Functional Properties of Polydomain Ferroelectric Oxide Thin Films

The properties of a ferroelectric, (001)-oriented, thin film clamped to a substrate are investigated analytically and numerically. The emphasis is on the tetragonal, polydomain, ferroelectric phase, using a three domain structure, as is observed experimentally, instead of the two-domain structure used in earlier literature. The previously used, very restrictive set of boundary conditions, arising from the domain walls, is relaxed, creating more modes for energy relaxation. It is argued that this approach gives a more realistic description of the clamped ferroelectric film. It is shown that for the ferroelectric oxides PbZr1−xTixO3, the tetragonal, polydomain phase is present over a wide range of substrate induced strains for x≥0.5, corresponding to the tetragonal side of the bulk phase diagram. A polydomain, rhombohedral phase is present for x<0.5, at the bulk rhombohedral side. Phase-temperature diagrams, and ferroelectric, dielectric, and piezoelectric properties as well as lattice parameters are calculated as function of substrate induced strain and applied field. The analytical formulation allows the decomposition of the numerically obtained values of these properties into three different causes: domain wall motion, field induced elastic effects, and piezoelectric effects. It is found that domain wall motion and polarization rotation of the in-plane oriented domains under an applied field contribute most to the properties, while the out-of-plane oriented domains hardly contribute.