STUDY OF LIGHT INDUCED CHANGES IN a-Si:H BY DETAILED COMPUTER MODELING OF ADMITTANCE AND DLTS

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STUDY OF LIGHT INDUCED CHANGES IN a-Si:H BY DETAILED COMPUTER MODELING OF

ADMITTANCE AND DLTS

J. Cohen, D. Lang, J. Harbison, A. Sergent

To cite this version:

J. Cohen, D. Lang, J. Harbison, A. Sergent. STUDY OF LIGHT INDUCED CHANGES IN a-Si:H

BY DETAILED COMPUTER MODELING OF ADMITTANCE AND DLTS. Journal de Physique

Colloques, 1981, 42 (C4), pp.C4-371-C4-374. �10.1051/jphyscol:1981479�. �jpa-00220936�

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STUDY OF LIGHT INDUCED CHANGES IN a-Si:H BY DETAILED COMPUTER MODELING O F ADMITTANCE AND DLTS

J . D . Cohen, D.V. Lang, J.P. H a r b i s o n and A.M. S e r g e n t Bell Laboratories, Murray H i Z Z , New Jersey 07974, U.S.A.

A b s t r a c t . - A f u l l n u m e r i c a l a n a l y s i s o f t h e n o n e q u i l i b r i u m r e s p o n s e of a Schottky- b a r r i e r space c h a r g e r e g i o n f o r a semiconductor w i t h an a r b i t r a r y d e n s i t y of s t a t e s , g ( E ) , h a s been developed. These methods a r e a p p l i e d t o measurements of a d m i t t a n c e t e m p e r a t u r e , t h e r m a l l y s t i m u l a t e d c a p a c i t a n c e (TSCAP), and 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) of n-type a-Si:K samples i n a 200°C d a r k a n n e a l e d s t a t e ( S t a t e A) a s w e l l a s s e v e r a l p a r t i a l l y d a r k a n n e a l e d s t a t e s follow- i n g i n i t i a l p r e p a r a t i o n i n a l i g h t s a t u r a t e d s t a t e ( S t a t e B ) . We f i n d t h a t changes i n t h e a d m i t t a n c e and DLTS s p e c t r a can be e x p l a i n e d i n d e t a i l by a lower- i n g of t h e

bulk

Fermi e n e r g y and a n i n c r e a s e i n t h e d e n s i t y of s t a t e s below midgap.

D e t a i l e d p i c t u r e s of g(E) o b t a i n e d from DLTS s p e c t r a of two p a r t i a l l y a n n e a l e d s t a t e s a r e p r e s e n t e d . The q u a n t i t a t i v e r e l a t i o n of t h e Fermi e n e r g y s h i f t s t o t h e o b s e r v e d changes i n g(E) i s d i s c u s s e d .

INTRODUCTION.- One of t h e more i n t r i g u i n g p r o p e r t i e s of a-Si:H a l l o y s d i s c o v e r e d i n r e c e n t y e a r s h a s been t h e a b i l i t y t o r e v e r s i b l y d e c r e a s e t h e d a r k c o n d u c t i v i t y by i n t e n s e l i g h t e x p o s u r e

-

t h e s o - c a l l e d Staebler-Wronski e f f e c t . [ l ] At p r e s e n t t h e mechanism f o r t h i s phenomenon remains u n e x p l a i n e d . C o n s i d e r a b l y c o n t r o v e r s y c e n t e r s upon whether t h e e f f e c t i s a b u l k p r o p e r t y [ 2 , 3 ] o r m e r e l y a r e s u l t of changes i n s u r f a c e c h a r g e w i t h t h e a t t e n d a n t m o d i f i c a t i o n of b a r r i e r h e i g h t s . [ 4 ] I n t h i s p a p e r we r e p o r t a n e x t e n s i v e s e r i e s of a d m i t t a n c e v s . t e m p e r a t u r e (C-T) and 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) measurements on PH3-doped a-Si:E f i l m s which p r o v e t h a t t h e b u l k p r o p e r t i e s of t h e s e f i l m s a r e indeed b e i n g m o d i f i e d by l i g h t exposure.

I n p a r t i c u l a r t h e s e measurements i n d i c a t e a s u b s t a n t i a l s h i f t i n t h e b u l k F e n i e n e r g y , EF, i n o u r samples and a m o d i f i c a t i o n of t h e u n d e r l y i n g d e n s i t y of s t a t e s , g (E)

.

EXPERIIIENTAL METKODS AND INTERPRETATION

P e v e r a l p r e l i m i n a r y a c c o u n t s f o r i n t e r p r e t i n g DLTS d a t a t o deduce g(E) have beer1 g i v e n . [ 5 , 6 ] More r e c e n t l y , we have developed a f u l l n u m e r i c a l a n a l y s i s o f t h e t r a n s i e n t and a c dynamic r e s p o n s e of a S c h o t t k y - b a r r i e r s p a c e c h a r g e r e g i o n f o r a semiconductcr w i t h a c o n t i n u o u s d i s t r i b u t i o n of gap s t a t e s . T h i s method a l l o w s u s t o r e l a t e DLTS, t h e r m a l l y s t i m u l a t e d c a p a c i t a n c e (TSCAP), complex a d m i t t a n c e v s . temp- e r a t u r e and f r e q u e n c y (C-T-&), o r c a p a c i t a n c e - v o l t a g e measurements (C-V) t o t h e u n d e r l y i n g g ( E ) . For many n-type samples we may o b t a i n a d e t a i l e d q u a n t i t a t i v e p i c t u r e of g(E) e x t e n d i n g o v e r a p p r o x i m a t e l y 70% of t h e m o b i l i t y gap. A f u l l account of t h i s work w i l l a p p e a r s h o r t l y . [ 7 ]

Our PH3-doped a-Si:E samples were p r e p a r e d by t h e s t a n d a r d r f d e c o n p o s i t i o n of SiHq a s d e s c r i b e d e l s e w h e r e . [ 5 ] A f t e r i n i t i a l c h a r a c t e r i z a t i o n i n t h e d a r k ' a n n e a l e d s t a t e ( S t a t e A), we s u b j e c t e d t h e sample t o i n t e n s e 2eV l a s e r i l l u m i n a t i o n ( l p s p u l s e s of 80 m ~ l c m 2 p e r 5 u l s e w i t h 1 5 p u l s e s / s e c o n d c o r r e s p o n d i n g t o an a v e r a g e power d e n s i t y of 1.2W/cm ) f o r 1 o r 2 h o u r s a t 300K. We t h e n performed i s o c h r o n a l a n n e a l i n g s t e p s i n t h e d a r k a t z e r o b i a s b e g i n n i n g a t 380K w i t h 10K t e m p e r a t u r e i n c r e m e n t s . A f t e r each a n n e a l i n g s t e p we performed C-T-w, C-V, and p e r i o d i c a l l y a l s o DLTS and TSCAP measurements. The f i n a l 470K a n n e a l produced r e s u l t s n e a r l y i d e n t i c d w i t h t h e o r i g i n a l d a r k a n n e a l e d S t a t e A.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1981479

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JOURNAL DE PHYSIQUE

F i g . 1: lOkIIz C a p a c i t a n c e Temperature a t 5 v o l t r e v e r s e b i a s f o l l o w i n g a s e r i e s of d a r k a n n e a l i n g s t e p s a s summarized i n T a b l e I below. The a c t i v a t i o n e n e r g i e s of t h e

" f r e e z e - o u t " of t h e a c r e s p o n s e f o r s e v e r a l c u r v e s i s ' i n d i c a t e d . The sample a r e a i s 2x10-3 cm2.

TEMPERATURE ( K )

RESULTS AND DISCUSS1ON

I n F i g . 1 one s u c h s e r i e s of lOkEz C-T measurements i s shown. The shape of t h e s e c u r v e s i n d i c a t e s t h a t g(E) h a s a minimum i n t h e upper h a l f of t h e gap a s p r e v i o u s l y d i s c u s s e d . [ 5 - 7 1 From accompanying measurements a t 100tiz, IkEIz, and lOOkEz we may deduce t h e a c t i v a t i o n e n e r g y of t h e b u l k a c c o n d u c t i v i t y a s i n d i c a t e d i n T a b l e I. These e n e r g i e s a r e not a f f e c t e d by v a r y i n g t h e a p p l i e d b i a s and a r e t h e r e - f o r e a l s o i n s e n s i t i v e t o p o s s i b l e s h i f t s i n f l a t - b a n d v o l t a g e s due t o changes i n s u r f a c e s t a t e o c c u p a t i o n . Supplemental C-V measurements i n d i c a t e t h a t t h e c a p a c i - t a n c e changes shown i n F i g . 1 c o r r e s p o n d s t o a change i n t h e t r u e b u l k of t h e mate- r i a l o v e r t h e r a n g e of a t t a i n a b l e space-charge d e p l e t i o n w i d t h s (up t o r o u g h l y O.3pm i n t o t h e sample). The b u l k n a t u r e of t h e e f f e c t is f u r t h e r v e r i f i e d by t h e g e n e r a l agreement between t h e s e d a t a and c a l c u l a t i o n s o f t h e p r e d i c t e d change i n C-T b e h a v i o r o c c u r r i n g from a s h i f t of EF f o r t h e b u l k d e n s i t y of s t a t e s we d e t e r m i n e by DLTS

( i n c l u d i n g t h e d r a m a t i c change between t h e s h a p e s of c u r v e s B 1 and B2 i n F i g . 1 ) .

Symbol Anneal Temperature Anneal Time

( K e l v i n ) (minutes)

Ea

( e l e c t r o n v o l t s )

T a b l e I: Summary of a n n e a l sequence of a d m i t t a n c e c u r v e s of F i g . 1 g i v i n g a c t i v a t i o n e n e r g i e s o f a c ( d a r k ) c o n d u c t i v i t y , Eo.

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I I I I I 1 I

1

50 (00 (50 200 250 300 350 400 450 TEMPERATURE (K )

F i g . 2: 10kKz C a p a c i t a n c e DLTS f o r two p a r t i a l a n n e a l s and t h e f i n a l a n n e a l ( S t a t e A ) u s i n g a l 0 s e c - l r a t e window and -5 v o l t ambient b i a s . The lower c u r v e 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 s t o z e r o b i a s of l O m s d u r a t i o n and t h e upper c u r v e s u s i n l u s 2eV l a s e r p u l s e s w i t h 5x10K6 photons/cm2 p e r p u l s e . The s o l i d l i n e s drawn t h r o u g h t h e B2 and B5 d a t a p o i n t s a r e t h e c a l c u l a t e d DLTS s i g n a l s d e r i v e d from t h e

d e n s i t i e s of s t a t e s i n F i g . 3

I

EF (STATE 95) +

-

60 min ANNEAL AT I ^II

loqT

-

380K (92)

.--.-ADDITION& 5 min ANNEAL I AT 420K (95)

l 0 ' 6 l 1 I I I I '

I

1.6 1 2 0.8 0.4 0

ENERGY BELOW Ec (ELECTRON VOLTS)

F i g . 3: D e n s i t i e s of S t a t e s d e t e r m i n e d from t h e DLTS d a t a of F i g . 2 f o r a n n e a l

" S t a t e s " B2 and B5. The p o s i t i o n s of t h e Fermi e n e r g i e s i n d i c a t e d a r e d e t e r - mined from t h e a c t i v a t i o n e n e r g i e s of t h e a c ( d a r k ) c o n d u c t i v i t y . The band t a i l below Ec ( d a s h e d ' l i n e ) i s

not

o b t a i n e d from t h e DLTS d a t a d i r e c t l y b u t i s i n f e r r e d from s u p p l e m e n t a l TSCAP d a t a .

The DLTS s p e c t r a f o r S t a t e A and t h e p a r t i a l l y a n n e a l e d " S t a t e s " ~ 2 and B5 i n F i g . 2 i n d i c a t e s i g n i f i c a n t changes i n t h e u n d e r l y i n g g(E). Some c a r e must be t a k e n i n t h e i n t e r p r e t a t i o n . Because t h e a c c o n d u c t i v i t y o f t h e f i l m changes d r a m a t i c a l l y a f t e r s e v e r a l a n n e a l i n g s t e p s , t h e q u a n t i t a t i v e r e l a t i o n s h i p of t h e DLTS d a t a t o g(E) a l s o v a r i e s . T h i s r e s u l t s p r i m a r i l y from changes i n t h e e f f e c t i v e a r e a of t h e b a r r i e r a t t h e a - S i : H / s u b s t r a t e i n t e r f a c e due t o c u r r e n t s p r e a d i n g i n t h e a-Si:H f i l m ( s e e Xef. 7 ) . While some a t t e m p t h a s been made t o c o r r e c t f o r t h i s , t h e remaining r e l a - t i v e u n c e r t a i n t y of 20-40% between S t a t e A and S t a t e B2 i s comparable t o t h e changes observed i n t h e DLTS s p e c t r a i n F i g . 2. For t h i s r e a s o n we s h a l l compare g(E) f o r t h e two p a r t i a l l y a n n e a l e d s t a t e s B2 and B5 where s u c h u n c e r t a i n t i e s have been d e t e r m i n e d t o b e a t most 15%.

The DLTS-determined g(E) r e s u l t s f o r s t a t e s B2 and B5 a r e shown i n F i g . 3 . The most s i g n i f i c a n t change a p p e a r s t o o c c u r a t 0.5eV above Ev, where, i n f a c t , most samples we have s t u d i e d e x h i b i t a f a i r l y w e l l d e f i n e d d e f e c t band in.DLTS. The change i n g(E) ( s e e F i g . 4 ) i s f a i r l y l a r g e - r o u g h l y 1018 cm-3. The above mentioned 15% un- c e r t a i n t y i n t h e r e l a t i v e c a l i b r a t i o n could a f f e c t t h i s d i f f e r e n c e by a t most a f a c t o r of two. To o b t a i n g(E) i n t h e lower h a l f of t h e gap we a l s o assumed a n e q u a l c a p t u r e a c r o s s s e c t i o n f o r e l e c t r o n and h o l e s . I f , a s may b e more r e a s o n a b l e , s t a t e s n e a r EV have a much l a r g e r h o l e c r o s s s e c t i o n , t h e d i f f e r e n c e spectrum would be a f a c t o r of two s m a l l e r .

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JOURNAL DE PHYSIQUE

F i g . 4: D i f f e r e n c e between g(E) f o r S t a t e s B2 and B5 from F i g . 3 p l o t t e d on a l i n e a r s c a l e i n d i c a t i n g a l a r g e change i n g(E) below midgap. E s t i m a t e d u n c e r t a i n t i e s and t h e r e l e v a n c e t o observed s h i f t s i n EF a r e d i s c u s s e d i n t h e t e x t .

ENERGY BELOW EC (ELECTRON VOLTS) The change i n o c c u p a t i o n of s t a t e s n e a r Ec due t o t h e a p p a r e n t 40meV s h i f t i n EF between S t a t e s B2 and B5 h a s been determined d i r e c t l y from t h e changes i n measured a d m i t t a n c e t o be 2.4

*

0.2~1016cm-3. T h i s i s much lower t h a n t h e a p n a r e n t changes i n g(E) i n t h e lower h a l f of t h e gap. T h i s i n d i c a t e s t h a t t h e change i n t h e

E, +

0.5 eV l e v e l i s most l i k e l y

not

r e s p o n s i b l e f o r t h e s h i f t i n EF ( i . e . , t h e s e s t a t e s a r e more p r o b a b l y d o n o r - l i k e ) . Eowever, o t h e r s m a l l changes i n g(E) c o u l d e a s i l y a c c o u n t f o r t h e s h i f t i n EF. For example, i f t h e d i f f e r e n c e between t h e raw DLTS s p e c t r a of S t a t e A and S t a t e B2 i s t a k e n a t f a c e v a l u e , t h e s h i f t i n EF i n o u r n-type samples may b e l i n k e d t o t h e a p p a r e n t changes i n g(E) i n t h e u p p e r h a l f of t h e gap ( a t a p p r o x i m a t e l y Ec-0.7eV). We e x p e c t t o b e a b l e t o unambiguously r e s o l v e t h i s i s s u e i n t h e n e a r f u t u r e .

ACKNOWLEDGEMENTS

We wish t o t h a n k A. J . Williams and A. Savage f o r v a l u a b l e a s s i s t a n c e i n sample ' growth and d i o d e f a b r i c a t i o n .

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