Effects of Acidic and Basic Interface Metal Oxides on H-ECRAM Performance

Three-terminal electrochemical random-access memory (ECRAM) is promising for energy-efficient neuromorphic computing, owing to device non-volatility and reversible, symmetric switching behavior. The conductance of a channel material, typically WO₃, represents analog states, changing in response to ion intercalation, protons in the case of H-ECRAM. Kinetic bottlenecks impede write voltage below 1 V and nanosecond programming. Typical performance enhancements are high ionic conductivity electrolytes and scaling of channel and electrolyte thicknesses.¹ As the device stack shrinks, the interfaces may also be potential kinetic bottlenecks.²<br/>We hypothesized that the interface reaction at the electrolyte-channel interface depends on the strength of proton bonding to oxygen ions. From acid-base scales, more basic metal oxides tend to have stronger O-H bonds, which may increase the barrier to proton transfer and suppress interface kinetics. To explore this concept, we deposited a range of thin, amorphous metal oxide coatings at the electrolyte-channel interface. We evaluated H-ECRAM device performance under cycling and extracted exchange current density (j₀) values with electrochemical impedance spectroscopy (EIS) to describe the interface kinetics. Both the magnitude of conductance change and j₀ values decreased with basic coatings (more basic relative to WO₃). This decrease followed the ordering of the metal oxide coatings on the acid-base scale used.³ X-ray photoelectron spectroscopy (XPS) showed that WO₃ film work functions decreased with more basic coatings, with only minor increases in oxidation state and sheet resistance. The absence of major changes in oxidation state and sheet resistance suggests these coatings may affect mainly the proton transfer at the interface, correlating with acid-base scale trends.<br/>These results show the tunability of H-ECRAM device performance at the electrolyte-channel interface. Acid-base scales may guide interface design that lowers write voltages further and supplements existing strategies to reduce write voltage for fast programming.<br/><br/>(1) M. Onen, N. Emond, B. Wang, D. Zhang, F.M. Ross, J. Li, B. Yildiz, and J.A. del Alamo, Science 377(6605), 539–543 (2022).<br/>(2) M. Huang, M. Schwacke, M. Onen, J. del Alamo, J. Li, and B. Yildiz, Advanced Materials 35(37), (2023).<br/>(3) D.W. Smith, J. Chem. Educ. 64(6), 480 (1987).