Current Localization and Redistribution as the Basis of Discontinuous Current Controlled Negative Differential Resistance in NbO_x

Abstract

In-situ thermo-reflectance imaging is used to show that the discontinuous, snap-back mode of current-controlled negative differential resistance (CC-NDR) in NbO x -based devices is a direct consequence of current localization and redistribution. Current localisation is shown to result from the creation of a conductive filament either during electroforming or from current bifurcation due to the super-linear temperature dependence of the film conductivity. The snap-back response then arises from current redistribution between regions of low and high current-density due to the rapid increase in conductivity created within the high current density region. This redistribution is further shown to depend on the relative resistance of the low current-density region with the characteristics of NbO x cross-point devices transitioning between continuous and discontinuous snap-back modes at critical values of film conductivity, area, thickness and temperature, as predicted. These results clearly demonstrate that snap-back is a generic response that arises from current localization and redistribution within the oxide film rather than a material-specific phase transition, thus resolving a longstanding controversy.Current-controlled negative differential resistance (NDR) is observed in a wide range of amorphous transition metal oxides (e.g. TiO x 1 , TaO x 2 , VO x 3 and NbO x 4,5 ) and is being used to fabricate devices for brain-inspired computing, including: trigger comparators 6 , self-sustained and chaotic oscillators 7-10 , threshold logic devices 11,12 and the emulation of biological neuronal dynamics 13,14 . In their simplest form such devices consist of simple metal-oxidemetal structures and exhibit a smooth transition from positive to negative differential resistance under current-controlled operation (hereafter referred to as S-type NDR) due to a rapid increase in device conductance, as shown in Figure 1a. In general this can arise from electronic, thermal or a combination of electronic and thermal processes but for amorphous

Negative Differential Resistance Characteristics in Forming‐Free NbOx with Crystalline NbO2 Phase

“…Various transition metal oxides such as TiO x , TaO x , VO x , and NbO x were reported to show NDR. [ 6–18 ] Among these oxides, NbO x films particularly exhibit two types of NDR characteristics, namely, S‐type [ 8–16 ] and snapback, [ 15–20 ] under current‐source measurement. Recent studies suggest that the continuous S‐type NDR in amorphous NbO x films is attributed to the temperature‐dependent Poole–Frenkel (P–F) model with local Joule heating.…”

Engineering the Threshold Switching Response of Nb2O5-Based Memristors by Ti Doping

“…We have previously shown that complex compound CC-NDR characteristics can be simulated using a parallel memristor (core−shell) model that includes both filamentary conduction and conduction in the surrounding oxide film. 34,42,55 However, in the present case, conduction dominated by the electroformed filament, and as a consequence, the model can be reduced to a single resistive element, where the resistance is determined by Poole−Frenkel conduction and positive feedback from local Joule heating. 56 The resistance of the filamentary conduction path is then given by…”

“…While memristive switching has been observed in a range of materials and device structures, two terminal metal–oxide–metal (MOM) devices based on niobium oxides have attracted particular attention because of their simple structure, reliability, and diverse switching characteristics. − The latter include non-volatile resistive switching, volatile threshold switching, and a combination of resistive and threshold switching, with the specific response depending on the film stoichiometry, crystallinity, electrode metals, device geometry, and operating conditions (e.g., maximum currents, bias polarity, etc.,). ,,− For example, devices based on NbO 2 and non-stoichiometric NbO x typically exhibit volatile threshold switching, − while those based on Nb 2 O 5 exhibit unipolar (or non-polar) switching when inert (e.g., Au and Pt) top and bottom electrodes are used , and threshold switching when one of the inert electrodes is replaced by a reactive metal. , The dependence on electrode metal highlights the fact that switching is sensitive to interface reactions and field-induced oxygen exchange between the electrode and oxide. ,, Understanding such dependencies is critical for engineering devices with well-defined switching characteristics and remains an active area of research.…”

Neuromorphic Dynamics at the Nanoscale in Silicon Suboxide RRAM

“…However, our observations in Figure 1 demonstrate that the background may also be dynamic, as the whole scan area was modified by electrical stress. NbO x RRAM devices have been reported to conduct via a core (filament) in parallel with a shell (background) (Li et al, 2019;Nandi et al, 2019). The combination of two parallel channels can give rise to complex oscillatory dynamics-important for neuromorphic applications.…”