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- (V₄), hexa- (V₆), and dodeca-vacancy (V₁₂), 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 ({κ}) rapidly drops with vacancy concentration (nᵥ) with an inverse power-law relation (κ∝nᵥ^-α, with α {≈} 0.7--1.1 depending on cluster size); the presence of 1.5% vacancies leads to a 95% reduction in {κ} as compared to the defect free c-Si. When nᵥ is low (1%), the reduction of {κ} with nᵥ appears to be a function of cluster size, and the size effect becomes unimportant as nᵥ 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.