The dependence of the oxidation enhancement of InP(100) surface on the coverage of the adsorbed Cs

We report the oxidation of the InP(100) surface promoted by adsorbed Cs by synchrotron radiation photoemission. Oxygen exposure causes reduction in the charge transferred to the InP substrate from Cs and the growth of indium oxide and phosphorous oxide. The oxide growth displays a clear dependence on the Cs coverage. The oxidation of phosphorous is negligible up to 1000 L of O2 exposure when the Cs coverage is less than half a monolayer (ML) but the formation of the second half-ML of Cs greatly accelerates the oxidation. This different enhancement of the InP oxidation by the first and the second half-ML of Cs is due to the double layer structure of the adsorbed Cs atoms, and consequently the higher 6s electron density in the Cs atoms when Cs coverage is larger than 0.5 ML.