Rare earth-doped polycrystalline zinc oxide electroluminescent ceramics

Trivalent rare earth ions (Eu3+, Tm3+, Er3+)-doped zinc oxide ceramics have been prepared. They were found to be luminescent when submitted to electric fields and the luminescence spectra are those of the trivalent rare earth ions. Compared to varistors which have the same structure (a polycrystalline semiconducting zinc oxide pellet sandwiched between two metallic solders) and a closely related chemical composition, these ceramics present a weak non-ohmic behaviour and they can be considered as electroluminescent systems with a metal-semiconductor-metal (M-S-M) structure. The mechanism which explains the observed luminescence is based on a hot electron impact-excitation of the trivalent rare earth ions inserted into the semiconducting lattice. According to the model of varistors, the electrons are generated by thermionic emission at the grain boundaries where double Schottky barriers are present. The variation of the luminescence intensity with the applied voltage allows an estimation of the size of the zinc oxide grains which has been compared to that measured from the scanning electron micrographs.