Polar phonons in polycrystalline SrTiO3 thin films: The role of the substrate

Abstract

Recently we have demonstrated that out-of-plane dielectric properties of polycrystalline SrTiO3 (ST) films depend on the strain/stress induced by the substrates due to thermal expansion misfit. Here, we present a complementary infra-red (IR) spectroscopy investigation of the role of Al2O3 and MgO substrates on the lattice dynamics and in-plane dielectric properties of ST thin films also prepared by sol-gel. We verify that ST films deposited on Al2O3 substrates, being under a tensile stress, present the soft mode behaviour similar to that of ST single crystals, deviating just below 150 K. In contrast, ST films deposited on MgO substrates, being under compressive stress, have higher soft mode frequency from 10 to 300 K. It is thus proved that for polycrystalline ST films the intrinsic in-plane relative permittivity may be tuned through an induced stress state.