Thickness-dependent twinning evolution and ferroelectric behavior of epitaxial BiFeO3 (001) thin films

With increasing film thickness of the epitaxial BiFeO3 (001) films deposited on SrTiO3 substrates from 30 to 720 nm, the crystal structure evolutes from a fully strained tetragonal lattice to partially relaxed monoclinic one with rotated twins. Although the mismatch strain results in a significant lattice distortion and twinning evolution, the polarization does not show a direct correlation with strain. Instead, there is a strong dependence of polarization on the body diagonal length of the distorted pseudocubic unit cell. The distortion in the polarization direction is the critical parameter, other than strain, that determines the polarization in the monoclinic phase ferroelectric thin films.