Electrical Transport Properties of Ultra-Thin Metallic Films: Percolation, Localization and Superconductivity.

Electrical transport measurements have been carried out on ultra-thin Sn films prepared by deposition from a molecular beam source onto substrates held at 15 K. Deposition techniques were similar to those used in molecular beam epitaxy (MBE). Shrouded molecular beam vapor sources combined with low deposition rates allowed studies as a function of thickness to be carried out. Between depositions the sample was retracted into a cryogenic environment for detailed transport measurements. The extremely low background pressure in the cryogenic environment allowed the sample to be repeatedly lowered into the growth chamber and re-exposed to the vapor source with very little surface contamination. With such repeated exposure to the molecular beam, film thickness could be changed in a semi-continuous manner. These studies revealed transition temperature oscillations as a function of thickness which are discussed within the context of simple models. Also investigated was the systematic development of both normal state conductivity and superconductivity in ultra-thin samples as a function of thickness. This evolution was mapped out in detail and explained in a qualitative manner using models based on percolation and localization theories.