IMPROVED ENHANCEMENT LAYER FOR ELECTROPHOTOGRAPHIC DEVICES AND METHOD FOR DECREASING CHARGE FATIGUE THROUGH THE USE OF SAID LAYER

Abstract: An improved enhancement layer (18) operatively disposed between the top protective layer (19) and the photoconductive layer (16) of an electrophotographic device (10). The enhancement layer (18) is specifically tailored from a semicondcutor alloy material designed to substantially prevent charge carriers from being caught in deep midgap traps as said carriers move toward the surface of the electrophotogrpahic device (10) from the photoconductive layer (16) thereof. A method of substantially improving charge fatigue characteristics through the use of such an improved enhancement layer (18) is also disclosed.

Description: This invention relates to an electrophotographic medium. More particularly the invention concerns an improved enhancement layer which is specifically tailored to substantially eliminate charge fatigue in electrophotographic photoreceptors by forming the enhancement layer from semiconductor alloy material which has been intentionally doped so as to substantially reduce charge carrier trapping in deep medgap states. An electrophotographic medium according to the opening part of claim 1 is described in EP-A-0 137 516. This reference teaches to make the difference between the energy level of the enhancement layer or intermediate layer on the one hand and the photoconductive layer on the other hand as small as possible. The energy difference should be lower than 0,2 in order to match the energy bands of the enhancement layer and of the top-protective layer of the photoconductive layer as good as possible. Nitrogen is used as a dopant for this purpose and boron is used for compensation and controlling the FERMI-level. Furthermore Patent Abstracts of Japan, vol 7, number 188 (p-127) (1333) of August 17, 1983 describes to insert an inhibiting layer between a substrate and a photoconductive layer in order to implantate electric charge. The instant invention relates to improved enhancement layers for use in electrophotographic imaging processes. It is an object to increase the density of defect states and to prevent deep trapping of the photogenerated charge barriers and hence to prevent image flow. The invention is claimed in claim 1. Preferred embodiments are claimed in sub-claims. The improved enhancement layer of the instant invention is fabricated from semiconductor alloy material, comprising a decreased number of deep midgap defect sites in which charge carriers can be trapped. By decreasing the number of deep traps the rate of charge carrier emission from traps is increased and the problem of charge fatigue which is prevalent in prior art electrophotographic media is virtually eliminated. Electrophotography, also referred to generically as xerography, is an imaging process which relies upon the storage and discharge of an electrostatic charge by a photoconductive material for its operation. A photoconductive material is one which becomes electrically conductive in response to the absorption of illumination; i.e., light incident thereupon, and generates electron-hole pairs (referred to generally as "charge carriers"), within the bulk of the photoconductive material. It is these charge carriers which permit the passage of an electrical current through that material for the discharge of the static electrical charge (which charge is stored upon the outer surface of the electrophotographic media in the typical electrophotographic process). A typical photoreceptor includes a cylindrically-shaped, electrically conductive substrate member, generally formed of a metal such as aluminum. Other substrate configurations, such as planar sheets, curved sheets or metallized flexible belts may likewise by employed. The photoreceptor also includes a photoconductive layer, which, as previously described, is formed of a photoresistive material having a relatively low electrical conductivity in the dark and a relatively high electrical conductivity under illumination. Disposed between the photoconductive layer and the substrate member is a blocking layer, formed either by the oxide naturally occuring on the substrate member, or from a deposited layer of semiconductor alloy material. As will be discussed in greater detail hereinbelow, the blocking layer functions to prevent the flow of unwanted charge carriers from the substrate member into the photoconductive layer in which layer they could then neutralize the charge stored upon the top surface of the photoreceptor. A typical photoreceptor also generally includes a top protective layer disposed upon the photoconductive layer to stabilize electrostatic charge acceptance against changes due to adsorbed chemical species and to improve the photoreceptor durability. Finally, a photoreceptor also generally includes a top protective layer disposed upon the photoconductive layer to stabilize electrostatic charge acceptance against changes due to adsorbed chemical species and to improve the photoreceptor durability. Finally, a photoreceptor also may include an enhancement layer operatively disposed between the photoconductive layer and the top protective layer, the enhancement layer adapted to substantially prevent charge carriers from being caught in deep traps and hence prevent charge fatigue in the photoreceptor. In order to obtain high resolution copies, it is desirable that the electrophotographic photoreceptor accept and retain a high static electrical charge in the dark; it must also provide for the flow of the charge carriers which form that charge from portions of the photoreceptor to the grounded substrate, or from the substrate to the charge portions of the photoreceptor under illumination; and it must retain substantially all of the initial charge for an appropriate period of time in the non-illuminated portions without substantial decay thereof. Image-wise discharge of the photoreceptor occurs through the photoconductive process previously described. However, unwanted discharge may occur via charge injection at the top or bottom surface and/or through bulk thermal charge carrier generation in the photoconductor material. A major source of charge injection is at the metal substrate/semiconductor alloy material interface. The metal substrate provides a virtual sea of electrons available for injection and subsequent neutralization of, for example, the positive static charge on the surface of the photoreceptor. In the absence of any impediment, these electrons would immediately flow into the photoconductive layer; accordingly, all practical electrophotographic media include a bottom blocking layer disposed between the substrate and the photoconductive member. One area of particular concern causing problems in the operation of prior art electrophotographic media results from the inherent property exhibited by the semiconductor alloy material from which the layers of prior art constructions were fabricated, e.g., the inherent property of that material to trap charge carriers in deep sites in the energy gap thereof as they reach the interface between the photoconductive layer and the top protective layer. This condition has become known as charge fatigue and occurs when the failure of the charge carriers to quickly vacate traps results in a breakdown of the blocking function of the top protective layer. Once the top protective layer breaks down, a flow of charge carriers is able to freely move therethrough in an attempt to neutralize the electrostatic charge residing on the surface of the electrophotographic medium. This problem, as well as Applicants' solution, will be explained in detail in the following paragraphs. In the course of operation of the typical electrophotographic process, a positive corona charge is placed on the outer surface (the exposed surface of the top protective layer) of the electrophotographic media. The initial reaction of the photoconductive layer of the electrophotographic media to the application of this positive charge to the top surface thereof is to have any free electrons from the bulk be swept toward that surface in an attempt to neutralize the positive charge residing thereon. However, in the movement of these electrons from the bulk of the photoconductive layer to the outer surface of the top protective layer (on which surface the positive charge carriers have accumulated), said electrons encounter deep trap sites such as midgap defect states. While these trap sites are located throughout the bulk of the photoconductive layer, they are of particular importance when they reside near the interface of the photoconductive layer and the top protective layer. This is because the blocking function (the inability of the positive charge carriers electrostatically positioned on the periphery of the top protective layer to penetrate that layer) will cease to be effective (will "breakdown") when an electrical field of sufficient strength is placed across the top protective layer. Obviously, a given density of negative charge carriers trapped near the aforementioned interface of the top protective layer and the photoconductive layer will generate a sufficiently strong electrical field across the top protective layer to cause breakdown, whereas the same number of negative charge carriers trapped in the bulk thereof will not. Further, trapping sites located deep in the energy gap of a semiconductor alloy material release trapped charge carriers at a much slower rate than do sites located closer to one of the bands. This results from the fact that more thermal energy is required, for example, to re-excite a trapped electron from the deep sites which exist near the middle of the energy gap to the conduction band than is required to re-excite an electron from the shallower sites which exist closer to the conduction band. The slow release rate from deep traps gives rise to a higher equilibrium trap occupancy and thus a higher electric field distribution. It is important to note that in the fabrication of the typical electrophotographic photoreceptor which operates with a positive corona charge applied to outer surface thereof, the photoconductive layer thereof is made from a "pi-type" silicon:fluorine:hydrogen:boron alloy. As used herein, "pi-type" will refer to semiconductor alloy material, the Fermi level of which has been displaced from its undoped position closer to the conductive band to a position approximately "midgap". Further note that as used herein, the term "midgap" will be used to define a point in the energy gap of a semiconductor alloy material which is positioned approximately half-way between the valence band and the conductive band (in the case of 1.8 eV amorphous silicon:fluroine:hydrogen:boron alloy this is about 0.9 eV from each of the bands). It is necessary to make the photoconductive layer of the photoreceptor pi-type because the typical "intrinsic" amorphous silicon:hydrogen:fluorine alloy as deposited in a glow discharge decomposition process is slightly "nu-type" (the Fermi level of that material is slightly closer to the conductive band than to the valence band) and in a positive corona charge electrophotographic process, the movement of charge carriers through the photoconductive layer under illumination must be maximized while minimizing the thermal generation of charge carriers. It is to be noted that when the Fermi level is positioned at midgap (as after the addition of the p-dopant to the silicon:fluorine:hydrogen alloy material), electrons moving through said pi-type material will encounter deep traps from which they cannot readily emerge. This is because the deepest electron trap sites in a layer of semiconductor alloy material lie at or near the Fermi level and in this Pi type material this energy coincides with midgap. The thermal energy required to release an electron from a deep trap is dependent on the depth of that trap. More particularly, the time which a trapped electron will wait, on average, before being thermally emitted from any trap is given by the formula: t = [ν₀ EXP(-ΔE/kT)] where "ν₀" is the number of electrons attempting to escape per second, "ΔE" is the energy required to move an electron from the Fermi lever to the conduction band edge, and kT is the absolute temperature multiplied by Boltzman's constant. "ν₀" may be assumed to have a value of approximately 10¹² electrons per second in most solids. For a Fermi level position of 0.9 eV (midgap) the emission time is therefore calculated to be 4 x 10³ seconds at room temperature. This slow escape time means that it takes approximately 1.2 hours for a electron to vacate the trap. Obviously, an electrophotographic photoreceptor cannot tolerate such a slow electron discharge rate. If electrons, once trapped, remain confined for such a lengthy period of time, a large concentration of electrons trapped at the photoconductor layer/top protective layer interface will build up and this space charge and the positive charge accumulated on the surface of the top protective layer will create a very high electric field distortion across said top protective layer, which field causes the top protective layer to "breakdown". As used herein, "breakdown" refers to the inability of the top protective layer to inhibit the flow of charge carriers therethrough. Applicants have discovered that this breakdown phenomena can be eliminated by reducing the number of defect states which give rise to deep charge carrier traps. The addition of an "enhancement layer" operatively disposed between the top protective layer and the photoconductive layer beneficially affects the performance of an electrophotographic device incorporating that layer. Applicants now have determined that the addition of the enhancement layer (as fabricated in the manner taught therein) operated to reduce the escape time of charge carriers caught in deep traps previously encountered at the interface of the photoconductive layer by reducing the overall density of defect states in the material from which the enhancement layer was formed. However, the enhancement layer described in the aforementioned copending application, decreased the overall density of defect states by depositing intrinsic semiconductor alloy material by r.f. glow discharge rather than by microwave glow discharge (since microwave deposition tends to create additional defect states). Therefore, the enhancement layer of said aforementioned application relied upon a reduction in the overall density of defect states present in undoped semiconductor alloy material to aid in reducing the number of deep traps in which charge carriers could be caught in order to reduce charge fatigue. However, no attempt or even suggestion of how to optimize the chemical composition of the enhancement layer in order to further prevent charge carriers from being caught in the deep midgap traps was discussed or suggested in said application. An important advantage obtained by following the teachings of the present invention resides in the optimization of the enhancement layer so as to prevent charge carrier fatigue and improve the operational cycling time of electrophotographic devices incorporating said optimized enhancement layer. Moreover, by utilizing the disclosure found herein, charge carriers are substantially inhibited from falling into the deep midgap traps. Only relatively shallow defect states remain in which charge carriers may be trapped and the rate of emission of charge carriers from these shallow traps can be measured in terms of seconds rather than in terms of days. Therefore, in its broadest form, the present application relates to the positioning of the Fermi level of the semiconductor alloy material from which the enhancement layer is formed to a position above midgap. This results in the deep mid-gap states being occupied by electrons and thus not being effective as electron traps. In this way electrons moving through the enhancement layer do not have to pass through a region in which there are effective deep midgap traps. This translates into an electron escape time of less than about 1 second for a 1.8 eV silicon:hydrogen:fluorine:phosphine alloy having the Fermi thereof positioned in the most favored range of 0.75 to 0.65 eV from the conduction band. Because of the quick release time there will be no substantial build up of trapped charge in this region and therefore no high field distortion. Similarly, in instances where negative charging is utilized, positioning the Fermi level of the enhancement layer 0.75 to 0.65 eV from the valence band will allow for a similar quick release of trapped carriers. It is noteworthy that the subject inventors do not claim to have invented the concept of fixing the Fermi level of the amorphous semiconductor alloy material from which one of the operative layers of an electrophotographic photoreceptor is fabricated. Rather, said inventors claim to be the first to recognize that it is possible to substantially prevent charge carriers from being caught in deep midgap traps by pinning the Fermi level of the semiconductor alloy material from which the enhancement layer is fabricated at a point approximately 0.8 to 0.5 eV from either the conduction or valence band. Applicants' discovery is to be sharply contrasted to a technique described by Mort , et al in a paper entitled "Field-effect Phenomena in Hydrogenated Amorphous Silicon Photoreceptors" published in the Journal of Applied Physics, April 16, 1984 at page 3197. In this paper, Mort , et al describe a process for the elimination of field effect in photoreceptors, which process was accomplished by the proper doping of the a-Si:H-insulator interface. Mort , et al observed Fermi level motion under the influence of the field generated by corona charging of the electrophotographic photoreceptor, the deleterious effects of which they proposed to counteract by doping. More particularly, Mort , et al proposed the addition of a boron-doped trapping layer interposed between the top surface of the photoconductive layer and the insulating layer (the top protective layer) for quenching the effects of the electric field and removing the effect of "field-inducing blurring" (commonly referred to as "image-flow"). In this manner, Mort , et al were able to counteract the problem of "image-flow". However, Mort , et al were not concerned with and failed to address the concurrently present problem of "charge fatigue". Moreover, Mort , et al , by adding boron dopant, shifted the Fermi level of the semiconductor alloy material toward the valence band. By so shifting the Fermi level of the semiconductor alloy material, they inherently caused electrons, attempting to move to the conduction band, to pass through the deep midgap states which are responsible for the problem of charge fatigue and which the subject application attempts to avoid. Note that Mort , et al specifically prohibit the use of phosphorous doping to shift the Fermi level of the enhancement layer toward the conduction band because such a shift would make the semiconductor alloy material thereof more conductive, thereby causing just the type of lateral electron flow they seek to avoid. In contrast thereto, Applicants first intentionally phosphorous doped the semiconductor alloy material of the enhancement layer which is interposed between the photoconductive layer and the top protective layer in order to shift the Fermi level thereof toward the conduction band. By so shifting the Fermi level of the semiconductor alloy material, the electrons do not have to move through and become caught in the deep midgap states present in the energy gap thereof. This substantially eliminates the problems of charge fatigue by keeping the electrons out of the deep midgap states. Applicants then introduce both boron dopant and phosphorus dopant so as to pin the Fermi level at that preselected position in the energy gap through the addition of defect states on both sides of the pinned Fermi level. The added defect states, being shallow, not only solve charge fatigue problems, but those states are sufficiently numerous to inhibit lateral electron flow, quench the field effect and hence simultaneously solve image flow problems. As should accordingly be apparent from the foregoing discussion, while Mort , et al propose a solution to the problem of image flow in electrophotographic media, they fail to consider the problem of charge fatigue which their solution to image flow inherently invokes. The subject invention, on the other hand, solves both problems by first appropriately shifting and then pinning the Fermi level of the semiconductor alloy material of a newly added enhancement layer. In light of the many definitions utilized for the terms "amorphous" and "microcrystalline" in the scientific and patent literature it will be helpful to clarify the definition of those terms as used herein. The term "amorphous", as used herein, is defined to include alloys or materials exhibiting long range disorder, although said alloys or materials may exhibit short or intermediate range order or even contain crystalline inclusions. As used herein the term "microcrystalline" is defined as a unique class of said amorphous materials characterized by a volume fraction of crystalline inclusions, said volume fraction of inclusions being greater than a threshold value at which the onset of substantial changes in certain key parameters such as electrical conductivity, band gap and absorption constant occur. It is to be noted that pursuant to the foregoing definitions, the microcrystalline, materials employed in the practice of the instant invention fall within the generic term "amorphous" as defined hereinabove. These and other objects and advantages of the instant invention will be apparent from the detailed description of the invention, the brief description of the drawings and the claims which follow. According to the present invention the electrophotographic medium comprises an electrically conductive substrate, a bottom layer overlying the substrate which is adapted to block the free flow of charge carriers from the substrate, a photoconductive layer overlying the bottom layer which is adapted to discharge an electrostatic charge, and an enhancement layer overlying the photoconductive layer which is adapted to substantially reduce the number of charge carriers caught in deep midgap traps, said enhancement layer formed of intentionally doped semiconductor alloy material and a top protective layer overlying the enhancement layer which is adapted to protect the photoconductive layer from ambient conditions and aid in the transport of charge carriers under illumination. The bottom blocking layer is preferably formed of a doped microcrystalline semiconductor alloy material which is selected from the group consisting essentially of chalcogens, amorphous silicon alloys, amorphous germanium alloys, amorphous silicon-germanium alloys, photoconductive organic polymers and combinations thereof. The enhancement layer is preferably fabricated from a material selected from the group consisting essentially of amorphous silicon alloys, amorphous germanium alloys and amorphous silicon-germanium alloys. The enhancement layer is yet more favorably fabricated from an amorphous silicon alloy and the Fermi level thereof is moved to within 0.5 to 0.8 eV of the conduction or valence band. In a yet more preferred embodiment, the Fermi level of the enhancement layer is moved to within 0.65 to 0.75 eV of the conduction or valence band. In this manner, the enhancement layer is fabricated from a material which has been specifically tailored so as to provide for the thermal emission of charge carriers from traps at the interface thereof with the top protective layer in approximately one second or less. The thickness of the enhancement layer is approximately 250 nm - 1 µm and preferably about 0,5 µm. The Fermi level of the enhancement layer may be pinned at a given location from the conduction band. The pinning of the Fermi level may be accomplished by including both phosphorus and boron, in the semiconductor alloy matrix for adding shallow states at the energy gap of the semiconductor matrix so as to pin said Fermi level at a preselected position. There is further disclosed herein a method of preventing charge fatigue in electrophotographic media of the type which include an electrically conductive substrate, a bottom charge injection blocking layer, a photoconductive layer and a top protective layer. The method includes the steps of forming an enhancement layer from an intentionally doped semiconductor alloy material and operatively disposing said enhancement layer between the photoconductive layer and the top protective layer so that the enhancement layer is adapted to substantially decrease the number of charge carriers caught in deep midgap traps as charge carriers approach the interface between said enhancement layer and the top protective layer. The method includes the further steps of forming the back blocking layer from a microcrystalline, boron doped silicon:hydrogen:fluorine alloy, the extent of boron doping being sufficient to make the material degenerate and (2) forming the enhancement layer from a material selected from the group consisting essentially of amorphous silicon alloys, amorphous germanium alloys and amorphous silicon-germanium alloys. In the preferred embodiment, the further step is included of moving the Fermi level of the enhancement layer to within 0.5 to 0.8 eV of the conduction or valence band and preferably to within 0.65 to 0.75 eV of the band. In this manner, the material from which the enhancement layer is fabricated is tailored so as to provide for the emission of charge carriers from said traps in approximately one second or less. The method may still include the further step of forming the enhancement layer to be approximately 250 nm - 1 µm thick and preferably approximately 0,5 µm thick. In the most preferred embodiment, the Fermi level of the semiconductor alloy material from which the enhancement layer is fabricated is pinned by introducing both boron and phosphorus, into the host semiconductor matrix thereof so as to add additional shallow states at both sides of the Fermi level in the energy gap thereof. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a partial cross-sectional view of an electrophotographic photoreceptor which includes the improved enhancement layer of the instant invention; and, Figure 2 is a schematic, cross-sectional view of a microwave glow discharge deposition apparatus as adapted for the manufacture of electrophotographic photoreceptors such as illustrated in Figure 1. Referring now to figure 1, there is illustrated in a partial cross-sectional view, a generally cylindrically shaped electrophotographic photoreceptor 10 of the type incorporating all of the innovative principles disclosed within the specification of the instant invention. The photoreceptor 10 includes a generally cylindrically shaped substrate 12 formed, in this embodiment, of aluminum, although other nondeformable metals such as stainless steel could also be employed as a preferred embodiment. The periphery of the aluminum substrate 12 is provided with a smooth, substantially defect free surface by any well known technique such as diamond machining and/or polishing. Disposed immediately atop the deposition surface of the substrate 12 is deposited a doped layer 14 of microcrystalline semiconductor alloy material which has been specifically designed and adapted to serve as the bottom blocking layer for said photoreceptor 10. In keeping with the teachings disclosed in US-A-4,582,773, the blocking layer 14 is formed of highly doped, highly conductive microcrystalline semiconductor alloy material. Disposed immediately atop the bottom blocking layer 14 is the photoconductive layer 16 which may be formed from a wide variety of photoconductive materials. Among some of the preferred materials are doped intrinsic amorphous silicon alloys, amorphous germanium alloys, amorphous silicon-germanium alloys, chalcogenide materials and organic photoconductive polymers. Disposed atop the photoconductive layer 16 is the improved enhancement layer 18 of the subject invention, said enhancement layer specifically designed to substantially reduce the problem of charge fatigue described in the Background section of this specification. Finally, the photoreceptor 10 includes a top protective layer 19 operatively disposed atop the enhancement layer 18, which protective layer 19 (1) protects the upper surface of the photoconductive layer 16 from ambient conditions and (2) separates the charge stored on the surface of the photoreceptor 10 from carriers generated in the photoconductive layer 16. In accordance with the principles of the first embodiment of the instant invention, the improved enhancement layer 18 is formed of an intentionally doped semiconductor alloy material. The purpose of intentionally doping the enhancement layer 18 is to move the Fermi level closer to the conduction band (in the case of a positive corona charge) of the semiconductor alloy material from which said layer is fabricated. Obviously, in the case of a negative surface charge, it would be desirable to intentionally dope the enhancement layer 18 so as to move the Fermi level of the semiconductor alloy material from which it is fabricated closer to the valence band. A wide variety of semiconductor alloy materials may be employed from which to fabricate the enhancement layer 18. Among some of the favored materials are silicon:hydrogen alloys, silicon:hydrogen:halogen alloys, germanium:hydrogen alloys, germanium:hydrogen:halogen alloys, silicon:germanium:hydrogen alloys, and silicon:hydrogen:halogen alloys. Among the halongenated materials, fluorinated alloys are particularly preferred. Doping of the semiconductor alloy material may be accomplished by any technique and employing any material which is well known to those of ordinary skill in the art. Because Applicants' previous enhancement layers were prepared with a reduced density of defect states, the charge carriers moving through that layer from the photoconductive layer 16 to neutralize charge located at the surface of the top protective layer 19 were not caught in as many deep midgap traps. The result was a reduction in the number of carriers which required the aforedescribed lengthy period of time required to be emitted from the deep traps. By employing the principles espoused in the subject application and employing an enhancement layer 18, the Fermi level of which is moved to a desired location and pinned so that charge carriers are able to avoid the deep midgap states present in the silicon alloy material from which the layer is fabricated, the residency time of charge carriers caught in traps is significantly decreased since only the traps accessible to the carriers are shallow traps. The absence of deep trapped carriers not only prevents a breakdown of the top protective layer 20, but significantly increases the cycle time in which the electrophotographic medium 10 is capable of recovering lost surface charge and readying itself for reproducing a further copy. While a wide variety of semiconductor materials may be employed from which to fabricate the photoconductive layer 16, the amorphous silicon alloys, amorphous germanium alloys and amorphous-silicon germanium alloys were found to be particularly advantageous. Such alloys and methods for their preparation are disclosed in the patents and applications referred to and incorporated by reference hereinabove. The conductivity type of the materials from which the blocking layer 14 and the photoconductive layer 16 are fabricated, are chosen so as to establish a blocking contact therebetween whereby injection of unwanted charge carriers into the bulk of the photoconductive layer 16 is effectively inhibited. In cases where the photoreceptor 10 is adapted to be electrostatically charged with a positive charge, the bottom blocking layer 14 will preferably be fabricated from a heavily p-doped alloy and the photoconductive layer 16 will be fabricated from an intrinsic semiconductor layer, an n-doped semiconductor layer or a lightly p-doped semiconductor layer. Combinations of these conductivity types will result in the substantial inhibition of electron flow from the substrate 12 into the bulk of the photoconductor layer 16. It should be noted that intrinsic, or lightly doped semiconductor layers are generally favored for the fabrication of the photoconductive layer 16 insofar as such materials will have a lower rate of thermal charge carrier generation than will more heavily doped materials. Layers of intrinsic semicondcutor alloy materials are most preferably favored insofar as such layers have the lowest number of defect states per unit volume and the most favorable discharge characteristics. In cases where the electrophotographic photoreceptor 10 is adapted for a negative charging, it will be desirable to prevent the flow of holes into the bulk of the photoconductive layer 16. In such instances the conductivity types of the layers of semiconductor alloy material referred to hereinabove will be reversed, although obviously, intrinsic materials will still have significant utility. The maximum electrostatic voltage which the photoreceptor 10 can sustain (V sat ) will depend upon the efficiency of the blocking layer 14 as well as the thickness of the photoconductive layer 16. For a given blocking layer efficiency, a photoreceptor 10 having a thicker photoconductive layer 16 will sustain a greater voltage. For this reason, charging capacity or charge acceptance is generally referred to in terms of volts per micron thickness of the photoconductive layer 16. For economy of fabrication and elimination of stress it is generally desirable to have the total thickness of the photoconductive layer 16 be 25 µm or less. It is also desirable to have as high a static charge maintained thereupon as possible. Accordingly, gains in barrier layer efficiency, in terms of volts per micron charging capacity, translate directly into improved overall photoreceptor performance. It has routinely been found that photoreceptors structured in accordance with the principles of the instant invention are able to sustain voltages of greater than 50 V/µm on up to a point nearing the dielectric breakdown of the semiconductor alloy material. The intentionally doped semiconductor alloy material of the enhancement layer of the instant invention is produceable by a wide variety of deposition techniques, all of which are well known to those skilled in the art. Said deposition techniques include, by way of illustration, and not limitation, chemical vapor deposition techniques, photoassisted chemical vapor deposition techniques, sputtering, evaporation, electroplating, plasma spray techniques, free radical spray techniques, and glow discharge deposition techniques. At present, glow discharge deposition techniques have been found to have particular utility in the fabrication of the enhancement layer of the instant invention. In glow discharge deposition processes, a substrate is disposed in a chamber maintained at less than atmospheric pressure. A process gas mixture including a precursor of the semiconductor alloy material (and dopants) to be deposited is introduced into the chamber and energized with electromagnetic energy. The electromagnetic energy activates the precursor gas mixture to form ions and/or radicals and/or other activated species thereof which species effect the deposition of a layer of semiconductor material upon the substrate. The electromagnetic energy employed may be dc energy, or ac energy such as radio frequency or microwave energy. Microwave energy has been found particularly advantageous for the fabrication of electrophotographic photoreceptors insofar as it allows for the rapid, economical preparation of successive layers of high quality semiconductor alloy material. Referring now to Figure 2, there is illustrated a cross-sectional view of one particular apparatus 20 adapted for the microwave energized deposition of layers of semiconductor material onto a plurality of cylindrical drums or substrate members 12. It is in an apparatus of this type that the electrophotographic photoreceptor 10 of Figure 1 may be advantageously fabricated. The apparatus 20 includes a deposition chamber 22, having a pump-out port 24 adapted for suitable connection to a vacuum pump for removing reaction products from the chamber and maintaining the interior thereof at an appropriate pressure to facilitate the deposition process. The chamber 22 further includes a plurality of reaction gas mixture input ports 26, 28 and 30 through which reaction gas mixtures are introduced into the deposition environment. Supported within the chamber 22 are a plurality of cylindrical drums or substrate members 12. The drums 12 are arranged in close proximity, with the longitudinal axes thereof disposed substantially mutually parallel and the outer surfaces of adjacent drums being closely spaced apart so as to define an inner chamber region 32. For supporting the drums 12 in this manner, the chamber 22 includes a pair of interior upstanding walls, one of which is illustrated at 34. The walls support thereacross a plurality of stationary shafts 38. Each of the drums 12 is mounted for rotation on a respective one of the shafts 38 by a pair of disc shaped spacers 42 having outer dimensions corresponding to the inner dimension of the drums 12, to thereby make frictional engagement therewith. The spacers 42 are driven by a motor and chain drive, not shown, so as to cause rotation of the cylindrical drums 12 during the coating process for facilitating uniform deposition of material upon the entire outer surface thereof. As previously mentioned, the drums 12 are disposed so that the outer surfaces thereof are closely spaced apart so as to form the inner chamber 32. As can be noted in figure 2, the reaction gases from which the deposition plasma will be formed are introduced into the inner chamber 32 through at least one of the plurality of narrow passages 52 formed between a given pair of adjacent drums 12. Preferably, the reaction gases are introduced into the inner chamber 32 through every other one of the narrow passages 52. It can be noted in the figure each pair of adjacent drums 12 is provided with a gas shroud 54 connected to one of the reaction gas input ports 26, 28 and 30 by a conduit 56. Each shroud 54 defines a reaction gas reservoir 58 adjacent to the narrow passage through which the reaction gas is introduced. The shrouds 54 further include lateral extensions 60 which extend from opposite sides of the reservoir 58 and along the circumfrence of the drums 12 to form narrow channel 62 between the shroud extension 60 and the outer surfaces of the drums 12. The shrouds 54 are configured as described above so as to assure that a large percentage of the reaction gas will flow into the inner chamber 32 and maintain uniform gas flow along the entire lateral extent of the drums 12. As can be noted in the figure, narrow passages 66 which are not utilized for reaction gas introduction into the chamber 32 are utilized for removing reaction products from the inner chamber 32. When the pump coupled to the pump out port 24 is energized, the interior of the chamber 22 and the inner chamber 32 is pumped out through the narrow passages 66. In this manner reaction products can be extracted from the chamber 22, and the interior of the inner chamber 32 can be maintained at a suitable pressure for deposition. To facilitate the production of precursor free radicals and/or ions and/or other activated species from the process gas mixture, the apparatus further includes a microwave energy source, such as a magnetron with a waveguide assembly or an antenna, disposed so as to provide microwave energy to the inner chamber 32. As depicted in figure 2, the apparatus 20 includes a window 96 formed of a microwave permeable material such as glass or quartz. The window 96 in addition to enclosing the inner chamber 32, allows for dispostion of the magnetron or other microwave energy source exteriorly of the chamber 22, thereby isolating it from the environment of the process gas mixture. During the deposition process it may be desirable to maintain the drums 12 at an elevated temperature. To that end, the apparatus 20 may further include a plurality of heating elements, not shown, disposed so as to heat the drums 12. For the deposition of amorphous semiconductor alloys the drums are generally heated to a temperature between 20°C and 400°C and preferrably about 225°C. EXAMPLE In this example, an electrophotographic photoreceptor was fabricated in a microwave energized glow discharge deposition system generally similar to that depicted with reference to Figure 2. A cleansed aluminum substrate was first operatively positioned in the deposition apparatus and then the chamber was evacuated and a gas mixture comprising 0,15 SCCM (standard cubic centimeters per minute) of a 10.8% mixture of BF₃ in hydrogen; 75 SCCM of 1000 ppm SiH₄ in hydrogen and 45 SCCM of hydrogen was introduced thereinto. The pumping speed was constantly adjusted to maintain a total pressure of approximately 100 µm in the chamber while the substrate was maintained at a temperature of approximately 300°C. A bias of +80 volts was established by disposing a charged wire in the plasma region. Microwave energy of 2,45 GHz was introduced into the deposition region. These conditions resulted in the deposition of the bottom blocking layer of boron doped microcrystalline silicon:hydrogen:fluorine alloy material. The deposition rate was approximately 2 nm/s and the deposition continued until the boron doped microcrystalline blocking layer obtained a total thickness of approximately 750 nm. At this point the microwave energy was terminated, and the reaction gas mixture flowing therethrough was changed to a mixture comprising 0,5 SCCM of a 0,18% mixture of BF₃ in hydrogen; 30 SCCM of SiH₄, 4 SCCM of SiF₄ and 40 SCCM of hydrogen. Pressure was maintained at 50 µm and microwave energy of 2,45 GHz was introduced into the apparatus. This resulted in the deposition of a layer of lightly p-doped amorphous silicon:hydrogen:fluorine alloy material. The deposition of this alloy material (which formed the photoconductive layer of the electrophotographic medium) occured at a rate of approximately 10 nm/s and continued until approximately 20 µm of the amorphous silicon alloy material was deposited. In order to deposit the amorphous silicon alloy from which the improved enhancement layer of the subject invention is fabricated, it is necessary to add sufficient amounts of phosphorous obtained from phosphine gas so as to move the Fermi level of the deposited alloy to approximately 0,75 to 0,65 eV from the conduction band thereof. In order to both accomplish this Fermi level movement and fix the Fermi level at this position so as to avoid splitting said level upon illumination, approximately equal quantities of phosphine and boron-trifluorine gas are introduced into the precursor gas mixture after the Fermi level has been moved to the 0,75 to 0,65 eV range. The remainder of the deposition parameters are kept the same as in the foregoing paragraph. A top protective layer of an amorphous silicon:carbon:hydrogen:fluorine alloy is deposited atop the improved enhancement layer. A gas mixture comprising 2 SCCM of SiH₄, 30 SCCM of CH₄ and 2 SCCM of SiF₄ is introduced into the deposition apparatus for depositing this layer. Next, the microwave energy source is energized and deposition of a layer of amorphous silicon:hydrogen:fluorine:carbon occured at a rate of approximately 4 nm/s. Deposition continued until approximately 500 nm of the protective layer was deposited at which time the microwave energy was terminated, the substrate was cooled to 100°C, the apparatus was raised to atmospheric pressure and the thus prepared electrophotographic photoreceptor was removed for testing. Obviously, the foregoing process could be modified to fabricate a photoreceptor adapted for negative charging by merely substituting opposite dopants in roughly equimolar quantities. That is to say, the bottommost blocking layer would be a phosphorous doped layer; the photoconductive layer would be inrinsic or slightly phosphorous doped; the enhancement layer would have its Fermi level positioned and pinned within 0,65 to 0,75 eV of the valence band. It should be understood that numerous modifications and variations should be made to the foregoing within the scope of the instant invention. While the foregoing example was oriented toward electrophotographic photoreceptors formed of amorphous silicon alloy materials, the instant invention is obviously not so limited but may be utilized in conjunction with the fabrication of photoreceptors which include a wide variety of photoconductive material such as chalcogenide photoconductive materials as well as organic photoconductive materials. The blocking layers, discussed herein, may be fabricated from a wide variety of microcrystalline semiconductor alloy materials in keeping in spirit of the instant invention.

Claims: Electrophotographic medium comprising: a) an electrically conductive substrate (12), b) a photoconductive layer (16) above said substrate (12), the photoconductive layer (16) being adapted to discharge an electrostatic charge and fabricated from a material selected from the group consisting of: chalcogenide photoconductors, amorphous silicon alloys, amorphous germanium alloys, amorphous silicon germanium alloys, photoconductive organic polymers and combinations thereof, c) an enhancement layer (18) fabricated from a semi-conductor alloy material selected from the group consisting of: amorphous silicon alloys, amorphous germanium alloys and amorphous silicon-germanium alloys; said enhancement layer (18) overlaying the photoconductive layer (16) and being doped by boron as a dopant for controlling the Fermi level thereof, d) a top-protective layer (19) overlaying the enhancement layer (18) said protective layer (19) being adapted to protect the photoconductive layer (16) from ambient conditions, characterized in that said enhancement layer (18) is adapted to substantially reduce the number of charge carriers caught in deep midgap traps for preventing charge fatigue by intentionally doping by phosphorous in addition to boron so as to move the Fermi level of said enhancement layer (18) to a predetermined position and to pin the Fermi level in order to avoid said deep trapping and image flow, and that e) a bottom layer (14) is inserted between said substrate (12) and said photoconductive layer (16) the bottom layer (14) being adapted to block the free flow of charge carriers from the substrate (12). An electrophotographic medium as claimed in claim 1, characterized in that said enhancement layer (18) has a thickness of between 250-1000 nm. An electrophotographic medium as claimed in claim 2, characterized in that said enhancement layer (18) has a thickness of about 500 nm. An electrophotographic medium as claimed in one of the preceding claims, characterized in that said enhancement layer (18) comprises halogenes. An electrophotographic medium as claimed in claim 5, characterized in that said enhancement layer (18) comprises fluorine.

Inventors: HUDGENS, STEPHEN J., JOHNCOCK, ANNETTE
Applicants: ENERGY CONVERSION DEVICES, INC.
Application year: 1986
Main classification: G03G5
IPC codes: G03G5