Structure dynamics of strongly reduced epitaxial BaTiO3−x studied by Raman scattering

Raman scattering technique was used to study structure dynamics of strongly reduced epitaxial barium titanate (BaTiO3−x) thin films grown on MgO (1 0 0) substrates by laser molecular beam epitaxy under different oxygen pressures from 10−2 to 10−5 Pa. X-ray diffraction and asymmetric rocking curves indicate that lattice parameters c and c/a ratio increase, and a slightly decreases with decreasing oxygen pressure, indicating increased lattice volume of BaTiO3−x thin film. Raman spectra confirm that BaTiO3−x thin films are in tetragonal phase with some deviated features, which maybe origin from tensile strain at film–substrate interface due to lattice parameter mismatch. Moreover, two weak peaks in Raman spectra of BaTiO2.52 thin film grown under 3.0 × 10−5 Pa may be induced by second-order two-phonon processes. Raman peaks shift towards lower frequencies with decreasing oxygen pressure during deposition, suggesting a decrease of the stress in BaTiO3−x thin films. In the meantime, Raman peaks become broadened, which may be attributed to higher degree of structural disorder in strongly reduced BaTiO3−x lattice structure.