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-lay 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 SiO2/Si interface. The extent of the structural strain induced in the interface region is found to be smaller for SiO2 grown at 1000 degrees C on Si(100) than 1000 degrees C SiO2 on Si(111) or 800 degrees C SiO2 on Si(100). It is also found that, from the onset of the energy loss signal for O-1s photoelectrons, the bandgap of the oxides thicker than similar to 2.3 nm is 8.95 +/- 0.05 eV irrespective of the oxide thickness, For oxides thinner than similar to 2.3 nm, a remarkable increase in the 5-9 eV energy loss signal for O-1s 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.