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

Abstract

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 Si${\mathrm{O}}_2$ with either high or low OH content, crystalline $\alpha$-quartz, and compacted aerosil powder. All show peaks at $E_{\mathrm{loss}}=3.3, 5.0, \mathrm{and} 6.8$ eV for excitation of valence-band electrons with low-energy ($E_p=175\mathrm{}$ 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 × ${10}^{20}$ electrons/${\mathrm{cm}}^2$ at 5 keV, (iii) in situ annealing in vacuum, ${\mathrm{O}}_2$ or ${\mathrm{H}}_2$, and (iv) ${\mathrm{Ar}}^{+}$ 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 $\mathrm{Si}{\mathrm{O}}_x$ ($1<x<\mathrm{}2\mathrm{}$). After irradiation sufficient to produce $x=1.5\mathrm{}$, 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 Si${\mathrm{O}}_2$.