Interactions of excited activators in rare earth and transition metal doped phosphors and their role in low energy cathodoluminescence

Abstract

We have measured the time decay of spectrally resolved, pulsed cathodoluminescence (CL) and photoluminescence (PL) in several phosphors activated by rare earth and transition metal impurities, including Y2O3:Eu, Y2SiO5:Tb, and Zn2SiO4:Mn. Activator concentrations ranged from ∼0.003% to 10%. The CL decay curves are almost always nonlinear on a log (CL)-linear(time) plot—i.e., they deviate from first-order decay kinetics. These deviations are always more pronounced at short times and larger activator concentrations and are largest at low beam energies where the decay rates are noticeably faster. PL decay is always slower than that seen for CL, but these differences disappear after most of the excited species have decayed. PL decays display higher order decay kinetics and resemble CL decay profiles when the laser excitation intensity is increased. We have also measured the dependence of steady state CL efficiency on beam energy and find that larger activator concentrations accelerate the drop in CL efficiency seen at low beam energies. These effects are largest for the activators which interact more strongly with the host lattice. While activator–activator interactions are known to limit PL and CL efficiency in most phosphors, the present data suggest that a more insidious version of this mechanism is partly responsible for poor CL efficiency at low beam energies. This “enhanced” concentration quenching is due to the interaction of nearby excited activators. These interactions can lead to nonradiative activator decay, hence lower steady state CL efficiency. We suggest that this excited state “clustering” arises from the large energy loss rate of low energy primary electrons.