Defect structure in homoepitaxial non-stoichiometric strontium titanate thin films

Abstract

The defect structure of non-stoichiometric strontium titanate thin films epitaxially grown on SrTiO₃(100) substrates by pulsed-laser deposition was studied using transmission electron microscopy and X-ray diffraction. The A-site-excess SrTiO₃ thin film adopted a completely unrelaxed pseudomorphic perovskite structure with an enlarged lattice parameter normal to the surface. Excess SrO was accommodated as Ruddlesden-Popper planar faults showing a peculiar three-dimensional mosaic structure, without forming secondary phases or dislocations due to non-stoichiometry. Biaxial compressive stress induced by lattice mismatch between the film and substrate was estimated at 9,7 GPa. Such a highly strained film is attributable to planar fault formation, which increases the cell volume and suppresses misfit dislocations. The B-site-excess SrTiO₃ thin-film microstructure consisted of crystalline SrTiO₃ and amorphous TiO₂-rich phases. The slight increase in the lattice parameter normal to the surface and crystallographic shear structure with a TiO₂ double layer suggest the low solubility of excess TiO₂.