Band alignment at Ag/ZnO(0001) interfaces: A combined soft and hard x-ray photoemission study

Abstract

Band alignment at the interface between evaporated silver films and Zn- or O-terminated polar orientations of ZnO is explored by combining soft and hard x-ray photoemissions on native and hydrogenated surfaces. Ultraviolet photoemission spectroscopy (UPS) is used to track variations of work function, band bending, ionization energy, and Schottky barrier during silver deposition. The absolute values of band bending and the bulk position of the Fermi level are determined on continuous silver films by hard x-ray photoemission spectroscopy (HAXPES) through a dedicated modeling of core levels. Hydrogenation leads to the formation of $\ensuremath{\sim}0.3$ monolayer of donorlike hydroxyl groups on both ZnO-O and ZnO-Zn surfaces and to the release of metallic zinc on ZnO-Zn. However, no transition to an accumulation layer is observed. On bare surfaces, silver adsorption is cationic on $\text{ZnO}(000\overline{1})\text{-O}$ [anionic on ZnO(0001)-Zn] at the earliest stages of growth as expected from polarity healing before adsorbing as a neutral species. UPS and HAXPES data appear quite consistent. The two surfaces undergo rather similar band bendings for all types of preparation. The downward band bending of ${V}_{bb,\text{ZnO-O}}=\ensuremath{-}0.4$ eV and ${V}_{bb,\text{ZnO-Zn}}=\ensuremath{-}0.6$ eV found for the bare surfaces is reinforced upon hydrogenation (${V}_{bb,\text{ZnO-O}+\mathrm{H}}=\ensuremath{-}1.1$ eV, ${V}_{bb,\text{ZnO-Zn}+\mathrm{H}}=\ensuremath{-}1.2$ eV). At the interface with Ag, a unique value of band bending of $\ensuremath{-}0.75$ eV is observed. While exposure to atomic hydrogen modulates strongly the energetic positions of the surface levels, a similar Schottky barrier of 0.5--0.7 eV is found for thick silver films on the two surfaces.