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 deg. 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 [11-bar0] direction of SrTiO3