Study of Local Valence Electronic States of SiO2 Ultrathin Films Grown on Si(111) by Using Auger Photoelectron Coincidence Spectroscopy: Upward Shift of Valence-Band Maximum Depending on the Interface Structure

To clarify the factors governing the local valence electronic states of SiO 2 ultrathin films, we have measured the Si- L 23 V V –Si n + -2 p 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 V V Auger electron spectra, measured as a function of SiO 2 thickness, taken in coincidence with Si 4+ -2 p 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 ≈1 ML 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...