Probing the electron trap-depth distribution in Sr4Al14O25:Eu2+,Dy3+

Abstract

Sr4Al14O25:Eu2+,Dy3+ is a superluminous persistent luminescence phosphor. Owing to its intrinsic and extrinsic point defects, it tends to luminescence incessantly after excitation, in some cases doing so for as long as 20 h. We report analysis for the energy distribution of its electron traps. The study was done using thermoluminescence (TL) measured on a sample irradiated to 2 Gy at 20 °C. A glow curve measured at 1 °C/s shows two glow peaks at 64 °C (labelled as P1) and at 252 °C (P2). The variation of intensity of peak P1 as a function of heating rate is consistent with competing radiative and non-radiative recombination processes. When the heating rate is faster than 1 °C/s, non-radiative recombinations dominate and the luminescence efficiency drops sharply beyond 70 °C. The activation energy for thermal quenching is estimated to be 0.69±0.06 eV. Once corrected for thermal quenching, the glow curves show a secondary peak (P1A) on the high temperature end of P1. The so-called Tm-Tstop method shows that peaks P1 and P2 gradually shift towards high temperature with preheating temperature Tstop. Indeed, the peak position Tm of both peaks increases with irradiation temperature Tirr. Together, these results suggest that peaks P1 and P2 are each composed of closely spaced overlapping peaks. Analysis of the fractional area between a pair of glow curves measured from the sample irradiated at different temperatures (Tirr) shows that the trap distribution associated with peak P2 is Gaussian whereas that associated with peak P1 follows a non-Gaussian distribution. The activation energy of the electron traps responsible for P1 are estimated to be in the range 0.63–0.68 eV. On the other hand, the Gaussian distribution of trap energy levels responsible for peak P2 suggests that most of the traps have an activation energy of ∼1.58 eV.