DEVELOPMENT OF THE SURFACE ELECTRONIC-STRUCTURE OF K AND CS OVERLAYERS ON SI(111)7X7

The interaction between the metallic Si(111)7×7 surface and evaporated alkali metals, K and Cs, has been studied by angle-integrated ultraviolet photoelectron spectroscopy and angle-resolved inverse photoemission. Changes observed in both the occupied and unoccupied surface electronic states show a transition from metallic to semiconducting surfaces taking place in both systems after submonolayer coverages. Both metals are found to interact preferentially with the silicon adatom dangling bounds, filling the empty state and lowering the energy of the filled one. At saturation of the work function both the K- and Cs-covered surfaces exhibit a 7×7 structure in low-energy electron diffraction and a narrow semiconducting gap. One K-induced and two Cs-induced empty surface states have developed at this stage. Further evaporation beyond the saturation coverage yields a metallic surface in the case of cesium, while the K-covered surface remains semiconducting. © 1990 The American Physical Society.