EFFECTS OF THE SUBSTRATE POTENTIAL ON THE INCORPORATION MANNER OF HYDROGEN AND IMPURITY IN a-Si : H FILMS

I n t r o d u c t i o n . I n hydrogenated amorphous S i f i l m s p r e p a r e d by plasma d e p o s i t i o n f r o m monos i lane , t h e r e a r e t o o many p a r a m e t e r s i n d e t e r m i n i n g t h e i n c o r p o r a t i o n manner o f hydrogen and i m p u r i t y atorns and t h e r e b y s t r u c t u a l and e l e c t r o n i c p r o p e r t i e s [ l , 2 , 3 , 4 ] . W h i l e , as demonst ra ted by FRITZSCHE[4], K N I G H T s [ ~ ] , TSAI [ 6 ] and OKAMOT0[7], s u r f a c e p o t e n t i a l a t t h e s u b s t r a t e a l s o has a s i g n i f i c a n t in f111enr.e o n t h e o v e r a l l p r o p e r t i e s o f a -S i :H f i l m s a s w e l l a s o t h e r c o n v e n t i o n a l d e p o s i t i o n parameters ; s u b s t r a t e tempera t u r e , gas p r e s s u r e , gas f l o w r a t e , r f power and so f o r t h . F o r a f u r t h e r d e t a i l e d r e c o g n i t i o n o f s u r f a c e p o t e n t i a l e f f e c t s , we have conduc ted a s y s t e m a t i c i n v e s t i g a t i o n s hy e m p l o y i n g t h e c r o s s f i e l d plasnia d e p o s i t i o n method [7 ] w h i c h makes i t p o s s i b l e t o c o n t r o l t h e s u b s t r a t e p o t e n t i a l s e p a r a t e l y f r o m o t h e r d e p o s i t i o n parameters . T h i s paper p r e s e n t s a s e r i e s o f e x p e r i m e n t a l d a t a o n t h e hydrogen and boron i n c o r p o r a t i o n , t h e e l e c t r i c a l , o p t i c a l and 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 undoped and i m p u r i t y doped a S i : H f i l m s d e p o s i t e d under t h e e x t e r n a l l y b i a s e d c o n d i t i o n . B e s i d e s , s i n c e optical-emission-spectroscopy(0ES) o f a s i l a n e g l o w d i s c h a r g e i s , i n t h e g e n e r a l concensus, expec ted t o be a u s e f u l t o o l f o r u n d e r s t a n d i n g t h e p lasma c h e m i s t r y [ 8 ] , we a l s o d i s c u s s t h e r e s u l t s o f OES d u r i n g t h e plasma d e c o m p o s i t i o n w i t h b i a s e d c o n d i t i o n i n conj u n c t i o n w i t h t h e f i l m p r o p e r t i e s . Cross f i e l d plasma de o s i t i o n system.F i g u r e I shows a s c h e P a t i c i I l u s t r a t :on o f t h e d e o o s i t i o n svstem ernDloved i n t h e work . The , , plesma was e x c i t e d by 13.56 M H ~ r f o s c i l l a t e r t h r o u g h t h e symmet r i c c o u p l i n g e l e c t r o d e s A and B s e t t l e d o u t s i d e t h e r e a c t i o n t u n n e l chamber o f 14 cm i n d i a m e t e r , 2 5 cm i n h i g h t . An a d d i t i o n a l dc e l e c t r i c f i e l d was super imposed p e r p e n d i c u l a r l y t o r f e l e c t r i c f i e l d by u s i n q two p a r a l e l l e l e c t r o d e s C and D s e p a r a t e d by 14 cm. S u b s t r a t e m a t e r i a l s were ?-. . &$. I ( . c h ~ ' z t i c d i n p u ~ ~f CIWSS fieLd h o l d e d o n t h e r o t a t i n g e l e c t r o d e D o f 10 cm i n d i a m e t e r . One o f t h e n o t i c e a b l e m e r i t s o f I; ? c c c iqssi <.Lo: r;nci:od. , >. , :' <::5*. .? .F.sp ;,..;-:-,s.:--. '--:-.r:;:," -.,-,. .-.,: ..,.... . . . A J ~ ~ : ~ ' . . . ; : "':. :: ..., r !cr: ' , : ; .c; : : , 3 : ; .... ,. . . , , . , . . " . . . . L,. !.CCZC .-.?-c,:.,:..Lch:L,,.:. :j:j.;';:AL L C , L Z . . , ?.<>.>.., ! . ! , sgo . Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19814138 C4-632 JOURKAL DE PHYSIQUE t h i s d e p o s i t i o n system i s t h a t t he s u b s t r a t e p o t e n t i a l w i t h respect t o t h e plasma can be c o n t r o l l e d independent ly f rom the energy d e n s i t y o f t h e plasma e x c i t e d by r f e l e c t r i c f i e l d [ 9 ] . The d i s t a n c e between the c e n t e r o f t he r f c o u p l i n g e l e c t r o d e s and s u b s t r a t e was f i x e d t o be 1 1 crn th roughout t he p resen t work. A dc b i a s v o l t a g e Vb a p p l i e d between the e l e c t r o d e s C and D, t h a t i s V D V C , was v a r i e d f rom -150 t o +150V. F igu re 2 shows the Ib-Vb c h a r a c t e r i s t i c s . As the e l e c t r o d e C i s l o c a t e d deep i n t he p o s i t i v e column, t he i on c u r r e n t i n t o t he -200 -100 e l e c t r o d e C cou ld cancel o u t t h e s a t u r a t i o n . e l e c t r o n c u r r e n t i n t o t he e l e c t r o d e D. Theref o r e , t he b i a s v o l t a g e Vb i s f a i r l y i d e n t i c a l w i t h the s u b s t r a t e p o t e n t i a l VD. Regarding t h i s 0 35;v c h a r a c t e r i s t i c s as t h a t o f LANGMUIR plasma 2 0 5 5 w I l r . 2 5 0 " ~ d i a g n o s t i c p robe [ lO ] , t he v o l t a g e Vb a t lb=0 i n 6 0 ~ ~ 2-3 i.,, Fig .2 i n d i c a t e s t h e f l o a t i n g p o t e n t i a l V f a t which equal number o f e l e c t r o n s and ions a r r i v e :.fe. 7 1: Y. c h r c c : e ~ ; s t i c s o f ~ i i i /si ??ow a t a sur face. The c h a r a c t e r i s t i c v o l t a g e V ; .. U 4 a'isc&zryr; as c .~,crmetcr of rjpower. so c a l l e d t he plasma p o t e n t i a l , above w h i c t a c u r r e n t l b (main ly e l e c t r o n c u r r e n t t o t h e e l e c t r o d e D) s a t u r a t e s i n t h e f i r s t quadrant i n F ig .2 i s t he space p o t e n t i a l i n t he plasma. I n t h e case o f a f l a t bed system t r i e d , f o r example,by KNIGHTS e t a l . [4,5,6] , t h e s u b s t r a t e p o t e n t i a l i s e s s e n t i a l l y modurated by t h e a p p l i e d r f v o l t a g e [ l 0 ] and c o u l d n o t be e x t e r n a l l y c o n t r o l l e d f r e e l y [ l l ] . On t h e c o n t r a r y , i n ou r system, as the e l e c t r o d e s C and D a r e e l e c t r i c a l l y i s o l a t e d from t h e r f c i r c u i t , t he s u b s t r a t e p o t e n t i a l can be d e f i n i t e l y c o n t r o l l e d s t a b l y by s e t t i n g o f Vb. Amorphous Si:H f j l r ns were depos i ted o n t o 7059 Corning g l a s s o r h i g h r e s i s t i v e c r y s t a l l i n e S i wafer f rom monosi lane, monosi lane/phosphine o r monos i lane/d iborane m i x t u r e d i l u t e d w i t h hydrogen gas t o 3-10 vo l .% a t t he s u b s t r a t e temperature o f 250°C. T o t a l gas f l o w was ma in ta ined t o be 60-80 sccm w i t h a pressure o f 1-2 t o r r and i n p f t power was 55W. Under these c o n d i t i o n s , t h e d e p o s i t i o n r a t e s were 0.7, 1.2 and 1 . 1 A/ sec f o r P-doped, B-doped and undoped f i l m s , r e s p e c t i v e l y . These d e p o s i t i o n r a t e s a r e almost independent o f t h e p o l a r i t y and magnitude o f t h e b ias v o l t a g e Vb l e s s than 4 150 V. Th is f a c t means t h a t t he b i a s v o l t a g e a p p l i e d between t h e e l e c t r o d e s D and C does not d i s t u r b t he plasma decomposi t ion process i n the p o s i t i v e column. E f f e c t s o f the b i a s vo l t aqe on hydrogen i n c o r p o r a t i o n . As the f i r s t s t e p i den t i f i c a t i o n o f a -S i :H f i l m s produced by c ross f i e l d method, i r abso rp t i on spec t ra measurements were made w i t h f i l m s depos i t ed on h i g h p u r i t y c r y s t a l l i n e S i wafer . F igu re 3 shows t h e hydrogen con ten t CH and a b s o r p t i o n c o e f f i c i e n t a a t t h e group) and 2000 cm-' ( i s o l a t e d monohydrides) as a f u n c t i o n o f Vb. As seen i n t h e f i g u r e , a c l e a r s t r e t c h i n g mode f requenc ies 2090 cm-1 ( d i hyd r i de and cont inuous in terchange o f i r a b s o r p t i o n peaks f rom 2090 t o 2000cm-~ i s observed i n co r res pondence w i t h t h e v a r i a t i o n o f Vb f rom t h e p o s i t i v e t o n e g a t i v e va lue, even a t t he s u b s t r a t e temperature o f 250°C. The bending mode a b s o r p t i o n peaks near 840 and 890cm-I ( d i h ~ d r i d e group) have been a l s o observed i n f i l m s depos i ted under the 5 5 W l i c m o i b i a s v o l t a g e more than +100V. Resu l ts o f P-doped 2 250 "C f i l m s a l s o e x h i b i t t h e same tendency w i t h the 0 . v a r i a t i o n o f Vb. These f a c t s show t h a t t h e nega-1 00 0 100 t i v e and p o s i t i v e b i a s v o l t a g e promote hydrogen B ias Vol!age ( v ) atoms t o be i nco rpo ra ted m a i n l y as monohydrides Fig.3 liyhogen content cH and absorption and d i hyd r i des, r e s p e c t i v e l y . I n c o n t r a s t t o t h i s , coe,Fficient at Si-H 2ibrationoL mode fpequCH a r e almost independent o f t he b i a s v o l t a g e and they a r e about 16, 17, 1851 a t .% f o r undoped, Pas a function O f bias voztage. doped and B-doped, r e s p e c t i v e l y . We have c a r r i e d ou t the e l e c t r i c a l , o p t i c a l and o p t o e l e c t r o n i c measurements i n o r d e r t o understand how these d i f f e r e n t i n c o r p o r a t i o n manner o f hydrogen atoms r e f l e c t on t h e p r o p e r t i e s o f a -S i :H f i l m s . F igu re 4 show5 the normal ized pho t o c o n d u c t i v i t y q g r under 1.9eV i l l u m i n a t i o n (3x 1014photons/sec/cm2) and t h e dark cond u c t i v i t y od a t room temperature as a f u n c t i o n o f Vb. As can be seen i n t he f i g u r e , t he re e x i s t s t he optimum b i a s v o l tage Vop f o r t h e photoconduct ive p r o p e r t y o f undoped f i l m s and a t t h i s b i a s v o l t a g e od e x h i b i t s minimum. Vop i s r e l a t e d w i t h r f power as a l s o shown i n F ig .4 , and the va lues o f Vop a r e about 0 and + 50 V f o r the power o f 35W and 55W, r e s p e c t i v e l y . The plasma p o t e n t i a l V, , as a l ready r e f e r e d P: -,.< ijoormaLizei s%otcconduc:i.;ic ;n,r cn.' &yi;( c y t i n t he p rev ious sec t i oh , was about +6OV and + l lOV f o r 35W and 55W, r e s p e c t i v e l y . I t i s uot ivi t? od n: rgom tr..nperai! p l . I'hys. L e t t . , 35 (171?) 2 4 4 . I61 TSAI C.C. a:ld FKITZSQ!!: 1 1 . . S o l a r I:nergy I l . ? ~ e r i n l s 1 (1979) 29. [71 OKWOTO I!. YAMACUC!!I T. SITTA Y. and IlrtV.AK,\Gh Y . , J . S o n c r y s ~ . Sc;lirls 356)6 (1980) 201. I81 K,\?IPAS Y . J . an(1 CilIFi'ITll R . G . , S o l a r C r l l s 2 (I 'J8Oj 385. (91 TAWAI)A Y., YIL'iAGUCIII T . , HOXOXlJRn S . , Ilo;';n S . , Oi:.LYOTO 11. 2nd IIA~L\KL\I<A Y . , J;,r. J . Appl. Pitys. 22 s11vp1.2 (1981) 213. . . [IO]COI3CKh' J . W . and KAY E . , J . Appl. I'11j.s. [a (1972) c.965. [IIIVOSSES J . L . and CUOHO J . J . , in " T i t i n Fi lm 2 r o c c s s " r d . VOSSEN .l .L. .md Zi:&Y b!., cns2p.Xl , Acndcntic P r e s s , New York (1978) 11 . (121CAKLSOS V.I<. et a1 . . SEKI Repol.: \:o. SLRJIPil-C-8254-3 (1980) . (1D]P(I\'rSCIlA A., SAKACAWA K . , Th\AKA S., YATSCWJKA S . , YA'USAKI S . , OKLSHI t!. and IIZI:% S., J . Non-crysi . S o l i d s a 6 2 (1980) 163.