Elastic recoil detection (ERD), Rutherford backscattering/channeling spectrometry (RBS/C) and transmission electron microscopy (TEM) techniques are applied to study the retention of ion implanted He atoms in Cz (1 0 0) Si wafers. The implantations were performed at room temperature using energies from 5 to 60 keV and fluences within 1 to 4 × 1016 cm−2. The retained fraction of He decreases from 100% at 60 keV to ≈5% at 15 keV. The retained He fraction is independent of the implanted fluence, of the accumulated lattice damage, and/or of the presence of TEM observable bubbles. Pure thermal diffusion cannot explain the present results which are applied to improve the strain relaxation of pseudomorphic SiGe/Si (1 0 0) ultra thin buffer layers with low threading dislocation densities.