WO1991007689A1

WO1991007689A1 - Improved light addressed liquid crystal light valve incorporating electrically insulating light blocking material

Info

Publication number: WO1991007689A1
Application number: PCT/US1990/006484
Authority: WO
Inventors: David E. Slobodin
Original assignee: Greyhawk Systems, Inc.
Priority date: 1989-11-14
Filing date: 1990-11-13
Publication date: 1991-05-30

Abstract

Apparatus and method for improving the performance and manufacturability of photoaddressed liquid crystal light valves (10) that employ hydrogenated amorphous silicon as a photoconductor. A germanium or tin alloy layer (14) is used as the light blocking layer. The germanium or tin alloy layer requires no special bonding to be usable with the hydrogenated amorphous silicon photoconductor (11) and dielectric mirror (13), and may be deposited using the same techniques as the photoconductor.

Description

IMPROVED LIGHT ADDRESSED LIQUID CRYSTAL LIGHT VALVE INCORPORATING ELECTRICALLY INSULATING LIGHT BLOCKING MATERIAL

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 amorphous hydrogenatedgermanium, germaniumalloy, orothergroup four alloy.

Summary 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 andoptical 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. Patent No. 4,019,807 for a Reflective Liquid Crystal Light Valve with Hybrid Field Effect Mode, issued April 16, 1977, by Boswell, et al., and assigned to Hughes Aircraft Company. The device of this patent utilizes a cadmium sulfide CdS photoconductor, a CdS/CdTe photoresponsive heterojunction, a cadmium telluride CdTe light blocking layer, and a MgF/ZnS multilayer dielectric mirror, explain 2:6 interdependence.

As light valve technology has progressed, it has become apparent to those skilled in the art that hydrogenated a orphous silicon (hereinafter "a-Si:H") has significant advantages over CdS as a photosensitive layer, particularly with regard to speed of light valve operation and reproducib- ility. There exist in the prior art numerous publications describing light valves which utilize an a-Si:H photosensi¬ tive 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. Patent No.4,799,773 for a Liquid Crystal Light Valve, issued January 24, 1989, by Sterling, R. , and assigned to Hughes Aircraft Company, describes an a-Si:H light valve which uses CdTe as the light-blocking layer and a Si0₂/Ti0₂ 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 '773 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, separatethin filmdeposition systems are required for photoconductor layer deposition and CdTe layer deposi¬ tion. Moreover, deposition of the CdTe light blocking layer must be carefully controlled to maintain precise Cd/Te stoichiometry so that the layer has a resistivity high enough for high resolution light valve applications. Second, the prior art light valve has performance disadvantages. According to the prior art preferred embodiment, a CdTe light blocking layer of 2 micrometers thickness is required. This layer is almost 3 times thicker than that required in the light valve of the present invention to achieve similar light valve gain. Consequently, for a given level of light valve gain and given liquid crystal cell structure, the prior art light valve has smaller dynamic range and poorer resolution relative to the light valve of the present invention.

SUMMARY OF THE INVENTION Accordingly, it is the 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 liquid crystal light valve having a light blocking layer whichcontains an amorphous hydrogenatedgroup four element.

It is yet another object of the present invention to provide a liquid crystal light valve having a light blocking layer made of an amorphous hydrogenated germanium or tin alloy.

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 alloy film as a light blocking layer. The alloy light blocking layers that may be used include amorphous hydrogenated germanium; amorphous hydrogenated alloys of germanium or tine and one or more of the following elements: Si, C, N or O; and unhydrogenated alloys of germanium and N and/or O. The significant advantages of this 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 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.

The attainment of the foregoing and related objects, advantages and features of the invention should be more readily apparent to those skilled in the art, after review of the followingmore detailed description of the invention, taken together with the drawings, in which:

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a cross sectional view of a liquid crystal light valve of the preferred embodiment.

Figure 2 is a first approximation equivalence circuit for the liquid crystal light valve of the preferred embodiment.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to Figure 1, a cross sectional view of the photoaddressed liquid crystal (or thermo optic) 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 layers 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 the photoconducting layer 11 and adjacent 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. Two alignment layer 25 are provided on each side of the liquid crystal 12. The alignment layers 25 are applied to induce the required molecular orientation of the liquid crystal 12. In operation, the photoconductor 11 electrically modulates the state on the liquid crystal layer 12. A voltage is applied across the electrodes 23 and 24 as a 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 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 photocon¬ ductor 11, respectively. The dielectricmirror 13 functions to reflect most of the read light 17 (used in projection of the image created in the liquid crystal 12) after the read light 17 has passes 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 in 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) .

Referring to Figure 2, an equivalence 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 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. For the light valve 10 to work effectively, two conditions must be met. One, 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. Two, 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.

A further 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 liquid crystal 12 impedance. If the impedance of these layers is too high, then when the photoconductor 11 is illuminated, most of the drive voltage falls across the dielectric mirror and light blocking layer instead of the liquid crystal 12. In light valve 10 design, it is generally desireable to maximize the ratio of liquid crystal voltage of the light valve 10 from when it is illuminated to when it is dark. 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 0.7 micrometer thickness is acceptable.

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 10⁶ 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.

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 for the light blocking layer 14 is not the most electrically conducting layer in the light valve. The following equation describes the relationship between light valve resolution and light blocking layer sheet resistivity, p:

P=(t/(C_pc+C )p)¹² where C and C_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 x 10¹² ohms/square in order to resolve a 10 micrometer 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 schematically compatible with the photoconductor and dielectric mirror.

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 semiconduc¬ tors, in the high absorption regime are described by Tauc's equation:

equation 1 (1) where is the optical absorption coefficient and E is referred to as the optical gap. The constant B in the Tauc expression depends on thewidth of the amorphous semiconduc¬ tor bandtails, E_w, and the minimum metallic conductivity, , as follows:

equation 2 (2) where c is the velocity of light and n is the refractive index for a film.

The optical density of a thin film is related to the optical absorption coefficient by the following (ignoring reflection) :

equation 3 (3) where d is the film thickness. Substituting equations (2) and (3) into the Tauc expression, equation (1) , and rearrang- ing provides OD as a function of optical gap:

equation 4 (4)

This clearly shows that the OD of any amorphous semiconductor decreases as optical gap is increased and OD is proportional to film thickness.

The electrical conductivity of an amorphous semiconductor is usually also related to the optical gap. Since the only resistive amorphous materials of interest are for light blocking applications, 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, , is related to the optical gap as follows: equation 5 (5) where k is the Boltzmann constant and T is the temperature. The sheet resistivity, , of a thin film is related to the conductivity as follows:

equation 6 (6)

The combination of equations (5) and (6) provides:

equation 7 (7)

This equation shows that sheet resistivity increases exponen¬ tially 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 forOD and sheet resistivity. Combining equations (5) and (7) provides the product of sheet resistivity and OD as a function of two material parameters Ew and Eopt:

equation 8 (8)

Since a figure of merit for light blocking layers is the product of sheet resistivity and OD, equation (8) shows that the smaller Ew is, the better the expected light blocking layer is. Ew is a measure of width of the bandtails in the amorphous semiconductor which in turn is related to the degree of structural disorder. Equation (8) can be used to solve for the optimum Eopt which will give desired light blocking layer properties for a given value of Ew. For example, if we want a light blocking layer with an OD of 4 at 550 nm (2.25 eV) and a sheet resistivity of lθ¹² ohms/square, (using Ew of 0.2 eV which is found in a-Si:H) we find that the optimum optical gap of the light blocking layer is 1.21 eV. Substituting this value of Eopt into equation (4) and using a value of 1 x 10³ (ohms-cm)^"1 for _min (typical for a-Si:H) in equation (3) , then solving for d, the film thickness in this case should be 0.3 micrometers, which is well within the requirement. Device quality a-Si:H has a relatively small Ew while other known amorphous materials typically have a somewhat higher Ew. A higher Ew will lead to a higher optimum Eopt. For example, if Ew is 0.4, the optimum optical gap is now 1.37 eV.

The optical gaps in the range of 1.1 to 1.6 eV are achievable in amorphous alloys of germanium and/or tin and one or more of the following elements: carbon, silicon, nitrogen, oxygen and hydrogen. The optical gap of an alloy is chosen so that the resulting light blocking layer has properties required for a particular light valve design. Not all alloys are useful for demanding light valve applications. A high performance light valve, that is, a light valve with gain in excess of 100,000, capable of resolving 15 micron elements, requires a light blocking layer with optical density of greater than 3 and sheet resistivity greater than 1 x 10¹² ohms/square. Only alloys with small bandtail widths and small photosensitivity (ratio of photoconductivity to dark conductivity less than 1) meet these requirements. The preferred embodiment describes several methods for fabricating light blocking layers that meet these requirements.

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 A of tin doped indium oxide (ITO) followed by 500 A of fluorine doped tin oxide using electron beam evaporation. The layer of tin oxide prevents indium from diffusing from the ITO into the amorphous photoconductor 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 of alloyed layers. In a preferred embodiment, the a-Si:H with high photo¬ sensitivity is deposited to thickness ranging from 1 to 20 micrometers by plasma enhance 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 0.7 micrometers. This is accomplished by using germane in combination with silane as a source has during PECVD.

The resulting layer is a hydrogenated amorphous silicon germanium alloy layer. Deposition conditions sufficient to produce the light blocking layer are as follows: germane:- silane gas flowratio=l:1, substrate temperature 200 degrees C and RF power=40 mW/cm². The discharge is run for 40 minutes to produce a film thickness of 0.65+/-0.07 micro¬ meters. The resulting alloyhas an optical gap as determined by the well known Tauc method of between 1.2 eV and 1.4 eV. This layer has electrical and optical properties required for high performance liquid crystal light valves 10, including a gain greater than 10,000; 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 x 10¹¹ 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=40 mW/cm2, 120 degrees C substrate temperature and gas flow of 40 seem. This would yield a hydrogenated amorphous germanium film with an optical gap of 1.3 eV.

The light blocking layer 14 may also be deposited using germane 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 requiredproperties. Plasma conditions similar to those previously state would be used.

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. 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 light valve 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 using conditions that are well known to those skilled in the art. The light blocking layer 13 is deposited by reactive sputter¬ ing of a germanium target using argon/nitrogen as the sputtering atmosphere. A light blocking layer with satisfac- tory 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 has and 40 seem total gas flow, and one hour duration. The resulting film has a thickness of 0.6+/-0.1 micrometers, an optical density of 2.1 at 630 nm, 3.5 at 550 nm and 4.0 at 4350 nm; and a sheet resistivity of 1.3 x 10¹³. 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 could be deposited without removal of the substrate. Usable light blocking layers could also use argon in combination with one or more of the following gases: 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. 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 con¬ templated. It is intended that the scope of the invention be defined by the Claims appended hereto and their equivalents.

Claims

WHAT IS CLAIMED IS:

1. A thermo optic light valve for reflection mode projection, comprising: photoconductor means having a photosensitive impedance; light blockingmeans bonded directly to saidphotoconductor means for protecting said photoconductor means from reflective mode light; dielectricmirrormeans locatedbetween said light blocking means and liquid crystal means for reflecting reflection mode light through thermo optic means; and liquid crystal means for yielding images under specific conditions, said reflection light reflecting off said dielectric mirror means and through said images in said liquid crystal means, thereby projecting said images.

2. A thermo optic light valve apparatus for reflection mode projection, comprising: a first and second substrate having several layer means therebetween, including, a first and second electrode means, said first electrode means being located to the interior of and adjacent said first substrate, said second electrode means being located to the interior of and adjacent to said second substrate; photoconductor means having a photosensitive impedance; light blocking means formed on said photoconductor means for protecting said photoconductor means from reflective mode light; dielectric mirror means located adjacent said light blocking means; and liquid crystal means located between said dielectricmirror means and said second electrode means; said light blocking means containing an amorphous group IV element.

3. The apparatus of claim 2 wherein said light blocking means includes an alloy from the class consisting of ger- manium containing alloy and tin containing alloy. 4. The apparatus of claim 2 wherein said light blocking means includes: an amorphous hydrogenated alloy from the class consisting of germanium and tin alloy; and at least one of the following elements: Si, C, N and 0.

5. The apparatus of claim 2 wherein said light blocking means includes: an unhydrogenated alloy of germanium; and an element of the class consisting of N and 0.

6. The apparatus of claim 2 wherein the light blocking means includes amorphous hydrogenated germanium.

7. In a liquid crystal light valve device, the structure comprising: a substrate; a transparent conductor^' formed on said substrate; an amorphous photoconducting layer formed on said substrate; and an amorphous light blocking layer formed on said photoconducting layer.

8. The light valve device of claim 7 wherein the light blocking layer includes an amorphous hydrogenated silicon/- germanium (Si/Ge) having a Ge/(Ge+Si) atomic ratio of between 0.1 and 1.0.

9. The light valve device of claim 7 wherein the light blocking layer includes an amorphous silicon/germanium

(Si/Ge) having a Ge/(Ge+Si) atomic ratio of between 0.1 and 1.0.

10. The light valve device of claim 7 wherein the light blocking layer includes an amorphous hydrogenated germanium having a hydrogen atomic fraction between 0.2 and 0.6. -lo¬ ll. The light valve device of claim 7 wherein the light blocking layer includes an amorphous hydrogenated germanium/- nitrogen (Ge/N) alloy havinga N/(N+Ge) atomic ratiobetween .03 and 0.3.

12. The light valve device of claim 7 wherein the light blocking layer includes an amorphous germanium/nitrogen (Ge/N) alloy having a N/(N+Ge) atomic ratio between .03 and 0.3.

13. The light valve device of claim 7 wherein the light blocking layer includes an amorphous hydrogenated silicon/tin

(Si/Sn) alloy having a SN/(Sn+Si) atomic ratio of 0.05 and 0.5.

14. The light valve device of claim 7 wherein the light blocking layer includes an amorphous silicon/tin (Si/Sn) alloy having a SN/(Sn+Si) atomic ratio of 0.05 and 0.5.

15. The light valve device of claim 7 wherein the light blocking layer includes amorphous hydrogenated germanium/- carbon (Ge/C) alloy having a C/(C+Ge) atomic ratio between 0.03 and 0.8.

16. The light valve device of claim 7 wherein the light blocking layer includes amorphous germanium/carbon (Ge/C) alloy having a C/(C+Ge) atomic ratio between 0.03 and 0.8.

17. The light valve device of claim 7 wherein the light blocking layer includes an amorphous hydrogenated germanium-

/oxygen (Ge/O) alloy having an 0/(0+Ge) atomic ratio between 0.03 and 0.3.

18. The light valve device of claim 7 wherein the light blocking layer includes an amorphous germanium/oxygen (Ge/O) alloy having an 0/(0+Ge) atomic ratio between 0.03 and 0.3. 19. The light valve device of claim 7 wherein the light blocking layer includes: an amorphous alloy from the class containing germanium alloy and tin alloy; and at least two of the following elements: Si, C, N, 0, H, F or Sn; said light blocking layer having an optical gap between 1.1 and 1.6 eV.

20. The light valve device of claim 7 wherein said photoconducting layer is hydrogenated amorphous silicon.

21. The light valve device of claim 7 wherein said photoconducting layer is a hydrogenated amorphous silicon/- geranium alloy.

22. The light valve device of claim 7 wherein said photoconducting layer is a'hydrogenated amorphous silicon/- carbon alloy.

23. The light valve device of claim 7 wherein said transparent conductor is a layer of conducting tin oxide deposited over a layer of tin doped indium oxide.

24. A method for producing a liquid crystal light valve for reflection mode image projection, comprising the steps of: providing a substrate having predefined coatings; applying a photoconducting layer of high photosensitivity a-Si:H to said substrate; applying a light blocking layer of containing germanium to said photoconducting layer; and applying a dielectric mirror layer to said light blocking layer.