Step instabilities in Fe/Cu(100) growth

Abstract

We examine the step instability observed in Fe growth on a Cu(100) vicinal substrate in which deposition of a relatively low coverage of Fe leads to a dramatic increase in the step roughness, with a change in the step morphology from relatively straight [100] steps to a mixture of [100] and [110] steps. Our temperature-accelerated dynamics (TAD) simulations and energetics calculations indicate that the dramatic change in the step morphology is due to a variety of unexpected complex multiatom interlayer diffusion (MID) processes near step edges which lead to a competition between [100] and [110] steps, whose barriers are significantly reduced due to the existence of strong Fe-Fe and Fe-Cu interactions as well as strain effects. In contrast, TAD simulations of vicinal Cu/Cu(100) growth do not lead to an instability, in good agreement with experiments. Our results also indicate that while the instability is driven by energetics, kinetics plays a crucial role. We also present the results of TAD simulations and energetics calculations for the case of Fe/Cu(100) growth on vicinal surfaces with [110] steps. Our results in this case indicate that a variety of effects, including MID processes as well as pinning of edge-diffusing atoms, lead to the formation of protrusions with [110] steps perpendicular to the original [110] steps. These results may also provide an explanation for the experimentally observed instabilities observed in growth on vicinal Ni/Cu(100) and Co/Cu(100) surfaces with [110] steps.