Widegap a-SI-H Prepared at Low Substrate Temperature

7/28/2019 Widegap a-SI-H Prepared at Low Substrate Temperature

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ELSEVIER Solar Energy Materials and Solar Cells 45 (1997) 115-126

S o l a r F.ner~ Mabff~lsand Solar Cells

W idegap a-Si:H f i lm s prepared at low substratetemperature

S . C . S a h a , S u k r it i G h o s h , S w a t i R a y *Energ y Research Unit , Indian Associatio n fo r the Cultivation of Science, Jadat,pur, Calcutta-700032, India

Received 20 Novem ber 1995

AbstractWide bandgap hydrogena ted amorphous s i l i con (a -S i :H) f i lms have been p repared by the

PE CV D m ethod at a low substra te temperature (80C), control l ing the incorporat ion of hydr ogen(bo nd ed with silicon) into the film. O ptim izing the depo sition parameters viz. hydro gen dilution,rf pow er, a-Si:H f i lm with Eg ~ 1 .90 eV and o 'ph> 10 -4 Scm - I has been developed. This filmexhibi ted bet ter optoelectronic propert ies compared to a-SiC:H of s imilar opt ical gap. Thequantum eff ic iency measurement on the Schottky barr ier solar cel l s t ructure showed a defini teenha ncem ent of blue response. Surfa ce reactio n as well as structural relaxation und er suitabledeposi t ion condit ion have b een cla im ed to be responsible for the developm ent of such materia l.Keywords: Am orphous silicon; PEC VD ; Optoelectronic properties; Scho ttky barrier solar cell

I . I n t r o d u c t i o nW i d e b a n d g a p h y d r o g e n a t e d a m o r p h o u s s i l i c o n m a t e r i a l s h a v e r e c e i v e d m u c h a t t e n -

t io n p a r t ic u l a rl y t o i m p r o v e t h e c o n v e r s i o n e f f i c i e n c y o f m u l t i ju n c t i o n a m o r p h o u ss i l i c o n ( a - S i ) s o l a r c e l l s [ 1 , 2 ] . H y d r o g e n a t e d a m o r p h o u s s i l i c o n c a r b i d e i s t h e c o n v e n -t i on a l c a n d i d a t e f o r t h is p u r p o s e [ 3 - 5 ] . T h o u g h i n c o r p o r a t i o n o f c a r b o n i s i n s t ru m e n t a li n i n c r e a s i n g t h e o p t i c a l t r a n s p a r e n c y , i t i n t r o d u c e s d e f e c t s a n d d i s o r d e r w h i c h d e g r a d et h e e l e c t r o n i c p r o p e r t i e s o f t h e a l l o y f i lm s . T h e r e a r e a l s o a f e w r e p o r t s [ 6 , 7 ] o n t h ea p p l i c a t i o n o f w i d e g a p a - S i O : H l a y e r s i n s o l a r ce l ls . B u t t h e u s e o f a ll o y m a t e r i a l sr e s u l ts i n a n i n c re a s e o f d i s o r d e r a n d d e e p d e f e c t d e n s i t y w h i c h d e g r a d e t h e p h o t o s e n s i -t i vi t y o f th e m a t e ri a l. R e c e n t l y p r e p a r a t io n o f w i d e b a n d g a p a - S i : H f i lm s h a v i n g g o o de l e c t r o n i c p r o p e r t i e s h a s b e e n r e p o r t e d [ 8 , 9] . T h e p r i n c i p l e b e h i n d t h e i n c r e a s e o f th eo p t i ca l g a p i n v o l v e s t h e c o m b i n a t i o n o f h y d r o g e n b o n d i n g a n d i ts c o n c e n t r a ti o n , t a k i n g

Corresponding author.0927-024 8/97/$17.00 Cop yright 1997 Elsevier Science B.V . All rights reserved.P II S 0 9 2 7 - 0 2 4 8 ( 9 6 ) 0 0 0 3 0 - X


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116 S. C Saha el al. / Solar Ene


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S.C. Saha et al. / Solar Energy Materials and Solar Cells 45 (1997) 115-126 11 72 .1

1 . 9

1 .71 6 3

'@t / )~, 1 6

f

& . .. - - - - - &

u

1 6 1 3 |0

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" o2 ' 0 & = OH 2 / S i H / . ,

Fig. 1 . Opt ica l gap (E~), dark conduct ivi ty (o 'd) , photoconduct ivi ty ( t~ph) and ac t ivat ion energy (A~) of darkconduc t iv i t y o f a-Si :H f i lms a s func t ions o f hydrogen to s il ane ra t i o ( f = H 2 /S iH 4) .

di lu t ion r a t i os ( f = H 2 /S iH 4) a t a subs t ra t e t empera tu re o f 80C whi l e o the r p rocessparamete r s v i z . p res sure and r f power dens i t y were kep t f i xed a t 1 . 0 Tor r and 20m w /c m 2, r e spec t ive ly . F ig . 1 show s the va r ia t ions o f op ti cal gap (Eg) , da rk conduc t iv -i ty ( t rd) , photoconduct ivi ty ( t rph) and act ivat ion energy (Ae) of dark conduct ivi ty ofa -S i :H fi lms as a func t i on o f hydrogen t o s i l ane r a ti o ( f ) . Wi th t he i nc rease o f f , E~inc reases monoton i ca l l y f rom 1 .74 t o 1 .95 eV. Such h igh op t i ca l bandgaps (> 1 .75 eV)

8 0 3 . 2

. ~ / / \ \

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A2 L 0 4 ' 0 2 .H 2 / S i H 4

Fig . 2 . Depen dence o f r a t e o f depos i t i on (R d) and re f rac t ive i ndex (n f ) o f a -Si :H f i lms on hydrogen d i l u ti onra t i o ( f ) .


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1 1 8 S . C . 5 k I e t a l / ' So l a r k .' H er v M a t e r a l . s am / 5 ; o h l r ( 'e ll ,~ 4 5 ( 1 9 9 7 ) 1 1 5 - 1 2 0

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F i g . 3 . V a r i a t i o n s o f o p t i c a l g a p { E , ; ) , , .l u rk , . 'o n d u c l i ,A t } ( ~ r / I p h o t o c o n d u c t i v i t 5 ( o ] , h ) a n d a c t i v a t i o n e n e r g y .{ . . X e ) o f d a r k c o n d u c t i v i t y o f a - S i : H f i lm s w i th r f p o w e r d e n s i t y ' ( ~ i , ) .

a r e g e n e r a l l y a c h i e v e d b y a l l o y in g s i l i c o n w i th c a r b o n [ 3 ,1 5 ] . Bo th o - a n d ~ ,h i n i t ia l l yin c r e a s e a s f i n c r e a s e s f r o m I t o 2 0 a n d t h e n ( rp h d e c r e a s e s a l it tl e c o m p a r e d t o as i g n i f i c a n t c h a n g e i n ( rd o n f u r t h e r d i l u t i o n ( l ) . T h e a c t i v a t i o n e n e r g y i n c r e a s e s s l o w l yo v e r t h e e n t ir e r a n g e o f d il u t io n . F i g. 2 s h o w s t h e d e p e n d e n c e o f th e d e p o s i t i o n r a t e( R d ) a n d r e f r a c ti v e i n d e x ( n / ) o f th e a - S i : H f i l m s a s th e f u n c t i o n o f h y d r o g e n t o s i la n er a t i o ( f ) . I t i s e v id e n t f r o m F ig . 2 t h a t R~ d e c r e a s e s w i th . f wh i l e n I i n c r e a s e s i n t h erang e 5 _< f < 30 an d th e rea f te r i t dec r ease s .

Th us , i t i s c lea r tha t the f i lm exh ib i t s h igh ~{,~, ( I .1 10 4 Sc m J) in co n ju nc t io nwi th l o w ( rd v a lu e ( 9 .9 1 0 j j S c m l ) a n d n , a s h ig h a s 3 .0 4 , e v e n a t a n o p t i c a l g a po f 1 .9 e V . T h e s e r e s u lt s a r e s u p e r i o r t o th o s e o b t a i n e d b y o t h e r g r o u p s f o r a - S i C : H a l l o y[ 1 5 - 1 7 ] . M a t s u d a e t a l. [ 1 6] u s i n g h y d r o g e n d i l u t io n f o r a - S i C : H o b t a i n e d p h o t o c o n d u c -t i v i t y a s h ig h a s - 1 .0 1 0 5 S c m ~ a t a n o p t i c a l g a p o f 1 .9 5 e V : b u t t h e d e p o s i t i o nr a te w a s t o o l o w ( ~ 6 A / m i n ) . M o r e o v e r . t h e p r e s e n t v a l u e o f o~,h i s be t te r than thep r e v i o u s v a l u e r e p o r te d b y H i s h i k a w a e t a l. [ 8 ] w h e r e t h e y c o u l d c l a im t h e m a x i m u m~ h O f -- 1 0 5 S c m i t o r a - S i : H f i h n w i t h E~= 1 . 9 0 e V .3 .1 .2 . E ~ ' c t ( " ' p o w e r

T h e a - S i : H f i l m s r e f e r r ed t o t h is s u b - s e c t i o n w e r e d e p o s i t e d u n d e r v a r i o u s r f p o w e r sk e e p in g s u b s t r a t e t e m p e r a tu r e , p r e s s u r e a n d h y d r o g e n d i l u t i o n r a t i o f i x e d a t 8 0 C , 1 .0T o r r a n d 3 0 , r e s p e c t i v e ly . F i g . 3 s h o w s t h e d e p e n d e n c i e s o f o p ti c a l g a p , d a r k - a n dp h o t o c o n d u c t i v i t y a n d t h e a c t i v a t io n e n e r g y o f d a rk c o n d u c t i v i t y a s a f u n c t i o n o f r f


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S.C. Saha et al. / Sola r Energy Materia ls and So lar Cells 45 (1997) 115 -126 11 93-2

2. 9

2 . 6

~0- o

2J0 4 0 610 80 9J0W p ( r o W / c m 2 )F i g . 4 . R a t e o f d e p o s i t i o n ( R d) a n d r e f r a c t i v e i n d e x ( n f ) o f a - S i : H f i l m s v e r s u s r f p o w e r d e n s i t y ( W p ) .

p o w e r d e n s i t y (Wp ) . I t i s c l e a r th a t E g i n c r e a s e s f r o m 1 . 90 e V t o 1 .9 5 e V w i t h Wv b u t~ph d e c r e a s e s b y o n e o r d e r o f m a g n i t u d e . M o r e o v e r , t h e d e c r e a s e o f n f ( F i g . 4 ) c o u l db e a t t r i b u t e d t o a n i n c r e a s e o f d e n s i t y o f v o i d s . T h e i n c r e a s e o f d e p o s i t i o n r a t e w i t h Wpi s o b v i o u s l y d u e t o i n c r e a s e o f r a d ic a l d e n s i t y w i th i n c r e a s e o f r f p o w e r d e n s i t y .

3.1.3. E ffect of substrate temperatureI n o r d e r t o i n v e s t i g a t e t h e r o l e o f th e s u b s t r a t e t e m p e r a t u r e ( T ~ ) o n t h e p r o p e r t i e s o f

t h e fi l m s , th e f i lm s w e r e d e p o s i t e d a t d i f f e re n t T r a n g i n g f r o m 6 0 t o 2 0 0 C u n d e r t w oh y d r o g e n d i l u t i o n s ( f ) v i z . 5 a n d 3 0 . H o w e v e r , i n b o t h t h e c a s e s c h a m b e r p r e s s u r e a n dr f p o w e r w e r e k e p t f i x e d a t 1 .0 T o r r a n d 2 0 m W / c m 2 , r e s p ec t iv e l y . T a b le 1 s u m m a r i z est h e s u b s t r a t e t e m p e r a t u r e d e p e n d e n c e o f o p t o e l e c t r o n i c p r o p e r t i e s t o g e t h e r w i t h g r o w t hr a t e o f a - S i :H f i l m s p r e p a r e d a t l o w H 2 d i lu t io n ( f ~ 5 ). I t c a n b e s e e n f r o m t h e t a b let h a t w i t h t h e i n c r e a s e o f s u b s t r a t e t e m p e r a t u r e t h e f i l m q u a l i t y i m p r o v e s a t t h e c o s t o fd e c r e a s e o f o p t i c al g a p . T h u s , a t l o w H 2 d i lu t i o n ( f ~ 5 ) t h e m a x i m u m E g is o b s e r v e dto be on ly 1 .79 eV wi th O 'ph~ 2 .9 10 -5 Sc m -1 a t T , ~ 8 0C.T his resu l t i s d i f fe r en t

T a b l e 1S u b s t r a t e t e m p e r a t u r e ( T ~ ) d e p e n d e n c e o f o p t o e l e c t r o n i c p r o p e r t i e s o f a - S i : H s a m p l e s : f = 5 . W p = 2 0m W / c m 2 , a n d P c = 1 ,0 T o r rT~ ( C ) R d ( A / m i n ) E g ( e V ) n f o" ( S c m - I ) O 'ph ( S c m - i ) A e ( e V )6 0 2 8 .3 1 . 76 2 . 6 0 2 . 0 1 0 - l 0 7 . 7 x 1 0 - 6 0 , 8 68 0 3 0 . 4 1 . 79 2 . 7 9 2 . 6 x 1 0 - 9 2 . 9 x 1 0 - 5 0 . 811 20 3 4 . 0 1 .7 7 2 . 8 2 1 . 7 1 0 - I 3 . 4 1 0 - 5 0 . 811 50 4 0 . 0 1 .7 4 2 . 9 4 1 . 0 X 1 0 - t 5 . 2 1 0 - 5 0 . 8 22 0 0 4 3 . 0 1 . 70 3 . 0 4 3 . 2 x 1 0 - 1 8 . 3 1 0 - 5 0 . 8 0


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120 S . C . 3a l t o e t a l . / So l a r Ene r k , y M a t e r i a l s a n d S o l a r C e l l x 4 5 ( 1 9 9 7 ) I 1 5 - 1 2 02 . 0 [

~ f1 . ? f -#ld a

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- 11lO

O " " ~ ' - - - - - O " - - ( T P h - C . .. . . . . . .. . . . - C ,i , o0 -8r

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L I50 100 150 2O 0t s l ' C ~Fig. 5. Variations of optical gap (l: '~). da rk conductivib (~q ). photoconductixity (~rph) and activation energy'( k s ) of dark condu ctivity of a-Si:H films with substrate temperature (T,).

f r o m p r e v i o u s r e p o r t [8 ], w h e r e bJ~ h a s b e e n r e p o r t e d t o b e a s h i g h a s 1 .9 5 e V , a t s i m i l a rh y d r o g e n d i l u t i o n a n d s u b s t r a te t e m p e r a t u r e . W e h a v e a l s o s tu d i e d th e t e m p e r a t u r ed e p e n d e n c e a t h i g h e r h y d r o g e n d i l u t i o n ( / ' - 3 0 ). F i g . 5 s h o w s t h a t p h o t o - a n d d a r k c o n -d u c t i v i t i e s a r e a l m o s t i n d e p e n d e n t o f t h e s u b s t r a te t e m p e r a t u r e . S i m i l a r to l o w d i l u t i o nc a s e , t h e r e i s a d e c r e a s e o f o p t i c a l g a p w i t h t h e i n c r e a s e o f s u b s t r a t e t e m p e r a t u r e , t h eh i g h e s t E~ b e i n g 1 .9 0 e V a t T - 8 0 C . I t i s n o t e w o r t h y t h a t h e r e t h e c h a n g e i n o p t i c a lg a p w i t h s u b s t r a t e t e m p e r a t u r e i s h i g h e r c o m p a r e d t o t h a t a t l o w H e d i l u t i o n . T h u s , i tm a y b e p o i n t e d o u t t h a t o p t i c a l g a p f l e x i b i l i ty e n h a n c e s s i g n i f i c a n t l y f o r h i g h h y d r o g e n

3 . 2 r ~ . . . . . . . . . . .

[28 i

E ~

i

5 0 - 1 --00 1 5 0 20 0T s ( C )

Fig. 6. R ate of dep osit on ( R j) and refractive index ( n, ) ol a-Si:H films versus sub strate tem perature (7",).


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S . C . S a h a e t a l. / S o l a r E n e r g y M a t e r i a l s a n d S o l a r C e l l s 4 5 ( 1 9 9 7 ) 1 1 5 - 1 2 6 121

d i lu t i on . T h e r e i s a m a r g i n a l i m p r o v e m e n t o f p h o t o c o n d u c t i v i t y a n d s o a l s o t h ep h o to sen s i t i v i ty w i th th e in c rease o f su b s t r a t e t emp era tu re . F ig . 6 d ep ic t s t h a t su b s t r a t et e m p e r a t u r e h a r d l y c h a n g e s t h e d e p o s i t i o n r a t e b u t t h e r e i s a l i t t l e i m p r o v e m e n t o fr e f r a c t i v e i n d e x .3.2. Bonding configuration

F i g . 7 d e p ic t s t h e S i - H s t r e tc h i n g m o d e s o f I R v i b r a t i o n a l s p e c t r a o f t h e a - S i: H f i l m sp r e p a r e d a t v a r i o u s d e p o s i t i o n c o n d i t io n s . I n o r d e r t o h a v e a q u a n t i t a t iv e i d e a a b o u t t h eb o n d i n g c o n f i g u r a t io n s , t h e m i c r o s t r u c t u r e p a ra m e t e r , R ( d e f i n e d a s t h e r a t io o f a b s o r p -t i o n p e a k i n t e n s i t y a t 2 09 0 c m - l t o th e t o ta l a b s o r p t io n p e a k i n te n s i ti e s a t 2 0 0 0 c m -a n d 2 0 9 0 c m - 1 ) [ 18 ], a n d t h e b o n d e d h y d r o g e n c o n t e n t ( C H ) h a v e b e e n c a l c u l a t e d . T h eR an d C H v a lu es a lo n g w i th th e i r p rep ara t io n co n d i t io n s a re g iv en in T ab le 2 . I t can b es e e n t h a t a s h y d r o g e n d il u t io n i s in c r e a s e d t h e r e i s a n in c r e a s e o f 2 0 0 0 c m - ~ p e a kc o r r e s p o n d in g t o S i l l s t re t c hi n g m o d e c o m p a r e d t o S i H 2 / ( S i H 2 ) . s t re t c hi n g m o d e

~ 0 0 0

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N

1 0 0 0

( 3 1

0 ~1 8 0 0 1 9 0 0 2 0 0 0 2 1 0 0 2 2 0 0 2 3 0 0W o v e n u m b e r cm 1 )

Fig . 7 . The IR abso rp t ion coe f f i c i en t (o r ) i n t he S i -H s t re t ch ing reg ion o f a -Si :H f i lms p repa red unde rd i f f e re n t d e p o s i t io n c o n d it i o n s . C u r v e 1 : f = 1 , W p = 2 0 m W / c m 2 ; C u r v e 2 : f = 3 0 , W p = 2 0 m W / c m 2 ;C u r v e 3 : f = 3 0 , W p = 4 8 m W / c m 2 .


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122 S.C. Sah d et al. / Sola r lsnerg, v Metlerml.s a~ul SM ar Cells 45 ( 1997~ 11 5- 12 6

Table 2Bonded hydrogen content (Cn), microstructure parame|cl { R L along with the properties of a-Si:H filmsprepared under different hydrogen dilutions (/) and rf powers: 7[ = 80C/ ( H e / S i l l 4 ) 14~, (m W /c m -~ &. (cV) n, C H (atC/~) R I '~ h ( S c I ] ] l )I 20 1.74 2 4 t} 16.1 0.57 2.9~ " 10 e,30 2{} 1.9{) :,0 4 25.7 0.3 0 1.1 >4 10 ~'30 4,"; 1.83 2.54 ~ '8.. {).37 8.1 ~ I0 s

a p p e a r i n g a t 2 0 9 0 c m ~ [ 19 ]. T h e r e is a s i g n i f i c a n t d e c r e a s e o f R v a l u e f r o m 0 . 5 7 t o0 . 3 0 f o r th e i n c r e a s e o f d il u t i o n f r o m I t o 3 0 . s u g g e s t i n g a l e s s p o r o u s a n d c o m p a c ts t r u c tu r e o f t h e f i l m p r e p a r e d u n d e r h i g h h y d r o g e n d i l u t i o n . T h e i n c r e a s e o f C H a n dd e c r e a s e o f R w i t h h y d r o g e n d i lu t io n a c c o u n t f o r i n c r e a se o f E,~ f r o m 1 . 74 e V t o 1 . 9 0e V . I n F i g . 7, t h e p e a k s h i f t o f c u r v e 3 t o w a r d s h i g h e n e r g y c o m p a r e d t o t h a t o f c u r v e 2i n d i c a t e s t h a t i n c r e a s e i n i-1" p o w e r c a u s e s a n i n c r e a s e o f p o l y h y d r i d e s . T h i s i s a l s os u b s t a n t i a t e d b y h i g h e r R v a l u e a n d b o n d e d h y d r o g e n c o n t e n t ( T a b l e 2 ) . T h u s in s pi te o fa l it tl e i n c r e a s e o f C H a t h i g h e r r f p o w e r , t h e d e c r e a s e o f E ~ f r o m 1 .9 0 e V t o 1 .8 3 e Vm a y b e a t tr i b u te d t o t h e i n c re a s e o f R . H o w e v e r . a n i n c r e a se o f r f p o w e r d o e s n o te n h a n c e t h e in c r e a s e o f S i l l , p e a k a t 2 0 9 0 c m ~ s i g n i f ic a n t l y t h o u g h i t h a s b e e no b s e r v e d b y o t h e r s [ 8 , 2 0 2 2 ], u s u a l l y in c a s e o f h i g h p o w e r d e p o s i t e d f i lm s .

3 . 3 . D e f e c t d e n s i t ~ a n d q t ta ~ Hl tl H < [ l i c i c v q v

T h e i m p r e s s i v e p h o t o c o n d u c t i v i t y o b s e r v e d i n h i g h g a p a - S i : H s a m p l e s a l l o w e d u s t om e a s u r e t h e su b g a p a b s o r p t i o n b y d ua l b e a m p h o t o c o n d u c t i v i ty ( D B P ) m e a s u r e m e n t . A si t is w e l l k n o w n , t h is m e a s u r e m e n t is f a i r l y i n s e n s i t i v e t o t h e s u r f a c e s t a t e s a n dc o n s e q u e n t l y s u i ta b l e f o r d e t e r m i n i n g t h e d e e p d e f e c t d e n s i ty ( N d ) in t h in s a m p l e s . T h es p e c tr a l d e p e n d e n c e o f o p ti c a l a b s o r p t i o n c o e f f i c i e n t , in th e l o w e n e r g y r e g i o n , m e a -s u r ed b y D B P o f a - S i : H g r o w n u n d e r d i f f e r e n t c o n d i t io n s a r e d e p i c t e d in F ig . 8 . T h ed e f e c t d e n s i ty o f t h e s a m p l e s w a s e s t i m a t e d f r o m t he e x c e s s s u b g a p a b s o r p t i o n w h i l e t h eU r b a c h e n e r g y , E , , , a m e a s u r e o f t h e w i d th o f th e v a l e n c e b a n d t ai l w a s d e t e r m i n e d b yi ts s l o p e . T h e d e e p d e f e c t s ( N d ), t h e U r b a c h e n e r g y o f s o m e s e l e c t e d s a m p l e s a l o n g w i t ht h e i r d a r k a n d p h o t o c o n d u c t i v i t i e s a r e p r o v i d e d i n T a b l e 3 . It c a n b e s e e n f r o m t h e ta b l et h at th e r e i s a s y s t e m a t i c d e c r e a s e o f N~ w i th h y d r o g e n d i l u t i o n w h i c h i s c o n s i s t e n t w i t h

Table 3Defecl density (Nd ), Urbach energ) ( L,, ) , ~r,h and L.._ ol a-N i:H lihns prepared at different hydrogen dilutions(1) and substrate temperatures (7-[). ~+'i,= 20 m W /c m ~, and t~ = 1.0 Tort/ ' (H _~ /SI H 4) T UC) f:'~ (eV) l :o (meV ) % h (S cm I ) Nd (cm ~)I HI) 1.74 52 2.9X 10 ~' 7. 9 I0 >2{) 8{) 1.80 58 9.4> ,'10 " 1. 8X l( ) ~'30 80 1.90 5~ 1.1 x I0 4 l a x IO n,30 250 1.81 59 1.9>(, I0 4 5 .2 x I0 ~('


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S . C . S a h a e t a l. / S o l a r E n e r g y M a t e r i a ls a n d S o l a r C e l l s 4 5 ( 1 9 9 7 ) 1 1 5 - 1 2 6

1 0 S r

1 2 3

l o 4

E~1o3C(~(J

: = l OOOEO~ . ( 1 :

/ ( 2 ), o 0 LI I I I /1 10 1 , 2 1 ' - 4 1 . 6 1 -8 , . 0 2 . 2P h o t o n E n e r g y ( e V )

Fig. 8. The absorption coefficient (~ ) versus photon energy (ht,) for a-Si:H films prepared und er differentdeposition conditions. Curve 1: f= 1, T, = 80C; Cu rve 2: f = 20, T, = 80C; Cu rve 3: f = 30, 7", = 80C;Cu rve 4: f = 30, T~ = 250C.

t h e i m p r o v e m e n t o f O -ph. B u t , t h e r e i s a n i n c r e a s e o f E o u p o n h y d r o g e n d i l u t io n .U s u a l l y , i n c a s e o f a l l o y m a t e r i a l s , N d i n c r e a s e s w i t h a n i n c r e a s e o f E g b u t t h e p r e s e n tr e s u l ts d o n o t i n d i c a t e in c r e a s e o f N a w i t h E g . W e r e p o r t th e d e f e c t d e n s i t y o f t h ea - S i : H f i l m , g r o w n a t 8 0 C , a s l o w a s 1 .4 x 1 01 6 c m - 3 a t a n o p t i c a l g a p o f 1 .9 0 e V .T h i s f i lm e x h i b i t s a h i g h p h o t o c o n d u c t i v i t y o f 1.1 1 0 - 4 S c m - 1 b e c a u s e o f l o w d e f e c td e n s i ty . S a n y o g r o u p [ 8] re p o r t e d p h o t o c o n d u c t i v i t y o f 1 0 - 5 S c m - 1 f o r a - S i : H f i l mp r e p a r e d a t t h e s a m e t e m p e r a t u r e w i t h d e f e c t d e n s i t y o f 3 . 2 1 0 t6 c m 3. It is t o b en o t e d t h a t i n c o m p a r i s o n t o l o w T ,, h i g h ~ ( 2 5 0 C ) r e s u l t s i n a n i n c r e a s e o f Nd u n d e rs u c h a h i g h h y d r o g e n d i l u t i o n b u t t h i s i s n o t r e f l e c t e d i n p h o t o c o n d u c t i v i t y , p o s s i b l yb e c a u s e o f d e c r e a s e o f E g.

T o u n d e r s t a n d t h e d e v i c e p e r f o r m a n c e o f th e o p t i m i z e d f i l m t h e s p e c tr a l r e s p o n s e o fq u a n t u m e f f ic i e n c y ( Q .E ) w a s t a k e n o n t h e S c h o t t k y b a r r ie r s o l ar ce l l w i th s s / n + a -


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124 S.C. Saha et a l. /S ol ar Energy Materials and Solar CelLs 45 (1997) 115-1261 0 0 . 0 0

8 0 . 0 0v

6 0 - 0 0OJU. J

E l . , 0 . 0 00o

2 0 . 0 0

/ f - ~ \ - - ( 1 )// , ) - ' ,

. \

/

i

0 0 0 1 L . . . . . . I3 0 0 4 0 0 5 0 0 6 0 0

( 2 )

i '

'k \\

700 800W a v e l e n g th ( n m )Fig. 9. Internal quantum efficiency' spectra of high band gap (1.90 eV) a-Si:H (curve I) and mo ral bandgap(1.75 eV) a-Si:H (curve 2),

S i : H / a - S i : H / P d s tr u ct u re . A c o m p a r i s o n o f Q . E o f t hi s fi lm w i t h t ha t o f no r m a l h i g ht e m p e r a tu r e d e p o s i t e d a - S i :H ( k ~ = 1 .7 5 e V ) h a s b e e n p r e s e n t e d i n F ig . 9 . C l e a r l y t h eo p t i m i z e d h i g h g a p a - S i : H s h o w s a h i g h e r Q . E t o w a r d s t h e l o w e r w a v e l e n g t h s i d ec o m p a r e d t o t h a t f o r n o r m a l o n e a n d a s a r e s ul t t he p e a k o f th e Q . E c u r v e e x h i b i t s as h if t s t o w a r d s h i g h e r e n e r g y .

4 . D i s c u s s i o n

T h e d e c r e a s e o f d e p o s i t i o n r a t e w i t h h y d r o g e n d i l u ti o n i s o b v i o u s l y d u e t o d e p l e t io no f f i lm g r o w i n g p r e c u r s o r s . A t h ig h d i l u ti o n , t h e e n h a n c e d c o n c e n t r a t i o n o f a t o m i ch y d r o g e n m a y c a u s e e t c h i n g f r o m t h e t o p g r o w i n g s u r fa c e a n d g r o w t h z o n e o f t h e f il mr e s u l ti n g i n a r e d u c e d r a t e o f d e p o s i t i o n . T h e m a x i m u m r e f r a c t iv e i n d e x a t t h e h y d r o g e nd i l u t io n r a ti o o f 3 0 i s an i n d i c a t i v e o f d e n s e n e t w o r k s t ru c t u r e. T h e g o o d q u a l i t y o f t hem a te r i a l i s f u r t h e r s u b s t a n t i a t e d b y t h e h ig h p h o to c o n d u c t i v i t y ( 1 .1 1 0 - 4 S c m ~) a swe l l a s p h o to s e n s i t i v i t y m o r e t h a n l 0 6 w i th a r e a s o n a b l e o p t i c a l g a p o f 1 .90 e V .A l t h o u g h a n o p t i c a l g a p o f 1 ,9 5 e V i s o b t a i n e d a t h y d r o g e n d i l u t i o n r a t io o f 4 0 , t h ea s s o c i a t e d e l e c t r o n i c p r o p e r t i es a r e n o t so e n c o u r a g i n g . I n c r e a s e o f p o w e r d e n s i t y w i t ho th e r d e p o s i t i o n c o n d i t i o n s s im i l a r t o 1 .9 0 e V f i lm r e s u l t e d i n f i lm b e s e t w i th i n c r e a s e o fm i c r o s t r u c t u r a l d e f e c t s , d e c r e a s e o f o pt i c a l g a p a n d d e t e r i o r a t io n o f o p t o e l e c t r o n i cp r o p e r ti e s . T h e s e a r e a s c ri b e d t o th e h i g h r a te o f d e p o s i t i o n a n d t h e b o m b a r d m e n t o f


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S . C . S a h a e t al . / S o l a r E n e r g y M a t e r i a l s a n d S o l a r C e l l s 4 5 ( 1 9 9 7 ) 1 1 5 - 1 2 6 125energe t i c i ons and e l ec t rons a t t he g rowing sur f ace making t he f i lm more de fec t i ve [23] .The mic ros t ruc tu re pa ramete r , R , i s dependen t on t he depos i t i on condi t i ons and r ap id lyd e c r e a s e s w h e n S i l l 4 i s d i l u t e d b y H 2 a n d t h e r f p o w e r i s r e d u c e d . P h o t o c o n d u c t i v i t y i sc lose ly r e l a t ed t o s t ruc tu ra l de fec t s such as dang l ing bond dens i t y and mic ros t ruc tu ra lde fec t s . Thu s , i n our p resen t s t udy we m ay in fe r t ha t a -S i :H f i lm prepared w i th 80C an da t hydrogen d i l u t i on r a t i o o f 30 exh ib i t s good op toe l ec t ron i c p roper t i e s . In f ac t i t sop toe l ec t ron i c p roper t i e s a r e be t t e r t han t ha t o f an a -S iC :H f i lm of s imi l a r op t i ca l gap(1 .90 eV) . A r e l a t i ve ly smal l i nc rease o f Urbach energy of t he a -S i :H f i lms wi th t heo p t i ca l g a p ( T a b l e 3 ) a c c o u n t s f o r t h e o b s e r v e d l o w d e f e c t d e n s it ie s c o m p a r e d t o t h o s efor a -S iC :H f i lm s of s imi l a r op t ica l gap [24] . The h igh pho tosens i t i v i t y (1 .1 X 1 0 6 ) o fth i s op t imized h igh gap a -S i :H i s a t t r i bu t ed t o l ow defec t dens i t y o f 1 .4 1 01 6 c m - 3 .S u c h l o w v a l u e o f d e f e c t d e n s i t y i n c o m p a r i s o n w i t h a - S i C : H f i l m i s c l e a r l y d u e t oa b s e n c e o f c a r b o n i n t h e f il m . T h e h i g h e r q u a n t u m e f f i c ie n c y (Q . E ) o f th e o p t im i z e df i lm, pa r t i cu l a r l y i n t he l ower wave l eng th s i de , t han t ha t fo r normal a -S i :H depos i t ed a th igh t em pera tu re , is t he s i gna ture o f enhan cem ent o f op ti ca l gap . The su f f i c i en t l y goo dQ . E s p e c t ru m o f t h e d e v e l o p e d f i l m i s i n g o o d a g r e e m e n t w i t h th e o b s e r v e d o p t o e l e c -t ronic proper t ies .

F r o m t h e a b o v e d i s c u s s i o n i t m a y b e c o n c l u d e d t h a t h i g h h y d r o g e n d i l u t i o n , l o ws u b s t r a t e t e m p e r a t u r e a n d l o w p o w e r a r e c o n d u c i v e t o o b t a i n g o o d q u a l i t y w i d e g a pa-S i :H f i lms . A l l the abo ve de pos i t i on pa ramete r s f avo ur to a t t a in low grow th r a te o f t hef i lm. Thus , i t may be c l a imed tha t depos i t i on r a t e has an impor t an t ro l e on t he e l ec t r i ca n d o p t i c a l p ro p e r ti e s o f a - S i :H f i l m s g r o w n b y P E C V D . T h e d e p o s i t io n o f a - S i : H f i lm sb y P E C V D i s c o n s i d e r e d t o b e c o n t r o l l e d b y g a s - p h a s e r e a c t i o n a n d s u r f a c e r e a c t i o n .H o w e v e r , t h e g r o w t h p r o c e s s o f a - S i : H u n d e r a b o v e d e p o s i t i o n c o n d i t i o n s m a y b et h o u g h t t o b e C V D l i k e [ 2 5 ] h a v i n g l o w g r o w t h r a t e , a c c o m p l i s h e d v i a t h e s u r f a c ereac t i on . When the f i lm depos i t i on r a t e i s l ow, more t ime i s ava i l ab l e fo r su r f acereac t i ons which f ac i l i t a t e fu r the r s t ruc tu ra l r e l axa t i on o f S i -S i ne twork of a -S i :H f i lm.On the con t r a ry , t he e f f ec t o f depo s i t ion r a te a t h igh TS i s no t so s i gn i f i can t s i nce a t sucht e m p e r a t u r e s u r f a c e re a c t io n i s i n d e e d h i g h . M o r e o v e r , a t h i g h h y d r o g e n d i l u ti o n , h i g h e rh y d r o g e n c o v e r a g e o f t h e g r o w i n g s u r f a c e h e lp s t h e d i f f u s i o n o f r a d i ca l s h a v i n g l o n g e rl i f e t im e to seek en erge t i ca l l y f avou rab l e s i te s [16 ,26] w hich he lps t he fo rma t ion o f densene twork s t ruc tu re . Fur the r i t i s be l i eved t ha t hydrogen e t ch ing pre fe ren t i a l l y e l imina t eenerge t i ca l l y unfavourab l e s i l i con bonding conf igura t i on , l eav ing beh ind t he s t ab l econf igura t i on o f h igh-qua l i t y f i lm [27] . Thus , pa r t i cu l a r l y a t l ow subs t r a t e t empera tu re ,h igh hydrogen d i l u t i on i s needed t o r ea l i ze t he sur f ace r eac t i on as we l l a s t he ne tworkr e la x a t io n . T h i s a c c o u n t s f o r p r o n o u n c e d o p t ic a l g a p f l e x ib i l it y a t h i g h h y d r o g e nd i l u ti o n . T h e o p t ic a l g a p w i d e n i n g i s p r i n c i p a ll y d u e t o h i g h b o n d e d h y d r o g e n c o n t e n t ,m o s t l y i n t h e f o r m o f m o n o h y d r i d e o f s i li c o n ; it s g o o d o p t o e le c t ro n i c p r o p e rt ie s c o u l db e a s c r ib e d t o t h e c o n s e q u e n c e o f d e n s e a n d r e l ax e d s tr u c tu r e .

5 . C o n c l u s i o n sW i d e b a n d g a p a - S i : H f i l m s h a v e b e e n p r e p a r e d a t l o w t e m p e r a t u r e u n d e r s u i t a b l eh y d r o g e n d i l u t io n . L o w s u b s tr a te t e m p e r a t u r e , h ig h h y d r o g e n d i l u t io n a n d l o w r f p o w e r


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12 6 S.C. Saha et al. / Solar l~m'rgv Materials and Solar Cells 45 (1997) 115 -12 6

a r e t h e r e c i pe t o de v e l o p s uc h f i l m , T h e o pt i m i z e d s a m pl e h a v i n g o pt i c a l g a p o f 1 . 9 0 e Vis found to possess be t ter optoe lec tronic proper t ie s compared to i t s a -SiC:H counterpar to f s imi lar opt ica l gap. This una l loyed f i lm thus prepared o f fers the promise o f i t s use a sph otosen s i t ive layer in a -Si so lar ce l l .

A c k n o w l e d g e m e n t sThe authors would like to thank Professor A.K. Barua for useful discussions and

active interest in the problem. One of us (SG) gratefully acknowledges financial supportby C.S.IR. of India,

R e f e r e n c e s[1 ] K . S . L i ra , M. K onag a i and K , T akahas h i , J . App l . P h~ s 56 (1984) 538 .[2] R.R. Arya, A. Cata lano und I) .E . C arls~u/, Proc . 6th Ira. PhL)/ow)ltuic Sdc uc c an d Eng ine erin g Co nf. .

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Photovo l t a i c Sc ience and E ng in ee r ing Conf . , L as Vegas , 1988. p . 79 .[6 ] Y . Ich ikawa , S . Fu ja j ikake , T . T akayama . S . Sa i to . H . Ohta , T . Y os h ida , T . Iha ra and H. Saka i , Proc .

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