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 triva lent rare earth ions. Compared to varistors which have the same struct ure (a polycrystalline semiconducting zinc oxide pellet sandwiched bet ween two metallic solders) and a closely related chemical composition, these ceramics present a weak non-ohmic behaviour and they can be con sidered as electroluminescent systems with a metal-semiconductor-metal (M-S-M) structure. The mechanism which explains the observed luminesc ence is based on a hot electron impact-excitation of the trivalent rar e earth ions inserted into the semiconducting lattice. According to th e model of varistors, the electrons are generated by thermionic emissi on 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 be en compared to that measured from the scanning electron micrographs.