High purity FZ-Si crystals with a very low secondary grown-in defects density (less than 10(2) cm(-2)) were grown under critical value V/G approximate to 2 x 10(-5) cm(2)/s K where V is the crystal growth rate in cm/s and G the axial temperature gradient in K/cm at the crystal-melt interface. Because of the coexistence self-interstitials and vacancies in this case, the structure of grown-in defects seems to be far from understood. To elucidate the point defect reaction at such a point we propose to use the data of in situ electron irradiation in a high resolution electron microscope (HREM) at room temperature which allow us to study a 'frozen' clustering of both types of point defects depending on the type of local point defect supersaturation in the thin FZ-Si samples covered with Si3N4 films. Here, we show that new types of extended defects are created upon an isolated and combined clustering of vacancies and self-interstitial atoms on {1 1 3} and {1 1 1} habit planes. The existence of an energy barrier against recombination of self-interstitials with the extended aggregates of vacancies results in the formation of unusual extended defects having a displacement vector close to zero. It is proposed that the combined clustering of point defects on {1 1 3} is able to provide the recombination of point defects at a higher temperature. However, the small energy of {1 1 1} defect and its highly ordered structure suggest the possibility of such a defect formation during vacancy-rich crystal growth.