JPH06342221A - Electrophotographic photoreceptor - Google Patents

Abstract

PURPOSE:To provide an amorphous electrophotographic photoreceptor with a photosensitive layer, a surface protective layer capable of satisfactorily and stably protecting the photosensitive layer without deteriorating image quality even after copying many times and a p-type amorphous silicon layer inhibiting the formation of a space-charge region. CONSTITUTION:A photosensitive layer 2 of amorphous silicon, a p-type amorphous silicon layer 4 and an insulating surface protective layer 3 made of an amorphous silicon nitride film are successively laminated on an electric conductive substrate 1 by plasma CVD. The dark resistivity of the photosensitive layer 2 is >=10<12>OMEGA.cm, the thickness of the p-type amorphous silicon layer 4 is 30-1,000Angstrom , the dark resistivity of the layer 4 is >=10<11>OMEGA.cm and the thickness of the surface protective layer 3 is 500-10,000Angstrom . The resulting electrophotoreceptor proves that it enables copying >=100,000 sheets by a copying test.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an electrophotographic photoconductor, and more particularly to an amorphous silicon electrophotographic photoconductor.

[0002]

2. Description of the Related Art Amorphous silicon (amorphous silicon, which may hereinafter be abbreviated as a-Si) obtained by a plasma CVD method using SiH ₄ (monosilane) gas is a hydrogen atom incorporated therein. In 1976, it was announced by Spear that Si could be a useful semiconductor material whose conduction type and carrier concentration can be controlled by reducing the localized level by bonding to Si dangling bonds. (Applied PhysicsLetter, Vol. .28, No.2, 1976, J
an.).

Subsequent research has revealed that a-Si is becoming an indispensable material for manufacturing semiconductor elements such as solar cells and thin film transistors, because a large-area film can be obtained at low cost. Since the a-Si film has excellent properties of no pollution, high sensitivity, and long life, its application to an electrophotographic photoreceptor has been considered. However, the resistance value of the a-Si film at the early stage of development was not as high as required for the photoconductor, so that the practical use of the a-Si film as the electrophotographic photoconductor was slowed down. That is, if the photoreceptor does not have a high resistance, even if the surface of the a-Si film is charged by corona discharge, the dark decay is large and the charge retention characteristic is deteriorated. Regarding this point, it is conceivable to form a pn junction near the surface layer to increase the resistance by utilizing the fact that the conductivity type can be controlled, but there are various problems and it has not yet been put to practical use.

[0004]

SUMMARY OF THE INVENTION The present inventors
Attempts were made to improve the charge retention characteristics by increasing the resistivity of the -Si film itself, and succeeded in obtaining an a-Si photosensitive member having a high resistance comparable to that of the Se-based photosensitive member. It was disclosed in Japanese Patent No. 37352. According to it, S
iH ₄ gas with N ₂ (nitrogen) gas, B ₂ H ₆ (diborane)
While mixing an appropriate amount of gas, a by the plasma CVD method
-Si film is obtained, and the a-Si film thus obtained is
Remarkably high resistance and good photosensitivity characteristics, and excellent image formation was actually achieved. However, from a practical point of view, the life is not always satisfactory.
The cause was considered to be as follows.

That is, in an apparatus such as a copying machine or a printer, the surface of the photoconductor is directly subjected to various stimuli. For example, ozone or nitride adsorption caused by corona discharge and chemical action caused by adhesion of chemically active species caused by moisture and toner in the air with these, physical action caused by scratching by cleaning plate or friction with paper, handling When the stimulus is used for a long period of time, white stripes, white spot defects, image blurring, fog, etc. may occur. Therefore, the image quality is significantly deteriorated.

Therefore, the present inventors use the same material gas in the same apparatus as the apparatus for manufacturing the a-Si photosensitive layer as a method for protecting the a-Si film, and simply change the operating conditions such as the flow rate and the supplied power value. A method for continuously forming an amorphous silicon nitride film on an a-Si film has been proposed and disclosed in Japanese Patent Laid-Open No. 58-145.
It was disclosed in Japanese Patent Publication No. 951. By forming this amorphous silicon nitride film as a surface protective layer, durability,
In terms of image formation and life, the a-Si photoreceptor disclosed in JP-A-57-37352 has reached the range of practical use. At present, not only an amorphous silicon nitride film but also a film of amorphous silicon oxide, amorphous silicon carbide or the like is being studied as the surface protection layer.

The relationship between the surface protective layer and the characteristics of the photoconductor will be further described with reference to the conventional structure with reference to FIG. Figure 2
2A is a partial cross-sectional view of a conventional a-Si photoconductor, FIG.
(B)-(d) is a schematic diagram of the energy band structure in different situations. 1 is a conductive substrate such as Al (aluminum), 2 is a thickness of 1 to 1 formed on the conductive substrate 1.
It is a 50-micrometer a-Si photosensitive layer. The a-Si photosensitive layer 2 is formed by decomposing SiH ₄ gas mixed with N ₂ gas, B ₂ H ₆ gas, and optionally PH ₃ (phosphine) gas by a plasma CVD method, and is a film containing hydrogen atoms. Yes 1
It has a high resistivity of 0 ¹² Ω · cm or more. Reference numeral 3 is a surface protective layer having a thickness of 0.01 to 1 μm and formed on the a-Si photosensitive layer 2 and having an insulating property. The surface protective layer 3 is an amorphous silicon nitride film continuously formed by using SiH ₄ gas and N ₂ gas, and as shown in the energy band structure diagram described below, the a-Si photosensitive layer is formed. Wider band than 2.

The energy band structure in the equilibrium state before use of the a-Si photoconductor of FIG. 2A is shown in FIG. 2B, and the surface of the a-Si photoconductor of FIG. The energy band structure when positively charged is shown in FIG. In the figure, E _F is the Fermi level, E _V is the top of the valence band, and E _C is the bottom of the conduction band. FIG. 2D shows a state of carrier generation when image light is incident on the a-Si photoconductor of FIG. 2A.
In the i photosensitive layer 2, electron-hole pairs are generated, electrons flow to the surface side and holes flow to the conductive substrate 1 side, and neutralize the charges on the conductive substrate 1 and the surface, respectively.

When the surface protective layer 3 is present, surface electrons can be neutralized when electrons move and pass through the surface protective layer 3 by the tunnel effect to reach the surface. However, for example, if the thickness of the surface protective layer 3 is large and electrons cannot pass through the barrier of the surface protective layer 3, the electrons are trapped at the interface S between the a-Si photosensitive layer 2 and the surface protective layer 3 and remain due to the surface charge that is not neutralized. The magnitude of the electric potential is determined. Initially, the surface protective layer 3 having an insulating property was emphasized for the function of protecting the surface of the a-Si photosensitive layer 2, but the a-Si photosensitive layer 2
Of the a-Si photosensitive layer 2 when the resistivity of the
The role as a blocking layer that prevents the surface charge from being neutralized by the transfer injection of the carriers from the above was also important. For this reason, if the surface protective layer 3 is formed thick, carrier tunneling cannot be performed and the residual potential becomes extremely large.

As a result, as shown in FIG.
A space charge region is formed in the vicinity of the interface S between the Si photosensitive layer 2 and the surface protective layer 3, which further blocks the movement of carriers, so that the thickness of the surface protective layer 3 is several tens of liters, and even if it is thick,
It had to be extremely thin, less than 000Å. Therefore, the surface protection function for the a-Si photosensitive layer 2 was not sufficient. On the contrary, when the thickness of the surface protective layer 3 exceeds a certain limit, the Carlson method can no longer be adopted and the NP
We have to consider a completely different copying method such as the law. However, the inventors of the present invention have sufficiently increased the resistivity of the a-Si photosensitive layer 2 by the method disclosed in the above-mentioned Japanese Patent Laid-Open No. 57-37352. Since a thick surface protection layer is formed and the residual potential can be reduced to a small extent, the ratio to the whole can be reduced, so that a sufficient S / N ratio can be obtained. Therefore, the Carlson method can be adopted,
The purpose of surface protection was achieved, and at the same time, the life was extended.

However, the inventors of the present invention formed an amorphous silicon nitride film as a surface protection layer, prototyped a copying machine, and further repeated accurate experiments. As a result, depending on the formation conditions of the amorphous silicon nitride film,
It turned out that an extremely inconvenient case might occur, and it was found that not only the thickness but also the composition was required as the surface protective film. That is, the present invention is for electrophotography having an insulating protective layer of an amorphous silicon nitride film having a structure and composition capable of sufficiently stably protecting a photoconductor even if it is used for a long period of time without degrading image quality. To provide a photoconductor.

[0012]

The present invention provides an amorphous silicon electrophotographic photoreceptor having an amorphous silicon photosensitive layer, a p-type amorphous silicon layer and a surface protective layer on a substrate, wherein the p-type amorphous silicon layer is the amorphous silicon. There is a thickness of 30 to 1000Å between the photosensitive layer and the surface protective layer, and the height of the bottom of the conduction band with respect to the Fermi level at the equilibrium state of the interface between the p-type amorphous silicon layer and the amorphous silicon photosensitive layer. Is not less than the height of the bottom of the conduction band with respect to the Fermi level of the amorphous silicon photosensitive layer in the equilibrium state of the interface, and the thickness of the surface protective layer is 5
The height of the bottom of the conduction band with respect to the Fermi level in the equilibrium state of the interface of the surface protective layer with the p-type amorphous silicon layer is the p-type in the equilibrium state of the interface. It is characterized in that it is higher than the height of the bottom of the conduction band with respect to the Fermi level of the amorphous silicon layer.

[0013]

The structure, function, etc. of the present invention will be described in detail below along with experimental examples. When the amorphous silicon nitride film is considered as the surface protective film, the most stable film chemically and structurally is considered to be the best stoichiometric Si ₃ H ₄ film. However, in the plasma CVD method, it is difficult to form a Si ₃ H ₄ film having a stoichiometric composition, and the composition ratio Si / N is 0.8 to 1.5 as well as hydrogen is included, which is stoichiometric. It is customary for Si to be slightly over the composition ratio Si / N = 0.75. In order to approximate the stoichiometric composition of Si ₃ H ₄ as much as possible, the ratio of N ₂ gas to SiH ₄ gas, the high frequency power, and the substrate temperature should be as high as possible. Therefore, SiH ₄ gas on the substrate,
After mixing an appropriate amount of N ₂ gas and B ₂ H ₆ gas to form an amorphous silicon photosensitive layer having a certain thickness under certain conditions,
By changing the generation conditions, an amorphous silicon nitride layer as a surface protective layer was formed on the layer with a constant thickness, and the actual copying performance was investigated. The results will be described below.

(A) First, an amorphous silicon nitride film having a stoichiometric composition of Si ₃ N ₄ was formed as close as possible. The thickness of the surface protective layer is 1500Å, the formed film has a composition ratio Si / N = 0.8, and a resistivity = 10 ¹⁵ Ω · c.
m, optical band gap E _o = 5 eV. When this was used as a photoconductor for copying, a clear image was obtained on the first sheet, but the image began to blur significantly from the second consecutive sheet, and after several sheets it was almost unusable. Atsuta This photoreceptor was charged with the opposite polarity and the surface potential was set to zero, and copying was carried out again. However, after the second copy, the same phenomenon as described above was exhibited, and in any case, the role as the surface protective layer was sufficiently stable. It was found that it is impossible to continuously obtain high image quality even if it can be achieved.

(B) Next, an amorphous silicon nitride film deviating from the stoichiometric composition of Si ₃ N ₄ was formed with the same 1500 Å thickness as in the experiment (A) by changing the gas flow rate and the high frequency power. This film has a composition ratio Si / N = 1.2, a resistivity = 2 × 10 ¹⁴ Ω · cm, and an optical band gap E _o =
It was 3.8 eV. When copying was performed using this as a photoconductor, a clearer image than the first sheet was obtained, and there was no particular problem in continuous copying. However, as a result of repeating the intermittent durability test, white streaks and image unevenness were conspicuous from about the 20,000th sheet, so that the life was not yet satisfactory. From the experimental results of (A) and (B) above, the closer the composition of the amorphous silicon nitride film as the surface protective layer is to the stable composition of Si ₃ N ₄ , the more the original characteristics of the photoreceptor are impaired. I found out. As a result of examination with reference to the results of other experiments, it was concluded that the following was the cause.

The forbidden band width of the a-Si film as the photosensitive layer is 1.
While the band gap is 7 to 1.9 eV, the forbidden band width of the amorphous silicon nitride film as the surface protective layer having an insulating property is larger than this, so that the interface S as shown in FIGS. When a barrier is formed in the photosensitive layer and the carrier generated in the photosensitive layer moves near the surface and the carrier tunnels through the surface protective layer to neutralize the surface charge, the barrier has a certain height and width. Must be: In the case where it is difficult for the carrier to quickly pass through the surface protective layer, the carrier is accumulated at the interface S between the a-Si photosensitive layer and the surface protective layer due to repeated irradiation of incident light. A space charge region as shown in (d) is formed.

Since the energy band is bent in this region, a new barrier is formed and the movement of the carrier to the surface is further blocked, so that the residual potential is increased, the photosensitivity is lowered, and the charge level is lowered. Bring Not only that, the carriers accumulated in the vicinity of the interface S form conductive channels parallel to the surface, so that the carriers are easily dispersed. Further, during transfer charging, the carrier is shaken and charge coupling is also performed through the insulating protective layer,
As a result, the resolution is lowered and the image is considered to be blurred. That is, as shown in FIG. 3A, it is possible to charge the surface of the photoconductor to a uniform potential V _S and to make a sharp change in potential when divided into bright and dark places at a constant spatial frequency. If so, the resolution is excellent and no screen blur occurs. However, if the surface protective layer is not suitable, the result shown in FIG.
It has been found that, as shown in (4), the change in light and dark becomes gradual, the resolution is inferior, the residual potential V _R is generated, and the image is blurred.

As a countermeasure against this, when the surface protective layer having the composition in the experiment (A) is used, a method for neutralizing the space charge for each copy, such as reverse characteristic charging or simultaneous exposure AC charging, is used. It is possible to do it. However, setting of working conditions for that purpose is complicated and precise, and is not practical. Further, in order to avoid this, if the conditions of the surface protective layer are loosened and the height of the barrier is lowered to give a slight conductivity as in the experiment (B), the copying characteristics are temporarily good. However, it was shown that the surface protection ability was lowered and it could not withstand long-term use.

As described above, the problems of the conventional photoconductor are as follows.
It was found that the height of the barrier is due to the difference in the forbidden band width of the surface protective layer compared with the forbidden band width of the photosensitive layer, and the abrupt change of the barrier and the generation of the space charge region due to the carriers trapped and accumulated at the interface. Therefore, the inventors of the present invention provide an amorphous silicon electrophotographic photoreceptor that can substantially avoid the formation of space charge regions and conductive channels by further providing a p-type amorphous silicon layer between the photosensitive layer and the surface protective layer. It came to invent.

[0020]

1 shows an embodiment of an amorphous silicon electrophotographic photosensitive member of the present invention. FIG. 1 (a) is a partial sectional view thereof, and FIG. 1 (b) is a schematic view of its energy band structure. ,
In the figure, reference numerals 1 to 3, E _C , E _F and E _V are the same as in FIG. Then, 4 is a p-type amorphous silicon layer formed between the amorphous silicon photosensitive layer 2 and the surface protective layer 3. The amorphous silicon photosensitive layer 2 on the conductive substrate 1 is made of N ₂ gas, B ₂ H ₆ gas, and optionally PH ₃ gas.
It is formed by a plasma CVD method using SiH ₄ gas mixed with gas. To form the p-type amorphous silicon layer 4 thereabove, B ₂ H ₆ gas or the like may be used as an impurity gas. However, when a strong p-type is used, a decrease in surface resistance due to p-type conduction adversely affects image quality. Therefore, it is necessary to have an appropriate resistivity and thickness. results of the experiment,
The resistivity of the p-type amorphous silicon layer 4 is 10 ¹¹ Ω · c.
It is desirable to have a dark resistivity of m or more, and the thickness is considered to be in the range of 30 to 1000Å.

The energy band structure of the photoconductor of this embodiment is shown in FIG. 1 (b). That is, the p-type amorphous silicon layer 4 and the amorphous silicon photosensitive layer 2
P-type level of the bottom portion E _C of the conduction band relative to the Fermi level of the amorphous silicon layer 4, the conduction band of the amorphous silicon photosensitive layer 2 in the equilibrium state of the interface S ₁ for the Fermi level in an equilibrium state of the interface S ₁ between the Is higher than the height of the bottom E _C of the. Further, the height of the bottom E _C of the conduction band with respect to the Fermi level of the surface protective layer 3 at the equilibrium state of the interface S ₂ between the surface protective layer 3 and the p-type amorphous silicon layer 4 is p at the equilibrium state of the interface S _2. The height is equal to or higher than the height of the bottom E _C of the conduction band with respect to the Fermi level of the amorphous silicon layer 4. Further, the height of the bottom E _C of the conduction band with respect to the Fermi level E _F in the equilibrium state of the surface protective layer 3 is
The height of the bottom E _C of the conduction band with respect to the Fermi level E _F of the amorphous silicon photosensitive layer 2 gradually increases from the interface S ₂ toward the surface of the surface protective layer 3. Although it is continuously and gradually increased in the figure, it may be formed gradually and gradually.

The method of forming the amorphous silicon photosensitive layer 2 and the thickness thereof are the same as those described in the operation. That is,
An amorphous silicon photosensitive layer 2 is formed on a conductive substrate 1 by using a SiH ₄ gas, an N ₂ gas, and a B ₂ H ₆ gas by a plasma CVD method. The dark resistivity of the amorphous silicon photosensitive layer 2 is 10 ¹² Ω · cm or more. In forming the amorphous silicon nitride film which is the surface protective layer 3, first, the production conditions were set to the same conditions as in the experiment (B) described in the operation, and the composition was changed by continuously changing the conditions. Finally, the production conditions were set to be the same as those in the experiment (A) described in the operation, and the amorphous silicon nitride film was set to 1500 Å
Formed to a thickness of. The composition ratio Si / N of this amorphous silicon nitride film changed substantially continuously from 1.2 to 0.8 toward the surface.

As a result of a copying test using the photoconductor of the present embodiment (the thickness of the amorphous silicon nitride film is 1500Å), a clearer image than that of the first sheet was obtained, and no problems occurred in continuous copying. It was Further, even in the long-term intermittent durability test, no problematic defect occurred up to the 100,000th sheet, and the life was completely satisfactory. As a result of various experiments, it was found that similar test results could be obtained if the thickness of the amorphous silicon nitride film was in the range of 500 to 10000Å and the band gap was widened in the range of 2.0 to 5.0 eV. It was

[0024]

As described above, in the electrophotographic photoreceptor of the present invention, by providing the p-type amorphous silicon layer between the amorphous silicon photosensitive layer and the surface protective layer which is an amorphous silicon nitride film, an image can be obtained. The formation of space charge regions and conductive channels that contribute to blur can be substantially avoided. Further, since the height of the bottom of the conduction band with respect to the Fermi level in the equilibrium state of the surface protective layer is formed so as to gradually increase toward the surface of the surface protective layer itself, it is sufficient for long-term use. With the endurance and formation of the p-type amorphous silicon layer, good copying can be achieved without causing deterioration of image quality.
In addition, it was demonstrated that the surface protection layer can protect the amorphous silicon photosensitive layer sufficiently stably, and can copy 100,000 sheets or more. Although the present invention has been described above focusing on the amorphous silicon nitride film, it goes without saying that the present invention is also applied to a surface protective layer using an amorphous silicon oxide film or an amorphous silicon carbide film.

[Brief description of drawings]

FIG. 1A is a partial cross-sectional view of an example of an electrophotographic photoreceptor of the present invention. (B) A schematic view of an energy band structure of an example of the electrophotographic photosensitive member of the present invention.

FIG. 2A is a partial cross-sectional view of a conventional a-Si photoconductor. (B) It is a schematic diagram of the energy band structure of the conventional a-Si photoreceptor. (C) It is a schematic diagram of the energy band structure of the conventional a-Si photoreceptor. (D) It is a schematic diagram of the energy band structure of the conventional a-Si photoreceptor.

FIG. 3A is a waveform diagram for explaining a charging characteristic, a resolving power, and a residual potential of a photoconductor (a waveform when there is no image blur). FIG. 6B is a waveform diagram for explaining the charging characteristics, resolution, and residual potential of the photoconductor (waveform when image blur occurs).

[Explanation of symbols]

1 the conductive substrate 2 amorphous silicon (a-Si) photosensitive layer 3 surface protective layer or insulating protective layer 4 p-type amorphous silicon layer E _F Fermi level E _V valence band bottom V _S uniform top E _C conduction band of Potential S interface S ₁ interface S ₂ interface

Claims (2)

[Claims] 1. An amorphous silicon photosensitive layer on a substrate,
An amorphous silicon electrophotographic photosensitive member having a p-type amorphous silicon layer and a surface protective layer, wherein p
The amorphous amorphous silicon layer has a thickness of 30 to 1000Å between the amorphous silicon photosensitive layer and the surface protective layer, and the Fermi quasi-state at the equilibrium state of the interface between the p-type amorphous silicon layer and the amorphous silicon photosensitive layer. The height of the bottom of the conduction band with respect to the position is equal to or higher than the height of the bottom of the conduction band with respect to the Fermi level of the amorphous silicon photosensitive layer in the equilibrium state of the interface, and the thickness of the surface protective layer is 500 to 10000Å. The height of the bottom of the conduction band with respect to the Fermi level at the equilibrium state of the interface of the surface protection layer with the p-type amorphous silicon layer is set to the range of the p-type amorphous silicon layer at the equilibrium state of the interface. Electrophotographic photosensitive material having a height equal to or higher than the bottom of the conduction band with respect to the Fermi level. . 2. The surface protective layer is formed of amorphous silicon nitride containing hydrogen and has a composition ratio Si / N of 0.75.
And the composition ratio Si / N.
2. The electrophotographic photosensitive member according to claim 1, wherein is gradually reduced from the interface with the p-type amorphous silicon layer toward the surface of the surface protective layer.