The photoluminescence from high-quality crystals, grown from the vapour phase, revealed a predominant Zn acceptor and a C acceptor; as well as an acceptor that appeared to have a smaller binding energy than that of C. It was shown that this shallowest acceptor was not a discrete chemical acceptor, having a smaller binding energy than C, but was instead due to a complex centre.

D.C.Reynolds, D.C.Look, B.Jogai, G.McCoy: Journal of Physics - Condensed Matter, 1997, 9[48], 10795-9