The behavior of radiation damage of Nb⁺ ion implanted sapphire after annealing at reducing atmosphere

Sapphire crystals with (0001) and (0112) orientations were implanted with 380 keV Nb⁺ ions to a dose of 5 × 10¹⁶ ions/cm² at room temperature and 100 K. The behavior of the radiation damage produced by ion implantation followed by annealing with a series of steps from 500 to 1100°C at reducing atmosphere is investigated using optical absorption technique. The absorption curves were evaluated by Gaussian fitting based on the well known F-type centers, which were confirmed by luminescence measurements. The results of optical density (OD) from the bands with annealing temperature and time show that the annealing behavior of the radiation damage produced by Nb⁺ ion implantation can be divided into three stages. The first stage is in the range of annealing from 500 to 800 to 900°C; the second is from 900 to 1000°C, the third is one above 1000°C. Defects were annealed within these stages due to different mechanisms which are proposed on the basis of the optical absorption measurements combined with the observation of SEM micrographs and colour variation of the sample after annealing steps with temperature.