Structural and optical properties of BaTiO₃ ultrathin films

Using laser molecular beam epitaxy technique, BaTiO₃ ultrathin films of 7, 12, 25, 750 monolayer (ML) thicknesses were obtained on MgO substrates. The effect of reducing the thickness of a BaTiO₃ film on its structural and optical properties was investigated by Raman scattering measurement, X-ray diffraction technique, and optical transmittance spectra. In situ reflection high-energy electron diffraction and X-ray diffraction observations have revealed that the BaTiO₃ films are epitaxially grown on MgO substrates from the initial stage without any other phase formation. Even in the sample of 7 ML, Raman scattering can reveal the structural information of the film. The BaTiO₃ ultrathin films have a highly c-axis–oriented tetragonal phase, and the formation of the perovskite phase is found in films as thin as 7 ML. An increase of the energy gap and the blue shifts of Raman modes are observed with reduction of the film thickness and the possible origin was analyzed.