Sung-Oong Kang

Large resistive switching in ferroelectric BiFeO3 nano-island based switchable diodes

O N The demand for non-volatile memory technologies that offer high speed, high storage density and low power consumption has stimulated extensive research into new functional materials and device physics. [ 1–5 ] Nano-ferronic devices based on multiferroic/ferroelectric materials have been emerging as nextgeneration nano-electronics, which deal with the interplay between ferroic orders (e.g. ferroelectricity and ferromagnetism) and electronic transport on the nanoscale. [ 6 ] Recent investigations into various multiferroic/ferroelectric materials have revealed remarkable polarization dependent electronic transport properties, which include signifi cant electroresistance effects in a switchable ferroelectric diode [ 7–11 ] and multiferroic/ferroelectric tunnel junctions (M/FTJs) [ 12–17 ] and intriguing charge conduction in ferroelectric domain/walls. [ 18 , 19 ] These conduction properties can be utilized for fast and non-destructive readout in emergent non-volatile memories such as resistive random access memory (RRAM) and memristor. [ 20 ] Especially, ferroelectric-resistive memories based on ferroelectric diode and tunnel junctions have demonstrated that it is possible to achieve high resistive ON/OFF ratio, high speed and low write power with a high reproducibility by controlling ferroelectric polarization. In a switchable ferroelectric diode, the Schottky-to-Ohmic contacts, forming at the interfaces between metal electrodes and semiconducting ferroelectric thin fi lms, are reversibly

Different fatigue behaviors of SrBi2Ta2O9 and Bi3TiTaO9 films: Role of perovskite layers

To investigate the role of the perovskite layers on fatigue behaviors, SrBi2Ta2O9(SBT) and Bi3TiTaO9 (BTT) films were prepared by pulsed laser deposition using 15% Bi-excess bulk targets. The SBT and the BTT films grown at the similar deposition conditions showed similar growth behaviors, electrical properties, and retention characteristics. However, these films showed very different fatigue behaviors. The difference should come from the oxygen stability in the perovskite layer. Our work demonstrates that oxygen stability of the perovskite layers, as well as the self-regulating adjustment of the Bi2O2 layers, should be considered in the search for new candidate materials for nonvolatile ferroelectric memory devices.

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.

Coexistence of bi-stable memory and mono-stable threshold resistance switching phenomena in amorphous NbOx films

Both bi-stable memory and mono-stable threshold switching are observed in amorphous NbOx films. In addition, the transition between memory and threshold switching can be induced by changing external electrical stress. Raman spectroscopy and transmission electron microscope data show that the NbOx film is self-assembled into a layered structure consisting of a top metal-rich region and a bottom oxygen-rich region. The volume ratio of the two regions depends on the film thickness. Our experimental results suggest that different characteristics of conducting filaments in the two regions result in thickness dependence of switching types and the transition between memory and threshold switching.

Unipolar resistive switching mechanism speculated from irreversible low resistance state of Cu2O films

We observed cathode-interfaced electroreduction and a variation of oxygen content in an irreversible low resistance state of highly oriented cuprous oxide (Cu2O) films. These local microstructural and stoichiometric changes in hard breakdown of film allow speculation on the unipolar resistive switching mechanism: formation of metallic filaments originating from generation/migration of oxygen ions (O2−) from the cathode and release of oxygen gas through the anode. Based on the as-proposed switching model, endurance switching properties could be modulated from tens up to over 104 switching cycles by controlling ambient gas or the interface between Cu2O and anode.

Memristor Behaviors of Highly Oriented Anatase TiO2 Film Sandwiched between Top Pt and Bottom SrRuO3 Electrodes

Bipolar resistive switching behaviors have been observed in Pt/TiO2/SrRuO3 structures whose TiO2 has a highly oriented anatase phase. The resistive switching behaviors reveal a strong dependence on the duration time of the switching pulse, top electrode size, and amplitude of the switching voltage. We have also analyzed the conduction mechanisms of each resistance state in both polarities. All the resistive switching characteristics of our Pt/TiO2/SrRuO3 structures can be explained by memristor behavior based on locally induced ion migration.

Effects of a Load Resistor on Conducting Filament Characteristics and Unipolar Resistive Switching Behaviors in a Pt/NiO/Pt Structure

With a simply modified circuit design connecting a Pt/NiO/Pt capacitor to a serial load resistor (RL), resistive switching (RS) characteristics are significantly improved. Distributions of current at low-resistance state and switching voltages during reset and set processes are much reduced, and RS endurance is much enhanced. Using percolation theory, we find that a multiply connected conducting filament (CF) is preferentially formed in a Pt/NiO/Pt capacitor serially connected to an RL of 1 kΩ. The multiply connected CF may provide uniform heat distribution to a Pt/NiO/Pt capacitor, resulting in reduced randomness during heat-assisted reset process and improved RS characteristics.

Fabrication and Memory Effect of Zr Nanocrystals Embedded in ZrO2 Dielectric Layer

A floating gate (FG) memory structure employing Zr nanocrystals (NCs) embedded in a ZrO2 dielectric layer was fabricated for the first time through a one-step process using dc-magnetron sputtering deposition. The unique FG memory stacked only with an NC-capping ZrO2 control oxide layer, without a tunneling oxide layer, was obtained under oxygen-deficient conditions during the sputtering process. Transmission electron microscopy (TEM) revealed that crystalline Zr-NCs with sizes ranging from 3 to 8 nm were embedded in the ZrO2 layer with densities of Zr-NC up to 1.22 ×1012 cm-2. Capacitance versus voltage (C–V) curves of a Zr-NC-based metal–oxide–semiconductor (MOS) capacitor were also obtained. The clockwise C–V hysteresis loops on the basis of the Zr-NC-based MOS structure fabricated in this work were understood by the electron trapping of Zr–O dangling bonds at the interface between Zr-NCs and the ZrO2 capping layer.