Large and constant absorption coefficient in NbxTi1−xO2 thin films throughout the visible range

Abstract

We investigated the optical absorption properties of NbxTi1−xO2 thin films, a solid solution combining the reduced titanium and niobium oxide phases TiO2 and NbO2. The optical absorption properties of NbxTi1−xO2 thin films prepared by pulsed laser deposition at 600 °C were shown to be large in magnitude at an almost constant value of ≈17 µm−1. Because this large absorption coefficient is nearly independent of incident photon energy in the visible range (400–700 nm), the NbxTi1−xO2 thin films appear optically black. Flat and homogenous, optically black coatings like these are desirable for color isolation in flat panel displays. The origin of flat wavelength dispersion in NbxTi1−xO2 is the coexistence of semiconducting absorption and metallic light absorption mechanisms. Localized Nb-Nb dimers in the metallic NbxTi1−xO2 phase open an optical band gap which gives rise to semiconducting behavior. We show here that reduction results in strong visible light sensitization in normally transparent Nb-Ti oxides.