Local atomic arrangements and lattice distortions in layered Ge-Sb-Te crystal structures

Ge-Sb-Te (GST) compounds are of high interest due to their technologically outstanding optical and electronic properties. Thin films of GST alloys are widely used as phase change materials (PCMs) in optical storage media1,2,3,4 and are also major contenders for the next generation non-volatile RAM5,6,7,8. The operating principle of conventional PCM devices is based on the reversible transformation between the amorphous and metastable crystalline phases triggered either by optical or electrical ultrafast pulses. Interfacial PCMs (iPCMs)9 or chalcogenide superlattices consisting of Sb2Te3 and GeTe multilayers are a promising candidate for data storage devices with reduced energy consumption since reversible transition between SET and RESET states is assumed to be constrained by motion of atoms in 1D instead of 3D as in the case of conventional PCM devices. Thus, the switching mechanism of iPCMs is determined by the local atomic arrangement in distinct layers10,11,12,13, which also defines the electronic properties of the materials14. In particular, theoretical simulations showed that iPCMs can be a 3-D topological insulator15 or a Dirac semimetal11. Investigations of atomic structure in such technologically relevant iPCM revealed that various layered GST crystal structures can be formed during iPCMs production16,17,18. Consequently, the knowledge of the proper local atomic arrangement in layered GST alloys is of paramount importance in order to understand the switching mechanism of iPCMs and their material properties. The overall structure of layered GST compounds consists of rocksalt-type building blocks with alternating cation (GeSb) and anion (Te) layers. These blocks are stacked along the c-axis and periodically separated from each other by intrinsic vacancy layers (van der Waals gaps, vdWg’s) between adjacent Te layers19. However, local atomic arrangements in GST alloys are controversial and still under discussion. Even a recent high-resolution STEM study was not fully able to reveal the real structure of GST225 and GST124 lattices since the cation layers were not resolved20. In addition, there is a structural similarity in the atomic arrangement between the technological important cubic GST and its trigonal phase. Thus, the aim of this work is to study the local atomic arrangement and lattice distortions in Ge-Sb-Te thin films consisting of layered Ge2Sb2Te5 (GST225), Ge1Sb2Te4 (GST124) and Ge3Sb2Te6 (GST326) crystal structures by using a combination of atomic-resolution aberration-corrected (Cs-corrected) high-angle annular dark-filed scanning transmission electron microscopy (HAADF-STEM) imaging and theoretical image simulation. The stacking sequences in layered GST crystal structures are considered in detail and the favourable sequences are identified and discussed. The approach used here can also be applied to the fast evaluation of stacking sequences in layered GST crystal structures grown by different methods.