JPH0117145B2 - - Google Patents

Description

Detailed Description of the Invention [Field of Application of the Invention] The present invention relates to a photoconductor layer used in an electrophotographic photosensitive plate, and in particular to dark decay characteristics and photosensitivity characteristics of a photoconductor layer using amorphous silicon. This is related to the improvement of.

[Background of the invention]

Conventionally, photoconductive materials used as electrophotographic photosensitive films include inorganic materials such as Se, CdS, and ZnO;
Organic substances include poly-N vinyl carbazole (PVK) and trinitrofluorenone (TNF). Although these materials have high photoconductivity, when a photoresist film is formed using these materials themselves or when a photoresist film is formed by dispersing powder of these materials in an organic binder, the hardness of the film is This was not sufficient and had the disadvantage that the surface of the film was scratched or abraded during use as an electrophotographic photosensitive film. Further, many of these materials are harmful to the human body, and it is not desirable for even a small amount to wear out and adhere to the copy paper. In order to improve these drawbacks, it has been proposed to use amorphous silicon as a photosensitive film (for example, Japanese Patent Application Laid-Open No. 78135/1983). Since the amorphous silicon film has higher hardness than the conventional photoresist film and has almost no toxicity, the drawbacks of the conventional photoresist film can be improved.
However, amorphous silicon films have too low dark resistance to be used as electrophotographic photosensitive films, and films with a high resistance of about 10 ¹⁰ Ω・cm have too low photoelectric gain, making them unsatisfactory as electrophotographic photosensitive films. Nakatsuta.
To overcome this drawback, n-type, n ⁺ type, p-type,
A film structure has been proposed in which two or more types of amorphous silicon films of different conductivity types, such as ^p Kaisho 54-121743
Publication No.). However, when forming a depletion layer by bonding two or more films with different conductivities, it is difficult to form a depletion layer on the surface of the photoresist film, so it is necessary to maintain the charge pattern. The specific resistance of the critical surface portion of the photoresist film decreases, causing a lateral flow of the charge pattern, which may result in a decrease in the resolution of electrophotography.

[Purpose of the invention]

The present invention eliminates the above-mentioned drawbacks, and provides an amorphous material that has no fear of resolution deterioration, has benign dark decay characteristics, and can increase sensitivity to long wavelength light.
The purpose of the present invention is to provide a silicon electrophotographic photosensitive film.

[Summary of the invention]

In order to achieve the above object, as an electrophotographic photosensitive film, a portion having a thickness of at least 10 nm from the surface (or interface) of the amorphous silicon photoconductor layer constituting the film toward the inside of the photoconductor layer is optically protected. It is made of amorphous silicon with a forbidden band width of 1.6 eV or more and a specific resistance of 10 ¹⁰ Ω・cm or more, and the optical band gap is small in the part that is not in contact with the surface (or interface).
It is preferable to adopt a structure made of amorphous silicon which has a voltage of 1.1 eV or more and does not exceed the optical band gap of the amorphous silicon layer forming the surface of the photoconductor layer.

An amorphous silicon film made purely of silicon element has a high local level density and has almost no photoconductivity. However, by adding hydrogen to this, the localized level can be significantly reduced,
High photoconductivity or adding impurities to p
It can be of a conductivity type such as type type or n type. Furthermore, by adding hydrogen to amorphous silicon, it is possible to significantly increase the optical band gap of amorphous silicon and increase the specific resistance.

A well-known method for forming hydrogen-containing amorphous silicon (usually denoted a-Si:H) is the glow discharge method by low-temperature decomposition of monosilane _SiH4 in a hydrogen-containing atmosphere. These methods include reactive sputtering, which performs sputter deposition of silicon, and ion plating. Amorphous silicon films prepared by these methods usually contain several to several tens of atomic percent of hydrogen, and their optical band gap is considerably larger than the 1.1 eV of pure silicon.

By the way, the localized level density in pure amorphous silicon that does not contain hydrogen is estimated to be about 10 ²⁰ /cm ³ , and when hydrogen is added to this, the hydrogen atoms increase the localized level in a 1:1 ratio. If it were to be quenched, all localized levels should be quenched by adding about 0.1 atomic percent of hydrogen. However, in reality, photoconductivity appears, a change in the optical bandgap width occurs, and amorphous silicon, which is useful as a photoconductor, is obtained when the hydrogen concentration is 1 atomic % for silicon of 50 atomic % or more. It becomes from around the time when it exceeds.

In order to change the hydrogen concentration in an amorphous silicon film, it is necessary to control the substrate temperature, hydrogen concentration in the atmosphere, input power, etc. when forming the film using the various film formation methods mentioned above. good. Among the above-mentioned film forming methods, the reactive sputtering method has particularly good process controllability and can easily produce a high-resistance, high-quality photoconductive amorphous silicon film.

The present inventors have obtained a-Si:H having a specific resistance of 10 ¹⁰ Ω·cm or more, which can be used as an electrophotographic photosensitive film, by reactive sputtering of silicon in a mixed atmosphere of argon and hydrogen. was completed.
This film has high photoconductivity as well as high resistance, and is a so-called intrinsic semiconductor with a Fermi level near the center of the forbidden band. In semiconductors with a constant forbidden band width, the intrinsic (i) usually has the highest resistivity.
Therefore, the amorphous silicon film produced by the above sputtering method is suitable as an electrophotographic photosensitive film. A method previously proposed by the present inventors utilized such properties of an amorphous silicon film.

In the first place, in an accumulation-type light-receiving device such as an electrophotographic photosensitive film, the resistivity of the photosensitive film must satisfy the following two requirements.

(1) The specific resistance of the film is 10 ¹⁰ Ω to prevent charges deposited on the photoresist film surface by corona discharge from discharging through the thickness of the film before exposure.
It needs to be about cm or more.

(2) After exposure, the charge pattern formed on the surface (and interface) of the photoresist film is caused by horizontal charge flow.
The surface resistance of the photosensitive film must also be sufficiently high so that it does not disappear before development. If this is converted into specific resistance, it will be approximately 10 ¹⁰ Ω・cm or more, as in the previous section.

In order to satisfy the condition in item 2 above, the resistivity near the surface of the photoresist film that retains charge must be approximately 10 ¹⁰ Ω・cm or more, but the resistivity in the thickness direction of the film must be uniformly 10 ¹⁰ Ω・cm. It is not necessarily necessary to have a specific resistance higher than that. The time constant of dark decay in the thickness direction of the film is τ, and the capacitance per unit area of the film is C,
If R is the resistance in the thickness direction per unit area, then the relationship τ = RC (1) holds, and it is sufficient that τ is sufficiently long compared to the time from charging to the phenomenon, and R is the resistance in the thickness direction of the film. It just needs to be large enough from a macro perspective.

According to the present inventors, in a high-resistance thin film device such as an electrophotographic photosensitive film, the macroscopic resistance in the film thickness direction includes not only the specific resistance of the film itself but also the charge injected from the interface with the electrode. It became clear that they played an important role.

In order to prevent charge injection from the substrate side that holds the photoresist film, a junction such as a pn junction is created in the amorphous silicon film close to the substrate and reverse biased by an external electric field. Another method is possible, but this method makes it difficult to satisfy the above-mentioned requirement (2).

Therefore, the inventors discovered that the original resistance film was 10 ¹⁰ Ω・
We thought of solving this problem by using amorphous silicon with a high resistance of more than cm. This high-resistance region is usually an intrinsic semiconductor (i-type), but this region serves as a layer to prevent charge injection from the electrode to the photoresist film, and at the same time is used as a charged charge retention layer on the surface. Then it is valid.
Incidentally, the thickness of this high-resistance amorphous silicon film must be 10 nm or more to prevent charges from passing through this region due to the tunnel effect. Furthermore, in order to effectively prevent charge injection from the electrode, SiO ₂ , CeO ₂ , Sb ₂ S ₃ , Sb ₂ Se ₃ ,
It is also effective to interpose a thin film of As ₂ S ₃ , As ₂ Se ₃ or the like.

Now, as already mentioned, as an electrophotographic photosensitive film, the specific resistance near the surface (or interface) of the photosensitive film must be sufficiently high, but the specific resistance inside the film does not necessarily need to be high. It is sufficient that the macroscopic resistance R of the photoresist film satisfies equation (1).

Utilizing this fact, the spectral sensitivity characteristics of the photoresist film can be improved. In other words, an a-Si:H film with a high specific resistance of 10 ¹⁰ Ω・cm or more usually
Since it has an optical band gap of about 1.7 eV, it has no sensitivity to light with wavelengths longer than the long wavelength region of visible light. This is very inconvenient when the a-Si:H film is used as a photosensitive film for a laser beam printer using a semiconductor radar having a wavelength around 800 nm.

In order to improve this point, by creating an amorphous silicon region with a small optical band gap in the inner part of the photoresist film, the sensitivity to incident light is increased in the narrow inner part of the optical band gap, and dark attenuation is achieved. A good electrophotographic photosensitive film can be provided by improving the characteristics by using an amorphous silicon layer with high resistance and a wide optical bandgap provided on the surface and the substrate interface side.

A band model of a membrane with such a structure is shown in FIG. The constricted part of the band in the figure is a region created to be sensitive to long wavelengths, and the long wavelength light absorbed at the constricted part generates electron-hole pairs inside the constricted part, and the generated carrier is initiated by an external electric field.

Although such a method was extremely effective in improving the spectral sensitivity characteristics of a photoresist film, there were technical difficulties in actually implementing it. In other words, in this method, the constriction was created by controlling the amount of hydrogen contained in the amorphous silicon film. Figure 2 shows the relationship between the hydrogen partial pressure in the atmospheric gas and the optical bandgap width of the a-Si:H film created using the reactive sputtering method. be. By utilizing this relationship and forming the film under a small amount of hydrogen partial pressure, it is possible to form the constricted portion in the amorphous silicon photoresist film.

However, in practice, this method has problems in the following points. One is that, as shown in Figure 2, in regions with a small amount of hydrogen, the optical band gap changes greatly due to slight movements in the hydrogen partial pressure, making it difficult to create the desired value constriction accurately. That's funny. Furthermore, if the amount of hydrogen contained in amorphous silicon is small, such as 1 atomic % or less, the localized levels within the amorphous silicon cannot be completely eliminated, and as a result, the photoconductivity deteriorates. It is something you end up doing. That is,
The amount of hydrogen in a-Si:H cannot be extremely reduced, and as a result, there is a limit to the size of the constriction of the forbidden band width.

The present invention aims to further improve this point, and is characterized by adding a halogen element to the amorphous silicon in the constricted portion. The amount of halogen element added must be 0.1 atomic% or more,
It is preferably 30 atomic % or less. On the other hand, hydrogen may or may not be added. When added, it is preferably 20 atomic % or less, particularly preferably 10 atomic % or less.

Elements that have the effect of reducing the localized level density in amorphous silicon include hydrogen, fluorine, chlorine, bromine, iodine, and other so-called halogen group elements.
Although it has the effect of reducing the localized level density in silicon, it does not significantly change the optical band gap of amorphous silicon. For this reason, for example, fluorine-doped amorphous silicon exhibits high photoconductivity and has an optical band gap of about 1.1 eV, making it suitable as a photoresist film for semiconductor lasers. However, although the localized levels of amorphous silicon doped with 1 atomic percent or more of fluorine are reduced, the specific resistance is lower than that of hydrogen-added silicon, making it difficult to obtain a film of 10 ¹⁰ Ω·cm. In this respect, it can be said that the most effective method for use in electrophotography is to sandwich the surface side and the substrate side between hydrogen-containing amorphous silicon as in the present invention.

[Embodiments of the invention]

The specific structure of an electrophotographic photosensitive film having an amorphous silicon photoconductor layer will be described below.

In FIG. 3, 1 is a substrate and 2 is a photosensitive layer comprising an amorphous silicon layer. The substrate 1 may be a metal plate such as aluminum, stainless steel, or nichrome plate, or an organic material such as polyimide, or glass or ceramics, but if the substrate is an electrical insulating film, an electrode is placed on the substrate as shown in 11. need to be covered. The electrodes are thin films of metal materials such as aluminum and chromium,
An oxide-based transparent electrode such as SnO ₂ In--Sn--O is used. A photosensitive layer 2 is provided on this,
When the substrate 1 is translucent and the electrode 11 is transparent, the light incident on the photosensitive layer 2 may be irradiated through the substrate 1. The photosensitive layer 2 includes a layer 2 for suppressing injection of excessive carriers from the substrate side.
2 can also be provided. Materials for layer 21 and layer 22 include SiO, SiO ₂ , Al ₂ O ₃ , CeO ₂ ,
Layers of highly resistive oxides, sulfides, and selenides such as V ₂ O ₃ , Ta ₂ O, As ₂ Se ₃ , As ₂ S ₃ and, in some cases, organic layers such as polyvinyl carbazole are added. used. Although these layers 21 and 22 are intended to improve the electrophotographic properties of the photoresist film of the present invention, they are not necessarily essential. Layers 23, 24, and 25 are all layers mainly composed of amorphous silicon, and each layer 23, 25 has an optical band gap of 1.6 eV or more, and a specific resistance of 10 ¹⁰ Ω・cm or more. It is a layer with a thickness of 10 nm or more. The layer 24 also has an optical bandgap width.
The layer has a voltage of 1.1 eV or more and does not exceed the optical band gap of layers 23 and 25, and has a film thickness of
It has a diameter of 10 nm or more. The specific resistance of the layer 24 is 10 ¹⁰ Ω・
Even if the thickness is less than cm, the presence of layers 23 and 25 does not adversely affect the dark decay characteristics of the electrophotographic photosensitive film. In addition, carbon, trace amounts of boron, germanium, etc. may be added to the amorphous silicon layer for the purpose of changing the electrical and optical properties of the film; The inclusion of any other element is necessary to ensure photoconductive properties, and as long as the requirement is satisfied, a film prepared even if it contains any other element is included within the scope of the present invention. Furthermore, layer 24
It is also within the scope of the present invention to control the optical band gap by including some hydrogen in the portion.

As mentioned at the beginning, known methods for forming an amorphous silicon film containing hydrogen include a method using decomposition of SiH ₄ by glow discharge, anti-performance sputtering method, and ion plating method. Regardless of the method used, a film with the best photoelectric conversion properties can be obtained when the substrate temperature during film formation is 150-250°C, but in the case of the glow discharge method, the film with good photoelectric conversion properties has a specific resistance. is 10 ⁶
Since it is low at ~10 ⁷ Ω·cm and is not suitable for electrophotography, consideration must be given to doping a small amount of boron to increase the resistivity. On the other hand, according to the reactive sputtering method, a film with good photoelectric conversion properties and a specific resistance of 10 ¹⁰ Ωcm or more can be obtained, and by using a sputtering target with a sufficiently large area. Since it is possible to form a uniform film over a large area, it can be said to be particularly useful for forming photosensitive films for electrophotography.

Usually, reactive sputtering is carried out using an apparatus as shown in Fig. 4, in which 31 is a bell gear, 32 is an exhaust system, 33 is a high frequency power supply, 34 is a sputtering target, 35 is a substrate holder,
36 is a substrate. Note that some sputtering equipment is not only for sputter deposition on a flat substrate as shown in this figure, but also has a structure that allows sputter deposition on a cylindrical drum-shaped substrate. You can use it properly.

Well, reactive sputtering is Bergier 31.
This is carried out by evacuating the inside of the chamber, introducing an inert gas such as hydrogen, silicon tetrafluoride, and argon into the chamber, and supplying a high frequency voltage from the high frequency power source 33 to cause discharge. The amount of hydrogen contained in the film formed at this time is approximately determined by the partial pressure of hydrogen present in the atmospheric gas during discharge, and the amorphous silicon film containing hydrogen suitable for the present invention is , hydrogen partial pressure during sputtering is 5×
10 ^-5 Torr to 5×10 ^-2 Torr, and the layer 24 is made of silicon tetrafluoride at 5×
It is obtained when the partial pressure is set in the range of 10 ^-5 Torr to 1×10 ^-2 Torr.

The present invention will be specifically explained below using examples.

Example 1 An aluminum cylinder with a mirror-polished surface was heated at 300°C for 2 hours in an oxygen atmosphere, and the surface of the cylinder was heated to a mirror-polished surface.
Forms an Al ₂ O ₃ film. Installed in the cylindrical rotary magnetron sputtering device, 1×10 ^-6 Torr
After evacuating the cylinder to 200°C,
In a mixed atmosphere of hydrogen at 10 ^-3 Torr and argon at 3 × 10 ^-3 Torr, a high frequency power of 350W
An amorphous silicon film with a resistivity of 10 ¹⁰ Ω·cm and an optical band gap of 1.95 eV is deposited to a thickness of 500 Å at a deposition rate of Å/sec. Over the next five minutes,
While keeping the partial pressure of argon constant, the partial pressure of hydrogen was gradually lowered to 1×10 ^-5 Torr, and at the same time, the partial pressure of silicon tetrafluoride was gradually increased to 1×
10 ^-4 Torr. The optical band gap of amorphous silicon at the maximum silicon tetrafluoride is
It is 1.3eV. Over a further 5 minutes, the hydrogen partial pressure was gradually increased while keeping the argon partial pressure constant, and the silicon tetrafluoride partial pressure was lowered to return each to the initial state. Sputtering was continued to reduce the overall film thickness. Set to 24 μn. This cylinder is used as a photosensitive drum for electrophotography.

Example 2 A SnO ₂ transparent electrode is formed on a hard glass cylinder by thermal decomposition of SnCl ₄ at 450°C. The cylinder is fixed in a glow discharge deposition tank. Inside the tank 5x
Evacuate to 10 ^-6 Torr and heat the cylinder to 200°C.
Next, silane gas is introduced into the tank at a partial pressure of 1×10 ^-2 Torr, and the exhaust valve is adjusted to bring the pressure inside the tank to 1 Torr. In this state, high-frequency power is applied to generate a glow discharge in the tank, and a 1 μm film is deposited in this state. In this way, boron (B) is incorporated into the amorphous silicon, so the resistivity decreases to 10 ¹⁰
When the resistance is Ω·cm or more, a-Si having photoconductivity can be obtained. Thereafter, the diborane and silane gases are gradually squeezed out, and the reaction is continued while increasing silicon tetrafluoride to the same value as the initial partial pressure of the silane gas.
A constricted part of the optical forbidden band width is formed over a thickness of 3000A. Thereafter, the silicon tetrafluoride is squeezed out again, and the silane gas and diborane gas are introduced to the initial level to continue the deposition reaction, so that the total film thickness is 20 μm. This cylinder is used as a photosensitive drum for electrophotography.

[Brief explanation of drawings]

Figure 1 shows the band model of the amorphous silicon photoresist film of the present invention, and Figure 2 shows the hydrogen partial pressure in the atmospheric gas and the optical band gap of the amorphous silicon film created in the reactive sputtering method. FIG. 3 is a sectional view showing the structure of the electrophotographic photosensitive film of the present invention, and FIG. 4 is an explanatory diagram of a reactive sputtering apparatus. 1: Substrate, 2: Photosensitive layer, 11: Electrode, 21: Excess carrier injection suppressing layer, 22, 23, 24: Layer mainly composed of amorphous silicon, 25: Charge injection controlling layer.

Claims (1)

[Scope of Claims] 1. An electrophotographic photosensitive film having an amorphous silicon photoconductor layer containing at least 50 atomic % or more of silicon on average within the film, wherein the photoconductor layer has an amorphous silicon photoconductor layer containing at least 50 atomic % or more of silicon on the surface or from the surface and interface of the photoconductor layer. at least
The 10 nm region contains 1 atomic % or more of hydrogen on average within the film, has an optical band gap of 1.6 eV or more, and has a specific resistance of 10 ¹⁰ Ω cm or more, and the photoconductor layer has a It has a constriction with an optical bandgap width of 10 nm or more in thickness, and the constriction of the optical bandgap width is
The film contains an average of 0.1% or more of a halogen element, and has an optical band gap of 1.1 eV or more, which exceeds the optical band gap of the amorphous silicon forming the surface of the photoconductor layer. 10nm film thickness
An electrophotographic photosensitive film characterized in that it is an amorphous silicon region as described above. 2. The electrophotographic photosensitive film according to claim 1, wherein the halogen element is fluorine.