Initial Stage of Oxidation of Si(001)-2×1 Surface Studied by X-Ray Photoelectron Spectroscopy

The surface composition of a Si(001)-2 ×1 reconstructed surface after various O2 exposures and the oxidation process in an atomic scale based on the experiments using X-ray photoelectron spectroscopy have been investigated. We have demonstrated that the initial oxidation process is explained by our modified random bonding layer-by-layer mechanism. The suboxide contents (Si1+:Si2+:Si3+) change from 1:0:0, to 2:1:0 to 3:2:1 with increasing O2 exposure. The total suboxide saturates to SiO5/6 which is an intermediate state of the layer-by-layer oxidation, and to a metastable state prior to the nucleation of the two-dimensional SiO2 islands. The ratio of the bridging oxygen atoms to the on-top oxygen atoms is approximately 4:1. The portion of oxygen at the on-top site decreases with increasing O2 exposure. The oxygen atoms preferentially insert into the back bond of the dimer down-atom.