OXIDE FORMATION ON SI(100)-2X1 SURFACES STUDIED BY SCANNING-TUNNELING-MICROSCOPY SCANNING TUNNELING SPECTROSCOPY

The oxide formation on Si(100)-2x1 surfaces at room temperature has be en studied by scanning tunneling microscopy (STM) and scanning tunneli ng spectroscopy (STS). The change in electronic structures due to oxid ation has been discussed based on the spatially resolved STS spectra. It was found in the initial stage of oxidation that the local density- of-states (LDOS) of back bond states and dangling bond sites decreases dramatically at the oxidation-induced bright protrusions. The STS spe ctra of the 2x1 region with an oxygen exposure of 4.5 L suggest that o xygen atoms adsorb on the back bond sites. At the average SiO2 thickne ss of 0. 2, nm, the surface states of Si(100)-2 x 1 vanish completely and the energy width of the gap near the Fermi level becomes very clos e to the band gap of bulk Si. In addition, the observed LDOS is spatia lly uniform in spite of the presence of bumps in the STM image. The ST S spectra indicate the formation of the valence band and the conductio n band of SiO2 at the oxide thickness of 0.4 nm.