Local order in amorphous Ge2Sb2Te5 and GeSb2Te4

The structure of amorphous Ge2Sb2Te5 and GeSb2Te4 phase-change alloys was investigated by high energy x-ray diffraction, extended x-ray-absorption fine structure, and neutron diffraction. The reverse Monte Carlo simulation technique was used to generate large scale atomic models compatible with all experimental data sets. Simulations revealed that Ge-Ge bonding is present already in GeSb2Te4. Ge-Sb bonding was also found to be significant for both compositions. Within experimental uncertainties, all atomic species satisfy formal valence requirements: Ge is fourfold coordinated, Sb has three neighbors, and Te is mostly twofold coordinated. The environment of Ge atoms was investigated in detail. The predominance of GeTe4 or Te3Ge-GeTe3 units in these compositions can be excluded. Instead, these alloys are characterized by a variety of local motifs.