Dislocations in monolayers and semiconductors.
Creators: name: Ren, Qiang.
Publisher: name: Université d'Ottawa / University of Ottawa
Year: 1995
Subjects: subject: Physics, Condensed Matter.

description: Four different aspects of the properties of dislocations in monolayer and semiconductors have been investigated: (i) Using atomic relaxation techniques, dislocation dipoles of various sizes and orientations have been studied for monolayers with the Lennard-Jones potential (LJP) and the nearest-neighbour piecewise linear force (PLF) interactions. In the WP system the lower energy vacancy dipoles have over a wide range of angles an energy which is mainly a function of the vacancy content of the dipole. There is a competition between the elastic forces and the topological constraints which favour a five-fold coordinate vacancy (FCV) at the centre of each core. For the short range PLF system the lattice usually compresses upon the introduction of a dislocation, a consequence of the soft core of the interaction potential, and interstitial dipoles are lower in energy. For the long range LJP system the dislocations are mobile whereas for the PLF system they are pinned. The relevance of these results to existing theories of melting are discussed. (ii) Using generalized stacking-fault (GSF) energies obtained from first-principles density-functional calculations, a zero-temperature model for dislocations in silicon is constructed within the framework of a Peierls-Nabarro (PN) model. Core widths, core energies, PN pinning energies, and stresses are calculated for various possible perfect and imperfect dislocations. Both shuffle and glide sets are considered. 90$\sp\circ$ partials are shown to have a lower Peierls stress (PS) than 30$\sp\circ$ partials in accord with experiment. (iii) We have also studied by atomic relaxation techniques the properties of dislocations in silicon, modelled by the empirical potential of Stillinger and Weber. In order to compare with the preceding calculation no reconstruction is allowed. We find no evidence of dissociation in the shuffle dislocations. Within this model shuffle dislocations glide along their slipping planes. On the other hand, glide sets are shown to glide only in dissociated form. The dislocation displacement fields are essentially planar. The PS is found to be isotropic within the (111) plane. In other words the minimum stress at 0K required to move the dislocation in any direction with in that plane has the same projection unto the Burgers vector, the PS of the dislocation. Our PS are in good agreement with those from (ii). (iv) Using a simple two dimensional UP model, relaxation mechanisms of the epitaxial strain layers (ESL) have been simulated for various misfits and layer thickness. In this model, the relationship of two competing relaxation mechanisms is found. At small misfit, strain is released by nucleating misfit dislocations from the edges of system. This process is more favourable for the thicker layer. At large misfit, stress is relaxed through surface instability, allowing easy generation of misfit dislocations from the surface. Those results are qualitatively in agreement with experiments.