MeV ion implantation in Si above a dose of 1014/cm2leads to secondary defect formation in a buried layer, which is rather stable. Annealing of Si(100) implanted with 2.0 MeV B+ions to a dose of 2.2×1014/cm2has been investigated by means of cross‐sectional transmission electron microscopy. After annealing at 900 °C for 15 min, dislocation loops elongated along [110] were found. A remarkable decrease in secondary defect formation has been observed if, in addition, 140 keV Si+was implanted prior to annealing.