Identification of two light-induced charge states of the oxygen vacancy in single-crystalline rutile TiO2

The nonstoichiometry of the main structural phase of TiO2 (rutile) and its natural n-type conductivity is due to oxygen deficiency, which has been explained in terms of native defects, i.e., oxygen vacancies and titanium interstitials. While the latter has been unambiguously identified by electron paramagnetic resonance (EPR) and attributed to a shallow donor, the oxygen vacancy lacks such identification, in fact, any identification at all. In this work, two charge states of the oxygen vacancy, i.e., the singly V-O(+) and doubly V-O(0) occupied charge states with S=0.5 and S=1, respectively, are identified by EPR under blue light excitation in fully oxidized TiO2 (rutile). It is shown that the vacancy is a very shallow donor (2.8 meV) and that both charge states suffer different strong lattice relaxations depending on charge.