Dielectric properties in heteroepitaxial Ba0.6Sr0.4TiO3 thin films: Effect of internal stresses and dislocation-type defects

A series of heteroepitaxial ${\mathrm{Ba}}_{\mathrm{0.6}}{\mathrm{Sr}}_{\mathrm{0.4}}{\mathrm{TiO}}_{\mathrm{3}}$ were grown on $\text{0.29(}{\mathrm{LaAlO}}_{\mathrm{3}}\text{):0.35(}{\mathrm{Sr}}_{\mathrm{2}}{\mathrm{TaAlO}}_{\mathrm{6}})$ substrates using pulsed-laser deposition. X-ray characterization revealed compressive in-plane stresses in the thinnest films, which were relaxed in a continuous fashion with increasing thickness. A theoretical treatment of the misfit strain was in good agreement with the measured out-of-plane lattice parameter. The low-frequency dielectric constant was measured to be significantly less than the bulk value and found to decrease rapidly for films less than 100 nm. A thermodynamic model was developed to understand the reduction in dielectric constant. By observing the microstructure using plan-view and cross-section transmission electron microscopy, we identified local strain associated with a threading dislocation density on the order of ${10}^{11} {\mathrm{cm}}^{\mathrm{-2}}$ as a possible mechanism for dielectric degradation in these films.