Substitutional Ni- in synthetic specimens was investigated by using electron paramagnetic resonance and photo-electron paramagnetic resonance techniques. The latter method showed that the Ni- electron paramagnetic resonance signal could be diminished by optical illumination. A threshold energy of 2.47eV suggested that the acceptor level was located at 3.03eV above the valence band. The energy dependence of the optical cross-section for electron ionization was deduced from an analysis of the electron paramagnetic resonance transient.

D.M.Hofmann, M.Ludwig, P.Christmann, D.Volm, B.K.Meyer, L.Pereira, L.Santos, E.Pereira: Physical Review B, 1994, 50[23], 17618-20