Investigation of competitive recombination processes in rare-earth activated garnet phosphors

Spectral, kinetic, and relative quantum intensity measurements are reported for a series of ${\mathrm{Y}}_3$ ${\mathrm{Al}}_5$ ${\mathrm{O}}_{12}$ (YAlG) crystals, both nominally undoped and activated by ${\mathrm{Ce}}^{3+}$ and ${\mathrm{Tb}}^{3+}$. The undoped crystal shows a complex emission band in the near uv with fast- and slow-decaying components, analogous to the alkali halides. The band shape appears to involve emission from an unrelaxed and a relaxed defect state of the lattice. In the doped crystals these defect states compete with the activator ions as recombination pathways. The essential properties of the rare-earth activated crystals are described by a simple kinetic model, and initial estimates of the relative magnitudes of the rate constants are made. Temperature-dependence measurements for the cathodoluminescence of the YAlG: Ce (0.01%) crystal show that energy trapped at the defect centers can be thermally released, leading to an interaction between the defects and the activator ions. The effective activation energy for release is approximately 1900 ${\mathrm{cm}}^{-1\mathrm{}}$ and an initial estimate for the thermalization time constant based on the kinetic model is approximately 160 nsec at room temperature. At low concentration the time constant for the initial defect trapping state is slower than this value because of multiple capture effects, $\tau \mathrm{}\left(D\right)\mathrm{}\sim 200$ nsec. The rise and decay curves for ${\mathrm{Ce}}^{3+}$ luminescence reflect this slow time constant, but annealing the crystals in an oxidizing atmosphere introduces an additional shunt path which reduces the luminescence efficiency but decreases the activator time constant to the intrinsic ${\mathrm{Ce}}^{3+}$ value $\tau \left(\mathrm{Ce}\right)\sim 60$ nsec. The significance of the results for phosphor action in these oxides is discussed.