Temperature dependence of the dielectric properties of strained barium strontium titanate films for tunable microwave applications

Understanding strain effects is critical to achieve desirable dielectric properties in ferroelectric films, which are of interest for tunable microwave applications. Sputter-deposited Ba$_{1-x}$Sr$_{x}$TiO$_{3}$ films on (001) MgO were studied in various strain states: in-plane or out-of-plane tetragonal lattice distortions. The optimal system calibration for microwave measurements changes greatly with temperature, requiring frequent recalibration. A temperature-dependent interpolation calibration technique was developed to increase the efficiency of measurements taken as a function of temperature. The films showed significant differences in the ferroelectric phase transition due to lattice distortions, with a strong temperature dependence of the in-plane dielectric behavior for films under tensile strain and a weak temperature dependence for films under compressive strain. We believe that films under tensile strain have polarizations aligned parallel to the applied electric field and so the in-plane dielectric properties are strongly coupled with the field, while films under compressive strain have polarizations perpendicular to the field, resulting in minimal influence on the in-plane dielectric behavior.