We propose a viable route for interstitial-mediated formation of arsenic-vacancy clusters that are primarily responsible for arsenic deactivation in the fabrication of ultrashallow junctions in Si, based on first-principles density functional calculations for the stability of arsenic-defect clusters. We present the atomic structures and binding energies of newly identified neutral arsenic-interstitial complexes (AsmIn, m <= 6 and n <= 3), with comparison to the energetics of arsenic-vacancy complexes (AsmVn, m <= 6 and n <= 2). Based on these results, we discuss the relative role of interstitials and vacancies in arsenic deactivation during ultrashallow junction formation. (c) 2006 The Electrochemical Society.