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 thesurface morphology is investigated for MgO films of 1–30 ML thickness epitaxially grown on Mo(001). Thestrain induced by the mismatch with the substrate is relieved between 1 and 7 ML MgO due to the formationof an ordered network of interfacial misfit dislocations aligned along the MgO (110) directions, particularlyevident after annealing the film at 1070 K. A dislocation periodicity of about 60 Å has been determined bymeans of grazing incidence x-ray diffraction. The dislocations induce a tilting of the surface that appears inelectron diffraction along the (100) MgO directions for thin films and changes to (110) directions when theoxide thickness increases. Scanning tunneling microscopy (STM) shows the presence of a regular pattern onthe surface below 7 ML thickness associated to the dislocation network. With increasing thickness, screwdislocations connected by nonpolar steps appear on the oxide surface. Thanks to the combination of differentdiffraction techniques and STM measurements, a comprehensive picture of the relaxation mechanisms in MgOfilms on Mo(001) can be drawn.