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 Se 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 Eeff, diameter and height values, and diameter-height correlation. Hillock formation appears with a low efficiency above a Se 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 Se above 10 keV/nm reaching 12.5 nm for the largest Se value investigated. Similarly, the efficiency grows versus Se achieving 100% for Se > 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 Se 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.