The adsorption site of oxygen on Si(100) determined by SEXAFS

We present a SEXAFS (surface extended X-ray absorption fine structure) investigation of oxygen adsorbed on Si(100) at the oxygen K absorption edge (EK=535 eV), in the photon energy range 500–800 eV. The oxygen uptake on Si(100) at room temperature reaches a saturation at about one monolayer coverage, where the (2×1) reconstruction of the clean surface is lost. Our results demonstrate the following: the oxygen atoms occupy two types of bridge positions, namely between Si atoms belonging to the first layer and between Si atoms belonging to different topmost layers. The bridge site is characterized by a bond distance of 1.65 A and a Si−O−Si bond angle of ∼120°. No evidence of other types, of adsorption geometries is found nor the presence of adsorbed molecular oxygen is observed.