Investigation of Indenter-Size-Dependent Nanoplasticity of Silicon by Molecular Dynamics Simulation

Silicon (Si) is commonly used in microelectronic devices and micro-/nanoelectromechanical systems, and the mechanical behavior of nanoscale silicon is very important. However, the origin of its nanoplasticity is still controversial, and it is not certain whether the elastic-plastic transition is determined by phase transformation or dislocation. Herein, molecular dynamics simulation is performed to reveal the nanoplasticity of Si at room temperature during nanoindentation with indenter diameters in the range of 20-100 nm. Our results predict an interesting indenter-size-dependent transition for the nanoplasticity of Si. When a small indenter is used, the high-pressure phase transformation (HPPT) is exclusively responsible for the elastic-plastic transition and subsequent incipient plasticity, giving rise to the strain-induced HPPT from the cubic diamond structure Si (dc-Si) to the β-Si and bct5 phases. However, for a large indenter, HPPT alone can no longer relieve the mechanical load, and dislocations are activated synchronously. In fact, the larger the indenter, the more prevailing are the dislocations. All the observed dislocations are the shuffle dislocation loops in the <110>{111} slip system emitted from the high-pressure phases/dc-Si interface. Moreover, a large indenter promotes a sharp pop-in singularity in the load-penetration depth curve, but a small indenter gives rise to a gentle and difficult-to-detect pop-in singularity. Our findings shed light on the elastic-plastic transition controversy of Si and provide knowledge about the nanoplasticity of Si.