High-resolution core-level study of initial oxygen adsorption on Si(001): Surface stoichiometry and anomalous Si 2p core-level shifts

The submonolayer oxygen adsorption on the Si(001) surface is studied by high-resolution Si $2p$ photoemission. Significant intensities of Si $2p$ components due to the Si²⁺ and Si³⁺ species are observed from the very early stage of adsorption at 120 K, which grow linearly with the oxygen coverage. This indicates an active agglomeration of oxygen adsorbates even for submonolayer adsorption at low temperatures. Annealing above {~}500 K of oxygen adlayers formed at 120 K induces not only changes of the adlayer stoichiometry but also shifts of the Si $2p$ binding energies for the Si²⁺ and Si³⁺ species by 0.14 and 0.23 eV, respectively. This change demonstrates the sensitivity of the Si $2p$ binding energies beyond the chemical shift.