Diffusion and Cluster Growth of Binary Alloys on Surfaces

Abstract

Metastable nanoclusters grown on surfaces by vapour deposition or molecular beam epitaxy techniques have become an active topic in surface science because of their potential to display new physical properties useful for applications. Atomistic modelling and Kinetic Monte Carlo (KMC) simulations of these processes are reviewed with emphasis on two-component adatom systems. The situation we consider is that two types of atoms are co-deposited to the substrate. In this field of binary growth, systematic theoretical investigation is only at its beginning. Surface diffusion and nucleation leads to the formation of two-dimensional islands, before two-dimensional cluster growth sets in. In distinction to well-known growth scenarios for one species of adatoms, a wealth of new aspects arises in binary systems. Already in the regime of submonolayer growth, differences in adatom diffusion coefficients and binding energies make it necessary to generalise traditional scaling relations for island densities. Surface segregation and compositional fluctuations in growing 3-D clusters are issues requiring renewed examination under the point of view that the atomic short-range order, which is frozen in the bulk, was generated through the surface kinetics during previous stages of growth. We discuss the current theoretical understanding and experimental implications of these problems, including a description of perpendicular magnetic anisotropy (PMA) in metastable CoPt3 nanoalloys, as detected recently.