Sun-Kyoung Seo

Reproducible resistance switching in polycrystalline NiO films

Negative resistance behavior and reproducible resistance switching were found in polycrystalline NiO films deposited by dc magnetron reactive sputtering methods. Oxygen to argon gas ratio during deposition was critical in deciding the detailed switching characteristics of either bi-stable memory switching or mono-stable threshold switching. Both metallic nickel defects and nickel vacancies influenced the negative resistance and the switching characteristics. We obtained a distribution of low resistance values which were dependent on the compliance current of high-to-low resistance switching. At 200°C, the low-resistance state kept its initial resistance value while the high-resistance state reached 85% of its initial resistance value after 5×105s. We suggested that the negative resistance and the switching mechanism could be described by electron conduction related to metallic nickel defect states existing in deep levels and by small-polaron hole hopping conduction.

Electrical observations of filamentary conductions for the resistive memory switching in NiO films

Experimental results on the bistable resistive memory switching in submicron sized NiO memory cells are presented. By using a current-bias method, intermediate resistance states and anomalous resistance fluctuations between resistance states are observed during the resistive transition from high resistance state to low resistance state. They are interpreted to be associated with filamentary conducting paths with their formation and rupture for the memory switching origin in NiO. The experimental results are discussed on the basis of filamentary conductions in consideration of local Joule heating effect.

Improvement of resistive memory switching in NiO using IrO2

For the development of resistive memory devices using NiO, improvements of several memory switching properties are required. In NiO memory cells with noble metal electrodes, broad dispersions of memory switching parameters are generally observed with continuous memory switchings. We report the improvements in minimizing the dispersions of all memory switching parameters using thin IrO2 layers between NiO and electrodes. The role of thin IrO2 layers on NiO growth and memory switching stabilization are discussed.

Conductivity switching characteristics and reset currents in NiO films

Conductivity switching phenomena controlled by external voltages have been investigated for various NiO films deposited by dc reactive sputtering methods. Pt∕NiO∕Pt capacitor structures with top electrodes of different diameters have showed increasing off-state current with the diameter of a top electrode and nearly the same on-state current independent of the diameter. Local conductivity switching behaviors have been observed in a series structure consisting of two Pt∕NiO∕Pt capacitors with different resistance values. By reasoning out conductivity switching mechanisms from the switching characteristics and introducing multilayers consisting of NiO layers with different resistance values, we have reduced the reset current by two orders of magnitude.

Electrode dependence of resistance switching in polycrystalline NiO films

We investigated resistance switching in top-electrode/NiO∕Pt structures where the top electrode was Au, Pt, Ti, or Al. For Pt∕NiO∕Pt and Au∕NiO∕Pt structures with ohmic contacts, the effective electric field inside the film was high enough to induce trapping or detrapping at defect states and thus resistance switching. For a Ti∕NiO∕Pt structure with well-defined Schottky contact at Ti∕NiO interface accompanied by an appreciable voltage drop, the effective electric field inside the NiO film was not enough to induce resistance switching. For an Al∕NiO∕Pt structure with a low Schottky barrier at the Al∕NiO interface, resistance switching could be induced at a higher voltage since the voltage drop at the Al∕NiO interface was not negligible but small.

Different resistance switching behaviors of NiO thin films deposited on Pt and SrRuO3 electrodes

We have compared resistance switching of NiO films deposited on Pt and SrRuO3 (SRO): unipolar switching in Pt/NiO/Pt and bipolar switching in Pt/NiO/SRO. Linear fitted current-voltage curves and capacitance-voltage results show that on- and off-states conductions in unipolar switching are dominated by inductive Ohmic behavior and Poole–Frenkel effect, respectively. However, the conductions of on- and off-states in bipolar switching follow capacitive Ohmic behavior and Schottky effect, respectively. Therefore, we infer that the mechanisms of the unipolar and bipolar switching behaviors in NiO films are related with changes in bulk-limited filamentary conduction and interfacial Schottky barrier, respectively.

Scaling behaviors of reset voltages and currents in unipolar resistance switching

The wide distributions of switching voltages in unipolar switching currently pose major obstacles for scientific advancement and practical applications. Using NiO capacitors, we investigated the distributions of the reset voltage and current. We found that they scaled with the resistance value Ro in the low resistance state and that the scaling exponents varied at Ro≈30 Ω. We explain these intriguing scaling behaviors and their crossovers by analogy with percolation theory. We show that the connectivity of conducting filaments plays a crucial role in the reset process.

The interlayer screening effect of graphene sheets investigated by Kelvin probe force microscopy

We report on the interlayer screening effect of graphene using Kelvin probe force microscopy (KPFM). By using a gate device configuration that enables the supply of electronic carriers in graphene sheets, the vertical screening properties were studied from measuring the surface potential gradient. The results show layer-dependence of graphene sheets, as the number of graphene layers increases, the surface potential decreases exponentially. In addition, we calculate the work function-related information of the graphene layers using KPFM.

Resistive memory switching in epitaxially grown NiO

Epitaxial NiO films have been fabricated on SrRuO3 films prepared on SrTiO3 single-crystal substrates. The x-ray diffraction spectra and transmission electron microscopy confirm the epitaxial growth of NiO with atomically flat surfaces on the SRO electrode. The I-V measurements of epitaxial NiO show the resistive memory switching behavior with a change in the polarity of the voltage bias, in contrast with the switching behavior of polycrystalline NiO by a single polarity. The I-V characteristics of epitaxial NiO prepared under various synthesis conditions and electrodes are presented, which suggests an important role of interfaces between NiO and electrodes on the resistive switching behavior.

Polarization dynamics and retention loss in fatigued PbZr0.4Ti0.6O3 ferroelectric capacitors

Short-time retention loss behaviors were investigated for fatigued Pt/PbZr0.4Ti0.6O3/Pt capacitors. In the short-time regime of t<1 s, fatigued capacitors showed a significant loss in retained polarization, which could be well described by a power-law function. This behavior was interpreted in terms of a superposition of polarization relaxations with a relaxation time distribution. The effects of the pulse voltage on the relaxation time distribution suggested that the retention loss should be activated by a depolarization field. As the fatigue stress was applied, the retention loss became worse. This effect can be explained in terms of the increase of the depolarization field, possibly due to the growth of an interfacial passive layer.