Accumulation of structural disorder in Si bombarded at -196C with 0.5MeV 209Bi and 1MeV 209Bi2 ions (the so-called molecular effect) was studied by Rutherford backscattering/channeling spectrometry. Results showed that the damage buildup was sigmoidal even for such heavy-ion bombardment at liquid N temperature. This strongly suggested that, for the implant conditions of this study, the build-up of lattice damage cannot be considered as an accumulation of completely disordered regions. Instead, damage-dose curves were 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 showed that Bi2 ions produce more lattice damage than Bi ions implanted to the same dose. The ratio of lattice disorder produced by Bi2 and Bi ions was 1.7 near the surface, decreased with depth, and finally became close to unity in the bulk defect peak region. Parameters of collision cascades obtained using ballistic calculations were in good agreement with experimental data. The molecular effect was attributed to a spatial overlap of (relatively dense) collision sub-cascades, which gave rise to (i) non-linear 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.

Damage Buildup in Si under Bombardment with MeV Heavy Atomic and Molecular Ions. A.I.Titov, S.O.Kucheyev, V.S.Belyakov, A.Y.Azarov: Journal of Applied Physics, 2001, 90[8], 3867-72