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EXAFS STUDIES OF THERMOSTRUCTURAL AND PHOTOSTRUCTURAL CHANGES OF
VAPOR-DEPOSITED AMORPHOUS As2S3 FILMS
C. Yang, J. Lee, M. Paesler, D. Sayers
To cite this version:
C. Yang, J. Lee, M. Paesler, D. Sayers. EXAFS STUDIES OF THERMOSTRUCTURAL AND PHO- TOSTRUCTURAL CHANGES OF VAPOR-DEPOSITED AMORPHOUS As2S3 FILMS. Journal de Physique Colloques, 1986, 47 (C8), pp.C8-387-C8-390. �10.1051/jphyscol:1986876�. �jpa-00226198�
JOURNAL DE PHYSIQUE
Colloque C8, supplement au n o 12, Tome 47, decernbre 1986
EXAFS STUDIES OF THERMOSTRUCTURAL AND PHOTOSTRUCTURAL CHANGES OF VAPOR-DEPOSITED AMORPHOUS As,S3 FILMS
C.Y. YANG, J.M. LEE, M.A. PAESLER and D.E. SAYERS Department of Physics, North Carolina State University, Raleigh, NC 27695-8202, U.S.A.
ABSTRACT
We r e p o r t t h e e f f e c t s of thermal-annealing and l i g h t - s o a k i n g on t h e s t r u c t u r e of amorphous ( a - ) As2S3 f i l m s . The EXAFS of n e a r - s t o c h i o m e t r i c f i l m s were examined f o r a s - d e p o s i t e d , thermally-annealed and l i g h t - s o a k e d samples. The p r i n c i p l e As-S f e a t u r e i n c r e a s e s slowly f o r s p e c t r a of a l l f i l m s , while a weak homopolar A s - A s f e a t u r e d e c r e a s e s w i t h b o t h thermal-annealing and subsequent l i g h t - s o a k i n g . S l i g h t changes a t e a l s o seen i n the higher s h e l l s . Our r e s u l t s imply t h a t s t r u c t u r a l changes i n a - As2S3 brought about by thermal-annealing and by l i g h t - s o a k i n g i n v o l v e breaking, r e a r r a n g i n g and remaking a s i g n i f i c a n t f r a c t i o n of t h e m a t e r i a l ' s bonds.
INTRODUCTION
The c o n t r o l and understanding of t h e o p t i c a l l y and thermally induced changes i n the p h y s i c a l and chemical p r o p e r t i e s of amorphous chalcogenide semiconductors a r e o f c o n s i d e r a b l e c u r r e d t i n t e r e s t . I t i s w e l l known [ I ] t h a t t h e s e changes depend s e n s i t i v e l y on t h e p r e p a r a t i o n method and subsequent t r e a t m e n t of t h e samples. The i n f l u e n c e of h e a t i n g and band gap i l l u m i n a t i o n on the s t r u c t u r e and o p t i c a l proper- t i e s i s p a r t i c u l a r l y pronounced i n vapor-deposited As2S3 f i l m s 121. The s t r u c t u r a l changes induced by h e a t and l i g h t a r e r e f e r r e d t o a s t h e r m o s t r u c t u r a l and photo- s t r u c t u r a l , r e s p e c t i v e l y .
EXPERIMENTAL
The a - As2S3 f i l m s were prepared by evaporating high p u r i t y bulk As2S3 g l a s s u s i n g a Mo boat. Films were d e p o s i t e d onto Eoom temperature kapton s u b s t r a t e s i n a base vacuum of 1 x 1 0 - ~ t o r r a t a r a t e of 80 A/sec. Experiments were performed on t h e f o l l o w i n g samples: ( a ) a s - d e p o s i t e d f i l m , ( b ) t h e a s - d e p o s i t e d f i l m a f t e r 2 h r thermal ly-annealing a t 220°C, and ( c ) the thermally-annealed f i l m a f t e r 5 h r l i g h t - soaking. The light-soaked f i l m was exposed to an argon l a s e r (488 and 514 nm, power d e n s i t y = 500 ~ a t t s / c m ~ ) while being held a t 77 K i n a dewar b e f o r e subsequently being r e turned t o room temperature.
The EXAFS measurements were c a r r i e d o u t a t the N a t i o n a l Synchrotron L i g h t Source (NSLS) on beam l i n e X - 1 1 A with a S i (111) double c r y s t a l monochromator a t t h e A s k-edge. The t h i c k n e s s of the f i l m s was about 3.6 pm and about 8 l a y e r s were used t o make samples of s u f f i c i e n t x-ray a b s o r p t i o n f o r t h e EXAFS measurements t o be made. Transmission measurements on each sample were made a t 80 K and room temperature.
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1986876
J O U R N A L D E PHYSIQUE
Fig. 1 EXAFS k 3 X (K) for (a) as- deposited film, (b) 2h thermally- annealed film and (c) 5h light- soaked film.
RESULTS
Fig. 2 Fourier transforms of k3 X(K) for (a) as-deposited film, (b) 2h thermally-annealed film and (c) 5h light-soaked film.
The experimental data were analyzed using standard procedures of background sub- traction, normalization and extraction of the EXAFS signal X (k). The X(k) data
measured a t 80 K f o r t h r e e samples used during t h i s s t u d y , m u l t i p l i e d by k3 a r e shown i n Fig. 1. It can be seen i n Fig. 1, t h a t t h e a s - d e p o s i f e d , thermally- annealed and light-soaked f i l m s have s i m i l a r s p e c t r a below 11 A-1; whereas the s p e c t r a change markedly a t v a l u e s of k above 11 1-1.
The c o r r e s p o ~ d i n g magnitude of t h e F o u r i e r transforms of k3 X (k) over a k range of 2.6 t o 15.8 A ' ~ a r e shown i n Fig. 2. The F o u r i e r transforms show t h a t t h e as- d e p o s i t e d , thermally-annealed, and light-soaked f i l m s a l l c o n t a i n c o n t r i b u t i o n s from both As-S and A s - A s bonds i n t h e f i r s t s h e l l . A major d i f f e r e n c e among t h e 3 samples i s due t o t h e change of t h e c o n t r i b u t i o n of t h e As-As bonds i n t h e f i r s t s h e l l . Other changes i n t h e h i g h e r s h e l l s t r u c t u r e a r e a l s o seen.
I n o r d e r t o enhance t h e c o n t r i b u t i o n s of t h e A s - A s bonds i n t h e f i r s t s h e l l , t h e F o u r i e r transforms of t h e EXAFS s p e c t r a were c a r r i e d o u t by removing t h e s u l f u r s c a t t e r e r p h a s e - s h i f t b e f o r e taking t h e transforms. The As-S phase s h i f t f u n c t i o n was e m p i r i c a l l y determined [ 3 ] from t h e f i r s t s h e l l of t h e As-S bonds i n c r y s t a l -
l i n e As2S3. The a d d i t i o n a l f e a t u r e s causing t h e s h o u l d e r of t h e f i r s t peak of the a s - d e p o s i t e d , thermally-annealed and l i g h t - s o a k e d f i l m s a r e thereby enhanced.
These r e s i d u a l f e a t u r e s could be w e l l f i t by using a s i n g l e A s - A s c o n t r i b u t i o n which d i m i n i s h e s c o n t i n o u s l y from t h e a s - d e p o s i t e d f i l m t o t h e light-soaked f i l m . Arso shown i n Fig. 2a i s a t e n t a t i v e assignment t o one h i g h e r s h e l l peak based on i t s p o s i t i o n a s compared t o t h e model compounds.
DISCUSSION
A. S t r u c t u r e of vapor d e p o s i t e d a-As7Sq f i l m s . The f i r s t peak i n t h e F o u r i e r transform shown i n Fig. 2a of t h e a s - d e p o s i t e d f i l m r e p r e s e n t s t h e As-S d i s t a n c e and i t s s h o u l d e r i n d i c a t e s a s u b s t a n t i a l number of homopolar bonds (As-As). The second peak could be due t o c o n t r i b u t i o n s from t h e A s - A s d i s t a n c e i n a S-As-S-As h e l i x o r i n i n t e r c h a i n As-S-As l i n k a g e s a s i n c- As2S3. Comparison of t h e a s - d e p o s i t e d f i l m w i t h c r y s t a l l i n e As4S4 ( r e a l g a r ) [3] and As-rich samples of A S , S ~ , ~ g l a s s e s [ 4 ] s u g g e s t s t h a t t h e As4Sq molecules a r e contained i n t h i s film. F u r t h e r comparison between t h i s f i l m and S - r i c h bulk AsxS g l a s s e s i n d i c a t e s t h a t t h e t h i r d peak may a r i s e from a n As-S-S l i n k a g e thus s u g g e s t i n g the p o s s i b i l i t y of homopolar S-S bonding a s well. These f e a t u r e s imply t h a t i n t h e process of vacuum e v a p o r a t i o n of As2S3 s e v e r a l molecular s p e c i e s may be p r e s e n t . L i k e l y c a n d i d a t e s i n c l u d e As2S3, As4S4 and S8. A c e r t a i n degree of the molecular i n t e g r i t y of t h e s e s p e c i e s i s presumed t o e x i s t i n a v i r g i n f i l m where t h e evaporated molecules may be bonded by van d e r Waals f o r c e s w i t h i n t h e network of t h e g l a s s .
B. E f f e c t s of thermal Annealing and L i g h t Soaking. F i g u r e 1 shows changes i n EXAFS s p e c t r a of d e p o s i t e d f i l m s which occur upon thermal-annealing and l i g h t - s o a k i n g . The p r i n c i p l e As-S f e a t u r e i n c r e a s e s slowly w h i l e t h e homopolar A s - A s f e a t u r e de- c r e a s e s a f t e r thermal-anneal.ing and l i g h t - s o a k i n g . There a r e a l s o s y s t e m a t i c changes i n t h e h i g h e r s h e l l . S i n c e changes a r e seen between t h e as-deposited and thermally-annealed a s w e l l a s between t h e thermally-annealed and l i g h t - s o a k e d f i l m s , t h e s e changes may be due t o an i r r e v e r s i b l e change induced by the post- d e p o s i t i o n processing. The same may be t r u e f o r t h e changes i n t h e h i g h e r s h e l l s , b u t p a r t i c u l a r a t t e n t i o n i n v o l v i n g f u r t h e r a n a l y s i s and more measurements w i l l be g i v e n t o t h e s e changes t o s e e i f t h e f e a t u r e s can be i d e n t i f i e d and r e l a t e d t o t h e 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 .
CONCLUSION
Our p r e l i m i n a r y r e s u l t s of t h e r m o s t r u c t u r a l and p h o t o s t r u c t u r a l changes i n a- As2S3 f i l m s show evidence of a r e o r g a n i z a t i o n of l o c a l bonding c o n f i g u r a t i o n s . Some of t h e changes, p a r t i c u l a r l y i n t h e f i r s t s h e l l , may be evidence of i r r e v e r s i b l e e f f e c t s , p a r t i c u l a r l y due t o non-stochiometry of t h e f i l m s . Other changes ( i . e . i n t h e h i g h e r s h e l l s ) may be evidence of t h e 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 change, b u t a more d e f i n i t i v e c o n c l u s i o n w i l l a w a i t f u r t h e r measurements and a n a l y s i s .
JOURNAL DE PHYSIQUE
ACKNOWLEDGEMENT
S u p p o r t f o r t h e development of beamline X-11A a t NSLS and p a r t i a l s u p p o r t f o r t h i s r e s e a r c h was provided by t h e Department of Energy under C o n t r a c t DE-AS05-80- ER10742. P a r t i a l s u p p o r t f o r t h i s r e s e a r c h was a l s o provided by t h e N a t i o n a l S c i e n c e Foundation u n d e r G r a n t DMR-8407265.
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