Ik-Su Byun

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.

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.

Friction Anisotropy–Driven Domain Imaging on Exfoliated Monolayer Graphene

Otherwise identical regions of supported graphene can be distinguished by changes in friction with sliding direction. Graphene produced by exfoliation has not been able to provide an ideal graphene with performance comparable to that predicted by theory, and structural and/or electronic defects have been proposed as one cause of reduced performance. We report the observation of domains on exfoliated monolayer graphene that differ by their friction characteristics, as measured by friction force microscopy. Angle-dependent scanning revealed friction anisotropy with a periodicity of 180° on each friction domain. The friction anisotropy decreased as the applied load increased. We propose that the domains arise from ripple distortions that give rise to anisotropic friction in each domain as a result of the anisotropic puckering of the graphene.

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.

Nanoscale Lithography on Monolayer Graphene Using Hydrogenation and Oxidation

Monolayer graphene is one of the most interesting materials applicable to next-generation electronic devices due to its transport properties. However, realization of graphene devices requires suitable nanoscale lithography as well as a method to open a band gap in monolayer graphene. Nanoscale hydrogenation and oxidation are promising methods to open an energy band gap by modification of surface structures and to fabricate nanostructures such as graphene nanoribbons (GNRs). Until now it has been difficult to fabricate nanoscale devices consisting of both hydrogenated and oxidized graphene because the hydrogenation of graphene requires a complicated process composed of large-scale chemical modification, nanoscale patterning, and etching. We report on nanoscale hydrogenation and oxidation of graphene under normal atmospheric conditions and at room temperature without etching, wet process, or even any gas treatment by controlling just an external bias through atomic force microscope lithography. Both the lithographically defined nanoscale hydrogenation and oxidation have been confirmed by micro-Raman spectroscopy measurements. Patterned hydrogenated and oxidized graphene show insulating behaviors, and their friction values are several times larger than those of graphene. These differences can be used for fabricating electronic or electromechanical devices based on graphene.

Resistive switching transition induced by a voltage pulse in a Pt/NiO/Pt structure

We have observed a switching transition between bistable memory switching and monostable threshold switching in Pt/NiO/Pt structure. Bistable memory switching could be changed to monostable threshold switching by applying a positive electrical pulse with height of 2 V and width between 10−2 and 10−4 s. The change is reversible by applying a negative electrical pulse with the same height and width. By considering polarity- and width-dependence of the switching transition and compositional difference on electrical properties in NiOx, we have proposed a model in which the migration of oxygen ions (O2−) is responsible for the switching transition in Pt/NiO/Pt structures.

Characteristics and Effects of Diffused Water Between Graphene and a SiO 2 Substrate

AbstractThe graphene/SiO2 system is a promising building block for next-generation electronic devices, integrating the high electromagnetic performance of graphene with the mature technology of Si-based electronic devices. It is well known that the electromagnetic performance of graphene/SiO2 is dramatically reduced by structural defects, such as wrinkles and folding, which are suspected to result from water droplets. Therefore, understanding water diffusion between graphene and SiO2 is required for controlling structural defects and thus improving the electromagnetic performance of this system. Although the behavior of water between graphene and atomically flat mica has been investigated, the characteristics and effects of diffused water between graphene and SiO2 remain unidentified. We have investigated water diffusion between monolayer graphene and SiO2 under high humidity conditions using atomic force microscopy. For a relative humidity of over 90%, water diffuses into graphene/SiO2 and forms an ice-like structure up to two layers thick. Liquid-like water can further diffuse in, stacking over the ice-like layer and evaporating relatively easily in the air causing graphene to wrinkle and fold. By similarly investigating water diffusion between graphene and mica, we argue that water-induced wrinkle formation depends on the hydrophilicity and roughness of the substrate.

Electrochemical growth and resistive switching of flat-surfaced and (111)-oriented Cu2O films

Flat-surfaced and fully (111)-oriented Cu2O films were grown through a chelate-assisted electrochemical approach. Based on key roles of chelating agent, the flat surface of films controlled over the columnar-grained growth was obtainable with a root-mean-square roughness value below 3 nm. Cu2O films treated by a rapid-thermal-annealing process at 200 °C exhibited unipolar switching I-V characteristics, presenting the bistable resistance states with a high resistance ratio (Roff/Ron) over 3 orders of magnitude and considerably stable switching properties within 100 switching cycles.

Giant and Stable Conductivity Switching Behaviors in ZrO2 Films Deposited by Pulsed Laser Depositions

ZrO2 films have been deposited on Pt/Ti/SiO2/Si substrates by pulsed laser deposition methods. Giant and stable conductivity switching behaviors with maximum on/off ratio of 106 and switching endurance of 105 times are observed in a typical Pt/ZrO2/Pt structure. The Pt/ZrO2/Pt structure exhibits two ohmic behaviors in the low-voltage region (V < 1.4 V) depending on the value of previously applied voltage and Schottky-type conduction in the high-voltage region (1.4 V< V <8.9 V). It seems that the conductivity switching behaviors result from the changes in both the Schottky barrier and the bulk conductivity controlled by applied voltages.

Different nonvolatile memory effects in epitaxial Pt/PbZr0.3Ti0.7O3/LSCO heterostructures

We found different nonvolatile memory effects between ferroelectric and resistive switching in Pt/PbZr0.3Ti0.7O3(PZT)/La0.5Sr0.5CoO3 (LSCO) heterostructures, depending on thickness of epitaxial PZT films. As the film thickness decreased below 34 nm, leakage and/or tunneling currents increased and hindered ferroelectric switching of films; alternatively, bipolar resistive switching was observed. Analysis using fitting plot on resistive switching behaviors suggested that variable Schottky barrier at the interface between Pt electrode and the film may be responsible for the different nonvolatile memory switching.