Association of the 6-eV Optical Band in Sapphire with Oxygen Vacancies

Arnold and Compton(1) have demonstrated that the prominent 6-eV optical absorption band observed in reactor irradiated crystalline sapphire by Levy and Dienes(2,3) and Mitchell et al.(4) is due to atomic displacement events and not to ionizing radiation alone, as x-ray and γ-irradiation will not produce this band. The orientationally averaged displacement threshold energies associated with the product ion of the 6-eV band were determined to be 90±5 and 50± eV when the absorption is associated with respectively anion or cation vacancies by a series of low temperature 0.6-to-1.8 MeV electron irradiations. However, they were unable to determine which damaged sublattice, anion or cation, is responsible for the uv optical absorption. Arnold(5,6) et al. and Evans(7) have subsequently shown that energetic light ions such as 50-keV and 200-keV H+, 100-keV D+ (and 100-keV 3He+ to lesser degree) at fluences of 1016 cm-2 also produce the intense 6-eV, whereas bombardment to the same fluence with heavier ions such as 220-keV 0+, 500-keV A+ and 200-keV Xe+ results in virtually none of these characteristic bands. Arnold(5) has offered an explanation of these observations based on defect center charge decoration.