Gallium nitride crystal, gallium nitride substrate, and method for manufacturing gallium nitride substrate

Abstract

The objective of this invention is to provide a GaN crystal with a long luminescence lifetime as measured by time-resolved photoluminescence, and to provide a high-quality GaN crystal and GaN substrate with fewer specific crystal defects affecting this luminescence lifetime. The gallium nitride crystal of this invention has a luminescence lifetime of 5 ps to 200 ps as measured by time-resolved photoluminescence, and satisfies at least one of the following requirements (i) and (ii): (i) The full half-maximum (FWHM) of the 004 diffraction X-ray rocking curve is less than 50 arcsec at at least one location in the crystal. (ii) The differential packing density is less than 5 × 10⁶ cm⁻².

Description

Technical Field

[0001] This invention relates primarily to gallium nitride crystallization. Prior Technology

[0002] Previously, gallium nitride (GaN) crystals had a wide bandgap and direct bandgap migration, so they were used as semiconductor materials for various semiconductor components such as light-emitting diodes for ultraviolet light and blue light, or light-emitting elements, electronic components, and semiconductor sensors for shorter wavelengths, such as semiconductor lasers. In recent years, in addition to its use in light-emitting devices, GaN crystals have also been used in power semiconductor devices (power devices) or high-frequency power devices. As a result, the development of GaN crystals that can withstand high voltage and high current has begun. Furthermore, these components are preferably manufactured using a high-quality semiconductor substrate (self-standing substrate) made of the same material and with few crystal defects. Currently, research is being actively conducted on manufacturing technologies for GaN crystals that can serve as such semiconductor substrates.

[0003] As methods for manufacturing GaN crystals, liquid-phase growth methods such as ammonothermal methods and gas-phase growth methods such as hydride vapor-phase growth methods (HVPE method) are known. The HVPE method involves introducing Ga chloride and NH3 into a furnace in a hydrogen stream to thermally decompose them, causing the resulting crystals to accumulate on a substrate. On the other hand, the ammonothermal method utilizes the dissolution-precipitation reaction of nitrogen-containing solvents such as ammonia in supercritical and / or subcritical states with raw materials to produce the desired crystalline material. When applied to crystal growth, it leverages the temperature dependence of the solubility of the raw material in nitrogen-containing solvents such as ammonia, creating a supersaturated state through a temperature difference to induce crystal precipitation. Specifically, the raw material or seed crystal is placed in a pressure-resistant container such as an autoclave and sealed, then heated with a heater, thereby creating a high-temperature zone and a low-temperature zone within the pressure-resistant container. This dissolves the raw material while simultaneously promoting crystal growth, thus producing crystals (see, for example, Patent Documents 1-3). The ammonothermal process has advantages over the HVPE process in terms of raw material utilization efficiency and can suppress manufacturing costs. Furthermore, the ammonothermal process can produce high-quality GaN crystals with larger diameters, and has been moving towards practical application in recent years. [Previous Technical Documents] [Patent Literature]

[0004] [Patent Document 1] Japanese Patent Application Publication No. 2003-277182 [Patent Document 2] Japanese Patent Application Publication No. 2005-8444 [Patent Document 3] Japanese Patent Application Publication No. 2011-68545 Summary of the Invention

[0005] [The problem that the invention aims to solve] As stated above, GaN crystal growth via the ammonothermal method is advantageous from the perspective of controlling manufacturing costs and achieving high-quality and large-diameter crystals. However, according to the inventors' research, GaN crystals obtained via the ammonothermal method have a shorter luminescence lifetime measured by time-resolved photoluminescence (TRPL) than GaN crystals obtained via the HVPE method. This suggests that GaN crystals obtained via the ammonothermal method contain a large number of specific crystal defects or impurities, which may affect the fabrication of high-performance semiconductor devices. Therefore, the research objective of this invention is to provide GaN crystals with long luminescence lifetimes as measured by TRPL, and to provide high-quality GaN crystals and GaN substrates with fewer specific crystal defects that affect the luminescence lifetime. [Methods for solving problems]

[0006] The inventors of this invention conducted detailed research to solve the aforementioned problems and discovered that the problems could be solved by suppressing the generation of midpoint defects in GaN crystals, thus completing this invention.

[0007] The embodiments of the present invention include the following, but are not limited thereto. [1] A gallium nitride crystal having a luminescence lifetime of 5 ps to 200 ps as measured by time-resolved photoluminescence, and satisfying at least one of the following requirements (i) and (ii): (i) The full half-maximum (FWHM) of the 004 diffraction X-ray rocking curve is below 50 arcsec at at least one point in the crystallization; (ii) The differential density is below 5×106cm-2. [2] The gallium nitride crystal described in [1] has a differential packing density of less than 1×106cm-2. [3] The gallium nitride crystals described in [1] or [2] have a hydrogen concentration of less than 2 × 10¹⁹ atoms / cm³. [4] The gallium nitride crystal described in any of [1] to [3] has an oxygen concentration of less than 2 × 10¹⁹ atoms / cm³. [5] A gallium nitride substrate, which is obtained by slicing gallium nitride crystals as described in any one of [1] to [4]. [6] A method for manufacturing a gallium nitride substrate, comprising the following steps: selecting a region with a luminescence lifetime of more than 5 ps and less than 200 ps as determined by time-resolved photoluminescence measurement, and cutting a gallium nitride crystal slice as described in any one of [1] to [4]. [7] A gallium nitride substrate, which is a gallium nitride substrate obtained by the manufacturing method described in [6], wherein the luminescence lifetime measured by time-resolved photoluminescence is more than 5 ps and less than 200 ps in more than 90% of the substrate surface area. [8] The gallium nitride substrate as described in [7], wherein when time-resolved photoluminescence measurements are performed at two or more different points on the main surface of the substrate, the difference in luminescence lifetime at each measurement point is less than 30 ps. [9] The gallium nitride substrate as described in [7], wherein at least two points on the main surface of the substrate have a difference of more than 1 ps in the luminescence lifetime of each measurement point when performing time-resolved photoluminescence measurement.

[10] A method for manufacturing a gallium nitride substrate includes the following steps: growing gallium nitride crystals with a film thickness of more than 500 μm on gallium nitride seed crystals by ammonothermal method, and cutting the grown gallium nitride crystals into plate shapes to manufacture a gallium nitride substrate; The luminescence lifetime of the aforementioned gallium nitride substrate, as measured by time-resolved photoluminescence, is between 5 ps and 200 ps. [Effects of the Invention]

[0008] According to the present invention, certain crystal defects that are frequently observed in GaN crystals obtained by the ammonothermal method are reduced, thereby providing high-quality GaN crystals and GaN substrates. Simple Explanation of the Diagram

[0009] Figure 1 is a schematic diagram of a crystallization manufacturing apparatus that can be used in an embodiment of the present invention. Figure 2 is a schematic diagram illustrating the measurement points in the GaN crystallization measurement of PL lifetime using Example 1. Figure 3 is a graph showing the results of measuring PL lifetime using GaN crystals from Example 2. Figure 4 is a graph showing the results of measuring PL intensity using GaN crystals from Example 1. Implementation

[0010] In gallium nitride crystallization, the (0001) and (000-1) crystal planes are collectively referred to as c-planes, the {10-10} crystal plane is called the m-plane, and the {11-20} crystal plane is called the a-plane. The crystal axis perpendicular to the c-plane is called the c-axis, the crystal axis perpendicular to the m-plane is called the m-axis, and the crystal axis perpendicular to the a-plane is called the a-axis. Unless otherwise specified, references to crystallization axis, crystallization plane, and crystallization orientation in this specification refer to the crystallization axis, crystallization plane, and crystallization orientation of GaN crystals. The Miller index (hkil) of a hexagonal crystal has the relationship h + k = i, so it is sometimes represented by a 3-digit number (hkl). For example, (0002) is represented by a 3-digit number (002). In this specification, for convenience, the (0001) crystal plane and the (000-1) crystal plane are collectively referred to as the (000±1) crystal plane. Also, in the diagram, the

[0001] direction is represented as the +c direction, and the [000-1] direction is represented as the -c direction. The following details gallium nitride according to embodiments of the present invention, its manufacturing method, and the crystallization apparatus and components used in the manufacturing process. The description of the constituent elements described below represents a representative embodiment of the present invention, but the present invention is not limited to such embodiments. Furthermore, the numerical range indicated by "~" in this specification refers to the range including the values ​​described before and after "~" as a lower and upper limit value.

[0011] Gallium nitride crystals The gallium nitride (GaN) crystal (hereinafter also referred to as "GaN crystal") of the embodiment of the present invention has a luminescence lifetime (hereinafter also referred to as "PL lifetime") of 5 ps or more and 200 ps or less as measured by time-resolved photoluminescence (TRPL), and satisfies at least one of the following requirements (i) and (ii). (i)004 The full amplitude of the diffraction X-ray rocking curve is below 50 arcsec at at least one point in the crystallization. (ii) The differential density is below 5×106cm-2. The luminescence lifetime of GaN crystals, measured by TRPL, is 5 ps to 200 ps, ​​with the specific system referring to areas in the crystal that have at least 5 ps to 200 ps.

[0012] The shape of GaN crystals is not particularly limited; they can be irregular blocks or plates. In the case of plates, the surface shape can be, for example, circular, or polygonal, such as quadrilateral, hexagonal, or octagonal. Furthermore, GaN crystals are preferably GaN single crystals. The following description uses a plate-shaped GaN crystal with a circular surface as an example to illustrate its structure and properties, but this description is also applicable to other shapes within the scope of its applicability.

[0013] There are no particular limitations on the thickness of GaN crystals, which can be 100μm or more, 150μm or more, 250μm or more, 300μm or more, 400μm or more, 500μm or more, 750μm or more, 1mm or more, 2mm or more, and can also be less than 5mm, less than 2mm, less than 1mm, less than 750μm, less than 500μm, less than 400μm, less than 300μm, less than 250μm, etc. There is no specific upper limit to this thickness, which is typically below 20mm. Furthermore, when GaN crystals are irregularly shaped blocks, the growth direction of the crystals preferably conforms to these numerical ranges. The diameter of the main surface of GaN crystals is not particularly limited, but typically it can be 45-55 mm (approximately 2 inches), 95-105 mm (approximately 4 inches), 145-155 mm (approximately 6 inches), 195-205 mm (approximately 8 inches), or 295-305 mm (approximately 12 inches). Furthermore, when GaN crystals are plate-shaped with a non-circular surface, the maximum length obtained on the surface preferably meets these numerical ranges, and more preferably, the minimum length obtained on the surface also meets these numerical ranges. Additionally, when GaN crystals are irregularly shaped blocks, the maximum length obtained in a cross-section perpendicular to the crystal growth direction preferably meets these numerical ranges. The surface area of ​​the main surface of the GaN crystal is not particularly limited, but it is preferably 15 cm² or more. It can also be 15 cm² or more but less than 50 cm², 50 cm² or more but less than 100 cm², 100 cm² or more but less than 200 cm², 200 cm² or more but less than 350 cm², 350 cm² or more but less than 500 cm², or 500 cm² or more but less than 750 cm². Furthermore, when the GaN crystal is an irregularly shaped block, it is preferable that the maximum area of ​​the cross-section perpendicular to the crystal growth direction satisfies these numerical ranges.

[0014] The crystallization state of the main surface of GaN crystals is not particularly limited; for example, it can be a c-plane, a-plane, or m-plane, with a c-plane being the most preferred.

[0015] There are no particular restrictions on the luminescence lifetime (PL lifetime) of GaN crystals, as measured by time-resolved photoluminescence (TRPL), as long as it is above 5 ps and below 200 ps. Preferably, the PL lifetime is above 6 ps, more preferably above 8 ps, even better above 10 ps, ​​and exceptionally better above 15 ps. More preferably, it is below 200 ps, ​​more preferably below 150 ps, ​​even better below 100 ps, ​​even better below 90 ps, ​​and exceptionally better below 50 ps. Specifically, the crystal preferably has a PL lifetime within the above range. By having the PL lifetime of GaN crystals exceed the lower limit of the above range, high-quality GaN crystals with excellent reduction of point defects can be obtained. These can be ideally used as high-performance GaN substrates that can withstand higher voltages and currents than previous GaN crystals, making them ideal for manufacturing semiconductor devices. Furthermore, the statement that it has a range of 5 ps to 200 ps means that when the TRPL of the GaN crystal being measured is performed, the PL lifetime is 5 ps to 200 ps at at least any measurement point of the GaN crystal, whether inside or on the surface. The photoluminescence (PL) lifetime of GaN crystals was measured using time-resolved photoluminescence (TRPR) measurements under the following conditions. GaN crystals in a nitrogen atmosphere were excited with the third high-modulation wave (photon energy 4.65 eV, repetition frequency 8 MHz) of an Al₂O₃:Ti laser with a pulse width of 100 fs. To observe the non-radiative recombination process, measurements were performed under weak excitation conditions at 295 K and an excitation density of 120 nJ / cm². To measure the decay time of the bandgap luminescence intensity of GaN, a synchronous scanning ultra-high-speed camera with a time resolution of approximately 1 ps was used.

[0016] The region satisfying the above-mentioned PL lifetime conditions is preferably 50% or more of the GaN crystal. From the viewpoint of achieving functional improvement of semiconductor devices, it is preferably 55% or more, 60% or more, 65% or more, 70% or more, 80% or more, 90% or more, 95% or more, and most preferably 100%. However, a preferred range that does not require a specific upper limit can also be 99.99% or less, 99.9% or less, 99% or less, or 98% or less. For example, when a bulk GaN crystal is cut into multiple plates (e.g., 6 pieces) and the PL lifetime is measured on the surface of each plate of GaN crystal, if more than half (e.g., 3 pieces) of the total number of obtained plate-shaped GaN crystals satisfy the above-mentioned PL lifetime conditions, it can be evaluated that more than 50% of the region of that bulk GaN crystal satisfies the above-mentioned PL lifetime conditions. As another example, when the PL lifetime is measured at multiple points (e.g., 6 points) on the surface of a GaN crystal, if more than half of the total number of measurement points (e.g., 3 points) meet the above-mentioned PL lifetime conditions, it can be evaluated that more than 50% of the area of ​​the bulk GaN crystal meets the above-mentioned PL lifetime conditions. According to the inventors' research, it was found that in GaN crystals grown using the ammonothermal method, the area near the seed crystal in the early stages of growth tends to have a short photoperiod lifetime (PL). As the crystal continues to grow, the PL lifetime increases, and if it continues to grow to a certain thickness, the PL lifetime value becomes fixed. Therefore, the proportion of this region can be adjusted by selecting the region obtained from the grown GaN crystal through slicing or other processing methods. Furthermore, the PL lifetime is usually almost the same in the region perpendicular to the growth direction of the GaN crystal (parallel to the growth plane). Therefore, when the GaN crystal is sliced ​​in a direction perpendicular to this growth direction, the PL lifetime on the sliced ​​surface of the resulting crystal becomes more uniform.

[0017] The inventors of this case discovered that by suppressing specific crystal defects that affect the lifetime of a photocatalytic polymer (PL), specifically by suppressing point defects caused by impurities mixed into GaN crystals grown using the ammonothermal crystallization method, the lifetime of a PL can be extended. Point defects are those where atoms constituting the crystal are removed, creating pores, and impurities enter (replace) these pores, forming point-like defects. The differential packing density, described later, is an indicator of the density of linear crystal defects, and its concept differs from that of point defects. When GaN crystals are grown using the HVPE method, the PL lifetime of these GaN crystals typically exceeds 200 ps, ​​reaching around 400 ps, ​​and can even reach 1000 ps in some cases. Therefore, GaN crystals obtained by the HVPE method are generally considered to have fewer point defects. On the other hand, as will be discussed later, there is generally a difference that "the differential packing density of GaN crystals obtained by the HVPE method is significantly higher than that of GaN crystals obtained by the ammonothermal method."

[0018] From the perspective of PL lifetime, the inventors of this case speculate that there is a correlation between specific impurity elements and point defects. In the GaN crystal of this embodiment, the concentration of impurity elements measured by secondary ion mass spectrometry (SIMS) is preferably within the following range. The hydrogen (H) concentration in the GaN crystal is preferably 2 × 10¹⁹ atoms / cm³ or less, more preferably 1 × 10¹⁹ atoms / cm³ or less, even more preferably 5 × 10¹⁸ atoms / cm³ or less, and particularly preferably 1 × 10¹⁸ atoms / cm³ or less. Furthermore, the GaN crystal of this embodiment is grown by ammonothermal method, therefore it can contain H (hydrogen) at a concentration of 1 × 10¹⁷ atoms / cm³ or more. Also, the oxygen (O) concentration in the GaN crystal is preferably 2 × 10¹⁹ atoms / cm³ or less, more preferably 1 × 10¹⁹ atoms / cm³ or less, even more preferably 5 × 10¹⁸ atoms / cm³ or less, and particularly preferably 1 × 10¹⁸ atoms / cm³ or less. Furthermore, the GaN crystal of this embodiment is grown by ammonothermal method, therefore it can contain H (hydrogen) at a concentration of 1 × 10¹⁷ atoms / cm³ or more and O (oxygen) at a concentration of 1 × 10¹⁷ atoms / cm³ or more. Furthermore, when using a fluorine (F) mineralizer to grow GaN crystals via ammonothermal methods, the concentration of F can be above 1×10¹⁵ atoms / cm³, but from the viewpoint of extending PL lifetime, it is preferable to have a concentration of 1×10¹⁸ atoms / cm³ or less, more preferably 5×10¹⁷ atoms / cm³ or less, and even more preferably 1×10¹⁷ atoms / cm³ or less. Preferably, the growth is carried out using an ammonothermal method that does not use alkali metal compounds as mineralizing agents, so the concentrations of lithium (Li), sodium (Na), and potassium (K) can be less than 1 × 10¹⁵ atoms / cm³. Furthermore, it is possible to achieve an Fe concentration of 10¹⁵ atoms / cm³ or less, and a Ni concentration of 10¹⁵ atoms / cm³ or less.

[0019] As long as either condition (i) or (ii) above is met, the differential packing density, which serves as an indicator of the line defect density in GaN crystals, is not particularly limited, but is preferably less than 5 × 10⁶ cm⁻², more preferably less than 1 × 10⁶ cm⁻², even more preferably less than 5 × 10⁵ cm⁻², particularly preferably less than 1 × 10⁵ cm⁻², and typically greater than 1 × 10² cm⁻². When GaN crystals are grown using the HVPE method, the differential packing density of the GaN crystals is typically on the order of 10⁶ cm⁻² or higher. Preferably, the GaN crystals contain at least a 1 mm × 1 mm region with the aforementioned differential packing density. Typically, it is preferred that at least a 1 mm × 1 mm region with the aforementioned differential packing density exists on the crystal surface or in any plane parallel to the opposite surface. Furthermore, any plane parallel to the opposite surface is obtained by, for example, slicing the crystal parallel to the GaN crystal surface to obtain a new surface. Also, the aforementioned differential packing density can be evaluated using the methods described in the embodiments described later.

[0020] Regarding the aforementioned requirement (i), in GaN crystals, the full half-maximum (FWHM) of the 004 diffraction X-ray rocking curve is preferably less than 50 arcsec at at least one location within the crystal. More preferably, it is less than 50 arcsec; even more preferably, less than 40 arcsec; particularly preferably, less than 30 arcsec; especially preferably, less than 20 arcsec; and typically, more than 5 arcsec. Furthermore, the statement that the FWHM of the 004 diffraction X-ray rocking curve is less than 50 arcsec at at least one location within the crystal means that when performing XRD measurements on the GaN crystal, whether inside or on the surface of the GaN crystal, the FWHM of the 004 diffraction X-ray rocking curve is less than 50 arcsec at at least any measurement location within the GaN crystal. Typically, it is preferable that the above-mentioned full-amplitude value is present at at least one measurement point on the crystal surface or in any plane parallel to the surface. Alternatively, the plane parallel to the surface can be, for example, a new surface obtained by slicing the crystal parallel to the GaN crystal surface. As a preferred example, on a GaN crystal surface, the full amplitude of the 004 diffraction X-ray rocking curve at at least one location on the surface is preferably less than 100 arcsec, more preferably less than 50 arcsec, even more preferably less than 30 arcsec, particularly preferably less than 20 arcsec, and typically more than 5 arcsec. The full half-peak amplitude of the 004 diffraction X-ray rocking curve is a parameter measured by ω scanning using CuKα1 radiation; the smaller this value, the better the crystal quality. It is sufficient if the full half-peak amplitude of this 004 diffraction X-ray rocking curve is satisfied at least at one point on the GaN crystal surface; ideally, it is satisfied across the entire surface. In the 004 diffraction X-ray rocking curve measurement, the X-ray tube was operated with a voltage of 45kV and a current of 40mA, and a Ge(440)4 crystal symmetric monochromator was used to inject monochromatic CuKα lines into the GaN crystal. The direction from which the X-rays were injected into the GaN crystal was not particularly limited; for example, the incident plane of the X-rays could be perpendicular to the a-axis. The beam size of the X-ray, when the incident angle (the angle formed by the X-ray and the reflecting surface) is set to 90°, i.e., when the X-ray is incident perpendicularly to the Ga polar surface as the measurement surface, the size of the irradiated area on the GaN crystal is set to 5 mm for the direction parallel to the ω-axis and 1 mm for the direction perpendicular to the ω-axis. The ω-axis is the rotation axis of the sample in the rocking curve measurement. With the X-ray beam size set in this way, in the 004 diffraction X-ray rocking curve measurement of GaN crystal, ω is approximately 36.5°, and the size of the irradiated area on the GaN crystal is approximately 1.7 × 5 mm2.

[0021] When the diameter of the GaN crystal exceeds 40 mm, an ω-scan is performed along a line on the GaN crystal at 1 mm intervals along the 40 mm length, under the conditions described above. This yields the 004 diffraction X-ray oscillation curves of 40 measurement points PM arranged side-by-side at 1 mm intervals along this line. In this case, during the ω-scan at each measurement point PM, the ω-axis is made perpendicular to the line L. That is, X-rays are incident on the GaN crystal with the X-ray incident plane parallel to the line.

[0022] In a preferred embodiment, when the measurement is performed along at least one line on a GaN crystal, the maximum value of the full-amplitude half-peak of the 004 diffraction X-ray rocking curve between all measurement points can be less than 50 arcsec. The average half-peak full amplitude of the 004 diffraction X-ray rocking curve between all measurement points can be less than 100 arcsec, further less than 50 arcsec, further less than 30 arcsec, further less than 20 arcsec, and typically more than 5 arcsec.

[0023] GaN crystals grown by the ammonothermal method exhibit a peak in the infrared absorption spectrum at 3140~3200 cm⁻¹, which is classified as a gallium pore-hydrogen complex.

[0024] Regarding the grown crystals, bandgap luminescence was confirmed near the emission wavelength of 350-380 nm (photon energy 3.2-3.6 eV). However, generally speaking, crystals with a peak emission value near the wavelength corresponding to the band gap width of the material (e.g., around 356 nm at a low temperature of around 10 K for strain-free GaN crystals; photon energy around 3.48 eV) or a narrow half-width at half-maximum (WHM) of the bandgap emission peak value indicate fewer impurities and are therefore preferred. Furthermore, higher bandgap luminescence intensity indicates higher purity and is also preferred. Generally, the quality of a crystal can be judged by the intensity ratio (YL / NBE) of the band-end emission (NBE) and the yellow band emission (YL) observed at an emission wavelength of 500-600 nm. Yellow band emission is observed when Ga defects are present, so it can be said that crystals with a lower YL / NBE value have fewer defects. The ratio of the intensity of band-end emission (NBE) to the intensity of yellow band emission (YL) observed at emission wavelengths of 500-600 nm (YL / NBE) of GaN crystals is preferably 10 or less, preferably 1 or less, and most preferably 0.8 or less. Furthermore, in the GaN crystal of this embodiment, a broad peak was observed near the photon energy of 2.9 eV (emission wavelength 400-470 nm) during low-temperature PL measurements, confirming the presence of a blue emission band (BL). The intensity ratio (BL / NBE) of the bandgap emission (NBE) of the GaN crystal to the intensity of the blue emission band (BL) observed at the emission wavelength of 400-470 nm is preferably 0.1 or less, preferably 0.01 or less, and preferably 0.007 or less. It is speculated that a low intensity ratio (BL / NBE) indicates a crystal with fewer point defects.

[0025] Generally speaking, GaN crystallization can be evaluated with high precision by measuring the 004 diffraction X-ray rocking curve mentioned above, but the following 002 diffraction X-ray rocking curve can also be used to evaluate GaN crystallization. In GaN crystals, the full half-maximum (FWHM) of the 002 diffraction X-ray rocking curve is preferably less than 50 arcsec at at least one location within the crystal. More preferably, it is less than 50 arcsec; even more preferably, less than 40 arcsec; particularly preferably, less than 30 arcsec; especially preferably, less than 20 arcsec; and typically, it is greater than 5 arcsec. Furthermore, the statement that the FWHM of the 002 diffraction X-ray rocking curve is less than 50 arcsec at at least one location within the crystal means that, when performing XRD measurements on the GaN crystal being measured, whether inside or on the surface of the GaN crystal, the FWHM of the 002 diffraction X-ray rocking curve is less than 50 arcsec at at least any measurement location within the GaN crystal. Typically, it is preferable that the above-mentioned full-amplitude value is present at at least one measurement point on the crystal surface or in any plane parallel to the opposite surface. Alternatively, the plane parallel to the opposite surface can be obtained, for example, by slicing the crystal parallel to the GaN crystal surface. As a preferred example, the full amplitude of the 002 diffraction X-ray rocking curve on the surface of GaN crystal is preferably less than 50 arcsec at at least one location on its surface, more preferably less than 40 arcsec, even more preferably less than 30 arcsec, particularly preferably less than 20 arcsec, and typically more than 5 arcsec. The full half-peak amplitude of the 002 diffraction X-ray rocking curve is a parameter measured by ω scanning using CuKα1 radiation; the smaller this value, the better the crystal quality. It is acceptable if at least one point on the surface of the GaN crystal meets the range of this full half-peak amplitude value of the 002 diffraction X-ray rocking curve, and preferably the entire surface meets the requirement. (002) In the X-ray swing curve measurement, the X-ray tube is operated by a voltage of 45kV and a current of 40mA, and a Ge(440)4 crystal symmetric monochromator is used to cause monochromatic CuKα lines to be incident on the GaN crystal. The direction of the X-ray incident on the GaN crystal is not particularly limited; for example, the incident surface of the X-ray can be perpendicular to the a-axis. The beam size of the X-ray, when the incident angle (the angle formed by the X-ray and the reflecting surface) is set to 90°, that is, when the X-ray is incident perpendicularly to the Ga polarity surface as the measurement surface, the size of the irradiated area in the GaN crystal is set as 5 mm in the direction parallel to the ω-axis and 1 mm in the direction perpendicular to the ω-axis. The ω-axis refers to the rotation axis of the sample in the rocking curve measurement. With the X-ray beam size set in this way, the ω in the 002 diffraction X-ray rocking curve measurement of GaN crystal is approximately 36.5°, therefore the size of the irradiated area on the GaN crystal is approximately 1.7 × 5 mm2.

[0026] If the diameter of the GaN crystal exceeds 40 mm, an ω-scan is performed along a line on the GaN crystal at 1 mm intervals along the 40 mm length, using the conditions described above. This yields the 002 diffraction X-ray oscillation curves of 40 measurement points PM arranged side-by-side at 1 mm intervals along this line. In this case, during the ω-scan at each measurement point PM, the ω-axis is made perpendicular to line L. That is, X-rays are incident on the GaN crystal with the X-ray incident plane parallel to the line.

[0027] In a preferred embodiment, when this measurement is performed along at least one line on a GaN crystal, the maximum value of the full-amplitude of the 002 diffraction X-ray rocking curve between all measurement points can be less than 50 arcsec. The average half-peak full amplitude of the 002 diffraction X-ray rocking curve between all measurement points can be less than 100 arcsec, further less than 50 arcsec, further less than 30 arcsec, further less than 20 arcsec, and typically more than 5 arcsec.

[0028] When measuring the 002 diffraction X-ray rocking curves at 1mm intervals along a GaN crystal line, it is preferable to draw at least one line segment of 10mm or more, wherein the difference between the maximum and minimum peak angles of the X-ray rocking curves between all measurement points is less than 0.2°. The difference between the maximum and minimum peak angles of the X-ray rocking curves between measurement points can be less than 0.2°, preferably less than 0.15°, and more preferably less than 0.1°.

[0029] The radius of curvature of GaN crystals can be 50 m or more, preferably 70 m or more, and even more preferably 80 m or more. There is no particular upper limit, and it can be, for example, below 1000 m. The radius of curvature can be obtained, for example, by the difference between the maximum and minimum peak angles of the X-ray rocking curves of 002 diffraction at 1 mm intervals on a line segment.

[0030] <Methods for manufacturing gallium nitride crystals> The above-mentioned method for manufacturing GaN crystals is not particularly limited as long as it includes the step of growing crystals by ammonothermal method, but it can be manufactured by, for example, the following method. Specifically, the process includes the following steps: GaN crystals grow on the surface of the seed crystal by placing a seed crystal with a hexagonal crystal structure, a nitrogen-containing solvent, raw materials, and a mineralizing agent in a reaction vessel, with the nitrogen-containing solvent being in a supercritical and / or subcritical state. From the perspective of obtaining GaN crystals with the expected PL lifetime, or further from the perspective of suppressing the generation of specific point defects related to PL lifetime in GaN crystallization, crystal growth is achieved by making the ratio of the volume of the reaction vessel to the internal surface area (internal volume (cm3) / internal surface area (cm2)) 2 (cm) or more, and by making the pressure inside the reaction vessel 5~200MPa or less. As another example of this disclosure, the following method is included. Specifically, a method for manufacturing gallium nitride crystals by growing gallium nitride crystals using an ammonothermal reaction vessel in which the volume-to-internal-surface-area ratio (volume (cm3) / internal-surface-area (cm2)) is 1.5 cm3 or more is provided.

[0031] (Mineralizing agent) The type of mineralizer is not particularly limited, but fluorine-containing mineralizers are preferred. Examples include: ammonium fluoride, hydrogen fluoride, and hydrocarbon ammonium fluoride; alkylammonium salts such as tetramethylammonium fluoride, tetraethylammonium fluoride, benzyltrimethylammonium fluoride, dipropylammonium fluoride, or isopropylammonium fluoride; fluorinated alkyl metals such as sodium fluoride; fluorinated alkaline earth metals; or fluorinated metals. Among these, fluorides of alkaline earth metals, metal fluorides, ammonium fluoride, or hydrogen fluoride are preferred; fluorides of alkaline earth metals, ammonium fluoride, or group 13 metals are even more preferred; and ammonium fluoride (NH4F) or gallium fluoride are particularly preferred. These mineralizers can be used alone or in combination of two or more. By using fluorine-containing mineralizers, it is generally possible to obtain high-quality crystals with the expected PL lifetime and good crystallinity. It is even more preferable to use mineralizers containing only fluorine as a halogen element.

[0032] The mineralizing agent may be a fluorine-containing mineralizing agent or a mixture of a fluorine-containing mineralizing agent and a fluorine-free mineralizing agent. Examples of fluorine-free mineralizing agents include: acidic mineralizing agents composed of halogens other than fluorine and ammonium ions, such as ammonium chloride, ammonium iodide, or ammonium bromide; neutral mineralizing agents composed of alkali metals and halogens, such as lithium chloride, lithium bromide, lithium iodide, sodium chloride, sodium bromide, sodium iodide, potassium chloride, potassium bromide, or potassium iodide; neutral mineralizing agents composed of alkaline earth metals and halogens, such as beryllium chloride, beryllium bromide, beryllium iodide, magnesium chloride, magnesium bromide, magnesium iodide, calcium chloride, calcium bromide, or calcium iodide; or alkaline mineralizing agents such as alkaline earth metal amides, rare earth amides, alkali metal nitrides, alkaline earth metal nitrides, azide compounds, or other hydrazine salts. When using fluorine-containing and fluorine-free mineralizers, it is preferable that the proportion of fluorine-containing mineralizers in all mineralizers is 50 mol% or more, more preferably 60 mol% or more, even more preferably 80 mol% or more, and may be less than 100 mol%. As for fluorine-free mineralizers, from the viewpoint of controlling the crystal growth shape and controlling the precipitation of impurities by utilizing the orientation dependence of the growth rate derived from the mineralizer, it is preferable to use mineralizers containing halogens other than fluorine. In this case, it is preferable that 50% or more of all halogens in the mineralizer are fluorine, more preferably 60% or more, even more preferably 80% or more, and may be less than 100 mol%.

[0033] The mineralizer is preferably one that exhibits negative solubility in nitrogen-containing solvents near the temperature at which GaN crystals grow. For example, ammonium fluoride exhibits negative solubility in ammonia at temperatures above 400°C. It is generally preferred that the temperature of the crystal growth region be set above 450°C; therefore, when using ammonium fluoride alone as a mineralizer, it exhibits negative solubility at the crystal growth temperature. On the other hand, when used in combination with ammonium chloride or similar substances that exhibit positive solubility at this temperature, the slope of the solubility curve varies depending on the mixing ratio and the solubility characteristics of each substance. If the absolute value of the slope of the solubility curve is small, the efficiency of crystal growth deteriorates; therefore, the mixing ratio is adjusted to produce a solubility curve with an appropriate slope.

[0034] The concentration of fluorine moles in the mineralizer is preferably 0.2 mol% or more, more preferably 0.27 mol% or more, even more preferably 1.0 mol% or more, particularly preferably 1.5 mol% or more, even more preferably 30 mol% or less, more preferably 20 mol% or less, even more preferably 10 mol% or less, and particularly preferably 2 mol% or less. Higher concentrations tend to increase the solubility of the raw material in the nitrogen-containing solvent and accelerate the growth rate, which is preferable. On the other hand, lower concentrations maintain solubility appropriately, thus suppressing spontaneous nucleation and maintaining less supersaturation, making it easier to control, which is also preferable.

[0035] (pressure) From the viewpoint of obtaining GaN crystals with the expected PL lifetime, and that is, from the viewpoint of suppressing point defects in GaN crystals, the pressure inside the reaction vessel during crystal growth is preferably 5 MPa or higher to 200 MPa to facilitate crystal growth. The pressure inside the reaction vessel is preferably 10 MPa or higher, more preferably 12 MPa or higher, even more preferably 15 MPa or higher, particularly preferably 20 MPa or higher, more preferably 150 MPa or lower, even more preferably 120 MPa or lower, and even more preferably 100 MPa or lower. The manufacturing method of this embodiment is characterized by the ability to efficiently grow GaN crystals even at lower pressures. If crystal growth is performed at lower pressures, the thickness of the pressure-resistant vessel can be reduced, energy efficiency can be improved, and costs can be suppressed. Furthermore, by performing crystal growth at a lower pressure below the aforementioned lower limit, in addition to obtaining the expected PL lifetime, there is also a tendency to obtain high-quality crystals with good crystallinity.

[0036] (Seed Crystal) The seed crystal (hereinafter also referred to as "seed crystal") is not particularly limited, but it is preferably a seed crystal with a hexagonal crystal structure, and particularly preferably a crystal with the same elemental composition as the target GaN crystal. As a seed crystal with a hexagonal crystal structure, it is preferably a nitride of an element in Group 13 of the periodic table. Examples include single crystals of nitrides such as gallium nitride or aluminum nitride. The seed crystals mentioned above can be determined by considering their solubility in the solvent and their reactivity with the mineralizer. For example, seed crystals for GaN can be single crystals obtained by epitaxial growth on dissimilar substrates such as sapphire and then peeling off; single crystals obtained by crystallization from metallic Ga using Na, Li, or Bi as flux; single crystals obtained by homogeneous / heterogeneous epitaxial growth using liquid phase epitaxy (LPE); single crystals produced by solution-based growth methods; and crystals obtained by cutting these. There are no particular limitations on the specific epitaxial growth methods mentioned above, and methods such as hydride vapor phase growth (HVPE), metal-organic chemical vapor deposition (MOCVD), liquid phase methods, or ammonothermal methods can be used.

[0037] (Nitrogen-containing solvent) As a solvent, a nitrogen-containing solvent is used. Examples of nitrogen-containing solvents include solvents that do not compromise the stability of GaN crystal growth. Examples of such solvents include ammonia, hydrazine, urea, amines (e.g., primary amines such as methylamine, secondary amines such as dimethylamine, tertiary amines such as trimethylamine, or diamines such as ethylenediamine), or melamine. These solvents can be used alone or in combination. The aim is to minimize the amount of water and oxygen contained in the solvent. The water and oxygen content in the solvent, individually and in total, is preferably below 1000 ppm by weight, more preferably below 10 ppm by weight, and even more preferably below 0.1 ppm by weight. There is no particular lower limit; it can be above 0 ppm by weight (0 ppm by weight means below the detection limit). When using ammonia as a solvent, its purity is typically above 99.9% by weight, preferably above 99.99% by weight, and even more preferably above 99.999% by weight. There is no particular upper limit; it can be below 100% by weight or even less than 100% by weight.

[0038] (raw material) As a raw material, a material containing elements constituting the GaN crystal to be grown on the seed crystal is used. Preferably, this is a polycrystalline raw material for GaN crystallization and / or the metal to be nitrided, more preferably gallium nitride and / or metallic gallium. The polycrystalline raw material does not need to be a complete nitride; depending on conditions, it may also contain a metallic component of a Group 13 element in its metallic state (zero valence). For example, in the case of gallium nitride crystallization, a mixture of gallium nitride and metallic gallium can be used. The aforementioned methods for manufacturing polycrystalline raw materials are not particularly limited. For example, nitride polycrystalline materials can be produced by reacting a metal or its oxide or hydroxide with ammonia in a reaction vessel in which ammonia gas has already flowed in. Furthermore, as more reactive metal compound raw materials, halides, amide compounds, amide compounds, or compounds with covalent MN bonds such as galazan can be used. Moreover, nitride polycrystalline materials prepared by reacting metals such as Ga with nitrogen under high temperature and pressure can also be used.

[0039] The amount of water and oxygen contained in polycrystalline raw materials used as raw materials should be low. The oxygen content in polycrystalline raw materials is usually below 10,000 ppm by weight, preferably below 1,000 ppm by weight, and even more preferably below 1 ppm by weight. There is no particular limit to the lower limit, and it can also be above 0 ppm by weight (0 ppm by weight refers to below the detection limit). The ease with which oxygen mixes into polycrystalline raw materials is related to the reactivity or absorption energy of water. The worse the crystallinity of the polycrystalline raw material, the more active groups such as NH groups exist on the surface, which may react with water to form oxides or hydroxides. Therefore, it is generally preferable to use polycrystalline raw materials with high crystallinity. Crystallinity can be evaluated by the half-width of powder X-ray diffraction. The half-width of the (100) diffraction line (2θ = approximately 32.5° in hexagonal gallium nitride) is usually below 0.25°, preferably below 0.20°, and even more preferably below 0.17°.

[0040] (Reaction vessel) The growth reaction of GaN crystals is carried out in a reaction vessel. A reaction vessel is a container used to manufacture GaN crystals in a supercritical and / or subcritical state where a nitrogen-containing solvent can directly contact its inner wall surface. Preferred examples include the internal structure of a pressure vessel or a cavity located within a pressure vessel.

[0041] The pressure-resistant part of the reaction vessel is preferably made of any one of Ni-based alloys, Fe-based alloys or cobalt-based alloys or a combination thereof, especially a Ni-Fe-based alloy (as a Ni-Fe-based alloy, especially with an Fe content of 30 to 40% by mass, and preferably containing Cr, Ti, Al or Nb as other elements).

[0042] Therefore, there is no particular limitation on the form of the reaction vessel constructed from such alloys. The reaction vessel can be formed by directly lining or coating the inner surface of the pressure-resistant part with a material with excellent corrosion resistance, or a cavity made of a material with excellent corrosion resistance can be placed inside the pressure-resistant vessel. The shape of the reaction vessel is primarily cylindrical, but it can be any shape. Furthermore, the reaction vessel can be installed upright, horizontally, or at an angle.

[0043] Platinum group metals or platinum group alloys can be used as the lining material and cavity for the aforementioned corrosion-resistant parts. Examples of platinum group metals include Pt, Au, Ir, Ru, Rh, Pd, and Ag. In this manufacturing method, regarding the mineralizing agent, Ag or Ag-containing alloys can be ideally used as the lining material by using a fluorine-containing compound alone as the mineralizing agent.

[0044] In the past, to stabilize manufacturing conditions such as temperature uniformity during crystal growth, or to ensure the uniformity of the resulting GaN crystals, the ratio of the reaction vessel's volume to its internal surface area (volume (cm3) / internal surface area (cm2)) was generally reduced. However, in the manufacturing of GaN crystals in this embodiment, from the viewpoint of obtaining GaN crystals with the expected PL lifetime, that is, from the viewpoint of suppressing point defects in GaN crystals, a larger ratio is preferred. This ratio is 2 (cm) or more, more preferably 3 (cm) or more, more preferably 4 (cm) or more, even more preferably 5 (cm) or more, particularly preferably 6 (cm) or more, and preferably 16 (cm) or less, even more preferably 12 (cm) or less, even more preferably 9 (cm) or less, and particularly preferably 7 (cm) or less. If this ratio is above the lower limit of the above range, impurities introduced due to contact with the inner wall of the reaction vessel can be suppressed, point defects in the GaN crystals are reduced, and GaN crystals with the expected PL lifetime are easily obtained. Furthermore, if the aforementioned ratio is below the upper limit of the above range, from the perspective of ease of temperature difference control and supersaturation control, it is easier to stabilize the quality of GaN crystals. For the same reasons as those mentioned above, the internal diameter of the reaction vessel is generally reduced. However, in the manufacturing of GaN crystals in this embodiment, from the viewpoint of suppressing point defects in GaN crystals, it is preferable to have a diameter of 100 mm or more, more preferably 120 mm or more, even more preferably 150 mm or more, and preferably 650 mm or less, even more preferably 550 mm or less, and even more preferably 450 mm or less.

[0045] (Crystallization growth) A seed crystal with a hexagonal crystal structure is placed in a reaction vessel containing a nitrogen-containing solvent, raw materials, and a mineralizing agent. GaN crystals grow on the surface of the seed crystal under controlled conditions, with the solvent in a supercritical and / or subcritical state. The pressure conditions at this time are as described above. The temperature during growth is set differently in the raw material dissolution region and the crystal growth region on the seed crystal. The temperature of the crystal growth region is preferably 450°C or higher, more preferably 500°C or higher, and even more preferably 550°C or higher. A higher temperature in the crystal growth region is also acceptable, but it can be preferably set to 700°C or lower, or 650°C or lower. When using a mineralizer with negative solubility characteristics, the temperature of the raw material dissolution region is set higher than that of the crystal growth region. When using a mineralizer with positive solubility characteristics, the temperature is set lower than that of the crystal growth region. The temperature difference between the raw material dissolution region and the crystal growth region is typically set to 30°C or higher, preferably 40°C or higher, and typically lower than 150°C, preferably lower than 120°C.

[0046] By using the conditions described above to grow GaN crystals, a faster growth rate can be achieved. Optimizing these conditions allows for crystal growth rates exceeding 300 μm / day, 500 μm / day, 700 μm / day, and even 900 μm / day. Furthermore, even under low-temperature and low-pressure conditions, a faster growth rate than previous methods can be achieved.

[0047] (Preparation Steps) In addition to the growth steps described above, a preparatory step may be performed prior to the aforementioned growth steps, in which seed crystals, a nitrogen-containing solvent, and raw materials are placed into the reaction vessel. The method of placing these materials into the reaction vessel is not particularly limited; examples include placing the seed crystals at the bottom of the reaction vessel and the raw materials at the top before the nitrogen-containing solvent flows in.

[0048] (Exhaust procedure) In addition to the growth steps described above, to obtain high-purity GaN crystals, it is preferable to include a venting step before the growth steps to heat and degas the reaction vessel. This venting step reduces oxygen levels within the reaction vessel, thereby adjusting the oxygen content of the GaN crystals. There are no particular limitations on the method of heating and exhausting; familiar methods can be used, such as heating while simultaneously exhausting air using a vacuum pump. The temperature during heating and exhaust is not particularly limited, but the preferred temperature range is above 80°C, more preferably above 120°C, even more preferably above 160°C, preferably below 300°C, even more preferably below 280°C, and even more preferably below 260°C. The higher the vacuum level during heating and exhaust, the better. Ideally, it should be below 5×10⁻⁴ Pa, even better, below 1×10⁻⁴ Pa, and even better, below 5×10⁻⁵ Pa. The longer the heating and exhaust time, the better the effect of reducing oxygen impurities; however, a shorter time can improve productivity and reduce costs. The preferred heating and exhaust time is more than 2 hours, more preferably more than 6 hours, even better than more than 12 hours, preferably less than 72 hours, more preferably less than 48 hours, and ideally less than 24 hours.

[0049] (Processing steps) In addition to the growth steps described above, a processing step of crystallizing GaN may also be performed after the aforementioned growth steps. As a processing step, for example, it is anticipated that the obtained GaN crystal will be sliced ​​to obtain only the portion with the desired characteristics, preferably only the portion satisfying the aforementioned PL lifetime range. The GaN crystal thus obtained can be used as a GaN substrate as described later. As mentioned above, when slicing is typically performed perpendicular to the growth direction of GaN crystals (in a direction parallel to the growth plane), the PL lifetime of the resulting crystal slices is uniform on the slice side surface.

[0050] <Gallium Nitride Substrates and Their Manufacturing Methods> Another embodiment of the present invention is a gallium nitride substrate (hereinafter also referred to as "GaN substrate") obtained by slicing the aforementioned GaN crystal and its manufacturing method. The diameter of GaN substrates is 50 mm or more, typically 50-55 mm (about 2 inches), 100-105 mm (about 4 inches), or 150-155 mm (about 6 inches). Because the strength of the GaN substrate is required to be sufficient not to impede operation, its thickness is usually 250 μm or more, and can be increased to accommodate larger diameters. When the diameter of the GaN substrate is about 2 inches, the thickness is preferably 250 μm or more, more preferably 300 μm or more, and preferably 450 μm or less, more preferably 400 μm or less. When the diameter of the GaN substrate is about 4 inches, the thickness is preferably 350 μm or more, more preferably 400 μm or more, and preferably 750 μm or less, more preferably 650 μm or less. When the diameter of the GaN substrate is about 6 inches, the thickness is preferably 450 μm or more, more preferably 550 μm or more, and preferably 800 μm or less, more preferably 700 μm or less. GaN substrates are well-suited for manufacturing nitride semiconductor devices. In the manufacturing process of GaN semiconductor devices, one or more nitride semiconductor layers are epitaxially grown on the main surface of a GaN substrate to form an epitaxial wafer. Preferred epitaxial growth methods include metal-organic vapor phase epitaxy (MOVPE), molecular beam epitaxy (MBE), pulsed excitation deposition (PXD), sputtering, and hydride vapor phase epitaxy (HVPE). Epitaxially grown nitride semiconductor layers can be doped to become n-type conductive, p-type conductive, or semi-insulating.

[0051] When performing time-resolved photoluminescence measurements at two or more different points on the main surface of a GaN substrate, the difference in photoluminescence lifetime (PL) between the measurement points is preferably less than 30 ps, ​​more preferably less than 20 ps, ​​even more preferably less than 10 ps, ​​and most preferably less than 5 ps. No lower limit needs to be set, but the optimal difference is greater than 0 ps. This means that regardless of which two points are selected on the main surface of the GaN substrate, the difference will be below the aforementioned values. A GaN substrate with such a small PL lifetime difference can, for example, be obtained by slicing the GaN crystals in a direction perpendicular to the crystal growth direction. Furthermore, as another example, when performing time-resolved photoluminescence measurements on the main surface of a GaN substrate, at least two measurement points exhibit a PL lifetime difference of 1 ps or more, or even 5 ps or more, or further, 10 ps or more, or even 15 ps or more. This GaN substrate with a PL lifetime difference exceeding a specific value can, for example, be obtained by slicing GaN crystals in a direction inclined relative to the crystal growth direction.

[0052] A preferred method for manufacturing a GaN substrate includes the following steps: selecting a region that satisfies the range of PL lifetime values ​​obtained from time-resolved photoluminescence measurements of the GaN crystal, and then slicing the GaN crystal. Specifically, it is preferred to select a region in the crystal that at least satisfies the specified range of values. Generally, when slicing is performed perpendicular to the growth direction of the GaN crystal (parallel to the growth plane), the PL lifetime on the surface of the sliced ​​crystal is uniform. On the other hand, when slicing is performed at a predetermined angle from a plane inclined at a plane perpendicular to the growth direction (parallel to the growth plane), the PL lifetime on the surface of the sliced ​​crystal has a distribution that reflects the variation along the growth direction. Furthermore, on the surface of the cut GaN substrate, preferably in an area of ​​more than 50% of the main surface, the photoluminescence lifetime obtained by time-resolved photoluminescence measurement of GaN crystallization is within the range of the aforementioned GaN crystallization photoluminescence lifetime. The proportion of this area on the surface of the GaN substrate is more preferably 70% or more, 80% or more, 90% or more, 95% or more, or 99% or more. Moreover, there is no need to set an upper limit; it can be less than 100% or less, or less than 99.99%.

[0053] Another embodiment of the present invention is a method for manufacturing a gallium nitride substrate (hereinafter also referred to as "GaN substrate"). A method for manufacturing a GaN substrate includes the steps of growing a GaN crystal with a thickness of 500 μm or more on a GaN seed crystal using an ammonothermal method, and cutting the grown GaN crystal into a plate shape to fabricate a GaN substrate. The GaN substrate has a luminous lifetime measured by time-resolved photoluminescence (TRPL) of 5 ps to 200 ps. Specifically, it is a method for manufacturing a GaN substrate having a luminous lifetime measured by TRPL of at least 5 ps to 200 ps. There are no particular limitations on the GaN seed crystals used in the above-mentioned crystal growth process; conventional ones can be used, but it is preferred to use a GaN substrate of another embodiment of the present invention. GaN crystals can be grown on the aforementioned GaN seed crystals using the ammonothermal method, and the growth conditions in the aforementioned GaN crystal manufacturing method can be applied. The thickness of the GaN crystals grown on the GaN seed crystals by ammonothermal method is 500 μm or more, preferably 600 μm or more, 700 μm or more, 800 μm or more, or 900 μm or more. The conditions for manufacturing GaN substrates can be applied, within the applicable range, to the conditions for GaN crystallization and GaN substrates described above.

[0054] Examples of nitride semiconductor devices that can be manufactured using GaN substrates include: light-emitting diodes (LEDs), laser diodes (LDs), and other light-emitting elements; rectifiers; bipolar transistors, field-effect transistors, high electron mobility transistors (HEMTs), and other electronic components; temperature sensors, pressure sensors, radiation sensors, visible-ultraviolet light detectors, and other semiconductor sensors; and solar cells. Other applications of GaN substrates include seed crystals used in the growth of bulk GaN crystals by methods such as HVPE, THVPE (Tri-Halide Vapor Phase Epitaxy), OVPE (Oxide Vapor Phase Epitaxy), ammonothermal method, Na flux method, or various other methods. [Example]

[0055] The following examples and comparative examples illustrate the features of the present invention in more detail. The materials, amounts, proportions, processing contents, and processing procedures shown in the following examples can be appropriately modified without departing from the spirit of the present invention. Therefore, the scope of the present invention should not be construed as limited to the specific examples described below.

[0056] <Evaluation Methods> (1) Impurity analysis of GaN crystals Impurity elements in GaN crystals were analyzed using secondary ion mass spectrometry (SIMS). The measuring apparatus used was a secondary ion mass analyzer.

[0057] (2) PL lifespan The photoluminescence (PL) lifetime of GaN crystals was measured using a time-resolved photoluminescence / fluorescence spectrophotometer. GaN crystals placed in a nitrogen atmosphere were excited with the third harmonic of an Al₂O₃:Ti laser with a pulse width of 100 fs (photon energy 4.65 eV, repetition frequency 8 MHz, dot size 94 μm φ). The GaN crystals were then kept under weak excitation conditions at 295 K and an excitation density of 120 nJ / cm², and the results were observed. Additionally, a synchronous scanning ultra-high-speed camera with a time resolution of approximately 1 ps was used to measure the decay time of the bandgap luminescence intensity of the GaN crystals.

[0058] (3) Full amplitude of the half-peak of the X-ray swing curve X-ray rocking curves of 002 and 004 diffraction were measured at a position away from the edge on the Ga polar surface. In the measurements, the linear focal length CuKα X-ray source of the X-ray diffraction apparatus [Spectris Co., Ltd. Panalytical X'Pert Pro MRD] was operated at 45 kV and 40 mA, and CuKα1 X-rays were obtained using a Ge(440)4 crystal symmetric monochromator. A parallel optical system was used, employing a 1 / 2 slit, an X-ray mirror, and a w1mm × h1mm cross slit on the incident side. The detector used was a PIXcel3D (registered trademark) semiconductor pixel detector in 0D mode. The angular analysis capability was 5–6 arcsec.

[0059] The beam size of the X-ray is set as follows: when the incident angle is set to 90°, that is, when the X-ray is incident perpendicularly to the Ga polar surface of the sample substrate, the size of the irradiated area on the Ga polar surface is 5 mm in the direction parallel to the ω axis and 1 mm in the direction perpendicular to the ω axis. In the rocking curve measurement, X-rays are incident on the sample from a direction perpendicular to the a-axis of the GaN crystal. In other words, the incident plane of the X-rays is parallel to the a-plane of the GaN crystal.

[0060] (4) Differential packing density Differential packing density can be measured by calculating etch pits. For the (0001) side of a C-plane GaN substrate, chemical mechanical polishing (CMP) is performed, followed by etching with 89% sulfuric acid heated to 270°C for 1 hour to form etch pits. Each etch pit corresponds to a through-packing, so the differential packing density can be calculated by observing the GaN substrate with etch pits using an optical microscope and counting their number. Alternatively, in the as-grown crystal stage, after crystal growth on the (000-1) side, etching is performed in supercritical ammonia during cooling to form etch pits. Therefore, the differential packing density can be roughly calculated by observing the (000-1) side of the as-grown crystal using an optical microscope.

[0061] <Example 1> In this embodiment, the reaction apparatus shown in Figure 1 is used to grow GaN crystals. The inner surface was lined with Ag, and a Ni-Fe based alloy autoclave (volume to inner surface area ratio of 2.3 (cm²)) was used as the pressure vessel for crystal growth. The vessel, which has an outer wall 3, was filled under atmospheric conditions.

[0062] Six hexagonal GaN single crystals (approximately 5 mm × 15 mm × 0.4 mm) grown by ammonothermal method were used as seed crystals 6. The (000-1) facet of the seed crystals was finished by chemical mechanical polishing (CMP) or by etching with KOH. These seed crystals 6 were suspended from a molybdenum seed crystallization support frame with 0.2 mm diameter tungsten wire and placed in the crystallization growth area 2 at the bottom of the autoclave.

[0063] A molybdenum buffer plate 5 is placed between the lower crystallization growth zone 2 and the upper raw material dissolution zone 1. Then, 5800g of polycrystalline GaN particles are weighed as raw material 4 and placed in the upper region (raw material dissolution zone 1) of the autoclave. Next, fully dried NH4F with a purity of 99.9% is weighed as a mineralizing agent source at a concentration of 10 mol% relative to the ammonia being filled and added to the pressure vessel.

[0064] Then close the lid of the autoclave equipped with the valve. Next, connect the conduit to the vacuum pump through the valve attached to the autoclave, and open the valve to purge the vacuum. Then, while maintaining the vacuum, cool the autoclave with dry ice and methanol solvent, and temporarily close the valve. Then, connect the conduit to the NH3 cylinder, and reopen the valve to continuously fill the autoclave with NH3 without contact with external gases, and close the valve again. Allow the autoclave to return to room temperature, allow the outer surface to dry thoroughly, and measure the weight of the autoclave. Confirm the NH3 filling amount using the mass flow meter reading and adjust it to a filling rate of 33%.

[0065] The autoclave was then placed inside an electric furnace consisting of upper and lower heaters. The temperature was increased by setting the temperature of the raw material melting zone 1 on the outer surface of the autoclave to 562.5°C and the temperature of the crystallization growth zone 2 to 586.4°C (temperature difference 23.9°C). After reaching the set temperature, this temperature was maintained for 6 days. The pressure inside the autoclave was 114 MPa. The temperature unevenness on the outer surface of the autoclave during maintenance was controlled to be below ±0.5°C.

[0066] Next, the valve attached to the autoclave was opened to remove NH3 from the autoclave. It was allowed to cool naturally until the temperature of the outer surface of the autoclave returned to room temperature. Then, the lid of the autoclave was opened. Upon inspection of the autoclave interior, gallium nitride crystals were found to be uniformly precipitated on the entire surface of a 5×15 mm square seed crystal. The N-face growth rate was 189 μm / day, and the thickness of the GaN crystal grown on the N-face side was 1.0 mm. SIMS analysis of the impurity concentration in the N-face growth region of this crystal (approximately 800 μm from the seed crystal surface of the obtained GaN crystal) revealed the following results: fluorine concentration 1.5 × 10¹⁷ atoms / cm³, oxygen concentration 8.0 × 10¹⁷ atoms / cm³, hydrogen concentration 9.3 × 10¹⁷ atoms / cm³, carbon concentration less than 3.0 × 10¹⁴ atoms / cm³ (detection limit), nickel concentration 4.5 × 10¹⁵ atoms / cm³, iron concentration 1.7 × 10¹⁵ atoms / cm³, and chromium concentration less than 5.0 × 10¹³ atoms / cm³ (detection limit). Furthermore, the differential packing density of the obtained GaN crystal was between 10³ and 10⁴, which was also the same in Examples 2-4 and Comparative Examples 1-3 below. Furthermore, as shown in Figure 2, the PL lifetimes were measured at 850 μm (Figure 2, 1), 600 μm (Figure 2, 2), 400 μm (Figure 2, 3), and 150 μm (Figure 2, 4) from the seed crystal surface of the obtained GaN crystal. The results were 40 ps, ​​29 ps, 17 ps, and 2 ps, respectively. Additionally, the PL lifetime of the outermost portion along the extension lines of these measurements was 38 ps. Furthermore, photoluminescence (PL) measurements were performed on the N-side growth surface of the obtained GaN crystal at 12K, and the results are shown in Figure 4. A broad peak was observed near 2.9 eV, confirming the presence of a blue emission band. The intensity ratio (BL / NBE) of the band-end emission (NBE) of the obtained GaN crystal to the intensity of the blue emission band (BL) observed at emission wavelengths of 400–470 nm was 0.006. The PL measurements were performed using a He-Cd laser with a wavelength of 325 nm and an output of 38 W / cm². The full-width values ​​of the 004 diffraction X-ray rocking curve at the measurement point are 18 arcsec (a-axis direction) and 19 arcsec (m-axis direction). The full half-peak amplitude of the 002 diffraction X-ray rocking curve at the measurement point is 19 arcsec (a-axis direction) and 19 arcsec (m-axis direction). The full half-peak amplitude of the 102 diffraction X-ray rocking curve is 10 arcsec (a-axis direction). The Δω of the 002 diffraction X-ray rocking curve is less than ±0.004°. The radius of curvature is 151m.

[0067] <Example 2> Regarding the crystal growth conditions, the temperature was increased to 562.6°C in the raw material dissolution zone on the outer surface of the autoclave and 585.8°C in the crystal growth zone (temperature difference of 23.2°C). After reaching the set temperature, the temperature was maintained for 14 days, and the pressure inside the autoclave was 115 MPa. Otherwise, GaN crystals were produced in the same manner as in Example 1. The growth rate of the N-side is 160 μm / day, and the thickness of the GaN crystal grown on the N-side is 2.2 mm. Furthermore, the PL lifetimes were measured at 2100 μm (measurement position 1), 1800 μm (measurement position 2), 1500 μm (measurement position 3), 1250 μm (measurement position 4), 1000 μm (measurement position 5), 700 μm (measurement position 6), 400 μm (measurement position 7), and 150 μm (measurement position 8) from the seed crystal surface of the obtained GaN crystal. The results were 34 ps, 36 ps, 35 ps, 34 ps, 34 ps, 33 ps, 14 ps, and 4 ps, respectively. These results are shown in Figure 3. The full half-peak amplitude of the 002 diffraction X-ray rocking curve at the measurement point is 18 arcsec (a-axis direction) and 19 arcsec (m-axis direction). The full half-peak amplitude of the 102 diffraction X-ray rocking curve is 9 arcsec (a-axis direction). The Δω of the 002 diffraction X-ray rocking curve is less than ±0.005°. The radius of curvature is 93m.

[0068] <Example 3> Regarding the crystal growth conditions, the temperature was increased to 571.8°C in the raw material dissolution zone on the outer surface of the autoclave and 575.2°C in the crystal growth zone (temperature difference of 3.4°C). After reaching the set temperature, the temperature was maintained for 6 days, and the pressure inside the autoclave was 114 MPa. Otherwise, GaN crystals were produced in the same manner as in Example 1. The N-face growth rate was 73 μm / day, and the thickness of the GaN crystal grown on the N-face was 0.5 mm. SIMS analysis of the impurity concentration in the N-face growth region (850 μm from the seed crystal surface of the obtained GaN crystal) revealed the following results: fluorine concentration 4.8 × 10¹⁶ atoms / cm³, oxygen concentration 4.1 × 10¹⁸ atoms / cm³, hydrogen concentration 2.4 × 10¹⁸ atoms / cm³, carbon concentration less than 3.0 × 10¹⁴ atoms / cm³ (detection limit), nickel concentration 3.9 × 10¹⁵ atoms / cm³, iron concentration 1.3 × 10¹⁵ atoms / cm³, and chromium concentration less than 5.0 × 10¹³ atoms / cm³ (detection limit). Furthermore, the PL lifetimes at 450 μm, 250 μm, and 100 μm from the seed crystal surface of the obtained GaN crystal were measured, and the results were 8 ps, 5 ps, and 3 ps, respectively. In addition, the PL lifetime of the outermost part of the extension line at these measurement points is 10 ps. The full-width value of the 004 diffraction X-ray rocking curve at the measurement point is 16 arcsec (a-axis direction) and 16 arcsec (m-axis direction). The full half-peak amplitude of the 002 diffraction X-ray rocking curve at the measurement point is 18 arcsec (a-axis direction) and 19 arcsec (m-axis direction). The full half-peak amplitude of the 102 diffraction X-ray rocking curve is 9 arcsec (a-axis direction). The Δω of the 002 diffraction X-ray rocking curve is less than ±0.003°. The radius of curvature is 485m.

[0069] <Example 4> Regarding the crystal growth conditions, the temperature was increased to 581.5°C in the raw material dissolution zone on the outer surface of the autoclave and 587.8°C in the crystal growth zone (temperature difference of 6.3°C). After reaching the set temperature, the temperature was maintained for 6 days, and the pressure inside the autoclave was 114.5 MPa. Otherwise, GaN crystals were produced in the same manner as in Example 1. The N-face growth rate was 202 μm / day, and the thickness of the GaN crystal grown on the N-face was 1.1 mm. SIMS analysis of the impurity concentration in the N-face growth region (200 μm from the seed crystal surface of the obtained GaN crystal) revealed the following concentrations: fluorine 4.5 × 10¹⁶ atoms / cm³, oxygen 1.9 × 10¹⁸ atoms / cm³, hydrogen 1.3 × 10¹⁸ atoms / cm³, carbon 3.0 × 10¹⁵ atoms / cm³, nickel 4.3 × 10¹⁵ atoms / cm³, iron 3.9 × 10¹⁵ atoms / cm³, chromium less than 1.0 × 10¹⁴ atoms / cm³ (detection limit), silicon less than 6.0 × 10¹³ atoms / cm³, and silver less than 1.0 × 10¹⁵ atoms / cm³ (detection limit). Furthermore, the photoperiod lifetimes (PL) were measured at 1050 μm, 800 μm, 600 μm, 400 μm, and 150 μm from the seed crystal surface of the obtained GaN crystal, and the results were 14 ps, 12 ps, 7 ps, 3 ps, and 2 ps, respectively. Additionally, the PL lifetime at the outermost surface portion along the extension lines of these measurement points was 15 ps. The full-width values ​​of the 004 diffraction X-ray rocking curve at the measurement point are 16 arcsec (a-axis direction) and 20 arcsec (m-axis direction). The full half-peak amplitude of the 002 diffraction X-ray rocking curve at the measurement point is 18 arcsec (a-axis direction) and 20 arcsec (m-axis direction). The full half-peak amplitude of the 102 diffraction X-ray rocking curve is 8 arcsec (a-axis direction). The Δω of the 002 diffraction X-ray rocking curve is less than ±0.003°. The radius of curvature is 105 m.

[0070] <Comparative Example 1> As the reaction vessel, an Ag-lined Ni-Fe-based alloy autoclave (volume to internal surface area ratio of 1.2 cm²) was used as the pressure vessel. NH₄F with a purity of ≥99.9% was added as a mineralizing agent at an amount of 1.25 mol% relative to the ammonia filling. Regarding the crystal growth conditions, the temperature was increased to 640°C in the raw material dissolution zone on the outer surface of the autoclave and 660°C in the crystal growth zone (temperature difference of 20°C). After reaching the set temperature, it was maintained at this temperature for 10 days, and the pressure inside the autoclave was 117.4 MPa. Otherwise, GaN crystals were produced in the same manner as in Example 1. The growth rate of the N-face is 120 μm / day, and the thickness of the GaN crystal grown on the N-face side is 1.2 mm. Furthermore, the PL lifetime was measured at a distance of 1000 μm from the seed crystal surface of the obtained GaN crystal, and the result was 4 ps.

[0071] <Comparative Example 2> A platinum alloy chamber was installed inside a Ni-Fe based alloy autoclave as a reaction vessel for crystal growth. The volume of the chamber was 1.3 cm² relative to its internal surface area. NH₄F (3 mol / L relative to the filled ammonia) and NH₄I (2 mol / L relative to the filled ammonia) with a purity of 99.99% or higher were used as mineralizers. The crystal growth conditions were as follows: the temperature of the raw material dissolution zone on the outer surface of the autoclave was 607°C, and the temperature of the crystal growth zone was 617°C (a temperature difference of 10°C). After reaching the set temperature, it was maintained at this temperature for 33 days, and the pressure inside the autoclave was 200 MPa. Otherwise, GaN crystals were produced in the same manner as in Example 1. The growth rate of the N-face is 100 μm / day, and the thickness of the GaN crystal grown on the N-face side is 3.3 mm. Furthermore, the PL lifetime was measured at a distance of 900 μm from the seed crystal surface of the obtained GaN crystal, and the result was 2 ps.

[0072] <Reference Example> The same reaction vessel as Comparative Example 1 (volume to internal surface area ratio of 1.2 cm²) was used as the pressure vessel. NH₄F with a purity of ≥99.9% was added as a mineralizing agent at a concentration of 2.5 mol% relative to the ammonia filling. Regarding the crystal growth conditions, the temperature was increased to 560°C in the raw material dissolution zone on the outer surface of the autoclave and 610°C in the crystal growth zone (temperature difference of 50°C). After reaching the set temperature, this temperature was maintained for 10 days, and the pressure inside the autoclave was 123.8 MPa. Otherwise, GaN crystals were produced in the same manner as in Example 1. The N-face growth rate was 300 μm / day, and the thickness of the GaN crystal grown on the N-face side was 2.6 mm. SIMS analysis of the impurity concentration in the N-face growth region (2300 μm from the seed crystal surface of the obtained GaN crystal) revealed the following concentrations: fluorine 1.3 × 10¹⁶ atoms / cm³, oxygen 1.6 × 10¹⁸ atoms / cm³, hydrogen 9.0 × 10¹⁷ atoms / cm³, carbon 2.9 × 10¹⁴ atoms / cm³ (within the detection limit), nickel 6.0 × 10¹⁶ atoms / cm³, iron 1.5 × 10¹⁶ atoms / cm³, and chromium 8.0 × 10¹³ atoms / cm³.

[0073] The conditions and characteristics of the GaN crystals obtained in the above embodiments and comparative examples are summarized in Table 1 below. Furthermore, the "maximum PL lifetime" in Table 1 refers to the maximum PL lifetime measured in the embodiments and comparative examples, respectively. Additionally, "-" in Table 1 indicates that no measurement was performed.

[0074]

Table 1

[0075] As shown in Table 1 above, GaN crystals with a PL lifetime of more than 5 ps can be obtained by increasing the ratio of the volume of the reaction vessel to the internal surface area and reducing the pressure during crystal growth. [Industry availability]

[0076] According to the manufacturing method of an embodiment of the present invention, GaN crystals with a PL lifetime of 5 ps to 200 ps can be manufactured. Furthermore, the manufacturing method of an embodiment of the present invention can also reduce energy costs. The GaN crystals efficiently manufactured according to the manufacturing method of an embodiment of the present invention can be used not only in blue light-emitting diodes (LEDs) or blue semiconductor lasers (LDs) made of nitride semiconductors of Group 13 elements in the periodic table, but also in a wide range of applications such as GaN substrates for power semiconductor devices (power devices) or high-frequency power devices. Therefore, the present invention has extremely high industrial applicability.

[0077] 1: Upper part (raw material dissolving area) 2: Lower part (crystallization growth region) 3:Outer wall 4: Raw materials 5: Buffer plate 6: Seed Crystal

Claims

1. A gallium nitride crystal having a luminescence lifetime of more than 5 ps and less than 200 ps as measured by time-resolved photoluminescence, and satisfying only the following requirement (ii), or simultaneously satisfying both the following requirements (i) and (ii): (i) the full half-maximum (FWHM) of the 004 diffraction X-ray rocking curve is less than 50 arcsec at at least one location of the crystal; (ii) the differential packing density is less than 1 × 10⁶ cm⁻².

2. The gallium nitride crystal as described in claim 1, wherein the hydrogen concentration is below 2 × 10¹⁹ atoms / cm³.

3. The gallium nitride crystal as described in claim 1, wherein the oxygen concentration is below 2 × 10¹⁹ atoms / cm³.

4. A gallium nitride substrate obtained by slicing a gallium nitride crystal as described in any one of claims 1 to 3.

5. A method for manufacturing a gallium nitride substrate, comprising the following steps: selecting a region with a luminescence lifetime of 5 ps to 200 ps as measured by time-resolved photoluminescence, and then slicing a gallium nitride crystal as described in any one of claims 1 to 3.

6. A gallium nitride substrate, which is obtained by the manufacturing method described in claim 5, wherein the luminescence lifetime measured by time-resolved photoluminescence is more than 5 ps and less than 200 ps in more than 90% of the substrate surface area.

7. The gallium nitride substrate as described in claim 6, wherein when time-resolved photoluminescence measurements are performed at two or more different points on the main surface of the substrate, the difference in luminescence lifetime at each measurement point is less than 30 ps.

8. The gallium nitride substrate as claimed in claim 6, wherein at least two points on the main surface of the substrate have a luminescence lifetime difference of more than 1 ps between the measurement points when time-resolved photoluminescence measurements are performed.

9. A method for manufacturing a gallium nitride substrate, comprising the following steps: growing gallium nitride crystals with a film thickness of 500 μm or more on gallium nitride seed crystals by ammonothermal method, and cutting the grown gallium nitride crystals into plate shapes to produce a gallium nitride substrate; wherein the luminescence lifetime of the aforementioned gallium nitride substrate, as measured by time-resolved photoluminescence, is 5 ps or more and 200 ps or less.

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