2D Nanovaristors at Grain Boundaries Account for Memristive Switching in Polycrystalline BiFeO₃

Abstract

Memristive switching in polycrystalline materials is widely attributed to the formation and rupture of conducting filaments, believed to be mediated by oxygen‐vacancy redistribution. The underlying atomic‐scale processes are still unknown, however, which limits device modeling and design. Here, experimental data are combined with multiscale calculations to elucidate the entire atomic‐scale cycle in undoped polycrystalline BiFeO3. Conductive atomic force microscopy reveals that the grain boundaries behave like 2D nanovaristors, while on the return part of the cycle, the decreasing current is through the grains. Using density‐functional‐theory and Monte Carlo calculations, the atomic‐scale mechanism of the observed phenomena is deduced. Oxygen vacancies in nonequilibrium concentrations are initially distributed relatively uniformly, but they are swept into the grain boundaries by an increasing voltage. A critical voltage, the SET voltage, then eliminates the barrier for hopping conduction through vacancy energy levels in grain boundaries. On the return part of the cycle, the grain boundaries are again nonconductive, but the grains show nonzero conductivity by virtue of remote doping by oxygen vacancies. The RESET voltage amounts to a heat pulse that redistributes the vacancies. The realization that nanovaristors are at the heart of memristive switching in polycrystalline materials may open possibilities for novel devices and circuits.

Highly Itinerant Atomic Vacancies in Phosphorene

“…For instance, mobile vacancies accumulated in regions of filament or grain boundaries in memresistive semiconductors are the underlying origin for resistive switching devices. [46][47][48][49] However, fabrication based on either conventional oxides 46,47 or recently developed anion-deficient 2D layered transition metal dichalcogenides [48][49][50] requires a properly aligned conducting filaments or grain boundaries connecting the electrodes, which clearly complicates the fabrication of those devices. Moreover, as shown in resistive switching devices based on oxide bulk materials and 2D MoS 2 , [48][49][50] aggregative vacancies are critically important for the formation of filaments, which are highly desired for highperformance resistive switching devices.…”

High On/Off Ratio Memristive Switching of Manganite/Cuprate Bilayer by Interfacial Magnetoelectricity

“…The memristive behavior of TMO films is often attributed to the diffusion of oxygen vacancies . This mechanism has also been invoked in the case of TMO bilayer and trilayer structures such as PCMO/YBCO junctions .…”

“…However this mechanism may be absent as the LCMO is a poor conductor and the PBCO barrier limits the current, which is further reduced when the bilayer is at the high resistance state. Recombination‐enhanced diffusion can enable the diffusion at low temperature and also can cause memristive behavior of oxides, but this effect is also unlikely present in the manganite/cuprate bilayers as there is no non‐equilibrium concentration of electrons and holes and thus no carrier recombination.…”

Resistive Switching in Nanodevices

“…Electronic mechanisms describe the switching as electrons being trapped and un-trapped inside the insulator storage medium, such that the differently charged systems would exhibit different resistivities [11][12][13]. Ion drift mechanisms, on the other hand, attribute the switching to a concerted diffusion of atoms/ions that leads to a controlled and reproducible formation and breaking of conductive filaments through the insulator storage medium [14][15][16][17][18][19][20][21][22]. The proposed electronic mechanisms suffer from the voltage-time dilemma [9] because the average lifetimes for the asserted metastable electron-trapping states are much shorter than typical memristor operation times, and ensueingly, such a system would spontaneously forget which resistive state it is in.…”