Method for manufacturing group III nitride semiconductors

Abstract

The present invention provides a method for manufacturing a group III nitride semiconductor that can improve the nucleation rate. [Solution] The method for manufacturing a group III nitride semiconductor involves supplying a nitrogen-containing gas to a mixed melt 101 obtained by mixing a group III metal and a flux, and growing a group III nitride semiconductor on a seed substrate 9. The seed substrate 9 has a substrate 1 and a plurality of seed crystals 2 made of group III nitride semiconductor provided on the substrate 1. The method for manufacturing a group III nitride semiconductor includes a nucleation step in which initial nuclei 3 made of group III nitride semiconductor are generated in the seed crystals 2, and a nucleation growth step in which the initial nuclei are grown after the nucleation step, and in the nucleation step, calcium is included in the mixed melt 101.

Description

Technical Field

[0001] The present invention relates to a method for manufacturing a group III nitride semiconductor.

Background Art

[0002] As a method for manufacturing GaN, the Na flux method is known. The Na flux method is a method of dissolving nitrogen in a mixed melt of Ga and Na and growing GaN in a liquid phase. In the Na flux method, generally, a seed substrate is placed in the mixed melt and GaN is grown on the seed substrate.

[0003] Also, as a seed substrate, a substrate provided with a plurality of seed crystals on it is used, and a method of generating initial nuclei from each seed crystal and growing and combining the initial nuclei is known. In this case, if there is variation in the size and shape of each initial nucleus, there is a problem that defects and cracks occur in the GaN crystal grown on the seed substrate.

[0004] In order to suppress the size and variation of each initial nucleus, it is conceivable to improve the generation rate of initial nuclei from each seed crystal. Patent Document 1 describes that by setting the temperature in the nucleation process to 870°C or lower, the supersaturation of nitrogen in the mixed melt is improved, and thereby the nucleation rate becomes 100%.

Prior Art Documents

Patent Documents

[0005]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0006] However, Patent Document 1 does not take into account the size of the seed crystal. According to the inventors' research, it was found that the size of the seed crystal actually has a significant impact on the nucleation rate. In particular, it was found that the nucleation rate decreases as the seed crystal size decreases. Therefore, it was found that even when using the method of Patent Document 1, the nucleation rate may not reach 100%.

[0007] This invention has been made in view of the above background, and aims to provide a method for manufacturing a group III nitride semiconductor that can improve the nucleation rate. [Means for solving the problem]

[0008] One aspect of the present invention is, A method for producing a group III nitride semiconductor, comprising supplying a nitrogen-containing gas to a mixed molten material obtained by mixing a group III metal and a flux, and growing a group III nitride semiconductor on a seed substrate, The seed substrate comprises a substrate and a plurality of seed crystals made of a group III nitride semiconductor provided on the substrate. A nucleation step is performed to generate initial nuclei made of a group III nitride semiconductor in the seed crystal, The process includes a nucleation growth step for growing the initial nucleus after the nucleation step, The nucleation step is a method for producing a group III nitride semiconductor, wherein the mixed melt contains calcium. [Effects of the Invention]

[0009] In the above embodiment, the mixed melt 101 contains calcium during the nucleation process. This reduces the driving force for crystal growth, improves the generation rate of each initial nucleus from various crystals, and synchronizes the generation timing of each initial nucleus. As a result, the shape and size of each initial nucleus can be standardized.

[0010] As described above, according to the above embodiment, it is possible to provide a method for manufacturing a group III nitride semiconductor that can improve the nucleation rate. [Brief explanation of the drawing]

[0011] [Figure 1] Figure 1 is a flowchart showing the method for manufacturing a group III nitride semiconductor in the first embodiment. [Figure 2] Figure 2 is a cross-sectional view of a seed substrate, and is a cross-sectional view perpendicular to the main surface of the substrate. [Figure 3] Figure 3 is a plan view of the seed substrate as seen from above. [Figure 4] Figure 4 is a cross-sectional view showing the structure of a seed crystal, and is a cross-sectional view perpendicular to the main surface of the substrate. [Figure 5] Figure 5 is a plan view of the seed crystal seen from above. [Figure 6] Figure 6 is a cross-sectional view of the seed substrate during the nuclear growth process, and is a cross-sectional view perpendicular to the main surface of the substrate. [Figure 7] Figure 7 is a cross-sectional view of the seed substrate during the planarization process, and is a cross-sectional view perpendicular to the main surface of the substrate. [Figure 8] Figure 8 is a cross-sectional view of the seed substrate during the thick-film formation process, and is a cross-sectional view perpendicular to the main surface of the substrate. [Figure 9] Figure 9 is a schematic diagram showing the FFC method in the planarization process. [Figure 10] Figure 10 is a SEM image of the early nucleus viewed obliquely from above. [Modes for carrying out the invention]

[0012] A method for producing a group III nitride semiconductor involves supplying a nitrogen-containing gas to a mixed molten mixture of a group III metal and a flux to grow a group III nitride semiconductor on a seed substrate, wherein the seed substrate comprises a substrate and a plurality of seed crystals made of group III nitride semiconductors provided on the substrate, and the method comprises a nucleation step for generating initial nuclei made of group III nitride semiconductors in the seed crystals, and a nucleation growth step for growing the initial nuclei after the nucleation step, wherein the mixed molten mixture contains calcium in the nucleation step.

[0013] In the method for manufacturing the group III nitride semiconductor, in the nucleation step, the calcium concentration in the mixed melt may be less than 0.001 mol%. Variations in the shape and size of the initial nuclei can be further suppressed.

[0014] In the method for manufacturing the group III nitride semiconductor, the mixed melt is held by a crucible, and by using a crucible made of alumina containing calcium as the crucible, the mixed melt may contain calcium in the nucleation step. Calcium can be easily added to the mixed melt.

[0015] In the method for manufacturing the group III nitride semiconductor, the calcium concentration in the mixed melt in the nucleation step may be 0.001 mol% or more and 0.1 mol% or less. Variations in the shape and size of the initial nuclei can be further suppressed.

[0016] In the method for manufacturing the group III nitride semiconductor, after the nucleation step, there are further a planarization step of filling the space between adjacent initial nuclei with an embedded layer made of a group III nitride semiconductor to planarize the crystal plane, and a thickening step of forming a flat layer made of a group III nitride semiconductor on the planarized crystal plane after the planarization step. In the thickening step, the mixed melt may contain calcium. The crystallinity of the flat layer can be improved.

[0017] In the method for manufacturing the group III nitride semiconductor, the calcium concentration in the mixed melt in the thickening step may be higher than the calcium concentration in the mixed melt in the nucleation step. The crystallinity of the flat layer can be further improved.

[0018] In the method for manufacturing the group III nitride semiconductor, the diameter of the seed crystal may be 150 μm or less. Even if the seed crystal 2 is small, variations in the shape and size of the initial nuclei can be suppressed.

[0019] (First Embodiment) 1. Outline of the Flux Method Figure 1 is a flowchart showing the method for manufacturing a group III nitride semiconductor in the first embodiment. In the method for manufacturing a group III nitride semiconductor in the first embodiment, the group III nitride semiconductor is grown by the flux method. The flux method is a method in which a gas containing nitrogen is supplied to a mixed molten material containing an alkali metal which will be used as a flux and a group III metal which will be used as a raw material, and the materials are dissolved, and the group III nitride semiconductor is epitaxially grown in the liquid phase.

[0020] The raw material is a Group III metal, which is at least one of gallium (Ga), aluminum (Al), or indium (In). The composition of the Group III nitride semiconductor grown can be controlled by the ratio of these metals, allowing for the growth of GaN, AlN, InN, AlGaN, InGaN, AlGaInN, and the like. This invention is particularly suitable for growing GaN.

[0021] The alkali metal flux is usually sodium (Na), but potassium (K) may also be used, or a mixture of Na and K.

[0022] Calcium (Ca) is added to the mixed molten material according to the manufacturing stage, as described later.

[0023] Carbon (C) may be added to the mixed melt. The addition of C can accelerate the crystal growth rate. In addition, dopants other than C may be added to the mixed melt for purposes such as controlling the properties of the group III nitride semiconductor being grown, such as conductivity and magnetism, as well as promoting crystal growth, suppressing miscellaneous crystals, and controlling the growth direction.

[0024] The nitrogen-containing gas is a gas containing nitrogen molecules or compounds such as ammonia that contain nitrogen as a constituent element, and may also be a mixture of these gases. Furthermore, the nitrogen-containing gas may be mixed with an inert gas such as a noble gas.

[0025] 2. Structure of the substrate In the first embodiment, a seed substrate 9 is placed in a mixed molten metal, and a group III nitride semiconductor is grown on the seed substrate 9. The seed substrate 9 may be placed in the mixed molten metal before heating and pressurizing, but it is preferable to heat and pressurize it to reach the growth temperature and growth pressure before placing it in the mixed molten metal. This can suppress the melt-back of the seed crystal 2 of the seed substrate 9.

[0026] A multi-point seed (MPS) substrate is used for the seed substrate 9. An MPS substrate is a substrate in which multiple dot-shaped seed crystals 2 are periodically arranged on the substrate 1. Figure 2 is a cross-sectional view of the seed substrate 9, which is a cross-section perpendicular to the main surface of the substrate. Figure 3 is a plan view of the seed substrate 9 seen from above.

[0027] The substrate 1 can be made of a group III nitride semiconductor, sapphire, aluminum oxynitride, SiC, Si, spinel, ZnO, gallium oxide, etc. When a sapphire substrate is used, its main surface can be, for example, the c-plane or the a-plane.

[0028] 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 made of a group III nitride semiconductor of any composition, such as GaN, AlGaN, or 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 made of GaN. The material of the seed crystal 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.

[0029] 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 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 a triangular lattice pattern is used, 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 coincidence. An angular deviation of about 10 degrees relative to the a-axis and m-axis directions is acceptable as an error. Preferably, the angular deviation is 1 degree or less relative to the a-axis and m-axis directions.

[0030] 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. A more preferred range for the distance L1 is 200 to 1500 μm, and even more preferably 300 to 1000 μm.

[0031] 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 of the seed crystal 2 seen from above. As shown in Figures 4 and 5, the seed crystal 2 has a disc-shaped portion and a regular frustum-shaped portion located in contact with the cylindrical portion, with a recess 2d in the center of the regular frustum-shaped portion.

[0032] 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. Furthermore, the opening in the mask has a circular shape in plan view. Therefore, after the mask is removed, the mask opening remains as a disc-shaped portion. This remaining portion is the disc.

[0033] The shape of the disc portion is the same as the shape of the mask opening when selectively growing seed crystal 2. Therefore, the thickness of the disc portion is approximately equal to the thickness of the mask. The diameter of the disc portion is approximately equal to the diameter of the mask opening. Also, the diameter D1 of the regular frustum of a hexagon is larger than the diameter of the disc portion. Since the shape of the disc portion is circular in plan view, when separating the substrate 1 after growing the group III nitride semiconductor by the flux method, stress can be distributed, and the occurrence of cracks in the grown crystal can be suppressed.

[0034] While it is possible to change the shape of the opening in the mask to make the disc portion a regular hexagon or other shape in plan view, a disc is preferred because it distributes stress as described above.

[0035] 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 is aligned with the a-axis) is preferred. Since group III nitride semiconductors are hexagonal, using a regular hexagon allows the group III nitride semiconductors grown from the frustum of the regular hexagon of various crystals 2 to be uniformly bonded together. However, it is not necessary to perfectly align with the a-axis, and an angular deviation of about 10 degrees relative to the a-axis is acceptable. Preferably, the angular deviation is 1 degree or less relative to the a-axis.

[0036] 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 in the mixed melt of the Na flux method. Therefore, the group III nitride semiconductor grows 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 each initial nucleus 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) plane as part of the (10-11) plane as an error.

[0037] 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.

[0038] The first embodiment is particularly effective when the diameter D1 of the seed crystal 2 is small. For example, it is effective when D1 is 150 μm or less, and especially 100 μm or less. Reducing the diameter of the seed crystal 2 makes it easier to separate the substrate 1 after the growth of the group III nitride semiconductor is complete, but it reduces the nucleation rate from the seed crystal 2, resulting in variations in the timing of the generation of each initial nucleus 3. This is because when the seed crystal 2 is small, the surface on which initial nuclei are easily generated is less exposed. Therefore, when the diameter D1 of the seed crystal 2 is small, it was difficult to make the shape and size of each initial nucleus 3 uniform. However, according to the first embodiment described later, even when the diameter D1 of the seed crystal 2 is small, the nucleation rate from the seed crystal 2 can be improved, and the timing of the generation of each initial nucleus 3 can be made uniform.

[0039] 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. A more preferable range for the height H1 is 20 to 60 μm, and an even more preferable range is 30 to 50 μm.

[0040] 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. A more preferable range for the height H1 is 0.1 to 0.35 times, and an even more preferable range is 0.15 to 0.3 times.

[0041] 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 group III nitride semiconductors.

[0042] 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.

[0043] 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.

[0044] 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. A more preferable range for the depth H2 is 20 to 60 μm, and an even more preferable range is 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. A more preferable range for the depth H2 is 0.6 to 0.8 times the height H1 of the seed crystal 2.

[0045] 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 can be etched 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 can cause dislocations of the seed crystal 2 to propagate to the upper surface. By creating a shape in which there is no c-plane on the upper surface, variations in the shape of each initial nucleus 3 can be suppressed. As a result, the propagation of dislocations of the seed crystal 2 to the upper surface can be suppressed.

[0046] 3. Method for manufacturing seed substrates 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.

[0047] Next, a buffer layer (not shown) and seed crystal 2 are selectively grown on the substrate exposed to the aperture using methods such as MOCVD or HVPE. Then, the mask is removed by wet etching with hydrofluoric acid or the like. The seed substrate 9 can be fabricated by the above steps.

[0048] 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, thereby allowing the shape of seed crystal 2 to be 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 of seed crystal 2 is determined by selective growth, the shapes of various crystals 2 can be made uniform.

[0049] 4. Method for manufacturing Group III nitride semiconductors Next, the method for manufacturing a group III nitride semiconductor in the first embodiment will be described with reference to the figure. As shown in Figure 1, the method for manufacturing a group III nitride semiconductor in the first embodiment includes a nucleation step S1, a nucleation growth step S2, a planarization step S3, and a thick film formation step S4.

[0050] First, the nucleation process S1 is performed. In the nucleation process S1, 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 inside the reactor.

[0051] Next, predetermined amounts of alkali metals and group III metals are weighed in a glove box where the atmosphere, including oxygen and dew point, is controlled. Then, the weighed amounts of alkali metals and group III metals are placed into crucible 100. Additives such as carbon may be added as needed.

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

[0053] Here, the mixed melt 101 is made to contain Ca. One way to add Ca to the mixed melt 101 is as follows:

[0054] Firstly, a crucible 100 made of a material containing Ca is used. Ca dissolves from the crucible 100 into the mixed molten material 101, thereby allowing the mixed molten material 101 to contain Ca. For example, a crucible 100 made of alumina containing 0.05 to 0.5 mol% Ca is suitable. Such a crucible 100 made of alumina containing Ca can be formed by casting using a plaster mold.

[0055] Secondly, Ca is added to Na and Ga beforehand. When Na and Ga melt upon heating to form a mixed melt 101, Ca also melts into the mixed melt 101, thereby allowing the mixed melt 101 to contain Ca.

[0056] Thirdly, when alkali metals and group III metals are added to crucible 100, Ca is also added. When Na and Ga melt due to heating to form a mixed melt 101, the Ca contained in Na and Ga also melts into the mixed melt 101, so Ca can be included in the mixed melt 101.

[0057] Next, 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, crystals of group III nitride semiconductor (initial nuclei 3) are generated from the various crystals 2 of the seed substrate 9.

[0058] Here, Ca is added to the mixed melt 101. This reduces the driving force for crystal growth, making it easier for initial nuclei 3 to form from the seed crystal 2. As a result, the timing of the formation of initial nuclei 3 from each type of crystal 2 becomes more synchronized. And by synchronizing the timing of the formation of initial nuclei 3, variations in the shape and size of each initial nucleus 3 can be suppressed. As a result, the occurrence of defects and cracks in the seed substrate 9 can be suppressed.

[0059] The Ca concentration in the mixed melt 101 is, for example, 0.001 to 0.1 mol%. By setting the Ca concentration to 0.001 mol% or higher, the timing of the emergence of initial nuclei 3 from seed crystal 2 can be more uniform. Conversely, by setting the Ca concentration to 0.1 mol% or lower, the emergence of miscellaneous crystals can be suppressed.

[0060] Next, the nucleation growth process S2 is performed. In the nucleation growth process S2, the growth temperature and growth pressure from the nucleation process S1 are maintained, and the initial nuclei 3 that were generated in the various crystals 2 in the nucleation process S1 are grown. The initial nuclei 3 grow into a truncated hexagonal pyramidal or regular hexagonal pyramidal shape. The growth of the initial nuclei 3 continues until adjacent initial nuclei 3 begin to fuse together (see Figure 6). Note that a gap remains between the initial nuclei 3 and the substrate 1.

[0061] In the nucleus growth process S2, the mixed melt 101 may be made Ca-free. By making it Ca-free, it becomes easier to make the initial nucleus 3 frustum-shaped, and the shape and size of the initial nucleus 3 can be made more uniform. Here, Ca-free means that the concentration is below the detection limit of general elemental analysis methods. Specifically, it is less than 0.001 mol%.

[0062] One way to ensure that the mixed molten material 101 does not contain Ca is to remove the seed substrate 9 from the crucible 100 and transfer it to another crucible. In this case, the other crucible should be made of a material that does not contain Ca. For example, a crucible made of alumina, which does not contain Ca, should be used. Furthermore, the other crucible should be used to hold the mixed molten material 101 that does not contain Ca.

[0063] 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 being etched. 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 side surface 2a while maintaining the (10-11) plane. Since the shapes of the various crystals 2 are uniform, the timing of the emergence of the initial nuclei 3 from the various crystals 2 is also uniform, and furthermore, the initial nuclei 3 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. As a result, the shape of each initial nucleus 3 can be made uniform.

[0064] 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.

[0065] 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 etched 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.

[0066] Next, the planarization process S3 is performed. Once adjacent initial nuclei 3 begin to fuse together, the process transitions from the nucleation growth process S2 to the planarization process S3. In the planarization process S3, crystal growth is carried out using the FFC (flux film coating) method. The FFC method involves repeatedly removing the seed substrate 9 from the mixed molten liquid 101 and adding it to the mixed molten liquid 101 at predetermined intervals (see Figure 9). 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 filling layer 5 to grow along the depressions 4 (see Figure 7).

[0067] 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 group III metal is also small, and crystal growth stops after a while. Therefore, the seed substrate 9 is again introduced into the mixed molten material 101, and the substrate is removed from the mixed molten material 101, thereby intermittently supplying the mixed molten material 101 containing group III metal to the depression 4. The FFC method is continued until the depression 4 is filled by the growth of the embedding layer 5. This makes it possible to grow crystals with flat c-planes.

[0068] It is not always necessary to form the embedded layer 5 by the FFC method, but it is preferable to use the FFC method to further improve the flatness of the crystal and reduce warping. Also, in the planarization step S3, the mixed melt 101 may or may not contain Ca.

[0069] Next, the thickening process S4 is performed. Once the depressions 4 are filled and a flat crystal surface is formed, the process moves from the planarization process S3 to the thickening process S4. In the thickening process S4, the seed substrate 9 is again added to the mixed melt 101. Then, a flat layer 6 is grown on the flat crystal surface to increase its thickness. The flat layer 6 is a layer made of a group III nitride semiconductor with a flat surface. Because the shape of each initial nucleus 3 is uniform, the flat layer 6 can also be formed uniformly within the plane. In addition, because the transfer of dislocations from the seed crystal 2 to the upper part is suppressed, a high-quality flat layer 6 can be formed.

[0070] In the thickening process S4, it is preferable that the mixed melt 101 contains Ca. This improves the wettability of the mixed melt 101, thereby suppressing macrostep growth. As a result, the crystallinity of the flat layer 6 can be improved. In this case, the Ca concentration in the thickening process S4 should be higher than the Ca concentration in the nucleation process S1. However, from the viewpoint of suppressing miscrystals, it is preferable that the Ca concentration in the thickening process S4 be 1 mol% or less.

[0071] Once the flat layer 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 group III nitride semiconductor. At this point, the gap between the initial nucleus 3 and the substrate 1 remains unfilled. Therefore, the substrate 1 can be naturally detached during cooling due to the difference in thermal expansion coefficients. In particular, if the diameter D1 of the seed crystal 2 is 150 μm or less, the substrate 1 can be detached more easily.

[0072] As described above, according to the method for manufacturing a group III nitride semiconductor in the first embodiment, since Ca is added to the mixed melt 101 in the nucleation process, the generation rate of each initial nucleus 3 from each crystal 2 can be improved. In addition, the generation timing of each initial nucleus 3 from each crystal 2 can be synchronized. As a result, the shape and size of each initial nucleus 3 can be made uniform. And by having the shape and size of each initial nucleus 3 uniform, defects and cracks in the seed substrate 9 can be suppressed.

[0073] Next, we will describe the experimental results relating to the first embodiment.

[0074] First, a seed substrate 9 was fabricated as follows. A mask made of SiO2 was formed on a substrate 1 made of c-plane sapphire by CVD. The mask had a pattern in which openings were arranged in a regular triangular grid. The shape of the openings was circular. The distance between the centers of adjacent openings was 550 μm, and the diameter of the openings was 175 μm. Next, a buffer layer was formed on the bottom surface of the openings by MOCVD, and then a seed crystal 2 made of GaN was formed on the buffer layer. The seed crystal 2 was grown at a temperature of 1140°C and a V / III ratio of 720. The seed substrate 9 was then fabricated.

[0075] Next, a crucible 100 made of alumina containing 0.1 mol% Ca was prepared. Then, seed substrate 9, sodium, and gallium were placed in crucible 100, and GaN crystals were grown on the seed substrate 9 by heating and pressurizing in a nitrogen atmosphere. Growth was carried out by growing initial nuclei 3 from seed crystal 2 until initial nuclei 3 from adjacent seed crystals 2 coalesced.

[0076] Figure 10 shows an SEM image of the grown initial nuclei 3. As shown in Figure 10, it was found that the shape and size of each initial nucleus 3 were uniform. This is thought to be because, during the generation stage of the initial nuclei 3, the Ca dissolved from the crucible 100 was included in the mixed melt 101, making it easier for the initial nuclei 3 to be generated from the seed crystal 2, and thus the growth timing of the initial nuclei 3 was synchronized. [Explanation of Symbols]

[0077] 1: Circuit board 2: Seed crystal 2a: Side 2b: Bottom 2c: Side 2d: recessed 3: Initial nucleus 4: Indentation 5: Embedding layer 6: Flat layer 9: Seed substrate

Claims

1. A method for producing a group III nitride semiconductor, comprising supplying a nitrogen-containing gas to a mixed molten material obtained by mixing a group III metal and a flux to grow a group III nitride semiconductor on a seed substrate, The seed substrate comprises a substrate and a plurality of seed crystals made of a group III nitride semiconductor provided on the substrate. A nucleation step is performed to generate initial nuclei made of a group III nitride semiconductor in the seed crystal, The process includes a nucleation growth step for growing the initial nucleus after the nucleation step, A method for producing a group III nitride semiconductor, wherein the nucleation step includes calcium in the mixed melt.

2. The method for producing a group III nitride semiconductor according to claim 1, wherein in the nucleation growth step, the calcium concentration of the mixed melt is less than 0.001 mol%.

3. The mixed molten liquid is held in a crucible. A method for producing a group III nitride semiconductor according to claim 1, wherein a crucible made of calcium-containing alumina is used as the crucible, so that the mixed molten liquid contains calcium in the nucleation step.

4. A method for producing a group III nitride semiconductor according to claim 1 or claim 2, wherein the calcium concentration of the mixed melt in the nucleation step is 0.001 mol% or more and 0.1 mol% or less.

5. After the nucleation growth process, a planarization process is performed to flatten the crystal plane by filling the spaces between adjacent initial nuclei with a filling layer made of a group III nitride semiconductor. The process further comprises a thick-film formation step, in which a flat layer made of a group III nitride semiconductor is formed on the planarized crystal surface after the planarization step, The method for producing a group III nitride semiconductor according to claim 1 or claim 2, wherein the thickening step includes calcium in the mixed melt.

6. The method for producing a group III nitride semiconductor according to claim 5, wherein the calcium concentration of the mixed melt in the thick-film formation step is higher than the calcium concentration of the mixed melt in the nucleation step.

7. The method for producing a group III nitride semiconductor according to claim 1, wherein the diameter of the seed crystal is 150 μm or less.

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