Apparatus and method for producing a gallium oxide crystal

Abstract

Apparatus (10) for producing a gallium oxide crystal, comprising a vertical Bridgman furnace with: a base body (12); a cylindrical furnace body (14) which has heat resistance and is arranged above the base body (12); a cover element (18) which closes the furnace body (14); a heater (20) arranged within the furnace body (14); a crucible shaft (24) which is provided to be vertically movable through the base body (12); and a crucible (30) arranged on the crucible shaft (24) and heated by the heater (20), wherein the crucible (30) comprises a Pt-Rh base alloy having a Rh content of 10 to 30 wt.% and the furnace body (14) has an inner wall which is designed as a heat-resistant wall (32) which contains a plurality of annular heat-resistant elements (32b) which each have a predetermined height and are stacked on top of one another, wherein the annular heat-resistant elements (32b) each contain a plurality of sections (32a) which are connected to one another to form the annular shape.

Description

Technical area

[0001] The present invention relates to an apparatus and a method for producing a gallium oxide crystal, which is a wide bandgap semiconductor for power devices and can be used as a crystal material of the post-silicon era. State of the art

[0002] Since the reports of Y. Tomm et al. (NPL 3 and 4) in 2000 on single crystal growth using the FZ method or the CZ method, gallium oxide single crystals (especially β-Ga2O3 single crystals, the following description refers to β-Ga2O3 single crystals) have been investigated for their crystal growth and were originally developed as a substrate for the fabrication of GaN thin films for LEDs.

[0003] In recent years, M. Higashiwaki et al. reported the realization of a FET for a power device using a β-Ga2O3 single crystal (NPL 11), and the fabrication of large, high-quality, and low-cost β-Ga2O3 single crystals for the realization of wide bandgap semiconductor substrates for power devices is attracting strong interest.

[0004] It is assumed that β-Ga2O3 single crystals for application in components can be prepared by processes such as floating zone melting (FZ), CZ, EFG, VB and HB processes as described in Fig. 18 can be bred.

[0005] Of these crystal growth methods, the FZ method, due to its crystal growth principle, does not require a vessel to hold a raw material melt, and therefore, heating to high temperatures (melting point) to melt the raw material can be carried out relatively easily. Various studies have been conducted in this regard (see NPL 1 to 3, 5, 7, and 8). However, due to its growth principle and temperature conditions, the FZ method encounters technical limitations in obtaining large, high-quality crystals in which structural defects, including dislocations, are suppressed. Although various studies have been conducted over the past decade (see NPL 1 to 3, 5, 7, and 8 and PTL 6), it can be concluded that the FZ method does not sufficiently meet the requirements for device applications.

[0006] As methods for growing large, high-quality single crystals suitable for industrial production, the CZ method and the EFG method have been widely used. Regarding the growth of large β-Ga2O3 single crystals, the CZ method (NPL 4 and 10) and the EFG method (NPL 9 and PTL 1 to 5) are believed to have undergone active development since 2000. However, to date, no high-quality large β-Ga2O3 single crystals have been produced at low cost and used in future power devices.

[0007] Patent Publication No. 7 describes a method for producing a lithium tantalate single crystal. Patent Publication No. 8 discloses the production of oxide single crystals. Patent Publication No. 9 describes a method and apparatus for producing Ga2O3 crystals and a crucible container. List of cited documentsPatent literature: PTL 1: JP 2013-237591 A PTL 2: JP 2011-190134 A PTL 3: JP 2011-190127 A PTL 4: JP 2011-153054 A PTL 5: JP 2006-312571 A PTL 6: JP 2004-262684 A PTL 7: US 4 144 117 A PTL 8: JP H02-267 184 A PTL 9: JP 2016-079 080 A Nichtpatentliteratur: NPL 1: N. Ueda, H. Hosono, R. Waseda, H. Kawazoe, Appl. Phys. Lett., 70 (1997) 3561. NPL 2: V.I. Vasyltsiv, Ya.I. Rym, Ya.M. Zakharo, Phys. Stat. Sol., B195 (1996) 653. NPL 3: Y. Tomm, J.M. Ko, A. Yoshikawa, T. Fukuda, Solar Energy mater. Solar Cells, 66 (2000) 369. NPL 4: Y. Tomm et.al; Czochralski grown Ga2O3 crystals, Journal of Crystal Growth, 220 (2000) 510-514. NPL 5: E.G. Villora et al.; Large-size β-Ga2O3 single crystals and wafers, Journal of Crystal Growth, 270 (2004) 420-426. NPL 6: M. Zinkevich et al.; Thermodynamic Assessment of the Gallium-Oxygen System, J. Am. Ceram. Soc., 87 [4] 683-91 (2004). NPL 7: J. Zhanga et al.; Growth and spectral characterization of β-Ga2O3 single crystals, Journal of Physics and Chemistry of Solids, 67 (2006) 2448-2451. NPL 8: J. Zhanga et al.; Growth and characterization of new transparent conductive oxides single crystals β-Ga2O3:Sn, Journal of Physics and Chemistry of Solids, 67 (2006) 1656-1659. NPL 9: H. AIDA et al.; Growth of β-Ga2O3 Single Crystals by the Edge-Defined, Film Fed Growth Method, Japanese Journal of Applied Physics, Vol. 47, No. 11, 2008, pp. 8506-8509. NPL 10: Z. Galazka et al.; Czochralski growth and characterization of β-Ga2O3 single crystals, Cryst. Res. Technol., 45, No.12 (2010) 1229-1236. NPL 11: M. Higashiwaki et al.; Gallium oxide (Ga2O3) metal-semiconductor field-effect transistors on single-crystal β-Ga2O3 (010) substrates, Appl. Phys. Lett., 100, (2012) 013504. Summary of the inventionTechnical problem

[0008] When growing crystals using the CZ and EFG methods, it is necessary to use a crucible to hold the raw material melt. Since β-Ga2O3 has a high melting point of approximately 1800°C, examples of materials used as crucible materials include high-melting-point metals such as Ir, Mo, and W.

[0009] However, it is known that Mo and W are clearly unsuitable for crucibles when β-Ga2O3 is melted at high temperatures above 1800°C in a Mo or W crucible, because Mo or W as a crucible material decomposes β-Ga2O3 by depriving it of oxygen and is oxidized due to the high reducing power of Mo and W. Consequently, it was recognized that Ir is the only high-melting-point metal suitable for use as a material for the crucible in the CZ process and for the crucible and nozzle in the EFG process. This can also be seen from the fact that the crucible material used in the cited documents for the CZ process (NPL 4 and 10) and the EFG process (NPL 9) is always Ir.

[0010] However, the inventors of the present invention have made it clear through various experiments and theoretical considerations that Ir still poses a significant problem when used as a crucible material for the CZ process and as a crucible material (including nozzle material) for the EFG process.

[0011] In particular, it was found that Ir undergoes oxidation reactions in a high-temperature furnace above 1800°C under an oxygen partial pressure of more than a few percent, making it difficult to use as a stable crucible material. It was also found that β-Ga2O3 undergoes decomposition reactions with loss of oxygen at high temperatures above 1800°C under an oxygen partial pressure of 10 percent or less, and does not readily exist as a stable β-Ga2O3 melt.

[0012] The above statements make it clear that the oxygen partial pressure required for β-Ga2O3 as the raw material melt in a high-temperature furnace is incompatible with the oxygen partial pressure required for the Ir crucible containing the raw material melt. Consequently, it is clear that Ir cannot be a suitable crucible material for containing a β-Ga2O3 raw material melt.

[0013] Furthermore, although β-Ga2O3 crystal growth can be achieved by the CZ method and EFG method using an Ir crucible within the narrow range of oxygen partial pressure in the furnace, it has been experimentally determined that the β-Ga2O3 crystal grown in this way has problems, including a high density of oxygen vacancies, which are often found in oxide crystals grown under oxygen deficiency, and evaporation losses and deterioration resulting from Ir oxidation. Furthermore, various problems arise in the realization of semiconductor devices; for example, the oxygen vacancies impede the formation of high concentrations of donors by the n-type impurities, making it extremely difficult to obtain p-type β-Ga2O3. Solution to the problem

[0014] The present invention solves these problems, and one of its objects is to provide an apparatus and a method for producing a gallium oxide crystal, which can provide large gallium oxide crystals of high quality, which can be used as a wide bandgap semiconductor material essential for the production of future power devices as a crystal material of the post-silicon era.

[0015] The invention relates to an apparatus for producing a gallium oxide crystal, comprising a vertical Bridgman furnace having: a base body; a cylindrical furnace body having heat resistance and arranged above the base body; a lid member that closes the furnace body; a heater arranged within the furnace body; a crucible shaft provided so as to be vertically movable through the base body; and a crucible arranged on the crucible shaft and heated by the heater. The crucible comprises a Pt-Rh base alloy with a Rh content of 10 to 30 wt.%, and the furnace body has an inner wall formed as a heat-resistant wall containing a plurality of annular heat-resistant elements, each having a predetermined height and stacked one upon another. The annular heat-resistant elements each include a plurality of sections that are connected to one another to form the annular shape.

[0016] The crucible comprises a Pt-Rh base alloy with a Rh content of 10 to 30 wt%.

[0017] The heat-resistant wall preferably contains zirconium dioxide.

[0018] The heater used can be a resistance heater or a high-frequency induction heater. Examples of resistance heaters include a resistance heater containing MoSi2 as the main material, and examples of high-frequency induction heaters include a heater containing a Pt-Rh-based alloy with a Rh content of 10 to 30 wt%.

[0019] The furnace body may include a cylinder support member formed of a heat-resistant material and disposed outside the heat-resistant wall, and a heat insulator disposed between the heat-resistant wall and the cylinder support member, and the lid member may be supported by the cylinder support member.

[0020] The cover member may include a heat insulator, and a reinforcing member may be arranged in the heat insulator.

[0021] In the invention described above, as a crucible vessel for growing a gallium oxide crystal at the high temperatures of the melting point of gallium oxide or higher under an oxygen atmosphere, a crucible made of a Pt-Rh base alloy having a Rh content of 10 to 30 wt% is used instead of Ir.

[0022] Fig. Figure 1 shows the high-temperature evaporation loss quantities of the Pt group elements that can be used as crucible materials at the high temperatures of the melting point of gallium oxide (β-Ga2O3) or above, in air. Fig. The data shown in Figure 1 are based on known data.

[0023] Based on the known data and the results of accurate melting experiments and accurate crystal growth experiments of β-Ga2O3 conducted by the inventors, the inventors have found that a platinum-based alloy, particularly an alloy of platinum (Pt) and rhodium (Rh), is suitable as a material for a crucible for use in producing a β-Ga2O3 crystal.

[0024] A Pt-Rh alloy has different melting points depending on the Rh content in Pt. Fig. Figure 2 shows the relationship between the composition of the Pt-Rh alloy (wt%) and the melting point, based on the known literature data and the experimental data of the inventors.

[0025] The experiment to measure the melting point of the Pt-Rh alloy was carried out in air (with an oxygen partial pressure of about 20%), and it was confirmed that the Fig. 2 do not change significantly even in an argon atmosphere (Ar atmosphere) with an oxygen partial pressure of 10 to 50% and in a nitrogen atmosphere (N2 atmosphere) with an oxygen partial pressure of 10 to 20%.

[0026] According to the melting test of β-Ga2O3 conducted by the inventors, β-Ga2O3 completely melts at about 1795°C. Accordingly, Pt, with a melting point of 1768°C, naturally cannot be used as a material for a crucible for melting and holding β-Ga2O3. However, a Pt-Rh alloy containing about 2 wt% or more of Rh has a melting point higher than that of β-Ga2O3 and can thus theoretically be used as a crucible for holding a β-Ga2O3 melt.

[0027] In the practical implementation of crystal growth of β-Ga2O3, the melting point of the Pt-Rh alloy crucible required to stably hold a β-Ga2O3 melt with a melting point of about 1795°C and to perform crystal growth varies depending on the crystal growth principle, the size of the crystal to be grown, the crystal growth conditions, etc.

[0028] In β-Ga2O3 crystal growth using the VB (Vertical Bridgman) method, it was found that the lower limit of the Rh content in a usable Pt-Rh alloy crucible is necessarily 10 wt% or more, and the crucible has a melting point of 1850°C or more. For growing a crystal with a diameter of 100 mm, it was found that it is sufficient if the Rh content is about 20 wt% and the crucible has a melting point of about 1900°C. In a Pt-Rh alloy crucible, the problem of Rh elution may occur if the Rh content is too high, so the Rh content should be 10 to 30 wt%.

[0029] Fig. Figure 2 shows the above-mentioned experimentally and empirically obtained range of the composition of the Pt-Rh alloy of the crucible used in a VB process. (in Fig. 2 marked with (b)) and the range of composition of a crucible for the CZ and EFG processes (in Fig. 2 marked with (a)).

[0030] The melting point of the crucible, which is necessary for a stable crystal growth process by preventing the crucible from suffering from problems such as local deterioration or local melting and complete melting, differs characteristically for each crystal growth method. In particular, the fact that the Rh content of the Pt-Rh alloy of the crucible used for the VB method is lower than that of the crucible used for the CZ method and the EFG method is related to the fact that the VB method is the crystal growth method in which the crystal diameter does not need to be controlled, and can thus be considered an adequate result.

[0031] According to the method and apparatus for producing a gallium oxide crystal (β-Ga2O3 crystal) of the present invention, the necessary and sufficient oxygen partial pressure (i.e., an oxygen partial pressure of 10 to 50%) required in view of the crystal growth conditions and the characteristics of the grown crystal can be applied, so that oxygen vacancies in the crystal, which are a significant problem in crystal growth methods using conventional Ir crucibles, are significantly reduced, whereby high-quality single crystals can be provided. Advantageous effects of the invention

[0032] According to the method and apparatus for producing a gallium oxide crystal of the present invention, a gallium oxide crystal (particularly a β-Ga2O3 crystal) can be advantageously grown in an oxygen atmosphere by using a crucible containing a Pt-Rh base alloy with a Rh content of 10 to 30 wt%, and large gallium oxide crystals of high quality and with fewer defects can be produced. When the inner wall of the furnace body is formed into a heat-resistant wall containing many stacked, annular heat-resistant elements, each having a predetermined height, and the annular heat-resistant elements each include many pieces joined together to form a ring shape, thermal expansion and contraction can be absorbed, and a gallium oxide crystal production apparatus with excellent durability can be provided. Short description of the drawings Fig. Figure 1 is a graph showing the high-temperature evaporation loss amounts of Pt group elements in air in a high-temperature range. Fig. Figure 2 is a graph showing the relationship between the composition of the Pt-Rh alloy (wt%) and the melting point based on the known literature data and the experimental data of the inventors. Fig. 3 is a cross-sectional view showing an example of the structure of the apparatus for producing a gallium oxide crystal. Fig. 4 is a perspective view showing an example of the ring-shaped heat-resistant member. Fig. 5 is a perspective view showing an example of the furnace body. Fig. 6 is a perspective view showing an example of the heater. Fig. 7 is a plan view showing an example of the lid member as an alumina plate (RF1800), wherein the dashed lines represent a sapphire tube. Fig. 8 is a schematic diagram of an example of the apparatus for producing a gallium oxide crystal using a high-frequency induction heating system. Fig. Figure 9 is a graph showing the actual measured data of the temperature profile of the crucible when β-Ga2O3 is added to the crucible and the temperature of the crucible is increased. Fig. Figure 10 is a graph showing the actually measured data of the temperature profile when the temperature of the crucible is gradually lowered after melting β-Ga2O3 in the crucible. The Fig. 11A and Fig. 11B are photographs showing the state of the β-Ga2O3 material before heating ( Fig. 11A) and after melting and solidification ( Fig. 11B). The Fig. 12A, Fig. 12B and Fig. Figure 12C are photographs showing the melting test of β-Ga2O3 using a Pt-Rh alloy crucible with a Pt / Rh ratio of 70 / 30 wt%. The Fig. 13A and Fig. Figure 13B are photographs showing the melting test of β-Ga2O3 using a Pt-Rh alloy crucible with a Pt / Rh ratio of 90 / 10 wt%. Fig. Figure 14 is a photograph showing the melting test of β-Ga2O3 carried out using a Pt-Rh alloy crucible with a Pt / Rh ratio of 90 / 10 wt% in an argon atmosphere. Fig. Figure 15 is a photograph showing the three types of typical crystals in unidirectional crucible solidification. The Fig. 16A, Fig. 16B, Fig. 16C and Fig. 16D are photographs showing the results of observation of the double-sided mirror-polished substrate using crossed Nicols, X-ray topography, and optical microscopy. Fig. Figure 17 is a photograph of a β-Ga2O3 crystal grown with a 2-inch inner diameter 80 / 20 wt% Pt / Rh alloy crucible using the method described in Fig. 3 shown large-scale atmospheric furnace with resistance heating. Fig. 18 is a diagram showing crystal growth methods (ie, FZ method, CZ method, EFG method, VB method and HB method). Description of embodimentsExample of a manufacturing device

[0033] In the apparatus for producing a gallium oxide crystal (β-Ga2O3 crystal) according to an embodiment of the invention, a crucible material other than Ir is used as the crucible material for growing the β-Ga2O3 crystal, namely a platinum-based alloy, and preferably an alloy material of platinum (Pt) and rhodium (Rh).

[0034] Fig. 3 shows an example of a gallium oxide crystal manufacturing apparatus 10 for growing a β-Ga2O3 crystal. The gallium oxide crystal manufacturing apparatus 10 is an apparatus for growing a β-Ga2O3 crystal by the VB (vertical Bridgman) method in an oxygen atmosphere (ie, in air).

[0035] The example of the apparatus 10 for producing a gallium oxide crystal is described schematically.

[0036] In Fig. 3, a furnace body 14 is arranged above a base body (base) 12. The base body 12 has a cooling mechanism 16 provided therein, through which cooling water is passed.

[0037] The furnace body 14 has an overall cylindrical shape and is designed to have a heat-resistant structure that can withstand high temperatures of up to about 1850°C.

[0038] The furnace body 14 has an opening which can be closed with a cover element 18.

[0039] The furnace body 14 has a bottom section 22 which contains a plurality of heat-resistant materials stacked in its lower part.

[0040] A heater 20 is arranged inside the furnace body 14. The heater in this embodiment is a resistance heater that releases heat when current is applied.

[0041] The bottom section 22 and the base body 12 have a vertically continuous through-hole, and a crucible shaft 24 is arranged through the through-hole. The crucible shaft 24 is vertically movable and rotatable about its central axis by a drive mechanism not shown in the figure. The crucible shaft 24 is also made of a heat-resistant material that can withstand high temperatures, such as aluminum oxide. A thermocouple 26 is arranged inside the crucible shaft 24, with which the temperature in the furnace body 14 can be measured.

[0042] An adapter 28 made of a heat-resistant material such as zirconium dioxide is mounted on the upper end of the crucible shaft 24, and a crucible 30 made of the Pt-Rh alloy is placed in the adapter 28. The crucible 30 is heated by means of a heater 20.

[0043] The parts of the device are described in detail below.

[0044] In the embodiment shown in the figure, the furnace body 14 has a four-layer structure including a heat-resistant wall 32 as the innermost wall and, starting from the inner layer in this order, a heat insulation layer 33, a cylinder support member 34, and a heat insulation layer 35. The outside of the heat insulation layer 35 is covered with an outer wall, which is not shown in the figure.

[0045] As in the Fig. 4 and Fig. As shown in Figure 5, the heat-resistant wall 32 is formed to have a cylindrical shape by vertically stacking a plurality of heat-resistant elements 32b, each of which includes six sections 32a connected to each other to form a ring shape with a predetermined height. The heat-resistant elements 32b formed into a ring shape are preferably stacked such that the sections 32a of the heat-resistant elements 32b that are adjacent to each other in the vertical direction are circumferentially offset from each other, as shown in Figure 5. Fig. 5 is made clear.

[0046] The heat-resistant member 32b is not particularly limited and is preferably formed of alumina or zirconia having a heat resistance up to a temperature of about 2000°C.

[0047] The cylinder support member 34 is arranged outside the heat-resistant wall 32 at a distance from the heat-resistant wall 32. The cylinder support member 34 is further formed to have a cylindrical shape by stacking many annular members 34a, each having a predetermined height. The annular members 34a, which are adjacent to each other in the vertical direction, are preferably fixed with a suitable connecting member not shown in the figure. A support ring 34b having inwardly projecting portions is mounted on the upper part of the cylinder support member 34, and the lid member 18 is supported by the support ring 34b.

[0048] The cylinder support member 34 functions as a mechanical structure and is preferably formed of alumina, which is heat-resistant while having excellent strength.

[0049] The heat insulation layer 33 is disposed between the heat-resistant wall 32 and the cylinder support member 34. The heat insulation layer 33 contains alumina fibers aggregated to a predetermined density and is formed to be porous in nature and possess heat resistance and heat-insulating properties.

[0050] The heat insulation layer 35, which is arranged outside the cylinder support member 34, is formed by filling with alumina fibers.

[0051] The lid member 18 includes a necessary number of stacked plates 18a, each of which includes alumina fibers aggregated to a predetermined density, comparable to the heat insulating layer 33. Accordingly, the lid member 18 is lightweight, and to enhance the strength in the stacked plates, a reinforcing member 37 formed of a sapphire tube or the like and having heat resistance is inserted.

[0052] Although the lid member 18 may be formed of high-density zirconia or alumina, the gallium oxide crystal manufacturing apparatus 10 according to the embodiment is heated internally to high temperatures of 1800°C or more, and thus may not be able to withstand its own weight with the lid member formed of high-density zirconia or alumina, leading to problems including deformation. This problem can be solved by using a lightweight lid member 18 formed of aggregated alumina fibers and compensating for the lack of strength with the reinforcing member 37.

[0053] Fig. 6 is a perspective view showing a specific structure of the heater 20.

[0054] The heater 20 used in the embodiment is a U-shaped molybdenum disilicide (MoSi2) resistance heater (trade name: Kanthal Super). As shown in Fig. 6, four heaters 20 are fixed to a frame-shaped support element 38 which is attached to the furnace body 14. In particular, as shown in Fig. 7, elongated holes 40 through which the heaters 20 are inserted are formed in the lid member 18, and the heaters 20 are inserted and arranged in the elongated holes 40 such that the heaters 20 are positioned to surround the crucible 30 in the furnace body 14. The location where the heater 20 is inserted into the elongated hole 40 has a high temperature, and therefore, a gap is formed at the location to prevent the heater 20 from being in direct contact with the elongated hole 40.

[0055] The support member 38 is fixed at a suitable position of the furnace body 14 (which is not shown in the figure).

[0056] To form the thermal insulation layer 41, the space between the support element 38 and the cover element 18 is filled with the same thermal insulation formed from aluminum oxide fibers as is used in the thermal insulation layer 35.

[0057] Kanthal Super (trade name) is made of molybdenum disilicide and can heat to high temperatures of up to approximately 1900°C. The heating temperature can be controlled by regulating the electrical power supplied to the heater 20. In addition to Kanthal Super (trade name), a Keramax heater (trade name) can also heat to high temperatures.

[0058] The gallium oxide crystal manufacturing apparatus 10 according to the embodiment is configured as described above and can grow a gallium oxide crystal in air by the conventional vertical Bridgman method. By using the crucible 30 formed of a Pt-Rh base alloy material with a Rh content of 10 to 30 wt%, unlike when Ir is used alone, the crucible 30 can be prevented from being oxidized even in an air atmosphere, and crystal growth can be performed in oxygen-rich air, so that a gallium oxide crystal can be grown without oxygen vacancies.

[0059] Although this embodiment uses a resistance heater as the heater, a high frequency induction heating system may also be used as the heater.

[0060] Fig. 8 is a schematic diagram of an example of the apparatus 10 for producing a gallium oxide crystal using a high frequency induction heating system.

[0061] The Fig. 8 shown furnace body 14 appears to be slightly different from Fig. 3, but is actually the same as in the Fig. 3 to 7.

[0062] The differences in this embodiment include a high-frequency coil 44 disposed outside the furnace body 14 and a heater 46 that heats by high-frequency induction heating, which is provided instead of the resistance heater 20 of the previous embodiment. The heater 46 used is preferably a heater formed of a Pt-based alloy material, particularly a Pt-Rh-based alloy material. As described above, for growing a gallium oxide crystal by the VB method, a crucible formed of a Pt-Rh-based alloy with a Rh content of 10 to 30 wt% is used as the crucible material, and the material for the heater 46 is preferably a Pt-Rh-based alloy material with a Rh content of about 30 wt%, that is, rich in Rh, which can withstand a higher temperature than the crucible 30.The gallium oxide crystal manufacturing apparatus 10 according to this embodiment can also grow a gallium oxide crystal without oxygen vacancies in air by the VB method, preventing oxidation of the crucible 30.

[0063] A melting and solidification test of β-Ga2O3 as raw material is shown below. Melting and solidification test of β-Ga2O3

[0064] Using the Fig. In the manufacturing apparatus 10 shown in Figure 8, a β-Ga2O3 raw material is placed in the crucible 30, and a melting test of β-Ga2O3 is performed. The crucible used is a vessel made of a Pt-Rh alloy (Pt / Rh: 90 / 10 wt%).

[0065] Fig. 9 is a graph showing the actually measured data of the temperature profile of the crucible 30 using the apparatus 10 in the case where the β-Ga2O3 raw material is placed in the crucible and the temperature inside the furnace body 14 is gradually increased from room temperature. Fig. 9 also shows the time elapsed during the temperature increase.

[0066] The Fig. The temperature profile shown in Figure 9 shows that although the rate of temperature increase appears constant from room temperature, the rate of temperature increase slows down once at 1789.2°C, so that the temperature increase is delayed, and then the original rate of temperature increase is restored at 1793.5°C. Thus, 1789.2°C, where the rate of temperature increase begins to slow down, is the temperature at which the β-Ga2O3 material begins to melt, and 1793.6°C, where the original rate of temperature increase is restored, is the temperature at which the β-Ga2O3 material in the crucible is completely melted.

[0067] Fig. Figure 10 is a graph showing the actual measured temperature profile data for the case where, after heating the crucible to 1800°C or more (1802°C), the temperature of the crucible is gradually lowered. The temperature profile shows that, at the time the temperature drops to 1772.2°C, the temperature rapidly increases from 1772.2°C to 1778.1°C. This temperature change occurs due to the heat generated by the solidification of the molten β-Ga2O3 (heat of solidification). Thus, it is shown that molten β-Ga2O3 solidifies at 1772.2°C, or in other words, the β-Ga2O3 placed in the crucible was completely melted and then solidified.

[0068] The Fig. 11A and Fig. 11B are photographs showing the β-Ga2O3 raw material placed in the crucible before heating ( Fig. 11A) and after melting and solidification ( Fig. 11B). Fig. Figure 11A shows the state where a mass of β-Ga2O3 material is placed in the crucible. Fig. Figure 11B shows that the β-Ga2O3 material melted completely to fill the crucible and then solidified.

[0069] The Fig. The melting and solidification test of β-Ga2O3 shown in Figure 9 is carried out with a precise temperature measurement and is significant in that the melting temperature of β-Ga2O3 was accurately determined and β-Ga2O3 was completely melted and solidified in the crucible.

[0070] Various values ​​have been reported for the melting point of β-Ga2O3 within a range of 1650 to 1800°C. In the melting experiment, the temperature at which the β-Ga2O3 material begins to melt, i.e., 1789.2°C, and the temperature at which the β-Ga2O3 material is completely melted in the crucible, i.e., 1793.5°C, were actually measured. The experiment thus identified the precise melting temperature of β-Ga2O3 for the first time. Accordingly, a β-Ga2O3 crystal can be reliably grown by selecting the material for the crucible and, based on the melting temperature of β-Ga2O3 obtained from the melting experiment, by controlling the temperature for crystal growth.

[0071] In the melting experiment, the crucible used was a vessel made of a Pt-Rh alloy (Pt / Rh: 90 / 10 wt%). The result of the experiment shows that a β-Ga2O3 crystal can be produced using a vessel made of a Pt-Rh alloy (Pt / Rh: 90 / 10 wt%). Melting test of β-Ga2O3 II

[0072] The Fig. 12A, Fig. 12B and Fig. Figure 12C shows photographs showing another example of a melting test of β-Ga2O3. In the melting test, β-Ga2O3 is melted using a Pt-Rh alloy with a Pt / Rh ratio of 70 / 30 wt% for the crucible.

[0073] Fig. Figure 12A shows the β-Ga2O3 raw material used in the experiment. The raw material used is a cylindrical sintered β-Ga2O3 material.

[0074] Fig. Figure 12B shows the state in which the β-Ga2O3 raw material is placed (where the β-Ga2O3 raw material is shot vertically).

[0075] Fig. Figure 12C shows the state of the crucible after raising the crucible temperature to approximately 1800 to 1860°C and then lowering it to room temperature. The β-Ga2O3 raw material is completely melted and solidified.

[0076] The result of the experiment shows that a crucible made of a Pt-Rh alloy with a Pt / Rh ratio of 70 / 30 wt% can be sufficiently used for crystal growth of β-Ga2O3.

[0077] Both melting experiments I and II were conducted in air (oxidative atmosphere). The results of the experiments demonstrate that crystal growth of β-Ga2O3 can be performed in air using a crucible vessel made of a Pt-Rh alloy. Melting test of β-Ga2O3 III

[0078] A melting test of β-Ga2O3 was conducted using the manufacturing apparatus 10 described above. A crucible vessel made of a Pt-Rh alloy with a Pt / Rh ratio of 90 / 10 wt% was used. The melting test was used to observe the condition when the temperature for heating the crucible was increased to a significantly higher temperature range than the melting temperature of β-Ga2O3.

[0079] Fig. Figure 13A shows the state where a mass of sintered β-Ga2O3 material is placed in the crucible before heating. Fig. Figure 13B shows the state where the crucible was raised to the melting temperature of β-Ga2O3 or more and then lowered to room temperature.

[0080] In this experiment, the crucible is expected to heat up to about 1800 to 1860°C, thereby completely melting the β-Ga2O3 raw material, while the crucible also partially melts.

[0081] The reason why the crucible partially melts is believed to be that the temperature of the crucible exceeds 1850°C, which is the melting point of a Pt-Rh alloy (Pt / Rh: 90 / 10 wt%).

[0082] Therefore, when crystal growth of β-Ga2O3 is performed using a Pt-Rh alloy (Pt / Rh: 90 / 10 wt%) as the crucible material, it is naturally necessary to control the crystal growth temperature to the melting temperature of the crucible or less. Melting test of β-Ga2O3 IV

[0083] The previously mentioned melting tests of β-Ga2O3 are tests in which a β-Ga2O3 raw material is melted using the melting methods described in Fig. 8 in the air (oxidative atmosphere). As a comparative example, an experiment was conducted in which a β-Ga2O3 raw material was melted using a growth furnace with an argon gas atmosphere.

[0084] In the crystal growth furnace with an argon gas atmosphere, a carbon heater is arranged outside the crucible, the crucible and a part of a support member of the crucible are hermetically shielded with the carbon heater and a heat insulation material, and the crucible is heated while argon gas flows into the area where the crucible is housed.

[0085] The crucible used in the melting test is a crucible made of a Pt-Rh alloy (Pt / Rh: 90 / 10 wt%).

[0086] Fig. Figure 14 shows the state where a β-Ga2O3 raw material is placed in the crucible. When the crucible is heated to 1700°C in an argon atmosphere and then cooled to room temperature, the β-Ga2O3 raw material is lost and the crucible vessel melts (not shown in the figure). This phenomenon shows that by heating the crucible to 1700°C in an argon atmosphere, the Ga2O3 is reductively decomposed, and the Ga metal and the Pt-Rh alloy of the crucible form an alloy with a lower melting temperature that melts at 1700°C.

[0087] The result of the experiment shows that in the case where a β-Ga2O3 raw material is melted in a crucible, it is necessary to carry out the crystal growth of β-Ga2O3 in an oxidative atmosphere, because in the high temperature range where β-Ga2O3 is melted, reductive decomposition of Ga2O3 occurs, thus preventing β-Ga2O3 from existing as a stable melt. Example of crystal growth of β-Ga2O3

[0088] An attempt was made to grow β-Ga2O3 crystals in a VB furnace by unidirectional solidification without seed / seed crystals.

[0089] A sintered β-Ga2O3 material was placed as raw material in a crucible made of a Pt-Rh base alloy with an inner diameter of 25 mm and a height of 50 mm and heated in a high-temperature air furnace (i.e., in the Fig. The crystal was completely melted at 1800°C or more (device shown in Figure 8), with its temperature distribution adjusted to bring the temperature gradient to 5 to 10°C / cm around the melting point of β-Ga2O3 (approximately 1795°C). Unidirectional solidification then occurred through the combination of crucible movement and temperature decrease in the furnace. After cooling, the crucible was released, and the grown crystal was removed.

[0090] Fig. Figure 15 is a photograph of crystals showing the three types of typical crystals obtained by crystal growth of β-Ga2O3 by means of unidirectional solidification. Crystal A shows the case where the entire material undergoes polycrystalline growth. Crystal B shows the case where polycrystalline growth quickly transitions to single crystal growth. Crystal C shows the case where single crystal growth occurred from the bottom to the top. X-ray diffraction and observation of the characteristic crystal habit reveal that both the single crystal region of crystal B and single crystal C undergo facet growth of the (100) plane in the <100> -direction, and that a (001) facet plane occurred at about 104° to the (100) plane and the direction perpendicular to the two facet planes was the <010> -direction was.It was confirmed that the (100)-level facet growth in the <100> -direction due to the strong anisotropy in the growth rate most likely took place without a seed / seed crystal, with the growth rate in the <010> -direction was about an order of magnitude higher than in the <100> -Direction.

[0091] From the resulting single crystal, a (100)-plane substrate was cut perpendicular to the growth direction to produce a double-sided mirror-polished substrate with a thickness of approximately 0.5 mm. The substrate sample was subjected to observation with crossed Nicols, X-ray topography, and optical microscopy after etching with KOH.

[0092] Fig. Figure 16A shows the results of observation using Nicol prisms. The observation revealed that the substrate was a single crystal without a detectable low-angle tilt boundary. Fig. Figure 16B shows the transmission X-ray topography of the same substrate. A transmission X-ray diffraction pattern was obtained except for a portion of the outer edge. The portion of the outer edge that does not show a pattern (white area) corresponds to a region with a high dislocation density or a low dislocation angle, which cannot be detected by the Nicol prism method. Fig. Figure 16C shows the dislocation pit line, which is locally located essentially in the <010> -direction. The pits had a density of about 1 × 10 4 per square centimeter. The area corresponding to the white section in the Fig. 16B, showed dislocation pits with a high density of about 5 × 10 5 per square centimeter. Fig. Figure 16D shows the defects, which range in size from 10 µm to several tens of micrometers in <010> -direction, which do not correspond to the X-ray topography. The defects were observed without etching and were considered linear defects.

[0093] Fig. Figure 17 is a photograph of the crystal obtained with an alloy crucible with a Pt / Rh ratio of 80 / 20 wt% with an inner diameter of approximately 5 cm (2 inches) using the method described in Fig. 3. In this example, a sintered β-Ga2O3 material was placed in the crucible as the raw material, completely melted, and then solidified starting from the lower part (thin section), and a β-Ga2O3 crystal with a diameter of approximately 5 cm (2 inches) was grown, although the crystal was not a complete single crystal.

Claims

[1] Apparatus (10) for producing a gallium oxide crystal, comprising a vertical Bridgman furnace with: a base body (12); a cylindrical furnace body (14) which has heat resistance and is arranged above the base body (12); a cover element (18) which closes the furnace body (14); a heater (20) arranged within the furnace body (14); a crucible shaft (24) which is provided to be vertically movable through the base body (12); and a crucible (30) arranged on the crucible shaft (24) and heated by the heater (20), wherein the crucible (30) comprises a Pt-Rh base alloy having a Rh content of 10 to 30 wt.% and the furnace body (14) has an inner wall which is designed as a heat-resistant wall (32) which contains a plurality of annular heat-resistant elements (32b) which each have a predetermined height and are stacked on top of one another, wherein the annular heat-resistant elements (32b) each contain a plurality of sections (32a) which are connected to one another to form the annular shape. [2] The apparatus (10) for producing a gallium oxide crystal according to claim 1, wherein the heat-resistant wall (32) contains zirconia. [3] The apparatus (10) for producing a gallium oxide crystal according to claim 1 or 2, wherein the heater (20) is a resistance heater (20). [4] The apparatus (10) for producing a gallium oxide crystal according to claim 3, wherein the resistance heater (20) contains MoSi2 as the main material. [5] The apparatus (10) for producing a gallium oxide crystal according to claim 1 or 2, wherein the heater (20) is a high frequency induction heater (20). [6] The apparatus (10) for producing a gallium oxide crystal according to claim 5, wherein the high frequency induction heater (20) contains a Pt-Rh base alloy. [7] The apparatus (10) for producing a gallium oxide crystal according to any one of claims 1 to 6, wherein the furnace body (14) includes a cylinder support member (34) formed of a heat-resistant material and disposed outside the heat-resistant wall (32), and a heat insulator (33) disposed between the heat-resistant wall (32) and the cylinder support member (34), and the lid member (18) is supported via the cylinder support member (34). [8] Apparatus (10) for producing a gallium oxide crystal according to any one of claims 1 to 7, wherein the lid member (18) contains a heat insulator and a reinforcing member (37) is arranged in the heat insulator. [9] A method for producing a gallium oxide crystal, comprising growing a gallium oxide crystal in an oxygen atmosphere using the apparatus (10) for producing a gallium oxide crystal according to any one of claims 1 to 8. [10] A method for producing a gallium oxide crystal according to claim 9, wherein the gallium oxide is β-Ga2O3.

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