Epitaxial growth and polarized Raman scattering of niobium dioxide films

We report the structural, electrical, and optical characterization of epitaxial niobium dioxide (NbO2) films fabricated on MgF2(001) substrates. The films were almost stoichiometric, had an indirect bandgap of 0.7 eV, and exhibited a phase transition at ∼1080 K. A polarized Raman scattering study of the films was conducted to investigate the Raman symmetry in the low-temperature phase. Based on the angular-dependent polarized Raman spectra, we assigned 13 modes to Ag symmetry and 14 to Bg symmetry. We also evaluated the Raman tensor elements of the Bg modes and found that the off-diagonal elements were nearly zero in most of the Bg modes, except for a phonon mode at 267 cm−1. This study aids understanding of the lattice dynamics of NbO2, which plays a critical role in the phase transition.