A confocal fluorescence microscopic study was made of monocrystals which had been grown by using molecular beam epitaxy and were 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. The 3-dimensional images clearly revealed not only the size and form of the illuminating defect centers but also their orientations with respect to the plane upon which the defects were grown. This was considered to be direct evidence for radiative recombination at the defect centers when excited by 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