Structural investigation of Ca/Si(111) surfaces

The surface structures of different Ca/Si(111) surfaces were studied by low-energy electron diffraction (LEED) and high-resolution core-level photoelectron spectroscopy. Five different phases were observed in LEED depending on the Ca coverage. The lowest coverage phase has both $(3\mathrm{}\times 2)$ and $c(6\mathrm{}\times 2)$ periodicities, and the highest coverage phase has a $(2\mathrm{}\times 1)$ periodicity. The LEED patterns of the three intermediate phases were $(5\mathrm{}\times 2),$ $(7\mathrm{}\times 1),$ and $(9\mathrm{}\times 1).$ In the Si $2p$ core-level spectra, three surface components were observed in both the lowest and highest coverage phases. These three surface components were also observed in each intermediate phase together with two other components. The presence of the two extra components indicates that the intermediate phases are not completely described by simple combinations of the two end phases as suggested in the literature. By considering the energy shift and intensity of each surface component, we conclude that the structure of the $(3\mathrm{}\times 2)$ phase is basically the same as that of the honeycomb-chain-channel model with a Ca coverage of 1/6 ML, and the $(2\mathrm{}\times 1)$ phase is formed by $\pi$-bonded Seiwatz Si chains with a coverage of 0.5 ML. Moreover, taking the energy shifts and intensities of the extra surface components into account, we propose a structural model of the $(5\mathrm{}\times 2)$ phase, whose Ca coverage is 0.3 ML.