Near-infrared absorption lines at 2916, 4066, 4113, 4139 and 4168/cm were observed in type-IIa samples which had been irradiated, and then annealed at temperatures above 1400C. A photochromic behavior of these defects under various illumination conditions was observed, and was explained in terms of a charge transfer model. It was concluded that the peaks probably arose from electonic transitions of vacancy clusters.

Y.Mita, Y.Shiraki, Y.Nisida: Solid State Communications, 1997, 102[9], 659-61