DIFFUSION AND INTERACTIONS OF POINT-DEFECTS IN SILICON - MOLECULAR-DYNAMICS SIMULATIONS

We have simulated the motion and clustering of vacancies and interstitials in silicon using molecular dynamics methods. The diffusion coefficients of isolated defects were calculated from atomic displacements in simulations performed over a wide range of temperatures. The results give an apparent migration energy barrier of E(V)((M)) = 0.43 eV for vacancies and E(I)((M)) = 0.9 eV for interstitials. The diffusion coefficients are between 10(-6) and 10(-5) cm(2)/s at 800 degrees C, and are in approximate agreement with recent first-principles calculations, although they are many orders of magnitude larger than the most direct experimental measurements. Simulations with high concentrations of defects show that, like defects aggregate into stable clusters, and that individual defects are bound to these clusters with energies in the range of 0.6-2.3 eV. Defect clusters have mobilities which can differ substantially from those of the individual defects. The di-interstitial has a dramatically smaller diffusion barrier, E(2I)((M)) approximate to 0.2 eV, whereas the tri-interstitial has a mobility which is so small that it is difficult to measure accurately by molecular dynamics simulations. We discuss some of the implications of these simulations for diffusion under silicon device-processing conditions.