Single-crystal aluminum nitride substrate and optoelectronic devices fabricated therefrom

Abstract

This disclosure provides a method for forming an aluminum nitride single crystal substrate, comprising growing an AlN single crystal boule and then cooling the AlN single crystal boule in three phases: a first cooling phase from the crystal growth temperature to a first intermediate temperature of about 1900°C to about 1800°C; a second cooling phase from the first intermediate temperature to a second intermediate temperature of about 1500°C to about 1400°C, characterized by a cooling rate of 5.0°C / min or less; and a third cooling phase from the second intermediate temperature to room temperature. An aluminum nitride single crystal substrate having a dimensionless figure of merit (FOM) of 0.4 or higher, and an optoelectronic device fabricated therefrom are also provided.

Description

[Technical Field]

[0001] This disclosure relates to a high-quality single-crystal aluminum nitride substrate exhibiting UV transparency, and to optoelectronic devices fabricated therefrom. [Background technology]

[0002] Among Group III nitride semiconductors, single-crystal AlN is a material characterized by a direct band gap of approximately 6 eV. In addition, because AlN has a larger band gap than other nitrides such as GaN and InN, it is possible to manipulate the band gap energy by alloying AlN with Ga and / or In. As a result, Group III nitride semiconductors are recognized as useful in the manufacture of white light LEDs, UV-LEDs for sterilization applications, and lasers for high-density light storage, communication, detection, and other applications, enabling short-wavelength emission in the ultraviolet (UV) spectral range. To form semiconductor devices such as light-emitting devices, it is necessary to form a multilayer structure including an active layer between an n-type semiconductor layer electrically connected to an n-electrode and a p-type semiconductor layer electrically connected to a p-electrode. It is important that all layers have high crystallinity with few dislocations and point defects that can adversely affect luminescence efficiency.

[0003] Sapphire substrates are often used for Group III nitride LEDs due to their stable supply, cost, and UV transmittance. By using highly transparent sapphire as the substrate, Group III nitride semiconductor devices can be obtained. However, due to the fact that there is a considerable difference in lattice constants between the Group III nitride LED device layer and the sapphire substrate, 10 9 cm -2It is a well-known problem in the art that large dislocation densities, typically exceeding the standard, are generated at the interface between the substrate and the device structure, and that this high dislocation density adversely affects the luminous efficiency and lifespan of LED devices. Therefore, it is desirable to use AlN single crystals or GaN single crystals as substrates for Group III nitride LEDs, because using these natural Group III nitride substrates minimizes the difference in lattice constants between the substrate and the device layer.

[0004] Achieving high UV transmittance in single-crystal AlN substrates is difficult, and elemental impurities (such as carbon and oxygen) have traditionally been considered the main cause of poor UV absorption. Therefore, attempts to maximize UV ​​transmittance in such substrates have generally focused on reducing such impurities.

[0005] One method for reducing elemental impurities involves depositing a single-crystal AlN layer by hydride vapor phase growth (HVPE) on an AlN substrate grown using sublimation methods such as physical vapor transport (PVT). When growing an AlN layer by HVPE, the crystal growth temperature in the HVPE reactor is much lower than in a sublimation growth reactor, making it relatively easy to reduce the level of impurities originating from the reactor's structural elements. See, for example, U.S. Patent No. 9,840,790 by Akinori et al. However, this method adds an additional crystal growth step, thereby increasing the cost and complexity of UV-transparent substrate manufacturing.

[0006] Other methods disclosed in the art for achieving high UV transmittance focus on controlling carbon and / or oxygen impurities, along with specific types of temperature control during the substrate manufacturing process. For example, Bondokov et al., U.S. Patent No. 10,954,608, suggests that good UV transmittance can be achieved by actively controlling the cooling of grown AlN crystals from high growth temperatures, combined with extremely low levels of oxygen and carbon impurities. -1The embodiments described in the patent as having an ultraviolet absorption coefficient less than about 3×10 17 cm -3 are also described as having a carbon concentration less than about 0.5 and a carbon-to-oxygen concentration ratio less than about 0.5.

[0007] Similarly, U.S. Patent No. 9,034,103 to Schujman et al. suggests controlling the oxygen impurity level of a single crystal AlN substrate to a very low level (e.g., about 5×10 17 cm -3 ) in addition to controlling the cooling after crystal growth to achieve high transmittance to ultraviolet light.

[0008] Furthermore, U.S. Patent No. 11,168,411 to Bondokov et al. suggests combining post-cooling high-temperature annealing of an AlN boule having a low carbon concentration of about 1.8×10 16 cm -3 to 5×10 17 cm -3 and a low oxygen concentration of about 1×10 17 cm -3 to 7.9×10 17 cm -3 (e.g., in a carbon-free furnace) to enhance UV transmittance.

Prior Art Documents

Patent Documents

[0009]

Patent Document 1

Patent Document 2

Patent Document 3

Patent Document 4

Summary of the Invention

Problems to be Solved by the Invention

[0010] Achieving consistently low levels of elemental impurities during the PVT crystal growth process is challenging due to the severe limitations on material selection that meet the requirements of a low-impurity growth environment at high temperatures. In this field, there is still a need for high-quality group III nitride substrates fabricated using PVT growth techniques that provide both the desired UV transmittance and structural quality, and are therefore useful for the manufacture of optoelectronic devices emitting light in the UV range. [Means for solving the problem]

[0011] This disclosure relates to a method for forming an aluminum nitride single-crystal substrate that, when determined by dimensionless figure of merit (FOM), results in a significant improvement in the crystal quality / uniformity and UV transmittance / uniformity of the resulting AlN substrate. Surprisingly, this result is achieved without strict control over elemental impurities in the substrate.

[0012] This disclosure includes, but is not limited to, the following embodiments:

[0013] Embodiment 1: The single crystal AlN seed and the AlN raw material are arranged in a crucible in a separated manner. A crucible is heated in a furnace so as to form a temperature gradient between a single crystal AlN seed and an AlN raw material, causing a portion of the AlN raw material to sublimate and deposit on the single crystal AlN seed to form an AlN single crystal boule, wherein the crystal growth temperature in the crucible adjacent to the single crystal AlN seed during heating is maintained at approximately 2200°C or lower, and A method for forming an aluminum nitride single crystal substrate, comprising cooling an AlN single crystal boule with three phases: a first cooling phase from the crystal growth temperature to a first intermediate temperature of approximately 1900°C to approximately 1800°C; a second cooling phase from the first intermediate temperature to a second intermediate temperature of approximately 1500°C to approximately 1400°C, characterized by a cooling rate of 5.0°C / min or less; and a third cooling phase from the second intermediate temperature to room temperature, wherein the cooling rate of one or both of the first and third cooling phases is greater than 5.0°C / min.

[0014] Embodiment 2: The method of Embodiment 1, wherein the second cooling phase is performed at a cooling rate of 2.0°C / min or less.

[0015] Embodiment 3: The method of Embodiment 1 or 2, wherein the second cooling phase is performed at a cooling rate of 1.5°C / min or less.

[0016] Embodiment 4: Any one of Embodiments 1 to 3, wherein one or both of the first cooling phase and the third cooling phase are performed at a rate exceeding 10°C / min, for example, exceeding 15°C / min, or exceeding 20°C / min, or exceeding 25°C / min, or exceeding 30°C / min.

[0017] Embodiment 5: Any one of Embodiments 1 to 4, wherein the temperature in the crucible adjacent to the AlN raw material during heating is maintained at approximately 2200°C or less, for example, approximately 2180°C or less.

[0018] Embodiment 6: Any one of Embodiments 1 to 5, wherein the temperature in the crucible adjacent to the single crystal AlN seed during heating is maintained at approximately 2150°C or lower, for example, 2120°C or lower.

[0019] Embodiment 7: Any one of Embodiments 1 to 6, wherein the pressure inside the crucible during heating is about 500 Torr or less, for example, about 100 to about 500 Torr.

[0020] Embodiment 8: Any one of Embodiments 1 to 7, wherein after the cooling of the AlN single crystal boule, the AlN single crystal boule is not subjected to further heating above 2000°C.

[0021] Embodiment 9: A 2-inch diameter substrate cut from an AlN single crystal boule has a dimensionless figure of merit (FOM) of 0.4 or greater, for example, 0.45 or greater, or 0.5 or greater, or 0.55 or greater, and the FOM has the following formula, according to any one of Embodiments 1 to 8: FOM=A*R*T*U (In the formula, 1×1mm 2When determined using a grid overlay, A = (surface area of ​​substrate without expansion defects / total surface area of ​​substrate), and the surface area of ​​substrate without expansion defects is the portion of the total surface area that is free of expansion defects. R = 15 arcsec / maximum (15 arcsec, median FWHM), where median FWHM is the median of all rocking curve Full Width at Half Maximum (FWHM) intensity measurements of the double axis rocking curve for the (002) and (102) crystal planes, expressed in arcsec. T = exp(-alpha_median * 0.01 cm), where alpha_median is the median absorption coefficient at 265 nm. U = 1 - 3(alpha_st.dev. / alpha_mean), where alpha_st.dev. is the standard deviation of the absorption coefficient measurements and alpha_mean is the average absorption coefficient at 265 nm.

[0022] Embodiment 10: A 2-inch diameter substrate cut from an AlN single crystal boule is (1) approximately 1 × 10 18 cm -3 The above carbon concentration; (2) approximately 1 × 10 18 cm -3 The above oxygen concentration; (3) approximately 1 × 10 18 cm -3 A method according to any one of Embodiments 1 to 9, characterized by the above silicon concentration, or any two or more of (1), (2), and (3).

[0023] Embodiment 11: A 2-inch diameter substrate cut from an AlN single crystal boule is (1) approximately 2 × 10 18 cm -3 The above carbon concentration; (2) approximately 2 × 10 18 cm -3 The above oxygen concentration; (3) approximately 2 × 10 18 cm -3A method according to any one of Embodiments 1 to 10, characterized by the above silicon concentration, or any two or more of (1), (2), and (3).

[0024] Embodiment 12: A 2-inch diameter substrate cut from an AlN single crystal boule is (1) approximately 3 × 10 18 cm -3 The above carbon concentration; (2) approximately 3 × 10 18 cm -3 The above oxygen concentration; (3) approximately 3 × 10 18 cm -3 A method according to any one of Embodiments 1 to 11, characterized by the above silicon concentration, or any two or more of (1), (2), and (3).

[0025] Embodiment 13: A 2-inch diameter substrate cut from an AlN single crystal boule has (i) the median of the total full width at half maximum (FWHM) intensity measurements of the biaxial rocking curves with respect to the (002) and (102) crystal planes for about 25 arcsec or less; and (ii) about 50 cm -1 The following is the median absorption coefficient at 265 nm; (iii) approximately 150 mm 2 One of the methods of Embodiments 1 to 12, characterized by the following extended defects contrast area; or any combination of two or more of (i), (ii), and (iii).

[0026] Embodiment 14: An aluminum nitride single crystal substrate having a diameter of 2 inches or more, wherein the 2-inch diameter portion of the substrate has a dimensionless figure of merit (FOM) of 0.4 or more, for example, 0.45 or more, or 0.5 or more, or 0.55 or more, and the FOM has the following formula: FOM=A*R*T*U (In the formula, 1×1mm 2 When determined using the grid overlay, A = (surface area of ​​substrate without expansion defects / total surface area of ​​substrate), and the surface area of ​​substrate without expansion defects is the portion of the total surface area that is free of expansion defects. R = 15 arcsec / maximum(15 arcsec, median FWHM), where median FWHM is the median of the total full width at half maximum (FWHM) intensity measurements of the biaxial rocking curves for the (002) and (102) crystal planes, expressed in arcsec. T = exp(-alpha_median * 0.01 cm), where alpha_median is the median absorption coefficient at 265 nm. U = 1 - 3(alpha_st.dev. / alpha_mean), where alpha_st.dev. is the standard deviation of the absorption coefficient measurements and alpha_mean is the average absorption coefficient at 265 nm.

[0027] Embodiment 15: (1) Approximately 1 × 10 18 cm -3 The above carbon concentration; (2) approximately 1 × 10 18 cm -3 The above oxygen concentration; (3) approximately 1 × 10 18 cm -3 An aluminum nitride single crystal substrate of Embodiment 14, characterized by the above silicon concentration, or two or more of (1), (2), and (3).

[0028] Embodiment 16: (1) Approximately 2 × 10 18 cm -3 The above carbon concentration; (2) approximately 2 × 10 18 cm -3 The above oxygen concentration; (3) approximately 2 × 10 18 cm -3 An aluminum nitride single crystal substrate of Embodiment 14 or 15, characterized by the above silicon concentration, or two or more of (1), (2), and (3).

[0029] Embodiment 17: (1) Approximately 3 × 10 18 cm -3 The above carbon concentration; (2) approximately 3 × 10 18 cm -3 The above oxygen concentration; (3) approximately 3 × 10 18 cm -3An aluminum nitride single crystal substrate according to any one of embodiments 14 to 16, characterized by the above silicon concentration, or any two or more of (1), (2), and (3).

[0030] Embodiment 18: (i) the median of the total full width at half maximum (FWHM) intensity measurements of the biaxial rocking curves for the (002) and (102) crystal planes for about 25 arcsec or less; (ii) about 50 cm -1 The following is the median absorption coefficient at 265 nm; (iii) approximately 150 mm 2 One aluminum nitride single crystal substrate of any one of embodiments 14 to 17, characterized by the following expanded defect contrast area; or any combination of two or more of (i), (ii), and (iii).

[0031] Embodiment 19: A photoelectronic device adapted to emit ultraviolet light, comprising one aluminum nitride single crystal substrate from any of Embodiments 14 to 18 and an ultraviolet light-emitting diode structure superimposed on the aluminum nitride single crystal substrate.

[0032] Embodiment 20: The photoelectronic device of Embodiment 19, wherein the ultraviolet light-emitting diode structure comprises a first electrode electrically connected to an n-type semiconductor layer and optionally a second electrode electrically connected to a p-type semiconductor layer.

[0033] Embodiment 21: The photoelectron device of Embodiment 19 or 20, wherein the emission wavelength of the photoelectron device is in the range of approximately 250 nm to 290 nm.

[0034] These and other features, aspects and advantages of this disclosure will become apparent from reading the following detailed description together with the accompanying drawings, which are briefly described below. The present invention includes any combination of two, three, four, or more of the embodiments described above, and any combination of two, three, four, or more features or elements described herein, whether or not such features or elements are expressly combined in the description of the particular embodiments herein. This disclosure is intended to be read in whole so that, in any of its various aspects and embodiments, any separable features or elements of the disclosed invention should be considered to be intended to be combinable unless the context clearly indicates otherwise.

[0035] As described above, this disclosure has been explained using the general terminology mentioned earlier, but here we will refer to the attached drawings, which are not necessarily drawn to a consistent scale. [Brief explanation of the drawing]

[0036] [Figure 1a] An exemplary cross-sectional view of a reactor hot zone that may be used in the process of this disclosure is shown. [Figure 1b] An exemplary cross-sectional view of a crucible that may be used in the process of this disclosure is shown. [Figure 2] This is a cross-sectional view of another exemplary reactor that may be used in the process of this disclosure. [Figure 3] This is a schematic diagram of the crystal growth method according to this disclosure. [Figure 4a] The experimental section provides XRT images of the AlN substrate from comparative example A. [Figure 4b] Figure 4a shows the same substrate, with the location of the expansion defect indicated by red shading. [Figure 5a] The experimental section provides an XRT image of the AlN substrate from comparative example B. [Figure 5b] Figure 5a shows the same substrate, with the location of the expansion defect indicated by red shading. [Figure 6a]The experimental section provides XRT images of the AlN substrate from Example 1. [Figure 6b] Figure 6a shows the same substrate, with the location of the expansion defect indicated by red shading. [Figure 7a] The experimental section provides a schematic diagram of the 00.2 XRRC map of FWHM measurements for an AlN substrate from Comparative Example A. [Figure 7b] The experimental section provides a schematic diagram of the 10.2 XRRC map of FWHM measurements for the AlN substrate from Comparative Example A. [Figure 8a] The experimental section provides a schematic diagram of the 00.2 XRRC map of FWHM measurements for the AlN substrate from Comparative Example B. [Figure 8b] The experimental section provides a schematic diagram of the 10.2 XRRC map of FWHM measurements for the AlN substrate from Comparative Example B. [Figure 9a] The experimental section provides a schematic diagram of the 00.2 XRRC map of FWHM measurements for AlN substrates from Example 1. [Figure 9b] The experimental section provides a schematic diagram of the 10.2 XRRC map of FWHM measurements for AlN substrates from Example 1. [Figure 10] A schematic diagram of the 265nm absorption coefficient map obtained from the AlN substrate of Comparative Example A is provided. [Figure 11] A schematic diagram of the 265nm absorption coefficient map obtained from the AlN substrate of Comparative Example B is provided. [Figure 12] A schematic diagram of the 265nm absorption coefficient map obtained from the AlN substrate of Example 1 is provided. [Modes for carrying out the invention]

[0037] Next, the Disclosure is described more fully below with reference to its exemplary embodiments. These exemplary embodiments are written so as to give thoroughness and completeness to the Disclosure and to adequately convey its scope to those skilled in the art. In fact, the Disclosure may be embodied in many different forms and should not be construed as being limited to the embodiments described herein. Rather, these embodiments are provided so as to satisfy the legal requirements to which the Disclosure is applicable. Where used herein and in the appended claims, the singular forms "a," "an," and "the" refer to multiple subjects unless the context clearly indicates otherwise.

[0038] The terms “about” or “approximately” are defined as being close to what is understood by those skilled in the art. In one non-limiting embodiment, these terms are defined as being within 10%, for example, within 5%, or within 1%, or within 0.5%.

[0039] The terms “comprising” (and any form of “comprising,” such as “comprise” and “comprises”), “having” (and any form of “having,” such as “have” and “has”), “including” (and any form of “including,” such as “includes” and “include”), or “containing” (and any form of “containing,” such as “contains” and “contain”) are comprehensive or non-exclusive and do not exclude additional unlisted elements or methods or processes.

[0040] In the context of AlN substrates, “single crystal” refers to a crystal having symmetrically ordered lattice atoms, which may be, for example, c-planes, m-planes, a-planes, or r-planes. Optoelectronic devices built on single-crystal substrates inherit the crystal orientation of the substrate. In contrast, “polycrystalline” refers to the presence of many orientations with many grain boundaries that are not present in single crystals. When applied to substrates of this disclosure, “single-crystal substrate” refers to at least the presence of at least one single-crystal surface available for the growth of optoelectronic devices, and includes substrates having a uniform single-crystal structure throughout its entire thickness, and substrates having a single-crystal layer bonded to a polycrystalline substrate. Single-crystal AlN is a group III nitride semiconductor material characterized by a direct bandgap of about 6 eV. This represents a larger bandgap than other nitrides such as GaN and InN, and therefore AlN can be alloyed with Ga and / or In to manipulate the bandgap energy.

[0041] Aluminum nitride substrate This disclosure provides an aluminum nitride single crystal substrate exhibiting both excellent structural quality and high UV transmittance, as well as a method for fabricating such a substrate. Surprisingly, in contrast to the methods outlined in the prior art, the strong performance of the aluminum nitride single crystal substrate of this disclosure is not based on extremely low concentrations of elemental impurities, such as carbon, oxygen, or silicon.

[0042] The AlN substrates of this disclosure can be characterized using various parameters related to either structural quality or UV transmittance. For example, the AlN substrates may be characterized by the presence of expansion defects such as small angle grain boundaries (LAGBs) and other defects such as scratches (e.g., shown in Figure 5b). In this disclosure, the surface area of ​​a substrate containing expansion defects is 1 × 1 mm². 2 The grid overlay is used to determine the size. In certain embodiments, the 2-inch diameter AlN substrate of the present disclosure is approximately 150 mm 2 For example, approximately 120mm 2 Below, or approximately 100mm 2 Below, or approximately 90mm 2 Below, or approximately 80mm 2Below (for example, approximately 20 to 150 mm) 2 , or approximately 50 to 100 mm 2 It has an expanded defect contrast area of ​​). References to expanded defect contrast area herein refer to measurements made using the X-ray topography (XRT) techniques described in the Experimental section.

[0043] High-resolution X-ray diffraction (HRXRD) is another standard method used to characterize lattice distortion in single-crystal substrates. See, for example, the NIST High Resolution X-Ray Diffraction Standard Reference Material: SRM 2000, link: http: / / www.nist.gov / manuscript-publication-search.cfm?pub_id=902585, whose entire disclosure is incorporated herein by reference. See also High Resolution X-Ray Diffractometry And Topography by DKBowen, BKTanner, CRC Press 1998, whose entire disclosure is incorporated herein by reference. Dislocation density can be characterized by the full width at half maximum (FWHM) intensity of the rocking curve (RC) peak in HRXRD measurements. A narrow peak indicates less lattice disorder in the crystal, which is directly related to a low dislocation density. In certain embodiments, the median values ​​of the total full width at half maximum (FWHM) intensity measurements of the biaxial rocking curves for the (002) and (102) crystal planes of a 2-inch diameter AlN substrate of this disclosure are approximately 25 arcsec or less, approximately 20 arcsec or less, approximately 18 arcsec or less, or approximately 15 arcsec or less. Exemplary median ranges include approximately 7 to approximately 25 arcsec, approximately 10 to approximately 20 arcsec, and approximately 10 to approximately 15 arcsec. References to FWHM intensity herein refer to measurements made using the X-ray diffraction techniques described in the Experimental section.

[0044] Light transmission measurements can be used to determine the light absorption coefficient of an AlN substrate material. In certain embodiments, the 2-inch diameter AlN substrate of this disclosure is approximately 50 cm². -1 Below, or approximately 40cm -1 Below, or approximately 30cm -1 Below, or approximately 25cm -1 Below, or approximately 20cm -1 Below, or approximately 15cm -1 The following median absorption coefficients are characterized by measurement wavelengths in the 265 nm UV range. The exemplary median absorption coefficient range is approximately 1 to 50 cm. -1 , about 5~30cm -1 , and approximately 10-20cm -1 This includes. References to absorption coefficients in this specification refer to measurements performed using the light transmission measurement techniques described in the Experimental section.

[0045] In light of the importance of both structural quality and UV transmittance in AlN substrates suitable for use in optoelectronic devices, the inventors have developed a dimensionless figure of merit (FOM) that combines these critical qualities into a single quality control measurement, thereby not only evaluating the overall structural quality or transmittance of the substrate but also capturing the uniformity of quality / transmittance across the entire surface of the substrate. The FOM can have a value between 0 and 1, where 1 represents the best possible combination of structural quality / uniformity and UV-C transmittance / uniformity. The FOM is defined as follows: FOM=A*R*T*U In the formula, 1×1mm 2 When determined using the grid overlay, A = (surface area of ​​substrate without expansion defects / total surface area of ​​substrate), and the surface area of ​​substrate without expansion defects is the portion of the total surface area that is free of expansion defects. R = 15 arcsec / maximum(15 arcsec, median FWHM), where median FWHM is the median of the total full width at half maximum (FWHM) intensity measurements of the biaxial rocking curves for the (002) and (102) crystal planes, expressed in arcsec. T = exp(-alpha_median * 0.01 cm), where alpha_median is the median absorption coefficient at 265 nm. U = 1 - 3(alpha_st.dev. / alpha_mean), where alpha_st.dev. is the standard deviation of the absorption coefficient measurements and alpha_mean is the average absorption coefficient at 265 nm.

[0046] Any measurement techniques necessary to calculate the FOM value are provided in the experimental section of this disclosure. In certain embodiments, a 2-inch diameter portion of the AlN substrate has an FOM value of 0.4 or greater, for example, 0.45 or greater, or 0.5 or greater, or 0.55 or greater. Exemplary ranges of FOM values ​​include 0.4 to about 0.9, or about 0.45 to about 0.8, or about 0.5 to about 0.65.

[0047] In contrast to conventional knowledge, this disclosure presents evidence that it is not necessary to control elemental impurities such as carbon, oxygen, and silicon to extremely low levels in order to manufacture AlN substrates with high structural quality and UV transmittance, such as substrates with the high FOM values ​​mentioned above. Instead, in certain embodiments, the concentration of one or more of carbon, oxygen, and silicon is 10 18 cm -3 It has been determined that the elemental impurities may be within this range. Elemental impurities can be measured to a depth of 3 μm using secondary ion mass spectrometry (SIMS), as described in the experimental section, provided by private laboratories such as Eurofins EAG Laboratories.

[0048] In certain embodiments of this disclosure, the carbon concentration in the AlN substrate is approximately 1 × 10⁻⁶ 18 cm -3 For example, approximately 2 × 10 18 cm -3 The above, or approximately 3 x 10 18 cm -3 The above, or approximately 4 x 10 18 cm -3 Above, or approximately 5 x 10 18 cm -3 The above, or approximately 6 x 10 18 cm -3The above may be true. Exemplary carbon concentration ranges are from about 1×10 18 cm -3 to about 10×10 18 cm -3 or from about 2×10 18 cm -3 to about 9×10 18 cm -3 or from about 4×10 18 cm -3 to about 8×10 18 cm -3 and include.

[0049] In certain embodiments of the present disclosure, the concentration of oxygen in the AlN substrate is about 1×10 18 cm -3 or more, for example, about 2×10 18 cm -3 or more, or about 3×10 18 cm -3 or more, or about 4×10 18 cm -3 or more, or about 5×10 18 cm -3 or more, or about 6×10 18 cm -3 or more. Exemplary oxygen concentration ranges are from about 1×10 18 cm -3 to about 9×10 18 cm -3 or from about 2×10 18 cm -3 to about 8×10 18 cm -3 or from about 3×10 18 cm -3 to about 7×10 18 cm -3 and include.

[0050] In certain embodiments of the present disclosure, the concentration of silicon in the AlN substrate is about 1×10 18 cm -3 or more, for example, about 2×10 18 cm -3 or more, or about 3×10 18 cm -3 or more, or about 4×10 18 cm -3It can be greater than or equal to this. An example silicon concentration range is approximately 1 × 10⁻⁶. 18 cm -3 ~about 6×10 18 cm -3 , or approximately 2 x 10 18 cm -3 ~Approx. 5×10 18 cm -3 , or approximately 2 x 10 18 cm -3 ~Approx. 4×10 18 cm -3 Includes.

[0051] The above FOM measurements apply to a substrate with a diameter of 2 inches, but it should be noted that the aforementioned FOM is not limited to this particular substrate diameter. For substrates of any other diameter, the intervals between the XRRC and optical wafer mapping steps should simply be scaled linearly with respect to the substrate diameter, and the number of individual topographic images should be adjusted to cover the entirety of any given substrate. As a result, all of the above definitions remain valid for substrate diameters other than 2 inches.

[0052] AlN crystal growth process and system The process for preparing single-crystal AlN substrates involves physical vapor transport, meaning that the process involves the physical transport of vapor of the desired material (i.e., AlN) from the AlN raw material to the deposition site in a crucible or other crystal growth chamber. The deposition region typically contains single-crystal AlN seed material, and the growth process is typically carried out in an induction heating reactor. Seeded PVT growth processes for growing single-crystal AlN substrates suitable for use in this disclosure are described, for example, in U.S. Patent No. 7,678,195 by Schlesser et al., each of which is incorporated herein by reference in its entirety; Ehrentraut, D., & Sitar, Z. (2009) Advances in bulk crystal growth of AlN and GaN, MRS Bulletin, 34(4), 259-265; Lu et al. (2009) Seeded growth of AlN bulk crystals in m- and c-orientation, Journal of Crystal Growth, 312(1), 58-63; and Herro et al. (2010) Growth of AlN single crystalline boules, Journal of Crystal Growth, 312(18), 2519-2521. Crucibles useful for PVT crystal growth processes, such as those formed from tantalum carbide, niobium carbide, or alloys thereof, are described in U.S. Patent No. 7,632,454 by Schlesser et al., incorporated herein by reference.

[0053] In certain embodiments, the AlN single crystal substrate described herein is formed by a seed growth process using a physical vapor transport process, in which the raw material and the seed are separated in a crucible and heated in a manner sufficient to sublimate the raw material, thereby transporting volatile species from the raw material to the seed and recondensing them on the seed. The method of the present disclosure can be carried out using any high-temperature reactor capable of producing crystal growth temperatures in the range of about 1900°C to 2400°C. In certain embodiments, the reactor should also be able to operate at a pressure of up to about 1000 Torr. The reactor should provide the ability to control the temperature distribution within the reactor. In particular, the reactor should be configured to be able to establish an axial temperature gradient within the reactor (e.g., along the axis of symmetry of a cylindrical crucible).

[0054] Various reactor designs are possible to satisfy the above requirements. In induction-heated reactors, changes in the relative position between the induction coil and the susceptor / crucible induce changes in the upper and lower temperatures of the crucible, and consequently, changes in the axial temperature gradient within the crucible. The relative position between the induction coil and the crucible can be changed by a mechanism that allows the induction coil to move axially in a controllable manner, or by a mechanism that allows the crucible to translate axially in a controllable manner within the reactor's hot zone, or by a combination of such mechanisms. In resistance-heated reactors, one way to influence the temperature gradient within the crucible involves moving the crucible within the reactor's hot zone with a non-uniform axial temperature profile. Such a temperature profile may be established by using concentric heaters of limited length, resulting in a temperature gradient at the heater boundary.

[0055] Figure 1a shows an exemplary radio frequency (RF) heated water-cooled reactor, and Figure 1b shows an exemplary crucible design. The reactor 10 includes a heating chamber 12 having a size sufficient to accommodate a crucible 20. As shown, the reactor 10 includes an insulating layer 14 surrounding the heating chamber 12. The insulating layer 14 may include carbon fibers (e.g., carbon-bonded carbon fiber composites (CBCFs)) or carbon-based materials such as graphite. As stated above, strict control of carbon impurities is not required to implement this disclosure, so it is not necessary to remove all carbon materials from the reactor to create a carbon-free environment. Cost-effective carbon-based insulating materials can be used instead of relatively expensive insulating materials such as bubble alumina or thermal shielding made from refractory materials. Furthermore, the reactor 10 includes a double quartz tube 16 surrounding the insulating layer 14 and an induction coil 18 enclosing the double quartz tube. Although not shown in Figure 1a, as described above, the reactor design includes a mechanism for changing the relative position of the induction coil and the crucible in order to establish an axial temperature gradient.

[0056] Referring to Figure 1b, an exemplary crucible 20 for use in the present disclosure includes a crucible body 22 defining an open-end chamber 24 in which AlN raw material 26 can be placed. As shown, the chamber 24 is defined by the walls and bottom of the crucible body 22. The crucible 20 further includes a removable cap 28. As shown, an AlN single crystal seed 30 can be fused to the cap 28. Although a cylindrical shape is shown in Figure 1b, other crucible shapes can be used without departing from the present disclosure. Typically, the crucibles used in the present disclosure have a height of about 1 to about 10 inches (e.g., about 1 to about 5 inches), a wall thickness of about 1 / 16 to about 5 / 8 inches (e.g., about 1 / 8 to about 1 / 4 inch), and a width or outer diameter of about 3 / 4 to about 5 inches (e.g., about 1 to about 5 inches).

[0057] Figure 2 shows a further exemplary reactor design for use in the present disclosure. Reactor 50 is induction heated by a coil 32 powered by an inverter (not shown), such as an air-cooled 10 kHz RF inverter available from Mesa Electronics, and features a double water-cooled quartz tube 34 for radially extracting heat around the reactor. Reactor 50 also includes cooling baffles 36 at the top and bottom of the heating chamber 38, through which cooling water can flow to axially extract heat from the top and bottom of the heating chamber. Thick carbon-based insulation 40 (e.g., CBCF) is placed inside reactor 50 to minimize the radial temperature gradient within the crucible 20. The thickness of the insulation 40 at the top and bottom of the heating chamber 38 and the relative positions of the coil 32 can be used to provide a desired temperature gradient within the reactor. t ) and lower temperature (T b Two infrared pyrometers, 42 and 44, are positioned to measure each of the following:

[0058] The inner quartz tube 34 may be fitted using a double O-ring seal assembly to limit leakage of ambient air or cooling water, and the volume between the O-rings may be emptied by a vacuum pump or flushed by a steady flow of a gas inert to the crystal growth process. A process gas, typically nitrogen, or a mixture of nitrogen / hydrogen / argon, may flow upward through the inner quartz tube. A mass flow controller (not shown) may control the gas flow rate, and an electronic upstream pressure controller (not shown) may be used to keep the reactor pressure constant. Temperature may be controlled passively using feedforward power control or actively using the IR pyrometer signal as a process variable in a feedback power control scheme (not shown).

[0059] Directly measuring the temperature of the seed crystal inside the crucible enclosure is difficult. However, as is common practice in the art, a non-contact infrared pyrometer (e.g., a two-color pyrometer from the Raytek Endurance series) can be used to accurately determine the surface temperature of a suitable blackbody target (e.g., a solid graphite disk) that is placed in direct thermal contact with the outer crucible surface adjacent to the seed location.

[0060] It should be understood that the pyrometer reading does not generally represent the seed temperature because there is a temperature gradient (e.g., inside the crucible wall, inside the pyrometer target) that causes a temperature offset between the pyrometer reading and the actual seed temperature. To minimize this unavoidable offset, it is desirable to place the pyrometer target as close as possible to the seed location.

[0061] Furthermore, by finite element analysis of the reactor hot zone (for example, using a commercially available software package such as COMSOL multiphysics software in conjunction with knowledge of the relevant physical properties of all hot zone components), the temperature offset can be modeled for any given crystal growth conditions, and once established, the seed temperature can be derived from the measured infrared temperature data.

[0062] The AlN raw material temperature can be derived using a second infrared pyrometer that measures a second target in thermal contact with the crucible surface adjacent to the raw material location. Thus, any seed temperature (i.e., crystal growth temperature) and AlN raw material temperature referred to herein represent the actual seed temperature and raw material temperature determined by finite element analysis.

[0063] The crystal growth process of this disclosure typically begins with an in-situ seed conditioning process adapted to remove contaminants and surface / subsurface damage by high-temperature evaporation of the seed material at an inverted temperature gradient, meaning the seed is kept at a higher temperature than the raw material. Thus, no net crystal growth occurs at the seed's location before the seed surface is cleaned and recrystallized. The amount of seed material removed is determined by the duration of the seed conditioning process, as well as the process temperature, process pressure, and the magnitude of the axial temperature gradient. If mechanical damage to the seed surface or subsurface must be removed, more seed material needs to be evaporated. Typically, the seed conditioning process involves evaporation of at least about 10 μm of seed thickness. In some embodiments, seed thicknesses of about 1 to about 500 μm are removed during this process. For the removal of surface oxide layers, evaporation of about 1 to about 10 μm of seed thickness is typical. For the removal of subsurface damage to the seed, evaporation of about 100 μm or more of seed thickness is typical. The process temperature is typically in the range of approximately 2000 to 2300°C, and the axial temperature gradient between the raw material and the seed is in the range of nearly isothermal to approximately 100°C / cm, with a typical temperature gradient of approximately 5 to 30°C / cm. The duration of the washing process can vary, but is typically in the range of approximately 15 minutes to 6 hours. The total reactor pressure can vary, but is typically in the range of approximately 100 Torr to 1000 Torr, for example, approximately 100 Torr to 760 Torr, or approximately 100 Torr to 500 Torr.

[0064] After the in-situ seed conditioning step, the process may immediately proceed to crystal growth without interrupting heating. Further inversion of the crucible / cap assembly in the reactor is not required before the start of the crystal growth step of the process. During the crystal growth step of the process, the temperature gradient is reversed compared to the seed washing step. This can be done by changing the temperature field in the reactor hot zone by external control. For example, this can be achieved by using a movable RF induction coil or by using one or more auxiliary heaters. Alternatively, the crucible can be physically displaced within the thermally heterogeneous reactor hot zone. It is important that the transition from the washing step to the growth step is continuous, meaning that the temperatures of the raw material and seed do not change abruptly during the reversal of the heat gradient. As understood in the art, the crystal material deposited during the seed growth step has the same crystallographic orientation as the seed. The single-crystal material 72 prepared during the crystal growth step of this method can be collected and subdivided for further use.

[0065] The same nitrogen-containing atmosphere, reactor pressure, and temperature range specified above for the seed washing process can be used in the crystal growth stage. However, instead of the seed temperature being higher than the raw material temperature, the direction of the temperature gradient is reversed so that the raw material temperature is higher than the seed temperature. In the crystal growth stage of the process, a gradient of the same magnitude as described above, with reference to the seed conditioning process, can be used.

[0066] However, it has been determined that the crystal growth temperature should be in a more moderate range in order to achieve the highest levels of structural quality and UV transmittance, as defined by even higher FOM values. For example, the crystal growth temperature in the crucible adjacent to the single-crystal AlN seed during the crystal growth process can be maintained at about 2200°C or less, for example, about 2150°C or about 2120°C or less. Exemplary crystal growth temperature ranges include about 2100°C to about 2200°C, or about 2100°C to about 2150°C, or about 2100°C to about 2120°C.

[0067] The temperature in the crucible adjacent to the AlN raw material during the crystal growth heating stage can be maintained, for example, below about 2200°C, for example, below about 2180°C. Exemplary AlN raw material temperature ranges include about 2100°C to about 2200°C, or about 2120°C to about 2200°C, or about 2150°C to about 2200°C.

[0068] The pressure inside the crucible during the crystal growth heating stage is typically less than or equal to about 500 Torr, for example, about 100 to about 500 Torr.

[0069] Figure 3 shows only one of the various shapes that can be used in the seed growth process. The raw material 70 and seed 68 can have various shapes. In the example in Figure 3, the single crystal boule 72 grows mainly axially along an axial temperature gradient. However, this disclosure can also be applied to other shape configurations, such as a long cylindrical seed surrounded by a hollow cylindrical raw material. The principle of the integrated process, including in-situ seed washing by evaporation and subsequent seed growth, is not limited by factors other than the ability to establish and control the required temperature gradient.

[0070] It has been found that the manner in which AlN single-crystal Boules are cooled after the crystal growth process can affect the quality and UV transmittance of AlN Boules. In particular, it has been found that it is important to cool the Boules in a relatively slow and controlled manner within an intermediate temperature range from a first seed temperature of approximately 1900°C to approximately 1800°C to a second temperature of approximately 1500°C to approximately 1400°C. Within this temperature range, it is important to maintain relatively slow cooling rates such as 5.0°C / min or less, or approximately 4.0°C / min or less, or approximately 3.0°C / min or less, or approximately 2.0°C / min or less, or approximately 1.5°C / min or less. Exemplary cooling rate ranges include approximately 0.5°C / min to 5.0°C / min, or approximately 1.0°C / min to approximately 4.0°C / min, or approximately 1.5°C / min to approximately 3.0°C / min. This cooling rate can be applied, for example, from a starting seed temperature of approximately 1900°C, 1850°C, or 1800°C to a finishing seed temperature of approximately 1500°C, 1450°C, or 1400°C.

[0071] While not bound by any particular operating theory, the relatively slow cooling rate within this intermediate temperature range is considered advantageous for promoting the formation of complexes between various elemental impurities (e.g., carbon, oxygen, or silicon) that do not degrade the UV transmittance of the AlN material. Thus, AlN substrates suitable for use in UV-range photoelectronic devices can be prepared despite the presence of impurities that were conventionally thought to inhibit the desired level of UV transmittance. By rearranging the elemental impurities into harmless complexes, it is considered possible to achieve a favorable range of UV transmittance, for example, transmittance at the commercially important UV wavelength of 265 nm.

[0072] The cooling rates applied to AlN Boolean in the temperature ranges above and below the intermediate range described above may be relatively uncontrolled. While not bound by any particular operating theory, the cooling rates in the temperature range from the crystal growth temperature to the upper end of the intermediate range described above (i.e., the high-temperature range) are considered to have little effect on UV transmittance, at least partially, because elemental impurities are fluid and therefore relatively unstable at high temperatures. At the lower end of the temperature range from the lower end of the intermediate range described above to room temperature (i.e., the low-temperature range), elemental impurities are relatively fixed, and the cooling rate is considered to no longer affect their form. For these reasons, the cooling rates in the high-temperature and low-temperature ranges are not particularly limited. For process efficiency, it is advantageous for the cooling rates in one or both of these ranges to exceed the cooling rates used in the intermediate range, e.g., above 5.0°C / min, or above 10°C / min, or above 15°C / min, or above 20°C / min, or above 25°C / min, or above 30°C / min. Exemplary cooling rate ranges in the high-temperature and low-temperature ranges include approximately 6.0°C / min to approximately 80°C / min, or approximately 10°C / min to approximately 60°C / min, or approximately 20°C / min to approximately 40°C / min.

[0073] Following the cooling process described above, no further heating / annealing of the AlN single crystal boule is required. Therefore, in certain embodiments, the cooled AlN single crystal boule is not subjected to further heating above 2000°C after being cooled as described above.

[0074] In the art, it is known that one or more gettering materials are included in the crucible or reactor environment to capture and remove elemental impurities such as carbon, oxygen, or silicon. Such materials must be relatively stable at the crystal growth temperature. Examples of gettering materials include boron (melting point about 2300°C), iridium (melting point about 2410°C), niobium (melting point about 2468°C), molybdenum (melting point about 2617°C), tantalum (melting point about 2996°C), rhenium (melting point about 3180°C), and / or tungsten (melting point about 3410°C). Such gettering materials are not required because such impurities do not need to be controlled to extremely low levels according to this disclosure. In certain embodiments, the interior of the crucible used in this disclosure is substantially free of gettering materials, meaning that such materials are not intentionally added to the interior of the crucible. In certain embodiments, the concentration of such gettering material is less than 0.5% by weight, or less than 0.1% by weight, or less than 0.01% by weight, based on the total weight of all materials placed in the crucible.

[0075] Light-emitting device structure Embodiments of this disclosure also relate to optoelectronic devices such as light-emitting diodes ("LEDs") and laser diodes ("LDs") that can operate in the UV spectral range. Optoelectronic devices fabricated on AlN substrates as described herein maintain the crystalline structure, including defects in the substrate beneath the device. Because the AlN single-crystal substrate materials described herein exhibit excellent structural quality and UV transmittance, optoelectronic devices fabricated using such substrates are expected to exhibit strong performance characteristics, including long lifespan and improved efficiency.

[0076] Semiconductor devices such as light-emitting devices have a multilayer structure formed on a base substrate. To increase luminescence efficiency, each layer is required to have high crystallinity with few dislocations and point defects. Generally, LEDs have a multilayer structure that includes a substrate base and an active region between an n-type semiconductor layer electrically connected to an n-electrode and a p-type semiconductor layer electrically connected to a p-electrode. For the efficiency and lifespan of nitride-based semiconductor devices, it is important to achieve a low defect density throughout the active region. As described above, the high-quality substrates described herein are used to construct optoelectronic devices with low defect density and desirable performance characteristics.

[0077] While the precise structure and preparation methods of light-emitting devices can vary, they typically involve epitaxial growth, mounting, and packaging processes known in the art. Exemplary epitaxial growth processes include molecular beam epitaxial growth (MBE), metal-organic chemical vapor deposition (MOCVD), hydride vapor deposition (HVPE), and liquid phase deposition (LPE). One advantage of using the above-mentioned high-quality AlN substrates to construct light-emitting devices is that less optimization of the light-emitting structure is required to obtain robust device performance. The exemplary LED and LD devices of this disclosure may be simple homojunction devices, double heterostructure devices, or multi-well active layer devices.

[0078] The LD structure is similar to that of an LED structure. The LD incorporates an additional layer that properly confines photons and forms a resonant cavity. In edge-emitter LDs, the resonant cavity is oriented perpendicular to the layer growth direction, and the semiconductor layer structure is cleaved or etched to form a mirror. In such embodiments, the layers above and below the active region are modified to act as cladding layers to ensure that emitted photons propagate perpendicular to the layer growth direction without significant absorption.

[0079] Methods for constructing light-emitting devices include, for example, U.S. Patent No. 8,080,833 by Grandusky et al.; U.S. Patent No. 8,222,650 by Schowalter et al.; U.S. Patent No. 9,299,883 by Xie et al.; and U.S. Patent No. 9,437,430 by Schowalter et al., all of which are incorporated herein by reference; as well as Dalmau et al. (2011) Growth and characterization of AlN and AlGaN epitaxial films on AlN single crystal substrates, Journal of the Electrochemical Society, 158(5), H530-H535; Collazo et al. (2011) 265 nm light-emitting diodes on AlN single crystal substrates: Growth and characterization, (2011 Conference on Lasers and Electro-Optics (CLEO)); Collazo et al. (2011) Progress on n-type doping of AlGaN alloys on AlN single crystal substrates for UV Optoelectronic applications, Physica Status Solidi C - Current Topics in Solid State Physics, 8, 7-8; and Grandusky et al. (2010) Performance and reliability of ultraviolet-C pseudomorphic light emitting diodes on bulk AlN substrates, Phys. Status Solidi C, 7:2199-2201.

[0080] The ultraviolet light emitters based on Group III nitrides of this disclosure are useful in any industry where UV light is used, such as disinfection and sterilization, currency authentication, identification and verification, photolithography, phototherapy, or detection of bodily fluids or other organic substances. For example, U.S. Patent Publication 2009 / 0250626, whose entire disclosure is incorporated herein by reference, discloses a liquid disinfection apparatus comprising one or more LEDs that emit electromagnetic radiation primarily at two or more different UV wavelengths. U.S. Patent Publication 2010 / 0314551, whose entire disclosure is incorporated herein by reference, discloses a method and system for purifying a liquid to flow so as to produce a desired germicidal effect by exposing the liquid to UV radiation using at least one UV LED.

[0081] Experimental substrate analysis technology There are no particular restrictions on sample thickness, but the sample thickness is measured, typically ranging from 50 μm to 500 μm, for example, typically around 400 μm. In addition, the m-direction of the sample and the off-cut angle of the crystal axis relative to the sample surface are measured beforehand. After coring, orientation, multi-diamond wire saw slicing, lapping, and polishing, a double-sided polished 2-inch c-plane substrate was obtained. The final chemical mechanical polishing (CMP) step produced an undamaged surface with a surface roughness of less than nm as determined by atomic force microscopy. The substrate was oriented toward the m-plane with a vicinal off-axis orientation of less than 1° from the c-plane, typically around 0.3° ± 0.15°. Prior to measurement, the substrate surface was subsequently washed in acetone, deionized water, and methanol. This preparation specification is intended to ensure the optical analysis is as accurate as possible.

[0082] X-ray topography (XRT) XRT is used to measure the distribution of dislocation arrays, also known as small-angle grain boundaries (LAGBs), within AlN crystals. These LAGBs consist of dislocations arranged in specific geometric patterns, representing regions of high dislocation (structural defect) density within the crystal. Due to lattice strains associated with the dislocations, the arrays can form closed, circular shapes where the enclosed region has a different crystallographic orientation from the surrounding material. In this configuration, the defects are also called grains. Additionally, contrast in XRT images can arise from additional extended defect-related mechanisms, such as surface scratches. Contrast resulting from artifacts caused by the substrate holder is excluded from extended defect area measurements.

[0083] The proposed FOM includes a parameter called usable area A, which is the ratio of the substrate area without expansion defects to the total substrate area. Usable area A is 1 × 1 mm 2 A is calculated from an XRT image with a grid overlay, where A = ((total substrate area - number of grids with extended defect contrast) / total substrate area). The XRT image is acquired under the following specified conditions. ·Equipment: Panalytical X'Pert Pro Materials Research Diffractometer • X-ray tube: copper target, long microfocus X-ray tube, line focus setting • Incident beam optics: 1 / 2° divergence slit, 0.1 mm topography mask, W / Si parabolic mirror • Diffraction beam optics: PIXcel 3D detector, static area detector mode AlN reflection: 11.4% glancing incidence • Substrate mounting: Mount a double-sided polished AlN substrate to an X-ray goniometer stage with the Al polar side facing upwards and the main plane at the 6 o'clock position. • Scan parameters: Acquire individual topographic scans using a static area detector with a 10-second acquisition time, 255 pixels in the equatorial direction (2.5108° in 2-theta) and 255 pixels in the axial direction (2.5108° in gamma). • Substrate Mapping: For a 2-inch AlN substrate, a 15x15 measurement point grid is obtained with a step size of 4.0mm between scans. This area covers and extends beyond the entire substrate surface. • Final image: 225 individually acquired 4x4mm images using image stitching software. 2 Stitch the topographic images onto a 2-inch wafer topograph.

[0084] X-ray rocking curves and maps The structural integrity of a crystalline substrate is generally assessed by X-ray diffraction measurements known as X-ray rocking curves (XRRCs). The width of the rocking curve peaks measured from a given X-ray reflection of the crystalline material explores the degree of lattice plane ordering of the crystal, relative to a given reflection. Several factors, including dislocations that generate strain fields and thus negatively affect lattice plane ordering, can broaden the rocking curve. Therefore, a rocking curve with peaks of narrow symmetry is an indicator of high structural integrity, i.e., a low density of dislocations. When XRRCs are measured as a function of position across the surface of the crystalline substrate, a rocking curve map is generated.

[0085] The proposed FOM includes a parameter called the rocking curve quality coefficient R, which is evaluated by first calculating the median of all recorded rocking curve FWHMs, expressed in arcsecs, then dividing 15 arcsecs by the median, and finally limiting the result to a value between 0 and 1. The XRRC map is acquired under the following specified conditions: ·Equipment: Panalytical X'Pert Pro Materials Research Diffractometer • X-ray tube: copper target, long microfocus X-ray tube, line focus setting • Incident beam optics: 1 / 2° divergent slit, 2mm mask, W / Si parabolic mirror, 4x Ge

[0220] monochromator, cross-slit collimator set to mask width 1.62mm and divergent slit height 10mm • Diffraction beam optics: PIXcel 3D detector, open detector mode • AlN reflection: Symmetry 00.2 and skew symmetry 10.2 • Substrate mounting: Mount a double-sided polished AlN substrate to an X-ray goniometer stage with the Al polar side facing upwards and the main plane at the 6 o'clock position. • Scan parameters: Acquire individual rocking curve scans over a range of 0.1°, in 0.0002° steps, and at a time of 0.05 seconds per step. • Substrate mapping: For a 2-inch AlN substrate, 30 x 30 mm corresponds to a 3.0 mm step between measurement points. 2 Acquire the XRRC of each AlN reflection on an 11x11 measurement grid across the square. Center the substrate map (0,0) center point to the center of the substrate geometry. • Scanning process: Peak width is expressed in arcsec units (3600 arcsec = 1°) as the full width at half maximum (FWHM) intensity of the XRRC peak corresponding to each measurement position.

[0086] AlN absorption coefficient (alpha) at 265 nm To calculate the light absorption coefficient of a material, light transmission measurements are commonly used. In a typical experiment, light of intensity I0 is incident on the sample, and the transmitted light intensity I is measured as a function of wavelength. The transmittance Tr is then given by the following: Tr = I / I0.[1]

[0087] The proposed FOM includes a parameter called idealized transmittance T, which consists of a transmittance normalized to a thickness of 0.01 cm, calculated from the median absorption coefficient determined from the substrate absorption coefficient map.

[0088] The proposed FOM also includes a parameter called the permeability uniformity coefficient U, which is defined by first evaluating the relative standard deviation (i.e., the standard deviation divided by the mean) of all values ​​in the absorption coefficient map, and then subtracting 3 times the relative standard deviation from 1. The absorption coefficient map is obtained under the following specified conditions: • Equipment: XY Auto Stage accessories and 3x3mm 2 Shimadzu SolidSpec UV3700 equipped with a square beam mask ·Measurement method: photometry ·Measurement wavelength: 265nm • Measurement parameter: Transmittance • Slit width: 20nm • Substrate Mapping: For a 2-inch AlN substrate, measurements are performed on a 7x7 measurement grid, and in addition, three additional points are measured for each of the four basic directions of the wafer edge, for a total of 61 measurement points. The step spacing between measurement points is 5.0 mm. The substrate map (0,0) center point is centered on the substrate geometric center.

[0089] Absorption coefficient calculation: Calculate the AlN absorption coefficient for each measurement point using the following formula. a = (-1 / d) * ln(Tr / c),[2] In the formula, a is cm -1 The absorption coefficient is in units, d is the substrate thickness in cm, ln is the natural logarithm, Tr is the measured transmittance, and c is a correction factor that explains the reflectance loss at the substrate / air interface due to the difference in refractive index between the substrate and air (for 265 nm light, c = 0.7).

[0090] Comparative example A AlN boules are grown using a single-crystal AlN seed separated from the AlN raw material in a crucible, using the crystal growth reactor shown in Figure 2. During crystal growth, the temperature of the AlN seed (i.e., the crystal growth temperature) is maintained at approximately 2260°C, and the temperature of the AlN raw material is maintained at approximately 2320°C to maintain the temperature gradient necessary for the sublimation of the AlN raw material and the deposition of the AlN crystal material onto the seed. The pressure inside the crystal growth reactor is maintained at approximately 600 Torr.

[0091] After crystal growth, the temperature of the AlN Boule is lowered at a rate of approximately 50°C / min from the crystal growth temperature until the Boule temperature reaches approximately 1400°C. Then, the Boule is cooled to room temperature in a relatively uncontrolled manner, with a cooling rate fluctuating from approximately 50°C / min around 1400°C to approximately 3°C / min around room temperature.

[0092] Comparative example B AlN boules are grown using a single-crystal AlN seed separated from the AlN raw material in a crucible, and the crystal growth reactor shown in Figure 2. During crystal growth, the temperature of the AlN seed (i.e., the crystal growth temperature) is maintained at approximately 2110°C, and the temperature of the AlN raw material is maintained at approximately 2170°C to maintain the temperature gradient necessary for the sublimation of the AlN raw material and the deposition of the AlN crystal material onto the seed. The pressure inside the crystal growth reactor is maintained at approximately 450 Torr.

[0093] After crystal growth, the temperature of the AlN Boule is lowered at a rate of approximately 50°C / min from the crystal growth temperature until the Boule temperature reaches approximately 1400°C. Then, the Boule is cooled to room temperature in a relatively uncontrolled manner, with a cooling rate fluctuating from approximately 50°C / min around 1400°C to approximately 3°C / min around room temperature.

[0094] Therefore, this crystal growth was carried out at a lower growth temperature than in Comparative Example A, but with the same relatively rapid cooling as Boolean.

[0095] Example 1 AlN boules are grown using a single-crystal AlN seed separated from the AlN raw material in a crucible, and the crystal growth reactor shown in Figure 2. During crystal growth, the temperature of the AlN seed (i.e., the crystal growth temperature) is maintained at approximately 2110°C, and the temperature of the AlN raw material is maintained at approximately 2170°C to maintain the temperature gradient necessary for the sublimation of the AlN raw material and the deposition of the AlN crystal material onto the seed. The pressure inside the crystal growth reactor is maintained at approximately 450 Torr.

[0096] After crystal growth, the temperature of the AlN Boule is lowered at a rate of approximately 1.5°C / min from the crystal growth temperature until the Boule temperature reaches approximately 1400°C. Then, the Boule is cooled to room temperature in a relatively uncontrolled manner, with a cooling rate that fluctuates from approximately 50°C / min around 1400°C to approximately 3°C / min around room temperature.

[0097] Therefore, although this crystal growth was carried out at the same low crystal growth temperature as Comparative Example B, it involved a much slower Boule cooling compared to Comparative Examples A or B.

[0098] Analysis of substrate materials for FOM calculations Two-inch diameter substrates were cut from the boules prepared by each of Comparative Example A, Comparative Example B, and Example 1, and prepared for the above analysis under the substrate analysis technique.

[0099] After preparation, each substrate was analyzed, and the four FOM parameters (A, R, T, U) were determined using the test methods described above under Substrate Analysis Techniques. The results are shown in Tables 1-3 below.

[0100] Table 1 shows the X-ray topography results and the calculation of the FOM parameter A, where 1 × 1 mm 2 When determined using a grid overlay, A = (surface area of ​​substrate without expansion defects / total surface area of ​​substrate), where the surface area of ​​substrate without expansion defects is the portion of the total surface area that is free of expansion defects.

[0101] Figure 4a provides an XRT image of an AlN substrate from Comparative Example A, showing dark contrast due to an expanding defect in a specific area. Figure 4b shows the same substrate, with the location of the expanding defect indicated by red shading. In the upper right corner of both images, contrast resulting from artifacts caused by the substrate holder is observed.

[0102] Figure 5a provides an XRT image of an AlN substrate from Comparative Example B, showing dark contrast due to an expanding defect in a specific area. Figure 5b shows the same substrate, with the location of the expanding defect indicated by red shading. In the upper right corner of both images, contrast resulting from artifacts caused by the substrate holder can be observed.

[0103] Figure 6a provides an XRT image of an AlN substrate from Example 1, showing dark contrast due to an expansion defect in a specific area. Figure 6b shows the same substrate, with the location of the expansion defect indicated by red shading. In the upper right corner of both images, contrast resulting from artifacts caused by the substrate holder can be observed. [Table 1]

[0104] As shown in Table 1, the lower crystal growth temperature in Comparative Example B compared to Comparative Example A minimized the impact on the expanded defect contrast area. However, the slower cooling process in Example 1 slightly increased the expanded defect contrast area compared to Comparative Examples A or B, but still resulted in an extremely high area of ​​defects-free material.

[0105] Table 2 shows the X-ray rocking curve (XRCC) results, expressed in arcsec as the median of the total rocking curve full width at half maximum (FWHM) intensity measurements for the (002) and (102) crystal planes, and the FOM parameter R is calculated, where R = 15 arcsec / maximum(15 arcsec, median FWHM).

[0106] Figure 7a is the 0.2 XRRC map of the FWHM measurements of the AlN substrate from Comparative Example A. Figure 7b is the 10.2 XRRC map of the FWHM measurements of the AlN substrate from Comparative Example A.

[0107] Figure 8a is the 0.2 XRRC map of the FWHM measurements of the AlN substrate from Comparative Example B. Figure 8b is the 10.2 XRRC map of the FWHM measurements of the AlN substrate from Comparative Example B.

[0108] Figure 9a is the 0.2 XRRC map of the FWHM measurements of the AlN substrate from Example 1. Figure 9b is the 10.2 XRRC map of the FWHM measurements of the AlN substrate from Example 1. [Table 2]

[0109] As shown in Table 2, the lower crystal growth temperature in Comparative Example B compared to Comparative Example A minimized the impact on the central FWHM. Furthermore, the slower cooling process in Example 1 also minimized the impact on the central FWHM.

[0110] Table 3 shows the median absorption coefficient at 265 nm and the relative standard deviation calculated as alpha_st.dev. / alpha_mean, where alpha_st.dev. is the standard deviation of absorption coefficient measurements and alpha_mean is the mean absorption coefficient at 265 nm. Table 3 also provides the T and U calculations for each example of FOM, where T = exp(-alpha_median*0.01cm), where alpha_median is the median absorption coefficient at 265 nm, and U = 1-3(alpha_st.dev. / alpha_mean), where alpha_st.dev. is the standard deviation of absorption coefficient measurements and alpha_mean is the mean absorption coefficient at 265 nm.

[0111] Figure 10 provides a schematic diagram of the 265nm absorption coefficient map obtained from the AlN substrate of Comparative Example A. Figure 11 provides a schematic diagram of the 265nm absorption coefficient map obtained from the AlN substrate of Comparative Example B. Figure 12 provides a schematic diagram of the 265nm absorption coefficient map obtained from the AlN substrate of Example 1. [Table 3]

[0112] As shown in Table 3, the lower crystal growth temperature in Comparative Example B compared to Comparative Example A resulted in a significant improvement in UV transmittance. Furthermore, the slower cooling process in Example 1 led to an even greater improvement in UV transmittance compared to Comparative Example B.

[0113] Based on the above results, the dimensionless FOM for each example was calculated as follows: FOM=A*R*T*U In the formula, 1×1mm 2 When determined using the grid overlay, A = (surface area of ​​substrate without expansion defects / total surface area of ​​substrate), and the surface area of ​​substrate without expansion defects is the portion of the total surface area that is free of expansion defects. R = 15 arcsec / maximum(15 arcsec, median FWHM), where median FWHM is the median of the total full width at half maximum (FWHM) measurements of the biaxial rocking curves for the (002) and (102) crystal planes, expressed in arcsec. T = exp(-alpha_median * 0.01 cm), where alpha_median is the median absorption coefficient at 265 nm. U = 1 - 3(alpha_st.dev. / alpha_mean), where alpha_st.dev. is the standard deviation of the absorption coefficient measurements and alpha_mean is the average absorption coefficient at 265 nm.

[0114] The results are shown in Table 4 below. [Table 4]

[0115] As shown in Table 4, the lower crystal growth temperature in Comparative Example B compared to Comparative Example A resulted in some improvement in FOM. However, the slower cooling process in Example 1 resulted in a significantly higher FOM value compared to Comparative Example B. This indicates that, considering crystal quality / uniformity and UV transmittance / uniformity together, the present disclosure provides a significantly improved AlN substrate. Analysis of elemental impurities in substrate materials

[0116] In the Group III nitride industry, it is standard practice to analyze elemental impurities using secondary ion mass spectrometry (SIMS). In the SIMS process, primary ions (usually O2) are analyzed. + or Cs + The substrate surface is sputtered / etched using a beam of ) and secondary ions formed during the sputtering process are extracted and analyzed using a mass spectrometer (quadrupole, magnetic sector, or time-of-flight). Secondary ions can be concentrated in a range from matrix levels to sub-ppm trace levels. SIMS analysis is typically performed at private laboratories such as Eurofins EAG Laboratories.

[0117] Multiple substrate samples cut from Boules prepared according to Comparative Example A, Comparative Example B, and Example 1 were analyzed using SIMS by Eurofins EAG Laboratories, employing standard depth profiling up to 3 μm. The concentrations of carbon, oxygen, silicon, and hydrogen were reported from the SIMS analysis. The reported ranges for each Boule preparation technique are shown in Table 5 below, where the indicated ranges represent the average concentrations measured at depths of 2.5–3 μm. [Table 5]

[0118] As shown in Table 5, despite the excellent performance of Example 1 in terms of both structural defects and UV transmission, as indicated by the FOM calculations, Example 1 has significantly lower levels of carbon, oxygen, or silicon impurities compared to Comparative Examples A and B. This result is surprising from the perspective of prior art, which has focused on achieving low levels of elemental impurities as a requirement for achieving UV transmission.

[0119] Those skilled in the art, who have an interest in the teachings presented in the foregoing description and the associated drawings, will likely recognize many modifications and other embodiments of the disclosure described herein. Therefore, it should be understood that the disclosure is not limited to any particular embodiment disclosed, and that modifications and other embodiments are intended to be included within the claims. Certain terms are used herein, but they are used in a general and descriptive sense only and not for limiting purposes.

Claims

1. The single crystal AlN seed and the AlN raw material are arranged in a crucible with a separation relationship. The crucible is heated in a furnace so that a temperature gradient is formed between the single crystal AlN seed and the AlN raw material, causing a portion of the AlN raw material to sublimate and deposit on the single crystal AlN seed to form an AlN single crystal boule, wherein the crystal growth temperature in the crucible adjacent to the single crystal AlN seed during heating is maintained at approximately 2200°C or lower. The method includes cooling the AlN single crystal boule with three phases: a first cooling phase from the crystal growth temperature to a first intermediate temperature of about 1900°C to about 1800°C; a second cooling phase from the first intermediate temperature to a second intermediate temperature of about 1500°C to about 1400°C, characterized by a cooling rate of 5.0°C / min or less; and a third cooling phase from the second intermediate temperature to room temperature, wherein the cooling rate of one or both of the first and third cooling phases is greater than 5.0°C / min. A method for forming an aluminum nitride single crystal substrate.

2. The method according to claim 1, wherein the second cooling phase is performed at a cooling rate of 2.0°C / min or less.

3. The method according to claim 1, wherein the second cooling phase is performed at a cooling rate of 1.5°C / min or less.

4. The method according to any one of claims 1 to 3, wherein one or both of the first cooling phase and the third cooling phase are performed at a rate exceeding 10°C / min, for example, exceeding 15°C / min, or exceeding 20°C / min, or exceeding 25°C / min, or exceeding 30°C / min.

5. The method according to claim 1, wherein the temperature inside the crucible adjacent to the AlN raw material during heating is maintained at approximately 2200°C or lower, for example, approximately 2180°C or lower.

6. The method according to claim 1, wherein the temperature inside the crucible adjacent to the single crystal AlN seed during heating is maintained at approximately 2150°C or lower, for example, 2120°C or lower.

7. The method according to claim 1, wherein the pressure inside the crucible during heating is about 500 Torr or less, for example, about 100 to about 500 Torr.

8. The method according to any one of claims 1 to 7, wherein, after the cooling of the AlN single crystal boule, the AlN single crystal boule is not subjected to further heating above 2000°C.

9. The method according to any one of claims 1 to 8, wherein a 2-inch diameter substrate cut from the AlN single crystal boule has a dimensionless figure of merit (FOM) of 0.4 or more, for example, 0.45 or more, or 0.5 or more, and the FOM has the following formula: FOM=A*R*T*U (In the formula, 1 x 1 mm ２ When determined using the grid overlay, A = (surface area of ​​substrate without expansion defects / total surface area of ​​substrate), and the surface area of ​​substrate without expansion defects is the portion of the total surface area that is free of expansion defects. R = 15 arcsec / maximum (15 arcsec, median FWHM), where median FWHM is the median of the total full width at half maximum (FWHM) intensity measurements of the biaxial rocking curves for the (002) and (102) crystal planes, expressed in arcsec. T = exp(-alpha_median * 0.01 cm), where alpha_median is the median absorption coefficient at 265 nm. U = 1 - 3 (alpha_st.dev. / alpha_mean), where alpha_st.dev. is the standard deviation of the absorption coefficient measurements, and alpha_mean is the average absorption coefficient at 265 nm.

10. A 2-inch diameter substrate cut from the aforementioned AlN single crystal boule is (1) approximately 1 × 10 １８ cm －３ The above carbon concentrations; (2) approximately 1 × 10 １８ cm －３ The oxygen concentration above; (3) approximately 1 × 10 １８ cm －３ The method according to any one of claims 1 to 9, characterized by the above silicon concentration, or any two or more of (1), (2), and (3).

11. A substrate with a diameter of 2 inches cut from the AlN single crystal boule has (1) a carbon concentration of about 2×10 １８ cm －３ or more; (2) an oxygen concentration of about 2×10 １８ cm －３ or more; (3) a silicon concentration of about 2×10 １８ cm －３ or more, or the method according to claim 10, characterized by any two or more of (1), (2) and (3).

12. A 2-inch diameter substrate cut from the aforementioned AlN single crystal boule is (1) approximately 3 × 10 １８ cm －３ The above carbon concentrations; (2) approximately 3 × 10 １８ cm －３ The above oxygen concentration; (3) approximately 3 × 10 １８ cm －３ The method according to claim 11, characterized by the above silicon concentration, or two or more of (1), (2), and (3).

13. A 2-inch diameter substrate cut from the aforementioned AlN single crystal boule yielded (i) the median of the total full width at half maximum (FWHM) intensity measurements of the biaxial rocking curves with respect to the (002) and (102) crystal planes, with a range of approximately 25 arcsecs; and (ii) approximately 50 cm². －１ The following is the median absorption coefficient at 265 nm: (iii) approximately 150 mm ２ The method according to any one of claims 1 to 12, characterized by the following expanded defect contrast areas; or any combination of two or more of (i), (ii), and (iii).

14. An aluminum nitride single crystal substrate having a diameter of 2 inches or more, wherein the 2-inch diameter portion of the substrate has a dimensionless figure of merit (FOM) of 0.4 or more, for example, 0.45 or more, or 0.5 or more, or 0.55 or more, and the FOM has the following formula: FOM=A*R*T*U (In the formula, 1 x 1 mm ２ When determined using the grid overlay, A = (surface area of ​​substrate without expansion defects / total surface area of ​​substrate), and the surface area of ​​substrate without expansion defects is the portion of the total surface area that is free of expansion defects. R = 15 arcsec / maximum (15 arcsec, median FWHM), where median FWHM is the median of the total full width at half maximum (FWHM) intensity measurements of the biaxial rocking curves for the (002) and (102) crystal planes, expressed in arcsec. T = exp(-alpha_median * 0.01 cm), where alpha_median is the median absorption coefficient at 265 nm. U = 1 - 3 (alpha_st.dev. / alpha_mean), where alpha_st.dev. is the standard deviation of the absorption coefficient measurements, and alpha_mean is the average absorption coefficient at 265 nm.

15. (1) Approximately 1×10 １８ cm －３ The above carbon concentrations; (2) approximately 1 × 10 １８ cm －３ The oxygen concentration above; (3) approximately 1 × 10 １８ cm －３ The aluminum nitride single crystal substrate according to claim 14, characterized by the above silicon concentration, or any two or more of (1), (2), and (3).

16. (1) Approximately 2 x 10 １８ cm －３ The above carbon concentrations; (2) approximately 2 × 10 １８ cm －３ The oxygen concentration above; (3) approximately 2 × 10 １８ cm －３ The aluminum nitride single crystal substrate according to claim 15, characterized by the above silicon concentration, or two or more of (1), (2), and (3).

17. (1) Approximately 3 x 10 １８ cm －３ The above carbon concentrations; (2) approximately 3 × 10 １８ cm －３ The above oxygen concentration; (3) approximately 3 × 10 １８ cm －３ The aluminum nitride single crystal substrate according to claim 16, characterized by the above silicon concentration, or two or more of (1), (2), and (3).

18. (i) The median of the total full width at half maximum (FWHM) intensity measurements of the biaxial rocking curves for the (002) and (102) crystal planes for approximately 25 arcsecs or less; (ii) Approximately 50 cm －１ The following is the median absorption coefficient at 265 nm: (iii) approximately 150 mm ２ An aluminum nitride single crystal substrate according to any one of claims 14 to 17, characterized by the following expanded defect contrast areas; or any combination of two or more of (i), (ii), and (iii).

19. An aluminum nitride single crystal substrate according to any one of claims 14 to 18, A UV light-emitting diode structure superimposed on the aluminum nitride single crystal substrate and A photoelectronic device equipped with and adapted to emit ultraviolet light.

20. The photoelectronic device according to claim 19, wherein the ultraviolet light-emitting diode structure comprises a first electrode electrically connected to an n-type semiconductor layer and optionally a second electrode electrically connected to a p-type semiconductor layer.

21. The photoelectronic device according to claim 19, wherein the emission wavelength of the photoelectronic device is in the range of approximately 250 nm to 290 nm.

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