Group III nitride single crystal growth method

Abstract

This invention provides a method for growing group III nitride single crystals with high crystal quality using an MPS substrate. [Solution] A method for growing a group III nitride single crystal, comprising: an alkali metal melt preparation step S1 of preparing an alkali metal melt from which oxygen atoms have been removed; a seed substrate preparation step S2 of preparing a seed substrate on which a plurality of seed crystals made of a group III nitride semiconductor are formed in a dot pattern; an initial nucleation step S3 of forming initial nuclei on the seed crystals by bringing a mixture of the alkali metal melt and a melt of a group III metal into contact with the surface of the seed crystals in a nitrogen-containing atmosphere; and a crystal growth step S4 of growing a group III nitride single crystal from the initial nuclei by bringing the mixture into contact with the initial nuclei in a nitrogen-containing atmosphere.

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 single crystal is grown on a GaN substrate by immersing the GaN substrate in a mixed melt of Ga and Na stored in a crucible. However, if the mixed melt contains oxygen atoms, polycrystals are generated inside the crucible, resulting in a decrease in the crystallinity of the GaN single crystal. In order to suppress such a decrease in crystallinity, Patent Document 1 discloses a configuration in which a liquid Na material serving as a raw material for the mixed melt is circulated between a high-temperature state and a low-temperature state by a Na circulation device, and oxygen atoms are precipitated and removed as Na2O.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] However, Patent Document 1 does not disclose a configuration for growing a GaN single crystal with high crystal quality from various crystals using an MPS (multi-point seed) substrate, which is a substrate in which a plurality of dot-shaped seed crystals are periodically arranged. Further, when using an MPS substrate, new problems occur in that the size and shape of nuclei generated on the seed crystals on the MPS substrate become non-uniform, and cracks and defects occur in the initial nuclei and group III nitride single crystals grown on the initial nuclei, but Patent Document 1 does not disclose anything about this point. On the other hand, it is conceivable to increase the supersaturation of N (nitrogen) in the mixed solution by lowering the temperature and increasing the pressure to promote nucleation on the seed crystals on the MPS substrate, but in this case, new problems occur in that polycrystals are generated and the crystal quality deteriorates.

[0005] This invention has been made in view of the above problems, and aims to provide a method for growing a group III nitride single crystal that can form a group III nitride single crystal with high crystal quality using an MPS substrate. [Means for solving the problem]

[0006] One aspect of the present invention is, A method for growing single crystals of group III nitrides, The alkali metal melt preparation process involves preparing an alkali metal melt from which oxygen atoms have been removed, A seed substrate preparation step involves preparing a seed substrate in which multiple seed crystals made of a group III nitride semiconductor are formed in a dot pattern on a substrate, An initial nucleation step is performed by bringing a mixture of the alkali metal molten liquid and the group III metal molten liquid into contact with the surface of the seed crystal in a nitrogen-containing atmosphere to form an initial nucleus on the seed crystal. A crystal growth step in which a group III nitride single crystal is grown from the initial nucleus by bringing the mixture into contact with the initial nucleus in a nitrogen-containing atmosphere, This relates to a single crystal growth method for group III nitrides. [Effects of the Invention]

[0007] In the above embodiment of the method for growing a group III nitride single crystal, an MPS substrate is used in which a group III nitride semiconductor is formed in the shape of multiple dots on the substrate. The group III nitride semiconductor on the MPS substrate is used as a seed crystal, and a mixture of an alkali metal molten liquid from which oxygen atoms have been removed and a group III metal molten liquid is brought into contact with the surface of the seed crystal to form initial nuclei on the seed crystal, after which a group III nitride single crystal is grown from the initial nuclei. As a result, because an alkali metal molten liquid from which oxygen atoms have been removed is used, the degree of supersaturation of N (nitrogen) in the mixture can be increased without lowering the temperature and pressure, and nucleation on the seed crystal on the MPS substrate can be promoted while suppressing the generation of miscellaneous crystals. As a result, the size and shape of the generated initial nuclei can be made uniform, so that cracks and defects are suppressed in the initial nuclei and the group III nitride single crystal grown on the initial nuclei, and the crystal quality can be improved.

[0008] As described above, according to the above embodiment, a method for growing a group III nitride single crystal that can form a group III nitride single crystal with high crystal quality using an MPS substrate can be provided. [Brief explanation of the drawing]

[0009] [Figure 1] A flowchart illustrating the method for growing a group III nitride single crystal in Embodiment 1. [Figure 2] A conceptual diagram showing the configuration of the sodium circulation device in Embodiment 1. [Figure 3] 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 4] A plan view showing the configuration of the seed substrate in Embodiment 1. [Figure 5] 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 6] A plan view showing the configuration of the seed crystal in Embodiment 1. [Figure 7] A conceptual diagram showing the state in which the seed substrate is not immersed in the mixed molten material in Embodiment 1. [Figure 8] A conceptual diagram showing the state in which a seed substrate has been added to the mixed molten material in Embodiment 1. [Figure 9] A top view of the jig and crucible in Embodiment 1. [Figure 10] (a) A diagram showing the growth state of the initial nuclei in the test example, and (b) A diagram showing the growth state of the initial nuclei in the comparative example. [Modes for carrying out the invention]

[0010] In the alkali metal melt preparation step, it is preferable to circulate the alkali metal melt between a first region where it is maintained at a first temperature and a second region where it is maintained at a second temperature lower than the first temperature, thereby removing oxygen atoms from the alkali metal melt in the second region. In this case, oxygen atoms can be efficiently removed from the alkali metal melt.

[0011] In the alkali metal melt preparation step, it is preferable to control the concentration of oxygen atoms in the alkali metal melt by controlling the second temperature. In this case, the concentration of oxygen atoms in the alkali metal melt can be maintained at a low level, which further promotes nucleation of seed crystals on the MPS substrate while suppressing the generation of miscellaneous crystals, and further promotes the uniformity of the size and shape of the initial nuclei that are generated. As a result, further improvement in crystal quality can be achieved.

[0012] Preferably, the second temperature is controlled within a range of 120°C to 300°C, and the first temperature is controlled to a temperature higher than the second temperature. In this case, the fluidity of the alkali metal melt can be maintained while further suppressing the generation of miscellaneous crystals, thereby further improving the crystal quality of the formed group III nitride single crystal.

[0013] The oxygen concentration in the alkali metal melt is preferably 30 ppm or less. In this case, since the oxygen concentration in the alkali metal melt is sufficiently small, the generation of twinned crystals can be further suppressed, and the crystal quality of the grown group III nitride single crystal can be further improved.

[0014] In the seed substrate preparation step, the diameter of the seed crystal formed on the substrate is preferably in the range of 30 to 300 μm. In this case, a group III nitride single crystal with few dislocations and warpage can be formed, the area of the side surface of the seed crystal can be widened, and the growth of primary nuclei from the side surface can be facilitated. Further, by setting the diameter of the seed crystal within the above range, after the growth of the group III nitride single crystal, the group III nitride single crystal and the seed crystal portion are easily separated, so that the separation of the seed crystal portion including the MPS substrate from the group III nitride single crystal becomes easy.

[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 serving as a raw material, and a group III nitride semiconductor is epitaxially grown in the liquid phase.

[0016] The method for growing a group III nitride single crystal according to Embodiment 1 includes an alkali metal melt preparation step S1, a seed substrate preparation step S2, an initial nucleus formation step S3, and a crystal growth step S4, as shown in FIG. 1. The crystal growth step S4 includes a planarization step S41 and a thickening step S42. Hereinafter, each step will be described in detail.

[0017] 2. Alkali metal melt preparation step S1 The alkali metal melt preparation step S1 involves preparing an alkali metal melt from which oxygen atoms have been removed. The alkali metal melt is obtained by melting an alkali metal that will serve as a flux. The alkali metal can be Na, Li, K, etc., and in this embodiment, Na is used as the alkali metal.

[0018] In this embodiment, in the alkali metal melt preparation step S1, as shown in Figure 2, the alkali metal melt is circulated between a first region 201 maintained at a first temperature and a second region 202 maintained at a second temperature lower than the first temperature. The circulation of the alkali metal melt can be performed by the Na circulation device 200 shown in Figure 2. The Na circulation device 200 is a device that circulates Na while controlling the purity of Na. The Na circulation device 200 is connected to the glove box 300. Therefore, the Na material whose purity has been increased by the Na circulation device 200 can be supplied to the inside of the glove box 300. The Na material is supplied to the glove box 300, where the dew point and atmosphere are controlled. Therefore, the Na material supplied to the glove box 300 is suppressed from reacting with oxygen and moisture.

[0019] The Na circulation device 200 includes a supply tank 210, a dump tank 220, a cold trap 230, an electromagnetic pump 240, an expansion tank 250, a metering tank 260, a Na collection port 270, and piping 281, 282, and 283. In this embodiment, the first region 201 includes the electromagnetic pump 240, the expansion tank 250, the metering tank 260, and piping 281, 282, and 283, and the second region 202 includes the cold trap 230. The Na circulation device 200 also includes other piping, a valve, and a heating device 290 for heating each part.

[0020] The Na circulation device 200 has a circulation path LP1 for flowing Na material in a liquid state. The circulation path LP1 includes a cold trap 230, an expansion tank 250, an electromagnetic pump 240, and piping 281, 282, and 283.

[0021] The supply tank 210 is a tank for supplying the initial Na material to the Na circulation device 200. The initial Na material is of a relatively high purity, but contains trace amounts of impurities. The initial Na material is solid. Because the supply tank 210 is heated, the Na material becomes liquid. The liquid Na material is then sent to the dump tank 220.

[0022] The dump tank 220 is designed to absorb reflected shock waves.

[0023] The cold trap 230 is designed to remove impurities from the sodium material. The cold trap 230 also functions as a sodium purity control unit, removing or adding impurities such as oxygen. Details of the cold trap 230 will be described later.

[0024] The electromagnetic pump 240 is used to return the Na material from the expansion tank 250 to the cold trap 230. The Na circulation device 200 purifies the Na material by circulating it between the cold trap 230 and the expansion tank 250.

[0025] The expansion tank 250 is a sodium storage section for temporarily storing sodium material from which impurities have been removed by the cold trap 230.

[0026] The weighing tank 260 is used to measure the amount of sodium material to be extracted from the sodium extraction port 270. The purity of the sodium material stored in the weighing tank 260 is sufficiently high.

[0027] The Na extraction port 270 is a supply port for supplying the purified Na material to the glove box 300.

[0028] The heating device 290 is for heating each part of the Na circulation device 200 to a predetermined temperature. The heating device 290 includes a first heating device 291 and a second heating device 292. The first heating device 291 can maintain a first temperature, which is the temperature of the first region 201, at a set temperature, and the second heating device 292 can maintain a second temperature, which is the temperature of the second region 202, at a set temperature.

[0029] 2-1. Operation of the Na circulation device In the alkali metal melt preparation step S1, first, solid Na material is supplied to the supply tank 210 of the Na circulation device 200. The Na material is heated in the supply tank 210 and becomes liquid Na material. The liquid Na material is sent to the dump tank 220. Then, the liquid Na material is gradually sent from the dump tank 220 to the circulation path LP1. The liquid Na material circulates through the circulation path LP1.

[0030] Here, the higher the temperature of the Na material, the higher the solubility of oxygen in the Na material. The cold trap 230 in the second region 202 is the coldest point in the circulation path LP1. Therefore, for example, Na2O precipitates at the cold trap 230. The Na2O can be removed by a filter or the like.

[0031] As the liquid Na material circulates through the circulation path LP1, it alternates between two states: a high-temperature state in the first region 201 and a low-temperature state in the second region 202. Therefore, Na2O precipitates during the low-temperature state, repeatedly removing oxygen from the liquid Na material. As a result, a highly pure Na material is purified. The Na material supplied by the Na extraction port 270 is liquid. The liquid Na material is poured into a container inside the glove box 300. The liquid Na material cools in the container and becomes solid.

[0032] 2-2. Temperature settings for the first region 201 and the second region 202 As described above, the temperature of the first region 201 is defined as the first temperature, and the temperature of the second region is defined as the second temperature, with the second temperature being lower than the first temperature. Both the first and second temperatures are higher than the melting point of Na (approximately 98°C) and lower than the boiling point of Na (890°C). The second temperature can be between 120°C and 300°C, preferably between 150°C and 180°C. If the second temperature is below 120°C, it is close to the melting point of Na, which is undesirable because it reduces the fluidity of the Na material. If the second temperature exceeds 300°C, the solubility of oxygen in Na increases, making it difficult for Na2O to precipitate, and reducing the amount of oxygen removed from the Na material. The first temperature of the first region 201 can be any temperature that is higher than or equal to the second temperature and lower than the boiling point of Na.

[0033] As mentioned above, the solubility of oxygen in the Na material depends on the temperature of the Na material. Therefore, the concentration of oxygen atoms in the reaction solution made of Na material can be controlled by controlling the second temperature. For example, by setting the second temperature between 120°C and 300°C, the oxygen concentration in the reaction solution can be reduced to 30 ppm or less. By setting the second temperature between 120°C and 180°C, the oxygen concentration in the reaction solution can be reduced to 10 ppm or less. By setting the second temperature between 120°C and 150°C, the oxygen concentration in the reaction solution can be reduced to 3 ppm or less. Therefore, it is preferable to set the second temperature between 150°C and 180°C, which allows for a lower oxygen concentration in the reaction solution while maintaining the fluidity of the Na material.

[0034] In this embodiment, the expansion tank 250, which forms the first region 201, and the surrounding piping 281, 282, and 283 are maintained at a first temperature of 200°C to 300°C by the first heating device 291, and the cold trap 230, which forms the second region 202, is maintained at a second temperature of 150°C to 180°C by the second heating device 292. This allows oxygen atoms in the Na material to be removed by the cold trap 230. Other impurities can also be removed. The Na material from which oxygen atoms and other impurities have been removed is supplied from the Na sampling port 270 to the glove box 300 and prepared as a reaction solution, thus completing the alkali metal melt preparation step S1. The inside of the glove box 300 is supplied with Ar gas and is slightly above 1 atmosphere.

[0035] 3. Seed substrate preparation process S2 In the seed substrate preparation step S2 shown in Figure 1, the seed substrate 9 shown in Figure 3(a) is prepared. The seed substrate 9 is an MPS (multipoint seed) substrate, in which a plurality of dot-shaped seed crystals 2 are periodically arranged on the substrate 1. Figure 3(a) is a cross-sectional view of the seed substrate 9, which is a cross-section perpendicular to the main surface of the substrate. Figure 4 is a plan view of the seed substrate 9 seen from above.

[0036] 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.

[0037] 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.

[0038] 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.

[0039] 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.

[0040] Next, the shape of the seed crystal 2 will be described in detail. Figure 5 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 6 is a plan view showing the structure of the seed crystal 2. As shown in Figures 5 and 7, 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.

[0041] 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.

[0042] 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.

[0043] 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.

[0044] In this embodiment, the diameter D1 of the seed crystal 2 is defined as the diameter D1 of the frustum hexagonal portion of the seed crystal 2 (the diameter of the circumscribed circle in a plan view), and the diameter D1 of the seed crystal 2 is preferably 30 to 300 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 hexagonal portion of the seed crystal 2 can be increased, making it easier to grow the initial nuclei 3 from the side surface 2a. The diameter D1 of the seed crystal 2 is more preferably 100 to 200 μm.

[0045] 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.

[0046] 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.

[0047] A recess 2d is provided in the center of the seed crystal 2. By providing the recess 2d, as shown in Figures 3(a) and 3(b), the initial nuclei 3 grown from the seed crystal 2 in the initial nucleation process S3 described later do not fill the recess 2d, and a void 7 (reference) is formed. The formation of the void 7 prevents dislocations of the seed crystal 2 from propagating upwards, enabling the growth of a high-quality GaN single crystal.

[0048] As shown in Figure 5, the bottom surface 2b of the recess 2d is flat and is the (0001) plane (c plane) of the group III nitride semiconductor. Also, 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. Furthermore, the shape of the bottom surface 2b in plan view does not need to be circular.

[0049] 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.

[0050] 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.

[0051] 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.

[0052] In the seed substrate preparation step S2, the seed substrate 9 can be manufactured, 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 pattern of regular triangular grids. 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.

[0053] 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.

[0054] 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 5 and 6. 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.

[0055] 4. Initial nucleation step S3 In the initial nucleation step S3 shown in Figure 1, an initial nucleus 3 is formed on the various crystals 2, as shown in Figure 3(b), by bringing a mixture of the alkali metal melt from which oxygen atoms have been removed (prepared in the alkali metal melt preparation step S1) and a group III metal melt into contact with the surface of the seed crystal 2 in a nitrogen-containing atmosphere.

[0056] In the initial nucleation step S3, first, as shown in Figure 7, a mixture 101 of an alkali metal molten liquid from which oxygen atoms have been removed and a group III metal molten liquid is stored in a crucible 100, and the seed substrate 9 is immersed in the mixture 101 using a jig 400. The jig 400 is placed inside the crucible 100 for growing semiconductor single crystals by the flux method, as shown in Figures 7 to 9, and can support the seed substrate 9 for growing group III nitride semiconductor single crystals inside the crucible 100. The jig 400 has a first leg portion 401, a second leg portion 402, a third leg portion 403, a connecting portion 404, and a lifting shaft 405. The material of each component of the jig 400 is alumina. As shown in Figures 7 to 9, the first leg portion 401, the second leg portion 402, and the third leg portion 403 are formed in a roughly rod shape and hang down from the corners of the roughly triangular, flat connecting portion 404 in a plan view, as shown in Figure 9.

[0057] As shown in Figures 7 and 8, a substrate support portion 410, consisting of a protrusion capable of supporting a seed substrate 9, is formed at the lower end of each of the first leg portion 401, the second leg portion 402, and the third leg portion 403 shown in Figure 9. As shown in Figures 7 and 8, the first leg portion 401 is formed to be longer than the second leg portion 402 and the third leg portion 403 (not shown). As a result, the substrate 1 supported by the substrate support portion 410 is supported in an inclined state with respect to the connecting portion 404. The connecting portion 404 is connected to the lifting shaft 405 so that it can assume an inclined position with respect to the lifting shaft 405. As a result, the seed substrate 9 supported by the substrate support portion 410 is in an inclined state with respect to the horizontal, as shown in Figure 7, before being immersed in the mixed liquid 101 stored in the crucible 100, and is in a horizontal state, as shown in Figure 8, when immersed in the mixed liquid 101 stored in the crucible 100.

[0058] The initial nucleation process S3 then involves replacing the reactor atmosphere with an inert gas, heating the reactor, and then evacuating it to sufficiently reduce outgassing components such as oxygen in the reactor.

[0059] Subsequently, predetermined amounts of Na and Ga are weighed in a glove box 300 where the atmosphere, including oxygen and dew point, is controlled. Then, the weighed amounts of Na and Ga are poured into an empty crucible 100 (see Figure 7) to form a mixture 101. If necessary, additive elements such as carbon may be added.

[0060] Next, the crucible 100 containing the raw materials and the seed substrate 9 are placed in the reaction vessel and evacuated, and then a gas containing nitrogen is supplied to the reaction vessel. Once the pressure inside the reaction vessel reaches the crystal growth pressure, the temperature inside the furnace is raised 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.

[0061] When the reaction vessel reaches the crystal growth temperature and pressure, and the nitrogen dissolved in the mixture 101 becomes supersaturated, the seed substrate 9 is added to the mixture 101 in the crucible 100. Then, GaN crystals (initial nuclei 3) begin to grow from the various crystals 2 on the seed substrate 9. The growth of the initial nuclei 3 continues until adjacent initial nuclei 3 begin to fuse together (see Figure 3(b)). Note that a gap remains between the initial nuclei 3 and the substrate 1.

[0062] Here, the (10-11) plane, which is the side surface 2a of the frustum of the regular hexagonal pyramid of the seed crystal 2, exists stably in the mixed solution 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.

[0063] 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 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.

[0064] 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 liquid 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.

[0065] 5. Crystal growth process S4 The crystal growth step S4 shown in Figure 1 involves bringing the mixed solution 101 into contact with the initial nuclei 3 in a nitrogen-containing atmosphere to grow a group III nitride single crystal from the initial nuclei 3. In this embodiment, the crystal growth step S4 includes a planarization step S41 and a thickening step S42.

[0066] 5-1. Flattening process S41 The planarization step S41 is a step in which crystal growth is carried out 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 liquid 101 stored in a crucible 100, and after being pulled out, the process of heating under a nitrogen atmosphere is repeated, 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.

[0067] In the FFC method in the planarization step S41 of this embodiment 1, as shown in Figure 7, the seed substrate 9 is repeatedly removed from the mixed liquid 101 at predetermined intervals, or immersed in the mixed liquid 101 as shown in Figure 8. As shown in Figure 3(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 liquid 101, the mixed liquid 101 accumulates in the depressions 4 between adjacent initial nuclei 3. This allows the crystal 5 to grow along the depressions 4 (see Figure 6(c)).

[0068] Here, because the mixed solution 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 solution 101 is small, the amount of Ga is also small, and crystal growth stops after a while. Therefore, as shown in Figure 8, the seed substrate 9 is immersed in the mixed solution 101 again, and as shown in Figure 7, the seed substrate 9 is removed from the mixed solution 101, thereby intermittently supplying the mixed solution 101 containing Ga to the depression 4. 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.

[0069] 5-2.Thickening process S42 As shown in Figure 1, the thickening process S42 is performed after the planarization process S41. The thickening process S42 is a process in which a GaN substrate, which is a seed substrate 9 having a crystal surface planarized in the planarization process S41, is immersed in a mixed solution 101 of Ga and Na stored in a crucible 100 to form a thickened group III nitride single crystal (GaN single crystal) 6 on the GaN substrate (seed substrate 9).

[0070] In the thickening process S42 of Embodiment 1, as shown in Figure 8, the seed substrate 9, which has a flat crystal surface, is immersed in the mixed liquid 101 while supported by the substrate support part 410. 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 S42 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.

[0071] 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.

[0072] 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.

[0073] In this embodiment, the planarization step S41 based on the FFC method is not necessarily required, but it is preferable to perform the planarization step S41 in order to further improve the flatness of the crystal and further reduce warping.

[0074] 6. Confirmation Test In this embodiment 1, the growth state of the initial nuclei was compared between a test example using an alkali metal melt from which oxygen atoms had been removed by the group III nitride single crystal growth method, and a comparative example using an alkali metal melt from which oxygen atoms had not been removed. In the test example, the second temperature in the alkali metal melt preparation step S1 was set to 180°C. In both the test and comparative examples, the seed substrate 9 had seed crystals 2 with a diameter in the range of 100 to 200 μm and a height of 5 to 40 μm. In both the test and comparative examples, the growth conditions of the initial nuclei other than the alkali metal melt were the same.

[0075] In the test example using alkali metal molten material from which oxygen atoms had been removed, no initial nucleus growth defects were detected within the captured area, as shown in Figure 10(a). However, in the comparative example using alkali metal molten material from which oxygen atoms had not been removed, multiple initial nucleus growth defects were detected within the captured area, as shown in Figure 10(b). Furthermore, the incidence rate of initial nucleus growth defects across the entire measurement range was approximately 1 / 10 that of the comparative example. This confirmed that removing oxygen atoms from alkali metal molten material can suppress initial nucleus growth defects.

[0076] 7. Effects The effects and advantages 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, an MPS substrate is used as the seed substrate 9, on which a Group III nitride semiconductor is formed in the shape of multiple dots. The Group III nitride semiconductor on the seed substrate 9 is used as the seed crystal 2, and a mixture 101 of an alkali metal molten liquid with oxygen atoms removed and a Group III metal molten liquid is brought into contact with the surface of the seed crystal 2 to form initial nuclei 3 on the seed crystal 2, after which a Group III nitride single crystal 6 is grown from the initial nuclei 3. As a result, because an alkali metal molten liquid with oxygen atoms removed is used, the degree of supersaturation of N (nitrogen) in the mixture 101 can be increased without lowering the temperature or increasing the pressure, and nucleation on the seed crystal 2 on the seed substrate 9 can be promoted while suppressing the generation of miscellaneous crystals. As a result, the size and shape of the generated initial nuclei 3 can be made uniform, so that cracks and defects are suppressed in the initial nuclei 3 and the Group III nitride single crystal 6 grown on the initial nuclei 3, and the crystal quality can be improved.

[0077] Furthermore, in this embodiment, in the alkali metal melt preparation step S1, the alkali metal melt is circulated between a first region 201 maintained at a first temperature and a second region 202 maintained at a second temperature lower than the first temperature, thereby removing oxygen atoms from the alkali metal melt in the second region 202. This suppresses the generation of miscellaneous crystals while further promoting nucleation of seed crystals 2 on the seed substrate 9, which is an MPS substrate, and further promotes the uniformity of the size and shape of the initial nuclei 3 that are generated. As a result, further improvement in crystal quality can be achieved.

[0078] Furthermore, in this embodiment, the concentration of oxygen atoms in the alkali metal melt can be controlled by controlling the second temperature in the alkali metal melt preparation step S1. This allows the concentration of oxygen atoms in the alkali metal melt to be maintained at a low level, thereby suppressing the generation of unwanted crystals and further improving the crystal quality of the formed group III nitride single crystal 6.

[0079] Furthermore, in this embodiment, the second temperature is controlled to a range of 120°C to 300°C, and the first temperature is controlled to a higher temperature than the second temperature. This allows for the removal of oxygen atoms while maintaining the fluidity of the alkali metal melt, thereby suppressing the generation of unwanted crystals and further improving the crystal quality of the formed group III nitride single crystal 6.

[0080] Furthermore, in this embodiment, the oxygen concentration in the alkali metal melt is 30 ppm or less. As a result, the oxygen concentration in the alkali metal melt is sufficiently low, which further suppresses the generation of miscellaneous crystals and further improves the crystal quality of the formed group III nitride single crystal 6.

[0081] Furthermore, in this embodiment, in the seed substrate preparation step S2, the diameter D1 of the seed crystal 2 formed on the substrate 1 is in the range of 30 to 300 μm. This makes it possible to form a group III nitride single crystal 6 with few dislocations and warping, and also makes it possible to increase the surface area of ​​the side surface of the seed crystal 2, thereby facilitating the growth of initial nuclei 3 from these side surfaces.

[0082] As described above, this embodiment provides a method for growing a group III nitride single crystal that can form a group III nitride single crystal with high crystal quality using an MPS substrate.

[0083] The present invention is not limited to the embodiments described above, and can be applied to various embodiments without departing from its spirit. [Explanation of symbols]

[0084] 1: Circuit board 2: Seed crystal 3: Initial nucleus 6: Group III nitride single crystal (GaN single crystal) 9: Seed substrate 100: Crucible 101: Mixed liquid 200:Na circulation device 201:First area 202:Second area 210: Supply Tank 220: Dump tank 230: Cold Trap 240: Electromagnetic pump 250: Expansion Tank 260: Measuring tank 270:Na sampling port 281~283: Piping 290: Heating device 300: Glove box 400: Jig

Claims

1. A method for growing single crystals of group III nitrides, The alkali metal melt preparation process involves preparing an alkali metal melt from which oxygen atoms have been removed, A seed substrate preparation step involves preparing a seed substrate in which multiple seed crystals made of a group III nitride semiconductor are formed in a dot pattern on a substrate, An initial nucleation step is performed by bringing a mixture of the alkali metal molten liquid and the group III metal molten liquid into contact with the surface of the seed crystal in a nitrogen-containing atmosphere to form an initial nucleus on the seed crystal. A crystal growth step in which a group III nitride single crystal is grown from the initial nucleus by bringing the mixture into contact with the initial nucleus in a nitrogen-containing atmosphere, A method for growing single crystals of group III nitrides, including the group III nitride.

2. The method for growing a group III nitride single crystal according to claim 1, wherein in the alkali metal melt preparation step, the alkali metal melt is circulated between a first region where it is maintained at a first temperature and a second region where it is maintained at a second temperature lower than the first temperature, thereby removing oxygen atoms from the alkali metal melt in the second region.

3. The method for growing a group III nitride single crystal according to claim 2, characterized in that the concentration of oxygen atoms in the alkali metal melt is controlled by controlling the second temperature in the alkali metal melt preparation step.

4. The method for growing a group III nitride single crystal according to claim 2 or 3, wherein the second temperature is controlled to be in the range of 120°C to 300°C, and the first temperature is controlled to be higher than the second temperature.

5. The method for growing a group III nitride single crystal according to any one of claims 1 to 3, wherein the oxygen concentration in the alkali metal melt is 30 ppm or less.

6. The method for growing a group III nitride single crystal according to any one of claims 1 to 3, wherein in the seed substrate preparation step, the diameter of the seed crystal formed on the substrate is in the range of 30 to 300 μm.

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