Observation of a Cs-induced state in the band gap of GaP(110): Alkali-metal bonding and Fermi-level pinning

The deposition of submonolayer quantities of Cs onto GaP(110) causes strong photoemission features in the region of the semiconductor fundamental band gap. This observation is interpreted in terms of emission from a hybrid state caused by the interaction of the Cs 6s level with the unoccupied dangling-bond state. This hybrid state has long been postulated in descriptions of the metal-semiconductor surface bond, and is responsible for the pinning of the Fermi level. The absence of dispersion in the state suggests that Cs/GaP(110) represents a realization of a Mott-Hubbard insulator, by comparison with results from other alkali-metal/compound-semiconductor systems