Microwave dielectric properties of strained Ba0.5Sr0.5TiO3 films with and without strain-induced permanent polarization at room temperature

At room temperature, Ba0.5Sr0.5TiO3 films can be distorted into in-plane or out-of-plane tetragonal structure depending on the type of film strain while the bulk ferroelectric is in a paraelectric phase (i.e., cubic). The film distortion can be accompanied with or without strain-induced permanent polarization depending on the intensity of film strain. The microwave dielectric constant and tuning of the distorted films are observed to be heavily dependent on the directions of both the tetragonal distortion and the permanent polarization with respect to the measurement direction. The experimentally observed anisotropic dielectric properties of the distorted films are interpreted based on the phenomenological thermodynamics of film strain effect depending on the presence of permanent polarization in the films at room temperature and are discussed with a physical model of polarization and dielectric tuning mechanisms in the strained films. (c) 2007 American Institute of Physics.