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

Abstract

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 6.0eV, 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 F22+ and F2+ 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 3.69eV were shown to be due to Mg-perturbed F22+ and F2+ centers, respectively. In addition, photoconversion of F22+ and F2+ centers was observed. In crystals containing large concentrations of F22+ centers, electrons excited by 5.0eV light are trapped by F22+ and F2+ centers resulting in the conversion of F22+ centers into F2+. A model of F-type centers was extended to F22+ and F2+ 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 F2-type centers are different in TCR Al2O3 crystals and in n-irradiated undoped crystals.