INTERACTION OF ALKALI-METALS WITH SI(001)-2X1

An extensive set of total-energy and force calculations has been performed for alkali metals such as Na and K on Si(001)-2 X 1 at different coverages. Full lattice relaxation has been carried out in the presence of adsorbed overlayers at competing sites to arrive at a definitive structure. These results are compared with other theoretical and experimental results. A complete discussion of the 2 X 3 low-energy electron-diffraction structure and atop site assignment by scanning tunneling microscopy at low coverages is given. The compatibility of the structures with the negative-electron-affinity activation of Si(001)-2 X 1 is also pursued. We address issues such as interface metallicity, charge transfer, and bonding, using the results of our electronic-structure calculations. The metallic behavior at 1/2 monolayer coverage of alkali metals is emphasized by showing the Fermi line in the two-dimensional surface Brillouin zone. We also examine in depth the issue raised by a recent cluster-model calculation where the widely used quasihexagonal adsorption site is found to be unstable towards a zigzag Peierls type of DELTA-x distortion of the alkali-metal overlayer. We studied in detail the energetics associated with this distortion for Na-Si(001)-2 X 1 at 1/2 a monolayer coverage but found no instability of the quasihexagonal adsorption site. The apparent lack of agreement is explained in terms of incomplete nesting of the Fermi surface and alkali-metal-Si bonding. These findings are further confirmed upon deliberately altering the bonding by replacing Na with Al. At a monolayer coverage the two competing structural models, the so-called (H-B) and (H-C), cannot be distinguished on the basis of total energy in the presence of complete lattice relaxation. There is a tendency to lift the surface 2 X 1 reconstruction by stretching the dimer bond, but a complete reversal to ideal surface structure is found to be energetically unfavorable. This is to be contrasted with Al, which lifts the reconstruction at about 1/2 a monolayer coverage. Our calculated electronic structure is compared with the angular-resolved photoemission data with K-saturated single-domain Si(001)-2 X 1. The observed reentrant behavior to an insulating state at the monolayer coverage of alkali metals can be explained equally well by both models.