Abstract
The two polar surfaces of ZnO were investigated by soft x-ray photoemission spectroscopy. Surface components due to variation in the Madelung energy were identified in photoemission core-level spectra. Sulfur adsorption was used to passivate the surfaces in order to enable separation of the bulk from the surface components. For the ZnO0001-Zn surface the observed photoemission peaks were consistent with a Zndeficient surface, exhibiting a high density of O-terminated step edges. The ZnO000–1-O surface is very reactive toward hydrogen adsorption and only above 650 K a hydrogen free surface was observed. For
hydrogen-free and small hydrogen coverage an electrostatic shift of the Fermi-level toward the band-gap center was observed. This indicates an incomplete compensation of the internal electrostatic potential by surface oxygen vacancies or charged adsorbates. Coadsorption of sulfur lowered the desorption temperature for hydrogen indicating the possibility to tune the chemical properties of these polar surfaces by dopants.