Effect of a metallic surfactant on the electrical percolation of gold films

The electrical transport of ultrathin films is an issue for modern electronics, since conduction properties of commonly used metals present important differences for thicknesses in the nanometer scale. This report shows that the sheet resistance of gold ultrathin films (between 1.2 nm and 3.8 nm) decreases up to 5 orders of magnitude when a chromium layer 0.8 nm thick is used as surfactant. The chromium layer changes the morphology of the gold films on mica, composed by large isolated grains, allowing the formation of a population of small grains between the large ones, therefore reducing the thickness at which the electrical percolation is achieved. The electrical percolation thickness, evidenced as a sharp decay in sheet resistance, occurs at 3.9 nm, for gold deposited directly on mica, while this value is reduced to 1.2 nm if a thin chromium layer is previously grown on the mica substrate. A deeper study of the electrical properties of this gold ultrathin films grown on Cr/mica show that above 2.8 nm, they already present a metallic behavior; while films near 2.2 nm show a zero temperature coefficient, meaning that the electrical conduction driven by tunneling still plays an important role in the electronic transport. Finally, is demonstrated, that the contribution of the chromium layer to the electrical conductivity of films can be neglected. Namely the influence of the chromium layer on metallic ultrathin films, is mainly as a surfactant, with no direct role in the electronic transport itself.