We report the effect of post-deposition annealing on the electrical transport properties of anatase Ti 0.94 Nb 0.06 O 2 (TNO) epitaxial thin films. Annealing TNO films in pure oxygen drastically suppressed the carrier density ( n e ). A high n e of the order of 10 21 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 Nb 5+ relative to that of Nb 4+ tends to increase with O 2 annealing, suggesting that carriers released by Nb donors are compensated by electron-killing impurity states created by O 2 annealing. Based on these findings, we propose that excess oxygen atoms incorporated by O 2 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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Nogawa et al. (2010) studied this question.
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