Photoconductor coupled liquid crystal light valve and method for manufacturing the same

Abstract: A photoconductor coupled liquid crystal light valve is disclosed comprising at least a pair of transparent electrodes (3), a liquid crystal layer (2) and a photoconductive layer (1) showing blocking properties both sandwiched between said transparent electrodes (3), wherein said photoconductive layer (1) includes a light absorbing layer (21) and a blocking layer (31) having a band gap wider than said light absorbing layer (21), both said light absorbing layer (21) and said blocking layer (31) being formed of non-alloy a-Si. The invention also includes a method for manufacturing said photoconductor coupled light valve wherein said photoconductive layer (1) is prepared by an electron cyclotron resonance method.

Description: Background of the Invention 1. Field of the Invention The present invention relates to a photoconductor coupled liquid crystal light valve and a method of manufacturing the same. More particularly it relates to a photoconductor coupled liquid crystal light valve using the photoconductive effect of a photoconductive material and the electro-optical effect of a liquid crystal and to a method for manufacturing the same. 2. Description of the Related Art FIG. 4 shows a general construction of liquid crystal light valves. Liquid crystal light valves comprise a photoconductive layer 10, a liquid crystal layer 2, a pair of transparent electrodes 3 disposed by sandwiching therebetween the photoconductive layer 10 and the liquid crystal layer 2, means 4 for applying voltage including said transparent electrodes 3, a reflecting layer 5 and an orientation treatment film 6 disposed between the photoconductive layer 10 and the liquid crystal layer 2. On the external surfaces of the transparent electrodes 3 are disposed a pair of glass substrates 7. A basic behavior of liquid crystal light valve devices is illustrated hereinbelow. For the sake of explanation, it is assumed that the resistance value of the reflecting layer 5 and the orientation treatment layer 6 is considerably smaller than that of the photoconductive layer 10 and the liquid crystal layer 2. Applying voltage V o to the photoconductive layer 10 and the liquid crystal layer 2 from means 4 for applying voltage energizes the photoconductive layer 10 and the liquid crystal layer 2 at a voltage corresponding to the resistance value of respective layers. When the photoconductive layer 10 is not irradiated with light, the resistance applied to the photoconductive layer 10 exhibits a large value to result in a low voltage applied to the liquid crystal layer 2, which is even lower than a threshold voltage V th generating an electro-optical effect. Then, applying a writing light 8 to the photoconductive layer 10 in a pattern formed by introducing or shielding light coming from the side of the photoconductive layer 10 reduces a resistance at a portion of the photoconductive layer 10 irradiated with light 9 whereas increasing voltage applied to a portion of the liquid crystal layer 2 irradiated with light coming from the side of the photoconductive layer 10. The voltage at this point becomes larger than the threshold voltage V th , an electro-optical effect generates in the liquid crystal layer 2, thereby allowing writing an image in the liquid crystal layer 2. This written image is fetched as an image 19 constituted by the absence or the presence of reflected light at the reflecting layer 5 by applying a reading light 9 from the side of the liquid crystal layer 2. A light valve operating in the above way must satisfy the following formula to store a favorable image with a clear contrast. In the formula, R PCL designates a resistance value of the photoconductive layer in a region irradiated with light, R LC a resistance value of the liquid crystal, R PCD a resistance value of the photoconductive layer in a region not irradiated with light; R PCL « R LC « R PCD Assuming that the photoconductive layer 10 (or 1 in Fig 1) and the liquid crystal layer 2 has approximately the same thickness, the formula can be represented by replacing resistance values with resistivity value. In other word, the following formula can be produced; ρ PCL « ρ LC « ρ PCD Considering the properties of the photoconductive layer, increasing ρ PCD will increase the value of ρ PCD /ρ PCL . As a method for attaining the object, the following two methods (a) and (b) can be considered. (a) Using an i type film having a large band gap as a blocking layer and further overlaying an i type light absorbing layer having a smaller band gap than the former i type film allows the formation of an i-i type hetero junction, thereby enlarging an actual value of ρ PCD . (b) Or a III group element such as boron (or V group element such as phosphorus) is doped into a film having a wide band gap to form a blocking layer. Subsequently an i-type light absorbing layer having a narrower band gap than that of the blocking layer is overlaid to provide a p-i junction (or n-i junction) with the photoconductive layer to which a reverse bias is applied to enlarge an effective ρ PCD . In other words, properties required of a material which forms the blocking layer include a wide band gap which inhibits the absorption of writing light. When the band gap of the blocking layer is small, the writing light is absorbed by the blocking layer before it is absorbed by the light absorbing layer with a result that the photosensitivity is deteriorated. To overcome such a drawback, an alloy type amorphous silicon such as a-SiC, a-SiN and a-SiO which has a wide band gap as a material for a blocking layer is used. Preparing an alloy film made of a-SiC, a-SiN and the like by plasma CVD method (hereinafter referred to as PCVD method) or sputtering method requires as a material gas a mixture such as SiH 4 +CH 4 and SiH 4 +NH 3 to complicate the preparation process, which constitutes a factor of increasing the cost. Besides, the process of laminating the alloy type blocking layer with the non-alloy type light absorbing layer suffers from a drawback that such process will cause the contamination of carbon, nitrogen and oxygen atoms in the blocking layer into the light absorbing layer, thereby failing in giving desired properties. On the other hand, Japanese Laid-Open Patent No. SHO 58-34435/1983 Publication discloses a three-layered photoconductor coupled liquid crystal light valve having a photoconductive layer formed with PCVD method. As an embodiment thereof, the following three methods are disclosed; (1) a method for forming a p-i-n structure consisting of an i-layer formed by using a-Si:H film, a p-layer and an n-layer provided on both side of said i-layer by doping phosphorus and boron respectively therein. (2) a method for forming a three-layer structure consisting an i-layer using an a-Si:H film, an alloy type blocking layer having a wide band gap such as a-Si:C:H film and a-Si:N:H film provided on the side of the transparent electrodes, and an n-layer doped with phosphorus and the like and provided on the opposite side of the transparent electrodes. (3) a method for forming a three-layer structure consisting an i-layer using an a-Si:H layer, a thin metal film formed of platinum, gold and molybdenum and the like provided on the side of the transparent electrodes, and an n-layer doped with phosphorus and the like and provided on the opposite side of the transparent electrodes. However, the above Publication does not disclose any method for overcoming drawbacks such as a complicated process for manufacturing the above liquid crystal light valve. The PCVD method used in forming a photoconductive layer inevitably results in producing (SiH 2 ) powders in the deposition chamber. The publication does not disclose any method for overcoming a drawback that defects are produced in formed films owing to the deposition of (SiH 2 ) powders on the substrate of formed films. Besides, Japanese Laid-Open Patent Publication No. HEI 3-126920/1991 discloses a photoconductor coupled liquid crystal light valve having a photoconductive layer formed with ECR method. Although the Publication discloses an improvement in inhibiting the production of (SiH 2 ) n powders in the deposition chamber, it does not disclose the provision of a blocking layer for storing an image with a favorable contrast. From EP-A-0 288 910 a liquid crystal light valve is known which comprises a pair of glass cover plates, transparent electrodes which are located on the inner faces of each cover plate and electrically connected to alternating current sources. Insulating films are placed on either side of liquid crystal material to provide electrical and chemical isolation between the liquid crystal and the electrodes. Positioned on the side of the liquid crystal from which writing light enters the light valve are respectively a dielectric mirror, a light blocking layer and a photoactive layer. The photoactive layer comprises hydrogenated amorphous silicon which is configured as a photodiode with a junction layer facing the writing light. The light blocking layer of this document clearly serves to block light, and the light blocking layer is disposed between the photoactive layer and the dielectric layer. That is, the function of the light blocking layer of this document is different from the function of the blocking layer according to the present invention. SUMMARY OF THE INVENTION The present invention has been made to overcome the above drawback. Thus, according to the invention both a blocking layer and a light absorbing layer in a liquid crystal light valve are made of non-alloy a-Si:H which eliminates the need of using CH 4 and NH 3 as material gas and allows reduction of the production cost. In addition, conventional problem of carbon, nitrogen and oxygen atoms in the blocking layer contaminating into the light absorbing layer can be prevented and a favorable image with a clear contrast can be stored in the liquid crystal light valve. Accordingly, the present invention provides a photoconductor coupled liquid crystal light valve comprising at least a pair of transparent electrodes, a liquid crystal layer and a photoconductive layer showing blocking properties both sandwiched between said transparent electrodes, wherein said photoconductive layer includes a light absorbing layer having a function of absorbing light incident to the photoconductive layer to lower the resistance in the photoconductive layer in a portion to which light is incident, and a blocking layer disposed on the transparent electrode side of the photoconductive layer and having a band gap wider than said light absorbing layer, both said light absorbing layer and said blocking layer being formed of non-alloy a-Si. A method of manufacturing a photoconductor coupled light valve according to the present invention is as claimed in claim 4. BRIEF DESCRIPTION OF THE DRAWINGS The present invention will be detailed by way of the drawings in which: FIG. 1 is a schematic view illustrating a construction of a photoconductor coupled liquid crystal light valve according to the present invention. FIG. 2 is a schematic view illustrating a construction of a film forming device by ECR method. FIG. 3 is a schematic view showing relations between the amount of doped boron in a photoconductive layer thus formed in the present invention, an optical band gap and dark conductivity. FIG. 4 shows a schematic view illustrating a construction of a general liquid crystal light valve. DESCRIPTION OF THE PREFERRED EMBODIMENT A photoconductor coupled liquid crystal light valve according to the present invention comprises at least one pair of transparent electrodes, a liquid crystal layer and a photoconductive layer. Such pair of transparent electrodes are formed of, for example, indium tin oxide (ITO) and tin oxide (SnO). The liquid crystal layer is formed of, for example, cholesteric nonanoate. The photoconductive layer includes a light absorbing layer and a blocking layer. The blocking layer is arranged on the side of the electrode whereas the light absorbing layer is arranged on the liquid crystal layer. When the blocking layer shows a band gap wider than the light absorbing layer, a favorable image with a clear contrast can be stored therein. The photoconductive layer is formed of a non-alloy a-Si, and preferably prepared with Electron Cyclotron Resonance method (ECR method). In other words, an a-Si film formed with ECR method is more rich hydrogen content than a-Si film formed with PCVD method. The a-Si film of the present invention is considered to be different from the conventional a-Si film in the bonding conditions between silicon atoms and hydrogen atoms just because the film contains a large amount of hydrogen. Owing to such bond different from the conventional counterparts, the blocking layer in the liquid crystal light valve of the present invention shows a wide band gape and impurities can be effectively doped therein. Besides, when the blocking layer is formed by using ECR method, a film with a sufficiently wide band gap can be formed of non-alloy a-Si and impurities can be doped therein. Therefore, a film formed of different material such as a-SiC and a-SiN need not be used to prepare the blocking layer. In other words, the kinds of material gas to be used in preparing the layer can be quite few in number, thereby enabling the simplification of the process for preparing the liquid crystal light valve to reduce the cost of production. Silicon compounds used for forming a film needed for the photoconductive layer include those containing hydrogen and a halogen such as SiH 4 , SiF 4 , Si 2 H 6 and Si 2 F 6 and the like. Impurities to be used for doping to prepare a p-type a-Si include boron and the like whereas impurities to be used for doping to prepare a n-type a-Si include phosphorus and the like. The blocking layer and the light absorbing layer must satisfy the conditions shown hereinbelow to form films with ECR method. A film for the blocking layer is formed under the following conditions: the silicon compound gas flow rate of 0.1 to 100sccm, or preferably 0.5 to 5sccm; inactivated gas flow rate of 10 to 200sccm, or preferably 50 to 150sccm; micro-wave output of 100 to 1000w, or preferably 500 to 700w; H 2 flow rate of 1 to 100sccm, or preferably 10 to 20 sccm; gas pressure of 0.1 to 10mTorr, or preferably 1.0 to 1.5mTorr. When doping impurities, impurity flow rate 0.1 to 100sccm (3000ppm when diluted with H 2 ), or preferably 0.5 to 50sccm. The thickness of the blocking layer prepared under the above conditions ranges between 10 to 1000Å, or preferably 50 to 200Å. The band gap in the blocking layer thus prepared 2.2 to 1.98eV. The film for the light absorbing layer is formed under the following conditions: silicon compound gas flow rate of 10 to 500sccm, or preferably 100 to 300 sccm, inactivated gas flow rate of 1 to 100sccm, or preferably 10 to 50sccm; micro-wave output of 1 to 5kw, or preferably 2 to 3kw, gas pressure of 0.1 to 100m Torr, or preferably 5 to 20mTorr. The thickness of the film ranges between 1000 and 200,000 Å, or preferably 10,000 to 1000,000 Å. The band gap of the light absorbing layer prepared under the above conditions ranges between 1.6 to 1.8 eV. The blocking layer prepared with ECR method shows a very large band gap value ranging between 2.2 to 1,98eV, though the layer is made of non-alloy material free of carbon, nitrogen, and oxygen atoms. The photoconductive layer having the above blocking layer and a light absorbing layer formed of an a-Si:H film with a band gap of 1.6 to 1.8 ev overlaid thereon shows a much larger effective ρ PCD / ρ PCL representing a ratio of specific resistance at the time of applying a wriring light to the counterpart at the time of not applying it than a photoconductive layer with no blocking layer. Besides, forming the blocking layer at non-alloy type a-Si:H allows the inhibition of deterioration in the photosensitivity owing to the inflow of carbon, nitrogen and oxygen atoms into the light absorbing layer. Further, the blocking layer can be doped with impurities. When B 2 H 6 /SiH 4 contained in the material gas is 5%, doping boron allows the formation of a blocking layer having a very wide band gap of 1.98eV. In addition, the value of dark conductivity can increase by 5 to 6 order. Even using a blocking layer doped with boron produces a much larger effective ρ PCD /ρ PCL representing a ratio of resistivity at the time of applying a writing light to the counterpart at the time of not applying it in the state of a reverse bias application of light to give the same effect as using a blocking layer non-doped with boron. Thus, forming both the blocking layer and the light absorbing layer of non-alloy a-Si eliminates the need of using CH 4 and NH 4 as a material gas, thereby reducing the production cost. Examples of the present invention will be detailed hereinbelow. Example 1 Initially, a construction of an a-Si film forming device using ECR method will be detailed hereinbelow. FIG. 2 shows a construction of a film forming device using ECR method. The film forming device is provided with a rectangular waveguide pipe 11, a plasma generating chamber 12, a deposition chamber 13 and an exhaust system 14. The plasma generating chamber 12 consists of a cavity resonator to which a microwave is introduced through the rectangular waveguide 11. Incidentally, a microwave introducing window 15 for introducing a microwave from the conducting pipe 11 is formed of quartz glass which easily transmits the microwave. In the vicinity of the plasma generating chamber 12 was disposed a magnetic coil 16 which drew into the deposition chamber 13 plasma generated in the plasma generating chamber 12 by applying a divergent magnetic field. In the deposition chamber 13 a substrate 18 was disposed. The substrate 18 is consisted of a glass substrate having a transparent electrode 3 overlaid thereon and transmitting a writing light 8. In the process of forming a film, the plasma generating chamber 12 and the deposition chamber 13 are evacuated through the exhaust system, followed by introducing a material gas. The material gas to be used in the process consisted of a silicon compound containing hydrogen or halogen such as SiH 4 and SiF 4 . A doping gas consisted of a mixture of B 2 H 6 diluted with hydrogen and added with a proper amount of inactivated gas such as Ar and the like. The gases were introduced into the deposition chamber 13. After introducing these gases, a microwave was introduced into the plasma generating chamber 12 with the gas pressure maintained at a predetermined value while applying a magnetic field, thereby exciting a plasma. Consequently, gases converted into plasma was introduced to the substrate with the divergent magnetic field to deposit a-Si layer on the substrate. In this example, the substrate was not heated. Incidentally, adjusting the position of a window 17 for drawing out plasma and the size thereof can improve the uniformity of the film thus prepared. Table 1 shows film forming conditions under which a blocking layer formed of a-Si using such a film forming device. Fig. 3 shows relations between the amount of doped boron and optical band gap as well as relations between the amount of doped boron and the dark conductivity which are represented by ○, . Table 2 shows film forming conditions using a PCVD method which is given as a comparative example. Fig. 3 shows film properties which represented by ▵,▴ like Fig. 3. film forming conditions by ECR method (films doped with boron) SiH 4 flow rate 1 sccm B 2 H 6 flow rate (diluted with hydrogen to 3000ppm) 0 to 17sccm (B 2 H 6 /SiH 4 = 0 to 5%) Ar flow rate 100 sccm micro wave power 560kW gas pressure 1.0mTorr film forming conditions by PCVD method non-doped film SiH 4 flow rate 25sccm B 2 H 6 flow rate 40sccm RF power 20W gas pressure 0.2 Torr substrate temperature 250°C film doped with boron SiH 4 flow rate 10sccm B 2 H 6 flow rate (diluted with hydrogen to 5.42%) 1.85 to 9.3sccm (B 2 H 6 /SiH 4 =1 to 5%) Ar flow rate 100sccm microwave power 560kW gas pressure 1.0mTorr substrawte temperature 250 to 300 °C Fig 3 shows that a film formed with ECR method is not doped with impurifies having a wice band gap of 2.2 eV which is larger than the counterpart of a film formed with sinral PCVD method. It further shows that cark conductivity σ d increases along with the increase of the amount of doped impurities. The band gap showed a very large value of 1.98eV in the case of B 2 H 6 /SiH 4 = 5% in the material gas. Films having such a wide band gap and allowing impurities to be doped therein cannot be prepared with a conventional PCVD method. It can be prepared only with ECR method. Besides, the blocking layer has a wide band gap and impurities can be effectively doped therein just because the layer contains a larger amount of hydrogen than an a-Si film prepared with a conventional PCVD method, and the state of bonding between Si atoms and H atoms in the blocking layer is different from the conventional counterpart. Example 2 Subsequently, an example of a photoconductor coupled liquid crystal light valve having a photoconducive layer showing blocking properties will be shown. FIG. 1 shows a schematic view of such example. Referring to FIG. 1, reference numeral 31 designates a blocking layer, 21 a light absorbing layer. Preparation example 1 : a photoconductor coupled liquid crystal light valve having a blocking layer formed of non-doped a-Si Example 2 of the photoconductor coupled liquid crystal light valve of the present invention was formed of the following elements under the conditions shown hereinbelow; Substrate glass: CONING #7059 Transparent electrode: ITO was vapor deposited on the substrate glass. Photoconductive layer Blocking layer: SiH 4 flow rate 1sccm Ar flow rate 100sccm microwave output 550w H 2 flow rate 15 sccm gas pressure 1mTorr a-Si:H film formed to a thickness of 100 Å under the above condition Light absorbing layer: SiH 4 flow rate 120sccm Ar flow rate 20sccm microwave output 2.5kw gas pressure 12mTorr a-Si:H film formed to a thickness ot about 3 to 10µm under the above conditions. Reflecting layer: formed of laminated material having different refractive index. The thickness of the reflective layer ranges between 0.05 to 0.1µm. Concretely, the reflecting layer consisted of a dielectric mirror having 10 to 15 layers of MgF-ZnS, Si-SiO 3 and the like. However, in the present invention, a combination of Si-SiO 3 is desirable to prevent the photoconductive layer from being contaminated with other atoms than Si. Orientation treatment film: After applying silane treatment agent on the reflecting layer, the layer was subjected to a high temperature treatment. Liquid crystal layer: A chiral agent (such as cholesteric nonanoate and the like) 7 to 8 wt.% was mixed with phenylcyclohexane series to form a liquid crystal layer having a thickness of 7 to 8 µm and a twisted pitch of about 20% of the thickness of the layer. Preparation example 2 : a photoconductor coupled liquid crystal light valve having a blocking layer doped with boron (B 2 H 6 /SiH 4 in the material gas = 5%) Conditions are the same with preparation example 1 except the conditions for forming a blocking layer. The conditions for the blocking layer in shown below. Photoconductive layer: Blocking layer: SiH 4 flow rate 1sccm Ar flow rate 100sccm B 2 H 6 (diluted to 3000 ppm with H 2 ) 17sccm (B 2 H 6 /SiH 4 in supplied gas=5%) microwave output 550w gas pressure 1mTorr a-Si:H film formed to a thickness to about 100 Å under the above conditions. Writing an image on the photoconductor coupled liquid crystal light valve thus prepared by applying a square wave pulse at 150V from means for applying voltage 4 using as a writing light 5mW semiconductor laser having a wavelength of 670nm or a white light such as a halogen light source gave a favorable contrast ratio to show that the photoconductive layer has an excellent photosensitivity. The photoconductor coupled liquid crystal light valve of the present invention has a blocking layer formed in a photoconductive layer so as to provide blocking properties in said blocking layer. The blocking layer comprises a non-doped a-Si having a wide band gap or an a-Si doped with impurities by ECR method. Such a construction of the liquid crystal light valve allows the preparation of the liquid crystal light valve having a sufficient photosensitivity and enlarged ratio of specific resistance in the dark state against the counterpart in the bright state to show a favorable contrast. Further, forming the blocking layer and the light absorbing layer of a-Si can simplify the process for preparing the liquid crytal light valve to reduce the cost of production. In addition, forming an a-Si layer with a conventional PCVD method or sputtering method cannot prevent the generation of (SiH 2 ) n powders in the deposition chamber. The powders thus generated deposits on the substrate for forming a film to cause a defect in the film thus prepared. On the other hand ECR method can inhibit the generation of such powders to prevent such defects in the formed film, thereby improving the yield ratio in the preparation of the device.

Claims: A photoconductor coupled liquid crystal light valve comprising at least a pair of transparent electrodes (3), a liquid crystal layer (2) and a photoconductive layer (1) both sandwiched between said transparent electrodes, wherein said photoconductive layer (1) includes a light absorbing layer (21) having a function of absorbing light incident to the photoconductive layer (1) to lower the resistance in the photoconductive layer (1) in a portion to which light is incident, characterised in that said photoconductive layer further includes a blocking layer (31) disposed on the transparent electrode (3) side of the photoconductive layer (1) and having a band gap wider than said light absorbing layer (21), both said light absorbing layer and said blocking layer (31) being formed of non-alloy a-Si. A photoconductor coupled liquid crystal light valve of claim 1 wherein said blocking layer (31) is formed of a-Si doped with impurities. A photoconductor coupled liquid crystal light valve of claim 2 wherein said impurity is boron or phosphorus. A method of manufacturing a photoconductor coupled light valve of claim 1 wherein it comprises a step of preparing said photoconductive layer by an electron cyclotron resonance method.

Inventors: ADACHI KATSUMI, HAYAKAWA TAKASHI, NARIKAWA SHIRO, ADACHI, KATSUMI, HAYAKAWA, TAKASHI, NARIKAWA, SHIRO
Applicants: SHARP KK, SHARP KABUSHIKI KAISHA
Application year: 1993
Main classification: G02F1
IPC codes: G02F1