Irradiation with low-energy (1keV) electrons resulted in decomposition of the crystal and in the formation of metallic clusters at the surface, and in a near-surface layer. The formation of surface clusters, their shapes and their transformations in ultra-high vacuum were described. From the evidence of various experiments, it was concluded that surface metallization was strongly related to the diffusive transport of irradiation-induced defects. The present experimental results could be explained, together with earlier observations, by assuming that the conversion of primary F and H center defects into charged Vk and I centers, as well as their electric-field induced diffusion, were controlled by the irradiation. The diffusion of F into the bulk led to the formation of sub-surface F gas bubbles that appeared as micron-sized surface elevations in scanning force microscopy. In the initial stages of metal cluster formation on the (111) surface, ordering phenomena were observed which indicated epitaxial growth. The surface topography changed, due to oxidation, when metal clusters were subjected to a residual gas atmosphere (10-9mbar) for more than 24h.

Bulk and Surface Processes in Low-Energy Electron-Induced Decomposition of CaF2. R.Bennewitz, D.Smith, M.Reichling: Physical Review B, 1999, 59[12], 8237–46