Surprisingly large apparent profile shifts of As and Sb markers in Si bombarded with ultra-low-energy Cs ion beams

Shallow arsenic implantation distributions and narrowly spaced antimony delta markers in silicon were used to explore near-surface changes of the sputtering yield under ultra-low-energy Cs+ bombardment (0.25–1 keV) at impact angles θ between 0° (normal incidence) and 60°, i.e. below the critical angle for ripple formation. In all cases, the profiles were found to be shifted towards the surface, with shifts ranging from about 1 nm (250 eV, 0–15°) up to 4 nm (1 keV, 0°). These artefacts are attributed to unexpectedly high sputtering yields during Cs build-up in silicon. In most cases, the yields became stationary at sputtered depths of less than 5 nm. Using the mean erosion rate between 0 and 5 nm and the stationary rate above 5 nm to define sputtering yields, the initial yield enhancement η=〈Y5 nm〉/Y∞ for 1 keV Cs+ was found to be as large as 3.2 at 15°, decreasing to about 1.6 at 60°. Between 30° and 60° the η-values for 250 and 500 eV were only slightly less than at 1 keV. Significantly lower values derived at angles θ<30° are probably due to the fact that the respective transition depths are significantly smaller than 5 nm. The results imply that depth profiles previously observed under the conditions of this study suffer from serious depth calibration errors.