Quantum and Classical Molecular Dynamics Studies of the Threshold Displacement Energy in Si Bulk and Nanowire
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1181-DD05-02
Quantum and Classical Molecular Dynamics Studies of the Threshold Displacement Energy in Si Bulk and Nanowire
E. Holmstr¨om1 , A. V. Krasheninnikov1,2 , and K. Nordlund1 1 Helsinki Institute of Physics and Department of Physics, P.O. Box 43, FI-00014 University of Helsinki, Finland 2 Department of Applied Physics, Helsinki University of Technology, P.O. Box 1100, FI02015, Finland ABSTRACT Using quantum mechanical and classical molecular dynamics computer simulations, we study the full three-dimensional threshold displacement energy surface in Si. We show that the SIESTA density-functional theory method gives a minimum threshold energy of 13 eV that agrees very well with experiments, and predicts an average threshold displacement energy of 36 eV. Using the quantum mechanical result as a baseline, we discuss the reliability of the classical potentials with respect to their description of the threshold energies. We also examine the threshold energies for sputtering in a nanowire, and find that this threshold depends surprisingly strongly on which layer the atom is in. INTRODUCTION The threshold displacement energy of a material Ed is the single most fundamental quantity in determining the primary state of radiation damage in both bulk [1, 2] and nanoscale materials [3]. Knowing Ed in silicon is essential not only for the well-known use of the material in the manufacturing of semiconductor devices, but also because of contexts such as particle accelerators and space missions, where Si-based radiation detectors are exposed to extensive hadron damage. In spite of this vast technological interest and extensive scientific study of this quantity, the Ed averaged over all lattice directions remains poorly known in the material. Experimental methods show a widely varying scale of results for the average Ed in the range of 10 - 30 eV [4–7], and simulations using classical potentials show a similarly wide range of results [8–12]. The large variation of the simulation results on Ed is clearly related to the uncertainty of the interatomic potentials in the interaction energy range which determines the threshold. Since threshold displacement energies are typically in the range 10 - 50 eV [13–15], the part of the interatomic potential that determines the threshold is roughly speaking 1 20 eV above the minimum of the potential well. Unfortunately, no other commonly available experimental quantity depends on interaction energies in this range, and hence in this energy range the shape of the potential is usually just an extrapolation of a fit to much lower-energy data of elastic properties. In metals, in fact, potentials are commonly fit to known threshold displacement energies [16], but in Si this approach is not useful due to the large experimental uncertainties in the quantity.
Computer capacity has increased sufficiently to allow for dynamic simulation of atom motion in small systems using quantum mechanical approaches such as density-functional theory [17]. Since simulation of the threshold displacement energy onl
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