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

Abstract

To provide a gallium nitride single crystal substrate with Ge doped at high density uniformly.SOLUTION: A gallium nitride single crystal substrate has a diameter of 50 mm or more, and a crystal face with a low index nearest a main plane is a (0001) face. A Ge concentration in the substrate is 3×1018 cm-3 or more. A first peak with a smallest diameter among peaks appearing in a histogram of a diameter of an etch pit formed when the main plane is etched by alkaline etching solution is a single peak without a shoulder.SELECTED DRAWING: Figure 4

Description

[Technical Field]

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

[0002] 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. In order to improve the quality (semiconductor characteristics, etc.) of semiconductor devices made of Group III nitride semiconductors, it is desirable to manufacture semiconductor stacks or nitride semiconductor freestanding substrates for use in manufacturing semiconductor devices with good crystal quality.

[0003] As a method for producing a GaN single crystal substrate, for example, Patent Document 1 discloses a method for producing a gallium nitride single crystal substrate by a VAS (Void-Assisted Separation) method. Also, Patent Document 2 discloses a method including a step of epitaxially growing a semiconductor layer made of GaN on a substrate and a step of slicing the GaN layer to produce a GaN freestanding substrate.

[0004] GaN substrates that are intentionally doped with impurities to give them conductivity are required to have low resistivity. In particular, in recent years, there has been a demand for reduced resistivity in n-type GaN substrates used in high-power laser diodes and power device applications. Conventionally, silicon (Si)-doped crystals have been widely used for n-type GaN substrates. However, when attempting to dope a large amount of Si to further reduce the resistivity of the substrate, the Si concentration in the crystals becomes 3×10 18 cm -3 It has been found that the crystallinity of GaN deteriorates above this level. Therefore, the use of germanium (Ge) doped crystals instead of Si is being considered. In the case of Ge, the crystallinity is 3×10 18 cm -3 Even if doped in excess of this limit, the crystallinity of GaN is hardly deteriorated (for example, Non-Patent Document 1). [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Laid-Open No. 2003-178984 [Patent Document 2] Japanese Patent Application Laid-Open No. 2008-156189 [Non-patent literature]

[0006] [Non-Patent Document 1] Y. Oshima, et al., J. Cryst. Growth 312 (2010) 3569 Summary of the Invention [Problem to be solved by the invention]

[0007] However, it has become clear that a GaN substrate doped with high concentrations of Ge has a problem of large variations in the Ge concentration distribution within the substrate surface, as will be described later.

[0008] An object of the present invention is to provide a gallium nitride single crystal substrate that is highly and uniformly doped with Ge. [Means for solving the problem]

[0009] According to one aspect of the present invention, A gallium nitride single crystal substrate having a diameter of 50 mm or more, and having a low-index crystal plane closest to the primary surface as the (0001) plane, The Ge concentration in the substrate is 3×10 18 cm -3 That's all, A gallium nitride single crystal substrate is provided, in which, among the peaks appearing in a histogram of the diameters of etch pits formed when the primary surface is etched with an alkaline etching solution, a first peak having the smallest diameter is a single peak without a shoulder.

[0010] According to another aspect of the present invention, A step (a) of preparing a base substrate made of gallium nitride single crystal, the low-index crystal plane closest to the primary surface being the (0001) plane; The primary surface of the base substrate is provided with a Ge concentration of 3×10 18 cm -3 a step (b) of epitaxially growing the above-described gallium nitride single crystal; 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 and the growth interface is periodically etched, thereby doping Ge uniformly into the crystal, thereby providing a method for producing a gallium nitride single crystal substrate. [Effects of the Invention]

[0011] According to the present invention, it is possible to provide a gallium nitride single crystal substrate that is highly and uniformly doped with Ge. [Brief explanation of the drawings]

[0012] [Figure 1] FIG. 1 is a flowchart showing a method for manufacturing a gallium nitride single crystal substrate according to one embodiment of the present invention. [Figure 2] 2(a) to 2(c) are schematic cross-sectional views showing a part of a method for producing a gallium nitride single crystal substrate according to one embodiment of the present invention. [Figure 3] FIG. 3 is an SEM image showing an example of an etch pit formed on the primary surface of a gallium nitride single crystal substrate according to one embodiment of the present invention. [Figure 4] FIG. 4 is a histogram of the diameters of etch pits on the substrate of Example 1. [Figure 5] FIG. 5 is a histogram of the diameters of etch pits on the substrate of Example 2. [Figure 6] FIG. 6 is a histogram of the diameters of etch pits on the substrate of Example 3. [Figure 7]FIG. 7 is a histogram of the diameters of etch pits at five locations within the surface of the substrate of Example 1. [Figure 8] FIG. 8 is a histogram of the diameters of etch pits on the substrate of Comparative Example 1. [Figure 9] FIG. 9 is a histogram of the diameters of etch pits on the substrate of Comparative Example 2. [Figure 10] FIG. 10 is a histogram of the diameters of etch pits on the substrate of Comparative Example 3. DETAILED DESCRIPTION OF THE INVENTION

[0013] <Insights gained by the inventor> First, the findings of the inventors will be described.

[0014] The inventors have repeatedly experimented with growing Ge-doped GaN crystals and found that Ge can indeed be doped at a higher concentration than Si, for example, 1×10 19 cm -3 Although it is possible to grow GaN as a single crystal even with this level of doping, we discovered that when the Ge doping concentration becomes too high, unique defects occur that cause variations in the performance of devices fabricated on the substrate. The inventors also investigated the size distribution of etch pits corresponding to dislocations obtained by etching the c-plane of a Ge-doped GaN substrate with an alkaline etching solution, and discovered that a unique feature appears in the histogram of etch pit sizes corresponding to dislocations in regions with a high Ge concentration. That is, in the histogram of several types of etch pits of different sizes detected by etching, the peak of the smallest etch pit has a shoulder or double peak. This is thought to be because in regions with a locally high Ge concentration, Ge is trapped at the dislocation core and diffuses abnormally above the dislocation core, generating Ge clusters and precipitates. This expands the strain field around the dislocation core and affects the size of the etch pits. It has also become clear that the factors that cause the above-mentioned variations in device performance are correlated with the generation of strain fields around the dislocation core due to the formation of Ge clusters and precipitates. According to the model described above, it is predicted that Ge clusters and precipitates are more likely to occur the more Ge is incorporated into the crystal and the lower the dislocation density in the crystal, as they tend to accumulate on each dislocation core. However, to reduce the resistivity of GaN substrates, it is desirable to dope as much Ge as possible while also minimizing the density of dislocations, which are crystal defects. To satisfy these requirements and suppress the formation of Ge clusters and precipitates, it is necessary to dope at a high concentration that does not cause Ge clusters or precipitates to form while minimizing variation in the Ge concentration in the crystal. Here, Ge is easily incorporated into the growing crystal unevenly, and when large-diameter GaN substrates (those exceeding 50 mm in diameter) are manufactured, regions with locally high Ge concentrations tend to occur within the substrate surface. This is because, compared to Si, the amount of Ge incorporated into the crystal varies more easily depending on the inclination of the crystal plane orientation at the crystal growth interface. When a morphology with a slightly inclined surface corresponding to the facet growth region appears at the crystal growth interface, the amount of Ge incorporated in that region increases. Therefore, when Ge is doped at a high concentration, concentration unevenness that exceeds the threshold for the formation of Ge clusters and precipitates is likely to occur. It was found that growth on large-diameter substrates with a wide growth interface area is likely to form large facet surfaces, and therefore form morphologies with a high degree of roughness, making it more likely that high-concentration regions will occur. The threshold Ge concentration at which Ge clusters and precipitates form is difficult to determine in general terms, as it depends not only on the Ge concentration incorporated into the crystal but also on various factors such as the density and type of dislocations present in that region, the crystal growth temperature and pressure, the concentrations of other impurities and point defects in the crystal, and the degree of residual stress. However, based on the above findings, it is possible to determine whether or not Ge clusters or precipitates have occurred by observing whether the first peak in the etch pit size histogram is a single peak without a shoulder. In other words, the etch pit size histogram can be used as a guide to measure the uniformity of Ge distribution.

[0015] The size of etch pits also depends significantly on etching conditions. Therefore, discussing the absolute value of etch pit size is meaningless. However, among the various types of etch pits detected by etching, the smallest etch pit is highly likely to correspond to an edge dislocation with the smallest strain field around the dislocation core. Therefore, the inventors investigated a histogram of etch pit diameters, normalized the diameter of the smallest peak among the peaks appearing in this histogram, and investigated the diameters and numbers of other etch pits. This method is highly useful for investigating the distribution of dislocation species and dislocation-related properties in gallium nitride single crystal substrates.

[0016] It is very difficult to confirm localized unevenness in the concentration of Ge in a GaN crystal. However, the method of the present invention can detect unevenness in a relatively large area (for example, 1 mm 2 By creating a histogram of the etch pit diameters using the above procedure, it is possible to check the unevenness of the Ge concentration.

[0017] Furthermore, the inventors conducted extensive research into methods for producing substrates that can produce the above-described histogram, i.e., gallium nitride single crystal substrates that are uniformly and highly doped with Ge. Variations in the incorporation of Ge into GaN crystals occur due to the occurrence of inclined surfaces at the growth interface during crystal growth, which are caused by facet growth. Facet growth at the growth interface is thought to be due to variations in the density of atomic steps formed by defects such as dislocations and the concentration of impurities remaining due to interfacial segregation. Therefore, in the process of growing a Ge-doped GaN layer, a gas containing HCl is intermittently introduced into the growth interface, and the surface of the growing crystal is periodically etched to maintain the flatness and cleanliness of the interface. This suppresses the formation of minute facets that serve as precursors to facet growth at the growth interface, thereby establishing a technology for uniformly doping Ge into the GaN crystal.

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

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

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

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

[0022] (S100: Base board preparation process) As shown in FIG. 2(a), in the base substrate preparation step S100, a base substrate 10 made of a gallium nitride single crystal is prepared, in which 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 (not a template, but a freestanding substrate) may be used as the base substrate 10, or the base substrate 10 made of a gallium nitride single crystal may be fabricated by the Void-Assisted Separation (VAS) method described in Patent Document 1. The base substrate 10 may be Ge-doped, Si-doped, or undoped, but is preferably Ge-doped. This reduces the difference in Ge concentration between the base substrate 10 and the growth layer 30, which will be described later.

[0023] (S110: Growth process) In the growth step S110, a GaN single crystal is epitaxially grown on the primary surface 10s of the base substrate 10 prepared in the base substrate preparation step S100, for example, using the c-plane 30c as the growth surface, as shown in Fig. 2(b). Specifically, for example, GaCl gas and NH gas are supplied to the heated base substrate 10 by the HVPE method, thereby causing epitaxial growth directly on the primary surface 10s of the base substrate 10, and growing a growth layer 30.

[0024] In the growth step S110, the growth layer 30 is grown under predetermined growth conditions, for example. In this embodiment, the growth conditions preferably include a growth temperature of 980° C. or higher and 1200° C. or lower. In addition, in the growth step S110, the ratio of the supply rate of NH gas as the nitriding agent gas to the supply rate of GaCl gas as the Group III source gas (hereinafter also referred to as the "V / III ratio") is preferably 0.1 or higher and 5.0 or lower.

[0025] In the growth step S110, GaCl gas, NH gas, and an n-type dopant gas, for example, tetrachlorogermane (GeCl) gas, are supplied to the base substrate 10, so that the Ge concentration becomes 3×10 18 cm -3 More than 1×10 20 cm -3 A GaN layer doped with a high concentration of Ge is epitaxially grown as follows. GeCl4 gas is supplied, for example, from a cylinder, or by placing the source container in a thermostatic chamber maintained at about 5°C and bubbling GeCl4 in liquid form using a carrier gas. The carrier gas can be either H2 gas or N2 gas, or both. In either case, it is preferable to control the flow rate using a mass flow controller so that the partial pressure of the GeCl4 gas is 0.3 to 30 Pa.

[0026] Other growth conditions in the growth step S110 are, for example, as follows: Growth pressure: 90 to 105 kPa, preferably 90 to 95 kPa GaCl gas partial pressure: 1.5 to 15 kPa Partial pressure of GeCl4 gas: 0.3 to 30 Pa N2 gas flow rate / H2 gas flow rate: 0 to 1

[0027] In the growth step S110, a gas containing HCl is intermittently introduced and the growth interface is periodically etched (also referred to as intermittent etching) to uniformly dope Ge into the crystal. Specifically, for example, during growth using the c-plane 30c as the growth surface, the supply of GaN source gas and n-type dopant gas is temporarily stopped, and HCl gas and a carrier gas (e.g., H2 gas) are introduced. This suppresses the formation of minute facets on the growth surface, allowing for uniform doping of Ge. Note that the introduction of the gas containing HCl is preferably performed, for example, at intervals of 5 to 10 minutes, for 0.5 to 1 minute per introduction.

[0028] In the growth step S110, the thickness of the growth layer 30 is preferably set to, for example, 1 mm or more and 20 mm or less. The c-plane warpage of the base substrate 10 is inherited by the growth layer 30 grown on it. Therefore, by growing the growth layer 30 to a certain thickness or more, stress is generated in the crystal to restore the warpage and flatten the c-plane, gradually reducing the magnitude of the c-plane warpage. If the growth layer 30 is less than 1 mm thick, the stress required to restore the c-plane warpage may be insufficient. Furthermore, it is difficult to obtain a GaN single crystal substrate with sufficient thickness in the subsequent slicing and processing step S120. In contrast, by setting the thickness of the growth layer 30 to 1 mm or more, the c-plane warpage can be reduced. Furthermore, a GaN single crystal substrate with sufficient thickness can be obtained in the subsequent slicing and processing step S120. On the other hand, if the thickness of the growth layer 30 exceeds 20 mm, excessive strain accumulates in the crystal due to the force attempting to restore the c-plane warpage, which may result in the proliferation of dislocations and the generation of microcracks. Furthermore, the increased internal residual stress in the GaN crystal makes the crystal more susceptible to cracking during the slicing and processing step S120. By limiting the thickness of the growth layer 30 to 20 mm or less, the internal residual stress in the GaN crystal can be reduced, preventing cracks and breakage. Furthermore, by performing intermittent etching during Ge-doped growth, the impurity distribution becomes smaller, reducing the internal residual stress in the crystal. As a result, it becomes possible to grow the crystal thicker than when intermittent etching is not performed, and the radius of curvature of the c-plane can be further increased.

[0029] (S120: Slicing and processing process) In the slicing and processing step S120, as shown in FIG. 1(c), the growth layer 30 is sliced ​​using a wire saw or the like along a cutting plane substantially parallel to the primary surface 30s (c-plane 30c) of the growth layer 30. This results in at least one gallium nitride single crystal substrate 50 (also referred to as substrate 50) having a diameter of 50 mm or more as an as-sliced ​​substrate. At this time, it is preferable to slice the substrate 50 so that the thickness is, for example, 300 μm or more and 500 μm or less.

[0030] When a GaN substrate produced by the VAS method is used as the base substrate 10, the c-plane 50c of the base substrate 10 typically has a curvature with a radius of curvature of approximately 5 m, with the surface being concave. The radius of curvature of the c-plane 50c of the substrate 50 cut out 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, for example, 10 m or more, and more preferably 20 m or more. This allows the variation in the off-angle θ of the c-axis relative to the normal to the major surface 50s of the substrate 50 (the maximum and minimum difference between 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.

[0031] Once the substrate 50 is obtained as an as-sliced ​​substrate, both surfaces of the substrate 50 may be polished using, for example, a polishing device.

[0032] In this embodiment, the substrate 50 obtained in the slicing and processing step S120 may be used as a new base substrate 10, and the growing step S110 and the slicing and processing step S120 may be repeated multiple times (e.g., four or more times). This type of growth method is referred to herein as "alternate-generation growth." By performing alternate-generation growth rather than growing a thick film all at once, the c-plane warpage is alleviated each time the substrate 50 is sliced, thereby reducing the internal residual stress of the GaN crystal. The reduced internal residual stress reduces the difference in the radius of curvature between the grown substrate and the base substrate 10, facilitating uniform Ge doping. The upper limit of the number of times alternate-generation growth can be performed is not particularly limited, but from the perspective of efficiently producing large-diameter GaN single crystal substrates, it is preferable to perform the process 10 or less times.

[0033] Through the above steps, the substrate 50 according to this embodiment is manufactured.

[0034] (Process for manufacturing semiconductor laminates and process for manufacturing semiconductor devices) After the substrate 50 is manufactured, a semiconductor functional layer made of, for example, a group III nitride semiconductor may be epitaxially grown on the substrate 50 to produce a semiconductor laminate. After the semiconductor laminate is produced, electrodes and the like may be formed on the semiconductor laminate, and the semiconductor laminate may be diced to cut out chips of a predetermined size. In this manner, a semiconductor device may be produced. Because the substrate 50 is doped with Ge at a high concentration and uniformly, it is suitable for producing vertical devices and the like.

[0035] (2) Gallium nitride single crystal substrate (freestanding nitride semiconductor substrate, nitride crystal substrate) Next, the gallium nitride single crystal substrate 50 according to this embodiment will be described.

[0036] In this embodiment, the substrate 50 obtained by the above-described manufacturing method is a free-standing substrate made of single crystal GaN.

[0037] The diameter of the substrate 50 is, for example, 50 mm or more, and the thickness of the substrate 50 is, for example, 300 μm or more and 1 mm or less.

[0038] The substrate 50 is n-type doped with Ge, and the Ge concentration in the substrate 50 is, for example, 3×10 18 cm -3 Over 1.0 x 10 20 cm -3 less than or equal to 5 × 10 18 cm -3 Over 1.0 x 10 20 cm -3 or less, more preferably 1×10 19 cm -3 Over 1.0 x 10 20 cm -3 The higher the Ge concentration in the substrate 50, the greater the variation in the Ge concentration within the substrate 50, which increases the likelihood that the peak of the smallest etch pit in the etch pit histogram will have a shoulder. Therefore, the more highly Ge-doped the substrate, the more effective it is to perform intermittent etching frequently during Ge doping growth.

[0039] The substrate 50 has, for example, a primary surface 50s that serves as an epitaxial growth surface. In this embodiment, the low-index crystal plane closest to the primary surface 50s is, for example, a c-plane 50c.

[0040] The main surface 50s of the substrate 50 is, for example, mirror-finished, and the root mean square roughness RMS of the main surface 50s of the substrate 50 is, for example, less than 1 nm.

[0041] Furthermore, in this embodiment, the impurity concentration in the substrate 50 obtained by the above-described manufacturing method is lower than that of a substrate obtained by liquid phase growth such as a flux method or an ammonothermal method.

[0042] Specifically, the hydrogen concentration in the substrate 50 is, for example, 1×10 17 cm -3 Less than 5 x 10 16 cm -3 The oxygen concentration in the substrate 50 is, for example, 5×10 16 cm -3 Less than or equal to 3 x 10 16 cm -3 The following is the result.

[0043] (Curving of the c-plane and variation in off-angle) The radius of curvature of the c-face 50c of the substrate 50 is, for example, larger than the radius of curvature of the c-face 10c of the base substrate 10. Specifically, the radius of curvature of the c-face 50c of the substrate 50 is, for example, preferably 10 m or more, and more preferably 20 m or more.

[0044] In this embodiment, the upper limit of the radius of curvature of the c-face 50c of the substrate 50 is not particularly limited, and the larger the radius, the better. When the c-face 50c of the substrate 50 is substantially flat, the radius of curvature of the c-face 50c can be considered to be infinite.

[0045] Furthermore, in this embodiment, since the radius of curvature of the c-plane 50c of the substrate 50 is large, the variation in the off-angle θ of the c-axis relative to the normal to the main surface 50s of the substrate 50 can be made smaller than the variation in the off-angle of the c-axis of the base substrate 10.

[0046] Specifically, when an X-ray rocking curve measurement is performed on the (0002) plane of the substrate 50 and the off-angle θ of the c-axis relative to the normal to the main surface 50s is measured based on the diffraction peak angle of the (0002) plane, the variation calculated as the maximum / minimum difference in the magnitude of the off-angle θ within a diameter of 25 mm from the center of the main surface 50s is, for example, 0.14° or less, preferably 0.07° or less, and more preferably 0.05° or less.

[0047] In this embodiment, the lower limit of the variation in the off-angle θ of the c-axis of the substrate 50 is not particularly limited, and the smaller the better. When the c-plane 50c of the substrate 50 is substantially flat, the variation in the off-angle θ of the c-axis of the substrate 50 can be considered to be 0°.

[0048] Furthermore, in this embodiment, the curvature of the c-plane 50c is isotropically reduced relative to the main surface 50s of the substrate 50, so the radius of curvature of the c-plane 50c has little directional dependency.

[0049] Specifically, the difference between the radius of curvature of c-face 50c in the direction along the a-axis, as determined by the above-described measurement method, and the radius of curvature of c-face 50c in the direction along the m-axis, is, for example, 50% or less, preferably 20% or less, of the larger of these radii of curvature.

[0050] (dislocation density) In this embodiment, the dislocation density at the surface of the substrate 50 is reduced compared to the dislocation density at the major surface 10s of the base substrate 10.

[0051] When a substrate 50 is manufactured using a base substrate 10 made of a high-purity GaN single crystal produced by the VAS method, there are few non-radiative centers caused by foreign matter or point defects in the substrate 50. Therefore, 95% or more (preferably 99% or more) of the dark spots observed when the primary surface of the substrate 50 of the present application is observed using a multi-photon excitation microscope or the like correspond to dislocations rather than non-radiative centers caused by foreign matter or point defects. Note that a "multi-photon excitation microscope" is sometimes called a two-photon excitation fluorescence microscope.

[0052] In this embodiment, crystal growth is carried out so as to maintain the flatness of the crystal growth interface, so a mechanism that causes localized concentration of propagating dislocations does not work. Therefore, no regions with particularly high dislocation density due to dislocation concentration are formed, and the distribution of dislocation density is uniform within the substrate surface. Note that, unlike Si, it has been confirmed that the dislocation density in GaN crystals does not increase even when doped at a high concentration with Ge.

[0053] (Etch pit histogram) When the main surface 50s of the substrate 50 is etched using an alkaline etching solution, etch pits as shown in FIG. 3 are formed. In this embodiment, the substrate 50 was immersed for 15 minutes in a molten solution (temperature: 470°C) prepared by mixing potassium hydroxide (KOH) and sodium hydroxide (NaOH) in a 1:1 ratio. The main surface 50s of the etched substrate 50 was then observed using an SEM. The diameter of each etch pit was calculated by image analysis. A histogram of the etch pit diameters was created from the obtained data. The etch pit diameter may be the maximum diameter of each etch pit obtained by image analysis, or the equivalent circle diameter. In this embodiment, the diameter of the smallest peak (hereinafter also referred to as the first peak) among the peaks appearing in the histogram of etch pit diameters was standardized as a, and the horizontal axis (increment) was set to 0.1a. The vertical axis represents the number (frequency) of etch pits within the measurement area.

[0054] The average density (number per unit area) of etch pits formed when the main surface 50s of the substrate 50 of this embodiment is etched under the above-mentioned conditions nearly matches the dislocation density determined from observations using a multiphoton excitation microscope. The size of the etch pits can be changed by adjusting the etching conditions, but if they are present at too high a density, adjacent etch pits will overlap, making it difficult to measure the diameter of the etch pits. Therefore, in order to perform meaningful measurements, the average density of etch pits should be less than 1×10 6 cm -2 Preferably, less than 5.5×10 5 cm -2 less than 3 x 10 5 cm -2 It is preferable that the average density of etch pits and the diameter of etch pits are not more than 1 mm. 2 It is preferable to measure from the above areas.

[0055] In the case of a substrate having Ge clusters or the like on the dislocation core, the peaks (particularly the first peak) of the histogram have a shoulder or become double peaks. In contrast, in the 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, the substrate 50 of this embodiment can be said to be a substrate uniformly doped with Ge.

[0056] In this specification, the first peak being a single peak without a shoulder means, for example, that when the diameter of the first peak is a, there are no other peaks in the range of 0.8a to 1.2a inclusive, and when the peak width at a height that is 1 / 10 of the peak height from the baseline of the first peak is W, and the width on the rising side of the peak obtained by bisecting the peak width of W by a perpendicular line drawn from the peak apex to the horizontal axis is f, the symmetry coefficient S defined by the following formula (1) is 0.8 to 1.2 inclusive. S = W / (2 × f) (1)

[0057] In the substrate 50 of this embodiment, when the frequency of the first peak is A, the number of peaks appearing in the diameter histogram with a frequency of A / 10 or more, including the first peak, is preferably two. The number of peaks appearing in the etch pit histogram can be said to correspond to the number of constituent types of dislocation defects corresponding to the etch pits. Crystals with many observed peaks are likely to have not only simple edge and mixed dislocations, but also a wide variety of impurities and point defects complexly associated with them, i.e., to have many impurity levels and defect levels. These crystals are likely to have large variations in characteristics when devices are fabricated, or in other words, to have a high probability of low device yield. In contrast, crystals with two peaks appearing in the histogram with a frequency of A / 10 or more, including the first peak, suggest that only two types of dislocations exist in the crystal: simple edge dislocations and mixed dislocations. This suggests that the crystals have small variations in characteristics when devices are fabricated, and therefore have a high probability of high device yield.

[0058] In the substrate 50 of this embodiment, when diameter histograms are created for multiple different regions (for example, three or more different regions, more preferably five or more different regions) on the main surface 50s, it is preferable that the first peak is a single peak without a shoulder in all of the multiple histograms. The fact that the first peak is a single peak without a shoulder in all of the multiple histograms indicates that Ge is uniformly doped not only within a microscopic region but also at every position on the surface.

[0059] In the substrate 50 of this embodiment, the total number of etch pits with a diameter exceeding 4a is preferably 1 / 1000 or less of the number of etch pits constituting the first peak. This can also be said to mean that there are substantially no etch pits with a diameter exceeding 4a. However, because scratches or foreign matter on the substrate 50 can sometimes form large etch pits, the expression 1 / 1000 or less is used. In the substrate 50 of this embodiment, the above-described manufacturing method prevents dislocations from piling up, and therefore multiple dislocations are not combined into one. Therefore, it can be said that there are substantially no dislocations with large accumulated strain that would deteriorate device characteristics. Therefore, the characteristics of devices fabricated on the substrate 50 can be improved.

[0060] In substrate 50 of this embodiment, the total number of etch pits having a diameter of 2a or more is preferably 1 / 10 or less, and more preferably 1 / 100 or less, of the number of etch pits constituting the first peak. Etch pits having a diameter of 2a or more may correspond to dislocations that affect the device characteristics of substrate 50. Therefore, by making the total number of etch pits having a diameter of 2a or more 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 further improved.

[0061] In the substrate 50 of this embodiment, the number of etch pits constituting the first peak is preferably 50% or more (more preferably 70% or more) of the total. In this embodiment, as described above, dislocation synthesis is unlikely to occur and no Ge clusters or precipitates are formed, so the proportion of dislocations with small strain fields is high. Since dislocations with small strain fields do not significantly affect the device characteristics of the substrate 50, 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 means that the characteristics of devices fabricated on the substrate 50 are good.

[0062] Furthermore, the substrate 50 of this embodiment preferably has a second peak with a diameter greater than a but less than 2a, and the number β of etch pits constituting the second peak is less than the number α of etch pits constituting the first peak. The fact that the diameter of the etch pits constituting the second peak is greater than a but less than 2a indicates that they are likely to correspond to simple mixed dislocations. This presupposes that the first peak is a single peak without a shoulder. However, a small number of etch pits constituting the second peak, in other words, a large number of etch pits constituting the first peak, indicates that the dislocations in the crystal are primarily composed of dislocations with minimal strain around the dislocation core. These characteristics mean that devices fabricated on the substrate 50 have good characteristics.

[0063] <Other embodiments> Although the embodiments of the present invention have been specifically described above, the present invention is not limited to the above-described embodiments and can be modified in various ways without departing from the spirit and scope of the present invention.

[0064] In the above embodiment, the growth step S110 was described as involving epitaxial growth of the growth layer 30 using the c-plane 30c as the growth surface. However, for example, a growth step using an inclined interface other than the c-plane as the growth surface may be interposed during the growth step S110. This allows dislocations to bend, propagate, meet, and disappear. However, since the Ge is likely to become non-uniform during growth using an inclined interface as the growth surface, it is preferable not to heavily dope Ge during growth at the inclined interface, but to return the growth interface to a flat surface using the c-plane 30c as the growth surface, and then perform intermittent etching to grow the layer with a high Ge doping concentration, and then obtain the substrate 50 from the crystalline region where intermittent etching was performed.

[0065] In the above embodiment, the growth layer 30 is sliced ​​using a wire saw in the slicing and processing step S120, but for example, an outer diameter blade slicer, an inner diameter blade slicer, an electric discharge machine, etc. may also be used. [Example]

[0066] Next, examples of the present invention will be described. These examples are merely examples of the present invention, and the present invention is not limited to these examples.

[0067] (1) Preparation of gallium nitride single crystal substrate Gallium nitride single crystal substrates of Examples 1 to 3 and Comparative Examples 1 to 3 were fabricated as follows.

[0068] [Conditions for producing a gallium nitride single crystal substrate in Example 1] (Base substrate) Material: GaN Preparation method: VAS method Diameter: 2 inches Thickness: 400 μm The closest low-index crystal plane to the main plane: c-plane No patterning of the main surface, such as a mask layer, is required. Root mean square roughness RMS of main surface: 2nm Off-angle of the main surface: 0.4° in the m direction (growth layer) Material: GaN Growth method: HVPE method Growth temperature: 980℃ or higher and 1,020℃ or lower V / III ratio: 2 or more and 20 or less Ge doping method: GeCl4 gas Partial pressure of GeCl4 gas: 0.35 Pa Growth layer thickness: 4500 μm (Intermittent etching conditions) Etching gas: HCl gas Etching interval: 10 minutes / time Etching time: 0.5 minutes / cycle (Slicing and processing conditions) Gallium nitride single crystal substrate thickness: 400 μm Kerf loss: 200 μm

[0069] The substrate 50 was fabricated under the above fabrication conditions. The Ge concentration of the substrate 50 was measured by secondary ion mass spectrometry (SIMS), and it was found to be 3×10 18cm -3 It was.

[0070] [Conditions for producing a gallium nitride single crystal substrate in Example 2] (Base substrate) Same as Example 1. (growth layer) Growth was performed by changing the GeCl4 gas partial pressure and intermittent etching conditions in order to increase the Ge concentration more than in Example 1. The conditions other than those listed below were the same as in Example 1. Partial pressure of GeCl4 gas: 1.2 Pa (Intermittent etching conditions) Etching interval: 7 minutes / time Etching time: 1 minute / cycle (Slicing and processing conditions) Same as Example 1.

[0071] The substrate 50 was fabricated under the above fabrication conditions. The Ge concentration of the substrate 50 was measured by secondary ion mass spectrometry, and the Ge concentration was 1×10 19 cm -3 It was.

[0072] [Conditions for producing a gallium nitride single crystal substrate in Example 3] (Base substrate) Same as Example 1. (growth layer) In order to further increase the Ge concentration compared to Example 2, growth was performed by changing the GeCl4 gas partial pressure and intermittent etching conditions. The conditions were the same as Example 1 except for the following: Partial pressure of GeCl4 gas: 12 Pa (Intermittent etching conditions) Etching interval: 5 minutes / time Etching time: 1 minute / cycle (Slicing and processing conditions) Same as Example 1.

[0073] The substrate 50 was fabricated under the above fabrication conditions. The Ge concentration of the substrate 50 was measured by secondary ion mass spectrometry, and the Ge concentration was 1×10 20 cm -3 It was.

[0074] [Conditions for producing gallium nitride single crystal substrate in Comparative Example 1] (Base substrate) Same as Example 1. (growth layer) Same as Example 1. (intermittent etching) In Comparative Example 1, intermittent etching was not performed. (Slicing and processing conditions) Same as Example 1.

[0075] The substrate 50 was fabricated under the above fabrication conditions. The Ge concentration of the substrate 50 was measured by secondary ion mass spectrometry, and found to be 3×10 18 cm -3 It was.

[0076] [Conditions for producing gallium nitride single crystal substrate in Comparative Example 2] (Base substrate) Same as Example 1. (growth layer) Same as Example 2. (intermittent etching) In Comparative Example 2, intermittent etching was not performed. (Slicing and processing conditions) Same as Example 1.

[0077] The substrate 50 was fabricated under the above fabrication conditions. The Ge concentration of the substrate 50 was measured by secondary ion mass spectrometry, and the Ge concentration was 1×10 19 cm -3 It was.

[0078] [Conditions for producing gallium nitride single crystal substrate in Comparative Example 3] (Base substrate) Same as Example 1. (growth layer) Same as Example 3. (intermittent etching) In Comparative Example 3, intermittent etching was not performed. (Slicing and processing conditions) Same as Example 1.

[0079] The substrate 50 was fabricated under the above fabrication conditions. The Ge concentration of the substrate 50 was measured by secondary ion mass spectrometry, and the Ge concentration was 1×10 20 cm -3 It was.

[0080] (2) Evaluation of etch pit histogram The main surfaces of the gallium nitride single crystal substrates of Examples 1 to 3 and Comparative Examples 1 to 3 were subjected to etching using an alkaline etching solution under the following conditions to form etch pits. Etching solution: Alkaline melt consisting of 500g KOH and 500g NaOH Melt temperature: 470℃ Soaking time: 15 minutes

[0081] The main surface of the substrate with etch pits was observed using a SEM (SU5000, Hitachi High-Tech Corporation). The measurement field was 127 μm × 95.3 μm (1000x magnification), and the etch pits were measured by image analysis of a combined image (1143 μm × 1048 μm) of 99 SEM images (9 horizontal × 11 vertical). Specifically, the image was binarized using image processing software (Image J), ​​and the etch pit area was filled in. The larger of the width and height of the etch pit area was calculated as the etch pit diameter. Note that when filling in the etch pit area, areas representing small particles and linear pit areas caused by processing scratches were removed.

[0082] A histogram of etch pit diameters was created from the obtained data on etch pit diameters. In this example, the diameter of the smallest peak (first peak) among the peaks appearing in the histogram of etch pit diameters was normalized as 1, and the horizontal axis was scaled in increments of 0.1.

[0083] Figures 4 to 6 show histograms measured at the center of the substrates of Examples 1 to 3, Figure 7 shows the distribution of histograms measured at five locations within the surface of the substrate of Example 1, and Figures 8 to 10 show histograms measured at the center of the substrates of Comparative Examples 1 to 3. In each histogram, the horizontal axis shows the normalized etch pit diameter, and the vertical axis shows the number (frequency) of etch pits within the measurement area.

[0084] As shown in Fig. 4, the first peak of the histogram was a single peak without a shoulder for the substrate of Example 1. On the other hand, as shown in Fig. 8, the first peak of the histogram was a double peak for the substrate of Comparative Example 1.

[0085] In addition, in the substrates of Example 2 (FIG. 5) and Example 3 (FIG. 6), which had a higher Ge concentration than the substrate of Example 1, the first peak of the histogram was a single peak without a shoulder. In addition, in the substrates of Comparative Example 2 (FIG. 9) and Comparative Example 3 (FIG. 10), which had a higher Ge concentration than the substrate of Comparative Example 1, the first peak of the histogram was a double peak. 18 cm -3 It was confirmed that when grown at the above concentration without intermittent etching, the first peak of the histogram becomes a double peak.

[0086] Furthermore, as shown in Figure 7, when histograms were checked at the center of the substrate in Example 1 and at five locations within the surface that were ±20 mm away from the center in the horizontal and vertical directions, the first peak in all histograms was a single peak with no shoulder.

[0087] From the above, we confirmed that by performing intermittent etching during the growth of the growth layer, it is possible to manufacture a substrate that produces a histogram in which the first peak is a single peak without a shoulder. Such a substrate can be said to be highly and uniformly doped with Ge, and is therefore suitable for the fabrication of vertical devices, etc.

[0088] <Preferred embodiment of the present invention> Preferred embodiments of the present invention will be described below.

[0089] (Appendix 1) A gallium nitride single crystal substrate having a diameter of 50 mm or more, and having a low-index crystal plane closest to the primary surface as the (0001) plane, The Ge concentration in the substrate is 3×10 18 cm -3 That's all, A gallium nitride single crystal substrate, wherein a first peak having the smallest diameter among peaks appearing in a histogram of the diameters of etch pits formed when the primary surface is etched with an alkaline etching solution is a single peak without a shoulder.

[0090] (Appendix 2) 2. The gallium nitride single crystal substrate according to claim 1, wherein, in a histogram of etch pit diameters, when the diameter of the first peak is taken as a, there are no peaks other than the first peak in the range of 0.8a to 1.2a inclusive, and when the peak width at a height 1 / 10 of the peak height from the baseline of the first peak is taken as W and the width on the rising side of the peak obtained by bisecting the peak width of W by a perpendicular line drawn from the peak apex to the horizontal axis is taken as f, the symmetry coefficient S defined by the following formula (1) is 0.8 to 1.2 inclusive: S = W / (2 × f) (1)

[0091] (Appendix 3) 2. The gallium nitride single crystal substrate according to claim 1, wherein, when the frequency of the first peak is A, there are two peaks in the histogram having a frequency of A / 10 or more, including the first peak.

[0092] (Appendix 4) 2. The gallium nitride single crystal substrate according to claim 1, wherein, when the histograms are created in a plurality of different regions on the primary surface, the first peak is a single peak without a shoulder in all of the plurality of histograms.

[0093] (Appendix 5) 2. The gallium nitride single crystal substrate according to claim 1, wherein, when the etch pit diameter of the first peak is a, in the histogram, the total number of etch pits having a diameter exceeding 4a is 1 / 1000 or less of the number of the etch pits constituting the first peak.

[0094] (Appendix 6) 2. The gallium nitride single crystal substrate according to claim 1, wherein in the histogram of etch pit diameters, the total number of etch pits having a diameter of 2a or more is 1 / 10 or less, more preferably 1 / 100 or less, of the number of etch pits constituting the first peak.

[0095] (Appendix 7) 2. The gallium nitride single crystal substrate according to claim 1, wherein in the histogram of etch pit diameters, the number of etch pits constituting the first peak is 50% or more of the total, and more preferably 70% or more.

[0096] (Appendix 8) 2. The gallium nitride single crystal substrate of claim 1, wherein a histogram of the etch pit diameters has a second peak in a diameter range greater than a and less than 2a, and the number β of etch pits constituting the second peak is less than the number α of etch pits constituting the first peak.

[0097] (Appendix 9) The average density of the etch pits is 1×10 6 cm -2 2. The gallium nitride single crystal substrate according to claim 1, wherein the gallium nitride single crystal substrate has a thickness of less than 100 μm.

[0098] (Appendix 10) The histogram has an area of ​​1 mm 2 10. A gallium nitride single crystal substrate according to any one of Supplementary Note 1 to Supplementary Note 9, wherein the measurement is performed from the above-mentioned region.

[0099] (Appendix 11) A step (a) of preparing a base substrate made of gallium nitride single crystal, the low-index crystal plane closest to the primary surface being the (0001) plane; The primary surface of the base substrate is provided with a Ge concentration of 3×1018 cm -3 a step (b) of epitaxially growing the above-described gallium nitride single crystal; 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 and the growth interface is periodically etched, thereby doping Ge uniformly into the crystal. [Explanation of symbols]

[0100] 10 Base substrate 30 growth layer 50 Gallium nitride single crystal substrate (substrate) S100 Base board preparation process S110 Growth process S120 Slicing and Processing

Claims

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

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

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

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

5. The average density of the etch pits is 1×10 ６ cm －２ The gallium nitride single crystal substrate according to claim 1 , wherein the thickness is less than 100 μm.

6. The histogram has an area of ​​1 mm ２ The gallium nitride single crystal substrate according to any one of claims 1 to 5, wherein the measurement is performed from the above-mentioned region.

7. a step (a) of preparing a base substrate made of gallium nitride single crystal, the low-index crystal plane closest to the primary surface being the (0001) plane; A Ge concentration of 3×10 １８ cm －３ a step (b) of epitaxially growing the above-described gallium nitride single crystal; 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 doping Ge uniformly into the crystal.

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