Carrier Compensation by Excess Oxygen Atoms in Anatase Ti0.94Nb0.06O2+delta Epitaxial Thin Films

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 (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 Nb5+ relative to that of Nb4+ 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. (C) 2010 The Japan Society of Applied Physics