Hyun-Mo Lee

Electrostatic actuation of microscale liquid-metal droplets

This paper reports sliding of micro liquid-metal droplets by electrostatic actuation for MEMS applications, bi-stable switching in particular. Basic theory concerning droplets on a plane solid surface is exposed followed by experimental study. Being a major parameter in the modeling of sliding droplets, the contact angle has been characterized in the case of mercury on an oxidized silicon wafer. The method used involves both traditional optical microscope and confocal laser imaging. The contact angle is found to be around 137/spl deg/ with an associated standard deviation of 8/spl deg/. The sample preparation is detailed. The droplets deposition method is based on selective condensation of mercury vapor on gold dots acting as preferred nucleation sites. This technique provides control of droplet dimensions and locations and is suitable for batch fabrication. Experimental study of electrostatic actuation coupled with finite-element method (FEM) analysis is described, leading to the determination of the sliding condition parameter, which represents a contact angle hysteresis of about 6/spl deg/. Experimental results also confirm the proportionality between minimum driving force and droplet dimension. Finally, a design optimization methodology is proposed, based on the use of finite-element model simulations.

Performance Improvement of Operating Three-Degree-of-Freedom Spherical Permanent-Magnet Motor

This paper describes performance of operating three-degree-of-freedom (3-D.O.F.) spherical permanent-magnet motor with a finite-element-analysis and experimental studies. Torque and efficiency comparison was made for two different models; the pure iron processed stator core model with a Guide-frame structure, and powder formed stator core model with a Guide-frame structure. As a post study to compensate the defect of the guide frame structure, a double-air-gap operating three-degree-of-freedom spherical permanent-magnet motor has been proposed. Double air-gap type has a single stator, double air-gap between the outer rotor and the inner rotor. Moreover, the double air-gap type, which does not have a guide frame, has a higher torque density, better efficiency, and greater applicability. Finally, the estimated performance has been proven via experiment conducted under the proposed efficiency measurement method.

Effect of mechanical stress on the stability of flexible InGaZnO thin-film transistors

ABSTRACT Demonstrated herein is the effect of mechanical stress on the device performance and stability of amorphous indium–gallium–zinc oxide thin-film transistors (TFTs) on a flexible polyimide substrate. Flexible TFTs were placed on jigs with various bending radii to apply different degrees of mechanical strain on them. When the tensile strain on the TFTs was increased from 0.19% to 0.93%, the threshold voltage shifted after a 10,000 s increase in bias–temperature–stress (BTS), under vacuum conditions. The BTS instability was further exacerbated when the device was exposed to the air ambient at a 0.93% strain. The device reliability deteriorated due to the increase in the subgap density of states as well as the enhanced ambient effects via the strain-induced gas permeation paths.

The resonant interaction between anions or vacancies in ZnON semiconductors and their effects on thin film device properties

Zinc oxynitride (ZnON) semiconductors are suitable for high performance thin-film transistors (TFTs) with excellent device stability under negative bias illumination stress (NBIS). The present work provides a first approach on the optimization of electrical performance and stability of the TFTs via studying the resonant interaction between anions or vacancies in ZnON. It is found that the incorporation of nitrogen increases the concentration of nitrogen vacancies (VN+s), which generate larger concentrations of free electrons with increased mobility. However, a critical amount of nitrogen exists, above which electrically inactive divacancy (VN-VN)0 forms, thus reducing the number of carriers and their mobility. The presence of nitrogen anions also reduces the relative content of oxygen anions, therefore diminishing the probability of forming O-O dimers (peroxides). The latter is well known to accelerate device degradation under NBIS. Calculations indicate that a balance between device performance and NBIS stability may be achieved by optimizing the nitrogen to oxygen anion ratio. Experimental results confirm that the degradation of the TFTs with respect to NBIS becomes less severe as the nitrogen content in the film increases, while the device performance reaches an intermediate peak, with field effect mobility exceeding 50 cm2/Vs.

Three-dimensional network of coaxial carbon nanotube/manganese oxides electrode for supercapacitors

A three-dimensional network of carbon nanotubes (3DNC) coaxially coated with manganese oxides (MnOx) is used as an electrode for supercapacitors. 3DNC serves as a stable and conductive framework for controlled electrochemical deposition (ECD), and provides sufficient voids for fast ionic transport and diffusion. The coated thin layer MnOx reduces ion diffusion and electron transport distance, enabling fast reversible faradic reactions.

Supreme performance of zinc oxynitride thin film transistors via systematic control of the photo-thermal activation process

Zinc oxynitride (ZnON) is a relatively novel class of material, often regarded as a promising alternative to oxide semiconductors, owing to its relatively high electron mobility and low concentration of oxygen-related defects that affect the device reliability. In the present study, thermal annealing of ZnON for thin film transistor (TFT) applications is performed in conjunction with a source of ultraviolet (UV) radiation, as an attempt to lower the heat treatment temperature. The oxygen radicals and ozone produced in this process appear to oxidize the ZnON surface. As the annealing temperature increases in the presence of UV light, chemically stable ZnO and non-stoichiometric ZnxNy bonds are formed without significant change in the oxygen/nitrogen ratio within the film. Such a phenomenon is accompanied by a slight reduction in the field effect mobility and device stability under positive bias stress, however under optimized photo-thermal annealing conditions, ZnON TFTs fabricated at a relatively low annealing temperature (150 °C) exhibit high field effect mobility values exceeding 50 cm2 V−1 s−1 and reasonable reliability, as examined under positive bias stress conditions.

Photothermally Activated Nanocrystalline Oxynitride with Superior Performance in Flexible Field-Effect Transistors

Photochemical reactions in inorganic films, which can be promoted by the addition of thermal energy, enable significant changes in the properties of films. Metaphase films depend significantly on introducing external energy, even at low temperatures. We performed thermal-induced, deep ultraviolet-based, thermal-photochemical activation of metaphase ZnOxNy films at low temperature, and we observed peculiar variations in the nanostructures with phase transformation and densification. The separated Zn3N2 and ZnO nanocrystalline lattice in amorphous ZnOxNy was stabilized remarkably by the reduction of oxygen defects and by the interfacial atomic rearrangement without breaking the N-bonding. On the basis of these approaches, we successfully demonstrated highly flexible, nanocrystalline-ZnOxNy thin-film transistors on polyethylene naphthalate films, and the saturation mobility showed more than 60 cm2 V-1 s-1.

Near-Infrared Photoresponsivity of ZnON Thin-Film Transistor with Energy Band-Tunable Semiconductor

Amorphous oxide semiconductors have attracted attention in electronic device applications because of their high electrical uniformity over large areas, high mobility, and low-temperature process. However, photonic applications of oxide semiconductors are highly limited because of their larger band gap (over 3.0 eV). Here, we propose low band gap zinc oxynitride semiconductors not only because of their high electrical performance but also their high photoresponsivity in the vis-NIR regions. The optical band gap of zinc oxynitride films, which is in the range of 0.95-1.24 eV, could be controlled easily by changing oxygen and nitrogen ratios during reactive sputtering. Band gap tuned zinc oxynitride-based phototransistors showed significantly different photoresponse following both threshold voltage and drain current changes due to variation in nitrogen-related defect sites.

Reduction of persistent photoconduction in Ge-Ga-In-O semiconductors by the incorporation of nitrogen

The effect of nitrogen incorporation in Ge-Ga-In-O (GGIO) semiconductors was investigated with respect to persistent photoconduction (PPC) and the associated thin-film transistor stability under negative bias illumination stress (NBIS). As the nitrogen partial pressure [pN2 = N2/(Ar + O2 + N2)] was increased from 0% to 40% during the reactive sputter growth of GGIO layers, the PPC phenomenon became less pronounced and higher device stability under NBIS was observed. X-ray photoelectron spectroscopy analyses suggest that the concentration of light-sensitive oxygen vacant sites in the GGIO semiconductors decreases as a result of nitrogen incorporation, hence the reduced PPC and higher device stability under NBIS.

Photoresponses of InSnGaO and InGaZnO thin-film transistors

The photoresponses of thin-film transistors (TFTs) using indium-based oxide semiconductors have been studied. The devices were fabricated using amorphous InSnGaO (ITGO) or InGaZnO (IGZO) as the active semiconducting layer. ITGO and IGZO TFTs showed typical electrical characteristics including high on/off ratios and low off currents. Both devices induced photocurrents upon exposure to ultraviolet light due to their wide band gaps. However, the recovery time of IGZO TFTs was almost 1 h due to the slow recombination of trapped charges in the oxide semiconductors. In contrast, the recovery time of ITGO TFTs was significantly reduced compared to that of IGZO TFTs. We found that the origin of the shorter recovery time of ITGO TFTs was the low electron binding energy of indium, which was obtained by replacing zinc with tin and by increasing the composition ratio of indium. This method may be a useful way to fabricate high-speed optoelectronics based on oxide semiconductors.