Seed crystal substrate and graphite susceptor with seed crystal substrate

Abstract

To provide a seed crystal substrate having high temperature controllability on a substrate surface.SOLUTION: A seed crystal substrate includes a substrate including a GaN seed crystal on the surface, an adhesion layer formed on the rear surface of the substrate and a carbon film covering the adhesion layer. The substrate consists of a GaN single crystal substrate or a sapphire substrate having a GaN film; the adhesion layer includes Si; thereby, the seed crystal substrate arranging the carbon film having high infrared emissivity on the side of the rear surface can sufficiently obtain heat dissipation due to radiation even on low pressure conditions; and this can enhance the temperature controllability of the substrate surface.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a seed crystal substrate and a graphite susceptor with a seed crystal substrate, and more particularly to a seed crystal substrate having a GaN seed crystal on its surface and a graphite susceptor with a seed crystal substrate having a GaN seed crystal on its surface. [Background technology]

[0002] Gallium nitride (GaN) has long been widely used, primarily in optical devices such as LEDs and lasers. Furthermore, because GaN is a wide-bandgap semiconductor (bandgap: 3.39 eV), has high dielectric breakdown strength, and allows for thinner drift layers, thereby reducing on-resistance, it is expected to have applications not only in optical devices but also in power devices.

[0003] Several methods are known for growing GaN crystals. For example, molecular beam epitaxy (MBE) and metal-organic vapor deposition (MOVPE) are used for growing crystals with relatively thin film thicknesses of a few micrometers. On the other hand, hydride vapor deposition (HVPE) is used for growing crystals on self-supporting substrates with film thicknesses exceeding 10 micrometers.

[0004] The HVPE method, with its high crystal growth rate (approximately 100 μm / h), is suitable for creating self-supporting substrates. However, autodoping from the quartz reaction vessel inevitably introduces oxygen impurities into the grown crystal. Since oxygen impurities act as donors in GaN crystals, this poses a problem for the quality of GaN single crystals.

[0005] Therefore, the halogen-free vapor phase epitaxy (HF-VPE) method described in Non-Patent Document 1 has been proposed as a crystal growth method for producing high-quality, large-diameter, and low-cost GaN self-supporting substrates. Non-patent document 1 contains: (1) A crucible that holds molten Ga and is kept at a high temperature, (2) NH3 gas supply source, (3) A seed crystal substrate having a GaN seed crystal on its surface and facing the crucible A configuration is disclosed that includes In Document 1, it is disclosed that in the HF-VPE method, the crystal growth rate is high (>100 μm / h) and the stability of the crystal growth rate is also high.

[0006] However, in the HF-VPE method in Non-Patent Document 1, since it is crystal growth under low-pressure conditions (<10 kPa), heat dissipation of the substrate may be delayed because it cannot rely on gas heat conduction. This is due to the extremely small infrared emissivity ε on the back surface side of the substrate (ε = 0.2 to 0.4) when the substrate is a GaN single crystal substrate or a sapphire substrate with a GaN film. That is, in a situation where heat dissipation of the substrate has to rely on radiation, due to the low infrared emissivity ε of the above substrate, heat is likely to accumulate excessively.

[0007] For this reason, the substrate may overheat and the surface temperature of the substrate may not be stable. Therefore, a countermeasure can be considered to make the seed crystal substrate contact with other members such as a graphite susceptor and conduct heat to the other members for heat dissipation. However, under low-pressure conditions, simply contacting with other members cannot obtain a sufficient heat dissipation effect. Fluctuations in the substrate surface temperature inhibit the crystal growth rate and the stability of the crystal growth rate. Therefore, further improvement is required for the control of the substrate surface temperature.

[0008] Therefore, various proposals as shown below have been made. In Patent Document 1, (1) A SiC substrate having a seed crystal on its surface, (2) A carbon film formed on the back surface of the SiC substrate, (3) A carbon fixing layer derived from a carbon-based adhesive that fixes the SiC substrate to the support portion and is inserted between the carbon film and a support portion arranged on the back surface side of the carbon film A support portion with a seed crystal substrate including the above is disclosed.

[0009] In Patent Document 2, (1) A SiC substrate having a seed crystal on its surface, (2) A SiC fixing layer derived from a polycarbonate adhesive is interposed between the back surface of the SiC substrate and the support portion to fix the SiC substrate to the support portion. A support portion with a seed crystal substrate is disclosed.

[0010] At this point, one might argue that the heat dissipation of the substrate can be improved by joining the substrate described in Non-Patent Document 1 to other components using the carbon-based adhesive described in Patent Document 1. However, substrates having GaN species crystals on their surface generally include sapphire substrates or GaN single-crystal substrates, both of which have very poor adhesion to carbon films. This is also true for carbon-based adhesives, making it difficult to join sapphire substrates and GaN single-crystal substrates to other components using carbon-based adhesives.

[0011] On the other hand, it is possible to bond sapphire substrates and GaN single crystal substrates to other components by using the polycarbonosilane adhesive described in Patent Document 2. However, polycarbonate adhesives generate Si-containing gas molecular species during GaN single crystal growth, which adhere to the GaN crystal surface as Si impurities, thus inhibiting the growth of high-quality GaN single crystals. [Prior art documents] [Patent Documents]

[0012] [Patent Document 1] Japanese Patent Publication No. 2013-124196 [Patent Document 2] Japanese Patent Publication No. 2013-237592 [Non-patent literature]

[0013] [Non-Patent Document 1] Nakamura, D.; Kimura, T.; Horibuchi, K., Halogen-free vapor phase epitaxy for high-rate growth of GaN bulk crystals, Appl. Phys. Express 2017, 10(4), 045504 [Overview of the Initiative] [Problems that the invention aims to solve]

[0014] The problem that this invention aims to solve is to provide a seed crystal substrate with high temperature controllability of the substrate surface. Another problem that the present invention aims to solve is to provide a graphite susceptor with a seed crystal substrate that offers high temperature control over the substrate surface. [Means for solving the problem]

[0015] To solve the above problems, the seed crystal substrate according to the first embodiment is A substrate having GaN species crystals on its surface, An adhesion layer formed on the back surface of the substrate, A carbon film covering the adhesion layer and Equipped with, The substrate consists of a GaN single crystal substrate or a GaN film-coated sapphire substrate. The adhesion layer is Si Consists of .

[0016] The seed crystal substrate according to the second embodiment is An expanded graphite sheet is placed on the back side of the carbon film, A bonding layer that joins the carbon film and the expanded graphite sheet. It also has the following features.

[0017] The graphite susceptor with a seed crystal substrate according to the first embodiment is, A seed crystal substrate according to the first embodiment, A graphite susceptor is placed on the back side of the seed crystal substrate, A fixed layer that fixes the seed crystal substrate to the graphite susceptor and Equipped with, The absolute value of the difference in average thermal expansion coefficients between the substrate and the graphite susceptor is 0.5 × 10⁻⁶. -6 K -1 It is as follows:

[0018] The graphite susceptor with a seed crystal substrate according to the second embodiment is, A seed crystal substrate according to the second embodiment, A graphite susceptor is placed on the back side of the seed crystal substrate, A fixed layer that fixes the seed crystal substrate to the graphite susceptor and It is equipped with. In the graphite susceptor with seed crystal substrate according to the second embodiment, the absolute value of the difference in average thermal expansion coefficients between the substrate and the graphite susceptor is 0.5 × 10⁻⁶. -6 K -1 The following are preferred: [Effects of the Invention]

[0019] [Seed crystal substrate] In the seed crystal substrate according to the present invention, a Si film is formed on the back surface of the substrate as an adhesion layer, and a carbon film is formed so as to cover this Si film. Here, the Si film has good adhesion to the GaN single crystal substrate and the sapphire substrate, and also has good adhesion to the carbon film. In other words, the seed crystal substrate has a carbon film with high infrared emissivity (ε=0.7~0.9) formed on its back side, which ensures sufficient heat dissipation by radiation even under low-pressure conditions. Therefore, it is possible to improve the controllability of the substrate surface temperature.

[0020] Furthermore, since the seed crystal substrate has a carbon film with good thermal conductivity on its back side, it is possible to suppress the occurrence of thermal unevenness within the surface of the seed crystal substrate. Therefore, it is possible to further improve the controllability of the substrate surface temperature.

[0021] [Graphite susceptor with seed crystal substrate] The graphite susceptor with a seed crystal substrate according to the present invention is obtained by fixing the seed crystal substrate according to the present invention to a graphite susceptor disposed on the back side via a fixed layer. This allows heat to be dissipated from the substrate through direct heat conduction due to the bonding between the seed crystal substrate and the graphite susceptor. Therefore, it is possible to improve the controllability of the substrate surface temperature. [Brief explanation of the drawing]

[0022] [Figure 1] This is a schematic cross-sectional view of the seed crystal substrate (first embodiment). [Figure 2] This is a schematic cross-sectional view of the seed crystal substrate (second embodiment). [Figure 3] This is a schematic cross-sectional view of a graphite susceptor with a seed crystal substrate (first embodiment). [Figure 4] This is a schematic cross-sectional view of a graphite susceptor with a seed crystal substrate (second embodiment). [Figure 5] This is a schematic cross-sectional view of a graphite susceptor with a seed crystal substrate (comparative example). [Figure 6] This diagram illustrates the relationship between the thickness of the adhesion layer and the state of adhesion. [Modes for carrying out the invention]

[0023] [Configuration 1] A substrate having GaN species crystals on its surface, An adhesion layer formed on the back surface of the substrate, A carbon film covering the adhesion layer and Equipped with, The substrate consists of a GaN single crystal substrate or a GaN film-coated sapphire substrate. The adhesion layer is Si Consists of Seed crystal substrate.

[0024] [Configuration 2] The adhesion layer is a seed crystal substrate as described in Configuration 1, having a thickness of 10 nm to 200 nm.

[0025] [Configuration 3] The carbon film is a seed crystal substrate according to configuration 1 or 2, having a thickness of 5 nm or more and 100 nm or less.

[0026] [Structure 4] An expanded graphite sheet is placed on the back side of the carbon film, A bonding layer that joins the carbon film and the expanded graphite sheet. A seed crystal substrate as described in any one of configurations 1 to 3, further comprising the above.

[0027] [Composition 5] The bonding layer is Graphite particles and, A carbon layer interposed between graphite particles A seed crystal substrate according to configuration 4, comprising the above.

[0028] [Composition 6] A seed crystal substrate as described in one of configurations 1 to 3, A graphite susceptor is placed on the back side of the seed crystal substrate, A fixed layer that fixes the seed crystal substrate to the graphite susceptor and Equipped with, The absolute value of the difference in average thermal expansion coefficients between the substrate and the graphite susceptor is 0.5 × 10⁻⁶. -6 K -1 The following is Graphite susceptor with seed crystal substrate.

[0029] [Composition 7] The fixed layer is, Graphite particles and, A carbon layer interposed between graphite particles A graphite susceptor with a seed crystal substrate as described in configuration 6.

[0030] [Structure 8] A seed crystal substrate as described in configuration 4 or 5, A graphite susceptor is placed on the back side of the seed crystal substrate, A fixed layer that fixes the seed crystal substrate to the graphite susceptor and A graphite susceptor with a seed crystal substrate.

[0031] [Composition 9] The absolute value of the difference in average thermal expansion coefficients between the substrate and the graphite susceptor is 0.5 × 10⁻⁶. -6 K -1 A graphite susceptor with a seed crystal substrate as described in configuration 8 below.

[0032] [Configuration 10] The fixed layer is, Graphite particles and, A carbon layer interposed between graphite particles A graphite susceptor with a seed crystal substrate as described in configuration 8 or 9.

[0033] One embodiment of the present invention will be described in detail below. [1. Seed crystal substrate] The seed crystal substrate according to the present invention is A substrate having GaN species crystals on its surface, An adhesion layer formed on the back surface of the substrate, A carbon film covering the adhesion layer and Equipped with, The substrate consists of a GaN single crystal substrate or a GaN film-coated sapphire substrate. The adhesion layer is Si Consists of .

[0034] The seed crystal substrate according to the present invention is An expanded graphite sheet is placed on the back side of the carbon film, A bonding layer that joins the carbon film and the expanded graphite sheet. It may also be equipped with additional features.

[0035] [1.1. Circuit Board] The substrate has GaN species crystals on its surface and is either a GaN single-crystal substrate or a GaN-coated sapphire substrate. Here, the surface orientation of the GaN species crystal is not particularly limited and may be a polar surface (0001) plane (Ga plane) or (000-1) plane (N plane), a nonpolar surface (11-20) plane (a plane) or (10-10) plane (m plane), or a semipolar surface (01-12) plane (r plane). For example, the surface orientation of the GaN seed crystal is preferably the (0001) plane, which allows for relatively less impurity incorporation during GaN single crystal growth and enables the acquisition of high-quality GaN single crystals.

[0036] [1.1.1. GaN single crystal substrate] The GaN single crystal substrate is preferably one that has been treated with CMP (Chemical Mechanical Polishing) on ​​the surface and lapping polishing on the back surface.

[0037] [1.1.2. GaN-coated sapphire substrate] The GaN-coated sapphire substrate is preferably one on which a heteroepitaxial GaN film (0001) surface, deposited by MOVPE to a thickness of 1 to 2 μm, is formed on a sapphire substrate having either the (0001) or (11-20) surface. Furthermore, it is preferable that the GaN-coated sapphire substrate has been subjected to lapping polishing on its back surface.

[0038] [1.2. Close contact layer] A bonding layer is required between the substrate and the carbon film, which will be discussed later. As mentioned earlier, the adhesion of the carbon film to GaN single-crystal substrates and sapphire substrates is very poor. Therefore, the material of the bonding layer must have adhesion not only to the GaN single-crystal substrate or sapphire substrate, but also to the carbon film. Si is one such material.

[0039] Possible methods for depositing a Si film as an adhesion layer include electron beam deposition, sputter deposition, chemical vapor deposition (CVD), and pulsed laser deposition (PLD). The resulting Si film may be amorphous or single crystal. Therefore, it is preferable to use an amorphous Si film obtained by electron beam deposition or sputter deposition as the Si film.

[0040] Furthermore, if the Si film thickness is too thin, there is a concern that the film may peel off due to a chemical reaction with the carbon film, as described later. Therefore, the thickness of the Si adhesion layer is preferably 10 nm or more, and more preferably 15 nm or more, 20 nm or more, or 25 nm or more. On the other hand, if the Si film thickness is too thick, there is a concern that the film deposition cost will increase and the film may peel off due to increased film stress. Therefore, the Si film thickness is preferably 200 nm or less, and more preferably 150 nm or less, or even 100 nm or less.

[0041] At this point, one might raise concerns that the Si film, like the polycarbonate adhesive mentioned earlier, could negatively affect GaN single crystal growth. However, since the Si film is expected to form passivated SiC with the carbon film (described later) during GaN single crystal growth, this is not a problem. In other words, the adhesion layer after GaN single crystal growth is thought to be a mixture of Si and SiC.

[0042] [1.3. Carbon film] As mentioned earlier, the carbon film dissipates heat through radiation, contributing to improved temperature control of the substrate surface. Possible methods for depositing carbon films include carbonization (heat) treatment of photoresist spin-coated films, sputter deposition, CVD, and PLD. The resulting carbon film may be amorphous or single-crystal. Here, a single-crystal carbon film is, for example, a laminate of graphene, which is deposited by CVD or similar methods. Therefore, it is preferable to use an amorphous carbon film obtained by carbonizing a spin-coated photoresist film as the carbon film. Furthermore, phenol resin solutions, novolac resin solutions, etc., can be used as photoresists.

[0043] Specifically, first, a photoresist is dropped onto the Si film, which is the adhesion layer on the back of the substrate, and a spin-coated film of 0.5 to 10 μm is formed by rotating it at a rotation speed of 1000 to 5000 rpm for 30 seconds. Next, the spin-coated substrate is subjected to heat treatment at 800°C for 1 hour in a vacuum or inert gas atmosphere to form a carbon film that covers the Si film on the back surface of the substrate.

[0044] If the carbon film thickness is too thin, there is a concern that the infrared emissivity will decrease. Therefore, the carbon film thickness is preferably 5 nm or more, and more preferably 10 nm or more, or even 15 nm or more. On the other hand, if the carbon film thickness is too thick, there is a concern that the film may peel off due to increased film stress. Therefore, the carbon film thickness is preferably 100 nm or less, and more preferably 50 nm or less, or 20 nm or less.

[0045] [1.4. Expanded Graphite Sheet] Expanded graphite sheets are highly processable and flexible, making them very easy to handle. While an expanded graphite sheet is not strictly necessary, placing it on the back side of the substrate can further improve the heat dissipation from the substrate through radiation. Furthermore, when interposed between the graphite susceptor (described later), its flexibility allows it to function as a stress buffer layer.

[0046] If the expanded graphite sheet is too thin, it becomes difficult to handle and may not function properly as a stress buffer layer. Therefore, the thickness of the expanded graphite sheet is preferably 10 μm or more, and more preferably 50 μm or more. On the other hand, if the expanded graphite sheet is too thick, it may actually hinder heat dissipation. Therefore, the thickness of the expanded graphite sheet is preferably 500 μm or less, and more preferably 200 μm or less.

[0047] [1.5. Bonding layer] The bonding layer connects the carbon film and the expanded graphite sheet. The bonding layer is derived from a carbon-based adhesive commonly used for bonding graphite materials, and consists of graphite particles and a carbon layer interposed between the graphite particles. Here, the carbon-based adhesive is a mixture comprising a carbon-based resin, graphite particles, a polymerization accelerator, a solvent, and the like.

[0048] Graphite particles are used as fillers, but the particle size and content are important. etc.The specifications are not particularly limited, and the most suitable ones should be selected as appropriate depending on the purpose. The particle size of the graphite particles is preferably 0.3 μm or more and 10 μm or less. In addition, the graphite particle content is preferably 10 mass% or more and 60 mass% or less. In addition, carbon-based adhesives such as phenolic resin adhesives, novolac resin adhesives, and furfuryl alcohol resin adhesives can be used.

[0049] Specifically, first, the carbon film and the expanded graphite sheet are bonded together with a carbon-based adhesive, and the carbon-based adhesive is cured by heating it in air at a temperature of 200°C for 20 minutes. Furthermore, the application method and pressure of the carbon-based adhesive are adjusted so that the thickness of the adhesive layer is between a few micrometers and over ten micrometers.

[0050] Next, the substrate to which the expanded graphite sheet is bonded via a carbon-based adhesive layer is subjected to heat treatment at 800°C for 1 hour in a vacuum or inert gas atmosphere, thereby carbonizing the adhesive layer and forming a bonded layer. Here, the carbon-based adhesive contains a carbon-based resin that becomes non-graphitizable carbon through carbonization treatment, and the bonding layer formed by the carbonization treatment of the carbon-based adhesive has a structure comprising graphite particles and non-graphitizable carbon interposed between the graphite particles.

[0051] Furthermore, during the carbonization process, oxygen-containing gases such as water are released from the solvent of the carbon-based adhesive, so the carbon film also functions as an oxidation-preventing film for the Si film. From this perspective as well, the thickness of the carbon film needs to be at least 5 nm. Furthermore, without the carbon film, the Si film will be oxidized during the carbonization process, and delamination will occur at the interface with the oxidized Si film.

[0052] The seed crystal substrate may, for example, be one used in an atmospheric pressure MOVPE apparatus. In this case, thermal conduction through contact with the susceptor is considered sufficient for the seed crystal substrate. Here, since the absorptivity is equal to the emissivity, it is considered that the absorptivity of the seed crystal substrate is also improved by the carbon film on the back surface.

[0053] Therefore, for example, even when temperature modulation is applied to the susceptor, the temperature of the substrate surface can be controlled with good responsiveness. This makes it possible to further improve the controllability of InGaN and AlGaN layers, where the incorporation of In and Al changes with temperature.

[0054] [1.6. Specific Examples] [1.6.1. Specific Example 1] Figure 1 shows a schematic cross-sectional view of a seed crystal substrate according to the first embodiment of the present invention. The seed crystal substrate 1 is A substrate 2 having GaN species crystals on its surface, The adhesion layer 3 formed on the back surface of the substrate 2, The carbon film 5 covering the adhesion layer 3 and It is equipped with. Here, substrate 2 is either a GaN single crystal substrate or a GaN-coated sapphire substrate. Furthermore, the adhesion layer 3 is Composed of Si . Note that in Figure 1, the dimensions of each part have been enlarged or reduced compared to the actual dimensions for easier viewing. The same applies to Figures 2 through 5.

[0055] [1.6.2. Specific Example 2] Figure 2 shows a schematic cross-sectional view of a seed crystal substrate according to the second embodiment of the present invention. The seed crystal substrate 11 is A substrate 12 having GaN species crystals on its surface, An adhesion layer 13 formed on the back surface of the substrate 12, The carbon film 15 covering the adhesion layer 13 and It is equipped with. Here, the substrate 12 is either a GaN single crystal substrate or a GaN-coated sapphire substrate. Furthermore, the adhesion layer 13 is made of Si It consists of.

[0056] And the seed crystal substrate 11 is An expanded graphite sheet 16 is placed on the back side of the carbon film 15, A bonding layer 18 that joins the carbon film 15 and the expanded graphite sheet 16. It also has the following features. Here, the expanded graphite sheet 16 functions as a radiation-enhancing layer.

[0057] [2. Graphite susceptor with seed crystal substrate] The graphite susceptor with a seed crystal substrate according to the present invention has the following configuration. A graphite susceptor with a seed crystal substrate is, The seed crystal substrate according to the present invention, A graphite susceptor is placed on the back side of the seed crystal substrate, A fixed layer that fixes the seed crystal substrate to the graphite susceptor and It is equipped with.

[0058] [2.1. Seed crystal substrate] Here, the seed crystal substrate according to the present invention is, (a) A seed crystal substrate according to the first embodiment, or (b) Seed crystal substrate according to the second embodiment It refers to. As the details of the seed crystal substrate have been described above, a detailed explanation will be omitted.

[0059] [2.2. Graphite Susceptor] Graphite is a material commonly used in high-temperature applications due to its high resistance to chemicals and high temperatures. Types of graphite include isotropic graphite, which has a low coefficient of thermal expansion and is uniform in all directions, and extruded graphite, which is formed by extrusion molding. In GaN single crystal growth using the HF-VPE method, crystal growth takes place in a high-temperature range of approximately 1100°C on the substrate surface, so a graphite susceptor is used. Furthermore, it is preferable to use an isotropic graphite susceptor.

[0060] "Isotropic graphite" refers to polycrystalline graphite material produced by cold isostatic pressing (CIP). Graphite belongs to the hexagonal crystal system, and therefore its properties are anisotropic. On the other hand, isotropic graphite has the characteristic that the crystal orientation of each grain is unoriented, so there is no difference in properties depending on the cutting direction.

[0061] Here, in the HF-VPE method, the graphite susceptor is a seed crystal substrate holder for holding the seed crystal substrate, a heated material that receives energy from a heating source, and also a heat radiating material that radiates heat received from a high-temperature crucible (~1250 °C). Therefore, by fixing the seed crystal substrate to the graphite susceptor with a fixing layer described later, heat radiation of the seed crystal substrate can be performed via the graphite susceptor, and controllability of the temperature on the substrate surface can be enhanced.

[0062] Note that the graphite susceptor may be coated with a SiC film or the like. Thereby, the durability of the graphite susceptor can be further improved even in a high-temperature environment exposed to NH3 gas.

[0063] [2.3. Fixing Layer] The fixing layer is for fixing the seed crystal substrate to the graphite susceptor. The fixing layer is derived from a carbon-based adhesive generally used for bonding graphite materials, and is composed of graphite particles and a carbon layer interposed between the graphite particles. Details are the same as those of the bonding layer described above, and thus the description is omitted. Note that the fixing layer and the bonding layer may be the same or different from each other.

[0064] [2.4. Difference in Average Thermal Expansion Coefficient] The average thermal expansion coefficient of isotropic graphite (the average value of the thermal expansion coefficient (CTE) from room temperature to 1000 °C, hereinafter referred to as the average CTE) is usually 3.8 to 7.0×10 -6 K -1 to some extent depending on its manufacturing method and composition. Also, the average CTE of the sapphire substrate is 7×10 -6 K -1 , and the average CTE of the GaN single crystal substrate is 5.5×10 -6 K -1 is.

[0065] Therefore, in the absence of an expanded graphite sheet between the carbon film and the graphite susceptor, in order to prevent the seed crystal substrate from detaching from the graphite susceptor during heating, etc., the absolute value of the difference in average CTE between the graphite susceptor and the seed crystal substrate should be 0.5 × 10⁻⁶. -6 K -1 The following is required: For example, in the case of a sapphire substrate, the average CTE of the graphite susceptor is 6.5 × 10⁻⁶. -6 K -1 ~7.5×10 -6 K -1 It is necessary to do so. Also, in the case of a GaN single crystal substrate, the average CTE of the graphite susceptor should be 5.0 × 10⁻⁶. -6 K -1 ~6.0×10 -6 K -1 It is necessary to do so. Furthermore, in the case of a sapphire substrate, the average CTE of the graphite susceptor is 6.8 × 10⁻⁶. -6 K -1 ~7.3×10 -6 K -1 It is more preferable to do so. Also, in the case of a GaN single crystal substrate, the average CTE of the graphite susceptor should be 5.3 × 10⁻⁶. -6 K -1 ~5.7×10 -6 K -1 It is preferable to do so.

[0066] Here, in the seed crystal substrate of the second embodiment, the flexible expanded graphite sheet functions as a stress buffer layer, so the absolute value of the difference in average CTE between the seed crystal substrate and the graphite susceptor is not necessarily 0.5 × 10 -6 K -1 It is not necessary to do the following. However, even when an expanded graphite sheet is provided, the absolute value of the difference in average CTE is 0.5 × 10⁻⁶. -6 K -1 The following measures can further suppress the decrease in durability caused by thermal stress.

[0067] [2.5. Specific Examples] [2.5.1. Specific Example 1] Figure 3 shows a schematic cross-sectional view of a graphite susceptor with a seed crystal substrate according to the first embodiment of the present invention. The graphite susceptor 21 with a seed crystal substrate is, Seed crystal substrate 1, A graphite susceptor 23 is placed on the back side of the seed crystal substrate 1, A fixed layer 25 that fixes the seed crystal substrate 1 to the graphite susceptor 23 and It is equipped with.

[0068] [2.5.2. Specific Example 2] Figure 4 shows a schematic cross-sectional view of a graphite susceptor with a seed crystal substrate according to a second embodiment of the present invention. The graphite susceptor 31 with a seed crystal substrate is, Seed crystal substrate 11, A graphite susceptor 33 is placed on the back side of the seed crystal substrate 11, A fixed layer 35 that fixes the seed crystal substrate 11 to the graphite susceptor 33 and It is equipped with. In this case, when the seed crystal substrate 11 is used as is, the expanded graphite sheet functions as a radiation-enhancing layer, but in the graphite susceptor 31 with a seed crystal substrate, the expanded graphite sheet 16 functions as a stress buffer layer.

[0069] [3. Effect] [3.1. Seed crystal substrate] In the seed crystal substrate according to the present invention, a Si film is formed on the back surface of the substrate as an adhesion layer, and a carbon film is formed so as to cover this Si film. Here, the Si film has good adhesion to the GaN single crystal substrate and the sapphire substrate, and also has good adhesion to the carbon film. In other words, the seed crystal substrate has a carbon film with high infrared emissivity (ε=0.7~0.9) formed on its back side, which ensures sufficient heat dissipation by radiation even under low-pressure conditions. Therefore, it is possible to improve the controllability of the substrate surface temperature.

[0070] [3.2. Graphite susceptor with seed crystal substrate] The graphite susceptor with a seed crystal substrate according to the present invention is obtained by fixing the seed crystal substrate according to the present invention to a graphite susceptor disposed on the back side via a fixed layer. This allows heat to be dissipated from the substrate through direct heat conduction due to the bonding between the seed crystal substrate and the graphite susceptor. Therefore, it is possible to improve the controllability of the substrate surface temperature. [Examples]

[0071] (Examples 1-3, Comparative Examples 1-3: Seed crystal substrates) [4.1. Sample Preparation] [4.1.1. Example 1] First, a 100 nm thick Si film (EB-Si) was formed as an adhesion layer on the back surface of a GaN single crystal substrate and a GaN-coated sapphire substrate using electron beam deposition. Next, a spin-coated film was formed using photoresist (THMR-IP5700, manufactured by Tokyo Ohka Kogyo Co., Ltd.) to cover the Si film. The spin-coating speed was 5000 rpm and the duration was 30 seconds. Next, the spin-coated substrate was subjected to heat treatment at 800°C for 1 hour in a vacuum atmosphere to carbonize the spin-coated film, thereby forming a 20 nm carbon film that covers the Si film on the back surface of the substrate (see Figure 1).

[0072] [4.1.2. Example 2] First, a 50 nm thick Si film (sputter-Si) was formed as an adhesion layer on the back surface of a GaN single crystal substrate and a GaN-coated sapphire substrate using sputter deposition. Next, a spin-coated film was formed using photoresist (THMR-IP5700, manufactured by Tokyo Ohka Kogyo Co., Ltd.) to cover the Si film. The spin-coating speed was 5000 rpm and the duration was 30 seconds. Next, the spin-coated substrate was subjected to heat treatment at 800°C for 1 hour in a vacuum atmosphere to carbonize the spin-coated film, thereby forming a 20 nm carbon film that covers the Si film on the back surface of the substrate (see Figure 1).

[0073] [4.1.3. Comparative Example 1] First, a 100 nm thick Ti film (EB-Ti) was formed as an adhesion layer on the back surfaces of a GaN single crystal substrate and a GaN-coated sapphire substrate using electron beam evaporation. Next, a spin-coated film was formed using photoresist (THMR-IP5700, manufactured by Tokyo Ohka Kogyo Co., Ltd.) to cover the Ti film. The spin-coating speed was 5000 rpm and the duration was 30 seconds. Next, the spin-coated substrate was subjected to heat treatment at 800°C for 1 hour in a vacuum atmosphere to carbonize the spin-coated film, thereby forming a 20 nm carbon film that covered the Ti film on the back surface of the substrate (see Figure 1).

[0074] [4.1.4. Comparative Example 2] First, a 100 nm thick Ni film (EB-Ni) was formed as an adhesion layer on the back surface of a GaN single crystal substrate and a GaN-coated sapphire substrate using electron beam deposition. Next, a spin-coated film was formed using photoresist (THMR-IP5700, manufactured by Tokyo Ohka Kogyo Co., Ltd.) to cover the Ni film. The spin-coating speed was 5000 rpm and the duration was 30 seconds. Next, the spin-coated substrate was subjected to heat treatment at 800°C for 1 hour in a vacuum atmosphere to carbonize the spin-coated film, thereby forming a 20 nm carbon film that covered the Ni film on the back surface of the substrate (see Figure 1).

[0075] [4.1.5. Comparative Example 3] First, a 50 nm SiC film (sputter-SiC) was formed as an adhesion layer on the back surface of a GaN single crystal substrate and a GaN-coated sapphire substrate by sputter deposition. Next, a spin-coated film was formed using photoresist (THMR-IP5700, manufactured by Tokyo Ohka Kogyo Co., Ltd.) to cover the SiC film. The spin-coating speed was 5000 rpm and the duration was 30 seconds. Next, the spin-coated substrate was subjected to heat treatment at 800°C for 1 hour in a vacuum atmosphere to carbonize the spin-coated film, thereby forming a 20 nm carbon film that covers the SiC film on the back surface of the substrate (see Figure 1).

[0076] [4.1.6. Example 3] First, a 100 nm thick Si film (EB-Si) was formed as an adhesion layer on the back surface of a GaN single crystal substrate and a GaN-coated sapphire substrate using electron beam deposition. Next, a spin-coated film was formed using photoresist (THMR-IP5700, manufactured by Tokyo Ohka Kogyo Co., Ltd.) to cover the Si film. The spin-coating speed was 5000 rpm and the duration was 30 seconds. Next, the spin-coated substrate was subjected to heat treatment at 800°C for 1 hour in a vacuum atmosphere to carbonize the spin-coated film, thereby forming a 20 nm carbon film that covers the Si film on the back surface of the substrate.

[0077] Next, a carbon film and a 100 nm expanded graphite sheet (PERMA-FOIR, manufactured by Toyo Tanso Co., Ltd.) were bonded together with a phenolic resin adhesive (ST-201 adhesive, manufactured by Nisshinbo Chemical Co., Ltd.), and the phenolic resin adhesive was cured by heating it in air at 200°C for 20 minutes. Next, the substrate with the expanded graphite sheets bonded to it was subjected to heat treatment in a vacuum atmosphere at a temperature of 800°C for 1 hour to carbonize the adhesive layer and form a bonded layer (see Figure 2).

[0078] (Examples 4-6, Comparative Examples 4-6: Susceptors with seed crystal substrates) [4.2. Sample Preparation] [4.2.1. Example 4] First, a seed crystal substrate was prepared in the same manner as in Example 1. Next, the obtained seed crystal substrate and the graphite susceptor placed on the back side of the seed crystal substrate were bonded together with a phenolic resin adhesive (ST-201 adhesive, manufactured by Nisshinbo Chemical Co., Ltd.), and the phenolic resin adhesive was cured by heating in air at 200°C for 20 minutes. The graphite susceptor was made of isotropic graphite. Next, the graphite susceptor to which the seed crystal substrate was bonded was subjected to heat treatment in a vacuum atmosphere at a temperature of 800°C for 1 hour to carbonize the adhesive layer and form a fixed layer (see Figure 3).

[0079] Here, the average CTE of the graphite susceptor applied to the GaN single crystal substrate is 5.5 × 10⁻⁶. -6 K -1 The average CTE of graphite susceptors applied to GaN-coated sapphire substrates is 7.1 × 10⁻⁶. -6 K -1 That's what I decided.

[0080] [4.2.2. Example 5] First, a seed crystal substrate was prepared in the same manner as in Example 1. Next, a 100 nm expanded graphite sheet (PERMA-FOIR, manufactured by Toyo Tanso Co., Ltd.) was bonded to the surface of the graphite susceptor using a phenolic resin adhesive (ST-201 adhesive, manufactured by Nisshinbo Chemical Co., Ltd.). The phenolic resin adhesive was then cured by heating in air at 200°C for 20 minutes. The graphite susceptor was made of isotropic graphite.

[0081] Next, the seed crystal substrate and the graphite susceptor placed on the back side were bonded together via an expanded graphite sheet using a phenolic resin adhesive (ST-201 adhesive, manufactured by Nisshinbo Chemical Co., Ltd.), and the phenolic resin adhesive was cured by heating it in air at 200°C for 20 minutes. Next, the graphite susceptor with the seed crystal substrate bonded to it was subjected to heat treatment in a vacuum atmosphere at a temperature of 800°C for 1 hour to carbonize the adhesive layer and form the bonding layer and the fixed layer (see Figure 4).

[0082] Here, the average CTE of the graphite susceptor applied to the GaN single crystal substrate is 5.5 × 10⁻⁶. -6 K -1 The average CTE of graphite susceptors applied to GaN-coated sapphire substrates is 7.1 × 10⁻⁶. -6 K -1 That's what I decided. Furthermore, the expanded graphite sheet functions as a stress buffer layer.

[0083] [4.2.3. Comparative Example 4] Figure 5 shows schematic cross-sectional views of susceptors with seed crystal substrates according to Comparative Examples 4 and 5. The graphite susceptor 41 with a seed crystal substrate is, Circuit board 2 and A graphite susceptor 43 is placed on the back side of the circuit board 2, A fixing layer 45 that fixes the substrate 2 to the graphite susceptor 43 and It is equipped with. Here, substrate 2 is either a GaN single crystal substrate or a GaN-coated sapphire substrate.

[0084] First, a GaN single crystal substrate and a GaN-coated sapphire substrate were bonded to graphite susceptors placed on their respective back surfaces using a phenolic resin adhesive (ST-201 adhesive, manufactured by Nisshinbo Chemical). The phenolic resin adhesive was then cured by heating in air at 200°C for 20 minutes. The graphite susceptors were made of isotropic graphite. Next, the graphite susceptor with the substrate bonded to it was subjected to heat treatment in a vacuum atmosphere at a temperature of 800°C for 1 hour to carbonize the adhesive layer and form a fixed layer (see Figure 5).

[0085] Here, the average CTE of the graphite susceptor applied to the GaN single crystal substrate is 5.5 × 10⁻⁶. -6 K -1 The average CTE of graphite susceptors applied to GaN-coated sapphire substrates is 7.1 × 10⁻⁶. -6 K -1 That's what I decided.

[0086] [4.2.4. Comparative Example 5] First, a GaN single crystal substrate and a GaN-coated sapphire substrate were bonded to graphite susceptors placed on their respective back surfaces using a polycarbonate adhesive (STARFIRESYSTEMS, AD-478). The adhesive was then cured by heating in air at 200°C for 20 minutes. The graphite susceptors were made of isotropic graphite. Next, the graphite susceptor with the substrate bonded to it was subjected to heat treatment in a vacuum atmosphere at a temperature of 800°C for 1 hour to carbonize the adhesive layer and form a fixed layer (see Figure 5).

[0087] Here, the average CTE of the graphite susceptor applied to the GaN single crystal substrate is 5.5 × 10⁻⁶. -6 K -1The average CTE of graphite susceptors applied to GaN-coated sapphire substrates is 7.1 × 10⁻⁶. -6 K -1 That's what I decided.

[0088] [4.2.5. Comparative Example 6] First, a seed crystal substrate was prepared in the same manner as in Example 1. Next, the obtained seed crystal substrate and the graphite susceptor placed on the back side of the seed crystal substrate were bonded together with a phenolic resin adhesive (ST-201 adhesive, manufactured by Nisshinbo Chemical Co., Ltd.), and the phenolic resin adhesive was cured by heating in air at 200°C for 20 minutes. The graphite susceptor was made of isotropic graphite. Next, the graphite susceptor with the seed crystal substrate bonded to it was subjected to heat treatment in a vacuum atmosphere at a temperature of 800°C for 1 hour to carbonize the adhesive layer and form a fixed layer.

[0089] Here, the average CTE of the graphite susceptor applied to the GaN single crystal substrate is 4.8 × 10⁻⁶. -6 K -1 The average CTE of graphite susceptors applied to GaN-coated sapphire substrates is 5.5 × 10⁻⁶. -6 K -1 That's what I decided.

[0090] [4.2.6. Example 6] First, a seed crystal substrate was prepared in the same manner as in Example 1. Next, a 100 nm expanded graphite sheet (PERMA-FOIR, manufactured by Toyo Tanso Co., Ltd.) was bonded to the surface of the graphite susceptor using a phenolic resin adhesive (ST-201 adhesive, manufactured by Nisshinbo Chemical Co., Ltd.). The phenolic resin adhesive was then cured by heating in air at 200°C for 20 minutes. The graphite susceptor was made of isotropic graphite.

[0091] Next, the seed crystal substrate and the graphite susceptor placed on the back side were bonded together via an expanded graphite sheet using a phenolic resin adhesive (ST-201 adhesive, manufactured by Nisshinbo Chemical Co., Ltd.), and the phenolic resin adhesive was cured by heating it in air at 200°C for 20 minutes. Next, the graphite susceptor with the seed crystal substrate bonded to it was subjected to heat treatment in a vacuum atmosphere at a temperature of 800°C for 1 hour to carbonize the adhesive layer and form the bonding layer and the fixed layer.

[0092] Here, the average CTE of the graphite susceptor applied to the GaN single crystal substrate is 4.8 × 10⁻⁶. -6 K -1 The average CTE of graphite susceptors applied to GaN-coated sapphire substrates is 5.5 × 10⁻⁶. -6 K -1 That's what I decided. Furthermore, the expanded graphite sheet functions as a stress buffer layer.

[0093] [4.3. Test Method] [4.3.1. Adhesion Test (Examples 1 and 2, Comparative Examples 1-3)] The adhesion of the bonding layer and carbon film laminate formed on the back surface of the substrate was evaluated by tape peeling (tape test JIS H8504). If the carbon film or other material adhered to the peeled tape surface, it was judged as a failure (×), and if the carbon film or other material did not adhere to the peeled tape surface, it was judged as a good result (○).

[0094] [4.3.2. Bonding Test (Examples 3-6, Comparative Examples 4-6)] The bonding state of the bonding layer or the fixed layer was evaluated. The bonding state was evaluated by visual inspection from the substrate surface. If good bonding was obtained in 80% or more of the bonding area, it was judged as good (○). If good bonding was obtained in an area of ​​30% to less than 80% of the bonding area, it was judged as partially good (△). If good bonding was obtained in less than 30% of the bonding area, it was judged as poor (×). Furthermore, because the adhesion layer and carbon film are thin, the bonding state of the bonding layer or the fixed layer can be visually confirmed from the substrate surface.

[0095] [4.3.3. Crystal Growth Test (Examples 1-6, Comparative Example 5)] GaN single crystals were grown using the HF-VPE method, and their crystallinity was evaluated. After crystal growth, the surface morphology was observed under a microscope, and the symmetry of the morphology was used to distinguish between single crystals and polycrystalline materials. Single crystals were judged as good (○), polycrystalline materials as poor (×), and materials with partial single crystal growth were judged as partially good (△).

[0096] [4.4. Results] The results are shown in Table 1. From Table 1, the following can be seen: In all of Comparative Examples 1 to 3, it was found that the adhesion between the GaN single crystal substrate and the sapphire substrate of the adhesion layer was poor. Furthermore, the materials Ti, Ni, and SiC used in the adhesion layer in Comparative Examples 1 to 3 were chosen because they do not disappear at the GaN single crystal growth temperature (≒1100°C) and also provide good adhesion to the carbon film.

[0097] Furthermore, in the crystal growth tests in Examples 1 to 3, GaN single crystal growth was performed with the seed crystal substrate held in place within the HF-VPE apparatus by a gripping portion that held its periphery. In other words, GaN single crystal growth was performed with the seed crystal substrate alone, without using a graphite susceptor or the like.

[0098] From the results of Examples 4 and 5, although the crystal growth test results were good in both cases, it was found that the adhesion between the bonding layer and the fixed layer was better when an expanded graphite sheet was interposed as a stress buffer layer between the seed crystal substrate and the graphite susceptor. Therefore, for long-term crystal growth, it is more preferable to interpose an expanded graphite sheet as a stress buffer layer.

[0099] The results from Comparative Example 4 confirmed that the adhesion of carbon-based adhesives to GaN single-crystal substrates and sapphire substrates is poor, as described above. Furthermore, the results of Comparative Example 5 showed that it was possible to bond sapphire substrates and GaN single crystal substrates to graphite susceptors using a polycarbonate adhesive. However, as mentioned above, it was confirmed that Si-containing gas molecular species are generated during GaN single crystal growth and adhere to the GaN crystal surface as Si impurities, inhibiting the growth of high-quality GaN single crystals.

[0100] The results from Comparative Example 6 showed that matching the average CTE between the substrate and the graphite susceptor is important. If the average CTE of the substrate and the graphite susceptor is not matched sufficiently, there is a risk that the seed crystal substrate will detach from the graphite susceptor during heating.

[0101] The results from Example 6 confirmed that even if the average CTE matching between the substrate and the graphite susceptor is insufficient, the expanded graphite sheet functions as a stress buffer layer. In conjunction with the results from Example 5, it is preferable to include an expanded graphite sheet as a stress buffer layer in a graphite susceptor with a seed crystal substrate.

[0102] [Table 1]

[0103] (Example 7, Influence of adhesion layer thickness on bonding properties) [4.5. Sample Preparation] First, Si films of 0, 10, 50, 100, 200, and 500 nm were formed as adhesion layers on the back surfaces of GaN single-crystal substrates and GaN-coated sapphire substrates using electron beam deposition.

[0104] Next, a spin-coated film was formed using photoresist (THMR-IP5700, manufactured by Tokyo Ohka Kogyo Co., Ltd.) to cover the Si film. The spin-coating speed was 5000 rpm and the duration was 30 seconds. Next, the spin-coated substrate was subjected to heat treatment at 800°C for 1 hour in a vacuum atmosphere, and a 20 nm carbon film was formed that covered the Si film on the back of the substrate by carbonizing the spin-coated film.

[0105] Next, a 100 nm expanded graphite sheet (PERMA-FOIR, manufactured by Toyo Tanso Co., Ltd.) was bonded to the surface of the graphite susceptor using a phenolic resin adhesive (ST-201 adhesive, manufactured by Nisshinbo Chemical Co., Ltd.). The phenolic resin adhesive was then cured by heating in air at 200°C for 20 minutes. The graphite susceptor was made of isotropic graphite.

[0106] Next, the seed crystal substrate and the graphite susceptor placed on the back side were bonded together via an expanded graphite sheet using a phenolic resin adhesive (ST-201 adhesive, manufactured by Nisshinbo Chemical Co., Ltd.), and the phenolic resin adhesive was cured by heating it in air at 200°C for 20 minutes. Next, the graphite susceptor with the seed crystal substrate bonded to it was subjected to heat treatment in a vacuum atmosphere at a temperature of 800°C for 1 hour to carbonize the adhesive layer and form the bonding layer and the fixed layer. Then, the bonding strength of the bonding layer was visually inspected.

[0107] [4.6. Results] The results are shown in Figure 6. From Figure 6, the following can be seen: (1) When the thickness of the adhesion layer was 200 nm or less, the area with good adhesion was 50% or more. (2) When the thickness of the adhesion layer was in the range of 10 nm to 100 nm, the area with good adhesion was 80% or more. (3) In the range where the thickness of the adhesion layer is less than 10 nm, the rate of change of the area with good adhesion relative to the film thickness was steep.

[0108] Therefore, in the range where the adhesion layer thickness is less than 10 nm, even a slight change in the adhesion layer thickness may cause a significant change in the area of ​​good adhesion, making it not a suitable range for use. Furthermore, if the thickness of the adhesion layer exceeds 200 nm, the area with good adhesion becomes less than half. Therefore, it was found that the suitable range for the adhesion layer thickness is 10 nm or more and 200 nm or less.

[0109] Although embodiments of the present invention have been described in detail above, the present invention is not limited in any way to the above embodiments, and various modifications are possible without departing from the spirit of the present invention. [Industrial applicability]

[0110] The seed crystal substrate and the graphite susceptor with seed crystal substrate according to the present invention can be used when growing GaN single crystals in the vapor phase. [Explanation of Symbols]

[0111] 1. Seed crystal substrate 2. Substrate 3. Adhesion layer 5. Carbon film 21 Susceptor with seed crystal substrate 23 Graphite susceptor 25 Fixed layer

Claims

1. A substrate having GaN seed crystals on its surface, An adhesion layer formed on the back surface of the substrate, A carbon film covering the adhesion layer and Equipped with, The substrate consists of a GaN single crystal substrate or a GaN film-coated sapphire substrate. The aforementioned adhesion layer is made of Si. Seed crystal substrate.

2. The seed crystal substrate according to claim 1, wherein the adhesion layer has a thickness of 10 nm or more and 200 nm or less.

3. The seed crystal substrate according to claim 1, wherein the carbon film has a thickness of 5 nm or more and 100 nm or less.

4. An expanded graphite sheet is disposed on the back side of the carbon film, A bonding layer that joins the carbon film and the expanded graphite sheet. A seed crystal substrate according to any one of claims 1 to 3, further comprising

5. The aforementioned bonding layer is Graphite particles and, The carbon layer interposed between the graphite particles The seed crystal substrate according to claim 4, comprising:

6. A seed crystal substrate according to any one of claims 1 to 3, A graphite susceptor is disposed on the back side of the seed crystal substrate, A fixed layer that fixes the seed crystal substrate to the graphite susceptor Equipped with, The absolute value of the difference in the average thermal expansion coefficients between the substrate and the graphite susceptor is 0.5 × 10⁻⁶. －６ K －１ The following is Graphite susceptor with seed crystal substrate.

7. The aforementioned fixed layer is Graphite particles and, The carbon layer interposed between the graphite particles A graphite susceptor with a seed crystal substrate according to claim 6, comprising the features described above.

8. The seed crystal substrate according to claim 4, A graphite susceptor is disposed on the back side of the seed crystal substrate, A fixed layer that fixes the seed crystal substrate to the graphite susceptor A graphite susceptor with a seed crystal substrate.

9. The absolute value of the difference in the average thermal expansion coefficients between the substrate and the graphite susceptor is 0.5 × 10⁻⁶. －６ K －１ The graphite susceptor with seed crystal substrate according to claim 8, which is as follows:

10. The aforementioned fixed layer is Graphite particles and, The carbon layer interposed between the graphite particles A graphite susceptor with a seed crystal substrate according to claim 8, comprising the features described above.

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