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THE PREPARATION AND PROPERTIES OF PECVD-MADE SnO2 THIN FILMS
Y. Chen, Z. Zheng, S. Li, H. Ma
To cite this version:
Y. Chen, Z. Zheng, S. Li, H. Ma. THE PREPARATION AND PROPERTIES OF PECVD- MADE SnO2 THIN FILMS. Journal de Physique Colloques, 1989, 50 (C5), pp.C5-605-C5-611.
�10.1051/jphyscol:1989571�. �jpa-00229603�
THE PREPARATION AND PROPERTIES OF PECVD-MADE SnOz THIN FILMS
Y.P. CHEN, Z.Q. ZHENG, S.Y. L1 and H.L. MA
Department -of P h y s i c s , Shandong U n i v e r s i t y , J i n a n , P.R. China
Resume Oes f i l m s de Sn02 n o n dop6 d e h a u t e s q u a l i t C s , b appl i c a t i o n s p o t e n t i e l l e s p o u r l e s c e l l u l e s s o l a i r e s au s i l i c i u m amorphe, o n t 6 t i d d p o s i s s u r un v e r r e s o d o c a l c i q u e Q b a s s e t e m p e r a t u r e [ 2 0 0 ~ C l b p a r t ir du systhme SnCI4-O2 dons un systhme a c t i v e p a r plasma. .Les f i lms o n t , en moyenne une r e s i s t a n c e de couche p a r c a r r 6 de 50 Q e t une t r a n s p a r e n c e opt ique super i e u r e b 90% dans I e domai ne 400-8oonm. L ' i n f l u e n c e d e s p r i nc i paux p a r a m k t r e s de depat s u r l e s p r o p r i e t 6 s o p t iques e t G l e c t r i ques a e t e e t u d i e e ,
Abstract
-
High q u a l i t y f i l m s o f undoped Sna2,
which have p o t e n t i a l app- l i c a t i o n s f o r a-Si s o l a r c e l l s , were deposited on soda lime glass a t t h e r e l a t i v e l y low temperature o f 2 0 0 ' ~ from t h e anhydrous Sn Cl4-O2 r e a c t i o n system by plasma-enhanced chemical vapour d e p o s i t i o n (PEcVD). Typically, t h e f i l m s have a sheet r e s i s t a n c e o f 50 ohm p e r square w i t h an average o p t i c a l transparence o f more t n a n 9046 i n t h e 400-800nm wavelength range.The i n f l u e n c e s
of
t h e main c o n t r o l d e p o s i t i o n parameters on e l e c t r i c a l and o p t i c a l p r o p e r t i e s o f t h e r e s u l t i n g S& f i l m s were studied.Sn02 t h i n f i l m s are c u r r e n t l y a c t i v e l y investigates s i n c e t h e i r high t r a n s p a r - ency and e l e c t r i c a l c o n d u c t i v i t y , and chemical s t a b i l i t y would be o f unique value i n numerous o p t o e l e c t r o n i c a p p l i c a t i o n s . These p r o p e r t i e s a r e shared w i t h o n l y a few o t h e r m a t e r i a l s , and many techniques f o r t h e i r d e p o s i t i o n s have t h e r e f o r e been developed. Deposition temperature u s u a l l y ranging from 450 t o 7 5 8 ~ are necessary i n t h e s e processes. S p u t t e r i n g i s one o f t h e most e x t e n s i v e - l y used t e c h n i q u e s f o r t h e d e p o s i t i o n o f t r a n s p a r e n t conducting oxide films.
Although lower temperature a r e used i n t h e r f s p u t t e r i n g , t h e p r o p e r t i e s o f t h e r e s u l t i n g f i l m s a r e very d i f f i c u l t t o c o n t r o l . Ohandi e t al. ( l ) have reported
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1989571
C5 -6 06 JOURNAL DE PHYSIQUE
h i g h i y t r a n s p a r e n t f l l m s o f Sn02 deposited a t room temperaturn and base on the r e a c t i o n o f r f plapnna-actived oxygen w i t h tetramethyltin. I n t h i s l e t t e r a new technique i s presented f o r t h e d e p o s i t i o n o f Sn02 f i l m s w i t h both high transm- i s s i o n and c o n d u c t i v i t y , based upon t h e r e a c t i o n o f Sn C l 4 w i t h oxymn a t lower temperature o f 2 0 0 ' ~ by plasma-enhanced chemical vapour d e p o s i t i o n (PEcVD).
2. Experimental
The d e p o s i t i o n o f Sn02 f i l m s was performed i n a v e r t i c a l r e a c t o r , 30cm i n diameter. r f e x c i t a t i o n a t 13.75 MHz was used t o c ~ e a t e t h e oxygen plasma i n this system. A mechanical pump r e s u l t e d i n a p r e s s u r e below I O - ~ t o r r . A t o t a l system p r e s s u r e o f l . 5 t o r r was maintained f o r all d e p o s i t i o n described i n t h i s work. TO o b t a i n uniform f l l m s , t h e grounded p e d e s t a l , on which t h e samples were l o c a t e d , r o t a t e d at t h e speed of 10 c y c l e s p e r minute d u r i n g grow.th. Nitrogen w a s bubbled through t h e s a t u r a t o r c o n t a i n i n g t h e anhydrous Sn C l 4 source which w a s maintained at t h e temperature o f '45'~. Each o f t h e gases used i n t h i s work w a s fed t o chamber through a needle valve, and monitored by means o f a flow gauge. Chamber p r e s s u r e was measured by a thermocouple gauge. A round polyflon plank, 45mm i n diameter, w a s placed between t h e two e l e c t r o d e p l a t e s . T h i s served t o e f f e c t i v e l y change t h e gas d i s t r i b u t i o n w i t h i n t h e r e a c t i o n chamber, and g r e a t l y improve t h e uniformity o f t h e deposited Sn02 f i l m s .
I n o p e r a t i o n , t h e system was pumped down t o a base p r e s s u r e o f below 10- 2 t o r r . Nitrogen c a r r y i n g Sn C l 4
,
oxygen and a d d i t i o n a l n i t r o g e n were t o g e t h e r introduced t o t h e chamber u n t i l t h e t o t a l system pressure o f 1.5 t o r r w a s achi- eved. Then, t h e r f power was applied. A p l a t e v o l t a g e and c u r r e n t o f 1200.V and 70mA,respectively, were maintained d u r i n g these experiments. T h i s was followed by plasma e x c i t a t i o n f o r t h e d e s i r e d d e p o s i t i o n time. 1.5rnm t h i c k 4.0 X5.0cm soda lime g l a s s e s , on which Sn02 f i l m s were deposited, were employed. The sub- s t r a t e temperature w a s monitored during t h e growth run f o r evidence o f change.For o u r work, t h i s temperature changed by l e s s t h a n Io0C o v e r an e n t i r e run.
3. R e s u l t s and Discussion
3.1 F i l m s o f undoped Sn02 prepared by PECVD at optimum growth c o n d i t i o n s a r e f a i r l y uniform. Scanning e l e c t mm mic m g r a p h s a t 10,000X m a g n i f i c a t i o n e x h i b i t a very fine-grained s i z e of about 0 . 2 ~ ~ . X-ray d i f f r a c t i o n p a t t e r n s r e v e a l
3.2 The e f f e c t s o f d e p o s i t i o n v a r i a b l e s on f i l m p r o p e r t i e s were explored t o f i - nd optimum p r e c e s s conditions. The v a r i a b l e s i n c l u d e source and s u b s t r a t e tem- ) e r a t u r e s , r e a c t a n t and c a r r i e r g a s flow r a t e s , and growth P a t e s of t h e f i l m s .
Ng.1 shows t h e r e l a t i o n between f i l m t h i c k n e s s and growth time f o r Sn C l 4 and O 2 flow o f 1.5 and 0.4 lmin-l, r e s p e c t i v e l y , and corresponds t o a growth r a t e o f 103 h n - I f o r t h i s s e t o f experimental conditions. N l m t h i c k n e s s was
FIG.l..Film thickness versus growth time
measured by means o f a multiple-beam i n t e r f e r o m e t e r . I n o u r system, t h e h i g h e r growth r a t e (about 100-1 30 Ijmin'l) a t low pressure i s p r i m a r i l y due t o t h e high e l e c t e o n temperature achieved because o f t h e r f e x c i t a t i o n , but changing t h e
s u b s t r a t e temperature was found t o have a s t r o n g influence on the c o n d u c t i v i t y of t h e f i l m . F i l m s deposited a t low s u b s t r a t e temperature showed high s h e e t r e s i s t a n c e Rsh and hence low figure o f m e r i t Q ( e q u a l t o Rah LnT ( 2), T-average v i s i b l e t r a n s m i s s i o n i n t h e 400-800nm wavelength range). The low c o n d u c t i v i t y may be due t o small f i l m t h i c k n e s s caused by low d e p o s i t i o n r a t e and presence o f unreacted s t a n n i c chloride. S u b s t r a t e temperatures i n e x c e s s o f 1 5 0 ~ ~ i n o u r
system were found necessarg t o o x i d i z e Sn C l 4 vapour f o r forming t i n oxide f i - l m s . 811 works were conduoted at a fixed temperature o f 2 0 0 ~ ~ . which w a s found t o y i e l d optimum d e p o s i t i o n r a t e s and e x c e l l e n t p r o p e r t i e s o f t h e f i l m s .
Fig. 2 g i v e s a p l o t o f d e p o s i t i o n r a t e v e r s u s n i t m e n flow c a r r y i n g Sn Cl4 vapour. The growth r a t e i n i t i a l l y i n c r e a s e s s h a r p l y with i n c r e a s i n g Sn C14f10w f r o m 0 t o 0.4 lmin-l. L a t e r i t s a t u r a t e s t o a value o f about 150
bin-l.
TheC5-608 JOURNAL DE PHYSIQUE
mechani m o f Sn02 film fonnation h a s been thoroughly examined by Gho shtagore ( 3 1.
1
O2 flow: 0.4 lrnin-l
0 U
U /
-
%-
W 5 0
a :
2 P 0 t
0.5 1 1-0 1.5 2.0, 2.5 1
o N2 FLOW CaiRYINC SnCLk (LMIN-l) FIC.2. Deposition r a t e v e r s u s N2 flow
c a r r y i n g SnC14 vapour.
According t o Rideal-Eley mechanism, t h e r e a c t i o n o f Sn C l 4 h a s been found t o t a k e p l a c e at t h e s u r f a c e o f t h e s u b s t r a t e w i t h an absorbed oxygen atom. obvi- ously, when t h e e x c i t a t i o n power i s given, i n c r e a s i n g Sn C l 4 vapour r e a c t s t o fixed atomic oxygen on t h e s u r f a c e of t h e s u b s t r a t e , forming Sn02 f i l m w i t h i n c r e a s i n g thickness. However, r e a c t o r o p e r a t i o n a t Sn C l 4 flow i n e x c e s s of 2.5 lmin-'led t o formation o f l a r g e amounts o f p a r t i c u l a t e m a t t e r i n t h e system,
r e s u l t i n g i n hazy films. T h i s a p p e a r s t o be the l i m i t i n g f a c t o r f o r t h e i n c r e a - s i n g i n NZ flow c a r r y i n g Sn Cl4.
Fig.3 o f f e r s t h e denpendence o f Rsh, T and Q on oxygen flow. A s seen i n Fig.', oxygen flow h a s t h e s t r o n g e r i n f l u e n c e on Rsh, and t h e weaker on T. To r e c e i v e h i g h figure of m e f i t , i t i s concluded t h a t t h e optirmun oxygen flows ran- ge between 0.2-0.6 lain-'.
reaction-time : 30min Sn~l,, flow: 1 ,S lmin- 1
FIC.3. Rsh ( A ) , T ( 0 ) and Q (.) v e r s u s
o2
f l o wthrough Sn C l 4 source. Films deposited at the low nitrogen flow ( i . e . 1 0 ~ Sn C l 4 concentration) show high Rsh and hence low Q-The f i l m s made between 1.5-2.5 lmin-I a r e s u p e r i o r with high transmission, low sheet r e s i s t a n c e and hence high
figure o f merit. Higher Sn C l 4 flows l e d t o t h e d e t e r i o r a t i o n o f t h e f i l m c h a r a c t e r i s t i c s a s s a i d above. reaction-time :,30min
100 ,log- O2 flow: 0.4 lrnin-'
FIG.4. R s h ( A ) , T ( 0 ) and Q (0) versus N2 flow carrying SnC14 vapour.
Fig. 5 d e p i c t s transmission s p e c t r a i n t h e wavelength o f 400-800m f o r t h r e e Sn02 f i l m s f a b r i c a t e d a t t h r e e d i f f e r e n t oxygen flow r a t e s : 0.2, 0.4, 0.85 lain", and t h e same Sn C l 4 flow o f 1.5 1min-l. F i l m s produced a t the oxygen
flow r a t e s between 0.2-0.7 lmin-I &ways have high
averse
l i g h t transmission o fo f more than 90% and low sheet r e s i s t a n c e / l e s s than 60 ohm pec square. A t h i - gher oxygen flow r a t e s (more than 1.0 lmin-' ), the d e p o s i t i o n r a t e was high and the f i l m s become foggy, r e s u l t i n g i n a i n c r e a s e i n r e s i s t a n c e , a decrease i n transmission and hence i n figure o f merit a s shown i n Fig.3.
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FIC.5. Transmission s p e c t r a f o r t h r e e SnO f i l m s made a t t h r e e d i f t e r e n t O2 f l o y l r a t e s :
a - - 4 . 2 lmin 1 b - - 4 . 4 lmin--l c---0.85 lmin
I t i s well-known t h a t the- o n l y way t o o b t a i n good t r a n s p a r e n t conductors i s t o c r e a t e e l e c t r o n degeneracy i n a wide band gap ( g r e a t e r t h a n 3eV) oxide by
c o n t r o l l a b l y i n t r o d u c i n g non-stoichiometry and/or a p p r o p r i a t e dopants. The n-type c o n d u c t i v i t y o f undoped S O 2 i s p r i m a r i l y due t o i t s non-stoichiometry.
But f o r o u r work, i n t h e d e p o s i t e d f i l m s fmm chlorde (Sn
cl4)
t h e c o r p o r a t i o n o f c h l o r i n e i o n s i n t o t h e l a t t i c e a l s o c o n t r i b u t e s t o t h e conductivity.F i n a l l y , fast and low temperature ( 4 0 0 ' ~ ) a n n e a l i n g w i t h forming gas i n an open q u a r t z tube system was e f f e c t i v e l y employed t o i n c r e a s e t h e c o n d u c t i v i t y o f t h e f i l m d e p o s i t e d n o t a t t h e optimum conditions. It w a s also observed t h a t a n n e a l i n g h a s a minor i n f l u e n c e s on t h e f i l m o f t h e optimum growth condition.
3.3 The s u r f a c e compositions o f Sn02 f i l m s were observed w i t h X-ray photoelec- t r o n spectmscopgr (xPs). From XPS measurements, i t i s found t h a t elements e x i s t i n g on t h e s u r f a c e o f Sn02 f i l m s were S n ( I V ) , Sn(11), Oand C. C h l o r i n e s were n o t observed on t h e s u r f a c e , which may be i n c o r p o r a t e d i n t o Sn02 l a t t i c e e i t h e r i n t e r s t i t i a l l y o r s u b s t i t u t i o n a l l y (49. According t o t h e t r a n s p o r t p r o p e r t y modelf 5
1,
f r e e c a r r i e r s may be trapped on s u r f a c e states. These s u r f - ace s t a t e s h e r e were i d e n t i f i e d t o be ~ n ( I 1 ) .3.4 F-doped S O 2 f i l m s were a l s o made and e x c e l l e n t results were obtained. Fr- eon, as F dopant, w a s used t o pmduce F-doped Sn02, which g r e a t l y decrease t h e
sheet r e s i s t a n c e , and i n c r e a s e t h e o p t i c a l t r a n m i s s i o n o f t h e f i l m s . 'fYpica1- l y , t h e F-doped Sn02 f i l m s have a s h e e t r e s i s t a n c e o f 20 ohm p e r square and an
ge. F u r t h e r e f f o r t s are been done on t h e F-doped Sn02 films.
T h i s p a p e r d e s c r i b e s a new CVD p r o c e s s f o r t h e d e p o s i t i o n o f h i g h l y conduc- t i v e Sn02 f i l m s based on t h e o x i d a t i o n o f Sn Cl4. Undoped f i l m s w i t h Rsh o f 50 ohm p e r square and average t r a n s m i s s i o n o f more t h a n 90% were o b t a i n e d at o p t i - mum o o n d i t i o n s . The F-doped Sn02 f i l m s e x h i b i t e x c e l l e n t s h e e t r e s i s t a n c e and o p t i c a l t r & m a i s s i o n . T h i s new t e c h n i q u e h a s t h e a d v a n t a g e s o f &mple a p p a r a t u s , low d e p o s i t i o n t e m p e r a t u r e , and good r e p m d u c i b i t y . We can conclude t h a t t h e Sn02 f i l m m a d e by PECVD, which are used as t r a n s p a r e n t e l e c t r i c a l l y c o n d u c t i n g a n t i r e f l e c t i o n f i l m s o f a-Si s o l a r c e l l s , a r e v e r y promising m a t e r i a l s .
REFERENCE S
I . S.K. Ghandi, R.Siviy and J.M. Borrego, Pgpl. Rhys. L e t t . , 34(1979)833 2. V.K. J a i n and A.P. Kulshreshtha, Sol Energy Mater., 4(1981)151
3. R.N. Ohoshtagore, J. Electrochem. Soc. , l 25(1978) 110 4. J.A. Aboaf and V.C. Marcotte, T h i s J o u r n a l , 120,701(9973)
5. D. B e l a n m r , J.P. Dodelet, B.A. Lombos, and J. I. Dickson,J.'Electrochem. Soc., 132, I 398( 1985)
