MORPHOLOGY AND RESISTIVITY OF CVD POLYCRYSTALLINE SILICON LAYERS CONTAINING CARBON

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MORPHOLOGY AND RESISTIVITY OF CVD POLYCRYSTALLINE SILICON LAYERS

CONTAINING CARBON

M. Hendriks, S. Radelaar, Th. de Keijser, R. Delhez

To cite this version:

M. Hendriks, S. Radelaar, Th. de Keijser, R. Delhez. MORPHOLOGY AND RESISTIVITY OF CVD

POLYCRYSTALLINE SILICON LAYERS CONTAINING CARBON. Journal de Physique Colloques,

1982, 43 (C1), pp.C1-307-C1-312. �10.1051/jphyscol:1982141�. �jpa-00221798�

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MORPHOLOGY AND RESISTIVITY OF CVD POLYCRYSTALLINE SILICON LAYERS CONTAINING CARBON

M. H e n d r i k s , S. R a d e l a a r , Th.H. de K e i j s e r and R. Delhez

Laboratory of Metallurgy, Delft University of Technology, Rotterdamseweg 1S7, 2628 AL Delft, The Netherlands

Résumé

Les auteurs ont étudié l'influence de l'addition de carbone sur la structure cristalline et la resistivité des couches minces de silicium, formées par la décomposition simultanée de SiH,, C^H» et PH. à 1000°C. Ils ont déterminé la teneur en carbone, la morphologie, la texture, la taille des grains et les dilatations de réseau. On a trouvé que le carbone a un effet prononcé sur la structure cristalline et sur la resistivité de ces couches minces. Il existe une corrélation entre la structure et la resistivité, qui s'explique qualitativement.

Abstract

The influence of the addition of carbon on the crystalline structure and resistivity of polycrystalline silicon layers grown by simultaneous decomposition of SiH,, C^H. a n d P HT a t '°"° c w a s studied. Carbon content, morphology, preferred orientation, crystallite size, lattice strains and resistivity were determined. It was found that carbon has a pronounced effect on the crystalline structure and

resistivity of the layers. A correlation exists between the structure and resistivity which can be understood qualitatively.

1. Introduction

Carbon can be used to influence the resistivity and grain size of polycrystalline silicon layers, as was recently reported by Bloem and Claassen [1]. Incorporation of small amounts of carbon led to a minimum in the resistivity of phosphorus-doped layers. Higher concentrations led to an increase in resistivity and a reduction in grain size accompanied by the formation of a second phase. Although substitutionally dissolved carbon is considered to be an electrically inactive impurity, it can influence the electrical properties of polycrystalline silicon by causing simultaneously:

(i) a change in grain size (L) and

(ii) a change in the density of traps (Q ) at the grain boundaries.

It has been firmly established [2, 3] that the trapping of charge carriers at grain boundaries plays the most important role in the electrical behaviour of polycrystalline silicon layers. The effects are most pronounced when the product of grain size L and the doping concentration N becomes approximately equal to the density of trapping states Q :

L.N = Qt (1)

For low dopant concentration (N « Q /L) nearly all the free charge carriers

introduced by doping will be trapped at the grain boundaries and the resistivity will be high due to the formation of potential barriers at the grain boundaries and the low concentration of free charge carriers. For high doping concentrations

(N » Q /L) only a small fraction of the charge carriers are trapped at the grain boundaries, the depletion zone adjacent to the grain boundaries will be small and

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1982141

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C1-308 JOURNAL DE PHYSIQUE

t h e r e s i s t i v i t y w i l l t h u s approach t h e v a l u e f o r s i n g l e c r y s t a l l i n e s i l i c o n . The most r a p i d changes of r e s i s t i v i t y with dopant c o n c e n t r a t i o n a r e t h u s expected t o occur i n t h e r e g i o n where eq. (1) i s approximately f u l f i l l e d . S e v e r a l a u t h o r s 13, 41 have r e f i n e d S e t o ' s model [ 2 ] . E.g. Graef [4] introduced a continuous d i s t r i b u t i o n of t r a p p i n g s t a t e s i n s t e a d of i d e n t i c a l t r a p s with c o n s t a n t energy E t , a s assumed by Seto. Fig. 1 shows t h e r e s i s t i v i t y a s a f u n c t i o n of g r a i n s i z e f o r t h r e e

F i g . 1 .

C a l c u l a t e d r e s i s t i v i t y v e r s u s g r a i n s i z e L according t o t h e c a r r i e r t r a p p i n g model with a continuous d i s t r i b u t i o n of t r a p s

t h u hout t h e gap with d e n s i t y 10f9(~V-1.cm-2), Graef [ 4 ] .

-

3 cm cm -3

d i f f e r e n t doping c o n c e n t r a t i o n s of 10'' 10;: and210 l 8 cm-3 r e s p e c t i v e l y , using a uniform t r a p d e n s i t y d i s t r i b u t i o n of 1012 eV .cm

.

^Iti s e v i d e n t t h a t t h e most r a p i d changes o c u r i n t h e range of g r a i n s i z e s t y p i c a l f o r CVD p o l y c r y s t a l l i n e

S

s i l i c o n (1

-

10 nm). Bloem and Claassen assumed t h a t s m a l l amounts of carbon diminish t h e n u m b e r o f t r a p s a t t h e g r a i n b o u n d a r i e s . I n t h i s paper we w i l l show t h a t small amounts of carbon can a l s o i n c r e a s e t h e c r y s t a l l o g r a p h i c p e r f e c t i o n of t h e layers,which can produce marked e f f e c t s on t h e r e s i s t i v i t y when the g r a i n s i z e i s i n t h e r e g i o n where L Q t / N .

2. Experimental techniques

P o l y c r y s t a l l i n e s i l i c o n l a y e r s were y w n by simultaneous decomposition of

(0.1 v o l . p e r c . ) , C H (up t o 9x10- v o l . p e r c . ) and pH3 (0 o r 8 . 4 x 1 0 - ~ v o l . p e r c . )

:?$

^{a t}^I ^{atm. and} 1 6 0 6 0 ~ i n a h o r i z o n t a l a i r cooled r e a c t o r a t P h i l i p s Research,

~ i n d i o v e n [ I ] . The s u b s t r a t e s were (100) S i c r y s t a l s , covered w i t h 0.15 pm Si02 o r Si3y4 and t h e poly-Si t h i c k n e s s v a r i e d between 3.0 um and 3.5

urn.

We r e p o r t on t h r e e s e r l e s , two of them were grown under p r a c t i c a l l y i d e n t i c a l c o n d i t i o n s w i t h i n an i n t e r v a l of about one y e a r .

The carbon c o n t e n t of t h e l a y e r s was determined by e l e c t r o n probe micro a n a l y s i s (EPMA). Because f i b e r type t e x t u r e s were p r e s e n t , t h e p r e f e r r e d o r i e n t a t i o n was s t u d i e d by X-ray d i f f r a c t i o n u s i n g $-scans: t h e ( h k l ) i n t e n s i t y measured a t $ i s p r o p o r t i o n a l t o t h e volume f r a c t i o n of c r y s t a l l i t e s w i t h ( h k l ) p l a n e s making an angle $ with t h e s p e c i m e n s u r f a c e . F r o m X - r a y d i f f r a c t i o n l i n e p r o f i l e a n a l y s i s (LPA) i n f o r m a t i o n was o b t a i n e d about ( i ) t h e e f f e c t i v e p a r t i c l e s i z e D and t h e r.m.s. of t h e short-range s t r a i n s ^Eand ( i i ) t h e long-range s t r a i n . The a n a l y s i s i s based on t h e f a c t t h a t i m p e r f e c t i o n s i n t h e c r y s t a l l i k e g r a i n boundaries, s t a c k i n g f a u l t s , twins ( e f f e c t i v e p a r t i c l e s i z e ) and d i s l o c a t i o n s (r.m.s. short-range s t r a i n ) cause a

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r e f l e c t i o n s [ 6 ] . The Warren-Averbach s i z e - s t r a i n a n a l y s i s 451 was a p p l i e d t o c a l c u l a t e D and s from two o r d e r s of a r e f l e c t i o n . The s i n IJJ method [6] was a p p l i e d t o determine l o n g range s t r e s s e s from peak s h i f t s . Since a d i f f r a c t o m e t e r w i t h Bragg-Brentano f o c u s s i n g geometry was used, a r e f l e c t i o n ( h k l ) o r i g i n a t e s from c r y s t a l l i t e s with ( h k l ) planes p a r a l l e l t o t h e specimen s u r f a c e . Consequently, D and s r e l a t e t o t h o s e c r y s t a l l i t e s o n l y a n d r e p r e s e n t t h e e f f e c t i v e p a r t i c 1 e s i z e a n d r . m . s . short-range s t r a i n i n t h e d i r e c t i o n normal t o t h e l a y e r s u r f a c e . Because a <110>

f i b e r t e x t u r e occurred i n t h e l a y e r s , t h e (220) and (440) r e f l e c t i o n s were chosen, t h u s p r o v i d i n g i n f o r m a t i o n about a r e l a t i v e l y l a r g e f r a c t i o n of c r y s t a l l i t e s . horphology was s t u d i e d by scanning and t r a n s m i s s i o n e l e c t r o n microscopy (SDi and TEM). We t h i n k t h e r e s u l t s of t h e l i n e p r o f i l e a n a l y s i s a r e very u s e f u l h e r e because they a r e a measure of t h e c r y s t a l l o g r a p h i c p e r f e c t i o n which r e l a t e s t o t h e

r e s i s t i v i t y , whereas SEN y i e l d s t h e o u t e r s i z e and shape of c l u s t e r s . R e s i s t i v i t y was determined by t h e f o u r p o i n t p;obe method. The number of i n c o r p o r a t e d P atoms

i n t h e poly-Si l a y e r s was determined from t h e r e s i s t i v i t y of a simultaneously grown m o n o c r y s t a l l i n e e p i t a x i a l l a y e r .

I r v i n p l o t s [7] were used t o r e l a t e t h e r e s i s t i v i t y of t h e monocrystalline l a y e r t o t h e number of dopant atoms and i t was assumed t h a t t h e number of i n c o r p o r a t e d dopant atoms i s t h e same f o r both types of l a y e r s .

3. R e s u l t s

The t o t a l carbon c o n c e n t r a t i o n p r e s e n t i n t h e l a y e r s a s measured by means of EPbIA i s shown i n F i g . 2. The carbon c o n c e n t r a t i o n i s n o t l i n e a i r l y r e l a t e d t o t h e C H

2 2

C

i

Carbon c o n t e n t C v e r s u s C H c o n c e n t r a t i o n

( a t . % ) 2 2

d u r i n g growth a t 1 0 0 0 ' ~ f o r poly-Si on Si3N4, no dope, t h i c k n e s s poly-Si = 3.5 pm.

3 6 9 .

vo1.Z C H 2 2

c o n c e n t r a t i o n b u t i n c r e a s e s r a p i d l y a f t e r a comparatively slow i n i t i a l i n c r e a s e . The change-over occurs a t about 3 x 1 0 - ~ v o l . p e r c . C H o r 1 atomic p e r c e n t carbon. This

2 2

a m o u n t g r e a t l y e x c e e d s t h e s o l u b i l i t y of carbon i n s i l i c o n a t t h i s temperature (= 1 0 ~ ~ c m - 3 ) , w h i ~ ~ corresponds t o c 2x10-5 a t . p e r c . C [ S ] . For C H c o n c e n t r a t i o n s h i g h e r than 3x10 v o l . p e r c . p r e c i p i t a t i o n of 6-Sic p a r t i c l e s at2t?ie g r a i n

boundaries was observed by means of TEN.

The l a y e r s without carbon showed a 1110> f i b e r t e x t u r e , g r a d u a l l y d i s a p p e a r i n g with i n c r e a s i n g carbon c o n t e n t . SEM r e v e a l e d l a r g e , f a c e t t e d , columnar g r a i n s and a rough s u r f a c e f o r l a y e r s without carbon, changing t o s m a l l e r , equi-axed g r a i n s and a r e l a t i v e l y smooth s u r f a c e f o r t h e l a y e r s w i t h a high carbon c o n t e n t . F i g . 3 shows t h e e f f e c t i v e p a r t i c l e s i z e a s a f u n c t i o n of t h e C H2 c o n c e n ~ f a t i g n f o r both

undoped l a y e r s and l a y e r s w i t h a phosphorus conten$ of ~ 3 x 1 0 cm

.

^The ⁱt r o d u c t i o n of carbon a t f i r s t does n o t a f f e c t t h e p a r t i c l e s i z e s o much b u t a t 3xlO-' v o l . p e r c . C 2 2 H a pronounced maximum i s found. A t h i g h e r C H c o n c e n t r a t i o n s a r a p i d d e c r e a s e i n e f f e c t i v e p a r t i c l e s i z e i s observed. ~ l t h o u g i $he g r a i n s i z e s a s determined from t h e SEM micrographs a r e i n t h e same range a s those from t h e l i n e p r o f i l e a n a l y s i s , t h e pronounced maximum could not be f o u n d i n t h e m i c r o g r a p h s . Fig. 3 f u r t h e r s h o w s t h a t a l a r g e r p a r t i c l e s i z e i s accompanied by a lower short-range s t r a i n , both i n d i c a t i n g a more p e r f e c t c r y s t a l l i n e s t r u c t u r e . The l a y e r s contained t e n s i l e s t r e s s e s a s high a s 2 . 4 ~ 1 0 ~ m a , which i s q u i t e high a s compared t o t h e breaking s t r e n g t h of S i i n

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C1-310 JOURNAL DE PHYSIQUE

F i g . 3. E f f e c t i v e p a r t i c l e s i z e D and r.m.s. s h o r t - r a n g e s t r a i n ^Eof poly-Si l a y e r s v e r s u s C2H2 c o n c e n t r a t i o n d u r i n g growth a t 1 0 0 0 ~ ~ i n H w i t h t h e f o l l o w i n g

c h a r a c t e r ~ s t i c s : 2

-

s u b s t r a t e S i N no dope, t h i c h n ? s poly-Si = 3.5 lim

ba,

^c

^-

s u b s t r a t e

sid

f'p-dope 3. cm

',

t h i c k n e s s p o l y - s i = 3.0

urn.

D and E a r e d e t e r m i n e 2 from X-ray d i f f r a c t i o n l i n e p r o f i l e a n a l y s i s .

F i g . 4 . R e s i s t i v i t y ( f o u r p o i n t probe) v e r s u s C2H2 c o n c e n t r a t i o n d u r i n g growth, s e e f u r t h e r c a p t i o n F i g . 3.

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appearance of a minimum i n p a t t h e C H2 c o n c e n t r a t i o n f o r which a maximum i n D occurs i s r a t h e r s t r i k i n g . For t h e unioped l a y e r s (Fig. 4a) t h e r e s i s t i v i t y remains p r a c t i c a l l y c o n s t a n t a f t e r t h e p r e c i p i t o u s d e c r e a s e around t h e C2H2 c o n c e n t r a t i o n of

~ X I O - ~ v o l . p e r c .

4. Discussion

The i n i t i a l i n c r e a s e i n t h e e f f e c t i v e p a r t i c l e s i z e D with i n c r e a s i n g carbon c o n c e n t r a t i o n i s r a t h e r unexpected. Ifeasurements of t h e average g r a i n s i z e by means of SEPi and TEPl only show a monotonous d e c r e a s e w i t h i n c r e a s i n g carbon c o n t e n t . One h a s t o keep i n mind however t h a t t h e s e techniques measure d i f f e r e n t c h a r a c t e r i s t i c s of a given assembly of g r a i n s . The e f f e c t i v e p a r t i c l e s i z e D i s a measure f o r t h e .iimensionsofcoherentlyscatteringdomains i n a d i r e c t i o n p e r p e n d i c u l a r t o t h e l a y e r , whereas t h e average g r a i n s i z e L determined from SEN o r TEM micrographs i s a

measure of t h e average d i s t a n c e between l a r g e a n g l e boundaries i n a d i r e c t i o n p a r a l l e l t o t h e l a y e r . The two measures D and L may d i f f e r s i g n i f i c a n t l y , e s p e c i a l l y f o r c o a r s e g r a i n e d m a t e r i a l s .

The i n i t i a l i n c r e a s e of t h e e f f e c t i v e p a r t i c l e s i z e D with i n c r e a s i n g carbon c o n t e n t i s n o t e a s y t o e x p l a i n . A p o s s i b l e cause f o r t h i s e f f e c t could be t h e i n f l u e n c e of carbon on t h e s t a c k i n g f a u l t energy i n s i l i c o n . Lowering of t h e s t a c k i n g f a u l t energy would l e a d t o an i n c r e a s e i n t h e d e n s i t y of twins. Twins l a y i n g p a r a l l e l t o t h e growth d i r e c t i o n p l a y a r o l e i n t h e growth of p o l y s i l i c o n and probably a r e t h e cause of t h e <I102 type f i b e r t e x t u r e commonly observed i n p o l y s i l i c o n l a y e r s grown by CVD.

The formation of B-Sic p a r t i c l e s f o r l a y e r s grown a t C2H c o n c e n t r a t i o n s above 3. v o l . p e r c . causes a s t r o n g r e d u c t i o n i n e f f e c t i v e p a r $ i c l e s i z e . The sudden i n c r e a s e i n carbon uptake f o r t h e s e l a y e r s i s a l s o l i k e l y due t o t h e presence of S i c . Apparently a c e r t a i n amount of s u p e r s a t u r a t i o n of C2H2 i s needed t o make t h e n u c l e a t i o n of S i c p o s s i b l e .

There i s a s t r i k i n g c o r r e l a t i o n between t h e maximum i n t h e e f f e c t i v e p a r t i c l e s i z e and t h e minimum i n t h e r e s i s t i v i t y of t h e f i l m s doped with phosphorus. Such a minimum i n t h e r e s i s t i v i t y as a f u n c t i o n of carbon c o n t e n t f o r n-type l a y e r s ( N 'L 1 0 ~ ~ c m - 3 ) ~ r o w n a t 8 5 0 ' ~ was a l s o observed by Bloem e t a l . [ I ] . These a u t h o r s e x p l a i n t h e e x i s t a n c e of a minimum by a compensation of d a n g l i n g bonds due t o t h e i n c o r p o r a t i o n of carbon i n t h e g r a i n boundaries. The subsequent i n c r e a s e i n t h e r e s i s t i v i t y i s thought t o be due t o t h e d e c r e a s e i n g r a i n s i z e induced by t h e carbon a d d i t i o n . This e x p l a n a t i o n i m p l i e s t h a t t h e c o n d i t i o n given by eq. (1) i s approximately f u l f i l l e d . Using the v a l u e of t h e g r a i n s i z e of 0.1 pm r e p o r t e d i n t h e i r paper

tP5

t_r?pping d e n s i t y n e a r t h e r e s i s t i v i t y minimum i s found t o be of t h e o r d e r of 1.01 i s v a l u e i s i n d e y j sqmewhat lower than t h e v a l u e s r e p o r t e d by Seto ( 3 . 3 ~ 1 0

"

ci-$ and Graef (3.10 cm ) . Our X-ray measurements show t h a t the p e r f e c t i o n of t h e c r y s t a l l i t e s i s g r e a t e s t around t h e r e s i s t i v i t y minimum. P a r t of t h e d e c r e a s e i n r e s i s t i v i t y w i l l t h u s be due t o t h i s e f f e c t . The r e s i s t i v i t y of undoped l a y e r s a l s o d e c r e a s e suddenly w i t h i n c r e a s i n g carbon c o n t e n t a t t h e p o i n t where t h e maximum i n D and t h e minimum i n E i s observed. This e f f e c t i n d i c a t e s t h a t carbon i t s e l f c o n t r a r y t o t h e e x p e c t a t i o n expressed i n t h e i n t r o d u c t i o n i s an e l e c t r i c a l l y a c t i v e impurity. It i s l i k e l y t h a t t h e r e i s a r e l a t i o n between t h i s e l e c t r i c a l a c t i v i t y and t h e presence of S i c o r of Sic-Si i n t e r f a c e s .

I t i s concluded t h a t carbon has a pronounced e f f e c t on t h e morphology and r e s i s t i v i t y of p o l y c r y s t a l l i n e s i l i c o n l a y e r s and t h a t t h e two e f f e c t s a r e c l o s e l y r e l a t e d .

Acknowledgement

The a u t h o r s thank p r o f . J. Bloem and d r . W.A.P. Claassen of P h i l i p s Research, Eindhoven f o r many s t i m u l a t i n g d i s c u s s i o n s ; they a l s o provided t h e specimens. The t e x t u r e was measured by Plr. N.M. van d e r Pers. Ir. D. Szhalkoord and M r . D. Nelemans performed t h e EPIA and SEM and M r . F. Verhagen t h e TEM. The long-range s t r a i n was measured i n c o o p e r a t i o n with Ir. P.F. Willemse of Twente U n i v e r s i t y of Technology.

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C1-312 JOURNAL DE PHYSIQUE

Mr. R. Spekhorst assisted with the resistivity measurements. This work is part of the research program of the Stichting voor Fundamenteel Onderzoek der Materie and was made possible by financial support from the Nederlandse Organisatie voor Zuiver- Wetenschappelijk Gnderzoek.

References

1. J. Bloem, W.A.P. Claassen, Appl.Phys.Letters c(1982)725.

2. J.Y.W. Seto, J.Appl.Phys. %(1975)5247.

3. G. Baccarani, B. Ricco and G. Spadini, J.Appl.Phys. 2(1978)5565.

4. M.W. M. Graef, Thesis Nijmegen (1980).

5. B.E. Warren, X-ray Diffraction, Addison-Wesley (1969).

6. B.D. Cullity, Elements of X-ray Diffraction, Addison-Wesley (1978), pp. 447-478.

7. J.C. Irvin, Bell System Techn.3. 41(1962)387-410.

8. A.R. Bean and R.C. Newman, J.~h~s.Chem.~olids z(1971) 121 1.

9. W.R. Runyan, Silicon Semiconductor Technology, McGraw-Hill, (1965), p. 213.