Abstract
A reaction model explaining (i) the hydrogen-mediated transformation of the vacancy-oxygen (VO) center into a vacancy–oxygen-hydrogen center (VOH*), with an energy level at 0.37 eV below the conduction-band edge (Ec), and (ii) the passivation of the divacancy center is presented. VOH* dissociates with a rate of 2×10−5 s−1 at 195 ∘ C, causing VO to recover after long duration (>104 min), while a similar evolution occurs at 300∘C on a time scale of the order of 10 min. The diffusivity of the monatomic hydrogen used in the model agrees closely with the established values for the diffusivity of protons. After the recovery of VO, further annealing at higher temperatures and/or longer durations transforms VO into the “ordinary” vacancy–oxygen-hydrogen center with an energy level at Ec−0.32 eV (VOH). VOH is subsequently transformed into VOH2. For temperatures above 250 ∘C, two additional hydrogen-related levels occur (∼0.17 and ∼0.58 eV below Ec) with a one-to-one ratio and a possible association with different charge states of a V2OH center is discussed.
