Abstract
We investigated the electronic structures of anatase Ti1−xNbxO2+δ (TNO) thin films as a function of Nb concentration x using photoemission spectroscopy (PES) measurements to elucidate the origin of the abrupt decrease in carrier activation in heavily Nb-doped regime. The existing intensity ratio of Nb5+ evaluated from Nb 3d core-level PES spectra maintained a constant value of ∼0.8 at x = 0.06–0.3, implying that electron carriers generated by Nb doping are compensated by p-type defects. Ti 2p-3d and O1s-2p resonant PES measurements of x = 0.06–0.3 films revealed that the in-gap states positioned ∼1 eV below the Fermi level (EF) have a mixed character of Ti 3d and O 2p orbitals, whereas the states at EF mainly have a Ti 3d nature. We proposed a carrier compensation mechanism that interstitial oxygen atoms strongly combined with surrounding Nb atoms kill conduction electrons in heavily Nb-doped anatase TiO2.
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