Polar and non-polar domain borders in MgO ultrathin films on Ag(0 0 1)

The morphology of ultrathin epitaxial MgO layers reactively grown at 470 K and at 1ML min_1 on Ag(001) wasinvestigated by scanning tunnelling microscopy (STM). At deposition of 1.3 and 2.6 ML, square MgO domains ofabout (10 • 10) nm2 form. On mono- and bi-layer domains, edges are mainly oriented along the h110i oxide direction,corresponding to polar (P) borders, while on multi-layer domains also non-polar (NP) borders appear, correspondingto the h100i orientation. By using periodic models and ab initio DFT calculations, the energetics of the two orientationswas studied. Interaction with the metal substrate was found to strongly reduce the instability of P borders, although notenough to make the two orientations equally stable. The results are discussed, also with reference to conflicting experimentalevidence from other laboratories._ 2005 Elsevier B.V. All rights reserved.