EXISTENCE OF METASTABLE MOLECULAR PRECURSORS TO DISSOCIATIVE OXYGEN-CHEMISORPTION ON SI(111) AND SI(100) AT 40-K

The presence of metastable molecular precursor to dissociative oxygen chemisorption has been identified, for the first time, on Si(100) and Si(111) surfaces using ultraviolet photoelectron spectroscopy and x-ra y photoelectron spectroscopy at 40 K. Density of valence states for su ch species shows three distinct peaks at 1.8, 3.4, and 4.9 eV below th e Fermi energy of substrates. The precursor stabilized on Si(100) at 4 0 K induces a large work-function increase of 1.5 eV at saturation cov erage. It also gives rise to a shoulder to O 1 s photoemission spectru m that is located at 1.2 eV lower binding energy than the O 1 s peak o f stable and dissociated oxygen species. It has been concluded that th e major reason of a larger work-function increment at 40 K is the exis tence of a molecular precursor, of peroxy radical type, on Si(100) and Si(III). These results strongly suggest that there exists a general m echanism of oxygen adsorption onto silicon surfaces.