阅读说明：本技术 基底基板 (Base substrate ) 是由 福井宏史 渡边守道 吉川润 于 2020-02-18 设计创作，主要内容包括：本发明提供高品质的基底基板,其具备用于13族元素的氮化物或氧化物结晶生长的取向层,该取向层中的结晶缺陷(位错)显著降低。该基底基板具备用于13族元素的氮化物或氧化物结晶生长的取向层,取向层的用于结晶生长一侧的表面由具有a轴长度和/或c轴长度比蓝宝石的a轴长度和/或c轴长度大的刚玉型结晶结构的材料构成,且在取向层中存在多个气孔。(The present invention provides a high-quality base substrate which is provided with an orientation layer for crystal growth of a nitride or oxide of a group 13 element, and crystal defects (dislocations) in the orientation layer are significantly reduced. The base substrate is provided with an orientation layer for crystal growth of a nitride or oxide of a group 13 element, the surface of the orientation layer on the side for crystal growth is made of a material having a corundum-type crystal structure in which the a-axis length and/or the c-axis length are greater than those of sapphire, and a plurality of pores are present in the orientation layer.)

1. A base substrate having an orientation layer for crystal growth of a nitride or oxide of a group 13 element,

the base substrate is characterized in that,

the surface of the orientation layer on the side for the crystal growth is composed of a material having a corundum-type crystal structure in which the a-axis length and/or the c-axis length are greater than those of sapphire,

and a plurality of air holes are present in the alignment layer.

2. The base substrate of claim 1,

in the alignment layer, the number of pores in a surface layer region including the surface is reduced as compared with the number of pores in a deep layer region away from the surface.

3. The base substrate according to claim 1 or 2,

for the a-axis length and/or c-axis length of the alignment layer, the a-axis length and/or c-axis length at the surface of the alignment layer and the a-axis length and/or c-axis length at the back surface of the alignment layer are different.

4. The base substrate according to any one of claims 1to 3,

the entire alignment layer is made of a material having the corundum-type crystal structure.

5. The base substrate according to any one of claims 1to 4,

the material having the corundum-type crystal structure includes: selected from the group consisting of alpha-Cr₂O₃、α－Fe₂O₃、α－Ti₂O₃、α－V₂O₃、α－Ga₂O₃、α－In₂O₃And alpha-Rh₂O₃A material selected from the group consisting of₂O₃、α－Cr₂O₃、α－Fe₂O₃、α－Ti₂O₃、α－V₂O₃、α－Ga₂O₃、α－In₂O₃And alpha-Rh₂O₃2 or more solid solutions in the group.

6. The base substrate according to any one of claims 1to 5,

the material with the corundum type crystal structure comprises alpha-Cr₂O₃Or containing alpha-Cr₂O₃And solid solutions of dissimilar materials.

7. The base substrate according to any one of claims 1to 5,

material bag with corundum type crystal structureComprises the following components: selected from the group consisting of alpha-Cr₂O₃、α－Fe₂O₃And alpha-Ti₂O₃1 or more kinds of materials in the group, or a solid solution thereof, or contains alpha-Al₂O₃And is selected from the group consisting of alpha-Cr₂O₃、α－Fe₂O₃And alpha-Ti₂O₃A solid solution of 1 or more materials in the group.

8. The base substrate according to any one of claims 1to 7,

the material having the corundum-type crystal structure at the surface has an a-axis length greater than that of the materialAnd is thatThe following.

9. The base substrate according to any one of claims 1to 8,

there is a gradient composition region in the orientation layer where the composition changes in the thickness direction.

10. The base substrate of claim 9,

the thickness of the gradient composition region is 20 [ mu ] m or more.

11. The base substrate according to any one of claims 1to 10,

the alignment layer has: a composition stabilizing region which is located in a position close to the surface and is stable in composition in a thickness direction; and a gradient composition region which is located away from the surface and whose composition varies in a thickness direction.

12. The base substrate according to any one of claims 9 to 11,

the gradient composition region is composed of a-Cr₂O₃And alpha-Al₂O₃The solid solution of (1).

13. The base substrate according to any one of claims 9 to 12,

in the gradient composition region, the Al concentration decreases toward the composition stable region in the thickness direction.

14. The base substrate according to any one of claims 1to 13,

the orientation layer is a heteroepitaxial growth layer.

15. The base substrate according to any one of claims 1to 14,

the alignment layer is further provided with a support substrate on the side opposite to the surface.

16. The base substrate of claim 15,

the supporting substrate is a sapphire substrate.

Technical Field

The present invention relates to a base substrate for crystal growth of a nitride or oxide of a group 13 element.

Background

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

In addition, a corundum-phase type of α -gallium oxide (α -Ga) having the same crystal structure as sapphire is actively performed₂O₃) Research and development of (1). In fact, alpha-Ga₂O₃The band gap of (A) is as high as 5.3eV, and is expected as a material for power semiconductor elements. For example, patent document 1 (jp-a 2014-72533) relates to a semiconductor device formed of 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, and discloses that α -Ga is formed on a sapphire substrate₂O₃A film is used as an example of the semiconductor layer. Further, patent document 2 (japanese patent application laid-open No. 2016-: in the examples, a metastable phase, that is, α -Ga having a corundum structure was formed on a c-plane sapphire substrate, and the examples include an n-type semiconductor layer containing a crystalline oxide semiconductor having a corundum structure as a main component, a p-type semiconductor layer containing an inorganic compound having a hexagonal crystal structure as a main component, and an electrode₂O₃The film is used as an n-type semiconductor layer and forms alpha-Rh having a hexagonal crystal structure₂O₃The film was used as a p-type semiconductor layer to fabricate a diode.

However, it is known that: in these semiconductor devices, good characteristics can be obtained with fewer crystal defects in the material. In particular, power semiconductors are required to have excellent withstand voltage characteristics, and therefore, it is desired to reduce crystal defectsAnd (5) sinking. This is because: the number of crystal defects influences the dielectric breakdown electric field characteristics. However, for GaN, alpha-Ga₂O₃A single crystal substrate having few crystal defects has not yet been put to practical use, and is generally formed by heteroepitaxial growth on a sapphire substrate having a lattice constant different from that of these materials. Therefore, crystal defects are easily generated due to the difference in lattice constant with sapphire. For example, a-Ga is doped on the c-plane of sapphire₂O₃In the case of film formation, sapphire (. alpha. -Al)₂O₃) A axis length ofAnd alpha-Ga₂O₃A axis length ofThe difference is about 5%, which difference constitutes the main cause of crystal defects.

As a method for reducing crystal defects by reducing the difference in lattice constant between the semiconductor layer and the semiconductor layer, there are reported: in the reaction of alpha-Ga₂O₃When forming a film, the film is formed on sapphire and alpha-Ga₂O₃Buffer layers are formed between the layers, whereby defects are reduced. For example, non-patent document 1(Applied Physics Express, vol.9, pages 071101-1to 071101-4) shows an example of a method of mixing sapphire and α -Ga₂O₃Introduction of (Al) between layers_x、Ga_1－x)₂O₃A layer (x is 0.2 to 0.9) as a buffer layer such that the edge dislocation and the screw dislocation are 3 × 10, respectively⁸/cm²And 6X 10⁸/cm²。

Documents of the prior art

Patent document

Patent document 1: japanese patent laid-open No. 2014-72533

Patent document 2: japanese patent laid-open publication No. 2016-25256

Non-patent document

Non-patent document 1: riena Jinno et Al, "Reduction in edge distribution density in rustum-structured. alpha. -Ga2O3 layers on sapphire substrates with quality-graded. alpha. - (Al, Ga)2O3 buffers", Applied Physics Express, Japan, The Japan Society of Applied Physics, June 1,2016, vol.9, pages 071101-1to 071101-4

Disclosure of Invention

However, the method of introducing a buffer layer disclosed in non-patent document 1 is insufficient for application to a power semiconductor requiring high dielectric breakdown electric field characteristics, and further reduction of crystal defects is desired.

The present inventors have recently found that a high-quality base substrate in which crystal defects (dislocations) in an alignment layer are significantly reduced can be provided by forming the surface on the side where a nitride or oxide crystal of a group 13 element is to be grown from a material having a corundum-type crystal structure in which the a-axis length and/or c-axis length are greater than those of sapphire and by providing a plurality of pores in the alignment layer formed from the material. Further, it is found that an excellent semiconductor layer can be formed by using such a high-quality base substrate.

Accordingly, an object of the present invention is to provide a high-quality base substrate which includes an alignment layer for crystal growth of a nitride or oxide of a group 13 element and in which crystal defects (dislocations) in the alignment layer are significantly reduced.

According to one aspect of the present invention, there is provided a base substrate including an orientation layer for crystal growth of a nitride or oxide of a group 13 element, wherein a surface of the orientation layer on a side of the crystal growth is made of a material having a corundum-type crystal structure in which an a-axis length and/or a c-axis length are greater than an a-axis length and/or a c-axis length of sapphire, and a plurality of pores are present in the orientation layer.

Drawings

Fig. 1 is a schematic diagram showing a structure of an Aerosol Deposition (AD) apparatus.

FIG. 2 is a schematic sectional view showing the structure of an atomizing CVD apparatus.

Detailed Description

Base substrate

According to the inventionThe base substrate is: a base substrate is provided with an orientation layer for crystal growth of a nitride or oxide of a group 13 element. That is, the base substrate is used for crystal growth of a semiconductor layer composed of a nitride or oxide of a group 13 element on the alignment layer. Here, the group 13 element is a group 13 element In the periodic table defined by IUPAC (international union of pure and applied chemistry), and specifically, is any one of boron (B), aluminum (Al), gallium (Ga), indium (In), thallium (Tl), and (Nh). Typical examples of the group 13 element nitride and oxide include gallium nitride (GaN) and α -gallium oxide (α -Ga)₂O₃)。

The alignment layer has a structure in which crystal orientations are substantially uniform in a substantially normal direction. By adopting such a configuration, a semiconductor layer having excellent quality, particularly excellent orientation, can be formed thereon. That is, when the semiconductor layer is formed on the alignment layer, the crystal orientation of the semiconductor layer substantially follows the crystal orientation of the alignment layer. Therefore, the semiconductor film can be an alignment film by configuring the base substrate to include the alignment layer. The orientation layer may be polycrystalline, a mosaic crystal (a collection of crystals having slightly different crystal orientations), or a single crystal. When the alignment layer is polycrystalline, it is preferably a biaxially alignment layer in which the twist direction (i.e., the rotation direction about the substrate normal line to which the orientation is given substantially perpendicular to the substrate surface) is substantially the same.

The surface of the orientation layer for the crystal growth side of the nitride or oxide of the group 13 element (hereinafter, sometimes simply referred to as "surface" or "orientation layer surface") is composed of sapphire having a ratio of the a-axis length and/or the c-axis length to the α -Al (α -Al)₂O₃) The a-axis length and/or the c-axis length of (2) is large. By controlling the lattice constant of the alignment layer in this manner, crystal defects in the semiconductor layer formed thereon can be significantly reduced. That is, the lattice constant of the oxide of the group 13 element constituting the semiconductor layer is larger than that of sapphire (α -Al)₂O₃) Has a large lattice constant. Actually, as shown in Table 1 below, α -Ga which is an oxide of a group 13 element₂O₃Has a lattice constant (a-axis length and c-axis length) ratio of alpha-Al₂O₃Crystal lattice ofThe constant is large. Therefore, by controlling the lattice constant of the alignment layer to be a-Al ratio₂O₃Is so large that, when the semiconductor layer is formed on the alignment layer, the mismatch of lattice constants between the semiconductor layer and the alignment layer is suppressed, and as a result, crystal defects in the semiconductor layer are reduced. For example, in the c-plane of sapphire, α -Ga is doped₂O₃In the case of film formation, α -Ga₂O₃Has a lattice length (a-axis length) substantially larger than that of sapphire, with a mismatch of about 5%. Therefore, by controlling the a-axis length of the alignment layer to the ratio α -Al₂O₃Has a large a-axis length, so that alpha-Ga₂O₃The crystal defects in the layer (b) are reduced. In m-plane of sapphire, alpha-Ga₂O₃In the case of film formation, α -Ga₂O₃Has a lattice length (c-axis length) substantially larger than that of sapphire, with a mismatch of about 3%. Therefore, by controlling the c-axis length of the alignment layer to the ratio α -Al₂O₃Has a large c-axis length of (a) so that alpha-Ga₂O₃The crystal defects in the layer (b) are reduced. In addition, the mismatch of lattice constants between GaN, which is a nitride of a group 13 element, and sapphire is also large. When GaN is formed on the c-plane of sapphire, the lattice length (a-axis length) of GaN in the in-plane direction is substantially larger than the lattice length of sapphire, and has a mismatch of about 16%. Therefore, by controlling the a-axis length of the alignment layer to the ratio α -Al₂O₃The a-axis length of (2) is large, so that crystal defects in the GaN layer are reduced. On the other hand, if these semiconductor layers are formed directly on a sapphire substrate, stress is generated in the semiconductor layers due to mismatch of lattice constants, and a large number of crystal defects may be generated in the semiconductor layers.

[ Table 1]

Table 1: lattice constant of group 13 oxides

With respect to the base substrate of the present invention,a plurality of pores are present in the alignment layer. The presence of these multiple air holes contributes to a significant reduction in crystal defects (dislocations) in the alignment layer. The mechanism is not yet determined, but it is thought that dislocations disappear by being taken up by the pores. The number of pores in the alignment layer is preferably 1.0X 10⁴～1.0×10⁹Per cm²More preferably 1.0X 10⁴～1.0×10⁷Per cm². In particular, in the orientation layer, it is preferable that the number Ns of pores in the surface layer region including the surface for crystal growth be reduced as compared with the number Nd of pores in the deep layer region away from the surface, in order to more effectively reduce crystal defects (dislocations). The number of pores Nd and Ns can be determined by observing the cross section of the alignment layer at 1cm per unit cross section²Number of pores (number/cm)²) And (4) showing. The surface layer region and the deep layer region may be relatively determined from the viewpoint of thickness and pore distribution, and need not be determined uniformly with a thickness as an absolute value, for example, the surface layer region is a region 20% thick from the surface with respect to the thickness of the alignment layer, and the deep layer region is a region 80% thick therebelow. For example, when the thickness of the alignment layer is 40 μm, the surface layer region is a region having a thickness of 8 μm from the surface of the alignment layer, and the deep layer region is a region having a thickness of 8 to 40 μm. The ratio of the number Nd of pores in the deep region to the number Ns of pores in the surface region including the surface (i.e., Nd/Ns) is preferably greater than 1.0, more preferably 2.0 or more, and there is no particular upper limit. Within the above range, dislocations are significantly reduced, and preferably can be almost or completely eliminated. On the other hand, the number of pores Ns in the surface layer region is preferably small. Therefore, there is no upper limit to the ratio of the number Nd of pores in the deep layer region to the number Ns of pores in the surface layer region including the surface (i.e., Nd/Ns), but it is actually preferably less than 60.

Preferably, the entire alignment layer is made of a material having a corundum-type crystal structure. This also provides an effect of reducing crystal defects in the alignment layer and the semiconductor layer. The alignment layer is preferably formed on the surface of the sapphire substrate. alpha-Al constituting sapphire substrate₂O₃Has a corundum-type crystal structure, and an alignment layer is formed of a material having a corundum-type crystal structureThe crystal structure of the material can be made the same as that of a sapphire substrate, and as a result, crystal defects due to mismatch of the crystal structure in the alignment layer are suppressed. In this regard, if the crystal defects in the alignment layer are reduced, the crystal defects in the semiconductor layer formed thereon are also reduced, which is preferable. This is because: if a large number of crystal defects are present in the alignment layer, the crystal defects are also inherited in the semiconductor layer formed thereon, and as a result, crystal defects are also generated in the semiconductor layer. Specific examples of the material having a corundum-type crystal structure include: alpha-Al₂O₃、α－Cr₂O₃、α－Fe₂O₃、α－Ti₂O₃、α－Ga₂O₃、α－V₂O₃、α－Ir₂O₃、α－Rh₂O₃、α－In₂O₃Or a solid solution (mixed crystal) thereof, and the like.

The material having a corundum-type crystal structure constituting the alignment layer preferably contains: selected from the group consisting of alpha-Cr₂O₃、α－Fe₂O₃、α－Ti₂O₃、α－V₂O₃、α－Ga₂O₃、α－In₂O₃And alpha-Rh₂O₃A material selected from the group consisting of₂O₃、α－Cr₂O₃、α－Fe₂O₃、α－Ti₂O₃、α－V₂O₃、α－Ga₂O₃、α－In₂O₃And alpha-Rh₂O₃2 or more solid solutions in the group, particularly preferably containing α -Cr₂O₃Or containing alpha-Cr₂O₃A material of solid solution with a dissimilar material. As shown in Table 1 above, these materials have a ratio of alpha-Al₂O₃A large lattice constant (a-axis length and/or c-axis length) and a lattice constant similar to that of a nitride or oxide of group 13 element constituting the semiconductor layer, i.e., GaN or α -Ga₂O₃Are relatively close in lattice constant, and therefore, canCrystal defects in the semiconductor layer are effectively suppressed. The solid solution may be either a substitutional solid solution or an invasive solid solution, but a substitutional solid solution is preferable. However, although the alignment layer is made of a material having a corundum-type crystal structure, it is not excluded that the alignment layer contains other trace components.

The material having a corundum-type crystal structure in the surface of the orientation layer on the side for crystal growth has an a-axis length greater than that of the materialAnd is thatHereinafter, more preferred isFurther preferred isIn addition, the c-axis length of the material having a corundum type crystal structure in the surface of the orientation layer on the side for crystal growth is larger thanAnd is thatHereinafter, more preferred isFurther preferred isBy controlling the a-axis length and/or the c-axis length of the surface of the alignment layer within the above range, the surface of the alignment layer can be made to be compatible with a nitride or oxide of a group 13 element constituting the semiconductor layer, particularly α -Ga₂O₃Are close in lattice constant (a-axis length and/or c-axis length).

The thickness of the alignment layer is preferably 10 μm or more, more preferably 40 μmm is more than m. The upper limit of the thickness is not particularly limited, but is typically 100 μm or less. The crystal defect density in the surface of the orientation layer is preferably 1.0X 10⁸/cm²Hereinafter, more preferably 1.0 × 10⁶/cm²Hereinafter, more preferably 4.0 × 10³/cm²Hereinafter, there is no particular lower limit. In the present specification, the crystal defect means: threading edge dislocations, threading screw dislocations, threading mixed dislocations and basal plane dislocations, and the crystal defect density is the total of the respective dislocation densities. In addition, basal plane dislocations pose a problem when the base substrate including the alignment layer has an off-angle, and do not pose a problem when there is no off-angle because they are not exposed to the surface of the alignment layer. For example, if the material contains threading edge dislocations of 3X 10⁸/cm²Threading screw dislocation 6X 10⁸/cm²4 x 10 threading mixed dislocation⁸/cm²Then, the crystal defect density is 1.3X 10⁹/cm²。

In the case where the alignment layer is formed of a sapphire substrate, it is preferable that a gradient composition region in which the composition changes in the thickness direction exists in the alignment layer. For example, the gradient composition region preferably has a region (gradient composition region) composed of a material selected from the group consisting of α -Cr₂O₃、α－Fe₂O₃、α－Ti₂O₃、α－V₂O₃、α－Ga₂O₃、α－In₂O₃And alpha-Rh₂O₃1 or more materials of the group and alpha-Al₂O₃And has alpha-Al₂O₃The gradient composition in which the amount of solid solution decreases from the sapphire substrate side toward the alignment layer surface side. The gradient composition region is particularly preferably composed of a-Al-containing material₂O₃、α－Cr₂O₃And alpha-Ti₂O₃Or a solid solution containing alpha-Al₂O₃、α－Cr₂O₃、α－Ti₂O₃And alpha-Fe₂O₃The solid solution of (1). That is, the alignment layer is preferably formed on the surface of the sapphire substrate and has a structure corresponding to the sapphire substrateStress due to a difference in lattice constant (a-axis length and/or c-axis length) between the plate and the alignment layer is relaxed, and crystal defects are suppressed. In other words, it is preferable that the a-axis length and/or the c-axis length are different between the surface and the back surface of the alignment layer, and it is more preferable that the a-axis length and/or the c-axis length of the surface of the alignment layer is greater than the a-axis length and/or the c-axis length of the back surface of the alignment layer. By adopting such a structure, the lattice constant changes in the thickness direction regardless of whether the orientation layer is a single crystal or a mosaic crystal, or a biaxial orientation layer. Therefore, a single crystal, a mosaic crystal, or a biaxially oriented layer can be formed on substrates having different lattice constants in a state where stress is relaxed. In the production of a base substrate described later, a gradient composition region such as this can be formed by heat-treating the sapphire substrate and the alignment precursor layer at a temperature of 1000 ℃. That is, if the heat treatment is performed at such a high temperature, a reaction occurs at the interface between the sapphire substrate and the alignment precursor layer, and the Al component in the sapphire substrate diffuses into the alignment precursor layer or the component in the alignment precursor layer diffuses into the sapphire substrate. As a result, alpha-Al is formed₂O₃A gradient composition region in which the amount of solid solution changes in the thickness direction. The gradient composition region is preferably thick because a thicker region tends to relax stress due to a difference in lattice constant. Therefore, the thickness of the gradient composition region is preferably 5 μm or more, and more preferably 20 μm or more. The upper limit of the thickness is not particularly limited, but is typically 100 μm or less. Further, by performing the heat treatment at 1000 ℃ or higher, the crystal defects reaching the surface of the alignment layer can be effectively reduced. The reason is not clear, but is considered to be because: the cancellation of the crystal defects from each other is promoted by the heat treatment at high temperature.

According to a more preferred aspect of the present invention, the alignment layer has: a composition stabilizing region which is located at a position close to the surface and in which the composition is stable in the thickness direction; and a gradient composition region which is located away from the surface and whose composition varies in the thickness direction. The compositional stability region refers to: a region where the content ratio of each metal element changes by less than 1.0 at%, and the gradient composition region means: variation of content ratio of each metal elementIs in the region of 1.0 at% or more. For example, the composition stable region and the gradient composition region may be determined as follows. First, a cross-sectional sample of the alignment layer was prepared, energy dispersive X-ray analysis (EDS) was performed at any 10 points in the vicinity of the surface of the alignment layer, and the average value of the content ratios (at%) of the detected metal elements was calculated. Next, EDS analysis was performed at any 10 points 2 μm away from the surface in the thickness direction, and the average value of the content ratio (at%) at 2 μm in thickness was calculated. At this time, the region from the surface to the thickness of 2 μm can be classified into either a composition stable region or a gradient composition region depending on whether the difference in the content ratio of at least 1 of all the detected metal elements is less than 1.0 at% or more by comparing the average values of the content ratios at the surface and the thickness of 2 μm. By the same method, the average value of the content ratio of the metal element is calculated every 2 μm in the thickness direction, and the assignment of the region between the positions can be determined by comparing the average value of the content ratio of the metal element between a certain thickness position and a position 2 μm away from the thickness position in the thickness direction. For example, for a region between a position with a thickness of 24 μm and a position with a thickness of 26 μm from the surface, the attribution can be determined by calculating and comparing the average values of the metal element content ratios at the respective positions. Then, for example, in the case where Al is contained in the gradient composition region, it is more preferable that the Al concentration decreases in the thickness direction toward the composition stable region. In this embodiment, the material having a corundum-type crystal structure preferably contains: selected from the group consisting of alpha-Cr₂O₃、α－Fe₂O₃And alpha-Ti₂O₃1 or more kinds of materials in the group, or a solid solution thereof, or contains alpha-Al₂O₃And is selected from the group consisting of alpha-Cr₂O₃、α－Fe₂O₃And alpha-Ti₂O₃A solid solution of 1 or more materials in the group. Particularly preferably, the gradient composition region consists of a material comprising alpha-Cr₂O₃And alpha-Al₂O₃The solid solution of (1). In addition, the composition stable region is a lattice constant (a-axis length and/or c-axis length) ratio alpha-Al₂O₃Lattice constant ofThe material can be large, can be a solid solution among a plurality of corundum-type materials, and can also be a single phase of the corundum-type material. That is, the material constituting the composition stable region is preferably: (i) selected from the group consisting of alpha-Cr₂O₃、α－Fe₂O₃、α－Ti₂O₃、α－V₂O₃、α－Ga₂O₃、α－In₂O₃And alpha-Rh₂O₃1 material selected from the group consisting of (ii) a material comprising a metal selected from the group consisting of alpha-Cr₂O₃、α－Fe₂O₃、α－Ti₂O₃、α－V₂O₃、α－Ga₂O₃、α－In₂O₃And alpha-Rh₂O₃(ii) a solid solution of 1 or more materials selected from the group consisting of (i), (ii) and (iii) a solid solution of a metal selected from the group consisting of₂O₃、α－Fe₂O₃、α－Ti₂O₃、α－V₂O₃、α－Ga₂O₃、α－In₂O₃And alpha-Rh₂O₃1 or more materials of the group and alpha-Al₂O₃A solid solution of (2). From the viewpoint of controlling the lattice constant, it is preferable that α -Al is not contained in the above-mentioned material₂O₃Or a solid solution thereof.

The material constituting the alignment layer is not particularly limited as long as it has alignment properties with respect to the surface of the base substrate, and examples thereof include c-axis alignment, a-axis alignment, and m-axis alignment. Accordingly, when a semiconductor layer is formed on a base substrate, the semiconductor film can be a c-axis alignment film, an a-axis alignment film, or an m-axis alignment film.

The orientation layer is preferably a heteroepitaxially grown layer. For example, when an alignment layer is grown on a sapphire substrate, since both the sapphire substrate and the alignment layer have a corundum-type crystal structure, when their lattice constants are close to each other, epitaxial growth may occur in which the crystal plane of the alignment layer is aligned following the crystal orientation of the sapphire substrate during heat treatment. By epitaxially growing the alignment layer in this manner, the alignment layer can be made to inherit the high crystallinity and crystal orientation unique to the single crystal of the sapphire substrate.

The arithmetic average roughness Ra of the surface of the alignment layer is preferably 1nm or less, more preferably 0.5nm or less, and still more preferably 0.2nm or less. Consider that: the surface of the alignment layer is smoothed in this manner, and the crystallinity of the semiconductor layer provided thereon is further improved.

The base substrate has a single side preferably of 20cm²Above, more preferably 70cm²Above, more preferably 170cm²The above area. By enlarging the area of the base substrate in this manner, the semiconductor layer formed thereon can be enlarged. Therefore, a plurality of semiconductor elements can be obtained from one semiconductor layer, and further reduction in manufacturing cost can be expected. The upper limit of the size is not particularly limited, and is typically 700cm on one side²The following.

The base substrate of the present invention preferably further includes a support substrate on the side opposite to the front surface (i.e., the back surface side) of the alignment layer. That is, the base substrate of the present invention may be: a composite base substrate is provided with a support substrate and an alignment layer provided on the support substrate. The support substrate is preferably a sapphire substrate or Cr₂O₃Isopoly-corundum single crystal, and a sapphire substrate is particularly preferable. By using a corundum single crystal as the support substrate, the support substrate can also serve as a seed crystal for heteroepitaxial growth of the alignment layer. In addition, by adopting the structure including the corundum single crystal in this manner, a semiconductor layer having excellent quality can be obtained. That is, the corundum single crystal has excellent mechanical properties, thermal properties, chemical stability, and the like. Particularly, sapphire has a thermal conductivity of 42W/mK at room temperature and is excellent in thermal conductivity. Therefore, by using a composite base substrate including a sapphire substrate, the thermal conductivity of the entire substrate can be made excellent. As a result, it is expected that: when a semiconductor layer is formed on a composite base substrate, unevenness in temperature distribution in the substrate surface is suppressed, and a semiconductor layer having a uniform film thickness can be obtained. In addition, a large area can be easily obtained for the sapphire substrate, the entire cost can be reduced, and a semiconductor layer having a large area can be obtained.

The sapphire substrate may have any orientation plane. That is, the plate may have a-plane, c-plane, r-plane, and m-plane, or may have a predetermined off angle with respect to these planes. In addition, in order to adjust the electrical characteristics, sapphire to which a dopant is added may be used. As the dopant, a known dopant can be used.

The semiconductor layer made of a nitride or oxide of a group 13 element can be formed using the alignment layer of the base substrate of the present invention. The method for forming the semiconductor layer may be a known method, but is preferably any of a vapor-phase film formation method such as various CVD methods, HVPE methods, MBE methods, PLD methods, and sputtering methods, and a liquid-phase film formation method such as a hydrothermal method and Na flux method. Examples of the CVD method include: thermal CVD, plasma CVD, atomized CVD, MO (organic metal) CVD, and the like. Among them, the atomized CVD method, the hydrothermal method, or the HVPE method is particularly preferable for forming the semiconductor layer made of an oxide of a group 13 element.

The base substrate of the present invention may be in the form of a free-standing substrate in which the alignment layer is independent, or may be in the form of a composite base substrate including a support substrate such as a sapphire substrate. Therefore, the alignment layer can be finally separated from the support substrate such as a sapphire substrate as necessary. The separation of the support substrate may be performed by a known method, and is not particularly limited. For example, there may be mentioned: a method of separating the alignment layer by applying mechanical impact, a method of separating the alignment layer by applying heat and thermal stress, a method of separating the alignment layer by applying vibration such as ultrasonic waves, a method of separating the alignment layer by etching an unnecessary portion, a method of separating the alignment layer by laser lift-off, a method of separating the alignment layer by machining such as cutting or polishing, and the like. In the case of a mode in which the alignment layer is heteroepitaxially grown on the sapphire substrate, the sapphire substrate may be separated and then the alignment layer may be provided on another support substrate. The material of the other support substrate is not particularly limited, and an appropriate material may be selected from the viewpoint of material properties. For example, from the viewpoint of thermal conductivity, there are: a metal substrate such as Cu, or a ceramic substrate such as SiC or AlN.

Manufacturing method

The base substrate of the present invention can be preferably manufactured by (a) preparing a sapphire substrate; (b) preparing a specified orientation precursor layer; (c) performing heat treatment on the sapphire substrate to convert at least a portion of the alignment precursor layer near the sapphire substrate into an alignment layer; (d) if desired, grinding, polishing, or the like is performed to expose the surface of the alignment layer. The alignment precursor layer is formed into an alignment layer by heat treatment, and includes: alpha-Al having a-axis length and/or c-axis length ratio₂O₃The a-axis length and/or the c-axis length of (A) is larger than that of (C) or (C) is a ratio of the a-axis length and/or the c-axis length of alpha-Al by heat treatment described later₂O₃The a-axis length and/or the c-axis length of (2) is large. The alignment precursor layer may contain a trace amount of components in addition to the material having a corundum-type crystal structure. According to this manufacturing method, the growth of the alignment layer can be promoted using the sapphire substrate as a seed crystal. That is, the high crystallinity and the crystal orientation peculiar to the single crystal of the sapphire substrate are inherited by the orientation layer.

(a) Preparation of sapphire substrate

To fabricate the base substrate, a sapphire substrate is first prepared. The sapphire substrate used may have any orientation plane. That is, the plate may have a-plane, c-plane, r-plane, and m-plane, or may have a predetermined off angle with respect to these planes. For example, in the case of c-plane sapphire, c-axis alignment is performed with respect to the surface, and therefore, an alignment layer of c-axis alignment can be easily heteroepitaxially grown thereon. In addition, a sapphire substrate to which a dopant is added may be used for adjusting the electrical characteristics. As the dopant, a known dopant can be used.

(b) Fabrication of alignment precursor layer

An orientation precursor layer is prepared which contains a material having a corundum-type crystal structure in which the a-axis length and/or the c-axis length are greater than the a-axis length and/or the c-axis length of sapphire, or a material which has a corundum-type crystal structure in which the a-axis length and/or the c-axis length are greater than the a-axis length and/or the c-axis length of sapphire as a result of heat treatment. The method for forming the alignment precursor layer is not particularly limited as long as the alignment layer having pores is formed after the heat treatment, and a known method can be used. The alignment precursor layer may have pores formed therein, or the alignment precursor layer may be dense, so that pores may be generated when the alignment layer is formed. However, from the viewpoint of controlling the state of formation of the air holes, it is preferable that the air holes be formed in the alignment precursor layer. Examples of a method for forming such an alignment precursor layer include: an AD (aerosol deposition) method, a sol-gel method, a hydrothermal method, a sputtering method, an evaporation method, various CVD (chemical vapor deposition) methods, a PLD method, and a CVT (chemical transport) method can be used, and a method of directly forming an alignment precursor layer on a sapphire substrate can be used. Examples of the CVD method include: thermal CVD, plasma CVD, atomized CVD, MO (organic metal) CVD, and the like. Alternatively, a method may be used in which a molded body of an alignment precursor is prepared in advance, and the molded body is placed on a sapphire substrate. The molded article can be produced by molding the material of the alignment precursor by a method such as tape casting or press molding. Further, a method may be employed in which polycrystalline bodies prepared in advance by various CVD methods, sintering, or the like are used as the alignment precursor layer and placed on the sapphire substrate. In this case, too, it is preferable that the polycrystalline body contains pores.

However, a method of directly forming an alignment precursor layer by an Aerosol Deposition (AD) method, various CVD methods, or a sputtering method is preferable. By using these methods, an alignment precursor layer can be formed in a relatively short time, and heteroepitaxial growth can be easily performed using a sapphire substrate as a seed crystal. In particular, the AD method does not require a high vacuum process, and is therefore preferable in terms of production cost. The sputtering method may be performed using a target made of the same material as the alignment precursor layer, but a reactive sputtering method may be used in which a film is formed using a metal target in an oxidizing atmosphere. A method of placing a previously fabricated molded body on sapphire is also preferable as a simple method. In the method of using a previously prepared polycrystal as the alignment precursor layer, two steps of preparing the polycrystal and performing heat treatment on the sapphire substrate are required. In addition, in order to improve the adhesion between the polycrystalline body and the sapphire substrate, it is necessary to sufficiently smooth the surface of the polycrystalline body in advance. In any of the methods, known conditions may be used, and a method of directly forming an alignment precursor layer by the AD method and a method of placing a previously prepared molded body on a sapphire substrate will be described below.

The AD method is a technique of mixing fine particles or a fine particle raw material with a gas, aerosolizing the mixture, ejecting the aerosol from a nozzle at a high speed, and colliding the aerosol with a substrate to form a coating film, and has a characteristic that the coating film can be formed at normal temperature. Fig. 1 shows an example of a film formation apparatus (aerosol deposition (AD) apparatus) used in the AD method. The film forming apparatus 20 shown in fig. 1 is configured to: an apparatus for use in an AD method in which a raw material powder is ejected onto a substrate in an atmosphere having a pressure lower than atmospheric pressure. The film forming apparatus 20 includes: an aerosol-generating unit 22 that generates an aerosol of raw material powder containing raw material components; and a film forming section 30 that forms a film containing a raw material component by spraying raw material powder onto the sapphire substrate 21. The aerosol-generating unit 22 includes: an aerosol generation chamber 23 that receives raw material powder and generates aerosol by receiving supply of carrier gas from a gas cylinder not shown; a raw material supply tube 24 for supplying the generated aerosol to the film forming section 30; and an exciter 25 for applying vibration to the aerosol-generating chamber 23 and the aerosol therein at a vibration frequency of 10 to 100 Hz. The film forming section 30 includes: a film formation chamber 32 that ejects an 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 X-Y table 33 which moves the substrate holder 34 in the X-Y axis direction. The film forming section 30 further includes: an injection nozzle 36 having a slit 37 formed at a tip thereof and injecting the aerosol toward the sapphire substrate 21; and a vacuum pump 38 that depressurizes the film forming chamber 32.

In the AD method, it is known that pores are generated in the film or the film becomes a powder compact depending on the film forming conditions. For example, the form of the AD film is easily affected by the collision speed of the raw material powder against the substrate, the particle size of the raw material powder, the aggregation state of the raw material powder in the aerosol, the ejection amount per unit time, and the like. The collision speed of the raw material powder against the substrate is affected by the differential pressure between the film forming chamber 32 and the injection nozzle 36, the opening area of the injection nozzle, and the like. Therefore, in order to control the number of pores in the alignment precursor layer, it is necessary to appropriately control these factors.

When a molded body in which an alignment precursor layer is prepared in advance is used, a raw material powder of an alignment precursor can be molded to prepare a molded body. For example, in the case of press molding, the alignment precursor layer is a press molded body. The raw material powder of the orientation precursor can be press-molded to prepare a press-molded body by a known method, for example, the raw material powder is put in a mold and molded so as to be preferably 100 to 400kgf/cm²More preferably 150 to 300kgf/cm²Pressing under the pressure of the pressure to manufacture the product. The molding method is not particularly limited, and casting, extrusion, blade method, and any combination of these methods may be used in addition to press molding. For example, in the case of tape casting, it is preferable that additives such as a binder, a plasticizer, a dispersant, and a dispersion medium are appropriately added to the raw material powder to form a slurry, and the slurry is passed through a narrow slit-shaped discharge port to be discharged and molded in a sheet form. The thickness of the molded article molded into a sheet is not limited, but is preferably 5 to 500 μm from the viewpoint of handling. In addition, when a thick alignment precursor layer is required, a plurality of the sheet molded bodies may be stacked and used at a desired thickness.

These molded articles are subjected to subsequent heat treatment on a sapphire substrate, whereby the portions near the sapphire substrate become alignment layers. As described above, in this method, the compact needs to be sintered in the heat treatment step described later to form predetermined pores and to achieve densification. Therefore, the molded body may contain a trace amount of components such as a sintering aid in addition to the material having or imparting the corundum-type crystal structure. In addition, a small amount of a known pore-forming agent may be added from the viewpoint of introducing pores into the alignment layer.

(c) Heat treatment of oriented precursor layer on sapphire substrate

The sapphire substrate having the alignment precursor layer formed thereon is subjected to a heat treatment at a temperature of 1000 ℃ or higher. By this heat treatment, at least a portion of the alignment precursor layer near the sapphire substrate can be converted into an alignment layer. Further, the orientation layer can be heteroepitaxially grown by the heat treatment. That is, by forming the alignment layer from a material having a corundum-type crystal structure, heteroepitaxial growth occurs in which the material having a corundum-type crystal structure is crystal-grown using the sapphire substrate as a seed crystal during the heat treatment. At this time, rearrangement of the crystals occurs, and the crystals are arranged following the crystal plane of the sapphire substrate. As a result, the sapphire substrate and the alignment layer can be aligned in crystal axis. For example, when a c-plane sapphire substrate is used, both the sapphire substrate and the alignment layer are c-axis aligned with respect to the surface of the base substrate. By this heat treatment, a gradient composition region can be formed in a part of the alignment layer. That is, during the heat treatment, a reaction occurs at the interface between the sapphire substrate and the alignment precursor layer, and the Al component in the sapphire substrate diffuses into the alignment precursor layer and/or the component in the alignment precursor layer diffuses into the sapphire substrate, and the composition containing α -Al is formed₂O₃A gradient composition region composed of a solid solution of (1).

Note that, known is: in various methods such as CVD, sputtering, PLD, and CVT, heteroepitaxial growth may occur on a sapphire substrate without heat treatment at 1000 ℃. However, the alignment precursor layer is preferably: amorphous or non-oriented polycrystal, which is in a non-oriented state at the time of production, is subjected to rearrangement of crystals using sapphire as a seed crystal at the time of the present heat treatment step. Accordingly, crystal defects reaching the surface of the alignment layer can be effectively reduced. The reason is not clear, but is considered to be because: the rearrangement of the crystal structure of the temporarily formed alignment precursor layer in a solid phase using sapphire as a seed crystal is also effective in reducing crystal defects.

The heat treatment is not particularly limited as long as it can obtain a corundum-type crystal structure and cause heteroepitaxial growth using a sapphire substrate as a seed crystal, and it can be performed in a known manner such as a tube furnace or a hot plateIs carried out in the heat treatment furnace. In addition, not only the heat treatment under the normal pressure (no pressure) described above, but also a heat treatment under pressure such as hot pressing or HIP, or a combination of the heat treatment under the normal pressure and the heat treatment under pressure may be used. The heat treatment conditions may be appropriately selected depending on the material used for the alignment layer. For example, the atmosphere for the heat treatment may be selected from the group consisting of air, vacuum, nitrogen, and an inert gas atmosphere. The preferred heat treatment temperature also varies depending on the material used for the alignment layer, but is, for example, preferably 1000 to 2000 ℃ and more preferably 1200 to 2000 ℃. The heat treatment temperature and the holding time are related to the thickness of the alignment layer generated in the heteroepitaxial growth and the thickness of the gradient composition region formed by diffusion with the sapphire substrate, and may be appropriately adjusted according to the kind of the material, the thickness of the target alignment layer, the thickness of the gradient composition region, and the like. However, when a previously prepared molded body is used as an alignment precursor layer, sintering is required in a heat treatment to form predetermined pores and densify the layer, and therefore, normal pressure firing at a high temperature, hot pressing, HIP, or a combination thereof is preferable. For example, when hot pressing is employed, the surface pressure is preferably 50kgf/cm from the viewpoint of densification²Above, more preferably 100kgf/cm²Above, 200kgf/cm is particularly preferable²The upper limit is not particularly limited. On the other hand, from the viewpoint of introducing pores into the alignment layer, it is preferable to appropriately reduce the pressing pressure. The firing temperature is not particularly limited as long as sintering and heteroepitaxial growth occur, and is preferably 1000 ℃ or higher, more preferably 1200 ℃ or higher, further preferably 1400 ℃ or higher, and particularly preferably 1600 ℃ or higher. The firing atmosphere may be selected from the group consisting of air, vacuum, nitrogen, and an inert gas atmosphere. As the firing jig such as the outer mold, a jig made of graphite or alumina, or the like can be used.

(d) Exposure of the surface of the alignment layer

An alignment precursor layer or a surface layer having poor alignment or no alignment may be present or remain on an alignment layer formed near a sapphire substrate by heat treatment. In this case, it is preferable to expose the surface of the alignment layer by applying a process such as grinding or polishing to the surface originating from the alignment precursor layer side. Accordingly, the material having excellent alignment properties is exposed on the surface of the alignment layer, and thus the semiconductor layer can be epitaxially grown thereon efficiently. The method for removing the alignment precursor layer and the surface layer is not particularly limited, and examples thereof include: grinding and lapping methods, and ion beam milling methods. The surface of the alignment layer is preferably polished by a polishing process using abrasive grains or Chemical Mechanical Polishing (CMP).

Semiconductor layer

With the base substrate of the present invention, a semiconductor layer containing a nitride or an oxide of a group 13 element can be formed. The method for forming the semiconductor layer may be a known method, but is preferably any of a vapor-phase film formation method such as various CVD methods, HVPE methods, sublimation methods, MBE methods, PLD methods, and sputtering methods, a hydrothermal method, a liquid-phase film formation method such as Na flux method, and particularly preferably an atomized CVD method, a hydrothermal method, or an HVPE method. Hereinafter, the aerosol CVD method will be described.

The aerosol CVD method is a method in which a raw material solution is atomized or made into droplets to generate a spray or droplets, the spray or droplets are transported to a film forming chamber provided with a substrate using a carrier gas, and the spray or droplets are thermally decomposed and chemically reacted in the film forming chamber to form a film on the substrate and grow the film. Fig. 2 shows an example of the atomizing CVD apparatus. The atomizing CVD apparatus 1 shown in fig. 2 includes: a susceptor 10 on which the substrate 9 is placed; a diluent gas source 2 a; a carrier gas source 2 b; a flow rate adjustment valve 3a for adjusting the flow rate of the dilution gas sent from the dilution gas source 2 a; a flow rate adjustment valve 3b for adjusting the flow rate of the carrier gas sent from the carrier gas source 2 b; an atomization generation source 4 that stores a raw material solution 4 a; a container 5 in which water 5a is placed; an ultrasonic oscillator 6 attached to the bottom surface of the container 5; a quartz tube 7 serving as a film forming chamber; a heater 8 provided at a peripheral portion of the quartz tube 7; and an exhaust port 11. The susceptor 10 is made of quartz, and a surface on which the substrate 9 is placed is inclined with respect to a horizontal plane.

The raw material solution 4a used in the atomized CVD method is not limited as long as it is a solution for obtaining a semiconductor layer containing a nitride or an oxide of a group 13 element, and examples thereof include: a solution obtained by dissolving Ga and/or an organometallic complex or halide of a metal forming a solid solution with Ga in a solvent. Examples of the organic metal complex include acetylacetone complexes. In addition, in the case where a dopant is added to the semiconductor layer, a solution of a dopant component may be added to the raw material solution. In addition, an additive such as hydrochloric acid may be added to the raw material solution. As the solvent, water, alcohol, or the like can be used.

Next, the obtained raw material solution 4a is atomized or made into droplets to generate a spray or droplets 4 b. A preferable example of the method of atomizing or forming droplets is a method of vibrating the raw material solution 4a by using the ultrasonic oscillator 6. The resulting spray or droplets 4b are then transported into the film forming chamber using a carrier gas. The carrier gas is not particularly limited, and may be one or two or more of an inert gas such as oxygen, ozone, or nitrogen, and a reducing gas such as hydrogen.

The film forming chamber (quartz tube 7) is provided with a substrate 9. The spray or droplets 4b delivered to the film forming chamber are thermally decomposed and chemically reacted therein, thereby forming a film on the substrate 9. The reaction temperature varies depending on the kind of the raw material solution, and is preferably 300 to 800 ℃, and more preferably 400 to 700 ℃. The atmosphere in the film formation chamber is not particularly limited as long as a desired semiconductor film is obtained, and may be an oxygen atmosphere, an inert gas atmosphere, a vacuum atmosphere, or a reducing atmosphere, and is preferably an atmospheric atmosphere.

For the semiconductor layer manufactured by using the base substrate of the present invention as described above, the crystal defect density of the surface is typically as low as 1.0 × 10⁶/cm²The following layers. The semiconductor layer having a significantly low crystal defect density has excellent dielectric breakdown electric field characteristics, and is suitable for use in power semiconductors. The crystal defect density of the semiconductor layer can be evaluated by plane TEM observation (top view) or cross-sectional TEM observation using a general Transmission Electron Microscope (TEM). For example, the transmission electron microscope employs HitachiWhen H-90001 UHR-I is observed in a plan view, it is sufficient to use an acceleration voltage of 300 kV. The test piece was cut out so as to include the film surface, and ion milling was performed so that the measurement visual field was 50 μm × 50 μm and the thickness of the test piece around the measurement visual field was 150 nm. 10 test pieces such as these were prepared, and TEM images of 10 fields in total were observed, whereby the crystal defect density could be evaluated with high accuracy. The crystal defect density is preferably 1.0X 10⁵/cm²Hereinafter, more preferably 4.0 × 10³/cm²Hereinafter, there is no particular lower limit.

To the knowledge of the present inventors, no known technique has been able to obtain a semiconductor layer having such a low crystal defect density. For example, non-patent document 1 discloses the use of sapphire and α -Ga₂O₃Between the layers is introduced (Al) as a buffer layer_x、Ga_1－x)₂O₃Forming alpha-Ga on substrate of layer (x is 0.2-0.9)₂O₃Layer, however, alpha-Ga obtained₂O₃The density of edge dislocations and screw dislocations of the layer is 3X 10⁸/cm²And 6X 10⁸/cm²。

Examples

The present invention will be described in more detail by the following examples.

Example 1

(1) Fabrication of alignment precursor layer

As a substrate, sapphire (diameter 50.8mm (2 inches), thickness 0.43mm, c-plane, off-angle 0.2 °) was used, and an Aerosol Deposition (AD) apparatus shown in fig. 1 was used to form an AD film on a seed substrate (sapphire substrate).

The crushing conditions of the raw material powders were changed to carry out AD film formation 2 times, and the film formation conditions were as follows.

(first AD film formation)

As the raw material powder, a powder obtained by using commercially available Cr₂O₃TiO is added into 100 weight portions of the powder₂4.4 parts by weight of a powder was wet-mixed to prepare a mixed powder, which was then pulverized by a jar millPulverizing to obtain powder with particle diameter D₅₀Is 0.3 μm. Carrier gas is set to N₂A ceramic nozzle having a slit with a long side of 5mm and a short side of 0.3mm was used. The scanning conditions of the nozzles were as follows: the scanning was repeated by moving 55mm in the direction perpendicular to the long side of the slit and advancing, 5mm in the long side direction of the slit, 55mm in the direction perpendicular to the long side of the slit and returning, and 5mm in the direction opposite to the initial position in the long side direction of the slit at a scanning speed of 0.5mm/s, and the scanning was repeated by moving the slit from the initial position by 55mm in the long side direction of the slit and returning the slit to the initial position in the direction opposite to the previous direction, and such a cycle was set to 1 cycle, and 150 cycles were repeated. In 1 cycle of film formation at room temperature, the set pressure of the transport gas was adjusted to 0.06MPa, the flow rate was adjusted to 6L/min, and the pressure in the chamber was adjusted to 100Pa or less.

(second AD film formation)

As the raw material powder, a powder obtained by mixing commercially available Cr₂O₃Powder and TiO₂After wet-mixing the powders, the resulting mixture was pulverized by a jar mill to give a particle diameter D₅₀And was 0.4 μm. Carrier gas is set to N₂A ceramic nozzle having a slit with a long side of 5mm and a short side of 0.3mm was used. The scanning conditions of the nozzles were as follows: the scanning was repeated by moving 55mm in the direction perpendicular to the long side of the slit and advancing, 5mm in the long side direction of the slit, 55mm in the direction perpendicular to the long side of the slit and returning, and 5mm in the direction opposite to the initial position in the long side direction of the slit at a scanning speed of 0.5mm/s, and the scanning was repeated by moving the slit from the initial position by 55mm in the long side direction of the slit and returning the slit to the initial position in the direction opposite to the previous direction, and such a cycle was set to 1 cycle, and 250 cycles were repeated. In 1 cycle of film formation at room temperature, the set pressure of the transport gas was adjusted to 0.06MPa, the flow rate was adjusted to 6L/min, and the pressure in the chamber was adjusted to 100Pa or less.

The total thickness of the AD films obtained by the first and second AD film formation was about 80 μm.

(2) Heat treatment of alignment precursor layer

The sapphire substrate having the AD film formed thereon was taken out from the AD apparatus and annealed at 1600 ℃ for 4 hours in a nitrogen atmosphere.

(3) Determination of thickness of crystal growth

A sample separately prepared by the same method as in (1) and (2) above was prepared and cut so as to pass through the center of the substrate in the direction perpendicular to the plate surface. The cut sample was subjected to polishing using diamond abrasive grains to smooth the cross section, and then subjected to mirror finishing using Chemical Mechanical Polishing (CMP) using colloidal silica. The obtained cross section was photographed by a scanning electron microscope (SU-5000, manufactured by Hitachi Kagaku Co., Ltd.). By observing the back-scattered electron image of the cross section after polishing, the orientation precursor layer and the orientation layer remaining in a polycrystalline form can be identified by the channel contrast due to the difference in crystal orientation. The thickness of each layer was estimated in this way, and as a result, the film thickness of the alignment layer was about 60 μm and the film thickness of the polycrystalline portion was about 20 μm.

(4) Grinding and lapping

The surface of the substrate obtained from the AD film side was ground with a grindstone having a grain size No. #2000 until the alignment layer was exposed, and then the surface of the substrate was further smoothed by polishing with diamond abrasive grains. Then, mirror finishing was performed by Chemical Mechanical Polishing (CMP) using colloidal silica, and a composite base substrate having an alignment layer on a sapphire substrate was obtained. The surface of the substrate from the AD film side is referred to as a "surface". The total of the polycrystalline portion and the alignment layer was about 40 μm for the amount of grinding and polishing, and the thickness of the alignment layer formed on the composite base substrate was about 40 μm.

(5) Evaluation of alignment layer

(5a) Section EBSD

The composite base substrate produced in the above (4) is cut so as to pass through the center of the substrate in a direction orthogonal to the plate surface. The cut sample was subjected to polishing using diamond abrasive grains to smooth the cross section, and then subjected to mirror finishing using Chemical Mechanical Polishing (CMP) using colloidal silica. Next, the inverse pole map of the cross section of the alignment layer was measured by the EBSD (Electron Back Scatter Diffraction patterns) method. Specifically, an inverse pole figure azimuth mapping of the cross section of the alignment layer was performed under the following conditions in a field of view of 50 μm × 100 μm using a scanning electron microscope (SU-5000, manufactured by Hitachi high tech Co., Ltd.) equipped with EBSD (Nordlys Nano).

< EBSD measurement Condition >

Acceleration voltage: 15kV

Spot strength: 70

Working distance: 22.5mm

Step size: 0.5 μm

Inclination angle of the sample: 70 degree

Measurement procedure: aztec (version3.3)

The inverse pole figure orientation map of the cross-section shows: the orientation layer is a biaxially oriented corundum layer, which is oriented in the same direction as the sapphire substrate in both the normal direction and the in-plane direction of the substrate.

(5b) Cross-sectional EDS

Next, a composition analysis of a cross section perpendicular to the main surface of the substrate was performed using an energy dispersive X-ray analyzer (EDS). As a result, Cr, O, Al and Ti were detected in a thickness range of about 40 μm from the substrate surface. In the thickness region of about 10 μm, the difference in the content ratio of each element of Cr, Al, and Ti between the planes separated by 2 μm in the thickness direction was less than 1.0 at%, and it was found that the Cr — Ti — Al oxide layer having a thickness of about 10 μm was formed as a composition stable region. Cr, O, Al and Ti were also detected in the range of about 30 μm in thickness of the lower layer of the Cr-Ti-Al oxide layer (i.e., in the thickness region of about 10 to 40 μm). In the thickness region of about 10 to 40 μm, it was confirmed that the content ratios of each element of Cr, Ti and Al greatly differ in the thickness direction, the Al concentration is high on the sapphire substrate side, and the Al concentration is low on the composition stable region side. In the thickness region of about 10 to 40 μm, Al between the surfaces separated by 2 μm in the thickness directionThe difference in the content ratio is 1.2 to 11.2 at%, and it is confirmed that: the reaction layer forms a gradient composition region. Further, in the gradient composition region, only O and Al are detected, and this is described as a sapphire substrate. In addition, the results of the inverse pole figure mapping and the EDS analysis of (5a) above show: the presence of Cr, Ti, Al and O in the biaxially oriented corundum layer suggests: the biaxially oriented layer is Cr₂O₃、Ti₂O₃、Al₂O₃A solid solution of (2).

(5c)XRD

The XRD in-plane measurement of the substrate surface was performed using a multifunctional high-resolution X-ray diffraction apparatus (D8 DISCOVER, manufactured by Bruker AXS Co., Ltd.). Specifically, after adjusting the Z axis according to the height of the substrate surface, Chi, Phi, ω, and 2 θ were adjusted with respect to the (11-20) crystal plane to establish axes, and 2 θ - ω measurement was performed under the following conditions.

Tube voltage: 40kV

Tube current: 40mA

The detector: tripple Ge (220) Analyzer

CuK α rays obtained by parallel monochromatization (half-value width 28 seconds) using a Ge (022) asymmetric reflection monochromator

Step width: 0.001 °

Scanning speed: 1.0 sec/step

The results show that: the a-axis length of the surface of the alignment layer isThe results of the EDS measurement indicate: the rear surface of the alignment layer (the interface with the sapphire substrate) contains almost no Cr and Ti. Therefore, the a-axis length of the back surface of the alignment layer was estimated asSuggesting that the a-axis length is different between the alignment layer surface and the back surface.

(5d) Plane TEM

In order to evaluate the crystal defect density of the alignment layer, a plane T was formed by a transmission electron microscope (H-90001 UHR-I, Hitachi, Ltd.)EM observation (top view). The sample piece was cut out parallel to the surface of the alignment layer (i.e., in the horizontal direction) so as to include the surface of the alignment layer, and was processed by ion milling so that the sample thickness (T) around the measurement field was 150 nm. The crystal defect density was evaluated by TEM observation of 10 fields with an acceleration voltage of 300kV and a measurement field of 50 μm × 50 μm, and the crystal defect density was 8.0 × 10³/cm²。

(5e) Pore observation

The pores of the cross-section polished sample prepared in (5a) were evaluated by a scanning electron microscope (SU-5000, manufactured by Hitachi height, Ltd.). Specifically, a secondary electron image was taken at a measurement magnification of 500 times (size of 1 field: 178 μm × 256 μm) for 25 fields for an arbitrary region of the biaxially oriented layer (thickness of about 40 μm), and the number Nd of pores in a deep region (region having a thickness of about 32 μm from the interface with the sapphire substrate) and the number Ns of pores in a surface region (region having a thickness of about 8 μm from the surface of the biaxially oriented layer) in the biaxially oriented layer were evaluated. Pores were formed in the part having a pore diameter of 0.3 μm or more, and the number of pores was counted by visual observation based on the secondary electron image taken, and the cross-sectional area per unit was 1cm²The number of pores in the steel sheet was evaluated. As a result, the number of pores (Nd + Ns) in the orientation layer region was 1.4X 10⁶Per cm²The ratio Nd/Ns of Nd to Ns was 3.7.

Example 2

In the above (1), Cr is used as a raw material powder for the first AD film formation₂O₃－TiO₂Particle diameter D of the pulverized mixed powder₅₀0.4 μm in size, and Cr was used as a raw material powder for the second AD film formation₂O₃－TiO₂Particle diameter D of the pulverized mixed powder₅₀A composite base substrate was produced and evaluated in the same manner as in example 1, except that the powder had a particle size of 0.5. mu.m.

The total thickness of the AD films obtained by the first and second AD film formation was about 80 μm as in example 1. When the cross-section of the AD film after the heat treatment was observed with a scanning electron microscope, the film thickness of the alignment layer was about 60 μm, and the film thickness of the polycrystalline portion was about 20 μm. A composite base substrate was prepared by grinding and polishing the surface in the same manner as in example 1. The total of the polycrystalline portion and the alignment layer was about 40 μm, and the thickness of the alignment layer formed on the composite base substrate was about 40 μm, as in example 1. The inverse pole map of the cross section measured with EBSD shows: the orientation layer is a biaxially oriented corundum layer, which is oriented in the same direction as the sapphire substrate in both the normal direction and the in-plane direction of the substrate.

Based on the cross-sectional EDS of the composite base substrate, Cr, Ti, O and Al were detected in a thickness region from the substrate surface to about 10 μm. In the thickness region of about 10 μm, the difference in the content ratio of each element of Cr, Al, and Ti between the planes separated by 2 μm in the thickness direction was less than 1.0 at%, and it was found that the Cr — Ti — Al oxide layer having a thickness of about 10 μm was formed as a composition stable region. Cr, O, Al and Ti were also detected in the range of about 30 μm in thickness of the lower layer of the Cr-Ti-Al oxide layer (i.e., in the thickness region of about 10 to 40 μm). In the thickness region of about 10 to 40 μm, it was confirmed that the content ratios of each element of Cr, Ti and Al greatly differ in the thickness direction, the Al concentration is high on the sapphire substrate side, and the Al concentration is low on the composition stable region side. In the thickness region of about 10 to 40 μm, the difference of Al ratio between the surfaces separated by 2 μm in the thickness direction is 1.2 to 11.2 at%, and thus it is confirmed that: the reaction layer forms a gradient composition region. Further, in the gradient composition region, only O and Al are detected, and this is described as a sapphire substrate. In addition, the results of the inverse pole map and EDS analysis show that: the presence of Cr, Ti, Al and O in the biaxially oriented corundum layer suggests: the biaxially oriented layer is Cr₂O₃、Ti₂O₃、Al₂O₃A solid solution of (2). The XRD measurement shows that: the a-axis length of the surface of the alignment layer isIncidentally, the results of the EDS measurement are said to beIt is clear that: the rear surface of the alignment layer (the interface with the sapphire substrate) contains almost no Cr and Ti. Therefore, the a-axis length of the back surface of the alignment layer was estimated asSuggesting that the a-axis length is different between the alignment layer surface and the back surface. The results of the planar TEM observation are: crystal defect density of 8.0X 10³/cm². The number of pores (Nd + Ns) in the orientation layer region was 3.6X 10 by observing pores from a secondary electron image of a scanning electron microscope⁵Per cm²The ratio Nd/Ns of Nd to Ns was 5.0.

Example 3

In the above (1), Cr is used as the raw material powder for the second AD film formation₂O₃－TiO₂Particle diameter D of the pulverized mixed powder₅₀Composite base substrates were produced and evaluated in the same manner as in example 1, except that the powder had a particle size of 0.2 μm.

The total thickness of the AD films obtained by the first and second AD film formation was about 80 μm as in example 1. When the cross-section of the AD film after the heat treatment was observed with a scanning electron microscope, the film thickness of the alignment layer was about 60 μm, and the film thickness of the polycrystalline portion was about 20 μm. A composite base substrate was prepared by grinding and polishing the surface in the same manner as in example 1. The total of the polycrystalline portion and the alignment layer was about 40 μm, and the thickness of the alignment layer formed on the composite base substrate was about 40 μm, as in example 1. The inverse pole map of the cross section measured with EBSD shows: the orientation layer is a biaxially oriented corundum layer, which is oriented in the same direction as the sapphire substrate in both the normal direction and the in-plane direction of the substrate.

Based on the cross-sectional EDS of the composite base substrate, Cr, Ti, O and Al were detected in a thickness region from the substrate surface to about 10 μm. In the thickness region of about 10 μm, the difference in the content ratio of each element of Cr, Al and Ti between the surfaces separated by 2 μm in the thickness direction was less than 1.0 at%, and it was found that the Cr-Ti-Al oxide layer having a thickness of about 10 μm was formed as a compositionally stable region. Cr, O, Al and Ti were also detected in the range of about 30 μm in thickness of the lower layer of the Cr-Ti-Al oxide layer (i.e., in the thickness region of about 10 to 40 μm). In the thickness region of about 10 to 40 μm, it was confirmed that the content ratios of each element of Cr, Ti and Al greatly differ in the thickness direction, the Al concentration is high on the sapphire substrate side, and the Al concentration is low on the composition stable region side. In the thickness region of about 10 to 40 μm, the difference of Al content ratio between the surfaces separated by 2 μm in the thickness direction is 1.2 to 11.2 at%, and thus it is confirmed that: the reaction layer forms a gradient composition region. Further, in the gradient composition region, only O and Al are detected, and this is described as a sapphire substrate. In addition, the results of the inverse pole map and EDS analysis show that: the presence of Cr, Ti, Al and O in the biaxially oriented corundum layer suggests: the biaxially oriented layer is Cr₂O₃、Ti₂O₃、Al₂O₃A solid solution of (2). The XRD measurement shows that: the a-axis length of the surface of the alignment layer isThe results of the EDS measurement indicate: the rear surface of the alignment layer (the interface with the sapphire substrate) contains almost no Cr and Ti. Therefore, the a-axis length of the back surface of the alignment layer was estimated asSuggesting that the a-axis length is different between the alignment layer surface and the back surface. The results of the planar TEM observation are: crystal defect density of 4.0X 10⁴/cm². The number of pores (Nd + Ns) in the orientation layer region was 2.0X 10 by observing pores from a secondary electron image of a scanning electron microscope⁶Per cm²The ratio Nd/Ns of Nd to Ns was 0.5.

Example 4

In the above (1), Cr is used as a raw material powder for the first AD film formation₂O₃－TiO₂Pulverizing the mixed powder to obtain a powder with a particle diameter D₅₀A composite base substrate was produced and evaluated in the same manner as in example 1 except that the powder had a particle size of 0.1. mu.mAnd (4) price.

The total thickness of the AD films obtained by the first and second AD film formation was about 80 μm as in example 1. When the cross-section of the AD film after the heat treatment was observed with a scanning electron microscope, the film thickness of the alignment layer was about 60 μm, and the film thickness of the polycrystalline portion was about 20 μm. A composite base substrate was prepared by grinding and polishing the surface in the same manner as in example 1. The total of the polycrystalline portion and the alignment layer was about 40 μm, and the thickness of the alignment layer formed on the composite base substrate was about 40 μm, as in example 1. The inverse pole map of the cross section measured with EBSD shows: the orientation layer is a biaxially oriented corundum layer, which is oriented in the same direction as the sapphire substrate in both the normal direction and the in-plane direction of the substrate.

Based on the cross-sectional EDS of the composite base substrate, Cr, Ti, O and Al were detected in a thickness region from the substrate surface to about 10 μm. In the thickness region of about 10 μm, the difference in the content ratio of each element of Cr, Al, and Ti between the planes separated by 2 μm in the thickness direction was less than 1.0 at%, and it was found that the Cr — Ti — Al oxide layer having a thickness of about 10 μm was formed as a composition stable region. Cr, O, Al and Ti were also detected in the range of about 30 μm in thickness of the lower layer of the Cr-Ti-Al oxide layer (i.e., in the thickness region of about 10 to 40 μm). In the thickness region of about 10 to 40 μm, it was confirmed that the content ratios of each element of Cr, Ti and Al greatly differ in the thickness direction, the Al concentration is high on the sapphire substrate side, and the Al concentration is low on the composition stable region side. In the thickness region of about 10 to 40 μm, the difference of Al content ratio between the surfaces separated by 2 μm in the thickness direction is 1.2 to 11.2 at%, and thus it is confirmed that: the reaction layer forms a gradient composition region. Further, in the gradient composition region, only O and Al are detected, and this is described as a sapphire substrate. In addition, the results of the inverse pole map and EDS analysis show that: the presence of Cr, Ti, Al and O in the biaxially oriented corundum layer suggests: the biaxially oriented layer is Cr₂O₃、Ti₂O₃、Al₂O₃A solid solution of (2). The XRD measurement shows that: orientation ofThe a-axis length of the layer surface isThe results of the EDS measurement indicate: the rear surface of the alignment layer (the interface with the sapphire substrate) contains almost no Cr and Ti. Therefore, the a-axis length of the back surface of the alignment layer was estimated asSuggesting that the a-axis length is different between the alignment layer surface and the back surface. The results of the planar TEM observation are: crystal defect density of 4.0X 10³/cm²The following. The number of pores (Nd + Ns) in the orientation layer region was 2.0X 10 by observing pores from a secondary electron image of a scanning electron microscope⁷Per cm²The ratio Nd/Ns of Nd to Ns was 58.1.

[ Table 2]

TABLE 2