Polar and non-polar domain borders in MgO ultrathin films on Ag(001)

The morphology of ultrathin epitaxial MgO layers reactively grown at 470 K and at 1 ML min(-1) on Ag(001) was investigated by scanning tunnelling microscopy (STM). At deposition of 13 and 2,6 ML. square MgO domains of about (10 x 10) nm(2) form. On mono- and bi-layer domains. edges are mainly oriented along the (110) oxide direction, corresponding to polar (P) borders. while on multi-layer domains also non-polar (NP) borders appear. corresponding to the (100) orientation. By using periodic models and ab initio DFT calculations, the energetics of the two orientations was studied. Interaction with the metal substrate was found to strongly reduce the instability of P borders, although not enough to make the two orientations equally stable. The results are discussed. also with reference to conflicting experimental evidence from other laboratories. (c) 2005 Elsevier B.V. All rights reserved.