Investigation of the silica surface via electron-energy-loss spectroscopy

Various aspects of the electronic structure of the silica surface and its response to electron irradiation have been studied. Electron-energy-loss spectra (ELS) were obtained for bulk high-purity synthetic SiO₂ with either high or low OH content, crystalline α-quartz, and compacted aerosil powder. All show peaks at Eₗₒₛₛ=3.3, 5.0, and 6.8 eV for excitation of valence-band electrons with low-energy (Eₚ=175 eV) primary beams. Aside from small changes in relative intensity, these features are independent of: (i) exposure to different atmospheres, (ii) electron irradiation up to levels of 5 ×{} 10²⁰ electrons/cm² at 5 keV, (iii) in situ annealing in vacuum, O₂ or H₂, and (iv) Ar⁺ sputtering at either 0.5 or 2 keV. Assignment of these transitions to intrinsic surface electronic states (as distinct from chemisorbed species or radiation-induced point defects) is discussed qualitatively. Electron irradiation is known to cause oxygen desorption, leading to a surface region of the form SiOₓ ($1<x<2$). After irradiation sufficient to produce $x=1.5$, as indicated by Auger spectroscopy, no strong evidence is found for interband or plasmon excitations characteristic of bulk elemental silicon. These results tend to favor the random-mixture model over the phase-separation model of the damage region. Arguments are also presented against the participation of surface point defects in the 70-100-eV Auger spectrum of electron-irradiated SiO₂.