High-resolution photoemission spectroscopic analysis of initial oxidation processes on Si(001) surfaces induced by kinetic energy of O2 molecules

The influence of incident O2 kinetic energy for oxidation processes of Si(001) surfaces has been studied by high-resolution photoemission spectroscopy with synchrotron radiation. The incident energy of O2 molecules was controlled up to 3 eV by a supersonic seeded molecular beam technique using a high temperature nozzle. Saturated oxygen amount after oxidation of partially-oxidized Si(001) surfaces showed two incident energy thresholds (1.0 eV and 2.6 eV). That after oxidation of clean surfaces, however, indicated monotonic increase as a function of incident energy. Si-2p photoemission spectra, measured in representative incident energies, revealed barrierless backbond oxidation for the clean surface case, and the incident energy dependent backbond oxidation of topmost dimers and a subsurface layer, caused by direct dissociative chemisorption of O2 molecules, for the partially-oxidized surface case. Moreover, the difference of chemical bonds for oxygen atoms was clarified to be low and high binding energy components in O-1s photoemission spectra, indicating the kinetic energy induced oxidation in backbonds. Especially, the real-time in-situ monitoring of Si-2p photoemission during initial oxidation revealed the characteristic variation of each oxide component every one minute for the first time. (author)