Observations of vacancy clusters formed in Czochralski (Cz) Si after high energy ion implantation are reported. Vacancy clusters were created by 2 MeV Si ion implantation of 1 X 10(15) ionS/cm(2) and after annealing between 600 and 650 degrees C. Doppler broadening measurements using a slow positron beam have been performed on the self-implanted Si samples, both as-implanted and after annealing between 200 and 700 degrees C for time intervals ranging from 15 to 120 min. No change in the S parameter was noted after the thermal treatment up to 500 degrees C. However, the divacancies (V-2) created as a consequence of the implantation were found to start agglomerating at 600 degrees C, forming vacancy clusters in two distinct layers below the surface; the first layer is up to 0.5 mu m and the second layer is up to 2 mu m. The S-W plots of the data suggest that clusters of the size of hexavacancies (V-6) Could be formed in both layers after an annealing for up to an hour at 600 T or half an hour at 650 T. After annealing for longer times, it is expected that vacancies are a mixture Of V-6 and V-2 with V-6 most probably dominating in the first layer. Further annealing for longer times or higher temperatures breaks up the vacancy clusters or anneals them away. (c) 2005 Elsevier B.V. All rights reserved.