Domain structure of epitaxial $Bi4Ti3O12$ thin films grown on (001) $SrTiO3$ substrates

The domain structure of epitaxial (001) $Bi4Ti3O12$ thin films grown on (001) $SrTiO3$ substrates by reactive molecular beam epitaxy was studied using transmission electron microscopy. It was found that the $Bi4Ti3O12$ thin films contain randomly distributed rotation domains of two different types, which are related by a 90° rotation around the c axis of $Bi4Ti3O12.$ These domains result from the difference in crystallographic symmetry between the $Bi4Ti3O12$ (001) plane and the $SrTiO3$ (001) surface. Moreover, out-of-phase boundaries were frequently observed in the epitaxial $Bi4Ti3O12$ films. Detailed quantitative high-resolution transmission electron microscopy studies showed that the growth of epitaxial $Bi4Ti3O12$ film on the $SrTiO3$ (001) surface begins with the energetically favorable central $TiO2$ layer in the middle of the triple perovskite block within $Bi4Ti3O12.$ As a result, a number of out-of-phase domain boundaries are formed at the atomic steps on the substrate surface. These studies suggest that $Bi4Ti3O12$ films grow on (001) $SrTiO3$ substrates through two-dimensional island growth mechanism, where individual domains nucleate with random orientations of their polar a axis along either [110] or $[11̄0]$ direction of $SrTiO3.$