Reactive coevaporation of YBaCuO superconducting films

Abstract

Growth conditions for YBaCuO thin films are investigated. Films have been made by reactive e-beam coevaporation using three metal sources. In the best cases, as-made films are superconducting with Tc's (R = 0) up to 90 K and Jc's (at 4.2 K) above 107 A/cm2. Oxygen pressure, deposition temperature, as well as compositional dependencies of the films are presented. It is found that in conditions of lower oxygen, pressure films with average composition off the 1–2–3 stoichiometry have higher Tc's. For pressure <10 mTorr, the highest Tc obtained is for Ba/Y deposition ratio ⋚1.4. The morphology and impurity phases of these films are examined. The Ba-deficient films have oriented CuYO2 and CuO as the dominant impurity phases. C-axis lattice parameters (c0) are also examined. It is found that for a given Tc, films made at lower pressure have c0's which are expanded compared to the films made at higher pressures (>100 mTorr). The expanded c0's for these films cannot be reduced by a low temperature oxygen anneal. We suggest that metal-atom point-like defects are quenched into these films and we discuss a particular Ba-for-Y substitution model.