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× 1015 to 8× 1016 He ions cm2 and energies ranging from 25 to 110 keV. Then, we implanted B ions (12 keV, 5× 1014 ions 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. © 2007 American Institute of Physics.