Φ140 mm sapphire crystal growth by temperature gradient techniques and its color centers

Sapphire crystals, 140 mm in diameter and 90 mm in height, have been grown by temperature gradient techniques (TGT). The growth direction of the boule was fixed by means of Laue X-ray diffraction. A prominent 204 nm absorption band in TGT-Al2O3, which does not appear in single crystals grown by Czochralski method has been studied. Analysis further substantiates the F-center model of this band. Two relatively weaker bands absorbing at 232 nm and 254 nm were ascribed to F+ centers. F-type centers concentration was determined using Smakula's equation.