Sandrine Brochard

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.

Dislocation nucleation from surface steps: Atomistic simulation in aluminium

Abstract The possible role of surface steps in the nucleation of dislocations from a free surface has been studied by means of a static atomistic simulation using a many-body potential for aluminium. The fcc crystal with a {100} free surface containing a monatomic step lying along a (110) dense direction is submitted to an increasing uniaxial stress along a direction belonging to the {100} plane. For a sufficiently high applied stress, well below the theoretical strength. dislocations are nucleated at the step and glide in the {111} planes emerging at the step. The effect of a stress orientation is examined. The type of dislocation formed. that is Shockley partials of 90° and 30° character or perfect dislocations, is rationalized by considering the resolved shear stress in the {111} planes. The plane where glide will occur is favoured well before nucleation; a shear of increasing amplitude and extension is progressively localized on this plane. The role of the stress field due to the step, in the formation of the localized shear, is discussed.

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.

Glissile dislocations with transient cores in silicon.

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.

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.

Effects of temperature and surface step on the incipient plasticity in strained aluminium studied by atomistic simulations

Atomistic simulations using an EAM potential are carried out to investigate the first stages of plasticity in aluminium slabs, in particular the effect of both temperature and step geometry on the nucleation of dislocations from surface steps. Temperature is shown to significantly reduce the elastic limit, and to activate the nucleation of dislocation half-loops. Twinning occurs by successive nucleations in adjacent glide planes. The presence of a kinked step is shown to have no influence on the nucleation mechanisms.

Onset of ductility and brittleness in silicon nanowires mediated by dislocation nucleation

Most studies show that materials at the nano-scale have different mechanical properties than in the bulk state. Semiconductors like silicon and germanium are brittle in the bulk state, but when their size is reduced to the nano-scale they appear to be ductile. Under tensile loading, we performed molecular dynamics simulations on silicon crystalline nanowires of different lengths. We present the details of the obtained mechanisms that led to ductility and brittleness. In the case of ductility, dislocation nucleation was observed with a signature of surface step formation on the surface and in the case of brittleness a cavity was formed after the distinct formation of a wedge-like shape on the surface. Interestingly, a common mechanism taking place behind ductility and brittleness is dislocation nucleation. We believe that the observed mechanisms reveal interesting information for understanding and explaining the size dependent brittle to ductile transition.

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.