Selective activation of memristive interfaces in TaO _x -based devices by controlling oxygen vacancies dynamics at the nanoscale

Abstract

The development of novel devices for neuromorphic computing and non-traditional logic operations largely relies on the fabrication of well controlled memristive systems with functionalities beyond standard bipolar behavior and digital ON-OFF states. In the present work we demonstrate for Ta O-based devices that it is possible to selectively activate/deactivate two series memristive interfaces in order to obtain clockwise or counterclockwise multilevel squared remanent resistance loops, just by controlling both the electroforming process and the (a)symmetry of the applied stimuli, and independently of the nature of the used metallic electrodes. Based on our thorough characterization, analysis and modeling, we show that the physical origin of this electrical behavior relies on controlled oxygen vacancies electromigration between three different nanoscopic zones of the active Ta 2 O 5−x layer: a central one and two quasi-symmetric interfaces with reduced TaO 2−h(y) layers. Our devices fabrication process is rather simple as it implies the room temperature deposition of only one CMOS compatible oxide-Ta-oxide-and one metal, suggesting that it might be possible to take advantage of these properties at low cost and with easy scability. The tunable opposite remanent resistance loops circulations with multiple-analogic-intermediate stable states allows mimicking the adaptable synaptic weight of biological systems and presents potential for non-standard logic devices.

Key Role of Oxygen-Vacancy Electromigration in the Memristive Response of Ferroelectric Devices

“…The latter figure can be interpreted (up to dimensional prefactors) as a HSL obtained under the assumption that the memristive effect relies exclusively on the modulation of both Schottky barriers by the switchable ferroelectric polarization. The curve is qualitatively similar to standard HSLs reported for symmetric memristive systems [19,38,55] in which the resistance switching is based on a completely different mechanism involving O-V electromigation.…”

“…However, a distinct feature is observed in both HSLs: the resistance of the top part of the "tables" presents a noticeable positive (negative) slope for negative (positive) voltages. This behavior is at odds with the one exhibited by (nonferroelectric) symmetric memristive complementary interfaces [19,38], where the "table with legs" HSLs exhibit a flat "table" (that is zero slope for both positive and negative voltages), as sketched by the horizontal dotted lines in Figs. 1(e) and 1(f).…”

“…In order to get further insight into the different scenarios and explain our experimental HLSs, we perform numerical simulations based on the voltage-enhanced O-V drift (VEOV) model, developed originally to describe nonferroelectric perovskites [19,42,43], further extended to simple oxide-based [38,44] memristive devices and here adapted to a symmetric metal-ferroelectric-metal system, as shown below.…”

“…metal-ferroelectric interfaces due to the presence of energy barriers [44] and disorder [38]. The only free parameter that is extracted from a quantitative fitting is R eff , as shown later.…”

On the key role of oxygen vacancies electromigration in the memristive response of ferroelectric devices

“…This behavior is at odd with the one exhibited by (non-ferroelectric) symmetric memristive complementary interfaces [17,32], where the "table with legs HSLs exhibit a flat "table " (that is zero slope for both positive and negative voltages), as it is sketched by the horizontal dotted lines in Figures 1(e) and (f). This quite unusual feature suggests that each interface resistance is modulated by the competition of ferroelectric memristive effects and the electromigration of OV.…”