Pierre Beauchamp

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.

Theoretical study of kinks on screw dislocation in silicon

Theoretical calculations of the structure, formation and migration of kinks on a non-dissociated screw dislocation in silicon have been carried out using density functional theory calculations as well as calculations based on interatomic potential functions. The results show that the structure of a single kink is characterized by a narrow core and highly stretched bonds between some of the atoms. The formation energy of a single kink ranges from 0.9 to 1.36 eV, and is of the same order as that for kinks on partial dislocations. However, the kinks migrate almost freely along the line of an undissociated dislocation unlike what is found for partial dislocations. The effect of stress has also been investigated in order to compare with previous silicon deformation experiments which have been carried out at low temperature and high stress. The energy barrier associated with the formation of a stable kink pair becomes as low as 0.65 eV for an applied stress on the order of 1 GPa, indicating that displacements of screw dislocations likely occur via thermally activated formation of kink pairs at room temperature.

First principles determination of the Peierls stress of the shuffle screw dislocation in silicon

The Peierls stress of the a/2⟨110⟩ screw dislocation belonging to the shuffle set is calculated for silicon using density functional theory. We have checked the effect of boundary conditions by using two models, the supercell method where one considers a periodic array of dislocations, and the cluster method where a single dislocation is embedded in a small cluster. The Peierls stress is underestimated with the supercell and overestimated with the cluster. These contributions have been calculated and the Peierls stress is determined in the range between 2.4 × 10−2 and 2.8 × 10−2 eV Å−3. When moving, the dislocation follows the {111} plane going through a low energy metastable configuration and never follows the 100 plane, which includes a higher energy metastable core configuration.

Dislocation motion in silicon: the shuffle-glide controversy revisited

Considering recently computed formation and migration energies of kinks on nondissociated dislocations, we have compared the relative mobilities of glide partial and shuffle perfect dislocations in silicon. We found that the latter should be more mobile over all the available stress range, invalidating the model of a stress driven transition between shuffle and glide dislocations. We discuss several hypotheses that may explain the experimental observations.

Calculations of dislocation mobility using nudged elastic band method and first principles DFT calculations

We present a new technique which makes it possible to determine mobility properties of dislocations with first principles accuracy without having to apply corrections for the influence of boundary conditions. The Nudged Elastic Band method is used together with periodic boundary conditions and all dislocations included in the simulated cell are coherently displaced during the calculations. The method is applied to the displacement of a non-dissociated shuffle screw dislocation in silicon along two different directions. Peierls energies as well as dislocation structure as a function of the dislocation position in the lattice have been obtained. We have determined the Peierls stresses for both directions, in excellent agreement with previous determinations. Finally, we discuss the advantages of the technique over other methods.

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.

Stability of undissociated screw dislocations in zinc-blende covalent materials from first-principle simulations

The properties of perfect screw dislocations have been investigated for several zinc-blende materials such as diamond, Si, β-SiC, Ge and GaAs, by performing first-principles calculations. For almost all elements, a core configuration belonging to shuffle set planes is favored, in agreement with low-temperature experiments. Only for diamond, a glide configuration has the lowest defect energy, thanks to an sp2 hybridization in the core.

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.