Theory of doping properties of Ag acceptors in ZnO

Doping properties of Ag in ZnO were analyzed by first-principles calculations within both the local-density and generalized gradient approximations. The ionization energy of AgZn, about 0.2 eV, is comparable to that of the commonly used group-V acceptors, and is lower than that of two other IB species, Cu and Au. Formation energy of Ag in the favorable O-rich conditions is 0.85 eV, which corresponds to the solubility limit of about 10¹⁸ cm^-3 at 700 ^∘C. Formation of Ag-rich second phases is predicted for high Ag concentrations. Energetics of the onset of this process is analyzed and AgZn display a tendency to form aggregates of AgO with the wurtzite structure. Formation of such nanoinclusions is shown to affect the lattice constant of ZnO:Ag. Two ``wrong'' incorporation channels, i.e., at the interstitial sites and at the oxygen sites as AgO, are predicted to be nonefficient due to the high formation energies. The calculated magnetic coupling between Ag ion reveals an unexpected dependence on the Ag-Ag distance; the interaction between the nearest-neighbor AgZn pair vanishes while that for the more distant pairs is weakly ferromagnetic.