Observation of charge transfer process for optical emission of rare-earth dopant using electric probing technique

Charge propagation dynamics for the excitation and de-excitation of samarium doped titanium dioxide (TiO₂:Sm) were observed by an electric measurement technique, namely, the phase response of complex impedance to step-function UV irradiation. The probing frequency for the impedance measurement was widely changed, and a critical frequency that emphasizes a specific charge propagation path was found. Above the critical frequency of 1 kHz the transitions from the photo-excited state to ground state progressed via a rapid, and a slow charge propagation path. Through a comparison of the frequency responses obtained under continuous UV light (photo-excited state) and in the dark (ground state), the rapid path was assigned to charge recombination in a defect-dopant complex in TiO₂:Sm, which is consistent with the persistence of luminescence observed in conventional optical measurements. The slow path that is not associated with the optical transition was explained by a dissipation of excited charges. Similar path consistent with the charge dissipation explanation was confirmed below the critical frequency <1 kHz.