Low-energy-electron escape lengths inSiO2

The electron escape length, λ, for low-energy (8–70 eV) electrons traveling through ${\mathrm{SiO}}_2$ was measured with core-level soft-x-ray photoemission. The intensity of the ${\mathrm{Si}}^{4+}$ component of the 2p core level was compared to the bulk ${\mathrm{Si}}^{0+}$ component as a function of the electron final-state energy. Spectra were collected in the constant-final-state mode in order to prevent escape-depth changes within a given spectrum. Two oxides of different thickness were compared, and the extra attenuation resulting from the additional oxide on the thicker sample was used to measure the escape length. The curves resulting from an analysis which employs an exponential-attenuation model shows a minimum at an electron kinetic energy of ∼35 eV, which is a result of scattering from the bulk plasmon. Below 35 eV, λ increases and reaches a local maximum at ∼20 eV, at which point λ begins to decrease again. The decrease in λ at low electron energies arises in part from electron-phonon scattering. This is confirmed by the shape of the bulk Si 2p photoemission peak observed on the thicker oxide sample, which is distorted by the phonon-induced energy-loss processes. In addition, a more complete analysis of the scattering processes is presented in which elastic scattering is explicitly considered. With this model, it is shown that the small value of λ measured at low kinetic energies is also, in part, a thin-film effect.