Spontaneous current constriction in threshold switching devices

Jonathan M Goodwill,Georg Ramer,Brian D Hoskins,Andrea Centrone,Dasheng Li,James A Bain,Jabez J Mcclelland,Marek Skowronski,Georges Pavlidis

doi:10.1038/s41467-019-09679-9

Abstract

Threshold switching devices are of increasing importance for a number of applications including solid-state memories and neuromorphic circuits. Their non-linear characteristics are thought to be associated with a spontaneous (occurring without an apparent external stimulus) current flow constriction but the extent and the underlying mechanism are a subject of debate. Here we use Scanning Joule Expansion Microscopy to demonstrate that, in functional layers with thermally activated electrical conductivity, the current spontaneously and gradually constricts when a device is biased into the negative differential resistance region. We also show that the S-type negative differential resistance I–V characteristics are only a subset of possible solutions and it is possible to have multiple current density distributions corresponding to the same value of the device voltage. In materials with steep dependence of current on temperature the current constriction can occur in nanoscale devices, making this effect relevant for computing applications.

Highlights

Threshold switching devices are of increasing importance for a number of applications including solid-state memories and neuromorphic circuits
The domains are commonly referred to as current filaments; here we adopt the nomenclature of current density domains and/or current constriction to differentiate this effect from permanent filaments found in memory devices
Answering those questions became more important recently as the current constriction is thought to be the first step in the electro-formation process in resistive switching devices, which critically depends on temperature and temperature gradient distributions[10–14]

Summary

Introduction

Threshold switching devices are of increasing importance for a number of applications including solid-state memories and neuromorphic circuits. Long disputed questions in S-NDR type devices concern the conditions under which the uniform current would constrict into domains and the size of such constriction Answering those questions became more important recently as the current constriction is thought to be the first step in the electro-formation process in resistive switching devices, which critically depends on temperature and temperature gradient distributions[10–14]. Kumar and Williams revisited the issue and proposed that the stability is determined by the minimum internal energy of the device[10] These authors argued that the domains could only form as a transient state during the threshold switching event when the differential circuit resistance decreases to zero and the device transitions between two points of the I–V at a constant source voltage (VSOURCE). The characteristics have multiple values of current corresponding to one value of voltage, similar to S-

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Conclusion