The valence-band and the O $2s$ core-level spectra of ultrathin (about 1 nm) ${\mathrm{SiO}}_2$ layers [which are formed at the initial stages of oxidation of hydrogen-terminated Si(100) substrates] were measured by high-resolution x-ray photoelectron spectroscopy. The energy difference between the valence band and the core level was found to be greater than that for the bulk ${\mathrm{SiO}}_2.$ A first-principle molecular-orbital calculation was performed on ${\mathrm{Si}}_2{\mathrm{O}}_7{\mathrm{H}}_6$ clusters in order to obtain the energy difference (which depends on the structure of ${\mathrm{SiO}}_2$ layers). A comparison of the experimental data and the molecular-orbital calculations indicates that the intertetrahedral bond angle, ${\theta }_{\mathrm{S}\mathrm{i}-\mathrm{O}-\mathrm{S}\mathrm{i}},$ is 135° to 140° in the ${\mathrm{SiO}}_2$ structural transition layers at the ${\mathrm{SiO}}_2/\mathrm{S}\mathrm{i}$ interfaces.

• Received 3 August 1998

DOI:https://doi.org/10.1103/PhysRevB.59.5617