Valence charges for ultrathin SiO2films formed on Si(100)

We measure the relative chemical shift between Si 1s and Si 2p, $\Delta $E$_{\rm 1s}- \Delta $E2p, for 0.20–1.96-nm-thick SiO2films formed on Si substrates using high-resolution high-energys x-ray radiation. It is found that $\Delta $E$_{\rm 1s} - \Delta $E2pis independent of SiO2film thickness for films thicker than 1.0 nm, whereas it is smaller for films thinner than 0.5 nm. The result, in conjunction with first principles molecular orbital calculations, indicates that the valence charge of the Si atom is larger for in ultrathin SiO2films than in the thicker SiO2films.