Early stages of YBa2Cu3O7−δ epitaxial growth on MgO and SrTiO3

Abstract

The initial stages of epitaxial growth of c-axis-perpendicular laser-ablated ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ (YBCO) thin films on ${\mathrm{SrTiO}}_3$ and MgO substrates were studied by scanning tunneling microscopy. Surface images of films grown on ${\mathrm{SrTiO}}_3$ with a sequential thickness variation reveal a transition from layerlike to island growth between 8 and 16 unit cells thickness, i.e., YBCO grows on ${\mathrm{SrTiO}}_3$ by a Stranski-Krastanov mechanism. Evidence also points to the existence of a slightly larger critical thickness for the introduction of screw dislocations into those films. In contrast, even the thinnest (∼8 cells thick) YBCO films were found to grow on MgO by an island mechanism, with screw-growth features clearly present.