Spontaneous current constriction in threshold switching devices
Jonathan M. Goodwill, Georg Ramer, Dasheng Li, Brian D. Hoskins, Georges Pavlidis, Jabez J. McClelland, Andrea Centrone, James A. Bain, Marek Skowronski
Threshold switching devices exhibit extremely non-linear current-voltage characteristics, which are of increasing importance for a number of applications including solid-state memories and neuromorphic circuits. It has been proposed that such non-linear characteristics are associated with a spontaneous current flow constriction i.e. formation of high current density domains that are volatile and dissolve with the termination of bias. The size and density of such domains and the mechanism underlying their formation is currently a subject of intense 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 interpret this effect is due to the coupling between heat and charge flow by simulation using a finite element model. We also show that the negative differential resistance type I-Vs are only a subset of possible solutions and it is possible to have multiple current density distributions corresponding to the same values of source voltage. In materials with steep dependence of current on temperature, like in oxides exhibiting insulator-to-metal transition, the current constriction can occur in nanoscale devices making them relevant for computing applications.