Thermal annealing and transformation of dimer F centers in neutron-irradiated Al2O3 single crystals

摘要

The precise study of the thermal annealing of the F-2-type dimer defects, being under discussion in the literature for a long time and responsible for the number of absorption bands below 4.5 eV, has been performed in corundum single crystals irradiated by fast neutrons with a fluence of 6.9 x 10(18) n/cm(2). The Gaussian components of the radiation-induced optical absorption with the maxima at 4.08, 3.45 and 2.75 eV have been considered as a measure of the F-2 , F-2(+) and F-2(2+) centers, respectively. In contrast to the F and F+ centers, the concentration of which continuously decreases at the sample heating up to 1100 K, the concentration of dimer defects with different charge states passes the increasing stages above 500 K starting from the F-2(2+) centers. The tentative mechanisms of such rise of the F-2(2+) centers as well as of the subsequent transformation/rise of dimer centers, F-2(2+) -> F-2(+) -> F-2 at 650-800 K are considered. The possible sources of carriers needed for the recharging of dimer centers are also analysed on the basis of thermally stimulated luminescence measurements up to similar to 850 K. (C) 2020 Elsevier B.V. All rights reserved.