Abstract
We report the effect of post-deposition annealing on the electrical transport properties of anatase Ti0.94Nb0.06O2 (TNO) epitaxial thin films. Annealing TNO films in pure oxygen drastically suppressed the carrier density (ne). A high ne of the order of 1021 cm-3 was recovered by successive annealing in pure hydrogen. Core-level X-ray photoemission spectroscopy revealed that Ti and Nb respectively exist as tetravalent and pentavalent ions in fully oxidized samples. The concentration of Nb5+ relative to that of Nb4+ tends to increase with O2 annealing, suggesting that carriers released by Nb donors are compensated by electron-killing impurity states created by O2 annealing. Based on these findings, we propose that excess oxygen atoms incorporated by O2 annealing occupy interstitial sites and behave as deep acceptor states, which compensate electron carriers generated by Nb doping. Resonant valence-band photoemission spectroscopy directly confirmed the formation of deep acceptor states associated with oxygen annealing.
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