JP2013126939A - Group iii nitride semiconductor substrate - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a nitride semiconductor substrate whose surface roughness is isotropic.SOLUTION: The group III nitride semiconductor substrate is composed of a group III nitride semiconductor single crystal containing one crystal plane and another crystal plane having a smaller hardness than the one crystal plane. The substrate surface has an absolute value of a surface roughness difference of ≤10 nm between the substrate surface in one direction parallel with the substrate surface and the substrate surface in a direction different from the one direction.

Description

  The present invention relates to a group III nitride semiconductor substrate.

  Conventionally, a GaN single crystal ingot is manufactured by vapor phase growth of a GaN single crystal on a substrate, and a GaN substrate is created by slicing the formed GaN single crystal ingot in a direction parallel to the growth direction. A GaN substrate manufacturing method including a slicing step is known (see, for example, Patent Document 1).

  According to the manufacturing method described in Patent Document 1, dislocations extend in the crystal growth direction of the GaN single crystal, and the ingot is sliced parallel to the direction in which the dislocations extend, so that the surface of the substrate obtained by slicing is sliced. Since dislocations exist in parallel, a GaN substrate having a surface with a low dislocation density can be obtained.

JP 2007-84435 A

  However, in the GaN substrate manufacturing method described in Patent Document 1, when the slicing speed is increased for the purpose of reducing the slicing time, the depth of crystal defects (damage layer depth) due to slicing increases. At the same time, due to saw marks formed on the surface exposed by slicing (slicing surface) along the direction parallel to the wire travel of the wire saw used for slicing, the surface roughness varies even after polishing the slicing surface. May occur.

  Accordingly, an object of the present invention is to provide a nitride semiconductor substrate in which the surface roughness of the substrate is isotropic.

  In order to achieve the above object, the present invention comprises a group III nitride semiconductor single crystal including one crystal plane and another crystal plane having a hardness smaller than the hardness of the one crystal plane, and a group III nitride having a substrate surface The substrate surface is a surface roughness of the substrate surface along one direction parallel to the substrate surface and another surface roughness of the substrate surface along another direction different from the one direction. A group III nitride semiconductor substrate having an absolute value of 10 nm or less is provided.

  The crystal orientation distribution in the substrate surface may have a center value of ± 0.05 ° or less.

Further, the iron concentration in the group III nitride semiconductor substrate may be 1 × 10 ¹⁶ cm ⁻³ or less.

  According to the present invention, it is possible to provide a nitride semiconductor substrate in which the surface roughness of the substrate is isotropic.

It is a figure which shows the surface orientation dependence of the Vickers hardness of the polar surface in a GaN single crystal. It is a figure which shows an example of the manufacturing flow of the group III nitride semiconductor substrate which concerns on embodiment of this invention. It is a figure which shows the result of the micro Raman spectroscopy measurement of the as-sliced wafer which concerns on Example 1. FIG. It is a figure which shows the result of the surface roughness measurement of the ground GaN substrate which concerns on Example 1. FIG. It is a figure which shows the result of the surface roughness measurement of the ground GaN substrate which concerns on Example 2. FIG. It is a figure which shows the result of the surface roughness measurement of the ground GaN substrate which concerns on the comparative example 1. It is a figure which shows the result of the surface roughness measurement of the ground GaN substrate concerning the comparative example 2.

  Group III nitride semiconductors (for example, boron nitride (BN), aluminum nitride (AlN), gallium nitride (GaN), indium nitride (InN), etc.) crystals are wurtzite (WZ) type, zinc blende (ZB) The crystal structure is either of the type or rock salt (RS) type. Here, in a group III nitride semiconductor having a WZ-type crystal structure, polarization occurs in the [0001] direction (c-axis direction) due to the difference between the electronegativity of the group III element and the electronegativity of the group V element. . For example, the plane perpendicular to the c-axis direction is classified into a group V polar plane and a group III polar plane. As an example, GaN, which is a group III nitride semiconductor, has a Ga polar face and an N polar face, but the N polar face is more easily etched than the Ga polar face.

  Here, the present inventor has obtained the following knowledge about GaN. That is, the present inventor measured the Vickers hardness of each of the Ga polar face and the N polar face, and found that the N polar face Vickers hardness was much softer than the Ga polar face Vickers hardness. The result is shown in FIG.

  FIG. 1 shows the plane orientation dependence of the Vickers hardness of a polar plane in a GaN single crystal.

  As can be seen from FIG. 1, in the GaN single crystal, the (000-1) plane, that is, the N-polar plane has a Vickers hardness of about 4.4 GPa, but the inclination from the (000-1) plane is 180 °. A Vickers hardness on a certain (0001) plane, that is, a Ga polar plane is about 7.9 GPa, which is harder than an N polar plane. And when the inclination from the (000-1) plane increases, the Vickers hardness increases. Specifically, the inclination from the (000-1) plane exceeds 30 °, and the Vickers hardness increases almost monotonically until the inclination is about 45 °. When the inclination exceeds 45 °, the degree of increase increases. It was shown that the Vickers hardness saturates as the inclination increases from 150 ° to 180 °. For example, in the Miller index notation such as (000-1), -n (n: natural number) means that it intersects a predetermined coordinate axis in a negative direction.

  Thus, the present inventors have found for the first time the fact that the mechanical hardness of the Ga polar face and the N polar face in GaN depends on the crystal orientation. The mechanism by which the mechanical hardness depends on the crystal orientation is not yet elucidated, but based on this fact, the present inventor has proposed that the crystal planes that are mechanically soft in group III nitride semiconductors including GaN single crystals. It was found that slicing from the mechanically hard crystal face (for example, Ga polar face in the case of GaN) to the side (as an example, N polar face in the case of GaN) is easy.

  That is, the present inventor, in a group III nitride semiconductor including one crystal face and another crystal face having a hardness smaller than the hardness of one crystal face, from another crystal face (mechanically soft crystal face) It was found that slicing toward one crystal plane (mechanically hard crystal plane) is easy. In addition, the present inventor also slices a bulk crystal made of a group III nitride semiconductor single crystal to produce a nonpolar substrate or a semipolar substrate, and is mechanically hard from the crystal plane side that is mechanically soft. It was found that slicing toward the crystal plane side is easy.

[Embodiment]
(Group III nitride semiconductor substrate manufacturing method)
FIG. 2 shows an example of a manufacturing flow of the group III nitride semiconductor substrate according to the embodiment of the present invention.

  In the manufacture of a group III nitride semiconductor substrate according to the present embodiment, a group III nitride semiconductor single crystal including one crystal plane and another crystal plane having a hardness smaller than the mechanical hardness of the one crystal plane. A bulk crystal preparation step for preparing a bulk crystal made of, and a group III nitridation that is not subjected to surface treatment by cutting the bulk crystal from the other crystal face side to the one crystal face side in the prepared bulk crystal The present embodiment is performed through a cutting process for obtaining a semiconductor substrate and a polishing process for polishing the front and back surfaces of the group III nitride semiconductor substrate obtained by cutting (hereinafter, also referred to as slice plane). A group III nitride semiconductor substrate according to the embodiment is manufactured.

  Hereinafter, an example of manufacturing a GaN substrate as a group III nitride semiconductor substrate will be described as an example.

(GaN ingot preparation process)
First, as an example, a thick GaN film is formed on a Ga polar surface or an N polar surface of a GaN single crystal substrate having a c-plane by using a Hide Vapor Phase Epitaxy (HVPE) method, thereby forming a GaN single crystal GaN film. An ingot is manufactured (step 100. Hereinafter, the step is referred to as “S”). Alternatively, pretreatment may be performed on the surface of the sapphire substrate using an epitaxial lateral overgrowth (ELO) method or the like. Then, a GaN thick film as a nitride semiconductor is formed by the HVPE method. Next, the sapphire substrate is removed by mechanical polishing or laser peeling. Thereby, a GaN ingot as a bulk crystal made of a GaN single crystal can be manufactured.

(Cutting process)
Next, the obtained GaN ingot is cut (hereinafter also referred to as “slice”). Specifically, the first crystal face as one crystal face having the first hardness and the second crystal face as another crystal face having a second hardness smaller than the first hardness are GaN. The ingot is included, and in the present embodiment, the slice is sliced from the second crystal face side to the first crystal face side (S110). For example, in the present embodiment, the GaN ingot is sliced from the N polar face to the Ga polar face. The slicing can be performed by, for example, a wire saw including a wire such as a piano wire (steel wire). The slicing can be performed at a slicing speed of 2 mm / h or more. As a result, a sliced GaN substrate (hereinafter sometimes referred to as “as-sliced wafer”) is manufactured.

  As long as slicing is performed from another crystal face toward one crystal face having a hardness larger than the hardness of the other crystal face, it is not limited to the case of cutting from the N polar face toward the Ga polar face. For example, the GaN ingot can be sliced at an angle between the cutting direction (slicing direction) and the + c-axis direction, that is, the [0001] direction, of 0 ° to less than 90 °, preferably 0 ° to 45 °.

(Polishing process)
Subsequently, both surfaces of the as-sliced wafer, that is, the front and back surfaces of the as-sliced wafer are mirror-polished (S120). In this polishing process, the damaged layer, which is a layer including damage such as crystal defects generated by the cutting process, is removed from the surface of the as-sliced wafer toward the inside of the as-sliced wafer. Since the depth of the damaged layer is 50 μm or less in the present embodiment, at least a depth of about 50 μm from the surface of the as-sliced wafer is polished and removed in the polishing step. As a result, a polished GaN substrate according to the present embodiment having an exposed polished surface from which the damaged layer has been removed is manufactured.

  The group III nitride semiconductor is not limited to GaN, and other group III nitride semiconductors having the same polar structure as GaN, such as AlN, can also be used. In addition, the GaN ingot according to the present embodiment can be obtained by metal organic chemical vapor deposition (MOVPE) method, molecular beam epitaxy (MBE) method, Na flux method, ammonothermal method, and the like. It can also be manufactured.

(Effect of embodiment)
According to the method for manufacturing a group III nitride semiconductor substrate according to the present embodiment, since the bulk crystal is sliced from another crystal plane toward one crystal plane having a hardness larger than the hardness of the other crystal plane, The thickness of the damaged layer of the sliced substrate, that is, the depth from the substrate surface to the end of the region including the portion where the crystal defect has occurred can be reduced. For example, even if the slicing speed is increased by using a bulk crystal made of a GaN single crystal that is harder and more brittle than a semiconductor crystal such as Si or GaAs, the thickness of the damaged layer can be reduced. Thereby, the thickness of the damaged layer can be reduced and the slicing speed can be improved.

  And since the thickness of a damage layer can be reduced, the quantity grind | polished in a grinding | polishing process can be reduced. Therefore, when the thickness of the damaged layer generated in the cutting process is added as a margin at the time of slicing to the thickness of the group III nitride semiconductor substrate manufactured through the polishing process, the thickness of the margin can be reduced. . As a result, the number of substrates that can be cut out from one ingot can be increased.

  Furthermore, even if the slicing speed is increased to a slicing speed of 2 mm / h or more as an example, the increase in the thickness of the damaged layer can be suppressed, so the time for manufacturing the group III nitride semiconductor substrate by slicing the bulk crystal is greatly increased. Can be shortened. For example, conventionally, since the slicing speed is 1 mm / h at most, the time required for slicing a bulk crystal having a diameter of 2 inches (50.8 mm) is two days or more. According to the manufacturing method, since the slicing speed can be increased twice, the time required for slicing a bulk crystal having a diameter of 2 inches can be reduced to ½ or less.

  Conventionally, streaky irregularities may be formed in a direction parallel to the traveling direction of a wire in a GaN crystal cut with a wire saw. This is called a saw mark and can be confirmed visually. Since the GaN crystal has a high hardness, the wire is bent during the cutting of the GaN ingot, the wire is easily shaken, and a saw mark is easily generated. Also, saw marks are more prominently generated as the slice speed is increased. GaN substrates with marked saw marks are likely to leave traces on the polished surface even after polishing, resulting in striped patterns resulting from differences in surface roughness on the polished surface, and shallow striped irregularities remaining on the polished surface. In some cases, the surface roughness of the polished surface is not isotropic (isotropic). However, according to the method for manufacturing a group III nitride semiconductor substrate according to the present embodiment, GaN which is a bulk crystal from another crystal plane toward one crystal plane having a hardness larger than the hardness of the other crystal plane Since the ingot is sliced, even if the sliced surface of the GaN substrate obtained by slicing is polished, the surface roughness of the polished surface can be made isotropic.

  Further, according to the method for manufacturing a group III nitride semiconductor substrate according to the present embodiment, the bulk crystal is sliced from another crystal plane toward one crystal plane having a hardness larger than the hardness of the other crystal plane. . That is, since the bulk crystal is sliced from the mechanically soft surface to the hard surface, the slicing time can be reduced even when the bulk crystal is hard. Thereby, the wear of the wire of a wire saw can be reduced and it can suppress that the material which comprises the wire produced by the wear of a wire contaminates the surface of the crystal | crystallization which sliced, ie, the surface vicinity of an as-sliced wafer. Therefore, a high-purity group III nitride semiconductor substrate can be manufactured.

  Further, for example, the off-angle of the surface of the GaN substrate used for the underlying substrate for epitaxial growth greatly affects the composition of the epitaxial growth layer such as InGaN or AlGaN. Therefore, precise control and management are required for the off-angle of the GaN substrate surface. However, conventionally, when a hard crystal such as a GaN crystal is sliced, if the wire is forcibly sliced at high speed, the wire may be bent or the wire may be displaced from the intended position of the GaN ingot. In this case, the crystal orientation distribution on the slice plane may become large. Although this slice plane with a large crystal orientation distribution can be corrected by obliquely polishing the slice plane, it is necessary to increase the initial cutout in order to ensure a predetermined finished thickness. On the other hand, according to the method for manufacturing a group III nitride semiconductor substrate according to the present embodiment, the bulk crystal is sliced from another crystal plane toward one crystal plane having a hardness larger than the hardness of the other crystal plane. Therefore, it is possible to appropriately slice at the intended position of the GaN ingot. Thereby, a substrate with good surface accuracy, that is, a highly accurate GaN substrate can be manufactured.

  First, a 2 inch c-plane GaN substrate was prepared. Subsequently, GaN was further grown on the Ga polar face of the prepared GaN substrate by the HVPE method. Thereby, a GaN ingot having a thickness of 20 mm was manufactured.

  Next, the GaN ingot manufactured using a wire saw was sliced along a direction parallel to the c-axis. That is, slicing was performed along the direction from the N polar face side to the Ga polar face side, that is, the [0001] direction and from the N polar face to the Ga polar face. Here, the wire of the wire saw was a piano wire having a diameter of 0.16 mm. Further, a coating agent in which diamond free abrasive grains having an average particle size of 5 μm were dispersed in an oil slurry was prepared, and sliced while supplying this coating agent to a slice (cut) portion of a GaN ingot by a wire saw. The temperature of the oil slurry was set at 30 ° C. The cutting speed was set to 4 mm / h.

  As a result, a GaN as-sliced wafer having slicing surfaces exposed by slicing on both sides was obtained from the GaN ingot. The obtained as-sliced wafer was a GaN substrate whose main surface was the M plane, that is, the (10-10) plane, and had a size of 20 mm × 40 mm and a thickness of 0.6 mm. The time required for cutting was about 5 hours. Further, no bending of the wire occurred during the GaN ingot slice. Here, when the cut surface of the as-sliced wafer, that is, the substrate surface exposed to the outside by slicing was visually observed, no saw mark was observed.

  The depth of the damage layer on the surface of the obtained as-sliced wafer was calculated by microscopic Raman spectroscopy. Here, the Raman peak position, which is the peak position of the Raman spectrum, shifts when the crystal lattice is distorted by the stress generated in the object to be measured. Therefore, the depth of the damage layer is determined by measuring the amount of shift of the Raman peak position with respect to the depth from the surface by microscopic Raman spectroscopy measurement of the cleavage plane of the as-sliced wafer from the surface side to the inside of the as-sliced wafer. Can be calculated.

  FIG. 3 shows the results of microscopic Raman spectroscopy measurement of the as-sliced wafer according to Example 1.

Referring to FIG. 3, in the vicinity of the slice surface of the as-sliced wafer, that is, in the region from the surface of the slice surface (the position where the depth is 0 μm in FIG. 3) to a depth of about 10 μm, the Raman peak is caused by damage such as crystal defects. The position was shifted to the low wavenumber side. As the depth from the surface of the slice surface becomes deeper, the Raman peak position gradually changed to the higher wavenumber side, and was saturated at a wavenumber of about 6.95 cm ⁻¹ when the depth was 40 μm or more. As a result, the depth of the damaged layer of the as-sliced wafer according to Example 1 was considered to be 40 μm or less.

  Subsequently, both sides of the as-sliced wafer according to Example 1 were polished to a mirror surface by 50 μm. As a result, a polished GaN substrate having a thickness of 0.5 mm and a mirror-finished surface was obtained. Since the ground GaN substrate according to Example 1 was formed by slicing a GaN ingot along the [0001] direction and then polishing the sliced surface, the principal surface was an M-plane, that is, (10-10) Surface.

  The surface roughness (RMS value) of the polished GaN substrate according to Example 1 was measured with an atomic force microscope (AFM). The surface roughness was measured in four directions inclined by 45 ° from this reference with reference to the traveling direction of the wire of the wire saw.

  FIG. 4 shows the results of measuring the surface roughness of the polished GaN substrate according to Example 1.

  Referring to FIG. 4, the RMS value of the surface of the polished GaN substrate showed a substantially constant value regardless of the measurement direction. Specifically, assuming that the RMS value measured along the traveling direction of the wire is a reference value (measured value at an angle of 0 ° in FIG. 4), this reference value and a direction inclined by 45 ° from the traveling direction of the wire The absolute value of the difference between the RMS value of the surface at 90 °, the RMS value of the surface in the direction inclined 90 °, and the RMS value of the surface in the direction inclined 135 ° was in the range of 10 nm or less. Thereby, the surface roughness of the surface of the ground GaN substrate according to Example 1 is isotropic, that is, the surface roughness stripes on the polished surface after polishing the slice surface in the wire traveling direction of the wire saw Confirmed that there is no phenomenon (hereinafter sometimes referred to as “the influence of saw marks”) in which unevenness occurs only in a specific direction, such as the occurrence of a pattern or the presence of striped unevenness. It was done.

  Further, the RMS value of the surface roughness in the region of 100 μm × 100 μm in the surface of the polished GaN substrate according to Example 1 was 5 nm. Further, the variation in crystal orientation distribution within the surface of the polished GaN substrate was measured by an X-ray diffraction method. As a result, it was confirmed that the variation of the crystal orientation distribution in the surface of the polished GaN substrate according to Example 1 was as small as the center value ± 0.01 °, and the GaN ingot could be sliced with high accuracy.

Next, GaN having a thickness of 3 μm was epitaxially grown on the surface of the polished GaN substrate using the MOVPE method to obtain a GaN substrate with an epitaxial layer. The depth direction profile of the iron (Fe) impurity concentration of the GaN substrate with an epitaxial layer was measured by SIMS analysis. As a result, it was confirmed that the Fe concentration was below the lower limit of detection (2 × 10 ¹⁵ cm ⁻³ ) in any of the GaN layer, the interface, and the GaN substrate.

  First, a GaN ingot having a thickness of 20 mm was manufactured in the same manner as in Example 1. Next, using the same conditions as in Example 1, the GaN ingot was sliced from the N polar face side to the Ga polar face side along the direction parallel to the (11-22) plane. However, in Example 2, the slice speed was set to 2 mm / h. Thereby, a GaN as-sliced wafer (size: 23.5 mm × 40 mm, thickness: 0.6 mm) according to Example 2 was obtained. Note that the time required for cutting was about 12 hours, and no bending of the wire occurred during the GaN ingot slice. Further, when the cut surface of the as-sliced wafer was visually observed, no saw mark was observed.

  The depth of the damaged layer on the surface of the as-sliced wafer according to Example 2 was calculated by microscopic Raman spectroscopy in the same manner as in Example 1. As a result, the depth of the damaged layer caused by slicing was considered to be 50 μm or less.

  Subsequently, both sides of the as-sliced wafer according to Example 2 were polished by 70 μm to be mirror-finished. As a result, a polished GaN substrate having a thickness of 0.46 mm and a polished surface that was a mirror surface was obtained. The polished GaN substrate according to Example 2 was formed by slicing a GaN ingot along a direction parallel to the (11-22) plane, and then polishing the sliced surface. 11-22) plane.

  The surface roughness (RMS value) of the polished GaN substrate according to Example 2 was measured with an atomic force microscope (AFM). The surface roughness was measured in four directions inclined by 45 ° from this reference with reference to the traveling direction of the wire of the wire saw.

  FIG. 5 shows the results of measuring the surface roughness of the polished GaN substrate according to Example 2.

  Referring to FIG. 5, the RMS value of the surface of the polished GaN substrate showed a substantially constant value regardless of the measurement direction. Specifically, assuming that the RMS value measured along the traveling direction of the wire is a reference value (measured value at an angle of 0 ° in FIG. 5), this reference value and a direction inclined by 45 ° from the traveling direction of the wire The absolute value of the difference between the RMS value of the surface at 90 °, the RMS value of the surface in the direction inclined 90 °, and the RMS value of the surface in the direction inclined 135 ° was in the range of 10 nm or less. As a result, it was confirmed that the surface roughness of the polished GaN substrate according to Example 2 was isotropic and that there was no influence of saw marks.

  Further, the variation in crystal orientation distribution in the surface of the polished GaN substrate according to Example 2 was measured by an X-ray diffraction method. As a result, it was confirmed that the variation of the crystal orientation distribution in the surface of the polished GaN substrate according to Example 2 was as small as the center value ± 0.02 °, and the GaN ingot could be sliced with high accuracy.

Next, GaN having a thickness of 3 μm was epitaxially grown on the surface of the polished GaN substrate according to Example 2 using the MOVPE method to obtain a GaN substrate with an epitaxial layer. The depth direction profile of the Fe impurity concentration of the GaN substrate with an epitaxial layer was measured by SIMS analysis. As a result, it was confirmed that the Fe concentration was below the lower limit of detection (2 × 10 ¹⁵ cm ⁻³ ) in any of the GaN layer, the interface, and the GaN substrate.

(Comparative Example 1)
First, a GaN ingot having a thickness of 20 mm was manufactured in the same manner as in Example 1. Next, the GaN ingot was sliced from the Ga polar face side to the N polar face side along the direction parallel to the c-axis, that is, the [000-1] direction. Other slicing conditions were the same as in Example 1. However, it took about 10 hours to completely cut the GaN ingot having a thickness of 20 mm because the wire was greatly bent during the GaN ingot slice. That is, in Comparative Example 1, the slice speed was substantially 2 mm / h. As a result, a GaN as-sliced wafer (size: 20 mm × 40 mm, thickness: 0.6 mm) according to Comparative Example 1 was obtained. When the cut surface of the as-sliced wafer was visually observed, saw marks were clearly observed.

  The depth of the damaged layer on the surface of the as-sliced wafer according to Comparative Example 1 was calculated by microscopic Raman spectroscopy in the same manner as in Example 1. As a result, the depth of the damaged layer produced by slicing was considered to be about 150 μm.

  Subsequently, both sides of the as-sliced wafer according to Comparative Example 1 were polished by 170 μm to be mirror-finished. As a result, a polished GaN substrate having a thickness of 0.26 mm and a polished surface that was a mirror surface was obtained. Since the ground GaN substrate according to Comparative Example 1 was formed by slicing a GaN ingot along the c-axis direction and then polishing the slice surface, the principal surface is an M (10-10) plane. In Comparative Example 1, the depth of the damaged layer is 150 μm, which is deeper than that of Example 1, so that a large amount of polishing is required to remove the damaged layer, and the thickness of the finally obtained GaN substrate is also increased. It was as thin as 0.26 mm.

  The surface roughness (RMS value) of the polished GaN substrate according to Comparative Example 1 was measured with an atomic force microscope (AFM). The surface roughness was measured in four directions inclined by 45 ° from this reference with reference to the traveling direction of the wire of the wire saw.

  FIG. 6 shows the result of measuring the surface roughness of the polished GaN substrate according to Comparative Example 1.

  Referring to FIG. 6, the RMS value of the surface of the polished GaN substrate tends to be larger in the direction perpendicular to the saw mark (90 °) than in the direction parallel to the saw mark (45 °, 135 °). It was. Specifically, the RMS value measured along the traveling direction of the wire is about 50 nm (in FIG. 6, the measured value is an angle of 0 °). The RMS value of the surface in the direction inclined by 45 ° from the traveling direction of the wire. Is about 170 nm, the RMS value of the surface in the direction inclined by 90 ° is about 330 nm, and the RMS value of the surface in the direction inclined by 135 ° is about 190 °. Thereby, it was confirmed that the surface roughness of the surface of the ground GaN substrate according to Comparative Example 1 is anisotropic and that there is an influence of saw marks.

  Further, the variation in crystal orientation distribution in the surface of the polished GaN substrate according to Comparative Example 1 was measured by an X-ray diffraction method. As a result, the variation in the crystal orientation distribution in the surface of the polished GaN substrate according to Comparative Example 1 was larger than that of Examples 1 and 2 at the center value ± 0.15 °. This is presumably because the wire was bent and the sliced surface was disturbed due to an attempt to forcefully cut the GaN ingot at high speed.

Next, GaN having a thickness of 3 μm was epitaxially grown on the surface of the polished GaN substrate according to Comparative Example 1 using the MOVPE method to obtain a GaN substrate with an epitaxial layer. The depth direction profile of the Fe impurity concentration of the GaN substrate with an epitaxial layer was measured by SIMS analysis. As a result, the Fe concentration was below the lower limit of detection (2 × 10 ¹⁵ cm ⁻³ ) in the epitaxially grown GaN film and in the GaN substrate. However, it was 2.2 × 10 ¹⁷ cm ⁻³ at the interface between the GaN substrate and the epitaxially grown GaN film, and a non-negligible concentration of Fe was detected when applied to an electronic device such as an optical device.

(Comparative Example 2)
First, a GaN ingot having a thickness of 24.5 mm was manufactured in the same manner as in Example 1. Next, the GaN ingot was sliced from the Ga polar face side toward the N polar face side along the direction parallel to the (11-22) plane. Other slicing conditions were the same as in Example 1. However, the slicing speed of the wire saw was set to 2 mm / h. However, it took about 20 hours to completely cut the GaN ingot having a thickness of 24.5 mm because the wire was greatly bent during the GaN ingot slice. That is, in Comparative Example 2, the slice speed was substantially 1.2 mm / h. As a result, a GaN as-sliced wafer (size: 23.5 mm × 40 mm, thickness: 0.6 mm) according to Comparative Example 2 was obtained. When the cut surface of the as-sliced wafer was visually observed, saw marks were clearly observed.

  The depth of the damaged layer on the surface of the as-sliced wafer according to Comparative Example 2 was calculated by microscopic Raman spectroscopy in the same manner as in Example 1. As a result, the depth of the damaged layer produced by slicing was considered to be about 100 μm.

  Subsequently, both sides of the as-sliced wafer according to Comparative Example 2 were polished by 120 μm to make a mirror surface. As a result, a polished GaN substrate having a thickness of 0.36 mm and a mirror-finished polishing surface was obtained. The ground GaN substrate according to Comparative Example 2 was formed by slicing a GaN ingot along a direction parallel to the (11-22) plane and then polishing the sliced surface. ) Surface. In Comparative Example 2, the depth of the damaged layer is 120 μm, which is deeper than that of Example 2, so that a large amount of polishing is required to remove the damaged layer, and the thickness of the finally obtained GaN substrate is also increased. It was as thin as 0.36 mm.

  The surface roughness (RMS value) of the ground GaN substrate according to Comparative Example 2 was measured with an atomic force microscope (AFM). The surface roughness was measured in four directions inclined by 45 ° from this reference with reference to the traveling direction of the wire of the wire saw.

  FIG. 7 shows the results of measuring the surface roughness of the polished GaN substrate according to Comparative Example 2.

  Referring to FIG. 7, it was observed that the RMS value of the surface of the polished GaN substrate tends to be larger in the direction perpendicular to the saw mark than in the direction parallel to the saw mark. Specifically, the RMS value measured along the traveling direction of the wire is about 50 nm (in FIG. 7, the measured value is an angle of 0 °). The RMS value of the surface in the direction inclined by 45 ° from the traveling direction of the wire. Is about 160 nm, the RMS value of the surface in the direction inclined by 90 ° is about 280 nm, and the RMS value of the surface in the direction inclined by 135 ° is about 160 °. Thereby, it was confirmed that the surface roughness of the surface of the ground GaN substrate according to Comparative Example 2 is anisotropic and that there is an influence of saw marks.

  Further, the variation in crystal orientation distribution in the surface of the polished GaN substrate according to Comparative Example 2 was measured by an X-ray diffraction method. As a result, the variation in the crystal orientation distribution in the surface of the polished GaN substrate according to Comparative Example 1 was larger than that of Examples 1 and 2 at the center value ± 0.15 °. This is presumably because the wire was bent and the sliced surface was disturbed due to an attempt to forcefully cut the GaN ingot at high speed.

Next, GaN having a thickness of 3 μm was epitaxially grown on the surface of the polished GaN substrate according to Comparative Example 2 using the MOVPE method to obtain a GaN substrate with an epitaxial layer. The depth direction profile of the Fe impurity concentration of the GaN substrate with an epitaxial layer was measured by SIMS analysis. As a result, the Fe concentration was below the lower limit of detection (2 × 10 ¹⁵ cm ⁻³ ) in the epitaxially grown GaN film and in the GaN substrate. However, it was 5.8 × 10 ¹⁶ cm ⁻³ at the interface between the GaN substrate and the epitaxially grown GaN film, and a non-negligible concentration of Fe was detected.

  While the embodiments and examples of the present invention have been described above, the embodiments and examples described above do not limit the invention according to the claims. It should be noted that not all combinations of features described in the embodiments and examples are necessarily essential to the means for solving the problems of the invention.

Claims (3)

A group III nitride semiconductor substrate comprising a group III nitride semiconductor single crystal including one crystal plane and another crystal plane having a hardness smaller than the hardness of the one crystal plane, and having a substrate surface,
The substrate surface has one surface roughness of the substrate surface along one direction parallel to the substrate surface and another surface roughness of the substrate surface along another direction different from the one direction. A group III nitride semiconductor substrate having an absolute difference of 10 nm or less.   The group III nitride semiconductor substrate according to claim 1, wherein a crystal orientation distribution in the substrate surface has a center value of ± 0.05 ° or less. 3. The group III nitride semiconductor substrate according to claim 2, wherein an iron concentration in the group III nitride semiconductor substrate is 1 × 10 ¹⁶ cm ⁻³ or less.

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