Nonmetallic conductivity of epitaxial monolayers of Ag at low temperatures

Epitaxial metallic monolayers are models for two-dimensional conduction. They provide well-ordered monatomic films with an atom distance as in the bulk. Measurements of the dc conductivity of epitaxial Ag films on a clean Si(111) 7×7 substrate at about 100 K reveal that the conductance for a thickness down to nearly a monolayer is well described by a simple Drude model with a mean free path given by the film thickness. For lower temperatures (down to nearly 4 K) the conductance of very thin films is reversibly decreased by orders of magnitude. Whereas annealing increases the mean free path for thick films up to twice the thickness, the films with a thickness of less than 2 monolayers show no annealing effect. The results may be discussed with models of amorphous or granular films. The lack of agreement with theoretically predicted temperature dependences may be due to the special structure of epitaxial films.