OXIDATION OF ULTRATHIN IRON LAYERS ON CU(100) - COMPOSITION OF THE OXIDE-FILMS

The oxidation of Cu(100)-Fe surfaces containing 0-2.7 ML of Fe was studied at room temperature using Auger electron spectroscopy, ellipsometry and low energy electron diffraction. The initial oxidation rate was independent of theta(Fe) which means that oxygen adsorption proceeds via a precursor mechanism. On all surfaces saturation of the oxidation occurred. For theta(Fe) < 2 all iron was oxidised to FeO1.43 +/- 0.07 (Fe2O3) and a surplus of theta(O) = 0.5 was present, probably at the Cu-Fe2O3 interface. For iron coverages exceeding 2 ML the oxidation stopped after the uptake of 3.3 +/- 0.3 ML of oxygen. LEED experiments on a surface containing 0.8 ML of oxidised iron gave evidence for the formation of ordered domains of oxide in two (perpendicular) orientations. The distances of the spots indicate that a strong contraction of the oxide lattice takes place in order to match the Cu(100) lattice.