Formation of Sim+ and SimCn+ clusters by C60+ sputtering of Si

The secondary ion mass spectrum of silicon sputtered by high energy C60+ ions in sputter equilibrium is found to be dominated by Si clusters and we report the relative yields of Sim+ (1 ≤ m ≤ 15) and various SimCn+ clusters (1 ≤ m ≤ 11 for n = 1; 1 ≤ m ≤ 6 for n = 2; 1 ≤ m ≤ 4 for n = 3). The yields of Sim+ clusters up to Si7+ are significant (between 0.1 and 0.6 of the Si+ yield) with even numbered clusters Si4+ and Si6+ having the highest probability of formation. The abundances of cluster ions between Si8+ and Si11+ are still significant (>1% relative to Si+) but drop by a factor of ∼100 between Si11+ and Si13+. The probability of formation of clusters Si13+–Si15+ is approximately constant at ∼5 × 10−4 relative to Si+ and rising a little for Si15+, but clusters beyond Si15 are not detected (Sim≥16+/Si+ < 1 × 10−4). The probability of formation of Sim+ and SimCn+ clusters depends only very weakly on the C60+ primary ion energy between 13.5 keV and 37.5 keV. The behaviour of Sim+ and SimCn+ cluster ions was also investigated for impacts onto a fresh Si surface to study the effects that saturation of the surface with C60+ in reaching sputter equilibrium may have had on the measured abundances. By comparison, there are very minor amounts of pure Sim+ clusters produced during C60+ sputtering of silica (SiO2) and various silicate minerals. The abundances for clusters heavier than Si2+ are very small compared to the case where Si is the target. The data reported here suggest that Sim+ and SimCn+ cluster abundances may be consistent in a qualitative way with theoretical modelling by others which predicts each carbon atom to bind with 3–4 Si atoms in the sample. This experimental data may now be used to improve theoretical modelling.