JPH05232501A - Liquid crystal display device - Google Patents

Abstract

PURPOSE:To suppress the blurring of the electric latent image patterns formed on a photoconductive layer and to improve the resolving power of the liquid crystal display element by disposing a semiconductor layer having low mobility or a semiconductor layer having a high defect level density on a liquid crystal layer side. CONSTITUTION:A first hydrogenated amorphous silicon film 31 and a second hydrogenated amorphous silicon film 32 are successively formed on a transparent conductive film 2 and the photoconductive layer 30 is formed of both these layers. The other surface of the second hydrogenated amorphous silicon film 32 is integrated with a reading-out reflection film 6. The second amorphous silicon layer 32 nearest the liquid crystal layer 4 is at least either of the layer which is higher in the defect level density incorporated into the layer than the other semiconductor layer and the layer having the small carrier mobility in the layer. Then, the blurring of the carriers accumulated between the photoconductive layer 30 and the liquid crystal layer 4 at the time of writing in an intra-surface direction is suppressed by the action of the second amorphous silicon layer 32 and consequently, the higher resolving power of the liquid crystal light valve is obtd.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical writing type liquid crystal display device used for a large-screen high-definition projection display or the like, and more particularly to improving its quality.

[0002]

2. Description of the Related Art FIG. 2 shows, for example, JP-A-59-2161.
A sectional view of a conventional liquid crystal display element disclosed in Japanese Patent No. 26 is shown. In the figure, 1 is a glass substrate arranged oppositely, 2 is a transparent conductive film formed on the inner surface of the glass substrate 1,
An electrode substrate is formed by these. Reference numeral 5 is an alignment film formed on one transparent conductive film 2, and 4 is a liquid crystal layer formed on the alignment film 5, and the light reflection film 6 is read on the liquid crystal layer 4 through the alignment film 5. Are formed. 7 is an adhesive, 3
Reference numeral 0 is a photoconductive layer made of a hydrogenated amorphous silicon film.

Next, the operation will be described. For writing to the liquid crystal display element, a predetermined bias is applied between the two transparent electrodes 2 facing each other, and at the same time, a write light pulse having an arbitrary light-dark spatial pattern from the right side of FIG. It is done by making it incident on. At this time, the photoconductive layer 30 in the region which is not irradiated with the writing light has a high resistance, but the resistance of the photoconductive layer 30 which is irradiated with the writing light becomes low. A carrier distribution corresponding to the spatial pattern of light occurs. Depending on the distribution, a spatial distribution occurs in the birefringence of the liquid crystal layer 4 or the magnitude of the optical rotatory power. For example, when a ferroelectric liquid crystal is used as the liquid crystal layer 4, a birefringence distribution of the liquid crystal layer 4 occurs according to the spatial distribution of input light.

The spatial distribution of the birefringence and the optical rotatory power of the liquid crystal layer 4 thus written is reflected by the reading light reflecting film when the reading light is incident from the left side of FIG. 2 through the polarizer. The light can be read out again by projecting the light on the screen through the polarizer. Using this liquid crystal display element, wavelength conversion of a two-dimensional image, incoherent-coherent conversion, and the like can be performed.

In order to realize the writing operation of the liquid crystal display element described above, the resistance of the photoconductive layer 30 when not irradiated with the writing light is sufficiently higher than that of the liquid crystal layer 4, and the liquid crystal layer 4 is irradiated with the light. Must be less than the resistance of. Normally, the liquid crystal display element is driven by applying a low-frequency AC bias of several tens Hz or less to the TN liquid crystal, and a pulse width of 100 μsec to 1 m for the ferroelectric liquid crystal.
It is performed by applying a bipolar pulse of about sec. As the semiconductor material forming the photoconductive layer 30, CdS, BSO, single crystal silicon, or the like is used in addition to amorphous silicon.

[0006]

The operation of the present liquid crystal display element is as described above, and the resolution of the present liquid crystal display element is as follows.
It is considered to be determined by factors such as (a), (b) and (c). (a) Diffusion of carriers generated by light irradiation in the in-plane direction during drift of the photoconductive layer 30. (b) Bleeding in the in-plane direction of the charge latent image pattern between the photoconductive layer 30 and the liquid crystal layer 4. (c) Drip of the liquid crystal potential from the charge latent image pattern due to the finite thickness of the liquid crystal layer 4. Of these, for (a), it is effective to increase the resistance and thin the semiconductor constituting the photoconductive layer. (c) is a document (IEEE Trans. Electron Device
s ED-21 , 8 (1974)), which is based on the Roarch model, and high resolution can be expected by thinning the liquid crystal layer.

Considering the factor (b), the carriers generated during irradiation of the writing light pulse cross the photoconductive layer 30 and reach the interface with the liquid crystal layer 4 to form a latent charge image pattern. Then, by the time the liquid crystal responds, the accumulated charge spreads in the plane by diffusion. The change in the potential between the photoconductive layer 30 and the liquid crystal layer 4 (in the xy plane) due to this diffusion is such that the exchange of charges between the photoconductive layer 30 and the liquid crystal layer 4 can be ignored, and D is the interface. Δ ² V / δx ² + δ ² V / δy, where R is the diffusion constant of charge diffusion, R is the sheet resistance of the photoconductive layer at the interface, and C is the combined capacitance per unit area between the interface and the two transparent electrodes. ² =-(1 / D)? V /? T = -RC? V /? T Equation (1) can be written. Here, for simplicity, it is considered that the liquid crystal display element does not include the reading light reflection film 6 and the liquid crystal layer 4 and the photoconductive layer 30 are in direct contact with each other.

From this equation, in order to suppress potential bleeding due to carrier diffusion, the resistance of the photoconductive layer 30 may be increased or the capacitance may be reduced by increasing the thickness of the photoconductive layer 30 or the liquid crystal layer 4. I understand. However, it is difficult to increase the resistance of the photoconductive layer 30 while maintaining the photosensitivity of the photoconductive layer 30, and it is necessary to suppress the deterioration of resolution due to (a) and (b). It is difficult to make the photoconductive layer 30 and the liquid crystal layer 4 extremely thin because it is necessary to perform impedance matching. Furthermore, as a liquid crystal layer,
When a ferroelectric liquid crystal is used, the thickness of the liquid crystal layer 4 is generally 2 in order to produce the effect of surface stabilization.
An appropriate value is about μm, and an extreme increase or decrease in thickness may cause difficulty in losing its ferroelectricity.

As described above, in the above-mentioned conventional liquid crystal display element, the in-plane diffusion of carriers, which is one of the determining factors of the spatial resolution, is effectively prevented without adversely affecting other characteristics of the device. It was difficult to reduce.

The present invention has been made to solve the above problems, and it is possible to suppress the bleeding of the electric latent image pattern formed on the photoconductive layer and improve the resolution of the liquid crystal display element. To aim.

[0011]

In the liquid crystal display element according to the present invention, the photoconductive layer has two or more semiconductor layers, of which the semiconductor layer closest to the liquid crystal layer is higher than other semiconductor layers. It is at least one of a layer having a high defect level density and a layer having a low carrier mobility contained in the layer.

[0012]

According to the present invention, the bleeding of the charge latent image pattern formed on the liquid crystal layer side of the photoconductive layer when a bias is applied under the irradiation of writing light has a low mobility on the liquid crystal layer side. It is reduced by disposing a semiconductor layer having a high density of defect states.

[0013]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below with reference to the drawings. FIG. 1 is a sectional view of a liquid crystal display device according to an embodiment of the present invention. In the figure, 32 is a semiconductor layer closest to the liquid crystal layer 4, which is a second hydrogenated amorphous silicon film in this example. Reference numeral 31 is another semiconductor layer, which is the first hydrogenated amorphous silicon film in this example. A first hydrogenated amorphous silicon film 31 and a second hydrogenated amorphous silicon film 32 are sequentially formed on the transparent conductive film 2, and a photoconductive layer 30 is formed by both of them. The other surface of the second hydrogenated amorphous silicon film 32 is integrated with the read light reflecting film 6.

In manufacturing the above-mentioned liquid crystal display element, an ITO film was formed as the transparent conductive film 2 on the glass substrate 1 by the sputtering method. The first hydrogenated amorphous silicon 31 was formed by the RF plasma CVD method. The film thickness was 3 μm. The second hydrogenated amorphous silicon 32 was formed by exposing the first hydrogenated amorphous silicon 31 to hydrogen rf plasma for 30 minutes. Plasma irradiation conditions are as follows: substrate 250 ° C., hydrogen gas pressure 1 Torr
Is. When the cross section was observed with an electron microscope, the surface of the first hydrogenated amorphous silicon 31 was about 50 nm.
The formation of the second hydrogenated amorphous silicon 32 having a coarse density was observed in the region of. Between the second hydrogenated amorphous silicon film 32 and the other transparent conductive film 2, a liquid crystal cell including an alignment film 5, a liquid crystal layer 4 and a read light reflection film 6 was formed. As the liquid crystal layer 4, a ferroelectric liquid crystal (CS-1014 manufactured by Chisso Corporation) was used. The alignment layer 5 is made of SiO.
An oblique vapor deposition film was used. The cell gap of the liquid crystal was 2 μm. A dielectric multilayer film formed by vapor deposition was used as the read light reflection film.

Next, the operation of the above embodiment will be described. The main operation for writing and reading is the same as in the conventional example. At the time of writing, when a bias is applied under the irradiation of writing light, the photocarriers become photoconductive layers 3
A charge latent image pattern is formed in the second amorphous silicon layer 32 disposed on the liquid crystal layer 4 side in the photoconductive layer 30 across 0. This second amorphous silicon layer 32
Is a rough film and therefore contains many defect levels and has a low carrier mobility. Therefore, most of the carriers are trapped in the defect level before they diffuse in the plane.
As a result, the sheet resistance value R of the equation (1) increases, and the diffusion of carriers into the plane is remarkably suppressed in the time period of about the pulse width of the bipolar pulse. Further, the accumulated carriers are easily swept out by applying a reverse bias.

As described above, due to the action of the second amorphous silicon film 32, the bleeding in the in-plane direction of the carriers stored between the photoconductive layer and the liquid crystal layer at the time of writing is suppressed, and as a result, the liquid crystal light valve. It is possible to increase the resolution.

In the above embodiment, the case where the first amorphous silicon film 31 has one layer has been described, but it goes without saying that it may have a plurality of layers.

The second amorphous silicon film 32
If the film thickness is too thick, the voltage drop cannot be ignored in that portion, and there is a concern that the drive voltage of the entire liquid crystal display element will increase. In this embodiment, the film thickness of the second amorphous silicon film 32 is at most 50 nm, which is sufficiently smaller than the film thickness of 3 μm of the first amorphous silicon film 31, so that the increase of the driving voltage can be ignored. In general,
In the structure of the liquid crystal display element shown in the above embodiment, the thickness of the second amorphous silicon film 32 is one tenth of the thickness of the first amorphous silicon film 31 in order to suppress the increase of the driving voltage by about 10%. Must be:

Further, in the above embodiment, since the second amorphous silicon film 32 is a film having a low mobility and a large number of defect levels, in-plane bleeding of carriers can be effectively suppressed. This second amorphous silicon film 3
No. 2 can be expected to have a bleeding suppressing effect as long as it has at least one of the above-mentioned properties of low mobility and high defect levels as compared with the first amorphous silicon 31. ..

Further, the second amorphous silicon film 32
Was formed by hydrogen plasma treatment on the surface of the first amorphous silicon film 31, but the same effect can be expected when a gas other than hydrogen is used. The same effect can be expected by modifying the surface by ion implantation instead of the plasma treatment. Further, such a second amorphous silicon film 32 may be formed by depositing it on the first amorphous silicon film 31 by CVD or the like.

In the above embodiment, the improvement of the resolution of the liquid crystal display element using amorphous silicon as a semiconductor is described. However, even when crystalline silicon, polycrystalline silicon or the like is used, the same processing as in this embodiment is performed. It is possible to improve the resolution.

Further, in this embodiment, the reading light reflection film 6 is used.
The so-called reflection type liquid crystal display device having the above has been described. The read light reflection film 6 is provided to increase the use efficiency of read light and to separate the photoconductive layer 3 and the read optical system. Therefore, the liquid crystal display element of the present invention is used as a transmissive liquid crystal display element by not forming the reading light reflection film 6 and using light outside the sensitivity of the photoconductive layer as the reading light. It is possible. Even in that case, the display characteristics can be improved by configuring the photoconductive layer 3 as in the present invention.

Further, although the ferroelectric liquid crystal is used as the liquid crystal layer 4, TN liquid crystal, guest-host type liquid crystal, liquid crystal dispersion type polymer or the like may be used.

[0024]

As described above, according to the present invention, the photoconductive layer has two or more semiconductor layers, of which the semiconductor layer closest to the liquid crystal layer is in the middle layer as compared with the other semiconductor layers. Since at least one of the layer having a high density of defect levels and the layer having a low carrier mobility contained in the photoconductive layer, the electric latent layer formed in the photoconductive layer is formed by the action of the closest semiconductor layer. The blurring of the image pattern is suppressed, and the resolution of the liquid crystal display element is improved.

[Brief description of drawings]

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

FIG. 2 is a cross-sectional view of a conventional liquid crystal display element.

[Explanation of symbols]

 1 Glass Substrate 2 Transparent Conductive Film 4 Liquid Crystal Layer 30 Photoconductive Layer 31 First Semiconductor Layer 32 Second Semiconductor Layer

Claims (2)

[Claims] 1. A photo-writing type liquid crystal display device comprising a photoconductive layer and a liquid crystal layer formed between a pair of electrode substrates facing each other, wherein the photoconductive layer has two or more semiconductor layers. The semiconductor layer closest to the liquid crystal layer is at least one of a layer having a higher defect level density and a layer having a smaller carrier mobility than the other semiconductor layers. A liquid crystal display device characterized by the above. 2. The liquid crystal display device according to claim 1, wherein the thickness of the semiconductor layer closest to the liquid crystal layer is one tenth or less of the entire photoconductive layer.