Disorder in order: A study of local and global order in Ge-rich Ge-Sb-Te alloys

Disorder in the amorphous state of phase-change alloys has been a focus of much research, however, more recently it has become clear that there is significant temperature activated disorder in the crystalline phases that may have important implications for electrical transport. We explore order on different length scales using a variety of techniques. Bragg diffraction (BD) to determine average order, X-ray absorption to analyze order on nearest neighbor length scales, and total scattering to investigate structure on intermediate length scales. We find that Ge-rich phase-change alloys lying along the technological important (GeTe)_x-(Sb₂Te₃)_1-x pseudobinary tie line exhibit a ferroelectric transition at a Curie temperature T_c from rhombohedral to cubic symmetry using BD while short-range order probes such as X-ray absorption spectroscopy indicate that local rhombohedral symmetry is maintained. Using total X-ray scattering, we demonstrate that this apparent discrepancy can be resolved by the introduction of disorder on intermediate length scales using a simple model.