Atomistic simulation study of misfit strain relaxation mechanisms in heteroepitaxial islands
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Atomistic simulation study of misfit strain relaxation mechanisms in heteroepitaxial islands Avinash M. Dongare and Leonid V. Zhigilei Department of Materials Science and Engineering, University of Virginia 116 Engineer’s Way, Charlottesville, VA 22904-4745 ABSTRACT The mechanisms of the misfit strain relaxation in heteroepitaxial islands are investigated in twodimensional molecular dynamics simulations. Stress distributions are analyzed for coherent and dislocated islands. Thermally-activated nucleation of misfit dislocations upon annealing at an elevated temperature and their motion from the edges of the islands towards the positions corresponding to the maximum strain relief is observed and related to the corresponding decrease of the total strain energy of the system. Differences between the predictions of the energy balance and force balance criteria for the appearance of misfit dislocations is discussed. Simulations of an island located at different distances form the edge of a mesa indicate that the energy of the system decreases sharply as the island position shifts toward the edge. These results suggest that there may be a region near the edge of a mesa where nucleation and growth of ordered arrays of islands is favored. INTRODUCTION The effect of the quantum confinement of charge carriers within zero-dimensional “quantum dots” has a promise of a remarkable advancement of electronic and optoelectronic devices. Realization of this promise hinges on the development of efficient methods for generation of ordered arrays of small, tens of nm in size, clusters or islands. One of the methods of quantum dot generation is lattice mismatched heteroepitaxial growth [1,2]. The electronic structure of a coherently strained island formed by heteroepitaxial growth is sensitive to the stress distribution in and around the island. Moreover, when the island size exceeds a certain critical size, the high misfit strain in the growing island can be released through the nucleation of misfit dislocations, which can also affect the electronic properties. Understanding the stress distributions and the mechanisms of stress relaxation are, therefore, important for the advancement of nanoscale electronic devices. While there have been significant theoretical and computational efforts aimed at analysis of stress/strain distributions in and around coherently strained heteroepitaxial islands, e.g. [3,4,5,6], the relaxation of the lattice mismatch stresses related to the misfit dislocations is much less studied. It is difficult to include the description of the processes of dislocation nucleation and migration into continuum models, whereas the scale of the atomistic simulations is not sufficient to study these processes. Experimentally, the transition from a coherently growing strained island to the one in which misfit dislocations partially relieve the strain energy has been observed via high resolution electron microscopy [7,8]. It has been found that mismatch dislocations typically nucleate at the corners of the island, the
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