Structural model for crystalline and amorphous Si-Ge alloys

There are serious contradictions between extended x-ray-absorption fine structure (EXAFS) measurements of the Si-Ge and Ge-Ge bond lengths in both crystalline and amorphous silicon-germanium alloys, and in theoretical and simulation predictions. In particular, results from EXAFS experiments show that the Si-Ge and Ge-Ge bond lengths are independent of the alloy composition, indicating that there is no topological rigidity in the lattice. These EXAFS results on Si-Ge alloys are in sharp disagreement with all previous EXAFS results on III-V and II-VI semiconductor alloys. We discuss the implications of the EXAFS results regarding the local and global structure of the alloys. We also propose a structural model to serve as a focus for further experiments. In order to satisfy the overall floppiness of the network, we suggest that the Si-Ge samples may contain a large density of planar cracks, lined with hydrogen, and separated by ∼10 Å. Some measurements that could confirm (or discredit) this model are suggested.