| Thermally evaporating a metal in vacuum and condensing the vapor on a T ≅ 4 K substrate, also known as quench condensation , has been used for fabricating 2-D disordered systems for important studies in superconductivity, localization and quantum phase transitions. Our understanding of some of these phenomena has been limited by a lack of direct structural measurements. The inherent randomness in the thermal evaporation process and the inhibited diffusion rates make these films structurally metastable, making direct morphology measurements challenging.;Scanning Tunneling Microscopy offers a suitable technique for studying the morphology of ultrathin films. In an effort to correlate the morphology and electronic properties of quench condensed films, we have designed and built a low temperature Scanning Tunneling Microscope (STM) system with in situ film deposition capabilities.;In situ STM measurements reveal that the morphology of quench condensed Au and Pb films depends on their mass deposited thicknesses. With increasing thickness, the film evolves from a smooth and nearly featureless morphology, to a single layer of nanoclusters, to a loose pack of multiple layers of nanoclusters. Combining our STM data with past observations, we propose a qualitative growth model for films deposited on cryogenically cooled substrates. According to this model, thinnest films are stabilized in an amorphous phase which with increasing film thickness, avalanches into a polycrystalline phase.;The factors controlling the transition from the insulating to the metallic phase in these films, are directly determined by the growth mechanism and the details of the film morphology mentioned above. Transport and STM experiments on quench condensed Pb films suggest that at the thickness at which they first conduct, they are in the polycrystalline phase, consisting of approximately two layers of nanoclusters. The implications of this result on past experiments remain to be seen. |
