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

The irradiation of CaF2, with low-energy (typically 1 keV) electrons result s 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 strong ly related to diffusive transport of irradiation-induced defects. We can co herently explain experimental results presented here, and earlier observati ons, by assuming that both the conversion of primarily created F and H cent er defects into charged Vk and I centers, as well as their electric-field-i nduced diffusion, are controlled by the electron irradiation. Fluorine diff usion into the bulk leads to the formation of subsurface fluorine gas bubbl es appearing as micron-sized surface elevations in scanning force microscop y images. For the initial stages of metal cluster formation on the (111) su rface, 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 x 10(-9) mbar for more than one day. [S0 163-1829(99)05508-3].