Nucleation kinetics for gold deposited onto mica substrates

Abstract

The nucleation behavior of gold deposited under ultra high vacuum conditions onto cleaved mica substrates has been measured over the temperature range 270–450 °C at impingement fluxes of 6 × 10 13 and 8 × 10 12 atoms cm −2sec −1. A step-kinetic technique was used wherein separate substrate areas were exposed for successively longer times to the vapor beam from a thermal source. The deposits were examined in a high resolution electron microscope. For mica cleaved in situ in UHV, the variation in number density of nuclei with time of exposure to the atom beam revealed that nucleation occurred rapidly on preferred sites which were exhausted within the first few seconds of exposure. After the initial saturation, nucleation appeared to occur randomly over the rest of the surface. The number density of preferred sites increased with decreasing temperature indicating that a spectrum of preferred sites of varying degrees of potency were active. The random nucleation rates could by analyzed satisfactorily using the atomistic model for nucleation. A critical sized nucleus consisting of one atom is consistent with the data. The characteristic energy for nucleation (2 ΔG des − ΔG sd), was found to be 1.1 eV. Nucleation behavior on mica substrates cleaved in air did not indicate the presence of active preferred sites, but nucleation was more rapid.