Ultra-thin conducting films of gold on platinum nucleating layers

Abstract

WE report a method for obtaining highly conducting films of noble metals between 11 and 50 Å thick on mica substrates. At the lower limit, the ultra-thin film (11 Å) may consist only of two or three monolayers each of gold and platinum. The minimum thickness at which pure gold films start to conduct is known to be 30 Å (ref. 1). Before this we had found that a pre-nucleating layer of silver, 5 to 7 Å thick, on mica yielded a highly conducting gold overlayer above a total (Au + Ag) thickness of about 32 Å (ref. 2). It was concluded that nucleating layers of suitable materials, of a thickness below which island formation commences, might lead to a higher conductivity in films of total thickness less than 50 Å. In this range, irregularities in the surface of the substrate, for example cleavage steps, become comparable to, or even greater than the film thickness. Cleaved mica surfaces are known to have hill-and-valley type microstructure. The cleavage step heights are about 20 Å or integer multiples thereof³. As film thickness decreases to these values, d.c. measurements of conductivity become progressively less accurate. To alleviate this difficulty, we have measured microwave sheet resistance (R_μ in Ω per square) at 9.7 GHz by Slater's method⁴. The corresponding wavelength is more than six orders of magnitude greater than any irregularity in the surface of the mica. We have used this method previously to estimate the conductivity of gold films thicker than 50 Å (ref. 5).