Liquid Crystal Display Device

The present invention relates to a thermally addressable liquid crystal display device, and more particularly to a laser-addressed high-resolution liquid crystal light valve.

Display devices used as the terminal devices of computers are required to have high resolution, due to the large capacities and increased capabilities of modern day large and small frame computers. In such computer applications such as image processing, newspaper composition, LSI design, etc., it is especially important that the display devices have both high resolution and discrete display element addressing. In a conventional cathode ray tube (CRT) such displays have not been realized due to the difficulty of enhancing resolution up to 2000 lines or more for a given display area. Accordingly, other display devices which could potentially produce higher resolution have been investigated. In an article entitled "Laser-Addressed Liquid Crystal Projection Displays" in the technical journal "Proceeding of the S.I.D." Vol. 19, No. 1, First Quarter 1978, p.p. 1-7, A. G. Dewey et al discusses a laser-addressed smectic liquid crystal light valve. Both a dielectric multi-layer film and a reflective electrode film are formed onto a rear glass plate. A front glass plate is provided with a transparent electrode film. Both front and rear glass plates are further provided with alignment films for alignment with respect to one another. A liquid crystal material is sandwiched between the rear and front glass plates. When a laser beam is focused onto the dielectric multi-layer films of the rear glass plate, it is absorbed and converted into heat. The heat is transmitted through both the reflective electrode film and the alignment film of the rear glass plate, raising the temperature of the liquid crystal material. A smectic liquid crystal is used as the liquid crystal material, such that as the temperature rises, the liquid crystal undergoes a nematic to liquid phase change. When the laser beam is subsequently removed, this phase change results in the random alignment of liquid crystal molecules in a frozen scattering mode. The scattering mode is "read out" by a light projected from the side of the front glass plate, and displayed on a screen.

According to this display technique, a line width of 10 .mu.m can be generated, such that over 5000 lines of information can be recorded in a single liquid crystal light valve of two inches square. Thus, it is possible to realize a display which has a high resolution as compared to the conventional CRT display devices of the prior art.

Since the liquid crystal light valve operates to record information by converting laser light into heat, the speed and the contrast of the liquid crystal display are closely related to the light adsorption characteristic of the dielectric multi-layer films. To accurately form the dielectric multi-layer films, it is necessary to vapor-deposit onto the rear glass plate inorganic materials having high refractive indices (such as zinc sulfide (ZnS), arsenic sulfide (As.sub.2 S.sub.3), etc. However, in such a multi-layer structure, gaps between layers occurs where the lattice constants of the respective layers have not been accurately matched. This serves to adversely affect the overall reliability of the display device.

An object of the present invention is to provide a reflective-type liquid crystal display device which is easy to manufacture.

Another object of the present invention is to provide a reflective-type laser-addressed liquid crystal light valve employing a single layer that is easily formed and is of high light absorption efficiency and reliability.

According to the present invention, a thermally addressable liquid crystal display device has a single layer of a compound semiconductor between a first transparent plate and a reflective electrode film. A transparent electrode film is provided on a second transparent plate, and a liquid crystal material is sandwiched between the pair of transparent plates. The semiconductor compound layer comprises a II-IV group compound containing magnesium (Mg) or calcium (Ca), a II-V group compound containing zinc (Zn) or either cadmium (Cd), or a II-VI group or a V-VI group compound containing tellurium.

Favorably, the thickness of the compound layer is chosen to be about 250 .ANG. to 2500 .ANG., and especially it is favorable to select the thickness to be equal to .lambda./4n, where .lambda. represents the wavelength of an addressing light beam, and n represents the refractive index of the compound layer.

FIG. 1 is a schematic cross-sectional view showing a liquid crystal display device according to one embodiment of the present invention;

FIG. 2 is a graph of the light transmissivity of a CdTe layer of 2000 .ANG. in thickness as a function of increasing wavelength;

FIG. 3 is a graph of the relation between the thickness and the light adsorption efficiency of a CdTe layer; and

FIG. 4 is a graph of the relation between the scanning speed and the contrast in a liquid crystal light valve according to the present invention.

A liquid crystal display device according to the present invention employs a single layer light adsorber which consists of a monolithic semiconductor having both a band gap (as converted into wavelength) smaller than the wavelength of an addressing laser beam and a sufficiently high layer absorption efficiency. In practice, if an argon laser beam of 5145 .ANG. in wavelength is used as the writing laser beam, semiconductor compounds having a band gap of 2.40 eV or less can be used. Examples of such semiconductor compounds are II-IV group or II-V group compounds containing magnesium (Mg) or calcium (Ca), such as Mg.sub.2 Si, Mg.sub.2 Ge, Mg.sub.2 Sn, Ca.sub.2 Si, Ca.sub.2 Sn, Ca.sub.2 Pb and Mg.sub.3 Sb.sub.2 ; II-V group or II-VI group compounds containing zinc (Zn) or Cadmium (Cd), such as ZnAs.sub.2, Zn.sub.3 As.sub.2, ZnSb, Cd.sub.3 As.sub.2, CdSb, CdS and CdSe; and II-VI group or V-VI group compound containing tellurium (Te) such as ZnTe, CdTe and Sb.sub.2 Te.sub.3.

Table 1 shows the above-mentioned groups of inorganic semiconductor compounds with their associated band gaps. As for the semiconductor compound layers having band gaps of 1.4 eV or less, it is favorable to use a GaAs semiconductor laser as a writing laser beam. Helium-neon gas laser can be used for the compound layer having a band gap of 1.8 eV or less. Cermet (a vitreous resistance film containing metal such as chromium (Cr), gold (Au), etc.) may also be employed as the semiconductor compound. Any of these materials can be easily formed into a monolithic layer on the first glass plate through vapor-deposition or sputtering, and a thin layer of such a material can serve as a strong absorber for a laser beam. Moreover, since the thermal conductivity of such a material is small, it eliminates the generated heat propagated along the layer surface which results in the decrease in resolution of liquid crystal display devices which utilize conventional dielectric multi-layer compositions.

An embodiment of a liquid crystal light valve having a CdTe layer as the absorption layer according to the present invention will now be described in greater detail. With reference to FIG. 1, a CdTe layer 3 is deposited by sputtering onto a glass plate 2. An aluminum reflective film 4 up to a thickness of 500 .ANG. is then deposited by evaporation onto the CdTe layer 3. A silicon oxide (SiO) film 8 as a parallel alignment film was obliquely evaporated onto the aluminum reflective film up to a thickness of 150 .ANG.. The alignment film align a liquid crystal molecule such that it is parallel to the glass plate. Note that a parallel alignment film in order to maximize display contrast is preferred to a perpendicular alignment film. In a second glass plate 7, a transparent electrode 6 of indium oxide is formed in the thickness of 360 .ANG., onto which a parallel alignment silicon oxide (SiO) film 81 is obliquely evaporated. A spacer 9 preferably of a polyester film 12 .mu.m in thickness is then disposed between the two glass plates. The assembly is then bonded and sealed with a bonding resin 11. An n-octyl cyanobiphenyl is injected into the space delineated by spacer 9 through an injection port (not shown) provided in the glass plate to form a smectic liquid crystal layer 5 under a vacuum. The injection port is sealed off with silicone resin. In this embodiment, a single layer of about 2000 .ANG. in thickness, obtained by sputtering CdTe within an argon gas atmosphere for 15 minutes, is used as the light absorbing layer 3. As shown in FIG. 2, the CdTe layer 3 presents a transmissivity of about 2% for an argon laser beam of 5145 .ANG. in wavelength, and thus can absorb more than 90% of the incident laser beam energy. A picture image is recorded in the liquid crystal light valve 10 by scanning a writing laser beam 1 on the first glass plate 2. The picture image is displayed as a reflection image by then irradiating the liquid crystal light valve 10 with projection light 12. Thus, a liquid crystal light valve having an excellent laser-recording sensitivity can be easily and reliably obtained by employing a single layer of inorganic semiconductor compound as the light absorbing layer.

In the above-described embodiment, by depositing a light absorbing CdTe layer of a thickness of about 2000 .ANG. onto the first glass plate, more than 90% of the incident laser beam energy can be absorbed. The thickness of the absorption layer may range from about 250 .ANG. to 2500 .ANG., depending upon the display contrast in the liquid crystal light valve that is desired. The display contrast of the display device is defined as the ratio of the reflection intensity of the non-addressed portion of surface of the transparent electrode 6 to that of its addressed portion or surface. The display contrast of the liquid crystal light valve itself ultimately depends upon its temperaure-induced phase change. Therefore, since all the proposed semiconductor compounds are thermal insulators, it is desirable to reduce the thickness of the light absorption layer. However, as the absorbing layer is thinned, its light absorption efficiency is decreased. Nevertheless, in the case of such a dielectric material having a high refractive index and absorption properties, the adsorption efficiency increases as the layer thickness is reduced to the point where t=.lambda./4n (t being the thickness, .lambda. being the light wavelength and n being the refractive index of the layer). This phenomena is due to light interference effect inherent in the absorption layer. FIG. 3 illustrates this tendency, wherein the layer thickness is taken along the abscissa and the light absorption efficiency is taken along the ordinate. When the light absorbing layer is as thick as 2000 .ANG., a sufficiently large light absorption efficiency is obtained. As the layer thickness is reduced to approximately 1000 .ANG. the absorption efficiency decreases. However, if the layer thickness is then further reduced, the absorption efficiency increases due to an interference effect, and becomes a maximum at a layer thickness in the proximity of t=.lambda./4n. If the layer thickness is reduced past this particular value, the absorption efficiency decreases rapidly. This maximum value is realized, for example, in the case of a CdTe layer of approximately of 400 .ANG.. Then, by selecting the layer thickness to be equal to or near t=.lambda./4n, it is possible to maintain a large light absorption efficiency while minimizing the layer thickness, which, as described previously, enhances the display contrast of the device.

FIG. 4 shows the display contrast in a recorded line when the liquid crystal light valve is irradiated with an argon laser beam (5145 .ANG. with .lambda.=) while maintaining a constant recording power of 75 mW. The display contrast is plotted, as a function of the scanning speed of the recording laser beam, with respect to the cases where the thickness of the CdTe layer 3 is 2000 .ANG. (curve 51) and 400 .ANG. (curve 52), respectively. Comparing the respective display contrasts at a scanning speed of about 1 meter per second, it can be seen that a layer thickness of .lambda./4n (400 .ANG. in this example) provides a display contrast about three-fold greater than that of a layer thickness of 2000 .ANG..

In the foregoing embodiments, it is contemplated an argon gas laser be used as the writing laser beam. This is because the light intensity of an argon gas laser is high, as compared to that of a semiconductor laser or He-Ne gas laser.

Furthermore, since the argon laser beam is visible, an addressing optical system can be more readily designed.

In a thermally addressable liquid crystal, such as that of the present invention, it is desirable to increase the utility factor of the laser beam energy while also providing a suffient display speed. In this respect, a light absorption layer should absorb at least 70%, and preferably should absorb up to 90% or more, of the laser beam. As shown in FIG. 3, in order to satisfy the 70% absorption minimum, the thickness of the absorption layer must be more than 250 .ANG., and the 90% or more absorption goal can be attained by a layer thickness to be either more than 1300 .ANG. or between 300 .ANG. and 600 .ANG.. By determining the layer thickness according to the relation t=.lambda./4n, more than 95% absorption can be obtained, and a sufficiently high display contrast can be attained.

In summary, a high display contrast can be attained by determining the layer thickness according to the relation t=(.lambda./4n).+-.(.lambda./20n), where the laser beam is absorbed in the order of 90%-95%. By providing a single absorption layer with a thickness that satisfies the above relation, the resulting liquid crystal display will have a greater resolution and higher reliability than those of the prior art.