Evaluation of possible mechanisms behind P gettering of iron

Several possible mechanisms for phosphorus diffusion gettering (PDG) in silicon (Si) are evaluated. Float Zone (FZ) monocrystalline Si samples were intentionally contaminated with iron by ion implantation followed by a 1 h anneal at 900°C to achieve a homogeneous iron distribution. Phosphorus gettering was then performed at different combinations of time and temperature. Depth-versus-concentration profiles of Fe, P, and O were measured by SIMS. The depth interval of Fe accumulation was found to be independent of the extension of the P profiles, unlike predictions from modeling of Fe x P y complex formation. The capture capability of P does not play an important role in PDG, still decreasing the P concentration below 6% in the surface source cause poor gettering efficiency. Hence accumulation of Fe close to the surface hinges on the presence of P, which is likely to be due to generation of vacancies during in-diffusion of high concentrations of P. The vacancies cause localized precipitation of oxygen in the highly P doped region. These oxygen precipitates act as gettering centers for Fe, as substantiated by a close correlation between the measured depth profiles of O and Fe after PDG. Our results suggest that interactions between oxygen, vacancies and metal impurities are the most crucial factors in PDG.