Effects of vacancy defects on thermal conductivity in crystalline silicon: A nonequilibrium molecular dynamics study

We examine the effects of vacancy defects on thermal conductivity in bulk crystalline silicon (c-Si) using nonequilibrium molecular dynamics simulations. While most vacancies are thought to remain in the form of clusters in bulk c-Si, recent theoretical studies have predicted that small vacancy clusters energetically prefer to be fourfold coordinated by nullifying dangling bonds. Hence, in this work, we consider three different-sized fourfold vacancy clusters, tetra-(V-4), hexa-(V-6), and dodeca-vacancy (V-12), with particular interest in studying how phonon transport is affected by vacancy concentration and cluster size in association with fourfold coordination-induced lattice distortions. Our simulations show that thermal conductivity (kappa) rapidly drops with vacancy concentration (n(v)) with an inverse power-law relation (kappa proportional to n(v)(-alpha), with alpha approximate to 0.7-1.1 depending on cluster size); the presence of 1.5% vacancies leads to a 95% reduction in kappa as compared to the defect free c-Si. When n(v) is low (< 1%), the reduction of kappa with n(v) appears to be a function of cluster size, and the size effect becomes unimportant as n(v) increases above 1%. We discuss the correlation between phone scattering and cluster size, based on the relative rates of phonon-vacancy scattering associated with defect-induced strain fields. We also estimate the dependence of phonon mean free path on vacancy concentration and cluster size.