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.