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

• Published in: Advanced electronic materials Vol. 7; no. 2
• Main Authors: Lee, Seungwoo, Banerjee, Writam, Lee, Sangmin, Sung, Changhyuck, Hwang, Hyunsang
• Format: Journal Article
• Language: English
• Published: 01.02.2021
• Subjects: AgTe alloys; conductive bridge random access memory; resistive random‐access memory; selectors; threshold switching
• ISSN: 2199-160X; 2199-160X
• DOI: 10.1002/aelm.202000869

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 HfO2 layers, the localized atomic scale movement of Ag ions can be effectively controlled in ultrathin bilayers. The stoichiometric HfO2 thickness is crucial to this. This study proposes defect‐ and density‐engineered bilayer TS with a 0.5 nm critical thickness of the stoichiometric HfO2 layer, which maximizes various switching characteristics. The unstable filament in the ultrathin stoichiometric HfO2 layer prevents the formation of stable Ag clusters owing to limited Ag injection into the dense and stoichiometric HfO2 layer. In addition, the substoichiometric HfO1.91 (1.5 nm) buffer layer prevents direct injection of Ag ions from the top electrode into the dense HfO2 layer. These factors enable the bilayer design to achieve a high turn‐off speed of 100 ns, excellent endurance above 107 cycles, low off current of ≈pA, and tight Vth 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. Density‐engineered bi‐layer solid electrolyte (HfO1.91/HfO2)–based threshold switching (TS) devices address disparities in turn‐on/turn‐off switching speeds using the density of a single solid electrolyte. Such bi‐layer TS devices can achieve fast turn‐off speed, over 103 times the endurance, and a threshold voltage distribution that is three times tighter than single‐electrolyte‐based TS. This can be useful in high‐density cross‐point arrays.