Growth of face-centered-cubic Fe films on Cu(100): Interdiffusion, surfactant effects, and island formation

Various factors complicating the growth of ultrathin films of ?-Fe on Cu(100) - interdiffusion, formation of surfactant layers, and island formation - have been investigated using density-functional calculations. It is shown that the formation of a surfactant Cu layer on top of the growing Fe films is energetically favored over both clean Fe surfaces and the formation of ordered surface alloys. However, the exchange process leading to the incorporation of the Fe atoms in or below the Cu top layer is characterized by a barrier of 1.45 eV/Fe atom, i.e., it will be kinetically hindered at low temperatures. In the initial stages of Fe deposition, as long as parts of the surface of the substrate remain uncovered by Fe, island formation is energetically preferred over the formation of atomically flat films. A surfactant Cu layer largely suppresses this tendency to surface roughening. Surface energies of 0.60 eV/atom for Cu(100) and of 0.84 eV/atom for ?-Fe(100) have been calculated and the difference is identified as the driving mechanism behind these phenomena.