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Growth and characterization of zinc sulfide films by conversion of zinc oxide films with H2S
M a t . R e s . B u l l . , V o l . 24, p p . 1 2 1 5 - 1 2 2 1 , 1 9 8 9 . Printed in the USA. 0 0 2 5 - 5 4 0 8 / 8 9 $ 3 . 0 0 + .00 Copyright ( c ) 1989 P e r g a m o n P r e s s p l c .
GROWTH AND CHARACTERIZATION OF ZINC SULFIDE FILMS BY CONVERSION OF ZINC OXIDE FILMS WITH 1-I.2S Y-M. Gao, P. Wu, J. B a g l i o + , K. D w i g h t a n d A. Wold Department of Chemistry, Brown University P r o v i d e n c e , RI 0 2 9 1 2 +GTE L a b s , Inc., W a l t h a m , MA 0 2 2 5 4 (Received
May 12,
1989;
Refereed)
ABSTRACT Zinc s u l f i d e f i l m s w e r e p r e p a r e d b y c o n v e r s i o n of z i n c o x i d e f i l m s i n t h e p r e s e n c e of h y d r o g e n s u l f i d e . The films which contained b o t h t h e h e x a g o n a l a n d c u b i c f o r m s of z i n c s u l f i d e w e r e s h o w n to b e u n i f o r m a n d g a v e a m e a s u r e d b a n d g a p of 3 . 6 5 eV. MATERIALS INDEX: Zinc, O x i d e s , S u l f i d e s
Introduction Zinc s u l f i d e is a n IR w i n d o w m a t e r i a l a n d a t r a n s p a r e n t w i t h a l a r g e d i r e c t b a n d gap. It also possesses piezoelectric, and electroluminescent properties. Thin films of zinc sulfide infrared antireflection coatings, light-emitting d i o d e s (LEDs), l u m i n e s c e n t (EL) d i s p l a y s , m u l t i l a y e r d i e l e c t r i c f i l t e r s , o p t i c a l and light guiding in integrated optics. In r e c e n t y e a r s , t h e r e a m o u n t of e f f o r t d i r e c t e d a t t h e g r o w t h of h i g h q u a l i t y f i l m s fabrication techniques have been employed, such as ion beam a t o m i c l a y e r e p i t a x y (ALE) (2), m o l e c u l a r b e a m e p i t a x y (MBE) c h e m i c a l v a p o r d e p o s i t i o n (4) a n d s p r a y p y r o l y s i s (5).
semiconductor photoconductive c a n be u t i l i z e d for electrophase modulation has been a large of ZnS. V a r i o u s s p u t t e r i n g (1), (3), m e t a l - o r g a n i c
Recently, a novel ultrasonic nebulization and pyrolysis method has been d e v e l o p e d i n t h i s l a b o r a t o r y (6). High q u a l i t y f i l m s of z i n c o x i d e c a n be g r o w n using this simple technique. P r e v i o u s l y , M a r u s k a (7) r e p o r t e d t h e c o n v e r s i o n of CdO f i l m s t o CdS f i l m s b y a n i o n e x c h a n g e p r o c e s s . It therefore appeared f e a s i b l e t o c o n v e r t ZnO f i l m s w i t h H2S i n t o ZnS f i l m s .
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of zinc s u l f i d e films
Zinc s u l f i d e f i l m s w e r e p r e p a r e d b y s u l f u r i z a t i o n o f z i n c o x i d e f i l m s w i t h hydrogen sulfide. Zinc o x i d e f i l m s o n s i l i c o n a n d s i l i c a s u b s t r a t e s w e r e p r e p a r e d by u l t r a s o n i c n e b u l i z a t i o n of zinc a c e t a t e s o l u t i o n s and t h e r m a l c o n v e r s i o n of t h e z i n c a c e t a t e t o z i n c o x i d e f i l m s . T h e d e t a i l e d p r o c e d u r e h a s b e e n d e s c r i b e d e l s e w h e r e (8). Zinc o x i d e f i l m s a s d e p o s i t e d w e r e a n n e a l e d in a m i x t u r e o f h y d r o g e n s u l f i d e a n d a r g o n (H2S:Ar = 1:1) in a h o r i z o n t a l t u b e furnace. T h e flow r a t e o f t h e g a s m i x t u r e w a s 50 c c / m i n . T h e t e m p e r a t u r e o f t h e f u r n a c e w a s r a i s e d g r a d u a l l y f r o m room t e m p e r a t u r e t o 500"C in 4 h r s . T h e f u r n a c e w a s m a i n t a i n e d a t 500"C f o r a n o t h e r 3 h r s a n d t h e n c o o l e d d o w n s l o w l y in t h e H2S/Ar a t m o s p h e r e . C o m p l e t i o n of t h e c o n v e r s i o n w a s v e r i f i e d b y x - r a y d i f f r a c t i o n a n a l y s i s a n d i n f r a r e d s p e c t r o s c o p y o f t h e films.
Characterization T h e t h i c k n e s s o f z i n c s u l f i d e f i l m s on s i l i c o n s u b s t r a t e s w a s d e t e r m i n e d by e l l i p s o m e t r y u s i n g a Rudolph R e s e a r c h Auto E I - I I e l l i p s o m e t e r . X - r a y d i f f r a c t i o n p a t t e r n s of t h e films were o b t a i n e d u s i n g a Philips d i f f r a c t o m e t e r and m o n o c h r o m a t e d h i g h i n t e n s i t y CuKa 1 r a d i a t i o n 0, = 1.5405A). Diffraction p a t t e r n s w e r e t a k e n w i t h a s c a n r a t e o f 1 ° 2 e / m i n o v e r t h e r a n g e 12 ° ( 2~ < 80". T h e s u r f a c e t o p o g r a p h y o f t h e f i l m s w a s s t u d i e d b y s c a n n i n g e l e c t r o n m i c r o s c o p y . P i c t u r e s w e r e t a k e n w i t h a n A m r a y 1000A S c a n n i n g E l e c t r o n M i c r o s c o p e o p e r a t i n g a t 20 KV. S e m i - q u a n t i t a t i v e (SQ) EDS a n a l y s i s w a s p e r f o r m e d on t h e JEOL 840 SEM w i t h a n e l e c t r o n b e a m e n e r g y of 20 keV. S p e c t r a w e r e a n a l y z e d on t h e TN5400 u s i n g t h e SQ p r o g r a m , w h i c h c a l c u l a t e s t h e k - r a t i o s from s t o r e d e l e m e n t a l s t a n d a r d s . O p t i c a l m e a s u r e m e n t s o f t h e f i l m s on s i l i c a s u b s t r a t e s w e r e p e r f o r m e d u s i n g a Cary Model 17 d u a l b e a m r a t i o r e c o r d i n g s p e c t r o p h o t o m e t e r i n t h e r a n g e o f 800 nm t o 1500 nm. T h e o p t i c a l b a n d g a p w a s d e t e r m i n e d f r o m t h e t r a n s m i t t a n c e n e a r t h e a b s o r p t i o n e d g e . I n f r a r e d s p e c t r a a t room t e m p e r a t u r e w e r e o b t a i n e d on a P e r k i n - E l m e r 580 s i n g l e - b e a m s c a n n i n g i n f r a r e d s p e c t r o p h o t o m e t e r a t a n i n s t r u m e n t a l r e s o l u t i o n of 2.8 cm-L T h e m e a s u r e m e n t s w e r e p e r f o r m e d in t h e t r a n s m i s s i o n mode o v e r t h e r a n g e 2.5 pm - 25 #m. T r a n s m i s s i o n t h r o u g h t h e sample was n o r m a l i z e d to t h e s i g n a l o b t a i n e d in t h e a b s e n c e of sample. Results and Discussion S m o o t h a n d h o m o g e n e o u s z i n c s u l f i d e f i l m s h a v e b e e n p r e p a r e d on b o t h silicon and silica s u b s t r a t e s by c o n v e r s i o n of zinc oxide films p r e p a r e d by an ultrasonic nebulization and pyrolysis technique. T h e f i l m s o f ZnS v a r i e d in t h i c k n e s s f r o m 0.1 pm t o 1 um a n d t h e t h i c k n e s s w a s u n i f o r m t o w i t h i n 1% a s i n d i c a t e d by e l l i p s o m e t r y m e a s u r e m e n t s . T h e s e f i l m s h a d good a d h e r e n c e t o b o t h substrates. They appeared uniform and shiny with bright colors varying with the thickness. T h e r e w a s l i t t l e d i f f e r e n c e i n t h e q u a l i t y of t h e i n i t i a l z i n c oxide and f i n a l zinc s u l f i d e films.
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X - r a y d i f f r a c t i o n a n a l y s i s w a s u s e d to d e t e r m i n e t h e a p p r o p r i a t e conditions for the conversion. It w a s d e s i r e d t h a t t h e c o n v e r s i o n s h o u l d be c a r r i e d o u t a t t h e l o w e s t p o s s i b l e t e m p e r a t u r e , p r o v i d e d t h e r e a c t i o n c a n be completed, tt w a s f o u n d t h a t a f t e r a n n e a l i n g in H2S a t 430°C f o r 1 1,'2 h r s , a f i l m o f l p m t h i c k n e s s s t i l l c o n t a i n e d a l a r g e a m o u n t of z i n c o x i d e . The peaks o f ZnO d i s a p p e a r e d in p a t t e r n s o f s u c h f i l m s a f t e r a n a n n e a l w a s p e r f o r m e d a t 500°C f o r 3 h r s in H2S" No i m p u r i t y p h a s e c o u l d be d e t e c t e d . X - r a y d i f f r a c t i o n a n a l y s i s w a s p e r f o r m e d on t h e z i n c s u l f i d e f i l m s a s d e p o s i t e d on t h e s u b s t r a t e s , The diffraction patterns showed that the films c o n s i s t e d of ZnS c r y s t a l l i z e d a s a m i x t u r e o f c u b i c a n d h e x a g o n a l p h a s e s . Both s t r u c t u r e s c o n s i s t of c l o s e - p a c k e d s u l f u r p l a n e s w i t h z i n c a t o m s o c c u p y i n g h a l f of t h e t e t r a h e d r a l s i t e s . T h e o n l y d i f f e r e n c e b e t w e e n t h e m is in t h e s t a c k i n g s e q u e n c e of t h e a n i o n p l a n e s . Hexagonal zinc sulfide films can therefore p a r t i a l l y c o n v e r t to t h e m o r e s t a b l e c u b i c z i n c s u l f i d e f o r m u n d e r t h e c o n d i t i o n s of film p r e p a r a t i o n . T h i s a p p a r e n t l y o c c u r s w i t h o u t a n y a p p r e c i a b l e r e d u c t i o n in t h e q u a l i t y of t h e f i n a l f i l m w h i c h is f o r m e d . Two p h a s e z i n c s u l f i d e f i l m s h a v e a l s o b e e n r e p o r t e d b y p r e v i o u s i n v e s t i g a t o r s (9). The prepared zinc sulfide films were examined with scanning electron microscopy. T h e y s h o w e d a u n i f o r m g r a i n t e x t u r e and t h e p a r t i c l e size grew l a r g e r w i t h t h i c k e r f i l m s . T h i s is s i m i l a r t o t h e p r o p e r t i e s o f t h e p r e c u r s o r z i n c o x i d e f i l m s (8). Fig. 1 s h o w s a p i c t u r e of a f i l m w i t h a t h i c k n e s s o f 1 am. T h e s u r f a c e of t h e film s h o w e d a s u b m i c r o n g r a i n t e x t u r e with t h e size of m o s t p a r t i c l e s b e i n g a b o u t 0.3 pro. Some f i l m s w e r e a n a l y z e d w i t h EDS. T h e r e s u l t s c o n f i r m t h e p r e s e n c e o f ZnS w i t h 5 0 . 2 a t o m i c p e r c e n t z i n c a n d 4 9 . 8 a t o m i c percent sulfur. ZnS f i l m s h a v e b e e n f o u n d s u i t a b l e a s s i n g l e - l a y e r a n t i r e f l e c t i o n c o a t i n g s i n t h e r e g i o n f r o m 8 t o 15 pm (10). F o r t h e ZnS f i l m s d e p o s i t e d i n t h i s s t u d y onto silicon, infrared spectroscopic measurements were performed. Fig. 2 s h o w s t h e m e a s u r e d t r a n s m i t t a n c e o f a 0.5 mm t h i c k s i l i c o n w a f e r u n c o a t e d a n d c o a t e d w i t h a s i n g l e l a y e r o f ZnS o f 1.15 um on e a c h s i d e . The transmittance a t I 0 pm ( n l d 1 = k / 4 ) is i n c r e a s e d f r o m 50 to 86% a n d r e m a i n s a b o v e 70% f r o m 7.3 pm to 15 um. T h e m a x i m u m t r a n s m i t t a n c e f o r s u c h a s a m p l e c a n be c a l c u l a t e d to be 93% a t k = 10 am (11). T h u s t h e a n t i r e f l e c t i o n p e r f o r m a n c e of o u r ZnS f i l m s w a s c l o s e to t h e t h e o r e t i c a l v a l u e . Fig. 3 s h o w s t h e i n f r a r e d s p e c t r a o f z i n c o x i d e a n d z i n c s u l f i d e f i l m s on s i l i c o n s u b s t r a t e s i n t h e r a n g e f r o m 2 0 0 cm -1 to 6 0 0 cm -1. T h e s t r o n g b a n d of t h e p r i n c i p a l l a t t i c e a b s o r p t i o n c a n be c l e a r l y s e e n a t 2 8 0 cm -1. Zinc o x i d e f i l m s h a v e a b r o a d a b s o r p t i o n b a n d w i t h t h e p e a k a t 4 1 0 c m - L w h i c h is c o n s i s t e n t w i t h C o l l i n s ' r e s u l t s o b t a i n e d f r o m t h e r e f l e c t i v i t y of a z i n c o x i d e s i n g l e c r y s t a l (12). T h e p e a k p o s i t i o n of t h e a b s o r p t i o n b a n d of t h e z i n c s u l f i d e f i l m s w a s a t 278 cm -~. K w a s n i e w s k i (13) r e p o r t e d a b r o a d a b s o r p t i o n b a n d c o v e r i n g 2 5 0 - 3 5 5 c m - ' in t h e i n f r a r e d t r a n s m i s s i o n s p e c t r u m o f c u b i c ZnS. Detailed information near the peak was not given because single crystals were used that were much thicker than thin films. B r a f m a n (14) r e p o r t e d t h a t t h e f u n d a m e n t a l r e s o n a n c e f r e q u e n c i e s of z i n c s u l f i d e w i t h t h e hexagonal and c u b i c s t r u c t u r e s w e r e 273 a n d 2 7 6 em -1, r e s p e c t i v e l y , w h i c h i s v e r i f i e d by t h e
1218
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Photomicrograph of 1 pm, The
1.
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of a zinc sulfide film with a measured grains average 0.3 pm in diameter.
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Transmission spectrum of a silicon wafter a 1.15 pm layer of ZnS on each side.
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Comparative infrared spectra of films of untreated zinc oxide, zinc oxide partially coverted to the sulfide, and of fully converted zinc sulfide.
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r e c e n t w o r k o f Memon (15). T h e s e r e s u l t s a r e all c o n s i s t e n t w i t h t h e r e s u l t s o b t a i n e d in t h i s i n v e s t i g a t i o n . I n f r a r e d s p e c t r a of zinc s u l f i d e films was used a s a m e a n s of v e r i f y i n g t h e c o m p l e t i o n o f c o n v e r s i o n o f ZnO t o ZnS. It is s h o w n i n Fig. 3 t h a t a film p a r t i a l l y c o n v e r t e d t o s u l f i d e h a s b o t h a b s o r p t i o n b a n d c h a r a c t e r i s t i c s of zinc s u l f i d e and zinc oxide. T h e o p t i c a l t r a n s m i s s i o n o f z i n c s u l f i d e f i l m s on s i l i c a s u b s t r a t e s w a s m e a s u r e d and t h e t r a n s m i s s i o n n e a r t h e a b s o r p t i o n edge was u s e d to g e n e r a t e t h e p l o t o f (ahu) 2 v e r s u s h u a s s h o w n in Fig. 4. T h e i n t e r c e p t of t h e p l o t g a v e t h e e n e r g y of t h e d i r e c t b a n d gap. It w a s f o u n d t h a t t h e d i r e c t b a n d g a p o f t h e p r e p a r e d z i n c s u l f i d e f i l m s w a s 3.65 eV, w h i c h is c o n s i s t e n t w i t h t h e v a l u e g i v e n i n t h e l i t e r a t u r e (4).
Conclusions This s t u d y h a s d e m o n s t r a t e d t h e p r e p a r a t i o n of zinc s u l f i d e films by t h e c o n v e r s i o n o f z i n c o x i d e f i l m s t o z i n c s u l f i d e in t h e p r e s e n c e o f H2S. Zinc oxide films were p r e p a r e d as a p r e c u r s o r by u l t r a s o n i c n e b u l i z a t i o n of zinc a c e t a t e s o l u t i o n followed by d e c o m p o s i t i o n of t h e zinc a c e t a t e to zinc oxide. U n i f o r m f i l m s o f ZnS w i t h good a d h e r e n c e h a v e b e e n o b t a i n e d u s i n g t h i s technique. The t r a n s p a r e n t p o l y c r y s t a l l i n e films h a v e submicron grain t e x t u r e w i t h a b a n d g a p o f 3.65 eV. T h e y s h o w e d p o t e n t i a l a s a n IR a n t i r e f l e e t i o n coating material.
Acknowledl~ements This r e s e a r c h was p a r t i a l l y s u p p o r t e d by t h e Office of Naval R e s e a r c h , by t h e N a t i o n a l S c i e n c e F o u n d a t i o n C o n t r a c t No. DMR 8 8 0 3 1 8 4 , b y GTE L a b s , Inc. W a l t h a m , MA, a n d b y E a s t m a n K o d a k C o m p a n y , R o c h e s t e r , NY. T h e i n f r a r e d s p e c t r a w e r e o b t a i n e d w i t h t h e a s s i s t a n c e o f Ted K i r s t .
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7.
H. P. M a r u s k a , A. R. Young, II, a n d C. R. W r o n s k i , 1 5 t h [EEE P h o t o v o l t a i c s S p e c i a l i s t s C o n f e r e n c e , p. 1030 (1981).
8.
P. Wu, Y-M. Gao, J. Baglio, R. K e r s h a w , K. D w i g h t a n d A. Wold, S u b m i t t e d to C h e m i s t r y o f M a t e r i a l s .
9.
B. G r e e n b e r g , W. K. Z ~ i c k e r a n d 1. C a d o f f , T h i n Solid Films, 141, 89 (1986).
10.
J. T. Cox a n d G. H a s s , P h y s i c s o f T h i n Films, ~., 239 (1964).
11.
O. S. H e a v e n s , P h y s i c s o f T h i n F i l m s , 2, 193 (1964).
12.
R. J. C o l l i n s a n d D. A. K l e i n m a n , J. P h y s . Chem. S o l i d s , 11, 190 (1959).
13.
E. A. K w a s n i e w s k i , E. S. K o t e l e s a n d W. R. D a t a r s , Can. J. P h y s . , 54, 1053 (1976).
14.
O. B r a f m a n a n d S. S. Mitra, P h y s . Rev., 171, 931 (1968).
15.
A. Memon a n d S. B. T a n n e r , P h y s . S t a r . Sol. B, 128, 49 (1985).
1221
GROWTH AND CHARACTERIZATION OF ZINC SULFIDE FILMS BY CONVERSION OF ZINC OXIDE FILMS WITH 1-I.2S Y-M. Gao, P. Wu, J. B a g l i o + , K. D w i g h t a n d A. Wold Department of Chemistry, Brown University P r o v i d e n c e , RI 0 2 9 1 2 +GTE L a b s , Inc., W a l t h a m , MA 0 2 2 5 4 (Received
May 12,
1989;
Refereed)
ABSTRACT Zinc s u l f i d e f i l m s w e r e p r e p a r e d b y c o n v e r s i o n of z i n c o x i d e f i l m s i n t h e p r e s e n c e of h y d r o g e n s u l f i d e . The films which contained b o t h t h e h e x a g o n a l a n d c u b i c f o r m s of z i n c s u l f i d e w e r e s h o w n to b e u n i f o r m a n d g a v e a m e a s u r e d b a n d g a p of 3 . 6 5 eV. MATERIALS INDEX: Zinc, O x i d e s , S u l f i d e s
Introduction Zinc s u l f i d e is a n IR w i n d o w m a t e r i a l a n d a t r a n s p a r e n t w i t h a l a r g e d i r e c t b a n d gap. It also possesses piezoelectric, and electroluminescent properties. Thin films of zinc sulfide infrared antireflection coatings, light-emitting d i o d e s (LEDs), l u m i n e s c e n t (EL) d i s p l a y s , m u l t i l a y e r d i e l e c t r i c f i l t e r s , o p t i c a l and light guiding in integrated optics. In r e c e n t y e a r s , t h e r e a m o u n t of e f f o r t d i r e c t e d a t t h e g r o w t h of h i g h q u a l i t y f i l m s fabrication techniques have been employed, such as ion beam a t o m i c l a y e r e p i t a x y (ALE) (2), m o l e c u l a r b e a m e p i t a x y (MBE) c h e m i c a l v a p o r d e p o s i t i o n (4) a n d s p r a y p y r o l y s i s (5).
semiconductor photoconductive c a n be u t i l i z e d for electrophase modulation has been a large of ZnS. V a r i o u s s p u t t e r i n g (1), (3), m e t a l - o r g a n i c
Recently, a novel ultrasonic nebulization and pyrolysis method has been d e v e l o p e d i n t h i s l a b o r a t o r y (6). High q u a l i t y f i l m s of z i n c o x i d e c a n be g r o w n using this simple technique. P r e v i o u s l y , M a r u s k a (7) r e p o r t e d t h e c o n v e r s i o n of CdO f i l m s t o CdS f i l m s b y a n i o n e x c h a n g e p r o c e s s . It therefore appeared f e a s i b l e t o c o n v e r t ZnO f i l m s w i t h H2S i n t o ZnS f i l m s .
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V o l . 24,
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Experimental Preparation
of zinc s u l f i d e films
Zinc s u l f i d e f i l m s w e r e p r e p a r e d b y s u l f u r i z a t i o n o f z i n c o x i d e f i l m s w i t h hydrogen sulfide. Zinc o x i d e f i l m s o n s i l i c o n a n d s i l i c a s u b s t r a t e s w e r e p r e p a r e d by u l t r a s o n i c n e b u l i z a t i o n of zinc a c e t a t e s o l u t i o n s and t h e r m a l c o n v e r s i o n of t h e z i n c a c e t a t e t o z i n c o x i d e f i l m s . T h e d e t a i l e d p r o c e d u r e h a s b e e n d e s c r i b e d e l s e w h e r e (8). Zinc o x i d e f i l m s a s d e p o s i t e d w e r e a n n e a l e d in a m i x t u r e o f h y d r o g e n s u l f i d e a n d a r g o n (H2S:Ar = 1:1) in a h o r i z o n t a l t u b e furnace. T h e flow r a t e o f t h e g a s m i x t u r e w a s 50 c c / m i n . T h e t e m p e r a t u r e o f t h e f u r n a c e w a s r a i s e d g r a d u a l l y f r o m room t e m p e r a t u r e t o 500"C in 4 h r s . T h e f u r n a c e w a s m a i n t a i n e d a t 500"C f o r a n o t h e r 3 h r s a n d t h e n c o o l e d d o w n s l o w l y in t h e H2S/Ar a t m o s p h e r e . C o m p l e t i o n of t h e c o n v e r s i o n w a s v e r i f i e d b y x - r a y d i f f r a c t i o n a n a l y s i s a n d i n f r a r e d s p e c t r o s c o p y o f t h e films.
Characterization T h e t h i c k n e s s o f z i n c s u l f i d e f i l m s on s i l i c o n s u b s t r a t e s w a s d e t e r m i n e d by e l l i p s o m e t r y u s i n g a Rudolph R e s e a r c h Auto E I - I I e l l i p s o m e t e r . X - r a y d i f f r a c t i o n p a t t e r n s of t h e films were o b t a i n e d u s i n g a Philips d i f f r a c t o m e t e r and m o n o c h r o m a t e d h i g h i n t e n s i t y CuKa 1 r a d i a t i o n 0, = 1.5405A). Diffraction p a t t e r n s w e r e t a k e n w i t h a s c a n r a t e o f 1 ° 2 e / m i n o v e r t h e r a n g e 12 ° ( 2~ < 80". T h e s u r f a c e t o p o g r a p h y o f t h e f i l m s w a s s t u d i e d b y s c a n n i n g e l e c t r o n m i c r o s c o p y . P i c t u r e s w e r e t a k e n w i t h a n A m r a y 1000A S c a n n i n g E l e c t r o n M i c r o s c o p e o p e r a t i n g a t 20 KV. S e m i - q u a n t i t a t i v e (SQ) EDS a n a l y s i s w a s p e r f o r m e d on t h e JEOL 840 SEM w i t h a n e l e c t r o n b e a m e n e r g y of 20 keV. S p e c t r a w e r e a n a l y z e d on t h e TN5400 u s i n g t h e SQ p r o g r a m , w h i c h c a l c u l a t e s t h e k - r a t i o s from s t o r e d e l e m e n t a l s t a n d a r d s . O p t i c a l m e a s u r e m e n t s o f t h e f i l m s on s i l i c a s u b s t r a t e s w e r e p e r f o r m e d u s i n g a Cary Model 17 d u a l b e a m r a t i o r e c o r d i n g s p e c t r o p h o t o m e t e r i n t h e r a n g e o f 800 nm t o 1500 nm. T h e o p t i c a l b a n d g a p w a s d e t e r m i n e d f r o m t h e t r a n s m i t t a n c e n e a r t h e a b s o r p t i o n e d g e . I n f r a r e d s p e c t r a a t room t e m p e r a t u r e w e r e o b t a i n e d on a P e r k i n - E l m e r 580 s i n g l e - b e a m s c a n n i n g i n f r a r e d s p e c t r o p h o t o m e t e r a t a n i n s t r u m e n t a l r e s o l u t i o n of 2.8 cm-L T h e m e a s u r e m e n t s w e r e p e r f o r m e d in t h e t r a n s m i s s i o n mode o v e r t h e r a n g e 2.5 pm - 25 #m. T r a n s m i s s i o n t h r o u g h t h e sample was n o r m a l i z e d to t h e s i g n a l o b t a i n e d in t h e a b s e n c e of sample. Results and Discussion S m o o t h a n d h o m o g e n e o u s z i n c s u l f i d e f i l m s h a v e b e e n p r e p a r e d on b o t h silicon and silica s u b s t r a t e s by c o n v e r s i o n of zinc oxide films p r e p a r e d by an ultrasonic nebulization and pyrolysis technique. T h e f i l m s o f ZnS v a r i e d in t h i c k n e s s f r o m 0.1 pm t o 1 um a n d t h e t h i c k n e s s w a s u n i f o r m t o w i t h i n 1% a s i n d i c a t e d by e l l i p s o m e t r y m e a s u r e m e n t s . T h e s e f i l m s h a d good a d h e r e n c e t o b o t h substrates. They appeared uniform and shiny with bright colors varying with the thickness. T h e r e w a s l i t t l e d i f f e r e n c e i n t h e q u a l i t y of t h e i n i t i a l z i n c oxide and f i n a l zinc s u l f i d e films.
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X - r a y d i f f r a c t i o n a n a l y s i s w a s u s e d to d e t e r m i n e t h e a p p r o p r i a t e conditions for the conversion. It w a s d e s i r e d t h a t t h e c o n v e r s i o n s h o u l d be c a r r i e d o u t a t t h e l o w e s t p o s s i b l e t e m p e r a t u r e , p r o v i d e d t h e r e a c t i o n c a n be completed, tt w a s f o u n d t h a t a f t e r a n n e a l i n g in H2S a t 430°C f o r 1 1,'2 h r s , a f i l m o f l p m t h i c k n e s s s t i l l c o n t a i n e d a l a r g e a m o u n t of z i n c o x i d e . The peaks o f ZnO d i s a p p e a r e d in p a t t e r n s o f s u c h f i l m s a f t e r a n a n n e a l w a s p e r f o r m e d a t 500°C f o r 3 h r s in H2S" No i m p u r i t y p h a s e c o u l d be d e t e c t e d . X - r a y d i f f r a c t i o n a n a l y s i s w a s p e r f o r m e d on t h e z i n c s u l f i d e f i l m s a s d e p o s i t e d on t h e s u b s t r a t e s , The diffraction patterns showed that the films c o n s i s t e d of ZnS c r y s t a l l i z e d a s a m i x t u r e o f c u b i c a n d h e x a g o n a l p h a s e s . Both s t r u c t u r e s c o n s i s t of c l o s e - p a c k e d s u l f u r p l a n e s w i t h z i n c a t o m s o c c u p y i n g h a l f of t h e t e t r a h e d r a l s i t e s . T h e o n l y d i f f e r e n c e b e t w e e n t h e m is in t h e s t a c k i n g s e q u e n c e of t h e a n i o n p l a n e s . Hexagonal zinc sulfide films can therefore p a r t i a l l y c o n v e r t to t h e m o r e s t a b l e c u b i c z i n c s u l f i d e f o r m u n d e r t h e c o n d i t i o n s of film p r e p a r a t i o n . T h i s a p p a r e n t l y o c c u r s w i t h o u t a n y a p p r e c i a b l e r e d u c t i o n in t h e q u a l i t y of t h e f i n a l f i l m w h i c h is f o r m e d . Two p h a s e z i n c s u l f i d e f i l m s h a v e a l s o b e e n r e p o r t e d b y p r e v i o u s i n v e s t i g a t o r s (9). The prepared zinc sulfide films were examined with scanning electron microscopy. T h e y s h o w e d a u n i f o r m g r a i n t e x t u r e and t h e p a r t i c l e size grew l a r g e r w i t h t h i c k e r f i l m s . T h i s is s i m i l a r t o t h e p r o p e r t i e s o f t h e p r e c u r s o r z i n c o x i d e f i l m s (8). Fig. 1 s h o w s a p i c t u r e of a f i l m w i t h a t h i c k n e s s o f 1 am. T h e s u r f a c e of t h e film s h o w e d a s u b m i c r o n g r a i n t e x t u r e with t h e size of m o s t p a r t i c l e s b e i n g a b o u t 0.3 pro. Some f i l m s w e r e a n a l y z e d w i t h EDS. T h e r e s u l t s c o n f i r m t h e p r e s e n c e o f ZnS w i t h 5 0 . 2 a t o m i c p e r c e n t z i n c a n d 4 9 . 8 a t o m i c percent sulfur. ZnS f i l m s h a v e b e e n f o u n d s u i t a b l e a s s i n g l e - l a y e r a n t i r e f l e c t i o n c o a t i n g s i n t h e r e g i o n f r o m 8 t o 15 pm (10). F o r t h e ZnS f i l m s d e p o s i t e d i n t h i s s t u d y onto silicon, infrared spectroscopic measurements were performed. Fig. 2 s h o w s t h e m e a s u r e d t r a n s m i t t a n c e o f a 0.5 mm t h i c k s i l i c o n w a f e r u n c o a t e d a n d c o a t e d w i t h a s i n g l e l a y e r o f ZnS o f 1.15 um on e a c h s i d e . The transmittance a t I 0 pm ( n l d 1 = k / 4 ) is i n c r e a s e d f r o m 50 to 86% a n d r e m a i n s a b o v e 70% f r o m 7.3 pm to 15 um. T h e m a x i m u m t r a n s m i t t a n c e f o r s u c h a s a m p l e c a n be c a l c u l a t e d to be 93% a t k = 10 am (11). T h u s t h e a n t i r e f l e c t i o n p e r f o r m a n c e of o u r ZnS f i l m s w a s c l o s e to t h e t h e o r e t i c a l v a l u e . Fig. 3 s h o w s t h e i n f r a r e d s p e c t r a o f z i n c o x i d e a n d z i n c s u l f i d e f i l m s on s i l i c o n s u b s t r a t e s i n t h e r a n g e f r o m 2 0 0 cm -1 to 6 0 0 cm -1. T h e s t r o n g b a n d of t h e p r i n c i p a l l a t t i c e a b s o r p t i o n c a n be c l e a r l y s e e n a t 2 8 0 cm -1. Zinc o x i d e f i l m s h a v e a b r o a d a b s o r p t i o n b a n d w i t h t h e p e a k a t 4 1 0 c m - L w h i c h is c o n s i s t e n t w i t h C o l l i n s ' r e s u l t s o b t a i n e d f r o m t h e r e f l e c t i v i t y of a z i n c o x i d e s i n g l e c r y s t a l (12). T h e p e a k p o s i t i o n of t h e a b s o r p t i o n b a n d of t h e z i n c s u l f i d e f i l m s w a s a t 278 cm -~. K w a s n i e w s k i (13) r e p o r t e d a b r o a d a b s o r p t i o n b a n d c o v e r i n g 2 5 0 - 3 5 5 c m - ' in t h e i n f r a r e d t r a n s m i s s i o n s p e c t r u m o f c u b i c ZnS. Detailed information near the peak was not given because single crystals were used that were much thicker than thin films. B r a f m a n (14) r e p o r t e d t h a t t h e f u n d a m e n t a l r e s o n a n c e f r e q u e n c i e s of z i n c s u l f i d e w i t h t h e hexagonal and c u b i c s t r u c t u r e s w e r e 273 a n d 2 7 6 em -1, r e s p e c t i v e l y , w h i c h i s v e r i f i e d by t h e
1218
Fig.
Y-M.
Photomicrograph of 1 pm, The
1.
GAO, et al.
of a zinc sulfide film with a measured grains average 0.3 pm in diameter.
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.
.
.
.
Vol. 24, No.
.
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thickness
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Transmission spectrum of a silicon wafter a 1.15 pm layer of ZnS on each side.
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i0
Vol. 24, No. i0
ZINC SULFIDE FILMS
1219
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Comparative infrared spectra of films of untreated zinc oxide, zinc oxide partially coverted to the sulfide, and of fully converted zinc sulfide.
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G A O , e t al.
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10
r e c e n t w o r k o f Memon (15). T h e s e r e s u l t s a r e all c o n s i s t e n t w i t h t h e r e s u l t s o b t a i n e d in t h i s i n v e s t i g a t i o n . I n f r a r e d s p e c t r a of zinc s u l f i d e films was used a s a m e a n s of v e r i f y i n g t h e c o m p l e t i o n o f c o n v e r s i o n o f ZnO t o ZnS. It is s h o w n i n Fig. 3 t h a t a film p a r t i a l l y c o n v e r t e d t o s u l f i d e h a s b o t h a b s o r p t i o n b a n d c h a r a c t e r i s t i c s of zinc s u l f i d e and zinc oxide. T h e o p t i c a l t r a n s m i s s i o n o f z i n c s u l f i d e f i l m s on s i l i c a s u b s t r a t e s w a s m e a s u r e d and t h e t r a n s m i s s i o n n e a r t h e a b s o r p t i o n edge was u s e d to g e n e r a t e t h e p l o t o f (ahu) 2 v e r s u s h u a s s h o w n in Fig. 4. T h e i n t e r c e p t of t h e p l o t g a v e t h e e n e r g y of t h e d i r e c t b a n d gap. It w a s f o u n d t h a t t h e d i r e c t b a n d g a p o f t h e p r e p a r e d z i n c s u l f i d e f i l m s w a s 3.65 eV, w h i c h is c o n s i s t e n t w i t h t h e v a l u e g i v e n i n t h e l i t e r a t u r e (4).
Conclusions This s t u d y h a s d e m o n s t r a t e d t h e p r e p a r a t i o n of zinc s u l f i d e films by t h e c o n v e r s i o n o f z i n c o x i d e f i l m s t o z i n c s u l f i d e in t h e p r e s e n c e o f H2S. Zinc oxide films were p r e p a r e d as a p r e c u r s o r by u l t r a s o n i c n e b u l i z a t i o n of zinc a c e t a t e s o l u t i o n followed by d e c o m p o s i t i o n of t h e zinc a c e t a t e to zinc oxide. U n i f o r m f i l m s o f ZnS w i t h good a d h e r e n c e h a v e b e e n o b t a i n e d u s i n g t h i s technique. The t r a n s p a r e n t p o l y c r y s t a l l i n e films h a v e submicron grain t e x t u r e w i t h a b a n d g a p o f 3.65 eV. T h e y s h o w e d p o t e n t i a l a s a n IR a n t i r e f l e e t i o n coating material.
Acknowledl~ements This r e s e a r c h was p a r t i a l l y s u p p o r t e d by t h e Office of Naval R e s e a r c h , by t h e N a t i o n a l S c i e n c e F o u n d a t i o n C o n t r a c t No. DMR 8 8 0 3 1 8 4 , b y GTE L a b s , Inc. W a l t h a m , MA, a n d b y E a s t m a n K o d a k C o m p a n y , R o c h e s t e r , NY. T h e i n f r a r e d s p e c t r a w e r e o b t a i n e d w i t h t h e a s s i s t a n c e o f Ted K i r s t .
References 1.
T. E. V a r i t i m o s a n d R. W. T u s t i s o n , T h i n Solid Films, 151, 27 ( ] 9 8 7 ) .
2.
A. H u n t e r a n d A. H. K i t a i , J. C r y s t . G r o w t h , 91,
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