Method for growing single crystals of group III nitrides and jig for growing single crystals of group III nitrides

Abstract

This invention provides a method for growing single crystals of group III nitrides that can improve crystal quality. [Solution] A method for growing a group III nitride single crystal, comprising: an initial nucleation step S1 in which a seed substrate having a plurality of seed crystals made of group III nitride single crystals formed on its upper surface is immersed in a mixed molten liquid stored in a crucible to form a plurality of initial nuclei; a planarization step S2 in which the seed substrate is repeatedly immersed in the mixed molten liquid and then lifted out, and heated in a nitrogen atmosphere to fill the spaces between adjacent initial nuclei with group III nitride single crystals and flatten the crystal plane; and a thickening step S3 in which the seed substrate having the flattened crystal plane is immersed in a mixed molten liquid stored in a crucible to form a thickened group III nitride single crystal on the seed substrate, wherein in the initial nucleation step S1, the mixed molten liquid does not contain alkali metals and alkaline earth metals other than Na, and in the thickening step S3, the mixed molten liquid contains alkali metals or alkaline earth metals other than Na.

Description

Technical Field

[0001] The present invention relates to a method for growing a group III nitride single crystal and a jig for growing a group III nitride single crystal.

Background Art

[0002] Conventionally, a flux method is known in which a GaN substrate is immersed in a Ga-Na melt, which is a mixed melt of Ga and Na, to grow a GaN single crystal on the GaN substrate and form a thick-film GaN single crystal. In this method, due to the three-dimensional growth of the GaN single crystal, crystal growth that forms a large step (macro step) on the order of several μm to several hundred μm called macro step growth is observed. In this macro step growth region, a lump of Ga-Na melt called an inclusion is formed in the crystal, which deteriorates the crystal quality.

[0003] In order to suppress the formation of such inclusions, Patent Document 1 discloses adding an alkaline earth metal such as Ca to the Ga-Na melt in advance to improve the wettability of the Ga-Na melt with respect to the GaN substrate, and repeating heating in a nitrogen atmosphere after immersing the GaN substrate in the Ga-Na melt to form a thick-film GaN single crystal on the GaN substrate. As a result, since macro step growth is suppressed, the formation of inclusions is suppressed and the crystal quality is improved.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0005] However, in the configuration disclosed in Patent Document 1, when the process of forming initial nuclei of GaN single crystals on a substrate is performed using a crucible containing the above-mentioned Ga-Na melt, a large amount of non-GaN single crystals are generated due to the presence of alkaline earth metals such as Ca, making it difficult to form the desired initial nuclei and resulting in a significant decrease in crystal quality.

[0006] This invention has been made in view of the above problems, and aims to provide a method for growing group III nitride single crystals and a jig for growing group III nitride single crystals that can improve crystal quality. [Means for solving the problem]

[0007] One aspect of the present invention is, An initial nucleation step involves immersing a seed substrate, on which multiple seed crystals made of group III nitride single crystals are formed on its upper surface, in a mixed molten solution of group III metal and Na stored in a crucible, thereby growing group III nitride single crystals from the multiple seed crystals and forming multiple initial nuclei; A planarization step is performed by repeatedly immersing the seed substrate on which the initial nuclei have been formed in the mixed molten liquid stored in the crucible, then removing it and heating it under a nitrogen atmosphere, thereby growing group III nitride single crystals from the initial nuclei and filling the spaces between adjacent initial nuclei with group III nitride single crystals to flatten the crystal plane. A thick-film formation step involves immersing the seed substrate having a planarized crystal surface in the mixed molten liquid stored in a crucible to form a thick-film group III nitride single crystal on the seed substrate, Includes, In the initial nucleation step, the mixed melt does not contain alkali metals or alkaline earth metals other than Na. In the aforementioned thickening step, the mixed melt contains an alkali metal or alkaline earth metal other than Na, in a method for growing group III nitride single crystals.

[0008] Other aspects of the present invention include: A jig for growing group III nitride single crystals, which supports a substrate for growing group III nitride single crystals inside a crucible, A substrate support portion is provided for supporting the substrate and is configured to allow the substrate to be immersed in a mixed molten liquid of group III metal and Na stored in the crucible. The device comprises a melt additive unit configured to add alkali metals other than Na or alkaline earth metals to the mixed melt when immersed in the mixed melt, The jig for growing a group III nitride single crystal is configured to allow switching between a first state in which the substrate supported by the substrate support is immersed in the mixed molten liquid and the molten liquid additive part is not immersed in the mixed molten liquid, and a second state in which the substrate supported by the substrate support and the molten liquid additive part are immersed in the mixed molten liquid. [Effects of the Invention]

[0009] In the above-described embodiment of the group III nitride single crystal growth method, the mixed melt does not contain alkali metals or alkaline earth metals other than Na in the initial nucleation step. Therefore, the generation of miscellaneous crystals is suppressed when forming the initial nuclei, and the desired initial nuclei can be formed with high precision. Then, in the thickening step, alkali metals or alkaline earth metals other than Na are included in the mixed melt. As a result, the growth of the group III nitride single crystal is promoted laterally from the initial nuclei, so the formation of macrosteps is suppressed and the formation of inclusions is suppressed. These factors improve the crystal quality of the formed group III nitride single crystal.

[0010] In the jig for growing group III nitride single crystals according to the other embodiment described above, the substrate supported by the substrate support is immersed in a mixed molten liquid stored in a crucible, and the jig is configured to allow switching between a first state in which the substrate supported by the substrate support and the melt addition part are not immersed in the mixed molten liquid, and a second state in which the substrate supported by the substrate support and the melt addition part are immersed in the mixed molten liquid. This makes it easy to ensure that, when immersing the substrate in the mixed molten liquid, alkali metals other than Na or alkaline earth metals are not added to the mixed molten liquid from the melt addition part during the initial nucleation process, and alkali metals other than Na or alkaline earth metals are added to the mixed molten liquid during the thick-film formation process. As a result, the crystal quality of the formed group III nitride single crystal can be improved.

[0011] As described above, according to the above aspect, it is possible to provide a method for growing a group III nitride single crystal capable of improving crystal quality and a jig for growing a group III nitride single crystal.

Brief Description of the Drawings

[0012] [Figure 1] Flow chart showing the method for growing a group III nitride single crystal in Embodiment 1. [Figure 2] Conceptual diagram showing a state where a seed substrate is immersed in a mixed melt in Embodiment 1 and an alumina plate is not immersed. [Figure 3] Conceptual diagram showing a state where a seed substrate and an alumina plate are not immersed in a mixed melt in Embodiment 1. [Figure 4] Conceptual diagram showing a state where a seed substrate and an alumina plate are immersed in a mixed melt in Embodiment 1. [Figure 5] Top view of the jig and crucible in Embodiment 1. [Figure 6] In Embodiment 1, (a) Conceptual diagram of a seed substrate on which a plurality of seed crystals are formed, (b) Conceptual diagram of a seed substrate on which a plurality of initial nuclei are formed, (c) Conceptual diagram showing a state where a GaN single crystal is formed between a plurality of initial nuclei, (d) Conceptual diagram showing a state where a GaN single crystal has grown on a flattened crystal surface. [Figure 7] Plan view showing the configuration of the seed substrate in Embodiment 1. [Figure 8] Cross-sectional view showing the configuration of the seed crystal in Embodiment 1, which is a cross-sectional view perpendicular to the main surface of the substrate. [Figure 9] Plan view showing the configuration of the seed crystal in Embodiment 1. [Figure 10] Conceptual diagram for explaining the initial nucleus formation process in Modified Form 1. <� [Figure 11] Conceptual diagram for explaining the thickening process in Modified Form 1. [Figure 12] Conceptual diagram for explaining the initial nucleus formation process in Modified Form 2. [Figure 13]Conceptual diagram for explaining the thick film formation process in deformation mode 2.

Embodiments for Carrying Out the Invention

[0013] In the method for growing a group III nitride single crystal according to the above aspect, in the thick film formation process, a member containing an alkali metal and an alkaline earth metal other than Na is immersed in the mixed melt, so that an alkali metal or an alkaline earth metal other than Na is eluted from the member into the mixed melt. In the initial nucleation process, it is preferable not to immerse the member containing an alkali metal and an alkaline earth metal other than Na in the mixed melt. In this case, in the thick film formation process, it becomes easy to add a trace amount of an alkali metal and an alkaline earth metal other than Na to the mixed melt.

[0014] In the method for growing a group III nitride single crystal according to the above aspect, in the planarization process, a member containing an alkali metal and an alkaline earth metal other than Na is immersed in the mixed melt, so that an alkali metal or an alkaline earth metal other than Na can be eluted from the member into the mixed melt. In this case, in the planarization process, the wettability of the mixed melt with respect to the seed substrate can be improved, and the planarization of the crystal plane can be promoted.

[0015] In the method for growing a group III nitride single crystal according to the above aspect, in the planarization process, it is possible not to immerse a member containing an alkali metal and an alkaline earth metal other than Na in the mixed melt. In this case, in the planarization process, the formation of twinned crystals is suppressed and the crystal quality is improved.

[0016] In the Group III nitride single crystal growth method according to the above embodiment, the component containing alkali metals other than Na and alkaline earth metals is preferably an alumina component containing alkali metals other than Na and alkaline earth metals. In this case, the alumina component containing alkali metals other than Na and alkaline earth metals can be gently dissolved by immersion in the mixed melt, making it easy to add trace amounts of alkali metals other than Na and alkaline earth metals to the mixed melt in the thick-film formation process.

[0017] In the above embodiment of the method for growing a group III nitride single crystal, the seed substrate is preferably supported by a jig and configured to be immersed in and withdrawn from the mixed molten metal via the jig, and the component containing alkali metals other than Na and alkaline earth metals constitutes part of the jig or is supported by the jig. In this case, the jig makes it easy to control whether or not alkali metals other than Na and alkaline earth metals are added to the mixed molten metal.

[0018] In the above embodiment of the method for growing a group III nitride single crystal, it is preferable to include a melt addition step in which alkali metals other than Na and alkaline earth metals are added to the mixed melt between the initial nucleation step and the thickening step, or together with the thickening step. In this case, it becomes easier to ensure that alkali metals other than Na and alkaline earth metals are not included in the mixed melt during the initial nucleation step, and that alkali metals other than Na and alkaline earth metals are included in the mixed melt during the thickening step.

[0019] In the Group III nitride single crystal growth method according to one embodiment described above, it is preferable that in the thickening step, the mixed melt contains alkali metals or alkaline earth metals other than Na in a ratio of 0.01 mol% to less than 0.05 mol%. In this case, since the proportion of alkali metals or alkaline earth metals other than Na contained in the mixed melt in the thickening step is extremely low, it is possible to suppress both inclusion formation and the formation of miscellaneous crystals.

[0020] In the jig for growing Group III nitride single crystals according to the other embodiment described above, the melt addition section holds a member containing an alkali metal or alkaline earth metal other than Na, and in the second state, it is preferable to dissolve the alkali metal or alkaline earth metal other than Na from the member containing the alkali metal or alkaline earth metal other than Na into the mixed melt. In this case, it is easy to switch between the first state in which the mixed melt does not contain an alkali metal or alkaline earth metal other than Na and the second state in which the mixed melt contains an alkali metal or alkaline earth metal other than Na.

[0021] (Embodiment 1) 1. Overview of the flux method The group III nitride single crystal growth method of Embodiment 1 is a method for growing a group III nitride single crystal by the flux method. This produces a group III nitride semiconductor. The flux method is a method in which a gas containing nitrogen is supplied to a mixed molten material containing an alkali metal which serves as the flux and a group III metal which serves as the raw material, and the group III nitride semiconductor is epitaxially grown in the liquid phase.

[0022] In this embodiment, the mixed melt contains Na as an alkali metal flux. The Group III metal raw material is at least one of gallium (Ga), aluminum (Al), and indium (In), and the composition of the formed Group III nitride single crystal can be controlled by the ratio of these metals, allowing for the formation of GaN, AlN, InN, AlGaN, InGaN, AlGaInN, etc. The present invention is particularly suitable for the formation of GaN, and this embodiment 1 is a method for growing GaN single crystals. Furthermore, Na is used as the flux, and such a flux method is specifically called the Na flux method. The nitrogen-containing gas is a nitrogen molecule or a gas of a compound containing nitrogen as a constituent element, such as ammonia, and may be a mixture of these gases, or the nitrogen-containing gas may be mixed with an inert gas such as a noble gas.

[0023] The method for growing a group III nitride single crystal according to Embodiment 1 includes an initial nucleation step S1, a planarization step S2, and a thickening step S3, as shown in Figure 1. In the mixed melt, the initial nucleation step S1 does not contain alkali metals or alkaline earth metals other than Na, while the thickening step S3 contains alkali metals or alkaline earth metals other than Na. Li can be used as the alkali metal other than Na, and Ca, Ba, Mg, or Sr can be used as the alkaline earth metal. The content ratio of alkali metals or alkaline earth metals other than Na in the mixed melt in the thickening step S3 can be less than 0.05 mol%, and in Embodiment 1, it is 0.01 mol% or more and less than 0.05 mol%. Each step S1 to S3 will be described in detail later.

[0024] 2. Fixtures for growing Group III nitride single crystals The jig 200 for growing a group III nitride single crystal in this embodiment 1 is placed inside a crucible 100 for growing a semiconductor single crystal by the flux method, as shown in Figures 2 and 3. The jig 200 can support a seed substrate 9 for growing a group III nitride semiconductor single crystal inside the crucible 100. The jig 200 has a first leg portion 201, a second leg portion 202, a third leg portion 203, a connecting portion 204, and a lifting shaft 205. The material of each component of the jig 200 is alumina. As shown in Figure 2, the first leg portion 201, the second leg portion 202, and the third leg portion 203 are formed in a substantially rod shape and hang down from the corners of the connecting portion 204, which is a substantially triangular flat plate in plan view, as shown in Figure 5.

[0025] At the lower ends of the first leg portion 201, the second leg portion 202, and the third leg portion 203, a substrate support portion 210 is formed, each consisting of a protrusion capable of supporting a substrate 9. As shown in Figures 2 and 3, the first leg portion 201 is formed to be longer than the second leg portion 202 and the third leg portion 203. As a result, the substrate 1 supported by the substrate support portion 210 is supported in an inclined state with respect to the connecting portion 204. The connecting portion 204 is connected to the lifting shaft 205 so that it can assume an inclined position with respect to the lifting rotation shaft 150. As a result, the substrate 1 supported by the substrate support portion 210 can be switched between the horizontal state shown in Figure 2 and the inclined state shown in Figure 3.

[0026] Furthermore, in the first leg portion 201, the second leg portion 202, and the third leg portion 203, a melt addition portion 220 is formed at a position vertically above the substrate support portion 210. In this embodiment 1, the melt addition portion 220 forms a step and supports the alumina plate 10 containing an alkali metal other than Na or an alkaline earth metal. In this embodiment 1, the alumina plate 10 contains Ca as the alkaline earth metal. The alumina plate 10 is supported parallel to the seed substrate 9 supported by the substrate support portion 210.

[0027] 3. Details of the Group III Nitride Single Crystal Growth Method The method for growing a group III nitride single crystal in Embodiment 1 includes an initial nucleation step S1, a planarization step S2, and a thickening step S3, as shown in Figure 1. Each step is described in detail below. Note that Embodiment 1 is a method for growing a GaN single crystal, which is a group III nitride single crystal.

[0028] 3-1. Initial nucleation step S1 The initial nucleation step S1 is a step in which a seed substrate 9 (see Figure 6(a)), on which a plurality of seed crystals 2 made of GaN single crystals are formed on its upper surface, is immersed in the aforementioned mixed melt 101 stored in the crucible, thereby growing GaN single crystals from the plurality of seed crystals and forming a plurality of initial nuclei 3.

[0029] In the initial nucleation step S1, first, as shown in Figure 2, the seed substrate 9 is placed on the substrate support portion 210 of the jig 200, and the substrate support portion 210 is immersed in the mixed melt 101 stored in the crucible 100. At this time, the seed substrate 9 is approximately horizontal. In this embodiment 1, in the initial nucleation step S1, an alumina plate 10 is placed in the melt addition portion 220 of the jig 200, but the alumina plate 10 is not immersed in the mixed melt 101. It is preferable to heat and pressurize the seed substrate 9 to reach the growth temperature and growth pressure before putting it into the mixed melt. This can suppress the meltback of the seed crystal 2 of the seed substrate 9.

[0030] A multi-point seed (MPS) substrate is used for the seed substrate 9. The MPC substrate is a substrate in which multiple dot-shaped seed crystals 2 are periodically arranged on the substrate 1. Figure 2 is a cross-sectional view of the seed substrate 9, which is a cross-section perpendicular to the main surface of the substrate. Figure 2 is a plan view of the seed substrate 9 seen from above.

[0031] Substrate 1 can be made of a group III nitride semiconductor, sapphire, aluminum oxynitride, SiC, Si, spinel, ZnO, gallium oxide, etc. In the case of a sapphire substrate, for example, it is a substrate with the c-plane or a-plane as the main surface.

[0032] Multiple seed crystals 2 are provided on the substrate 1 via a buffer layer (not shown). The seed crystals 2 are arranged in a regular triangular lattice pattern. The buffer layer and seed crystals 2 are group III nitride semiconductors of any composition, such as GaN, AlGaN, and AlN. The material of the buffer layer is selected appropriately depending on the material of the seed crystals 2. For example, if the seed crystal 2 is GaN, the buffer layer is preferably GaN. The material of the seed crystals is usually a group III nitride semiconductor with the same composition as the group III nitride semiconductor to be grown by the flux method. The seed crystals 2 may be grown by any method such as MOCVD, HVPE, or MBE, but MOCVD and HVPE are preferred in terms of crystallinity and growth time.

[0033] The arrangement of seed crystals 2 is a triangular lattice pattern, as shown in Figure 3. While any periodic arrangement is acceptable, not limited to a triangular lattice, highly symmetrical patterns such as square or triangular lattices are preferred. This allows for the uniform bonding of group III nitride semiconductors grown from various crystals 2, resulting in the growth of group III nitride semiconductors with fewer dislocations and warping. When using a triangular lattice pattern, it is preferable that the arrangement direction coincides with the a-axis and m-axis directions of seed crystals 2. Here, "coincidence" does not mean perfect agreement; an angular deviation of about 10 degrees is acceptable as an error. Preferably, the angular deviation is 1 degree or less.

[0034] The distance L1 between the centers of adjacent seed crystals 2 is preferably 100 to 2000 μm. Within this range, a group III nitride semiconductor with fewer dislocations and warping can be grown. More preferably, it is 200 to 1500 μm, and even more preferably 300 to 1000 μm.

[0035] Next, the shape of the seed crystal 2 will be described in detail. Figure 8 is a cross-sectional view showing the structure of the seed crystal 2, and is a cross-sectional view perpendicular to the main surface of the substrate. Figure 9 is a plan view showing the structure of the seed crystal 2. As shown in Figures 8 and 9, the seed crystal 2 has a disc-shaped disc portion and a regular frustum-shaped hexagonal portion located in contact with the cylindrical portion, with a recess 2d in the center of the regular frustum-shaped hexagonal portion.

[0036] As described later, seed crystal 2 is formed by selective growth using a mask, with crystal growth occurring laterally from the opening in the mask. The opening pattern of the mask is circular. Therefore, after the mask is removed, the mask opening remains as a disc-shaped portion. This remaining portion is the disc. The shape of the disc portion is the same as the shape of the mask opening used when selectively growing seed crystal 2. Also, the diameter D1 of the regular hexagonal truncated pyramidal portion is larger than the diameter of the disc portion. Since the disc portion is circular in plan view, stress can be distributed when separating the substrate 1 after GaN single crystal growth by flux method, thereby suppressing the occurrence of cracks in the grown crystal. Although the shape of the mask opening pattern can be changed to a regular hexagonal plate or other shape instead of the disc portion, the disc is preferred from the standpoint of stress distribution as described above.

[0037] The base of the frustum of the seed crystal is a regular hexagon. In particular, a regular hexagon in which each side is aligned with the m-plane of the seed crystal 2 (each side coincides with the a-axis direction) is preferred. Since group III nitride semiconductors are hexagonal, using a regular hexagon allows for the uniform bonding of group III nitride semiconductors grown from the frustum of the hexagon of various crystals 2. However, it is not necessary for it to perfectly coincide with the a-axis, and an angular deviation of about 10 degrees is acceptable. Preferably, the angular deviation is 1 degree or less.

[0038] The six sides 2a of the frustum hexagonal pyramidal portion of seed crystal 2 are (10-11) planes of the group III nitride semiconductor. The (10-11) planes are stable planes in the mixed melt of the Na flux method. Therefore, the initial nuclei 3, described later, grow from the sides 2a of the frustum hexagonal pyramidal portion of seed crystal 2 while maintaining the (10-11) planes. As a result, the shape of the initial nuclei 3 can be made uniform. Note that the entire surface of side 2a does not need to be a (10-11) plane, but it is preferable that 95% or more of the entire surface is a (10-11) plane. Furthermore, the (10-11) planes referred to here include planes that form an angle of -5 to 5 degrees with respect to the (10-11) planes as part of the (10-11) plane as an error.

[0039] The diameter D1 (diameter of the circumscribed circle in a plan view) of the frustum of the regular hexagonal pyramid of the seed crystal 2 is preferably 10 to 500 μm. Within this range, it is possible to grow a group III nitride semiconductor with fewer dislocations and warping. In addition, the area of ​​the side surface 2a of the frustum of the regular hexagonal pyramid of the seed crystal 2 can be increased, making it easier to grow the initial nuclei 3 from the side surface 2a. More preferably, it is 50 to 300 μm, and even more preferably 100 to 200 μm.

[0040] Furthermore, the height H1 of the seed crystal 2 is preferably 30 μm or more. Within this range, the area of ​​the side surface 2a can be sufficiently wide, and crystals can be grown uniformly from each side surface 2a. As a result, the shape of the initial nuclei 3 that grow from each type of crystal 2 can be made uniform. However, if H1 is too high, problems such as the formation of the seed crystal 2 taking a long time arise, so it is preferable to keep it at 100 μm or less. More preferably it is 20 to 60 μm, and even more preferably 30 to 50 μm.

[0041] Furthermore, for the same reasons as above, the height H1 of the seed crystal 2 is preferably 0.01 to 0.6 times the diameter D1 of the seed crystal 2. More preferably 0.1 to 0.35 times, and even more preferably 0.15 to 0.3 times.

[0042] A recess 2d is provided in the center of the seed crystal 2. By providing the recess 2d, the initial nuclei 3 grown from the seed crystal 2 do not fill the recess 2d, and voids 7 are formed. The formation of voids 7 prevents dislocations in the seed crystal 2 from propagating upwards, enabling the growth of high-quality GaN single crystals.

[0043] The bottom surface 2b of the recess 2d is flat and is the (0001) plane (c plane) of the group III nitride semiconductor. Furthermore, the bottom surface 2b is approximately circular in plan view. However, the bottom surface 2b does not need to be flat and may have irregularities. Also, the shape of the bottom surface 2b in plan view does not need to be circular.

[0044] The side surface 2c of the recess 2d has numerous irregularities and, overall, has a slope similar to that of the (10-11) plane. By giving side surface 2c such an irregular shape, side surface 2c becomes the starting point for crystal growth of the group III nitride semiconductor, making it easier to fill the upper part of the seed crystal 2 with the group III nitride semiconductor. Note that side surface 2c may also be a flat surface.

[0045] The depth H2 of the recess 2d is preferably 10 to 100 μm. This range makes it easier to form voids 7, allowing for the growth of higher quality group III nitride semiconductors. More preferably, it is 20 to 60 μm, and even more preferably 30 to 50 μm. Also, for similar reasons, the depth H2 of the recess 2d is preferably 0.3 to 1.0 times the height H1 of the seed crystal 2, and more preferably 0.6 to 0.8 times.

[0046] The diameter of the upper surface of the recess 2d is such that the seed crystal 2 has no upper surface, and the side surface 2c of the recess 2d and the side surface 2a of the seed crystal 2 are connected at an angle. Therefore, there is no c-plane on the upper surface of the seed crystal 2. The c-plane may melt back in the mixed melt of the Na flux method, which can cause variations in the shape of each initial nucleus 3. In addition, crystal growth from the c-plane may cause dislocations of the seed crystal 2 to propagate upwards. Therefore, by creating a shape in which there is no c-plane on the upper surface, it is possible to suppress variations in the shape of each initial nucleus 3 and suppress the upward propagation of dislocations of the seed crystal 2.

[0047] In the initial nucleation step S1, the seed substrate 9 can be fabricated, for example, as follows. First, a mask having a plurality of openings is formed on the substrate 1. The plurality of openings are arranged in a triangular grid pattern. The shape of the openings is circular. Other shapes such as regular hexagons are also acceptable, but it is preferable to use circles as in the embodiment in order to form a disc portion and suppress cracks when the substrate is peeled off. The material of the mask can be any material that can suppress the growth of group III nitride semiconductors on the mask, for example, SiO2.

[0048] Next, a buffer layer (not shown) and seed crystal 2 are selectively grown sequentially on the substrate exposed to the aperture using methods such as MOCVD or HVPE. Then, the mask is removed by melt-back with hydrofluoric acid or the like. The seed substrate 9 can be fabricated by the above steps.

[0049] Here, when selectively growing seed crystal 2 from the opening of the mask, the group III nitride semiconductor can be faceted by appropriately controlling the growth conditions, and the shape of seed crystal 2 can be made as shown in Figures 4 and 5. For example, the growth temperature can be set to 1120-1145°C and the V / III ratio to 970-1020. Furthermore, since the shape is determined by selective growth, the shapes of various crystals 2 can be made uniform.

[0050] In this embodiment 1, the initial nucleation process S1 can be carried out as follows. First, the furnace atmosphere is replaced with an inert gas, the furnace is heated, and then the furnace is evacuated to sufficiently reduce outgassing components such as oxygen in the furnace.

[0051] Next, predetermined amounts of Na and Ga are weighed in a glove box where the atmosphere, including oxygen and dew point, is controlled. Then, the weighed amounts of Na and Ga are placed into an empty crucible 100 (see Figure 2). If necessary, additive elements such as carbon may be added.

[0052] Next, the crucible 100 containing the raw materials and the seed substrate 9 are placed in the reaction vessel, and after vacuuming, a gas containing nitrogen is supplied to the reaction vessel. Once the pressure inside the reaction vessel reaches the crystal growth pressure, the furnace is heated to the crystal growth temperature. The crystal growth temperature is, for example, between 700°C and 1000°C, and the crystal growth pressure is, for example, between 2 MPa and 10 MPa. During the heating process, the solid Na and Ga in the crucible 100 melt into liquids, forming a mixed melt 101. At this stage, the seed substrate 9 is not yet added to the mixed melt 101.

[0053] When the reaction vessel reaches the crystal growth temperature and pressure, and the nitrogen dissolved in the mixed molten liquid 101 becomes supersaturated, the seed substrate 9 is added to the mixed molten liquid 101 in the crucible 100. Then, GaN crystals (initial nuclei 3) begin to grow from the various crystals 2 of the seed substrate 9. The growth of the initial nuclei 3 continues until adjacent initial nuclei 3 begin to fuse together (see Figure 6(b)). Note that a gap remains between the initial nuclei 3 and the substrate 1.

[0054] Here, the (10-11) plane, which is the side surface 2a of the frustum of the regular hexagon of the seed crystal 2, exists stably in the mixed melt 101 without melting back. Also, the height H1 of the seed crystal 2 is 30 μm or more, and the side surface 2a has a sufficiently large area. Therefore, the initial nuclei 3 grow from the side surface 2a while maintaining the (10-11) plane. Since the shapes of the various crystals 2 are uniformly consistent and they grow uniformly from the seed crystal 2 while maintaining the (10-11) plane, variations in the shape of each initial nucleus 3 can be suppressed, and the shapes of each initial nucleus 3 can be made uniform.

[0055] Furthermore, because a recess 2d is formed in the center of the seed crystal 2, the initial nucleus 3 does not completely fill the recess 2d, and a void 7 is formed. The mixed molten liquid 101 is trapped inside the void 7. Because the void 7 is formed above the seed crystal 2, the transfer of dislocations from the seed crystal 2 to the upper part can be suppressed.

[0056] Furthermore, by making the diameter of the recess 2d wider, the seed crystal 2 has a shape in which there is no top surface (c-face). The c-face is an unstable surface that can be melted back in the mixed melt 101. Since there is no crystal growth from such an unstable surface, the variation in the shape of each initial nucleus 3 can be further suppressed. In addition, since there is no crystal growth from the c-face, the transfer of dislocations from the seed crystal 2 to the upper surface can be further suppressed.

[0057] 3-2. Flattening process S2 As shown in Figure 1, a planarization step S2 is performed after the initial nucleation step S1. The planarization step S2 in Embodiment 1 is a step in which crystal growth is performed using the FFC (flux film coating) method, in which a seed substrate 9 on which initial nuclei 3 have been formed is immersed in a mixed molten liquid stored in a crucible 100, and then repeatedly heated under a nitrogen atmosphere, thereby growing GaN single crystals from the initial nuclei 3 and filling the spaces between adjacent initial nuclei 3 with GaN single crystals to planarize the crystal plane.

[0058] In the FFC method in the planarization step S2 of this embodiment 1, as shown in Figure 3, the seed substrate 9 is repeatedly removed from the mixed melt 101 at predetermined intervals, or immersed in the mixed melt 101 as shown in Figure 2. As shown in Figure 6(b), when adjacent initial nuclei 3 begin to fuse together, depressions 4 are formed on the fused surface. When the seed substrate 9 is removed from the mixed melt 101, the mixed melt 101 accumulates in the depressions 4 between adjacent initial nuclei 3. This allows the crystals 5 to grow along the depressions 4 (see Figure 6(c)).

[0059] Here, because the mixed molten liquid 101 accumulated in the depression 4 is thin, it easily becomes supersaturated with nitrogen. Therefore, the rate of crystal growth can be accelerated. On the other hand, because the amount of accumulated mixed molten liquid 101 is small, the amount of Ga is also small, and crystal growth stops after a while. Therefore, as shown in Figure 2, the seed substrate 9 is immersed in the mixed molten liquid 101 again, and as shown in Figure 3, the seed substrate 9 is removed from the mixed molten liquid 101, thereby intermittently supplying the depression 4 with mixed molten liquid 101 containing Ga. The FFC method is continued until the depression 4 is filled by the growth of crystal 5. This makes it possible to grow crystals with flat c-planes.

[0060] 3-3.Thickening process S3 As shown in Figure 1, the thickening process S3 is performed after the planarization process S2. The thickening process S3 is a process in which a GaN substrate, which is a seed substrate 9 having a planarized crystal surface formed in the planarization process S2, is immersed in a mixed melt of Ga and Na 101 stored in a crucible 100 to form a thickened GaN single crystal 6 on the GaN substrate (seed substrate 9).

[0061] In the thickening process S3 of Embodiment 1, as shown in Figure 4, the seed substrate 9, which has a flat crystal surface, is immersed in the mixed melt 101 while being supported by the substrate support part 210, and the alumina plate 10 is immersed in the mixed melt 101 while being supported by the melt addition part 220. Here, as mentioned above, the alumina plate 10 contains Ca, so in the thickening process S3, Ca dissolved from the alumina plate 10 is added to the mixed melt 101. Therefore, in Embodiment 1, the melt addition process is carried out together with the thickening process S3. As a result, the inclusion of Ca in the mixed melt 101 in the thickening process S3 promotes the lateral growth of the GaN single crystal 6.

[0062] On the other hand, in the thickening process S3, the Ca content in the mixed melt 101 is less than 0.05 mol%, and in the initial nucleation process S1 and the planarization process S2, the Ca content is even lower. As a result, the formation of miscellaneous crystals other than GaN single crystals 6 is suppressed in each process. When using a large-capacity crucible 10, in the thickening process S3, instead of immersing the alumina plate 10 in the mixed melt 101, Ca may be directly added to the mixed melt 101 so that the Ca content in the mixed melt 101 is less than 0.05 mol%.

[0063] Once the GaN single crystal 6 has grown to the desired thickness, the temperature is lowered to room temperature, and the pressure is also lowered to atmospheric pressure to terminate the growth of the GaN single crystal 6. The duration of the thickening process S3 can be appropriately set according to the desired thickness of the GaN single crystal 6. At this point, the gap between the initial nucleus 3 and the substrate 1 remains unfilled. Therefore, the substrate 1 can be naturally peeled off during cooling due to the difference in thermal expansion coefficients.

[0064] As described above, according to the GaN single crystal growth method in Embodiment 1, the side surface of the seed crystal 2 is composed of (10-11) planes. Therefore, variations in the shape of each initial nucleus 3 can be suppressed, and a uniform, high-quality GaN single crystal 6 can be formed.

[0065] Furthermore, a recess 2d is provided in the center of the seed crystal 2, resulting in a shape with no upper surface. Therefore, during the growth of the initial nucleus 3, the upper part of the seed crystal 2 is not filled, and a void 7 is formed. As a result, the propagation of dislocations in the seed crystal 2 to the upper part can be suppressed, and a high-quality GaN single crystal 6 can be formed.

[0066] In this embodiment, the planarization step S2 based on the FFC method is not necessarily required, but it is preferable to perform the planarization step S2 in order to further improve the flatness of the crystal and further reduce warping.

[0067] 4. Effects The effects of the Group III nitride single crystal growth method in this embodiment 1 are described below. In the Group III nitride single crystal growth method in this embodiment 1, the mixed melt 101 does not contain alkali metals or alkaline earth metals other than Na in the initial nucleation step S1. Therefore, the generation of miscellaneous crystals is suppressed when forming the initial nuclei 3, and the desired initial nuclei 3 can be formed with high precision. Then, in the thickening step S3, alkali metals or alkaline earth metals other than Na are included in the mixed melt 101. As a result, the growth of the Group III nitride single crystal 6 from the initial nuclei 3 in the lateral direction is promoted, so the formation of macrosteps is suppressed and the formation of inclusions is suppressed. As a result, the crystal quality of the formed Group III nitride single crystal 6 can be improved.

[0068] Furthermore, in this embodiment 1, in the thickening process S3, the alumina plate 10, which is a component containing alkali metals other than Na and alkaline earth metals, is immersed in the mixed melt 101, thereby eluting alkali metals other than Na or alkaline earth metals from the alumina plate 10 into the mixed melt 101. This makes it easy to add trace amounts of alkali metals other than Na and alkaline earth metals to the mixed melt 101 in the thickening process S3.

[0069] Furthermore, in this embodiment 1, in the planarization step S2, the alumina plate 10, which is a component containing alkali metals other than Na and alkaline earth metals, is not immersed in the mixed melt 101. As a result, the formation of miscellaneous crystals is suppressed in the planarization step S2, and the crystal quality is improved.

[0070] In addition, in the planarization step S2, as shown in Figure 4, the alumina plate 10, which is a component containing alkali metals other than Na and alkaline earth metals, may be immersed in the mixed melt 101 to dissolve the alkali metals other than Na or alkaline earth metals into the mixed melt 101 from the alumina plate 10. In this case, the wettability of the mixed melt to the seed substrate 9 can be improved in the planarization step S2, thereby promoting the planarization of the crystal plane.

[0071] Furthermore, in this embodiment 1, the component containing alkali metals other than Na and alkaline earth metals is an alumina plate 10, which is an alumina component containing alkali metals other than Na and alkaline earth metals. As a result, the alumina plate 10 containing alkali metals other than Na and alkaline earth metals can be gently dissolved by immersion in the mixed melt 101, making it easy to add trace amounts of alkali metals other than Na and alkaline earth metals to the mixed melt 101 in the thickening process S3.

[0072] Furthermore, in this embodiment 1, the seed substrate 9 is supported by a jig 200 and is configured to be immersed in and withdrawn from the mixed melt 101 via the jig 200, while the alumina plate 10, which is a component containing alkali metals other than Na and alkaline earth metals, is supported by the melt addition section 220 of the jig 200. This makes it easy to control whether or not alkali metals other than Na and alkaline earth metals are added to the mixed melt 101 using the jig 200.

[0073] Furthermore, in this embodiment 1, along with the thickening step S3, a melt addition step is included in which alkali metals other than Na and alkaline earth metals are added to the mixed melt 101. In this case, it becomes easy to ensure that the mixed melt does not contain alkali metals other than Na and alkaline earth metals in the initial nucleation step, and to ensure that the mixed melt contains alkali metals other than Na and alkaline earth metals in the thickening step.

[0074] Furthermore, in this embodiment 1, in the thickening process S3, alkali metals other than Na or alkaline earth metals are mixed into the mixed melt 101 in a ratio of 0.01 mol% to less than 0.05 mol%. As a result, the proportion of alkali metals other than Na or alkaline earth metals contained in the mixed melt 101 in the thickening process S3 is extremely low, making it possible to suppress both inclusion formation and the formation of miscellaneous crystals.

[0075] Furthermore, in the jig 200 of this embodiment 1, the seed substrate 9 supported by the substrate support part 210 is immersed in the mixed melt 101 stored in the crucible 100, and the melt addition part 220 is not immersed in the mixed melt 101. The jig is configured to allow switching between these two states. This makes it easy to set the initial nucleation step S1 in which no alkali metals other than Na or alkaline earth metals are added to the mixed melt 101 from the melt addition part 220, and the thickening step S3 in which alkali metals other than Na or alkaline earth metals are added to the mixed melt 101. As a result, the crystal quality of the formed GaN single crystal 6 can be improved.

[0076] Furthermore, in this embodiment, the jig 200 has a melt addition section 220 that holds an alumina plate 10 which is a component containing an alkali metal or alkaline earth metal other than Na, and in the second state, the alkali metal or alkaline earth metal other than Na is dissolved from the alumina plate 10 into the mixed melt 101. This makes it easy to switch between the first state in which the mixed melt 101 does not contain an alkali metal or alkaline earth metal other than Na, and the second state in which the mixed melt 101 contains an alkali metal or alkaline earth metal other than Na.

[0077] In this embodiment, as shown in Figure 4, the jig 200 supports the alumina plate 10 in the melt addition section 220, and in the thickening process S3, alkali metals other than Na or alkaline earth metals are added to the mixed melt 101 from the alumina plate 10. Alternatively, as shown in the modified form 1 in Figure 10, the jig 200 holds alumina powder 11 containing alkali metals other than Na or alkaline earth metals in the melt addition section 220, and as shown in Figure 11, in the thickening process S3, alkali metals other than Na or alkaline earth metals are added from the alumina powder 11 held in the melt addition section 220 immersed in the mixed melt 101. Furthermore, when using a large-capacity crucible 100, instead of adding alkali metals other than Na or alkaline earth metals from the alumina powder 11 in the thickening process S3, alkali metals other than Na or alkaline earth metals are added directly to the mixed melt 101.

[0078] Furthermore, as shown in the modified form 2 in Figure 12, the melt addition section 220 may be constructed by forming a part of the second leg portion 202 of the jig 200 with an alumina material containing an alkali metal or alkaline earth metal other than Na. In this modified form 2, as shown in Figure 13, in the thickening process S3, an alkali metal or alkaline earth metal other than Na may be added from the alumina material constituting the melt addition section 220 which is immersed in the mixed melt 101.

[0079] Although not shown in the figures, instead of the jig 200 being equipped with a melt addition section 220, a melt addition step may be performed between the initial nucleation step S1 and the thickening step S3, in which an alumina material containing an alkali metal or alkaline earth metal other than Na is added to the mixed melt 101.

[0080] As described above, according to this embodiment and its modified form, it is possible to provide a method for growing group III nitride single crystals and a jig 200 for growing group III nitride single crystals that can improve crystal quality.

[0081] The present invention is not limited to the above-described embodiments and variations, and can be applied to various embodiments without departing from its spirit. [Explanation of Symbols]

[0082] 1: Circuit board 2: Seed crystal 2a: Side 2b: Bottom 2c: Side 2d: recessed 3: Initial nucleus 4: Indentation 5, 6: Crystal 9: Seed substrate

Claims

1. An initial nucleation step involves immersing a seed substrate, on which multiple seed crystals made of group III nitride single crystals are formed on its upper surface, in a mixed molten solution of group III metal and Na stored in a crucible, thereby growing group III nitride single crystals from the multiple seed crystals and forming multiple initial nuclei; A planarization step is performed by repeatedly immersing the seed substrate on which the initial nuclei have been formed in the mixed molten liquid stored in the crucible, then removing it and heating it under a nitrogen atmosphere, thereby growing group III nitride single crystals from the initial nuclei and filling the spaces between adjacent initial nuclei with group III nitride single crystals to flatten the crystal plane. A thick-film formation step involves immersing the seed substrate having a planarized crystal surface in the mixed molten liquid stored in a crucible to form a thick-film group III nitride single crystal on the seed substrate, Includes, In the initial nucleation step, the mixed melt does not contain alkali metals or alkaline earth metals other than Na. In the thickening step, the mixed melt contains an alkali metal or alkaline earth metal other than Na, in a method for growing a group III nitride single crystal.

2. In the thickening process, a member containing alkali metals other than Na and alkaline earth metals is immersed in the mixed melt, thereby eluting alkali metals other than Na or alkaline earth metals from the member into the mixed melt. The method for growing a group III nitride single crystal according to claim 1, wherein in the initial nucleation step, a member containing alkali metals other than Na and alkaline earth metals is not immersed in the mixed melt.

3. The method for growing a group III nitride single crystal according to claim 2, wherein in the planarization step, a member containing an alkali metal other than Na and an alkaline earth metal is immersed in the mixed melt to dissolve the alkali metal other than Na or an alkaline earth metal into the mixed melt from the member.

4. The method for growing a group III nitride single crystal according to claim 2, wherein in the planarization step, the member containing alkali metals other than Na and alkaline earth metals is not immersed in the mixed melt.

5. The method for growing a group III nitride single crystal according to any one of claims 2 to 4, wherein the member containing alkali metals other than Na and alkaline earth metals is an alumina member containing alkali metals other than Na and alkaline earth metals.

6. The aforementioned substrate is supported by a jig and is configured to be immersed in the mixed molten liquid and withdrawn from the mixed molten liquid via the jig. The method for growing a group III nitride single crystal according to claim 5, wherein the member containing alkali metals other than Na and alkaline earth metals constitutes part of the jig or is supported by the jig.

7. The method for growing a group III nitride single crystal according to claim 1, further comprising a melt addition step of adding alkali metals other than Na and alkaline earth metals to the mixed melt between the initial nucleation step and the thickening step, or together with the thickening step.

8. The method for growing a group III nitride single crystal according to any one of claims 1 to 4, wherein in the thickening step, the mixed melt contains an alkali metal or alkaline earth metal other than Na in a ratio of 0.01 mol% to less than 0.05 mol%.

9. A jig for growing a group III nitride single crystal, which supports a substrate for growing a group III nitride single crystal inside a crucible, A substrate support portion is provided for supporting the substrate and is configured to allow the substrate to be immersed in a mixed molten liquid of Group III metal and Na stored in the crucible. The device comprises a melt additive unit configured to add alkali metals other than Na or alkaline earth metals to the mixed melt when immersed in the mixed melt, A jig for growing a group III nitride single crystal, configured to allow switching between a first state in which the substrate supported by the substrate support is immersed in the mixed molten liquid and the molten liquid additive section is not immersed in the mixed molten liquid, and a second state in which the substrate supported by the substrate support and the molten liquid additive section are immersed in the mixed molten liquid.

10. The molten material addition section holds a member containing an alkali metal or alkaline earth metal other than Na, and in the second state, the alkali metal or alkaline earth metal other than Na is dissolved from the member containing the alkali metal or alkaline earth metal other than Na into the mixed molten material, as described in claim 9, for growing a group III nitride single crystal.

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