A molecular beam study of alkali promotion of O2 sticking on Ge(100) and Si(100)

The alkali (Cs) promotion of O₂ sticking on Si(100) and Ge(100) has been studied by a molecular beam scattering method. In the curves of initial sticking probabilities versus alkali coverages, a threshold coverage for the alkali promotion is found around 0.2 ML. It is suggested that the appearance of the threshold coverage is a result of an abrupt change in the electronic state of the adatom from ionic to covalent or metallic bond with the substrates. For Ge(100) the alkali promotion is further increased as the incident energy is decreased, while an opposite trend is observed in Si(100). Alkali promotion effect on the physisorption-migration mediated process is discussed.