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Multilayered photoreceptors based in hydrogenated amorphous silicon
JournaJ of Non-CrystallineSolids 77 & 78 (1985) 1433-1436 North-Holland, Amsterdam
1433
MULTILAYERED PHOTORECEPTORS BASED ON HYDROGENATEDAMORPHOUSSILICON
S.M. PAASCHE, G.H. BAUER I n s t i t u t fuer Physikalische Elektronik, Universitaet Stuttgart, Pfaffenwaldring 47, D 7000 Stuttgart 80, F. R. Germany For xerographic applications an optimized two layer structure has been realized (AI/a-Si:H/a-SiC:H), which shows a saturated surface potential of -500V. In cyclic operation of charging and l i g h t induced discharging the surface potential drops to -200V. With help of a third, highly n-doped layer introduced at the Al/a-Si:H interface,this reduction can be avoided and a saturated surface potential of -550V, in cyclic operation of -460V has been achieved. I. INTRODUCTION Recently, growing attention has been paid to the application of a-Si:H to photoreceptor drums1-3. The use of a-Si:H for electrophotographic applications compared with the convential materials like amorphous selenium and i t s compounds has several advantages, like high photosensitivity in the 7OOnmrange, strong mechanical surface hardness (1500-2000 Vickers strength) 4 and excellent thermal stability. However, the dark conductivity of a-Si:H (10-10-10-11~-1cm-I) is too high, to get acceptable decay times and sufficient surface potentials. Several suggestions have been made to decrease the dark conductivity by O- and B-doping5 to 10-13_10-14~-1cm -1 ' and to avoid carrier injection 2'6 into the a-Si:H from the surface or metal/semiconductor interface. Another p o s s i b i l i t y consists in a separation of the function of charge storage and that of sufficient photosensitivity in the visible range which can be transferred each to a well suited and optimized semiconductor: e.g. to a high resistive a-SiC:H-alloy showing excellent surface hardness and to a photoconductive a-Si:H-film which provides the injection of photocarriers into the a-SiC:H. In this case the thickness of the passivation layer, which stands high field-strengths (107-108 V/m) without affecting reasonable decay times, has to be increased to (2-4)Mm. 2. PREPARATION Two different structures ( f i g . 1,2) of photoreceptors have been worked out: a two layer system AI/a-Si:H/a-SiC:H and a three layer system Al/n+-a-Si:H/a-Si:H/a-SiC:H. 0022-3093/85/$03.30 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
S.M. Paasche, G.H. Bauer / Multilayered photoreceptors
1434
o-SiC:H
~5Nm
a-Si:H
15Nrn
a-SiC:H
2,5~m
o-Si:H
15Nrn
n*-a-,9:H
FIGURE I Xerographic two layer structure
300A
FIGURE 2 Xerographicthree layer structure
The samples have been prepared in a dc glow discharge apparatus, placed overhead at a heater, which acts also as an anode. Mass flow controlled gas supply and pumping are on opposite sides of the reactor. Onto an aluminum substrate f i r s t l y amorphous silicon is deposited at 550 K (15pm, 5~/s) and at a silane pressure of 0.5 mbar, the addition of methane to the silan (gas flow ratio SiH4/CH4 = 0.4-2) provides the deposition of the passivation layer. For highly n-doped a-Si:H layer (Eac~O.2 eV) the doping gas flow ratio (PH3/SiH4) has been I-5x10-2. 3. ELECTROPHOTOGRAPHICPROPERTIES Both types of photoreceptors exhibit a saturated surface potential of -500 V (two layer system), and -550V (three layer system) and a residual potential of -30V after exposure to an illumination of I=l~J/cm2 (502 nm). However, in cyclic operation of charging and light induced discharging, the surface potential of the two layer system drops to -200V at Ucor=-7kV (fig. 3) . Including a highly n-doped a-Si:H-layer (Eact~O.2 eV, 300 X) at the metal/a-Si:H interface, the light induced reduction of the surface potential in cyclic mode of operation turned out to be much smaller and the surface potential only drops from -550V to -460V at Ucor=-7kV (fig. 4). The residual potential and the photosensitivity of this three layer structure is by no means affected by the addition of a thin highly doped film. 4. DISCUSSION The influence of the a-SiC:H passivation layer thickness d on the photosensitivity is outlined in fig. 5. For increasing d to>4Mmthe transport of photogenerated holes from the a-Si:H layer through the a-SiC:H film is
1435
S.M. Paasche, G.H. Bauer/Multilayered photoreceptors
600
600
4 O0
200
w~U.c~ - -gkV
400
, ,,cycK: photoinduced chomjng d~chogng ond by ,, rechorgng c o n ~ ', by cc~ono --= , I , , 0 50 i00 t[sec]
Ucc~=-7kV
200
cyclic l:/noloinduced
!
0
Surface p o t e n t i a l structure
•
chQrg,-g by coR~o ,
0 0
FIGURE 3 Vs of a two l a y e r
Surface p o t e n t i a l structure
I 101
Oschoma~ond
, 50
by corona , 100 tIsec)
FIGURE 4 Vs of a t h r e e l a y e r
c o n s i d e r a b l y embarrassed by t h e i r
low
m o b i l i t y and l i f e t i m e (Schubweg)(phT h =10-8-10 -9 cm2v-l). Since Vr in both
~E u 10° ~
configurations
is o n l y about -30V, we
can assume t h a t i t , 10-I
resulting
from
trapped h o l e s , m a i n l y drops along the a-SiC:H l a y e r . Furthermore an optimized
10_2
thickness d
can be estimated from I/2 d =~U~hVr ) amounting to (2-6) ~m *
i0-3
0
,
~ , 2
, 4
,
, 6
:
: 8
which corresponds well to the thickness
:
10 "-"
dependence of the p h o t o s e n s i t i v i t y . The d i f f e r e n c e s
dQ-s'IxCx:HIpm]
surface p o t e n t i a l
in a c h i e v a b l e during c y c l i c
o p e r a t i o n in dark and under i l I u Photosensitivity l a y e r thickness
FIGURE 5 versus p a s s i v a t i o n
the metal as w e l l as by the f i e l d
m i n a t i o n o b v i o u s l y r e s u l t from dynamic discharge hole currents governed by the interface barriere at
dependent r e s i s t a n c e of the a-SiC:H l a y e r .
In the two l a y e r s t r u c t u r e the discharging hole c u r r e n t and by t h i s the a c h i e v a b l e n e g a t i v e surface p o t e n t i a l
in l i g h t / d a r k
cycles is dominated by the
c y c l i c change in occupation of deep traps at the metal/semiconductor i n t e r f a c e ,
1436
~M. Paasche, ~H. Bauer/Multilayeredphotoreceptom
which in the charging process act as a time dependent carrier source. In steady state operation (surface potential -200V) a contribution of holes originating from the interface to the discharge current w i l l reduce the surface potential. Increasing the corona voltage ends up in a diminishing of this effect ( f i g . 3). In the three layer structure the n+-layer forces, that the deep traps are thermally occupied by electrons, and obviously blocks the hole current injection. Consequently the surface potential reaches higher values and shows excellent temporal s t a b i l i t y . 5. CONCLUSION The reduction of the surface potential in cyclic operation of an a-Si:H photoreceptor can be strongly diminished by adding a t h i r d highly n-doped a-Si:H film at the Al/a-Si:H-interface to the system, without changing the photosensit i v i t y of the photoreceptor. In addition to the high surface hardness of a-SiC:H 8 (5000 knoop) the samples are showing excellent electrophotographic properties for high-speed copiers. REFERENCES 1) l.Shimizu, S.Shirai and E.Inoue, J.Appl.Phys. 52(4) (1981) 2776. 2) l.Shimizu, T.Komatsu and E.Inoue, Photographic Science and Engineering voi.24, no.5 (1980) 251. 3) J.Mort, S.Grammatica, J.C.Knights and R.Lujan, Photographic Science and Engineering voi.24, no.5 (1980) 241. 4) l.Shimizu, Electrophotography, in: Semiconductors and Semimetals voi.21, Hydrogenated Amorphous Silicon, Part D, ed. J.l.Pankove (Academic Press 1984) p. 58. 5) K.Wakita, Y.Nakayama and T.Kawamura, Photographic Science and Engineering voi.26, no.4 (1982) 183. 6) R.M.Schaffert, Electrophotography (Focal Press, New York 1975) p. 265. 7) G.H.Bauer et a l . , Proceedings 5 EC PVSEC, London (U.K.) (1985). 8) M.A.Bayne et a l . , Thin Solid Films (1983), 201.
1433
MULTILAYERED PHOTORECEPTORS BASED ON HYDROGENATEDAMORPHOUSSILICON
S.M. PAASCHE, G.H. BAUER I n s t i t u t fuer Physikalische Elektronik, Universitaet Stuttgart, Pfaffenwaldring 47, D 7000 Stuttgart 80, F. R. Germany For xerographic applications an optimized two layer structure has been realized (AI/a-Si:H/a-SiC:H), which shows a saturated surface potential of -500V. In cyclic operation of charging and l i g h t induced discharging the surface potential drops to -200V. With help of a third, highly n-doped layer introduced at the Al/a-Si:H interface,this reduction can be avoided and a saturated surface potential of -550V, in cyclic operation of -460V has been achieved. I. INTRODUCTION Recently, growing attention has been paid to the application of a-Si:H to photoreceptor drums1-3. The use of a-Si:H for electrophotographic applications compared with the convential materials like amorphous selenium and i t s compounds has several advantages, like high photosensitivity in the 7OOnmrange, strong mechanical surface hardness (1500-2000 Vickers strength) 4 and excellent thermal stability. However, the dark conductivity of a-Si:H (10-10-10-11~-1cm-I) is too high, to get acceptable decay times and sufficient surface potentials. Several suggestions have been made to decrease the dark conductivity by O- and B-doping5 to 10-13_10-14~-1cm -1 ' and to avoid carrier injection 2'6 into the a-Si:H from the surface or metal/semiconductor interface. Another p o s s i b i l i t y consists in a separation of the function of charge storage and that of sufficient photosensitivity in the visible range which can be transferred each to a well suited and optimized semiconductor: e.g. to a high resistive a-SiC:H-alloy showing excellent surface hardness and to a photoconductive a-Si:H-film which provides the injection of photocarriers into the a-SiC:H. In this case the thickness of the passivation layer, which stands high field-strengths (107-108 V/m) without affecting reasonable decay times, has to be increased to (2-4)Mm. 2. PREPARATION Two different structures ( f i g . 1,2) of photoreceptors have been worked out: a two layer system AI/a-Si:H/a-SiC:H and a three layer system Al/n+-a-Si:H/a-Si:H/a-SiC:H. 0022-3093/85/$03.30 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
S.M. Paasche, G.H. Bauer / Multilayered photoreceptors
1434
o-SiC:H
~5Nm
a-Si:H
15Nrn
a-SiC:H
2,5~m
o-Si:H
15Nrn
n*-a-,9:H
FIGURE I Xerographic two layer structure
300A
FIGURE 2 Xerographicthree layer structure
The samples have been prepared in a dc glow discharge apparatus, placed overhead at a heater, which acts also as an anode. Mass flow controlled gas supply and pumping are on opposite sides of the reactor. Onto an aluminum substrate f i r s t l y amorphous silicon is deposited at 550 K (15pm, 5~/s) and at a silane pressure of 0.5 mbar, the addition of methane to the silan (gas flow ratio SiH4/CH4 = 0.4-2) provides the deposition of the passivation layer. For highly n-doped a-Si:H layer (Eac~O.2 eV) the doping gas flow ratio (PH3/SiH4) has been I-5x10-2. 3. ELECTROPHOTOGRAPHICPROPERTIES Both types of photoreceptors exhibit a saturated surface potential of -500 V (two layer system), and -550V (three layer system) and a residual potential of -30V after exposure to an illumination of I=l~J/cm2 (502 nm). However, in cyclic operation of charging and light induced discharging, the surface potential of the two layer system drops to -200V at Ucor=-7kV (fig. 3) . Including a highly n-doped a-Si:H-layer (Eact~O.2 eV, 300 X) at the metal/a-Si:H interface, the light induced reduction of the surface potential in cyclic mode of operation turned out to be much smaller and the surface potential only drops from -550V to -460V at Ucor=-7kV (fig. 4). The residual potential and the photosensitivity of this three layer structure is by no means affected by the addition of a thin highly doped film. 4. DISCUSSION The influence of the a-SiC:H passivation layer thickness d on the photosensitivity is outlined in fig. 5. For increasing d to>4Mmthe transport of photogenerated holes from the a-Si:H layer through the a-SiC:H film is
1435
S.M. Paasche, G.H. Bauer/Multilayered photoreceptors
600
600
4 O0
200
w~U.c~ - -gkV
400
, ,,cycK: photoinduced chomjng d~chogng ond by ,, rechorgng c o n ~ ', by cc~ono --= , I , , 0 50 i00 t[sec]
Ucc~=-7kV
200
cyclic l:/noloinduced
!
0
Surface p o t e n t i a l structure
•
chQrg,-g by coR~o ,
0 0
FIGURE 3 Vs of a two l a y e r
Surface p o t e n t i a l structure
I 101
Oschoma~ond
, 50
by corona , 100 tIsec)
FIGURE 4 Vs of a t h r e e l a y e r
c o n s i d e r a b l y embarrassed by t h e i r
low
m o b i l i t y and l i f e t i m e (Schubweg)(phT h =10-8-10 -9 cm2v-l). Since Vr in both
~E u 10° ~
configurations
is o n l y about -30V, we
can assume t h a t i t , 10-I
resulting
from
trapped h o l e s , m a i n l y drops along the a-SiC:H l a y e r . Furthermore an optimized
10_2
thickness d
can be estimated from I/2 d =~U~hVr ) amounting to (2-6) ~m *
i0-3
0
,
~ , 2
, 4
,
, 6
:
: 8
which corresponds well to the thickness
:
10 "-"
dependence of the p h o t o s e n s i t i v i t y . The d i f f e r e n c e s
dQ-s'IxCx:HIpm]
surface p o t e n t i a l
in a c h i e v a b l e during c y c l i c
o p e r a t i o n in dark and under i l I u Photosensitivity l a y e r thickness
FIGURE 5 versus p a s s i v a t i o n
the metal as w e l l as by the f i e l d
m i n a t i o n o b v i o u s l y r e s u l t from dynamic discharge hole currents governed by the interface barriere at
dependent r e s i s t a n c e of the a-SiC:H l a y e r .
In the two l a y e r s t r u c t u r e the discharging hole c u r r e n t and by t h i s the a c h i e v a b l e n e g a t i v e surface p o t e n t i a l
in l i g h t / d a r k
cycles is dominated by the
c y c l i c change in occupation of deep traps at the metal/semiconductor i n t e r f a c e ,
1436
~M. Paasche, ~H. Bauer/Multilayeredphotoreceptom
which in the charging process act as a time dependent carrier source. In steady state operation (surface potential -200V) a contribution of holes originating from the interface to the discharge current w i l l reduce the surface potential. Increasing the corona voltage ends up in a diminishing of this effect ( f i g . 3). In the three layer structure the n+-layer forces, that the deep traps are thermally occupied by electrons, and obviously blocks the hole current injection. Consequently the surface potential reaches higher values and shows excellent temporal s t a b i l i t y . 5. CONCLUSION The reduction of the surface potential in cyclic operation of an a-Si:H photoreceptor can be strongly diminished by adding a t h i r d highly n-doped a-Si:H film at the Al/a-Si:H-interface to the system, without changing the photosensit i v i t y of the photoreceptor. In addition to the high surface hardness of a-SiC:H 8 (5000 knoop) the samples are showing excellent electrophotographic properties for high-speed copiers. REFERENCES 1) l.Shimizu, S.Shirai and E.Inoue, J.Appl.Phys. 52(4) (1981) 2776. 2) l.Shimizu, T.Komatsu and E.Inoue, Photographic Science and Engineering voi.24, no.5 (1980) 251. 3) J.Mort, S.Grammatica, J.C.Knights and R.Lujan, Photographic Science and Engineering voi.24, no.5 (1980) 241. 4) l.Shimizu, Electrophotography, in: Semiconductors and Semimetals voi.21, Hydrogenated Amorphous Silicon, Part D, ed. J.l.Pankove (Academic Press 1984) p. 58. 5) K.Wakita, Y.Nakayama and T.Kawamura, Photographic Science and Engineering voi.26, no.4 (1982) 183. 6) R.M.Schaffert, Electrophotography (Focal Press, New York 1975) p. 265. 7) G.H.Bauer et a l . , Proceedings 5 EC PVSEC, London (U.K.) (1985). 8) M.A.Bayne et a l . , Thin Solid Films (1983), 201.