JP7712380B2 - AlN single crystal - Google Patents

Description

The present invention relates to an AlN single crystal.

In recent years, aluminum nitride (AlN) single crystals have been attracting attention as a base substrate for deep ultraviolet light-emitting devices using AlN-based semiconductors. For example, AlN and AlGaN are used as AlN-based semiconductors. These AlN-based semiconductors have a direct transition band structure, making them suitable for light-emitting devices, and can be applied to deep ultraviolet LEDs (Light Emitting Diodes) and LDs (Laser Diodes).

Known methods for producing AlN single crystals include HVPE (halide vapor phase epitaxy), mist CVD (chemical vapor phase epitaxy), sublimation, etc. For example, Non-Patent Document 1 (Kamata Hiroyuki et al., "Aluminum Nitride Single Crystals," Fujikura Technical Report, 2008, Vol. 2, No. 114, pp. 42-47) discloses a method for growing AlN single crystals on a seed substrate using sublimation.

In addition, the surface of the AlN single crystal thus produced is then flattened and mirror-finished by grinding and polishing. For example, Patent Document 1 (Patent Publication No. 4511801) discloses a polishing method for obtaining a high-quality substrate for GaN single crystal, which belongs to the same group 13 element nitride as AlN.

Patent No. 4511801

Hiroyuki Kamata et al., "Aluminum Nitride Single Crystal", Fujikura Technical Report, 2008, Vol. 2, No. 114, pp. 42-47

However, in the AlN single crystal produced by the method disclosed in Non-Patent Document 1, chipping (defects such as chips and cracks) is likely to occur in the grinding process, resulting in a problem of reduced yield of the AlN single crystal. In addition, the polishing process (i.e., mechanical polishing process and chemical mechanical polishing (CMP) process) for mirror-finishing the surface of the AlN single crystal also has a similar problem. As mentioned above, Patent Document 1 discloses a method for polishing a GaN single crystal, but not only is there no mention of suppressing the occurrence of chipping, but it does not disclose anything about the AlN single crystal. Therefore, it is desirable to suppress chipping that occurs in the AlN single crystal when polishing the AlN single crystal.

The inventors have now discovered that by endowing an AlN single crystal with the property that its transmittance decreases at a constant rate in a specific wavelength range in the transmission spectrum, it is possible to provide an AlN single crystal that is less susceptible to chipping when ground or polished.

Therefore, the object of the present invention is to provide an AlN single crystal that is less likely to chip when ground or polished.

According to the present invention, the following aspects are provided.
[Aspect 1]
An AlN single crystal, wherein in a transmission spectrum of the AlN single crystal, an average value of the transmittance in the range of 640 to 660 nm is lower than an average value of the transmittance in the range of 540 to 560 nm and an average value of the transmittance in the range of 780 to 800 nm, and the average value of the transmittance in the range of 640 to 660 nm is lower than the average value of the transmittance in the range of 780 to 800 nm by 5 to 20 percentage points (%pt).
[Aspect 2]
In the transmission spectrum of the AlN single crystal, there is an absorption peak with a half-width of 50 to 150 nm within a wavelength range of 540 to 800 nm,
The AlN single crystal according to aspect 1, wherein the half width is defined as (i) the width of a wavelength range that gives a transmittance lower than the transmittance calculated from _T2 + ( _T1 - _T2 ) / 2, where _T1 is the smaller of the average transmittance in the range of 540 to ₅₆₀ nm and the average transmittance in the range of 780 to 800 nm, and T2 is the average transmittance in the range of 640 to 660 nm.
[Aspect 3]
3. The AlN single crystal according to aspect 1 or 2, wherein in a transmission spectrum of the AlN single crystal, an average value of the transmittance in the range of 640 to 660 nm is lower by 1 to 10% pt than an average value of the transmittance in the range of 540 to 560 nm.
[Aspect 4]
The AlN single crystal according to any one of aspects 1 to 3, wherein in a transmission spectrum of the AlN single crystal, the average transmittance in the range of 640 to 660 nm is 6 to 18% pt lower than the average transmittance in the range of 780 to 800 nm.
[Aspect 5]
The AlN single crystal according to any one of aspects 2 to 4, wherein the half-width is 70 to 120 nm.
[Aspect 6]
The AlN single crystal according to aspect 3, wherein in a transmission spectrum of the AlN single crystal, an average value of the transmittance in the range of 640 to 660 nm is 3 to 8% pt lower than an average value of the transmittance in the range of 540 to 560 nm.

FIG. 2 is a schematic cross-sectional view showing the configuration of a heat treatment apparatus used for producing AlN raw material powder. FIG. 1 is a schematic cross-sectional view showing the configuration of a crystal growth apparatus used in a sublimation method. FIG. 1 is a schematic cross-sectional view showing the configuration of a crystal growth apparatus used in the HVPE method. 1 is a transmission spectrum of the AlN single crystal produced in Example 1.

AlN single crystal The AlN single crystal according to the present invention has a lower average transmittance (%) at 640 to 660 nm than the average transmittance (%) at 540 to 560 nm and the average transmittance (%) at 780 to 800 nm in the transmission spectrum of the AlN single crystal. Moreover, the average transmittance (%) at 640 to 660 nm of this AlN single crystal is 5 to 20 percentage points (%pt) lower than the average transmittance (%) at 780 to 800 nm. In this way, by giving the AlN single crystal a characteristic that the transmittance decreases at a constant rate in a specific wavelength range in the transmission spectrum, it is possible to provide an AlN single crystal that is less likely to chip when ground or polished. Therefore, by subjecting such an AlN single crystal to polishing or the like, it is possible to manufacture an AlN single crystal substrate with a high yield. That is, as described above, the surface of the AlN single crystal is flattened and mirror-finished by subsequent grinding or polishing. However, conventional AlN single crystals have a problem in that chipping is likely to occur in the AlN single crystal after undergoing a grinding or polishing process. The AlN single crystal of the present invention can advantageously solve this problem.

Here, the "average transmittance" of a specific wavelength region in the transmission spectrum is obtained by dividing the sum of the transmittance (%) of each wavelength (nm) measured in a specific wavelength range (e.g., 540-560 nm, 640-660 nm, 780-800 nm, etc.) by the number of measurement points. For example, if the transmittance is measured at 1 nm intervals in the 540-560 nm region and the sum of the transmittances is 1050, the average transmittance (e.g., 1050/21=50%) can be obtained by dividing the sum of the transmittances by 21, which is the number of measurement points. In addition, it is preferable to use the transmittance T _{100 μm} when the thickness of the AlN single crystal is converted to 100 μm. This is because if there is variation in the thickness of the AlN single crystal to be measured, the transmittance will also change accordingly. For example, if the AlN single crystal is thick, the transmittance will be low, and if the AlN single crystal is thin, the transmittance will be high.

The transmittance in the transmission spectrum can be calculated, for example, by the following calculation method. The total light transmittance T _a of the AlN single crystal is measured using a spectrophotometer. The absorption coefficient α of the AlN single crystal is calculated using the measured value of T _a and the theoretical transmittance T _t of the AlN single crystal. Then, the transmittance T _{100 μm} is calculated when the thickness of the AlN single crystal is converted to 100 μm. At this time, α and T _{100 μm} are calculated by the following formula:
α=-1/t×ln(T _a /T _t ), and T _100μm =exp(-α/100)
(where t represents the actual thickness (cm) of the AlN single crystal sample). For AlN single crystal samples with low transmittance and for which it is difficult to calculate the absorption coefficient α, the actual thickness may be reduced to measure the total light transmittance _Ta . In this way, a transmission spectrum based on the transmittance T100 _μm converted to a thickness of 100 μm can be obtained.

In the transmission spectrum of the AlN single crystal, the average transmittance at 640 to 660 nm is preferably 1 to 10% pt lower, and more preferably 3 to 8% pt lower, than the average transmittance at 540 to 560 nm. In addition, in the transmission spectrum of the AlN single crystal, the average transmittance at 640 to 660 nm is preferably 5 to 20% pt lower, and more preferably 6 to 18% pt lower, and more preferably 8 to 13% pt lower than the average transmittance at 780 to 800 nm.

The AlN single crystal of the present invention preferably has an absorption peak with a half-width of 50 to 150 nm in the wavelength range of 540 to 800 nm in the transmission spectrum, more preferably with a half-width of 70 to 120 nm. The "absorption peak" means an absorption band in which the transmittance is minimal (i.e., the absorbance is maximal) in the wavelength range of 540 to 800 nm in the transmission spectrum with the horizontal axis being wavelength (nm) and the vertical axis being transmittance (%), and in such an absorption band, the shape of the transmission spectrum is concave downward (e.g., valley-like). The half-width is defined as the width of a wavelength range that gives a transmittance lower than the transmittance calculated from (i) T ₂ + (T ₁ -T ₂ )/ ₂ , where T ₁ is the smaller of the average transmittance in the range of 540 to 560 nm and the average transmittance in the range of 780 to 800 nm, and T 2 is the average transmittance in the range of 640 to 660 nm.

The AlN single crystal in the present invention is preferably an oriented layer oriented in both the c-axis direction and the a-axis direction, and may contain mosaic crystals. Mosaic crystals are a collection of crystals that do not have clear grain boundaries, but whose orientation direction is slightly different from one or both of the c-axis and a-axis. Such an oriented layer has a configuration in which the crystal orientation is roughly aligned in the approximately normal direction (c-axis direction) and the in-plane direction (a-axis direction). With such a configuration, it is possible to form a semiconductor layer with excellent quality, particularly excellent orientation, thereon. That is, when a semiconductor layer is formed on the oriented layer, the crystal orientation of the semiconductor layer roughly follows the crystal orientation of the oriented layer. Therefore, it is easy to form a semiconductor film formed on the AlN single crystal into an oriented film.

The method for evaluating the orientation of the AlN single crystal in the present invention is not particularly limited, but known analytical methods such as EBSD (Electron Back Scatter Diffraction Patterns) method and X-ray pole figures can be used. For example, when using the EBSD method, inverse pole figure mapping and crystal orientation mapping of the surface (plate surface) or a cross section perpendicular to the plate surface of the AlN single crystal are measured. In the obtained inverse pole figure mapping, (A) the crystal is oriented in a specific direction (first axis) approximately normal to the plate surface, (B) the crystal is oriented in a specific direction (second axis) approximately in the plate surface direction perpendicular to the first axis, and in the obtained crystal orientation mapping, (C) the inclination angle from the first axis is distributed within ±10°, and (D) the inclination angle from the second axis is distributed within ±10°. When these four conditions are satisfied, the crystal orientation can be defined as being oriented in two axes, approximately normal and approximately plate surface directions. In other words, when the above four conditions are satisfied, it can be determined that the crystal is oriented along two axes, the c-axis and the a-axis. For example, when the approximately normal direction of the plate surface is oriented along the c-axis, the approximately in-plane direction of the plate may be oriented along a specific direction (e.g., the a-axis) perpendicular to the c-axis. The AlN single crystal may be oriented along two axes, the approximately normal direction and the approximately in-plane direction, but it is preferable that the approximately normal direction is oriented along the c-axis. The smaller the inclination angle distribution in the approximately normal direction and/or the approximately in-plane direction, the smaller the mosaic property of the AlN single crystal, and the closer it is to zero, the closer it is to a perfect single crystal. Therefore, from the viewpoint of the crystallinity of the AlN single crystal, it is preferable that the inclination angle distribution in both the approximately normal direction and the approximately in-plane direction is small, for example, ±5° or less, and more preferably ±3° or less.

The AlN single crystal has an area of preferably 20 ^cm2 or more, more preferably 70 ^cm2 or more, and even more preferably 170 ^cm2 or more on one side. By increasing the area of the AlN single crystal substrate in this way, it becomes possible to increase the area of the semiconductor layer formed thereon. Therefore, it becomes possible to obtain a large number of semiconductor elements from one semiconductor layer, and a reduction in manufacturing costs is expected. The upper limit of the size is not particularly limited, but is typically 710 ^cm2 or less on one side.

Manufacturing method The AlN single crystal of the present invention can be manufactured by various methods as long as it is an AlN single crystal whose transmittance decreases by a predetermined amount in a specific wavelength range in the transmission spectrum. A seed substrate may be prepared and epitaxially grown thereon, or an AlN single crystal may be directly manufactured by spontaneous nucleation without using a seed substrate. In addition, an AlN substrate may be used as the seed substrate so as to achieve homoepitaxial growth, or a substrate other than the seed substrate may be used for heteroepitaxial growth. Any of the vapor phase film-forming method, liquid phase film-forming method, and solid phase film-forming method may be used for the growth of the single crystal, but it is preferable to use the vapor phase film-forming method to form the AlN single crystal, and then grind and remove the seed substrate portion as necessary, thereby making it possible to obtain an AlN single crystal having the desired transmission spectrum characteristics. Examples of the vapor phase deposition method include MOVPE (metal organic vapor phase epitaxy), various CVD (chemical vapor deposition) methods (such as thermal CVD and plasma CVD), sputtering, hydride vapor phase epitaxy (HVPE), molecular beam epitaxy (MBE), sublimation, and pulsed laser deposition (PLD), among which sublimation or HVPE is preferred. Examples of the liquid phase deposition method include solution growth (such as flux). In addition, even if the AlN single crystal is not directly formed on the seed substrate, it is possible to obtain the AlN single crystal by a process of forming an orientation precursor layer, a process of converting the orientation precursor layer into an AlN single crystal layer by heat treatment, and a process of grinding and removing the seed substrate. Examples of the manufacturing method for forming the orientation precursor layer include the AD (aerosol deposition) method and the HPPD (supersonic plasma particle deposition) method.

Although any of the above-mentioned solid phase deposition method, vapor phase deposition method, and liquid phase deposition methods can use known conditions, the following describes a method for producing an AlN single crystal using, for example, a sublimation method. Specifically, the AlN single crystal is produced by (a) heat treatment of an AlN polycrystalline powder, (b) deposition of an AlN single crystal layer, and (c) grinding and removing the seed substrate and polishing the surface of the AlN single crystal layer.

(a) Heat Treatment of AlN Polycrystalline Powder This step is a step of heat treating AlN polycrystalline powder to obtain AlN raw material powder. As shown in FIG. 1, AlN powder 12 is placed in a boron nitride (BN) sheath 10 as a raw material for AlN single crystal, and heat treated in a _N2 atmosphere. At this time, a BN crucible 16 containing graphite powder 14 is also placed in the sheath 10 so as not to directly contact the AlN powder 12. This BN crucible 16 is of a size that can be stored in the sheath 10. The amount of graphite powder to be added is preferably 0.015 to 0.130 g, more preferably 0.020 to 0.120 g, per 100 g of AlN powder. When the amount of graphite powder to be added is within the above range, for example, it is possible to obtain an AlN single crystal in which the transmittance in a specific wavelength range in the transmission spectrum is reduced by a predetermined amount. The pressure inside the sheath 10 is preferably 0.1 to 10 atm, more preferably 0.5 to 5 atm. The heat treatment temperature is preferably 2150°C to 2300°C, and more preferably 2200°C to 2250°C.

(b) Deposition of AlN single crystal layer This step is a step of depositing an AlN single crystal on a seed substrate in a crystal growth apparatus. An example of a crystal growth apparatus used in the sublimation method is shown in FIG. 2. The deposition apparatus 20 shown in FIG. 2 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 an AlN raw material powder 28 in its lower portion, and a seed substrate 30 on which a sublimate of the AlN raw material powder 28 is deposited in its upper portion. The inside of the crucible 22 is pressurized under an _N2 atmosphere, and the crucible 22 is heated by the coil 26 to sublimate 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 provided so that the temperature in the vicinity of the seed substrate 30 in the upper part of the crucible 22 is lower than the temperature in the vicinity of the AlN raw material powder 28 in the lower part of the crucible 22. For example, the portion of the crucible 22 in the vicinity of the AlN raw material powder 28 is preferably heated to 1900 to 2250°C, more preferably 2000 to 2200°C, and the portion of the crucible 22 in the vicinity of the seed substrate 30 is preferably heated to 1400 to 2150°C, more preferably 1500 to 2050°C. At this time, the temperature of the portion in the vicinity of the seed substrate 30 is preferably lower by 100 to 500°C, more preferably 200 to 400°C, relative to the portion in the vicinity of the AlN raw material powder 28. The heating is preferably maintained for 2 to 100 hours, more preferably 4 to 90 hours. Temperature control can be performed by measuring the temperatures of the upper and lower parts of the crucible 22 with a radiation thermometer (not shown) through holes in the heat insulating material 24 covering the crucible 22 and feeding the measured temperatures back to the temperature adjustment. In this manner, a SiC single crystal is placed as the seed substrate 30, and AlN is reprecipitated on the surface thereof to form an AlN single crystal layer 32.

(c) Grinding the seed substrate and polishing the surface of the AlN single crystal layer This process includes a grinding process for grinding the seed substrate to expose the AlN single crystal layer, and a polishing process for removing irregularities and defects on the AlN single crystal surface. Since the AlN single crystal layer produced through the above steps (a) and (b) using a SiC substrate as the seed substrate still has SiC single crystals remaining, the surface of the orientation layer is exposed by grinding. In addition, in order to mirror-finish the surface of the AlN single crystal layer after film formation, the plate surface is smoothed by lapping using diamond abrasive grains, and then polished by chemical mechanical polishing (CMP) using colloidal silica or the like. In this way, the AlN single crystal can be produced.

As a method other than the sublimation method, a method for producing AlN single crystals using the HVPE method is described below. As a specific example, it is produced by (d) forming an AlN single crystal layer on a seed substrate, (e) grinding off the seed substrate, (f) heat treating the AlN single crystal layer, and (g) polishing the surface of the AlN single crystal layer.

(d) Deposition of AlN single crystal layer on seed substrate This step is a step of depositing an AlN single crystal layer on a seed substrate. FIG. 3 shows a vapor phase growth apparatus using the HVPE method. The vapor phase growth apparatus using the HVPE method includes a reactor 40, a susceptor 44 on which a base substrate 42 for film formation is placed, a carrier gas supply source 46, an Al source supply source 48, a metal Al powder source 50 disposed in the Al source supply source 48, a heater 52, and a gas exhaust section 54. It is preferable to use a SiC substrate as the base substrate 42 for film formation. The substrate 42 is placed on the susceptor 44, and AlCl ₃ gas obtained by supplying HCl gas to the heated Al source supply source 48 and NH ₃ gas from the carrier gas supply source 46 are mixed and supplied to the base substrate 42 for film formation. At this time, the heating temperature of the Al source supply source 48 is preferably 500 to 700°C, more preferably 550 to 650°C. The flow rate ratio of _AlCl3 gas to _NH3 gas is preferably 1:1 to 1:500, more preferably 1:10 to 1:200. The pressure in the reaction furnace 40 is preferably 1 to 100 Torr, more preferably 10 to 30 Torr. The growth temperature is preferably 1100 to 1400°C, more preferably 1150 to 1250°C.

(e) Grinding and Removing the Seed Substrate This is a grinding step in which the seed substrate is ground and removed to expose the AlN single crystal layer. Since the AlN single crystal layer produced through the above step (d) using the SiC substrate as the seed substrate still has SiC single crystal remaining, the surface of the orientation layer is exposed by grinding.

(f) Heat treatment of the AlN single crystal layer The AlN single crystal is placed in a BN sheath together with graphite powder and heat treated in a _N2 atmosphere. At this time, the pressure inside the furnace is preferably 0.1 to 10 atm, more preferably 0.5 to 5 atm. The heat treatment temperature is preferably 2150°C to 2300°C, more preferably 2200 to 2250°C.

(g) Polishing the surface of the AlN single crystal layer This is a polishing process for mirror-finishing the surface of the AlN single crystal layer. In order to mirror-finish the surface of the AlN single crystal layer after the heat treatment, the plate surface is smoothed by lapping using diamond abrasive grains, and then polished by chemical mechanical polishing (CMP) using colloidal silica or the like. In this way, the AlN single crystal substrate can be produced.

The present invention will be further illustrated by the following examples.

Example 1
(1) Preparation of AlN single crystals (1a) Heat treatment of AlN polycrystalline powder As shown in FIG. 1, a commercially available AlN powder 12 (Tokuyama AlN powder, F grade) used as a raw material for AlN single crystals was placed in a BN sheath 10, and a BN crucible 16 containing commercially available graphite powder 14 (graphite powder made by SEC Carbon, SGP grade) was placed so as not to directly touch the AlN powder 12. This BN crucible 16 is large enough to be stored in the sheath 10. This BN sheath 10 was heat treated at 2200° C. in a N ₂ atmosphere at 0.1 to 10 atm in a graphite heater furnace. At this time, the graphite powder was added at a ratio of 0.052 g per 100 g of AlN powder. In this way, the AlN polycrystalline powder was heat treated to prepare an AlN raw material powder.

(1b) Formation of AlN single crystal layer As shown in Fig. 2, a crucible 22 was used as a crystal growth container, and a SiC substrate was placed in the crucible as a base material (seed substrate) 30, and the AlN raw material powder 28 prepared in (1a) above was placed in the crucible so as not to come into contact with the SiC substrate. The crucible 22 was pressurized at 50 kPa in a _N2 atmosphere, and the portion in the crucible 22 near the AlN raw material powder 28 was heated to 2100°C by high-frequency induction heating, while the portion in the crucible 22 near the SiC substrate 30 was heated to a lower temperature (temperature difference of 200°C) and held at that temperature, thereby re-precipitating an AlN single crystal layer 32 on the SiC substrate 30. The holding time was 10 hours.

(1c) Grinding and Removal of SiC Substrate and Polishing of AlN Single Crystal Layer Surface The SiC substrate on which AlN was reprecipitated obtained in (1b) above was ground using a grindstone with a grit size of up to #2000 until the AlN single crystal was exposed, and then the plate surface was further smoothed by lapping using diamond abrasive grains. After that, the plate surface was mirror-finished by chemical mechanical polishing (CMP) using colloidal silica. In this way, an AlN single crystal was produced.

(2) Evaluation of AlN Single Crystal (2a) EBSD Measurement EBSD measurements were performed on the front and back surfaces of the AlN single crystal, and it was found that the AlN crystal was oriented in both the c-axis and a-axis directions.

(2b) Transmission spectrum The total light transmittance T _a of the AlN single crystal, including the wavelength range of 200 to 800 nm, was measured using a spectrophotometer (UH4150, manufactured by Hitachi High-Tech Science). After determining the absorption coefficient α of the AlN single crystal using the measured value of T _a and the theoretical transmittance T _t of the AlN single crystal, the transmittance T _{100 μm} was calculated when the thickness of the AlN single crystal was converted to 100 μm. α and T _{100 μm} are expressed by the following formula:
α=-1/t×ln(T _a /T _t ), and T _100μm =exp(-α/100)
(where t represents the actual thickness (cm) of the AlN single crystal sample)
It was obtained by. In this way, a transmission spectrum based on the transmittance T _{100 μm} when the thickness was converted to 100 μm was obtained. Based on the obtained transmission spectrum, the difference (%pt) of the average value of the transmittance (%) at 640 to 660 nm to the average value of the transmittance (%) at 540 to 560 nm (hereinafter referred to as Δ(T _av640-660 -T _av540-560 )) and the difference (%pt) of the average value of the transmittance (%) at 640 to 660 nm to the average value of the transmittance (%) at 780 to 800 nm (hereinafter referred to as Δ(T _av640-660 -T _av540-560 )) were calculated. In addition, the half-width (nm) of the absorption peak was obtained in the wavelength range of 540 to 800 nm. This half-width was determined by (i) determining the smaller of the average transmittance (%) from 540 to 560 nm and the average transmittance (%) from 780 to 800 nm as _T1 , and determining the average transmittance (%) from 640 to 660 nm as _T2 , and then (ii) reading from the transmission spectrum the width of the wavelength range that gives a transmittance lower than the transmittance calculated from the formula: _T2 + ( _T1 - _T2 )/2. The results are shown in Table 1.

(2c) Confirmation of chipping The surface of the AlN single crystal after grinding and polishing in (1c) above was observed with an optical microscope, and the presence or absence of chipping was confirmed, regarding chips and cracks with a maximum length of 50 μm or more as chipping. A total of 10 AlN single crystals were produced in the same manner as in (1) above, and it was confirmed how many of them had chipping, and a rating evaluation was performed according to the evaluation criteria shown below. The results are shown in Table 1.
<Evaluation criteria>
- Evaluation A: 9 to 10 AlN single crystal substrates were free of chipping. - Evaluation B: 6 to 8 AlN single crystal substrates were free of chipping. - Evaluation C: 3 to 5 AlN single crystal substrates were free of chipping. - Evaluation D: Chips were observed on all AlN single crystal substrates.

Examples 2 to 6
In the above (1a), an AlN single crystal was produced and evaluated in the same manner as in Example 1, except that the amount of graphite powder added during the heat treatment of the AlN polycrystalline powder was the amount shown in Table 1. In addition, when EBSD measurements were performed on the front and back surfaces of the AlN single crystal, it was found that the AlN crystal was oriented in both the c-axis direction and the a-axis direction. The results are shown in Table 1.

Examples 7 and 8 (Comparative)
In the above (1a), an AlN single crystal was produced and evaluated in the same manner as in Example 1, except that the amount of graphite powder added during the heat treatment of the AlN polycrystalline powder was the amount shown in Table 1. In addition, when EBSD measurements were performed on the front and back surfaces of the AlN single crystal, it was found that the AlN crystal was oriented in both the c-axis direction and the a-axis direction. The results are shown in Table 1.

Example 9 (Comparison)
An AlN single crystal was produced and evaluated in the same manner as in Example 1, except that the above step (1a) was not carried out and a commercially available AlN powder (Tokuyama AlN powder, F grade) was used as the AlN raw material powder. In addition, EBSD measurements were carried out on the front and back surfaces of the AlN single crystal, and it was found that the AlN crystal was oriented in both the c-axis direction and the a-axis direction. The results are shown in Table 1.

Claims (6)

An AlN single crystal containing carbon , wherein in a transmission spectrum of the AlN single crystal, an average value of the transmittance in the range of 640 to 660 nm is lower than an average value of the transmittance in the range of 540 to 560 nm and an average value of the transmittance in the range of 780 to 800 nm, and the average value of the transmittance in the range of 640 to 660 nm is lower than the average value of the transmittance in the range of 780 to 800 nm by 5 to 20 percentage points (%pt). In the transmission spectrum of the AlN single crystal, there is an absorption peak with a half-width of 50 to 150 nm within a wavelength range of 540 to 800 nm,
The AlN single crystal according to claim ₁ , wherein the half-width is defined as (i) the width of a wavelength range that gives a transmittance lower than the transmittance calculated from _T2 + ( _T1 - T2)/ ₂ , where T1 is the smaller of the average transmittance in the range of 540 to 560 nm and the average transmittance in the range of 780 to ₈₀₀ nm, and T2 is the average transmittance in the range of 640 to 660 nm. The AlN single crystal according to claim 1 or 2, wherein in the transmission spectrum of the AlN single crystal, the average transmittance at 640 to 660 nm is 1 to 10%pt lower than the average transmittance at 540 to 560 nm. The AlN single crystal according to claim 1 or 2, wherein in the transmission spectrum of the AlN single crystal, the average transmittance at 640 to 660 nm is 6 to 18% pt lower than the average transmittance at 780 to 800 nm. The AlN single crystal according to claim 2, wherein the half-width is 70 to 120 nm. 4. The AlN single crystal according to claim 3, wherein in a transmission spectrum of the AlN single crystal, an average value of the transmittance in the range of 640 to 660 nm is lower by 3 to 8% pt than an average value of the transmittance in the range of 540 to 560 nm.