Han-Cheol Ryu

An analysis of “non-lattice” oxygen concentration effect on electrical endurance characteristic in resistive switching MnOx thin film

Electrical endurance characteristic of resistive switching MnOx thin film was investigated associated with various oxygen concentrations. From experimental results of various top electrode changing on the examined devices and oxygen concentration during the post-deposition annealing process, it was revealed that electrical endurance characteristic can be significantly improved by possessing high “non-lattice oxygen” concentration in resistive switching thin film and minimizing out-diffusion of oxygen during resistive switching. Finally, a 250 nm-diameter via-hole structure device, composed of TiN/MnOx/Pt, was fabricated and the promising electrical endurance and retention characteristics and the impressively narrow distribution of resistive switching operation parameters were confirmed in the MnOx thin film.

Electrically controllable terahertz square-loop metamaterial based on VO2 thin film

An electrically controllable square-loop metamaterial based on vanadium dioxide (VO2) thin film was proposed in the terahertz frequency regime. The square-loop shaped metamaterial was adopted to perform roles not only as a resonator but also as a micro-heater for the electrical control of the VO2. A dual-resonant square-loop structure was designed to realize band-pass characteristics in the desired frequency band. The measured Q-factors of the basic and scaled-down metamaterials fabricated on VO2 thin films were 2.22 and 1.61 at the center frequencies of 0.44 and 1.14 THz in the passbands, respectively. The transmittances of the proposed metamaterial were successfully controlled by applying a bias voltage without an external heater. The measured transmittance on-off ratios of the metamaterials were over 40 at the center frequencies in the passbands. In the future, electrically controllable terahertz metamaterial based on VO2 metamaterial could be employed as high-performance active filters or sensors.

Direct evidence on Ta-Metal Phases Igniting Resistive Switching in TaO(x) Thin Film.

A Ta/TaOx/Pt stacked capacitor-like device for resistive switching was fabricated and examined. The tested device demonstrated stable resistive switching characteristics including uniform distribution of resistive switching operational parameters, highly promising endurance, and retention properties. To reveal the resistive switching mechanism of the device, micro structure analysis using high-resolution transmission electron microscope (HR-TEM) was performed. From the observation results, two different phases of Ta-metal clusters of cubic α-Ta and tetragonal β-Ta were founded in the amorphous TaOx mother-matrix after the device was switched from high resistance state (HRS) to low resistance state (LRS) by externally applied voltage bias. The observed Ta metal clusters unveiled the origin of the electric conduction paths in the TaOx thin film at the LRS.

The interfacial layer effect on bi-stable resistive switching phenomenon in MnOx thin film

Resistive switching behavior of MnOx thin films was comparatively investigated. Two different top electrode materials of Ti and Pt over the common MnOx/Pt structure showed significantly distinct electrical endurance characteristics. Various structural and electrical analyses revealed that the interfacial oxide layer associated with Ti in the Ti/MnOx/Pt structure affected the improved electrical endurance characteristic. Finally, an 8 × 8 crossbar array with 100 nm-width interconnection line was fabricated to confirm the scalability and the stability of the resistive switching performances in the Ti/MnOx/Pt structure.

Discrimination of Cynanchum wilfordii and Cynanchum auriculatum by terahertz spectroscopic analysis

INTRODUCTION Precise identification of botanical origin of plant species is crucial for the quality control of herbal medicine. In Korea, the root part of Cynanchum auriculatum has been misused for C. wilfordii in the herbal drug market due to their morphological similarities. Currently, DNA analysis using the polymerase chain reaction (PCR) method is employed to discriminate between these species. OBJECTIVE In order to develop a new analytical tool for the rapid discrimination of C. wilfordii and C. auriculatum, terahertz (THz) spectroscopy was employed. METHODOLOGY Authentic samples of C. wilfordii and C. auriculatum were provided from the National Institute and standardized pellets for each species were prepared to get optimum results with terahertz time-domain spectroscopy (THz-TDS) in frequency range 0.2-1.20 THz. RESULTS The C. wilfordii pellet showed longer time delay compare to the sample of C. auriculatum and this was due to the difference in permittivity. The pellet samples of C. wilfordii and C. auriculatum showed a permittivity difference of about 0.08 at 0.2-1.20 THz. CONCLUSION The experimental results indicated that THz-TDS analysis can be an effective and rapid method for the discrimination of C. wilfordii and C. auriculatum, and this application can be expanded for the discrimination of other similar herbal medicines.

Terahertz radiation based on fiber-pigtailed InGaAs photoconductive antenna pumped by 1030 nm mode-locked Yb-doped fiber laser

We present a terahertz (THz) radiation pumped by a passively mode-locked Yb-doped fiber laser using two fiber-pigtailed log-spiral-based low-temperature-grown (LTG) InGaAs photoconductive antenna (PCA) modules. The mode-locked fiber laser produces over 220 mW of the average optical power with positively chirped of 1.49 ps pulses. In order to generate THz radiation using the fiber-pigtailed PCA modules, the mode-locked optical pulses are pre-chirped with 538 fs using two diffraction gratings. We successfully achieved THz radiation over 2.0 THz using the pre-chirped pulses. We successfully observed the various absorption lines of water vapor dips in the free space of 120 mm.

Design of a Broadband Sleeve Monopole Antenna by Using Matching Characteristics of the Sleeve

The optimization design method for the broadband operation of the sleeve monopole was proposed to unify the multiple antennas essential to the multi-functional broadband wireless communication systems into one antenna. The structure of the sleeve part was optimized to enhance the impedance matching characteristics based on the theoretical analysis that sleeve part can works as the open stub. The thick monopole was used for the broadband operation. The radius of the sleeve and the permittivity of the dielectric inside the sleeve was optimized to enhance the impedance matching characteristics for the broadband operation. The optimized sleeve monopole having thick monopole shows broadband characteristics over 3:1 bandwidth, from 0.8 to 2.43 GHz, which is suitable for the commercial wireless communication system. The proposed broadband sleeve monopole can reduce multiple antennas essential to the multi-functional broadband systems to one antenna.