JP7855060B2 - Group III element nitride substrate and method for manufacturing a Group III element nitride substrate - Google Patents

Description

This invention relates to a group III element nitride substrate and a method for manufacturing a group III element nitride substrate.

Group III element nitride substrates are used as substrates for various devices such as light-emitting diodes, semiconductor lasers, and power ICs.

The above-mentioned Group III element nitride substrate can be obtained, for example, by epitaxially growing Group III element nitride crystals on a substrate, as described in Patent Document 1.

Japanese Patent Publication No. 2010-168226

However, the group III element nitride substrates obtained by the above epitaxial growth method are prone to cracking, and an improvement in yield is desired.

In view of the above, the main objective of the present invention is to provide a group III element nitride substrate in which crack occurrence is suppressed.

1. The group III element nitride substrate according to an embodiment of the present invention is a group III element nitride substrate having a first main surface and a second main surface facing each other, wherein the thermal conductivity of the group III element nitride in the m-axis direction at a predetermined portion within the substrate surface is greater than the thermal conductivity of the group III element nitride in the a-axis direction.
2. In the group III element nitride substrate described in item 1 above, the thermal conductivity in the m-axis direction at a predetermined location within the substrate surface may be 2% or more greater than the thermal conductivity in the a-axis direction.
3. In the Group III element nitride substrate described in 1 or 2 above, the difference between the average value of the thermal conductivity in the m-axis direction and the average value of the thermal conductivity in the a-axis direction at multiple locations within the plane of the substrate may be 2% or more. Here, the difference between the average value of the thermal conductivity in the m-axis direction and the average value of the thermal conductivity in the a-axis direction is calculated from the formula: (average value of thermal conductivity in the m-axis direction - average value of thermal conductivity in the a-axis direction) / average value of thermal conductivity in the m-axis direction.
4. The group III element nitride substrate described in any of items 1 to 3 above may be disc-shaped, and its diameter may be 75 mm or more.
5. The Group III element nitride substrate described in any of items 1 to 4 above may be a self-supporting substrate of a Group III element nitride crystal.

6. A method for manufacturing a Group III element nitride substrate according to another embodiment of the present invention comprises preparing a base substrate having an upper surface and a lower surface facing each other, and growing a Group III element nitride crystal on the base substrate by a flux method, wherein the crystal growth by the flux method is carried out such that the flow of the raw material solution in a first direction within the plane of the base substrate is faster than the flow of the raw material solution in a second direction within the plane of the base substrate.
7. In the manufacturing method described in item 6 above, the substrate may contain a group III element nitride.
8. In the manufacturing method described in 6 or 7 above, the Group III element nitride substrate may have a first main surface and a second main surface facing each other, and the thermal conductivity of the Group III element nitride in the m-axis direction at a predetermined location within the substrate surface may be greater than the thermal conductivity of the Group III element nitride in the a-axis direction.

According to embodiments of the present invention, a group III element nitride substrate in which crack formation is suppressed can be provided.

This is a schematic cross-sectional view showing the general structure of a group III element nitride substrate according to one embodiment of the present invention. Figure 1A is a plan view of a group III element nitride substrate. This is a diagram illustrating the measurement points for thermal conductivity. This figure shows the manufacturing process of a group III element nitride substrate according to one embodiment. This figure follows Figure 3A. This figure follows Figure 3B. This is a top view of the substrate placed inside the crucible in the first embodiment. This is a top view of the substrate placed inside the crucible in the second embodiment. This is a schematic cross-sectional view showing the general configuration of an element substrate according to one embodiment of the present invention.

The embodiments of the present invention will be described below with reference to the drawings, but the present invention is not limited to these embodiments. Furthermore, in order to clarify the explanation, the drawings may schematically represent the width, thickness, shape, etc., of each part compared to the embodiments, but these are merely examples and do not limit the interpretation of the present invention.

A. Group III Element Nitride Substrate Figure 1A is a schematic cross-sectional view showing the general configuration of a Group III element nitride substrate according to one embodiment of the present invention, and Figure 1B is a plan view of the Group III element nitride substrate shown in Figure 1A. The Group III element nitride substrate 10 is plate-shaped and has a first main surface 11 and a second main surface 12 facing each other, which are connected via a side surface 13.

In the illustrated example, the Group III element nitride substrate is disc-shaped (wafer), but it is not limited to this and can be any suitable shape. The size of the Group III element nitride substrate can be appropriately set according to the purpose. The diameter of the disc-shaped Group III element nitride substrate is, for example, 50 mm or more and 300 mm or less, preferably 75 mm or more, and more preferably 100 mm or more. A larger Group III element nitride substrate (for example, with a diameter of 75 mm or more) can improve the productivity of larger devices, for example.

The thickness of the Group III element nitride substrate is, for example, 250 μm or more and 800 μm or less, preferably 300 μm or more and 750 μm or less, and more preferably 350 μm or more and 725 μm or less.

Group III element nitride substrates are composed of Group III element nitride crystals. Examples of Group III elements used to make up Group III element nitrides include aluminum (Al), gallium (Ga), and indium (In). These can be used individually or in combination of two or more. Specific examples of Group III element nitrides include aluminum nitride (Al _x N), gallium nitride (Ga _y N), indium nitride (In _z N), aluminum gallium nitride (Al _x Ga _y N), gallium gallium nitride (Ga _y In _z N), aluminum gallium nitride (Al _x In _z N), and aluminum gallium indium nitride (Al _x Ga _y In _z N). In the chemical formulas in parentheses, typically x + y + z = 1.

The above-mentioned Group III element nitrides may contain dopants. Examples of dopants include p-type dopants such as beryllium (Be), magnesium (Mg), strontium (Sr), cadmium (Cd), iron (Fe), manganese (Mn), and zinc (Zn), and n-type dopants such as silicon (Si), germanium (Ge), tin (Sn), and oxygen (O). These can be used individually or in combination of two or more.

In the above-described Group III element nitride crystal, typically, the <0001> direction is the c-axis direction, the <1-100> direction is the m-axis direction, and the <11-20> direction is the a-axis direction. Furthermore, the crystal plane perpendicular to the c-axis is the c-plane, the crystal plane perpendicular to the m-axis is the m-plane, and the crystal plane perpendicular to the a-axis is the a-plane. In this embodiment, the thickness direction of the Group III element nitride substrate 10 is parallel or substantially parallel to the c-axis, the first main surface 11 is the Group III element polarity plane on the (0001) plane side, and the second main surface 12 is the nitrogen polarity plane on the (000-1) side.

Typically, the first main surface 11 may be parallel to the (0001) plane, or it may be inclined with respect to the (0001) plane. The inclination angle of the first main surface 11 with respect to the (0001) plane is, for example, 10° or less, may be 5° or less, may be 2° or less, or may be 1° or less. The second main surface 12 may be parallel to the (000-1) plane, or it may be inclined with respect to the (000-1) plane. The inclination angle of the second main surface 12 with respect to the (000-1) plane is, for example, 10° or less, may be 5° or less, may be 2° or less, or may be 1° or less.

The in-plane thermal conductivity of the Group III element nitride substrate 10 is, for example, 170 W/mK to 220 W/mK. In any or predetermined location within the plane of the Group III element nitride substrate, the thermal conductivity in the m-axis direction of the Group III element nitride is greater than the thermal conductivity in the a-axis direction perpendicular to the m-axis direction. Preferably, in the first location 51 situated in the central part 10a in a plan view, the thermal conductivity in the m-axis direction is greater than the thermal conductivity in the a-axis direction, and in the second location 52 situated in the peripheral part 10b in a plan view, the thermal conductivity in the m-axis direction is greater than the thermal conductivity in the a-axis direction. The peripheral part 10b may mean, for example, a region within 50% of the radius of the Group III element nitride substrate 10 radially inward from the edge 10c of the Group III element nitride substrate 10. Alternatively, it may mean a region greater than 50% of the radius of the Group III element nitride substrate 10 radially outward from the center of the Group III element nitride substrate 10.

Thermal conductivity can be calculated by measuring the thermal diffusivity using measurement methods such as the laser flash method or periodic heating radiation thermometer, and then obtaining the result from the thermal diffusivity.

In a predetermined portion of the plane of a group III element nitride substrate, the thermal conductivity in the m-axis direction is preferably 2% or more greater than the thermal conductivity in the a-axis direction, more preferably 3% or more greater, even more preferably 5% or more greater, and particularly preferably 6% or more greater. In the first portion described above, the thermal conductivity in the m-axis direction is preferably 2% or more greater than the thermal conductivity in the a-axis direction, more preferably 3% or more greater, even more preferably 5% or more greater, and particularly preferably 6% or more greater. In the second portion described above, the thermal conductivity in the m-axis direction is preferably 2% or more greater than the thermal conductivity in the a-axis direction, more preferably 3% or more greater, even more preferably 5% or more greater, and particularly preferably 6% or more greater.

In one embodiment, at five points on the disc-shaped substrate surface shown in Figure 2, specifically the center and the points extending 60% of the substrate radius radially outward from the center (left, top, center, right, bottom), the thermal conductivity in the m-axis direction is preferably 2% or more greater than the thermal conductivity in the a-axis direction, more preferably 3% or more greater, even more preferably 5% or more greater, and particularly preferably 6% or more greater.

The difference between the average value of the thermal conductivity in the m-axis direction and the average value of the thermal conductivity in the a-axis direction at multiple locations within the plane of a group III element nitride substrate is preferably 2% or more, more preferably 3% or more, even more preferably 5% or more, and particularly preferably 6% or more. Here, the difference between the average value of the thermal conductivity in the m-axis direction and the average value of the thermal conductivity in the a-axis direction is calculated from the formula: (average value of thermal conductivity in the m-axis direction - average value of thermal conductivity in the a-axis direction) / average value of thermal conductivity in the m-axis direction. For example, the difference between the average value of the thermal conductivity in the m-axis direction at the first location in the central part and the thermal conductivity in the m-axis direction at the second location in the peripheral part, and the average value of the thermal conductivity in the a-axis direction at the first location in the central part and the thermal conductivity in the a-axis direction at the second location in the peripheral part, is preferably 2% or more, more preferably 3% or more, even more preferably 5% or more, and particularly preferably 6% or more.

In one embodiment, the difference between the average value of the thermal conductivity in the m-axis direction and the average value of the thermal conductivity in the a-axis direction at five points (left, top, center, right, bottom) on the disc-shaped substrate surface shown in Figure 2 is preferably 2% or more, more preferably 3% or more, even more preferably 5% or more, and particularly preferably 6% or more.

The Group III element nitride substrate according to the embodiment of the present invention can satisfy the above relationship of thermal conductivity and effectively suppress crack formation. While crack formation tends to increase with increasing size (e.g., diameter) of the Group III element nitride substrate, according to the embodiment of the present invention, crack formation can be effectively suppressed even when the Group III element nitride substrate is large.

B. Manufacturing Method A method for manufacturing a group III element nitride substrate according to one embodiment of the present invention includes preparing a base substrate and growing a group III element nitride crystal on the base substrate.

Figures 3A to 3C show the manufacturing process of a group III element nitride substrate according to one embodiment. Figure 3A shows a base substrate 20 having an upper surface 20a and a lower surface 20b facing each other.

As the base substrate mentioned above, for example, a substrate having a shape and size that allows for the manufacture of a Group III element nitride substrate of the desired shape and size is used. Typically, the base substrate is disc-shaped with a diameter of 50 mm to 350 mm. The thickness of the base substrate is, for example, 300 μm to 2000 μm.

The substrate is composed of a material having the same composition (chemical composition) as the Group III element nitride crystal layer to be grown, and may contain Group III element nitride. Specifically, a Group III element nitride substrate can be used as the substrate. Details of the Group III element nitride are as described above. In one embodiment, a gallium nitride substrate is used.

The above-mentioned Group III element nitride substrate can be obtained, for example, by forming a seed crystal film on a growth substrate, growing a Group III element nitride crystal on the seed crystal film side of the seed crystal substrate, and then separating the formed growth layer from the growth substrate. The growth substrate can be made of any suitable material. Typically, a sapphire substrate is used as the growth substrate.

The thickness of the seed crystal film is, for example, 0.2 μm to 10 μm, preferably 1 μm to 5 μm. Typical materials used to constitute the seed crystal film include group III element nitrides. Details of the group III element nitrides are as described above. In one embodiment, gallium nitride is used.

Seed crystal films can be deposited by any suitable method. Typical methods for depositing seed crystal films include vapor deposition. Specific examples of vapor deposition methods include metal-organic chemical vapor deposition (MOCVD), hydride vapor deposition (HVPE), pulsed excitation deposition (PXD), molecular beam epitaxy (MBE), vapor deposition, and sublimation. Among these, MOCVD is preferred.

A Group III element nitride crystal can be grown by any suitable method to obtain a Group III element nitride substrate. The method for growing the Group III element nitride crystal is not particularly limited, as long as it can achieve a crystal orientation that generally follows the crystal orientation of the seed crystal film. Specific examples of methods for growing Group III element nitride crystals will be described later.

Next, a group III element nitride crystal is grown on the base substrate 20 to form a group III element nitride crystal layer 16, and a laminated substrate 30 is obtained as shown in Figure 3B. The degree of growth of the group III element nitride crystal (thickness of the group III element nitride crystal layer 16) can be adjusted according to the desired thickness of the group III element nitride substrate. Any appropriate direction can be selected for the growth direction of the group III element nitride crystal depending on the application, purpose, etc. Specific examples include the normal directions of the c-plane, a-plane, and m-plane, respectively, and the normal directions of the planes inclined to the c-plane, a-plane, and m-plane.

Group III element nitride crystals can be grown by any suitable method. The method for growing Group III element nitride crystals is not particularly limited, as long as it can achieve a crystal orientation that generally follows the crystal orientation of the underlying substrate. Specific examples of methods for growing Group III element nitride crystals include vapor phase growth methods such as metal-organic chemical vapor deposition (MOCVD), hydride vapor deposition (HVPE), pulsed excitation deposition (PXD), molecular beam epitaxy (MBE), and sublimation; and liquid phase growth methods such as flux, amonothermal, hydrothermal, and sol-gel methods. These can be used individually or in combination of two or more.

Preferably, a flux method (e.g., the Na flux method) is used as a method for growing Group III element nitride crystals. Details of such a growth method are described, for example, in Japanese Patent Publication No. 5244628, and the crystals may be grown by adjusting the various conditions of the described growth method as appropriate. Specifically, the growth of Group III element nitride crystals can be carried out by adjusting various conditions using a crystal manufacturing apparatus comprising a pressure vessel capable of supplying pressurized nitrogen gas, a turntable rotatable within the pressure vessel, and an outer vessel placed on the turntable.

The growth of Group III element nitride crystals by the flux method is typically carried out using a crucible as the growth container. Specifically, the seed crystal substrate is placed in a predetermined position within the crucible, and then the raw materials are filled in. The crucible containing the seed crystal substrate is typically placed with a lid on, under a nitrogen-containing atmosphere, and under predetermined pressure and temperature, and subjected to the growth process.

The above raw materials are, for example, a molten composition containing flux, a Group III element, and optionally a dopant. The flux preferably contains at least one of an alkali metal and an alkaline earth metal, and more preferably contains metallic sodium. Typically, the flux and the metal raw material are used in mixture form. As the metal raw material, elemental metals, alloys, metal compounds, etc., can be used, but from the viewpoint of handling, elemental metals are preferably used.

The crucible (including the lid) can be formed from any suitable material that can be used in the flux process. Examples of crucible materials include alumina, yttria, and YAG (yttrium aluminum garnet). The crucible material may be single crystal or polycrystalline (ceramic). The ceramic may have its relative density increased by HIP treatment or other methods, giving it so-called translucency.

As described above, growth can be carried out under a nitrogen-containing atmosphere. The growth atmosphere may contain other gases in addition to nitrogen. Inert gases such as argon, helium, and neon are preferably used as the other gases.

The atmospheric pressure during cultivation can be set to any appropriate pressure. From the viewpoint of preventing flux evaporation, the atmospheric pressure during cultivation is preferably 10 atmospheres or higher, and more preferably 30 atmospheres or higher. On the other hand, from the viewpoint of preventing the cultivation apparatus from becoming large-scale, the atmospheric pressure during cultivation is preferably 2000 atmospheres or lower, and more preferably 500 atmospheres or lower.

The ambient temperature during cultivation can be set to any appropriate temperature. Preferably, the ambient temperature during cultivation is 700°C to 1000°C, and more preferably 800°C to 900°C.

Cultivation is preferably carried out while rotating the crucible. For example, a lidded crucible is placed in the outer container and placed on the rotating platform, and the crucible is rotated by rotating the platform. The direction of rotation can be set to any appropriate direction, but in a preferred embodiment, it alternates between clockwise and counterclockwise rotations at a predetermined period. The rotation speed is preferably 5 rpm to 40 rpm.

The growth is preferably carried out by rotating the seed crystal substrate so that the flow of the raw material solution in the first direction within the plane of the substrate is faster than the flow of the raw material solution in the second direction. With this configuration, a group III element nitride substrate having the above-mentioned thermal conductivity can be obtained successfully. Specifically, by improving the agitation in a predetermined direction (first direction), crystals can be grown with strong bonds in the first direction, and a crystal layer with higher thermal conductivity than in other directions (second direction) can be formed.

The first direction described above may correspond to the m-axis direction of the resulting Group III element nitride substrate, and the second direction described above may correspond to the a-axis direction of the resulting Group III element nitride substrate. The first direction may correspond to the m-axis direction of the Group III element nitride underlayment, and the second direction may correspond to the a-axis direction of the Group III element nitride underlayment. By orienting the first direction perpendicular to the m-plane, which is the easily cleavable surface of the Group III element nitride underlayment, crack generation can be suppressed (for example, when cooling the laminated substrate 30 after forming the Group III element nitride crystal layer 16), and the yield can be improved.

The flow of the raw material solution described above can be controlled, for example, by adjusting the positional relationship between the substrate (seed crystal substrate) on which crystals are grown in the crucible and the wall surrounding the substrate. Figure 4A is a top view of the substrate arranged in the crucible in the first embodiment. In this embodiment, the flow of the raw material solution is controlled using a crucible 90 that is approximately elliptical in plan view. In this case, the direction of the long side of the ellipse may be the first direction L1, and the direction of the short side of the ellipse may be the second direction L2. Specifically, the substrate (seed crystal substrate) 20 on which crystals are grown is placed in the center of the crucible, and the distance d1 between the substrate 20 and the inner wall 90a of the crucible 90 in the direction of the long side (first direction L1) is arranged to be greater than the distance d2 between the substrate 20 and the inner wall 90a of the crucible 90 in the direction of the short side (second direction L2). The ratio of distance d1 to distance d2 (d1/d2) is preferably 1.5 or more.

Figure 4B is a top view of the substrate placed inside the crucible in the second embodiment. In this embodiment, the flow of the raw material solution is controlled by placing adjustment walls 92 inside the crucible 90. In this case, the direction along the adjustment walls 92 may be the first direction L1, and the direction substantially perpendicular to the adjustment walls 92 may be the second direction L2. Specifically, the substrate (seed crystal substrate) 20 on which crystals are grown is placed so as to be sandwiched between a pair of adjustment walls 92, and the distance d1 between the substrate 20 and the inner wall 90a of the crucible 90 in the direction along the adjustment walls 92 (first direction L1) is set to be greater than the distance d2 between the substrate 20 and the adjustment walls 92 in the direction substantially perpendicular to the adjustment walls 92 (second direction L2). The ratio of distance d1 to distance d2 (d1/d2) is preferably 1.5 or more. The length of the adjustment walls 92 may be formed to be longer than the diameter of the substrate 20. The material used for the adjustment wall 92 may be the same as the material used for the crucible described above. In this embodiment, the shape of the crucible 90 is not particularly limited, as long as the flow of the raw material solution can be controlled by the adjustment wall 92. In the illustrated example, the crucible 90 is approximately circular in plan view, but it may be approximately elliptical, for example, as in the first embodiment.

After growing the Group III element nitride crystals, as shown in Figure 3C, the Group III element nitride crystals (Group III element nitride crystal layer 16) may be separated from the base substrate 20 to obtain a self-supporting substrate 32. Alternatively, unlike the illustrated example, the laminated substrate 30 may be used as a self-supporting substrate as is. The Group III element nitride crystals can be separated from the base substrate by any suitable method. Examples of methods for separating the Group III element nitride crystals include a method that utilizes the difference in thermal shrinkage between the Group III element nitride crystals and the base substrate during the cooling process after growth to allow for spontaneous separation, a method of separation by chemical etching, and a laser lift-off method using laser light irradiation. When separating the Group III element nitride crystals by the laser lift-off method, typically, laser light is irradiated from the lower surface 20b side of the base substrate 20 of the laminated substrate 30. Alternatively, a self-supporting substrate may be obtained by grinding or cutting using a cutting machine such as a wire saw.

The self-supporting substrate can be used as is to obtain the above-mentioned Group III element nitride substrate, but typically, the self-supporting substrate is subjected to any appropriate processing to obtain the above-mentioned Group III element nitride substrate.

One example of processing performed on the above-mentioned self-supporting substrate is grinding of the peripheral edge (for example, grinding using a diamond grinding wheel). Typically, grinding is performed to process the substrate into the desired shape and size (for example, a disc shape with a desired diameter).

Another example of processing performed on the above-mentioned self-supporting substrate is grinding and polishing of the main surfaces (top and bottom surfaces) (e.g., lapping, chemical mechanical polishing (CMP)). Typically, grinding and polishing are used to thin and flatten the substrate to the desired thickness.

Furthermore, examples of processing performed on the above-mentioned self-supporting substrate include chamfering of the outer edge, removal of the processed altered layer, and removal of residual stress that may be caused by the processed altered layer.

C. Applications A functional layer can be formed on the above-mentioned Group III element nitride substrate. Figure 5 is a schematic cross-sectional view showing the general configuration of an element substrate according to one embodiment of the present invention. The element substrate 40 has a Group III element nitride substrate 10 and a functional layer 42 formed on the first main surface (Group III element polar surface) 11 of the Group III element nitride substrate 10. The functional layer 42 is typically formed by epitaxial growth of crystals.

The above-mentioned functional layer can function, for example, as a light-emitting layer, a rectifier layer, a switching element layer, or a power semiconductor layer. In one embodiment, a group III element nitride crystal is used as the material constituting the above-mentioned functional layer. Examples of group III elements that constitute the group III element nitride include Ga (gallium), Al (aluminum), and In (indium). These can be used individually or in combination of two or more.

Furthermore, while the functional layer 42 is formed on the group III element nitride substrate 10 (in the state of being an element substrate 40), the second main surface (nitrogen polar surface) 12 of the group III element nitride substrate 10 may be subjected to processing such as grinding or polishing.

The present invention will be specifically described below with reference to examples, but the present invention is not limited to these examples.

[Comparative Example 1]
(Preparation of the base substrate)
A 6-inch diameter c-plane self-supporting gallium nitride substrate was prepared.

(Growth of gallium nitride crystals)
The growth of gallium nitride crystals was carried out using a crystal manufacturing apparatus comprising a pressure vessel capable of supplying pressurized nitrogen gas, a rotating platform that can rotate within the pressure vessel, and an outer container placed on the rotating platform.
The above substrate was placed in the center of a 200 mm diameter alumina crucible inside a glove box under a nitrogen atmosphere. Next, metallic gallium and metallic sodium were filled into the crucible so that the atomic ratio Ga/(Ga+Na) (mol%) was 15 mol%, and the crucible was covered with an alumina plate. In this state, the crucible was placed in a stainless steel inner container, and then placed in a stainless steel outer container capable of housing this inner container, and the outer container was closed with a lid equipped with a nitrogen introduction pipe. In this state, the outer container was placed on a rotating platform installed in the heating section of the crystal manufacturing apparatus, which had been pre-baked under vacuum, and the pressure vessel of the crystal manufacturing apparatus was sealed with a lid.
Next, the pressure vessel was evacuated to 0.1 Pa or less using a vacuum pump. Subsequently, the heating section (upper heater, middle heater, and lower heater) was operated to heat the heating space to 870°C, while nitrogen gas was introduced into the pressure vessel from a nitrogen gas cylinder until the pressure reached 4.0 MPa. The outer vessel was then rotated clockwise and counterclockwise at a constant period of 20 rpm (stirring speed) around its central axis. This state was maintained for 40 hours.
Afterward, the mixture was allowed to cool naturally to room temperature and then reduced to atmospheric pressure. The lid of the pressure vessel was opened and the crucible was removed. The solidified metallic sodium inside the crucible was removed, and the substrate on which the gallium nitride crystals had grown was recovered. In this way, a gallium nitride substrate (wafer) was obtained.

[Example 1]
A gallium nitride substrate was fabricated in the same manner as in Comparative Example 1, except that the crucible used for growing gallium nitride crystals was changed to an elliptical alumina crucible with a long side of 260 mm and a short side of 180 mm, the base substrate (c-plane self-supporting gallium nitride substrate) was placed in the crucible so that its m-axis direction was aligned with the long side direction and its a-axis direction was aligned with the short side direction (d1/d2: 3.67 as shown in Figure 4A), and the stirring speed during gallium nitride crystal growth was changed to 10 rpm.

[Example 2]
A gallium nitride substrate was fabricated in the same manner as in Example 1, except that the stirring speed was changed to 20 rpm during the growth of the gallium nitride crystal.

[Example 3]
A gallium nitride substrate was fabricated in the same manner as in Example 1, except that the stirring speed was changed to 30 rpm during the growth of the gallium nitride crystal.

<Evaluation>
The following evaluations were performed on the examples and comparative examples.
1. Thermal Conductivity The thermal diffusivity and specific heat of the obtained gallium nitride substrate were determined at five points (left, top, center, right, and bottom) as shown in Figure 2, and the thermal conductivity was calculated. The thermal diffusivity was measured using a laser flash thermal constant measuring device, and the specific heat was measured using a differential scanning calorimeter, both at room temperature. The density obtained by weight measurement was 6.06 g/ ^cm³ . The thermal diffusivity was measured in the a-axis and m-axis directions of a test piece cut to a size of 10 mm × 10 mm, including the specified locations, after grinding the gallium polar surface of the substrate obtained using a grinder (#1000) to a thickness of 500 μm. Furthermore, the gallium polar surface of the substrate obtained using a grinder (#1000) was ground to a thickness of 300-400 μm, and three 3 mm x 3 mm test pieces were cut from a 10 mm x 10 mm piece including the predetermined area, and these were stacked to a thickness of 1 mm or more and used for measuring specific heat (in the c-axis direction).
The evaluation results are shown in Tables 1 to 4. The value in the lower right corner of each table is calculated by subtracting the average thermal conductivity in the a-axis direction from the average thermal conductivity in the m-axis direction at five points, and then dividing this result by the average thermal conductivity in the m-axis direction at five points.
2. Yield In each example and comparative example, 20 wafers were fabricated, and the presence or absence of cracks was visually checked. The number of wafers without cracks was counted, and the yield was calculated. All cracks that were found (cracks on the m-plane of the underlying substrate) occurred after the growth of the gallium nitride crystal.
The evaluation results are shown in Table 5.

In the examples, crack occurrence was suppressed, and a high yield (over 70%) was confirmed.

The Group III element nitride substrate according to the embodiment of the present invention can be used, for example, as a substrate for various semiconductor devices.

10 Group III element nitride substrate 11 First main surface 12 Second main surface 13 Side surface 16 Group III element nitride crystal layer 20 Underlayment substrate 20a Top surface 20b Bottom surface 30 Laminated substrate 32 Self-supporting substrate 40 Element substrate 42 Functional layer 90 Crucible 92 Adjustment wall

Claims (5)

A group III element nitride substrate having a first principal surface and a second principal surface facing each other,
The inclination angle of the first main surface with respect to the (0001) surface is 10° or less.
The inclination angle of the second main surface with respect to the (000-1) surface is 10° or less.
The thermal conductivity of the group III element nitride in the m-axis direction at a predetermined location within the substrate surface is 2% or more greater than the thermal conductivity of the group III element nitride in the a-axis direction.
Group III element nitride substrate. The difference between the average value of the thermal conductivity in the m-axis direction and the average value of the thermal conductivity in the a-axis direction at multiple locations within the plane of the substrate is 2% or more, according to claim 1 :
Here, the difference between the average value of the thermal conductivity in the m-axis direction and the average value of the thermal conductivity in the a-axis direction is calculated using the formula: (average value of the thermal conductivity in the m-axis direction - average value of the thermal conductivity in the a-axis direction) / average value of the thermal conductivity in the m-axis direction. A group III element nitride substrate according to claim 1 or 2, having a disc shape and a diameter of 75 mm or more. A group III element nitride substrate according to claim 1 or 2, which is a self-supporting substrate of group III element nitride crystals. A method for manufacturing a group III element nitride substrate,
Prepare a base substrate having an upper surface and a lower surface that face each other, and
This includes growing a group III element nitride crystal on the aforementioned substrate by a flux method,
The crystal growth by the flux method is carried out such that the flow of the raw material solution in the first direction within the plane of the substrate is faster than the flow of the raw material solution in the second direction within the plane of the substrate .
The aforementioned substrate is a Group III element nitride substrate,
The first direction is perpendicular to the m-plane of the substrate.
The group III element nitride substrate has a first main surface and a second main surface that face each other.
The inclination angle of the first main surface with respect to the (0001) surface is 10° or less.
The inclination angle of the second main surface with respect to the (000-1) surface is 10° or less.
In a predetermined location within the surface of the group III element nitride substrate, the thermal conductivity of the group III element nitride in the m-axis direction is 2% or more greater than the thermal conductivity of the group III element nitride in the a-axis direction.
A method for manufacturing a group III element nitride substrate.