Light addressed liquid crystal light valve incorporating electrically insulating light blocking material of a-SiGe:H

An improved light addressed liquid crystal light valve incorporating electrically insulating light blocking material is disclosed. The light valve has a hydrogenated amorphous silicon photosensitive layer and a germanium containing or tin containing alloy film as a light blocking layer. The light blocking layer that may be used is a selected alloy which includes one or more elements from the group consisting of germanium and tin, one or more elements from the group consisting of hydrogen, nitrogen and oxygen, and zero or more elements from the group consisting of silicon and carbon. The light blocking layer has an optical density per unit thickness approximately equal to or greater than 3 OD/micron for visible light and a sheet resistivity approximately equal to or greater than 10.sup.10 ohms/square.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a liquid crystal light valve containing a 
photoconductor and light blocking layer. More specifically, the present 
invention relates to a light valve having a light blocking layer comprised 
of an amorphous germanium-containing or tin-containing alloy. 
Specifically, the light blocking layer may contain a mixture of an element 
from the group of germanium and tin, at least one element from the group 
of hydrogen, oxygen and nitrogen, and zero or more elements from the group 
of carbon and silicon. In the preferred embodiment, this light blocking 
layer contains an amorphous hydrogenated alloy of germanium and silicon. 
2Summary of the Prior Art 
The prior art is replete with liquid crystal light valves. These light 
valves are used in high resolution displays, electronic imaging and 
optical computing applications. With respect to the present invention, 
those light valves of most interest employ a photoconductor layer and 
operate in reflection mode. One light valve of this type is found in U.S. 
Pat. No. 4,019,807 for a Reflective Liquid Crystal Light Valve with Hybrid 
Field Effect Mode, issued Apr. 16, 1977, by Boswell et al. The device of 
this patent utilizes a cadmium sulfide CdS photoconductor, a cadmium 
telluride CdTe light blocking layer, a CdS/CdTe photoresponsive 
heterojunction, and a magnesium fluoride/zinc sulfide MgF/ZnS multilayer 
dielectric mirror. 
As light valve technology has progressed, it has become apparent to those 
skilled in the art that hydrogenated amorphous silicon (hereinafter 
"a-Si:H") has significant advantages over CdS as a photosensitive layer, 
particularly with regard to speed of light valve operation and 
reproducibility. There exist in the prior art numerous publications 
describing light valves which utilize an a-Si:H photosensitive layer, but 
which have no light-blocking layer or dielectric mirror. Thus, light 
valves incorporating an a-Si:H photosensitive layer, a light-blocking 
layer, and a dielectric mirror are less common in the prior art. 
U.S. Pat. No. 4,799,773 for a Liquid Crystal Light Valve, issued Jan. 24, 
1989, by Sterling, describes an a-Si:H photoconductor light valve which 
uses CdTe as the light-blocking layer and a silicon dioxide/titanium 
dioxide SiO.sub.2 /TiO.sub.2 multilayer dielectric mirror. In this light 
valve, a special multilayer intermediate bonding structure is required to 
bond the CdTe light blocking layer to the CdS photoconductor layer. In the 
absence of this extraneous layer, peeling of the light blocking layer from 
the photoconductor layer, and vice versa, occurred. The extraneous 
multilayer structure also facilitated device repeatability. A significant 
disadvantage of this type of light valve structure, however, is that a 
rather complex and lengthy fabrication is required to produce the multiple 
and chemically unique layers. 
More specifically, in the device of the Sterling patent, the special 
multilayer structure is required to bond the CdTe layer to the 
photoconductor because CdTe does not adhere well when directly deposited 
on the a-Si:H photoconductor. Fabrication of the bonding structure 
requires four processing steps and a dedicated thin film deposition 
system. In addition, separate thin film deposition systems are required 
for photoconductor layer deposition and CdTe deposition. Moreover, 
deposition of the CdTe light blocking layer must be carefully controlled 
to maintain precise CdTe stoichiometry so that the layer has a resistivity 
high enough for high resolution light valve applications. 
The prior art light valve has performance disadvantages. It is desirable to 
construct a light valve having as thin a light blocking layer as possible. 
As will be explained, the thinner the light blocking layer is while 
satisfying the optical density requirement, as well as other requirements, 
the better the performance of the light valve system. According to the 
prior art preferred embodiment of Sterling, a CdTe light blocking layer of 
2 micrometers thickness is required. The present invention is a light 
blocking layer which is easier to deposit than CdTe and which performs the 
same functions with a thinner layer. 
The placement of amorphous alloys of silicon and germanium, and other 
elements, in contact with amorphous silicon is generally known, although 
not for use to form a light blocking layer in a photoaddressed liquid 
crystal light valve. Amorphous silicon germanium alloy has been deposited 
on amorphous silicon in tandem solar cells as a photovoltaic layer and in 
electrophotographic devices as a photosensitive layer, for example. The 
prior art of photocells and electrophotographic devices does not address 
or touch on a light blocker's three essential requirements of low 
impedance, high optical density, and high sheet resistivity because there 
is no need to maintain resolution of an optical signal or block light. 
Furthermore, the material characteristics necessary for amorphous silicon 
germanium alloys which are used in light blockers are different from the 
material characteristics necessary in photocells and electrophotographic 
devices. 
U.S. Pat. No. 4,723,838 for a Liquid Crystal Display Device, issued Feb. 9, 
1988, by Aoki et al, describes an amorphous silicon germanium alloy layer 
placed adjacent to a photosensitive silicon layer for the purpose of 
blocking light. There are substantial reasons why Aoki et al is not 
applicable to the technology of spatial light modulators and/or lacks a 
teaching necessary for the construction of a light blocking layer usable 
in a spatial light modulator. 
The fact that amorphous materials are usable as light blockers in TFT 
matrices, as in Aoki et al, does not imply that they are usable as light 
blockers in spatial light modulators. In the light valve, the light 
blocking layer must simultaneously meet the following three critical 
factors to satisfy operating capability: the optical density must be high 
(3 OD or more) to achieve good light absorption the sheet resistance must 
be high (10.sup.10 ohms/square or more) to achieve high resolution, and 
the impedance must be low (less than that of the liquid crystal layer) so 
that substantially all of the voltage falls across the liquid crystal 
instead of the light blocking layer and to achieve a large voltage swing 
across the liquid crystal for a good dynamic range. In fact, the lower the 
impedance of the light blocking layer, the better the dynamic range that 
can be achieved. 
In the prior art, Aoki's sheet resistivity is not a concern because the 
light blocking material is disposed in individual separate and distinct 
elements below each pixel. Charge spreading does not occur when the 
elements are separated, and therefore maintaining resolution by sheet 
resistivity and preventing charge spreading is again not addressed. In the 
present invention the light blocking layer is a continuous sheet across 
the light valve in which sheet resistivity must be kept high to maintain 
resolution. 
In the prior art of Aoki et al, impedance is not a concern. The impedance 
of the light blocking layer does not need to be small because the electric 
field created by the pixel electrodes will not be crossing the liquid 
crystal material in the region where the light blocking layer is located, 
and therefore no reduction in dynamic range would occur. Because the 
impedance and sheet resistivity are not a concern, the necessary optical 
density of light blocking layer can be achieved by depositing an 
arbitrarily thick layer. In summary, there is no requirement for Aoki et 
al to have a thin light blocking layer. 
Furthermore, the optical density required by Aoki et al is not discussed. 
The optical density required by Aoki et al may be less than that required 
by the present invention. In Aoki, the optical density is sufficient to 
reduce the ambient read light so as to allow proper functioning of the 
circuit. In the present invention, the optical density must be 
sufficiently high to make sure that the intense projection read light is 
reduced so much that the dim write light is not washed out by the read 
light. Thus, the optical densities required by these two applications are 
not necessarily the same. 
A further matter not discussed in the prior art is the ratio of optical 
density per unit thickness. Part of the present invention is the ability 
to create the necessary optical density with a reduced thickness. As 
argued above, there is no necessity in Aoki et al to achieve a high 
optical density, while at the same time reducing thickness. As mentioned 
above, in the prior art of Sterling the thickness of the light blocking 
layer is described as 2 microns (column 3, line 59). Assuming that the 
optical density of Sterling is about the same as the present invention, 
because they have similar operating requirements, then Sterling has a 
ratio of optical density to thickness of (3-5 OD/2 microns) 1.5-2.5 OD per 
micron. In the present invention, the thickness of the light blocking 
layer is approximately 1 micron or less. Thus the ratio of optical density 
to thickness in the present invention is (3-5 OD/1 microns) approximately 
3 OD per micron or greater. 
The present invention is a light blocking layer two or more times thinner 
than the light valve of the prior art which achieves similar light valve 
gain and which is easier to deposit. Consequently, for a given level of 
light valve gain and given liquid crystal cell structure, the present 
light valve has a larger dynamic range and better resolution relative to 
the light valves of the prior art. 
SUMMARY OF THE INVENTION 
Accordingly, it is an object of the present invention to provide a light 
blocking layer in a photoaddressed liquid crystal light valve that is 
capable of direct and efficient bonding to the photosensitive layer. 
It is another object of the present invention to provide a light blocking 
layer which is suitable for use in a liquid crystal light valve. 
It is yet another object of the present invention to provide a light 
blocking layer made of an amorphous germanium or tin alloyed with at least 
on other element. 
It is still another object of the present invention to provide a light 
blocking layer with an optical density per unit thickness of 3 OD per 
micron or greater. 
It is another object of the present invention to provide a light blocking 
layer with a sheet resistivity greater than 10.sup.10 ohms/square. 
The attainment of these and related objects may be achieved through use of 
the novel light valve herein disclosed. A light valve in accordance with 
this invention has an a-Si:H photosensitive layer and a germanium 
containing or tin containing alloy film as a light blocking layer. The 
light blocking layer that may be used is a selected alloy which includes 
one or more elements from the group consisting of germanium and tin, one 
or more elements from the group consisting of hydrogen, nitrogen and 
oxygen, and zero or more elements from the group consisting of silicon and 
carbon. The significant advantages of this selected alloy structure are: 
(1) no special bonding layer is required between the photoconductor and 
the light blocking layer so fabrication is simplified; and (2) the light 
blocking layer may be deposited using the same equipment as used to 
deposit the photoconductor, further simplifying light valve fabrication. 
In addition, the germanium or tin alloy light blocking layers can be made 
to have electrical and optical properties which result in light valves 
with gain and resolution equal to or better than the prior art. This, in 
turn, allows the required impedance and sheet resistivity to be achieved. 
The result of the Applicant's invention is a light blocking layer composed 
of a particular alloy which equals or surpasses the prior art in several 
stringent requirements necessary for use in a photoaddressed liquid 
crystal light valve. These requirements include optical density, 
impedance, and sheet resistivity. An important part of applicant's 
invention is that the required optical density for a light blocking layer 
usable in a light valve can be achieved in a thickness less than that 
known in the prior art. 
Applicant's invention is also, in part, the recognition that the B-value is 
a critical criteria that should be considered for the creation of a 
suitable light blocking layer for use in a liquid crystal light valve. 
Prior to the applicant's invention it was not realized that the competing 
processing requirements for B-value and for .sigma..sub.0 should be 
deliberately controlled and optimized in order to provide the light 
blocking material with the desired electrical and optical properties. 
The attainment of the foregoing and related objects, results, advantages 
and features of the invention should be more readily apparent to those 
skilled in the art, after review of the following more detailed 
description of the invention, taken together with the drawings, in which:

DETAILED DESCRIPTION OF THE INVENTION 
Referring to FIG. 1, a cross sectional view of the photoaddressed liquid 
crystal light valve 10 of the preferred embodiment is shown. A liquid 
crystal material 12 is enclosed between two glass substrates 21 and 22. 
Transparent conducting electrodes 23 and 24 are located to the interior of 
the glass substrates 21 and 22. The purpose of these electrodes and the 
application of voltages across the light valve are well known. On 
electrode 23 a photoconductor layer 11 is located. A light blocking layer 
14 is adjacent to the photoconducting layer 11 and adjacent to the light 
blocking layer 14 is a dielectric mirror 13. The remaining space, between 
the dielectric mirror 13 and the electrode 24 is a liquid crystal layer 
12. Of course, the present invention is not limited solely to liquid 
crystal materials. Other electro-optic materials such as PDLC or 
electrochromic materials which can modulate the projection light 17 can be 
substituted for liquid crystal layer 12. The preferred embodiment of this 
device uses a liquid crystal material which can modulate polarization. Two 
alignment layer 25 are provided on each side of the liquid crystal 12 to 
induce the required molecular orientation of the liquid crystal 12. 
In operation, the photoconductor 11 electrically modulates the state of the 
liquid crystal layer 12. A voltage is applied by voltage generator 27 
across the electrodes 23 and 24 as light is impinged upon the 
photoconductor 11. Since the impedance of the photoconductor 11 is light 
sensitive, a spatially varying light pattern, such as an image, will 
produce a spatially varying electric field across the liquid crystal 
material 12, thereby creating an image in the liquid crystal (through well 
known methods). The dielectric mirror 13 and a light blocking layer 14 are 
placed between the photoconductor 11 and the liquid crystal 12, in essence 
to reflect projection light through the liquid crystal and to protect the 
photoconductor 11, respectively. The dielectric mirror 13 functions to 
reflect most of the projection (or read) light 17 (used in projection of 
the image created in the liquid crystal 12) after the projection light 17 
has passed through the liquid crystal layer 12. The light blocking layer 
14 prevents most of the small percentage of light that actually does pass 
through the dielectric mirror 13 from impinging upon the photoconductor 
11. The light blocking layer 14 is significant because it blocks light 
that may otherwise interfere with or overwhelm the low intensity write 
light 19 incident on the other side of the photoconductor 11 (i.e., used 
to created the initial image in the liquid crystal layer 12). 
In the preferred embodiment, the light blocking layer 14 is disposed as a 
single continuous layer directly onto the photoconductor layer 11. Of 
course, techniques explained in the prior art, such as the insertion of a 
bonding layer between the light blocking layer 14 and the photoconductor 
11 could still be utilized. A thin 0.1 micron layer of silicon oxide could 
be placed between the light blocking layer 14 and the photoconductor layer 
11. It should also be noted that the light blocking layer 14 need not be a 
single layer. The light blocking layer 14 can have several germanium or 
tin alloy layers separated by silicon oxide layers. Although in the 
preferred embodiment the dielectric mirror 13 is disposed directly onto 
the light blocking layer 13, the light blocking layer 14 could be 
integrated with the dielectric mirror 13 by replacing titanium oxide 
layers in the dielectric mirror with the germanium or tin alloy light 
blocking material. 
Referring to FIG. 2, an equivalent circuit for the liquid crystal light 
valve 10 of the preferred embodiment is shown. There are at least four 
important constraints on the light blocking layer 14. The first 
requirement is that the light blocking layer 14 must be of suitably small 
thickness. The light valve 10 to a first approximation acts like a set of 
capacitors in series and the photoconductor acts as a variable capacitor 
whose impedance decreases as illumination level is increased. An AC (e.g. 
square wave) voltage is applied across the light valve 10 between 
transparent conducting layers 23 and 24. For the light valve 10 to work 
effectively, two conditions must be met. First, when the light valve 10 is 
not exposed to light, the impedance of the photoconductor layer 11 must be 
greater than that of the liquid crystal 12, so that only a small voltage 
drop appears across the liquid crystal. Second, when the light valve 10 is 
exposed to light, the impedance of the photoconductor 11 must be less than 
that of the liquid crystal 12 so that most of the voltage appears across 
the liquid crystal 12. 
Thus, a requirement for effective operation of the light valve 10 is that 
the impedance of the dielectric mirror 13 and the light blocking layer 14 
must be much less than the impedance of liquid crystal material 12. If the 
impedance of these layers is too high, then when the photoconductor Il is 
illuminated, most of the drive voltage falls across the dielectric mirror 
and light blocking layer instead of the liquid crystal 12. In the design 
of light valve 10, it is generally desirable to maximize the ratio of 
liquid crystal voltage of the light valve 10 from when it is illuminated 
to when it is dark so as to increase the dynamic range of the electric 
field that can be applied across the liquid crystal layer 12. Thus, it is 
an important design criterion to have a light blocking layer 13 with low 
impedance. The impedance of the light blocking layer 14 or the liquid 
crystal layer 12 is approximately equal to layer thickness divided by 
layer dielectric constant. Thus, it is desirable for the light blocking 
layer to be thinner than the liquid crystal layer. Typically, the 
dielectric constant of the light blocking layer 14 is two or three times 
that of the liquid crystal 12 so if, for example, the liquid crystal layer 
12 in a light valve 10 is 3 micrometers thick, then a light blocking layer 
14 of about 1.0 micrometer thickness is acceptable, while a thickness of 
0.7 micrometers is preferable. In fact, the thinner the layer that can be 
utilized while still providing a sufficient light blocking capability, the 
better the light valve performance, and, in particular, the better the 
dynamic range. 
The second requirement is that the layer should be very efficient in 
absorbing light. For example, in a light valve 10 with a designed gain 
(ratio of read light to write light intensity) of 10.sup.6 and a 
dielectric mirror 13 with 1% optical transmission, a light blocking layer 
14 which absorbs 99.99% of incident light is required. A measure of the 
efficiency of light absorption of the light blocking layer is the optical 
density (OD) defined as --log(transmission). A light blocking layer 14 
might typically have an OD between 3 and 5. As will be discussed, the 
total optical density of a layer is proportional to the thickness of that 
layer. A good measure of the effectiveness of a material at blocking light 
is the ratio of optical density per unit thickness. For example the 
present invention is able to achieve a total optical density of 3 in a 
thickness of 1 micron, giving 3 OD/micron. In the preferred embodiment, 
the light blocking layer has an OD/micron of 4, and still more preferred 
is 5 OD/micron or more (e.g. 3.5 OD in 0.7 microns). Because the 
projection light is primarily visible light from a lamp to be used in a 
color video projector, the required optical density applies to the visible 
light range of wavelengths. In this context, visible light is considered 
to range from about 400 nm to about 650 nm. 
The third requirement is the light blocking layer 14 must have a 
sufficiently high electrical sheet resistivity. The resolution of a light 
valve 10 is determined by the most conducting layer in the device (other 
than the transparent electrode layers). The lower the sheet resistivity of 
the light blocking layer, the faster the spatially varying electric field 
induced by the photoconductor will defocus with time and the lower the 
light valve resolution will be. Therefore, it is important that the light 
blocking layer 14 is not the most electrically conducting layer in the 
light valve. The following equation describes the approximate relationship 
between light valve resolution and light blocking layer sheet resistivity, 
.rho..sub..quadrature. : 
EQU P=(t/(C.sub.PC +C.sub.LC).rho..sub..quadrature.).sup.1/2 (1) 
where C.sub.PC and C.sub.LC are dependent on the photoconductor and liquid 
crystal layer thickness, t is the liquid crystal switching time, and P is 
the dimension of the light valve resolution element. Using typical values 
for light valves, the light blocking layer 14 should have a sheet 
resistance of approximately 1.times.10.sup.12 ohms/square in order to 
resolve a less than 10 micron element. As a rough estimate, a sheet 
resistivity of 10.sup.11 ohms/square is required for resolving a 15 micron 
element, and a sheet resistivity of approximately 10.sup.10 will resolve a 
45 micron element. 
The fourth requirement is that the light blocking layer must have proper 
material properties. It must have low intrinsic stress and good adhesion 
to neighboring layers so that it does not peel or crack. It also should be 
electrically and chemically compatible with the photoconductor and 
dielectric mirror. 
A fifth requirement is that the sheet resistivity and the impedance of the 
light blocking layer not change when light impinges it. This means that 
the layer should have a low photosensitivity. The standard measurement of 
photosensitivity is .sigma..sub.L /.sigma..sub.D, or the ratio of the 
conductivity of the material in light, .sigma..sub.L to dark, 
.sigma..sub.D. A ratio .sigma..sub.L /.sigma..sub.D of about 1 is 
appropriate, less than 2 is preferred, and less than 10 is essential. The 
sheet resistivity .rho..sub.58 can be kept high by maintaining the Fermi 
level at substantially midgap. The photoconductivity can be kept low by 
having a high number of recombination centers. The number of recombination 
centers is controlled by material composition and processing conditions. 
In summary, a high performance light valve, that is, a light valve with 
gain in excess of 100,000 (assuming a dielectric mirror with 99% 
efficiency), capable of resolving less than 10 micron elements, requires a 
light blocking layer with optical density of greater than 3 and sheet 
resistivity greater than 1.times.10.sup.12 ohms/square. A slightly lower 
performance light valve, also with 45 micron resolution, would have a 
sheet resistivity greater than 1.times.10.sup.10 ohms/square. Furthermore, 
only alloys with small photosensitivity (ratio of photoconductivity to 
dark conductivity much less than 10) are of interest. The preferred 
embodiment described several methods for fabricating light blocking layers 
that meet these requirements. 
Physical Parameters 
In constructing a liquid crystal light valve 10, and more specifically, in 
constructing the light blocking layer 14, the following physical 
parameters must be considered. 
The optical absorption spectrum for many amorphous semiconductors, and 
nearly all group IV amorphous semiconductors, in the high absorption 
regime are described by Tauc's equation: 
EQU .alpha.h.nu.=B(h.nu.-E.sub.OPT).sup.2 (2) 
where .alpha. is the optical absorption coefficient, .nu. is the frequency 
of the light photon to be absorbed, and E.sub.OPT is referred to as the 
optical gap. 
The optical density of a thin film, ignoring reflection, is related to the 
optical absorption coefficient by the following equation: 
EQU OD=.alpha.d log.sub.10 e (3) 
where d is the film thickness and e is the logarithmic constant. 
Substituting equation (3) into the Tauc expression, equation (2), and 
rearranging provides OD as a function of optical gap: 
##EQU1## 
This clearly shows that the OD of any amorphous semiconductor decreases as 
optical gap is increased (because the photon energy h.nu. must be larger 
than the optical gap E.sub.OPT) and OD is proportional to film thickness. 
The electrical conductivity of an amorphous semiconductor is usually also 
related to the optical gap. Since only resistive amorphous materials of 
interest are for light blocking applications, and because it is desirable 
to avoid conduction by variable range hopping- as this leads to sheet 
resistivity which is too low, the concern is only for amorphous material 
whose conductivity is dominated by thermally activated extended state 
transport and whose Fermi level lies near mid-gap. Under these conditions, 
the electrical conductivity, .sigma., is related to the optical gap as 
follows: E1 ? 
##STR1## 
where k is the Boltzmann constant and T is the temperature. The sheet 
resistivity, .rho..sub..quadrature., of a thin film is related to the 
conductivity as follows: 
##EQU2## 
The combination of equations (5) and (6) provides: 
##EQU3## 
This equation shows that sheet resistivity increases exponentially with 
optical gap and it is inversely proportional to film thickness. Equations 
(4) and (7) show that there is a tradeoff between OD and sheet 
resistivity. A satisfactory amorphous light blocking layer 14 should have 
an optical gap and thickness such that it meets the previously stated 
requirements for OD and sheet resistivity. 
The requirement for high optical density and high sheet resistivity put 
opposing requirements on the optical gap E.sub.OPT. The optical gap is 
determined primarily by the material's composition, and secondarily by 
processing conditions. Although selection of atomic ratio of the various 
elements can control the optical gap, it was not clear, before the present 
invention, that any optical gap could satisfy the conflicting requirements 
for high optical density and high sheet resistivity, and therefore, the 
material composition to give a correct optical gap was not known. 
Applicant has found that material compositions resulting in an optical gap 
between 1.0 eV and 1.5 eV are adequate, an optical gap between 1.1 and 
1.45 are preferred, and an optical gap between 1.2 and 1.4 is more 
preferred. If the optical gap is greater than 1.5 then the optical density 
is too low, whereas if the optical gap is less than 1.0, then the sheet 
resistivity is too low. 
In order to obtain high optical density in less than 1 micron at 600 nm to 
650 nm light, the standard red light in a video projector (inspection of 
equation 4 shows that the optical density is most difficult to achieve at 
long wavelengths) a large B-value and a low optical gap E.sub.OPT are 
required. The lower the B-value, the lower the optical gap must be to 
compensate. The needed B-value is given by equations (8a,b) below: 
##EQU4## 
Equation (8a) was derived simply from equation (4) , while equation (8b) 
added the assumptions that the desired optical density is 3 within 1 
micron thickness at a wavelength of 650 nm. Because the highest B-value 
typically obtained in amorphous Group IV materials is 6.times.10.sup.5, 
the optical gap must be about 1.5 eV or less. As seen from equation (4), 
the higher the B-value, the better the optical density. The B-value should 
be greater than 2.times.10.sup.5, still more preferred is for the B-value 
to be greater than 4.times.10.sup.5. The B-value is dependent on the 
material network density and, to a lesser degree, structural order. A high 
network density results in a high B-value. In order to achieve a high 
network density, deposition conditions should have a high substrate 
temperature, low growth rate, and low ion-damage. 
A further requirement is that the material have low photosensitivity and 
high sheet resistivity. As seen from equation (7), high sheet resistivity 
requires low .rho..sub.0 and high optical gap E.sub.OPT, and the higher 
.rho..sub.0, the higher the optical gap E.sub.OPT must be to compensate. 
The necessary .rho..sub.0 is given approximately, assuming that the Fermi 
level is at midgap as discussed previously, by equations (9a,b) below: 
##EQU5## 
Equation (9a) was derived simply from equation (7), while equation (9b) 
added the assumptions that at room temperature 2kT is approximately 0.518 
eV and that the sheet resistivity goal is 10.sup.10 ohms/square for a 1 
micron thick film. A .rho..sub.0 less than 8.times.10.sup.4 
(ohm-cm).sup.-1 is usable, less than 8.times.10.sup.3 is preferred, less 
than 8.times.10.sup.2 is more preferred. In order to obtain both the low 
.rho..sub.0 and the low photosensitivity, carrier mobility and carrier 
lifetime must be low, and the Fermi level of the material should be 
approximately midgap (that is the material is neither substantially p-type 
or n-type). If care is taken to exclude dopant type impurities during 
material preparation, the midgap Fermi level can be achieved. The carrier 
mobility is related to the material network density. A low network density 
results in a low carrier mobility. A low substrate temperature, high 
growth rate, and higher ion-damage are the proper conditions for the low 
network density. 
As described in the previous two paragraphs, the requirement for low 
carrier mobility, which needs low network density, is in direct 
competition with the requirement for a high B-value, which needs high 
network density. It was not clear, before the present invention, that 
these conflicting requirements for a high performance light valve could be 
achieved simultaneously. 
The following table indicates the required physical properties of amorphous 
group IV semiconductors in the prior art and for the present invention: 
______________________________________ 
Solar Electro- TFT Light 
Cell photography (Aoki) Valve 
______________________________________ 
B-value high high n/a high 
.sigma..sub.0 
high low low low 
.sigma..sub.L /.sigma..sub.D 
high high low low 
______________________________________ 
The light valve must achieve the requirements of simultaneous high B, low 
.rho..sub.0, and low .rho..sub.L /.rho..sub.D. Other prior art deposition 
processes have not needed to meet these requirements simultaneously. 
Because the intended characteristics are different for the light valve 
application compared to other applications, the essential processing 
conditions to achieve the light blocking layer are not taught from the 
other applications. 
To achieve both a B high enough for and a .rho..sub.0 low enough for the 
present invention, one must carefully balance the competing requirements 
on the processing steps and material composition described above. The 
inventive method by which this is achieved is now described. 
The high B-value and low .rho..sub.0 in an amorphous material is related to 
the amount of disorder in the structure. To achieve both a B-value high 
enough and a .rho..sub.0 low enough for the present invention, very 
specific and limited material composition and processing conditions must 
be imposed. The proper growth conditions can be discovered. They are 
highly sensitive to the method and system of deposition, as well as the 
deposition parameters of gas mixture, substrate temperature, glow 
discharge power and such. The parameters may be very different for each 
particular material composition. Applicant has discovered that, within the 
known ranges for any particular composition and deposition system, the 
parameters may be qualitatively described as slow deposition rates, low 
ion damage, and low temperature substrates. Applicant has further 
discovered that the primary parameters to fine tune the material 
characteristics are the substrate temperature and deposition rate. 
First, a material is selected to meet the optical gap and other 
requirements. The material composition which gives the desired amorphous 
light blocking layer must include a semi-metal component (germanium or 
tin) for the required low optical gap and a terminator (hydrogen, 
nitrogen, or oxygen, and possibly fluorine or chlorine) to yield material 
dominated by extended state transport. The composition may also include a 
semi-insulator component (silicon or carbon). The basic optical gap must 
be established with the semi-metal component, because the optical gaps of 
the semi-insulator components are too high. In order to make the light 
blocking layer be dominated by extended state transport and have a Fermi 
level near midgap, a terminator is added. In addition, because the 
competing requirements of B-value and .rho..sub.0 on the network density 
must be balanced, the addition of the terminator to adjust this network 
density is essential to the proper functioning of the light blocking 
layer. In the embodiment using a germanium semi-metal component, hydrogen 
is the preferred terminator, and it should have an atomic fraction in the 
alloy between 0.2 and 0.6 if no semi-insulator components are present, and 
an atomic fraction between 0.05 and 0.3 if a semi-insulator component is 
present. If other terminators such as nitrogen or oxygen are used, they 
should be present with an atomic fraction between 0.03 and 0.3. Addition 
of the terminator will also increase the optical gap of the material. If 
the optical gap must still be increased, then the semi-insulator can be 
added. Applicants have found that the ratio of the semi-insulator to the 
total amount of group IV elements must be less than 0.35 if the semi-metal 
is germanium, and less than 0.90 if the semi-metal is tin. Of course, 
traces of other elements might be included in the above mixture without 
altering its basic nature. Specifically, small amounts of various dopants, 
phosphorous or boron, for example, might be added to the light blocking 
layer to shift the Fermi level closer to midgap. 
Some additional factors that should be considered are as follows. First, if 
all four of the group IV elements are present, excess disorder may occur. 
Second, tin may require the presence of a semi-insulator component to 
ensure that 4-fold bonding takes place so that the tin acts in the same 
chemical manner as the other group IV elements. 
The second step is to maximize the B-value by setting the deposition rate. 
One starts with high enough temperature and low enough deposition rate of 
the light blocking layer to obtain a material with a maximized network 
density. Applicant has found that low deposition rates, in the range of 1 
to 4 .ANG. per second, are preferred in order to achieve a high network 
density at normally used substrate temperature less than 450.degree. for 
plasma deposition conditions. This yields a layer with a maximized B-value 
and a .rho..sub.0 that is often higher than desired. 
The third step is to reduce .rho..sub.0 by reducing the substrate 
temperature until the resistivity requirement is met. The substrate 
temperature is lowered to reduce the network density enough to lower 
.rho..sub.0 without significantly reducing the B-value. Applicant has 
discovered that as temperature is lowered, reduction of .rho..sub.0 occurs 
before the reduction of B-value. Applicant has found that substrate 
temperatures somewhere between 100.degree. and 250.degree. typically 
provide a low .rho..sub.0 while maintaining a high B-value. 
The final step is to adjust the material composition slightly, if required. 
If the resistivity is too low, the optical gap is increased, it the 
optical density is to high, the optical gap is decreased. If the network 
density needs to be changed, the amount of terminator may be adjusted. 
The likely course of action which is known in the prior art in order to 
increase the optical density would be to decrease the optical gap 
E.sub.OPT. Examination of Equation 7 shows that this course of action 
would result in an decreased sheet resistivity. In a TFT application such 
as Aoki et al, the decrease in sheet resistivity is not a problem because 
the light blocking areas are not continuous sheets, as discussed above, 
whereas in the present invention the increased sheet resistivity is a 
problem. It is the applicant's invention to achieve a high B-value while 
maintaining a low conductivity .rho..sub.0 in order to obtain high optical 
density with a lower thickness while maintaining high sheet resistivity. 
Although Aoki et al mentions an optical gap E.sub.OPT, the only requirement 
for the optical gap is that it be less than that of the adjacent 
semiconductor. Aoki et al is deficient in that it does not teach, suggest, 
or recognize the importance of control of the B-value or any associated 
parameter. Even though Aoki et al teaches the use of an amorphous silicon 
germanium alloy to block light, by utilizing the teachings of Aoki et al, 
one would not have constructed a light blocking layer with all of the 
critical factors recited above, and particularly the required high optical 
density and low thickness. Aoki et al does not teach that E.sub.OPT can be 
specifically selected so as to simultaneously maximize both the optical 
density and sheet resistivity, nor does Aoki et al teach the importance of 
controlling the network density. Furthermore Aoki et al teaches a material 
in which the ratio of germanium (semi-metal) to total group IV material is 
0.2. Such a composition does not provide a suitable light blocking layer 
for the light valve application, in which the germanium must be at least 
0.35, and more preferably approximately 0.5. 
Procedure for Fabrication 
The procedure for fabricating the preferred light valve 10 is as follows. 
The glass substrate 21 is cleaned and then coated with 500 .ANG. of tin 
doped indium oxide (ITO) followed by 500 .ANG. of fluorine doped tin oxide 
using electron beam evaporation. The layer of tin oxide 23b prevents 
indium from diffusing from the ITO 22a into the amorphous photoconductor 
11 during photoconductor deposition. Indium diffusion into the 
photoconductor has deleterious effects on light valve 10 performance. The 
resultant transparent coating has a sheet resistivity of approximately 50 
ohms/square. Following this step, the substrate is coated with 
hydrogenated amorphous silicon photoconductor which may include doped 
and/or alloyed layers. 
In a preferred embodiment, the a-Si:H with high photosensitivity 
sensitivity is deposited to thickness ranging from 1 to 20 microns by 
plasma enhanced chemical vapor deposition (PECVD) using silane, for 
example, as a source gas. The conditions required to deposit highly 
photosensitive a-Si:H using silane PECVD are well known. 
Immediately following this step, and without removing the substrate from 
the PECVD system, the germanium alloy light-blocking layer is deposited to 
thickness of 0.1 to 1.0 microns. A thickness of approximately 0.7 microns 
is preferred. In one embodiment, this is accomplished by using germane in 
combination with silane as a source gas during PECVD. The resulting layer 
is a hydrogenated amorphous silicon germanium alloy layer. 
The PECVD processing conditions sufficient to produce the light blocking 
layer are as follows: germane to silane gas flow ratio of 1:1, substrate 
temperature 200.degree. C and RF power at 40 mW/cm.sup.2. The discharge is 
run for 30 minutes to produce a film thickness of 0.65.+-.0.07 
micrometers. The resulting alloy has an optical gap as determined by the 
well known Tauc method of approximately 1.28 eV. This layer has electrical 
and optical properties required for high performance liquid crystal light 
valves 10, including a gain (with a dielectric mirror of 99% efficiency) 
greater than 100,000; an OD of 3 at 630 nm, 4.4 and 550 nm, and greater 
than 5 at 450 nm; and a sheet resistivity of 8.times.10.sup.11 
ohms/square. Furthermore, there is excellent adhesion between the light 
blocking layer and the a-Si:H so no special bonding layer is required. 
The light blocking layer 14 of this embodiment could also be deposited 
using germane by itself as a source gas under the following conditions: RF 
power of 40 mW/cm.sup.2, 120.degree. C. substrate temperature, and gas 
flow of 40 sccm. This would yield a hydrogenated amorphous germanium film 
with an optical gap of approximately 1.3 eV. 
The light blocking layer 14 may also be deposited using germane or 
tetramethyltin in combination with one or more of the following gases: 
methane, oxygen, or ammonia, to yield a film with an optical gap of 1.3 eV 
and the required properties. Plasma conditions would be derived using the 
above described method. 
Following the creation of the light blocking layer 14, the dielectric 
mirror 13 is deposited. The dielectric mirror 13 may be made from any 
multilayer stack of alternating high and low refractive index material 
layers. Silicon oxide and titanium oxide are often used for these 
alternating layers. The structure and fabrication of these dielectric 
mirrors is known in the art. 
In the final processing step, alignment layers are applied and the light 
valve 10 is assembled and filled with liquid crystal 12 using procedures 
established in the prior art of liquid crystal cell fabrication. 
In an alternative embodiment, the photosensitive a-Si:H is deposited by 
means of reactive sputtering of a silicon target with an argon/hydrogen 
sputtering atmosphere conditions that are well known to those skilled in 
the art. The light blocking layer 13 is deposited by reactive sputtering 
of a germanium target using argon/nitrogen as the sputtering atmosphere. A 
light blocking layer with satisfactory properties is obtained using the 
following conditions: 2 inch diameter target, pressure=5 mTorr, 500 W RF 
power, 100 degrees C. substrate temperature, 9% nitrogen in argon 
sputtering gas and 40 sccm total gas flow, and one hour duration. The 
resulting film has a thickness of 0.6.+-.0.1 microns, an optical gap of 
approximately 1.12 eV, an optical density of 2.1 at 630 nm, 3.5 at 550 nm 
and 4.0 at 450 nm; and a sheet resistivity of 1.3.times.10.sup.10. These 
properties are satisfactory for a light valve with a gain of greater than 
1000 and a resolution of 10 line pairs/mm. 
Deposition of the photoconductor and the light blocking layer 13 are 
preferably carried out in a single sputtering system with multiple targets 
so that both layers can be deposited without removal of the substrate. 
Usable light blocking layers could also be fabricated by reactive 
sputtering of a germanium or tin target using argon in combination with 
one or more of the following gases as the sputtering atmosphere: oxygen, 
hydrogen, methane or silane. 
The foregoing descriptions of specific embodiments of the present invention 
have been presented for purposes of illustration and description. They are 
not intended to be exhaustive or to limit the invention to the precise 
forms disclosed, and obviously many modifications and variations are 
possible in light of the above teaching. Specifically, the possibility of 
adding small amounts of various dopants, phosphorous or boron, for 
example, to the light blocking layer to shift the Fermi level closer to 
midgap, is intended to falling within the scope of the claims. The 
embodiments were chosen and described in order to best explain the 
principles of the invention and its practical application, to thereby 
enable others skilled in the art to best utilize the invention and various 
embodiments with various modifications as are suited to the particular use 
contemplated. It is intended that the scope of the invention be defined by 
the claims appended hereto and their equivalents.