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

Angle-resolved photoemission experiment was performed on the single-domain $3\times 2$ and $2\times 1$ surfaces induced by Ca adsorption on a Si(111) surface. For the $3\times 2$ 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 $3\times 2$ phase induced by Ba and are roughly consistent with the theoretical calculation for the $3\times 1$ phase of Li on the same substrate. This result generally favors the honeycomb-chained channel model for the $3\times 2$ surface induced by alkali-earth adsorbates with a common Si topmost layer (“host”) reconstruction to the $3\times 1$ surface by alkali metals. In addition, we observe a rather obvious $\times 2$ symmetry in the surface-state dispersions, which differentiates the surface band structure of the $3\times 2$ phase from that of the host $3\times 1$ reconstruction. The apparent $\times 2$ periodicity, which is also clear in electron diffraction, is attributed to the alternating occupation of the $T4\mathrm{}$ sites along the Si channels by Ca adatoms with a coverage of 1/6 ML. In case of the $2\times 1$ 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 $(\times 1)$ direction, which agrees reasonably with the theoretical prediction for the $\pi$-bonded Seiwatz chain structure of the bare $\mathrm{Si}\mathrm{}\left(111\right)2\times 1$ surface. This result supports the recent structure model of the $2\times 1$ surface with one-dimensional Ca chains in between $\pi$-bonded 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.