Group III nitride single crystal growth method

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 GaN single crystal on a GaN substrate 9, comprising a thickening step of forming a thick GaN single crystal on the GaN substrate 9 by repeatedly immersing the GaN substrate 9 in a mixed molten solution 101 of Ga and Na stored in a crucible 100, pulling it out, and then heating it under a nitrogen atmosphere. The crucible 100 is an alumina crucible mainly composed of alumina and containing 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.

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, three-dimensional growth of the GaN single crystal results in crystal growth that forms large steps (macrosteps) on the order of several μm to several hundreds of μm, called macrostep growth. In this macrostep growth region, lumps of the Ga-Na melt, called inclusions, are formed in the crystal, deteriorating 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 repeatedly heating the GaN substrate 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, macrostep growth is suppressed, so 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, the amount of alkaline earth metal added to the Ga-Na melt beforehand is 0.05 mol% or more, so a large amount of miscellaneous crystals other than GaN single crystals are formed during crystal growth. Therefore, there is room for improvement in order to improve the crystal quality of the GaN single crystal.

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

[0007] One aspect of the present invention is, A method for growing a group III nitride single crystal on a group III nitride substrate, The process includes a thick-film formation step in which the group III nitride substrate is repeatedly immersed in a mixed molten solution of group III metal and Na stored in a crucible, then withdrawn and heated under a nitrogen atmosphere, thereby forming a thick-film group III nitride single crystal on the GaN substrate. The crucible is an alumina crucible mainly composed of alumina and containing alkali metals or alkaline earth metals other than Na, in a method for growing group III nitride single crystals. [Effects of the Invention]

[0008] In the above embodiment of the method for growing a group III nitride single crystal, by storing a mixed molten mixture of a group III metal and Na in an alumina crucible containing an alkali metal or alkaline earth metal other than Na, a very small amount of the alkali metal or alkaline earth metal other than Na dissolves into the mixed molten mixture. This improves the wettability of the mixed molten mixture to the group III nitride substrate, suppressing the formation of inclusions. Furthermore, since the amount of alkali metal or alkaline earth metal other than Na in the mixed molten mixture is extremely small, the formation of miscellaneous crystals is also suppressed. As a result, both the suppression of inclusion formation and the suppression of miscellaneous crystal formation are achieved, thereby improving the crystal quality of the group III nitride single crystal.

[0009] As described above, according to the above embodiment, a method for growing a group III nitride single crystal that can improve crystal quality can be provided. [Brief explanation of the drawing]

[0010] [Figure 1] A flowchart illustrating the GaN single crystal growth method in Embodiment 1. [Figure 2] The first embodiment shows (a) a conceptual diagram of a seed substrate on which multiple seed crystals are formed, (b) a conceptual diagram of a seed substrate on which multiple initial nuclei are formed, (c) a conceptual diagram showing a state in which a GaN single crystal is formed between multiple initial nuclei, and (d) a conceptual diagram showing a state in which a GaN single crystal is grown on a planarized crystal plane. [Figure 3] A plan view showing the configuration of the seed substrate in Embodiment 1. [Figure 4] A cross-sectional view showing the structure of a seed crystal in Embodiment 1, wherein the cross-sectional view is perpendicular to the main surface of the substrate. [Figure 5] A plan view showing the configuration of the seed crystal in Embodiment 1. [Figure 6] (a) First conceptual diagram illustrating the initial nucleation process in Embodiment 1, and (b) Second conceptual diagram. [Figure 7] A conceptual diagram illustrating the planarization process in Embodiment 1. [Figure 8] A conceptual diagram illustrating the thick-film formation process in Embodiment 1. [Figure 9] A cross-sectional view perpendicular to the main surface of the substrate after the thickening process in Embodiment 1. [Modes for carrying out the invention]

[0011] Preferably, the mixed melt contains alkali metals or alkaline earth metals other than Na that dissolved from the crucible in the thick-film formation process 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 thick-film formation process is extremely low, it is possible to further suppress both inclusion formation and the formation of miscellaneous crystals.

[0012] In the thick-film formation process, it is preferable to set the immersion time, which is the time the group III nitride substrate is immersed in the mixed molten material, to within the range of 1 to 10 minutes, and the gas-phase heating time, which is the time the group III nitride substrate is heated in a nitrogen atmosphere after being removed from the mixed molten material, to within the range of 10 to 120 minutes. The immersion time is relatively short but sufficient to supply the mixed molten material to the upper surface of the group III nitride substrate, and the gas-phase heating time is sufficient to grow a single crystal of group III nitride using the mixed molten material supplied to the upper surface of the group III nitride substrate as the raw material. Therefore, in the thick-film formation process, it is possible to efficiently grow a single crystal of group III nitride while suppressing the formation of macrosteps.

[0013] The process includes an initial nucleation step in which a seed substrate, on which a plurality of seed crystals made of group III nitride single crystals are formed on its upper surface, is immersed in the mixed molten liquid stored in the crucible to grow group III nitride single crystals from the plurality of seed crystals and form a plurality of initial nuclei, and a planarization step in which the seed substrate on which the initial nuclei are formed is repeatedly immersed in the mixed molten liquid stored in the crucible and then heated in a nitrogen atmosphere to grow group III nitride single crystals from the initial nuclei and fill the spaces between adjacent initial nuclei with group III nitride single crystals to flatten the crystal plane, and in the thickening step, the seed substrate having the planarized crystal plane formed in the planarization step can be used as the group III nitride substrate. In this case, a thickened group III nitride single crystal with high crystal quality can be formed on the seed substrate.

[0014] Preferably, in the order of the initial nucleation step, the planarization step, and the thickening step, the ratio of an alkali metal or an alkaline earth metal other than Na in the mixed melt increases. In this case, in the initial nucleation step, even if the wettability of the mixed melt with respect to the seed substrate is improved, the influence on crystal formation is small. Therefore, by reducing the ratio of an alkali metal or an alkaline earth metal other than Na in the mixed melt, the formation of abnormal crystals in the initial nucleation step can be suppressed. Further, in the planarization step, by improving the wettability of the mixed melt with respect to the seed substrate, the mixed melt is likely to be retained on the seed substrate when the seed substrate is pulled up from the mixed melt, so that the planarization of the crystal surface can be promoted. Further, in the thickening step, by further improving the wettability of the mixed melt, it is possible to further achieve both suppression of inclusion formation and suppression of abnormal crystal formation.

[0015] (Embodiment 1) 1. Outline of the flux method The method for growing a group III nitride single crystal according to Embodiment 1 is a method for growing a group III nitride single crystal by the flux method. Thereby, a group III nitride semiconductor is manufactured. The flux method is a method in which a gas containing nitrogen is supplied and dissolved in a mixed melt containing an alkali metal serving as a flux and a group III metal as a raw material, and a group III nitride semiconductor is epitaxially grown in the liquid phase.

[0016] In this embodiment, the mixed melt contains Na as an alkali metal that serves as a flux. The group III metal as a raw material is at least one of gallium (Ga), aluminum (Al), and indium (In). The composition of the group III nitride single crystal formed can be controlled by the ratio, and GaN, AlN, InN, AlGaN, InGaN, AlGaInN, etc. can be formed. The present invention is particularly suitable for the formation of GaN, and Embodiment 1 is a method for growing a single crystal of GaN. Also, Na is used as a flux, and such a flux method is particularly called the Na flux method. The gas containing nitrogen is a gas of nitrogen molecules or a compound containing nitrogen as a constituent element such as ammonia, and a mixed gas thereof may be used. Further, the gas containing nitrogen may be mixed with an inert gas such as a noble gas.

[0017] 2. Configuration of crucible In Embodiment 1, an alumina crucible containing an alkali metal or alkaline earth metal other than Na and having alumina as a main component is used as a crucible for storing a mixed melt for growing a GaN single crystal. Examples of the crucible that can be used in this embodiment are given below. Note that the crucible used in this embodiment is not limited to the following configuration.

[0018] The crucible that can be used in this embodiment is a ceramic (sintered body of aluminum oxide) mainly composed of alumina and has high heat resistance and alkali resistance. It is advisable to provide a lid on the crucible. For the crucible, alumina grains with abnormal grain growth are observed on its inner wall surface, and those with a maximum particle size of the abnormally grown alumina grains of 10 μm or more are used. The particle size is defined as the diameter of the circumcircle of the particle. The number density of the abnormally grown alumina grains on the inner wall surface is approximately 10 to 10,000 grains / 1 mm square (the number of abnormally grown alumina grains per 1 mm square).

[0019] Here, abnormal grain growth refers to the growth of alumina grains that are larger in size than normal alumina grains that have not undergone abnormal growth (for example, grain sizes that are on the order of an order of magnitude larger). If the grain size of the alumina grains is uniform before sintering, the grain size of the alumina grains should also be uniform after sintering. However, if abnormal grain growth occurs, alumina grains that are larger than normal will be produced. When abnormal grain growth does not occur, the grain size of the alumina grains is approximately uniform and circular, and the grain size distribution is a bell-shaped distribution with one peak.

[0020] On the other hand, when abnormal grain growth occurs, alumina grains with larger diameters than those without abnormal growth are produced, and their shapes are not only circular but also closer to elongated ovals. Furthermore, the grain size distribution is such that there is a peak in the grain size of alumina grains without abnormal growth, and a peak in the grain size of alumina grains with abnormal growth at a position where the grain size is larger than the peak of alumina grains without abnormal growth. In addition, the grain size of alumina grains with abnormal growth is roughly uniform.

[0021] Furthermore, the crucible used is one that has been manufactured by casting using a plaster mold. Specifically, it is manufactured as follows: First, alumina powder, the raw material, and water are mixed to make a slurry (suspension), which is then poured into a plaster mold made of plaster. A wet jet mill treatment is used to produce the slurry. Then, it is left for a predetermined time to allow the plaster mold to absorb the moisture and deposit a certain thickness of alumina powder on the surface of the plaster mold. Next, the liquid slurry remaining in the plaster mold is discarded, leaving only the layer of alumina powder deposited on the surface of the plaster mold. The remaining layer of alumina powder is dried for a predetermined time to solidify the deposited layer of alumina powder and obtain a molded body. Then, the molded body is removed from the plaster mold and fired at 1570°C in air to sinter it, thereby manufacturing a crucible, which is a sintered alumina body.

[0022] In this manufacturing method, impurities such as Ca, C, and O dissolve from the gypsum mold into the slurry, and the molded body contains these impurities. As a result, the crucible of this embodiment 1 contains at least Ca, which is an alkaline earth metal. These impurities are considered to be a factor that causes abnormal grain growth during sintering. Note that impurities are components that have a sufficiently small content ratio compared to the main component, and the main component refers to the component that has the highest content ratio.

[0023] Therefore, when a crucible is formed by casting using a plaster mold, alumina grains that have undergone abnormal grain growth may be present. Thus, in Embodiment 1, multiple crucibles are produced by casting using a plaster mold, and from among these crucibles, one that satisfies the following conditions is selected for use. These conditions are that alumina grains that have undergone abnormal grain growth are present on the inner wall surface of the crucible, and the maximum particle size of these abnormally grown alumina grains is 10 μm or larger.

[0024] The particle size of alumina grains can be easily measured by microscopic observation. In particular, it is desirable to observe the bottom surface of the crucible's inner wall and measure the maximum particle size, as the bottom surface of the crucible is always in contact with the molten material. Furthermore, when selecting crucibles, it is not necessary to measure the particle size of alumina grains with abnormal growth on the entire inner wall surface of the crucible; it is sufficient to measure the particle size of alumina grains with abnormal growth that are present in a randomly selected predetermined range. For example, one can randomly select a range of 100 μm square or 1 mm square, search for alumina grains with abnormal growth within that range, measure their particle size, and select crucibles with a maximum particle size of 10 μm or more.

[0025] When GaN is grown using the Na flux method with such a crucible, macrostep growth can be suppressed, as described later, and the crystal quality of GaN can be improved. To further enhance the effect of suppressing macrostep growth, it is preferable to use alumina grains with a maximum particle size of 10 to 150 μm. More preferably, it is 40 to 100 μm.

[0026] Furthermore, it is desirable to use a crucible in which the number density of alumina grains that have undergone abnormal grain growth on the inner wall surface (especially the bottom surface) is 10 to 10,000 grains / 1 mm square. This is because macrostep growth is further suppressed, and GaN crystals with good crystallinity can be obtained. However, it is undesirable if the number density exceeds 10,000 grains / 1 mm square because it reduces the strength of the crucible. More preferably, it is 50 to 2,000 grains / 1 mm square, and even more preferably, 100 to 1,000 grains / 1 mm square.

[0027] In the above method, the alumina particles undergo abnormal growth naturally due to the incorporation of impurities such as Ca from the gypsum mold. However, abnormal growth can also be artificially induced by adding alkali metals other than Na or alkaline earth metals to the slurry. Li can be used as the alkali metal other than Na, and in addition to Ca, Ba, Mg, and Sr can be used as alkaline earth metals.

[0028] Furthermore, the various conditions for the casting process may be as follows: When preparing the slurry, it is preferable to use alumina powder with an average particle size of 0.1 to 1 μm. This makes the alumina particles in the crucible more dense, improving its strength.

[0029] Furthermore, although water was used as the solvent for the slurry, any solvent conventionally used for slurries, such as alcohols, ketones, or amines, may be used. However, water is preferred from the standpoint of solvent absorption into the mold, environmental impact, and the absence of solvent residue in the crucible. Distilled water is also preferred from the viewpoint of suppressing the inclusion of impurities.

[0030] Furthermore, while plaster molds are used, any material can be used as long as it can absorb the solvent of the slurry and allow impurities to be mixed into the slurry. Alternatively, if a mold made of a material that does not release many impurities into the slurry is used, impurities can be added to the slurry to artificially induce abnormal grain growth. However, it is preferable to use plaster molds for slip casting because they are inexpensive and allow for easy mixing of impurities into the slurry.

[0031] Furthermore, it is desirable to fire the molded body at a temperature of 1500-1600°C. This is to make the alumina grains dense and prevent them from becoming too large.

[0032] Furthermore, the slurry contains sintering aids to promote sintering, and alumina powder is evenly dispersed in the solvent. Dispersants may be added to achieve this. If a dispersant is added, a heat treatment to remove the dispersant from the molded body may be performed at a temperature lower than the sintering temperature of the molded body before firing.

[0033] Furthermore, if it is possible to cause abnormal grain growth in the alumina grains of the crucible, the crucible may be formed by a method other than casting.

[0034] 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.

[0035] 3-1. Initial nucleation step S1 The initial nucleation step S1 is a step in which a seed substrate 9 (see Figure 2(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 stored in the crucible, thereby growing GaN single crystals from the plurality of seed crystals and forming a plurality of initial nuclei 3. 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 melt-back of the seed crystals 2 on the seed substrate 9.

[0036] 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. Figures 2(a) to 2(d) are cross-sectional views of the seed substrate 9, which are cross-sections perpendicular to the main surface of the substrate. Figure 3 is a plan view of the seed substrate 9 seen from above.

[0037] 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.

[0038] 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.

[0039] 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.

[0040] 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.

[0041] Next, the shape of the seed crystal 2 will be described in detail. Figure 4 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 5 is a plan view showing the structure of the seed crystal 2. As shown in Figures 4 and 5, the seed crystal 2 has a disc-shaped disc portion and a regular frustum-shaped hexagonal pyramidal portion located in contact with the cylindrical portion, with a recess 2d in the center of the regular frustum-shaped hexagonal pyramidal portion.

[0042] 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.

[0043] 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.

[0044] 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.

[0045] 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.

[0046] 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.

[0047] 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.

[0048] 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.

[0049] 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.

[0050] 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.

[0051] 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.

[0052] 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.

[0053] 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.

[0054] 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.

[0055] 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.

[0056] In the initial nucleation process S1, the initial nucleus 3 can be formed as follows: First, the reactor atmosphere is replaced with an inert gas, the reactor is heated, and then the reactor is evacuated to sufficiently reduce outgassing components such as oxygen in the reactor.

[0057] 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 6). If necessary, additive elements such as carbon may be added.

[0058] 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.

[0059] 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.

[0060] 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.

[0061] 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.

[0062] 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.

[0063] 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.

[0064] In the FFC method in the planarization step S2 of this embodiment 1, as shown in Figure 7, the seed substrate 9 is repeatedly removed from the mixed molten liquid 101 and immersed in the mixed molten liquid 101 at predetermined intervals. When the seed substrate 9 is removed from the mixed molten liquid 101, it is slightly tilted relative to the horizontal plane. At the stage when adjacent initial nuclei 3 begin to fuse together, depressions 4 are formed on the fusion surface. When the seed substrate 9 is removed from the mixed molten liquid 101, the mixed molten liquid 101 accumulates in the depressions 4 between adjacent initial nuclei 3. This allows the crystals 5 to grow along the depressions 4 (see Figure 2(c)).

[0065] Here, because the mixed molten material 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 material 101 is small, the amount of Ga is also small, and crystal growth stops after a while. Therefore, the seed substrate 9 is immersed in the mixed molten material 101 again and then removed from the mixed molten material 101, thereby intermittently supplying the depression 4 with mixed molten material 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.

[0066] The duration of the planarization process S2 is not limited, but in this embodiment 1, the cycle of immersing and removing the seed substrate 9 from the mixed molten liquid 101 is repeated for 100 hours, with each set consisting of immersion for 1 minute, removal, and heating in a nitrogen atmosphere for 30 minutes, thereby filling the depressions 4. In the planarization process S2, the immersion time in one set is not limited and can be appropriately set within a range of approximately 1 to 10 minutes, and the heating time in a nitrogen atmosphere is also not limited and can be appropriately set within a range of approximately 10 to 120 minutes.

[0067] 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, and then repeatedly removed and heated under a nitrogen atmosphere to form a thickened GaN single crystal 6 on the GaN substrate (seed substrate 9).

[0068] In the thickening process S3 of Embodiment 1, as shown in Figure 8, the seed substrate 9, which has a flat crystal surface, is repeatedly immersed in the mixed melt 101 and removed from the mixed melt 101 at predetermined intervals. When the seed substrate 9 is removed from the mixed melt 101, it is slightly tilted with respect to the horizontal plane. Here, as mentioned above, the crucible 100 contains a trace amount of Ca as an impurity, so in the thickening process S3, the mixed melt 101 contains a very small amount of Ca dissolved from the crucible 100, and the Ca content ratio in the thickening process S3 is less than 0.05 mol%. The Ca content ratio in the mixed melt 101 that has dissolved from the crucible 100 increases with the passage of time after the mixed melt 101 is added to the crucible 100. Therefore, when comparing the content ratio of Ca dissolved from the crucible 100 in the mixed melt 101, the initial nucleation step S1 is smallest in the initial nucleation step S1, followed by the planarization step S2 and then the thickening step S3.

[0069] Furthermore, the wettability of the mixed melt 101 to the crystal plane of the seed substrate 9 depends on the content ratio of alkali metals or alkaline earth metals other than Na in the mixed melt 101, and the higher the content ratio, the higher the wettability. Therefore, in the thick-film formation process S3, the wettability of the mixed melt 101 to the crystal plane of the seed substrate 9 is relatively high. As a result, when the seed substrate 9 immersed in the mixed melt 101 is removed from the mixed melt 101, a thin layer of the mixed melt 101 is formed on the flat crystal plane of the seed substrate 9, as shown in Figure 8.

[0070] 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 the GaN single crystal 6 is suppressed in each process.

[0071] As shown in Figure 8, the thin layer of mixed melt 101 formed on the flat crystal plane of the seed substrate 9 is thin, so nitrogen tends to become supersaturated, similar to the planarization process S2. Therefore, the rate of crystal growth can be accelerated. On the other hand, because the amount of this thin layer is small, the amount of Ga is also small, and crystal growth stops after a while. Therefore, the seed substrate 9 is immersed in the mixed melt 101 again and the substrate is removed from the mixed melt 101, thereby intermittently supplying the mixed melt 101 containing Ga to the flat crystal plane. By repeating this, the GaN single crystal 6 is grown and thickened (see Figure 2(c)). Because the shape of each initial nucleus 3 is uniform, the GaN single crystal 6 can also be formed uniformly within the plane. In addition, since the transfer of dislocations from the seed crystal 2 to the upper part is suppressed, the GaN single crystal 6 can be formed with high quality.

[0072] In the thickening process S3, the Ca content in the mixed melt 101 is less than 0.05 mol%, the immersion time, which is the time the GaN substrate (seed substrate 9) is immersed in the mixed melt 101, is, for example, within the range of 1 to 10 minutes, and the gas phase heating time, which is the time the GaN substrate (seed substrate 9) is heated in a nitrogen atmosphere after being removed from the mixed melt 101, can be, for example, within the range of 10 to 120 minutes, and the above immersion of the GaN substrate (seed substrate 9) in the mixed melt 101 and the above gas phase heating can be repeated as one set for a predetermined time.

[0073] In this embodiment 1, the thickening process S3 involved a Ca content ratio of 0.01 mol% to less than 0.05 mol% in the mixed melt 101, an immersion time of 1 minute for immersing the GaN substrate (seed substrate 9) in the mixed melt 101, and a gas-phase heating time of 30 minutes for heating under a nitrogen atmosphere after removing the GaN substrate (seed substrate 9) from the mixed melt 101. This process of immersion in the mixed melt 101 and gas-phase heating was repeated as one set for 50 hours. As a result, it was confirmed that a large number of layers of GaN single crystals 6, each approximately 1 μm thick, were stacked in the region indicated by the symbol A in Figure 9, forming a GaN single crystal 6 with a thickness of approximately 100 μm.

[0074] The duration of the thickening process S3 can be appropriately set according to the desired thickness of the GaN single crystal 6. Furthermore, the immersion time per set and the heating time under a nitrogen atmosphere in the thickening process S3 can also be appropriately set.

[0075] 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. At this point, the gap between the initial nucleus 3 and the substrate 1 remains unfilled. Therefore, the difference in thermal expansion coefficients allows the substrate 1 to be naturally delaminated during the cooling process.

[0076] 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.

[0077] 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.

[0078] 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.

[0079] 3. Effects The effects of the Group III nitride single crystal growth method in this embodiment 1 are described below. According to the Group III nitride single crystal growth method of this embodiment 1, by storing a mixed molten mixture 101 of Ga and Na, which are Group III metals, in an alumina crucible 100 containing an alkali metal or alkaline earth metal other than Na, an extremely small amount of the alkali metal or alkaline earth metal other than Na dissolves into the mixed molten mixture 101. This improves the wettability of the mixed molten mixture 101 to the GaN substrate (seed substrate 9), and suppresses the formation of inclusions. Furthermore, since the amount of alkali metal or alkaline earth metal other than Na contained in the mixed molten mixture 101 is extremely small, the formation of miscellaneous crystals is also suppressed. As a result, both the suppression of inclusion formation and the suppression of miscellaneous crystal formation are achieved, thus improving the crystal quality of the GaN single crystal 6.

[0080] Furthermore, in this embodiment 1, the mixed melt 101 contains alkali metals or alkaline earth metals other than Na that have dissolved from the crucible 100 in the thickening process S3, in a ratio of 0.01 mol% to less than 0.05 mol%. As a result, the proportion of alkali metals or alkaline earth metals other than Na contained in the mixed melt 101 in the thickening process S3 is extremely low, which further enhances the suppression of inclusion formation and the suppression of miscellaneous crystal formation.

[0081] Furthermore, in this embodiment 1, in the thick-film formation process S3, the immersion time, which is the time the GaN substrate 9 is immersed in the mixed molten liquid 101, is set to a range of 1 to 10 minutes, and the gas-phase heating time, which is the time the GaN substrate 9 is heated in a nitrogen atmosphere after being removed from the mixed molten liquid 101, is set to a range of 10 to 120 minutes. The above immersion time is relatively short but is sufficient to supply the mixed molten liquid to the upper surface of the GaN substrate, and the above gas-phase heating time is sufficient to grow GaN single crystals 6 using the mixed molten liquid supplied to the upper surface of the GaN substrate as raw material. Therefore, in the thick-film formation process S3, the growth of GaN single crystals 6 can be efficiently carried out while suppressing the formation of macrosteps.

[0082] Furthermore, in this embodiment 1, the process includes an initial nucleation step S1 in which a seed substrate 9, on which a plurality of seed crystals 2 made of GaN single crystals are formed on its upper surface, is immersed in a mixed melt 101 stored in a crucible 100 to grow GaN single crystals from the plurality of seed crystals 2 and form a plurality of initial nuclei 3, and a planarization step S2 in which the seed substrate 9 on which the initial nuclei 3 are formed is repeatedly immersed in the mixed melt 101 stored in the crucible 100, pulled out, and heated under a nitrogen atmosphere to grow GaN single crystals from the initial nuclei 3 and fill the spaces between adjacent initial nuclei 3 with GaN single crystals to flatten the crystal plane. Then, in the thickening step S3, the seed substrate 9 having the planarized crystal plane formed in the planarization step S2 is used as the GaN substrate. As a result, a thickened GaN single crystal 6 with high crystal quality can be formed on the seed substrate 9.

[0083] Furthermore, in this embodiment 1, the proportion of alkali metals other than Na or alkaline earth metals in the mixed melt 101 increases in the order of initial nucleation step S1, planarization step S2, and thickening step S3. As a result, even if the wettability of the mixed melt 101 with respect to the seed substrate 9 is improved in the initial nucleation step S1, the effect on crystal formation is small, and by lowering the proportion of alkali metals other than Na or alkaline earth metals in the mixed melt 101, the formation of miscellaneous crystals in the initial nucleation step S1 can be suppressed. In addition, by improving the wettability of the mixed melt 101 with respect to the GaN substrate (seed substrate 9) in the planarization step S2, the mixed melt 101 is more easily retained on the GaN substrate (seed substrate 9) when the GaN substrate (seed substrate 9) is pulled out from the mixed melt 101, thereby promoting crystal plane planarization. Furthermore, in the thickening step S3, by further improving the wettability of the mixed melt 101, it is possible to further suppress both inclusion formation and miscellaneous crystal formation.

[0084] As described above, this embodiment and its modified form provide a method for growing group III nitride single crystals that can improve crystal quality.

[0085] The present invention is not limited to the embodiments and variations described above, and can be applied to various embodiments without departing from its spirit. [Explanation of Symbols]

[0086] 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. A method for growing a group III nitride single crystal on a group III nitride substrate, The process includes a thick-film formation step in which the group III nitride substrate is repeatedly immersed in a mixed molten solution of group III metal and Na stored in a crucible, then withdrawn and heated under a nitrogen atmosphere, thereby forming a thick-film group III nitride single crystal on the group III nitride substrate. The crucible is an alumina crucible mainly composed of alumina and containing alkali metals or alkaline earth metals other than Na, in a method for growing a group III nitride single crystal.

2. The method for growing a group III nitride single crystal according to claim 1, wherein the mixed melt contains alkali metals or alkaline earth metals other than Na that have dissolved from the crucible in the thick-film formation step in a ratio of 0.01 mol% to less than 0.05 mol%.

3. The method for growing a group III nitride single crystal according to claim 1 or 2, wherein in the thick-film formation step, the immersion time, which is the time for immersing the group III nitride substrate in the mixed melt, is within the range of 1 to 10 minutes, and the gas-phase heating time, which is the time for heating the group III nitride substrate in a nitrogen atmosphere after removing it from the mixed melt, is within the range of 10 to 120 minutes.

4. 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 the mixed molten liquid stored in the 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. Includes, The method for growing a group III nitride single crystal according to claim 1 or 2, wherein in the thickening step, the seed substrate having a planarized crystal plane formed in the planarization step is used as the group III nitride substrate.

5. The method for growing a group III nitride single crystal according to claim 4, wherein the proportion of alkali metals or alkaline earth metals other than Na in the mixed melt increases in the order of the initial nucleation step, the planarization step, and the thickening step.

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