Ferroelectric Thin Films and Multilayer Structures Based on Them

Abstract

A brief review is presented of the results of recent research into ferroelectric films and their multilayer structures. The main attention is paid to theoretical calculation of the physical properties that characterize ferroelectric materials (electric polarization, phase transition temperature, dielectric response) in thick and thin films and their multilayer structures. Within the phenomenological theory it is shown that the main reason for a decrease in film symmetry is internal mechanical stress connected with the mismatch in lattice constants, difference in thermal expansion coefficients of the substrate and film, and also growth imperfections. These stresses lead to a change (decrease or increase) in the para-ferroelectric transition temperature that is actually observed in thick films. In thin films, where it is necessary to consider polarization gradients, a ferroelectric transition develops whose temperature depends on film thickness (thickness induced phase transition). The polarization and dielectric permittivity of films and their multilayer structures are calculated. It is demonstrated that permittivity becomes infinitely great close to the thickness induced phase transition temperature. The theory fits well with the recently observed huge dielectric permittivity and its temperature dependence in a multilayer structure of thin films of PbTiO₃ and Pb_0.72La_0.28TiO₃.