Clustering of N impurities in ZnO

Ab initio density functional theory and quasiparticle calculations for the incorporation of nitrogen atoms on oxygen sites in ZnO are presented. It is demonstrated that clustering of N atoms is energetically favored over the isolated N ₀ substitutional impurity. Tetrahedrons of N ₀ give rise to promising quasiparticle band structures with impurity states slightly above the valence band maximum (VBM), which, however, shift to higher energies with increasing negative ionization. The lowest recharging level ε(0/-) tends to a value 0.4 eV above the VBM, which is too deep for anything other than a weak p-doping.