Abstract: An epitaxial composite comprising a thin film of single crystal Group III-V wide band-gap compound semiconductor or semiconductor alloy on single crystal, electrically insulating oxide substrates such as sapphire, spinel, BeO, ThO.sub.2, or the like, and on III-V semiconductors or alloys. The thin film may be produced in situ on a heated substrate by reaction of an organic compound containing the Group III constituent, typically tfhe alkyl metal organic, such as trimethylgallium and/or triethylgallium with a Group V hydride such as arsine, phosphine and/or stibine.