First shell bond lengths in Si_xGe_1-x crystalline alloys

J.C. Aubry, T. Tyliszczak, A.P. Hitchcock
Brockhouse Institute for Materials Research, McMaster University, Hamilton, Canada L8S 4M1

J.-M. Baribeau and T.E. Jackman
Institute for Microstructural Sciences, National Research Council, Ottawa, Canada

      Si and Ge K-edge X-ray absorption fine structure (XAFS) spectra of strained and relaxed Si_xGe_1-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. [S0163-1829(99)00316-1]

c) American Physical Society (1999)