Symmetric versus asymmetric collisions in ion-induced Auger emission from silicon

We investigate the production of Si 2p Auger electrons during bombardment of Si surfaces by 1.0-5.1 keV Ar+ ions. Yields of Auger electrons emitted at ≅ 146deg with respect to the ion beam are measured as a function of ion energy. We observe a threshold of ≅ 3.8 keV for formation of a double L vacancy in Si, which is due to Ar-Si collisions. This value is higher than those of previous studies, which were presumably influenced by contamination of the Ar+ beam with multiply charged ions, as follows from our measurements of the variation of the Auger yields with the energy of the electrons in the ion source. We conclude that Auger emission in our energy range is dominated by symmetric collisions involving a fast Si recoil, with excitation in asymmetric, Ar-Si collisions growing in importance above the ≅ 3.8 keV threshold. The conclusion is also supported by the appearance of additional, Doppler shifted. Auger peaks at high projectile energies. Our threshold value for Ar-Si collisions is in good agreement with previous studies of gas-phase collisions, and should be used to reevaluate recent computer simulations which fail to predict the observations. (orig.)