Note: Descriptions are shown in the official language in which they were submitted.
~ ` 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.
8ACKGROUND OF THE INVENTION
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
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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
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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
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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
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. ~ , . .
1~71076
1406
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
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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,
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. 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.
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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.
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lX~107~;
1406
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.
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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
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~ 271~76
1406
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,
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.
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
-16-
~ ~ 7 ~ ~ 7
1406
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
-17-
~ 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
-18-
- 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
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~Q
. . .
,. ...... , ` `
. . . ..
~ 07~
1406
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,
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-- ~27~07~
1406
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
I -21-
-
`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
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127~7t~
1406
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
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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
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~` ~ 7~L~6
1406
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
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`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
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1406
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.
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