Impact ionization mechanism in rare earth activated sulfides

In this letter we present the theoretical estimation of the 3+■2+ ionization energies of rare earth (RE) ions in sulfides. It is shown that for Yb, Eu, Sm, Tm, and Pr the RE2+ energy level is located in the forbidden gap of wide-gap sulfides [ZnS (except Pr), CaS, SrS, BaS, MgS]. For these RE ions the possibility of a new efficient excitation mechanism (impact ionization) of electroluminescence is indicated. Highly efficient electroluminescence can be obtained if the ionized RE center rapidly recaptures a free carrier either directly or indirectly via the RE3+ excited state. This prediction is confirmed by the recent experimental results for Pr-doped SrS. Further experimental results supporting the presented calculations are discussed.