Compound semiconductor substrate, semiconductor device, and processes for producing them

10 2 10 2 10 2 10 2 A compound semiconductor substrate according to the present invention is comprised of a Group III nitride and has a surface layer containing a chloride of not less than 200×10atoms/cmand not more than 12000×10atoms/cmin terms of Cl and an oxide of not less than 3.0 at % and not more than 15.0 at % in terms of O, at a surface. The inventors conducted elaborate research and newly discovered that when the surface layer at the surface of the compound semiconductor substrate contained the chloride of not less than 200×10atoms/cmand not more than 12000×10atoms/cmin terms of Cl and the oxide of not less than 3.0 at % and not more than 15.0 at % in terms of O, Si was reduced at an interface between the compound semiconductor substrate and an epitaxial layer formed thereon and, as a result, the electric resistance at the interface was reduced.