We have investigated the effect of preannealing in the 400-800° C range and carbon atoms on formation of oxygen aggregates and new donors upon prolonged annealing at 650° C. Suppression of both oxygen aggregation and new donor formation occurs even in high-carbon-content silicon, which was preannealed at 700° C for a long period. Enhancement occurs in silicon preannealed at 500° C irrespective of carbon concentration. The suppression and the enhancement are caused by absence and presence of agglomeration of silicon self-interstitials (Si)is, respectively. Absence of agglomeration of (Si)is is due to less excess of (Si)is emitted from a small number of large oxygen aggregates formed at 700° C. Accordingly, the emission of (Si)is and oxygen aggregation are so suppressed as not to increase the energy due to the (Si)i formation. Hence, the large oxygen aggregates are transformed to platelike oxygen precipitates during prolonged annealing at 650° C to decrease the strain energy. Agglomeration of (Si)is is due to greater excess of (Si)is emitted from a number of small oxygen aggregates formed at 500° C. Further emission of (Si)is can proceed during annealing at 650° C; thus, oxygen aggregation is enhanced, and agglomerates of (Si)is develop into rodlike defects and dislocation dipoles.