The origin of commonly observed and much debated blue luminescence in Mg doped GaN was uncovered by a combined first-principle calculation and X-ray spectroscopic study. A number of atomistic defect candidates was considered and their XANES signatures were theoretically calculated using DFT and full multiple scattering analysis and compared with the experimental data. It was found that a defect complex consisting of an interstitial Mg (Mgi) and substitutional Mg (MgGa) forms a deep donor state in the band gap and leads to the observed blue luminescence by a donor-acceptor pair transition.
