In situ characterization of vacancy ordering in Ge-Sb-Te phase-change memory alloys

Tailoring the degree of structural disorder in Ge-Sb-Te alloys is important for the development of non-volatile phase-change memory and neuro-inspired computing. Upon crystallization from the amorphous phase, these alloys form a cubic rocksalt-like structure with a high content of intrinsic vacancies. Further thermal annealing results in a gradual structural transition towards a layered structure and an insulator-to-metal transition. In this work, we elucidate the atomic-level details of the structural transition in crystalline GeSb₂Te₄ by in situ high-resolution transmission electron microscopy experiments and ab initio density functional theory calculations, providing a comprehensive real-time and real-space view of the vacancy ordering process. We also discuss the impact of vacancy ordering on altering the electronic and optical properties of GeSb₂Te₄, which is relevant to multilevel storage applications. The phase evolution paths in Ge-Sb-Te alloys and Sb₂Te₃ are illustrated using a summary diagram, which serves as a guide for designing phase-change memory devices.