Molecular Dynamics Simulations of Low Energy Displacement Cascades in Silicon
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MOLECULAR DYNAMICS SIMULATIONS OF LOW ENERGY DISPLACEMENT CASCADES IN SILICON. A.M.MAZZONE CNR - Istituto
LAMEL,
Via de'Castagnoli 1 - I 40126 Bologna.
ABSTRACT Displacement cascades formed in a silicon crystal by As+ and Si+ ions with energy in the range 50-500 eV are constructed with a molecular dynamics and a Monte Carlo simulation method. Conspicuous spike effects are observed with the molecular dynamics simulation. INTRODUCTION It is known that the evolution of a displacement cascade proceeds through a collisional phase, dominated by two-body collisions and a relaxation phase, during which the deposited energy is radiated by the excited phonons outside the volume of the damaged region. In a silicon target many-body interactions (spike effects) occurring on the wake of projectiles forming dense cascades are thought to be responsible for an anomalous disordering well above the one produced during the collisional phase [1]. However the form and the extent of such interactions are uncertain and the subject hotly contended. In this work a molecular dynamics simulation method is used to illustrate the excitation of the silicon lattice atoms resulting from the passage of a silicon and an arsenic ion with energy in the range 50-500 eV. The comparison with the cascade structure obtained with a Monte Carlo simulation method, which is the standard way of treating damage events, indicates the formation of a more diffused disorder sustained by the electrostating coupling between the lattice atoms. MOLECULAR DYNAMICS AND MONTE CARLO SIMULATION METHODS The simulation method is the one described in (2]. In such calculations, after a cycle of lattice equilibration, an ion, endowed with the wanted energy and direction of motion, is ejected from a lattice location. The evolution of the atom assembly is then determined by integrating the classical equations of motion. Owing to the primary energies considered in this work the cascade is formed by atoms with very different properties. The most energetic recoils have velocities well above the thermal range and are unlikely to share the structural properties of the silicon target. Their energy losses are determined by hard collisions at distances shorter than the normal solid state separation. For such collisions screened coulomb potentials are generally used in Monte Carlo and molecular dynamics simulations [3]. Cascade atoms with energy in the range 1 eV or below are likely to have the characteristics of excited phonons and maintain the structural properties of the silicon target. There has been considerable recent interest in an atomistic simulation of the structural properties of tetragoMat. Res. Soc. Symp. Proc. Vol. 128. ' 1989 Materials Research Society
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nal semiconductors and classical three-body potentials have been developed which can stabilize the silicon lattice. Generally these potentials have the disadvantage of a greater computational complexity with respect to a simple central potential. A computational advantageous interatomic potential which can also stabil
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