Base substrate

Abstract

Provided is a base substrate comprising an alignment layer used for crystal growth for an α-Ga2O3-based or an α-Cr2O3 -based semiconductor film. The base substrate allows an improved yield (non-defective product rate) by reducing the hetero-phase formation probability of a semiconductor film to be formed on the base substrate. The base substrate comprises an alignment layer used for crystal growth for a semiconductor film, the alignment layer being composed of at least one selected from the group consisting of α-Ga2O3, α-Ga2O3-based solid solution, α-Cr2O3, and α-Cr2O3-based solid solution. A surface of the alignment layer is on the side used for crystal growth and is constituted of a material having a corundum crystal structure having a larger a-axis length and / or c-axis length than sapphire. The alignment layer has a composition stable region in which the composition is stable in the thickness direction and an inclined composition region in which the composition changes in the thickness direction. The composition stable region is thicker than the inclined composition region.

Description

Base substrate

[0001] The present disclosure relates to α-Ga ２ O ３ system or α-Cr ２ O ３ The present invention relates to a base substrate having an orientation layer used for crystal growth of a silicon-based semiconductor film.

[0002] In recent years, semiconductor devices using gallium nitride (GaN) have been put to practical use. For example, a mass-produced device has been developed in which an n-type GaN layer, a multi-quantum well (MQW) layer consisting of an n-type GaN layer, an InGaN quantum well layer, and a GaN barrier layer alternately stacked on a sapphire substrate, and a p-type GaN layer are stacked in this order.

[0003] In addition, α-gallium oxide (α-Ga) of the corundum phase type, which has the same crystal structure as sapphire, ２ O ３ ) is also being actively researched and developed. ２ O ３ Its band gap is as large as 5.3 eV, and it is attracting attention as a material for power semiconductor elements. For example, Patent Document 1 (JP 2014-72533 A) describes a semiconductor device formed from a base substrate having a corundum type crystal structure, a semiconductor layer having a corundum type crystal structure, and an insulating film having a corundum type crystal structure, in which an α-Ga ２ O ３ In addition, Patent Document 2 (JP 2016-25256 A) discloses an example in which a semiconductor device is provided with an n-type semiconductor layer containing as a main component a crystalline oxide semiconductor having a corundum structure, a p-type semiconductor layer containing as a main component an inorganic compound having a hexagonal crystal structure, and electrodes, and in the examples, an α-Ga GaN layer having a corundum structure, which is a metastable phase, is formed on a c-plane sapphire substrate. ２ O ３ The p-type semiconductor layer is an α-Rh layer having a hexagonal crystal structure. ２ O ３ Forming a film to create a diode is disclosed.

[0004] Incidentally, it is known that the fewer the crystal defects in these semiconductor devices, the better their characteristics will be. In particular, in power semiconductors, the dielectric breakdown field characteristics are affected by the amount of crystal defects, so it is desirable to significantly reduce the crystal defects. Note that the crystal defects referred to here refer to threading edge dislocations, threading screw dislocations, threading mixed dislocations, and basal plane dislocations, and the crystal defect density is the sum of the densities of each dislocation. However, in α-Ga ２ O ３ Since α-Ga is a metastable phase, single crystal substrates with few crystal defects have not been put to practical use, and it is common for it to be formed by heteroepitaxial growth on a sapphire substrate or the like. In such cases, stress is applied to the semiconductor film due to the difference in lattice constant with sapphire, and many crystal defects may be formed. For example, when α-Ga is grown on a c-plane sapphire, ２ O ３ When forming a film of sapphire (α-Al ２ O ３ ) a-axis length (4.754 Å) and α-Ga ２ O ３ The a-axis length (4.983 Å) differs by about 5%, and this difference is the main cause of crystal defects.

[0005] α-Ga ２ O ３ As an approach to reduce the difference in lattice constant with sapphire, it is known to use an orientation layer containing a material having a corundum-type crystal structure with a longer a-axis length and / or c-axis length than sapphire. For example, Patent Document 3 (Japanese Patent No. 7159449) describes a base substrate provided with an orientation layer used for crystal growth of a nitride or oxide of a Group 13 element, in which the surface of the orientation layer used for crystal growth is made of a material having a corundum-type crystal structure with a longer a-axis length and / or c-axis length than sapphire, and the orientation layer is made of α-Al ２ O ３ , α-Cr ２ O ３ , α-Fe ２ O ３ , α-Ti ２ O ３ , α-V ２ O ３ , and α-Rh ２ O ３Patent Document 3 also discloses an underlying substrate including a solid solution containing two or more species selected from the group consisting of:

[0013] Furthermore, Patent Document 3 also discloses that the alignment layer has a compositionally stable region located near the surface, where the composition is stable in the thickness direction, and a compositionally graded region located farther from the surface, where the composition changes in the thickness direction.

[0006] JP 2014-72533 A JP 2016-25256 A Japanese Patent No. 7159449 A

[0007] It is known that gallium oxide can have other crystal structures such as β-phase and ε-phase in addition to the α-phase of the corundum structure. ２ O ３ α-Ga on top ２ O ３ Even when forming a film, the above-mentioned heterogeneous phases may be formed locally. The formation of such heterogeneous phases reduces the yield. In industrial production, a yield of 95% or more is desirable.

[0008] The present inventors have now discovered that α-Ga ２ O ３ system or α-Cr ２ O ３ The orientation layer used for the crystal growth of the α-Ga-based semiconductor film has a composition stable region where the composition is stable in the thickness direction and a composition gradient region where the composition changes in the thickness direction at a predetermined thickness ratio, so that the α-Ga-based semiconductor film to be formed thereon can be grown. ２ O ３ system or α-Cr ２ O ３ It has been found that it is possible to provide a base substrate that can reduce the probability of hetero-phase formation in a silicon-based semiconductor film and improve yield (good product rate).

[0009] Therefore, the object of the present invention is to ２ O ３ system or α-Cr ２ O ３ A base substrate having an orientation layer used for crystal growth of an α-Ga-based semiconductor film, ２ O ３ system or α-Cr ２ O ３The present invention aims to provide a base substrate capable of reducing the probability of hetero-phase formation in a silicon-based semiconductor film and improving the yield (non-defective product rate).

[0010] The present disclosure provides the following aspects: [Aspect 1] A base substrate having an orientation layer used for crystal growth of a semiconductor film, the semiconductor film being an α-Ga ２ O ３ , α-Ga ２ O ３ system solid solution, α-Cr ２ O ３ , and α-Cr ２ O ３ A base substrate, the base substrate comprising at least one selected from the group consisting of crystalline solid solutions, wherein the surface of the orientation layer on the side used for the crystal growth is made of a material having a corundum-type crystal structure with an a-axis length and / or a c-axis length longer than that of sapphire, the orientation layer having a stable composition region in which the composition is stable in the thickness direction and a gradient composition region in which the composition changes in the thickness direction, the stable composition region being thicker than the gradient composition region. [Aspect 2] A base substrate according to Aspect 1, wherein the thickness of the stable composition region is 1.5 times or more the thickness of the gradient composition region. [Aspect 3] A base substrate according to Aspect 1 or 2, wherein the thickness of the stable composition region is 2.0 times or more the thickness of the gradient composition region. [Aspect 4] A base substrate according to any one of Aspects 1 to 3, wherein the thickness of the stable composition region is 30 μm or more. [Aspect 5] A base substrate, the stable composition region being α-Cr ２ O ３ A starting substrate according to any one of Aspects 1 to 4, comprising: [Aspect 6] The starting substrate according to any one of Aspects 1 to 5, wherein the stable composition region is located near the surface used for the crystal growth, and the gradient composition region is located far from the surface used for the crystal growth. [Aspect 7] The gradient composition region is α-Cr ２ O ３ and α-Al ２ O ３and a base substrate according to any one of Aspects 1 to 6, which is composed of a solid solution containing the above. [Aspect 8] The base substrate according to Aspect 7, in which, in the gradient composition region, the Al concentration decreases in the thickness direction toward the stable composition region. [Aspect 9] The base substrate according to any one of Aspects 1 to 8, in which the alignment layer is a heteroepitaxially grown layer. [Aspect 10] The base substrate according to any one of Aspects 1 to 9, further comprising a support substrate on the side opposite to the surface of the alignment layer. [Aspect 11] The base substrate according to Aspect 10, in which the support substrate is a sapphire substrate. [Aspect 12] The base substrate according to any one of Aspects 1 to 11, in which the alignment layer is a heteroepitaxially grown layer based on a sapphire substrate.

[0011] It is a schematic cross-sectional view showing an example of a base substrate according to the present invention. It is a schematic cross-sectional view showing the configuration of an aerosol deposition (AD) apparatus. It is a schematic cross-sectional view showing the configuration of a mist CVD apparatus. It is a diagram for explaining the positions of the center point and four outer peripheral points on the surface of an alignment layer.

[0012] 1 shows an example of a substrate according to the present invention. The substrate 10 shown in FIG. 1 is provided with an orientation layer 12 used for crystal growth of a semiconductor film. This semiconductor film is an α-Ga ２ O ３ , α-Ga ２ O ３ system solid solution, α-Cr ２ O ３ , and α-Cr ２ O ３ A film composed of at least one selected from the group consisting of α-Ga-based solid solutions (hereinafter referred to as α-Ga ２ O ３ system or α-Cr ２ O ３ That is, the base substrate 10 has an α-Ga-based semiconductor film formed on the alignment layer 12. ２ O ３ system or α-Cr ２ O ３The orientation layer 12 is used for crystal growth of an α-Ga-based semiconductor film. The surface of the orientation layer 12 used for crystal growth is made of a material having a corundum-type crystal structure with a longer a-axis length and / or c-axis length than sapphire. The orientation layer 12 has a stable composition region 12a in which the composition is stable in the thickness direction, and a gradient composition region 12b in which the composition changes in the thickness direction. The stable composition region 12a is thicker than the gradient composition region 12b. In this way, α-Ga ２ O ３ system or α-Cr ２ O ３ The orientation layer 12 used for the crystal growth of the α-Ga-based semiconductor film has a composition stable region 12a in which the composition is stable in the thickness direction and a composition gradient region 12b in which the composition changes in the thickness direction at a predetermined thickness ratio, so that the α-Ga-based semiconductor film to be formed thereon can be grown. ２ O ３ system or α-Cr ２ O ３ The probability of hetero-phase formation in the semiconductor film can be reduced, and the yield (good product rate) can be improved.

[0013] As mentioned above, it is known that gallium oxide can have other crystal structures such as β-phase and ε-phase in addition to the α-phase of the corundum structure. ２ O ３ α-Ga on top ２ O ３ Even when forming a film, the heterogeneous phases described above may be formed locally. The formation of such heterogeneous phases reduces the yield. In industrial production, a yield of 95% or more is desirable. This problem is successfully solved by the present invention by making the stable composition region 12a in the orientation layer 12 thicker than the gradient composition region 12b. That is, when α-Ga is deposited on such an orientation layer 12, ２ O ３ system or α-Cr ２ O ３ When a semiconductor film containing ZnO is formed, the probability of hetero-phase formation can be reduced, and the yield (non-defective product rate) can be improved.

[0014] The orientation layer 12 typically has a structure in which the crystal orientation is generally aligned in the direction of the normal. This structure allows for the formation of a semiconductor film with excellent quality, particularly excellent orientation, on the layer. That is, when a semiconductor film is formed on the orientation layer 12, the crystal orientation of the semiconductor film generally follows the crystal orientation of the orientation layer 12. Therefore, by providing the base substrate 10 with the orientation layer 12, the semiconductor film can be an oriented film. The orientation layer 12 may be a polycrystalline or mosaic crystalline (a collection of crystals with slightly misaligned crystal orientations), or it may be a single crystalline. If the orientation layer 12 is polycrystalline, it is preferably a biaxially oriented layer in which the twist direction (i.e., the direction of rotation about the substrate normal, which is oriented generally perpendicular to the substrate surface) is also generally aligned.

[0015] The surface of the orientation layer 12 used for crystal growth (hereinafter, sometimes simply referred to as the "surface" or the "orientation layer surface") is made of sapphire (α-Al ２ O ３ The lattice constant of the orientation layer 12 is controlled to significantly reduce crystal defects in the semiconductor film formed thereon. ２ O ３ , α-Ga ２ O ３ system solid solution, α-Cr ２ O ３ , or α-Cr ２ O ３ The lattice constant of the solid solution is the same as that of sapphire (α-Al ２ O ３ ) is larger than that of α-Ga as shown in Table 1 below. ２ O ３ is a structure in which the lattice constants (a-axis length and c-axis length) are α-Al ２ O ３ Therefore, the lattice constant of the orientation layer 12 is set to be larger than that of α-Al. ２ O ３By controlling the lattice constant to be larger, when a semiconductor film is formed on the orientation layer 12, the mismatch in lattice constant between the semiconductor film and the orientation layer 12 is suppressed, and as a result, crystal defects in the semiconductor film are reduced. ２ O ３ When a film is formed, α-Ga ２ O ３ The lattice length (a-axis length) in the in-plane direction of α-Al is larger than that of sapphire, and there is a mismatch of about 4.8%. ２ O ３ By controlling the α-Ga ２ O ３ The crystal defects in the layer are reduced. ２ O ３ When a film is formed, α-Ga ２ O ３ The lattice lengths (c-axis length and a-axis length) in the in-plane direction of α-Al are larger than those of sapphire, and there is a mismatch of about 3.4% in the c-axis length and about 4.8% in the a-axis length. ２ O ３ By controlling the α-Ga ２ O ３ The crystal defects in the layer are reduced. ２ O ３ The lattice constant mismatch between α-Cr and sapphire is large. ２ O ３ When a film is formed, α-Cr ２ O ３ The lattice length (a-axis length) in the in-plane direction of α-Al is substantially larger than that of sapphire, and there is a mismatch of about 4.4%. ２ O ３ By controlling the α-Cr ２ O ３ The crystal defects in the layer are reduced. ２ O ３ system or α-Cr ２ O ３When a silicon-based semiconductor film is formed directly on a sapphire substrate, stress is generated in the semiconductor film due to a mismatch in lattice constant, which may result in a large number of crystal defects in the semiconductor film.

[0016] It is preferable that the entire orientation layer 12 is made of a material having a corundum-type crystal structure. This makes it possible to reduce crystal defects in the orientation layer 12 and the semiconductor film. It is desirable that the orientation layer 12 is formed on the surface of a sapphire substrate. The α-Al that constitutes the sapphire substrate ２ O ３ has a corundum-type crystal structure, and by forming the orientation layer 12 from a material having a corundum-type crystal structure, its crystal structure can be made the same as that of the sapphire substrate, thereby suppressing the occurrence of crystal defects in the orientation layer 12 due to a mismatch in crystal structure. In this regard, reducing the crystal defects in the orientation layer 12 is preferable because it also reduces the crystal defects in the semiconductor film formed thereon. This is because if a large number of crystal defects exist in the orientation layer 12, the crystal defects will be inherited in the semiconductor film formed thereon, resulting in the occurrence of crystal defects in the semiconductor film as well.

[0017] The material having a corundum type crystal structure that constitutes the orientation layer 12 is α-Cr ２ O ３ , or α-Cr ２ O ３ As shown in Table 1 above, these materials preferably contain α-Al-based solid solutions. ２ O ３ α-Ga, which has a larger lattice constant (a-axis length and / or c-axis length) and constitutes a semiconductor film ２ O ３ and α-Cr ２ O ３ Since the lattice constant is relatively close to or identical to that of the crystalline silicon, it is possible to effectively suppress crystal defects in the semiconductor film. Such a solid solution may be either a substitutional solid solution or an interstitial solid solution, but a substitutional solid solution is preferred. While the orientation layer 12 is made of a material having a corundum-type crystal structure, this does not exclude the inclusion of other trace components or microcrystals.

[0018] The a-axis length of the material having a corundum type crystal structure on the surface of the orientation layer 12 used for crystal growth is greater than 4.754 Å and not greater than 5.157 Å, more preferably 4.850 to 5.000 Å, and even more preferably 4.900 to 5.000 Å. The c-axis length of the material having a corundum type crystal structure on the surface of the orientation layer 12 used for crystal growth is greater than 12.990 Å and not greater than 13.998 Å, more preferably 13.000 to 13.800 Å, and even more preferably 13.400 to 13.600 Å. By controlling the a-axis length and / or c-axis length of the surface of the orientation layer 12 within such ranges, the α-Ga ions constituting the semiconductor film can be controlled. ２ O ３ , α-Ga ２ O ３ system solid solution, α-Cr ２ O ３ , or α-Cr ２ O ３ It becomes possible to make the lattice constant (a-axis length and / or c-axis length) close to or match that of the solid solution.

[0019] The thickness of the orientation layer 12 is preferably 10 μm or more, more preferably 40 μm or more. The upper limit of the thickness is not particularly limited, but is typically 1000 μm or less. When the orientation layer 12 is used as a stand-alone substrate, the orientation layer 12 may be thicker, for example, 1 mm or more, from the perspective of ease of handling, but from a cost perspective, it is preferably 2 mm or less. In this way, by increasing the thickness of the orientation layer 12, it is possible to reduce crystal defects on the surface of the orientation layer 12. When the orientation layer 12 is formed on a sapphire substrate, the lattice constants of the sapphire substrate and the orientation layer 12 are slightly different, and as a result, crystal defects are likely to occur at their interface, i.e., below the orientation layer. However, by increasing the thickness of the orientation layer 12, the influence of such crystal defects occurring below the orientation layer can be reduced on the surface of the orientation layer. Although the reason for this is unclear, it is believed that crystal defects occurring below the orientation layer do not reach the surface of the thick orientation layer and disappear. Furthermore, by thickening the alignment layer 12, it is possible to form a semiconductor film on the alignment layer 12, peel the semiconductor film, and reuse the base substrate 10. The crystal defect density on the surface of the alignment layer 12 is preferably 1.0×10 ８ / cm ２ or less, more preferably 1.0 × 10 ６ / cm ２ More preferably, 4.0 × 10 ３ / cm ２ In this specification, the crystal defects refer to threading edge dislocations, threading screw dislocations, threading mixed dislocations, and basal plane dislocations, and the crystal defect density refers to the total density of each dislocation. For example, if a material has 3×10 threading edge dislocations, ８ / cm ２ , threading screw dislocations 6 × 10 ８ / cm ２ , threading mixed dislocations 4 × 10 ８ / cm ２ If it is included, the crystal defect density is 1.3 × 10 ９ / cm ２ Basal plane dislocations become a problem when the base substrate 10 including the alignment layer 12 has an off-axis angle, but do not become a problem when there is no off-axis angle, since they are not exposed to the surface of the alignment layer 12.

[0020] The material constituting the alignment layer 12 is not particularly limited as long as it has an orientation relative to the surface of the base substrate 10, and may be, for example, a c-axis, a-axis, or m-axis orientation. In this way, when a semiconductor film is formed on the base substrate 10, the semiconductor film can be a c-axis, a-axis, or m-axis oriented film.

[0021] The orientation layer 12 is preferably a heteroepitaxially grown layer. For example, when the orientation layer 12 is grown on a sapphire substrate, since both the sapphire substrate and the orientation layer 12 have a corundum-type crystal structure, if their lattice constants are close to each other, epitaxial growth may occur during heat treatment, in which the crystal planes of the orientation layer 12 are aligned to the crystal orientation of the sapphire substrate. By epitaxially growing the orientation layer 12 in this way, it is possible for the orientation layer 12 to inherit the high crystallinity and crystal orientation specific to the single crystal of the sapphire substrate.

[0022] As described above, the orientation layer 12 has a stable composition region 12a, in which the composition is stable in the thickness direction, and a gradient composition region 12b, in which the composition changes in the thickness direction. Preferably, the stable composition region 12a is located near the surface used for crystal growth, and the gradient composition region 12b is located farther from the surface used for crystal growth. The stable composition region 12a refers to a region in which the change in the content ratio of each metal element is less than 1.0 at%. The gradient composition region 12b refers to a region in which the change in the content ratio of each metal element is 1.0 at% or more. For example, the stable composition region 12a and the gradient composition region 12b can be determined as follows. First, a cross-sectional sample of the orientation layer 12 is prepared, and energy dispersive X-ray analysis (EDS) is performed at 10 random locations near the orientation layer surface, and the average content ratio (at%) of the detected metal elements is calculated. Next, EDS analysis is performed at 10 random locations 2 μm away from the surface in the thickness direction, and the average content ratio (at%) at the 2 μm thickness point is calculated. At this time, the average values ​​of the content ratios at the surface and at a point 2 μm thick are compared, and if the difference in the content ratio of at least one of all detected metal elements is less than 1.0 at% or more than 1.0 at% then the region from the surface to a thickness of 2 μm can be assigned to either the stable composition region 12a or the gradient composition region 12b. Using a similar method, the average value of the content ratio of metal elements is calculated every 2 μm in the thickness direction, and the average value of the content ratio of metal elements between a certain thickness point and a point 2 μm away from that point in the thickness direction can be compared to determine the assignment of the region between the points. For example, the assignment of the region between a point 24 μm thick from the surface and a point 26 μm thick can be determined by calculating and comparing the average value of the content ratio of metal elements at each point. Furthermore, for example, if the orientation layer 12 contains Al, it is more preferable that the Al concentration in the gradient composition region 12b decreases in the thickness direction toward the stable composition region 12a. In this embodiment, the gradient composition region 12b is α-Cr. ２ O ３ and α-Al ２ O ３ It is preferable that the stable composition region 12a is made up of a solid solution containing α-Cr. ２ O ３ (typically as a major phase), and more preferably α-Cr ２O ３ , or α-Cr ２ O ３ system solid solution (e.g., α-Cr ２ O ３ and α-Al ２ O ３ and a solid solution containing

[0023] The gradient composition region 12b can be formed by heat treating the sapphire substrate and the texture precursor layer at a temperature of 1000°C or higher in the manufacturing process of the base substrate 10, which will be described later. When heat treated at such a high temperature, a reaction occurs at the interface between the sapphire substrate and the texture precursor layer, causing the Al component in the sapphire substrate to diffuse into the texture precursor layer, or the component in the texture precursor layer to diffuse into the sapphire substrate. As a result, α-Al ２ O ３ A gradient composition region 12b is formed in which the amount of solid solution of changes in the thickness direction.

[0024] As described above, in the orientation layer 12, the compositionally stable region 12a is thicker than the compositionally graded region 12b in order to reduce the probability of heterophase formation. Specifically, the thickness of the compositionally stable region 12a is greater than 1.0 times the thickness of the compositionally graded region 12b, preferably 1.5 times or more, and more preferably 2.0 times or more. While the upper limit of this thickness ratio is not particularly limited, the thickness of the compositionally stable region 12a is typically 10 times or less, more typically 5 times or less, of the compositionally graded region 12b. From the viewpoint of reducing the probability of heterophase formation, the thickness of the compositionally stable region 12a is preferably 30 μm or more, more preferably 50 μm or more, even more preferably 100 μm or more, and particularly preferably 150 μm or more. While the upper limit of the thickness of the compositionally stable region 12a is not particularly limited, it is typically 400 μm or less, more typically 300 μm or less. On the other hand, from the viewpoint of alleviating stress due to the difference in lattice constants, the thickness of the gradient composition region 12 b is preferably 20 μm or more, more preferably 30 μm or more, even more preferably 40 μm or more, and particularly preferably 50 μm or more. There is no particular upper limit to the thickness of the gradient composition region 12 b, but it is typically 200 μm or less, more typically 150 μm or less.

[0025] The arithmetic mean roughness Ra of the surface of the alignment layer 12 is preferably 1 nm or less, more preferably 0.5 nm or less, and even more preferably 0.2 nm or less. By smoothing the surface of the alignment layer 12 in this way, it is believed that the crystallinity of the semiconductor film formed thereon is further improved.

[0026] The base substrate 10 has one side preferably having a thickness of 20 cm ２ More preferably, 70 cm ２ More preferably, 170 cm ２ By increasing the area of ​​the base substrate 10 in this way, it is possible to increase the area of ​​the semiconductor film formed thereon. Therefore, it becomes possible to obtain a large number of semiconductor elements from one semiconductor film, and further reduction in manufacturing costs is expected. There is no particular upper limit on the size, but typically, it is 700 cm per side. ２ The following is the result.

[0027] The base substrate 10 of the present invention preferably further comprises a support substrate 14 on the side opposite (i.e., the backside) to the surface of the orientation layer 12 (the surface used for crystal growth). That is, the base substrate 10 of the present invention may comprise a support substrate 14 and an orientation layer 12 provided on the support substrate 14. The support substrate 14 may be a sapphire substrate, a Cr substrate, or a Cr ２ O ３Corundum single crystals such as those mentioned above are preferred, with sapphire substrates being particularly preferred. Using a corundum single crystal for the support substrate 14 allows the orientation layer 12 to double as a seed crystal for heteroepitaxial growth. Furthermore, a configuration including a corundum single crystal like this makes it possible to obtain a high-quality semiconductor film. Corundum single crystals possess excellent mechanical properties, thermal properties, chemical stability, and other characteristics. Sapphire, in particular, has a high thermal conductivity of 42 W / m·K at room temperature, providing excellent thermal conductivity. Therefore, using a sapphire substrate as the base substrate 10 can improve the thermal conductivity of the entire substrate. As a result, when a semiconductor film is formed on the base substrate 10, uneven temperature distribution within the substrate surface is suppressed, and it is expected that a semiconductor film with a uniform thickness can be obtained. Furthermore, sapphire substrates are easily available in large areas, which reduces overall costs and allows for the production of large-area semiconductor films.

[0028] The sapphire substrate used as the support substrate 14 may have any oriented plane. That is, it may have an oriented plane such as the a-plane, c-plane, r-plane, or m-plane, or may have a predetermined off-angle relative to these planes. In addition, the sapphire may be doped with a dopant to adjust the electrical properties. Known dopants can be used as such dopants.

[0029] The base substrate 10 of the present invention may be in the form of a freestanding substrate containing only the alignment layer 12, or may be in the form of a base substrate 10 accompanied by a support substrate 14 such as a sapphire substrate. Therefore, if necessary, the alignment layer 12 may be finally separated from the support substrate 14 such as a sapphire substrate. Separation of the support substrate 14 may be performed by any known method, and is not particularly limited. Examples of such methods include a method of separating the alignment layer 12 by applying mechanical impact, a method of separating the alignment layer 12 by applying heat and utilizing thermal stress, a method of separating the alignment layer 12 by applying vibration such as ultrasonic waves, a method of separating the alignment layer 12 by etching unnecessary portions, a method of separating the alignment layer 12 by laser lift-off, and a method of separating the alignment layer 12 by mechanical processing such as cutting or polishing. Furthermore, when the alignment layer 12 is grown heteroepitaxially on a sapphire substrate, the alignment layer 12 may be placed on another support substrate 14 after separating the sapphire substrate. The material of the other support substrate 14 is not particularly limited, and a suitable material may be selected from the viewpoint of the material's physical properties. For example, from the viewpoint of thermal conductivity, metal substrates such as Cu substrates and ceramic substrates such as SiC and AlN substrates are available.

[0030] Manufacturing Method The base substrate of the present invention can be preferably manufactured by (a) preparing a sapphire substrate, (b) preparing a predetermined textured precursor layer, (c) heat-treating the textured precursor layer on the sapphire substrate to convert at least a portion thereof near the sapphire substrate into a textured layer, (d) repeating steps (b) and (c) on the substrate manufactured in step (c) any number of times (one or more times), and then (e) smoothing the surface of the textured layer by grinding, polishing, or other processing. This textured precursor layer becomes a textured layer upon heat treatment and includes a material having a corundum-type crystal structure in which the a-axis length and / or c-axis length is greater than that of sapphire, or a material that becomes a corundum-type crystal structure in which the a-axis length and / or c-axis length is greater than that of sapphire upon heat treatment, as described below. This manufacturing method can promote the growth of a textured layer using the sapphire substrate as a seed crystal. In other words, the high crystallinity and crystal orientation characteristic of the single crystal of the sapphire substrate are inherited by the textured layer.

[0031] (a) Preparation of Sapphire Substrate To prepare a base substrate, first, a sapphire substrate is prepared. The sapphire substrate used may have any oriented plane. That is, it may have an oriented plane such as the a-plane, c-plane, r-plane, or m-plane, and may have a predetermined off-angle relative to these planes. For example, when c-plane sapphire is used, the c-axis is oriented relative to the surface, making it easy to heteroepitaxially grow a c-axis oriented oriented layer on it. In addition, it is also possible to use a sapphire substrate to which a dopant has been added in order to adjust the electrical properties. Known dopants can be used as such dopants.

[0032] (b) Preparation of the First Textured Precursor Layer An textured precursor layer is prepared containing a material having a corundum-type crystal structure in which the a-axis length and / or c-axis length is greater than that of sapphire, or a material that will become a corundum-type crystal structure in which the a-axis length and / or c-axis length is greater than that of sapphire upon heat treatment. The method for forming the textured precursor layer is not particularly limited, and known methods can be used. Examples of methods for forming the textured precursor layer include aerosol deposition (AD), hydrothermal, sputtering, vapor deposition, various CVD (chemical vapor deposition) methods, HVPE, PLD, CVT (chemical vapor transport), and sublimation. Examples of CVD methods include thermal CVD, plasma CVD, mist CVD, and metal-organic (MO) CVD. Alternatively, a textured precursor compact may be prepared in advance and then placed on a sapphire substrate. Such a compact can be prepared by molding the textured precursor material using methods such as tape casting or press molding. Alternatively, a polycrystalline material previously prepared by various CVD methods or sintering may be used as the orientation precursor layer, and the polycrystalline material may be placed on the sapphire substrate.

[0033] However, AD, various CVD methods, or sputtering are preferred. These methods allow for the formation of a dense textured precursor layer in a relatively short time, facilitating heteroepitaxial growth using a sapphire substrate as a seed crystal. In particular, AD does not require a high-vacuum process and has a relatively fast film formation rate, making it preferable in terms of manufacturing costs. When using sputtering, deposition can be performed using a target made of the same material as the textured precursor layer. Alternatively, reactive sputtering, in which a metal target is used to form the layer under an oxygen atmosphere, can also be used. A method in which a pre-prepared compact is placed on sapphire is also preferred as a simple method, but because the textured precursor layer is not dense, a densification process is required in the heat treatment step described below. The method of using a pre-prepared polycrystalline body as the textured precursor layer requires two steps: the step of preparing the polycrystalline body and the step of heat treatment on the sapphire substrate. Furthermore, to improve adhesion between the polycrystalline body and the sapphire substrate, it is necessary to take measures such as sufficiently smoothing the surface of the polycrystalline body. Although both methods can use known conditions, the method of directly forming an orientation precursor layer using the AD method and the method of placing a pre-prepared molded body on a sapphire substrate will be described below.

[0034] The AD method is a technology in which fine particles or fine particle raw material are mixed with a gas to form an aerosol, and this aerosol is then sprayed at high speed from a nozzle to collide with a substrate, forming a coating. The AD method is characterized by its ability to form a dense coating at room temperature. An example of a film formation apparatus (aerosol deposition (AD) apparatus) used in this AD method is shown in FIG. 2. The film formation apparatus 20 shown in FIG. 2 is configured as an apparatus used in the AD method, in which raw material powder is sprayed onto a substrate under an atmosphere at a pressure lower than atmospheric pressure. This film formation apparatus 20 includes an aerosol generation unit 22 that generates an aerosol of raw material powder containing raw material components, and a film formation unit 30 that sprays the raw material powder onto a sapphire substrate 21 to form a film containing the raw material components. The aerosol generation unit 22 includes an aerosol generation chamber 23 that contains raw material powder and generates aerosol by receiving a supply of carrier gas from a gas cylinder (not shown), a raw material supply pipe 24 that supplies the generated aerosol to the film formation unit 30, and a vibrator 25 that applies vibrations at a frequency of 10 to 100 Hz to the aerosol generation chamber 23 and the aerosol therein. The film formation unit 30 includes a film formation chamber 32 that sprays the aerosol onto the sapphire substrate 21, a substrate holder 34 that is disposed inside the film formation chamber 32 and fixes the sapphire substrate 21, and an XY stage 33 that moves the substrate holder 34 in the X-axis and Y-axis directions. The film formation unit 30 also includes an injection nozzle 36 that has a slit 37 formed at its tip and sprays the aerosol onto the sapphire substrate 21, and a vacuum pump 38 that depressurizes the film formation chamber 32.

[0035] It is known that the AD method can control the film thickness, film quality, etc. by adjusting the film formation conditions. For example, the morphology of an AD film is easily affected by the collision speed of the raw material powder with the substrate, the particle size of the raw material powder, the agglomeration state of the raw material powder in the aerosol, the amount sprayed per unit time, etc. The collision speed of the raw material powder with the substrate is affected by the pressure difference between the film formation chamber 32 and the spray nozzle 36, the opening area of ​​the spray nozzle, etc. If appropriate conditions are not used, the coating may become a compact or may have pores, so it is desirable to appropriately control these factors.

[0036] When using a molded body in which the orientation precursor layer has been previously prepared, the molded body can be prepared by molding the raw material powder of the orientation precursor. For example, when press molding is used, the orientation precursor layer is a press-molded body. The press-molded body can be prepared by press-molding the raw material powder of the orientation precursor based on a known method. For example, the raw material powder is placed in a mold and pressurized, preferably at 100 to 400 kgf / cm. ２ , more preferably 150 to 300 kgf / cm ２ The film can be produced by pressing at a pressure of 1000 psi. The molding method is not particularly limited, and in addition to press molding, tape casting, slip casting, extrusion molding, doctor blade molding, and any combination thereof can be used. For example, when tape molding is used, additives such as binders, plasticizers, dispersants, and dispersion media are appropriately added to the raw material powder to form a slurry, and this slurry is preferably discharged and molded into a sheet by passing it through a narrow slit-shaped discharge port. The thickness of the sheet-shaped molded body is not limited, but from the viewpoint of handling, a thickness of 5 to 500 μm is preferable. If a thick alignment precursor layer is required, a number of these sheet-shaped bodies can be stacked to obtain the desired thickness.

[0037] These compacts are subsequently heat-treated on a sapphire substrate, and the portion near the sapphire substrate becomes an orientation layer. As mentioned above, this method requires sintering and densifying the compact in the heat treatment step described below. For this reason, the compact may contain trace components such as sintering aids in addition to materials that have or can give rise to a corundum-type crystal structure.

[0038] (c) Heat Treatment of the Textured Precursor Layer on the Sapphire Substrate The sapphire substrate on which the textured precursor layer is formed is heat-treated at a temperature of 1000°C or higher. This heat treatment converts at least the portion of the textured precursor layer near the sapphire substrate into a dense textured layer. In other words, this heat treatment enables heteroepitaxial growth of the textured layer. That is, the presence of the textured precursor layer on the sapphire substrate causes heteroepitaxial growth, in which a material having a corundum-type crystal structure grows using the sapphire substrate as a seed crystal during heat treatment. During this process, crystal rearrangement occurs, and the crystals are aligned along the crystal planes of the sapphire substrate. As a result, the crystal axes of the sapphire substrate and the textured layer can be aligned. For example, using a c-plane sapphire substrate makes it possible to achieve a c-axis orientation of both the sapphire substrate and the textured layer relative to the surface of the base substrate. Furthermore, this heat treatment makes it possible to form a gradient composition region in part of the textured layer. That is, during the heat treatment, a reaction occurs at the interface between the sapphire substrate and the texture precursor layer, and the Al component in the sapphire substrate diffuses into the texture precursor layer and / or the component in the texture precursor layer diffuses into the sapphire substrate, resulting in the formation of α-Al ２ O ３ A gradient composition region is formed, which is composed of a solid solution containing

[0039] It is known that heteroepitaxial growth can occur on a sapphire substrate without heat treatment at 1000°C or higher using various CVD, sputtering, HVPE, PLD, CVT, sublimation, and other methods. However, the oriented precursor layer is not oriented at the time of preparation, i.e., amorphous or unoriented polycrystalline. It is preferable to use sapphire as a seed crystal to cause crystal rearrangement during this heat treatment process. This effectively reduces crystal defects that reach the surface of the oriented layer. Although the reason for this is unclear, it is believed to be due to the tendency for crystal defects generated in the lower part of the oriented layer to annihilate each other.

[0040] The heat treatment method is not particularly limited as long as a corundum-type crystal structure is obtained and heteroepitaxial growth occurs using the sapphire substrate as a seed. It can be performed in a known heat treatment furnace, such as a tubular furnace or a hot plate. In addition to these atmospheric (pressless) heat treatments, pressurized heat treatments such as hot pressing and HIP, or a combination of atmospheric and pressurized heat treatments, can also be used. The heat treatment conditions can be appropriately selected depending on the material used for the orientation layer. For example, the heat treatment atmosphere can be selected from air, vacuum, nitrogen, and inert gas atmospheres. The preferred heat treatment temperature also varies depending on the material used for the orientation layer, but is preferably 1000 to 2000°C, and more preferably 1200 to 2000°C. The heat treatment temperature and holding time are related to the thickness of the orientation layer generated by heteroepitaxial growth and the thickness of the gradient composition region formed by diffusion with the sapphire substrate, and can be appropriately adjusted depending on the type of material, the desired orientation layer, the thickness of the gradient composition region, etc. However, when a pre-prepared molded body is used as the orientation precursor layer, it is necessary to sinter and densify it during the heat treatment, and high-temperature atmospheric pressure firing, hot pressing, HIP, or a combination thereof is suitable. For example, when hot pressing is used, the surface pressure is 50 kgf / cm. ２ More preferably, 100 kgf / cm ２ More preferably, 200 kgf / cm ２ The upper limit is not particularly limited. The firing temperature is also not particularly limited as long as sintering, densification, and heteroepitaxial growth occur, but is preferably 1000°C or higher, more preferably 1200°C or higher, even more preferably 1400°C or higher, and particularly preferably 1600°C or higher. The firing atmosphere can also be selected from air, vacuum, nitrogen, and inert gas atmospheres. Firing jigs such as molds can be made of graphite, alumina, or the like.

[0041] After the heat treatment, an orientation precursor layer or a surface layer with poor orientation or no orientation may be present or remain on the orientation layer. In this case, it is preferable to expose the surface of the orientation layer by grinding, polishing, or the like on the surface derived from the orientation precursor layer, and smooth the surface of the orientation layer. By doing so, a material with excellent orientation is exposed on the surface of the orientation layer, and a new orientation layer can be effectively epitaxially grown thereon in the step (d) described below. The method for removing the orientation precursor layer or the surface layer is not particularly limited, but examples include grinding and polishing and ion beam milling. The surface of the orientation layer is preferably polished by lapping using abrasive grains or chemical mechanical polishing (CMP).

[0042] (d) Repeated preparation of orientation precursor layer and heat treatment The preparation of the orientation precursor layer and heat treatment (and optionally processing such as grinding and polishing) according to (b) and (c) above are repeated any number of times (one or more times) for the substrate obtained in (c). That is, a new orientation precursor layer is prepared on the orientation layer obtained in (c) above (i.e., on the orientation layer prepared on the sapphire substrate) and heat treatment is performed. The preparation and heat treatment of the new orientation precursor layer can be performed in the same manner as described in (b) and (c) above.

[0043] (e) Processing such as grinding and polishing As mentioned above, an orientation precursor layer or a surface layer with poor orientation or no orientation may exist or remain on the orientation layer. In this case, it is preferable to perform processing such as grinding or polishing on the surface derived from the orientation precursor layer to expose the surface of the orientation layer and smooth the surface of the orientation layer. By doing so, a material with excellent orientation is exposed on the surface of the orientation layer, allowing a semiconductor film to be effectively epitaxially grown thereon. The method for removing the orientation precursor layer or the surface layer is not particularly limited, but examples include grinding and polishing and ion beam milling. Polishing of the surface of the orientation layer is preferably performed by lapping using abrasive grains or chemical mechanical polishing (CMP).

[0044] Semiconductor film: Using the base substrate 10 of the present invention, α-Ga is grown on the orientation layer 12 of the base substrate 10. ２ O３ , α-Ga ２ O ３ system solid solution, α-Cr ２ O ３ , and α-Cr ２ O ３ A semiconductor film composed of at least one selected from the group consisting of crystalline solid solutions can be formed with a low probability of heterophase formation (i.e., with a high yield or good product rate). The method for forming the semiconductor film is not particularly limited, and known methods are possible. However, any of various CVD methods, HVPE, sublimation, MBE, PLD, sputtering, and other vapor-phase film-forming methods, and hydrothermal and Na-flux methods are preferred. More preferred are mist CVD, HVPE, MBE, MOCVD, hydrothermal, and sputtering, with mist CVD being particularly preferred. The mist CVD method involves atomizing or condensing a raw material solution to generate mist or droplets, transporting the mist or droplets to a film-forming chamber equipped with a substrate using a carrier gas, and thermally decomposing and chemically reacting the mist or droplets in the film-forming chamber to form and grow a film on the substrate. This method does not require a vacuum process and allows for the production of a large number of samples in a short period of time. The mist CVD method will be described in detail in the following examples.

[0045] The present invention will be explained in more detail by the following examples, but the present invention is not limited to the following examples.

[0046] Example 1 (1) Preparation of the first orientation layer (1a) Preparation of the first orientation precursor layer Commercially available Cr ２ O ３ The powder was pulverized in a pot mill to obtain Cr particles with a particle size D50 of 0.3 μm. ２ O ３ The raw material powder was prepared. ２ O ３ Using the raw material powder and a sapphire substrate (diameter 50.8 mm (2 inches), thickness 0.43 mm, c-plane, off-angle 0.2°), Cr was deposited on the seed substrate (sapphire substrate) by the aerosol deposition (AD) apparatus shown in FIG. ２ O ３ The AD film (orientation precursor layer) was formed from the aerosol deposition (AD) device 20, which has the same configuration as described above.

[0047] The AD film formation conditions were as follows: the carrier gas was N ２ A ceramic nozzle with a 5 mm long x 0.3 mm short slit was used. The nozzle scanning conditions were a scan speed of 0.5 mm / s, with the nozzle moving 55 mm in a direction perpendicular to the long side of the slit and forward, 5 mm in the direction of the long side of the slit, 55 mm in a direction perpendicular to the long side of the slit and returning, and 5 mm in the direction of the long side of the slit and opposite the initial position, repeatedly. After moving 55 mm from the initial position in the direction of the long side of the slit, the nozzle repeated a scan in the opposite direction and returned to the initial position. This cycle was repeated 200 times. In one film formation cycle at room temperature, the set pressure of the carrier gas was 0.06 MPa, the flow rate was 6 L / min, and the chamber pressure was 100 Pa or less.

[0048] (1b) Heat Treatment of the Orientation Precursor Layer The sapphire substrate on which the AD film was formed was removed from the AD apparatus and annealed at 1700° C. for 1 hour in a nitrogen atmosphere.

[0049] (1c) Measurement of Crystal Growth Thickness A sample prepared separately using the same method as in (1a) and (1b) above was prepared and cut perpendicular to the plate surface, passing through the center of the substrate. The cross section of the cut sample was smoothed by lapping using diamond abrasive grains, and a mirror finish was achieved by chemical mechanical polishing (CMP) using colloidal silica. The obtained cross section was photographed with a scanning electron microscope (SU-5000, manufactured by Hitachi High-Technologies Corporation). Observing the backscattered electron image of the cross section after polishing, the channeling contrast due to differences in crystal orientation allowed the identification of the oriented precursor layer that remained as a polycrystal and the oriented layer with crystalline orientation. The thickness of each layer was estimated in this way, resulting in a film thickness of approximately 40 μm for the oriented layer and approximately 20 μm for the polycrystalline portion.

[0050] (1d) Grinding and Polishing The surface of the obtained substrate originating from the AD film was ground using a grindstone with a grit size up to #2000 until the orientation layer was exposed, and then the surface was further smoothed by lapping using diamond abrasive grains. Then, a mirror finish was applied by chemical mechanical polishing (CMP) using colloidal silica to obtain a composite base substrate with an orientation layer on the sapphire substrate. The surface of the substrate originating from the AD film was designated as the "surface". The amount of grinding and polishing, including the polycrystalline portion and the orientation layer, was approximately 30 μm, and the thickness of the orientation layer formed on the sapphire substrate was approximately 30 μm.

[0051] (2) Preparation of second alignment layer (2a) Preparation of second alignment precursor layer Cr was deposited on the alignment layer prepared in (1d) above by the same method (AD method) as in (1a) above. ２ O ３ An AD film (orientation precursor layer) consisting of the above was formed.

[0052] (2b) Heat Treatment of the Orientation Precursor Layer The sapphire substrate on which the AD film was formed was taken out of the AD apparatus and annealed at 1600° C. for 1 hour in a nitrogen atmosphere.

[0053] (2c) Measurement of Crystal Growth Thickness A sample prepared separately using the same method as in (2a) and (2b) above was prepared and cut perpendicular to the plate surface, passing through the center of the substrate. The cross section of the cut sample was smoothed by lapping using diamond abrasive grains, and a mirror finish was achieved by chemical mechanical polishing (CMP) using colloidal silica. The obtained cross section was photographed using a scanning electron microscope (SU-5000, manufactured by Hitachi High-Technologies Corporation). When observing the backscattered electron image of the cross section after polishing, the channeling contrast due to differences in crystal orientation allowed the identification of the polycrystalline remaining oriented precursor layer and the oriented layer, respectively. Furthermore, the thickness of each layer newly formed through (2a) and (2b) above was estimated, and the film thickness of the oriented layer was approximately 30 μm, and the film thickness of the polycrystalline portion was approximately 30 μm. At this time, the thickness of the newly formed alignment layer was determined by subtracting the thickness of the alignment layer estimated in (1d) above (i.e., approximately 30 μm) from the measured total thickness of the alignment layer (i.e., approximately 60 μm).

[0054] (2d) Grinding and Polishing The surface of the obtained substrate originating from the AD film was ground using a grindstone with a grit size up to #2000 until the orientation layer was exposed, and then the surface was further smoothed by lapping using diamond abrasive grains. Then, a mirror finish was applied by chemical mechanical polishing (CMP) using colloidal silica to obtain a composite base substrate with an orientation layer on a sapphire substrate. The surface of the substrate originating from the AD film was designated as the "surface". The amount of grinding and polishing, including the polycrystalline portion and the orientation layer, was approximately 35 μm, and the thickness of the orientation layer formed on the composite base substrate was approximately 25 μm.

[0055] (3) Evaluation of the Oriented Layer (3a) Cross-Section EBSD The composite base substrate prepared in (2) above was cut so as to pass through the center of the substrate in a direction perpendicular to the plate surface. The cross section of the cut sample was smoothed by lapping using diamond abrasive grains, and then mirror-finished by chemical mechanical polishing (CMP) using colloidal silica. Next, inverse pole figure mapping of the orientation layer cross section was measured using the EBSD (Electron Back Scatter Diffraction Patterns) method. Specifically, using a scanning electron microscope (Hitachi High-Technologies Corporation, SU-5000) equipped with an EBSD (Oxford Instruments Nordlys Nano), inverse pole figure orientation mapping of the orientation layer cross section was performed under the following conditions in a field of view of 50 μm × 100 μm.

[0056] <EBSD measurement conditions> Acceleration voltage: 15 kV Spot intensity: 70 Working distance: 22.5 mm Step size: 0.5 μm Sample tilt angle: 70° Measurement program: Aztec (version 3.3)

[0057] The inverse pole figure orientation map of the cross section showed that the orientation in the oriented layer was the same as that of the sapphire substrate in both the substrate normal direction and in-plane direction, and that the oriented layer was a biaxially oriented corundum layer.

[0058] (3b) Cross-Sectional EDS Next, a compositional analysis of a cross section perpendicular to the substrate main surface was performed using an energy dispersive X-ray analyzer (EDS). As a result, Cr, O, and Al were detected in the thickness region from the composite substrate surface (the surface on the orientation layer side) to approximately 30 μm. In this thickness region, the difference in the content ratio of each element of Cr and Al between surfaces separated by 2 μm in the thickness direction was less than 1.0 at%. This indicated that a Cr-Al oxide layer having a thickness of approximately 30 μm was formed as a compositionally stable region. Furthermore, Cr, O, and Al were also detected in the thickness range of approximately 25 μm below this Cr-Al oxide layer (i.e., a thickness region from the surface to a depth of approximately 30 to 55 μm). Within this depth region of approximately 30 to 55 μm, the content ratio of each element of Cr and Al varied significantly in the thickness direction, with a high Al concentration on the sapphire substrate side and a decrease in the Al concentration on the side closer to the compositionally stable region. Within this thickness region of approximately 30 to 55 μm in depth, the difference in Al content between planes separated by 2 μm in the thickness direction was 1.2 to 11.2 at%, confirming that this reaction layer constituted a gradient composition region. Furthermore, only O and Al were detected below this gradient composition region, indicating that it was a sapphire substrate. Furthermore, the results of the inverse pole figure map and the EDS analysis indicated that Cr, Al, and O were present in the biaxially oriented corundum layer, and the biaxially oriented layer was Cr. ２ O ３ and Al ２ O ３ It was suggested that it was a solid solution with

[0059] (4) Formation of Gallium Oxide Film (4a) Mist CVD The mist CVD apparatus 61 used in this example is illustrated in Figure 3. The mist CVD apparatus 61 includes a susceptor 70 on which a substrate 69 is placed, a dilution gas source 62a, a carrier gas source 62b, a flow control valve 63a for adjusting the flow rate of the dilution gas sent from the dilution gas source 62a, a flow control valve 63b for adjusting the flow rate of the carrier gas sent from the carrier gas source 62b, a mist source 64 containing a raw material solution 64a, a container 65 containing water 65a, an ultrasonic vibrator 66 attached to the bottom of the container 65, a quartz tube 67 serving as a film formation chamber, a heater 68 installed around the quartz tube 67, and an exhaust port 71. The susceptor 70 is made of quartz, and the surface on which the substrate 69 is placed is inclined relative to the horizontal plane.

[0060] (4b) Preparation of raw material solution Metallic gallium was added to hydrochloric acid and stirred at room temperature for 4 weeks to obtain a gallium chloride solution with a gallium ion concentration of 3 mol / L. Water was added to the obtained gallium chloride solution to adjust the aqueous solution to a gallium ion concentration of 65 mmol / L. Ammonium hydroxide was further added to this aqueous solution to adjust the pH to 4.0, thereby obtaining a raw material solution.

[0061] (4c) Film Formation Preparation The obtained raw material solution 64a was placed in the mist generating source 64. The composite base substrate obtained in (2d) above was placed on the susceptor 70 as the substrate 69, with the orientation layer facing outward, and the heater 68 was operated to raise the temperature inside the quartz tube 67 to 450°C. Next, the flow control valves 63a and 63b were opened to supply dilution gas and carrier gas from the dilution gas source 62a and carrier gas source 62b into the quartz tube 67. After the atmosphere inside the quartz tube 67 was sufficiently replaced with the dilution gas and carrier gas, the flow rates of the dilution gas and carrier gas were adjusted to 0.5 L / min and 1 L / min, respectively. Nitrogen gas was used as the dilution gas and carrier gas.

[0062] (4d) Film Formation Next, the ultrasonic vibrator 66 was vibrated at 2.4 MHz, and the vibration was propagated through the water 65a to the raw material solution 64a, thereby misting the raw material solution 64a and generating mist 64b. This mist 64b was introduced into the quartz tube 67, which is a film formation chamber, by the dilution gas and carrier gas, and reacted within the quartz tube 67, and a film was deposited on the substrate 69 by a CVD reaction on the surface of the substrate 69. In this way, a crystalline semiconductor film (hereinafter referred to as a gallium oxide film) having a thickness of 1.5 μm was obtained, composed of gallium oxide. The film formation time was 120 minutes.

[0063] (5) Evaluation of Gallium Oxide Film (5a) EBSD of Film-Deposited Surface Inverse pole figure orientation mapping of the gallium oxide film surface was performed in a field of view of 50 μm × 50 μm using an SEM (SU-5000, manufactured by Hitachi High-Technologies Corporation) equipped with an electron backscatter diffraction (EBSD) device (Nordlys Nano, manufactured by Oxford Instruments). The conditions for this EBSD measurement were as follows.

[0064] <EBSD measurement conditions> Acceleration voltage: 15 kV Spot intensity: 70 Working distance: 22.5 mm Step size: 0.2 μm Sample tilt angle: 70° Measurement program: Aztec (version 3.3)

[0065] (5b) Yield Rate (Probability of Occurrence of Heterogeneous Phases) 100 composite base substrates were prepared using the same manufacturing method as in (1) and (2) above, and a gallium oxide film was formed under the same conditions as in (4) above to obtain a composite base substrate with a gallium oxide film (hereinafter referred to as wafer). After film formation, EBSD measurement was performed on the surface of the gallium oxide film. As shown in FIG. 4, this EBSD measurement was performed at a total of five locations consisting of the center point of the substrate and four outer periphery points on a line approximately 20 mm away from the center point when two straight lines intersecting at right angles from the center point were drawn on the substrate, plus 20 arbitrarily selected locations (not shown). In this EBSD measurement, the crystalline phase was automatically determined by the device. As a result, wafers in which no heterogeneous phases were detected at any location were determined to be "acceptable," and wafers in which a heterogeneous phase was detected at even one location were determined to be "unacceptable." The value obtained by dividing the number of wafers that were "acceptable" by the total number of wafers (100) was used as an index of the yield rate R. Therefore, the closer the yield rate R is to 1, the lower the probability of occurrence of a different phase.

[0066] As a result of evaluating 100 wafers obtained in this example, a heterogeneous phase was detected in the gallium oxide film of five wafers, and the yield rate R was calculated to be 0.95.

[0067] Example 2: In the above (2b), the second heat treatment of the AD film was performed by annealing it in a nitrogen atmosphere at 1500°C for 4 hours, but in the same manner as in Example 1, a composite base substrate and a gallium oxide film thereon were prepared and evaluated. The details are shown below together with the results.

[0068] The total thickness of the AD film obtained by the first AD film formation was about 60 μm, similar to Example 1. Cross-sectional observation of the heat-treated AD film using a scanning electron microscope revealed that the film thickness of the orientation layer was about 40 μm and the film thickness of the polycrystalline portion was about 20 μm. As in Example 1, surface grinding and polishing were performed to obtain a composite base substrate. The amount of grinding and polishing was about 30 μm, including the polycrystalline portion and the orientation layer, similar to Example 1, and the film thickness of the orientation layer formed on the sapphire substrate was about 30 μm. The inverse pole figure orientation map of the cross section measured using EBSD showed that the orientation layer was oriented in the same direction as the sapphire substrate in both the substrate normal direction and the in-plane direction, indicating that the orientation layer was a biaxially oriented corundum layer.

[0069] The total thickness of the newly formed AD film by the second AD film formation was about 60 μm, similar to Example 1. Cross-sectional observation of the heat-treated AD film using a scanning electron microscope revealed that the film thickness of the orientation layer was about 40 μm and the film thickness of the polycrystalline portion was about 20 μm. As in Example 1, the surface was ground and polished to form a composite base substrate. The grinding and polishing amount, including the polycrystalline portion and the orientation layer, was about 30 μm, and the thickness of the newly formed orientation layer on the composite base substrate (on which an orientation layer with a thickness of about 30 μm was formed in the first film formation) was about 30 μm. The inverse pole figure orientation map of the cross section measured using EBSD showed that the orientation layer was oriented in the same direction as the sapphire substrate in both the substrate normal direction and the in-plane direction, indicating that the orientation layer was a biaxially oriented corundum layer.

[0070] An energy dispersive X-ray analyzer (EDS) detected Cr, O, and Al in the thickness region from the composite substrate surface (the surface on the orientation layer side) to approximately 35 μm. In this thickness region, the difference in the content ratio of each element of Cr and Al between surfaces separated by 2 μm in the thickness direction was less than 1.0 at %, indicating that a Cr-Al oxide layer having a thickness of approximately 35 μm was formed as a compositionally stable region. Cr, O, and Al were also detected in the thickness range of approximately 25 μm below this Cr-Al oxide layer (i.e., a thickness region from the surface to a depth of approximately 35 to 60 μm). Within this depth region of approximately 35 to 60 μm, the content ratio of each element of Cr and Al varied significantly in the thickness direction, with a high Al concentration on the sapphire substrate side and a low Al concentration on the side closer to the compositionally stable region. Within this thickness region of approximately 35 to 60 μm in depth, the difference in Al content between planes separated by 2 μm in the thickness direction was 1.2 to 11.2 at%, confirming that this reaction layer constituted a gradient composition region. Furthermore, only O and Al were detected below this gradient composition region, indicating that it was a sapphire substrate. Furthermore, the results of the inverse pole figure map and the EDS analysis indicated that Cr, Al, and O were present in the biaxially oriented corundum layer, and the biaxially oriented layer was Cr. ２ O ３ and Al ２ O ３ It was suggested that it was a solid solution with

[0071] Finally, the crystal phase of a gallium oxide film formed on an orientation layer under the same conditions as in Example 1 was evaluated, and the yield rate R was 0.95.

[0072] Example 3: In the above (2a), the number of cycles of the second AD film (see the above cited (1a)) was 300. Except for this, a composite base substrate and a gallium oxide film thereon were prepared and evaluated in the same manner as in Example 1. The details are shown below together with the results.

[0073] The total thickness of the AD film obtained by the first AD film formation was about 60 μm, similar to Example 1. Cross-sectional observation of the heat-treated AD film using a scanning electron microscope revealed that the film thickness of the orientation layer was about 40 μm and the film thickness of the polycrystalline portion was about 20 μm. As in Example 1, surface grinding and polishing were performed to obtain a composite base substrate. The amount of grinding and polishing was about 30 μm, including the polycrystalline portion and the orientation layer, similar to Example 1, and the film thickness of the orientation layer formed on the sapphire substrate was about 30 μm. The inverse pole figure orientation map of the cross section measured using EBSD showed that the orientation layer was oriented in the same direction as the sapphire substrate in both the substrate normal direction and the in-plane direction, indicating that the orientation layer was a biaxially oriented corundum layer.

[0074] The total thickness of the newly formed AD film by the second AD film formation was approximately 80 μm. Cross-sectional observation of the heat-treated AD film using a scanning electron microscope revealed that the film thickness of the orientation layer was approximately 50 μm and the film thickness of the polycrystalline portion was approximately 30 μm. As in Example 1, the surface was ground and polished to form a composite base substrate. The total amount of grinding and polishing, including the polycrystalline portion and the orientation layer, was approximately 40 μm, and the thickness of the newly formed orientation layer on the composite base substrate (on which an orientation layer approximately 30 μm thick was formed in the first film formation) was approximately 40 μm. The inverse pole figure orientation map of the cross section measured using EBSD showed that the orientation layer was oriented in the same direction as the sapphire substrate in both the substrate normal direction and the in-plane direction, indicating that the orientation layer was a biaxially oriented corundum layer.

[0075] An energy dispersive X-ray analyzer (EDS) detected Cr, O, and Al in the thickness region from the composite substrate surface (the surface on the orientation layer side) to approximately 45 μm. In this thickness region, the difference in the content ratio of each element of Cr and Al between surfaces separated by 2 μm in the thickness direction was less than 1.0 at %, indicating that a Cr-Al oxide layer having a thickness of approximately 45 μm was formed as a compositionally stable region. Cr, O, and Al were also detected in the thickness range of approximately 25 μm below this Cr-Al oxide layer (i.e., a thickness region from the surface to a depth of approximately 45 to 70 μm). Within this depth region of approximately 45 to 70 μm, the content ratio of each element of Cr and Al varied significantly in the thickness direction, with a high Al concentration on the sapphire substrate side and a low Al concentration on the side closer to the compositionally stable region. Within this thickness region of approximately 45 to 70 μm in depth, the difference in Al content between planes separated by 2 μm in the thickness direction was 1.2 to 11.2 at%, confirming that this reaction layer constituted a gradient composition region. Furthermore, only O and Al were detected below this gradient composition region, indicating that it was a sapphire substrate. Furthermore, the results of the inverse pole figure map and the EDS analysis indicated that Cr, Al, and O were present in the biaxially oriented corundum layer, and the biaxially oriented layer was Cr. ２ O ３ and Al ２ O ３ It was suggested that it was a solid solution with

[0076] Finally, the crystal phase of the gallium oxide film formed on the alignment layer under the same conditions as in Example 1 was evaluated, and the yield rate R was 0.97.

[0077] Example 4 In the above (2a), the number of cycles of the second AD film (see the above cited (1a)) was set to 300, and in the above (2b), the second heat treatment of the AD film was performed by annealing in a nitrogen atmosphere at 1500°C for 4 hours. Except for this, a composite base substrate and a gallium oxide film thereon were prepared and evaluated in the same manner as in Example 1. The details are shown below together with the results.

[0078] The total thickness of the AD film obtained by the first AD film formation was about 60 μm, similar to Example 1. Cross-sectional observation of the heat-treated AD film using a scanning electron microscope revealed that the film thickness of the orientation layer was about 40 μm and the film thickness of the polycrystalline portion was about 20 μm. As in Example 1, surface grinding and polishing were performed to obtain a composite base substrate. The amount of grinding and polishing was about 30 μm, including the polycrystalline portion and the orientation layer, similar to Example 1, and the film thickness of the orientation layer formed on the sapphire substrate was about 30 μm. The inverse pole figure orientation map of the cross section measured using EBSD showed that the orientation layer was oriented in the same direction as the sapphire substrate in both the substrate normal direction and the in-plane direction, indicating that the orientation layer was a biaxially oriented corundum layer.

[0079] The total thickness of the newly formed AD film by the second AD film formation was approximately 80 μm. Cross-sectional observation of the heat-treated AD film using a scanning electron microscope revealed that the film thickness of the orientation layer was approximately 60 μm and the film thickness of the polycrystalline portion was approximately 20 μm. As in Example 1, the surface was ground and polished to form a composite base substrate. The total amount of grinding and polishing, including the polycrystalline portion and the orientation layer, was approximately 30 μm, and the thickness of the newly formed orientation layer on the composite base substrate (on which an orientation layer approximately 30 μm thick was formed in the first film formation) was approximately 50 μm. The inverse pole figure orientation map of the cross section measured using EBSD showed that the orientation layer was oriented in the same direction as the sapphire substrate in both the substrate normal direction and the in-plane direction, indicating that the orientation layer was a biaxially oriented corundum layer.

[0080] An energy dispersive X-ray analyzer (EDS) detected Cr, O, and Al in the thickness region from the composite substrate surface (the surface on the orientation layer side) to approximately 55 μm. In this thickness region, the difference in the content ratio of each element of Cr and Al between surfaces separated by 2 μm in the thickness direction was less than 1.0 at %, indicating that a Cr-Al oxide layer having a thickness of approximately 55 μm was formed as a compositionally stable region. Cr, O, and Al were also detected in the thickness range of approximately 25 μm below this Cr-Al oxide layer (i.e., a thickness region from the surface to a depth of approximately 55 to 80 μm). Within this depth region of approximately 55 to 80 μm, the content ratio of each element of Cr and Al varied significantly in the thickness direction, with a high Al concentration on the sapphire substrate side and a low Al concentration on the side closer to the compositionally stable region. Within this thickness region of approximately 55 to 80 μm in depth, the difference in Al content between planes separated by 2 μm in the thickness direction was 1.2 to 11.2 at%, confirming that this reaction layer constituted a gradient composition region. Furthermore, only O and Al were detected below this gradient composition region, indicating that it was a sapphire substrate. Furthermore, the results of the inverse pole figure map and the EDS analysis indicated that Cr, Al, and O were present in the biaxially oriented corundum layer, and the biaxially oriented layer was Cr. ２ O ３ and Al ２ O ３ It was suggested that it was a solid solution with

[0081] Finally, the crystal phase of the gallium oxide film formed on the alignment layer under the same conditions as in Example 1 was evaluated, and the yield rate R was found to be 0.99.

[0082] Example 5: A composite base substrate and a gallium oxide film thereon were prepared and evaluated in the same manner as in Example 1, except that the third and subsequent AD depositions were performed and the sapphire substrate was removed to obtain a freestanding substrate consisting of an orientation layer. The details are shown below along with the results.

[0083] The total thickness of the AD film obtained by the first AD film formation was about 60 μm, similar to Example 1. Cross-sectional observation of the heat-treated AD film using a scanning electron microscope revealed that the film thickness of the orientation layer was about 40 μm and the film thickness of the polycrystalline portion was about 20 μm. As in Example 1, surface grinding and polishing were performed to obtain a composite base substrate. The amount of grinding and polishing was about 30 μm, including the polycrystalline portion and the orientation layer, similar to Example 1, and the film thickness of the orientation layer formed on the sapphire substrate was about 30 μm. The inverse pole figure orientation map of the cross section measured using EBSD showed that the orientation layer was oriented in the same direction as the sapphire substrate in both the substrate normal direction and the in-plane direction, indicating that the orientation layer was a biaxially oriented corundum layer.

[0084] The total thickness of the newly formed AD film by the second AD film formation was approximately 60 μm. Cross-sectional observation of the heat-treated AD film using a scanning electron microscope revealed that the film thickness of the orientation layer was approximately 30 μm and the film thickness of the polycrystalline portion was approximately 30 μm. As in Example 1, the surface was ground and polished to form a composite base substrate. The total amount of grinding and polishing, including the polycrystalline portion and the orientation layer, was approximately 35 μm, and the thickness of the newly formed orientation layer on the composite base substrate (on which an orientation layer approximately 30 μm thick was formed in the first film formation) was approximately 25 μm. The inverse pole figure orientation map of the cross section measured using EBSD showed that the orientation layer was oriented in the same direction as the sapphire substrate in both the substrate normal direction and the in-plane direction, indicating that the orientation layer was a biaxially oriented corundum layer.

[0085] As the third and subsequent AD film formation processes, the series of steps (2a), (2b), and (2d) for thickening (i.e., AD film formation, annealing, and grinding and polishing) were repeated 15 more times. As a result, the thickness of the composite base substrate after the final polishing was 0.43 mm. In this way, a composite base substrate with a thickened orientation layer was obtained. The surface on which the AD film was formed will be referred to as the "surface."

[0086] (Self-standing of the orientation layer) The obtained composite base substrate was fixed to a ceramic surface plate, and the surface (back surface) opposite to the surface (front surface) derived from the AD film, i.e., the surface on the sapphire substrate side, was ground using a grindstone to remove the sapphire substrate. Then, the surface of the orientation layer on the side from which the sapphire substrate was removed was ground to #2000 using a grindstone to flatten the plate surface. Next, the plate surface was smoothed by lapping using diamond abrasive grains. At this time, the lapping was performed while gradually reducing the size of the diamond abrasive grains from 3 μm to 0.5 μm, thereby improving the flatness of the plate surface. The amount of grinding and polishing including sapphire was 430 μm, and the thickness of the substrate after polishing was 0.43 mm. The obtained substrate was a free-standing substrate composed only of the orientation layer.

[0087] An energy dispersive X-ray analyzer (EDS) detected Cr, O, and Al in a thickness region extending from the freestanding substrate surface (the surface not in contact with the sapphire substrate) to approximately 280 μm. In this thickness region, the difference in the content ratios of Cr and Al between surfaces separated by 2 μm in the thickness direction was less than 1.0 at %, indicating that a Cr-Al oxide layer having a thickness of approximately 280 μm was formed as a compositionally stable region. Cr, O, and Al were also detected in a thickness range of approximately 150 μm below this Cr-Al oxide layer (i.e., a thickness region extending from the surface to a depth of approximately 280 to 430 μm). Within this thickness region of approximately 280 to 430 μm, the content ratios of Cr and Al varied significantly in the thickness direction, with a higher Al concentration on the sapphire substrate side and a lower Al concentration on the side closer to the compositionally stable region. In this thickness region of approximately 280 to 430 μm deep, the difference in Al content between the surfaces separated by 2 μm in the thickness direction was 1.2 to 11.2 at%, which confirmed that this reaction layer formed a gradient composition region. Furthermore, the results of the inverse pole figure map and the EDS analysis indicated that Cr, Al, and O were present in the biaxially oriented corundum layer, and the biaxially oriented layer was Cr. ２ O ３ and Al ２ O ３ It was suggested that it was a solid solution with

[0088] Finally, the crystal phase of a gallium oxide film formed on a free-standing substrate (orientation layer) under the same conditions as in Example 1 was evaluated, and the yield rate R was found to be 0.97.

[0089] Example 6 (Comparative) A composite base substrate and a gallium oxide film thereon were prepared and evaluated in the same manner as in Example 1, except that in (2b) above, the second heat treatment of the AD film was performed by annealing it in a nitrogen atmosphere at 1500°C for 1 hour. The details are shown below along with the results.

[0090] The total thickness of the AD film obtained by the first AD film formation was about 60 μm, similar to Example 1. Cross-sectional observation of the heat-treated AD film using a scanning electron microscope revealed that the film thickness of the orientation layer was about 40 μm and the film thickness of the polycrystalline portion was about 20 μm. As in Example 1, surface grinding and polishing were performed to obtain a composite base substrate. The amount of grinding and polishing was about 30 μm, including the polycrystalline portion and the orientation layer, similar to Example 1, and the film thickness of the orientation layer formed on the sapphire substrate was about 30 μm. The inverse pole figure orientation map of the cross section measured using EBSD showed that the orientation layer was oriented in the same direction as the sapphire substrate in both the substrate normal direction and the in-plane direction, indicating that the orientation layer was a biaxially oriented corundum layer.

[0091] The total thickness of the AD film obtained by the second AD film formation was approximately 60 μm. Cross-sectional observation of the heat-treated AD film using a scanning electron microscope revealed that the film thickness of the orientation layer was approximately 20 μm and the film thickness of the polycrystalline portion was approximately 40 μm. As in Example 1, the surface was ground and polished to form a composite base substrate. The total amount of grinding and polishing, including the polycrystalline portion and the orientation layer, was approximately 45 μm, and the thickness of the newly formed orientation layer on the composite base substrate (on which an orientation layer approximately 30 μm thick was formed in the first film formation) was approximately 15 μm. The inverse pole figure orientation map of the cross section measured using EBSD showed that the orientation layer was oriented in the same direction as the sapphire substrate in both the substrate normal direction and the in-plane direction, indicating that the orientation layer was a biaxially oriented corundum layer.

[0092] An energy dispersive X-ray analyzer (EDS) detected Cr, O, and Al in the thickness region from the composite substrate surface (the surface on the orientation layer side) to approximately 20 μm. In this thickness region, the difference in the content ratio of each element of Cr and Al between surfaces separated by 2 μm in the thickness direction was less than 1.0 at %, indicating that a Cr-Al oxide layer having a thickness of approximately 20 μm was formed as a compositionally stable region. Cr, O, and Al were also detected in the thickness range of approximately 25 μm below this Cr-Al oxide layer (i.e., a thickness region from the surface to a depth of approximately 20 to 45 μm). Within this depth region of approximately 20 to 45 μm, the content ratio of each element of Cr and Al varied significantly in the thickness direction, with a high Al concentration on the sapphire substrate side and a low Al concentration on the side closer to the compositionally stable region. Within this thickness region of approximately 20 to 45 μm in depth, the difference in Al content between planes separated by 2 μm in the thickness direction was 1.2 to 11.2 at%, confirming that this reaction layer constituted a gradient composition region. Furthermore, only O and Al were detected below this gradient composition region, indicating that it was a sapphire substrate. Furthermore, the results of the inverse pole figure map and the EDS analysis indicated that Cr, Al, and O were present in the biaxially oriented corundum layer, and the biaxially oriented layer was Cr. ２ O ３ and Al ２ O ３ It was suggested that it was a solid solution with

[0093] Finally, the crystal phase of the gallium oxide film formed on the alignment layer under the same conditions as in Example 1 was evaluated, and the yield rate R was found to be 0.91.

[0094] Results Table 2 shows the results obtained in Examples 1-6.

[0095]

Claims

1. A base substrate having an orientation layer used for crystal growth of a semiconductor film, wherein the semiconductor film is α-Ga ２ O ３ , α-Ga ２ O ３ system solid solution, α-Cr ２ O ３ , and α-Cr ２ O ３ a substrate comprising at least one material selected from the group consisting of crystalline solid solutions, wherein the surface of the orientation layer used for the crystal growth is made of a material having a corundum-type crystal structure with an a-axis length and / or a c-axis length greater than those of sapphire, and the orientation layer has a stable composition region in which the composition is stable in the thickness direction and a gradient composition region in which the composition changes in the thickness direction, and the stable composition region is thicker than the gradient composition region.

2. The base substrate according to claim 1, wherein the thickness of said stable composition region is at least 1.5 times the thickness of said gradient composition region.

3. The base substrate according to claim 1, wherein the thickness of said stable composition region is at least 2.0 times the thickness of said gradient composition region.

4. The base substrate according to any one of claims 1 to 3, wherein the thickness of the composition stable region is 30 µm or more.

5. The compositional stability region is α-Cr ２ O ３ The base substrate according to any one of claims 1 to 3, comprising:

6. A base substrate according to any one of claims 1 to 3, wherein the stable composition region is located near the surface used for the crystal growth, and the gradient composition region is located far from the surface used for the crystal growth.

7. The gradient composition region is α-Cr ２ O ３ and α-Al ２ O ３ The base substrate according to any one of claims 1 to 3, which is composed of a solid solution containing 8. The starting substrate according to claim 7, wherein in said gradient composition region, the Al concentration decreases in the thickness direction toward said stable composition region.

9. The base substrate according to any one of claims 1 to 3, wherein the alignment layer is a heteroepitaxially grown layer.

10. The base substrate according to any one of claims 1 to 3, further comprising a support substrate on the side opposite the surface of the alignment layer.

11. The base substrate according to claim 10, wherein the support substrate is a sapphire substrate.

12. The base substrate according to any one of claims 1 to 3, wherein the orientation layer is a heteroepitaxially grown layer based on a sapphire substrate.

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