The atomic geometries, electronic structures, and formation energies of neutral nitrogen im- purities in ZnO have been investigated by first-principles calculations. The nitrogen impuri- ties are always deep acceptors, thus having no contributions to p-type conductivity. Among all the neutral nitrogen impurities, nitrogen substituting on an oxygen site has the lowest formation energy and the shallowest acceptor level, while nitrogen .substituting on a zinc site has the second-lowest formation energy in oxygen-rich conditions. Nitrogen interstitials are unstable at the tetrahedral site and spontaneously relax into a kick-out configuration. Though nitrogen may occupy the octahedral site, the concentrations will be low for the high formation energy. The charge density distributions in various doping cases are discussed, and self-consistent results are obtained.
First-principles calculations have been performed to clarify the differences of the electronic structures of Ga-doped ZnO and ZnS. Results show the local density approximation and local density approximation＋U calculations are in good qualitative agreement with each other. After doping, impurity states appear near the Fermi level in both ZnO and ZnS cases. When ZnO is doped, the impurity states are delocalized in the whole conduction band. On the contrary, when ZnS is doped, though the p state of Ga is also delocalized, the s state is localized near the Fermi level. Partial charge density distributions of the frontier orbital show the same information. After an exchange of the crystal structures of ZnO and ZnS, results remain unchanged. The localized Ga s state accounts for the bad electrical properties of Ga-doped ZnS.
