Scanning Tunnelling Microscopy and Spectroscopy of Adsorbates on Aluminium and Silicon

Abstract: The main topic of this thesis is the investigation of atomic andmolecular adsorbates on single crystal surfaces of silicon and aluminiumusing the techniques of scanning tunnelling microscopy (STM) andspectroscopy (STS). The emphasis has been to use atomic-resolutionspectroscopy and imaging to determine the relationship between thesurfaces' geometric and electronic structures. The STM results have beenprofitably compared with synchrotron-based valence band and core levelphotoelectron spectroscopy (PES) data to provide a more complete picturethan any single technique could provide. On Si(001) a peak seen in valence band PES at Li coverages up to 0.4 MLwas explained by STM images showing a local perturbation of theSi(001)-2x1 reconstruction by the Li atoms. STM also showed that Liadsorption on p-type substrates created localised electron traps whichinduced negative differential resistance in tunnel spectra. The formation of 3D islands of metallic MoSi₂ was shown to be a generalfeature of annealing 0.1 - 10 ML Mo films on Si(001) and Si(111) at 800 C.STM and STS showing the growth of the silicide islands at the expense ofatomic-scale features formed at lower temperatures was correlated withPES data showing the changes in the Si bonding on the surface. STS of Rb on Al(111) revealed that the (Root(3)xRoot(3))R30 overlayer formedat room temperature confined electrons to a 2D quantum well state. STM ofthe growing layer supported existing models for the (Root(3)xRoot(3))R30reconstruction and a complex structure formed at lower coverages, as well asdemonstrating the existence of kinetic barriers to atomic motion on thesurface. STM of C₆₀ on Al(111) showed details of the growth of a (2Root(3)x2Root(3))R30ordered overlayer at room temperature. Spectroscopy of an annealedsurface proved that inequivalent molecules in its (6x6) unit cell havedifferent molecular energy levels and bonding to the substrate.