Optically-active defects in undoped epilayers which had been grown onto sapphire, by using metal-organic chemical vapor epitaxy, were investigated by means of photoluminescence techniques. A new metastable defect which emitted blue light was found, as well as the well-known yellow luminescence centers. Upon excitation by a 325nm He-Cd laser, this metastable defect, at low temperatures, exhibited a luminescence fatigue effect, with a decay time of about 360s. When the temperature was increased to room temperature, it recovered its optically-active state. The yellow band emission increased in intensity as the blue-band emission intensity decreased. Analysis showed that this metastable center was a hole trap, with the Ga vacancy being the most probable candidate.

S.J.Xu, G.Li, S.J.Chua, X.C.Wang, W.Wang: Applied Physics Letters, 1998, 72[19], 2451-3