Study of local valence electronic states of SiO _2 ultrathin films grown on Si(111) by using Auger photoelectron coincidence spectroscopy: Upward shift of valence-band maximum depending on the interface structure

摘要

To clarify the factors governing the local valence electronic states of SiO _2 ultrathin films, we have measured the Si-L _(23)VV- Si ~(n+)-2p Auger-electron photoelectron coincidence spectra (n = 0, 1, 2, 3, 4, where n represents the number of oxygen atoms bonded to Si) of SiO _2 thermally grown on a Si(111)-7×7 surface [SiO _2/Si(111)]. The results indicate that the valence electronic states in the vicinity of the Si ~(n+) sites shift to deeper binding energies as n increases. Furthermore, Si ~(4+)-L _(23)VV Auger electron spectra, measured as a function of SiO _2 thickness, taken in coincidence with Si ~(4+)-2p photoelectron emission show that the valence-band maximum (VBM) of the SiO _2 layer shifts by 2.7 ± 1.0 eV toward the Fermi level when SiO _2 thickness is decreased to ≈1 monolayer (ML). This upward shift is much larger than that for a SiO _2 layer with a thickness of ≈1ML thermally grown on Si(100)-2×1 (about 1.6 eV). We attribute the large shift in the VBM of SiO _2/Si(111) with a 1-ML-thick SiO _2 layer to the formation of SiO _2 islands on the 7×7 structure and to the presence of Si ~(1+) atoms adjacent to the SiO _2 sites.