A NEW CHALCOGENIDE PHOTOCONDUCTOR FOR A DIODE LASER BEAM PRINTER

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A NEW CHALCOGENIDE PHOTOCONDUCTOR FOR A DIODE LASER BEAM PRINTER

Y. Taniguchi, H. Yamamoto, S. Horigome, S. Saito, E. Maruyam

To cite this version:

Y. Taniguchi, H. Yamamoto, S. Horigome, S. Saito, E. Maruyam. A NEW CHALCOGENIDE PHO-

TOCONDUCTOR FOR A DIODE LASER BEAM PRINTER. Journal de Physique Colloques, 1981,

42 (C4), pp.C4-971-C4-974. �10.1051/jphyscol:19814213�. �jpa-00220842�

JOURNAL D E PHYSIQUE

CoZZoque C4, suppl6rnent au nO1O, Tome 42, octobre 7981 page C & - 9 7 1

A NEW CHALCOGENIDE PHOTOCONDUCTOR FOR A D I O D E LASER BEAM P R I N T E R

Y. T a n i g u c h i , H. Yamamoto, S. Horigorne, S. S a i t o and E . Maruyama Central Research Laboratory, il'itachi Ltd., Kokubunji, Tokyo 1 8 5 , Japan

Abstract.- A highly sensitive photoconductive layer with low dark decay has been fabricated for electrophotography using Se-As-Te amorphous chalcogenide. This photoconductor consists of four layers: a Se-rich blocking layer, a Te-rich ir-sensitive layer, an As-rich space-charge layer and a Se-rich voltage- sustaining layer. It has a sensitivity to beams having a wavelength of 750 nm almost as high as that of pure Se at a wavelength of 442 nm.

Introduction.- There is increasing need for electrophotographic applications in such fields as copying and printing. There have been many approaches to obtaining photoconductors which have the sensitivity in the long wavelength region above 750 nm. However, a photoconductor that is both highly sensitive to diode laser beams and stable in printing operations has not yet been developed.

On the other hand, the s a t i c o a image pick-up tube(1) with chalcogenide-glass photodiode has been developed. The photoconductor used in this image sensor has high sensitivity in the visible wavelength region. It has been recently improved for use in the near-infrared wavelength region. This article summarizes our success in obtaining a chalcogenide photoconductor with low dark decay and high sensitivity in the 750 nm to 800 nm range.

Experimental.- In preparing the photoconductor, the Se-As-Te layers were deposited, in a computer controlled evaporation system, on a glass or aluminum substrate.

The electrophotographic properties were estimated from the surface potential decay of the photoconductor after it had been charged by corona discharge. In order to avoid experimental error, a computer controlled pulse light exposure technique was employed.

Photoconductor was exposed to a 9 2 msec laser pulse 2 sec after charging of the layer, and to a continuous laser beam 2 sec after that. The initial suface potential, Vo, was the potential 1 sec after charging. The rate of dark decay was calculated from the straight line approximating the semi-logarithmic plot for the first 1 sec. The residual surface potential, VR, was estimated by extrapolation of the approximate straight line in the semi-logarithmic plot between 12 and 15 sec, when the photo-decay almost disappears, back to the initial time point.

Photo-energy, E, m-yuird. to reduce surface potential to half its initial value was calculated from the potential decay caused by

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19814213

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h e r e , s i n c e t h e b a n d g a p o f t h e s e c o n d S e l a y e r i s 1 . 6 eV a n d t h e b a n d g a p o f t h e f o u r t h S e l a y e r i s a b o u t 2 . 0 e V . A t h i r d S e l a y e r i s t h u s formed t o r e d u c e t h e e f f e c t o f t h i s e n e r g y b a r r i e r .

I f A s ' i s d o p e d i n t o S e , t h e b a n d g a p d e c r e a s e s w i t h t h e i n c r e a s e i n A s c o n c e n t r a t i o n . When t h e S e c o n t a i n s 40 % by w e i g h t o f A s , t h e b a n d g a p i s a b o u t 1 . 7 e V . I n t h e t h i r d S e l a y e r , t h i s A s c o n c e n t r a t i o n is g r a d u a l l y r e d u c e d f r o m t h e h i g h e s t c o n t e n t t o a low l e v e l . I f t h e Te c o n c e n t r a t i o n i n t h e s e c o n d S e l a y e r i s 45 % by w e i g h t , t h e maximum c o n c e n t r a t i o n o f A s i n t h e t h i r d l a y e r s h o u l d b e a d j u s t e d t o 40 % by w e i g h t .

I f A s i s d o p e d i n t o S e , a l o c a l i z e d s t a t e i s b r o u g h t a b o u t i n t h e i n t e r i o r o f t h e b a n d g a p , and e l e c t r o n s a r e r e a d i l y t r a p p e d . The l a y e r c o n t a i n i n g a h i g h A s c o n t e n t d e v e l o p s a n e g a t i v e s p a c e c h a r g e . T h i s n e g a t i v e s p a c e c h a r g e i n t e n s i f i e s t h e e l e c t r i c f i e l d a p p l i e d t o t h e s e c o n d S e l a y e r and t h e h o l e g e n e r a t e d i n t h e i n t e r i o r o f t h e s e c o n d S e l a y e r a r e r e a d i l y a t t r a c t e d i n t o t h e i n t e r i o r o f t h e t h i r d S e l a y e r .

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The h o l e i s g u i d e d i n t o t h e i n t e r i o r o f t h e f o u r t h S e l a y e r by t h e t h i r d S e l a y e r and is e x t i n g u i s h e d by r e c o m b i n a t i o n w i t h a c h a r g e o n t h e n e g a t i v e l y c h a r g e d s u r f a c e o f t h e f o u r t h S e l a y e r . T h u s , t h e f i r s t S e l a y e r , a n d t h e f o u r t h Se l a y e r , a c t a s t r a n s p o r t l a y e r s f o r e l e c t r o n s , a n d h o l e s , r e s p e c t i v e l y .

When t h e p h o t o c o n d u c t o r is u s e d i n a n e l e c t r o p h o t o g r a p h i c d e v i c e o r l a s e r beam p r i n t e r , t h e t h i c k n e s s o f t h e S e l a y e r i s a d j u s t e d t o a b o u t 4 0 p m i n v i e w o f t h e s u r f a c e p o t e n t i a l . I f t h e t e m p e r a t u r e a t f o r m a t i o n o f t h i s l a y e r i s a d j u s t e d t o a r a n g e o f f r o m 40 t o 60°C, t h e r e s i d u a l s u r f a c e p o t e n t i a l i s r e d u c e d t o b e l o w 1 / 2 0 o f t h e p o t e n t i a l o b s e r v e d when room t e m p e r a t u r e i s a d o p t e d .

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JOURNAL DE PHYSIOUE

V a r i o u s a d v a n t a g e s c a n b e a t t a i n e d i f a n i n s u l a t i n g l a y e r o f a n n - t y p e o x i d e , s u c h a s c e r i u m o x i d e , i s i n t e r p o s e d a s t h e c a r r i e r b l o c k i n g l a y e r b e t w e e n t h e c o n d u c t o r and t h e f i r s t S e l a y e r . One a d v a n t a g e i s t h a t i n j e c t i o n o f h o l e s i n t o t h e f i r s t S e l a y e r f r o m t h e s u b s t r a t e i s p r e v e n t e d , r e s u l t i n g i n r e d u c e d d a r k d e c a y . A n o t h e r a d v a n t a g e i s t h a t d i f f u s i o n o f i m p u r i t i e s c o n t a i n e d i n t h e s u b s t r a t e i n t o t h e f i r s t S e l a y e r is p r e v e n t e d .

S p e c t r a l s e n s i t i v i t y c h a r a c t e r i s t i c s f o r a p h o t o c o n d u c t o r i n w h i c h t h e Te c o n t e n t i s a d j u s t e d t o 4 5 3 by w e i g h t and f o r a n amorphous S e p h o t o c o n d u c t o r c o m p r i s e d o f S e a l o n e a r e shown i n F i g . 3 . The s e n s i t i v i t y o f t h e p h o t o c o n d u c t o r c o n t a i n i n g Te i n c r e a s e s , a s t h e l a s e r beam w a v e l e n g t h s h o r t e n s i n t h e w a v e l e n g t h r e g i o n o f t h e d i o d e l a s e r . The s e n s i t i v i t y o f t h i s p h o t o c o n d u c t o r t o beams h a v i n g a w a v e l e n g t h o f 750 nm h a s b e e n shown t o b e a b o u t 300 m L / ~ . I t i s a l m o s t a s h i g h a s t h a t o f t h e p u r e S e p h o t o c o n d u c t o r t o beams h a v i n g a w a v e l e n g t h o f 442 nm.

The d a r k d e c a y r a t e i s i n t h e r a n g e o f f r o m 5 t o 8 %/sec. The i n i t i a l s u r f a c e p o t e n t i a l i s a b o u t 700 v o l t s . The r a t i o o f t h e r e s i d u a l s u r f a c e p o t e n t i a l and t h e i n i t i a l s u r f a c e p o t e n t i a l i s a b o u t 4 3 , S i n c e t h e d a r k d e c a y r a t e and r e s i d u a l s u r f a c e p o t e n t i a l a r e a t a l o w l e v e l , E and S a r e o f a l m o s t t h e same v a l u e .

The s e c o n d l a y e r , a c t i n g a s t h e p h o t o e l e c t r i c c o n v e r s i o n l a y e r , is l o c a t e d d e e p i n a n i n n e r p o r t i o n o f t h e p h o t o c o n d u c t o r . The a d v a n t a g e o f t h i s i s t h a t e v e n i f t h e p h o t o c o n d u c t o r i s damaged by f r i c t i o n a l c o n t a c t w i t h r e c o r d i n g p a p e r d u r i n g t h e t r a n s f e r s t e p , t h e s e n s i t i v i t y is n o t d e g r a d e d .

The s e n s i t i v i t y was r e d u c e d by a b o u t 1 0 % o f t h e i n i t i a l l e v e l a f t e r b e i n g s t o r e d a t room t e m p e r a t u r e f o r two y e a r s . T h i s i s c a u s e d by t h e d i f f u s i o n o f Te m o l e c u l e s . However, t h e i n i t i a l s u r f a c e p o t e n t i a l , d a r k d e c a y and r e s i d u a l s u r f a c e p o t e n t i a l h a v e b e e n k e p t a t t h e i r same l e v e l s f o r two y e a r s .

O p e r a t i o n o f a l a s e r beam p r i n t e r u s i n g a d i o d e l a s e r was c a r r i e d o u t a s a n a p p l i c a t i o n o f t h e p h o t o c o n d u c t o r o b t a i n e d i n t h e p r e s e n t work. A t y p i c a l p r i n t i n g p a t t e r n i s shown i n F i g .

-

4 . The o p t i c a l power r e q u r e d f o r p r i n t i n g was 20 m J / m Z a t t h e s u r f a c e o f t h e p h o t o c o n d u c t o r w i t h beams h a v i n g a w a v e l e n g t h o f 770 nm.

T h i s p h o t o c o n d u c t o r h a s f a v o r a b l e e l e c t r o p h o t o g r a p h i c c h a r a c t e r i s t i c s f o r p r a c t i c a l u s e i n a l a s e r beam p r i n t e r .

F i g . 4 . T y p i c a l p r i n t i n g p a t t e r n f r o m a d i o d e l a s e r beam p r i n t e r .

Acknowledgments.- W e e x p r e s s o u r s i n c e r e t h a n k s t o Dr. A. A r i m o t o , Mr. K . T a t s u n o , Mr. K . K a t a o k a and Mr. Y. M o r i f o r t h e i r u s e f u l l e x p e r i m e n t s and a c t i v e d i s c u s s i o n s .

R e f e r e n c e . - (1) E . Maruyama, T. H i r a i , T. ~ u j i t a , N. G o t o , Y.

I s o z a k i and K . S h i d a r a , J . J a p . S o c . A p p l . P h y s .

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