Origin of Cu-rich precipitate formation on superconducting films: A competition between nucleation, oxidation, and growth kinetics

We have unraveled the growth mechanism of Cu-rich precipitates on the surface of c-axis-oriented DyBa2Cu3O7 thin films. Despite the strong oxidation conditions present, the surface of c-axis DyBa2Cu3O7 acts as an ideal nucleation site for Cu2O crystallites, whose (110) planes are parallel to the substrate surface. Once nucleated, they act as effective sinks for the rest of the deposited copper and form large crystals that revert to monoclinic CuO upon cooling. This result has important implications for all currently used thin film growth processes and indicates that smooth surfaces can only be prepared under stringent composition control.