The role of activator–activator interactions in reducing in low-voltage-cathodoluminescence efficiency in Eu and Tb doped phosphors

Abstract

High resolution measurements of spectrally resolved cathodoluminescence (CL) decay have been made in several commercial and experimental phosphors doped with Eu and Tb at beam energies ranging from 0.8 to 4 keV. CL emission from the lowest two excited states of both rare-earth activators was compared to the decay of photoluminescence (PL) after pulsed laser excitation. We find that, at long times after the cessation of electron excitation, the CL decay rates are comparable to those measured in PL; at short times, the decay process is considerably faster and has a noticeable dependence on the energy of the electron beam. These beam energy effects are largest for the higher excited states and for phosphors with larger activator concentrations. Measurements of the experimental phosphors over a range of activator fractions from 0.1 to 0.002 show that the beam energy dependence of the steady-state CL efficiency is larger for higher excited states and weakens as the activator concentration is reduced; the latter effect is strongest for Y2SiO5:Tb, but also quite evident in Y2O3:Eu. We suggest that the electron beam dependence of both the decay lifetimes and the steady state CL efficiency may be due to interaction of nearby excited states which occurs as a result of the large energy deposition rate for low energy electrons. This picture for nonradiative quenching of rare-earth emission is an excited state analog of the well-known (ground state-excited state) concentration quenching mechanism.