JP2019151542A - Semiconductor substrate, gallium nitride single crystal, and method of producing gallium nitride single crystal - Google Patents

Abstract

To provide a single crystal film of gallium nitride having less crystal defects, a semiconductor substrate including the same, and a method of producing the gallium nitride single crystal.SOLUTION: There is provided a semiconductor substrate 1 comprising: a sapphire substrate 20; an intermediate layer 11 formed of gallium nitride having random crystal directions and provided on the sapphire substrate; and at least one semiconductor layer 10 formed of a gallium nitride single crystal and provided on the intermediate layer. Further, there is provided a method of producing the gallium nitride single crystal, including the steps of: heating metal gallium and iron nitride up to a reaction temperature at which at least the iron nitride and the metal gallium react in a nitrogen atmosphere; and holding them at a temperature within a range of the reaction temperature for 20 hours or longer after the heating up to the reaction temperature, wherein the sapphire substrate is provided in the heated metal gallium and iron nitride, the iron nitride includes one or more of tetrairon mononitride, triiron mononitride, and diiron mononitride, and the reaction temperature is 700-1,000°C.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a semiconductor substrate, a gallium nitride single crystal, and a method for manufacturing a gallium nitride single crystal.

  In recent years, gallium nitride (GaN) has attracted attention as a semiconductor material for forming blue light emitting diodes, semiconductor lasers, and high voltage / high frequency power supply ICs (Integrated Circuits).

  When gallium nitride is formed as a single crystal thin film on a sapphire substrate or the like, generally, vapor phase growth methods such as hydride vapor phase epitaxy and metal organic vapor phase epitaxy are used. In the hydride vapor phase epitaxial growth method, specifically, ammonia gas and gallium chloride vapor are reacted on a substrate.

  In such a vapor phase growth method, a gallium nitride crystal is grown on a substrate formed of a different material such as sapphire. For this reason, in order to alleviate the difference in thermal expansion coefficient between the substrate and gallium nitride or the lattice mismatch, a technique of providing a buffer layer as disclosed in Patent Documents 1 and 2 below is known.

  However, even when the techniques disclosed in Patent Documents 1 and 2 below are employed, gallium nitride crystals synthesized by vapor phase growth have a number of crystal defects, and therefore have the desired characteristics when incorporated in a device. It was difficult to get. Further, in the vapor phase growth method, ammonia gas having high reactivity is used as a nitrogen source, so that a harmful gas removal facility is required, the manufacturing process and the manufacturing apparatus are easily complicated, and the manufacturing cost is increased.

  Here, as a method for producing a gallium nitride crystal in which crystal defects are unlikely to occur, a method for producing a gallium nitride crystal using liquid phase growth has been studied as disclosed in Patent Document 3 below. In such a method, since the gallium nitride crystal can be grown in a step-like manner by crystal precipitation from the liquid phase, lattice mismatching is unlikely to occur and generation of crystal defects in the gallium nitride crystal can be suppressed. It is said.

JP-A-8-310900 JP 2000-269605 A JP2015-71529A

  However, even when the technique disclosed in Patent Document 3 is adopted, it is still difficult to form a gallium nitride crystal in which crystal defects are sufficiently reduced.

  Accordingly, the present invention has been made in view of the above problems, and an object of the present invention is to provide a semiconductor substrate, a gallium nitride single crystal, and a gallium nitride single crystal having a gallium nitride crystal with reduced crystal defects. The object is to provide a method for producing crystals.

  In order to solve the above problems, according to an aspect of the present invention, a sapphire substrate, an intermediate layer made of gallium nitride having a random crystal direction, and provided on the sapphire substrate, and a gallium nitride single crystal are provided. And a semiconductor substrate provided with at least one semiconductor layer provided on the intermediate layer.

  The tilt width of gallium nitride constituting the intermediate layer and the semiconductor layer may be 0.05 ° or more and 0.4 ° or less.

  The twist width of gallium nitride constituting the intermediate layer and the semiconductor layer may be not less than 0.1 ° and not more than 0.7 °.

  The surface orientation of the surface on which the intermediate layer of the sapphire substrate is provided may be c-plane or a-plane.

The gallium nitride constituting the intermediate layer and the semiconductor layer contains iron atoms, and the concentration distribution of the iron atoms in the thickness direction of the gallium nitride is 1.0 × 10 ¹⁶ atoms / cm ³ or more and 1.0 × It may be included in a range of 10 ²² atoms / cm ³ or less.

  The concentration distribution of the iron atoms in the thickness direction of the gallium nitride is higher in the surface region of the gallium nitride and the interface region with the sapphire substrate, and lower in the intermediate region between the surface region and the interface region. It may have a shape distribution.

  The maximum value of the iron atom concentration in the interface region may be 10 times or more the minimum value of the iron atom concentration in the intermediate region.

  The maximum value of the iron atom concentration in the surface region may be 10 times or more the minimum value of the iron atom concentration in the intermediate region.

  In the thickness direction of the gallium nitride, the difference in concentration of the iron atoms in the intermediate region at any two points may be 50% or less of the concentration of the iron atom in any one of the two points. .

  In the in-plane direction of the gallium nitride, the difference in concentration of the iron atoms in the intermediate region at any two points may be 50% or less of the concentration of the iron atom at any one of the two points. .

  The interface region is a region within 2 μm in the thickness direction of the gallium nitride from the interface with the sapphire substrate, and the surface region is a region within 1 μm in the thickness direction of the gallium nitride from the surface of the gallium nitride. It may be.

  In order to solve the above-described problem, according to another aspect of the present invention, at least one or more layers are provided on a sapphire substrate via an intermediate layer composed of gallium nitride having a random crystal direction. A gallium nitride single crystal is provided.

  The tilt width of the gallium nitride constituting the intermediate layer and the gallium nitride single crystal may be 0.05 ° or more and 0.4 ° or less.

  The twist width of the gallium nitride constituting the intermediate layer and the gallium nitride single crystal may be not less than 0.1 ° and not more than 0.7 °.

  The surface orientation of the sapphire substrate on which the intermediate layer is provided may be a c-plane or a-plane.

  In order to solve the above problems, a gallium nitride single crystal provided in the semiconductor substrate or a method for manufacturing the gallium nitride single crystal, wherein the metal gallium and the iron nitride are at least in the nitrogen atmosphere at least the iron nitride. Heating the metal gallium and the iron nitride to the reaction temperature, and then heating the metal gallium and the iron nitride to a temperature within the range of the reaction temperature. Holding the sapphire substrate in the heated metal gallium and the iron nitride, wherein the iron nitride is composed of tetrairon mononitride, triiron mononitride or dinitride mononitride. There is provided a method for producing a gallium nitride single crystal, comprising any one or more of iron, wherein the reaction temperature is higher than 700 ° C. and lower than 1000 ° C.

  With the above structure, it is possible to suppress the generation of crystal defects due to lattice mismatch between the sapphire substrate, which is a different material, and the semiconductor layer by the intermediate layer.

  As described above, according to the present invention, it is possible to provide a gallium nitride crystal with further reduced crystal defects.

It is a mimetic diagram explaining the section composition which cut the semiconductor substrate concerning one embodiment of the present invention in the thickness direction. It is explanatory drawing which illustrates typically the tilt measurement in X-ray diffraction. It is explanatory drawing which illustrates typically the twist measurement in X-ray diffraction. It is a schematic diagram explaining the structure of the reaction apparatus used for manufacture of the gallium nitride single crystal which concerns on one Embodiment of this invention. It is a perspective view which shows more concretely the holder of the board | substrate shown in FIG. 4 is a TEM bright-field image obtained by observing a cross section in the vicinity of an interface between a sapphire substrate, an intermediate layer, and a semiconductor layer of a semiconductor substrate according to Example 1. It is a TEM bright field image which observed the cross section of the interface vicinity of the sapphire substrate of the semiconductor substrate which concerns on the comparative example 1, an intermediate | middle layer, and a semiconductor layer. 3 is a result of tilt measurement of a single crystal constituting a semiconductor layer of a semiconductor substrate according to Example 1. FIG. 3 is a result of twist measurement of a single crystal constituting a semiconductor layer of a semiconductor substrate according to Example 1. FIG. It is a scatter diagram which shows the correlation of tilt width and twist width. In evaluation of in-plane distribution, it is a schematic diagram which shows arrangement | positioning of the several measurement point in the 2-inch sapphire board | substrate which makes a crystal plane (0001) a main surface. 2 is an AFM image in an observation area 5 μm × 5 μm of a sample according to Example 1. FIG. 2 is an SEM image of a sample according to Example 1. It is the reverse pole figure which looked at the analysis result by the EBSD method of the sample concerning Example 1 from the Z-axis direction. 3 is a graph showing the SIMS measurement results of the gallium nitride crystal obtained in Example 1. FIG. It is a graph which shows the SIMS measurement result of the gallium nitride crystal formed on the sapphire substrate by vapor phase growth.

  Exemplary embodiments of the present invention will be described below in detail with reference to the accompanying drawings. In addition, in this specification and drawing, about the component which has the substantially same function structure, duplication description is abbreviate | omitted by attaching | subjecting the same code | symbol.

  In the drawings referred to in the following description, the size of some constituent members may be exaggerated for convenience of description. Therefore, the relative sizes of the constituent members illustrated in the drawings do not necessarily accurately represent the magnitude relationship between the actual constituent members. In the following description, the stacking direction is expressed as the up-down direction, and the direction in which the sapphire substrate exists is expressed as the down direction.

<1. Semiconductor substrate>
First, a configuration of a semiconductor substrate according to an embodiment of the present invention will be described with reference to FIG. FIG. 1 is a schematic diagram illustrating a cross-sectional configuration of the semiconductor substrate according to the present embodiment cut in the thickness direction.

  As shown in FIG. 1, the semiconductor substrate 1 includes a sapphire substrate 20, an intermediate layer 11, and a semiconductor layer 10. Among these, the intermediate layer 11 and the semiconductor layer 10 are made of gallium nitride.

The sapphire substrate 20 is a plate-like support made of single crystal sapphire, for example. Single crystal sapphire is a crystalline body having a corundum structure made of α-alumina (α-Al ₂ O ₃ ), and has excellent mechanical and thermal properties, chemical stability, and light transmittance. Therefore, the sapphire substrate 20 can be suitably used as a substrate for manufacturing, for example, a blue light emitting diode and a semiconductor laser. The thickness of the sapphire substrate 20 may be about 0.4 mm, for example.

  In addition, as for the sapphire substrate 20, it is preferable that the surface orientation of the surface in which the intermediate | middle layer 11 is provided becomes c surface or a surface. In the semiconductor substrate 1 according to the present embodiment, the gallium nitride crystal can be grown more epitaxially by stacking the intermediate layer 11 and the semiconductor layer 10 on the c-plane or a-plane of the sapphire substrate 20.

  The intermediate layer 11 is provided on the sapphire substrate 20 and is made of gallium nitride having a random crystal orientation. The intermediate layer 11 adjusts the lattice constants of the sapphire substrate 20 and the semiconductor layer 10 to assist the epitaxial growth of the gallium nitride single crystal constituting the semiconductor layer 10 with fewer crystal defects. The thickness of the intermediate layer 11 may be 100 nm or less, for example.

  The semiconductor layer 10 is provided on the intermediate layer 11 and is composed of a gallium nitride single crystal. In other words, the semiconductor layer 10 is provided on the intermediate layer 11 as a single crystal film made of gallium nitride. For example, the gallium nitride single crystal constituting the semiconductor layer 10 is provided by epitaxial growth on the sapphire substrate 20. The thickness of the semiconductor layer 10 may be about several μm, for example.

  The semiconductor layer 10 may be provided in a plurality of layers on the intermediate layer 11. That is, two or more semiconductor layers 10 may be stacked on the intermediate layer 11. The upper limit of the number of stacked semiconductor layers 10 is not particularly provided, but when the number of stacked semiconductor layers 10 becomes excessive, the semiconductor substrate 1 may be warped in the stacking direction of the semiconductor layers 10. Therefore, the upper limit of the number of stacked semiconductor layers 10 may be, for example, 10 layers.

<2. Characteristics of Gallium Nitride Crystal>
Below, the 1st characteristic and 2nd characteristic with which the intermediate | middle layer 11 and the semiconductor layer 10 (namely, gallium nitride crystal) which are provided in the semiconductor substrate 1 which concern on this embodiment are demonstrated.

(First characteristic)
First, with reference to FIG. 2A and FIG. 2B, the 1st characteristic with which a gallium nitride crystal is provided is demonstrated in detail.

  According to the present embodiment, the semiconductor layer 10 composed of a gallium nitride single crystal with further reduced crystal defects can be formed on the sapphire substrate 20. For example, thin film X-ray diffraction can be used to analyze the crystallinity of such a single crystal film. The thin film X-ray diffraction can be measured by, for example, a fully automatic horizontal multipurpose X-ray analyzer “SmartLab 3XG” manufactured by Rigaku Corporation.

  Specifically, an index indicating the crystallinity in the crystal stacking direction (that is, the degree of alignment of the crystal orientation in the crystal stacking direction) is generally referred to as tilt and is obtained by X-ray diffraction as shown in FIG. 2A. Can be measured. FIG. 2A is an explanatory diagram schematically illustrating tilt measurement in X-ray diffraction.

  As shown in FIG. 2A, in X-ray diffraction tilt measurement, measurement is performed in an out-of-plane arrangement in which X-ray incidence and diffraction are detected in a plane perpendicular to the surface of the sample. Such measurement is also referred to as symmetrical reflection measurement because the incident angle and the outgoing angle of X-rays with respect to the surface of the sample are equal. In tilt measurement of X-ray diffraction, a lattice plane parallel to the surface of the sample is measured. Specifically, as shown in FIG. 2A, using a parallel beam optical system, the detector is fixed at the diffraction angle (2θ), and the diffraction intensity is measured by swinging the sample around the ω axis. To do. In tilt measurement of X-ray diffraction, the orientation distribution of the orientation axis in the stacking direction of the crystal as a sample can be evaluated from the change in diffraction intensity at this time.

  Note that the penetration depth of the X-ray into the sample at this time is about several tens of μm. Therefore, the value of the peak intensity in the tilt measurement of X-ray diffraction is presumed to depend on, for example, the volume (average thickness) of the intermediate layer 11 and the semiconductor layer 10 (that is, gallium nitride crystal) existing in the X-ray irradiation region. The

  An index indicating the crystallinity in the in-plane direction of the crystal (that is, the degree of alignment of the crystal orientation in the in-plane direction of the crystal) is generally referred to as twist and measured by X-ray diffraction as shown in FIG. 2B. can do. FIG. 2B is an explanatory diagram schematically illustrating twist measurement in X-ray diffraction.

  As shown in FIG. 2B, in the X-ray diffraction twist measurement, a parallel beam optical system is used, and measurement is performed with an in-plane arrangement in which X-ray incidence and diffraction are detected within the surface of the sample. Done. In the twist measurement of X-ray diffraction, X-rays are incident on the surface of the sample, and the diffraction from the lattice plane orthogonal to the surface of the sample is measured. Specifically, as shown in FIG. 2B, the detector is fixed to the value of 2θχ calculated using the Bragg equation from the lattice spacing of the sample to be measured, and the sample is rotated around the in-plane rotation angle φ axis. The diffraction intensity is measured by a method of swinging the lens. In the twist measurement of X-ray diffraction, the orientation distribution of the orientation axis in the in-plane direction of the crystal as a sample can be evaluated from the fluctuation of the diffraction intensity at this time.

  In the twist measurement, it is possible to relatively compare the peak intensity values by unifying the conditions of the measurement optical system such as the incident angle, the incident slit width, and the length limiting slit width, and matching the X-ray irradiation areas. In such a case, it is presumed that the value of the peak intensity depends on the volume (average thickness) of the intermediate layer 11 and the semiconductor layer 10 (that is, gallium nitride crystal) existing in the X-ray irradiation region, for example.

  The intermediate layer 11 and the semiconductor layer 10 made of gallium nitride are formed by heteroepitaxial growth on a sapphire substrate 20 made of a different material. However, gallium nitride has a completely different crystal structure from sapphire and has a lattice mismatch (also referred to as mismatch) of about 16%, so that the crystal structure tends to be disturbed near the interface. Therefore, it is important to evaluate the crystallinity or the degree of crystal defects of the intermediate layer 11 and the semiconductor layer 10 provided on the sapphire substrate 20.

  Further, in the gallium nitride single crystal, the growth anisotropy is greatly different in the stacking direction and the in-plane direction. Therefore, it is important to evaluate the crystallinity of the semiconductor layer 10 composed of a gallium nitride single crystal by carrying out precise analysis by separating the crystal stacking direction and the in-plane direction.

  The full width at half maximum (also referred to as tilt width) of the tilt peak of the gallium nitride crystals constituting the intermediate layer 11 and the semiconductor layer 10 measured by the X-ray diffraction described above is 0.05 ° or more and 0.4 ° or less. possible. Further, the full width at half maximum (also referred to as twist width) of the twist peak of the gallium nitride crystal constituting the intermediate layer 11 and the semiconductor layer 10 may be 0.1 ° or more and 0.7 ° or less. Since the intermediate layer 11 and the semiconductor layer 10 included in the semiconductor substrate 1 according to this embodiment have high crystallinity, the above-described tilt width and twist width can be reduced.

  Further, according to the present embodiment, the semiconductor layer 10 made of gallium nitride single crystal and having a better in-plane distribution of crystallinity can be formed on the sapphire substrate 20. The in-plane distribution of crystallinity of the semiconductor layer 10 can be measured as follows using X-ray diffraction.

  Specifically, as shown in FIG. 2A, the in-plane distribution of crystallinity of the semiconductor layer 10 is an out-of-plane arrangement in which X-ray incidence and diffraction are detected in a plane perpendicular to the surface of the sample. Measurement is performed at. Such measurement is also referred to as symmetrical reflection measurement because the incident angle and the outgoing angle of X-rays with respect to the surface of the sample are equal. However, in the measurement of the in-plane distribution of the semiconductor layer 10, unlike the tilt measurement described above, the minute portion condensing optical system is used, and the measurement is performed in conjunction with the diffraction angle (2θ) and the sample axis (θ). By performing such measurement at a plurality of points on the sapphire substrate 20, calculating the average value and standard deviation, and calculating the standard deviation / average value of the peak intensity of the (0002) plane of gallium nitride at each measurement point. In-plane variation in crystallinity of the semiconductor layer 10 can be evaluated.

  As shown in the examples described later, in the intermediate layer 11 and the semiconductor layer 10 included in the semiconductor substrate 1 according to this embodiment, it can be understood that the in-plane distribution of crystallinity is good because the index is low.

  Further, according to the present embodiment, the semiconductor layer 10 made of gallium nitride single crystal and having few threading dislocations can be formed on the sapphire substrate 20. In addition, about the threading dislocation density of the semiconductor layer 10, it can measure as follows using the etch pit method.

  Specifically, after immersing the semiconductor substrate 1 in a heated basic solution or the like, the semiconductor substrate 1 is cleaned with a diluted acidic solution, and the surface of the semiconductor layer 10 is observed with an atomic force microscope (AFM). To do. Since threading dislocations in the semiconductor layer 10 become holes eroded by hot alkali, the threading dislocation density per unit area of the semiconductor layer 10 is calculated by measuring the number of holes per observation field. Can do.

  As shown in the examples described later, in the semiconductor layer 10 provided in the semiconductor substrate 1 according to the present embodiment, the threading dislocation density is extremely low, and the quality is equal to or higher than that of a general gallium nitride crystal by vapor phase growth. I can understand. Therefore, it can be understood that the semiconductor layer 10 included in the semiconductor substrate 1 according to the present embodiment has few threading dislocations and extremely few crystal defects.

  According to this embodiment, the semiconductor layer 10 made of a gallium nitride single crystal and having little variation in crystal orientation can be formed on the sapphire substrate 20. The in-plane distribution of the crystal orientation of the semiconductor layer 10 can be measured as follows using the crystal orientation analysis function of a scanning electron microscope (SEM).

  Specifically, the crystal orientation analysis function of the SEM is a function of observing a microscopic crystal structure of the sample by electron back scattered diffraction pattern (EBSD). For example, when an electron beam is irradiated while tilting the sample greatly in the SEM, if the sample is crystalline, electron beam diffraction occurs in the sample. The crystal orientation can be obtained by indexing the orientation using a diffraction pattern called a Kikuchi pattern for this electron beam diffraction. Furthermore, by scanning the sample with an electron beam and mapping the Kikuchi pattern, it is possible to obtain a crystal orientation map or crystal grain information on the sample surface.

  As shown in the examples described later, in the semiconductor layer 10 included in the semiconductor substrate 1 according to this embodiment, almost the entire surface of the semiconductor layer 10 is oriented in the c-axis direction of <0001>. Therefore, it can be understood that the semiconductor layer 10 included in the semiconductor substrate 1 according to the present embodiment has a uniform crystal orientation and a good in-plane distribution.

(Second characteristic)
Next, the second characteristic of the gallium nitride crystal will be described in more detail.

  According to this embodiment, the intermediate layer 11 and the semiconductor layer 10 that are made of gallium nitride crystal doped with iron atoms and having few crystal defects can be formed on the sapphire substrate 20.

  Gallium nitride crystals are expected to be applied to next-generation functional electronic components such as light-emitting diodes, high-frequency power supply transistors, or high-voltage power supply transistors. When considering application to such an electronic component, it is important that the gallium nitride crystal has a semi-insulating property. Thus, a technique for imparting semi-insulating properties by doping iron atoms into a gallium nitride crystal has been studied.

  For example, a technique for doping a gallium nitride crystal with iron atoms by thermal diffusion has been studied. However, since this method is batch processing, throughput is low and productivity is low. Further, in this method, since it is difficult to quantitatively monitor the amount of iron atoms doped in the gallium nitride crystal, it is difficult to uniformly dope iron atoms into the gallium nitride crystal. Furthermore, this method has a problem that a crystal defect is caused in the gallium nitride crystal by the thermally diffused iron atoms.

  A technique for doping iron atoms into a gallium nitride crystal by ion implantation has been studied. However, in this method, since the ion implantation depth and concentration are affected by the crystal orientation, it is difficult to uniformly dope the gallium nitride crystal with iron atoms. In addition, this method has a problem of causing crystal defects in the gallium nitride crystal by ion implantation.

  In the semiconductor substrate 1 according to the present embodiment, a gallium nitride crystal constituting the intermediate layer 11 and the semiconductor layer 10 is formed by liquid phase growth using a melt obtained by melting iron nitride containing iron atoms and metal gallium. Yes. Therefore, according to the present embodiment, it is possible to uniformly form a gallium nitride crystal in which iron atoms are naturally taken into the crystal in the course of crystal growth.

Specifically, the gallium nitride crystals constituting the intermediate layer 11 and the semiconductor layer 10 may contain iron atoms at a concentration of 1.0 × 10 ¹⁶ atoms / cm ³ or more and 1.0 × 10 ²² atoms / cm ³ or less. Good.

However, the gallium nitride crystal formed on the semiconductor substrate 1 has different insulation properties depending on the application. Therefore, the gallium nitride crystal formed on the semiconductor substrate 1 may contain an iron atom having an appropriate concentration depending on the application. For example, the gallium nitride crystal of the semiconductor substrate 1 for low resistance use may contain iron atoms having a concentration of 1.0 × 10 ¹⁶ atoms / cm ³ or more and 1.0 × 10 ¹⁹ atoms / cm ³ or less. In addition, the gallium nitride crystal of the semiconductor substrate 1 for high resistance use may contain iron atoms having a concentration of 1.0 × 10 ¹⁷ atoms / cm ³ or more and 1.0 × 10 ²² atoms / cm ³ or less.

More specifically, the concentration distribution of iron atoms in the gallium nitride crystals constituting the intermediate layer 11 and the semiconductor layer 10 is 1.0 × 10 ¹⁶ atoms / cm ³ or more and 1.0 × 10 ²² atoms / cm ³ or less. It may be included in the range and may have a bathtub shape in the thickness direction of the gallium nitride crystal.

  The bathtub shape is a shape in which the central portion is flat and both end portions are lifted. That is, the concentration of iron atoms in the gallium nitride crystal increases in the thickness direction of the gallium nitride crystal in the surface region of the gallium nitride crystal and in the interface region with the sapphire substrate 20. It may have a distribution in which the concentration of iron atoms is low in the intermediate region between.

  The reason why the concentration distribution of iron atoms takes such a shape in the thickness direction of the gallium nitride crystal can be considered as follows.

  The gallium nitride crystal in the interface region with the sapphire substrate 20 is an initial stage of growth of the gallium nitride crystal. Therefore, it is considered that the gallium nitride crystal in the interface region with the sapphire substrate 20 incorporates more iron atoms into the crystal in order to eliminate the crystal lattice mismatch between the sapphire substrate 20 and gallium nitride. Further, the gallium nitride crystal in the surface region is a starting point for the growth of the gallium nitride crystal on the sapphire substrate 20. Therefore, it is considered that the gallium nitride crystal in the surface region takes in more iron atoms as a catalyst for gallium nitride crystal formation into the crystal as the gallium nitride crystal is generated. On the other hand, since the gallium nitride crystal in the intermediate region between the interface region and the surface region grows at a constant rate, it is considered that iron atoms are taken into the crystal at a substantially constant concentration.

  For example, in the thickness direction of the gallium nitride crystal, the concentration distribution of iron atoms is such that the maximum value of the concentration of iron atoms in the interface region with the sapphire substrate 20 is 10 times or more the minimum value of the concentration of iron atoms in the intermediate region. The distribution may be as follows. Further, the concentration distribution of iron atoms may be such that the maximum value of the iron atom concentration in the surface region of the gallium nitride crystal is 10 times or more the minimum value of the iron atom concentration in the intermediate region. Good.

  By growing a gallium nitride crystal from a melt obtained by melting iron nitride and metal gallium by liquid phase growth, the gallium nitride crystal formed on the semiconductor substrate 1 has a concentration distribution of iron atoms contained therein. Distribution. According to this, the gallium nitride crystal formed on the semiconductor substrate 1 can contain iron atoms without generating crystal defects.

  The concentration of iron atoms in the gallium nitride crystal can be controlled, for example, by the amount of iron nitride added to the melt during liquid phase growth. Specifically, the concentration of iron atoms in the gallium nitride crystal can be increased overall by increasing the amount of iron nitride added to the melt during liquid phase growth. Conversely, by reducing the amount of iron nitride added to the melt during liquid phase growth, the concentration of iron atoms in the gallium nitride crystal can be reduced overall. The concentration of iron atoms in the gallium nitride crystal can be controlled by controlling the growth rate of the gallium nitride crystal during liquid phase growth. This is because by controlling the growth rate of the gallium nitride crystal, it is possible to control the rate of incorporation of iron atoms into the gallium nitride crystal and the discharge rate of impurities from the gallium nitride crystal.

  Here, the concentration distribution of the iron atoms in the gallium nitride crystal in the intermediate region has high uniformity because the iron atoms are substantially constant and taken into the crystal. For example, in the thickness direction of the gallium nitride crystal, the concentration difference of iron atoms in any two intermediate regions can be 50% or less of the concentration of any one of the two iron atoms. That is, in the thickness direction of the gallium nitride crystal, the variation in the concentration of iron atoms in the gallium nitride crystal in the intermediate region is extremely small and uniform compared to the increase in the concentration of iron atoms in the interface region or the surface region.

  In the semiconductor substrate 1 according to the present embodiment, the concentration distribution of iron atoms in the gallium nitride crystal has high uniformity even in the in-plane direction of the gallium nitride crystal. For example, in the in-plane direction of the gallium nitride crystal, the difference in iron atom concentration between any two intermediate regions can be 50% or less of the concentration of any one of the two iron atoms. That is, in the in-plane direction of the gallium nitride crystal, the variation in the concentration of iron atoms in the gallium nitride crystal in the intermediate region is compared with the increase in the concentration of iron atoms in the interface region or surface region, as in the thickness direction of the gallium nitride crystal. Very small and uniform.

  Therefore, according to the present embodiment, the intermediate layer 11 and the semiconductor layer 10 composed of gallium nitride crystals doped with iron atoms can be formed without causing crystal defects. In addition, according to the present embodiment, the gallium nitride crystal in the intermediate region between the surface region of the gallium nitride crystal and the interface region with the sapphire substrate 20 can be doped with a substantially uniform concentration.

  Note that the interface region in the gallium nitride crystal refers to a region until the increase in the iron atom concentration distribution at the interface between the gallium nitride crystal and the sapphire substrate 20 is reduced in the thickness direction of the gallium nitride crystal and becomes substantially constant. The interface region in the gallium nitride crystal may be, for example, a region within 2 μm from the interface with the sapphire substrate 20 in the thickness direction of the gallium nitride crystal. In addition, the surface region in the gallium nitride crystal refers to a region until the increase in the iron atom concentration distribution on the surface of the gallium nitride crystal is reduced in the thickness direction of the gallium nitride crystal and becomes substantially constant. The surface region in the gallium nitride crystal may be, for example, a region within 1 μm from the surface of the gallium nitride crystal in the thickness direction of the gallium nitride crystal.

<3. Method for producing gallium nitride crystal>
Then, the manufacturing method of the gallium nitride single crystal with which the semiconductor substrate which concerns on one Embodiment of this invention is provided is demonstrated.

  The method for producing a gallium nitride single crystal according to the present embodiment heats and melts metal gallium and iron nitride, and uses the nitriding action of iron nitride to form gallium nitride on the sapphire substrate 20 via the intermediate layer 11. This is a method for producing a single crystal film (semiconductor layer 10). According to the above method, a gallium nitride single crystal can be grown on a sapphire substrate by liquid phase epitaxial growth under a low pressure (for example, normal pressure) environment as compared with conventional liquid phase growth.

(Reaction material)
First, reaction materials used in the method for producing a gallium nitride single crystal according to the present embodiment will be described. Metal gallium and iron nitride are used as the reaction material.

  It is preferable to use a metal gallium having a high purity. For example, a metal gallium having a purity of 99.99% or more can be used.

Specifically, iron nitride may be tetrairon mononitride (Fe ₄ N), triiron mononitride (Fe ₃ N), diiron mononitride (Fe ₂ N), or a mixture of two or more of these. it can. It is preferable to use high-purity iron nitride. For example, commercially available iron nitride having a purity of 99.9% or more can be used.

  Iron atoms in iron nitride are mixed with metal gallium and heated to function as a catalyst, and generate active nitrogen from nitrogen atoms in iron nitride or nitrogen molecules in the melt. The generated active nitrogen reacts with metal gallium to generate a gallium nitride crystal.

  Specifically, when tetrairon mononitride is used as the iron nitride, iron nitride and metal gallium react by the nitriding action of tetrairon mononitride to generate gallium nitride (Reaction Formula 1).

Fe ₄ N + 13Ga → GaN + 4FeGa ₃ ... Reaction formula 1

  Further, iron nitride and metal gallium dissolved in the melt from a nitrogen atmosphere react with each other when iron atoms function as a catalyst to generate gallium nitride (Reaction Formula 2).

2Ga + N ₂ + Fe → 2GaN + Fe (2)

  In addition, it is preferable that the ratio of the iron nitride in a reaction material is 2 mol% or less with respect to the total number of moles of metal gallium and iron nitride. When the ratio of iron nitride exceeds 2 mol%, the amount of alloy formation between iron atoms in iron nitride and metal gallium increases, and the viscosity of the melt obtained by melting the reaction material increases, so that the gallium nitride crystal This is not preferable because the growth rate decreases.

(Reactor)
Next, with reference to FIG. 3, a reaction apparatus used in the method for producing a gallium nitride single crystal according to the present embodiment will be described. FIG. 3 is a schematic diagram illustrating the configuration of a reaction apparatus used for manufacturing a gallium nitride single crystal.

  As shown in FIG. 3, the reactor 100 includes an electric furnace 113, a heater 114 provided on a side surface of the electric furnace 113, a gas inlet 131, a gas outlet 132, a lifting shaft 122, and a lifting shaft 122. And a sealing material 123 that ensures airtightness between the electric furnaces 113. Further, inside the electric furnace 113, a pedestal 112 for placing the reaction vessel 111 charged with the raw material melt 110 of the reaction material is provided, and a substrate 140 on which a gallium nitride crystal is grown is formed at one end of the pulling shaft 122. A holding tool 120 is provided for holding the. That is, the reaction apparatus 100 is an apparatus for epitaxially growing a gallium nitride crystal film on a substrate 140 immersed in a raw material melt 110 obtained by melting a reaction material.

  The electric furnace 113 has a sealed structure, and accommodates the reaction vessel 111 therein. For example, the electric furnace 113 may have a cylindrical structure having an inner diameter (diameter) of about 200 mm and a height of about 800 mm. The heater 114 is disposed on a side surface in the longitudinal direction of the electric furnace 113 and heats the inside of the electric furnace 113.

The gas inlet 131 is provided below the electric furnace 113 and introduces atmospheric gas (for example, nitrogen (N ₂ ) gas) into the electric furnace 113. The gas discharge port 132 is provided above the electric furnace 113 and discharges the atmospheric gas inside the electric furnace 113. Due to the gas inlet 131 and the gas outlet 132, the pressure inside the electric furnace 113 is maintained at substantially normal pressure (that is, atmospheric pressure).

  The gantry 112 is a member that supports the reaction vessel 111. Specifically, the gantry 112 supports the reaction vessel 111 so that the reaction vessel 111 is evenly heated by the heater 114. For example, the height of the gantry 112 may be such a height that the reaction vessel 111 is positioned at the center of the heater 114.

  The reaction vessel 111 is a vessel for holding the raw material melt 110 in which the reaction material is melted. The reaction vessel 111 may be, for example, a cylindrical vessel having an outer diameter (diameter) of about 100 mm, a height of about 90 mm, and a thickness of about 5 mm. The reaction vessel 111 is made of, for example, carbon, but may be made of other materials such as aluminum oxide as long as it does not react with metal gallium at a high temperature around 1000 ° C.

  The raw material melt 110 is a liquid in which a reaction material is melted. Specifically, the raw material melt 110 is a liquid obtained by heating and melting metal gallium and iron nitride, which are reaction materials, with a heater 114. In addition, as for the mixing ratio of the metal gallium and iron nitride which are reaction materials, it is preferable that the number of moles of iron nitride is 2% or less with respect to the total number of moles of metal gallium and iron nitride as mentioned above.

  The substrate 140 is a substrate on which a gallium nitride crystal film can be stacked. Specifically, the substrate 140 may be a sapphire substrate. The substrate 140 may have any shape, but may be, for example, a substantially flat plate shape, a substantially disk shape, or the like.

The sealing material 123 is provided between the pulling shaft 122 and the electric furnace 113 to ensure airtightness in the electric furnace 113. Since the outside of the electric furnace 113 is prevented from flowing into the electric furnace 113 by the sealing material 123, the inside of the electric furnace 113 has a gas atmosphere (for example, nitrogen (N ₂ )) introduced from the gas inlet 131. Atmosphere).

  The pulling shaft 122 immerses the substrate 140 in the raw material melt 110 and pulls up the substrate 140 from the raw material melt 110. Specifically, the lifting shaft 122 is provided through the upper surface of the electric furnace 113. A holding tool 120 that holds the substrate 140 is provided at one end of the pulling shaft 122 in the electric furnace 113. Therefore, the substrate 140 held by the holder 120 can be immersed in the raw material melt 110 or pulled up by moving the pulling shaft 122 up and down.

  The pulling shaft 122 may be provided so as to be rotatable about the shaft. In such a case, by rotating the pulling shaft 122, the holder 120 and the substrate 140 can be rotated, and the raw material melt 110 can be stirred. Thereby, since the nitrogen concentration distribution in the raw material melt 110 can be made more uniform, a uniform gallium nitride single crystal film can be grown on the substrate 140.

  The holding tool 120 includes a plurality of scissors and holds the substrate 140 horizontally. By holding the substrate 140 horizontally, the influence of the nitrogen concentration distribution in the depth direction of the raw material melt 110 on the substrate 140 can be reduced. The holder 120 may be made of carbon, similar to the reaction vessel 111, but may be made of other materials such as aluminum oxide as long as it does not react with metallic gallium even at a high temperature around 1000 ° C. .

  With the above configuration, the reaction apparatus 100 can grow the gallium nitride single crystal film on the substrate 140 by immersing the substrate 140 in the raw material melt 110 by moving the pulling shaft 122 up and down.

  Here, with reference to FIG. 4, the more specific shape of the holder 120 is demonstrated. FIG. 4 is a perspective view showing the holder 120 of the substrate 140 shown in FIG. 3 more specifically.

  As shown in FIG. 4, the holder 120 has a structure in which both ends of support columns 126 and 127 that are two columnar members are connected by beam portions 124 and 125, respectively. In addition, at least one shelf board 128 is provided in the space formed by the column parts 126 and 127 and the beam parts 124 and 125. The shelf board 128 is provided perpendicular to the support columns 126 and 127 to hold the substrate 140 horizontally.

  The holder 120 may include a plurality of shelf boards 128. In such a case, the holder 120 can synthesize the crystal films of gallium nitride on the plurality of substrates 140 by simultaneously immersing the plurality of substrates 140 in the raw material melt 110 in the reaction vessel 111. In addition, the space | interval of each shelf board 128 should just be about 10 mm, for example.

  According to the above configuration, the reactor 100 can stack the crystal film of gallium nitride having the same crystal orientation as the substrate 140 (that is, epitaxially grown) on the substrate 140.

(Reaction process)
Next, the flow of the method for manufacturing a gallium nitride single crystal according to this embodiment will be described.

  First, powders of metal gallium and iron nitride are mixed and filled in the reaction vessel 111 described above, and the reaction vessel 111 is placed in an electric furnace 113.

  Subsequently, nitrogen gas is introduced into the electric furnace 113 from the gas inlet 131 to make the inside of the electric furnace 113 a nitrogen atmosphere, and the reaction material in the reaction vessel 111 is heated by the heater 114. In addition, since the nitrogen gas introduced into the electric furnace 113 from the gas inlet 131 is discharged from the gas outlet 132, the electric furnace 113 is maintained at a substantially normal pressure.

  Here, the reaction material in the reaction vessel 111 is heated at least to a reaction temperature at which metal gallium and iron nitride react. Specifically, such reaction temperature is more than 700 ° C. and 1000 ° C. or less.

  The reaction material in the reaction vessel 111 is preferably maintained at a temperature within the above reaction temperature range for 20 hours or more while the gallium nitride single crystal is grown on the substrate 140. While the temperature is maintained, the temperature of the reaction material does not need to be constant and may vary as long as it is within the reaction temperature range (for example, more than 700 ° C. and 1000 ° C. or less). .

  After the reaction material in the reaction vessel 111 is melted to become the raw material melt 110, the substrate 140 held by the holder 120 is immersed in the raw material melt 110 by operating the pulling shaft 122. Thereby, a uniform single crystal film of gallium nitride can be grown on the substrate 140 immersed in the raw material melt 110.

  However, the reaction product obtained in the above process may contain byproducts such as iron and gallium intermetallic compounds. Therefore, by-products are removed from the substrate 140 on which the gallium nitride crystal is deposited through a purification process.

  In the purification step, for example, acid washing using an acid such as aqua regia can be used. Thereby, an intermetallic compound of iron and gallium or the like can be dissolved in an acid, and the gallium nitride single crystal can be purified.

  According to the above process, a gallium nitride single crystal can be efficiently produced by liquid phase growth under a low-pressure nitrogen atmosphere such as normal pressure.

  Hereinafter, the semiconductor substrate, the gallium nitride single crystal, and the method for manufacturing the gallium nitride single crystal according to the present embodiment will be specifically described with reference to examples and comparative examples. In addition, the Example shown below is an example to the last, and the manufacturing method of the semiconductor substrate which concerns on this embodiment, a gallium nitride single crystal, and a gallium nitride single crystal is not limited to the following example.

  In the following Examples and Comparative Examples, 7N pure metal gallium (manufactured by DOWA Electronics) and triiron mononitride (manufactured by High Purity Chemical Research Laboratory) having a purity of 99% or more were used. Further, the crystal growth substrate has a diameter of about 2 inches and a thickness of 0.4 mm, and the plane orientations of the planes on which the single crystal films of gallium nitride are laminated are c plane (0001) and a plane (11-20), respectively. ) Or r-plane (1-102) sapphire substrates (manufactured by Shinko) were used.

Example 1
First, each raw material of metallic gallium (Ga) and triiron mononitride (Fe ₃ N) is added to a reaction vessel installed inside the reactor in a ratio of Ga: Fe ₃ N = 99.8 mol%: 0.2 mol%. And mixed. Further, a sapphire substrate having a c-plane (0001) plane orientation of the surface on which the semiconductor layer is formed was placed on the shelf plate of the holder.

  Subsequently, by controlling the internal temperature of the reactor with the following temperature profile, metal gallium and triiron mononitride were reacted to produce a gallium nitride single crystal.

  Specifically, first, the internal temperature of the reactor was manually increased to 200 ° C., and then the internal temperature was increased to 700 ° C. at a rate of 100 ° C. per hour. Thereafter, the internal temperature of the reactor was held at 700 ° C. for 20 hours, then increased to 900 ° C. at a rate of 100 ° C. per hour, and held at 900 ° C. for 40 hours. At this time, the melt was stirred by rotating the holder at a speed of 10 revolutions per minute with the pulling shaft as the center. Then, it cooled naturally until the inside of reaction container returned to room temperature by natural heat radiation. The extracted sapphire substrate was washed with aqua regia to remove residual metal gallium or an intermetallic compound of iron and gallium.

  When the cross section of the semiconductor substrate according to Example 1 was confirmed with a transmission electron microscope (TEM) or the like, an intermediate layer composed of gallium nitride crystals and a semiconductor layer were formed on the sapphire substrate. It was confirmed that The result is shown in FIG. 5A. 5A is a TEM bright-field image obtained by observing a cross section in the vicinity of the interface between the sapphire substrate, the intermediate layer, and the semiconductor layer of the semiconductor substrate according to Example 1. FIG.

  In FIG. 5A, the slightly bright area on the right side of the figure is the sapphire substrate 20, and the slightly dark area on the left side is the gallium nitride crystal (intermediate layer 11 and semiconductor layer 10). As shown in FIG. 5A, it can be confirmed that an intermediate layer 11 having a disordered lattice arrangement is formed at a thickness of about 20 nm at the interface between the sapphire substrate 20 and the gallium nitride crystal.

  Subsequently, the tilt measurement and the twist measurement described above were performed on the sapphire substrate on which the intermediate semiconductor layer (gallium nitride crystal film) according to Example 1 was stacked. The measurement results are shown in FIGS. 6A and 6B.

  FIG. 6A shows the result of tilt measurement of the gallium nitride crystal constituting the intermediate layer and the semiconductor layer of the semiconductor substrate according to Example 1. Tilt measurement (ω scan) measures changes or variations in crystal orientation in the crystal stacking direction, and the narrower the peak width, the smaller the crystal orientation variation and the better the crystallinity.

  Using the above-described fully automatic horizontal multi-purpose X-ray analyzer “SmartLab 3XG” manufactured by Rigaku Corporation, 2θ = 34.6 in an arrangement in which X-ray incidence and diffraction detection are performed in a plane perpendicular to the surface of the sample. The measurement was performed by ω-scanning by placing a detector at ° (value calculated using the Bragg equation from the lattice spacing of the (0002) plane of gallium nitride). At this time, the penetration depth of X-rays into the sample is about several tens of μm.

  As shown in FIG. 6A, a peak was observed in the vicinity of ω = 17.3 degrees. The full width at half maximum (tilt width) of the intensity of the peak was 0.13 °.

  6B is a result of twist measurement of the gallium nitride crystal constituting the intermediate layer and the semiconductor layer of the semiconductor substrate according to Example 1. FIG. Twist measurement (φ scan) is a measurement of a change or variation in crystal orientation in the in-plane direction of the crystal. The narrower the peak width, the smaller the crystal orientation variation and the better the crystallinity.

  Using the above-mentioned fully automatic horizontal multi-purpose X-ray analyzer “SmartLab 3XG” manufactured by Rigaku Corporation, X-ray incidence and diffraction detection are performed within the surface of the sample, so that the sample surface is very shallow. X-rays were incident at an angle. At this time, the penetration depth of X-rays into the sample is about several μm. In the twist measurement, an incident angle of 0.25 °, an incident slit width of 0.1 mm, and a length limiting slit width of 5 mm are used as conditions of the measurement optical system, and 2θχ = 57.91 ° (gallium nitride (11-20) plane) The value was calculated from the lattice spacing using the Bragg equation) and the measurement was performed by φ scan. As a result, an intensity peak was observed at a 6-fold symmetry position (in-plane rotation at intervals of 60 °).

In the twist measurement which is an in-plane measurement, since the φ axis is a relative value, the peak maximum point position is shown as φ = 0 °. As shown in FIG. 6B, the full width at half maximum (twist width) of the peak intensity was 0.42 °. Moreover, the intensity value of the gallium nitride (11-20) plane in the twist measurement was 7 × 10 ⁴ cps.

(Example 2)
A gallium nitride single crystal was grown on the sapphire substrate in the same manner as in Example 1 except that the temperature profile was as follows.

  Specifically, first, the internal temperature of the reactor was manually increased to 200 ° C., and then the internal temperature was increased to 780 ° C. at a rate of 100 ° C. per hour. Next, the internal temperature of the reactor was held at 780 ° C. for 40 hours. Then, it cooled naturally until the inside of reaction container returned to room temperature by natural heat radiation.

  In addition, when the cross section of the semiconductor substrate which concerns on Example 2 was confirmed by SEM or TEM etc., the intermediate | middle layer and semiconductor layer which were comprised on the sapphire substrate were laminated | stacked on the sapphire substrate like Example 1. It was confirmed (not shown).

Tilt measurement and twist measurement were performed on the sapphire substrate on which the intermediate layer and the semiconductor layer (gallium nitride crystal film) according to Example 2 were stacked in the same manner as in Example 1. As a result, the tilt width was 0.33 ° and the twist width was 0.66 °. Further, the intensity value of the gallium nitride (11-20) plane in the twist measurement was 2 × 10 ⁴ cps.

(Example 3)
A gallium nitride single crystal was grown on the sapphire substrate in the same manner as in Example 1 except that a sapphire substrate having a plane (11-20) of the surface on which the semiconductor layer was formed was used.

  According to the X-ray analysis, in the sapphire substrate on which the intermediate layer and the semiconductor layer (gallium nitride crystal film) according to Example 3 were formed, a (0001) plane gallium nitride single crystal was grown in parallel with the surface of the sapphire substrate. It was confirmed that

  When the cross section of the semiconductor substrate according to Example 3 was confirmed by SEM, TEM, or the like, as in Example 1, an intermediate layer and a semiconductor layer made of gallium nitride crystals were stacked on the sapphire substrate. It was confirmed (not shown).

Tilt measurement and twist measurement were performed on the sapphire substrate on which the intermediate layer and the semiconductor layer (gallium nitride crystal film) according to Example 3 were formed in the same manner as in Example 1. As a result, the tilt width was 0.15 °, and the twist width was 0.54 °. Further, the strength value of the gallium nitride (11-20) plane in the twist measurement was 5 × 10 ⁴ cps.

(Comparative Example 1)
A gallium nitride single crystal was grown on the sapphire substrate in the same manner as in Example 1 except that the temperature profile was as follows.

  Specifically, first, the internal temperature of the reactor was manually increased to 200 ° C., and then the internal temperature was increased to 620 ° C. at a rate of 100 ° C. per hour. Next, after maintaining the internal temperature of the reactor at 620 ° C. for 10 hours, the temperature was raised to 680 ° C. at a rate of 0.5 ° C. per hour and held at 680 ° C. for 3 hours. Then, it cooled naturally until the inside of reaction container returned to room temperature by natural heat radiation.

  In addition, when the cross section of the semiconductor substrate which concerns on the comparative example 1 was confirmed by SEM, the crystal grain of the gallium nitride was discretely formed on the sapphire substrate in the island form of about several micrometers-several dozen micrometer ( Not shown).

  Moreover, the result of having confirmed the cross section of the semiconductor substrate which concerns on the comparative example 1 with TEM is shown to FIG. 5B. FIG. 5B is a TEM bright-field image obtained by observing a cross section near the interface between the sapphire substrate, the intermediate layer, and the semiconductor layer of the semiconductor substrate according to Comparative Example 1.

  In FIG. 5B, the slightly brighter region on the right side of the figure is the sapphire substrate 20, and the slightly darker region on the left side is the gallium nitride crystal (semiconductor layer 10). As shown in FIG. 5B, in the cross section of the semiconductor substrate according to Comparative Example 1, the gallium nitride crystal was polycrystalline and the orientation of crystal grains was random. It was confirmed that the intermediate layer 11 was not clearly confirmed or extremely thin, and only the semiconductor layer 10 was formed on the sapphire substrate 20.

Tilt measurement and twist measurement were performed on the sapphire substrate on which the semiconductor layer (gallium nitride single crystal film) according to Comparative Example 1 was formed in the same manner as in Example 1. As a result, the tilt width was 0.40 ° and the twist width was 1.18 °. Moreover, the intensity value of the gallium nitride (11-20) plane in the twist measurement was 5 × 10 ² cps.

(Comparative Example 2)
A gallium nitride single crystal was grown on the sapphire substrate in the same manner as in Example 1 except that the temperature profile was as follows.

  Specifically, first, the internal temperature of the reactor was manually increased to 200 ° C., and then the internal temperature was increased to 700 ° C. at a rate of 100 ° C. per hour. Next, after maintaining the internal temperature of the reaction apparatus at 700 ° C. for 40 hours, the temperature was increased to 900 ° C. at a rate of 100 ° C. per hour and held at 900 ° C. for 3 hours. Then, it cooled naturally until the inside of reaction container returned to room temperature by natural heat radiation.

  In addition, when the cross section of the semiconductor substrate which concerns on the comparative example 2 was confirmed by SEM or TEM etc., similarly to the comparative example 1, on the sapphire substrate, it is discretely made into the island shape of about several micrometers-several dozen micrometer. Gallium nitride crystal grains were formed. Therefore, in the semiconductor substrate according to Comparative Example 2, a polycrystalline gallium nitride crystal having a random crystal grain orientation is formed, the intermediate layer is not clearly confirmed, and only the semiconductor layer is formed. Confirmed (not shown).

Tilt measurement and twist measurement were performed on the sapphire substrate on which the semiconductor layer (gallium nitride crystal film) according to Comparative Example 2 was formed in the same manner as in Example 1. As a result, the tilt width was 0.51 ° and the twist width was 0.59 °. Moreover, the intensity value of the gallium nitride (11-20) plane in twist measurement was 1 × 10 ³ cps.

(Comparative Example 3)
A gallium nitride single crystal was grown on the sapphire substrate in the same manner as in Example 1 except that a sapphire substrate having an r-plane (1-102) plane orientation for forming the semiconductor layer was used.

  In addition, when the cross section of the semiconductor substrate which concerns on the comparative example 3 was confirmed by SEM or TEM etc., similarly to the comparative example 1, on the sapphire substrate, it is discretely made into the island shape of about several micrometers-dozens of micrometers. Gallium nitride crystal grains were formed. Therefore, in the semiconductor substrate according to Comparative Example 3, a polycrystalline gallium nitride crystal having a random crystal grain orientation is formed, the intermediate layer is not clearly confirmed, and only the semiconductor layer is formed. Confirmed (not shown).

  According to the X-ray analysis, the sapphire substrate on which the semiconductor layer (gallium nitride crystal film) according to Comparative Example 3 is formed has a low X-ray diffraction intensity, but a single (0001) gallium nitride plane parallel to the surface of the sapphire substrate. It was confirmed that crystals were growing.

Tilt measurement and twist measurement were performed on the sapphire substrate on which the semiconductor layer (gallium nitride crystal film) according to Comparative Example 3 was formed in the same manner as in Example 1. As a result, the tilt width was 0.69 ° and the twist width was 2 °. Moreover, the intensity value of the gallium nitride (11-20) plane in the twist measurement was 3 × 10 ² cps.

  As a result of the tilt width and the twist width measured in Examples 1 to 3 and Comparative Examples 1 to 3, the intensity values of the gallium nitride (11-20) plane in the twist measurement are summarized in Table 1 and FIG. Show. FIG. 7 is a scatter diagram showing the correlation between the tilt width and the twist width.

  The tilt width is an index indicating the crystallinity in the stacking direction of crystals (the degree of alignment of crystal orientations in the stacking direction), and the smaller the value, the better the crystallinity. If the tilt width is approximately 0.4 ° or less, it is determined that the tilt width is good. The twist width is an index indicating the crystallinity in the in-plane direction of the crystal (the degree of alignment of crystal orientations in the in-plane direction). The smaller the value, the better the crystallinity. The twist width is determined to be good if it is approximately 0.7 ° or less.

  Referring to the results in Table 1, since Examples 1 to 3 each have a small tilt width and twist width, single crystals having good orientation and crystallinity are formed in both the stacking direction and the in-plane direction. I understand.

  In Examples 1 to 3 and Comparative Examples 1 to 3, since the conditions of the measurement optical system such as the incident angle, the incident slit width, and the length limiting slit width are unified and the X-ray irradiation region is matched, the value of the peak intensity Can be compared relative to each other. Therefore, it is presumed that the volume (average thickness) oriented in the (11-20) plane of the gallium nitride crystal film increases as the relative peak intensity value of gallium nitride (11-20) in the twist measurement increases.

According to this, Examples 1-3 show favorable crystallinity since the tilt width is in the range of 0.13 ° to 0.33 ° and the twist width is in the range of 0.42 ° to 0.66 °. I understand. In Examples 1 to 3, since the relative peak intensity in twist measurement is 10 ⁴ cps or more, it is presumed that a sufficient volume (thickness) gallium nitride crystal film can be formed.

On the other hand, in Comparative Examples 1 to 3, since the tilt width is in the range of 0.40 ° to 0.69 ° and the twist width is in the range of 0.59 ° to 2 °, the tilt and twist have large variations in crystal orientation. I understand. In Comparative Examples 1 to 3, the peak intensity relative value in twist measurement is 10 ³ cps or less, so that it is presumed that a sufficient volume (thickness) gallium nitride crystal film cannot be formed.

  Further, when Examples 1 and 3 and Comparative Example 3 are compared, the surface orientation of the sapphire substrate on which the gallium nitride crystal is formed is different, so that even when the same temperature profile condition is used, the formed gallium nitride crystal It can be seen that there is a difference in the membrane.

  Specifically, the sapphire substrate regardless of whether the plane orientation of the sapphire substrate forming the gallium nitride crystal is a c-plane (0001), a-plane (11-20), or r-plane (1-102). The (0001) plane gallium nitride was formed in parallel with the surface.

  However, as in Examples 1 and 3, when the plane orientation of the sapphire substrate is the c-plane (0001) and the a-plane (11-20), both the tilt width and the twist width are small, and the peak intensity relative value in the twist measurement It can be seen that also becomes larger. Thus, in Examples 1 and 3, single crystals having good orientation and crystallinity are formed in both the stacking direction and the in-plane direction, and a sufficient volume (thickness) gallium nitride single crystal film can be formed. It is inferred that

  On the other hand, as in Comparative Example 3, when the surface orientation of the sapphire substrate is the r-plane (1-102), it can be seen that both the tilt width and the twist width are large and the peak intensity relative value in the twist measurement is small. Thus, in Comparative Example 3, it is presumed that the crystal orientation of gallium nitride having a large volume (thickness) and a sufficient volume (thickness) variation cannot be formed in both tilt and twist.

(Evaluation of in-plane distribution of crystallinity)
Subsequently, in Example 1 and Comparative Example 2, the in-plane distribution of crystallinity of the gallium nitride single crystal film (semiconductor layer) formed on the sapphire substrate was evaluated.

  Specifically, in X-ray diffraction, measurement was performed in an out-of-plane arrangement in which X-ray incidence and diffraction are detected in a plane perpendicular to the surface of the sample. However, a sample horizontal multi-purpose X-ray diffractometer Ultima IV-MAJ (manufactured by Rigaku) was used, and a small portion condensing optical system CBO-f optical element and a small portion sample stage attachment were used. Further, a Cu tube was used, irradiated with X-rays under conditions of a voltage of 40 kV and a current of 40 mA, and the divergence longitudinal slit was 2 mm. The X-ray irradiation region on the sample surface was an elliptical region having a major axis of 2.0 × minor axis of 1.5 mm.

  Under the above-described apparatus conditions, measurement was performed in an out-of-plane arrangement in which X-ray incidence and diffraction are detected in a plane perpendicular to the surface of the sample. Such measurement is also referred to as symmetrical reflection measurement because the incident angle and the outgoing angle of X-rays with respect to the surface of the sample are equal. Note that the difference from the tilt measurement described above is that the diffraction angle (2θ) and the sample axis (θ) are interlocked with each other by using a minute portion condensing optical system. Accordingly, in 2θ / θ scanning measurement, 2θ is scanned in a 0.02 ° step within a range of 30.0 ° to 44.0 °, and measured with a counting time of 0.5 seconds. 2θ = 34.6 ° The intensity of the diffraction peak of gallium nitride (0002) appearing in the vicinity was determined.

  The above measurement was performed at a plurality of points on a 2-inch sapphire substrate having a crystal plane (0001) as a main surface as shown in FIG. The sapphire substrate was arranged so that the directions of X-ray incidence and emission were included in the direction parallel to the orientation flat surface (OF surface (11-20)).

  Measurement points were set as indicated by the positions of the black circles shown in FIG. 8, and the same measurement was performed at a total of 13 points in a vertical and horizontal grid pattern every 10 mm. Furthermore, the peak intensity of the (0002) plane of gallium nitride was determined at each measurement point, and the average value and standard deviation of the peak intensity of the (0002) plane of gallium nitride at 13 measurement points were determined. As a measurement result of the in-plane intensity distribution with respect to Example 1 and Comparative Example 2, the average value of the peak intensity on the (0002) plane of gallium nitride and the standard deviation / average value (an index indicating in-plane intensity variation) Are shown in Table 2 below.

  Referring to the results in Table 2, it can be seen that in Example 1, the standard deviation / average value of the peak intensity of the (0002) plane of gallium nitride is as low as 29%, and the in-plane intensity variation is small. On the other hand, in Comparative Example 2, the standard deviation / average value of the peak intensity of the (0002) plane of gallium nitride is as high as 73%, and it can be seen that the in-plane intensity variation is large.

(Evaluation of threading dislocation density)
Next, in Example 1, since a single crystal film of gallium nitride having a flat region having a sufficient thickness and a sufficient area was obtained, the threading dislocation density was evaluated using an etch pit method. In Comparative Examples 1 to 3, the single crystal film of gallium nitride has an island shape of about several μm to several tens of μm, and a flat region having a sufficient area cannot be secured. Therefore, the threading dislocation density can be evaluated. could not.

  Specifically, first, a sample of about 10 mm square was cut out from the semiconductor substrate according to Example 1 after the gallium nitride single crystal film was formed. Next, an etching solution in which 86% by mass of potassium hydroxide solution and 90% by mass of sodium hydroxide solution are mixed at 1: 1 (mol ratio) is prepared, put into a Zr crucible, and the Zr crucible together. Heated to 500 ° C. in a box oven.

  Subsequently, a sample around which a φ0.2 mm Inconel wire was wound was suspended and immersed in an etching solution heated to 500 ° C. for 1 minute. Thereafter, the sample was taken out from the etching solution and allowed to stand at room temperature for 3 minutes. Next, after immersing the sample in diluted hydrochloric acid obtained by diluting 38% by mass of hydrochloric acid with pure water at a mass ratio of 1: 1, the sample was washed with pure water for 1 minute and dried.

  The surface of the gallium nitride single crystal of the sample after drying was observed with AFM (Nano Scope IIIa manufactured by Digital Instruments). The result is shown in FIG. FIG. 9 is an AFM image in the observation area 5 μm × 5 μm of the sample according to Example 1.

As shown in FIG. 9, the black shadow portion of the AFM image is a hole portion eroded by the hot alkali of the etching solution, and corresponds to a place where threading dislocations exist. By measuring the number of holes in the observation field, the density of threading dislocations per unit area of the gallium nitride single crystal film can be calculated. In the sample according to Example 1, the threading dislocation density was estimated to be 3 × 10 ⁸ pieces / cm ² . This level of threading dislocation density is a quality level equivalent to that of a single crystal film of gallium nitride formed by general vapor phase growth.

(Evaluation of crystal orientation distribution)
Subsequently, in Example 1, the distribution of crystal orientation was evaluated using the electron beam backscatter diffraction (EBSD) method of SEM. In Comparative Examples 1 to 3, the single crystal film of gallium nitride has an island shape of about several μm to several tens of μm, and a flat region having a sufficient area cannot be secured. I could not.

  Specifically, by observing the sample according to Example 1 with an SEM (JEOL JSM-7100F) and analyzing it with an electron backscatter diffraction (EBSD) method, the crystal orientation distribution on the sample surface and the crystal Grain information was obtained.

  FIG. 10 shows an SEM image of the sample according to Example 1. The acceleration voltage during SEM observation was 15 kV, and the sample was tilted at an angle of 70 °. Since the surface of the sample was not polished and was observed as it was after the gallium nitride single crystal was formed, irregularities or depressions were observed on the sample surface.

  FIG. 11 shows an inverse pole figure (inverse pole figure map: IPF map) of the analysis result of the sample according to Example 1 as viewed from the Z-axis direction. In the IPF map, the direction of crystal orientation is represented by color, and in FIG. 11, it can be seen that almost the entire surface is the same color (001: c-axis) and the crystals are oriented in the same orientation. In FIG. 11, the darker part than the part is represented in black on the color image, and is a part where the crystal phase could not be identified. Since this portion is also black in the IPF map viewed from other directions, it is presumed that data has been lost due to the effect of surface irregularities.

  In the IPF map shown in FIG. 11, in the sample according to Example 1, regions oriented in different crystal orientations or crystal grain boundaries were not observed. Therefore, from a microscopic viewpoint, it was confirmed that the single crystal film (semiconductor layer) of gallium nitride included in the semiconductor substrate according to Example 1 was a crystal thin film uniformly oriented in the c-axis direction.

(Evaluation of iron atom concentration distribution)
Next, the concentration distribution of iron atoms in the gallium nitride crystal obtained in Example 1 was measured using SIMS (Secondary Ion Mass Spectrometry). Specifically, the concentration distribution of each atom in the thickness direction of the gallium nitride crystal obtained in Example 1 was measured using SIMS. The iron atoms (Fe) were measured at three different points in the in-plane direction of the gallium nitride crystal.

The SIMS measurement result of the gallium nitride crystal obtained in Example 1 is shown in FIG. Note that the horizontal axis of FIG. 12 indicates the depth from the surface of the gallium nitride crystal. As shown in FIG. 12, in the gallium nitride crystal obtained in Example 1, the concentration distribution of iron atoms is 1.0 × 10 ¹⁶ atoms / cm ³ or more and 1.0 × 10 ²² atoms / cm ³ or less. You can see that it is within the range.

  In the gallium nitride crystal obtained in Example 1, the concentration distribution of iron atoms is such that the concentration of the interface region with the sapphire substrate and the concentration of the surface region of the gallium nitride crystal are between the interface region and the surface region. It can be seen that it has a bathtub shape higher than the density of the intermediate region. Specifically, the maximum value of the concentration of iron atoms in the interface region with the sapphire substrate is 10 times or more the minimum value of the concentration of iron atoms in the intermediate region, and iron in the surface region of the gallium nitride crystal. It can be seen that the maximum value of the concentration of atoms is 10 times or more the minimum value of the concentration of iron atoms in the intermediate region.

  Furthermore, in the gallium nitride crystal obtained in Example 1, it can be seen that the concentration of iron atoms in the intermediate region is substantially the same in the thickness direction of the gallium nitride crystal. Specifically, in the thickness direction of the gallium nitride crystal, the concentration difference of iron atoms in any two intermediate regions is 50% or less of the concentration of any one of the two iron atoms. I understand that. Therefore, according to Example 1, it turns out that the gallium nitride crystal by which the iron atom was uniformly doped by the intermediate region was manufactured.

  In the gallium nitride crystal obtained in Example 1, the concentration distribution of iron atoms measured at three different points in the in-plane direction of the gallium nitride crystal is substantially the same. Specifically, in the in-plane direction of the gallium nitride crystal, the concentration difference of iron atoms in any two intermediate regions is 50% or less of the concentration of any one of the two iron atoms. I understand that. Therefore, according to Example 1, it can be seen that a gallium nitride crystal in which the concentration of iron atoms is extremely uniform in the in-plane direction is manufactured.

  On the other hand, FIG. 13 shows the SIMS measurement results of a gallium nitride crystal (commercially available from Ostend) formed on a sapphire substrate by vapor phase growth. Note that the horizontal axis of FIG. 13 indicates the depth from the surface of the gallium nitride crystal.

As shown in FIG. 13, in the gallium nitride crystal formed by vapor phase growth, the measured value of the concentration of iron atoms shows the vicinity of 5 × 10 ¹⁴ atoms / cm ³ , which is the detection limit. It turns out that it is not doped. Therefore, when doping iron atoms into a gallium nitride crystal formed by vapor phase growth, the thermal diffusion method or ion implantation method is used as described above, so the gallium nitride crystal after doping with iron atoms is crystallized. It turns out that it produces a defect.

  As described above, according to the present embodiment, the gallium nitride formed on the sapphire substrate by using the liquid phase method has better crystallinity, sufficient thickness, and in-plane crystallinity. A single crystal film with good distribution can be obtained.

  The preferred embodiments of the present invention have been described in detail above with reference to the accompanying drawings, but the present invention is not limited to such examples. It is obvious that a person having ordinary knowledge in the technical field to which the present invention belongs can come up with various changes or modifications within the scope of the technical idea described in the claims. Of course, it is understood that these also belong to the technical scope of the present invention.

DESCRIPTION OF SYMBOLS 1 Semiconductor substrate 10 Semiconductor layer 11 Intermediate layer 20 Sapphire substrate 100 Reactor 110 Raw material melt 111 Reaction vessel 112 Mount 113 Electric furnace 114 Heater 120 Holder 122 Lifting shaft 123 Sealing material 124, 125 Beam part 126, 127 Post part 128 Shelf Plate 131 Gas inlet 132 Gas outlet 140 Substrate

Claims (16)

A sapphire substrate,
The crystal direction is composed of random gallium nitride, an intermediate layer provided on the sapphire substrate,
A semiconductor layer made of gallium nitride single crystal and provided on the intermediate layer at least one layer;
A semiconductor substrate comprising:   The semiconductor substrate according to claim 1, wherein a tilt width of gallium nitride constituting the intermediate layer and the semiconductor layer is 0.05 ° or more and 0.4 ° or less.   2. The semiconductor substrate according to claim 1, wherein a twist width of gallium nitride constituting the intermediate layer and the semiconductor layer is not less than 0.1 ° and not more than 0.7 °.   The semiconductor substrate according to any one of claims 1 to 3, wherein a surface orientation of the surface of the sapphire substrate on which the intermediate layer is provided is a c-plane or an a-plane. The gallium nitride constituting the intermediate layer and the semiconductor layer contains iron atoms,
The concentration distribution of the iron atoms in the thickness direction of the gallium nitride are included in the scope of 1.0 × ¹⁰ 16 Atoms / cm ³ or more 1.0 × ¹⁰ 22 Atoms / cm ³ or less, of claims 1 to 4 The semiconductor substrate as described in any one.   The concentration distribution of the iron atoms in the thickness direction of the gallium nitride is higher in the surface region of the gallium nitride and the interface region with the sapphire substrate, and lower in the intermediate region between the surface region and the interface region. The semiconductor substrate according to claim 5, having a shape distribution.   The semiconductor substrate according to claim 6, wherein a maximum value of the iron atom concentration in the interface region is 10 times or more a minimum value of the iron atom concentration in the intermediate region.   8. The semiconductor substrate according to claim 6, wherein a maximum value of the iron atom concentration in the surface region is 10 times or more a minimum value of the iron atom concentration in the intermediate region.   The difference in concentration of the iron atoms in the intermediate region at any two points in the thickness direction of the gallium nitride is 50% or less of the concentration of the iron atoms in any one of the two points. The semiconductor substrate as described in any one of 6-8.   The in-plane direction of the gallium nitride has a difference in concentration of the iron atoms in the intermediate region at any two points that is 50% or less of the concentration of the iron atoms in any one of the two points. The semiconductor substrate as described in any one of 6-9. The interface region is a region within 2 μm in the thickness direction of the gallium nitride from the interface with the sapphire substrate,
11. The semiconductor substrate according to claim 6, wherein the surface region is a region within 1 μm from a surface of the gallium nitride in a thickness direction of the gallium nitride.   A gallium nitride single crystal comprising at least one layer on a sapphire substrate via an intermediate layer made of gallium nitride having a random crystal direction and composed of a single crystal.   The gallium nitride single crystal according to claim 12, wherein a tilt width of the gallium nitride constituting the intermediate layer and the gallium nitride single crystal is 0.05 ° or more and 0.4 ° or less.   The gallium nitride single crystal according to claim 12, wherein a twist width of the gallium nitride constituting the intermediate layer and the gallium nitride single crystal is not less than 0.1 ° and not more than 0.7 °.   The gallium nitride single crystal according to any one of claims 12 to 14, wherein the surface orientation of the sapphire substrate on which the intermediate layer is provided is a c-plane or an a-plane. It is a manufacturing method of the gallium nitride single crystal with which the semiconductor substrate as described in any one of Claims 1-11 is equipped, or the gallium nitride single crystal as described in any one of Claims 12-15,
Heating the metal gallium and iron nitride to a reaction temperature at which the iron nitride and the metal gallium react at least in a nitrogen atmosphere;
Holding the metal gallium and the iron nitride to the reaction temperature and then holding the metal gallium and the iron nitride at a temperature within the reaction temperature range for 20 hours or more;
Including
In the heated metal gallium and the iron nitride, the sapphire substrate is provided,
The iron nitride includes one or more of tetrairon mononitride, triiron mononitride, or diiron mononitride,
The method for producing a gallium nitride single crystal, wherein the reaction temperature is higher than 700 ° C. and lower than 1000 ° C.