Kyung Soo Suh

Charge-transfer nature in luminescence of YNbO4:Bi blue phosphor

Bismuth doped yttrium niobate (YNbO4:Bi) is a potential blue phosphor for field emission displays application. It is observed after introducing Bi ions that cathodoluminescence efficiency of YNbO4:Bi becomes lower than that of YNbO4. From the calculations of critical distance (RC) of energy transfer and Huang–Rhys parameter (SHR) of YNbO4:Bi, it is found that the energy transfer by a dipolar-type interaction is unlikely compared with that of a short-range interaction possible in the luminescence of YNbO4:Bi. Thus, it is believed that the luminescence property of YNbO4:Bi is mainly affected by the host lattice YNbO4 having self-luminescent NbO43− complex. By time-resolved spectroscopy, it is found that luminescence characteristics of activator Bi3+ in YNbO4:Bi shows a charge-transfer behavior.

Emission Band Shift of the Cathodoluminescence of Y2SiO5:Ce Phosphor Affected by Its Activator Concentration

The high-temperature phase (X2-phase) and the low-temperature phase (X1 phase) of a potential low-voltage blue phosphor Y2SiO5:Ce have been studied. It is observed that as the concentration of Ce increased, the emission band of the X2 phase was shifted to the longer wavelength, toward the emission band of the X1 phase, when the phosphor synthesis was performed under reduction atmosphere. This phenomenon seems to be related to a preferential site of Ce in Y2SiO5 as the Ce concentration is increased. It appears that when the number of a large-size ions, Ce3+, is increased under reduction atmosphere, their coordination number is affected. One of the cation sites (B1 site) in its high-temperature phase (X2 phase) is preferentially occupied, and shows emission similiar to that of the low-temperature phase (X1 phase).

Device Characteristics of Pentacene Dual-Gate Organic Thin-Film Transistor

We fabricated pentacene organic thin-film transistors (OTFTs) with a dual-gate structure, in which 300-nm-thick thermally grown SiO2 and 500-nm-thick parylene were used as a bottom-gate and a top-gate dielectric, respectively. The threshold voltage (Vth) of the dual-gate OTFT changed systematically with the application of voltage bias to the top-gate electrode. When voltage bias from -20 to 20 V was applied to the top-gate electrode, Vth changed from 9.4 to -9.3 V. The range of Vth shift in the dual-gate OTFT with a thin 10 nm pentacene layer was much wider than that with a thick 500 nm pentacene layer. This shift of Vth due to the body effect allows the change from an enhancement- to a depletion-mode transistor, which is beneficial for the fabrication of organic circuits.

Electrical and chemical properties of photo-cross-linked polymeric insulating materials

Photosensitive polymeric insulating films, which were prepared by mixing host polymers and a photosensitizer, exhibit high chemical resistance and excellent electrical properties. These polymer films formed by the spin-coating method were exposed through a mask to patterned radiation of UV light. The nonexposed areas of these films were dissolved by an appropriate solvent. Leakage current densities for these films are very low, approximately 5 ×10-7 ~3 ×10-8 A/cm2 at a bias of 10.0 V. These polymer insulators are particularly suitable for plastic-based electronic device applications.

Cathodoluminescence Change of YNbO4:Bi Phosphor after Acid Treatment

The reason for the cathodoluminescence (CL) change of YNbO4:Bi after acid treatment is determined. It is observed that after acid treatment, the CL intensity of YNbO4:Bi decreased to a value below that of the untreated one at excitation energies lower than 1 keV. However, YNbO4:Bi was brighter than the untreated one at an excitation energy of 1 keV or higher. Meanwhile, ZnGa2O4 was compared with YNbO4:Bi in terms of luminescence properties. After acid treatment, ZnGa2O4 showed the same trend as YNbO4:Bi at excitation energies lower than 1 keV. However, the luminescence intensity of ZnGa2O4 did not increase at an excitation energy of 1 keV or higher. It was found that surface modification using an acid increases the luminescence efficiency of YNbO4:Bi by realizing stoichiometric composition at the phosphor surface.

Compositional dependence of luminescence of lithium zinc gallate phosphor

The photoluminescence (PL) and cathodoluminescence (CL) characteristics of lithium gallate and lithium zinc gallate were investigated as a function of composition and firing temperature. As Li/Ga of lithium gallate decreased, the emission peak shifted from about 420 to 390 nm under ultraviolet (UV) excitation. Similar results were obtained under cathode ray (CR) excitation. PL spectra of lithium gallates fired at high temperature were most similar to those of Li-deficient lithium gallate. This result clearly indicates that lithium gallate fired at high temperature is Li deficient. The PL and CL emission spectra of lithium zinc gallate consisted of a single broadband under UV and CR excitation. The emission peak of the composition of (1−x)Li0.5Ga2.5O4–xZnGa2O4 shifted from 406 to 440 nm as x increased under UV excitation and the peak intensity decreased with x. However, the intensity increased with x under CR excitation. When the composition of lithium zinc gallate was 0.4Li0.5Ga2.5O4–0.6ZnGa2O4, the brigh...

Threshold voltage control of pentacene thin‐film transistor with dual‐gate structure

Abstract This paper presents a comprehensive study on threshold voltage (Vth) control of organic thin‐film transistors (OTFTs) with dual‐gate structure. The fabrication of dual‐gate pentacene OTFTs using plasma‐enhanced atomic layer deposited (PEALD) 150 nm thick Al2O3 as a bottom gate dielectric and 300 nm thick parylene or PEALD 200 nm thick Al2O3 as both a top gate dielectric and a passivation layer was investigated. The Vth of OTFT with 300 nm thick parylene as a top gate dielectric was changed from 4.7 V to 1.3 V and that with PEALD 200 nm thick Al2O3 as a top gate dielectric was changed from 1.95 V to ‐9.8 V when the voltage bias of top gate electrode was changed from ‐10 V to 10 V. The change of Vth of OTFT with dual‐gate structure was successfully investigated by an analysis of electrostatic potential.

Longevity of plastic-based organic light-emitting devices by means of protective oils as passivation layer

For the encapsulation of plastic-based organic light-emitting devices (OLEDs), an inert oil as a liquid passivation layer is applied on the periphery of the device. The electroluminescence (EL) and rate of degradation of such devices are examined to compare the electrical and emissive properties of the devices before and after forming the inert liquid passivation layer. The presence of the liquid layer firmly prevents the occurrence and growth of dark spots and enhances the lifetime of the devices in air. Driven at 27.45 mA/cm2 (Lo~1780 cd/m2), a driving lifetime of 195.0 h is achieved, which is affected by the type of inert oil used.

MOCVD growth of InGa(Al)AsInAlAs multilayer heterostructures for long wavelength DBRs

Abstract We have investigated the InGa(Al)As InAlAs multilayers grown on InP substrates by low-pressure MOCVD to obtain high-quality distributed Bragg reflectors (DBRs) operating at 1.55 μm. Primarily, our aim was to work on a trade-off between interface sharpness and crystal quality for optimal growth of the heterostructure. In situ laser beam reflectometry was used to monitor the real growth time of Bragg layers for precise and reproducible stop-band position. The In 0.53 Ga 0.47 As In 0.52 Al 0.48 As DBR grown at 650°C yields the optimized mirror performance. In order to avoid the band-edge absorption near 1.55 μm, a quaternary layer containing a small content of Al has been proposed. A more symmetric band shape and higher reflectivities of 86% for 10 periods and 99% for 25 periods have been achieved for 1.55 μm quaternary In 0.53 Ga 0.41 Al 0.06 As In 0.52 Al 0.48 As DBR.

Fabrication of Black/White Electronic Ink Using High Mobility Particles

Black and white electronic ink containing white nano particles with high mobility and organic black pigment particles dispersed in a dielectric fluid were prepared. A charge control agent affects the electrophoretic zeta potentials of the white particles, which show the maximum value of the zeta potential. The electronic ink panel fabricated with the charged white particles and the black particles exhibits a contrast ratio of more than 19:1 at 10 V.