Molecular Dynamics studies of Dislocations in SI
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MOLECULAR DYNAMICS STUDIES OF DISLOCATIONS IN SI M.S.
DUESBERY,
D.J.
MICHEL* AND B.
JOOS***
*GeoCenters Inc., Fort Washington, MD 20744 **Naval Research Laboratory, Washington, D.C. 20375 ***Ottawa-Carleton Institute for Physics, University of Ottawa Campus, Ottawa, Ontario, Canada KIN 9B4 ABSTRACT The mobility of dislocations in a model Silicon lattice is examined at an atomistic level using molecular dynamics. Straight and double-kinked 300 and 900 partial dislocation glide-set dipoles are modelled in a strain-free environment: reconstruction and antiphase defects are found to be present for 300 partial dislocations. The effects of applied shear strains and of temperatures up to the melting point are considered. INTRODUCTION Plasticity in Silicon is typical of elemental and many compound covalent materials, in that dislocation mobility is highly anisotropic and extremely sensitive to dopants, point defects and impurities [1]. Experimental EPR and DLTS [1] and theoretical [2) evidence suggests that this may be due to strong coupling of the dislocation core to the electronic structure, via deep levels localized either along the dislocation line or at secondary defects, such as kinks [3) or anti-phase defects (APDs) [4]. Therefore it is unlikely that any theoretical treatment which does not allow for electron redistribution will provide a completely successful description of the core. Nevertheless, the computational load implicit in a self-consistent electron-ion calculation is sufficiently large that it is worthwhile to pursue the simpler avenue of pure ionic interactions, if only to establish bounds and identify states which merit further examination. The present paper will be concerned only with mechanical properties, specifically the dislocation mobility, using the valence force field potential developed by Stillinger and Weber [5). Similar work, with emphasis on the static core energy, has been performed by Nandedkar and Narayan [6,7]; Heggie and Jones [8] have considered static double kinks. Other calculations which place more emphasis on the electronic structure are referenced elsewhere [1,2,9]. METHODOLOGY The Stillinger-Weber potential [5) defines the total energy E of a covalent solid by an expression of the form E = E ( Z f 2 ( rj I1 + X Z f 3 ( Irtj 1, rikI,'jik)
)
(1)
in which r-. is the vector joining ions i and j, Ojtk is the angle between the vectors rij, rik and the summations are assumed to run over all enclosed indices. The parameter e normalizes the potential and A specifies the strength of the angular term. Details of the functional forms of f 2 and f 3 , and of the Si parametrization, are contained in (5]. Mat. Res. Soc. Symp. Proc. Vol. 163. ©1990 Materials Research Society
942
An advantage of the form (1) is that the strength of the angular term can be varied simply by changing X. For X = 0, the potential reduces to a central pair form. In this limit the diamond cubic lattice is not stable, of course, but for values of X down to X = 0.i)o, where Xo is the value appropriate for Si [
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