JP2008162886A - Method for manufacturing substrate - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for manufacturing a substrate, by which the crystallinity of a crystal layer of a group III nitride semiconductor can be improved. <P>SOLUTION: The method for manufacturing the substrate includes a chromium layer depositing process for depositing a chromium layer 20 on a ground substrate 10 and a nitriding process for nitriding the chromium layer 20 at a temperature of ≥1,000°C to convert it into a chromium nitride film 30. In the nitriding process, an intermediate layer is formed between the ground substrate 10 and the chromium nitride film 30. Further, the method includes a crystal layer growth process for growing a crystal layer of a group III nitride semiconductor on the chromium nitride film 30. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a method for manufacturing a substrate.

Conventionally, as shown in Non-Patent Document 1, there is a technique of forming a chromium layer by MBE and nitriding the chromium layer with an N ₂ plasma source. In the technique of Non-Patent Document 1, a chromium nitride film is formed by nitriding a chromium layer at a low temperature. A gallium nitride crystal layer is obtained by epitaxially growing gallium nitride on the chromium nitride film.
Li Asahi and 7 others, “Growth of GaN using low temperature CrxN buffer layer by MBE method”, Proceedings of the Japan Society of Applied Physics, page 364

  An electronic device such as an LED (Light Emitting Diode) may be formed on a semiconductor substrate including a crystalline layer of gallium nitride. Here, in order to improve the characteristics of the electronic device, it is necessary to improve the crystallinity of the crystal layer of gallium nitride. In order to improve the crystallinity of the gallium nitride crystal layer, the gallium nitride crystal layer is not formed directly on the base substrate, but a low temperature buffer layer is formed on the base substrate. It is common to form a crystalline layer of gallium nitride on top. The low-temperature buffer layer is a layer obtained by growing gallium nitride at a temperature lower than the temperature for forming the gallium nitride crystal layer.

  In general, the base substrate includes a sapphire crystal. In this case, the lattice mismatch is large between the base substrate (sapphire) and the low-temperature buffer layer (gallium nitride), and the difference in thermal expansion coefficient is large. As a result, dislocations and internal stress are generated in the low-temperature buffer layer grown on the base substrate, so that the crystallinity of the gallium nitride crystal layer grown thereon may not be improved. A technique for mitigating lattice mismatch and thermal expansion coefficient difference between the base substrate (sapphire) and the low-temperature buffer layer (gallium nitride) is desired.

  On the other hand, the present inventor has proposed a technique of forming a chromium layer on a base substrate and nitriding the chromium layer to form a chromium nitride film. The lattice spacing of the chromium nitride film has a value between the lattice spacing of the base substrate (sapphire) and the lattice spacing of the low-temperature buffer layer (gallium nitride). The thermal expansion coefficient of the chromium nitride film has a value between the thermal expansion coefficient of the base substrate (sapphire) and the thermal expansion coefficient of the low-temperature buffer layer (gallium nitride).

  Here, if the crystallinity of the chromium nitride film is good, a lattice mismatch between the base substrate (sapphire) and the low temperature buffer layer (gallium nitride) is formed by forming a low temperature buffer layer (gallium nitride) on the chromium nitride film. And the difference in thermal expansion coefficient can be reduced. However, if the crystallinity of the chromium nitride film is not good, the crystallinity of the low-temperature buffer layer (gallium nitride) grown on the chromium nitride film decreases, and the crystal of the crystal layer of gallium nitride grown on the low-temperature buffer layer further May also decrease.

  Non-Patent Document 1 merely discloses that the chromium layer is nitrided at a low temperature, and there is a disclosure as to what temperature the nitride layer can be nitrided to improve the crystallinity of the chromium nitride film. Absent. A method for improving the crystallinity of the chromium nitride film is desired.

  An object of the present invention is to provide a substrate manufacturing method capable of improving the crystallinity of a chromium nitride film.

  In addition, as a group III nitride semiconductor, Ga and In type semiconductors can be cited. Group III nitride semiconductors are, for example, GaN-based, AlGaN-based, and AlInGaN-based, but are not limited thereto.

  The substrate manufacturing method according to the first aspect of the present invention includes a chromium layer forming step of forming a chromium layer on a base substrate, and a nitriding step of nitriding the chromium layer into a chromium nitride film. In the nitriding step, an intermediate layer is formed between the base substrate and the chromium nitride film.

  In addition to the features of the substrate manufacturing method according to the first aspect of the present invention, the substrate manufacturing method according to the second aspect of the present invention grows a group III nitride semiconductor crystal layer on the chromium nitride film. And a crystal layer growing step.

  The substrate manufacturing method according to the third aspect of the present invention is characterized in that, in addition to the features of the substrate manufacturing method according to the first or second aspect of the present invention, the upper surface of the base substrate has hexagonal and pseudo-hexagonal crystals. It is characterized by being any (0001) plane of the system or cubic (111) plane.

  The substrate manufacturing method according to the fourth aspect of the present invention includes a reducing gas atmosphere containing nitrogen in addition to the characteristics of the substrate manufacturing method according to any one of the first to third aspects of the present invention. The chrome layer is nitrided.

  In the substrate manufacturing method according to the fifth aspect of the present invention, in addition to the features of the substrate manufacturing method according to the fourth aspect of the present invention, the reducing gas containing nitrogen includes at least one of ammonia and hydrazine. It is characterized by that.

The substrate manufacturing method according to the sixth aspect of the present invention is characterized in that, in addition to the characteristics of the substrate manufacturing method according to any one of the first to fifth aspects of the present invention, in the nitriding step, the chromium nitride film is formed. A substrate manufacturing method according to a seventh aspect of the present invention is characterized in that a plurality of triangular pyramid-shaped microcrystalline portions are formed on the surface. In addition to the features of the substrate manufacturing method according to the sixth aspect of the present invention, In each of the microcrystalline portions of the chromium nitride film, each side of the bottom surface has a [10-10] direction, a [01-10] direction, and a [−1100] direction on the (0001) plane of the base substrate. It extends along either of these.

  In the substrate manufacturing method according to the eighth aspect of the present invention, in addition to the characteristics of the substrate manufacturing method according to the seventh aspect of the present invention, the base substrate includes sapphire, and the intermediate layer includes aluminum nitride. It is characterized by that.

  The substrate manufacturing method according to the ninth aspect of the present invention is characterized in that, in addition to the features of the substrate manufacturing method according to any one of the sixth to eighth aspects of the present invention, the triangular pyramid-shaped microscopic structure of the chromium nitride film. The crystal part is characterized in that one side has a length of 10 nm to 300 nm.

  The substrate manufacturing method according to the tenth aspect of the present invention is characterized in that, in addition to the features of the substrate manufacturing method according to any one of the second to ninth aspects of the present invention, the chromium nitride film is etched to form the III The method further includes a separation step of separating the crystal of the group nitride semiconductor from the base substrate.

The substrate manufacturing method according to the eleventh aspect of the present invention is characterized in that, in addition to the features of the substrate manufacturing method according to any one of the first to tenth aspects of the present invention, the upper surface of the group III nitride semiconductor crystal layer The pit density is 10 ⁵ / cm ² or less.

  According to the present invention, the crystallinity of the chromium nitride film can be improved.

  In this specification, the “film” may have, for example, a flat surface or a surface including unevenness. The film having a surface including irregularities may be, for example, a film in which laterally arranged triangular pyramid-shaped microcrystals are in contact with each other at least at one point.

  A method for manufacturing a substrate according to an embodiment of the present invention will be described with reference to FIGS. In the following, GaN as a group III nitride semiconductor will be described as an example of the crystal layer, but the same applies to other group III nitride semiconductors. As will be described later, considering that the crystal layer is used as a free-standing substrate and applied to a diode or the like, the group III nitride semiconductor used as the material of the crystal layer is preferably GaN.

  1 and 2 are process cross-sectional views illustrating a substrate manufacturing method according to an embodiment of the present invention. 3 and 4 are XRD (X-Ray Diffraction) charts. FIG. 6 is an SEM photograph of the sample surface. FIG. 10 is a photomicrograph of the sample surface. FIG. 8 is a schematic diagram of the surface morphology of the chromium nitride film. FIG. 9 is a diagram showing the crystal orientation of the microcrystalline portion of the chromium nitride film.

  In the step shown in FIG. 1A, a base substrate 10 is prepared. The base substrate 10 is formed of a single crystal of sapphire. The upper surface 10a of the base substrate 10 is a (0001) plane of sapphire single crystal. A single crystal of sapphire has a pseudo hexagonal crystal structure.

  Note that the base substrate may be formed of a material other than sapphire as long as it has a hexagonal, pseudo-hexagonal, or cubic crystal structure. When the base substrate is a cubic system, the (111) plane is used as the upper surface in the following description.

  In the step shown in FIG. 1B, a chromium layer 20 is formed on the upper surface 10 a of the base substrate 10. That is, the chromium layer 20 is formed on the (0001) plane of the sapphire crystal. Specifically, first, the base substrate 10 is cleaned by a normal semiconductor substrate cleaning method (degreasing by organic cleaning, removal of contaminants / particles by acid / alkali / pure water cleaning), and the cleanliness of the surface 10a. Secure. Next, a chromium metal layer 20 is formed on the surface 10a in which cleanliness is ensured by forming a metal Cr film by sputtering in an inert gas atmosphere, for example, an Ar gas atmosphere.

  Here, the average film thickness of the chromium layer 20 is preferably a value within a range of 7 nm to 45 nm, and more preferably a value of 10 nm to 40 nm. By making the average layer thickness of the Cr layer 7 nm or more and less than 45 nm, it becomes possible to grow a crystalline layer having good crystallinity, and further by reducing the pit density of the crystal layer by making it 10 nm or more and 40 nm or less. Is possible.

  The chromium layer 20 may be formed by chemical vapor deposition (CVD) using an alkyl compound containing metal or chloride, or may be formed by metal organic chemical vapor deposition (MOCVD). Alternatively, the film may be formed by vacuum (thermal) vapor deposition.

  When the XRD analysis of the sample obtained in this step is performed, for example, the result shown in FIG. 3 is obtained. In FIG. 3, the vertical axis indicates the peak intensity (arbitrary unit), and the horizontal axis indicates the diffraction angle 2θ. Thus, it can be seen that the (0001) plane of the base substrate 10 (sapphire) and the (110) plane of the chromium layer 20 are aligned in parallel. The chromium layer 20 is a metal having a body-centered cubic structure.

  In the step shown in FIG. 1C, the base substrate 10 on which the chromium layer 20 is formed is transferred to an apparatus for growing GaN crystals. Then, the base substrate 10 on which the chromium layer 20 is formed is heated and nitrided in a reducing gas atmosphere containing nitrogen. The reducing gas containing nitrogen preferably contains at least one of ammonia and hydrazine. At that time, the heating temperature is preferably 1000 ° C. or higher (1273 K or higher) and 1300 ° C. or lower, more preferably 1040 ° C. or higher and 1300 ° C. or lower, and further preferably 1060 ° C. or higher and 1300 ° C. or lower. By nitriding at a heating temperature of 1000 ° C. or more and 1300 ° C. or less, the chromium layer 20 is almost entirely nitrided to become the chromium nitride film 30, and the intermediate layer 60 (see FIG. 7) is replaced with the base substrate 10 and the chromium nitride film. 30.

  For example, when the base substrate 10 includes sapphire, Al atoms diffuse from the base substrate 10 and N atoms diffuse from the chromium nitride film 30 by nitriding at a heating temperature of 1000 ° C. to 1300 ° C. Thereby, the intermediate layer 60 containing aluminum nitride is formed between the base substrate 10 and the chromium nitride film 30. As will be described later, the intermediate layer 60 is considered to support rearrangement with respect to the base substrate 10 in a state where the crystal lattice of the chromium nitride film 30 is aligned in a specific direction.

  As a result, the chromium nitride film 30 has a plurality of triangular pyramid-shaped microcrystalline portions 31 on the surface.

  Here, the composition of the chromium nitride film 30 is preferably CrN. The thickness of the intermediate layer 60 is sufficient to assist the rearrangement with respect to the base substrate 10 in a state where the crystal lattice of the chromium nitride film 30 is aligned in a specific direction. .5 nm. Note that the thickness of the intermediate layer 60 can be controlled by changing the heating temperature at the time of performing the heat nitriding treatment or the time for performing the heat nitriding treatment.

The length of one side of the triangular pyramid-shaped microcrystalline portion 31 varies depending on the thickness of the Cr film before nitriding and the heating temperature condition, but is in the range of 10 nm to 300 nm. By nitriding at a heating temperature of 1040 ° C., the pit density on the surface 50a of a GaN crystal layer 50 to be grown later is reduced to a level of 10 ² to 10 ³ / cm ² . By nitriding at a heating temperature of 1060 ° C., the pit density on the surface 50a of the GaN crystal layer 50 described later is reduced to a few / cm ² level. This is probably because the higher the heating temperature, the more the irregular shape of the triangular pyramid is resolved.

  However, if the temperature is excessively high, there is a problem of deterioration of the members of the apparatus due to an increase in the thermal load. Therefore, the heating temperature is preferably 1300 ° C. or less.

  Here, the average film thickness of the chromium nitride film 30 is preferably a value in the range of 22.5 nm to 75 nm, and more preferably a value of 29 nm to 31 nm.

  When the XRD analysis of the sample obtained in this step is performed, for example, the result shown in FIG. 4 is obtained. FIG. 4 is an XRD chart for the sample shown in FIG. In FIG. 4, the vertical axis indicates the peak intensity (arbitrary unit), and the horizontal axis indicates the diffraction angle 2θ. In the XRD chart of FIG. 4, sapphire peaks and CrN peaks are observed, but no Cr peaks are observed. Thereby, it can be seen that the chromium layer 20 is almost entirely nitrided to form the chromium nitride film 30.

  Further, regarding CrN, only the peaks of the (111) plane and the (222) plane are observed, and the half-value width is narrow. Thereby, it can be seen that the chromium nitride film 30 is in a state in which the orientation of the (111) plane is aligned in parallel with the (0001) plane of the sapphire substrate.

  When the surface of the sample obtained in this step is observed by SEM, for example, the result shown in FIG. 6 is obtained. FIG. 6 is an SEM photograph of the sample surface.

  6 that the chromium nitride film 30 has a plurality of triangular pyramid-shaped microcrystalline portions 31 on the surface. It can also be seen that the microcrystalline portions 31 of the chromium nitride film 30 have a substantially uniform size and are distributed at substantially uniform intervals. Each microcrystalline portion 31 of the chromium nitride film 30 is distributed over substantially the entire surface 10 a of the base substrate 10.

  Here, in order to confirm that the intermediate layer 60 is formed between the base substrate 10 and the chromium nitride film 30, the cross section of the sample obtained by the process shown in FIG. . As a result, as shown in FIG. 7, by performing electron beam diffraction analysis of each layer having different contrasts in the cross-sectional TEM image of the sample, between the base substrate 10 (sapphire) and the chromium nitride film 30 (CrN). It was confirmed that the intermediate layer 60 (AlN) was formed. The thickness of the intermediate layer 60 was about 2.5 nm.

  Thereafter, the chromium nitride film 30 was selectively etched using a chemical solution to expose the intermediate layer 60. When a GIXRD (Granching Incident X-ray Diffraction) analysis of the sample obtained by this is performed, the result shown in FIG. 5 will be obtained, for example. FIG. 5 is a GIXRD chart for a sample in which the intermediate layer 60 is exposed after the intermediate layer 60 is formed between the base substrate 10 and the chromium nitride film 30. In FIG. 5, the vertical axis indicates the peak intensity (arbitrary unit), and the horizontal axis indicates the diffraction angle 2θ. Here, the apparatus used for the measurement is a spring 8. The wavelength of the X-ray at the time of measurement is 0.09987 nm. The incident angle of X-rays with respect to the sample surface is 0.5 °. The intensity of X-ray is 8 GeV. In the GIXRD chart of FIG. 5, not only sapphire peaks but also AlN peaks are observed. Thereby, it was confirmed that the intermediate layer 60 was formed between the base substrate 10 and the chromium nitride film 30. Further, the XRD chart of FIG. 5 has a peak of (11-20) plane of sapphire and a peak of (10-10) plane as m-plane (plane perpendicular to the crystal lattice plane) of aluminum nitride. Has been observed. Thus, it was confirmed that the (0001) plane of aluminum nitride was rotated in-plane by 30 ° with respect to the (0001) plane of sapphire. That is, the (0001) plane of aluminum nitride that is the intermediate layer 60 has a stable crystal lattice of chromium nitride in a direction in which the (111) plane of chromium nitride is rotated by 30 ° with respect to the (0001) plane of sapphire. It is thought that it assists to rearrange.

The above-mentioned chemical solution (etchant) is preferably a mixed aqueous solution of perchloric acid (HClO ₄ ) and ceric ammonium nitrate (Ce (NH ₄ ) ₂ (NO ₃ ) ₆ ), for example. It is not limited.

  As shown in FIG. 8, each microcrystalline portion 31 of the chromium nitride film 30 has bottom sides in the [10-10] direction, the [01-10] direction, and the [−1100] direction of the base substrate 10. It extends along either.

  Further, a cross section of the sample obtained in the step shown in FIG. As a result, it was found that the three side surfaces (facet surfaces other than the bottom surface) of each microcrystalline portion 31 were formed of {100} plane groups.

  Thus, each microcrystalline portion 31 of the chromium nitride film 30 is a single crystal and a microcrystalline (multi-twin) aggregate having two crystal orientations. As shown in FIG. 6 and FIG. 8, the direction of the base of the triangular pyramid has two kinds of crystal orientations (multi-twin) rotated in the plane of 180 °. Therefore, the grown group III nitride semiconductor crystal becomes a single crystal, so there is no problem. That is, as shown in FIG. 9, the lattice spacing of the microcrystalline portions 31 of the chromium nitride film 30 (the spacing of the black circles of the equilateral triangular portion shown in FIG. 9) is the lattice spacing of the base substrate 10 (sapphire) (FIG. 9). Different from the white circle interval shown in FIG. As a result, atoms (black circles shown in FIG. 9) constituting each microcrystalline portion 31 are stably present at positions between sapphire lattices (white circles shown in FIG. 9). Thereby, each microcrystal part 31 becomes a microcrystal (multi-twin) in which patterns of crystal lattices indicated by black circles are repeatedly arranged, and has a plurality of triangular pyramid-shaped microcrystal parts on the surface. Each microcrystalline portion 31 has a base extending along any one of the [10-10] direction, the [01-10] direction, and the [-1100] direction, and the side surface is a {100} plane group. Thereby, the crystal orientation deviation due to the in-plane rotation of the microcrystals becomes extremely small. The direction from the center of gravity of the bottom surface of each microcrystal portion 31 toward the upper end is perpendicular to the (0001) plane of the base substrate, that is, the orientation parallel to the C axis of the sapphire crystal.

  Here, the average film thickness of the chromium nitride film 30 is preferably a value within a range of 10 nm to 68 nm, and more preferably a value of 15 nm to 60 nm. Here, the average film thickness of the CrN film can be obtained by measuring unevenness with a cross-sectional TEM, and it was confirmed that it corresponds to 1.5 times the average layer thickness of the Cr layer before nitriding.

  When the average film thickness of the chromium nitride film 30 is less than 10 nm, that is, when the chromium layer thickness is less than 7 nm, the surface 10a of the base substrate 10 may be partially exposed. In the process, the GaN buffer layer starts to grow from both the base substrate 10 and the chromium nitride film 30. As a result, the crystal orientation differs between the GaN buffer layer grown from the base substrate 10 and the GaN buffer layer grown from the chromium nitride film 30, so that an improvement in crystal quality is expected in the process of FIG. There is a possibility that it may not be possible, or there is a possibility that pits will increase on the surface of GaN after crystal growth in the step of FIG. Further, when the average film thickness of the chromium nitride film 30 exceeds 68 nm, the chromium nitride film 30 does not progress uniformly on the underlying substrate 10 in the above-described heat nitriding process, and the chromium nitride film 30 does not proceed uniformly. 30 tends to be polycrystalline. As a result, GaN grown on the chromium nitride film 30 in the later-described step of FIG. 2A becomes mosaic or polycrystalline, and improvement in crystal quality cannot be expected in the later-described step of FIG. 2B. There is a fear.

  Note that the step shown in FIG. 1B and the step shown in FIG. 1C may be performed by the same apparatus or different apparatuses. It is preferable to perform without opening to the atmosphere between the step shown in FIG. 1B and the step shown in FIG.

  Next, in the step shown in FIG. 2A, the base substrate temperature is lowered to 900 ° C., and a buffer layer 40 of group III nitride (for example, GaN) is formed by HVPE. The film thickness of the buffer layer 40 is about 10 μm, for example.

  Here, the buffer layer 40 has triangular pyramid-shaped microcrystals (microcrystal portion 31) of the triangular pyramid-shaped chromium nitride film 30 as growth nuclei (as nucleation sites), from each of the {100} facet plane groups. Grows laterally. Thereby, it is possible to suppress the dislocation (threading dislocation) generated at the interface (growth interface) between the chromium nitride film 30 and the buffer layer 40 from propagating upward. The triangular pyramid shape is not limited to those having an acute angle or having a straight line on one side, but generally refers to a triangular pyramid shape. It also includes an object whose shape is artificially processed or made into a polyhedron during the growth process.

  In addition, since the crystal orientations of the microcrystals (microcrystal part 31) of the chromium nitride film 30 are aligned, in-plane rotation occurs when crystals grown from different directions are combined in the lateral growth of the group III nitride. Misalignment due to, and crystal axis misalignment (C axis misalignment) in the growth thickness direction can be reduced. As a result, the crystallographic orientations can be merged, so that the occurrence of group III nitride dislocations can be suppressed in the portion where the crystals grown from different directions merge.

  Further, the microcrystalline portions 31 that become nucleation sites have a substantially uniform size on the base substrate 10 and are distributed at uniform intervals. Thereby, since the buffer layer 40 grows in a uniform direction on the chromium nitride film 30, the occurrence of dislocation can be suppressed also from this point.

  In the step shown in FIG. 2B, the temperature of the base substrate is raised to 1040 ° C., and the GaN crystal layer 50 is grown. The film thickness of the crystal layer 50 during growth is, for example, about 10 μm. As a result, the structure 1 including the base substrate 10, the chromium nitride film 30, the buffer layer 40, and the crystal layer 50 is formed.

Since the crystal layer 50 is grown on the buffer layer 40 with reduced dislocations as described above, the dislocation density of the crystal layer 50 is reduced to 10 ⁷ to 10 ⁸ ; / cm ² . That is, the dislocation density is reduced by 1 to 2 digits compared to the so-called low temperature buffer layer technology.

  When the surface of the sample obtained in this step is observed with a microscope, for example, the result shown in FIG. 10 is obtained. FIG. 10 is a photomicrograph of the sample surface.

According to the micrograph of FIG. 10, it can be seen that the surface 50a of the crystal layer 50 has almost no pits. That is, the surface pit density is reduced to a level of 0 to 10 ² / cm ² . That is, when a method using a metal buffer layer (for example, a method using Al, Au, Ag, Ni, Ti, Cu disclosed in JP-A-2002-284600 is used, the surface pit density is 10 ⁴ to 10 ⁵ / cm. Compared with ² ), the surface pit density of the epitaxially grown film can be reduced by 3 to 4 digits or more. As a result, the yield is not reduced due to the pits. It is also estimated that the dislocation density in the crystal layer 50 can be reduced.

  In the step shown in FIG. 2C, the chromium nitride film 30 is selectively etched using a chemical solution. The GaN substrate SB can be separated from the base substrate 10. That is, the GaN substrate SB can be obtained as a free-standing substrate. Here, the substrate SB includes the buffer layer 40 and the crystal layer 50.

  As a secondary effect, a recess 41 corresponding to the microcrystalline portion 31 of the chromium nitride film 30 is formed on the back surface of the buffer layer 40. Since the recess 41 has an inverted triangular pyramid shape of several tens of nm to several hundreds of orders, the light extraction efficiency of the light emitting diode can be improved when used in a device. Moreover, since the internal quantum efficiency of the light emitting diode is improved by reducing the crystal defect density, an effect that the overall light emitting efficiency of the light emitting diode is greatly improved can be obtained.

  An excellent semiconductor element can be obtained by further laminating a group III-based nitride semiconductor layer on the buffer layer 40 to obtain an element structure.

Comparative example

  Next, a method for manufacturing a substrate according to a comparative example will be described with reference to FIGS. Below, it demonstrates centering on a different part from the manufacturing method of the board | substrate which concerns on embodiment of this invention, and abbreviate | omits description about the same part.

  In the step shown in FIG. 11A, the base substrate 10 on which the chromium layer 20 is formed is transferred to an apparatus for growing GaN crystals. Then, the base substrate 10 on which the chromium layer 20 is formed is heated and nitrided in a reducing gas atmosphere containing nitrogen. This reducing gas containing nitrogen contains at least one of ammonia and hydrazine. In that case, heating temperature shall be 900 degreeC. Thereby, the vicinity of the surface of the chromium layer 20 is nitrided, and the chromium nitride film 130 having the substantially flat surface 130a is formed.

  When the XRD analysis of the sample obtained in this step is performed, for example, the result shown in FIG. 12 is obtained. FIG. 12 is an XRD chart for the sample shown in FIG. In FIG. 12, the vertical axis indicates the peak intensity (arbitrary unit), and the horizontal axis indicates the diffraction angle 2θ. In the XRD chart of FIG. 12, not only sapphire peaks and CrN peaks but also Cr peaks are observed. Thereby, it can be seen that the chromium layer 20 is partially nitrided to form the chromium nitride film 130.

  For CrN, only the peaks on the (111) plane and the (222) plane are observed, and the full width at half maximum is widened. Thereby, it can be seen that the chromium nitride film 130 is in a state in which the orientation of the (111) plane varies with respect to the (0001) plane of the sapphire substrate.

  Here, the average film thickness of the chromium nitride film 130 is, for example, 5 nm. Further, since the temperature for performing the heat nitriding treatment is low, the intermediate layer 60 (see FIG. 7) is not formed between the base substrate 10 and the chromium nitride film 130. For example, when the base substrate 10 includes sapphire, since it is nitrided at a heating temperature of 900 ° C., Al atoms do not diffuse from the base substrate 10 and N atoms do not diffuse from the chromium nitride film 130. Thereby, the intermediate layer 60 containing aluminum nitride is not formed between the base substrate 10 and the chromium nitride film 130. Since the intermediate layer 60 is not formed between the base substrate 10 and the chromium nitride film 130, it is difficult to rearrange the base substrate 10 with the crystal lattice of the chromium nitride film 30 aligned in a specific direction. It is done. For this reason, the chromium nitride film 130 is in a state in which the orientation of the (111) plane varies with respect to the (0001) plane of the sapphire substrate.

  When the surface of the sample obtained in this step is observed with an SEM, for example, the result shown in FIG. 13 is obtained. FIG. 13 is an SEM photograph of the sample surface.

  According to the SEM photograph of FIG. 13, it can be seen that the chromium nitride film 130 has a flat surface 130a. That is, triangular pyramid-shaped microcrystals (microcrystal parts) are not formed on the surface 130 a of the chromium nitride film 130.

  Next, in the step shown in FIG. 11B, the GaN buffer layer 140 is formed by the HVPE method with the substrate temperature kept at 900.degree. The film thickness of the buffer layer 140 is about 10 μm, for example.

  Here, the buffer layer 140 is grown on the flat surface 130 a of the chromium nitride film 130. Thereby, dislocations generated at the interface (growth interface) between the chromium nitride film 130 and the buffer layer 140 are likely to propagate upward.

  Further, since the crystal orientation of the chromium nitride film 130 varies, when the crystals of the buffer layer 140 grown thereon are combined, the orientation misalignment due to in-plane rotation and the crystal axis misalignment in the growth thickness direction (C axis) Is likely to occur. As a result, coalescence may occur in a state where the crystal orientation varies, so dislocations are likely to occur in the portion where crystals grown from different directions merge.

  Even if atomic level flatness is ensured with respect to the surface 130a of the chromium nitride film 130, there is a lattice irregularity between GaN and CrN, so that high-density dislocations tend to occur at the growth interface. Further, since the growth does not involve lateral growth, there is a possibility that the dislocation density cannot be reduced.

  In the step shown in FIG. 11C, the substrate temperature is raised to 1040 ° C., and the GaN crystal layer 150 is grown. The film thickness of the crystal layer 150 during growth is, for example, about 10 μm.

  As described above, since the crystal layer 150 is grown on the buffer layer 140 where dislocation is likely to occur, the dislocation density of the crystal layer 150 tends to increase.

  When the surface of the sample obtained in this step is observed with a microscope, for example, the result shown in FIG. 14 is obtained. FIG. 14 is a photomicrograph of the sample surface.

  According to the micrograph in FIG. 14, it can be seen that many pits are generated on the surface 150 a of the crystal layer 150. As a result, the yield may be reduced due to the pits. It is also estimated that the dislocation density in the crystal layer 150 is high.

  As described above, the shape of the obtained chromium nitride film changes depending on the temperature (nitriding temperature) at which the chromium layer 20 is nitrided. As a result, the crystallinity of the buffer layer and the crystal layer grown thereon changes. Therefore, a sample was prepared by the same process as that shown in FIGS. Here, when the nitriding temperature (heating temperature in the step shown in FIG. 1C) is changed, the FWHM of the GaN peak is evaluated from the XRD analysis result of the crystal layer, and the SEM photograph or microscope of the crystal layer surface The pit density on the surface of the crystal layer was evaluated from the photograph. The result is shown in FIG. In FIG. 15, the white circle plot represents the change in peak half width, and the black square plot represents the surface pit density. The smaller the peak half-value width, the better the crystallinity.

  From the results shown in FIG. 15, in the heat treatment in a reducing gas atmosphere containing nitrogen, the heating temperature is preferably 1000 ° C. or higher, more preferably 1040 ° C. or higher, and more preferably 1060 ° C. or higher. Is more preferable.

  That is, by nitriding at a heating temperature of 1000 ° C. or more, the chromium layer 20 is almost entirely nitrided, and a triangular pyramid-shaped microcrystal (microcrystal part) is formed on the surface of the chromium nitride film. It can be seen that the pit density on the surface of the crystal layer is reduced by almost one digit as compared with the case where the heating temperature is 900 ° C.

For example, nitriding at a heating temperature of 1040 ° C. reduces the pit density on the surface 50a of the crystal layer 50 to a level of 10 ² to 10 ³ / cm ² . This shows that the pit density on the surface of the crystal layer is reduced as compared with the case of nitriding at a heating temperature of 1000 ° C.

For example, nitriding at a heating temperature of 1060 ° C. reduces the pit density on the surface 50a of the crystal layer 50 to a few / cm ² level. Thereby, it can be seen that the pit density on the surface of the crystal layer is reduced as compared with the case of nitriding at a heating temperature of 1040 ° C.

  As described above, it is considered that the higher the heating temperature is, the more the irregular shape of the triangular pyramid shape is eliminated, so that the pit density on the surface of the crystal layer is reduced. However, if the temperature is excessively high, there is a problem of deterioration of the members of the apparatus due to an increase in the thermal load. Therefore, the heating temperature is preferably 1300 ° C. or less.

Process sectional drawing which shows the manufacturing method of the board | substrate which concerns on embodiment of this invention. Process sectional drawing which shows the manufacturing method of the board | substrate which concerns on embodiment of this invention. XRD chart of the sample. XRD chart of the sample. Sample GIXRD chart. SEM photograph of the sample surface. The photograph which shows the cross-sectional TEM image of a sample. Schematic diagram of surface morphology. The figure which shows the crystal orientation of the microcrystal part of a chromium nitride film | membrane. A photomicrograph of the sample surface. Process sectional drawing which shows the manufacturing method of the board | substrate which concerns on the comparative example of this invention. XRD chart of the sample. SEM photograph of the sample surface. A photomicrograph of the sample surface. The figure which shows the relationship between nitriding temperature and crystallinity.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 Base substrate 20 Chromium layer 30,130 Chromium nitride film | membrane 40,140 Buffer layer 50,150 Crystal layer

Claims (11)

A chromium layer forming step of forming a chromium layer on the base substrate;
A nitriding step of nitriding the chromium layer to form a chromium nitride film;
With
In the nitriding step, an intermediate layer is formed between the base substrate and the chromium nitride film. 2. The method of manufacturing a substrate according to claim 1, further comprising a crystal layer growth step of growing a crystal layer of a group III nitride semiconductor on the chromium nitride film. 3. The substrate according to claim 1, wherein the upper surface of the base substrate is a hexagonal or pseudo-hexagonal (0001) plane, or a cubic (111) plane. 4. Production method. 4. The substrate manufacturing method according to claim 1, wherein, in the nitriding step, the chromium layer is nitrided in a reducing gas atmosphere containing nitrogen. 5. 5. The substrate manufacturing method according to claim 4, wherein the reducing gas containing nitrogen contains at least one of ammonia and hydrazine. 6. The substrate manufacturing method according to claim 1, wherein in the nitriding step, a plurality of triangular pyramid-shaped microcrystalline portions are formed on a surface of the chromium nitride film. Each of the microcrystalline portions of the chromium nitride film has any one of the [10-10] direction, the [01-10] direction, and the [−1100] direction with each side of the bottom surface on the (0001) plane of the base substrate. The method of manufacturing a substrate according to claim 6, wherein the substrate extends along the gap. The base substrate includes sapphire,
The method for manufacturing a substrate according to claim 7, wherein the intermediate layer includes aluminum nitride. 9. The method of manufacturing a substrate according to claim 6, wherein the triangular pyramid-shaped microcrystalline portion of the chromium nitride film has a side length of 10 nm or more and 300 nm or less. The substrate according to any one of claims 2 to 9, further comprising a separation step of etching the chromium nitride film to separate the group III nitride semiconductor crystal from the base substrate. Production method. 11. The method of manufacturing a substrate according to claim 1, wherein a pit density on an upper surface of the crystal layer of the group III nitride semiconductor is 10 ⁵ / cm ² or less.