Valence charges for ultrathin SiO2 films formed on Si(100)

We measure the relative chemical shift between Si Is and Si 2p, ΔE 1s -ΔE 2p , for 0.20-1.96-nm-thick SiO 2 films formed on Si substrates using high-resolution high-energys x-ray radiation. It is found that ΔE 1s -ΔE 2p is independent of SiO 2 film 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 SiO 2 films than in the thicker SiO 2 films.