The amorphous structure of the phase change materials (PCM) GCT-9110 (Ge45Cu5Te50), GCT-7310 (Ge35Cu15Te50) and GCT-112 (Ge25Cu25Te50) located on the pseudo-binary tie line between GeTe and CuTe are investigated using anomalous x-ray scattering (AXS) and x-ray absorption fine structure (XAFS) measurements.
Supported by x-ray near edge structure (XANES) simulations and constrained by the experimental data, large three dimensional models of these compositions are generated using reverse Monte-Carlo simulations (RMC). In the low Cu-concentration regime (GCT-9110 and GCT-7310) the Cu atoms agglomerate to clusters, embedded into the amorphous matrix and linked via Cu-Te bonds. The resulting relative Te-enrichment of the amorphous matrix increases with the Cu-concentration which explains the rising crystallization temperature in this Cu-concentration regime.
Additionally, the amorphous matrix exhibits a dualistic topology, tetrahedral and defect-octahedral sites coexist resulting in weak extended range ordering. With rising Cu concentration the number of defect-octahedral sites diminishes. Contrary to this, in the high Cu concentration regime (GCT-112) an amorphous Ge-Cu-Te network is formed which is solely characterized by tetrahedral-like sites shaping a distinct extended range ordering compared to the low Cu concentration regime.