Plasticity in crystalline-amorphous core-shell Si nanowires controlled by native interface defects

Abstract

Experimentally the silicon nanowires or nanopillars are naturally recovered by a thin oxide layer as soon as they are exposed to the air or present an amorphous layer of silicon when they are milled by focused ion beam (FIB) techniques. Here we investigate the role of the silicon amorphous shell on the plasticity of Si nanowires (NWs), thanks to molecular dynamics simulations. It is shown that the yield strain for the nucleation of the first dislocation is decreased for NWs with an amorphous shell when compared to pristine nanowires. For NWs with circular cross-sections, it is shown that the shell thickness has no influence on the yield strain. Besides, through the investigation of various rhombic cross-sections we observe that when an amorphous shell is present, the yield strain is independent of the cross-section shape. All these results can be explained by the presence of native atomic defects at the crystalline/amorphous interface, as revealed by a detailed atomistic analysis. These defects act as seeds for the dislocation nucleation. As a consequence this work raises the question about the role of point defects created in micro-and nanopillars milled by FIB techniques, in particular when they are used to study the mechanical properties at the nanoscale.