Composite substrate, manufacturing method therefor, epitaxial wafer, manufacturing method therefor, device, and manufacturing method therefor

Abstract

A composite substrate according to the present embodiment comprises a first single crystal substrate layer, and a second single crystal substrate layer adhered to the first single crystal substrate layer. The resistivity of the first single crystal substrate layer is less than 10 mΩ·cm, and the resistivity of the second single crystal substrate layer is 10 mΩ·cm or more.

Description

Composite substrate and method for manufacturing the same, epitaxial wafer and method for manufacturing the same, device and method for manufacturing the same 【0001】 This disclosure relates to composite substrates and methods for manufacturing the same, epitaxial wafers and methods for manufacturing the same, devices and methods for manufacturing the same. 【0002】 Silicon carbide (SiC) has a dielectric breakdown field an order of magnitude larger and a band gap three times larger than silicon (Si). Furthermore, its thermal conductivity is approximately three times higher than that of silicon (Si). Due to these properties, silicon carbide (SiC) is expected to have applications in power devices, high-frequency devices, and other fields. In addition, devices using silicon carbide (SiC) can operate at high temperatures of 150°C or higher. In recent years, SiC epitaxial wafers have come into use in semiconductor devices such as those described above. 【0003】 Semiconductor devices using SiC are called SiC devices. SiC devices are fabricated using SiC epitaxial wafers. SiC epitaxial wafers are manufactured by stacking SiC epitaxial layers on the surface of a SiC substrate. SiC substrates are cut from SiC ingots. SiC ingots are SiC single crystals processed into a cylindrical shape. 【0004】 For example, Patent Document 1 describes a semiconductor substrate comprising a single-crystal silicon carbide substrate and a single-crystal silicon carbide layer laminated on the side surface of the single-crystal silicon carbide substrate, wherein the quality index of the single-crystal silicon carbide substrate is lower than that of the single-crystal silicon carbide layer. Furthermore, Patent Document 1 states that the resistivity of the single-crystal silicon carbide wafer may be in the range of 0.010 ohmcm (Ω·cm) to 0.030 Ω·cm, and the resistivity of the single-crystal silicon carbide substrate may be in the range of 0.010 Ω·cm to 0.030 Ω·cm. 【0005】 Japanese Patent Publication No. 2023-501646 【0006】Conventionally, in epitaxial wafers formed by stacking epitaxial layers on a single-crystal substrate layer, the high resistivity of the single-crystal substrate layer has sometimes been a problem. Specifically, in devices equipped with elements formed on the epitaxial layer of an epitaxial wafer, sufficient device characteristics could not be obtained due to the high resistivity of the single-crystal substrate layer. 【0007】 One possible solution to this problem is to use doped ingots as the material for the single-crystal substrate layer of the epitaxial wafer. However, if the concentration of dopant elements in the ingot is increased in order to obtain a single-crystal substrate layer with sufficiently low resistivity, defects are more likely to occur in the epitaxial layer that is laminated onto the single-crystal substrate layer cut from the ingot. As a result, the reliability of devices formed using epitaxial wafers decreases. 【0008】 The present invention has been made in view of the above circumstances, and aims to provide a composite substrate and a method for manufacturing the same that can form an epitaxial layer with sufficiently low resistivity and few defects. 【0009】 Furthermore, the present invention aims to provide an epitaxial wafer comprising an epitaxial layer laminated on the composite substrate of the present invention, and a method for manufacturing the same. Furthermore, the present invention aims to provide a device comprising an epitaxial layer laminated on the composite substrate of the present invention and an element formed on the epitaxial layer, and a method for manufacturing the same. 【0010】To solve the above problems and realize a substrate capable of forming an epitaxial layer with sufficiently low resistivity and few defects, the inventor focused on a composite substrate in which two single-crystal substrate layers are bonded together, and conducted diligent studies as described below. That is, the composite substrate can be manufactured by bonding a first single-crystal substrate layer and a second single-crystal substrate that will become the second single-crystal substrate layer, which are manufactured separately. For this reason, for example, when manufacturing a composite substrate in which an epitaxial layer is deposited on the second single-crystal substrate layer, the first single-crystal substrate layer before bonding can be formed using an ingot doped with a high concentration of dopant elements without affecting the quality of the second single-crystal substrate that will become the second single-crystal substrate layer. 【0011】 Furthermore, in a composite substrate formed by bonding a first single-crystal substrate layer and a second single-crystal substrate layer, defects in the first single-crystal substrate layer are less likely to spread to the epitaxial layer deposited on the second single-crystal substrate layer. Therefore, even if the first single-crystal substrate layer is highly doped with dopant elements and has many defects, if the second single-crystal substrate layer is not doped with dopant elements, or is doped at a low concentration, and has fewer defects compared to the first single-crystal substrate layer, it is presumed that a defect-free epitaxial layer can be formed by depositing an epitaxial layer on the second single-crystal substrate layer. 【0012】 Based on these findings, the inventors conducted extensive research on the resistivity of the first and second single-crystal substrate layers forming the composite substrate. As a result, they discovered that a composite substrate consisting of a first single-crystal substrate layer with a resistivity of less than 10 mΩ·cm and a second single-crystal substrate layer with a resistivity of 10 mΩ·cm or more should be bonded together, leading to the invention of this invention. The present invention provides the following means. 【0013】 [1] A composite substrate comprising a first single-crystal substrate layer and a second single-crystal substrate layer bonded to the first single-crystal substrate layer, wherein the resistivity of the first single-crystal substrate layer is less than 10 mΩ·cm and the resistivity of the second single-crystal substrate layer is 10 mΩ·cm or more. 【0014】[2] The composite substrate according to [1], wherein the resistivity of the second single-crystal substrate layer is 25 mΩ·cm or more. [3] The composite substrate according to [1], wherein the resistivity of the composite substrate is 15 mΩ·cm or less. 【0015】 [4] The composite substrate according to [1], wherein the inclination of the (0001) plane with respect to a plane perpendicular to the thickness direction of the first single crystal substrate layer is different from the inclination of the (0001) plane with respect to a plane perpendicular to the thickness direction of the second single crystal substrate layer. 【0016】 [5] The composite substrate according to [1], wherein the difference between the inclination of the (0001) plane with respect to a plane perpendicular to the thickness direction of the first single crystal substrate layer and the inclination of the (0001) plane with respect to a plane perpendicular to the thickness direction of the second single crystal substrate layer is 1.0° or more. 【0017】 [6] The composite substrate according to [1], wherein the inclination of the (0001) plane with respect to a plane perpendicular to the thickness direction of the first single crystal substrate layer is 0° or more and 3.5° or less, and the inclination of the (0001) plane with respect to a plane perpendicular to the thickness direction of the second single crystal substrate layer is greater than 0° and 4.5° or less. 【0018】 [7] The composite substrate according to [1] wherein the total dislocation density of the first single-crystal substrate layer is higher than the total dislocation density of the second single-crystal substrate layer. [8] The difference between the basal plane dislocation density of the first single-crystal substrate layer and the basal plane dislocation density of the second single-crystal substrate layer is 1000 dislocations / cm². ２ The composite substrate according to [1] wherein the basal plane dislocation density of the second single crystal substrate layer is lower than the basal plane dislocation density of the first single crystal substrate layer. [9] The basal plane dislocation density of the second single crystal substrate layer is 500 dislocations / cm ２ The composite substrate according to [1], wherein the basal plane dislocation density of the second single crystal substrate layer is less than the basal plane dislocation density of the first single crystal substrate layer. 【0019】

[10] A composite substrate comprising a first single-crystal substrate layer, a second single-crystal substrate layer, and a junction layer disposed between the first single-crystal substrate layer and the second single-crystal substrate layer, wherein the resistivity of the first single-crystal substrate layer is less than 10 mΩ·cm and the resistivity of the second single-crystal substrate layer is 10 mΩ·cm or more. 【0020】

[11] A method for manufacturing a composite substrate, comprising: preparing a first single crystal substrate layer having a resistivity of less than 10 mΩ·cm and a second single crystal substrate having a resistivity of 10 mΩ·cm or more; and bonding the first single crystal substrate layer and the second single crystal substrate together. 【0021】

[12] An epitaxial wafer comprising the composite substrate according to [1] and an epitaxial layer laminated on the second single crystal substrate layer. 【0022】

[13] A method for manufacturing an epitaxial wafer, comprising: preparing the composite substrate according to [1]; and forming an epitaxial layer on the second single crystal substrate layer of the composite substrate. 【0023】

[14] A device comprising the composite substrate according to [1], an epitaxial layer laminated on the second single crystal substrate layer, and an element formed on the epitaxial layer. 【0024】

[15] A method for manufacturing a device, comprising: performing the method for manufacturing an epitaxial wafer according to

[13] ; and forming an element on the epitaxial layer. 【0025】 Since the resistivity of the first single crystal substrate layer of the composite substrate of the present invention is less than 10 mΩ·cm, the resistivity is sufficiently low. Moreover, since the composite substrate of the present invention includes a second single crystal substrate layer having a resistivity of 10 mΩ·cm or more bonded to the first single crystal substrate layer, an epitaxial layer with fewer defects can be formed on the second single crystal substrate layer. 【0026】 A plan view showing an example of the composite substrate of the present invention. A cross-sectional view of the composite substrate shown in FIG. 1. A cross-sectional view showing an example of the epitaxial wafer of the present invention. A cross-sectional view showing an example of the device of the present invention. 【0027】The composite substrate and its manufacturing method, epitaxial wafer and its manufacturing method, and device and its manufacturing method of the present invention will be described in detail below with appropriate reference to the drawings. In the drawings used in the following description, characteristic parts may be enlarged for convenience in order to make the features of this embodiment easier to understand. Therefore, the dimensional ratios of each component may differ from those of the actual product. 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 them as appropriate without changing the gist of the invention. 【0028】 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. 【0029】 "Composite Substrate" Figure 1 is a plan view showing an example of the composite substrate of the present invention. Figure 2 is a cross-sectional view of the composite substrate shown in Figure 1. As shown in Figure 1, the composite substrate 1 is a wafer that is substantially circular in plan view. As shown in Figure 2, the composite substrate 1 of this embodiment comprises a first single crystal substrate layer 11 and a second single crystal substrate layer 12. 【0030】 First, we define the directions. In this embodiment, the thickness direction of the composite substrate 1 (the thickness direction of the first single crystal substrate layer 11 and the second single crystal substrate layer 12) is defined as the Z direction. One direction of the plane perpendicular to the Z direction is defined as the X direction. One direction perpendicular to both the Z direction and the X direction is defined as the Y direction. The X direction is, for example, the <11-20> direction of the first single crystal substrate layer 11 and / or the second single crystal substrate layer 12. The Y direction is, for example, the <1-100> direction of the first single crystal substrate layer 11 and / or the second single crystal substrate layer 12. 【0031】 The diameter of the composite substrate 1 in this embodiment 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. 【0032】The diameter of the composite substrate 1 is, for example, 145 mm or more, preferably 149 mm or more. The diameter of the composite substrate 1 is, for example, 155 mm or less, preferably 151 mm or less. The diameter of the composite substrate 1 is, for example, 195 mm or more, preferably 199 mm or more. The diameter of the composite substrate 1 is, for example, 205 mm or less, preferably 201 mm or less. The diameter of the composite substrate 1 is, for example, 245 mm or more, preferably 249 mm or more. The diameter of the composite substrate 1 is, for example, 255 mm or less, preferably 251 mm or less. The diameter of the composite substrate 1 is, for example, 295 mm or more, preferably 299 mm or more. The diameter of the composite substrate 1 is, for example, 305 mm or less, preferably 301 mm or less. 【0033】 As shown in Figure 1, the composite substrate 1 may have a notch n for determining the direction of the crystal axis when viewed from the Z direction. The notch n is a groove cut out from the outer circumference of the composite substrate 1 toward the inside of the wafer. The notch n is located, for example, in the <1-100> direction from the center of the composite substrate 1 toward the second single crystal substrate layer 12. The composite substrate 1 may have an orientation flat instead of a notch n. 【0034】 As shown in Figure 2, the first single-crystal substrate layer 11 of the composite substrate 1 is in contact with the second single-crystal substrate layer 12 and is bonded to the second single-crystal substrate layer 12. The first single-crystal substrate layer 11 and the second single-crystal substrate layer 12 were originally separate substrates and were bonded together during the manufacturing process. 【0035】 The first single-crystal substrate layer 11 and the second single-crystal substrate layer 12 both contain, for example, SiC. The polytype of the first single-crystal substrate layer 11 and the second single-crystal substrate layer 12 is not particularly limited and may be any of 2H, 3C, 4H, or 6H. For example, the first single-crystal substrate layer 11 and the second single-crystal substrate layer 12 are 4H-SiC. 【0036】The first single-crystal substrate layer 11 is made of a dopant-doped SiC single crystal. The dopant is, for example, nitrogen. The second single-crystal substrate layer 12 may be made of an undoped SiC single crystal or a dopant-doped SiC single crystal. The dopant is, for example, nitrogen. For example, the first single-crystal substrate layer 11 and the second single-crystal substrate layer 12 are n-type SiC single crystals. 【0037】 In this embodiment, the composite substrate 1 has a resistivity of less than 10 mΩ·cm for the first single-crystal substrate layer 11, which is sufficiently low. Therefore, the composite substrate 1 in this embodiment has a sufficiently low resistivity. Thus, for example, a device comprising an epitaxial layer laminated on the second single-crystal substrate layer 12 of the composite substrate 1 in this embodiment and an element formed on the epitaxial layer can be prevented from having its device characteristics impaired due to the high resistivity of the composite substrate 1. Furthermore, since the resistivity of the first single-crystal substrate layer 11 in this embodiment is less than 10 mΩ·cm, even if a part of the first single-crystal substrate layer 11 is removed during the slimming process when forming the device, it is possible to prevent the device characteristics from being impaired due to the high resistivity of the composite substrate 1. In addition, when the resistivity of the composite substrate 1 is sufficiently low, the amount of the first single-crystal substrate layer 11 that needs to be removed in the slimming process to reduce the resistivity of the composite substrate 1 can be reduced. Therefore, the slimming process can be simplified or even omitted in some cases. The resistivity of the first single-crystal substrate layer 11 may be 5 mΩ·cm or less, or 3 mΩ·cm or less. 【0038】 The resistivity of the first single-crystal substrate layer 11 is preferably 0.01 mΩ·cm or higher. This is because it can be easily manufactured by using a first single-crystal substrate layer 11 cut from an ingot doped with a high concentration of dopant elements before bonding. The resistivity of the first single-crystal substrate layer 11 is more preferably 0.05 mΩ·cm or higher, and even more preferably 0.1 mΩ·cm or higher. 【0039】The resistivity of the first single crystal substrate layer 11 of the composite substrate 1 of this embodiment can be calculated by peeling the first single crystal substrate layer 11 from the composite substrate 1, planarizing the peeled surface, measuring the sheet resistance using a non-contact eddy current method, measuring the thickness of the measurement part, and using the results. 【0040】 It is preferable that the first single crystal substrate layer 11 is doped with nitrogen as a dopant element. The nitrogen concentration of the first single crystal substrate layer 11 is determined according to the resistivity of the first single crystal substrate layer 11. In order to make the first single crystal substrate layer 11 have a resistivity of less than 10 mΩ·cm, it is preferably 1×10 １９ atoms / cm ３ or more, more preferably 2×10 １９ atoms / cm ３ or more, and even more preferably 3×10 １９ atoms / cm ３ or more. The nitrogen concentration of the first single crystal substrate layer 11 may be 5×10 ２０ atoms / cm ３ or less, and may also be 2×10 ２０ atoms / cm ３ or less. 【0041】 The nitrogen concentration of the first single crystal substrate layer 11 can be measured using the secondary ion mass spectrometry (SIMS) method. 【0042】 The resistivity of the second single crystal substrate layer 12 is 10 mΩ·cm or more. Therefore, for example, by using a method of making the second single crystal substrate layer 12 doped with a dopant element at a high concentration, it is not necessary to lower the resistivity of the second single crystal substrate layer 12. Therefore, the second single crystal substrate layer 12 with few defects caused by the doping of the dopant element can be obtained. The resistivity of the second single crystal substrate layer 12 is preferably 25 mΩ·cm or more, more preferably 30 mΩ·cm or more, and even more preferably 35 mΩ·cm or more. 【0043】The resistivity of the second single-crystal substrate layer 12 is preferably 500 mΩ·cm or less. This is because the resistivity of the composite substrate 1 in this embodiment tends to be lower. Furthermore, if the resistivity of the second single-crystal substrate layer 12 is 500 mΩ·cm or less, it is possible to suppress the deterioration of the quality of the epitaxial layer caused by the resistivity of the second single-crystal substrate layer 12 being too high. The resistivity of the second single-crystal substrate layer 12 is more preferably 300 mΩ·cm or less, and even more preferably 100 mΩ·cm or less. 【0044】 The resistivity of the second single-crystal substrate layer 12 of the composite substrate 1 in this embodiment can be calculated by peeling the second single-crystal substrate layer 12 from the composite substrate 1, flattening the peeled surface, measuring the sheet resistance using a non-contact eddy current method, and measuring the thickness of the measurement area, and then using the results. 【0045】 The second single-crystal substrate layer 12 may be doped with nitrogen as a dopant element. When the second single-crystal substrate layer 12 is doped with nitrogen, the nitrogen concentration of the second single-crystal substrate layer 12 is 5 × 10⁻¹⁰ １６ atoms / cm ３ Preferably, it is 1 x 10 １７ atoms / cm ３ It is more preferable that the above conditions are met, 5 × 10 １７ atoms / cm ３ It is even more preferable that the above conditions are met. The nitrogen concentration of the second single crystal substrate layer 12 is 5 × 10 １９ atoms / cm ３ The following may also be true: 1 x 10 １９ atoms / cm ３ The following is also acceptable. 【0046】 The nitrogen concentration in the second single-crystal substrate layer 12 can be measured using secondary ion mass spectrometry (SIMS). 【0047】The resistivity of the composite substrate 1 in this embodiment is determined by the sheet resistance and thickness of the first single-crystal substrate layer 11 and the second single-crystal substrate layer 12. The resistivity of the composite substrate 1 is preferably 15 mΩ·cm or less. This is because, for example, a device comprising an epitaxial layer laminated on the second single-crystal substrate layer 12 of the composite substrate 1 in this embodiment and an element formed on the epitaxial layer will not have its device characteristics impaired by a high resistivity of the composite substrate 1. The resistivity of the composite substrate 1 is more preferably 10 mΩ·cm or less, and even more preferably 5 mΩ·cm or less. 【0048】 The resistivity of the composite substrate 1 in this embodiment is preferably 0.1 mΩ·cm or higher. The reason for this is that if the resistivity of the composite substrate 1 is too low, it becomes necessary to perform thinning processing to reduce the thickness of the high-resistivity second single-crystal substrate layer 12 after bonding the first single-crystal substrate layer 11 and the second single-crystal substrate layer 12. As a result, material loss due to thinning processing increases, and productivity deteriorates. The resistivity of the composite substrate 1 is more preferably 0.5 mΩ·cm or higher, and even more preferably 1.0 mΩ·cm or higher. 【0049】 The resistivity of the composite substrate 1 in this embodiment can be calculated by measuring the sheet resistance using a non-contact eddy current method, measuring the thickness of the measurement area, and using the results. 【0050】In this embodiment, it is preferable that the inclination of the (0001) plane of the first single crystal substrate layer 11 with respect to the plane perpendicular to the thickness direction (hereinafter sometimes referred to as the "offset angle with respect to the plane perpendicular to the thickness direction") is different from the offset angle of the second single crystal substrate layer 12 with respect to the plane perpendicular to the thickness direction. The reason for this is that, for example, when the second single crystal substrate layer 12 is used as the surface on which the epitaxial layer is formed, defects in the first single crystal substrate layer 11 are less likely to spread to the epitaxial layer. As a result, in this embodiment, even if the first single crystal substrate layer 11 is doped with a high concentration of dopant elements and has many defects, a better epitaxial wafer layer with fewer defects can be formed by depositing an epitaxial layer on the second single crystal substrate layer 12. In other words, the planes perpendicular to the thickness direction of the first single-crystal substrate layer 11 and the second single-crystal substrate layer 12 are the planes parallel to the mounting surface on which the composite substrate 1 is placed. 【0051】 Furthermore, if the offset angle of the first single-crystal substrate layer 11 with respect to a plane perpendicular to the thickness direction is different from the offset angle of the second single-crystal substrate layer 12 with respect to a plane perpendicular to the thickness direction, for example, by performing X-ray diffraction measurements on the first and second surfaces of the composite substrate 1, the surface on which the first single-crystal substrate layer 11 is located and the surface on which the second single-crystal substrate layer 12 is located can be distinguished. Therefore, for example, when depositing an epitaxial layer on the second single-crystal substrate layer 12, it is possible to prevent accidentally depositing the epitaxial layer 21 on the first single-crystal substrate layer 11. 【0052】In this embodiment, the composite substrate 1 preferably has a difference of 1.0° or more between the offset angle of the first single crystal substrate layer 11 with respect to a plane perpendicular to the thickness direction and the offset angle of the second single crystal substrate layer 12 with respect to a plane perpendicular to the thickness direction, and more preferably 3.0° or more. The reason for this is that, for example, when the second single crystal substrate layer 12 is used as the surface on which the epitaxial layer is formed, defects in the first single crystal substrate layer 11 are less likely to spread to the epitaxial layer. Furthermore, by performing X-ray diffraction measurements on the first and second surfaces of the composite substrate 1, for example, the surface on which the first single crystal substrate layer 11 is located and the surface on which the second single crystal substrate layer 12 is located can be more reliably distinguished. 【0053】 In this embodiment, the first single-crystal substrate layer 11 of the composite substrate 1 preferably has an offset angle with respect to a plane perpendicular to the thickness direction of, for example, 0° or more and 3.5° or less, more preferably 0° or more and 0.5° or less, and the closer to 0°, the better. In other words, it is most preferable that the first single-crystal substrate layer 11 has no offset angle with respect to a plane perpendicular to the thickness direction. The reason for this is that the closer the offset angle of the main surface of the first single-crystal substrate layer 11 is to 0°, the more first single-crystal substrate layers 11 can be obtained from one ingot before bonding using a laser peeling method. 【0054】 The main surface of the first single crystal substrate layer 11 may or may not have an offset angle in the <11-20> direction with respect to the (0001) plane. If the main surface of the first single crystal substrate layer 11 has an offset angle in the <11-20> direction with respect to the (0001) plane, it is preferable that the offset angle is 0° or more and 0.5° or less. This is because the offset angle with respect to the plane perpendicular to the thickness direction of the first single crystal substrate layer 11 tends to be close to 0°. 【0055】Furthermore, the main surface of the first single crystal substrate layer 11 may or may not have an offset angle in the <1-100> direction with respect to the (0001) plane. If the main surface of the first single crystal substrate layer 11 has an offset angle in the <1-100> direction with respect to the (0001) plane, it is preferable that the offset angle is 0° or more and 0.5° or less. This is because the offset angle with respect to the plane perpendicular to the thickness direction of the first single crystal substrate layer 11 tends to be close to 0°. 【0056】 The second single-crystal substrate layer 12 preferably has an offset angle of more than 0° and 4.5° or less with respect to a plane perpendicular to the thickness direction, and more preferably 3.5° or more and 4.5° or less. The second single-crystal substrate layer 12 having an offset angle of more than 0° with respect to a plane perpendicular to the thickness direction is less likely to contain heterogeneous polymorphs internally. Therefore, when an epitaxial layer is grown on the main surface of the second single-crystal substrate layer 12, the generation of heterogeneous polymorphs in the epitaxial layer can be suppressed. Accordingly, the second single-crystal substrate layer 12 having an offset angle of more than 0° with respect to a plane perpendicular to the thickness direction within the above range can be suitably used as the surface on which the epitaxial layer is formed. 【0057】 The main surface of the second single crystal substrate layer 12 may or may not have an offset angle in the <11-20> direction with respect to the (0001) plane. Preferably, the main surface of the second single crystal substrate layer 12 has an offset angle of more than 0° and 4.5° or less in the <11-20> direction with respect to the (0001) plane, and more preferably 3.5° or more and 4.5° or less. This is because it is easier to obtain a composite substrate 1 in which the offset angle with respect to the plane perpendicular to the thickness direction of the second single crystal substrate layer 12 is 3.5° or more and 4.5° or less. 【0058】The main surface of the second single crystal substrate layer 12 may or may not have an offset angle in the <1-100> direction with respect to the (0001) plane. Preferably, the main surface of the second single crystal substrate layer 12 has an offset angle of 0° or more and 0.5° or less in the <1-100> direction with respect to the (0001) plane. If the offset of the main surface of the second single crystal substrate layer 12 in the <1-100> direction is large, the deviation angle between the direction of the offset of the (0001) plane relative to <0001> and the <11-20> direction becomes large. As a result, for example, in the etching process performed when manufacturing a device using the composite substrate 1 of this embodiment, variations in the etching rate due to the above-mentioned deviation angle in the second single crystal substrate layer 12 occur. When the main surface of the second single crystal substrate layer 12 has an offset angle of 0.5° or less in the <1-100> direction with respect to the (0001) plane, variations in the etching rate caused by the above-mentioned displacement angle in the second single crystal substrate layer 12 can be suppressed. 【0059】 In this embodiment, the composite substrate 1 may have the <11-20> direction of the main surface of the first single crystal substrate layer 11 aligned with the <11-20> direction of the main surface of the second single crystal substrate layer 12, or they may be different. When the <11-20> direction of the main surface of the first single crystal substrate layer 11 and the <11-20> direction of the main surface of the second single crystal substrate layer 12 are different, for example, when the second single crystal substrate layer 12 is used as the surface on which the epitaxial layer is formed, defects in the first single crystal substrate layer 11 are less likely to spread to the epitaxial layer, which is preferable. Furthermore, for example, by performing X-ray diffraction measurements on the first and second surfaces of the composite substrate 1, it is possible to more reliably distinguish between the surface on which the first single crystal substrate layer 11 is located and the surface on which the second single crystal substrate layer 12 is located, which is also preferable. 【0060】The angle between the <11-20> direction of the main surface of the first single crystal substrate layer 11 and the <11-20> direction of the main surface of the second single crystal substrate layer 12 is preferably 15° to 345°, and more preferably 30° to 330°. The reason for this is that when the angle is 15° to 345°, for example, when the second single crystal substrate layer 12 is used as the surface on which the epitaxial layer is formed, defects in the first single crystal substrate layer 11 are less likely to spread to the epitaxial layer. 【0061】 In this embodiment, the offset angles of the first single-crystal substrate layer 11 and the second single-crystal substrate layer 12 of the composite substrate 1 with respect to a plane perpendicular to the thickness direction, the offset angle in the <11-20> direction with respect to the (0001) plane, and the offset angle in the <1-100> direction with respect to the (0001) plane can all be easily, non-contact, and accurately measured in a short time by X-ray diffraction measurement. 【0062】 In particular, when a notch n (or orientation flat) is provided in the <1-100> direction in the first single crystal substrate layer 11 and the second single crystal substrate layer 12, and the offset angle in the <1-100> direction is small, the offset angle with respect to the plane perpendicular to the thickness direction, the offset angle in the <11-20> direction with respect to the (0001) plane, and the offset angle in the <1-100> direction with respect to the (0001) plane can be easily measured by X-ray diffraction measurement. The reason for this is that when a notch n (or orientation flat) is not provided in the <1-100> direction in the first single crystal substrate layer 11 and the second single crystal substrate layer 12, and / or when the offset angle in the <1-100> direction in the first single crystal substrate layer 11 and the second single crystal substrate layer 12 is large, it is not necessary to specify the <1-100> direction by performing X-ray diffraction measurement on the side surface of the composite substrate 1. 【0063】 Ideally, the first single-crystal substrate layer 11 and the second single-crystal substrate layer 12 should not contain defects and dislocations, but they do contain defects and dislocations. Dislocations are caused by the displacement of atoms within the crystal. It is preferable that the total dislocation density of the first single-crystal substrate layer 11 of the composite substrate 1 is higher than the total dislocation density of the second single-crystal substrate layer 12. 【0064】 The total dislocation density can be calculated by dividing the total number of each type of dislocation—basal plane dislocations (BPDs), through edge dislocations (TEDs), through helical dislocations (TSDs), through mixed dislocations (TMDs), and micropipe dislocations (MPs)—by the area. These dislocations can be observed, for example, using synchrotron radiation topography or X-ray topography. They can also be observed using an optical microscope after etching with, for example, KOH or NaOH. Basal plane dislocations (BPDs) are, as the name suggests, dislocations that exist on the (0001) plane (=c plane), which is the basal plane of a single crystal. 【0065】 In this embodiment, the composite substrate 1 has a difference of 1,000 dislocations / cm² between the basal plane dislocation density of the first single crystal substrate layer 11 and the basal plane dislocation density of the second single crystal substrate layer 12. ２ In addition, it is preferable that the basal plane dislocation density of the second single crystal substrate layer 12 is lower than the basal plane dislocation density of the first single crystal substrate layer 11. In this case, the basal plane dislocation density of the second single crystal substrate layer 12 will be sufficiently lower than that of the first single crystal substrate layer 11. As a result, for example, when an epitaxial layer is grown on the main surface of the second single crystal substrate layer 12, a composite substrate 1 is obtained in which an epitaxial layer with fewer dislocations is obtained compared to when an epitaxial layer is grown on the main surface of the first single crystal substrate layer 11. 【0066】 Furthermore, because the composite substrate 1 has a high basal plane dislocation density in the first single crystal substrate layer 11, the ingot for obtaining the first single crystal substrate layer 11 before bonding can be manufactured more easily and efficiently than the ingot for obtaining the second single crystal substrate before bonding, which will become the second single crystal substrate layer 12. As a result, for example, the productivity of the composite substrate 1 can be improved while maintaining the quality of the second single crystal substrate layer 12 used as the surface on which the epitaxial layer is formed. 【0067】 The difference between the basal plane dislocation density of the first single crystal substrate layer 11 and the basal plane dislocation density of the second single crystal substrate layer 12 is 1000 dislocations / cm². ２ It is more preferable that the number be greater than or equal to 5000 pieces / cm². ２ It is even more preferable that the above conditions are met. 【0068】 The basal plane dislocation (BPD) density of the second single crystal substrate layer 12 is 500 dislocations / cm³. ２ It is preferable that the basal plane dislocation density of the second single crystal substrate layer 12 is lower than that of the first single crystal substrate layer 11. In this case, for example, growing an epitaxial layer on the main surface of the second single crystal substrate layer 12 makes it easier to obtain an epitaxial layer with fewer dislocations, which is preferable. Also, basal plane dislocations cause stacking faults when an electric current is applied to a SiC device including the composite substrate 1 of this embodiment. ２ If the value is less than this, it is possible to suppress the adverse effects of basal plane dislocations of the second single crystal substrate layer 12 on the SiC device including the composite substrate 1 of this embodiment. 【0069】 The basal plane dislocation density of the second single crystal substrate layer 12 is more preferably 100 dislocations / cm³. ２ The following is the result: 50 pieces / cm ２ The following is even more preferable: 【0070】 The thickness of the first single-crystal substrate layer 11 of the composite substrate 1 is preferably 250 μm or more and 360 μm or less. A thickness of 250 μm or more for the first single-crystal substrate layer 11 is preferable because it provides sufficient strength, making it easier to handle the first single-crystal substrate layer 11 before bonding. Furthermore, a thickness of 250 μm or more for the first single-crystal substrate layer 11 is preferable because it makes it easier to obtain a composite substrate 1 with a resistivity of 15 mΩ·cm or less. Furthermore, a thickness of 360 μm or less for the first single-crystal substrate layer 11 allows for a larger number of pre-bonded first single-crystal substrate layers 11 to be obtained from a single ingot. A thickness of 300 μm or more and 330 μm or less for the first single-crystal substrate layer 11 is more preferable. 【0071】The thickness of the first single-crystal substrate layer 11 can be measured using a contact-type thickness measuring instrument. The thickness of the first single-crystal substrate layer 11 can also be measured using a non-contact measuring instrument such as a capacitive or laser displacement type. The thickness of the first single-crystal substrate layer 11 can be calculated from the thickness of the original single-crystal substrate and the amount of processing during the manufacturing process of the composite substrate 1. When measuring the thickness of the first single-crystal substrate layer 11 in a bonded state with the second single-crystal substrate layer 12, a method using light reflectance can be applied. That is, the total thickness is measured in the bonded state, and the difference in light reflectance is measured when light is incident from the first single-crystal substrate layer 11 and when it is incident from the opposite second single-crystal substrate layer 12. Then, by comparing the relationship between the reflectance and a pre-prepared matrix table of carrier concentration and plate thickness with the measurement results, the thickness of the first single-crystal substrate layer 11 can be calculated non-destructively. If destructive measurement is permitted, it is also possible to directly measure the thickness from cross-sectional observation in the thickness direction. Furthermore, if the thickness of the composite substrate 1 and the thickness of the second single-crystal substrate layer 12 are known, the difference between these two can be used to determine the thickness of the first single-crystal substrate layer 11. Alternatively, when determining the thickness of the second single-crystal substrate layer 12, the thickness of the first single-crystal substrate layer 11 can be subtracted from the thickness of the composite substrate 1. 【0072】 The thickness of the second single-crystal substrate layer 12 of the composite substrate 1 is preferably 5 μm or more and less than 100 μm. A thickness of 5 μm or more for the second single-crystal substrate layer 12 is preferable because defects in the first single-crystal substrate layer 11 are less likely to spread to the epitaxial layer formed on the second single-crystal substrate layer 12. A thickness of less than 100 μm for the second single-crystal substrate layer 12 is preferable because it is easier to obtain a composite substrate 1 with a resistivity of 15 mΩ·cm or less. A thickness of 20 μm or more and less for the second single-crystal substrate layer 12 is more preferable. 【0073】 The thickness of the second single-crystal substrate layer 12 can be measured using a contact-type thickness measuring instrument. The thickness of the second single-crystal substrate layer 12 can also be measured using a non-contact measuring instrument such as a capacitive or laser displacement type. 【0074】The composite substrate 1 has a bonding layer between the first single-crystal substrate layer 11 and the second single-crystal substrate layer 12. The bonding layer is a layer made of amorphous SiC located between the first single-crystal substrate layer 11 and the second single-crystal substrate layer 12. The bonding layer is formed by bonding the first single-crystal substrate layer 11 and the second single-crystal substrate layer 12 together. The presence of the bonding layer can be confirmed, for example, by magnified observation of the substrate side (end face) and / or by observation of the cross-section using a transmission electron microscope (TEM). The thickness of the bonding layer is, for example, 0.5 nm to 10 nm. The thickness of the bonding layer can sometimes be measured directly by TEM observation of the cross-section, or by scanning the Si-C bonding state using X-ray photoelectron spectroscopy (XPS). 【0075】 "Method for Manufacturing a Composite Substrate" Next, a method for manufacturing the composite substrate 1 according to this embodiment will be described. The method for manufacturing the composite substrate 1 includes, for example, a first single crystal preparation step, a second single crystal preparation step, and a bonding step. 【0076】 (First single crystal preparation step) In the first single crystal preparation step, the first single crystal that will become the first single crystal substrate layer 11 before bonding is prepared. The first single crystal is, for example, an ingot made of a SiC single crystal having a predetermined dislocation density. The first single crystal may be made by sublimation, gas method, or solution method. 【0077】 The nitrogen concentration of the first single crystal is set to 1 × 10 in order to obtain a first single crystal substrate layer 11 with a resistivity of less than 10 mΩ·cm. １９ atoms / cm ３ The above is preferable. The nitrogen concentration can be adjusted by the amount of nitrogen supplied when producing the first single crystal. In the first single crystal preparation step, the first single crystal is sliced ​​to a desired thickness to obtain the first single crystal substrate layer 11 before bonding. The method for slicing the first single crystal and cutting out the first single crystal substrate layer 11 from the first single crystal can be carried out by known methods, such as using a wire saw. 【0078】In this embodiment, when cutting the first single crystal substrate layer 11 from the first single crystal, the offset angle with respect to the plane perpendicular to the thickness direction of the first single crystal substrate layer 11 is set to a predetermined angular range. Specifically, it is preferable to cut the first single crystal substrate layer 11 from the first single crystal such that the offset angle with respect to the plane perpendicular to the thickness direction of the first single crystal substrate layer 11 is close to 0°. 【0079】 A notch n may or may not be provided on the side surface of the first single crystal (ingot) from which the first single crystal substrate layer 11 is cut. Alternatively, an orientation flat may be provided instead of a notch n. It is preferable that the notch n be provided on the side surface of the first single crystal from which the first single crystal substrate layer 11 is cut, as this allows for efficient manufacturing of the composite substrate 1 in fewer steps. 【0080】 Furthermore, when manufacturing a composite substrate 1 in which, for example, the <11-20> direction of the main surface of the first single crystal substrate layer 11 and the <11-20> direction of the main surface of the second single crystal substrate layer 12 are different, it is preferable that the first single crystal does not have a notch n. In this case, it is preferable to form a notch n (see Figure 1) on the bonded body obtained after the bonding process, or on a part of the second single crystal substrate separated from the bonded body, indicating that the <1-100> direction of the main surface of the second single crystal substrate layer 12 is the Y direction. The position where the notch n is provided on the side surface of the first single crystal from which the first single crystal substrate layer 11 is cut can be, for example, a position in the <1-100> direction. 【0081】 (Second Single Crystal Preparation Process) In the second single crystal preparation process, the second single crystal, which is the material for the second single crystal substrate before bonding, is prepared to become the second single crystal substrate layer 12. The second single crystal is, for example, an ingot made of a SiC single crystal having a predetermined dislocation density. The second single crystal may be produced by sublimation, gas method, or solution method. 【0082】 The nitrogen concentration of the second single crystal is 5 × 10 in order to obtain a second single crystal substrate layer 12 with a resistivity of 10 mΩ·cm or more. １９ atoms / cm ３The following is preferable. The nitrogen concentration can be adjusted by the amount of nitrogen supplied when producing the second single crystal. 【0083】 The second single crystal preferably has a lower dislocation density than the first single crystal from which the first single crystal substrate layer 11 is cut. For this reason, the second single crystal is preferably manufactured by sublimation using a seed crystal with fewer dislocation defects than the first single crystal. A seed crystal with fewer dislocation defects can be manufactured, for example, using the RAF (Repeated a-face method) method. Alternatively, a second single crystal with a low dislocation density may be manufactured, for example, by using a seed crystal with a high dislocation density and performing crystal growth to a sufficient length to cause annihilation of nearby dislocations. 【0084】 In the second single crystal preparation step, the second single crystal is sliced ​​to a desired thickness to obtain the second single crystal substrate layer 12 before bonding. The method for slicing the second single crystal and cutting out the second single crystal substrate from it can be done by known methods, such as a laser peeling method or a wire saw method. Using a laser peeling method allows for efficient acquisition of the second single crystal substrate from the second single crystal. 【0085】 In this embodiment, when cutting the second single crystal substrate from the second single crystal, the offset angle with respect to the plane perpendicular to the thickness direction of the second single crystal substrate is set to a predetermined angular range. Specifically, it is preferable that the offset angle with respect to the plane perpendicular to the thickness direction of the second single crystal substrate is larger than the offset angle with respect to the plane perpendicular to the thickness direction of the first single crystal substrate layer. For example, it is preferable to cut the second single crystal substrate from the second single crystal such that the offset angle with respect to the plane perpendicular to the thickness direction of the second single crystal substrate is in the range of 3.5° to 4.5°. 【0086】A notch n may or may not be provided on the side surface of the second single crystal (ingot) from which the second single crystal substrate is cut. Alternatively, an orientation flat may be provided instead of a notch n. It is preferable that the notch n be provided on the side surface of the second single crystal from which the second single crystal substrate is cut, as this allows for efficient manufacturing of the composite substrate 1 in fewer steps. 【0087】 Furthermore, when manufacturing a composite substrate 1 in which, for example, the <11-20> direction of the main surface of the first single crystal substrate layer 11 and the <11-20> direction of the main surface of the second single crystal substrate layer 12 are different, it is preferable that the first single crystal does not have a notch n. The position where the notch n is provided on the side surface of the second single crystal from which the second single crystal substrate is cut can be, for example, a position in the <1-100> direction. 【0088】 (Bonding process) Next, the first single crystal substrate layer 11 and the second single crystal substrate are bonded together. In this embodiment, the angle between the <11-20> direction of the main surface of the first single crystal substrate layer 11 and the <11-20> direction of the main surface of the second single crystal substrate layer 12 is set to a predetermined angle, so that the angle between the <11-20> direction of the main surface of the first single crystal substrate layer 11 and the <11-20> direction of the main surface of the second single crystal substrate layer 12 is set to a predetermined angle. Therefore, the <11-20> direction of the main surface of the first single crystal substrate layer 11 and the <11-20> direction of the main surface of the second single crystal substrate may be aligned, or they may be aligned to be different. 【0089】 As a method for bonding the first single-crystal substrate layer 11 and the second single-crystal substrate, for example, a method can be used in which the bonding surfaces of the first single-crystal substrate layer 11 and the second single-crystal substrate are activated, and then they are stacked and pressure is applied. As a method for activating the bonding surfaces, for example, a method of irradiating the bonding surfaces with an Ar beam can be used. By performing the bonding process, a bonded substrate is obtained in which the first single-crystal substrate layer 11 and the second single-crystal substrate are joined together. 【0090】Next, in this embodiment, it is preferable to perform a bonding substrate polishing step in which one or both sides of the bonding substrate are polished. Specifically, for example, a method can be used in which both sides of the bonding substrate, the side facing the first single crystal substrate layer 11 and the side facing the second single crystal substrate layer 12, are polished using a CMP slurry. The amount of material removed from one side of the bonding substrate by performing the bonding substrate polishing step can be, for example, 0.1 μm to 10 μm. By performing the bonding substrate polishing step, one or both surfaces of the bonding substrate can be flattened, and damage caused by the bonding step, such as transport and / or adsorption on the side opposite to the bonding surface, can be removed. Through the above steps, the composite substrate 1 of this embodiment is obtained. 【0091】 The composite substrate 1 of this embodiment comprises a first single-crystal substrate layer 11 and a second single-crystal substrate layer 12 bonded to the first single-crystal substrate layer 11, wherein the resistivity of the first single-crystal substrate layer 11 is less than 10 mΩ·cm. As a result, the composite substrate 1 has a sufficiently low resistivity. Therefore, for example, a device comprising an epitaxial layer laminated on the second single-crystal substrate layer 12 of the composite substrate 1 and an element formed on the epitaxial layer can be prevented from having its device characteristics impaired due to the high resistivity of the composite substrate 1. 【0092】 Furthermore, the second single-crystal substrate layer 12 of the composite substrate 1 in this embodiment has a resistivity of 10 mΩ·cm or more. Therefore, the second single-crystal substrate layer 12 can be made with fewer defects caused by the doping of dopant elements. 【0093】 Furthermore, in the composite substrate 1 of this embodiment, the second single-crystal substrate layer 12 is bonded to the first single-crystal substrate layer 11. Therefore, for example, when an epitaxial layer is formed on the second single-crystal substrate layer 12, defects in the first single-crystal substrate layer 11 are less likely to spread to the epitaxial layer formed on the second single-crystal substrate layer 12. Consequently, even if the first single-crystal substrate layer 11 is highly doped with dopant elements and has many defects, if the second single-crystal substrate layer 12 has fewer defects compared to the first single-crystal substrate layer 11, an epitaxial layer with fewer defects can be formed by forming an epitaxial layer on the second single-crystal substrate layer 12. 【0094】 Furthermore, the manufacturing method of the composite substrate 1 of this embodiment includes the steps of preparing a first single-crystal substrate layer 11 having a resistivity of less than 10 mΩ·cm and a second single-crystal substrate having a resistivity of 10 mΩ·cm or more, and bonding the first single-crystal substrate layer 11 and the second single-crystal substrate. Thus, the composite substrate 1 of this embodiment can be manufactured. 【0095】 "Epitaxial Wafer" Figure 3 is a cross-sectional view showing an example of an epitaxial wafer of the present invention. The epitaxial wafer 2 shown in Figure 3 comprises a composite substrate 1 in which a first single crystal substrate layer 11 and a second single crystal substrate layer 12 are bonded together, and an epitaxial layer 21. The composite substrate 1 is the same as that of the embodiment described above. 【0096】 As shown in Figure 3, the epitaxial layer 21 is laminated on the second single-crystal substrate layer 12 of the composite substrate 1. The epitaxial layer 21 is, for example, SiC. The epitaxial layer 21 may be undoped SiC or dopant-doped SiC. For example, nitrogen-doped n-type SiC is preferred for the epitaxial layer 21. 【0097】 "Method for Manufacturing Epitaxial Wafers" The epitaxial wafer 2 of this embodiment shown in Figure 3 can be manufactured, for example, by the method described below. First, a composite substrate 1 is prepared. Next, an epitaxial layer 21 is deposited on the second single-crystal substrate layer 12 of the composite substrate 1. The deposition of the epitaxial layer 21 can be carried out by known methods. 【0098】 In this embodiment, an epitaxial layer 21 is formed on a second single-crystal substrate layer 12 bonded to a first single-crystal substrate layer 11 of the composite substrate 1. Therefore, defects in the first single-crystal substrate layer 11 are less likely to spread to the epitaxial layer. As a result, even if the first single-crystal substrate layer 11 has a high total dislocation density and / or basal plane dislocation density, if the second single-crystal substrate layer 12 has fewer defects compared to the first single-crystal substrate layer 11, a good epitaxial layer 21 with few defects can be formed on the second single-crystal substrate layer 12. By going through the above steps, the epitaxial wafer 2 of this embodiment is obtained. 【0099】The epitaxial wafer 2 obtained in this manner according to this embodiment may be polished on its surface if necessary. Specifically, for example, both the surface of the epitaxial wafer 2 facing the first single crystal substrate layer 11 and the surface facing the epitaxial layer 21 may be polished simultaneously using a CMP slurry. 【0100】 The epitaxial wafer 2 of this embodiment comprises the composite substrate 1 of this embodiment and an epitaxial layer 21 laminated on the second single-crystal substrate layer 12 of the composite substrate 1. Therefore, the epitaxial wafer 2 of this embodiment comprises a composite substrate 1 with sufficiently low resistivity and an epitaxial layer 21 with few defects. 【0101】 "Device" Figure 4 is a cross-sectional view showing an example of the device of the present invention. The device 3 of this embodiment shown in Figure 4 comprises a first single crystal substrate layer 31, a second single crystal substrate layer 32, an epitaxial layer 33, and an element 34 formed on the epitaxial layer 33. 【0102】 The first single-crystal substrate layer 31 is a chipped version of the first single-crystal substrate layer 11 in the epitaxial wafer 2 of this embodiment. The first single-crystal substrate layer 31 may also be formed by slimming the back surface of the first single-crystal substrate layer 11 to create a thin film. The second single-crystal substrate layer 32 is a chipped version of the second single-crystal substrate layer 12 in the epitaxial wafer 2 of this embodiment. The second single-crystal substrate layer 32 is equivalent to the second single-crystal substrate layer 12, except that it is a chipped version. 【0103】 The epitaxial layer 33 is a chipped version of the epitaxial layer 21 in the epitaxial wafer 2 of this embodiment. The epitaxial layer 33 is equivalent to the epitaxial layer 21, except that it is chipped. 【0104】 The element 34 is formed in the epitaxial layer 33. The element is a combination of, for example, a transistor, capacitor, inductor, resistor, wiring, etc. In Figure 4, a transistor is shown as an example of element 34. 【0105】"Method for Manufacturing the Device" The device 3 shown in Figure 4 can be manufactured, for example, by the method described below. The device 3 can be manufactured, for example, by performing an element formation process, a slimming process, and a chipping process. (Element Formation Process) In the element formation process, a plurality of elements 34 are formed on the epitaxial layer 21 of the epitaxial wafer 2 of this embodiment. The formation of the elements 34 can be carried out by known methods. 【0106】 (Slimming process) In the slimming process, a portion of the first single-crystal substrate layer 11 of the epitaxial wafer 2 is removed by a known method. In this embodiment, it is preferable that the second single-crystal substrate layer 12 of the epitaxial wafer 2 is of higher quality, less productive and more expensive than the first single-crystal substrate layer 11. This is because, since the portion removed in the slimming process is the first single-crystal substrate layer 11, the amount of high-quality, less productive and more expensive second single-crystal substrate layer 12 used in the manufacture of the device 3 can be reduced without affecting the quality of the device 3. Furthermore, the high-quality, less productive and more expensive second single-crystal substrate layer 12 is not wasted by being removed in the slimming process. 【0107】 (Chip Forming Process) In the chip forming process, the epitaxial wafer 2 is cut into chips for each element 34 using a known method. By forming chips, multiple devices 3 shown in Figure 4 are obtained from the epitaxial wafer 2. 【0108】 The device 3 according to this embodiment comprises a first single-crystal substrate layer 31 and a second single-crystal substrate layer 32, which are chipped composite substrates 1 according to this embodiment; an epitaxial layer 33, which is a chipped epitaxial layer 21 of the epitaxial wafer 2 according to this embodiment; and an element 34 formed on the epitaxial layer 33. Therefore, the device 3 according to this embodiment has an epitaxial layer 33 with few defects, and the high resistivity of the composite substrate 1 prevents interference with the device characteristics, resulting in excellent reliability. 【0109】While preferred embodiments of this disclosure have been described in detail above, this disclosure is not limited to any particular embodiment, and various modifications and changes are possible within the scope of the gist of this disclosure as described in the claims. 【0110】 For example, in the above embodiment, the case in which both the composite substrate 1 and the epitaxial layer 21 are made of SiC is illustrated, but the elements constituting them are not limited to SiC. For example, semiconductors such as GaN and GaInAs may be used for the composite substrate 1 and the epitaxial layer 21. Furthermore, the first single crystal substrate layer 11 and the second single crystal substrate layer 12 may be made of different elements. 【0111】 "Example 1" (First single crystal preparation step) It consists of a SiC single crystal with a diameter of 200 mm (8 inches) and a nitrogen concentration of 5 × 10 １９ atoms / cm ３ The first single crystal ingot was sliced, polished, and planarized to obtain the first single crystal substrate layer 11 before bonding. 【0112】 For the obtained first single-crystal substrate layer 11, the resistivity, thickness, offset angle with respect to a plane perpendicular to the thickness direction, offset angle in the <11-20> direction with respect to the (0001) plane, offset angle in the <1-100> direction with respect to the (0001) plane, total dislocation density, and basal plane dislocation density were measured using the method described above. The results are shown in Table 1. 【0113】 【0114】 (Second single crystal preparation process) It consists of a SiC single crystal with a diameter of 200 mm (8 inches) and a nitrogen concentration of 5 × 10 １８ atoms / cm ３ The second single crystal ingot was sliced, polished, and planarized to obtain the second single crystal substrate before bonding, which would become the second single crystal substrate layer 12. 【0115】For the obtained second single crystal substrate, the resistivity, thickness, offset angle with respect to the plane perpendicular to the thickness direction, offset angle in the <11-20> direction with respect to the (0001) plane, offset angle in the <1-100> direction with respect to the (0001) plane, total dislocation density, and basal plane dislocation density were measured using the method described above. The results are shown in Table 2. 【0116】 【0117】 (Bonding Process) Next, the bonding surfaces of the first single crystal substrate layer 11 and the second single crystal substrate were activated by irradiating them with an Ar beam. Then, the first single crystal substrate layer 11 and the second single crystal substrate were stacked with their bonding surfaces facing each other, and aligned so that the angle between the <11-20> direction of the main surface of the first single crystal substrate layer 11 and the <11-20> direction of the main surface of the second single crystal substrate was the angle shown in Table 3, to form a laminate. A pressure of 5t was applied in the thickness direction of the obtained laminate to bond the first single crystal substrate layer 11 and the second single crystal substrate, obtaining a bonded substrate. 【0118】 Next, a bonding substrate polishing process was performed, in which both sides of the bonding substrate were polished using a CMP slurry. The amount of material removed from both sides of the bonding substrate by the bonding substrate polishing process, specifically the side facing the first single crystal substrate layer 11 and the side facing the second single crystal substrate layer 12, was 1 μm each. This obtained the composite substrate 1 of Example 1. The resistivity of the composite substrate 1 of Example 1 obtained in this way was measured using the method described above. The results are shown in Table 3. 【0119】 Table 3 also shows the difference between the first single-crystal substrate layer 11 and the second single-crystal substrate layer 12 in terms of the offset angle with respect to a plane perpendicular to the thickness direction and the basal plane dislocation density. 【0120】 【0121】"Examples 2 to 10, Comparative Examples 1 to 2" As the first single crystal substrate layer 11, a substrate with the resistivity, thickness, offset angle, and dislocation density shown in Table 1 was used. Also, as the second single crystal substrate, a substrate with the resistivity, thickness, offset angle, and dislocation density shown in Table 2 was used. Then, the composite substrates 1 of Examples 2 to 10 and Comparative Examples 1 to 2 were obtained in the same manner as in Example 1, except that the first single crystal substrate layer 11 and the second single crystal substrate of Examples 2 to 10 and Comparative Examples 1 to 2 were used. 【0122】 The resistivity of the composite substrates 1 obtained from Examples 2 to 10 and Comparative Examples 1 to 2 was measured in the same manner as the composite substrate 1 of Example 1. The results are shown in Table 3. 【0123】 (Fabrication of Epitaxial Wafers) Epitaxial layers 21 were grown on the second single-crystal substrate layer 12 of the composite substrates 1 of Examples 1 to 10 and Comparative Examples 1 to 2 obtained in this manner, to fabricate epitaxial wafers 2. The epitaxial layer 21 is SiC containing nitrogen as a dopant. The dopant concentration of the epitaxial layer 21 is 1 × 10⁻⁶ １６ atoms / cm ３ The thickness of the epitaxial layer 21 was set to 10 μm. 【0124】 For the epitaxial wafers 2 obtained in Examples 1 to 10 and Comparative Examples 1 to 2, the defect density of the epitaxial layer 21 was calculated using the method described below and evaluated according to the criteria shown below. The results are shown in Table 3. 【0125】 [Method for Calculating Defect Density] Using a SiC wafer defect inspection device (product name: SICA, manufactured by Lasertec Corporation), crystal defects within the epitaxial layer 21 were detected on the entire surface of the epitaxial wafer 2 using the photoluminescence (PL) method. The detected crystal defects were then classified, and the number of defects caused by crystal defects in the second single-crystal substrate layer 12 of the composite substrate 1 was measured. Using these results, the defect density of the epitaxial layer 21 was calculated. 【0126】 [Evaluation Criteria] ◎; Defect density of epitaxial layer 21 is 0.5 / cm ２Less than ○; Defect density of epitaxial layer 21 is 0.5 / cm² ２ 1.0 / cm or more ２ Less than △; Defect density of epitaxial layer 21 is 1.0 / cm² ２ 2.0 / cm or more ２ Less than ×; Defect density of epitaxial layer 21 is 2.0 / cm² ２ That's all. 【0127】 As shown in Table 3, epitaxial wafers 2 using the composite substrates 1 of Examples 1 to 10, in which the resistivity of the first single-crystal substrate layer 11 is less than 10 mΩ·cm and the resistivity of the second single-crystal substrate layer 12 is 10 mΩ·cm or more, had a low number of defects in the epitaxial layer 21, and the evaluation of the number of defects in the epitaxial layer 21 was ◎, ○, or △. In particular, epitaxial wafers 2 using the composite substrates 1 of Examples 6 and 7, in which the resistivity of the second single-crystal substrate layer is 40 mΩ·cm or more, had a very low number of defects in the epitaxial layer 21, and the evaluation of the number of defects in the epitaxial layer 21 was ◎. 【0128】 In contrast, as shown in Table 3, the epitaxial wafer 2 of Comparative Example 2, which used a composite substrate 1 in which the resistivity of the second single-crystal substrate layer 12 was less than 10 mΩ·cm, had a large number of defects in the epitaxial layer 21, resulting in a defect count evaluation of ×. On the other hand, the epitaxial wafer 2 of Comparative Example 1, which used a composite substrate 1 in which the resistivity of the first single-crystal substrate layer 11 was 10 mΩ·cm or more, had a small number of defects in the epitaxial layer 21, resulting in a defect count evaluation of ○. However, in Comparative Example 1, because the resistivity of the first single-crystal substrate layer 11 and the composite substrate 1 is high, sufficient device characteristics may not be obtained when manufacturing a device equipped with elements formed on the epitaxial layer 21. 【0129】 1. Composite substrate 2. Epitaxial wafer 3. Device 11, 31. First single-crystal substrate layer 12, 32. Second single-crystal substrate layer 21, 33. Epitaxial layer 34. Element n notch

Claims

1. A composite substrate comprising a first single-crystal substrate layer and a second single-crystal substrate layer bonded to the first single-crystal substrate layer, wherein the resistivity of the first single-crystal substrate layer is less than 10 mΩ·cm and the resistivity of the second single-crystal substrate layer is 10 mΩ·cm or more.

2. The composite substrate according to claim 1, wherein the resistivity of the second single-crystal substrate layer is 25 mΩ·cm or more.

3. The composite substrate according to claim 1, wherein the resistivity of the composite substrate is 15 mΩ·cm or less.

4. The composite substrate according to claim 1, wherein the inclination of the (0001) plane with respect to a plane perpendicular to the thickness direction of the first single crystal substrate layer is different from the inclination of the (0001) plane with respect to a plane perpendicular to the thickness direction of the second single crystal substrate layer.

5. The composite substrate according to claim 1, wherein the difference between the inclination of the (0001) plane with respect to a plane perpendicular to the thickness direction of the first single crystal substrate layer and the inclination of the (0001) plane with respect to a plane perpendicular to the thickness direction of the second single crystal substrate layer is 1.0° or more.

6. The composite substrate according to claim 1, wherein the inclination of the (0001) plane with respect to a plane perpendicular to the thickness direction of the first single crystal substrate layer is 0° or more and 3.5° or less, and the inclination of the (0001) plane with respect to a plane perpendicular to the thickness direction of the second single crystal substrate layer is greater than 0° and 4.5° or less.

7. The composite substrate according to claim 1, wherein the total dislocation density of the first single-crystal substrate layer is higher than the total dislocation density of the second single-crystal substrate layer.

8. The difference between the basal plane dislocation density of the first single-crystal substrate layer and the basal plane dislocation density of the second single-crystal substrate layer is 1000 dislocations / cm². ２ The composite substrate according to claim 1, wherein the basal plane dislocation density of the second single crystal substrate layer is lower than the basal plane dislocation density of the first single crystal substrate layer.

9. The basal plane dislocation density of the second single crystal substrate layer is 500 dislocations / cm³. ２ The composite substrate according to claim 1, wherein the basal plane dislocation density of the second single crystal substrate layer is less than the basal plane dislocation density of the first single crystal substrate layer.

10. A composite substrate comprising a first single-crystal substrate layer, a second single-crystal substrate layer, and a junction layer disposed between the first single-crystal substrate layer and the second single-crystal substrate layer, wherein the resistivity of the first single-crystal substrate layer is less than 10 mΩ·cm and the resistivity of the second single-crystal substrate layer is 10 mΩ·cm or more.

11. A method for manufacturing a composite substrate, comprising the steps of: preparing a first single-crystal substrate layer having a resistivity of less than 10 mΩ·cm; preparing a second single-crystal substrate having a resistivity of 10 mΩ·cm or more; and bonding the first single-crystal substrate layer and the second single-crystal substrate together.

12. An epitaxial wafer comprising a composite substrate according to claim 1 and an epitaxial layer laminated on the second single crystal substrate layer.

13. A method for manufacturing an epitaxial wafer, comprising the steps of: preparing a composite substrate according to claim 1; and forming an epitaxial layer on the second single-crystal substrate layer of the composite substrate.

14. A device comprising: a composite substrate according to claim 1; an epitaxial layer laminated on the second single crystal substrate layer; and an element formed on the epitaxial layer.

15. A method for manufacturing a device, comprising the steps of: performing the method for manufacturing an epitaxial wafer according to claim 13; and forming an element on the epitaxial layer.

Images