In-Situ Growth of YBaCuO Superconducting Thin Films at Low Oxygen Pressure

Abstract

We discuss growth of YBa2Cu3Oy thin films in low oxygen pressure. Thermodynamic stability requirements for YBa2Cu3Oy, as presently understood, are presented first. We compare these with the reported conditions for successful in-situ growth of thin films. We then review results on films made by reactive e-beam coevaporation using three metal sources at low oxygen pressure (0.1 – 1 Pa). In the best cases, these films are superconducting with T c ’s (R=0) up to 90K and J c ’s (at 4.2K) above 107 A/cm2. However, several anomalous results have been found for these low pressure films. Films that are made with an average composition off the 1:2:3 stoichiometry, and Ba deficient, have higher T c ’s and less expanded c-axis lattice parameters, c o . The expanded c o ’s for these films cannot be reduced by a low temperature anneal in oxygen. Additionally, the films show evidence that they are hole-doped compared to the ideal material. We suggest that metal-atom point-like defects are quenched into these films. A model based on the presence of Ba-for-Y substitutions is discussed and found to be consistent with the experimental results presented here. The proposed mechanism for this extended cation solubility in the Y layers is based on charge compensation for crystal growth in a reduced oxygen potential. We suggest that these defects are important for superconducting and other properties of the films. We also discuss some implications for low pressure growth.