Doppler broadening spectrosopy (DBS) coupled to a slow positron beam has been used to investigate the formation of Hecavities in the presence of high vacancy concentrations in Cz-Si (111). Si samples were first implanted with MeV Si ions in order to create a damaged Si layer. DBS measurements show the presence of divacancy (S nu(2)/S-Silattice 1.052,W nu(2)/W-Silattice 0.83) from the surface up to 4.2 mu m depth with a concentration higher than 10(18) cm(-3). The thickness of this damaged layer was confirmed by spreading resistance measurements. In the second step, samples were implanted with 50 keV He-3 with fluence of 10(16) cm(-2). DBS results show that the apparent divancancy concentration decreases at He-3 implantation depth similar to 435 nm due to He-3 passivation of vacancies that occurs during the implantation process. After 900 degrees C annealing, large defects are detected at depth up to 2 mu m and (S, W) values suggest the detection of cavities at the implantation depth. We also report the possible presence of impurity complexes. The formation of these complexes is attributed to the gettering of metallic impurities present in the Si sample. (c) 2005 Elsevier B.V. All rights reserved.