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.

Pulsed and quasi-static current-voltage (I-V) characteristics of threshold switching in TiN/TaO/TiN crossbar devices were measured as a function of stage temperature (200-495 K) and oxygen flow during the deposition of TaO. A comparison of the pulsed and quasi-static characteristics in the high resistance part of the I-V revealed that Joule self-heating significantly affected the current and was a likely source of negative differential resistance (NDR) and thermal runaway. The experimental quasi-static I-V's were simulated using a finite element electro-thermal model that coupled current and heat flow and incorporated an external circuit with an appropriate load resistor. The simulation reproduced the experimental I-V including the OFF-state at low currents and the volatile NDR region. In the NDR region, the simulation predicted spontaneous current constriction forming a small-diameter hot conducting filament with a radius of 250 nm in a 6 μm diameter device.

Oxide‐based resistive‐switching devices hold promise for solid‐state memory technology. Information encoding is accomplished by electrically switching the device between two nonvolatile states with low and high resistance states (LRS/HRS). It is generally accepted that the change between these states is due to the motion of oxygen vacancies forming a continuous (LRS) or gapped (HRS) filament between the electrodes. Direct assessments of filaments are rare due to their small size and the difficulty of locating the filament. Electron microscopy experiments reveal the filament structure and chemistry in TaO2.0 ± 0.2‐based 150 × 150 nm2 devices with cross‐sectional geometry after forming with power dissipation lower than 1 mW. The filaments appear to be roughly hourglass‐shaped with a diameter of less than 10 nm and are composed of Ta‐rich and O‐poor mostly amorphous material with local compositions as Ta‐rich as TaO0.4. The as‐formed HRS has a gap up to 10 nm wide located next to the anode and composed of nearly stoichiometric TaO2.5. The tantalum and oxygen distribution is consistent with filaments formed by the motion of both Ta and O driven by temperature gradients (Soret effect) and an electric field. This interpretation points towards a new compact model of resistive‐switching devices.