Thickness dependence of crystal and electronic structures within heteroepitaxially grownBiFeO3thin films

Crystal and electronic structures of $\mathrm{BiFe}{\mathrm{O}}₃$ thin films ($\ensuremath{\sim}10$ and \ensuremath{\sim}300 nm) grown on $\mathrm{SrTi}{\mathrm{O}}₃$ substrate have been investigated in terms of $\mathrm{BiFe}{\mathrm{O}}₃$ film thickness dependence using the advanced transmission electron microscopy (TEM) technique. Electron diffraction patterns of both $\mathrm{BiFe}{\mathrm{O}}₃$ thin films acquired along ${[011]}_{\mathrm{SrTi}{\mathrm{O}}₃}$ cross sections clearly exhibited the existence of extra Bragg's reflections which are absent in that from $\mathrm{SrTi}{\mathrm{O}}₃$. Structure factor calculations unambiguously revealed that the electron diffraction pattern corresponds to the [211] net pattern of rhombohedral $\mathrm{BiFe}{\mathrm{O}}₃$. High-resolution TEM images combined with multislice simulation also demonstrated that the crystalline structure of both $\mathrm{BiFe}{\mathrm{O}}₃$ films is rhombohedral. Electron energy loss spectroscopy results for both $\mathrm{BiFe}{\mathrm{O}}₃$ thin films showed spectra with the characteristics of bulk $\mathrm{BiFe}{\mathrm{O}}₃$, i.e., rhombohedral. The lattice mismatch of 2.5% between $\mathrm{BiFe}{\mathrm{O}}₃$ and $\mathrm{SrTi}{\mathrm{O}}₃$ found in a particular epitaxial relationship is considered to be the reason that $\mathrm{BiFe}{\mathrm{O}}₃$ can grow by maintaining its bulk crystalline, i.e., rhombohedral, structure.