Photon-stimulated desorption of H + and F + from BeO, Al 2O 3 and SiO 2: Comparison of near edge structure to photoelectron yield

Abstract

Photon-stimulated desorption (PSD) spectra of H + and F + from BeO, Al 2O 3 and SiO 2 are compared to surface and/or bulk photoelectron yield (PEY) spectra at photon energies near the Be(K), Al(L) and Si(L) edges, respectively, as a demonstration of the spectral content of the PSD spectra. The PEY spectra can serve as fingerprints of local geometry and electronic structure from which to deduce the local environment of the desorbed species. This is especially true for these materials in which the near edge structure is determined primarily by the first coordination shell of the excited atom. The main features in the spectra consist of core excitons at threshold followed by a series of “inner-well resonances” whose structure is characteristic of the local environment. Spectral shifts and structural differences are observed between H + and F + spectra and between the PSD spectra and PEY spectra. At the Si(L 23) edge in SiO 2 there is ~0.5 eV shift in the excition peak between the H + and F + spectra. The peaks in the spectra closely resemble those for the “first monolayer” of oxide on Si(111) and display significant shifts relative to those of “bulk” SiO 2. On Al 2O 3, H appears to be bonded at a tetrahedrally coordinated Al site as seen in α-Al 2O 3 while F is bonded at a site with spectral features having both tetrahedral and octahedral characteristics. On BeO both H and F bonded Be sites appear to be essentially identical to bulk sites. A contaminant on the BeO surface in the form of Si fluoride first appears with a Si spectral shape line SiO and then transforms with time to an SiO 2-like (tetrahedral) spectrum, indicating that the Si has reacted from SiF to SiF 4. The implications for these kinds of measurements for this class of materials is discussed.