Method for producing gallium oxide crystal

Abstract

An aqueous solution containing Ga ions is brought to a supercritical state at a temperature of 400 °C or higher and a pressure of 22.1 MPa or higher, whereby α- or β-Ga2O3 crystals are obtained.

Description

Technical Field

[0001] The present invention relates to a method for producing gallium oxide crystals. Background Art

[0002] In recent years, gallium oxide (Ga 2 O 3 ) has attracted much attention as a semiconductor material. It is known that gallium oxide has five crystal forms: α, β, γ, δ, and ε. Among them, the metastable α-Ga 2 O 3 has a very large band gap of 5.3 eV and is highly expected as a power semiconductor material. For example, Patent Document 1 discloses a semiconductor device including: a substrate substrate having a corundum crystal structure, a semiconductor layer having a corundum crystal structure, and an insulating film having a corundum crystal structure, and describes an example in which an α-Ga 2 O 3 film is formed as the semiconductor layer on a sapphire substrate. In addition, Patent Document 2 discloses a semiconductor device including: an n-type semiconductor layer containing a crystalline oxide semiconductor having a corundum structure as a main component, a p-type semiconductor layer mainly composed of an inorganic compound having a hexagonal crystal structure, and an electrode. In an embodiment of Patent Document 2, it is disclosed that an α-Ga 2 O 3 film having a corundum structure in a metastable phase is formed as the n-type semiconductor layer on a c-plane sapphire substrate, and an α-Rh 2 O 3 film having a hexagonal crystal structure is formed as the p-type semiconductor layer to fabricate a diode. In addition, the application of α-Ga 2 O 3 in phosphors is also expected. For example, as described in Non-Patent Document 1, β-Ga 2 O 3 is expected as a semiconductor material for high-performance power devices. In addition, the applications of β-Ga 2 O 3 in gas sensors, transparent conductive films, deep ultraviolet light detectors, EL light-emitting elements, catalysts, etc. are also expected.

[0003] It is well known that the hydrothermal synthesis method is a method capable of synthesizing high-quality crystals at relatively low temperatures and costs (for example, Non-Patent Document 2). As the hydrothermal synthesis of Ga 2 O 3 , as in Non-Patent Document 3, the following method is known. First, GaOOH is synthesized, and then α-Ga 2 O 3 or β-Ga 2 O 3Crystals. In addition, as described in Non-Patent Document 4, a method for producing Ga crystals in supercritical water at a temperature of 365 - 384°C and a pressure of 235 atm (about 23.8 MPa) is also known. 2 O 3 crystals.

[0004] Prior Art Documents

[0005] Patent Documents

[0006] Patent Document 1: Japanese Patent Application Laid-Open No. 2014 - 72533

[0007] Patent Document 2: Japanese Patent Application Laid-Open No. 2016 - 25256

[0008] Non-Patent Documents

[0009] Non-Patent Document 1: IEEE ELECTRON DEVICE LETTERS, VOL.40, NO.3, MARCH 2019, pp.431 - 434

[0010] Non-Patent Document 2: Ann.Chim.Sci.Mat., 2002, 27(6), pp.15 - 36

[0011] Non-Patent Document 3: Crystal Growth&Design, Vol.8, No.4, 2008, pp.1282 - 1287

[0012] Non-Patent Document 4: Russian Journal of Physical Chemistry A, 2011, Vol.85, No.3, pp.377 - 382 Summary of the Invention

[0013] However, in the method of Non-Patent Document 3, since α- or β-GaO crystals are produced by heat treatment of GaOOH, a large number of fine pores considered to be caused by the detachment of OH groups are generated in the obtained crystals. If such GaO crystals are applied to semiconductor devices or the like, it may cause problems such as leakage, which is not desirable. In addition, in the method of Non-Patent Document 4, although GaO crystals are obtained, their crystal form is γ-GaO crystals for which there is no report as a semiconductor for power devices. Therefore, it is desired to produce α- or β-GaO crystals with almost no fine pores. 2 O 3 crystals. If such GaO crystals are applied to semiconductor devices or the like, it may cause problems such as leakage, which is not desirable. In addition, in the method of Non-Patent Document 4, although GaO crystals are obtained, their crystal form is γ-GaO crystals for which there is no report as a semiconductor for power devices. Therefore, it is desired to produce α- or β-GaO crystals with almost no fine pores. 2 O 3 crystals. If such GaO crystals are applied to semiconductor devices or the like, it may cause problems such as leakage, which is not desirable. In addition, in the method of Non-Patent Document 4, although GaO crystals are obtained, their crystal form is γ-GaO crystals for which there is no report as a semiconductor for power devices. Therefore, it is desired to produce α- or β-GaO crystals with almost no fine pores. 2 O 3 crystals, but their crystal form is γ-GaO crystals for which there is no report as a semiconductor for power devices. 2 O 3 crystals. Therefore, it is desired to produce α- or β-GaO crystals with almost no fine pores. 2 O 3 crystals.

[0014] The present invention is implemented to solve the above problems, and its main object is to produce α- or β-Ga 2 O 3 crystals with almost no fine pores.

[0015] Regarding the method for producing gallium oxide crystals of the present invention, an aqueous solution containing Ga ions is brought into a supercritical state at a temperature of 400 °C or higher and a pressure of 22.1 MPa or higher, thereby obtaining α- or β-Ga 2 O 3 crystals.

[0016] According to this production method, α- or β-Ga crystals with almost no fine pores can be obtained. 2 O 3 It is known that water is in a supercritical state at a temperature of 374 °C or higher and a pressure of 22.1 MPa or higher. Even when an aqueous solution containing Ga ions is brought into a supercritical state, as in Non-Patent Document 4, under the conditions of a temperature of 365 - 384 °C and a pressure of 23.8 MPa, only γ-Ga 2 O 3 crystals are obtained. However, if the conditions are set to a temperature of 400 °C or higher and a pressure of 22.1 MPa or higher as in the production method of the present invention, then α- or β-Ga 2 O 3 crystals are obtained. In addition, according to the production method of the present invention, α- or β-Ga 2 O 3 crystals with almost no fine pores are obtained. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Figure 1 is a longitudinal sectional view of the pressure-resistant container 10.

[0018] Figure 2 is a schematic explanatory view of the gallium oxide manufacturing apparatus 20.

[0019] Figure 3 is an XRD pattern of the product obtained in Example 1.

[0020] Figure 4 is an SEM image of the product obtained in Example 1.

[0021] Figure 5 is an XRD pattern of the product obtained in Example 2. DETAILED DESCRIPTION OF THE INVENTION

[0022] Hereinafter, with reference to the drawings, preferred embodiments of the present invention will be described. Figure 1 is a longitudinal sectional view of the pressure-resistant container 10, Figure 2 is a schematic explanatory view of the gallium oxide manufacturing apparatus 20.

[0023] For the method for producing gallium oxide of the present embodiment, an aqueous solution containing Ga ions is brought into a supercritical state at a temperature of 400 °C or higher and a pressure of 22.1 MPa or higher, whereby α- or β-Ga 2 O 3 crystals are obtained.

[0024] Examples of the aqueous solution containing Ga ions include: an aqueous solution of gallium halide, an aqueous solution of gallium nitrate, an aqueous solution of gallium sulfate, an aqueous solution of gallium hydroxide, etc. Examples of the gallium halide include: gallium chloride, gallium bromide, gallium iodide, etc. The aqueous solution containing Ga ions may contain alkali metal ions. For example, a solution obtained by adjusting the pH of an acidic aqueous solution containing Ga ions with a pH adjuster containing alkali metal ions can be used. At this time, as the pH adjuster, an aqueous solution of an alkali metal hydroxide (for example, an aqueous solution of KOH) can be used. The aqueous solution containing Ga ions may contain ammonium ions. For example, a solution obtained by adjusting the pH of an acidic aqueous solution containing Ga ions with a pH adjuster containing ammonium ions can be used. At this time, as the pH adjuster, an aqueous solution containing ammonium ions (for example, ammonia water) can be used. The Ga ion concentration of the aqueous solution containing Ga ions is not particularly limited, and for example, it can be 0.1 M or more and 10 M or less.

[0025] In order to bring the aqueous solution containing Ga ions into a supercritical state at a temperature of 400 °C or higher and a pressure of 22.1 MPa or higher, it is preferable to place the aqueous solution containing Ga ions in a pressure-resistant container and set it to a temperature of 400 °C or higher and a pressure of 22.1 MPa or higher. A temperature of 400 °C or higher is sufficient, and it can be set to 400 °C or higher and 800 °C or lower. Even if the temperature exceeds 800 °C, it can be carried out, but the price of the pressure-resistant container that can be used under such temperature and pressure conditions is high. A pressure of 22.1 MPa or higher is sufficient, and it can be 28.0 MPa or higher. There is no particular upper limit for the pressure. If it is 100 MPa or higher, the price of the pressure-resistant container is high. The pressure is determined according to the internal volume of the pressure-resistant container, the liquid volume of the aqueous solution placed in the pressure-resistant container, the temperature inside the pressure-resistant container, and the setting of the pressure regulating valve. The reaction time is not particularly limited, and for example, it can be 0.5 hours or more and 100 hours or less. After the reaction is completed, the temperature inside the pressure-resistant container is lowered, and the produced gallium oxide is taken out from the pressure-resistant container. When the aqueous solution containing Ga ions contains alkali metal ions, there is a tendency to selectively generate α-Ga 2 O 3 ; when the aqueous solution containing Ga ions contains ammonium ions, there is a tendency to selectively generate β-Ga 2 O 3 . In addition, when the pH of the aqueous solution containing Ga ions is 7.0 or higher, particularly when the pH is 9.0 or higher, there is a tendency to selectively generate α-Ga2 O 3 tendency; when the pH of an aqueous solution containing Ga ions is less than 7.0, especially less than 6.5, there is a tendency to selectively form β-Ga 2 O 3 .

[0026] The seed substrate can be pre-impregnated in an aqueous solution containing Ga ions. Examples of the seed substrate include: sapphire substrate, a substrate having a lattice constant closer to that of α- or β-Ga 2 O 3 oxide. In addition, examples include: α- or β-Ga 2 O 3 single crystal substrate, a substrate having an α- or β-Ga 2 O 3 single crystal film formed on the surface, etc. When a sapphire substrate is used, Ga 2 O 3 is formed in a particulate state on the sapphire substrate. On the other hand, when using an α- or β-Ga 2 O 3 single crystal substrate or any one of the substrates having an α- or β-Ga 2 O 3 single crystal film formed on the surface, the generated Ga 2 O 3 has the same crystal form as the seed and is formed in a film state on the seed substrate. At this time, if the size of the pressure-resistant container is increased to increase the size of the oxide substrate, a larger Ga 2 O 3 film corresponding to the size of the substrate can be obtained. In addition, seed particles can be dispersed in the aqueous solution containing Ga ions. Examples of the seed particles include α- or β-Ga 2 O 3 crystallites, etc., and preferably seed particles having the same crystal form as the Ga 2 O 3 produced by the method for producing gallium oxide of the present embodiment can be used. When using such seed particles, the Ga 2 O 3 to be generated is formed on the seed particles.

[0027] When it is desired to obtain α- or β-Ga 2 O 3 containing a dopant, it is only necessary to make the aqueous solution containing Ga ions contain ions corresponding to the dopant. Examples of the dopant include: Group 14 elements such as carbon (C), silicon (Si), germanium (Ge), tin (Sn), lead (Pb), etc. By making α- or β-Ga 2 O 3Contains dopants that can control α- or β-Ga 2 O 3 conductivity.

[0028] An example of a pressure-resistant container is shown in Figure 1 . Figure 1 The pressure-resistant container 10 is made of stainless steel, and an internal thread is provided at the opening of a bottomed cylindrical container body 11, and a cap 12 having a protrusion 12a provided with an external thread is screwed into the internal thread. An aqueous solution 14 containing Ga ions is placed in the container body 11 of the pressure-resistant container 10. A seed crystal substrate 18 supported in an upright state by a substrate support jig 16 made of Pt is immersed in the aqueous solution 14.

[0029] An example of a gallium oxide production apparatus is shown in Figure 2 .for Figure 2 The manufacturing device 20 has a pressure sensor 22, a temperature sensor 24 and a pressure regulating valve 26 installed in an airtight manner on the pressure-resistant container 10, and fixed to an electric furnace 28. The electric furnace 28 is used to heat the entire pressure-resistant container 10 so that the temperature inside the container is above 400°C and the pressure inside the container is above 22.1 MPa. The pressure inside the container is determined by the internal volume of the pressure-resistant container 10, the amount of the aqueous solution 14 placed in the pressure-resistant container 10, and the setting of the temperature inside the container and the pressure regulating valve 26. Therefore, the amount of the aqueous solution 14 placed in the pressure-resistant container 10 is adjusted in such a way that the pressure inside the container is above 22.1 MPa when the temperature inside the container is set to above 400°C. This state is maintained for a specified time, and then, after the temperature inside the container is cooled to room temperature, the α- or β-Ga 2 O 3 The seed crystal substrate 18 is taken out from the pressure-resistant container 10, rinsed with pure water, and then dried in a dryer.

[0030] Figure 1 In the pressure-resistant container 10, the substrate support jig 16 and the seed crystal substrate 18 may be omitted. In this case, the α- or β-Ga 2 O 3 The generated α- or β-Ga is not attached to the seed crystal substrate 18 but is generated in the form of particles in the solution. 2 O 3 The particles are taken out from the pressure-resistant container 10, rinsed with pure water, and then dried using a dryer.

[0031] According to the method for producing gallium oxide of the present embodiment described above, α- or β-GaO2 having almost no fine pores can be produced at a relatively low cost. 2 O 3 crystallization.

[0032] It should be noted that the present invention is not limited by any of the above embodiments. Of course, as long as it belongs to the technical scope of the present invention, it can be implemented in various ways.

[0033] Example

[0034] Hereinafter, embodiments of the present invention will be described. It should be noted that the following embodiments do not limit the present invention in any way.

[0035] [Example 1]

[0036] 1. Hydrothermal synthesis

[0037] A 0.1 M aqueous solution of gallium nitrate octahydrate (manufactured by Kishida Chemical) was prepared, and 1 M KOH aqueous solution was used as a pH adjuster to adjust the pH to 7 to obtain a raw material solution. Next, as Figure 1 shown, using a Pt substrate support jig 16, a 10 mm square c-plane sapphire substrate (seed substrate 18) was placed upright in a SUS316 pressure-resistant container 10 (inner diameter 16 mm, internal volume 10 mL). Furthermore, 6 mL of the above-mentioned raw material solution (aqueous solution 14) was added and sealed. Next, as Figure 2 shown, a pressure sensor 22, a temperature sensor 24, and a pressure regulating valve 26 were airtightly installed on the pressure-resistant container 10, and it was placed in an electric furnace 28. Then, the entire pressure-resistant container 10 was heated by the electric furnace 28 to make the temperature inside the container (maximum temperature) 420 °C. At this time, the pressure inside the container was 29.0 MPa. It was maintained in this state for 5 hours. Then, the temperature inside the container was cooled to room temperature to complete the hydrothermal synthesis process. After that, the substrate 18 with particles attached to its surface was taken out from the pressure-resistant container 10, rinsed with pure water, and dried using a dryer.

[0038] 2. Evaluation

[0039] An XRD device (manufactured by Rigaku, RINT-TTR III) was used to obtain the XRD pattern of the particles attached to the surface of the sapphire substrate under the conditions of tube voltage 50 kV, tube current 300 mA, and 2θ = 20° - 80° to identify the crystal phase. It should be noted that in order to prevent damage to the XRD detector caused by strong diffraction peaks, the measurement was performed except for the vicinity (40° - 43°) of the diffraction peak of the (006) crystal plane of sapphire (α-Al 2 O 3 ). As a result, as shown in the XRD pattern of Figure 3 , α-Ga 2 O 3 was detected as the main phase of the product. In addition, the particles attached to the surface of the sapphire substrate were observed using SEM (magnification 2000 times, the same below), and as a result, it was observed that Figure 4The hexagonal plate-like crystals brought about by the corundum structure of α-Ga 2 O 3 were observed. No pores were observed in the crystals. The maximum diameter (major axis) of the grains was more than 10 μm for the larger ones.

[0040] [Example 2]

[0041] 1. Hydrothermal synthesis

[0042] As the pH regulator of the raw material solution, ammonia water (manufactured by Dasheng Chemical Industry) was used. Except for this, hydrothermal synthesis was carried out in the same manner as in Example 1 to obtain a sapphire substrate with particles attached to its surface.

[0043] 2. Evaluation

[0044] In the same manner as in Example 1, XRD was used to identify the crystal phase of the particles attached to the surface of the sapphire substrate. As a result, as shown in the Figure 5 XRD pattern, β-Ga 2 O 3 was identified. In addition, the particles attached to the surface of the sapphire substrate were observed by SEM. As a result, no pores were observed in the generated β-Ga 2 O 3 crystals.

[0045] [Example 3]

[0046] The pH of the raw material solution was set to 10.0, and neither the Pt fixture nor the sapphire substrate was immersed. In addition, the pressure adjustment valve was used to adjust the pressure in the pressure-resistant container to 22.5 MPa at the highest temperature. Except for this, hydrothermal synthesis treatment was carried out in the same manner as in Example 1. The generated particles were taken out of the pressure-resistant container, rinsed with pure water, and then dried using a dryer. For the obtained particles, an XRD pattern was obtained in the same manner as in Example 1. As a result, as the main phase, α-Ga 2 O 3 was identified. In addition, the obtained particles were observed by SEM. As a result, no pores were observed in the generated α-Ga 2 O 3 crystals. The maximum diameter of the grains was more than 50 μm for the larger ones, and the shape was approximately hexagonal plate-like. However, there were many particles with a thickness / diameter larger than that in Example 1 and a relatively isotropic shape.

[0047] [Example 4]

[0048] Set the maximum temperature to 400 °C, and adjust the pressure regulating valve so that the pressure inside the pressure-resistant container is 25.0 MPa at the maximum temperature. Other than that, perform the hydrothermal synthesis treatment in the same manner as in Example 3. Similarly to Example 3, obtain an XRD pattern for the resulting particles. As a result, α-Ga was identified as the main phase. 2 O 3 . In addition, observe the resulting particles using SEM. As a result, pores were not observed in the generated α-Ga 2 O 3 crystals. The maximum diameter and shape of the crystal grains were at the same level as in Example 3.

[0049] [Example 5]

[0050] Set the pH of the raw material solution to 6.0, and set the maximum temperature to 400 °C. In addition, adjust using the pressure regulating valve so that the pressure inside the pressure-resistant container is 30.0 MPa at the maximum temperature. Other than that, perform the hydrothermal synthesis treatment in the same manner as in Example 3. Similarly to Example 3, obtain an XRD pattern for the resulting particles. As a result, β-Ga was identified as the main phase. 2 O 3 . In addition, observe the resulting particles using SEM. As a result, pores were not observed in the generated β-Ga 2 O 3 crystals.

[0051] [Example 6]

[0052] For the raw material solution, add 5 mg of the particles obtained under the conditions of Example 3 as seeds, and synthesize particles under the same conditions as in Example 3. For the resulting particles, obtain an XRD pattern in the same manner as in Example 1. As a result, α-Ga was identified as the main phase. 2 O 3 . In addition, observe the resulting particles using SEM. As a result, pores were not observed in the generated α-Ga 2 O 3 crystals. The maximum diameter of the crystal grains of the larger ones exceeded 100 μm, and the shape was approximately hexagonal plate-like. However, there were many particles with a thickness / diameter greater than that of Example 1 and a relatively isotropic shape.

[0053] This application is based on Japanese Patent Application No. 2019-200475 filed on November 5, 2019, and the entire content thereof is incorporated herein by reference.

[0054] Industrial Applicability

[0055] The present invention can be used, for example, as a material for power semiconductors.

[0056] Symbol Explanation

[0057] 10 Pressure-resistant container, 11 Container main body, 12 Lid, 12a Protrusion, 14 Aqueous solution containing Ga ions, 16 Substrate support jig, 18 Seed substrate, 20 Gallium oxide manufacturing apparatus, 22 Pressure sensor, 24 Temperature sensor, 26 Pressure regulating valve, 28 Electric furnace.

Claims

1. A method for preparing gallium oxide crystals, wherein, An aqueous solution containing Ga ions and alkali metal ions is brought into a supercritical state at a temperature of 400 °C or higher and 800 °C or lower and a pressure of 25.0 MPa or higher and lower than 100 MPa, whereby α-Ga 2 O 3 crystals are obtained.

2. The method for preparing gallium oxide crystals according to claim 1, wherein, As a seed substrate, a sapphire substrate is immersed in the aqueous solution.

3. The method for preparing gallium oxide crystals according to claim 1 or 2, wherein, The aqueous solution is a solution obtained by adjusting the pH of an acidic aqueous solution containing Ga ions with a pH regulator containing alkali metal ions.

4. A method for preparing gallium oxide crystals, wherein, An aqueous solution containing Ga ions and ammonium ions is brought to a supercritical state at a temperature of 400 °C or higher and 800 °C or lower and a pressure of 25.0 MPa or higher and lower than 100 MPa, thereby obtaining β-Ga 2 O 3 crystals.

5. The method for preparing gallium oxide crystals according to claim 4, wherein, As a seed substrate, a sapphire substrate is immersed in the aqueous solution.

6. The method for preparing gallium oxide crystals according to claim 4 or 5, wherein, The aqueous solution is a solution obtained by adjusting the pH of an acidic aqueous solution containing Ga ions with a pH regulator containing ammonium ions.

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