Underlying substrate

A ground 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 a plurality of pores are present in the orientation layer and the number of pores in the orientation layer is 1.0×104/cm2 to 1.0×109/cm2,
• wherein the material having the corundum-type crystal structure contains one or more materials selected from the group consisting of α-Cr2O3, α-Fe2O3, and α-Ti2O3 or a solid solution thereof, or a solid solution containing α-Al2O3 and one or more materials selected from the group consisting of α-Cr2O3, α-Fe2O3, and α-Ti2O3,
• wherein in the orientation layer, the number of pores in a surface layer region including the front surface is smaller than the number of pores in a deep layer region away from the front surface, and
• wherein the surface layer region represents 20% of a thickness of the orientation layer, and the deep layer region represents 80% of the remaining thickness of the orientation layer.