The Relationship Between Concentration and Efficiency in Rare Earth Activated Phosphors

Abstract

The concentration‐dependence of cathodoluminescence efficiency for the rare earth (RE) activators Ce³⁺, Eu³⁺, Gd³⁺, or Tb³⁺ in YAG is compared at 300° and 25°K and extends earlier work by the authors. Competitive recombination processes in the garnet phosphors include recombination at bulk lattice defect centers detected by near‐u.v. luminescence and recombination through some shunt pathway dominant at 300°K. The experimental data are interpreted using a kinetic model, and except for the activator Gd³⁺, can be fitted using reasonable parameters. The model includes a specific mechanism for activator excitation (a bound exciton state) and the effects of impurity scattering on the rate of recombination at lattice defect centers. It is shown that one of the most important factors governing the general shape of the efficiency vs. concentration curve is the activator thermalization factor, . This in turn depends on the depth of the impurity potential and the rate of energy transfer in the impurity‐exciton complex. The general theory as developed is useful in rationalizing earlier measurements of efficiency vs. concentration for RE activators in a number of oxide and oxysulfide lattices.