The impact of oxygen vacancy defect density on the nonlinearity and short-term plasticity of TiO₂-based exponential selector

Abstract

The readout margin of the one selector-one RRAM (1S1R) crossbar array architecture is strongly dependent on the nonlinearity of the selector device. In this work, we demonstrated that the nonlinearity of Pt/TiO2/Pt exponential selectors increases with decreasing oxygen vacancy defect density. The defect density is controlled by modulating the sputtering pressure in the oxide deposition process. Our results reveal that the dominant conduction mechanisms of the Pt/TiO2/Pt structure transit from Schottky emission to Poole-Frenkel emission with the increase of sputtering pressure. Such transition is attributed to the rise of oxygen vacancy concentration. In addition, the short-term plasticity feature of the Pt/TiO2/Pt selector is shown to be enhanced with a lower defect density. These results suggest that low defect density is necessary for improved exponential selector performances.

Memory Switching versus Threshold Memory Switching: Finding a Promising Synaptic Device for Brain-Inspired Artificial Learning Systems

“…14−16 Especially, IMT materials, such as NbO 2 , VO 2 , TiO 2 , etc., stand out as an excellent choice for the selector layer due to their ability to provide a high threshold to OFF-state current, fast switching speed, and promising integration capability with the MS device. 17,18 Although recent research efforts have mostly focused on understanding the analog synaptic behavior of the selector based on TiO 2 , 19,20 a few studies have been dedicated to understand its performance at the CPA level. 21 Particularly interesting, a recent qualitative investigation on the CPA architecture using exponential and threshold selector cells demonstrates that the threshold-type selector layer could potentially enhance learning accuracy in the synaptic CPA by facilitating more linear conductance modulation.…”

“…As an alternative solution, the two-terminal passive selector layer with a low OFF-state current presents an attractive option to integrate with the memory switching (MS) layer. The integration of selector layers, such as metal ions, ovonics, chalcogenides, semiconductors, and insulator-to-metal transition (IMT) materials, with MS cells, has emerged as a promising strategy to mitigate sneak path current in cross-point memory arrays. − Especially, IMT materials, such as NbO 2 , VO 2 , TiO 2 , etc., stand out as an excellent choice for the selector layer due to their ability to provide a high threshold to OFF-state current, fast switching speed, and promising integration capability with the MS device. , Although recent research efforts have mostly focused on understanding the analog synaptic behavior of the selector based on TiO 2 , , a few studies have been dedicated to understand its performance at the CPA level . Particularly interesting, a recent qualitative investigation on the CPA architecture using exponential and threshold selector cells demonstrates that the threshold-type selector layer could potentially enhance learning accuracy in the synaptic CPA by facilitating more linear conductance modulation. , However, their results fell short of reaching experimental benchmarks due to the qualitative nature of threshold device modeling.…”