Molecular‐beam epitaxy and deposition of high‐T_c superconductors

We have grown thin, highly oriented, polycrystalline DyBa₂Cu₃O_7−x films using molecular‐beam epitaxy (MBE) techniques that show the onset of superconductivity at temperatures above 90 K and complete transitions at temperatures as high as 87 K. These films have critical current densities as high as 5×10⁵ A/cm² at 4.2 K. Films were grown in a modified Varian 360 MBE machine using effusion sources containing the metal constituents, along with a gaseous oxygen source. The early stages of deposition were monitored with reflection high‐energy electron diffraction (RHEED). The best films were obtained on SrTiO₃ substrates at substrate temperatures of 600–750 °C. At these temperatures, the initial stage of growth is dominated by epitaxy of copper islands. At lower temperatures, the growth is amorphous, while at higher temperatures, copper may not be incorporated into the film. Copper incorporation is also affected by oxygen flux. In all cases, the films are semiconducting or insulating as grown, and become superconducting only after high‐temperature oxygen anneals. We comment on the prospects for obtaining true epitaxy superconducting phase in situ.