Growth modes transition induced by strain relaxation in epitaxial MgO thin films on SrTiO3 (001) substrates

MgO thin films on SrTiO3 (001) substrates with large lattice mismatch (f = 8%) were deposited by laser molecular beam epitaxy method. Growth modes transition induced by strain relaxation in epitaxial MgO thin films was investigated by using in situ reflection high-energy electron diffraction. Surface morphology was analyzed by atomic force microscopy. The initial deposition stage resulted in the growth of a uniformly strained MgO wetting layer. After reaching a critical thickness, the Stranski-Krastanow growth mode allows the highly strained 2D layer to relax via coherently strained 3D islands. This can induce formation of MgO quantum dots. Moreover, the MgO lattice was found to be misoriented with respected to each other, giving rise to a biaxially textured MgO layer at this stage. The results could be explained using a kinematical electron scattering model combined with a geometrical distribution of the MgO lattice. Finally, strain was relieved by introducing misfit dislocations at the film/substrate interface, which could flatten the film surface. Our research provides the basis for the growth of high-quality single crystal MgO ultra thin films and a valuable approach for further understanding the relationship between strain relaxation and growth modes transition.