Barrier-height imaging of Cs-adsorbed Si(111)

We have carried out STM and barrier-height imaging on the Cs-adsorbed Si(111) 7 7 surface. At small coverages, Cs atoms adsorbed on Si(111) at room temperature tend to form clusters, as previously observed by Hashizume et al. [J. Vac. Sci. Technol. B 9 (1991) 745]. Although they report empty-state images showing anomalous contrast, no such images are observed in our experiment. The measured barrier height decreases locally above Cs sites and exhibits no long-range variation around each adsorption site. The average reduction in the barrier height at Cs sites is found to be Df ˆ 0:87 eV. # 2001 Elsevier Science B.V. All rights reserved.