Strain Effect on the Properties of Polar Dielectric Thin Films

Abstract

Low cost scalable processing and substrates are critical for optimized polar dielectric performance of functional oxide thin films if they are to achieve commercialization. Here, we present a comprehensive investigation of the role low-cost MgO, Al2O3, SrTiO3 and Si substrates on the structural and electrical properties of sol-gel derived SrTiO3 (ST) and K0.5Na0.5NbO3 (KNN) thin films. The substrate is found to have a strong effect on the stress/stain state and, consequently, on the dielectric and ferroelectric response of the films. A tensile stress induced in-plane by the thermal expansion mismatch between the substrates and the films observed for ST and KNN films deposited on platinized Al2O3 and Si substrates, respectively, lowers the relative permittivity and remanent polarization values in the parallel plate capacitor geometry. In contrast, a compressive stress/strain observed for ST films deposited on MgO/Pt and KNN films on SrTiO3/Pt substrates result in superior polarization and dielectric permittivity, corresponding to enhanced out-of-plane displacement of Ti4+ ions in ST films and Nb5+ ions in KNN films. It is thus demonstrated that for polycrystalline polar dielectric thin films the relative permittivity and polarization may be optimized through an induced compressive stress state.