a-Si : H IN ELECTROPHOTOGRAPHY AND VIDICON DEVICES

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a-Si : H IN ELECTROPHOTOGRAPHY AND VIDICON DEVICES

I. Shimizu, S. Oda, K. Saito, H. Tomita, E. Inoue

To cite this version:

I. Shimizu, S. Oda, K. Saito, H. Tomita, E. Inoue. a-Si : H IN ELECTROPHOTOGRAPHY AND VIDICON DEVICES. Journal de Physique Colloques, 1981, 42 (C4), pp.C4-1123-C4-1130.

�10.1051/jphyscol:19814243�. �jpa-00220875�

JOURNAL DE PHYSIQUE

CoZZoque C4, suppZ6ment au nOIO, Tome 42, octobre 1981 page C 4 - 1 1 2 3

a - S i : H I N ELECTROPHOTOGRAPHY AND V I D I C O N DEVICES

I . S h i m i z u , S . O d a , K . S a i t o , H. T o m i t a a n d E . I n o u e

Imaging Science and Engineering Laboratory, Tokyo Institute of TechnoZogy, Nagatsuta, Midori-ku, Yokohma, Japan

A b s t r a c t . - Experimental s t u d i e s i n a p p l i c a t i o n o f a-Si:H t o a photoreceptor o f electrophotography and v i dicon t a r g e t o f image pickup tube are summerized.

Two types o f b l o c k i n g contacts, homojunction and heterophase b l o c k i n g , have been proposed t o s u s t a i n the s u f f i c i e n t charges on the photoreceptor and a low c u r r e n t l e v e l o f v i d i c o n t a r g e t i n the dark. An " i n t r i n s i c " a-Si:H prepared i n the o p t i m i z e d c o n d i t i o n s i s a p p l i c a b l e as the photoconductive l a y e r o f t h e b o t h devices. Excel l e n t photoresponse can be achieved i n the charge d e p l e t i o n devices w i t h the b l o c k i n g c o n t a c t s i n whole v i s i b l e region. W i t h i n t h e frame- work o f o u r l a b o r a t o r y scale study, we conclude t h a t a-Si:H i s a promising m a t e r i a l as a photoconductor o f image devices.

I n t r o d u c t i o n . - Resul t i n from r e c e n t e x t e n s i v e s t u d i e s , i t has been r e v e a l e d c l e a r l y t h a t amorphous s i l i c o n ?a-Si :H) prepared by glow discharge technique i s a promising m a r e r i a l i n various f i e l d s o f a p p l i c a t i o n , i. e., p h o t o v o l t a i c s o l a r - c e l l s , FET t r a n s i s t o r a r r a y s f o r f l a t screen d i s p l a y and imaging technology. The a-Si:H has a t t r a c t i v e advantages, e. g., excel l e n t p h o t o c o n d u c t i v i t y , s t r o n g o p t i c a l a b s o r p t i o n f o r v i s i b l e l i g h t , a b i l i t y t o f a b r i c a t e a l a r g e area o f the t h i n f i l m w i t h s u p e r i o r two-dimentional homogeniety, mechanical s t r e n g t h and h e a t d u r a b i l i t y i n a d d i t i o n t o t h e a b i l i t y t o c o n t r o l the e l e c t r i c p r o p e r t i e s by doping w i t h f o r e i g n elements, which have s t i m u l a t e d us i n development o f some new types o f imaging systems w i t h a-Si:H and t h e r e l a t e d m a t e r i a l s .

The main purpose o f t h i s paper i s t o i n t r o d u c e o u r r e c e n t attempts a t a p p l i c a t i o n o f a-Si :H t o a photoconductor i n conventional electrophotography and a v i d i c o n - t y p e image pickup tube. The former technique i s a well-known imaging procedure f o r dup- l i c a t i o n o f " t h e s t a t i c images" and t h e l a t t e r uses a t y p i c a l scanning image techno- l o g y which i s a p p l i c a b l e t o t r e a t i n g " t h e dynamic images". The demands f o r the p h o t o - e l e c t r i c p r o p e r t i e s o f photoconductive m a t e r i a l s i n o t h e r a p p l i c a t i o n s , e. g., f a c s i m i l e s , h i g h speed p r i n t e r s , some s o l i d state-imaging devices, must be i n c l u d e d i n the r e q u i rements f o r b o t h techniques and the knowledges a t t a i n e d i n these s t u d i e s a r e expected t o be a p p l i c a b l e t o extending the f i e l d s o f a p p l i c a t i o n s o f a-Si:H and the r e l a t e d m a t e r i a l s .

M a t e r i a l Preparation.- A photoconductive l a y e r w i t h wide area, a t l e a s t 30cm X 30cm, i s r e q u i r e d as a photoreceptor o f an e l e c t r o p h o t o g r a p h i c copy machine. I t has been e s t a b l i s h e d t h a t an a-Si:H f i l m prepared by RF glow discharge o f SiH* showed excel- l e n t homogeniety i n the thickness and t h e o p t i c a l o r the e l e c t r i c a l p r o p e r t i e s o v e r t h e two-dimentional area o f 30cm X 30cm by c o n t r o l l i n g p r e p a r a t i o n parameters.

Yamamoto e t a l ! ) f a b r i c a t e d an a-Si:H:O f i l m on a c y l i n d r i c a l s u b s t r a t e ( A l ) by the i n d u c t i v e - c o u p l e d RF glow discharge o f SiH4 and took t h e measurements o f t h e imaging

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19814243

C4-1124 JOURNAL DE PHYSIQUE

c h a r a c t e r i s t i c s i n a p r a c t i c a l copying c o n d i t i o n . The r e c e n t progress i n the f a b r i - c a t i o n technology w i t h t h e RF glow discharge has made a l a r g e s i z e o f the a-Si:H f i l m showing excel l e n t f e a t u r e s p r a c t i c a l .

A p a r t from the c o s t problem the r a p i d growth o f the f i l m i s an i m p o r t a n t demand from t h e p r a c t i c a l p o i n t o f view. R e l a t i v e l y t h i c k f i l m o f about 1 0 ~ m t h i c k i s r e - q u i r e d as a photoreceptor and thus many hours, more than 10 hours, must be spent t o prepare the photoconductive l a y e r as f a r as we would s t a y i n the p o i n t e s t a b l i s h e d by ~ n i ghtsz'from h i s systematic s t u d i e s on RF glow discharge technique i n p r e p a r i n g a-Si:H. However, some conspicuous progress has been made i n the growth r a t e . Knights e t a1 ?'recently presented t h e f a c t t h a t h i g h d e p o s i t i o n r a t e s ( 3 ~ 5 p m l h r . ) c o u l d be achieved c o n c u r r e n t l y w i t h low d e f e c t d e n s i t i e s when helium was used as a d i l u e n t o f SiH4. S c o t t e t al:"proposed a r a p i d growth ( -14 A/sec) o f a-Si:H pre- pared from SizHs. I n r e c e n t years, we have succeeded i n f a b r i c a t i n g 1Oym t h i c k f i l m w i t h good s u r f a c e w i t h i n one and a h a l f h o u r ' s d e p o s i t i o n from gaseous m i x t u r e of S i F 4 and SiH4. Moreover, a-Si:H f i l m showing s u f f i c i e n t p h o t o - e l e c t r i c features c o u l d be f a b r i c a t e d i n a growth r a t e o f more than 60 A/sec by RF glow discharge o f h i g h e r s i l a n e s . The d e t a i l s w i l l be pursued i n another study. The a-Si:H photore- ceptor, t h e r e f o r e , becomes hopeful from the p r a c t i c a l p o i n t o f view.

Requirements o f the B l o c k i n g Layer.- The photoreceptor o f t h e conventional e l e c t r o - photography and the v i d i c o n - t y p e t a r g e t o f the image pickup tube a r e operated i n a

" b l o c k i n g c o n t a c t " c o n d i t i o n . E s p e c i a l l y t h e photoreceptor must have b l o c k i n g c o n t a c t t o e x t r a c t a l l thermal c a r r i e r s from the l a y e r s , and thus are termed the charge-depletion device?) The e l e c t r i c a l c o n d u c t i v i t y a t room temperature

(6~

tnonj') o f the o p t i m a l a-Si:H f i l m prepared by glow discharge can be c o n t r o l l e d b more than n i n e orders of magnitude by doping w i t h phosphor o r boron. The minimum & o f - 1 0 ~ "

(&m)-' can be a t t a i n e d i n a f i l m prepared fro111 a gaseous m i x t u r e o f B2H6/SiH4-loppm ( i n volume p a r t s p e r m i l l i o n ) . The d i e l e c t r i c r e l a x a t i o n time

( f r

= E

WC*)

e s t i - mated from t h e e l e c t r i c a l c o n d u c t i v i t y was s h o r t e r than 0.1 sec and had been con- s i d e r e d t o be too h i g h conductive t o apply these f i l m s t o t h e image-devices. Seve- r a l attempts have been c a r r i e d o u t t o f a b r i c a t e a h i g h i n s u l a t i v e a-Si:H by Rf sput- tering6>or by RF glow discharge o f gaseous m i x t u r e s o f SiHy + 02!) The photoconducti- v i t y , however, tends t o become poor i n c o n c u r r e n t w i t h an increase i n t h e r e s i s t i v i - t y . We have e s t a b l i s h e d t h a t such h i g h r e s i s t i v i t y i s n o t the e s s e n t i a l l y r e q u i r e d f o r t h e imaging devices o p e r a t i n g i n the b l o c k i n g c o n d i t i o n ? ) I n the d e p l e t i o n con- d i t i o n the dark c u r r e n t must be c o n t r o l l e d by t h e r m a l l y generated c a r r i e r s . T i e d j e e t a l g j p r e s e n t e d frequency dependent capacitance o f t h e d e p l e t i o n r e g i o n and i l l u s t - r a t e d these d i s p e r s i v e f e a t u r e i n terms o f a d i s t r i b u t i o n o f t r a p release times.

The r e l e a s e t i m e (

-kr

sec) i s given by

t r = G E+/*T

here i s a c o n s t a n t and E t i s t h e depth o f the l o c a l i z e d s t a t e s . The t r o f several seconds i s a t t a i n e d from t h i s r e l a t i o n as t h e release time o f c a r r i e r s from the s t a t e s i n the. mi d-gap o f a-Si:H a t room temperature ( b e i n g a p a r t by 0.8eV from the m o b i l i t y edge). I n f a c t , t h e discharge time o f the photoreceptor c o n s i s t i n g o f "undoped a- Si:HH whose ?& value i s about 10-9 (ncm)-' i s about 6 sec i n the dark a t room tem- p e r a t u r e when the b l o c k i n g c o n t a c t i s provided. Two types, namely, ( a ) homojunction and ( b ) heterophase j u n c t i o n a r e a p p l i c a b l e as the b l o c k i n g c o n t a c t a t t h e i n t e r f a c e between a conductive s u b s t r a t e and t h e " i n t r i n s i c " a-Si:HH.')

(a) Hornojunction u s i n g a t h i n a-Si:H doped w i t h P ( o r B )

The p h o t o r e c e p t o r s e n s i t i z e d w i t h n e g a t i v e corona must have t h e same b l o c k i n g behaviors as t h a t o f t h e v i d i c o n type o f t a r g e t , where the i n j e c t i o n o f holes f r o m the conductive s u b s t r a t e must be p r o h i b i t e d . A t h i n f i l m ( 0 . 1 ~ 0 . 2 p t h i c k ) o r n- type a-Si:H doped w i t h P i s an e f f i c i e n t m a t e r i a l f o r t h i s purpose. I n case o f p o s i - t i v e corona, p-type a-Si:H doped w i t h B o f f e r s s u f f i c i e n t b l o c k i n g e f f e c t a g a i n s t e l e c t r o n s . The b l o c k i n g e f f e c t o f t h e homojunction can be seen i n the c u r r e n t VS.

v o l t a g e r e l a t i o n s h i p a t t a i n e d f r o m t h e v i d i c o n t a r g e t s as shown i n F i g . l - a . I n a t a r g e t w i t h o u t the b l o c k i n g layer(O), the dark c u r r e n t i s p r o p o r t i o n a l t o the t a r g e t voltages ( V r ) b u t i n the one w i t h t h e b l o c k i n g l a y e r , t h e r e i s no corresponding i n - crease i n the dark c u r r e n t d e s p i t e the increase i n the VT u n t i l a c e r t a i n v o l t a g e and an a b r u p t increase i n c u r r e n t a r e observed when t h e VT exceeds the t h r e s h o u l d

'

a-s~:nts-~orr-+m n-trr. 02rm

l 0

J

l 0 100

TARGET VOLTAGE (V)

v01 tage ( V T ) . S The

V;

value depends g r e a t l y upor e i t h e r the thickness o r the doping l e v e l o f t h e b l o c k i n g l a y e r . We r e f e r t o the doping l e v e l as the volume r a t i o (vppm) o f pH3 added t o SiH4.

The s a t u r a t e d c u r r e n t showed i n t h i s f i g u r e ( a ) i n c l u d e d some e r r o r s a r i s i n g from p h o t o c u r r e n t caused by ill umination w i t h l i g h t e m i t t e d from the f i l a m e n t o f the electron-beam-gun o f t h e de- mountable v i d i c o n t e s t e r . A low c u r r e n t l e v e l ( 1 - 2 nA) was a t t a i n a b l e i n these t a r g e t s con- s i s t i n g o f a-Si:H f i l m doped s l i g h t l y w i t h B (doping l e v e l 10 vppm) i n sealed image pickup tubes. The b l o c k i n g e f f e c t by the n-type a-Si:H i s i n t e r p r e t e d i n terms o f a s h o r t Schubwegk-tE) o f holes i n the l a y e r , deducing from t h e f a c t t h a t the t h r e s h o u l d v01 tage c o r r e l a t e d w i t h the onset o f r i s i n g c u r r e n t depends g r e a t l y upon t h e thickness and the doping l e v e l . The dark c u r r e n t l e v e l i s c o n t r o l l e d by t h e r m a l l y generated c a r - r i e r s i n t h e photoconductive l a y e r and i s main- t a i n e d a t the low l e v e l as f a r as the " i n t r i n s i c "

a-Si:H, namely, the f i l m doped s l i g h t l y w i t h B (doping l e v e l , < 5 0 vppm) o r un-doped f i l m , i s used. A p a r t from these region, the dark c u r r e n t l e v e l r i s e s d r a m a t i c a l l y . The p-type a-Si :H f i l m performs the e f f e c t i v e b l o c k i n g i n the photoreceptor as i t i s s e n s i t i z e d w i t h p o s i t i v e

.-

Fig. Z ^: Current vs. voztage for .a-Si:H vidicon targets with the homojunetion ( a ) and the hetero- phase blocking ( b )

The numbers i n ( a ) and f b ) indicate the doping l e v e l s and the temperature

0.1

,+od,

TARGET VOLTAGEIVI

b th. dark

- - - _ _ _

I-.

0 1 0 2 0

10' 10' to'

ELECTRIC FIELD (V k m )

F i g . 2 : Photodischarging charac-

t e r i s t i c s of a-Si:H photoreceptor o f sensitized with positive corona corona.

( b ) Heterophase B l o c k i n g

The v i dicon t a r g e t must be d i s t i n g u i s h e d from the photoreceptor o f electrophotography by the d i r e c t i o n o f l i g h t i l l u m i n a t i o n , t h a t i s , i n

t h e v i dicon t a r g e t , l i g h t impinges upon the photoconductor through the s u b s t r a t e . The s t r o n g o p t i c a l a b s o r p t i o n o f the b l o c k i n g l a y e r a t the c o n t a c t w i t h t h e conductive s u b s t r a t e r e s u l t s i n reducing the p h o t o s e n s i t i v i t y . A heterophase b l o c k i n g using t r a n s p a r e n t d i e l e c t r i c t h i n f i l m has been developed t o a v o i d t h e f i l t e r i n g e f f e c t due t o the o p t i c a l a b s o r p t i o n o f the b l o c k i n g l a y e r . Thin f i l m o f a-Si,N,-, prepared by glow discharge o f gaseous m i x t u r e o f s i l a n e and n i t r o g e n i s a p p l i c a b l e as the d i - e l e c t r i c f i l m . The c u r r e n t vs. v o l t a g e curves i n the dark a r e shown i n F i g . l - b f o r the a-Si:H v i d i c o n t a r g e t s , v a r y i n g t h e temperature. The dark c u r r e n t corresponding t o the t h e r m a l l y generated c a r r i e r s ( 1 - 2 nA) i s maintained up t o the a p p l y i n g f i e l d o f 2x10' v/cm and r i s e d a b r u p t l y w i t h an increase i n the f i e l d when t h e a p p l y i n g v o l - tage exceedes a c e r t a i n v o l t a g e a t room temperature. The onset v o l t a g e i s indepen- dent o f t h e thickness o f a-Si,N,-* i n t h e r e g i o n from 30 A t o 300 A. When t h e t h i c k - ness exceedes 1000 A, the s a t u r a t e d dark c u r r e n t trends t o decrease due t o t h e re-

C4-1126 JOURNAL DE PHYSIQUE

d u c t i o n i n t h e e f f e c t i v e v o l t a g e on t h e p h o t o c o n d u c t i v e l a y e r . The r a p i d i n c r e a s e i n t h e c u r r e n t a t h i g h v o l t a g e r e g i o n i s i n t e r p r e t e d i n terms o f t h e e m i s s i o n - l i m i t e d c u r r e n t c r o s s e d t h e d i e l e c t r i c l a y e r . These heterophase b l o c k i n g a t t h e c o n t a c t w i t h t h e c o n d u c t i v e s u r f a c e i s e s s e n t i a l l y e f f e c t i v e i n t h e e l e c t r o p h o t o g r a p h i c r e c e p t o r which can b e s e n s i t i z e d w i t h corona o f b o t h p o l a r i t i e s .

Photoresponse.-

1 P h o t o r e c e p t o r o f E l e c t r o p h o t o g r a p h y

When an a-Si:H p h o t o r e c e p t o r w i t h t h e b l o c k i n g l a y e r a t t h e c o n t a c t w i t h t h e c o n d u c t i v e s u b s t r a t e i s s e n s i t i z e d w i t h p o s i t i v e corona, t h e acceptance v01 tage o f a b o u t 40 v/pm i s a t t a i n a b l e . The f a c t t h a t t h e s a t u r a t e d acceptance v o l t a g e (Vso v o l t ) i s p r o p o r t i o n a l t o t h e p h o t o r e c e p t o r t h i c k n e s s v e r i f i e s t h e e f f e c t i v e b l o c k i n g o f t h e a-Si:H s u r f a c e a g a i n s t t h e p o s i t i v e corona. I l l u m i n a t i o n w i t h l i g h t causes p h o t o d i s c h a r g e o f t h e s u r f a c e v01 tages (Vs) and forms an e l e c t r o s t a t i c l a t e n t image on t h e p h o t o r e c e p t o r . T y p i c a l p h o t o d i s c h a r g e curves a r e shown i n F i g . 2 as a p l o t o f Vs vs. exposure t i m e . ( a ) , v a r y i n g t h e l i g h t i n t e n s i t y . The p h o t o c u r r e n t g i v e n f r o m t h e t i m e d e r i v a t i v e V s ( t ) i s p l o t t e d as a f u n c t i o n o f Vs i n l o g - l o g p l o t s ( b ) . The p h o t o d i s c h a r g e r a t e ( V s ( t ) v01 t / s e c ) i s independent o f t h e f i e l d down t o t h e 0 . 8 . V ~ ~ i n t h e e m i s s i o n - l i m i t e d c o n d i t i o n . I n o t h e r words, t h e p h o t o c u r r e n t i s s a t u r a t e d d e s p i t e t h e i n c r e a s e i n t h e e l e c t r i c f i e l d . The s a t u r a t e d 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 i n c i d e n t l i g h t f l u x and t h e r e s i d u a l v o l t a g e i s n e g l i g i b l e . A p h o t o r e c e p t o r o f more t h a n 1 0 p m t h i c k i s r e q u i r e d , f r o m t h e p r a c t i c a l p o i n t o f view, f o r t h e purpose o f s u s t a i n i n g t h e s u r f a c e v o l t a g e o f h i g h e r t h a n 400 v o l t s i n o r d e r t o develop t h e l a t e n t image w i t h c o n v e n t i o n a l t o n o r . The p h o t o d i s c h a r g e c h a r a c t e r i s t i c s , s t a r t i n g a t d i f f e r e n t i n i t i a l v o l t a g e s , a r e shown F i g . 3-a f o r t h e p h o t o r e c e p t o r o f 10,Um t h i c k . The c u r r e n t i s s a t u r a t e d a t h i g h e l e c t r i c f i e l d and tends t o decrease g r a d u a l l y w i t h t h e decrease i n t h e f i e l d b e f o r e t h e o n s e t o f t h e s t r o n g f i e l d - d e p e n d e n t c u r r e n t r e g i o n . The shape o f t h e curve, l o g J p vs. l o g VS, depends c l e a r l y on t h e i n i t i a l s u r f a c e v o l t a g e a t t h e l o w e l e c t r i c f i e l d . The c u r r e n t i n t h i s r e g i o n can be i n t e r - p r e t e d i n terms o f t h e space charge l i m i t e d c u r r e n t (scLC) and t h e o n s e t v01 tage must b e r e l a t e d t o t h e amount o f space charge accumulated i n t h e p h o t o c o n d u c t o r . The g r a d u a l decrease i n t h e c u r r e n t a t t h e i n t e r m e d i a t e r e g i o n i s r e l e v a n t t o t h e range- l i m i t e d current.I0) We c o n c l u d e t h a t e x c e l l e n t p h o t o d i s c h a r g e can be a c h i e v e d i n t h e a-Si:H p h o t o r e c e p t o r o p e r a t e d w i t h p o s i t i v e corona due t o t h e f a c t t h a t t h e p h o t o h o l e s i n t h e " i n t r i n s i c " a-Si:H have t h e range

(FT)

s u f f i c i e n t f o r p a s s i n g t h r o u g h t h e l a y e r o f a b o u t 10)tmthick. The p h o t o c o n d u c t i v i t y g a i n (G) g i v e n b y G = Jp/eF a r e p l o t t e d as a f u n c t i o n o f wavelengths ( s e e F i g . 3-b). The d o t t e d l i n e shows t h e o p t i c a l ab- s o r p t i o n o f t h e a-Si:H. A comparison between t h e o p t i c a l a b s o r p t i o n and t h e photo- c o n d u c t i v e g a i n r e v e a l s t h a t t h e quantum y i e l d o f t h e c a r r i e r g e n e r a t i o n i s indepen- d e n t o f t h e photon energy, a t l e a s t u n d e r t h e a p p l y i n g v01 t a g e down t o 1 0 - ~ v / c m . T h i s f a c t can be c o u n t e d as a g r e a t advantage o f a-Si:H i n comparison w i t h o t h e r amorphous photoconductors.

The f r e e s u r f a c e o f a-Si:H, on t h e o t h e r hand, i s u n s t a b l e w i t h r e s p e c t t o t h e b l o c k i n g e f f e c t a g a i n s t n e g a t i v e corona. E s p e c i a l l y t h e acceptance v o l t a g e f o r ne- g a t i v e corona depends g r e a t l y o n t h e h u m i d i t y . T a n i e l i a n e t al!')pointed o u t t h a t t h e w a t e r m o l e c u l e adsorbed on a-Si:H h a d a b e h a v i o r as t h e donor and made t h e s u r -

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d e v i c e as w e l l as t h e p h o t o r e c e p t o r o f e l e c t r o - photography. Both t y p e s o f t h e b l o c k i n g c o n t a c t s can be a p p l i c a b l e t o p r o h i b i t i o n a g a i n s t t h e i n - j e c t i o n o f c a r r i e r s f r o m t h e t r a n s p a r e n t I T 0

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tages. A schematic c r o s s s e c t i o n o f a-Si:H v i d i c o n t a r g e t i s i l l u s t r a t e d i n F i g . 5. On

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i s d e p o s i t e d t o a v o i d s w a r i n g image due t o t h e l a t e r a l d i f f u s i o n o f e l e c t r o n s . The t o p l a y e r i s n o t e s s e n t i a l l y

r e q u i r e d t o p r o h i b i t t h e i n j e c t i o n o f e l e c t r o n ^{cRT In,O,} h ( I T 0 1

f r o m t h e s u r f a c e o f a-Si:H. T h i s l's a d i s t i n - U*5.$ fLCE %Am

g u i s h a b l e d i f f e r e n c e f r o m t h e r e s u l t r e p o r t e d

b y Imamura e t a1 ?who used a-Si:H p r e p a r e d b y Fig. 5 : Schematic diagram o f

RF s p u t t e r i n g . a-Si:H uidicon

i n j e c t e d i n t o t h e b u l k f r o m t h e s u r f a c e by t h e

a i d o f t h e e x s i s t e n c e o f t h e n - t y p e s u r f a c e s t a t e s . Id7- We must a v o i d t h e s e e f f e c t s t o complete t h e b l o c k - i n g c o n t a c t a t t h e s u r f a c e when t h e p h o t o r e c e p t o r i s s e n s i t i z e d w i t h n e g a t i v e corona. T h i n f i l m s of chalcogenides, e. g:, a-Se, a - A s r S g , o r a-As-

3

Se-Te a c t as t h e b l o c k ~ n g l a y e r on t h e t o p o f t h e

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The t a r g e t c u r r e n t - v o l t a g e c h a r a c t e r i s t i c s u n d e r l i g h t i l l u m i n a t i o n a r e shown i n F i g . 6 . I n t h e t a r g e t w i t h o u t t h e b l o c k i n g l a y e r "NONE", t h e p h o t o c u r r e n t i n c r e a s e s g r a d u a l l y w i t h r i s i n g t h e t a r g e t v o l t a g e s as w e l l as t h e c u r r e n t i n t h e d a r k . A l l t h e a-Si:H t a r g e t s e x c e p t one w i t h o u t t h e b l o c k i n g l a y e r a r e c h a r a c t e r i z e d by an ab- r u p t i n c r e a s e i n t h e p h o t o c u r r e n t a n d a s a t u r a t i o n a t t h e t a r g e t v o l t a g e s o f l e s s t h a n 10 v o l t . T h i s i m p l i e s t h a t t h e r e q u i r e m e n t f o r photoconductor, i

.

e., t h e h o l e Schuweg(bL2E) must b e g r e a t e r t h a n t h e t h i c k n e s s , i s accomplished b y v i r t u e o f b l o c k - i n g c o n t a c t . The t h i c k n e s s o f t h e p h o t o c o n d u c t i v e l a y e r must be determined, t a k i n g t h e " l a g " performance i n t o account. A c c o r d i n g t o t h e p h o t o c u r r e n t c u r v e s o f t h e t a r g e t s w i t h t h e h o m o j u n c t i o n - t y p e b l o c k i n g ( l e f t ) , t h e shapes o f curves a r e a l t e r e d s l i g h t l y b y v a r y i n g t h e d o p i n g - l e v e l o f t h e b l o c k i n g l a y e r . The v a l u e o f t h e s a t u - r a t e d c u r r e n t tends t o be reduced i n t h e t a r g e t s w i t h t h e heavy-doped f i l m . I t can b e i n t e r p r e t e d i n terms o f t h e " f i l t e r i n g e f f e c t " f o r t h e i n c i d e n t l i g h t ; i. e., c a r r i e r s g e n e r a t e d i n t h e b l o c k i n g l a y e r a r e p a r t l y i n e f f e c t i v e as t h e photoresponse due t o t h e i r s h o r t h o l e range. The photoresponse s p e c t r a showed i n F i g . 7 ( l e f t ) l e n d s u p p o r t t o t h i s e l u c i d a t i o n . The p h o t o c o n d u c t i v i t y g a i n i n t h e s h o r t wavelengths r e g i o n i s a f f e c t e d g r e a t l y by t h e t h i c k n e s s and t h e doping l e v e l o f t h e b l o c k i n g l a y e r . I n t h e v i d i c o n t a r g e t u s i n g t h e t r a n s p a r e n t b l o c k i n g c o n t a c t , on t h e o t h e r hand, t h e p h o t o c u r r e n t vs. v01 tage c u r v e ( s e e F i g . 6 ( r i g h t ) ) i s i n d e p e n d e n t o f t h e c o l o r o f

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p h o t o d i s c h a r g e s i m a l t a n e o u s l y . T y p i c a l l o g J p vs. l o g Vs c u r v e s a r e shown i n F i g . 4 f o r t h e pho- t o r e c e p t o r w i t h t h e s u r f a c e b l o c k i n g (B) a n d t h a t

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( ) r ~ d l ~ ~ t o n ~ c d s e c w i t h o u t any b l o c k i n g a t t h e s u r f a c e ( A ) . A d i s - 10 100 I 500

t i n c t p r o g r e s s has been o f f e r e d i n t h e p h o t o d i s - VOLTAGE ( V

charge p r o p e r t i e s b y a p p l y i n g t h e s u r f a c e b l o c k i n g B c o n t a c t w i t h a t h i n d i e l e c t r i c f i l m .

2 V i d i c o n T a r g e t

I n t h e image p i c k u p tubes, t h e photoconduc- .(P-250)

t i v e f i l m i s charged w i t h a scanninq e l e c t r o n

C4-1128 JOURNAL DE PHYSIQUE

looo

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^{F i q . 6 :} Photocurrent v s . voltage for vidicon with homo junction f l e f t ) and heterophase blocking f r i g h t ) R. G and B denote Light of red, green and blue, res- pectively

TARGET VOLTAGE ( V ) TnRGET VOLTAGE ( V

l i g h t (R, red; G, green; and B, b l u e ) and the thickness o f t h e b l o c k i n g l a y e r i n the onset v01 tage causing the ab- range o f 50 A-300 A. The r u p t increase i n the photo- d u r r e n t i s s h i f t e d t o h i g h v01 tage as the thickness o f t h e b l o c k i n g l a y e r exceeds the range o f 500 A. Moreover, some undesirable e f f e c t s ,

"blooming" o r "charge-up" Y D wvarffim Y Drnnr ~ X ) ~ UUVELEffiTH ~ (nm)

tend to appear

in the device- Fig.

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^: Spectral photoresponse of vidicon with homo- The photoresponse junetzon ( l e f t ) and heterophase blocking f r i g h t )

( F i g . 7 ( r i g h t ) ) i s indepen-

dent o f the thickness o f a-Si:H l a y e r and is r e t a i n e d a t h i g h l e v e l i n the whole v i s i b l e r e g i o n .

Turning t o the photoresponse curves o f t h e t a r g e t s , the current-v01 tage r e l a t i o n can be c l a s s i f i e d i n t h r e e regions as shown i n Fig. 8; i. e., i ) one c a r r i e r SCLC a t t h e low voltages, i i ) the s a t u r a t e d c u r r e n t r e g i o n a t h i g h voltages where t h e photo- c o n d u c t i v i t y gain approaches towards u n i t y and i i i ) the i n t e r m e d i a t e region. Under a c o n s t a n t V ~ ( 1 0 v o l t ) , t h e s a t u r a t e d c u r r e n t i s p r o p o r t i o n a l t o the i n c i d e n t l i g h t f l u x u n t i l i t behaves as t h e SCLC where the p h o t o c u r r e n t i s independent o f the i n - c i d e n t o f l i g h t f l u x . The observed photocurrent-v01 tage r e l a t i o n s h i can be we1 l described i n terms o f the SCLC model proposcd by F. J. du Chatenie:'!nder a hypothe- s i s o f e x i s t a n c e o f shallow traps. By f i t t i n g t h e t h e o r e t i c a l curves t o t h e e x p e r i - mental r e s u l t , we have a t t a i n e d the,u-c p r o d u c t o f 1 .0 X 10-S cm2/v and t h e P / Q value o f 8.8 X 10-@ cmz/v.s., where @ i s t h e r a t i o o f the t o t a l number o f holes t o t h a t o f the trapped ones. A p a r t from the q u a n t i t a t i v e a n a l y s i s , these r e s u l t s l e a d us t o an i m p o r t a n t conclus.ion as f o l l o w s ;

( 1 ) A h i g h h o l e range @-C) o f a-Si :H i s r e q u i r e d as the photoconductive l a y e r o f the v i d i c o n t a r g e t w i t h respect t o the photoresponse and t h e dynamic range o f the v i dicon device.

( 2 ) A b l o c k i n g c o n t a c t i s a l s o r e q u i r e d t o s u s t a i n i n g the low dark c u r r e n t and the t h e r m a l l y generated c u r r e n t o f the " i n t r i n s i c " a-Si:H i n the charge-depletion c o n d i t i o n i s low enough t o f a b r i c a t e t h e device.

I n a d d i t i o n t o t h e photoresponse, t h e image r e s o l t i o n and the 7ag c h a r a c t e r i s t i c s a r e the i m p o r t a n t f u n c t i o n s t o e v a l u a t e t h e a-Si:H vidicon. The h o l i z o n t a l r e s o l u - t i o n o f more than 800 TV l i n e s f o r 1 i n c h tubes and about 750 TV l i n e s f o r 2/3-inch tubes a r e a t t a i n e d as shown i n F i g . 9. The image r e s o l u t i o n i s p r i m a r i l y depend upon t h e n a t u r e o f the top l a y e r o f the t a r g e t . I n t h e t a r g e t w i t h o u t the top l a y e r o r

w i t h a h i g h r e s i s t i v e a-Si:H prepared by glow d i s - charge a t low s u b s t r a t e temperature (150°C), images tend t o be smeared and consequently show very poor r e s o l u t i o n . Thin f i l m s o f chalcogenides, e. g., Sb,S3 o r As-Se-Te, a r e adequate m a t e r i a l s as t h e

t o p l a y e r o f a-Si:H v i d i c o n due t o t h e h i g h l a t e r a l ^V,

-

^{I O V}

r e s i s t i v i t y and the s t r o n g e l e c t r o n c o l l e c t i v e f o r c e .

10 loo

I n Fig. 10, the decay l a g c h a r a c t e r i s t i c s are LIGHT INTENSITY (wb u x t )

i l l u s t r a t e d . The measurements were made w i t h 2/3- i n c h dia. t a r g e t s (scanning area 8.8 x 6.6 mm), v a r y i n g the thickness o f a-Si:H. The c u r r e n t , a f t e r cessation o f i l l u m i n a t i o n ( s i g n a l c u r r e n t 200 nA)

,

i s p l o t t e d as a f u n c t i o n o f the f i e l d number. The b i a s l i g h t l e v e l i s 5 nA and t h e t a r g e t voltage o f 15 v o l t i s applied. The l a g c h a r a c t e r i s t i c s a r e g r e a t l y a f f e c t e d by t h e thickness o f t h e t a r g e t ,

i m p l y i n g t h a t t h e general p o r t i o n o f t h e l a g curve

5

i s l i k e l y t o be r u l e d by the c a p a c i t i v e lag. I n a d d i t i o n t o the f a s t component, some l o n g t a i l , which i s a t t r i b u t e d t o t h e t r a p p i n g e f f e c t , i s ob- served. The decay l a g i s about 5% a t 50ms. ( 3 f i e l d s ) i n the t a r g e t comprised o f a-Si :H o f 4pm t h i c k .

Summary

.-

As t h e photoconductive m a t e r i a l f o r the charge-depletion devices, e. g., e l e c t r o p h o t o g r a p h i c photoreceptors o r v i d i c o n - t y p e image pickup tubes, the " i n t r i n s i c " a-Si:H prepared i n the optimum de-

p o s i t i o n c o n d i t i o n s gave t h e b e s t r e s u l t s i n t h e ^{1 10 100} aspect o f the photocurrent response. A h i g h r e s i s - TARGET MLTAGE (v) t i v e f i l m r e s u l t i n g from an increase i n t h e gap- ^: photocurrent

s t a t e s does n o t make anv sense. for vidicon varying iZZwninution

F i g . 9 : Image recmstracted from the video- signal using a-Si:H vidicon

--

FIELD NUMm

Fig. 20 : Decay Zag characteristics o f vidicons, vanyiYlg bhe thickness of photoconductive layer

The dark c u r r e n t o f t h e " i n t r i n s i c " a-Si:H i n t h e charge depleted c o n d i t i o n i s determined by the thermalized c a r r i e r s from t h e s t a t e s a t the mid-gap. Consequently a s u f f i c i e n t l o n g r e t e n t i o n time o f 10-20 sec can be achieved i n the photoreceptor u s i n g undoped and s l i g h t l y B-doped f i l m .

In

t h e v i d i c o n t a r g e t , the low dark cur- r e n t l e v e l o f 1 - 2 nA i s a t t a i n a b l e under t h e t a r g e t voltages o f up t o 50 v o l t s . The acceptance v o l t a g e o f a-Si

:H

photoreceptor i s 35-40 v o l ts/pm f o r p o s i t i v e corona

and 3 0 b 3 5 v o l t s / r m f o r negative corona and thus t h e l a t e n t images on the photorecep-

C4-1130 JOURNAL DE PHYSIQUE

t o r o f 10 -15pm t h i c k a r e developable w i t h the conventional powdery tonor. A h a l f o f charges on the photoreceptor can be/ discharged by exposure o f 4 - 6 ergs/cm2 i n %he whole v i s i b l e r e g i o n (400 nm-760 nm wavelength). The r e s i d u a l charges a f t e r t h e

l i g h t i l l u m i n a t i o n a r e n e g l i g i b l e . A good s t a b i l i t y o f these photoreceptor has been v e r i f i e d i n t h e l a b o r a t o r y - s c a l e experiment b u t much e f f o r t s must be maid t o e s t a b l i s h t h e s t a b i l i t y i n p r a c t i c a l copying c o n d i t i o n s .

I n case o f vidicon, a l l performances described above l e a d us a conclusion t h a t a-Si :H v i d i c o n can be competable w i t h o t h e r m a t e r i a l s p r a c t i c a l l y used.

I n a d d i t i o n t o these conventional image techniques, some new types o f image devices a r e expected by u s i n g these remarkable p h o t o e l e c t r i c p r o p e r t i e s o f a-Si:H and\ the f e a s i b i l i t y o f c o n t r o l l i n g the e l e c t r i c p r o p e r t i e s .

The authors a r e deeply i n d e b t e d t o Drs. E. Mavuyam and T. H i r a i o f f o r k i n d l y h e l p f o r t h e measurements o f v i d i c o n .

t h e Hoso Bunka Foundation (NHK) and by a G r a n t - i n - A i d f o r Science Research from t h e M i n i s t r y o f Education, Science and C u l t u r e .

f i e f e r e n c e s .

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Appl. ~ h y s . " ~ e t t . , 37, 727 (1980)

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