In this paper, we point out the critical role of oxygen stoichiometry in the solid state epitaxy process used for obtaining 5 A EOT in SrTiO3 gate dielectrics on Si. Incomplete oxygenation of the TiO2 fraction of the amorphous SrO.TiO2 film enhances the inherent tendency of TiO2 to react with Si to yield TiSi2 and SiO2. O2 excess is needed to obtain a crystalline interface after UHV crystallization. The solid state epitaxy process results in heavily oxygen deficient films, whose insulating properties can be recovered by an atomic oxygen treatment.