Surface electrical conduction due to carrier doping into a surface-state band on Si(111)-3×3-Ag

Photoemission spectroscopy has shown that each Ag atom in its two-dimensional adatom gas (2DAG) phase deposited on the Si(111)-3×3-Ag surface at room temperature donates one electron into an antibonding surface-state band of this substrate, resulting in a steep increase in electrical conductance through the band. The surface space-charge layer makes no contribution to the conductance increase by the 2DAG adsorption, estimated from the band-bending measurements. When the 2DAG nucleates into three-dimensional Ag microcrystals by further deposition beyond a critical supersaturation coverage, the carrier-doping effect vanishes, returning to a lower conductance. These results reveal that the surface state acts as a surface conduction band. The electron mobility in this band is estimated to be on the order of 10 cm²/V s.