Theoretical determination of the adsorption geometry of Na on the Si(001) surface

Ab initio total-energy calculations for Na on the Si(001) 2X1 surface at co verages ranging from one-fourth to one monolayer were performed using the d ensity-functional method with a plane-wave basis set and a slab with a 2X2 surface cell. The results of these electronic structure calculations are di scussed and compared with other theoretical and experimental findings. At t he highest coverage of one monolayer, we obtained the lowest surface energy for the double-layer model with combined occupation of the hollow and pede stal sites. For lower coverages, the structures are more complicated, as th e hollow site becomes unstable and adsorption at the cave and the pedestal site is preferred. In particular, for half a monolayer a distinct larger ad sorption energy is obtained for the combination of cave and pedestal sites. [S0163-1829(98)08348-9].