Ground substrate and method for producing same

1. A base substrate comprising an orientation layer used for crystal growth of a nitride or oxide of a Group 13 element,

wherein a front surface of the orientation layer on a side used for the crystal growth is composed of a material having a corundum-type crystal structure having an a-axis length and/or c-axis length larger than that of sapphire,

wherein the orientation layer contains a material selected from the group consisting of 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 3 , or a solid solution containing two or more selected from the group consisting 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 3 , and

wherein a half width of an X-ray rocking curve of a (104) plane of the corundum-type crystal structure on the front surface of the orientation layer on the side used for crystal growth is 500 arcsec. or less.

2. The base substrate according to claim 1 , wherein a half width of an X-ray rocking curve of a (006) plane of the corundum-type crystal structure on the front surface of the orientation layer on the side used for crystal growth is 100 arcsec. or less.

3. The base substrate according to claim 1 , wherein when an X-ray diffraction (XRD) φ scan measurement is performed on the (104) plane of the corundum-type crystal structure on the front surface of the orientation layer on the side used for crystal growth, a proportion of a peak intensity of a sub-peak caused by a rotation domain to a peak intensity of a main peak is 5.0% or less.

4. The base substrate according to claim 1 , wherein a half width of an X-ray rocking curve of a (006) plane of the corundum-type crystal structure on the front surface of the orientation layer on the side used for crystal growth is 50 arcsec. or less.

5. The base substrate according to claim 1 , wherein a crystal defect density of the front surface of the orientation layer on the side used for crystal growth is 1×10 6 /cm 2 or less.

6. The base substrate according to claim 1 , wherein the base substrate is used for crystal growth of a semiconductor layer composed of α-Ga 2 O 3 or an α-Ga 2 O 3 solid solution, and the orientation layer is composed of a material containing α-Cr 2 O 3 or a material containing a solid solution of α-Cr 2 O 3 and a different material.

7. The base substrate according to claim 1 , wherein a value of the a-axis length and/or c-axis length of the orientation layer on the front surface of the orientation layer is larger than a value of an a-axis length and/or c-axis length on a rear surface of the orientation layer by 4.0% or more.

8. The base substrate according to claim 1 , wherein the material having the corundum-type crystal structure contains one or more materials selected from the group consisting of α-Cr 2 O 3 , α-Fe 2 O 3 , and α-Ti 2 O 3 or solid solutions thereof, or a solid solution containing α-Al 2 O 3 and one or more selected from the group consisting of α-Cr 2 O 3 , α-Fe 2 O 3 , and α-Ti 2 O 3 .

9. The base substrate according to claim 1 , wherein an entirety of the orientation layer is composed of the material having the corundum-type crystal structure.

10. The base substrate according to claim 1 , wherein the a-axis length of the material having the corundum-type crystal structure on the front surface is larger than 4.754 Å and 5.157 Å or less.

11. The base substrate according to claim 10 , wherein the a-axis length is 4.850 to 5.000 Å.

12. The base substrate according to claim 1 , wherein a gradient composition region having a composition varying in a thickness direction is present in the orientation layer.

13. The base substrate according to claim 1 , wherein the orientation layer has a composition stable region located near the front surface and having a composition stable in a thickness direction, and a gradient composition region located far from the front surface and having a composition varying in the thickness direction.

14. The base substrate according to claim 12 , wherein the gradient composition region is composed of a solid solution containing α-Cr 2 O 3 and α-Al 2 O 3 .

15. The base substrate according to claim 12 , wherein in the gradient composition region, an Al concentration decreases in the thickness direction toward the composition stable region.

16. The base substrate according to claim 1 , wherein the orientation layer is a heteroepitaxial growth layer.

17. The base substrate according to claim 1 , further comprising a support substrate on a side opposite to the front surface of the orientation layer.

18. The base substrate according to claim 17 , wherein the support substrate is a sapphire substrate.

19. The base substrate according to claim 1 , wherein the orientation layer is a heteroepitaxial growth layer of a sapphire substrate.

20. A method for producing the base substrate according to claim 1 , comprising:

providing a sapphire substrate;

forming an orientation precursor layer on a surface of the sapphire substrate, wherein the orientation precursor layer contains a material having a corundum-type crystal structure having an a-axis length and/or c-axis length larger than that of sapphire, or a material capable of having a corundum-type crystal structure having an a-axis length and/or c-axis length larger than that of sapphire by heat treatment; and

heat-treating the sapphire substrate and the orientation precursor layer at a temperature of 1000° C. or greater.