FACTS AND ARTIFACTS IN THE CHARACTERIZATION OF SI/SIGE MULTILAYERS WITH SIMS

A number of Si/Si1-xGe(x) heterostructures, grown by molecular beam epitaxy (MBE) and vapour phase epitaxy (VPE), were examined with secondary ion mass spectrometry (SIMS). A variety of experimental conditions was employed, limited to positive secondary ions and moderate to excellent depth resolution. The most elaborate results were obtained for O2+ primary ion bombardment at energies of 2-8 keV. These demonstrate (for x < 0.4): (1) a minimal erosion rate variation with germanium content x; (2) the near-absence of matrix effects, i.e. x dependence, of the secondary ion yields for impurities like B, Ga, Sn, Sb and others; (3) an approximately concentration-independent sensitivity for germanium. Furthermore, a surface-energy-difference-driven, Ge-segregation mechanism operative during MBE growth is discussed. This phenomenon is just one manifestation of the fundamentally different processes during MBE and VPE. Such processes may be responsible for the fact that interfaces in MBE samples, as obtained with SIMS, are less abrupt than for VPE material. A simple explanation in terms of an inferior substrate-cleaning procedure cannot be ruled out, however.