ELECTRONIC PROPERTIES OF a-Si : H FROM MEASUREMENTS ON DEVICES

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ELECTRONIC PROPERTIES OF a-Si : H FROM MEASUREMENTS ON DEVICES

R. Crandall

To cite this version:

R. Crandall. ELECTRONIC PROPERTIES OF a-Si : H FROM MEASUREMENTS ON DEVICES.

Journal de Physique Colloques, 1981, 42 (C4), pp.C4-413-C4-422. �10.1051/jphyscol:1981489�. �jpa-

00220946�

JOURNAL DE PHYSIQUE

CoZZoque C4, suppZ6ment au n0lO, Tome 42, octobre 1981 page C4-413

ELECTRONIC PROPERTIES O F a-Si:H FROM MEASUREMENTS ON DEVICES*

R.S. C r a n d a l l

RCA Laboratories, Princeton, NJ, 08540, U. S . A.

A b s t r a c t . - Some o f t h e p a r a m e t e r s t h a t can b e determined from b o t h d c and t r a n s i e n t c u r r e n t measurements on s o l a r c e l l d e v i c e s c o n s t r u c t e d from hydrogenated amorphous s i l i c o n (a-Si:H) w i l l b e reviewed. T h i s w i l l i n c l u d e u s i n g t h e p h o t o c u r r e n t t o d e t e r m i n e t h e a b s o r p t i o n co- e f f i c i e n t f o r weakly a b s o r b i n g g a p s t a t e s ; d e t e r m i n a t i o n of t h e sum o f t h e e l e c t r o n a n d h o l e d r i f t l e n g t h s from t h e f i e l d dependence of t h e p h o t o c u r r e n t ; d e t e r m i n a t i o n o f t h e sum of t h e e l e c t r o n and h o l e mobil- i t i e s from p h o t o c a p a c i t a n c e measurements; d e t e r m i n a t i o n 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 and d e n s i t y of gap s t a t e s from t r a n s i e n t c u r r e n t measure- ments.

I. I n t r o d u c t i o n . - Measurements on semiconductor d e v i c e s can o f t e n y i e l d - v a l u a b l e p h y s i c a l p a r a m e t e r s . For s o l a r c e l l d e v i c e s f a b r i c a t e d from a-Si:H a number of i m p o r t a n t p a r a m e t e r s have been o b t a i n e d i n t h i s way. I n t h i s a r t i c l e I would l i k e t o o u t l i n e some o f t h e t e c h n i q u e s t h a t have been u s e d and t h e r e s u l t s t h a t have been o b t a i n e d . These w i l l i n c l u d e d e t e r m i n a t i o n of t h e a b s o r p t i o n c o e f f i c i e n t f o r s t a t e s i n t h e g a p , d e n s i t y of s t a t e s i n t h e gap, e l e c t r o n and h o l e m o b i l i t y a s w e l l a s t h e e l e c t r o n a n d h o l e d r i f t l e n g t h s .

I n t h e f i r s t p a r t I w i l l r e v i e w t h e p a r a m e t e r s t h a t can b e o b t a i n e d from p h o t o c o n d u c t i v i t y measurements on s o l a r c e l l s . The most i n f o r m a t i o n c a n be o b t a i n e d from measurements on p-i-n s t r u c t u r e s b e c a u s e i n t h i s c a s e t h e s p a c e c h a r g e i n t h e i l a y e r i s low enough s o t h a t t h e e l e c t r i c f i e l d i s n e a r l y uniform. T h i s s i m p l i f i e s t h e a n a l y s i s . I n t h e second p a r t t r a n s i e n t c u r r e n t measurements on S c h o t t k y b a r r i e r c e l l s w i l l b e p r e s e n t e d a s a method t o d e t e r m i n e t h e e l e c t r o n d r i f t m o b i l i t y . A DLTS d e t e r m i n a t i o n o f t h e d e n s i t y of gap s t a t e s a s w e l l a s a d e f e c t a s s o c i a t e d f o r t h e Staebler-Wronski e f f e c t w i l l a l s o be d i s c u s s e d .

11. S t e a d y S t a t e P h o t o c o n d u c t i v i t y . - The ~ o t e n t i a l energy diagram f o r a p-i-n s t r u c t u r e i s s k e t c h e d i n f i g . 1 . When t h e c e l l i s i l l u m i n a t e d w i t h weakly a b s o r b e d l i g h t e l e c t r o n - h o l e p a i r s w i l l be produced u n i f o r m l y i n t h e i l a y e r . A t l a r g e f i e l d s , r e c o m b i n a t i o n i s u n i m p o r t a n t and o n e e l e c t r o n f l o w s i n t h e e x t e r n a l c i r c u i t f o r e a c h a b s o r b e d photon. T h i s i s b e c a u s e t h e n - i i n t e r f a c e is a b l o c k i n g c o n t a c t f o r e l e c t r o n s . T h i s c a n b e e x p l o i t e d t o measure t h e number of photons a b s o r b e d by t h e i l a y e r . For wavelengths l o n g e r t h a n t h e a b s o r p t i o n edge p h o t o c o n d u c t i v i t y measurements c a n b e u s e d t o measure t h e

a b s o r p t i o n c o e f f i c i e n t when o p t i c a l t r a n s m i s s i o n measurements a r e u n s u c c e s s f u l (1).

*Research r e p o r t e d h e r e i n was s u p p o r t e d by S o l a r Energy R e s e a r c h ~ n s t i t ' u t e , under C o n t r a c t No. XG-0-9372-1, and by RCA L a b o r a t o r i e s , P r i n c e t o n , N J , 08540. U.S.A.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1981489

C4-4 14 J9UiUJA.L DE PHYSIQUE

Because t h e c o n t a c t s a r e b l o c k i n g , t h e r e w i l l be more h o l e s n e a r t h e i-p i n t e r f a c e a n d more e l e c t r o n s n e a r t h e n - i i n t e r f a c e . T h i s s p a c e c h a r g e w i l l g i v e a c o n t r i b u t i o n t o t h e c a p a c i t a n c e . S i n c e t h e d i s t r i b u t i o n of s p a c e c h a r g e r e s u l t s from t h e d r i f t i n t h e e l e c t r i c f i e l d t h e c a p a c i t a n c e w i l l be a measure of t h e e l e c t r o n and h o l e m o b i l i t i e s ( 2 ) . At low e l e c t r i c f i e l d s recombina- t i o n becomes i m p o r t a n t and t h e s p a c e c h a r g e d e n s i t y i n t h e i l a y e r a s w e l l a s t h e c u r r e n t i n t h e e x t e r n a l c i r c u i t a r e d e t e r m i n e d by t h e recombinat i o n r a t e s of e l e c t r o n s and h o l e s . I n t h i s s i t u a t i o n a n a n a l y s i s of t h e photo- c u r r e n t can b e u s e d t o measure t h e sum of t h e e l e c t r o n and h o l e d r i f t l e n g t h s ( 3 ) .

Fig. 1. P o t e n t i a l e n e r g y v e r s u s d i s t a n c e f o r a n i d e a l i z e d p-i-n s t r u c t u r e i n t h e s h o r t c i r c u i t c o n f i g u r a t i o n .

( a ) A b s o r p t i o n C o e f f i c i e n t

F i g . 2 i s a n example of t h e a b s o r p t i o n c o e f f i c i e n t d e t e r m i n e d from t h e p h o t o c o n d u c t i v i t y . The dashed c u r v e was o b t a i n e d from t r a n s m i s s i o n measure- ments. The s o l i d c u r v e s were o b t a i n e d from p h o t o c o n d u c t i v i t y measurements made a t l a r g e r e v e r s e b i a s where a l l t h e e l e c t r o n h o l e p a i r s a r e c o l l e c t e d . The agreement between t h e two t e c h n i q u e s i n t h e i r r e g i o n of o v e r l a p i s good.

ENERGY ( e V )

F i g , 2. A b s o r p t i o n c o e f f i c i e n t

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a , a s a f u n c t i o n o f photon e n e r g y

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measured by t r a n s m i s s i o n on a t h i c k undoped f i l m ; c u r v e A, a determined from p h o t o c o n d u c t i v i t y on a n undoped f i l m ; c u r v e B , a determined from p h o t o c o n d u c t i v i t y on a phosphorus doped f i l m ; o a measured by photo- t h e r m a l d e f l e c t i o n t e c h n i q u e on a n undoped f i l m ( 5 ) ; a measured by p h o t o t h e r m a l d e f l e c t i o n t e c h n i q u e on a phosphorus doped Xerox f i l m ( 5 ) .

A new t e c h n i q u e h a s been d e v i s e d t h a t p e r m i t s a n e x t e n s i o n of o p t i c a l a b s o r p t i o n measurements t o low a b s o r p t i o n c o e f f i c i e n t s on t h i n f i l m s ( 4 ) . T h i s t e c h n i q u e , termed p h o t o t h e r m a l d e f l e c t i o n s p e c t r o s c o p y , PDS

,

h a s been u s e d t o measure t h e a b s o r p t i o n c o e f f i c i e n t of t h i n f i l m s of a-Si:H ( 5 ) .

T h i s a l l o w s a check on t h e p h o t o c u r r e n t t e c h n i q u e . U n f o r t u n a t e l y , t h e two t y p e s

of measurements have n o t been made on t h e same sample. N e v e r t h e l e s s , t h e a g r e e - ment i s s a t i s f a c t o r y . T h i s i s shown by PDS (5). The open c i r c l e s a r e f o r a n undoped Xerox f i l m . The p r o d u c t i o n t e c h n i q u e f o r t h i s f i l m i s s i m i l a r t o that used t o p r e p a r e t h e d e v i c e measured f o r c u r v e A . The d o t s a r e f o r a phosphorus doped f i l m grown a t Xerox which c o n t a i n e d r o u g h l y t h e same doping a s t h e f i l m u s e d f o r c u r v e B. The p h o t o c o n d u c t i v i t y measurement would b e e x p e c t e d t o g i v e a lower a b s o r p t i o n c o e f f i c i e n t because. it d e t e c t s o n l y t h e t r a n s i t i o n s l e a d i n g t o f r e e e l e c t r o n s and h o l e s .

A d i f f i c u l t y i n d e t e r m i n i n g t h e a b s o r p t i o n c o e f f i c i e n t from t h e s o l a r c e l l p h o t o c u r r e n t is t h a t u n d e r some c o n d i t i o n s i n t e r n a l p h o t o e m i s s i o n can compete w i t h p h o t o c o n d u c t i v i t y a s a n a d d i t i o n a l s o u r c e of c u r r e n t . T h i s i s p a r t i c u l a r l y t h e c a s e w i t h t h i c k c e l l s o r i f p a l l a d i u m S c h o t t k y b a r r i e r s a r e used ( 6 ) . T h i s problem can b e minimized by u s i n g p-i-n s t r u c t u r e s w i t h I T 0 o r some o t h e r t r a n s p a r e n t c o n t a c t .

P h o t o c u r r e n t measured on a t h i n f i l m r a t h e r t h a n i n a d e v i c e s t r u c t u r e h a s a l s o been used t o d e t e r m S n a t h e a b s o r p t i o n c o e f f i c i e n t of gap s t a t e s ( 7 ) . I n t h e s e measurements t h e c u r r e n t i s p r o p o r t i o n a l t o t h e p r o d u c t of t h e a b s o r p t i o n c o e f f i c i e n t and t h e m a j o r i t y c a r r i e r l i f e t i m e . Thus a n assumption i s made t h a t t h e l i f e t i m e i s independent of photon energy. T h i s d o e s n o t a p p e a r t o b e a good assumption i n a l l c a s e s (8). U s u a l l y a peak i n t h e d e n s i t y of s t a t e s i s found around 1 eV which h a s n o t been d e t e c t e d by PDS ( 5 ) .

(b) P h o t o c a p a c i t a n c e

Within t h e i l a y e r t h e f r e e e l e c t r o n and h o l e d e n s i t i e s a r e n o t u n i f o r m b u t r a t h e r a r e d e t e r m i n e d by t h e d r i f t away from t h e b l o c k i n g c o n t a c t s under t h e a c t i o n of t h e e l e c t r i c f i e l d . T h e i r d e n s i t i e s a r e s k e t c h e d -in f i g . 3 . S i n c e h o l e s a r e c o n c e n t r a t e d t h e p - i i n t e r f a c e and e l e c t r o n s n e a r t h e n - i i n t e r f a c e , t h e r e i s a nonuniform s p a c e c h a r g e i n t h e i l a y e r which c o n t r i b u t e s t o t h e c a p a c i t a n c e . T h i s c a p a c i t a n c e , C p , which i s i n a d d i t i o n t o t h e

g e o m e t r i c a l c a p a c i t a n c e , i s g i v e n by ( 2 )

where e is t h e e l e m e n t a l c h a r g e , G t h e g e n e r a t i o n r a t e of e l e c t r o n h o l e p a i r s p e r second p e r u n i t volume, E t h e e l e c t r i c f i e l d , 1-1, t h e e l e c t r o n m o b i l i t y ,

up

t h e h o l e m o b i l i t y , and L t h e i l a y e r t h i c k n e s s . Cp i s compared w i t h e x p e r i m e n t a l d a t a i n f i g , 4a where Cp i s shown a s a f u n c t i o n of l i g h t i n t e n s i t y (G i s p r o p o r t i o n a l t o t h e l i g h t i n t e n s i t y ) t o show i t s l i n e a r dependence on l i g h t i n t e n s i t y . I n f i g . 4b, Cp i s p l o t t e d i n a manner t o show i t s i n v e r s e dependence on t h e s q u a r e of t h e v o l t a g e .

F i g , 3 . The l i n e s a r e t h e d e n s i t i e s of f r e e e l e c t r o n s and h o l e s i n t h e i l a y e r r e s u l t i n g from i l l u m i n a t i o n w i t h u n i f o r m l y absorbed l i g h t a t a h i g h enough e l e c t r i c f i e l d s o t h a t r e c o m b i n a t i o n i s u n i m p o r t a n t .

C4-4 16 JOURNAL DE PHYSIQUE

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l i g h t i n t e n s i t y f o r a p-i-n s o l a r Applied Bias (V) c e l l a t 1 V r e v e r s e b i a s i l l u m i n a t e d

a t 674 nm r a d i a t i o n . F i g . 4b. R e c i p r o c a l o f t h e s q u a r e r o o t of t h e p h o t o c a p a c i t a n c e v e r s u s

a p p l i e d v o l t a g e f o r t h e c o n d i t i o n s i n f i g . 4a.

( c ) E l e c t r o n and Hole D r i f t L e n g t h s

A t low f i e l d s a c r o s s t h e i l a y e r , r e c o m b i n a t i o n o f e l e c t r o n h o l e p a i r s becomes i m p o r t a n t and dominates t h e t r a n s p o r t i n t h e c e l l . I n t h i s c a s e t h e c o n t a c t s no l o n g e r l i m i t t h e c u r r e n t . It i s g i v e n by t h e u s u a l e x p r e s s i o n f o r a photoconductor which is

where T, i s t h e e l e c t r o n l i f e t i m e and T t h e h o l e l i f e t i m e . The Ohms l a w b e h a v i o r p r e d i c t e d by eq. ( 2 ) i s shown

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t h e d a t a i n f i g . 5. Here t h e p h o t o c u r r e n t i n a p-i--n s t r u c t u r e i s p l o t t e d v e r s u s t h e a p p l i e d v o l t a g e . A t t h e f l a t band v o l t a g e , Vf, t h e f i e l d i n t h e i l a y e r and t h e p h o t o c u r r e n t a r e z e r o .

I n t h e neighborhood o f Vf t h e f i e l d i s low and t h e c u r r e n t a b e y s Ohms law. At h i g h e r f i e l d s ( d e c r e a s i n g a p p l i e d v o l t a g e ) t h e p h o t o c u r r e n t m a k e s a t r a n s i t i o n t o s a t u r a t i o n a t t h e v a l u e eGL. A d e t a i l e d c a l c u l a t i o n (3) of t h e c u r r e n t - v o l t a g e c u r v e s ( s o l i d c u r v e s ) shows that no i n f o r m a t i o n o t h e r t h a n t h a t c o n t a i n e d i n eq. (2) can b e o b t a i n e d . The e n t i r e c u r v e ts c h a r a c - t e r i z e d by t h e sum o f t h e e l e c t r o n and h o l e d r i f t l e n g t h s .

E x p r e s s i o n s (1) and (2) c o n t a i n t h e t r a n s p o r t p a r a m e t e r s t h a t a r e i m p o r t a n t f o r s o l a r c e l l o p e r a t i o n . U n f o r t u n a t e l y , t h e y o c c u r i n p a i r s and a d d i t i o n a l measurements a r e n e c e s s a r y t o d e t e r m i n e t h e i n d i v i d u a l p a r a m e t e r s . The f a c t that t h e p a r a m e t e r s always o c c u r i n p a i r s shows t h a t b o t h e l e c t r o n s and h o l e s must c o n t r i b u t e t o s o l a r c e l l o p e r a t i o n .

111. T r a n s i e n t Measurements.- I n a d d i t i o n t o d c p h o t o c u r r e n t measurements, -

v a r i o u s t r a n s i e n t t e c h n i q u e s c a n b e u s e d t o s t u d y a-Si:H. I n t h i s s e c t i o n I s h a l l d e s c r i b e how t r a n s i e n t c u r r e n t and c a p a c i t a n c e measurements on S c h o t t k y b a r r i e r a n d p-i-n s t r u c t u r e s can b e u s e d t o d e t e r m i n e t h e e l e c t r o n d r i f t m o b i l i t y a s w e l l a s t h e d e n d i t y o f gap s t a t e s . Also new i n f o r m a t i o n a b o u t t h e d e f e c t s a s s o c i a t e d w i t h t h e Staebler-Wronski e f f e c t ( 9 ) w i l l b e p r e s e n t e d .

To u n d e r s t a n d how t h e s e t r a n s i e n t measurements c a n be a p p l i e d ,

f i g . 6a-6d shows t h e p o t e n t i a l e n e r g y diagrams f o r a n i d e a l n-type S c h o t t k y b a r r i e r on a-Si:H b e f o r e , d u r i n g and f o l l o w i n g a forward b i a s c u r r e n t p u l s e .

Fig. 5. P h o t o c u r r e n t a s a f u n c t i o n o f a p p l i e d b i a s , VA, f o r two p-i-n s o l a r c e l l s i l l u m i n a t e d w i t h 674 nm r a d i a t i o n . The i n t e r c e p t on t h e

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v o l t a g e a x i s c o r r e s p o n d s t o V f . The d a t a a r e f o r c e l l s made i n two d i f - f e r e n t systems. Zero a p p l i e d v o l t a g e c o r r e s p o n d s t o s h o r t c i r c u i t c o n d i t i o n s . The d o t s and c i r c l e s a r e d a t a ; t h e c u r v e s a r e t h e o r e t i c a l from r e f . ( 3 ) .

F i g . 6 a i l l u s t r a t e s t h e e l e c t r o n p o t e n t i a l diagram f o r t h e s t e a d y s t a t e , z e r o c u r r e n t f l o w , r e v e r s e b i a s c o n f i g u r a t i o n . A u n i f o r m p o s i t i v e s p a c e c h a r g e f i l l s t h e d e p l e t i o n w i d t h (w). The a p p l i e d b i a s i s Va. The S c h o t t k y b a r r i e r and Ohmic c o n t a c t t o t h e semiconductor a r e l a b e l e d MI and M2, r e s p e c t i v e l y .

Fig. 6a-b. P o t e n t i a l energy v e r s u s d i s t a n c e f o r a S c h o t t k y b a r r i e r s o l a r c e l l a t d i f f e r e n t s t a g e s of t r a p f i l l i n g and emptying. ( a ) r e v e r s e b i a s s h a l l o w and deep t r a p s empty. (6) d u r i n g a forward b i a s t r a p f i l l i n g p u l s e . (c) a f t e r t h e forward b i a s p u l s e i s t e r m i n a t e d , b u t b e f o r e a l l t h e s h a l l o w t r a p s have emptied, ( d ) a f t e r t h e s h a l l o w s t a t e s have emptied b u t t h e d e e p s t a t e s a r e s t i l l emptying.

C4-4 18 JOURNAL DE PHYSIQUE

I f a b i a s p u l s e o f magnitude Va i s a p p l i e d s o t h a t t h e bands c a n j u s t f l a t t e n , t h e n e l e c t r o n s w i l l f l o w from t h e b u l k r e g i o n of t h e semiconductor i n t o t h e r e g i o n c o n t a i n i n g s p a c e c h a r g e and n e u t r a l i z e i t , s h r i n k i n g t h e d e p l e t i o n w i d t h t o z e r o . The e l e c t r o n s w i l l d i s t r i b u t e t h e m s e l v e s between s h a l l o w t r a p s and t h e c o n d u c t i o n band. I f t h e b i a s i s r e t u r n e d t o i t s o r i g i n a l v a l u e t h e n e l e c t r o n s w i l l f l o w o u t of t h e semiconductor t h r o u g h c o n t a c t MI. S i n c e MI is a b l o c k i n g c o n t a c t , no c h a r g e can e n t e r . The s p a c e c h a r g e w i l l b e uncovered and t h e d e p l e t i o n w i d t h w i l l widen u n t i l i t r e a c h e s t h e v a l u e W when t h e c u r r e n t h a s decayed t o z e r o . The c h a r a c t e r i s t i c t i m e f o r t h i s p r o c e s s depends on whether i t i s l i m i t e d by e m i s s i o n from deep t r a p s o r by t h e t r a n s i t t i m e o f t h e e l e c t r o n s a c r o s s t h e d e p l e t i o n w i d t h .

I n t h i s m a t e r i a l t h e e m i s s i o n l i m i t e d p r o c e s s o c c u r s a t l o n g t i m e s compared w i t h t h e t r a n s i t t i m e e f f e c t s c o n s i d e r e d h e r e . F u r t h e r m o r e , s i n c e t h e s h a l l o w s t a t e s a r e more numerous t h a n t h e deep s t a t e s , t h e y m a i n l y

d e t e r m i n e t h e d e p l e t i o n width governed by t h e e x t r a c t i o n o f e l e c t r o n s from t h e s h a l l o w s t a t e s i n t h e d e p l e t i o n w i d t h movement. By measuring t h e t r a n s i e n t c u r - r e n t a t s h o r t t i m e s where i t i s governed by t h e e x t r a c t i o n o f e l e c t r o n s from t h e s h a l l o w s t a t e s i n t h e d e p l e t i o n w i d t h , it i s p o s s i b l e t o d e t e r m i n e t h e e l e c t r o n d r i f t m o b i l i t y ( 1 0 ) . I n t h e f i r s t s e c t i o n I w i l l o u t l i n e how t h i s c a n b e done and g i v e a n example f o r n-type a-Si:H. I n t h e second s e c t i o n DLTS (11) r e s u l t s w i l l b e u s e d t o d e t e r m i n e t h e d e n s i t y of g a p s t a t e s (12,13) a s w e l l a s g i v e

i n f o r m a t i o n about t h e d e f e c t a s s o c i a t e d w i t h t h e Staebler-Wronski e f f e c t ( 9 ) . ( a ) D r i f t M o b i l i t y *

I n i t i a l l y t h e d e v i c e i s i n t h e s t a t e shown by f i g . 6a. A s h o r t forward b i a s p u l s e i s a p p l i e d t o n e u t r a l i z e t h e p o s i t i v e c h a r g e i n t h e d e p l e t i o n w i d t h ( f i g

.

^{6 b ) . A t} t = O i t i s removed. E l e c t r o n s now l e a v e and t h e d e p l e t i o n w i d t h r e e s t a b l i s h e s i t s e l f . A t a n i n t e r m e d i a t e t i m e b e f o r e t h e c u r r e n t h a s decayed t o z e r o and t h e d e p l e t i o n w i d t h h a s widened f u l l y , t h e s i t u a t i o n w i l l b e a s d e p i c t e d i n f i g . 6 ( c ) . The boundary between t h e n e u t r a l h u l k and t h e s p a c e c h a r g e r e g i o n i s l o c a t e d a t t h e c o o r d i n a t e XB ( t ) . AS e l e c t r o n s l e a v e t h e s p a c e c h a r g e r e g i o n I t h e boundary moves toward t h e p o i n t w.

It i s t h e dynamics o f t h i s movement t h a t d e t e r m i n e s t h e t r a n s i e n t c u r r e n t i n t h e e x t e r n a l c i r c u i t . The v e l o c i t y of t h i s boundary i s d e t e r m i n e d by s u c h p a r a m e t e r s a s t h e e l e c t r o n d r i f t m o b i l i t y p, t h e s p a c e c h a r g e d e n s i t y and t h e e l e c t r i c f i e l d i n r e g i o n 11.

Because t h e r e g i o n I1 e x t e n d i n g from t h e p o i n t XB ( t ) t o t h e c o n t a c t M2 i s n e u t r a l , t h e f i e l d i n i t i s uniform. Region I c o n t a i n s s p a c e c h a r g e . The f i e l d h e r e i s nonuniform and can b e found by s o l v i n g P o i s s o n ' s e q u a t i o n . The e l e c t r o n c u r r e n t i n r e g i o n I i s a p u r e d i s p l a c e m e n t c u r r e n t whereas

r e g i o n I1 c o n t a i n s b o t h c o n d u c t i o n a n d d i s p l a c e m e n t c u r r e n t s . C u r r e n t c o n t i n u i t y r e q u i r e s t h a t t h e s e two c u r r e n t s be e q u a l . T h i s g i v e s enough c o n d i t i o n s t o o b t a i n a n e q u a t i o n f o r t h e movement of t h e boundary XB ( t ) between r e g i o n I and I1 and t h u s t h e t i m e dependence of t h e c u r r e n t which i s (10)

J = Jo s e c h 2 ( t / ~ ) [ l - ~ t a n h ( t / ~ / L l ; Jo = ndevVo/L (3) and

i s t h e t r a n s i t t i m e a c r o s s t h e d e p l e t i o n width. The s h a l l o w donor d e n s i t y is nd-

I n F i g . 7 Eq. (3) i s compared w i t h t h e measured c u r r e n t f o r two d i f f e r e n t v o l t a g e s on a n n-type S c h o t t k y b a r r i e r d e v i c e c o n t a i n i n g a b o u t 1 x 1017

s h a l l o w d o n o r s .

TIME ( p s )

Fig. 7. C u r r e n t t r a n s i e n t s f o r two a p p l i e d v o l t a g e s a t room t e m p e r a t u r e . The p o i n t s a r e d a t a ; t h e c u r v e s a r e e q . ( 3 ) . The d e p l e t i o n w i d t h s and t i m e

c o n s t a n t s a r e shown i n t h e f i g u r e .

S i n c e t h e o n l y unknown p a r a m e t e r i n (3) i s t h e m o b i l i t y , t h e d a t a c a n b e u s e d t o f i n d

u.

I t s t e m p e r a t u r e dependence i s shown i n F i g , 8. The r e s u l t s a r e i n s u b s t a n t i a l agreement w i t h m o b i l i t y v a l u e s o b t a i n e d on undoped a-Si:H by t i m e o f f l i g h t measurements. The p r e s e n t t r a n s i e n t t e c h n i q u e , however, works on doped m a t e r i a l where t h e d i e l e c t r i c r e l a x a t i o n t i m e i s t o o

s h o r t f o r t i m e of f l i g h t measurements. Below 200K t h e s h a l l o w s t a t e s now behave l i k e deep s t a t e s and t h e p r o c e s s i s l i m i t e d by e m i s s i o n r a t h e r t h a n t r a n s i t time.

(b) DLTC D e n s i t y of S t a t e s

C u r r e n t t r a n s i e n t s a t t i m e s l o n g e r t h a n t h e c h a r a c t e r i s t i c t i m e f o r e l e c t r o n t r a n s i t a c r o s s t h e d e p l e t i o n w i d t h , eg. (5) a r e c o n t r o l l e d by e m i s s i o n from deep t r a p s . Thus, t h e y can b e used t o d e t e r m i n e t h e number o f t h e s e t r a p s a s w e l l a s t h e i r e n e r g y l e v e l ( 1 2 ) . T h i s i s o n e o f t h e forms of t h e powerfuE Deep L e v e l T r a n s i e n t S p e c t r o s c o p y (DLTS) t e c h n i q u e (14). It i s more common t o measure c a p a c i t a n c e t r a n s - i e n t s b e c a u s e t h e s i g n o f t h e t r a p p e d c h a r g e c a n b e d e t e r m i n e d . However, t h e r e a r e problems a s s o c i a t e d w i t h c a p a c i t a n c e measurements i n a-Si:H (15).

The measuring f r e q u e n c y must b e low enough s o t h a t it i s l e s s t h a n t h e i n v e r s e o f t h e d i e l e c t r i c r e l a x a t i o n t i m e , A c o n d i t i o n sometimes h a r d t o s a t i s f y , e s ~ e c i a l l y _{- -} - f o r undoped m a t e r i a l . N e v e r t h e l e s s , c a p a c i t a n c e Fig. 8. The l o g a r i t h m o f t h e d r i f t

m o b i l i t y v e r s u s t h e r e c i p r o c a l o f t r a n s i e n t s have been u s e d (13) t o map o u t t h e d e n s i t y of s t a t e s f o r t h e a b s o l u t e t e m p e r a t u r e .

h e a v i l y doped n - t y p e a-Si:H. The r e s u l t s , where t h e y can b e compared w i t h c u r r e n t t r a n s i e n t s g i v e s u b s t a n t i a l l y t h e same d e n s i t y o f s t a t e s (16).

For undoped m a t e r i a l , however, t h e d i e l e c t r i c r e l a x a t i o n t i m e i s t o o l o n g f o r

C4-420 JOURNAL DE PHYSIQUE

t h e c a p a c i t a n c e t r a n s i e n t t e c h n i q u e t o be u s e f u l f o r S c h o t t k y b a r r i e r s t r u c t u r e s . N e v e r t h e l e s s , p-i-n s t r u c t u r e s c a n b e used b e c a u s e i n t h i s c a s e i t is t h e

d e p l e t i o n w i d t h s i n t h e n and p l a y e r s t h a n change i n r e s p o n s e t o t h e t r a p emptying in t h e i l a y e r . It i s t h e d i e l e c t r i c n e l a x a t i o n t i m e i n t h e n and p l a y e r s t h a n d e t e r m i n e s t h e measuring frequency.

The d e n s i t y o f s t a t e s d e t e r m i n e d by DLTS i s much l e s s t h a n t h a t d e t e r m i n e d from f i e l d e f f e c t measurements ( 1 7 ) . T h i s i s i l l u s t r a t e d i n f i g , ( 9 ) . Curve a showing t h e h i g h e s t d e n s i t y o f s t a t e s was measured by f i e l d e f f e c t ( 1 7 ) . The r e m a i n i n g c u r v e s a r e DLTS d e n s i t y of s t a t e s . Curve b was o b t a i n e d by c a p a c i t i v e t r a n s i e n t s on n-type m a t e r i a l ( 1 3 ) ; c u r v e c by c u r r e n t t r a n s i e n t s on n-type m a t e r i a l (12); c u r v e d on undoped m a t e r i a l u s i n g c u r r e n t t r a n s i e n t s (12).

The f a c t t h a t t h e c u r r e n t t r a n s i e n t s , which measure a l l t h e c h a r g e t r a p p e d in t h e d e p l e t i o n width, do n o t i n d i c a t e more s t a t e s t h a n t h e c a p a c i t a n c e t r a n s i e n t s i s good e v i d e n c e t h a t t h e c a p a c i t a n c e t r a n s i e n t s a r e measuring a l l t h e s t a t e s . C a p a c i t a n c e sum r u l e arguments (16) l e a d t o t h e same c o n d l u s i o n . Thus DLTS measures a l l t h e s t a t e s i n t h e gap. The l a r g e r d e n s i t y o f s t a t e s measured by f i e l d e f f e c t must be d u e t o t h e measuring t e c h n i q u e o r t h e f a c t t h a t i t i s p r i m a r i l y s e n s i t i v e t o s t a t e s n e a r and on t h e s u r f a c e (18).

Fig. 9. D e n s i t y o f s t a t e s p e r u n i t e n e r g y p e r u n i t volume v e r s u s energy f o r a-Si:H a s d e t e r m i n e d by f i e l d e f f e c t a n d DLTS

--

f i e l d e f f e c t r e f . (17) ; c u r v e b DLTS r e f . (13) ; c u r v e c DLTS r e f . ( 1 2 ) ; c u r v e d DLTS r e f . (12).

ENERGY ( e V )

( c ) Staebler-Wronski E f f e c t

C a p a c i t a n c e t r a n s i e n t s g i v e u s e f u l i n f o r m a t i o n a b o u t s t a t e s i n t h e gap t h a t a r e produced d u r i n g prolonged i l l u m i n a t i o n o f a-Si:H ( 9 ) . Prolonged i l l u m i n - a t i o n o f a-Si:H f i l m s r e s u l t s in c h a n g e s in t h e i r t r a n s p o r t p r o p e r t i e s . These changes, which can be r e v e r s e d by h i g h t e m p e r a t u r e a n n e a l i n g a r e u s u a l l y r e f e r r e d t o a s t h e Staebler-Wronski e f f e c t ( 9 ) . The c u r r e n t u n d e r s t a n d i n g of t h e e f f e c t i s that i t i s p r i m a r i l y a s u r f a c e e f f e c t t h a t i s d r i v e n by a d e f e c t i n t h e b u l k ( 1 8 ) . Prolonged i l l u m i n a t i o n (19) o r e l e c t r o n beam i r r a d i a t i o n (20) a l s o p r o d u c e re- v e r s i b l e changes i n t h e luminescence. It i s b e l i e v e d t h a t t h e d e f e c t r e s p o n s i b l e f o r t h i s i s t h e same a s t h a t c a u s i n g t h e Staebler-Wronski e f f e c t .

The normal way t o s t u d y t h i s d e f e c t u s i n g DLTS would b e t o l o o k f o r changes in t h e DLTS s i g n a l c a u s e d by prolonged i l l u m i n a t i o n of a S c h o t t k y b a r r i e r d e v i c e .

T h i s h a s n o t t u r n e d o u t t o b e v e r y u s e f u l ; t h e changes a r e s m a l l and n o t e a s i l y i n t e r p r e t a b l e . However, i f t h e d e f e c t i s produced d u r i n g t h e forward b i a s c u r r e n t p u l s e o r by a n i n t e n s e l i g h t p u l s e and i t s s u b s e q u e n t decay o b s e r v e d by e i t h e r a c u r r e n t o r c a p a c i t a n c e t r a n s i e n t , t h e n b o t h i t s decay r a t e and p r o d u c t i o n r a t e can b e measured. T h i s e n t a i l s making t r a n s i e n t measurements n e a r t h e a n n e a l i n g t e m p e r a t u r e o f t h e d e f e c t ; above 1500C depending on t h e d e f e c t .

Fig. 1 0 shows a c a p a c i t a n c e t r a n s i e n t DLTS s i g n a l on a S c h o t t k y b a r r i e r s o l a r c e l l . The t i m e window i s Is and t h e measuring f r e q u e n c y 1000 Hz. The n e g a t i v e s i g n a l c o r r e s p o n d i n g t o s t a t e s t h a t t r a p e l e c t r o n s were o b t a i n e d u s i n g v o l t a g e p u l s e e x c i t a t i o n . To o b t a i n s i g n a l s d u e t o h o l e t r a p p i n g 674 nm l a s e r p u l s e e x c i t a t i o n was u s e d , The DLTS s i g n a l below 400K r e p r e s e n t s a d e n s i t y of e l e c t r o n and h o l e t r a p s s i m i l a r t o t h o s e shown in f i g . 9.

ELECTRONS Fig. 10. C a p a c i t a n c e t r a n s i e n t DLTS s i g n a l a s a f u n c t i o n of t e m p e r a t u r e f o r undoped n-type S c h o t t k y b a r r i e r / / * s o l a r c e l l

--

forward b i a s v o l t a g e

0 + HOLES

! k2kl

^{200 300 400 500} p u l s e p u l s e e x c i t a t i o n e x c i t a t i o n .

---

⁶⁷⁴ nm l a s e r TEMP ( K )

The w e l l - d e f i n e d peak n e a r 440K is t h e same whether l i g h t o r v o l t a g e e x c i t a t i o n i s u s e d . The d e n s i t y of e l e c t r o n t r a p s r e s p o n s i b l e f o r t h i s peak i s 6 x 1014 ~ m - ~ . The energy l e v e l f o r t h e t r a p c a n b e found i n t h e u s u a l manner ( 1 4 ) . It i s 0.93 eV which i s a l a r g e e n e r g y f o r a t r a p . What i s u n i q u e a b o u t t h i s t r a p i s t h a t t h e e l e c t r o n c a p t u r e i s t h e r m a l l y a c t i v a t e d w i t h a l a r g e a c t i v a t i o n energy, a b o u t 1 eV. T h e r e f o r e , p r o l o n g e d l a s e r o r v o l t a g e e x c i t a t i o n i s r e q u i r e d t o p o p u l a t e t h e s e t r a p s a t room t e m p e r a t u r e . S i m i l a r e l e c t r o n t r a p s a r e found i n o t h e r samples. I n some c a s e s t h e d e n s i t y i s a s h i g h a s 4 x 1016 Because t h e s e t r a p s a r e p o p u l a t e d under t h e same c o n d i t i o n s a s t h o s e r e q u i r e d f o r t h e Staebler-Wronski e f f e c t , i t i s t e m p t i n g t o a s s o c i a t e them w i t h t h e e f f e c t . The t r a p emptying o c c u r s a t t h e a n n e a l i n g t e m p e r a t u r e f o r t h e Staebler-Wronski e f f e c t ( 9 ) . A deep e l e c t r o n t r a p h a s b e e n observed i n n-type a-Si:H i n t h e a n n e a l e d s t a t e (13). However, a c t i v a t e d e l e c t r o n c a p t u r e was n o t o b s e r v e d . T h e r e f o r e , i t i s n o t t h e same a s t h e one d i s c u s s e d above

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Williams, R., Q u a r t e r l y Report No. 4 , SERI/PR-0-8254-3, p r e p a r e d u n d e r S u b c o n t r a c t No. XJ-9-8254.

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