Strain effects on the stability and structure of vacancy clusters in Si: A first-principles study

Using first-principles density-functional theory calculations, we investigate the influence of both biaxial and uniaxial strain (-4%<=epsilon <= 4%) on the stability and structure of small, neutral vacancy clusters (V-n, n <= 12) on Si (100). A thorough understanding of vacancy clusters under strain is an important step toward elucidation of the evolutionary life cycle of native defects, especially during semiconductor manufacturing. Fourfold-coordinated (FC) structures are more favorable than "partial hexagonal ring" (PHR) structures in the size regime of our study under strain-free conditions; however, FC structures are also more rigid and consequently more sensitive to strain. Our calculation results indicate that PHR structures can be thermodynamically more favorable than FC structures in the presence of specific strain conditions. In addition, we identify orientation effects in which the cluster symmetry and its alignment within the strain field dictate cluster stability; in consequence, both configuration and orientation are essential factors in the identification of minimum-energy vacancy structures in strained Si. Furthermore, highlights of our simulation results suggest that minimum-energy cluster configurations formed under strain are often different than minimum-energy cluster configurations formed in the absence of strain.