On the nature of niobium substitution in niobium doped titania thin films by AACVD and its impact on electrical and optical properties

Abstract

Niobium doped TiO₂ thin films were deposited on silica coated glass substrate using aerosol assisted chemical vapour deposition (AACVD) from hexane solution, at 500 °C. The as-deposited films appeared blue, were transparent in the visible, were reflective in the IR region at around 30% and were electrically conductive (n = 1.23 × 10¹⁹ cm⁻³, μ = 18.9 cm² V⁻¹ s⁻¹, sheet resistance = 120 Ω □⁻¹). The structure of the films was investigated using XRD, Raman spectroscopy, XPS and X-ray absorption spectroscopy (XAS). No visible phase segregation was found by XRD nor by Raman, though analysis of the Nb K-edge using XANES and EXAFS revealed the presence of niobium both incorporated into the titanium dioxide lattice as well as present in the form of Nb₂O₅. The high resolution TEM imaging showed sub 4 nm Nb₂O₅ crystals within the lattice. This work questions the solubility limit of niobium in the TiO₂ lattice and suggests previous literature on Nb-doped TiO₂ may have overestimated the degree of niobium substitution.