CA1271076A - Enhancement layer for electrophotographic devices and method for decreasing charge fatigue through the use of said layer - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

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

Description

~ ` 1 406 i.;~71()7~i FIELD OF THE INVENTION

This invention relates generally to electrophotographic devices and more particularly to an improved enhancement layer which is specifically tailored to substantially ellminate 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 midgap states.

The instant invention relates to improved enhancement layers for use in electrophotographic imaging processes. The improved enhancement layer of the instant invention is fabricated from semiconductor alloy material, said material character1zed by a decreased number of deep midgap defect sites in whlch charge carriers can be trapped. By decreaslng 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 generlcally as xerography, is an imaging process which relies upon the stor~age 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 , -1-c, ;1 .7", 1406 1 2 7 1 0 7 ~

these charge carriers which perm1t 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 lO aluminum. Other substrate configurations, such as planar sheets, curved sheets or metallized flexible belts ~ay likewise be 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 20 oxide naturally occuring on the substrate member, or from a deposited layer of semiconductor alloy material. As will be dlscussed in greater detail J hereinbelow, the blocklng layer functions to prevent the flow of unwanted charge carriers from the substrate member into the photoconductive layer in whlch layer they could then neutralize the charge stored upon the top surface of the photoreceptor. A
typ1cal photoreceptor also generally includes a top protective layer disposed upon the photoconductive 30 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 ~, -2-: , 1406 ~L~ 7 ~ 0 7 ~

carriers from being caught 1n 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 electr1cal 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 lQ charged port~ons of the photoreceptor under illuminatlon; and it must retain substantially all of the in~tial 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 generatlon in the photoconductor material.
A ma~or source of charge inject10n is at the metal substrate/semiconductor alloy material interface. The metal substrate provides a virtual sea of electrons available for injection and subsequent neutral1zat10n 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 electrophotgraphic 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 sem1conductor alloy material from which the layers of prior art 1406 127~L076 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 in1tial 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 ~n an attempt to neutrallze the pos1tive charge residing thereon. However, ln 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 protect~ve layer.
This 1s because the blocking functlon (the inability . ~ , . .

1~71076 of the posltive 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 w111 generate a lQ sufficiently strong electrical f~eld 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 ex1st closer to the conduction band. The slow release rate from deep traps glves rise to a higher equilibrium trap occupancy and thus a higher electric field distr~bution.
It is important to note that in the fabrication of the typical electrophotographic photoreceptor which operates with a positlve corona charge applied to outer surface thereof, the photoconductive layer thereof ~s made from a ~pi-type" silicon:fluorine:hydrogen:boron alloy. As used herein, "pi-type" will refer to semiconductor alloy material, the Ferml level of which has been displaced from its undoped position closer to the conduction band to a position approx~mately 1406 lZ7~07~

"midgap". Further note that as used herein, the term ~midgap" will be used to define a point ln the energy gap of a sem~conductor alloy material which is positioned approximately half-way between the valence band and the conduction band (in the case of 1.8 eV
amorphous silicon:fluorine: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"
10 amorphous s~licon:hydrogen:fluorine alloy as deposited in a glow discharge decomposition process ls sl~ghtly ~nu-type~ (the Fermi levél of that material is sl~ghtly closer to the conduction band than to the valence band) and in a posltive corona charge electr~ophotographic process, the movement of charge carriers through the photoconductive layer under lllumination must be maximized wh~le miminizing the thermal generation of charge carrlers.
It is to be noted that when the Fermi level 20 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. Thls is because the deepest electron trap sltes in a layer of semiconductor alloy material lie at or near the Fermi level and in this Pt type material this energy coincides with midgap.
The thermal energy required to release an electron from a deep trap ~s dependent on the depth of that 30 trap. More particularly, the time which a trapped 1406 127~07~

electron will wait, on average, before be~ng thermally em~tted from any trap ~s given by the formula:
t - [~o EXP(-~E/~T)~
where " ~0" is the number of electrons attempting to escape per second, "~E" is the energy required to move an electron from the Ferm~ lever to the conduction band edge, and kT ~s the absolute temperature multiplied by Boltzman's constant. " ~0"
may be assumed to have a value of approximately 1012 electrons per second tn most solids. For a Ferml level posit~on 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 ~nterface w~ll bu~ld up and this space charge and the pos~t~ve charge accumulated on the surface of the top protective layer wlll create a very high electric field distortion across aid top protective layer, whlch 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 g1ve rise to deep charge carrier traps. As tauqht in Applicant's U.S.
Patents 4,619,729 and 4,715r927, A~

. 1406 ~27107~

the addition of an "enhancement layer~ operatively disposed between the top protect~ve layer and the photoconduct1ve layer benef~c1ally affects the performance of an electrophotograph1c device incorporat~ng that layer.
Wh~le at the t~me of f~l~ng said ~atents, the reason for the phys~cal behav~or of the enhancement layer was unknown, Appl1cants now have determ~ned that the addit~on of the enhancement layer 10 (2S fabr1ca~ed 1n the manner taught thereln) operated to reduce the escape t1me of charge carr1ers caught 1n deep traps prev10usly encountered at the 1nterface of the photoconductive layer by reduc1ng the overall dens1ty of defect states 1n the mater1al from which the enhancement layer was formed. However, the enhancement layer descr1bed 1n the aforement10ned patents, decreased the overall density of defect states by depos1t1ng 1ntr1ns1c sem1conductor alloy mater1al by r.f. glow d1scharge rather than by m1crowave glow d1scharge (s1nce m1crowave depos1tlon tends to create addlt10nal defect states). Therefore, the enhancement l?yer of sa1d aforement10ned patents relied upon a reduct10n 1n the overall dens~ty of defect states present 1n undoped sem~conductor alloy materlal to a1d 1n reduc1ng the number of deep traps 1n which charge carr1ers could be caught 1n order to reduce charge fat1gue. However, no attempt or even suggest10n of how to opt1mize the chem1cal compos1t10n of the enhancement layer 1n order to further prevent charge carr1ers from being caught in the deep m1dgap traps was d1scussed or suggested in sa1d patents.

~ A

1 406 lX7107~i An important advantage obtained by following the teachlngs 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 falllng into the deep mldgap traps. Only relatlvely shallow defect states remain in which charge carrlers may be trapped and the rate of emlsslon of charge carrlers from these shallow traps can be measured ln terms of seconds rather than in terms of tays. Therefore, ln its broadest form, the present appllcation relates to the positlonlng of the Ferml level of the sem1conductor alloy material from whlch the enhancement layer ls formed to a position above mldgap. Thls results ln the deep mid-gap states belng occupled by electrons and thus not being effectlve as electron traps. In thls way electrons moving through the enhancement layer do not have to pass through a reglon ln whlch there are effective deep mldgap traps. Thls translates into an electron escape tlme of less than about l second for a 1.8 eV
slllcon:hydrogen:fluorine:phosph~ne alloy having the Ferml thereof positloned ln the most favored range of 0.75 to 0.65 eV from the conductlon band. Because of the qulck release tlme there wlll be no substantial bulld up of trapped charge in this region and therefore no hlgh field distortlon. Similarly, in lnstances where negative charglng ls utllized, positionlng the Ferml level of the enhancement layer 0.75 to 0.65 eV from the valence band wlll allow for a slmilar qulck release of trapped carriers.
It ls noteworthy that the sub~ect inventors do not clalm to have invented the concept of fixing _g_ 1406 1~7~L07~

- 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 entltled ~Fleld-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 insulat1ng layer (the top protective layer) for quenching the effects of the electric field and removing the effect of ~field-induced blurring"
(commonly referred to as ~image-flow"). In this manner, Mort, et al were able to counteract the problem of ~image-flowH.

lX~107~;

However, Mort, et al were not concerned w~th 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 applicatlon 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 ~ust the type of lateral electron flow they seek to avoid.
In contrast thereto, Applicants first intentlonally phosphorous doped the semlconductor alloy material of the enhancement layer which is interposed between the photoconductive layer and the top protectlve layer in order to shift the Fermi level thereof toward the conduct~on band. By so shlftlng 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 ~n the energy gap thereof. This substantially el1minates 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 p1nned Fermi level. The added defect states, being shallow, not only solve charge fatigue problems, but 1406 1 ~ 7~L0~ 6 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 disrussion, while Mort, et al propose a solution to the problem of image flow in e1ectrophotographic 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 appropr1ately shifting and then pinning the Fermi level of the sem~conductor alloy material of a newly added enhancement layer.
In l~ght of the many defin1tions utilized for the terms Ramorphous" and "microcrystalline" in the scientific and patent literature it will be helpful to clarlfy the definition of those terms as used herein. The term Uamorphous~, 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 ln 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 ~amorphousU as def1ned hereinabove.

:

1406 1~7iL07~

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.

BRIEF SUMMARY OF THE INVENTION

There ~s disclosed herein electrophotographic med~a comprising an electrically conduct~ve substrate, a bottom layer overlying the substrate wh~ch is adapted to block the free flow of charge carriers from the substrate, a photoconductive layer overlying the bottom layer wh~ch is adapted to discharge an electrostatic charge, an enhancement layer overlying the photoconductive layer which is adapted to substant~ally reduce the number of charge carr1ers caught in deep mldgap 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 ~n 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 s~licon alloys, amorphous germanium alloys, amorphous silicon-germanium alloys, photoconductive organ~c polymers and combinations thereof. The enhancement layer is preferably fabricated from a mater~al selected from the group consisting essentially of amorphous sil~con alloys, amorphous germanlum alloys and amorphous silison-germanium alloys. The enhancement layer ls yet more favorably fabricated from an amorphous 1406 1X710~

silicon alloy and the Fermi level thereof is moved to within 0.5 to 0.8 eY 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 2,500 to lO,000 angstroms and preferably about 5,000 angstroms. 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 in~ection blocking layer, a photoconductive layer and a top protective layer. The method includes the steps of forming an enhancement layer from an intentionally doped sem1conductor 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 , ~ 271~76 from a m~crocrystalline, boron doped silicon:hydrogen:fluorine alloy, the extent of boron doping being suff~c~ent to make the material degenerate and (2) form~ng the enhancement layer from a mater~al 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 ~s 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 mater1al from wh~ch the enhancement layer is fabr~cated ls ta~lored so as to provide for the em~ss~on of charge carriers from sa~d traps in approx~mately one second or less. The method may still 1nclude the further step of form~ng the enhancement layer to be approx~mately 2,500 to 10,000 angstroms th~ck and preferably approxlmately 5,000 angstroms th~ck. In the most preferred embod~ment, the Ferm1 level of the sem~conductor alloy mater~al from wh~ch the enhancement layer is fabricated is p~nned by 1ntroduc~ng both boron and phosphorus, into the host sem~conductor matr~x thereof so as to add add~t~onal shallow states at both s~des of the Fermi level ln the energy gap thereof.

BRIEF DESCRIPTION OF THE DRAWINGS
.
Flgure l is a part~al cross-sectional view of an electrophotographic photoreceptor which includes the ~mproved enhancement layer of the ~nstant ~nvent~on; and, .

1406 1~7~7~

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 Flgure 1.

DETAILED DESCRIPTION OF THE DRAWINGS

Referrlng now to flgure 1, there is illustrated ln a partlal cross-sectional view, a generally cyllndrically shaped electrophotographic photoreceptor 10 of the type incorporatlng all of the lnnovatlve princlples disclosed wlthin the specif1catlon of the lnstant lnventlon. The photoreceptor 10 lncludes a generally cylindrically shaped substrate 12 formed, ln this embodiment, of alumlnum, although other nondeformable metals such as stainless steel could also be employed as a preferred embodlment. The perlphery of the alumlnum substrate 12 ls provlded with a smooth, substantlally defect free surface by any well known technlque such as diamond machinlng andlor pollshlng. Disposed immediately atop the deposltion surface of the substrate 12 is deposlted a doped layer 14 of microcrystalline semiconductor alloy material whlch has been speclflcally deslgned and adapted to serve as the bottom blocklng layer for sald photoreceptor lO. In keeping with the teachlngs dlsclosed ln commonly asslgned Patent No. 4,582,773, the blocklng layer 14 is formed of hlghly doped, hlghly conductive mlcrocrystalllne semiconductor alloy material.
Dlsposed lmmedlately atop the bottom blocking layer 14 ls the photoconductlve layer 16 which may be formed from a wlde varlety of photoconductive materlals. Among some of the preferred materlals are doped ~ntrlnslc amorphous slllcon alloys, amorphous ~ ~ 7 ~ ~ 7 germanium alloys, amorphous sil~con-germanlum alloys, chalcogen~de materials and organic photoconductive polymers. Disposed atop the photoconductive layer 16 ls 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 lO
includes a top protective layer l9 operatively disposed atop the enhancement layer 18, which protective layer l9 (l) protects the upper surface of the photoconductive layer 16 from ambient conditions and (2) separates the charge stored on the surface of the photoreceptor lO from carriers generated in the photoconductive layer 16.
In accordance with the principles o~ the f~rst embodlment of the ~nstant invention, the improved enhancement layer 18 is formed of an intentionally doped sem1conductor alloy material.
The purpose of intentionally doping the enhancement -~ layer 18 1s to move the Ferml level closer to the conductlon band (in the case of a positive corona charge) of the semiconductor alloy materlal from which said layer is fabr~cated. Obviously, in the case of a negative surface charge, it would be des1rable to intentionally dope the enhancement layer 18 so as to move the Fermi level of the semiconductor alloy materlal from which it is fabricated closer to the valence band. A wide variety of semiconductor alloy materials may be employed from wh1ch 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 s~l~con:hydrogen:halogen alloys. Among the ~ 1406 127107~

halongenated mater~als, fluorinated alloys are part~cularly preferred.
Dop~ng of the semiconductor alloy mater~al may be accompllshed by any technlque and employing any mater~al wh~ch ~s well known to those of ord~nary sk~ n the art. Because Applicants' prev~ous enhancement layers. as described ~n sa~d patents 4,619,729 and 4,715,927 were prepared with a reduced dens1ty of defect states, the charge carr~ers movlng through that layer from the photoconduct~ve layer 16 to neutral~ze charge located at the surface of the top protect~ve layer 19 were not caught ~n as many deep m1dgap traps. The result was a reduct~on ~n the number of carr~ers which requ~red the aforedescr~bed lengthy per~od of t~me required to be em~tted from the deep traps. By employ~ng the pr~nc~ples espoused in the sub~ect appl~cation and employ~ng an enhancement layer 18, the Fermi level of wh~ch ~s moved to a deslred locat~on and pinned so that charge carr~ers are able to avo~d the deep m~dgap states present ~n the s~l~con alloy mater~al from wh~ch the layer ~s fabr~cated, the res~dency t~me of charge carr~ers caught 1n traps is s1gn~f~cantly decreased s~nce only the traps access~ble to the carr~ers are shallow traps. The absence of deep trapped carriers not only prevents a breakdown of the top protect1ve layer 20, but slgn1f~cantly ~ncreases the cycle time ~n whlch the --electrophotograph1c med~um 10 ~s capable of recover~ng lost surface charge and ready~ng ~tself for reproduc~ng a further copy.
Whlle a wide var~ety of semlconductor mater~als may be employed from wh~ch to fabricate the photoconduct~ve layer 16, the amorphous s~l~con alloys, amorphous german~um alloys and amorphous s11~con german~um alloys were found to be - 1406 ~ 2 7 ~ 0 7 ~

part1cularly advantageous. Such alloys and methods for the1r preparation are disclosed 1n the patents refer~e~ to hereinabove.

The conduct1v1ty type of the mater1als from wh~ch the block~ng layer 14 and the photoconduct~ve layer 16 are fabr1cated are chosen so as to establ1sh a block~ng contact therebetween whereby 1n~ect10n of unwanted charge carr1ers into the bulk of the photoconduct1ve layer 16 1s effect1vely 1nh1bited. In cases where the photoreceptor lO 1s adapted to be electrostat1cally charged w1th a pos~t1ve charge the bottom block1ng layer 14 will preferably be fabr~cated from a heav~ly p-doped alloy and the photoconduct1ve layer 16 w~ll be fabr1cated from an 1ntr~ns1c sem1conductor layer an n-doped sem~conductor layer or a 11ghtly p-doped sem1conductor layer. Comb~nat~ons of these conduct1vlty types w111 result 1n the substantial 1nh~b1t10n of electron flow from the substrate 12 lnto the bulk of the photoconductor layer 16. It should be noted that 1ntr1ns1c or 11ghtly doped sem1conductor layers are generally favored for the fabr1cat10n of the photoconduct1ve layer 16 ~nsofar as such mater~als w111 have a lower rate of thermal charge carr1er generat10n than w111 more heav11y doped mater1als. Layers of 1ntr1ns1c sem1conductor alloy mater1als are most preferably favored 1nsofar as such layers have the lowest number of defect states per un1t volume and the most favorable d1scharge character1st1cs.
In cases where the electrophotograph1c photoreceptor lO 1s adapted for a negat1ve charg1ng 1t w111 be des~rable to prevent the flow of holes 1nto the bulk of the photoconductlve layer 16. In such 1nstances the conduct1v1ty types of the layers -19' .~
~Q

. . .
,. ...... , ` `
. . . ..

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of semiconductor alloy material referred to hereinabove w~ll be reversed, although obviously, intrinsic materials will still have significant utility.
The max~mum electrostatic voltage which the photoreceptor lO can sustain (Vsat) w~ll depend upon the efficiency of the blocking layer 14 as well as the thickness of the photoconductive layer 16.
- For a given block~ng layer efflciency, a photoreceptor lO having a th~cker photoconductive layer 16 will sustaln 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 fabricat~on and elimination of stress it ~s generally desirable to have the total thickness of the photoconductlve layer 16 be 25 microns or less.
It is ilso desirable to have as high a static charge maintained thereupon as possible. Accordingly, gains in barrier layer efficiency, in terms of volts per micron charglng capac1ty, translate directly into ~mproved overall photoreceptor performance. It has rout1nely been found that photoreceptors structured ~n accordance wlth the principles of the ~nstant ~nvent~on are able to sustain voltages of greater than 50 volts per m~cron on up to a point nearing the d~electric breakdown of the semiconduc~or alloy material.
The intentionally doped semiconductor alloy material of the enhancement layer of the instant inventlon is produceable by a wide varlety of deposit~on techn~ques, all of which are well known to those skilled in the art. Sa~d deposition techniques include, by way of illustration, and not limitation, chem~cal vapor deposition techniques, photoassisted chem1cal vapor depos~tion techniques, sputterlng, ... .....

-- ~27~07~

evaporation, electroplating, plasma spray techniques, free radical spray techniques, and glow discharge deposition techniques.
At present, glow discharge deposition technlques have been found to have particular utillty 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 fabr1cation of electrophotographic photoreceptors insofar as it a110ws for the rapid, economical preparation of success1ve layers of high quality semiconductor alloy mater1al. Referring now to Figure 2, there is illustrated a cross-sect10nal 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 thls type that the electrophotographic photoreceptor 10 of Flgure 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 -`1406 ~27~076 and maintaining the ~nterior thereof at an appropr~ate pressure to facilitate the deposition process. The chamber 22 further inctudes a plurality of reactlon gas mixture input ports 26, 28 and 30 through which reactlon gas mixtures are introduced ~nto 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 ad~acent drums being closely spaced apart so as to define an inner chamber reg~on 32. For support1ng the drums 12 ln this manner, the chamber 22 lncludes a palr of interior upstanding walls, one of whlch ls illustrated at 34. The walls support thereacross a plurality of statlonary shafts 38.
Each of the drums 12 ls mounted for rotation on a respectlve one of the shafts 38 by a palr of disc shaped spacers 42 hav~ng outer dlmensions corresponding to the lnner dimenslon of the drums 12, to thereby make frlctlonal engagement therewith. The spacers 42 are drlven by a motor and chaln drive, not shown, so as to cause rotat~on of the cyl1ndrlcal drums 12 during the coatlng process for facllitatlng uniform deposit10n of materlal upon the entire outer surface thereof.
As previously mentloned, 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 whlch the deposltion plasma wlll be formed are introduced into the inner chamber 32 through at least one of the plurality of narrow passages 52 formed between a glven palr of ad~acent drums 12.
Preferably, the reactlon gases are lntroduced into 127~7t~

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 inter10r 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.
f 30 To facilitate the production of precursor free radicals and/or ions and/or other activated spec1es 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 1n figure 2, the -1406 ~'71076 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 20C and 400C and preferrably about 225C.

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 flrst operatively positioned in the deposition apparatus and then the chamber was evacuated and a gas mixture comprising .15 SCCM
(standard cubic centimeters per minute) of a 10.8 mixture of BF3 in hydrogen; 75 SCCM of 1000 ppm SiH4 ln hydrogen and 45 SCCM of hydrogen was introduced thereinto. The pumping speed was constantly adjusted to maintain a total pressure of approximately 100 microns in the chamber while the substrate was maintained at a temperature of approximately 300C. A bias of ~80 volts was established by disposing a charged wire in the plasma region. Microwave energy of 2.45 GHz was introduced ~` ~ 7~L~6 into the deposition region. These conditions resulted in the deposition of the bottom blocking layer of boron doped microcrystalline silicon:hydrogen:fluorlne alloy material. The deposition rate was approx~mately 20 Angstroms per second and the deposition continued until the boron doped microcrystalline blocking layer obtained a total thickness of approximately 7500 Angstroms.
At this point the microwave energy was term~nated, and the reaction gas m~xture flowing therethrough was changed to a mixture comprising .5 SCCM of a 0.18X m~xture of BF3 in hydrogen; 30 SCCM
of SiH4, 4 SCCM of SiF4 and 40 SCCM of hydrogen.
Pressure was maintained at 50 microns 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 depositlon of this alloy material (which formed the photoconductive layer of the electrophotographic medium) occured at a rate of approximately lO0 Angstroms per second and continued until approximately 20 microns of the amorphous silicon alloy material was deposited.
In order to deposit the amorphous silicon alloy from which the improved enhancement layer of the sub~ect 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 accompllsh this Fermi level movement and fix the Fermi level at this position so as to avoid splitting sald level upon illumlnation, approximately equal quantitles of phosphine and boron-trifluorine gas are introduced into the precursor gas mixture after the `1406 1~107~

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 ~s 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 40 Angstroms per second. Deposition contlnued until approximately 5000 Angstroms of the protective layer was deposited at which time the microwave energy was terminated, the substrate was cooled to 100C, 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 charg1ng by merely substituting opposite dopants ln roughly equimolar quantities. That is to say, the bottommost blocking layer would be a phosphorous doped layer; the photoconductive layer would be inrlnsic 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 variatlons should be made to the foregoing w~thin the scope of the instant invention.
~hile the foregoing example was oriented toward electrophotographic photoreceptors formed of amorphous silicon alloy materials, the instant invention ls obviously not so limlted but may be ~~ 1 2 7 ~(Y-~

utilized 1n conjunct10n with the fabrication of photoreceptors which 1nclude a wide var1ety of photoconductive material such as chalcogenide photoconduct~ve materials as well as organic photoconductive mater1als. 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.
The preceeding drawings, descr1ption, d1scuss10n and examples are merely meant to be 111ustrative of the instant 1nvention and are not meant to be 11m1tat10ns upon the practice thereof.
It 1s the follow1ng cla1ms, 1nclud1ng all J equ1valents, wh1ch Appl1cants def1ne the instant 1nvent1on.

~ -27-:

Claims (26)

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

1. An electrophotographic medium comprising:
an electrically conductive substrate;
a bottom layer overlying the substrate, the bottom layer adapted to block the free flow of charge carriers from the substrate;
a photoconductive layer overlying the bottom layer, the photoconductive layer adapted to discharge an electrostatic charge;
an enhancement layer overlying the photoconductive layer, the enhancement layer adapted to substantially reduce the number of charge carriers caught in deep mid-gap traps for preventing charge fatigue; said enhancement layer formed of semiconductor alloy material which is intentionally doped so as to avoid said deep trapping and prevent image flow; and a top protective layer overlying the enhancement layer, said protective layer adapted to protect the photoconductive layer from ambient conditions.

2. A medium as in Claim 1, wherein the photoconductive layer is fabricated from a material selected from the group consisting essentially of:
chalcogens, amorphous silicon alloys, amorphous germanium alloys, amorphous silicon-germanium alloys, photoconductive organic polymers and combinations thereof.

3. A medium as in Claim 1, wherein the bottom blocking layer is formed of a doped microcrystalline semiconductor alloy material.

4. A medium as in Claim 3, wherein the microcrystalline bottom blocking layer is fabricated from a material selected from the group consisting essentially of: silicon alloys, germanium alloys, and silicon-germanium alloys.

5. A medium as in Claim 4, wherein said microcrystalline back blocking layer is fabricated from a boron doped silicon:hydrogen:fluorine alloy.

6. A medium as in Claim 4, wherein said microcrystalline back blocking layer is sufficiently doped so as to become substantially electrically degenerate.

7. A medium as in Claim 1, wherein the enhancement layer is fabricated from a material selected from the group consisting essentially of:
amorphous silicon alloys, amorphous germanium alloys and amorphous silicon-germanium alloys.

8. A medium as in Claim 7, wherein the enhancement layer is fabricated from an amorphous silicon alloy and the Fermi level of the enhancement layer is moved to within approximately 0.5 to 0.8 eV
of the conduction band thereof.

9. A medium as in Claim 8, wherein the Fermi level of the enhancement layer is moved to within approximately 0.65 to 0.75 eV of the conduction band.

10. A medium as in Claim 1, wherein the enhancement layer is fabricated from an amorphous semiconductor alloy material which has been specifically tailored so as to provide for the emission of charge carriers from traps at the interface thereof with the top protective layer in approximately one second or less.

11. A medium as in Claim 1, wherein the thickness of the enhancement layer is approximately 2500 to 10,000 angstroms.

12. A medium as in Claim 11, wherein the thickness of the enhancement layer is approximately 5,000 angstroms.

13. A medium as in Claim 8, wherein the Fermi level of the semiconductor alloy material from which the enhancement layer fabricated is pinned.

14. A medium as in Claim 13, wherein the enhancement layer includes phosphorus and boron for adding non-deep-trapping states in the band gap of the amorphous silicon alloy material, said states adapted to pin said Fermi level.

15. A method of preventing charge fatigue and image flow in electrophotographic medium of the type which include an electrically conductive substrate, a bottom blocking layer, a photoconductive layer and a top protective layer; said method including the steps of:
forming an enhancement layer from an intentionally doped semiconductor alloy material;
operatively disposing said enhancement layer between the photoconductive layer and the top protective layer, said enhancement layer adapted to substantially decrease the number of charge carriers caught in deep traps present in the middle of the energy gap of the semiconductor alloy material from which said enhancement layer is fabricated.

16. A method as in Claim 15, including the further step of forming the photoconductive layer from a material selected from the group consisting essentially of chalcogens, amorphous silicon alloys, amorphous germanium alloys, amorphous silicon-germanium alloys, photoconductive organic polymers and combinations thereof.

17. A method as in Claim 15, including the further step of: forming the blocking layer from a doped microcrystalline material selected from the group consisting essentially of silicon alloys, germanium alloys and silicon-germanium alloys.

18. A method as in Claim 17, including the further step of forming the blocking layer from a boron-doped silicon:hydrogen:fluorine alloy, the extent of boron-doping being sufficient to make said silicon alloy degenerate.

19. A method as in Claim 18, including the further step of 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.

20. A method as in Claim 19, including the further step of moving the Fermi level of the semiconductor alloy material from which the enhancement layer is fabricated to within approximately 0.5 to 0.8 eV of the conduction band.

21. A method as in Claim 20, including the further step of moving the Fermi level of the semiconductor alloy material from which the enhancement layer is fabricated to within approximately 0.65 to 0.75 eV of the conduction band.

22. A method as in Claim 20, including the further step of tailoring the semiconductor alloy material from which the enhancement layer is fabricated so as to substantially prevent charge carriers from being caught in midgap traps which said charge carriers cannot vacate in approximately one second or less.

23. A method as in Claim 19, including the further step of forming the enhancement layer to a thickness of approximately 2,500 to 10,000 angstroms.

24. A method as in Claim 23, including the further step of forming the enhancement layer to a thickness of approximately 5,000 angstroms.

25. A method as in Claim 15, including the further step of pinning, at approximately 0.5 to 0.8 eV, the Fermi level of the semiconductor alloy material from which the enhancement layer is fabricated.

26. A method as in Claim 25, including the further steps of introducing boron and phosphorus into the semiconductor alloy material from which the enhancement layer is fabricated so as to (1) move the Fermi level to the desired location in the energy gap and (2) pin the Fermi level at that location by adding non-trapping states on both sides thereof.