Structure and electronic properties of epitaxial metallic monolayers

Epitaxial metallic monolayers are the ideal candidates for fundamental studies of two-dimensional conduction, when they are grown on an insulating surface. Therefore, Ag and Pb films with a thickness of a few monolayers have been grown on Si(111) surfaces in ultrahigh vacuum and studied in situ at low temperatures, since silicon is an insulator at low temperatures. Conductivity depends heavily on defects; therefore, those films have been analysed with spot profile analysis of low-energy electron diffraction (SPA-LEED). They are epitaxial with a mosaic structure +/-3 degrees and a grain size that is dependent on thermal history. The conductivity at T<100 K is described within the Drude model using a mean free path equal to the film thickness. For films up to a thickness of about 2 ML and T<20 K a transition to an insulating state is observed. The plasma resonance of the Ag films also shows for the continuous films a localization within the grains of the mosaic structure. The results will be discussed with respect to possible conduction and localization mechanisms in ultrathin films. (C) 1999 Elsevier Science B.V. All rights reserved.