Basic mechanisms of structural relaxation and diffusion in amorphous silicon
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Basic mechanisms of structural relaxation and diffusion in amorphous silicon G. T. Barkema1 , Normand Mousseau2† , R.L.C. Vink3 and Parthapratim Biswas4 (1) Theoretical Physics, Utrecht University, Utrecht, The Netherlands (2) Department of Physics and Astronomy and CMSS, Ohio University, Athens, OH 45701, USA (3) Instituut Fysische Informatica, Utrecht University, Utrecht, The Netherlands (4) Debye Institute, Utrecht University, Utrecht, The Netherlands ABSTRACT The low-temperature dynamics in amorphous silicon occurs through a sequence of discrete, activated events that reorganize the topology of the network. In this review, we present some recent work done to understand better the nature of these events and the associated dynamics in a-Si. Using the activation-relaxation technique (ART), we generated more than 8000 events in a 1000-atom model of a-Si, providing an extensive database of relaxation and diffusion mechanisms. The generic properties of these events, such as the number of involved atoms and the activation energies, were investigated and found to be in agreement with experimental data. As it turns out, the bond-transposition mechanism proposed by Wooten, Winer and Weaire (WWW) some time ago plays an important role in the events generated by ART. We have therefore turned to an optimized version of the WWW algorithm to generate the best overall configurations of a-Si available today. We discuss the details of the optimization and present the structural and electronic properties of the resulting models. INTRODUCTION At temperatures well below melting, the microscopic evolution of materials occurs through a sequence of activated processes, in which the energy barriers crossed are high compared to the average thermal energy. The high degree of symmetry characterizing crystalline materials implies that only a small set of atomistic mechanisms are responsible for the activated dynamics. The situation is completely different in disordered materials: as a consequence of the lack of symmetry, the nature of the activated mechanisms varies as a function of the local environment. The standard approach to study these microscopic mechanisms, introduced by Weber and Stillinger [1], is to simulate a material at a high temperature and to quench configurations at regular intervals. At a sufficiently high temperature, and sufficiently long time intervals, the quenches will result in distinct configurations, and their sequence may provide a basis for the reconstruction of activated dynamics. Difficulties arise, however, since (1) it is unclear whether the reconstructed path is the one that the configuration would follow if simulated at lower temperature and (2) the prefactor of the diffusion constant often favors different mechanisms at high and low temperatures [2]. A different avenue to explore microscopic mechanisms is provided by the activation-relaxation technique [3, 4, 5]. In the first part of this manuscript we review the results obtained with this method for the study of amorphous silicon (a-Si). Applying the activ
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