Group III element nitride semiconductor substrate

Abstract

Provided is a group-III element nitride semiconductor substrate which, even when the off-orientation varies, exhibits little behavior change in crystal growth during epitaxial growth, and in which in-plane morphological disturbance and characteristic variation are unlikely to occur. A group-III element nitride semiconductor substrate according to one embodiment of the present invention is provided with a first surface and a second surface. A c-surface is inclined with respect to the orientation of the first surface. The direction of the inclination is oriented between the <1-100> direction and the <11-20> direction.

Description

[Technical Field]

[0001] The present invention relates to a Group III element nitride semiconductor substrate, and more particularly to a Group III element nitride semiconductor substrate having a first surface and a second surface, the orientation of the first surface being tilted with respect to the c-plane. [Background technology]

[0002] Group III element nitride semiconductor substrates such as gallium nitride (GaN) wafers and aluminum nitride (AlN) wafers are used as substrates for light-emitting devices such as light-emitting diodes and semiconductor lasers (see, for example, Patent Document 1). Furthermore, high-frequency devices and power devices using these substrates are being actively developed for practical use.

[0003] The Group III element nitride semiconductor substrate has a first surface and a second surface. When the first surface is the main surface and the second surface is the back surface, the main surface is typically a Group III element polar surface, and the back surface is typically a nitrogen polar surface. Epitaxial crystals can be grown on the main surface, and various devices can be fabricated on the main surface.

[0004] When using a GaN substrate whose principal plane orientation coincides with the crystal plane (c-plane), growing a thin film on the principal plane can result in a wavy surface that does not necessarily result in a smooth plane. For this reason, devices are often fabricated by growing a thin film on a GaN substrate slightly tilted from the c-plane. This slight tilt of the plane from a low plane index is called an off-angle, and such a substrate is called an off-angle substrate, with the inclination angle being called the off-angle. The tilt direction (off-orientation) of the principal plane is generally the <1-100> direction (m-axis direction) or the <11-20> direction (a-axis direction) of the GaN crystal lattice (Patent Documents 1-3). [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Patent No. 3888374 [Patent Document 2] Patent No. 4952547 [Patent Document 3] JP 2019-77600 A Summary of the Invention [Problem to be solved by the invention]

[0006] Group III nitride semiconductors have a hexagonal wurtzite crystal structure. Therefore, the a-axis and m-axis are the orientations within the c-plane that have the most distinct properties and structures, and the a-axis and m-axis directions also have different crystal growth behaviors. For example, when a group III nitride semiconductor is grown in solution, the m-plane develops, resulting in a hexagonal basal shape, with the a-axis direction corresponding to the vertices of the hexagon and the m-axis direction corresponding to the edges of the hexagon. Therefore, if the off-orientation of a group III nitride semiconductor substrate deviates from the a-axis or m-axis direction within the plane of the substrate, the behavior of the crystal growth during epitaxial growth changes significantly, resulting in in-plane morphological disturbances and characteristic variations in the epitaxial film. On the other hand, group III nitride semiconductor substrates are often fabricated by processing crystals with significant warpage due to heteroepitaxial growth, which can easily lead to in-plane variations in the off-orientation of the group III nitride semiconductor substrate.

[0007] An object of the present invention is to provide a Group III element nitride semiconductor substrate in which, even if the off-orientation varies, the behavioral change of the crystal growth during epitaxial growth is small, and in-plane morphology disturbance and characteristic variations are unlikely to occur. [Means for solving the problem]

[0008] A Group III element nitride semiconductor substrate according to one embodiment of the present invention comprises: A group III element nitride semiconductor substrate having a first surface and a second surface, the c-plane is inclined with respect to the orientation of the first surface, The tilt direction is between the <1-100> direction and the <11-20> direction.

[0009] In one embodiment, the tilt direction is within a range of ±15° from a direction midway between the <1-100> direction and the <11-20> direction.

[0010] In one embodiment, the tilt direction is within a range of ±12.5° from a direction midway between the <1-100> direction and the <11-20> direction.

[0011] In one embodiment, the tilt direction is within a range of ±7.5° from a direction midway between the <1-100> direction and the <11-20> direction.

[0012] In one embodiment, the area of the region occupied in the direction of the inclination is 50% or more of the total area of the first surface.

[0013] In one embodiment, the angle of inclination is 0.2° to 0.8°.

[0014] A Group III element nitride semiconductor substrate according to one embodiment of the present invention comprises: A group III element nitride semiconductor substrate having a first surface and a second surface, the c-plane is inclined with respect to the orientation of the first surface, It has an orientation flat parallel to the <11-20> direction, The tilt direction is within a range of +45°±15°, or −45°±15°, or +135°±15°, or −135°±15° from the <1-100> direction.

[0015] In one embodiment, the tilt direction is within a range of +45°±12.5°, or −45°±12.5°, or +135°±12.5°, or −135°±12.5° from the <1-100> direction.

[0016] In one embodiment, the tilt direction is within a range of +45°±7.5°, or −45°±7.5°, or +135°±7.5°, or −135°±7.5° from the <1-100> direction. [Effects of the Invention]

[0017] According to the present invention, it is possible to provide a Group III element nitride semiconductor substrate in which, even if the off-orientation varies, the behavioral change of the crystal growth during epitaxial growth is small, and in-plane morphology disturbance and characteristic variations are unlikely to occur. [Brief explanation of the drawings]

[0018] [Figure 1] 1 is a schematic representative perspective view of a group III element nitride semiconductor substrate according to an embodiment of the present invention. [Figure 2] 1 is a schematic diagram illustrating plane orientations and crystal planes in the crystal structure of a group III nitride semiconductor substrate according to an embodiment of the present invention. [Figure 3] FIG. 2 is a schematic diagram illustrating the off-orientation in a group III nitride semiconductor substrate according to an embodiment of the present invention. [Figure 4] 1 is a schematic representative cross-sectional view of a group III element nitride semiconductor substrate according to an embodiment of the present invention. [Figure 5] 1A to 1C are schematic explanatory views showing a method for manufacturing a group III element nitride semiconductor substrate according to an embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0019] When the expression "weight" appears in this specification, it may be read as "mass," which is a commonly used SI unit indicating weight.

[0020] The Group III nitride semiconductor substrate according to the embodiment of the present invention is typically a free-standing substrate made of Group III nitride crystal. In the description of the present invention, the term "free-standing substrate" refers to a substrate that can be handled as a solid object without being deformed or broken by its own weight when handled. The free-standing substrate can be used as a substrate for various semiconductor devices such as light-emitting elements and power control elements.

[0021] The group III nitride semiconductor substrate according to the embodiment of the present invention is typically in the form of a wafer (approximately a perfect circle), but may be processed into other shapes, such as a rectangle, as necessary.

[0022] The diameter of the Group III element nitride semiconductor substrate according to the embodiment of the present invention may be any appropriate diameter as long as the effects of the present invention are not impaired. In terms of further exhibiting the effects of the present invention, the diameter of the Group III element nitride semiconductor substrate according to the embodiment of the present invention is preferably 50 mm or more, more preferably 75 mm or more, and even more preferably 100 mm or more. In particular, if the Group III element nitride semiconductor substrate according to the embodiment of the present invention is a so-called large-diameter Group III element nitride semiconductor substrate having a diameter of 75 mm or more, the Group III element nitride semiconductor substrate according to the embodiment of the present invention can be easily applied to high-frequency / high-power electronic devices, and in particular, can be easily applied to devices that handle high power (so-called power devices) with large element sizes.

[0023] Specific examples of large-diameter Group III element nitride semiconductor substrates include 4-inch wafers, 6-inch wafers, 8-inch wafers, and 12-inch wafers.

[0024] The Group III nitride semiconductor substrate according to the embodiment of the present invention preferably has a thickness (if the thickness is not constant, the thickness at the thickest point) of 300 μm to 1000 μm.

[0025] Representative examples of Group III element nitrides include gallium nitride (GaN), aluminum nitride (AlN), indium nitride (InN), and mixed crystals thereof, which may be used alone or in combination of two or more.

[0026] Group III element nitrides include, specifically, GaN, AlN, InN, Ga x Al 1-x N(1>x>0), Ga x In 1-x N(1>x>0), Al x In 1-x N(1>x>0), Ga x Al y In z N(1>x>0, 1>y>0, x+y+z=1), which may be doped with various n-type or p-type dopants.

[0027] Typical p-type dopants include beryllium (Be), magnesium (Mg), strontium (Sr), and cadmium (Cd), and these may be used alone or in combination of two or more.

[0028] Typical examples of n-type dopants include silicon (Si), germanium (Ge), tin (Sn), and oxygen (O). These may be used alone or in combination of two or more.

[0029] A Group III element nitride semiconductor substrate according to an embodiment of the present invention is a Group III element nitride semiconductor substrate having a first surface and a second surface. When the first surface is the main surface and the second surface is the back surface, if the surface orientation of the Group III element nitride semiconductor substrate is the c-plane side, the main surface is typically a Group III element polar surface and the back surface is typically a nitrogen polar surface. However, the main surface may be a nitrogen polar surface, or the back surface may be a Group III element polar surface. Epitaxial crystals can be grown on the main surface, and various devices can be fabricated on it. The back surface can be held by a susceptor or the like, allowing the Group III element nitride semiconductor substrate according to an embodiment of the present invention to be transported.

[0030] In the description of the Group III nitride semiconductor substrate according to the embodiment of the present invention, the first surface will be referred to as the main surface and the second surface will be referred to as the back surface. Therefore, in this specification, the "main surface" may be read as the "first surface," the "first surface" may be read as the "main surface," the "back surface" may be read as the "second surface," and the "second surface" may be read as the "back surface."

[0031] 1 is a schematic perspective view of a group III nitride semiconductor substrate 100 according to an embodiment of the present invention. As shown in FIG. 1, the group III nitride semiconductor substrate 100 according to the embodiment of the present invention has a plane orientation of 100 .times.1 with respect to a normal vector A of its first surface. <0001> That is, the III-nitride semiconductor substrate 100 according to the embodiment of the present invention has a plane orientation (c-axis) tilted. <0001> The substrate is an off-angle substrate having an off-angle tilted from the substrate.

[0032] 2 is a schematic diagram illustrating the crystal orientation and crystal planes in the crystal structure of a Group III nitride semiconductor substrate according to an embodiment of the present invention. <0001> The c-axis direction is the <1-100> direction, the m-axis direction is the <11-20> direction, and the a-axis direction is the <11-20> direction. The top surface of the hexagonal crystal, which can be considered as a regular hexagonal prism, is the c-plane, and the side wall surface of the regular hexagonal prism is the m-plane.

[0033] In the group III nitride semiconductor substrate according to the embodiment of the present invention, the c-plane is inclined with respect to the orientation of the first surface. In other words, in the group III nitride semiconductor substrate according to the embodiment of the present invention, with respect to the normal vector of the first surface (normal vector A in FIG. 1), <0001> The direction (c-axis direction) is tilted.

[0034] The angle of inclination is preferably 0.2° to 0.8°, more preferably 0.3° to 0.8°, even more preferably 0.4° to 0.8°, and particularly preferably 0.5° to 0.7°, in order to further demonstrate the effects of the present invention.

[0035] In the group III element nitride semiconductor substrate according to the embodiment of the present invention, the tilt direction is between the <1-100> direction and the <11-20> direction. That is, in the group III element nitride semiconductor substrate according to the embodiment of the present invention, the tilt direction (off-orientation) is between the m-axis direction and the a-axis direction. In the group III element nitride semiconductor substrate according to the embodiment of the present invention, the off-orientation is between the <1-100> direction and the <11-20> direction in this way, and thus the effects of the present invention can be more effectively exhibited.

[0036] 3 is a schematic diagram illustrating the off-orientation in a Group III element nitride semiconductor substrate according to an embodiment of the present invention. Conventional Group III element nitride semiconductor substrates generally have an off-orientation in the <1-100> direction (m-axis direction) or the <11-20> direction (a-axis direction), as described in the above-mentioned Patent Documents 1 to 3. However, the off-orientation in the Group III element nitride semiconductor substrate according to an embodiment of the present invention is between the <1-100> direction and the <11-20> direction (the direction within the range indicated by the solid curved arrow), as shown in FIG.

[0037] In the Group III nitride semiconductor substrate according to the embodiment of the present invention, in order to further exhibit the effects of the present invention, the direction of the tilt is preferably within a range of ±15° from the midpoint between the <1-100> direction and the <11-20> direction, more preferably within a range of ±12.5° from the midpoint between the <1-100> direction and the <11-20> direction, even more preferably within a range of ±10° from the midpoint between the <1-100> direction and the <11-20> direction, particularly preferably within a range of ±7.5° from the midpoint between the <1-100> direction and the <11-20> direction, and most preferably within a range of ±5° from the midpoint between the <1-100> direction and the <11-20> direction. Here, the direction intermediate between the <1-100> direction and the <11-20> direction is the direction at an angle of 45° from the <1-100> direction toward the <11-20> direction (or vice versa), and is the direction indicated by the dashed arrow in FIG.

[0038] In the Group III nitride semiconductor substrate according to the embodiment of the present invention, the area of the region occupied by the above-mentioned tilt direction (a tilt direction within a range of preferably ±15° (more preferably within a range of ±12.5°, even more preferably within a range of ±10°, particularly preferably within a range of ±7.5°, and most preferably within a range of ±5°) from a direction midway between the <1-100> direction and the <11-20> direction)) is preferably 50% or more, more preferably 60% or more, and even more preferably 70% or more of the entire area of the first surface, in order to more effectively exhibit the effects of the present invention. This means that in the Group III nitride semiconductor substrate according to the embodiment of the present invention, the majority (preferably 50% or more) of the directions in which the c-plane tilts with respect to the orientation of the first surface are preferably within a range of ±15° (more preferably within a range of ±12.5°, even more preferably within a range of ±10°, particularly preferably within a range of ±7.5°, and most preferably within a range of ±5°) from a direction midway between the <1-100> and <11-20> directions, and the wider the area occupied by these tilt directions, the more preferable it is (more preferably 60% or more, and even more preferably 70% or more). Note that the upper limit of the ratio of the area occupied by the above tilt directions (tilt directions preferably within a range of ±15° (more preferably within a range of ±12.5°, even more preferably within a range of ±10°, particularly preferably within a range of ±7.5°, and most preferably within a range of ±5°) from a direction midway between the <1-100> and <11-20> directions) to the total area of the first surface is preferably 100% or less in practice.

[0039] In a Group III nitride semiconductor substrate according to an embodiment of the present invention, the tilt direction of the c-plane with respect to the orientation of the first surface at the center of the substrate is between the <1-100> direction and the <11-20> direction, and preferably within a range of ±15° (more preferably within a range of ±12.5°, even more preferably within a range of ±10°, particularly preferably within a range of ±7.5°, and most preferably within a range of ±5°) from the midpoint between the <1-100> direction and the <11-20> direction. Note that in a Group III nitride semiconductor substrate according to an embodiment of the present invention, as long as most of the tilt direction of the c-plane with respect to the orientation of the first surface is within the above range, part of the tilt direction of the c-plane with respect to the orientation of the first surface may be tilted in the <1-100> direction (m-axis direction) or the <11-20> direction (a-axis direction).

[0040] A group III nitride semiconductor substrate according to an embodiment of the present invention may have an orientation flat parallel to the <11-20> direction, in which the tilt direction is within the range of +45°±15°, −45°±15°, +135°±15°, or −135°±15° from the <1-100> direction, in order to further exhibit the effects of the present invention.

[0041] The tilt direction is preferably within a range of +45°±12.5°, −45°±12.5°, +135°±12.5°, or −135°±12.5° from the <1-100> direction, in order to further enhance the effects of the present invention, more preferably within a range of +45°±10°, −45°±10°, +135°±10°, or −135°±10° from the <1-100> direction, even more preferably within a range of +45°±7.5°, −45°±7.5°, +135°±7.5°, or −135°±7.5° from the <1-100> direction, and particularly preferably within a range of +45°±5°, −45°±5°, +135°±5°, or −135°±5° from the <1-100> direction. By designing in this manner, the tilt direction can be within a range of ±15° (preferably within a range of ±12.5°, more preferably within a range of ±10°, even more preferably within a range of ±7.5°, and particularly preferably within a range of ±5°) from the midpoint between the <1-100> direction and the <11-20> direction.

[0042] In the Group III nitride semiconductor substrate according to the embodiment of the present invention, the c-plane is inclined with respect to the orientation of the first surface as described above, and the direction of the inclination is between the <1-100> direction and the <11-20> direction. The mechanism by which the effects of the present invention can be exhibited is considered as follows.

[0043] When forming an epitaxial film on a Group III nitride semiconductor whose primary surface is the c-plane, the a-plane does not develop in the lateral direction of the film, but the m-plane tends to develop instead, because the crystal growth rate in the m-axis direction is slower than that in the a-axis direction.

[0044] On a so-called just substrate, which has no off-axis angle, the growth of the epitaxial film progresses everywhere on the substrate surface, resulting in non-uniform crystal growth, which tends to result in unevenness in the film thickness and large irregularities in the epitaxial film.

[0045] On the other hand, on an off-axis substrate, an epitaxial film grows by so-called step-flow growth, in which minute steps called atomic steps are formed on the substrate, and raw materials (metal ions, etc.) precipitate on the substrate as if these atomic steps advance in the direction of the off-axis tilt, resulting in the formation of a smooth, highly crystalline epitaxial film.

[0046] In c-plane group III nitride semiconductor substrates off the m-axis, the atomic steps tend to be flat (linear) when viewed from above. This is because the atomic steps move in the m-axis direction, resulting in a progressively developed m-plane. On the other hand, if there is a difference in the rate of advancement of the atomic steps due to some reason (e.g., temperature variations or raw material concentration variations), fast steps tend to advance faster and slow steps tend to advance slower. This can lead to fast steps catching up with slower steps and merging, resulting in large step-like bunching. Giant steps, where bunching occurs, can also occur. Bunching and giant steps can lead to uneven incorporation of dopants and other impurities, resulting in variations in the properties of epitaxial films. Furthermore, when the m-plane is cleaved during the device fabrication process, irregularities in the cleavage plane can occur, leading to variations in device properties.

[0047] On the other hand, in c-plane group III nitride semiconductor substrates off the a-axis, atomic steps tend to form irregularities when viewed from above. This is because although the atomic steps progress in the a-axis direction, the a-plane is difficult to develop due to the fast crystal growth rate along the a-axis. Because the a-plane does not develop, the atomic steps do not progress in a uniform direction, and a wavy morphology accompanied by undulations is likely to occur. When this wavy morphology occurs, the surface irregularities of the epitaxial film become significant, hindering device fabrication.

[0048] As described above, although the formation of a smooth epitaxial film can be expected on an off-m-axis substrate, the crystal growth conditions must be strictly controlled to prevent bunching. On the other hand, on an off-a-axis substrate, the direction of step progression is unstable, and undulations tend to occur in the epitaxial film.

[0049] In the Group III nitride semiconductor substrate according to the embodiment of the present invention, as described above, the c-plane is inclined with respect to the orientation of the first surface, and the direction of the inclination is between the <1-100> and <11-20> directions. This positions the off-orientation between the m-axis and the a-axis. First, because the direction of atomic steps is not parallel to the m-axis, the atomic steps are not perfectly linear. Instead, cranks, which serve as the starting points for step advancement, are formed. This reduces the unevenness of the step advancement speed and bunching. Furthermore, because the direction of atomic steps has a constant inclination with respect to the a-axis, they are less likely to meander, resulting in less wavy morphology. Therefore, it is possible to form an epitaxial film with minimal unevenness over a wider range of conditions.

[0050] According to the present invention, as described above, it is possible to provide a Group III element nitride semiconductor substrate that is less susceptible to in-plane morphology disturbances and characteristic variations, thereby improving the yield of devices fabricated using this substrate. For example, when fabricating a power device using the Group III element nitride semiconductor substrate of the present invention, it is possible to eliminate variations in withstand voltage and improve the yield.

[0051] Furthermore, according to the present invention, the characteristic crystal growth behavior in the a-axis and m-axis directions is alleviated, making it possible to provide a Group III nitride semiconductor substrate in which waviness, bunching, and giant steps on the surface of the epitaxial film are suppressed.

[0052] Furthermore, the present invention enables the fabrication of lasers with low loss and excellent characteristics. With an off-m-axis substrate, bunching and giant steps cause irregularities in the cleavage planes, preventing the laser end faces from being formed parallel, resulting in loss. On the other hand, with an off-a-axis substrate, undulations due to wavy morphology occur in the epitaxial film, resulting in loss within the laser waveguide. However, with the present invention, the laser end faces can be formed approximately parallel, and undulations in the epitaxial film are suppressed, enabling the fabrication of lasers with low loss and excellent characteristics.

[0053] 4 is a schematic cross-sectional view of a group III element nitride semiconductor substrate according to an embodiment of the present invention. As shown in FIG. 4, a group III element nitride semiconductor substrate 100 according to an embodiment of the present invention typically has a primary surface (group III element polar surface) 10 and a back surface (nitrogen polar surface) 20. The group III element nitride semiconductor substrate 100 according to an embodiment of the present invention may also have a side surface 30.

[0054] The end portion of the Group III element nitride semiconductor substrate according to the embodiment of the present invention may have any suitable shape as long as the effect of the present invention is not impaired. Examples of the end portion of the Group III element nitride semiconductor substrate according to the embodiment of the present invention include a shape chamfered so that the chamfered portions on the main surface side and the back surface side are flat, a shape chamfered so that the chamfered portions on the main surface side and the back surface side are rounded, a shape chamfered so that only the chamfered portion on the main surface side of the end portion is flat, and a shape chamfered so that only the chamfered portion on the back surface side of the end portion is flat.

[0055] When the edge of the group III nitride semiconductor substrate according to the embodiment of the present invention is chamfered, the chamfered portion may be provided along the entire circumference of the outer periphery, or may be provided along only a portion of the outer periphery.

[0056] From the viewpoint of obtaining semiconductor devices in which the device layer is epitaxially grown to produce devices with good characteristics and little variation in device characteristics between devices, it is preferable that the main surface be a surface from which the process-affected layer has been substantially removed and which has small surface roughness in the microscopic region.

[0057] From the above viewpoints, the surface roughness (Ra) of the main surface measured using an AFM over a 90 μm square is preferably 1.0 nm or less, more preferably 0.6 nm or less, and even more preferably 0.3 nm or less, and the surface roughness (Ra) of the main surface measured using an AFM over a 10 μm square is preferably 0.6 nm or less, more preferably 0.4 nm or less, and even more preferably 0.2 nm or less.

[0058] The Group III nitride semiconductor substrate according to the embodiment of the present invention can be manufactured by any appropriate method as long as the effects of the present invention are not impaired. In the following, a preferred method for manufacturing the Group III nitride semiconductor substrate according to the embodiment of the present invention will be described in terms of further demonstrating the effects of the present invention.

[0059] 5(a), a Group III element nitride semiconductor substrate according to an embodiment of the present invention is typically formed by forming a seed crystal film 2 on a major surface 1a of a base substrate 1, and then forming a Group III element nitride layer 3 on a Group III element polar surface 2a of the seed crystal film 2. Next, the Group III element nitride layer (seed crystal film 2+Group III element nitride layer 3) that will become a free-standing substrate is separated from the base substrate 1, thereby obtaining a free-standing substrate 100' having a major surface 10' and a back surface 20'.

[0060] Any suitable material can be used as the base substrate as long as it does not impair the effects of the present invention. Examples of such materials include sapphire, crystal-oriented alumina, gallium oxide, and Al. x Ga 1-x Examples include N(0≦x≦1), GaAs, and SiC.

[0061] In order to efficiently obtain a Group III nitride semiconductor substrate according to an embodiment of the present invention, it is preferable that the off-orientation of the base substrate be such that the off-orientation of the Group III nitride crystal grown on the base substrate is between the <1-100> direction (m-axis direction) and the <11-20> direction (a-axis direction). In the Group III nitride semiconductor substrate according to an embodiment of the present invention, in order to further demonstrate the effects of the present invention, the tilt direction is preferably within a range of ±15° from the midpoint between the <1-100> and <11-20> directions of the Group III nitride, more preferably within a range of ±10° from the midpoint between the <1-100> and <11-20> directions of the Group III nitride, and even more preferably within a range of ±5° from the midpoint between the <1-100> and <11-20> directions of the Group III nitride. Here, the direction intermediate between the <1-100> direction and the <11-20> direction is the direction at an angle of 45° from the <1-100> direction toward the <11-20> direction (or vice versa).

[0062] Methods for obtaining a base substrate whose main surface is inclined and whose off-orientation is between the <1-100> direction (m-axis direction) and the <11-20> direction (a-axis direction) of the Group III element nitride crystal grown on the base substrate include, for example, a method in which a cylindrical boule is produced from a single crystal of the material of the base substrate grown by a pulling method (Czochralski method) in the c-axis direction, and the orientation tilted a specified amount from the c-axis direction toward the design direction is determined using X-ray diffraction, and the boule is fixed to a specified jig and cut out with a wire saw; a method in which the boule is produced by hollowing out a large single crystal grown by the chiroporus method; and a method in which a plate-shaped single crystal is produced by the EFG method using a seed crystal with a specified orientation and then shaped.

[0063] However, even when a base substrate having an inclined primary surface and an off-orientation between the <1-100> direction (m-axis direction) and the <11-20> direction (a-axis direction) of the Group III nitride crystal grown on the base substrate is not used, a Group III nitride semiconductor substrate according to an embodiment of the present invention can be efficiently obtained by processing a freestanding substrate (Group III nitride crystal layer) fabricated off the a-axis or off the m-axis into a disk shape by tilting it to a desired orientation.

[0064] Any suitable material can be used as the material for the seed crystal film as long as it does not impair the effects of the present invention. x Ga 1-x N(0≦x≦1) and In x Ga 1-x N (0≦x≦1), preferably gallium nitride. The seed crystal film is more preferably made of gallium nitride, which exhibits a yellow luminescence effect when observed under a fluorescent microscope. The yellow luminescence is a peak (yellow luminescence (YL) or yellow band (YB)) that appears in the range of 2.2 to 2.5 eV in addition to band-to-band exciton transition (UV).

[0065] As a method for forming the seed crystal film, any appropriate formation method can be adopted as long as it does not impair the effects of the present invention. Examples of such formation methods include vapor phase growth methods, and preferred examples include metal-organic chemical vapor deposition (MOCVD), hydride vapor phase epitaxy (HVPE), pulsed electron beam deposition (PXD), MBE, and sublimation. Among these, metal-organic chemical vapor deposition (MOCVD) is more preferred as a method for forming the seed crystal film.

[0066] The seed crystal film is preferably formed by MOCVD by depositing a low-temperature grown buffer layer to a thickness of 20 nm to 50 nm at 450°C to 550°C, and then laminating a film to a thickness of 2 μm to 4 μm at 1000°C to 1200°C.

[0067] As a method for forming the Group III element nitride crystal layer, any suitable method can be adopted as long as it produces a crystal orientation that roughly matches the crystal orientation of the seed crystal film, without impairing the effects of the present invention. Examples of such a method include vapor phase growth methods such as metal-organic chemical vapor deposition (MOCVD), hydride vapor phase epitaxy (HVPE), pulsed photoexcitation deposition (PXD), molecular beam epitaxy (MBE), and sublimation; liquid phase growth methods such as Na flux, ammonothermal, hydrothermal, and sol-gel; powder growth methods (solid phase growth methods) that utilize solid phase growth (grain growth) of powder; and combinations thereof.

[0068] When the Na flux method is employed as a method for forming a Group III element nitride crystal layer, it is preferable to carry out the Na flux method in accordance with the manufacturing method described in Japanese Patent No. 5244628, by appropriately adjusting the conditions etc. so as to more effectively exhibit the effects of the present invention.

[0069] Next, the Group III element nitride crystal layer is separated from the base substrate, thereby obtaining a free-standing substrate including the Group III element nitride crystal layer.

[0070] Any suitable method can be used to separate the Group III element nitride crystal layer from the starting substrate as long as it does not impair the effects of the present invention. Examples of such methods include: spontaneous separation of the Group III element nitride crystal layer from the starting substrate using differential thermal contraction during a cooling step after growth of the Group III element nitride crystal layer; chemical etching to separate the Group III element nitride crystal layer from the starting substrate; laser lift-off, as shown in Figure 5(a), in which a laser beam is irradiated from the back surface 1b of the starting substrate 1 in the direction of arrow A; and grinding to separate the Group III element nitride crystal layer from the starting substrate. Alternatively, a freestanding substrate including the Group III element nitride crystal layer may be obtained by slicing the Group III element nitride crystal layer using a wire saw or the like.

[0071] Next, the outer periphery of the free-standing substrate is ground to form a circular shape with a desired diameter.

[0072] The size of the freestanding substrate may be any appropriate size as long as it does not impair the effects of the present invention, such as 25 mm (about 1 inch), 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), and 295 to 305 mm (about 12 inches).

[0073] Next, the main surface and / or the back surface are removed by surface processing such as grinding, lapping, or polishing to obtain a free-standing substrate that is thinned and flattened to a desired thickness.

[0074] When performing surface processing such as grinding, lapping, or polishing, the free-standing substrate is usually attached to a processing platen using wax, etc. At this time, the pressure with which the free-standing substrate is attached to the processing platen, specifically, the pressure applied to the free-standing substrate when attaching it to the processing platen, is appropriately adjusted.

[0075] The thickness of the free-standing substrate after polishing (if the thickness is not constant, the thickness at the thickest point) is preferably 300 μm to 1000 μm.

[0076] If necessary, the outer peripheral edge of the free-standing substrate is chamfered by grinding. If a process-affected layer remains on the main surface, the process-affected layer is substantially removed. Furthermore, if residual stress resulting from the process-affected layer remains on the back surface, the residual stress is removed, and finally, a group III nitride semiconductor substrate 100 according to an embodiment of the present invention is obtained.

[0077] In the Group III nitride semiconductor substrate according to the embodiment of the present invention, chamfering can be performed by any appropriate chamfering method as long as the effects of the present invention are not impaired. Examples of such chamfering methods include grinding using a diamond grindstone, polishing using tape, and chemical mechanical polishing (CMP) using a slurry such as colloidal silica and a nonwoven polishing pad.

[0078] The obtained Group III element nitride semiconductor substrate 100 can have crystals epitaxially grown on its main surface (Group III element polarity surface) 10, and as shown in Figure 5(c), a functional layer 4 is formed to obtain a functional device 5. 20 is the back surface (nitrogen polarity surface).

[0079] Examples of epitaxial crystals grown on the obtained Group III element nitride semiconductor substrate include gallium nitride, aluminum nitride, indium nitride, and mixed crystals thereof. Specific examples of such epitaxial crystals include GaN, AlN, InN, Ga x Al 1-x N(1>x>0), Ga x In1-x N(1>x>0), Al x In 1-x N(1>x>0), Ga x Al y In z N (1>x>0, 1>y>0, x+y+z=1). Examples of functional layers to be provided on the obtained Group III element nitride semiconductor substrate include a rectifying element layer, a switching element, and a power semiconductor layer, in addition to a light-emitting layer. After providing a functional layer on the Group III element polarity surface of the obtained Group III element nitride semiconductor substrate, the nitrogen polarity surface can be processed, for example, by grinding or polishing, to reduce the thickness and thickness distribution of the free-standing substrate. [Example]

[0080] The present invention will be specifically described below using examples, but the present invention is not limited to these examples. The test and evaluation methods used in the examples are as follows. The term "parts" means "parts by weight" unless otherwise specified, and the term "%" means "% by weight" unless otherwise specified.

[0081] <Measurement of off-angle and off-orientation> Measurements were carried out using an energy-dispersive X-ray diffractometer (D2 CRYSO, manufactured by Bruker AXS). For the measurement of the off-angle, the diffraction peak from the (0002) plane of a group III nitride semiconductor with a hexagonal unit cell was used. For the measurement of the crystal orientation and the off-orientation, the diffraction peak from the (10-15) plane of a group III nitride semiconductor with a hexagonal unit cell was used. The angular relationship between the direction in which the off-angle is maximized, i.e., the inclination direction of the c-plane with respect to the main plane, and the crystal orientation within the c-plane was measured, and the <1-100> direction and the <11-20> direction were defined so as to sandwich the direction in which the off-angle is maximized. These directions can also be described as [1-100] and [11-20], respectively, and each is parallel to the m-axis and the a-axis and perpendicular to each other. When multiple <1-100> directions and <11-20> directions are defined so as to sandwich the direction in which the off-angle is maximized, each direction was defined so that the direction in which the off-angle is maximized is closer to the middle of the two directions.

[0082] <Observation of bunching and wavy morphology> The surface morphology of the epitaxial film was observed at magnifications of 100 times, 200 times, and 500 times using a differential interference microscope (DM8000M, manufactured by Leica).

[0083] <Observation by AFM and measurement of surface roughness (Ra)> Performed using an AFM (Atomic Force Microscope). The measurement range was 90 μm square or 10 μm square.

[0084] [Example 1] A cylindrical boule was produced from a sapphire single crystal grown by the c-axis pulling method (Chokralski method), and an orientation inclined by a predetermined amount from the c-axis direction to the designed direction was derived by X-ray diffraction method and fixed to a predetermined jig, and a 6-inch sapphire substrate with a main plane orientation inclined 0.5° from the c-plane was produced by a method of cutting out with a wire saw. The inclination direction of the main plane (off-angle orientation) was set to be exactly in the middle between the a-axis and the m-axis. Specifically, it is the direction rotated 45° from the <11-20> direction within the c-plane to the <1-100> direction. A seed crystal film made of gallium nitride was formed to a thickness of 2 μm on the prepared sapphire substrate by MOCVD to obtain a seed crystal substrate. The obtained seed crystal substrate was placed in an alumina crucible in a glove box under a nitrogen atmosphere. Next, metallic gallium and metallic sodium were filled into the crucible so that Ga / (Ga+Na) (mol%) = 15 mol%. This crucible was then placed in a heat-resistant metal container and placed on a rotatable platform for the crystal growth furnace. The crystal growth furnace was heated and pressurized to 870°C and 4.0 MPa, and then held for 100 hours while rotating the container to stir the solution and grow crystals. The furnace was then gradually cooled to room temperature and depressurized to atmospheric pressure. The growth container was then removed from the crystal growth furnace. The solidified metallic sodium in the crucible was washed away with alcohol, and a crack-free gallium nitride crystal layer (thickness 1 mm) was obtained on the seed crystal substrate. The base substrate was peeled off by the LLO (laser lift-off) method, and the gallium nitride crystal layer was separated to obtain a freestanding gallium nitride substrate. The outer periphery of the gallium nitride freestanding substrate was ground to form a circular gallium nitride freestanding substrate with a diameter of 100 mm. The free-standing substrate was attached to a ceramic processing platen using wax, and the Ga polarity surface was ground and lapped. For final finishing, the surface was mirror-finished using diamond abrasive grains with a particle size of 0.1 μm. Next, the Ga-polar surface of the freestanding substrate was turned over and fixed to a ceramic processing platen using wax, and the Nitrogen-polar surface was ground and lapped. For the final finish, diamond abrasive grains with a particle size of 0.1 μm were used to achieve a mirror finish. In this way, a wafer (1) was produced as a freestanding gallium nitride substrate. The thickness of the wafer (1) was 500 μm. The off-orientation of the obtained wafer (1) was evaluated by X-ray diffraction. Measurement of the off-orientation and off-angle at the center of the substrate revealed that it was tilted by 0.6° midway between the a-axis and the m-axis. The wafer (1) was used to form an epitaxial film by MOCVD, with the film formation conditions (temperature, source gas flow rate, and time) adjusted so that the epitaxial film had a thickness of 1 μm. The surface of the epitaxial film was observed using a differential interference contrast microscope. Observation at 200x magnification revealed no obvious bunching or wavy morphology on the epitaxial film surface. It was then evaluated using an AFM. Evaluation over a 90 μm x 90 μm field of view revealed uniform contrast, with an Ra of 0.9 nm. Furthermore, observation over a 10 μm x 10 μm field of view revealed atomic steps on the surface, with an Ra of 0.2 nm.

[0085] Example 2 A wafer (2) was produced in the same manner as in Example 1, except that the tilt direction (off-angle orientation) of the primary surface of the sapphire substrate was tilted in a direction rotated 37.5° from the <11-20> direction to the <1-100> direction within the c-plane. Using the prepared wafer (2), an epitaxial film was formed in the same manner as in Example 1. The surface of the epitaxial film was observed using a differential interference microscope. When observed at 200x magnification, no clear bunching or wavy morphology was observed on the epitaxial film surface. Next, it was evaluated using an AFM. When evaluated over a 90μm x 90μm field of view, uniform contrast was obtained, and Ra was 1.0nm.

[0086] Example 3 A wafer (3) was produced in the same manner as in Example 1, except that the tilt direction (off-angle orientation) of the primary surface of the sapphire substrate was tilted in a direction rotated 57.5° from the <11-20> direction to the <1-100> direction within the c-plane. Using the prepared wafer (3), an epitaxial film was formed in the same manner as in Example 1. The surface of the epitaxial film was observed using a differential interference microscope. When observed at 200x magnification, no clear bunching or wavy morphology was observed on the epitaxial film surface, but gentle unevenness was observed. Next, the surface was evaluated using an AFM. When evaluated over a 90μm x 90μm field of view, a faint contrast was observed, with an Ra of 1.2nm.

[0087] Comparative Example 1 A wafer (C1) was produced in the same manner as in Example 1, except that the tilt direction (off-angle direction) of the main surface of the sapphire substrate was tilted toward the a-axis direction. An epitaxial film was formed in the same manner as in Example 1 using the produced wafer (C1). The surface of the epitaxial film was observed using a differential interference microscope. When observed at 200x magnification, a wavy morphology accompanied by undulations was observed on the epitaxial film surface. Next, the surface was evaluated using an AFM. When evaluated in a 90μm x 90μm field of view, a contrast with shading corresponding to the undulations was obtained, and Ra was 1.3nm.

[0088] Comparative Example 2 A wafer (C2) was produced in the same manner as in Example 1, except that the tilt direction (off-angle orientation) of the main surface of the sapphire substrate was tilted toward the m-axis direction. Using the prepared GaN substrate, an epitaxial film was formed in the same manner as in Example 1. The surface of the epitaxial film was observed with a differential interference microscope. Giant steps were observed at 200x magnification. The surface was then evaluated using an AFM. Evaluation in a 90μm x 90μm field of view revealed contrast with shading corresponding to bunching, with an Ra of 2.1nm. [Industrial Applicability]

[0089] The group III nitride semiconductor substrate according to the embodiment of the present invention can be used as a substrate for various semiconductor devices. [Explanation of symbols]

[0090] 100 Group III element nitride semiconductor substrate 100' freestanding board 1. Base substrate 1a Main surface of base substrate 1 1b Back surface of base substrate 1 2 Seed crystal film 2a Group III element polar surface of seed crystal film 2 3 Group III element nitride layer 4 Functional Layer 5 Functional elements 10 Main Surface 10' Main surface 20 Back side 20' back side 30 Side

Claims

1. A group III element nitride semiconductor substrate having a first surface and a second surface, the Group III element nitride semiconductor substrate is a free-standing substrate having a thickness of 300 μm to 1000 μm; the c-plane is inclined with respect to the orientation of the first surface, the angle of inclination is 0.2° to 0.8°; The tilt direction is within a range of ±12.5° from the midpoint between the <1-100> direction and the <11-20> direction. Group III element nitride semiconductor substrate.

2. 2. The Group III element nitride semiconductor substrate according to claim 1, wherein the tilt direction is within a range of ±7.5° from a direction midway between the <1-100> direction and the <11-20> direction.

3. 3. The Group III nitride semiconductor substrate according to claim 1, wherein an area of the region occupied by the inclined direction is 50% or more of the entire area of the first surface.

4. A group III element nitride semiconductor substrate having a first surface and a second surface, the Group III element nitride semiconductor substrate is a free-standing substrate having a thickness of 300 μm to 1000 μm; the c-plane is inclined with respect to the orientation of the first surface, the angle of inclination is 0.2° to 0.8°; It has an orientation flat parallel to the <11-20> direction, The tilt direction is within a range of +45°±12.5°, −45°±12.5°, +135°±12.5°, or −135°±12.5° from the <1-100> direction. Group III element nitride semiconductor substrate.

5. 5. The Group III element nitride semiconductor substrate according to claim 4, wherein the tilt direction is within a range of +45°±7.5°, −45°±7.5°, +135°±7.5°, or −135°±7.5° from the <1-100> direction.

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