Electrical transport and thermometry of electroformed titanium dioxide memristive switches

Abstract

We investigated the electrical transport of electroformed titanium dioxide memristive switches from liquid helium to room temperatures in order to better understand their internal states. After electroforming, we observed a continuous transition between two distinct limiting behaviors: a nearly Ohmic "ON"-state and an "OFF"-state characterized by conduction through a barrier. We interpret our data in terms of a model in which the electroforming step creates a conducting channel that does not completely bridge the metal contacts on the titanium dioxide film. The switching then occurs as a result of voltage-induced changes in the oxygen vacancy concentration in the gap between the tip of the channel and the adjacent metal contact. We used the metallic resistivity of the conduction channel as an in situ thermometer to measure the local device temperature, thus revealing an important implicit state variable.

Intrinsic constrains on thermally-assisted memristive switching

“…2 can be used to estimate the radius of the conducting channel in a memristive device given Joule heating information deduced from the experimentally measured resistance increase [2,18] and/or information about the mobile species and reset transition dynamics (i.e. the activation energy for mobile ions and the reset switching time) [25].…”

“…For example, the internal temperature during the reset operation of various unipolar devices has been estimated by the increase in resistance of the high conductance (metallic) state to be over 1000 K in the case of NiO films [18]. Significantly less heating was observed during switching of bipolar TiO 2−x [2] and SrTiO 3 [11] devices, which may be due to a lower activation energy for the drift of mobile dopants (vacancies or ions) [24,25] that cause the resistance changes. (Note that elevated temperatures during switching are related to state retention and switching speed characteristics of the device since they provide significant nonlinearity of the ionic drift with applied voltage [25,28].)…”

Impact of electroforming polarity on <i>TiO</i>₂ based memristor

“…With the small CC of 70 uA in the experiments, the incomplete filament is formed and the non-linear I-V relation is induced. As a result, the switching region sites at the tunnelling gap between the electrode and the tip of the longest incomplete filament [18]. With regard to devices after positive electroforming, the gap stands near the bottom electrode.…”

Phenomenological modeling of memristive devices

“…1c) is smooth and features an effective voltage threshold, which justifies using 0.5 V bias for non-disturbing read, and convergence to zero for both very high and very low resistances. These features are likely related to Joule heating-assisted resistive switching [49], e.g., super-linear dependence of temperature increase on applied voltage, redistribution of dissipated power from active region to series resistance upon decrease in resistance [7], and decrease in total dissipated power when resistance increases. Instead of relying on accurate physical model, we use fitting functions sinh v…”

“…The physical explanation for linear behavior at small voltages is self-evident, while at high voltages, it is likely due to the dominant effect of series resistance in memristive devices [7]. Given G(v, w) from Eq.…”