Mechanism of swelling in low-energy ion-irradiated silicon

The phenomenon of swelling (surface expansion) in low-energy self-ion implanted silicon has been investigated using atomic force microscopy and transmission electron microscopy, for a wide range of fluence and postimplant annealing conditions. The swelling height in excess to that contributed by implanted ions shows approximately a cube root dependence on the Si1-ion fluence. Postimplantation annealing exhibits a marked reduction in the swelling at 650 °C. Both the fluence dependence and the annealing characteristics of the excess swelling suggest the involvement of vacancy clusters in the amorphous layer. We propose that the excess swelling in low-energy implanted Si results from the migration and segregation of the displaced Si atoms from the bulk to the surface leaving behind corresponding vacancies in the lattice. We assume that during irradiation, the interstitials are mobile even in the damaged layer. From the measured swelling, we estimate a density reduction of about 3.1% for the amorphous phase with respect to the crystalline phase.