ELECTRONIC-STRUCTURE OF THE RB-ADSORBED SI(100)2X1 SURFACE STUDIED BYDIRECT 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, w ith angle-resolved direct and inverse photoemission (IPES). At very lo w 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 high er Rb coverages, the surface electronic structure changes significantl y. In the IPES spectra, a new, rubidium-induced peak appears at covera ges above 0.3 ML. With increasing Rb coverages, it moves downwards in energy, until it reaches the Fermi level at the 1-ML saturation covera ge, causing a metallization of the surface. The dispersion of the empt y overlayer state was measured along the main crystallographic directi ons. 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 o bserved in both the <(Gamma)over bar>-(J) over bar and <(Gamma)over ba r>-(J) over bar' directions, showing the metallic character of the ove rlayer and the strong Rb-Rb interaction in both directions. These resu lts provide further evidence for the double-layer model for alkali-met al adsorption on Si(100)2x1, as well as a mainly covalent bonding pict ure. Our data are compared to previous studies of Li, Na, and K adsorp tion on the Si(100)2x1 surface. It is shown that although the electron ic structures of the different adsorption systems are similar, systema tic differences appear that can be attributed to the sizes of the alka li-metal atoms. In particular, a larger alkali-metal atom leads to a s tronger alkali-alkali interaction and a weaker alkali-metal-Si interac tion.