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 7ML. 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.