We present the results of atomistic and ab initio simulation of several different tilt grain boundaries (GB) in silicon. The boundary structures obtained with genetic algorithm turned out to have no coordination defects, i.e. all silicon atoms restored their tetrahedral coordination during the structure optimisation. That concerns previously known symmetric Σ5 (130), Σ3 (211) and Σ29 (520) boundaries as well as previously unknown asymmetric Σ9 (255)/(211), Σ3 (255)/(211) and Σ13 (790)/(3 11 0) structures. We have performed an extensive study of defect segregation on the boundaries, including neutral vacancy and carbon, phosphorus and boron impurities. A clear correlation between the segregation energy of the defect and local geometry of the boundary site where the defect is segregated has been revealed. We suggest a simple purely geometric model for evaluation of approximate segregation energies of the listed defects.

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