Role of C in the formation and kinetics of nanovoids induced by He+ implantation in Si.

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, all the C atoms have been displaced from substitutional sites. No more strain is present in the C-doped layer and the presence of large cavities in its neighborhood is strongly inhibited. This work shows how localized strain in epitaxial films can be effectively used to drive nanovoid formation and evolution. [ABSTRACT FROM AUTHOR]