A New Non-Equilibrium Molecular Dynamics Simulation Method for Rapid Solidification
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A NEW NON-EQUILIBRIUM MOLECULAR DYNAMICS SIMULATION METHOD FOR RAPID SOLIDIFICATION DHANRAJ K. CHOKAPPA AND PAULETTE CLANCY School of Chemical Engineering, Cornell University, Ithaca, NY 14853, U.S.A.
ABSTRACT A new non-equilibrium Molecular Dynamics (NEMD) computer simulation method has been developed to study ultra-rapid melting and resolidification processes, e.g. laser annealing, ion implantation, etc. An atomic-level description of the material is combined with a new simulation technique to produce thermodynamic, structural and kinetic information as a function of time. Experimentally realistic values of the energy fluence, pulse duration and substrate temperature are used as input to the simulation. Rapid heat transfer simulating the action of the energy input is then set up allowing a complete prediction of the undercooling and associated kinetic properties. As such this new method offers the most realistic simulation model for rapid thermal processing to date.
SIMULATION CELL The simulation cell is a regular parallelopiped of approximate dimensions, 13o x 13o x 250 (where o is the Lennard-Jones parameter governing atomic size). The cell contains of the order of 10004000 particles, sufficient that the effects of the periodic boundary conditions in stabilizing the solid are significantly diminished. The cell consists of four distinct regions: above the solid surface are a few vapor particles, the top 20 atomic planes or so consist of a so-called "dynamic solid" region wherein the atoms are free to move under the influences of the constituent interatomic forces imposed by the heating and cooling. Below this are 3-4 layers of the substrate which act as a heat sink, wherein the velocities of the particles are scaled to slowly maintain or restore the system to the pre-set substrate temperature. Below this, at the bottom of the cell are 4-5 atomic planes of a static lattice where particles interact with the system, but are not allowed to move. These latter regions simulate the presence of the bulk substrate. This is the same configuration used by Landman and co-workers [11 in their NEMD studies of a Lennard-Jones system.
THE NON-EQUILIBRIUM MD SIMULATION METHOD The key problem in devising a non-equilibrium simulation method appropriate to this study lies in producing a realistic model for the rapidly varying, large heat gradient which would be produced in the real physical system. In our model, a thermal gradient is produced within the simulation cell by allowing energy transfer agents (ETAs) to distribute a given amount of energy, e.g. an experimentally-known value of the fluence of a laser, for a chosen pulse duration (ps-ns). The ETAs are particles which appear less than one lattice plane width above the original solid/vapor interface, in line with the action of a laser which heats only the solid substrate. The ETAs have no volume and virtually no mass (typically 10-6 that of the substrate atoms). They collide elastically with the top half of the solid (depending on their energy and number), transfer their e
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