Method for producing an AlN single crystal substrate and an AlN single crystal substrate

Abstract

A method is provided for the high-productivity, crack-free production of an AIN single-crystal substrate, and it is also a technique suitable for increasing the diameter of the AIN single-crystal substrate. The method for producing an AIN single-crystal substrate includes the steps of providing an AIN composite body, composed of an AIN seed crystal and an AIN sintered body, and subjecting the AIN composite body to heat treatment to grow an AIN single crystal from the AIN seed crystal.

Description

TECHNICAL AREA

[0001] The present disclosure relates to a process for producing an AIN single crystal substrate and an AIN single crystal substrate. TECHNICAL BACKGROUND

[0002] In recent years, AIN single-crystal substrates have been developed as base substrates for deep ultraviolet LEDs. A sublimation process has been investigated as a method for producing an AIN single crystal. For example, patent literature 1 (JP2019-19042A) discloses a method for producing an AIN single crystal in which the sublimation process is used to grow an AIN single crystal on a SiC seed substrate in at least two separate steps. LIST OF QUOTES, PATENT LITERATURE

[0003] Patent Literature 1: JP2019-19042A BRIEF SUMMARY OF THE INVENTION

[0004] When an AIN single crystal is produced by the sublimation process described above, cracking can occur due to a difference in the coefficient of thermal expansion between the SiC seed substrate and the AIN single crystal. This problem becomes more pronounced with increasing substrate size. A possible solution to this problem could be to perform the sublimation process using an AIN single crystal with a diameter of 50.8 mm (2 inches) instead of the SiC seed substrate. This is because the sublimation process is characterized by the AIN single crystal gradually increasing in size with each application of the sublimation process, and therefore the diameter of the AIN single-crystal substrate can be increased by exploiting this property.However, this technique suffers from low productivity because the sublimation process has to be repeated many times so that, for example, the AIN single crystal has a size (diameter) of 50.8 mm the first time, 60 mm the second time, 70 mm the third time, etc.

[0005] The inventors of the present invention have now discovered that growing an AIN single crystal using an AIN composite body composed of an AIN seed crystal and an AIN sintered body makes it possible to produce an AIN single crystal substrate with high productivity without cracking, and is a method suitable for increasing the diameter of the AIN single crystal substrate.

[0006] Accordingly, it is an object of the present invention to provide a method by which an AIN single crystal substrate can be produced with high productivity without the formation of cracks, which also represents a suitable technique for increasing the diameter of the AIN single crystal substrate.

[0007] The present revelation provides the following aspects. [Aspect 1]

[0008] The process for producing an AIN single crystal substrate, comprising the steps: Providing an AIN composite body composed of an AIN seed crystal and an AIN sintered body; and Subjecting the AIN composite body to heat treatment in order to grow an AIN single crystal from the AIN seed crystal. [Aspect 2]

[0009] The process for producing an AIN single-crystal substrate according to aspect 1, wherein the AIN sintered body is provided by a process comprising the steps of: (a1) Mixing an AIN powder and a powder containing at least one rare earth element to produce a mixed powder with an AIN content of 95% by weight or more; (a2) Shaping the mixed powder into a predetermined shape to produce a green body, and (a3) Firing of the green body to produce an AIN sintered body with an average crystal grain size of 1 to 40 µm. [Aspect 3]

[0010] The process for producing an AIN single crystal substrate according to aspect 1 or 2, wherein the step of growing an AIN single crystal from the AIN seed crystal includes growing an AIN single crystal over an entire region of the AIN sinter body. [Aspect 4]

[0011] The AIN single crystal substrate, produced by the process according to one of aspects 1 to 3. [Aspect 5]

[0012] The AIN single crystal substrate according to aspect 4, wherein the AIN single crystal substrate has a size with a diameter of 100 mm or more. [Aspect 6]

[0013] The AIN single crystal substrate according to aspect 4 or 5, wherein the half-width of the X-ray rocking curve of a (002) plane of the AIN single crystal on at least one surface of the AIN single crystal substrate is 20 to 350 arcseconds. [Aspect 7]

[0014] The AIN single crystal substrate according to one of aspects 4 to 6, wherein the half-width of the X-ray seesaw curve of a (102) plane of the AIN single crystal on at least one surface of the AIN single crystal substrate is 20 to 500 arcsec. [Aspect 8]

[0015] The AIN single crystal substrate according to one of aspects 4 to 7, wherein at least one surface of the AIN single crystal substrate has a defect density of 1.0 × 10 3 up to 1.0 × 10 7 cm -2 exhibits. [Aspect 9]

[0016] A device comprising the AIN single crystal substrate according to one of aspects 4 to 8. BRIEF DESCRIPTION OF THE DRAWING Fig. Figure 1 is a process diagram that schematically shows the process for producing an AIN single crystal substrate according to the present invention. DESCRIPTION OF THE EXECUTION FORMS

[0017] The present invention relates to a method for producing an AIN single-crystal substrate. As described in Fig. As shown in Figure 1, this process comprises the steps of providing an AIN composite body 16, composed of an AIN seed crystal 12 and an AIN sintered body 14, and subjecting the AIN composite body 16 to heat treatment to grow an AIN single crystal 18 from the AIN seed crystal 12. This process enables the production of an AIN single-crystal substrate 20 with high productivity and without cracking, while simultaneously being suitable for increasing the diameter of the AIN single-crystal substrate.

[0018] As described above, cracking can occur during the fabrication of an AIN single crystal by sublimation due to a difference in the coefficient of thermal expansion between the SiC seed substrate and the AIN single crystal. This problem becomes even more pronounced with increasing substrate size. One possible solution to this problem could be to perform the sublimation process using an AIN single crystal with a diameter of 50.8 mm (2 inches) instead of the SiC seed substrate; however, this method suffers from low productivity because the sublimation process must be repeated many times to increase the substrate size. In this respect, the method of the present invention can avoid such low-productivity repetition of the sublimation process, resulting in improved productivity.For example, the sublimation process is only used to produce the AIN seed crystal 12; subsequently, the AIN seed crystal 12 is transferred to the AIN sintered body, and an AIN single crystal is grown from the AIN seed crystal in the region of the AIN sintered body. Thus, it is only necessary to perform the deposition of the AIN seed crystal 12 by the sublimation process to achieve a thickness that does not cause cracking even at a large size (e.g., a diameter of 100 mm or more), and the subsequent growth of the AIN single crystal 18 from the AIN seed crystal 12 is achieved by transforming the AIN sintered body 14 (AIN polycrystalline body) into the AIN single crystal 18, instead of using the sublimation process.In this case, the coefficients of thermal expansion of the AIN seed crystal 12 and the AIN single crystal 18 are very similar, so that the possibility of cracks occurring due to a difference in the coefficient of thermal expansion, as in the case of using a SiC seed substrate, is extremely low. Therefore, according to the present invention, the AIN single crystal substrate 20 can be obtained in a large size (for example, with a diameter of 100 mm or more) with good quality without performing a step that is likely to lead to cracking.

[0019] Therefore, the AIN single-crystal substrate 20 has a diameter of 100 mm or more, and preferably a diameter of 150 mm or more, or 200 mm or more. According to the method of the present invention, the diameter of the AIN single crystal 18 can be easily increased by enlarging the AIN seed crystal 12 and the AIN sintered body 14. Therefore, although the upper limit of the diameter of the AIN single-crystal substrate 20 is not specifically limited, the diameter of the AIN single-crystal substrate 20 is typically 300 mm or less, and typically 250 mm or less. The AIN single-crystal substrate 20 typically has a circular shape. As used here, the “circular shape” need not be a perfectly circular shape, but may be a substantially circular shape that is generally recognizable as a circular shape overall.For example, the shape may be one in which a section of the circle is cut out to identify the crystal orientation or for other purposes (for example, a circular shape that includes an orientation surface or a notch).

[0020] The individual steps of the process for producing an AIN single crystal substrate are described below. (1) Provision of an AIN composite body

[0021] As in Fig. As shown in Figure 1(ii), the AIN composite body 16 is provided, which is composed of the AIN seed crystal 12 and the AIN sintered body 14. The AIN sintered body 14 is a polycrystalline AIN body and thus a ceramic material composed of a plurality of interconnected AIN crystal grains. The AIN composite body 16 has a diameter of preferably 100 mm or more, and more preferably 150 mm or more, or 200 mm or more. The diameter of the AIN composite body 16 is typically 300 mm or less, and more typically 250 mm or less. The thickness of the AIN seed crystal 12 is not specifically limited, but is preferably 0.1 to 700 µm, more preferably 0.5 to 450 µm, and most preferably 1 to 5 µm. The thickness of the AIN sintered body 14 is not specifically limited, but is preferably 200 to 700 µm, more preferably 250 to 650 µm and particularly preferably 350 to 550 µm.

[0022] The AIN composite body 16 can be provided by any method, as long as it is composed of the AIN seed crystal 12 and the AIN sintered body 14. Preferably, the AIN composite body 16 is produced by a method that is described in the following steps (a) to (c) and in the Fig. 1(i) and (ii) are shown. The order of steps (a) and (b) can be reversed. (a) Providing an AIN sintered body

[0023] The AIN sintered body 14 is provided. For example, the AIN sintered body 14 can be produced by mixing an AIN powder and a sintering aid, shaping the resulting mixed powder, and firing the resulting green body. In particular, the step of providing the AIN sintered body 14 (step (a)) is preferably carried out by (a1) producing a mixed powder; (a2) producing a green body; and (a3) ​​sintering the green body, as described below. It should be noted that the AIN sintered body 14 can be provided by various known techniques, which are not limited to the process with the following steps (a1) to (a3). (a1) Production of a mixed powder

[0024] First, an AIN powder and a powder containing at least one rare earth element are mixed to produce a blended powder with an AIN content of 95 wt.% or more. The AIN powder is a major component of the blended powder, while the powder containing the rare earth element is used as a sintering aid. Preferred examples of the rare earth element are Y, La, Ce, Sm, Eu, Gd, Dy, and Yb. These rare earth elements are preferably present in the powder in the form of oxides, carbonates, hydroxides, or composite oxides. The sintering aid is not limited to the powder containing the rare earth element and can also be a powder containing an alkaline earth element such as Mg or Ca. The AIN content in the blended powder is 95 wt.% or more, typically 96 wt.% or more, even more typically 97 wt.% or more, and still more typically 98 wt.% or more.The content of the sintering aid in the mixed powder is not specifically limited, but is preferably 0.1 to 5.0 wt.%, more preferably 0.2 to 4.0 wt.%, even more preferably 0.4 to 3.0 wt.% and particularly preferably 0.5 to 2.0 wt.%. (a2) Production of a green body

[0025] The resulting mixed powder is formed into a predetermined shape to produce a green body. The forming process is not specifically limited, and the forming can be carried out by pressing (e.g., uniaxial pressing) or sheet forming (e.g., doctor blade forming). (a3) Sintering of the green body

[0026] The resulting green body is fired to produce the AIN sintered body 14 with an average crystal grain size of 1 to 40 µm. The firing is preferably carried out by hot pressing, wherein the green body is held at a predetermined firing temperature and pressure for a predetermined time. The firing temperature during hot pressing is preferably 1500 to 2000°C, more preferably 1600 to 1900°C, and particularly preferably 1650 to 1800°C. The holding time at the firing temperature (i.e., the firing duration) is preferably 1 to 10 hours, more preferably 2 to 8 hours, and particularly preferably 4 to 6 hours. The pressing force during hot pressing is preferably 0 to 30 MPa, more preferably 0 to 20 MPa, and particularly preferably 0 to 10 MPa.The average crystal grain size of the AIN sintered body 14 (the average grain size of a plurality of AIN crystal grains from which the AIN sintered body 14 is composed) is 1 to 40 µm, preferably 2 to 25 µm and more preferably 3 to 10 µm. The average crystal grain size can be measured using the method described below in the section “Examples”. (b) Providing an AIN template

[0027] As in Fig. As shown in Figure 1(i), an AIN template 13 is provided. The AIN template 13 is a composite material comprising the AIN seed crystal 12 and a base substrate 10 supporting the AIN seed crystal 12. The base substrate 10 is preferably a substrate on which the AIN seed crystal 12 can be formed. Preferred examples of the base substrate 10 are a SiC substrate, a sapphire substrate, and a Si substrate. The AIN seed crystal 12 is preferably formed on the base substrate 10 by a gas-phase process. Examples of the gas-phase process are sublimation, chemical vapor deposition (CVD), sputtering, and hydride vapor deposition (HVPE), with sublimation being preferred. (c) Bonding the AIN sinter body to the AIN seed crystal and removing the base substrate

[0028] As in the Fig. As shown in Figures 1(i) and (ii), the AIN sintered body 14 is bonded to the AIN seed crystal 12 of the resulting AIN template 13, and the base substrate 10 is removed to obtain the AIN composite body 16. This bonding can be carried out by a surface activation method or by plasma bonding. In the case of the surface activation method, the bonding can preferably be carried out by the following procedure. First, the surfaces of the AIN sintered body 14 are polished to a mirror finish. Next, a surface of the AIN seed crystal 12 and a surface of the AIN sintered body 14 are activated. The surfaces can be activated by irradiation with a neutralized beam, for example, an argon beam. Then the AIN template 13 and the AIN sintered body 14 are placed on top of each other so that the activated surfaces of the AIN seed crystal 12 and the AIN sintered body 14 are in contact with each other to form a stacked body.The stacked body is bonded under vacuum by applying a load of 100 to 20,000 N. After bonding, the no-longer-needed base substrate 10 is removed to expose the AIN seed crystal 12, yielding the AIN composite body 16, which is composed of the AIN seed crystal 12 and the AIN sintered body 14. The base substrate 10 can be removed using a known technique such as grinding or reactive ion etching (RIE).

[0029] The surface activation described above using a neutralized beam can be carried out by introducing an inert gas into a chamber and applying a high voltage from a DC power source to an electrode located in the chamber. In this setup, electrons are moved by an electric field generated between the electrode (positive electrode) and the chamber (negative electrode), causing beams of atoms and ions to be generated by the inert gas. Of the beams reaching a grid, the ion beams are neutralized at the grid, resulting in the emission of neutral atom beams from a high-speed atom beam source. The atomic species forming the beams are preferably an inert gas element (e.g., Ar, Ne, Kr, He, N, or Xe). During activation by beam irradiation, the voltage is, for example, 0.5 to 2.0 kV and the current, for example, 50 to 200 mA.

[0030] While the AIN sintered body 14 in the example shown in the figure is bonded only on one side of the AIN seed crystal 12, the AIN sintered body 14 can be bonded on both sides of the AIN seed crystal 12. (2) Growing an AIN single crystal

[0031] As in Fig. As shown in Figures 1(ii) and (iii), the AIN composite body 16 is subjected to heat treatment to grow the AIN single crystal 18 from the AIN seed crystal 12. That is, the AIN composite body 16 is heat treated, whereby the AIN sintered body 14 is gradually transformed from the section in contact with the AIN seed crystal 12 into the AIN single crystal 18, resulting in the growth of the AIN single crystal 18. In contrast to the sublimation process, where cracks can occur due to the use of a SiC seed substrate with a different coefficient of thermal expansion than that of the AIN single crystal, in this technique for growing the AIN single crystal 18 the coefficients of thermal expansion of the AIN seed crystal 12 and the AIN single crystal 18 are very similar, so that the AIN single crystal 18 can be grown to a desired thickness without cracking, even at large sizes (e.g. a diameter of 100 mm or more).This heat treatment is preferably carried out in an inert gas atmosphere such as nitrogen. The firing temperature during the heat treatment is preferably 2000 to 2300°C, more preferably 2100 to 2250°C, and particularly preferably 2150 to 2200°C. The holding time at the firing temperature (i.e., the firing duration) is preferably 1 to 50 hours, more preferably 3 to 45 hours, and particularly preferably 5 to 40 hours. This heat treatment can be carried out by firing at atmospheric pressure or by hot pressing. The pressing force during the heat treatment is preferably 0 to 30 MPa, more preferably 0 to 20 MPa, and particularly preferably 0 to 10 MPa. Two plates of the AIN composite body 16 can be stacked and then subjected to the heat treatment.

[0032] In this case, it is as in Fig.As shown in Figures 1(ii) and (iii), it is preferred to grow the AIN single crystal 18 over an entire region of the AIN sintered body 14, as this can prevent cracking more effectively. However, as long as no cracking occurs, the AIN single crystal 18 can be grown in the center of the AIN sintered body 14, leaving a section of the AIN sintered body 14 exposed (in which case it is desirable to remove the remaining section of the AIN sintered body 14, for example, by grinding). The freestanding AIN single-crystal substrate 20 can be obtained by grinding and polishing the surface of the AIN single crystal 18 thus obtained. AIN single crystal substrate

[0033] According to a preferred embodiment of the present invention, the AIN single-crystal substrate 20, which is produced by the method described above, is also provided. The AIN single-crystal substrate 20 can exhibit good crystallinity and a low defect density, and is also free of cracks, even when produced in a large size (for example, with a diameter of 100 mm or more). As described above, the AIN single-crystal substrate 20 has a diameter of preferably 100 mm or more, and more preferably 150 mm or more, or 200 mm or more. While the upper limit of the diameter of the AIN single-crystal substrate 20 is not specifically limited, the diameter of the AIN single-crystal substrate 20 is typically 300 mm or less, and typically 250 mm or less. The AIN single-crystal substrate 20 typically has a circular shape.The thickness of the AIN single crystal substrate 20 is not specifically limited, but is preferably 200 to 700 µm, more preferably 250 to 680 µm and particularly preferably 300 to 650 µm.

[0034] The good crystallinity can be evaluated by measuring the profile of an X-ray rocking curve (hereinafter referred to as "XRC") of a (002) or (102) plane of the AIN single crystal and by evaluating its full width at half maximum (FWHM). In particular, the XRC FWHM of the (002) plane of the AIN single crystal on at least one surface of the AIN single-crystal substrate 20 is preferably 20 to 350 arcseconds, more preferably 100 to 300 arcseconds, and most preferably 100 to 280 arcseconds. If the XRC FWHM of the (002) plane lies within these ranges, this has the advantage that, in the fabrication of an apparatus, the film formed exhibits good performance without dislocations. Furthermore, the XRC half-width of the (102) plane of the AIN single crystal on at least one surface of the AIN single crystal substrate 20 is preferably 20 to 500 arcsec, more preferably 200 to 450 arcsec and particularly preferably 200 to 400 arcsec.If the XRC half-width of the (102) plane lies within these regions, this has the advantage that, in the fabrication of a device, the film formed exhibits good performance without dislocations. The surface with an XRC half-width of the (102) plane within the aforementioned regions is preferably the same as the surface with an XRC half-width of the (002) plane within the aforementioned regions. Preferably, both surfaces of the AIN single-crystal substrate 20 have an XRC half-width of the (102) plane within the aforementioned regions and / or an XRC half-width of the (002) plane within the aforementioned regions. The measurement of an XRC profile of the (002) or (102) plane of the AIN single crystal can be performed using a commercially available XRD instrument (e.g. D8 DISCOVER, manufactured by Bruker-AXS) and the associated XRD analysis software (e.g. “LEPTOS” Ver4).03, manufactured by Bruker-AXS) based on the procedure described in the section “Examples” below.

[0035] At least one surface of the AIN single crystal substrate 20 has a defect density of preferably 1.0 × 10 3 up to 1.0 × 10 7 cm -2 , preferred 1.0 × 10 6 up to 8.0 × 10 6 cm -2 and even more preferred 1.0 × 10 6 up to 6.0 × 10 6 cm -2If the defect density lies within these ranges, this has the advantage that, in the fabrication of a device, the film formed exhibits good performance without dislocations. The surface with a defect density within the aforementioned ranges is preferably the same surface of the AIN single-crystal substrate 20 as the surface with an XRC half-width of the (102) plane within the aforementioned numerical ranges and the surface with an XRC half-width of the (002) plane within the aforementioned numerical ranges. Preferably, both surfaces of the AIN single-crystal substrate 20 have a defect density within the aforementioned numerical ranges. The defect density can be measured based on the method described in the "Examples" section below. device

[0036] The AIN single-crystal substrate 20 obtained by the method of the present invention exhibits, as described above, good crystallinity and a low defect density and is also suitable for large dimensions, making it suitable for use in various devices. Therefore, according to a preferred embodiment of the present invention, a device comprising the AIN single-crystal substrate 20 is provided. Preferred examples of such devices include deep ultraviolet LEDs, ultraviolet lasers, power devices, MEMS devices, and HMETs (high electron mobility transistors). EXAMPLES

[0037] The present invention is described in more detail with reference to the following examples. However, the present invention is not limited to the following examples. Examples 1 to 11(1) Production of an AIN sintered body

[0038] Aluminum nitride powder (manufactured by Tokuyama Corporation, Grade F), yttrium oxide powder (manufactured by Shin-Etsu Chemical Co., Ltd.), dysprosium oxide powder (manufactured by Shin-Etsu Chemical Co., Ltd.), and samarium oxide powder (manufactured by Nippon Yttrium Co., Ltd.) were provided. The aluminum nitride powder was mixed with the yttrium oxide powder (Examples 1 to 6 and 9 to 11), the dysprosium oxide powder (Example 7), or the samarium oxide powder (Example 8) in the weight ratios specified in Table 1 to obtain a blended powder. This blended powder was subjected to uniaxial pressing and subsequently to hot pressing by heating it to the maximum temperature and pressure specified in Table 1 and holding it at the maximum temperature and pressure for the time specified in Table 1.Both surfaces of the resulting hot-pressed body were polished to a mirror finish to obtain a plate-shaped AIN sintered body (AIN polycrystal).

[0039] The average crystal grain size of the resulting AIN sinter body was measured according to the following procedure and the result shown in Table 1 was obtained. (Measurement of average crystal grain size)

[0040] The grain size of the sintered body was determined using the intersection method by polishing a cross-section of the sintered body to a mirror finish and examining the microstructure in the 64 µm × 48 µm range with a scanning electron microscope (SEM) (JSM-IT500LA, manufactured by JEOL Ltd.) at 2000x magnification. Specifically, in the SEM image showing the polished surface of the sintered body, a certain number of line segments with a length of 40 µm or more were plotted on the SEM image scale, and the number n of crystal grains intersected by these line segments was determined. It should be noted that if one end of a line segment was located within a crystal grain, the crystal grain was counted as 1 / 2. A value obtained by dividing the length L of each line segment by n was declared the average crystal grain size (i.e.,average cut length) I is defined, and a value obtained by multiplying I by a coefficient of 1.5 was defined as the average sinter grain size. (2) Production of an AIN template by the sublimation process

[0041] A silicon dioxide (SiC) substrate was placed in a crucible serving as a crystal growth vessel, and an AIN raw material powder was added, avoiding contact with the SiC substrate. The growth vessel was pressurized to 50 kPa in a nitrogen atmosphere, and the section of the growth vessel near the AIN raw material powder was heated to 2100°C by radiofrequency induction heating, while the section near the SiC substrate was heated to a temperature 200°C below this (i.e., 1900°C). By maintaining the aforementioned heating temperature for 30 minutes, an AIN seed crystal was formed on the SiC substrate. The surface of this AIN seed crystal was then shaped to a mirror finish to create a SiC substrate with the AIN seed crystal as the AIN template. (3) Bonding of the AIN sinter body to the AIN seed crystal and removal of the base substrate

[0042] The surface of the AIN sintered body and the AIN seed crystal side surface of the AIN template were irradiated for 70 seconds with a fast argon neutral atom beam (acceleration voltage: 1 kV, argon flow rate: 60 sccm) to activate these surfaces. The AIN sintered body and the AIN template were placed face-to-face so that the AIN sintered body and the AIN seed crystal were in contact, and the AIN sintered body and the AIN template were bonded under vacuum by applying a load of 1000 N. The SiC substrate was removed from the resulting composite body by grinding with a #2000 size grinding wheel, and the surface was subsequently further smoothed by lapping with diamond abrasives to obtain an AIN composite body composed of the AIN sintered body and the AIN seed crystal.The AIN composite body was disk-shaped and had a diameter of 100 mm in Examples 1 to 8 and a diameter of 150 mm in Examples 9 and 10. The AIN seed crystal had a thickness of 2 µm in Examples 1 to 10 and 4 µm in Example 11. In all Examples 1 to 11, the AIN sintered body had a thickness of 400 µm. (4) Growth of an AIN single crystal

[0043] The resulting AIN composite body was hot-pressed for 40 hours under conditions of 2160°C and 13 MPa in an N2 atmosphere to grow an AIN single crystal from the AIN seed crystal over the entire region of the AIN sinter body. (5) Next step (grinding and polishing)

[0044] In Examples 1 to 5 and 7 to 10, a predetermined amount of the surface of the resulting AIN single crystal was ground and polished to obtain a freestanding AIN single-crystal substrate with a thickness of 0.3 mm. In Example 6, the surface of the AIN single crystal was also ground and polished in the same manner as above to obtain a freestanding AIN single-crystal substrate with a thickness of 0.3 mm; however, cracks appeared in the AIN single-crystal substrate. In Example 11, the AIN single crystal fractured during the growth process and was therefore not ground and polished. (6) Evaluation of the AIN single crystal

[0045] The resulting AIN single crystal was evaluated as follows. (6a) Evaluation of the production of the AIN single crystal substrate

[0046] The state of the AIN single crystal during the growth step of an AIN single crystal ((4) above) and in the subsequent step ((5) above) was observed and evaluated based on the following criteria. The results are presented in Table 1. • Rating A: The AIN single crystal grew without cracks. Furthermore, the AIN single crystal did not break in the subsequent step (grinding and polishing). • Rating B: The AIN single crystal grew without cracks. Although cracks did appear in the subsequent step (grinding and polishing), it was found that the cracks could be prevented by changing the conditions to milder conditions in the subsequent step. • Rating C: The AIN single crystal broke during the growth process and a freestanding AIN single crystal substrate could not be obtained. (6b) Half-width of the X-ray rocking curve

[0047] The XRC measurement of the (002) plane of a surface (the surface opposite the side that was previously the AIN seed crystal) of the AIN single-crystal substrate was performed using a multifunctional high-resolution X-ray diffractometer (D8 DISCOVER, manufactured by Bruker-AXS). The conditions for the XRC measurement were as follows. <xrd-messbedingungen> • Tube voltage: 40 kV • Tube current: 40 mA • Detector: Triple Ge (220) analyzer • CuKα radiation, converted into parallel monochromatic light (half-width: 28 seconds) using an asymmetric reflection monochromator Ge (022) • Step width: 0.001° • Scan speed: 0.5 s / step

[0048] In practice, axial alignment was achieved by adjusting 2θ, ω, χ, and φ until a peak of the (002) plane of the AIN single crystal appeared. The range from ω = 14.5° to 19.5° was then measured at an anti-scattering slit of 3 mm. The full width at half maximum (FWHM) of the XRC profile of the (002) plane of the resulting AIN single crystal was determined using XRD analysis software (“LEP-TOS” Ver4.03, manufactured by Bruker-AXS) by peak search after profile smoothing. The FWHM of the XRC profile of the (002) plane of the AIN single crystal substrate surface is shown in Table 1.

[0049] Furthermore, an XRC measurement of the (102) plane of a surface (the surface opposite the side that was previously the AIN seed crystal) of the AIN single-crystal substrate was also performed. Using the D8 DISCOVER XRD instrument, manufactured by Bruker-AXS, axial alignment was achieved by adjusting 2θ, ω, χ, and φ until a peak of the (102) plane of the AIN single crystal appeared, and then the measurement was performed at ω = 24.5 to 29.5°. The remaining conditions and the analytical procedure were identical to those of the XRC measurement of the (002) plane. As a result, the full width at half maximum (FWHM) of the XRC profile of the (102) plane of the surface of the AIN single-crystal substrate was as shown in Table 1. (6c) Defect density

[0050] The defect density of the resulting AIN single-crystal substrate (on the surface opposite the side that was previously the AIN seed crystal) was evaluated by measuring the entire surface region using X-ray topography (XRTmicron, manufactured by Rigaku Corporation). If the defect density is 1.0 × 10 5 cm -2 Since the number of etch pits is greater than or greater than this, it is difficult to accurately calculate the number of pits using X-ray topography. Therefore, an etch pit evaluation was performed using molten KOH etchant to measure the defect density on the surface of the AIN single-crystal substrate. Specifically, in the etch pit evaluation, the surface of the AIN single-crystal substrate was immersed for 5 minutes in a molten mixture obtained by mixing KOH and NaOH in a weight ratio of KOH : NaOH = 1:1 and heating to 450°C, and then etched. The defect density was subsequently measured using an optical microscope. Example 12 (Comparison)

[0051] An AIN single crystal was prepared by sublimation as follows and then evaluated as in Examples 1 to 11. The results are shown in Table 1. (Preparation of an AIN single crystal substrate)

[0052] A 100 mm diameter SiC substrate was placed in a crucible serving as a crystal growth vessel, and an AIN raw material powder was added, avoiding contact with the SiC substrate. The crystal growth vessel was pressurized to 50 kPa in a nitrogen atmosphere, and the area near the AIN raw material powder within the crystal growth vessel was heated to 2100°C by high-frequency induction heating, while the area near the SiC substrate was heated to 200°C below this temperature (i.e., 1900°C). An AIN single crystal was deposited and grown on the SiC substrate by maintaining the aforementioned heating temperature for 10 hours. A SiC substrate containing the AIN single crystal was obtained. After cooling, the examination of the AIN single crystal showed that cracks (fracture points) had appeared.It is assumed that the cracks were caused by thermal stress due to the difference in thermal expansion that arose during the temperature drop, since there is a significant difference in thermal expansion between the SiC substrate and the AIN single crystal. [Table 1] Table 1 QUOTES INCLUDED IN THE DESCRIPTION

[0000] This list of documents cited by the applicant was automatically generated and is included solely for the reader's convenience. The list is not part of the German patent or utility model application. The DPMA accepts no liability for any errors or omissions. Cited patent literature

[0000] JP 2019-19042A [0002, 0003]

Claims

[1] Method for the preparation of an AIN single crystal substrate, comprising the steps: Providing an AIN composite body composed of an AIN seed crystal and an AIN sintered body; and Subjecting the AIN sintered composite body to heat treatment in order to grow an AIN single crystal from the AIN seed crystal. [2] Method for producing an AIN single crystal substrate according to claim 1, wherein the AIN sintered body is provided by a method comprising the steps: (a1) Mixing an AIN powder and a powder containing at least one rare earth element to produce a mixed powder with an AIN content of 95% by weight or more; (a2) Shaping the mixed powder into a predetermined shape to produce a green body, and (a3) Firing the green body to produce an AIN sintered body with an average crystal grain size of 1 to 40 µm. [3] Method for producing an AIN single crystal substrate according to claim 1, wherein the step of growing an AIN single crystal from the AIN seed crystal comprises growing an AIN single crystal over an entire region of the AIN sintered body. [4] AIN single crystal substrate produced by the process according to any one of claims 1 to 3. [5] AIN single crystal substrate according to claim 4, wherein the AIN single crystal substrate has a size with a diameter of 100 mm or more. [6] AIN single crystal substrate according to claim 4, wherein the half-width of the X-ray rocking curve of a (002) plane of the AIN single crystal on at least one surface of the AIN single crystal substrate is 20 to 350 arcsec. [7] AIN single crystal substrate according to claim 4, wherein the half-width of the X-ray rocking curve of a (102) plane of the AIN single crystal on at least one surface of the AIN single crystal substrate is 20 to 500 arcsec. [8] AIN single crystal substrate according to claim 4, wherein at least one surface of the AIN single crystal substrate has a defect density of 1.0 × 10 3 up to 1.0 × 10 7 cm -2 exhibits. [9] Device comprising the AIN single crystal substrate according to claim 4.

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