Structure and electronic states of ultrathin SiO2 thermally grown on Si(100) and Si(111) surfaces

The chemical and electronic structures of ultrathin SiO2 thermally grown on Si(100) and Si(111) have been investigated by using Fourier-transform infrared attenuated total reflection (FT-IR-ATR) and X-ray or ultraviolet excited photoelectron spectroscopy (XPS/UPS), respectively. A red-shift of the p-polarized LO phonon peak observed for oxides thinner than 2 nm indicates that compressively strained SiOSi bonds exist near the SiO2Si interface. The extent of the structural strain induced in the interface region is found to be smaller for SiO2 grown at 1000°C on Si(100) than 1000°C SiO2 on Si(111) or 800°C SiO2 on Si(100). It is also found that, from the onset of the energy loss signal for O1s photoelectrons, the bandgap of the oxides thicker than ∼2.3 nm is 8.95 ± 0.05 eV irrespective of the oxide thickness. For oxides thinner than ∼2.3 nm, a remarkable increase in the 5–9 eV energy loss signal for O1s photoelectrons is observed. This could be attributed to not only the contribution of suboxides at the interface but also the built-in stress in the interface region which causes the band edge tailing.