Oxidation of ultrathin iron layers on Cu(100)

The oxidation of Cu(100)--Fe surfaces containing 0-2.7 ML on iron was studied at room temperature using Auger electron spectroscopy, ellipsometry and low energy electron diffraction. The initial oxidation rate was independent of theta sub Fe which means that oxygen adsorption proceeds via a precursor mechanism. On all surfaces saturation of the oxidation occurred. For theta sub Fe < 2 all Fe was oxidised to FeO sub 1.43 plus/minus 0.07 (Fe sub 2 O sub 3 ) and a surplus of theta sub O = 0.5 was present, probably at the Cu/Fe sub 2 O sub 3 interface. For Fe coverages exceeding 2 ML the oxidation stopped after the uptake of 3.3 plus/minus 0.3 ML of oxygen. LEED experiments on a surface containing 0.8 ML of oxidised Fe 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.