Gallium nitride single crystal substrate, and gallium nitride single crystal substrate production method

Abstract

To restrain cracks generated during growth and processing, when producing a gallium nitride single crystal substrate with low dislocation density.SOLUTION: A gallium nitride single crystal substrate has a diameter of 50 mm or more, and a crystal face with a low index nearest a main plane is a (0001) face. Average density of an etch pit by etching solution is less than 1×106 cm-2. Among peaks of an etch pit diameter histogram in plural different regions on the main plane, when a first peak diameter with the smallest diameter is a, a second peak diameter with the second smallest diameter is b, a first peak frequency is A, a second peak frequency is B, a first peak constituting etch pit number is α and a second peak constituting etch pit number is β, any of following formula (1), (2) and (3) is satisfied. In the plural histograms, (1) variation of a value of a / b is within an average value of ±5%. (2) Variation of a value of A / B is within an average of ±15%. (3) Variation of a value of α / β is within an average of ±30%.SELECTED DRAWING: Figure 5

Description

[Technical Field]

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

[0002] For example, Patent Document 1 discloses a process for preparing a base substrate made of a single crystal of a group III nitride semiconductor, having a mirror-finished primary surface, with the lowest index crystal plane closest to the primary surface being the (0001) plane; epitaxially growing a single crystal of a group III nitride semiconductor having a top surface exposing the (0001) plane directly on the primary surface of the base substrate; generating a plurality of recesses on the top surface, which are made up of inclined interfaces other than the (0001) plane; gradually expanding the inclined interfaces upward on the primary surface of the base substrate; The method for manufacturing a gallium nitride single crystal substrate is disclosed, which comprises: a first step of growing a first layer whose surface is composed only of inclined interfaces by eliminating the (0001) plane from the top surface; and a second step of epitaxially growing a single crystal of a Group III nitride semiconductor on the first layer, eliminating the inclined interfaces, and growing a second layer having a mirror-finished surface; in the first step, multiple recesses are generated on the top surface of the single crystal, and the (0001) plane is eliminated, thereby forming multiple valleys and multiple peaks on the surface of the first layer.

[0003] Furthermore, for example, Non-Patent Document 1 discloses a method for producing a gallium nitride single crystal substrate by a VAS (Void-Assisted Separation) method. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Publication No. 2020-33211 [Non-patent literature]

[0005] [Non-Patent Document 1] Jpn. J. Appl. Phys. Vol. 42 (2003) pp. L 1-L 3 Summary of the Invention [Problem to be solved by the invention]

[0006] An object of the present invention is to suppress cracks that occur during growth and processing when producing a gallium nitride single crystal substrate with low dislocation density. [Means for solving the problem]

[0007] According to one aspect of the present invention, A gallium nitride single crystal substrate having a diameter of 50 mm or more, and having a low-index crystal plane closest to the primary surface as the (0001) plane, The average density of etch pits formed when etching the main surface with an alkaline etching solution is 1×10 6 cm -2 is less than A histogram of the diameters of the etch pits is created in each of a plurality of different regions on the main surface, and among the peaks appearing in the histogram, the diameter of the first peak, which is the smallest diameter, is defined as a, the diameter of the second peak, which is the second smallest diameter, is defined as b, the frequency of the first peak (the number of etch pits) is defined as A, the frequency of the second peak is defined as B, the number of the etch pits constituting the first peak is defined as α, and the number of the etch pits constituting the second peak is defined as β. This provides a gallium nitride single crystal substrate that satisfies at least one of the following conditions (1), (2), and (3): (1) The variation in the a / b values ​​in the multiple histograms is within ±5% of the average value. (2) The variation in the A / B values ​​in the multiple histograms is within ±15% of the average value. (3) The variation in the α / β values ​​in the multiple histograms is within ±30% of the average value.

[0008] According to another aspect of the present invention, a step (a) of preparing a base substrate made of gallium nitride single crystal, the low-index crystal plane closest to the primary surface being the (0001) plane, and the primary surface having a uniform in-plane dislocation density distribution; (b) epitaxially growing a gallium nitride single crystal on the primary surface of the base substrate; and step (c) of obtaining a gallium nitride single crystal substrate having a diameter of 50 mm or more from the gallium nitride single crystal epitaxially grown in step (b), In the step (b), the gallium nitride single crystal is epitaxially grown using only the (0001) plane as the growth plane throughout the growth period without generating any inclined interface other than the (0001) plane; There is provided a method for producing a gallium nitride single crystal substrate, in which the steps (b) and (c) are repeated at least once using the gallium nitride single crystal substrate obtained in the step (c) as a new base substrate. [Effects of the Invention]

[0009] According to the present invention, when a gallium nitride single crystal substrate with low dislocation density is produced, cracks that occur during growth and processing can be suppressed. [Brief explanation of the drawings]

[0010] [Figure 1] FIG. 1 is a flowchart showing a method for manufacturing a gallium nitride single crystal substrate according to one embodiment of the present invention. [Figure 2] 2(a) to 2(c) are schematic cross-sectional views showing a part of a method for producing a gallium nitride single crystal substrate according to one embodiment of the present invention. [Figure 3] FIG. 3 is an SEM image showing an example of an etch pit formed on the primary surface of a gallium nitride single crystal substrate according to one embodiment of the present invention. [Figure 4] FIG. 4 is a histogram of the etch pit diameters of the VAS substrate. [Figure 5] FIG. 5 is a histogram of the diameters of etch pits on the substrate of Example 1. [Figure 6]FIG. 6 is a histogram of the diameters of etch pits on the substrate of Comparative Example 1. DETAILED DESCRIPTION OF THE INVENTION

[0011] <Insights gained by the inventor> First, the findings of the inventors will be described.

[0012] It is known that when the c-plane of a gallium nitride (GaN) single crystal substrate is etched using, for example, an alkaline etching solution, etch pits corresponding to dislocations are formed. Furthermore, methods for identifying the type of dislocation (edge, mixed, screw) based on the size (diameter) of the etch pit are known to those skilled in the art, but these methods are not necessarily universally applicable. The size of the etch pit formed by alkaline etching corresponds to the magnitude of distortion in the crystal lattice around the dislocation core. Therefore, the size of the etch pit is determined not only by the type of dislocation, but also by the impurities trapped in the dislocation, precipitates on the dislocation, and the distance to other nearby dislocations.

[0013] For the reasons mentioned above, it is difficult to identify the type of dislocation (edge, mixed, screw) based on the size of the etch pit, but it can be said that dislocations that form large etch pits also have large strain fields around the dislocation core. Dislocations with large strain fields are likely to have large Burgers vectors, and such dislocations tend to have hollow-core micropipe defects that act as anomalous diffusion paths for impurities.

[0014] The size of etch pits also depends heavily on etching conditions. Therefore, discussing the absolute value of etch pit size is meaningless. However, among the various types of etch pits detected by etching, the smallest etch pit is highly likely to correspond to an edge dislocation with the smallest strain field around the dislocation core. Therefore, the inventors developed a technique for determining the presence or absence of dislocation defects that adversely affect the characteristics of devices fabricated on gallium nitride single crystal substrates by examining a histogram of etch pit diameters and using the smallest peak diameter as a reference. The inventors then discovered that, when the diameter of the reference etch pit is defined as a, etch pits with a diameter exceeding 4a correspond to dislocations with the aforementioned large accumulated strain, and that such dislocations are likely to adversely affect device characteristics.

[0015] Furthermore, as described in Patent Document 1, when the dislocation density is reduced by forcibly changing the propagation direction of dislocations and annihilating them through growth using an inclined interface as the growth plane (hereinafter also referred to as 3D growth), it has been found that although the dislocation density of the entire substrate is reduced, the type and density of dislocations are likely to vary within the plane. When the type and density of dislocations vary within the plane, a large stress field is formed locally, making it easier for cracks to be introduced into the crystal.

[0016] The inventors conducted extensive research into the relationship between histograms of etch pit diameters formed by alkaline etching and crack occurrence. As a result, they created diameter histograms for multiple different regions within a surface, and found that, where a is the diameter of the first peak (the smallest diameter), b is the diameter of the second peak (the second smallest diameter), A is the frequency of the first peak (the number of etch pits), B is the frequency of the second peak, α is the number of etch pits that make up the first peak, and β is the number of etch pits that make up the second peak, if the variations in the values ​​of a / b, A / B, or α / β in the multiple histograms fall within a specified range, the occurrence of cracks can be dramatically reduced.

[0017] Furthermore, the inventors conducted extensive research into a method for producing a substrate with a uniform in-plane histogram as described above (in other words, a substrate with uniformly reduced dislocations in its plane). As a result, they discovered that a substrate with a uniform in-plane histogram can be produced by using a substrate with a uniform in-plane dislocation density distribution as a starting seed substrate, growing a gallium nitride single crystal using only the (0001) plane as the growth plane throughout the growth period without generating any inclined interfaces other than the (0001) plane (hereinafter also referred to as 2D growth), and further performing generation-alternating growth (described in detail below) without growing a thick film all at once.

[0018] <One embodiment of the present invention> Hereinafter, an embodiment of the present invention will be described with reference to the drawings.

[0019] In the following, in a GaN crystal having a wurtzite structure, <0001> The axis is called the "c-axis" and the (0001) plane is called the "c-plane." The (0001) plane is sometimes called the "+c-plane (gallium polarity plane)" and the (000-1) plane is sometimes called the "-c-plane (nitrogen (N) polarity plane)." The <1-100> axis is called the "m-axis" and the {1-100} plane is called the "m-plane." The m-axis can also be written as the <10-10> axis. The <11-20> axis is called the "a-axis" and the {11-20} plane is called the "a-plane."

[0020] (1) Method for manufacturing gallium nitride single crystal substrate The method for manufacturing a gallium nitride single crystal substrate according to this embodiment will be described with reference to Figure 1 and Figures 2(a) to 2(c). Figure 1 is a flowchart showing the method for manufacturing a gallium nitride single crystal substrate according to this embodiment. Figures 2(a) to 2(c) are schematic cross-sectional views showing a part of the method for manufacturing a gallium nitride single crystal substrate according to this embodiment.

[0021] As shown in FIG. 1, the method for manufacturing a gallium nitride single crystal substrate according to this embodiment includes, for example, a base substrate preparation step S100, a 2D growth step S110, and a slicing and processing step S120.

[0022] (S100: Base board preparation process) As shown in FIG. 2(a), in the base substrate preparation step S100, a base substrate 10 is prepared, which is made of a gallium nitride single crystal and in which the low-index crystal plane closest to the primary surface 10s is the (0001) plane (c-plane 10c), and the primary surface 10s has a uniform in-plane dislocation density distribution. Specifically, the base substrate 10 made of the gallium nitride single crystal is preferably fabricated by the Void-Assisted Separation (VAS) method described in Non-Patent Document 1. GaN substrates fabricated by the VAS method are characterized by a uniform in-plane dislocation density distribution, but the c-plane in the crystal has a warp that forms a concave surface toward the primary surface 10s. To fabricate a substrate with a uniform distribution of dislocation species, which is a characteristic of the gallium nitride single crystal substrate according to this embodiment, the dislocations in the base substrate 10, which serves as the starting substrate, must not have undergone a 3D growth process such as an ELO (Epitaxial Lateral Overgrowth) method, and the dislocations must not accumulate locally, i.e., the dislocations must be uniformly distributed in the plane. The VAS method is a suitable means for obtaining such substrates, but substrates produced by the VAS method have a fair amount of c-plane warpage, the magnitude of which is in a trade-off relationship with the in-plane uniformity of dislocation density. In this embodiment, emphasis is placed on the in-plane uniformity of dislocation density, and measures are taken to alleviate the c-plane warpage in the subsequent crystal growth process (for example, the generational growth described below), thereby allowing c-plane warpage in the base substrate 10.

[0023] The base substrate 10 may be a substrate doped with a dopant such as germanium (Ge), silicon (Si), or oxygen (O), or may be a so-called undoped substrate that is not intentionally doped with a dopant.

[0024] (S110:2D growth process) In the 2D growth step S110, a GaN single crystal is epitaxially grown on the primary surface 10s of the base substrate 10 prepared in the base substrate preparation step S100, with the c-plane 30c serving as the growth surface, as shown in Fig. 2(b). Specifically, for example, by HVPE, GaCl gas and NH gas are supplied to the heated base substrate 10, thereby causing epitaxial growth directly on the primary surface 10s of the base substrate 10, thereby growing a growth layer 30.

[0025] Depending on the growth conditions, as described in Patent Document 1, an inclined interface other than the c-plane may appear on the growth surface, resulting in 3D growth. There are three types of crystal growth modes in the initial stage of epitaxial crystal growth, and these modes vary depending on the crystal growth conditions. Of the three modes, the VW (Volmer-Weber) mode and the SK (Stranski-Krastanov) mode are growth modes that induce 3D growth. In contrast, in the 2D growth step S110, the GaN single crystal must be epitaxially grown (i.e., grown in 2D) using only the c-plane as the growth surface throughout the entire growth period, without creating an inclined interface other than the c-plane. To achieve 2D growth, the crystal growth conditions should be adjusted to achieve the FM (Frank-van der Merwe) mode, which is different from the above growth mode. In 3D growth, the direction of propagation of dislocations is forcibly bent and they accumulate locally, which allows multiple dislocations to be combined and effectively reduces the dislocation density. However, dislocations with large Burgers vectors tend to occur, and dislocation species with various crystal strains are scattered across the substrate surface. In such regions, a large stress field is formed locally, which can cause cracks during crystal growth or cooling. In 2D growth, the direction of propagation of dislocations is not forcibly bent. However, in the process of growing a thick crystal, there is a certain probability that propagating dislocations that are tilted in the direction of propagation will meet and disappear, or dislocation loops will form and propagation will stop, thereby gradually reducing the dislocation density. Thus, although 2D growth takes longer to reduce dislocations than 3D growth, it can reduce dislocations uniformly within the plane because it does not change the propagation direction of dislocations or force dislocations to combine. Another advantage is that dislocations with large Burgers vectors, which are formed by combining multiple dislocations, are less likely to occur.

[0026] In the 2D growth step S110, the growth layer 30 is grown under predetermined growth conditions so that crystal growth proceeds in the FM mode. In this embodiment, the growth conditions preferably include a growth temperature of, for example, 980°C or higher and 1200°C or lower. Furthermore, in the 2D growth step S110, the ratio of the supply rate of NH gas as a nitriding agent gas to the supply rate of GaCl gas as a Group III source gas (hereinafter also referred to as "V / III ratio") is preferably, for example, 0.1 or higher and 5 or lower.

[0027] In the 2D growth step S110, GaCl gas, NH gas, and dichlorosilane (SiH2Cl2) gas as an n-type dopant gas may be supplied to the base substrate 10 to epitaxially grow a Si-doped GaN layer as the growth layer 30. Alternatively, tetrachlorogermane (GeCl4) gas may be supplied as the n-type dopant gas instead of SiH2Cl2 gas to epitaxially grow a Ge-doped GaN layer. Note that the growth layer 30 may be a GaN layer doped with iron (Fe), manganese (Mn), carbon (C), or the like.

[0028] Other growth conditions in the 2D growth step S110 are, for example, as follows: Growth pressure: 90 to 105 kPa, preferably 90 to 95 kPa GaCl gas partial pressure: 1.5 to 15 kPa N2 gas flow rate / H2 gas flow rate: 0 to 1

[0029] In the 2D growth step S110, the thickness of the growth layer 30 is preferably set to, for example, 3 mm or more and 20 mm or less. If the thickness of the growth layer 30 is less than 3 mm, the dislocation reduction effect may be insufficient. Furthermore, since the c-plane warpage of the base substrate 10 is inherited by the growth layer 30 grown on top of it, growing the growth layer 30 to a certain thickness or more generates stress in the crystal that attempts to restore the warpage and flatten the c-plane, gradually reducing the magnitude of the c-plane warpage. If the thickness of the growth layer 30 is less than 3 mm, the stress required to restore the c-plane warpage may be insufficient. Furthermore, it is difficult to obtain a GaN single crystal substrate with sufficient thickness in the subsequent slicing and processing step S120. In contrast, by setting the thickness of the growth layer 30 to 3 mm or more, dislocations can be uniformly reduced within the plane and the c-plane warpage can be reduced. Furthermore, a GaN single crystal substrate with sufficient thickness can be obtained in the subsequent slicing and processing step S120. On the other hand, if the thickness of the growth layer 30 exceeds 20 mm, excessive strain accumulates in the crystal due to the force attempting to restore the c-plane warpage, which can lead to the proliferation of dislocations and the occurrence of microcracks. Furthermore, the internal residual stress of the GaN crystal increases, making the crystal more susceptible to cracking in the slicing and processing step S120. In contrast, by setting the thickness of the growth layer 30 to 20 mm or less, the internal residual stress of the GaN crystal can be reduced, suppressing cracking and breakage.

[0030] (S120: Slicing and processing process) In the slicing and processing step S120, as shown in FIG. 1(c), the growth layer 30 is sliced ​​using a wire saw or the like along a cutting plane substantially parallel to the primary surface 30s (c-plane 30c) of the growth layer 30. This results in at least one gallium nitride single crystal substrate 50 (also referred to as substrate 50) having a diameter of 50 mm or more as an as-sliced ​​substrate. At this time, it is preferable to slice the substrate 50 so that the thickness is, for example, 300 μm or more and 500 μm or less.

[0031] As described above, by growing the growth layer 30 to a certain thickness, stress is generated during growth that attempts to restore the warpage of the c-plane, and therefore the radius of curvature of the c-plane 50c of the substrate 50 cut out from the growth layer 30 becomes larger than the radius of curvature of the c-plane 10c of the base substrate 10. Furthermore, by separating the growth layer 30 from the base substrate 10 in the slicing and processing step S120, the constraints imposed on the growth layer 30 by the warpage of the c-plane from the base substrate 10 are released, and therefore the radius of curvature of the c-plane 50c of the cut-out substrate 50 becomes even larger than before the base substrate 10 was separated. This also reduces the internal residual stress in the cut-out substrate 50.

[0032] Specifically, the radius of curvature of the c-plane 50c of the substrate 50 is preferably, for example, 60 m or more, and more preferably 80 m or more, which makes it possible to make the variation in the off-angle θ of the c-axis relative to the normal to the main surface 50s of the substrate 50 (the maximum and minimum difference in the off-angle θ within the substrate plane) smaller than the variation in the off-angle of the c-axis of the base substrate 10.

[0033] Once the substrate 50 is obtained as an as-sliced ​​substrate, both surfaces of the substrate 50 may be polished using, for example, a polishing device.

[0034] (generational growth) In this embodiment, the substrate 50 obtained in the slicing and processing step S120 is used as a new base substrate 10, and the 2D growth step S110 and the slicing and processing step S120 are repeated at least once (preferably four times or more). This type of growth method is referred to herein as generation-altering growth. By performing generation-altering growth rather than growing a thick film all at once, the warpage of the c-plane is alleviated each time the substrate 50 is sliced. This method can eliminate the drawbacks of using a substrate with a uniform dislocation density distribution as the base substrate 10 at the expense of c-plane warpage. Furthermore, generation-altering growth can further reduce the internal residual stress of the resulting GaN crystal. The reduced internal residual stress makes it easier to maintain the dislocation propagation direction in the new 2D growth step S110 using the substrate 50 as the base substrate 10, thereby facilitating the reduction of dislocations uniformly across the surface. Specifically, by repeating generation-altering growth and increasing the cumulative growth thickness of the growth layer 30 to 12 mm or more, the dislocation density can be reduced to 5×10 5 cm -3 Although there is no particular upper limit to the number of times that generation alternation growth can be performed, from the viewpoint of efficiently producing large-diameter GaN single crystal substrates, it is preferable to set the number of times to, for example, 10 or less.

[0035] Through the above steps, the substrate 50 according to this embodiment is manufactured.

[0036] (Process for manufacturing semiconductor laminates and process for manufacturing semiconductor devices) After the substrate 50 is manufactured, a semiconductor functional layer made of, for example, a Group III nitride semiconductor may be epitaxially grown on the substrate 50 to produce a semiconductor laminate. After the semiconductor laminate is produced, electrodes and the like may be formed on the semiconductor laminate, and the semiconductor laminate is diced to cut out chips of a predetermined size. In this manner, a semiconductor device may be produced. Since the substrate 50 has uniformly reduced dislocations in its surface, the characteristics of the semiconductor device produced on the substrate 50 are uniform in its surface, and reliability can be improved.

[0037] (2) Gallium nitride single crystal substrate (freestanding nitride semiconductor substrate, nitride crystal substrate) Next, the gallium nitride single crystal substrate 50 according to this embodiment will be described.

[0038] In this embodiment, the substrate 50 obtained by the above-described manufacturing method is a free-standing substrate made of single crystal GaN.

[0039] The diameter of the substrate 50 is, for example, 50 mm or more, and the thickness of the substrate 50 is, for example, 300 μm or more and 1 mm or less.

[0040] The conductivity of the substrate 50 is not particularly limited. However, when the substrate 50 is used to manufacture a semiconductor device as a vertical Schottky barrier diode (SBD), the substrate 50 is, for example, n-type, the n-type impurity in the substrate 50 is, for example, Si or Ge, and the n-type impurity concentration in the substrate 50 is, for example, 1.0×10 18 cm -3 Over 1.0 x 10 20 cm -3 The following is the result.

[0041] The substrate 50 has, for example, a primary surface 50s that serves as an epitaxial growth surface. In this embodiment, the low-index crystal plane closest to the primary surface 50s is, for example, a c-plane 50c.

[0042] The main surface 50s of the substrate 50 is, for example, mirror-finished, and the root mean square roughness RMS of the main surface 50s of the substrate 50 is, for example, less than 1 nm.

[0043] Furthermore, in this embodiment, the impurity concentration in the substrate 50 obtained by the above-described manufacturing method is lower than that of a substrate obtained by liquid phase growth such as a flux method or an ammonothermal method.

[0044] Specifically, the hydrogen concentration in the substrate 50 is, for example, 1×10 17 cm -3 Less than 5 x 10 16 cm -3 The oxygen concentration in the substrate 50 is, for example, 5×10 16 cm -3Less than or equal to 3 x 10 16 cm -3 The following is the result.

[0045] (Curving of the c-plane and variation in off-angle) In this embodiment, substrate 50 is manufactured by alternate generation growth, and therefore the radius of curvature of c-face 50c of substrate 50 is larger than, for example, the radius of curvature of c-face 10c of base substrate 10 used in the above-described method of manufacturing substrate 50. Specifically, the radius of curvature of c-face 50c of substrate 50 is preferably, for example, 60 m or more, and more preferably 80 m or more.

[0046] In this embodiment, the upper limit of the radius of curvature of the c-face 50c of the substrate 50 is not particularly limited, and the larger the radius, the better. When the c-face 50c of the substrate 50 is substantially flat, the radius of curvature of the c-face 50c can be considered to be infinite.

[0047] Furthermore, in this embodiment, since the radius of curvature of the c-plane 50c of the substrate 50 is large, the variation in the off-angle θ of the c-axis relative to the normal to the main surface 50s of the substrate 50 can be made smaller than the variation in the off-angle of the c-axis of the base substrate 10.

[0048] Specifically, when an X-ray rocking curve measurement is performed on the (0002) plane of the substrate 50 and the off-angle θ of the c-axis relative to the normal to the main surface 50s is measured based on the diffraction peak angle of the (0002) plane, the variation calculated as the maximum and minimum difference in the magnitude of the off-angle θ within a diameter of 25 mm from the center of the main surface 50s is, for example, 0.024° or less, preferably 0.018° or less.

[0049] In this embodiment, the lower limit of the variation in the off-angle θ of the c-axis of the substrate 50 is not particularly limited, and the smaller the better. When the c-plane 50c of the substrate 50 is substantially flat, the variation in the off-angle θ of the c-axis of the substrate 50 can be considered to be 0°.

[0050] Furthermore, in this embodiment, the curvature of the c-plane 50c is isotropically reduced relative to the main surface 50s of the substrate 50, so the radius of curvature of the c-plane 50c has little directional dependency.

[0051] Specifically, the difference between the radius of curvature of c-face 50c in the direction along the a-axis, as determined by the above-described measurement method, and the radius of curvature of c-face 50c in the direction along the m-axis, is, for example, 50% or less, preferably 20% or less, of the larger of these radii of curvature.

[0052] (dislocation density) In this embodiment, the dislocation density in the surface of substrate 50 is reduced below the dislocation density in major surface 10s of base substrate 10 by the manufacturing method described above.

[0053] When a substrate 50 is manufactured using a base substrate 10 made of a high-purity GaN single crystal produced by the VAS method, there are few non-radiative centers caused by foreign matter or point defects in the substrate 50. Therefore, 95% or more (preferably 99% or more) of the dark spots observed when the primary surface of the substrate 50 of the present application is observed using a multi-photon excitation microscope or the like correspond to dislocations rather than non-radiative centers caused by foreign matter or point defects. Note that a "multi-photon excitation microscope" is sometimes called a two-photon excitation microscope.

[0054] In this embodiment, the dislocations are reduced uniformly within the surface by the above-described manufacturing method, so that no regions with particularly high dislocation density due to dislocation concentration are formed, and regions with low dislocation density are formed uniformly. Specifically, in this embodiment, when the main surface 50s of the substrate 50 is observed with a multiphoton excitation microscope in a field of view of 250 μm square and the dislocation density is calculated from the dark spot density, the dislocation density is 1×10 6 cm -2 There is no region where the dislocation density exceeds 5×10 5 cm -2 The area where this is less than 80% or more of the main surface 50s, preferably 90% or more, and more preferably 95% or more.

[0055] In other words, in this embodiment, the dislocation density averaged over the entire main surface 50s of the substrate 50 is, for example, 1×10 6 cm -2 less than 5.5 x 10 5 cm -2 and more preferably less than 3×10 5 cm -2 The following is the result.

[0056] (Etch pit histogram) When the main surface 50s of the substrate 50 is etched using an alkaline etching solution, etch pits as shown in FIG. 3 are formed. In this embodiment, the substrate 50 was immersed for 20 minutes in a molten solution (temperature: 470°C) prepared by mixing potassium hydroxide (KOH) and sodium hydroxide (NaOH) in a 1:1 ratio. The etched main surface 50s of the substrate 50 was then observed using an SEM. The diameter of each etch pit was calculated by image analysis. A histogram of the etch pit diameters was created from the obtained data. The etch pit diameter may be the maximum diameter of each etch pit region analyzed by image analysis, or the equivalent circle diameter. In this embodiment, the diameter of the smallest peak (hereinafter also referred to as the first peak) among the peaks appearing in the histogram of etch pit diameters was standardized as a, and the horizontal axis (increment) was set to 0.1a. The vertical axis represents the number (frequency) of etch pits within the measurement region.

[0057] The average density (number per unit area) of etch pits formed when the main surface 50s of the substrate 50 of this embodiment is etched under the above-mentioned conditions nearly matches the dislocation density determined from observation using a multiphoton excitation microscope. The size of the etch pits can be changed by adjusting the etching conditions, but if they are present at too high a density, adjacent etch pits will overlap, making it difficult to measure the diameter of the etch pits. Therefore, in order to perform meaningful measurements, the average density of etch pits should be less than 1×10 6 cm -2 Preferably, less than 5.5×10 5 cm -2less than 3 x 10 5 cm -2 It is preferable that the average density of etch pits and the diameter of etch pits are not more than 1 mm. 2 It is preferable to measure from the above areas.

[0058] Histograms of etch pit diameters are created in multiple different regions on the main surface 50s of the substrate 50, and among the peaks appearing in the histogram, the diameter of the first peak, which is the smallest diameter, is denoted as a, the diameter of the second peak, which is the second smallest diameter, is denoted as b, the frequency of the first peak (number of etch pits) is denoted as A, the frequency of the second peak is denoted as B, the number of etch pits constituting the first peak is denoted as α, and the number of etch pits constituting the second peak is denoted as β.The substrate 50 of this embodiment satisfies at least one of the following conditions (1), (2), and (3). (1) The variation of the a / b values ​​in multiple histograms is within ±5% of the average value. (2) The variation of the A / B values ​​in multiple histograms is within ±15% of the average value. (3) The variation of the α / β values ​​in multiple histograms is within ±30% of the average value.

[0059] A substrate 50 that provides such a histogram can be said to have uniformly reduced dislocations within the surface and to be a substrate that is less susceptible to cracking during crystal growth or processing. On the other hand, a substrate with variations exceeding conditions (1), (2), and (3) can be said to have a large stress field generated locally and to be more susceptible to cracking during crystal growth or processing. To accurately determine conditions (1), (2), and (3), it is preferable to create histograms of etch pit diameters in three or more (more preferably five or more) different regions on the main surface 50s.

[0060] The substrate 50 preferably satisfies two of the conditions (1), (2), and (3), and more preferably satisfies all of the conditions (1), (2), and (3), which can dramatically reduce the rate of crack occurrence during crystal growth and processing.

[0061] Furthermore, the substrate 50 preferably satisfies at least one of the following conditions (4), (5) and (6), more preferably satisfies two of them, and particularly preferably satisfies all of them. (4) The difference between the maximum and minimum values ​​of a / b in multiple histograms is 0.1 or less. (5) The difference between the maximum and minimum values ​​of A / B in multiple histograms is 0.5 or less. (6) The difference between the maximum and minimum values ​​of α / β in multiple histograms is 0.8 or less.

[0062] Furthermore, the substrate 50 preferably satisfies at least one of the following conditions (7), (8) and (9), more preferably satisfies two of them, and particularly preferably satisfies all of them. (7) The standard deviation of a / b in multiple histograms is 0.03 or less. (8) The standard deviation of A / B in multiple histograms is 0.20 or less. (9) The standard deviation of β / α in multiple histograms is 0.3 or less.

[0063] Furthermore, it is preferable that the substrate 50 satisfy at least one of the following conditions (10) and (11), and it is more preferable that the substrate 50 satisfy both of them. (10) In a plurality of histograms, A / α is 0.5 or more, and more preferably 0.7 or more. (11) In a plurality of histograms, B / β is 0.5 or more, and more preferably 0.7 or more.

[0064] Furthermore, in the plurality of histograms, the number of etch pits in the substrate 50 is preferably 90% or more, and more preferably 95% or more.

[0065] In the substrate 50 of this embodiment, the total number (total number) of etch pits with a diameter exceeding 4a is preferably α / 1000 or less. This can also be said to mean that there are substantially no etch pits with a diameter exceeding 4a, but since scratches or foreign matter on the substrate 50 can form large etch pits, the expression α / 1000 or less is used. In the substrate 50 of this embodiment, dislocations are unlikely to combine with each other due to the manufacturing method described above, so it can be said that there are substantially no dislocation species or dislocation accumulation regions that generate large localized stress. Therefore, the occurrence of cracks during crystal growth or processing can be suppressed.

[0066] In substrate 50 of this embodiment, the total number of etch pits with a diameter of 2a or more is preferably 1 / 10 or less, and more preferably 1 / 100 or less, of the number of etch pits constituting the first peak. Since etch pits with a diameter of 2a or more may correspond to dislocations that generate large local stresses, the occurrence of cracks can be suppressed by setting the total number of etch pits with a diameter of 2a or more to 1 / 10 or less (more preferably 1 / 100 or less) of the number of etch pits constituting the first peak.

[0067] In the substrate 50 of this embodiment, the number of etch pits constituting the first peak is preferably 50% or more (more preferably 70% or more) of the total. In this embodiment, as described above, dislocation synthesis is unlikely to occur, so the proportion of dislocations with small strain fields is high. This makes it possible to suppress the occurrence of cracks.

[0068] In the substrate 50 of this embodiment, when the frequency of the first peak is A, the number of peaks appearing in the diameter histogram with a frequency of A / 10 or more (the first peak and the second peak) is preferably two. The number of peaks appearing in the etch pit histogram corresponds to the number of constituent types of dislocation defects corresponding to the etch pits. Crystals with many observed peaks are likely to have not only simple edge and mixed dislocations, but also a wide variety of impurities and point defects complexly associated with them, i.e., to have many impurity levels and defect levels. This creates a large localized stress field, making them prone to cracking during crystal growth or processing. In contrast, crystals with only two peaks appearing in the histogram with a frequency of A / 10 or more (the first peak and the second peak) suggest that the only types of dislocations present in the crystal are simple edge dislocations and mixed dislocations. This suggests that the substrate is less susceptible to cracking during crystal growth or processing.

[0069] Furthermore, the substrate 50 of this embodiment preferably has a second peak with a diameter in the range of more than a but less than 2a, and the number β of etch pits constituting the second peak is less than the number α of etch pits constituting the first peak. The fact that the diameter of the etch pits constituting the second peak is in the range of more than a but less than 2a indicates that they are likely to correspond to simple mixed dislocations. A small number of etch pits constituting the second peak, in other words, a large number of etch pits constituting the first peak, indicates that the dislocations in the crystal are primarily composed of dislocations with little strain around the dislocation core. These characteristics mean that the substrate 50 is a substrate that is less susceptible to cracking during crystal growth or processing.

[0070] <Other embodiments> Although the embodiments of the present invention have been specifically described above, the present invention is not limited to the above-described embodiments and can be modified in various ways without departing from the spirit and scope of the present invention.

[0071] In the above-described embodiment, the case where the substrate 50 is mainly n-type has been described, but the substrate 50 may be p-type or semi-insulating. For example, when the substrate 50 is used to manufacture a semiconductor device as a high electron mobility transistor (HEMT), the substrate 50 is preferably semi-insulating.

[0072] In the above embodiment, the growth layer 30 is sliced ​​using a wire saw in the slicing and processing step S120, but for example, an outer diameter blade slicer, an inner diameter blade slicer, an electric discharge machine, etc. may also be used. [Example]

[0073] Next, examples of the present invention will be described. These examples are merely examples of the present invention, and the present invention is not limited to these examples.

[0074] (1) Preparation of gallium nitride single crystal substrate Gallium nitride single crystal substrates of Example 1 and Comparative Example 1 were fabricated as follows.

[0075] [Conditions for producing a gallium nitride single crystal substrate in Example 1] (Base substrate) Material: GaN Preparation method: VAS method Diameter: 2 inches Thickness: 400 μm The closest low-index crystal plane to the main plane: c-plane No patterning of the main surface, such as a mask layer, is required. Root mean square roughness RMS of main surface: 2nm Off-angle of the main surface: 0.4° in the m direction Radius of curvature of main surface: 5m Dislocation density on the main surface: 3×10 6 cm -2 (growth layer) Material: GaN Growth method: HVPE method, 2D growth Growth temperature: 980℃ or higher and 1,200℃ or lower V / III ratio: 0.1 or more and 5 or less Growth layer thickness: 3mm (Slicing conditions) Gallium nitride single crystal substrate thickness: 400 μm (generational growth) Number of generations of growth: 4

[0076] [Conditions for producing gallium nitride single crystal substrate in Comparative Example 1] (Base substrate) Same as Example 1. (1st layer) Material: GaN Growth method: HVPE method, 3D growth First growth condition: The growth temperature was set to 980°C or higher and 1,020°C or lower, and the V / III ratio was set to 2 or higher and 20 or lower. Thickness from the main surface of the base substrate to the surface of the first layer: 1 mm (2nd layer) Material: GaN Growth method: HVPE method, 2D growth Second growth condition: The growth temperature was set to 1,050°C or higher and 1,080°C or lower, and the V / III ratio was set to 2 or higher and 5 or lower. Thickness from the main surface of the base substrate to the surface of the second layer: 2 mm (Slicing conditions) Same as Example 1. (generational growth) In Comparative Example 1, generational alternation growth was not performed.

[0077] (2) Evaluation of etch pit histogram The main surfaces of the base substrate (VAS substrate), the gallium nitride single crystal substrate of Example 1, and the gallium nitride single crystal substrate of Comparative Example 1 were etched using an alkaline etching solution under the following conditions to form etch pits. Etching solution: Alkaline melt consisting of 500g KOH and 500g NaOH Melt temperature: 470℃ Soaking time: 20 minutes

[0078] The main surface of the substrate with etch pits was observed using a SEM (SU5000, Hitachi High-Tech Corporation). The measurement field was 127 μm × 95.3 μm (1000x magnification), and the etch pits were measured by image analysis of a combined image (1143 μm × 1048 μm) of 99 SEM images (9 horizontal × 11 vertical). Specifically, the image was binarized using image processing software (Image J), ​​and the etch pit area was filled in. The larger of the width and height of the etch pit area was calculated as the etch pit diameter. Note that when filling in the etch pit area, areas representing small particles and linear pit areas caused by processing scratches were removed.

[0079] A histogram of etch pit diameters was created from the obtained data on etch pit diameters. In this example, the diameter of the peak (first peak) appearing on the smallest diameter side of the peaks appearing in the histogram of etch pit diameters was normalized as 1, and the horizontal axis was scaled in increments of 0.1. The histogram was created for five regions: the substrate center (0 mm, 0 mm), the right side of the substrate (20 mm, 0 mm), the left side of the substrate (-20 mm, 0 mm), the upper side of the substrate (0 mm, 20 mm), and the lower side of the substrate (0 mm, -20 mm), with the substrate center as the origin, the left-right direction in plan view as the x direction, and the up-down direction in plan view as the y direction.

[0080] Figure 4 shows a histogram for the VAS substrate, Figure 5 shows a histogram for Example 1, and Figure 6 shows a histogram for Comparative Example 1. In each histogram, the horizontal axis shows the normalized etch pit diameter, and the vertical axis shows the number (frequency) of etch pits in the measurement area. Table 1 also shows the average etch pit density, a / b value, A / B value, α / β value, etc. in each histogram for the VAS substrate, Example 1, and Comparative Example 1.

[0081] [Table 1]

[0082] As shown in Fig. 4 and Table 1, the histogram shape of the VAS substrate was uniform across the surface, satisfying the above conditions (2) and (3). In other words, although the average density of etch pits (in other words, the dislocation density) of the VAS substrate is somewhat large, it can be said that the VAS substrate is a base substrate with a uniform dislocation density distribution across the surface.

[0083] Furthermore, as shown in FIG. 5 and Table 1, the histogram shape of the substrate of Example 1 was uniform across the surface, satisfying all of the above-mentioned conditions (1), (2), and (3). Furthermore, the average density of etch pits was smaller than that of the VAS substrate. In other words, the substrate of Example 1 can be said to be a substrate in which dislocations are uniformly reduced across the surface. Furthermore, the variations from the average values ​​of the a / b value, the A / B value, and the α / β value of the substrate of Example 1 were smaller than those of the VAS substrate.

[0084] 6 and Table 1, the histogram shape varied within the surface of the substrate of Comparative Example 1, and did not satisfy the above conditions (1), (2), and (3). In other words, the substrate of Comparative Example 1 can be said to be a substrate in which the dislocation density was reduced but the density and type of dislocations varied within the surface.

[0085] Table 2 shows the occurrence rates of cracks (growth cracks) during substrate fabrication and cracks (processing cracks) during substrate processing for the VAS substrate, the substrate of Example 1, and the substrate of Comparative Example 1. As shown in Table 2, it can be said that the higher the in-plane uniformity of the histogram of a substrate, the lower the occurrence rate of cracks.

[0086] [Table 2]

[0087] (3) Evaluation of the radius of curvature of the c-plane The radius of curvature of the c-plane was measured for the base substrate (VAS substrate), the substrate of Example 1, and the substrate of Comparative Example 1. As a result, the radius of curvature was 5 m for the VAS substrate, 60 m for the substrate of Example 1, and 40 m for the substrate of Comparative Example 1.

[0088] From the above, it was confirmed that a substrate with uniform in-plane dislocation density distribution can be manufactured by using a VAS substrate with a uniform in-plane dislocation density distribution as a base substrate and repeating 2D growth and generation-altering growth. It was also confirmed that the higher the in-plane uniformity of the histogram of the substrate, the lower the rate of crack occurrence. It was also confirmed that the substrate of Example 1, which underwent repeated generation-altering growth, had a larger radius of curvature of the c-plane than the VAS substrate, which was the starting seed substrate, and than the substrate of Comparative Example 1, which did not undergo generation-altering growth.

[0089] <Preferred embodiment of the present invention> Preferred embodiments of the present invention will be described below.

[0090] (Appendix 1) A gallium nitride single crystal substrate having a diameter of 50 mm or more, and having a low-index crystal plane closest to the primary surface as the (0001) plane, The average density of etch pits formed when etching the main surface with an alkaline etching solution is 1×10 6 cm -2 is less than A gallium nitride single crystal substrate that satisfies at least one of the following conditions (1), (2), and (3): when histograms of the diameters of the etch pits are created in a plurality of different regions on the main surface, and when the diameter of a first peak, which is the smallest diameter among the peaks appearing in the histograms, is defined as a, the diameter of a second peak, which is the second smallest diameter, is defined as b, the frequency of the first peak (the number of etch pits), is defined as A, the frequency of the second peak, is defined as B, the number of the etch pits constituting the first peak is defined as α, and the number of the etch pits constituting the second peak is defined as β. (1) The variation in the a / b values ​​in the multiple histograms is within ±5% of the average value. (2) The variation in the A / B values ​​in the multiple histograms is within ±15% of the average value. (3) The variation in the α / β values ​​in the multiple histograms is within ±30% of the average value.

[0091] (Appendix 2) A gallium nitride single crystal substrate according to appendix 1, which satisfies two of the conditions (1), (2) and (3).

[0092] (Appendix 3) A gallium nitride single crystal substrate according to appendix 1, which satisfies all of the above conditions (1), (2), and (3).

[0093] (Appendix 4) A gallium nitride single crystal substrate according to appendix 1, which satisfies at least one of the following conditions (4), (5), and (6): (4) The difference between the maximum and minimum values ​​of a / b in the multiple histograms is 0.1 or less. (5) The difference between the maximum and minimum values ​​of A / B in the multiple histograms is 0.5 or less. (6) The difference between the maximum and minimum values ​​of α / β in the multiple histograms is 0.8 or less. More preferably, two of the conditions (4), (5) and (6) are satisfied, and particularly preferably, all of the conditions (4), (5) and (6) are satisfied.

[0094] (Appendix 5) A gallium nitride single crystal substrate according to appendix 1, which satisfies at least one of the following conditions (7), (8), and (9): (7) The standard deviation of a / b in the multiple histograms is 0.03 or less. (8) The standard deviation of A / B in the multiple histograms is 0.20 or less. (9) The standard deviation of β / α in the plurality of histograms is 0.3 or less. More preferably, two of the conditions (7), (8) and (9) are satisfied, and particularly preferably, all of the conditions (7), (8) and (9) are satisfied.

[0095] (Appendix 6) A gallium nitride single crystal substrate according to appendix 1, which satisfies at least one of the following conditions (10) and (11): (10) In the plurality of histograms, A / α is 0.5 or more, and more preferably 0.7 or more. (11) In the plurality of histograms, B / β is 0.5 or more, and more preferably 0.7 or more. More preferably, both of the conditions (10) and (11) are satisfied.

[0096] (Appendix 7) 2. The gallium nitride single crystal substrate according to claim 1, wherein in a plurality of said histograms, α+β accounts for 90% or more of the total number of etch pits. More preferably, α+β accounts for 95% or more of the total number of etch pits.

[0097] (Appendix 8) 2. A gallium nitride single crystal substrate according to claim 1, wherein the total number of etch pits having a diameter exceeding 4a in the histogram is α / 1000 or less.

[0098] (Appendix 9) 2. The gallium nitride single crystal substrate according to claim 1, wherein the radius of curvature of the (0001) plane is 60 m or more.

[0099] (Appendix 10) The average density of etch pits and the histogram are 2 10. A gallium nitride single crystal substrate according to any one of Supplementary Note 1 to Supplementary Note 9, wherein the measurement is performed from the above-mentioned region.

[0100] (Appendix 11) a step (a) of preparing a base substrate made of gallium nitride single crystal, the low-index crystal plane closest to the primary surface being the (0001) plane, and the primary surface having a uniform in-plane dislocation density distribution; (b) epitaxially growing a gallium nitride single crystal on the primary surface of the base substrate; and step (c) of obtaining a gallium nitride single crystal substrate having a diameter of 50 mm or more from the gallium nitride single crystal epitaxially grown in step (b), In the step (b), the gallium nitride single crystal is epitaxially grown using only the (0001) plane as the growth plane throughout the growth period without generating any inclined interface other than the (0001) plane; A method for producing a gallium nitride single crystal substrate, wherein the steps (b) and (c) are repeated at least once using the gallium nitride single crystal substrate obtained in the step (c) as a new base substrate.

[0101] (Appendix 12) 12. The method for producing a gallium nitride single crystal substrate according to claim 11, wherein the base substrate prepared in the step (a) is a substrate produced by the VAS method.

[0102] (Appendix 13) 12. The method for producing a gallium nitride single crystal substrate according to claim 11, wherein in the step (b), the gallium nitride single crystal is grown to a thickness of at least 3 mm.

[0103] (Appendix 14) 12. The method for producing a gallium nitride single crystal substrate according to claim 11, wherein each time the steps (b) and (c) are repeated, the dislocation density of the resulting gallium nitride single crystal substrate decreases compared to the starting substrate, and the radius of curvature of the (0001) plane increases compared to the starting substrate. [Explanation of symbols]

[0104] 10 Base substrate 30 growth layer 50 Gallium nitride single crystal substrate (substrate) S100 Base board preparation process S110 2D growth process S120 Slicing and Processing

Claims

1. A gallium nitride single crystal substrate having a diameter of 50 mm or more, and a low-index crystal plane closest to the primary surface being the (0001) plane, The average density of etch pits formed when etching the main surface with an alkaline etching solution is 1×10 ６ cm －２ is less than A gallium nitride single crystal substrate that satisfies at least one of the following conditions (1), (2), and (3): when histograms of the diameters of the etch pits are created in a plurality of different regions on the main surface, and when the diameter of a first peak, which is the smallest diameter of peaks appearing in the histograms, is denoted as a, the diameter of a second peak, which is the second smallest diameter, is denoted as b, the frequency of the first peak, is denoted as A, the frequency of the second peak, is denoted as B, the number of the etch pits constituting the first peak is denoted as α, and the number of the etch pits constituting the second peak is denoted as β: (1) The variation in the a / b values ​​in the multiple histograms is within ±5% of the average value. (2) The variation in the A / B values ​​in the multiple histograms is within ±15% of the average value. (3) The variation in the α / β values ​​in the multiple histograms is within ±30% of the average value.

2. 2. The gallium nitride single crystal substrate according to claim 1, which satisfies two of the conditions (1), (2) and (3).

3. 2. The gallium nitride single crystal substrate according to claim 1, which satisfies all of the above conditions (1), (2), and (3).

4. 2. The gallium nitride single crystal substrate according to claim 1, wherein the total number of etch pits having a diameter exceeding 4a in said histogram is α / 1000 or less.

5. 2. The gallium nitride single crystal substrate according to claim 1, wherein the radius of curvature of the (0001) plane is 60 m or more.

6. The average density of etch pits and the histogram are ２ The gallium nitride single crystal substrate according to any one of claims 1 to 5, wherein the measurement is performed from the above-mentioned region.

7. a step (a) of preparing a base substrate made of a gallium nitride single crystal, the low-index crystal plane closest to the main surface being the (0001) plane, and the main surface having a uniform in-plane dislocation density distribution; (b) epitaxially growing a gallium nitride single crystal on the primary surface of the base substrate; and step (c) of obtaining a gallium nitride single crystal substrate having a diameter of 50 mm or more from the gallium nitride single crystal epitaxially grown in step (b), In the step (b), the gallium nitride single crystal is epitaxially grown using only the (0001) plane as the growth plane throughout the growth period without generating any inclined interface other than the (0001) plane; A method for producing a gallium nitride single crystal substrate, comprising repeating steps (b) and (c) at least once using the gallium nitride single crystal substrate obtained in step (c) as a new base substrate.

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