Improved Threshold Switching and Endurance Characteristics Using Controlled Atomic‐Scale Switching in a 0.5 nm Thick Stoichiometric HfO₂ Layer

Abstract

Abstract Electrochemical metallization cell–based threshold switching (TS) devices are promising candidates for selectors in high‐density cross‐point memory arrays. However, TS characteristics in density‐ and stoichiometry‐engineered solid electrolyte systems have not been studied. By adopting TS‐based stoichiometric and substoichiometric solid electrolyte HfO 2 layers, the localized atomic scale movement of Ag ions can be effectively controlled in ultrathin bilayers. The stoichiometric HfO 2 thickness is crucial to this. This study proposes defect‐ and density‐engineered bilayer TS with a 0.5 nm critical thickness of the stoichiometric HfO 2 layer, which maximizes various switching characteristics. The unstable filament in the ultrathin stoichiometric HfO 2 layer prevents the formation of stable Ag clusters owing to limited Ag injection into the dense and stoichiometric HfO 2 layer. In addition, the substoichiometric HfO 1.91 (1.5 nm) buffer layer prevents direct injection of Ag ions from the top electrode into the dense HfO 2 layer. These factors enable the bilayer design to achieve a high turn‐off speed of 100 ns, excellent endurance above 10 7 cycles, low off current of ≈pA, and tight V th distributions even for sub‐2 nm devices. These exceptional results demonstrate the possibility of designing density‐graded bilayer TS devices with high stability, fast switching, and high endurance.