Atomic Reconfiguration of van der Waals Gaps as the Key to Switching in GeTe/Sb₂Te₃ Superlattices

Nonvolatile memory, of which phase-change memory (PCM) is a leading technology, is currently a key element of various electronics and portable systems. An important step in the development of conceptually new devices is the class of van der Waals (vdW)-bonded GeTe/Sb₂Te₃ superlattices (SLs). With their order of magnitude faster switching rates and lower energy consumption compared to those of alloy-based devices, they are widely regarded as the next step in the implementation of PCM. In contrast to conventional PCM, where the SET and RESET states arise from the crystalline and amorphous phases, in SLs, both the SET and RESET states remain crystalline. In an earlier work, the superior performance of SLs was attributed to the reduction of entropic losses associated with the one-dimensional motion of interfacial Ge atoms located in the vicinity of Sb₂Te₃ quintuple layers. Subsequent experimental studies using transmission electron microscopy revealed that GeTe and Sb₂Te₃ blocks strongly intermix during the growth of the GeTe phase, challenging the original proposal but at the same time raising new fundamental issues. In this work, we propose a new approach to switching in SLs associated with the reconfiguration of vdW gaps accompanied by local deviation of stoichiometry from the GeTe/Sb₂Te₃ quasibinary alloys. The model resolves in a natural way the existing controversies, explains the large conductivity contrast between the SET and RESET crystalline states, is not compromised by Ge/Sb intermixing, and provides a new perspective for the industrial development of memory devices based on such SLs. The proposed concept of vdW gap reconfiguration may also be applicable to designing a broad variety of engineered two-dimensional vdW solids.