ORIGIN OF THE SEMICONDUCTING NATURE OF THE NA-ADSORBED SI(111) SURFACE

Abstract

The change of electronic properties induced by Na adsorption on the Si(111)-(3x1) surface has been investigated by using high-resolution electron-energy-loss spectroscopy. We find that the initially (3X1) surface at 1/3 monolayer (ML) develops into an ordered (6X1) phase at 1/2 ML while keeping the surface semiconducting. The surface further transforms into another (3X1) phase at 2/3 ML still exhibiting the semiconducting nature before turning into a metallic phase at saturation. We ascribe this semiconducting nature of the surface above 1/3 ML to the bipolaronic effect primarily caused by the onsite electron-electron correlation.