Residual Stresses and Micro-voids Propel Metal Diffusion for Filament-Based Memristors

Abstract

Metal filamentation based mechanisms have the advantage of a high switching current ratio, yet typically require high switching voltages to activate the memristive device due to the primary mechanism of atomic vacancy filling and movement. Herein, Introducing non-reactive nitrogen gas during plasma sputtering of silver is shown to prime the overlying metal nitride layer to achieve low threshold switching at applied biases of below 60 millivolts. Residual nitrogen species within the silver under-layer promote the creation of nano-sized void defects within the superjacent dielectric layer, which, coupled with residual stresses in the gigapascal range, enable sub-micron filamentation growth. These memristor devices function similarly to potassium ion channels, displaying current growth and relaxation patterns that align with the Hodgkin-Huxley model, and as such are amenable to the development of artificial neuron structures. Further, a diverse set of neuromorphic behaviors not seen within typical metal filamentation based memristors is observed. This includes multi-peak synaptic weight changes in the device's response to spiked stimuli. Both the switching voltages and neuromorphic properties are linked to the nitrogen-argon pressure during silver deposition. Interestingly, these devices also exhibit lateral growth of silver filamentation across the surface of the metal nitride thin film layer with gaps of more than a hundred micrometers, suggesting that the underlying silver undergoes accumulation and breakthrough. The filling of large micro-voids with Ag generates large nanoparticles that easily propagate, enabling a large diffusion front and faster filamentation time, whereas small micro-voids create a bottleneck in the filamentation process. Additionally, the introduction of residual stresses in conventional diffusion theory indicates greater dendritic interconnectivity and thus electrode to electrode connection. This study demonstrates that the facile incorporation of non-reactive gases during the sputter-deposition of a metal electrode opens a path to unique material mechanisms that facilitate the development of versatile memristors.