Nitride semiconductor epitaxial substrate, method for manufacturing the same, and nitride semiconductor device.

Abstract

This nitride semiconductor epitaxial substrate (100) is provided with a Si substrate (101), a nitride semiconductor epitaxial layer formed over the Si substrate (101), and a crystal layer (102) of Si and a Group 3 metal element arranged between the Si substrate (101) and the nitride semiconductor epitaxial layer and including a high concentration of C. The C concentration in the crystal layer (102) is equal to or higher than 1.0×10+21cm-3, and the transition metal element concentration in the crystal layer (102) is equal to or lower than 5.0×10+16cm-3.

Description

Technical Field

[0001] The present disclosure relates to a nitride semiconductor epitaxial substrate using a Si substrate, a method for manufacturing the same, and a nitride semiconductor device typified by a field effect transistor manufactured using the nitride semiconductor epitaxial substrate.

Background Art

[0002] Nitride semiconductors have a larger bandgap than compound semiconductors such as Si semiconductors or GaAs, have a high breakdown voltage and a high saturation drift velocity, and are applied to electronic devices such as high-voltage power devices and high-speed high-power transistors.

[0003] As substrates for growing nitride semiconductors applied to electronic devices, sapphire substrates, SiC substrates, Si substrates, etc. are used. However, since Si substrates have established manufacturing technology for large-diameter substrates and are also excellent in cost, they are most advantageous for mass production.

[0004] However, since Si substrates have a large lattice mismatch and a large difference in thermal expansion coefficient with nitride semiconductors compared to other substrates, it is difficult to form a nitride semiconductor epitaxial layer with excellent crystallinity above the Si substrate. Since the crystallinity of the nitride semiconductor epitaxial layer greatly affects the characteristics and reliability of electronic devices, forming a nitride semiconductor epitaxial layer with excellent crystallinity above the Si substrate is a major issue.

[0005] Regarding the formation of a nitride semiconductor epitaxial layer above a Si substrate, the nitride semiconductor layer structure and the introduction of a buffer layer have been studied. For example, in Patent Document 1 and Patent Document 2, by doping C or Fe at a high concentration in the initial layer of the nitride semiconductor epitaxial layer formed above the Si substrate, reduction of leakage current and improvement of high-frequency characteristics are achieved. In Patent Document 3, by forming a nitride semiconductor epitaxial layer above a semiconductor substrate in which a single-crystalline SiC film is formed on the surface of the Si substrate, the crystallinity of the nitride semiconductor epitaxial layer is improved. [Prior art documents] [Patent Documents]

[0006] [Patent Document 1] Japanese Patent Publication No. 2018-46207 [Patent Document 2] Japanese Patent Publication No. 2011-166067 [Patent Document 3] Patent No. 6156833 [Overview of the project] [Problems that the invention aims to solve]

[0007] However, in the semiconductor substrates described in Patent Documents 1 and 2, the crystallinity of the nitride semiconductor epitaxial layer is the same as or lower than that of the substrate in which the initial layer of the nitride semiconductor epitaxial layer is not doped with C or Fe. In other words, there remains a challenge in improving the quality of the nitride semiconductor epitaxial layer.

[0008] The semiconductor substrate described in Patent Document 3 presents challenges such as difficulty in achieving high-quality single-crystal SiC on top of the Si substrate, and a significant increase in cost.

[0009] In view of the above issues, this disclosure aims to realize a nitride semiconductor epitaxial substrate having a layer with excellent crystallinity formed on a Si substrate, a method for manufacturing the same, and a nitride semiconductor device using the nitride semiconductor epitaxial substrate. [Means for solving the problem]

[0010] To solve the above problems, a nitride semiconductor epitaxial substrate according to one aspect of the present disclosure comprises a Si substrate, a nitride semiconductor epitaxial layer formed above the Si substrate, and a mixed crystal layer of Si and a group III metal element containing a high concentration of C, disposed between the Si substrate and the nitride semiconductor epitaxial layer, wherein the C concentration in the mixed crystal layer is 1.0 × 10⁻⁶ +21 cm-3 The above results indicate that the transition metal element concentration in the mixed crystal layer is 5.0 × 10⁻⁶. +16 cm -3 The following applies:

[0011] Furthermore, a nitride semiconductor epitaxial substrate according to one aspect of the present disclosure comprises a Si substrate, a heterostructure epitaxial layer including a nitride semiconductor epitaxial layer formed above the Si substrate, and a mixed crystal layer of Si and a group III metal element containing a high concentration of C, disposed between the Si substrate and the nitride semiconductor epitaxial layer, wherein the C concentration in the mixed crystal layer is higher than the C concentration of each layer contained in the heterostructure epitaxial layer, and the transition metal element concentration in the mixed crystal layer is 5.0 × 10⁻⁶. +16 cm -3 The following applies:

[0012] Furthermore, a nitride semiconductor device according to one aspect of this disclosure is formed using the above-mentioned nitride semiconductor epitaxial substrate.

[0013] Furthermore, a method for manufacturing a nitride semiconductor epitaxial substrate according to one aspect of the present disclosure includes the steps of: raising the temperature of a Si substrate to 500°C or higher; supplying a C raw material to the surface of the Si substrate; growing a first nitride semiconductor layer on top of the Si substrate; maintaining the temperature of the Si substrate and the first nitride semiconductor layer at 900°C or higher to diffuse a group III metal element from the first nitride semiconductor layer into the Si substrate; and growing a second nitride semiconductor layer on top of the first nitride semiconductor layer. [Effects of the Invention]

[0014] This disclosure provides a nitride semiconductor epitaxial substrate and the like, which has a layer with excellent crystallinity formed on a Si substrate. [Brief explanation of the drawing]

[0015] [Figure 1] Cross-sectional view of a nitride semiconductor epitaxial substrate according to Embodiment 1 of this disclosure. [Figure 2]Schematic diagram of C concentration, Fe concentration, and Si concentration in each component according to Embodiment 1 of this disclosure [Figure 3] This figure shows the results of evaluating the dependence of the C concentration in the mixed crystal layer and the Fe concentration (a transition metal element) on the crystallinity of the AlN layer. [Figure 4] This figure shows the secondary ion mass spectrometry results for the C concentration and Si concentration in each component according to Embodiment 1 of this disclosure. [Figure 5] Cross-sectional view of a nitride semiconductor epitaxial substrate according to a modified example 1 of Embodiment 1 of this disclosure. [Figure 6A] Schematic diagram of C concentration and Si concentration in each component according to Modification 1 of Embodiment 1 of this disclosure [Figure 6B] Cross-sectional view of a nitride semiconductor epitaxial substrate according to a modified example 2 of Embodiment 1 of this disclosure. [Figure 7] Cross-sectional view of a nitride semiconductor heterostructure epitaxial substrate according to Embodiment 2 of this disclosure. [Figure 8] This figure shows the results of evaluating the effect of carbon concentration in the mixed crystal layer and carbon concentration in the heterostructured epitaxial layer on the crystallinity of the GaN channel layer. [Figure 9] Cross-sectional view of a nitride semiconductor device according to Embodiment 3 of this disclosure [Modes for carrying out the invention]

[0016] The embodiments of this disclosure will be described in detail below with reference to the drawings. Note that the embodiments described below are either comprehensive or specific examples. The numerical values, shapes, materials, components, arrangement positions of components, and connection configurations shown in the following embodiments are examples only and are not intended to limit this disclosure. Furthermore, components in the embodiments of this disclosure that are not described in an independent claim will be described as optional components.

[0017] Please note that each figure is a schematic diagram and not necessarily a strictly accurate representation. Furthermore, in each figure, substantially identical components are denoted by the same reference numerals, and redundant explanations may be omitted or simplified.

[0018] Furthermore, in the following embodiments, the term "upwards" does not refer to the upward direction (vertically upward) in absolute spatial perception. The term "upwards" also applies not only when two components are spaced apart and another component exists between them, but also when two components are placed in close proximity and touching each other.

[0019] (Embodiment 1) [composition] First, the configuration of the nitride semiconductor epitaxial substrate 100 according to this embodiment will be described.

[0020] Figure 1 is a cross-sectional view of a nitride semiconductor epitaxial substrate 100 according to Embodiment 1 of the present disclosure. As shown in Figure 1, in the nitride semiconductor epitaxial substrate 100 according to this embodiment, a nitride semiconductor epitaxial layer, for example, an AlN layer 103, is epitaxially grown on top of a Si substrate 101 via a mixed crystal layer 102 mainly composed of Al, Si, and C. In other words, the nitride semiconductor epitaxial substrate 100 according to this embodiment comprises a Si substrate 101, a mixed crystal layer 102 provided on top of the Si substrate 101, and a nitride semiconductor epitaxial layer (here, an AlN layer 103) provided on top of the mixed crystal layer 102. That is, the mixed crystal layer 102 is arranged between the Si substrate 101 and the AlN layer 103, in contact with the Si substrate 101 and the AlN layer 103.

[0021] The Si substrate 101 is a substrate made of silicon.

[0022] The mixed crystal layer 102 is a layer containing a group III metallic element (in this case, Al) and Si, and furthermore, a layer containing a high concentration of C. The term "main component" above means that in the mixed crystal layer 102, the ratio of the total amount of group III metallic elements, Si, and C to the total amount of elements in the mixed crystal layer 102 is, for example, 50% or more. This ratio may also mean 90% or more.

[0023] Here, the constituent elements of the mixed crystal layer 102 are not limited to Al, which is an example of a group III metallic element. The constituent elements of the mixed crystal layer 102 may be other group III metallic elements (for example, Ga and In) instead of Al, and may consist of one or more of these elements.

[0024] The mixed crystal layer 102 is a polycrystalline layer mainly composed of Al, Si, and C, and the lattice constant of the mixed crystal layer 102 is closer to the lattice constant of AlN than to the lattice constant of Si. Therefore, by using this polycrystalline layer (mixed crystal layer 102) as a buffer layer and epitaxially growing the AlN layer 103 above the mixed crystal layer 102, it is possible to realize a nitride semiconductor epitaxial layer (AlN layer 103) with superior crystallinity compared to conventional methods.

[0025] The mixed crystal layer 102 is formed in the growth furnace where the AlN layer 103, which is a nitride semiconductor epitaxial layer, is formed. Therefore, the group III metal elements that make up the mixed crystal layer 102 are preferably the same as one or more of the group III metal elements that make up the nitride semiconductor epitaxial layer formed above it. In other words, in this embodiment, the group III metal elements of the mixed crystal layer 102 are the same as the group III metal elements of the nitride semiconductor epitaxial layer. This makes it possible to realize the nitride semiconductor epitaxial substrate 100 according to this embodiment without significantly increasing the number of steps.

[0026] The group III metal elements in the AlN layer 103, which is a nitride semiconductor epitaxial layer, are not limited to Al. Other group III metal elements (e.g., Ga and In) may be used instead of Al, and the layer may consist of one or more of these. However, to prevent abnormal growth due to the reaction between Ga and Si, it is preferable that the group III metal elements in the portion of the AlN layer 103 in contact with the mixed crystal layer 102 be Al only. The mixed crystal layer 102 may also contain N or O, which are present on the surface of the Si substrate 101 as constituent elements of the oxide or nitride film.

[0027] Figure 2 is a schematic diagram of the C concentration, Fe concentration, and Si concentration in each component according to Embodiment 1 of the present disclosure. As shown in Figure 2, the C concentration contained in the mixed crystal layer 102 is 1.0×10 +21 cm -3 or more, and the Fe concentration is 5.0×10 +16 cm -3 or less. Further, the C concentration contained in the mixed crystal layer 102 is 1.0×10 +22 cm -3 or less. Note that the C concentration contained in the mixed crystal layer 102 is defined as the highest value in the mixed crystal layer 102. Also, in the present embodiment, the mixed crystal layer 102 contains C at a high concentration. Here, "high concentration" means that the C concentration is 1.0×10 +21 cm -3 or more as described above. Thus, the mixed crystal layer 102 according to the present embodiment contains the above Group III metal element (here Al), Si, C, and further a transition metal element (here Fe). Also, when referring to the transition metal element contained in the mixed crystal layer 102, it may be described as the transition metal element in the mixed crystal layer 102.

[0028] The transition metal element contained in the mixed crystal layer 102 is not limited to Fe. The transition metal element contained in the mixed crystal layer 102 is preferably any one of Cr, Cu, Ni, Mn, and Co, which are transition metal elements that may be mixed in a MOCVD furnace, which is an example of a growth furnace for forming the AlN layer 103. Note that MOCVD means Metal Organic Chemical Vapor Deposition. By configuring the mixed crystal layer 102 in this way, the crystallinity stability of the AlN layer 103 can be ensured. That is, in the present embodiment, the crystallinity stability of the AlN layer 103 can be ensured by the transition metal element in the mixed crystal layer 102 being at least one of Fe, Cr, Cu, Ni, Mn, and Co.

[0029] Also, when the transition metal element contained in the mixed crystal layer 102 is at least one of Fe, Cr, Cu, Ni, Mn, and Co, the concentration of any of the transition metal elements is 5.0×10 +16 cm -3The following measures can further ensure the crystalline stability of the AlN layer 103. For example, if the mixed crystal layer 102 according to this embodiment contains the above-mentioned Fe and also a transition metal element, Cr, the Fe concentration is 5.0 × 10 +16 cm -3 The following applies, and the Cr concentration is also 5.0 × 10⁻⁶. +16 cm -3 The following applies: In other words, if there is only one transition metal element in the mixed crystal layer 102, the concentration of that one transition metal element is 5.0 × 10⁻⁶. +16 cm -3 The following applies: If there are two or more transition metal elements in the mixed crystal layer 102, the concentration of each of the two or more transition metal elements is 5.0 × 10⁻⁶. +16 cm -3 The following applies: Thus, the transition metal element in the mixed crystal layer 102 is at least one of Fe, Cr, Cu, Ni, Mn, and Co, and the elemental concentration of each of these transition metal elements is 5.0 × 10⁻⁶. +16 cm -3 The following is the explanation. This further ensures the crystallinity stability of the AlN layer 103. The following explains the specifications for the C concentration and the numerical values ​​of transition metal elements including Fe in the mixed crystal layer 102.

[0030] Since the mixed crystal layer 102 is formed by supplying C and Al raw materials to the surface of the Si substrate 101, it is predicted that it will not adequately contribute as a buffer layer if the supply of these materials is insufficient. Furthermore, the concentration of transition metal elements contained in the mixed crystal layer 102 is also thought to affect the crystal quality of the AlN layer 103. Therefore, this effect will be explained using Figure 3. Figure 3 shows the results of evaluating the dependence of the C concentration and the Fe concentration (a transition metal element) in the mixed crystal layer on the crystallinity of the AlN layer. Here, the crystallinity was evaluated for samples 1, 2, and 3.

[0031] Sample 1 is a nitride semiconductor epitaxial substrate according to Comparative Example 1. Sample 1 has the same configuration as the nitride semiconductor epitaxial substrate 100 according to this embodiment, except that the C concentration in the mixed crystal layer of Sample 1 is different from the C concentration in the mixed crystal layer 102 according to this embodiment. Similarly, Sample 2 is a nitride semiconductor epitaxial substrate according to Comparative Example 2. Sample 2 has the same configuration as the nitride semiconductor epitaxial substrate 100 according to this embodiment, except that the Fe concentration in the mixed crystal layer of Sample 2 is different from the Fe concentration in the mixed crystal layer 102 according to this embodiment. Sample 3 means the nitride semiconductor epitaxial substrate 100 according to this embodiment.

[0032] The crystallinity of the AlN layers in Sample 1 and Sample 2, and the AlN layer 103 of Sample 3, was evaluated by the full width at half maximum of the XRD rocking curve of the (0002) plane. XRD stands for X-ray diffraction. The evaluation results for Sample 1 and Sample 2 in Figure 3 are an example of the results of evaluating the dependence on the carbon concentration in the mixed crystal layer 102. In other words, by comparing Sample 1 and Sample 2 in Figure 3, the influence of the carbon concentration in the mixed crystal layer on the crystallinity of the AlN layer can be evaluated.

[0033] The carbon concentration in the mixed crystal layer of Sample 1 is 1.5 × 10⁻⁶. +18 cm -3 The carbon concentration in the mixed crystal layer of sample 2 is 1.8 × 10⁻⁶. +21 cm -3 In Sample 1, the AlN(0002) FWHM was 1750 arcsec, which is the same as before, but in Sample 2, the AlN(0002) FWHM was 1400 arcsec, showing improvement. Furthermore, in order to confirm the dependence on the Fe concentration in the mixed crystal layer, Sample 3 was prepared. The Fe concentration in the mixed crystal layer of Sample 2 was 1.0 × 10⁻⁶. +17 cm -3 The Fe concentration in the mixed crystal layer 102 of sample 3 is 2.0 × 10⁻⁶. +16 cm -3As can be seen from Figure 3, a significant improvement was observed in the AlN(0002) full width at half maximum (FWHM) of sample 3, which was 1020 arcsecs, compared to samples 1 and 2. As a result of this evaluation of the AlN(0002) FWHM, the C concentration in the mixed crystal layer 102 was 1.0 × 10⁻¹⁰. +21 cm -3 In summary, the transition metal element concentration is 5.0 × 10⁻⁶. +16 cm -3 The following results confirmed a significant improvement in the crystallinity of AlN layer 103.

[0034] As described above, in this embodiment, the C concentration in the mixed crystal layer 102 is 1.0 × 10 +21 cm -3 The above results indicate that the transition metal element concentration in the mixed crystal layer 102 is 5.0 × 10⁻⁶. +16 cm -3 The following is the case. As a result, the AlN layer 103 provided above the mixed crystal layer 102 has excellent crystallinity. The nitride semiconductor epitaxial substrate 100 according to this embodiment comprises a Si substrate 101 and such an AlN layer 103 (an example of a nitride semiconductor epitaxial layer) above the Si substrate 101. In other words, as shown in this embodiment, a nitride semiconductor epitaxial substrate 100 is realized that has a layer formed above the Si substrate 101 and has excellent crystallinity.

[0035] The amount of transition metal elements in the mixed crystal layer 102 here is the sum of the amount of intentional doping using dopant material and the amount of automatic doping from the furnace environment. The mixed crystal layer 102 is a polycrystalline Al-Si-C that realizes a nitride semiconductor epitaxial layer with excellent crystallinity, and its main components are Al, Si, and C. Therefore, the C concentration in the mixed crystal layer 102 is 4.0 × 10⁻¹⁶, which is the C concentration of single-crystal SiC. +22 cm -3 This will be a smaller value compared to [the previous value].

[0036] The numerical values ​​of the C concentration in the mixed crystal layer 102 are described below. Figure 4 shows the results of secondary ion mass spectrometry (SIMS) of the C concentration and Si concentration in each component according to Embodiment 1 of this disclosure. The mixed crystal layer 102 is formed when C raw material is supplied to the surface of the Si substrate 101 and C is diffused towards the Si substrate 101 by thermal diffusion. Therefore, as shown in Figure 4, the C concentration distribution in the mixed crystal layer 102 is such that the C concentration is highest near the interface between the mixed crystal layer 102 and the AlN layer 103, and the C concentration decreases continuously toward the Si substrate 101. The area near the interface between the mixed crystal layer 102 and the AlN layer 103 refers, for example, to the region of the mixed crystal layer 102 within 20 nm of the interface. In other words, the carbon concentration distribution in the mixed crystal layer 102 is a distribution that continuously decreases, with the carbon concentration being higher on the nitride semiconductor epitaxial layer (AlN layer 103) side and lower on the Si substrate 101 side. Note that, only when describing the carbon concentration distribution in the mixed crystal layer 102 in this way, "carbon concentration" refers to the carbon concentration at that specific location, unlike the definition above. For other explanations, the carbon concentration in the mixed crystal layer 102 is defined as the highest value within the mixed crystal layer 102, as described above. Furthermore, the film thickness of the mixed crystal layer 102 is such that the carbon concentration is 1.0 × 10⁻⁶. +21 cm -3 The above range is defined as follows. For example, in this embodiment, the C concentration in the mixed crystal layer 102 is 1.8 × 10 +21 cm -3 The thickness of the mixed crystal layer 102 is 40 nm. The thickness of the mixed crystal layer 102 can be controlled by the amount of C diffusion, for example, by the thermal diffusion temperature or thermal diffusion time. A thickness of 50 nm or less for the mixed crystal layer 102 is desirable from the viewpoint of the crystallographic stability of the AlN layer 103, which is the nitride semiconductor epitaxial layer. In other words, in this embodiment, the thickness of the mixed crystal layer 102 is 50 nm or less, thereby ensuring further crystallographic stability of the AlN layer 103, which is the nitride semiconductor epitaxial layer.

[0037] Furthermore, the transition metal elements in the mixed crystal layer 102 also exhibit a similar concentration distribution (i.e., a concentration distribution that continuously decreases, with higher transition metal element concentrations on the AlN layer 103 side and lower concentrations on the Si substrate 101 side). Note that, only when describing the transition metal element concentration distribution in the mixed crystal layer 102, "transition metal element concentration" refers to the concentration of the transition metal element at that specific location. In other explanations, the highest value in the mixed crystal layer 102 is defined as the transition metal element concentration in the mixed crystal layer 102. Since the group III metal elements in the mixed crystal layer 102 are supplied by thermal diffusion, similar to carbon, the group III metal element concentration is highest near the interface with the AlN layer 103 and decreases towards the Si substrate 101 side. By forming the mixed crystal layer 102 through thermal diffusion in this way, the process can be simplified.

[0038] [Manufacturing method] Furthermore, the manufacturing method for the nitride semiconductor epitaxial substrate 100 according to Embodiment 1 will be specifically explained here with reference to Figure 1.

[0039] The Si substrate 101 is set in a metal-organic vapor deposition (MOCVD) furnace, and the temperature of the Si substrate 101 is raised to 500°C or higher. After this, carbon (C) and aluminum (Al) raw materials are supplied to the surface of the Si substrate 101 at a temperature of 500°C or higher. As an example of the carbon raw material, trimethylaluminium (TMA), triethylaluminium (TEA), carbon tetrabromide (CBr4), or propane (C3H8), which are organometallic raw materials, can be used. These are raw materials provided in the MOCVD furnace as sources of Group III elements or dopant sources, and the process can be simplified. TMA or TEA is used as the Al raw material. H2, N2, or a mixture thereof is used as a carrier gas, and the above-mentioned carbon and Al raw materials are supplied to the surface of the Si substrate 101. As a result, the carbon and Al separated by the thermal decomposition reaction on the surface of the Si substrate 101 are adsorbed onto the surface of the Si substrate 101. Subsequently, by maintaining the temperature of the Si substrate 101 at 900°C or higher in an atmosphere supplied with NH3 and a carrier gas, the C and Al supplied to the surface of the Si substrate 101 are thermally diffused from the surface to the back of the Si substrate 101. This forms a mixed crystal layer 102 mainly composed of Al, Si, and C. Since the mixed crystal layer 102 is formed by the thermal diffusion of C and Al adsorbed on the surface of the Si substrate 101, the concentration distribution of C and Al in the mixed crystal layer 102 is highest near the interface with the AlN layer 103, and decreases toward the Si substrate 101. The highest value of C concentration in the mixed crystal layer 102 can be controlled by the amount of C raw material supplied. Regarding the transition metal elements in the mixed crystal layer 102, in addition to intentional doping control by, for example, the amount of Cp2Fe, which is the Fe raw material, the concentration of transition metal elements can be controlled by autodoping control by the reactor atmosphere such as growth temperature, growth pressure, and carrier gas flow rate. Cp2Fe refers to ferrocene.

[0040] Subsequently, the Si substrate 101 is heated to the growth temperature of the AlN layer 103, for example, 1000°C or higher. Then, TMA or TEA is supplied as the Al raw material, NH3 as the N raw material, and H2, N2, or a mixture thereof are supplied as carrier gases. Through this process, the AlN layer 103 is formed as a nitride semiconductor epitaxial layer.

[0041] In this way, the structure of Embodiment 1 (i.e., the nitride semiconductor epitaxial substrate 100) can be manufactured. Furthermore, by continuously forming the mixed crystal layer 102 and the nitride semiconductor epitaxial layer (for example, the AlN layer 103) in an MOCVD furnace, the nitride semiconductor epitaxial substrate 100 according to Embodiment 1 can be realized without significantly increasing the number of steps.

[0042] (Modification 1 of Embodiment 1) Next, a nitride semiconductor epitaxial substrate 500 according to a modification 1 of Embodiment 1 will be described with reference to Figure 5. The nitride semiconductor epitaxial substrate 500 has the same configuration as the nitride semiconductor epitaxial substrate 100 according to Embodiment 1, except that the structure of the AlN layer 503, which is an example of a nitride semiconductor epitaxial layer, differs from that of the AlN layer 103 according to Embodiment 1.

[0043] [composition] Figure 5 is a cross-sectional view of a nitride semiconductor epitaxial substrate 500 according to Modification 1 of Embodiment 1 of this disclosure. As shown in Figure 5, in this modification, an AlN layer 503, which is an example of a nitride semiconductor epitaxial layer, is epitaxially grown on top of a Si substrate 501 via a mixed crystal layer 502 mainly composed of Al, Si, and C. Similar to the mixed crystal layer 102 in Embodiment 1, the C concentration in the mixed crystal layer 502 is 1.0 × 10⁻¹⁴. +21 cm -3 In summary, the Fe concentration is 5.0 × 10⁻⁶. +16 cm -3The following applies: The AlN layer 503 consists of a first AlN layer 504 and a second AlN layer 505. The carbon concentration of the first AlN layer 504 is higher than that of the second AlN layer 505. The carbon concentration of both the first AlN layer 504 and the second AlN layer 505 is 1.0 × 10⁻¹⁴. +21 cm -3 That's all.

[0044] In other words, in this modified example, the nitride semiconductor epitaxial layer (AlN layer 503) consists of a first nitride semiconductor layer (first AlN layer 504) and a second nitride semiconductor layer (second AlN layer 505). Furthermore, the carbon concentration of the first nitride semiconductor layer is higher than that of the second nitride semiconductor layer. Also, in this modified example, the nitride semiconductor epitaxial layer (AlN layer 503) has an AlN layer (first AlN layer 504) on the mixed crystal layer 502 side.

[0045] Figure 6A is a schematic diagram of the C concentration and Si concentration in each component according to Modification 1 of Embodiment 1 of this disclosure. As shown in Figure 6A, the C concentration of the first AlN layer 504 is 1.0 × 10⁻⁶ +19 cm -3 The C concentration in the second AlN layer 505 is 1.0 × 10⁻⁶. +16 cm -3 The control of the C concentration in the first AlN layer 504 and the second AlN layer 505 to the above concentration can be achieved, for example, by growing the first AlN layer 504 at a low temperature and the second AlN layer 505 at a high temperature.

[0046] In this modified example, the initial layer of the AlN layer 503 (i.e., the first AlN layer 504) is composed of AlN with a high C concentration. This provides a nitride semiconductor epitaxial substrate 500 in which the formation of a low-resistance layer at the interface between the AlN layer 503 (more specifically, the first AlN layer 504) and the mixed crystal layer 502 is suppressed. Furthermore, power transistors fabricated using this nitride semiconductor epitaxial substrate 500 can reduce leakage current. In addition, by growing the initial layer of the AlN layer 503 at a low temperature, a nitride semiconductor epitaxial substrate 500 with high reproducibility and excellent productivity can be realized.

[0047] [Manufacturing method] Furthermore, a manufacturing method for the nitride semiconductor epitaxial substrate 500 according to a modified example 1 of Embodiment 1 will be specifically described here with reference to Figure 5.

[0048] The Si substrate 501 is set in an MOCVD furnace and heated to over 500°C. This is the process of raising the temperature of the Si substrate 501 to over 500°C. After this, a process of supplying C raw materials to the surface of the Si substrate 501 is carried out. More specifically, C raw materials and Al raw materials are supplied to the surface of the Si substrate 501 at a temperature of over 500°C. As an example of the C raw material, TMA, TEA, CBr4, or C3H8 can be used. TMA or TEA is used as the Al raw material. H2, N2, or a mixture thereof is used as a carrier gas and the above C raw materials and Al raw materials are supplied to the surface of the Si substrate 501. Furthermore, TMA or TEA is used as the Al raw material, NH3 as the N raw material, and H2, N2, or a mixture thereof is used as a carrier gas to form the first AlN layer 504. This is the process of growing the first nitride semiconductor layer on top of the Si substrate 501. Subsequently, the temperature of the Si substrate 501 is maintained at 900°C or higher in an atmosphere supplied with NH3 and a carrier gas, thereby thermally diffusing the C and Al supplied to the surface of the Si substrate 501 from the surface to the back. This is the process of maintaining the temperature of the Si substrate 501 and the first nitride semiconductor layer at 900°C or higher to diffuse the group III metal elements from the first nitride semiconductor layer into the Si substrate 501. This forms a mixed crystal layer 502 mainly composed of Al, Si, and C. The formation of the first AlN layer 504 is preferably performed before the thermal diffusion process at 900°C or higher in order to stabilize the surface, and is preferably performed at a temperature at least 100°C lower than the thermal diffusion temperature. Furthermore, the film thickness of the first AlN layer 504 is preferably 10 nm or less from the viewpoint of improving crystallinity. The highest value of C concentration in the mixed crystal layer 502 can be controlled by the amount of C raw material supplied. Regarding the transition metal elements in the mixed crystal layer 502, the concentration of transition metal elements can be controlled not only by intentional doping control, for example by adjusting the supply amount of Cp2Fe, which is the Fe raw material, but also by auto-doping control, which is controlled by the reactor atmosphere, such as growth temperature, growth pressure, and carrier gas flow rate.

[0049] Subsequently, the Si substrate 501 is heated to the growth temperature of the second AlN layer 505, for example, 1000°C or higher, and TMA or TEA is supplied as the Al raw material, NH3 as the N raw material, and H2, N2, or a mixture thereof as the carrier gas. This is the process of growing a second nitride semiconductor layer (nitride semiconductor epitaxial layer) on top of the first nitride semiconductor layer. As a result, the second AlN layer 505 is formed.

[0050] In this way, the structure of the nitride semiconductor epitaxial substrate 500 according to the modified example 1 of Embodiment 1 can be manufactured.

[0051] As described above, the manufacturing method for the nitride semiconductor epitaxial substrate 500 according to this embodiment realizes an AlN layer 503 with excellent crystallinity. The nitride semiconductor epitaxial substrate 500 according to this embodiment comprises a Si substrate 501 and such an AlN layer 503 (an example of a nitride semiconductor epitaxial layer) above the Si substrate 501. In other words, as shown in this embodiment, a manufacturing method for a nitride semiconductor epitaxial substrate 500 having a layer with excellent crystallinity formed above the Si substrate 501 is realized.

[0052] (Modification 2 of Embodiment 1) Next, a nitride semiconductor epitaxial substrate 500a according to a modification 2 of Embodiment 1 will be described with reference to Figure 6B. The nitride semiconductor epitaxial substrate 500a has the same configuration as the nitride semiconductor epitaxial substrate 500 according to Modification 1, except that the structure of the AlN layer 503a, which is an example of a nitride semiconductor epitaxial layer, differs from that of the AlN layer 503 according to Modification 1.

[0053] [composition] Figure 6B is a cross-sectional view of a nitride semiconductor epitaxial substrate 500a according to a modified example 2 of Embodiment 1 of this disclosure. As shown in Figure 6B, in this modified example, an AlN layer 503a, which is an example of a nitride semiconductor epitaxial layer, is epitaxially grown on top of a Si substrate 501 via a mixed crystal layer 502 mainly composed of Al, Si, and C. Similar to the mixed crystal layer 102 in Embodiment 1, the C concentration in the mixed crystal layer 502 is 1.0 × 10⁻¹⁴. +21 cm -3 In summary, the Fe concentration is 5.0 × 10⁻⁶. +16 cm -3 The following applies: The AlN layer 503a consists of a first AlN layer 504a and a second AlN layer 505a. The carbon concentration in the first AlN layer 504a is higher than that in the second AlN layer 505a. Furthermore, the carbon concentration distribution of the first AlN layer 504a is characterized by a distribution in which the carbon concentration continuously decreases from the mixed crystal layer 502 toward the second AlN layer 505a within the first AlN layer 504a.

[0054] Similar to the first modification of Embodiment 1, by controlling the growth temperature, for example, the C concentration in the first AlN layer 504a is 1.0 × 10⁻⁶. +19 cm -3 The C concentration in the second AlN layer 505a is 1.0 × 10⁻⁶. +16 cm -3 Therefore, by changing the growth conditions without interrupting the growth of the first AlN layer 504a, a concentration distribution in which the C concentration in the first AlN layer 504a continuously decreases is realized.

[0055] In this modified example, the initial layer of the AlN layer 503a (i.e., the first AlN layer 504a) is composed of AlN with a high C concentration, and the C concentration is continuously varied from the mixed crystal layer 502 upwards to the AlN layer with a lower C concentration (the second AlN layer 505a). This provides a nitride semiconductor epitaxial substrate 500a with excellent surface flatness and a low defect count, in which a low-resistance layer is not formed at the interface between the AlN layer 503a (more specifically, the first AlN layer 504a) and the mixed crystal layer 502.

[0056] Furthermore, power transistors fabricated using this nitride semiconductor epitaxial substrate 500a can reduce leakage current and improve yield by reducing the number of defects. In addition, by growing the initial layer of the AlN layer 503a at a low temperature, a nitride semiconductor epitaxial substrate 500a with high reproducibility and excellent productivity can be realized.

[0057] [Manufacturing method] Furthermore, a manufacturing method for the nitride semiconductor epitaxial substrate 500a according to a modified example 2 of Embodiment 1 will be specifically explained here with reference to Figure 6B.

[0058] The Si substrate 501 is set in an MOCVD furnace and heated to over 500°C. After this, carbon (C) and aluminum (Al) raw materials are supplied to the surface of the Si substrate 501 at a temperature of over 500°C. As an example, TMA, TEA, CBr4, or C3H8 can be used as the carbon raw material. TMA or TEA is used as the Al raw material. H2, N2, or a mixture thereof is used as a carrier gas to supply the above-mentioned carbon and Al raw materials to the surface of the Si substrate 501. Furthermore, TMA or TEA is used as the Al raw material, NH3 as the N raw material, and H2, N2, or a mixture thereof is used as a carrier gas to form the first AlN layer 504a. Subsequently, the temperature of the Si substrate 501 is raised and maintained at over 900°C to thermally diffuse the C and Al supplied to the surface of the Si substrate 501 from the surface to the back surface. This forms a mixed crystal layer 502 mainly composed of Al, Si, and C. During this thermal diffusion process, the supply of NH3, TMA or TEA, and carrier gas is continued. This allows the thermal diffusion process to be carried out without interrupting the formation of the first AlN layer 504a. Therefore, a carbon concentration distribution can be achieved in which the carbon concentration decreases continuously from the mixed crystal layer 502 side to the second AlN layer 505a side in the first AlN layer 504a. The highest value of carbon concentration in the mixed crystal layer 502 can be controlled by the amount of carbon raw material supplied. Regarding the transition metal elements in the mixed crystal layer 502, the concentration of transition metal elements can be controlled not only by intentional doping control by the amount of Cp2Fe supplied as a Fe raw material, but also by auto-doping control by the reactor atmosphere, such as growth temperature, growth pressure, and carrier gas flow rate.

[0059] Subsequently, the Si substrate 501 is heated to the growth temperature of the second AlN layer 505a, for example, 1000°C or higher, and TMA or TEA is supplied as the Al raw material, NH3 as the N raw material, and H2, N2, or a mixture thereof as the carrier gas. This forms the second AlN layer 505a. In this way, the structure of the nitride semiconductor epitaxial substrate 500a according to the modified example 2 of Embodiment 1 can be manufactured.

[0060] (Embodiment 2) In order to utilize a nitride semiconductor epitaxial substrate with excellent crystallinity as a semiconductor device, it is necessary to form a heterostructure epitaxial layer on top of the nitride semiconductor epitaxial substrate according to the purpose. In this disclosure, a nitride semiconductor epitaxial substrate equipped with a heterostructure epitaxial layer is specifically referred to as a nitride semiconductor heterostructure epitaxial substrate. Below, a specific example of a nitride semiconductor heterostructure epitaxial substrate used in a power transistor will be described. Here, a nitride semiconductor heterostructure epitaxial substrate 700 according to Embodiment 2, which is an example of a nitride semiconductor epitaxial substrate, will be described with reference to Figure 7.

[0061] Figure 7 is a cross-sectional view of a nitride semiconductor heterostructure epitaxial substrate 700 according to Embodiment 2 of this disclosure.

[0062] The nitride semiconductor heterostructure epitaxial substrate 700 according to this embodiment includes a Si substrate 701 having the same configuration as the Si substrate 101 according to Embodiment 1. Furthermore, the nitride semiconductor heterostructure epitaxial substrate 700 includes a mixed crystal layer 702 having a configuration that differs from the mixed crystal layer 102 according to Embodiment 1 only in its C concentration. In addition, in this embodiment, an AlN layer 703, an example of a nitride semiconductor epitaxial layer, is provided above the Si substrate 701 via a mixed crystal layer 702 mainly composed of Al, Si, and C. x Ga 1-x A buffer layer 706, a GaN channel layer 707, and an AlGaN barrier layer 708, each consisting of one or more layers of N (0 ≤ x ≤ 1), are formed as a heterostructure by epitaxial growth. In other words, the heterostructure epitaxial layer 720 of the nitride semiconductor heterostructure epitaxial substrate 700 according to this embodiment includes an AlN layer 703, a buffer layer 706, a GaN channel layer 707, and an AlGaN barrier layer 708. Furthermore, the heterostructure epitaxial layer 720 is arranged in contact with the mixed crystal layer 702 from above and is constructed by stacking the AlN layer 703, buffer layer 706, GaN channel layer 707, and AlGaN barrier layer 708 in that order. Also, the buffer layer 706 is Al x Ga 1-xWhen composed of multiple layers of N(0≦x≦1), the value of x may differ for each layer. In this embodiment, the GaN channel layer 707 is an example of a third nitride semiconductor layer formed above the nitride semiconductor epitaxial layer (AlN layer 703). The AlGaN barrier layer 708 is an example of a fourth nitride semiconductor layer formed above the third nitride semiconductor layer (GaN channel layer 707). Furthermore, at the interface between the GaN channel layer 707 and the AlGaN barrier layer 708, a high-concentration two-dimensional electron gas is formed due to the effects of piezoelectric polarization and spontaneous polarization. In other words, the nitride semiconductor heterostructure epitaxial substrate 700 according to this embodiment has a two-dimensional electron gas at the interface between the third nitride semiconductor layer and the fourth nitride semiconductor layer. The buffer layer 706 contains a maximum of 1.0 × 10⁻¹⁶ electrons. +20 cm -3 The buffer layer 706 is doped with carbon, resulting in high resistance. The carbon concentration in the buffer layer 706 is the highest carbon concentration in the heterostructured epitaxial layer 720.

[0063] The carbon concentration of the mixed crystal layer 702 according to this embodiment is higher than the carbon concentration of each layer included in the heterostructure epitaxial layer 720. As described above, the layers included in the heterostructure epitaxial layer 720 are the AlN layer 703, the buffer layer 706, the GaN channel layer 707, and the AlGaN barrier layer 708. In other words, the carbon concentration of the mixed crystal layer 702 is higher than the carbon concentration of any of the layers: the AlN layer 703, the buffer layer 706, the GaN channel layer 707, and the AlGaN barrier layer 708. Note that the carbon concentration of each layer (AlN layer 703, buffer layer 706, GaN channel layer 707, and AlGaN barrier layer 708) is defined as the highest value in each respective layer.

[0064] The following describes the results of an investigation into the influence of the relationship between the C concentration in the mixed crystal layer 702 and the highest C concentration in the buffer layer 706 on the crystallinity of the GaN channel layer 707. The crystallinity of the GaN channel layer 707 was evaluated by the full width at half maximum of the XRD rocking curves of the (0002) and (10-11) planes. The results are shown in Figure 8. Figure 8 shows the results of evaluating the influence of the C concentration in the mixed crystal layer 702 and the C concentration in the heterostructure epitaxial layer 720 on the crystallinity of the GaN channel layer 707. Here, the maximum C concentration in the buffer layer 706 was set to 1.0 × 10⁻⁶. +20 cm -3 The carbon concentration in the mixed crystal layer 702 is set to 1.5 × 10⁻⁶ each. +18 cm -3 and 1.8 × 10 +21 cm -3 Figure 8 shows the crystallinity evaluation results for Sample A and Sample B. More specifically, Sample A is a nitride semiconductor heterostructure epitaxial substrate according to Comparative Example 1. Sample A has the same configuration as the nitride semiconductor heterostructure epitaxial substrate 700 according to this embodiment, except that the C concentration in the mixed crystal layer of Sample A is different from the C concentration in the mixed crystal layer 702 according to this embodiment. Sample B means the nitride semiconductor heterostructure epitaxial substrate 700 according to this embodiment.

[0065] As can be seen from Figure 8, in sample B, where the C concentration in the mixed crystal layer 702 is sufficiently higher than the C concentration in the buffer layer 706, a significant improvement was observed in both the (0002) plane half-width and the (10-11) plane half-width. In this embodiment, the buffer layer 706 is doped with a high concentration of C to increase its resistance, but in applications where high resistance of the buffer layer 706 is not required, the C concentration may be lower.

[0066] If the carbon concentration in the buffer layer 706 is low, the crystallinity of the GaN channel layer 707 tends to improve. Therefore, if the carbon concentration in the mixed crystal layer 702 is higher than that of the buffer layer 706, it will be effective in improving the crystallinity of the GaN channel layer 707. In this embodiment as well, the transition metal element concentration contained in the mixed crystal layer 702 is 5.0 × 10⁻¹⁶. +16 cm -3 The following applies:

[0067] In this way, by forming a heterostructured epitaxial layer 720 with a mixed crystal layer 702 mainly composed of Al, Si, and C as a buffer layer, a nitride semiconductor heterostructured epitaxial substrate 700 with excellent crystallinity of the active layer (in this case, the GaN channel layer 707) can be realized.

[0068] In summary, the nitride semiconductor heterostructure epitaxial substrate 700 is an example of a nitride semiconductor epitaxial substrate. Furthermore, the carbon concentration in the mixed crystal layer 702 is higher than the carbon concentrations in each layer contained in the heterostructure epitaxial layer 720, and the transition metal element concentration in the mixed crystal layer 702 is 5.0 × 10⁻⁶. +16 cm -3 The following is the case. As a result, the active layer (in this case, the GaN channel layer 707) provided above the mixed crystal layer 702 has excellent crystallinity. In other words, as shown in this embodiment, a nitride semiconductor epitaxial substrate (nitride semiconductor heterostructure epitaxial substrate 700) is realized that has a layer with excellent crystallinity formed above the Si substrate 701.

[0069] Furthermore, by forming power transistors using the nitride semiconductor heterostructure epitaxial substrate 700, which has excellent crystallinity, device failure caused by crystal defects can be suppressed, and a nitride semiconductor device with high reliability can be realized.

[0070] (Embodiment 3) Next, the nitride semiconductor device 900 according to Embodiment 3 will be described with reference to Figure 9. The nitride semiconductor device 900 is an example of a semiconductor device comprising a nitride semiconductor heterostructure epitaxial substrate.

[0071] FIG. 9 is a cross-sectional view of a nitride semiconductor device 900 according to Embodiment 3 of the present disclosure. The nitride semiconductor device 900 according to the present embodiment includes a Si substrate 901 and a mixed crystal layer 902, which have the same configuration as the Si substrate 701 and the mixed crystal layer 702 according to Embodiment 2.

[0072] Furthermore, in the present embodiment, via a mixed crystal layer 902 mainly composed of Al, Si, and C above the Si substrate 901, an AlN layer 903, Al x Ga 1-x a buffer layer 906, a GaN channel layer 907, and an AlGaN barrier layer 908 each composed of a single layer or a plurality of layers of N (0 ≤ x ≤ 1) are formed by epitaxial growth as a hetero structure. That is, the hetero structure epitaxial layer 920 included in the nitride semiconductor device 900 according to the present embodiment includes an AlN layer 903, a buffer layer 906, a GaN channel layer 907, and an AlGaN barrier layer 908. Furthermore, the hetero structure epitaxial layer 920 is disposed in contact with the upper side of the mixed crystal layer 902 and is configured by being stacked in the order of the AlN layer 903, the buffer layer 906, the GaN channel layer 907, and the AlGaN barrier layer 908. Also, when the buffer layer 906 is composed of a plurality of layers of Al x Ga 1-x N (0 ≤ x ≤ 1), the value of x may be different for each layer. At the interface between the GaN channel layer 907 and the AlGaN barrier layer 908, a high-concentration two-dimensional electron gas is formed due to the effects of piezoelectric polarization and spontaneous polarization. The mixed crystal layer 902 has a higher concentration of C than any layer of the hetero structure epitaxial layer 920. That is, the C concentration of the mixed crystal layer 902 according to the present embodiment is higher than the C concentration of each layer included in the hetero structure epitaxial layer 920. For example, a maximum of 1.0×10 +20 cm -3 of C is doped in the buffer layer 906, and the C concentration contained in the mixed crystal layer 902 is 1.0×10 +21 cm -3 or more, and the transition metal element concentration is 5.0×10 +16 cm -3 or less.

[0073] Furthermore, the nitride semiconductor device 900 according to this embodiment includes a gate electrode 911, a source electrode 909, and a drain electrode 910. More specifically, the source electrode 909 and the drain electrode 910 are formed above the AlGaN barrier layer 908, spaced apart to the left and right of the gate electrode 911. In other words, the nitride semiconductor device 900 according to this embodiment is a semiconductor device using a nitride semiconductor epitaxial substrate having a layer with excellent crystallinity formed above the Si substrate 901.

[0074] In this way, by forming a power transistor using a nitride semiconductor heterostructure epitaxial substrate with excellent crystallinity, it is possible to suppress device failure caused by crystal defects and realize a nitride semiconductor device 900 with excellent reliability.

[0075] (Other embodiments) Although embodiments have been described above, this disclosure is not limited to the embodiments described above.

[0076] Furthermore, this disclosure also includes forms obtained by applying various modifications to each embodiment that a person skilled in the art could conceive, or forms realized by arbitrarily combining the components and functions of each embodiment without departing from the spirit of this disclosure. [Industrial applicability]

[0077] This disclosure enables the realization of high-quality nitride semiconductor epitaxial substrates, thereby improving the device performance and extending the device lifespan of nitride semiconductor devices using these substrates. [Explanation of Symbols]

[0078] 100, 500, 500a Nitride Semiconductor Epitaxial Substrates 101, 501, 701, 901 Si substrates 102, 502, 702, 902 mixed crystal layers 103, 503, 503a, 703, 903 AlN layer 504, 504a First AlN layer 505, 505a Second AlN layer 700 Nitride Semiconductor Heterostructure Epitaxial Substrate 706, 906 buffer layers 707, 907 GaN channel layer 708, 908 AlGaN barrier layer 720, 920 heterostructured epitaxial layer 900 Nitride semiconductor equipment 909 Source electrode 910 Drain electrode 911 NG

Claims

1. Si substrate and A nitride semiconductor epitaxial layer formed above the Si substrate, Displaced between the Si substrate and the nitride semiconductor epitaxial layer is a mixed crystal layer of Si and a group III metal element containing a high concentration of C. The carbon concentration in the mixed crystal layer is 1.0 × 10 ＋２１ cm －３ That's all. The transition metal element concentration in the mixed crystal layer is 5.0 × 10 ＋１６ cm －３ The following is Nitride semiconductor epitaxial substrate.

2. The transition metal element is at least one of Fe, Cr, Cu, Ni, Mn, and Co. The nitride semiconductor epitaxial substrate according to claim 1.

3. The transition metal element is at least one of Fe, Cr, Cu, Ni, Mn, and Co. The elemental concentrations of each of the aforementioned transition metal elements are 5.0 × 10 ＋１６ cm －３ The following is A nitride semiconductor epitaxial substrate according to claim 1 or 2.

4. The aforementioned Group III metal element is the same as the Group III metal element in the nitride semiconductor epitaxial layer. A nitride semiconductor epitaxial substrate according to any one of claims 1 to 3.

5. The carbon concentration distribution in the mixed crystal layer is such that the carbon concentration is high on the nitride semiconductor epitaxial layer side and decreases continuously on the Si substrate side. A nitride semiconductor epitaxial substrate according to any one of claims 1 to 4.

6. The thickness of the mixed crystal layer is 50 nm or less. A nitride semiconductor epitaxial substrate according to any one of claims 1 to 5.

7. The carbon concentration in the mixed crystal layer is 1.0 × 10 ＋２２ cm －３ The following is A nitride semiconductor epitaxial substrate according to any one of claims 1 to 6.

8. The nitride semiconductor epitaxial layer has an AlN layer on the mixed crystal layer side. A nitride semiconductor epitaxial substrate according to any one of claims 1 to 7.

9. The nitride semiconductor epitaxial layer consists of a first nitride semiconductor layer and a second nitride semiconductor layer. The first nitride semiconductor layer is arranged on the mixed crystal layer side. The second nitride semiconductor layer is positioned above the first nitride semiconductor layer. The carbon concentration of the first nitride semiconductor layer is higher than the carbon concentration of the second nitride semiconductor layer. A nitride semiconductor epitaxial substrate according to any one of claims 1 to 8.

10. Si substrate and A heterostructured epitaxial layer including a nitride semiconductor epitaxial layer formed above the Si substrate, Displaced between the Si substrate and the nitride semiconductor epitaxial layer is a mixed crystal layer of Si and a group III metal element containing a high concentration of C. The carbon concentration in the mixed crystal layer is higher than the carbon concentration in each of the layers contained in the heterostructured epitaxial layer. The concentration of the transition metal element in the mixed crystal layer is 5.0×10 ＋１６ cm －３ or less Nitride semiconductor epitaxial substrate.

11. A third nitride semiconductor layer formed above the nitride semiconductor epitaxial layer, A fourth nitride semiconductor layer formed above the third nitride semiconductor layer, Furthermore, The interface between the third nitride semiconductor layer and the fourth nitride semiconductor layer has a two-dimensional electron gas. A nitride semiconductor epitaxial substrate according to any one of claims 1 to 10.

12. Formed using the nitride semiconductor epitaxial substrate described in claim 11 Nitride semiconductor equipment.

13. A step of raising the temperature of the Si substrate to 500°C or higher, A step of supplying a carbon raw material to the surface of the Si substrate, A step of growing a first nitride semiconductor layer on top of the Si substrate, A step of maintaining the temperature of the Si substrate and the first nitride semiconductor layer at 900°C or higher while diffusing a group III metal element from the first nitride semiconductor layer into the Si substrate, The process includes the step of growing a second nitride semiconductor layer on top of the first nitride semiconductor layer. A method for manufacturing nitride semiconductor epitaxial substrates.

14. The aforementioned raw material C is trimethylaluminum, triethylaluminum, carbon tetrabromide, or propane. A method for manufacturing a nitride semiconductor epitaxial substrate according to claim 13.

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