Atomistic modelling of fracture mechanisms in semiconductor nanowires under tension [electronic resource]

The fracture behavior of silicon (Si) and germanium (Ge) nanowires (NWs) in tension is studied by molecular dynamics (MD) simulations. First, the MD simulation results depend on the inter-atomic potential models used. At room temperature and for NWs of diameter D = 5 nm, while some potentials predict brittle fracture initiated by crack nucleation from the surface, most potentials show ductile fracture initiated by dislocation nucleation and slip. A ductility parameter, defined as the ratio between the ideal tensile strength and the ideal shear strength, multiplied by the appropriate Schmid factor, is found to correlate well with the observed brittle versus ductile fracture behaviors of the NWs for all the potentials used in this study. The ductility parameter is then computed by ab initio methods, which predict brittle fracture at room temperature. Because the ductility parameter of the modified embedded-atom-method (MEAM) potential is closest to that of it ab initio methods, the MEAM potential is considered to be the most reliable model among the interatomic potentials tested here for the fracture simulations. These results also suggest that it is important to monitor the ductility parameter during the development of inter-atomic potentials, if they are to be used in fracture simulations. Using the MEAM potential, more MD simulations are performed for [110]-oriented Si NWs of different diameters under a constant strain rate in tension until failure, at various temperatures. The fracture behavior of the NWs is observed to depend on both temperature and NW diameter. For NWs with diameter larger than 4 nm, cleavage fracture on the transverse (110) plane is predominantly observed at temperatures below 1000 K. At higher temperatures, the same NWs shear extensively on inclined {111} planes prior to fracture, analogous to the brittle-to-ductile transition (BDT) in bulk Si. Surprisingly, NWs with diameter less than 4 nm fail by shear regardless of temperature. Detailed analysis reveals that cleavage fracture is initiated by the nucleation of a crack, while shear failure is initiated by the nucleation of a dislocation, both from the surface. The BDT mechanism in semiconductor nanowires is different from that in the bulk, due to the lack of a pre-existing macrocrack that is always assumed in bulk BDT models. While dislocation mobility is believed to be the controlling factor of BDT in bulk Si, our analysis shows that the change of failure mechanism in Si NWs with decreasing diameters is nucleation-controlled. The simulation results are compared with tensile experiments of Si NWs. To reveal the mechanism behind the size effect on the fracture behavior of Si NWs in tension, the energy barriers for dislocation nucleation and crack nucleation in Si NWs of different diameters are calculated. The rough energy landscape in Si potential models makes the conventional minimum-energy-path (MEP) search algorithm unstable. A modified string method is developed and successfully obtains the MEP for dislocation and crack nucleation in Si NWs. The energy barrier of crack nucleation changes little with different NW diameters, while that of dislocation nucleation reduces almost by half as the diameter decreases from D = 7 nm to D = 3 nm. This trend may explain the MD observation that thinner Si NWs fail by shear fracture for all temperature range, while thicker Si NWs show temperature-dependent fracture behavior.

Keywords: Mechanical Engineering
Advisor: Cai Wei
Darve Eric advisor. Thesis advisor
Nix William D. advisor. Thesis advisor