Microstructures and surface step-induced antiphase boundaries in epitaxial ferroelectric Ba _0.6Sr _0.4TiO ₃ thin film on MgO

Ba _0.6Sr _0.4TiO ₃ thin films were epitaxially grown on (001) MgO substrates using pulsed laser ablation. Cross-sectional and plan-view transmission electron microscopy have been employed to study the microstructures and the interface behavior of the as-grown thin films. The 110-nm-thick Ba _0.6Sr _0.4TiO ₃ thin films have a flat surface and sharp interface. The entire thin film has a single-crystal cubic structure with an interface relationship of (001) Ba _0.6Sr _0.4TiO ₃//(001) _MgO and 〈100〉 Ba _0.6Sr _0.4TiO ₃//〈100〉 _MgO with respect to the substrate. The 6.4% lattice mismatch between Ba _0.6Sr _0.4TiO ₃ and MgO was completely released at the interface by forming equally spaced misfit dislocations with a distance of ∼3.2 nm. High-resolution transmission electron microscopy investigation shows that the initial grown layer of the film is the TiO ₂ monolayer. The growth models of "substrate surface-terrace induced defects" for perovskite on a rock-salt system have been developed to understand the as-grown features where the conservative and nonconservative antiphase boundaries can be formed.