Jung Ho Yoon

Highly Improved Uniformity in the Resistive Switching Parameters of TiO2 Thin Films by Inserting Ru Nanodots

Limiting the location where electron injection occurs at the cathode interface to a narrower region is the key factor for achieving a highly improved RS performance, which can be achieved by including Ru Nanodots. The development of a memory cell structure truly at the nanoscale with such a limiting factor for the electric-field distribution can solve the non-uniformity issue of future ReRAM.

Study on the electrical conduction mechanism of bipolar resistive switching TiO2 thin films using impedance spectroscopy

The electrical conduction mechanism within a resistive switching TiO2 film in its bipolar high resistance state was examined by ac impedance spectroscopy and dc current-voltage measurements. Bipolar switching, which can be initiated from a unipolar high resistance state, was attributed to both modulation of the Schottky barrier height at the film-electrode interface and the electronic energy state in the film. Numerical fittings of the impedance data revealed two distinct RC domains in series, which were attributed to an interfacial barrier (activation energy ∼0.1 eV) and a nonconducting layer (activation energy ∼0.5 eV), respectively.

Pt/Ta2O5/HfO2−x/Ti Resistive Switching Memory Competing with Multilevel NAND Flash

Pt/Ta2 O5 /HfO2- x /Ti resistive switching memory with a new circuit design is presented as a feasible candidate to succeed multilevel-cell (MLC) NAND flash memory. This device has the following characteristics: 3 bit MLC, electroforming-free, self-rectifying, much higher cell resistance than interconnection wire resistance, low voltage operation, low power consumption, long-term reliability, and only an electronic switching mechanism, without an ionic-motion-related mechanism.

Memristive tri-stable resistive switching at ruptured conducting filaments of a Pt/TiO2/Pt cell

A tri-stable memristive switching was demonstrated on a Pt/TiO₂/Pt device and its underlying mechanism was suggested through a series of electrical measurements. Tri-stable switching could be initiated from a device in unipolar reset status. The unipolar reset status was obtained by performing an electroforming step on a pristine cell which was then followed by unipolar reset switching. It was postulated that tri-stable switching occurred at the location where the conductive filament (initially formed by the electroforming step) was ruptured by a subsequent unipolar reset process. The mechanism of the tri-stable memristive switching presented in this article was attributed to the migration of oxygen ions through the ruptured filament region and the resulting modulation of the Schottky-like interfaces. The assertion was further supported by a comparison study performed on a Pt/TiO₂/TiO(2-x)/Pt cell.

Improved endurance of resistive switching TiO2 thin film by hourglass shaped Magnéli filaments

A modified biasing scheme was adopted to improve the electrical endurance characteristics of conducting filamentary resistive switching (RS) in a Pt/TiO2/Pt RS cell. The modified bias scheme included the application of bias voltages with alternating polarity, even though RS proceeds in non-polar mode, which results in the stable distribution of each resistance states as well as improved endurance. This was attributed to the minimized consumption of oxygen ions in the TiO2 film, which can be induced by the formation of hourglass-shaped conducting filament (HSCF). The presence of a HSCF was confirmed by high-resolution transmission electron microscopy.

Real-time identification of the evolution of conducting nano-filaments in TiO2 thin film ReRAM

Unipolar resistance switching (RS) in TiO2 thin films originates from the repeated formation and rupture of the Magnéli phase conducting filaments through repeated nano-scale phase transitions. By applying the Johnson-Mehl-Avrami (JMA) type kinetic model to the careful analysis on the evolution of transient current in a pulse-switching, it was possible to elucidate the material specific evolution of the Magnéli phase filament. This methodology was applied to the two types of TiO2 films grown by plasma-enhanced atomic layer deposition (PEALD) and sputtering. These two samples have structurally and electrically distinctive properties: PEALD film exhibited high variability in switching parameters and required an electroforming while sputtered film showed higher uniformity without distinct electroforming process. The JMA-type kinetic analysis of the RS behaviors revealed that the rejuvenation of the filament is accomplished by repeated one-dimensional nucleation followed by a two-dimensional growth in PEALD samples, whereas one-dimensional nucleation-free mechanism dominates in sputtered films.

Role of Ru nano-dots embedded in TiO2 thin films for improving the resistive switching behavior

Ru nano-dots were embedded in a Pt/TiO2/Pt resistive switching cell to improve the uniformity of the switching parameters. The TiO2 film grown on the Ru nano-dots had a rutile structure locally whereas other parts of the TiO2 film had an anatase structure. The rutile-structured TiO2 formed conducting filaments easily and their rupture was much more uniform than the randomized ones in anatase TiO2. This largely improved the resistance uniformity at the reading voltage during the repeated resistance switching events. The improvement was also attributed to the high leakage current of the pristine sample at the reading voltage.

Schottky diode with excellent performance for large integration density of crossbar resistive memory

A Schottky diode (SD) with Au/Pt/TiO2/Ti/Pt stacked structure were fabricated for its application to crossbar type resistive switching (RS) memory. The SDs showed a highly promising rectification ratio (∼2.4 × 106 @ ±2 V) between forward and reverse state currents and a high forward current density (∼3 × 105 A/cm2 @ 2 V), which is useful for highly integrated crossbar RS memory. The SD has local forward current conduction paths, which provides extremely scaled devices with an advantage. The minimization of interconnection line resistance is also important to provide sufficient current to achieve stable operation of RS memory.

Scanning probe based observation of bipolar resistive switching NiO films

The switching mechanism of the bipolar resistive switching behavior on NiO films was examined using local probe based measurements. Unlike the unipolar switching normally observed on a metal-insulator-metal structure, repetitive bipolar switching was observed on NiO films when a local probe was used as the top electrode. Surface potential and current maps obtained after the anodic/cathodic bias application through the scanning probe both in air and under high vacuum suggested that the resistive switching is caused mainly by the electrochemical redox reaction at the electrode-film interface rather than by charge drift within the NiO film.

Understanding structure-property relationship of resistive switching oxide thin films using a conical filament model

The relations between the reset current IR, room temperature filament resistance R0, and third harmonic coefficient B0 were evaluated by a conical filament model. It was found that IR∼1/R0 when the filament is either very weak, where the filament is more conical, or quite strong, where the filament is more cylindrical. The physical implication of the B0 was also understood from the materials properties. The coherence between the model expectations for the bulkier conical filaments, typically found in TiO2, and the more random-network like filaments, typically found in NiO, suggests a common switching mechanism works in both materials.