Dependence of YBa2Cu3O7-x Ultrathin Film Surface Morphology on Substrate and Growth Temperature

We have studied the influence of substrate matching and growth temperature on the morphology of YBa2Cu3O7- x ultrathin film. For films grown on SrTiO3 and NdGaO3 substrates, we observed a temperature-dependent surface morphology. The film changed from a smooth epitaxial layer at growth temperatures equal to or below 750° C, into one with several unit-cell-high islands at 800° C. It was also found that the density of these islands differs between films on SrTiO3 and NdGaO3 substrates. This substrate-dependent island density was interpreted in terms of the Stransky-Krastanov-type film structure model together with an assumed temperature dependence of the epitaxial layer thickness.