A Tight-Binding Molecular Dynamics Simulation of the Melting and Solidification of Silicon
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A TIGHT-BINDING MOLECULAR DYNAMICS SIMULATION OF THE MELTING AND SOLIDIFICATION OF SILICON ANDREW HORSFIELD and PAULETTE CLANCY * *School of Chemical Engineering, Olin Hall,Cornell University, Ithaca,NY 14853
ABSTRACT The melting of crystalline silicon and the cooling of liquid silicon are investigated using Molecular Dynamics. Both the Stillinger-Weber (SW) potential and the Tight-Binding Bond Model are used to calculate the forces. The electrical properties are investigated using an empirical pseudopotential method with a plane wave basis. The melting point of the solid is found to be about 2300K. The dependency of this temperature with cell size is investigated. On cooling, there are changes in some of the properties of the liquid: the energy per particle decreases, tile diffusion constant decreases, and the low frequency electrical conductivity decreases slightly as the temperature decreases. Between 1180K and 980K the liquid undergoes a transition to a glassy phase. There are large changes in the pair correlation function, the SW three-body energy distribution, the diffusion constant, the density of electron single particle states and the electrical conductivity. All of these changes are consistent with increased tetrahedral bonding.
INTRODUCTION There are a number of reasons for investigating the properties of liquid silicon. First, there is a technological motivation due to the extensive use of silicon in the electronics industry. At various points during the manufacturing process the silicon can be melted (such as for purification). Knowledge of the structural properties is relevant in this case. Second, there is a purely scientific motivation: liquid silicon is a metal, thus it is a system that allows the electronic properties of liquid metals to be further investigated. Third, there is a motivation from simulation methodology. There is currently a great deal of interest in improving the methods used for performing computer simulations, particularly for ab initio methods. Silicon has been well studied already by a variety of methods, and thus provides a natural testing ground for new methods. The questions to be addressed here fall into two categories: structural and electronic. In the structural category the questions are as follows. First, at what temperatures do the meltilg and solidification transitions occur, and what is the nature of these phase transitions; second, in the liquid phase, what is the nature of the local atomic coordination, and how does this vary with temperature; third, how does the diffusion constant vary with temperature? With regard to the electronic properties, the questions are as follows. First, how does the density of states of the electrons vary with temperature; second, how does the frequency-dependent electrical conductivity vary with temperature?
METHODS USED Atomic structure Molecular Dynamics is used in this work to explore the phase space needed to evaliaate tie. averages of quantities such as the pair correlation function and the diffusion constant. Bothi the Stillin
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