ALKALI-METALS ON SI(100)2 X-1 - COMPARATIVE-STUDY OF THE SURFACE ELECTRONIC-STRUCTURES FOR LI, NA AND K ADSORPTION

We have studied the electronic structures of lithium, sodium and potassium-adsorbed Si(100)2 x 1 surfaces with angle-resolved direct and inverse photoemission. An empty, overlayer-derived state appeared for all three adsorbates at submonolayer coverage and shifted downwards in energy for increasing coverage. It is assigned to alkali-metal np(z) or mixed ns-np(z) orbitals. For potassium it showed a strong dispersion and the minimum of this band reached the Fermi level (E(F)) at saturation and made the surface metallic. For sodium and even more for lithium it showed weaker dispersions and smaller energy shifts with the energy positions 0.7 and 2.0 eV above E(F), respectively, for the saturated 2 x 1 surfaces. These differences are assigned to alkali-metal (AM) size effects, where a smaller size results in a stronger AM-Si interaction and a weaker AM-AM interaction on the 2 x 1-reconstructed surface.