Self-purification in Si nanocrystals: An energetics study

We examine the energetics of dopants in Si nanocrystals to understand the phenomena of ``self-purification,'' i.e., a process by which extrinsic defects in the interior of a nanocrystal are expelled to the surface. Specifically, we calculate the changes in the total energy of a dopant atom in a Si nanocrystal with respect to position. We consider typical dopant atoms such as P, B, and Li. We find these dopants exhibit different variations in total energies as they move from the center toward the surface of a nanocrystal. These differences can be explained by the change in electronic binding energy and the interaction of the dopant with the surface, i.e., the interaction of a dopant-induced strain with the nanocrystal surface energy.