Role of surface nanovoids on interstitial trapping in He implanted crystalline Si

The effect of interstitial trapping by surface nanovoids, induced by He ion implantation in crystalline Si, is described. The difference with respect to the effects induced by the deep void layer is evidenced and discussed. Interstitial trapping is investigated by studying the diffusion and the electrical activation of shallow boron implanted in Si. B and He ion implantations were performed on Czochralski bulk and silicon-on-insulator samples in order to isolate the void surface region from the deep void layer. A remarkable reduction of B diffusion is recorded near half the projected range (R-p) of He implantation, which leads to a boxlike shape in the distribution of mobile and electrically active B. Surface nanovoids (2-3 nm in size) cause the observed B diffusivity reduction through an enhanced recombination of self-interstitials. Moreover, these surface nanovoids do not hinder the B electrical activation, being the electrically active B dose comparable for the He implanted and the He-free samples. This peculiar effect of He coimplantation at about half the R-p could be used as an efficient interstitial trapping in defect engineering during Si based device processing.