Method for manufacturing a group III nitride semiconductor, and apparatus for manufacturing a group III nitride semiconductor

Abstract

This invention provides a method for producing group III nitride semiconductors that can grow high-quality crystals. [Solution] The seed substrate 9 comprises a substrate 1 and a plurality of seed crystals 2 made of a group III nitride semiconductor provided on the substrate 1. The process includes a nucleation step S1 in which initial nuclei 3 made of a group III nitride semiconductor are grown from the seed crystals 2, a planarization step S2 after the nucleation step S1 in which the spaces between adjacent initial nuclei 3 are filled with a filling layer 5 made of a group III nitride semiconductor to flatten the crystal plane, and a thickening step after the planarization step S2 in which a flattened layer 6 made of a group III nitride semiconductor is formed on the flattened crystal plane. In the nucleation step S1, a first crucible made of alumina that does not contain calcium is used and the mixed molten material is held in the first crucible so that the mixed molten material does not contain calcium. In the thickening step S3, a second crucible made of alumina that contains calcium is used and the mixed molten material is held in the second crucible so that the mixed molten material contains calcium.

Description

Technical Field

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

Background Art

[0002] As a method for manufacturing GaN, the Na flux method is known. In the Na flux method, nitrogen is dissolved in a mixed melt of Ga and Na to grow GaN in the 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] Further, Patent Document 1 discloses a method in which a substrate having a plurality of seed crystals arranged thereon is used as a seed substrate, initial nuclei are generated from the respective seed crystals, and then the initial nuclei are grown and united.

[0004] Further, Patent Document 2 discloses that a crucible made of alumina is produced by casting using a gypsum mold, and the crucible is used to hold a mixed melt in the Na flux method. By using such a crucible, Ca can be dissolved out from the crucible, thereby suppressing macrostep growth.

Prior Art Documents

Patent Documents

[0005]

Patent Document 1

Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0006] However, according to the study by the inventors, it has been found that when Ca is added to the mixed melt in the method of Patent Document 1, it has an adverse effect on the shape of the initial nuclei. As a result, it has been difficult to grow high-quality crystals.

[0007] This invention has been made in view of the above background, and aims to provide a method for producing a group III nitride semiconductor capable of growing high-quality crystals, and an apparatus for producing a group III nitride semiconductor. [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 arranged on the substrate. A nucleation step is performed to grow an initial nucleus made of a group III nitride semiconductor from the aforementioned seed crystal, After the nucleation step, a planarization step 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 includes a thick-film formation step in which a flat layer made of a group III nitride semiconductor is formed on the flattened crystal surface after the planarization step. In the nucleation step, a first crucible made of alumina with a calcium concentration of 0.01 mol% or less is used, and the mixed molten material is held in the first crucible so that the calcium concentration of the mixed molten material becomes 0.001 mol% or less. The method for producing a group III nitride semiconductor involves using a second crucible made of alumina with a calcium concentration higher than 0.01 mol% in the thick-film formation step, and holding the mixed molten material in the second crucible so that the calcium concentration of the mixed molten material becomes higher than 0.001 mol%.

[0009] Another 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 arranged on the substrate. A nucleation step is performed to grow an initial nucleus made of a group III nitride semiconductor from the aforementioned seed crystal, After the nucleation step, a planarization step 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 includes a thick-film formation step in which a flat layer made of a group III nitride semiconductor is formed on the flattened crystal surface after the planarization step. In the nucleation step, a first crucible made of alumina with a calcium concentration of 0.01 mol% or less is used, and the mixed molten material with a calcium concentration of 0.001 mol% or less is held in the first crucible. The present invention relates to a method for producing a group III nitride semiconductor, wherein the thick-film formation step involves using a second crucible made of alumina and holding the mixed molten material having a calcium concentration higher than 0.001 mol% in the second crucible.

[0010] Another aspect of the present invention is: A manufacturing apparatus for a group III nitride semiconductor, which involves 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 system has a plurality of jigs for holding the seed substrate and for immersing and removing the seed substrate from the mixed melt, Each of the aforementioned jigs is configured to allow the seed substrate to be immersed in and removed from the mixed molten liquid independently of each other, and is part of a manufacturing apparatus for group III nitride semiconductors. [Effects of the Invention]

[0011] In the above aspect, by using crucibles made of different materials in the nucleation step and the thick film formation step, the calcium concentration in the mixed melt is set to 0.001 mol% or less in the nucleation step, and the calcium concentration in the mixed melt is set to be higher than 0.001 mol% in the thick film formation step. Since the calcium concentration in the mixed melt is 0.001 mol% or less in the nucleation step, initial nuclei can be formed in a pyramidal or frustum of a pyramid shape. Also, since the calcium concentration in the mixed melt is higher than 0.001 mol% in the thick film formation step, macrostep growth of the group III nitride semiconductor can be suppressed. As a result, high-quality group III nitride semiconductor crystals can be grown.

[0012] As described above, according to the above aspect, it is possible to provide a method for manufacturing a group III nitride semiconductor capable of growing high-quality crystals and a manufacturing apparatus for a group III nitride semiconductor.

Brief Description of the Drawings

[0013] [Figure 1] It is a flowchart showing a method for manufacturing a group III nitride semiconductor in the first embodiment. [Figure 2] It is a cross-sectional view of a seed substrate, which is a cross-sectional view perpendicular to the main surface of the substrate. [Figure 3] It is a plan view of the seed substrate as viewed from above. [Figure 4] It is a cross-sectional view showing the structure of a seed crystal, which is a cross-sectional view perpendicular to the main surface of the substrate. [Figure 5] It is a plan view of the seed crystal as viewed from above. [Figure 6] It is a diagram schematically showing a state where the seed substrate is replaced into different crucibles. [Figure 7] It is a cross-sectional view of the seed substrate in the nucleation step, which is a cross-sectional view perpendicular to the main surface of the substrate. [Figure 8] It is a cross-sectional view of the seed substrate in the planarization step, which is a cross-sectional view perpendicular to the main surface of the substrate. [Figure 9] It is a cross-sectional view of the seed substrate in the thick film formation step, which is a cross-sectional view perpendicular to the main surface of the substrate. [Figure 10]It is a diagram schematically showing the FFC method in a planarization process. [Figure 11] It is a schematic diagram showing the configuration of a jig in a manufacturing apparatus for a group-III nitride semiconductor. [Figure 12] It is a diagram showing the relationship between the arrangement of a first crucible, a second crucible, and a third crucible and the arrangement of three jigs, and is a plan view seen from directly above in the vertical direction.

Embodiments for Carrying Out the Invention

[0014] A method for manufacturing a group-III nitride semiconductor is a method for manufacturing a group-III nitride semiconductor in which a gas containing nitrogen is supplied to a mixed melt obtained by mixing a group-III metal and a flux to grow a group-III nitride semiconductor on a seed substrate. The seed substrate has a substrate and a plurality of seed crystals made of a group-III nitride semiconductor arranged on the substrate. It has a nucleation step of growing an initial nucleus made of a group-III nitride semiconductor from the seed crystal, 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 after the nucleation step, 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 nucleation step, a first crucible made of alumina with a calcium concentration of 0.01 mol% or less is used, and a mixed melt with a calcium concentration of 0.001 mol% or less is held in the first crucible. In the thickening step, a second crucible made of alumina is used, and a mixed melt with a calcium concentration higher than 0.001 mol% is held in the second crucible.

[0015] Furthermore, the method for manufacturing a group III nitride semiconductor involves 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, wherein the seed substrate comprises a substrate and a plurality of seed crystals made of group III nitride semiconductors arranged on the substrate, and the method includes a nucleation step in which initial nuclei made of group III nitride semiconductors are grown from the seed crystals, and after the nucleation step, filling the spaces between adjacent initial nuclei with group III nitride semiconductors. The process comprises a planarization step in which layers are filled to flatten the crystal plane, and a thickening step in which a flat layer made of a group III nitride semiconductor is formed on the flattened crystal plane after the planarization step. In the nucleation step, a first crucible made of alumina with a calcium concentration of 0.01 mol% or less is used, and a mixed molten material with a calcium concentration of 0.001 mol% or less is held in the first crucible. In the thickening step, a second crucible made of alumina is used, and a mixed molten material with a calcium concentration higher than 0.001 mol% is held in the second crucible.

[0016] In the above method for manufacturing a group III nitride semiconductor, in the planarization step, a third crucible made of alumina with a calcium concentration higher than 0.01 mol% is used, and the mixed molten material is held in the third crucible so that the calcium concentration of the mixed molten material becomes higher than 0.001 mol%, and the calcium concentration of the mixed molten material in the thick-film formation step may be higher than the calcium concentration of the mixed molten material in the planarization step.

[0017] In the above method for producing a group III nitride semiconductor, a third crucible made of alumina is used in the planarization step, and a mixed molten material with a calcium concentration higher than 0.001 mol% is held in the third crucible. The calcium concentration of the mixed molten material in the thick-film formation step may be higher than the calcium concentration of the mixed molten material in the planarization step.

[0018] In the above method for manufacturing a group III nitride semiconductor, a second crucible may be used in the planarization step.

[0019] In the above method for producing a group III nitride semiconductor, in the thick film formation step, a fourth crucible made of alumina with a calcium concentration higher than 0.01 mol% may be used, and the mixed molten material may be held in the fourth crucible so that the calcium concentration of the mixed molten material becomes higher than 0.001 mol%. After forming a flat layer using the second crucible, a flat layer may be formed using the fourth crucible.

[0020] In the above method for producing a group III nitride semiconductor, in the thick film formation step, a fourth crucible made of alumina may be used, and a mixed molten material with a calcium concentration higher than 0.001 mol% may be held in the fourth crucible. After forming a flat layer using the second crucible, a flat layer may be formed using the fourth crucible.

[0021] The Group III nitride semiconductor manufacturing apparatus is a apparatus for growing a Group III nitride semiconductor on a seed substrate by supplying a nitrogen-containing gas to a mixed molten mixture of a Group III metal and a flux. The apparatus has multiple jigs for holding the seed substrate and for immersing and removing the seed substrate from the mixed molten mixture, and each jig is configured to be able to immerse and remove the seed substrate from the mixed molten mixture independently of each other.

[0022] In the above-described apparatus for manufacturing group III nitride semiconductors, each jig may be arranged at equal intervals on a circumference when viewed from above in the vertical direction, and each jig may be configured to rotate integrally around the center of the circumference.

[0023] (First Embodiment) 1. Overview 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. In the flux method, a gas containing nitrogen is supplied to a mixed molten material containing an alkali metal which will be used as the flux and a group III metal which will be used as the raw material, causing them to dissolve, and the group III nitride semiconductor is epitaxially grown in the liquid phase.

[0024] 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.

[0025] The alkali metal flux is usually sodium (Na), but potassium (K) may also be used, or a mixture of Na and K.

[0026] Calcium (Ca) is added to the molten mixture according to the manufacturing stage, as described later. Furthermore, three different types of crucibles are prepared to hold the molten mixture, each made of a different material, as described later, depending on the manufacturing stage.

[0027] 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.

[0028] 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.

[0029] 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.

[0030] A multi-point seed (MPS) substrate is used for the seed substrate 9. In the MPS substrate, 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, taken perpendicular to the main surface of the substrate. Figure 3 is a plan view of the seed substrate 9 seen from above.

[0031] 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.

[0032] Multiple seed crystals 2 are arranged 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 crystals 2 are made of GaN, the buffer layer is preferably made of GaN. The material of the seed crystals is usually a group III nitride semiconductor with the same composition as the group III nitride semiconductor to be grown by the flux method. The seed crystals 2 may be grown by any method such as MOCVD, HVPE, or MBE, but MOCVD and HVPE are preferred in terms of crystallinity and growth time.

[0033] The arrangement of seed crystals 2 has a triangular lattice pattern, as shown in Figure 3. The arrangement of seed crystals 2 is not limited to a triangular lattice; any periodic arrangement is acceptable, but it is preferable to have a highly symmetrical pattern such as a square lattice or a triangular lattice. The group III nitride semiconductors grown from various crystals 2 can be uniformly bonded, and as a result, group III nitride semiconductors with fewer dislocations and warping can be grown. When the arrangement of seed crystals 2 has a triangular lattice pattern, it is preferable that the arrangement direction coincides with the a-axis and m-axis directions of the seed crystals 2. Here, "coincidence" does not mean a perfect coincidence. An angular deviation of about 10 degrees in the arrangement direction with respect to the a-axis and m-axis directions is acceptable as an error. Preferably, the angular deviation of the arrangement direction with respect to the a-axis and m-axis directions is 1 degree or less.

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

[0035] 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. Furthermore, the seed crystal 2 has a shape in which a recess 2d is formed in the center of the regular frustum-shaped portion.

[0036] As described later, seed crystal 2 is formed by selective growth using a mask, with crystal growth occurring laterally from the opening in the mask. 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 corresponds to the disc.

[0037] 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 hexagons is larger than the diameter of the disc portion. Since the shape of the disc portion is circular in plan view, stress can be distributed when separating the substrate 1 after growing the group III nitride semiconductor by the flux method, and crack formation in the grown crystal can be suppressed.

[0038] 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.

[0039] The base of the frustum of a regular hexagon of the seed crystal has a regular hexagonal shape. Preferably, the base has a regular hexagonal shape in which each side is aligned with the m-plane of the seed crystal 2 (each side is aligned with the a-axis direction). Since group III nitride semiconductors have a hexagonal crystal structure, the regular hexagonal shape allows for the uniform bonding of group III nitride semiconductors grown from the frustum of a regular hexagon of various crystals 2. However, it is not necessary for each side of the base to perfectly coincide with the a-axis; an angular deviation of about 10 degrees with respect to the a-axis is acceptable. Preferably, each side of the base has an angular deviation of 1 degree or less with respect to the a-axis.

[0040] The six sides 2a of the frustum hexagonal pyramidal portion of the seed crystal 2 have (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 the 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 the side 2a does not need to have (10-11) planes. Preferably, more than 95% of the entire surface of the side 2a has (10-11) planes. 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.

[0041] 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.

[0042] The first embodiment is particularly effective when the diameter D1 of the seed crystal 2 is small. For example, when the diameter D1 is 150 μm or less, 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 method for manufacturing a group III nitride semiconductor in the first embodiment described later, the shape and size of each initial nucleus 3 can be made uniform even when the diameter D1 of the seed crystal 2 is small.

[0043] 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 large, 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 the height H1 is too high, problems arise such as the formation of the seed crystal 2 taking a long time. For this reason, the height H1 is preferably 100 μm or less. The height H1 is more preferably 20 to 60 μm, and even more preferably 30 to 50 μm.

[0044] 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, the height H1 is 0.1 to 0.35 times the diameter D1 of the seed crystal 2, and even more preferably, 0.15 to 0.3 times the diameter D1 of the seed crystal 2.

[0045] A recess 2d is formed in the center of the seed crystal 2. By forming the recess 2d, the initial nuclei 3 grown from the seed crystal 2 do not fill the recess 2d, and voids are formed. The formation of voids prevents dislocations in the seed crystal 2 from propagating upwards, enabling the growth of high-quality group III nitride semiconductors.

[0046] The bottom surface 2b of the recess 2d is formed flat and has a (0001) plane (c plane) of the group III nitride semiconductor. Furthermore, the bottom surface 2b has an approximately circular shape 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.

[0047] 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.

[0048] The depth H2 of the recess 2d is preferably 10 to 100 μm. This range makes it easier to form voids, allowing for the growth of higher quality group III nitride semiconductors. The depth H2 is more preferably 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. The depth H2 is more preferably 0.6 to 0.8 times the height H1 of the seed crystal 2.

[0049] 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.

[0050] 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 have a pattern arranged in a regular triangular grid. The shape of the openings is circular. The shape of the openings may be other than circular, such as a regular hexagon, but they are preferably circular as in the embodiment in order to form a disc portion and suppress cracks when the substrate is peeled off. The mask can be made of any material that can suppress the growth of group III nitride semiconductors on the mask, for example, SiO2.

[0051] 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.

[0052] 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.

[0053] 4. Crucible to use As shown in Figure 1, the method for manufacturing a group III nitride semiconductor in the first embodiment includes a nucleation step S1, a planarization step S2, and a film thickening step S3. Here, three different crucibles made of different materials are used to hold the mixed melt in the nucleation step S1, the planarization step S2, and the film thickening step S3. Hereinafter, the crucible used in the nucleation step S1 will be referred to as the first crucible 100A, and the mixed melt held in the first crucible 100A will be referred to as the mixed melt 101A. Similarly, the crucible used in the planarization step S2 will be referred to as the second crucible 100B, and the mixed melt held in the second crucible 100B will be referred to as the mixed melt 101B. Furthermore, the crucible used in the thickening process S3 is designated as the third crucible 100C, and the mixed melt held in the third crucible 100C is designated as mixed melt 101C.

[0054] The first crucible 100A is made of alumina that does not contain Ca. Here, "Ca-free" includes having a concentration of Ca below the detection limit of general elemental analysis methods. The same applies hereafter. Specifically, the Ca concentration is 0.001 mol% or less. When a mixed molten material 101A that does not contain Ca is held in the first crucible 100A made of such a Ca-free material, the mixed molten material 101A does not contain Ca.

[0055] The second crucible 100B is made of alumina containing Ca. Here, "containing Ca" means having a concentration higher than the detection limit of Ca by general elemental analysis methods. Specifically, the Ca concentration is greater than 0.001 mol%. The same applies hereafter. For example, alumina has a Ca concentration greater than 0.001 mol% and less than or equal to 0.1 mol%. When the mixed molten material 101B is held in the second crucible 100B made of such a Ca-containing material, Ca dissolves from the second crucible 100B into the mixed molten material 101B, and as a result the mixed molten material 101B becomes Ca-containing. The second crucible 100B made of alumina containing Ca can be formed by casting using a plaster mold.

[0056] The third crucible 100C consists of alumina containing Ca. For example, the alumina has a Ca concentration of 0.05 to 0.5 mol%. Similar to the case using the second crucible 100B, using the third crucible 100C results in the mixed melt 101C containing Ca. The Ca concentration of the alumina in the third crucible 100C should be higher than that of the second crucible 100B. This allows the Ca concentration in the mixed melt 101C to be higher than that in the mixed melt 101B, thereby enabling the growth of high-quality group III nitride semiconductors.

[0057] 5. 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 planarization step S2, and a thick-film step S3.

[0058] 5-1.Nucleation process 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.

[0059] 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 predetermined amounts of alkali metals and Group III metals are placed into the first crucible 100A, the second crucible 100B, and the third crucible 100C, respectively. Additives such as carbon may be added as needed. The ratio of alkali metals to Group III metals is the same in the first crucible 100A, the second crucible 100B, and the third crucible 100C.

[0060] Next, the first crucible 100A, the second crucible 100B, and the third crucible 100C containing the raw materials, along with the seed substrate 9, are placed in the reaction vessel and evacuated. Then, 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 first crucible 100A, the second crucible 100B, and the third crucible 100C melt into liquids, forming mixed melts 101A to 101C. At this stage, the seed substrate 9 is not yet added to the mixed melt 101A.

[0061] Next, once the reaction vessel reaches the crystal growth temperature and pressure, and the nitrogen dissolved in the mixed molten material 101A becomes supersaturated, the seed substrate 9 is added to the mixed molten material 101A in the first crucible 100A, as shown in Figure 6. Then, crystals of group III nitride semiconductor (initial nuclei 3) are generated and grow from the various crystals 2 of the seed substrate 9. 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 merge (see Figure 7). Note that a gap remains between the initial nuclei 3 and the substrate 1.

[0062] Here, since the first crucible 100A is made of alumina that does not contain Ca, the mixed melt 101A also does not contain Ca. By making the mixed melt 101A not contain Ca, it becomes easier to make the shape of the initial nucleus 3 hexagonal pyramidal or truncated hexagonal cone, and the shape and size of the initial nucleus 3 can be made more uniform.

[0063] Furthermore, the (10-11) plane on the side surface 2a of the frustum hexagonal portion of the seed crystal 2 remains stable in the mixed melt 101A 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 the side surface 2a while maintaining the (10-11) plane. Since the shapes of the various crystals 2 are uniformly consistent, and 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 shapes of each initial nucleus 3 can be made uniformly consistent.

[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 is formed. The mixed molten liquid 101A is trapped inside the void. Because a void 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 susceptible to etching in the mixed melt 101A and is therefore unstable. 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] 5-2. Flattening process Next, the planarization process S2 is performed. Once adjacent initial nuclei 3 begin to fuse together, as shown in Figure 6, the seed substrate 9 is removed from the first crucible 100A and placed into the mixed molten material 101B held in the second crucible 100B. This transitions the process from the nucleation process S1 to the planarization process S2.

[0067] In the planarization process S2, crystal growth is carried out using the FFC (flux film coating) method. In the FFC method, as shown in Figure 10, the seed substrate 9 is repeatedly removed from and added to the mixed molten material 101B at predetermined intervals. 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 material 101B, the mixed molten material 101B accumulates in the depressions 4 between adjacent initial nuclei 3. This allows the filling layer 5 to grow along the depressions 4 (see Figure 8).

[0068] Here, because the mixed molten liquid 101B accumulated in the depression 4 is thin, it easily becomes supersaturated with nitrogen. Therefore, the rate of crystal growth can be accelerated. On the other hand, because the amount of accumulated mixed molten liquid 101B 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 liquid 101, and the substrate is removed from the mixed molten liquid 101, thereby intermittently supplying the mixed molten liquid 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.

[0069] Furthermore, in the planarization step S2, since the second crucible 100B is made of alumina containing Ca, Ca dissolves from the second crucible 100B into the mixed melt 101B. As a result, the mixed melt 101B contains Ca. The inclusion of Ca in the mixed melt 101B improves its wettability, thereby improving the crystallinity of the embedded layer 5. The Ca concentration of the mixed melt 101B is preferably 0.001 to 0.1 mol%. By setting the Ca concentration within this range, the crystallinity of the embedded layer 5 can be further improved, and the formation of unwanted crystals can be suppressed.

[0070] In the planarization step S2, it is not always necessary to form the embedded layer 5 by the FFC method, but it is preferable to use the FFC method in order to further improve the flatness of the crystal and reduce warping.

[0071] 5-3.Thickening process Next, the thickening process S3 is performed. Once the depressions 4 are filled with the filling layer 5 and a flat crystal surface is formed, as shown in Figure 6, the seed substrate 9 is removed from the second crucible 100B and placed into the mixed molten liquid 101C held in the third crucible 100C. This transitions the process from the planarization process S2 to the thickening process S3.

[0072] In the thickening process S3, the seed substrate 9 is immersed in the mixed molten material 101C to grow a flat layer 6 on a flat crystal plane, thereby increasing its thickness. The flat layer 6 is made of a group III nitride semiconductor with a flat surface. Because the shape of each initial nucleus 3 is uniformly formed, the flat layer 6 can also be formed uniformly within the plane. Furthermore, because the transfer of dislocations from the seed crystal 2 to the upper layer is suppressed, the flat layer 6 can be formed with high quality.

[0073] Furthermore, in the thickening process S3, since the third crucible 100C is made of alumina containing Ca, Ca dissolves from the third crucible 100C into the mixed melt 101C. As a result, the mixed melt 101C contains Ca. The inclusion of Ca in the mixed melt 101C improves its wettability, thereby suppressing the macrostep growth of the group III nitride semiconductor. As a result, the crystallinity of the flat layer 6 can be improved. In this case, the Ca concentration of the mixed melt 101C in the thickening process S3 should be higher than the Ca concentration of the mixed melt 101B in the planarization process S2. The Ca concentration is preferably 0.001 to 0.1 mol%. However, from the viewpoint of suppressing miscrystals, the Ca concentration in the thickening process S3 is preferably 0.05 mol% or less.

[0074] Once the flat layer 6 has grown to the desired thickness, the seed substrate 9 is removed from the third crucible 100C, and then the temperature is lowered to room temperature and the pressure to atmospheric pressure to complete 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 difference in thermal expansion coefficients allows the substrate 1 to be naturally detached from the grown crystal during the cooling process. In particular, if the diameter D1 of the seed crystal 2 is 150 μm or less, the substrate 1 can be detached more easily.

[0075] As described above, the method for manufacturing a group III nitride semiconductor in the first embodiment uses three types of crucibles: a first crucible 100A, a second crucible 100B, and a third crucible 100C, each with a different Ca content. This allows the mixed melt 101A to be Ca-free in the nucleation step S1, while the mixed melts 101B and 101C to contain Ca in the planarization step S2 and the thick-film step S3. As a result, high-quality group III nitride semiconductor crystals can be grown.

[0076] Furthermore, since different crucibles are used in the nucleation process S1, the planarization process S2, and the thickening process S3, depletion of raw materials such as Ga can be prevented.

[0077] (First modified form of the first embodiment) In the first embodiment, the mixed melt 101B contained Ca in the planarization step S2, but the mixed melt 101B does not need to contain Ca. In this case, the second crucible 100B may be made of alumina that does not contain Ca.

[0078] (Second variant of the first embodiment) In the first embodiment, three types of crucibles, the first crucible 100A, the second crucible 100B, and the third crucible 100C, are used. However, it is also possible to use the first crucible 100A and the third crucible 100C, and then use either the first crucible 100A or the third crucible 100C in the planarization step S2. This reduces the number of crucibles used and makes it easier to grow group III nitride semiconductors.

[0079] (Third variant of the first implementation) Furthermore, in the first embodiment, the Ca content of the three crucibles, the first crucible 100A, the second crucible 100B, and the third crucible 100C, is varied, thereby changing the Ca content of the mixed melts 101A to 101C. However, the Ca content of the mixed melts 101A to 101C may be changed by other methods.

[0080] For example, the first crucible 100A, the second crucible 100B, and the third crucible 100C may be made of alumina without Ca, and the Ca content of the mixed melts 101A to 101C may be changed by changing the amount of Ca added when loading the raw materials into the first crucible 100A, the second crucible 100B, and the third crucible 100C. Alternatively, the Ca content of the mixed melts 101A to 101C may be changed by using solid Na or solid Ga with different Ca content as raw materials.

[0081] (Fourth variant of the first embodiment) Alternatively, by using three seed substrates 9 and rotating the nucleation process S1, planarization process S2, and thickening process S3 in sequence, it is possible to grow a group III nitride semiconductor on three seed substrates 9 in a single manufacturing process. That is, once the nucleation process S1 is completed for the first seed substrate 9, the second seed substrate 9 is placed in the first crucible 100A to perform the nucleation process S1, and the first seed substrate 9 is placed in the second crucible 100B to perform the planarization process S2. Once the planarization process S2 is completed for the first seed substrate 9 and the nucleation process S1 is completed for both seed substrates 9, the third seed substrate 9 is placed in the first crucible 100A to perform the nucleation process S1, the second seed substrate 9 is placed in the second crucible 100B to perform the planarization process S2, and the first seed substrate 9 is placed in the third crucible 100C to perform the thickening process S3. By rotating the substrates in the same manner, it is possible to grow a group III nitride semiconductor on three seed substrates 9.

[0082] (Fifth variant of the first embodiment) A fourth crucible made of calcium-containing alumina may be prepared, and after performing the planarization process using the third crucible 100C for a predetermined time, the planarization process may be further performed using the fourth crucible. In the thickening process S3, raw materials such as Ga tend to be depleted in order to form a thick flat layer 6. Therefore, using the fourth crucible can prevent the depletion of raw materials. Of course, a fifth or sixth crucible may be prepared and the process repeated in the same manner. Alternatively, as in the third modified form, a fourth crucible made of calcium-free alumina may be used, and the calcium-containing mixed molten material may be held in the fourth crucible.

[0083] (Second Embodiment) Figures 11 and 12 show the configuration of a group III nitride semiconductor manufacturing apparatus in the second embodiment. The group III nitride semiconductor manufacturing apparatus in the second embodiment can be used, for example, in a method for manufacturing a group III nitride semiconductor in the fourth variant of the first embodiment.

[0084] The manufacturing apparatus for a group III nitride semiconductor in the second embodiment includes three jigs 200. Figure 11 is a schematic diagram showing the configuration of the jig 200 in the manufacturing apparatus for a group III nitride semiconductor. Figure 12 is a plan view taken from above in the vertical direction, showing the relationship between the arrangement of the first crucible 100A, the second crucible 100B, and the third crucible 100C and the arrangement of the three jigs 200.

[0085] The three jigs 200 are placed inside the reaction vessel together with the first crucible 100A, the second crucible 100B, and the third crucible 100C. As shown in Figures 11 and 12, the jig 200 has a first leg portion 201, a second leg portion 202, a third leg portion 203, a connecting portion 204, and a lifting shaft 205. Each component of the jig 200 is made of alumina. The first leg portion 201, the second leg portion 202, and the third leg portion 203 are formed in a roughly rod shape and hang down from the corners of the connecting portion 204, which is a flat plate that is roughly triangular in plan view, as shown in Figure 12.

[0086] The lower ends of the first leg 201, the second leg 202, and the third leg 203 each have protrusions capable of supporting a seed substrate 9. The connecting part 204 is connected to the lifting shaft 205. The lifting shaft 205 can move up and down in the vertical direction. This makes it possible to put the seed substrate 9, supported by the first leg 201, the second leg 202, and the third leg 203, into and out of the crucible. In addition, the lifting shafts 205 of the three jigs 200 can move up and down independently of each other.

[0087] The three jigs 200 are arranged to form an equilateral triangle in plan view, as shown in Figure 12. Let O be the circumcenter of this equilateral triangle. The first crucible 100A, the second crucible 100B, and the third crucible 100C are arranged to form an equilateral triangle opposite each other to the three jigs 200. The three jigs 200 can be rotated around point O. By rotating the three jigs 200 in this way, a particular jig 200 can be moved to face any of the first crucible 100A, the second crucible 100B, or the third crucible 100C.

[0088] When a jig 200 and a first crucible 100A are facing each other, the lifting shaft 205 of the jig 200 is moved up and down, thereby moving the seed substrate 9 held by the jig 200 up and down, which allows control of immersion and removal of the seed substrate 9 into the mixed molten liquid 101A held in the first crucible 100A. Immersion and removal of the seed substrate 9 into the mixed molten liquid 101B held in the second crucible 100B and immersion and removal of the seed substrate 9 into the mixed molten liquid 101C held in the third crucible 100C can also be controlled independently.

[0089] As described above, by using the Group III nitride semiconductor manufacturing apparatus of the second embodiment, the Group III nitride semiconductor manufacturing method of the fourth modified form of the first embodiment can be easily realized.

[0090] Although Embodiment 2 uses three jigs 200, the same method can be applied when using two jigs 200 or four or more jigs 200. In other words, in a plan view, multiple jigs 200 should be arranged at equal intervals on a circumference, and the multiple jigs should be configured to rotate integrally around the center of the circumference. [Explanation of Symbols]

[0091] 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 100A: Crucible 1 100B:Second crucible 100C: 3rd crucible

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 arranged on the substrate. A nucleation step of growing an initial nucleus made of a group III nitride semiconductor from the aforementioned seed crystal, After the nucleation step, a planarization step 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 includes a thick-film formation step, after the planarization step, in which a flat layer made of a group III nitride semiconductor is formed on the planarized crystal surface. In the nucleation step, a first crucible made of alumina with a calcium concentration of 0.01 mol% or less is used, and the mixed molten material is held in the first crucible so that the calcium concentration of the mixed molten material becomes 0.001 mol% or less. A method for producing a group III nitride semiconductor, comprising: in the thick-film formation step, using a second crucible made of alumina with a calcium concentration higher than 0.01 mol%, and holding the mixed molten material in the second crucible so that the calcium concentration of the mixed molten material becomes higher than 0.001 mol%.

2. 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 arranged on the substrate. A nucleation step of growing an initial nucleus made of a group III nitride semiconductor from the aforementioned seed crystal, After the nucleation step, a planarization step 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 includes a thick-film formation step, after the planarization step, in which a flat layer made of a group III nitride semiconductor is formed on the planarized crystal surface. In the nucleation step, a first crucible made of alumina with a calcium concentration of 0.01 mol% or less is used, and the mixed molten material with a calcium concentration of 0.001 mol% or less is held in the first crucible. A method for producing a group III nitride semiconductor, comprising: using a second crucible made of alumina in the thick-film formation step; and holding the mixed molten material having a calcium concentration higher than 0.001 mol% in the second crucible.

3. In the planarization step, a third crucible made of alumina with a calcium concentration higher than 0.01 mol% is used, and the mixed molten material is held in the third crucible so that the calcium concentration of the mixed molten material becomes higher than 0.001 mol%. A method for producing a group III nitride semiconductor according to claim 1 or 2, wherein the calcium concentration of the mixed melt in the thickening step is higher than the calcium concentration of the mixed melt in the planarization step.

4. In the planarization step, a third crucible made of alumina is used, and the mixed molten material having a calcium concentration higher than 0.001 mol% is held in the third crucible. A method for producing a group III nitride semiconductor according to claim 1 or 2, wherein the calcium concentration of the mixed melt in the thickening step is higher than the calcium concentration of the mixed melt in the planarization step.

5. A method for producing a group III nitride semiconductor according to claim 1 or claim 2, wherein the second crucible is used in the planarization step.

6. In the thickening process, a fourth crucible made of alumina with a calcium concentration higher than 0.01 mol% is used, and the mixed molten material is held in the fourth crucible so that the calcium concentration of the mixed molten material becomes higher than 0.001 mol%. A method for manufacturing a group III nitride semiconductor according to claim 1 or 2, wherein the flat layer is formed using the second crucible, and then the flat layer is formed using the fourth crucible.

7. In the aforementioned thickening process, a fourth crucible made of alumina is used, and the mixed molten material having a calcium concentration higher than 0.001 mol% is held in the fourth crucible. A method for manufacturing a group III nitride semiconductor according to claim 1 or 2, wherein the flat layer is formed using the second crucible, and then the flat layer is formed using the fourth crucible.

8. A apparatus for manufacturing 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 system has a plurality of jigs for holding the seed substrate and for immersing and removing the seed substrate from the mixed melt, A manufacturing apparatus for group III nitride semiconductors, wherein each jig is configured to allow independent immersion and removal of the seed substrate into the mixed molten liquid.

9. The apparatus for manufacturing a group III nitride semiconductor according to claim 8, wherein each of the jigs is arranged at equal intervals on a circumference when viewed from above in the vertical direction, and each of the jigs is configured to be rotatable as a whole around the center of the circumference.

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