Studies on the surface swelling of ion-irradiated silicon: Role of defects

Low-energy (keV) Si ions are implanted on mask-pattered Si wafer at doses above amorphization threshold for a variety of implantation conditions (ion dose, energy, temperature) and post-implantation annealing. Precise measurement of the surface swelling (step-height) on the amorphous Si layer was carried out using an atomic force microscope (AFM). Thickness and microstructure of the ion-damaged layer was evaluated by transmission electron microscopy (TEM) measurements. The measured step-height in excess to that contributed by implanted ions shows a cube root dependence on the Si ion dose. The swelling heights studied for two different energies of Si ions show identical step-heights. Implantation at low temperature (77 K) shows a reduced step-height, indicating possible contribution of migrated Si atoms in the swelling. We propose that excess swelling is caused by migration and segregation of the displaced Si atoms from the bulk to the surface leaving behind corresponding vacancies in the lattice. Ellipsometric studies of the similarly damaged layer profile provide supporting evidence for surface segregation of Si atoms over the amorphous layer.