Characterization of swift heavy ion tracks in CaF$_2$ by scanning force and transmission electron microscopy

Single crystals of fluorite (CaF$_2$) 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 $\sim$5 keV/nm. The mean height of these hillocks is approximately constant ($\sim$1 nm) between 5 and 10 keV/nm and increases linearly with S$_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 $\sim$13 nm, which is significantly larger than 6 nm wide tracks observed by transmission electron microscopy