PHOTOEMISSION-STUDY OF THE SI(111)6X1-CS SURFACE

We report results of a photoemission study, using synchrotron uv and x-ray radiation, for Cs adsorption on the clean Si(111)7X7 surface at an elevated temperature with an emphasis on the Si(111)6X1-Cs surface. From the analysis of the Cs 4d and Si 2p core-level spectra, we find that even at 540 degrees C Cs atoms are adsorbed at adatom and rest atom sites at the initial stage, and successive deposition of Cs induces a reconstruction of the substrate to the 6X1 phase via 3X1. In the case of the 6X1 surface there are two surface components, S-1' and S-2', originated from the Si bonded to Cs atoms and the unbonded Si atoms, respectively. The saturation Cs coverage of the 6X1 surface is estimated to be 2/3 ML. We have also observed a surface state SS1' at about 0.49 eV below the Fermi edge in the valence band and the semiconducting nature of the 6X1 surface. The physical implications of these experimental observations are discussed within the framework of the previously proposed structural models.