Surface electronic structures of Ca-induced reconstructions on Si(111)

Abstract

Angle-resolved photoemission experiment was performed on the single-domain 3 X 2 and 2 X I surfaces induced by Ca adsorption on a Si(111) surface. For the 3 X 2 surface, we identify three fully occupied surface states within the Si bulk-band gap with clear dispersions of 0.3-0.6 eV. These surface-state bands resemble those observed for the similar 3×2 phase induced by Ba and are roughly consistent with the theoretical calculation for the 3 X I phase of Li on the same substrate. This result generally favors the honeycomb-chained channel model for the 3×2 surface induced by alkali-earth adsorbates with a common Si topmost layer (host) reconstruction to the 3×1 surface by alkali metals. In addition, we observe a rather obvious ×2 symmetry in the surface-state dispersions, which differentiates the surface band structure of the 3×2 phase from that of the host 3×I reconstruction. The apparent ×2 periodicity, which is also clear in electron diffraction, is attributed to the alternating occupation of the T4 sites along the Si channels by Ca adatoms with a coverage of l/6 ML. In case of the 2×I reconstruction formed at a higher coverage, we observe only one surface state within the bulk-band-gap region at a binding energy of 1.1-1.5 eV. This state exhibits a larger dispersion along the chain (×1) direction, which agrees reasonably with the theoretical prediction for the π-bonded Seiwatz chain structure of the bare Si(111)2 X I surface. This result supports the recent structure model of the 2×1 surface with one-dimensional Ca chains in between π-honded Si chains. It is deduced from the present observations that the surface electronic band structures of the Si surface phases with alkali-earth adsorbates are largely and rigidly determined by the reconstruction of the Si topmost layer itself.