GAN substrate, method for producing GAN crystal, and method for producing GAN substrate

Abstract

Disclosed is a method for producing a GaN substrate, the method including: growing a GaN crystal by an ammonothermal method through a mask opening of a first pattern mask on a first base substrate that has the first pattern mask disposed on the nitrogen polar surface thereof so as to obtain a multilayer structure in which the GaN crystal is superposed on the nitrogen polar surface of the first base substrate through the first pattern mask; using the GaN crystal separated from the multilayer structure as a second base substrate and disposing a second pattern mask on the nitrogen polar surface so that at least a part of a region grown above the mask opening of the first pattern mask on the nitrogen polar surface is covered with the mask part of the second pattern mask; and growing a GaN crystal by an ammonothermal method through a mask opening of the second pattern mask. Also disclosed is a GaN substrate which is obtained by the above-described production method. This GaN substrate achieves improvement in the yield or reduction in crystal defects over a wide range when used as a c-plane GaN substrate during the production of a device.

Description

GaN substrate, GaN crystal manufacturing method, and GaN substrate manufacturing method

[0001] The present invention relates to a gallium nitride (GaN) substrate, a method for manufacturing a GaN crystal, and a method for manufacturing a GaN substrate.

[0002] GaN substrates obtained from GaN crystals are used as substrates for manufacturing laser diodes (LDs). Recently, GaN substrates have been considered promising as substrates for vertical power semiconductor devices (PDs).

[0003] GaN substrates with reduced crystal defects are required for improving device performance as GaN substrates for LDs and vertical PDs. Patent Document 1 describes a method for growing GaN crystals using an ammonothermal method on a GaN seed provided with a pattern mask, and the GaN crystals obtained thereby. Patent Document 2 describes GaN crystals obtained using an HVPE method with GaN crystals obtained by the ammonothermal method as seeds.

[0004] WO2018 / 030311 WO2020 / 036047

[0005] M. Fukuzawa, Doctoral Dissertation "Study on Photoelastic Evaluation of Compound Semiconductors", Kyoto Institute of Technology (1997); M. Fukuzawa, T. Chu, and M. Yamada, J. Soc. Mat. Sci., Japan., 51, 9, 966, (2002); M. Fukuzawa, R. Kashiwagi, and M. Yamada, Phys. Status Solidi C 8, No. 2, 432, (2011).

[0006] As described above, GaN crystals obtained by growing GaN crystals using the ammonothermal method on GaN seeds with patterned masks, and GaN crystals grown using the ammonothermal method with such seeds, have room for improvement in terms of yield when used as c-plane GaN substrates in device fabrication. Furthermore, as described above, GaN crystals obtained by growing GaN crystals using the ammonothermal method on GaN seeds with patterned masks still have room for improvement in reducing crystal defects over a wide range. Therefore, an object of a first aspect of the present invention is to provide a GaN substrate that can improve yield when used as a c-plane GaN substrate in device fabrication. Furthermore, an object of a second aspect of the present invention is to provide a GaN crystal and a GaN substrate with reduced crystal defects over a wide range.

[0007] As a result of intensive research into the above-mentioned problem, the inventors have found that one of the causes of the decrease in yield is strain locally present on the primary surface of the GaN crystal, which has led to the completion of the first embodiment of the present invention. Furthermore, as a result of intensive research into the above-mentioned problem, the inventors have found that a required manufacturing method can be used to realize GaN crystals with reduced crystal defects over a wide range, which has led to the completion of the second embodiment of the present invention.

[0008] The embodiments of the present invention include the following: <1> A silicon substrate having an inclination of 0 to 20 degrees from the (0001) plane and an area of ​​15 cm ２ A GaN substrate having a primary surface of 0.275 cm or more, ２ an observation region having an area of ​​1.0×10 ４ cm －２ <2> The GaN substrate, wherein the observation region includes a unit region A and a unit region B, and the dislocation density of the unit region A is less than 1.0×10 ２ cm －２ The dislocation density of the unit area B is less than 1.0 × 10 ３ cm －２<3> The GaN substrate according to <1>, wherein the observation area is an observation area of ​​5.5 mm × 5.0 mm, and the observation area is divided into 10 unit areas of 5.5 mm × 0.5 mm, and the dislocation density is 1.0 × 10 ２ cm －２ The GaN substrate according to <1> or <2>, wherein the number of unit regions where the (004) XRD rocking curve FWHM is less than 20 arcsec is one or more. <4> The GaN substrate according to any of <1> to <3>, wherein the (004) XRD rocking curve FWHM of the primary surface is 20 arcsec or less. <5> The GaN substrate according to any of <1> to <4>, wherein, when a normal direction of the primary surface is defined as a z direction, and two directions that are orthogonal to the z direction and are orthogonal to each other are defined as an x ​​direction and a y direction, respectively, a fluctuation range of the x-direction component of the offcut angle on a first line that passes through the center of the primary surface and extends in the x direction, and a fluctuation range of the y-direction component of the offcut angle on a second line that passes through the center of the primary surface and extends in the y direction are each 0.2 degree or less within a 40 mm long section. <6> The GaN substrate according to <1> or <2>, wherein the concentrations of Li, Na, K, Mg, and Ca are all 1×10 １６ atoms / cm ３ <7> The GaN substrate according to any one of <1> to <5>, wherein the H concentration is less than 5×10. <7> The GaN substrate according to any one of <1> to <6>, containing F. <8> The GaN substrate according to <7>, containing, in addition to F, one or more halogens selected from Cl, Br, and I. <9> The GaN substrate according to <8>, containing F and I. <10> The GaN substrate according to <7>, wherein the halogen element contained is substantially only F. <11> The GaN substrate according to <1>, wherein the H concentration is 5×10 １７ atoms / cm ３ 1x10 or more ２０ atoms / cm ３ <12> The GaN substrate according to any one of <1> to <10>, wherein the GaN substrate has an infrared absorption peak attributable to gallium vacancy-hydrogen complexes in the range of 3140 to 3200 cm －１ The GaN substrate according to any one of <1> to <11>, which is made of a GaN crystal having the following characteristics:

[0009] <13> A method for producing a GaN crystal, comprising the following steps: preparing a first undersubstrate having a nitrogen-polar surface; arranging a first pattern mask including a mask portion and a mask opening on the nitrogen-polar surface of the first undersubstrate; growing GaN crystals through the mask openings of the first pattern mask by an ammonothermal method to obtain a layered structure in which GaN crystals are layered on the nitrogen-polar surface of the first undersubstrate via the first pattern mask; separating at least a portion of the GaN crystals grown on the nitrogen-polar surface of the first undersubstrate from the layered structure; and planarizing the nitrogen-polar surface of the separated GaN crystals. a step of using the separated GaN crystal as a second base substrate and arranging a second pattern mask on the planarized nitrogen-polar surface, the second pattern mask being arranged so that at least a portion of a region of the nitrogen-polar surface of the GaN crystal serving as the second base substrate that has grown above the mask opening of the first pattern mask is covered by the mask portion of the second pattern mask; and a step of growing a GaN crystal through the mask opening of the second pattern mask by an ammonothermal method. <14> The method for producing a GaN crystal according to <13>, wherein the second pattern mask is arranged so that a coreless surface of the nitrogen-polar surface of the GaN crystal serving as the second base substrate that has been coreless on the mask portion of the first pattern mask and a region grown thereover are covered by the mask portion of the second pattern mask. <15> The method for producing a GaN crystal according to <13> or <14>, wherein the second pattern mask is arranged such that, on the nitrogen-polar surface of the GaN crystal serving as the second base substrate, a region grown above the mask opening of the first pattern mask (base mask opening region) and a region where the base mask opening region and the mask opening of the second pattern mask overlap (base opening exposed region) satisfy the following relational expression (I):<16> The method for producing a GaN crystal according to any one of <13> to <15>, wherein the second pattern mask is arranged so that there is no area on the nitrogen-polar surface of the GaN crystal serving as the second base substrate where a region grown on the mask opening of the first pattern mask overlaps with a mask opening of the second pattern mask. <17> A method for producing a GaN crystal, comprising using a GaN crystal produced by the GaN crystal production method according to any one of <13> to <16> as a third base substrate, growing a GaN crystal on a gallium-polar surface by HVPE. <18> A method for producing a GaN substrate, comprising slicing a GaN crystal produced by the GaN crystal production method according to any one of <13> to <17> to obtain a GaN substrate.

[0010] <19> The inclination from the (0001) plane is 0 to 20 degrees, and the area is 15 cm ２ a GaN substrate having a primary surface having a dislocation density of 5.0×10 or more; ４ cm －２ or less, and the strain evaluation value calculated based on the following formula (1) by photoelastic measurement of the main surface is 5×10 －５ a GaN substrate, wherein the area (S1) exceeding 1.0% is 2.5% or less of the area of ​​the entire main surface; Each term in formula (1) is expressed by the following formulas (2) and (3): In the formula, δ and ψ are the phase difference and the principal axis azimuth angle, which are the birefringence amounts due to strain, respectively, λ is the light source wavelength of the photoelasticity measuring device, d is the sample thickness, n ０ is the distortion-free refractive index for the light source wavelength, |p １１ -p １２ | are the photoelastic tensor components, among which λ, n ０ and |p １１ -p １２ | satisfies the following condition: λ = 633 nm n ０ = 2.355 | p １１ -p １２ |=0.13

[0011] <20> The strain evaluation value calculated based on the formula (1) by photoelastic measurement of the main surface is 1×10－４ <21> The GaN substrate according to <19>, wherein the area (S2) where the area exceeds 1 / 2 is 1% or less of the entire area of ​​the main surface. <21> The GaN substrate according to <19> or <20>, wherein the FWHM of a (004) XRD rocking curve of the main surface is 20 arcsec or less. <22> The GaN substrate according to any of <19> to <21>, wherein, when a normal direction of the main surface is defined as the z direction, and two directions that are orthogonal to the z direction and are orthogonal to each other are defined as the x direction and the y direction, respectively, a fluctuation range of the x-direction component of the offcut angle on a first line that passes through the center of the main surface and extends in the x direction, and a fluctuation range of the y-direction component of the offcut angle on a second line that passes through the center of the main surface and extends in the y direction are each 0.2 degree or less within a 40 mm long section. <23> The GaN substrate according to any one of <19> to <22>, which satisfies the following condition (a) regarding impurity concentration: Condition (a): O concentration is 3×10 １６ atoms / cm ３ <24> The GaN substrate according to any one of <19> to <23>, which satisfies the following condition (b) regarding impurity concentration: Condition (b): H concentration is 1×10 １７ atoms / cm ３ The following is the result.

[0012] According to one aspect of the present invention, a GaN substrate can be obtained in which dislocations on the primary surface and strain locally present within the primary surface are reduced, thereby reducing yield degradation when used as a c-plane GaN substrate in device fabrication.Furthermore, according to one aspect of the present invention, a GaN crystal with reduced crystal defects over a wide range can be obtained by a required manufacturing method, thereby suppressing yield degradation due to degradation of device performance when such a GaN crystal is used as a c-plane GaN substrate for device fabrication.

[0013] FIG. 1 is a perspective view showing an example of a c-plane GaN substrate according to this embodiment. FIG. 2 is a diagram illustrating that the offcut angle of a c-plane GaN substrate can be decomposed into components in two directions perpendicular to each other. FIG. 3 is a plan view showing a GaN substrate according to an embodiment. FIG. 4 is a plan view showing a GaN substrate according to an embodiment. FIG. 5 is a flowchart showing a GaN crystal growth method. FIG. 6(a) is a perspective view showing a base substrate, and FIG. 6(b) is a perspective view showing the base substrate after a patterned mask has been placed on the nitrogen-polar surface. FIG. 7 is a plan view showing a portion of the nitrogen-polar surface side of the base substrate after the patterned mask has been placed on it. FIG. 8 is a plan view showing a portion of the nitrogen-polar surface side of the base substrate after the patterned mask has been placed on it. FIGS. 9(a) to 9(d) are plan views showing a base substrate with a patterned mask placed on its nitrogen-polar surface, respectively. FIGS. 10(e) to 10(h) are plan views showing a base substrate with a patterned mask placed on its nitrogen-polar surface, respectively. FIGS. 11(i) to 11(l) are plan views showing a base substrate with a patterned mask disposed on its nitrogen-polar surface. FIGS. 12(a) to 12(f) are plan views showing portions of a patterned mask formed on the nitrogen-polar surface of a base substrate. FIGS. 13(a) to 13(f) are plan views showing portions of a patterned mask formed on the nitrogen-polar surface of a base substrate. FIGS. 14(a) to 14(e) are schematic cross-sectional views showing the process of growing a GaN crystal. FIG. 15 shows a crystal growth apparatus that can be used for growing a GaN crystal by the ammonothermal method. FIG. 16(a) is a schematic cross-sectional view of a layered structure obtained by growing a GaN crystal by the ammonothermal method through a mask opening in a patterned mask formed on the nitrogen-polar surface of a base substrate. FIG. 16(b) is a schematic cross-sectional view showing a base substrate with a patterned mask disposed thereon and the separated GaN crystal after the GaN crystal has been separated from the base substrate with the patterned mask disposed thereon. Fig. 17(a) is a schematic cross-sectional view of a layered structure obtained by growing GaN crystals by the ammonothermal method through mask openings in a pattern mask formed on the nitrogen-polar surface of a base substrate, and Fig. 17(b) is a schematic cross-sectional view of a GaN crystal obtained by separating the GaN crystal grown in Fig. 17(a) from the base substrate on which the pattern mask was placed.FIG. 17(c) is a schematic cross-sectional view of a GaN crystal in FIG. 17(b) with a second pattern mask disposed thereon. FIG. 17(d) is a schematic cross-sectional view of a stacked structure obtained by growing a GaN crystal through the opening of the second pattern mask by ammonothermal growth on the nitrogen-polar surface of the GaN crystal with the second pattern mask disposed thereon. FIGS. 18(a) and 18(b) are schematic views of a second pattern mask disposed on the nitrogen-polar surface of a second base substrate. FIG. 19 is a schematic cross-sectional view showing the basic configuration of an HVPE apparatus. FIG. 20(a) is a schematic cross-sectional view showing a seed and an edge cover set on a susceptor, and FIG. 20(b) is a schematic cross-sectional view showing a GaN crystal grown on the seed shown in FIG. 20(a). FIG. 21 is a schematic cross-sectional view showing a seed and an edge cover set on a susceptor. Fig. 22 is a schematic diagram illustrating a 5.5 mm x 5.0 mm observation area (OE) and 5.5 mm x 0.5 mm unit areas (UE) obtained by dividing the observation area into 10. Fig. 23 is a schematic diagram illustrating the apparatus used to perform the photoelasticity measurement.

[0014] Unless otherwise specified, GaN crystals referred to in this specification refer to hexagonal GaN crystals having a wurtzite crystal structure. In GaN crystals, the crystal axis parallel to

[0001] and [000-1] is called the c-axis, the crystal axis parallel to <10-10> is called the m-axis, and the crystal axis parallel to <11-20> is called the a-axis. The crystal plane perpendicular to the c-axis is called the c-plane, the crystal plane perpendicular to the m-axis is called the m-plane, and the crystal plane perpendicular to the a-axis is called the a-plane. In this specification, the (0001) crystal plane and the (000-1) crystal plane are collectively referred to as the c-plane. Hereinafter, when crystal axes, crystal planes, crystal orientations, etc. are referred to, unless otherwise specified, they refer to the crystal axes, crystal planes, crystal orientations, etc. of the GaN crystal. The Miller indices (hkil) of a hexagonal crystal are sometimes written in three digits as (hkl) because there is a relationship of h + k = -i. For example, (0004) can be written in three digits as (004). Unless otherwise specified, dislocations referred to in this specification mean threading dislocations. In this specification, screw dislocations, mixed dislocations, and edge dislocations are not distinguished from one another and are collectively referred to as dislocations. Hereinafter, embodiments of the present invention will be described with reference to the drawings as appropriate.

[0015] 1. GaN Substrate 1.1. First Embodiment The GaN substrate according to the first embodiment has an inclination of 0 to 20 degrees from the (0001) plane and an area of ​​15 cm ２ a main surface (first main surface) having a dislocation density of 5.0×10 or more; ４ cm －２ or less, and the strain evaluation value calculated based on the following formula (1) by photoelastic measurement of the first main surface is 5×10 －５ The area (S1) where the area exceeds 2.5% or less of the area of ​​the entire main surface.

[0016]

[0017] Each term in formula (1) is expressed by the following formulas (2) and (3).

[0018]

[0019] where δ and ψ are the phase difference and the principal axis azimuth angle, which are the birefringence amounts due to strain, respectively, λ is the light source wavelength of the photoelasticity measuring device, d is the sample thickness, and n ０ is the distortion-free refractive index for the light source wavelength, |p １１ -p １２ | are the photoelastic tensor components, among which λ, n ０ and |p １１ -p １２ | satisfies the following condition: λ = 633 nm n ０ = 2.355 | p １１ -p １２ |=0.13

[0020] Photoelasticity measurement can quantitatively measure local strain present in the primary surface of a GaN substrate. The amount of strain is expressed as the magnitude of the strain evaluation value.

[0021] The principles of photoelastic measurement are described in detail below. When strain due to residual stress exists in a crystal, or when an external force is applied, birefringence occurs due to the photoelastic effect. By measuring the birefringence (phase difference δ) and the principal axis azimuth angle ψ in relation to the strain in the crystal substrate using the photoelastic method and using the anisotropic elastic constants of the crystal, it is possible to quantitatively evaluate the strain component. Gallium nitride crystals are uniaxial crystals, and in addition to strain-induced birefringence, they also exhibit natural birefringence. However, natural birefringence does not appear for light incident in the normal direction to the c-plane. Therefore, when measuring gallium nitride c-plane substrates, quantitative evaluation of in-plane strain is possible without the influence of natural birefringence.

[0022] The photoelasticity method mainly uses linear polarimeters and circular polarimeters. These involve placing a sample between a polarizer and an analyzer, passing light through the sample, and observing light and dark fringe patterns, such as isoclinic fringes that appear when the direction of the main axis of birefringence coincides with the direction of the birefringence, and isochromatic fringes that appear when the phase difference of birefringence is 2πn (n is an integer). Furthermore, the phase difference of birefringence is proportional to the sample thickness (the length of the light transmission path).

[0023] However, if the strain that causes the photoelastic effect is minute or the sample is thin (compound semiconductor substrates are generally several hundred microns thick), and the phase difference caused by the strain is less than 2π, the above-mentioned stripe pattern cannot be observed.On the other hand, it is possible to measure the amount of birefringence using the rotating analyzer method with a circular polarimeter.

[0024] This measurement and strain analysis utilize the photoelasticity method published in Non-Patent Documents 1 and 2. This method is similar to a linear polarimeter, but it is structured so that the polarizer and analyzer are rotated synchronously while maintaining an arbitrary relative angle precisely, and the intensity of light transmitted through the sample can be collected as a function of the polarization angle φ. By analyzing this polarization characteristic, it is possible to quantitatively evaluate strain taking into account the elastic anisotropy of the crystal. Another advantage is that this method is not affected by the polarization characteristics of the light used in this measurement or the reflectivity of the sample.

[0025] The measurement method is described below. The transmitted light intensity of the sample when the polarizer and analyzer are oriented perpendicular and parallel is expressed as I ⊥ (φ), I ｜｜ (φ), the transmitted light intensity ratio I ｒ (φ) is expressed by the following equation using the phase difference δ, which is the amount of birefringence due to strain, and the principal axis azimuth angle ψ.

[0026]

[0027] In actual transmitted light measurement, the polarization angle φ is measured at intervals of 2π / J (J is the number of divisions), and each I ｒ (φ ｊ ) where φ ｊ = 2πj / J (j = 0, 1, 2, ..., J-1), and the measurement accuracy improves as the division number J increases. ｒ By performing sin and cos transformation on (φ) as shown below, δ and ψ are obtained.

[0028]

[0029] Next, the relationship between the strain component in the crystal due to the photoelastic effect and the refractive index is expressed by the following equation:

[0030]

[0031] In the above formula, n ｉｊis the refractive index tensor, p ｉｊｋｌ is the photoelastic constant, S ｋｌ is the strain tensor.

[0032] However, in the aforementioned Non-Patent Document 1, it is argued that, due to the anisotropy of the photoelastic effect, δ and the strain in the crystal may not be simply proportional, and that even if the strain is the same value, if the strain tensor components are different, the generated δ and ψ will be different, so it is more appropriate to express them in terms of in-plane strain components rather than δ and ψ. Therefore, in Non-Patent Document 3, by tensor analysis of the photoelastic effect caused by incident light in the normal direction of the c-plane of gallium nitride, the phase difference δ and the principal axis azimuth angle ψ, which are the amount of birefringence derived from strain, are expressed as |S ｙｙ -S ｘｘ | and 2 | S ｘｙ Converted to |.

[0033]

[0034] |S ｙｙ -S ｘｘ | is the difference in expansion and contraction strain along the x-axis and y-axis, which are the crystallographic axes, and 2|S ｘｙ | is the shear strain between the x-axis and y-axis. λ is the wavelength of the light source of the photoelasticity measuring device, d is the sample thickness, and n ０ is the distortion-free refractive index for the light source wavelength. In addition, in the experiment described in Non-Patent Document 3, the photoelastic tensor component |p １１ -p １２ | has been determined to be 0.13.

[0035] For a circularly processed substrate, the difference in expansion and contraction strain between the radial and tangential directions in the cylindrical coordinate system is |S ｒ -S ｔ | is |S ｙｙ -S ｘｘ | and 2 | S ｘｙ | is expressed by the following equation.

[0036]

[0037] The inventors have defined the above formula as formula (1), and |S ｒ -S ｔ The value of | is used as the distortion evaluation value. The specific configuration of the measurement device used to find the distortion evaluation value can be the configuration and method described below.

[0038] When a GaN substrate is used as a device substrate, particularly as a substrate for an LD (especially a substrate for a long-wavelength LD), a low dislocation density in the substrate's main surface is generally required. However, it has been found that the presence of local strain within the main surface prevents the formation of desired end faces when the wafer is separated into bars. Because the end face shape significantly affects device characteristics, local strain leads to a decrease in yield in device manufacturing. In contrast, the GaN substrate of this embodiment has a reduced dislocation density and effectively reduces local strain within the main surface, so when used as a c-plane substrate for devices, it can sufficiently reduce the decrease in yield in device manufacturing.

[0039] The smaller the area of ​​the region with large local strain in the main surface, the more preferable it is from the viewpoint of suppressing deterioration in yield in device manufacturing. －５ The area (S1) exceeding 1 / 2 is preferably 2% or less of the area of ​​the entire main surface, more preferably 1% or less, and particularly preferably 0.5% or less.

[0040] Furthermore, from the viewpoint of suppressing deterioration in yield in device manufacturing, the strain evaluation value of the GaN substrate according to the first embodiment measured by photoelasticity measurement is set to 1×10 －４ The area (S2) exceeding 1% is preferably 1% or less of the area of ​​the entire main surface, more preferably 0.5% or less, even more preferably 0.1% or less, particularly preferably less than 0.1%, and most preferably 0%.

[0041] Furthermore, from the viewpoint of suppressing deterioration in yield in device manufacturing, the maximum value of the strain evaluation value on the principal surface of the GaN substrate according to the first embodiment is set to 1×10 －４ For the same reason, the maximum value of the strain evaluation value on the main surface is preferably 8×10 －５ More preferably, it is 5×10 or less. －５ Less than is even more preferred.

[0042] 1.2. Second Embodiment A GaN substrate according to a second embodiment of the present invention has an inclination of 0 to 20 degrees from the (0001) plane and an area of ​​15 cm ２and a main surface (first main surface) having a thickness of 0.275 cm or more. ２ an observation region having an area of ​​1.0×10 ４ cm －２ There is at least one observation area within the major surface that is less than 0.275 cm as defined above. ２ and the dislocation density in the observation area is 1.0×10 ４ cm －２ The presence of at least one observation region within the primary surface where the defect size is less than 0.275 cm means that the crystal defects have been reduced to an extremely low level over a very wide range on the primary surface of the GaN substrate. By confirming the presence of at least one observation region, it can be confirmed that the crystal defects have been reduced to an extremely low level over a very wide range on the primary surface of a substrate made of GaN crystal. It is preferable that the presence of two or more observation regions can be confirmed at positions that do not overlap each other, and it ... over a very wide range on the primary surface of any 0.275 cm ２ (excluding the area less than 5 mm from the outer periphery of the substrate and the facet growth area) was used as the observation area, and the dislocation density was 1.0 × 10 ４ cm －２ It is more preferable that it is less than 10 ...

[0043] When a GaN substrate is used as a substrate for an LD or a substrate for a power device, dislocations in the primary surface of the substrate may lead to a decrease in the yield of the LD or electronic device, or to a decrease in the breakdown voltage of the electronic device. In contrast, the GaN substrate of this embodiment has an extremely low dislocation density over a wide range, so when used as a C-face substrate for an LD or a C-face substrate for a power device, it is possible to reduce the decrease in yield in device manufacturing and suppress a decrease in device performance.

[0044] From the above viewpoint, the dislocation density is 8.0×10 ３ cm －２ It is more preferable that at least one observation region having a dislocation density of 5.0×10 or less is present in the main surface. ３ cm －２It is more preferable that at least one observation region having the following structure is present within the main surface.

[0045] The observation area has a dislocation density of 1.0 × 10 ２ cm －２ It is preferable that the dislocation density is less than 1.0×10 ２ cm －２ The fact that the dislocation density is less than 1.0×10 means that there are almost no dislocations in that unit area. The more such areas are present, the more the yield deterioration in device manufacturing can be reduced and the deterioration of device performance can be suppressed when the substrate is used as a C-face substrate for LD or a C-face substrate for power devices. Furthermore, when the observation area includes unit area A, it typically includes a unit area with a higher dislocation density than unit area A. For example, when the observation area includes unit area A, the observation area typically includes a unit area with a higher dislocation density than unit area A, for example, a unit area with a dislocation density of 1.0×10 ２ cm －２ That's it, 1.0 x 10 ２ cm －３ or more, or 1.0 x 10 ４ cm －２ More specifically, when the observation region includes unit region A, the dislocation density is 1.0×10 or more. ３ cm －２ The GaN substrate of this embodiment may include a unit region B having a dislocation density of 100 or more. Although a higher dislocation density may occur in a unit region including a coreless region, the GaN substrate of this embodiment may have a configuration in which dislocations are almost absent in other regions. In this specification, the term "unit region" refers to each region obtained by dividing the observation region one-dimensionally or two-dimensionally into 2 to 100 equal parts. Each of the unit region A and the unit region B consists of one of the divided regions described above, and there may be a plurality of unit regions A and a plurality of unit regions B in the observation region. Furthermore, the plurality of unit regions A may have different dislocation densities from each other, and the plurality of unit regions B may have different dislocation densities from each other.

[0046] For example, unit regions A may be periodically present on substantially the entire main surface (for example, all regions excluding regions less than 5 mm away from the outer periphery of the substrate and facet growth regions). Here, the term "periodic" does not mean that the periodicity is strictly equal, and does not mean that unit regions A and other unit regions alternate. The periodicity may be one-dimensional or two-dimensional. An example of a form in which unit regions A are periodically present is a form in which unit regions A are present along the periodic opening pattern of a pattern mask used in a GaN substrate manufacturing method described later. When the GaN substrate is manufactured by a GaN substrate manufacturing method described later, the dislocation density may be 1.0 × 10 ２ cm －２ The unit area A, which is less than 1.0×10, is formed in correspondence with the periodic opening pattern of the pattern mask arranged on the second base substrate. According to the manufacturing method described below, the number of dislocation defects inherited from the second base substrate is small, so that the dislocation density is 1.0×10 ２ cm －２ Furthermore, for example, if the observation area is a rectangular observation area of ​​5.5 mm × 5.0 mm, and the observation area is divided into 10 divided areas of 5.5 mm × 0.5 mm, it is possible to realize a unit area in which the dislocation density is less than 1.0 × 10 ２ cm －２ In other words, the primary surface of the GaN substrate according to the second embodiment of the present invention preferably has a dislocation density of 1.0×10 ２ cm －２ At least one region of 5.5 mm × 0.5 mm can be found as a unit region A in which the dislocation density is less than 1.0 × 10. In addition, a 5.5 mm × 5.0 mm region including this region as one of the 10 divided regions can be found. ４ cm －２ In the rectangular observation area of ​​5.5 mm × 5.0 mm, it is preferable that there is a region where the dislocation density is less than 1.0 × 10 ２ cm －２ The number of unit regions that is less than 100 is more preferably 3 or more, and even more preferably 5 or more.

[0047] 1.3. GaN Substrates of First and Second Embodiments The GaN substrates according to the above two embodiments will now be described in further detail.

[0048] The inclination of the first main surface from the (0001) plane is 0 to 20 degrees, but may be 0.2 degrees or more, or may be 10 degrees or less, less than 5 degrees, less than 2.5 degrees, less than 1.5 degrees, less than 1.0 degree, or less than 0.5 degrees. The area of ​​the first main surface is 15 cm ２ It is enough if it is more than 18cm. ２ It may be 400 cm or more. ２ It may be the following:

[0049] An example of a GaN substrate according to this embodiment is shown in FIG. 1. The substrate 100 shown in FIG. 1 is a free-standing GaN wafer and has two main surfaces (large-area surfaces) facing in opposite directions, i.e., a first main surface 101 and a second main surface 102. The first main surface 101 and the second main surface 102 are preferably parallel to each other. The first main surface 101 is the main surface used for epitaxial growth of a nitride semiconductor, such as when manufacturing a nitride semiconductor device, i.e., the "front surface." The second main surface 102 is the "back surface."

[0050] In this specification, the first main surface 101 is Ga-polar, and the other, second main surface 102 is N-polar. The inclination of the second main surface 102 with respect to the (000-1) crystal plane is preferably 0 degrees or more and 20 degrees or less. The inclination may be 0.2 degrees or more, or may be 10 degrees or less, less than 5 degrees, less than 2.5 degrees, less than 1.5 degrees, less than 1.0 degree, or less than 0.5 degrees.

[0051] The diameter of the substrate 100 is such that the area of ​​the first main surface 101 is 15 cm ２ There are no particular limitations as long as it is equal to or greater than this, but it is usually 45 mm or greater, and may be 95 mm or greater, or 145 mm or greater, and is typically 50 to 55 mm (about 2 inches), 100 to 105 mm (about 4 inches), 150 to 155 mm (about 6 inches), etc.

[0052] The thickness of the substrate 100 is designed so that the substrate 100 can stand on its own and be easily handled. When the diameter of the substrate 100 is about 2 inches, the thickness is preferably 250 μm or more, more preferably 300 μm or more, and preferably 500 μm or less, more preferably 450 μm or less. When the diameter of the substrate 100 is about 4 inches, the thickness is preferably 400 μm or more, more preferably 500 μm or more, and preferably 800 μm or less, more preferably 650 μm or less. When the diameter of the substrate 100 is about 6 inches, the thickness is preferably 500 μm or more, more preferably 600 μm or more, and preferably 850 μm or less, more preferably 750 μm or less.

[0053] The offcut angle of the substrate 100 can be resolved into two mutually orthogonal components in the first main surface 101, that is, an x-direction component and a y-direction component. This will be explained below with reference to FIG. 2. When the normal direction of the first main surface 101 is the z-direction and a vector parallel to the c-axis is a vector Vc, the offcut angle of the wafer 20 is equal to the inclination θ of the vector Vc from the z-direction. This vector Vc is the x-direction component of the vector Vc. ｘ and the y-direction component of the vector Vc ｙ The orthogonal projection of vector Vc on the xz plane is vector Vc ｘ and the orthogonal projection of vector Vc on the yz plane is vector Vc ｙ When the vector Vc is decomposed in this way, the vector Vc ｘ The inclination from the z direction is the x-direction component θ of the offcut angle θ ｘ and the vector Vc ｙ The inclination from the z direction is the y direction component θ of the offcut angle θ ｙ is.

[0054] In the substrate 100, it is preferable that the fluctuation range of the x-direction component of the offcut angle on a first line passing through the center of the first main surface 101 and extending in the x-direction, and the fluctuation range of the y-direction component of the offcut angle on a second line passing through the center of the first main surface 101 and extending in the y-direction, are each 0.2 degrees or less within a 40 mm section. The x-direction may be parallel to one of the a-planes, and in this case, the y-direction is parallel to one of the m-planes. The fluctuation range here refers to the difference between the maximum and minimum values ​​of the offcut angle, and a fluctuation range of 0.2 degrees or less means that the fluctuation from the median value of the offcut angle is within ±0.1 degrees.

[0055] The smaller the fluctuation range of the x-direction component of the offcut angle on the first line and the fluctuation range of the y-direction component of the offcut angle on the second line, the more preferable. These fluctuation ranges may be 0.16 degrees or less, further 0.14 degrees or less, further 0.12 degrees or less, further 0.10 degrees or less, further 0.08 degrees or less, further 0.06 degrees or less, or even 0.04 degrees or less within a 40 mm long section. When evaluating the fluctuation range of the offcut angle, portions that are less than 5 mm away from the outer periphery of the substrate may be excluded.

[0056] The first main surface 101, which is the front surface of the substrate 100, is usually mirror-finished, and its root mean square (RMS) roughness, measured with an atomic force microscope (AFM) in a measurement range of 2 μm×2 μm, is preferably less than 2 nm, and may be less than 1 nm or less than 0.5 nm. The second main surface 102, which is the back surface, may be mirror-finished or matte-finished.

[0057] The edges of the substrate 100 may be chamfered. Various markings may be provided on the substrate 100 as needed, such as an orientation flat or notch that indicates the crystal orientation, or an index flat that makes it easy to distinguish between the front and back surfaces. The main surface of the substrate 100 is circular, but is not limited to a circular shape and may be changed to a square, rectangle, hexagon, octagon, or any other shape.

[0058] The substrate 100 may be a GaN crystal grown by HVPE. GaN grown in a conventional HVPE apparatus equipped with a quartz reactor may satisfy one or more of the following conditions (a) to (c) regarding impurity concentration: Condition (a) Si concentration is 5×10 １６ atoms / cm ３ Condition (b): O concentration is 3×10 １６ atoms / cm ３ Condition (c): H concentration is 1×10 １７ atoms / cm ３ The following is the result.

[0059] The substrate 100 may satisfy two or more of the above conditions (a) to (c). In some cases, the substrate 100 may satisfy both the above conditions (a) and (b), both the above conditions (b) and (c), or both the above conditions (a) and (c). In some cases, the substrate 100 may satisfy all of the above conditions (a) to (c).

[0060] The substrate 100 may be comprised of a GaN crystal grown ammonothermally in a Pt (platinum) capsule using ammonium halides such as NH4F, NH4Cl (ammonium chloride), NH4Br (ammonium bromide), and NH4I as mineralizers. In such GaN crystals, unless intentionally added, the concentrations of alkali metals such as Li (lithium), Na (sodium), and K (potassium), and alkaline earth metals such as Mg (magnesium) and Ca (calcium) are less than 1×10 for each element. １６ atoms / cm 3 It is usually less than that.

[0061] GaN crystals grown ammonothermally using ammonium halides as mineralizers may contain halogens from the mineralizer. For example, GaN crystals grown using NH4F as mineralizer may contain halogens from 5×10 １４ atoms / cm 3 1x10 or more １６ atoms / cm 3 Less than 1 x 10 １６ atoms / cm 3 1x10 or more１７ atoms / cm 3 The inventors have confirmed through experiments that the I (iodine) concentration in GaN crystals grown ammonothermally using NH4F and NH4I as mineralizers is typically 1×10 １６ atoms / cm 3 is less than.

[0062] GaN crystals grown ammonothermally using ammonium halide as a mineralizer have a crystal size of 5 × 10 １７ atoms / cm ３ There is a possibility that H (hydrogen) is contained at a concentration of 10 or more. The H concentration in such GaN crystal is usually 10 ２１ atoms / cm ３ is less than or equal to 5×10 ２０ atoms / cm ３ Below, 1 x 10 ２０ atoms / cm ３ or less, or 5 x 10 １９ atoms / cm ３ GaN crystals grown by the ammonothermal method generally exhibit an infrared absorption peak at 3140-3200 cm, which is attributed to the gallium vacancy-hydrogen complex. －１ Such an infrared absorption peak is not observed in GaN crystals grown by the HVPE method or the Na flux method.

[0063] The substrate 100 has an alkali metal and alkaline earth metal concentration of preferably 1×10 16 atoms / cm 3 less than 1×10 15 atoms / cm 3 is less than.

[0064] The substrate 100 may be made of unintentionally doped GaN (UID-GaN). UID-GaN typically exhibits n-type conductivity due to unintentional doping with donor impurities. UID-GaN has a carrier concentration of 5×10 １７ atoms / cm３ The room temperature resistivity can be as high as about 0.04 Ω cm, and the room temperature resistivity can be as low as about 0.04 Ω cm. This is because the Si concentration of UID-GaN is 5×10 １７ atoms / cm ３ The O concentration can be as high as 2×10 １７ atoms / cm ３ This can be as high as

[0065] In one example, intentional doping can be used to reduce the room temperature resistivity of the substrate 100 to less than 0.03 Ω cm, or even less than 0.02 Ω cm, or even less than 0.015 Ω cm, or even less than 0.010 Ω cm. To achieve a sufficient reduction in resistivity, the carrier concentration of the substrate 100 at room temperature can be reduced to 1×10 １８ atoms / cm ３ More than that, and even 2 x 10 １８ atoms / cm ３ The carrier concentration is preferably 3×10 or more. １８ atoms / cm ３ Above, and further 4 x 10 １８ atoms / cm ３ From the viewpoint of electrical properties, there is no particular upper limit to the carrier concentration, but in order not to deteriorate the productivity of the substrate 100, the carrier concentration may be 1×10 １９ atoms / cm ３ It is recommended to set it to 8 x 10 １８ atoms / cm ３ Below, further 5 x 10 １８ atoms / cm ３ It may be set to the following value, because strong doping increases the frequency of abnormal growth of GaN crystals.

[0066] The dopants used to reduce the resistivity of the substrate 100 are preferably donor impurities, since donor impurities generally exhibit higher activation ratios than acceptor impurities, where activation ratio is the ratio of carrier concentration to dopant concentration in doped GaN.

[0067] Elements that act as donors in GaN crystals include Group 14 elements such as Si (silicon) and Ge (germanium), and Group 16 elements such as O (oxygen) and S (sulfur), with Si and Ge being preferred as intentional dopants. GaN crystals intentionally doped with Ge may contain Si as an unintentional dopant at a concentration on the same order as Ge.

[0068] When evaluating the dislocation density on the first main surface 101 of the substrate 100, a 360 μm×360 μm (129600 μm) area was measured at each measurement point. ２ When the observation area is 360 μm×360 μm or more, the maximum value of the dislocation density in the first main surface 101 does not exceed four times the minimum value, and is usually three times or less, and may even be two times or less.

[0069] In the first embodiment, the dislocation density on the first main surface 101 of the substrate 100 is 5.0×10 ４ cm －２ In any embodiment, 4.0 × 10 ４ cm －２ Preferably, it is equal to or less than 3.0 × 10 ４ cm －２ More preferably, 2.0 × 10 ４ cm －２ More preferably, 1.0 × 10 ４ cm －２ The dislocation density is usually 10 cm －２ In the second embodiment, the dislocation density on the first main surface 101 is 1.0×10 ４ cm －２or less. Dislocations appear as dark spots in photoluminescence (PL) imaging (PLI) images of GaN crystals, so the dislocation density can be measured by observing the PLI image. When the first main surface 101 is Ga-polar, the density of etch pits on the first main surface 101 after etching the substrate 100 for one hour with 89% sulfuric acid heated to 270°C may be considered to be the dislocation density. When evaluating the dislocation density on the first main surface 101, regions less than 5 mm away from the outer periphery of the substrate and facet growth regions may be excluded as exceptional regions. In this specification, a facet growth region refers to a closed region in which crystals have grown with a plane orientation different from that of the main surface 1. A facet growth region has a concave shape and is typically 100 to several hundred microns in diameter, making it observable with the naked eye or a microscope.

[0070] Specifically, the dislocation density can be measured by the following method. The normal direction to the main surface is defined as the z direction, and the direction perpendicular to the z direction and parallel to one of the a-planes is selected as the x direction. A first line passing through the center of the main surface and extending in the x direction, and a second line passing through the center of the main surface and extending in the y direction perpendicular to the x direction are set. Dislocation density is measured by PLI image observation at a total of 17 points: the intersection of the first and second lines (= the center of the main surface), eight points on the first line excluding the intersection, and eight points on the second line excluding the intersection. The interval between adjacent measurement points is 5 mm. A square area of ​​360 μm × 360 μm is observed at each measurement point, and the number of dislocations found in the square area is counted based on its area (129,600 μm ２ The dislocation density was calculated for each measurement point by dividing the average value among the 17 measurement points by the average value of the dislocation density of the main surface of the c-plane GaN substrate.

[0071] The quality of the crystal 100 is CuKα １ The (004) XRD rocking curve FWHM measured by ω scan using radiation can be used as an index for evaluation. The better the crystal quality, the narrower the (004) XRD rocking curve FWHM. In the (004) XRD rocking curve measurement, the X-ray tube is operated at a voltage of 45 kV and a current of 40 mA, and a monochromated CuK αThe X-rays are made incident on the first main surface 11. There are no particular limitations on the direction from which the X-rays are made incident on the first main surface 11, and for example, the plane of incidence of the X-rays may be perpendicular to the a-axis.

[0072] The X-ray beam size is set so that when the angle of incidence (the angle between the reflecting surface and the X-ray) is 90°, that is, when the X-ray is incident perpendicularly to the (004) reflecting surface, the size of the irradiation area on the first main surface 11 is 5 mm in the direction parallel to the ω axis and 1 mm in the direction perpendicular to the ω axis. The ω axis is the rotation axis of the sample in rocking curve measurement. When the X-ray beam size is set in this way, in the (004) XRD rocking curve measurement of GaN, ω is about 36.5°, so the size of the irradiation area on the first main surface 11 is about 1.7 × 5 mm ２ The (004) XRD rocking curve FWHM measured in this manner can be 20 arcsec or less, further 18 arcsec or less, further 16 arcsec or less, further 14 arcsec or less, or further 12 arcsec or less.

[0073] When the diameter of the first main surface 11 exceeds 40 mm, as shown in FIG. 3, by performing ω scanning on the first main surface 101 along one line L for a length of 40 mm at intervals of 1 mm under the above conditions, 40 measurement points P are arranged at 1 mm pitches on the line L. Ｍ In this case, the (004) XRD rocking curve at each measurement point P Ｍ In the ω scan, the ω axis is set perpendicular to the line L. That is, the X-ray is made incident on the crystal 10 so that the X-ray incidence plane and the line L are parallel to each other.

[0074] In a preferred example, when such measurements are made along at least one line on the first main surface 101, the maximum value of the FWHM of the (004) XRD rocking curve among all measurement points may be 20 arcsec or less. The average value of the FWHM of the (004) XRD rocking curve among all measurement points may be 18 arcsec or less, further 16 arcsec or less, further 14 arcsec or less, further 12 arcsec or less, or even 10 arcsec or less.

[0075] When the diameter of the first main surface 101 exceeds 40 mm, two lines L perpendicular to each other as shown in FIG. １ and L ２ By performing ω scanning under the above conditions every 1 mm over a length of 40 mm along each of the lines L, the (004) XRD rocking curves at 40 measurement points arranged at 1 mm pitch were obtained. １ , L ２ In this case, the line L １ In the ω scan at each measurement point above, the ω axis is drawn along the line L １ and perpendicular to the line L ２ In the ω scan at each measurement point above, the ω axis is drawn along the line L ２ and perpendicular to it.

[0076] In a preferred example, when such measurements are made along at least two mutually perpendicular lines on the first main surface 101, the maximum value of the (004) XRD rocking curve FWHM among all measurement points on each line may be 20 arcsec or less. That is, the maximum value among 40 measurement points on one of the two lines and the maximum value among 40 measurement points on the other of the two lines may both be 20 arcsec or less. The average value of the (004) XRD rocking curve FWHM among all measurement points on each line may be 18 arcsec or less, further 16 arcsec or less, further 14 arcsec or less, further 12 arcsec or less, or even 10 arcsec or less.

[0077] From the crystal 10, at least one anomalous X-ray transmission image of a 10 mm x 10 mm square area can be obtained in X-ray topography using the Lang method. In the Lang method, an X-ray source is placed on one main surface of a plate-shaped test piece, and an X-ray detector is placed on the other main surface. The anomalous X-ray transmission is also called the Borrmann effect, and is a phenomenon in which X-rays pass through a crystal with a thickness that would normally prevent them from passing through due to absorption. For example, if a transmission image is obtained from a 344 μm thick GaN crystal in X-ray topography using MoKα (wavelength 0.71073 Å) as the X-ray source, it is an anomalous transmission image. This is because the absorption coefficient μ of GaN is 290.40 cm when the X-ray source is MoKα.-1 When the wafer thickness t is 344 μm, μ t = 10.0, and if there is no anomalous transmission, no transmission image will be obtained under the condition μ t ≧ 10. Since anomalous transmission images are not observed when the crystal is of low perfection, a GaN crystal that can obtain an anomalous transmission image in X-ray topography can be said to be of good quality. It is preferable that at least one anomalous X-ray transmission image of a 15 mm × 15 mm square area be obtained from crystal 10, and it is even more preferable that at least one anomalous X-ray transmission image of a 20 mm × 20 mm square area be obtained.

[0078] 2. GaN Substrate Manufacturing Method 2.1. GaN Crystal Growth Method A GaN crystal growth method that can be preferably used to manufacture a GaN substrate according to the embodiment will now be described. This GaN crystal growth method includes the following three steps S1 to S6, as shown in the flowchart of FIG. 5 : S1: Preparing a first undersubstrate having a nitrogen-polar surface. S2: Placing a first patterned mask including a mask portion and a mask opening on the nitrogen-polar surface of the first undersubstrate prepared in step S1. S3: Growing a GaN crystal through the mask opening of the first patterned mask by ammonothermal method. S4: Separating the GaN crystal from the first undersubstrate on which the first patterned mask is disposed. S5: Using the separated GaN crystal as a second undersubstrate, placing a second patterned mask on the nitrogen-polar surface. S6: Growing a GaN crystal through the mask opening of the second patterned mask by ammonothermal method.

[0079] Each step is described in detail below. (1) Step S1 In step S1, a first base substrate having a nitrogen-polar surface is prepared. A preferred base substrate is a C-plane GaN substrate obtained by processing a bulk GaN crystal grown by the HVPE method or the acidic ammonothermal method. In a C-plane GaN substrate, the main surface on the

[0001] side is a gallium-polar surface, and the main surface on the [000-1] side is a nitrogen-polar surface. The orientation of the nitrogen-polar surface of the base substrate is preferably within 2° of [000-1], more preferably within 1° of [000-1].

[0080] The area of ​​the nitrogen polar surface of the base substrate is preferably 15 cm ２ or more, and 15 cm ２ More than 50cm ２ Less than 50cm ２ More than 100cm ２ Less than 100cm ２ More than 200cm ２ Less than 200cm ２ More than 350cm ２ Less than 350cm ２ More than 500cm ２ Less than 500cm ２ More than 750cm ２ When the nitrogen-polar surface of the starting substrate is circular, its diameter is preferably 45 mm or more. The diameter is typically 45 to 55 mm (about 2 inches), 95 to 105 mm (about 4 inches), 145 to 155 mm (about 6 inches), 195 to 205 mm (about 8 inches), 295 to 305 mm (about 12 inches), etc. For example, when the starting substrate is a C-plane GaN substrate with a diameter of 50 mm, its thickness is preferably 300 μm or more, and the larger the diameter, the larger the preferable lower limit of the thickness becomes. There is no particular upper limit to the thickness of the starting substrate, but it is usually 20 mm or less.

[0081] The size of the base substrate is determined taking into consideration the size of the GaN crystal to be grown in the subsequent steps S3 and S6. For example, if a C-plane GaN substrate measuring 45 mm in the [1-100], [10-10], and [01-10] directions is to be cut out from the GaN crystal to be grown, it is necessary to grow the GaN crystal so that the sizes in the [1-100], [10-10], and [01-10] directions are all 45 mm or greater. To grow a GaN crystal measuring 45 mm in the [1-100], [10-10], and [01-10] directions, it is preferable to use a base substrate whose sizes in the [1-100], [10-10], and [01-10] directions are all 45 mm or greater. The nitrogen-polar surface of the base substrate is planarized by polishing or grinding. Preferably, the damaged layer introduced by the planarization process is removed from the nitrogen-polar surface by CMP (Chemical Mechanical Polishing) and / or etching.

[0082] (2) Step S2 In step S2, a first pattern mask including a mask portion and a mask opening is placed on the nitrogen-polar surface of the first base substrate prepared in step S1. The material forming the surface of the pattern mask is preferably a platinum group metal, i.e., a metal selected from Ru (ruthenium), Rh (rhodium), Pd (palladium), Os (osmium), Ir (iridium), and Pt (platinum), with Pt being particularly preferred. The pattern mask may be a single-layer film made of a platinum group metal or its alloy, but is preferably a multilayer film formed by laminating a platinum group metal layer as a surface layer on an underlayer made of a metal that has better adhesion to GaN crystals than platinum group metals. Examples of materials for the underlayer include, but are not limited to, W (tungsten), Mo (molybdenum), Ti (titanium), and alloys containing at least one selected from these.

[0083] A pattern mask includes a mask portion where the nitrogen-polar surface of the base substrate is covered by the mask and a mask opening portion where the nitrogen-polar surface of the base substrate is exposed without the mask. The mask opening portion typically consists of linear openings. The mask is called a pattern mask because it has a periodic opening pattern. An example will be described with reference to FIGS. 6 and 7. FIG. 6( a) is a perspective view of the base substrate. The base substrate 20 is a disc-shaped C-plane GaN substrate having a gallium-polar surface 21, a nitrogen-polar surface 22, and a side surface 23. FIG. 6( b) is a perspective view of the base substrate 20 after a pattern mask 30 has been placed on the nitrogen-polar surface 22. The pattern mask 30 has multiple linear mask openings 31 arranged parallel to each other. The periodic opening pattern formed by the linear mask openings 31 is a stripe pattern. FIG. 7 is a plan view of a portion of the nitrogen-polar surface 22 side of the base substrate 20 after the pattern mask 30 has been placed. 7, a pattern mask 30 has a plurality of linear mask openings 31 arranged parallel to one another at a constant pitch P, and the nitrogen-polar surface 22 of the base substrate 20 is exposed inside each linear mask opening 31. The pitch refers to the distance between the center lines of adjacent parallel linear mask openings with a non-opening (i.e., masked portion) of the pattern mask sandwiched therebetween.

[0084] In order to reduce dislocation defects that the GaN crystal grown in the subsequent step S3 inherits from the base substrate 20, it is advantageous for the line width W of the linear mask opening 31 to be narrow. Therefore, the line width W is preferably 0.5 mm or less, more preferably 0.2 mm or less, and even more preferably 0.1 mm or less. From the viewpoint of manufacturing efficiency, it is preferable for the line width W of the linear mask opening 31 to be appropriately wide. This is because this increases the growth rate in the initial stage when the GaN crystal is grown in the subsequent step S3. Therefore, the line width W is preferably 5 μm or more, more preferably 10 μm or more, and even more preferably 15 μm or more.

[0085] In order to reduce dislocation defects inherited from the base substrate 20 by the GaN crystal grown in the subsequent step S3, it is advantageous to have a larger pitch P between the linear mask openings 31. Therefore, the pitch P is preferably 1 mm or more, more preferably 2 mm or more, more preferably 3 mm or more, and even more preferably 4 mm or more. On the other hand, the larger the pitch P between the linear mask openings 31, the longer it takes for the through-holes formed above the mask portion of the pattern mask to close when the GaN crystal is grown in the subsequent step S3. Therefore, from the viewpoint of manufacturing efficiency, the pitch P is preferably 10 mm or less, and can also be 4 mm or less, 3 mm or less, or even 2 mm or less. When the direction of the intersection between the nitrogen-polar surface 22 and the A-plane [(11-20) plane, (2-1-10) plane, or (1-210) plane] of the base substrate 20 is defined as a reference direction, the angle θ between the longitudinal direction of the linear mask openings 31 and this reference direction is preferably 0°±5°. The angle θ may be 0°±3°, 0°±2°, or 0°±1°. Orienting the linear mask openings in this way can reduce the probability of dislocation defects in the underlying substrate penetrating through.

[0086] The pattern mask may be provided with a periodic opening pattern composed of linear mask openings, including intersections. An example will be described with reference to FIG. 8 . FIG. 8 is a plan view showing a portion of the nitrogen-polar surface 22 side of the base substrate 20 after the pattern mask 30 has been placed. The pattern mask 30 has linear mask openings 31, and the nitrogen-polar surface 22 of the base substrate is exposed inside the linear mask openings 31. The linear mask openings 31 provided in the pattern mask 30 are of two types: first linear mask openings 311 and second linear mask openings 312, which have different longitudinal directions. A square lattice pattern is formed by a plurality of the first linear mask openings 311 and a plurality of the second linear mask openings 312. The pitch P1 between the first linear mask openings 311 and the pitch P2 between the second linear mask openings 312 are both constant. The pitch refers to the center-to-center distance between adjacent parallel linear mask openings across the mask portion of the pattern mask. Although the pitch P1 and the pitch P2 may be the same, the inventors have found through experiments that if the pitch P1 and the pitch P2 are different, the through-holes formed above the mask portion of the pattern mask tend to be more easily blocked when growing GaN crystals in the subsequent step S3. Therefore, it is preferable that one of the pitches P1 and P2 is 1.5 times or more, and more preferably 2 times or more, the other.

[0087] The square lattice pattern provided in the pattern mask 30 includes intersections K formed between the first linear mask openings 311 and the second linear mask openings 312. As will be described later, providing intersections in the opening pattern is advantageous in promoting the blocking of through-holes that occur above the mask portions of the pattern mask when GaN crystals are grown in the subsequent step S3. From this perspective, the number density of intersections included in the pattern mask is preferably 1 cm －２ On the other hand, in order to increase the number density of intersections, it is necessary to increase the density of linear mask openings, and considering that as the density of linear mask openings is increased, the number of dislocation defects that the GaN crystal grown in the subsequent step S3 inherits from the base substrate increases, the number density of intersections is preferably 20 cm －２Less than 15 cm, more preferably －２ Less than 10 cm, more preferably －２ The following is the result.

[0088] In order to reduce dislocation defects that the GaN crystal grown in the subsequent step S3 inherits from the base substrate 20, it is advantageous for the line width W1 of the first linear mask opening 311 and the line width W2 of the second linear mask opening 312 to be narrow. Therefore, the line widths W1 and W2 are preferably 0.5 mm or less, more preferably 0.2 mm or less, and even more preferably 0.1 mm or less. The line widths W1 and W2 may be the same or different. From the viewpoint of manufacturing efficiency, it is preferable that the line width W1 of the first linear mask opening 311 and the line width W2 of the second linear mask opening 312 are appropriately wide. This is because this increases the growth rate in the initial stage when the GaN crystal grows in the subsequent step S3. Therefore, the line width W １ and W ２ is preferably 5 μm or more, more preferably 10 μm or more, and more preferably 15 μm or more.

[0089] To reduce dislocation defects inherited from the base substrate 20 by the GaN crystal grown in the subsequent step S3, it is advantageous to have a larger pitch P1 between the first linear mask openings 311 and a larger pitch P2 between the second linear mask openings 312. Therefore, the pitches P1 and P2 are preferably 1 mm or greater, more preferably 2 mm or greater, more preferably 3 mm or greater, and even more preferably 4 mm or greater. On the other hand, smaller pitches P1 and P2 can reduce the time required for the through-holes formed above the mask portion of the pattern mask to close when the GaN crystal is grown in the subsequent step S3. Therefore, from the standpoint of manufacturing efficiency, it is preferable to set at least one of the pitches P1 and P2 to 10 mm or less. In one example, one or both of the pitches P1 and P2 can be set to 4 mm or less, 3 mm or less, or even 2 mm or less. In a preferred example, taking into consideration both the reduction of inherited dislocation defects and improved manufacturing efficiency, only one of the pitches P1 and P2 can be set to 4 mm or less, 3 mm or less, or 2 mm or less.

[0090] The orientations of the first linear mask opening 311 and the second linear mask opening 312 can be conveniently expressed by expressing one of the directions of the intersection line between the nitrogen polar surface 22 and the A-plane as a first reference direction and the other as a second reference direction. For example, when the first reference direction is the direction of the intersection line between the nitrogen polar surface 22 and the (11-20) plane, the second reference direction is the direction of the intersection line between the nitrogen polar surface 22 and the (2-1-10) plane or the (1-210) plane. In one preferred example, the angle θ between the longitudinal direction of the first linear mask opening 311 and the first reference direction is １ and the angle θ that the longitudinal direction of the second linear mask opening 312 makes with the second reference direction. ２ When the total length of the first linear mask opening 311 is equal to or greater than the total length of the second linear mask opening 312, at least the angle θ １ In other words, it is preferable that the longitudinal direction of the linear mask opening 31 in a portion that accounts for 50% or more of the total length forms an angle of 0°±5° with respect to the direction of the intersection line between the nitrogen polarity surface and the A-plane of the base substrate. In a more preferable example, the angle θ １ and angle θ ２ In other words, the longitudinal direction of the entire linear mask opening 31 forms an angle of 0±5° with respect to the direction of the intersection line between the nitrogen polarity surface and the A-plane of the underlying substrate. １ and θ ２ Alternatively, the angle may be 0±3°, 0±2°, or 0±1°. If the linear mask openings are oriented as described above, the through holes formed above the mask portions of the pattern mask are more likely to be closed when GaN crystals are grown in the subsequent step S3.

[0091] The periodic opening pattern that can be provided in the pattern mask placed on the nitrogen-polar surface of the base substrate in step S2 is not limited to the stripe pattern or square lattice pattern described above. Each of the drawings included in Figures 9 to 11 is a plan view showing the base substrate 20 after the pattern mask 30 has been placed on the nitrogen-polar surface 22, and illustrates various periodic opening patterns that can be provided in the pattern mask, but the opening patterns that can be employed are not limited to these. In Figure 9(a), the linear mask openings 31 form a zigzag stripe pattern. In Figure 9(b), the linear mask openings 31 form a type of lattice pattern. In Figure 9(c), the linear mask openings 31 form a tilted brick lattice pattern. In Figure 9(d), the linear mask openings 31 form a tilted square lattice pattern.

[0092] In Figure 10(e), the linear mask openings 31 form a herringbone lattice pattern. In Figure 10(f), the linear mask openings 31 form a lattice pattern that is a combination of a tilted brick lattice and a tilted square lattice. In Figure 10(g), the linear mask openings 31 form a triangular lattice pattern. In Figure 10(h), the linear mask openings 31 form a flattened honeycomb lattice pattern. In Figure 11(i), the linear mask openings 31 form a Bishamon-tortoiseshell lattice pattern. In each of Figures 11(j) and (k), the linear mask openings 31 form a cubic pattern. In Figure 11(l), the linear mask openings 31 form a Y-shaped pattern.

[0093] In all of the examples shown in Figures 9 to 11, the periodic opening pattern provided in the pattern mask 30 includes intersections. Several types of intersections are shown in Figures 12(a) to (f) and Figures 13(a) to (f). An intersection in which two or more linear mask openings with different longitudinal directions are connected, including those shown in Figures 12(a) to (f), is referred to as a continuous intersection in this specification. Unless otherwise specified, the intersection referred to in this specification includes not only continuous intersections but also discontinuous intersections such as those exemplified in Figures 13(a) to (f). A discontinuous intersection can be considered as an intersection obtained by modifying a continuous intersection so that the connection between the linear mask openings is severed. In a discontinuous intersection, the distance between two linear mask openings separated by a mask portion is 300 μm or less, preferably 200 μm or less.

[0094] In all examples of Figures 9 to 11 except for Figure 9(b), the intersections included in the periodic opening pattern are arranged two-dimensionally. If the periodic opening pattern includes intersections, when GaN crystals are grown in the subsequent step S3, through-holes that are formed above the mask portion of the pattern mask are more likely to be blocked. This effect is more pronounced when the intersections in the periodic opening pattern are arranged two-dimensionally, and is furthermore more pronounced by increasing the number density of the intersections. For this reason, it is preferable that the intersections in the periodic opening pattern are arranged two-dimensionally, and in that case, the number density of the intersections included in the pattern mask is preferably 1 cm －２ However, in order to increase the number density of intersections, it is necessary to increase the density of linear mask openings, and considering that as the density of linear mask openings is increased, the number of dislocation defects that the GaN crystal grown in the subsequent step S3 inherits from the base substrate increases, the number density of intersections is preferably 20 cm －２ Less than 15 cm, more preferably －２ Less than 10 cm, more preferably －２ The following is the result.

[0095] When providing the various periodic opening patterns shown in Figures 9 to 11 on a pattern mask, the preferred designs for the orientation, line width, and pitch of the linear mask openings are as follows. It is preferable that the longitudinal direction of at least a portion of the linear mask openings forms an angle of 0±5° with respect to the direction of the intersection line between the nitrogen-polar surface and the A-plane of the underlying substrate. More preferably, the longitudinal direction forms an angle of 0±5° with respect to the direction of the intersection line between the nitrogen-polar surface and the A-plane of the underlying substrate in a portion accounting for 50% or more of the total length of the linear mask openings, and even in all of the linear mask openings. The line width of the linear mask openings is preferably 0.5 mm or less, more preferably 0.2 mm or less, and more preferably 0.1 mm or less, and is preferably 5 μm or more, more preferably 10 μm or more, and more preferably 15 μm or more. The line width does not need to be the same throughout the entire linear mask opening.

[0096] (3) Step S3 In step S3, GaN crystals are grown by ammonothermal growth on the nitrogen-polar surface of the first base substrate prepared in step S1 through the mask openings of the first pattern mask formed in step S2, resulting in a layered structure in which GaN crystals are stacked on the nitrogen-polar surface of the first base substrate via the first pattern mask. The GaN crystal growth process in step S3 will be described with reference to FIG. 14. FIG. 14(a) is a cross-sectional view showing the state before crystal growth begins. A pattern mask 30 having linear mask openings 31 is provided on the nitrogen-polar surface 22 of the base substrate 20. FIG. 14(b) shows the state in which GaN crystals 40 have begun to grow on the nitrogen-polar surface 22 exposed inside the linear mask openings 31 provided in the pattern mask 30. 14(c), the GaN crystal 40 grows not only in the [000-1] direction but also in the lateral direction (the direction parallel to the nitrogen-polar surface 22), but a gap G is formed between the GaN crystal 40 and the pattern mask 30. As a result, the orientation disorder of the GaN crystal 40, which may occur due to contact with the pattern mask 30, is suppressed.

[0097] At the growth stage shown in FIG. 14( c), the GaN crystal 40 has a through-hole T above the mask portion of the pattern mask 30. As the GaN crystal 40 continues to grow, the gap G is gradually filled, but not completely, and the through-hole T closes, leaving a void V, as shown in FIG. 14( d). After the through-hole T closes, the GaN crystal 40 is further grown in the [000-1] direction, as shown in FIG. 14( e). It is believed that the stress generated between the base substrate 20 and the GaN crystal 40 is alleviated by the void V, thereby reducing strain in the GaN crystal 40. The amount of growth of the GaN crystal 40 in the [000-1] direction after the through-hole T is closed is preferably 1 mm or more, more preferably 2 mm or more, and more preferably 3 mm or more, with no particular upper limit. It should be noted that in step S3, GaN crystals also grow on the gallium polar surface 21 of the base substrate 20, but this is not shown in FIG.

[0098] When the through-hole T closes at the stage of FIG. 14(d), dislocations occur at the coreless surface, or the dislocations are all bent in the [000-1] direction at the coreless surface. For either or both reasons, a dislocation array appears on the main surface of the C-plane GaN substrate cut from the GaN crystal formed at the stage of FIG. 14(e). The shape of the dislocation array is roughly the shape of the line of intersection formed between the extension plane of the coreless surface in the [000-1] direction and the main surface of the C-plane GaN substrate. This line of intersection may include straight portions, curved portions, bends, and branches. Note that the coreless surface here refers to the surface where GaN crystals that started growing from two adjacent mask openings join together as a result of further growth in the lateral direction on the mask portion of the pattern mask. In this specification, this joining may be referred to as "coreless." The region grown above the coreless surface may also be referred to as the "coreless region." Because the coreless surface is formed above the mask portion of the pattern mask, when the pattern mask has multiple closed mask portions, multiple dislocation arrays appear discretely on the main surface of the C-plane GaN substrate. When the arrangement of the multiple closed mask portions in the pattern mask is periodic, the arrangement of the multiple dislocation arrays on the main surface of the C-plane GaN substrate is also periodic. When the arrangement of the multiple closed mask portions in the pattern mask is two-dimensional, the arrangement of the multiple dislocation arrays on the main surface of the C-plane GaN substrate is also two-dimensional. A closed mask portion is a mask portion whose periphery is surrounded by linear mask openings, and can also be referred to as a mask portion whose outline forms a ring. Of the various examples shown in Figures 9 to 11, the pattern masks with closed mask portions are shown in Figures 9(c) and (d), Figures 10(e) to (h), and Figures 11(i) to (j). In these examples, the arrangement of the closed mask portions in the pattern mask is periodic and two-dimensional.

[0099] A crystal growth apparatus of the type shown in FIG. 15 can be preferably used for growing GaN crystals by the ammonothermal method in step S3. Referring to FIG. 15, the crystal growth apparatus 200 includes an autoclave 201 and a Pt capsule 202 placed therein. The capsule 202 has a raw material dissolution zone 202a and a crystal growth zone 202b separated from each other by a Pt baffle 203. A feedstock FS is placed in the raw material dissolution zone 202a. A seed S suspended by a Pt wire 204 is placed in the crystal growth zone 202b. A gas line connected to a vacuum pump 205, an ammonia cylinder 206, and a nitrogen cylinder 207 is connected to the autoclave 201 and the capsule 202 via a valve 208. NH ３ When adding ammonia, NH 3 supplied from an ammonia cylinder 206 is used. ３ The amount of the gas can be confirmed by the mass flow meter 209.

[0100] The feedstock contains gaseous GaCl obtained by contacting elemental Ga (metallic gallium) with HCl (hydrogen chloride) gas under heating, and NH ３ Polycrystalline GaN produced by reacting NH 3 with NH 4 is preferably used. ４ Cl (ammonium chloride), NH ４ Br (ammonium bromide) and NH ４ It is preferable to use a combination of one or more ammonium halides selected from ammonium iodide and NH4F (ammonium fluoride). It is particularly preferable to use NH4F and NH4I in combination.

[0101] When using a growth temperature of 650°C or less, it is not recommended to use only ammonium halides other than NH4F as the mineralizer. This is because the growth direction of GaN crystals is essentially limited to the [000-1] direction, and lateral growth does not occur. On the other hand, when NH4F is used alone as the mineralizer, lateral growth is strongly promoted. If the lateral growth of GaN crystals is promoted too much, it becomes difficult to grow GaN crystals in the form shown in FIG. 14, i.e., to grow GaN crystals so that gaps are formed between the GaN crystals and the pattern mask.

[0102] When growing GaN crystals on the seed S, NH ３ After placing the feedstock FS in the autoclave 201, the capsule 202 is heated from the outside by a heater (not shown) to bring the inside of the capsule 202 into a supercritical or subcritical state. Until the feedstock FS is fully dissolved and the solvent reaches a saturated state, etching also occurs on the surface of the seed S. If necessary, a temperature reversal period can be provided before the start of growth, in which the temperature gradient between the source material dissolution zone 202a and the crystal growth zone 202b is reversed from that during crystal growth, in order to promote etch-back of the seed S. The growth temperature is preferably 550°C or higher. While growth temperatures of 1000°C or higher are not prohibited, it is possible to grow sufficiently high-quality GaN crystals even at temperatures below 700°C. The growth pressure can be set, for example, within the range of 100 to 250 MPa, but is not limited thereto.

[0103] As an example, NH ４ F and NH ４ I, NH ３GaN can be grown under the following conditions: the molar ratios of the feedstock to the GaN crystal are 0.5% and 4.0%, respectively; the pressure is approximately 220 MPa; the average temperature Ts of the feedstock dissolution zone and the temperature Tg of the crystal growth zone are approximately 600°C; and the temperature difference Ts - Tg between these two zones is approximately 5°C (Ts > Tg). The growth rate of GaN crystal can be increased by increasing the temperature difference between the feedstock dissolution zone and the crystal growth zone; however, if the growth rate is too high, it may be difficult for the GaN crystal growth to progress from the stage shown in Figure 14(c) to the stage shown in Figure 14(d), i.e., the through-holes in the GaN crystal may be difficult to close. In step S3, the capsule is replaced each time the feedstock is used up, and GaN crystal regrowth can be repeated.

[0104] (4) Step S4 In step S4, at least a portion of the GaN crystal grown in step S3 is separated from the layered structure obtained in step S3. The separation process in step S4 will be described with reference to FIG. 16. FIG. 16(a) is a schematic cross-sectional view of a layered structure obtained in step S3 by growing GaN crystals by ammonothermal growth on the nitrogen-polar surface of the base substrate prepared in step S1 through the mask openings of the first pattern mask formed in step S2. FIG. 16(b) is a schematic cross-sectional view of the base substrate on which the pattern mask is disposed and the separated GaN crystal after the GaN crystal grown in step S3 is separated from the base substrate on which the first pattern mask is disposed in step S4. That is, in step S3, a layered structure in which GaN crystals are layered on the nitrogen-polar surface of the first base substrate via the first pattern mask is obtained, and in step S4, a portion consisting only of the GaN crystals layered as described above is separated from the layered structure. The GaN crystal to be separated may be at least a portion of the GaN crystal grown by the ammonothermal method in step S3. While the portion to be separated is not particularly limited, it is preferably a portion of the stacked structure farthest from the first undersubstrate. This is because it minimizes the number of dislocation defects inherited from the undersubstrate 20. Furthermore, the GaN crystal is preferably separated in sheet form. The number of GaN crystals to be separated is not particularly limited. For example, one or more sheets may be separated, but one is preferred. By separating the GaN crystal grown by the ammonothermal method in step S3, the GaN crystal grown in step S3 can be used as a undersubstrate to suitably perform the ammonothermal method, particularly the ammonothermal method in a crystal growth apparatus such as that shown in FIG. 15 . While known methods for slicing GaN crystals can be used for the separation, it is also preferable to slice only the GaN crystal from the stacked structure using, for example, a wire saw. A single-wire saw or a multi-wire saw can be used as the wire saw.

[0105] The shape and area of ​​the nitrogen-polar surface of the GaN crystal to be separated preferably roughly coincide with the shape and area of ​​the nitrogen-polar surface of the base substrate prepared in step S1. The area of ​​the nitrogen-polar surface of the GaN crystal to be separated is preferably 15 cm. 2 or more, and 15 cm 2 More than 50cm 2 Less than 50cm 2 More than 100cm 2 Less than 100cm 2 More than 200cm 2 Less than 200cm 2 More than 350cm 2 Less than 350cm 2 More than 500cm 2 Less than 500cm 2 More than 750cm ２ When the nitrogen-polar surface of the GaN crystal to be separated is circular, its diameter is preferably 45 mm or more. The diameter is typically 45 to 55 mm (about 2 inches), 95 to 105 mm (about 4 inches), 145 to 155 mm (about 6 inches), 195 to 205 mm (about 8 inches), 295 to 305 mm (about 12 inches), etc. When the nitrogen-polar surface of the GaN crystal to be separated is circular and has a diameter of about 50 mm, for example, its thickness is preferably 300 μm or more, and the larger the diameter, the larger the preferable lower limit of the thickness becomes. There is no particular upper limit to the thickness of the GaN crystal to be separated, but it is usually 20 mm or less.

[0106] In step S5, which will be described later, a second pattern mask is placed on the nitrogen-polar surface of the separated GaN crystal. Before placing the second pattern mask on the nitrogen-polar surface of the separated GaN crystal, a step of planarizing the nitrogen-polar surface is performed. Planarizing the nitrogen-polar surface facilitates accurate formation of the second pattern mask on the surface. Preferably, the nitrogen-polar surface is planarized by polishing or grinding. It is more preferable to remove the damaged layer introduced by the planarization process from the nitrogen-polar surface by CMP (Chemical Mechanical Polishing) and / or etching. The planarization step is performed after the separation step. By performing the planarization step after the separation step, the nitrogen-polar surface can be accurately planarized without being affected by issues such as the strength of the underlying substrate on which the pattern mask is placed and the layered structure including the GaN crystal grown thereon. By separating at least a portion of the GaN crystal grown from the layered structure and polishing the nitrogen-polar surface of the separated GaN crystal, the second pattern mask can be accurately formed.

[0107] (5) Step S5 In step S5, the GaN crystal separated from the first base substrate on which the first pattern mask was placed in step S4 is used as a second base substrate, and a second pattern mask is placed on the nitrogen-polar surface of the second base substrate. The second pattern mask may be the same as that described in section (2) Step S2. The second pattern mask may be the same as or different from the first pattern mask.

[0108] The second pattern mask is arranged so that at least a portion of the region of the nitrogen-polar surface of the GaN crystal grown in step S2 that has grown on the mask opening of the first pattern mask (underlying mask opening region) is covered by the mask portion of the second pattern mask. It is preferable that the second pattern mask is arranged so that 50% or more of the underlying mask opening region is covered by the mask portion of the second pattern mask. It is more preferable that the second pattern mask is arranged so that 80% or more of the underlying mask opening region is covered by the mask portion of the second pattern mask. It is even more preferable that the second pattern mask is arranged so that the entire underlying mask opening region is covered by the mask portion of the second pattern mask.

[0109] By arranging the base mask opening regions of the nitrogen-polar surface of the GaN crystal so that they are covered by the mask portions of the second pattern mask, the GaN crystal obtained by growing the GaN crystal through the mask openings of the second pattern mask by the ammonothermal method in the subsequent step S6 can have fewer dislocations and reduced regions where large local strain exists. Furthermore, the GaN crystal obtained in this manner can have a reduced dislocation density over a wide range of the main surface.

[0110] The inventors consider the reason for this as follows. The following description will be given with reference to the appropriate figures. FIG. 17( a) is a schematic cross-sectional view of a layered structure obtained in step S3 by growing GaN crystals on the nitrogen-polar surface of an underlying substrate through the mask openings in the first pattern mask by the ammonothermal method. FIG. 17( b) is a schematic cross-sectional view of a GaN crystal obtained by separating the GaN crystal grown in FIG. 17( a) in step S4. The GaN crystal β300 grown on the nitrogen-polar surface of the underlying substrate β100 through the mask openings in the first pattern mask β200 in step S3 includes a region β301 grown above the mask openings in the first pattern mask (underlying mask opening region) and a region β302 grown above the mask portion of the first pattern mask. As explained in the section on step S3, the GaN crystal β300 growing on the nitrogen-polar surface exposed inside the mask opening in the pattern mask grows not only in the GaN [000-1] direction but also in the lateral direction (direction parallel to the nitrogen-polar surface) after passing through the pattern mask, and GaN crystals also grow above the mask portion. Therefore, the GaN crystals in the underlying mask opening region β301 are mainly grown in the [000-1] direction from the nitrogen-polar surface of the underlying substrate β100 exposed from the mask opening, and the GaN crystals in the underlying mask portion region β302 grown above the mask portion are mainly grown simultaneously from the mask opening in the GaN [000-1] direction and the lateral direction (direction parallel to the nitrogen-polar surface). That is, the underlying mask opening region β301 and the underlying mask portion region β302 are grown under different environments, but the inventors' studies have found that the residual strain of the GaN crystal is greater in the underlying mask opening region β301 than in the underlying mask portion region β302. As a result, the GaN crystal β300 obtained in step S3 can have effectively reduced dislocation density due to the mechanism explained in the section on step S3, but on the other hand, the residual strain remains relatively large in the underlying mask opening region β301.

[0111] 17(c) is a schematic cross-sectional view of a GaN crystal of FIG. 17(b) with a second pattern mask disposed thereon. FIG. 17(d) is a schematic cross-sectional view of a stacked structure obtained by growing a GaN crystal by ammonothermal growth through the mask openings of the second pattern mask on the nitrogen-polar surface of the GaN crystal with the second pattern mask disposed thereon. The GaN crystal β300 obtained in step S3 is used as a base substrate, and the mask portion β400 of the second pattern mask is provided on the nitrogen-polar surface. Furthermore, by covering part or all of the base mask opening region β301 of the GaN crystal with the mask portion β400 of the second pattern mask so that the base mask opening region β301 is not exposed by the mask opening β401 of the second pattern mask, the GaN crystal β500 grown through the mask opening β401 of the second pattern mask can be realized with reduced residual strain derived from the base mask opening region β301 and fewer dislocations.

[0112] In terms of reducing residual strain resulting from the underlying mask opening region β301 in the GaN crystal β500 grown through the mask opening β401 of the second pattern mask, it is preferable that the area of ​​the underlying mask opening region β301 exposed through the mask opening β401 of the second pattern mask on the nitrogen-polar surface of the GaN crystal after the second pattern mask is disposed be as small as possible. This will be explained with reference to FIG. 18 . FIG. 18( a) is a schematic cross-sectional view of the second pattern mask disposed on the nitrogen-polar surface of the second underlying substrate. In FIG. 18( a), the entire underlying mask opening region β301 on the nitrogen-polar surface of the GaN crystal β300, which is the second underlying substrate, is covered by the mask portion β400 of the second pattern mask. That is, in FIG. 18( a), there is no region where the underlying mask opening region and the mask opening of the second pattern mask overlap (hereinafter also referred to as the "exposed underlying opening region"). 18(b), a part of the base mask opening region β301 on the nitrogen-polar surface of the GaN crystal β300, which is the second base substrate, is exposed through the mask opening β401 of the second pattern mask. In other words, an exposed base opening region exists in FIG.

[0113] For this reason, the second pattern mask is preferably arranged so that the regions of the nitrogen-polar surface of the GaN crystal obtained in step S4 that have grown on the mask openings of the first pattern mask, i.e., the underlying mask opening regions, and the regions where the underlying mask opening regions and the mask openings of the second pattern mask overlap (exposed underlying opening regions) satisfy the following relational expression (I): Area of ​​exposed underlying opening region / Area of ​​underlying mask opening region≦10% (I).

[0114] From the same viewpoint, it is more preferable that the second pattern mask is disposed so as to satisfy the following relationship (II): Area of ​​exposed region of base opening / Area of ​​base mask opening region≦1% (II).

[0115] From the same viewpoint, it is more preferable that the second pattern mask is arranged so that there is no exposed base opening region. Note that the absence of an exposed base opening region is synonymous with satisfying the following relationship (III): Area of ​​exposed base opening region / Area of ​​base mask opening region=0 (III).

[0116] As described above, when the second pattern mask is placed on the nitrogen-polar surface of the GaN crystal in step S5, it is preferable to place it so that part or all of the underlying mask opening region of the GaN crystal is covered by the mask portion of the second pattern mask. In this case, it is preferable to confirm the position of the underlying mask opening region of the GaN crystal before placing the second pattern mask, and to determine the position of the second pattern mask using information on the confirmed position of the underlying mask opening region. Because the GaN crystal grown in step S3 is separated from the underlying substrate on which the first pattern mask is placed in step S4, the position of the underlying mask opening region of the GaN crystal is often not necessarily clear. For this reason, it is more preferable to estimate the position of the base mask opening region from the location where the residual strain on the nitrogen-polar surface of the GaN crystal is large using the above-mentioned photoelastic measurement, or to estimate the position of the base mask opening region from the location where the residual strain on the nitrogen-polar surface of the GaN crystal is relatively large compared to the surrounding area by observing crossed Nicol images using a polarized distortion inspection device, or to confirm the positional relationship of the base mask opening region of the GaN crystal by using the shape of the substrate, etc. as a landmark.

[0117] It is also preferable that the second pattern mask be arranged so that the coreless surface of the nitrogen-polar surface of the GaN crystal grown in step S2, which has been coreless above the mask portion of the first pattern mask, and the region (coreless region) grown using that coreless surface, are covered by the mask portion of the second pattern mask. By arranging the second pattern mask so that the coreless surface of the nitrogen-polar surface of the GaN crystal, which has been coreless above the mask portion of the first pattern mask, and the region (coreless region) grown using that coreless surface, are covered by the mask portion of the second pattern mask, the GaN crystal obtained by ammonothermal growth through the mask openings of the second pattern mask in the subsequent step S6, can have an extremely reduced dislocation density over a wide range of its main surface.

[0118] The inventors consider the reason for this as follows. Referring again to FIG. 17( a), the GaN crystal β300 grown in step S3 has an underlying mask opening region β301 and an underlying mask portion region β302, as described above. Here, the underlying mask portion region β302 is grown primarily from the mask opening of the first pattern mask simultaneously in the GaN [000-1] direction and the lateral direction (the direction parallel to the nitrogen-polar surface), and a coreless surface, where the GaN crystals grown in the lateral direction form a coreless surface, is formed above the mask portion. As explained in detail in step S3, dislocation arrays appear in the coreless surface and in the GaN crystal grown above it (coreless region β303). For this reason, dislocation arrays corresponding to the coreless surface may be present in the coreless region β303 of the GaN crystal β300 grown in step S3.

[0119] In contrast, referring to Figures 17(c) and (d), the GaN crystal β300 obtained in step S3 is used as a base substrate, and a mask portion β400 of a second pattern mask is provided on its nitrogen-polar surface.The second pattern mask is then positioned so that the coreless region β303 of the GaN crystal β300 is covered with the mask portion β400 of the second pattern mask so that the coreless region β303 is not exposed through the mask opening β401 of the second pattern mask.This allows the GaN crystal β500 grown through the mask opening β401 of the second pattern mask to have extremely few dislocations over a wide area, while reducing the dislocation array originating from the coreless region β303.

[0120] For the reasons mentioned above, it is preferable that the second pattern mask is disposed so that at least a portion of the underlying mask opening region and the underlying coreless region of the nitrogen-polar surface of the GaN crystal grown in step S2 are covered by the mask portion of the second pattern mask, thereby reducing regions with large residual strain and obtaining a GaN crystal with reduced dislocation density over a wide range of the main surface.

[0121] (6) Step S6 In step S6, GaN crystals are grown by ammonothermal method through the mask openings of the second pattern mask on the nitrogen-polar surface of the second base substrate on which the second pattern mask was placed in step S5. The specific method for growing GaN crystals in step S6 can be the same as that described in step S3 above.

[0122] The GaN crystal grown in step S6 may be used as a third base substrate for further growth of a new GaN crystal. For example, the GaN crystal grown in step S6 may be used as a third base substrate to grow a GaN crystal on the primary surface of the GaN crystal by the HVPE method, thereby obtaining a new GaN crystal. The GaN substrate of the first embodiment or the GaN crystal of the second embodiment described above can also be obtained from the new GaN crystal obtained in this manner.

[0123] 3. GaN Crystal Growth Method by HVPE Method A preferred method for growing GaN crystal by HVPE method will be described below.

[0124] 3.1 HVPE Apparatus The basic configuration of an HVPE apparatus that can be used to produce GaN crystals is shown in Figure 19. Referring to Figure 19, the HVPE apparatus 300 includes a hot-wall reactor 301, a gallium reservoir 302 and a susceptor 303 that are placed inside the reactor, and a first heater 304 and a second heater 305 that are placed outside the reactor. The first heater 304 and the second heater 305 each surround the reactor 301 in an annular shape.

[0125] The reactor 301 is a quartz tube chamber. Inside the reactor 301, there is a first zone Z, which is heated mainly by a first heater 304. １ and a second zone Z heated mainly by the second heater 305. ２ There is a problem with the exhaust pipe P Ｅ is the second zone Z ２ The first zone Z is connected to the reactor end on the side １ The gallium reservoir 302 located in the second zone Z is a quartz vessel having a gas inlet and a gas outlet. ２The susceptor 303 is made of, for example, graphite. A mechanism for rotating the susceptor 103 can be provided as desired.

[0126] A seed is placed on the susceptor 303. Preferably, as shown in FIG. 20( a), an edge cover that covers the edge of the seed is placed on the susceptor together with the seed. The edge cover is made of, for example, graphite, and its height h is set so that the top surface of the GaN thick film grown on the seed is not positioned above the top end of the edge cover at the end of growth, as shown in FIG. 20( b). The edge cover may also cover the outer periphery of the main surface of the seed in addition to the edge of the seed, as shown in FIG. 21.

[0127] Returning to FIG. 19 again, when growing GaN crystals, the inside of the reactor 301 is heated by the first heater 304 and the second heater 305, and NH ３ (ammonia) through the ammonia inlet pipe P １ Through the second zone Z ２ HCl (hydrogen chloride) diluted with carrier gas is supplied to the hydrogen chloride supply pipe P ２ This HCl reacts with metallic gallium in the gallium reservoir 302, and the resulting GaCl (gallium chloride) is introduced through the gallium chloride introduction pipe P ３ Through the second zone Z ２ The second zone Z ２ At NH ３ and GaCl react with each other, and the resulting GaN crystallizes on the seeds placed on the susceptor 303 .

[0128] When doping the GaN crystal growing on the seed with impurities, the doping gas diluted with the carrier gas is introduced through the dopant introduction pipe P ４ through the second zone Z in the reactor 301 ２ Ammonia introduction pipe P １ , hydrogen chloride introduction pipe P ２ , gallium chloride introduction tube P ３ and dopant introduction tube P ４The portion disposed in the reactor 301 can be made of quartz.

[0129] In FIG. 19, the distance from the nozzle to the susceptor 303 is １ and gallium chloride introduction tube P ３ The ammonia introduction pipe P １ Insert the nozzle into the gallium chloride introduction tube P ３ 19, the ammonia supply pipe P may be opened at a position farther from the susceptor 303 (upstream side) than the nozzle of １ Nozzle and gallium chloride introduction tube P ３ The nozzles may be integrated into one body to form a double-tube nozzle with the former as an outer tube and the latter as an inner tube.

[0130] In FIG. 19, the gallium chloride introduction pipe P ３ and dopant introduction tube P ４ Although the nozzles in the first and second zones are depicted as separate, this is not limiting. For example, to uniformly dope the growing GaN crystal, GaCl and doping gases may be mixed and then injected into the second zone Z through a common nozzle. ２ The dopant introduction tube P ４ Insert the nozzle into the gallium chloride introduction tube P ３ It may also have an opening inward.

[0131] When doping the growing GaN crystal with Si, the doping gas is SiH ４ (silane), SiH ３ Cl (monochlorosilane), SiH ２ Cl ｓ (dichlorosilane), SiHCl ３ (trichlorosilane) or SiCl ４ (tetrachlorosilane) can be preferably used. When growing GaN crystals, GeH is used as the doping gas. ４ (German), GeH ３ Cl (monochlorogermane), GeH ２ Cl ２ (dichlorogermane), GeHCl ３ (trichlorogermane) or GeCl ４(tetrachlorogermane) can be preferably used. When the growing GaN crystal is doped with Sn, SnH ４ (Stannan), SnH ３ Cl (monochlorostannane), SnH ２ Cl ２ (Dichlorostannane), SnHCl ３ (trichlorostannane), or SnCl ４ (Tetrachlorostannane) can be preferably used. The grown GaN crystal may contain O and Si even though it is not intentionally doped. The unintentional Si probably comes from the quartz that constitutes the reactor or piping, and the unintentional O probably comes from either or both of the quartz and moisture that has infiltrated into the reactor from the outside.

[0132] The components arranged in the reactor 301, including those omitted in FIG. 19, include those made of quartz and carbon, as well as SiC (silicon carbide), SiN ｘ (silicon nitride), BN (boron nitride), alumina, W (tungsten), Mo (molybdenum), etc. can be used. By doing so, the concentration of each impurity except for Si, O, and H in the GaN crystal grown on the seed is 5×10 unless intentional doping is performed. １５ atoms / cm ３ It can be as follows:

[0133] 3.2. Undersubstrate The undersubstrate used for growing the GaN crystal is the GaN crystal obtained by steps S1 to S6, as described above.

[0134] 3.3 Growth Conditions Preferred conditions for growing GaN crystals on the above-mentioned base substrate by HVPE are as follows: The temperature of the gallium reservoir is, for example, 500 to 1000°C, preferably 700°C or higher, and preferably 900°C or lower. The susceptor temperature is, for example, 900 to 1100°C, preferably 930°C or higher, more preferably 950°C or higher, and preferably 1080°C or lower, more preferably 1050°C or lower.

[0135] NH in the reactor ３The V / III ratio, which is the ratio of the partial pressure of GaCl to the partial pressure of Ni, can be, for example, 1 to 20, but is preferably 2 or more, more preferably 3 or more, and is preferably 10 or less. If the V / III ratio is too large or too small, it will cause deterioration in the surface morphology of the growing GaN. Deterioration in surface morphology will cause deterioration in crystal quality and an increase in the concentration of oxygen unintentionally incorporated into the GaN crystal.

[0136] The growth rate of GaN crystals is controlled by the NH ３ The rate can be controlled using the product of the partial pressure of GaCl and the GaCl partial pressure as a parameter. The rate is, for example, 20 to 300 μm / h, preferably 30 μm / h or more, more preferably 40 μm / h or more, and preferably 200 μm / h or less, more preferably 150 μm / h or less, and even more preferably 100 μm / h or less. The inventors have found that when growing GaN crystal by HVPE on a high-crystal-quality base substrate made of GaN crystal grown by steps S1 to S6, if the growth rate is set too low, a problem occurs in which localized areas where growth has stopped are formed on the growth surface.

[0137] When a low growth rate of 20 μm / h to 60 μm / h is adopted, the generation of new threading dislocations at the interface between the base substrate made of the GaN crystal obtained in steps S1 to S6 and the GaN crystal grown on the base substrate by HVPE can be preferably prevented. The growth rate may be increased during growth to improve production efficiency. Experiments by the present inventors suggest that when the initial growth rate is 40 μm / h and the growth rate is increased to 80 μm / h or 120 μm / h, the threading dislocation density of the GaN crystal grown at 80 μm / h is the same as that of the base substrate made of the GaN crystal obtained in steps S1 to S6, and even in the GaN crystal grown at 120 μm / h, the threading dislocation density is at most twice that of the base substrate.

[0138] NH ３ The carrier gas for diluting each of the HCl and doping gas is H ２ (hydrogen gas), N ２ (nitrogen gas) or H ２ and N ２A mixed gas of H in a carrier gas can be preferably used. ２ The molar ratio of H in the carrier gas affects the impurity concentration of the growing GaN crystal. ２ The molar ratio is calculated based on the flow rate of each gas species supplied as a carrier gas from outside the reactor into the reactor.

[0139] 4. Applications of GaN Substrates The c-plane GaN substrate according to the embodiment can be used in the manufacture of various nitride semiconductor devices, including light-emitting devices such as light-emitting diodes (LEDs) and laser diodes (LDs), and electronic devices such as rectifiers, bipolar transistors, field-effect transistors, and high electron mobility transistors (HEMTs). Nitride semiconductors are also called nitride-based III-V compound semiconductors, group III nitride-based compound semiconductors, GaN-based semiconductors, etc., and include GaN as well as compounds in which part or all of the gallium in GaN is substituted with other group 13 elements of the periodic table (e.g., B, Al, In). A nitride semiconductor device is a semiconductor device in which the main part of the device structure is formed from a nitride semiconductor.

[0140] The results of experiments conducted by the present inventors are described below. Example 1-1: A C-plane GaN substrate cut from a GaN crystal grown by HVPE was prepared as a first base substrate. This C-plane GaN substrate had a nitrogen-polar surface and a gallium-polar surface, each of which had been finished by CMP. The orientation of the nitrogen-polar surface was within 1° of [000-1]. A first pattern mask consisting of a laminated film having a 150 nm-thick Pt layer on a 50 nm-thick TiW layer was formed on the nitrogen-polar surface of the C-plane GaN substrate by lift-off. The pattern mask had a stripe pattern consisting of linear mask openings. The line width of the linear openings was 20 μm, and the pitch between the linear openings was 2 mm. The stripe direction was parallel to the intersection of the A-plane and the nitrogen-polar surface of the first base substrate.

[0141] After forming the first pattern mask, GaN crystals were grown on the nitrogen-polar surface of the base substrate through the openings in the first pattern mask by an acidic ammonothermal method using a crystal growth apparatus of the type shown in Figure 15. The feedstock consisted of gaseous GaCl obtained by contacting elemental Ga with HCl gas under heating, and NH ３ Polycrystalline GaN was synthesized by reacting it with NH as a mineralizer. ４ F and NH ４ I was used in combination. ４ F and NH ４ The amount of I is NH ３ The molar ratios to NH were 5.0% and 3.5%, respectively. ４ I is NH ３ The synthesis was carried out by introducing HI (hydrogen iodide) into the Pt capsule after placing the above in the capsule.

[0142] The growth conditions were as follows: the average value of the temperature Tg in the crystal growth zone and the temperature Ts in the raw material dissolving zone was 598°C, the temperature difference between the crystal growth zone and the raw material dissolving zone was 15°C (Ts > Tg), and the pressure was 220 MPa. After 40 days had passed since the start of growth, the capsule was opened and a layered structure consisting of the base substrate on which the first pattern mask had been placed and the GaN crystal grown thereon was taken out.

[0143] The GaN crystal was separated from the laminated structure by slicing the GaN crystal on the base substrate on which the first pattern mask was disposed using a wire saw.

[0144] The GaN crystal separated in the above process was processed to produce a 500 μm-thick first C-plane GaN substrate. Specifically, the GaN crystal was sliced ​​parallel to the C-plane using a multi-wire saw. Both main surfaces of the resulting blank substrate were ground and flattened, and then further polished by CMP to remove damaged layers. This first C-plane GaN substrate was then used as a second base substrate to grow GaN crystals using an acidic ammonothermal method. A second pattern mask consisting of a laminated film with a 150 nm-thick Pt layer on a 50 nm-thick TiW layer was formed on the nitrogen-polar surface of the second base substrate using a lift-off method. The pattern mask had a stripe pattern consisting of linear mask openings. The line width of the linear openings was 20 μm, and the pitch between the linear openings was 2 mm. The stripe direction was parallel to the intersection of the A-plane and the nitrogen-polar surface of the second base substrate. Before placing the second pattern mask, the position of the region grown on the mask opening of the first pattern mask (base mask opening region) on the nitrogen-polar surface of the second base substrate was confirmed by observing crossed Nicols images using a polarization distortion inspection device, LED Rainbow Viewer (manufactured by Mitate Imaging Co., Ltd.). Using the confirmed position information of the base mask opening region, the position of the second pattern mask was adjusted and determined so that at least a portion of the base mask opening region was covered by the mask portion of the second pattern mask, and the second pattern mask was then placed. The exposure rate of the region where the base mask opening region and the mask opening of the second pattern mask overlap (base opening exposed region), i.e., (area of ​​base opening exposed region) / (area of ​​base mask opening region), was 0%.

[0145] After forming the second pattern mask, GaN crystals were grown on the nitrogen-polar surface of the second base substrate through the openings in the second pattern mask by an acidic ammonothermal method using a crystal growth apparatus of the type shown in Figure 15. The feedstock consisted of gaseous GaCl obtained by contacting elemental Ga with HCl gas under heating, and NH ３ Polycrystalline GaN synthesized by reacting with gas was used.

[0146] Mineralizers include NH ４ F and NH ４ I was used.４ F and NH ４ The amount of I is NH ３ The molar ratio of each to NH was 1.0%. ４ I is NH ３ The synthesis was performed by introducing HI into a Pt capsule after placing the GaN crystals in the capsule. The growth conditions were as follows: the average temperature (Tg) of the crystal growth zone and the temperature (Ts) of the raw material dissolution zone was 605-610°C, the temperature difference between the crystal growth zone and the raw material dissolution zone was 15-20°C (Ts > Tg), and the pressure was 220 MPa. After 28 days of growth, GaN crystals grew 1.8 mm in the [000-1] direction on the nitrogen-polar surface of the second base substrate. The grown GaN crystals were then processed to produce a second C-plane GaN substrate (hereinafter also referred to as "Sample E-1") with a diameter of 50 mm. Both main surfaces of the GaN substrate were ground to flatten them, and then further polished by CMP to remove damaged layers. The final substrate thickness was 350 μm.

[0147] The dislocation density on the Ga-polar main surface of sample E-1 was investigated by measuring the etch pit density (EPD). The substrate was etched for 1 hour with 89% sulfuric acid heated to 270°C, and the etch pits were observed using an optical microscope. FIG. 22 is a schematic diagram illustrating a 5.5 mm x 5.0 mm observation area (OE) and 5.5 mm x 0.5 mm unit areas (UE) obtained by dividing the observation area into 10. A 5.5 mm x 5.0 mm rectangular area was defined as the observation area, and 5.5 mm x 0.5 mm rectangular areas obtained by dividing this observation area into 10 were used as unit areas. The observation area was located inside the substrate at a distance of 5 mm from the outer periphery, with the long side (5.5 mm side) of the rectangle parallel to the stripe direction of the first pattern mask during manufacture. The dislocation density in each unit area was calculated by measuring the etch pit density in each unit area. The average dislocation density in each of the 10 unit areas constituting the observation area was measured using a 5.5 mm × 5.0 mm (area: 0.275 cm ２ The dislocation density in the observation area of ​​5.5 mm × 5.0 mm was calculated as 2.4 × 10 ３ cm －２The 10 unit regions constituting the observation area had the following dislocation densities from one end to the other (unit regions 1 to 10), and the dislocation density in the unit region was 1.0 × 10 ２ cm －２ The number of unit areas that were less than 0 cm was 5. Unit area 1: 0 cm －２ Unit area 2: 0 cm －２ Unit area 3: 1.3 x 10 ４ cm －２ Unit area 4: 6.2 x 10 ２ cm －２ Unit area 5: 0 cm －２ Unit area 6: 0 cm －２ Unit area 7: 1.0 x 10 ４ cm －２ Unit area 8: 2.9 x 10 ２ cm －２ Unit area 9: 0 cm －２ Unit area 10: 1.1 x 10 ２ cm －２

[0148] For sample E-1, the dislocation density was also measured in a different observation area from the above observation area. The dislocation density in the 5.5 mm × 5.0 mm observation area was 4.8 × 10 ３ cm －２ That is, in sample E-1, the presence of two or more of the above observation regions was confirmed at positions that did not overlap each other. In addition, the 10 unit regions that make up the observation region, from one end to the other end (unit regions 1 to 10), each had the dislocation density shown below, and the dislocation density in the unit region was 1.0 × 10 ２ cm －２ The number of unit areas that were less than 0 cm was 4. Unit area 1: 0 cm －２ Unit area 2: 3.6 x 10 １ cm －２ Unit area 3: 1.9 x 10 ４ cm －２ Unit area 4: 1.8 x 10 ２ cm －２ Unit area 5: 0 cm －２ Unit area 6: 0 cm －２ Unit area 7: 2.9 x 10４ cm －２ Unit area 8: 3.6 x 10 ２ cm －２ Unit area 9: 0 cm －２ Unit area 10: 7.3 x 10 １ cm －２

[0149] Comparative Example 1-1 A C-plane GaN substrate was obtained that was equivalent to the first C-plane GaN substrate used in the manufacturing process of Example 1. In other words, the C-plane GaN substrate was obtained from GaN crystals obtained by a manufacturing method that included the steps of preparing a first base substrate having a nitrogen-polar surface, arranging a first patterned mask including a mask portion and a mask opening on the nitrogen-polar surface of the first base substrate, growing GaN crystals through the mask openings of the first patterned mask by an ammonothermal method, and separating the GaN crystals from the first base substrate on which the first patterned mask was arranged, but did not include the steps of arranging a second patterned mask on the nitrogen-polar surface of the separated GaN crystals as a second base substrate, and growing GaN crystals through the mask openings of the second patterned mask by an ammonothermal method. The observation area was 5.5 mm × 5.0 mm, and was located inside the substrate at a distance of 5 mm from the outer periphery, where the long side (5.5 mm side) of the rectangle was parallel to the stripe direction of the first pattern mask during manufacture. The dislocation density in the observation area was determined in the same manner as in Example 1-1, and was found to be 1.4 × 10 ４ cm －２ The 10 unit regions constituting the observation area had the following dislocation densities from one end to the other (unit regions 1 to 10), and the dislocation density in the unit region was 1.0 × 10 ２ cm －２ The number of unit areas less than 1.9 × 10 was 0. Unit area 1: 1.9 × 10 ４ cm －２ Unit area 2: 1.3 x 10 ４ cm －２ Unit area 3: 4.4 x 10 ４ cm －２ Unit area 4: 5.5 x 10 ３ cm －２ Unit area 5: 7.6 x 10 ３ cm－２ Unit area 6: 2.9 x 10 ３ cm －２ Unit area 7: 3.2 x 10 ４ cm －２ Unit area 8: 6.4 x 10 ３ cm －２ Unit area 9: 9.4 x 10 ３ cm －２ Unit area 10: 2.4 x 10 ３ cm －２

[0150] Example 2-1 A C-plane GaN substrate fabricated in the same manner as the second C-plane GaN substrate in Example 1-1 above was used as a seed (third base substrate), and a GaN thick film was grown on the Ga polar plane by HVPE using a vapor phase growth apparatus having the same basic configuration as the HVPE apparatus shown in Figure 19. The procedure is roughly as follows. First, the seed was set on a susceptor. No edge cover was used. Next, N ２ , H ２ and N.H. ３ were supplied into the reactor so that the partial pressures thereof became 0.80 atm, 0.15 atm, and 0.05 atm, respectively, while the reactor was heated by a heater installed outside the reactor.

[0151] After the susceptor temperature reached 1020° C., the susceptor temperature was kept constant to grow GaN. The temperature of the Ga reservoir was set to 800° C. The carrier gas supplied into the reactor was H 2 O 3 , which accounted for 51% in molar ratio. ２ and the remainder is N ２ For 60 minutes immediately after the start of growth, GaCl and NH ３ The partial pressure of each is 7.2 × 10 －３ atm and 6.9 x 10 －２ The gas was supplied into the reactor so as to attain a pressure of 1000 MPa atm, and no doping gas was intentionally supplied.

[0152] 60 minutes after the start of growth, GeCl ４ (tetrachlorogermane) supply was started. ４ The feed rate of GeCl was gradually increased over 10 minutes.４ After the supply rate of GaCl, NH ３ and GeCl ４ The partial pressure of each is 1.1 × 10 －２ atm, 8.0 x 10 －２ atm and 2.1 × 10 －５ The Ge-doped GaN thick film was grown to a thickness of about 5.4 mm by supplying the gas into the reactor at a pressure of about 1000 MPa. The growth rate of the Ge-doped GaN thick film calculated from the thickness and growth time was about 83 μm / hr.

[0153] Next, this GaN thick film was sliced ​​parallel to the c-plane, and the Ga polarity surface of the resulting wafer was flattened by grinding and then subjected to CMP finishing. Slicing damage on the N polarity surface of the wafer was removed by etching. The thickness was 400 μm, and the main surface area was approximately 20 cm. ２ Thus, a third C-plane GaN substrate (hereinafter also referred to as "sample E-2") was completed.

[0154] The photoelastic strain in the principal surface of the Ga-polar principal surface of the third C-plane GaN substrate (sample E-2) was measured by photoelastic measurement. The conditions for the photoelastic measurement were as follows. The photoelastic device used was a device developed by the authors of the aforementioned Non-Patent Documents 1 to 3. A schematic diagram of the device is shown in Figure 22.

[0155] In the photoelasticity measurement, a 633 nm LED was used as the incident light source. Light passed through a rotating polarizer was incident along the c-plane normal of the c-plane gallium nitride substrate, which was the substrate under test. The transmitted light intensity at various locations within the surface was detected by a CCD camera through a rotating analyzer. The substrate under test must be polished on both sides to allow light to pass through. Note that if the c-plane GaN substrate sample is unpolished on both sides or polished on one side, the incident light may not pass through the GaN substrate, making proper measurement impossible. In this case, photoelasticity measurement is possible by polishing the unpolished main surface of the GaN substrate to allow the incident light to pass through.

[0156] In the photoelasticity measurement, the transmitted light intensity is measured while rotating the rotating polarizer and rotating analyzer synchronously under orthogonal and parallel conditions, but the amount of light is adjusted so that the transmitted light intensity does not exceed the detection intensity of the CCD camera and become saturated, and the number of divisions of the polarization angle of the rotating polarizer and rotating analyzer is set to 25 or more. The in-plane resolution of the CCD camera, or the so-called pixel size, is 167 μm both vertically and horizontally.

[0157] The phase difference δ, which is the amount of birefringence at various points in the plane, and the principal axis azimuth angle ψ are determined by polarization analysis using the formula below mentioned above.

[0158]

[0159] The thickness of the substrate to be measured and the distortion-free refractive index of gallium nitride for the light source wavelength of 633 nm are set to 2.355, and the photoelastic tensor component |p １１ -p １２ |=0.13, and the distortion evaluation value |S ｒ -S ｔ The in-plane distribution of | was determined and evaluated.

[0160]

[0161] The measurements were carried out at room temperature (around 25°C). The refractive index of gallium nitride is known to be temperature dependent, and if the temperature during measurement is significantly different from room temperature, proper measurements may not be possible. For this reason, it is preferable to carry out photoelastic measurements at room temperature (around 25°C).

[0162] The dislocation density on the Ga-polarity side main surface of sample E-2 was also investigated by photoluminescence (PLI) image observation. The normal direction to the main surface was defined as the z-direction, and the direction perpendicular to the z-direction and parallel to one of the a-planes was defined as the x-direction. Dislocation density measurements were performed on a first line passing through the center of the main surface and extending in the x-direction, and on a second line passing through the center of the main surface and extending in the y-direction perpendicular to the x-direction. Measurements were performed at a total of 17 points: the intersection of the first and second lines (= the center of the main surface), eight points on the first line excluding the intersection, and eight points on the second line excluding the intersection. The interval between adjacent measurement points was 5 mm. A square area of ​​360 μm × 360 μm was observed at each measurement point, and the number of dislocations found within the square area was calculated based on its area (129,600 μm２ The dislocation density was calculated for each measurement point by dividing the dislocation density by the average value for the 17 measurement points. The dislocation density of the main surface of the c-plane GaN substrate was determined as the average value for the 17 measurement points.

[0163] In addition, the fluctuation range of the x-direction component of the offcut angle within a 40 mm section on the first line and the fluctuation range of the y-direction component of the offcut angle within a 40 mm section on the second line were examined using an X-ray diffraction device [PANalytical X'Pert Pro MRD, manufactured by Spectris Co., Ltd.].

[0164] The (004) XRD rocking curve of sample E-2 was measured at a position on the Ga polar plane away from the edge. The measurement was performed using a line-focus CuKα radiation source in an X-ray diffractometer [PANalytical X'Pert Pro MRD manufactured by Spectris Inc.] operated at 45 kV and 40 mA, and a Ge (440) tetracrystalline symmetric monochromator was used to measure CuKα. １ The optical system used was a parallel optical system, with a 1 / 2 slit, an X-ray mirror, and a cross slit of 1 mm wide x 1 mm high on the incident side. The detector was a PIXcel semiconductor pixel detector. 3D The 0D mode of the NIRS (registered trademark) was used. The angular resolution was 5 to 6 arcsec.

[0165] The X-ray beam size was set so that when the incident angle was 90°, i.e., when the X-rays were incident perpendicularly to the Ga-polar surface of the sample substrate, the size of the irradiation area on the Ga-polar surface was 5 mm in the direction parallel to the ω-axis and 1 mm in the direction perpendicular to the ω-axis. In the rocking curve measurement, the X-rays were incident on the sample from a direction perpendicular to the a-axis of the GaN crystal. In other words, the incident plane of the X-rays was parallel to the a-plane of the GaN crystal.

[0166] Example 2-2: A seed used for growing a GaN thick film by HVPE was one that had already been reused twice, after growing a GaN thick film under the same conditions as in Example 2-1, separating the grown layer, and planarizing the main surface. In other words, a seed that had been used three times was used. A C-plane GaN substrate was fabricated in the same manner as in Example 2-1, except for the above points.

[0167] Example 2-3 A C-plane GaN substrate was fabricated in the same manner as in Example 2-1, except that a C-plane GaN substrate obtained under the same conditions as in Example 2-1 was used as a seed for growing a GaN thick film by HVPE.

[0168] Comparative Example 2-1: A C-plane GaN substrate obtained by a method similar to that for the first C-plane GaN substrate used in the manufacturing process of Example 1-1 was used as a seed for growing a GaN thick film by HVPE. In other words, a GaN crystal obtained by a manufacturing method including the steps of preparing a first base substrate having a nitrogen-polar surface, placing a first patterned mask including a mask portion and a mask opening on the nitrogen-polar surface of the first base substrate, growing a GaN crystal through the mask opening of the first patterned mask by an ammonothermal method, and separating the GaN crystal from the first base substrate on which the first patterned mask is placed, but not including the steps of placing a second patterned mask on the nitrogen-polar surface of the separated GaN crystal as a second base substrate, and growing a GaN crystal through the mask opening of the second patterned mask by an ammonothermal method, was used as a seed for growing a GaN thick film by HVPE. The HVPE growth conditions were the same as in Example 2-1, and a C-plane GaN substrate was fabricated.

[0169] The evaluation results of the C-plane GaN substrates obtained in Examples 2-1 to 2-3 and Comparative Example 2-1 are shown in Table 1 below.

[0170]

[0171] As shown in Table 1, the GaN substrates of Examples 2-1 to 2-3 have low dislocation densities, and when converted into c-plane GaN substrates, the local strain in the main surface is significantly reduced. In other words, these GaN crystals can be said to achieve both reduced local strain in the main surface and excellent crystal quality. This makes it possible to improve the manufacturing yield when the GaN substrates of Examples 2-1 to 2-3 are used as device substrates. It is believed that the GaN substrates of Examples 2-1 to 2-3 can improve the manufacturing yield of long-wavelength LDs in particular. On the other hand, the GaN substrate of Comparative Example 2-1 has a low dislocation density but many regions with large local strain in the main surface. The "variation range of the offcut angle" in Table 1 refers to the largest variation range between the variation range of the x-direction component and the variation range of the y-direction component.

[0172] While the present invention has been described above with reference to specific embodiments, these embodiments are presented as examples and do not limit the scope of the present invention. Each embodiment described in this specification can be modified in various ways without departing from the spirit of the invention, and can be combined with features described in other embodiments to the extent possible.

[0173] This application is based on a Japanese patent application (Patent Application No. 2024-048031) filed on March 25, 2024, the contents of which are incorporated herein by reference.

[0174] 100 Substrate 101 First main surface 102 Second main surface L Line segment LS1 First line segment LS2 Second line segment 20 Base substrate 21 Gallium polar surface 22 Nitrogen polar surface 23 Side surface 30 Pattern mask 31 Linear mask opening 40 GaN crystal G Gap T Through hole V Void K Intersection 200 Crystal growth apparatus 201 Autoclave 202 Capsule 202a Raw material dissolution zone 202b Crystal growth zone 203 Baffle 204 Pt wire 205 Vacuum pump 206 Ammonia cylinder 207 Nitrogen cylinder 208 Valve 209 Mass flow meter S Seed FS Feedstock β100 Base substrate β200 Mask portion of first pattern mask β300 GaN crystal β301 Base mask opening region β302 Base mask portion region β303 Coreless region β400 Mask portion of second pattern mask β401 Mask opening of second pattern mask β403 Base opening exposed region β500 GaN crystal 300 HVPE apparatus 301 Reactor 302 Gallium reservoir 303 Susceptor 304 First heater 305 Second heater

Claims

1. The tilt from the (0001) plane is 0 to 20 degrees and the area is 15 cm ２ A GaN substrate having a primary surface of 0.275 cm or more, ２ an observation region having an area of ​​1.0×10 ４ cm －２ a GaN substrate having at least one observation region within the primary surface, the observation region being less than 100 nm; 2. The observation area includes unit area A and unit area B, and the dislocation density of unit area A is 1.0 × 10 ２ cm －２ The dislocation density of the unit area B is less than 1.0 × 10 ３ cm －２ The GaN substrate according to claim 1 .

3. The observation area is 5.5 mm x 5.0 mm, and the observation area is divided into 10 unit areas of 5.5 mm x 0.5 mm, and the dislocation density is 1.0 x 10 ２ cm －２ The GaN substrate of claim 1 , wherein the number of unit regions that is less than 1 is one or more.

4. A GaN substrate according to any one of claims 1 to 3, wherein the (004) XRD rocking curve FWHM on said primary surface is 20 arcsec or less.

5. A GaN substrate according to any one of claims 1 to 3, wherein, when the normal direction to said primary surface is defined as the z direction, and two directions that are orthogonal to said z direction and are orthogonal to each other are defined as the x direction and the y direction, respectively, the variation range of the x direction component of the offcut angle on a first line that passes through the center of said primary surface and extends in the x direction, and the variation range of the y direction component of the offcut angle on a second line that passes through the center of said primary surface and extends in the y direction, are each 0.2 degrees or less within an interval that is 40 mm long.

6. The concentrations of Li, Na, K, Mg, and Ca are all 1 x 10 １６ atoms / cm ３ The GaN substrate according to any one of claims 1 to 3, wherein the thickness is less than 100 nm.

7. The GaN substrate according to any one of claims 1 to 3, which contains F.

8. The GaN substrate according to claim 7, which contains, in addition to F, one or more halogens selected from Cl, Br, and I.

9. The GaN substrate according to claim 8, containing F and I.

10. The GaN substrate according to claim 7, which contains substantially only F as a halogen element.

11. H concentration is 5 x 10 １７ atoms / cm ３ 1x10 or more ２０ atoms / cm ３ 4. The GaN substrate according to claim 1, wherein:

12. The infrared absorption peak attributable to gallium vacancy-hydrogen complexes is in the range of 3140 to 3200 cm －１ 4. The GaN substrate according to claim 1, comprising a GaN crystal having the following structure:

13. A method for manufacturing a GaN crystal, comprising the following steps: preparing a first undersubstrate having a nitrogen-polar surface; arranging a first pattern mask including a mask portion and a mask opening on the nitrogen-polar surface of the first undersubstrate; growing GaN crystals through the mask openings of the first pattern mask by an ammonothermal method to obtain a layered structure in which GaN crystals are layered on the nitrogen-polar surface of the first undersubstrate via the first pattern mask; separating at least a portion of the GaN crystal grown on the nitrogen-polar surface of the first undersubstrate from the layered structure; planarizing the nitrogen-polar surface of the separated GaN crystal; using the separated GaN crystal as a second undersubstrate, and arranging a second pattern mask on the planarized nitrogen-polar surface, the second pattern mask being arranged so that at least a portion of the region of the nitrogen-polar surface of the GaN crystal to be used as the second undersubstrate that is grown above the mask openings of the first pattern mask is covered by the mask portion of the second pattern mask; and ammonothermally growing GaN crystals through the mask openings of the second pattern mask.

14. A method for producing a GaN crystal as described in claim 13, wherein the second pattern mask is positioned so that the coreless surface of the nitrogen-polar surface of the GaN crystal to be used as the second base substrate, which is coreless on the mask portion of the first pattern mask, and the region grown above it, are covered by the mask portion of the second pattern mask.

15. The method for producing a GaN crystal according to claim 13, wherein the second pattern mask is arranged so that, of the nitrogen-polar surface of the GaN crystal serving as the second base substrate, the region grown above the mask opening of the first pattern mask (base mask opening region) and the region where the base mask opening region and the mask opening of the second pattern mask overlap (exposed base opening region) satisfy the following relational expression (I): Area of ​​exposed base opening region / Area of ​​base mask opening region≦10% (I) 16. A method for producing a GaN crystal as described in claim 13, wherein the second pattern mask is positioned so that there is no area on the nitrogen-polar surface of the GaN crystal that serves as the second base substrate where the area grown on the mask opening of the first pattern mask overlaps with the mask opening of the second pattern mask.

17. A method for producing a GaN crystal, comprising using a GaN crystal produced by the method for producing a GaN crystal according to claim 13 as a third base substrate, and growing a GaN crystal on a gallium-polar surface by the HVPE method.

18. A method for producing a GaN substrate, comprising slicing a GaN crystal produced by the method for producing a GaN crystal according to any one of claims 13 to 17 to obtain a GaN substrate.

19. The tilt from the (0001) plane is 0 to 20 degrees and the area is 15 cm ２ a GaN substrate having a primary surface having a dislocation density of 5.0×10 or more; ４ cm －２ or less, and the strain evaluation value calculated based on the following formula (1) by photoelastic measurement of the main surface is 5×10 －５ a GaN substrate, wherein the area (S1) exceeding 1.0% is 2.5% or less of the area of ​​the entire main surface; Each term in formula (1) is expressed by the following formulas (2) and (3): In the formula, δ and ψ are the phase difference and the principal axis azimuth angle, which are the birefringence amounts due to strain, respectively, λ is the light source wavelength of the photoelasticity measuring device, d is the sample thickness, n ０ is the distortion-free refractive index for the light source wavelength, |p １１ -p １２ | are the photoelastic tensor components, among which λ, n ０ and |p １１ -p １２ | satisfies the following condition: λ = 633 nm n ０ = 2.355 | p １１ -p １２ |=0.13 20. The strain evaluation value calculated based on formula (1) by photoelastic measurement of the principal surface is 1 x 10 －４ 20. The GaN substrate according to claim 19, wherein the area (S2) exceeding 1% or less of the area of ​​the entire primary surface.

21. The GaN substrate according to claim 19, wherein the (004) XRD rocking curve FWHM on the primary surface is 20 arcsec or less.

22. The GaN substrate according to claim 19, wherein, when the normal direction of the primary surface is defined as the z direction, and two directions that are orthogonal to the z direction and are orthogonal to each other are defined as the x direction and the y direction, respectively, the fluctuation range of the x-direction component of the offcut angle on a first line that passes through the center of the primary surface and extends in the x direction, and the fluctuation range of the y-direction component of the offcut angle on a second line that passes through the center of the primary surface and extends in the y direction, are each 0.2 degrees or less within an interval that is 40 mm long.

23. The GaN substrate according to any one of claims 19 to 22, which satisfies the following condition (a) regarding impurity concentration: Condition (a): O concentration is 3×10 １６ atoms / cm ３ The following is the result.

24. The GaN substrate according to any one of claims 19 to 22, which satisfies the following condition (b) regarding impurity concentration: Condition (b): H concentration is 1×10 １７ atoms / cm ３ The following is the result.

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