WO1991007689A1 - Modulateur de lumiere ameliore a cristaux liquides photoadresse comportant un materiau reflechissant isole electriquement - Google Patents

Modulateur de lumiere ameliore a cristaux liquides photoadresse comportant un materiau reflechissant isole electriquement Download PDF

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Publication number
WO1991007689A1
WO1991007689A1 PCT/US1990/006484 US9006484W WO9107689A1 WO 1991007689 A1 WO1991007689 A1 WO 1991007689A1 US 9006484 W US9006484 W US 9006484W WO 9107689 A1 WO9107689 A1 WO 9107689A1
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WO
WIPO (PCT)
Prior art keywords
light
light blocking
alloy
light valve
blocking layer
Prior art date
Application number
PCT/US1990/006484
Other languages
English (en)
Inventor
David E. Slobodin
Original Assignee
Greyhawk Systems, Inc.
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by Greyhawk Systems, Inc. filed Critical Greyhawk Systems, Inc.
Publication of WO1991007689A1 publication Critical patent/WO1991007689A1/fr

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Classifications

    • G—PHYSICS
    • G02—OPTICS
    • G02F—OPTICAL DEVICES OR ARRANGEMENTS FOR THE CONTROL OF LIGHT BY MODIFICATION OF THE OPTICAL PROPERTIES OF THE MEDIA OF THE ELEMENTS INVOLVED THEREIN; NON-LINEAR OPTICS; FREQUENCY-CHANGING OF LIGHT; OPTICAL LOGIC ELEMENTS; OPTICAL ANALOGUE/DIGITAL CONVERTERS
    • G02F1/00—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics
    • G02F1/01—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour 
    • G02F1/13—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour  based on liquid crystals, e.g. single liquid crystal display cells
    • G02F1/133—Constructional arrangements; Operation of liquid crystal cells; Circuit arrangements
    • G02F1/135—Liquid crystal cells structurally associated with a photoconducting or a ferro-electric layer, the properties of which can be optically or electrically varied
    • G—PHYSICS
    • G02—OPTICS
    • G02F—OPTICAL DEVICES OR ARRANGEMENTS FOR THE CONTROL OF LIGHT BY MODIFICATION OF THE OPTICAL PROPERTIES OF THE MEDIA OF THE ELEMENTS INVOLVED THEREIN; NON-LINEAR OPTICS; FREQUENCY-CHANGING OF LIGHT; OPTICAL LOGIC ELEMENTS; OPTICAL ANALOGUE/DIGITAL CONVERTERS
    • G02F1/00—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics
    • G02F1/01—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour 
    • G02F1/13—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour  based on liquid crystals, e.g. single liquid crystal display cells
    • G02F1/133—Constructional arrangements; Operation of liquid crystal cells; Circuit arrangements
    • G02F1/135—Liquid crystal cells structurally associated with a photoconducting or a ferro-electric layer, the properties of which can be optically or electrically varied
    • G02F1/1351—Light-absorbing or blocking layers

Definitions

  • 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.
  • 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.
  • a-Si:H hydrogenated a orphous silicon
  • a-Si:H light valve which uses CdTe as the light-blocking layer and a Si0 2 /Ti0 2 multilayer dielectric mirror.
  • a special multilayer intermediate bonding structure is required to bond the CdTe light blocking layer to the CdS photoconductor 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 is that a rather complex and lengthy fabrication is required to produce the multiple and chemically unique layers.
  • 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.
  • 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.
  • 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.
  • 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.
  • 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.
  • FIG. 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.
  • 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) .
  • an equivalence circuit for the liquid crystal light valve 10 of the preferred embodiment is shown.
  • 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.
  • 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.
  • the light blocking layer it is desirable for the light blocking layer to be thinner than the liquid crystal layer.
  • 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.
  • 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) .
  • OD optical density
  • 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:
  • the light blocking layer 14 should have a sheet resistance of approximately 1 x 10 12 ohms/square in order to resolve a 10 micrometer element.
  • 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.
  • Equation 1 (1) where is the optical absorption coefficient and E is referred to as the optical gap.
  • 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.
  • 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:
  • 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 (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) 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.
  • 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 10 12 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.
  • 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.
  • the substrate is coated with hydrogenated amorphous silicon photoconductor which may include doped and of alloyed layers.
  • 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.
  • PECVD plasma enhance chemical vapor deposition
  • silane for example, as a source gas.
  • the conditions required to deposit highly photosensitive a-Si:H using silane PECVD are well known.
  • 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.
  • the discharge is run for 40 minutes to produce a film thickness of 0.65+/-0.07 micro ⁇ meters.
  • the resulting alloy has 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 10 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 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.
  • 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.
  • 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.
  • 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.
  • 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 10 13 . 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.

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  • Physics & Mathematics (AREA)
  • Nonlinear Science (AREA)
  • Mathematical Physics (AREA)
  • Chemical & Material Sciences (AREA)
  • Crystallography & Structural Chemistry (AREA)
  • General Physics & Mathematics (AREA)
  • Optics & Photonics (AREA)
  • Liquid Crystal (AREA)

Abstract

Appareil et méthode permettant d'améliorer l'efficacité de la fabrication de modulateurs de lumière (10) à cristaux liquides photoadressés (10) utilisant du silicium amorphe hydrogéné comme photoconducteur. On utilise une couche d'alliage au germanium ou à l'étain (14) comme couche réfléchissante. Ladite couche n'exige pas de liaison spéciale pour être utilisable avec le photoconducteur en silicium amorphe hydrogéné (11) et le miroir diélectrique (13), et l'on peut la déposer en utilisant les mêmes techniques que pour le photoconducteur.
PCT/US1990/006484 1989-11-14 1990-11-13 Modulateur de lumiere ameliore a cristaux liquides photoadresse comportant un materiau reflechissant isole electriquement WO1991007689A1 (fr)

Applications Claiming Priority (2)

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US43640089A 1989-11-14 1989-11-14
US436,400 1989-11-14

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Cited By (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
EP0506442A2 (fr) * 1991-03-29 1992-09-30 Victor Company Of Japan, Ltd. Modulateur spatial de lumière
EP0556985A1 (fr) * 1992-02-04 1993-08-25 Ngk Insulators, Ltd. Modulateur spatial de lumière et sa méthode de fabrication
EP0621645A1 (fr) * 1993-04-19 1994-10-26 Sharp Kabushiki Kaisha Elément de commutation optique et dispositif d'affichage à balayage optique l'utilisant

Citations (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US4723838A (en) * 1984-12-10 1988-02-09 Hosiden Electronics Co., Ltd. Liquid crystal display device
US4799773A (en) * 1987-08-27 1989-01-24 Hughes Aircraft Company Liquid crystal light valve and associated bonding structure
US4862227A (en) * 1985-02-27 1989-08-29 Kanegafuchi Kagaku Kogyo Kabushiki Kaisha Semiconductor device and its manufacturing method
US4941735A (en) * 1989-03-02 1990-07-17 University Of Colorado Foundation, Inc. Optically addressable spatial light modulator

Patent Citations (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US4723838A (en) * 1984-12-10 1988-02-09 Hosiden Electronics Co., Ltd. Liquid crystal display device
US4862227A (en) * 1985-02-27 1989-08-29 Kanegafuchi Kagaku Kogyo Kabushiki Kaisha Semiconductor device and its manufacturing method
US4799773A (en) * 1987-08-27 1989-01-24 Hughes Aircraft Company Liquid crystal light valve and associated bonding structure
US4941735A (en) * 1989-03-02 1990-07-17 University Of Colorado Foundation, Inc. Optically addressable spatial light modulator

Cited By (8)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
EP0506442A2 (fr) * 1991-03-29 1992-09-30 Victor Company Of Japan, Ltd. Modulateur spatial de lumière
EP0506442A3 (en) * 1991-03-29 1993-01-20 Victor Company Of Japan, Ltd. Spatial light modulator
US5268779A (en) * 1991-03-29 1993-12-07 Victor Company Of Japan, Ltd. Spatial light modulator with composite film formed by vapor deposition of light blocking semiconductor material and insulation material
EP0556985A1 (fr) * 1992-02-04 1993-08-25 Ngk Insulators, Ltd. Modulateur spatial de lumière et sa méthode de fabrication
US5471331A (en) * 1992-02-04 1995-11-28 Nippon Hoso Kyokai Spatial light modulator element with amorphous film of germanium, carbon and silicon for light blocking layer
EP0722112A2 (fr) * 1992-02-04 1996-07-17 Ngk Insulators, Ltd. Modulateur spatial de lumière et sa méthode de fabrication
EP0722112A3 (fr) * 1992-02-04 1996-07-31 Ngk Insulators Ltd
EP0621645A1 (fr) * 1993-04-19 1994-10-26 Sharp Kabushiki Kaisha Elément de commutation optique et dispositif d'affichage à balayage optique l'utilisant

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