Chemical trends in Schottky barriers: Charge transfer into adsorbate-induced gap states and defects

The chemical trends reported for barrier heights in metal-GaAs contacts are explained by a charge transfer between the metal and adsorbate-induced gap states, which are identified as the virtual gap states of the complex band structure of GaAs, as well as fabrication-induced defects of donor type. Following the concept of the ionicity of chemical bonds, the charge transfer is described by the difference in the electronegativities of overlayer and substrate atoms. The density of fabrication-induced defects varies considerably.