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 (similar to 30 nm) Si1-xGex (x similar to 0.3) layer. The conditions used were near-normal incidence O-2(+) 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 (C) 2010 John Wiley & Sons, Ltd.