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C8.19.1

Atomistic Modeling and Simulation of Impurity Atmosphere in Silicon and Edge Dislocation Locking Effects A. Karoui Materials Science and Engineering Dept. North Carolina State University, Raleigh, NC 27695-7916 [email protected], ABSTRACT A theoretical study of edge dislocation locking by impurities in silicon is presented. Three groups of impurities are considered: (i) light atoms O, N, and C, (ii) large atoms Ga, and Ge, and (iii) small dopant atoms B, P, and Al. Based on impurity size effect model, these three groups produce distinct different dislocation locking effects. Atoms from the first group strongly bind with edge dislocations. The O, N, and C atmospheres are similar, with a slightly stronger occupancy probability for O and N in the vicinity of the dislocation core. For the second group, Ge loosely binds to dislocation and resists at most 1/3 of the separation shear stress that the first group can withstand. Germanium has only a small chance to reach the dislocation core. The third impurity group does not resist shear any separation stress from edge dislocations. Moreover, B and P atoms can not be trapped at all by edge dislocations. At a local atomic fraction of 10-4, edge dislocation-impurity binding energy varies from 0.008 eV/Å for P to 1.7 eV/Å for N and 1. 8 eV/Å for O. In addition, using molecular mechanics on system of 34552 atoms the self-energy of an edge dislocation was calculated and found equal to 156 meV/Å. INTRODUCTION It is well known that oxygen in Czochralski silicon increases drastically the material hardness. Subsequent to the recent tendency to reduce oxygen level, nitrogen doping was introduced to compensate the hardness decrease. Meanwhile, this option has been adopted for the originally fragile float zone silicon, used for high efficiency solar cells. The understanding of material hardening,1,2 issues has been improved by the early work of Cottrel and Bilby3 on point defect interactions with dislocation stress field. Subsequently, gettering issues and electrical activities are explored by EBIC and interpreted in connection to Cottrel atmosphere.4 It is believed that for a fully reconstructed clean dislocation the size-effect arising from impurity volume mismatch with the host crystal atoms is the primary factor in the dislocation-impurity interactions. In this paper we investigate the composition of edge dislocation atmosphere and the interaction energy due to the atomic size mismatch. We have varied the impurity type, the atomic fraction within the formed impurity atmosphere, and the temperature. Variations in impurity-dislocation interactions are discussed. MODEL FOR IMPURITY-DISLOCATION STRESS FIELD INTERACTIONS In addition to the self-energy barrier for dislocation creation, the binding energy of foreign atoms (including dopants) to the dislocation must be considered when determining the activation energy for dislocation migration. The major part of this energy barrier comes from the strain compensation by foreign atoms and includes "size interaction" and “shape defo