Three-dimensional atom probe tomography was used to characterize the segregation of B dopant atoms to grain boundaries in polycrystalline Si after flash-assisted rapid thermal annealing. Tomographic reconstructions allowed direct measurement of segregation coefficients, which were found to be greater at lower flash temperatures with thermal budgets that limit grain growth. Hall measurements confirmed the deactivation of B at the grain boundaries, while secondary ion mass spectrometry was used to measure B diffusion in the film. Experimental parameters were then simulated in a diffusion model which accurately predicts the diffusion of B in polycrystalline Si at flash temperatures of 1150° C, 1250 °C, and 1350 °C, as well as with conventional rapid thermal annealing. © 2012 American Institute of Physics.