Julien Godet

Evidence of two plastic regimes controlled by dislocation nucleation in silicon nanostructures

We performed molecular dynamics simulations of silicon nanostructures submitted to various stresses and temperatures. For a given stress orientation, a transition in the onset of silicon plasticity is revealed depending on the temperature and stress magnitude. At high temperature and low stress, partial dislocation loops are nucleated in the {111} glide set planes. But at low temperature and very high stress, perfect dislocation loops are formed in the other set of {111} planes called shuffle. This result confirmed by three different classical potentials suggests that plasticity in silicon nanostructures could be controlled by dislocation nucleation.

Dislocation formation from a surface step in semiconductors: An ab initio study

The role of a simple surface defect, such as a step, for relaxing the stress applied to a semiconductor, has been investigated by means of large-scale first-principles calculations. Our results indicate that the step is the privileged site for initiating plasticity, with the formation and glide of 60 degrees dislocations for both tensile and compressive deformations. We have also examined the effect of surface and step termination on the plastic mechanisms.

Comparison between classical potentials and ab initio methods for silicon under large shear

The homogeneous shear of the {111} planes along the direction of bulk silicon has been investigated using ab initio techniques, to better understand the strain properties of both shuffle and glide set planes. Similar calculations have been done with three empirical potentials, Stillinger–Weber, Tersoff and EDIP, in order to find the one giving the best results under large shear strains. The generalized stacking fault energies have also been calculated with these potentials to complement this study. It turns out that the Stillinger–Weber potential better reproduces the ab initio results, for the smoothness and the amplitude of the energy variation as well as the localization of shear in the shuffle set.

A new parametrization of the Stillinger–Weber potential for an improved description of defects and plasticity of silicon

A new parametrization of the widely used Stillinger-Weber potential is proposed for silicon, allowing for an improved modelling of defects and plasticity-related properties. The performance of the new potential is compared to the original version, as well as to another parametrization (Vink et al 2001 J. Non-Cryst. Solids, 282 248), in the case of several situations: point defects and dislocation core stability, threshold displacement energies, bulk shear, generalized stacking fault energy surfaces, fracture, melting temperature, amorphous structure, and crystalline phase stability. A significant improvement is obtained in the case of dislocation cores, bulk behaviour under high shear stress, the amorphous structure, and computation of threshold displacement energies, while most of the features of the original version (elastic constants, point defects) are retained. However, despite a slight improvement, a complex process like fracture remains difficult to model.

Deformation of silicon nanowires studied by molecular dynamics simulations

The effect of various parameters such as temperature or surface state on the very first stages of plasticity in semiconductor nanowires has been investigated by molecular dynamics simulations. In particular, the role of edge and surface reconstructions has been analyzed and discussed in detail. To this end, square nanowires with the [0 0 1] and [1 2 3] axis and various side surfaces have been considered. In general, the onset of plasticity appears from the NW edges at very high stresses. However, when surface reconstructions make surface steps at the intersection of the slip plane and the NW surface, the step can favor the dislocation nucleation from surfaces. This study raises the role of the detailed geometry of the surfaces and edges on the onset of plasticity in nanostructures.

Dislocation cores in silicon: new aspects from numerical simulations

Recent theoretical investigations of the properties of dislocation cores in silicon are reviewed. New results, obtained from numerical simulations for the non-dissociated screw and 60° dislocations, are presented and discussed in relation with experiments.

Determination of activation parameters for the core transformation of the screw dislocation in silicon

The non-dissociated screw dislocation in a model covalent material like silicon is known to exist in three possible stable core configurations. We performed calculations combining the nudged elastic band technique and a semi-empirical description in order to determine mechanisms and activation parameters for transforming one core into another. Our results showed that a glide core is necessarily reconstructed, since the energy barrier for reconstruction is easily overcome by thermal activation. Conversely, a transformation between a shuffle and a glide core appears unlikely at low temperature, which raises questions about the existence of the double-period glide configuration.

Surface step effects on Si (100) under uniaxial tensile stress, by atomistic calculations

Abstract This paper reports a study of the influence of the step at a silicon surface under an uniaxial tensile stress, using an empirical potential. Our aim was to find conditions leading to nucleation of dislocations from the step. We obtained that no dislocations could be generated with such conditions. This behaviour, different from the one predicted for metals, could be attributed either to the covalent bonding or to the cubic diamond structure.

Investigation of the interaction between hydrogen and screw dislocation in silicon by first-principles calculations

The stability of atomic and molecular hydrogen in the vicinity of a screw dislocation in silicon has been investigated using first-principles calculations. The lowest energy configurations are obtained for H atoms located in the dislocation core, suggesting that the segregation of hydrogen is favoured in the dislocation core. It is found that a spontaneous dissociation of H(2) could occur in the dislocation core. Finally, the variation of the interaction energy between hydrogen and the dislocation core as a function of the separation distance has been calculated. There is no sizeable interaction variation for H(2). However, in the case of a single H, an inverse law has been obtained, which can be explained by the anisotropic stress field generated by the insertion of H in the silicon lattice.

Influence of strain on dislocation core in silicon

Abstract First principles, density functional-based tight binding and semi-empirical interatomic potentials calculations are performed to analyse the influence of large strains on the structure and stability of a 60 dislocation in silicon. Such strains typically arise during the mechanical testing of nanostructures like nanopillars or nanoparticles. We focus on bi-axial strains in the plane normal to the dislocation line. Our calculations surprisingly reveal that the dislocation core structure largely depends on the applied strain, for strain levels of about 5%. In the particular case of bi-axial compression, the transformation of the dislocation to a locally disordered configuration occurs for similar strain magnitudes. The formation of an opening, however, requires larger strains, of about 7.5%. Furthermore, our results suggest that electronic structure methods should be favoured to model dislocation cores in case of large strains whenever possible.