Ground substrate and method for producing same
Provided is a base substrate including an orientation layer used for crystal growth of a nitride or oxide of a Group 13 element. A front surface of the orientation layer on a side used for 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. The orientation layer contains 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 .
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 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 ,
wherein a gradient composition region having a composition varying in a thickness direction is present in the orientation layer, and
wherein the a-axis length and/or the c-axis length on the front surface of the orientation layer is longer than an a-axis length and/or a c-axis length on a rear surface of the orientation layer by 2.5% or more.
2. 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 a solid solution of α-Cr 2 O 3 and a different material.
3. The base substrate according to claim 1 , wherein the material having the corundum-type crystal structure contains a solid solution containing two or more selected from the group consisting of α-Cr 2 O 3 , α-Fe 2 O 3 , and α-Ti 2 O 3 , 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 .
4. 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.
5. 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.
6. The base substrate according to claim 5 , wherein the a-axis length is 4.850 to 5.000 Å.
7. The base substrate according to claim 1 , wherein the orientation layer has a composition stable region having a composition stable in a thickness direction, and a gradient composition region having a composition varying in the thickness direction, and wherein the composition stable region is located at a position closer to the front surface than the gradient composition region.
8. The base substrate according to claim 1 , wherein the gradient composition region is composed of a solid solution containing α-Cr 2 O 3 and α-Al 2 O 3 .
9. The base substrate according to claim 7 , wherein in the gradient composition region, an Al concentration decreases in the thickness direction toward the composition stable region.
10. The base substrate according to claim 1 , wherein the orientation layer is a heteroepitaxial growth layer.
11. The base substrate according to claim 1 , further comprising a support substrate on a side opposite to the front surface of the orientation layer.
12. The base substrate according to claim 11 , wherein the support substrate is a sapphire substrate.
13. The base substrate according to claim 1 , wherein the orientation layer is a heteroepitaxial growth layer of a sapphire substrate.
14. 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.