EARLY STAGES OF YBA2CU3O7-DELTA EPITAXIAL-GROWTH ON MGO AND SRTIO3

The initial stages of epitaxial growth of c-axis-perpendicular laser-ablated YBa2Cu3O7-delta (YBCO) thin films on SrTiO3 and MgO substrates were studied by scanning tunneling microscopy. Surface images of films grown on SrTiO3 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 SrTiO3 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 (approximately 8 cell thick) YBCO films were found to grow on MgO by an island mechanism, with screw-growth features clearly present.