Microstructure and electrical properties of La2Ti2O7 thin films on SrTiO3 substrates

Abstract La2Ti2O7 has recently received extensive interest due to its attractive ferroelectric and photocatalytic properties. Here, high-quality La2Ti2O7 thin films on (110) SrTiO3 substrates are prepared via oxidation of LaTiO3 thin films that are fabricated by pulsed-laser deposition. X-ray diffraction analyses reveal that the La2Ti2O7 thin film grows epitaxially on the (110) SrTiO3 substrate and has a good crystallinity. Atomic force microscopy investigations suggest that the surface of La2Ti2O7 thin film turns into a stepped and terraced structure after the annealing. X-ray photoelectron spectroscopy analyses indicate that the La2Ti2O7 thin film has a high purity and contain only Ti4+ ions (i.e., without Ti3+ ions). The band gap of the La2Ti2O7 film is measured to be ∼ 3.2 eV by UV-visible spectra. Atomic and electronic structures of the La2Ti2O7/SrTiO3 heterointerface have been studied by a combination of transmission electron microscopy and first-principles calculations. The La2Ti2O7/SrTiO3 heterointerface is atomically abrupt and coherent. Electronically, this heterointerface contributes to the decrease of band gap in the La2Ti2O7 film. The reduction of spontaneous polarization in La2Ti2O7 is localized in two layers of TiO6 octahedra near the interface due to the discontinuous Ti-O bonds between the perovskite layers of La2Ti2O7.