chern 1985 spin coated

8/2/2019 CHERN 1985 Spin Coated

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Thin Sol id Films, 123 (1985) 289 296ELECTRONICS AND OPTICS 289

S P I N - C O A T E D A M O R P H O U S C H A L C O G E N I D E F I L M S :P H O T O I N D U C E D E F F E C T SG. C. CHERN* AND I. LAUKS'~Cen ter fo r Ch emic al Electronics, University o[" Pennsylvania, M oo re Sc ho ol of Eh, ctrical Engineering,Phi lade lphia, PA 19104 (U.S.A .)K . H . NORIANDep artme nt of E lec tr ical an d Comp uter Engineering, 304 Pa cka rd Labo ratory 19, Lehigh Univers ity,B e th l ehe m , P A 18015 ( U .S .A . )(Received Ju ne 11. 1984; accep ted No vem ber 23. 1984)

P h o t o i n d u c e d e f fe c ts i n s p i n - c o a t e d a r s e n i c s u lf id e fi lm s w e r e in v e s t i g a t e d b yU V - v i s i b l e s p e c t r o s c o p y , X - r a y d i f f r a c t o m e t r y , t r a n s m i s s i o n e l e c t r o n m i c r o s c o p ya n d I R s p e c t r o s c o p y . R e v e r s i b l e ef fe c ts as w e l l a s i rr e v e r s i b l e ef fe c ts w e r e o b s e r v e d .T h e p h o t o i n d u c e d d e c r e as e in t h e j o i n t d e n s i t y o f s t a te s b e t w e e n t h e c o n d u c t i o n a n dv a l e n c e b a n d s , w h i c h c a n b e r e c o v e r e d b y t h e r m a l a n n e a l i n g , i s b e l i e v e d t o b ec o m p e n s a t e d b y a n i n c r e a se i n t he d e f ec ts in t h e e n e r g y g ap . T h e d e c o m p o s i t i o n ,c r y s ta l l iz a t io n , o x i d a t i o n a n d g r a i n g r o w t h o f t h e m a t e r i a l u n d e r U V i l lu m i n a t i o nw e r e a s s o c i a t e d w i t h t h e i r re v e r s i b l e p h o t o b l e a c h i n g e ff ec t.

1. INTRODUCTIONI n r e c e n t y e a r s a m o r p h o u s c h a l c o g e n i d e t h i n f i l m s h a v e a t t r a c t e d r e s e a r c h

a t t e n t i o n b e c a u s e o f t h e i r p o t e n t i a l i n a n u m b e r o f t e c h n o l o g i c a l a p p l i c a t io n s .P h o t o g r a p h i c e ff ec ts o n t h in f il m c h a lc o g e n i d e g l a s se s p r e p a r e d b y m e l tq u e n c h i n g 1"2 o r v a p o r d e p o s i t i o n 2 -1 2 h a v e b e e n i n v e s t ig a t e d . S o m e r e s e a r c h e r sh a v e r e p o r t e d t h a t t h e a b s o r p t i o n c o e f f i c i e n t s a t t h e a b s o r p t i o n e d g e a n d t h er e f r a c ti v e i n d e x o f A s 2 S 3 f il m s i n c r e a s e a f t e r b a n d g a p i l l u m i n a t i o n 1 -5 . B r a n d e s e ta l . ~ a t t r i b u t e d t h e t r a n s m i s s i o n l o s s t o a n i n c r e a s e i n s c a tt e r in g . K e n e m a n 3s u g g e s t ed t h a t t h e p h o t o s t o r a g e o f h o l o g r a m s in A s / S 3 t h i n f i lm s c a n b e e r a s e d b yh e a t i n a r e v e r s i b l e r e a c t i o n . B e r k e s e t a l . 4 a n d O h m a c h i a n d I g o 5 s p e c u l a te d t h a tt h e p h o t o d e c o m p o s i t i o n r e s u l t i n g in t h e g r o w t h o f a r s e n i c c lu s t e r s in t h e fi lm isr e s p o n s i b l e f o r t h e p h o t o d a r k e n i n g e ff ec t. D e N e u f v i l l e e t a l . 2 p r o p o s e d t w o k i nd s o fp h o t o e f f e c t s : o n e is a p h o t o s t r u c t u r a l c h a n g e , w h i c h a c c o u n t s f o r th e s h if ts i n b o t ht h e a b s o r p t i o n e d g e a n d t h e r e fr a c ti v e i n d e x a n d is c a u s e d b y t h e p o l y m e r i z a t i o n o ft h e a s - d e p o s i t e d m o l e c u l a r ( A s 4 S 6 o r A s 4 S e 6 ) g l as s th r o u g h i l lu m i n a t i o n ; t h e o t h e ris a p h o t o d a r k e n i n g e ff ec t, w h i c h a c c o u n t s f o r t h e s h if t i n th e a b s o r p t i o n e d g ew i t h o u t a s i g n if i c an t c h a n g e i n t h e r e f r a c ti v e in d e x a n d i s c a u s e d b y e x c es s t r a p p e d* Present address: C omm odore Sem iconductor Systems,950 Rittenhous e Road, N orristown, PA 19403,U.S.A.t Present address: Integrated lonics Inc., 1011 State Road, P rinceton, NJ 08540, U.S.A.0040-6090/85/$3.30 (C), Else vier Se quoia/P rinte d in Th e Ne ther land s


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290 G . C . C H E R N , I . L A U K S , K . H . N O R I A N

n o n - e q u i l i b r i u m e l e c t r o n s a n d h o l e s w h o s e e l e c t r o s t a t i c a t t r a c t i o n c o n s t i t u t e s al o c a l d e f o r m a t i o n a n d c o u l d a f fe c t t h e j o i n t d e n s i t y o f l o n e p a i r a n d c o n d u c t i o nb a n d s t a t e s a t e n e r g ie s n e a r t h e o p t i c a l g a p e n e r g y .

H o w e v e r , T a n a k a a n d c o w o r k e r s 6 12 f o u n d b o t h a d e c r ea s e a n d a n i n c r ea s e int h e a b s o r p t i o n c o e ff i ci e nt a n d r e f ra c t iv e i n d e x o f a r s e n ic s u l f id e f il m s d e p e n d i n g o nt h e ir p r e p a r a t i o n . T h e p h o t o b l e a c h i n g e f fe ct s w e r e a t t r i b u t e d t o p h o t o d e c o m p o s i -t i o n a n d t h e n o x i d a t i o n o f t h e f il m 6 ' 7. T h e p h o t o d a r k e n i n g e f fe c ts w e r e e x p l a i n e d b yt h e r e a r r a n g e m e n t o f t h e a t o m s w i th i n t h e d is t a n ce o f s h o r t - r a n g e o r d e r t h r o u g hi l l u m i n a t i o n , r e s u l t i n g in a q u a s i - e q u i l i b r i u m a t o m i c c o n f i g u r a t i o n o f t h e s y s t e mw i t h a s h o r t e r a v e r a g e m o l e c u l a r d i s t a n c e s. T h e d y n a m i c c h a n g e i n th e r e f r a c t iv ei n d e x o f A s ~ S l o o x f ib n s i n d u c e d b y b a n d g a p il l u m i n a t i o n , w h i c h c a n b e r e c o v e r e db y i r r a d i a t i o n a t a l o w e r p h o t o e n e r g y , is p o s i t i v e f o r A s x S l o o x , x > 3 5 , a n d i sn e g a t i v e o t h e r w i s e 9, T h e y s p e c u l a t e d t h a t t h e e x i s t e n c e o f a d e f e c t c a u s i n g a s i n g leb r o a d t r a p l e v e l n e a r t h e c o n d u c t i o n b a n d is r e s p o n s i b l e f o r t h e c h a n g e i n r e f ra c t i v ei n d e x a c c o m p a n i e d b y p h o t o d a r k e n i n g e ff ec ts 9 12. A c o n f i g u r a t i o n d i a g r a m o f as i n g l e - w e l l e l e c t r o n e x c i t e d s t a t e a n d a d o u b l e - w e l l e l e c t r o n g r o u n d s t a t e w i t h ab a r r i e r h e i g h t o f 0 . 5 - 1 .5 e V w a s p r o p o s e d 11 ,1 2

I n a n u m b e r o f p r e v i o u s p u b l i c a t i o n s 13 17 w e h a v e d e m o n s t r a t e d t h a tc h a l c o g e n i d e g l a s s e s c a n a l s o b e d e p o s i t e d b y s p i n c o a t i n g f r o m s o l u t i o n . T h em e c h a n i s m o f d i s s o lu t i o n o f ch a l c o g e n id e s i n a m i n e s ( R N H 2 ) h a s b e e n i n-v e s t i g a t e d 1 3'1 ~. T h e r e s u lt s f a v o r a m e c h a n i s m o f d i s s o l u t i o n i n w h i c h t h ec o n t i n u o u s n e t w o r k o f t h e s ol id c h a l c o g e n i d e is a t t a c k e d b y t h e a m i n e b y t h eb r e a k i n g o f w e a k b o n d s a t d e f ec t s i te s w i t h t h e b o n d r u p t u r e t h e n e x t e n d i n g t on e i g h b o r i n g b o n d s i n a c o n t i n u o u s m a n n e r . I n t h i s w a y m a c r o m o l e c u l e s o fc h a l c o g e n i d e ( w i th m o l e c u l a r w e i g h t s o f a b o u t 1 0 0 0 00 ) a r e f o r m e d . T h e s em a c r o m o l e c u l e s a r e t e rm i n a t e d a t th e ir s u rf a c es b y - - S - R N H 3 g r o u p s . W e c al lt h is m a t e r i a l a l k y l a m m o n i u m a r s en i c su lf id e. D u r i n g b a k e o u t a f t e r s p in d e p o s i t i o no f th e s e m a t e r i a ls , R N H 2 w a s e v o l v e d l e a v in g h y d r o g e n a t e d c h a l c o g e n i d e c l us t er si n t h e s o li d p h a se . F u r t h e r a n n e a l i n g o f t h e m a t e r i a l a t a h i g h e r t e m p e r a t u r e c a u s e dc r o s s - l i n k i n g o f t h e c l u s t er s w i t h r e l e a s e o f H2S 15-17

I n t h is p a p e r p h o t o b l e a c h i n g , d e c o m p o s i t i o n , c r y s ta l l iz a t io n a n d o x i d a t i o n o fs p i n - c o a t e d a r s e n i c s u l f i d e f i l m s a r e i n v e s t i g a t e d b y t r a n s m i s s i o n e l e c t r o n m i c r o s -c o p y ( T E M ) , X - r a y d i f f r a c to m e t r y a n d I R a n d U V - v i s i b l e s p e c t r o s c o p y .2. P R E P A R A T I O N O F M A T E R I A L S

F o r T E M e x p e r i m e n t s a s o l u ti o n o f 1 g As2S 3 i n 1 00 m l b u t y l a m i n e w a s s p u no n t o o x i d i z e d p o l i s h e d s i l ic o n w a f e r s a t a b o u t 5 0 00 r e v m i n 1 f o r 30 s. T h i s r e s u l t e din As2S 3 f il m s 5 0 n m t h ic k . T h e s e w e r e s e p a r a t e d f r o m t h e s u b s t r a t e b y i m m e r s i o n i nd e i o n i ze d w a t e r a n d w e r e p la c e d o n t o c o p p e r s p e c i m e n g ri d s b o t h b e f o re a n d a f te rU V e x p o su r e .S a m p l e s f o r X - r a y d i f fr a c ti o n w e r e p r e p a r e d b y d r o p p i n g s o m e s o l u t io n o f 2.5 gAszS 3 i n 1 0 m l b u t y l a m i n e o n t o S i(1 00 ) s u b s t r a t e s a n d t h e n b a k i n g a t 4 0 C f o r 2 4 ha n d a t 9 0 C f o r 0.5 h in a v a c u u m o v e n . T h i s r e s u l t e d i n a f i lm w i t h a t h i c k n e s s o f af e w t e n t h s o f a m i l l i m e t e r .

M a t e r i al s f o r I R s p e c t r o s c o p y w e r e p r e p a r e d f r o m a s o l u t io n o f 2 g As2S 3 in


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1 0 m l b u t y l a m i n e b y s p i n n i n g a t 2 0 0 0 r e v m i n 1 f o r 2 0 s o n K R S - 5 s u b s t r a t e s a n dt h e n a n n e a l i n g a t 5 0 C f o r 1 00 m i n .

F o r U V - v i s i b l e t r a n s m i s s i o n m e a s u r e m e n t s f il m s o f A s2 S 3 0 .3 lam t h i ck w e r ep r e p a r e d o n q u a r t z s u b s t r a t e s f r o m a s o l u t i o n o f 1 g As2S 3 in I 0 m l b u t y l a m i n e b ys p i n n i n g a t a b o u t 3 0 0 0 r ev m i n - 1 f o r 3 0 s. T h e f i lm s w e r e b a k e d a t 4 5 C f o r 1 2 h a n da t 9 0 C f o r 0 .5 h i n a v a c u u m o v e n .3. E X P E R I M E N T S A N D R E S U L T S

T h e t h i n fi lm m o r p h o l o g y w a s e x a m i n e d w i th a P h i li p s E M 4 0 0 T t r a n s m i s s i o ne l e c t r o n m i c r o s c o p e . F i g u r e l ( a ) s h o w s t h e r e s u l t f o r t h e a s - d e p o s i t e d m a t e r i a l .C l e a r l y r e s o lv e d w e r e d a r k c l u st e rs c o r r e s p o n d i n g t o h e a v y a t o m s s u c h a s a r se n i ca n d s u lf u r w i t h d i m e n s i o n s o f 5 - 7 n m i n t e rs p e r s e d a m o n g l i gh t ar e a s c o r r e s p o n d i n gt o l ig h t a t o m s o f t h e i n t e rc l u s t e r o r g a n i c c o n t e n t o r v o id s . T h i s a g r e e s w i t h p r e v i o u sr e s u l ts 16. A f t e r U V e x p o s u r e a t 1 2 J c m - 2 g r a i n g r o w t h a n d c r y s t a l l i z a t i o n o f t h em a t e r i a l w e r e o b s e r v e d ( F i g , l ( b ) ) . S e l e c t e d a r e a t r a n s m i s s i o n e l e c t r o n d i f f r a c t i o nr e s u l t s f o r t h e a s - d e p o s i t e d m a t e r i a l s h o w h a l o s ( F i g . 2 ( a ) ) c o r r e s p o n d i n g t oa m o r p h o u s m a t e r i a l, w h il e t h e re s u lt s fo r t h e U V - e x p o s e d m a t e r i a l s h o w s h a r ps p o t s ( F i g . 2 ( b ) ) i d e n t i f i e d a s c r y s t a l l i n e a r s e n i c ( s y n t h e t i c h e x a g o n a l ; s p a c e g r o u pR3m).

(a) (b)F i g . 1. T r a n s m i s s i o n e l e c t ro n m i c r o g r a p h s o f b u t y l a m m o n i u m a r s e n i c s ul f id e fi lm s : ( a) a s - d e p o s i t e d ; ( b )a f t e r U V e x p o s u r e a t 1 2 J c m - 2.

X - r a y d i f fr a c ti o n d a t a w e r e o b t a i n e d u s i n g a P h i l ip s d i f f r a c t o m e t e r w it h C u K sr a d i a t i o n . F i g u r e 3 s h o w s t h e r e s u l t s . T h e u n e x p o s e d m a t e r i a l e x h i b i t s t h r e e b r o a da m o r p h o u s p e a k s , c o n s i s t e n t w i t h t h e p r e v i o u s r e s u l ts 13. A f t e r 4 h o f U V e x p o s u r e an u m b e r o f s h a r p p e a k s c o r r e s p o n d i n g t o c r y s ta l l iz a t io n o f t h e m a t e r i a l a p p e a r e d i n


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2 9 2 C;. C. CHE RN, 1. LAU KS, K. H. N ORI AN

(a ) (b )Fig. 2. Selected a rea transmission electron diffraction patterns of bu tyl am m on iu m arsenic sulfide f ilms:(a) as-deposited; (b) after U V exposure at 12 J cm 2.

tO 0

C

-~

cc

, 7 ' 1

8O

6 0

40

2 0

I I I I I I7 0 s o 5 0 4 o 3 0 2 0 lo2 0 ( d e g )

F i g . 3 , X - r a y diffraction spectra of buty l am m on ium arsenic sulf ide before ( - - ) and after ( - - ) U Vexposure at 250 J cm 2.

a d d i t i o n t o t h e t h r e e a m o r p h o u s p e a k s . P e a k s a t 2 0 = 1 3 .7 , 2 7 . 4 , 2 7 . 9 a n d 3 2 . 3 a r e i d e n t i c a l w i t h t h o s e o f m o n o c l i n i c a n d c u b i c A s 2 0 3 . P e a k s a t 20 = 3 2 .2 , 4 8 . 4 a n d 5 9 .3 c o r r e s p o n d t o s y n t h e t i c h e x a g o n a l a r s e n ic . A l s o s h o w n a r e u n i d e n t i f i e dp e a k s a t 2 0 = 2 0 . 1 , 3 5 . 2 , 3 9 . 7 , 4 2 . 3 , 4 6 . 3 , 5 5 . 0 , 5 7 . 6 a n d 6 4 . 7 .

T h e e f f ec t o f U V i l l u m i n a t i o n o n t h e I R t r a n s m i s s i o n s p e c t r u m w a s s t u d i e d


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S P I N - C O A T E D A M O R P H O U S C H , C O G E N I D E F I LM S 293

using a Beckman IR 4250 sp,. rom ete r. Figure 4 shows the IR transmission spectraof the material before and aft ;" UV exposure at about 40 J c m 2 . The main changein the IR transmission spect.: um in the range 4 00 0- 65 0c m- ~ is the presence of a,~tcong absorptio n peak at 800 cm -1 after UV exposure, which pro bably corre-

ads t o A s 2 0 3 1 8 . In the range 650 -200 cm- ~ the material without UV exposures a broad absorption peak at 305 cm ~ corresponding to antisymmetric

st~ ', :h in g of the As2S3 unit 19'2, a shoulder at 340cm ~ cor respondin g tosymmetric stretching of the As2S3 unit, and unassigned absorption peaks at 415,479and 544 cm 1. After UV exposure at abou t 40 J cm 2 new peaks appear at 565 and625 cm-~, and there are changes in the absorpt ions at 305, 340, 415, 479 and 544cm-~ . The absorptio n peaks at 305 and 415 cm ~ decrease while the absorpt ionpeaks at 479 and 544 cm ~ increase, and the shoulder at 340 cm ~ becomes a strongabsorpt ion peak which might indicate the decomp osition of the AszS 3 network.

1 0 0 12 0 1 0 1- I I I I

~a

I I I I3600 2800 200 0 1600 1200 800 600 400WAVENUMBER(cm-')F i g. 4i I R t r a n s m i s s i o n s p e c t r a o f b u t y l a m m o n i u m a r s e n i c s u l f id e b e f o re ( - ) a n d a f te r ( - - - ) U V

e x p o s u r e a t 4 0 J c m 2.

The results of UV visible transmission measured on a Pe rki n-E lme r 553spectr'~photometer are shown in Fig. 5. The inset shows the quantity (~ho~)~/2plotted against h~o, where h(,~ is the photon energy and ~ the absorpt ion coefficient.The results are in accordance with the often observed band edge behavior inchalcogenide glasses

B(ho E o p t ) 2- (1)h~o

where E o p I is the extrapola ted optical band gap 21. The absorp tion coefficient wasobtained as described previously 16.

The extrapolated optical band gap decreases from 2.42 eV for the 45 C soft-baked material to 2.33 eV after annealing at 90 ~C, in agreement with the resultsreported previously 16. After a 72 J cm - 2 UV exposure the abs orp tio n edge shifts tohigher photon energy showing the photobleaching effect. Further annealing of thesample at 100 ~C for 3 h in a vac uum oven shifts the ab sorption edge back to lowerphoton energy. These effects are opposi te to those repor ted by other researchers 1 5.These results do not correspond to a simple reversible photographic effect since thethickness of the material was reduced after thermal annealing t6 and no significantchange in thickness was observed after UV exposure.


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8090 ~ . , ..... ..7o / / " : < , . . ; ! . Z ; ~ - - -. ._ . . . . ,~l i / . . ~ . . . / . / . . . . . . . . . .,z. . . . . . . . . . . . .

/ / i . i // f l - - l ' " _ _ _ 2 4 I 3i " / I ~ o u ~ , , / - ,I I ! # , t ; " , . "

t l' ' , , ' = '

// I . / ~ e o o % / ~ / , /I I; / 2 "' / l / . ' ", o : . i i o - : / 7 , _ , _ , . , , ,. ! / / 2 .~ 2 . z ~ .s ~ .5. / / r v . . ~ e V )

0 L " - " " ~ " ~ ' / ' I I I IJO00 4 0 0 0 5 0 0 0 6 0 0 0 ? (3 00 8 0 0 0WAVELENGTH(A)Fig. 5. UV visible transmission spectra of butyl amm on iu m arsenic sullide after a sequence of anneal s atvarious tempe ratu res and UV exposure: -- , after a 45 C anneal for 0.5 h; - - - , after a 90 C anneal for0 .5h ; , a f t e rUVex posur ea t7 2Jcm- 2 ; . . . . . , a f t e r a l0 0 C annea l for 3 h,

6 O

Z~ 4 C

I . - - - -

4. DISCUSSIONWe have shown that UV illumination on the spin-cast arsenic sulfide film

produces photobleaching, crystallization, oxidation and grain growth. On the basisof these results we suggest a model for the mechanism of the photographic effectswhich is similar to that presented by Tan aka and Kikuchi 6. Deco mpositi on underUV exposure of As2S3 and oxidation of arsenic occur according to the reactions

AsES 3 ~ As 2 + As 2 _xS3 0 < x < 2 (2)4A s+ 302 ~ 2As203 (3)

We assume that othe r un kno wn reactions associated with the amine salt compone ntof the material account for the unidentified crystallization peaks of the X-raydiffraction spectrum. Dur ing the decomposition, the liberation of arsenic atoms isfollowed by their migration in the film to form small crystals which then oxidize intoAs203 crystals. These crystals are sufficiently small and scattered in the film thatthey have almost no effect on the measurement of the overall UV-visibl etransmission 6. The rest of the material consists of clusters containing amo rph oussulfur-rich arsenic sulfide, which is consistent wi)h the shift in the absorp tion edge toa higher photon energy after UV exposure. This agrees with observations on the


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S P I N - C O A T E D A M O R P H O U S C H A L C O G E N I D E F IL M S 295

a b s o r p t i o n e d g e o f b u l k a r s e n i c s u l f id e g la s s w h i c h s h if ts to a h i g h e r p h o t o n e n e r g yw i t h a n i n c r e a s e i n t h e s u l f u r c o n t e n t 22.

T a n a k a a n d K i k u c h i 6 p o s t u l a t e d t h a t t h e i n c id e n t p h o t o n s a b s o r b e d b y th ef il m e x c it e v a le n c e e l e c t r o n s f r o m t h e i r b o n d i n g s t a t e s a n d l o c a l l y p r o d u c e a w e a k e rc h e m i c a l b o n d t h a n t h e in i ti a l u n i r r a d i a t e d b o n d i n t h e n e t w o r k . S i n c e o u r m a t e r i a lh a s a s t r u c t u r e c o n t a i n i n g c l us te r s, p h o t o d e c o m p o s i t i o n m a y o c c u r m o r e e a s il y b ye x c it in g A s - - S a n d S - - o r g a n i c b o n d s a t cl u s te r b o u n d a r i e s , a n d t h e e n s u i n gr e a c t io n is re s p o n s i b l e f or t he f o r m a t i o n o f u n k n o w n c r y s ta l s a n d c r o s s - l i n k i n gb e t w e e n c l u s t e r s c a u s i n g g r a i n g r o w t h . F u r t h e r i n v e s t i g a t i o n o f t h e f il m p r o p e r t i e s isn e c e s s a r y to i d e n ti fy t h e u n k n o w n c r y s ta l s ca u s e d b y U V i l lu m i n a t i o n .

A l t h o u g h t h e a b o v e p h o t o d e c o m p o s i t i o n , c r y s ta l l iz a t io n a n d o x i d a t i o n r e a c -t i o n s w h i c h o c c u r i n s p i n - c o a t e d A s 2 S 3 f i l m s a r e i r r e v e r s i b l e , o t h e r r e v e r s i b l ec h a n g e s a l s o o c c u r i n t h e f il m s . A s s h o w n i n F i g . 5, t h e d e c r e a s e i n th e s l o p e o f t h e(o th~o) 1 /2 ve rs us h~o p l o t a f t e r U V e x p o s u r e i n d i c a t e s a d e c r e a s e in t h e j o i n t d e n s i t y o fs t a t e s b e t w e e n c o n d u c t i o n a n d v a l e n c e b a n d s . T h i s d e c r e a s e in t h e j o i n t d e n s i t y o fs t a t e s i s p o s s i b l y c o m p e n s a t e d b y a n i n c r e a s e i n d e f e c t s t a t e s i n t h e e n e r g y g a p ,w h i c h w e p r e s u m e t o b e t h e s a m e C 3 + C 1 - p a i r s d i s c u s s e d e l s e w h e r e 17 f o r t h e s i lv e rp h o t o d o p i n g p h e n o m e n a . T h e i n c re a s e d d ef e ct s t at e s m a y m o d i f y t h e e n e r g y b a n ds t r u c t u r e n e a r b a n d t a i l s . W e b e l i e v e t h a t t h i s w i l l n o t i n f l u e n c e t h e v a l u e o f E o p tw h i c h is a n e x t r a p o l a t e d v a l u e b a s e d o n e q n . ( 1 ) . T h i s e q u a t i o n is d e r i v e d b ya s s u m i n g p a r a b o l i c b a n d s a n d is v a l id o n l y f o r e n e r g i es n o t t o o c l o se to t h e b a n dt a i l s . T h e i n c r e a s e i n s t a t e s n e a r b a n d t a i l s s h o u l d n o t d e c r e a s e t h e " e x t r a p o l a t e d "e n e r g y g a p b u t s h o u l d c a u s e a n i n c r e a s e i n t h e a b s o r p t i o n c o e f f i c i e n t s i n t h e l o wa b s o r p t i o n r e g i o n , i .e . a t p h o t o n e n e r g i e s n e a r E o p t. I n t h i s r e g i o n t h e c h a n g e i na b s o r p t i o n is d i ff ic u lt t o o b s e r v e i n o u r m e a s u r e m e n t s o n t h i n f i l m s b e c a u s e o f l i g h ti n t e r f er e n c e c a u s e d b y m u l t i p l e r e f le c ti o n s . H o w e v e r , o t h e r s h a v e a t t r i b u t e d t h e E optd e c r e a s e s o f t h e p h o t o d a r k e n i n g e ff ec t t o a c h a n g e i n b a n d t ai l d e n s i t y 9 11. T h e r m a la n n e a l i n g o f t h e m a t e r ia l a f t er U V e x p o s u r e n o t o n l y i n c re a s e d t h e a b s o r p t i o n b u ta l s o i n c r e a s e d t h e s l o p e o f t h e ( 0 ~ h ( t ) ) 1 / 2 v e r su s h~o p l o t a l m o s t b a c k t o i t s o r i g i n a lv a l u e, T h i s m a y i n d i c a t e a n i n c r e a se i n t h e j o i n t d e n s i t y o f s t a t e s w i th t h e r m a la n n e a l i n g a f t e r U V e x p o s u r e a n d is b e l ie v e d t o b e a c c o m p a n i e d b y a d e c r e a s e in t h ed e f e c t s t a t e d e n s i t y .A C K N O W L E D G M E N T S

T h e a u t h o r s w o u l d l ik e t o t h a n k D r s . S . R a b i i , B. S i n g h a n d J . N . Z e m e l f o r t h e i rv a l u a b l e d i s c u s s io n s .

T h i s w o r k is s u p p o r t e d b y t h e U . S . A i r F o r c e O f f ic e o f S c ie n ti fi c R e s e a r c h u n d e rG r a n t 8 1 - 0 2 0 1 .R E F E R E N C E S

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chern 1985 spin coated

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