A study was made of the changes which occurred in the photoluminescence spectra of crystals and films when they were exposed to low levels of visible laser radiation. New photoluminescence features, at 1.73 and 1.76eV, were seen to appear at low temperatures during the laser irradiation (to low fluences) of crystals which had been grown from purified powder. In the case of rapidly grown crystals, the 1.76eV peak predominated in the photoluminescence spectrum. These new peaks were attributed to emission from metastable defects (X-traps) which were either photo-induced or were produced during growth. By analogy with solid anthracene emissions, it was suggested that the observed variations in the positions and relative intensities of the photoluminescence peaks arose from differences that were due to the presence of chemical impurities or physical imperfections, to the growth method which was used, to irradiation with light, to temperature cycling, and to strain in the crystal.

U.D.Venkateswaran, M.G.Schall, Y.Wang, P.Zhou, P.C.Eklund: Solid State Communications, 1995, 96[12], 951-5