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Mechanism of insulator-to-metal transition in heavily Nb doped anatase TiO2

Heavily Nb-doped anatase TiO2 (TNO) thin films were prepared by pulsed dc magnetron sputtering using an Nb-doped TiO2 target. The as-grown films exhibit high resistivity whose resistance decreases by ∼2 × 104-fold upon vacuum annealing. The ∼40% Nb-doped anatase TiO2 film shows a low resistivity of 5.7 × 10−4 Ω cm and a high electron concentration of 3.07 × 1021 cm−3. Combined in situ x-ray photoelectron spectroscopy (XPS) measurement and density-functional theory (DFT) calculations show that oxygen interstitial (Oint) and Nb interstitial (Nbint) defect clusters introduce localized shallow p-type accepter states that trap the electrons and reduce the conductivity. These defect clusters can be eliminated by vacuum annealing which is companied by outward diffusion of Nb. As a result, the trapped electrons backfill the Ti sites which are delocalized to promote conductivity.