Contribution of Electronic Energy Deposition to the Atomic Cascade Damage in Nanocrystals
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Contribution of Electronic Energy Deposition to the Atomic Cascade Damage in Nanocrystals Marie Backman1 , Flyura Djurabekova1,2 , Olli H. Pakarinen1 , Kai Nordlund1 , and Marcel Toulemonde3 1
Helsinki Institute of Physics and Department of Physics, P.O. Box 43, FI-00014 University of Helsinki, Finland 2 Arifov Institute of Electronics, Durmon yuli 33, 100125 Tashkent, Uzbekistan 3 Centre Interdisciplinaire de Recherche sur les Ions, les Mat´eriaux et la Photonique (CIMAP), Caen, France ABSTRACT Using Molecular Dynamics we study the role of electronic excitations in the radiation damage caused by an energetic ion in Ge nanocrystals embedded in amorphous SiO2 . The electronic effects are included as heating along the ion path modeled by the thermal spike model. In an ion energy regime where the electronic stopping power is larger than the nuclear, we find that the electronic effects enhance the defect creation significantly. We conclude that the electronic excitations below the track production threshold due to an energetic ion cannot be disregarded as a source of radiation damage. INTRODUCTION Swift heavy ions passing through a material can leave permanent damage in the form of ion tracks, which can be observed by for instance channeling Rutherford backscattering or electron microscopy. At these high energies the ions lose energy primarily by interactions with electrons in the target, leaving an area around the ion path with a high density of excited electrons. As the excess energy in the electronic system is transferred to the atoms as thermal vibrations, a molten region is formed that upon freezing forms the ion track. One necessary condition for the formation of an ion track is that the electronic stopping power (dE/dx)e is above a threshold value which is dependent on the ion and target material [1–3]. Below this threshold value the primary source of damage are atomic cascades due to interactions between ion and target nuclei. In this energy regime there are thus no visible tracks, but electronic heating remains present, although not causing melting of the material. The contributions to irradiation effects of the nuclear cascade and electronic excitations are hard to distinguish in experiments, but the separate mechanisms are easily isolated using computer simulations. From Molecular Dynamics (MD) studies it is known that electronic energy losses affect the outcome of bombarding ions, like for instance sputtering yield [4–7] and irradiation damage [8, 9]. Most studies are, however, concerned with electronic excitations in metals, where the electronic system can transport the heat away from the hot cascade region thereby enhancing the damage by quenching present defects. Few experimental studies have addressed this issue, but Toulemonde et al. found evidence that the swelling of irradiated quartz can be described as a linear superposition of nuclear and electronic damage within an energy regime where nuclear and electronic stopping powers are of comparable
magnitude [10]. A synergetic effect betw
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