Electroforming-Free Low Switching Operation in CeO₂-based 1S1R Memristor Device for Neuromorphic Computing Applications

Resistive random-access memory (RRAM) devices with crossbar array (CBA) configuration have been considered as promising next-generation non-volatile memory as well as future computing paradigms, such as in-memory computing and neuromorphic computing [1]. However, the generation of sneak path currents through unselected cells in a CBA architecture affects the efficiency and power consumption of the cells. Therefore, the sneak current issues can be resolved by vertical integration of a memristor (R) with a selector (S) in a 1S1R crossbar array [2]. However, it is also essential that the material composition and electrical properties of both the memristor and selector should be compatible for the proper functioning of 1S1R integrated device. Among various oxide switching layers, here, we have considered CeO₂-based rare-earth oxide as a dielectric switching layer for memory and selector devices due to its large bandgap, high oxygen ionic conductivity, and strong oxygen-getting ability of Ce [3]. For this, we have fabricated a memristor with TiN/CeO₂/TiN structure and vertically stacked it with a selector device made of Ag/CeO₂/SiO₂/TiN through a shadow mask in a capacitor structure. The selector (S) device with Ag as the top electrode (TE) acts as a volatile memory under a low current operation of 1 µA and could help in reducing the leakage current on integration with the memory (R) device. The memory device shows analog switching characteristics under a bias voltage of ±4 V with a leakage current of 430 nA at a read voltage of 1V. In memory devices with TiN electrodes, the possibility of the formation of a TiO<i>_x</i>N<i>_y </i>interface layer during the fabrication process can serve as a reservoir of oxygen vacancies which may act as a leaky dielectric layer. Upon repeating +V sweep from 0 to +4 V, the current increased by more than 10 times indicating a potential application of the memory device for synaptic weight update. However, the continuous negative voltage sweep does not return the device current to its initial stage which could be due to the formation of different interface barriers between top TiN/CeO₂interface and bottom CeO₂/TiN interface although the device is structurally symmetric. In comparison, the 1S1R device also shows an analog switching with ±4 V and requires no electroforming process prior to the switching operation. Due to the addition of a selector device, the leakage current was suppressed by nearly four orders to 28 pA at a read voltage of 1V. Initially, both memory and selector devices are at HRS and most of the voltage drop is on the selector due to its large off-state resistance. The device shows memory switching after the selector turns on which reduces the leakage current. The present 1S1R device shows reliable data retention of 68.7% after 30 min which has the potential for studying its long-term plasticity behavior. A detailed analysis will be carried out by fabricating 1S1R in crossbar array architecture to study the synaptic behavior for the implementation of a hardware neural network.<br/><br/><b>References:</b><br/>1. Ielmini, D., & Wong, H. S. P. <i>Nature electronics</i> 1, 333-343, (2018).<br/>2. Linn, E., Rosezin, R., Kügeler, C., & Waser, R. <i>Nature materials</i> 9, 403-406, (2010).<br/>3. Skorodumova, N. V., Ahuja, R., Simak, S. I., Abrikosov, I. A., Johansson, B., & Lundqvist, B. I., <i>Physical Review B </i>64, 115108, (2001).