Electronic interface states at grain boundaries in ZnO:Pr varistors by single grain boundary measurements

Electronic interface states at grain boundaries in ZnO:Pr varistors were directly observed by single grain boundary measurement. Photo-capacitance spectroscopy and photo-isothermal capacitance transient spectroscopy(photo-ICTS) were applied to these experiments. Photo-capacitance spectrum in the near infrared region showed a peak at 1380 nm, i.e. 0.9 eV. Since this peak is originated by electron excitation from the interface states at the grain boundary, the energy for this peak indicates that the electronic states at the grain boundaries are located at 0.9 eV below the top of the double Schottky barrier. This result is the direct evidence which shows the value of the energy level of the interface states in ZnO varistors. Moreover the ICTS peak shifted to shorter range by the irradiation of 1380 nm light. The photo-intensity change of ICTS peaks revealed the linear relation between emission rate of the interface states and the light intensity.