Substitutional Co-mediated island-edge decoration in Co/Cu(001): A kinetic Monte Carlo study

Abstract

Film morphologies of heteroepitaxial systems are as complex as they are diverse and in some cases may involve step decoration of substrate islands by adsorbate atoms as well as the presence of intermixed, heterogeneously nucleated islands. Given that epitaxial growth takes place under far-from-equilibrium conditions, it is not totally unreasonable to assume that kinetics—differences in surface mobilities—is an important driver for island-edge decoration. Indeed, this is believed to be the case in our heteroepitaxial model system, Co/Cu(001), in which deposited Co in the submonolayer regime transforms into substitutional Co atoms through a thermally activated process known as atomic exchange. Here, a Co atom from the adlayer descends into a subsurface site, dislodging a substrate Cu atom, which then moves up to the surface—in the process creating a substitutional Co atom and a Cu atom. In this kinetic Monte Carlo simulation study, we propose that while sCo serves as pinning centers for both Cu and Co atoms alike, it is only very weak in the case of the former and does not have a stabilizing effect on Cu-dominated islands by itself. Thus, the classic picture of Cu islands nucleating around sCo sites and agglomerating with each additional fast diffusing Cu atom, and whose edges are subsequently decorated by the slower moving Co atoms, needs to be revisited. On the contrary, our analysis indicating the presence of both Co and sCo atoms at the rapidly shifting island periphery suggests that island-edge decoration is ephemeral and that sCo itself has a less intuitive, outsize role in Cu-dominated island's rapid growth. We also found that differences in surface mobilities alone lead to intermixing rather than island-edge decoration.