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JOUR NAL OF M AT E R I AL S SC I E NC E L E T T E R S 9 ( 1 9 9 0 ) 1 3 5 1 - 1 3 5 3

P re pa ra t ion a nd e le c t r i c a l p rope r t i e s o f undope d z inc ox ide f i lmsb y C V D *M. F. OGAWA, Y. NATSUME, T. HIRAYAMADepartment of Industria l Chemistry, Tokai University, 111 7, Kitakaname, Hiratsuka-shLKanagawa 259-12, JapanH. SAKATAResearch and Developm ent General Division, As ahi G lass Co., Ltd., 2- 1-2 , MarunouchL Chiyoda-ku,Tokyo 100, Japan

Z i n c o x i d e f i l m s h a v e b e e n a p p l i e d t o s o l a r c e l l s ,v a r i s t o r , a n d g a s s e n s o r s [ 1] , b e i n g a n n - t y p e s e m i c o n -d u c t o r . T h e f il m s o r i e n t e d a l o n g t h e e - a x is w e r e u s e di n s u r f a c e a c o u s t i c w a v e d e v i c e s [2 ] d u e t o h i g h p i e z o -e l e c t r i c i t y [ 3 ] . S i n c e Z n O t h i n f i l m s a r e e x p e c t e d f o ru s e i n d i f f e r e n t e l e c t r o n i c a p p l i c a t i o n s , v a r i o u s f i l mf o r m i n g m e t h o d s h a v e b e e n s t u d i e d, s u c h a s s p u t t e r i n g[ 4] , i o n i z e d - c l u s t e r b e a m d e p o s i t i o n [ 5 ], s p r a y p y r o l y s i s[ 6 ] , a n d c h e m i c a l v a p o u r d e p o s i t i o n [ 7 , 8 ] . K a m a t aa n d M a t s u m o t o [9] s t u di e d Z n O f il m f o r m a t i o n b y aC V D m e t h o d , s u b l i m a t i n g z in c a c e t y l a c e t o n a t e inw a t e r v a p o u r a t m o s p h e r e .T h i s c o m m u n i c a t i o n d e s c ri b es t h e p r e p a r a t i o n a n de l e ct r ic a l p r o p e r t i e s o f Z n O t h i n f i lm s u s i n g n o r m a lp r e ss u r e C V D m e t h o d u n d e r o x y g e n a t m o s p h e r e a n do x y g e n p l u s w a te r v a p o u r a t m o s p h e r e . T h e C V Da p p a r a t u s c o n s i s t ed o f a h o r i z o n t a l e l e c t r ic f u r n a ce .T h e r e a c t i o n c h a m b e r w a s o f a m u l l i t e t u b e w i th a ni n s i d e d i a m e t e r o f 3 0 m m w h e r e r e a c t i o n g a s f l o w e dn o r m a l t o t h e s u b s t r a t e s u r f a c e . G a s e s p r o v i d e d t o t h i ss y s te m w e r e d e h y d r a t e d a r g o n a n d o x y g e n . T h e s u b -s t r a t e m a t e r i a l w a s w e l l - c l e a n e d P y r e x g l a s s o f2 0 x 5 0 x 3 r a m i n d i m e n s i o n s . C l e a n i n g w a s c a r -r i e d o u t s u c c e s s i v e l y w i t h w a t e r , i o n - e x c h a n g e d w a t e r ,a c e t o n e a n d e t h a n o l i n a n u l t r a s o n i c c l e a n e r f o r3 0 m i n , r e s p e c t i v e l y , a n d t h e r e a f t e r d r y i n g i n a n e l e c -t r i c o v e n . T h e s o u r c e m a t e r i a l w a s B i s ( 2 , 4 - p e n t a n e -d i o n a t o ) Z i n c , i . e. , z in c a c e t y l a c e t o n a t e . A b o u t 0 .2 go f z in c a c e t y l a c et o n a t e ( T o k y o K a s e i, R G ) w a s c o n -t a i n e d i n a g l a s s b o a t m a i n t a i n e d a t 1 6 0 C a n d t h es u b l i m a t e d v a p o u r w a s t r a n s p o r t e d i n t o t h e r e a c t i o nc h a m b e r t o g e t h e r w i t h a r g o n g a s . A t t h e s a m e t i m ea l s o o x y g e n g a s w a s i n t r o d u c e d t h r o u g h a s e p a r a t ep a r t o f t h e r e ac t i o n ch a m b e r . T h e a r g o n a n d o x y g e nw e r e u s e d a s c a r r i e r g a s a n d r e a c t i o n g a s , r e s p e c ti v e l y .T h e f l o w r a t e o f t h e f o r m e r a n d t h e l a t t e r w a s 5 0 0 a n d1 0 m l m i n - l , r e s p e ct i v el y . T h e Z n O f il m s w e r ep r e p a r e d f o r 1 h o n P y r e x g l as s s u b s t r a t e f o r v a r i o u ss u b s t r a t e t e m p e r a t u r e s r a n g i n g f r o m 5 0 0 t o 6 0 0 C a ta n i n t e r v a l o f 2 5 C , u n d e r t h e c o n d i t i o n s o f o x y g e na t m o s p h e r e a n d o x y g e n p lu s w a t e r v a p o u r a t m o s -p h e r e . T h u s , 2 0 0 t o 6 0 0 n m t h i c k Z n O f il m s w e r eo b t a i n e d f o r 1 h d e p o s i t i o n , w h i c h w e r e f o u n d b yS E M t o b e h i g h l y u n i f o r m i n s u r f a c e t e x t u r e .

E l e c t ri c a l p r o p e r t i e s o f t h e d e p o s i t e d Z n O f il m s a n d

t h e i r a c t i v a t i o n e n e r g y i n e l e c t r i c a l c o n d u c t i o n w e r et h e n i n v e s t i g a t e d .

A l l t h e e x p e r i m e n t a l s a m p l e s w e r e u n d o p e d i n t h es e n s e t h a t t h e y d o n o t c o n t a i n a n y i m p u r i t y d o p a n t sd u r i n g t h e f i l m d e p o s i t i o n . T h e f i l m s w e r e c h a r a c -t e r i z e d b y X - r a y d i f f r a c t i o n a n d A u g e r s p e c t r o s c o p y .F i g . 1 s h o w s X - r a y d i f f r a c t o g r a m s f o r t h e Z n O f il md e p o s i t e d a t s u b s t r a t e t e m p e r a t u r e s r a n g i n g f r o m 5 0 0t o 6 0 0 C i n t h e ca s e o f f o r m i n g c o n d i t i o n s i n a no x y g e n a t m o s p h e r e , t o g e th e r w i th t h a t f o r Z n O p o w -d e r a s a r ef e r e n c e . T h i s a n d o t h e r X - r a y d a t a o nt h e s e f i l m s s h o w e d t h a t t h e f i l m s i m p r o v e d h i g h l yt h e i r c - a x i s o r i e n t a t i o n w i t h i n c r e a s e i n d e p o s i t i o nt e m p e r a t u r e , b e c a u s e t h e ( 0 0 2 ) p e a k w a s r e m a r k a b l e .T h e c - ax i s o r i e n t a t i o n o f t h e f i lm s t r u c t u r e w a s h a r d l yi n f l u e n c e d b y t h e o x y g e n a t m o s p h e r e o r t h e o x y g e np l u s w a t e r v a p o u r a t m o s p h e r e i n f i l m f o r m i n g . T h i s

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Figure 1 X-ray diffractionof ZnO films and po wder as a referencein an oxygen atmosphere.*Part of the pap er presented at the 1989 International Chem ical Cong ress of P acific Basin Societies, Hon olulu, Hawaii, USA ., 17-22December 1989.0261-8028/90 $03.00 + .12 1990 Chapman and Hall Ltd. ] 351

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. . . . ' ' '

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1 .6 n rn I ~S p u t te r - I / " ~ . .

8 .0 n m / ~ \ \ ' , ,S p u t t e r . / / ~ " ~/ ~ . - - ~ " ~ ~ , , ~ . _ ,

259 261 263 265 267 269B i n d in g e n e r g y ( e V )

Figure 2 Zn(L3M45M45)Auger transitions for Z nO films depositedin different atmospheres: ( ) oxygen and ( - - - ) oxygen pluswater va pour. Substrate tem perature: 550 C.

r e s u lt is t he s a m e a s t h a t o f d e p o s i t i o n b y t h e o t h e rau th o r s [1 0 ] .

A u g e r s p e c t r o s c o p y m e a s u r e m e n t s w e r e p e r f o r m e do n t h e Z n O f il m s to c h e c k p r e s e n c e o f i m p u r it i e s .F ig . 2 sh o ws Z n ( L 3M 4 5 M 4 5 ) A u g e r t r a n s i t i o n f o r t h ea s - d e p o s i t e d Z n O f il m , in d i c a t in g a c l e a r p r e s e n t a t i o no f Z n O b i n d i n g e n e r g y . E v e n w h e n t h e f il m s w e r es p u t t e r - e t c h e d , t h e A u g e r p e a k p o s i t i o n d i d n o tc h a n g e . F o r t h e f il m s m a d e i n t h e o x y g e n a t m o s p h e r eo r o x y g e n p l u s w a t e r v a p o u r a t m o s p h e r e , n o d i f -f e r en c e s w e re f o u n d i n b in d i n g e n e r g y i n Z n O a n dm e t a l l i c z i n c . A s s h o w n i n t h e f i g u r e t h e r e w e r e n os u r f a c e c o n t a m i n a t i o n s , a s t he p r e s e n c e o f c a r b o n .T h e r e f o r e , i t c a n b e a d m i t t e d t h a t a p e r f e c t Z n O b o n dw a s f o r m e d b y t h i s C V D .

F i l m r e s i s ti v it y m e a s u r e m e n t s w e r e c a rr i e d o u t b yt h e f o u r - p o i n t p r o b e m e t h o d . T h e v a r i a t i o n o f t h er e s is t iv i ty w i t h s u b s t r a t e t e m p e r a t u r e s i s s h o w n i nF i g . 3 . A m i n i m u m r e s i s t iv i t y ( 2 . 44 D c m ) w a so b t a i n e d f o r a fi lm d e p o s i t e d a t a s u b s t r a t e t e m -p e r a t u r e o f 5 50 C u n d e r o x y g e n a t m o s p h e r e , a n d f o r

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Figure 3 R e s i s t i v i ty o f Z n O f i lm s a s a f u n c t i o n o f s u b s t r a t e t e m -p e r a t ur e . ( ) o x y g e n a n d ( . . . ) o x y g e n p l u s w a t e r v a p o u ra t m o s p h e r e .1 3 5 2

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T -1 x 10 3 (K)Figure 4 C o n d u c t i v i t y a g a i n s t I/T f o r Z n O f i l m s p r e p a r e d a t d i f-f e r en t s u b s t r a t e t e m p e r a t u r e s i n a n o x y g e n a t m o s p h e r e : ( O ) 5 0 0,( z x ) 5 5 0 a n d ( E l ) 6 0 0 C .

a n o t h e r a m i n i m u m v a l u e o f 2 .2 0 D c m w a s f o u n d f o ra d e p o s i t io n u n d e r t h e o x y g e n pl u s w a t e r v a p o u ra t m o s p h e r e . A t a s u b s t r a t e t e m p e r a t u r e o f 5 00 C , t h er e s i s t iv i t y w a s a b o u t 5 0 f~ c m a n d w i t h i n c r e a s e i n t h et e m p e r a t u r e , i t d e c r e a s e d t o a m i n i m u m r e s i s t i v i t y t o5 5 0 C i n t h e o x y g e n a t m o s p h e r e . F u r t h e r , f o r a h i g ht e m p e r a t u r e o f 5 7 5 C t h e r e s is t iv i ty a g a i n i n c r e a se d .T h i s t e n d e n c y i s a l s o s h o w n i n t h e f i l m f o r m i n g c o n -d i t i o n s w i t h o x y g e n p l u s w a t e r v a p o u r a t m o s p h e r e s .T h e r e s i s ti v i ti e s o f th e a s - d e p o s i t e d f i l m s a r e h i g h a n dt h i s i s p r o b a b l y d u e t o t h e g r a i n b o u n d a r y e f f e c ts s i n c et h e f i l m s a r e p o l y c r y s t a l l i n e i n s t r u c t u r e [ 1 1 ] .

S o m e s a m p l e s f o r v a r i o u s d e p o s i t io n t e m p e r a t u r e sw e r e s u b j e c t e d t o o b t a i n a c t i v a t i o n e n e r g y i n e l e c t r i c a l

1o 0

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Figure 5 C o n d u c t i v i t y a g a i n s t I/T f o r Z n O f i lm s p r e p a r e d a t d i f-f e r e nt s u b s t r a t e t e m p e r a t u r e s i n o x y g e n p l u s w a t e r v a p o u r a t m o s -p h e r e s : ( O ) 5 0 0 , ( z x ) 5 5 0 a n d ( E l) 6 0 0 C .

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cond uction and their temperature dependen ce of theelectrical conductivity were studied in a temperaturerange from 81 to 264K as shown in Figs 4 and 5.From the linear parts of these curves, the apparentactiva tion energies E~ in the range 0.03 t o 0.09 eV wereobtained for different atmospheric condit ions in thef ilm format ion . T hese data are comparab le w i th thosefor ZnO films prepared by spray pyrolysis [11] . H ow -ever, these curves c learly deviate from the l inearrelation ship for low er temperatures (-%< 00 K ). Th issuggests that there is a possibi l i ty of variable rangehop ping occurring [12, 13] of local ized e lectrons in thefi lms, since in case of variable range h opping condu c-t iv ity i s expec ted to obey e x p ( - A T -~/("+~)) where Aand n are a constant and dimensional number incond uction , respectively. Further invest igations onthis point are under way.R e f e r e n c e s

1. S. PIZZ INI, N. BUTTA, D. NAR DUCC I and M. PAL-LADINO, J. Electrochem. Soc. 136 (7) (1989) 1945.

2. G. S. KINO and R. S. WAG ERS, J. Appl . Phys. 44(1973) 1480.

3. A. R. HUT SON , Ph ys . Rev . Le t t . 4 (1960) 505.4. T. MINAM1 , H. NAN TO and S. TAKA TA, Th i n S o l i d

Fi l m s 124 (1985) 43.5. T. TAKAG I, I. YAMADA, K. MATSU BARA a n d

H. TAKAOKA, J. Cryst . Growth 45 (1978) 318.6. S. MAJOR , A. BANE RJEE and K. L. CHOP RA, Thin

S o l i d F i l m s 1 0 8 (1983) 333.7. M. SHIM IZU, T. HORI I, T. SHIO SAKI a n d

A. KAWABATA, ibid. 96 (1982) 149.8. J . R. SHEALY, B.J . BALIGA, R. J. FIELD a n d

S. K. GHANDHI, J. Eleetrochem. Soc. 128 (1981) 558.9. K. KAMATA and S. MATSUM OTO, Yo g yo -Kyo ka i - sh i

8 9 (1981) 337.10. S. K. GH AN DI and R. J. FIEL D, Ap p l . Ph ys . Le t t . 37

(1980) 449.11. M. N. ISLAM , M. O. HAK IM and H. RAH MAN , J.

Ma t . S c i . 22 (1987) 1379.12. N. F. MOTT , Ph i l. Ma g . 19 (1969) 835.13. W. BRENIG, G. H. DOHLE R and H. HEYS ZENAU ,

ibid. 27 (1973) 1093.Received 7 March 1990and accepted 12 June 1990

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