  Based on density functional theory calculations, we systematically
investigate the behaviors of a H atom in Ag-doped ZnO, involving the preference
sites, diffusion behaviors, the electronic structures and vibrational
properties. We find that a H atom can migrate to the doped Ag to form a Ag-H
complex by overcoming energy barriers of 0.3 - 1.0 eV. The lowest-energy site
for H location is the bond center of a Ag-O in the basal plane. Moreover, H can
migrate between this site and its equivalent sites with energy cost of less
than 0.5 eV. In contrast, dissociation of such a Ag-H complex needs energy of
about 1.1 - 1.3 eV. This implies that the Ag-H complexes can commonly exist in
the Ag-doped ZnO, which have a negative effect on the desirable p-type carrier
concentrations of Ag-doped ZnO. In addition, based on the frozen phonon
calculation, the vibrational properties of ZnO with a Ag-H complex are
predicted. Some new vibrational modes associated with the Ag-H complex present
in the vibrational spectrum of the system.
