JPS5919915B2 - Liquid phase epitaxial growth method for electro-optic crystals - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for producing a thin film epitaxial single crystal of an electro-optic crystal such as a bismuth sirenite group single crystal.

Bismuth sirenite group single crystal (J bismuth oxide (Bi2
It is a compound of O3) and other oxides, and has remarkable electro-optical properties, piezoelectricity, and photoconductivity, so it is used as an optical image input device, spatial filtering device, optical memory, and optical communication in optical information technology. It is an interesting material as a material for optical waveguide elements and optical modulation elements in technology.

For any of the above applications, it would be more effective if a thin film-like single crystal of bismuth sirenite group single crystal could be obtained.

For example, 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 the above-mentioned single crystal has been proposed; Appl by Feinleib et al.
ledOpticsVOl. ll, Waterfall 12, Dec. 1
972, P. It was announced in 2752. The device announced there has a configuration as shown in Figure 1.

In Fig. 1, 1 is a bismuth silicon oxide single crystal, 2 (J transparent insulating film, 3, 3' are transparent electrodes. A constant voltage is applied between the electrodes 3, 3', and the image is imaged in that state. Alternatively, a character pattern can be projected on one side.Using the wavelength dependence of the photoconductive effect of bismuth silicon oxide single crystal, short wavelength light of 500 nm or less, which has a particularly remarkable photoconductive effect, can be used as light for writing images. I use it, but
In fact, white light from a xenon or tungsten lamp is sufficient.In the area irradiated with light, photogenerated electrons and hole pairs move toward both poles within the crystal, and at the interface with the insulating film 2. Because the crystal is trapped, the potential gradient inside the crystal is smaller than that of the part that is not irradiated with light.In this state, the next step is to use a He-Ne laser beam as a readout light, for example, with a wavelength of 633 nm or more due to the photoconductive effect. By uniformly irradiating the device surface with linearly polarized light whose polarization direction is at an angle of 45 degrees to the crystal axis, it is possible to respond to the magnitude of the potential gradient after transmission. The retardation δ due to the electro-optic effect changes, and, for example, by passing the light through an analyzer, an intensity-modulated coherent light image is obtained. δ is as follows.

(In the formula, λ (1 optical wavelength, NO represents the refractive index, R4l represents the electro-optic coefficient, and V represents the voltage.

) Bismuth silicon oxide is a body-centered cubic crystal and has point group symmetry 23, so it returns as shown in equation }1(1) for an electric field perpendicular to the crystal plane (100) and a light beam in the incident direction. The dimension δ is independent of the crystal thickness.

In this case, when the thickness of the bismuth silicon oxide single crystal is reduced, the image resolution improves as a result of the thinning itself as an optical image conversion element. Effects such as obtaining a sharp image can be achieved.

As described above, thin film single crystallization of the bismuth sirenite family (
This is an important issue, and attempts have been made to grow thin film single crystals using various methods.

For example, V. J. Silvestri et al., Journa
lCrystalGrOwth, Volume 2O (1973) P
．． According to No. 165-168, 0 or T. No. 1, which attempts to grow substrate materials such as sapphire, spinel or quartz by vapor phase reaction. Mitsuya et al., Journa
lElec-TrOchem. SOc. Japl976
．． P. According to 94, B by high frequency sputtering.
We are investigating methods for growing i,2GeO2O thin film single crystals on glass substrates and silicon substrates. However, these studies have reported that good single crystal growth could not be achieved on any of the substrate crystals. The present inventors (as a result of repeated research on a method for growing a J bismuth sirenite group single crystal to obtain a thin film single crystal) found that it is sufficient to epitaxially grow a bismuth sirenite group single crystal on a specific substrate material. This discovery led to the present invention.

The present invention provides an optical image input device which enables thin film epitaxial growth by selecting an appropriate single crystal substrate when epitaxially growing a bismuth sirenite group single crystal, and which can obtain clear images with good image resolution. The present invention aims to provide a method for manufacturing electro-optical elements such as spatial filtering elements, optical memory optical waveguide elements, and optical conversion elements. Conditions required for a single crystal substrate during epitaxial growth include having a crystal structure similar to that of the target thin film single crystal and having a similar lattice constant. The method of the present invention provides a single crystal substrate made of bismuth silicon oxide or bismuth germanium oxide, or bismuth silicon oxide or bismuth germanium oxide doped with a material selected from the group consisting of aluminum, gallium, calcium, barium and iron. Select and combine the composition of the liquid phase epitaxial layer,
It is characterized by liquid phase epitaxial growth.

According to the present invention, after a thin layer is grown on a single crystal substrate by liquid phase epitaxial growth, another thin layer can be further grown thereon by liquid phase epitaxial growth. In the present invention, bismuth silicon oxide (or bismuth germanium oxide) refers to Bl2O3 and SiO2 (or GeO2).
This does not mean only compounds having a composition with a molar ratio of 6:1 to 2), but also includes compounds having other molar ratios.

Furthermore, in the present invention, when an element selected from the group consisting of aluminum, gallium, calcium, barium, and iron is added to a bismuth silicon oxide single crystal or a bismuth germanium oxide single crystal, it does not affect the crystal structure or lattice constant. without reducing its photoconductive properties. The present invention as described above was obtained through consideration of the photoconductive properties of bismuth sirenite group single crystals, as shown below.

Compounds of bismuth oxide (Bl2O3) and other oxides (RO) usually have a body-centered cubic crystal structure,
For example, bismuth silicon oxide, which is a typical material of the bissilenite group, is made up of Bi2O3 and silicon dioxide (SiO
2) in a 6:1 molar ratio, resulting in a semiconductor with a forbidden band interval of 3.25e.

Photoelectron pairs are generated inside the single crystal by the incidence of photons with energy greater than the forbidden band spacing, but the holes are captured by the inherent hole capture center, and the capture center in the captured state Since it becomes a negatively charged body, recombination of free electrons and holes is prevented, resulting in a remarkable photoconductive effect. Such a trapping center is called a sensitizing center in a photoconductor, and in particular, in the case of a bismuth sirenite group single crystal, a composite vacancy of element R vacancies (Va-cancy) and oxygen atoms is sensitizing. It is estimated that the center is C. It is known that the photoconductive properties of the bismuth sirenite group are greatly influenced by impurity additives as well as by the inherent sensitizing centers. For example, aluminum, gallium, calcium added to bismuth silicon oxide single crystal,
Photoconductivity properties may be reduced by barium, iron, etc. That is, the added Calo element acts complementary to the sensitizing center,
It is estimated that this results in a reduction in the photoconductive effect. When applied to optical image conversion devices, a material without photoconductive properties is required as a substrate material for epitaxial growth.

One of the present inventions is that aluminum, gallium, calcium,
A single crystal made of a material whose photoconductive properties are suppressed by adding barium or iron is used as a substrate for epitaxial growth, and a single crystal thin film having the same composition as the substrate without the above elements is epitaxially grown on it. This is a manufacturing method that

The present invention also enables a manufacturing method in which the compositions of the substrate and epitaxial thin layer described above are reversed. Next, we will discuss the composition of the oxide, which is an important point when manufacturing by epitaxial growth.

Bismuth silicon oxide (Bil.SiO2O) (
Bi2O3- related to the compound phase of (hereinafter referred to as BSO)
The SiO2 phase diagram is shown in FIG. When a BSO single crystal is used as the epitaxial growth substrate, in the liquid phase growth method, the substrate Dj comes into direct contact with the reaction melt, so the temperature of the reaction melt must be lower than the melting point of BSO, 910°C.

Since the melting point of a reaction melt containing additive elements with the same composition is also 910°C, it is generally considered that a reaction melt having the same composition as the substrate cannot be used.
In the phase diagram shown in the figure, the reaction melt composition is 0.
If it is limited to 5 mol % to 12 mol % or 18 mol % to 27 mol %, it is the melting point at which the substrate does not melt, and the composition is within the above range. It can be seen that it is possible to crystallize the phase. Furthermore, even if the SiO2 content of the reaction melt is in the range of 12 to 18 mol %, epitaxial growth can be achieved by keeping the melt in a supercooled state. The present invention takes advantage of these facts to obtain the desired epitaxial growth for the first time.In addition, with supercooling of 5'C or more,
' Other phase compounds such as cubic structures other than Bcc will precipitate. Next, bismuth germanium oxide (B
ll2GeO2O) (hereinafter referred to as BGO) will be described.

The phase diagram of BGO is shown in FIG. This is BSO
This is the same phase diagram as in the case of , but the melting points are different. B.G.O.
In order to deposit an epitaxial growth layer containing additive elements using a single crystal as a substrate, the composition of the melt containing additive elements was set to 0,
Even when the value is within the range of 5 mol% to 32 mol%, the temperature is 70°C from the melting point of each composition, as in the case of BSO above.
It is deposited on a substrate immersed in the melt in a supercooled state of less than or equal to 100 mL.

Note that with supercooling above 70°C, other cubic phases precipitate (C).
Put it away. Next, examples of the present invention will be described.

Embodiment 1 FIG. 4 is a sectional view of a liquid phase epitaxial growth apparatus used in an embodiment of the present invention.

First, 10m7! Lφ, thickness 0.57 nm 0) The front and back surfaces of a (100) plane wafer of a BSO single crystal substrate 4 are polished, and it is horizontally attached to a platinum holder 5 and fixed to a C lifting shaft 6. The composition of the reaction melt 7 is Bl2O3: 96 mol SiO2
: 4 mol %, and 100 p. of aluminum A was added to this.
p. m. It has been added. Platinum crucible 8 installed in the center of the furnace
is 50mmφ, length 50m7! High-purity Bi2O3 and high-purity SiO2 having the above-mentioned composition ratio were added thereto, and the temperature was raised to 89°C with a heater 9 to melt them together with the additives. 10 is a crucible support stand.

Next, the temperature of the reaction melt 7 was maintained at 870° C., and the substrate 4 was immersed therein, and after about 5 minutes, it was pulled up and taken out from the melt 7. During this time, the board 4 was heated at 20r. p. The rotation of m was maintained. A 1BS0 single crystal layer was grown on the surface of the substrate 4 to a thickness of about 10 μm.
As a result of measuring a part of the epitaxially grown layer by mass spectrometry, it was found that about 50p. p. An additional element of m, aluminum, was detected.

Only bismuth and silicon were detected. Instead of the aluminum added to the reaction melt 7 in the above example, 100p of barium was added. p. m. Using reaction melt 7 to which 87
The temperature was maintained at 0°C, and the same BSO single crystal substrate 4 as in the above example was immersed in this in the same way, and when it was pulled out after about 5 minutes, an epitaxial growth layer with a thickness of about 10 μm was obtained on the surface of the substrate 4. Ta.

It was confirmed that the photoconductivity of the epitaxially grown layer doped with aluminum or barium was reduced to about 1/1000 for visible light, particularly for light in the wavelength range of blue light.
Similarly to the above example, 500 p of calcium was used as the reaction melt 7.
．． p. A BSO single crystal substrate was similarly immersed in the melt containing 500 m and maintained at 87°C, and pulled out after about 5 minutes, resulting in an epitaxial growth layer with a thickness of about 10 μm on the surface of the substrate 4. It was done.

Similarly to the above example, 1000p of iron was used as the reaction melt 7. p.
m. Using a melt containing added
A BSO single crystal substrate was immersed in the same manner. When the substrate 4 was pulled up after about 5 minutes, an epitaxially grown layer with a thickness of about 10 μm was obtained on the surface of the substrate 4.

It was found that the photoconductivity of the epitaxially grown layer doped with iron (in the case of the iron-added epitaxial growth layer) was reduced to about 1/1000 in almost the entire wavelength range of visible light. Since the flux used in other homogeneous epitaxial growths is not used, there is no contamination of the grown layer by flux, making it an ideal method for producing a highly pure grown layer.

Example 2 10m7 by the same method as Example 1! Lφ. Thickness 0.
The front and back surfaces of a (100) plane wafer of a 5 mm BGO single crystal substrate 4 were polished, attached to a platinum holder 5, and fixed to a pulling shaft 6.

The composition of the reaction melt 7 is 396 mol% of Bl2O, SiO24
In mole %, 150 p. of gallium is added to this. p. m was added.

The temperature of the reaction melt 7 was raised to 900°C, and after about 1 hour 8
It was kept at 80°C. After that, the substrate 4 was heated for 20r. p. It was immersed in the melt 7 while being rotated by m, and was pulled out of the melt 7 after being left for about 5 minutes. On the surface of the board No. 4 (approximately 20 cm thick)
It was confirmed by an X-ray microanalyzer that a BGO single crystal layer of μ was deposited. Epitaxially grown layers doped with gallium, like those doped with aluminum or barium, are sensitive to visible light, particularly in the blue wavelengths.
It was confirmed that the photoconductivity sensitivity of 1 was reduced to about 1/1000. In both Examples 1 and 2, the photoconductive properties of the epitaxially grown layer are significantly different from those of the substrate layer, and in the case of the one obtained in Example 1, at a wavelength λ = 600 nm when the irradiation light intensity is 10 μW, 1010Ω in the growth layer?
, 5X101 long Ω・CfL in the substrate layer, wavelength λ=480
In nm, 7X109Ω・α in the growth layer and 1′2×108Ω・? in the substrate layer. As shown, the wavelength dependence of the photoconductive properties of the grown layer almost disappeared, which was clearly different from the large wavelength dependence of the substrate layer.

The same was true in Example 2. As described above, it has been found that the present invention can provide an epitaxial growth layer suitable for, for example, a photoimage conversion device.

In Examples 1 and 2, aluminum, gallium (hereinafter referred to as Group 1B of the periodic table), calcium, barium (hereinafter referred to as Group A of the periodic table), or iron (transition element) was added to the reaction melt. Although we have described the case where each is added alone, one or more elements selected from group 1B elements, group A elements, and transition elements of the periodic table including other elements are added. It is thought that similar effects can be obtained.

Next, Examples 1 and 2 and combinations in which the substrate and the epitaxial growth layer are reversed will be described. Example 3 A useful image conversion element can be realized by using a BSO or BGO single crystal containing additive elements as a substrate and growing a single crystal layer having the same composition as the substrate without additive elements thereon.

That is, the method of the present invention can easily realize a thin active layer that has both photoconductive properties and electro-optic properties. In this case, thinning the film brings about improvements in device characteristics such as improvement in image accuracy (resolution) and reduction in light loss during readout. In these cases, the single crystal substrate used is a single crystal substrate grown in advance from a melt mixed with additive elements by the Czyochralski-growth method, and sliced from the grown single crystal ingot.

For this liquid phase epitaxial growth on the substrate, a high purity material composition having the same composition of the reaction melt 7 as shown in Example 1 or 2 and containing no additive elements is used. The epitaxial growth conditions are the same as in Example 1 or 2. Example 4 Next, by combining the methods of Example 3 and 1 or 2 of the present invention, an image conversion element without an insulating film consisting of three layers as shown in FIG. 5 was constructed. can do.

In Figure 5, BSO (or BGO) containing additive elements
A single crystal is used as a substrate 11, and a BSO (or BGO) thin layer 12 without additive elements is first grown thereon, and then a (BSO or BGO) thin layer 13 containing additive elements is grown by liquid phase epitaxial growth.

The operating principle of the image conversion element as shown in FIG. 5 will be explained. The substrate 11 and the thin layer 13 do not exhibit photoconductivity to light with a wavelength of 400 to 640 nm due to additive elements such as gallium. Only the BSO (or BGO) of the thin layer 12 is photoconductive when irradiated, so that it is possible to write images on it. Since the right side of the substrate 11 and the thin layer 13 are insulators with a resistivity of 101 and a high resistance of Ω·α, as shown in FIG. Even if a voltage is applied by the DC power supply 15 to the 11/12 and 12/13 interfaces, the carriers generated in the thin layer 12 will be trapped. Therefore, the carrier density changes depending on the magnitude of the light intensity corresponding to the shading of the image, and these move to the valley electrode, but 11/12, 1
Since it is trapped at the 2/13 interface, an electrostatic image is accumulated. In reading this image, after writing, the electrodes 14, 1
4 is short-circuited, the internal electric field becomes as shown in FIG.

In other words, the potential corresponds to the magnitude of the light intensity, and where the light intensity is high, a large potential gradient such as α remains in the thin layer 12, and where the light intensity is low, a small potential gradient such as β remains. There is. As shown in FIG. 7, light A1 having a long wavelength and low photoconductivity sensitivity is incident on the element as read light.

Since layers 11, 12 and 13 each have the same electro-optic effect, if the thicknesses of layers 11 and 13 are made equal at potential gradients α7 and α/', retardation due to the potential difference in each layer will be reduced. are canceled out, retardation occurs substantially only at α, and even if BSO (or 1BG0) containing an additive element having an electro-optic effect is used as an insulating layer, it does not affect the image conversion element characteristics at all. I understand. Above, we have mainly discussed combinations in which the substrate and the epitaxial growth layer have the same composition (with or without additive elements).
Another aspect of the present invention is a combination of these proboscises of different species, which is possible.

That is, a bismuth silicon oxide (or bismuth germanium oxide) single crystal containing additive elements is used as a substrate,
This is a method in which bismuth germanium oxide (or bismuth silicon oxide) without any of the above elements added thereto is grown by liquid phase epitaxial growth.Of course, the combination of the substrate and the epitaxial growth layer can be reversed to the above combination. Alternatively, it is also possible to make a three-layer structure by epitaxially growing it once and then epitaxially growing it again.Example 5 A BSO layer was grown on a BGO substrate containing additive elements.

First, a BGO single crystal containing additive elements was grown in advance, and then a sliced wafer was used as a substrate and directly immersed in a reaction melt having the composition shown below.

The composition was 0.5 mol% to 32 mol% of Bi2O3-SlO2-based SiO2. Example 6 A BSO layer was grown on a BGO substrate containing an additive element, and a BGO layer containing an additive element was further grown thereon.

The process up to the growth of the BSO layer was the same as in Example 5, and in order to grow BGO containing additional elements on top of it, it was necessary to immerse it at a temperature lower than the melting point of BSO, so it was supercooled to the necessary degree. A reaction melt having the following composition was used.

The composition is Bl2O3-GeO2-based Ge containing additive elements.
O2 was 0.5 mol% to 32 mol%. Example 7 A BGO layer was grown on a BSO substrate containing additive elements.

The composition of the reaction melt BGO (is the same as in Example 6, GeO2 is 0)
．． The content was 5 mol % to 32 mol %, and the other conditions were the same as in Example 5. Example 8 A BGO layer was grown on a BSO substrate containing additive elements, and a BSO layer containing additive elements was further grown on top of the BGO layer.

Until the BGO layer is grown (1) in the same manner as in Example 7, and in order to further grow BSO containing additional elements, the composition of the reaction melt is the same as in Example 5, Bl2O3-SlO2-based Si.
O2 was 0.5 mol% to 32 mol%. Unlike the homogeneous junctions as in Examples 1 to 3, the lattice spacing is BSO:10.10A.BG0: 1
0.14 A (!l) Although slightly different, it was sufficient for liquid phase epitaxial growth, and a uniform epitaxial growth layer was obtained.

As described above, in the method of the present invention, when growing bismuth silicon oxide or bismuth germanium oxide single crystal by liquid phase epitaxial growth, a homologous single crystal with equal or close lattice spacing is used as a substrate, and By using a reaction melt with an appropriate composition that has a low melting point, these liquid phase epitaxial growths are possible, and thin electro-optic single crystal layers can be achieved. The device has the advantage of improved image resolution and the ability to obtain clear images.

[Brief explanation of the drawing]

FIG. 1 is a sectional view showing the structure of an optical image conversion element. Figure 2 is the phase diagram of the Bl2O3-SiO2 system, and Figure 3 is the phase diagram of the Bi2O3-SiO2 system.
2O3-GeO2. FIG. 4 shows a liquid phase epitaxial growth apparatus used in an embodiment of the present invention. FIG. 5 is a sectional view showing the structure of a three-layer image conversion element manufactured according to an embodiment of the present invention, and FIG. 6 is an operational circuit diagram thereof. FIG. 7 is a diagram showing a potential gradient within the element shown in FIG. 6. 1... Bismuth silicon oxide single crystal, 2... Transparent insulating film, 3, 31... Electrode, 4... Single crystal substrate, 5... ...Platinum holder 6...Pulling shaft, 7...Reaction melt, 8...Platinum crucible, 9...Heater 10... - Crucible support stand, 11... Single crystal substrate, 12, 13... Thin layer, 14...
・Transparent electrode.

Claims (1)

[Claims] 1. A single crystal of bismuth silicon oxide or bismuth germanium oxide doped with an element selected from the group consisting of aluminum, gallium, calcium, barium, and iron is used as a substrate, and bismuth silicon oxide or bismuth germanium oxide is deposited on the substrate. A method for liquid phase epitaxial growth of an electro-optic crystal, characterized by growing a layer by liquid phase epitaxial growth. 2 The reaction melt for liquid phase epitaxial growth of a bismuth silicon oxide layer on a bismuth silicon oxide single crystal substrate doped with additive elements is Bi_2O_3-S.
The liquid phase epitaxial growth method of an electro-optic crystal according to claim 1, characterized in that in iO_2, SiO_2 is 0.5 to 27 mol%. 3 The reaction melt for liquid phase epitaxial growth of a bismuth germanium oxide layer on a bismuth germanium oxide single crystal substrate doped with additive elements is Bi_2O_
2. The method for liquid phase epitaxial growth of an electro-optic crystal according to claim 1, wherein GeO_2 is 0.5 to 32 mol% in 3-GeO_2. 4 The reaction melt for liquid phase epitaxial growth of a bismuth germanium oxide layer on a bismuth silicon oxide single crystal substrate doped with additive elements is Bi_2O_3-
The liquid phase epitaxial growth method of an electro-optic crystal according to claim 1, characterized in that GeO_2 is 0.5 to 32 mol% in GeO_2. 5 The reaction melt for liquid phase epitaxial growth of a bismuth silicon oxide layer on a bismuth germanium oxide single crystal substrate doped with additive elements is Bi_2O_3-
2. The liquid phase epitaxial growth method of an electro-optic crystal according to claim 1, wherein SiO_2 is 0.5 to 32 mol%. 6 Using bismuth silicon oxide or bismuth germanium oxide single crystal as a substrate, growing a bismuth silicon oxide or bismuth germanium oxide layer doped with an element selected from the group consisting of aluminum, gallium, calcium, barium, and iron by liquid phase epitaxial growth on the substrate. A method for liquid phase epitaxial growth of electro-optic crystals, characterized in that: 7 The reaction melt for liquid phase epitaxial growth of a bismuth silicon oxide layer doped with additive elements on a bismuth silicon oxide single crystal substrate is Bi_2O_3-S.
7. The method for liquid phase epitaxial growth of an electro-optic crystal according to claim 6, wherein in iO_2, SiO_2 is 0.5 to 27 mol%. 8 The reaction melt for liquid phase epitaxial growth of a bismuth germanium oxide layer containing additive elements on a bismuth germanium oxide single crystal substrate is Bi_2O_
7. The method for liquid phase epitaxial growth of an electro-optic crystal according to claim 6, wherein GeO_2 is 0.5 to 32 mol% in 3-GeO_2. 9 The reaction melt for liquid phase epitaxial growth of a bismuth germanium oxide layer containing additive elements on a bismuth silicon oxide single crystal substrate is Bi_2O_3-
7. The method for liquid phase epitaxial growth of an electro-optic crystal according to claim 6, characterized in that GeO_2 is 0.5 to 32 mol% in GeO_2. 10 The reaction melt for liquid phase epitaxial growth of a bismuth silicon oxide layer doped with additive elements on a bismuth germanium oxide single crystal substrate is Bi_2O_3.
-0.5 to 32 mol% of SiO_2 in SiO_2
7. A method for liquid phase epitaxial growth of an electro-optic crystal according to claim 6.