MBE process for preparing oxide superconducting films

A process for preparing a film of oxide superconductor having a layered crystal structure by depositing constituent layers of said layered crystal structure on a substrate successively by molecular beam epitaxy (MBE) method while an oxygen-containing gas which is excited by irradiation of microwave is introduced in the neighborhood of a surface of the substrate, comprising the steps of depositing preselected numbers of constituent layers which correspond to one unit crystal or less than one unit crystal, interrupting the film-forming operation by the MBE method temporarily, maintaining the deposited constituent layers in an activated oxygen atmosphere in order to effect a crystallization promotive operation and then restarting a next film-forming operation by the MBE method after completion of said crystallization promotive operation which is terminated when a desired pattern is observed in a reflection high energy electron diffraction (RHEED) analyzer, a combination of said film-forming operation and said crystallization promotive operation being repeated for desired times until a desired film thickness is obtained.