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The present work deals with a detailed analysis of the small-angle X-ray scattering in amorphous silicon–germanium alloy using the simulation technique. We envisage the two-phase alloy model consisting in a mixture of Ge-rich and Ge-poor domains at the nanoscale. By substituting Ge atoms for Si atoms in a continuous random network, compositionally nanoheterogeneous 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 X-ray scattering pattern. Using a smoothing procedure, the computed small-angle X-ray scattering patterns are corrected for the termination errors due to the finite size of the model, allowing so, for the first time at our best knowledge, a rigorous quantitative analysis of this scattering. Our simulation shows that the relative alloy composition of the two phases is a determinant parameter for the occurrence of small-angle X-ray scattering. A composition-contrast threshold, independent of the overall composition of the alloy as well as the relative volume fraction of the phases, has been demonstrated. Thus, two-phase silicon–germanium alloy with composition-contrast smaller than 0.3 behaves as compositionally homogeneous alloy in the context of small-angle X-ray scattering experiment. Our simulation also shows that the experimentally derived characteristics of the compositional heterogeneity in the alloy, such as radius of gyration and correlation volume, are composition-contrast dependent; they are generally smaller or equal to the actual values. [ABSTRACT FROM AUTHOR]

Abstract: A detailed microstructural analysis of amorphous silicon–germanium alloys with Ge fraction ranging from 0.1 to 0.5 is performed by means of a numerical modeling technique. By substituting Ge atoms for Si atoms in the nanoporous paracrystalline network of amorphous silicon, several amorphous silicon–germanium structures have been generated. Our main aim in the present work is to study the effect of compositional heterogeneities on the structural properties of amorphous silicon–germanium alloys in comparison with the standard case, that of a homogeneous random distribution of the atoms. We have limited ourselves here to the borderline case, that of segregation of Ge atoms at the nanoscale. The microstructure of our structural models is analyzed by examining the macroscopic mass density, the intensity of X-ray diffraction, the pair distribution functions, the bond lengths and the coordination numbers within the first coordination shell of Si and Ge atoms. Our structural models account for the experimentally derived mass densities regardless of the Ge distribution pattern. They also account for the intense small-angle X-ray scattering observed for some amorphous silicon–germanium samples. The short-range compositional disorder, reflected in the bond lengths and the coordination numbers within the first coordination shell of Ge atoms, is found to be very sensitive to how these atoms are arranged in the alloys. [Copyright &y& Elsevier]