Comparison of the surface electronic structures of H-adsorbed ZnO surfaces: An angle-resolved photoelectron spectroscopy study

Synchrotron-radiation angle-resolved photoelectron spectroscopy has been utilized to study the interaction of atomic H with the nonpolar ZnO(101̄0) surface and polar ZnO(0001̄) and (0001) surfaces. H adsorption leads to the semiconductor-to-metal transition on the ZnO(101̄0) and (0001̄) surfaces. Metallization is a consequence of the formation of a single metallic band within the potential well between the surface/vacuum interface barrier and the edge of the conduction band, which is bent downwardly at the surface. The electrons confined in the potential well exhibit a free-electron-like behavior along the surface parallel, realizing a two-dimensional electron gas system. For the H/ZnO(0001) system, on the other hand, no feature associated with the metallic band is observed. Higher reactivity of the ZnO(0001) surface toward H than the other two ZnO surfaces is responsible for the different behavior for the modification of the surface electronic structure by H adsorption.