Surface colloid evolution during low-energy electron irradiation of CaF2(111)

The effects of 1 keV electron irradiation (current density typically 40 mu A cm(-2)) on the surface structure of CaF2 (111) are studied by scanning force microscopy (SFM) to investigate the role of diffusion in the process of electron-induced surface metal colloid Formation. Systematic variation of beam voltage, irradiation time, intensity and dosage is investigated in regard to metal formation on CaF2 surfaces. The main features observed in an experiment with constant irradiation intensity are colloids on the surface growing from an average size of 50 nm at a dosage density of 0.66 mC cm(-2) to 200 nm at 85 mC cm(-2). The surface metal volume first increases quadratically in time and saturates at dosage densities above 6 mC cm(-2) due to the increasing coverage of the surface by metal. Such a quadratic dependence confirms that surface metal is created by electron-stimulated F-center diffusion from the bulk. By varying current density we also find a saturation in F-centers arriving at the surface. A competing mechanism of metal creation directly at the surface becomes dominant for current densities above 50 mu A cm(-2). In this intensity regime we find dosage-dependent metallization features showing a transition from metallic clusters of 30 nm diameter to larger aggregates and formation of mu m-sized blisters with increasing dosage. For highest irradiation dosages, the main features are large irregularly shaped metal platelets with folded ridges of approximately 150 nm elevation. We propose that these result from the collapse of blisters that were previously filled with fluorine gas resulting from the radiolysis of the halide crystal. Furthermore, the CaF2 surface is investigated after removal of the metallic deposits by water treatment. SFM images reveal the existence of holes about 30 nm in diameter, which grow into a random network of larger grooves at higher dosages.