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 (100) 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*0x10(--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.