Epitaxial FeO films from ultrathin fe on MgO(001) studied by he-atom scattering

With He-atom scattering, we investigated ultrathin iron oxide prepared by oxidation of an in-situ grown metal film that was deposited onto ultrahigh vacuum (UHV)-cleaved MgO(001). Three-dimensional metal island growth of Fe was suppressed by evaporation of a first atomic layer at 140 K and of a second layer at room temperature. This epitaxial two-monolayer iron film almost completely covers the substrate. However, a low specular intensity indicates poor crystalline quality. Exposure to oxygen and subsequent heating to 950 K of this ultrathin Fe film yields pseudomorphic FeO with high crystalline quality as proved by high specular intensity as well as by sharp inelastic features due to surface phonons. Only one surface phonon branch with a dispersion different to that of the MgO(001) surface mode was observed. This 'one-mode behaviour' should be correlated to the fact that chemical bonds in FeO, MgO, and at their mutual interface are very similar. (C) 1999 Elsevier Science B.V. All rights reserved.