Energy loss spectroscopy study of Si(111)–alkali metal interfaces at low temperatures

Studies are made at ∼150 K under ultrahigh vacuum conditions on a wide range of alkali metal coverages on Si(111)-7×7. Negative second-derivative backscattered electron energy loss spectroscopy is used with 100 eV primary electrons. The interaction of the alkali metals with the silicon substrate goes through two stages as a function of alkali coverage: In the initial coverages, for less than ∼0.3 monolayer of alkali atoms, the basic reaction is that of charge transfer from the alkali atoms to the Si surface with a loss peak at ∼3.3 eV associated with the charge transfer states. The second stage of reaction—starting after the depletion of all the Si surface states—falls in a coverage range between ∼0.3 and ∼1 monolayer, in which the formation of a metallic layer with a coverage-dependent loss feature at about 2 eV is observed. At still higher coverages, multiple surface and bulk plasmon excitations and their combinations are dominant. In the overall scattering processes most of the parallel momentum (∼3 Å−1) is transferred to the sample during the elastic backscattering from the surface, and all the losses are essentially attributed to the forward inelastic scattering before and/or after the elastic process takes place near the metal/Si interface.