ELECTROPHOTOGRAPHIC MULTI-LAYERED PHOTOSENSITIVE MEMBER HAVING A TOP LAYER OF AMORPHOUS SILICON CARBINE AND METHOD FOR FABRICATING THE SAME

Abstract: An electrophotographic multi-layered photosensitive member comprises a top layer (11) of hydrogenated amorphous silicon carbide formed over a photoconductive layer (3) and having an atomic ratio of carbon to carbon plus silicon C/(Si+C) ranging from 0.17 to 0.45 and a ratio of number of hydrogen atoms bonded to silicon atoms per silicon atom, to number of hydrogen atoms bonded to carbon atoms per carbon atom, (Si-H)/Si / (C-H)/C , ranging from 0.3 to 1.0. The top layer is formed on a photosensitive layer of hydrogenated amorphous silicon by employing a glow discharge CVD method. In one embodiment, the gaseous mixture is composed of disilane (Si₂H₆) and propane (C₃H₈) mixed with a mol ratio C₃H₈/(Si₂H₆+ C₃H₈) ranging from 0.2 to 0.6. In another embodiment, the gaseous mixture comprises disilane (Si₂H₆) gas, propane (C₃H₈) gas, and hydrogen (H₂) gas, the mixing mol ratio C₃H₈/ (Si₂H₆+C₃H₈) ranging from 0.2 to 0.7, and the mixing mol ratio H₂/(Si₂H₆+C₃H₈ ranging froml 1 to 10.

Description: The present invention relates to an electrophotographic printing apparatus. More particularly, it relates to a multi-layered photosensitive member formed on a substrate and having a top layer formed atop of the photosensitive member. The invention also relates to a method for fabricating the top layer. Electrophotography is a well-known technology, and electrophotographic printing apparatuses are widely used. They include a photosensitive member disposed on the surface of a printing drum. At a first step of the electrophotographic printing process, the photosensitive member is sensitized by being charged to a uniform potential by means of an electrostatic charger such as a corona discharger. The charged portion of the photosensitive surface is exposed to a light image of an original document to be reproduced. This records an electrostatic latent image on the photosensitive member corresponding to the original document. Thereafter, the latent image is developed by bringing a developing material such as toner powder into contact therewith. A powder image is thus formed on the surface of the photosensitive member which is transferred onto a recording sheet. Thereafter, the powder image is permanently affixed to the recording sheet in the image configuration. Finally, at a next cleaning station, residual toner particles are removed from the surface of the photosensitive member. This is a typical one cycle of a conventional electrophotographic printing process, taking approximately 20 seconds or so for a standard level apparatus, and several seconds for a high speed apparatus. The photosensitive member of the printing drum described above consists of a photosensitive, chargeable material such as selenium or a chalcogenide glass (arsenic-selenium alloys and compounds). It is also known to utilize organic photosensitive materials therefor. Recently, however, amorphous silicon has become widely used, as illustrated in U.S. Patent No. 4, 507, 375, for example, issued on March 26, 1985. The requirements for materials used in an electrophotographic printing apparatus are as follows. The material of the surface layer of the photosensitive member formed on the printing drum must have a high photosensitivity in the spectral range of the employed light source, such as a laser source. The material must also have a specific electrical impedance in darkness (dark resistance) of magnitude higher than 10¹² ohm-cm, in order to substantially retain an electrostatic latent image thereon during at least one cycle of the printing operation, approximately 20 seconds as described above. The material must also have properties which remain unaltered with a continuous loading and unloading, i.e. which operates in a fatigue-proof manner and which is sufficiently resistant to abrasion during the printing operation, and to various environmental hazards such as high humidity and damages caused by corona discharge. It is difficult to satisfy these requirements with a single photoconductive layer. For example, a photosensitive material having a high dark resistance and a high light conductance at the same time, is rarely found. In order to fulfill the above requirements, multi-layered photosensitive members of amorphous silicon have been developed, being now a well-known technology. One such photosensitive member is disclosed in U.S. Patent No. 4,452, 874 issued on June 5, 1984 to Ogawa et al which member is provided with a top layer of amorphous silicon carbide containing silicon atoms, carbon atoms, and hydrogen atoms. Also disclosed is a method for forming the layer by electrically decomposing a gaseous mixture of hydrogenated silicon gases and hydrocarbon gases. In a great number of examples, a gas mixture of silane (SiH₄) and methane (CH₄) is used. However, the top layers formed in prior art electrophotographic printing apparatuses have proved to show some defects which cause problems during use of the apparatuses. It is therefore an object of the invention to provide a multi-layered photosensitive member with an improved top layer having high environmental resistance and electrical impedance whereby an electrophotographic printing apparatus can be obtained of high reliability and quality and capable of reproducing a clear information image. It is also an object of the invention to provide a method for fabricating the improved top layer. According to the invention, the multi-layered photosensitive member has a top layer consisting of hydrogenated amorphous silicon carbide which has an atomic ratio of carbon to carbon plus silicon C/(Si+C) ranging from 0.17 to 0.45 and a ratio of the number of hydrogen atoms bonded to silicon atoms per silicon atom to the number of hydrogen atoms bonded to carbon atoms per carbon atom, {(Si-H)/Si}/{(C-H)/C}, ranging from 0.3 to 1.0, where C and Si designate the number of carbon atoms and the number of silicon atoms, respectively, and (Si-H) and (C-H) designate the number of hydrogen atoms bonded to a silicon atom and the number of hydrogen atoms bonded to a carbon atom, respectively. According to another aspect of the invention, there is provided a method for fabricating an electrophotographic multi-layered photosensitive member comprising the steps of : evacuating a vacuum tight chamber in which a photosensitive member comprising a photoconductive layer on a substrate is disposed in a predetermined position; introducing a gaseous mixture into said vacuum tight chamber; and decomposing said gaseous mixture, and depositing the resulting material on said photoconductive layer to form said top layer of hydrogenated amorphous silicon carbide (a-SiC:H) thereon, wherein said gaseous mixture comprises disilane (Si₂H₆) gas and propane (C₃H₈) gas, and has a mixing mol ratio C₃H₈/(Si₂H₆+C₃H₈) ranging from 0.2 to 0.6, where C₃H₈ and Si₂H₆ designate the numbers of propane molecules and disilane molecules, respectively. A variant of the method for fabricating an electrophotographic mulit-layered photosensitive member according to the invention comprises the steps of : evacuating a vacuum tight chamber in which a photosensitive member comprising a photoconductive layer on a substrate is disposed in a predetermined position; introducing a gaseous mixture into said vacuum tight chamber; and decomposing and depositing said gaseous mixture to form said top layer of hydrogenated amorphous silicon carbide (a-SiC:H) over said photosensitive member, wherein said gaseous mixture comprises disilane (Si₂H₆) gas, propane (C₃H₈) gas and hydrogen (H₂) gas, and has a mixing mol ratio C₃H₈(Si₂H₆+C₃H₈) ranging from 0.2 to 0.7, and a mixing mol ratio of said hydrogen gas to the remaining gas, H₂/(Si₂H₆+C₃H₈) ranging from 1 to 10, where C₃H₈, Si₂H₆ and H₂ designate the numbers of propane molecules, disilane molecules, and hydrogen molecules, respectively. The invention will be more readily understood from the reading of the following description made with reference to the accompanying drawings wherein: Fig. 1 is a partial cross-sectional view of a prior art photosensitive member formed on a printing drum of an electrophotographic apparatus; Fig. 2 is a partial cross-sectional view of a photosensitive member of a printing drum of an electrophotographic apparatus according to the present invention; Fig. 3 is a schematic cross-sectional view of a glow discharge CVD apparatus illustrating a CVD chamber and its associated gas feeding and exhausting system for carrying out a first embodiment of the method according to the invention; Fig. 4 is a diagram illustrating the relation between the atomic ratio of carbon to silicon in hydrogenated amorphous silicon carbide (a-SiC:H) layers formed by a glow discharge CVD method, according to said first embodiment of the method of the invention, and the gas ratio of the gaseous mixture of disilane and propane; Fig. 5 is a diagram illustrating the relation between the ratio of hydrogen atoms bonded to a silicon atom to those bonded to a carbon atom in an hydrogenated amorphous silicon carbide (a-SiC:H) layer, formed according to said first embodiment of the method of the invention, and the gas ratio of gaseous mixture of disilane and propane; Fig. 6 is a diagram illustrating the relation between the dark resistance of an hydrogenated amorphous silicon carbide layer and the mixing ratio of propane to disilane of gaseous mixture used for forming the layer by a glow discharge CVD process according to the method of the invention; Fig. 7 is a diagram illustrating the empirical results of moisture durability of a layer formed according to the first embodiment of the method of the invention; Fig. 8 is a schematic cross-sectional view of a glow discharge CVD apparatus illustrating a CVD chamber and its associated gas feeding and exhausted system for carrying out a second embodiment of the method according to the invention; Fig. 9 is a diagram illustrating the relation between the atomic ratio of carbon to silicon of the hydrogenated amorphous silicon carbide (a-SiC:H) layer, formed by a glow discharge CVD method, according to the second embodiment of the method of the invention, and the gas ratio of the gaseous mixture of disilane and propane; Fig. 10 is a diagram illustrating the relation between the ratio of hydrogen atoms bonded to a silicon atom to those bonded to a carbon atom in the hydrogenated amorphous silicon carbide (a-SiC:H) layer, formed according to the second embodiment of the method of the invention, and the gas mixing ratio of the gaseous mixture of disilane and propane used for forming said layer and Fig. 11 is a diagram illustrating the empirical results of moisture durability of the layer formed according to the second embodiment of the invention. In all the drawings, the same references are used to designate identical or similar parts. A prior art multi-layered photosensitive member will be first described with reference to Fig. 1. On a cylindrical drum base 1 of conductive material such as aluminium, a charge blocking layer 2 of highly p-type doped hydrogenated amorphous silicon (a-Si:H) is formed by a conventional method such as by glow discharge CVD (chemical vapor deposition) for decomposing and depositing a gaseous mixture of silane (SiH₄) and diborane (B₂H₆) using electrical energy. Over the charge blocking layer 2, a photoconductive layer 3 of slightly p-type doped hydrogenated amorphous silicon (a-Si:H) is formed by the same CVD method employing a similar gaseous mixture but with different gas ratio. The photoconductive layer 3 has a high electrical conductance under exposure of light (light conductance) but not so high dark resistance. Thereafter, a top layer 4 is formed on the photoconductive layer 3, not only for protecting the surface thereof from various environment hazards, but also for retaining the charges of the electrostatic latent image formed thereon and preventing the latent image from being dispersed and weakened. The top layer 4 is formed of a photosensitive material having a high dark resistance such as hydrogenated amorphous silicon oxide (a-SiO:H), hydrogenated amorphous silicon nitride (a-SiN:H), or hydrogenated amorphous silicon carbide (a-SiC:H). The top layer 4 also has a high resistance to abrasion to protect the surface from exterior mechanical damage during operation. The charge blocking layer 2 has rectifying characteristics due to the difference of doping density between the charge blocking layer 2 and the photoconductive layer 3. Consequently, the injection of electrical carriers from the drum base 1 into the photosensitive member under dark condition is blocked and excess charges generated in the photoconductive layer 3 under exposure of light are allowed to flow from the photoconductive layer 3 to the drum base 1. Thus, by the aid of the charge blocking layer 2 and the top layer 4, the entire surface of the photosensitive member has a high dark resistance, being immune from any image flow or image weakening. Among the already mentioned requirements for a photosensitive member formed on a printing drum, the following are directed to the top layer 4: charge retaining capability for maintaining charges of a latent image recorded therein, endurance to corona discharge during the charging process, and resistance to abrasion and moisture caused by the exterior hazards. However, these requirements have not been fully satisfied with the prior art top layers, causing some problems during use of the electrophotographic printing apparatus. The problems may be attributed to some defects in the top layer 4, such as small pin holes. Such defects of the top layer 4 are considered to be caused by some defective structure of the material of the top layer 4. Generally, the structure defects, namely, local distortion of silicon network, of amorphous silicon or amorphous silicon compounds such as amorphous silicon carbide, are caused by the presence of dangling bonds, i.e. non-terminated bonds of silicon atoms. For example, in intrinsic amorphous silicon (a-Si), the distribution density of the dangling bonds is approximately 10²⁰cm⁻³. In hydrogenated amorphous silicon, these non-terminated bonds are intended to be bonded to hydrogen atoms (H). It is reported that the density of the dangling bonds can be reduced to approximately 10¹⁵cm⁻³ with an adequate fabrication method. However, the hydrogen atoms tend to be bonded to silicon or other atoms non-uniformly. Particularly, in hydrogenated amorphous silicon carbide, hydrogen atoms are attracted and bonded to carbon atoms and a uniform distribution of bonded hydrogen atoms is desirable for reducing the structure defects in the amorphous silicon compound material. Thus, a material having less dangling bonds therein and a method for fabricating the material become a key point for improving the photosensitive member of the printing drum, and solving the above-mentioned problems. Various efforts are directed to solving the above problem regarding a top layer for an electrophotographic photosensitive member. The quality and the production efficiency depends on the combination and composition of the gas mixture used. In view of further improvement, namely, in order to achieve faster film formation rate and an increased resistance against the damage of the surface of the substrate (hereby, a photoconductive layer 3) caused by the glow discharge plasma during glow discharge CVD process, a new gaseous mixture has been studied. In the above and following description, a glow discharge CVD method is mentioned for the formation of a photosensitive member. However, other conventional methods, such as a sputtering method, or a laser assisted CVD method, are available for the same purpose, the selection of the method used depending in particular on the quantity of production, variety of products, and investment for installations. The problems arising in a prior art electrophotographic printing apparatus are successfully solved by the multi-layered photosensitive member according to the present invention. Fig. 2 is a partial cross-sectional view of such a the photosensitive member. In comparison with the prior art illustrated in Fig. 1, a newly improved top layer 11 is employed which is formed on a photoconductive layer 3 of slightly doped p-type hydrogenated amorphous silicon (a-Si:H) provided on a drum base 1 with the interposition of a blocking layer 2. As described before, hydrogenated amorphous silicon carbide (a-SiC:H) has been utilized as a material for a top layer of a photosensitive member formed on a printing drum. According to the present invention, the composition of the material is adequately selected and the material is formed employing a suitable fabricating method, in order to reduce defects contained in the top layer material, and achieve a more effective and reliable fabrication method. The hydrogenated amorphous silicon carbide used for the top layer 11 has an atomic ratio of carbon to carbon plus silicon C/(Si+C) ranging 0.17 to 0.45 and a ratio of the number of hydrogen atoms bonded to silicon atoms per silicon atom, to the number of hydrogen atoms bonded to carbon atoms per carbon atom, {(Si-H)/Si}/{(C-H)/C}, ranging from 0.3 to 1.0. The top layer 11 is formed on a photoconductive layer 3 of p⁻ type doped hydrogenated amorphous silicon (a-Si:H) which is formed on a drum base 1 through a blocking layer 2. These layers are deposited employing a well-known glow discharge CVD method, but the gaseous mixture to be decomposed is improved. Unlike in the method known from US 4,452,874, more chemically active disilane (Si₂H₆) is used as a hydrogenated silicon gas to achieve a higher film formation rate and a lower discharging power which results in less plasma damage of the associated substrate surface. As the best combination with disilane gas, propane (C₃H₈) gas is selected from various hydrocarbon gases. The mixing gas ratio in the gaseous mixture is determined by a number of experiments. Thus, the top layer 11 is deposited from a gaseous mixture composed of disilane (Si₂H₆) and propane (C₃H₈) mixed with a mol ratio C₃H₈/(Si₂H₆ + C₃H₈) ranging from 0.2 to 0.6. As a result, a satisfactory top layer 11 is obtained, having a sufficiently small number of defects to fulfill the above-mentioned requirements for an electrophotographic printing apparatus. As a further improved method of forming the top layer, a gaseous mixture comprising disilane (Si₂H₆) gas, propane (C₃H₈) gas, and hydrogen gas is used, the mixing mol ratio of the propane gas to the disilane gas plus propane gas C₃H₈/(Si₂H₆+C₃H₈) ranging from 0.2 to 0.7, and the mixing mol ratio of the hydrogen gas to the remaining gas, H₂/(Si₂H₆+C₃H₈), ranging from 1 to 10. The quality of the top layer, namely, number of structural defects thereof, depends on the method for forming the top layer, particularly, on the gas mixing ratio of the gaseous mixture of component gases. The forming method will now be described in detail. Fig. 3 is a schematic cross-sectional view of a glow discharge CVD apparatus, with a CVD chamber and its associated gas feeding and exhausting system. Heating means 23, comprising sheathed heaters arranged on a cylindrical surface, a rotatable holder 22 driven by a driving means 47, a gas ejecting cylinder 26 having ejecting holes opened therein, and a cylindrical discharge electrode 25 are coaxially arranged in a vacuum tight chamber 21 in the recited order outward from the center of said chamber. The whole chamber 21 is exhausted by exhausting means through an exhausting tube 49, and is fed with reaction gasses received from a gas feeding system through a gas feeding tube 27. At first, a cylindrical drum base 24 is set coaxially within chamber 21 by means of the holder 22. A vacuum valve 28 is opened and the chamber 21 is pre-evacuated by means of a mechanical booster pump 29 and a rotary pump 30 to a vacuum degree such as 1x10⁻³ Torr (0,13 N/m²), sufficient to reach the back pressure of an oil diffusion pump 32. Then, the vacuum valve 28 is closed, and other vacuum valves 31 and 33 are opened to reduce the pressure within chamber 21 to a higher vacuum degree such as 1x10⁻⁶ Torr (0,13.10⁻³ N/m²) by the aid of a rotary pump 34 and of the oil diffusion pump 32. After evacuation of the residual air or gas from chamber 21, the vacuum valves 31 and 33 are closed and the evacuation operation is switched back to the former vacuum circuit comprising mechanical booster pump 29 by opening the vacuum valve 28. The drum base 24 is rotated by the driving means 47, and pre-heated up to 150 to 350°C, preferably up to 200 to 300°C, by the heating means 23 positioned inside of the drum base 24. Thereafter, gas valves 35, 37, 39, and 41 are opened, allowing disilane gas (Si₂H₆) and diborane gas (B₂H₆) to flow in from respective bombs 38 and 42. The flow rate of each gas is controlled by a respective mass flow controller 36, 40. The mixing ratio of diborane (B₂H₆) to disilane (Si₂H₆) is selected to be 100 to 1000 ppm. The opening of vacuum valve 28 is adjusted to keep the gas pressure inside the chamber 24 at 0.005 to 5.0 Torr (0,67 to 6,7 10² N/m², preferably at 0.1 to 3.0 Torr (0,13.10² to 4.10² N/m²) In this state, power supplied from a power source 48, having a high frequency, such as 13.56 MHz, and a power density of 5 to 500 mW.cm⁻², preferably 10 to 200 mW.cm⁻², is applied between the discharge electrode 25 and the drum base 24 to generate a glow discharge therebetween. Consequently, the gaseous mixture is decomposed by the glow discharge, and a charge blocking layer 2 is deposited on the surface of the drum base 24. The resulting layer 2 is composed of p⁺ doped, hydrogenated amorphous silicon layer having a thickness of 0.01 to 1.00 micron. At the next step, in the same way, a photoconductive layer 3 of slightly p-type doped hydrogenated amorphous silicon layer having a high light conductance and a thickness of 5 to 30 microns, is deposited from a gaseous mixture having a ratio of diborane (B₂H₆) to disilane (Si₂H₆) ranging 1.0 to 10 ppm. Thereafter, gas valves 35, 37, 39, and 41 are closed and disilane (Si₂H₆) bomb 38 and diborane (B₂H₆) bomb 42 are cut away from the chamber 21, and the evacuation system is operated to evacuate the chamber 21. Then, the gas valves 35,37, 43 and 45 are opened and disilane (Si₂H₆) gas and propane (C₃H₈) gas are supplied from respective bombs 38 and 46. The flow rate of each gas is controlled by a respective mass flow controller 36, 44 to form a gaseous mixture having a mixing ratio C₃H₈/(Si₂H₆+C₃H₈) of 0.2 to 0.6. The pressure of the gaseous mixture is kept at 0.1 to 3.0 Torr (0,13.10² to 4.10² N/m²) In the same way as for the formation of the former layers, a top layer 11 of hydrogenated amorphous silicon carbide (a-SiC:H) having a thickness of 0.01 to 1.0 micron is formed over the photoconductive layer 3. Thus, the photo-sensitive member of the printing drum is completed. The structural, optical and electrical properties of hydrogenated amorphous silicon carbide (a-Si:H) which is formed by a glow discharge CVD method, are dependent upon the gaseous mixture containing the carbon and silicon sources and the decomposition conditions. These properties of the layer formed according to the present invention will now be studied and examined. The atomic ratio of carbon atoms to silicon atoms is measured in hydrogenated amorphous silicon carbide layers deposited from different gaseous mixtures of disilane (Si₂H₆) and propane (C₃H₈), the gas mixing ratio being varied in some range, and being controlled by means of mass flow controllers. For each formed layer, the atomic ratio is determined by an electron spectroscopy for chemical analysis (ESCA) method or by X-ray photo-emission spectroscopy (XPS).The number of hydrogen atoms bonded to a silicon or carbon atom is measured by a conventional Fourier transform infrared absorption spectroscopy (FT-IR) method. The result is plotted in the diagram of Fig.4 where the gas mixing ratio (mol ratio) of C₃H₈/(Si₂H₆+C₃H₈) is noted on the abscissa axis, and the atomic ratio of C/(Si+C) is noted on the ordinate axis. As shown in the diagram, the range of gas mixing ratio from 0.2 to 0.6, corresponds to the range of the atomic ratio from 0.17 to 0.40. This atomic ratio range is favourable for the electrical and environmental characteristics of the top layer 11 as described later. Generally, in a hydrogenated amorphous silicon carbide layer, hydrogen atoms tend to be bonded to carbon atoms forming CH₂, CH₃, and CH₄ bonds, and leaving silicon atoms in non-bonded state, namely dangling bond states. This is undesirable to reduce the structural defects of the hydrogenated amorphous silicon carbide. Fig.5 is a diagram illustrating the relation between the ratio of hydrogen atoms bonded to a silicon atom to those bonded to a carbon atom in the formed hydrogenated amorphous silicon carbide (a-SiC:H) layer, and the gas ratio in the gaseous mixture of disilane (Si₂H₆) and propane (C₃H₈). The number of hydrogen atoms bonded to a silicon or carbon atom is measured by a conventional FT-IR method. The glow discharge conditions of the CVD process are as follows. The frequency of the high frequency glow discharge power is fixed at 13.56 MHz and the flow rate of the gaseous mixture is fixed at 15 SCCM (standard cubic centimeter per minute). For curve A, the other glow discharge conditions are a total gas pressure of the discharge gas of 3 Torrs (4.10² N/m²) a discharge power of 200 W. For curve B, the discharge conditions are 1 Torr (1,3 10² N/m²), and 30W respectively. As can be seen from Fig.5, the atomic ratio of hydrogen atoms bonded to a silicon atom to those bonded to a carbon atom falls in the range of 0.3 to 1.0 for a gas mixing ratio of 0.2 to 0.6. Curve C is taken for reference and relates to an hydrogenated amorphous silicon carbide layer formed with a prior art technology employing a gaseous mixture of silane (SiH₄) gas and methane (CH₄) gas. As seen from curve C, the atomic ratio of hydrogen atoms bonded to a silicon atom to those bonded to a carbon atom falls below 0.2. As a result, in the prior art layer, there are less hydrogen atoms bonded to silicon atoms, namely a larger number of dangling bonds of silicon atoms than that in the layer formed according to the present invention. Curves A,B, and C imply that the hydrogenated amorphous silicon carbide layer according to the present invention contains much less dangling bonds than the layer formed by the prior art method. For checking purposes, the resistance characteristics of an hydrogenated amorphous silicon carbide layer formed according to the present invention are measured. Fig.6 is a diagram showing the variation of dark resistivity of the layer as a function of the mixing ratio of propane to disilane of the gaseous mixture used for forming the layer by glow discharge CVD process. The achieved resistivity is higher than 10¹² ohm-cm, which is sufficient to maintain a high charged potential on the surface of the top layer. The empirically obtained curve indicates that the gas mixing ratio ranging from 0.2 to 0.6 is suitable for obtaining a top layer capable of sustaining a clear electrostatic image on the photosensitive layer. Reduction of the number of dangling bonds or structural defects in a top layer will serve to increase moisture durability of the surface, resulting in keeping charged potential applied to the surface of the layer exposed to a high moisture. The moisture durability of a top layer formed of hydrogenated amorphous silicon carbide according to the present invention, is empirically studied with a number of specimens. Fig.7 is a diagram illustrating the empirical results of the moisture durability tests, the gas mixing ratio of propane (C₃H₈) to disilane (Si₂H₆) being noted on the abscissa axis and the charged potential of the surface of the associated specimen being noted on the ordinate axis. For each curve shown, the corresponding relative humidity to which the specimens are exposed is given as a parameter. Each specimen is kept in a moist environment of designated relative humidity, at room temperature of 35°C, for approximately 2 hrs, and after that, the surface potential is measured after charging up by corona discharge by means of a discharger charged under a voltage of 5.5 kV. The resulting charged potential over 400 V is obtained for the most severe environmental condition of 90% relative humidity with respect to the gas mixing ratio ranging from 0.2 to 0.6. As shown in Fig.7, however, gas mixing ratio higher than approximately 0.6, results in substantially poor moisture durability of the layer. Although not shown in Fig.7, a prior art top layer has a so poor moisture durability that the top layer kept in a moist environment of 70% to 80% relative humidity shows a remarkable drop of charged potential by 50% or more. As described before, the capability of keeping high charged potential of the surface of a top layer of a photosensitive member serves to achieve a clear reproduced recorded image having high OD (optical density). In view of this, a practical experiment is performed with photosensitive members having top layers of hydrogenated amorphous silicon carbide formed by the method according to the present invention. The quality evaluation of the photosensitive members are conducted by practical printing operation employing a document printer, type M3071A, manufactured by the applicant. Different top layers of hydrogenated amorphous silicon carbide are prepared using a gaseous mixture of propane and disilane with various mixing ratio. The experiments are performed under environments of various humidity, and their results are evaluated and tabulated in Table 1 for each gas mixing ratio. In the last column, there is tabulated, as sample No. 10, an experimental result obtained with a prior art top layer deposited from a gaseous mixture of silane and methane. The evaluations are classified in four classes, namely, excellent denoted by a letter O, fairly good by a letter C, good by a letter A, and no good by a letter X. The standard of the classification is indicated ahead of Table 1. An initial charged potential is provided by a corona discharger with a charging voltage of 5.5 kV, and the wavelength of irradiating light to the photosensitive member is 780 nm. When using a gaseous mixture containing less propane gas, resulting hydrogenated amorphous silicon carbide layer contains less carbon atoms, and tends to have a characteristic similar to that of intrinsic hydrogenated amorphous silicon, lower dark resistivity, which produces problems such as image flow or ambiguous reproduction of the printed image. Thus, it is concluded that a gas mixing ratio ranging from 0.2 to 0.6, most preferably from 0.3 to 0.5 is the best selection for glow discharge CVD to form an hydrogenated amorphous silicon carbide top layer. A second embodiment of the method according to the present invention for forming a top layer of hydrogenated amorphous silicon carbide, will now be described. Briefly speaking, this second embodiment distinguishes over the first one in that hydrogen gas is added to the gaseous mixture for glow discharge CVD for forming the top layer. Some of the hydrogen gas molecules in a glow discharge field are activated to radical hydrogen atoms which react with amorphous silicon carbide and bond to silicon atoms, serving to reduce dangling bonds of the silicon atoms. Fig.8 is a schematic cross-sectional view of a glow discharge CVD apparatus employed for the second embodiment of the method according to the invention. The apparatus illustrated in Fig.8 is almost similar to that of Fig;3, except the addition of a hydrogen gas feeding system comprising two gas valves 50 and 51, a mass flow controller 52, and a hydrogen bomb 53. As shown in Fig.8, the hydrogen gas feeding system is connected to a vacuum chamber 21 in parallel with feeding systems for the other depositing gases. The gaseous mixture for glow discharge CVD is composed of propane, disilane, and hydrogen. The mixing ratio of propane to disilane plus propane, C₃H₈/(Si₂H₆+C₃H₈) is selected to range from 0.2 to 0.7, and the mixing ratio of hydrogen to propane plus disilane, H₂/(Si₂H₆+C₃H₈) is selected to range from 1 to 10. Other conditions such as pressure of the gaseous mixture, and high frequency power density for glow discharge, are the same as those in the first embodiment of the method according to the invention. Furthermore, the hydrogen gas feeding system is connected or disconnected to the vacuum tight chamber 21 in the same way as the feeding systems for propane and disilane gases and at the same processing step. A further description of the operation of the apparatus of Fig.8 is therefore not necessary. The values of the atomic ratio of carbon atoms to silicon atoms for different gas mixing ratios are measured in the hydrogenated amorphous silicon carbide layers formed according to the second embodiment of the method according to the invention. The results are plotted in the diagram of Fig.9. The gas mixing mol ratio of C₃H₈/(Si₂H₆+C₃H₈) is noted on the abscissa axis, and the atomic ratio of C/(Si+C) is noted on the ordinate axis. As shown in the diagram, the gas ratio range from 0.2 to 0.7 corresponds to the range of the atomic ratio from 0.17 to 0.45. This gas ratio is then favourable for the electrical and environmental characteristics of the top layer 11 as described before. A mixing ratio of hydrogen to the remaining gases ranging from 1 to 15 does not affect the atomic ratio of the layer. It may be noted that the characteristics of Fig.9 is almost the same as Fig.4. Fig.10 is a diagram illustrating the relation between the atomic ratio of hydrogen atoms bonded to a silicon atom to those bonded to a carbon atom, in the top layer formed according to the second embodiment of the method, and the gas ratio of the gaseous mixture of disilane (Si₂H₆) and propane (C₃H₈). The frequency of the high frequency glow discharge power used in the CVD process is fixed at 13.56 MHz and the flow rate of the gaseous mixture is 15 SCCM. The other glow discharge conditions, namely, the total gas pressure of the discharge gas and the discharge power are 3 Torrs (4. 10²N/m²) and 200 W respectively. For curve A, the gas mixture contains no hydrogen gas, and for curves A-1, A-2, A-3, and A-4 the mixing ratio of hydrogen gas to the remaining gases is respectively, 1, 5, 10 and 15. As can be seen from Fig. 10, the atomic ratio of hydrogen atoms bonded to a silicon atom to those bonded to a carbon atom increases as the hydrogen content in the gaseous mixture increases. With a mixing ratio (mol ratio) C₃H₈/(Si₂H₆+C₃H₈) not exceeding 0.7 and a gas mixing ratio (mol ratio) of H₂/(Si₂H₆+C₃H₈) at least equal to 1, the atomic ratio of hydrogen atoms bonded to a silicon atom to those bonded to a carbon atom is higher than 0.3, where hydrogen atoms are bonded to carbon and silicon atoms almost uniformly and the layer contains little structural defects. However, when the content of hydrogen, H₂/(Si₂H₆+C₃H₈) is higher than 10, the deposition rate of the hydrogenated amorphous silicon layer becomes substantially slow, decreasing production efficiency. Therefore, a gas mixing ratio of hydrogen H₂/(Si₂H₆+C₃H₈) below 10 is desirable in practice. Curve C is taken for reference and relates to an hydrogenated amorphous silicon carbide layer formed with a prior art technology employing a gaseous mixture of silane (SiH₄) gas and methane (CH₄)gas with a mixing ratio of 0.1 to 0.9. The relation between the mixing ratio of the gaseous mixture and the resistivity of the formed hydrogenated amorphous silicon carbide layer is the same as the one obtained with the first embodiment of the method, as shown in Fig.6. Table 2 indicates evaluation results for various specimens obtained with the second embodiment of the method. The experiments and the evaluation standards are the same as those hereinabove defined with respect to the first embodiment. Comparing both results tabulated in Table 1 and table 2 with each other, the hydrogenated amorphous silicon carbide layer obtained with the second embodiment shows somewhat superior characteristics to that obtained with the first embodiment. It is concluded that a mixing ratio of propane to disilane plus propane C₃H₈/(Si₂H₆+C₃H₈) ranging from 0.2 to 0.7, and a mixing ratio of hydrogen to the total of other gases H₂/(Si₂H₆+C₃H₈), ranging from 1 to 10 constitute the best selection for glow discharge CVD to form an hydrogenated amorphous silicon carbide top layer. Fig.11 is a diagram illustrating the empirical result of moisture durability with respect to a top layer formed according to the second embodiment under the condition of a room temperature of 35°C and a charging voltage of 5.5 Kv. Compared with the result shown in Fig.7, the measured durability is on almost the same level.

Claims: An electrophotographic multi-layered photosensitive member formed on a substrate (1), having a top layer (11) formed atop of said photosensitive member, characterized in that said top layer (11) consists of hydrogenated amorphous silicon carbide which has an atomic ratio of carbon to silicon plus carbon C/(Si+C) ranging from 0.17 to 0.45 and a ratio of the number of hydrogen atoms bonded to silicon atoms per silicon atom to the number of hydrogen atoms bonded to carbon atoms per carbon atom, {(Si-H)/Si}/{(C-H)/C}, ranging from 0.3 to 1.0, where C and Si designate the number of carbon atoms and the number of silicon atoms, respectively, and (Si-H) and (C-H) designate the number of hydrogen atoms bonded to a silicon atom and the number of hydrogen atoms bonded to a carbon atom, respectively. An electrophotographic multi-layered photosensitive member according to claim 1, characterized in that said substrate is a cylindrical printing drum (1) of electrically conductive material. An electrophotographic multi-layered photosensitive member according to any one of claims 1 and 2, characterized in that said photosensitive member includes a photoconductive layer (3) of hydrogenated amorphous silicon (a-Si:H), and said top layer (11) is formed over said photoconductive layer. A method for fabricating an electrophotographic multi-layered photosensitive member according to claim 1, comprising the steps of : evacuating a vacuum tight chamber in which a photosensitive member comprising a photoconductive layer on a substrate is disposed in a predetermined position; introducing a gaseous mixture into said vacuum tight chamber; and decomposing said gaseous mixture, and depositing the resulting material on said photoconductive layer to form said top layer of hydrogenated amorphous silicon carbide (a-SiC:H) thereon, said method being characterized in that said gaseous mixture comprises disilane (Si₂H₆) gas and propane (C₃H₈) gas, and has a mixing mol ratio C₃H₈/(Si₂H₆+C₃H₈) ranging from 0.2 to 0.6, where C₃H₈ and Si₂H₆ designate the numbers of propane molecules and disilane molecules, respectively. A method according to claim 4, whereby the photoconductive layer is formed by the steps of : evacuating the vacuum tight chamber in which the substrate is disposed in a predetermined position; introducing a gaseous mixture containing silicon component and hydrogen component into said vacuum tight chamber; decomposing said gaseous mixture and depositing a hydrogenated amorphous silicon photoconductive layer on said substrate; and evacuating said gaseous mixture remaining in said chamber; and whereby the top layer is formed according to the steps of claim 4. A method for fabricating an electrophotographic multi-layered photosensitive member according to claim 1, comprising the steps of : evacuating a vacuum tight chamber in which a photosensitive member comprising a photoconductive layer on a substrate is disposed in a predetermined position; introducing a gaseous mixture into said vacuum tight chamber; and decomposing and depositing said gaseous mixture to form said top layer of hydrogenated amorphous silicon carbide (a-SiC:H) over said photosensitive member, said method being characterized in that said gaseous mixture comprises disilane (Si₂H₆) gas, propane (C₃H₈) gas and hydrogen (H₂) gas, and has a mixing mol ratio C₃H₈/(Si₂H₆+C₃H₈) ranging from 0.2 to 0.7, and a mixing mol ratio of said hydrogen gas to the remaining gas, H₂/(Si₂H₆+C₃H₈) ranging from 1 to 10, where C₃H₈, Si₂H₆ and H₂ designate the numbers of propane molecules, disilane molecules, and hydrogen molecules, respectively. A method according to claim 6, whereby the photoconductive layer is formed by the steps of : evacuating the vacuum tight chamber in which said substrate is disposed in a predetermined position; introducing a gaseous mixture containing silicon component and hydrogen component into said vacuum tight chamber; decomposing said gasous mixture and depositing a hydrogenated amorphous silicon photoconductive layer on said substrate; and whereby the top layer is formed according to claim 6. A method according to any of claims 4 to 7, characterized in that said decomposing of said gaseous mixtures is performed by a glow discharge chemical vapour deposition method.

Inventors: ARAKI, SHIN MANHAIMU-NAKAYAMA 602, KAMAJI, HIDEKI FUJITSU TODOROKIRYO, KIYOTA, KOHEI, NO, HIROSHI FUJITSU DAI 3 ICHIGAORYO
Applicants: FUJITSU LIMITED
Application year: 1986
Main classification: G03G5
IPC codes: G03G5