Electroluminescence of Ho3+ ions in a ZnO varistor-type structure

The characteristic luminescence of Ho3+ ions has been observed in a ZnO varistor-type structure containing holmium oxide. This luminescence evidences the presence of hot electrons as responsible for the excitation of the trivalent rare earth ions. The relationships between the emitted light intensity, the applied potential and the current flow were studied. They show that hot electrons can be generated at a relatively low voltage which corresponds to the prebreakdown region of the varistor. The results are consistent with the model of electrical breakdown at the grain boundaries. In addition, the possibility to use zinc oxide varistors as electroluminescent devices is demonstrated.