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TEMPERATURE DEPENDENCE OF THE ELECTRON DRIFT MOBILITY IN HYDROGENATED a-Si PREPARED BY
SPUTTERING
T. Tiedje, T. Moustakas, J. Cebulka
To cite this version:
T. Tiedje, T. Moustakas, J. Cebulka. TEMPERATURE DEPENDENCE OF THE ELECTRON
DRIFT MOBILITY IN HYDROGENATED a-Si PREPARED BY SPUTTERING. Journal de
Physique Colloques, 1981, 42 (C4), pp.C4-155-C4-158. �10.1051/jphyscol:1981431�. �jpa-00220887�
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CoZZoque C4, suppZe'ment au nOIO, Tome 42, octobre 1981 page C 4 - 1 5 5
TEMPERATURE DEPENDENCE OF THE ELECTRON DRIFT MOBILITY IN HYDROGENATED a-Si PREPARED BY SPUTTERING
T. T i e d j e , T.D. M o u s t a k a s a n d J . M . C e b u l k a
Corporate Research Laboratory, Emon Research and Engineering Co., Linden, NJ 07036, U.S.A.
Abstract.-The temperature dependence of t h e e l e c t r o n d r i f t mobility and i t s d i s p e r s i o n have been measured i n s p u t t e r e d a-SiHx f i l m s with d i f f e r e n t H contents. The d a t a a r e c o n s i s t e n t with t h e m u l t i p l e trapping model of d i s p e r s i v e t r a n s p o r t and demonstrate t h a t t h e e l e c t r o n t r a n s p o r t i s i n s e n s i t i v e t o H c o n t e n t i n t h e 14-19.5% range.
Hydrogen plays a c r u c i a l r o l e i n t h e e l e c t r o n i c ? r o o e r t i e s of hydrolenated amornhous s i l i c o n (a-SiHx). For example i t reduces t h e d e n s i t y of deep pa? s t a t e s t o very low l e v e l s by t h e e l i m i n a t i o n of dangling bonds. Also e l e c t r o n i c s t r u c t u r e calcu- l a t i o n s l and photoemission experiments2 show an SiH bondina o r b i t a l deep i n t h e valence band. However, very l i t t l e i s known about t h e e f f e c t of H on t h e s t a t e s near t h e band edges except t h a t t h e a d d i t i o n of H tends t o widen t h e o p t i c a l ga?.
In t h i s paper we explore t h e e f f e c t of tI on t h e conduction band edge by time-of-
f l i g h t measurements of t h e e l e c t r o n d r i f t mobi 1 i t y i n r e a c t i v e l y s p u t t e r e d a - ~ i ~ , ~ prepared with d i f f e r e n t H c o n t e n t s .
The films discussed here were deposited by r f diode s p u t t e r i n g a t an Ar p r e s s u r e of 5mT, s u b s t r a t e temperature of 325OC and v a r i a b l e p a r t i a l p r e s s u r e of hydrogen.
The 1.5um t h i c k undoped f i l m s were deposited on 5331 of n+ a-SiH, a s an ohmic con- t a c t t o s t a i n l e s s s t e e l s u b s t r a t e s . The P doped n+ l a y e r was ?redeposited t o avoid P contamination of t h e undoped l a y e r . Semitransparent 501 t h i c k 2mm2 P t d o t s were s p u t t e r e d onto t h e top of t h e undoped l a y e r a s blocking c o n t a c t s f o r t h e d r i f t mobility measurements.
The e l e c t r o n d r i f t mobility was measured with a t i m e - o f - f l i g h t technique i n which e l e c t r o n - h o l e p a i r s were generated near t h e P t c o n t a c t by an 8ns f l a s h of 457nm l i g h t from a N2 pumped dye l a y e r running a t a r e p e t i t i o n r a t e of 2 hz. The P t Schottky diode s t r u c t u r e was reverse biased by a voltage ~ u l s e t h a t was longer than t h e e l e c t r o n t r a n s i t time t~ but much s h o r t e r than t h e i n t e r v a l between l a s e r f l a s h e s . The photocurrent i n t h e sample was recorded by a Tektronix 7912AD t r a n - s i e n t d i g i t i z e r and s i g n a l averaged several hundred times. The signal-averaged c u r r e n t decay was converted by a desk-top c a l c u l a t o r t o a 10s c u r r e n t vs. log time p l o t , from which t h e t r a n s i t times and slopes of t h e c u r r e n t decays before and a f t e r t h e t r a n s i t time t~ were determined manually.
The room temperature e l e c t r o n d r i f t mobility p~ i s shown i n Fig. 1 a s a f u n c t i o n of hydrogen p a r t i a l pressure P H , i n t h e s p u t t e r i n g gas;. Note t h e absence of any s i g n i f i c a n t dependence of p~ on P H . The o p t i c a l gap and hydrogen content of these samples increased monotonically from 1.72 t o 1.83eV and 14 t o 19.5% r e s ? e c t i v e l y , with i n c r e a s i n g All of t h e samples i n Fig. 1 e x h i b i t e d d i s p e r s i v e t r a n s p o r t so t h a t t~ was defined by t h e i n t e r s e c t i o n of l i n e a r f i t s t o t h e f i r s t ( t < t T )
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1981431
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and second ( t > t T ) branches o f l o g I- l o g t p l o t s . 5 T y p i c a l photocurrent decay data a r e shown as i n s e t s i n Figs. 2(a) and ( b ) . Also shown i n these f i g u r e s i s t h e temperature de?endence o f t h e d r i f t m o b i l i t y , f o r t h r e e d i f f e r e n t b i a s v o l - tages, f o r two o f t h e samples i n F i g . 1, one prepared a t PH = 3.7mT and t h e o t h e r a t 1.3mT. tlote t h e s t r o n g f i e l d depen- dence a t low temperatures c h a r a c t e r i s t i c o f d i s p e r s i v e t r a n s p o r t 5 and t h e small o r n o n - e x i s t e n t e f f e c t o f H on t h e tem- p e r a t u r e dependence.
I n t h e m u l t i p l e - t r a ? p i n g (P'iT) model o f
o d i s p e r s i v e transport,6 i n f o r m a t i o n about
0.4 0.6 0.8 1 .0 the microscopic p r o p e r t i e s o f the PH (~TORR) l o c a l i z e d s t a t e d i s t r i b u t i o n near t h e
m o b i l i t y edge can be i n f e r r e d from pO(T). I n t h e s,pecial case where t h e d i s t r i b u t i o n o f l o c a l i z e d s t a t e s below Fig. 1: Room tem?erature e l e c t r o n d r i f t t h e conduction band m o b i l i t y edge i s m o b i l i t y as a f u n c t i o n o f Hz p a r t i a l ex?onential, t h e d i s p e r s i o n parameter a pressure i n s p u t t e r i n g gas. i s equal t o T/Tc where Tc i s a character-
i s t i c temperature d e s c r i b i n g t h e w i d t h o f t h e band t a i l .7 The parameter a can be estimated from t h e experimental data i n two ways: e i t h e r from t h e slope o f t h e c u r r e n t decay on a l o g p l o t o r from t h e f i e l d dependence o f t ~ .
The experimental temperature dependence o f a ? determined b o t h ways, i s shown i n Figs. 3(a) and ( b ) f o r t h e same two samples as i n Figs. 2 ( a ) and ( b ) . The s o l i d c i r c l e s were determine fr m t h e f i e l d dependence o f t h e t r a n s i t time, assumed t o be o f t h e form t~
*
E-fi\p
The o t h e r data i n F i g . 3 was d e r i v e d from t h e slope o f t h e p h o t o c u r r e n t decays on t h e l o g - l o g p l o t s . The a values from t h e f i r s t branch (slope a l - 1 ) and second branch ( s l o p e -a,-1) were averaged. I n general a 1 > a2 although t h e d i f f e r e n c e was o n l y m a r g i n a l l y g r e a t e r than t h e experimental uncer- t a i n t y . T h i s d i f f e r e n c e i n a f o r t h e two branches may i n d i c a t e t h a t t h e d e n s i t y o f s t a t e s f a l l s o f f more s l o w l y than e x p o n e n t i a l l y towards midgap. T h i s hypothesis i s c o n s i s t e n t w i t h t h e f a c t t h a t a increases w i t h b i a s (see Fig. 3 ) . A t h i g h f i e l d s ( s h o r t times) t h e e l e c t r o n s sample the r a p i d l y v a r y i n g d e n s i t y o f s t a t e s c l o s e t o t h e m o b i l i t y edge so t h a t a i s l a r g e and a t low f i e l d s ( l o n g times) they sample the more s l o w l y v a r y i n g d e n s i t y o f s t a t e s c l o s e t o t h e middle o f t h e gap and a i s small. Nevertheless a t any given f i e l d , t h e depth t h a t an average e l e c t r o n s i n k s i n t o t h e t r a p d i s t r i b u t i o n a t t~ i s independent o f temperature, t o a f i r s t approximation.7 Thus a(T) f o r a given f i e l d r e f l e c t s t h e slope of t h e d e n s i t y o f s t a t e s a t a p a r t i c u l a r t r a ? deyth.I n o r d e r t o estimate t h e Tc f o r t h e t r a p d i s t r i b u t i o n vie f i t t h e a(T) data obtained from t h e f i e l d dependence o f t ~ , shown i n Fig. 3(a) and ( b ) , by t h e l i n e s through the o r i g i n , a l s o shown i n the f i g u r e . The corresponding values f o r Tc (411K and 419K f o r t h e 0.7mT and 1 .OmT samples r e s p e c t i v e l y ) , t o g e t h e r w i t h t h e experimental values o f vD(T) enable us t o e s t i m a t e the f r e e c a r r i e r m o b i l i t y po and t h e attempt r a t e v f o r thermal emission from t h e l o c a l i z e d states. I n t h e HT model W D i s given by7
f o r a < 1, where L i s t h e sample thickness, V i s t h e a p ~ l i e d b i a s and to = L~/N,V i s the f r e e c a r r i e r t r a n s i t time. The attempt r a t e v f o r thermal emission from l o c a l i z e d s t a t e s i s assumed t o be independent o f temperature. Using Eq. ( I ) , t h e Tc values discussed above and a d j u s t i n g v and pd f o r a b e s t f i t t o t h e 4V d r i f t
1000/T Fig. 2(a)
Fig. 2(a) ,(b).- Temperature dependence o f t h e e l e c t r o n d r i f t m o b i l i t y , w i t h t y p i c a l photocurrent decays as i n s e t s . The s o l i d .nes a r e f i t s t o t h e 4V data w i t h T = 4 1 1 K (419K) u = 2.4 x 1 0 1 3 s - l (1.0 x 1313 s - l ) and YO = 20 C ~ ~ / V S
(24 cm$/vS) f o r the 3.7mT (l.OnT) samples. The broken l i n e s a r e c a l c u l a t e d curves f o r the o t h e r voltages i n d i c a t e d .
F i g . 3 ( a ) , ( b ) . - Temperature dependence o f d i s p e r s i o n parameter determined from c u r r e n t decays and f i e l d dependence o f t ~ . The l i n e s are f i t s t o the f i e l d dependence data.
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mobility d a t a a s a function of temperature, we obtain t h e f i t s shown a s s o l i d l i n e s i n Figs. 2 ( a ) and (b). The broken l i n e s were c a l c u l a t e d from t h e same parameters, t h e only d i f f e r e n c e being t h a t t h e b i a s voltage was a d j u s t e d t o match t h e r e s t of t h e exyerimental d a t a . The c a l c u l a t e d curves a r e a l l i n e x c e l l e n t agreement with experiment.
In a d d i t i o n t h e r e s u l t i n g values of t h e f i t t i n parameters i n d i c a t e d i n t h e f i g u r e
4
c a p t i o n s ( u o = 20-24 c m e / ~ s , u = 1-2 x 1313 s - ) a r e p h y s i c a l l y p l a u s i b l e . I t i s i n t e r e s t i n g t o note t h a t both a r e l a r g e r than t h e c o r r e s onding parameters f o r glow
f'
discharge material (po = 13 C ~ ~ / V S and v = 4.6 x 1011 s- ) . 8 The band t a i l widths a r e a l s o l a r g e r
-
411K o r 419K compared with 312K. These d i f f e r e n c e s may be a consequence of t h e apparent n o n - e x ~ o n e n t i a l c h a r a c t e r of t h e l o c a l i z e d s t a t e d i s t r i - bution i n s p u t t e r e d m a t e r i a l , a c h a r a c t e r i s t i c t h a t was not observed i n t h e glow discharge m a t e r i a l . On t h e o t h e r hand t h e d i f f e r e n c e s may r e f l e c t r e a l v a r i a t i o n si n t h e l o c a l i z e d s t a t e s , t h e f r e e c a r r i e r mobility o r t h e l o c a t i o n of t h e mobility edge.
In conclusion t h e d r i f t mobility r e s u l t s presented here demonstrate t h a t hydrogen c o n t e n t i n t h e 14-19.5% range has very l i t t l e e f f e c t on t h e e l e c t r o n d r i f t mobility i n s p u t t e r e d f i l m s even thou h t h e d e n s i t y of s t a t e s near midgap changes by
several o r d e r s of magnitude17 and t h e o p t i c a l gap i n c r e a s e s by 0 - l l e V . We conclude t h a t t h e conduction band t a i l s t a t e s a r e not H r e l a t e d e i t h e r through SiH a n t i - bonding o r b i t a l s 1 99 o r Si-H r , c h b a r r i e r regions.10
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