Structure and morphology of thin MgO films on Mo(001)

We report on the structural evolution versus thickness of MgO thin films grown epitaxially on Mo(001) and on the correlation between structure and surface morphology. The misfit strain induced by the mismatch with the substrate is relieved between 1 and 7 ML MgO due to the formation of an ordered network of interfacial misfit dislocations aligned along the MgO <110> directions, particularly evident after annealing the film at 1070 K. A dislocation periodicity of about 60 Å has been determined by means of Grazing Incidence X-ray Diffraction (GIXD). The dislocations induce a tilting of the surface that appears in electron diffraction along the <100> MgO directions for thin films and changes to <110> directions when the oxide thickness increases. Scanning Tunnelling Microscopy (STM) shows the presence of a regular pattern on the surface below 7 ML thickness associated to the dislocation network. With increasing thickness screw dislocations with non-polar steps appear on the oxide surface.