The formation and growth of nanovoids in a C-doped Si layer after He+ implantation and thermal annealing are reported. A structure consisting of 240 nm of Si, 20 nm of Si doped with C at 0.8 at. %, and 240 nm of Si cap was realized by molecular beam epitaxy onto a (100) Si Czochralsky substrate. Three sets of samples were implanted with He+ at 30 keV and different doses of 8×1015, 3×1016, and 5×1016 cm−2 and subsequently annealed at 800 °C in N2 atmosphere. Cross-section transmission electron microscopy was used to determine the void size and location. The tensile strain of the C-doped layer was measured by high-resolution x-ray diffraction. Our studies report the double role of C in the formation and evolution of nanovoids. After the low dose implantation, the C-doped layer still shows tensile strain due to substitutional C, and voids are localized only within this layer. At higher implantation doses …