The structural and electronic properties of rare-earth (Eu, Er and Tm)-related defect pairs in GaN were investigated theoretically. Based on LDA+U total-energy calculations, their possible role in the luminescence process was discussed. In all charge states, the lanthanides showed a strong preference for the Ga-lattice site, either as isolated substitutional or complexed with intrinsic defects. With respect to the electronic valence, a proper description of correlation effects of the strongly localized 4f electrons was shown to be crucial, especially if the REGa was paired with donors like the Ga interstitial or the N vacancy. The pairs formed by REGa substitutionals and vacancies or interstitials lower the symmetry and were found to locally distort the environment. By this, they were quite effective in relaxing the selection rules for the luminescent intra-4f-shell transitions. While for n-type GaN, the next-nearest-neighbour pair REGaVGa pair was energetically favoured, for p-type GaN, the REGaVN pair provided the most stable configuration and introduced shallow levels close to the conduction band, which could act as assistant levels in the luminescence process

Rare-Earth Defect Pairs in GaN: LDA+U Calculations. S.Sanna, W.G.Schmidt, T.Frauenheim, U.Gerstmann: Physical Review B, 2009, 80[10], 104120