A comparative study of primary ion energy dependence of secondary ion yields from Si and Ge surfaces under inert and reactive ion bombardment

Primary ion energy variation curves (In+ α E, where E = 3–10 keV; n=1,2) for secondary ions emitted from Si and Ge targets at different target current densities (Jp ≈ 60–400 μA/cm2) have been studied, both under inert (Ar+) and reactive ion (N2+, O2+) bombardment. It is interesting to note that while In+ increases with E under Ar+ bombardment the same parameter under the same bombardment conditions exhibits a steady fall under N2+ or O2+ bombardment, at least for E up to ∼ 5 keV. Not only this, the changes in the slopes of the above curves with Jp in the two cases are also in the opposite direction. In the former case, the increase in the slopes of the curves with Jp can be accounted for by taking into account target current density effects which are now known to occur even for Jp < 1 mA/cm2, due to ionization of sputtered neutrals above the target surface. In the latter case however, the results can only be interpreted in terms of our recent publication which presents some evidence regarding gradual build-up of primary atom concentration at the target surface with increase in target current density — a phenomenon, which is rather unexpected from theoretical considerations.