JPS5845690B2 - Optical image conversion element - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an optical spatial filter element and an optical image conversion element using a single crystal having a photoconductive effect and an electro-optic effect.

Conventionally, B i having electro-optical properties, piezoelectric properties, and photoconductive properties has been used as an optical image input device in optical information technology.
An element that converts an incoherent light (incoherent light) image into a coffeelet light (coherent light) image using a bismuth sirenite group single crystal such as 12S 102o and B i 1□Gem2o has been proposed. J.

Applied Opt by Fe1nleibe et al.
ics, Vol.

11, A12. Dec, 1972. The device disclosed in P., 2752 has a structure as shown in FIG.

In Figure 1, 1 is bismuth silicon oxide (B1
12S1020) Single crystal, 2 and 2' are transparent insulating films, 3.
3' is a transparent electrode.

Here, the insulating layers 2, 2' have been created by depositing thin films of organic or inorganic materials onto the single crystal 1.

For example, according to the above-mentioned J. Feinleibe, there is an example in which a method is used to form a film of paraxylene polymer polyparaxylene as the organic material.

Furthermore, there is an example in which a MgF2 vapor deposited film is used as the inorganic material.

As the transparent electrodes 3 and 3', single metals such as Au and Pt or vapor deposited films such as In 203 y S n 02 are used, and I, which has particularly excellent transparency and electrical properties, is used.
A mixed composition of n203 and Sn02 is used.

In the optical image conversion element having such a structure, the electrodes 3,
A constant voltage is applied between 3' and an image or character pattern is projected onto one side in that state.

Utilizing the wavelength dependence of the photoconductive effect of bismuth silicon oxide scientific crystal, we have developed a wavelength 50 that has a particularly remarkable photoconductive effect.
Light with a short wavelength of 0 nm or less is used as the image writing light, but in reality, white light from a xenon or tungsten lamp is sufficient.

In the area irradiated with light, the photogenerated electron-hole pairs move toward both poles within the crystal and are trapped at the interface with the insulating film 2, so the potential gradient within the crystal changes to the area not irradiated with light. Comparatively small.

In such a state, the next step is to use a He-Ne laser beam, for example, with a wavelength of 633, as the readout light.
By uniformly irradiating the element surface with linearly polarized light having a polarization direction at an angle of 45 degrees with respect to the crystal axis, the coherent light of long wavelength light that does not contribute to the photoconduction effect of μm or more can be used to increase the potential after transmission. The retardation δ due to the electro-optic effect changes depending on the magnitude of the gradient, and an intensity-modulated coherent light image can be obtained by passing the light through an analyzer, for example.

δ is as follows. 2π δ= 7 ・n□” +, r4s ・V (r ad
) (1) (where λ is the optical wavelength, n□ is the refractive index, and r
41 represents an electro-optic coefficient, and ■ represents a voltage.

) Bismuth silicon oxide is a body-centered cubic crystal and has point group symmetry23, so for an electric field in a direction perpendicular to the crystal plane (ioo) and a light beam in the incident direction, retardation occurs as shown in equation (1). δ is independent of crystal thickness.

Now, an important characteristic required of the optical image conversion element having the above structure is image resolution.

This resolution improves in almost inverse proportion to the thickness of bismuth silicon oxide 1, which is the photoconductive layer.

As the thickness increases, the region where optical carriers are generated by the projected light expands in the depth direction, and these carriers have a concentration distribution that corresponds to the intensity of the projected light, resulting in carrier diffusion due to spatial concentration differences, and the projection surface resolution within the range is reduced.

In other words, reducing the thickness of the photoconductive layer is an important factor for improving resolution, but the photoconductive layer in the configuration shown in Figure 1 is thinner than the single crystal obtained by the conventional pulling method from melt. It is a flat plate with a (100) plane that has been cut and polished, and with current cutting and polishing technology, it is 10~30+1! ? The minimum thickness that can be used to finish a bismuth sirenite group single crystal such as +1 [1 bismuth silicon oxide or the like to a flatness of λ/10 and parallelism of 1σ without disturbing optical information is about 150μ.

It is impossible to reduce the crystalline temperature to less than that because the mechanical strength of the single crystal is insufficient, resulting in cracking and chipping.

The present invention achieves the following by growing an epitaxial layer with remarkable photoconductive properties on a substrate with extremely poor photoconductive properties.
Obtaining a photoconductive layer of arbitrary thickness from 10μ to 100μ,
The present invention provides a high-resolution optical image conversion device that could not be obtained with a device using a photoconductive layer formed by conventional mechanical processing methods.

According to the present invention, a thin film of highly insulating organic material is required on only one side, the image storage/erasing time can be selected depending on the polarity of the voltage applied to the element, and an antireflection film is formed on the epitaxial substrate side. Therefore, it has many advantages, such as being able to eliminate multiple interference fringes that occur during readout using coherent light, which has been a problem in conventional elements.

FIG. 2 shows the cross-sectional structure of the optical image conversion element according to the present invention.

4 is a substrate single crystal having a significantly small photoconductive effect, and 5 is a layer epitaxially grown on 4 and having a significant photoconductive property.

The most important factor in epitaxial growth is to minimize the mismatch between the lattice constants of the substrate and the grown layer.

A case where B 112 S 1020 or B i1□Ge O20 is used as the epitaxial layer will be described.

In this case, B 112 S! 020 or B t
It is well known that when producing 12G e O2° crystals by pulling, the photoconductive properties can be significantly reduced by adding elements belonging to Group ⅠA, Group nA, or transition elements of the periodic table. It is a fact that (S
, L., Hou et al., J., Appl., Phys., Vol.
44 (1973) 2652).

Figure 3 shows one example, and Bt1□SiO2
Bl 1 in which Al was added in an amount of several moles or more relative to Si.
2 S 1020: (Al) has a light wavelength of 450 nm
It exhibits almost no photoconductive properties until

Furthermore, since the addition of impurities in this case does not change the crystal structure and lattice constant, it can be used as the substrate single crystal of the present invention.

Exactly the same conditions hold for Bi1□GeO20, which is possible as a single crystal substrate.

Next, the epitaxial growth method will be explained.

FIG. 4 is an explanatory diagram of the epitaxial growth apparatus.

8. `` is a heater that keeps the reaction melt at the required temperature.

9 is a support stand for the platinum crucible, which is made of a heat insulating material such as alumina porcelain.

10 holds the reaction melt, and a platinum crucible is used to melt the bismuth sirenite group.

11 is a reaction melt forming an epitaxial layer.

Reference numeral 12 denotes a pulling shaft, which rotates and moves up and down the substrate single crystal 14 held by a substrate holder 13 made of platinum.

Here, the substrate holder 13 has a structure that allows epitaxial growth to occur only on one side of the substrate.

Furthermore, by rotating the substrate, the epitaxial growth layer becomes more uniform within the plane.

The problem here is that the substrate single crystal B112Si020 doped with impurities and Bi1□5 without doped impurities.
There is almost no difference in the melting points of 102g, but this point is solved as follows.

Figure 5 is the phase diagram of the Bi2O3-8in system, and B t
12 S t 02° is Bt 203: S 102
= 6: Congruendo at a composition of 1 molar ratio, and melts at about 910°C.

By selecting one of these compositions in the range of 0.5 to 27 moles of SiO2 and supercooling the melt, epitaxial growth is possible without melting the substrate single crystal.

As can be seen from the phase diagram, Bi1□SiO□ is formed by supercooling within 50°C from the melting point of the substrate.

It is possible to crystallize the phase, and supercooling at 50° C. or higher is inappropriate because phase ratio substances other than bcc will precipitate.

The same is true for Bil□GeO20, and Fig. 6 shows the Bt203-GeO2 system phase diagram, but in this case, the possible composition range for epitaxial growth is 0.5 to 32 moles of Ge02, and during epitaxial growth. The degree of supercooling is within 70°C.

By the method described above, the substrate 4 and epitaxial layer 5 shown in FIG. 2 were formed.

Next, a photoconductive layer 5 formed by epitaxial growth
An insulating film 6 is coated thereon.

The purpose of this insulating layer 6 is to trap electrons, which are majority carriers, among the carriers generated in the photoconductive layer by the projection of the optical image, at the interface A with the photoconductive layer 5, thereby creating an electric field corresponding to the brightness of the optical image. This is to produce a difference in the photoconductive layer 5 having an electro-optical effect.3 As the insulating film 6, a film made of a highly insulating organic material such as the already mentioned xylene film or epoxy resin is used.

Next, as shown in FIG. 2, transparent electrodes 7.1' are deposited on both end faces by a method such as vapor deposition or sputtering.

The material is the same as for conventional elements, but
Preferably, a semiconductor thin film layer such as In203tSnO2 has excellent electrical conductivity and mechanical strength.

The optical image conversion element obtained as described above performs the following operations.

Figure 7a shows the DC voltage ■. is applied, and at this time ■.

is 4,5.6 Voltage distribution determined by dielectric constant ε and thickness d of each layer V4. V.

■It becomes 6.

At this time ■. The insulating film side is made to have ten polarity.

When irradiated with light of 500 to 450 nm in this state, carriers generated in the photoconductive layer reach interface A due to the applied voltage and are trapped there, and as a result, the internal electric field of the photoconductive layer decreases significantly, resulting in the same As shown in the figure ■4',
■5′.

■The voltage distribution becomes 6', and the retardation given to the 633 nm readout light is the voltage V4 before irradiation.
It is determined by the voltage v4'+v,' which is significantly smaller than +V.

Here, the substrate layer does not have a photoconductive effect, but has an electro-optic effect, so the adhesive tarpion as a whole is 4
It is determined by +■5 (or ■4'+■,').

However, since the substrate layer does not have a photoconductive effect, the voltage change thereon is very small, and the difference in retardation is mostly determined by the voltage change in the photoconductive layer.

In other words, it performs the same operation as a conventional optical image conversion element.

Here, the reason why the insulating film side is set to + polarity is to trap electrons, which are majority carriers, at the interface A of the insulating film with high resistance (1015 to 1016g-c10l6) for the time required for reading. When the sides are made depolar, electrons are trapped at the interface B, but since the insulation resistance of the substrate is as low as 1012 to 10138-CrrL, the written optical image disappears in a short time.

In other words, by reversing the polarity, one hour of optical image memory can be selected, which is also one of the features of the present invention.

In the above operation, one of the features of the present invention, which is the improvement in resolution, is achieved by using a 150μ thick B
i1□SiO□.

The resolution of the optical image conversion element using single crystal is 2001pA
1L, whereas for a device with a 50μ thick photoconductive layer grown under optimal conditions and grown by the method of the present invention, it was 400L.
A high resolution of 11p/mπ can be obtained.

Furthermore, regarding the possibility of adding an antireflection layer, which is one of the features of the present invention, in the structure shown in FIG.
For example, when In2O3 is used as the layer 5, one side of the substrate 4 is coated with a multilayer film such as MgF2.
, 4, 16, 7' (for example, He
-The reflectance for the 633 nm light of the -Ne laser) can be suppressed to 0.1% or less as a whole, and the reflectance of Bi1□5iO21), which occurs in the conventional configuration shown in Fig. The occurrence of interference fringes due to multiple reflections can be reduced to an extent that does not pose a problem in use.

Here, the features of the present invention will be explained in detail by making a detailed comparison with some conventionally proposed optical image conversion elements.

Fig. 9a is the same as Fig. 1, but since the photoconductive layer 1 cannot be made into a thin film, the resolution is limited and two insulating layers 2 and 2' are required. Because highly insulating organic materials are used, coating with an antireflection film, which requires heating to about 300° C., is difficult.

The structure shown in the figure is an asymmetric structure in which a highly insulating organic material is adhered to only one side of the polished photoconductive layer 1, as in a, but as in a, it is difficult to make the film thin, and it is difficult to achieve high resolution. Elements cannot.

Furthermore, in this type of element, since the transparent electrode 3' is attached directly to one side of the photoconductive layer, carriers that reach the electrode depending on the polarity of the applied electric field flow to the electrode without being trapped.

In other words, one of the carriers generated by photoconduction hardly contributes to the effect of changing the electric field of the photoconductive layer, causing a decrease in the sensitivity of the device.

In this respect, although the device of the present invention is asymmetrical, by having insulating layers on both sides, carriers generated by the photoconductive effect can be effectively utilized.

In the configuration shown in FIG. 4C, an epitaxial layer 5゜5' that does not have a photoconductive effect is grown on a substrate 4 that has photoconductive properties to serve as an insulating layer, or the substrate 4 is made of a material that does not have a photoconductive effect. The epitaxial layer 5 with photoconductivity is formed using
5' to grow.

In an element with this configuration, since both the substrate and the epitaxial layer have a high melting point (approximately 900 to 930°C), the electrode T, 7'
It is possible to coat the antireflection layer before depositing the epitaxial layer 5. When the photoconductive layer is used as the photoconductive layer, resolution can be improved by making the film thinner.

However, the resistivity of the insulating layer without photoconductive effect in this configuration is IQ, 12-1013 Ω-;
Highly insulating organic material (resistivity 1015~1016Q-CIr
Compared to the case where L) is used, the image storage time is shorter, and its use is as a high-speed writing and reading element.

As described above, the conventional elements shown in FIG. 9 a = c each have their own advantages and disadvantages, so their applications are naturally limited, but the elements of the present invention can be used in a wider range of applications by eliminating the disadvantages of the conventional elements. Can be used.

In other words, its characteristics are: (1) Epitaxial growth of the photoconductive layer enables high-resolution devices.

■ As the insulating layer is an asymmetric type with a substrate layer of 1012 to 1013 Ω- and a highly insulating organic material of 1015 to 10" J7- ) or as a short-time (within several tens of seconds) image storage element.

(2) By forming an anti-reflection layer on the other surface of the substrate after epitaxial growth, readout using coherent light without multiple interference is possible.

■ By forming the anti-reflection layer described in (■), the electrode on the side of the organic insulating film has good transparency and low reflection, but the reflectance is high even without attaching In2O3, etc., which tend to deteriorate the organic insulating film due to heating during vapor deposition. Materials such as Au and Pt, which can be deposited at near room temperature, can be used.

(2) By making the photoconductive layer thinner, absorption of readout light can be reduced and image quality can be improved.

Next, examples of the present invention will be shown.

As a substrate single crystal, B l 20s' S t 0
2 = 6: 1 mole of raw material with 6 atoms of Al added to Si is melted by high frequency heating to <11
A wafer with a (100) plane was cut from a single crystal pulled by the Czochralski method using a seed crystal in the 1> direction, and this was sliced into 2011t11LO, 150μ thick pieces.
It was optically polished (flatness λ/10, parallelism 10'').

Next, raw materials having a molar ratio of Bi2O3:5i02=96:4 were heated to 890°C and melted, then cooled to 870°C, the above substrate was immersed, and the substrate was pulled out after about 15 minutes.

Thereafter, the furnace was cooled at 50° C./hour and the substrate was taken out after reaching room temperature.

As a result, it was confirmed by cross-sectional analysis using an X-ray microanalyzer that a Ri1□SiO□0 single crystal with a thickness of 50 μm had grown on the substrate.

Next, on the epitaxial layer, a 5 μm thick layer of paraxylene was deposited to form an insulating layer.

Finally, Au was deposited to a thickness of about 200λ as a transparent electrode.

Applying 1.500V to this element, the light from a 500WXe lamp passed through an interference filter that passes 500 nm.
Writing was performed for 0 seconds, and reading was performed using 633 nm light from a 5 mW He--Ne laser.

As a result of measuring the resolution under the same conditions as a conventional element with a photoconductive layer of 150 μm thickness (Fig. 1), it was found that the resolution of the conventional element was 200 μm.
lp/mm, the element according to the invention has 400 lp
It was confirmed that it has a high resolution of /mx.

Regarding the image storage time, when the parylene side was set to 10 polarity, there was no noticeable deterioration in the image quality for about 20 minutes with continuous readout using the He-Ne laser beam.

When the polarity was reversed, a complete image was accumulated after 2 seconds by taking a photograph of the readout image, but it was confirmed that after 10 seconds, most of the original image shape had been lost.

The above is a device having the structure shown in FIG. 2, but after epitaxial growth was performed under the same conditions, a device having the structure shown in FIG. 8 with an antireflection layer 16 was created.

The electrode on the antireflection layer side is In2O3, and is made of an organic insulator (
The electrode on the Parylene side was made of An (200 mm thick).

In terms of resolution, it exhibited the same performance as the configuration shown in FIG. 2, and no interference fringes were found even when read out with a He-Ne laser, confirming an improvement in image quality.

[Brief explanation of drawings]

Figure 1 is a cross-sectional view showing the structure of a conventional optical image conversion element, Figure 2 is a cross-sectional view showing the structure of an optical image conversion element according to the present invention, and Figure 3 is a cross-sectional view showing the structure of a conventional optical image conversion element. Figure 4 is an explanatory diagram of an apparatus for forming an epitaxial growth layer on a single crystal substrate of an optical image conversion element according to the present invention, and Figure 5 is a diagram showing changes in resistivity versus wavelength of input light of a Bi2O3- The phase diagram of 8i02, Fig. 6 is the phase diagram of 'Bi2'03'GeO2, Fig. 7a,
b is a diagram showing the potential gradient within the device when no light is applied to the optical image conversion element of the present invention and when light is applied to the optical image conversion element, and Figure 8 is a diagram showing the optical image conversion element of the present invention with an antireflection layer. FIG. 9a is a sectional view of an embodiment of the present invention. b and c show various transformations of the conventional one shown in FIG. 1...Bismuth silicon oxide, 2゜1...Insulating film, 3,3'...Transparent electrode, 4...Single crystal substrate, 5 ...Single crystal thin film, 6...Insulating film, 7°7'...Transparent electrode, 8. Drama... Heater 9... Crucible support stand, 10... Platinum crucible, 11...
...Reaction melt, 12... Pulling shaft, 13...
...Substrate holder 14...Substrate single crystal, 15.
...DC power supply, 16...Antireflection layer, A
......Interface between photoconductive layer and insulating film, B...
Interface between photoconductive layer and single crystal substrate. For sex

Claims (1)

[Claims] 1. A thin film layer of the bismuth sirenite group, which has a remarkable photoconductive effect, is epitaxially grown on a single crystal substrate of the bismuth sirenite group, which has an extremely small photoconductive effect, and an organic insulating layer is further grown on this epitaxially grown layer. An optical image conversion element characterized by being coated with a thin film. 2. As an epitaxial growth layer with a remarkable photoconductive effect, single crystals of bismuth sirenite group are used as bismuth germanium oxide (Bi1□Ge02g) or bismuth.
The optical image conversion element according to claim 1, which is a single crystal of silicon oxide (B 11□S 1020 ). 3. As a substrate for a bismuth sirenite group single crystal with extremely small photoconductive effect, a single crystal obtained by adding an element belonging to group 1A, group 11A or transition metal element in the periodic table to a bismuth sirenite group single crystal. The optical image conversion element according to claim 1, characterized in that the optical image conversion element uses: