Damage buildup in Si under bombardment with MeV heavy atomic and molecularions

Accumulation of structural disorder in Si bombarded at -196 degreesC with 0 .5 MeV Bi-209(1) and 1 MeV Bi-209(2) ions (the so-called molecular effect) is studied by Rutherford backscattering/channeling spectrometry. Results sh ow that the damage buildup is sigmodal even for such heavy-ion bombardment at liquid nitrogen temperature. This strongly suggests that, for the implan t conditions of this study, the buildup of lattice damage cannot be conside red as an accumulation of completely disordered regions. Instead, damage-do se curves are well described by a cascade-overlap model modified to take in to account a catastrophic collapse of incompletely disordered regions into an amorphous phase after damage reaches some critical level. Results also s how that Bi-2 ions produce more lattice damage than Bi-1 ions implanted to the same dose. The ratio of lattice disorder produced by Bi-2 and Bi-1 ions is 1.7 near the surface, decreases with depth, and finally becomes close t o unity in the bulk defect peak region. Parameters of collision cascades ob tained using ballistic calculations are in good agreement with experimental data. The molecular effect is attributed to a spatial overlap of (relative ly dense) collision subcascades, which gives rise to (i) nonlinear energy s pike processes and/or (ii) an increase in the defect clustering efficiency with an effective increase in the density of ion-beam-generated defects. (C ) 2001 American Institute of Physics.