Contrary to bulk materials, in which plasticity is often linked to the presence of Franck-Read sources, nanostructured materials are too small for such sources to operate. Plasticity is then mainly due to the nucleation of dislocations from surfaces and interfaces. In free surfaces, irregularities such as steps are expected to be favored sites for the formation of dislocations. Nucleation, happening at small spatial and temporal scales, is difficult to observe experimentally. An alternative is atomistic simulations, allowing to investigate the very first stages of plasticity.

We present here an analysis of the dislocation nucleation activation from a surface step in a crystal under stress, studied by atomistic calculations. Two model materials are considered: a face centered cubic metal (aluminum) and a diamond-like semiconductor (silicon), both of them being modeled with semi-empirical potentials.

Finite temperature simulations in aluminum lead to the formation of dislocation half-loops, and allow the determination of saddle-point configurations and the associated activation energies. In order to investigate a wide range of applied stress, different methods have been used: molecular dynamics simulations, and nudged elastic band calculations. The obtained results are then compared with those deduced from elasticity.

In the case of silicon, the determination of the activation parameters is more complicated because of the numerous different types of dislocation that can be obtained, depending essentially on the temperature. Nevertheless, the simulations bring important information on a possible change of nucleation mechanisms with temperature, namely nucleation of perfect dislocations in the shuffle set at low temperatures and partial dislocations in the glide set at high temperatures.

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1.  Nucleation and dislocations cores in semi conductors:  from macrostructures to nanostructures