Locking Tetrahedral Ge Coordination Enables Stable Multilevel Phase-Change Memory

As data-intensive technologies continue to scale, phase-change random access memory (PCRAM) has emerged as a leading contender for next-generation nonvolatile memory. However, its practical deployment remains hindered by limited storage density and resistance drift. Herein, we report a superlattice-like structure comprising [GeTe(24 nm)/Ge(2 nm)/Ge₂Sb₂Te₅(24 nm)]₁, where the incorporation of an ultrathin Ge buffer layer promotes the formation of tetrahedral Ge units, effectively suppressing the tetrahedral-to-octahedral ("umbrella-flip") coordination transition in Ge₂Sb₂Te₅, while simultaneously stabilizing the structural evolution of the intermediate-resistance state to ensure reliable multilevel storage. This structural stabilization facilitates a well-defined sequential crystallization pathway, evolving from the amorphous state through an intermediate face-centered cubic (FCC) phase, and finally transitioning into a stable hexagonal (HEX) configuration. These findings highlight the potential of Ge-buffered superlattice-like structures as a robust platform for high-density multilevel memory applications.