Tunneling process in thermally stimulated luminescence of mixed LuxY1-xAlO3 : Ce crystals

A thermally stimulated luminescence (TSL) study in the temperature range 20 -300 degrees C has been performed after x-ray irradiation on mixed LuxY1-xA lO3:Ce crystals (x=0.1-0.3) and on YAlO3:Ce. Several glow peaks have been d etected, namely at 50 degrees C, 100-130 degrees C (composite structure), 1 75 degrees C and 225 degrees C, whose intensities are dependent upon the cr ystal composition. The spectrum of the emitted light features one band at 3 .45 eV due to the 5d-->4f transition of the Ce3+ ion, in good agreement wit h the UV excited emission spectrum. A detailed analysis of the trap depths of the peaks has been performed in the case of Lu0.2Y0.8AlO3:Ce by partial cleaning of the glow curve. A constant energy value of approximately 1.15 e V has been found for all TSL peaks. From this result, and from the lack of a thermally stimulated current signal in the whole temperature range, it is possible to propose that TSL recombination is governed by a thermally assi sted tunneling process from one trap level found at different distances fro m cerium emitting centers. A strong increase of TSL intensity has been obse rved after a prolonged annealing treatment in a vacuum atmosphere, suggesti ng that the electron traps are probably related to oxygen vacancies. Under this hypothesis, the relation between the frequency factors of the TSL peak s and the shortest O-Ce distances in the lattice has been investigated: an exponential dependence has been found, in accordance with what is expected for the thermally assisted tunneling recombination process.