Angular dependence of silicon oxide formation and gold segregation due to low-energy O2+ implantation

High-resolution Rutherford backscattering spectrometry was used to investigate the effect of the angle of incidence on the composition of silicon bombarded with 4 keV O2+ ions. The redistribution of gold impurities, initially present as a thin surface layer, was studied simultaneously. Complete oxidation, i.e. the formation of SiO2, could be achieved at impact angles between 0° and 30° to the surface normal. Assuming a density equivalent to that of thermally grown oxides, the thicknesses of the oxides generated by implantation were found to range from 13 nm (0°) to 10 nm (30°). At impact angles exceeding 30° the near-surface region of silicon was only incompletely oxidized to SiOx, with x < 2. Comparison with the angular dependence of the silicon sputtering yield Y suggests that complete oxidation is achieved at angles where Y < 0.5 Si atoms per O atom. At angles above 60° the oxidation state appeared to be enhanced owing to the incorporation of oxygen from the residual gas, the effect being noticeable because of moderate working pressures and relatively low mean current densities. This supposition was substantiated by studying oxygen incorporation during bombardment with 4 keV Xe+ ions. The gold impurities were found to segregate on the large-depth side of the SiO2 layer generated by oxygen implantation. O2+ bombardment of a fully oxidized layer at 45° showed that the internal profile of the segregated gold atoms largely retains its shape while being shifted towards the sputter-eroded surface.