PHOTOLUMINESCENCE STUDY OF ZNO VARISTOR STABILITY

Photoluminescence (PL) measurements were carried out on commercial ZnO varistor samples that were electrically stressed and/or annealed at d ifferent temperatures. Changes in the intensity of green and yellow lu minescence centers were studied as a function of annealing treatment. It was found that the ZnO luminescence (green and yellow) decrease wit h increase in annealing temperature, reach a minimum at 700 degrees C, and increase again beyond 800 degrees C. Furthermore, these green and yellow luminescence bands observed in the PL spectra are quenched in the ZnO varistor samples, compared to pure ZnO. In an electrically str essed ZnO varistor sample, the luminescence intensity was found to be higher compared to the as-sintered varistor sample. Annealing of the s tressed varistor sample resulted in a decrease of the luminescence int ensity. These PL observations are consistent with previous deep level transient spectroscopy and doppler positron annihilation spectroscopy results. All of the experimental results are consistent with the ion m igration model of degradation and can be explained using a grain bound ary defect model.