GAN crystal and method for producing GAN crystal

Abstract

The present invention relates to a GaN crystal including a Ni-doped GaN layer, wherein the Ni-doped GaN layer has a Ni concentration of 1.0×1016 atoms / cm3 or more and 1.0×1020 atoms / cm3 or less, and the Ni-doped GaN layer satisfies at least one of (1) a total donor impurity concentration of less than 2.0×1017 atoms / cm3, or (2) a full width at half maximum of the (004) rocking curve by X-ray diffraction of 50 arcsec or less.

Description

GaN crystal and method for manufacturing GaN crystal

[0001] The present invention relates to a gallium nitride (GaN) crystal and a method for producing the same.

[0002] In recent years, GaN-based high electron mobility transistors (HEMTs) (hereinafter sometimes referred to as "GaN-HEMTs") have been actively developed for use in devices for high frequencies ranging from several tens of GHz to several hundreds of GHz.

[0003] GaN-HEMTs are constructed by growing GaN crystals on a substrate. While silicon, silicon carbide (SiC), GaN, and other materials are being considered for use as substrates for GaN-HEMTs, GaN-based GaN-HEMTs are expected to offer superior performance compared to those using other substrates.

[0004] GaN-HEMTs repeatedly turn current on and off at high frequencies, but ideally, no current should flow when the device is in the OFF state. Therefore, the GaN substrate used in GaN-HEMTs must have high resistance, and GaN substrates are often made semi-insulating to form semi-insulating substrates (SI substrates).

[0005] One known method for making a GaN substrate semi-insulating is to dope the GaN crystal with transition metal elements. Transition metal elements act as acceptors, compensating for background donors unintentionally introduced into the GaN substrate and reducing the carrier concentration within the GaN crystal. Such transition metal elements are called compensating impurities.

[0006] For example, in the invention described in Non-Patent Document 1, a GaN crystal doped with iron (Fe) or manganese (Mn) as a compensating impurity is obtained.

[0007] M Iwinska et al., 'Iron and manganese as dopants used in the crystallization of highly resistive HVPE-GaN on native seeds', Japanese Journal of Applied Physics, Volume 58 SC1047, 2019

[0008] Conventionally, the resistivity of silicon-insulated substrates has been discussed in terms of values ​​near room temperature, but it is known that the resistivity decreases as the temperature increases. Non-Patent Document 1 also shows that the resistivity of Mn-doped GaN crystals decreases as the temperature increases (see Fig. 8(a)). Meanwhile, the fields in which high-frequency devices are used have expanded in recent years, and as a result, opportunities for their use in harsh environments such as high-temperature environments have also increased. Therefore, realizing high-frequency devices that can withstand high-temperature environments is one of the important challenges for improving operational reliability and large-volume information processing and controllability in a wide range of application environments.

[0009] Therefore, an object of the present invention is to provide a GaN crystal that has a high resistivity even in a high-temperature environment.

[0010] The present inventors have conducted extensive research into the above-mentioned problems, and have found that resistivity can be increased by providing a GaN crystal including a Ni-doped GaN layer in which Ni is doped so as to fall within a specific concentration range, and that better results can be obtained and the above-mentioned problems can be solved by setting the total donor impurity concentration and the full half maximum of the rocking curve of the (004) plane in the Ni-doped GaN layer within specific ranges, thereby completing the present invention.

[0011] That is, the gist of the present invention is as follows: [1] A GaN crystal including a Ni-doped GaN layer, wherein the Ni-doped GaN layer has an Ni concentration of 1.0×10 １６ atoms / cm ３ Above 1.0 x 10 ２０ atoms / cm ３A GaN crystal that satisfies at least one of the following requirements (1) and (2): (1) the Ni-doped GaN layer has a total donor impurity concentration of 2.0×10 １７ atoms / cm ３ (2) The Ni-doped GaN layer has a full width at half maximum of a rocking curve measured by (004) X-ray diffraction of 50 arcsec or less. [2] The Ni-doped GaN layer has a dislocation density of 1×10 ７ cm －２ [3] The GaN crystal according to [1] above, wherein the Ni-doped GaN layer has an activation energy Ea of 0.5 eV or more, which is calculated by substituting the resistivity into the following formula (I): Resistivity (Ωcm)=A×EXP(−Ea / kT) (I) (In formula (I), A is a proportionality constant, EXP is an exponential function, Ea is the activation energy of resistivity (eV), and k is Boltzmann's constant (8.617×10 －５ eV / K), and T is the temperature in Kelvin (K). [4] The GaN crystal according to [3], wherein the activation energy Ea of the resistivity is 0.7 to 1.2 eV. [5] The activation energy Ea of the resistivity is determined by a least squares method using the resistivity at two or more temperatures, and the coefficient of determination R of an approximation curve when exponential approximation is performed by the least squares method is ２ The GaN crystal according to any one of [3] to [5] above, wherein the activation energy Ea of the resistivity is a value obtained by substituting the resistivity measured at intervals of 20 K in a temperature range from 700 K to 800 K into formula (I) and applying the least squares method. [7] The Ni-doped GaN layer has an O (oxygen) concentration and a Si (silicon) concentration of 1.0×10 １５ atoms / cm ３ [8] The GaN crystal according to any one of [1] to [6], wherein the resistivity of the Ni-doped GaN layer at 700 K is 5×10 or more. ５ The GaN crystal according to any one of the above [1] to [7], wherein the resistivity of the Ni-doped GaN layer at 800 K is 5×10 ４

[10] The GaN crystal according to any one of [1] to [9] above, wherein the Ni-doped GaN layer has a thickness of 50 μm or more.

[11] The GaN crystal according to any one of [1] to

[10] above, wherein the Ni-doped GaN layer has a surface that is inclined at an angle of 10 degrees or less from the (0001) crystal plane.

[0012]

[12] Ni concentration is 1.0 × 10 １６ atoms / cm ３ Above 1.0 x 10 ２０ atoms / cm ３ A method for producing a GaN crystal including a Ni-doped GaN layer by vapor phase growth, the Ni-doped GaN layer being:

[13] The method for producing a GaN crystal according to

[12] above, wherein the vapor phase growth is an HVPE method.

[0013] The present invention provides a GaN crystal that can maintain high resistivity even in high-temperature environments, making it highly suitable as a substrate for nitride semiconductor devices with a lateral device structure, such as GaN-HEMTs.

[0014] FIG. 1 is a plan view showing the (0001) surface of a GaN crystal according to this embodiment divided into 5 mm x 5 mm cells by a square lattice. FIG. 2 is a cross-sectional view showing one embodiment of a GaN crystal according to this embodiment. FIG. 3 is a cross-sectional view showing one embodiment of a GaN crystal according to this embodiment. FIG. 4 is a cross-sectional view illustrating steps in a method for manufacturing a GaN crystal according to this embodiment, where FIG. 4(a) shows a state of a c-plane GaN seed, FIG. 4(b) shows a state in which a GaN layer is stacked on the c-plane GaN seed, and FIG. 4(c) shows a state in which the GaN layer, which is a stacked structure, has been thinned. 5A and 5B are cross-sectional views illustrating the steps of a method for producing a c-plane GaN seed used in the production of GaN crystals according to this embodiment, with (a) in FIG. 5 representing the state of the seed wafer, (b) in FIG. 5 representing the state in which a GaN thick film has been grown on the c-plane GaN seed, and (c) in FIG. 5 representing the state in which the GaN thick film has been processed to obtain at least one c-plane GaN seed. FIG. 6 is a schematic diagram showing the basic configuration of an HVPE apparatus. FIG. 7 is a graph showing the measurement results of the resistivity of the GaN crystal of Example 1. FIG. 8 is a graph showing the measurement results of the resistivity of the GaN crystal of Example 2.

[0015] The present invention will be described in detail below, but the present invention is not limited to the following embodiments and can be implemented with various modifications within the scope of the gist thereof. In this specification, the crystal axis parallel to the

[0001] axis is called the c-axis, the crystal axis parallel to the <10-10> axis is called the m-axis, and the crystal axis parallel to the <11-20> axis is called the a-axis. A crystal plane perpendicular to the c-axis is called the c-plane, a crystal plane perpendicular to the m-axis is called the m-plane, and a crystal plane perpendicular to the a-axis is called the a-plane. The Miller indices (hkil) of a hexagonal crystal are sometimes written in three digits as (hkl) because of the relationship h + k = -i. For example, (0004) can be written in three digits as (004). In this specification, when a crystal axis, crystal plane, crystal orientation, etc. are mentioned, unless otherwise specified, they respectively refer to the crystal axis, crystal plane, crystal orientation, etc. of a GaN substrate or GaN layer. In this specification, each concentration, such as Ni concentration, C concentration, O concentration, and Si concentration at a specific position, is a value determined by the amount detected using secondary ion mass spectrometry (SIMS). In this specification, when the expression "to" is used, it is used as an expression including the numerical value or physical property value before and after it. In other words, "A to B" means A or more and B or less. In this specification, mass ppm and weight ppm are synonymous.

[0016] [GaN Crystal] The GaN crystal according to this embodiment includes a Ni-doped GaN layer. The Ni-doped GaN layer has a Ni concentration of 1.0×10 １６ atoms / cm ３ Above 1.0 x 10 ２０ atoms / cm ３ The Ni-doped GaN layer preferably satisfies at least one of the following requirements (1) and (2), and more preferably satisfies both requirements (1) and (2): (1) The Ni-doped GaN layer has a total donor impurity concentration of 2.0 × 10 １７ atoms / cm ３ (2) The Ni-doped GaN layer has a full width at half maximum of a rocking curve measured by (004) X-ray diffraction of 50 arcsec or less.

[0017] The Ni-doped GaN layer is composed of a crystalline layer of Ni-doped GaN. The specific concentration of Ni increases the resistance of the GaN layer, and the specific resistance can be maintained high even in a high-temperature environment. This is more pronounced when the Ni-doped GaN layer satisfies at least one of the above requirements (1) and (2). In this specification, a high-temperature environment refers to an environment of, for example, 500 K or higher.

[0018] In one aspect of the GaN crystal according to this embodiment, the Ni concentration in the entire crystal is 1.0×10 １６ atoms / cm ３ Above 1.0 x 10 ２０ atoms / cm ３ That is, the GaN crystal is composed of only the Ni-doped GaN layer.

[0019] In addition, another aspect of the GaN crystal according to this embodiment is a GaN crystal having a Ni concentration of 1.0×10 １６ atoms / cm ３ Above 1.0 x 10 ２０ atoms / cm ３ In this case, the thickness of the Ni-doped GaN layer in the c-axis direction does not need to match the thickness of the entire GaN crystal in the c-axis direction.

[0020] For example, if the thickness of the entire GaN crystal is 400 μm, the thickness of the Ni-doped GaN layer is sufficient if it is 50 μm from the surface of the GaN crystal. Of course, the thickness of the entire GaN crystal and the thickness of the Ni-doped GaN layer are not limited to the above, and cases where they are thicker or thinner than that are not excluded in any way. Furthermore, the GaN crystal may be a GaN crystal in which the entire thickness direction of the GaN crystal is composed of a Ni-doped GaN layer. In that case, the Ni concentration in the entire GaN crystal as mentioned above is 1.0×10 １６ atoms / cm ３ Above 1.0 x 10 ２０ atoms / cm ３ This applies if the following is true:

[0021] Ni concentration is 1.0 × 10 １６ atoms / cm ３ Above 1.0 x 10２０ atoms / cm ３ The Ni-doped GaN layer described below may have various thicknesses and may take various forms, such as a free-standing substrate, an epitaxial layer grown on another GaN crystal, or a GaN layer laminated on a support substrate by bonding technology. For example, from the viewpoint of ease of handling, such as being able to obtain a free-standing GaN substrate by processing, the thickness of the Ni-doped GaN layer is preferably 50 μm or more, more preferably 80 μm or more, and even more preferably 100 μm or more. The upper limit of the thickness is not particularly limited, but is preferably, for example, 200 μm or less.

[0022] The Ni concentration of the Ni-doped GaN layer is 1.0×10 １６ atoms / cm ３ Above 1.0 x 10 ２０ atoms / cm ３ The compensating impurity Ni contributes to high resistance, and the Ni concentration is 1.0×10 １６ atoms / cm ３ As a result, the Ni-doped GaN layer can achieve high resistivity even in a high-temperature environment. The reason for this is not yet clear, but it is presumed that the Ni-doped GaN layer has a high resistivity activation energy Ea, as will be described later. In addition, the Ni concentration of the Ni-doped GaN layer is 1.0×10 ２０ atoms / cm ３ By satisfying the condition below, it is possible to maintain good crystal quality.

[0023] The Ni concentration in the Ni-doped GaN layer is 1.0×10 ２０ atoms / cm ３ The following is a stepwise １９ atoms / cm ３ Below, 5.0 x 10 １９ atoms / cm ３ Below, 3.0 x 10 １９ atoms / cm ３ Below, 1.0 x 10 １９ atoms / cm ３ Below, 5.0 x 10 １８ atoms / cm ３The Ni concentration of the Ni-doped GaN layer is preferably 1.0×10 １６ atoms / cm ３ That's all, but in the following steps, 3.0 x 10 １６ atoms / cm ３ That's it, 5.0 x 10 １６ atoms / cm ３ That's it, 1.0 x 10 １７ atoms / cm ３ That's it, 2.0 x 10 １７ atoms / cm ３ The above is preferable.

[0024] The C concentration in the Ni-doped GaN layer is 1.0×10 １８ atoms / cm ３ It is preferable that the C concentration in the Ni-doped GaN layer is less than 0.05. While the compensating impurity C can contribute to high resistance, if the C content is greater than a certain amount, there is a risk of current collapse occurring when a GaN crystal including a Ni-doped GaN layer is used in a device. Therefore, from the viewpoint of reducing the risk of causing the current collapse phenomenon, it is preferable that the C concentration in the Ni-doped GaN layer is low.

[0025] The C concentration of the Ni-doped GaN layer is preferably 1.0×10 １８ atoms / cm ３ The following is a stepwise １７ atoms / cm ３ Below, 5.0 x 10 １７ atoms / cm ３ Below, 3.0 x 10 １７ atoms / cm ３ Below, 1.0 x 10 １７ atoms / cm ３ Below, 5.0 x 10 １６ atoms / cm ３ Below, 2.0 x 10 １６ atoms / cm ３ The following is preferred: The lower limit of the C concentration in the GaN crystal is not particularly limited, and is usually equal to the detection limit.

[0026] Known impurities that act as donors to GaN include O (oxygen), Si (silicon), S (sulfur), Ge (germanium), and Sn (tin), and are referred to as donor impurities. Donor impurities inhibit the effect of compensating impurities in increasing the resistance. Therefore, the sum of the concentrations of donor impurities in the Ni-doped GaN layer, i.e., the total donor impurity concentration, is set to 2.0×10, from the viewpoint that the resistivity of the Ni-doped GaN layer is less susceptible to fluctuations in the donor impurity concentration and that high resistivity can be achieved even with a relatively low Ni concentration. １７ atoms / cm ３ The donor impurity concentration is preferably less than 1.0×10 １７ atoms / cm ３ More preferably, 5.0 x 10 １６ atoms / cm ３ More preferably, it is less than 4.0 × 10 １６ atoms / cm ３ Even more preferably, less than 3.0 x 10 １６ atoms / cm ３ Since the amount of donor impurities is preferably as small as possible, there is no particular lower limit to the total donor impurity concentration.

[0027] As will be described later, the Ni-doped GaN layer in this embodiment can be grown, for example, preferably by vapor phase epitaxy, more preferably by hydride vapor phase epitaxy (HVPE). The Ni-doped GaN layer obtained by the above method contains donor impurities, O (oxygen) and Si (silicon), at concentrations of 10 １５ atoms / cm ３ That is, in the Ni-doped GaN layer of this embodiment, the O (oxygen) concentration and the Si (silicon) concentration can both be 1.0×10 １５ atoms / cm ３ It could be more than that.

[0028] In this embodiment, the O (oxygen) concentration of the Ni-doped GaN layer is 2.0×10 １５ atoms / cm ３ or more, or 5.0 x 10 １５atoms / cm ３ or more, or 8.0 x 10 １５ atoms / cm ３ That's it, 1.0 x 10 １６ atoms / cm ３ The upper limit of the O (oxygen) concentration is 5.0×10 １６ atoms / cm ３ Below, 4.0 x 10 １６ atoms / cm ３ It can be the following:

[0029] On the other hand, the only time that a Ni-doped GaN layer contains donor impurities other than O and Si at non-negligible concentrations is when the layer is intentionally doped with such donor impurities. Note that "intentional doping" refers to the addition of a target element, either as a single element or as a compound, to the Ni-doped GaN layer.

[0030] Therefore, unless the Ni-doped GaN layer is intentionally doped with donor impurities other than O and Si, the total donor impurity concentration of the Ni-doped GaN layer may be considered to be equal to the sum of the O concentration and the Si concentration. Whether the Ni-doped GaN layer is doped with donor impurities other than O and Si can be confirmed by elemental analysis or the like.

[0031] The Ni-doped GaN layer may contain compensating impurities other than Ni, such as Fe (iron), Co (cobalt), Mn (manganese), etc., as long as this does not cause any practical problems. Alternatively, the Ni-doped GaN layer may be substantially free of compensating impurities other than Ni. The total concentration of compensating impurities other than Ni in the Ni-doped GaN layer is, for example, 5×10 １６ atoms / cm ３ In one preferred embodiment, the Ni-doped GaN layer has a total concentration of compensating impurities other than Ni and C (carbon) of 5×10 or less. １６ atoms / cm ３ The following is the result.

[0032] The Ni-doped GaN layer may contain H (hydrogen) in addition to the donor impurities and compensating impurities mentioned above. The concentration of H is, for example, 10 １６ ~10１７ atoms / cm ３ However, in the case of the Ni-doped GaN layer obtained by the HVPE method described above, the upper limit of the H (hydrogen) concentration is 3.0 × 10 １７ atoms / cm ３ or less, or 2.0 x 10 １７ atoms / cm ３ or less, or 1.0 x 10 １７ atoms / cm ３ It can be the following:

[0033] The Ni-doped GaN layer may also contain other impurities, such as alkali metal elements such as Li (lithium), Na (sodium), and K (potassium), and halogen elements such as F (fluorine), Cl (chlorine), Br (bromine), and I (iodine). However, in the case of the Ni-doped GaN layer obtained by the above-mentioned HVPE method, the content of these impurities can be kept low. In this case, the Ni-doped GaN layer has a total halogen element content of 1×10 １６ atoms / cm ３ It can be the following:

[0034] In this embodiment, the Ni-doped GaN layer preferably has an alkali metal concentration at the crystal surface of 10 mass ppm or less. The Ni-doped GaN layer satisfying the above requirement means that the Ni-doped GaN layer does not have any regions with a high alkali metal concentration. The absence of any regions with a high alkali metal concentration suppresses the effect on the lattice length of the GaN crystal, and can suppress cracking when the GaN crystal including the Ni-doped GaN layer is used as a substrate.

[0035] The alkali metal concentration on the crystal surface is more preferably 8 ppm by mass or less, even more preferably 5 ppm by mass or less, and the lower the better, but is usually 0.1 ppm by mass or more. Note that the alkali metal concentration in this specification means the alkali metal concentration in any 100 μm × 100 μm square lattice on the crystal surface, and can be measured, for example, by scanning electron microscope-energy dispersive X-ray spectroscopy (SEM-EDX).

[0036] In this embodiment, the Ni-doped GaN layer has a Ni concentration of 1.0×10 １６atoms / cm ３ Above 1.0 x 10 ２０ atoms / cm ３ In addition, the C concentration is 1.0 × 10 １８ atoms / cm ３ Hereinafter, the donor impurity concentration is 2.0 × 10 １７ atoms / cm ３ (Requirement 1), the sum of the concentrations of compensating impurities other than Ni is less than 5 × 10 １６ atoms / cm ３ Below, H concentration is 3.0 × 10 １７ atoms / cm ３ The total content of halogen elements is 1 x 10 １６ atoms / cm ３ and an alkali metal concentration on the crystal surface of 10 ppm by mass or less, it is preferable that one or more of the following conditions be satisfied, more preferably two or more, even more preferably three or more, even more preferably four or more, particularly preferably five or more, and particularly preferably all six of the following conditions be satisfied.

[0037] The Ni-doped GaN layer in this embodiment preferably does not contain alkali metal inclusions. When present inside the crystal, alkali metal inclusions refer to closed spaces containing alkali metal formed in the Ni-doped GaN layer, and when present on the crystal surface, they refer to the closed spaces exposed on the crystal surface as depressions.

[0038] When a GaN crystal containing a Ni-doped GaN layer containing alkali metal inclusions inside or on the crystal surface is used as a substrate, when a GaN crystal layer is epitaxially grown on the substrate, the alkali metal, e.g., Na, contained in the closed spaces may volatilize, causing volume expansion, which may cause the substrate to crack or degrade the crystal quality of the epitaxially grown GaN crystal layer. It is generally known that such closed spaces can occur in GaN crystals obtained by the flux method. The presence or absence of alkali metal inclusions can be determined using an optical microscope or SEM.

[0039] It is also preferable that the Ni-doped GaN layer in this embodiment does not emit light whose peak does not lie in the 2.64 to 2.82 eV region when irradiated with light having an energy greater than the bandgap energy of GaN. It is known that when irradiated with the above light, GaN crystals produced by, for example, the Na flux method among the flux methods, generally emit broad light with a peak between 2.64 and 2.82 eV. The presence of this peak indicates that the GaN crystal was produced by the Na flux method. As mentioned above, GaN crystals produced by the flux method may have regions with high alkali metal concentrations on the crystal surface or may contain alkali metal inclusions, which may make them undesirable as semi-insulating GaN crystals for devices requiring high reliability.

[0040] The full width at half maximum of the rocking curve obtained by (004) X-ray diffraction of the Ni-doped GaN layer is preferably 50 arcsec or less (Requirement 2). The rocking curve is a diffraction intensity distribution obtained in X-ray diffraction measurement when the direction of the incident X-rays and the position of the detector are fixed and only the crystal sample is rotated, and is one of the indicators of crystal quality.

[0041] The smaller the full width at half maximum of the rocking curve, the fewer the crystal defects and the better the crystal quality. The full width at half maximum of the rocking curve by (004) X-ray diffraction is more preferably 40 arcsec or less, even more preferably 30 arcsec or less, and even more preferably 20 arcsec or less. The lower limit of the full width at half maximum is not particularly limited, but is usually 5 arcsec or more. In this specification, the full width at half maximum is synonymous with what is generally referred to as the half width. That is, the maximum intensity f of the peak ｍａｘ For half the strength, 1 / 2f ｍａｘ This means the distance between the positions indicated.

[0042] The full width at half maximum can be adjusted by the growth method (vapor phase method, liquid phase method, or the like) of the GaN crystal layer in the Ni-doped GaN layer, the crystal characteristics of the seed substrate used when growing the GaN crystal layer, the crystal growth conditions, the selection of the growth surface, the impurity content, etc.

[0043] The rocking curve measurement by (004) X-ray diffraction uses CuKα1 radiation. In the measurement, the X-ray tube is operated at a voltage of 45 kV and a current of 40 mA, for example. When X-rays are incident on the (0001) surface, the plane of incidence of the X-rays can be perpendicular to the a-axis or m-axis.

[0044] The X-ray beam size can be set so that when the angle of incidence (the angle between the reflecting surface and the X-ray) is 90°, that is, when the X-ray is incident perpendicularly to the (004) reflecting surface, the size of the irradiation area on the (0001) surface is 5 mm in the direction parallel to the ω axis and 1 mm in the direction perpendicular to the ω axis. The ω axis is the rotation axis of the sample in rocking curve measurement, and ω represents the angle between the incident X-ray and the crystal surface. When the X-ray beam size is set in this way, ω is approximately 36.5° in rocking curve measurement by X-ray diffraction of (004), so the size of the irradiation area is approximately 1.7 × 5 mm. ２ is.

[0045] Similar to the measurement of the (004) plane, rocking curves of other crystal planes can also be measured. For example, the (201) plane is often measured. The full width at half maximum in the rocking curve of the (004) plane is an index for observing the uniformity of a plane parallel to the c-axis, i.e., the c-plane. On the other hand, the full width at half maximum in the rocking curve of the (201) plane is an index for observing the rotation uniformity around the c-axis of a plane not parallel to the c-axis, i.e., the c-plane.

[0046] Rocking curve measurements on a plane parallel to the c-axis are called on-axis measurements, while rocking curve measurements on a plane not parallel to the c-axis are called off-axis measurements. The former correlates with the dislocation density with a screw dislocation component, while the latter correlates with the dislocation density with an edge dislocation component. To evaluate crystal quality, it is preferable to perform both on-axis and off-axis measurements.

[0047] The threading dislocation density (hereinafter sometimes referred to as "dislocation density") of the Ni-doped GaN layer is 1×10 ７ cm －２ From the viewpoint of crystallinity, it is preferable that the average particle size is less than 5 × 10 ６ cm －２More preferably, 1×10 ６ cm －２ The lower limit of the threading dislocation density is not particularly limited, but is usually 1×10 ２ cm －２ That's all.

[0048] Furthermore, when the (0001) surface 11 of the GaN crystal 10 according to this embodiment is divided into 5 mm x 5 mm cells by a square lattice as shown in Figure 1, it is particularly preferable that, for the Ni-doped GaN layer, there is at least one 100 μm x 100 μm square region free of threading dislocations within each 5 mm x 5 mm cell.

[0049] The dislocation density can be adjusted by the growth method of the GaN crystal layer, such as a vapor phase method or a liquid phase method, the crystal characteristics of the seed substrate used when growing the GaN crystal layer, the crystal growth conditions, the selection of the growth surface, the impurity content, etc. Dislocation density is synonymous with the dark spot density measured by the cathodoluminescence (CL) method. There are three types of threading dislocations: edge, screw, and mixed. In this specification, these are not distinguished and are collectively referred to as threading dislocations.

[0050] The presence and density of threading dislocations in a Ni-doped GaN layer can be examined using a multiphoton excitation microscope. Dark spots observed in multiphoton excitation microscope images are confirmed by examining the correspondence between the dark spots and dark spots appearing in cathodoluminescence (CL) images.

[0051] As described above, the GaN crystal according to this embodiment has a Ni concentration of 1.0×10 １６ atoms / cm ３ Above 1.0 x 10 ２０ atoms / cm ３ The Ni-doped GaN crystal may be composed of only the Ni-doped GaN layer, but the Ni concentration may be 1.0×10 or less on the seed crystal layer made of GaN. １６ atoms / cm ３ Above 1.0 x 10 ２０ atoms / cm ３ The following Ni-doped GaN layer may be stacked.

[0052] When a GaN crystal has a Ni-doped GaN layer and a seed crystal layer, the GaN crystal includes a first region 110 and a second region 120, as shown in Figure 2. Figure 2 shows a cross section of a GaN crystal 100 cut along a plane perpendicular to the (0001) surface 101. Of the GaN crystal 100, the first region 110 including the (0001) surface 101 is preferably made of a Ni-doped GaN layer. The GaN crystal 100 has a second region 120 on the (000-1) surface 102 side, and the second region 120 has a room temperature resistivity of 1 x 10 ５ The GaN crystal layer may be a GaN crystal layer having a resistivity of less than Ωcm, that is, a GaN crystal layer that is not semi-insulating. In this specification, room temperature means 293K.

[0053] The total concentration of compensating impurities in the second region 120 is typically lower than that in the first region 110. The second region 120 may have a region near the boundary with the first region 110 in which the total concentration of compensating impurities increases stepwise or continuously with increasing proximity to the first region 110. GaN crystal 100 having first region 110 and second region 120 can be manufactured by forming second region 120 on first region 110 by epitaxial growth, or by forming first region 110 on second region 120 by epitaxial growth.

[0054] An example of a GaN crystal according to this embodiment has a (0001) surface 101 with Ga polarity and a (000-1) surface 102 with N polarity. When the GaN crystal 100 includes a seed crystal layer and a Ni-doped GaN layer, as shown in Figure 2, the surface of the first region 110, which is the Ni-doped GaN layer, is the (0001) surface 101, and the surface of the second region 120, which is the seed crystal layer, is the (000-1) surface 102. When the GaN crystal 100 is entirely composed of only a Ni-doped GaN layer, as shown in Figure 3, one surface of the Ni-doped GaN layer, which is the first region 110, is the (0001) surface 101, and the other surface is the (000-1) surface 102.

[0055] The Ni-doped GaN layer in this embodiment preferably has a surface inclined at an angle of 10 degrees or less from the (0001) crystal plane. In this specification, "a surface inclined at an angle of 10 degrees or less from the (0001) crystal plane" may be collectively referred to as "a (0001) surface." In addition, an inclination of 10 degrees or less is a concept that also includes an angle of 0 degrees.

[0056] The GaN crystal according to this embodiment may have any shape, but is preferably one that can produce a GaN substrate, as described below. The area of ​​the (0001) surface of the Ni-doped GaN layer is, for example, 1 cm ２ It may be more than 5 cm ２ or more, or 10 cm ２ or more, or 18 cm ２ or more, or 75 cm ２ or above, or 165cm ２ The upper limit is not particularly limited, but for example, 350 cm ２ It may be the following:

[0057] The GaN crystal according to this embodiment may be grown in any direction, but is preferably grown on a plane tilted at an angle of 10 degrees or less (including 0 degrees) from the (0001) crystal plane.

[0058] In the GaN crystal according to this embodiment, the (0001) surface is preferably the main surface, i.e., the large-area surface. Furthermore, the area ratio of the (0001) surface to the crystal growth surface is preferably 60% or more, more preferably 70% or more, and even more preferably 80% or more, with the upper limit being 100%. If the area of ​​the (0001) surface is large, as will be described later, it becomes possible to keep the total donor impurity concentration low, and it becomes possible to have semi-insulating properties even with a relatively low Ni concentration. The same applies when the area ratio of the (0001) surface to the crystal growth surface is within the above range.

[0059] When the GaN crystal according to this embodiment is disk-shaped, its diameter is typically 20 mm or more, and can be set in steps of 25 mm or more, 40 mm or more, 50 mm or more, and 90 mm or more. The diameter can also be set to any size, such as 25 to 27 mm (approximately 1 inch), 50 to 55 mm (approximately 2 inches), 100 to 105 mm (approximately 4 inches), or 150 to 155 mm (approximately 6 inches). When the GaN crystal has a shape other than a disk, a size can be adopted such that the diameter of the disk having the same area as the main surface is the above-mentioned size.

[0060] When used as a GaN substrate, the thickness t of the GaN crystal according to this embodiment is set to a value that allows for easy handling of the GaN substrate, depending on the diameter. For example, when the diameter of the GaN crystal is approximately 2 inches, the thickness t of the GaN crystal is preferably 250 to 500 μm, and more preferably 300 to 450 μm. Here, the thickness t is preferably 250 μm or more, more preferably 300 μm or more, and is preferably 500 μm or less, and more preferably 450 μm or less.

[0061] In the case where the GaN crystal according to this embodiment has a structure in which the second region 120 made of the seed crystal layer and the first region 110 made of the Ni-doped GaN layer are stacked as shown in FIG. 2, the thickness t １ is preferably 50 to 200 μm, more preferably 80 to 200 μm, and even more preferably 100 to 150 μm. １ is preferably 50 μm or more, more preferably 80 μm or more, and even more preferably 100 μm or more, and is preferably 200 μm or less, and more preferably 150 μm or less.

[0062] When the GaN crystal according to this embodiment is used as a GaN substrate, of the two large-area surfaces of the GaN crystal, the (0001) surface 101 is used as the front surface for epitaxial growth of the nitride semiconductor layer. The (0001) surface 101 is mirror-finished, and its root-mean-square roughness (RMS) measured with an atomic force microscope (AFM) is typically less than 2 nm, preferably less than 1 nm, and more preferably less than 0.5 nm in a measurement range of 2 μm × 2 μm. The (000-1) surface 102 is the back surface, and so may be mirror-finished or matte (roughened) finished.

[0063] The edges of the GaN substrate made of GaN crystal according to this embodiment may be chamfered. The GaN substrate may be provided with various markings as needed, such as an orientation flat or notch that indicates the crystal orientation, or an index flat that makes it easy to distinguish between the front and back surfaces.

[0064] The shape of the GaN substrate is not particularly limited, and the shape of the (0001) surface and the (000-1) surface may be disc-shaped, square, rectangular, hexagonal, octagonal, elliptical, or irregular.

[0065] In the GaN crystal of this embodiment, the Ni-doped GaN layer preferably has an activation energy Ea of 0.5 eV or more, more preferably 0.5 to 1.2 eV, which is calculated by substituting the resistivity of the Ni-doped GaN layer into the following formula (I): Resistivity (Ωcm) = A × EXP (-Ea / kT) (I) (In formula (I), A is a proportionality constant, EXP is an exponential function, Ea is the activation energy of resistivity (eV), and k is Boltzmann's constant (8.617 × 10 －５ eV / K), and T is the temperature in Kelvin (K).

[0066] When the activation energy Ea of resistivity is 0.5 eV or more, the decrease in resistivity at high temperatures is suppressed, and a GaN substrate that maintains high resistivity in a high-temperature environment tends to be obtained. From the same viewpoint, the activation energy Ea of resistivity is preferably 0.7 eV or more, more preferably 0.8 eV or more, and even more preferably 0.9 eV or more, and is preferably 1.2 eV or less, and more preferably 1.1 eV or less.

[0067] The activation energy Ea of the resistivity is preferably determined by the least squares method from a scatter diagram of resistivity and 1 / T using a temperature T of 2 or more.

[0068] Although the temperature range for determining the activation energy Ea of resistivity is not particularly limited, from the viewpoint of data reliability, it is preferable to determine the Ea of resistivity using a value of resistivity measured at a temperature of 450 K or higher. This is because the Ni-doped GaN layer has a very high resistivity, making it difficult to perform accurate measurement at room temperature, i.e., around 293 K. In contrast, by setting the measurement temperature to 450 K or higher, the resistivity is reduced to a measurable range, allowing a more accurate Ea of resistivity to be determined.

[0069] When determining the activation energy Ea of resistivity by the least squares method, the coefficient of determination R of the approximation curve when performing exponential approximation by the least squares method is ２ is preferably 0.85 or more, and the closer to 1 the better, from the viewpoint of higher reliability of the value of activation energy Ea of resistivity.

[0070] Specifically, the activation energy Ea of the resistivity can be determined by substituting the resistivity measured at intervals of 20 K in the temperature range of 700 K to 800 K into the above formula (I) and applying the least squares method.

[0071] The resistivity and the carrier type, which will be described later, can be determined by Hall effect measurement.

[0072] In the GaN crystal of this embodiment, the resistivity of the Ni-doped GaN layer at 700 K is 5×10 ５It is preferable that the resistivity is 8×10 Ωcm or more, because the GaN substrate can be operated in a wide range of application environments without a decrease in resistivity even when used in a high-temperature environment. ５ It is more preferable that the resistance is Ωcm or more, and 1×10 ６ More preferably, 1.5×10 Ωcm or more ６ A resistivity of Ωcm or more is particularly preferred, and the higher the resistivity, the better, so there is no particular upper limit.

[0073] In the GaN crystal of this embodiment, the resistivity of the Ni-doped GaN layer at 800 K is 5×10 ４ It is preferable that the resistivity at 800K is 1×10 Ωcm or more, because the GaN substrate can be operated in a wide range of application environments without a decrease in resistivity even when used in a high-temperature environment. ５ More preferably, it is 2×10 Ωcm or more. ５ More preferably, 3×10 Ωcm or more ５ A resistivity of Ωcm or more is particularly preferred, and the higher the resistivity, the better, so there is no particular upper limit.

[0074] In this embodiment, the Ni-doped GaN layer preferably has a p-type carrier type. If the crystal quality of the GaN single crystal layer is poor, even if the GaN layer is doped with Ni to form a Ni-doped GaN layer, the layer tends to become n-type because electrons cannot be sufficiently compensated due to unintended impurities such as Si and O. Therefore, the p-type carrier type of the Ni-doped GaN layer means that the GaN single crystal layer constituting the GaN crystal has good crystallinity.

[0075] [GaN Crystal Manufacturing Method] The present invention relates to a method for manufacturing a GaN crystal as described above in [GaN Crystal]. That is, one aspect of the manufacturing method according to this embodiment is a method for manufacturing a GaN crystal in which the Ni concentration is 1.0×10 １６ atoms / cm ３ Above 1.0 x 10 ２０ atoms / cm ３ The present invention relates to a method for producing a GaN crystal including a Ni-doped GaN layer having a Ni concentration of 1.0×10 or less. １６atoms / cm ３ Above 1.0 x 10 ２０ atoms / cm ３ Requirement (1): the total donor impurity concentration is 2.0 × 10 or less １７ atoms / cm ３ and requirement (2): the full width at half maximum of the rocking curve measured by (004) X-ray diffraction is 50 arcsec or less.

[0076] The Ni-doped GaN layer can be manufactured by, for example, vapor phase growth or liquid phase growth. Examples of vapor phase growth include the HVPE method, and examples of liquid phase growth include the acidic ammonothermal method and the basic ammonothermal method. Among these, the HVPE method is preferred from the viewpoint of ease of manufacturing.

[0077] The method for producing a GaN crystal according to this embodiment is a method for producing the above-mentioned Ni-doped GaN layer by vapor phase epitaxy, in which nickel dichloride is preferably used as a doping precursor, and production by the HVPE method is more preferred.

[0078] The method for producing a GaN crystal according to this embodiment will be described below using the HVPE method as an example.

[0079] The GaN crystal manufacturing method according to this embodiment using the HVPE method preferably includes the following first step (i). This method is applicable to the manufacture of GaN crystals having an N-polarity region and a Ga-polarity region sandwiching a regrowth interface, and more preferably, the resistivity is increased in at least a portion of the Ga-polarity side. (i) A first step of growing a (0001)-oriented Ni-doped GaN layer on a c-plane GaN seed by the HVPE method to form the Ni-doped GaN layer. Note that in this specification, "on the substrate" is synonymous with "on the surface of the substrate."

[0080] The first step will now be described in more detail.

[0081] In the first step, as shown in Figures 4(a) and 4(b), a (0001) oriented GaN layer 6 is grown on the Ga polar plane of a c-plane GaN seed 5 by the HVPE method to obtain a GaN layer having a stacked structure. ６ｇ At this time, a regrowth interface is formed between the c-plane GaN seed 5 and the GaN layer 6.

[0082] The method for obtaining the c-plane GaN seed 5 used in the first step is not particularly limited. For example, a seed wafer 1 shown in FIG. 5A is prepared, and a (0001)-oriented GaN thick film 2 made of intentionally undoped GaN is grown thereon by the HVPE method, as shown in FIG. 5B. Furthermore, as shown in FIG. 5C, the GaN thick film 2 is processed to obtain at least one substrate, a c-plane GaN seed 3.

[0083] An example of the seed wafer 1 is a c-plane sapphire wafer, preferably one having a peeling layer on the main surface. Alternatively, a GaN thick film may be grown by HVPE on the thus obtained c-plane GaN seed 3, and the resulting c-plane substrate may be used as the c-plane GaN seed in the first step.

[0084] Typically, the Ga polar surface of the c-plane GaN seed 5 undergoes a planarization process, i.e., a process of flattening the surface by appropriate use of techniques such as grinding, polishing, or CMP (Chemical Mechanical Polishing), a processing technique using a chemical abrasive and a polishing pad, before growing the GaN layer 6. The planarized Ga polar surface may then undergo a roughening process, i.e., a process of roughening the surface by etching, before growing the GaN layer 6.

[0085] In the first step, a Ni-doped GaN layer is formed by doping at least a portion of the GaN layer 6 with Ni, which is a compensating impurity. Note that doping with Fe, which is a compensating impurity, in addition to Ni is not excluded in any way.

[0086] The doping of Ni can be performed using a method similar to that used for doping with a conventional compensating impurity, but for example, the HVPE method is preferred. When the HVPE method is used, as described above, nickel dichloride is preferably used as the dopant precursor, because this allows the production of GaN crystals with better crystal quality.

[0087] When nickel dichloride is used as the source gas, the growth rate is preferably 20 μm / hr or more and 200 μm / hr or less. From the viewpoint of high productivity, the growth rate is preferably 20 μm / hr or more, more preferably 40 μm / hr or more. From the viewpoint of stable crystal growth, the growth rate is preferably 200 μm / hr or less, more preferably 150 μm / hr or less, and even more preferably 100 μm / hr or less. If the growth rate is too high, the surface morphology of the grown crystal may deteriorate, and the incorporation of donor impurities into the crystal may become excessive. The nickel dichloride partial pressure is adjusted so that the nickel concentration in the resulting Ni-doped GaN layer falls within the aforementioned range.

[0088] Growth thickness t of GaN layer 6 ６ｇ may be set according to the designed thickness of the Ga-polar region of the GaN crystal to be manufactured.

[0089] In the GaN crystal, even if the diameter of the GaN crystal is 4 inches or 6 inches, the growth thickness t ６ｇ It is possible to suppress the thickness to 500 μm or less.

[0090] After the first step, if necessary, a thinning step is performed to thin the GaN layer 6, which is a stacked structure, as shown in FIG. 4C. In FIG. 4C, the thickness of the c-plane GaN seed 5 is reduced to the initial thickness t ５ｉ to the final thickness t ５ｆ and the thickness of the GaN layer 6 is reduced to the initial thickness t ６ｉ to the final thickness t ６ｆ , however, only one of the c-plane GaN seed 5 and the GaN layer 6 may be processed in the thinning step.

[0091] When the offcut orientation of the GaN crystal to be manufactured is the same as the offcut of the c-plane GaN seed 5, the back surface of the c-plane GaN seed 5 (the N-polar surface of the laminated structure) can be used as a reference for the plane orientation during thinning. When the offcut orientation of the GaN crystal to be manufactured is different from the offcut of the c-plane GaN seed 5, i.e., when at least one of the offcut angle and the offcut direction is different, the crystal orientation of the GaN layer that will become the laminated structure is confirmed using an X-ray diffraction device before thinning. The processing technique used in the thinning step can be appropriately selected from grinding, lapping, CMP, dry etching, wet etching, etc.

[0092] By using the manufacturing method described above, the GaN crystal of this embodiment can be produced with a high yield.

[0093] Next, an example of an HVPE apparatus that can be used in the first step included in the above-mentioned manufacturing method will be described below with reference to Fig. 6. The HVPE apparatus 20 shown in Fig. 6 includes a hot-wall type reactor 21, a gallium reservoir 22 and a susceptor 23 disposed within the reactor 21, and a first heater 24 and a second heater 25 disposed outside the reactor 21. The first heater 24 and the second heater 25 each surround the reactor 21 in an annular shape.

[0094] The reactor 21 is a quartz tube chamber. Inside the reactor 21, there is a first zone Z, which is mainly heated by a first heater 24. １ and a second zone Z2 which is heated mainly by the second heater 25. Ｅ is the second zone Z ２ The first zone Z is connected to the reactor end on the side １ The gallium reservoir 22 located in the second zone Z is a quartz vessel having a gas inlet and a gas outlet. ２ The susceptor 23 is made of, for example, graphite. A mechanism for rotating the susceptor 23 can be optionally provided.

[0095] To grow GaN in the HVPE apparatus 20, a seed is placed on the susceptor 23, and the inside of the reactor 21 is heated by the first heater 24 and the second heater 25. At the same time, NH 3 diluted with a carrier gas is heated. ３ (ammonia) through the ammonia inlet pipe P １ Through the second zone Z ２ HCl (hydrogen chloride) diluted with carrier gas is supplied to the hydrogen chloride supply pipe P ２ This HCl reacts with metallic gallium in the gallium reservoir 22, and the resulting GaCl (gallium chloride) is introduced through the gallium chloride introduction pipe P ３ Through the second zone Z ２ It is transported to.

[0096] Second Zone Z ２ At NH ３ and GaCl react with each other, and the resulting GaN crystallizes on the seed placed on the susceptor 23. When the growing GaN is to be intentionally doped, a doping gas diluted with a carrier gas is introduced through a dopant introduction pipe P ４ through the second zone Z in the reactor 21 ２ Ammonia introduction pipe P １ , hydrogen chloride introduction pipe P ２ , gallium chloride introduction tube P ３ and dopant introduction tube P ４ The part of the NH 3 that is placed inside the reactor 21 is made of quartz. ３ The carrier gas for diluting each of the HCl and doping gas is H ２ (hydrogen gas), N ２ (nitrogen gas) or H ２ and N ２ In particular, a mixture of NH ３ The carrier gas for diluting each of the HCl and doping gas is N ２ It is more preferable to use only H gas from the viewpoint of Ni doping efficiency and obtaining good quality crystals. ２ This is thought to be because, when gas is present, a decomposition reaction of nickel dichloride gas may occur.

[0097] Preferred conditions for growing GaN using the HVPE apparatus 20 are as follows: The temperature of the gallium reservoir is, for example, 500 to 1000°C. Here, the temperature of the gallium reservoir is preferably 500°C or higher, more preferably 700°C or higher, and preferably 1000°C or lower, more preferably 900°C or lower. The susceptor temperature is, for example, 900 to 1100°C. Here, the susceptor temperature is preferably 900°C or higher, more preferably 930°C or higher, even more preferably 950°C or higher, and preferably 1100°C or lower, more preferably 1050°C or lower, even more preferably 1030°C or lower, and still more preferably 1020°C or lower.

[0098] NH in the reactor ３ The V / III ratio, which is the ratio of the partial pressure of GaCl to the partial pressure of GaCl, is, for example, 1 to 20. Here, the V / III ratio is preferably 1 or more, more preferably 2 or more, and even more preferably 3 or more, and is preferably 20 or less, and more preferably 10 or less.

[0099] A V / III ratio that is too large or too small can cause the morphology of the GaN growth surface to deteriorate. Poor morphology of the growth surface can lead to a decrease in crystal quality. The incorporation efficiency of certain impurities into GaN crystals strongly depends on the crystal orientation of the growth surface. The concentration uniformity of such impurities decreases inside GaN crystals grown under conditions where the growth surface morphology is poor. This is because a growth surface with poor morphology has facets with various orientations.

[0100] A typical example of an impurity whose incorporation efficiency into GaN crystals differs significantly depending on the crystal orientation of the growth surface is oxygen (O). Because O is a donor impurity, a decrease in the uniformity of its concentration leads to a decrease in the uniformity of the resistivity.

[0101] In addition, using a V / III ratio that is too low increases the concentration of nitrogen vacancies in the growing GaN crystal. Although the effect of nitrogen vacancies on GaN crystals, GaN substrates using such crystals, or nitride semiconductor devices formed on such GaN substrates is not yet clear, since nitrogen vacancies are point defects, it is believed that the concentration of such nitrogen vacancies should be kept as low as possible.

[0102] The growth rate of GaN is preferably 40-200 μm / h, and the NH ３ The product of the partial pressure and the GaCl partial pressure can be controlled as a parameter. An excessively high growth rate deteriorates the growth surface morphology of the grown GaN.

[0103] When doping the GaN layer 6 in the first step described above, it is preferable to gradually increase the supply rate of the doping gas to a predetermined value over several minutes to several tens of minutes from the start of supply to prevent deterioration of the morphology of the growth surface. For the same reason, it is preferable to start supplying the doping gas when the GaN layer 6 has grown at least several μm. Specifically, when forming the GaN layer in the first step, it is preferable to grow the GaN layer as an undoped layer without supplying the doping gas for at least 5 μm or more, preferably at least 10 μm or more, of the initial GaN layer growth. This prevents new dislocations from occurring at the interface between the undoped layer and the doped layer, making it possible to more appropriately maintain the quality of the seed crystal.

[0104] Although there is no limitation on the method for doping the GaN layer 6 with Ni and optionally other compensating impurities, a method of introducing a doping gas into the HVPE apparatus is usually employed.

[0105] The doping gas for Ni doping is as described above. When nickel dichloride is used as the source gas, metallic Ni is placed in the introduction tube, and nickel dichloride gas can be prepared on the spot by heating it and introducing hydrogen chloride gas.

[0106] The shape of the metallic Ni is not particularly limited, but a wire shape is preferable. Furthermore, when preparing nickel dichloride gas in situ, it is also preferable to increase the contact area between the metallic Ni and hydrogen chloride gas so that the concentration of the nickel dichloride gas is uniform. For example, the metallic Ni can be crushed into small pieces with a diameter of several millimeters, packed into a quartz pipe, and diluted hydrogen chloride gas is passed through the pipe, thereby increasing the gas / solid contact area. This is preferable because it can further improve the uniformity of the crystal quality of the Ni-doped GaN layer.

[0107] GaN crystals grown using the HVPE apparatus 20 may contain O and Si at concentrations detectable by SIMS (Secondary Ion Mass Spectroscopy) even when not intentionally doped. The Si source is quartz (SiO ２ ), and the O source is either or both of the quartz and moisture remaining or having infiltrated into the reactor.

[0108] 6, the components arranged in the reactor 21 can be made of quartz and carbon, as well as silicon carbide (SiC), silicon nitride (SiNx), boron nitride (BN), alumina, tungsten (W), molybdenum (Mo), etc. In this way, the concentrations of the impurity elements other than Si, O, and H in the GaN crystal grown using the HVPE apparatus 20 are independently 5×10 or less unless intentionally doped. １６ atoms / cm ３ The total donor impurity concentration can be 2.0×10 or less. １７ atoms / cm ３ It may be less than.

[0109] Up to this point, an example of a method for manufacturing a GaN crystal having a layered structure has been described as a method for manufacturing a GaN crystal according to this embodiment, but the GaN crystal is not limited to a GaN crystal having a layered structure. For example, such a GaN crystal can also be obtained by growing the GaN layer 6 to a certain thickness or more in the first step described above to form a GaN thick film, and then slicing a GaN crystal that is a c-plane GaN crystal from this GaN thick film. In this case, the obtained GaN crystal will be composed only of the GaN layer 6 described above.

[0110] When the Ni-doped GaN layer having a high Ni concentration is selected as the region to slice the c-plane GaN crystal, the Ni concentration in all regions of the resulting GaN crystal is high, i.e., a GaN crystal can be obtained that does not have the c-plane GaN seed 5 and is composed only of the Ni-doped GaN layer of this embodiment.

[0111] [Uses of GaN Crystal] The GaN crystal according to this embodiment can be preferably used in the manufacture of nitride semiconductor devices, particularly nitride semiconductor devices with a lateral device structure. Specifically, the nitride semiconductor device can be manufactured, for example, by a method comprising the steps of preparing the above-described GaN crystal, preparing a substrate from the prepared GaN crystal, and epitaxially growing one or more nitride semiconductor layers on the substrate.

[0112] Nitride semiconductors are also called nitride-based Group III-V compound semiconductors, Group III nitride-based compound semiconductors, GaN-based semiconductors, etc., and include GaN as well as compounds in which part or all of the gallium in GaN is substituted with other Group 13 elements of the periodic table (B, Al, In, etc.).

[0113] A typical example of a nitride semiconductor device with a lateral device structure is a GaN-HEMT, but the lateral device structure can also be adopted in electronic devices other than HEMTs, such as bipolar transistors, and in light-emitting devices such as light-emitting diodes (LEDs) and laser diodes (LDs).

[0114] The present invention will be specifically described below with reference to examples, but the present invention is not limited to these examples.

[0115] Example 1 1. Epitaxial Growth of GaN Crystals Using a vapor phase growth apparatus equipped with a quartz hot-wall reactor, GaN crystals were epitaxially grown by HVPE on freestanding GaN seeds measuring 20 mm square and 400 μm thick. The growth surface of the freestanding GaN seed was inclined at 0.35 degrees from the (0001) plane. This process involved the following steps: (1) a temperature increase step, (2) a GaN layer growth step, and (3) a cooling step, as described below.

[0116] (1) Temperature-raising step First, a free-standing GaN seed was placed in a reactor. Next, the reactor temperature was raised from room temperature to 1030°C while supplying ammonia and a carrier gas to the free-standing GaN seed. Nitrogen gas was used as the carrier gas.

[0117] (2) Ni-doped GaN Layer Growth Step: While maintaining the reactor temperature at 1030° C., a mixed gas containing ammonia, gallium chloride, and nickel dichloride was supplied as a source gas under the following growth conditions to grow a Ni-doped GaN layer to a thickness of 500 μm on the freestanding GaN seed. The growth rate was 100 μm / hr, and the growth conditions were: reactor pressure 101 kPa, ammonia partial pressure 15.781 kPa, GaCl partial pressure 1.263 kPa, nitrogen gas partial pressure 83.83 kPa, and nickel dichloride partial pressure 1.14×10 －３ The nickel dichloride was produced by passing hydrogen chloride gas through multiple 10 mm long metal Ni wires heated to 840°C. Note that for the first 20 minutes of growth, hydrogen chloride gas was not passed through the Ni wires to form an undoped layer. The gas partial pressure (P Ｇ ) is the ratio (r) of the volumetric flow rate of the gas in question to the total volumetric flow rate of all gases supplied into the reactor, expressed as a function of the reactor pressure (P Ｒ ), that is, P Ｇ = r × P Ｒ The value is expressed as:

[0118] (3) Cooling Step After completing the Ni-doped GaN layer growth step (2) above, the supply of gallium chloride to the freestanding GaN seeds was stopped, and the reactor heating was stopped to allow the reactor temperature to drop to room temperature. The gases flowing into the reactor were ammonia and nitrogen gas until the temperature dropped to 500°C, and then only nitrogen gas. The as-grown GaN crystal surface removed from the reactor was mirror-finished and flat across the entire surface.

[0119] 2. Fabrication of c-plane GaN Substrates: The as-grown GaN crystal obtained in step 1 above was subjected to laser coring to obtain square epitaxial substrates with sides measuring 8 mm. Next, the +c-plane and -c-plane were sequentially lapped and polished to complete a c-plane GaN substrate consisting of a GaN crystal containing a Ni-doped GaN layer. During the processing step, the seed portion on the -c-plane side was completely removed by grinding. The GaN crystal serving as the c-plane GaN substrate had a total thickness of 350 μm and consisted solely of a Ni-doped GaN layer. In other words, the Ni-doped GaN layer was also 350 μm thick. One of the principal surfaces of the c-plane GaN substrate was tilted at 0.35 degrees from the (0001) crystal plane.

[0120] 3. Evaluation of c-plane GaN substrate <Measurement of each element concentration> The Ni concentration, C concentration, Si concentration, O concentration, and H concentration of the c-plane GaN substrate obtained above were measured by SIMS. As a result, the Ni concentration in the c-plane GaN substrate was 1.5 × 10 １８ atoms / cm ３ The C concentration was 2 × 10 １６ atoms / cm ３ The Si concentration was 8×10 １６ atoms / cm ３ The H concentration was 1×10 １７ atoms / cm ３ The O concentration was 2 × 10 １６ atoms / cm ３ That is, the Ni concentration in the Ni-doped GaN layer was 1.5×10 １８ atoms / cm ３ and the C concentration is 2×10 １６ atoms / cm ３and the O concentration is 2×10 １６ atoms / cm ３ and the H concentration is 1×10 １７ atoms / cm ３ and the Si concentration is 8×10 １６ atoms / cm ３ and the total donor impurity concentration is 1.0×10 １７ atoms / cm ３ It was.

[0121] <Measurement of dislocation density> The dislocation density of the c-plane GaN substrate obtained above, i.e., the Ni-doped GaN layer, was 1×10 ６ cm －２ This was equivalent to the freestanding GaN seed used, indicating that Ni doping did not increase dislocations. Dislocation density measurements were performed using the following method. A Ni-doped GaN layer was grown for 20 minutes, followed by an undoped layer for 20 minutes. The dislocation distributions on the crystal surface and growth interface of the GaN crystal were observed using a multiphoton excitation microscope. A femtosecond pulse laser with a wavelength of 1035 nm was used as the excitation light, and the 350-370 nm emission from the crystal was imaged using a 50x objective lens. The reason for adding an undoped layer is that the Ni-doped GaN layer does not emit light in the 350-370 nm range due to the excitation light. Dislocation density was measured at two points within a 115 μm square observation field of view and the average was calculated. It was found that the dislocation distributions on the crystal surface and the growth interface were nearly identical, indicating that Ni doping did not increase dislocations.

[0122] <Measurement of resistivity and carrier type> For Hall measurements, 30 nm of Ti and 100 nm of Au were successively vacuum-deposited on the surface of the c-plane GaN substrate obtained above, i.e., the Ni-doped GaN layer. The Hall measurements were performed using the four-terminal Van der Pauw method. In the Hall measurements, the resistivity was measured at six points at 20 K intervals while changing the measurement temperature from 700 K to 800 K. The results are shown in Figure 7. In the graph of Figure 7, 1000 / T (1 / K) is plotted on the x-axis and resistivity (Ωcm) on the y-axis. The resistivity was 1.87 x 10 at 700 K. ６ Ωcm, 3.20 × 10 at 800K ５Ωcm and decreased as the temperature increased.

[0123] The resistivity values ​​at the six points were substituted into the formula (I) [resistivity (Ωcm) = A × EXP (-Ea / kT)] and fitted using the least squares method. The activation energy Ea of the resistivity was found to be 0.91 eV. The coefficient of determination R of the approximation curve obtained by exponential approximation using the least squares method was also found to be 0.91 eV. ２ was 0.88. Here, A is a proportionality constant, k is the Boltzmann constant (8.617 × 10 －５ eV / K), and T is the temperature in Kelvin.

[0124] The Hall measurement results showed that the c-plane GaN substrate, i.e., the Ni-doped GaN layer, exhibited a large activation energy value for resistivity. This large value is thought to contribute to the large resistivity of the c-plane GaN substrate. The carrier type determined by the Hall measurement was also p-type.

[0125] <Measurement of Rocking Curves by X-ray Diffraction> Using an X-ray diffractometer [SmartLab, manufactured by Rigaku Corporation] using CuKα1 radiation as an X-ray source, rocking curves (XRC) were measured for the (004) and (201) reflections of the c-plane GaN substrates prepared above. A Ge(440) tetracrystal monochromator was used as the incident optical system of the X-ray diffractometer. The size of the longitudinal limiting slit of the X-ray beam was 2 mm. The X-ray tube was operated at a voltage of 45 kV and a current of 40 mA. The full width at half maximum (FWHM) of the rocking curve for the (004) reflection by X-ray diffraction was 16.3 arcsec. The full width at half maximum (FWHM) of the rocking curve for the (201) reflection by X-ray diffraction was 27.6 arcsec.

[0126] Example 2: The Ni concentration in the Ni-doped GaN layer was 3.0×10 １７ atoms / cm ３ A c-plane GaN substrate was fabricated in the same manner as in Example 1, except that the Ni concentration in the c-plane GaN substrate obtained above was measured by SIMS. As a result, the Ni concentration in the c-plane GaN substrate was 3.0 × 10 １７ atoms / cm ３The C concentration was 2 × 10 １６ atoms / cm ３ The Si concentration was 3×10 １６ atoms / cm ３ The H concentration was 6×10 １６ atoms / cm ３ The O concentration was 9 x 10 １５ atoms / cm ３ That is, the Ni concentration in the Ni-doped GaN layer was 3.0 × 10 １７ atoms / cm ３ and the C concentration is 2×10 １６ atoms / cm ３ and the O concentration is 9×10 １５ atoms / cm ３ and the H concentration is 6×10 １６ atoms / cm ３ and the Si concentration is 3×10 １６ atoms / cm ３ and the total donor impurity concentration is 3.9×10 １６ atoms / cm ３ The total thickness of the GaN crystal, which was the c-plane GaN substrate, was 350 μm, and it was composed only of the Ni-doped GaN layer. That is, the thickness of the Ni-doped GaN layer was also 350 μm. The resistivity and carrier type measurements, and the activation energy Ea of the resistivity, were calculated in the same manner as in Example 1. The results are shown in FIG. 8. In the graph of FIG. 8, 1000 / T (1 / K) is plotted on the x-axis and resistivity (Ωcm) on the y-axis. The resistivity was 3.63×10 at 700 K. ６ Ωcm, 8.43 × 10 at 800K ５ The resistivity was Ωcm, which decreased as the temperature increased. The activation energy Ea of the resistivity was 0.70 eV. The coefficient of determination R of the approximation curve obtained by exponential approximation using the least squares method was ２ The full width at half maximum (FWHM) of the rocking curve obtained by X-ray diffraction of the (004) reflection was 18.0 arcsec, and the full width at half maximum (FWHM) of the rocking curve obtained by X-ray diffraction of the (201) reflection was 26.2 arcsec.

[0127] Although the present invention has been described in detail and with reference to specific embodiments, it will be apparent to those skilled in the art that various changes and modifications can be made without departing from the spirit and scope of the present invention. This application is based on a Japanese patent application (Patent Application No. 2023-130116) filed on August 9, 2023, the contents of which are incorporated herein by reference.

[0128] REFERENCE SIGNS LIST 1 seed wafer 2 GaN thick film 3 c-plane GaN seed 5 c-plane GaN seed 6 GaN layer 10 GaN crystal 11 (0001) surface 20 HVPE device 21 reactor 22 gallium reservoir 23 susceptor 24 first heater 25 second heater 100 GaN crystal 101 (0001) surface 102 (000-1) surface 110 first region 120 second region

Claims

1. A GaN crystal including a Ni-doped GaN layer, wherein the Ni-doped GaN layer has a Ni concentration of 1.0×10 １６ atoms / cm ３ Above 1.0 x 10 ２０ atoms / cm ３ A GaN crystal that satisfies at least one of the following requirements (1) and (2): (1) the Ni-doped GaN layer has a total donor impurity concentration of 2.0×10 １７ atoms / cm ３ (2) The Ni-doped GaN layer has a full width at half maximum of a rocking curve measured by (004) X-ray diffraction of 50 arcsec or less.

2. The Ni-doped GaN layer has a dislocation density of 1×10 ７ cm －２ The GaN crystal of claim 1 , wherein the GaN crystal has a thickness of less than 100 nm.

3. The GaN crystal according to claim 1 or 2, wherein the Ni-doped GaN layer has an activation energy Ea of 0.5 eV or more, calculated by substituting the resistivity into the following formula (I): Resistivity (Ωcm) = A × EXP (-Ea / kT) ... (I) (In formula (I), A is a proportional constant, EXP is an exponential function, Ea is the activation energy of resistivity (eV), and k is the Boltzmann constant (8.617 × 10 －５ eV / K), and T is the temperature in Kelvin (K).

4. The GaN crystal according to claim 3, wherein the activation energy Ea of the resistivity is 0.7 to 1.2 eV.

5. The activation energy Ea of the resistivity is determined by a least squares method using the resistivity at two or more temperatures, and the coefficient of determination R of the approximation curve when exponential approximation is performed by the least squares method is ２ The GaN crystal according to claim 3 , wherein is 0.85 or more.

6. The GaN crystal according to claim 3, wherein the activation energy Ea of the resistivity is a value obtained by substituting resistivity values ​​measured at intervals of 20 K in a temperature range from 700 K to 800 K into formula (I) and applying the least squares method.

7. The Ni-doped GaN layer has an O (oxygen) concentration and a Si (silicon) concentration of 1.0×10 １５ atoms / cm ３ The GaN crystal according to claim 1 or 2.

8. The resistivity of the Ni-doped GaN layer at 700 K is 5×10 ５ 3. The GaN crystal according to claim 1, having a resistivity of Ωcm or more.

9. The resistivity of the Ni-doped GaN layer at 800 K is 5×10 ４ 3. The GaN crystal according to claim 1, having a resistivity of Ωcm or more.

10. The GaN crystal according to claim 1 or 2, wherein the Ni-doped GaN layer has a thickness of 50 μm or more.

11. The GaN crystal according to claim 1 or 2, wherein the Ni-doped GaN layer has a surface that is inclined at 10 degrees or less from the (0001) crystal plane.

12. Ni concentration is 1.0 x 10 １６ atoms / cm ３ Above 1.0 x 10 ２０ atoms / cm ３ A method for producing a GaN crystal including a Ni-doped GaN layer by a vapor phase epitaxy method, the method comprising: growing the Ni-doped GaN layer using nickel dichloride as a doping precursor.

13. The method for producing a GaN crystal according to claim 12, wherein the vapor phase growth method is an HVPE method.