TRANSIENT PHOTOCONDUCTIVITY AND SCHOTTKY BARRIER PROFILE DETERMINATION IN a-Si:H

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TRANSIENT PHOTOCONDUCTIVITY AND

SCHOTTKY BARRIER PROFILE DETERMINATION IN a-Si:H

T. Datta, M. Silver

To cite this version:

T. Datta, M. Silver. TRANSIENT PHOTOCONDUCTIVITY AND SCHOTTKY BARRIER PRO-

FILE DETERMINATION IN a-Si:H. Journal de Physique Colloques, 1981, 42 (C4), pp.C4-563-C4-

566. �10.1051/jphyscol:19814122�. �jpa-00220740�

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JOURNAL DE PHYSIQUE

CoZZoque C4, suppZ6ment au nOIO, Tome 42, octobre 1981 page C4-563

TRANSIENT PHOTOCONDUCTIVITY AND SCHOTTKY BARRIER PROFILE DETERMINATION

I N a - S i : H

T. D a t t a and M. S i l v e r

Physics and Astronomy Department, University o f North CaroZina, a t Chapel H i Z Z , North CaroZina, 27514, U.S.A.

A b s t r a c t . - C a r r i e r d r i f t s t u d i e s on submicron t h i c k a-Si a l l o y s sandwiched between b l o c k i n g m e t a l and i n j e c t i n g c o n t a c t s r e v e a l anomalous b e h a v i o r . Using a p u l s e d e x t e r n a l f i e l d and a t u n e a b l e - p u l s e d , nano-second l a s e r , we o b s e r v e t h a t t h e t r a n s i e n t p h o t o c u r r e n t c a n n o t b e e x p l a i n e d by t h e c u r r e n t r e s p o n s e of a s h e e t of d r i f t i n g c h a r g e s . The non d r i f t c o n t r i b u t i o n s a p p e a r t o o r i g i n a t e from t h e t i m e dependent r e l a x a t i o n of t h e S c h o t t k y b a r r i e r a s a r e s u l t of t h e p h o t o e x c i t a t i o n . T h i s e f f e c t i s more o b v i o u s i n t h e s m a l l forward b i a s (2+1.0 v o l t s ) mode, when a r e v e r s a l of t h e p h o t o r e s p o n s e i s o b t a i n e d . Also, we h a v e d e v i s e d a n o v e l t e c h n i q u e t o d e t e r m i n e t h e j u n c t i o n p o t e n t i a l p r o f i l e . T h i s i s done by measuring t h e e x t e r n a l f i e l d r e q u i r e d t o n u l l t h e i n i t i a l photo c u r r e n t a s a f u n c t i o n of t h e e x c i t a t i o n wave l e n g t h (hence, t h e p e n e t r a t i o n d e p t h ) .

We a d d r e s s t h e q u e s t i o n of t h e d i s c r e p a n c y between t h e r e p o r t e d d r i f t m o b i l i t y v a l u e s between t h i n (<11~m) and t h i c k ( > l p m ) samples i n a - s i a l l o y s ( 1 ) . R e c e n t l y o u r r e s u l t s ( 2 ) of t r a n s i e n t p h o t o - c u r r e n t measurements i n t h i n a-Si:H:F f i l m s showed a n o n - t r a n s p o r t component i n t h e p h o t o r e s p o n s e . These s i g n a l s may e a s i l y b e m i s -

t a k e n a s bona f i d e t r a n s i e n t p u l s e s and from which o n e would d e r i v e e r r o n e o u s v a l u e s f o r t h e m o b i l i t y . I n t h e p r e s e n t p a p e r we a n a l y z e s i m i l a r d a t a i n S i l a n e samples.

Also, we w i l l r e p o r t o n t h e r e s u l t s of s i m p l e measurements which y i e l d t h e p o t e n t i a l p r o f i l e i n t h e S c h o t t k y b a r r i e r between a m e t a l c o n t a c t and a-Si:H. These d a t a were o b t a i n e d v i a t h e t e c h n i q u e d i s c u s s e d by D a t t a and S i l v e r ( 3 ) .

The d e t a i l s of o u r p h o t o - i n j e c t i o n and t r a n s i e n t measurements h a v e been de- s c r i b e d i n r e f e r e n c e ( 2 ) . B r i e f l y , t h e c a r r i e r s were c r e a t e d by l a s e r e x i t a t i o n s o f a b o u t 6 nS d u r a t i o n a t 1 0 HZ i n a sub-micron t h i c k a-Si:H (W d e p o s i t e d ) l a y e r sandwiched between b l o c k i n g m e t a l and l n j e c t i n g c o n t a c t s . For t h e d r i f t experiment t h e l a s e r wave l e n g t h was chosen t o b e 500nm, b u t f o r p o t e n t i a l p r o f i l i n g t h e wave l e n g t h was v a r i e d between 450nm and 600nm, i n a number of c o n v e n i e n t d i s c r e t e s t e p s . The photon f l u x was a d j u s t e d s u c h t h a t t h e i n i t i a l peak s i g n a l v a r i e d d i r e c t l y w i t h t h e l i g h t i n t e n s i t y .

Fig. 1 shows t h e t r a n s i e n t s i g n a l s f o r a-Si:H. The t o p c u r v e i s t h e r e s p o n s e f o r l . O V , and t h e ' m i d d l e c u r v e i s t h a t f o r 0.5V. Both t h e s e p u l s e s h a v e t h e g e n e r a l a p p e a r a n c e o f t h e p r e v i o u s l y r e p o r t e d d r i f t s i g n a l s by T i e d j e ( l ) , namely a r e l a - t i v e l y f l a t t o p f o l l o w e d by a f a s t decay. The b r e a k o r knee of t h e c u r v e , is nor- m a l l y t a k e n t o makr t h e a r r i v a l of t h e d r i f t i n g c h a r g e s a t t h e c o l l e c t i n g e l e c t r o d e . The slow i n i t i a l r i s e i s u n i v e r s a l l y observed i n t h e s e m a t e r i a l s and i s d u e t o a s l o w r e l e a s e o f c a r r i e r s from n e a r s u r f a c e t r a p s , S i l v e r ( 4 ) . The l o w e s t c u r v e i s a r e - p l o t of T i e d j e ' s 1.OV d a t a f o r a 3.5 pm t h i c k a-Si:H sample. N o t i c e , t h a t on a n ab- s o l u t e t i m e s c a l e t h e r e s u l t s of o u r measurements a r e t h e same a s t h o s e o b t a i n e d by T i e j g e ; n o t w i t h s t a n d i n g , t h e a l m o s t 9 : l r a t i o i n t h e specimen t h i c k n e s s . I n p o i n t of f a c t , t o d a t e a l l l a b o r a t o r i e s h a v e r e p o r t e d a l m o s t i d e n t i c a l t r a n s i t t i m e s e v e n though t h e t h i c k n e s s of t h e samples d i f f e r e d by 1 5 t o 1.. W e do n o t b e l i e v e t h i s is a concidence. As we have d i s c u s s e d i n d e t a i l i n r e f e r e n c e ( 2 ) , t h e photo-response c a n n o t b e s o l e l y due t o a " d r i f t i n g s h e e t " of c h a r g e s . T h i s i s t r u e , d e s p i t e t h e

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19814122

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C4-564 JOURNAL DE PHYSIQUE

f a c t t h a t t h e apparent t r a n s i e n t time v a r i e s i n v e r s e l y w i t h t h e a p p l i e d v o l t a g e . The non-transport component of t h e t r a n s i e n t p u l s e i s more a p p a r e n t when observed under small forward b i a s . Fig. 2 shows t h e r e s p o n s e f o r 1 . 3 v o l t s forward b i a s . Here we

Fig. .l:

T r a n s i e n t photo- response i n a-Si:H f i l m s . Top curve l . O V , middle curve 0 . 5 ~ pulsed r e v e r s e d b i a s . Lowest c u r v e i s a r e - p l o t of T i e d j e ' s d a t a ( 2 ) , and c l e a r l y shows t h e c o i n c i d e n c e i n t h e two time s c a l e s d e s p i t e a l a r g e d i f f e r e n c e i n t h e sample t h i c k n e s s .

0 100 200 300 400

1 0 ' ~ SEC

s e e t h e s t r a n g e r e v e r s a l i n t h e p h o t o c u r r e n t , j u s t a s was s e e n i n a-Si:H:F. The presence of t h i s u n f a m i l i a r phenomena i n a-Si:H a s i n a-Si:H:F cannot b e explained i n a s i m p l e d r i f t model, because n e i t h e r t h e amplitude nor t h e time of r e v e r s a l , s c a l e s c o r r e c t l y w i t h t h e a p p l i e d f i e l d . We conclude, t h a t t h e dominent c o n t r i b u t i o n t o t h e photo-response i n t h i n samples r e s u l t s from a s p a c e c h a r g e r e - o r i e n t a t i o n due t o t h e p h o t o - e x c i t a t i o n w i t h i n t h e Schottky b a r r i e r . Since t h e space c h a r g e r e l a x a - t i o n p u l s e width i s i n s e n s i t i v e t o t h e sample t h i c k n e s s , a "constant time" would b e i n f e r r e d and t h e r e f o r e , t h e deduced m o b i l i t y w i l l appear t o i n c r e a s e w i t h L ~ / V in- s t e a d of d e c r e a s i n g w i t h t h i c k n e s s a s p r e d i c t e d v i a d i s p e r s i v e t r a n s p o r t (5).

This i n t e r p r e t a t i o n i s r e i n f o r c e d by t h e r e s u l t s r e c e n t l y r e p o r t e d by Nielson e t a l . ( 1 ) . The observed a n i n c r e a s e i n t h e a p p a r e n t d r i f t m o b i l i t y i n a-Si:H, ranging from 10-'cm2/Vsec f o r LSlpm t o 1.2 ~ m - ~ i v s e c f o r L = i'.5lym. These a r e i n agreement w i t h t h e r e s u l t s and d i s c u s s i o n s p r e s e n t e d above. The more r e c e n t i n t e r - p r e t a t i o n of d i s p e r s i v e t r a n s p o r t i n a-Si, i s i n terms of a n e x p o n e n t i a l d i s t r i b u t i o n of t r a p s ( 6 , 7 ) . This model t o o p r e d i c t s a n a p p a r e n t m o b i l i t y which e i t h e r remains c o n s t a n t o r d e c r e a s e s w i t h t h i c k n e s s , depending upon t h e sample temperature. Thus, one i s hard pressed t o i n t e r p r e t ~ i e l s o n ' s d a t a (1) except through a n e x t r a n e o u s near s u r f a c e space c h a r g e c o n t r i b u t i o n .

A s a p a r t of our s t u d i e s , we found t h a t we could e x p e r i m e n t a l l y determine t h e p o t e n t i a l , o r t h e i n t e r n a l f i e l d p r o f i l e of t h e j u n c t i o n between t h e blocking c o n t a c t and t h e amorphous i n s u l a t i n g f i l m . I n t h e l i g h t of t h e r o l e of t h e j u n c t i o n r e s p o n s e i n t h e t r a n s i e n t d r i f t m o b i l i t y measurement i n t h e s e systems, i t i s a p p r o p r i a t e t o r e p o r t on some r e c e n t l y obtained p o t e n t i a l p r o f i l e d a t a i n a-Si:H. A s d i s c u s s e d i n d e t a i l i n r e f e r e n c e ( 3 ) , t h e p h y s i c a l b a s i s of t h i s measurement is t o n u l l t h e i n i - t i a l photo-current .l(t=of), by v a r y i n g t h e a p p l i e d forward b i a s f o r a g i v e n

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Fig. 2:

Anomalous t r a n - s i e n t response i n a a-Si:H f i l m ,

(under a small forward b i a s ) . Notice, t h e photo c u r r e n t r e v e r s e s s i g n from i n i t i a l l y n e g a t i v e t o a f i n a l p o s i t i v e v a l u e .

a b s o r p t i o n depth of t h e i n c i d e n t pulsed l i g h t . Where

I n e q u a t i o n 1, e i s t h e e l e c t r o n i c charge,

un

and

v

a r e t h e e l e c t r o n i c and h o l e mo- b i l i t i e s r e s p e c t i v e l y , n(x) t h e excess f r e e c a r r i e r p d e n s i t y . E(x) is t h e t o t a l f i e l d , which f o r s h o r t v o l t a g e p u l s e t i m e s , is:

E(x) = E.(x)

- - v

L

...

²

Where E.(x) i s t h e r e q u i r e d i n t e r n a l f i e l d and V t h e a p p l i e d p u l s e d e x t e r n a l v o l t a g e a c r o s s h e sample of t h i c k n e s s L. The photo generated f r e e c a r r i e i ; d e n s i t y is:

n ( x ) = n e-ax

...

3

Where a i s t h e a b s o r p t i o n c o e f f i c i e n t ; t h e r e f o r e t h e ~ ( t = o + ) i s z e r o f o r an e x t e r n a l f i e l d V(cr)null when:

L

I

L ^(Ei(x)

-

^-V ( a ) n ~ l l ) ~ - @ x ~ ~ _L =

...

⁴

S i n c e a i s a f u n c t i o n of t h e e x c i t a t i o n wave l e n g t h , by v a r y i n g t h e wavelength of t h e l a s e r l i g h t s a y m times, one o b t a i n s m i n t e g r a l o r f u n c t i o n a l e q u a t i o n s t h e type;

-%-L Vnull(ak) 1

-

e K

L _"k = lLEi ( X ) exp 1-akx] dx

. ^. . ^{. . .}

⁵

where k r u n s from 1 t o m. Modeling E . ( x ) by a f u n c t i o n a l form c o n t a i n i n g a t most m parameters t h e m e q u a t i o n (of t h e t$pe 5) may b e i n v e r t e d t o o b t a i n a r e a s o n a b l e e s t i m a t e f o r Ei(x). We found t h a t our d a t a could b e b e s t f i t t e d t o an a n a l y t i c form shown below,

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JOURNAL DE PHYSIQUE

F i g . 3:

Vnull a s a f u n c t i o n of e x c i t a t i o n wave l e n g t h . The e r r o r b a r s i n d i c a t e t h e s c a t t e r s o v e r s e v e r a l measurement s.

I n F i g . 3 we show t h e e x p e r i m e n t a l d a t a f o r t h e Vnull VS L a s e r wave l e n g t h . The s o l i d c u r v e i s t h e model f i t o f p r e d i c t e d Vnull f o r e q u a t i o n 6 w i t h E. = 7x104v/cm and 8 - l = 600 A". It a p p e a r s t h a t t h e s e a r e r e a s o n a b l e v a l u e s and y i e l d a

S c h o t t k y b a r , r i e r p o t e n t i a l of around 0 . 4 ~ .

Acknowledgements: We w i s h t o t h a n k D r . A. Madan of Energy Conversion Devices f o r t h e p r e p a r a t i o n of t h e specimens and a l s o f o r h i s i n v a l u a b l e a d v i c e . T h i s r e - s e a r c h i s p a r t i a l l y s u p p o r t e d by t h e NSF G r a n t IIDMR 7920023, which we acknowledge s i n c e r e l y .

R e f e r e n c e s :

1. LeComber P.G., Madan A . , and Spear W.G.J. Non compt. S o l 11, (1972), 219.

Moore A.R. App. Phys. L e t t . 31, (1977), 762.

T i e d j e T . , Abeles B. M o r r e l D. L. e t a l . App. Phys. L e t t . 36, (1980), 695.

N i e l s e n P. and D a l a l V. Bal. Phys. Soc. 26, (1981).

2. D a t t a T. and S i l v e r M. t o a p p e a r i n Soc. S t . Comm. (1981).

3 . D a t t a T. and S i l v e r M. t o a p p e a r i n App. Phys. L e t t . (1981).

4. S i l v e r M., Dy K. S. and Huang I. L. PRL. 27, (1971), 21.

5. S c h e r H. and M o n t r o l e E. W. PREi 1 6 , (1977), 4466.

6. S i l v e r M. and Cohen L. PRB 1 5 , (1977), 3276.

7. T i e d j e T. and Rose A. S o l . S t . Comm. 37, (1981), 49.