Electronic structure of the Rb-adsorbed Si(100)2×1 surface studied by direct and inverse angle-resolved photoemission

We have studied the development of the surface electronic structure of a rubidium-adsorbed Si(100)2×{}1 surface for increasing Rb coverages, with angle-resolved direct and inverse photoemission (IPES). At very low coverages, up to 0.2 ML, the $5s$ electrons of the Rb atoms fill the minimum of the normally empty substrate-derived surface band. For higher Rb coverages, the surface electronic structure changes significantly. In the IPES spectra, a new, rubidium-induced peak appears at coverages above 0.3 ML. With increasing Rb coverages, it moves downwards in energy, until it reaches the Fermi level at the 1-ML saturation coverage, causing a metallization of the surface. The dispersion of the empty overlayer state was measured along the main crystallographic directions. A single-domain surface was obtained using vicinal samples, which showed a 2×{}1 periodicity at the Rb saturation coverage. Large upward paraboliclike dispersions from the minimum at Γ were observed in both the Γ-J̄ and Γ-J^' directions, showing the metallic character of the overlayer and the strong Rb-Rb interaction in both directions. These results provide further evidence for the double-layer model for alkali-metal adsorption on Si(100)2×{}1, as well as a mainly covalent bonding picture. Our data are compared to previous studies of Li, Na, and K adsorption on the Si(100)2×{}1 surface. It is shown that although the electronic structures of the different adsorption systems are similar, systematic differences appear that can be attributed to the sizes of the alkali-metal atoms. In particular, a larger alkali-metal atom leads to a stronger alkali-alkali interaction and a weaker alkali-metal--Si interaction.