Amorphous photoreceptor with high sensitivity to long wavelengths

The present invention is a photoconductive member which includes a layer of amorphous semiconductor, adjacent to a multilayered amorphous semiconductor material, the composite material sandwiched between two blocking layers. The entire structure is supported by a layer of metal or glass depending on whether the photoconductive member is used in electrophotography or in an image pickup tube.

BACKGROUND OF THE INVENTION 
The present invention is related to photoreceptors. In particular, the 
invention is related to photoreceptive materials which are sensitive to 
wavelengths up to .lambda..about.0.9 .mu.m. 
The spectral response of photoreceptors is very important in determining 
the type of imaging system in which they can be used. Their overall 
performance is determined by their spectral sensitivity, or bandgap, and 
their electronic properties. For example, in systems which utilize light 
emitted by GaAs lasers or photodiodes it is necessary to have a 
photoreceptor with a large sensitivity to light of wavelength &gt;0.85 .mu.m. 
This requires materials with bandgaps which are &lt;1.45 eV. The high 
sensitivity beyond 0.7 .mu.m also improves the performance of the 
photoreceptor when used in conjunction with solid state lasers and 
photodiodes emitting light at .lambda..about.0.8. 
Amorphous silicon passivated by hydrogen (a-Si:H) or other elements such as 
fluorine have been used as photoconductive receptors, electrophotography 
and image pickup tubes, see, e.g., U.S. Pat. No. 4,394,426, Maruyama et 
al, Journal of Non-Crystalline Solids 59 & 60 (1983) 1247-1254, North 
Holland Publishing Company and Imamura et al, Proceedings of the 11th 
Conference (1979 International) on Solid State Devices, Tokyo, 1979; 
Japanese Journal of Applied Physics, Volume 19 (1980) Supplement 19-1, pp. 
573-577. However, because it has a bandgap of .about.1.7 eV, it has low 
optical absorption for .lambda..gtoreq.0.7 .mu.m and hence rapidly loses 
its photosensitivity at these wavelengths. To increase optical absorption 
of a semiconductor it is necessary to decrease its bandgap and there have 
been attempts to obtain amorphous semiconductors that have smaller 
bandgaps and good electronic properties. Amorphous semiconductors based on 
a-Si:H exist with smaller bandgaps such as a-Si.sub.x Ge.sub.1-x :H alloys 
in which a bandgap of .about.1.5 eV is obtained with .about.50% of Ge 
present in the films. In order to decrease the bandgap, E.sub.g, below 1.5 
eV it is necessary to have more than .about.50% Ge in the films. 
However, it is found that when the fraction of Ge exceeds .about.50% by 
volume and when E.sub.g decreases below .about.1.5 eV there is a large 
degradation in the electronic properties of the alloy films, see, e.g., G. 
Nakamura et al Japan J. Appl. Phys. 20, 1981, Suppl. 20-1, p. 291-296; R. 
L. Weisfield, J. Appl. Phys., 54, (1983) 6401. This degradation seriously 
limits any potential performance of a-Si:H/a-Ge:H alloys as photoreceptors 
having high sensitivity to wavelengths from 0.8 to 0.9 .mu.m. 
In a preferred embodiment of the present invention, a photoreceptor member 
is described which includes material whose bandgap can be selected to make 
it sensitive to infrared light. The bandgap of the photosensitive material 
can be reduced below 1.5 eV with a ratio of Ge to Si which is .ltorsim.50% 
Ge and with sufficiently good electronic properties to make it highly 
photosensitive to wavelengths up to 0.9 .mu.m. 
SUMMARY OF THE INVENTION 
The present invention is a photoconductive member which includes a layer of 
material capable of carrying charge, adjacent to a multilayered amorphous 
semiconductor material. The multilayered material is formed from 
tetrahedrally bonded elements or alloys containing tetrahedrally bonded 
elements. The composite material may have an adjacent blocking layer on 
either side or may be sandwiched between two blocking layers. The entire 
structure is supported by a layer of metal or glass depending on whether 
the photoconductive member is used in electrophotography or in an image 
pickup tube.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The present invention is a photoreceptive member with enhanced sensitivity 
to infrared radiation. This sensitivity is achieved by incorporating into 
the photoreceptive member a multilayered material which has high infrared 
optical absorption and good electronic properties. In a preferred 
embodiment, the layered material includes alternating thin layers formed 
from tetrahedrally bonded elements or alloys containing tetrahedrally 
bonded elements. In a more preferred embodiment, the multilayered material 
includes alternating thin layers of 1-Si:H and a-Ge:H. Such a material is 
sometimes referred to as a superlattice, see, e.g., Solid-State 
Superlattices by G. Dohler, Scientific American, November 1974. If the 
layers 1, 3, 5 . . . and 2, 4, 6 . . . of the structure have the same 
thickness then the structure will have a repeat distance of the thickness 
of layer 1 plus the thickness of layer 2. The present invention shall be 
illustrated and described by a superlattice having a-Si:H and A-Ge:H 
layers. 
The photoreceptive member is used in electrography and infrared image 
pickup tubes. The photoreceptive member of the present invention is 
particularly useful when it is combined with a system that uses a laser or 
photodiode source of light whose wavelength is in the infrared. 
FIG. 1 shows a schematic diagram of a preferred embodiment of the 
photoreceptive member of the present invention. The elements of the member 
include blocking layers 2 and 8, a material 4 that will carry charge, a 
multilayered amorphous superlattice 6 and a support 10. Both blocking 
layers 2 and 8 may not be necessary depending on the other elements of the 
device as discussed below. As shown in FIG. 1, the photoreceptive member 
is sensitive to light incident upon blocking layer 2. The layer 6 is 
substantially thinner than layer 4. 
The layer 4 can be an organic or inorganic material which need not be 
photosensitive to wavelengths beyond 0.7 .mu.m. It must be a material 
capable of carrying charge. That is, a material into which charge created 
by light absorbed in layer 6 can be injected efficiently. Furthermore, 
this injected charge can be transported across the layer 4 to the opposite 
surface. The layer 4 may be an organic polymer such as polyester, 
polycarbonate, or PVK which has charge transport molecules incorporated 
into it. If in these materials the charge transport is to be by holes then 
this transport must occur at energy levels which are close to the valence 
band of layer 6 so that the photogenerated holes in layer 6 can be easily 
injected into these levels. The layer 4 can also be inorganic material 
like Se, SeTe, SeTeAs or similar alloys. If the transport in these 
materials is to be by holes, the valence band of layer 6 must be close to 
that of layer 6 for efficient injection of holes photogenerated in layer 
6. If the material of layer 4 has sufficient resistance 
(.gtorsim.10.sup.12 cm.sup.-1) under operating conditions than it may not 
be necessary to having a blocking layer adjacent to it (top or bottom). 
The photoreceptive member is made sensitive to infrared light by varying 
the thicknesses of the a-Si and the a-Ge layers of the superlattice 
between .about.5 .ANG. and .about.100 .ANG.. The optical gap changes from 
.about.1.5 ev to .about.1.1 eV when the repeat distance of the 
superlattice is increased from .about.10 .ANG. to .about.100 .ANG. (see 
FIG. 2). Note that in FIG. 2 d.sub.r .about.30 .ANG. gives E.sub.g 
.about.1.3 eV. 
The increase in optical absorption that occurs as the layer thickness is 
decreased leads to increased absorption of infrared light. Large fractions 
of incident light at wavelengths from 0.8 to 0.9 .mu.m are absorbed in 
films which are only 1 .mu.m thick. This is illustrated in FIG. 3 which 
shows the percentage of the incident light of wavelengths 0.8 and 0.9 
.mu.m that is absorbed in a .about.1 .mu.m thick film superlattice 
materials having different values of d.sub.r. (The thicknesses of the 
a-Si:H and a-Ge:H in the case shown are in the ratio 1.1 to 1.0). Also 
indicated in the figure are the corresponding values for a 1 .mu.m thick 
a-Si:H film having a bandgap of .about.1.7 eV. 
As may be observed from FIG. 3, only a thin film of a-Si:H/a-Ge:H 
superlattice material is required to absorb a very large fractions of 
light at these wavelengths. The sufficiency of such thin films relaxes the 
requirements on the electronic properties of the superlattices for 
efficient extraction of photogenerated carriers. The requirement for 
efficient extraction of carriers is that the photogenerated carrier 
lifetime .perspectiveto. the carrier transit times. 
The decrease in bandgap described herein can be also achieved by not having 
equal thicknesses of the individual a-Si:H, a-Ge:H layers but by having a 
suitable ratio of d.sub.Ge (thickness of Ge) to d.sub.Si (thickness of 
Si). 
The thickness of Si in the superlattice, however, is to be kept to less 
than .about.50 .ANG. and preferably .about.30 .ANG.. This is in order to 
allow efficient electron tunneling between adjacent a-Ge layers across the 
intervening a-Si layer. This is necessary because the bandgap changes in 
these layered materials result primarily from the movement of the 
conduction band (electron extended states). 
As a consequence, the discontinuity between the valence bands in the Ge and 
Si are .ltorsim.0.1 eV and the rest of the bandgap difference occurs at 
the conduction band. 
Efficient tunneling occurs for silicon thicknesses of up to .about.30 .ANG. 
with ohmic currents up to average electric fields of 
.about.5.times.10.sup.4 V/cm. At higher fields enhanced transport of 
electrons through the Si layers is obtained. 
The superlattice layer 6 and the semiconductor or insulator 4 may be 
interchanged thereby placing the superlattice layer 6 adjacent to a 
blocking layer 2 which is necessary. In this case, because of the 
dependence of the perpendicular conductivity on tunneling, the 
conductivity in the plane of the layers can be very much higher than the 
conductivity perpendicular to them. For high resolution, this parallel 
conductivity must be low. This can be achieved by having charge depletion 
in the a-Ge layers or by donor-acceptor compensation so that the Fermi 
level is at midgap of the a-Ge superlattice. 
The photosensitivity of the a-Ge/a-Si superlattice layers in photoreceptors 
is determined by the optical absorption, transport and lifetimes of the 
carriers in the perpendicular (transverse) direction to the layers. The 
properties of the a-Si/a-Ge layered structures, having the individual 
thicknesses specified, satisfy the requirements that are necessary for 
enhancing the IR sensitivity of a-Si photoreceptors used in 
electrophotography. Specifically, a member .ltorsim.1 .mu.m comprised of 
these alternating layers can be used to efficiently absorb light from 
solid state lasers and photodiodes emitting light up to .about.0.9 .mu.m. 
Furthermore, these photogenerated carriers can be extracted from such 
members with collection efficiency close to 100% at fields 
.ltorsim.1.times.10.sup.5 V/cm. 
As described above, the layer 4 may be an amorphous material such as a-Si. 
In addition to a-Si, the layer 4 may be a wide bandgap photoreceptor 
material with E.sub.g .gtoreq.2 eV which has good transport properties and 
appropriate band alignment, i.e., its valence band must match the valence 
band of the superlattice sufficiently to pass the required hole current. 
The layer 4 must allow the injected photogenerated carriers to cross the 
layer to reach the charged surface. The thickness of layer 4 should be 
between .about.10 .mu.m and .about.50 .mu.m. Such materials presently used 
in electrophotography include amorphous silicon and selenium and organic 
polymers. 
The substrate 10 depends on the manner of use for the photoreceptor. If the 
photoreceptor is used in electrophotography then the substrate 10 is metal 
(may be a metal drum or belt). If the photoreceptor is used in image 
pickup tubes, the substrate 10 is a transparent material such as glass 
with a transparent conductivity oxide electrode such as SnO, ITO, CdO, 
etc. 
The properties of the blocking contacts 2 and 8 also depend on how the 
photoreceptor material is used. Long wavelength radiation will generate 
electron hole pairs in the layered superlattice 6. Then if, for example, 
the photoreceptor is used in electrophotography devices wherein a negative 
corona is used to deposit negative ions on the surface of layer 2, then 
the layer 2 will be an electron blocking contact (not hole blocking) and 
layer 8 will be a hole blocking contact (not electron blocking). If the 
resistivity of layer 4 is sufficient to maintain charge on the surface 
then it may not be necessary to have a blocking contact 2. 
The blocking contact adjacent to the substrate can be a thin layer of doped 
a-Si:H or an appropriate thin insulating layer such as SiO.sub.2, SiC, 
SiN. The doped a-Si:H must be n.sup.+ doped to be blocking for holes and 
p.sup.+ doped to be blocking for electrons. The blocking contact for the 
corona has to be an appropriate thin insulating layer in order to maintain 
resolution. Such a blocking contact however may not be necessary if the 
contact between layer 4 and the corona itself is sufficiently blocking. 
If a positive charge is imparted to the surface of layer 2, then layer 2 
will be a hole blocking layer and layer 8 will be an electron blocking 
layer. 
The photogenerated electrons and holes will cross the amorphous 
semiconductor layer 4 just as in present conventional electrophotography 
devices. 
In the embodiment shown in FIG. 1, if the photoreceptor is used with a 
negative corona, the electrons will have no barrier in going from the 
superlattice layer 6 to layer 8 and metal substrate 10. With a-Si as 
material 4 the holes will have a slight barrier, .about.0.1 ev, in going 
from the superlattice layer 6 to the amorphous semiconductor layer 4. If 
the corona is positive, the layer 2 is hole blocking and layer 8 is 
electron blocking. In this case, the electrons will have to overcome a 
small barrier up to .about.0.4 eV in order to enter the a-Si amorphous 
semiconductor layer 4. A large potential across the photoreceptor and high 
electric fields help the electron to go over this barrier. In addition, by 
changing the superlattice layer thicknesses the bandgap can be graded 
(e.g., from .about.1.3 to 1.7 eV) to make this transition easier. 
An alternate embodiment of the photoreceptor includes the same structure as 
shown in FIG. 1 except that the superlattice layer 6 may be interposed 
between layer 2 and layer 4 instead of between layer 8 and layer 4. The 
operation would be similar as for the structure shown in FIG. 1 depending 
on whether a positive or negative corona was used. With negative corona 
the electrons would be injected into the amorphous semiconductor layer 4 
and with positive corona the holes would be injected into layer 4. 
If the photoreceptor material is used in an image pickup tube, then the 
substrate 10 is glass with a transparent conducting oxide electrode and 
light is incident on the material through the glass substrate 10. The 
layer 2 is charged by an electron beam. The operation is similar to a 
negative corona electrophotography device described earlier except the 
electron beam scans the layer 2 every 1/30 of a second. 
The thickness of the photoreceptor in this case is from .about.2 to 5 .mu.m 
and, therefore, the superlattice layer 6 is &lt;1 .mu.m. 
The transport of photogenerated electrons in the direction parallel to the 
incident light and perpendicular to the alternating layers is determined 
by their ability to tunnel from the adjacent a-Ge layers through the 
a-Si:H intermediate layers and on their recombination rates. The tunneling 
depends on the thickness of a-Si:H and E.sub.g. The recombination is 
determined by the densities of recombination centers in the a-Ge and the 
a-Si/a-Ge interfaces (recombination in a-Si is negligible because of the 
low densities of defects). Therefore, it is very important to have low 
densities of interface defects because of the many a-Si/a-Ge interfaces 
present in this member. This can be achieved by layers fabricated under 
conditions such as described by B. Abeles and T. Tiedje, B. Abeles, et 
al., Journal of Non-Crystalline Solids, Vol. 66, (1984), p. 351, however, 
it is not limited to such preparation conditions. In particular it may not 
be necessary for the layered material to be prepared under fabrication 
conditions which result in interfaces as sharp as those described by 
Abeles and Tiedje. 
Because of the dependence of the carrier transport on tunneling it is 
important to optimize the thicknesses of the a-Si layer for given E.sub.g. 
This thickness cannot exceed 20-30 .ANG. for an E.sub.g of .about.1.3 eV. 
Also, the a-Ge has to be deposited under conditions which result in low 
densities of deep gap states (.ltorsim.10.sup.17 cm.sup.-3 eV.sup.-1). 
This requires "passivators" such as H or F to saturate dangling bonds. 
Electron transport of photogenerated carriers determines the 
photoconductivity of the layered structures shown in FIG. 1. The effective 
electron .sub.e e products (for the parallel and transverse directions) 
for the layered materials having equal thicknesses of a-Si:H and a-Ge:H 
are shown in FIG. 4 as function of E.sub.g. These .mu..sub.e .tau..sub.e 
products were determined from photoconductive measurements with ohmic 
contacts. Note the decrease below 1.4 eV in effective .mu..sub.e 
.tau..sub.e for the transverse direction results from the decrease in 
electron tunneling that corresponds to a-Si:H thickness .about.20 .ANG. . 
Note also, that these .mu..sub.e .tau..sub.e products &gt;10.sup.-8 cm.sup.2 
V.sup.-1 are comparable to those in high quality a-Si. 
The photogenerated hole transport is also determined by recombination. 
However, because of the small potential barrier between the valence bands 
of the a-Ge and a-Si (.about.0.1 eV) the transport at ambient temperatures 
can proceed by thermal emission over this low barrier. In undoped 
a-Si/a-Ge layered structures the holes are minority carriers and do not 
contribute significantly to photoconductivity. However, they become just 
as important as electrons in determining the operation of the 
photoreceptors described in our invention. In the invention described 
herein, high quantum efficiencies at long wavelengths are obtained when 
both electrons and holes are swept out efficiently from the layered member 
into the a-Si, into the substrate and to the charged surface of the 
photoreceptors (similar to solar cells). The ability to sweep out these 
carriers is determined by the recombination lifetimes of both carriers and 
the electric field. This ability to be collected can be expressed by a 
collection width, X.sub.c, which is thickness of a layered member from 
which both photogenerated carriers can be collected. Because with blocking 
contacts a positive space charge is generated in the layered member, the 
electric field, E, is not uniform and both its magnitude and the distance 
over which is extends depends on the voltage. 
The ability to efficiently utilize IR long wavelength light in the present 
invention was demonstrated by measuring the quantum efficiency in films 
.about.1 .mu.m thick which comprised of the a-Si:H/a-Ge:H layers with 
evaporated platinum as a blocking, Schottky barrier contact (rather than 
insulating layers with or without thick a-Si films). In these structures 
efficient carrier collection could be achieved even without any external 
bias because of the built in potential resulting from the Schottky barrier 
(.about.0.4 volts). The thicknesses of the a-Si/a-Ge layered structures 
were from 0.6 to 1.0 .mu.m. The thicknesses from which photogenerated 
carriers could be collected depends on the bias. The voltages that could 
be applied in these experiments were limited by the blocking 
characteristics of the Schottky barriers which are inferior to thin 
insulating heterojunction type blocking contacts. The quantum 
efficiencies, i.e., number of carriers collected per photon entering the 
layered film for .mu.=0.8 and .lambda.=0.9 .mu.m with 0 and -2V reverse 
bias to the platinum are shown in FIGS. 5 and 6, respectively. The maximum 
quantum efficiencies that could be obtained in a-Si:H films at these 
wavelengths for a 1 .mu.m thick a-Si:H film are 0.3% at .lambda.=0.8 .mu.m 
and 0.01% at .lambda.=0.9 .mu.m. It can be seen that for the E.sub.g =1.3 
eV, even with no external bias, for .lambda.=0.8 .mu.m the efficiency is 
15% and for .lambda.=0.9 .mu.m with 2 volts reverse bias applied the 
efficiency is 1000 times higher than for 1 .mu.m of a-Si:H (i.e., 
1000=10/0.01). 
The high photosensitivity of the layered material over a wide range of 
wavelengths &gt;0.7 .mu.m is illustrated in FIG. 8. FIG. 8 shows the quantum 
efficiency as a function of wavelengths for material having E.sub.g =1.36 
eV. These efficiencies were obtained with -2 volts applied to the platinum 
on a 0.8 .mu.m thick film. 
The collection efficiencies as a function of applied reverse bias for the 
E.sub.g =1.31 eV film having a-Si:H/a-Ge:h layers of 17 .ANG. and 15 .ANG. 
respectively is shown in FIG. 7. Note that the collection efficiencies 
increase monotonically with applied voltage across the 0.8 .mu.m film, and 
with 5 volts the collection efficiency for .lambda.=0.8 .mu.m is over 60% 
and for .lambda.=0.9 .mu.m is over 20%. Because the collection width 
X.sub.c with 5 volts is 0.5 .mu.m, the mean electric field across X.sub.c 
corresponds to 1.times.10.sup.5 v/cm. (This also means that the same 
quantum efficiency would be obtained with a layered member which is only 
0.5 .mu.m thick). The data indicates that if 10 volts were applied with a 
better blocking contact, the QE for .lambda.=0.8 .mu.m would be close to 
100% and for .lambda.=0.9 .mu.m about 35% (170 times higher than for 
a-Si:H 20 .mu.m layer and 3,500 higher than for a 1 .mu.m a-Si:H layer). 
In the invention described herein, with an a-Si layer 4 of 20 .mu.m 
thickness, it would be necessary to charge the receptor to 200 volts in 
order to obtain these fields of .about.10.sup.5 V/cm across the member. In 
the invention, because of the close alignment of the valence bands between 
the layered member and the a-Si, there is no problem to be expected with 
extraction of photogenerated carriers into a-Si. Also to optimize the 
extraction of photogenerated carriers from the layered material 6, its 
thickness should be made equal to the X.sub.c that is present under the 
operating conditions of the photoreceptor.