Thermally Assisted Atomic-Scale Intermixing and Ordering in GeTe-Sb₂Te₃ Superlattices

Interfacial phase change memory (iPCM) devices have been shown to switch with significantly reduced power consumption, compared with conventional phase-change memory devices. These iPCMs are based on a periodic structure of nanometer-sized layers of chalcogenides called a chalcogenide superlattice (CSL). Strong temperature increases have been observed within the CSL during the switching procedure, questioning the stability of the CSL structure. In this study, we conduct a detailed quantitative analysis to investigate the evolution of the structure and composition of the sputter-deposited GeTe-Sb₂Te₃ CSL upon a temperature increase using atom probe tomography. We find that GeTe-Sb₂Te₃ CSLs already feature significant interdiffusion during the synthesis, with a considerable fraction of Sb found in GeTe and Ge in Sb₂Te₃. Upon heating the atoms rearrange considerably and form layers of stable Ge₂Sb₂Te₅ and Ge₃Sb₂Te₆ phases, which can be described as a layered solid of GeTe and Sb₂Te₃ blocks, i.e., Ge₂Sb₂Te₅ = 2 × GeTe + Sb₂Te₃, while Ge₃Sb₂Te₆ = 3 × GeTe + Sb₂Te₃. Moreover, these layered solids form in such a way as to preserve and maximize the number of van der Waals (vdW)-like contacts. Interestingly, our electrothermal simulations indicate that the transformation of the original CSL structure into layered stacks of Ge₂Sb₂Te₅ and Ge₃Sb₂Te₆ will have a beneficial effect on device performance. Finally, we discuss the mechanism behind the interdiffusion and phase formation and its implications for iPCM devices. In doing so, the applicability of atom probe tomography to directly investigate intermixing and phase formation on the nanoscale in phase change and related memory devices is demonstrated.