Ion irradiation effects in silicon nanowires

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Ion irradiation effects in silicon nanowires K. Nordlund1 , S. Hoilijoki1 , and E. Holmstr¨om1 Helsinki Institute of Physics and Department of Physics, P.O. Box 43, FI-00014 University of Helsinki, Finland

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ABSTRACT Ion irradiation effects in nanowires are of increasing interest due to potential applications of the wires as e.g. current-carrying elements in transistors or as efﬁcient light emitters. Although several experiments have already demonstrated such functionalities, very few theoretical studies on the fundamental mechanisms of ion irradiation have been carried out. To shed light on the basic mechanisms of nanowire irradiation, we have simulated 0.03 - 10 keV Ar ion irradiation of Si nanowires with a < 111 >-oriented axis and with all side facets being < 112 >. We compare the results with those for Si surfaces and bulk. The results show that the damage production in the nanowire is strongly inﬂuenced by surface effects. INTRODUCTION Ion implantation is a standard method for introducing dopants into semiconductors. While conventional implantation of course deals with irradiation of bulk materials, recent development of transistors using Si nanowires as the active component [1, 2] raise the question of how ion implantation affects this low-dimensional system. Since a nanowire has a huge surface-area-to-volume ratio, one could expect on one hand that sputtering and transmission is much more pronounced than in bulk Si, on the other hand that the surface might destabilize the system. Radiation effects in nanowires have been studies to some extent experimentally (for a review see [3]), and major differences (such as a much larger radiation hardness in some nanowire materials) to bulk materials have been reported. In the particular case of Si nanowires, it appears that as in its bulk counterpart, also the Si nanowire can be strongly damaged by the irradiation, and annealing is necessary to obtain functional devices. However, the understanding of damage in Si nanowires is very poor. As a step to obtain such understanding, we have carried out molecular dynamics computer simulations of the primary damage states produced by Ar ion irradiation of Si nanowires. To be able to assess whether ﬁnite-size effects contribute signiﬁcantly to the damage production, we also carried out simulations of bulk Si (by Si self-recoils) and Si thin ﬁlms of the same thickness as the nanowires. SIMULATION METHOD We simulated the irradiation using classical molecular dynamics [4] with the PARCAS code [5]. For modeling the Si–Si interactions, the analytical Stillinger-Weber (SW) threebody potential was used [6]. This potential was chosen because recent comparisons of

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Figure 1: Snapshots of a 1 keV Ar impact on a Si nanowire. The left side shows a cross section of the whole simulation cell, plotting atoms and covalent bonds. The right si

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