[en] The present work deals with a detailed analysis of the anomalous small-angle X-ray scattering in amorphous silicon–germanium alloy using the simulation technique. We envisage the nanoporous two-phase alloy model consisting in a mixture of Ge-rich and Ge-poor domains and voids at the nanoscale. By substituting Ge atoms for Si atoms in nanoporous amorphous silicon network, compositionally heterogeneous alloys are generated with various composition-contrasts between the two phases. After relaxing the as-generated structure, we compute its radial distribution function, and then we deduce by the Fourier transform technique its anomalous X-ray scattering pattern. Using a smoothing procedure, the computed X-ray scattering patterns are corrected for the termination errors due to the finite size of the model, allowing so a rigorous quantitative analysis of the anomalous small-angle scattering. Our simulation shows that, as expected, the anomalous small-angle X-ray scattering technique is a tool of choice for characterizing compositional heterogeneities coexisting with structural inhomogeneities in an amorphous alloy. Furthermore, the sizes of the compositional nanoheterogeneities, as measured by anomalous small-angle X-ray scattering technique, are X-ray energy independent. A quantitative analysis of the separated reduced anomalous small-angle X-ray scattering, as defined in this work, provided a good estimate of their size

[en] A detailed crystal-field analysis, based on the Racah' theory, was carried out for the so-called A, B and G1 isolated charge-compensation centers of Er³⁺ ion doped in CaF₂ crystal. Three sets of crystal-field parameters were obtained by a least-squares fitting of the optical data of Er³⁺ ion diluted in epitaxial Ca_1-xEr_xF_2+x thin film. This theoretical analysis confirms the expected C_4ν site symmetry for the A center and the C_3ν site symmetry for the G1 center. For the B center, however, the site symmetry is not exactly C_3ν in contrast to what is believed. (orig.)

[en] Short-range structural and chemical ordering in Si-rich chemically deposited a-Si_1-xC_x thin films have been investigated via Raman scattering and the numerical modelling technique. Raman spectra have been presented over a wide frequency range including both Stokes and anti-Stokes scattering. The interpretation of the spectra is performed in terms of the whole density of vibrational states. In order to determine the latter, the structure of the a-Si_1-xC_x (x<0.5) system has been modelled and the corresponding dynamical properties have been computed in the harmonic approximation using the valence-force-field model. By integrating the Stokes and anti-Stokes, first-order, and multiple-order processes, a fit of the experimental Raman spectra has been achieved. As expected, our analysis shows a tendency to chemical ordering into a tetrahedrally coordinated network for Si-rich alloys. Nevertheless, a total chemical ordering is not achieved since homonuclear C-C bonds coexist with Si-Si and Si-C ones in these alloy compounds. (author)