Solid-State Electrochemical Random Access Memory for in-Memory Computing

Abstract

Novel approaches to computing are needed to accommodate the rapid increases in data processing needs. Standard von Neumann architectures may not be optimal for operations like machine learning due to the high energy costs of moving data between separate processor and memory elements. In-memory computing with combined logic and memory functionality is a promising energy-efficient solution. Filament-based phase-change memory and resistive random access memory are well-investigated approaches to in-memory computing; however, they suffer from unreliable switching due to the stochastic motion of ions within the filament. This results from the positive feedback process associated with filament-based memories.In this work, we develop electrochemical random-access memory (ECRAM) that uses the 3D concentration of oxygen vacancy ions to store information1,2. We sandwich an ion-conducting, electron-blocking solid electrolyte between the reservoir and switching layers, both mixed ionic and electronic conductors (MIECs). Due to essential zero electronic conductivity, the ion conductor blocks the nonuniform joule heating positive feedback that results in filament formation. Instead, oxygen vacancies are uniformly distributed in the 3D bulk. This electrochemical memory cell is therefore sensitive to the average statistical behavior of all ions, as opposed to the stochastic behavior of a small number of ions in the filament. This average ion concentration yields the analogue information state.We show that small, short electrochemical voltage pulses (+/-1.5V, 2 μs) can be used to shuttle vacancies between the MIECs; this enables reliable, linear, and deterministic switching behavior with hundreds of information states. At the same time, the materials are CMOS compatible with long endurance and information retention. The switching and retention times are dependent upon the ability of the materials to allow reversible changes in the oxygen stoichiometry, analogous to an “oxygen battery,” as well as the ionic resistance of the electrolyte. These ECRAM cells not only solve the longstanding problems within resistive memory, but also present significant opportunities for future research onto different nonstoichiometric transition metal oxides.Reference:1 Y. Li et al. “Filament-free bulk resistive memory enables deterministic analogue switching,” Adv Mater, 32, 202003984 (2020)2. Y. Li et al. "Low-Voltage, CMOS-Free Synaptic Memory Based on LiXTiO2 Redox Transistors," ACS Appl. Mater. Interfaces, 11, 42, 38982–38992 (2019) Figure 1