Thickness dependence of crystal and electronic structures within heteroepitaxially grown BiFe O3 thin films

Crystal and electronic structures of BiFeO3 thin films (∼10 and ∼300 nm) grown on SrTiO3 substrate have been investigated in terms of BiFeO3 film thickness dependence using the advanced transmission electron microscopy (TEM) technique. Electron diffraction patterns of both BiFeO3 thin films acquired along [011]SrTiO3 cross sections clearly exhibited the existence of extra Bragg's reflections which are absent in that from SrTiO3. Structure factor calculations unambiguously revealed that the electron diffraction pattern corresponds to the [211] net pattern of rhombohedral BiFeO3. High-resolution TEM images combined with multislice simulation also demonstrated that the crystalline structure of both BiFeO3 films is rhombohedral. Electron energy loss spectroscopy results for both BiFeO3 thin films showed spectra with the characteristics of bulk BiFeO3, i.e., rhombohedral. The lattice mismatch of <2.5% between BiFeO3 and SrTiO3 found in a particular epitaxial relationship is considered to be the reason that BiFeO3 can grow by maintaining its bulk crystalline, i.e., rhombohedral, structure.