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PLASMA-DEPOSITED HYDROGENATED CHALCOGENIDE GLASSES
H. Fritzsche, V. Šmíd, H. Ugur, P. Gaczi
To cite this version:
H. Fritzsche, V. Šmíd, H. Ugur, P. Gaczi. PLASMA-DEPOSITED HYDROGENATED CHALCOGENIDE GLASSES. Journal de Physique Colloques, 1981, 42 (C4), pp.C4-699-C4-702.
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H . F r i t z s c h e , V . Sm:dr, H. Ugur and P . J . G a c z i
Department o f Physics and James h a n c k I n s t i t u t e , The University o f Chicago, Chicago, I i
6 0 6 3 7 , U.S.A.A b s t r a c t . - H y d r o g e n a t e d c h a l c o g e n i d e g l a s s e s s p a n n i n g t h e b i n a r y s y s t e m s As-S and As-Se h a v e b e e n p r e p a r e d by p l a s m a d e c o m p o s i t i o n of t h e c o n s t i t u e n t hy- d r i d e s . The i n f r a r e d s p e c t r a show t h a t H i s bonded p r e d o m i n a n t l y t o e l e m e n t s p r e s e n t . i n e x c e s s o f s t o i c h i o m e t r y . The r e . ~ e r s i b l e p h o t o - i n d u c e d s h i f t o f t h e a b s o r p t i o n e d g e a n d t h e number anrl ESR spectra o: p h o t o - i n d u c e d s p i n s a r e t h e same a s i i ~ g l a s s e : ; n o t c o n t a i n i n g h y d r o g e n .
I n t r o d u c t i o n . - H y d r o g e n a t e d amorphous S i p r e p a r e d b y r . f . p l a s m a d e p o s i t i o n e x h i b i t s many u n i q u e p r o p e r t i e s b e c a u s e t h i s m a t e r i a l c o n t a i n s a much l o w e r d e n s i t y o f l o c a l - i z e d g a p s t a t e s t h a n amorphous S i f i l m s which d o n o t c o n t a i n h y d r o g e n [ I ] . We do n o t expec.t t h a t h y d r o g e n p l a y s t h e same r o l e i n c h a l c o g e n i d e g l a s s e s as i t d o e s i n t e t r a h e d r a l l y c o o r d i n a t e d amorphous s e m i c o n d u c t o r s b e c a u s e t h e d e f e c t c h e m i s t r y d i f - f e r s g r e a t l y i n t h e s e two m a t e r i a l s y s t e m s [2-41. N e v e r t h e l e s s , a s t u d y o f h y d r o - g e n a t e d c h a l c o g e n i d e g l a s s e s m i g h t s h e d some l i g h t o n t h e b a s i c p h y s i c a l p r o p e r t i e s o f t h e s e m a t e r i a l s . As a n e x t e n s i o n o f o u r e a r l i e r s t u d y [ 5 ] we r e p o r t h e r e t h e p r o p e r t i e s of h y d r o g e n a t e d f i l m s o f t h e As-S and As-Se b i n a r y s y s t e m s s p a n n i n g a w i d e r a n g e o f c o m p o s i t i o n s .
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V a r i a n A s s o c . , P a l o A l t o , CA 9 4 3 0 3Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19814155
C4-700 JOURNAL DE PHYSIQUE
Sample P r e p a r a t i o n . - The r e a c t a n t g a s e s , AsH3 d i l u t e d w i t h SO% Ar and e i t h e r H2S o r H2Se w e r e a d m i t t e d i n t o o u r s t a i n l e s s s t e e l r . f . p l a s m a chamber d e s c r i b e d e a r l i e r ( 6 1 . A p r e s s u r e o f 0 . 2 T o r r was m a i n t a i n e d a t a f l o w r a t e o f % 1 2 s c c / m i n . The s u b s t r a t e s w e r e n o t h e a t e d and r e a c h e d 2. 50°C u n d e r t h e a p p l i e d 13.6 MHZ r . f . power o f 1 6 w a t t s . The c a t h o d e C s e l f - b i a s e d t o a n e g a t i v e p o t e n t i a l o f a b o u t 220 V . The unused r e a c t a n t s w e r e c o l l e c t e d i n a 78K c o o l e d t r a p b e f o r e t h e pump; a f t e r dep- o s i t i o n t h e y w e r e p a s s e d t h r o u g h a s a t u r a t e d s o l u t i o n of KMn04 and a 1 5 m o l e % s o - l u t i o n o f NaOH. The f i l m c o m p o s i t i o n was d e t e r m i n e d by a t o m i c a b s o r p t i o n s p e c t r o - p h o t o m e t r i c a n a l y s i s . S i n c e a d i r e c t v a l u e o f t h e [ S ] c o n c e n t r a t i o n c o u l d n o t b e o b t a i n e d , t h e c o m p o s i t i o n o f o u r As-S f i l m s w e r e c a l c u l a t e d from t h e m e a s u r e d [ A s ] c o n c e n t r a t i o n w h i c h r e s u l t e d i n l a r g e e r r o r s f o r s m a l l [ A s ] . A l t h o u g h t h e d e p o s i - t i o n c o n d i t i o n s h a v e n o t y e t b e e n o p t i m i z e d , we g e n e r a l l y o b s e r v e d t h a t t h e d e p o s i - t i o n r a t e a s w e l l a s t h e c o m p o s i t i o n o f A ~ ~ s 1 - x f i l m s d e p e n d s t r o n g l y o n t h e p o s i - t i o n z o f t h e s u b s t r a t e a l o n g t h e g a s f l o w d i r e c t i o n a s shown i n f i g u r e s l a and l b . C l o s e t o t h e g a s i n l e t ( z = 2 . 5 ) t h e d e p o s i t i o n r a t e and t h e As c o n t e n t i n t h e f i l m s w e r e c o n s i d e r a b l y h i g h e r t h a n n e a r t h e pump p o r t ( P - 2 . 5 ) . T h i s t e n d e n c y t o g e t h e r w i t h p l a s m a i n h o m o g e n e i t i e s n e a r t h e w a l l . y i e l d t h e c o m p l e x b e h a v i o r o f f i g u r e l a . B e c a u s e o f t h i s , o n l y s m a l l s a m p l e a r e a s w e r e s t u d i e d . The AsxSel-x f i l m s w e r e much more homogeneous.
The r e l a t i v e i n t e n s i t i e s , w i d t h s , and p o s i t i o n s o f t h e two d i f f u s e p e a k s i n t h e X-ray d i f f r a c t i o n s p e c t r a a g r e e w i t h t h o s e of t h e r e s p e c t i v e b u l k g l a s s e s and o f a n n e a l e d e v a p o r a t e d o r s p u t t e r e d f i l m s . The d i f f r a c t i o n s p e c t r a d i d n o t c h a n g e a f t e r a n n e a l i n g a t 160°C f o r s e v e r a l h o u r s o r a f t e r e x p o s u r e t o l i g h t .
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a t 1980 cm-l. c a t h o d e f i l m s .
I n f r a r e d S p e c t r a . - F i g s . 2 and 3 show t h e i n f r a r e d t r a n s m i t t a n c e o f f i l m s d e p o s i t e d o n c r y s t a l l i n e S i w a f e r s f o r t h e A S , S ~ - ~ and As,Sel-, s y s t e m , r e s p e c t i v e l y . The f i l m t h i c k n e s s d and As- c o n t e n t x a r e i n d i c a t e d f o r e a c h c u r v e . A and C d e n o t e f i l m s d e p o s i t e d on t h e g r o u n d e d a n o d e and t h e f l o a t i n g c a t h o d e p l a t e . S-H, Se-H, a n d As-H s t r e t c h i n g modes a r e o b s e r v e d a t 2480, 2180, and 1980 cm-l, r e s p e c t i v e l y .
t h e h y d r o g e n c o n c e n t r a t i o n [ H I r e a c h e s a minimum n e a r x = 0 . 5
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0.05 r a t h e r t h a n a t t h e s t o i c h i o r n e t r i c c o m p o s i t i o n . N u c l e a r r e s o n a n c e r e a c t i o n a n a l y s i s w i t h 1 5 ~ i o n s y i e l d e d [HI a 7 a t . % f o r S- and Se- r i c h f i l m s a n d 2. 1 2 a t . % f o r As- r i c h f i l m s . No n o t i c e a b l e c h a n g e i n [HI was o b s e r v e d a f t e r a n n e a l i n g a t 100°C f o r s e v e r a l h o u r s . 0 t i c a l Ga.-
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d e p e n d s on p h o t o n e n e r g y a s u h v = B(fiv-E0)' w y t h B +U 6 . 5 x 1 0 5 (cm e v ) - l , a common v a l u e f o r t h e s e g l a s s e s , e x c e p t n e a r x = 0 . 5 ? 0 . 0 5 w h e r e B = 3 . 5 x
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(cm e ~ ) - l was f o u n d . The o p t i c a l g a p Eo i s a b o u t 0 . 1 eV l a r g e r f o r h y d r o g e n a t e d AsXS1-, f i l m s t h a n f o r b u l k o r e v a p o r a t e d a n d a n - n e a l e d g l a s s e s , a s shown i n f i g u r e 4 . The r e f r a c t i v e i n d e x n a n d t h e d e n s i t y o f o u r f i l m s a g r e e w e l l w i t h t h o s e o f n o n h y d r o g e n a t e d g l a s s e s e x c e p t f o r x = 0 . 9 9 w h e r e we o b t a i n e d n = 3 . 2 . F i g . 5 shows t h e p h o t o - i n d u c e d r e d s h i f t of a g l o w - d i s c h a r g es a m p l e w i t h x = 0 . 3 5 . The f i l m was i l l u m i n a t e d t h r o u g h a n IR f i l t e r a t 300K f o r 1 8 h o u r s w i t h a 300W t u n g s t e n - h a l o g e n lamp f r o m a d i s t a n c e o f 1 5 cm. A n n e a l i n g f o r 1 h o u r a t 140°C r e s t o r e s t h e o r i g i n a l s t a t e . The r e v e r s i b l e s h i f t i s e s s e n t i a l - l y t h e same a s f o r e v a p o r a t e d As2S3 f i l m s [ 7 ] .
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1'250 mbl/cm2 we o b t a i n e d s p i n d e n s i r i e s Ns 2. 5 x 1017 i n A S ~ S ~ - ~ w i t h 0 . 2 5 < x < 0 . 7 , w h i c h i s t h z c o m p o s i t i o n r a n g e s t u d i e d . The p h o t o - i n d u c e d ESR was b l e a c h a b l e a t 12K w i t h 6 4 7 1 A l i g h t . The p h o t o - i n d u c e d s p i n c o n c e n t r a t i o n a s w e l l a s t h e ESR l i n e s h a p e s w e r e i d e n t i c a l t o t h o s e o b s e r v e d i n b u l k s a m p l e s o f c o r - r e s p o n d i n g c o m p o s i t i o n s and n o t c o n t a i n i n g h y d r o g e n . The ESR c e n t e r s i n x > 0 . 4 s a m p l e s w e r e i d e n t i f i e d w i t h n e u t r a l u n d e r c o o r d i n a t e d A s a t o m s and t h o s e i n x < 0 . 3 5 s a m p l e s w i t h n e u t r a l S a t o m s bonded t o n e i g h b o r i n g t w o - f o l d c o o r d i n a t e d S a t o m s [ 8 , 9 ] . Near x = 0 . 3 7 t h e d e n s i t i e s o f As a n d S s p i n s a r e t h e s a n e . T h e s e p h o t o - i n d u c e d s p i n c e n t e r s w e r e e i t h e r c r e a t e d by t h e l i g h t [ l o ] o r p r o d u c e d upon c a p t u r e o f p h o t o - e x c i t e d c a r r i e r s b y a l r e a d y e x i s t i n g p a r a m a g n e t i c d e f e c t c e n t e r s w h i c h h a v e a l l t h e p r o p e r t i e s o f v a l e n c e - a l t e r n a t i o n c e n t e r s [ 8 , 9 ] .
Summary a n d C o n c l u s i o n s . - Up t o 1 0 0 Um t h i c k h y d r o g e n a t e d c h a l c o g e n i d e g l a s s f i l m s h a v e k e n d e p o s i t e d by p l a s m a d e c o m p o s i t i o n o f t h e c o n s t i t u e n t h y d r i d e s . The compo-
C4-702 JOURNAL DE PHYSIQUE
s i t i o n and growth r a t e o f AsxSl-,films depend s t r o n g l y on t h e plasma c o n d i t i o n s and a p p e a r t o i n v o l v e n o n - l o c a l e f f e c t s o f q u i t e l o n g r a n g e . The hydrogen c o n t e n t r e a c h e s 8 a t . X and 1 2 a t . X f o r c h a l c o g e n - and As- r i c h c o m p o s i t i o n s and i s v e r y s m a l l n e a r x = 0 . 5 . The f i l m s show a r e v e r s i b l e photo-induced s h i f t o f t h e a b s o r p - t i o n e d g e a s w e l l a s photo-induced ESR s p e c t r a which a r e t h e same a s t h o s e i n c h a l - c o g e n i d e g l a s s e s n o t c o n t a i n i n g hydrogen. We c o n c l u d e t h a t t h e p r e s e n c e o f hydrogen does n o t n o t i c e a b l y a f f e c t t h e d e f e c t c h e m i s t r y of c h a l c o g e n i d e g l a s s e s . T h i s a g r e e s w i t h t h e p r e d i c t i o n s o f t h e v a l e n c e - a l t e r n a t i o n d e f e c t model [ 3 . 4 ] .
We a r e most i n d e b t e d t o W i l l i a m A . L a n f o r d f o r t h e n u c l e a r r e s o n a n c e r e a c t i o n a n a l y s i s . The work was s u p p o r t e d i n p a r t by NSF DMR 8009225 and by t h e NSF t h r o u g h t h e M a t e r i a l s R e s e a r c h L a b o r a t o r y a t The U n i v e r s i t y of Chicago.
m e n c e s
[ l ] F r i t z s c h e H . , S o l a r Energy M a t e r i a l s
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(1980) 447, and r e f e r e n c e s t h e r e i n . [ 2 ] S t r e e t R.A. and Mott N.F., Phys. Rev. L e t t .2
(1975) 1293.[ 3 ] K a s t n e r M . , A d l e r D. and F r i t z s c h e H . , Phys. Rev. L e t t .
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(1976) 1504.[ 4 ] K a s t n e r M. and F r i t z s c h e H . , P h i l . Mag.
B37
(1978) 199.[ 5 ] :mid V . and F r i t z s c h e H . , S o l . S t a t e . Commun.
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(1980) 735.[ 6 ] T s a i C.C. and F r i t z s c h e H . , S o l a r Energy M a t e r i a l s
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(1979) 29 [ 7 ] Tanaka K . , J. Non-Cryst. S o l i d s35136
(1980) 1023.[ 8 ] G a c z i P . J . and F r i t z s c h e H . , S o l . S t a t e Commun. ( i n p r e s s ) . [ 9 ] G a c z i P . J . , P h i l . Mag, ( i n p r e s s ) .
[ l o ] B i e g e l s e n D.K. and S t r e e t R . A . , I'hys. Rev. L e t t .
