SHI induced surface re-organization of non-amorphisable nanodimensional fluoride thin films

Surface re-organization in nanodimensional fluoride (LiF and BaF₂) thin films is observed under dense electronic excitation produced by swift heavy ion (SHI) irradiation. The irradiation was performed at an angle of less than 15° with respect to the film surface while keeping the sample at liquid nitrogen temperature. The surface of the irradiated samples was characterized by atomic force microscopy (AFM) and scanning electron microscopy (SEM) complemented by energy dispersive X-ray spectroscopy (EDX). Detailed analyses indicate that the surface starts cracking at lower fluence. With an increase in the ion fluence, the materials shrinking and surface re-structuring lead to lamellae periodic structures. The average width of the wall decreases, while the separation and the height of the structures increase with the fluence. The composition of the lamellae walls and the gap in between were analyzed by EDX. At the highest fluence of irradiation, a strong signal of the substrate and negligible signals of F and Ba are observed between the walls of the lamellae structures, which shows that the entire deposited material is removed and the Si substrate is completely exposed to the ion beam. It is also observed that the substrate remains unaffected by SHI irradiation and does not undergo any structural transformation as evident by cross-sectional SEM micrographs. Such surface re-organization is not expected in fluoride thin films due to their non-amorphizable nature even at very high fluence SHI irradiation. The concept of grain rotation under SHI irradiation is used to explain the re-organization phenomena in such non-amorphizable materials.