First-principles study of the origin of retarded diffusion of boron in silicon in the presence of germanium

Dopant diffusion plays an important role in the formation of ultrashallow junctions. To avoid unwanted transient enhanced diffusion of dopants, especially boron, other species (e.g., Ge, C, N, and F) are introduced into the Si substrate. Here, ab initio calculations have been carried out using a density functional theory code, DFT++, to investigate the origin of the experimentally observed retardation of boron diffusion in SiGe alloys. The formation energies of individual Si, B, and Ge point defects, and Si-B, Ge-B, and Si-Ge pair defects were calculated. Based on these calculations, the energetics of Si:Ge:B systems suggest that one mechanism to retard boron diffusion involves the effect of Ge atoms to decrease the number of available Si self-interstitials by increasing the migration energy and, concomitantly, to increase the migration energy of boron atoms. Our results also show that it is difficult for a Ge interstitial to approach a substitutional boron atom compared with a Si self-interstitial.