STRUCTURAL STUDY OF THE SI-ALKALI-METAL INTERFACE WITH X-RAY STANDING WAVES

X-ray standing-wave measurements on Cs and Rb adlayers on the Si(111)7 X 7 surface are presented. Low [almost-equal-to 0. 06 monolayer (ML)] and medium (almost-equal-to 0. 15 ML) coverages have been analyzed. The (111) and the (111BAR) reflections have been used. Since the experimental results clearly indicate that multisite occupancy takes place, the threefold hollow (H-3), the threefold filled (T4), and all the dangling-bond sites have been considered as possible adsorption sites. Distinction has been made between the faulted and the unfaulted half of the 7 X 7 cell. The Si-alkali-metal bond length has been assumed to be the sum of the covalent radii of the two elements. Our results indicate that both Rb and Cs adsorb preferentially on the H-3 and the T4 sites. The remaining atoms adsorb most at the atop sites for Rb and at the adatom sites (i.e., on top of the silicon adatom) for Cs.