Depth resolved lattice-charge coupling in epitaxial BiFeO(3) thin film

For epitaxial films, a critical thickness (t(c)) can create a phenomenological interface between a strained bottom layer and a relaxed top layer. Here, we present an experimental report of how the t(c) in BiFeO(3) thin films acts as a boundary to determine the crystalline phase, ferroelectricity, and piezoelectricity in 60 nm thick BiFeO(3)/SrRuO(3)/SrTiO(3) substrate. We found larger Fe cation displacement of the relaxed layer than that of strained layer. In the time-resolved X-ray microdiffraction analyses, the piezoelectric response of the BiFeO(3) film was resolved into a strained layer with an extremely low piezoelectric coefficient of 2.4 pm/V and a relaxed layer with a piezoelectric coefficient of 32 pm/V. The difference in the Fe displacements between the strained and relaxed layers is in good agreement with the differences in the piezoelectric coefficient due to the electromechanical coupling.