Structure and electronic properties of epitaxial metallic monolayers

Epitaxial metallic monolayers are the ideal candidates for fundamental stud ies of two-dimensional conduction, when they are grown on an insulating sur face. 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 l ow temperatures, since silicon is an insulator at low temperatures. Conduct ivity 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 th at is dependent on thermal history. The conductivity at T<100 K is describe d 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 i nsulating state is observed. The plasma resonance of the Ag films also show s for the continuous films a localization within the grains of the mosaic s tructure. 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.