Gallium nitride single crystal substrate and method for producing gallium nitride single crystal substrate

Abstract

Provided is a gallium nitride single crystal substrate which is uniformly doped with Ge at a high concentration. The gallium nitride single crystal substrate has a diameter of 50 mm or more, the low-index crystal plane closest to the main surface is a (0001) plane, the Ge concentration in the substrate is 3 * 1018 cm-3 or more, and among peaks appearing in a histogram of the diameters of etch pits formed when etching is performed with an alkali-based etching solution on the main surface, the first peak having the smallest diameter is a single peak having no shoulder.

Description

Technical Field

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

[0002] Group III nitride semiconductors, typified by gallium nitride (GaN), are widely used as materials for semiconductor devices such as light-emitting devices and electronic devices. To improve the quality (semiconductor properties, etc.) of semiconductor devices composed of Group III nitride semiconductors, it is desirable to manufacture semiconductor stacks or nitride semiconductor self-supporting substrates with high crystal quality.

[0003] As a method for manufacturing a GaN single crystal substrate, for example, Patent Document 1 discloses a method for manufacturing a gallium nitride single crystal substrate using the VAS (Void-Assisted Separation) method. Furthermore, Patent Document 2 discloses a method comprising a step of epitaxially growing a semiconductor layer composed of GaN on a substrate and a step of slicing the GaN layer to produce a GaN self-supporting substrate.

[0004] The GaN substrate, which is intentionally doped with impurities to impart conductivity, is being sought to have a lower resistivity. In recent years, in particular, there has been a demand for lower resistivity in n-type GaN substrates used in high-performance laser diodes and power devices. For n-type GaN substrates, silicon (Si)-doped crystals have been used. However, when a large amount of Si is doped to further reduce the resistivity of the substrate, it is known that when the Si concentration in the crystal exceeds 3×10 18 cm -3 When the crystallinity of GaN deteriorates, the crystallinity of GaN deteriorates. Therefore, the use of crystals doped with germanium (Ge) instead of Si is studied. If Ge is used, even if the doping exceeds 3×10 18 cm -3 The addition of Ge rarely deteriorates the crystallinity of GaN (for example, Non-Patent Document 1).

[0005] Prior art literature

[0006] Patent Literature

[0007] Patent Document 1: Japanese Patent Application Laid-Open No. 2003-178984

[0008] Patent Document 2: Japanese Patent Application Laid-Open No. 2008-156189

[0009] Non-patent literature

[0010] Non-patent literature 1: Y. Oshima, et al., J. Cryst. Growth 312 (2010) 3569 Summary of the Invention

[0011] Problems to be solved by the invention

[0012] However, it is known that a GaN substrate doped with Ge at a high concentration has a problem in that the Ge concentration distribution within the substrate surface, which will be described later, has a large deviation.

[0013] An object of the present invention is to provide a gallium nitride single crystal substrate doped with Ge at a high concentration and uniformly.

[0014] Solutions to Problems

[0015] One embodiment of the present invention provides a gallium nitride single crystal substrate having a diameter of 50 mm or more, wherein the low-index crystal plane closest to the main surface is the (0001) plane, wherein:

[0016] The Ge concentration in the substrate is 3×10 18 cm -3 above,

[0017] Among the peaks appearing in the histogram of the diameters of the etch pits formed when the main surface is etched with an alkaline etching solution, the first peak with the smallest diameter is a single peak without a shoulder.

[0018] In addition, another embodiment of the present invention provides a method for manufacturing a gallium nitride single crystal substrate, wherein:

[0019] The method for manufacturing the gallium nitride single crystal substrate comprises:

[0020] Step (a) of preparing a base substrate composed of a gallium nitride single crystal whose low-index crystal plane closest to the main surface is the (0001) plane;

[0021] Step (b) is to make the Ge concentration 3×10 18 cm -3 The above gallium nitride single crystal is epitaxially grown on the main surface of the base substrate; and

[0022] Step (c) of obtaining a gallium nitride single crystal substrate having a diameter of 50 mm or more from the gallium nitride single crystal epitaxially grown in step (b);

[0023] In the step (b), a gas containing HCl is intermittently introduced to periodically etch the growth interface, thereby uniformly doping Ge into the crystal.

[0024] Effects of the Invention

[0025] According to the present invention, a gallium nitride single crystal substrate doped with Ge at a high concentration and uniformly can be provided. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1This is a flowchart showing a method for manufacturing a gallium nitride single crystal substrate according to one embodiment of the present invention.

[0027] Figure 2A This is a schematic cross-sectional view showing a portion of a method for manufacturing a gallium nitride single crystal substrate according to one embodiment of the present invention.

[0028] Figure 2B This is a schematic cross-sectional view showing a portion of a method for manufacturing a gallium nitride single crystal substrate according to one embodiment of the present invention.

[0029] Figure 2C This is a schematic cross-sectional view showing a portion of a method for manufacturing a gallium nitride single crystal substrate according to one embodiment of the present invention.

[0030] Figure 3 This is an SEM image showing an example of an etch pit formed on the main surface of a gallium nitride single crystal substrate according to one embodiment of the present invention.

[0031] Figure 4 This is a histogram of the diameters of the etch pits on the substrate of Example 1.

[0032] Figure 5 This is a histogram of the diameters of the etch pits on the substrate of Example 2.

[0033] Figure 6 This is a histogram of the diameters of the etch pits on the substrate of Example 3.

[0034] Figure 7 This is a histogram of the diameters of etch pits at five locations within the surface of the substrate of Example 1.

[0035] Figure 8 This is a histogram of the diameters of the etch pits of the substrate of Comparative Example 1.

[0036] Figure 9 It is a histogram of the diameter of the etch pit of the substrate of Comparative Example 2.

[0037] Figure 10 It is a histogram of the diameter of the etch pit of the substrate of Comparative Example 3.

[0038] Description of Reference Numerals

[0039] 10. Substrate

[0040] 30 Growth Layer

[0041] 50 Gallium nitride single crystal substrate (substrate)

[0042] S100 Substrate Preparation Process

[0043] S110 Growth Process

[0044] S120 Slicing / Processing DETAILED DESCRIPTION

[0045] Insights Gained by the Inventors

[0046] First, the findings obtained by the inventors will be described.

[0047] The inventors discovered the following phenomenon during repeated attempts to grow Ge-doped GaN crystals: Ge can indeed be doped at a higher concentration than Si. For example, even with a doping concentration of 1×10 19 cm -3 The above amount of Ge can also grow GaN as a single crystal, but if the Ge doping concentration reaches a high level, a special defect will occur that causes fluctuations in the performance of devices manufactured on the substrate.

[0048] Furthermore, the inventors investigated the size distribution of etch pits corresponding to dislocations, obtained by etching the c-plane of a Ge-doped GaN substrate using an alkaline etchant. They discovered that a unique characteristic was present in the histogram of the sizes of etch pits corresponding to dislocations located in regions with high Ge concentrations. Specifically, in the histogram of several etch pits of varying sizes detected by etching, the peak of the smallest etch pit had a shoulder or formed a double peak. This is believed to be because, in regions where Ge concentration is locally high, Ge is captured by dislocation cores and diffuses abnormally on the dislocation cores, generating Ge clusters and precipitates. This, in turn, expands the strain field around the dislocation cores, affecting the size of the etch pits.

[0049] Furthermore, it was found that the main cause of the variation in the performance of the above-mentioned devices is related to the generation of a strain field around the dislocation core due to the formation of Ge clusters and precipitates.

[0050] According to the above model, for Ge clusters and precipitates, the more Ge enters the crystal and the lower the dislocation density in the crystal, the easier it is to gather on each dislocation core, so it is predicted that they will be easily generated. However, there is a desire to dope as much Ge as possible to reduce the resistivity of the GaN substrate, and to minimize the density of dislocations as crystal defects. On the basis of meeting the above expectations, in order to suppress the generation of Ge clusters and precipitates, it is necessary to perform a maximum high-concentration doping to ensure that the Ge concentration in the crystal does not deviate and that Ge clusters and precipitates are not generated.

[0051] Here, the way Ge enters the growing crystal is prone to unevenness. In particular, when manufacturing large-diameter GaN substrates with a diameter exceeding 50 mm, a tendency is observed to easily produce areas with locally high Ge concentrations within the substrate surface. This is because, compared to Si, Ge has the property that the amount of Ge entering the crystal easily changes depending on the inclination of the crystal plane orientation at the crystal growth interface. Therefore, if a morphology with a slightly inclined surface corresponding to a facet growth region appears at the crystal growth interface, the amount of Ge entering this region increases. Therefore, when Ge is doped at a high concentration, concentration unevenness that exceeds the threshold for generating Ge clusters and precipitates is likely to occur. In the growth of large-diameter substrates with a large area of the growth interface, larger facets are easily formed, resulting in a morphology with a large degree of concavity and convexity, which easily produces higher concentration regions.

[0052] It should be noted that the threshold value of Ge concentration for the generation of Ge clusters and precipitates is not only affected by the concentration of Ge entering the crystal, but also by various factors such as the density and type of dislocations existing in the region, the crystal growth temperature, pressure, the concentration of other impurities existing in the crystal, the point defect concentration, and the degree of residual stress. Therefore, it is difficult to determine it in general terms.

[0053] However, based on the above findings, if the first peak of the pit size histogram is observed to be a single peak without a shoulder, it is possible to determine whether Ge clusters or precipitates have occurred. In other words, the pit size histogram can be used as a criterion for measuring the uniformity of Ge distribution.

[0054] The size of the etch pits also depends significantly on the etching conditions. Therefore, discussing the absolute value of the etch pit size is meaningless. However, among the several etch pits of varying sizes detected by etching, the smallest etch pit is highly likely to correspond to an edge dislocation with the smallest strain field around the dislocation core. Therefore, the inventors investigated a histogram of etch pit diameters, normalized the diameter of the smallest peak in the histogram, and investigated the diameter and number of the remaining etch pits. This method significantly contributes to investigating the distribution of dislocation seeds and the distribution of dislocation-related properties in gallium nitride single crystal substrates.

[0055] It is very difficult to confirm the local concentration unevenness of Ge in GaN crystal. However, according to the method of the present invention, by measuring the concentration unevenness of Ge in a relatively large area (e.g., 1 mm 2 By making a histogram of the diameter of the etch pits (as above), the uneven concentration of Ge can be confirmed.

[0056] Furthermore, the inventors have conducted in-depth research on methods for manufacturing substrates that can obtain the above-mentioned histogram, that is, gallium nitride single crystal substrates that are highly and uniformly doped with Ge. The deviation in the way Ge enters the GaN crystal occurs because an inclined surface caused by facet growth is generated at the growth interface during crystal growth. It is believed that the reason for the facet growth at the growth interface is that the density of atomic steps formed due to defects such as dislocations, the concentration of impurities remaining due to interface segregation, etc., produce deviations. Therefore, in the process of growing the Ge-doped GaN layer, a gas containing HCl is intermittently introduced into the growth interface, and the surface of the growing crystal is regularly etched to maintain the flatness and cleanliness of the interface, thereby suppressing the formation of tiny facets at the growth interface that serve as a prelude to facet growth, thereby establishing a technology for uniformly doping Ge into the GaN crystal.

[0057] <One embodiment of the present invention>

[0058] Hereinafter, one embodiment of the present invention will be described with reference to the drawings.

[0059] It should be noted that, hereinafter, in GaN crystals having a wurtzite structure, the <0001> axis is referred to as the "c-axis," and the (0001) plane is referred to as the "c-plane." It should be noted that the (0001) plane is sometimes referred to as the "+c-plane (gallium polar plane)," and the (000-1) plane is sometimes referred to as the "-c-plane (nitrogen (N) polar plane)." Furthermore, the <1-100> axis is referred to as the "m-axis," and the {1-100} plane is referred to as the "m-plane." It should be noted that the m-axis can also be expressed as the <10-10> axis. Furthermore, the <11-20> axis is referred to as the "a-axis," and the {11-20} plane is referred to as the "a-plane."

[0060] (1) Method for manufacturing gallium nitride single crystal substrate

[0061] use Figure 1 as well as Figure 2A to Figure 2C , a method for manufacturing a gallium nitride single crystal substrate according to this embodiment is described. Figure 1 1 is a flowchart showing a method for manufacturing a gallium nitride single crystal substrate according to this embodiment. Figure 2A to Figure 2C This is a schematic cross-sectional view showing a part of the method for manufacturing a gallium nitride single crystal substrate according to the present embodiment.

[0062] like Figure 1 As shown, the method for manufacturing a gallium nitride single crystal substrate according to the present embodiment includes, for example, a base substrate preparation step S100 , a growth step S110 , and a slicing / processing step S120 .

[0063] (S100: Base substrate preparation process)

[0064] like Figure 2AAs shown, in the base substrate preparation step S100, a base substrate 10 composed of a gallium nitride single crystal is prepared, wherein the low-index crystal plane closest to the primary surface 10s is the (0001) plane (c-plane 10c). Specifically, a commercially available gallium nitride single crystal substrate (a free-standing substrate, not a template) can be used as the base substrate 10, or a base substrate 10 composed of a gallium nitride single crystal can be produced using the VAS (Void-Assisted Separation) method described in Patent Document 1. It should be noted that the base substrate 10 can be Ge-doped, Si-doped, or undoped, with Ge-doping being particularly preferred. This reduces the Ge concentration difference between the base substrate 10 and the growth layer 30 described later.

[0065] (S110: Growth process)

[0066] In the growth step S110, Figure 2B As shown, for example, a GaN single crystal is epitaxially grown on the primary surface 10s of the base substrate 10 prepared in the base substrate preparation step S100, using the c-plane 30c as the growth plane. Specifically, for example, GaCl gas and NH3 gas are supplied to the heated base substrate 10 by the HVPE method, thereby epitaxially growing the growth layer 30 directly on the primary surface 10s of the base substrate 10.

[0067] In the growth step S110, for example, the growth layer 30 is grown under predetermined growth conditions. In this embodiment, the growth conditions are preferably set at a growth temperature of, for example, 980° C. or higher and 1200° C. or lower. Furthermore, in the growth step S110, the ratio of the supply rate of NH 3 gas, which is a nitriding gas, to the supply rate of GaCl gas, which is a Group III source gas (hereinafter also referred to as the "V / III ratio") is preferably set at, for example, 0.1 or higher and 5.0 or lower.

[0068] In the growth step S110 , GaCl gas, NH 3 gas, and germanium tetrachloride (GeCl 4 ) gas as an n-type dopant gas are supplied to the base substrate 10 so that the Ge concentration is 3×10 18 cm -3 Above and 1×10 20 cm -3The following highly concentrated Ge-doped GaN layer is epitaxially grown. GeCl4 gas is supplied, for example, from a gas cylinder, or by placing the raw material container in a thermostat maintained at approximately 5°C and bubbling the GeCl4 with a carrier gas while the GeCl4 is in a liquid state. The carrier gas can be either H2 gas or N2 gas, or both H2 gas and N2 gas. Preferably, in either case, the flow rate is controlled by a mass flow controller so that the partial pressure of the GeCl4 gas is 0.3 to 30 Pa.

[0069] It should be noted that other growth conditions in the growth step S110 are as follows, for example.

[0070] Growth pressure: 90~105kPa, preferably 90~95kPa

[0071] Partial pressure of GaCl gas: 1.5~15kPa

[0072] Partial pressure of GeCl4 gas: 0.3~30Pa

[0073] N2 gas flow rate / H2 gas flow rate: 0~1

[0074] In the growth step S110, a gas containing HCl is intermittently introduced to periodically etch the growth interface (also known as intermittent etching), thereby uniformly doping Ge into the crystal. Specifically, for example, during the growth with the c-plane 30c as the growth plane, the supply of GaN raw material gas and n-type dopant gas is temporarily stopped, and HCl gas and carrier gas (such as H2 gas) are introduced. As a result, the formation of tiny facets generated on the growth surface is suppressed, thereby enabling uniform Ge doping. It should be noted that the introduction of the gas containing HCl is preferably performed at intervals of 0.5 to 1 minute per time at intervals of 5 to 10 minutes per time, for example.

[0075] In the growth step S110, the thickness of the growth layer 30 is preferably set to, for example, not less than 1 mm and not more than 20 mm. The warping of the c-plane of the base substrate 10 is also inherited by the growth layer 30 grown thereon, so by growing the growth layer 30 thicker than a certain extent, a stress is generated that causes the warping to return to the crystal and flatten the c-plane, and the size of the warping of the c-plane gradually decreases. When the thickness of the growth layer 30 is less than 1 mm, it may not be possible to obtain sufficient stress to return the warping of the c-plane. In addition, in the subsequent slicing / processing step S120, it is difficult to obtain a GaN single crystal substrate of sufficient thickness. In contrast, by making the thickness of the growth layer 30 not less than 1 mm, the warping of the c-plane can be reduced. In addition, in the subsequent slicing / processing step S120, a GaN single crystal substrate of sufficient thickness can be obtained. On the other hand, when the thickness of the growth layer 30 exceeds 20 mm, excessive strain accumulates in the crystal due to the force to return the warping of the c-plane, sometimes causing dislocations to multiply or microcracks to form in the crystal. In addition, since the internal residual stress of the GaN crystal increases, the crystal is easily broken during the slicing / processing step S120. In contrast, by making the thickness of the growth layer 30 less than 20 mm, the internal residual stress of the GaN crystal can be reduced, and cracks and breakage can be suppressed. In addition, by performing intermittent etching during Ge-doped growth, the impurity distribution becomes smaller and the internal residual stress of the crystal becomes smaller. As a result, compared with the case where intermittent etching is not performed, it can grow thicker and the radius of curvature of the c-plane can be further increased.

[0076] (S120: Slicing / Processing Process)

[0077] In the slicing / processing step S120, as Figure 2C As shown, for example, the growth layer 30 is sliced using a wire saw or the like along a cut plane substantially parallel to the primary surface 30s (c-plane 30c) of the growth layer 30. This results in at least one gallium nitride single crystal substrate 50 (also referred to as substrate 50) having a diameter of 50 mm or greater, as an as-cut substrate. At this time, the slicing is preferably performed so that the thickness of the substrate 50 is, for example, 300 μm to 500 μm.

[0078] When a GaN substrate produced using the VAS method is used as the base substrate 10, the c-plane 50c of the base substrate 10 typically has a curvature radius of approximately 5 μm in the direction of the surface becoming concave. The radius of curvature of the c-plane 50c of the substrate 50 cut from the growth layer 30 tends to be larger than the radius of curvature of the c-plane 10c of the base substrate 10. Specifically, the radius of curvature of the c-plane 50c of the substrate 50 is preferably 10 μm or greater, and more preferably 20 μm or greater. This allows the variation in the off-angle θ of the c-axis relative to the normal to the primary surface 50s of the substrate 50 (the range of the off-angle θ within the substrate plane) to be smaller than the variation in the off-angle of the c-axis of the base substrate 10.

[0079] After the substrate 50 is obtained as an as-cut substrate, both surfaces of the substrate 50 may be polished using a polishing device, for example.

[0080] It should be noted that, in the present embodiment, the substrate 50 obtained by the slicing / processing step S120 can also be used as a new base substrate 10, and the growth step S110 and the slicing / processing step S120 can be repeated multiple times (for example, more than 4 times). In this specification, such a growth method is referred to as replacement growth. By performing replacement growth instead of thick film growth at one time, the warping of the c-plane is alleviated each time the substrate 50 is cut out, so that the internal residual stress of the GaN crystal can be reduced. Since the internal residual stress is reduced, the difference in curvature radius between the grown substrate and the base substrate 10 becomes smaller, so it is easy to uniformly dope Ge. It should be noted that the upper limit of the number of times replacement growth is performed is not particularly limited. From the perspective of efficiently manufacturing large-diameter GaN single crystal substrates, it is preferably set to 10 times or less.

[0081] Through the above steps, the substrate 50 of this embodiment is manufactured.

[0082] (Process for Manufacturing a Semiconductor Stack and Process for Manufacturing a Semiconductor Device)

[0083] After manufacturing substrate 50, a semiconductor functional layer composed of, for example, a Group III nitride semiconductor can be epitaxially grown on substrate 50 to produce a semiconductor stack. After the semiconductor stack is produced, electrodes and other components are further formed on the semiconductor stack, and the semiconductor stack is cut into chips of a predetermined size. This also allows the production of semiconductor devices. Since substrate 50 is uniformly doped with high concentrations of Ge, it is suitable for the production of vertical devices, etc.

[0084] (2) Gallium nitride single crystal substrates (nitride semiconductor self-supporting substrates, nitride crystal substrates)

[0085] Next, the gallium nitride single crystal substrate 50 of this embodiment will be described.

[0086] In the present embodiment, the substrate 50 obtained by the above-described manufacturing method is a self-supporting substrate composed of a GaN single crystal.

[0087] The diameter of the substrate 50 is, for example, not less than 50 mm. The thickness of the substrate 50 is, for example, not less than 300 μm and not more than 1 mm.

[0088] The substrate 50 is n-type and is doped with Ge. The Ge concentration in the substrate 50 is, for example, 3×10 18 cm -3 Above and 1.0×10 20 cm-3 Below, preferably 5×10 18 cm -3 Above and 1.0×10 20 cm -3 Below, more preferably 1×10 19 cm -3 Above and 1.0×10 20 cm -3 As the Ge concentration in the substrate 50 increases, the variation in Ge concentration within the substrate 50 increases, and thus the probability that the peak of the smallest etch pit in the etch pit histogram will have a shoulder increases. Therefore, the more highly Ge-doped the substrate is, the more effective it is to perform intermittent etching during Ge-doped growth.

[0089] The substrate 50 has, for example, a principal surface 50s serving as an epitaxial growth surface. In this embodiment, the low-index crystal plane closest to the principal surface 50s is, for example, the c-plane 50c.

[0090] It should be noted that the principal surface 50 s of the substrate 50 is, for example, mirror-finished, and the root mean square roughness RMS of the principal surface 50 s of the substrate 50 is, for example, less than 1 nm.

[0091] Furthermore, in the present embodiment, the impurity concentration in the substrate 50 obtained by the above-described manufacturing method is lower than that in a substrate obtained by liquid phase growth such as a flux method or an ammonothermal method.

[0092] Specifically, the hydrogen concentration in the substrate 50 is, for example, less than 1×10 17 cm -3 , preferably 5×10 16 cm -3 Below. In addition, the oxygen concentration in the substrate 50 is, for example, 5×10 16 cm -3 Below, preferably 3×10 16 cm -3 the following.

[0093] (C-plane curvature and deviation of the off-angle)

[0094] The curvature radius of the c-plane 50c of the substrate 50 is, for example, larger than the curvature radius of the c-plane 10c of the base substrate 10. Specifically, the curvature radius of the c-plane 50c of the substrate 50 is, for example, preferably 10 μm or more, more preferably 20 μm or more.

[0095] In this embodiment, the upper limit of the curvature radius of the c-plane 50c of the substrate 50 is preferably as large as possible and is not particularly limited. When the c-plane 50c of the substrate 50 is substantially flat, the curvature radius of the c-plane 50c may be considered to be infinite.

[0096] In this embodiment, the c-plane 50 c of the substrate 50 has a large curvature radius, so that the deviation of the off angle θ of the c-axis with respect to the normal to the principal surface 50 s of the substrate 50 can be made smaller than the deviation of the off angle of the c-axis of the base substrate 10 .

[0097] Specifically, when an X-ray rocking curve of the (0002) plane of the substrate 50 is measured and the deviation angle θ of the c-axis relative to the normal of the main surface 50s is measured based on the diffraction peak angle of the (0002) plane, the deviation calculated based on the extreme difference in the magnitude of the deviation angle θ from the center of the main surface 50s to within a diameter of 25 mm is, for example, less than 0.14°, preferably less than 0.07°, and more preferably less than 0.05°.

[0098] In this embodiment, the lower limit of the deviation of the off angle θ of the c-axis of the substrate 50 is preferably as small as possible and is not particularly limited. When the c-plane 50c of the substrate 50 is substantially flat, the deviation of the off angle θ of the c-axis of the substrate 50 is considered to be 0°.

[0099] Furthermore, in the present embodiment, the curvature of the c-plane 50 c is isotropically reduced with respect to the principal surface 50 s of the substrate 50 , and therefore the direction dependence of the radius of curvature of the c-plane 50 c is small.

[0100] Specifically, the difference between the curvature radius of the c-plane 50c along the a-axis and the curvature radius of the c-plane 50c along the m-axis obtained by the above-mentioned measurement method is, for example, less than 50% of the larger curvature radius, preferably less than 20%.

[0101] (Dislocation density)

[0102] In the present embodiment, the dislocation density in the surface of the substrate 50 is lower than the dislocation density in the main surface 10 s of the base substrate 10 .

[0103] When the substrate 50 is manufactured using the base substrate 10 composed of a high-purity GaN single crystal produced by the VAS method, the substrate 50 contains fewer non-luminescent centers caused by foreign matter or point defects.

[0104] Therefore, when observing the main surface of the substrate 50 of the present application using a multiphoton excitation microscope or the like, more than 95% (preferably more than 99%) of the dark spots observed are not non-luminescent centers caused by foreign matter or point defects, but rather correspond to dislocations. It should be noted that "multiphoton excitation microscopy" is sometimes also referred to as two-photon excitation fluorescence microscopy.

[0105] In this embodiment, since crystal growth is performed while maintaining the flatness of the crystal growth interface, the mechanism of localized concentration of propagating dislocations does not operate. Consequently, regions with exceptionally high dislocation density due to concentration of dislocations are not formed, and the dislocation density distribution is uniform across the substrate surface. It should be noted that, unlike Si, Ge, even high-concentration doping has been shown to not increase the dislocation density in GaN crystals.

[0106] (Erosion pit histogram)

[0107] When the main surface 50s of the substrate 50 is etched with an alkaline etching solution, Figure 3 The etching pits are as shown. In this embodiment, the substrate 50 is immersed in a melt (temperature 470°C) of potassium hydroxide (KOH) and sodium hydroxide (NaOH) mixed in a ratio of 1:1 for 15 minutes and then etched. Then, the main surface 50s of the etched substrate 50 is observed using an SEM, and the diameter of each etching pit is calculated by image analysis. A histogram of the diameters of the etching pits is produced based on the data of the obtained etching pit diameters. It should be noted that the diameter of the etching pit can be either the maximum diameter of each etching pit obtained by image analysis or the equivalent circle diameter. In this embodiment, the diameter of the peak (hereinafter also referred to as the first peak) that appears on the side with the smallest diameter among the peaks that appear in the histogram of the etching pit diameter is set to a and standardized, and the level (scale width) of the horizontal axis is set to 0.1a scale. The vertical axis is the number (frequency) of etching pits in the measurement area.

[0108] The average density (number per unit area) of etch pits formed when etching the principal surface 50s of the substrate 50 of this embodiment under the aforementioned conditions roughly matches the dislocation density determined by observation using a multiphoton excitation microscope. The size of the etch pits can vary depending on the etching conditions, but if the etch pits are present at an excessively high density, adjacent etch pits overlap, making it difficult to measure the pit diameter. Therefore, for meaningful measurement, the average density of the etch pits is preferably less than 1×10 6 cm -2 , more preferably less than 5.5×10 5 cm -2 , more preferably 3×10 5 cm -2 It should be noted that the average density of the pits and the histogram of the pit diameter are preferably calculated based on an area of 1 mm. 2 The above area measurement.

[0109] Here, in substrates with Ge clusters or the like on dislocation cores, the histogram peak (particularly the first peak) has a shoulder or is double-peaked. In contrast, for substrate 50 of this embodiment, the first peak in the histogram of etch pit diameters is a single peak without a shoulder. In other words, substrate 50 of this embodiment can be said to be uniformly doped with Ge.

[0110] It should be noted that, in this specification, the first peak being a single peak without a shoulder means that, for example, when the diameter of the first peak is set to a, no peak other than the first peak exists within the range of 0.8a to 1.2a, and when the peak width at a height from the baseline of the first peak to 1 / 10 of the peak height is set to W, and the width on the rising side of the peak obtained by dividing the peak width W into two using a perpendicular line drawn from the peak apex to the horizontal axis is set to f, the symmetry coefficient S defined by the following formula (1) is 0.8 to 1.2.

[0111] S=W / (2×f) …(1)

[0112] For substrate 50 of this embodiment, when the frequency of the first peak is set to A, the number of peaks appearing in the diameter histogram with a frequency of A / 10 or greater, including the first peak, is preferably two. The number of peaks appearing in the pit histogram can be said to correspond to the number of types of dislocation defects corresponding to the pits. Crystals in which multiple peaks are observed have many impurity or point defect deformations in addition to simple edge and mixed dislocations. In other words, these crystals are likely to have a high probability of generating a high number of impurity or defect energy levels, resulting in a large variation in characteristics during device fabrication. In other words, these crystals are likely to have a low device yield. In contrast, crystals in which two peaks appearing in the histogram with a frequency of A / 10 or greater, including the first peak, indicate that the dislocations present in the crystal are limited to simple edge and mixed dislocations. In other words, these crystals have a low variation in characteristics during device fabrication, resulting in a high device yield.

[0113] For the substrate 50 of this embodiment, when diameter histograms are prepared for a plurality of different regions (e.g., three or more different regions, more preferably five or more different regions) on the main surface 50s, the first peak in all of the plurality of histograms is preferably a single peak without a shoulder. The fact that the first peak in all of the plurality of histograms is a single peak without a shoulder indicates that Ge is uniformly doped not only in microscopic regions but also at all locations within the surface.

[0114] For the substrate 50 of this embodiment, the total number (aggregate count) of etch pits with a diameter exceeding 4 Å is preferably 1 / 1000 or less of the number of etch pits constituting the first peak. This can also be expressed as 1 / 1000 or less, as there are no actual etch pits exceeding 4 Å in diameter. However, larger etch pits may form due to damage or foreign matter on the substrate 50, which is why the number is expressed as 1 / 1000 or less. The substrate 50 of this embodiment, through the aforementioned manufacturing method, does not cause dislocations to aggregate, so multiple dislocations are not combined into a single one. Therefore, it can be said that there are essentially no dislocations that accumulate large strains, which could degrade device characteristics. Consequently, the characteristics of devices fabricated on the substrate 50 can be improved.

[0115] For substrate 50 of this embodiment, the total number of etch pits with a diameter of 2 Å or greater is preferably 1 / 10 or less, and more preferably 1 / 100 or less, of the number of etch pits constituting the first peak. Since etch pits with a diameter of 2 Å or greater may correspond to dislocations that affect the device characteristics of substrate 50, by limiting the total number of etch pits with a diameter of 2 Å or greater to 1 / 10 or less (more preferably 1 / 100 or less) of the number of etch pits constituting the first peak, the characteristics of devices fabricated on substrate 50 can be improved.

[0116] For substrate 50 of this embodiment, the number of etch pits constituting the first peak preferably accounts for at least 50% (more preferably at least 70%) of the total substrate. In this embodiment, as described above, dislocation synthesis is less likely to occur, and Ge clusters and precipitates are not generated. Therefore, the prevalence of dislocations with a small strain field is high. Dislocations with a small strain field do not significantly affect the device properties of substrate 50. Therefore, the fact that the first peak is a single peak without a shoulder and that the proportion of etch pits constituting the first peak is high indicates that the device fabricated on substrate 50 has good properties.

[0117] Furthermore, substrate 50 of this embodiment preferably has a second peak within a diameter range exceeding a and less than 2a, and the number of pits β constituting the second peak is less than the number of pits α constituting the first peak. Having a diameter exceeding a and less than 2a for the pits forming the second peak indicates a high probability of corresponding to simple mixed dislocations. Assuming the first peak is a single peak without a shoulder, a smaller number of pits constituting the second peak, or in other words, a larger number of pits constituting the first peak, indicates that the dislocations in the crystal are primarily composed of dislocations with less strain surrounding the dislocation core. These characteristics indicate that devices fabricated on substrate 50 will have excellent characteristics.

[0118] <Another embodiment>

[0119] While the embodiments of the present invention have been described in detail above, the present invention is not limited to the above-described embodiments and various modifications can be made without departing from the spirit and scope of the present invention.

[0120] In the above-described embodiment, in the growth step S110, the case of epitaxially growing the growth layer 30 with the c-plane 30c as the growth plane is described. However, for example, a growth step with an inclined interface other than the c-plane as the growth plane can also be performed in the middle of the growth step S110. As a result, dislocations are easily propagated, converged, and disappeared in a curved manner. However, during the growth with the inclined interface as the growth plane, Ge tends to become uneven. Therefore, it is preferable not to dope Ge at a high concentration during the growth with the inclined interface, and after returning the growth interface to a flat surface with the c-plane 30c as the growth plane, intermittent etching is performed while the growth with a high concentration of Ge is performed, and the substrate 50 is obtained from the crystal region where the intermittent etching is performed.

[0121] In the above embodiment, a case where a wire saw is used to slice the growth layer 30 in the slicing / machining step S120 is described. However, for example, an outer peripheral blade slicer, an inner peripheral blade slicer, an electric discharge machine, or the like may be used.

[0122] [Example]

[0123] Next, examples of the present invention will be described. These examples are examples of the present invention, and the present invention is not limited to these examples.

[0124] (1) Fabrication of GaN single crystal substrate

[0125] Gallium nitride single crystal substrates of Examples 1 to 3 and Comparative Examples 1 to 3 were produced as follows.

[0126] [Conditions for Fabricating Gallium Nitride Single Crystal Substrate in Example 1] (Base Substrate)

[0127] Material: GaN

[0128] Production method: VAS method

[0129] Diameter: 2 inches

[0130] Thickness: 400μm

[0131] Low-index crystal plane closest to the main surface: c-plane

[0132] There is no patterning of a mask layer or the like on the main surface.

[0133] Root mean square roughness RMS of the main surface: 2nm

[0134] Offset angle of the main surface: 0.4° in the m direction (growth layer)

[0135] Material: GaN

[0136] Growth method: HVPE method

[0137] Growth temperature: above 980℃ and below 1020℃

[0138] V / III ratio: 2 or more and 20 or less

[0139] Ge doping method: GeCl4 gas

[0140] Partial pressure of GeCl4 gas: 0.35 Pa

[0141] Growth layer thickness: 4500 μm (intermittent etching conditions)

[0142] Etching gas: HCl gas

[0143] Etching interval: 10 minutes / time

[0144] Etching time: 0.5 minutes / time (slicing / processing conditions)

[0145] Thickness of GaN single crystal substrate: 400μm

[0146] Kerf loss: 200 μm

[0147] The substrate 50 was manufactured under the above-mentioned manufacturing conditions. When the Ge concentration of the substrate 50 was measured by secondary ion mass spectrometry (SIMS), the Ge concentration was 3×10 18 cm -3 .

[0148] [Conditions for Fabricating Gallium Nitride Single Crystal Substrate in Example 2] (Base Substrate)

[0149] Same as Example 1.

[0150] (Growth layer)

[0151] In order to increase the Ge concentration compared to Example 1, the growth was performed by changing the GeCl4 gas partial pressure and the intermittent etching conditions. The conditions were the same as those in Example 1 except for the following.

[0152] Partial pressure of GeCl4 gas: 1.2 Pa (intermittent etching conditions)

[0153] Etching interval: 7 minutes / time

[0154] Etching time: 1 minute / time (slicing / processing conditions)

[0155] Same as Example 1.

[0156] The substrate 50 was manufactured under the above-mentioned manufacturing conditions. When the Ge concentration of the substrate 50 was measured by secondary ion mass spectrometry, the Ge concentration was 1×10 19 cm -3 .

[0157] [Conditions for Fabricating Gallium Nitride Single Crystal Substrate in Example 3] (Base Substrate)

[0158] Same as Example 1.

[0159] (Growth layer)

[0160] In order to further increase the Ge concentration compared to Example 2, the growth was performed by changing the GeCl4 gas partial pressure and the intermittent etching conditions. The conditions other than the following were the same as those of Example 1.

[0161] Partial pressure of GeCl4 gas: 12 Pa (intermittent etching conditions)

[0162] Etching interval: 5 minutes / time

[0163] Etching time: 1 minute / time (slicing / processing conditions)

[0164] Same as Example 1.

[0165] The substrate 50 was manufactured under the above-mentioned manufacturing conditions. When the Ge concentration of the substrate 50 was measured by secondary ion mass spectrometry, the Ge concentration was 1×10 20 cm -3 .

[0166] [Conditions for Fabricating Gallium Nitride Single Crystal Substrate in Comparative Example 1] (Base Substrate)

[0167] Same as Example 1.

[0168] (Growth layer)

[0169] Same as Example 1.

[0170] (Intermittent etching)

[0171] In Comparative Example 1, intermittent etching was not performed.

[0172] (Slicing / Processing Conditions)

[0173] Same as Example 1.

[0174] The substrate 50 was manufactured under the above-mentioned manufacturing conditions. When the Ge concentration of the substrate 50 was measured by secondary ion mass spectrometry, the Ge concentration was 3×10 18 cm -3 .

[0175] [Conditions for Fabricating Gallium Nitride Single Crystal Substrate in Comparative Example 2] (Base Substrate)

[0176] Same as Example 1.

[0177] (Growth layer)

[0178] Same as Example 2.

[0179] (Intermittent etching)

[0180] In Comparative Example 2, intermittent etching was not performed.

[0181] (Slicing / Processing Conditions)

[0182] Same as Example 1.

[0183] The substrate 50 was manufactured under the above-mentioned manufacturing conditions. When the Ge concentration of the substrate 50 was measured by secondary ion mass spectrometry, the Ge concentration was 1×10 19 cm -3 .

[0184] [Conditions for Fabricating Gallium Nitride Single Crystal Substrate in Comparative Example 3] (Base Substrate)

[0185] Same as Example 1.

[0186] (Growth layer)

[0187] Same as Example 3.

[0188] (Intermittent etching)

[0189] In Comparative Example 3, intermittent etching was not performed.

[0190] (Slicing / Processing Conditions)

[0191] Same as Example 1.

[0192] The substrate 50 was manufactured under the above-mentioned manufacturing conditions. When the Ge concentration of the substrate 50 was measured by secondary ion mass spectrometry, the Ge concentration was 1×10 20 cm -3 .

[0193] (2) Evaluation of the pit histogram

[0194] The main surfaces of the gallium nitride single crystal substrates of Examples 1 to 3 and Comparative Examples 1 to 3 were etched with an alkaline etching solution under the following conditions to form etch pits.

[0195] Etching solution: Alkaline solution consisting of 500g KOH and 500g NaOH

[0196] Melt temperature: 470°C

[0197] Soaking time: 15 minutes

[0198] The main surface of the substrate with etch pits was observed using an SEM (manufactured by Hitachi High-Technologies Corporation, SU5000). The measurement field of view was set to 127μm×95.3μm (magnification 1000 times), and the image (1143μm×1048μm) formed by combining 9 horizontal × 11 vertical SEM images (a total of 99 images) was analyzed to measure the etch pits. Specifically, the image was binarized using image processing software (Image J), the etch pit area was filled, and the larger of the width and height of the etch pit area was calculated as the diameter of the etch pit. It should be noted that when filling the etch pit area, areas representing smaller garbage or linear pit areas caused by processing damage were removed.

[0199] A histogram of pit diameters was created based on the obtained pit diameter data. In this example, the diameter of the peak with the smallest diameter (the first peak) among the peaks appearing in the pit diameter histogram was set to 1 and normalized, with the horizontal axis scaled to 0.1.

[0200] exist Figures 4 to 6 The histograms measured at the center of the substrates of Examples 1 to 3 are shown in FIG. Figure 7 The distribution of the histogram measured at 5 locations within the surface of the substrate of Example 1 is shown in FIG. Figures 8 to 10 ] shows histograms measured at the center of the substrates of Comparative Examples 1 to 3. In each histogram, the horizontal axis represents the normalized pit diameter, and the vertical axis represents the number (frequency) of pits in the measurement area.

[0201] like Figure 4 As shown in FIG. 1 , in the substrate of Example 1, the first peak of the histogram is a single peak without a shoulder. Figure 8 As shown, in the substrate of Comparative Example 1, the first peak of the histogram becomes a double peak.

[0202] In addition, in Example 2 ( Figure 5 ) and Example 3 ( Figure 6 ) substrate, the first peak of the histogram becomes a single peak without a shoulder. In addition, in Comparative Example 2 ( Figure 9 )、Comparative Example 3( Figure 10 ) substrate, the first peak of the histogram becomes a double peak. 18 cm -3 When the concentration is above 0.05 and the growth is performed without performing intermittent etching, it can be confirmed that the first peak of the histogram becomes a double peak.

[0203] And, as Figure 7As shown, in the substrate of Example 1, when the histograms at the center of the substrate and five locations within ±20 mm from the center in the horizontal and vertical directions were checked, the first peak in any of the histograms was a single peak without a shoulder.

[0204] Based on the above, it was confirmed that by performing intermittent etching during the growth of the growth layer, a substrate can be produced in which the first peak is a single peak with no shoulder. Such a substrate can be said to be highly and uniformly doped with Ge, making it suitable for the production of vertical devices, etc.

[0205] <Preferred embodiment of the present invention>

[0206] Preferred embodiments of the present invention are described below.

[0207] (Note 1)

[0208] A gallium nitride single crystal substrate having a diameter of 50 mm or more, wherein the low-index crystal plane closest to the main surface is the (0001) plane, wherein:

[0209] The Ge concentration in the substrate is 3×10 18 cm -3 above,

[0210] Among the peaks appearing in the histogram of the diameters of the etch pits formed when the main surface is etched with an alkaline etching solution, the first peak with the smallest diameter is a single peak without a shoulder.

[0211] (Note 2)

[0212] Based on the gallium nitride single crystal substrate described in Supplementary Note 1,

[0213] In the histogram of the pit diameters, when the diameter of the first peak is set to a, no peak other than the first peak exists within the range of 0.8a to 1.2a, and when the peak width from the baseline of the first peak to a height of 1 / 10 of the peak height is set to W, and the width of the peak rising side obtained by dividing the peak width W into two parts using a vertical line drawn from the peak apex to the horizontal axis is set to f, the symmetry coefficient S defined by the following formula (1) is greater than or equal to 0.8 and less than or equal to 1.2.

[0214] S=W / (2×f) …(1)

[0215] (Note 3)

[0216] Based on the gallium nitride single crystal substrate described in Supplementary Note 1,

[0217] When the frequency of the first peak is set to A, there are two peaks appearing in the histogram with a frequency of A / 10 or more, including the first peak.

[0218] (Note 4)

[0219] Based on the gallium nitride single crystal substrate described in Supplementary Note 1,

[0220] When the histograms are created for a plurality of different regions on the main surface, the first peak is a single peak without a shoulder in all of the plurality of histograms.

[0221] (Note 5)

[0222] Based on the gallium nitride single crystal substrate described in Supplementary Note 1,

[0223] When the pit diameter of the first peak is defined as a, in the histogram, the total number of pits having a diameter exceeding 4a is 1 / 1000 or less of the number of the pits constituting the first peak.

[0224] (Note 6)

[0225] Based on the gallium nitride single crystal substrate described in Supplementary Note 1,

[0226] In the histogram of pit diameters, the total number of pits having a diameter of 2a or greater is 1 / 10 or less, more preferably 1 / 100 or less, of the number of pits constituting the first peak.

[0227] (Note 7)

[0228] Based on the gallium nitride single crystal substrate described in Supplementary Note 1,

[0229] In the histogram of the pit diameters, the number of pits constituting the first peak accounts for 50% or more of the total, and preferably 70% or more.

[0230] (Note 8)

[0231] Based on the gallium nitride single crystal substrate described in Supplementary Note 1,

[0232] The histogram of the pit diameters has a second peak within a range where the diameter exceeds a and is smaller than 2a, and the number β of the pits constituting the second peak is smaller than the number α of the pits constituting the first peak.

[0233] (Note 9)

[0234] Based on the gallium nitride single crystal substrate described in Supplementary Note 1,

[0235] The average density of the etch pits is less than 1×10 6 cm -2 .

[0236] (Note 10)

[0237] Based on the gallium nitride single crystal substrate described in any one of Supplementary Notes 1 to 9,

[0238] The histogram is based on an area of 1 mm 2 The above area measurement.

[0239] (Note 11)

[0240] A method for manufacturing a gallium nitride single crystal substrate, wherein:

[0241] The method for manufacturing the gallium nitride single crystal substrate comprises:

[0242] Step (a) of preparing a base substrate composed of a gallium nitride single crystal whose low-index crystal plane closest to the main surface is the (0001) plane;

[0243] Step (b) is to make the Ge concentration 3×10 18 cm -3 The above gallium nitride single crystal is epitaxially grown on the main surface of the base substrate; and

[0244] Step (c) of obtaining a gallium nitride single crystal substrate having a diameter of 50 mm or more from the gallium nitride single crystal epitaxially grown in step (b);

[0245] In the step (b), a gas containing HCl is intermittently introduced to periodically etch the growth interface, thereby uniformly doping Ge into the crystal.

Claims

1. A gallium nitride single crystal substrate having a diameter of 50 mm or more, wherein the low-index crystal plane closest to the main surface is the (0001) plane, wherein: The Ge concentration in the substrate is 3×10 18 cm -3 above, Among the peaks appearing in the histogram of the diameters of the etch pits formed when the main surface is etched with an alkaline etching solution, the first peak with the smallest diameter is a single peak without a shoulder.

2. The gallium nitride single crystal substrate according to claim 1, wherein When the frequency of the first peak is set to A, there are two peaks appearing in the histogram with a frequency of A / 10 or more, including the first peak.

3. The gallium nitride single crystal substrate according to claim 1, wherein When the histograms are created for a plurality of different regions on the main surface, the first peak is a single peak without a shoulder in all of the plurality of histograms.

4. The gallium nitride single crystal substrate according to claim 1, wherein When the pit diameter of the first peak is defined as a, in the histogram, the total number of pits having a diameter exceeding 4a is 1 / 1000 or less of the number of the pits constituting the first peak.

5. The gallium nitride single crystal substrate according to claim 1, wherein The average density of the etch pits is less than 1×10 6 cm -2 .

6. The gallium nitride single crystal substrate according to any one of claims 1 to 5, wherein The histogram is based on an area of 1 mm 2 The above area measurement.

7. A method for manufacturing a gallium nitride single crystal substrate, wherein: The method for manufacturing the gallium nitride single crystal substrate comprises: Step (a) of preparing a base substrate composed of a gallium nitride single crystal whose low-index crystal plane closest to the main surface is the (0001) plane; Step (b) is to make the Ge concentration 3×10 18 cm -3 The above gallium nitride single crystal is epitaxially grown on the main surface of the base substrate; and Step (c) of obtaining a gallium nitride single crystal substrate having a diameter of 50 mm or more from the gallium nitride single crystal epitaxially grown in step (b); In the step (b), a gas containing HCl is intermittently introduced to periodically etch the growth interface, thereby uniformly doping Ge into the crystal.

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