Yoon-Jae Baek

Digital versus analog resistive switching depending on the thickness of nickel oxide nanoparticle assembly

The thickness-dependent digital versus analog resistive switching of nickel oxide (NiOx) nanoparticle assemblies was investigated in a Ti/NiOx/Pt structure. The NiOx nanoparticles were chemically synthesized with ∼5 nm diameter. The Ti/NiOx/Pt structure with assembly thickness of ∼60 nm exhibited the digital-type bipolar resistive switching. However, the assembly with a thickness of ∼90 nm presented analog resistive switching with gradually decreasing resistance when sweeping −V while increasing resistance after applying +V. Repeating −V pulses decreased the resistance sequentially, but the high resistance was restored sequentially by repeating +V pulses, which is analogous to the potentiation and depression of adaptive synaptic motion.

Multimode threshold and bipolar resistive switching in bi-layered Pt-Fe2O3 core-shell and Fe2O3 nanoparticle assembly

The bias-polarity dependent multimode threshold and bipolar resistive switching characteristics in bi-layered Pt-Fe2O3 core-shell and γ-Fe2O3 nanoparticles assembly were investigated. The Ti/Pt-Fe2O3-core-shell-nanoparticles (∼20 nm)/γ-Fe2O3-nanoparticles (∼40 nm)/Pt structure exhibited a threshold switching upon applying −V at Ti electrode. However, the filaments were formed at +V and subsequently ruptured at −V, featured to be bipolar switching. After rupturing filaments, it returned to threshold switching mode. The presence of core-shell nanoparticles facilitates the threshold switching either by temporary formation of filaments or enhanced charge transport. Also, the oxygen reservoir role of Ti electrode was essential to form stable filaments for bipolar switching.

Investigation of analog memristive switching of iron oxide nanoparticle assembly between Pt electrodes

The analog memristive switching of iron oxide (γ-Fe2O3) nanoparticle assembly was investigated. The γ-Fe2O3 nanoparticles were chemically synthesized with ∼10 nm in diameter and assembled to be a continuous layer as a switching element in Pt/nanoparticles/Pt structure. It exhibited the analog switching that the resistance decreased sequentially as repeating −V sweeps and pulses while increased as applying +V. The capacitance-voltage curves presenting hysteresis with flatband voltage shift and distortion of their shapes with respect to the applied voltage supported the redistribution of space charges in nanoparticle assembly that might induce resistive switching. The polarity-dependent analog resistance change proportional to pulse voltage, time, and number of pulses was analogy to potentiation and depression of adaptive synaptic motion.

Resistive switching characteristics of maghemite nanoparticle assembly on Al and Pt electrodes on a flexible substrate

Resistive switching characteristics of maghemite (γ-Fe2O3) nanoparticle assembly were investigated in structures of top-electrode (Al,Pt)/γ-Fe2O3-NPs (~ 30 nm-thick)/bottom electrode (Al,Pt) on a flexible polyethersulfone substrate. The assembled NP layer with Al electrodes showed both unipolar and bipolar switchings with abrupt resistance change in multiple levels associated with formation and sequential rupture of conducting filaments, which is ascribed to Fe enrichment by the interfacial reaction. On the other hand, the NP layer with Pt electrodes exhibited memristive switching with hysteresis in current–voltage characteristics dependent on bias polarity, gradually changing the resistance with respect to bias conditions, and preserved resistance until a new state was developed by subsequent biasing.

Analog and bipolar resistive switching in pn junction of n-type ZnO nanowires on p-type Si substrate

A pn junction consisting of n-type ZnO nanowires (NWs) on p-type Si substrate exhibits analog resistive switching dependent on the polarity of applied voltage before forming operation for the bipolar switching. The current-voltage curves of Ti/ZnO-NWs/ZnO-seed-layer/p+-Si substrate show diode characteristics with hysteresis in the reverse bias condition, presenting a gradually increasing and then saturated current with repeated voltage sweeps. The current is then further increased with sweeping –V and decreased during the subsequent +V sweep. This polarity-dependent analog switching remains the same during pulse measurement. The analog switching is thought to originate from gradual redistribution of oxygen vacancies, trapping and detrapping of charges in the ZnO NWs, which modulate the depletion width and space charge density. Consequently, the resistance of the pn junction is changed in an analog fashion. After the forming operation, bipolar switching is observed with a transition from high to low resistance states (SET) at +V and reverse transition (RESET) at –V, originating from the formation and rupture of filaments. These results demonstrate multiple features of the ZnO NWs based pn junction, including diode characteristics, analog-type resistive switching before forming operation, and digital-type bipolar switching after forming.

Voltage-dependent resistive switching characteristics in mixed layer consisting of γ-Fe2O3 and Pt-Fe2O3 core-shell nanoparticles

A mixed layer of γ-Fe2O3 and Pt-Fe2O3 core-shell nanoparticles between Ti and Pt electrodes exhibited both analog and digital bipolar resistive switching depending on applied voltage. The resistance sequentially reduced with repeated −1.5 V sweeps and pulses for 100 ms and increased as repeating +1.5 V. After forming at +3 V, digital bipolar switching was achieved with SET transition from high to low resistance at +1 to 2 V and reverse RESET transition at −1 V. Pulse operation at ±2 V led to identical bipolar switching, associated with facilitated interconnection of filament segments between Pt cores in nanoparticle layer.

Memcapacitive characteristics in reactive-metal (Mo, Al)/HfOX/n-Si structures through migration of oxygen by applied voltage

Memcapacitive characteristics were investigated in metal-oxide-semiconductor (MOS) structure of reactive electrode (Mo, Al) and hafnium oxide (HfOX) on n-type Si substrate. The capacitance-voltage curves exhibited sequentially changing capacitance with memory function as repeating voltage sweeps, featured the memcapacitive behaviors. The saturation capacitance was decreased by repeating +V sweeps, while barely changed by −V sweeps. Also, the capacitance-time curves disclosed the same tendency. However, the MOS structure with inert Pt electrode did not show the capacitance change. The memcapacitive behaviors were induced by the migration of oxygen ions from HfOX to reactive electrodes by applied voltage, which altered the permittivity of HfOX.

Analog Memristive and Memcapacitive Characteristics of Pt-Fe 2 O 3 Core-Shell Nanoparticles Assembly on p + -Si Substrate

Analog memristive and memcapacitive switching characteristics were investigated in Pt-Fe2O3 core-shell nanoparticles (NPs) assembly on p+-Si substrate. The Ti/NPs/p+-Si structure exhibited gradually changing resistance (memristive) and capacitance (memcapacitive) at the same time as repeating the application of voltage with respect to the polarity of voltage. As applying negative voltage at top Ti electrode, the resistance decreased and the capacitance increased due to the increase of diffusion capacitance at n-NPs/p+ -Si junction. On the other hand, applying the positive voltage increased resistance and decreased capacitance by increasing depletion width at the junction. The polarity-dependent resistance and capacitance changes are thought to be ascribed to the charging of the NPs assembly that alters the potential of the assembly. The concurrent analog memristive and memcapacitive characteristics also emulated the biological synaptic potentiation and depression motions, which is indicative of potential application to neuromorphic devices as well as analog nonvolatile memory and circuits.

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The memristor ratioed logic (MRL)-based NAND operation is demonstrated using a combination of two Ag/γ-Fe2O3 nanoparticles assembly/Pt memristors in series with one CMOS inverter. This letter finds the MRL operational frequency to be restricted due to an inconsistency between SET and RESET and by an imbalance of the resistance between the two memristors. It is also shown that optical tuning resistance is a promising technique for use in improving the operational frequency of the MRL operation. MRL-based NAND logic demonstrated in this letter manifests itself as a potential candidate for memristor-based logic compatible with CMOS digital/analog circuitry.

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Hysteresis and threshold switching characteristics were investigated in the indium-gallium-zinc-oxide (IGZO) thin-film-transistors (TFTs) with inserted Pt-Fe2O3 core–shell nanocrystals (NCs) layer between source/drain and IGZO channel. The output curves showed the hysteresis with threshold drain voltage and the transfer curves showed the hysteresis with the shift of threshold gate voltage. These hysteresis, threshold switching, and shift of threshold voltage in both output and transfer curves are caused by charging of inserted NCs. These unique features demonstrated the memory and on/off switching operation by controlling both threshold gate and drain voltages through charging NCs.