In this paper, we present an approach of measuring the SET kinetics of redox-based resistive memories at timescales below 100 ps. Automatic measurements with an RF pulse generator and a source measure unit allow the consecutive application of short electrical pulses and the precise detection of the device resistance. In addition, a statistical evaluation of the SET kinetics has been performed. By increasing the pulse duration in small steps, varying the pulse amplitude and collecting a comprehensive dataset, the transient resistance of a device can be determined at a picosecond timescale. With this setup, we measured the SET kinetics of two different valence change memory-based resistive switching oxides, namely, TaOx and ZrOx, between 50 ps and 250 ps. Two characteristic times were measured: the SET time, being the delay after which the transition to the low resistance state sets in, and the transition time, which is the timespan during which the resistance shifts from the high to the low resistive state. We measured SET times down to 50 ps and transition times below 15 ps for both materials. The intrinsic maximum switching speed is not reached yet, which is limited by the ion migration in the oxides, possibly corresponding to the phonon THz frequency. Although charging times and heating times potentially slow down the measured SET times, they still allow 50 ps writing times at voltages of less than 5.0 V.

This study investigates resistive switching in amorphous undoped and Cr-doped vanadium oxide thin films synthesized by sputtering deposition at low oxygen partial pressure. Two different volatile threshold switching characteristics can occur as well as a non-volatile bipolar switching mechanism, depending on device stack symmetry and Cr-doping. The two threshold switching types are associated with different crystalline phases in the conduction filament created during an initial forming step. The first kind of threshold switching, observed for undoped vanadium oxide films, was, by its temperature dependence, proven to be associated with a thermally triggered insulatorto-metal transition in a crystalline VO 2 phase, whereas the threshold switch observed in chromium doped films is stable up to 90 C and shows characteristics of an electronically induced Mott transition. This different behaviour for undoped versus doped films has been attributed to an increased stability of V 3þ due to the Cr 3þ doping (as evidenced by X-ray photoelectron spectroscopy analysis), probably favouring the creation of a crystalline Cr-doped V 2 O 3 phase (rather than a Cr-doped VO 2 phase) during the energetic forming step. The symmetric Pt/a-(VCr)O x /Pt device showing high temperature stable threshold switching may find interesting applications as a possible new selector device for resistive switching memory (ReRAM) crossbar arrays.

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Current-voltage characteristics of oxide-based resistive switching memories often show a pronounced asymmetry with respect to the voltage polarity in the high resistive state (HRS), where the HRS after the RESET is more conducting than the one before the SET. Here, we report that most of this HRS asymmetry is a volatile effect as the HRS obtained from a read operation differs from the one taken from the switching cycle at identical polarity and voltages. Transitions between the relaxed and the volatile excited states can be achieved via voltage sweeps, which are named subloops. The excited states are stable over time as long as a voltage is applied to the device and have a higher conductance than the stable relaxed state. Experimental data on the time and voltage dependence of the excitation and decay are presented for Ta/TaO/Pt and Ta/ZrO/Pt devices. The effect is not limited to one oxide or electrode material but is observed with different magnitudes (up to 10× current change) in several oxide systems. These observations describe an additional state variable of the memristive system that is controlled in a highly polarity dependent manner.