Physical properties of the Ba-adsorbed Si(111) surface at elevated temperatures

We have investigated the adsorption of Ba on the Si(111) surface at elevated temperatures by using high-resolution electron-energy-loss spectroscopy, low-energy-electron diffraction, and photoelectron spectroscopy with synchrotron photons. We found two new ordered phases 2 x 1a and 2 x 1b with increasing Ba coverage in addition to other ordered phases reported earlier. All the ordered surfaces were found to remain semiconducting with a hybridization band gap of similar to 1.1 eV almost independent of Ba coverage. We discuss evidence for the evolution of a Ba s - s hybridization band for Ba coverage beyond 0.5 monolayers and propose structural models for the three ordered phases, which are quite consistent with our experimental data.