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Dopant atoms can segregate to SiO 2 /Si͑001͒ interfaces and be deactivated there. Using phosphorus as a typical example of a donor and guided by results of ab initio calculations, we present a model of donor segregation. We find that P is trapped at the interface in the form of threefold-coordinated atoms. The atomic detailed configuration and the process of P incorporation depend on P concentration C P in the vicinity of the interface. At low C P , phosphorus atoms prefer to substitute Si atoms with dangling bonds. At high C P , phosphorus pairs are formed. At intermediate C P , ͑around 10 17-10 19 cm Ϫ3 ͒ segregation occurs to sites associated with interface roughness and to interface Si-Si bridges, and is mediated by diffusion and annihilation of Si dangling bonds and by reoxidation during oxide annealing. Making diffusion of dangling bonds more difficult ͑for example, by nitridation͒ should, therefore, reduce the trapping efficiency of SiO 2 /Si͑001͒ in the technologically important regime of intermediate C P .

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