Bulk and surface processes in low-energy-electron-induced decomposition ofCaF2

Abstract

The irradiation of ${\mathrm{CaF}}_{2}$ with low-energy (typically 1 keV) electrons results in a decomposition of the crystal and the formation of metallic clusters on its surface, and in a near-surface layer. In this paper we describe the formation of surface clusters and their typical shapes and transformations they undergo in an ultrahigh-vacuum environment. From the evidence found in a variety of experiments, we conclude that surface metallization is strongly related to diffusive transport of irradiation-induced defects. We can coherently explain experimental results presented here, and earlier observations, by assuming that both the conversion of primarily created F and H center defects into charged ${V}_{k}$ and I centers, as well as their electric-field-induced diffusion, are controlled by the electron irradiation. Fluorine diffusion into the bulk leads to the formation of subsurface fluorine gas bubbles appearing as micron-sized surface elevations in scanning force microscopy images. For the initial stages of metal cluster formation on the (111) surface, we observe ordering phenomena indicating an epitaxial growth. Due to oxidation the surface topography changes when metal clusters are subjected to a residual gas atmosphere of $1\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}9}\mathrm{mbar}$ for more than one day.