Optical properties of vacancies in thermochemically reduced Mg-doped sapphire single crystals

Optical absorption and emission experiments were used to characterize defects and defect aggregates in Mg-doped Al2O3 crystals due to thermochemical reduction at high temperatures. Oxygen vacancies and higher-order defects are produced much more readily in Mg-doped than in undoped Al2O3 crystals. F+ and F centers (oxygen vacancies with one or two electrons, respectively) were monitored by their optical absorption bands at about 4.8 and 6.0 eV, respectively. In contrast with undoped crystals, where the reduction produces primarily F centers and a small amount of F+ centers, in Mg-doped crystals both F and F+ centers are created in comparable concentrations. These thermally generated F and F+ centers are much more stable than those produced in undoped crystals irradiated with neutrons. Clustering of individual oxygen vacancies forming higher-order defects, such as anion divacancy F-2(2+) and F-2(+) centers, was investigated by low temperature absorption and luminescence experiments, in conjunction with UV irradiation and thermal treatments. The strong absorption bands at 2.87 and 3.69 eV were shown to be due to Mg-perturbed F-2(2+) and F-2(+) centers, respectively. In addition, photoconversion of F-2(2+) and F-2(+) centers was observed. In crystals containing large concentrations of F-2(2+) centers, electrons excited by 5.0 eV light are trapped by F-2(2+) and F-2(+) centers resulting in the conversion of F-2(2+) centers into F-2(+). A model of F-type centers was extended to F-2(2+) and F-2(+) centers. The calculated optical parameters are in very good agreement with those determined experimentally. A simple analysis of the lattice energy suggests that the environments of the F-2-type centers are different in TCR Al2O3 crystals and in n-irradiated undoped crystals. (c) 2007 American Institute of Physics.