Stimulation Rate-Dependent 1S1R Synapse with Short-Term Plasticity TiO_x-Based Exponential Selector

Short-term plasticity (STP) is an important synaptic characteristic in hardware-based artificial neural networks, as it is suitable for temporal information processing (TIP).^[^1–3^] However, the STP feature is difficult to be demonstrated in a single non-volatile memory because it requires a certain degree of volatility. Generally, selector devices are implemented with a non-volatile resistive random-access memory (RRAM) in one selector-one RRAM (1S1R) configuration to suppress the sneak current in a crossbar array.^[^4,5^] In this work, a TiO_x-based selector is introduced not only to suppress the sneak current but also to enable the TIP feature in the 1S1R synaptic device, which consists of a Pt/TiO_x/Pt exponential selector and a Pt/HfO_x/Ti RRAM device. Our measurements reveal that the exponential selector exhibits the STP characteristic, while the RRAM device enables the long-term memory capabilities of the synapse. With the corresponding 1S1R device, an on/off ratio of up to ~ 18 can be achieved under optimised reading conditions. Furthermore, 5000-cycle writing endurance was demonstrated with uniform switching. Moreover, the potentiation and depression synaptic characteristics are realised in the 1S1R device. Thereafter, we experimentally demonstrated the stimulation rate-dependent multilevel switching in the 1S1R device. 4 multilevel states were achieved in the 1S1R synapse using different frequencies of pulses at 100 000 Hz, 50 000 Hz and 100 Hz, demonstrating the TIP capability in the 1S1R device. This work therefore provides a reliable means of producing the TIP capability in a 1S1R device.<br/><br/><b>References:</b><br/>(1) Cho, H.; Kim, S. Enhancing Short-Term Plasticity by Inserting a Thin TiO2 Layer in WOx-Based Resistive Switching Memory. <i>Coatings </i>. 2020.<br/>(2) Lee, Y.; Mahata, C.; Kang, M.; Kim, S. Short-Term and Long-Term Synaptic Plasticity in Ag/HfO2/SiO2/Si Stack by Controlling Conducting Filament Strength. <i>Appl. Surf. Sci.</i> <b>2021</b>, <i>565</i>, 150563.<br/>(3) Ohno, T.; Hasegawa, T.; Tsuruoka, T.; Terabe, K.; Gimzewski, J. K.; Aono, M. Short-Term Plasticity and Long-Term Potentiation Mimicked in Single Inorganic Synapses. <i>Nat. Mater.</i> <b>2011</b>, <i>10</i> (8), 591–595.<br/>(4) Dananjaya, P. A.; Loy, D. J. J.; Chow, S. C. W.; Lew, W. S. Unidirectional Threshold Switching Induced by Cu Migration with High Selectivity and Ultralow OFF Current under Gradual Electroforming Treatment. <i>ACS Appl. Electron. Mater.</i> <b>2019</b>, <i>1</i> (10), 2076–2085.<br/>(5) Lee, W.; Park, J.; Kim, S.; Woo, J.; Shin, J.; Choi, G.; Park, S.; Lee, D.; Cha, E.; Lee, B. H.; Hwang, H. High Current Density and Nonlinearity Combination of Selection Device Based on TaOx/TiO2/TaOx Structure for One Selector–One Resistor Arrays. <i>ACS Nano</i> <b>2012</b>, <i>6</i> (9), 8166–8172.