Strong defect-specific low-frequency peaks were detected in the low-temperature Raman spectra of hexagonal material which had been grown, by using molecular beam epitaxy, onto sapphire substrates. The intensity of these peaks was found to be enhanced by excitation in resonance with yellow luminescence transitions. Their attribution to electronic Raman scattering was confirmed. The results implied that the observed electronic Raman scattering peaks were related to shallow donors which were not necessarily hydrogenic. A very low (0.0117eV) frequency Raman peak was instead attributed to a pseudo-localized vibrational mode.

D.S.Jiang, M.Ramsteiner, K.H.Ploog, H.Tews, A.Graber, R.Averbeck, H.Riechert: Applied Physics Letters, 1998, 72[3], 365-7