Ag and N acceptors in ZnO: Anab initiostudy 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 quasimolecular 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.