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

Abstract

The precise study of the thermal annealing of the F2-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 × 1018n/cm2. 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 F2, F2+and F22+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 F22+centers. The tentative mechanisms of such rise of the F22+centers as well as of the subsequent transformation/rise of dimer centers, F22+→F2+→F2at 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 ~850 K.