Gallium oxide crystal manufacturing method

Abstract

α– or β–Ga2O3 crystals are obtained by placing an aqueous solution containing Ga ions into a supercritical state having a temperature of at least 400°C and a pressure of at least 22.1 MPa.

Description

[Technical field]

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

[0002] In recent years, gallium oxide (Ga2O3) has been attracting attention as a semiconductor material. Gallium oxide is known to have five crystal forms, α, β, γ, δ, and ε. Among these, α-Ga2O3, which is a metastable phase, has a very large band gap of 5.3 eV and is expected to be used as a power semiconductor material. For example, Patent Document 1 discloses a semiconductor device having a base substrate with a corundum type crystal structure, a semiconductor layer with a corundum type crystal structure, and an insulating film with a corundum type crystal structure, and describes an example in which an α-Ga2O3 film is formed as a semiconductor layer on a sapphire substrate. Patent Document 2 discloses a semiconductor device having an n-type semiconductor layer mainly composed of a crystalline oxide semiconductor having a corundum structure, a p-type semiconductor layer mainly composed of an inorganic compound having a hexagonal crystal structure, and an electrode. In the examples of this patent document 2, it is disclosed that an α-Ga2O3 film having a metastable corundum structure as an n-type semiconductor layer and an α-Rh2O3 film having a hexagonal crystal structure as a p-type semiconductor layer are formed on a c-plane sapphire substrate to fabricate a diode. In addition, α-Ga2O3 is expected to be applied to phosphors. β-Ga2O3 is expected to be a semiconductor material for high-performance power devices, as described in Non-Patent Document 1, for example. In addition, β-Ga2O3 is expected to be applied to gas sensors, transparent conductive films, deep ultraviolet light detectors, EL light-emitting elements, catalysts, etc.

[0003] Hydrothermal synthesis is generally known as a method capable of synthesizing high-quality crystals at a relatively low temperature and low cost (for example, Non-Patent Document 2). As a hydrothermal synthesis of Ga2O3, a method is known in which GaOOH is first synthesized and then heat-treated to produce α-Ga2O3 or β-Ga2O3 crystals, as in Non-Patent Document 3. In addition, a method is known in which Ga2O3 crystals are produced in supercritical water at a temperature of 365-384°C and a pressure of 235 atm (approximately 23.8 MPa), as in Non-Patent Document 4. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] JP 2014-72533 A [Patent Document 2] JP 2016-25256 A [Non-patent literature]

[0005] [Non-Patent Document 1] IEEE ELECTRON DEVICE LETTERS, VOL. 40, NO. 3, MARCH 2019, pp. 431-434 [Non-Patent Document 2] Ann. Chim. Sci. Mat., 2002, 27(6), pp. 15-36 [Non-Patent Document 3] Crystal Growth & Design, Vol. 8, No. 4, 2008, pp. 1282-1287 [Non-Patent Document 4] Russian Journal of Physical Chemistry A, 2011, Vol. 85, No. 3, pp. 377-382 Summary of the Invention [Problem to be solved by the invention]

[0006] However, in the method of Non-Patent Document 3, since α- or β-Ga2O3 crystals are produced by heat treatment of GaOOH, many fine holes are generated in the obtained crystals, which are thought to be due to the elimination of OH groups. If such Ga2O3 crystals are applied to semiconductor devices, it is not preferable because it may cause leakage. In addition, although Ga2O3 crystals are obtained by the method of Non-Patent Document 4, the crystal form is γ-Ga2O3 crystals, which have not been reported as power device semiconductors. For these reasons, it has been desired to produce α- or β-Ga2O3 crystals with almost no fine holes.

[0007] The present invention has been made to solve such problems, and a main object of the present invention is to produce α- or β-Ga2O3 crystals that are almost free of fine pores. [Means for solving the problem]

[0008] The method for producing gallium oxide crystals of the present invention involves bringing an 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, thereby obtaining α- or β-Ga2O3 crystals.

[0009] According to this manufacturing method, α- or β-Ga2O3 crystals with almost no fine pores can be obtained. It is known that water becomes supercritical at a temperature of 374°C or higher and a pressure of 22.1 MPa or higher. Even if an aqueous solution containing Ga ions is made supercritical, only γ-Ga2O3 crystals can be obtained under the conditions of temperature 365-384°C and pressure 23.8 MPa as in Non-Patent Document 4, but α- or β-Ga2O3 crystals can be obtained under the conditions of temperature 400°C or higher and pressure 22.1 MPa or higher as in the manufacturing method of the present invention. In addition, according to the manufacturing method of the present invention, α- or β-Ga2O3 crystals with almost no fine pores can be obtained. [Brief description of the drawings]

[0010] [Figure 1] FIG. [Diagram 2] FIG. 2 is a schematic explanatory diagram of an apparatus 20 for producing gallium oxide. [Diagram 3] XRD profile of the product obtained in Example 1. [Figure 4] SEM image of the product obtained in Example 1. [Diagram 5] XRD profile of the product obtained in Example 2. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0011] A preferred embodiment of the present invention will be described below with reference to the drawings, in which: Fig. 1 is a vertical cross-sectional view of a pressure vessel 10, and Fig. 2 is a schematic explanatory view of an apparatus 20 for producing gallium oxide.

[0012] The method for producing gallium oxide according to the present embodiment involves bringing an 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, thereby obtaining α- or β-Ga2O3 crystals.

[0013] Examples of the aqueous solution containing Ga ions include an aqueous gallium halide solution, an aqueous gallium nitrate solution, an aqueous gallium sulfate solution, and an aqueous gallium hydroxide solution. Examples of the gallium halide solution include gallium chloride, gallium bromide, and gallium iodide. The aqueous solution containing Ga ions may contain an alkali metal ion, and may be, for example, an acidic aqueous solution containing Ga ions whose pH has been adjusted with a pH adjuster containing an alkali metal ion. In this case, the pH adjuster may be an aqueous solution of an alkali metal hydroxide (for example, a KOH aqueous solution). The aqueous solution containing Ga ions may contain an ammonium ion, and may be, for example, an acidic aqueous solution containing Ga ions whose pH has been adjusted with a pH adjuster containing an ammonium ion. In this case, the pH adjuster may be an aqueous solution containing ammonium ions (for example, ammonium water). The Ga ion concentration of the aqueous solution containing Ga ions is not particularly limited, and may be, for example, 0.1 M or more and 10 M or less.

[0014] In order to bring the aqueous solution containing Ga ions into a supercritical state at a temperature of 400°C or more and a pressure of 22.1 MPa or more, it is preferable to put the aqueous solution containing Ga ions into a pressure-resistant container and set the temperature to 400°C or more and the pressure to 22.1 MPa or more. The temperature may be 400°C or more, and may be 400°C or more and 800°C or less. The temperature can be more than 800°C, but the pressure-resistant container that can be used under those temperature and pressure conditions is expensive. The pressure may be 22.1 MPa or more, and may be 28.0 MPa or more. There is no particular upper limit to the pressure, but if it is 100 MPa or more, the pressure-resistant container will be expensive. The pressure is determined by the internal volume of the pressure-resistant container, the amount of the aqueous solution to be put into the pressure-resistant container, the temperature in the pressure-resistant container, and the setting of the pressure adjustment valve. The reaction time is not particularly limited, but may be, for example, 0.5 hours or more and 100 hours or less. After the reaction is completed, the temperature in the pressure-resistant container is lowered, and the product gallium oxide is taken out from the pressure-resistant container. When an aqueous solution containing Ga ions contains alkali metal ions, α-Ga2O3 tends to be selectively generated, and when an aqueous solution containing Ga ions contains ammonium ions, β-Ga2O3 tends to be selectively generated. Also, when the pH of the aqueous solution containing Ga ions is 7.0 or more, especially when the pH is 9.0 or more, α-Ga2O3 tends to be selectively generated, and when the pH of the aqueous solution containing Ga ions is less than 7.0, especially when the pH is less than 6.5, β-Ga2O3 tends to be selectively generated.

[0015] A seed crystal substrate may be immersed in the aqueous solution containing Ga ions. Examples of the seed crystal substrate include a sapphire substrate and an oxide substrate with a lattice constant closer to α- or β-Ga2O3 than sapphire. Other examples include an α- or β-Ga2O3 single crystal substrate and a substrate with an α- or β-Ga2O3 single crystal film formed on the surface. When a sapphire substrate is used, Ga2O3 is generated in the form of particles on the sapphire substrate. On the other hand, when either an α- or β-Ga2O3 single crystal substrate or a substrate with an α- or β-Ga2O3 single crystal film formed on the surface is used, the Ga2O3 generated has the same crystal form as the seed crystal and is generated in the form of a film on the seed crystal substrate. At this time, if the size of the pressure-resistant container is increased to increase the size of the oxide substrate, a large Ga2O3 film corresponding to the size of the substrate can be obtained. Also, seed crystal particles may be dispersed in the aqueous solution containing Ga ions. The seed crystal particles include α- or β-Ga2O3 crystal particles, and those having the same crystal form as Ga2O3 produced by the method for producing gallium oxide of the present embodiment can be preferably used. When such seed crystal particles are used, the Ga2O3 produced is produced on the seed crystal particles.

[0016] If one wishes to obtain α- or β-Ga2O3 containing a dopant, one may add ions corresponding to the dopant to the aqueous solution containing Ga ions. Examples of dopants include Group 14 elements such as carbon (C), silicon (Si), germanium (Ge), tin (Sn), and lead (Pb). By adding a dopant to α- or β-Ga2O3, the electrical conductivity of α- or β-Ga2O3 can be controlled.

[0017] An example of a pressure-resistant vessel is shown in Fig. 1. The pressure-resistant vessel 10 in Fig. 1 is made of stainless steel, and has a lid 12 with a male-threaded protrusion 12a screwed onto a female screw provided at the opening of a bottomed cylindrical vessel body 11. The vessel body 11 of the pressure-resistant vessel 10 contains an aqueous solution 14 containing Ga ions. A seed crystal substrate 18 supported in an upright position by a Pt substrate support jig 16 is immersed in this aqueous solution 14.

[0018] An example of a gallium oxide manufacturing apparatus is shown in FIG. 2. In the manufacturing apparatus 20 in FIG. 2, a pressure sensor 22, a temperature sensor 24, and a pressure adjustment valve 26 are airtightly attached to a pressure-resistant container 10, which is then set in an electric furnace 28. The entire pressure-resistant container 10 is heated by the electric furnace 28 so that the temperature inside the container is 400° C. or higher and the pressure inside the container is 22.1 MPa or higher. The pressure inside the container is determined by the internal volume of the pressure-resistant container 10, the amount of the aqueous solution 14 to be put into the pressure-resistant container 10, the temperature inside the container, and the setting of the pressure adjustment valve 26. Therefore, the amount of the aqueous solution 14 to be put into the pressure-resistant container 10 may be adjusted so that the pressure inside the container is 22.1 MPa or higher when the temperature inside the container is 400° C. or higher. After maintaining this state for a predetermined time, the temperature inside the container is cooled to room temperature, and then the seed crystal substrate 18 to which α- or β-Ga2O3 is attached is taken out of the pressure-resistant container 10, rinsed with pure water, and then dried in a dryer.

[0019] 1, the substrate support jig 16 and the seed crystal substrate 18 may be omitted. In that case, since α- or β-Ga2O3 is generated in the form of particles in the solution without adhering to the seed crystal substrate 18, the generated α- or β-Ga2O3 particles are taken out of the pressure vessel 10, rinsed with pure water, and then dried in a dryer.

[0020] According to the method for producing gallium oxide of the present embodiment described above, α- or β-Ga2O3 crystals having almost no micropores can be produced relatively inexpensively.

[0021] It goes without saying that the present invention is not limited to the above-described embodiment, and can be embodied in various forms within the technical scope of the present invention. EXAMPLES

[0022] EXAMPLES The present invention will be described below with reference to examples, but the present invention is not limited to the following examples in any way.

[0023] [Example 1] 1. Hydrothermal synthesis A 0.1M aqueous solution of gallium nitrate octahydrate (Kishida Chemical) was prepared, and the pH was adjusted to 7 using a 1M KOH aqueous solution as a pH adjuster to obtain a raw material solution. Then, as shown in FIG. 1, a 10 mm square c-plane sapphire substrate (seed crystal substrate 18) was placed in a pressure-resistant container 10 (inner diameter 16 mm, internal volume 10 mL) made of SUS316 in an upright state using a Pt substrate support jig 16, and 6 mL of the raw material solution (aqueous solution 14) was further placed and sealed. Then, as shown in FIG. 2, a pressure sensor 22, a temperature sensor 24, and a pressure adjustment valve 26 were airtightly attached to the pressure-resistant container 10, which was then set in an electric furnace 28. Then, the entire pressure-resistant container 10 was heated by the electric furnace 28, and the temperature inside the container (maximum temperature) was set to 420° C. At this time, the pressure inside the container was 29.0 MPa. This state was maintained for 5 hours. After that, the temperature inside the container was cooled to room temperature, and the hydrothermal synthesis process was completed. Thereafter, the substrate 18 with the particles adhering to its surface was removed from the pressure-resistant container 10, rinsed with pure water, and then dried in a dryer.

[0024] 2. Evaluation Using an XRD device (Rigaku, RINT-TTR III), the XRD profile of the particles attached to the surface of the sapphire substrate was obtained under the conditions of a tube voltage of 50 kV, a tube current of 300 mA, and 2θ = 20°-80°, and the crystal phase was identified. In order to prevent damage to the XRD detector due to strong diffraction peaks, the measurement was performed excluding the diffraction peak of the (006) plane of sapphire (α-Al2O3) (40°-43°). As a result, α-Ga2O3 was detected as the main phase of the product, as shown in the XRD profile in Figure 3. In addition, the particles attached to the surface of the sapphire substrate were observed by SEM (magnification 2000 times, the same below), and hexagonal plate-shaped crystals due to the corundum structure of α-Ga2O3 were observed as shown in Figure 4. No holes were observed in the crystals. The maximum diameter (longest diameter) of the crystal particles exceeded 10 μm for the largest ones.

[0025] [Example 2] 1. Hydrothermal synthesis Hydrothermal synthesis was carried out in the same manner as in Example 1, except that ammonia water (manufactured by Ohmori Kako Co., Ltd.) was used as a pH adjuster for the raw material solution, to obtain a sapphire substrate having particles attached to its surface.

[0026] 2. Evaluation As in Example 1, the crystal phase of the particles attached to the surface of the sapphire substrate was identified by XRD, and β-Ga2O3 was identified as shown in the XRD profile in Figure 5. In addition, as a result of observing the particles attached to the surface of the sapphire substrate by SEM, no holes were observed in the generated β-Ga2O3 crystals.

[0027] [Example 3] The pH of the raw material solution was set to 10.0, the Pt jig and the sapphire substrate were not immersed, and the pressure inside the pressure vessel at the highest temperature was adjusted to 22.5 MPa by the pressure adjustment valve. The generated particles were taken out of the pressure vessel, rinsed with pure water, and then dried in a dryer. The XRD profile of the obtained particles was obtained by the same method as in Example 1, and α-Ga2O3 was identified as the main phase. In addition, the obtained particles were observed by SEM, and no holes were observed in the generated α-Ga2O3 crystals. The maximum diameter of the crystal particles exceeded 50 μm for the largest ones, and the shape was approximately hexagonal plate-like, but the thickness / diameter was larger than that of Example 1, and many of them had a relatively isotropic shape.

[0028] [Example 4] A hydrothermal synthesis treatment was carried out in the same manner as in Example 3, except that the maximum temperature was set to 400°C and the pressure control valve was adjusted so that the pressure inside the pressure-resistant container at the maximum temperature was 25.0 MPa. As in Example 3, an XRD profile was obtained for the obtained particles, and α-Ga2O3 was identified as the main phase. In addition, the obtained particles were observed by SEM, and no holes were observed in the generated α-Ga2O3 crystals. The maximum diameter and shape of the crystal particles were equivalent to those in Example 3.

[0029] [Example 5] The hydrothermal synthesis treatment was carried out in the same manner as in Example 3, except that the pH of the raw material solution was set to 6.0, the maximum temperature was set to 400°C, and the pressure inside the pressure-resistant container at the maximum temperature was set to 30.0 MPa by a pressure control valve. As in Example 3, an XRD profile was obtained for the obtained particles, and β-Ga2O3 was identified as the main phase. In addition, the obtained particles were observed by SEM, and no holes were observed in the generated β-Ga2O3 crystals.

[0030] [Example 6] Particles were synthesized under the same conditions as in Example 3, except that 5 mg of the particles obtained under the conditions in Example 3 were added to the raw material solution as seed crystals. The XRD profile of the obtained particles was obtained by the same method as in Example 1, and α-Ga2O3 was identified as the main phase. In addition, the obtained particles were observed by SEM, and no holes were observed in the generated α-Ga2O3 crystals. The maximum diameter of the crystal particles exceeded 100 μm for the largest ones, and the shape was approximately hexagonal plate-like, but the thickness / diameter was larger than in Example 1, and many of them had a relatively isotropic shape.

[0031] This application claims priority from Japanese Patent Application No. 2019-200475, filed on November 5, 2019, the entire contents of which are incorporated herein by reference. [Industrial Applicability]

[0032] The present invention can be used, for example, as materials for power semiconductors. [Explanation of symbols]

[0033] 10 pressure-resistant vessel, 11 vessel body, 12 lid, 12a protrusion, 14 aqueous solution containing Ga ions, 16 substrate support jig, 18 seed crystal substrate, 20 gallium oxide manufacturing apparatus, 22 pressure sensor, 24 temperature sensor, 26 pressure adjustment valve, 28 electric furnace.

Claims

1. The 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, thereby producing α- or β-Ga. 2 O 3 Obtaining crystals How gallium oxide crystals are made.

2. Immersing a sapphire substrate as a seed crystal substrate in the aqueous solution; A method for producing the gallium oxide crystal according to claim 1.

3. The aqueous solution contains alkali metal ions. A method for producing the gallium oxide crystal according to claim 1 or 2.

4. The aqueous solution contains ammonium ions. A method for producing the gallium oxide crystal according to claim 1 or 2.

Images