Photoluminescence techniques were used to study residual transition-metal contaminants in nitride layers which had been grown, using the sandwich technique, onto 6H-SiC or sapphire substrates. Three no-phonon lines (1.3, 1.19, 1.047eV) were observed, in the near-infrared optical region, which were caused by 3d transition metals. The appearance of GaN-related host modes in the phonon side-band of these emissions proved that the luminescence centers were incorporated into the hexagonal nitride layers. Here, attention was especially focused upon a luminescence band with a no-phonon line at 1.047eV. Temperature-dependent photoluminescence measurements revealed an excited-state splitting of 0.008eV. By using photoluminescence excitation spectroscopy, a further excited state was observed at 1.6eV; with a fine-structure splitting. The appearance of this excited state in n-type samples suggested that the defect already existed in its luminescent charge state, even without illumination. The experimental results for the 1.047eV emission were consistent with a 4T2(F)  4A2(F) internal electronic transition of a metal with a 3d7 electronic configuration.

K.Pressel, S.Nilsson, R.Heitz, A.Hoffmann, B.K.Meyer: Journal of Applied Physics, 1996, 79[6], 3214-8