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ELECTRICAL AND OPTICAL PROPERTIES OF HYDROGENATED AMORPHOUS CARBON
D. Jones, A. Stewart
To cite this version:
D. Jones, A. Stewart. ELECTRICAL AND OPTICAL PROPERTIES OF HYDROGENATED AMORPHOUS CARBON. Journal de Physique Colloques, 1981, 42 (C4), pp.C4-1085-C4-1088.
�10.1051/jphyscol:19814238�. �jpa-00220869�
D . I . J o n e s and A.D. Stewart
Camegie Laboratory of Physics, The University, ~ u n d e e , DD1 4HN, ScotZand, U.K.
Abstract.- Carbon f i l m s have been prepared by t h e decomposition of methane, e t h a n e , e t h y l e n e and a c e t y l e n e i n a n r . f . glow d i s c h a r g e . E l e c t r i c a l conducti- v i t y and o p t i c a l a b s o r p t i o n measurements show a s t r o n g dependence on s u b s t r a t e temperature and on t h e gas used, whereas t h e hydrogen c o n t e n t of t h e f i l m s i s found t o be independent of t h e s e parameters. The a d d i t i o n of phosphine o r d i b o r a n e d u r i n g p r e p a r a t i o n i n c r e a s e s t h e c o n d u c t i v i t y .
1 . I n t r o d u c t i o n .
During r e c e n t y e a r s t h e r e h a s been a growing i n t e r e s t i n t h e d e p o s i t i o n of amorphous f i l m s by t h e glow d i s c h a r g e decomposition of a s u i t a b l e gas o r mixture of gases. I n p a r t i c u l a r , t h e s u c c e s s f u l doping of amorphous s i l i c o n ( 1 ) h a s added t o t h e importance of t h e glow d i s c h a r g e t e c h n i q u e f o r producing amorphous semiconducting d e v i c e s .
Unlike s i l i c o n o r germanium, which a r e g e n e r a l l y prepared by t h e decomposition of s i l a n e o r germane, hydrogenated carbon f i l m s have been produced by t h e decomposi- t i o n of a v a r i e t y of hydrocarbon g a s e s such a s a c e t y l e n e (2) ( 3 ) , methane ( 4 ) , buta- n e ( 4 ) and e t h y l e n e (5). The p r o p e r t i e s of such f i l m s can d i f f e r widely, b u t under a p p r o p r i a t e c o n d i t i o n s , f i l m s may be made which a r e mechanically extremely hard and chemically i n e r t .
I n t h e i r r e c e n t work Meyerson and Smith (3) observed a s t r o n g dependence of t h e e l e c t r i c a l c o n d u c t i v i t y o on t h e s u b s t r a t e temperature Td, which was a s c r i b e d t o a lower hydrogen c o n c e n t r a t i o n a t t h e h i g h e r Td. They have a l s o modified o by a f a c t o r of 105 on adding 1% phosphine o r 10% d i b o r a n e t o t h e a c e t y l e n e p r i o r t o dep- o s i t i o n ( 6 ) . I n t h i s paper we p r e s e n t t h e r e s u l t s of d . c . c o n d u c t i v i t y and o p t i c a l a b s o r p t i o n measurements on carbon f i l m s prepared from v a r i o u s hydrocarbon g a s e s i n a r . f . glow d i s c h a r g e .
2. Sample p r e p a r a t i o n .
The carbon f i l m s i n v e s t i g a t e d were d e p o s i t e d i n a c a p a c i t i v e l y coupled r . f . glow d i s c h a r g e p r e p a r a t i o n u n i t s i m i l a r t o t h a t used i n t h e p r e p a r a t i o n of amorphous s i l i c o n ( 7 ) . The gases used were methane, e t h a n e , e t h y l e n e and a c e t y l e n e and t h e specimens were d e p o s i t e d on s u b s t r a t e s of q u a r t z o r 7059 g l a s s a t t e m p e r a t u r e s i n t h e range from 1500C t o 3200C. Film t h i c k n e s s e s l a y between 0 . 1 um and 2.5 pm and d e p o s i t i o n r a t e s v a r i e d from 0.05 AS-' t o 1.1
AS-^.
The l a t t e r depended on Td and t h e gas used; t h e lowest d e p o s i t i o n r a t e s were found f o r h i g h Td specimens from methane. Specimens were a l s o prepared from gas mixtures c o n t a i n i n g up t o 1% phos- phine o r di5orane.3 . Experimental r e s u l t s . 3.1 E l e c t r i c a l c o n d u c t i v i t y .
The e l e c t r i c a l c o n d u c t i v i t y 5 f o r most specimens was measured a s a f u n c t i o n of temperature between 500C and 3700C. Evaporated aluminium e l e c t r o d e s were used i n a gap c e l l c o n f i g u r a t i o n whenever p o s s i b l e , b u t f o r specimens w i t h a c o n d u c t i v i t y
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19814238
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F i g . 1 . E l e c t r i c a l c o n d u c t i v i t y U a s a f u n c t i o n of 1 0 3 / ~ f o r specimens p r e p a r e d from methane a t d i f f e r e n t s u b s t r a t e temp- e r a t u r e s T
d .
lower t h a n t h a t of g l a s s o r q u a r t z , measurements were made i n a sandwich c e l l w i t h e v a p o r a t e d chromium o r molybdenum a s a bottom e l e c t r o d e and aluminium a s a t o p c o n t - a c t .
F i g . 1 shows t h e v a r i a t i o n of a w i t h 1/T f o r a s e r i e s of specimens p r e p a r e d from methane a t d i f f e r e n t Td; s i m i l a r c u r v e s were o b t a i n e d f o r specimens d e p o s i t e d from
t h e o t h e r g a s e s . Most l o g o v s 1/T c u r v e s show some c u r v a t u r e i n t h e t e m p e r a t u r e r a n g e i n v e s t i g a t e d , which i s more pronounced f o r t h e lower Td samples. F o r t h e pur- pose o f c o r r e l a t i o n 02.0, t h e c o n d u c t i v i t y measured a t
1 0 ~ 1 ~
= 2.0 (T = 2270C), i s p l o t t e d a s a f u n c t i o n of Td i n f i g . 2 f o r methane (M), e t h a n e ( E ) , a c e t y l e n e (A) and e t h y l e n e ( E y l ) . ~ 2 i s s t r o n g l y dependent on Td, t h e g r e a t e s t o b s e r v e d change . ~ o c c u r s i n specimens p r e p a r e d from e t h y l e n e , where ~ 2i n c r e a s e s from 1 0 - ' ~ G - l . ~ cmm1 t o 10-3 G - l cm-l a s Td i n c r e a s e s from 2300C t o 3000C.The lower graph i n f i g . 3 shows t h e a c t i v a t i o n e n e r g y E, a s a f u n c t i o n of Td f o r t h e f o u r g a s e s . E, i s d e t e r m i n e d from t h e a l m o s t l i n e a r p o r t i o n of t h e c o n d u c t i v i t y c u r v e s a t t e m p e r a t u r e s above 227OC. I t i s dependent on Td and o n t h e p r e p a r a t i o n g a s and changes b y a s much a s 0 . 7 eV o v e r t h e r a n g e of Td. I n s p i t e of t h e s t r o n g dep- endence on Td, a n n e a l i n g up t o 370% d i d n o t change a o r E,.
F i g . 2 . E l e c t r i c a l c o n d u c t i v i t y measured a t F i g . 3 . Eopt and E, a s f u n c t i o n s of 227%' a s f u n c t i o n s of s u b s t r a t e t e m p e r a t u r e s u b s t r a t e t e m p e r a t u r e T
Td f o r specimens p r e p a r e d from methane ( M ) , d .
e t h a n e ( E ) , a c e t y l e n e (A) and e t h y l e n e ( E y l ) .
Specimens have been prepared from methane, e t h a n e and e t h y l e n e c o n t a i n i n g up t o 1% phosphine and from methane w i t h up t o 1% d i b o r a n e . The changes found i n t h e e l e c - t r i c a l c o n d u c t i v i t y were dependent on Td; f o r example, f o r Td = 1800C t h e i n c r e a s e was t y p i c a l l y by two o r d e r s of magnitude i n 02.0, accompanied by a drop of about 0 . 2 eV i n E ~ .For h i g h e r v a l u e s of Td t h e i n c r e a s e i n 02.0 was reduced t o one o r d e r of magnitude. The o p t i c a l gap of such specimens was unchanged. S i m i l a r changes i n 5 were observed when l i t h i u m was d i f f u s e d i n t o undoped samples.
3.4 Hydrogen Content.
The hydrogen c o n t e n t , CH = nH/(nc
+
nH), of a number of carbon f i l m s prepared from t h e d i f f e r e n t g a s e s has been measured by t h e p r e v i o u s l y used e v o l u t i o n tech- nique ( 8 ) . Table 1 shows CH f o r a s e l e c t i o n of specimens. For a l l specimens CH v a l u e s were between 30% and 45% which compare favourably w i t h p r e v i o u s l y p u b l i s h e d v a l u e s ( 4 ) ( 9 ) ( 1 0 ) . No s y s t e m a t i c v a r i a t i o n i n CH was observed between f i l m s prepared under v a r i o u s c o n d i t i o n s .TABLE 1.
Gas
Methane
Ethane
Ethylene
Acetylene
4 . Discussion.
The s i g n i f i c a n t f e a t u r e of t h e r e s u l t s i s t h e s t r o n g dependence of o and E, on Td, not u n l i k e t h e r e s u l t s of Meyerson and Smith ( 3 ) who s t a t e t h a t t h e dependence i s due t o a d e c r e a s i n g i n c o r p o r a t i o n of hydrogen as Td i s i n c r e a s e d . This c o n c l u s i o n i s n o t supported by t h e CH v a l u e s shown i n Table 1. The t a b l e a l s o g i v e s t h e temperat- u r e Tmin a t which hydrogen was f i r s t evolved. T,;, i s dependent on t h e gas and t e n d s t o i n c r e a s e w i t h i n c r e a s i n g Td. This could mean t h a t a s i n a-Si (11) t h e hydrogen may be bonded i n d i f f e r e n t c o n f i g u r a t i o n s . Changes i n Td a r e l i k e l y t o a l t e r t h e i r r e l a t i v e p r o p o r t i o n s .
Although E, and a r e dependent on b o t h Td and t h e g a s , f i g . 4 shows t h a t
-
remains f a i r l y c o n s t a n t f o r a l l t h e specimens. T h i s , t o g e t h e r w i t h t h e f a c t t h a t t h e t h e r m o e l e c t r i c power i s n e g a t i v e f o r undoped specimens, s u g g e s t s t h a t t h e energy l e v e l diagram i n f i g . 4 may b e a p p r o p r i a t e . I t i m p l i e s t h a t t h e p o s i t i o n of t h e Fermi l e v e l , E F remains f i x e d r e l a t i v e t o t h e v a l e n c e band ( 1 2 ) , whereas t h e? o s i t i o n of t h e conduction p a t h , CP, i s s t r o n g l y dependent on Td and t h e gas. The p r e e x p o n e n t i a l f a c t o r s U,, c a l c u l a t e d from 02.0 and c. l i e i n t h e range 1 0 - ~ S ? - ~ c m - ~ t o 1 R-' cm-', although i n t h e h i g h e r Td methane specimens oo exceeds 10 S?-I cm-l.
I t i s t h e r e f o r e p o s s i b l e t h a t f o r some of t h e s e specimens t h e conduction p a t h a t h i g h e r temperatures l i e s i n t h e extended s t a t e s . However, w i t h d e c r e a s i n g T, t h e predominant t r a n s p o r t p a t h i s l i k e l y t o move downward towards oF i n a l l specimens.
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Conduction i s t h e n by t h e r m a l l y a c t i v a t e d hopping i n a f a i r l y h i g h d e n s i t y o f l o c a l - i z e d s t a t e s . The l a t t e r i s s u p p o r t e d b y f i e l d e f f e c t measuremencs which l e a d t o h i g h v a l u e s f o r and t h u s a c c o u n t f o r t h e r e l a t i v e i n s e n s i t i v i t y o f t h e mat- e r i a l t o doping.
€ "P'
- c
1.2(eV) C.8 0.6- 0 4 0.2
Acknowledgements.
The a u t h o r s a r e i n d e b t e d t o t h e l a t e W . King f o r h i s h e l p w i t h specimen p r e p a r - a t i o n and c o n d u c t i v i t y measurements. They would a l s o l i k e t o t h a n k W.E. S p e a r and P.G. Le Comber f o r many h e l p f u l d i s c u s s i o n s and DRES f o r s u p p o r t o f t h i s p r o j e c t .
O153 ' MO 2% 300
T~ ('C)
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-
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