Dislocation formation from a surface step in semiconductors: Anab initiostudy

Abstract

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...

Effect of surface roughness on elastic limit of silicon nanowires

“…In this work, we skip re-explaining how localized lattice reconstructions of notches' roots affect the initiation of dislocations, which can be referred in the works on plasticity induced by surface steps [36][37][38]. Similar to surface step, crack in bulk silicon is closely related to stress concentration and its opening angle is often useful to characterize stress intensive factor.…”

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

“…7 In bulk, dislocations are multiplied by mechanisms such as Frank-Read, 8 but in nanostructures such as nanowires or in thin layers, the small dimensions prevent those mechanisms and dislocations are preferentially nucleated from surface irregularities. [9][10][11][12][13][14][15] An additional complexity comes from the silicon cubic diamond structure, which is composed of two sets of ͕111͖ slip planes: the shuffle and the glide ͑Fig. 1͒.…”

Relationship between monolayer stacking faults and twins in nanocrystals

“…This field can nucleate dislocations without an outstanding grain boundary flaw; however, very large stresses would be required to do so. Furthermore, for the nanoscale range of D probed here (up to 100 nm), twins have been found to nucleate from pre-existing defects in grain boundaries [53][54][55][56]. To best represent this scenario and isolate the nucleation event, we chose not to insert a priori a particular twinning, partial or extended full dislocation.…”