Composition modulation in amorphous metal-germanium alloys detected by anomalous small-angle X-ray scattering

The tunability of synchrotron radiation was used to perform anomalous small-angle X-ray scattering (ASAXS) experiments on amorphous metal-germanium (a-M_xGe_100-x) alloys with the aim of detecting and characterizing composition modulation and phase separation. The combination of SAXS with the anomalous-dispersion effect allows the composition modulation to be distinguished from density fluctuations arising from voids and defects. Data gathered from a-Fe_xGe_100-x, a-W_xGe_100-x and a-Mo_xGe_100-x samples in the low metal concentration regime show nanometer-scale composition modulation. In these samples the density of Ge atoms is uniform throughout. The small-angle X-ray scattering arises from fluctuations in the density of metal atoms. The position of the SAXS peak moves towards smaller values of the scattering vector with increased metal concentration indicating an increase in the correlation length of the composition fluctuations with increased metal content.