Structural investigations of amorphous phase change materials Ge${}_{50-x}$Cu${}_x$Te${}_{50}$ by means of scattering and absorption of X-rays

The amorphous structure of the phase change materials (PCM) GCT-9110 (Ge${}_{45}$Cu${}_5$Te${}_{50}$), GCT-7310 (Ge${}_{35}$Cu${}_{15}$Te${}_{50}$) and GCT-112 (Ge${}_{25}$Cu${}_{25}$Te${}_{50}$) 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.