First-shell bond lengths in SixGe1-x crystalline alloys

Si and Ge K-edge x-ray absorption fine structure (XAFS) spectra of strained and relaxed SixGe1-x crystalline alloys grown by molecular beam epitaxy on Si(001) substrates are reported. For alloys with less than 30% Si, fluorescence yield detection is shown to be essential to avoid distortions of the Si K-edge XAFS signal caused by the underlying Ge L-edge XAFS signal from the majority Ge species. The average first shell structure has been deduced using simultaneous fitting of all data for relaxed alloys, while imposing physically reasonable constraints. The Ge-Ge, Ge-Si, and Si-Si first-shell distances are found to vary with composition. The results are compared with other experimental results and theoretical predictions in the literature. Our results are generally consistent with other experimental studies but they differ from recent theoretical predictions based on macroscopic elastic properties in that we observe a different compositional dependence [i.e., slope of R(x) lines] for the Ge-Ge and Si-Si bond lengths. The slope for the Ge-Si bond length composition line was found to be intermediate between that of Si-Si and Ge-Ge.