Absorption coefficient spectra were determined by means of optical transmission studies of undoped nitride films. It was found that the absorption of the n-type nitride at photon energies that were lower than the band edge could be attributed to the local deep defect levels. For the photo-emission of electrons to the conduction band, 2 optical threshold energies were detected. These were approximately 2.1eV from one deep level and about 1.1eV from one or several deep levels. It was suggested that these levels might participate in the familiar 2.2eV defect luminescence transitions.

L.Balagurov, P.J.Chong: Applied Physics Letters, 1996, 68[1], 43-5