GRAIN BOUNDARIES INTRODUCED DEEP LEVELS IN POLYSILICON

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GRAIN BOUNDARIES INTRODUCED DEEP LEVELS IN POLYSILICON

B. Lombos, S. Yee, M. Pietrantonio, M. Averous

To cite this version:

B. Lombos, S. Yee, M. Pietrantonio, M. Averous. GRAIN BOUNDARIES INTRODUCED DEEP LEVELS IN POLYSILICON. Journal de Physique Colloques, 1982, 43 (C1), pp.C1-199-C1-206.

�10.1051/jphyscol:1982127�. �jpa-00221783�

GRAIN BOUNDARIES INTRODUCED DEEP LEVELS IN POLYSILICON

B.A. Lombos, S . Yee, M . Pietrantonio and M . Averous*

Department of EZectrical Engineering, Concordia University, Montreal, P. Q., H3G 1M8, Canada

*Centre drEtudes d'Electronique des Solides, Université des Sciences e t Techniques du Languedoc, 34060 MontpeZZier, France

RESUME

Des f i l m s de p o l y s i l i c i u m o n t é t é obtenus dans un f o u r à résistance, par dépo- s i t i o n chimique en phase vapeur de s i l a n e . Pour m i n i m i s e r l a r é a c t i o n de l a phase gazeuse, un système d ' e n t r é e r e f r o i d i a é t é développé. Le c o n t r ô l e de l a r é a c t i o n de s u r f a c e permet une croissance en colonne. Le raie des j o i n t s de g r a i n s c a r a c t é r i s a n t l e s p o l y c r i s t a u x e s t examiné. Pour l e s mesures de , p r o p r i é t é de t r a n s p o r t , l e s cou- ches de p o l y s i l i c i u m o n t é t é i s o l é e s par une d é p o s i t i o n en phase vapeur d'un f i l m de d i o x i d e de s i l i c i u m sur l e S i s u b s t r a t .

Les niveaux profonds q u i p i è g e n t l e s é l e c t r o n s e t l e s t r o u s sont r e l i é s aux dimensions des grains. Les minima de m o b i l i t é c a r a c t é r i s t i q u e s en f o n c t i o n de l a c o n c e n t r a t i o n sont r e l lé s au passage du niveau de Fermi à t r a v e r s l e niveau profond associé. Le c r i t è r e de l a f o r m a t i o n d'une bande d'impuretés associée aux l i a i s o n s pendantes i n t r o d u i t e s p a r l e s j o i n t s de g r a i n s e s t examiné. Les m o b i l i t é s c a l c u l é e s c a r a c t é r i s a n t l e s p r o p r i é t é s de t r a n s p o r t dans un système a l é a t o i r e , i n t r o d u i t dans ce cas par l a formation d'une bande d'impuretés, sont r e l i é e s aux v a l e u r s mesurées aux minima de m o b i l i t é .

ABSTRACT

P o l y s i l i c o n f i l m s have been grown, i n a r e s i s t a n c e heated furnace, by chemical vapor d e p o s i t i o n o f s i l a n e . To minimize t h e gas-phase r e a c t i o n a cooled e n t r y sys- tem has been developed. The dominance o f surface r e a c t i o n c o n t r o l r e s u l t e d i n colum- n a r growth. The r o l e o f t h e g r a i n boundaries, c h a r a c t e r i z i n g p o l y c r y s t a l s , were examined. For t r a n s p o r t p r o p e r t i e s measurements t h e p o l y s i 1 ic o n l a y e r s were i s o l a - t e d by a chemical vapor deposited s i l i c o n d i o x i d e f i l m on a s i l i c o n substrate. Deep l y i n g e l e c t r o n and h o l e t r a p p i n g center concentrations were r e l a t e d t o g r a i n s i z e s . The c h a r a c t e r i s t i c measured m o b i l i t y minima as a f u n c t i o n of c a r r i e r concentrations, were associated t o t h e c r o s s i n g o f t h e Fermi energy t h e a p p r o p r i a t e deep l y i n g impu- r i t y l e v e l

.

The c r i t e r i a o f i m p u r i t y band formation, associated t o g r a i n boundaries introduced dangling bonds, were examined. The c a l c u l a t e d mobil i t i e s , c h a r a c t e r i z i n g t h e t r a n s p o r t p r o p e r t i e s o f a random system, introduced i n t h i s case by t h e forma- t i o n o f an i m p u r i t y band, were c o r r e l a t e d t o t h e measured values a t t h e m o b i l i t y minima, i n p o l y c r y s t a l l i n e s i l i c o n .

INTRODUCTION

Chemical Vapor Deposited (C.V.D. ) p o l y s i l i c o n has i n c r e a s i n g a p p l i c a t i o n i n s o l i d s t a t e e l e c t r o n i c devices ( 1 ) . Due t o i t s t r a n s p o r t p r o p e r t i e s , however, i t s u t i l i t y f o r a c t i v e devices i s l i m i t e d and i t i s m a i n l y used f o r passive components.

But i n t h e case of l a r g e area, low c o s t and l a r g e q u a n t i t y p h o t o v o l t a i c power gen- e r a t o r s , p y r o l i t i c C.V.D. p o l y s i l i c o n f i l m s are having more and more promising pos- s i b i l i t i e s (2-7). The a n a l y s i s of t h e t r a n s p o r t p r o p e r t i e s o f p o l y s i l i c o n m i g h t b r i d g e t o a b e t t e r understanding o f amorphous s i l i c o n , through t h e e l u c i d a t i o n o f random system introduced impuri t y band conduction ( 8 , l O ) .

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1982127

JOURNAL DE PHYSIQUE

I n t h i s i n v e s t i g a t i o n low c o s t substrates were used t o d e p o s i t p o l y s i l i c o n by C.V.D. u t i l i z i n g t h e p y r o l y s i s o f s i l a n e . The substrates were g r a p h i t e and f o r t h e t r a n s p o r t p r o p e r t i e s measurements they were s i l i c o n , covered by a t h i n C.V.D. grown s i l i c o n d i o x i d e f i l m . The s t r u c t u r e o f t h e p o l y s i l i c o n f i l m s as a f u n c t i o n o f deposi t i o n c o n d i t i o n s (temperature, dopi ng concentrations) was i n v e s t i g a t e d . The t r a n s p o r t p r o p e r t i e s o f t h e f i l m s were measured. The obtained experimental data were analysed by a mode1 based on g r a i n boundaries introduced d o n o r - l i k e and accep- t o r - l i k e deep l e v e l s . This model assumes t h a t t h e g r a i n boundaries introduced t r a p - ping s t a t e s are monovalent and c o n s i s t s o f s t a t e s w i t h a u n i f o n n d e n s i t y . The deep l y i n g acceptors being i n t h e upper h a l f o f t h e band gap w h i l e t h e corresponding donors i n t h e lower one according t o t h e suggestions o f several authors (11,12,13, 14). T h i s model

,

c l o s e l y r e l a t e d t o t h e s o - c a l l e d segregation theory (11-14), was p r e f e r r e d since t h e t r a p p i n g s t a t e introduced d e p l e t i o n l a y e r approximation ceases t o be a p p l i c a b l e i f t h e g r a i n s i z e s exceed 60 nm (1 1 ) . This deep l e v e l model was developed f o r s e m i - i n s u l a t i n g g a l l i u m arsenide t o i n t e r p r e t i t s measured t r a n s p o r t p r o p e r t i e s (15), and was s u c c e s s f u l l y a p p l i e d t o c l a r i f y t h e anomalies" i n t h e t r a n s p o r t p r o p e r t i e s o f mercury t e l l u r i d e (16), as w e l l .

The e x p e r i m e n t a l l y detennined m o b i l i t y minima as a f u n c t i o n o f c a r r i e r concen- t r a t i o n s , are r e l a t e d t o t h e c r o s s i n g o f t h e Fermi l e v e l t h e a p p r o p r i a t e deep l y i n g i m p u r i t y l e v e l . These are, i n t u r n , r e l a t e d t o t h e g r a i n boundaries introduced t r a p p i n g centers.

The p o s s i b i l i t y o f i m p u r i t y band conduction, c h a r a c t e r i z e d by a random system (8,9), and r e l a t e d t o deep l y i n g l e v e l s i n p o l y s i l i c o n m i g h t l e a d t o some i n t e r - e s t i n g a p p l i c a t i o n s . I n t h e case o f s o l a r c e l l s , however, t h i s phenomenon might have t o be avoided, s i n c e t h e t r a n s p o r t p r o p e r t i e s o f t h e deposited f i l m s need t o be optimized t o approach those o f a s i n g l e c r y s t a l t o maximize t h e power conversion e f f i c i e n c y o f t h e prepared s o l a r c e l l s . I n t h e case o f i n t e g r a t e d c i r c u i t a p p l i c a - t i o n s o f t h e p o l y s i l i c o n l a y e r s , however, i f t h e mechanism and c o n d i t i o n s o f impu- r i t y band conduction can be substantiated, some i n t e r e s t i n g p r o p e r t i e s o f t h e pas- s i v e components might be achieved.

EXPERIMENTAL PROCEDURES AND RESULTS

High p u r i t y (99.9999%) s i l a n e (SiH4) d i l u t e d i n hydrogen (0.5% v o l SiH4) w i t h n i t r o g e n as a c a r r i e r gas was used as t h e feedstock. The doping o f the p o l y s i l i c o n f i l m s was achieved by mixing diborane (0.05% v o l B2H6 i n Hz), and phosphine (0.05%

v o l PH3 i n Hz) t o t h e feedstock f o r p- and n- t y p e f i l m s , r e s p e c t i v e l y . The s i l i c o n d i o x i d e p a s s i v a t i n g f i l m s were grown i n t h e same system using s i l a n e i n n i t r o g e n

(0.5% v o l SiH4 i n Np) and o x i d i z e d by pure oxygen. Al1 gas m i x t u r e s were o f u l t r a h i g h p u r i t y grade. The d e p o s i t i o n temperature f o r p o l y s i l i c o n f i l m s ranged from 500°C t o 973"C, w h i l e t h e s i l i c o n d i o x i d e f i l m s were grown a t 500°C (17).

The substrates were g r a p h i t e and s i n g l e c r y s t a l s i l i c o n . They were about1.5mm t h i c k p o l i s h e d t o m i r r o r f i n i s h w i t h alumina powder. These substrates were degreased and then degased a t 1000°C i n hydrogen ambient.

The Chemical Vapor Deposition s e t up has been described p r e v i o u s l y (26). A t temperatures above 600°C t h e s i z e s o f t h e g r a i n s , forming t h e f i l m s , increased mono- t o n i c a l l y wi t h i n c r e a s i n g deposi t i o n temperatures. A t around 800°C, t y p i c a l g r a i n sizes were about 3pm i n c r e a s i n g t o 6pm a t 900°C and t o 10 t o 15pm a t about 950°C and about 20um a t 967OC.

To demonstrate t h e s t r u c t u r e o f t h e g r a i n s across t h e f i l m , a c r o s s - s e c t i o n a l Scanning E l e c t r o n Microscope (S.E.M.) photograph o f i t i s shown i n Fig. 1 deposited a t about 967OC.

g r a i n s i z e s o f about 20pm.

TRANSPGRT PROPERTIES MEASUREMENTS

H a l l e f f e c t measurements were performed on t h e deposited p o l y s i l i c o n f i l m s . The samples were prepared according t o A.S.T.M. s p e c i f i c a t i o n s (18).

The H a l l e f f e c t measurements were c a r r i e d o u t a t room temperature. The mag- n e t i c f i e l d dependence o f t h e H a l l c o e f f i c i e n t was v e r i f i e d and i t was found t h a t i n t h e above mentioned temperature range, i t became f i e l d independent above 0.1 Tesla.

Therefore a l 1 measurements were performed a t 0.5 Tesla.

DISCUSSION

To analyse t h e t r a n s p o r t p r o p e r t i e s o f p o l y c r y s t a l l i n e s i l i c o n f i l m s t h e r o l e o f t h e g r a i n boundaries i s o f fundamental importance. T h e i r behavior has been s t u d i e d by a number o f i n v e s t i g a t o r s i n t h e cases o f germanium (19) and s i l i c o n (11,20). The segregation (13,14) and the t r a p p i n g (11,21) t h e o r i e s assume a 6 - shaped d e n s i t y o f s t a t e s a t t h e g r a i n boundaries c h a r a c t e r i z i n g t h e t r a p p i n g states.

These a r e taken t o be monovalent and peaking c l o s e t o the midgap. Furthermore, i t i s assumed t h a t these t r a p s c o n s i s t o f s t a t e s w i t h uniform d e n s i t y (11,Zl) l o c a t e d i n t h e case o f t h e deep l y i n g donors i n t h e lower h a l f o f t h e band gap, 0.37 eV above the valence band. I n t h e case o f t h e deep l y i n g acceptors, t h e p o s i t i o n o f the a p p r o p r i a t e l e v e l i s assumed t o be i n t h e upper h a l f o f the band gap and t h i s i n v e s t i g a t i o n takes i t s l o c a t i o n t o be a t 0.37 eV below the conduction band edge was s u b s t a n t i a t e d by d e f e c t r e l a t e d deep l e v e l s found i n s i 1 ic o n (23). According t o the d e p l e t i o n theory (11,21 ,22) these t r a p p i n g s t a t e s a r e reducing t h e f r e e c a r r i e r con- c e n t r a t i o n . This approximation gave good r e s u l t s up t o g r a i n s i z e s o f about 60 nm.

However, i n a l 1 t h i s type of c a l c u l a t i o n s a surface t r a p p i n g c o n c e n t r a t i o n o f about 3.3 x 10~2,crn-2 was r e q u i r e d t o f i t t h e experimental d a t a (11,20,21,24).

I n t h i s i n v e s t i g a t i o n t h e columnar g r a i n s i z e s a r e reaching 20um and i n a l 1 the f o l 1 owing c a l c u l a t i o n s t h e c h a r a c t e r i s t i c surface t r a p p i n g center c o n c e n t r a t i o n ought t o be taken equal t o t h e atomic d e n s i t y o f t h e (111) plane i n s i l i c o n 7.8 x 1014, cm-2. I t i s assumed t h a t t h e g r a i n boundaries a r e c r e a t i n g a number of d e f e c t s i n t h e c r y s t a l due t o incomplete atomic bonding. The r e s u l t i n g h y b r i d i z e d dangl i n g bonds a r e randomly d i s t r i b u t e d and a r e forming e l e c t r o n and h o l e t r a p s .

JOURNAL DE PHYSIQUE

These deep l y i n g l e v e l s i n t h e band gap are t r e a t e d then t h e same way as donors and acceptors w i t h t h e i r a p p r o p r i a t e i o n i z a t i o n p o t e n t i a l s , as i n t h e case o f semi- i n s u l a t i n g GaAs (15) and i n o t h e r semiconductors (24). This mode1 i s schematically represented i n Fig. 2.

' Nad-

+ + + +

^+Ndd+

-

^, _a ^,

₊

^,

_{+ +} -

^{, -Nas-} _{+ p +}

;

^037eV

^j-.

Figure 2: Schematic r e p r e s e n t a t i o n o f t h e p o s i t i o n o f the deep-1 y i ng acceptor

,

Idad;

and deep-lying cionor, ~ ~ d ' , i n t h e band gap, Eg, o f s i l i c o n .

The c o n d u c t i v i t y o f t h e system can be analysed then by computing t h e p o s i t i o n o f t h e Fermi l e v e l based on t h e steady s t a t e n e u t r a l i t y c o n d i t i o n :

where t h e mobile h o l e c o n c e n t r a t i o n i s pi w h i l e Mdif and Nai- represents t h e i o n i z e d shallow and deep l y i n g donor and acceptor concentrations and the mobile e l e c t r o n concentration, n-, i s given by t h e corresponding c l a s s i c a l expression as i t has been d e t a i l e d i n (15). For any g i v e n i m p u r i t y and t r a p p i n g c e n t e r concentration a t a g i v e n temperature, t h e c h a r a c t e r i s t i c Fermi l e v e l w i l l be given when t h e n e u t r a l i t y c o n d i t i o n i n equ. (1) i s s a t i s f i e d .

A simple example i s shown i n Fig. 3, t o demonstrate t h e computation using, f o r example,the data c h a r a c t e r i z i n g a d e p o s i t i o n a t 800°C where the boron concentration, being shallow acceptors, i s ilas- = 5.3 x 1018,cm-3, t h e donor t r a p p i n g center concen- t r a t i o n i s ~ d= 1.56 ~ +x 1019, cm-3 then a t T = 300°C, t h e Fermi l e v e l i s c a l c u l a t e d t o be a t E ( f ) = 0.37 eV above t h e conduction band.

F i g u r e 3: The c a l c u l a t e d i o n i z e d impuri t y Nasr,

N ~

and

~ +

c a r r i e r concentrations n-, p+, as t h e f u n c t i o n o f energy, w i t h the computed Fermi energy corresponding t o t h e n e u t r a l i t y c o n d i t i o n (equ. ( 1 ) ) .

d e n s i t y i n s i 1 ic o n w i l l be of 7.8 x 1014,crn-~. The corresponding t r a p p i n g center c o n c e n t r a t i o n can be computed t o be Ndt = 1.56 x 1019,cm-~. This i s t h e value used i n t h e c a l c u l a t i o n s i n Fig. 3. I f one assumes average g r a i n s i z e s o f 6vm and 12vm char- a c t e r i z i n g t h e f i l m s deposited at9009C and 9 5 O Q C r e s p e c t i v e l y t h e n t h e correspondingtrap- ping center concentrations can be computed t o be 7.8 x 1018, cm-3 and 3.9 x 1018,cm-3 r e s p e c t i v e l y (see Table 1 ) . Using t h e m u l t i l e v e l model o u t l i n e d above (15), based on t h e n e u t r a l i t y c o n d i t i o n , equ. ( l ) , t h e p o s i t i o n o f t h e Fermi l e v e l as t h e f u n c t i o n o f acceptor concentrations f o r t h e t h r e e d i f f e r e n t c h a r a c t e r i s t i c t r a p p i n g center concentrations can be computed. The r e s u l t s a r e shown i n F i g . 4. The c r i t i c a l acceptor concentrations, where t h e Fermi l e v e l i s c r o s s i n g t h e deep l y i n g donor l e v e l are c o l l e c t e d i n Table 1 under NaS-. The corresponding i o n i z e d deep donor con- c e n t r a t i o n s ~ d d + are g i v e n w i t h t h e average i m p u r i t y separation, R(nm) and t h e mobil- i t y minima, pmin, cm2 s - l , as w i l l be discussed l a t e r .

F i g u r e 4: Fermi energies as t h e func- t i o n o f s h a l l ow-lying accep- t o r concentrations, Nas-, w i t h d i f f e r e n t t r a p p i n g c e n t e r concentrations corre- sponding t o t h e g r a i n sizes of 3, 6 and 12pm, as c o l - l e c t e d i n Table 1.

based on t h i s treatment t h e t y p i c a l l a r g e v a r i a t i o n o f c o n d u c t i b i l i t y o f p o l y s i l i c o n , observed by a number o f i n v e s t i g a t o r s (11 ;13,14,21), c o u l d be c l a r i f ie d q u a n t i t a - t i v e l y (26). Here t h e e x p e r i m e n t a l l y measured and c a l c u l a t e d r e s i s t i v i t i e s as t h e f u n c t i o n o f doping concentrations are depicted. T h i s i s t o v e r i f y whether t h e char- a c t e r i s t i c l a r g e v a r i a t i o n , by a f a c t o r o f

l06(ohm-cmf of

t h e r e s i s t i v i t i e s a r e prop- e r l y r e f l e c t e d by the deep-lying t r a p p i n g centers compensation model. The r e s u l t s a r e shown i n Fig. 5 w i t h t h e ones obtained by Seto ( I l ) , which are t h e p o i n t s of 1.

Curve 2 i s c a l c u l a t e d by t h e same author ( I l ) , w h i l e curve 3 d e p i c t s t h e c a l c u l a t e d values obtained i n t h i s i n v e s t i g a t i o n f o r a p-type f i l m deposited a t 950°C. The p o i n t s 4 are t h e corresponding e x p e r i m e n t a l l y measured data o f t h i s p-type f i l m . Curve 5 represents the c a l c u l a t e d values o f an n-type f i l m (26). I n a l 1 cases good e x p e r i m e n t a l l y measured r e s i s t i v i t i e s can be seen.

10'5 1017 lof9 m21 ACC. Concentration ( c m 3 )

Figure 5: R e s i s t i v i t i e s (ohm-cm) as the f u n c t i o n o f doping con- c e n t r a t i o n (cm-3) f o r 1 : experimental p o i n t o f (10);

2: t h e o r e t i c a l curve of (10);

3: c a l c u l a t e d curve f o r p- type f i l m ; 4: experimental p o i n t s o f p-type f i l m ; 5:

c a l c u l ated curve of n-type f i l m .

JOURNAL DE PHYSIQUE

TABLE 1

IMPURITY BAND FORMATION

There i s another i n t e r e s t i n g consequence o f t r e a t i n g the t r a p p i n g centers as deep-lying i m p u r i t y - l i k e compensating species. Since t h e i m p u r i t i e s i n cgeneral a r e randomly d i s t r i b u t e d i n the c r y s t a l l a t t i c e t h e average i m p u r i t y separation can be estimated by R = ( 3 / 4 r ~ ~ ~ + ) 1 / 3 , (see Table 1 ) . A c h a r a c t e r i s t i c Bohr radius,

a(*),

representing these ion-ized deep-lying impuri t i e s can be computed by a* = 'fi2K/p*e2 where ^K i s t h e product o f the absolute and r e l a t i v e d i e l e c t r i c constants and e i s e l e c t r o n i c charge and p* i s t h e h o l e e f f e c t i v e mass. I n a p-type p o l y s i l i c o n , t a k i n g i n t o c o n s i d e r a t i o n t h e above mentioned deep-lying compensating i o n i z e d donor concentrations (see Table 1 )

,

t h e average impuri t y separations a r e R(1) = 3.58, R(2) = 4.58 and R(3) = 5.6 nm f o r c h a r a c t e r i s t i c g r a i n sizes o f 3,6 and 12rim, respec- t i v e l y . The Bohr r a d i u s , c h a r a c t e r i z i n g these deep-lying i o n i z e d States can be given by a(*) = 0.62 nm. Then, f o l l o w i n g t h e arguments of M o t t (8,9), i f the average i m p u r i t y separation, R, and t h e Bohr r a d i u s , a(*), o f t h e i o n i z e d i m p u r i t i e s a r e i n t h e range of: 3a*<R, t h e formation o f a m e t a l l i c i m p u r i t y band can be assumed (9). I n t h i s case t h e overlapping wave f u n c t i o n s o f t h e i o n i z e d i m p u r i t i e s , forming t h e i m p u r i t y band, couTd reach through t h e g r a i n boundaries, s h o r t i n g t h e i r e f f e c t . When t h e Fermi l e v e l i s i n t h e i m p u r i t y band, t h e propagation o f t h e charge c a r r i e r s can be regarded as a d i f f u s i o n process i n a random system. Consequently t h e l a t t i c e p e r i o d i c i t y would l o s e i t s e f f e c t on t h e t r a n s p o r t p r o p e r t i e s r e s u l t i n g i n t h e replacement o f t h e e f f e c t i v e mass w i t h t h e r e s t mass o f t h e charge c a r r i e r s and i n a c h a r a c t e r i s t i c low m o b i l i t y (6,9). I n t h e case o f s i 1 ic o n then t h e c o n d i t i o n o f t h e formation o f an i m p u r i t y band can be given as: 1.19 nm<R. Comparing t h e above mentioned average i m p u r i t y separations (see Table 1) t h e formation o f a m e t a l l i c

i m p u r i t y band can be assumed.

Following M o t t i n t h a t case a c h a r a c t e r i s t i c low m o b i l i t y can be expected t o be approximated by (8,9) :

2a e R~

'min <

6h

Taking i n t o c o n s i d e r a t i o n the above estimated average i m p u r i t y separations these minimum m o b i l i t i e s , i n d i c a t i n g t h e c r o s s i n g o f t h e Fermi l e v e l t h e i m p u r i t y band, would be l e s s than approximately: 32, 53 and 79 c m 2 ~ - 1 s-1 r e s p e c t i v e l y as they are c o l l e c t e d i n Table 1. The e x p e r i m e n t a l l y measured m o b i l i t y minima as i t can be seen i n Fig. 6, are s a t i s f y i n g t h e t r e n d i n d i c a t e d i n Table 1, and Fig. 4 r e l a t e d t o t h e c r o s s i n g o f t h e Fermi l e v e l t h e deep l y i n g l e v e l a t h i g h e r and h i g h e r shallow i m p u r i t y concentrations w i t h decreasing g r a i n sizes. These r e s u l t s m i g h t be considered t o strengthen t h e a p p l i c a b i l i t y o f t h e deep-lying i m p u r i t y nodel and t h e concept o f i m p u r i t y band conduction t o p o l y c r y s t a l l i n e s i l i c o n as w e l l . S i m i l a r phenornena were demonstrated t o e x i s t i n germanium (10,24) and i n mercury t e l l u r i d e

(16).

,a";@

- # /

-

-

I

:

I

i

I I

- _{1 !}

1 1 1 1 I I I

lds I d ' Ids

1021 MC. Concentration (cm-3)

t e r i z i n g t h e samples deposited a t 750 and 950°C.

CONCLUSIONS

P o l y c r y s t a l l i n e s i l i c o n f i l m s were deposited by t h e p y r o l y s i s of s i l a n e . A t about 500°C t h e f i l m s were amorphous. I n c r e a s i n g d e p o s i t i o n temperature increased t h e c r y s t a l l i n i t y o f t h e chemical vapor deposited l a y e r s . The t r a n s p o r t p r o p e r t i e s o f these f i l m s were measured t o e l u c i d a t e t h e r o l e o f t h e g r a i n boundaries charac- t e r i z i n g p o l y s i l i c o n f i l m s . The obtained data were analyzed by a deep-lying impu- r i t y l e v e l s model. The g r a i n boundaries introduced dangling bonds were t r e a t e d as compensating d o n o r - l i k e and a c c e p t o r - l i k e i m p u r i t i e s . Then, based on t h e n e u t r a l i t y c o n d i t i o n , t h e p o s i t i o n o f t h e Fermi energy was c a l c u l a t e d , i n t h e case o f d i f f e r e n t deep-lying t r a p p i n g center concentrations, as a f u n c t i o n of s h a l l o w - l y i n g doping concentrations, i n the case o f f i l m s deposited a t 800, 900 and 950°C. T h i s model, accounting s a t i s f a c t o r i l y f o r t h e measured t r a n s p o r t p r o p e r t i e s , l e d t o t h e recogni- t i o n o f t h e p o s s i b l e existance o f m e t a l l i c i m p u r i t y band conduction i n p o l y s i l i c o n . These deep-lying i m p u r i t y bands c o u l d e l u c i d a t e t h e e x p e r i m e n t a l l y observed m o b i l i t y minima, as a f u n c t i o n o f doping concentrations, a t d i f f e r e n t t r a p p i n g center

concentrations. These m o b i l i t y minima, c h a r a c t e r i s t i c t o random systems c o i n c i d e w i t h the crossing o f t h e Fermi l e v e l t h e i m p u r i t y band, created by t h e g r a i n bound- a r i e s introduced t r a p p i n g centers. This model c l a r i f i e s , as w e l l , t h e e x i s t a n c e of s e m i - i n s u l a t i n g p o l y c r y s t a l l i n e s i l i c o n t h e same way as i t was found t o be t h e case i n s e m i - i n s u l a t i n g GaAs.

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