X-ray photoelectron spectroscopy of silica in theory and experiment

Fourier deconvolved X-ray photoelectron spectroscopy (XPS) valence band spectra obtained from crystalline and amorphous silica, used in conjuction with the results of quantum chemical calculations of the SiO₄ tetrahedral unit and other spectrometric measurements (soft X-ray emission, UV absorption and reflectivity, photoconductivity, photoinjection and energy loss spectroscopy), suggest a reinterpretation of the electronic band structure of silica that is consistent with all the data. A unique method for pinning the Fermi level of insulators to that of a metal calibrant is described, resulting in the ability to obtain absolute binding energies of the electronic levels in wide bandgap insulators. Observe peaks in UV reflectivity and energy loss spectra of silica are all assigned to direct interband transitions, and no excitonic states need be involved to explain the data. Upper and lower limits for the bandgap of dry crystalline (α-quartz) and amorphous (Corning Code 7940 glass) silica are adjusted downward from the 8.9 eV bandgap proposed by DiStefano and Eastman 1] to 7.8–5.55 eV for α-quartz and 7.3–5.05 eV for fused silica, respectively. This in no way compromises the obvious insulating properties of silica in MOS devices, since the conductivity is governed by the high barrier height (～3.8 eV in the case of gold) for metal-insulator electron transfer. The lowered bandgap results from increased low-energy electron density in the valence band, which we ascribe to the 1t₁ molecular orbital predicted by various quantum formalisms, but heretofore not detected experimentally in bulk (thick) silica. Disappearances of this orbital and rearrangement of the non-binding 5t₂ and 1e orbitals in silicas rich in silanols (OH), as may be the case for thin-film silica on Si metals, would increase the bandgap to 8.3 eV, in better agreement with previous determinations.