Surfactant properties of oxygen in the homoepitaxial growth of Fe: a MDS study

The growth of ultra thin iron films (up to a thickness of 5-6 ML) on O(1 x 1)-Fe/Ag(001) has been investigated by means of He reflectivity (R-He) and metastable de-excitation spectroscopy. The presence of oxygen induces a quasi-ideal layer-by-layer growth at variance with the case of the homoepitaxial growth of iron on Fe(001). The surface electronic density of states suddenly changes upon the deposition of the first half of a monolayer. After the sudden change in the first stages of the growth, the surface density of states of both O-2p and Fe-3d states remains essentially unchanged, irrespectively of the thickness of the deposited film. This provides a clear indication that oxygen floats at the surface acting as a surfactant for the growth of iron on O(1 x 1)-Fe/Ag(001). The stationary fraction of oxygen that remains on the topmost layer as growth proceeds depends on the substrate temperature. Post-growth annealing up to 650 K restores the initial coverage of oxygen and the ordered O(1 x 1) phase.