Young-Hwan Kim

A study on the UWB antenna with band-rejection characteristic

This paper proposes a novel ultra-wideband antenna that has the omni-directional pattern in azimuth and the band-rejection characteristic of the 5.15/spl sim/5.825 GHz band limited by IEEE 802.11a and HIPERLAN/2. The proposed antenna is the circular monopole type with two slits and fed by a CPW line, and it satisfies the UWB band, 3.1/spl sim/10.6 GHz for VSWR < 2.

Compositional investigation of liquid crystal alignment on tantalum oxide via ion beam irradiation

The homogeneously aligned liquid crystal display on Ta2O5 via ion beam (IB) irradiation was first embodied with controllability of pretilt angle depending on incident angle of the IB. As a result of x-ray photoelectron spectroscopic analysis, the intensity of Ta–O and O–Ta bondings as a function of incident angle behaved reversely with the pretilt angle and the lowest amplitude was observed at 45°. It revealed that the creation of pretilt angle was attributed to the irradiation of the IB by breaking Ta–O and O–Ta bonding so orientational order was generated by directional IB. Comparable electro-optical characteristics to rubbed polyimide were also achieved.

Surface reformation and electro-optical characteristics of liquid crystal alignment layers using ion beam irradiation

The surface modification characteristics of liquid crystal (LC) alignment layers irradiated with various argon (Ar) ion beam (IB) energies were investigated as a substitute for rubbing technology. Various pretilt angles were created on the IB-irradiated polyimide (PI) surfaces after IB irradiation, but the Ar ions did not alter the morphology on the PI surface, indicating that the pretilt angle was not due to microgrooves. The chemical bonding states of the IB-irradiated PI surfaces were analyzed in detail by x-ray photoelectron spectroscopy to verify the compositional behavior for the LC alignment. Chemical structure analysis showed that the alignment ability of LCs was due to the preferential reorientation of the carbon network due to the breaking of C=O double bonds in the imide ring parallel to the incident IB direction. The potential of applying nonrubbing technology to display devices was further supported by the superior electro-optical characteristics compared to rubbed PI.

High-performance ZnO thin-film transistor fabricated by atomic layer deposition

We report the fabrication and characteristics of a ZnO thin-film transistor (TFT) using a 50 nm thick ZnO film as an active layer on an Al2O3 gate dielectric film deposited by atomic layer deposition. Lowering the deposition temperature allowed the control of the carrier concentration of the active channel layer (ZnO film) in the TFT device. The ZnO TFT fabricated at 110 °C exhibited high-performance TFT characteristics including a saturation field-effect mobility of 11.86 cm2 V−1 s−1, an on-to-off current ratio of 3.09 × 107 and a sub-threshold gate-voltage swing of 0.72 V decade−1.

No-Bias-Bend Liquid-Crystal Display with an Intermediate Angle on Blended Polyimide via Ion-Beam Irradiation

We created a no-bias-bend (NBB) pi cell on a blended polyimide (PI) layer with intermediate pretilt angles using the ion-beam method. Various pretilt angles were easily obtained depending on the concentration of homeotropic PI. The plot of surface tension corresponded to the variation of the pretilt angle. We also observed a uniform alignment of the liquid crystals on the blended PI layer. The NBB pi cell had a low threshold voltage of 0.9 V and a fast response time of 5.4 ms.

Zinc oxide nanolevel surface transformation for liquid crystal orientation by ion bombardment

This paper introduces the characteristics of the zinc oxide (ZnO) inorganic film deposited by radio-frequency magnetron sputtering as an alternative alignment layer for liquid crystal display (LCD) applications. The crystalline structure related to the texture formation of ZnO (101¯3) was observed with a tilt angle of approximately 28.1° to the ZnO (0001) plane, leading to a smooth surface and high-density structure. Ion beam (IB) bombardment at various incident angles was used to induce liquid crystal (LC) alignment and cause the measured pretilt angle on ZnO films to assume a triangular contour. The orientation order of liquid crystal molecules was due to the van der Waals force for the vertical alignment of LCs with selective breaking of O–Zn bonds by IB bombardment. The contact angle contour as a function of the IB incident angle resembled the behavior of the pretilt angle. The pretilt angle is controllable by adjusting the surface features on ZnO films with IB bombardment. The electro-optic character...

Ultra wideband tapered slot antenna with band-stop characteristic

This paper proposes a novel ultra wideband (UWB) tapered slot antenna (TSA) to achieve both enhanced impedance bandwidth for UWB RF front-ends and band-stop characteristics in the 5 GHz WLAN band, limited by IEEE802.11a and HIPERLAN/2. To obtain these two characteristics of wideband and band-stop in the TSA, a broadband microstrip-slotline transition with multi-arm stubs and /spl lambda//4 short stubs are used respectively. From the measured results, it can be shown that a wide bandwidth of 1.99 octaves from 2.8 to 11.09 GHz for a VSWR<2 is obtained while the 5.0 /spl sim/5.93 GHz band is rejected.

UWB wide slot antenna with band-notch function

A novel UWB wide slot antenna that has both enhanced impedance bandwidth enough to cover UWB systems and band-notch function of 5 GHz WLAN band limited by IEEE 802.11a and HIPERLAN/2 is proposed. To achieve these two characteristics of wide bandwidth and partial band-notch, a fork-shaped and tapered microstrip feed line and conductor lines in the slot are used, respectively. From the results, it is observed that wide bandwidth from 3.1 to 10.6 GHz for the VSWR<2 is achieved, while 5.15 ~ 5.825 GHz is band-notched

Superior optical properties of homogeneous liquid crystal alignment on a tin (IV) oxide surface sequentially modulated via ion beam irradiation

We first investigated the alignment characteristics of tin (IV) oxide (SnO(2)) thin films deposited by radio-frequency (RF) magnetron sputtering. This study demonstrates that liquid crystal (LC) molecules could be aligned homogeneously by controlling the Ion Beam (IB) irradiation energy densities. We also show that the pretilt angle of the LC molecules has a close relation with the surface energy. X-ray photoelectron spectroscopy (XPS) indicates that a non-stoichiometric SnO(2-x) surface converted by ion beam irradiation can horizontally align the LC molecules. The measured electro-optical (EO) characteristics showed high performance, comparable with those of rubbed and ion-beam irradiated polyimide (PI) layers.

Liquid Crystal Alignment Effects for Nematic Liquid Crystal on Homeotropic Polyimide Surface Using New Ion-Beam Source

We have studied the liquid crystal (LC) alignment and tilt angle generation of a nematic liquid crystal (NLC) on a homeotropic polyimide (PI) surface using a new ion beam source. The tilt angle of the NLC on the homeotropic PI surface for all incident angles is about 38° and this angle has a stabilization trend. A good LC alignment of the NLC on the homeotropic PI surface by to exposure ion beam of 45° incidence angle was observed. Also, the tilt angle of the NLC on the homeotropic PI surface by to exposure ion beam of 45° incidence angle had a tendency to decrease as ion beam energy density increased. Thus, we demonstrated that the tilt angle could be controlled from vertical to horizontal directions. Also, the alignment character of the NLC on the homeotropic PI surface with respect to ion beam energy was good at more than 1500 eV. Finally, a superior LC alignment thermal stability on the homeotropic PI surface subject to ion beam exposure was achieved.