ADSORPTION OF NA ON SI(100)2X1 AT ROOM-TEMPERATURE STUDIED WITH PHOTOELECTRON-SPECTROSCOPY

Room-temperature adsorption of Na on the Si(100)2x1 surface was studie d with high-resolution core-level spectroscopy and angle-resolved phot oemission. Photoelectron spectra from the Si 2p and Na 2p core levels and from the Si valence band have been analyzed for increasing Na cove rage. An abrupt decrease of the band bending by similar to 0.3 eV was observed for the initial adsorption due to the onset of occupation of a surface band above the surface band gap. Further Na deposition onto the surface results in a smeared-out appearance of the Si 2p spectra, indicating multiple surface shifts corresponding to an inhomogeneous s urface. Evidence for two adsorption sites was observed in the Na 2p sp ectra for higher Na exposures. At coverages around 1/3 of the saturati on coverage a good 4x1 periodicity was observed by low-energy electron diffraction (LEED). The Si 2p spectrum of the Na-saturated Si(100)2x1 surface shows a strong, well-resolved, Na-induced component with a su rface core-level shift of similar to 0.33 eV toward lower binding ener gies. Based on its intensity and the 2x1 LEED pattern observed for the saturated surface this component is assigned to Si atoms forming symm etric dimers on the surface.