Thin Film Synthesis of Superconducting Ceramics

Highly oriented, epitaxial Y sub 1 Ba sub 2 Cu sub 3 O sub (7-x) (001) thin films were prepared on MgO(100) and SrTiO sub 3 (100) by molecular beam epitaxy. The in-situ growth was achieved by incorporating reactive oxygen species produced by a remote microwave plasma in a flow-tube reactor. The epitaxial growth occurred in a substrate temperature of 550-720C. The oxygen partial pressure near the substrates is ~ mid 10 sup (-3) torr, while the main chamber pressure is maintained at 5x10 sup (-6) torr. After growth the substrates were cooled to 450C for 20 min in an oxygen pressure increased to 4 times as high. In-situ reflection high energy electron diffraction indicated a highly ordered, atomically smooth film surface. Consistently, optical microscopy also revealed a smooth, featureless film morphology on the submicron scale. The crystallinity of the epitaxial (001) orientation was demonstrated by X-ray diffraction, and ion channeling. Reproducibly good superconducting properties were obtained in the as-grown films, even for samples as thin as 500angstroms. Typical electrical properties for the 500angstrom films are rho(300K)= 230 - 300 microohm-cm, rho(300K)/rho(100K=2.8 - 3.0, T sub c (onset) = 91K, and T sub c (R=0) = 86.5K. The dependence of superconducting properties on the growth parameters including substrate materials, deposition temperature, and film thickness will be discussed.