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, fluores cence 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 t he majority Ge species. The average first shell structure has been deduced using simultaneous fitting of all data for relaxed alloys, while imposing p hysically reasonable constraints. The Ge-Ge, Ge-Si, and Si-Si first-shell d istances are found to vary with composition. The results are compared with other experimental results and theoretical predictions in the literature. O ur results are generally consistent with other experimental studies but the y differ from recent theoretical predictions based on macroscopic elastic p roperties in that we observe a different compositional dependence [i.e., sl ope 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 tha t of Si-Si and Ge-Ge. [S0163-1829(99)00316-1].