Critical test of the structure of the ordered phase in epitaxially grown SixGe1-x films

Two different structural models have been recently proposed to explain the ordering observed in SixGe1-x alloys grown at low temperatures on Si(001). We show that, through dark field imaging of the different domains of the ordered phase, it is possible to differentiate between the two structures un- equivocally. In this way, we determine that the ordered phase corresponds to ordering along a single set of {111} planes, where Ge-rich double layers alternate with Si-rich double layers. We also show that this conclusion is consistent with all other experimental data reported so far. © 1993 The American Physical Society.