Gallium nitride sintered body and method for producing same

Abstract

The present invention provides at least one of a gallium nitride sintered body which has a smoother surface than conventional gallium nitride sintered bodies and therefore enables epitaxial growth of a gallium nitride film, and a method for producing the gallium nitride sintered body. A gallium nitride sintered body 10 according to the present embodiment comprises a high-density layer 1 and a low-density layer 2 which has an area density lower than that of the high-density layer 1, wherein the area density of the high-density layer is 84.0 area% or more, and the Ga / (Ga+N) ratio, which is the atomic ratio of gallium to the total of gallium and nitrogen, is 0.55 or less.

Description

Gallium nitride sintered body and method for manufacturing the same

[0001] This disclosure relates to a gallium nitride sintered body and a method for manufacturing the same.

[0002] Gallium nitride is attracting attention as a material for realizing next-generation power devices. Conventionally, gallium nitride films were formed by epitaxial growth on a silicon substrate via an insulating buffer layer or the like. However, in this case, cracks sometimes occurred in the gallium nitride film because the thermal expansion coefficients of the silicon substrate and the gallium nitride film were different. For this reason, in recent years, QST™ substrates have been used instead of silicon substrates (see, for example, Non-Patent Document 1 below). Since the thermal expansion coefficient of QST™ substrates is equivalent to that of gallium nitride films, gallium nitride films formed on QST™ substrates are less likely to crack.

[0003] Furthermore, Patent Document 1 discloses a Group III nitride-based epitaxial growth substrate used for the epitaxial growth of gallium nitride films, comprising: a support substrate having a structure in which a core made of nitride ceramics is encased in a sealing layer with a thickness of 0.05 μm to 1.5 μm; a planarization layer provided on the upper surface of the support substrate and having a thickness of 0.5 μm to 3.0 μm; and a single crystal seed crystal layer provided on the upper surface of the planarization layer and having a thickness of 0.04 μm to less than 0.1 μm.

[0004] However, since the substrates described above are composed of compounds with a different composition from gallium nitride, there is a risk that impurities originating from the substrate may be introduced into the gallium nitride film, for example, by diffusing yttrium additives contained in the aluminum nitride core layer into the gallium nitride film. Therefore, from the viewpoint of suppressing the introduction of impurities into the gallium nitride film, it is preferable to use a gallium nitride substrate instead of the above-mentioned substrates.

[0005] As a gallium nitride substrate, it is known to use single-crystalline gallium nitride. However, since it is difficult to manufacture a large-sized single-crystalline gallium nitride bulk body without defects by the Na flux method or the HVPE method, it is difficult to increase the size. Furthermore, since both methods require time for the crystal growth of gallium nitride, the manufacturing takes time. Therefore, when using a gallium nitride substrate, from the viewpoints of increasing the size and mass productivity, it is preferable to use a gallium nitride sintered body as the gallium nitride substrate.

[0006] Patent Document 2 discloses a high-density sintered body suitable as a sputtering target as a gallium nitride sintered body.

[0007] International Publication No. 2023 / 127249, International Publication No. 2023 / 243566

[0008] J. h. Leach et al., "Towards Manufacturing Large Area GaN Substrates from QST (registered trademark) Seeds", [online], May 7 - 10, 2018, Proceedings Compd. Semicond. Manuf. Technol, [searched on November 11, 2024], Internet <https: / / csmantech.org / digests / ?digest=2018&thesession=4320>

[0009] However, the single-layer gallium nitride sintered body disclosed in Patent Document 2 does not have sufficient smoothness on its surface, and when applied as a substrate for epitaxial growth of a gallium nitride film, a gallium nitride film with a sufficiently smooth surface could not be obtained.

[0010] An object of the present disclosure is to provide at least one of a gallium nitride sintered body having a smoother surface compared to a conventional gallium nitride sintered body, thereby enabling epitaxial growth of a gallium nitride film, and a manufacturing method thereof.

[0011] This disclosure focuses on the relationship between the structure and density of gallium nitride sintered bodies. As a result, it was found that the surface smoothness of the gallium nitride sintered body can be improved by providing a structure in which gallium nitride layers of different densities are stacked, and furthermore, that such a gallium nitride sintered body is suitable for the epitaxial growth of gallium nitride films. In other words, the present invention is as defined in the claims, and the gist of this disclosure is as follows.

[0012] [1] A gallium nitride sintered body having a high-density layer and a low-density layer, wherein the area density of the high-density layer is 84.0 area% or more, and the Ga / (Ga+N) ratio, which is the atomic ratio of gallium to the sum of gallium and nitrogen, is 0.55 or less. [2] The gallium nitride sintered body according to [1], wherein the area density of the low-density layer is 65.0 area% or more. [3] The gallium nitride sintered body according to [1] or [2], wherein the average surface roughness of the high-density layer is 2.50 μm or less. [4] The gallium nitride sintered body according to any one of [1] to [3], wherein the Ga / (Ga+N) ratio is 0.50 or less. [5] The gallium nitride sintered body according to any one of [1] to [4], which is plate-shaped. [6] The gallium nitride sintered body according to [5], wherein the high-density layer forms one main surface and the low-density layer forms the other main surface. [7] A gallium nitride sintered body according to any one of [1] to [6] above, wherein the thickness of the gallium nitride sintered body is 0.50 mm or more and 3.0 mm or less. [8] A gallium nitride sintered body according to any one of [1] to [7] above, wherein the thickness of the high-density layer is 0.05 mm or more and 1.0 mm or less. [9] A gallium nitride sintered body according to any one of [1] to [8] above, wherein the thickness of the low-density layer is 0.2 mm or more and 2.9 mm or less.

[10] A gallium nitride sintered body according to any one of [1] to [9] above, which is a substrate for epitaxial growth of a gallium nitride film.

[11] A method for manufacturing a gallium nitride sintered body, comprising: providing a first powder containing gallium nitride and metallic gallium, wherein the Ga / (Ga+N) ratio, which is the atomic ratio of gallium to the total of gallium and nitrogen, is greater than 0.59 and less than or equal to 0.65; and providing a second powder containing gallium nitride and metallic gallium, wherein the Ga / (Ga+N) ratio is greater than or equal to 0.50 and less than or equal to 0.59; stacking a first powder layer composed of the first powder and a second powder layer composed of the second powder to obtain a molded body; and sintering the molded body in a nitriding atmosphere.

[12] The method according to

[11] above, satisfying the following relationship: 0.01 ≤ (Ga / (Ga + N) ratio of the first powder) - (Ga / (Ga + N) ratio of the second powder) ≤ 0.15

[13] The method according to

[11] or

[12] above, including polishing the surface formed from the first powder after sintering the formed body in a nitrogen atmosphere.

[14] The method according to any one of

[11] to

[13] above, for manufacturing the gallium nitride sintered body according to any one of [1] to

[10] above.

[0013] According to the present disclosure, it is possible to provide at least one of a gallium nitride sintered body having a smooth surface as compared with a conventional gallium nitride sintered body, thereby enabling epitaxial growth of a gallium nitride film, and a method for manufacturing the same.

[0014] FIG. 1 is a conceptual diagram showing a high-density layer and a low-density layer of the gallium nitride sintered body of the present embodiment. FIG. 2 is a SEM observation view (magnification: 500 times) of the high-density layer (FIG. 1(a)) and the low-density layer (FIG. 1(b)) of Example 1.

[0015] An example of an embodiment of the present disclosure will be described in detail. However, this embodiment is not limited to the following embodiments. Further, this embodiment includes any combination of each configuration and parameter disclosed in this specification, and also includes a range of any combination of the upper limit and the lower limit of the values disclosed in this specification.

[0016] <<Gallium Nitride Sintered Body>> The gallium nitride sintered body of the present embodiment has a high-density layer and a low-density layer, the area density of the high-density layer is 84.0 area% or more, and the Ga / (Ga + N) ratio, which is the atomic ratio of gallium to the total of gallium and nitrogen, is 0.55 or less.

[0017] This embodiment relates to a gallium nitride (GaN) sintered body. The gallium nitride (GaN) sintered body is a sintered body whose main component (matrix, host phase) is gallium nitride, and may be a polycrystalline gallium nitride, or a sintered body mainly composed of gallium nitride. In this embodiment, having gallium nitride as the main component means that the mass ratio of gallium and nitrogen in the gallium nitride sintered body is 95% (95 mass%) or more. Preferably, the mass ratio of gallium and nitrogen in the gallium nitride sintered body is 98 mass% or more, more preferably 99 mass% or more, and may also be 100 mass% or less, or less than 100 mass%. The mass ratio of gallium and nitrogen in the gallium nitride sintered body may be the mass ratio calculated from formula (1) described later. The gallium nitride sintered body of this embodiment may contain components other than gallium nitride, such as metallic gallium.

[0018] Specifically, for example, as shown in Figure 1, the gallium nitride sintered body 10 of this embodiment may be in the form of a plate having a high-density layer 1 and a low-density layer 2.

[0019] In the gallium nitride sintered body of this embodiment, the area density of the high-density layer is sufficiently high that when the surface of the high-density layer is polished, there are no large voids on the polished surface that would affect its smoothness. Therefore, it is possible to make the surface smoother compared to conventional gallium nitride sintered bodies. Such a smooth surface is preferable for applying the gallium nitride sintered body of this embodiment to a substrate for epitaxial growth of gallium nitride. Furthermore, in the gallium nitride sintered body of this embodiment, by having a high-density layer and a low-density layer, the low-density layer can function as a support layer for the high-density layer. In addition, compared to a gallium nitride sintered body in which the entire body is sintered as a high-density layer, the gallium nitride sintered body of this embodiment having a high-density layer and low-density layer allows for more uniform nitriding of the high-density layer, resulting in a higher density high-density layer and a smoother surface, i.e., a polished surface. Furthermore, by using the low-density layer as a support, the amount of Ga used can be reduced, thereby reducing raw material costs.

[0020] Furthermore, the presence of a high-density layer and a low-density layer (i.e., a layer with a smaller area density than the high-density layer) in the gallium nitride sintered body of this embodiment can be confirmed by scanning electron microscope (SEM) observation of the cross-section of the gallium nitride sintered body. Specifically, by observing with SEM a region of 15±5% from one surface (hereinafter also referred to as the "first region") in the direction from one surface toward the opposing surface (hereinafter also referred to as the "thickness direction") and a region of -15±5% from the opposing surface in the thickness direction (hereinafter also referred to as the "second region"), it can be confirmed that the first region and the second region have different area densities, thereby confirming the presence of a high-density layer and a low-density layer.

[0021] Here, SEM observation can be performed using a general scanning electron microscope (e.g., VE-9800, manufactured by KEYENCE) under the following conditions: Acceleration voltage: 10 kV Magnification: 500x or more, preferably 1000x ± 500x

[0022] As an example, Figures 2(a) and 2(b) show SEM observations of the high-density and low-density layers of Example 1, respectively. As shown in Figure 2, the first region has a dense microstructure (Figure 2(a)), and the second region has a sparse microstructure (Figure 2(b)). From this, it can be confirmed that the gallium nitride sintered body of Example 1 has two layers of different densities, namely a high-density layer and a low-density layer.

[0023] (Area density of the high-density layer) In the gallium nitride sintered body of this embodiment, the area density of the high-density layer may be 84.0 area% or more, 84.5 area% or more, 85.0 area% or more, 85.5 area% or more, or 86.0 area% or more, and may also be less than 95.0 area%, 94.0 area% or less, 93.0 area% or less, 92.0 area% or less, 91.0 area% or less, or 90.0 area% or less. Furthermore, the area density of the high-density layer may be 84.0 area% or more but less than 95.0 area%, 84.0 area% or more but 94.0 area%, 84.5 area%, or more but 93.0 area%, 85.0 area%, or more but less than 95.0 area%, 85.0 area%, or more but 94.0 area%, 85.0 area%, or more but 93.0 area%, 85.0 area%, or more but 92.0 area%, 85.5 area%, or more but 91.0 area%, or 86.0 area%, or more but 90.0 area%, or less.

[0024] The area density of the gallium nitride sintered body of this embodiment can be measured as follows.

[0025] Prior to measurement, the gallium nitride sintered body is processed and polished as a pretreatment. Specifically, a cross-section is obtained by cutting the gallium nitride sintered body, and this cross-section is polished with sandpaper (in the order of #600, #1000, and #1500). Then, the measurement surface is obtained by polishing it at room temperature under the following conditions using a general polishing device (e.g., LaboForce-100, manufactured by Strürs): Rotation speed: 300 rpm Polishing compound: POLIPLA304M (manufactured by Fujimi Incorporated) Processing time: 10 minutes

[0026] The area density is determined by SEM observation of the measurement surface obtained by pretreatment, specifically by observing the high-density layer and the low-density layer regions. For example, the region 15±5% from one surface in the thickness direction of the gallium nitride sintered body, and the region -15±5% from the opposite surface in the thickness direction, are observed using SEM to obtain an SEM observation map, which is then analyzed. The observation is performed using an SEM (e.g., VE-9800, manufactured by KEYENCE) under the following conditions: Acceleration voltage: 10kV Magnification: 500x Field of view: 3 fields

[0027] Next, using general image analysis software (for example, Image-Pro 10, manufactured by Hakuto Co., Ltd.), the obtained SEM observation image is binarized, and vacancies are detected by distinguishing between non-vacancies (i.e., crystal particles constituting the gallium nitride sintered body) and vacancies, and their area [μm²] is determined. ２ The following formula is used to determine the area [μm²] of the SEM observation image. For binarization, the SEM observation image is imported into image analysis software as a grayscale image with 256 levels of black and white, and discriminant analysis is used. After detecting voids, the observation area [μm²] of the SEM observation image is calculated using the following formula. ２ Area of ​​the region excluding the voids [μm²] ２ From this, calculate the area density [area %]: Area density [area %] = {(area of ​​the part excluding voids) / (observed area of ​​the SEM observation map)} × 100 Also, calculate the area density in three fields of view and use the average of these values.

[0028] (Degree of orientation of the surface of the high-density layer) In the gallium nitride sintered body of this embodiment, the degree of orientation of the surface of the high-density layer (hereinafter also simply referred to as "degree of orientation") is preferably 20% or more, 25% or more, or 30% or more. When the degree of orientation is of the above value, the degree of orientation of the seed crystal layer tends to be high when a seed crystal layer is placed on the surface of the high-density layer. A higher degree of orientation is preferable, but upper limits include 50% or less, 40% or less, or 35% or less. The degree of orientation may also be 20% or more and 50% or less, 25% or more and 40% or less, or 30% or more and 35% or less.

[0029] Here, the degree of orientation can be determined by performing X-ray diffraction (hereinafter also referred to as "XRD") measurements on the surface of the gallium nitride sintered body and obtaining the diffraction peaks corresponding to the 002 and 004 planes of the resulting XRD pattern.

[0030] Prior to measuring the degree of orientation, the gallium nitride sintered body is polished as a pretreatment. Specifically, the surface of the high-density layer of the gallium nitride sintered body is polished using sandpaper (in the order of #600, #1000, and #1500), and then this surface is polished using a general polishing device (for example, LaboForce-100, manufactured by Strürs) to obtain the measurement surface. The polishing conditions may be the same as those for measuring the area density.

[0031] XRD measurements can be performed using a standard X-ray diffractometer (e.g., RINT Ultima III, manufactured by Rigaku Corporation) under the following conditions: Source: CuKα rays (λ = 1.5405 Å) Measurement mode: Continuous scan Step width: 0.02° Scan speed: 2° / min Measurement range: 2θ = 20° to 80°

[0032] From the diffraction peaks corresponding to the 002 and 004 planes, which have peak tops at 2θ = 34.56 ± 0.08° and 72.90 ± 0.08° in the obtained XRD pattern, the Rottgering factor F(002,004) can be determined using the following formula, and this can be used as the degree of orientation: F(002,004) = (ρ - ρ ０ ) / (1-ρ) ０ ) Here, ρ and ρ ０ The following is true: ρ: The ratio of the peak heights of diffraction peaks with peak tops at 2θ = 34.56 ± 0.08° and 72.90 ± 0.08° to the total peak height of diffraction peaks in the XRD pattern of the gallium nitride sintered body. ０ The ratio of the peak heights of diffraction peaks with peak tops at 2θ = 34.56 ± 0.08° and 72.90 ± 0.08° to the total peak height of diffraction peaks in the XRD pattern of gallium nitride shown in PDF card No. 01-073-7289.

[0033] (Composition) Furthermore, according to the gallium nitride sintered body of this embodiment, the Ga / (Ga+N) ratio is 0.55 or less, that is, the degree of nitriding of gallium is high, so when applied to a substrate for epitaxial growth of gallium nitride, it can have a coefficient of thermal expansion similar to that of gallium nitride that is epitaxially grown.

[0034] The Ga / (Ga+N) ratio of the gallium nitride sintered body in this embodiment is preferably 0.50 or less. With such a Ga / (Ga+N) ratio, the gallium nitride sintered body in this embodiment is substantially composed of gallium nitride only.

[0035] The Ga / (Ga+N) ratio of the gallium nitride sintered body in this embodiment may be 0.55 or less, 0.53 or less, 0.50 or less, less than 0.50, or 0.49 or less, and may also be 0.45 or more, 0.46 or more, or 0.47 or more. Furthermore, this Ga / (Ga+N) ratio may be 0.45 or more and 0.53 or less, 0.45 or more and 0.50 or less, 0.46 or more and less than 0.50, or 0.47 or more and 0.49 or less.

[0036] When the gallium nitride sintered body of this embodiment is applied to a substrate for epitaxial growth of gallium nitride, it is preferable that the gallium nitride sintered body of this embodiment contains a low amount of elements other than nitrogen and gallium in order to prevent contamination of the gallium nitride with impurities to be epitaxially grown. Examples of impurities contained in the gallium nitride sintered body of this embodiment include oxygen. Furthermore, the gallium nitride sintered body of this embodiment may contain metallic impurities as long as its effectiveness is not impaired. Examples of metallic impurities include at least one of aluminum (Al) and indium (In). Metallic impurities may be contained as metals or as metallic compounds. It is preferable that the metallic impurities are substantially absent. Therefore, the content of metallic impurities may be 50 ppm by mass or less, 10 ppm by mass or less, or 5 ppm by mass or less as the mass ratio of metallic impurities [mass ppm] of the total mass of elements determined by glow discharge mass spectrometry to the total mass of elements determined by glow discharge mass spectrometry. The lower limit of the metal impurity content is 0 ppm or more by mass, greater than 0 ppm by mass, or 1 ppm or more by mass. Furthermore, the metal impurity content is 0 ppm or more and 50 ppm or less by mass, or greater than 0 ppm and 10 ppm or less by mass.

[0037] The gallium nitride sintered body of this embodiment may have an oxygen content of 5.00 atm% or less, 4.00 atm% or less, 3.00 atm% or less, 2.00 atm% or less, 1.50 atm% or less, 1.00 atm% or less, 0.50 atm% or less, 0.40 atm% or less, or 0.30 atm% or less, and may also be 0.01 atm% or more, 0.02 atm% or more, 0.05 atm% or more, 0.10 atm% or more, 0.15 atm% or more, 0.18 atm% or more, or 0.20 atm% or more. Further, the oxygen content may be 0.01 atm% or more and 5.00 atm% or less, 0.01 atm% or more and 4.00 atm% or less, 0.02 atm% or more and 3.00 atm% or less, 0.02 atm% or more and 2.00 atm% or less, 0.05 atm% or more and 1.50 atm% or less, 0.10 atm% or more and 1.00 atm% or less, 0.15 atm% or more and 0.50 atm% or less, 0.18 atm% or more and 0.40 atm% or less, or 0.20 atm% or more and 0.30 atm% or less.

[0038] The composition of the gallium nitride sintered body of this embodiment can be substantially represented by the following formula (1): 100 [mass%] = W Ｇａ [mass%] + W Ｏ [mass%] + W Ｎ [mass%]... (1) Here, W Ｇａ 、W Ｏ and W Ｎ are the mass ratios of gallium, oxygen, and nitrogen in the gallium nitride sintered body, respectively.

[0039] W Ｏ and W Ｎ are values measured by a thermal decomposition method (inert gas fusion - infrared absorption method) in which the gallium nitride sintered body is thermally decomposed using a general oxygen and nitrogen analyzer (for example, LECO ON736, manufactured by LECO Corporation). W Ｇａ is a value obtained from the measured values of W Ｏ and W Ｎ according to formula (1).

[0040] If metallic impurities are present, their content is expressed as the mass ratio [mass ppm] of the metallic element to the total mass of the elements determined by glow discharge mass spectrometry. Due to differences in measurement methods, if the gallium nitride sintered body of this embodiment contains metallic impurities, its composition may appear to exceed 100% by mass.

[0041] Furthermore, the oxygen content [atm%] is measured according to the method in accordance with JIS H 1695 and is a value obtained from the following formula (2): Oxygen content [atm%] = (W Ｏ / M Ｏ ) / {(W Ｇａ / M Ｇａ ) + (W Ｎ / M Ｎ ) + (W Ｏ / M Ｏ )} × 100 … (2) Here, M Ｏ The atomic weight of oxygen is 16.00 [g / mol], M Ｇａ The atomic weight of gallium is 69.72 [g / mol], and M Ｎ The atomic weight of nitrogen is 14.01 [g / mol].

[0042] The Ga / (Ga+N) ratio is a value that can be obtained from the following equation (3): Ga / (Ga+N) ratio = (W Ｇａ / M Ｇａ ) / {(W Ｇａ / M Ｇａ ) + (W Ｎ / M Ｎ )} ... (3)

[0043] (Area density of the low-density layer) In the gallium nitride sintered body of this embodiment, the area density of the low-density layer may be 65.0 area% or more.

[0044] In the gallium nitride sintered body of this embodiment, the strength of the low-density layer tends to be higher because the area density of the low-density layer is 65.0 area% or more. Furthermore, this reduces the difference in thermal expansion coefficients between the low-density layer and the high-density layer, making it easier to suppress distortion of the shape of the gallium nitride sintered body due to the difference in thermal expansion coefficients, such as warping of the gallium nitride sintered body.

[0045] For example, in the gallium nitride sintered body of this embodiment, the area density of the low-density layer may be 65.0 area% or more, 70.0 area% or more, or 75.0 area% or more, and may also be less than 84.0 area%, 83.0 area% or less, or 82.0 area% or less. Furthermore, the area density of the low-density layer may be 65.0 area% or more and 84.0 area% or less, 70.0 area% or more and 83.0 area% or less, or 75.0 area% or more and 82.0 area% or less.

[0046] (Average surface roughness of the high-density layer) In the gallium nitride sintered body of this embodiment, the average surface roughness of the high-density layer (hereinafter also referred to as "average surface roughness") may be 2.50 μm or less. The surface of the high-density layer of the gallium nitride sintered body of this embodiment may be polished.

[0047] The gallium nitride sintered body of this embodiment has a low average surface roughness, making it suitable for use as a substrate for epitaxial growth of gallium nitride.

[0048] For example, in the gallium nitride sintered body of this embodiment, the average surface roughness may be 2.50 μm or less, 2.00 μm or less, 1.80 μm or less, 1.50 μm or less, 1.20 μm or less, or 1.00 μm or less, and may also be 0.10 μm or more, 0.20 μm or more, 0.30 μm or more, 0.40 μm or more, or 0.50 μm or more. Furthermore, the average surface roughness may be 0.10 μm or more and 2.50 μm or less, 0.20 μm or more and 2.00 μm or less, 0.20 μm or more and 1.80 μm or less, 0.30 μm or more and 1.80 μm or less, 0.30 μm or more and 1.50 μm or less, 0.40 μm or more and 1.80 μm or less, 0.40 μm or more and 1.20 μm or less, 0.50 μm or more and 1.80 μm or less, or 0.50 μm or more and 1.00 μm or less.

[0049] The average surface roughness is measured using a general-purpose three-dimensional shape measuring machine (for example, the VR-6000 one-shot 3D shape measuring machine, manufactured by KEYENCE). Specifically, prior to measurement, the gallium nitride sintered body is subjected to the same pretreatment as for the degree of orientation to obtain the measurement surface. Using an observation map of the measurement surface at a magnification of 160x, the surface roughness is determined using analysis software (for example, the analysis software included with the three-dimensional shape measuring machine), with the reference plane set to the least-squares plane. The average surface roughness is calculated by determining the surface roughness in three fields of view and taking the average of these values.

[0050] (Average crystal grain size) In the gallium nitride sintered body of this embodiment, the average crystal grain size of the high-density layer may be 0.1 μm or more, 0.2 μm or more, or 0.3 μm or more, and may also be 200 μm or less, 100 μm or less, or 50 μm or less. Furthermore, the average crystal grain size of the high-density layer may be 0.1 μm or more and 200 μm or less, or 0.2 μm or more and 100 μm or less.

[0051] In the gallium nitride sintered body of this embodiment, the average grain size of the low-density layer may be 0.1 μm or more, 0.2 μm or more, or 0.3 μm or more, and may also be 200 μm or less, 100 μm or less, or 50 μm or less. Furthermore, the average grain size of the low-density layer may be 0.1 μm or more and 200 μm or less, or 0.2 μm or more and 100 μm or less.

[0052] Regarding the gallium nitride sintered body of this embodiment, the crystal grain size and average crystal grain size are determined by scanning electron microscopy-electron backscatter diffraction (hereinafter also referred to as "SEM-EBSD") of the cross-section of the gallium nitride sintered body. SEM-EBSD observation can be performed using a general EBSD (e.g., Symmetry, Oxford Instruments) to obtain an SEM observation map in the same manner as the area density observation described above. The conditions for EBSD observation are shown below. Prior to measurement, the measurement surface is obtained by pretreatment in the same manner as for area density. Acceleration voltage: 15 kV Sample tilt: 70 degrees Tilt correction: 0° Magnification: 500x Step size: 0.2 μm

[0053] A position where the crystal orientation is tilted by 5° or more is considered a grain boundary, and the region enclosed by such grain boundaries is considered a crystal grain. The diameter of the circle corresponding to the area of ​​each crystal grain is calculated and is defined as the crystal grain size. SEM-EBSD observation is performed on the crystal grains observed in three fields of view, and the average crystal grain size in each field of view is calculated by image analysis. The average of these averages is then defined as the average crystal grain size.

[0054] (Shape) The gallium nitride sintered body of this embodiment only needs to have a shape that can be used as an epitaxial growth substrate, and may be plate-shaped, particularly disc-shaped. Furthermore, if the gallium nitride sintered body of this embodiment is plate-shaped, a high-density layer may form one main surface and a low-density layer may form the other main surface.

[0055] The gallium nitride sintered body of this embodiment, being plate-shaped, and especially disc-shaped, can be handled in the same way as silicon wafers and the like used in existing semiconductor manufacturing processes. Specifically, for example, the aspect ratio of the gallium nitride sintered body of this embodiment, i.e., the ratio of the diameter in the plane direction to the thickness, may be 25 or more, 50 or more, 100 or more, 200 or more, or 300 or more, and may also be 800 or less, 700 or less, 600 or less, 500 or less, or 400 or less. Furthermore, this aspect ratio may be 25 to 800, 50 to 700, 100 to 600, 200 to 500, or 300 to 400. In this embodiment, "diameter in the plane direction" means the diameter of a circle equivalent to the plane direction of the gallium nitride sintered body, i.e., the diameter of a circle having an area equal to the plane direction area of ​​the gallium nitride sintered body.

[0056] (Thickness) The thickness of the gallium nitride sintered body in this embodiment may be 0.5 mm or more and 3.0 mm or less.

[0057] For example, the thickness of the gallium nitride sintered body in this embodiment is preferably the thickness suitable for use as an epitaxial growth substrate, and may be 0.5 mm or more, 0.6 mm or more, 0.7 mm or more, 0.8 mm or more, or 0.9 mm or more, and may also be 3.0 mm or less, 2.8 mm or less, 2.5 mm or less, 2.0 mm or less, 1.5 mm or less, or 1.3 mm or less. Furthermore, this thickness may be 0.5 mm or more and 2.8 mm or less, 0.6 mm or more and 2.5 mm or less, 0.7 mm or more and 2.0 mm or less, 0.8 mm or more and 1.5 mm or less, or 0.9 mm or more and 1.3 mm or less.

[0058] Here, the thickness of the gallium nitride sintered body is measured using a micrometer. For example, the thickness of the gallium nitride sintered body is determined by measuring four equal points on the outer circumference of the gallium nitride sintered body, and the center of gravity of the gallium nitride sintered body as the measurement point. The thickness of each measurement point is determined, and the average of these five points is calculated.

[0059] The thickness of the high-density layer in the gallium nitride sintered body of this embodiment (hereinafter also referred to as "thickness of the high-density layer") may be 0.05 mm or more and 1.0 mm or less. The thickness of the high-density layer is determined by performing image analysis on an SEM observation map obtained under the same pretreatment and measurement conditions as for the area density using general image analysis software (for example, Image-Pro 10, manufactured by Hakuto Co., Ltd.).

[0060] For example, the thickness of this high-density layer may be 0.05 mm or more, 0.1 mm or more, or 0.2 mm or more, and may also be 1.0 mm or less, 0.7 mm or less, or 0.5 mm or less. Furthermore, the thickness of the high-density layer may be 0.05 mm or more and 1.0 mm or less, 0.1 mm or more and 0.7 mm or less, or 0.2 mm or more and 0.5 mm or less.

[0061] The thickness of the low-density layer in the gallium nitride sintered body of this embodiment (hereinafter also referred to as "thickness of the low-density layer") may be 0.2 mm or more and 2.9 mm or less. The thickness of the low-density layer can be determined in the same manner as the thickness of the high-density layer.

[0062] For example, the thickness of this low-density layer may be 0.2 mm or more, 0.3 mm or more, 0.4 mm or more, or 0.5 mm or more, and may also be 2.9 mm or less, 2.5 mm or less, 2.0 mm or less, 1.5 mm or less, or 1.0 mm or less. Furthermore, the thickness of the low-density layer may be 0.2 mm or more and 2.9 mm or less, 0.3 mm or more and 2.5 mm or less, 0.4 mm or more and 2.0 mm or less, 0.4 mm or more and 1.5 mm or less, or 0.5 mm or more and 1.0 mm or less.

[0063] The low-density layer may be thicker or thinner than the high-density layer, but in order for the low-density layer to function as a support layer for the high-density layer, it is preferable that the low-density layer be thicker than the high-density layer.

[0064] For example, the thickness of the low-density layer may be more than 1.0 times, 1.1 times or more, 1.5 times or more, or 2.0 times or more than the thickness of the high-density layer, and may also be 5.0 times or less, 4.0 times or less, 3.0 times or less, or 2.5 times or less. Furthermore, the thickness of the low-density layer may be more than 1.0 times and 5.0 times or less, 1.1 times or more and 4.0 times or less, 1.5 times or more and 3.0 times or less, or 2.0 times or more and 2.5 times or less than the thickness of the high-density layer.

[0065] The thickness of the high-density layer does not need to be unnecessarily thicker than the thickness of the gallium nitride sintered body. In addition to being within the above-mentioned thickness range, the thickness of the high-density layer may be 0.02 times or more, 0.05 times or more, 0.1 times or more, 0.15 times or more, or 0.2 times or more of the thickness of the gallium nitride sintered body, or it may be 0.5 times or less, 0.45 times or less, or 0.4 times or less. Furthermore, the thickness of the high-density layer may be 0.02 times or more and 0.5 times or less, 0.05 times or more and 0.45 times or less, or 0.1 times or more and 0.4 times or less of the thickness of the gallium nitride sintered body.

[0066] (Bulk density) The bulk density of the gallium nitride sintered body of this embodiment is 4.5 g / cm³. ３ The above is sufficient. A high bulk density of the gallium nitride sintered body means that there are few voids contained in the gallium nitride sintered body. This bulk density is 4.5 g / cm³. ３ Above, 4.6g / cm ３ The above, or 4.7 g / cm³ ３The above is sufficient, and also 6.0 g / cm³. ３ Below, 5.8g / cm ３ Below, 5.6g / cm ３ The following, or 5.4 g / cm³ ３ The following is acceptable. Furthermore, this bulk density is 4.5 g / cm³. ３ 6.0g / cm or more ３ Below, 4.6g / cm ３ 5.8g / cm or more ３ Below, 4.7g / cm ３ 5.6g / cm or more ３ The following, or 4.7 g / cm³ ３ 5.4g / cm or more ３ The following is acceptable. The true density of gallium nitride is approximately 6.15 g / cm³. ３ That is the case.

[0067] The bulk density of the gallium nitride sintered body in this embodiment can be measured by a method in accordance with JIS R 1634. Here, the pretreatment for bulk density measurement can be performed using a vacuum method with distilled water.

[0068] (Lamination) The high-density layer and the low-density layer may each consist of a single layer or multiple layers. If the high-density layer or the low-density layer consists of multiple layers, the multiple layers constituting the high-density layer or the low-density layer as a whole may satisfy the requirements of the high-density layer or the low-density layer described above.

[0069] (Applications) As described above, the gallium nitride sintered body of this embodiment is preferred for use as a substrate for epitaxial growth of gallium nitride. Therefore, the gallium nitride sintered body of this embodiment may be a substrate for epitaxial growth of gallium nitride films.

[0070] When the gallium nitride sintered body of this embodiment is used as a substrate for epitaxial growth of a gallium nitride film, the gallium nitride sintered body of this embodiment may have a seed crystal layer (orientation layer) on the surface of the high-density layer. The seed crystal layer may be one or more layers selected from the group consisting of Si<111>, SiC, sapphire, aluminum nitride, aluminum gallium nitride, and gallium nitride. With these seed crystal layers, there is little risk of impurity elements diffusing into the gallium nitride film. The thickness of the seed crystal layer may be 0.04 μm or more, or less than 0.10 μm. For details of the seed crystal layer, please refer to the description in Patent Document 1.

[0071] (Manufacturing Method) The gallium nitride sintered body of this embodiment can be manufactured by any method, for example, by the method of this embodiment shown below.

[0072] <<Method for Manufacturing Gallium Nitride Sintered Body>> The method of this embodiment for manufacturing a gallium nitride sintered body comprises the following steps: providing a first powder containing gallium nitride and metallic gallium, and having a Ga / (Ga+N) ratio, which is the atomic ratio of gallium to the total of gallium and nitrogen, greater than 0.59 and less than or equal to 0.65; and a second powder containing gallium nitride and metallic gallium, and having the above Ga / (Ga+N) ratio of 0.50 or more and less than or equal to 0.59; stacking a first powder layer composed of the first powder and a second powder layer composed of the second powder to obtain a molded body; and sintering the molded body in a nitriding atmosphere.

[0073] According to the method of this embodiment, by forming a molded body from powder containing gallium nitride and metallic gallium, metallic gallium is interposed between the gallium nitride powder particles, which is thought to increase the bonding strength between the powder particles. In other words, the higher the metallic gallium content, the higher the density of the molded body. Therefore, according to the method of this embodiment for producing a gallium nitride sintered body, a high-density layer can be formed from a first powder layer with a large Ga / (Ga+N) ratio, i.e., a high metallic gallium content, and a low-density layer can be formed from a second powder layer with a small Ga / (Ga+N) ratio, i.e., a low metallic gallium content.

[0074] Furthermore, according to the method of this embodiment, the atmospheric gas such as ammonia constituting the nitriding atmosphere nitrides the gallium contained in the first powder layer for forming the high-density layer from the surface side of the high-density layer. At the same time, nitriding also occurs from the low-density layer side of the high-density layer through the voids in the low-density layer. Therefore, according to the method of this embodiment, it is possible to manufacture a gallium nitride sintered body that is sufficiently nitrided, in particular the gallium nitride sintered body of this embodiment, even though it has a high-density layer that is difficult to nitride overall.

[0075] <Steps for Providing the First and Second Powders> The method of this embodiment includes providing a first powder containing gallium nitride and metallic gallium, wherein the Ga / (Ga+N) ratio, which is the atomic ratio of gallium to the total of gallium and nitrogen, is greater than 0.59 and less than or equal to 0.65, and a second powder containing gallium nitride and metallic gallium, wherein the Ga / (Ga+N) ratio is greater than or equal to 0.50 and less than or equal to 0.59 (hereinafter also referred to as the "steps for providing the first and second powders").

[0076] (Average particle size) The average particle size of the first powder and the second powder in this embodiment is 0.1 μm or more or 0.2 μm or more, and can also be 150 μm or less, 50 μm or less, 20 μm or less, 15 μm or less, or 10 μm or less. Preferred average particle sizes include 0.1 μm or more and 150 μm or less, 0.2 μm or more and 50 μm or less, 0.2 μm or more and 20 μm or less, or 0.2 μm or more and 10 μm or less.

[0077] In this embodiment, the average particle diameter is determined from the area of ​​primary particles observed in an SEM observation map obtained using a general scanning electron microscope (e.g., VE-9800, manufactured by KEYENCE) under the following conditions: Acceleration voltage: 10 kV; Magnification: 50 to 5000x.

[0078] The obtained SEM observation image is binarized and analyzed using general image analysis software (e.g., Image-Pro 10, manufactured by Hakuto Co., Ltd.). 1200 ± 400 primary particles whose contours are observed without interruption in the SEM observation image are extracted, and the longest diameter of each primary particle is measured and defined as the particle diameter of each primary particle. The average particle diameter can then be calculated by taking the average of the particle diameters of each primary particle.

[0079] The first and second powders may be powders obtained by passing them through a sieve, or they may be powders in their original state without passing them through a sieve, but it is preferable that they be powders obtained by passing them through a sieve. In this case, variations in the particle size of the first and second powders are less likely to occur, making it easier to suppress local strain within the molded body.

[0080] The sieve diameter is not particularly limited, but it is preferably 400 μm or less. A sufficiently small sieve diameter makes it particularly difficult for variations in powder particle size to occur, and it becomes easier to effectively suppress local strain within the molded body. Note that the sieve diameter is the opening of the sieve. That is, if the sieve mesh is square, the sieve diameter is the length of one side of the square; if the sieve mesh is rectangular, it is the length of the shorter side of the rectangle; and if the sieve mesh is circular, it is the diameter of the circle.

[0081] The sieve diameter may be 500 μm or less, 300 μm or less, 250 μm or less, or 200 μm or less, and may also be 20 μm or more, 50 μm or more, 80 μm or more, or 100 μm or more. Furthermore, the sieve diameter may be 20 μm or more and 500 μm or less, 50 μm or more and 300 μm or less, 80 μm or more and 250 μm or less, or 100 μm or more and 200 μm or less.

[0082] (Composition) In the method of this embodiment, the Ga / (Ga+N) ratio of the first powder is greater than 0.59 and less than or equal to 0.65. This ratio may also be 0.64 or less, 0.63 or less, or 0.62 or less, and may be 0.60 or more. This ratio may also be 0.60 or more and less and less and less and less and less and less and less and less and less and less and less and less and less and less and less and less and less and less and less and less and less and less and less and less and less and less and less and less and less and less and less and less and less and less and less

[0083] In the method of this embodiment, the Ga / (Ga+N) ratio of the second powder is 0.50 or more and 0.59 or less. This ratio may be 0.50 or more, 0.51 or more, 0.52 or more, or 0.53 or more, and may also be 0.59 or less, less than 0.59, 0.58 or less, or 0.57 or less. Furthermore, this ratio may be 0.50 or more and 0.59 or less, 0.51 or more and less than 0.59, 0.52 or more and 0.58 or less, or 0.53 or more and 0.57 or less.

[0084] In the method of this embodiment, the difference between the Ga / (Ga+N) ratio of the first powder and the Ga / (Ga+N) ratio of the second powder, i.e., (Ga / (Ga+N) ratio of the first powder) - (Ga / (Ga+N) ratio of the second powder), may be 0.01 or more, 0.02 or more, or 0.03 or more, and may also be 0.15 or less, 0.12 or less, or 0.10 or less. Furthermore, this difference may be 0.01 or more and 0.15 or less, 0.02 or more and 0.12 or less, or 0.02 or more and 0.10 or less. In the method of this embodiment, by setting this difference as described above, the area density of the high-density layer obtained from the first powder tends to become larger.

[0085] To reduce the oxygen content of the resulting gallium nitride sintered body, the oxygen content of the first powder and the second powder is preferably less than 0.4 atm%, less than 0.35 atm%, or less than 0.3 atm%. The molded body is preferably oxygen-free (i.e., has an oxygen content of 0 atm%), but may contain oxygen to an extent that does not affect the properties of the resulting gallium nitride sintered body. Examples of oxygen content in the molded body include 0.005 atm% or more, or 0.01 atm% or more. Furthermore, the oxygen content of the molded body may be 0.005 atm% or more and less than 0.4 atm%, or 0.01 atm% or more and less than 0.35 atm%.

[0086] In the method of this embodiment, the oxygen content of the first powder and the second powder is a value calculated from the method and formula (2) described above.

[0087] <Molding Process> The method of this embodiment includes a process of laminating a first powder layer composed of a first powder and a second powder layer composed of a second powder to obtain a molded body (hereinafter also referred to as the "molding process"). This provides a molded body for sintering.

[0088] (Lamination) The lamination of the first powder and the second powder can be carried out by any method. For example, the lamination of the first powder and the second powder can be carried out by filling the mold with the first powder, scraping the top surface to make the top surface of the first powder flat, and then filling the mold with the second powder on top of the first powder, scraping the top surface to make the top surface of the second powder flat. Lamination can be carried out by filling the mold with the first powder first and then the second powder, or by filling the mold with the second powder first and then the first powder, with the latter being preferable. In addition, the mold may be vibrated after filling the mold with the first powder or the second powder, for example, tapping can be used as an example.

[0089] (Pressing) In the molding process, the first powder layer and the second powder layer can be laminated and then pressed to form the product. Examples of pressing methods include uniaxial press molding and cold isostatic pressing (hereinafter also referred to as "CIP") molding, with uniaxial press molding being preferred. A molded body (compacted powder) can be obtained by these molding methods. It is preferable to perform the molding without heating the first and second powders, and it is preferable to perform uniaxial press molding at room temperature (25 ± 10°C).

[0090] In the molding process, a molded body made of the first powder may be obtained by filling a mold with the first powder and performing a uniaxial press, and a molded body made of the first and second powders may be obtained by filling the molded body with the second powder and performing a uniaxial press.

[0091] The molding pressure is not particularly limited, but is preferably 300 MPa or higher. By applying sufficient molding pressure, the voids in the molded body can be sufficiently reduced, and the density of the gallium nitride sintered body can be sufficiently increased. From the viewpoint of further increasing the density of the gallium nitride sintered body, the molding pressure is preferably 350 MPa or higher, and particularly preferably 400 MPa or higher. Alternatively, the molding pressure may be 1000 MPa or less, 800 MPa or less, or 600 MPa or less.

[0092] A preferred forming method is uniaxial press forming, and preferred conditions for uniaxial press forming include a forming pressure of 350 MPa to 1000 MPa, and 400 MPa to 800 MPa.

[0093] (Thickness) In the method of this embodiment, the thickness of the molded body obtained by laminating a first powder layer composed of a first powder and a second powder layer composed of a second powder may be 0.5 mm or more and 3.0 mm or less.

[0094] For example, the thickness of the molded body may be 0.5 mm or more, 0.6 mm or more, 0.7 mm or more, 0.8 mm or more, or 0.9 mm or more, and may also be 3.0 mm or less, 2.8 mm or less, 2.5 mm or less, 2.0 mm or less, 1.5 mm, or 1.3 mm or less. Furthermore, the thickness of the molded body may be 0.5 mm or more and 2.8 mm or less, 0.6 mm or more and 2.5 mm or less, 0.7 mm or more and 2.0 mm or less, 0.8 mm or more and 1.5 mm or less, or 0.9 mm or more and 1.3 mm or less. Here, the thickness of the molded body can be measured using a micrometer. For example, the thickness of the molded body can be determined by measuring the thickness at four equally spaced measurement points on the outer circumference of the molded body, and the center of gravity of the molded body, and then taking the average of these measurements.

[0095] Furthermore, the thickness of the first powder layer in the molded body may be between 0.05 mm and 1.0 mm. By setting the thickness of the first powder layer within the above range, the first powder layer becomes more easily nitrided. The thickness of the first powder layer can be determined by observing the cross-section of the molded body under the same measurement conditions as for area density, and then performing image analysis on the resulting SEM observation image using general image analysis software (for example, Image-Pro 10, manufactured by Hakuto Co., Ltd.).

[0096] For example, the thickness of the first powder layer may be 0.05 mm or more, 0.1 mm or more, or 0.2 mm or more, and may also be 1.0 mm or less, 0.7 mm or less, or 0.5 mm or less. Alternatively, the thickness of the first powder layer may be 0.05 mm or more and 1.0 mm or less, 0.1 mm or more and 0.7 mm or less, or 0.2 mm or more and 0.5 mm or less.

[0097] The thickness of the second powder layer in the molded body may be between 0.2 mm and 2.9 mm. The thickness of the second powder layer can be determined in the same manner as the thickness of the first powder layer.

[0098] For example, the thickness of the second powder layer may be 0.2 mm or more, 0.3 mm or more, 0.4 mm or more, or 0.5 mm or more, and may also be 2.9 mm or less, 2.5 mm or less, 2.0 mm or less, 1.5 mm or less, or 1.0 mm or less. Furthermore, the thickness of the second powder layer may be 0.2 mm or more and 2.9 mm or less, 0.3 mm or more and 2.5 mm or less, 0.4 mm or more and 2.0 mm or less, 0.4 mm or more and 1.5 mm or less, or 0.5 mm or more and 1.0 mm or less.

[0099] The second powder layer may be thicker or thinner than the first powder layer, but in order for the low-density layer obtained from the second powder layer to function as a support layer for the high-density layer obtained from the first powder layer, it is preferable that the second powder layer be thicker than the first powder layer.

[0100] For example, the thickness of the second powder layer may be more than 1.0 times, 1.1 times or more, 1.5 times or more, or 2.0 times or more than the thickness of the first powder layer, and may also be 5.0 times or less, 4.0 times or less, 3.0 times or less, or 2.5 times or less. Furthermore, this ratio may be more than 1.0 times and 5.0 times or less, 1.1 times or more and 4.0 times or less, 1.5 times or more and 3.0 times or less, or 2.0 times or more and 2.5 times or less.

[0101] The thicknesses of the first powder layer and the second powder layer can be determined based on the thicknesses of the high-density layer and the high-density layer obtained from them, respectively. In this embodiment, the ratio of the thickness of the gallium nitride sintered body obtained from the molded body to the thickness of the molded body can be exemplified as 0.90 or more and 1.05 or less. The ratio of the thickness of the gallium nitride sintered body obtained from the molded body to the thickness of the molded body may exceed 1. This is thought to be because when the molded body is sintered in a nitriding atmosphere, the metallic gallium contained in the first powder and the second powder is nitrided to gallium nitride, causing volume expansion. The ratio of the thickness of the gallium nitride sintered body obtained from the molded body to the thickness of the molded body may be less than 1. This is thought to be because volume contraction occurs when grain growth and densification of gallium nitride occur during sintering.

[0102] (Measured density) The measured density of the molded body is 3.00 g / cm³. ３ or more, or 3.60 g / cm³ ３ That is all, and also 5.80 g / cm³ ３ The following or 5.50 g / cm³ ３ The following is a possible explanation, and the measured density of the molded body is 3.00 g / cm³. ３ 5.50g / cm or more ３ The following, or 3.60 g / cm³ ３ 5.80g / cm or more ３ The following can be given as examples.

[0103] The measured density of the molded body in this embodiment is a value measured by a method compliant with JIS Z 8807. Air compliant with JIS Z 8807 may be used as the reference substance for measuring the measured density.

[0104] (Shape) The shape of the molded body may be any desired shape, and may be one or more selected from the group consisting of disc-shaped, cylindrical, rectangular, polyhedral, and conical shapes, or any other shape depending on the purpose and application.

[0105] Examples of specific molded body shapes include disc shapes with aspect ratios of 25 to 800, 50 to 700, 100 to 600, 200 to 500, or 300 to 400.

[0106] <Sintering Process> The method of this embodiment includes sintering a molded body obtained by laminating a first powder layer and a second powder layer in a nitriding atmosphere (hereinafter also referred to as the "sintering process").

[0107] The sintering process is a process of sintering a molded body to obtain a gallium nitride sintered body. This process leads to densification of the molded body. In the sintering process, the molded body may be sintered without being placed in a mold to obtain a gallium nitride sintered body.

[0108] In this embodiment, the nitriding atmosphere is an atmosphere in which the nitriding reaction proceeds, and more specifically, an atmosphere in which the nitriding reaction proceeds but the oxidation reaction does not. Therefore, the nitriding atmosphere includes not only a nitrogen atmosphere but also atmospheres containing substances other than nitrogen. An example of a nitriding atmosphere is an atmosphere containing at least one of nitrogen and nitrogen compounds. The gas constituting the nitriding atmosphere is preferably one or more selected from the group consisting of a mixture of nitrogen and hydrogen, ammonia gas, hydrazine gas, and alkylamine gas, and more preferably at least one of ammonia gas and a mixture of nitrogen and hydrogen, and even more preferably ammonia gas. From the viewpoint of nitriding metallic gallium and improving the purity of the gallium nitride sintered body, the gas constituting the nitriding atmosphere is particularly preferably ammonia gas.

[0109] Particularly preferred nitriding atmospheres include flow atmospheres, nitrogen compound flow atmospheres, and ammonia flow atmospheres. In a flow atmosphere, that is, an atmosphere in which an atmospheric medium such as a nitrogen compound-containing gas flows, the nitriding reaction of metallic gallium is promoted. This results in a more uniform composition of the resulting gallium nitride sintered body. A flow atmosphere is one in which the atmospheric medium is flowing, for example, with a flow rate of 0.1 mL / min or more or 1 L / min or more. On the other hand, the flow rate can be appropriately set according to the performance of the molded body to be processed and the sintering furnace, and examples include 20 L / min or less or 10 L / min or less. The flow rate can be 0.1 mL / min or more and 20 L / min or less, or 1 L / min or more and 20 L / min or less.

[0110] In the sintering process, it is preferable to sinter the molded body (the material to be sintered) without applying pressure, and it is preferable to sinter the molded body without applying molding pressure.

[0111] The holding temperature in the sintering process can be any temperature at which the sintering of gallium nitride progresses, but the holding temperature is preferably 1100°C or lower, more preferably 1050°C or lower, and particularly preferably 1000°C or lower. The holding temperature is preferably 800°C or higher, more preferably 850°C or higher, and particularly preferably 900°C or higher.

[0112] In the sintering process, the molded body may be sintered without performing a warp suppression treatment to suppress warping, but it is preferable to perform such a treatment before sintering. Examples of warp suppression treatments include sintering while holding the molded body, specifically, by sandwiching the molded body between a pair of plate-like members. The plate-like members are preferably breathable members that allow for airflow. Examples of breathable members include mesh plates.

[0113] The holding time at the holding temperature is preferably 0.5 hours or more, more preferably 1 hour or more, and particularly preferably 2 hours or more. Examples of holding times include 20 hours or less, 10 hours or less, or 5 hours or less. Furthermore, sintering can be carried out in two stages of heat treatment. That is, the sintering process may have a first firing stage and a second firing stage. In this case, the density of the gallium nitride sintered body can be increased in the first stage (first firing stage), and the Ga / (Ga+N) ratio can be adjusted by further promoting the nitriding of the gallium nitride sintered body in the second stage (second firing stage). When sintering is carried out in two stages, the sintering conditions for the first stage and the sintering conditions for the second stage may be the same or different, but it is preferable that they be different. It is preferable that the holding temperature of the second stage of sintering is higher than the holding temperature of the first stage of sintering in order to further promote the nitriding of the gallium nitride sintered body.

[0114] The gallium nitride sintered body obtained by the method of this embodiment changes in color tone depending on the degree of nitriding. Specifically, the color tone of the surface of the gallium nitride sintered body, particularly the L color of the surface on the high-density layer side, changes. ＊ a ＊ b ＊ In the color system, if the following condition is met, the gallium nitride sintered body can be considered to be sufficiently nitrided: L ＊ : 50.00 or more and less than 56.00, and a ＊ : -1.00 or greater and 1.00 or less, and b ＊ :2.00 or more and 7.00 or less.

[0115] Here, the surface color of the gallium nitride sintered body can be measured by a method in accordance with JIS Z 8722. Prior to measurement, the gallium nitride sintered body is subjected to the same pretreatment as in the measurement of the degree of orientation to obtain the measurement surface. For measurement, a general colorimeter (e.g., Spectrophotometer SD 3000, manufactured by Nippon Denshoku Industries Co., Ltd.) is used, with a black board as the background, the sample is placed on it, and the measurement can be performed under the following conditions: Light source: D65 light source Viewing angle: 10° Measurement method: SCE method (a method that removes specular reflection and measures diffuse reflection)

[0116] (Polishing) In the method of this embodiment, after sintering the molded body in a nitriding atmosphere, the surface formed from the first powder, i.e., the surface of the high-density layer, may be further polished, particularly to a mirror finish. This makes it possible to reduce the average surface roughness of the high-density layer of the resulting gallium nitride sintered body, particularly to 2.50 μm or less, and even further to 1.80 μm or less.

[0117] <Other> For further details of the method of this embodiment, please refer to the description in Patent Document 2.

[0118] The present disclosure will be described below with reference to examples and comparative examples. However, the present disclosure is not limited to these examples.

[0119] (Area Density) Prior to measurement, the gallium nitride sintered body was processed and polished as a pretreatment. Specifically, the gallium nitride sintered body was processed into a rectangular parallelepiped shape with a width of 5 ± 1 mm, a length of 5 ± 1 mm, and a thickness of 1 ± 0.1 mm using a diamond cutter. After polishing the cross-section of the gallium nitride sintered body with sandpaper, the measurement surface was obtained by polishing it at room temperature under the following conditions using a polishing device (device name: LaboForce-100, manufactured by Strürs Inc.): Rotation speed: 300 rpm Polishing agent: POLIPLA304M (manufactured by Fujimi Incorporated) Processing time: 10 minutes

[0120] The area density was determined by SEM observation of the gallium nitride sintered body at a 15% region from one surface in the thickness direction and at a -15% region from the opposite surface in the thickness direction, on the measurement surface obtained by pretreatment. SEM observation maps were obtained, and the obtained SEM observation maps were determined by image analysis. Observation was performed using an SEM (device name: VE-9800, manufactured by KEYENCE) under the following conditions: Acceleration voltage: 10kV Observation magnification: 500x Observation field of view: 3 fields of view

[0121] Next, using image analysis software (software name: Image-Pro 10, manufactured by Hakuto Co., Ltd.), the obtained SEM observation image is binarized, and vacancies are detected by distinguishing between non-vacancies (i.e., crystal particles constituting the gallium nitride sintered body) and vacancies, and their area [μm²] is determined. ２ The area [μm²] of the SEM observation was calculated. For the binarization process, the SEM observation image was imported into image analysis software as a grayscale image representing the shades of black and white in 256 levels, and discriminant analysis was used. After detecting voids, the observation area [μm²] of the SEM observation image was calculated using the following formula. ２ Area of ​​the region excluding the voids [μm²] ２ The area density [area %] was calculated from the following. The area density was calculated by determining the area density in three fields of view and taking the average of these values. Area density [area %] = {(area excluding voids) / (observed area of ​​the SEM observation map)} × 100

[0122] (Bulk Density) The bulk density of the gallium nitride sintered body was measured according to the method in accordance with JIS R 1634. Pretreatment was performed using a vacuum method with distilled water.

[0123] (Composition) The mass percentages of oxygen [mass%] and nitrogen [mass%] in the gallium nitride sintered body were measured using an oxygen and nitrogen analyzer (instrument name: LECO ON736, manufactured by Leco) by inert gas fusion-infrared absorption method. From the obtained mass percentages of oxygen and nitrogen, the mass percentage of gallium was determined from equation (1) above, the oxygen content [atm%] from equation (2) above, and the Ga / (Ga+N) ratio from equation (3) above.

[0124] (Thickness of gallium nitride sintered body) The thickness of the gallium nitride sintered body was measured using a micrometer. The thickness of the gallium nitride sintered body was determined by measuring four equal points on the outer circumference of the sintered body, and the center of gravity of the gallium nitride sintered body as the measurement point. The thickness of each measurement point was determined, and the average of these five points was calculated.

[0125] (Orientation) The degree of orientation was determined by irradiating the measurement surface of the gallium nitride sintered body with X-rays and performing X-ray diffraction measurement, and obtaining the 002 and 004 peaks. Prior to the measurement, the gallium nitride sintered body was processed and polished as a pretreatment. Specifically, the gallium nitride sintered body was processed into a rectangular parallelepiped shape with a width of 5 ± 1 mm × length of 5 ± 1 mm × thickness of 1 ± 0.1 mm using a diamond cutter. After polishing the surface of the high-density layer of the gallium nitride sintered body with sandpaper, the measurement surface was obtained by polishing using a polishing device (device name: LaboForce-100, manufactured by Strürs). The polishing conditions were the same as those for area density.

[0126] XRD measurements were performed using an X-ray diffractometer (device name: RINT Ultima III, manufactured by Rigaku Corporation) under the following conditions: Radiation source: CuKα rays (λ = 1.5405 Å) Measurement mode: Continuous scan Step width: 0.02° Scan speed: 2° / min Measurement range: 2θ = 20° to 80°

[0127] The Rottgering factor F(002,004) was determined from the diffraction peaks of 002 and 004, which have peak tops at 2θ = 34.56 ± 0.08° and 72.90 ± 0.08° of the obtained XRD pattern, using the following formula, and this was taken as the degree of orientation. F(002,004) = (ρ - ρ ０ ) / (1-ρ) ０ )

[0128] Here, ρ is the ratio of the intensity of the diffraction peaks having peak tops at 2θ = 34.56 ± 0.08° and 72.90 ± 0.08° to the total intensity of the diffraction peaks in the XRD pattern of the gallium nitride sintered body. ０ This is the ratio of the intensity of diffraction peaks with peak tops at 2θ = 34.56 ± 0.08° and 72.90 ± 0.08° to the total intensity of diffraction peaks in the XRD pattern of gallium nitride shown in PDF card No. 01-073-7289.

[0129] (Color Tone) The surface color tone of the gallium nitride sintered body was measured according to the method conforming to JIS Z 8722. A colorimeter (device name: Spectrophotometer SD 3000, manufactured by Nippon Denshoku Industries Co., Ltd.) was used for measurement, with the sample placed on a black board as the background. The following measurement conditions were used. Prior to measurement, the gallium nitride sintered body was pre-treated in the same way as for the degree of orientation to obtain the measurement surface. Light source: D65 light source Viewing angle: 10° Measurement method: SCE method (a method that removes specular reflection and measures diffuse reflection)

[0130] (Average surface roughness) The average surface roughness was measured using a three-dimensional shape measuring machine (product name: One-Shot 3D Shape Measuring Machine VR-6000, manufactured by KEYENCE). Prior to measurement, the gallium nitride sintered body was subjected to the same pretreatment as for the degree of orientation to obtain the measurement surface.

[0131] Using observation images of the measurement surface at a magnification of 160x, the surface roughness was determined using analysis software attached to the 3D shape measuring machine, with the reference plane set to the least-squares plane. For three fields of view, the surface roughness was determined for each field of view, and the average value of these values ​​was used.

[0132] <Example 1> As the first powder, a mixed powder of metallic gallium and gallium nitride with a Ga / (Ga+N) ratio of 0.60 was prepared, and as the second powder, a mixed powder of metallic gallium and gallium nitride with a Ga / (Ga+N) ratio of 0.53 was prepared. Each mixed powder was passed through a sieve with a diameter of 300 μm in accordance with JIS Z 8801 to obtain a mixed powder with a particle size of less than 300 μm.

[0133] Next, the first powder was filled into a cylindrical mold with a diameter of 30 mm, and the top surface was scraped to make it flat. Then, the second powder was filled on top of the first powder, and the top surface was scraped to make it flat. After that, the molded body was obtained by uniaxial press molding at a pressure of 500 MPa.

[0134] A molded body was placed on an alumina setter with the surface made of the first powder facing downwards, and this was sintered in a nitriding atmosphere furnace under the heat treatment conditions consisting of the following first and second sintering steps to obtain the gallium nitride sintered body of this embodiment.

[0135] (First sintering process) Heat treatment atmosphere: Ammonia flow atmosphere (ammonia gas flow rate 6000 mL / min) Holding temperature: 950°C Holding time: 2 hours

[0136] (Second sintering process) Heat treatment atmosphere: Ammonia flow atmosphere (ammonia gas flow rate 6000 mL / min) Holding temperature: 950°C Holding time: 2 hours

[0137] Table 1 shows the manufacturing conditions for the gallium nitride sintered body of this embodiment. Table 2 shows the physical properties of the gallium nitride sintered body of this embodiment.

[0138] <Example 2> The gallium nitride sintered body of this example was obtained in the same manner as in Example 1, except that the Ga / (Ga+N) ratio of the first and second powders was changed as shown in Table 1, and the holding temperature in the second sintering step was changed as shown in Table 1. The manufacturing conditions for the gallium nitride sintered body of this example are shown in Table 1. The physical properties of the gallium nitride sintered body of this example are shown in Table 2.

[0139] <Example 3> The gallium nitride sintered body of this example was obtained in the same manner as in Example 1, except that the Ga / (Ga+N) ratio of the first and second powders was changed as shown in Table 1, and the holding temperature in the second sintering step was changed as shown in Table 1. The manufacturing conditions for the gallium nitride sintered body of this example are shown in Table 1. The physical properties of the gallium nitride sintered body of this example are shown in Table 2.

[0140] <Example 4> The gallium nitride sintered body of this example was obtained in the same manner as in Example 1, except that the Ga / (Ga+N) ratio of the first and second powders was changed as shown in Table 1, and the holding temperature in the second sintering step was changed as shown in Table 1, and the heat treatment was performed twice. The manufacturing conditions for the gallium nitride sintered body of this example are shown in Table 1. The physical properties of the gallium nitride sintered body of this example are shown in Table 2.

[0141] <Example 5> Except for changing the Ga / (Ga+N) ratio of the first and second powders as shown in Table 1, changing the sieve diameter as shown in Table 1, and changing the holding temperature in the second sintering step as shown in Table 1 and performing heat treatment twice, the gallium nitride sintered body of this example was obtained in the same manner as in Example 1. The manufacturing conditions for the gallium nitride sintered body of this example are shown in Table 1. The physical properties of the gallium nitride sintered body of this example are shown in Table 2.

[0142] <Comparative Example 1> A mixed powder of metallic gallium and gallium nitride with a Ga / (Ga+N) ratio of 0.56 was prepared, filled into a cylindrical mold with a diameter of 30 mm, and the top surface was scraped off. Then, a molded body was obtained by uniaxial press molding at a pressure of 300 MPa.

[0143] A gallium nitride sintered body of this comparative example was obtained by placing a molded body on an alumina setter and sintering it in a nitriding atmosphere furnace under the following conditions: Heat treatment atmosphere: Ammonia flow atmosphere (ammonia gas flow rate 6000 mL / min) Holding temperature: 950°C Holding time: 2 hours

[0144] Table 1 shows the manufacturing conditions for the gallium nitride sintered body of this comparative example. Table 2 shows the physical properties of the gallium nitride sintered body of this comparative example.

[0145] <Comparative Example 2> A sintered body was obtained in the same manner as in Comparative Example 1, except that a second firing step was added and the Ga / (Ga+N) ratio and molding pressure were changed as shown in Table 1.

[0146] <Comparative Example 3> A sintered body was obtained in the same manner as in Comparative Example 1, except that the Ga / (Ga+N) ratio, molding pressure, and firing temperature were changed as shown in Table 1.

[0147]

[0148]

[0149] In Examples 1 to 5, it was confirmed that the gallium nitride sintered bodies had a high-density layer and a low-density layer, and that the area density of the high-density layer was 84.0% or higher. Furthermore, in all examples, the average surface roughness was 2.5 μm or less, confirming that the average surface roughness was low. Thus, it was confirmed that the gallium nitride sintered bodies of the examples have high surface smoothness and can be applied as substrates for epitaxial growth of gallium nitride.

[0150] Comparative Example 1, obtained solely from a mixed powder of metallic gallium and gallium nitride with a Ga / (Ga+N) ratio of 0.56, had a surface area density of 83.3% and therefore lacked a high-density layer. Furthermore, Comparative Example 1 had an average surface roughness of 2.51 μm, confirming its low surface smoothness.

[0151] Comparative Examples 2 and 3, obtained solely from a mixed powder of metallic gallium and gallium nitride with a Ga / (Ga+N) ratio of 0.61, had surface area densities of 84.8% and 79.5%, respectively. Furthermore, Comparative Examples 2 and 3 had average surface roughness of 1.84 μm and 2.64 μm, respectively, confirming low surface smoothness. Compared to the sintered body of Comparative Example 2, the sintered body of Comparative Example 3, which was sintered at a higher temperature to promote nitriding, showed a smaller Ga / (Ga+N) ratio, but a decreased area density in the high-density layer and increased surface roughness.

[0152] In the examples, it was confirmed that the average surface roughness tended to decrease as the area density of the high-density layer increased.

[0153] A comparison of Example 1 and Example 2 confirms that even if the Ga / (Ga+N) ratio of the first powder for forming the high-density layer is the same, increasing the Ga / (Ga+N) ratio of the second powder for forming the low-density layer, that is, reducing the difference between the Ga / (Ga+N) ratio of the first powder for forming the high-density layer and the Ga / (Ga+N) ratio of the second powder for forming the low-density layer, increases the area density of the high-density layer.

[0154] A comparison between Example 1 and Example 3 confirms that even if the Ga / (Ga+N) ratio of the second powder used to form the low-density layer is the same, increasing the Ga / (Ga+N) ratio of the first powder used to form the high-density layer increases the area density of the high-density layer.

[0155] A comparison between Example 2 and Example 4 confirms that the area density of the high-density layer remains approximately the same even when the thickness of the first powder layer and the thickness of the second powder layer are changed.

[0156] A comparison between Example 4 and Example 5 confirms that reducing the sieve diameter of the sieves used to sift the first and second powders increases the area density of the high-density layer.

[0157] The entire contents of the specification, claims, drawings, and abstract of Japanese Patent Application No. 2024-227483, filed on December 24, 2024, are incorporated herein by reference as the disclosure of the specification.

[0158] 1. High-density layer 2. Low-density layer 10. Gallium nitride sintered body

Claims

1. A gallium nitride sintered body having a high-density layer and a low-density layer, wherein the area density of the high-density layer is 84.0 area% or more, and the Ga / (Ga+N) ratio, which is the atomic ratio of gallium to the total of gallium and nitrogen, is 0.55 or less.

2. The gallium nitride sintered body according to claim 1, wherein the area density of the low-density layer is 65.0 area% or more.

3. The gallium nitride sintered body according to claim 1, wherein the average surface roughness of the high-density layer is 2.50 μm or less.

4. The gallium nitride sintered body according to claim 1, wherein the Ga / (Ga+N) ratio is 0.50 or less.

5. The gallium nitride sintered body according to claim 1, which is in the form of a plate.

6. The gallium nitride sintered body according to claim 5, wherein the high-density layer forms one main surface and the low-density layer forms the other main surface.

7. The gallium nitride sintered body according to claim 1, wherein the thickness of the gallium nitride sintered body is 0.50 mm or more and 3.0 mm or less.

8. The gallium nitride sintered body according to claim 1, wherein the thickness of the high-density layer is 0.05 mm or more and 1.0 mm or less.

9. The gallium nitride sintered body according to claim 1, wherein the thickness of the low-density layer is 0.2 mm or more and 2.9 mm or less.

10. A gallium nitride sintered body according to any one of claims 1 to 9, which is a substrate for epitaxial growth of a gallium nitride film.

11. A method for manufacturing a gallium nitride sintered body, comprising: providing a first powder containing gallium nitride and metallic gallium, wherein the Ga / (Ga+N) ratio, which is the atomic ratio of gallium to the total amount of gallium and nitrogen, is greater than 0.59 and less than or equal to 0.65; and providing a second powder containing gallium nitride and metallic gallium, wherein the Ga / (Ga+N) ratio is greater than or equal to 0.50 and less than or equal to 0.59; obtaining a molded body by laminating a first powder layer composed of the first powder and a second powder layer composed of the second powder; and sintering the molded body in a nitriding atmosphere.

12. The method according to claim 11, satisfying the following relationship: 0.01 ≤ (the Ga / (Ga+N) ratio of the first powder) - (the Ga / (Ga+N) ratio of the second powder) ≤ 0.15 13. The method according to claim 11, comprising sintering the molded body in a nitriding atmosphere and then polishing the surface formed from the first powder.

14. The method according to any one of claims 11 to 13 for producing the gallium nitride sintered body described in claim 1.

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