EXPERIMENTAL AND SIMULATED TUNNELING SPECTRA OF THE POLAR ZNO SURFACES

Scanning tunneling microscopy was used to study the geometric and electronic structure of the two polar surfaces of zinc oxide in ultra-high vacuum. Even though the topographies of the surfaces are similar, the tunneling spectra differ on each surface plane (Zn-terminated or O-terminated), differ depending on the proximity to mesas, and are affected by the adsorption of O2. Calculations of tunneling spectra allow these data to be interpreted within the framework of established models for tunneling to semiconductor surfaces. These calculations demonstrate that changes in the spectra are due to changes in the local electronic properties of the surface. For example, features in tunneling spectra recorded at the edge of mesas can be related to spatially localized electronic states in the band-gap region, and changes induced by the adsorption of O2 can be explained by a reduction in the surface charge density. Negative differential conductance (NDC) was observed and a model which uses a varying density of states combined with tip-induced band bending is presented.