morphology and resistivity of cvd polycrystalline silicon ... - CiteSeerX

JOURNAL DE PHYSIQUE

Colloque

Cl,

supplément

au n°10,

Tome 43, octobre

1982

page

Cl-307

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 , Laboratory of Metallurgy, 1S7, 2628 AL Delft, The

Th.H. de K e i j s e r and R. Delhez Delft University Netherlands

of Technology,

Rotterdamseweg

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 H T 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 = Q t

(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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t h e r e s i s t i v i t y w i l l thus approach t h e value 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 thus expected t o occur i n t h e region where eq. (1) i s approximately f u l f i l l e d . Several authors 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 trapping 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

Fig. 1 . Calculated r e s i s t i v i t y versus 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

-3 cm

d i f f e r e n t doping concentrations 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 It i s evident t h a t t h e most rapid 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 small 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 . In 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 region where L Qt/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- vol.perc.) and pH3 (0 o r 8 . 4 x 1 0 - ~ vol.perc.) 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 with 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 conditions w i t h i n an i n t e r v a l of about one y e a r . The carbon content of the 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 using $-scans: t h e (hkl) 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 with (hkl) planes making an angle $ with t h e specimensurface.FromX-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 (LPA) information was obtained 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 the short-range s t r a i n s E and ( 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 imperfections 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

:?$

broadening of t h e Bragg r e f l e c t i o n s [ > I , whereas a long-range s t r a i n causes a change i n t h e average l a t t i c e parameter and thus a s h i f t i n t h e p o s i t i o n of t h e Bragg 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 applied t o c a l c u l a t e D and s from two orders of a r e f l e c t i o n . The s i n IJJ method [6] was applied t o determine long 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 with Bragg-Brentano focussing geometry was used, a r e f l e c t i o n (hkl) o r i g i n a t e s from c r y s t a l l i t e s with (hkl) planes p a r a l l e l t o the specimen s u r f a c e . Consequently, D and s r e l a t e t o those c r y s t a l l i t e s onlyand 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 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, thus providing information 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 transmission 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 incorporated 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 monocrystalline 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 the r e s i s t i v i t y of the monocrystalline l a y e r t o t h e number of dopant atoms and it was assumed t h a t t h e number of incorporated dopant atoms i s t h e same f o r both types of l a y e r s .

3. Results 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 the l a y e r s a s measured by means of EPbIA i s shown i n Fig. 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 (at.%)

i 3

6 vo1.Z C H

Carbon content C versus C H concentration 2 2 d u r i n g growth a t 1000'~ f o r poly-Si on Si3N4, no dope, thickness poly-Si = 3.5 pm.

9.

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 increase. The change-over occurs a t about 3 x 1 0 - ~v o l . p e r c . C H o r 1 atomic percent carbon. This 2 2 amountgreatlyexceeds 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 hcorresponds i~~ t o c 2x10-5 a t . p e r c . C [S]. For C H concentrations vol.perc. 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 h i g h e r than 3x10 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 , gradually disappearing with i n c r e a s i n g carbon c o n t e n t . SEM revealed 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 with a high carbon content. Fig. 3 shows the 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 nf o r both The i t r o d u c t i o n undoped l a y e r s and l a y e r s with a phosphorus conten$ of ~ 3 x 1 0 cm of carbon a t f i r s t does not 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-' vol.perc. C H a pronounced maximum i s found. A t h i g h e r C H concentrations a r a p i d decrease 2 2 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 , the 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 furthershows 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 0m~a , which is 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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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. short-range s t r a i n E of 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 2 character~stics: - s u b s t r a t e S i N no dope, thichn? s poly-Si = 3.5 lim 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 determine2 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 .

ba,

',

Fig. 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.

2

compression: 5 . 0 ~ 1 0 PPa [ 9 ] . The r e s i s t i v i t y curves (Pig. 4) show a s t r o n g c o r r e l a t i o n with t h e curves f o r the e f f e c t i v e p a r t i c l e s i z e . I n p a r t i c u l a r the appearance of a minimum i n p a t the C H2 concentration 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 the 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 the p r e c i p i t o u s decrease around the C2H2 concentration of ~ X I O - vol.perc. ~

4. Discussion The i n i t i a l increase 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 decrease with i n c r e a s i n g carbon content. One has 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 perpendicular 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 angle boundaries i n a d i r e c t i o n p a r a l l e l t o the 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 coarse grained 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 content i s not easy 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 the influence 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 play 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