Mechanism for the molecular effect in Si bombarded with clusters of light atoms

Abstract

We study the molecular effect (ME) in damage buildup in Si bombarded at room temperature with 40 keV/atom N{sub 1}{sup +} and N{sub 2}{sup +} ions. Structural disorder is monitored by a combination of Rutherford backscattering and channeling spectrometry and a method based on electron backscatter diffraction. In particular, the dependence of the ME efficiency on ion beam flux (varied in the range of 1.5x10{sup 11}-6.2x10{sup 12} atoms cm{sup -2} s{sup -1}) is studied. Results show that the ME efficiency decreases with increasing ion flux. For the highest beam flux used (6.2x10{sup 12} atoms cm{sup -2} s{sup -1}), the ME is absent. This finding indicates that the conventional mechanism based on the overlap of collision subcascades with the formation of local energy spikes is not valid for the case of light-ion bombardment of Si. We develop a model that takes into account the dependence of the defect clustering efficiency on the effective density of collision cascades. We demonstrate that our model can at least qualitatively describe experimental data.