In-plane lattice constant tuning of an oxide substrate with Ba _1-xSr_xTiO₃ and BaTiO₃ buffer layers

Oxide thin films are usually grown on substrates which offer the smallest mismatch with the desired structure of the film. The choice of high-quality substrates with suitable lattice constants is unfortunately very limited. Coherently grown films are therefore always under tensile or compressive in-plane strain. We report on the growth of a Ba_1-xSr _xTiO₃/BaTiO₃ bilayer buffer on a SrTiO ₃ substrate. Changing the Sr/Ba ratio in the Ba_1-xSr _xTiO₃ layer can be used to select a desired in-plane lattice constant in the 3.9 to 4.0 Å range. The thinner BaTiO₃ film serves as a compliant layer while the thicker Ba_1-xSr _xTiO₃ film determines the final surface lattice constant. This is achieved by a combination of low-temperature deposition and annealing at 1350°C. X-ray diffraction and high-resolution transmission electron microscopy were used to show that the final buffer layer surface is fully relaxed, cubic, and essentially defect free, while all lattice mismatch between the substrate and the buffer is relieved by misfit dislocations at the compliant BaTiO₃/SrTiO₃ interface.