Surface states and alkali-to-semiconductor charge transfer in the K/Si(111)(√3 × √3 )R(30°)-B system

High-resolution core-level x-ray photoemission and photoabsorption studies of potassium adsorption on the Si(111)(√3 × √3 )R(30°)-B surface show a large core-level shift (∼1.2 eV) for both B(1s) and Si(2p) states, in contrast to results from alkali adsorption on other Si surfaces where little or no shift is observed for Si(2p) states. We interpret these large shifts in terms of a large charge transfer from potassium to the Si-substrate dangling-bond state, possible because of the B-induced removal of filled clean-surface band-gap states. These new results suggest that, in general, charge transfer in the alkali-semiconductor surface bonding is determined by the occupation of band-gap states at the clean surface.