Jinsik Choi

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.

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.

The effect of K and Na excess on the ferroelectric and piezoelectric properties of K0.5Na0.5NbO3 thin films

We have fabricated K0.5Na0.5NbO3 (KNN) thin films on Pt substrates by a chemical solution deposition method and investigated the effect of K and Na excess (0?30?mol%) on ferroelectric and piezoelectric properties of KNN thin film. It was found that with increasing K and Na excess in a precursor solution from 0 to 30?mol%, the leakage current and ferroelectric properties were strongly affected. KNN thin film synthesized by using 20?mol% K and Na excess precursor solution exhibited a low leakage current density and well saturated ferroelectric P?E hysteresis loops. Moreover, the optimized KNN thin film had good fatigue resistance and a piezoelectric constant of 40?pm?V?1, which is comparable to that of polycrystalline PZT thin films.

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.

Observation of room-temperature ferroelectricity in tetragonal strontium titanate thin films on SrTiO3 (001) substrates

The authors investigated the ferroelectric properties of strontium titanate (STO) thin films deposited on SrTiO3 (001) substrates with SrRuO3 electrodes. The STO layer was grown coherently on the SrTiO3 substrate without in-plane lattice relaxation, but its out-of-plane lattice constant increased with a decrease in the oxygen pressure during deposition. Using piezoresponse force microscopy and P-V measurements, the authors showed that the tetragonal STO films possess room-temperature ferroelectricity. The authors discuss the possible origins of the observed ferroelectricity.

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.

Direct investigation on conducting nanofilaments in single-crystalline Ni/NiO core/shell nanodisk arrays

We report resistive switching characteristics of single-crystalline Ni/NiO core/shell nanodisk arrays, in which the conducting filaments are highly localized on the surface of nanostructure. The local current distributions observed in such a single-grained nanodisk demonstrate that the contact area and the contact time between the conductive tip of conducting atomic force microscopy and the surface of nanodisk critically influence the voltage-stress-induced electroforming behaviors of nanofilaments in NiO switching nanoblocks. These contact parameters, such as the contact area and the contact time, are interpreted to the electrode size and the voltage-stress time for the formation of filaments in metal oxides.

Defect-related room-temperature ferroelectricity in tensile-strained SrTiO3 thin films on GdScO3 (110) substrates

We investigate room-temperature (RT) ferroelectricity in tensile-strained SrTiO3 (STO) thin films grown on GdScO3 (110) substrates. To separate the strain and the defect dipole effect, we apply an electric field to measure the polarization in the direction perpendicular to the elongation axis, and the RT ferroelectric polarization is found to be perpendicular to that axis. These results clearly demonstrate the importance of the contribution of defect dipoles to the RT ferroelectricity observed in STO thin films.