Nucleation and growth of gold on defined silver faces

Abstract

A study has been made of nucleation and nucleus growth of Au by evaporation on (111) faces of silver crystals produced by a method which gives atomically smooth surface regions between monoatomic steps at distances up to 1 μm. In the temperature range 20–150°C time-dependent adsorption exists, and re-evaporation of the gold atoms can be neglected. By measurement of the critical time at which the first gold nuclei can be observed electron-microscopically, the critical adatom concentration necessary for nucleation can be determined. After a sufficiently long deposition period, a maximum nucleus density is reached. The growth processes at maximum nucleus density is treated with regard to the most simple case of growth of parallel steps. Provided that in the stationary case the maximum adatom concentration between steps where the nuclei no longer can be observed is equal to the critical concentration necessary for the nucleation, the diffusion coefficient of Au on a (111) Ag face can be calculated from the critical nucleation time and the maximum step distance. During continuous growth the gold nuclei degenerate dendritically to a high degree. An estimation shows that the gold nuclei can have a thickness of only a few atom layers.