Tunneling process in thermally stimulated luminescence of mixed ${\mathrm{Lu}}_x{\mathrm{Y}}_{1-x}{\mathrm{AlO}}_3:\mathrm{C}\mathrm{e}$ crystals

A thermally stimulated luminescence (TSL) study in the temperature range 20–300 °C has been performed after x-ray irradiation on mixed ${\mathrm{Lu}}_x{\mathrm{Y}}_{1-x}{\mathrm{AlO}}_3:\mathrm{C}\mathrm{e}$ crystals $(x=0.1\mathrm{}–0.3)$ and on ${\mathrm{YAlO}}_3:\mathrm{C}\mathrm{e}.$ Several glow peaks have been detected, namely at 50 °C, 100–130 °C (composite structure), 175 °C and 225 °C, whose intensities are dependent upon the crystal composition. The spectrum of the emitted light features one band at 3.45 eV due to the $5\overrightarrow{d}4f$ transition of the ${\mathrm{Ce}}^{3+}$ ion, in good agreement with the UV excited emission spectrum. A detailed analysis of the trap depths of the peaks has been performed in the case of ${\mathrm{Lu}}_{0.2}{\mathrm{Y}}_{0.8}{\mathrm{AlO}}_3:\mathrm{C}\mathrm{e}$ by partial cleaning of the glow curve. A constant energy value of approximately 1.15 eV 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 assisted tunneling process from one trap level found at different distances from cerium emitting centers. A strong increase of TSL intensity has been observed after a prolonged annealing treatment in a vacuum atmosphere, suggesting that the electron traps are probably related to oxygen vacancies. Under this hypothesis, the relation between the frequency factors of the TSL peaks 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.