Multiple slopes in the negative differential resistance region of NbOx-based threshold switches

Niobium oxide devices exhibit threshold switching behavior which enables their use as selectors in memory arrays or as locally active devices for neuromorphic computing. Among the basic dynamical phenomena appearing in non-linear circuits, the oscillations generated in a relaxation oscillator, which is making use of the negative differential resistance (NDR) effect of a threshold switching device, are of special significance for the design of neuromorphic electronic systems. Here, the necessary requirements for the emergence of oscillations of this kind in a simple relaxation oscillator circuit and their influence on the shape of the measured quasi-static I-Vm characteristic of the threshold switch are examined. In the corresponding experiments multiple NDR regions were found to appear in the quasi-static I-Vm characteristic of the threshold switch concurrently with the occurrence of oscillations. The observed 'multiple NDR phenomenon' is therefore merely a measurement artefact due to the averaging effect associated to the operating principles of the source measure unit (SMU) utilized to measure the device current and voltage. In this work, we analyzed how the emergence of oscillatory behavior in the relaxation oscillator depends upon the device layer stack composition. The probability of the appearance of oscillations within a large current range can be increased by decreasing the oxygen content in the sub-stoichiometric bottom layer of a niobium oxide bi-layer stack. It is shown that this trend is caused by the resulting decrease in the value of the product between thermal capacitance and thermal resistance of the threshold switching device. Furthermore, the changed stack composition reduces the variability and changes the forming voltage, which goes hand in hand with a change of the threshold voltage.