Rheed studies of epitaxial growth of YBCO-films prepared by thermal co-evaporation

We prepare YBCO films of high quality by reactive co-evaporation of the metals. The rates are controlled by individual quartz crystal monitors, or else by a quadrupole mass spectrometer. On the standard substrates MgO, LaAlO₃ and SrTiO₃ we obtain T_c (R = 0) = 92 K, j_c = 3.10⁶ A/cm² at 77 K and surface resistances of 30 mΩ at 87 GHz and 77 K. These results are comparable to those of the best alternative techniques. Moreover we achieve good quality even on bare silicon substrates, probably because our substrate temperature required for the in-situ-formation of the perovskite structure is only 600-650 ° C. The further improvement of the films requires detailed information on the epitaxial growth conditions of the films. This can be obtained by in-situ-RHEED even with the oxygen present. Our RHEED studies demonstrate that the crystallinity of the substrate is the relevant parameter which decides upon the successful epitaxy. On good MgO-substrates which have Kikuchi lines in their RHEED patterns, we observe the perovskite structure of the YBCO films already when they are as thin as 0.6 nm. These films are superconductors of high quality. In contrast, we do not observe epitaxial growth on more disordered MgO substrates without Kikuchi lines, and the resulting films are poor superconductors. The growth properties on LaAlO₃ and on bare Si are also discussed.