Formation of Si m+ and Si mC n+ clusters by C 60+ sputtering of Si

Abstract

The secondary ion mass spectrum of silicon sputtered by high energy C 60 + ions in sputter equilibrium is found to be dominated by Si clusters and we report the relative yields of Si m + (1 ≤ m ≤ 15) and various Si m C n + clusters (1 ≤ m ≤ 11 for n = 1; 1 ≤ m ≤ 6 for n = 2; 1 ≤ m ≤ 4 for n = 3). The yields of Si m + clusters up to Si 7 + are significant (between 0.1 and 0.6 of the Si + yield) with even numbered clusters Si 4 + and Si 6 + having the highest probability of formation. The abundances of cluster ions between Si 8 + and Si 11 + are still significant (>1% relative to Si +) but drop by a factor of ∼100 between Si 11 + and Si 13 +. The probability of formation of clusters Si 13 +–Si 15 + is approximately constant at ∼5 × 10 −4 relative to Si + and rising a little for Si 15 +, but clusters beyond Si 15 are not detected (Si m≥16 +/Si + < 1 × 10 −4). The probability of formation of Si m + and Si m C n + clusters depends only very weakly on the C 60 + primary ion energy between 13.5 keV and 37.5 keV. The behaviour of Si m + and Si m C n + cluster ions was also investigated for impacts onto a fresh Si surface to study the effects that saturation of the surface with C 60 + in reaching sputter equilibrium may have had on the measured abundances. By comparison, there are very minor amounts of pure Si m + clusters produced during C 60 + sputtering of silica (SiO 2) and various silicate minerals. The abundances for clusters heavier than Si 2 + are very small compared to the case where Si is the target. The data reported here suggest that Si m + and Si m C n + 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.