Microscopic model of the operation of the single-chalcogenide X-point memory

Ovonic threshold switching is the key process for several applications of chalcogenide alloys including phase change memories and selector elements in cross-points arrays. Very recently, it has been shown that the threshold switching voltage VT depends on the polarity of the applied field. This feature has been already exploited in the realization of the Single Chalcogenide X-point Memory (SXM) in which a single film of a chalcogenide alloy can serve as both a memory and selector unit. In this work, we provide a microscopic understanding of the polarity-dependent VT by leveraging electrical and physical measurements, numerical simulations based on technology computer aided design (TCAD) and electronic structure calculations based on density functional theory (DFT). We developed a Graded Band Gap (GBG) model in which an inhomogeneous distribution of localized electronic states in the gap is established by the opposite effect of a strong electric field at the cathode and a high density of electrons in the conduction band at the anode. The model is suitable to reproduce several features of the programming window, including its dependence on temperature, thickness and composition of the chalcogenide alloy. The microscopic understanding that we gained on the SXM operation lays the foundation for important improvements in the memory design and in the selection of better performing alloys for applications in enabling memory technologies. Ovonic threshold switching between high-resistive and conductive states in chalcogenide alloys is crucial for phase change memories and selector elements, yet its voltage dependence on field polarity remains underexplored. Here, the authors elucidate this phenomenon using electrical measurements, numerical simulations based on Technology Computer Aided Design, and DFT calculations, proposing a Graded Band Gap model that enhances memory design by predicting programming window characteristics.