Substrate diffusion in metastable ultrathin films: Iron on Cu(001)

Temperature stability and diffusion properties of ultrathin iron films grown at room temperature on a Cu(001) single-crystal surface have been investigated using low-energy ion scattering. None of the as-grown films is found to be thermally stable. Instead an enrichment of the surface with substrate atoms is observed. Below two monolayers, where the Cu substrate is not completely covered by Fe, the onset of Cu diffusion occurs at 300 K. Iron films wetting the substrate entirely appear to be more resistant against thermal treatment. It is shown explicitly that initially a single monolayer of copper migrates onto the films, leaving the underlying Fe layers mainly intact. This overlayer adopts the same crystal lattice as the as-grown films; in particular a monolayer of Cu atoms is found in bcc lattice sites on top of iron films exhibiting bcc structures.