Gate Tuning of Synaptic Functions Based on Oxygen Vacancy Distribution Control in Four-Terminal TiO2\u2212x Memristive Devices

Zenya Nagata,Tsuyoshi Isaka,Takuma Shimizu,Akira Sakai,Tetsuya Tohei,Nobuyuki Ikarashi

doi:10.1038/s41598-019-46192-x

Zenya Nagata, Tsuyoshi Isaka + Show 4 more

Open Access

https://doi.org/10.1038/s41598-019-46192-x

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Journal: Scientific Reports	Publication Date: Jul 10, 2019
Citations: 11	License type: open-access

Affiliation: Osaka University, Nagoya University

Abstract

Recent developments in artificial intelligence technology has facilitated advances in neuromorphic computing. Electrical elements mimicking the role of synapses are crucial building blocks for neuromorphic computers. Although various types of two-terminal memristive devices have emerged in the mainstream of synaptic devices, a hetero-synaptic artificial synapse, i.e., one with modulatable plasticity induced by multiple connections of synapses, is intriguing. Here, a synaptic device with tunable synapse plasticity is presented that is based on a simple four-terminal rutile TiO2−x single-crystal memristor. In this device, the oxygen vacancy distribution in TiO2−x and the associated bulk carrier conduction can be used to control the resistance of the device. There are two diagonally arranged pairs of electrodes with distinct functions: one for the read/write operation, the other for the gating operation. This arrangement enables precise control of the oxygen vacancy distribution. Microscopic analysis of the Ti valence states in the device reveals the origin of resistance switching phenomena to be an electrically driven redistribution of oxygen vacancies with no changes in crystal structure. Tuning protocols for the write and the gate voltage applications enable high precision control of resistance, or synaptic plasticity, paving the way for the manipulation of learning efficiency through neuromorphic devices.

Highlights

By exploiting electrocoloring phenomena[34,35], we can visualize the oxygen vacancy distribution in TiO2−x, which affects the electrical properties of the memristive device: regions with a higher concentration of oxygen vacancies are colored and more conductive
In the case of the positive applied voltage, a colored region bridging T1 and T3 (T2 and T4) was observed, while the resistance between T1 and T3 decreased. This is likely because positively charged oxygen vacancies are repelled from T2 and T4 (T1 and T3), and accumulate around T1 and T3 (T2 and T4) owing to drift motion induced by the electric field under the positive voltage V2,4
Depending on the morphology of the colored region, the device can be configured to be in a low resistance state (LRS) or high resistance state (HRS), which correspond to the colored region bridging or separating T1 and T3, respectively

Summary

Introduction

Www.nature.com/scientificreports suggested the feasibility of a device with multilevel resistance on the basis of modification of the oxygen vacancy distribution, which would allow continuous weighting of synaptic devices for neuromorphic computing. Consecutive positive write pulses Vwrite are applied to T1 until the device resistance reaches a target value in HRS for the depression process.

Results

Conclusion