Many crystallographic imperfections termed microdefects form in silicon crystals during their Czochralski growth. These are the aggregates of vacancies, of self-interstitials, or of oxygen (silicon dioxide). The distribution of microdefects can be strongly influenced and controlled by the addition of impurities such as nitrogen to the crystal. A model describing the Czochralski defect dynamics in the presence of nitrogen and oxygen is proposed and solved. The reactions between vacancies and self-interstitials, nitrogen monomers and dimers, nitrogen and vacancies, and the reactions involving vacancies, oxygen, and complexes of vacancies and oxygen are incorporated, along with the formation of various microdefects. All microdefects are approximated as spherical clusters. The formation of all clusters is described by the classical nucleation theory. The clusters, once formed, grow by diffusion-limited kinetics. The microdefect distributions in Czochralski crystals growing under steady state as well as unsteady state are discussed. (C) 2007 Elsevier B.V. All rights reserved.