In this letter, it is shown how the application of a Z-contrast scanning transmission electron microscopy technique to the study of the dopant signal in ultrashallow junctions in Si can lead, in the case of As in Si, to a quantitative determination of the dopant depth profile at subnanometer resolution. The quantitative results thus obtained demonstrate that As accumulates on the Si side of the SiO2∕Si interface with a negligible loss of dopant into the oxide. Modeling of the effect indicates that segregation occurring during the recrystallization of the implanted layer is the dominant cause of this dopant pileup.