SURFACE COLLOID EVOLUTION DURING LOW-ENERGY-ELECTRON IRRADIATION OF CAF2(111)

The effects of 1 keV electron irradiation (current density typically 4 0 mu A cm(-2)) on the surface structure of CaF2 (111) are studied by s canning force microscopy (SFM) to investigate the role of diffusion in the process of electron-induced surface metal colloid Formation. Syst ematic variation of beam voltage, irradiation time, intensity and dosa ge is investigated in regard to metal formation on CaF2 surfaces. The main features observed in an experiment with constant irradiation inte nsity are colloids on the surface growing from an average size of 50 n m 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 satura tes at dosage densities above 6 mC cm(-2) due to the increasing covera ge of the surface by metal. Such a quadratic dependence confirms that surface metal is created by electron-stimulated F-center diffusion fro m the bulk. By varying current density we also find a saturation in F- centers arriving at the surface. A competing mechanism of metal creati on directly at the surface becomes dominant for current densities abov e 50 mu A cm(-2). In this intensity regime we find dosage-dependent me tallization 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 fea tures are large irregularly shaped metal platelets with folded ridges of approximately 150 nm elevation. We propose that these result from t he 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 hi gher dosages.