The influence of beam energy on apparent layer thickness using ultralow energy O2+ SIMS on surface Si1−xGex

A technique that employs energy sequencing, i.e. depth profiling a sample for a range of beam energies and then extrapolating the profile shape to zero beam energy, has been proposed as a means for removing the effects of atomic mixing and surface transient behavior. Here we have tested this approach in an attempt to establish the accurate thickness of a superficial (∼30 nm) Si1−xGex (x ∼ 0.3) layer. The conditions used were near-normal incidence O2+ and the energy range 0.25–2.5 keV. Energy-dependent apparent layer thicknesses were extracted for numerous points (90, 85, 75, 50, 21 and 10%) across the decaying Ge signal between the SiGe and underlying Si. A monotonic increase in apparent layer thickness was found as the energy was reduced across the range 2.5–0.4 keV. However, for beam energies < 0.4 keV, the apparent layer thickness was found to decrease suggesting that an increase in the relative surface-to-bulk erosion rate had outweighed any decrease in the true width of the transient region. This effect may be inescapable or caused by a nanometre-scale Ge rich layer at the surface. The behavior found here shows that the ‘zero beam energy’ profile shape is not necessarily closest to the truth. Copyright © 2010 John Wiley & Sons, Ltd