A confocal fluorescence microscopic study was carried out on GaN monocrystals grown by molecular beam epitaxy and doped with Mn. The samples were irradiated with mid-bandgap radiation (488nm) rather than ultra-violet, and it was found that a strong yellow radiation was emitted by defect centers. Three-dimensional images clearly revealed not only the size and the form of the illuminating defect centers but also their orientations with respect to the plane on which these defects were grown. This was a direct evidence for the radiative recombination at the defect centers when excited by the mid-gap radiation.

Role of Defect Centers in Recombination Processes in GaN Monocrystals. N.V.Joshi, A.Cros, A.Cantarero, H.Medina, O.Ambacher, M.Stutzmann: Applied Physics Letters, 2002, 80[16], 2824-6