Volatile to Non-Volatile Switching Transition in Chalcogenides

Abstract

Over the past 60 years, three distinct electrical switching behaviors have been discovered in chalcogenides: ovonic threshold switch (OTS), ovonic memory switch (OMS), and phase-change switch (PCS). The first two have been successfully utilized in commercialized 3D Xpoint chips, serving as selector and memory cells, respectively. However, the relationships among these three behaviors remain unclear. Here, it is demonstrated that the Ge-Te binary system exhibits these three switching mechanisms. Specifically, the switching behavior transforms from PCS-like to OTS-like within the composition range from GeTe₈ to GeTe₆, while the shift from volatile (OTS) to non-volatile switching behavior (OMS) occurs between the compositions GeTe₂ and GeTe. The PCS-to-OTS transition is primarily driven by enhancements in glass-forming ability, with the Turnbull parameter increasing from 0.58 to 0.6, and the crystallization temperature exceeding 145 °C, while the shift from OTS to OMS behavior is largely due to the significantly accelerated crystallization speed, from microseconds to nanoseconds. Most importantly, the GeTe₁₆ PCS stands out, with a large ON current (0.6 mA), low leakage current (<2.5 × 10⁻⁸ A), high endurance (>6 × 10⁹ cycles), and high-temperature back-end-of-line compatibility. The crystalline nature of this PCS material addresses the composition and toxicity issues in conventional OTS materials. Moreover, GeTe₁₆ PCSs have been successfully integrated with GeTe OMSs, forming selector/memory arrays with a read margin of 0.4 V. These findings not only reveal the mechanisms underlying the volatile to non-volatile transition but also provide an alternative solution for high-density 3D memory, potentially replacing the OTS/OMS stack approach.