Thickness-dependent strain in epitaxial MgO layers on Ag(0 0 1)

Thin MgO films (1–20 ML) were synthesised by evaporating the metallic component in oxygen atmosphere on Ag(0 0 1). The reduced lattice misfit (3.1%) between the (0 0 1) surface unit meshes of the Ag fcc and MgO rock salt structure leads to the formation of epitaxial MgO layers with the (0 0 1)MgO∥(0 0 1)Ag and [1 0 0]MgO∥[1 0 0]Ag orientation relationships. The oxide layers were structurally characterized by LEED and modulated electron emission, while their surface morphology was studied by STM. In spite of the weak interaction between the oxide overlayer and the substrate, the occurrence of a significant tetragonal distortion of the MgO structure has been detected. We investigated in detail the strain in the oxide layer as a function of the thickness. Thin (<6 ML) films exhibit an expansion along the growth axis with respect to the equilibrium phase. This vertical strain progressively reduces as the thickness increases and disappears at about 9 ML. A Poisson ratio larger than the bulk MgO value has been obtained for the 3 ML film, while the bulk value is appropriate films thicker than 6 ML. STM data indicate that an almost complete substrate coverage is obtained at 3 ML deposition.