Characterization of swift heavy ion tracks in CaF2 by scanning force and transmission electron microscopy

Single crystals of fluorite (CaF₂) were exposed to various swift heavy ions (Ca up to U) of energy 1–11.1 MeV per nucleon, covering a large range of electronic stopping power S_e between 4.6 and 35.5 keV/nm. The irradiated (1 1 1) cleaved surfaces were investigated by means of scanning force microscopy in tapping mode. Nanometric hillocks produced by the ion projectiles were analyzed in terms of creation efficiency E_eff, diameter and height values, and diameter–height correlation. Hillock formation appears with a low efficiency above a S_e threshold of 5 keV/nm. The mean height of these hillocks is approximately constant (1 nm) between 5 and 10 keV/nm and increases linearly with S_e above 10 keV/nm reaching 12.5 nm for the largest S_e value investigated. Similarly, the efficiency grows versus S_e achieving 100% for S_e > 13 keV/nm where each projectile produces an individual hillock. Above 13 keV/nm, the hillock height and diameter are strongly correlated. The diameter was deduced by graphical deconvolution of the scanning-tip curvature that is determined experimentally for each set of measurements. In the entire S_e regime, the mean diameter exhibits a constant value of 13 nm, which is significantly larger than 6 nm wide tracks observed by transmission electron microscopy.