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PHOTOELECTRIC PROPERTIES OF GD a-Si : H MONOLAYER FILMS FOR
ELECTROPHOTOGRAPHIC APPLICATIONS
N. Yamamoto, K. Wakita, Y. Nakayama, T. Kawamura
To cite this version:
N. Yamamoto, K. Wakita, Y. Nakayama, T. Kawamura. PHOTOELECTRIC PROPERTIES OF GD
a-Si : H MONOLAYER FILMS FOR ELECTROPHOTOGRAPHIC APPLICATIONS. Journal de
Physique Colloques, 1981, 42 (C4), pp.C4-495-C4-498. �10.1051/jphyscol:19814105�. �jpa-00220720�
JOURNAL DE PHYSIQUE
CootZoque C4, suppZdment au nOIO, Tome 42, octobre 1981 page C4-495
PHOTOELECTRIC PROPERTIES OF GD a-Si:H MONOLAYER FILMS FOR ELECVROPHOTOGRAPHIC APPLICATIONS
N. Yamamoto, K. Wakita, Y. Nakayama and T. Kauamura
College of Engineering, University o f Osaka Prefecture, Mozu, Sakai, Osaka 591, Japan
Abstract.- Photoluminescence, i n f r a r e d absorption, and electrophotographic charge acceptance and s e n s i t i v i t y a r e measured as a f u n c t i o n o f RF power o f d e p o s i t i o n and oxygen c o n t e n t on a-Si:H f i l m s . A c l e a r c o r r e l a t i o n between photoluminescence and i n f r a r e d a b s o r p t i o n spectra i s observed. I t i s found t h a t photoluminescence measurement can be used as a good m o n i t o r f o r evalu- a t i n g t h e electrophotographic q u a l i t y as w e l l as i n f r a r e d absorption.
I n t r o d u c t i o n . - Since t h e f i r s t success o f c o n t r o l l e d doping o f t h e hydrogenated amorphous s i l i c o n ( 1 )
,
considerable e f f o r t s t o o b t a i n low r e s i s t i v e a-Si :H f i l m s were made f o r r e a l i z i n g a h i g h e f f i c i e n t and low c o s t s o l a r c e l l . I n s p i t e o f t h e excel l e n t photoconductive s e n s i t i v i t y o f a-Si :H over t h e v i s i b l e s p e c t r a l range, however, i t was o n l y r e c e n t l y t h a t a-Si:H photoreceptors w i t h m u l t i - l a y e r e d con- s t r u c t i o n were proposed f o r t h e e l e c t r o p h o t o g r a p h i c appl ications(2,3). I n e v i t a b l y prepared b l o c k i n g l a y e r i n these- -
photoreceptors was j u s t necessary because h i g h r e s i s t i v e a-Si:H f i l m s (>10'~ohrn-cm) w i t h s u f f i c i e n t photoconductive s e n s i t i v i t y have never been prepared.Recently(4), t h e authors have obtained v e r y h i g h r e s i s t i v e ( >l0 13 ohm-cm) and h i g h photoconductive a-Si:H f i l m s by doping w i t h B2H6 and a s l i g h t content o f oxy- gen. Using t h i s monolayer a-Si:H f i l m as a photoreceptor, b o t h p o s i t i v e and nega- t i v e reproduced images have been f i r s t demonstrated(4).
I n t h i s work, we have s t u d i e d on t h i s monolayer a-Si:H f i l m $0 examine some i n t e r r e l a t i o n s among photoluminescence (PL) spectra, i n f r a r e d a b s o r p t i o n (IR) spec- t r a , and e l e c t r o p h o t o g r a p h i c charge acceptance (CA) and s e n s i t i v i t y (ES) as a func- t i o n o f RF power and oxygen content. I t i s found t h a t a good c o r r e l a t i o n between PL and IR spectra e x s i s t s , and CA and ES 'are a l s o c l o s e l y r e l a t e d t o t h e PL peaks and i n t e n s i t y .
Experimental.- Amorphous Si:H f i l m s i n t h i s work were prepared u s i n g an i n d u c t i v e coupled RF glow discharge system as i s shown i n F i g . l. Substrate i s e i t h e r A l - p l a t e ( f o r PL, CA and ES measurements) o r S i s i n g l e c r y s t a l ( f o r I R measurement).
The s u b s t r a t e i s placed a t t h e c e n t e r o f a pyrex plasma r e a c t o r tube o f 200mm i n diameter and 800mm i n h i g h t , around which a water cooled RF c o i l i s arranged. The s u b s t r a t e i s heated by a ceramic h e a t e r and i s r o t a t e d a t t h e r a t e o f 50rpm f o r homogeneous d e p o s i t i o n . I n i t i a l l y , t h e plasma r e a c t o r tube i s evacuated up t o 5 X
1 0 T o r r b y an o i l d i f f u s i o n pump, and then t h e r e a c t i o n gases a r e i n t r o d u c e d ~ ~ under evacuation by r o t a r y pump. Doping gas r a t i o ( B ~ H ~ / S ~ H ~ ) , as w e l l as oxygen i n a separate path, i s c o n t r o l l e d by mass f l o w c o n t r o i l e r s . RF i n p u t power and oxygen p a r t i a l pressure a r e v a r i e d as parameters, and d e p o s i t i o n c o n d i t i o n t y p i c a l - l y used i s summarized i n Table 1.
IR a b s o r p t i o n spectra were measured w i t h Hitachi-260
IR
spectrometer using S i s i n g l e c r y s t a l as a 100 % transmission standard.Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19814105
JOURNAL DE PHYSIQUE
MASS FLOW CONnrYLEO
I
DLSMUNG I I I E YI
Table 1, D i p o s i t i o n parametersF i g . 1 , Schematic diagram o f the i n d u c t i v e coupled RF glow discharge system.
RF power Back pressure Gas pressure
B2H6/Si H4
O2 Flow r a t e Substrate temp.
Deposition r a t e Thickness
13.56 MHz, 0.4-3.0 kW 5 X 1 0 - ~ T o r r
I O - ~ , 1.0 T o r r (10 % i n H ~ ) 0-0.10 T o r r 150 sccm 20o0c
PL spectra were measured a t 8OK using He-Cd l a s e r (441 .€nm) as an e x c i t o r . T o t a l o f about 12mW l a s e r beam was i r r a d i a t e d t o t h e sample w i t h o u t any f o c u s i n g i n o r d e r t o be a b l e t o compare PL i n t e n s i t y among d i f f e r e n t samples. Luminescent l i g h t was dispersed by a Nikon 6-250 spectrometer and detected by a Hamamatsu R-406 photo- m u l t i p l i e r (S-l s p e c t r a l response). Because our samples were r a t h e r t h i c k , any
i n t e r f e r e n c e - e f f e c t on luminescence spectra was n o t observed and t h e c o r r e c t i o n t o t h e spectrometer and p h o t o m u l t i p l i e r responses was e a s i l y c a r r i e d o u t .
Dark- and photo-decay curves were measured on A l - s u b s t r a t e d photoreceptor. CA i s determined as the i n i t i a l s u r f a c e charge p o t e n t i a l p e r u n i t thickness when a colona charging by a constant v o l t a g e i s performed. ES i s estimated from t h e photo- decay curve as t h e r e c i p r o c a l o f t h e product o f exposure l i g h t i n t e n s i t y and t h e time r e q u i r e d f o r h a l f decay o f t h e surface p o t e n t i a l ( 4 ) .
& s u l t s and discussion.- I R and PL spectra o f B2H6 doped a-Si:H f i l m s as a f u n c t i o n o f RF i n p u t power a r e shown i n F i g . 2 (a) and (b), r e s p e c t i v e l y . For these samples, oxygen was n o t doped i n t e n t i o n a l l y , however, a s l i g h t c o n t e n t o f oxygen up t o 0.01 wt.% was detected by a spark source mass spectroscopic a n a l y s i s , although Si02 I R mode i s n o t seen i n F i g . 2 (a). A t low RF power (0.4-0.8kW), IR peak e x s i s t s a t 2000cm-I (SSH mode) and P 1 peak a t 1.2eV. A t 1.5kW RF power, I R peak shows a broadened one c o n s t r u c t e d w i t h comparable m i x t u r e s o f 2000cm-l and 2 0 9 0 c i 1 (SiH2 mode), and t h e second PL peak a t 1.45eV appears. For h i g h e r RF power (3.0kW), I R peak s h i f t s t o 2090cm-~ w i t h a shoulder o f 2000cm-~, where PL shows two peaks a t 1.3eV and 1.45eV b u t w i t h considerably decreased i n t e n s i t y .
Such a c o r r e l a t i o n between I R and PL spectra shows t h a t 1.2-1.3eV and 1.45eV PL peaks a r e c l o s e l y r e l a t e d w i t h SiH and SiH2 modes o f hydrogen bondings as was suggested by Ni t t a e t a l . ( 5 ) . For these samples, CA and ES were a l s o a f u n c t i o n o f RF power. A t low RF power, where o n l y SiH mode i n I R and 1.2eV peak i n PL a r e observed, CA i s low whereas
ES
i s enough. For t o o h i g h RF power, say 3.0kW, CA i s enough b u t ES decreases. Optimum c o n d i t i o n f o r CA and ES may be r e a l i z e d a t a c r i - t i c a l RF power, i.e., a c r i t i c a l r a t i o o f SiH,/SiH I R modes m i x t u r e . I t seems t h a tL
PL i n t e n s i t y shows maximum a t t h e optimum c o n d i t i o f l ; - I t i s t h e r e f o r e considered t h a t a p p r o p r i a t e m i x t u r e o f SiH2 mode may be necessary f o r enough CA f o r p o s i t i v e
- 3
-
l!
3
WAVE NUMBER [ciK1l
WAVELENGTH tnml
n w m o n a mo
-
a-Si.H:B1.1 1.2 1.3 1.1 15 1.6 1.7 15 PHOTON ENERGY teV1
Fig. 2, Infrared absorption (a) and photoluminescence (b) spectra as a function of RF power for the a-Si:H films doped with B2H6.
charge, and ES shows maximum where PL i n t e n s i t y shows maximum. However, CA f o r negative charge i s always v e r y poor i n these f i l m s doped w i t h o n l y B2H6.
I R and PL spectra o f B2H6 and oxygen doped a-Si:H f i l m s as a f u n c t i o n o f oxy- gen c o n t e n t a r e shown i n Fig. 3 (a) and (b), r e s p e c t i v e l y . Oxygen content i s an averaged value determined by spark source mass spectroscopy. For these samples, I R spectra shows t h e a p p r o p r i a t e m i x t u r e s o f SiH and SiH2 modes and two PL peaks a t 1.3eV and 1.45eV a r e observed i n a l l samples. As t h e oxygen c o n t e n t increases, Si02 mode a t 9 5 0 - 1 0 5 0 ~ 6 ' i n 1 R increases w h i l e t h e t h i r d peak a t 1 -6eV i n PL a l s o appears. For t o o h i g h oxygen content (0.04 wt%), PL i n t e n s i t y decreases r a p i d l y .
In
F i g . 4 a r e shown t h e dependences o f CA and ES on oxygen c o n t e n t f o r b o t h charge p o l a r i t i e s . I t should be noted t h a t n e g a t i v e CA increases w i t h a l a r g e r slope than t h a t o f p o s i t i v e CA w i t h t h e oxygen c o n t e n t increases, i.e., t h e f i l m sWAVE NUMBER (CM')
WAVELENGTH Inml
l M ) t Z O O m m x , 9m m
m
- 8 0 -
0
> 60-
2
2 0 -PHOTON ENERGY IeV)
Fig. 3, Infrared absorption (a) and photoluminescence (b) spectra as a function of oxygen content for the a-Si:H films doped with B2H6 and oxygen.
JOURNAL DE PHYSIQUE
becomes bi-chargable by oxygen doping. Both p o s i t i v e and n e g a t i v e ESYs are, however decreasing f u n c t i o n , f i r s t s l o w l y up t o 0.02 wt% o f oxygen and r a p i d l y over t h a t . Optimum oxygen c o n t e n t where bi-chargable p r o p e r t i e s a r e a v a i l a b l e w i t h enough ES e x s i s t s near 0.02 wt% oxygen content. PL i n t e n s i t y may shows maximum a l s o there.
P r o p e r t i e s o f oxygen doped a-Si :H f i l m s have been examined by b i g h t e t a1 . ( G ) w i t h p~ and IR, and i t s behaviaur l i k e as hydrogen t o occupy a dangling bond i s suggested. Knight e t a1.(6) a l s o observed a l a r g e s h i f t o f PL peak energy w i t h t h e increase o f oxygen content. I t i s n o t known whether o u r t h i r d PL peak a t 1.6eV may correspond t o t h a t by K n i g h t e t a l . , because t h e behaviours a r e d i f f e r e n t and B2H6 i s n o t doped i n t h e i r samples.
I n Fig. 5, we show an example o f reproduced image made by o u r monolayer a-Si:H photoreceptor. High s e n s i t i v i t y than ever used photoreceptor(4) opens t o h i g h speed processing and/or low exposure l i g h t i n t e n s i t y apparatus. Low f a t i g u e e f f e c t i n CA and ES and h i g h surface hardness matche f o r l o n g l i f e performance. The most important p r o p e r t y i s i t s bi-chargable one, however, a c r i t i c a l c o n t r o l e o f t h e d e p o s i t i o n c o n d i t i o n should be r e q u i r e d as described above.
v
OXYGEN CONCENTRATION (wP/el a06) @ M a @ g
(10.5%)(14s) 0 1 2 3 4 5 6 7 8 9 A B C D E F G H I J K L M N O P Q R S T U V W X Y Z
Fig. 4 , E l e c t r o p h o t o g r a p h i c charge P i g . 5 , An example of reproduced a c c e p t a n c e and s e n s i t i v i t y as a image made by a-Si:H monolayer f u n c t i o n of oxygen c o n t e n t . p h o t o r e c e p t o r .
Conclusions.- A c l e a r c o r r e l a t i o n between IR and PL s p e c t r a was observed, i .e., 1.2 -1.3eV, 1.45eV and 1.6eV peaks i n PL a r e r e l a t e d t o SiH, SiH2 and Si02 modes i n IR.
Appropriate m i x t u r e o f SiH2 mode seems t o be necessary t o o b t a i n enough CA f o r nega- t i v e charge and optimum oxygen c o n t e n t near 0.02 wt% f o r bi-chargable p r o p e r t y . PL spectra and i n t e n s i t y are u s e f u l i n d i c a t o r f o r e v a l u a t i n g t h e electrophotographic q u a l i t y o f t h e a-Si:H photoreceptor.
References
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3) Mort, J., Grammatica, S., Knight, J.C. and Lujan, R., Photogr.Sci.Eng.
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