N-type GAN substrate and n-type GAN crystal

Abstract

A high carrier concentration and a uniform in-plane carrier concentration distribution are exhibited by an n-type GaN substrate that has two main surfaces, is doped with Ge, has an average value for the carrier concentration of at least 1 x 1018 cm-3, and satisfies at least one selected from the group consisting of (1) to (3) when the carrier concentration is measured at measurement points taken uniformly within a circular region with a radius of 15 mm from the center on the at least one main surface: (1) the standard deviation / average value of the measurement values for measurement points on straight lines passing through said center at directions of 0 degrees, 45 degrees, 90 degrees, and 135 degrees is not more than 0.25; (2) the percentage, with reference to the total number of measurement points, for the number of measurement points having a carrier concentration of at least 2 x 1018 cm-3 is at least 90%; and (3) the percentage, with reference to the total number of measurement points, for the number of measurement points that exhibit a value (A) less than or equal to 60% / (B) less than or equal to 50% / (C) less than or equal to 40% of the maximum measurement value for all measurement points is (A) not more than 10% / (B) not more than 9% / (C) not more than 8%.

Description

n-type GaN substrate and n-type GaN crystal

[0001] The present invention relates to a Ge-doped n-type GaN substrate and an n-type GaN crystal.

[0002] The substrates used in currently commercially produced InGaN-based laser diodes (LDs) are conductive n-type GaN substrates doped with donor impurities such as Si and Ge. Recently, research and development of vertical GaN power devices (PDs) using such GaN substrates has been active. Patent Document 1 discloses n-type GaN crystals and c-plane GaN wafers with improved crystallinity.

[0003] International Publication No. 2020 / 036047

[0004] In recent years, there has been a growing demand for GaN substrates with higher carrier concentrations than conventional GaN substrates for laser diodes (LDs) and vertical photodiodes (PDs). The inventors' research has revealed that high doping of Si, a commonly used donor impurity, to obtain n-type GaN substrates with high carrier concentrations results in problems such as the formation of SiN films during crystal growth and the resulting GaN substrates becoming more susceptible to cracking. On the other hand, using Ge as the donor impurity did not pose the problems associated with using Si as a dopant. However, it has been found that GaN substrates using Ge as the donor impurity exhibit significant variations in the in-plane carrier concentration distribution. This was also the case with the Ge-doped GaN substrates described in Patent Document 1. Therefore, a first object of the present invention is to provide an n-type GaN substrate having a high carrier concentration and a uniform in-plane carrier concentration distribution. A second object of the present invention is to provide an n-type GaN crystal useful for manufacturing n-type GaN substrates having a high carrier concentration and a uniform in-plane carrier concentration distribution.

[0005] The present inventors conducted extensive research to solve the above-mentioned problems and attempted to elucidate the cause of the above-mentioned variations in carrier concentration distribution. In light of the presumed cause, they investigated the GaN crystal growth process and conditions during substrate manufacturing, and completed the present invention.

[0006] Specifically, the present invention provides the following: [1] An n-type GaN substrate having two primary surfaces, containing Ge as a donor impurity, and having an average carrier concentration of 1×10 １８ cm －３ or more, and on at least one of the two principal surfaces, a line segment (line segment A) of 30 mm in length, the midpoint of which coincides with the center of the principal surface, and a line segment (line segment B) of 30 mm in length, the midpoint of which coincides with the center of the principal surface and is perpendicular to line segment A, are drawn, and when a carrier concentration is measured using square lattice points at 0.5 mm intervals, with line segment A and line segment B set as two axes, and the square lattice points within a circular region with a radius of 15 mm from the center of the principal surface as measurement points, the n-type GaN substrate satisfies at least one of the following (1), (2), and (3): (1) the average value and standard deviation of the carrier concentration at all measurement points on line segment A, line segment B, and two line segments that are the bisectors of the angle between line segment A and line segment B, satisfy the relationship of the following formula (i): standard deviation / average value≦0.25 (i); (2) the carrier concentration is 2×10 １８ cm －３ (3) At least one selected from the group consisting of the following conditions (A), (B), and (C) is satisfied: (A) the ratio of the number of measurement points showing a value that is 60% or less of the maximum value among all the measurement points to the total number of measurement points is 10% or less; (B) the ratio of the number of measurement points showing a value that is 50% or less of the maximum value among all the measurement points to the total number of measurement points is 9% or less; (C) the ratio of the number of measurement points showing a value that is 40% or less of the maximum value among all the measurement points to the total number of measurement points is 8% or less. [2] An n-type GaN substrate having two main surfaces, doped with Ge as a donor impurity, and having an average carrier concentration of 1×10 １８ cm －３the n-type GaN substrate, wherein, on at least one of the two main surfaces, an axis approximately parallel to any one of the crystal orientations parallel to the n-type GaN substrate surface and an axis perpendicular to the axis are taken, lattice points are set at intervals of 0.5 mm from the center of the main surface with respect to each axis, and carrier concentration is measured using lattice points within a circular region having a radius of 15 mm from the center of the main surface as measurement points, the n-type GaN substrate satisfies at least one of the following conditions (1), (2), and (3): (1) the standard deviation and average value of the measured values ​​at all measurement points on a line that passes through the center of the main surface and forms angles of 0 degrees, 45 degrees, 90 degrees, and 135 degrees with the crystal orientation satisfy the following formula (i): standard deviation / average value≦0.25 (i); (2) the carrier concentration is 2×10 １８ cm －３(3) The ratio of the number of measurement points showing a value equal to or less than 60% of the maximum value among all measurement points to the total number of measurement points is 90% or less; (3) At least one selected from the group consisting of the following conditions (A), (B), and (C) is satisfied: (A) The ratio of the number of measurement points showing a value equal to or less than 60% of the maximum value among all measurement points to the total number of measurement points is 10% or less; (B) The ratio of the number of measurement points showing a value equal to or less than 50% of the maximum value among all measurement points to the total number of measurement points is 9% or less; (C) The ratio of the number of measurement points showing a value equal to or less than 40% of the maximum value among all measurement points to the total number of measurement points is 8% or less. [3] The n-type GaN substrate according to [2], which has an orientation flat that allows the crystal orientation to be identified. [4] The n-type GaN substrate according to any of [1] to [3], which satisfies (1). [5] The n-type GaN substrate according to any of [1] to [3], which satisfies (2). [6] The n-type GaN substrate according to any of [1] to [3], which satisfies (3). [7] An n-type GaN substrate according to any one of [1] to [3], which satisfies all of (1), (2), and (3). [8] An n-type GaN substrate according to any one of [1] to [7], which includes a first region on a main surface side that satisfies at least one selected from the group consisting of (1), (2), and (3), and a second region on the other main surface side that has a lower carrier concentration than the first region. [9] An n-type GaN substrate according to any one of [1] to [8], in which the donor impurity contained in the n-type GaN substrate at the highest concentration is Ge.

[10] An n-type GaN substrate according to any one of [1] to [9], which has a diameter of 40 mm or more.

[11] An n-type GaN substrate according to any one of [1] to

[10] , in which one of the two main surfaces is Ga-polar and has an inclination of 0 to 10 degrees with respect to the (0001) crystal plane.

[12] A dislocation density in at least one main surface is 5×10 ５ cm －２

[13] An n-type GaN substrate according to any one of [1] to

[11] , which is an n-type GaN crystal having two main surfaces, doped with Ge as a donor impurity, and having an average carrier concentration of 1×10 １８ cm －３

[14] An n-type GaN crystal according to

[13] , wherein when ten sub-square regions of 2 mm × 2 mm are randomly selected on at least one of the two main surfaces, the average value of the root mean square roughness of ten sub-square regions measured in each of the sub-square regions is 1 μm or less.

[0007] From another aspect of the present invention, there is provided a method for manufacturing an n-type GaN substrate having two main surfaces, the n-type GaN substrate being doped with Ge as a donor impurity, the method including epitaxially growing a GaN crystal on the Ga-polar main surface of a c-plane GaN wafer, and the inclination of the Ga-polar main surface with respect to a (0001) crystal plane is 0±0.1 degrees.

[0008] From another aspect of the present invention, there is provided a method for manufacturing an n-type GaN substrate having two main surfaces, wherein the n-type GaN substrate is doped with Ge as a donor impurity, the method including epitaxially growing a GaN crystal on the Ga-polar main surface of a c-plane GaN wafer, and the inclination of the Ga-polar main surface with respect to a (0001) crystal plane is 0.5 degrees or more.

[0009] The n-type GaN substrate of the present invention has a high carrier concentration and a highly uniform in-plane carrier concentration distribution, making it suitable as a GaN substrate for use in laser diodes and vertical GaN power devices.

[0010] FIG. 1( a) is a plan view of an n-type GaN substrate having an orientation flat, viewed from one principal surface. FIG. 1( b) is a diagram of a circular region with a radius of 15 mm drawn from the center of the principal surface of the substrate of FIG. 1( a). FIG. 1( c) is a diagram explaining measurement points on the substrate of FIG. 1( a). FIG. 2 is a plan view of an example of an n-type GaN substrate with the above-mentioned four line segments drawn. FIG. 3 is a diagram explaining measurement points on the four line segments of an example of an n-type GaN substrate. FIG. 4 is a perspective view of an n-type GaN substrate according to an embodiment. FIG. 5 is a cross-sectional view of an n-type GaN substrate according to an embodiment. FIG. 6 is a diagram explaining that the offcut angle of a c-plane GaN substrate can be decomposed into components in two directions perpendicular to each other. FIG. 7 is a plan view of an n-type GaN substrate according to an embodiment. FIG. 8 is a plan view of an n-type GaN substrate according to an embodiment. FIG. 9 is a schematic diagram showing the basic configuration of an HVPE apparatus. FIG. 10(a) is a cross-sectional view showing a seed and an edge cover set on a susceptor, and FIG. 10(b) is a cross-sectional view showing a GaN crystal grown on the seed shown in FIG. 10(a). FIG. 11 is a cross-sectional view showing a seed and an edge cover set on a susceptor. FIG. 12 is a diagram showing the basic configuration of a crystal growth apparatus used for growing GaN crystals by the ammonothermal method. FIG. 13 is a cross-sectional view showing a GaN crystal layer grown by the ammonothermal method on the N-polar surface of a first c-plane GaN wafer. FIG. 14 is a carrier concentration mapping image obtained by Raman spectroscopy of the n-type GaN substrate of Example 1. FIG. 15 is a carrier concentration mapping image obtained by Raman spectroscopy of the n-type GaN substrate of Comparative Example 1. FIG. 16 is a carrier concentration mapping image obtained by Raman spectroscopy of the n-type GaN substrate of Example 2.

[0011] The present invention will be described in detail below, but the present invention is not limited to the following embodiments and can be practiced with various modifications within the scope of the gist thereof. In this specification, the (0001) crystal plane and the (000-1) crystal plane are collectively referred to as the c-plane, the {10-10} crystal plane is referred to as the m-plane, and the {11-20} crystal plane is referred to as the a-plane. The crystal axis perpendicular to the c-plane is referred to as the c-axis, the crystal axis perpendicular to the m-plane is referred to as the m-axis, and the crystal axis perpendicular to the a-plane is referred to as the a-axis. In this specification, when referring to a crystal axis, crystal plane, crystal orientation, etc., they refer to the crystal axis, crystal plane, crystal orientation, etc. of the GaN substrate unless otherwise specified. The Miller indices (hkil) of a hexagonal crystal are sometimes expressed in three digits as (hkl) because of the relationship h + k = -i. For example, (0004) can be expressed in three digits as (004). Hereinafter, the present invention will be described based on embodiments, with appropriate reference to the drawings.

[0012] 1. n-type GaN substrate 1.1. n-type GaN substrate An n-type GaN substrate according to one embodiment of the present invention (hereinafter, sometimes referred to as "this embodiment") contains Ge as a donor impurity, has two main surfaces, and has an average carrier concentration of 1×10 １８ cm －３ or more, and when a line segment (line segment A) having a length of 30 mm whose midpoint coincides with the center of the main surface and a line segment (line segment B) having a length of 30 mm whose midpoint coincides with the center of the main surface and is perpendicular to line segment A are drawn on at least one of the two main surfaces, and line segment A and line segment B are set as two axes, and square lattice points are set at 0.5 mm intervals, and the square lattice points are located within a circular region having a radius of 15 mm from the center of the main surface as measurement points, at least one selected from the group consisting of the following (1), (2), and (3) is satisfied: (1) the average value and standard deviation of the carrier concentration at all measurement points on line segment A, line segment B, and two line segments that are the bisectors of the angle between line segment A and line segment B satisfy the following relationship: standard deviation / average value≦0.25 (i); (2) the carrier concentration is 2×10 １８ cm －３(3) At least one selected from the group consisting of the following conditions (A), (B), and (C) is satisfied: (A) the ratio of the number of measurement points showing a value that is 60% or less of the maximum value among all measurement points to the total number of measurement points is 10% or less; (B) the ratio of the number of measurement points showing a value that is 50% or less of the maximum value among all measurement points to the total number of measurement points is 9% or less; (C) the ratio of the number of measurement points showing a value that is 40% or less of the maximum value among all measurement points to the total number of measurement points is 8% or less.

[0013] The n-type GaN substrate of this embodiment is particularly doped with Ge as a donor impurity, has two main surfaces, and has an average carrier concentration of 1×10 １８ cm －３ The above is true, and when, on at least one of the two main surfaces, an axis that is approximately parallel to any one of the crystal orientations parallel to the substrate surface and an axis perpendicular to the axis are taken, lattice points are set at intervals of 0.5 mm from the center of the main surface with respect to each axis, and the carrier concentration is measured using the lattice points within a circular region with a radius of 15 mm from the center of the main surface as measurement points, at least one selected from the group consisting of (1), (2), and (3) above is satisfied.

[0014] As a result of investigations by the present inventors, it was found that when the GaN crystal constituting a GaN substrate has multiple hillocks, Ge incorporation is specifically inhibited at the boundary regions between the hillocks, which can result in large in-plane variations in carrier concentration in the Ge-doped GaN substrate. Based on this finding, by suppressing the generation of multiple hillocks during the growth process of the GaN crystal constituting the GaN substrate, it is possible to realize a GaN substrate with small in-plane variations in carrier concentration, even when Ge is doped to achieve a higher carrier concentration than conventional methods. The present inventors have found that an n-type GaN substrate with a highly uniform in-plane carrier concentration distribution, specifically an n-type GaN substrate satisfying at least one of the requirements selected from the group consisting of (1), (2), and (3) above, can be manufactured by, for example, adjusting the seed crystal preparation process, the off-angle of the seed crystal, the composition ratio and flow rate of the source gas, etc., during GaN crystal growth.

[0015] The n-type GaN substrate of this embodiment may satisfy any one, any two, or all of the conditions selected from the group consisting of (1), (2), and (3). The n-type GaN substrate of the present invention preferably satisfies at least (1), more preferably satisfies (1) and (2), or (1) and (3), and even more preferably satisfies all of (1), (2), and (3).

[0016] 1.2. Carrier concentration The average carrier concentration of the GaN substrate of this embodiment is 1×10 １８ cm －３ The average carrier concentration is 1×10 or more. １８ cm －３ From the viewpoint of further reducing the resistance, the average carrier concentration of the GaN substrate is set to 2×10 １８ cm －３ More than 4 × 10 is preferable. １８ cm －３ More preferably, 5 × 10 １８ cm －３ More preferably, 6 × 10 １８ cm －３ More preferably, the crystal quality is reduced to 5×10 １９ cm －３ Preferably, 2 x 10 １９ cm －３ More preferably, 1×10 １９ cm －３ The following is even more preferred:

[0017] In this specification, the average carrier concentration of a GaN substrate refers to the average carrier concentration measured in a circular region with a radius of 15 mm from the center of the GaN substrate. Specifically, the average carrier concentration is the average carrier concentration obtained by Raman spectroscopy measurement at 10,812 measurement points present in a circular region with a radius of 15 mm from the center of the substrate. The measurement points are defined as follows:

[0018] On at least one of the two principal surfaces of an n-type GaN substrate, a 30 mm long line segment (line segment A) whose midpoint coincides with the center of the principal surface and a 30 mm long line segment (line segment B) whose midpoint coincides with the center of the principal surface and is perpendicular to line segment A are drawn. Line segments A and B are set as two axes, and square lattice points are set at 0.5 mm intervals. The square lattice points located within a circular region with a radius of 15 mm from the center of the principal surface are measurement points. For example, an axis approximately parallel to one of the crystal orientations parallel to the n-type GaN substrate surface and an axis perpendicular to the axis can be set, and lattice points can be set at 0.5 mm intervals from the center of one of the principal surfaces of the substrate. In this case, the measurement points are those lattice points located within a circular region with a radius of 15 mm from the center of the substrate. In one embodiment, the crystal orientation may be the orientation of an orientation flat formed on the substrate.

[0019] An example of a measurement point on a substrate having an orientation flat (OF) will be specifically described using Figure 1. Figure 1(a) is a plan view of a substrate having an OF as viewed from one main surface. Figure 1(b) is a diagram of a circular region of radius 15 mm drawn from the center of the main surface of the substrate of Figure 1(a). As shown in Figure 1(c), lines approximately parallel to the OF and lines perpendicular to the axis are drawn at intervals of 0.5 mm, each of which includes a line passing through the center of the main surface. The points where these lines intersect are lattice points. Among the lattice points, those within the circular region are taken as measurement points.

[0020] The n-type GaN substrate according to this embodiment satisfies at least one requirement selected from the group consisting of (1), (2), and (3). The above-mentioned "measurement point" is common to all of the requirements (1), (2), and (3).

[0021] 1.3. Requirement (1) Requirement (1) is as follows: (1) The average value and standard deviation of the carrier concentration at all measurement points on line segment A, line segment B, and two line segments that are the bisectors of the angle between line segment A and line segment B satisfy the relationship of the following formula (i): standard deviation / average value≦0.25 (i) For example, it is sufficient that the standard deviation and average value of the measured values ​​at all measurement points on a line that passes through the center of at least one of the two main surfaces of the n-type GaN substrate and forms angles of 0 degrees, 45 degrees, 90 degrees, and 135 degrees with the above-mentioned crystal orientation satisfy the relationship of formula (i). In other words, when 30 mm long line segments are drawn that pass through the center of at least one of the two main surfaces of an n-type GaN substrate and that form angles of 0, 45, 90, or 135 degrees with any one of the crystal orientations parallel to the substrate surface so that their midpoints coincide with the center of the main surface, the standard deviation and average value of the measured values ​​at all measurement points (lattice points, 43 or 61 points per line segment) on the four line segments should satisfy the relationship shown in formula (i). In this specification, the above "standard deviation / average value" may also be referred to as the coefficient of variation of the carrier concentration.

[0022] When the coefficient of variation of the carrier concentration defined above is 0.25 or less, the in-plane variation of the carrier concentration is small, and the substrate is excellent as an LD substrate.

[0023] In requirement (1), the n-type GaN substrate according to this embodiment preferably has an orientation flat (OF) that indicates the above-mentioned crystal orientation.

[0024] Regarding requirement (1), the n-type GaN substrate according to this embodiment has a carrier concentration variation coefficient of 0.25 or less, as defined above. The carrier concentration variation coefficient of a GaN substrate can be determined as follows. First, on at least one principal surface of the GaN substrate, the carrier concentration is measured by Raman spectroscopy at all measurement points on a total of four lines: line segment A, line segment B, and two line segments that bisect the angle between line segment A and line segment B. Next, the average and standard deviation of the measured carrier concentrations obtained on the four line segments are calculated. The "standard deviation / average value" is calculated from the calculated average and standard deviation to calculate the carrier concentration variation coefficient. The four line segments may be, for example, 30 mm long line segments that pass through the center of the principal surface and form angles of 0 degrees, 45 degrees, 90 degrees, and 135 degrees with the above-mentioned crystal orientation of the substrate, with the midpoints of the line segments coinciding with the center of the principal surface. FIG. 2 shows a plan view of an example of an n-type GaN substrate with the above four line segments drawn thereon.

[0025] On line segments A and B (for example, on line segments that form angles of 0 and 90 degrees with respect to the reference crystal orientation: see Figure 3), the interval between measurement points of the carrier concentration is 0.5 mm, and the number of measurement points on the line segments is 59. On two line segments that are bisectors of the angle between line segments A and B (for example, on line segments that form angles of 45 and 135 degrees with respect to the reference crystal orientation: see Figure 3), the interval between measurement points of the carrier concentration is 0.5 mm × √2, and the number of measurement points on the line segments is 43.

[0026] The coefficient of variation of the carrier concentration may be 0.25 or less, but from the viewpoint of further reducing the variation in the carrier concentration, it is preferably 0.2 or less, more preferably 0.15 or less, and even more preferably 0.1 or less. The smaller the coefficient of variation of the carrier concentration, the better, so there is no particular lower limit, but it is usually 0.001 or more.

[0027] A small coefficient of variation of the carrier concentration as defined above means that the variation in carrier concentration is small across the entire surface of the GaN substrate. For example, when the coefficient of variation of the carrier concentration is calculated from the results of measuring the carrier concentration along a single line segment with a length of 30 mm passing through the center of the GaN substrate, the proportion of the line segment that is the measurement point relative to the entire main surface of the GaN substrate is small, and the coefficient of variation is not considered to be appropriate as an index for measuring the variation in carrier concentration within the surface of the GaN substrate. In contrast, the coefficient of variation of the carrier concentration as defined above is appropriate as an index for measuring the variation in carrier concentration within the surface of the GaN substrate.

[0028] 1.4. Requirement (2) Requirement (2) is as follows: (2) In at least one of the two principal surfaces of the n-type GaN substrate, in a circular region with a radius of 15 mm from the center of the principal surface, the carrier concentration with respect to the area of ​​the circular region is 2×10 １８ cm －３ Specifically, the carrier concentration measured by Raman spectroscopy is 2×10 １８ cm －３ The ratio of the number of measurement points that are equal to or greater than this to the total number of measurement points is 90% or more.

[0029] In a circular region with a radius of 15 mm from the center of the main surface, the carrier concentration with respect to the area of ​​the circular region is 2×10 １８ cm －３ When the area where the carrier concentration is equal to or greater than 90% is 90% or more, the in-plane variation of the carrier concentration is small, and the substrate is excellent as an LD substrate.

[0030] Regarding requirement (2), the n-type GaN substrate according to this embodiment has a carrier concentration of 2×10 on at least one of its principal surfaces. １８ cm －３ The ratio of the number of measurement points where the carrier concentration is 2×10 or more to the total number of measurement points is 90% or more. １８ cm －３ From the viewpoint of reducing the resistance of the GaN substrate, the ratio of the number of measurement points that is equal to or greater than this to the total number of measurement points is preferably 92% or greater, more preferably 94% or greater, and particularly preferably 95% or greater, with the upper limit being 100%.

[0031] The carrier concentration in the area of ​​a circular region with a radius of 15 mm from the center of the main surface is 2×10 １８ cm －３ Specifically, the ratio of the area where the carrier concentration is 2×10 or more among all measurement points is calculated as follows: １８ cm －３ The carrier concentration is calculated by dividing the number of these points by the total number of measurement points. 10,812 lattice points within a circular region with a radius of 15 mm from the center of the substrate are used as measurement points, and the carrier concentration at the 10,812 points is measured by Raman spectroscopy. １８ cm －３ This is calculated by dividing the number of points by 10,812.

[0032] 1.5. Requirement (3) Requirement (3) is as follows: (3) The n-type GaN substrate, on at least one primary surface, satisfies at least one selected from the group consisting of the following conditions (A), (B), and (C): Condition (A) the ratio of the number of measurement points showing a value that is 60% or less of the maximum value among all measurement points to the total number of measurement points is 10% or less; Condition (B) the ratio of the number of measurement points showing a value that is 50% or less of the maximum value among all measurement points to the total number of measurement points is 9% or less; Condition (C) the ratio of the number of measurement points showing a value that is 40% or less of the maximum value among all measurement points to the total number of measurement points is 8% or less.

[0033] In other words, condition (A) means that in a circular region having a radius of 15 mm from the center of the main surface, the area of ​​region (X1) defined below is 10% or less of the area of ​​the circular region, condition (B) means that in a circular region having a radius of 15 mm from the center of the main surface, the area of ​​region (X2) defined below is 9% or less of the area of ​​the circular region, and condition (C) means that in a circular region having a radius of 15 mm from the center of the main surface, the area of ​​region (X3) defined below is 8% or less of the area of ​​the circular region. Region (X1): A region in which the carrier concentration is 60% or less of the maximum carrier concentration on the main surface. Region (X2): A region in which the carrier concentration is 50% or less of the maximum carrier concentration on the main surface. Region (X3): A region in which the carrier concentration is 40% or less of the maximum carrier concentration on the main surface.

[0034] By satisfying at least one of the above conditions (A) to (C), the in-plane variation of the carrier concentration is small, making the substrate excellent for an LD.

[0035] Regarding requirement (3), the n-type GaN substrate according to this embodiment satisfies at least one of the above conditions (A) to (C) on at least one of its principal surfaces. Condition (A) requires that the ratio of the number of measurement points showing values ​​equal to or less than 60% of the maximum measured value to the total number of measurement points be equal to or less than 10%. The maximum measured value is the maximum value of the carrier concentration in a circular region with a radius of 15 mm from the center of the principal surface, and is the maximum value among the carrier concentration measurements at all measurement points.

[0036] Under condition (A), the ratio of the number of measurement points showing values ​​60% or less of the maximum measurement value to the total number of measurement points may be 10% or less. However, from the viewpoint of further reducing the variation in carrier concentration within the main surface, 8% or less is more preferable, 5% or less is even more preferable, and 3% or less is particularly preferable. Under condition (A), the ratio of the number of measurement points showing values ​​60% or less of the maximum measurement value to the total number of measurement points is specifically calculated as follows: The maximum carrier concentration is defined as the value measured at the measurement points. Next, the ratio is calculated by dividing the number of measurement points showing a carrier concentration 60% or less of the maximum carrier concentration by the total number of measurement points. In one embodiment, any one of the crystal orientations parallel to the substrate surface may be an orientation flat orientation. 10,812 lattice points within a circular region with a radius of 15 mm from the substrate center are used as measurement points, and the carrier concentrations at these 10,812 points are measured by Raman spectroscopy. The carrier concentration is calculated by dividing the number of measurement points where the carrier concentration is 60% or less of the maximum carrier concentration among the 10,812 measurement points by 10,812.

[0037] Condition (B) requires that the ratio of the number of measurement points showing values ​​50% or less of the maximum measurement value to the total number of measurement points be 9% or less. Under condition (B), the ratio of the number of measurement points showing values ​​50% or less of the maximum measurement value to the total number of measurement points may be 9% or less, but from the viewpoint of further reducing the variation in carrier concentration within the main surface, it is more preferably 7% or less, even more preferably 5% or less, and particularly preferably 3% or less. The ratio of the number of measurement points showing values ​​50% or less of the maximum measurement value to the total number of measurement points can be calculated in the same manner as the ratio under condition (A).

[0038] Condition (C) requires that the ratio of the number of measurement points showing values ​​40% or less of the maximum measurement value to the total number of measurement points be 8% or less. Under condition (C), the ratio of the number of measurement points showing values ​​40% or less of the maximum measurement value to the total number of measurement points may be 8% or less, but from the viewpoint of further reducing the variation in carrier concentration within the main surface, it is more preferably 6% or less, even more preferably 4% or less, and particularly preferably 2% or less. The ratio of the number of measurement points showing values ​​40% or less of the maximum measurement value to the total number of measurement points can be calculated in the same manner as the ratio under condition (A).

[0039] Regarding requirement (3), it is sufficient that the n-type GaN substrate according to this embodiment satisfies at least one of the above conditions (A) to (C) on at least one of its primary surfaces. However, it is preferable that the n-type GaN substrate satisfies any one of conditions (A) and (B), conditions (B) and (C), or conditions (C) and (A), and it is more preferable that the n-type GaN substrate satisfies all of conditions (A) to (C).

[0040] 1.6. Donor Impurities In this embodiment, the n-type GaN substrate contains donor impurities. For example, the n-type GaN substrate is doped with Ge as a donor impurity. Although it is not prohibited to further dope the n-type GaN substrate with an element other than Ge as a donor impurity in addition to Ge, it is preferable that the donor impurity contained in the GaN substrate at the highest concentration is Ge.

[0041] The n-type GaN substrate may be entirely made up of regions intentionally doped with Ge, or may be made up of a GaN substrate in which Ge is intentionally doped only in a portion thereof.

[0042] 1.7. Primary Surface The n-type GaN substrate according to this embodiment has two primary surfaces. While there are no particular limitations on the plane orientation of the two primary surfaces, it is preferable that one of the two primary surfaces be Ga-polar and tilted at an angle of 0 to 10 degrees relative to the (0001) crystal plane. In this specification, a GaN substrate tilted at an angle of 0 to 10 degrees relative to the (0001) crystal plane or the (000-1) crystal plane may be referred to as a "c-plane GaN substrate." It is preferable that the n-type GaN substrate according to this embodiment satisfies at least one of the conditions (1), (2), and (3) above, at least for the primary surface that is Ga-polar and tilted at an angle of 0 to 10 degrees relative to the (0001) crystal plane.

[0043] In the n-type GaN substrate according to this embodiment, the "first main surface" is the main surface intended to be used for epitaxial growth of a nitride semiconductor, for example, when using the GaN substrate to manufacture a nitride semiconductor device, i.e., the "front surface." The first main surface is mirror-finished, and its root-mean-square (RMS) roughness, measured with an atomic force microscope (AFM), is preferably less than 2 nm in a measurement range of 2 μm × 2 μm, and may be less than 1 nm or less than 0.5 nm. The "second main surface" is the "rear surface," and may be mirror-finished or matte-finished. The n-type GaN substrate according to this embodiment preferably satisfies at least one condition selected from the group consisting of (1), (2), and (3) above, at least on the first main surface.

[0044] 1.8. Specific Examples and Other Information The GaN substrate 20 shown in FIG. 4 is an example of an n-type GaN substrate according to this embodiment. The GaN substrate 20 is a c-plane GaN substrate, and its two opposing principal surfaces (large-area surfaces), i.e., the first principal surface 21 and the second principal surface 22, are Ga-polar and N-polar, respectively. The Ga-polar principal surface is sometimes referred to as the Ga-polar surface, and the N-polar principal surface is sometimes referred to as the N-polar surface. When the first principal surface 21 is Ga-polar, the inclination of the first principal surface 21 with respect to the (0001) crystal plane is 0 degrees or more and 10 degrees or less. The inclination may be 0.2 degrees or more, and may be less than 5 degrees, 2.5 degrees or less, 1.5 degrees or less, 1 degree or less, or 0.5 degrees or less. When the first principal surface 21 is N-polar, the inclination of the first principal surface 21 with respect to the (000-1) crystal plane is 0 degrees or more and 10 degrees or less. The inclination may be less than 5 degrees, less than 2.5 degrees, less than 1.5 degrees, less than 1 degree, or less than 0.5 degrees. The first major surface 21 and the second major surface 22 are preferably parallel to each other.

[0045] The diameter D of the n-type GaN substrate according to this embodiment Ｗ is usually 40 mm or more, 45 mm or more, 95 mm or more, or may be 145 mm or more, and is typically 50 to 55 mm (about 2 inches), 100 to 105 mm (about 4 inches), 150 to 155 mm (about 6 inches), etc. The thickness t of the GaN substrate Ｗ is the diameter D Ｗ For example, the diameter D of the GaN substrate is Ｗ When the thickness t is about 2 inches, Ｗ is preferably 250 to 500 μm, more preferably 300 to 450 μm, and the diameter D Ｗ When the thickness t is about 4 inches, Ｗ The diameter D of the GaN substrate is preferably 400 to 800 μm, more preferably 500 to 650 μm. Ｗ When the thickness t is about 6 inches, Ｗ is preferably 500 to 850 μm, more preferably 600 to 750 μm.

[0046] In the example shown in FIG. 5, the GaN substrate 20 has a first region R made of intentionally doped GaN (ID-GaN). １ a second region R made of unintentionally doped GaN (UID-GaN) on the Ga-polar first main surface 21 side; ２ The first region R １ is the second region R ２ The n-type GaN substrate according to this embodiment has at least the first region R １ It is preferable that the Ga-polar first main surface 21 of the first region R satisfies at least one condition selected from the group consisting of (1), (2), and (3) above. １ and the second region R ２ An intermediate region (not shown) having a carrier concentration intermediate between the N-polarity side and the Ga-polarity side may be provided between the N-polarity side and the Ga-polarity side. In this intermediate region, the carrier concentration may increase continuously or stepwise from the N-polarity side to the Ga-polarity side.

[0047] First area R １ Thickness t １ The thickness of the first region R can be, for example, 5 μm or more and less than 50 μm, 50 μm or more and less than 100 μm, 100 μm or more and less than 150 μm, or 150 μm or more and 250 μm or less. １ and the second region R ２ 5 may be manufactured by completing a c-plane GaN wafer made of UID-GaN, and then growing an ID-GaN layer on the Ga-polar primary surface of the c-plane GaN wafer by HVPE.

[0048] For reference, GaN grown by HVPE (hydride vapor phase epitaxy) has an infrared absorption spectrum of 3150-3250 cm －１ The absorption coefficient is 0.5 cm －１It can be distinguished from ammonothermally grown GaN by the absence of an infrared absorption peak above 1000 MHz, which is related to the high concentration of gallium vacancies in ammonothermally grown GaN [S. Suihkonen, et al., Applied Physics Letters 108, 202105 (2016); W. Jiang, et al., Applied Physics Express 10, 075506 (2017)].

[0049] In the GaN substrate 20 shown in FIG. １ The room temperature resistivity in the first region R may be less than 0.03 Ω cm, less than 0.02 Ω cm, less than 0.015 Ω cm, or less than 0.010 Ω cm. １ The carrier concentration at room temperature is preferably 1×10 １８ cm －３ or more, more preferably 2 × 10 １８ cm －３ That is 3×10 １８ cm －３ Above, and further 4 x 10 １８ cm －３ The first region R １ The intentional dopant contained in the first region R １ In a preferred embodiment, the donor impurity contained in the first region R at the highest concentration is Ge. １ In the second region R, the total concentration of donor impurities excluding Ge can be set to 10% or less of the Ge concentration, further 5% or less, or further 1% or less. ２ In the case where the Si concentration is 5×10 １７ atoms / cm ３ Below, the O concentration is 3 × 10 １６ atoms / cm ３ Below, 2 x 10 １６ atoms / cm ３ or less or 1 x 10 １６ atoms / cm ３ Hereinafter, the concentration of each impurity except for Si, O, and H is 5×10 １５ atoms / cm ３The second region R ２ In this case, the carrier concentration is 5×10 １７ The room temperature resistivity may be less than 0.04 Ω·cm or more.

[0050] The offcut angle of the n-type GaN substrate according to this embodiment can be resolved into two components in the x-direction and y-direction, which are orthogonal to each other within the main surface. Explaining this with reference to FIG. 6, when the normal direction of the first main surface 21 is the z-direction and a vector parallel to the c-axis is the vector Vc, the offcut angle of the wafer 20 is equal to the inclination θ of the vector Vc from the z-axis. This vector Vc is the x-direction component of the vector Vc. ｘ and the y-direction component of the vector Vc ｙ The orthogonal projection of vector Vc on the xz plane is vector Vc ｘ and the orthogonal projection of vector Vc on the yz plane is vector Vc ｙ When the vector Vc is decomposed in this way, the vector Vc ｘ The inclination from the z-axis is the x-direction component θ of the offcut angle θ ｘ and the vector Vc ｙ The inclination from the z-axis is the y-axis component θ of the offcut angle θ ｙ is.

[0051] In the n-type GaN substrate according to this embodiment, the variation range of the x-direction component of the offcut angle on a line passing through the center of at least one of the two principal surfaces and extending in the x-direction, and the variation range of the y-direction component of the offcut angle on a line passing through the center of the same principal surface and extending in the y-direction, can each be 0.15 degrees or less, preferably 0.1 degrees or less, and more preferably 0.08 degrees or less, within a 40 mm section. This evaluation may exclude portions of the GaN substrate that are less than 5 mm from the outer periphery when viewed from above. The variation range is the difference between the maximum and minimum values. For example, a variation range of 0.15 degrees or less means that the variation from the median is within ±0.075 degrees. The x-direction may be parallel to one of the a-planes, and the y-direction is parallel to one of the m-planes.

[0052] The edges of the n-type GaN substrate according to this embodiment may be chamfered. The n-type GaN substrate according to this embodiment may be provided with various markings as needed, such as an orientation flat (OF) indicating the crystal orientation, a notch, or an index flat for easily distinguishing between the front and back surfaces. The primary surface of the n-type GaN substrate according to this embodiment is circular, but is not limited to a circular shape and may be changed to a square, rectangle, hexagon, octagon, or any other shape.

[0053] The n-type GaN substrate according to this embodiment, or at least the high carrier concentration region of the n-type GaN substrate according to this embodiment (for example, the first region R in the example shown in FIG. 5) １ ) is the lower limit of the total concentration of donor impurities is 2 × 10 １８ atoms / cm ３ That's it, 4 x 10 １８ atoms / cm ３ or more, or 6 x 10 １８ atoms / cm ３ The upper limit of the total concentration of donor impurities is 5×10 to avoid a significant deterioration in crystal quality. １９ atoms / cm ３ Below, further 2 x 10 １９ atoms / cm ３ Below, further 1 x 10 １９ atoms / cm ３ In the n-type GaN substrate or its high carrier concentration region according to this embodiment, the variation in carrier concentration along the c-axis direction is preferably within ±25% of the median value, more preferably within ±20%, even more preferably within ±15%, and still more preferably within ±10%. When it is difficult to measure the carrier concentration, it may be substituted with the total concentration of donor impurities.

[0054] The room temperature resistivity of the n-type GaN substrate or its high carrier concentration region according to this embodiment may be less than 0.03 Ω cm, less than 0.02 Ω cm, less than 0.015 Ω cm, or less than 0.010 Ω cm. The carrier concentration at room temperature is preferably 1×10 １８ cm －３ or more, more preferably 2 × 10 １８ cm－３ That is 3×10 １８ cm －３ Above, and further 4 x 10 １８ cm －３ It may be more than that.

[0055] The n-type GaN substrate or its high carrier concentration region according to this embodiment preferably satisfies one or more of the following conditions (a) to (c) regarding impurity concentration: (a) Si concentration is 5×10 １６ atoms / cm ３ (b) The O concentration is 3×10 or more. １６ atoms / cm ３ (c) The H concentration is 1×10 or less. １７ atoms / cm ３ The following is the result.

[0056] The dislocation density on at least one of the principal surfaces of the n-type GaN substrate according to this embodiment is preferably 2×10 ５ cm －２ Less than 1×10, more preferably 1×10 ５ cm －２ More preferably, 5 × 10 ４ cm －２ The dislocation density may be less than 5 mm. When evaluating the dislocation density, portions of the GaN substrate that are less than 5 mm away from the outer periphery when viewed from above may be excluded. The dislocation density is measured as follows: That is, the number of dark spots in a 4 mm x 4 mm area is counted using PL imaging, with the measurement center being a point 8 mm away from the center of the substrate, and the dislocation density is calculated by dividing the count by the measurement area.

[0057] The crystal quality of the GaN substrate is determined by CuKα １ The (004) XRD rocking curve FWHM measured by ω scan using radiation can be used as an index for evaluation. The better the crystal quality, the narrower the (004) XRD rocking curve FWHM. In the (004) XRD rocking curve measurement, the X-ray tube is operated at a voltage of 45 kV and a current of 40 mA, and a monochromated CuK α The ray is incident on the first major surface 21 .

[0058] The X-ray beam size is set so that when the angle of incidence (the angle between the reflecting surface and the X-ray) is 90°, that is, when the X-ray is incident perpendicularly to the (004) reflecting surface, the size of the irradiation area on the main surface is 5 mm in the direction parallel to the ω axis and 1 mm in the direction perpendicular to the ω axis. The ω axis is the rotation axis of the sample in rocking curve measurement. When the X-ray beam size is set in this way, ω is about 36.5° in the (004) XRD rocking curve measurement of GaN, so the irradiation area on the main surface is about 1.7 × 5 mm. ２ is.

[0059] In the n-type GaN substrate according to this embodiment, when a first line segment having a length of 40 mm is drawn on at least one principal surface and an ω-scan is performed every 1 mm along the first line segment over a length of 40 mm to obtain (004) XRD rocking curves at 40 measurement points arranged at 1 mm intervals on the first line segment, it is preferable that the maximum value of the (004) XRD rocking curve FWHM among all measurement points is 30 arcsec or less. Furthermore, the average value of the (004) XRD rocking curve FWHM among all measurement points may be 20 arcsec or less, 18 arcsec or less, further 16 arcsec or less, further 14 arcsec or less, further 12 arcsec or less, or even 10 arcsec or less.

[0060] A specific example is shown in Fig. 7. By performing ω scanning every 1 mm along a line L of 40 mm on the first main surface 21 of the GaN substrate 20, 40 measurement points P arranged at 1 mm pitches on the line L are obtained. Ｍ The (004) XRD rocking curve can be obtained at each measurement point P Ｍ In the ω scan at , the ω axis is set perpendicular to the line L. In other words, the X-rays are made incident on the GaN substrate 20 so that the X-ray incidence plane and the line L are parallel to each other.

[0061] The n-type GaN substrate according to this embodiment may further include, on at least one primary surface, a second line segment having a length of 40 mm that is perpendicular to the first line segment, and performing an ω-scan every 1 mm over the 40 mm length along the second line segment to obtain a (004) XRD rocking curve at 40 measurement points arranged at 1 mm intervals on the second line segment. The maximum value among the 40 measurement points on one of the two line segments and the maximum value among the 40 measurement points on the other line segment may both be 20 arcsec or less. Furthermore, the average value of the (004) XRD rocking curve FWHM among all measurement points on the first and second line segments may be 18 arcsec or less, further 16 arcsec or less, further 14 arcsec or less, further 12 arcsec or less, or even 10 arcsec or less.

[0062] A specific example is shown in Fig. 8. On the first main surface 21 of the GaN substrate 20, two lines L １ and L ２ By performing ω scanning under the above conditions every 1 mm over a length of 40 mm along each of the lines L, the (004) XRD rocking curves at 40 measurement points arranged at 1 mm pitch were obtained. １ , L ２ In this case, the line L １ In the ω scan at each measurement point above, the ω axis is drawn along the line L １ and perpendicular to the line L ２ In the ω scan at each measurement point above, the ω axis is drawn along the line L ２ and perpendicular to it.

[0063] 2. Applications The n-type GaN substrate according to this embodiment can be preferably used as a substrate for the manufacture of various nitride semiconductor devices. A nitride semiconductor device is a semiconductor device in which the main portion of the device structure is formed of a nitride semiconductor. Nitride semiconductors are also called nitride III-V compound semiconductors, group III nitride compound semiconductors, GaN-based semiconductors, etc., and include GaN as well as compounds in which part or all of the gallium in GaN is replaced with other Group 13 elements of the periodic table (e.g., B, Al, In). Representative examples of nitride semiconductor devices that can be manufactured using the n-type GaN substrate according to this embodiment include, but are not limited to, light-emitting devices such as light-emitting diodes (LEDs) and laser diodes (LDs), and electronic devices such as rectifiers, bipolar transistors, field-effect transistors, and high electron mobility transistors (HEMTs).

[0064] 3. GaN Crystal Growth Method A GaN crystal growth method that can be preferably used to manufacture an n-type GaN substrate according to this embodiment will be described below. International Publication No. 2020 / 036047 can be referenced for the manufacturing method. 3.1. HVPE Apparatus FIG. 9 shows the basic configuration of an HVPE apparatus that can be used to manufacture an n-type GaN substrate according to this embodiment. Referring to FIG. 9, the HVPE apparatus 100 includes a hot-wall reactor 101, a gallium reservoir 102 and a susceptor 103 disposed within the reactor, and a first heater 104 and a second heater 105 disposed outside the reactor. The first heater 104 and the second heater 105 each surround the reactor 101 in an annular shape.

[0065] The reactor 101 is a quartz tube chamber. Inside the reactor 101, there is a first zone Z, which is heated mainly by a first heater 104. １ and a second zone Z heated mainly by the second heater 105. ２ There is an exhaust pipe P Ｅ is the second zone Z ２ The first zone Z is connected to the reactor end on the side １The gallium reservoir 102 located in the second zone Z is a quartz vessel having a gas inlet and a gas outlet. ２ The susceptor 103 is made of, for example, graphite. A mechanism for rotating the susceptor 103 can be optionally provided.

[0066] A seed is placed on the susceptor 103. Preferably, an edge cover that covers the edge of the seed is placed on the susceptor together with the seed, as shown in FIG. 10( a). The edge cover is made of, for example, graphite, and its height h is set so that the top surface of the GaN thick film grown on the seed is not positioned above the top end of the edge cover at the end of growth, as shown in FIG. 10( b). The edge cover may also cover the outer periphery of the main surface of the seed in addition to the edge of the seed, as shown in FIG. 11.

[0067] When growing thick GaN crystal films on large area seeds, it is preferable to use edge covers.

[0068] Returning to FIG. 9 again, when growing GaN crystals, the inside of the reactor 101 is heated by the first heater 104 and the second heater 105, and NH ３ (ammonia) through the ammonia inlet pipe P １ Through the second zone Z ２ HCl (hydrogen chloride) diluted with carrier gas is supplied to the hydrogen chloride supply pipe P ２ This HCl reacts with metallic gallium in the gallium reservoir 102, and the resulting GaCl (gallium chloride) is introduced through the gallium chloride introduction pipe P ３ Through the second zone Z ２ The second zone Z ２ At NH ３ and GaCl react with each other, and the resulting GaN crystallizes on the seeds placed on the susceptor 103 .

[0069] When doping the GaN crystal growing on the seed with impurities, the doping gas diluted with the carrier gas is introduced through the dopant introduction pipe P ４through the second zone Z in the reactor 101 ２ Ammonia introduction pipe P １ , hydrogen chloride introduction pipe P ２ , gallium chloride introduction tube P ３ and dopant introduction tube P ４ The portion disposed in the reactor 101 can be made of quartz.

[0070] In FIG. 9, the distance from the nozzle to the susceptor 103 is １ and gallium chloride introduction tube P ３ The ammonia introduction pipe P １ Insert the nozzle into the gallium chloride introduction tube P ３ 9, the ammonia supply pipe P may be opened at a position farther from the susceptor 103 (upstream side) than the nozzle of １ Nozzle and gallium chloride introduction tube P ３ The nozzles may be integrated into one body to form a double-tube nozzle with the former as an outer tube and the latter as an inner tube.

[0071] In FIG. 9, the gallium chloride introduction pipe P ３ and dopant introduction tube P ４ Although the nozzles in the first and second zones are depicted as separate, this is not limiting. For example, to uniformly dope the growing GaN crystal, GaCl and doping gases may be mixed and then injected into the second zone Z through a common nozzle. ２ The dopant introduction tube P ４ Insert the nozzle into the gallium chloride introduction tube P ３ It may also have an opening inward.

[0072] When growing GaN crystals, Ge is doped into the doping gas. ４ (German), GeH ３ Cl (monochlorogermane), GeH ２ Cl ｓ (dichlorogermane), GeHCl ３ (trichlorogermane) or GeCl ４(tetrachlorogermane) can be preferably used. The grown GaN crystal may contain O and Si even though it is not intentionally doped. The unintentional Si probably comes from the quartz that constitutes the reactor or piping, and the unintentional O probably comes from either or both of the quartz and moisture that has entered the reactor from the outside.

[0073] The components arranged in the reactor 101, including those omitted in FIG. 9, include quartz and carbon, as well as SiC (silicon carbide), SiN ｘ (silicon nitride), BN (boron nitride), alumina, W (tungsten), Mo (molybdenum), etc. can be used. By doing so, the concentration of each impurity except for Si, O, and H in the GaN crystal grown on the seed is 5×10 unless intentional doping is performed. １５ atoms / cm ３ It can be the following:

[0074] 3.2. Seed A suitable example of a seed used in growing GaN crystals in the manufacture of an n-type GaN substrate according to this embodiment is NH ４ F and NH ４ The c-plane GaN wafer is grown by the acidic ammonothermal method using I as a mineralizer, and its manufacturing method can be found in WO 2018 / 030311. This c-plane GaN wafer is often grown by 10 １８ atoms / cm ３ Although the oxygen concentration is above 3000kJ / cm2, the oxygen concentration on the Ga-polar main surface is 3×10 １６ atoms / cm ３ The following GaN crystals can be grown by HVPE with little strain:

[0075] The inclination of the main surface relative to the (0001) crystal plane (sometimes referred to herein as the "off-angle") is preferably 0±0.1 degrees, more preferably 0±0.05 degrees, and particularly preferably 0±0.01 degrees. The inventors have discovered that an off-angle within the above range can minimize in-plane variations in carrier concentration. This is believed to be because an off-angle within the above range can suppress the formation of multiple hillocks on the main surface, thereby reducing the boundary region between hillocks. It is common knowledge among those skilled in the art that, for epitaxial growth of GaN crystals, a seed substrate should have an off-angle of, for example, 0.3 degrees or more (see, for example, JP 2019-151518 A). However, the inventors have discovered that even when a substrate with an off-angle of 0±0.1 degrees is used as a seed, crystal growth can be achieved by HVPE, and in-plane variations in carrier concentration can also be suppressed. The off-angle can be adjusted by the angle at which the substrate is cut out.

[0076] On the other hand, it is also preferable that the off-angle of the primary surface is 0.5 degrees or greater. The inventors have found that even when the off-angle is within the above range, the in-plane variation in carrier concentration can be suppressed. While the reason for this is still unclear, they believe it is as follows: The increase in in-plane variation in carrier concentration when doped with Ge at a relatively high concentration is thought to be due in part to the plane orientation dependence of the Ge incorporation amount. Hillocks are formed when epitaxial growth is performed using a c-plane GaN substrate as a seed. Hillocks primarily originate from screw dislocations and have a roughly hexagonal pyramidal shape, which is strongly correlated with the off-angle of the underlying substrate. When the off-angle is 0±0.1 degrees, regular hexagonal pyramidal hillocks are formed. Because each crystal plane formed on the hexagonal pyramid is equivalent, the amount of Ge incorporation is uniform, suppressing in-plane variation. On the other hand, as the off-angle increases, the hillocks tilt in the off-axis direction, and the formed planes no longer become equivalent planes. As a result, in-plane variations in Ge incorporation begin to occur, but the hillocks disappear when the off-angle exceeds a certain value. Once the hillocks disappear, equivalent planes are formed again on the crystal surface, which is thought to enable uniform Ge doping.

[0077] 3.3 Growth Conditions Preferred conditions for growing GaN crystals on seeds by HVPE are as follows: The temperature of the gallium reservoir is, for example, 500 to 1000°C, preferably 700°C or higher, and preferably 900°C or lower. The susceptor temperature is, for example, 900 to 1100°C, preferably 930°C or higher, more preferably 950°C or higher, and preferably 1050°C or lower, more preferably 1020°C or lower.

[0078] NH in the reactor ３ The V / III ratio, which is the ratio of the partial pressure of GaCl to the partial pressure of Ni, can be, for example, 1 to 20, but is preferably 2 or more, more preferably 3 or more, and is preferably 10 or less. If the V / III ratio is too large or too small, it will cause deterioration in the surface morphology of the growing GaN. Deterioration in surface morphology will cause deterioration in crystal quality and an increase in the concentration of oxygen unintentionally incorporated into the GaN crystal.

[0079] The incorporation efficiency of some impurities into GaN crystals strongly depends on the orientation of the facets exposed on the GaN surface during growth. The uniformity of the concentration of such impurities is low inside GaN crystals with poor surface morphology during growth, because facets of various orientations exist on surfaces with poor morphology. A typical example of such an impurity is O (oxygen), but the inventors have found that Ge (germanium) also has a similar tendency. As will be described later, when intentionally growing Ge-doped GaN crystals, the H ２ This is why it is better not to lower the molar ratio too much.

[0080] Additionally, using too low a V / III ratio increases the concentration of nitrogen vacancies in the growing GaN crystal. Although the effect of nitrogen vacancies on GaN wafers and nitride semiconductor devices formed on GaN wafers is not yet clear, their concentration should be kept as low as possible because they are point defects.

[0081] The growth rate of GaN crystals is controlled by the NH ３The growth rate can be controlled using the product of the partial pressure of HCl and the GaCl partial pressure as a parameter. The growth rate is, for example, 20 to 200 μm / h, preferably 30 μm / h or more, more preferably 40 μm / h or more, and preferably 120 μm / h or less, more preferably 100 μm / h or less, and even more preferably 80 μm / h or less. When growing GaN crystals by HVPE on high-quality seeds made of GaN crystals grown by the acidic ammonothermal method, it is preferable to adopt a growth rate of 20 μm / h to 50 μm / h in the initial stage of crystal growth. This is because this can prevent the generation of new threading dislocations at the interface between the seed made of GaN crystals grown by the acidic ammonothermal method and the GaN crystals grown on the seeds by HVPE.

[0082] To improve production efficiency, the growth rate may be increased during growth. For example, the initial growth rate may be 40 μm / h, and the growth rate may be increased to 80 μm / h to 120 μm / h during growth. The threading dislocation density of GaN crystals grown at 80 μm / h is equivalent to that of seeds made of GaN crystals grown by the acidic ammonothermal method, and even in GaN crystals grown at 120 μm / h, the threading dislocation density is thought to be at most twice that of such seeds. When increasing the growth rate, it should be done within a range that does not deteriorate the surface morphology of the growing GaN crystal. The problems that can arise due to deterioration of the surface morphology have been described above.

[0083] NH ３ The carrier gas for diluting each of the HCl and doping gas is H ２ (hydrogen gas), N ２ (nitrogen gas) or H ２ and N ２ A mixed gas of H in a carrier gas can be preferably used. ２ The molar ratio of H in the carrier gas affects the impurity concentration of the growing GaN crystal. ２ The molar ratio is calculated based on the flow rate of each gas species supplied as a carrier gas from outside the reactor into the reactor.

[0084] 4. n-Type GaN Crystal Another embodiment of the present invention relates to an n-type GaN crystal. The n-type GaN crystal is doped with Ge as a donor impurity, has two main surfaces, and has an average carrier concentration of 1×10 １８ cm －３ The above is true, and when ten 2 mm × 2 mm sub-square regions are randomly selected on at least one of the two principal surfaces, the average value of the root mean square roughness of the ten sub-square regions measured in each sub-square region is 1 μm or less. The n-type GaN crystal described above is doped with Ge, has a relatively high carrier concentration, and has a small surface roughness on the principal surface of the crystal. As a result of the inventors' studies, they found that when attempting to produce a lower-resistance n-type GaN substrate by doping a relatively high-carrier-concentration n-type GaN crystal with Ge as a donor impurity, variations in the carrier concentration on the principal surface of the resulting n-type GaN crystal occur. They believe that the roughness of the crystal surface is one of the factors that influence this variation. While widely used Si doping does not result in variations in carrier concentration even when the same level of surface roughness is present, they found that with Ge doping, even slight roughness significantly affects the amount of incorporation. An n-type substrate produced by slicing such a crystal has large in-plane variations in carrier concentration, which adversely affects device characteristics, such as variations in resistance between chips, when a device is manufactured using the n-type substrate. When ten 2 mm × 2 mm sub-square regions are randomly selected on the main surface of the n-type GaN crystal, the average root mean square (RMS) roughness of the ten sub-square regions measured in each sub-square region is 1 μm or less, thereby reducing surface roughness and resulting in an n-type GaN substrate with reduced carrier concentration variation. From this perspective, the average RMS roughness of the ten sub-square regions measured in each sub-square region is preferably 0.8 μm or less, more preferably 0.6 μm or less, and even more preferably 0.5 μm or less. The RMS roughness here is a value measured using a confocal laser microscope. The n-type GaN crystal can be obtained, for example, by setting the off-angle of the seed to 0.5 degrees or more in the aforementioned GaN crystal manufacturing method. It is preferable that the principal surface of the n-type GaN crystal, including the sub-square region, is not subjected to planarization treatment.

[0085] The present invention will be specifically described below with reference to examples, but the present invention is not limited to these examples.

[0086] In the experiments, a crystal growth apparatus whose basic configuration is shown in Figure 12 was used to grow GaN crystals by the ammonothermal method. This crystal growth apparatus comprises an autoclave and a capsule made of a Pt-Ir alloy placed inside it. The capsule has a melting zone and a growth zone separated from each other by a Pt baffle. When growing GaN crystals, a feedstock is placed in the melting zone together with a mineralizer (not shown), and a seed is suspended in the growth zone by a Pt wire. Gas lines connected to a vacuum pump, an ammonia cylinder, and a nitrogen cylinder are connected to the autoclave and the capsule via valves. NH ３ When adding ammonia, NH3 supplied from an ammonia cylinder is used. ３ The amount of gas can be checked using a mass flow meter.

[0087] NH in capsules ３ The pressure inside the capsule when heated to a predetermined temperature is determined by the amount of NH. In order to balance the pressure inside and outside the capsule, NH is added not only inside the capsule but also in the space between the autoclave and the capsule. ３ Seeds, feedstock and mineralizers are installed and NH ３ After the introduction of the HCl solution is completed, the capsule is sealed and heated from the outside of the autoclave by a heater (not shown) to bring the inside of the autoclave into a supercritical state. To create a temperature gradient between the dissolution zone and the growth zone, the upper and lower parts of the autoclave are heated separately by multiple heaters.

[0088] 1. Example 1 (1) Seed Preparation First, a 0.4 mm thick c-plane GaN wafer was prepared, grown by the ammonothermal method using NH4F and NH4I as mineralizers. A stripe-patterned selective growth mask was formed on the mirror-finished N-polar surface of the first c-plane GaN wafer using a lift-off method. The mask consisted of a two-layer sputtered film consisting of a 100 nm thick TiW underlayer and a 100 nm thick Pt surface layer. The linear openings in the selective growth mask had a line width of 50 μm and a pitch of 2 mm. The stripe direction was parallel to the a-plane of the GaN crystal constituting the wafer.

[0089] Using a first c-plane GaN wafer with such a selective growth mask formed on its N-polar surface as a seed, GaN crystals were grown by the ammonothermal method. In the ammonothermal process, polycrystalline GaN was used as the feedstock, and NH4F and NH4I were used as mineralizers. The polycrystalline GaN was produced by contacting elemental Ga with HCl gas under heating to generate gaseous GaCl, which was then reacted with NH3 gas. The mineralizers were charged in amounts of 1.0% for each of NH4F and NH4I, in terms of molar ratio to the NH3 solvent. The NH4I mineralizer was synthesized by reacting HI (hydrogen iodide) gas with NH3 in a Pt-Ir capsule reaction vessel.

[0090] In the ammonothermal process, the temperature T of the dissolution zone １ and the temperature of the growth zone T ２ The average value of is between 600 and 620°C, and the temperature difference between the two zones is T １ -T ２ (T １ >T ２The temperature was between 5°C and 10°C, and the pressure inside the capsule was between 200 MPa and 230 MPa. These conditions were maintained for more than 30 days. As shown in Figure 13, GaN crystals grew in layers on the N-polar surface of the first c-plane GaN wafer, with a thickness t of 3 mm. Voids were formed adjacent to the selective growth mask, and the height h of the voids, i.e., the distance from the top surface of the selective growth mask to the top edge of the void, was between 0.5 mm and 1 mm. It should be noted that GaN crystals also grew on the Ga-polar surface of the first c-plane GaN wafer, but this is not shown in Figure 13.

[0091] The grown GaN crystal layer was then processed to form a second c-plane GaN wafer with a thickness of 0.4 mm. The Ga-polar surface of the second c-plane GaN wafer was mirror-flattened by grinding and then finished by CMP to remove damaged layers. The off-axis angle of the Ga-polar surface of the second c-plane GaN wafer was set to 0 degrees from the (0001) plane.

[0092] (2) Preparation of Ge-doped c-plane GaN substrate: The second c-plane GaN wafer was cut into pieces, each having a Ga polarity plane with an area of ​​approximately 7 cm. ２ Using a 0.4 mm thick c-plane GaN wafer as a seed, a GaN thick film was grown on the Ga polarity surface by HVPE using a vapor phase growth apparatus having the same basic configuration as the HVPE apparatus shown in Figure 9. The procedure is roughly as follows: First, the seed was set on a susceptor. An edge cover was used. Next, N ２ , H ２ and N.H. ３ were supplied into the reactor so that the partial pressures thereof became 0.30 atm, 0.65 atm, and 0.05 atm, respectively, while the reactor was heated by a heater installed outside the reactor.

[0093] After the susceptor temperature reached 1005° C., the susceptor temperature was kept constant to grow GaN. The temperature of the Ga reservoir was set to 800° C. The carrier gas supplied into the reactor was 38% H in molar ratio. ２ and the remainder is N ２ After the growth started, GaCl and NH ３The partial pressure of each is 9.5 × 10 －３ atm and 3.8 × 10 －２ The doping gas was not intentionally supplied for 300 minutes after the start of growth.

[0094] 300 minutes after the start of growth, GeCl ４ (tetrachlorogermane) supply was started. ４ The supply rate of Ge was gradually increased over 30 minutes. A Ge-doped GaN thick film was grown to a thickness of approximately 2.6 mm. The growth rate of the Ge-doped GaN thick film calculated from the thickness and growth time was approximately 48 μm / hr.

[0095] Next, this Ge-doped GaN thick film was sliced ​​parallel to the c-plane, and the Ga-polar surface of the resulting wafer was flattened by grinding, followed by CMP finishing. Slicing damage on the N-polar surface of the wafer was removed by etching. The wafer was then cut to a thickness of 335 μm and a main surface area of ​​approximately 7 cm. ２ A Ge-doped c-plane GaN substrate (hereinafter also referred to as "sample E-1") was completed.

[0096] (3) Evaluation <Carrier Concentration Distribution> Using a Raman spectrometer (HORIBA, LabRam HR800), a range of Φ30 mm from the center of the main surface on the gallium polar surface side of sample E-1 was measured at 0.5 mm intervals to obtain a carrier concentration mapping image ( FIG. 14 ). The number of data points (measurement points) in the carrier concentration mapping image was 10,812. The Raman measurement was performed in the vicinity of the surface on the gallium polar surface side of the substrate. The average carrier concentration at the data points in the carrier concentration mapping image included in a circular region with a radius of 15 mm from the center of the main surface of sample E-1 was 2.9×10 １８ cm －３ The maximum value of the carrier concentration at the data points of the carrier concentration mapping image included in the circular region was 3.3 × 10 １８ cm －３ The carrier concentration was 2×10 １８ cm －３The ratio of the number of data points where the carrier concentration is 2 × 10 or more to the total number of data points (the ratio of the carrier concentration to the area of ​​a circular region with a radius of 15 mm from the center of the main surface to the total number of data points) １８ cm －３ The percentage of the area where the area is equal to or greater than this was 97%.

[0097] On the carrier concentration mapping image, four line segments, each 30 mm long, were drawn, passing through the center of the main surface and forming angles of 0°, 45°, 90°, and 135° with the OF direction of the substrate, and the average value and standard deviation of the carrier concentration at data points on the line segments were calculated. The coefficient of variation of the carrier concentration (standard deviation / average value) was calculated from the calculated average value and standard deviation, and was found to be 0.06.

[0098] On the line segments that form angles of 0 and 90 degrees with the OF direction of the substrate, 59 measurement points are used to measure the carrier concentration at 0.5 mm intervals. On the line segments that form angles of 45 and 135 degrees with the OF direction of the substrate, 43 measurement points are used to measure the carrier concentration at 0.5 × √2 mm intervals.

[0099] 60% of the maximum value among the measured values ​​at all data points of sample E-1 (i.e., 2.0 × 10 １８ cm －３ The ratio of the number of data points showing values ​​equal to or less than 50% (i.e., 1.65 × 10) of the maximum value among the measured values ​​at all data points of sample E-1 to the total number of data points was 2.6%. １８ cm －３ The ratio of the number of data points showing values ​​equal to or less than 40% (i.e., 1.3 × 10) of the maximum value among the measured values ​​at all data points of sample E-1 to the total number of data points was 2.3%. １８ cm －３ The ratio of the number of data points showing values ​​below 0.05 to the total number of data points was 1.9%.

[0100] <Dislocation Density> For the main surface on the gallium polarity side of the substrate, the number of dark spots in a 4 mm x 4 mm area was counted using a PL imaging image, with the measurement center being a point 8 mm away from the center of the substrate, and the dislocation density was calculated as the value divided by the measurement area. As a result, the dislocation density was 3.5 x 10４ cm －２ It was.

[0101] 2. Comparative Example 1 (1) Preparation of Ge-doped c-plane GaN substrate The off-angle of the Ga polar plane of the second c-plane GaN wafer was changed to 0.35 degrees from the (0001) plane in the <1-100> direction. After the susceptor temperature reached 1005°C, 36% of the carrier gas was H ２ and the remainder is N ２ After the growth started, GaCl and NH ３ The partial pressure of each is 8.6 x 10 －３ atm and 3.9 × 10 －２ A Ge-doped c-plane GaN substrate (hereinafter also referred to as "Sample C-1") was fabricated in the same manner as in Example 1, except that the Ge was supplied into the reactor to a pressure of 1 atm, the Ge supply partial pressure was increased by 1.2 times, the Ge-doped GaN thick film was grown to a thickness of approximately 2.8 mm, and the growth rate of the Ge-doped GaN thick film calculated from the thickness and growth time was approximately 45 μm / hr.

[0102] (2) Evaluation <Carrier Concentration Distribution> Using a Raman spectrometer (HORIBA, LabRam HR800), measurements were taken at 0.5 mm intervals within a Φ30 mm range from the center of the main surface on the gallium polar surface side of sample E-1 to obtain a carrier concentration mapping image ( FIG. 15 ). The number of data points in the carrier concentration mapping image was 10,812. The Raman measurement was performed in the vicinity of the surface on the gallium polar surface side of the substrate. The average carrier concentration at the data points in the carrier concentration mapping image included in a circular region with a radius of 15 mm from the center of the main surface of sample C-1 was 3.6×10 １８ cm －３ The maximum value of the carrier concentration at the data points of the carrier concentration mapping image included in the circular region was 4.4 × 10 １８ cm －３ It was.

[0103] Carrier concentration is 2 x 10 １８ cm －３The ratio of the number of data points where the carrier concentration is 2 × 10 or more to the total number of data points (the ratio of the carrier concentration to the area of ​​a circular region with a radius of 15 mm from the center of the main surface to the total number of data points) １８ cm －３ The percentage of the area where the area is equal to or greater than this was 89%.

[0104] On the carrier concentration mapping image, a total of four line segments, each 30 mm long, were drawn passing through the center of the main surface and forming angles of 0°, 45°, 90°, and 135° with the OF direction of the substrate, and the average value and standard deviation of the carrier concentration at data points on the line segments were calculated. The coefficient of variation of the carrier concentration (standard deviation / average value) was calculated from the calculated average value and standard deviation, and was found to be 0.28.

[0105] On the line segments that form angles of 0 and 90 degrees with the OF direction of the substrate, 59 measurement points were used to measure the carrier concentration at 0.5 mm intervals. On the line segments that form angles of 45 and 135 degrees with the OF direction of the substrate, 43 measurement points were used to measure the carrier concentration at 0.5 × √2 mm intervals.

[0106] 60% of the maximum value among the measured values ​​at all data points of sample C-1 (i.e., 2.6 × 10 １８ cm －３ The ratio of the number of data points showing values ​​equal to or less than 50% (i.e., 2.2 × 10) of the maximum value among all the data points of sample C-1 to the total number of data points was 12%. １８ cm －３ The ratio of the number of data points showing values ​​equal to or less than 40% (i.e., 1.8 × 10) of the maximum value among the measured values ​​at all data points of sample C-1 to the total number of data points was 11%. １８ cm －３ ) The ratio of the number of data points showing values ​​below this to the total number of data points was 11%.

[0107] <Dislocation Density> For the main surface on the gallium polarity side of the substrate, the number of dark spots in a 4 mm x 4 mm area was counted using a PL imaging image, with the measurement center being a point 8 mm away from the center of the substrate, and the dislocation density was calculated as the value divided by the measurement area. As a result, the dislocation density was 4.1 x 10 ４ cm－２ It was.

[0108] 3. Example 2 (1) Seed Preparation A second c-plane GaN wafer having a thickness of 0.4 mm was formed in the same manner as in Example 1. The Ga polar surface of the second c-plane GaN wafer was mirror-flattened by grinding, and then finished by CMP to remove damaged layers. However, in Example 2, the off-angle of the Ga polar surface of the second c-plane GaN wafer was changed to 0.55 degrees in the <1-100> direction from the (0001) plane.

[0109] (2) Preparation of Ge-doped c-plane GaN substrate: A Ge-doped c-plane GaN substrate with a diameter of approximately 2 inches and an area of ​​the Ga polar surface of approximately 20 cm was prepared from the second c-plane GaN wafer. ２ Using a 0.4 mm thick c-plane GaN wafer as a seed, a GaN thick film was grown on the Ga polarity surface by HVPE using a vapor phase growth apparatus having the same basic configuration as the HVPE apparatus shown in Figure 9. The procedure is roughly as follows: First, the seed was set on a susceptor. An edge cover was used. Next, N ２ , H ２ and N.H. ３ were supplied into the reactor so that the partial pressures thereof became 0.30 atm, 0.65 atm, and 0.05 atm, respectively, while the reactor was heated by a heater installed outside the reactor.

[0110] After the susceptor temperature reached 1005° C., the susceptor temperature was kept constant to grow GaN. The temperature of the Ga reservoir was set to 800° C. The carrier gas supplied into the reactor was 38% H in molar ratio. ２ and the remainder is N ２ After the growth started, GaCl and NH ３ The partial pressure of each is 9.5 × 10 －３ atm and 3.8 × 10 －２ The doping gas was not intentionally supplied for 300 minutes after the start of growth.

[0111] 300 minutes after the start of growth, GeCl ４ (tetrachlorogermane) supply was started. ４The supply rate was gradually increased over 30 minutes. A Ge-doped GaN thick film was grown to approximately 2.6 mm. The growth rate of the Ge-doped GaN thick film calculated from the thickness and growth time was approximately 48 μm / hr. The RMS roughness of the Ga-polar surface of the GaN crystal having the Ge-doped GaN thick film was calculated using a confocal laser microscope (model number VK-X250). Specifically, ten 2 mm × 2 mm sub-square regions were randomly selected on the Ga-polar surface of the GaN crystal, and the RMS roughness of each sub-square region was measured. The average RMS roughness of the ten sub-square regions measured was 0.5 μm.

[0112] Next, this Ge-doped GaN thick film was sliced ​​parallel to the c-plane, and the Ga-polar surface of the resulting wafer was flattened by grinding, followed by CMP finishing. Slicing damage on the N-polar surface of the wafer was removed by etching. The wafer was then cut to a thickness of 335 μm, a diameter of approximately 2 inches, and a main surface area of ​​approximately 20 cm. ２ A Ge-doped c-plane GaN substrate (hereinafter also referred to as "sample E-2") was completed.

[0113] (3) Evaluation <Carrier Concentration Distribution> Using a Raman spectrometer (HORIBA, LabRam HR800), measurements were taken at 0.5 mm intervals within a Φ30 mm range from the center of the main surface of the gallium polar surface side of sample E-2 to obtain a carrier concentration mapping image ( FIG. 16 ). The number of data points (measurement points) in the carrier concentration mapping image was 10,812. The Raman measurement was performed in the vicinity of the surface on the gallium polar surface side of the substrate. The average carrier concentration at the data points in the carrier concentration mapping image included in a circular region with a radius of 15 mm from the center of the main surface of sample E-2 was 6.3×10 １８ cm －３ The maximum value of the carrier concentration at the data points of the carrier concentration mapping image included in the circular region was 7.3 × 10 １８ cm －３ The carrier concentration was 2×10 １８ cm －３The ratio of the number of data points where the carrier concentration is 2 × 10 or more to the total number of data points (the ratio of the carrier concentration to the area of ​​a circular region with a radius of 15 mm from the center of the main surface to the total number of data points) １８ cm －３ The ratio of the area where the area is equal to or greater than this was 100%.

[0114] On the carrier concentration mapping image, four line segments, each 30 mm long, were drawn, passing through the center of the main surface and forming angles of 0°, 45°, 90°, and 135° with the OF direction of the substrate, and the average value and standard deviation of the carrier concentration at data points on the line segments were calculated. The coefficient of variation of the carrier concentration (standard deviation / average value) was calculated from the calculated average value and standard deviation, and was found to be 0.06.

[0115] On the line segments that form angles of 0 and 90 degrees with the OF direction of the substrate, 59 measurement points are used to measure the carrier concentration at 0.5 mm intervals. On the line segments that form angles of 45 and 135 degrees with the OF direction of the substrate, 43 measurement points are used to measure the carrier concentration at 0.5 × √2 mm intervals.

[0116] 60% of the maximum value among all data points of sample E-2 (i.e., 4.4 × 10 １８ cm －３ The ratio of the number of data points showing values ​​equal to or less than 50% (i.e., 3.7 × 10) of the maximum value among the measured values ​​at all data points of sample E-2 to the total number of data points was 0%. １８ cm －３ The ratio of the number of data points showing values ​​equal to or less than 40% (i.e., 2.9 × 10) of the maximum value among the measured values ​​at all data points of sample E-2 to the total number of data points was 0%. １８ cm －３ ) The ratio of the number of data points showing values ​​below this to the total number of data points was 0%.

[0117] <Dislocation Density> For the main surface on the gallium polarity side of the substrate, the number of dark spots in a 4 mm x 4 mm area was counted using a PL imaging image, with the measurement center being a point 8 mm away from the center of the substrate, and the dislocation density was calculated as the value divided by the measurement area. As a result, the dislocation density was 5.1 x 10４ cm －２ It was.

[0118] 4. Comparative Example 2 A GaN crystal having a Ge-doped GaN thick film was obtained in the same manner as in Comparative Example 1. The RMS roughness of the Ga polar surface of the GaN crystal having the Ge-doped GaN thick film was calculated in the same manner as in Example 2. Specifically, ten 2 mm × 2 mm sub-square regions were randomly selected on the Ga polar surface of the GaN crystal, and the RMS roughness of each sub-square region was measured. The average RMS roughness of the ten sub-square regions was 1.1 μm.

[0119] 5. Example 3 (1) Seed Preparation In the same manner as in Example 2, a second c-plane GaN wafer having a thickness of 0.4 mm was formed.

[0120] (2) Preparation of Ge-doped c-plane GaN substrate: A Ge-doped c-plane GaN substrate with a diameter of approximately 2 inches and an area of ​​the Ga polar surface of approximately 20 cm was prepared from the second c-plane GaN wafer. ２ Using a 0.4 mm thick c-plane GaN wafer as a seed, a GaN thick film was grown on the Ga polar surface by HVPE using a vapor phase growth apparatus having the same basic configuration as the HVPE apparatus shown in FIG. ４ Growth was carried out under the same conditions as in Example 2, except that the supply amount of was changed.

[0121] (3) Evaluation <Carrier concentration distribution> Using a Raman spectrometer (HORIBA, LabRam HR800), a range of Φ30 mm from the center of the main surface on the gallium polar surface side of sample E-2 was measured at 0.5 mm intervals to obtain a carrier concentration mapping image. The number of data points (measurement points) in the carrier concentration mapping image was 10,812. The Raman measurement was performed in the vicinity of the surface on the gallium polar surface side of the substrate. The average value of the carrier concentration at the data points in the carrier concentration mapping image included in a circular region with a radius of 15 mm from the center of the main surface of sample E-2 was 4.2 × 10 １８ cm －３ The maximum value of the carrier concentration at the data points of the carrier concentration mapping image included in the circular region was 4.5 × 10 １８ cm －３The carrier concentration was 2×10 １８ cm －３ The ratio of the number of data points where the carrier concentration is 2 × 10 or more to the total number of data points (the ratio of the carrier concentration to the area of ​​a circular region with a radius of 15 mm from the center of the main surface to the total number of data points) １８ cm －３ The ratio of the area where the area is equal to or greater than this was 100%.

[0122] On the carrier concentration mapping image, four line segments, each 30 mm long, were drawn, passing through the center of the main surface and forming angles of 0°, 45°, 90°, and 135° with the OF direction of the substrate, and the average value and standard deviation of the carrier concentration at data points on the line segments were calculated. The coefficient of variation of the carrier concentration (standard deviation / average value) was calculated from the calculated average value and standard deviation, and was found to be 0.11.

[0123] On the line segments that form angles of 0 and 90 degrees with the OF direction of the substrate, 59 measurement points are used to measure the carrier concentration at 0.5 mm intervals. On the line segments that form angles of 45 and 135 degrees with the OF direction of the substrate, 43 measurement points are used to measure the carrier concentration at 0.5 × √2 mm intervals.

[0124] 60% of the maximum value among all data points of sample E-2 (i.e., 2.7 × 10 １８ cm －３ The ratio of the number of data points showing values ​​equal to or less than 50% (i.e., 2.3 × 10) of the maximum value among the measured values ​​at all data points of sample E-2 to the total number of data points was 7.3%. １８ cm －３ The ratio of the number of data points showing values ​​equal to or less than 40% (i.e., 1.8 × 10) of the maximum value among the measured values ​​at all data points of sample E-2 was 1.6%. １８ cm －３ ) The ratio of the number of data points showing values ​​below this to the total number of data points was 0%.

[0125] <Dislocation Density> For the main surface on the gallium polarity side of the substrate, the number of dark spots in a 4 mm x 4 mm area was counted using a PL imaging image, with the measurement center being a point 8 mm away from the center of the substrate, and the dislocation density was calculated as the value divided by the measurement area. As a result, the dislocation density was 2.8 x 10 ５ cm －２ It was.

[0126] While the present invention has been described above with reference to specific embodiments, these embodiments are presented as examples and do not limit the scope of the present invention. Each embodiment described in this specification can be modified in various ways without departing from the spirit of the invention, and can be combined with features described in other embodiments to the extent possible.

[0127] 20 GaN substrate 21 First main surface 22 Second main surface 100 HVPE apparatus 101 Reactor 102 Gallium reservoir 103 Susceptor 104 First heater 105 Second heater Z １ First Zone Z ２ Second Zone P １ Ammonia introduction pipe P ２ Hydrogen chloride introduction pipe P ３ Gallium chloride introduction tube P ４ Dopant introduction tube P Ｅ exhaust pipe

Claims

1. An n-type GaN substrate having two main surfaces, Contains Ge as a donor impurity. The average carrier concentration is 1 × 10⁻⁶ １８ cm －３ That's all. In at least one of the two main surfaces, A line segment (line segment A) with a length of 30 mm whose midpoint coincides with the center of the main surface, The midpoint of the line segment coincides with the center of the main surface, and a line segment (line segment B) with a length of 30 mm is drawn perpendicular to line segment A. When measuring the carrier concentration at square grid points spaced 0.5 mm apart, with line segments A and B set as two axes, and the square grid points within a circular region with a radius of 15 mm from the center of the main surface as measurement points, at least one selected from the group consisting of (1), (2), and (3) below is satisfied, The n-type GaN substrate wherein the donor impurity present in the highest concentration is Ge: (1) The mean and standard deviation of carrier concentrations at all measurement points on line segment A, line segment B, and the two line segments that are the angle bisectors of the angle between line segment A and line segment B are given by the following formula (i): Standard deviation / Mean ≤ 0.25 ... (i) Satisfying the relationship; (2) Carrier concentration is 2 × 10 １８ cm －３ The ratio of the number of measurement points meeting the above criteria to the total number of measurement points is 90% or more; (3) Satisfying at least one selected from the group consisting of the following conditions (A), (B), and (C); (A) The ratio of the number of measurement points that show a value of 60% or less of the maximum value among all measurement points to the total number of measurement points is 10% or less. (B) The ratio of the number of measurement points that show a value of 50% or less of the maximum value among all measurement points to the total number of measurement points is 9% or less. (C) The ratio of the number of measurement points that show a value of 40% or less of the maximum value among all measurement points to the total number of measurement points is 8% or less.

2. The n-type GaN substrate according to claim 1, wherein the line segment A is substantially parallel to any one crystal orientation parallel to the n-type GaN substrate surface.

3. The n-type GaN substrate according to claim 2, having an orientation flat capable of identifying the aforementioned crystal orientation.

4. (1) The n-type GaN substrate according to claim 1.

5. (2) The n-type GaN substrate according to claim 1.

6. (3) The n-type GaN substrate according to claim 1.

7. The n-type GaN substrate according to claim 1, satisfying all of (1), (2), and (3).

8. An n-type GaN substrate according to any one of claims 1 to 7, comprising a first region on the main surface side that satisfies at least one selected from the group consisting of (1), (2), and (3), and a second region on the other main surface side with a carrier concentration lower than that of the first region.

9. An n-type GaN substrate according to any one of claims 1 to 7, having a diameter of 40 mm or more.

10. An n-type GaN substrate according to any one of claims 1 to 7, wherein one of the two main surfaces is Ga polar and (0001) has an inclination of 0 degrees or more and 10 degrees or less with respect to the crystal plane.

11. The dislocation density on at least one principal plane is 5 × 10 ５ cm －２ The n-type GaN substrate according to any one of claims 1 to 7, which is as follows:

12. A nitride semiconductor device comprising an epitaxially grown nitride semiconductor on at least one of the two main surfaces of an n-type GaN substrate according to any one of claims 1 to 7.

13. A method for manufacturing a nitride semiconductor device, comprising the step of epitaxially growing a nitride semiconductor on at least one of the two main surfaces of an n-type GaN substrate according to any one of claims 1 to 7.