Charge-exchange processes in titanium-doped sapphire crystals. II. Charge-transfer transition states, carrier trapping, and detrapping

From the time-resolved emissions of the ${\mathrm{Ti}}^{4+}$:${\mathrm{Al}}_2$ ${\mathrm{O}}_3$ charge-transfer transition, two types of ${\mathrm{Ti}}^{4+}$ are found. The temperature-dependent lifetimes of the excited ${\mathrm{Ti}}^{4+}$ are well explained by a three-level system with a lower triplet excited state and a higher singlet excited state. The trapping of charge carriers following the charge exchange between Ti and the host is shown by the thermal release of trapped electrons and holes. These are revealed by thermally stimulated conductivity and thermoluminescence. The possible identities of these traps are discussed.