Understanding the Non-Volatile Resistive Switching Behavior of Lithium Titanium Oxide during Spinel to Rock Salt Conversion for Neuromorphic Computing

Abstract

Lithium based synaptic device research has been a growing new field in the last few years for enabling robust neuromorphic hardware systems. It can realize parallel analogue computation which is beyond the conventional von Neumann computing architecture which is slow, energy inefficient and expensive to execute complex tasks. In this study, we showcase the remarkable potential of Lithium Titanium Oxide (Li4Ti5O12) spinel, both computationally through Density of States (DoS) analysis using Density Functional Theory (DFT), and experimentally via direct current (DC) polarization investigation of electrochemically lithiated Li4Ti5O12 thin film, examining diverse states of discharge (SoD) in both in-plane and out-of-plane configurations. The observed conductivity jump spans up to six orders of magnitude, transitioning from the Li4Ti5O12 spinel phase to the fully lithiated Li7Ti5O12 rock salt phase. Raman Spectroscopy validates the transformation from the Li4Ti5O12 spinel phase to the Li7Ti5O12 rock salt phase. Moreover, the onset of increase in conductivity during this transformation is corroborated by changes in the oxidation state of Titanium (Ti), as explained with the help of X-Ray Photoelectron Spectroscopy. To actualize these findings, we engineered a three-terminal artificial synaptic device integrating Lithium Phosphorous Oxynitride (LiPON) solid-state electrolyte as the lithium ion source and conductor, while employing Lithium Titanium Oxide (Li4Ti5O12) spinel as the channel material. In device testing, we maintained a source-drain voltage (VSD) of 0.5 V, while sweeping the gate voltage (VG) from -3 V to 3 V, resulting in switching spanning up to 4 orders of magnitude, consistently replicable across multiple cycles. Widening the gate voltage window from ±3 V to ±7V induced an increase in conductance. Furthermore, non-volatile testing affirmed the sustained retention of the conductance state across all tested gate voltage windows, showcasing the device's stability over time. These compelling results portray a promising trajectory for the development of a robust synaptic device architecture reliant on lithium-based all solid-state materials, catalyzing the evolution of high-precision analogue neuromorphic computing systems.