Alkali-Atom Adsorption on Semiconductor Surfaces: Metallization and Schottky-Barrier Formation

Alkali metals deposited on weakly ionic semiconductors are neither reactive nor form large three-dimensional islands, offering an ideal system in which Schottky junctions can be analyzed. In this paper, the alkali-metal-semiconductor interface is reviewed with a special emphasis on the formation of the Schottky barrier. Two regimes are clearly differentiated for the deposition of AMs on a semiconductor: in the high-coverage limit the Schottky barrier is shown to depend, for not very defective interfaces, on the semiconductor charge neutrality level. For low coverages, different one- and two-dimensional structures appear on the semiconductor surface presenting an insulating behavior. For depositions around a metal monolayer, a Mott metal-insulator transition appears; then, the interface Fermi energy is pinned by the metallic density of states at the position determined by the semiconductor charge neutrality level. This situation defines the Schottky barrier height of a thick-metal overlayer.