Jeong In Han

Effect of Zinc/Tin Composition Ratio on the Operational Stability of Solution-Processed Zinc–Tin–Oxide Thin-Film Transistors

Variation of Zn:Sn atomic composition ratio in zinc-tin-oxide (ZTO) thin films induced a dramatic change in the microstructure and also strongly influenced the device performance and operational stability of ZTO thin-film transistors (TFTs). The large variation of threshold voltage shift under gate bias stress appears to be closely correlated to the excessive or deficient Sn content and the oxidation potentials of the metallic components as well as environmental effects. It is noted that the optimum Zn:Sn atomic composition ratio in ZTO films can improve the device performance and operational stability of the solution-processed ZTO TFTs.

Fast and Stable Solution-Processed Transparent Oxide Thin-Film Transistor Circuits

Fast and stable zinc-tin-oxide (ZTO) thin-film transistor (TFT) circuits were fabricated by simple and effective solution processing. The solution-processed ZTO TFTs have shown saturation mobility >;2.5 ± 0.29 cm2/V · s (W/L = 100/10 μm) and subthreshold slope <;0.4 ± 0.122 V/dec . The ZTO seven-stage ring oscillators have shown an oscillation frequency of 800 kHz with a supply voltage VDD = 60 V, corresponding to a propagation delay of <; 90 ns per stage. In addition, with appropriate passivation onto the semiconductor channel area, the circuits have shown relatively stable operation even at a gate and source/drain bias voltage of >; 50 V for several hours.

Effect of liquid crystal concentration on electro-optical properties of polymer dispersed liquid crystal lens for smart electronic glasses with auto-shading and auto-focusing function

Polymer dispersed liquid crystal lenses were prepared from a mixture of prepolymer (NOA 65) and E7 liquid crystal. The mixture of polymer dispersed liquid crystal was polymerized by ultraviolet (UV) curing in the polymerization induced phase separation process. With liquid crystal concentration, electro-optical properties of polymer dispersed liquid crystal lens devices including transmittance, driving voltage, response times, contrast ratio and slope of the linear region of the transmittance-voltage were measured and optimized for smart electronic glasses. The optimum concentration for polymer dispersed liquid crystal lens was NOA 65 of 40% and E7 liquid crystal concentration of 60%. This is the first report of the use of the polymer dispersed liquid crystal lens for smart electronic glasses with auto-shading and/or auto-focusing functions.

Effect of UV intensity on the electro-optical properties of polymer dispersed liquid crystal lens for smart electronic glasses

Polymer dispersed liquid crystal (PDLC) lenses were made from a mixture of prepolymer (NOA 65) and E7 liquid crystal (LC). The mixture of polymer dispersed in LC was polymerized by ultraviolet (UV) irradiation in the polymerization induced phase separation process. With varying UV curing intensity in this process, the electrooptical properties of PDLC lens device such as transmittance, driving voltage, response times, contrast ratio (C/R) and slope of the linear region of the transmittance-voltage were measured and optimized for application to smart electronic glasses with auto-shading and auto-focusing functions. The optimum UV intensity for the PDLC lenses was more than 580 µW/cm2. These results were improved compared to our previously reported data[1] for the application of these PDLC lenses to smart electronic glasses with auto-shading and/or auto-focusing functions.

Effect of cell gap on electro-optical properties of polymer dispersed liquid crystal lens for smart electronic glasses

Polymer dispersed liquid crystal (PDLC) lenses with a cell gap of 11 μm and 30 μm were made from a uniformly dispersed mixture of 40% prepolymer (NOA 65, Norland optical adhesive 65) and 60% E7 liquid crystal. PDLC’s mixture between two ITO coated glasses was polymerized by UV (ultraviolet) curing in the polymerization induced phase separation (PIPS) process. Decline of cell gap is a physical approach to improve the electrooptical properties, while cooling or doping of SiO2 nanoparticles is the microstructural approach to enhance the properties, because the electric field applied to the liquid crystal molecules in LC droplets is inversely proportional to the cell gap. A smaller cell gap significantly and effectively increases the electric field applied to PDLCD devices. The driving voltages and slope for the sample with a cell gap of 11 μm and 30 μm were drastically improved. The driving voltage and the slope of the linear region of PDLC lens with narrow cell gap of 11 μm were drastically enhanced compared to those of the samples with 30 μm cell gap and the cooled and doped samples. These improvements were due to the increase of the applied electric field. However, the response time and contrast ratio were deteriorated. It seems that this deterioration was caused by the sticking or fixing of liquid crystal molecules in LC (liquid crystal) droplets by the intensive electric field applied to the PDLC device.

Effects of oxide electron transport layer on quantum dots light emitting diode with an organic/inorganic hybrid structure

We report on the effects of an oxide semiconductor as an electron transport layer (ETL) on a quantum dots light emitting diode (QD-LED). To improve the properties of QD-LED, we optimized the process parameters for the deposition and post-annealing steps of an oxide ETL. When zinc tin oxide (ZTO) was deposited by radio-frequency magnetron sputtering in a gas mixture of argon and oxygen (Ar : O2 = 2 : 1) and then annealed under 760 Torr O2 for 10 min, our QD-LED showed improved luminescence characteristics. Additionally, to overcome the problem of non-uniform luminescence, we optimized the concentration and process conditions of colloidal quantum dot materials. Finally, we fabricated QD-LED devices with luminescence of 4,874 cd/m2 and luminous efficiency of 2.68 cd/A.

Fabrication of thermally evaporated Al thin film on cylindrical PET monofilament for wearable computing devices

During the initial development of wearable computing devices, the conductive fibers of Al thin film on cylindrical PET monofilament were fabricated by thermal evaporation. Their electrical current-voltage characteristics curves were excellent for incorporation into wearable devices such as fiber-based cylindrical capacitors or thin film transistors. Their surfaces were modified by UV exposure and dip coating of acryl or PVP to investigate the surface effect. The conductive fiber with PVP coating showed the best conductivities because the rough surface of the PET substrate transformed into a smooth surface. The conductivities of PET fiber with and without PVP were 6.81 × 103 Ω−1cm−1 and 5.62 × 103 Ω−1cm−1, respectively. In order to understand the deposition process of Al thin film on cylindrical PET, Al thin film on PET fiber was studied using SEM (Scanning Electron Microscope), conductivities and thickness measurements. Hillocks on the surface of conductive PET fibers were observed and investigated by AFM on the surface. Hillocks were formed and grown during Al thermal evaporation because of severe compressive strain and plastic deformation induced by large differences in thermal expansion between PET substrate and Al thin film. From the analysis of hillock size distribution, it turns out that hillocks grew not transversely but longitudinally.

Electrical characterization and thermal admittance spectroscopy analysis of InGaN/GaN MQW blue LED structure

Characterizations of InGaN/GaN-quantum wells based LED heterostructure were undertaken by static and dynamic electrical measurements at different temperatures. The analysis of the current-voltage (I-V) characteristics demonstrated different mechanisms involved in the current charge transport in the LED device. Experimental admittance spectra have been investigated in broad frequency range, at various temperature and different direct current biases. A specific extraction of the quantum well conductance, based on Nicollian and Goetzberger’s model related to interface state conductance in Metal-Insulator-Semiconductor structure, has shown the effect of the quantum structure on the electric transport, and hence a correlation between the I-V electrical characteristics and the admittance spectroscopy has revealed the different conduction mechanisms involved in the charge transport in the InGaN/GaN LED. Activation energies and carrier capture velocity obtained from Arrhenius plots, determined from the thermally activated quantum well conductance peaks which are revealed with the used model, have confirmed that quantum well parameters are related to the carrier emission from confined levels in quantum wells.

Enhanced Stability of All Solution-Processed Organic Thin-Film Transistors Using Highly Conductive Modified Polymer Electrodes

Enhanced stability of all solution-processed organic thin-film transistors (OTFTs) has been achieved by replacing metallic electrodes with glycerol-modified poly(3,4-ethylene dioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) polymer electrodes. The modified PEDOT:PSS showed a substantially low electrical resistivity of 6.4×10-3 Ω cm with improved environmental stability and water-resistant characteristics, which are crucial for highly reliable applications. Additionally, the modified PEDOT:PSS electrodes were highly stable under intense mechanical stress, allowing their application to flexible electronics. Particularly, all solution-processed flexible and transparent OTFTs with the modified PEDOT:PSS electrodes showed a field-effect mobility decrease of only 2.7% after a tensile mode mechanical fatigue test, while OTFTs with metallic electrodes showed a mobility decrease of 56.6% under identical test conditions.

Size effect on negative capacitance at forward bias in InGaN/GaN multiple quantum well-based blue LED

Size effect of InGaN/GaN multiple quantum well (MQW) blue light emitting diodes (LEDs), on electrical characteristics in forward bias voltage at high injection current in light emission regime, is observed to induce a substantial dispersion in the current density and normalized negative capacitance (NC) (i.e., capacitance per chip area). The correction of normalized NC by considering the LED p-n junction series resistance has been found to be independent of chip area size with lateral dimensions ranging from 100 µm × 100 µm to 400 µm × 400 µm. This fact, confirms that the inductive effect which is usually behind the NC apparition is homogeneously and uniformly distributed across the entire device area and hence the dispersive characteristics are not related to local paths. From the characteristics of NC dependence on temperature, frequency and direct current bias, a mechanism based on the electrons/holes charge carriers conductivity difference is proposed to be responsible for the transient electron-hole pair recombination process inducing NC phenomenon. Direct measurement of light emission brightness under modulated frequency demonstrated that modulated light output evolution follows the same behavioral tendency as measured in NC under alternating current signal modulation. Thus it is concluded that the NC is valuable information which would be of practical interest in improving the characteristics and parameters relevant to LED p-n junction internal structure.