Electronic structure of the Rb-adsorbed Si(100)2x1 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)2x1 surface for increasing Rb coverages, with angle-resolved direct and inverse photoemission (IPES). At very low coverages, up to 0.2 hit, 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 2x1 periodicity at the Rb saturation coverage. Large upward paraboliclike dispersions from the minimum at <(Gamma)over bar> were observed in both the <(Gamma)over bar>-(J) over bar and <(Gamma)over bar>-(J) over bar' 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)2x1, 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)2x1 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.