Memristive Behavior in Core-Shell Nanowire Networks for Neuromorphic Architectures

Abstract

Continuous enhancement of performance and processing powers of computing devices will soon reach a technological and physical limit and to overcome this, systems emulating the human brain are being developed. Presently, mathematical models known as artificial neural networks (ANNs) are designed to simulate the computational abilities of the brain. Due to large simulation times, the current digital ANNs cannot be efficiently scaled. By contrast, analog (as opposed to digital) neuromorphic devices with dedicated and adaptable synapses are expected to rival the scale and efficiency of the brain. Memristors are two-terminal electrical component where the resistance is a function of the amount and direction of current and therefore they can uniquely emulate biological synapse behavior. Here, we discuss memristive behavior of Ti/HFO2/Pt core shell nanowire systems and networks for neuromorphic architectures and discuss their potential application as ANNs. In prior studies, memristive architectures for neuromorphic computing are connected through a crossbar array of neurons. With a limit on the number of neurons and regular connectivity, the networks lack sparsity and randomness. As a result, they have high wiring cost and poor functional connectivity. To address such limitations, we have fabricated a random array of core-shell memristive wires to form the connectivity matrix for neuromorphic hardware. In the systems listed above, the conductive core (Pt or Ti) serves as the bottom electrode with a memristive shell (HfO2) that can be electroformed with a set of top electrodes (Ti or Pt) deposited on the surface. Prior to miniaturization, as proof of concept, the core-shell wire networks were fabricated using 20 μm tungsten fibers as conductive cores with tungsten oxide as memristive shells. After imprinting of top silver electrodes and electroforming, IV measurements revealed memristive behavior with switching between resistive states (i.e., LRS-HRS) at ±1 V. Subsequently, nanowire networks based on the Ti/HFO2/Pt system were fabricated using electron beam lithography (EBL). For example, in one embodiment, over 90 overlapping core (40 nm)-shell (5 nm) nanowires were written on a 200 μm × 200 μm write field. To complete the connectivity matrix, 64 nodes and individual vias connecting the resulting network to electrode pads were written and deposited. Network quantifying simulations revealed a small-world coefficient of 2.89, shortest path length of 3.61 and clustering coefficient of 0.057. After electroforming, the core-shell structures exhibited switching between LRS and HRS at ±7 V. In this presentation, the fabrication process and the memristive characteristics (endurance, resistance retention, pulse measurements, etc.) of the fabricated nanowire network structures will be discussed in detail.