Influence of oxygen desorption on in situ analysis of the surface composition during O2+ bombardment of Si

The use of an internal O18 implant to quantify the surface oxygen concentration, presented in Franzreb et al. [K. Franzreb, J. Lorincik, P. Williams, Surf. Sci. 573 (2) (2004) 291], is critically evaluated, in particular when applied for bombardment conditions leading to high depth resolution measurements. It is shown that this methodology gives no direct access to the surface composition, but only to the O/Si ratio in the total emission flux, ϕO16/ϕSi, which reflects the total amount of oxygen that passes through the surface. When applied under conditions relevant for high depth resolution studies (oxygen bombardment at low energies and small angles), extremely large ϕO16/ϕSi -ratios are observed (>50). These large values are interpreted in terms of the oxygen that leaves the surface by sputtering as well as by alternative processes such as by desorption. The latter is included in the total flux ratio ϕO16/ϕSi, but has little interaction with the sample surface and the departing Si-atoms. Therefore the use of these ϕO16/ϕSi ratios in ionization mechanism studies becomes inappropriate unless the desorbed oxygen fraction ϕOdesorb can be calculated quantitatively or is negligible. Based on the angular dependence of the useful ion yield of oxygen, we make a rough estimate of the desorbed oxygen fraction ϕOdesorb and show that this becomes a dominant fraction (up to 70%) in the emitted flux at low bombardment energies.