In a semiconductor device 100, it is possible to prevent C from piling up at a boundary face between an epitaxial layer 22 and a group III nitride semiconductor substrate 10 by the presence of 30×1010 pieces/cm2 to 2000×1010 pieces/cm2 of sulfide in terms of S and 2 at % to 20 at % of oxide in terms of O in a surface layer 12. By thus preventing C from piling up, a high-resistivity layer is prevented from being formed on the boundary face between the epitaxial layer 22 and the group III nitride semiconductor substrate 10. Accordingly, it is possible to reduce electrical resistance at the boundary face between the epitaxial layer 22 and the group III nitride semiconductor substrate 10, and improve the crystal quality of the epitaxial layer 22. Consequently, it is possible to improve the emission intensity and yield of the semiconductor device 100.