Great-Britaln

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1986871

J O U R N A L

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3. Resu:ts and d i s c u s s i o n a) Reversible PS e f f e c t

The r e s u l t s f o r glassy As2S3 are shown i n f i g . 1, where t h e p a i r d i s t r i b u t i o n

I I I I I I

- r

^-

2

-

.

d ----.-

L ANNEALED

5

- -

- rr

a

-

rn

10

- -

Fig.1 P a i r d i s t r i b u t i o n f u n c t i o n obtained by F o u r i e r t r a n s f o r m a t i o n o f the EXAFS amplitude f o r t h e As K-edge f o r glassy As2S3 i n annealed, i l l u m i n a t e d and ( p a r t , i a l l y ) reannealed states.

f u n c t i o n s (PDF's) obtained from F o u r i e r transforms o f the As K-edge EXAFS measurements f o r a sample i n t h e s e r i e s o f s t a t e s anneal e d - i 1 lumi nated-reannealed are shown; a l l EXAFS spectra were subjected t o the same background s u b t r a c t i o n treatment. I t can be seen t h a t t h e r e i s no s i g n i f i c a n t change i n t h e f i r s t s h e l l , i n d i c a t i n g t h a t t h e r e appears t o be no photo-induced change i n e i t h e r t h e As-S bond l e n g t h o r the nearest-neighbour c o o r d i n a t i o n number about t h e As, a t l e a s t t o w i t h i n the s e n s i t i v i t y o f t h e technique. On t h i s basis, t h e r e f o r e , bond-breaki ng mechanisms i n v o l v i n g s e l f - t r a ped e x c i t o n s (3) which r e s u l t i n a change i n the As c o o r d i n a t i o n ( i .e. g i v i n g As: o r As2- c e n t r e s ) appear t o be r u l e d out. However, t h e r e are d i s t i n c t photo-induced changes i n t h e r e g i o n o f the second s h e l l o f t h e As p a i r - d i s t r i b u t i o n f u n c t i o n (PDF) as shown i n f i g . 1; t h e peak p o s i t i o n s h i f t s t o a h i g h e r value o f r and broadens so t h a t t h e minimum beyond t h e second peak i s p a r t i a l l y f i l l e d i n . (The changes were only p a r t i a l l y annealed o u t because t h e sample c o u l d only be heated t o 50°C r a t h e r than t o i t s Tg. These r e s u l t s can be understood on t h e b a s i s o f a change i n As-As distances r e s u l t i n g e i t h e r from a photo-induced increase i n t h e As-S-As bond angle (and a concomitant increase i n t h e

! i s t r i b u t i o n o f such angles) o r from a photo-induced increase i n separation o f non-bonded" As neighbours. These two e f f e c t s c o u l d a r i s e from a double-potential w e l l model (1,4), where a chalcogen atom moves from i t s s t a b l e p o s i t i o n c h a r a c t e r i s t i c o f t h e annealed s t a t e t o another metastable p o s i t i o n under t h e a c t i o n o f l i g h t , o r from a photo-induced weakening o f "i n t e r - 1 ayer" back-bonding i n t e r a c t i o n s , r e s p e c t i v e l y . Unfortunately, t h e present EXAFS second-shell data do n o t a l l o w a q u a n t i t a t i v e i n t e r p r e t a t i o n (through c u r v e - f i t t i n g ) t o be drawn, and so we are unable t o decide between the two a l t e r n a t i v e p o s s i b l e models f o r t h e r e v e r s i b l e PS e f f e c t i n glassy As2S3.

I n c o n t r a s t t o t h e above case, t h e EXAFS s p e c t r a f o r t h e germanium

chalcogenide glasses GeSe, and GeSz do show c l e a r evidence f o r a change i n 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 on i r r a d i a t i o n ( 5 ) . Figs. 2(a,b) show t h e F o u r i e r - f i l t e r e d

k3-weighted EXAFS data f o r t h e Ge and Se K-edges, r e s p e c t i v e l y , f o r the case of

glassy GeSe2. It can be seen t h a t t h e r e i s a s i g n i f i c a n t decrease i n t h e EXAFS on

i l l u m i n a t i o n f o r t h e Se edge whereas t h e r e i s an increase f o r t h e Ge edge; t h e

photo-induced e f f e c t i s completely r e v e r s i b l e (see f i g . 2 ( a ) ) .

Fig.2 EXAFS amplitudes f o r glassy GeSe, beyond a) t h e Ge edge and b ) t h e Se edge i n both annealed - and i 11 umi nated ---- ^states. The r e v e r s i b i 1 i ty on reanneal i ng . i s shown i n ( a ) .

Whether these changes a r e due t o changes i n c o o r d i n a t i o n number (CN) and/or Debye-Waller (DW) f a c t o r can be a s c e r t a i n e d from p l o t t i n g the r a t i o o f t h e EXAFS amp1 i tudes ( f o r ill uminated and annealed s t a t e s ) 1 o g a r i thmical l y versus k2. I n t h e plane-wave approximation, and assuming o n l y 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 , such a p l o t should y i e l d a s t r a i g h t l i n e having e i t h e r a zero slope i f t h e r e i s a change i n CN (whose i n t e r c e p t y i e l d s an e s t i m a t e f o r t h e CN d i f f e r e n c e ) , o r a f i n i t e slope i f t h e r e i s a change i n D W f a c t o r (whose g r a d i e n t y i e l d s an estimate f o r the DW d i f f e r e n c e ) .

Fig. 3 shows such p l o t s f o r t h e Ge edges o f a-GeSe, and a-GeS, and t h e Se edge

Fig.3 P l o t of ln(xill/xann)

k 2 f o r t h e Ge K-(+) and Se K-edges ( x ) i n a-GeSe, and f o r t h e Ge K-edge (0) i n a-GeS2.

o f a-GeSe2, whence i t can be seen t h a t t h e r e appears t o be c l e a r evidence f o r an

increase i n the c o o r d i n a t i o n o f Ge ( i n GeS,) by AN = 0.5 and a decrease i n the

c o o r d i n a t i o n o f Se ( i n GeSe ) by AN = 0.15 on i l l u m i n a t i o n . The case o f Ge i n

a-GeSe i s l e s s c l e a r - c u t ; t 6 e data appear t o suggest a decrease i n t h e D W f a c t o r

o f

= 1 . 0 ~ 1 0 - ~ a2. I n a d d i t i o n , examination o f t h e EXAFS spectra f o r a l l t h e

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edges shows t h a t t h e r e i s l i t t l e , i f any, change i n t h e nearest-neighbour bond length, since t h e p e r i o d i c i t i e s o f the EXAFS o s c i l l a t i o n s are p r a c t i c a l l y t h e same before and a f t e r i l l u m i n a t i o n (see Fig. 2).

The r e s u l t s f o r a-GeSe, ( a decrease i n t h e nearest-neighbour CN f o r Se and a decrease i n t h e D W f a c t o r f o r t h e nearest-neigbour bond l e n g t h f o r Gel a r e d i f f i c u l t t o understand on bond-breaking o r o t h e r models i f t h e m a t e r i a l i s completely chemically ordered i n t h e annealed s t a t e . However, i f chemical d i s o r d e r i s present i n t h e form o f Se-Se (and Ge-Ge) bonds, i n d i c a t e d by Mossbauer r e s u l t s (61, then t h e two apparently c o n t r a d i c t o r y r e s u l t s can be p a r t l y r e c o n c i l e d w i t h i n t h e framework o f a bond-breaking model. I f i t i s assumed t h a t Se-Se bonds are p r e f e r e n t i a l l y broken under t h e a c t i o n o f 1 ig h t (reasonable because i t i s l i k e l y t h a t t h e Se lone-pai r e l e c t r o n s a r e p r e f e r e n t i a1 l y e x c i t e d ) and subsequently move a p a r t somewhat, t h e average CN o f Se would decrease ( b e a r i n g i n mind t h a t Ge and Se atoms have almost t h e same b a c k s c a t t e r i n g amplitudes and t h e r e f o r e appear t o be almost i d e n t i c a l as f a r as EXAFS i s concerned); the CN o f t h e Ge atoms i s unchanged i n t h i s process. However, i t i s p o s s i b l e t h a t Ge-Ge bonds a l s o break and form Ge-Se bonds w i t h the t e r m i n a l Se atoms; i n t h i s way t h e s t a t i c D W f a c t o r f o r the Ge-Se bond would increase. The observation t h a t i n a-GeS, t h e Ge c o o r d i n a t i o n increases on i l l u m i n a t i o n imp1 i e s t h a t i n t h i s m a t e r i a l , a t l e a s t , a qua1 i t a t i v e l y d i f f e r e n t bond-breaking model m i g h t be operating. I f t h e c o o r d i n a t i o n o f Ge i s completely t e t r a h e d r a l i n t h e annealed s t a t e ( i .e. no dangl i n g bonds), i r r e s p e c t i v e o f t h e degree o f chemical ( d i s l o r d e r i n t h e m a t e r i a l , t h i s r e s u l t i n d i c a t e s t h a t photo-induced o v e r c o o r d i n a t i o n o f t h e Ge i s t a k i n g place, perhaps towards octahedral c o o r d i n a t i o n employing d-orbi t a l s o f t h e Ge ( 5 ) . Octahedral c o o r d i n a t i o n o f Ge i s known i n a l k a l i germanium oxide glasses (7), and i t i s n o t unreasonable t o p o s t u l a t e t h a t a s i m i l a r s i t u a t i o n m i g h t occur a t c e r t a i n s i t e s i n glassy GeS, since t h e S atom i s a1 so q u i t e small ; packing c o n s t r a i n t s would t e n d t o d i s f a v o u r overcoordination o f Ge by Se because o f t h e l a r g e r s i z e o f Se.

b) I r r e v e r s i b l e PS e f f e c t .

I r r e v e r s i b l e PS changes are l a r g e r i n magnitude than t h e i r r e v e r s i b l e counterparts; as a r e s u l t , such changes should i n p r i n c i p l e be e a s i e r t o d e t e c t u s i n g l o c a l s t r u c t u r a l probes, such as EXAFS.

The r e s u l t s from As K-edge EXAFS measurements on evaporated a-ASS f i l m s a r e shown i n Fig. 4, where t h e PDF's i n both as-deposited and i l l u m i n a t e 8 s t a t e s are shown. It can be seen t h a t t h e r e i s a s u b s t a n t i a l change i n t h e r e g i o n o f t h e f i r s t , as w e l l as t h e second, c o o r d i n a t i o n s h e l l . The F o u r i e r - f i l t e r e d data f o r the f i r s t s h e l l i n k-space u s i n g the curved-wave theory ( 8 ) y i e l d e d f o r t h e as-deposited case a c o o r d i n a t i o n s h e l l c o n t a i n i n g N

2.5 S atoms a t a distance R1

=

2.251, t o y e t h e r w i t h an As sub-shell (Nll = i . 1 4 . 7 ) a t a s l i g h t l y l a r g e r d i s t a n c e f R I = 2.48W); s i m i l a r r e s u l t s have been r e p o r t e d p r e v i o u s l y ( 9 ) . These f i n d i n g s can be understood by n o t i n g t h a t the s t a b l e vapour species above a m e l t o f As,S3 are mostly As,S molecules ( l o ) , and t h a t t h e r e f o r e condensed f i l m s i n t h e as-deposited s t a t e w i v l a l s o c o n t a i n a preponderance o f such molecular species weakly bonded t o g e t h e r by Van der Waals forces. Each As atom i n an As S, molecule has 2 S near neighbours a t 2.238, and a f u r t h e r As near neighbour a t 2.$98,. I n t h e present case, t h e r e i s reason t o b e l i e v e t h a t the evaporated f i l m s were s l i g h t l y As-rich w i t h respect t o the s t a r t i n g stoichiometry, As,S3, and t h a t t h e r e f o r e t h i s t o o would g i v e r i s e t o As-As c o r r e l a t i o n s a t R12.51

The e f f e c t o f i l l u m i n a t i o n i s seen t o be an increase i n t h e i n t e n s i t y o f t h e

f i r s t eak i n t h e PDF and a s h i f t i n weight t o s l i g h t l y h i g h e r values o f R (see

f i g . 4 ! , together w i t h a d i m i n u t i o n o f t h e high-R shoulder. These observations are

borne o u t by c u r v e - f i t t i n g ; t h e CN of the main S s h e l l increases t o N1 = 2.8, a t an

average d i s t a n c e Rl = 2.261, a1 though changes i n t h e As sub-shell c o u l d n o t be

q u a n t i f i e d . There are a l s o changes e v i d e n t i n the r e g i o n o f t h e second s h e l l o f

t h e PDF (see f i g . 4). The model we have f o r t h e i r r e v e r s i b l e PS e f f e c t i n

amorphous As-chalcogenide f i l m s i s as shown i n f i g . 5, and i n v o l v e s a

p o l y m e r i z a t i o n process, whereby t h e weaker As-As b r i d g i n g bonds are broken and As-S

bonds are formed instead, thereby forming a more f u l l y connected 3D network. A

simi 1 ar mechanism has been proposed e a r l i e r

t o account f o r thermal l y - i nduced s t r u c t u r a l changes; here we a s c r i b e t h e bond-breaking t o o p t i c a l r a t h e r than t o thermal means.

Fig.4 P a i r d i s t r i b u t i o n f u n c t i o n obtained by F o u r i e r t r a n s f o r m a t i o n o f t h e EXAFS amplitude f o r t h e As K-edge f o r an a-Assx f i l m b e f o r e and a f t e r i l l u m i n a t i o n .

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Fig. 5,Schematic i l l u s t r a t i o n o f t h e photo-induced p o l y m e r i z a t i o n which i s proposed t o occur i n as-deposi t e d amorphous As chal cogeni de f i 1 ms, causing t h e i r r e v e r s i b l e PS e f f e c t .

+ V)

.-

C

d

5 20

c ) Metal p h o t o d i s s o l u t i o n .

I t has been known f o r some time t h a t c e r t a i n metals (e.g. Ag) d i s s o l v e i n t o chalcogenide glasses upon i r r a d i a t i o n by bandgap 1 ig h t ( o r o t h e r i o n i z i n g r a d i a t i o n s ) . T h i s phenomenon has p o t e n t i a l t e c h n o l o g i c a l appl i c a t i o n s i n t h e f i e 1 ds o f h i g h - r e s o l u t i o n 1 i thography and o p t i c a l storage. However, t h e fundamental mechanism r e s p o n s i b l e f o r t h e p h o t o d i s s o l u t i o n process i s s t i l l n o t understood, and so a knowledge o f t h e l o c a l s t r u c t u r a l environment o f the photo-reacted product should shed some l i g h t on the mechanism involved. We have t h e r e f o r e undertaken EXAFS measurements on m e t a l l i c Ag(3OOA)/a-GeSe, (0.511m) f i l m couples a t t h e Ag K-edge i n conventional transmission mode, and a l s o a t g l a n c i ng-angl e incidence u s i n g f l u o r e s c e n t d e t e c t i o n i n order t o probe s e l e c t i v e l y the s t r u c t u r e a t various depths from t h e surface o f the Ag f i l m . P r e l i m i n a r y r e s u l t s from the glancing-angle experiments ( t a k e n a t j u s t above t h e c r i t i c a l angle) are shown i n f i g . 6, where t h e EXAFS amplitude and i t s F o u r i e r t r a n s f o r m (FT) are shown b o t h b e f o r e and a f t e r the f i l m has been photo-reacted t o completion. By comparison w i t h the FT o f t h e transmission EXAFS spectrum o f a bulk Ag f o i l , we a s c r i b e t h e peak a t

2-68, t o Ag-Ag (metal) c o r r e l a t i o n s , and we a s c r i b e t h e l a r g e r peak l y i n g a t = 1.3A i n the FT t o Ag-0 ( o x i d e ) c o r r e l a t i o n s ; the discrepances between t h e peak p o s i t i o n s i n t h e FT and t h e t r u e i n t e r a t o m i c separations (R(Ag-Ag)=2.89A, R(Ag-O)=O 2.04A) a r e i n accord w i t h c o n s i d e r a t i o n s of t h e e f f e c t o f t h e r e l e v a n t phase s h i f t s .

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-

-

The most apparent d i f f e r e n c e between t h e s p e c t r a i s t h a t a f t e r photo-dissol u t i o n t h e EXAFS signal has p r a c t i c a l l y disappeared, i n d i c a t i n g t h a t t h e r e remains only a v e s t i g i a l ,amount o f Ag w i t h i n t h e depth (a few tens o f A) probed by t h e evanescent X-ray wave; t h e photo-reacted Ag product extends f o r a depth o f t y p i c a l l y 2000A. W h i l s t l i t t l e q u a n t i t a t i v e i n f o r m a t i o n can be gleaned from these p r e l i m i n a r y r e s u l t s , nevertheless they do demonstrate t h e f e a s i b i l i t y o f t h e glancing-angle approach as a d e p t h - s e n s i t i v e s t r u c t u r a l probe, t h e use o f which should be o f g r e a t u t i l i t y i n i n - s i t u k i n e t i c s experiments.

- s

n

-

w m

1.0 2.0 3.0 4.0

R

1x1

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Fig.6. Glancing-angle Ag K-edge EXAFS spectra o f p h o t o - d i s s o l u t i o n o f Ag i n a-GeSe, : a) EXAFS amplitude; b) F o u r i e r transform.

4. Conclusions.

We have demonstrated t h a t l o c a l photo-induced s t r u c t u r a l changes i n chalcogenide glasses can be detected u s i n g EXAFS. I n a d d i t i o n , we have demonstrated the f e a s i b i l i t y o f fluorescence-detected glancing-angle EXAFS as a depth-sensitive s t r u c t u r a l probe which should be o f considerable use i n m o n i t o r i n g t h e k i n e t i c s i n s i t u o f t h i n p h o t o - d i s s o l v i n g metal f i l m s i n amorphous chal cogeni des .

Acknowledgements

We a r e g r a t e f u l t o the SERC f o r p r o v i s i o n o f beam-time a t t h e SRS, Daresbury Laboratory.

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