  This study combines surface-sensitive photoemission experiments with density
functional theory (DFT) to give a microscopic description of H
adsorption-induced modifications of the ZnO(10${\overline{1}}$0) surface
electronic structure. We find a complex adsorption behavior caused by a strong
coverage dependence of the H adsorption energies: Initially, O--H bond
formation is energetically favorable and H acting as an electron donor leads to
the formation of a charge accumulation layer and to surface metallization. The
increase of the number of O--H bonds leads to a reversal in adsorption energies
such that Zn--H bonds become favored at sites close to existing O--H bonds,
which results in a gradual extenuation of the metallization. The corresponding
surface potential changes are localized within a few nanometers both laterally
and normal to the surface. This localized character is experimentally
corroborated by using sub-surface bound excitons at the
ZnO(10${\overline{1}}$0) surface as a local probe. The pronounced and
comparably localized effect of small amounts of hydrogen at this surface
strongly suggests metallic character of ZnO surfaces under technologically
relevant conditions and may, thus, be of high importance for energy level
alignment at ZnO-based junctions in general.
