Electrophotographic Recording Material and Method of Producing It

The present invention relates to a method of producing an electrophotographic recording material which includes applying a layer of amorphous silicon on a substrate using cathode sputtering.

An electrophotographic layer of amorphous silicon, hereinafter called an a-Si layer, is required to have a high dark resistance greater than 10.sup.12 .OMEGA. cm, a charging field intensity greater than 40 V/.mu.m and good chargeability.

Such layers are coated on relatively large-area copier drums which are adapted in size to accommodate standard paper sizes (DIN A4, A3).

German patent document (published without examination) No. 3,117,035 discloses the production of electrophotographic a-Si layers by means of a silane glow discharge process; in this case, a small amount of diborane and oxygen are added to the silane atmosphere. European Pat. No. 0,045,204 corresponding to U.S. Pat. No. 4,365,013 discloses the use of high frequency magnetron sputtering for the production of a-Si layers. In this process, cathode sputtering takes place in an atmosphere of argon and hydrogen and the resulting material is composed of a plurality of layers, with one of the layers having Si-O bonds.

The large-area a-Si layer applied to the copier drum should advisably have a thickness greater than or equal to 10 .mu.m. In apparatus operating with a silane glow discharge for the application of such photoconductive layers, a deposition rate up to 10 .mu.m/h is possible.

The Journal of Non-Crystalline Solids, Nos. 59 & 60, 1983, pages 1231-1234, discloses that a deposition rate up to 10 .mu.m/h can be achieved by means of silane glow discharge. Silane is a spontaneously combustible gas which flows through the reactor at a high flow rate of 1000 standard cubic centimeter per minute, hereinafter sccm. To achieve good electrophotographic characteristics for the a-Si layer, the layer must additionally be doped with boron from a gas mixture of B.sub.2 H.sub.6 +SiH.sub.4. Diborane is a very toxic gas.

German patent document (published without examination) No. 3,245,500 mentions the fact that an electrophotographic recording material can be produced with high growth rates by the use of magnetron cathode sputtering processes operating at high frequencies or with direct current. Tests have now shown that, with justifiable expenditures for engineering and labor, apparatus operating with high frequency magnetron sputtering produce deposition rates of only about 3 .mu.m/h. The publication does not reveal any further details regarding direct current magnetron sputtering.

A considerable amount of time is required to produce a large-area a-Si layer having a thickness greater than 10 .mu.m. The large amount of time involved has in the past prevented the mass production of a-Si layers for copier drums of the above-mentioned thickness by means of direct current magnetron sputtering.

It is an object of the invention to improve the method of producing an electrophotographic material in such a way that large-area a-Si layers having good electrophotographic characteristics and deposition rates of greater than 10 .mu.m/h can be produced with significantly less expenditures of time and money.

This is accomplished by the present invention which provides a method of producing an electrophotographic recording material, in which an aluminum substrate is coated with a blocking layer. The blocking layer is coated with a layer of amorphous silicon by direct current magnetron cathode sputtering. At least one sputter target containing silicon is used, and a power density of from about 2.0 W/cm.sup.2 to about 30 W/cm.sup.2 is used for the sputtering. The sputtering is performed in an atmosphere containing hydrogen and an inert gas, with a total pressure of inert gas and hydrogen being in a range from about 1.times.10.sup.-3 to about 10.times.10.sup.-3 mbar. This produces a layer of amorphous silicon having a hydrogen content of more than 40 atom %, an inert gas content in a range from 0.01 to 10 atom %, and in which the relative peak heights of the low and high temperature peaks during hydrogen effusion are approximately equal. The amorphous silicon layer is then coated with a cover layer.

The present invention has the following advantages.

At deposition rates of about 10 .mu.m/h, the recording material exhibits charging field intensities up to 80 V/.mu.m; its dark resistance at room temperature is 10.sup.12 to 10.sup.14 .OMEGA. cm. The resulting layers are homogeneous and amorphous and can be charged positively as well as negatively. A material rich in voids is obtained so that the a-Si layer adheres very well to the substrate. Due to the increased deposition rate, the material can be produced more economically.

FIG. 1 shows the hydrogen effusion curves of an a-Si layer produced by means of the method according to the invention and of an a-Si layer produced by means of high frequency magnetron cathode sputtering.

FIG. 2 shows the hydrogen effusion curves of an a-Si layer produced by means of the method according to the invention at a deposition rate of greater than 10 .mu.m/h and of an a-Si layer produced by means of direct current magnetron cathode sputtering and at a significantly lower deposition rate.

FIG. 3 shows, in a ten thousand times enlargement, the microstructure of a broken edge of an a-Si layer produced by means of high frequency magnetron cathode sputtering.

FIG. 4 shows, in a ten thousand times enlargement, the microstructure of a broken edge of an a-Si layer produced according to the method of the invention.

FIG. 5 is a diagram showing the charging potential, the dark discharge and the drop in exposure of an a-Si layer produced according to the method of the invention.

FIG. 6 is a diagram showing the change in dark conductivity of an a-Si layer produced according to the method of the invention as a function of the substrate temperature.

FIG. 7 is a diagram showing the drop in dark decay after 1 second following charging in a structure of Al/SiO.sub.x /a-Si as a function of the influx of hydrogen.

FIG. 8 is a cross-sectional representation of an electrophotographic recording material.

Manufacturing process:

Photoconductive layers of a structure of Al/SiO.sub.x /a-Si are produced on Al substrates; the SiO.sub.x forms a blocking layer.

The a-Si layer was produced under the following conditions:

To avoid difficulties during the direct current discharge, the resistance of the target is advisably selected to be less than or equal to 10 .OMEGA. cm.

Under these conditions, deposition rates greater than 10 .mu.m/h were obtained with acceptable electrophotographic data.

The a-Si layers produced according to the method of the invention may have an oxygen and carbon content of 0 to 10 atom % each; this results in greater photoconductivity and chargeability of the layers.

The material produced according to the invention and the material produced by means of high frequency magnetron cathode sputtering were subjected to hydrogen effusion measurements for the purpose of examining their structure and determining the hydrogen content in the a-Si layers.

In the diagram of FIG. 1, the hydrogen effusion curve A relates to an a-Si layer produced according to the method of the invention while curve B relates to an a-Si layer produced by means of high frequency magnetron sputtering. The ordinate marked DCM applies to curve A and the ordinate marked HFM applies to curve B; the temperature of the sample is plotted on the abscissa.

Curve A indicates a hydrogen content of 41.4 atom % for the a-Si layer produced by means of direct current magnetron cathode sputtering and curve B indicates a hydrogen content of 19.2 atom % for the a-Si layer produced by means of high frequency magnetron cathode sputtering.

The a-Si produced by direct current magnetron cathode sputtering contains inert gas from the plasma produced during cathode sputtering in an amount from 0.01 to 10 atom %, and is preferably 0.01 to 0.1 atom % of argon. As can be seen from the shape of curve A, the hydrogen in the a-Si layer produced according to the method of the invention effuses at temperatures around 400.degree. C. (low temperature peak C) and around 700.degree. C. (crystallization peak D). The low temperature peak C indicates that the material is rich in voids. The hydrogen discharged at about 400.degree. C. is present in the voids within the a-Si layer and contributes to the saturation of the inner surfaces of the layer. This saturation results in the good electronic and optical characteristics of the amorphous silicon. Particularly for relatively thick a-Si layers (>10 .mu.m), material rich in voids is preferred because it is more elastic and adheres better to the substrate. The crystallization peak D around 700.degree. C. indicates the percentage of bound hydrogen in the a-Si layer; a high percentage of bound hydrogen is significant for achieving good electrophotographic characteristics in the material.

To realize an a-Si layer produced by means of direct current magnetron sputtering with good electrophotographic characteristics, the relative heights of peaks C and D must be approximately the same. The hydrogen percentages that can be derived from peaks C and D indicate a high total hydrogen content greater than 40 atom % in the a-Si layer.

As can be seen from the shape of curve B, in an a-Si layer produced by means of high frequency magnetron cathode sputtering, the hydrogen effuses essentially around 400.degree. C. (low temperature peak C) and not additionally around 700.degree. C. The absence of a crystallization peak D at this temperature indicates a significantly lower percentage of hydrogen bound in the a-Si layer, thus explaining its poorer electrophotographic characteristics.

In the diagram according to FIG. 2, the hydrogen effusion curve A relates to an a-Si layer produced by the method according to the invention and at a deposition rate greater than 10 .mu.m/h (the shape of curve A corresponds to that of curve A of FIG. 1), while the hydrogen effusion curve B relates to an a-Si layer also grown by means of direct current magnetron cathode sputtering but at a lower deposition rate of about 4 .mu.m/h.

In both cases, there also is a low temperature peak C around 400.degree. C. and a crystallization peak C around 700.degree. C. The shape of curve B indicates that for the a-Si layer produced at the lower deposition rate, the relative heights of peaks C and D are not identical; peak C is noticeably lower than peak D. Thus, the resulting material is less elastic, lower in voids than the material on which curve A is based and there is less unbound hydrogen in the a-Si layer. The electrophotographic characteristics of the a-Si layer produced by means of direct current magnetron sputtering at a deposition rate of about 4 .mu.m/h (curve B) are clearly worse than those of the a-Si layer produced at the higher deposition rate (>10 .mu.m/h).

The improvement in the structure of a material produced with the method according to the invention is evident from a comparison of the microstructures shown in FIGS. 3 and 4; in each case, the substrate is marked 1 and the broken edge of the material is marked 2.

The microstructure shown in FIG. 3 is associated with an a-Si layer produced by means of high frequency magnetron cathode sputtering and exhibits a columnar structure with deep troughs which extend far into the layer; these troughs result in great degradation. As can be clearly seen in FIG. 4, the microstructure produced by the method of the present invention is significantly more refined and is highly homogeneous; columns and troughs are no longer in evidence, which in turn results in less degradation of the layer characteristics.

In the diagram of FIG. 5, the charging voltage U.sub.A in volts acting in the a-Si layer is plotted on the ordinate and the time in seconds is plotted on the abscissa.

If the a-Si layer is given a positive charge to a charging potential A of about 800 V, the dark discharge B is 14.5% in 1 second, while the drop in exposure C is 525 V for a measured illumination at a wavelength of 650 nm and an illumination intensity of 5.8 .mu.W/cm.sup.2. Subsequent illumination D with white light and an illumination intensity of 1 mW/cm.sup.2 produces a negligible residual potential of a few volts.

The data for the a-Si layer obtained by means of direct current magnetron sputtering at a relatively high deposition rate indicate that this layer is well suited for electrophotographic use. The power density for the direct current cathode sputtering process is 2.0 W/cm.sup.2 to 30 W/cm.sup.2, preferably up to 13 W/cm.sup.2.

In the diagram of FIG. 6, the dark conductivity of the a-Si layer is plotted logarithmically on the ordinate and the reciprocal temperature is plotted on the abscissa.

As can be seen from curve A, at room temperature and with a hydrogen content of 2.times.10.sup.-3 mbar in an argon-hydrogen gas mixture of 5.times.10.sup.-3 mbar total pressure, a dark conductivity of 3.5.times.10.sup.-14 .OMEGA..sup.-1 cm.sup.-1 results and, according to curve B, with a hydrogen content of 2.6.times.10.sup.-3, a dark conductivity of 2.5.times.10.sup.-13 .OMEGA..sup.-1 cm.sup.-1 results.

FIG. 7 shows the dark decay in percent after 1 second subsequent to charging the Al/SiO.sub.x -aSi structure plotted against the influx of hydrogen regulated by flow meters. The measurement was made for positive and negative charges (indicated in the drawing figure by + and - symbols). Curves A in each case show the values immediately after completion of the structure, curves B reflect measurements taken after 17 to 23 days and document the degradation of the structure. As can be seen, with a larger supply of hydrogen in the gas mixture, the dark decay value decreases; this can be explained by the greater hydrogen content in the a-Si layer. The dark decay reaches values up to 10%. The degradation is less for positive as well as negative charges if the hydrogen supply in the gas mixture is greater.

Referring to FIG. 8, a recording material produced according to the above-described method is composed of an aluminum substrate 10 and a blocking layer 11 which supports an amorphous silicon layer 12 on which there is disposed a cover layer 13. This silicon layer contains an inert gas at 0.01 to 10 atom %, preferably argon in a range from 0.01 to 0.1 atom %. Moreover, the hydrogen content is more than 40 atom %. The relative peak heights of the low and high temperature peaks during hydrogen effusion are approximately the same.

The blocking layer is preferably composed of SiO.sub.x or SiC.sub.x or of amorphous carbon (a-C:H) or doped, amorphous silicon. The cover layer is preferably composed of SiO.sub.x or SiC.sub.x or amorphous carbon or SiN.sub.x. The recording material is preferably employed for electrophotographic purposes.

The present disclosure relates to the subject matter disclosed in Federal Republic of Germany Application No. P 37 17 727.3 on May 26th, 1987, the entire specification of which is incorporated herein by reference.

It will be understood that the above description of the present invention is susceptible to various modifications, changes and adaptations, and the same are intended to be comprehended within the meaning and range of equivalents of the appended claims.