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

Using a combination of reciprocal and real-space techniques, the structural evolution and its effect on the surface morphology is investigated for MgO films of 1-30 ML thickness epitaxially grown on Mo(001). The 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 A has been determined by means of grazing incidence x-ray diffraction. 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 tunneling 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 connected by nonpolar steps appear on the oxide surface. Thanks to the combination of different diffraction techniques and STM measurements, a comprehensive picture of the relaxation mechanisms in MgO films on Mo(001) can be drawn.