A combined optical absorption, magnetic circular dichroism of optical absorption, optically detected electron paramagnetic resonance and positron annihilation spectroscopic study of undoped semi-insulating material showed that the concentrations of paramagnetic EL2, EL2+ and near-bandgap reverse contrast absorptions, a paramagnetic GaAs-related defect in trigonal symmetry, and the concentrations of As vacancies, were related.

K.Krambrock, C.Le Berre, C.Corbel, K.Saarinen, P.Hautojärvi: Materials Science Forum, 1995, 196-201, 195-200