JPS59182678A - Image converting element - Google Patents

Abstract

PURPOSE:To make the performance of an image converting element high by making an extracting electrode of the image converting element as a slit form and arranging the electrode with a gradient in the direction of surface acoustic wave propagation so as to apply sufficient surface acoustic wave into a photoconductive film. CONSTITUTION:Interdigital electrodes 11, 12 are formed on the surface of a piezoelectric base plate 13 being a surface acoustic wave propagating medium. Further, the photoconductive thin film 14 is formed to the base plate 13 via an insulation layer and heat-treated. The slit-shaped extracting electrodes 25, 26 are separated to form a thin film 14 with an electrode gap 27. The slit-shaped extracting electrodes 25, 26 are arranged while being cut off with each other by the electrode gap 27 in an angle theta with the direction of elastic wave propagation. Then, the performance of the image converting element is made high by applying sufficient surface acoustic wave in the inside of the thin film 14.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of the Invention The present invention relates to optical image conversion elements. In particular, it relates to the configuration of a conversion element that processes an optical image using bone waves.

Structure of a conventional example and its problems FIG. 1 shows the structure of a conventional optical image conversion element. In the same figure, 11 and 12 are interdigital electrodes for transmitting elastic waves provided on the piezoelectric substrate 13;
4 indicates a photoconductive thin film provided on the piezoelectric substrate 13, and 15.16 indicates an electrode provided on the photoconductive thin film 14. Next, the operating principle will be briefly explained. First, an optical image is projected onto the photoconductive thin film 14. Extraction electrode 15.16
Apply DC bias to. In such a situation,
When a voltage with a frequency fI * f2 is applied to the interdigital electrodes 11.12 to generate an elastic wave and transmitted to the photoconductive thin film 14, a current with a frequency (r, +fz) is optically generated between the extraction electrodes 15.16. It flows depending on the image.

In such a conventional image conversion element, the electrode 16
．． Due to the wide spacing between the photoconductive thin films 14, the DC resistance of the photoconductive thin film 14 was high, making it difficult to flow a sufficient DC current.

Therefore, the S/N ratio was poor.

Another drawback was that the elastic wave transmitted to the photoconductive thin film 14 by the optical image was significantly attenuated, and the output current at the frequency (fll+f2) was extremely small.

In other words, in this type of image conversion element, since the interaction between carriers in the semiconductor (photoconductor) and elastic waves is used, it is necessary to supply sufficient elastic waves into the semiconductor. In this element structure, it is difficult to supply sufficient elastic waves, and the S/
The N ratio was also very poor.

OBJECTS OF THE INVENTION The object of the present invention is to provide a high-performance image conversion element with low direct current resistance, high S/N ratio of optical images, and little attenuation of elastic waves, which eliminates the drawbacks of conventional elements. On offer.

Structure of the Invention The present invention includes a substrate serving as an elastic wave propagation medium, an electrode for generating elastic waves provided on the substrate, and a photoconductive thin film provided on an elastic wave propagation portion of the substrate. In the image conversion element having a take-out electrode provided on the photoconductive thin film, the take-out electrode is a slit-shaped electrode to lower the direct current resistance of the photoconductive thin film, and the longitudinal direction of the slit is the propagation direction of the elastic wave. By tilting it relative to the surface, a high-performance image conversion element with small propagation loss of elastic waves can be realized. Furthermore, by using the electrode gap in the direction perpendicular to the propagation direction of the elastic wave of the slit-shaped electrode, the S/N of the optical image can be reduced by making it less than the effective wavelength of the elastic wave.
This is to improve the ratio.

That is, by forming the extraction electrode into a slit-shaped electrode with a narrow gap, the direct current resistance can be lowered and a large direct current can be obtained, thereby realizing an improvement in the S/N ratio of the image conversion element.

Furthermore, by arranging the electrode gap at an angle with respect to the propagation direction of the elastic wave, the propagation loss of the elastic wave is reduced and sufficient elastic waves are supplied over the entire effective width of the photoconductive thin film. . That is, in the electrode gear, the elastic waves lose their elastic energy as a result of interacting with photoelectrons induced in the photoconductive thin film by the optical image. Therefore, when the electrode gap coincides with the propagation direction of the elastic wave, over the entire effective width of the photoconductive thin film, that is, over the entire elastic wave propagation direction,
It becomes difficult to supply sufficient elastic waves. Therefore, according to the present invention, by providing the slit-shaped extraction electrode obliquely to the propagation direction of the elastic wave, sufficient elastic waves can be supplied to the entire effective width of the photoconductive film. This makes it possible to realize a high-performance image conversion element.

In other words, in the case of the slit-shaped electrode according to the present invention, there is no interaction in areas other than the electrode gap area even when there is an optical image, so there is sufficient space for the entire photoconductive thin film to overcome the energy attenuation force of the elastic wave. It can supply elastic waves.

DESCRIPTION OF EMBODIMENTS A practical example of an image conversion element according to the present invention will be described with reference to FIG.

As the substrate 13, a LiNbO3 single crystal plate was used.

The plane orientation is 128°R-Y plane, and the propagation direction of the elastic wave is X.
direction. After cleaning the substrate, an insulating layer made of silicon oxide of 0.05 to 0.0
．． A photoconductive thin film 14 containing CdS as a main component was formed by vacuum evaporation. The film thickness (was approximately 0.6 μm. At this time, the inventors confirmed that the presence of the insulating layer significantly improves the adhesive strength of the photoconductive thin film 140 to the substrate 13. After forming the transparent thin film 14, powder method is used to form the 450 to 550
The photoconductive thin film 14 was activated by heat treatment with 'C. The electrodes 11 and 12 for transmitting elastic waves were formed using aluminum by ordinary photolithography technology. Electrode 11. ．． The effective width of 12 was planned to be 2m7. That is, the effective width of the elastic wave is 2 mar. The slit-shaped electrodes 25 and 26 were formed using indium. The length of the photoconductive thin film 14 in the direction of elastic wave propagation was 2' Omm. That is, the effective width of the photoconductive thin film is 20 mm. The electrode gap 27 was 100 μm. In this persimmon example, the inclination 28 between the propagation direction of the elastic wave and the slit-shaped electrode was approximately 5.7 degrees. Further, the length of the electrode gap in the elastic wave propagation direction was about 1.6 mm, which was less than 1.6 mm compared to the effective width of the photoconductive thin film.

FIG. 3 shows the results of measuring the elastic wave characteristics of the image conversion element based on the present invention produced in this way. In the figure, the vertical axis represents the insertion loss when an elastic wave is transmitted from the electrode 11 or 12 and received by the electrode 12 or 11. The horizontal axis shows the illuminance of light. The solid line shows the characteristics of the image conversion element according to the invention. The broken line shows the results of the conventional image conversion element with an electrode gap of 2 m.

From the figure, it can be seen that in the device based on the present invention, the insertion loss is significantly reduced compared to the conventional device, and the characteristics are significantly improved. That is, it is shown that sufficient elastic energy can be supplied into the photoconductive film in an element in which a slit-shaped electrode is formed based on the present invention.

FIG. 4 shows the output voltage characteristics according to the optical image obtained by the optical image conversion element based on the present invention. The horizontal axis is the frequency difference (Δf: f2-f) of the high-frequency power transmitted from the in-digital power % 11.12.

shows. Here, f is 37 mm2, and f2 is 374
It was varied within a range of 1.5MH2. The vertical axis indicates the output voltage of the frequency component ' (f, +f2) induced in the extraction@poles 21 and 22. In addition, the optical image (
It was in the shape of a slit with a width of 3.7 m. The illuminance of the light is 100
It was about ~3001x. Even with illuminance of 2o1x or more,
I got enough output. As is clear from the above, the band was 4.5 MHz, and therefore the minimum wavelength of the effective elastic wave was approximately 0.9 mm from fλ-υ, assuming a propagation speed of v-3980 m/sec.

It is considered that a sufficient S/N ratio can be obtained by making the electrode gap shorter than the minimum wavelength of the effective elastic wave.

The reason for this is that if the gap between the extraction electrodes is shorter than the wavelength of the effective elastic wave, the resistance change that is spatially modulated to the wavelength of the elastic wave by the optical image and the elastic wave in the photoconductive thin film can be efficiently detected. It is thought that That is,
When the electrode gap is wider than the wavelength of the elastic wave, it is thought that the above resistance change becomes difficult to detect due to the diffusion effect within the thin film.

Furthermore, the inventors have also discovered that a highly sensitive image conversion element can be realized by forming a photoconductive film using a CdS-based film. Furthermore, Cd'e in CaS-based thin films
We also found that the wavelength dependence of photosensitivity can be controlled by mixing them.

FIG. 5 shows the relationship between the composition of the photoconductive thin film and the wavelength at which the photosensitivity reaches its maximum value. The horizontal axis represents the film Cd51-xCdSe.
The composition when denoted by x (denotes xi, i.e., X =
0.4, the maximum sensitivity is 550 nm. Therefore, depending on the composition ratio, the sensitivity of the photoconductive film and furthermore the optical image conversion element can be made to match the luminous sensitivity.

It has also been found that the same elastic wave propagation characteristics and optical image conversion characteristics as described above can be obtained even if the material is provided at the interface between the piezoelectric substrate and the piezoelectric base. It was also confirmed that the above characteristics would not deteriorate even if the extraction electrode was made of metals such as indium, cadmium, aluminum, and chromium. In this case, the difference is that if the acoustic wave transmission quadrupole and the extraction electrode are made of the same material, it is clear that the process of manufacturing the element will be greatly simplified, which is a major manufacturing advantage. becomes.

Effects of the Invention As described above, the present invention provides an image conversion element having an elastic wave propagation medium and a photoconductive thin film provided on the medium, in which the take-out electrode is slit-shaped, and with respect to the elastic wave propagation direction, By arranging them at an angle, sufficient elastic waves can be supplied into the photoconductive film, making it possible to realize a high-performance image conversion element.

[Brief explanation of the drawing]

FIG. 1 is a perspective view of a conventional optical image conversion element, FIG. 3 is a diagram showing the elastic wave propagation characteristics of the optical image conversion element, and FIG. 4 is a diagram showing the output waveform of the optical image conversion element based on the present invention. FIG. 5 is a diagram showing the wavelength dependence of photosensitivity on composition. 11.12...Interdigital electrode, 13...
... Piezoelectric substrate, 14... Photoconductive thin film, 25° 26 ... Extraction electrode, 27... Electrode gap,
28 ・Angle between the electrode gap and the propagation direction of elastic waves ・Name of agent: Patent attorney Toshio Nakao and 1 other person No. 1
Figure G Figure 2

Claims (3)

[Claims] (1) A substrate serving as an elastic wave propagation medium, an elastic wave generation electrode provided on the substrate, a photoconductive thin film provided on the elastic wave propagation portion of the substrate, and the photoconductive thin film. What is claimed is: 1. An image conversion element having a take-out electrode provided therein, wherein the take-out electrode is a slit-shaped electrode, and the longitudinal direction of the sult is inclined with respect to the propagation direction of an elastic wave. (2) The image according to claim 1, characterized in that the slit-shaped electrode is provided on the diagonal of a rectangle whose two sides are the effective width of the elastic wave and the effective width of the photoconductive thin film. conversion element. (3) The image conversion element according to claim 1, wherein the dimension of the electrode gap in the direction perpendicular to the elastic wave propagation direction of the slit-shaped electrode is equal to or less than the effective wavelength of the elastic wave used.