Composite substrate, and substrate for epitaxially growing group 13 element nitride

A composite substrate comprising:

• a group 13 nitride semiconductor substrate comprising a first main surface and a second main surface;
• and
• a supporting substrate having a bonding surface bonded with said first main surface of said group 13 nitride semiconductor substrate,
• wherein said supporting substrate comprises a bonding region, and
• wherein said bonding region is composed of silicon carbide having an average micropipe density of 10 cm−2 or higher and 100 cm−2 or lower at said bonding surface of said supporting substrate or composed of synthetic diamond having an atomic ratio of nitrogen to carbon atoms of 500 ppm or higher and 2000 ppm or lower.