He induced nanovoids for point-defect engineering in B-implanted crystalline Si

In this paper we present a systematic study on the formation of He ion implantation induced nanovoids in Si and how they influence the self-interstitial $(Is)$ supersaturation, thus affecting the diffusion and electrical activation of implanted boron in crystalline silicon. We implanted He ions into (100)-oriented Si wafers, with doses ranging from $5×1015to8×1016Heions∕cm2$ and energies ranging from $25to110keV$⁠. Then, we implanted B ions (⁠$12keV$⁠, $5×1014ions∕cm2$⁠). All samples were annealed at $800°C$ in $N2$ atmosphere. We demonstrated the role of nanovoids in reducing B diffusion already at the first stages of postimplantation annealing. The effect has been attributed to the $Is$ trapping by the nanovoids that forces B to assume a boxlike profile. Moreover, we studied the nanovoid distribution as a function of He-implanted dose and energy, demonstrating, by means of Cu gettering experiments, the beneficial effect of increasing dose or decreasing energy of He implantation on the B diffusion and electrical activation. In fact, if the nanovoid density is high in the proximity of implanted B, implantation-related damage can annihilate at the internal dangling bonds of nanovoids, thus consuming the nanovoid layer. The potential of He coimplantation as a method for controlling point-defect distributions in crystalline Si is presented and critically discussed.