a-Si : H IN ELECTROPHOTOGRAPHY AND VIDICON DEVICES

Experimental s tud ies 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 lock ing contacts, homojunction and heterophase b lock ing, have been proposed t o susta in 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 id i con ta rge t i n the dark. An " i n t r i n s i c " a-Si:H prepared i n the opt imized condi t ions i s app l i cab le as the photoconductive l a y e r o f the both devices. Excel l e n t photoresponse can be achieved i n the charge deplet ion devices w i t h the b lock ing contacts i n whole v i s i b l e region. Wi th in the framework o f ou r labora to ry scale study, we conclude t h a t a-Si:H i s a promising mater ia l as a photoconductor o f image devices. In t roduc t ion . Resul t i n from recent extens ive s tud ies, i t has been revealed 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 marer ia l i n various f i e l d s o f app l i ca t ion , i. e., pho tovo l ta i c so la r -ce l l s , FET t r a n s i s t o r ar rays f o r f l a t screen d isp lay 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 photoconduct iv i ty , s t rong o p t i c a l absorpt ion 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 super io r two-dimentional homogeniety, mechanical s t reng th and heat d u r a b i l i t y i n a d d i t i o n t o the a b i l i t y t o con t ro l the e l e c t r i c p roper t ies by doping w i t h fo re ign elements, which have s t imu la ted us i n development o f some new types o f imaging systems w i t h a-Si:H and the r e l a t e d mater ia ls . The main purpose o f t h i s paper i s t o in t roduce our recent 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 id icontype image pickup tube. The former technique i s a well-known imaging procedure f o r dupl i c a t i o n o f " the s t a t i c images" and the l a t t e r uses a t y p i c a l scanning image technology which i s app l i cab le t o t r e a t i n g " t h e dynamic images". The demands f o r the pho to -e lec t r i c p roper t ies o f photoconductive mate r ia l s i n o ther app l i ca t ions , e. g., facs imi les, h igh speed p r i n t e r s , some s o l i d stateimaging devices, must be inc luded i n the requi rements f o r both techniques and the knowledges a t t a i n e d i n these s tudies are expected t o be app l i cab le t o extending the f i e l d s o f app l i ca t ions o f a-Si:H and the r e l a t e d mater ia ls . Mate r ia l Preparat ion.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 requ i red as a photoreceptor o f an e lect rophotographic copy machine. I t has been es tab l i shed t h a t an a-Si:H f i l m prepared by RF glow discharge o f SiH* showed excell e n t homogeniety i n the thickness and the o p t i c a l o r the e l e c t r i c a l p roper t ies over the two-dimentional area o f 30cm X 30cm by c o n t r o l l i n g preparat ion parameters. Yamamoto e t a l ! ) fabr icated an a-Si:H:O f i l m on a c y l i n d r i c a l subst rate ( A l ) by the induct ive-coupled RF glow discharge o f SiH4 and took the measurements o f the 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 condi t ion. The recent progress i n the f a b r i c a t i o n technology w i t h the 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 features p r a c t i c a l . Apar 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 impor tant demand from the 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 requ i red as a photoreceptor and thus many hours, more than 10 hours, must be spent t o prepare the photoconduct ive l a y e r as f a r as we would s tay i n the p o i n t es tab l i shed by ~ n i ghtsz'from h i s systematic s tud ies on RF glow discharge technique i n prepar ing a-Si:H. However, some conspicuous progress has been made i n the growth r a t e . Knights e t a1 ?'recently presented the f a c t t h a t h igh deposi t ion rates ( 3 ~ 5 p m l h r . ) cou ld be achieved concur ren t l y w i t h low defect dens i t i es when hel ium 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 prepared from SizHs. I n recent 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 sur face w i t h i n one and a h a l f hour ' s deposi t ion from gaseous mixture 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 pho to -e lec t r i c features cou ld 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 lanes. The d e t a i l s w i l l be pursued i n another study. The a-Si:H photoreceptor, therefore, 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 lock ing Layer.The photoreceptor o f the conventional e l e c t r o photography and the v id icontype t a r g e t o f the image pickup tube are operated i n a "b lock ing contact" cond i t i on . Espec ia l l y the photoreceptor must have b lock ing con tac t t o e x t r a c t a l l thermal c a r r i e r s from the layers, and thus are termed the charge-deplet ion 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 opt imal 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 ine 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 mixture 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 t ime ( f r = E WC*) e s t i mated from the 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 cons ide red t o be too h igh conduct ive t o apply these f i l m s t o the image-devices. Sever 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 igh i n s u l a t i v e a-Si:H by Rf sputtering6>or by RF glow discharge o f gaseous mixtures o f SiHy + 02!) The photoconduct iv i t y , however, tends t o become poor i n concurrent w i t h an increase i n the r e s i s t i v i t y . We have es tab l i shed t h a t such h igh 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 requ i red f o r the imaging devices opera t ing i n the b lock ing condi t ion?) I n the dep le t ion cond 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 thermal ly generated c a r r i e r s . T ied je e t a lg jpresented frequency dependent capacitance o f the deplet ion reg ion and i l l u s t r a t e d these d ispers ive fea tu re 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 re lease t ime ( -kr sec) i s given by