Fundamental Efficiency Limitations for Low Electron Energy Cathololuminescence

The design of field emission displays is severely constrained by the universally poor cathodoluminescence (CL) efficiency of most phosphors at low excitation energies. As part of the effort to understand this phenomenon, the authors have measured the time decay of spectrally-resolved, pulsed CL and photoluminescence (PL) in several phosphors activated by rare earth and transition metal impurities, including Y{sub 2}O{sub 3}:Eu, Y{sub 2}SiO{sub 5}:Tb, and Zn{sub 2}SiO{sub 4}:Mn. Activator concentrations ranged from {approximately}0.25 to 10%. The CL decay curves are always non-linear 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. They have also measured the dependence of steady state CL efficiency on beam energy. They 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 non-radiative activator decay, hence lower steady state CL efficiency. Excited state clustering, which may be caused by the large energy loss rate of low energy primary electrons, appears to enhance these interactions. In support of this idea, they find that PL decays obtained at high laser pulse energies resemble the non-linear decays seen in the CL data