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Herein, the operation of a GeSe ovonic threshold switch (OTS) is studied as a self‐selecting memory cell based on the polarity effect. From the observed operating current (Iop) dependence and area scaling behavior, the critical role of Joule heating in ensuring exceptionally large memory window in this material is confirmed. The underlying mechanism is further investigated by means of chemical analysis and is confirmed to be caused by polarity‐dependent atomic migration under high‐Iop regime, consistent with elemental segregation due to electronegativity contrast. More specifically, selective diffusion of Ge atoms through the TiN layer into negatively biased top electrode stack is observed. At the same time, there is no sign of a similar process for Se atoms under opposite voltage polarity. Based on these observations, a novel memory concept utilizing a selective diffusion barrier is proposed. Furthermore, under low‐Iop regime, no major composition change is observed, leaving room for alternative interpretation of the polarity effect under such conditions. Finally, it is demonstrated that functional GeSe OTS‐only memory is fabricated with atomic layer deposition, making it suitable for vertical 3D integration to enable low‐cost applications. [ABSTRACT FROM AUTHOR]

Amorphous chalcogenide‐based ovonic threshold switch (OTS) selectors are an important part of the crosspoint memory arrays. It is known that the threshold voltage (Vth) of the OTS device can be affected by operating conditions, such as pulse amplitude and ramp rate. Herein, the impact of pulse polarity on OTS parameters is investigated. Recent findings on the polarity‐induced Vth shift observed in SiGeAsTe and SiGeAsSe materials are summarized. This effect is manifested as a stable and reversible change in Vth resulting from the reversal of applied pulse polarity, thus allowing Vth to be electrically controlled. Herein, for the first time exceptionally large polarity‐induced Vth shift in GeSe OTS is reported. The behavior observed in binary GeSe and quaternary SiGeAs(Te/Se) material systems is compared and the dependence of polarity effects on the composition of OTS devices is discussed. The impact of film thickness, interface, and stoichiometry in GeSe OTS is investigated. [ABSTRACT FROM AUTHOR]

Density functional theory simulations are used to identify the structural factors that define the material properties of ovonic threshold switches (OTS). They show that the nature of mobility‐gap trap states in amorphous Ge‐rich Ge50Se50 is related to GeGe bonds, whereas in Se‐rich Ge30Se70 the Ge valence‐alternating‐pairs and Se lone‐pairs dominate. To obtain a faithful description of the electronic structure and delocalization of states, it is required to combine hybrid exchange–correlation functionals with large unit‐cell models. The extent of localization of electronic states depends on the applied external electric field. Hence, OTS materials undergo structural changes during electrical cycling of the device, with a decrease in the population of less exothermic GeGe bonds in favor of more exothermic GeSe. This reduces the amount of charge traps, which translates into coordination changes, an increase in mobility‐gap, and subsequently changes in the selector‐device electrical parameters. The threshold voltage drift process can be explained by natural evolution of the nonpreferred GeGe bonds (or "chains"/clusters thereof) in Ge‐rich GexSe1–x. The effect of extrinsic doping is shown for Si and N, which introduce strong covalent bonds into the system, increase both mobility‐gap and crystallization temperature, and decrease the leakage current. [ABSTRACT FROM AUTHOR]