GB2145530A

GB2145530A - Amorphous silicon photoreceptor

Info

Publication number: GB2145530A
Application number: GB08421208A
Authority: GB
Inventors: Hideo Nojima; Yoshimi Kojima; Takashi Hayakawa; Shiro Narikawa; Eiji Imada; Toshiro Matsuyama; Shaw Ehara
Original assignee: Sharp Corp
Current assignee: Sharp Corp
Priority date: 1983-08-23
Filing date: 1984-08-21
Publication date: 1985-03-27
Also published as: GB8421208D0; GB2145530B; DE3430940C2; JPS6045258A; DE3430940A1

Abstract

An a-Si photoreceptor for an electrophotographic copying machine comprises a substrate 1, an amorphous silicon charge carrier transport layer 2, an intermediate layer 4, and a surface layer 3. The intermediate layer has an energy band gap which is intermediate between those of the amorphous silicon charge carrier transport layer and the surface layer. The surface layer may be amorphous silicon which contains nitrogen. The intermediate layer may be amorphous silicon which contains less nitrogen that the surface layer. <IMAGE>

Description

SPECIFICATION Amorphous silicon photoreceptor for electrophotography BACKGROUND OF THE INVENTION The present invention relates to a photoreceptor for an electrophotographic copying machine and, more particularly, to an amorphous silicon photoreceptor for an electrophotographic copying machine.

In an electrophotographic copying process, an electrostatic latent image is produced onto a photoreceptor corresponding to a pattern image on a document such as a manuscript or book to be copied. Toner particles are electrostatically adhered to the latent image, so that the latent image becomes visible as a toner image. The toner image on the photoreceptor is transferred onto a copy paper by a transference charger.

Such a photoreceptor should have a high resistance and high photosensitivity. Therefore, the conventional material of the photoreceptor is selected to be a dispersion type in which cadmium sulfide particles are dispersed in an organic resin, or an amorphous material such as amorphous selenium (a-Se) and amorphous arsenic selenium (a-As2Se3) etc.

However, these materials are known as poisonous and harmful, so that recently amorphous silicon (a-Si) has been drawn attention as, probably, an ideal photoreceptor material since it is highly photosensitive, extremely hard, and pollution-free. Nevertheless, the a-Si per se cannot retain sufficient electric charges with a sufficient resistance, so that it cannot serve as a photoreceptor without any modification.

SUMMARY OF THE INVENTION Accordingly, it is an object of the present invention to provide an improved photoreceptor layer for electrophotography.

It is another object of the present invention to provide an improved amorphous silicon layer for a photoreceptor for electrophotography to provide good electric and mechanical characteristics.

It is a further object of the present invention to provide improved laminated layers comprising a surface layer, an intermediate layer, and an amorphous silicon carrier transport layer for a photoreceptor for electrophotography, in which the matching of energy levels of these layers is good in order to improve the overall performance of an a-Si photoreceptor to attain good image and stability.

Briefly described, in accordance with the present invention, a photoreceptor for an electrophotographic copying machine comprises a substrate, an amorphous silicon carrier transport layer, an intermediate layer, and a surface layer. The intermediate layer has an energy band gap which is intermediate between those of the amorphous silicon carrier transport layer and the surface layer.

BRIEF DESCRIPTION OF THE DRAWINGS The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings which are given by way of illustration only, and thus are not limitative of the present invention and wherein: The single drawing is a schematic cross sectional view of laminated layers of a photoreceptor for electrophotography according to the present invention.

DESCRIPTION OF THE INVENTION The single drawing is a schematic cross sectional view of a photoreceptor for an electrophotographic copying machine according to the present invention.

The layers comprises a substrate 1, an amorphos silicon (a-Si) carrier transport layer 2, a surface layer 3, and an intermediate layer 4, in which the latter three layers are composed of a-Si and a-Si compounds.

The substrate 1 is made of an electrically conductive material such as aluminum, stainless steel or the like. The a-Si layer 2 serves as a carrier transport layer. To improve charge retaining characteristics and provide passivation, the surface layer 3 is deposited on the light exposure side, which has a large energy band gap. Between the a-Si layer 2 and the surface layer 3, the intermediate layer 4 is interposed in order to provide better electrical and mechanical matching therebetween. The intermediate layer 4 is devised to have an energy band gap to be a certain value in the range between those of the surface layer 3 and the carrier transport layer 2. Thus, a-Si compounds with certain ingredients for the above-mentioned three layers are successively deposited onto the substrate 1 to provide the photoreceptor.

The reason why the surface layer 3 having a larger energy band gap is not directly on the a-Si layer 2 and, instead, the intermediate layer 4 is interposed between the surface layer 3 and the a-Si layer 2 will be described as follows. If the surface layer 3 is directly deposited on the a-Si carrier transport layer 2, the following problems happen: 1. Mechanical Unstability: Mechanical strain between the surface layer 3 and the carrier transport layer 2 due to the difference of the thermal expansion coefficients between these three layers causes mechanical unstability, which often detaches the deposited layers.

2. Electrical Problem: When, during electrophotography, light is exposed to the photoreceptor whose surface is preliminarily charged, electric charges with the opposing polarity can emerge on the a-Si carrier transport layer 2 due to the light exposure. The thus light produced charges can drift in the a-Si layer 2, so that they can electrostatically cancel the surface charge.

However, when the energy band gap of the surface layer 3 is large as it is the case, the gap at the boundary becomes very large, to thereby hinder the smooth charge drift. The charge may be left at the boundary between the surface layer 3 and the a-Si layer 2 as the residual potential, undesirably. Increase of the residual potential causes the deterioration of the photoreceptor characteristics. Further, the stored charge at the boundary may disperse laterally, resulting in smearing of the image.

Now turning back to the description of the present invention, since the composition of the intermediate layer 4 is selected to be substantially intermediate between the compositions of the a-Si layer 2 and the surface layer 3, the intermediate layer 4 has substantially an intermediate thermal expansion coefficient between those of the layers 2 and 3.

An article reports that the thermal expansion coefficient of the a-Si carrier transport layer 2 is about 1.9 x 10~6/dex and that of the surface layer 3 of a -SiNx is about 2.5 X lO-B/deg. The intermediate layer 4 is considered to have a middle thermal coefficient of these values. Although the thermal expansion is different between the a-Si layer 2 and the surface layer 3, any strain due to this difference can be thereby reduced to provide mechanical stability. Further, the providing of the intermediate layer 4 can also improve electrical characteristics for a photoreceptor.That is, the undesirable residual potential of the photoreceptor can be reduced by providing the intermediate layer 4 of P-type for a positive surface charge (or N-type for a negative surface charge) layer having a somewhat larger energy band yap than that of the a-Si carrier transport layer 2.

Therefore, the conduction type and the level of the Fermi energy of the intermediate layer 4 is deliberately designed in order to attain the above-mentioned performance.

More particularly, the a-Si layer 2 formed on the electrically conductive layer 1 is deposited from monosilane gas (SiH4) by an induction type plasma CVD (p-CVD) method.

During the deposition, the substrate 1 is heated at about 200-300 degrees Centigrade. In the case of a positive surface charge, a small amount of diborane gas (B2H8) is mixed to the raw-material gas (SiH4), in order to improve the carrier transport properties. After the carrier transport layer 2 with a thickness of about 20 ,um is formed, the intermediate layer 4 having the composition of a-SiNO 1 which has an intermediate energy band is formed with the raw-material gas of monosilane (six4) and ammonia (NH3) gases.

Thus, nitrogen of about 10% is added into hydrogenated amorphous silicon (a-Si) to form a hydrogenated amorphous silicon nitride film of a-SiNO as the intermediate layer 4. The thickness of the intermediate layer 4 is about 1,us. Further, a hydrogenated amorphous silicon nitride film a-SiNx:H (x < 4/3) having a large nitrogen amount than that of the intermediate layer 4 is deposited as the surface layer 3 with a thickness of about 1 dum or less, preferably, about 0. 1-0.3 CLm on the intermediate layer 4. The a-SiNx:H film of the surface layer 3 is deposited also by the inductive type p-CVD method with SiH4 and NH3.

The amount of nitrogen can be controlled by a mass flow controller (MFC) changing a flow amount ratio of these gases.

The a-Si carrier transport layer 2 is devoid of nitrogen. Preferably, the a-Si carrier transport layer 2 should have nitrogen of about 10-20%. Further, when the energy gap of the surface layer 3 is 2.5 eV or more, the ratio (N/Si) of nitrogen with silicon in the surface layer 3 is 70% or more of a-SiN, (x > 0.7).

When the above-structured photoreceptor is used for a conventional electrophotography, no substantial deterioration of the characteristics can be noticed after it is subjected to copying of about 200,000 pages to thereby maintain its initial good properties.

TABLE 1 shows characteristics of the abovedescribed photoreceptor of the present invention.

1. Enegy Band Gap: the carrier transport layer: 1.7 eV the intermediate layer: 1.8 eV the surface layer: 2.5 eV or more 2. Chaging Voltage: about 40 V/,um 3. Dark Decay Time: about 10 seconds 4. Half Value Exposure: 3 Ix.s (at 45 Ix white light exposure) 5. Spectral Sensitivity: peak sensitivity wavelength 725 nm 6. Copyable Pages: more than 200,000 TABLE I Since the peak sensitivity wavelength is 725 nm so as to be sensitive to long wavelength light, it is suitable for a laser beam printer (LBP) with a semiconductor laser.

The energy gap of the a-Si carrier transport layer 2 is constant to be about 1.7 eV without any modification. Preferably, that of the intermediate layer 4 is 1.8-1.9 eV and that of the surface layer 3 is about 2.5-3.0 eV.

TABLE II shows characteristics of the conventional photoreceptor without the intermediate layer 4.

1. Enegy Band Gap: the carrier transport layer: 1.7 eV the surface layer: about 2 eV 2. Charging Voatage: about 10 V/um 3. Dark Decay Time: 1 second 4. Half Value Exposure: 6 Ix.s 5. Spectral Sensitivity: peak sensitivity wavelength 725 nm 6. Copyable Pages: 10,000-50,000 pages TABLE II In the case of a photoreceptor comprising the carrier transport layer having an energy band gap of 1.7 eV and the surface layer having that of 2.5 eV or more without the intermediate layer 4 of the present invention, copy images are smeared and not clear. During aging, the residual potential may be increased.

In place of nitrogen increasing the energy band gap, carbon can be used with the rawmaterial gas of methane (CH3). Further, the induction type p-CVD apparatus as stated above can be replaced by a capacitive type apparatus.

While only certain embodiments of the present invention have been described, it will be apparent to those skilled in the art that various changes and modifications may be made therein without departing from the sprit and scope of the present invention as claimed.

Claims

1. A photoreceptor for electrophotography comprising: a substrate of an electrical conductive material; a carrier transport layer composed of amorphous silicon, said layer being formed on said substrate; an intermediate layer formed on said amorphous silicon carrier transport layer; and a surface layer on said intermediate layer, said surface layer having a larger energy band gap and said intermediate layer having an intermediate energy band gap between those of said amorphous silicon layer and said surface layer.

2. A photoreceptor of claim 1, wherein said surface layer is made of amorphous silicon added with nitrogen and said intermediate layer contains nitrogen whose amount is intermediate between those of said amorphous silicon and said surface layer.

3. The photoreceptor of claim 1, wherein said surface layer contains carbon.

4. The photoreceptor of claim 1, wherein said amorphous silicon layer has an energy band gap of about 1.7 eV, said intermediate layer has an energy band gap of about 1.8 eV, and said surface layer has an energy band gap of 2.5 eV or more.

5. The photoreceptor of claim 1, wherein said amorphous silicon carrier transport layer has a thickness of about 20 ym, said intermediate layer has a thickness of about 1 um, and said surface layer has a thickness of 1 ym or less.

6. The photoreceptor of claim 2, wherein the amount of nitrogen in said intermediate layer is about 10-20% and that of nitrogen in said surface layer is 70% or more.

7. A photoreceptor substantially as herein described with reference to, and as shown in, the accompanying drawings.