Trapping mechanism in the afterglow process of the rare-earth activated Y2O2S phosphors

Phosphorescence properties are investigated in Y2O2S phosphors doped with rare-earth (lanthanoid, Ln) ions. Luminescence afterglow with a decay time of several ten milliseconds is observed at room temperature in the phosphors activated by Nd, Sm, Eu, Dy, Ho, Tm, Er, and Yb. The depths (thermal activation energies) of the traps causing the afterglow are measured with the transient luminescence method. It is concluded that the excited electron and the hole in the conduction and valence bands are trapped separately in the states (impurity levels) located in the vicinity of the Ln3+ ion. The trapping depths of the level range from 0.3 to 1.1 eV and are dependent on the electron affinity of the Ln3+ ion estimated from the energy difference between the 4fn+1 and the 4fn configurations in the 4f shell of the ion.