Formation of Hillock on Muscovite Mica Surface Induced by Energetic Ion near Electronic Energy Loss Threshold

Muscovite mica sheets were bombarded with He⁺ ions of 0.65 MeV. The irradiated surfaces were analyzed in the air with atomic force microscopy (AFM) in the tapping mode. It reveals the ion-induced hillocks with height from less than 1 nm to a few nm at various temperatures. The electronic energy loss threshold for the hillock formation is found to be below 0.44 keV/nm by using He⁺ ions at the room temperature. The surfaces were also bombarded at various elevated temperatures up to 973 K to confirm the observed hillocks. It is found that the statistical distribution of the diameter and height of the hillocks gets more divergent and spreads with larger values at higher temperatures due to the thermal annealing, compared with that at the room temperature. The mean diameter and height of the hillocks do not show any significant change. The two thermal spike model calculations were compared with the experiment near the threshold. The analytical thermal spike model and the two-temperature thermal spike model were taken into account in both the electronic and nuclear energy loss. The threshold value calculated by analytical thermal spike model is quite beyond the experimental value. It is found that the experimental threshold is little below the calculated value if only the electronic energy loss is considered. Taking into account the small nuclear energy loss, the hillocks can be formed within the framework of the two temperature thermal spike model.