Comparison of molecular dynamics and binary collision approximation simulations for atom displacement analysis

L Bukonte,F Djurabekova,J Samela,K Nordlund,S.A Norris,M.J Aziz

doi:10.1016/j.nimb.2012.12.014

Abstract

Molecular dynamics (MD) and binary collision approximation (BCA) computer simulations are employed to study surface damage by single ion impacts. The predictions of BCA and MD simulations of displacement cascades in amorphous and crystalline silicon and BCC tungsten by 1keV Ar+ ion bombardment are compared. Single ion impacts are studied at angles of 50°, 60° and 80° from normal incidence. Four parameters for BCA simulations have been optimized to obtain the best agreement of the results with MD. For the conditions reported here, BCA agrees with MD simulation results at displacements larger than 5Å for amorphous Si, whereas at small displacements a difference between BCA and MD arises due to a material flow component observed in MD simulations but absent from a regular BCA approach due to the algorithm limitations. MD and BCA simulation results for crystalline W are found to be in a good agreement even at small displacements, while in crystalline Si there is some difference due to displacements in amorphous pockets.

Highlights

Studying ion irradiation induced damage at surfaces is important for the construction of shielding materials for applications in which the the materials are subject to small particle bombardment [1, 2]
Single 1 keV Ar+ ion irradiation impacts of amorphous and crystalline Si cells and a crystalline W cell were simulated with classical molecular dynamics code PARCAS [32, 33] and compared to the Monte Carlo binary collision approximation (BCA) code CASWIN [34], in which for every collision scattering integral is solved and distance to colliding atom is chosen randomly taking into account the density corresponding to Molecular dynamics (MD) simulation cell
The comparison between BCA and MD simulations of collision cascades in amorphous silicon by individual 1 keV Ar+ ion impact at three different incidence angles reveals a significant shortcoming of the BCA approach when applied to materials that are amorphous or become amorphous during ion irradiation

Summary

Introduction

Studying ion irradiation induced damage at surfaces is important for the construction of shielding materials for applications in which the the materials are subject to small particle bombardment [1, 2]. Ion bombardment is an effective approach for generation of self-organized, periodic nano-structures.[3, 4, 5, 6, 7] Both smoothening and nano-pattern formation by ion beam irradiation were recently concluded, by both theoretical [8] and experimental [9] lines of reasoning, to be a coherent effect of he impact-induced displacements of the atoms that are not sputtered away, but come to rest at new locations within or on the solid. The theoretical approach requires knowledge of the “crater function” – the average surface height-change profile induced by Craters induced by single ion impacts have been intensively studied over the last decades. [12, 13, 14, 15] Previous studies have shown that small changes in the shape of craters can lead to significant changes in macroscopic pattern-forming behavior. [16] well-converged, accurate simulations are an essential input to crater function theory for drawing conclusions about surface stability or large-scale pattern formation

Methods

Results

Conclusion