Defects in unirradiatedα-Al2O3

Single-crystal samples of $\alpha$-${\mathrm{Al}}_2$ ${\mathrm{O}}_3$ have been studied that were colored during growth. The samples initially contained a single absorption band at 6.1 eV with a half-width of 0.68 eV. The sample had to be heated to a over 1400°C to remove this band. The 6.1-eV band could be optically bleached at 10 K, which introduced bands at 4.1, 4.8, and 5.4 eV, and which shifted the high-energy peak to 6.05 eV. Bleaching the 6.1-eV band (at 10 K also) produced photoconductivity and an emission band at 3.0 eV, with a half-life of about 10 msec. The spectral dependence of the photoresponse and the excitation spectrum of the 3.0-eV band were similar to the 6.1-eV absorption. After bleaching an emission band at 3.8 eV was produced which was similar to a band produced in particle-irradiated material. The lifetime of the 3.8-eV emission was less than 50 nsec, and the excitation spectrum contained peaks at 4.8 and 5.4 eV, with a small shoulder at 5.9 eV. The temperature dependence of the half-width of the 3.8-eV emission followed a hyperbolic cotangent relation with an effective frequency of 340±20 ${\mathrm{cm}}^{-1\mathrm{}}$.