Nitride Semiconductor Substrate

Abstract

To improve crystal quality of a nitride semiconductor substrate.SOLUTION: A method for manufacturing a nitride semiconductor substrate comprises: the step of preparing a base substrate; forming a first base layer on the base substrate; forming a metal layer on the first base layer; performing heating to form voids in the first base layer; epitaxially growing a second base layer above the first base layer to form the main surface of the second base layer into a mirror surface; the three-dimensional growth step of directly epitaxially growing a single crystal of a group III nitride semiconductor having a top surface having an exposed (0001)plane on the main surface of the second base layer, forming a plurality of recessed parts constituted of inclination interfaces except the (0001)plane on the top surface, gradually expanding the inclination interfaces as the same go above the second base layer and eliminating the (0001)plane from the top surface to grow a first layer having a surface constituted of only the inclination interfaces; and the flattening step of epitaxially growing the single crystal of the group III nitride semiconductor on the first layer to eliminate the inclination interfaces and growing a second layer having a surface formed into a mirror surface.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to a method for manufacturing a nitride semiconductor substrate, a nitride semiconductor substrate, and a stacked structure. [Background technology]

[0002] Various methods have been disclosed for obtaining a free-standing substrate made of a single crystal of a Group III nitride semiconductor (hereinafter also referred to as a "nitride semiconductor substrate") (for example, Patent Document 1). [Prior art documents] [Patent documents]

[0003] [Patent Document 1] JP 2003-178984 A Summary of the Invention [Problem to be solved by the invention]

[0004] An object of the present invention is to improve the crystal quality of a nitride semiconductor substrate. [Means for solving the problem]

[0005] According to one aspect of the present invention, providing a base substrate; forming a first underlayer made of a Group III nitride semiconductor on the underlayer; forming a metal layer on the first underlayer; performing a heat treatment to form voids in the first underlayer; epitaxially growing a second underlayer made of a single crystal of a Group III nitride semiconductor and having a primary surface whose nearest low-index crystal plane is the (0001) plane on the first underlayer, and mirror-finishing the primary surface of the second underlayer; a three-dimensional growth step of epitaxially growing a single crystal of a Group III nitride semiconductor having a top surface with an exposed (0001) plane directly on the main surface of the second underlayer, generating a plurality of recesses on the top surface constituted by inclined interfaces other than the (0001) plane, gradually expanding the inclined interfaces toward the top of the second underlayer, causing the (0001) plane to disappear from the top surface, and growing a first layer whose surface is constituted only by the inclined interfaces; a planarization step of epitaxially growing a single crystal of a Group III nitride semiconductor on the first layer to eliminate the inclined interface and grow a second layer having a mirror-finished surface; have A method for manufacturing a nitride semiconductor substrate is provided.

[0006] According to another aspect of the present invention, In the method for producing a nitride semiconductor substrate according to the above aspect, a nitride semiconductor substrate is obtained by slicing the second layer. A nitride semiconductor substrate is provided.

[0007] According to yet another aspect of the present invention, A nitride semiconductor substrate having a diameter of 2 inches or more and a primary surface in which the nearest low-index crystal plane is the (0001) plane, When the principal surface is irradiated with Cu Kα1 X-rays through a Ge (220) plane two-crystal monochromator and a slit, and an X-ray rocking curve measurement of (0002) plane diffraction is performed, the difference FWHMa-FWHMb obtained by subtracting the half-width FWHMb of the (0002) plane diffraction when the width of the slit in the ω direction is 0.1 mm from the half-width FWHMa of the (0002) plane diffraction when the width of the slit in the ω direction is 1 mm is 30% or less of FWHMa. A nitride semiconductor substrate is provided.

[0008] According to another aspect of the present invention, A nitride semiconductor substrate having a diameter of 2 inches or more, When the main surface of the nitride semiconductor substrate is observed with a multiphoton excitation microscope in a field of view of 250 μm square and the dislocation density is calculated from the dark spot density, the dislocation density is 3×10 6 cm -2 and the dislocation density is not greater than 1×10 6 cm -2 The area where the ratio is less than 80% of the main surface is A nitride semiconductor substrate is provided.

[0009] According to yet another aspect of the present invention, A nitride semiconductor substrate having a primary surface in which the nearest low-index crystal plane is a (0001) plane, An X-ray rocking curve of the (0002) plane is measured at each position on a line passing through the center of the main surface, and a peak angle ω between the X-ray incident on the main surface and the main surface is plotted against the position on the line. When the peak angle ω is approximated by a linear function of the position, the radius of curvature of the (0001) plane, which is calculated by the inverse of the slope of the linear function, is 10 m or more; The error of the measured peak angle ω with respect to the linear function is less than or equal to 0.05°. A nitride semiconductor substrate is provided.

[0010] According to yet another aspect of the present invention, an underlayer made of a single crystal of a Group III nitride semiconductor, having a mirror-finished primary surface, the lowest-index crystal plane closest to the primary surface being the (0001) plane; a first layer, the first layer having a surface composed only of the inclined interfaces, formed by epitaxially growing a single crystal of a Group III nitride semiconductor directly on the main surface of the underlayer, the single crystal having a top surface with the (0001) plane exposed, generating a plurality of recesses on the top surface composed of inclined interfaces other than the (0001) plane, and gradually expanding the inclined interfaces toward the top of the underlayer until the (0001) plane disappears from the top surface; a second layer having a mirror-finished surface formed by epitaxially growing a single crystal of a Group III nitride semiconductor on the first layer to eliminate the inclined interface; have A laminate structure is provided. Effect of the Invention

[0011] According to the present invention, the crystal quality of the nitride semiconductor substrate can be improved. [Brief description of the drawings]

[0012] [Figure 1] 1 is a flowchart showing a method for manufacturing a nitride semiconductor substrate according to an embodiment of the present invention. [Diagram 2] 1 is a flowchart showing a base structure manufacturing process. [Diagram 3] 1(a) to 1(e) are schematic cross-sectional views showing a part of a method for manufacturing a nitride semiconductor substrate according to an embodiment of the present invention. [Figure 4] 1(a) to 1(c) are schematic cross-sectional views showing a part of a method for manufacturing a nitride semiconductor substrate according to one embodiment of the present invention. [Diagram 5] 1 is a schematic perspective view showing a part of a method for manufacturing a nitride semiconductor substrate according to an embodiment of the present invention. [Figure 6] 1(a) and 1(b) are schematic cross-sectional views showing a part of a method for manufacturing a nitride semiconductor substrate according to one embodiment of the present invention. [Figure 7] 1(a) and 1(b) are schematic cross-sectional views showing a part of a method for manufacturing a nitride semiconductor substrate according to one embodiment of the present invention. [Figure 8] 1A to 1C are schematic cross-sectional views showing a part of a method for manufacturing a nitride semiconductor substrate according to an embodiment of the present invention. [Figure 9] FIG. 1(a) is a schematic cross-sectional view showing the growth process under reference growth conditions in which the inclined interface and the c-plane do not expand or contract, and FIG. 1(b) is a schematic cross-sectional view showing the growth process under first growth conditions in which the inclined interface expands and the c-plane contracts. [Figure 10] 1A-1C are schematic cross-sectional views showing the growth process under second growth conditions in which the inclined interface shrinks and the c-plane expands. [Figure 11]FIG. 1A is a schematic top view showing a nitride semiconductor substrate according to one embodiment of the present invention; FIG. 1B is a schematic cross-sectional view along the m-axis of the nitride semiconductor substrate according to one embodiment of the present invention; and FIG. 1C is a schematic cross-sectional view along the a-axis of the nitride semiconductor substrate according to one embodiment of the present invention. [Figure 12] FIG. 1(a) is a schematic cross-sectional view showing the diffraction of X-rays against a curved c-plane, and (b) and (c) are diagrams showing the fluctuation of the diffraction angle of the (0002) plane versus the radius of curvature of the c-plane. [Figure 13] FIG. 2 is a view showing an image of a cross section of a laminated structure of an example observed with a fluorescent microscope. [Figure 14] FIG. 2 is a view of a main surface of a nitride semiconductor substrate according to an example, observed using a multiphoton excitation microscope. [Figure 15] FIG. 1( a ) is a diagram showing the results of X-ray rocking curve measurement of (0002) plane diffraction along the m-axis direction for each of the nitride semiconductor substrates of the example and comparative example 1, and FIG. 1( b ) is a diagram showing the results of X-ray rocking curve measurement of (0002) plane diffraction along the a-axis direction for each of the nitride semiconductor substrates of the example and comparative example 1. [Figure 16] FIG. 1(a) shows a normalized X-ray diffraction pattern when X-ray rocking curve measurements were performed with different slits on the nitride semiconductor substrate of the example, and FIG. 1(b) shows a normalized X-ray diffraction pattern when the same measurement as in the example was performed on the nitride semiconductor substrate of Comparative Example 1. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0013] <Knowledge gained by the inventors> First, the findings of the inventors will be described.

[0014] The method described in the above-mentioned Patent Document 1 is called a Void-assisted Separation Method (VAS). The VAS method is carried out, for example, as follows.

[0015] First, a first crystal layer made of a group III nitride semiconductor is formed on a predetermined base substrate. After the first crystal layer is formed, a metal layer is formed on the first crystal layer. After the metal layer is formed, a heat treatment is performed in an atmosphere containing a predetermined gas to form fine holes in the metal layer and to form voids in the first crystal layer through the mesh (nano-net) of the metal layer. After the voids are formed in the first crystal layer, a second crystal layer made of a group III nitride semiconductor is formed above the first crystal layer. At this time, some of the voids remain. After the second crystal layer is formed, the second crystal layer is peeled off from the base substrate due to the remaining voids when the temperature is lowered from the growth temperature of the second crystal layer. After the second crystal layer is peeled off, the second crystal layer is sliced ​​and polished to obtain a nitride semiconductor substrate with high crystal quality.

[0016] Here, when a crystal layer made of a single crystal of a Group III nitride semiconductor is grown thick with the c-plane as the growth plane, the dislocation density on the surface of the crystal layer tends to be inversely proportional to the thickness of the crystal layer.

[0017] Therefore, in the above-mentioned VAS method, in order to reduce the dislocation density of the second crystal layer, it is conceivable to grow the second crystal layer thicker above the first crystal layer.

[0018] However, in the above-mentioned VAS method, tensile stress generated by the attraction of the initial crystal nuclei accumulates on the undersubstrate side of the second crystal layer. Meanwhile, the c-plane of the second crystal layer is curved into a concave spherical shape due to the tensile stress generated in the second crystal layer. When the second crystal layer is grown thick on the concavely curved c-plane, the stress applied to the second crystal layer gradually changes to compressive stress as the second crystal layer becomes thicker. Therefore, the stress difference between the undersubstrate side and the surface side of the second crystal layer gradually increases. If the stress difference becomes excessive, cracks may occur in the second crystal layer.

[0019] Thus, in the VAS method, it is difficult to grow a thick second crystal layer using the c-plane as the growth plane.

[0020] The present invention is based on the above findings made by the inventors.

[0021] <One embodiment of the present invention> Hereinafter, an embodiment of the present invention will be described with reference to the drawings.

[0022] (1) Method for manufacturing a nitride semiconductor substrate A method for manufacturing a nitride semiconductor substrate according to this embodiment will be described with reference to FIGS. FIG. 1 is a flowchart showing a method for manufacturing a nitride semiconductor substrate according to this embodiment. FIG. 2 is a flowchart showing an underlayer structure fabrication process. FIGS. 3(a)-(e), 4(a)-(c), and 6(a)-8 are schematic cross-sectional views showing a part of the method for manufacturing a nitride semiconductor substrate according to this embodiment. FIG. 5 is a schematic perspective view showing a part of the method for manufacturing a nitride semiconductor substrate according to this embodiment. Note that the underside of the underlayer substrate 1 is omitted in FIGS. 3(a)-(e). FIG. 5 corresponds to a perspective view at the time of FIG. 4(b), and shows a part of the first layer 30 grown on the underlayer structure 10. In FIG. 6(b), the thin solid line in the second layer 40 indicates a crystal plane during growth, and in FIG. 4(c) and FIG. 6(a)-FIG. 8, the dotted line indicates a dislocation.

[0023] In the following, in a crystal of a group III nitride semiconductor having a wurtzite structure, <0001> The axis (e.g., the

[0001] axis) is called the "c-axis" and the (0001) plane is called the "c-plane". The (0001) plane is sometimes called the "+c-plane (III group element polar plane)" and the (000-1) plane is sometimes called the "-c-plane (nitrogen (N) polar plane)". The <1-100> axis (e.g., the [1-100] axis) is called the "m-axis" and the {1-100} plane is called the "m-plane". The m-axis may also be written as the <10-10> axis. The <11-20> axis (e.g., the [11-20] axis) is called the "a-axis" and the {11-20} plane is called the "a-plane".

[0024] As shown in FIG. 1, the method for manufacturing a nitride semiconductor substrate according to this embodiment includes, for example, a base structure fabrication step S100, a three-dimensional growth step S200, a planarization step S300, a slicing step S400, and a polishing step S500.

[0025] (S100: Base structure fabrication process) First, the base structure 10 is fabricated by, for example, the above-mentioned VAS method.

[0026] Specifically, the base structure fabricating step S100 includes, for example, a base substrate preparing step S110, a first base layer forming step S120, a metal layer forming step S130, a void forming step S140, and a second base layer forming step S150.

[0027] (S110: Base board preparation process) First, as shown in Fig. 3(a), a base substrate 1 is prepared. The base substrate 1 is made of, for example, a material different from a group III nitride semiconductor. Specifically, the base substrate 1 is, for example, a sapphire substrate. The base substrate 1 may be, for example, a Si substrate or a gallium arsenide (GaAs) substrate.

[0028] The diameter of the starting substrate 1 is, for example, 2 inches (50.8 mm) or more, and the thickness of the starting substrate 1 is, for example, 300 μm or more and 1 mm or less.

[0029] The base substrate 1 has, for example, a primary surface 1s which serves as an epitaxial growth surface. The low-index crystal plane closest to the primary surface 1s is, for example, the c-plane.

[0030] In this embodiment, the c-plane of the base substrate 1 is inclined with respect to the primary surface 1s. The c-axis of the base substrate 1 is inclined at a predetermined off angle with respect to the normal to the primary surface 1s. The off angle within the primary surface 1s of the base substrate 1 is uniform across the entire primary surface 1s. The magnitude of the off angle at the center of the primary surface 1s of the base substrate 1 is, for example, greater than 0° and equal to or less than 1°. The off angle within the primary surface 1s of the base substrate 1 affects the off angle at the center of the primary surface 6s of the second base layer 6 described below.

[0031] (S120: First base layer formation step) Next, as shown in FIG. 3(b), a first underlayer (first crystal layer) 2 made of a Group III nitride is formed on the primary surface 1s of the underlayer 1.

[0032] Specifically, for example, a group III source gas and a nitrogen source gas are supplied to a base substrate 1 heated to a predetermined growth temperature by metal organic vapor phase epitaxy (MOVPE). For example, trimethylgallium (TMG) gas as the group III source gas and ammonia gas (NH 3 ) is supplied to grow a low-temperature grown GaN buffer layer and a GaN layer in this order as the first underlayer 2 on the primary surface 1s of the underlayer 1. During the growth of the GaN layer, for example, monosilane (SiH 4 ) gas may be supplied to dope the GaN layer with n-type impurities.

[0033] At this time, the growth conditions of the low-temperature grown GaN buffer layer as the first underlayer 2 are adjusted so as to obtain the desired crystal quality of the GaN layer. Specifically, the growth temperature of the low-temperature grown GaN buffer layer is set to, for example, 400° C. or more and 600° C. or less.

[0034] At this time, the growth conditions of the GaN layer as the first underlayer 2 are adjusted so that desired voids are formed in the void formation step S140 described later. Specifically, the growth temperature of the GaN layer is set to, for example, 1,000° C. or more and 1,200° C. or less.

[0035] In addition, at this time, for example, the surface of the first underlayer 2 is mirror-finished. Note that the "mirror surface" here refers to a surface in which the difference in surface irregularities is equal to or less than the wavelength of visible light, and the surface of the first underlayer 2 may have hillocks in which facets other than the c-plane are exposed. Specifically, the root-mean-square roughness RMS of the surface of the first underlayer 2 is set to, for example, less than 10 nm, preferably less than 1 nm. By mirror-finishing the surface of the first underlayer 2 in this way, the appearance of voids can be made substantially uniform over the entire main surface 1s of the undersubstrate 1 in the void formation step S140 described later.

[0036] (S130: Metal layer formation process) 3(c), a metal layer 3 is formed on the first underlayer 2. For example, the metal layer 3 is formed by vacuum deposition or sputtering.

[0037] The metal layer 3 is preferably made of a material configured to form fine holes therein by heat treatment in the void formation step S140 described later, promote decomposition of the first underlayer 2, and form voids in the first underlayer 2. Specifically, examples of the metal layer 3 that satisfy this condition include titanium (Ti), scandium (Sc), yttrium (Y), zirconium (Zr), hafnium (Hf), vanadium (V), niobium (Nb), tantalum (Ta), chromium (Cr), molybdenum (Mo), tungsten (W), rhenium (Re), iron (Fe), ruthenium (Ru), osmium (Os), cobalt (Co), rhodium (Rh), iridium (Ir), nickel (Ni), palladium (Pd), manganese (Mn), copper (Cu), platinum (Pt), and gold (Au). In this embodiment, the metal layer 3 is, for example, a Ti layer.

[0038] At this time, the thickness of the metal layer 3 is, for example, 1 μm or less, preferably 300 nm or less, more preferably 100 nm or less. By setting the thickness of the metal layer 3 to 1 μm or less, preferably 300 nm or less, more preferably 100 nm or less, it is possible to suppress the decrease in flatness of the metal layer 3 (metal nitride layer 5) in the void formation step S140 described later. This makes it possible to suppress the decrease in the crystal quality of the second underlayer 6 formed on the metal nitride layer 5. The lower limit of the thickness of the metal layer 3 is not particularly limited. However, from the viewpoint of stably forming the metal nitride layer 5 in the void formation step S140 described later, it is preferable that the thickness of the metal layer 3 is, for example, 0.5 nm or more.

[0039] (S140: Void formation process) After the metal layer 3 is formed, a predetermined heat treatment is carried out to form voids in the first underlayer 2.

[0040] Specifically, the above-mentioned starting substrate 1 is placed in an electric furnace, and the starting substrate 1 is placed on a susceptor having a predetermined heater. After the starting substrate 1 is placed on the susceptor, the starting substrate 1 is heated by the heater and heat-treated in an atmosphere containing a predetermined gas.

[0041] In this case, for example, nitrogen (N 2 The heat treatment is performed in an atmosphere containing at least one of a nitrogen-containing gas and a nitrogen-containing gas. 3 Gas and hydrazine (N 2 H 4 ) gas. This allows the metal layer 3 to be aggregated and nitrided, and a mesh-like metal nitride layer 5 (also called a nanonet of the metal nitride layer 5) having a high density of fine holes (through holes) on the surface can be formed.

[0042] Furthermore, at this time, for example, hydrogen (H 2 The heat treatment is performed in an atmosphere containing at least one of the above-mentioned NH 3 Gas and N 2 H 4As a result, a part of the first underlayer 2 can be etched through the mesh of the metal nitride layer 5, and a high density of voids can be formed in the first underlayer 2.

[0043] At this time, the step of forming the nanonet of the metal nitride layer 5 and the step of forming voids in the first underlayer 2 may be carried out simultaneously, or may be carried out separately in different atmospheres.

[0044] At this time, the heat treatment conditions are adjusted so as to obtain a predetermined void ratio in the first underlayer 2. Specifically, the partial pressure ratio of the hydride gas in the atmosphere is, for example, 10% to 95%, preferably 50% or less. The heat treatment temperature is, for example, 1,000°C to 1,100°C. The heat treatment time is, for example, 1 minute to 100 minutes.

[0045] Through the above void forming step S140, a void-containing first underlayer 4 is formed as shown in FIG. 3(d).

[0046] (S150: Second base layer formation step) After the void-containing first underlayer 4 is formed, a second underlayer (second crystal layer) 6 made of a single crystal of a Group III nitride semiconductor is epitaxially grown on the void-containing first underlayer 4, as shown in FIG. 3(e).

[0047] Specifically, for example, the base substrate 1 is loaded into a predetermined vapor phase growth apparatus. Next, a group III source gas and a nitrogen source gas are supplied to the base substrate 1 heated to a predetermined growth temperature by a hydride vapor phase epitaxy (HVPE) method. For example, gallium chloride (GaCl) gas and NH 3 By supplying the gas, a GaN layer as the second underlayer 6 is epitaxially grown on the void-containing first underlayer 4 and the metal nitride layer 5. During the growth of the GaN layer as the second underlayer 6, for example, dichlorosilane (SiH 2 Cl 2 ) gas and tetrachlorogermane (GeCl 4) gas may be supplied to dope the GaN layer with n-type impurities.

[0048] In the initial stage of growth of the second underlayer 6, initial nuclei of island-shaped crystals are generated. At this time, the frequency of island-shaped crystals in the second underlayer 6 depends on the degree of supersaturation when growing on the nanonet of the metal nitride layer 5 and the variation in the opening width of the nanonet. As the second underlayer 6 is grown, the initial nuclei of island-shaped crystals grow laterally. Thereafter, when the distance between adjacent initial nuclei approaches, it is more energetically stable if the surfaces of the initial nuclei are bonded together than if there are two surfaces of the initial nuclei. Therefore, adjacent initial nuclei are forcibly attracted to each other (meet). The initial nuclei meet each other throughout the entire second underlayer 6, forming the main surface 6s of the second underlayer 6.

[0049] At this time, for example, the main surface 6s of the second underlayer 6 is mirror-finished. As described above, the "mirror surface" here refers to a surface in which the difference in surface irregularities is equal to or less than the wavelength of visible light, and the main surface 6s of the second underlayer 6 may have hillocks in which facets other than the c-plane are exposed. Specifically, the root-mean-square roughness RMS of the main surface 6s of the second underlayer 6 is, for example, less than 10 nm, preferably less than 1 nm. Although the details will be described later in (4), by mirror-finishing the main surface 6s of the second underlayer 6, the growth form of the first layer 30 described later can be changed from the growth form of island-like crystals generated in the early stage of growth of the second underlayer 6. As a result, the average distance between the nearest apexes of the first layer 30 described later can be controlled based on the first growth conditions in the three-dimensional growth step S200, and the distance between the nearest apexes can be increased.

[0050] In this case, since the low-index crystal plane closest to the primary surface 1s of the undersubstrate 1 is the c-plane, the low-index crystal plane closest to the mirror-finished primary surface 6s of the second underlayer 6 is also the c-plane.

[0051] At this time, the growth conditions of the second underlayer 6 are adjusted so that initial nuclei of crystals that grow in islands are generated at a predetermined frequency. Specifically, the growth temperature of the second underlayer 6 is set to, for example, 1,000°C or higher and 1,100°C or lower. In addition, the ratio of the partial pressure of NH as a nitrogen source gas to the partial pressure of GaCl gas as a Group III source gas during the growth of the second underlayer 6 is set to, for example, 1,000°C or higher and 1,100°C or lower. 3 The ratio of partial pressures of the gas flow rates (hereinafter also referred to as the "V / III ratio") is set to, for example, 1 or more and 50 or less.

[0052] In addition, at this time, the thickness of the second underlayer 6 is, for example, 100 μm or more and 1.5 mm or less, preferably 200 μm or more and 500 μm or less. If the thickness of the second underlayer 6 is less than 100 μm, it is difficult to mirror-finish the main surface 6s of the second underlayer 6. In contrast, by making the thickness of the second underlayer 6 100 μm or more, the main surface 6s of the second underlayer 6 can be easily mirror-finished. This reliably changes the growth form of the first layer 30 from the growth form of the island-like crystals generated in the early stage of growth of the second underlayer 6, and easily lengthens the distance between the nearest tops of the first layer 30 that grows three-dimensionally in the three-dimensional growth step S200 described later. Furthermore, by making the thickness of the second underlayer 6 200 μm or more, the main surface 6s of the second underlayer 6 can be stably mirror-finished. On the other hand, if the thickness of the second underlayer 6 exceeds 1.5 mm, there is a possibility that cracks will occur in the second underlayer 6. In contrast, by setting the thickness of the second underlayer 6 to 1.5 mm or less, it is possible to suppress the occurrence of cracks in the second underlayer 6. Furthermore, by setting the thickness of the second underlayer 6 to 500 μm or less, it is possible to stably suppress the occurrence of cracks in the second underlayer 6.

[0053] At this time, the second underlayer 6 grows on the void-containing first underlayer 4 and the metal nitride layer 5 through the holes in the metal nitride layer 5 from the void-containing first underlayer 4. Some of the voids in the void-containing first underlayer 4 are filled by the second underlayer 6, but other parts of the voids in the void-containing first underlayer 4 remain. A flat gap is formed between the second underlayer 6 and the metal nitride layer 5 due to the voids remaining in the void-containing first underlayer 4. This gap causes the second underlayer 6 to peel off in a peeling step S380 described later.

[0054] In addition, at this time, tensile stress is introduced into the second underlayer 6 by the initial nuclei generated during the growth process attracting each other. Therefore, due to the tensile stress generated in the second underlayer 6, internal stress acts so that the c-plane of the second underlayer 6 is recessed. In addition, the dislocation density on the main surface 6s side of the second underlayer 6 is low, while the dislocation density on the base substrate 1 side of the second underlayer 6 is high. Therefore, even due to the difference in dislocation density in the thickness direction of the second underlayer 6, internal stress acts so that the c-plane of the second underlayer 6 is recessed. Therefore, the c-plane of the second underlayer 6 is curved into a concave spherical shape with respect to the main surface 6s. The term "spherical" here means a curved surface that is approximated by a spherical surface. The term "spherical approximation" here means that the surface is approximated to a perfect spherical surface or an elliptical spherical surface within a predetermined range of error. As described above, because the c-plane is curved, the off-angle that the c-axis makes with respect to the normal to the center of the main surface 6s of the second underlayer 6 has a predetermined distribution.

[0055] By the above-described base structure fabrication step S100, the base structure 10 is obtained.

[0056] (S200: 3D growth process (1st layer growth process)) 4(b), 4(c), and 5, a single crystal of a Group III nitride semiconductor having a top surface 30u with an exposed c-plane 30c is epitaxially grown directly on the primary surface 6s of the second underlayer 6. This causes a first layer (three-dimensionally grown layer) 30 to grow.

[0057] At this time, a plurality of recesses 30p surrounded by inclined interfaces 30i other than the c-plane are generated on the top surface 30u of the single crystal, and the inclined interfaces 30i are gradually enlarged and the c-planes 30c are gradually reduced toward the top of the second underlayer 6. This causes the c-planes 30c to disappear from the top surface 30u. As a result, the first layer 30 is grown whose surface is composed only of the inclined interfaces 30i.

[0058] That is, in the three-dimensional growth step S200, the first layer 30 is three-dimensionally grown so as to intentionally roughen the main surface 6s of the mirror-finished second underlayer 6. Even if the first layer 30 is grown in such a growth form, it is grown as a single crystal as described above. In this respect, the first layer 30 is different from a so-called low-temperature grown buffer layer that is formed as an amorphous or polycrystalline state on a heterogeneous substrate such as sapphire before epitaxially growing a Group III nitride semiconductor on the heterogeneous substrate.

[0059] In this embodiment, for example, a layer made of the same Group III nitride semiconductor as that constituting the base substrate 1 is epitaxially grown as the first layer 30. Specifically, for example, the base substrate 1 is heated by the HVPE method, and GaCl gas and NH 3 By supplying the gas, a GaN layer is epitaxially grown as the first layer 30 .

[0060] Here, in the three-dimensional growth step S200, in order to realize the above-mentioned growth process, for example, the first layer 30 is grown under a predetermined first growth condition.

[0061] First, a reference growth condition in which the inclined interface 30i and the c-plane 30c do not expand or contract will be described with reference to Fig. 9(a). Fig. 9(a) is a schematic cross-sectional view showing a growth process under the reference growth condition in which the inclined interface and the c-plane do not expand or contract.

[0062] In Fig. 9(a), the thick solid line indicates the surface of the first layer 30 per unit time. The inclined interface 30i shown in Fig. 9(a) is the inclined interface most inclined with respect to the c-plane 30c. In Fig. 9(a), the growth rate of the c-plane 30c of the first layer 30 is G c0 The growth rate of the inclined interface 30i of the first layer 30 is G i 9(a), the angle between the c-plane 30c and the inclined interface 30i in the first layer 30 is assumed to be α. Also, in FIG. 9(a), the first layer 30 is assumed to grow while maintaining the angle α between the c-plane 30c and the inclined interface 30i. Note that the off-angle of the c-plane 30c of the first layer 30 is assumed to be negligible compared to the angle α between the c-plane 30c and the inclined interface 30i.

[0063] 9(a), when the inclined interface 30i and the c-plane 30c do not expand or contract, the locus of intersections between the inclined interface 30i and the c-plane 30c is perpendicular to the c-plane 30c. Therefore, the reference growth condition under which the inclined interface 30i and the c-plane 30c do not expand or contract satisfies the following formula (a). G c0 =G i / cosα (a)

[0064] Next, a first growth condition in which the inclined interface 30i expands and the c-plane 30c shrinks will be described with reference to Fig. 9(b). Fig. 9(b) is a schematic cross-sectional view showing a growth process under the first growth condition in which the inclined interface expands and the c-plane shrinks.

[0065] 9(b), as in FIG. 9(a), the thick solid line indicates the surface of the first layer 30 per unit time. The inclined interface 30i shown in FIG. 9(b) is also the inclined interface most inclined with respect to the c-plane 30c. In FIG. 9(b), the growth rate of the c-plane 30c of the first layer 30 is G c1 The progression rate of the path of the intersection between the inclined interface 30i and the c-plane 30c in the first layer 30 is R 1 In addition, the narrower angle between the path of intersections between the inclined interface 30i and the c-plane 30c and the c-plane 30c is defined as α R1 Let us assume that R1 Direction and G i If the angle between the direction is α', then α'=α+90-α R1 It is assumed that the off-angle of the c-plane 30c of the first layer 30 is negligible compared to the angle α between the c-plane 30c and the inclined interface 30i.

[0066] As shown in FIG. 9(b), the progression rate R 1 is expressed by the following equation (b). R 1 =G i / cosα' (b)

[0067] In addition, the growth rate G c1 is expressed by the following equation (c). G c1 =R 1 sin α R1 (c)

[0068] By substituting equation (b) into equation (c), G c1 is G i Using this, it is expressed by the following equation (d). G c1 =G i sin α R1 / cos(α+90-α R1 ) (d)

[0069] In order for the inclined interface 30i to expand and the c-plane 30c to shrink, α R1 Therefore, the first growth condition under which the inclined interface 30i expands and the c-plane 30c shrinks is expressed by the formula (d) and α R1 <90°, it is preferable that the following formula (1) is satisfied. G c1 >G i / cosα (1) However, as mentioned above, G i is the growth rate of the inclined interface 30i that is most inclined with respect to the c-plane 30c, and α is the angle between the inclined interface 30i that is most inclined with respect to the c-plane 30c and the c-plane 30c.

[0070] Or, G under the first growth condition c1 is G under standard growth conditions. c0 It can also be considered that it is preferable for G c1 >G c0 By substituting equation (a) into equation (1), equation (1) can be derived.

[0071] Since the growth condition for expanding the inclined interface 30i that is most inclined with respect to the c-plane 30c is the strictest condition, if the first growth condition satisfies formula (1), it is possible to expand the other inclined interfaces 30i as well.

[0072] Specifically, for example, when the inclined interface 30i most inclined with respect to the c-plane 30c is the {10-11} plane, α=61.95°. Therefore, it is preferable that the first growth condition satisfies, for example, the following formula (1′). G c1 >2.13G i (1')

[0073] Alternatively, as described later, for example, when the inclined interface 30i is a {11-2m} plane with m≧3, the inclined interface 30i most inclined with respect to the c-plane 30c is the {11-23} plane, and therefore α=47.3°. Therefore, it is preferable that the first growth condition satisfies, for example, the following formula (1″). G c1 >1.47G i ...(1")

[0074] As the first growth condition in this embodiment, for example, the growth temperature in the three-dimensional growth step S200 is set lower than the growth temperature in the flattening step S300 described later. Specifically, the growth temperature in the three-dimensional growth step S200 is set to, for example, 980° C. or more and 1,020° C. or less, preferably 1,000° C. or more and 1,020° C. or less.

[0075] As the first growth condition of this embodiment, for example, the V / III ratio in the three-dimensional growth step S200 may be set to be larger than the V / III ratio in the planarization step S300 described later. Specifically, the V / III ratio in the three-dimensional growth step S200 is set to, for example, 2 to 20, preferably 2 to 15.

[0076] In practice, at least one of the growth temperature and the V / III ratio is adjusted within the above-mentioned ranges so as to satisfy the formula (1) as the first growth condition.

[0077] Other conditions among the first growth conditions in this embodiment are, for example, as follows. Growth pressure: 90 to 105 kPa, preferably 90 to 95 kPa GaCl gas partial pressure: 1.5~15kPa N 2 Gas flow rate / H 2 Gas flow rate: 0~1

[0078] Here, the three-dimensional growth step S200 of this embodiment is classified into two steps, for example, based on the morphology of the first layer 30 during growth. Specifically, the three-dimensional growth step S200 of this embodiment includes, for example, an inclined interface expansion step S220 and an inclined interface maintenance step S240. Through these steps, the first layer 30 includes, for example, an inclined interface expansion layer 32 and an inclined interface maintenance layer 34.

[0079] (S220: Inclined interface expansion process) First, as shown in FIGS. 4(b), 4(c) and 5, the gradient interface expansion layer 32 of the first layer 30 made of a single crystal of a Group III nitride semiconductor is epitaxially grown on the second underlayer 6 under the above-mentioned first growth conditions.

[0080] In the initial stage of growth of the inclined interface spreading layer 32, the inclined interface spreading layer 32 grows in the normal direction (direction along the c-axis) of the main surface 1s of the base substrate 1, with the c-plane 30c as the growth plane.

[0081] By gradually growing the inclined interface expansion layer 32 under the first growth conditions, as shown in Figures 4(c) and 5, a plurality of recesses 30p consisting of inclined interfaces 30i other than the c-plane are generated on the top surface 30u where the c-plane 30c is exposed of the inclined interface expansion layer 32. The plurality of recesses 30p consisting of inclined interfaces 30i other than the c-plane are formed randomly on the top surface 30u. This forms an inclined interface expansion layer 32 in which the c-plane 30c and the inclined interfaces 30i other than the c-plane are mixed on the surface.

[0082] The term "inclined interface 30i" means a growth interface inclined with respect to the c-plane 30c, and includes low-index facets other than the c-plane, high-index facets other than the c-plane, or inclined faces that cannot be expressed by a plane index. Examples of facets other than the c-plane include {11-2m} and {1-10n}, where m and n are integers other than 0.

[0083] In this embodiment, the inclined interface enlarging layer 32 is grown on the above-mentioned mirror-finished second underlayer 6, and the first growth conditions are adjusted to satisfy formula (1), so that the inclined interface 30i can have, for example, a {11-2m} plane with m≧3. This makes it possible to make the inclination angle of the {11-2m} plane relative to the c-plane 30c gentle. Specifically, the inclination angle can be set to 47.3° or less.

[0084] By further growing the inclined interface expansion layer 32 under the first growth conditions, as shown in Fig. 4(c), in the inclined interface expansion layer 32, the inclined interfaces 30i other than the c-plane are gradually enlarged and the c-plane 30c is gradually reduced upward from the second underlayer 6. Note that at this time, the inclination angle of the inclined interfaces 30i with respect to the main surface 1s of the undersubstrate 1 gradually decreases upward from the second underlayer 6. As a result, most of the inclined interfaces 30i eventually become the {11-2m} planes with m ≥ 3 as described above.

[0085] As the inclined interface spreading layer 32 is further grown, the c-plane 30c of the inclined interface spreading layer 32 disappears from the top surface 30u, and the surface of the inclined interface spreading layer 32 is composed only of the inclined interface 30i. This results in the formation of the inclined interface spreading layer 32 in which the pyramids are continuously bonded.

[0086] In this way, by forming a plurality of recesses 30p consisting of inclined interfaces 30i other than the c-plane on the top surface 30u of the inclined interface expansion layer 32 and eliminating the c-plane 30c, a plurality of valleys 30v and a plurality of apexes 30t are formed on the surface of the inclined interface expansion layer 32 as shown in FIG. 4(c). Each of the plurality of valleys 30v is an inflection point of a downward convexity on the surface of the inclined interface expansion layer 320, and is formed above the position where each of the inclined interfaces 30i other than the c-plane is generated. On the other hand, each of the plurality of apexes 30t is an inflection point of an upward convexity on the surface of the inclined interface expansion layer 320, and is formed at or above the position where the c-plane 30c (lastly) disappeared, sandwiching a pair of inclined interfaces 30i that have expanded in opposite directions. The valleys 30v and apexes 30t are alternately formed in a direction along the main surface 1s of the base substrate 1.

[0087] In this embodiment, in the second underlayer formation step S150, a second underlayer 6 having a mirror-finished main surface 6s is grown on the main surface 1s of the undersubstrate 1, and then in the inclined interface expansion step S220, an inclined interface 30i other than the c-plane is generated on the surface of the inclined interface expansion layer 32. As a result, a plurality of valleys 30v are formed at positions spaced upward from the main surface 6s of the second underlayer 6.

[0088] Due to the above-described growth process of the inclined interface expansion layer 32, dislocations are bent and propagated as follows. Specifically, as shown in FIG. 4(c), a plurality of dislocations extending in the direction along the c-axis in the second underlayer 6 propagate from the second underlayer 6 toward the direction along the c-axis of the inclined interface expansion layer 32. In the region of the inclined interface expansion layer 32 that has grown with the c-plane 30c as the growth plane, dislocations propagate toward the direction along the c-axis of the inclined interface expansion layer 32. However, when a dislocation propagated in the direction along the c-axis of the inclined interface expansion layer 32 is exposed at the inclined interface 30i, the dislocation is bent and propagated toward a direction substantially perpendicular to the inclined interface 30i at the position where the inclined interface 30i is exposed. That is, the dislocation is bent and propagated in a direction inclined with respect to the c-axis. As a result, in the inclined interface expansion step S220 and subsequent steps, dislocations are locally collected above the substantially center between the pair of apexes 30t. As a result, the dislocation density on the surface of the second layer 40, which will be described later, can be reduced.

[0089] In this embodiment, when an arbitrary cross section perpendicular to the main surface 1s of the base substrate 1 is viewed, the average distance L (also referred to as the "average distance between nearest apexes") between the pair of nearest apexes 30t among the plurality of apexes 30t sandwiching one of the plurality of valleys 30v in the direction along the main surface 1s of the base substrate 1 is set to, for example, more than 100 μm. When the average distance L between nearest apexes is 100 μm or less, such as when fine hexagonal pyramidal crystal nuclei are generated on the second base layer 6 from the initial stage of the inclined interface expansion step S220, the distance over which dislocations bend and propagate in the steps after the inclined interface expansion step S220 becomes short. Therefore, dislocations are not sufficiently collected above the approximate center between the pair of apexes 30t in the inclined interface expansion layer 32. As a result, there is a possibility that the dislocation density on the surface of the second layer 40 described later is not sufficiently reduced. In contrast, in this embodiment, by setting the average distance L between the nearest apexes to more than 100 μm, the distance over which dislocations bend and propagate can be ensured to be at least more than 50 μm in the steps following the inclined interface expansion step S220. This allows dislocations to be sufficiently concentrated above the approximate center between a pair of apexes 30t of the inclined interface expansion layer 32. As a result, the dislocation density on the surface of the second layer 40 described below can be sufficiently reduced.

[0090] In this embodiment, for example, it is preferable that there is no portion in the first layer 30 where the nearest apex distance is 100 μm or less. In other words, when viewed in any cross section perpendicular to the main surface 1s of the base substrate 1, it is preferable that the nearest apex distance is more than 100 μm over the entire surface of the first layer 30. This makes it possible to reduce the dislocation density approximately uniformly over the entire surface of the second layer 40 described below.

[0091] On the other hand, in the present embodiment, the average distance L between the closest tops is less than 800 μm. When the average distance L between the closest tops is 800 μm or more, the height from the valley portion 30v to the top portion 30t of the inclined interface expansion layer 32 becomes excessively high. Therefore, in the planarization step S300 described later, the thickness until the second layer 40 becomes mirror-finished becomes thick. In contrast, in the present embodiment, by setting the average distance L between the closest tops to less than 800 μm, the height from the valley portion 30v to the top portion 30t of the inclined interface expansion layer 32 can be lowered. Thereby, in the planarization step S300 described later, the second layer 40 can be mirror-finished earlier.

[0092] Also, at this time, in the inclined interface expansion layer 320, based on the difference in the growth surface during the growth process, a first c-plane growth region 60 that has grown with the c-plane 30c as the growth surface and an inclined interface growth region 70 (gray portion in the figure) that has grown with an inclined interface 30i other than the c-plane as the growth surface are formed.

[0093] Also, at this time, in the first c-plane growth region 60, a valley portion 60a is formed at the position where the inclined interface 30i has occurred, and a peak portion 60b is formed at the position where the c-plane 30c has disappeared. Also, in the first c-plane growth region 60, on both sides sandwiching the peak portion 60b, a pair of inclined portions 60i are formed as the locus of the intersection of the c-plane 30c and the inclined interface 30i.

[0094] Also, at this time, by satisfying the first growth condition with the formula (1), the angle β formed by the pair of inclined portions 60i is, for example, 70° or less.

[0095] Details of these regions will be described later.

[0096] (S240: Inclined Interface Maintenance Step) After the c-plane 30c has disappeared from the surface of the inclined interface spreading layer 32, the growth of the first layer 30 is continued over a predetermined thickness while maintaining a state in which the inclined interface 30i occupies more of the surface than the c-plane 30c, as shown in Fig. 6(a). In this way, an inclined interface maintaining layer 34 having a surface in which the inclined interface 30i occupies more of the surface than the c-plane 30c is formed on the inclined interface spreading layer 32. By forming the inclined interface maintaining layer 34, the c-plane 30c can be reliably eliminated over the entire surface of the first layer 30.

[0097] At this time, the c-plane 30c may appear again in a part of the surface of the inclined interface maintaining layer 34, but it is preferable to mainly expose the inclined interface 30i on the surface of the inclined interface maintaining layer 34 so that the area ratio occupied by the inclined interface growth region 70 is 80% or more on the surface cross section along the main surface 1s of the base substrate 1. Note that the area ratio occupied by the inclined interface growth region 70 on the surface cross section is preferably as high as possible, and is preferably 100%.

[0098] At this time, the growth conditions in the inclined interface maintaining step S240 are maintained at the above-mentioned first growth conditions, similar to the inclined interface expanding step S220. This allows the inclined interface maintaining layer 34 to grow using the inclined interface 30i as a growth surface.

[0099] In addition, at this time, by growing the inclined interface maintaining layer 34 under the first growth conditions with the inclined interface 30i as the growth surface, as described above, the dislocations that have propagated while bending in a direction inclined with respect to the c-axis at the position where the inclined interface 30i is exposed in the inclined interface expansion layer 32 continue to propagate in the same direction in the inclined interface maintaining layer 34.

[0100] By the above three-dimensional growth step S200, the first layer 30 having the gradient interface enlarging layer 32 and the gradient interface maintaining layer 34 is formed.

[0101] In the three-dimensional growth step S200 of this embodiment, the height from the main surface 6s of the second underlayer 6 to the top 30t of the first layer 30 (the maximum height in the thickness direction of the first layer 30) is, for example, more than 100 μm and less than 1.5 mm.

[0102] (S300: Planarization process (second layer growth process)) After the first layer 30 from which the c-plane 30c has disappeared has been grown, a single crystal of a Group III nitride semiconductor is further epitaxially grown on the first layer 30, as shown in FIG. 6(b) and FIG. 7(a).

[0103] At this time, the inclined interface 40i is gradually reduced and the c-plane 40c is gradually expanded toward the top of the first layer 30. This causes the inclined interface 30i formed on the surface of the first layer 30 to disappear. As a result, the second layer (flattening layer) 40 having a mirror-finished surface is grown.

[0104] In this embodiment, the second layer 40 is epitaxially grown, for example, as a layer mainly composed of the same Group III nitride semiconductor as the Group III nitride semiconductor constituting the first layer 30. In the planarization step S300, GaCl gas, NH 3 SiH as a gas and n-type dopant gas 2 Cl 2 By supplying the gas, a silicon (Si)-doped GaN layer is epitaxially grown as the second layer 40. Note that the n-type dopant gas is SiH 2 Cl 2 Instead of gas, GeCl 4 Gas or the like may be supplied.

[0105] Here, in the planarization step S300, in order to realize the above-mentioned growth process, for example, the second layer 40 is grown under predetermined second growth conditions.

[0106] The second growth condition under which the inclined interface 40i is reduced and the c-plane 40c is expanded will be described with reference to Fig. 10. Fig. 10 is a schematic cross-sectional view showing the growth process under the second growth condition under which the inclined interface is reduced and the c-plane is expanded. Fig. 10 shows the process in which the second layer 40 grows on the first layer 30 in which the inclined interface 30i most inclined with respect to the c-plane 30c is exposed.

[0107] 10, similarly to FIG. 9(a), the thick solid line indicates the surface of the second layer 40 per unit time. In addition, in FIG. 10, the growth rate of the c-plane 40c of the second layer 40 is expressed as G c2 The growth rate of the inclined interface 40i of the second layer 40 is G i The progression rate of the locus of intersections between the inclined interface 40i and the c-plane 40c in the second layer 40 is R 2 In addition, the narrower angle between the path of intersections between the inclined interface 40i and the c-plane 40c and the c-plane 30c is defined as α R2 Let us assume that R 2 Direction and G i If the angle between the direction is α”, then α”=α-(90-α R2 10, the second layer 40 is grown while maintaining the angle α between the c-plane 30c and the inclined interface 30i in the first layer 30. It is assumed that the off-angle of the c-plane 40c of the second layer 40 is negligible compared to the angle α between the c-plane 30c and the inclined interface 30i.

[0108] As shown in FIG. 10, the progression rate R 2 is expressed by the following equation (e). R 2 =G i / cosα” ···(e)

[0109] In addition, the growth rate G c2 is expressed by the following equation (f). G c2 =R 2 sin α R2 (f)

[0110] By substituting equation (e) into equation (f), G c2 is G i Using this, it is expressed by the following formula (g). G c2 =G i sin α R2 / cos(α+α R2 -90) ···(g)

[0111] In order for the inclined interface 40i to shrink and the c-plane 40c to expand, α R2 Therefore, the second growth condition under which the inclined interface 40i is reduced and the c-plane 40c is expanded is expressed by the formula (g) and α R2 <90°, it is preferable that the following formula (2) is satisfied. G c2 <G i / cosα (2) However, as mentioned above, G i is the growth rate of the inclined interface 40i that is most inclined with respect to the c-plane 40c, and α is the angle between the inclined interface 40i that is most inclined with respect to the c-plane 40c and the c-plane 40c.

[0112] Alternatively, the growth rate of the c-plane 30c of the second layer 40 under the reference growth conditions is G c0 Then, G under the second growth condition c2 is G under standard growth conditions. c0 It can also be considered that it is preferable that G c2 <G c0 By substituting equation (a) into equation (2), equation (2) can be derived.

[0113] Since the growth condition for reducing the inclined interface 40i that is most inclined with respect to the c-plane 40c is the strictest condition, if the second growth condition satisfies formula (2), it is possible to reduce the other inclined interfaces 40i as well.

[0114] Specifically, when the inclined interface 40i most inclined with respect to the c-plane 40c is the {10-11} plane, it is preferable that the second growth condition satisfies the following formula (2'). G c2 <2.13G i (2')

[0115] Alternatively, for example, when the inclined interface 30i is a {11-2m} plane with m≧3, the inclined interface 30i most inclined with respect to the c-plane 30c is the {11-23} plane, and therefore, it is preferable that the second growth condition satisfies, for example, the following formula (2″). G c2<1.47G i (2")

[0116] As the second growth condition in this embodiment, the growth temperature in the planarization step S300 is set to be higher than the growth temperature in the three-dimensional growth step S200. Specifically, the growth temperature in the planarization step S300 is set to be, for example, 990° C. or higher and 1,120° C. or lower, preferably 1,020° C. or higher and 1,100° C. or lower.

[0117] As a second growth condition of this embodiment, the V / III ratio in the planarization step S300 may be adjusted. For example, the V / III ratio in the planarization step S300 may be made smaller than the V / III ratio in the three-dimensional growth step S200. Specifically, the V / III ratio in the planarization step S300 is set to, for example, 1 to 10, preferably 1 to 5.

[0118] In practice, at least one of the growth temperature and the V / III ratio is adjusted within the above ranges as the second growth condition so as to satisfy the formula (2).

[0119] Other conditions among the second growth conditions in this embodiment are, for example, as follows. Growth pressure: 90 to 105 kPa, preferably 90 to 95 kPa GaCl gas partial pressure: 1.5~15kPa N 2 Gas flow rate / H 2 Gas flow rate: 1~20

[0120] Here, the planarization step S300 of this embodiment is classified into two steps, for example, based on the morphology of the second layer 40 during growth. Specifically, the planarization step S300 of this embodiment includes, for example, a c-plane expansion step S320 and a main growth step S340. Through these steps, the second layer 40 includes, for example, a c-plane expansion layer 42 and a main growth layer 44.

[0121] (S320: c-plane expansion process) As shown in FIG. 6(b), a c-plane expansion layer 42 of a second layer 40 made of a single crystal of a Group III nitride semiconductor is epitaxially grown on the first layer 30 under the above-mentioned second growth conditions.

[0122] At this time, as one moves upward in the first layer 30, the c-plane 40c is enlarged, while the inclined interface 40i other than the c-plane is reduced.

[0123] Specifically, by growth under the second growth conditions, the c-plane expansion layer 42 grows from the inclined interface 30i of the inclined interface maintaining layer 34 in a direction perpendicular to the c-axis (i.e., in a longitudinal or lateral direction) with the inclined interface 40i as the growth plane. As the c-plane expansion layer 42 is grown in the lateral direction, the c-plane 40c of the c-plane expansion layer 42 begins to be exposed again above the top portion 30t of the inclined interface maintaining layer 34. This results in the formation of the c-plane expansion layer 42 in which the c-plane 40c and the inclined interface 40i other than the c-plane are mixed on the surface.

[0124] When the c-plane expansion layer 42 is further grown laterally, the c-plane 40c gradually expands and the inclined interfaces 40i of the c-plane expansion layer 42 gradually shrink. As a result, the recesses 30p formed by the multiple inclined interfaces 30i on the surface of the first layer 30 are gradually filled.

[0125] Thereafter, when the c-plane extension layer 42 is further grown, the inclined interfaces 40i of the c-plane extension layer 42 disappear completely, and the recesses 30p formed by the multiple inclined interfaces 30i are completely filled in the surface of the first layer 30. As a result, the surface of the c-plane extension layer 42 becomes a mirror surface (flat surface) formed only by the c-plane 40c.

[0126] At this time, dislocations are locally collected during the growth process of the first layer 30 and the c-plane extension layer 42, so that the dislocation density can be reduced. Specifically, dislocations that have propagated while bending in a direction inclined to the c-axis in the first layer 30 continue to propagate in the same direction in the c-plane extension layer 42. As a result, dislocations are locally collected at the meeting point of the adjacent inclined interfaces 40i, approximately above the center between the pair of apexes 30t, in the c-plane extension layer 42. Among the multiple dislocations collected at the meeting point of the adjacent inclined interfaces 40i in the c-plane extension layer 42, dislocations having mutually opposite Burgers vectors disappear when they meet. In addition, some of the multiple dislocations collected at the meeting point of the adjacent inclined interfaces 40i form loops, and are suppressed from propagating in a direction along the c-axis (i.e., toward the surface side of the c-plane extension layer 42). The other part of the dislocations collected at the meeting part of the adjacent inclined interfaces 40i in the c-plane extension layer 42 changes its propagation direction again from the direction inclined with respect to the c-axis to the direction along the c-axis, and propagates to the surface side of the second layer 40. By eliminating some of the dislocations in this way or suppressing the propagation of some of the dislocations to the surface side of the c-plane extension layer 42, the dislocation density on the surface of the second layer 40 can be reduced. Furthermore, by collecting the dislocations locally, a low dislocation density region can be formed above the part of the second layer 40 where the dislocations propagate in the direction inclined with respect to the c-axis.

[0127] At this time, in the c-plane expansion layer 42, the c-plane 40c gradually expands, and a second c-plane growth region 80 (described later) that grows using the c-plane 40c as a growth surface is formed while gradually expanding toward the top in the thickness direction.

[0128] On the other hand, in the c-plane expansion layer 42, the inclined interface 40i gradually shrinks, so that the inclined interface growth region 70 gradually shrinks upward in the thickness direction and terminates at a predetermined position in the thickness direction. Due to such a growth process of the c-plane expansion layer 42, a valley portion 70a of the inclined interface growth region 70 is formed at a position where the c-plane 40c appears again in a cross-sectional view. Also, during the process where the recess formed by the inclined interface 40i is gradually filled, a peak portion 70b of the inclined interface growth region 70 is formed at a position where the inclined interface 40i disappeared in a cross-sectional view.

[0129] In the c-plane expansion process S320, the surface of the c-plane expansion layer 42 becomes a mirror surface composed only of the c-plane 40c, so that the height in the thickness direction of the c-plane expansion layer 42 (maximum height in the thickness direction) is, for example, greater than or equal to the height from the valley 30v to the top 30t of the inclined interface maintaining layer 34.

[0130] (S340: Main growth process (c-plane growth process)) When the inclined interface 40i disappears in the c-plane expansion layer 42 and the surface becomes a mirror finish, as shown in Fig. 7(a), a main growth layer 44 is formed on the c-plane expansion layer 42 to a predetermined thickness with the c-plane 40c as the growth surface. This forms a main growth layer 44 that does not have the inclined interface 40i and has only the c-plane 40c on its surface.

[0131] At this time, the growth conditions in the main growth step S340 are maintained at the above-mentioned second growth conditions, similar to the c-plane expansion step S320. This allows the main growth layer 44 to be grown by step flow growth using the c-plane 40c as the growth surface.

[0132] At this time, the main growth layer 44 is grown using only the c-plane 40c as the growth surface without exposing the inclined interface 40i, so that the entire main growth layer 44 becomes a second c-plane growth region 80, which will be described later.

[0133] In the main growth step S340, the thickness of the main growth layer 44 is set to, for example, 300 μm or more and 10 mm or less. By setting the thickness of the main growth layer 44 to 300 μm or more, at least one substrate 50 can be sliced ​​from the main growth layer 44 in the slicing step S400 described later. On the other hand, by setting the thickness of the main growth layer 44 to 10 mm, the final thickness is set to 650 μm, and when slicing substrates 50 having a thickness of 700 μm from the main growth layer 44, at least ten substrates 50 can be obtained even taking into account a kerf loss of about 200 μm.

[0134] In this manner, the second layer 40 including the c-plane expansion layer 42 and the main growth layer 44 is formed.

[0135] (S380: Peeling process) After the growth of the second layer 40 is completed, the layered structure 90 having the second underlayer 6, the first layer 30 and the second layer 40 is peeled off from the undersubstrate 1, as shown in FIG. 7(b).

[0136] Specifically, in the process of cooling the inside of the chamber of the vapor phase growth apparatus, stress is generated between the above-mentioned laminated structure 90 and the base substrate 1 due to the difference in linear expansion coefficients therebetween. For example, the base substrate 1 made of sapphire has a larger linear expansion coefficient than the laminated structure 90 mainly made of GaN, and therefore generates stress that causes the base substrate 1 to contract relative to the laminated structure 90.

[0137] As a result, the above-mentioned laminated structure 90 is naturally peeled off from the base substrate 1 due to the flat gap formed between the metal nitride layer 5 and the second base layer 6. As a result, the laminated structure 90 can be peeled off from the base substrate 1 without causing cracks in the laminated structure 90.

[0138] Through the above steps, the laminated structure 90 of this embodiment is obtained.

[0139] The above steps from the second underlayer formation step S150 to the planarization step S300 are performed consecutively in the same vapor phase growth apparatus without exposing the undersubstrate 1 to the atmosphere. This makes it possible to suppress the formation of unintended high oxygen concentration regions (regions having an oxygen concentration excessively higher than that of the inclined interface growth region 70) at the interface between the second underlayer 6 and the first layer 30 and the interface between the first layer 30 and the second layer 40.

[0140] (S400: Slicing process) 8, the main growth layer 44 is sliced ​​by, for example, a wire saw along a cutting plane substantially parallel to the surface of the main growth layer 44. This forms at least one nitride semiconductor substrate 50 (also referred to as substrate 50) as an as-sliced ​​substrate. At this time, the thickness of the substrate 50 is set to, for example, 300 μm or more and 700 μm or less.

[0141] At this time, the radius of curvature of the c-plane 50c of the substrate 50 can be made larger than the radius of curvature of the c-plane of a nitride semiconductor substrate (i.e., a nitride semiconductor substrate obtained by the conventional VAS method) in the case where the second underlayer is grown to the same thickness as the second layer and the second underlayer is sliced ​​without performing the three-dimensional growth step and the planarization step. This makes it possible to make the variation in the off-angle θ of the c-axis 50ca with respect to the normal to the main surface 50s of the substrate 50 smaller than the variation in the c-axis off-angle of a nitride semiconductor substrate obtained by the conventional VAS method.

[0142] (S500: Polishing process) Next, a polishing device is used to polish both sides of the substrate 50. At this time, the final thickness of the substrate 50 is set to, for example, 250 μm or more and 650 μm or less.

[0143] Through the above steps S100 to S500, the substrate 50 according to the present embodiment is manufactured.

[0144] (Fabrication process of semiconductor laminate and fabrication process of semiconductor device) After the substrate 50 is manufactured, for example, a semiconductor functional layer made of a group III nitride semiconductor is epitaxially grown on the substrate 50 to manufacture a semiconductor laminate. After the semiconductor laminate is manufactured, electrodes and the like are formed using the semiconductor laminate, and the semiconductor laminate is diced to cut out chips of a predetermined size. In this way, a semiconductor device is manufactured.

[0145] (2) Laminated structure Next, a laminated structure 90 according to this embodiment will be described with reference to FIG.

[0146] The laminated structure 90 of the present embodiment includes, for example, a second underlayer 6, a first layer 30, and a second layer 40. The second underlayer 6 can be simply referred to as an "underlayer".

[0147] The second underlayer 6 is made of, for example, a single crystal of a group III nitride. The second underlayer 6 has, for example, a mirror-finished main surface 6s (a trace of such a surface). The low-index crystal plane closest to the main surface 6s of the second underlayer is, for example, the c-plane.

[0148] The first layer 30 is grown on the second underlayer 6, for example.

[0149] The first layer 30 has, for example, a plurality of valleys 30v and a plurality of apexes 30t formed by forming a plurality of recesses 30p composed of inclined interfaces 30i other than the c-plane on a top surface 30u of a single crystal of a Group III nitride semiconductor and eliminating the c-plane 30c. When any cross section perpendicular to the main surface 6s of the second underlayer 6 is viewed, the average distance between the nearest apexes is, for example, more than 100 μm.

[0150] Furthermore, the first layer 30 has a first c-plane growth region (first low oxygen concentration region) 60 and an inclined interface growth region (high oxygen concentration region) 70, for example, based on the difference in the growth surface during the growth process.

[0151] The first c-plane growth region 60 is a region grown using the c-plane 30c as a growth plane. The first c-plane growth region 60 has, for example, a plurality of valleys 60a and a plurality of peaks 60b in a cross-sectional view. In addition, each of the valleys 60a and the peaks 60b here means a part of the shape observed based on the difference in emission intensity when the cross section of the laminated structure 90 is observed with a fluorescent microscope or the like, and does not mean a part of the shape of the outermost surface generated during the growth of the first layer 30. Each of the multiple valleys 60a is a downwardly convex inflection point in the first c-plane growth region 60 in a cross-sectional view, and is formed at a position where the inclined interface 30i occurs. At least one of the multiple valleys 60a is provided at a position away from the main surface 6s of the second underlayer 6 upward. On the other hand, each of the multiple peaks 60b is an upwardly convex inflection point in the first c-plane growth region 60 in a cross-sectional view, and is formed at a position where the c-plane 30c (lastly) disappears between a pair of inclined interfaces 30i that expand in opposite directions. The valleys 60a and the peaks 60b are alternately formed in a direction along the main surface 6s of the second underlayer 6.

[0152] The first c-plane growth region 60 has, in a cross-sectional view, a pair of inclined portions 60i provided as a locus of intersections between the c-plane 30c and the inclined interfaces 30i on both sides of one of the multiple peaks 60b. Note that the inclined portions 60i referred to here refer to a part of a shape observed based on a difference in emission intensity when the cross section of the stacked structure 90 is observed with a fluorescent microscope or the like, and do not refer to the inclined interface 30i on the outermost surface that occurs during the growth of the first layer 30.

[0153] In a cross-sectional view, the angle β between the pair of inclined portions 60i is, for example, 70° or less, preferably 20° to 65°. The angle β between the pair of inclined portions 60i being 70° or less is a growth rate G of the inclined interface 30i that is most inclined with respect to the c-plane 30c of the first layer 30 under the first growth conditions. i The growth rate G of the c-plane 30c of the first layer 30 c1 Ratio of G c1 / G iThis means that the angle was high. This allows the inclined interface 30i other than the c-plane to be easily generated. As a result, it is possible to easily bend dislocations at the positions where the inclined interface 30i is exposed. In addition, by setting the angle β between the pair of inclined portions 60i to 70° or less, it is possible to easily generate a plurality of valleys 30v and a plurality of peaks 30t above the second underlayer 6. Furthermore, by setting the angle β between the pair of inclined portions 60i to 65° or less, it is possible to more easily generate an inclined interface 30i other than the c-plane, and it is possible to more easily generate a plurality of valleys 30v and a plurality of peaks 30t above the second underlayer 6. In addition, by setting the angle β between the pair of inclined portions 60i to 20° or more, it is possible to suppress the height from the valleys 30v to the peaks 30t of the first layer 30 from increasing, and to suppress the thickness of the second layer 40 from becoming mirror-finished.

[0154] On the other hand, the inclined interface growth region 70 is a region grown using an inclined interface 30i other than the c-plane as a growth plane. The lower surface of the inclined interface growth region 70 is formed, for example, following the shape of the first c-plane growth region 60. The inclined interface growth region 70 is provided continuously along the main surface 6s of the second underlayer 6.

[0155] The inclined interface growth region 70 is more likely to take in oxygen than the 1c-plane growth region 60. Therefore, the oxygen concentration in the inclined interface growth region 70 is higher than the oxygen concentration in the 1c-plane growth region 60. Note that the oxygen taken in by the inclined interface growth region 70 is, for example, oxygen unintentionally mixed into the vapor phase growth apparatus, or oxygen released from a member (such as a quartz member) constituting the vapor phase growth apparatus.

[0156] The oxygen concentration in the first c-plane growth region 60 is, for example, 5×10 16 cm -3 Less than or equal to 3×10 16 cm -3 On the other hand, the oxygen concentration in the inclined interface growth region 70 is, for example, 3×10 18 cm -3 5×10 or more 19 cm -3 The following is the result.

[0157] The second layer 40 has an inclined interface growth region 70 and a second c-plane growth region (second low oxygen concentration region) 80, for example, based on the difference in the growth plane during the growth process.

[0158] The upper surface of the inclined interface growth region 70 in the second layer 40 has, for example, a plurality of valleys 70a and a plurality of peaks 70b in a cross-sectional view. The valleys 70a and peaks 70b here refer to a portion of a shape observed based on a difference in luminescence intensity when the cross-section of the laminated structure 90 is observed with a fluorescent microscope or the like, and do not refer to a portion of the shape of the outermost surface generated during the growth of the second layer 40. The plurality of valleys 70a of the inclined interface growth region 70 are formed at a position where the c-plane 40c occurs again in a cross-sectional view, as described above. Also, the plurality of valleys 70a of the inclined interface growth region 70 are each formed above the plurality of peaks 60b of the first c-plane growth region 60 in a cross-sectional view. On the other hand, the plurality of peaks 70b of the inclined interface growth region 70 are formed at a position where the inclined interface 40i disappears in a cross-sectional view, as described above. Moreover, the multiple peaks 70b of the inclined interface growth region 70 are formed above the multiple valleys 60a of the first c-plane growth region 60, respectively, in a cross-sectional view.

[0159] Furthermore, a surface of the second layer 40 that is approximately parallel to the main surface 6s of the second underlayer 6 at the upper end of the inclined interface growth region 70 becomes a boundary surface 40b at the position where the inclined interface 40i in the second layer 40 disappears.

[0160] The second c-plane growth region 80 is a region grown using the c-plane 40c as a growth plane. In the second c-plane growth region 80, oxygen uptake is suppressed compared to the inclined interface growth region 70. Therefore, the oxygen concentration in the second c-plane growth region 80 is lower than the oxygen concentration in the inclined interface growth region 70. The oxygen concentration in the second c-plane growth region 80 is, for example, 5×10 16 cm -3 Less than or equal to 3×10 16 cm -3 The following is the result.

[0161] In this embodiment, in the growth process of the first layer 30, at a position where the inclined interface 30i other than the c-plane is exposed, dislocations bend and propagate in a direction approximately perpendicular to the inclined interface 30i, so that in the second layer 40, some of the multiple dislocations disappear or are suppressed from propagating to the surface side of the c-plane expansion layer 42. As a result, the dislocation density in the surface of the second layer 40 is reduced below the dislocation density in the main surface 6s of the second underlayer 6.

[0162] Additionally, in this embodiment, the entire surface of the second layer 40 is composed of +c planes, and the first layer 30 and the second layer 40 each do not include an inversion domain. In this respect, the laminated structure 90 of this embodiment differs from a laminated structure formed by the so-called DEEP (Dislocation Elimination by the Epitaxial-growth with inverse-pyramidal Pits) method, that is, from a laminated structure that includes an inversion domain in the core located at the center of the pit.

[0163] (3) Nitride semiconductor substrates (free-standing nitride semiconductor substrates, nitride crystal substrates) Next, a nitride semiconductor substrate 50 according to this embodiment will be described with reference to Fig. 11. Fig. 11(a) is a schematic top view showing the nitride semiconductor substrate according to this embodiment, (b) is a schematic cross-sectional view along the m-axis of the nitride semiconductor substrate according to this embodiment, and (c) is a schematic cross-sectional view along the a-axis of the nitride semiconductor substrate according to this embodiment.

[0164] In this embodiment, the substrate 50 obtained by slicing the second layer 40 by the above-mentioned manufacturing method is, for example, a free-standing substrate made of a single crystal of a Group III nitride semiconductor. In this embodiment, the substrate 50 is, for example, a free-standing GaN substrate.

[0165] The diameter of the substrate 50 is, for example, 2 inches or more. The thickness of the substrate 50 is, for example, 300 μm or more and 1 mm or less.

[0166] The conductivity of the substrate 50 is not particularly limited. However, when the substrate 50 is used to manufacture a semiconductor device as a vertical Schottky barrier diode (SBD), the substrate 50 is, for example, an n-type, the n-type impurity in the substrate 50 is, for example, Si or germanium (Ge), and the n-type impurity concentration in the substrate 50 is, for example, 1.0×10 18 cm -3 Above 1.0×10 20 cm -3 The following is the result.

[0167] The substrate 50 has, for example, a primary surface 50s which serves as an epitaxial growth surface. In this embodiment, the low-index crystal plane closest to the primary surface 50s is, for example, a c-plane 50c.

[0168] The main surface 50s of the substrate 50 is, for example, mirror-finished, and the root-mean-square roughness RMS of the main surface 50s of the substrate 50 is, for example, less than 1 nm.

[0169] Furthermore, in this embodiment, the impurity concentration in the substrate 50 obtained by the above-mentioned manufacturing method is lower than that of a substrate obtained by the flux method or ammonothermal method.

[0170] Specifically, the hydrogen concentration in the substrate 50 is, for example, 1×10 17 cm -3 Less than 5 x 10 16 cm -3 The following is the result.

[0171] In this embodiment, the substrate 50 is formed by slicing the main growth layer 44 grown using the c-plane 40c as the growth surface, and therefore does not include the inclined interface growth region 70 grown using the inclined interface 30i or the inclined interface 40i as the growth surface. In other words, the entire substrate 50 is composed of a low oxygen concentration region.

[0172] Specifically, the oxygen concentration in the substrate 50 is, for example, 5×10 16 cm -3 Less than or equal to 3×10 16 cm -3The following is the result.

[0173] (Curving of the c-plane and variation in off-angle) As shown in Figures 11(b) and (c), in this embodiment, the c-plane 50c, which is the low-index crystal plane closest to the primary surface 50s of the substrate 50, is curved into a concave spherical shape with respect to the primary surface 50s, for example, due to the manufacturing method of the substrate 50 described above.

[0174] In this embodiment, the c-plane 50c of the substrate 50 has a curved surface that is approximated as a sphere in both a cross section along the m-axis and a cross section along the a-axis.

[0175] In this embodiment, since the c-plane 50f of the substrate 50 is curved into a concave spherical shape as described above, at least a part of the c-axes 50ca is inclined with respect to the normal to the main surface 50s. The off-angle θ, which is the angle that the c-axis 50ca makes with respect to the normal to the main surface 50s, has a predetermined distribution within the main surface 50s.

[0176] In addition, the component of the off angle θ of the c-axis 50ca with respect to the normal to the main surface 50s along the m-axis is referred to as “θ m " and the direction component along the a-axis is "θ a " Note that θ 2 =θ m 2 +θ a 2 It is.

[0177] In this embodiment, since the c-plane 50c of the substrate 50 is curved into a concave spherical shape as described above, the off-angle m-axis component θ m and the off-axis component θ a can be approximately expressed as a linear function of x and a linear function of y, respectively.

[0178] Specifically, for example, an X-ray rocking curve of the (0002) plane is measured at each position on a line passing through the center of the main surface 50s, and the peak angle ω formed by the X-ray incident on the main surface 50s and the main surface 50s is plotted against the position on the line, and the peak angle ω can be approximated by a linear function of the position. Note that the "peak angle ω" here refers to the angle formed by the X-ray incident on the main surface 50s and the main surface 50s, and is the angle at which the diffraction intensity is maximum. The radius of curvature of the c-surface 50c can be calculated from the inverse of the slope of the linear function approximated as described above.

[0179] In this embodiment, the radius of curvature of the c-plane 50c of the substrate 50 is larger than, for example, the radius of curvature of the c-plane of a nitride semiconductor substrate obtained by the conventional VAS method described above.

[0180] Specifically, when the peak angle ω is approximated by a linear function of position in an X-ray rocking curve measurement of the c-face 50c, the radius of curvature of the c-face 50c calculated by the inverse of the slope of the linear function is, for example, 10 m or more, preferably 15 m or more, more preferably 19 m or more, and even more preferably 40 m or more.

[0181] In this embodiment, the upper limit of the radius of curvature of c-face 50c of substrate 50 is not particularly limited, since the larger the better. When c-face 50c of substrate 50 is substantially flat, the radius of curvature of c-face 50c may be considered to be infinite.

[0182] In this embodiment, since the radius of curvature of the c-plane 50c of the substrate 50 is large, the variation in the off-angle θ of the c-axis 50ca relative to the normal to the main surface 50s of the substrate 50 can be made smaller than the variation in the off-angle of the c-axis of nitride semiconductor substrates produced by the conventional VAS method.

[0183] In this embodiment, when the peak angle ω is approximated by a linear function of the position in the X-ray rocking curve measurement of the c-plane 50c, the error of ω with respect to the linear function of the position is small. The error of ω in this embodiment can be made smaller than that of a substrate obtained from a crystal layer grown on a base substrate processed by the ELO method using a mask layer, or a substrate obtained from a second layer when the c-plane does not disappear in a three-dimensional growth process.

[0184] Specifically, the error of the measured peak angle ω with respect to the linear function approximated as described above is, for example, 0.05° or less, preferably 0.02° or less, and more preferably 0.01° or less. Note that since at least some peak angles ω may coincide with the linear function, the minimum value of the error is 0°.

[0185] (scotoma) Next, a dark spot on the main surface 50s of the substrate 50 of this embodiment will be described. The term "dark spot" used here means a point with low luminescence intensity observed in an observation image of the main surface 50s with a multiphoton excitation microscope or a cathodoluminescence image of the main surface 50s, and includes not only dislocations but also non-luminescent centers caused by foreign matter or point defects. The "multiphoton excitation microscope" is sometimes called a two-photon excitation fluorescence microscope.

[0186] In this embodiment, the substrate 50 is manufactured using the base structure 10 produced by the VAS method, and therefore there are few non-radiative centers caused by foreign matter or point defects in the substrate 50. Therefore, 95% or more, preferably 99% or more of the dark spots observed when the main surface of the substrate 50 is observed with a multiphoton excitation microscope or the like are not non-radiative centers caused by foreign matter or point defects but dislocations.

[0187] Moreover, in this embodiment, the above-described manufacturing method reduces the dislocation density in the surface of the second layer 40 compared to the dislocation density in a nitride semiconductor substrate obtained by the conventional VAS method. As a result, dislocations are also reduced on the main surface 50s of the substrate 50 formed by slicing the second layer 40.

[0188] Furthermore, in this embodiment, the three-dimensional growth step S200 and the planarization step S300 are performed using the unprocessed base structure 10 by the above-mentioned manufacturing method, so that on the main surface 50s of the substrate 50 formed by slicing the second layer 40, no regions of high dislocation density due to dislocation concentration are formed, and regions of low dislocation density are formed uniformly.

[0189] Specifically, in this embodiment, when the main surface 50s of the substrate 50 is observed with a multiphoton excitation microscope in a field of view of 250 μm square and the dislocation density is calculated from the dark spot density, the dislocation density is 3×10 6 cm -2 There is no region exceeding 1×10 6 cm -2 The area where this is less than 100% accounts for 80% or more of the main surface 50s, preferably 90% or more, and more preferably 95% or more.

[0190] In other words, in this embodiment, the dislocation density averaged over the entire main surface 50s of the substrate 50 is, for example, 1×10 6 cm -2 less than 5.5 x 10 5 cm -2 More preferably, it is less than 3×10 5 cm -2 The following is the result.

[0191] The main surface 50s of the substrate 50 of this embodiment includes dislocation-free regions of at least 50 μm square based on the average distance L between the nearest apexes in the above-mentioned three-dimensional growth step S200. The main surface 50s of the substrate 50 of this embodiment includes dislocation-free regions of at least 50 μm square that do not overlap, for example, at a density of 100 / cm. 2 It has a density of more than 100 nm.

[0192] Next, the Burgers vector of dislocations in the substrate 50 of this embodiment will be described.

[0193] In this embodiment, since the dislocation density is low on the main surface 6s of the second underlayer 6 used in the above-described manufacturing method, multiple dislocations are unlikely to combine (mix) when the first layer 30 and the second layer 40 are grown on the second underlayer 6. This makes it possible to suppress the generation of dislocations with a large Burgers vector in the substrate 50 obtained from the second layer 40.

[0194] Specifically, in the substrate 50 of this embodiment, for example, the Burgers vector is <11-20> / 3, <0001> In addition, the "Burgers vector" here can be measured, for example, by large angle convergent beam electron diffraction (LACBED) using a transmission electron microscope (TEM). Dislocations with a Burgers vector of <11-20> / 3 are edge dislocations, and Burgers vectors of <0001> A dislocation with a Burgers vector of <11-23> / 3 is a mixed dislocation that is a mixture of an edge dislocation and a screw dislocation.

[0195] In this embodiment, when 100 dislocations are randomly selected from the main surface 50s of the substrate 50, the Burgers vector is <11-20> / 3, <0001> or <11-23> / 3 is, for example, 50% or more, preferably 70% or more, and more preferably 90% or more. Note that dislocations with a Burgers vector of 2<11-20> / 3 or <11-20> may be present in at least a portion of the main surface 50s of the substrate 50.

[0196] (X-ray rocking curve measurement with different slit widths) Here, the inventors discovered that by performing X-ray rocking curve measurements with different slit widths on the entrance side, it is possible to simultaneously evaluate both the mosaicity of the crystals constituting the substrate 50 of this embodiment and the curvature (warping) of the c-plane 50c described above.

[0197] First, we explain the effect of crystal mosaicity on X-ray rocking curve measurements.

[0198] Here, "crystal mosaicity" refers to the variation in crystal plane orientation. The more dislocations there are in a crystal, the more randomly the crystal plane orientation tilts, and the higher the crystal mosaicity tends to be. In particular, when multiple dislocations are arranged linearly and form lineages, the crystal plane orientations of adjacent subgrains are misaligned through the lineage, and the crystal mosaicity is likely to be high. In such cases where the crystal mosaicity is high, when an X-ray rocking curve measurement is performed, the fluctuation (variation, distribution width) of the diffraction angle of the crystal plane becomes large due to the mosaicity.

[0199] Next, the influence of the curvature of the c-plane 50c on the X-ray rocking curve measurement will be described with reference to Fig. 12(a), which is a schematic cross-sectional view showing the diffraction of X-rays with respect to the curved c-plane.

[0200] The width of the slit on the X-ray entrance side is a, the irradiation width (footprint) of the X-ray irradiated on the main surface of the substrate is b, and the Bragg angle of the crystal is θ B Then, the irradiation width b of the X-ray on the main surface of the substrate is calculated by the following formula (h). b=a / sinθ B (h)

[0201] 12(a), when the c-plane of the substrate is curved, the radius of curvature of the c-plane is R, and half the central angle formed by the curved c-plane within the range of the X-ray irradiation width b is γ, the radius of curvature R of the c-plane is very large compared to the X-ray irradiation width b. Therefore, the angle γ can be calculated by the following formula (i). γ = sin -1 (b / 2R) ≒ b / 2R (i)

[0202] At this time, at the end of the c-plane of the substrate on the incident side of the region irradiated with X-rays (the right end in the figure), the diffraction angle with respect to the main surface of the substrate is θ B +γ=θ B The result is +b / 2R.

[0203] On the other hand, at the end of the c-plane of the substrate on the light receiving side of the region irradiated with X-rays (the left end in the figure), the diffraction angle with respect to the main surface of the substrate is θ B -γ=θ B The result is -b / 2R.

[0204] Therefore, the fluctuation in the diffraction angle of the X-ray with respect to the curved c-plane is b / R, which is due to the difference between the diffraction angle with respect to the main surface of the substrate at the end of the c-plane on the incident side and the diffraction angle with respect to the main surface of the substrate at the end of the c-plane on the light receiving side.

[0205] 12(b) and (c) are diagrams showing the fluctuation of the diffraction angle of the (0002) plane with respect to the radius of curvature of the c-plane. Note that the vertical axis of Fig. 12(b) is a logarithmic scale, and the vertical axis of Fig. 12(c) is a linear scale.

[0206] As shown in Figures 12(b) and (c), when the width a of the slit on the X-ray incidence side is increased, i.e., the X-ray irradiation width b is increased, the fluctuation of the diffraction angle of the (0002) plane increases according to the X-ray irradiation width b. In addition, as the curvature radius R of the c-plane decreases, the fluctuation of the diffraction angle of the (0002) plane gradually increases. In addition, the difference in the fluctuation of the diffraction angle of the (0002) plane when the X-ray irradiation width b is changed increases as the curvature radius R of the c-plane decreases.

[0207] In fact, when X-ray rocking curve measurements are performed on a substrate with low crystal mosaicity, when the width a of the slit on the entrance side is narrow, the component due to the curvature of the c-plane in the fluctuation of the diffraction angle of the (0002) plane is small, and the component due to the mosaicity of the crystal is dominant. However, when the width a of the slit on the entrance side is wide, both the component due to the mosaicity of the crystal and the component due to the curvature of the c-plane are superimposed in the fluctuation of the diffraction angle of the (0002) plane. Therefore, if X-ray rocking curve measurements are performed with different widths a of the slit on the entrance side, it becomes possible to simultaneously evaluate both the mosaicity of the crystal and the curvature (warping) of the c-plane.

[0208] Here, the characteristics of the substrate 50 of this embodiment when an X-ray rocking curve measurement is performed will be described.

[0209] In the following, when Cu Kα1 X-rays are irradiated onto the main surface 50s of the substrate 50 through a Ge (220) plane double crystal monochromator and a slit, and an X-ray rocking curve measurement of the (0002) plane diffraction is performed, the half-width of the (0002) plane diffraction when the slit width in the ω direction is 1 mm is defined as "FWHMa," and the half-width of the (0002) plane diffraction when the slit width in the ω direction is 0.1 mm is defined as "FWHMb." Note that the "ω direction" refers to the rotation direction (circumferential direction) when the substrate 50 is rotated around an axis that passes through the center of the substrate 50 and is parallel to the main surface of the substrate 50 in the X-ray rocking curve measurement.

[0210] As described above, the substrate 50 of this embodiment has few dislocations and low crystal mosaicity over a wide range of the main surface 50s.

[0211] As a result, when X-ray rocking curve measurements of (0002) plane diffraction were performed at multiple measurement points set at 5 mm intervals on the main surface 50s of substrate 50 in this embodiment with a slit width in the ω direction of 0.1 mm, the half-width FWHMb of the (0002) plane diffraction is, for example, 80 arcsec or less, preferably 50 arcsec or less, at 95% or more, preferably 100%, of all measurement points.

[0212] Furthermore, in the substrate 50 of this embodiment, the diffraction spectrum of the (0002) plane when X-ray rocking curve measurement is performed with the slit width on the entrance side widened tends not to be narrower than the diffraction spectrum of the (0002) plane when X-ray rocking curve measurement is performed with the slit width on the entrance side narrowed.

[0213] As a result, in the substrate 50 of this embodiment, the half-width FWHMa of the (0002) plane diffraction when the slit width in the ω direction is 1 mm can be greater than or equal to the half-width FWHMb of the (0002) plane diffraction when the slit width in the ω direction is 0.1 mm, for example.

[0214] In the substrate 50 of the present embodiment, as described above, over a wide range of the main surface 50s, there are few dislocations and the crystal mosaicity is low. Further, the curvature of the c-plane 50c of the substrate 50 is small and the radius of curvature of the c-plane 50c is large. Due to these, in the substrate 50 of the present embodiment, even if the X-ray rocking curve measurement is performed with the incident-side slit width widened, the fluctuation of the diffraction angle of the (0002) plane does not increase significantly. Also, even if the X-ray rocking curve measurement is performed with different incident-side slit widths, the difference in the fluctuation of the diffraction angle of the (0002) plane becomes small.

[0215] As a result, at a predetermined measurement point (for example, the center of the main surface) of the substrate 50 of the present embodiment, the difference FWHMa - FWHMb obtained by subtracting the full width at half maximum FWHMb of the (0002) plane diffraction when the width of the slit in the ω direction is 0.1 mm from the full width at half maximum FWHMa of the (0002) plane diffraction when the width of the slit in the ω direction is 1 mm is, for example, 30% or less of FWHMa, preferably 22% or less.

[0216] Also, at a plurality of measurement points set at 5 mm intervals within the main surface 50s of the substrate 50 of the present embodiment, when the X-ray rocking curve measurement of the (0002) plane diffraction is performed with different widths of the slit in the ω direction, for example, at 95% or more, preferably 100% of all the measurement points, FWHMa - FWHMb is, for example, 30% or less of FWHMa, preferably 22% or less.

[0217] Note that in the substrate 50 of the present embodiment, even if FWHMa < FWHMb, |FWHMa - FWHMb| is 30% or less.

[0218] For reference, in the nitride semiconductor substrate obtained by the conventional VAS method described above, relatively, the curvature of the c-plane 10c is large and the radius of curvature of the c-plane 10c is small. Therefore, the difference FWHMa - FWHMb in the nitride semiconductor substrate obtained by the conventional VAS method is, for example, 50% or more of FWHMa.

[0219] (4) Effects obtained by the present embodiment According to this embodiment, one or more of the following advantages can be obtained.

[0220] (a) By three-dimensionally growing the first layer 30 on the base structure 10, a stress cancellation effect (stress relaxation effect) can be obtained in which the tensile stress accumulated in the second base layer 6 of the base structure 10 is cancelled out by the first layer 30.

[0221] One of the reasons why the first layer 30 has a stress canceling effect is as follows, for example.

[0222] As described above, in the three-dimensional growth step S200, the first layer 30 is three-dimensionally grown using the inclined interface 30i other than the c-plane as the growth plane, thereby forming the inclined interface growth region 70. The inclined interface growth region 70 is more likely to take in oxygen than the first c-plane growth region 60. Therefore, the oxygen concentration in the inclined interface growth region 70 is higher than the oxygen concentration in the first c-plane growth region 60. In other words, the inclined interface growth region 70 can be considered as a high-oxygen concentration region.

[0223] In this way, by incorporating oxygen into the high oxygen concentration region, the lattice constant of the high oxygen concentration region can be made larger than the lattice constant of other regions other than the high oxygen concentration region (reference: Chris G. Van de Walle, Physical Review B vol.68, 165209 (2003)). The second underlayer 6 or the first c-plane growth region 60 of the first layer 30 grown with the c-plane 30c as the growth surface is subjected to stress concentrated toward the center of curvature of the c-plane due to the curvature of the c-plane of the second underlayer 6. In contrast, by relatively increasing the lattice constant of the high oxygen concentration region, stress that spreads the c-plane 30c outward in the creeping direction can be generated in the high oxygen concentration region. This makes it possible to cancel out the stress concentrated toward the center of curvature of the c-plane 30c below the high oxygen concentration region and the stress that spreads the c-plane 30c of the high oxygen concentration region outward in the creeping direction.

[0224] By obtaining the stress canceling effect by the first layer 30 in this way, even if the second layer 40 is grown thick in a state where tensile stress is accumulated in the second underlayer 6, it is possible to suppress the occurrence of a stress difference between the underlayer 10 side and the surface side of the second layer 400. This makes it possible to suppress the occurrence of cracks and the like in the second layer 40.

[0225] (b) In this embodiment, by obtaining the above-described stress canceling effect by the first layer 30, the radius of curvature of the c-plane 50c of the substrate 50 obtained from the second layer 40 can be made larger than the radius of curvature of a nitride semiconductor substrate obtained by the conventional VAS method. This makes it possible to make the variation in the off-angle θ of the c-axis 50ca with respect to the normal to the main surface 50s of the substrate 50 smaller than the variation in the off-angle of the c-axis of a nitride semiconductor substrate obtained by the conventional VAS method.

[0226] (c) In the three-dimensional growth step S200, by generating an inclined interface 30i other than the c-plane on the surface of the single crystal constituting the first layer 30, dislocations can be bent and propagated in a direction substantially perpendicular to the inclined interface 30i at the position where the inclined interface 30i is exposed. This allows dislocations to be collected locally. By collecting dislocations locally, dislocations having mutually opposite Burgers vectors can be eliminated. Alternatively, the locally collected dislocations form a loop, which can suppress the dislocations from propagating to the surface side of the second layer 40. In this way, the dislocation density on the surface of the second layer 40 can be reduced. As a result, a substrate 50 can be obtained in which the dislocation density is reduced compared to that of a nitride semiconductor substrate obtained by the conventional VAS method.

[0227] (d) As described above, by eliminating some of the dislocations during the growth of the second layer 40 or by suppressing the propagation of some of the dislocations to the surface side of the second layer 40, the dislocation density can be reduced more rapidly than in the case where a crystal layer made of a single crystal of a Group III nitride semiconductor is grown using only the c-plane as the growth surface. As a result, the substrate 50 with reduced dislocation density can be efficiently obtained, and the productivity can be improved.

[0228] (e) In the three-dimensional growth step S200, the c-plane 30c is caused to disappear from the top surface 30u of the first layer 30. This makes it possible to form a plurality of valleys 30v and a plurality of tops 30t on the surface of the first layer 30. As a result, dislocations propagating from the second underlayer 6 can be reliably bent at the positions where the inclined interfaces 30i in the first layer 30 are exposed.

[0229] Here, let us consider the case where the c-plane remains in the three-dimensional growth process. In this case, in the part where the c-plane remains, dislocations propagating from the base substrate propagate almost vertically upward without being bent, and reach the surface of the second layer. Therefore, above the part where the c-plane remains, dislocations are not reduced, and a high dislocation density region is formed.

[0230] In contrast, according to the present embodiment, in the three-dimensional growth step S200, by eliminating the c-plane 30c from the top surface 30u of the first layer 30, the surface of the first layer 30 can be configured with only the inclined interface 30i other than the c-plane, and a plurality of valleys 30v and a plurality of apexes 30t can be formed on the surface of the first layer 30. This allows dislocations propagating from the second underlayer 6 to be reliably bent over the entire surface of the first layer 30. By reliably bending the dislocations, it is possible to make it easier to eliminate some of the plurality of dislocations, or to make it difficult for some of the plurality of dislocations to propagate to the surface side of the second layer 40. As a result, it is possible to reduce the dislocation density over the entire main surface 1s of the substrate 50 obtained from the second layer 40.

[0231] (f) In this embodiment, a second underlayer formation step S150 is performed between the void formation step S140 and the three-dimensional growth step S200 to mirror-finish the main surface 6s of the second underlayer 6. This makes it possible to change the growth form of the first layer 30 from the growth form of island-like crystals formed in the early stage of growth of the second underlayer 6.

[0232] Here, if the three-dimensional growth step S200 is performed without performing the second underlayer formation step S150 immediately after the void formation step S140, the first layer grows as island-like crystals above the void-containing first underlayer through the metal nitride layer from the initial growth stage of the first layer. In this case, the frequency of island-like crystals in the first layer depends on the degree of supersaturation when the metal nitride layer grows on the nanonet and the variation in the opening width of the nanonet, as described above. For this reason, the apexes of the first layer are densely formed, and the average distance between the nearest apexes of the first layer is shortened. When the average distance between the nearest apexes of the first layer is shortened, dislocations are not sufficiently collected. As a result, there is a possibility that the dislocation density on the surface of the second layer cannot be sufficiently reduced.

[0233] In contrast, in the present embodiment, by mirror-finishing the main surface 6s of the second underlayer 6 after the void formation process S140, the growth morphology of the first layer 30 can be changed from the growth morphology of the island-like crystals that occurred in the early stage of the growth of the second underlayer 6, as described above.

[0234] That is, the first layer 30 grown on the mirror-finished second underlayer 6 can be grown three-dimensionally depending on the above-mentioned first growth condition in the three-dimensional growth step S200, rather than being grown as island-like crystals from the initial stage of growth. At this time, the average distance between the nearest apexes of the first layer 30 is determined by the growth rate G in the c-axis direction of the c-plane 30c as the above-mentioned first growth condition. c0 and the growth rate G in the inclination direction of the inclined interface 30i i This makes it possible to control the average distance between the nearest apexes of the first layer 30 based on the first growth conditions, and to increase the distance between the nearest apexes.

[0235] In this embodiment, by mirror-finishing the main surface 6s of the second underlayer 6 after the void formation step S140, the morphology of the main surface 6s of the second underlayer 6 can be made substantially uniform over the entire surface, regardless of the state of the void-containing first underlayer 4 and the metal nitride layer 5. By making the morphology of the main surface 6s of the second underlayer 6 substantially uniform, the generation state of the inclined interface 30i can be made substantially uniform over the entire surface of the first layer 30 in the three-dimensional growth step S200. This suppresses the formation of an area with a short nearest apex distance in a part of the surface of the first layer 30, and the nearest apex distance can be made substantially uniformly long over the entire surface of the first layer 30.

[0236] As a result, in this embodiment, the formation of a region with high dislocation density in a portion of the surface of the second layer 40 can be suppressed, and the dislocation density can be reduced over the entire surface of the second layer 40.

[0237] (g) The number of steps in this embodiment can be reduced compared to the case where a three-dimensional growth step and a planarization step are performed using a free-standing substrate obtained by slicing and polishing a crystal layer made of a single crystal of a Group III nitride semiconductor. That is, the slicing step and the polishing step for obtaining a free-standing substrate can be omitted. This makes it possible to reduce the manufacturing cost while improving the yield of this embodiment.

[0238] (h) In this embodiment, the three-dimensional growth step S200 is performed on the base structure 10 in a state where neither the formation of a mask layer on the main surface 6s of the second base layer 6 nor the formation of a concave-convex pattern on the main surface 6s is performed. The "mask layer" here means a mask layer made of silicon oxide or the like and having a predetermined opening, which is used in the so-called ELO (Epitaxial Lateral Overgrowth) method. The "convex-convex pattern" here means at least one of a trench and a ridge that are directly patterned on the main surface, which are used in the so-called pendeoepitaxy method. The height difference of the concave-convex pattern here is, for example, 100 nm or more.

[0239] By performing the three-dimensional growth step S200 and the planarization step S300 without having the above-mentioned structure, it is possible to suppress the formation of regions with high dislocation density due to dislocation concentration on the main surface 50s of the substrate 50 formed by slicing the second layer 40, and to uniformly form regions with low dislocation density.

[0240] Furthermore, by performing the three-dimensional growth step S200 and the planarization step S300 without the above-mentioned structure, it is possible to eliminate the need for certain processing steps (such as a mask layer formation step and a photolithography step), thereby reducing the number of steps in this embodiment. This allows the manufacturing cost to be reduced while improving the yield of this embodiment.

[0241] (i) In this embodiment, the first layer 30 is grown on the mirror-finished second underlayer 6, and the first growth conditions are adjusted to satisfy formula (1) in the three-dimensional growth step S200, so that the {11-2m} plane with m≧3 can be generated as the inclined interface 30i in the three-dimensional growth step S200. This makes it possible to make the inclination angle of the {11-2m} plane relative to the c-plane 30c gentle. Specifically, the inclination angle can be set to 47.3° or less. By making the inclination angle of the {11-2m} plane relative to the c-plane 30c gentle, the period of the multiple apexes 30t can be lengthened. Specifically, when any cross section perpendicular to the main surface 1s of the base substrate 1 is viewed, the average distance L between the nearest apexes can be made to exceed 100 μm.

[0242] For reference, when etch pits are usually formed in a nitride semiconductor substrate using a specific etchant, etch pits composed of {1-10n} planes are formed on the surface of the substrate. In contrast, in this embodiment, {11-2m} planes, where m≧3, can be formed on the surface of the first layer 30 grown under specific conditions. Therefore, it is considered that, compared to normal etch pits, in this embodiment, an inclined interface 30i specific to the manufacturing method is formed.

[0243] (j) In this embodiment, when viewing any cross section perpendicular to the main surface 1s of the base substrate 1, the average distance L between the nearest apexes is set to more than 100 μm, so that the distance over which dislocations bend and propagate can be ensured to be at least more than 50 μm. This allows dislocations to be sufficiently concentrated above the approximate center between a pair of apexes 30t of the first layer 30. As a result, the dislocation density on the surface of the second layer 40 can be sufficiently reduced.

[0244] (k) In the three-dimensional growth step S200, after the c-plane 30c is eliminated from the surface of the first layer 30, the first layer 30 is allowed to continue growing over a predetermined thickness while maintaining a state in which the inclined interface 30i occupies more of the surface than the c-plane 30c. This allows sufficient time to bend dislocations at the positions where the inclined interface 30i is exposed after the c-plane 30c has disappeared. Here, if the c-plane growth is performed immediately after the c-plane has disappeared, the dislocations may not be bent sufficiently and may propagate in a substantially vertical direction toward the surface of the second layer. In contrast, in this embodiment, by ensuring sufficient time to bend dislocations at the positions where the inclined interface 30i other than the c-plane is exposed, dislocations in particular near the apex 30t of the first layer 30 can be reliably bent, and the propagation of dislocations from the second underlayer 6 to the surface of the second layer 40 in a substantially vertical direction can be suppressed. This allows the concentration of dislocations to be suppressed above the apex 30t of the first layer 30.

[0245] <Other embodiments> Although the embodiment of the present invention has been specifically described above, the present invention is not limited to the above embodiment, and various modifications can be made without departing from the spirit and scope of the present invention.

[0246] In the above embodiment, the case where the substrate 50 is a GaN free-standing substrate has been described. However, the substrate 50 is not limited to a GaN free-standing substrate. For example, the substrate 50 may be a Group III nitride semiconductor such as aluminum nitride (AlN), aluminum gallium nitride (AlGaN), indium nitride (InN), indium gallium nitride (InGaN), or aluminum indium gallium nitride (AlInGaN). x In yGa 1-x-y It may be a free-standing substrate made of a Group III nitride semiconductor represented by the composition formula N(0≦x≦1, 0≦y≦1, 0≦x+y≦1).

[0247] In the above embodiment, the substrate 50 is of n-type, but the substrate 50 may be of p-type or semi-insulating. For example, when the substrate 50 is used to manufacture a semiconductor device as a high electron mobility transistor (HEMT), the substrate 50 is preferably semi-insulating.

[0248] In the above-described embodiment, in the three-dimensional growth process S200, the case where the growth temperature is mainly adjusted as the first growth condition has been described. However, as long as the first growth condition satisfies formula (1), the first growth condition may be adjusted by adjusting a growth condition other than the growth temperature, or by combining the growth temperature with a growth condition other than the growth temperature.

[0249] In the above embodiment, in the flattening step S300, the case where the growth temperature is mainly adjusted as the second growth condition has been described. However, as long as the second growth condition satisfies the formula (2), the second growth condition may be adjusted by adjusting a growth condition other than the growth temperature, or by combining the growth temperature with a growth condition other than the growth temperature.

[0250] In the above embodiment, the growth conditions in the inclined interface maintaining step S240 are maintained at the above-mentioned first growth conditions, similar to the inclined interface expanding step S220. However, as long as the growth conditions in the inclined interface maintaining step S240 satisfy the first growth conditions, the growth conditions in the inclined interface maintaining step S240 may be different from the growth conditions in the inclined interface expanding step S220.

[0251] In the above embodiment, the growth conditions in the main growth process S340 are maintained at the above-mentioned second growth conditions, similar to the c-plane expansion process S320, as described above. However, as long as the growth conditions in the main growth process S340 satisfy the second growth conditions, the growth conditions in the main growth process S340 may be different from the growth conditions in the c-plane expansion process S320.

[0252] In the above embodiment, the case where the main growth layer 44 is sliced ​​using a wire saw in the slicing step S400 has been described, but for example, an outer circumferential blade slicer, an inner circumferential blade slicer, an electric discharge machine, or the like may also be used.

[0253] In the above embodiment, the case where the substrate 50 is obtained by slicing the main growth layer 44 of the laminated structure 90 has been described, but the present invention is not limited to this case. For example, the laminated structure 90 may be used as is to manufacture a semiconductor laminate for manufacturing a semiconductor device. Specifically, after the laminated structure 90 is manufactured, in a semiconductor laminate manufacturing process, a semiconductor functional layer is epitaxially grown on the laminated structure 90 to manufacture a semiconductor laminate. After the semiconductor laminate is manufactured, the back side of the laminated structure 90 is polished, and the second underlayer 6, the first layer 30, and the c-plane expansion layer 42 of the laminated structure 90 are removed. As a result, a semiconductor laminate having the main growth layer 44 and the semiconductor functional layer is obtained, as in the above embodiment. In this case, the slicing process S400 and the polishing process S500 for obtaining the substrate 50 can be omitted. EXAMPLES

[0254] Various experimental results that support the effects of the present invention will now be described.

[0255] (1) Experiment 1 (1-1) Preparation of nitride semiconductor substrate The nitride semiconductor substrates of the example and comparative example 1, and the stacked structure of comparative example 2 were fabricated as follows. Note that for the examples, stacked structures before slicing the nitride semiconductor substrate were also fabricated. In the following, the "nitride semiconductor substrate" may be abbreviated to "substrate."

[0256] [Conditions for producing nitride semiconductor substrate in the embodiment] (Base substrate) Material: Sapphire Diameter: 2 inches Thickness: 400μm The closest low-index crystal plane to the main plane: c-plane No processing such as mask layers is required on the main surface. (1st base layer) Low-temperature grown buffer layer: Material: GaN Growth temperature: 550℃ Thickness: 50nm GaN layer: Material: Undoped (unintentionally doped) GaN Growth temperature: 1,050℃ Thickness: 350nm The surface of the GaN layer was made mirror-finished. (metal layer) Material:Ti Thickness: 20nm (Heat treatment conditions) A two-stage heat treatment was carried out. First heat treatment: Atmosphere: N 2 Gas 80%, NH 3 Gas 20% Temperature: 1,050℃ Time: 10 minutes Second heat treatment: Atmosphere:H 2 Gas 80%, NH 3 Gas 20% Temperature: 1,050℃ Time: 20 minutes (2nd base layer) Material: Undoped (unintentionally doped) GaN Growth temperature: 1,050℃ Thickness: Approx. 200 μm The main surface of the second underlayer was made mirror-finished. (1st layer) Material: GaN Growth method: HVPE method First growth condition: The growth temperature was set to 980° C. or more and 1,020° C. or less, and the V / III ratio was set to 2 or more and 20 or less. At least one of the growth temperature and the V / III ratio was adjusted within the above range so that the first growth condition satisfied formula (1). Height from the main surface of the second underlayer to the top of the first layer: Approximately 450 μm (2nd layer) Material: GaN Growth method: HVPE method Growth temperature: 1,050℃ V / III ratio: 2 The second growth condition satisfies formula (2). Thickness from the top of the first layer to the surface of the second layer: Approximately 800 μm (Peeling conditions) Spontaneous peeling occurs when the temperature drops from the second layer growth temperature. (Slicing and polishing conditions) Slice position: The second main growth layer Kerf loss when slicing: 200μm Double-sided polishing Final thickness of nitride semiconductor substrate: 400μm

[0257] In the examples, a plurality of nitride semiconductor substrates were fabricated under similar conditions.

[0258] [Conditions for preparing the nitride semiconductor substrate in Comparative Example 1] In Comparative Example 1, a substrate was prepared under the same conditions as those for the conventional VAS method. (Base substrate) Same as in the embodiment. (First crystal layer, metal layer, and heat treatment conditions) The first underlayer, metal layer, and heat treatment conditions were the same as those in the example. (Second crystal layer) The conditions were the same as those for the second underlayer in the example, except that the thickness was 600 μm. (1st and 2nd layers) none. (Peeling conditions) Spontaneous peeling occurs when the temperature drops from the second crystal layer growth temperature. (Slicing and polishing conditions) The conditions were the same as those in the example, except that the slice position was the second crystal layer.

[0259] [Conditions for preparing nitride semiconductor substrate in Comparative Example 2] (Base substrate) Same as in the embodiment. (First crystal layer, metal layer, and heat treatment conditions) The first underlayer, metal layer, and heat treatment conditions were the same as those in the example. (Second crystal layer) The conditions were the same as those for the second underlayer in the example, except that the thickness was 1700 μm. (1st and 2nd layers) none.

[0260] (1-2) Evaluation (Observation by fluorescence microscope) In the examples, a cross section of the laminated structure before the nitride semiconductor substrate was sliced ​​was observed using a fluorescent microscope.

[0261] (Observation by multiphoton excitation microscope) A multiphoton excitation microscope was used to observe the main surfaces of the nitride semiconductor substrates of Example and Comparative Example 1. At this time, the dislocation density was measured by measuring the dark spot density over the entire main surface for every 250 μm field of view. It was confirmed that all the dark spots in these substrates were dislocations by measuring with the focus shifted in the thickness direction. At this time, the dislocation density for the total number of measurement areas in the field of view of 250 μm was 1×10 6 cm -2 The percentage of the number of regions where the dislocation density was less than 100 was calculated.

[0262] (X-ray rocking curve measurement) For each of the nitride semiconductor substrates of Example and Comparative Example 1, the following two types of X-ray rocking curve measurements were carried out.

[0263] For the X-ray rocking curve measurement, Spectris' "X'Pert-PRO MRD" was used, and the "hybrid monochromator" was used as the monochromator on the incident side. The hybrid monochromator has, in order from the X-ray light source side, an X-ray mirror and two crystals of Ge (220) surface. In this measurement, first, the X-ray emitted from the X-ray light source is made into parallel light by the X-ray mirror. This makes it possible to increase the number of X-ray photons used (i.e., X-ray intensity). Next, the parallel light from the X-ray mirror is made into monochromatic light of Cu Kα1 by the two crystals of Ge (220) surface. Next, the monochromatic light from the two crystals of Ge (220) surface is narrowed to a specified width through a slit and made to enter the substrate. The resolution when this hybrid monochromator is used is about 24 arcsec.

[0264] In this measurement, the X-rays incident on the substrate are parallel light directed toward the substrate in a cross section along the ω direction, but are not parallel light in a cross section along a direction perpendicular to the ω direction (the direction of the rotation axis of the substrate). Therefore, while the width of the X-rays in the ω direction is almost constant from the slit to the substrate, the width of the X-rays in the direction perpendicular to the ω direction increases. Therefore, in X-ray rocking curve measurement, the half-width of the X-rays diffracted by a specified crystal plane depends on the width of the ω direction of the slit on the entrance side where the X-rays become parallel light.

[0265] (X-ray rocking curve measurement 1) The width of the entrance slit in the ω direction was set to 0.1 mm, and the X-ray rocking curves of the (0002) plane of each of the nitride semiconductor substrates of the example and comparative example 1 were measured. The measurements were performed at a number of measurement points set at 5 mm intervals on a line passing through the center within the main surface of each substrate and along each of the m-axis direction and the a-axis direction. As a result of the measurements, the peak angle ω between the X-ray incident on the main surface and the main surface was obtained. The peak angle ω was then plotted against the position on the line, and the peak angle ω was approximated by a linear function of the position. The radius of curvature of the c-plane was calculated from the inverse of the slope of the linear function.

[0266] In addition, at each measurement point, the half-width FWHMb of the (0002) plane diffraction was determined when the width of the entrance slit in the ω direction was set to 0.1 mm.

[0267] (X-ray rocking curve measurement 2) The width of the entrance slit in the ω direction was set to 1 mm, and X-ray rocking curve measurements were performed on each of the nitride semiconductor substrates of Example and Comparative Example 1. The measurements were performed at the center of the main surface of each substrate. As a result of the measurements, the half-width FWHMa of the (0002) plane diffraction when the width of the entrance slit in the ω direction was set to 1 mm was obtained. Furthermore, the ratio of FWHMa-FWHMb to FWHMa was obtained at the center of the main surface of each substrate.

[0268] In X-ray rocking curve measurements 1 and 2, when X-rays are incident on the main surface of each substrate at a Bragg angle of 17.28° on the (0002) plane, the footprint of the X-rays is approximately 0.337 mm when the width of the slit in the ω direction is 0.1 mm, and the footprint of the X-rays is approximately 3.37 mm when the width of the slit in the ω direction is 1 mm.

[0269] (1-3) Results The results are shown in Table 1.

[0270] [Table 1]

[0271] (Comparison between Example and Comparative Example 2) First, the embodiment and the comparative example 2 are compared.

[0272] In Comparative Example 2, when the temperature was lowered to room temperature after the crystal layer was grown, the second crystal layer was found to have fine cracks. In addition, abnormal crystal growth was observed from the cross section of the cracked second crystal layer. From this, it is believed that the second crystal layer was cracked during the growth of the second crystal layer. In Comparative Example 2, a second crystal layer having sufficient thickness and diameter could not be obtained, so a nitride semiconductor substrate was not produced.

[0273] In contrast, in the example, when the temperature was lowered to room temperature after the growth of the second layer was completed, the laminated structure peeled off from the base substrate at the boundary between the metal nitride layer and the second base layer, and no cracks were observed in the laminated structure after peeling.

[0274] (Comparison between Example and Comparative Example 1) Table 1 and Figs. 13 to 16 are used to compare Example and Comparative Example 1. Fig. 13 is a diagram showing an observation image of a cross section of the stacked structure of Example observed by a fluorescent microscope. Fig. 14 is a diagram showing a main surface of the nitride semiconductor substrate of Example observed by a multiphoton excitation microscope. Fig. 15(a) is a diagram showing the results of X-ray rocking curve measurement of (0002) plane diffraction along the m-axis direction for each of the nitride semiconductor substrates of Example and Comparative Example 1, and Fig. 15(b) is a diagram showing the results of X-ray rocking curve measurement of (0002) plane diffraction along the a-axis direction for each of the nitride semiconductor substrates of Example and Comparative Example 1. In Figs. 15(a) and (b), the black circles indicate Example and the white circles indicate Comparative Example 1. Fig. 16(a) is a diagram showing a normalized X-ray diffraction pattern when X-ray rocking curve measurements were performed with different slits on the nitride semiconductor substrate of the Example, and (b) is a diagram showing a normalized X-ray diffraction pattern when the same measurement as in the Example was performed on the nitride semiconductor substrate of Comparative Example 1. Note that Figs. 16(a) and (b) show the measurement results in the direction along the m-axis. In addition, in the figure, "Line width" refers to the above-mentioned X-ray footprint.

[0275] As shown in FIG. 13, in the laminated structure of the example, the first layer had a first c-plane growth region grown using the c-plane as the growth surface and an inclined interface growth region grown using an inclined interface as the growth surface based on the difference in the growth surface during the growth process (i.e., the difference in oxygen concentration). The first c-plane growth region had a plurality of recesses and a plurality of protrusions. The average angle formed by a pair of inclined portions in the first c-plane growth region was approximately 45.1°. The average distance between the nearest apexes was approximately 109 μm. The inclined interface growth region was formed continuously along the main surface of the base substrate.

[0276] As shown in Table 1, the average dislocation density on the main surface of the substrate of the example is significantly reduced to 5.5 × 10 compared to the substrate of Comparative Example 1. 5 cm -2 It was less than.

[0277] In the example substrate, the dislocation density is 3×10 6 cm -2 Even in the region with the highest dislocation density, the dislocation density was 1.6×10 6 cm -2 In the substrate of the embodiment, the dislocation density was less than 1×10 6 cm -2 The dislocation density in the low dislocation density region was less than 1.3 × 10 5 ~7.6×10 5 cm -2 It was.

[0278] As shown in the rectangular frame in Fig. 14, the main surface of the substrate of the example included at least a 50 µm square dislocation-free region. Also, the main surface of the substrate of the example included non-overlapping 50 µm square dislocation-free regions at a density of 100 / cm. 2 It had a density of more than 100 nm.

[0279] Furthermore, as shown in Table 1 and FIGS. 15(a) and (b), in the substrate of the example, the radius of curvature of the c-plane was larger than that of the substrate of Comparative Example 1, being 19 m or more.

[0280] In the substrate of the embodiment, when the peak angle ω was approximated by a linear function of the position in the X-ray rocking curve measurement of the c-plane to obtain the radius of curvature of the c-plane, the error of the linear function was small. Specifically, the error of the measured peak angle ω with respect to the linear function approximated as described above was 0.01° or less.

[0281] Furthermore, as shown in Table 1 and Figures 15(a) and (b), in the substrate of the embodiment, the half-width FWHMb of the (0002) plane diffraction when the width of the slit in the ω direction was 0.1 mm was 50 arcsec or less at all measurement points (i.e., 100%).

[0282] As shown in FIG. 16(b), in the substrate of Comparative Example 1, when the width of the slit in the ω direction was 0.1 mm, the diffraction spectrum of X-rays was narrow, but when the width of the slit in the ω direction was 1 mm, the diffraction spectrum of X-rays was broad.

[0283] Therefore, as shown in Table 1, in the substrate of Comparative Example 1, FWHMa-FWHMb was 50% or more of FWHMa.

[0284] In contrast, as shown in FIG. 16(a), in the substrate of the embodiment, even when the width of the slit in the ω direction was increased from 0.1 mm to 1 mm, the diffraction spectrum of the X-rays broadened slightly, but the broadening was small.

[0285] As a result, as shown in Table 1, in the substrate of the example, FWHMa-FWHMb was 0% or more and 30% or less of FWHMa.

[0286] (summary) According to the above examples, the first layer was three-dimensionally grown on the base structure, and thus a stress canceling effect was obtained by the first layer. It was confirmed that the stress canceling effect by the first layer could suppress the occurrence of cracks in the second layer, even if the second layer was grown thick in a state where tensile stress was accumulated in the second base layer.

[0287] Further, according to the embodiment, in the three-dimensional growth process, the first growth conditions were adjusted so as to satisfy formula (1). As a result, in the growth process of the first layer, the c-plane could be surely eliminated. By surely eliminating the c-plane, dislocations could be surely bent at the positions where the inclined interfaces in the first layer were exposed. As a result, it was confirmed that the dislocation density on the main surface of the nitride semiconductor substrate could be efficiently reduced.

[0288] Further, according to the embodiment, in the second underlayer formation process, by mirror-polishing the main surface of the second underlayer, the growth mode of the first layer was changed from the growth mode of island crystals generated in the initial stage of growth of the second underlayer, and the nearest-neighbor top distance of the first layer that grows three-dimensionally in the three-dimensional growth process could be increased. As a result, it was confirmed that the formation of a high dislocation density region in a part of the main surface of the substrate was suppressed, and the dislocation density could be made low over the entire main surface of the substrate.

[0289] Further, according to the embodiment, by growing the first layer on the above-described mirror-polished second underlayer and adjusting the first growth conditions so as to satisfy formula (1), the average nearest-neighbor top distance could be made more than 100 μm. As a result, it was confirmed that the dislocation density on the main surface of the substrate could be sufficiently reduced. Further, it was confirmed that by making the average nearest-neighbor top distance more than 100 μm, a dislocation-free region having an angle of at least 50 μm square could be formed.

[0290] Further, according to the embodiment, by obtaining the stress cancellation effect by the first layer described above, it was confirmed that the radius of curvature of the c-plane of the substrate of the embodiment could be made larger than the radius of curvature of the c-plane of the substrate of Comparative Example 1 corresponding to the conventional VAS method.

[0291] Further, according to the embodiment, as described above, there were few dislocations over a wide range of the main surface of the substrate, and the crystal mosaicity in the substrate was low. As a result, it was confirmed that in the substrate of the embodiment, the FWHMb was 50 arcsec or less over a wide range of the main surface.

[0292] In addition, according to the embodiment, as described above, the crystal mosaicity was low and the curvature radius of the c-plane of the substrate was large. As a result, in the embodiment, it was confirmed that when X-ray rocking curve measurements were performed with different slit widths on the entrance side, the difference in half-width FWHMa-FWHMb was 30% or less of FWHMa.

[0293] <Preferred embodiment of the present invention> Preferred aspects of the present invention will be described below.

[0294] (Appendix 1) providing a base substrate; forming a first underlayer made of a Group III nitride semiconductor on the underlayer; forming a metal layer on the first underlayer; performing a heat treatment to form voids in the first underlayer; epitaxially growing a second underlayer made of a single crystal of a Group III nitride semiconductor and having a primary surface whose nearest low-index crystal plane is the (0001) plane on the first underlayer, and mirror-finishing the primary surface of the second underlayer; a three-dimensional growth step of epitaxially growing a single crystal of a Group III nitride semiconductor having a top surface with an exposed (0001) plane directly on the main surface of the second underlayer, generating a plurality of recesses on the top surface constituted by inclined interfaces other than the (0001) plane, gradually expanding the inclined interfaces toward the top of the second underlayer, causing the (0001) plane to disappear from the top surface, and growing a first layer whose surface is constituted only by the inclined interfaces; a planarization step of epitaxially growing a single crystal of a Group III nitride semiconductor on the first layer to eliminate the inclined interface and grow a second layer having a mirror-finished surface; have A method for manufacturing a nitride semiconductor substrate.

[0295] (Appendix 2) In the three-dimensional growth step, forming the plurality of recesses on the top surface of the single crystal and eliminating the (0001) plane to form a plurality of valleys and a plurality of peaks on a surface of the first layer; When viewed in any cross section perpendicular to the main surface, the average distance between a pair of apexes that are closest to each other across one of the plurality of valleys in a direction along the main surface is set to be more than 100 μm. 2. A method for manufacturing the nitride semiconductor substrate according to claim 1.

[0296] (Appendix 3) In the three-dimensional growth step, The average distance between the closest pair of apexes is less than 800 μm. 3. A method for producing the nitride semiconductor substrate according to claim 2.

[0297] (Appendix 4) In the three-dimensional growth step, After the (0001) plane has disappeared from the surface, the first layer is allowed to continue growing to a predetermined thickness while maintaining a state in which the inclined interface occupies more of the surface than the (0001) plane. 4. A method for producing a nitride semiconductor substrate according to any one of claims 1 to 3.

[0298] (Appendix 5) After the planarization step, slicing at least one nitride semiconductor substrate from the second layer; In the step of slicing the nitride semiconductor substrate, The radius of curvature of the (0001) plane of the nitride semiconductor substrate is made larger than the radius of curvature of the (0001) plane of the nitride semiconductor substrate when the second underlayer is grown to the same thickness as the second layer and the second underlayer is sliced ​​without performing the three-dimensional growth step and the planarization step. 5. A method for producing a nitride semiconductor substrate according to any one of claims 1 to 4.

[0299] (Appendix 6) The radius of curvature of the (0001) plane of the nitride semiconductor substrate is set to 10 m or more. 6. A method for producing the nitride semiconductor substrate according to claim 5.

[0300] (Appendix 7) The method further includes a peeling step of peeling a laminated structure having the second underlayer, the first layer, and the second layer from the undersubstrate after the planarizing step. 7. A method for producing a nitride semiconductor substrate according to any one of claims 1 to 6.

[0301] (Appendix 8) In the three-dimensional growth step, The inclined interface is a {11-2m} plane where m≧3. 8. A method for producing a nitride semiconductor substrate according to any one of claims 1 to 7.

[0302] (Appendix 9) In the three-dimensional growth step, the first layer is grown under first growth conditions that satisfy formula (1), In the planarization step, the second layer is grown under second growth conditions that satisfy formula (2). 9. A method for producing a nitride semiconductor substrate according to any one of claims 1 to 8. G c1 >G i / cosα (1) G c2 <G i / cosα (2) (wherein the growth rate of the (0001) plane of the first layer is G c1 and the growth rate of the (0001) plane of the second layer is G c2 is defined as the growth rate of the inclined interface that is most inclined with respect to the (0001) plane in each of the first layer and the second layer, and i and the angle between the inclined interface that is most inclined with respect to the (0001) plane in each of the first layer and the second layer and the (0001) plane is α.

[0303] (Appendix 10) The steps from forming the second underlayer to the planarization step are carried out continuously in the same vapor phase growth apparatus. 10. A method for producing a nitride semiconductor substrate according to any one of claims 1 to 9.

[0304] (Appendix 11) In the method for producing a nitride semiconductor substrate according to any one of claims 1 to 10, the nitride semiconductor substrate is obtained by slicing the second layer. Nitride semiconductor substrate.

[0305] (Appendix 12) A nitride semiconductor substrate having a diameter of 2 inches or more and a primary surface in which the nearest low-index crystal plane is the (0001) plane, When the principal surface is irradiated with Cu Kα1 X-rays through a Ge (220) plane two-crystal monochromator and a slit, and an X-ray rocking curve measurement of (0002) plane diffraction is performed, the difference FWHMa-FWHMb obtained by subtracting the half-width FWHMb of the (0002) plane diffraction when the width of the slit in the ω direction is 0.1 mm from the half-width FWHMa of the (0002) plane diffraction when the width of the slit in the ω direction is 1 mm is 30% or less of FWHMa. Nitride semiconductor substrate.

[0306] (Appendix 13) When the main surface is observed with a multiphoton excitation microscope in a field of view of 250 μm square and the dislocation density is calculated from the dark spot density, the dislocation density is 3×10 6 cm -2 and the dislocation density is not greater than 1×10 6 cm -2 The area where the ratio is less than 80% of the main surface is 13. The nitride semiconductor substrate according to claim 12.

[0307] (Appendix 14) A nitride semiconductor substrate having a diameter of 2 inches or more, When the main surface of the nitride semiconductor substrate is observed with a multiphoton excitation microscope in a field of view of 250 μm square and the dislocation density is calculated from the dark spot density, the dislocation density is 3×10 6 cm -2 and the dislocation density is not greater than 1×10 6cm -2 An area less than [value] exists in an amount of 80% or more of the main surface A nitride semiconductor substrate

[0308] (Appendix 15) The main surface includes a dislocation-free region with a side length of at least 50 μm The nitride semiconductor substrate according to any one of Appendices 12 to 14

[0309] (Appendix 16) Perform X-ray rocking curve measurement of the (0002) plane at each position on the straight line passing through the center within the main surface, plot the peak angle ω formed by the X-ray incident on the main surface and the main surface against the position on the straight line, and when approximating the peak angle ω with a linear function of the position, The radius of curvature of the (0001) plane obtained by the reciprocal of the slope of the linear function is 10 m or more, The error of the measured peak angle ω with respect to the linear function is 0.05° or less The nitride semiconductor substrate according to any one of Claims 12 to 15

[0310] (Appendix 17) A nitride semiconductor substrate having a main surface whose closest low-index crystal plane is the (0001) plane, Perform X-ray rocking curve measurement of the (0002) plane at each position on the straight line passing through the center within the main surface, plot the peak angle ω formed by the X-ray incident on the main surface and the main surface against the position on the straight line, and when approximating the peak angle ω with a linear function of the position, The radius of curvature of the (0001) plane obtained by the reciprocal of the slope of the linear function is 10 m or more, The error of the measured peak angle ω with respect to the linear function is 0.05° or less A nitride semiconductor substrate

[0311] (Appendix 18) Composed of a single crystal of a group III nitride semiconductor, having a polished main surface, and a base layer whose closest low-index crystal plane to the main surface is the (0001) plane, a first layer, the first layer having a surface composed only of the inclined interfaces, formed by epitaxially growing a single crystal of a Group III nitride semiconductor directly on the main surface of the underlayer, the single crystal having a top surface with the (0001) plane exposed, generating a plurality of recesses on the top surface composed of inclined interfaces other than the (0001) plane, and gradually expanding the inclined interfaces toward the top of the underlayer until the (0001) plane disappears from the top surface; a second layer having a mirror-finished surface formed by epitaxially growing a single crystal of a Group III nitride semiconductor on the first layer to eliminate the inclined interface; have Laminated structure.

[0312] (Appendix 19) The first layer is a first c-plane growth region grown using the (0001) plane as a growth plane; an inclined interface growth region grown using the inclined interface as a growth surface; having The second layer has a second c-plane growth region grown with the (0001) plane as a growth plane. 19. The laminate structure of claim 18.

[0313] (Appendix 20) The inclined interface growth region is provided continuously along the main surface of the underlayer. 20. The laminate structure of claim 19.

[0314] (Appendix 21) The first c-plane growth region is a protrusion provided at a position where the (0001) plane has disappeared; a pair of inclined portions provided on both sides of the convex portion as a locus of intersections between the (0001) plane and the inclined interface; having The angle between the pair of inclined portions is 70° or less. 21. The laminate structure according to claim 19 or 20. [Explanation of symbols]

[0315] 1. Base substrate 2 1st base layer 3 metal layer 4. Void-containing first undercoat layer 5 Metal nitride layer 6 Second base layer 10 Base structure 30 1st layer 40 2nd layer 50 Nitride semiconductor substrate (substrate)

Claims

1. A nitride semiconductor substrate having a diameter of 2 inches or more and a primary surface in which the nearest low-index crystal plane is the (0001) plane, The main surface is formed with 100 dislocation-free regions of 50 μm square not overlapping each other at a density of 100 / cm ２ With a density of more than At a plurality of measurement points set at 5 mm intervals on a line passing through the center within the principal surface and extending in each of the <1-100> axis direction and the <11-20> axis direction, X-ray rocking curve measurements of the (0002) plane were performed under the condition that the width of the entrance side slit in the direction of the angle between the X-rays incident on the principal surface and the principal surface was 0.1 mm, and the peak angle ω at which the diffraction intensity is maximum and which is the angle between the X-rays incident on the principal surface and the principal surface was plotted against the position on the line, and when the peak angle ω was approximated by a linear function of the position, the radius of curvature of the (0001) plane calculated by the reciprocal of the slope of the linear function is 10 m or more; an error of the peak angle ω measured at each of the plurality of measurement points with respect to the linear function is 0.05° or less; The oxygen concentration in the nitride semiconductor substrate is 5×10 １６ cm －３ Is less than or equal to Nitride semiconductor substrate.

2. The error of the peak angle ω measured at each of the plurality of measurement points with respect to the linear function is 0.01° or less. The nitride semiconductor substrate according to claim 1 .

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