ELECTRONIC PROPERTIES OF MICROCRYSTALLINE SILICON FILMS PREPARED IN A GLOW DISCHARGE PLASMA

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ELECTRONIC PROPERTIES OF

MICROCRYSTALLINE SILICON FILMS PREPARED IN A GLOW DISCHARGE PLASMA

W. Spear, G. Willeke, P. Le Comber, A. Fitzgerald

To cite this version:

W. Spear, G. Willeke, P. Le Comber, A. Fitzgerald. ELECTRONIC PROPERTIES OF MICRO-

CRYSTALLINE SILICON FILMS PREPARED IN A GLOW DISCHARGE PLASMA. Journal de

Physique Colloques, 1981, 42 (C4), pp.C4-257-C4-260. �10.1051/jphyscol:1981454�. �jpa-00220911�

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JOURNAL DE PHYSIQUE

CoZZoque C4, suppzgment au nO1O, Tome 42, octobre 1981 page C4-257

ELECTRONIC PROPERTIES OF MICROCRYSTALLINE SILICON FILMS PREPARED I N A GLOW DISCHARGE PLASMA

W.E. Spear, G. Willeke, P.G. Le Comber and A.G. F i t z g e r a l d

Camegie Laboratory of Physics, the University, Dundee DDL 4HN, Scotland, U.K.

A b s t r a c t .

-

The p r o p e r t i e s of glow d i s c h a r g e uc S i have been i n v e s t i g a t e d by c o n d u c t i v i t y , H a l l e f f e c t and f i e l d e f f e c t measurements a s a f u n c t i o n of c r y s t - a l l i t e s i z e and phosphorus doping r a t i o . It i s concluded t h a t t h e l a r g e inc- r e a s e i n a over a-Si i s almost e n t i r e l y caused by an i n c r e a s e d c a r r i e r d e n s i t y r e s u l t i n g from d e l o c a l i s e d e l e c t r o n t a i l s t a t e s . The s i g n of t h e H a l l c o n s t a n t remains normal down t o an e x t r a p o l a t e d c r y s t a l l i t e s i z e of about 20A.

1. I n t r o d u c t i o n .

The work of Vepr'ek and h i s c o l l a b o r a t o r s (1) has shown t h a t by chemical t r a n s - p o r t i n a hydrogen glow d i s c h a r g e plasma, m i c r o s c y s t a l l i n e (pc) l a y e r s of S i can b e d e p o s i t e d a t a s u b s t r a t e temperature as low as 80°C. The Zurich group suggested t h a t under c o n d i t i o n s whereby a chemical e q u i l i b r i u m i s e s t a b l i s h e d a t t h e plasma-solid i n t e r f a c e , s t a b l e n u c l e i a r e l i k e l y t o b e formed, l e a d i n g t o m i c r o c r y s t a l l i n e growth.

Recently work i n o t h e r l a b o r a t o r i e s (2) ( 3 ) (4) h a s shown t h a t P C f i l m s can a l s o b e d e p o s i t e d from t h e s i l a n e glow d i s c h a r g e , widely used i n t h e p r e p a r a t i o n of a-Si specimens. Chemical e q u i l i b r i u m i n t h i s case i s approached by s t r o n g hydrogen d i l - u t i o n . The p o s s i b i l i t y of c o n t r o l l a b l e pc plasma d e p o s i t i o n could b e of c o n s i d e r a b l e a p p l i e d i n t e r e s t ; i t a l s o p e r m i t s an e x t e n s i o n of fundamental s t u d i e s on t h e e l e c t - r o n i c p r o p e r t i e s j u s t beyond t h e a-phase a s a f u n c t i o n of s t r u c t u r a l o r d e r i n g . I n t h e following we should l i k e t o r e p o r t some of o u r r e c e n t work i n t h i s d i r e c t i o n . 2. P r e p a r a t i o n and S t r u c t u r a l Measurements.

Specimens were prepared a t a s u b s t r a t e temperature of 2 8 0 ' ~ i n a c a p a c i t a t i v e l y coupled d e p o s i t i o n u n i t . The gas mixture c o n s i s t e d of 3% ( o r 2% i n some c a s e s ) of s i l a n e i n hydrogen w i t h t h e r e q u i r e d a d d i t i o n of phosphine i n t h e case of t h e doped specimens. I n t h e l i g h t of Vepfek's i n t e r p r e t a t i o n , t h e s t r o n g d i l u t i o n of t h e s i l - ane by hydrogen enhances t h e r e v e r s e r e a c t i o n ( e r o s i o n ) a t t h e growing i n t e r f a c e and moves t h e r e a c t i o n c l o s e r towards e q u i l i b r i u m . The r e s u l t i n g d e c r e a s e i n t h e depos- i t i o n r a t e h a s been p a r t l y compensated by u s i n g a somewhat h i g h e r R.F. power of 18-25 W . This l e d t o a d e p o s i t i o n r a t e of about 0 . 1 ~ s - l .

The average c r y s t a l l i t e s i z e 6 was determined f o r each d e p o s i t i o n run from t h e half-width of t h e (111) e l e c t r o n d i f f r a c t i o n r i n g by means of t h e S c h e r r e r formula.

The use of e l e c t r o n d i f f r a c t i o n i n t h i s connection was checked by X-ray measurements.

The r e s u l t s show t h a t c r y s t a l l i t e s i z e s i n t h e range from about 18A t o 60A can b e o b t a i n e d from t h e glow d i s c h a r g e . 6 depends c r i t i c a l l y on plasma c o n d i t i o n s , t h e amount of hydrogen d i l u t i o n and, somewhat unexpectedly, on t h e e x c i t a t i o n frequency.

Of t h e 20 glow d i s c h a r g e m i c r o c r y s t a l l i n e (gd-uc) specimens s o f a r s t u d i e d , most of t h e l a r g e r c r y s t a l l i t e s i z e s were produced a t t h e h i g h e s t frequency of 40 MHz.

The p r o p e r t i e s of t h e r m a l l y c r y s t a l l i s e d (th-pc) glow d i s c h a r g e specimens have been i n v e s t i g a t e d i n p r e v i o u s work ( 5 ) . For comparison we have included some measurements on such specimens c r y s t a l l i s e d a t 800°C. T h e i r c r y s t a l l i t e s i z e s l i e above t h o s e of t h e gd-vc samples, g e n e r a l l y between 60A and 100A. On t h e o t h e r end of t h e s c a l e , t h e e l e c t r o n d i f f r a c t i o n l i n e width of t y p i c a l a-Si specimens p l a c e s them a t a nominal 6 between 10 and 12A on t h e S c h e r r e r c o r r e l a t i o n graph. This i s merely an e x t r a p o l a t i o n and no p h y s i c a l s i g n i f i c a n c e should b e a t t a c h e d t o t h i s r e s u l t .

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1981454

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The e l e c t r o n d i f f r a c t i o n p i c t u r e s i n f i g s . l a , b , c demonstrate very c l e a r l y t h e d i f f e r e n c e between t h e a- and t h e gd-PC-phases. ( a ) and (b) compare t h e r e s u l t s from an a-Si and a gd-uc specimen ( 6 s 5 0 A ) taken i n t h e m i c r o d i f f r a c t i o n mode i n a STEM. (b) i s a (110) p a t t e r n from a s i n g l e m i c r o c r y s t a l l i t e and i t s twin o r i e n t a t - i o n . However, f o r t h e d e t e r m i n a t i o n of 6 a high r e s o l u t i o n d i f f r a c t i o n s t a g e was used i n which a l a r g e number of c r y s t a l l i t e s (over an a r e a measuring about 60 urn) c o n t r i b u t e t o t h e r i n g p a t t e r n shown i n ( c ) , o b t a i n e d f o r t h e same specimen as i n (b).

F i g . 1 . E l e c t r o n d i f f r a c t i o n r e s u l t s . ( a ) a-Si, (b) and (c) gd-PC ( 6

-

^50A).

3 . Conductivity.

Fig.2 summarises t h e room temperature c o n d u c t i v i t y gRT as a f u n c t i o n of t h e doping r a t i o f o r a-Si, gd-PC-Si and th-pc-Si. R e s u l t s f o r s e v e r a l undoped specimens (U) have a l s o been i n c l u d e d . I t can b e seen t h a t t h e c o n d u c t i v i t y of t h e gd-uc

specimens i s over two o r d e r s of magnitude h i g h e r t h a n t h a t of t h e doped a-Si and reach- e s 2 0 ( ~ c m ) - ~ a t t h e h i g h e s t doping l e v e l . A l l t h e r e s u l t s r e f e r t o specimens d e p o s i t e d a t 40 MHz. The s p r e a d i n t h e experimental p o i n t s i s caused mainly by d i f f e r e n t c r y s t a l l - i t e s i z e s which, as we s h a l l s e e i n t h e next s e c t i o n , a r e a determining f a c t o r i n t h e c a r r i e r m o b i l i t y . The s o l i d l i n e r e f e r s t o 6 50A, and some of t h e lower c r y s t a l l i t e s i z e s a r e given i n f i g . 2 . Thermally c r y s t a l l - i s e d specimens l e a d t o bRT v a l u e s a t l e a s t an o r d e r of magnitude h i g h e r t h a n those c r y s t a l l - i s e d i n t h e glow d i s c h a r g e . The P C specimens show a c o n d u c t i v i t y a c t i v a t i o n energy which i s t h e sum of E~ and E ~ t h e a c t i v a t i o n ,

- e n e r g i e s a s s o c i a t e d w i t h c a r r i e r d e n s i t y and lo' Ha11 m o b i l i t y . These w i l l b e d i s c u s s e d i n

t h e n e x t s e c t i o n . Fig.2. Room temperature c o n d u c t i v i t y

p l o t t e d a g a i n s t doping r a t i o f o r a-Si, gd-.pc and th-pc S i

.

^Udenotes undoped specimens.

4. H a l l E f f e c t .

E x t e n s i v e H a l l e f f e c t measurements have been c a r r i e d o u t on t h e P C specimens between 440K and 210K, u s i n g t h e technique d e s c r i b e d p r e v i o u s l y i n measurements on a-Si ( 6 ) . The H a l l e f f e c t i n p o l y c r y s t a l l i n e semiconductors has r e c e i v e d a consid- e r a b l e amount of a t t e n t i o n i n t h e l i t e r a t u r e and i s reviewed i n a r e c e n t a r t i c l e by Orton and Powell ( 7 ) . The i n t e r p r e t a t i o n depends t o some e x t e n t on t h e s t r u c t u r a l model assumed f o r t h e m a t e r i a l . As we know l i t t l e about t h e s t r u c t u r a l d e t a i l s , we s h a l l regard t h e e l e c t r o n d e n s i t y n , and e l e c t r o n m o b i l i t y pH, deduced from t h e H a l l e f f e c t a s approximate average v a l u e s over t h e pc specimen.

4.1 C a r r i e r d e n s i t y .

F i g . 3 shows t h e dependence of t h e room temperature c a r r i e r d e n s i t y on t h e gaseous doping r a t i o PH3/SiH4 expressed i n vppm ( s e e t o p s c a l e ) f o r both gd-pc and

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Fig.3. C a r r i e r d e n s i t y (20°C) f o r gd-uc Fig.4. A c t i v a t i o n energy ^E, p l o t t e d specimens p l o t t e d a g a i n s t donor d e n s i t y a g a i n s t n .

(ND)G e s t i m a t e d from t h e gas phase.

th-uc samples. The lower s c a l e i n f i g . 3 g i v e s corresponding donor d e n s i t i e s (ND)G, c a l c u l a t e d on t h e assumption t h a t t h e gaseous doping r a t i o i s maintained i n t h e s o l i d . The r e s u l t s show c l e a r l y t h a t c a r r i e r d e n s i t i e s o b t a i n e d f o r a given doping r a t i o a r e independent of t h e method of c r y s t a l l i s a t i o n and of t h e g r a i n s i z e . The l i n e ND = 0 . 5 ( N ~ ) ~ g i v e s an approximate f i t over s e v e r a l o r d e r s of magnitude. I n agreement w i t h previous r e s u l t s (8) we t h e r e f o r e s u g g e s t t h a t about h a l f t h e phosphine molecules i n t h e plasma i n t r o d u c e a P - s i t e i n t o t h e s o l i d , most of which a c t a s donors i n t h e uc p h a s e . ( 5 ) . A t t h e h i g h e s t ND, p o i n t s tend t o l i e c o n s i s t e n t l y below t h i s l i n e ; t h e e f f e c t a r i s e s most probably from t h e movement of ^~f i n t o t h e

donor band, s o t h a t N ; / N ~ < ~ .

I n f i g . 4 t h e measured a c t i v a t i o n energy E,, a s s o c i a t e d w i t h t h e c a r r i e r den- s i t y , has been p l o t t e d a a i n s t n . We b e l i e v e t h a t t h e i n i t i a l d e c r e a s e of

s n ( =

-

^{f o r n}^<10kacm-3 r e f l e c t s t h e movement of E~ i n t o t h e donor band, b u t t h a t t h e subsequent p a r t of t h e curve shows mainly t h e d e c r e a s e i n t h e donor i o n i s a t i o n energy ( 9 ) . The l a t t e r v a n i s h e s a t ND 1 5 x 1 0 l ~ c m - ~ . 4.2 Haul M o b i l i t y .

Room temperature 1 . 1 ~ v a l u e s f o r t h e gd-uc specimens g e n e r a l l y l i e below lcm2 V - l s e c - l , and a r e l a r g e l y independent of t h e doping l e v e l . However, t h e c o r r e l a t i o n i n f i g . 5 shows t h a t UH d e c r e a s e s w i t h d e c r e a s i n g 6 . The H a l l c o e f f i c - i e n t i n gd-PC-Si i s n e g a t i v e , t h e expected s i g n f o r e l e c t r o n t r a n s p o r t on t h e macroscopic t h e o r y .

u~

e x t r a p o l a t e s t o zero a t a c r i t i c a l g r a i n s i z e of about 20A

and t h e l i n e then p a s s e s through t h e H a l l m o b i l i t y measured i n a-Si w i t h anomolous s i g n ( 6 ) a t t h e nominal 6 = 12A. This r e s u l t i s of c o n s i d e r a b l e i n t e r e s t ; i t pro- v i d e s t h e l i m i t i n g s i z e of t h e ordered r e g i o n s i n t h i s m a t e r i a l f o r which t h e c l a s s - i c a l approach t o t h e H a l l e f f e c t i s a p p l i c a b l e . A t 6 < 20A t h e microscopic i n t e r p - r e t a t i o n d i s c u s s e d by Friedman (10) and Emin (11) become r e l e v a n t .

The temperature dependence of pH i s found t o b e of t h e form:

E ~ which f o r t h e gd-uc specimens l i e s between 40 and 100 meV, h a s o f t e n been ,

i n t e r p r e t e d a s a measure of t h e average b a r r i e r h e i g h t between c r y s t a l l i t e s ( 7 ) . i s a measure of t h e H a l l m o b i l i t y i n t h e c r y s t a l l i t e having an average s i z e 6 . I t l i e s between 1 and 10 cm2v-Is-I and, l i k e pH, depends on 6. U, v a l u e s a r e t h e r e f o r e q u i t e comparable t o extended s t a t e m o b i l i t i e s i n a-Si, deduced from d r i f t m o b i l i t y experiments (12). This s u g g e s t s t h a t t h e formation of t h e small

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c r y s t a l l i n e r e g i o n s d i s c u s s e d h e r e does n o t g r e a t l y change t h e t r a n s p o r t p r o p e r t i e s a s com- p a r e d t o t h e a-phase.

F i n a l l y , H a l l e f f e c t measurements on th-pc specimens l e a d t o p ~ v a l u e s abou6 t e n times h i g h e r t h a n t h o s e f o r gd-PC samples. The d i f f e r e n c e between t h e two

eRT

c u r v e s i n f i g . 2 i s t h e r e f o r e e n t i r e l y accounted f o r by t h e d i f f e r e n c e i n m o b i l i t y .

Fig.5. H a l l m o b i l i t y i n gd-pc specimens p l o t t e d a g a i n s t c r y s t a l l i t e s i z e . 5 . D i s c u s s i o n .

The r e s u l t s of t h e p r e v i o u s s e c t i o n have e s t a b l i s h e d t h a t t h e i n c r e a s e d cond- u c t i v i t y observed i n t h e pc specimens a s compared t o t h e a-phase i s mainly a s s o c i a t - ed w i t h a l a r g e r c a r r i e r c o n c e n t r a t i o n . We s h o u l d l i k e t o s u g g e s t t h a t i n t h e p c phase t h e i n c r e a s e d o r d e r i s s u f f i c i e n t t o d e l o c a l i s e an a p p r e c i a b l e r a n g e of t h e t a i l s t a t e s . I n t h a t c a s e cC w i l l now l i e c l o s e t o t h e donor band and w i t h

( e c

-

e f ) O s E~ ( s e e f i g . 4 ) t h e i n c r e a s e i n n by two o r d e r s of magnitude o r more c a n e a s i l y b e accounted f o r , T h i s i n t e r p r e t a t i o n i s s u p p o r t e d by f i e l d e f f e c t measurements which have been c a r r i e d o u t o n a number of n-type specimens. They i n d i c a t e

t h a t cc h a s been s h i f t e d by a t l e a s t 0.15 eV from i t s p o s i t i o n i n a-Si, t h u s d e l o c a l - i s i n g most of t h e t a i l s t a t e s .

Acknowledgements: The a u t h o r s would l i k e t o t h a n k S t e w a r t Kinmond a n d A l l y F a l c o n e r f o r p r e p a r i n g t h e specimens and e x p r e s s t h e i r g r a t i t u d e t o t h e EEC f o r a s c i e n t i f i c and t e c h n i c a l g r a n t g i v e n t o one o f u s (G.W.).

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