Electrophotographic member with .alpha.-Si and H

Disclosed is an electrophotographic member having an amorphous-silicon photoconductive layer, wherein the distance between a portion in which light illuminating the photoconductor is absorbed therein until its intensity decreases to 1% of that at incidence and the interface of the photoconductor opposite to the light incidence side thereof is at most 5 .mu.m, whereby the residual potential of the photoconductive layer can be reduced. That part of the photoconductive layer constituting the electrophotographic member which is at least 10 nm thick inwardly of the photoconductive layer from the surface thereof to store charges is made of amorphous silicon which has an optical forbidden band gap of at least 1.6 eV and a resistivity of at least 10.sup.10 .OMEGA..multidot.cm. Further, within such photoconductive layer, a region of amorphous silicon which has an optical forbidden band gap smaller than that of the amorphous silicon forming the surface part is disposed at a thickness of at least 10 nm. By forming the region of the narrower optical forbidden band gap within the photoconductive layer in this manner, the sensitivity of the photoconductive layer to light of longer wavelengths can be enhanced.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to an electrophotographic member for use as an 
electrophotographic plate. More particularly, it relates to improvements 
in an electrophotographic member which employs amorphous silicon for a 
photoconductive layer. 
2. Description of the Prior Art 
As photoconductive materials to be used for electrophotographic members, 
there have heretofore been inorganic substances such as Se, CdS and ZnO 
and organic substances such as polyvinyl carbazole (PVK) and 
trinitrofluorenone (TNF). Although they exhibit high photoconductivities, 
they have the disadvantage that the layers of the substances exhibit 
insufficient hardnesses, so they have their surfaces flawed or wear away 
during the operations as the electrophotographic members. In addition, 
many of these materials are substances harmful to the human body. It is 
therefore unfavorable that the layers wear away to adhere on copying paper 
even if in small amounts. In order to improve these disadvantages, it has 
been proposed to employ amorphous silicon for the photoconductive layer 
(refer to, for example, the official gazette of Japanese Laid-open Patent 
Application No. 54-78135). In general, however, the amorphous silicon 
layer exhibits a dark resistivity which is too low for the 
electrophotographic member. The amorphous silicon layer having a high 
resistivity on the order of 10.sup.10 .OMEGA..multidot.cm exhibits a 
photoelectric gain being too low, and only an unsatisfactory one has been 
obtained as the electrophotographic member. 
SUMMARY OF THE INVENTION 
This invention has for its object to provide an electrophotographic member 
which is free from the fear of degradation in the resolution and which 
exhibits a low residual potential. 
Useful for accomplishing the object is a method in which the internal 
structure of an amorphous photoconductive layer is defined so that the 
region thereof in an electrophotographic member where illuminating light 
is sufficiently absorbed may be brought near to a substrate electrode in a 
manner to be spaced therefrom at most 5 .mu.m. The same object can also be 
accomplished by confining the wavelength of illuminating light to be used 
for erasing charges, though this measure results in a similar construction 
to the above. 
More specifically, in an electrophotographic member having an 
amorphous-silicon photoconductive layer, the distance between the portion 
of the photoconductive layer in which light illuminating the 
photoconductive layer is absorbed therein until its intensity decreases 
down to 1% of that at incidence and the interface of the photoconductive 
layer opposite to the light incidence side is made at most 5 .mu.m. 
Further, in the photoconductive layer of amorphous silicon constituting the 
electrophotographic member, a part at least 10 nm thick inwardly of the 
photoconductive layer from the surface thereof on the side on which 
charges are stored has its optical forbidden band gap made at least 1.6 eV 
and its resistivity made at least 10.sup.10 .OMEGA..multidot.cm. Further, 
within the photoconductive layer, a region of amorphous silicon whose 
optical forbidden band gap is smaller than that of amorphous silicon 
forming the surface region is provided to a thickness of at least 10 nm. 
By forming the region of the narrower optical forbidden band gap within 
the photoconductive layer in this manner, the sensitivity of the 
photoconductive layer to light of longer wavelengths can be enhanced. 
Regarding the improvements of extending the sensitivity of the 
electrophotographic member to the longer wavelength region, patent 
applications are pending in U.S.A., etc.

DETAILED DESCRIPTION OF THE INVENTION 
The inventors produced an electrophotographic member by the use of an 
amorphous silicon layer having a resistivity of at least 10.sup.10 
.OMEGA..multidot.cm over the entire layer, and investigated the residual 
potential characteristics thereof in detail. The result is shown in FIG. 
1. It has been revealed from the result that the magnitude of the residual 
potential depends greatly upon the wavelength of illuminating light. That 
is, the residual potential is conspicuously great on the side on which the 
wavelength of the incident light is shorter than a certain value, and it 
is extremely small on the side on which the wavelength is longer. This 
tendency becomes especially remarkable in case where the thickness of the 
sample exceeds 5 .mu.m. 
As to this phenomenon, the inventors conjecture as follows. Amorphous 
silicon containing hydrogen differs in the quantity of hydrogen contained 
therein and the form of the coupling between hydrogen and silicon, 
depending upon the producing conditions of the layer, and has a peculiar 
optical forbidden band gap accordingly. Therefore, light of energy lower 
than energy corresponding to the optical forbidden band gap is transmitted 
through the layer without generating carriers based on the photo 
excitation. Regarding the amorphous silicon layer used in the measurement 
of FIG. 1, results obtained by measuring the absorption of light of each 
wavelength within the layer are shown in FIG. 2. 
It is understood from FIG. 2 that, as the wavelength of the illuminating 
light is shorter, the light is absorbed in the closer vicinity of the 
surface. Among the carriers generated by the absorbed light, those of the 
opposite sign to that of charges stored on the surface side of the 
photoconductive layer flow towards the surface to neutralize the charges, 
whereas those of the same sign must migrate to an electrode underlying the 
layer. Since the amorphous silicon layer in which the carriers migrate 
contains hydrogen, localized states within the layer have been 
considerably extinguished, but carrier trap levels are still existent in 
the amorphous silicon more than in crystalline silicon, so that the 
property of the layer allowing the migration is inferior. When, in 
consequence, the generated photo-carriers remain without being 
sufficiently drawn out, they will form a cause for increasing the residual 
potential. 
Referring here to FIG. 1, the residual potential is extremely low in cases 
where the wavelength of the illuminating light is longer than the certain 
value. In view of FIG. 2, the wavelength of the boundary between the high 
and low residual potentials has been revealed to correspond to energy at 
which the illuminating light passes through the whole photoconductive 
layer to reach the underlying substrate. 
In the energy band structure, as shown by a curve (b) in FIG. 3, the 
amorphous silicon containing hydrogen has a state density spreading inside 
the optical forbidden band gap (E.sub.opt) in contrast to the crystalline 
silicon (illustrated by a curve (a) in FIG. 3). Such foot of the state 
density forms one cause for the trap level stated above. However, when 
this part is illuminated by the longer wavelength light, i.e., light of 
energy somewhat lower than the optical forbidden band gap, it is cleared 
by the illuminating light, and the property permitting the carriers to 
migrate increases. 
It is construed that the foregoing is the ground on which the 
wavelength-dependency as shown in FIG. 1 appears, and that among the 
photo-carriers, those flowing onto the surface side can reach the surface 
owing to the good migration property, whereas those flowing onto the 
underlying substrate side migrate through the part of the inferior 
migration property not illuminated by the light, so they form the residual 
potential component without reaching the substrate. Therefore, when the 
layer becomes thick beyond a certain degree and the migration region on 
the underlying substrate side becomes long, the residual potential 
increases. 
As the allowable limit of the residual potential, since the present-day 
toner adheres to a photoconductive layer at approximately 600 V and 
separates away therefrom at or below 200 V, it is considered that a value 
of at most approximately 40% is desirable and that a value of at most 20% 
is more desirable. In order to attain this value, it has been necessary to 
make the distance from a region, in which at least 99% of the illuminating 
light is absorbed, to the underlying substrate at most 5 .mu.m, more 
desirably at most 2 .mu.m. 
An actual electrophotographic member sometimes needs to have a thickness of 
10-20 .mu.m or above in order to withstand a voltage enough to hold the 
toner, and the light absorbing part must be made considerably close to the 
substrate side at that time. 
One expedient therefor is to limit the wavelength of the illuminating 
light. The wavelength corresponding to the optical forbidden band gap 
E.sub.opt is: 
##EQU1## 
Letting .lambda..sub.2 denote the wavelength of the light which, as stated 
previously, is absorbed 99% before it reaches the distance of 5 .mu.m from 
the underlying substrate side in the amorphous silicon layer used, the 
incident light may be confined within the following range: 
EQU .lambda..sub.2 .ltoreq.(wavelength of incident light).ltoreq..lambda..sub.1 
(2) 
Then, the electrophotographic member can have its residual potential 
suppressed sufficiently low while holding its photoconductivity high. 
Table 1 lists examples of the relationship between the hydrogen content and 
optical forbidden band gap (E.sub.opt) of amorphous silicon. Regarding 
hydrogen contents other than those of the concrete examples, 
interpolations may be satisfactorily made. 
TABLE 1 
______________________________________ 
Hydrogen 
content (at.-%) E.sub.opt (eV) 
.lambda..sub.1 (nm) 
______________________________________ 
5 1.3 950 
10 1.45 850 
15 1.6 775 
20 1.8 690 
25 2.0 620 
______________________________________ 
In case of using the photoconductive layer of this invention in a laser 
beam printer etc., the laser wavelength may be selected so as to fulfill 
the condition of Expression (2). According to the result of the inventors' 
study, the wavelength .lambda..sub.2 depends upon the thickness of the 
photoconductive layer. It is approximately 100-150 nm shorter than the 
wavelength .lambda..sub.1. It is experimentally determined. 
In a system wherein the photoconductive layer of this invention is 
illuminated by an ordinary white lamp or the like, it is satisfactory in 
practical use to utilize the illuminating light by passing it through a 
filter which cuts the side of wavelengths shorter than .lambda..sub.2 to 
the amount of at least 80%. For the betterment of the residual potential, 
it is important that the illuminating light principally includes the 
component of wavelengths longer than .lambda..sub.2. In case where the 
spectral width of the illuminating light is great, a satisfactory 
sensitivity is attained by the component fulfilling Expression (2). It is 
particularly important that, among the principal spectral components of 
the illuminating light, one on the longer wavelength side meets the 
aforecited condition. 
In addition, there is an expedient in which the sensitivity to light of 
longer wavelengths is enhanced by forming a region of narrowed optical 
forbidden band gap within the photoconductive layer. This expedient will 
be described. 
In a light receiving device of the storage mode such as the 
electrophotographic member, the resistivity of the photoconductive layer 
must satisfy the following two required values: 
(1) The resistivity of the photoconductive layer needs to be above 
approximately 10.sup.10 .OMEGA..multidot.cm lest charges stuck on the 
surface of the layer by the corona discharge or the like should be 
discharged in the thickness direction of the layer before exposure. 
(2) Also the sheet resistance of the photoconductive layer must be 
sufficiently high lest a charge pattern formed on the surface (as well as 
the interface) of the photoconductive layer upon the exposure should 
disappear before developing on account of the lateral flow of the charges. 
In terms of the resistivity, this becomes above approximately 10.sup.10 
.OMEGA..multidot.cm as in the preceding item. 
Amorphous silicon having such resistivity is, in general, a material whose 
optical forbidden band gap is at least 1.6 eV. 
In order to meet the conditions of the two items, the resistivity of and 
near the surface of the photoconductive layer to hold the charges must be 
above approximately 10.sup.10 .OMEGA..multidot.cm, but the resistivity of 
at least 10.sup.10 .OMEGA..multidot.cm need not be possessed uniformly in 
the thickness direction of the layer. Letting .tau. denote the time 
constant of the dark decay in the thickness direction of the layer, C 
denote the capacitance per unit area of the layer and R denote the 
resistance in the thickness direction per unit area of the layer, the 
following relation holds: 
EQU .tau.=RC (3) 
The time constant .tau. may be sufficiently long as compared with the 
period of time from the electrification to the developing, and the 
resistance R may be sufficiently great with the thickness direction of the 
layer viewed macroscopically. 
The inventors have revealed that, as a factor which determines the 
macroscopic resistance in the thickness direction of the layer in a 
high-resistivity thin-film device such as the electrophotographic member, 
charges to be injected from the interface with an electrode play an 
important role besides the resistivity of the layer itself. 
The inventors have considered to solve this problem by employing 
high-resistivity amorphous silicon which has a resistivity of at least 
10.sup.10 .OMEGA..multidot.cm as its layer. Ordinarily, such 
high-resistivity region is the intrinsic semiconductor (i-type). This 
region functions as a layer which blocks the injection of carriers from 
the electrode into the photoconductive layer, and can simultaneously be 
effectively used as a layer which holds the surface charges. In this case, 
the resistivity of that part of the photoconductive layer which does not 
adjoin the surface (or interface) need not always be as high as in the 
vicinity of the surface (or interface). That is, the macroscopic 
resistance R of the photoconductive layer may satisfy Expression (3). 
Therefore, the sensitivity can be extended to the light of the longer 
wavelength region owing to a structure in which the optical forbidden band 
gap of the interior of the photoconductive layer is narrowed, so that the 
amorphous silicon layer can also be used as a photoconductive layer for a 
semiconductor laser beam printer. 
In order to more effectively block the injection of the carriers from the 
electrode, it is also effective to interpose a thin layer of SiO.sub.2, 
CeO.sub.2, Sb.sub.2 Sb.sub.3, Sb.sub.2 Se.sub.3, As.sub.2 S.sub.3, 
As.sub.2 Se.sub.3 or the like between the electrode and the amorphous 
silicon layer. 
When the region of narrow forbidden band gap has been formed within the 
photoconductive layer in this manner, the longer wavelength light is 
absorbed in this region to generate electron-hole pairs. The situation is 
illustrated as an energy band model in FIG. 4. Since, in both the region 
of wide forbidden band gap and the region of narrow forbidden band gap, 
the resistances of the portions themselves are desired to be as high as 
possible, the photoconductive layer should more preferably be fully 
intrinsic (i-type). At this time, the energy band model becomes a shape 
constricted vertically with respect to the Fermi level. Photo-carriers 
generated in the constriction or the region of narrowed forbidden band gap 
are captured in the region by a built-in field existing therein. In order 
to draw the photo-carriers out of the region of narrowed forbidden band 
gap with an external electric field and to utilize them as effective 
photo-carriers, the external electric field must be greater than the 
built-in field of the region of narrowed forbidden band gap. Conversely 
stated, in case of forming the region of narrowed forbidden band gap, the 
built-in field to arise therein must become smaller than the external 
electric field. The built-in field of the region of narrowed forbidden 
band gap depends upon the depth (potential difference) D and the width W 
of the region. An abrupt change of the band gap generates a great built-in 
field, whereas a gentle change of the band gap generates a small built-in 
field. When the shape of the region of narrowed forbidden band gap is 
approximated by an isosceles triangle, the condition for drawing out the 
photo-carriers is: 
EQU E.sub.a .gtoreq.2D/W (4) 
where E.sub.a denotes the external electric field. 
Within the amorphous-silicon photoconductive layer, the part in which the 
region of narrowed forbidden band gap exists should preferably be disposed 
in the portion of at most 5 .mu.m from the interface (for example, the 
underlying substrate side) opposite to the light incidence side as stated 
before. In order to generate effective photo-carriers in the region of 
narrowed forbidden band gap, the width W of this region needs to be, in 
effect, at least 10 nm. The maximum limit of the width of the region of 
narrowed forbidden band gap is, of course, the whole thickness of the 
amorphous silicon layer, but the width W of the region is desired to be at 
most 1/2 of the whole thickness of the layer in order to keep the total 
resistance R in the thickness direction sufficiently high. 
The whole thickness of the amorphous-silicon photoconductive layer is 
determined by the surface potential, which in turn varies depending upon 
the kind of toner used and the service conditions of the photoconductive 
layer. However, the withstand voltage of the amorphous silicon layer is 
considered to be 10 V-50 V per .mu.m. Accordingly, when the surface 
potential is 500 V, the entire layer thickness becomes 10 .mu.m-50 .mu.m. 
Values of the entire layer thickness exceeding 100 .mu.m are not 
practical. 
In case of applying the method of this invention to the photoconductive 
layer of such structure, with note taken of the fact that the absorption 
of light occurs principally in the region of narrowed optical forbidden 
band gap, the place in which this region exists should preferably be 
disposed in the vicinity of the underlying substrate side spaced therefrom 
only the distance d of at most 5 .mu.m. This situation is elucidated in 
FIG. 4. The figure illustrates a case where positive charges are stored on 
the surface, and electrons 42 and holes 43 generated by incident light 41 
flow so as to neutralize the charges. 
Hereunder will be described the concrete structure of an 
electrophotographic member having an amorphous-silicon photoconductive 
layer. 
Referring to FIG. 5, numeral 1 designates a substrate, and numeral 2 a 
photoconductive layer including an amorphous silicon layer. The substrate 
1 may be any of a metal plate such as aluminum, stainless steel or 
nichrome plate, an organic material such as polyimide resin, glass 
ceramics etc. In case where the substrate is an electrical insulator, an 
electrode 11 needs to be deposited on the substrate. Used as the electrode 
is a thin film of a metal material such as aluminum and chromium, or a 
transparent electrode of an oxide such as SnO.sub.2 and In--Sn--O. The 
photoconductive layer 2 is disposed on the electrode. In case where the 
substrate 1 is light-transmissive and the electrode 11 is transparent, 
light to enter the photoconductive layer 2 is sometimes projected through 
the substrate 1. The photoconductive layer 2 can be provided with a layer 
21 for suppressing the injection of excess carriers from the substrate 
side, and a layer 22 for suppressing the injection of charges from the 
surface side. As the layers 21 and 22, layers of a high-resistivity oxide, 
sulfide or selenide such as SiO, SiO.sub.2, Al.sub.2 O.sub.3, CeO.sub.2, 
V.sub.2 O.sub.3, Ta.sub.2 O, As.sub.2 Se.sub.3 and As.sub.2 S.sub.3 are 
used, or layers of an organic substance such as polyvinyl carbazole are 
sometimes used. Although these layers 21 and 22 serve to improve the 
electrophotographic characteristics of the photoconductive layer of this 
invention, they are not always absolutely indispensable. All layers 23, 24 
and 25 are layers whose principal constituents are amorphous silicon. Each 
of the layers 23 and 25 is an amorphous silicon layer which satisfies the 
characteristics of this invention described before and which has a 
thickness of at least 10 nm. Even when the resistivity of the layer 23 is 
below 10.sup. 10 .OMEGA..multidot.cm, no bad influence is exerted on the 
dark decay characteristics as the electrophotographic member owing to the 
presence of the layers 21 and 22. Although, in FIG. 5, the amorphous 
silicon layer has the three-layered structure, it may of course be a 
uniform amorphous-silicon layer generally meeting the requirement of this 
invention as well. It is sometimes the case that the amorphous silicon 
layer is doped with carbon or a very small amount of boron or germanium in 
order to vary the electrical and optical characteristics of the layer. 
However, it is necessary for ensuring photoconductive characteristics that 
at least 50 atomic-% of silicon is contained on the average within the 
layer. As long as this requirement is fulfilled, produced layers fall 
within the scope of this invention whatever other elements they may 
contain. 
As methods for forming the amorphous silicon layer containing hydrogen, the 
process exploiting the decomposition of SiH.sub.4 by the glow discharge, 
the reactive sputtering process, the ion-plating process etc. have been 
known. With any of the methods, a layer having the best photoelectric 
conversion characteristics is obtained when the substrate temperature 
during the formation of the layer is 150.degree.-250.degree. C. In case of 
the glow discharge process, a layer of good photoelectric conversion 
characteristics has as low a resistivity as 10.sup.6 -10.sup.7 
.OMEGA..multidot.cm and is unsuitable for electrophotography. Therefore, 
such a consideration as doping the layer with a slight amount of boron to 
raise its resistivity is necessary. In contrast, the reactive sputtering 
process can produce a layer having a resistivity of at least 10.sup.10 
.OMEGA..multidot.cm besides good photoelectric conversion characteristics, 
and moreover, it can form a uniform layer of large area by employing a 
sputtering target of sufficiently large area. It can therefore be said 
particularly useful for forming the photoconductive layer for 
electrophotography. 
Usually, the reactive sputtering is performed by the use of an equipment as 
shown in FIG. 6. Referring to the figure, numeral 31 designates a bell 
jar, numeral 32 an evacuating system, numeral 33 a radio-frequency power 
source, numeral 34 a sputtering target, numeral 35 a substrate holder, and 
numeral 36 a substrate. Sputtering equipment include, not only the 
structure which serves to perform the sputter-evaporation on the flat 
substrate as exemplified in the figure, but also a structure which can 
perform the sputter-evaporation on a cylindrical or drum-shaped substrate. 
Therefore, they may be properly employed according to intended uses. 
The reactive sputtering is carried out by evacuating the bell jar 31, 
introducing hydrogen and such an inert gas as argon thereinto, and 
supplying a radio-frequency voltage from the radio-frequency power source 
33 to cause a discharge. The quantity of hydrogen which is contained in a 
layer to be formed at this time is determined principally by the pressure 
of hydrogen existent in the atmosphere gas during the discharge. The 
amorphous silicon layer containing hydrogen as is suited to this invention 
is produced when the hydrogen pressure during the sputtering lies in a 
range of from 5.times.10.sup.-5 Torr to 9.times.10.sup.-3 Torr. 
The localized state density in the pure amorphous silicon containing no 
hydrogen is presumed to be on the order of 10.sup.20 /cm.sup.3. Supposing 
that hydrogen atoms extinguish the localized states at 1:1 in case of 
doping such amorphous silicon with hydrogen, all the localized states 
ought to be extinguished with a hydrogen-doping quantity of approximately 
0.1 atomic-%. However, it is for the first time when the hydrogen content 
exceeds approximately 1 atomic-% that amorphous silicon useful as a 
photoconductor is actually obtained owing to the appearance of the 
photoconductivity and to the occurrence of the variation of the optical 
forbidden band gap. Hydrogen can be contained up to approximately 50 
atomic-%, but a content of at most 40 atomic-% is common and especially a 
content of at most 30 atomic-% is practical. 
A material in which part of silicon is substituted by germanium, carbon or 
the like can also be used for the electrophotographic member. Useful as 
the quantity of the substitution by germanium or carbon is within 30 
atomic-%. 
In order to vary the hydrogen content of the amorphous silicon layer, there 
may be controlled the substrate temperature, the concentration of hydrogen 
in an atmosphere, the input power, etc. in the case of forming the layer 
by the use of any of the layer forming methods already stated. 
Among the layer forming methods mentioned above, one which is excellent in 
the process controllability and which can readily produce a 
photoconductive amorphous silicon layer of high resistivity and good 
quality is the reactive sputtering process. 
By doping the amorphous silicon layer with an impurity, it can be turned 
into a conductivity type such as p-type and n-type. 
Referring to FIG. 7, the electrophotographic plate according to the present 
invention is formed on the surface of a rotary drum 51. When the rotary 
drum 51 is formed of a conductor such as aluminum, the rotary drum 51 per 
se may be used as the conductor substrate of the electrophotographic 
member according to the present invention. When a rotary drum formed of 
glass or the like is used, a conductor such as a metal is coated on the 
surface of the rotary drum of glass, and a plurality of predetermined 
amorphous Si layers are laminated thereon. Beams 55 from a light source 52 
such as a semiconductor laser pass through a beam collecting lens 53 and 
impinge on a polyhedral mirror 54, and they are reflected from the mirror 
54 and reach the surface of the drum 51. 
Charges induced on the drum 51 by a charger 56 are neutralized by signals 
imparting to the laser beams to form a latent image. The latent image 
region arrives at a toner station 57 where a toner adheres only to the 
latent image area irradiated with the laser beams. This toner is 
transferred onto a recording paper 59 in a transfer station 58. The 
transferred image is thermally fixed by a fixing heater 60. Reference 
numeral 61 represents a cleaner for the drum 51. 
There may be adopted an embodiment in which a glass cylinder is used as the 
drum, a transparent conductive layer is formed on the glass cylinder and 
predetermined Se layers are laminated thereon. 
In this embodiment, the writing light source may be disposed in the 
cylindrical drum. In this case, beams are incident from the conductor side 
of the electrophotographic plate. 
Needless to say, applications of the electrophotographic member are not 
limited to the above-mentioned embodiments. 
In the instant specification and appended claims, by the term 
"electrophotographic member" is meant one that is used for an 
electrophotographic device, a laser beam printer equipment and the like in 
the fields of electrophotography, printing, recording and the like. 
Hereunder, this invention will be concretely described in conjunction with 
examples. 
EXAMPLE 1 
FIG. 8 is a sectional view of an electrophotographic member of this 
example. 
An aluminum cylinder whose surface was mirror-polished was heated at 
300.degree. C. in an oxygen atmosphere for 2 hours, to form an Al.sub.2 
O.sub.3 film 21 on the surface of the cylinder 1. The cylinder was 
installed in a rotary magnetron type sputtering equipment, the interior of 
which was evacuated up to 1.times.10.sup.-6 Torr. Thereafter, whilst 
holding the cylinder at 200.degree. C., an amorphous silicon film 2 having 
an optical forbidden band gap of 1.95 eV and a resistivity of 10.sup.11 
.OMEGA..multidot.cm was deposited thereon to a thickness of 20 .mu.m at a 
deposition rate of 2 A/sec by a radio-frequency output of 350 W in a mixed 
atmosphere consisting of 2.times.10.sup.-3 Torr of hydrogen and 
3.times.10.sup.-3 Torr of argon. The cylinder thus prepared was used to 
fabricate an electrophotographic device of a system in which the amorphous 
silicon film was illuminated by a He--Ne laser beam (wavelength: 630 nm) 
or by a white lamp through a filter adapted to cut light of and below 500 
nm to the amount of at least 90%. 
Owing to such construction, especially the residual potential can be 
sharply lowered. 
More specifically, in an electrophotographic member having an 
amorphous-silicon photoconductive layer which contains at least 50 
atomic-% of silicon and at least 1 atomic-% of hydrogen on the average 
within the layer, the distance between a portion in which light 
illuminating the photoconductive layer (light contributive to create 
photo-carriers within the photoconductor) is absorbed within the 
photoconductor until its intensity becomes 1% of that at incidence and the 
interface of the photoconductive layer opposite to the light incidence 
side thereof is at most 5 .mu.m. 
Table 2 lists examples in which amorphous silicon containing 12 atomic-% of 
hydrogen to form an electrophotographic member was set at various 
thicknesses, and the residual potentials of the samples were measured. 
Illuminating light was 575 nm. 
TABLE 2 
______________________________________ 
Residual 
potential/ 
Thick- 
Remaining Distance Initial ness of 
light in- to opposite potential 
layer 
No. tensity (%) 
interface (.mu.m) 
(%) (.mu.m) 
______________________________________ 
1 1 3 5 13 
2 1 5 5 15 
3 1 10 38 20 comp. 
ex. 
4 1 15 40 25 comp. 
ex. 
______________________________________ 
Table 3 indicates the relations between the residual potential and the 
value of the remaining light intensity at a position of 5 .mu.m from the 
interface of a photoconductive layer opposite to the light incidence side 
thereof as represented by the percentage with respect to incident light 
((remaining light intensity/incident light intensity).times.100). As in 
the foregoing examples, the photoconductive layers were of amorphous 
silicon containing 12 atomic-% of hydrogen. 
TABLE 3 
______________________________________ 
Residual 
potential/ 
Thick- 
Remaining Distance Initial ness of 
light in- to opposite potential 
layer 
No. tensity (%) 
interface (.mu.m) 
(%) (.mu.m) 
______________________________________ 
1 0.5 5 3 13 
2 1 5 5 15 
3 5 5 30 16 comp. 
ex. 
4 10 5 40 18 comp. 
ex. 
______________________________________ 
From the results of Tables 2 and 3, it is understood that the residual 
potential can be sharply lowered by making the intensity of the incident 
light on the photoconductive layer at most 1% of the intensity at the 
incidence in the portion 5 .mu.m distant from the interface of the 
photoconductive layer opposite to the light incidence side thereof. 
EXAMPLE 2 
This example will be described with reference to FIG. 9. 
On a hard glass cylinder 1, a transparent electrode of SnO.sub.2 11 was 
formed by the thermodecomposition of SnCl.sub.4 at 450.degree. C. The 
resultant cylinder was installed in a rotary sputtering equipment, the 
interior of which was evacuated up to 2.times.10.sup.-6 Torr. 
Subsequently, whilst holding the cylinder at 250.degree. C., an amorphous 
silicon film (hydrogen content: 17.5 atomic-%) 23 having an optical 
forbidden band gap of 1.95 eV and a resistivity of 10.sup.11 
.OMEGA..multidot.cm was deposited to a thickness of 18 .mu.m at a 
deposition rate of 1 A/sec by a radio-frequency power of 300 W (at a 
frequency of 13.56 MHz) in a mixed atmosphere consisting of 
2.times.10.sup.-3 Torr of hydrogen and 2.times.10.sup.-3 Torr of argon. 
Thereafter, whilst holding the pressure of argon constant, the pressure of 
hydrogen was gradually lowered down to 3.times.10.sup.-5 Torr over a 
period of time of 20 minutes. The amorphous silicon at the minimum 
hydrogen pressure (hydrogen content: 9 atomic-%) had an optical forbidden 
band gap of 1.6 eV and a resistivity of 10.sup.8 .OMEGA..multidot.cm. 
Further, the hydrogen pressure was gradually raised up to 
2.times.10.sup.-3 Torr again over 20 minutes. Under this state, the 
sputtering was continued to form amorphous silicon layers 24 and 25 into 
the whole thickness of 20 .mu.m. A blocking layer of As.sub.2 Se.sub.3 or 
the like may well be inserted on the transparent electrode 11. A blocking 
layer as stated before may well be disposed on the photoconductive layer 
25. The thickness of the region whose optical forbidden band gap was less 
than 1.95 eV was approximately 2,500 A, and the distance thereof from the 
underlying substrate was 1.7 .mu.m. The cylinder was used as an 
electrophotographic drum. 
Also in case of such construction in which the region of narrow forbidden 
band gap is included in the photoconductive layer, this invention is 
effective for reducing the residual potential. 
The electrophotographic member above described had its sensitivity and 
residual potential characteristics measured with a semiconductor laser 
having an emission wavelength of 760 nm. The results are indicated in 
Table 4. 
TABLE 4 
______________________________________ 
Residual potential/ 
Sample Sensitivity (1/erg) 
Initial potential (%) 
______________________________________ 
This example. 
0.07 5 
The region of 
0.07 15 
narrow band gap 
is located in 
the middle of 
the layer. 
The region of 
&lt;0.001 &gt;80 
narrow band gap 
is not formed. 
______________________________________ 
As shown in FIG. 10, the laser beam was projected onto the photoconductive 
layer from inside the drum. In the figure, numeral 1 designates a glass 
substrate, and numeral 2 a photoconductive layer. The details of the 
construction of the electrophotographic member, such as the electrode 
etc., are omitted from the illustration. Numeral 52 indicates a 
semiconductor laser, numeral 53 a beam collecting lens, numeral 54 a 
mirror, and numeral 55 a laser beam. From the results of Table 4, it is 
understood that the present invention is extraordinarily excellent in the 
sensitivity and especially in the residual potential characteristics. 
EXAMPLE 3 
This example is a case where amorphous silicon doped with carbon is used at 
the surface and interface of a conductive layer. The fundamental structure 
is as shown in FIG. 9. 
On a polyimide film 1 a chrome film 11 was vacuum-evaported to a thickness 
of 400 A, to prepare a substrate. The resultant layer was installed in a 
sputtering equipment, the interior of which was evacauted up to 
5.times.10.sup.-7 Torr. Thereafter, whilst holding the substrate at 
150.degree. C. and by employing a target of polycrystalline silicon 
containing 10% of carbon, a film of amorphous silicon--carbon 22 having an 
optical forbidden band gap of 2.0 eV and a resistivity of 10.sup.13 
.OMEGA..multidot.cm was formed 3 .mu.m at a deposition rate of 3 A/sec 
under a radio-frequency power of 350 W in a gaseous mixture consisting of 
1.times.10.sup.-3 Torr of hydrogen and 4.times.10.sup.-3 Torr of argon. 
The hydrogen content of this film was approximately 14 atomic-%. 
Thereafter, sputtering was performed by the use of a target made up of 
silicon only and in a gaseous mixture consisting of 2.times.10.sup.-3 Torr 
of argon and 2.times.10.sup.-3 Torr of hydrogen, to form a film of 
amorphous silicon 23 having a thickness of 60 nm and exhibiting an optical 
forbidden band gap of 1.85 eV as well as a resistivity of 10.sup.11 
.OMEGA..multidot.cm. Further, on the film 23, a film 24 of the first 
amorphous silicon--carbon was formed 5 .mu.m. 
An electrophotographic member having a satisfactory resolution and good 
residual potential characteristics with respect to incident light at 650 
nm could be realized. 
EXAMPLE 4 
Reference is had to FIG. 5. 
On a hard glass cylinder 1, an SnO.sub.2 transparent electrode 11 was formd 
by the thermodecomposition of SnCl.sub.4 at 450.degree. C. The resultant 
cylinder was installed in a rotary sputtering equipment, the interior of 
which was evacuated up to approximately 2.times.10.sup.-6 Torr. 
Subsequently, whilst holding the cylinder at 250.degree. C., an amorphous 
silicon film (hydrogen content: 17.5 atomic-%) 22 was deposited 20 A by a 
radio-frequency power of 13.56 MHz and 300 W in a mixed atmosphere 
consisting of 2.times.10.sup.-3 Torr of hydrogen and 2.times.10.sup.-3 
Torr of argon. The optical forbidden band gap of this film was 1.95 eV, 
and the resistivity was 10.sup.11 .OMEGA..multidot.cm. Thereafter, using a 
sputtering target in which silicon and germanium were juxtaposed, a 
germanium-containing amorphous silicon film 23 was deposited to a 
thickness of 0.1 .mu.m. The sputtering was a gaseous mixture consisting of 
1.times.10.sup.-3 Torr of hydrogen and 2.times.10.sup.-3 Torr of argon. 
The content of germanium was 30 atomic-%, and that of hydrogen was 10 
atomic-%. In addition, the optical forbidden band gap was approximately 
1.40 eV, and the resistivity was approximately 10.sup.9 
.OMEGA..multidot.cm. Subsequently, an amorphous silicon film 24 was formed 
3 .mu.m under the same conditions as those of the first amorphous silicon 
film. The optical forbidden band gap of the film 24 was 1.95 eV, and the 
resistivity was 10.sup.11 .OMEGA..multidot.cm. When the 
germanium-containing amorphous silicon was used in this manner, an 
electrophotographic member having a satisfactory resolution and good 
residual potential characteristics with respect to illuminating light of 
580 nm projected from inside the cylinder could be realized.