SiC COMPOSITE SUBSTRATE, SiC EPITAXIAL WAFER, AND SiC DEVICE

Abstract

A SiC substrate according to the present embodiment comprises a first single crystal substrate layer and a second single crystal substrate layer that is bonded to the first single crystal substrate layer. The nitrogen concentration of the first single crystal substrate layer is 1x1020 atoms / cm3. The nitrogen concentration of the second single crystal substrate layer is 1x1016 atoms / cm3 to 1.4x1018 atoms / cm3.

Description

SiC Composite Substrate, SiC Epitaxial Wafer, and SiC Device 【0001】 The present disclosure relates to a SiC composite substrate, a SiC epitaxial wafer, and a SiC device. 【0002】 Silicon carbide (SiC) has a breakdown electric field that is one order of magnitude larger and a bandgap that is three times larger than that of silicon (Si). In addition, silicon carbide (SiC) has characteristics such as a thermal conductivity that is about three times higher than that of silicon (Si). Therefore, silicon carbide (SiC) is expected to be applied to power devices, high-frequency devices, etc. In addition, devices using silicon carbide (SiC) can operate in a high temperature range of 150°C or higher. In recent years, SiC epitaxial wafers have been used for semiconductor devices as described above. 【0003】 A semiconductor device using SiC is referred to as a SiC device. A SiC device is manufactured using a SiC epitaxial wafer. A SiC epitaxial wafer is obtained by laminating a SiC epitaxial layer on the surface of a SiC substrate. A SiC substrate is a substrate before a SiC epitaxial layer is laminated. A SiC substrate is cut out from, for example, a SiC ingot (also referred to as a SiC boule). A SiC ingot is a SiC single crystal processed into a cylindrical shape. 【0004】 Consideration has been given to joining a plurality of SiC substrates. For example, Patent Document 1 discloses a composite substrate in which a SiC single crystal substrate is bonded to a SiC polycrystalline substrate. Patent Document 1 also discloses that, since the current conduction direction after device manufacturing is in the thickness direction of the composite substrate, the back surface of the SiC polycrystalline substrate is subjected to a back grinding process for the purpose of reducing substrate resistance. 【0005】 Japanese Unexamined Patent Application Publication No. 2024-121436 【0006】 Wafers used for manufacturing SiC devices are required to have various performances. 【0007】For example, reducing substrate resistance is one of the required performance characteristics of a wafer in order to ensure current conduction in the thickness direction of a SiC device. Similarly, reducing defects within the SiC epitaxial layer is another required performance characteristic of a wafer in order to ensure the reliability of a SiC device. 【0008】 For example, increasing the impurity concentration (dopant concentration) of a wafer to lower its substrate resistance makes it more prone to stacking faults, which are one of the killer defects in devices, thus reducing the reliability of SiC devices. 【0009】 For example, to reduce the substrate resistance of a wafer, the substrate can be thinned (for example, as described in Patent Document 1), but backgrinding hard SiC is a process that places a heavy burden on the process. Also, as described in Patent Document 1, backgrinding can create other problems such as wafer warping. 【0010】 This disclosure has been made in view of the above-mentioned problems and aims to provide a SiC composite substrate, SiC epitaxial wafer, and SiC device that ensure reliability after device processing and have low resistance. 【0011】 After diligent research, the inventors have discovered that a composite substrate, formed by bonding single-crystal substrates together, can achieve both low defect density and low resistance. 【0012】 This disclosure provides the following means to solve the above problems. 【0013】 (1) The SiC composite substrate according to the first embodiment comprises a first single crystal substrate layer and a second single crystal substrate layer bonded to the first single crystal substrate layer. The nitrogen concentration of the first single crystal substrate layer is 1 × 10 ２０ atoms / cm ３ That concludes the explanation. The nitrogen concentration of the second single-crystal substrate layer is 1 × 10⁻⁶. １６ atoms / cm ３ The above 1.4 x 10 １８ atoms / cm ３ The following applies: 【0014】(2) In the SiC composite substrate according to the above aspect, the first single crystal substrate layer may include stacking defects. The area ratio of the stacking defects on the first surface of the first single crystal substrate layer on the second single crystal substrate layer side is, for example, 30% or less. 【0015】 (3) In the SiC composite substrate according to the above aspect, the total dislocation density on the surface of the second single crystal substrate layer is 3.0×10 ３ per cm ２ or less may be acceptable. 【0016】 (4) In the SiC composite substrate according to the above aspect, the basal plane dislocation density on the surface of the second single crystal substrate layer is 500 per cm ２ or less may be acceptable. 【0017】 (5) In the SiC composite substrate according to the above aspect, the total dislocation density of the through screw dislocations and the through mixed dislocations on the surface of the second single crystal substrate layer is 500 per cm ２ or less may be acceptable. 【0018】 (6) The SiC composite substrate according to the above aspect may further include a third single crystal substrate layer. The third single crystal substrate layer is located between the first single crystal substrate layer and the second single crystal substrate layer. The nitrogen concentration of the third single crystal substrate layer is 1.4×10 １８ atoms / cm ３ or more and 1×10 ２０ atoms / cm ３ or less. 【0019】 (7) In the SiC composite substrate according to the above aspect, the third single crystal substrate layer may have a gradient in nitrogen concentration in the thickness direction. 【0020】 (8) The SiC composite substrate according to the above aspect may have a bonding layer between the first single crystal substrate layer and the second single crystal substrate layer. 【0021】 (9) In the SiC composite substrate according to the above aspect, both the first single crystal substrate layer and the second single crystal substrate layer may include a single crystal of SiC. 【0022】(10) The SiC epitaxial wafer according to the second embodiment comprises a SiC composite substrate according to the first embodiment and a SiC epitaxial layer laminated on the SiC composite substrate. 【0023】 (11) A SiC epitaxial wafer according to a third embodiment comprises a first single crystal substrate layer, a second single crystal substrate layer bonded to the first single crystal substrate layer, and a SiC epitaxial layer laminated on the second single crystal substrate layer, wherein the nitrogen concentration of the first single crystal substrate layer is 10 times or more that of the second single crystal substrate layer, and the warp is 49 μm or less. 【0024】 (12) In the SiC epitaxial wafer according to the above embodiment, the nitrogen concentration of the first single crystal substrate layer may be 100 times or more the nitrogen concentration of the second single crystal substrate layer. 【0025】 (13) A SiC device according to a fourth embodiment comprises a first single crystal substrate layer, a second single crystal substrate layer bonded to the first single crystal substrate layer, a SiC epitaxial layer laminated on the second single crystal substrate layer, and an element formed on the SiC epitaxial layer. The nitrogen concentration of the first single crystal substrate layer is 1 × 10 ２０ atoms / cm ３ That concludes the explanation. The nitrogen concentration of the second single-crystal substrate layer is 1 × 10⁻⁶. １６ atoms / cm ３ The above 1.4 x 10 １８ atoms / cm ３ The following applies: 【0026】 (14) In the SiC device according to the above embodiment, both the first single crystal substrate layer and the second single crystal substrate layer may contain a single crystal of SiC. 【0027】 The SiC composite substrate and SiC epitaxial wafer according to the above embodiment can be used to manufacture low-resistance, high-quality devices. The SiC device according to the above embodiment has low resistance and is less prone to defects. 【0028】This is a plan view of the SiC composite substrate according to this embodiment. This is a cross-sectional view of the SiC composite substrate according to this embodiment. This is a diagram for explaining the definition of Warp. This is a cross-sectional view of the composite substrate according to the first modified example. This is a cross-sectional view of the SiC epitaxial wafer according to this embodiment. This is a cross-sectional view of the SiC device according to this embodiment. 【0029】 This embodiment will now be described in detail with reference to the drawings as appropriate. The drawings used in the following description may be enlarged for convenience to clearly illustrate the features of this embodiment, and the dimensional ratios of each component may differ from those of the actual components. The materials, dimensions, etc., exemplified in the following description are examples only, and this disclosure is not limited to them. It is possible to modify and implement these examples as appropriate without altering the essence of the invention. 【0030】 In this specification, individual orientations are indicated by [], collective orientations by <>, individual planes by (), and collective planes by {}. While crystallography dictates that negative exponents are represented by a "-" (bar) above the number, in this specification, the negative sign is placed before the number. 【0031】 First, let's define the directions. The thickness direction of the SiC composite substrate 10 is defined as the Z direction. The Z direction may be the <0001> direction, or it may be tilted by an offset angle relative to the <0001> direction. One direction of the plane perpendicular to the Z direction is defined as the X direction, and the direction perpendicular to both the X and Z directions is defined as the Y direction. For example, the X direction is <11-20>. For example, the Y direction is the <1-100> direction. 【0032】 "SiC Composite Substrate" Figure 1 is a plan view of the SiC composite substrate 10 according to this embodiment. The SiC composite substrate 10 is a wafer that is approximately circular in plan view. 【0033】 The diameter of the SiC composite substrate 10 is not particularly limited. The diameter of the SiC composite substrate 10 is, for example, 6 inches or more, preferably 8 inches or more, more preferably 10 inches or more, and even more preferably 12 inches or more. 【0034】The diameter of the SiC composite substrate 10 is, for example, 145 mm or more, preferably 149 mm or more. The diameter of the SiC composite substrate 10 is, for example, 155 mm or less, preferably 151 mm or less. The diameter of the SiC composite substrate 10 is, for example, 195 mm or more, preferably 199 mm or more. The diameter of the SiC composite substrate 10 is, for example, 205 mm or less, preferably 201 mm or less. The diameter of the SiC composite substrate 10 is, for example, 245 mm or more, preferably 249 mm or more. The diameter of the SiC composite substrate 10 is, for example, 255 mm or less, preferably 251 mm or less. The diameter of the SiC composite substrate 10 is, for example, 295 mm or more, preferably 299 mm or more. The diameter of the SiC composite substrate 10 is, for example, 305 mm or less, preferably 301 mm or less. 【0035】 The SiC composite substrate 10 may have a notch for determining the orientation of the crystal axis when viewed from the Z direction. The notch is a groove cut out from the outer periphery of the SiC composite substrate 10 inward. The notch is located, for example, in the [1-100] direction from the center of the SiC composite substrate 10. The center is the center of the circumscribed circle that circumscribes the outer periphery of the SiC composite substrate 10. The SiC composite substrate 10 may have an orientation flat instead of a notch. 【0036】 Figure 2 is a cross-sectional view of the SiC composite substrate 10 according to this embodiment. The SiC composite substrate 10 comprises a first single crystal substrate layer 1 and a second single crystal substrate layer 2. The first single crystal substrate layer 1 is bonded to the second single crystal substrate layer 2. The SiC composite substrate 10 is a composite substrate in which the first single crystal substrate layer 1 and the second single crystal substrate layer 2 are bonded together during the manufacturing process and each consists of a separate substrate. 【0037】The first single-crystal substrate layer 1 contains a single crystal of SiC. The first single-crystal substrate layer 1 is, for example, made of a single crystal of SiC. The polytype of the first single-crystal substrate layer 1 is not particularly limited and may be any of 2H, 3C, 4H, or 6H. The first single-crystal substrate layer 1 is, for example, 4H-SiC. When the first single-crystal substrate layer 1 is a single crystal of SiC, the first single-crystal substrate layer 1 becomes transparent in the visible light range. When the first single-crystal substrate layer 1 is transparent, there are fewer process constraints when fabricating SiC devices. 【0038】 The first single-crystal substrate layer 1 is doped with nitrogen as a dopant element. The nitrogen concentration of the first single-crystal substrate layer 1 is 1 × 10⁻¹⁶. ２０ atoms / cm ３ That concludes the explanation. The nitrogen concentration in the first single-crystal substrate layer 1 is 1.0 × 10⁻⁶. ２０ atoms / cm ３ The above 3.0 x 10 ２１ atoms / cm ３ Preferably, it is 2.0 × 10 ２０ atoms / cm ３ The above 5.0 x 10 ２０ atoms / cm ３ The following is more preferable: 【0039】 The nitrogen concentration of the first single-crystal substrate layer 1 may be measured on the first surface 1A, on the second surface 1B, or at the midpoint in the thickness direction. The first surface 1A is the surface of the first single-crystal substrate layer 1 facing the second single-crystal substrate layer 2, and is, for example, the bonding surface that joins with the second single-crystal substrate layer 2. The second surface 1B is the surface of the first single-crystal substrate layer 1 facing the first surface 1A. From the viewpoint of ease of measurement, it is preferable to measure the nitrogen concentration of the first single-crystal substrate layer 1 on the second surface 1B. The nitrogen concentration of the first single-crystal substrate layer 1 can be measured, for example, using secondary ion mass spectrometry (SIMS). 【0040】 The nitrogen concentration of the first single-crystal substrate layer 1 is the average value of the measurement results at multiple measurement points. The measurement points are, for example, the center of the substrate, multiple points spaced 10 mm apart in the <11-20> direction relative to the center, and multiple points spaced 10 mm apart in the <1-100> direction relative to the center. 【0041】A high nitrogen concentration in the first single-crystal substrate layer 1 results in low resistance. A high nitrogen concentration in the first single-crystal substrate layer 1 also increases the probability of having stacking faults internally. However, as will be described later, the first single-crystal substrate layer 1 can be used even if it has stacking faults internally. 【0042】 Ideally, the first single-crystal substrate layer 1 should not contain defects and dislocations, but it does contain defects and dislocations. For example, the first single-crystal substrate layer 1 contains stacking faults. The area ratio of stacking faults on the first surface 1A of the first single-crystal substrate layer 1 is, for example, 30% or less. The area ratio of stacking faults on the first surface 1A may be, for example, 5% or more. The area ratio of stacking faults on the first surface 1A is preferably, for example, 2% to 25%, and more preferably 5% to 20%. Nitrogen concentration 1.0 × 10⁻⁶ ２０ atoms / cm ３ When the first single-crystal substrate layer 1 is fabricated under the crystal growth conditions described above, there is a high frequency in obtaining a first single-crystal substrate layer 1 that satisfies the above area ratio. Such a first single-crystal substrate layer 1 can be used in the SiC composite substrate 10 according to this embodiment, and such a first single-crystal substrate layer 1 is readily available. 【0043】 The area ratio of stacking faults can be determined from the transmission X-ray topographic image of the first surface 1A. In the transmission X-ray topographic image, the areas with stacking faults have a different contrast from the surrounding normal areas, and the total area of ​​stacking faults can be determined from the image. By dividing this total area of ​​stacking faults by the area of ​​the first surface 1A, the area ratio of stacking faults can be determined. 【0044】 Since stacking faults can become killer defects in devices, it is preferable to have a low density of stacking faults. On the other hand, since the first single-crystal substrate layer 1 is bonded to the second single-crystal substrate layer 2, stacking faults in the first single-crystal substrate layer 1 are not carried over to the second single-crystal substrate layer 2. Therefore, the effect of stacking faults in the first single-crystal substrate layer 1 on the quality of the SiC epitaxial layer laminated on the second single-crystal substrate layer 2 is small. 【0045】When a SiC epitaxial layer is deposited on a SiC substrate, stacking faults in the SiC substrate are transferred to the SiC epitaxial wafer. Therefore, if the area ratio of stacking faults in the SiC substrate exceeds 5%, the yield rate of SiC devices decreases significantly. In contrast, the first single-crystal substrate layer 1 has little impact on the SiC epitaxial layer, so it can be used even if the area ratio of stacking faults in the first single-crystal substrate layer 1 exceeds 5%. 【0046】 The thickness of the first single-crystal substrate layer 1 is, for example, 50 μm to 500 μm, and preferably 100 μm to 300 μm. The thickness of the first single-crystal substrate layer 1 can be calculated from the thickness of the original single-crystal substrate and the amount of processing at each stage of the manufacturing process. When measuring from the bonded state, a method using light reflectance can be applied. The total thickness is measured, and the difference in light reflectance is measured when light is incident from the first single-crystal substrate layer 1 and when it is incident from the opposite second single-crystal substrate layer 2. By comparing the relationship between a pre-prepared matrix table of carrier concentration and plate thickness and reflectance with the measurement results, the thickness of the first single-crystal substrate layer 1 can be calculated non-destructively. If measurement involving destructive action is permitted, the thickness can also be measured directly from cross-sectional observation in the thickness direction. If the thickness of the SiC composite substrate 10 and the thickness of the second single-crystal substrate layer 2 are known, the thickness of the first single-crystal substrate layer 1 may be determined using the difference between these two. Alternatively, when determining the thickness of the second single-crystal substrate layer 2, the thickness of the first single-crystal substrate layer 1 may be subtracted from the thickness of the SiC composite substrate 10. 【0047】 The thickness of the first single-crystal substrate layer 1 is, for example, the average radial thickness measured along a straight line extending in the <11-20> direction. The thickness is measured, for example, at the center and at multiple measurement points arranged at 10 mm intervals from the center. The intervals between the multiple measurement points may be 15 mm, 20 mm, 25 mm, or 30 mm. 【0048】The main surface of the first single crystal substrate layer 1 may or may not have an offset angle with respect to the (0001) plane in the <11-20> direction. Preferably, the main surface of the first single crystal substrate layer 1 has an offset angle in the <11-20> direction. A single crystal whose main surface has an offset angle is less likely to contain heteromorphs internally. The main surface may be the first surface 1A or the second surface 1B. 【0049】 The offset angle is the angle between the plane perpendicular to the Z direction, which is the thickness direction of the SiC composite substrate 10, and the (0001) plane. The offset angle in the <11-20> direction is, for example, greater than 0° and 10° or less, preferably 0.1° or more and 8° or less, more preferably 3.5° or more and 4.5° or less, and even more preferably 4°. 【0050】 The main surface of the first single crystal substrate layer 1 may or may not have an offset angle with respect to the (0001) plane in the <1-100> direction. Preferably, the main surface of the first single crystal substrate layer 1 does not have an offset angle in the <1-100> direction. The offset angle of the main surface of the first single crystal substrate layer 1 in the <1-100> direction may be, for example, 0.1° or more and less than 1.0°. 【0051】 The second single-crystal substrate layer 2 is bonded to the first single-crystal substrate layer 1. The second single-crystal substrate layer 2 is in contact with the first single-crystal substrate layer 1 and is bonded to the first single-crystal substrate layer 1. 【0052】 The second single-crystal substrate layer 2 contains a single crystal of SiC. The second single-crystal substrate layer 2 is, for example, made of a single crystal of SiC. Preferably, the polytype of the second single-crystal substrate layer 2 is the same as that of the first single-crystal substrate layer 1. The second single-crystal substrate layer 2 is, for example, 4H-SiC. 【0053】 The second single-crystal substrate layer 2 is doped with nitrogen as a dopant element. The nitrogen concentration in the second single-crystal substrate layer 2 is lower than that in the first single-crystal substrate layer 1. The nitrogen concentration in the second single-crystal substrate layer 2 is 1 × 10⁻⁶. １６ atoms / cm ３ The above 1.4 x 10 １８ atoms / cm ３ The following is true: The nitrogen concentration in the second single-crystal substrate layer 2 is 1.0 × 10⁻⁶.１６ atoms / cm ３ The above 1.0 x 10 １８ atoms / cm ３ Preferably, it is 1.0 × 10 １７ atoms / cm ３ The above 2.5 x 10 １７ atoms / cm ３ The following is more preferable: 【0054】 When the nitrogen concentration of the second single-crystal substrate layer 2 satisfies the above range, lattice mismatch between the SiC epitaxial layer laminated on the second single-crystal substrate layer 2 and the second single-crystal substrate layer 2 becomes less likely. Furthermore, if the nitrogen concentration of the second single-crystal substrate layer 2 is too low, the probability of heteromorphs existing within the second single-crystal substrate layer 2 increases, but if the nitrogen concentration is within the above range, the risk of heteromorphs occurring can also be reduced. 【0055】 The nitrogen concentration of the second single-crystal substrate layer 2 may be, for example, 1 / 10 or less of the nitrogen concentration of the first single-crystal substrate layer 1. That is, the nitrogen concentration of the first single-crystal substrate layer 1 may be, for example, 10 times or more of the nitrogen concentration of the second single-crystal substrate layer 2. Alternatively, the nitrogen concentration of the second single-crystal substrate layer 2 may be, for example, 1 / 100 or less of the nitrogen concentration of the first single-crystal substrate layer 1. That is, the nitrogen concentration of the first single-crystal substrate layer 1 may be, for example, 100 times or more of the nitrogen concentration of the second single-crystal substrate layer 2. It is preferable that the nitrogen concentration of the second single-crystal substrate layer 2 is close to the nitrogen concentration of the SiC epitaxial layer in order to suppress lattice mismatch. If the nitrogen concentration of the first single-crystal substrate layer 1 is higher than the nitrogen concentration of the second single-crystal substrate layer 2, the resistivity of the entire SiC composite substrate 10 can be reduced while suppressing lattice mismatch. 【0056】The nitrogen concentration of the second single-crystal substrate layer 2 may be measured on the first surface 2A, on the second surface 2B, or at the midpoint in the thickness direction. The second surface 2B is the surface of the second single-crystal substrate layer 2 that faces the first single-crystal substrate layer 1, and is, for example, the bonding surface that is in contact with the first single-crystal substrate layer. The first surface 2A is the surface of the second single-crystal substrate layer 2 that faces the second surface 2B and is the surface that is exposed to the outside. From the viewpoint of ease of measurement, it is preferable to measure the nitrogen concentration of the second single-crystal substrate layer 2 on the first surface 2A. The nitrogen concentration of the second single-crystal substrate layer 2 can be measured, for example, using secondary ion mass spectrometry (SIMS). 【0057】 The nitrogen concentration of the second single-crystal substrate layer 2 is the average value of the measurement results at multiple measurement points. The measurement points are, for example, the center of the substrate, multiple points spaced 10 mm apart in the <11-20> direction relative to the center, and multiple points spaced 10 mm apart in the <1-100> direction relative to the center. 【0058】 Ideally, the second single-crystal substrate layer 2 should not contain defects and dislocations, but it does contain defects and dislocations. The total dislocation density on the surface of the second single-crystal substrate layer 2 is, for example, 3.0 × 10⁻¹⁴. ３ pieces / cm ２ The following is true: 1.5 × 10 ３ pieces / cm ２ The following is preferable: The surface is the first surface 2A. 【0059】 The total dislocation density is determined by dividing the total number of basal plane dislocations (BPDs), through edge dislocations (TEDs), through helical dislocations (TSDs), through mixed dislocations (TMDs), and micropipes (MPs) by the area of ​​the second single-crystal substrate layer 2. These dislocations can be observed, for example, using synchrotron radiation topography or X-ray topography. Alternatively, the surface of the second single-crystal substrate layer 2 can be etched with KOH or NaOH and observed with an optical microscope. 【0060】 The basal plane dislocation density on the surface of the second single crystal substrate layer 2 is, for example, 500 ３ pieces / cm ２ The following applies: 100 pieces / cm ２The following is preferable: Basal plane dislocations cause stacking faults when current is applied to a SiC device. By keeping the basal plane dislocation density sufficiently low, adverse effects on the SiC device can be suppressed. 【0061】 The total dislocation density of through-helic dislocations and through-mixed dislocations on the surface of the second single-crystal substrate layer 2 is, for example, 500 dislocations / cm³. ２ The following is true: 300 pieces / cm ２ The following is preferable: Through-helic dislocations and through-mixed dislocations are said to have a greater adverse effect on SiC devices than through-edge dislocations. By keeping the density of these dislocations sufficiently low, the adverse effects on SiC devices can be suppressed. 【0062】 The thickness of the second single-crystal substrate layer 2 is, for example, 50 μm or more and 300 μm or less, and preferably 100 μm or more and 250 μm or less. The thickness of the second single-crystal substrate layer 2 is determined by measuring the thickness of the original second single-crystal substrate and understanding the amount of processing at each step, similar to the first single-crystal substrate layer 1. When measuring the thickness in the bonded state, it can be measured using the same method as the first single-crystal substrate layer 1. The measurement locations for the thickness of the second single-crystal substrate layer 2 are the same as the measurement locations for the thickness of the first single-crystal substrate layer 1. 【0063】 The main surface of the second single crystal substrate layer 2 may or may not have an offset angle with respect to the (0001) plane in the <11-20> direction. Preferably, the main surface of the second single crystal substrate layer 2 has an offset angle in the <11-20> direction. A single crystal whose main surface has an offset angle is less likely to contain heterogeneous polymorphs internally, and the occurrence of heterogeneous polymorphs in the SiC epitaxial layer growing on the main surface can be suppressed. The main surface is, for example, the first surface 2A. 【0064】 The offset angle in the <11-20> direction of the main surface of the second single crystal substrate layer 2 is, for example, greater than 0° and 10° or less, preferably 0.1° or more and 8° or less, more preferably 3.5° or more and 4.5° or less, and even more preferably 4°. 【0065】The main surface of the second single crystal substrate layer 2 may or may not have an offset angle with respect to the (0001) plane in the <1-100> direction. Preferably, the main surface of the second single crystal substrate layer 2 does not have an offset angle in the <1-100> direction. The offset angle of the main surface of the second single crystal substrate layer 2 in the <1-100> direction may be, for example, -1.0° or more and less than 1.0°. 【0066】 The offset angle of the second single-crystal substrate layer 2 may or may not match the offset angle of the first single-crystal substrate layer 1. Since the second single-crystal substrate layer 2 is bonded to the first single-crystal substrate layer 1, the offset angles often do not perfectly match. 【0067】 In the <11-20> direction, the difference between the offset angle of the first single crystal substrate layer 1 and the offset angle of the second single crystal substrate layer 2 is preferably less than 1.0°, and more preferably 0.5° or less. In the <11-20> direction, the difference between the offset angle of the first single crystal substrate layer 1 and the offset angle of the second single crystal substrate layer 2 may be -0.5° or more. When the difference in offset angles between the first single crystal substrate layer 1 and the second single crystal substrate layer 2 is small, a SiC composite substrate 10 that is less prone to warping even after processing can be obtained. 【0068】 In the <1-100> direction, the difference between the offset angle of the first single crystal substrate layer 1 and the offset angle of the second single crystal substrate layer 2 is preferably 0.5° or less, and more preferably 0.2° or less. In the <1-100> direction, the difference between the offset angle of the first single crystal substrate layer 1 and the offset angle of the second single crystal substrate layer 2 may be -0.2° or more. When the difference in offset angles between the first single crystal substrate layer 1 and the second single crystal substrate layer 2 is small, a SiC composite substrate 10 that is less prone to warping even after processing can be obtained. 【0069】The SiC composite substrate 10 may have a bonding layer between the first single-crystal substrate layer 1 and the second single-crystal substrate layer 2. The bonding layer is located between the first single-crystal substrate layer 1 and the second single-crystal substrate layer 2. The bonding layer contains a compound of Si and C. For example, the bonding layer is a layer in which 90 atoms or more of the constituent elements are Si and C, and Si and C are present in approximately a 1:1 ratio. The bonding layer may be crystalline, amorphous, or a mixture of both. The bonding layer may, in some cases, contain noble gas elements such as Ar and Ne, elements used for etching such as H, and dopant elements such as N, P, and B. The total amount of elements other than Si and C contained in the bonding layer is 10 atoms or less. 【0070】 The boundary between the first single-crystal substrate layer 1 and the second single-crystal substrate layer 2 may contain voids. If there are too many voids, the bonding state may not be maintained when heated, so it is desirable to minimize the presence of voids. Preferably, the voids are less than 5% of the bonding area (approximately equivalent to the area of ​​the second surface 1B or the first surface 2A) in each interface region. The bonding layer is a layer formed by bonding the first single-crystal substrate layer 1 and the second single-crystal substrate layer 2 together. 【0071】 The bonding layer can sometimes be confirmed by, for example, magnifying the side (edge) of the substrate or by observing the cross-section with a transmission electron microscope (TEM). The thickness of the bonding layer is, for example, between 0.5 nm and 10 nm. The thickness of the bonding layer can sometimes be measured directly by TEM observation of the cross-section, or it can be measured by scanning the Si-C bonding state using X-ray photoelectron spectroscopy (XPS). 【0072】 Furthermore, among the through-dislocations on the first surface 1A of the first single-crystal substrate layer 1, those whose coordinates coincide with those on the second surface 2B of the second single-crystal substrate layer 2 are, for example, 0.5% or less. The coordinates are the XY coordinates with the center of the SiC composite substrate 10 as the origin. Through-dislocations include through-helical dislocations, through-wavy dislocations, through-mixed dislocations, and micropipes. 【0073】Since the first single-crystal substrate layer 1 and the second single-crystal substrate layer 2 are bonded together, the coordinates of the threading dislocations on the first surface 1A and the coordinates of the threading dislocations on the second surface 2B hardly coincide. 【0074】 The coordinates of the penetrating dislocations on the first surface 1A can be determined by measuring the first surface 1A after it has been exposed. For example, the location of the penetrating dislocations on the first surface 1A can be identified by classifying the etch pits formed by KOH etching the first surface 1A. Alternatively, the location of the penetrating dislocations can be identified from the reflected X-ray topography image of the first surface 1A. 【0075】 The coordinates of the threading dislocations on the second surface 2B can be determined by measuring them from the coordinates of the threading dislocations on the first surface 2A. Threading dislocations extend along the c-axis. Therefore, the coordinates of the threading dislocations on the second surface 2B can be determined using the coordinates of the threading dislocations on the first surface 2A, the film thickness of the second single-crystal substrate layer 2, and the offset angle. For example, if the coordinates on the first surface 2A are (X1, Y1), the second single-crystal substrate layer 2 has an offset angle θ in the X direction, and the film thickness of the second single-crystal substrate layer 2 is T, then the coordinates of the second single-crystal substrate layer 2 on the second surface B will be (X1 - T / tanθ, Y1). 【0076】 The warp of the SiC composite substrate 10 is preferably 30 μm or less, and more preferably 20 μm or less. Warp is one of the parameters that represent the degree of warping of the substrate. 【0077】 Figure 3 schematically illustrates the method for evaluating the shape (deformation) of a substrate W using Warp. Warp is the distance in the thickness direction between the highest point hp and the lowest point lp of the first surface Wa. The larger the Warp, the more deformed the substrate W is considered to be. First, the substrate W is placed on three support points set on a flat surface F. A virtual surface Slp is determined that passes through the lowest point lp on the first surface Wa and is parallel to the flat surface F, and a virtual surface Shp is determined that passes through the highest point hp on the first surface Wa and is parallel to the flat surface F. Warp is determined as the distance in the height direction between the virtual surface Slp and the virtual surface Shp. The height direction is perpendicular to the flat surface F and away from the flat surface F. 【0078】Next, a method for manufacturing the SiC composite substrate 10 according to this embodiment will be described. The method for manufacturing the SiC composite substrate 10 includes, for example, a first single crystal preparation step, a second single crystal preparation step, a bonding step, and a separation step. 【0079】 In the first single crystal preparation step, the first single crystal, which will become the first single crystal substrate layer 1 described above, is prepared. The first single crystal is a SiC single crystal. The first single crystal may be prepared by sublimation, gas method, or solution method. 【0080】 The nitrogen concentration of the first single crystal is 1 × 10⁻⁶ ２０ atoms / cm ３ The above is complete. The nitrogen concentration can be adjusted by the amount of nitrogen supplied when the first single crystal is prepared. By slicing the first single crystal to the desired thickness, the first single crystal substrate layer 1 is obtained. The first single crystal can be sliced ​​using known methods such as a wire saw. 【0081】 In the second single crystal preparation step, the second single crystal, which will become the second single crystal substrate layer 2 described above, is prepared. The second single crystal is a single crystal of SiC. The second single crystal may be prepared by sublimation, gas method, or solution method. Since the second single crystal preferably has a low defect density, it is preferable to use a seed crystal with few dislocation defects and prepare it by gas method or sublimation method. 【0082】 The nitrogen concentration of the second single crystal is 1 × 10⁻⁶ １６ atoms / cm ３ The above 1.4 x 10 １８ atoms / cm ３ The following procedure is followed. The nitrogen concentration can be adjusted by the amount of nitrogen supplied when preparing the second single crystal. A second single crystal substrate is obtained by slicing the second single crystal to the desired thickness. Slicing the second single crystal can be done by known methods such as a wire saw. 【0083】 Next, ions are implanted into the second single-crystal substrate. The ions implanted are, for example, hydrogen ions. By keeping the ion implantation energy constant, ion implantation regions can be formed at the same height. By changing the ion implantation energy, the position where the ion implantation regions are formed can be freely controlled. 【0084】 In the bonding process, the first single-crystal substrate layer 1 and the second single-crystal substrate are bonded together. The first single-crystal substrate layer 1 and the second single-crystal substrate can be bonded together by activating the bonding surface, then stacking them and applying pressure. The bonding surface can be activated, for example, by irradiating the bonding surface with Ar ions. By performing the bonding process, a bonded body is obtained in which the first single-crystal substrate layer 1 and the second single-crystal substrate are joined together. 【0085】 In the separation process, a portion of the second single-crystal substrate is separated from the bonded body. The second single-crystal substrate is separated in the thickness direction. For example, by heating the bonded body, the second single-crystal substrate is separated along the ion-implanted portion. By separating the second single-crystal substrate in the thickness direction, a thin film layer 2 of the second single-crystal substrate is obtained. 【0086】 By going through the process described above, a SiC composite substrate 10 is obtained in which a second single crystal substrate layer 2 is bonded to a first single crystal substrate layer 1. 【0087】 In this embodiment, the SiC composite substrate 10 has a high nitrogen concentration in the first single-crystal substrate layer 1. Therefore, the first single-crystal substrate layer 1 has low resistance, eliminating the need for backgrinding to reduce substrate resistance. Furthermore, the nitrogen concentration of the second single-crystal substrate layer 2 is set to match that of the SiC epitaxial layer laminated on the second single-crystal substrate layer 2, making lattice mismatch between the second single-crystal substrate layer 2 and the SiC epitaxial layer less likely to occur. Consequently, the SiC composite substrate 10 in this embodiment can be suitably used to manufacture high-quality SiC epitaxial wafers and can also achieve low resistance in SiC devices. 【0088】 While we have described an example of a SiC composite substrate so far, SiC composite substrates are not limited to this example. Within the scope of the gist of this disclosure, various modifications and changes are possible for SiC composite substrates. 【0089】For example, Figure 4 is a cross-sectional view of a SiC composite substrate 11 according to the first modified example. The SiC composite substrate 11 differs from the SiC composite substrate 10 in that it has a third single-crystal substrate layer 3. In the SiC composite substrate 11, components similar to those in the SiC composite substrate 10 are denoted by the same reference numerals and their descriptions are omitted. 【0090】 The SiC composite substrate 11 comprises a first single-crystal substrate layer 1, a second single-crystal substrate layer 2, and a third single-crystal substrate layer 3. The third single-crystal substrate layer 3 is located between the first single-crystal substrate layer 1 and the second single-crystal substrate layer 2. The first single-crystal substrate layer 1 is bonded to the third single-crystal substrate layer 3. The second single-crystal substrate layer 2 is bonded to the third single-crystal substrate layer 3. The first single-crystal substrate layer 1, the second single-crystal substrate layer 2, and the third single-crystal substrate layer 3 are each made from separate substrates. 【0091】 The third single-crystal substrate layer 3 contains a single crystal of SiC. The third single-crystal substrate layer 3 is, for example, made of a single crystal of SiC. Preferably, the polytype of the third single-crystal substrate layer 3 is the same as that of the first single-crystal substrate layer 1 and the second single-crystal substrate layer 2. The third single-crystal substrate layer 3 is, for example, 4H-SiC. 【0092】 The third single-crystal substrate layer 3 is doped with nitrogen as a dopant element. The nitrogen concentration of the third single-crystal substrate layer 3 is 1.4 × 10⁻⁶. １８ atoms / cm ３ The above 1 x 10 ２０ atoms / cm ３ The following is true: The nitrogen concentration in the third single-crystal substrate layer 3 is 2.0 × 10⁻⁶. １８ atoms / cm ３ The above 9.0 x 10 １９ atoms / cm ３ Preferably, it is 9.0 × 10 １８ atoms / cm ３ The above 4.0 x 10 １９ atoms / cm ３ The following is more preferable: 【0093】The nitrogen concentration of the third single-crystal substrate layer 3 may be measured on the first surface 3A, on the second surface 3B, or at the midpoint in the thickness direction. The first surface 3A is the bonding surface between the third single-crystal substrate layer 3 and the second single-crystal substrate layer 2. The second surface 3B is the bonding surface between the third single-crystal substrate layer 3 and the first single-crystal substrate layer 1. The nitrogen concentration of the third single-crystal substrate layer 3 can be measured, for example, using secondary ion mass spectrometry (SIMS). 【0094】 The nitrogen concentration of the third single-crystal substrate layer 3 is the average value of the measurement results at multiple measurement points. The measurement points are, for example, the center of the substrate, multiple points spaced 10 mm apart in the <11-20> direction relative to the center, and multiple points spaced 10 mm apart in the <1-100> direction relative to the center. 【0095】 The nitrogen concentration on the first surface 3A and the nitrogen concentration on the second surface 3B may be different. For example, the nitrogen concentration on the first surface 3A may be lower than that on the second surface 3B. The third single crystal substrate layer 3 may have a nitrogen concentration gradient between the first surface 3A and the second surface 3B. For example, the nitrogen concentration on the third single crystal substrate layer 3 may gradually decrease as you move from the second surface 3B towards the first surface 3A. 【0096】The thickness of the third single-crystal substrate layer 3 is, for example, 50 μm to 200 μm, and preferably 100 μm to 150 μm. The thickness of the third single-crystal substrate layer 3 can be calculated from the thickness of the original single crystal and the amount of processing at each stage of the manufacturing process, similar to the first single-crystal substrate layer 1 and the second single-crystal substrate layer 2. When measuring from the bonded state, a method using light reflectance can be applied. The total thickness is measured, as well as the difference in light reflectance when light is incident from the first single-crystal substrate layer 1 and when it is incident from the opposite second single-crystal substrate layer 2. By comparing the measurement results with a matrix table of carrier concentration and plate thickness in a three-layer structure prepared in advance and the relationship between reflectance, the thickness of the third single-crystal substrate layer 3 can be calculated non-destructively. If measurement involving destructive action is permitted, the thickness of the third single-crystal substrate layer 3 can also be directly measured from cross-sectional observation in the thickness direction. The measurement locations for the thickness of the third single-crystal substrate layer 3 are the same as those for the thickness of the first single-crystal substrate layer 1. Alternatively, the thickness of the third single-crystal substrate layer 3 may be determined by subtracting the thickness of the other layers from the total thickness of the SiC composite substrate 10. 【0097】 Preferably, the main surface of the third single crystal substrate layer 3 has an offset angle with respect to the (0001) plane in the <11-20> direction, and does not have an offset angle with respect to the (0001) plane in the <1-100> direction. 【0098】 The method for manufacturing the SiC composite substrate 11 differs from the method for manufacturing the SiC composite substrate 10 in that it includes a third single crystal preparation step and has two bonding steps. 【0099】 In the third single crystal substrate preparation step, the third single crystal, which will become the third single crystal substrate layer 3 described above, is prepared. The third single crystal is a single crystal of SiC. The third single crystal may be prepared by sublimation, gas method, or solution method. 【0100】 The nitrogen concentration of the third single crystal is 1.4 × 10⁻⁶. １８ atoms / cm ３ The above 1 x 10 ２０ atoms / cm ３The nitrogen concentration of the third single crystal may be changed during the crystal growth process. By changing the nitrogen concentration of the third single crystal during the growth process, a nitrogen concentration gradient can be created in the thickness direction of the third single crystal substrate layer 3. The nitrogen concentration can be adjusted by the amount of nitrogen supplied when the third single crystal is fabricated. The third single crystal substrate layer 3 can be obtained by slicing the third single crystal to a desired thickness. Slicing the third single crystal can be done by known methods such as a wire saw. Alternatively, ions may be implanted into the third single crystal and heat-treated to separate the third single crystal in the thickness direction at the ion implantation sites, thereby fabricating a thin film third single crystal substrate layer 3. 【0101】 The bonding process is divided into two stages. In the first stage, the third single crystal substrate layer 3 is bonded to the first single crystal substrate layer 1. In the second stage, the second single crystal substrate is bonded to the third single crystal substrate layer 3. The bonding method is the same as the bonding process described above. 【0102】 The SiC composite substrate 11 has a first single-crystal substrate layer 1 and a second single-crystal substrate layer 2 with controlled nitrogen concentrations, and therefore achieves the same effects as the SiC composite substrate 10. Furthermore, the SiC composite substrate 11 has a third single-crystal substrate layer 3 having an intermediate nitrogen concentration between the first single-crystal substrate layer 1 and the second single-crystal substrate layer 2, which suppresses warping of the SiC composite substrate 11. The warp of the SiC composite substrate 11 is preferably 30 μm or less, and more preferably 20 μm or less. 【0103】 "SiC Epitaxial Wafer" Figure 5 is a cross-sectional view of the SiC epitaxial wafer 20 according to this embodiment. The plan view shape of the SiC epitaxial wafer 20 is the same as the plan view shape of the SiC composite substrate 10. 【0104】 The SiC epitaxial wafer 20 has a SiC composite substrate 10 and a SiC epitaxial layer 21. The SiC composite substrate 10 of the SiC epitaxial wafer 20 can also be replaced with a SiC composite substrate 11. The SiC composite substrate 10 and SiC composite substrate 11 are as described above. 【0105】The SiC epitaxial layer 21 is formed on the second single-crystal substrate layer 2 of the SiC composite substrate 10. The SiC epitaxial layer 21 is, for example, SiC. The SiC epitaxial layer 21 may be undoped SiC or dopant-doped SiC. The SiC epitaxial layer 21 is preferably, for example, nitrogen-doped n-type SiC. 【0106】 If the SiC epitaxial layer 21 contains nitrogen, the nitrogen concentration of the SiC epitaxial layer 21 is, for example, 1.0 × 10⁻⁶. １４ atoms / cm ３ The above 1.0 x 10 １７ atoms / cm ３ The following is true: 5.0 × 10 １４ atoms / cm ３ The above 9.0 x 10 １６ atoms / cm ３ Preferably, it is 1.0 × 10 １５ atoms / cm ３ The above 5.0 x 10 １６ atoms / cm ３ The following is more preferable: 【0107】 The thickness of the SiC epitaxial layer 21 is, for example, 3 μm or more, preferably 5 μm or more, more preferably 10 μm or more, and even more preferably 15 μm or more. The thickness of the SiC epitaxial layer 21 may also be, for example, 200 μm or less. 【0108】 The thickness of the SiC epitaxial layer 21 is the average value of the thickness of the SiC epitaxial layer 21 measured at different points in the radial direction of the SiC epitaxial layer 21. The measurement points for the thickness of the SiC epitaxial layer 21 are the same as the measurement points for the thickness of the SiC composite substrate 10. The thickness of the SiC epitaxial layer 21 can be measured, for example, using a spectrophotometer. The thickness of the SiC epitaxial layer 21 can also be determined using the thickness of the original SiC composite substrate 10 and information from the film deposition process design. 【0109】Alternatively, the thickness of the SiC epitaxial layer 21 may be determined by optical interference analysis using FTIR (Fourier transform infrared spectrophotometer). FTIR is a method that irradiates a sample with infrared light and calculates the thickness from the reflection spectrum, but measurement can be difficult when the thickness is less than a few micrometers. In this case, it may be possible to measure the thickness using secondary ion mass spectrometry (SIMS). SIMS is a destructive test, but it measures the thickness of the SiC epitaxial layer 21 by measuring the thickness direction profile on the order of nanometers. If the total thickness of the second single crystal substrate layer 2 and the SiC epitaxial layer 21 exceeds 10 μm, the resolution decreases, so the sample may be cut at an angle and the line profile of the exposed cut surface may be measured. 【0110】 The warp of the SiC epitaxial wafer 20 is preferably 49 μm or less, and more preferably 40 μm or less. A smaller warp is expected to reduce the probability of suction errors during automated transport. 【0111】 The SiC epitaxial wafer 20 is obtained by depositing a SiC epitaxial layer 21 on the second single-crystal substrate layer 2 of the SiC composite substrate 10. A known method can be used to deposit the SiC epitaxial layer 21. 【0112】 The SiC epitaxial wafer 20 according to this embodiment has low resistance in the thickness direction due to the high nitrogen concentration of the first single crystal substrate layer 1. Therefore, the SiC epitaxial wafer 20 according to this embodiment does not require backgrinding to reduce substrate resistance. Furthermore, the SiC epitaxial wafer 20 according to this embodiment is of high quality. This is because the lattice matching between the SiC epitaxial layer 21 and the second single crystal substrate layer 2 is high, and defects are less likely to occur in the SiC epitaxial layer 21. The SiC epitaxial wafer 20 according to this embodiment can be used to manufacture high-quality, low-resistance SiC devices. 【0113】"SiC Device" Figure 6 is a cross-sectional view of the SiC device 30 according to this embodiment. The SiC device 30 is formed by creating a chip from the SiC epitaxial wafer 20 by forming an element 34 on the SiC epitaxial layer 21 of the SiC epitaxial wafer 20. 【0114】 The SiC device 30 includes a first single-crystal substrate layer 31, a second single-crystal substrate layer 32, a SiC epitaxial layer 33, and an element 34. 【0115】 The first single-crystal substrate layer 31 is a chipped version of the first single-crystal substrate layer 1 described above. The first single-crystal substrate layer 31 is equivalent to the first single-crystal substrate layer 1, except that it is a chipped version. 【0116】 The second single-crystal substrate layer 32 is a chipped version of the second single-crystal substrate layer 2 described above. The second single-crystal substrate layer 32 is equivalent to the second single-crystal substrate layer 2, except that it is a chipped version. 【0117】 The SiC epitaxial layer 33 is a chipped version of the SiC epitaxial layer 21 described above. The SiC epitaxial layer 33 is equivalent to the SiC epitaxial layer 21, except that it is chipped. 【0118】 The element 34 is formed in the SiC epitaxial layer 33. The element is a combination of components such as a transistor, capacitor, inductor, resistor, wiring, etc. Figure 6 shows a transistor as an example of the element 34. 【0119】 The SiC device 30 according to this embodiment has low resistance in the thickness direction because the nitrogen concentration of the first single crystal substrate layer 31 is high. Therefore, the SiC device has low resistance even without backgrinding. Furthermore, the SiC device 30 according to this embodiment has high lattice matching between the second single crystal substrate layer 32 and the SiC epitaxial layer 33, resulting in fewer defects in the SiC epitaxial layer 33 and a high yield. 【0120】The SiC device 30 can also be manufactured using the above-described SiC composite substrate 11. The SiC device 30 may have a third single-crystalline substrate layer between the first single-crystalline substrate layer 31 and the second single-crystalline substrate layer 32. This third single-crystalline substrate layer corresponds to the above-described third single-crystalline substrate layer 3 formed into a chip. 【0121】 As described above, the preferred embodiments of the present disclosure have been described in detail. However, the present disclosure is not limited to specific embodiments, and various modifications and changes are possible within the scope of the gist of the present disclosure described within the scope of the claims. 【0122】 "Example 1" A first single crystal having a diameter of 200 mm (8 inches) was prepared. The first single crystal is a SiC single crystal having a nitrogen concentration of 1.0×10 ２０ atoms / cm ３ From the first single crystal, a first single-crystalline substrate layer 1 having a film thickness of 300 μm was obtained. Also, a transmission X-ray topographic image of the first surface of the first single-crystalline substrate layer 1 was obtained, and the area ratio of stacking defects was determined. In the first single-crystalline substrate layer 1 of Example 1, the area ratio of stacking defects was 0%. 【0123】 Next, separately, a second single crystal having a diameter of 200 mm (8 inches) was prepared. The second single crystal is a SiC single crystal having a nitrogen concentration of 8.0×10 １６ atoms / cm ３ Hydrogen ions were implanted at a position 50 μm in the depth direction from the surface of the second single crystal. 【0124】 The first single-crystalline substrate layer 1 and the second single crystal were bonded and heat-treated to separate a part of the second single crystal. The second single crystal became a second single-crystalline substrate layer 2 having a thickness of 50 μm, and a SiC composite substrate 10 in which the first single-crystalline substrate layer 1 and the second single-crystalline substrate layer 2 were bonded was manufactured. The main surface of the second single-crystalline substrate layer 2 was measured by X-ray topography, and the total dislocation density of the second single-crystalline substrate layer 2 was determined. The total dislocation density of the second single-crystalline substrate layer 2 was 1500 pieces / cm －２ . 【0125】 Also, the Warp of the SiC composite substrate 10 of Example 1 was measured. The Warp of the SiC composite substrate 10 of Example 1 was 15 μm. 【0126】Next, a SiC epitaxial layer 21 was grown on the second single-crystal substrate layer 2 of the SiC composite substrate 10 to fabricate a SiC epitaxial wafer 20. The SiC epitaxial layer 21 is SiC containing nitrogen as a dopant. The dopant concentration of the SiC epitaxial layer 21 is 5.0 × 10⁻¹⁶. １５ atoms / cm ３ The thickness of the SiC epitaxial layer 21 was set to 10 μm. 【0127】 Furthermore, the warp of the SiC epitaxial wafer 20 of Example 1 was measured. The warp of the SiC epitaxial wafer 20 of Example 1 was 27 μm. 【0128】 Next, multiple SiC devices 30 were fabricated using a SiC epitaxial wafer 20. The processing yield when fabricating the SiC devices 30 from the SiC epitaxial wafer 20 and the long-term reliability yield, which indicates the long-term reliability of the SiC devices 30, were determined, and the device yield of the SiC devices 30 was calculated. The device yield is calculated as the product of the processing yield and the long-term reliability yield. 【0129】 The processing yield is calculated by dividing the number of SiC devices actually fabricated by the number of SiC devices 30 that can be obtained from the SiC epitaxial wafer 20. The long-term reliability yield is calculated by dividing the number of good SiC devices after long-term reliability testing by the number of SiC devices actually fabricated. For the long-term reliability testing, a TDDB (Time-Dependent Dielectric Breakdown) test was employed, which involves applying a voltage (electric field) higher than the normal operating environment and measuring the change in leakage current. 【0130】 Examples 2 to 5, Comparative Examples 1 to 3: Examples 2 to 5 and Comparative Examples 1 to 3 differ from Example 1 in that the nitrogen concentration and defect density of the first single-crystal substrate layer 1 and the second single-crystal substrate layer 2 are different. Other conditions were the same as in Example 1, and the same evaluation was performed as in Example 1. 【0131】The results of Examples 1 to 5 and Comparative Examples 1 to 3 are summarized in Table 1 below. In Table 1, the yield of SiC devices is based on Comparative Example 1. Devices that are superior to Comparative Example 1 in both processing yield and long-term reliability yield are classified as "best," devices that are superior to Comparative Example 1 in either processing yield or long-term reliability yield and at the same level as Comparative Example 1 are classified as "good," and devices that are inferior to Comparative Example 1 in both processing yield and long-term reliability yield are classified as "NG." 【0132】 【0133】 Examples 1 to 5 described above showed a higher yield of SiC devices than Comparative Examples 1 to 3. 【0134】 Similar experiments were also conducted using a substrate with a diameter of 150 mm (6 inches), and the same results were obtained. 【0135】 1, 31 First single crystal substrate layer 1A, 2A, 3A First surface 1B, 2B, 3B Second surface 2, 32 Second single crystal substrate layer 3 Third single crystal substrate layer 10, 11 SiC composite substrate 20 SiC epitaxial wafer 21, 33 SiC epitaxial layer 30 SiC device 34 Element

Claims

It comprises a first single crystal substrate layer and a second single crystal substrate layer bonded to the first single crystal substrate layer, The nitrogen concentration of the first single-crystal substrate layer is 1 × 10⁻¹⁶ ２０ atoms / cm ３ That's all. The nitrogen concentration of the second single-crystal substrate layer is 1 × 10⁻¹⁶ １６ atoms / cm ３ The above 1.4 x 10 １８ atoms / cm ３ The following is a SiC composite substrate. 　 The first single-crystal substrate layer contains stacking faults, The SiC composite substrate according to claim 1, wherein the area ratio of the stacking faults on the first surface of the first single crystal substrate layer on the second single crystal substrate layer side is 30% or less. 　 The total dislocation density on the surface of the second single-crystal substrate layer is 3.0 × 10⁻⁶. ３ pieces / cm ２ The SiC composite substrate according to claim 1, which is as follows: 　 The basal plane dislocation density on the surface of the second single crystal substrate layer is 500 pieces / cm ２ The SiC composite substrate according to claim 1, wherein the density is 500 pieces / cm or less. 　 The total dislocation density of through-helic dislocations and through-mixed dislocations on the surface of the second single-crystal substrate layer is 500 dislocations / cm³. ２ The SiC composite substrate according to claim 1, which is as follows: 　 It further comprises a third single-crystal substrate layer, The third single crystal substrate layer is located between the first single crystal substrate layer and the second single crystal substrate layer. The nitrogen concentration of the third single-crystal substrate layer is 1.4 × 10⁻⁶. １８ atoms / cm ３ The above 1 x 10 ２０ atoms / cm ３ The SiC composite substrate according to claim 1, which is as follows: 　 The SiC composite substrate according to claim 6, wherein the third single-crystal substrate layer has a nitrogen concentration gradient in the thickness direction. 　 The SiC composite substrate according to claim 1, wherein a bonding layer is provided between the first single crystal substrate layer and the second single crystal substrate layer. 　 Both the first single-crystal substrate layer and the second single-crystal substrate layer contain a single crystal of SiC. The SiC composite substrate according to claim 1. 　 The SiC composite substrate according to claim 1, A SiC epitaxial wafer comprising a SiC epitaxial layer laminated on the SiC composite substrate. 　 It comprises a first single crystal substrate layer, a second single crystal substrate layer bonded to the first single crystal substrate layer, and a SiC epitaxial layer laminated on the second single crystal substrate layer. The nitrogen concentration of the first single-crystal substrate layer is 10 times or more that of the second single-crystal substrate layer. A SiC epitaxial wafer with a warp of 49 μm or less. 　 The SiC epitaxial wafer according to claim 11, wherein the nitrogen concentration of the first single-crystal substrate layer is 100 times or more than the nitrogen concentration of the second single-crystal substrate layer. 　 The device comprises a first single-crystal substrate layer, a second single-crystal substrate layer bonded to the first single-crystal substrate layer, a SiC epitaxial layer laminated on the second single-crystal substrate layer, and an element formed on the SiC epitaxial layer. The nitrogen concentration of the first single-crystal substrate layer is 1 × 10⁻¹⁶ ２０ atoms / cm ３ That's all. The nitrogen concentration of the second single-crystal substrate layer is 1 × 10⁻¹⁶ １６ atoms / cm ３ The above 1.4 x 10 １８ atoms / cm ３ The following are SiC devices. 　 The first single-crystal substrate layer and the second single-crystal substrate layer both contain a single crystal of SiC. The SiC device according to claim 13.

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