Effect of an increase in the density of collision cascades on the efficiency of the generation of primary displacements during the ion bombardment of Si

The depth distributions of structural damage induced in Si at room temperature by the implantation of P and PF$_{4}$ with energies from 0.6 to 3.2 keV/amu are experimentally studied in a wide range of doses. It is found that, in all cases, the implantation of molecular PF$_{4}$ ions forms practically single-mode defect distributions, with maximum at the target surface. This effect is caused by an increase in the generation of primary defects at the surface of the target. Individual cascades formed by atoms comprising molecule effectively overlap in the surface vicinity; this overlap gives rise to nonlinear processes in combined cascades due to a high density of displacements in such cascades. Quantitative estimation of increase of effectiveness of point defect generation by PF$_{4}$ ions in respect to P ions is done on the base of experimental data.