AlN single crystal substrate and device

Abstract

Provided is an AlN single crystal substrate which is not susceptible to edge breakage when processed (grinding, polishing, cutting, etc.). The present invention relates to an AlN single crystal substrate comprising two layers composed of one AlN single crystal and capable of being divided into a first layer and a second layer in the thickness direction from the perspective of impurity concentration, the AlN single crystal substrate containing boron as an impurity, the boron atom concentration in the first layer being 9.4 * 1018 cm <-3 > or more, and the boron atom concentration in the second layer being 9.4 * 1018 cm <-3 > or more, and the boron atom concentration in the third layer being 9.4 * 1018 cm <-3 > or more. The boron atom concentration in the first layer is 10 times or more of the boron atom concentration in the second layer.

Description

TECHNICAL FIELD

[0001] The present application relates to an AlN single crystal substrate and a device having the AlN single crystal substrate. BACKGROUND

[0002] In recent years, an aluminum nitride (AlN) single crystal has attracted attention as a base substrate for a deep ultraviolet light emitting element using an AlN-based semiconductor. For example, AlN, AlGaN, or the like is used as the AlN-based semiconductor. These AlN-based semiconductors have a direct transition type energy band structure, and thus are suitable for light emitting devices, and can be applied to LEDs (Light Emitting Diodes) or LDs (Laser Diodes) in a deep ultraviolet region which can be used for sterilization or the like.

[0003] In such a light emitting device, in order to achieve high transmittance in the ultraviolet region, it is desirable that the impurity concentration in the base substrate be low. Patent Document 1 (Japanese Patent No. 6080148) discloses an AlN single crystal containing oxygen atoms and carbon atoms, in which the concentration of oxygen atoms is 5 x 10 17 cm －3 and 5 x 10 18 cm －3 Hereinafter, the concentration of carbon atoms is 4 x 10 17 cm －3 and 4 x 10 18 cm －3 Hereinafter, the concentration of oxygen atoms is higher than the concentration of carbon atoms. It is described in this document that in order to reduce the amount of impurities, highly controlled or special devices are required at the time of single crystal growth, and at this time, the above-described single crystal in which the concentrations of oxygen atoms and carbon atoms are controlled so that the ultraviolet light transmittance becomes good. In addition, Patent Document 2 (Japanese Patent Application Publication No. 2009-78971) discloses an AlN single crystal substrate having a composition of AlN, a total impurity density of 1 x 10 17 cm －3 and an absorption coefficient of 50 cm －1 or less in a full wavelength range of 350 to 780 nm.

[0004] Further, regarding impurities in an AlN single crystal, Patent Literature 3 (Japanese Patent No. 4811082) discloses an n-type AlN crystal which is an n-type AlN crystal having a structure in which a part of Al atoms of an AlN crystal is substituted with a group IIIa element or / and a group IIIb element, and one of adjacent N atoms is simultaneously substituted with an O atom, wherein the group IIIa element or / and the group IIIb element is one or more elements selected from the group consisting of Y, Sc, La, Ce, and Ga. In this document, a relationship between a total concentration of the group IIIa element or / and the group IIIb element and an oxygen concentration is described. In addition, Patent Literature 4 (Japanese Patent No. 6932995) discloses an AlN single crystal having a wurtzite crystal structure, and a content of boron is 0.5 mass ppm or more and 251 mass ppm or less.

[0005] Prior Art Documents

[0006] Patent Literature

[0007] Patent Literature 1: Japanese Patent No. 6080148

[0008] Patent Literature 2: Japanese Patent Application Laid-Open No. 2009-78971

[0009] Patent Literature 3: Japanese Patent No. 4811082

[0010] Patent Literature 4: Japanese Patent No. 6932995 SUMMARY

[0011] As described above, in order to exhibit a higher deep ultraviolet light transmittance or a controlled property of an n-type conduction of an AlN single crystal, it is considered to control the amount relationship of impurities present in the AlN single crystal or the like. However, the AlN single crystal substrate like that disclosed in Patent Literatures 1 to 4 can be likely to have a chipping (a defect such as a notch or a crack) when being processed (grinding, polishing, cutting, or the like), and there is room for improvement in terms of improving a yield of a finished product. Therefore, it is desired to suppress the chipping of the AlN single crystal substrate when the AlN single crystal substrate is processed.

[0012] The inventors of the present application have recently obtained the following insight: an AlN single crystal substrate composed of two layers of an entire body composed of one AlN single crystal in which a concentration of boron atoms as impurities is controlled is less likely to have a chipping when the AlN single crystal substrate is processed (grinding, polishing, cutting, or the like).

[0013] Therefore, an object of the present application is to provide an AlN single crystal substrate which is less likely to have a chipping when being processed (grinding, polishing, cutting, or the like).

[0014] According to the present application, the following solutions are provided.

[0015] [Scheme 1]

[0016] An AlN single crystal substrate which is an AlN single crystal substrate composed of two layers of a first layer and a second layer that can be distinguished in a thickness direction from the viewpoint of impurity concentration, the entire substrate being composed of one AlN single crystal,

[0017] The AlN single crystal substrate is characterized in that,

[0018] The AlN single crystal substrate contains boron as an impurity,

[0019] The boron atom concentration in the first layer is 9.4 x 10 18 cm －3 or more,

[0020] The boron atom concentration in the first layer is 10 times or more the boron atom concentration in the second layer.

[0021] [Scheme 2]

[0022] The AlN single crystal substrate according to Scheme 1 is characterized in that,

[0023] The carbon atom concentration in the second layer is 10 times or more the carbon atom concentration in the first layer.

[0024] [Scheme 3]

[0025] The AlN single crystal substrate according to Scheme 1 or 2 is characterized in that,

[0026] The oxygen atom concentration in the second layer is 2 times or more the oxygen atom concentration in the first layer.

[0027] [Scheme 4]

[0028] The AlN single crystal substrate according to any one of Schemes 1 to 3 is characterized in that,

[0029] The boron atom concentration in the first layer is 9.4 x 10 18 to 8.6 x 10 19 cm －3 , and the boron atom concentration in the second layer is less than 7.0 x 10 17 cm －3 .

[0030] [Scheme 5]

[0031] The AlN single crystal substrate according to any one of Schemes 1 to 4 is characterized in that,

[0032] The carbon atom concentration in the first layer is 1.0 x 10 18 to 5.0 x 10 19cm －3 and the oxygen atom concentration in the second layer is 1.0 x 10 19 ~ 5.0 x 10 20 cm －3 .

[0033] [Scheme 6]

[0034] The AlN single crystal substrate according to any one of the schemes 1 to 5, characterized in that

[0035] the oxygen atom concentration in the first layer is 1.0 x 10 18 ~ 7.0 x 10 19 cm －3 and the oxygen atom concentration in the second layer is 1.0 x 10 19 ~ 8.0 x 10 20 cm －3 .

[0036] [Scheme 7]

[0037] The AlN single crystal substrate according to any one of the schemes 1 to 6, characterized in that

[0038] the area of the AlN single crystal substrate is 75 ~ 18500 mm 2 and the thickness of the AlN single crystal substrate is 0.10 ~ 1.00 mm.

[0039] [Scheme 8]

[0040] The AlN single crystal substrate according to any one of the schemes 1 to 7, characterized in that

[0041] the thickness of the second layer is larger than the thickness of the first layer.

[0042] [Scheme 9]

[0043] The AlN single crystal substrate according to any one of the schemes 1 to 8, characterized in that

[0044] the surface of the first layer is an Al-polar surface and the surface of the second layer is an N-polar surface.

[0045] [Scheme 10]

[0046] A device provided with the AlN single crystal substrate according to any one of the schemes 1 to 9. BRIEF DESCRIPTION OF DRAWINGS

[0047] Figure 1 is a cross-sectional diagram showing an example of the configuration of a heat treatment device for producing an AlN raw material powder.

[0048] Figure 2is a cross-sectional diagram showing an example of a configuration of a heat treatment apparatus for producing an AlN raw material powder.

[0049] Figure 3 is a cross-sectional diagram showing a configuration of a film formation apparatus used in a sublimation method. DETAILED DESCRIPTION

[0050] AlN single crystal substrate

[0051] The AlN single crystal substrate of the present application is an AlN single crystal substrate of two layers composed of one AlN single crystal, which can be distinguished into a first layer and a second layer in the thickness direction from the viewpoint of impurity concentration. In the AlN single crystal substrate, boron is contained as an impurity. At this time, the boron atom concentration in the first layer is 9.4 x 1018atoms / cm3or more, and the boron atom concentration in the second layer is 9.4 x 1017atoms / cm3or less. 18 cm －3 The above. In addition, the boron atom concentration in the first layer is 10 times or more the boron atom concentration in the second layer. By making the AlN single crystal substrate of two layers composed of one AlN single crystal in which the concentration of boron atoms as an impurity is controlled as described above, chipping is less likely to occur when the AlN single crystal substrate is processed (ground, polished, cut, etc.). Therefore, by processing the AlN single crystal substrate, an AlN single crystal substrate can be manufactured at a high yield. That is, as described above, the conventional AlN single crystal substrate can have chipping occur when processed (ground, polished, cut, etc.), and has room for improvement in terms of improving the yield. Accordingly, the AlN single crystal substrate of the present application can well eliminate the above-described problem.

[0052] Here, "an AlN single crystal substrate consisting entirely of a single AlN single crystal, distinguishable as a first layer and a second layer in the thickness direction from the perspective of impurity concentration" refers to an AlN single crystal substrate that, while difficult to discern as two layers when observing a cross section, can be easily and conceptually distinguished as two layers (these two layered regions may be referred to as the first region and the second region, as appropriate) based on the level of impurity concentration measured using known methods. Furthermore, it refers to an AlN single crystal substrate that, while distinguishable as two layers based on the level of impurity concentration, lacks grain boundaries between the layers, allowing the two layers to be collectively identified as a single crystal. Here, the AlN single crystal substrate can be distinguished into two layers based on the level of impurity concentration by measuring the impurity concentration as follows. For example, the first and second layers can be distinguished by measuring the impurity concentration in the thickness direction of the AlN single crystal substrate using dynamic secondary ion mass spectrometry (D-SIMS). Specifically, after measuring the impurity concentration on the surface of the AlN single crystal substrate, the single crystal substrate is polished to a predetermined thickness (for example, 1 / 10 of the thickness of the single crystal substrate, or 30 μm), and the operation of measuring the impurity concentration on the surface caused by polishing is repeated to obtain the distribution of the impurity concentration in the thickness direction. In addition, the surface with the highest impurity concentration inside the AlN single crystal substrate is determined. Next, starting from the surface with the highest impurity concentration, the area with an impurity concentration lower than 1 / 10 of the impurity concentration is set as the second layer, and the boundary between the first layer and the second layer is determined. Based on this, the first and second layers of the AlN single crystal substrate can be distinguished. At this time, the impurity concentration of the first layer adopts the average value of the impurity concentration measured in the area of ​​the first layer, and the impurity concentration of the second layer also adopts the average value of the impurity concentration measured in the area of ​​the second layer. It should be noted that as the impurity concentration used to distinguish the first and second layers, it is preferable to measure the boron atom concentration.

[0053] The area and thickness of the AlN single crystal substrate are not particularly limited as long as the concentration of boron atoms as impurities is controlled as described above and the substrate is composed of two layers of a single AlN single crystal. Therefore, the area of ​​the AlN single crystal substrate is typically 75 to 18,500 mm. 2 , and the thickness of the AlN single crystal substrate is typically 0.10 to 1.00 mm.

[0054] When boron is contained as an impurity in the AlN single crystal substrate, the boron atomic concentration in the first layer is 9.4×10 18 cm －3 Above, preferably 9.4×10 18 ~8.6×10 19 cm －3 , more preferably 9.7×10 18 ~6.0×1019 cm －3 , more preferably 1.0×10 19 ~4.5×10 19 cm －3 On the other hand, the boron atomic concentration in the second layer is preferably less than 7.0×10 17 cm －3 By making the boron content of the first layer as an impurity higher than that of the second layer, an AlN single crystal substrate can be made that is less likely to chip during processing. Therefore, it is preferred that the boron atomic concentration in the first layer is 9.4×10 18 ~8.6×10 19 cm －3 , and the boron atomic concentration in the second layer is less than 7.0×10 17 cm －3 From this viewpoint, the boron atomic concentration in the first layer is 10 times or more, preferably 13 to 100 times, more preferably 20 to 80 times, and even more preferably 30 to 60 times the boron atomic concentration in the second layer.

[0055] The AlN single crystal substrate preferably contains carbon atoms as impurities. In this case, the carbon atom concentration in the first layer is preferably 1.0×10 18 ~5.0×10 19 cm －3 , more preferably 3.0×10 18 ~3.0×10 19 cm －3 , more preferably 5.0×10 18 ~1.0×10 19 cm －3 On the other hand, the carbon atom concentration in the second layer is preferably 1.0×10 19 ~5.0×10 20 cm －3 , more preferably 5.0×10 19 ~2.0×10 20 cm －3 , more preferably 7.0×10 19 ~1.0×10 20 cm －3 By setting the carbon atom concentration in each of the first layer and the second layer to the above range, an AlN single crystal substrate that is less likely to chip during processing can be produced. Therefore, it is preferred that the carbon atom concentration in the first layer is 1.0×10 18 ~5.0×10 19 cm －3 , and the carbon atom concentration in the second layer is 1.0×10 19 ~5.0×10 20 cm－3 .

[0056] Furthermore, the first layer preferably has a higher boron content than the second layer, while the second layer preferably has a higher carbon content than the first layer. This allows for an AlN single crystal substrate that is less susceptible to edge chipping during processing. From this perspective, the carbon atomic concentration in the second layer is preferably 10 times or more of the carbon atomic concentration in the first layer, more preferably 11 to 40 times, and even more preferably 12 to 30 times.

[0057] The AlN single crystal substrate preferably contains oxygen atoms as impurities. In this case, the oxygen atom concentration in the first layer is preferably 1.0×10 18 ~7.0×10 19 cm －3 , more preferably 4.0×10 18 ~5.0×10 19 cm －3 , more preferably 9.0×10 18 ~3.0×10 19 cm －3 On the other hand, the oxygen atomic concentration in the second layer is preferably 1.0×10 19 ~8.0×10 20 cm －3 , more preferably 1.5×10 19 ~4.0×10 20 cm －3 , more preferably 1.9×10 19 ~1.0×10 20 cm －3 By setting the oxygen atomic concentration in each of the first layer and the second layer to the above range, an AlN single crystal substrate that is less likely to chip during processing can be produced. Therefore, it is preferred that the oxygen atomic concentration in the first layer is 1.0×10 18 ~7.0×10 19 cm －3 , and the oxygen atomic concentration in the second layer is 1.0×10 19 ~8.0×10 20 cm －3 .

[0058] Furthermore, the first layer preferably has a higher boron content than the second layer, while the second layer preferably has a higher oxygen content than the first layer. This allows for an AlN single crystal substrate that is less susceptible to edge chipping during processing. From this perspective, the oxygen atomic concentration in the second layer is preferably at least twice the oxygen atomic concentration in the first layer, more preferably 3 to 20 times, and even more preferably 4 to 10 times.

[0059] As described above, by controlling the impurity concentration of each of the first layer and the second layer constituting the AlN single crystal substrate, an AlN single crystal substrate that is less likely to be chipped when processed can be produced. The thickness of each of the first layer and the second layer is not particularly limited, and the thickness of the second layer is preferably greater than the thickness of the first layer. In addition, it is preferable that the surface of the AlN single crystal substrate (the surface of the first layer) be an Al-polar surface and the back surface of the AlN single crystal substrate (the surface of the second layer) be an N-polar surface.

[0060] The first layer and the second layer constituting the AlN single crystal substrate are AlN single crystals, and can also be referred to as orientation layers. The AlN single crystal in the present application refers to a crystal that is oriented in both the c-axis direction and the a-axis direction, and includes mosaic crystals. The mosaic crystal refers to a collection of crystals in which the orientation direction of the crystal is slightly different from one or both of the c-axis and the a-axis, although the crystal does not have a clear grain boundary. An orientation layer like this has a configuration in which the crystal orientation is substantially uniform in the substantially normal direction (c-axis direction) and the in-plane direction (a-axis direction). By adopting a configuration like this, a semiconductor layer that is excellent in quality, and in particular, excellent in orientation, can be formed thereon. That is, when a semiconductor layer is formed on an orientation layer, the crystal orientation of the semiconductor layer substantially imitates the crystal orientation of the orientation layer. Therefore, the semiconductor film formed on the AlN single crystal substrate can be set as an oriented film.

[0061] In the first layer and the second layer, the AlN crystal is oriented in both the c-axis direction and the a-axis direction. The evaluation method of the orientation is not particularly limited, and for example, a known analysis method such as an EBSD (Electron Back Scatter Diffraction Patterns) method, an X-ray pole figure, or the like can be used. For example, in the case of using the EBSD method, the inverse pole figure imaging, the crystal orientation imaging of the surface (the plate surface) or the cross section orthogonal to the plate surface of the AlN single crystal layer are measured. When the following four conditions are satisfied, it can be defined that the two axes in the approximately normal line direction and the approximately plate surface direction are oriented, that is, in the obtained inverse pole figure imaging, (A) a specific azimuth (a first axis) in the approximately normal line direction of the plate surface is oriented, (B) a specific azimuth (a second axis) in the approximately plate surface direction orthogonal to the first axis is oriented; and in the obtained crystal orientation imaging, (C) the tilt angle distribution with respect to the first axis is within ±10°, and (D) the tilt angle distribution with respect to the second axis is within ±10°. In other words, in the case where the above four conditions are satisfied, it can be determined that the two axes in the c-axis and the a-axis are oriented. For example, in the case where the approximately normal line direction of the plate surface is oriented in the c-axis, the approximately plate surface direction can be oriented in a specific azimuth (for example, the a-axis) orthogonal to the c-axis. The AlN single crystal can be oriented in both the approximately normal line direction and the approximately plate surface direction, and it is preferable that the approximately normal line direction be oriented in the c-axis. When the tilt angle distribution in the approximately normal line direction and / or the approximately plate surface direction is small, the mosaicity of the AlN single crystal becomes small, and the closer to zero, the closer to a perfect single crystal. Therefore, from the viewpoint of the crystallinity of the AlN single crystal, it is preferable that the tilt angle distribution be small in both the approximately normal line direction and the approximately plate surface direction, for example, it is preferable to be within ±5°, and more preferably within ±3°.

[0062] Manufacturing method

[0063] With respect to the AlN single crystal substrate of the present application, as long as the boron atom concentration in the first layer is 9.4 x 10 18 cm －3The above, and the first layer in which the boron atom concentration is 10 times or more of the boron atom concentration in the second layer, the 2-layer substrate composed of the entire AlN single crystal can be manufactured using various methods. A seed substrate can be prepared and epitaxially formed thereon, or an AlN single crystal substrate can be directly manufactured by spontaneous nucleation without using a seed substrate. In addition, the seed substrate used can be an AlN substrate used in a homoepitaxial growth manner, or a substrate other than this can be used to allow heteroepitaxial growth. The growth of the single crystal can use any of a vapor deposition method, a liquid deposition method, and a solid phase deposition method, and the AlN single crystal is preferably formed using a vapor deposition method, and then the seed substrate portion is ground and removed as needed, thereby obtaining the desired AlN single crystal substrate. As examples of the vapor deposition method, various CVD (Chemical Vapor Deposition) methods (for example, a thermal CVD method, a plasma CVD method, a MOVPE method, and the like), a sputtering method, a hydride vapor phase epitaxy (HVPE) method, a molecular beam epitaxy (MBE) method, a sublimation method, a pulsed laser deposition (PLD) method, and the like can be given, and the sublimation method or the HVPE method is preferable. As examples of the liquid deposition method, a solution growth method (for example, a flux method), and the like can be given. In addition, even if the AlN single crystal is not directly formed on the seed substrate, an AlN single crystal substrate can be obtained by a process of forming an oriented precursor layer, a process of setting the oriented precursor layer to an AlN single crystal layer by heat treatment, and a process of grinding and removing the seed substrate. As a method of forming the oriented precursor layer at this time, an AD (Aerosol Deposition) method, an HPPD (High Power Pulsed Deposition) method, and the like can be given.

[0064] Any of the above-mentioned solid phase deposition method, vapor deposition method, and liquid deposition method can use known conditions, and for example, a method of manufacturing an AlN single crystal substrate using a sublimation method will be described below. Specifically, the manufacturing is performed by (a) production of an AlN raw material powder for the second layer, (b) production of an AlN raw material powder for the first layer, (c) formation of an AlN single crystal layer, and (d) grinding and removal of a seed substrate and polishing of the surface of the AlN single crystal layer.

[0065] (a) Production of an AlN Raw Material Powder for the Second Layer

[0066] This process is a process of obtaining an AlN raw material powder for forming the second layer by heat treatment of an AlN polycrystal powder. As the AlN polycrystal powder, for example, an AlN polycrystal powder obtained by a solid phase deposition method, a vapor deposition method, or a liquid deposition method can be used. Figure 1As shown, AlN powder 12 as a raw material of AlN single crystal is disposed in the crucible 10, and heat treatment is performed in a N2 atmosphere. At this time, the crucible 16 filled with graphite powder 14 is disposed in the crucible 10 in a manner not to directly contact the AlN powder 12. The crucible 16 is sized to be receivable in the crucible 10. The pressure in the furnace of the crucible 10 is preferably 0.1 to 10 atm, and more preferably 0.5 to 5 atm. The heat treatment temperature is preferably 1900°C to 2300°C, and more preferably 2000 to 2200°C. As a preferable example of the material constituting the crucible and the crucible, mention can be made of tantalum carbide, tungsten, molybdenum, and carbon, and more preferably carbon.

[0067] (b) Production of AlN raw material powder for the first layer

[0068] This step is a step of obtaining the AlN raw material powder for the first layer formation by heat treating the AlN polycrystal powder. As shown in FIG. 1, the AlN powder 12 as a raw material of AlN single crystal is disposed in the crucible 10, and heat treatment is performed in a N2 atmosphere. At this time, the crucible 16 filled with graphite powder 14 is disposed in the crucible 10 in a manner not to directly contact the AlN powder 12. The crucible 16 is sized to be receivable in the crucible 10. The pressure in the furnace of the crucible 10 is preferably 0.1 to 10 atm, and more preferably 0.5 to 5 atm. The heat treatment temperature is preferably 1900°C to 2300°C, and more preferably 2000 to 2200°C. As a preferable example of the material constituting the crucible and the crucible, mention can be made of tantalum carbide, tungsten, molybdenum, and carbon, and more preferably carbon. Figure 2

[0069] (c) Formation of AlN single crystal layer

[0070] This step is a step of forming an AlN single crystal on a seed substrate in a crystal growth apparatus. An example of the crystal growth apparatus used in the sublimation method is shown in FIG. 2. Figure 3 . Figure 3 ​The film forming apparatus 20 shown includes a crucible 22, a heat insulating material 24 for insulating the crucible 22, and a coil 26 for heating the crucible 22 to a high temperature. The crucible 22 includes AlN raw material powder 28 at its lower portion and a seed substrate 30 at its upper portion for precipitating sublimates of the AlN raw material powder 28. The crucible 22 is pressurized under an N2 atmosphere and heated by the coil 26 to sublime the AlN raw material powder 28. The pressure is preferably 10 to 100 kPa, more preferably 20 to 90 kPa. At this time, a temperature gradient is applied so that the temperature near the seed substrate 30 at the upper portion of the crucible 22 is lower than the temperature near the AlN raw material powder 28 at the lower portion of the crucible 22. For example, the portion of the crucible 22 near the AlN raw material powder 28 is preferably heated to 1900-2250°C, more preferably 2000-2200°C. The portion of the crucible 22 near the seed substrate 30 is preferably heated to 1400-2150°C, more preferably 1500-2050°C. At this time, the temperature near the seed substrate 30 is preferably 100-500°C lower than that near the AlN raw material powder 28, more preferably 200-400°C. This heating is preferably maintained for 2-100 hours, more preferably 4-90 hours. Temperature management can be achieved by measuring the temperatures of the upper and lower portions of the crucible 22 using a radiation thermometer (not shown) through a hole in the heat insulating material 24 covering the crucible 22, and then adjusting the temperature using feedback. In this manner, by placing, for example, a SiC single crystal as the seed substrate 30 and reprecipitating AlN on its surface, an AlN single crystal layer 32 can be formed.

[0071] (d) Grinding and removal of the substrate and polishing of the AlN single crystal layer surface

[0072] This process includes a grinding step to remove the seed substrate and expose the AlN single crystal layer, and a polishing step to remove irregularities and defects on the AlN single crystal surface. Since the AlN single crystal layer produced using a SiC substrate as the seed substrate through steps (a) to (c) above contains residual SiC single crystal, grinding is performed to expose the surface of the AlN single crystal layer. Furthermore, to mirror-finish the surface of the formed AlN single crystal layer, the plate surface is smoothed using lapping with diamond abrasives, followed by polishing using chemical mechanical polishing (CMP) or other methods using colloidal silica. This method produces an AlN single crystal substrate.

[0073] Device

[0074] A device can also be produced using the AlN single crystal substrate of the present application. That is, a device provided with an AlN single crystal substrate is preferably provided. As examples of such a device, a deep ultraviolet laser diode, a deep ultraviolet diode, a power electronic device, a high frequency device, a heat sink, and the like can be given. The method of producing a device using an AlN single crystal substrate is not particularly limited, and the device can be produced using a publicly known method.

[0075] Example

[0076] The present application is further specifically described by way of the following examples.

[0077] Examples 1 to 9

[0078] (1) Production of AlN single crystal substrate

[0079] (1a) Production of AlN raw material powder for second layer

[0080] The configuration of a heat treatment apparatus for producing an AlN raw material powder is shown in Figure 1 . As shown in Figure 1 , commercially available AlN powder 12 having an average particle diameter of 1 μm used as a raw material for an AlN single crystal is disposed in a carbon pot 10. Commercially available graphite powder 14 having an average particle diameter of 1 μm is disposed in a carbon crucible 16 at a ratio of 6 parts by weight to 100 parts by weight of the AlN powder. The carbon crucible 16 is disposed in the carbon pot 10 in such a manner as not to directly contact the AlN powder 12. The carbon crucible 16 is sized so as to be receivable in the carbon pot 10. The carbon pot 10 is heat treated in a N2atmosphere at 0.1 atm to 10 atm and at 2200°C in a graphite heating furnace. In this way, the AlN powder 12 as an AlN polycrystal powder is heat treated to produce an AlN raw material powder for forming a second layer.

[0081] (1b) Production of AlN raw material powder for first layer

[0082] The configuration of a heat treatment apparatus for producing an AlN raw material powder is shown in Figure 2 . As shown in Figure 2As shown, AlN powder 12 of 1 μm average particle diameter, which is commercially available and used as a raw material for AlN single crystal, is disposed in the BN crucible 11. Graphite powder 14 of 1 μm average particle diameter, which is commercially available, is disposed in the BN crucible 17 at a rate of 6 parts by weight for 100 parts by weight of the AlN powder. The BN crucible 17 is disposed in the BN crucible 11 in such a manner as not to be in direct contact with the AlN powder 12. Further, a BN crucible 18 containing BN powder 15 of 3 μm average particle diameter is disposed in the BN crucible 11 in such a manner as not to be in direct contact with the AlN powder 12. At this time, in Example 1, the BN powder 15 is disposed in the BN crucible 18 at a rate of 3 parts by weight for 100 parts by weight of the AlN powder. In Examples 2 to 9, the content of the BN powder 15 relative to 100 parts by weight of the AlN powder is adjusted in such a manner that the impurity concentration in the first layer constituting the finally obtained AlN single crystal substrate is the concentration shown in Table 1. The BN crucibles 17 and 18 are of a size that can be accommodated in the BN crucible 11. The BN crucible 11 is heat-treated in a graphite heating furnace in an N2atmosphere at 0.1 to 10 atm and at 2200°C. In this way, the AlN powder 12, which is AlN polycrystal powder, is heat-treated to produce AlN raw material powder for forming the first layer.

[0083] (1c) Formation of AlN single crystal layer

[0084] The configuration of a film formation apparatus used in the sublimation method is shown in Figure 3 . The film formation apparatus 20 is provided with a heat insulating material 24 for thermally insulating a crucible 22, which is a crystal growth container, and a coil 26 for heating the crucible 22. As shown in Figure 3 , the crucible 22, in which the AlN raw material powder 28 for forming the second layer produced in (1a) above is accommodated, is disposed inside the film formation apparatus 20. Further, a SiC substrate, which is a seed substrate 30 for causing the sublimate of the AlN raw material powder 28 to be deposited, is disposed in the upper portion of the crucible 22 in such a manner as not to be in contact with the AlN raw material powder 28. Next, the crucible 22 is pressurized at 50 kPa in an N2atmosphere, and the portion near the AlN raw material powder 28 in the crucible 22 is heated to 2100°C by high frequency induction heating using the coil 26. On the other hand, by heating the portion near the SiC substrate in the crucible 22 to a lower temperature (temperature difference of 200°C) and maintaining it, an AlN single crystal layer 32 (second layer) is again deposited on the SiC substrate. The holding time is set to 10 hours. Next, the second layer thus formed on the SiC substrate is set as the seed substrate 30, and the AlN raw material powder 28 for forming the first layer is used to cause an AlN single crystal layer 32 (first layer) to be again deposited on the second layer by the same method as above.

[0085] (1d) Polishing removal of SiC substrate and polishing of AlN single crystal layer surface

[0086] The SiC substrate obtained by reprecipitating the AlN obtained in the above (lc) was ground with a grindstone having a particle size number of #2000 or less to expose the AlN single crystal, and thereafter, the surface was further smoothed by lapping using diamond abrasive grains. Then, mirror finishing was performed on the surface by chemical mechanical polishing (CMP) using colloidal silica. In this way, a circular AlN single crystal substrate composed of two layers was produced, which had an area and a thickness shown in Table 2, and which was composed of one AlN single crystal as a whole and could be distinguished into a first layer and a second layer in the thickness direction from the viewpoint of impurity concentration.

[0087] (2) Evaluation of the AlN single crystal substrate

[0088] (2a) EBSD measurement

[0089] EBSD measurement was performed on the front and back surfaces of the AlN single crystal substrate, and as a result, it was found that the AlN crystal was oriented in both the c-axis direction and the a-axis direction.

[0090] (2b) Concentration of each atom in the AlN single crystal substrate

[0091] Dynamic secondary ion mass analysis (D-SIMS) was performed on the polished surface of the AlN single crystal substrate. The analysis device used was IMS-7f manufactured by CAMECA, and the measurement was performed using C S ＋ as the primary ion species, a primary acceleration voltage of 15 kv, and a detection area of 25 μm x 25 μm. With respect to this measurement, 10 measurement sites on the polished surface of the AlN single crystal substrate were measured. These 10 measurement sites were determined as follows: on the surface of the circular substrate, (i) 10 straight lines were drawn radially in such a manner that the circular substrate was divided into 10 equal parts from the center of the substrate toward the outer periphery (i.e., in such a manner that the angle formed by adjacent straight lines was 36 degrees), and (ii) the distance from the center of the substrate for each of the 10 straight lines was 50% of the radius of the substrate.

[0092] With respect to Examples 1 to 7, the average value of the boron atom concentration at a position 1 to 3 μm in depth from the surface of the substrate (surface) (region of the first layer) was measured at each measurement site, the average value at the 10 sites was calculated, and thereby the boron atom concentration (cm －3 ) in the first layer was obtained. The carbon atom concentration (cm －3 ) and the oxygen atom concentration (cm －3) are also solved in the same manner as described above. In addition, at each measurement site, the boron atom concentration, the carbon atom concentration, and the oxygen atom concentration at a position (a region of the second layer) at a depth of 1 to 3 μm from the surface (back surface) of the substrate are also solved in the same manner as described above. In this way, the boron atom concentration (cm －3 ), the carbon atom concentration (cm －3 ), and the oxygen atom concentration (cm －3 ) in each of the first layer and the second layer are solved. Further, the ratio of the boron atom concentration in the first layer to the boron atom concentration in the second layer, the ratio of the carbon atom concentration in the second layer to the carbon atom concentration in the first layer, and the ratio of the oxygen atom concentration in the second layer to the oxygen atom concentration in the first layer are solved. The results are shown in Table 1.

[0093] Note that the region of the first layer and the region of the second layer in the AlN single crystal substrate are determined as follows. That is, for the AlN single crystal substrate, the boron atom concentration measurement is performed at positions every 30 μm in the thickness direction from the surface, and the surface having the highest concentration in the inside of the AlN single crystal substrate is determined. Further, with respect to the surface having the highest boron atom concentration, a region having a boron atom concentration lower than 1 / 10 of the boron atom concentration is set as the second layer, and a region other than this is set as the first layer.

[0094] On the other hand, in Examples 8 and 9, the first layer and the second layer described above could not be determined. Therefore, at the above 10 measurement sites on the surface of the AlN single crystal substrate, each atom concentration was measured, and the average value at the 10 sites was set as the boron atom concentration, the carbon atom concentration, and the oxygen atom concentration in the first layer. Further, at the above 10 measurement sites on the back surface of the AlN single crystal substrate, each atom concentration was measured, and the average value at the 10 sites was set as the boron atom concentration, the carbon atom concentration, and the oxygen atom concentration in the second layer. Note that in this measurement, the lower limit value of the detection of the boron atom concentration was 7.0 x 10 17 cm －3 , the lower limit value of the detection of the carbon atom concentration was 1.0 x 10 16 cm －3 , and the lower limit value of the detection of the oxygen atom concentration was 5.0 x 10 17 cm －3 . The results are shown in Table 1.

[0095] Further, in Examples 1 to 7, the first layer and the second layer were determined using the above method, and the thickness of each layer was measured, and as a result, it was found that the thickness of the second layer was larger than the thickness of the first layer.

[0096] (2c) Confirmation of Edge Collapse

[0097] The surface of the AlN single crystal substrate after grinding and polishing in the above (1d) was observed using an optical microscope, and it was confirmed whether or not there was a chipped edge having a maximum length of 50 μm or more. Ten AlN single crystal substrates in total were produced using the same method as in the above (1), and it was confirmed that several of the AlN single crystal substrates had chipped edges. The AlN single crystal substrates were evaluated based on the evaluation criteria given below. The results are shown in Table 2.

[0098] < Evaluation Criteria >

[0099] - Evaluation A: 9 to 10 AlN single crystal substrates without chipped edges

[0100] - Evaluation B: 6 to 8 AlN single crystal substrates without chipped edges

[0101] - Evaluation C: 3 to 5 AlN single crystal substrates without chipped edges

[0102] - Evaluation D: Chipped edges were observed in all of the AlN single crystal substrates

[0103] (2d) Confirmation of Polarity

[0104] The surface of the AlN single crystal substrate after grinding and polishing in the above (1d) was polished using CMP, and the polarity was confirmed. In the case where the surface was an Al-polar surface, the polishing rate was slow, and in the case where the surface was an N-polar surface, the polishing rate was fast. Thus, it was confirmed that the surface of the AlN single crystal substrate (the surface of the first layer) was an Al-polar surface, and the back surface of the AlN single crystal substrate (the surface of the second layer) was an N-polar surface.

[0105] [Table 1]

[0106]

[0107] [Table 2]

[0108]

Claims

1. An AlN single crystal substrate having a two-layer structure consisting entirely of a single AlN single crystal and capable of being divided into a first layer and a second layer in a thickness direction from the viewpoint of impurity concentration; The AlN single crystal substrate is characterized in that: The AlN single crystal substrate contains boron as an impurity. The boron atomic concentration in the first layer is 9.4×10 18 cm －3 above, The boron atomic concentration in the first layer is 10 times or more the boron atomic concentration in the second layer.

2. The AlN single crystal substrate according to claim 1, wherein The carbon atom concentration in the second layer is 10 times or more the carbon atom concentration in the first layer.

3. The AlN single crystal substrate according to claim 1 or 2, characterized in that The oxygen atomic concentration in the second layer is at least twice the oxygen atomic concentration in the first layer.

4. The AlN single crystal substrate according to claim 1 or 2, characterized in that The boron atomic concentration in the first layer is 9.4×10 18 ~8.6×10 19 cm －3 , and the boron atomic concentration in the second layer is less than 7.0×10 17 cm －3 .

5. The AlN single crystal substrate according to claim 1 or 2, characterized in that The carbon atom concentration in the first layer is 1.0×10 18 ~5.0×10 19 cm －3 , and the carbon atom concentration in the second layer is 1.0×10 19 ~5.0×10 20 cm －3 .

6. The AlN single crystal substrate according to claim 1 or 2, characterized in that The oxygen atomic concentration in the first layer is 1.0×10 18 ~7.0×10 19 cm －3 , and the oxygen atomic concentration in the second layer is 1.0×10 19 ~8.0×10 20 cm －3 .

7. The AlN single crystal substrate according to claim 1 or 2, characterized in that: The area of ​​the AlN single crystal substrate is 75 to 18500 mm 2 , and the thickness of the AlN single crystal substrate is 0.10 to 1.00 mm.

8. The AlN single crystal substrate according to claim 1 or 2, characterized in that The second layer has a thickness greater than that of the first layer.

9. The AlN single crystal substrate according to claim 1 or 2, characterized in that The surface of the first layer is an Al polar surface, and the surface of the second layer is an N polar surface. 10 . A device comprising the AlN single crystal substrate according to claim 1 .

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