PROBING THE SURFACE-CHEMISTRY OF POLYCRYSTALLINE ZNO WITH SCANNING TUNNELING MICROSCOPY AND TUNNELING SPECTROSCOPY

Scanning tunneling microscopy and spatially resolved tunneling spectroscopy have been used to examine polycrystalline ZnO surfaces in ultrahigh vacuum after bake-out, after a low temperature anneal that cleaned the surface, after a high temperature anneal, which segregated bismuth to the surface, after being dosed with O2 and H2O, and after exposure to air. The tunneling spectra depend both on the proximity to structural features, such as grain boundaries, and on the chemical composition of the surface. For example, the segregation of bismuth to the surface causes the tunneling spectra to have a p-type rectification. Our results also indicate that the rectification of tunneling spectra acquired in air is caused by surface hydration and that images of surfaces that have not been heated in vacuum have inferior resolution due to a reduction in the height of the apparent tunnel barrier. Spatially resolved tunneling spectroscopy has been used to demonstrate that surface hydration has a greater effect on the crystallite surfaces than on the grain boundary surfaces.