Nanovoid Formation and Dynamics in He⁺-Implanted Nanocrystalline Silicon

Abstract

Helium implantation in single crystal silicon is known to lead, after a proper thermal treatment, to the formation of voids with diameters ranging between 10 nm and 30 nm. Formation of voids is governed by the coalescence of vacancies created by implantation, initially trapping helium atoms. At high temperatures (${\ge}700^{\circ }\hbox {C}$), helium leaves the nanobubbles and outdiffuses, while the now empty voids grow in size and eventually change their shape to form tetrakaidecahedra (Wulff construction). In this communication, we report how He⁺ implantation in heavily boron-doped nanocrystalline silicon shows a completely different dynamics. Annealing at $500^{\circ }\hbox {C}$ leads to the formation of large voids, located around grain boundaries, along with a large number of nanovoids with an average diameter of 2–4 nm and an estimated density of $3\times 10^{17}\,\hbox {cm}^{-3}$ distributed throughout the grains. Annealing at higher temperature (up to $1000^{\circ }\hbox {C}$) also induces a decrease of the void size with a change in their density, finally accounting to $2\times 10^{18}\,\hbox {cm}^{-3}$. The high temperature annealing also causes vacancy evaporation down to a depth of 80–100 nm from the outer surface. The possibility of obtaining a stable, uniform distribution of nanometer-sized voids is of major relevance as a novel tool for phonon and electron engineering in thermoelectric materials.