Gallium nitride single crystal substrate and method for manufacturing the same

Abstract

This invention provides a manufacturing technology for gallium nitride single crystal substrates in which stress at the edges has been reduced. [Solution] The gallium nitride single crystal substrate is a circular substrate composed of single crystal gallium nitride, having concentric striations that are observed as light and dark stripes when the main surface is observed with a fluorescence microscope, wherein the substrate center, which is the center of the circular substrate, is positioned offset from the striation center, which is the center of the concentric circles of the striations.

Description

Technical Field

[0001] The present invention relates to a gallium nitride single crystal substrate and a method for manufacturing the same.

Background Art

[0002] As a technique for manufacturing a single crystal substrate of gallium nitride (GaN), a method is performed in which a gallium nitride single crystal member as a base material, which is larger than the substrate, is grown, and the outer peripheral portion of the base material is removed to obtain the substrate (see, for example, Patent Document 1).

[0003] In the plane of the gallium nitride single crystal substrate, stress has a distribution. When handling the gallium nitride single crystal substrate, for example, when performing a process of forming a notch such as an orientation flat, the large stress at the edge of the substrate makes it easy to cause problems such as cracking at the edge.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0005] One object of the present invention is to provide a manufacturing technique for a gallium nitride single crystal substrate in which stress reduction at the edge is achieved.

Means for Solving the Problems

[0006] According to one aspect of the present invention, a circular substrate made of single crystal gallium nitride and having concentric striations that are observed in a light and dark striped pattern when the main surface is observed with a fluorescence microscope, A gallium nitride single crystal substrate in which the center of the substrate, which is the circular center of the substrate, is positioned offset from the center of the striations, which is the center of the concentric circles of the striations. It will be provided.

[0007] According to another aspect of the present invention, The process involves forming a base material by growing a gallium nitride single crystal on a circular seed substrate having a larger diameter than the obtained circular gallium nitride single crystal substrate, while at least rotating the seed substrate. A step of obtaining the gallium nitride single crystal substrate by processing the base material such that the center of the substrate, which is the circular center of the gallium nitride single crystal substrate, is located at a position different from the center of rotation of the gallium nitride single crystal during its growth, A method for manufacturing a gallium nitride single crystal substrate having the following characteristics It will be provided. [Effects of the Invention]

[0008] A manufacturing technology for gallium nitride single crystal substrates is provided, which reduces stress at the edges. [Brief explanation of the drawing]

[0009] [Figure 1] Figure 1 is a schematic plan view showing a substrate 10 according to a first embodiment of the present invention. [Figure 2] Figure 2 is a graph showing the distribution of equivalent stress in the base material 100 and substrate 10 with respect to the shear direction in the first embodiment. [Figure 3] Figure 3 is a graph showing the distribution of the equivalent stress of the substrate 10 in the first comparative form and the equivalent stress of the substrate 10 in the first embodiment, with respect to the shear direction. [Figure 4] Figure 4 is a graph showing how the equivalent stress at the edge of the substrate 10 changes depending on the amount of displacement d. [Figure 5] Figure 5 is a schematic plan view showing a first example of the substrate 10 according to the second embodiment. [Figure 6]Figure 6 is a graph showing the distribution of equivalent stress in the shear direction for a substrate 10 without a notch 20 formed according to the first embodiment, and for a substrate 10 with a notch 20 formed on the substrate center C1 side according to the first example of the second embodiment. [Figure 7] Figure 7 is a graph showing the distribution of the equivalent stress in the direction of displacement of the substrate 10 in which the notch 20 is formed according to the second comparative embodiment, and the equivalent stress of the substrate 10 in which the notch 20 is formed on the substrate center C1 side according to the first example of the second embodiment. [Figure 8] Figure 8(a) is a schematic plan view showing the in-plane distribution of equivalent stress in a substrate 10 with a notch 20 formed on the substrate center C1 side, according to the first example of the second embodiment. Figure 8(b) is a schematic plan view showing the in-plane distribution of equivalent stress in a substrate 10 with a notch 20 formed, according to the second comparative embodiment. [Figure 9] Figure 9 is a schematic plan view showing a second example of the substrate 10 according to the second embodiment. [Figure 10] Figure 10 is a graph showing the distribution of equivalent stress in the shear direction for a substrate 10 without a notch 20 formed according to the first embodiment, and for a substrate 10 with a notch 20 formed on the striation center C0 side according to the second example of the second embodiment. [Figure 11] Figure 11 is a schematic plan view showing the distribution of deflection in the eccentric substrate 10. [Figure 12] Figure 12 is a schematic plan view showing the substrate 10 according to the first comparative configuration. [Figure 13] Figure 13 is a graph showing the distribution of equivalent stress in the base material 100 and substrate 10 with respect to the shear direction in the first comparative configuration. [Figure 14] Figure 14 is a schematic plan view showing the substrate 10 according to the second comparative configuration. [Figure 15] Figure 15 is a graph showing the distribution of equivalent stress in the shear direction for a substrate 10 without a notch 20, according to the first comparison configuration, and for a substrate 10 with a notch 20, according to the second comparison configuration. [Figure 16]Figs. 16(a) to 16(c) are schematic cross-sectional views schematically showing a method for manufacturing a base material 100 and a substrate 10. [Figure 17] Fig. 17 is a fluorescence micrograph showing an example of striations 30. [Figure 18] Fig. 18(a) is a schematic plan view showing a GaN substrate in an experiment in which an orientation flat was formed on a GaN substrate with the shift amount d changed to examine the ease of crack generation, and Fig. 18(b) is a table summarizing the results of this experiment.

Mode for Carrying Out the Invention

[0010] <First Embodiment> A gallium nitride single crystal substrate 10 (hereinafter also simply referred to as substrate 10) according to the first embodiment of the present invention will be described. Fig. 1 is a schematic plan view showing the substrate 10 according to the first embodiment. The substrate 10 is a member obtained by cutting out from a base material 100. The outline of the base material 100 is shown by a two-dot chain line, and the outline of the substrate 10 is shown by a solid line. For ease of illustration, the outline of the substrate 10 is shown by a thick solid line.

[0011] The substrate 10 is made of single crystal gallium nitride and is a circular substrate having concentric striations 30 that are observed in a striped pattern of light and dark when the main surface 11 is observed with a fluorescence microscope. The main surface 11 of the substrate 10 is the c-plane.

[0012] The base material 100 is made of single crystal gallium nitride and is circular. As will be described later regarding the manufacturing methods of the base material 100 and the substrate 10, the center C0 of the concentric circles of the striations 30 (hereinafter also referred to as the striation center C0) is arranged at the center of the circle of the base material 100.

[0013] The substrate 10 has a radius r. The radius r of the substrate 10 is, for example, 25 mm or more (the diameter 2r is, for example, 50 mm or more). The substrate 10 according to this embodiment is characterized in that the circular center C1 of the substrate 10 (hereinafter also referred to as the substrate center C1) is positioned offset from the striation center C0. The distance of the substrate center C1 from the striation center C0 is referred to as the offset amount d. The direction from the striation center C0 toward the substrate center C1 is referred to as the offset direction. The configuration in which the substrate center C1 is offset from the striation center C0 according to this embodiment is also referred to as the eccentric configuration.

[0014] Figure 12 is a schematic plan view showing a substrate 10 according to the first comparative embodiment, which corresponds to the conventional technology. In the first comparative embodiment, the substrate 10 is obtained by cutting it from the base material 100 such that the striation center C0 and the substrate center C1 coincide. The embodiment in which the substrate center C1 coincides with the striation center C0 is also called the concentric embodiment.

[0015] The inventors of this invention investigated, through simulation, how the stress distribution in a substrate 10 with a concentric configuration differs from the stress distribution in a substrate 10 with an eccentric configuration. The simulation method and conditions are as follows: As a numerical calculation method, steady-state heat conduction structure coupled analysis using the finite element method was used. As analysis software, Ansys was used. As the analysis model, a full model of the substrate was used. As material properties, the elastic modulus, thermal conductivity, and coefficient of linear expansion of gallium nitride were set. As analysis conditions, the analysis model was divided into four sections (90 degrees apart), out-of-plane deformation was constrained with respect to the cut surfaces, and the center was fixed. A concentric temperature distribution was set on the surface of the substrate, and a uniform temperature was set on the back surface. The temperature conditions were fitted with the amount of warping that occurs in actual substrates.

[0016] The radius r of the substrate 10 was set to 25 mm (diameter 2r = 50 mm). The radius of the base material 100 was set to 35 mm (diameter = 70 mm). The radius of the base material 100 is not particularly limited, as long as it is large enough to obtain the substrate 10 from the base material 100.

[0017] Here, we consider the concentric configuration as the case where the displacement amount d is 0, and the eccentric configuration as the case where the displacement amount d is not 0, and explain both configurations together. The center C1 of the substrate 10 was set at a position separated by a displacement amount d from the center C0 of the base material 100. The displacement amount d was varied to 0 mm, 2.5 mm, 5 mm, and 9 mm, and the in-plane stress distribution of the substrate 10 was calculated for each displacement amount d. The deflection distribution was also calculated.

[0018] In the concentric configuration, the striation center C0 and the substrate center C1 coincide, and the direction of displacement is not defined as in the eccentric configuration. In this simulation, the direction of displacement set in the eccentric configuration is also referred to as the direction of displacement for the concentric configuration for convenience.

[0019] Figure 2 is a graph showing the distribution of equivalent stress in the base material 100 and substrate 10 with respect to the shear direction in the eccentric configuration (first embodiment). The results for a shear amount d of 5 mm are shown. The stress distribution of the base material 100 is shown by a dashed line, and the stress distribution of the substrate 10 is shown by a solid line.

[0020] A radius position of 0 mm indicates the center C0 of the base material 100. The side on which the center C1 of the substrate 10 is located relative to the center C0 of the base material 100 is the positive side in the shift direction. A radius position of +5 mm indicates the center C1 of the substrate 10. The base material 100 corresponds to a radius position range of -35 mm to +35 mm, and the substrate 10 corresponds to a radius position range of -20 mm to +30 mm.

[0021] The stress distribution of the base material 100 and the stress distribution of the substrate 10 have generally similar shapes. The common characteristics of both are as follows. Note that when explaining the common characteristics of the stress distribution of the base material 100 and the stress distribution of the substrate 10, both the base material 100 and the substrate 10 may be referred to as the substrate. A maximum of equivalent stress exists in the center of the substrate. The equivalent stress decreases toward the edges, reaches a minimum at an intermediate radial position, then increases toward the edges, and is greatest at the edges of the substrate.

[0022] The difference between the stress distribution of the base material 100 and the stress distribution of the substrate 10 is as follows: Compared to the base material 100, the substrate 10 cut from the base material 100 has a smaller magnitude of stress (absolute value of stress).

[0023] It can also be considered that the base material 100 is a substrate with a large radius, and substrate 10 is a substrate with a small radius. By comparing the stress distribution of the base material 100 and the stress distribution of substrate 10, it can be seen that the larger the radius of the substrate, the greater the stress tends to be at the edges of the substrate.

[0024] Figure 13 is a graph showing the distribution of equivalent stress in the base material 100 and substrate 10 with respect to the shear direction in a concentric configuration (first comparative configuration). The stress distribution of the base material 100 is shown by a dashed line, and the stress distribution of the substrate 10 is shown by a solid line.

[0025] The stress distribution of the base material 100 is the same as in the eccentric configuration. In the concentric configuration, the radius position 0 mm corresponds to the center C1 of the substrate 10, and the substrate 10 corresponds to a radius position in the range of -25 mm to +25 mm. In the concentric configuration as well, the similarities and differences between the stress distribution of the base material 100 and the stress distribution of the substrate 10 are the same as those described in the eccentric configuration.

[0026] Figure 3 is a graph showing the distribution of equivalent stress in the substrate 10 in a concentric configuration (first comparative configuration) (Figure 13) and in an eccentric configuration (first embodiment) (Figure 2) with respect to the direction of displacement. Here, for both the concentric and eccentric configurations, the center C1 of the substrate 10 is set to radius position 0 mm, and the graphs for both configurations are shown superimposed. The stress distribution for the concentric configuration is shown by a dashed line, and the stress distribution for the eccentric configuration is shown by a solid line.

[0027] Compared to the stress distribution in the concentric configuration, the stress distribution in the eccentric configuration shows a decrease in stress at both edges in the direction of displacement (specifically, within a width of about 2-3 mm from the edge). In the portion closer to the center from the edges, where no particular decrease in stress is observed, the stress distribution in the eccentric configuration is almost identical to that of the concentric configuration.

[0028] In the concentric configuration, the stress distribution is symmetrical with respect to the center C1 of the substrate 10. That is, the stress values ​​at both edges of the substrate 10 in the direction of shearing are equivalent. In contrast, in the eccentric configuration, the stress distribution exhibits asymmetry, with slightly different stress values ​​at both edges (the edge on the striation center C0 side and the edge on the substrate center C1 side).

[0029] Furthermore, the reduction in stress at the edges in the eccentric configuration is not limited to the edges on both sides in the shear direction. In the eccentric configuration, the tendency for stress to decrease at the edges was observed throughout the entire circumferential direction of the substrate 10.

[0030] Thus, in the eccentric configuration according to this embodiment, that is, by acquiring the substrate 10 from the base material 100 such that the substrate center C1 is offset from the striation center C0, the stress at the edge of the substrate 10 can be reduced compared to the concentric configuration according to the comparative configuration. As a result, when handling the substrate 10, for example, when performing processing to form a notch 20 such as an orientation flat as in the second embodiment described later, cracking at the edge of the substrate 10 can be suppressed.

[0031] As described above, the larger the radius r of the substrate 10, the greater the stress at the edge of the substrate 10 tends to be. Therefore, the stress reduction effect at the edge obtained in this embodiment is more favorably obtained when the radius r of the substrate 10 is larger.

[0032] Figure 4 is a graph showing how the equivalent stress at the edge of the substrate 10 changes depending on the amount of displacement d. The horizontal axis represents the amount of displacement d, and the vertical axis represents the equivalent stress. The values ​​on the horizontal and vertical axes are shown as relative values, as explained below. In the following explanation, these relative values ​​may be shown in parentheses.

[0033] Regarding the direction of displacement, the equivalent stress at the edge on the striation center C0 side (left side in Figure 1) is shown by a square plot, and the equivalent stress at the edge on the substrate center C1 side (right side in Figure 1) is shown by a circle plot. The average equivalent stress between the edge on the striation center C0 side and the edge on the substrate center C1 side is shown by an X-shaped plot.

[0034] Hereinafter, the equivalent stress at the edge on the C0 side of the striation center will be referred to as the C0-side stress, the equivalent stress at the edge on the C1 side of the substrate center will be referred to as the C1-side stress, and the average equivalent stress of the C0-side stress and the C1-side stress will be referred to as the mean stress.

[0035] The horizontal axis shows the displacement amount d as a ratio to the diameter 2r of the substrate 10, which is set to 100%. In this example, the diameter 2r of the substrate 10 is 50 mm, and the displacement amounts d of the plotted samples are 0 mm (0%), 2.5 mm (5%), 5 mm (10%), and 9 mm (18%).

[0036] The vertical axis shows the equivalent stress as a ratio to the equivalent stress when the displacement d is 0 mm, which is set to 100%. Specifically, it shows the ratio to the average stress when the displacement d is 0 mm, which is set to 100%.

[0037] As the equivalent stress at the edge, the maximum value of the equivalent stress near the edge is used. For example, with a displacement of 5 mm (10%), the equivalent stress at both edges is evaluated as follows. As shown in Figure 3, the distribution of stress on the C0 side has a maximum. Therefore, as the stress on the C0 side, the value of the equivalent stress at the maximum (not a smaller value obtained from the maximum to the edge) is used. In contrast, the distribution of stress on the C1 side increases to the edge (does not have a maximum). Therefore, as the stress on the C1 side, the value of the equivalent stress at the edge is used.

[0038] Furthermore, the maximum distribution of equivalent stress at the edge may or may not occur depending on the amount of displacement d. Also, whether the maximum occurs at the edge on the striation center C0 side or the edge on the substrate center C1 side may change depending on the amount of displacement d. In addition, whether the stress on the C0 side or the C1 side becomes larger may also change depending on the amount of displacement d. The reason why such phenomena occur is not clear, but these are new findings obtained from this simulation.

[0039] With a displacement of 2.5 mm (5%), the C0 side stress does not have a maximum and shows the value at the edge, while the C1 side stress has a maximum and shows the value at that maximum. With a displacement of 5 mm (10%), as described above, the C0 side stress has a maximum and shows the value at that maximum, while the C1 side stress does not have a maximum and shows the value at the edge. With a displacement of 9 mm (18%), neither the C0 side stress nor the C1 side stress has a maximum and shows the value at the edge.

[0040] In the concentric configuration, that is, when the displacement d is 0 mm (0%), the stress on the C0 side is 14.5 MPa (100%), the stress on the C1 side is 14.5 MPa (100%), and the mean stress is 14.5 MPa (100%).

[0041] The results for eccentric configurations are as follows: With a displacement d of 2.5 mm (5%), the C0 side stress was 13.6 MPa (93.9%), the C1 side stress was 12.1 MPa (83.7%), and the mean stress was 12.8 MPa (88.8%). With a displacement d of 5 mm (10%), the C0 side stress was 12.4 MPa (85.5%), the C1 side stress was 13.1 MPa (90.8%), and the mean stress was 12.7 MPa (88.1%). With a displacement d of 9 mm (18%), the C0 side stress was 14.5 MPa (100%), the C1 side stress was 12.7 MPa (88.1%), and the mean stress was 13.6 MPa (94.2%).

[0042] The C0-side stress, C1-side stress, and mean stress all exhibit a general trend of decreasing as the displacement d increases from 0 to a certain point, and increasing as the displacement d increases further beyond a certain point.

[0043] When examining the mean stress in a typical manner, the following trend can be observed. The mean stress is 100% when the displacement d is 0%, decreases to about 90% when the displacement d is around 5-10%, then increases, reaching about 100% when the displacement d is around 26%. From the perspective of reducing the mean stress by changing the eccentric configuration compared to the concentric configuration, it is preferable to use a range of displacement d such that the mean stress is less than 100%.

[0044] The following are guidelines for the lower limit of the displacement amount d. To make the mean stress less than 100%, 98% or less, 96% or less, 94% or less, 92% or less, and 90% or less, it is preferable to set the displacement amount d to be greater than 0%, 0.9% or more, 1.8% or more, 2.7% or more, 3.6% or more, and 4.5% or more, respectively.

[0045] While the mean stress can be reduced to less than 100% by setting the displacement amount d to more than 0%, it is preferable to set the displacement amount d to 0.5% or more in order to reduce the mean stress to less than 99% by clearly shifting the substrate center C1 from the striation center C0.

[0046] The following are guidelines for the upper limit of the displacement amount d. To keep the mean stress below 100%, below 98%, below 96%, below 94%, below 92%, and below 90%, it is preferable to set the displacement amount d to below 25.7%, below 23.0%, below 20.4%, below 17.8%, below 15.1%, and below 12.5%, respectively.

[0047] When examining the C0-side stress and C1-side stress in order to consider more favorable conditions, the following trends can be observed. In the concentric configuration where the displacement d is 0, the C0-side stress and C1-side stress are the same, and therefore the mean stress is also the same. In contrast, in the eccentric configuration where the displacement d is not 0, a discrepancy (asymmetry) is observed between the C0-side stress and the C1-side stress.

[0048] In the samples with a displacement d of 5% and 18%, the stress on the C0 side was greater than the stress on the C1 side, and in the sample with a displacement d of 10%, the stress on the C1 side was greater than the stress on the C0 side. Furthermore, the difference between the stress on the C0 side and the stress on the C1 side was greater in the samples with a displacement d of 5% and 18% than in the sample with a displacement d of 10%.

[0049] When comparing samples with shift amounts d of 5%, 10%, and 18%, the sample with a shift amount d of 10% had the lowest average stress, and furthermore, the difference between the stress on the C0 side and the stress on the C1 side was the smallest. From this viewpoint, it is preferable to set the shift amount d to be around 10%, for example, between 5% and 15%.

[0050] In the sample with a displacement of d of 18%, the stress on the C1 side and the mean stress were sufficiently small, but the stress on the C0 side reached 100%. From this perspective, it is preferable to keep the displacement of d to 18% or less.

[0051] <Second Embodiment> Next, a substrate 10 according to the second embodiment will be described. Figure 5 is a schematic plan view showing a first example of the substrate 10 according to the second embodiment, and Figure 9 is a schematic plan view showing a second example of the substrate 10 according to the second embodiment. The substrate 10 according to the second embodiment is a substrate in which a notch 20 indicating the crystal orientation of the gallium nitride single crystal constituting the substrate 10 is formed on the edge of the substrate 10 according to the first embodiment (i.e., the eccentric configuration). The notch 20 is, for example, an orientation flat, or for example, a notch.

[0052] The substrate 10 according to the second embodiment is also circular. Here, "circular" includes not only the form without a notch 20 on the edge, as in the substrate 10 according to the first embodiment, but also the form with a notch 20 on the edge, as in the substrate 10 according to the second embodiment.

[0053] The substrate 10 according to the second embodiment has at least one notch 20. If necessary, additional notches may be formed on the edge of the substrate 10. In this example, a configuration in which one orientation flat is formed as the notch 20 is shown.

[0054] The notch 20 (orientation flat or notch) is formed in a symmetrical shape across a predetermined radial direction of the substrate 10. The position of the notch 20 on the substrate 10 is determined by the center position of the notch 20 (the orientation in that radial direction). Details on how the position of the notch 20 is determined (what values ​​represent the position of the notch 20) will be described later with reference to Figure 11.

[0055] The substrate 10 according to the second embodiment is manufactured by preparing the substrate 10 according to the first embodiment and forming a notch 20 (orientation flat or notch) at a predetermined position on the edge of the substrate 10 according to the first embodiment. Various known methods can be used to form the notch 20.

[0056] As described in the first embodiment, the eccentric configuration reduces the stress at the edge of the substrate 10 compared to the concentric configuration. This offers advantages such as suppressing cracking at the edge of the substrate 10 during the formation of the notch 20.

[0057] In the second embodiment, as the first example (see Figure 5), a configuration in which the notch 20 is formed on the substrate center C1 side in the shifting direction will be described, and as the second example (see Figure 9), a configuration in which the notch 20 is formed on the striation center C0 side in the shifting direction will be described.

[0058] Figure 14 is a schematic plan view showing a substrate 10 according to the second comparative configuration, which corresponds to the conventional technology. The substrate 10 according to the second comparative configuration is a substrate in which a notch 20 is formed on the edge of the substrate 10 according to the first comparative configuration (i.e., the concentric configuration).

[0059] The inventors of this application investigated, through simulation, how the stress distribution in a substrate 10 without a notch 20 (first comparative form and first embodiment) differs from the stress distribution in a substrate 10 with a notch 20 (second comparative form and second embodiment) for both concentric and eccentric configurations. An orientation flat with a radial notch width of 5 mm (10% of the substrate 10's diameter of 50 mm) was formed as the notch 20.

[0060] In the eccentric configuration, the notch 20 on the substrate center C1 side was formed when the displacement amount d was 2.5 mm, 5 mm, and 9 mm, and the notch 20 on the striation center C0 side was formed when the displacement amount d was 5 mm. In the concentric configuration, the notch 20 was formed at the same position as the notch 20 on the substrate center C1 side in the eccentric configuration.

[0061] First, the results of the first example of the second embodiment (see Figure 5), in which a notch 20 is formed on the substrate center C1 side, will be described. Figure 6 is a graph showing the distribution of equivalent stress in the direction of displacement for a substrate 10 without a notch 20 (first embodiment) and a substrate 10 with a notch 20 formed on the substrate center C1 side (first example of the second embodiment) in an eccentric configuration. The results for a displacement amount d of 5 mm are shown. The stress distribution of the substrate 10 without a notch 20 is shown by a dashed line, and the stress distribution of the substrate 10 with a notch 20 formed on the substrate center C1 side is shown by a solid line. Note that the stress distribution of the substrate 10 without a notch 20, shown by a dashed line in Figure 6, is the same as the stress distribution of the substrate 10 shown by a solid line in Figure 2 of the first embodiment.

[0062] A radius position of +5mm indicates the center C1 of both substrates 10. For substrates 10 without a notch 20, the radius position corresponds to a range of -20mm to +30mm, while for substrates 10 with a notch 20 formed on the substrate center C1 side, the radius position corresponds to a range of -20mm to +25mm.

[0063] The difference between the stress distribution of the substrate 10 with the notch 20 and the stress distribution of the substrate 10 without the notch 20 can be interpreted as a change in stress distribution associated with the formation of the notch 20.

[0064] On the side of the substrate center C1 opposite the notch 20, there is little change in the stress distribution associated with the formation of the notch 20. On the side of the substrate center C1 that is the notch 20, an increase in stress at the edge is observed with the formation of the notch 20. Furthermore, a tendency for the minimum stress value to decrease is also observed.

[0065] Figure 15 is a graph showing the distribution of equivalent stress in the shear direction for a substrate 10 without a notch 20 (first comparative form) and a substrate 10 with a notch 20 (second comparative form) in a concentric configuration. The stress distribution of the substrate 10 without a notch 20 is shown by a dashed line, and the stress distribution of the substrate 10 with a notch 20 is shown by a solid line. Note that the stress distribution of the substrate 10 without a notch 20, shown by a dashed line in Figure 15, is the same as the stress distribution of the substrate 10 shown by a solid line in Figure 13 of the first comparative form.

[0066] The radius position 0 mm indicates the center C1 of both substrates 10. Substrates 10 without the notch 20 have a radius position ranging from -25 mm to +25 mm, while substrates 10 with the notch 20 have a radius position ranging from -25 mm to +20 mm.

[0067] Even in the concentric configuration, the differences between the stress distribution of the substrate 10 with the notch 20 and the stress distribution of the substrate 10 without the notch 20 (i.e., the trend of changes in the stress distribution associated with the formation of the notch 20) are the same as those described in the eccentric configuration.

[0068] Figure 7 is a graph showing the distribution of equivalent stress (Figure 15) for a concentric substrate 10 with a notch 20 formed thereon (second comparative form), and the distribution of equivalent stress (Figure 6) for an eccentric substrate 10 with a notch 20 formed on the substrate center C1 side (first example of the second embodiment), with respect to the direction of displacement. Here, for both the concentric and eccentric forms, the center C1 of the substrate 10 is set to radius position 0 mm, and the graphs for both forms are shown superimposed. The stress distribution for the concentric form is shown by a dashed line, and the stress distribution for the eccentric form is shown by a solid line.

[0069] In the first embodiment, that is, the eccentric configuration without the notch 20, a decrease in stress was observed at both edges compared to the concentric configuration. In contrast, in the second embodiment, that is, the eccentric configuration with the notch 20, a decrease in stress was observed at the edge on the side without the notch 20 compared to the concentric configuration, while at the edge on the side with the notch 20, an increase in stress equivalent to that of the concentric configuration was observed due to the formation of the notch 20.

[0070] Thus, in the eccentric configuration in which the notch 20 is formed, no particular stress reduction effect is observed at the edge on the side where the notch 20 is formed compared to the concentric configuration. However, as described above, the eccentric configuration provides a stress reduction effect at the edge before the notch 20 is formed. Furthermore, at the edge on the side where the notch 20 is not formed, a stress reduction effect similar to that of the concentric configuration is obtained (maintained).

[0071] Figure 8(a) is a schematic plan view showing the in-plane distribution of equivalent stress for an eccentric substrate 10 (first example of the second embodiment) in which the notch 20 is formed on the substrate center C1 side. Figure 8(b) is a schematic plan view showing the in-plane distribution of equivalent stress for a concentric substrate 10 (second comparative form) in which the notch 20 is formed. Regions with higher stress are shown in darker colors, and regions with lower stress are shown in brighter colors.

[0072] In the eccentric configuration shown in Figure 8(a), where the notch 20 is formed on the substrate center C1 side (first example of the second embodiment), the stress near the corner 21 formed at the end of the notch 20 (orientation flat) is reduced compared to that in the concentric configuration shown in Figure 8(b), where the notch 20 is formed (second comparative configuration).

[0073] Summarizing the (one-dimensional) stress distribution in the displacement direction shown in Figure 7 and the (two-dimensional) stress distribution in the plane shown in Figure 8(a), the following trend can be observed in the equivalent stress near the notch 20 in the eccentric configuration in which the notch 20 is formed on the substrate center C1 side (first example of the second embodiment).

[0074] In the offset direction, at the edge on the side where the notch 20 is formed (i.e., the longitudinal center position of the orientation flat), an increase in stress equivalent to that in the concentric configuration is observed with the formation of the notch 20. However, the stress near the corner 21 formed at the end of the notch 20 (orientation flat) is decreased compared to the concentric configuration.

[0075] Next, the results of a second example of the second embodiment (see Figure 9), in which a notch 20 is formed on the striation center C0 side, will be described. Figure 10 is a graph showing the distribution of equivalent stress in the direction of displacement for a substrate 10 without a notch 20 (first embodiment) and a substrate 10 with a notch 20 formed on the striation center C0 side (second example of the second embodiment) in an eccentric configuration. The results for a displacement amount d of 5 mm are shown. The stress distribution of the substrate 10 without a notch 20 is shown by a dashed line, and the stress distribution of the substrate 10 with a notch 20 formed on the striation center C0 side is shown by a solid line. Note that the stress distribution of the substrate 10 without a notch 20, shown by a dashed line in Figure 10, is the same as the stress distribution of the substrate 10 shown by a solid line in Figure 2 of the first embodiment.

[0076] A radius position of +5mm indicates the center C1 of both substrates 10. For substrates 10 without a notch 20, the radius position corresponds to a range of -20mm to +30mm, while for substrates 10 with a notch 20 formed on the striation center C0 side, the radius position corresponds to a range of -15mm to +30mm.

[0077] Even in the eccentric configuration where the notch 20 is formed on the striation center C0 side, the differences between the stress distribution of the substrate 10 with the notch 20 and the stress distribution of the substrate 10 without the notch 20 (i.e., the trend of changes in the stress distribution associated with the formation of the notch 20) are the same as those described in the eccentric configuration where the notch 20 is formed on the substrate center C1 side. In other words, an increase in equivalent stress at the edge (and a decrease in the minimum value) is observed on the side of the substrate center C1 where the notch 20 is formed.

[0078] Furthermore, in the eccentric configuration in which the notch 20 is formed on the striation center C0 side, similar to the eccentric configuration in which the notch 20 is formed on the substrate center C1 side (see Figure 7), a decrease in stress was observed at the edge on the side where the notch 20 is not formed, compared to the concentric configuration. At the edge on the side where the notch 20 is formed, an increase in stress equivalent to that of the concentric configuration was observed due to the formation of the notch 20.

[0079] Figure 11 is a schematic plan view showing the distribution of deflection in an eccentric substrate 10. Regions where the amount of deflection of the substrate 10 is equal are distributed in concentric circles centered on the striation center C0. Furthermore, the amount of deflection increases with distance from the striation center C0. The positions where the amount of deflection is equal are indicated by concentric lines.

[0080] A azimuth angle θ is defined with respect to the striation center C0, where the direction from the striation center C0 toward the substrate center C1 (i.e., the positive side of the shift direction) is defined as azimuth angle 0°. The region where the azimuth angle θ is in the range of -90° or more and 90° or less (the region to the right of the striation center C0 in Figure 11) is region R1. Outside of region R1, that is, the region where the azimuth angle θ is greater than -180° but less than -90°, and greater than 90° but 180° or less (the region to the left of the striation center C0 in Figure 11) is region R2.

[0081] The position of the notch 20 is determined by the azimuth angle θ that indicates the center position of the notch 20. The first example of the second embodiment, in which the notch 20 is formed on the substrate center C1 side, is an example in which the notch 20 is positioned at position P1 where the azimuth angle θ is 0°, and the notch 20 is located on the edge of the main surface 11 in the range (region R1) where the azimuth angle θ is between -90° and 90°. The second example of the second embodiment, in which the notch 20 is formed on the striation center C0 side, is an example in which the notch 20 is positioned at position P2 where the azimuth angle θ is 180°, and the notch 20 is located in the range (region R2) where the azimuth angle θ is greater than -180° and less than -90°, and greater than 90° and less than or equal to 180°.

[0082] The amount of deflection at the edge of the substrate 10 in region R1 is greater than the amount of deflection at the edge of the substrate 10 in region R2. Therefore, in the embodiment where the notch 20 is formed in region R1, the portion of the substrate 10 with a large amount of deflection is cut out when the notch 20 is formed, compared to the embodiment where the notch 20 is formed in region R2. As a result, in the embodiment where the notch 20 is formed in region R1, the average amount of deflection in the substrate 10 where the notch 20 is formed can be reduced compared to the embodiment where the notch 20 is formed in region R2.

[0083] The closer the azimuth angle θ is to 0°, the further the edge of the substrate 10 moves away from the striation center C0, resulting in a greater amount of deflection at the edge of the substrate 10. From the viewpoint of reducing the average amount of deflection of the substrate 10 due to the formation of the notch 20, it is more preferable to form the notch 20 in a range where the azimuth angle θ is close to 0°.

[0084] From this viewpoint, it is preferable that the (at least one) notch 20 is formed in the range of -90° to 90°, more preferably in the range of -60° to 60°, even more preferably in the range of -45° to 45°, and still more preferably in the range of -30° to 30°.

[0085] The manufacturing methods for the base material 100 and the substrate 10 will now be described. Figures 16(a) to 16(c) are schematic cross-sectional views illustrating the manufacturing methods for the base material 100 and the substrate 10. Figure 16(a) shows the growth process of the base material 100, that is, the growth process of the gallium nitride single crystal that constitutes the base material 100. The base material 100 is grown, for example, by the void formation exfoliation (VAS) method.

[0086] A circular seed substrate 200 having a larger diameter than the substrate 10 obtained from the base material 100 is prepared. The base material 100 is formed by growing a gallium nitride single crystal on the seed substrate 200 in the c-axis direction while at least rotating the seed substrate 200. The seed substrate 200 rotates around its circular center, that is, around the circular center of the base material 100. If necessary, the base material 100 may be grown while revolving in addition to rotating. Due to the periodic fluctuations in the growth conditions of the gallium nitride caused by rotation, a structure is formed in which the physical properties (e.g., impurity concentration) change periodically in the thickness direction of the grown gallium nitride.

[0087] As described later, striations detected by a fluorescence microscope are specifically variations in impurity concentration, that is, structures in which areas of high and low impurity concentration alternate. For striations to be clearly detected, the concentration of the areas with high impurity concentration must be, for example, 1 × 10⁻⁶. 17 / cm 3The above is preferable. Impurities may be intentionally added or may be unavoidably introduced. Examples of impurities include silicon (Si) and germanium (Ge) intentionally added during HVPE growth, and oxygen (O) unavoidably introduced during OVPE growth.

[0088] Figure 16(b) shows the matrix material 100 exfoliated from the seed substrate 200. Due to the tensile stress introduced during the growth of gallium nitride, curvature has occurred in the c-plane of the gallium nitride single crystal in the exfoliated matrix material 100. By using a growth method in which the distribution of initial nuclei is uniform in the plane, such as the VAS method, the c-plane in the matrix material 100 will be curved to have a uniform radius of curvature in the plane, i.e., spherical.

[0089] Figure 16(c) shows the process of cutting out the substrate 10 from the base material 100 (removing unnecessary parts to obtain the substrate 10). For example, grinding, ultrasonic machining, electrical discharge machining, and laser machining can be used as processing methods for cutting out the substrate 10 from the base material 100. If necessary, one substrate 10 may be obtained from the base material 100, or multiple substrates 10 may be obtained by slicing the material cut out from the base material 100.

[0090] The substrate 10 obtained from the base material 100 is flat, and the main surface 11 is processed to be planar. The substrate 10 is obtained such that the main surface 11 is approximately perpendicular to the axis of rotation during the growth of the gallium nitride single crystal. In other words, the substrate 10 is a c-plane substrate, and the normal direction of the main surface 11 is approximately parallel to the c-axis direction of the gallium nitride single crystal (the magnitude of the deviation between the normal direction of the main surface 11 and the c-axis direction is, for example, 1° or less). The end faces of each c-plane, which are stacked in a spherically curved manner in the base material 100, are exposed concentrically within the main surface 11 of the substrate 10. Consequently, the periodic structure that was formed in the thickness direction of the base material 100 is exposed concentrically within the main surface 11.

[0091] In this way, striations 30 are formed within the main surface 11, which are structures in which the physical properties (e.g., impurity concentration) change in a concentric manner. The center C0 of the concentric circles of the striations 30 indicates the center of rotation of the gallium nitride during its growth, and is also located at the circular center of the base material 100.

[0092] In the first comparative form (and the second comparative form), (that is, in the conventional technique), the substrate 10 is obtained by cutting out the substrate 10 so that the center C1 of the substrate 10 coincides with the center C0 of the striation 30 (see Figure 12).

[0093] In the first embodiment (and the second embodiment), the substrate 10 is obtained by cutting out the substrate 10 such that the center C1 of the substrate 10 is shifted by a predetermined amount d from the center C0 of the striation 30 (that is, by processing the base material 100 so that the substrate center C1 is located at a position different from the center of rotation during the growth of the gallium nitride single crystal) (see Figure 1).

[0094] In order for striations 30 to be formed in a concentric circular structure, it is preferable to grow the gallium nitride single crystal so that a spherical c-plane with suppressed distortion is obtained. To perform such growth, it is preferable to increase the uniformity of the growth conditions, specifically, for example, by performing the following type of growth.

[0095] The seed substrate 200 and the base material 100 are circular, and it is preferable that the center of the circular seed substrate 200 and the base material 100 be the center of rotation of the gallium nitride during growth. Furthermore, it is preferable to use a growth method that uniformly distributes the initial nuclei within the plane of the seed substrate 200. For this reason, growth methods that generate initial nuclei non-uniformly within the plane by using a mask pattern, such as Epitaxial Lateral Overgrowth (ELO), are undesirable.

[0096] Furthermore, the fact that the striations 30 are formed in a concentric manner indicates that a gallium nitride material having isotropic properties in the circumferential direction with respect to the center C0 of the striations 30 has been obtained. For this reason, in the eccentric configuration, if the notch 20 is not provided, the direction (orientation) in which the center C1 of the substrate 10 is shifted from the center C0 of the striations 30 may be any direction (orientation).

[0097] When providing a notch 20, it is preferable to determine the direction (orientation) of shifting the center C1 of the substrate 10 by working backward from the position of the notch 20, such that the notch 20 is positioned on the edge of the main surface 11 where the azimuth angle centered on the center C0 of the striations 30 is in the range of -90° to 90° (preferably -60° to 60°, more preferably -45° to 45°, and even more preferably -30° to 30°), with the direction from the center C0 of the striations 30 toward the center C1 of the substrate 10 being 0°. In other words, when providing a notch 20, the substrate 10 of the first embodiment (substrate 10 before the notch 20 is formed) is prepared after the direction of shifting is predetermined by working backward from the position of the notch 20. Note that the position of providing the notch 20 may be appropriately selected according to the design, and specifically, it is determined according to the customer's requirements for the substrate 10, so it cannot be determined to a single crystal orientation.

[0098] Figure 17 is a fluorescence microscope image showing an example of striations 30. The striations 30 are observed as light and dark stripes when the main surface 11 of the substrate 10 is observed with a fluorescence microscope. Specifically, the difference in light and dark observed with the fluorescence microscope is due to the difference in the concentration of impurities contained in the gallium nitride.

[0099] As illustrated in Figure 17, when the diameter of the concentric circles in the observation area is very large (compared to the size of the observation area), the striations 30 appear as nearly parallel stripes rather than concentric circles (because the observation area is narrow in a single field of view). In such cases, by moving the microscope stage and tracking the same line of striations 30, it is possible to confirm that the line is circular and detect the center of the circle, i.e., the striation center C0.

[0100] The striation center C0 may also be detected by other methods. For example, multiple locations on the main surface 11 may be observed with a fluorescence microscope, and the striation center C0 may be detected as the intersection of directions perpendicular to the stripes (diameter directions of the concentric circles) for multiple striped patterns. Here, the multiple locations to be observed are selected such that the directions perpendicular to the stripes are not parallel to each other (they intersect). Alternatively, the striation center C0 may be detected by directly observing the central region of the concentric circles.

[0101] The substrate center C1 is detected based on the circular shape of the substrate 10. In the case of an eccentric substrate 10, the substrate center C1 is positioned at a location offset from the striation center C0 (see Figure 1).

[0102] <Examples> The following describes an experiment in which orientation flats (OFs) were formed on GaN substrates with varying displacement amounts d, and the susceptibility to crack formation was investigated. Figure 18(a) is a schematic plan view of the GaN substrate used in this experiment.

[0103] A c-plane GaN substrate was fabricated using the VAS method with a sapphire substrate with a diameter of 76.2 mm. The diameter of the as-grown substrate (i.e., the base material) was approximately 70 mm. The base material was processed to obtain a substrate with a diameter of 50 mm and a thickness of 400 μm, and an OF (Optical Fiber) was formed on it. The OF was formed on the a-plane. The length of the OF was 16 mm.

[0104] Four types of substrates were fabricated by varying the displacement amount d to 0 mm, 1 mm, 2 mm, and 5 mm. The displacement direction was set so that the substrate center C1 was eccentric, with the striation center (center of the base material) C0 being on the OF side.

[0105] After crystal growth, a circular substrate was fabricated by setting a predetermined displacement amount d and performing a circular cutout to create a substrate before OF formation. After grinding both sides of the circular substrate before OF formation, chamfering and OF processing were performed to create a substrate with OF formed. Finally, the substrate with OF formed was mirror-polished to produce a finished substrate.

[0106] Circular cutouts were performed using a diamond electroplated core drill with a horizontal grinding machine (horizontal cutout machine). Surface grinding was performed using the same machine with a vitrified grinding wheel. Chamfering and OF processing were performed using a resin bond grinding wheel with a chamfering machine.

[0107] Figure 18(b) is a table summarizing the results of this experiment. When the displacement amount d was 0 mm (i.e., concentric configuration), cracks occurred in 3 out of 20 pieces (crack occurrence rate 3 / 20). Cracks occurred during hollowing and chamfering. When the displacement amount d was 1 mm (2% of the substrate diameter), cracks occurred in 1 out of 20 pieces (crack occurrence rate 1 / 20). Microcracks occurred at the edge of the OF. When the displacement amount d was 2 mm (4% of the substrate diameter), no cracks occurred in 156 pieces. When the displacement amount d was 5 mm (10% of the substrate diameter), no cracks occurred in 20 pieces.

[0108] In the configuration where the displacement amount d is not zero (eccentric) (specifically, in this experiment, when the displacement amount is 2% or more of the substrate diameter), the crack occurrence rate is lower compared to the configuration where the displacement amount d is zero (concentric). Furthermore, while crack occurrence was observed at a displacement amount d of 1 mm (2% of the substrate diameter), no crack occurrence was observed at a displacement amount d of 2 mm (4% of the substrate diameter) and a displacement amount d of 5 mm (10% of the substrate diameter) (i.e., when the displacement amount is 4% or more of the substrate diameter).

[0109] <Preferred Embodiments of the Invention> Preferred embodiments of the present invention are described below.

[0110] (Note 1) A circular substrate (preferably with a radius of 25 mm or more) composed of single-crystal gallium nitride, having concentric striations that are observed as light and dark stripes when the main surface is observed with a fluorescence microscope, A gallium nitride single crystal substrate in which the substrate center, which is the circular center of the substrate, is positioned offset from the striation center, which is the center of the concentric circles of the striation.

[0111] (Note 2) The gallium nitride single crystal substrate according to Appendix 1, wherein the distance from the center of the substrate to the striation center is 0.5% or more of the diameter of the substrate (preferably 0.9% or more, more preferably 1.8% or more, even more preferably 2.7% or more, even more preferably 3.6% or more, and even more preferably 4.5% or more).

[0112] (Note 3) A gallium nitride single crystal substrate as described in Appendix 1, wherein the distance from the center of the substrate to the center of the striations is less than 25.7% of the diameter of the substrate (preferably 23.0% or less, more preferably 20.4% or less, even more preferably 17.8% or less, even more preferably 15.1% or less, and even more preferably 12.5% ​​or less).

[0113] (Note 4) A gallium nitride single crystal substrate as described in Appendix 1, having a notch indicating the crystal orientation of the gallium nitride.

[0114] (Note 5) The gallium nitride single crystal substrate according to Appendix 4, wherein, when the direction from the striation center toward the substrate center is defined as an azimuth angle of 0°, the azimuth angle centered on the striation center is in the range of -90° or more and 90° or less (preferably -60° or more and 60° or less, more preferably -45° or more and 45° or less, and even more preferably -30° or more and 30° or less), and the notch portion is located on the edge of the main surface.

[0115] (Note 6) The process involves forming a base material by growing a gallium nitride single crystal on a circular seed substrate having a larger diameter than the obtained circular gallium nitride single crystal substrate, while at least rotating the seed substrate. A step of obtaining the gallium nitride single crystal substrate by processing the base material such that the center of the substrate, which is the circular center of the gallium nitride single crystal substrate, is located at a position different from the center of rotation of the gallium nitride single crystal during its growth, A method for manufacturing a gallium nitride single crystal substrate having the following characteristics. [Explanation of Symbols]

[0116] 10...Gallium nitride single crystal substrate, 11...Main surface, 20...Notch, 21...Corner, 30...Striation, 100...Base material, 200...Seed substrate

Claims

1. A circular substrate composed of single-crystal gallium nitride, having concentric striations observed as light and dark stripes when the main surface is observed with a fluorescence microscope, A gallium nitride single crystal substrate in which the substrate center, which is the circular center of the substrate, is positioned offset from the striation center, which is the center of the concentric circles of the striation.

2. The gallium nitride single crystal substrate according to claim 1, wherein the distance from the center of the substrate to the striation center is 0.5% or more of the diameter of the substrate.

3. The gallium nitride single crystal substrate according to claim 1, wherein the distance from the center of the substrate to the striation center is less than 25.7% of the diameter of the substrate.

4. The gallium nitride single crystal substrate according to claim 1, having a notch indicating the crystal orientation of the gallium nitride.

5. The gallium nitride single crystal substrate according to claim 4, wherein, when the direction from the striation center toward the substrate center is defined as an azimuth angle of 0°, the notch is located on the edge of the main surface in a range of -90° or more and 90° or less, with respect to the azimuth angle centered on the striation center.

6. The process involves forming a base material by growing a gallium nitride single crystal on a circular seed substrate having a larger diameter than the obtained circular gallium nitride single crystal substrate, while at least rotating the seed substrate. A step of obtaining the gallium nitride single crystal substrate by processing the base material such that the center of the substrate, which is the circular center of the gallium nitride single crystal substrate, is located at a position different from the center of rotation of the gallium nitride single crystal during its growth, A method for manufacturing a gallium nitride single crystal substrate having the following characteristics.

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