Coexistence of Memristance and Negative Differential Resistance in a Nanoscale Metal‐Oxide‐Metal System

Abstract

Memristive devices are nonlinear dynamical systems [ 1 ] that exhibit continuous, reversible and nonvolatile resistance changes that depend on the polarity, magnitude and duration of an applied electric fi eld. The memristive properties of metal/ metal oxide/metal (MOM) materials systems were discovered [ 2 , 3 ] in the 1960s and studied extensively for decades without reaching a consensus [4][5][6] on the physical switching mechanism. Recent research revealed that memristive switching is caused by electric fi eld-driven motion of charged dopants that defi ne the interface position between conducting and semiconducting regions of the metal oxide fi lm. [7][8][9][10][11][12] There have also been multiple reports of current-controlled negative differential resistance (CC-NDR) in electroformed MOM devices since the early 1960s (e.g. oxides of V, [13][14][15][16][17] Nb, [ 18 , 19 ] Ta, [ 20 ] Ti, [20][21][22][23] and Fe [ 24 ] ), and there have been a variety of proposals for the physical mechanism. Current work presents persuasive evidence that CC-NDR in these materials is due to a Joule-heating induced metal-insulator transition (MIT). [ 25 , 26 ] When the device is locally self-heated past the critical MIT temperature the resistivity drops abruptly, which has an unstable positive feedback effect on the current and results in the formation of a metallic phase conductive fi lament, [ 15 , 16 ] a necessary condition [ 27 ] for bulk CC-NDR. Independent researchers have recently shown [ 28 , 29 ] that the Magnéli phase Ti 4 O 7 [ 30 , 31 ] can be present in electroformed fi lms of memristive TiO 2 and may act as the source and sink for oxygen vacancies during memristive switching. Ti 4 O 7 is also known to exhibit a metal insulator transition at 155 K, [32][33][34] which opens the possibility to study nanoscale devices that simultaneously exhibit both CC-NDR and memristance.As shown in Figure 1 a, we have observed that electroformed titanium dioxide MOM devices can simultaneously exhibit both memristance and CC-NDR when immersed in liquid He. Here we derive an analytical model for the coexistence of both phenomena, which can be described by two independent mechanisms: (a) at all temperatures the memristance is due to the fi eld-driven motion of oxygen vacancies, and (b) at low temperatures the CC-NDR is caused by an insulator-to-metal phase transition triggered by Joule heating [ 13 , 14 , 25 , 26 ] of an electroformed conduction channel, probably the Magnéli phase Ti 4 O 7 . [ 28 , 29 ] Additionally, we analyze the electrical oscillations that arise from the CC-NDR effect in order to characterize the dynamics of the MIT. Finally, we demonstrate a notable application of such a device: a tunable voltage-controlled oscillator with conversion effi ciency greater than 1% that is capable of injecting AC energy into nanoscale oxide-based circuits.Schematic diagrams and an equivalent circuit of the model are presented in Figure 1 . We have previously reported a model for memristance in TiO 2[ 35 ] that accounts for t...

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