Abstract: An improved semiconductor substrate comprises at least one insulating layer comprising (a) a thin film of amorphous silicon dioxide formed on a base plate comprising single crystal silicon and (b) a thin film of single crystal sapphire superimposed on the silicon dioxide film. A semiconductor substrate may be prepared by forming a first thin film of single crystal sapphire on a base plate comprising single crystal silicon, converting, if desired, the upper surface layer of the silicon base plate into a first thin film of amorphous silicon dioxide by thermal oxidation through the sapphire film, forming a second thin film comprising single crystal silicon on the first sapphire film, forming a second thin film comprising single crystal sapphire on the second silicon film, and then thermally oxidizing the second silicon film through the second sapphire film to form a second film comprising amorphous silicon dioxide.

Abstract: A single crystal of Al.sub.2 O.sub.3 is epitaxially grown on an Si-single crystal of a semiconductor device by a vapor growth method. This vapor growth method employs starting materials of HCl, Al and CO.sub.2. Further, this method advantageouslyemploys a carrier gas to carry the gaseous product of the reaction of HCl with Al. An apparatus for the production of the above-mentioned semiconductor device comprises a chamber means for the reaction of the gaseous product and the single crystal, a reaction chamber for the reaction of Al and HCl, and an introducing tube for introducing CO.sub.2 in the proximity of the Si-single crystal.