Ag and N acceptors in ZnO: ab initio study of acceptor pairing, doping   efficiency, and the role of hydrogen
  Efficiency of ZnO doping with Ag and N shallow acceptors, which substitute respectively cations and anions, was investigated. First principles calculations indicate a strong tendency towards formation of nearest neighbor Ag-N pairs and N-Ag-N triangles. Binding of acceptors stems from the formation of quasi-molecular bonds between dopants, and has a universal character in semiconductors. The pairing increases energy levels of impurities, and thus lowers doping efficiency. In the presence of donors, pairing is weaker or even forbidden. However, hydrogen has a tendency to form clusters with Ag and N, which favors the Ag-N aggregation and lowers the acceptor levels of such complexes. 
O.Volnianska, P. Boguslawski and E. Kaminska
cond-mat.mtrl-sci