International audienceLarge-scale atomistic calculations, using empirical potentials for modeling semiconductors, have been performed on a stressed system with linear surface defects like steps. Although the elastic limits of systems with surface defects remain close to the theoretical strength, the results show that these defects weaken the atomic structure, initializing plastic deformations, in particular dislocations. The character of the dislocation nucleated can be predicted considering both the resolved shear stress related to the applied stress orientation and the Peierls stress. At low temperature, only glide events in the shuffle set planes are observed. Then they progressively disappear and are replaced by amorphization/melting zones at a temperature higher than 900 K

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.

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• dislocations for both tensile and compressive deformations. We have also examined the effect of surface and step termination on the plastic mechanisms. The plasticity of semiconductors has been extensively studied for the last decades in both fundamental and applied research, leading to significant progresses in the understanding of the key mechanisms involved. Several issues remain unsolved, however, one of the most essential being the formation of dislocations in nanostructured semiconductors such as nano-grained materials, or nanolayers in heteroepitaxy, systems extensively used in devices. While in bulk materials the few native dislocations are able to multiply via Frank-Read type mechanisms to ensure plasticity, the situation is different in nanostructured materials where dimensions are too small to allow dislocation multiplication [1]. The presence of dislocations in these materials appears to be more controlled by nucleation than by multiplication processes. It has been proposed that surfaces and interfaces, which become prominent for small dimensions, play a major role. Several observations support this assumption, especially for strained layers and misfit dislocations at interfaces [2,3,4]. The formation at surfaces is also relevant where large stresses exist, like near a crack [5,6,7,8,9].Since in situ experimental observations of dislocation nucleation is not yet possible due to the very small dimensions and short observation timescales, the formation of dislocations at surfaces has been mainly investigated theoretically, particularly with continuum models and elasticity theory [10,11,12]. However, in these approaches, the predicted activation energy is very large, in disagreement with experiments. It has been proposed that surface defects, such as steps, help the formation by lowering the activation energy. This is supported by experimental facts in the context of dislocation nucleation at or near crack fronts, with dislocation sources located on the cleavage surface and coinciding with cleavage ledges [13,14,15,16]. In addition, it has been shown that, in a stressed solid, a surface step is a source of local stress concentration [17,18,19], although not as efficient as a crack tip. Therefore, a number of continuum models have been developed, taking into account the energy gain associated to the step elimination in the process of dislocation nucleation [20,21,22,23]. Atomistic calculations have also been performed for characterizing the energetics, the processes involved, and the role of surface defects [24,25,26,27,28,29,30].These studies led to a better knowledge of the dislocation formation from surface steps or cleavage ledges, but we are still far from a complete understanding of the phenomenon. Fu...

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We study proton diffusion in amorphous SiO2 from the atomic scale to the long-range percolative regime. Ab initio molecular dynamics suggest that the dominant atomic process consists in cross-ring interoxygen hopping assisted by network vibrations. A statistical analysis accounting for the disorder in amorphous SiO2 yields relations between transition energies and interoxygen distances for both cross-ring and nearest-neighbor hopping. The percolative regime is then addressed through large-size model systems reproducing these relations. Cross-ring hopping is confirmed as the dominant diffusion mechanism and supported by a good agreement with experiment for the activation energy.

We report an unexpected characteristic of dislocation cores in silicon. Using first-principles calculations, we show that all of the stable core configurations for a nondissociated 60 degrees dislocation are sessile. The only glissile configuration, previously obtained by nucleation from surfaces, surprisingly corresponds to an unstable core. As a result, the 60 degrees dislocation motion is solely driven by stress, with no thermal activation. We predict that this original feature could be relevant in situations for which large stresses occur, such as mechanical deformation at room temperature. Our work also suggests that postmortem observations of stable dislocations could be misleading and that mobile unstable dislocation cores should be taken into account in theoretical investigations.

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.

Abstract.The homogeneous shear of the {111} planes along the < 110 > 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 for complementing 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 localisation of shear in the shuffle set.