CA1249476A - Low field electrophotographic process - Google Patents

Abstract

-i-LOW FIELD ELECTROPHOTOGRAPHIC PROCESS

ABSTRACT OF THE DISCLOSURE

An electrophotographic process in which a photoconductive insulating element, comprising a layer of intrinsic hydrogenated amorphous silicon in electrical contact with a layer of doped hydrogenated amorphous silicon, is electrostatically charged to a low level of surface voltage, such as, for example, a level of ten volts, provides an advantageous combination of very high electrophotographic sensitivity with minimal electrical noise.

Description

~24~76 LOW FIELD ELECTROPHOTOGRAPHIC PROCESS

FIELD OF THE INVENTION

This invention relates in general to electrophotography and in particular to a novel low field electrophotographic process. More specifically, this invention relates to a low field electrophotographic process employing a photoconductive insulating element which exhibits high quantum efficiency at low voltage.

BACKGROUND OF THE INVENTION

Photoconductive elements comprise a conducting support bearing a layer of a photo-conductivP material which is insulating in the dark but which becomes conductive upon exposure to radiation. A common technique for forming images with such elements is to uniformly electrostatically charge the surface of the element and ~hen imagewise expose it to radiation. In areas where the photo-conductive layer is irradiated, mobile charge carriers are generated which migrate to the surface of the element and there dissipate the surface charge. This leaves behind a charge pattern in nonirradiated areas, referred to as a latent electro-static image. This latent electrostatic image can then be developed, either on the surface on which it is formed, or on another surface to which it has been transferred, by application of a liquid or dry developer composition which contains electroscopic marking particles. These particles are selectively attracted to and deposit in the charged areas or are repelled by the charged areas and selectiveiy ~9$
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deposited in the uncharged areas. The pattern of marking particles can be fixed to the surface on which they are deposited or they can be transferred to another surface and fixed there.
Photoconductive elements can comprise a single active layer, containing the photoconductive material, or they can comprise multiple active layers~ Elements with multiple active layers (sometimes referred to as multi-active elements) have at leas't one charge-generating layer and ~t least one charge-transport layer. The charge-generating layer responds to radiation by generating mobile charge carriers and the charge-transport layer facilitates migration of the charge carriers to the surface of the element, where they dissipate the uniform electrostatic charge in light-struck areas and form ~he latent electrostatic image.
l'he photoreceptor properties that determine the radiation necessary to form the latent image are the quantum efficiency, the thickness, the dielectric constant, and the existence of trapping~
In the simplest case, where trapping can be neglected, the exposure can be expressed as:
k ~ ~V
Le~ J
where E is the exposure in ergs/cm2, ~ the relative dielectric constant, L the thickness in cm, e the electronic charge in esu~ A the wavelength in nm, ~ the quantum efficiency, k a constant equal to 5.2 X 10 133 and ~V the voltage difference between the image and background area, Vi - Vb. The quan~um efficiency, which'cannot exceed unity, represents the fraction of incident photons that are absorbed and result in free electron-hole pairs.

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a99~76 For electrophotographic processes known heretofore, ~V is typi-cally ~00-500 V. ~ssuming typical values oE ~ = 3.0, ~ = 500 nm, and ~ =
10- cm, the above equation predicts an exposure energy of 11.8 to 14.7 ergs/cm . This assumes that there is no trapping and is based on the absorbed radiation. In practice, the radiation is not completely absorbed, and the exposure is correspondingly larger. Thus, most photoreceptors require exposures in the range of 20-100 ergs/cm2 to form an electrostatic image. These are equivalent to ASA ratings between 0.1 and 0.02. In contrast, the exposure required to form a latent image in conventional silver halide photography is 15 in the range of 10 2 to 10 1 ergs/cm2, or less, and, accordingly, the radiation sensitivity of electrophotography is less than that of conventional silver halide photography by a factor of at least While increases in electrophotographic sensitivity can be realized by increases in thickness or quantum efficiency, these effects are limited. Increases in photoreceptor thickness tend to result in trapping, which gives rise to a sharp decrease in sensitivity. Since the quantum efficiency cannot exceed unity, increases in efficiency are limited. For the example discussed in the preceeding paragraph, the maximum increase in sensitivity would be a Eactor of about 5. In practice, absorption and reflection losses, photogeneration efficiencies of less than unity, etc., would limit the increase to probably no more than a factor of about 3. Consequently, if the sensitivity is to be significantly increased, the magnitude of the voltage difference between the image and background areas must be reduced.

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Moreover, if the sensitivity is to be increased without a concurrent increase in electrostatic noise, the magnitude of Vb must also be reduced, since a reduction in ~ without a corresponding reduction in Vb results in a very low signal to - noise (S/N) ratio.
A reduction in both ~V and Vb requires that the photoreceptor be initially charged to very low voltages, e.g., VO = 10 volts. However, with photoconductive elements of both the single-active-layer and mult;ple-active layer types, the quantum efficiency typically decreases sharply with decreasing voltage. [See D. M. Pai and R. C.
Enc~, Phys. Rev. 11, 5163, (1975); P. J. Melz, J. Chem. Phys. 57, 1694, (1972); and P. M.
Borsenberger and D. C. Hoesterey, J. Appl. Phys. 51, 4248 (1980)]. As a result, electrophotographic processes typically employ a high initial voltage, such as 500 volts, and electrostatic latent image formation typically requires exposures oE the order of 20 to 100 ergs/cm2.
It is toward the objective of providing a high speed electrophotographic process which exhibits minimal electrical noise, and, in particular, a low field process employing a very low initial voltage, such as a voltage of 10 volts, that the present invention is directed.

SU~IARY OF THE INVENTION
. . . _ The electrophotographic process of this invention comprises the steps of:
(1) providing a photoconductive insulating element comprising:
(a) an electrically-conductive support, (b) a barrier layer `overlying the support, and (c) a photoconductive stratum overlying the barrier layer which comprises a layer of intrinsic hydrogenated amorphous silicon in - electrical contact with a layer of doped hydrogenated amorphous silicon and in which the doped layer is very thin in relation to the thickness of the intrinsic layer;

(2) uniformly electrostatically charging the element to a surface voltage in the range of from S to S0 volts, and (3) lmage-wise exposing the element to activating radiation to thereby form a latent electrostatic image on the sur~ace thereof.

The term "activating radiation" as used herein is defined as electromagnetic radiation which is capable of generating electron-hole pairs in the photoconductive insulating element upon exposure thereof.
Use of a very low initial voltage in the process of this invention, that is a voltage in the range of 5 to 50 volts, in combination with use of an amorphous silicon element of the particular 25 structure described herein has been unexpectedly found to provide the desired characteristics of very high electrophotographic sensitivity without excessive electrical noise. The low Vb and low ~V
which characterize the process are rendered feasible 30 by the unique electrophotographic properties of the aforesaid element, which provides high quantum efficiency at low voltage.

~L2~947 BRIEF l)ESCRIPTIO~ OF-THE D~AWINGS
, FIGURE 1 is a logarithmic plot of quantum efficiency versus electric field for a photoconductive insulating element that is useful in - 5 the process of this invention and for a control element.
FIGU~E 2 is a ~-logE plot for the test element and control element of FIGU~F~ 1.

I)Esc~Ipl~IoN OF THE PREFERRED ~ sODll~ENTS

The preparation of thin films of amorphous silicon, hereinafter referred to as ~ -Si, by the glow discharge decomposition of silane gas, SiH4, has been known for a number of years. (See, for example, R. C. Chittick, J. H. Alexander and ~. F.
Sterling, J. Electrochem. Soc., 116, 77, lg69 and R. C. Chittick, J. N-Cryst. Solids, 3, 255, 1970).
It is also known that the degree of conductivity and conductivity type of these thin films can be varied by doping with suitable elements in a manner 20 analogous to that observed in crystalline semiconductors. (See, for example, W. E. Spear and P. G. LeComber, Solid.State Commun., 17, 1193, 1975). Furthermore, it is widely recognized that the presence of atomic hydrogen plays a major role 25 in the electrical and optical properties of these materials (see, for example, M. H. Brodsky, Thin Solid Films, 50, 57~ 1978) and thus there is widespread current interest in the properties and uses of thin films of so-called "hydrogenated 30 amorphous silicon," hereinafter referred to as O~-Si(H~.
The field oE electrophotography is one in which there is extensive current interest in the utilization of thin films of ~-Si(H). To date, the ~;~4~3~7~ii art has disclosed a wide variety of photoconductive insulating elements, comprising thin films of intrinsic and/or doped oC-Si(H), which are adapted for use in electrophotographic processes. (As used 5 hereing the term "a doped oc-Si(H) layer" refers to -- . a layer of hydrogenated amorphous silicon that has been doped with one or more elements to a degree sufficient to render it either n-type or p-type-).
Included among the many patents and published patent applications describing photoconductive insulating elements containing layers of intrinsic and/or doped oc-Si(H) are the following:
Misumi et al, U. K. Patent Application No.
2 018 446 A, published October 17, 1979.
15Kempter, U. S. patent 4,225,222, issued September 30, 1980.
Hirai et al, U. S. patent 4,265,991, issued May 5, 1981.
Fukuda et al, U.S. patent 4,359,512, issued 20 November 16, 1982.
Shimizu et al, U. S. patent 4,359,514, issued November 16, 1982.
Ishioka et al, U. S. patent 4,377,628, issued March 22, 1983.
25Shimizu et al, U. S. patent 4,403,026, issued September 6, 1983.
Shimizu et al, U. S. patent 4,409,308, issued October 11, 1983.
Kanbe et al, U. S. patent 4,443,529, issued 30 April 17, 1984.
As hereinabove described, the present invention makes use of a particular type of photoconductive insulating element, characterized by the presence of both doped and intrinsic layers of o~~Si~H)~ in an electrophotographic process in which the element is electrostatically charged to a ~Z~76 low surface voltage, that is a voltage in the range of from 5 to 50 volts. More specifically, the photoconductive insulating element utilized in the electrophotographic process of this invention 5 comprises:
- (a) an electrically-conductive support, by which is meant a support material which is itself electrically conductive or which is comprised of an electrically-insulating material 10 coated with an electrically-conductive layer, (b) a barrier layer overlying the support, by which is meant a layer which serves to prevent the migration of charge-carriers from the support into the photoconductive layers of the 15 element, and (c) a photoconductive stratum overlying the ba~rier layer which comprises a layer of intrinsic o~-Si(H) in electrical contact ~ith a layer of doped oC-Si(H) and in which the doped layer 20 is very thin in relation to the thickness of the intrinsic layer.
It is critical to the invention that the photoconductive stratum comprise both an intrinsic oG-Si(H) layer and a doped ~-Si(H) layer, since 25 use of an intrinsic ~-Si(H) layer alone would not be an effective means of generating the necessary charge carriers when employing a low surface voltage; while use of a doped v~-Si(H) layer alone would result in too high a dark conductivity for the 30 element to be useful in the low field process of this invention. It is also very important that the doped layer be very much thinner than the intrinsic layer9 since, if this were not the case, the dark conductivity would be excessively high for use in 35 the low field process of this invention.

9 ~Z4~6 It is also critical to the invention that the element be electrostatically charged to a very low surface voltage, that is a voltage in the range of ~rom 5 to 50 volts. Only by the use of such a low voltage is it possible to achieve very high electropho~ographic sensitivity -- a sensitivity which is so high that the element can be reason~bly characterized as a camera-speed material -- without the generation of e~cessive electrical noise. It is this use of very low voltage which specifically distinguishes the process of this invention from conventional electrophotographic processes ~hich utilize much higher voltages.
Photoconductive insulating elements,-whether of the single-active-layer or multiple-active-layer types, typically exhibit a quantum efficiency at low voltage which is much less than they exhibit at high voltage. However, the photoconductive insulating elements described herein exhibit a quantum efficiency at low voltage which is substantially the same a~ that at high voltage. It is this characteristic which renders them especially suitable for use in the novel low field electrophotographic process of this invention.
The elements employed in the process of this invention utilize an electrically-conductive support, and such support can be either an electrically-conductive material or a composite material comprised of an electrically-insulating substrate coated with one or more conductive layers. The electrically-conductive support should be a relatîvely rigid material and preferably one that has a thermal expansion coefficient that is fairly close to that of a layer of ~c-Si(H). Particularly 35 useful materials include aluminum, steel, and glass that has been coated with a suitable conductive 7~

coating. Preferably, the support is fabricated in a drum or tube configuration, since such configurations are most aperopriate for use with a relatively brittle and fragile material such as -Si(H).
A particularly important ~eature of the photoconductive insulating element employed in the process of this invention is the barrier layer. It serves to prevent the injection of charge carriers from the substrate into the photoconductive stratum.
Specifically, it prevents the injection of holes from the substrate when the photoreceptor is charged to a negative potential, and it prevents the injection of electrons from the substrate when the photoreceptor is charged to a positive potential. Either positive or negative charging can, of course, be used in the process of this invention, as desired. Inclusion of a barrier layer in the element is necessary in ordee for the element to provide adequate charge acceptance.
A number o~ materials are known to be useful to form a barrier layer in an amorphous silicon photo~onductive insulating alement. For example, useful materials include oxides such as silicon oxide (SiO) or aluminum oxide (A1203). Preferably, the barrier layer is a layer of -d -si ~H) which has been heavily doped with a suitable doping agent. The term "heavily doped", as used herein, is intended to mean a concentration of doping agent of at least 100 ppm.
The term "a photoconductive stratum" is used herein to refer to the combination of an intrinsic ~ -Si(~) layer and a doped ~ -Si(H) layer in electrical contact therewith. Since the essential requirement is merely that the activating radiation be incident upon the doped layer, the `:

Z~4~6 particular order of these layers in the photoconductive stratum-is not ordinarily critical.
For example, the doped layer can be the outermost layer and the exposure can be from the front side of the element, or the order of the doped and intrinsic - layers can be reversed and the exposure can be from the rear side.
The layer of intrinsic o~-Si(H) can be formed by processes which are well known in the art. Most commonly, the process employed is a gas phase reaction, known as plasma-induced dissociation, using a silane (for example SiH4) as the starting material. The hydrogen content of the intrinsic G~-Si(H) layer can be varied over a broad 15 range to provide particular characteristics as desired. Generally, the hydrogen content is in the range of 1 to 50 percent and preferably in ~he range of 5 to 25 percent (the content of hydrogen being defined in atomic percentage~.
The layer of doped ~-Si(H) can be formed in the same manner as the layer of intrinsic ~G Si(H), except that one or more doping elements are utilized in the layer forming process in an amount sufficient to render the layer n-type or p-type. (Doping elements can also be used in the formation of the intrinsic layer since a layer of hydrogenated amorphous silicon, as typically prepared by the plasma-induced dissociation of SiH4, is slightly n-type and a slight degree of p-doping is typically employed to render it intrinsic.) The hydrogen concentration in the doped layer can be in the same general range as in the intrinsic layer.
Many different doping agents are known in the art to be of utility in advantageously modifying the chaFacteristics of a layer of o~~Si(H).

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~24~76 Included among such doping agents Hre the elements of Group VA of the Periodic Table, namely N, P, As, Sb and Bi, which provide sn n-type layer - that is, one which exhiblts ~ preference for conduction of 5 negative charge carriers (electrons) - Rnd the elements of Group IIIA of the Periodic T~ble, namely B, Al, Ga, In and Tl, which provide a p-type layer - that is one which exhibits a preference for conduction of positive charge carriers (holes). The 10 preferred doping agent for forming an n-type layer is phosphorus, and it is conveniently utilized ~n the plasma-induced dissociation in the form of phosphine ~as (PH3). The preferred doping a~ent for forming a p-type layer is boron, and it is conveniently utili~ed 15 in the plasma-induced dissociatlon in the form of diborane gas (B2H6 ) .
The concentration of doping ~gent employed in forming the doped d~-Si(H) layer can be varied over a 20 very broad r~nge. Typically, the doping agent is employed in sn amount of up to about 1,000 ppm in the gaseous composition used to form the dopPd layer, and preferably in an amount of about 15 to about 150 ppm.
When a doped o~-Si(H) layer is util~zPd as the 25 barrier layer ~n the element, it is typically a heavily doped layer, for example, A layer formed from a composition containing 500 to 5,000 ppm of the doping agent.
A particularly advantageous process, for use 30 in formins the doped ~ -Sl~H) l~yer that is an essential component of the photoconductive insulating element employed in the method of this invention, is the process described in United States Patent 4,540,647, entitled "Method For The Manufacture Of 35 Photoconductive Insulating Elements With A Broad -13- ~24~76 Dynamic Exposure Range," by P. M. Borsenberger. As described in this patent, a ma~or improvement in the process of forming ~ doped ~-Si(H) lsyer by plssma-induced dissociatlon of a gaseous mixtu~e of a 5 silane and a doping agent is achieved by controlling the temperature of the dissociation process ~o ~hat an initial ms~or portion of the layer of doped d~-Si(H) is formed at a temperature in the range of from 200 C to 400 C and a final minor portion of 10 the layer of doped ~-Si(H) is formed at a temperature in the range of from 125C to 175C.
This improvement in the manufacturing process leads to the important benefit of a greatly extended dyn~mic exposure r~nge.
The dynamic exposure range is a very important factor in electrophotographic processes.
The usual method for evaluating this range is b~sed on a technique employed in conventionfll photography.
This technique involves the following steps:
(l) The surface potential in volts is plotted versus the logarithm of the exposing radiation for a given initial potential, VO~ to thereby provide a V-logE curve.
(2) The derivative of the curve is then 25 determined ~nd plotted on the same exposure axis. The derivatiYe is expressed in units of volts/logE and defined as the contrast,2r.

(3) The dynamic exposure range, in units of logE, is then defined as the ratio of the initial 30 potential, VO~ to the maximum contrast, armax Defined in this manner~ the experimental values of the dynamic exposure rsnge very closely approximAte the range of optical densities that can be accurately reproduced by the photoreceptor surface potential.
Photoconductive ins~lating element~ -5 compris1ng a layer of doped d~-Si(H) exhibit a r~ther high contrsst ~nd thus a rather nsrrow dynamic exposure range, typically a range of ~bout 0.7 to about 0.8 logE. While values of this m~gnitude are usually sufflcient for the 10 reproduction of digital information (line copy, for example), they are not sufficient for continuous tone reproduction (pictorial informa~ion, for exflmple). The invention disclosed in the aforesAid copending patent application is capable of extendin~
15 the dynamic exposure range to a value of flS high as 1.4 logE, or higher, and thus greatly enhances the utility of the resulting element.
In a preferred example of the process of the aforesaid United States Patent 4,540,647, an -Si(H) layer that is doped with boron is prepared by incorporating 15 ppm of diborane g~s in the s~lane gas, and the temper~ture of the deposition process is controlled so that ~bout eighty percent of the thickness of the doped d~-Si(H) layer is 25 formed st a temperature of about 250 C and the remaining twenty percent is formed at a temperature of about 150C. It ls not known with certainty why ~uch process provides the benefit of extended dynamic exposure range. The initial 3tep in 30 pla~ma-induced dissociation reactions is the transfer of the plasma energy to the gas phase.
Provided the plasma energy is sufficiently hlgh, new chemical species are formed that are the intermediate pecies in ~he formation of more ~able 35 compounds. In the dissociation of SlH4, the intermediate spec~es are believed to be the positive ~Z~76 ion fragments SiH, SiH2 and SiH3. Control of the temperature in the aforesaid manner may result in the formation of a "hydrogen profile," that is a variation in hydrogen concentration across the thickness of the layer, or it may alter the relative proportions of intermediate spécies that are formed and thereby alter the character of the layer that is deposited.
The thickness of the various layers making 10 up the photoconductive insulating elements employed in the process of this invention can be varied widely. The barrier layer will typically have a thickness in the range of from about 0.01 to about 5 microns, and preferably in the range o~ from about 15 0.05 to about 1 microns. The intrinsic ~--Si(H) layer will typically have a thickness in the range of from about 1 to about 50 microns, and pre~erably in the range of from about 3 to about 30 microns.
The doped ~-Si(H) layer will typically have a 20 thickness in the range of from about 0.01 to about 0.2 microns, and preferably in the range of from about 0.02 to about 0.1 microns.
The doped o~-Si(H) layer must be . sufficiently thin to provide the element with a high 25 degree of dark resistivity, generally a dark resistivity of at least 1011 ohm-cm, and most typically in the range of 1011 to 1014 ohm-cm.
While the exact ratio of the thickness of the doped layer to the thickness of the intrinsic layer is not 30 critical, the doped layer is typically very thin in relation to the thickness of the intrinsic layer.
It is preferred that the ratio of the thickness o~
the doped v~-Si(H) layer to the thickness of the intrinsic dG-Si(H) layer be less than 0.01 and 35 particularly preferred that it be in the range of from 0.001 to 0.005.

~9~76 As previously indicated, the preferred doping agent for forming an n-type layer is phosphorus, and the preferred doping agent for forming a p-type layer is boron. These agents are preferably utilized in the doped layer at a concentration of about 15 to about 150 ppm.
The amount of doping agent utilized needs to be carefully controlled to achieve optimum results. For example, an amount of doping agent lO which is too low will result in an undesirably low quantum efficiency, while an amount of doping agent that is too great will result in an excessively high dark conductivity.
In addition to the essential layers 15 described hereinabove, the photoconductive insulating elements employed in the process of this invention can contain certain optional layers. For example, they can contain anti-reflection layers to reduce reflection and thereby increase efficiency.
20 Silicon nitride is a particularly useful material for forming an anti-reflection layer, and-is advantageously employed at a thickness of about 0.1 to about 0.5 microns.
In the process of this invention, the 25 photoconductive insulating element is electrostatically charged to a surface voltage of 5 to 50 volts, and most preferably of 10 to 20 volts.
Charging to this low voltage provides the basis for a very high speed electrophotographic process. The 30 process is also advantageous in that the element has an extremely fast response time, exhibits sensitometry which is essentially temperature independent, and can be readily adapted to provide panchromatic sensitivity through appropriate control 35 of the hydrogen content.

~Z~L9~76 The invention is further illustrated by the following e~ample of its practice.
A photoconductive insulating element was prepared with the following layers arranged in the indicated order:
(1) a glass substrate, (Z) a vacuum-deposited layer of aluminum, (3) a barrier layer consisting of a 0.15 micron thick D layer of SiO, ~) a 10 micron thick layer of intrinsic ~ -Si(H), and (5) a 0.03 miceon thick layer of ~-Si(H) which had been doped with phosphoeous by incorporating phosphine gas at a concentration of 100 ppm in the silane composition used to form the layer.
Using a positive surface potential and exposure to activating radiation at a wavelength of 400 nm, the quantum efficiency was determined in relation to the magnitude of the surface potential. (The quantum efficiency is defined as the O ratio of the decrease in the surface charge density to ~he absorbed photon flux, assuming the charge density is related to the surface voltage by the geometrical capacitance.) The results are shown in Figure 1, which also provides the results for an other~ise identical control element which did not have the dnped -Si(~) layer. In the figure, which is a loga~ithmic plot of quantum efficiency (~) versus electric field, the results for the test element of the invention are shown by open circles, while those for the control element are shown by solid circles. As shown in Figure 1, the quantum efficiency of the control element O decreased substantially with decreasing surface voltage, while the quantum efficiency of the test element was substantially .~,.

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- independent of surface voltage over a wide range of voltages. With both the control and test elements, the quantum efficiency at high voltage was unity.
As demonstrated by Figure 1, the thin layer of doped ~C-Si(H) is a critical component of the photoconductive insulating elements which are useful -~ in the method of this invention, as this layer strongly reduces the field dependence of the photogeneration efficiency and thereby gives rise to the high sensitivity that is observed at low fields.
The exposure dependence of the surface voltage for the control and test elements described abov~, with an initial potential of 10 volts, is shown in Figure 2. In obtaining these data, the exposure wavelength was 400 nm, the exposure duration was 160 microseconds, and the voltage was sampled 0.5 seconds after the cessation of exposure. As shown by Figure 2, the control element exhibited discharge from VO to Vo/2 with an exposure of 0.29 ergs/cm , corresponding to an ASA
rating of about 12, while the test element required only 0.11 ergs/cm2, corresponding to an ASA rating of about 30.
The invention has been described in detail with particular reference to preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention.

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Claims (13)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A method of electrophotographic imaging which comprises:

(1) providing a photoconductive insulating element comprising:
(a) an electrically-conductive support, (b) a barrier layer overlying said support, and (c) a photoconductive stratum overlying said barrier layer, said stratum comprising a layer of intrinsic ?-Si(H) in electrical contact with a layer of doped ?-Si(H), said doped ?-Si(H) layer being very thin in relation to the thickness of said intrinsic ?-Si(H) layer, (2) uniformly electrostatically charging said element to a surface voltage in the range of from 5 to 50 volts, and (3) image-wise exposing said doped ?-Si(H) layer to activating radiation to thereby form a latent electrostatic image on the surface of said element.

2. The method of claim 1 wherein said surface voltage is in the range of from 10 to 20 volts.

3. The method of claim 1 wherein said doped ?-Si(H) layer is doped with an element of Group III A or Group VA of the Periodic Table.

4. The method of claim 1 wherein said doped ?-Si(H) layer is doped with phosphorus.

5. The method of claim 4 wherein the phosphorus is present in said doped .alpha.-Si(H) layer at a concentration of about 15 to about 150 ppm.

6. The method of claim 1 wherein the ratio of the thickness of said doped .alpha.-Si(H) layer to the thickness of said intrinsic .alpha.-Si(H) layer is less than 0.01.

7. The method of claim 1 wherein the ratio of the thickness of said doped .alpha.-Si(H) layer to the thickness of said intrinsic .alpha.-Si(H) layer is in the range of from 0.001 to 0.005.

8. The method of claim 1 wherein the hydrogen concentration in both said intrinsic .alpha.-Si(H) layer and said doped .alpha.-Si(H) layer is in the range of 5 to 25 percent.

9. The method of claim 1 wherein the thickness of said intrinsic .alpha.-Si(H) layer is in the range of about 3 to about 30 microns.

10. The method of claim 1 wherein the thickness of said doped .alpha.-Si(H) layer is in the range of about 0.02 to about 0.1 microns.

11. A method of elctrophotographic imaging which comprises:
(1) providing a photoconductive insulating element comprising:
(a) an electrically-conductive support, (b) a barrier layer overlying said support, and (c) a photoconductive stratum overlying said barrier layer, said stratum comprising a layer of intrinsic ?-Si(H) with a thickness of about 10 microns in electrical contact with a layer of phosphorus-doped ?-Si(H) with a thickness of about 0.03 microns, (2) uniformly electrostatically charging said element to a surface voltage of about 10 volts, and (3) image-wise exposing said layer of phosphorus-doped ?-Si(H) to activating radiation to thereby form a latent electrostatic image on the surface of said element.

12. The method of claim 1 wherein said doped ?-Si(H) layer has been formed by a process of plasma-induced dissociation of a gaseous mixture of a silane and a doping agent in which the temperature has been controlled so that an initial major portion of said layer of doped ?-Si(H) was formed at a temperature in the range of from 200°C to 400°C, and a final minor portion of said layer of doped ?-Si(H) was formed at a temperature in the range of from 125°C to 175°C.

13. The method of claim 12 wherein about eighty percent of the thickness of said layer of doped ?-Si(H) was formed at a temperature of about 250°C and the remainder was formed at a temperature of about 150°C.