Damage buildup in Si under bombardment with MeV heavy atomic and molecular ions

Accumulation of structural disorder in Si bombarded at −196 °C with 0.5 MeV Bi1209 and 1 MeV Bi2209 ions (the so-called molecular effect) is studied by Rutherford backscattering/channeling spectrometry. Results show that the damage buildup is sigmodal even for such heavy-ion bombardment at liquid nitrogen temperature. This strongly suggests that, for the implant conditions of this study, the buildup of lattice damage cannot be considered as an accumulation of completely disordered regions. Instead, damage-dose curves are well described by a cascade-overlap model modified to take into account a catastrophic collapse of incompletely disordered regions into an amorphous phase after damage reaches some critical level. Results also show that Bi2 ions produce more lattice damage than Bi1 ions implanted to the same dose. The ratio of lattice disorder produced by Bi2 and Bi1 ions is 1.7 near the surface, decreases with depth, and finally becomes close to unity in the bulk defect peak region. Parameters of collision cascades obtained using ballistic calculations are in good agreement with experimental data. The molecular effect is attributed to a spatial overlap of (relatively dense) collision subcascades, which gives rise to (i) nonlinear energy spike processes and/or (ii) an increase in the defect clustering efficiency with an effective increase in the density of ion-beam-generated defects.