Bo Soo Kang

Electrical manipulation of nanofilaments in transition-metal oxides for resistance-based memory.

The fabrication of controlled nanostructures such as quantum dots, nanotubes, nanowires, and nanopillars has progressed rapidly over the past 10 years. However, both bottom-up and top-down methods to integrate the nanostructures are met with several challenges. For practical applications with the high level of the integration, an approach that can fabricate the required structures locally is desirable. In addition, the electrical signal to construct and control the nanostructures can provide significant advantages toward the stability and ordering. Through experiments on the negative resistance switching phenomenon in Pt-NiO-Pt structures, we have fabricated nanofilament channels that can be electrically connected or disconnected. Various analyses indicate that the nanofilaments are made of nickel and are formed at the grain boundaries. The scaling behaviors of the nickel nanofilaments were closely examined, with respect to the switching time, power, and resistance. In particular, the 100 nm x 100 nm cell was switchable on the nanosecond scale, making them ideal for the basis for high-speed, high-density, nonvolatile memory applications.

Oxide double-layer nanocrossbar for ultrahigh-density bipolar resistive memory.

S. H. Chang , S. B. Lee , S. M. ang , Y S. C. Chae , H. K. oo , Y Prof. W Noh . . TReCFI, Department of Physics and Astronomy Seoul National University Seoul 151-747, Korea E-mail: twnoh@snu.ac.kr D. Jeon , . Y S. J. Park , Prof. G. Kim . TSchool of Electrical Engineering Korea University, Seoul 136-701, Korea Prof. B. S. Kang Department of Applied Physics Hanyang University Ansan, Gyeonggi-do 426-791, Korea Dr. M.-J. Lee Semiconductor Device Laboratory Samsung Advanced Institute of Technology Yongin, Gyeonggi-do 446-712, Korea

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.

Fractal Dimension of Conducting Paths in Nickel Oxide (NiO) Thin Films During Resistance Switching

A resistance-switching model in nickel oxide thin film is proposed based on Poisson distribution of electrical switching power. Conductive percolating network in soft breakdown surface may be the source of resistance switching. The main body of network may remain unchanged, but a portion of network is broken and healed repeatedly during switching. Dependence of reset current on electrode area is explained by fractal dimension.

Stabilizing the forming process in unipolar resistance switching using an improved compliance current limiter

The high reset current IR in unipolar resistance switching is a major obstacle to practical applications in memory devices. In particular, the first IR value after the forming process is so high that the capacitors sometimes do not exhibit reliable unipolar resistance switching. We find that the compliance current Icomp is a critical parameter for reducing IR values in polycrystalline Pt/NiOw/Pt, Pt/SrTiOx/Pt, Ti/SrTiOx/Pt, Pt/TiOy/Pt and Pt/FeOz/Pt capacitors, which show unipolar resistance switching. We therefore introduce an improved, simple and easy-to-use Icomp limiter that stabilizes the forming process by drastically decreasing the current overflow so as to precisely control the Icomp and subsequent IR values.

Dynamic switching mechanism of conduction/set process in Cu/a-Si/Si memristive device

The conduction/set processes of resistive switching have been systemically investigated for Cu/a-Si/Si electrochemical memristive devices. Experimental results indicate that the set process was driven by two different mechanisms, depending on the programming pulse amplitude: a purely electrical dielectric breakdown and a thermally assisted dielectric breakdown. For the latter process, we observe that the set time decreased exponentially with the increase in the programming pulse amplitude, whereas the former process shows amplitude independence. Through the temperature-dependent set transition characteristics, we argue that the filament growth in set process could be dominated by cation transport in the dielectric film. The thermal activation energy of Cu hopping in a-Si is extracted to be 0.16 eV.

Multilevel Programmable Oxide Diode for Cross-Point Memory by Electrical-Pulse-Induced Resistance Change

A multilevel one-time programmable (OTP) oxide diode for cross-point memory is introduced. The oxide diode is composed of a thin-film p-CuO/n-InZnOx (IZO). By applying negative electrical pulses, the p-CuO/n-IZO diode exhibited multilevel resistance states, and such characteristics of the p-CuO/n-IZO diode could be utilized as the cell of OTP cross-point memory. The resistance-change properties of the p-CuO/n-IZO diode originated possibly from a back-to-back diode phenomenon by oxygen ion migration in the IZO thin film.

Investigation for Resistive Switching by Controlling Overflow Current in Resistance Change Nonvolatile Memory

In recent years, resistance changes random access memory (RRAM) which shows reversible bistable resistance states by applied voltage has been studied as one of the alternatives of next-generation nonvolatile memory due to its excellent device characteristics including scalability, speed, and retention. Here, we report on the noncharge-based NiO RRAM device characteristics with load resistor as well as the simulation results of controlled conducting filament configuration. The RRAM device with load resistor showed super performances including highly reduction of switching current (~1 order) and significantly improved switching voltage distribution (30% reduction).

Effects of the fluctuation in a singly-connected conducting filament structure on the distribution of the reset parameters in unipolar resistance switching

The reset current (Ireset), voltage (Vreset), and resistance of the low resistance state, as functions of the compliance current (CC), were investigated in a Pt/NiO/Pt structure that showed unipolar resistance switching. Interestingly, the Ireset and the Vreset measured at low CCs were found to be widely distributed. In order to explain the behavior of the reset parameters for the singly-connected conducting filament (CF) structure, a simple model of CFs was employed whose width variation follows the Gaussian distribution. The wide distribution of the reset parameters can be attributed to the fluctuation in the number and/or the width of the CFs.

Mechanism of the reset process in bipolar-resistance-switching Ta/TaOx/Pt capacitors based on observation of the capacitance and resistance

The capacitance (C) and the resistance (R) were measured at various states as the reset process progressed in bipolar-resistance-switching Ta/TaOx/Pt thin film capacitors. The reset process was found to undergo three sequential stages where C and R showed different behavior: increasing C and constant R before an abrupt reset transition, the rapid increase of both C and R upon transition, and saturated C thereafter. These behaviors can be explained in terms of the annihilation of the oxygen vacancies followed by rupture of the conducting channels.