Adsorption of Cs on Si(111)7×7: Studies of photoemission from surface states and core levels

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Abstract
We have studied the development of the electronic structure at the early stages of interface formation of Cs on the Si(111)7×7 surface. Low-energy electron diffraction shows that the 7×7 surface symmetry is conserved throughout the sub-saturation-coverage region. The changes in the surface electronic states characteristic of the clean silicon surface and the development of Cs-induced surface electronic states with increasing Cs coverage have been studied with angle-resolved ultraviolet photoelectron spectroscopy. Changes observed predominantly in the emission from the adatom dangling-bond state suggest that bonding of Cs occurs primarily at the adatom site on the Si(111)7×7 surface. At two-thirds of the saturation coverage, a strong Cs-induced surface state dominates the valence-band region. This state exhibits a downward dispersion along the high-symmetry lines in going away from normal emission, thus proving the semiconducting state of this well-ordered Si(111)7×7-Cs surface. Continued adsorption takes place at the rest-atom sites, making the surface metallic as the coverage reaches saturation. The metallicity is evident from the Fermi cutoff in valence-band photoemission and from the asymmetric broadening of the Cs 4d states, characteristic of metallic materials. Studies of the Si 2p level reveal changes in the line shape, which can be modeled with changes in surface core-level shifts and introduction of asymmetric broadening. These observations suggest that metallization occurs through the development of states at the Fermi level of both Cs and Si character. Studies of the Si 2p level further show an upward band bending with a maximum of 0.38 eV at the saturation coverage.