First principles modeling of arsenic and fluorine behavior in crystalline silicon during ultrashallow junction formation
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The 2005 International Technology Roadmap for Semiconductors predicts ultrashallow junctions (USJs) less than 7 nm deep with unprecedented dopant activation levels will be required for silicon transistors to be manufactured in 2010. To meet these requirements, it is necessary to have a better understanding of the dopant transient enhanced diffusion (TED) and clustering behaviors that undermine the achievement of these manufacturing specifications. Arsenic (As) is a commonly used n-type dopant in USJ formation and fluorine (F) is an impurity commonly co-implanted with dopants to reduce dopant diffusion and clustering during USJ formation. In this dissertation, density functional theory within the generalized gradient approximation is used to understand the behavior of As and F in crystalline silicon during USJ formation. In the first part of this dissertation, the influence of silicon interstitials on As behavior during thermal annealing that follows dopant implantation is investigated. As a result of dopant implantation, a net excess of silicon interstitial defects exist in the silicon. First, it is shown that silicon interstitials can easily annihilate existing Asvacancy complexes in silicon with negligible recombination energy barriers. Second, experimentally observed As TED mediated by interstitials is explained by the formation of a highly mobile As-silicon interstitial pair that can exist in positive, neutral, and negative charge states. Finally, it is shown that large As-silicon interstitial complexes may form when excess interstitials are present and provide a kinetic route to As clustering that leads to As deactivation. In the second part of this dissertation, the interaction of F impurities with silicon interstitials and B dopants is investigated. First, the formation and diffusion of a highly mobile fluorine-silicon interstitial pair which has been suggested by experiment is detailed. Second, an immobile B-Sii-F structure is identified in which B has a deactivated configuration. This structure may play a role in deactivating and immobilizing B when implanted B and F profiles coincide. This research provides fundamental insight into the behavior of As dopants and F impurities during USJ formation. As the future of silicon-based devices relies on the ability to perform precise doping, these findings should be of great importance to device manufacturers.