Soon-Ju Kwon

High performance thin film transistor with low temperature atomic layer deposition nitrogen-doped ZnO

High performance thin film transistor (TFT) with atomic layer deposition (ALD) nitrogen doped ZnO (ZnO:N) as an active layer is demonstrated. The electrical properties of ZnO thin films were effectively controlled by in situ nitrogen doping using NH4OH as a source for reactants. Especially, the electron concentration in ZnO was lowered to below 1015cm−3. Good device characteristics were obtained from the inverted staggered type TFTs with ZnO:N channel and ALD Al2O3 gate insulator; μsat=6.7cm2∕Vs, Ioff=2.03×10−12A, Ion∕off=9.46×107, and subthreshold swing=0.67V∕decade. The entire TFT fabrication processes were carried out at below 150°C, which is a favorable process for plastic based flexible display.

Effect of interlamellar spacing on cementite dissolution during wire drawing of pearlitic steel wires

Cold drawing is an effective process to increase the strength of fully pearlitic steels with an acceptable level of ductility. Microstructural changes and deformation behavior of the pearlite during wire drawing are closely related to the initial microstructure of the pearlite. The main features of the changes while increasing drawing strain are a progressive alignment of lamellae along the drawing axis, a reduction of interlamellar spacing and a thinning of the lamellar cementite. Likewise, the behavior of the cementite dissolution would be influenced by microstructural features of the pearlite before drawing such as interlamellar spacing, colony size, etc. However, it is hard to find a report investigating the effect of microstructural features on the cementite dissolution during drawing. The purpose of this investigation is to examine the effect of microstructural features such as interlamellar spacing and colony size on the cementite dissolution during wire drawing, using Moessbauer spectroscopy, for fully pearlitic eutectoid steels.

Magnetic and electronic properties of transition-metal-substituted perovskite manganites—La0.7Ca0.3Mn0.95X0.05O3 (X=Fe,Co,Ni)

The magnetic and electronic properties of Fe-, Co-, Ni-substituted La0.7Ca0.3MnO3 were studied. Ferromagnetic–paramagnetic and metal–insulator transitions were significantly affected by Mn-site substitution although no observable differences were found in their crystal structures from x-ray diffraction analysis. Valence-band photoemission studies also showed that there were no appreciable changes in the electronic structures of each sample. Peak temperatures (Tp) at maximum resistivity decreased from 265 K of virgin LaCaMnO to 217, 203, and 171K for Ni-,Co-, and Fe-substituted LaCaMnO samples, respectively. As the Tp decreased, the electrical resistivity increased and the saturation magnetization decreased. The overall experimental results were well explained by considering superexchange interactions induced by substitution with magnetic ions having different magnetic moments. From our study, it is suggested that the magnetic properties of substituting elements should also be considered to deal with the p...

Multifunctional layer-by-layer self-assembly of conducting polymers and magnetic nanoparticles

Multifunctional thin films having both electrical conductivity and ferrimagnetic properties were successfully fabricated by the layer-by-layer self-assembling (LBL-SA) method, i.e. successive ionic adsorption of polypyrrole (PPy) and ferrite nanoparticles from their aqueous solutions. Using an ellipsometer, a constant increase of the film thickness by each deposition was characterized, implying a controllable process of the LBL-SA for multifunctional thin films. Uniformly distributed ferrite nanoparticles in the thin film were also observed in scanning and transmission electron microscopic images. The thin film consisting of six PPy and two ferrite nanoparticle layers had a conductivity of 0.18 S/cm and simultaneously showed a magnetic hysteresis.

Atomic Layer Deposition ZnO:N Thin Film Transistor: The Effects of N Concentration on the Device Properties

The electrical properties of atomic layer deposition (ALD) nitrogen-doped ZnO (ZnO:N) thin films were investigated as a function of incorporated nitrogen concentration. The nitrogen concentrations in the films were controlled by using different concentrations of NH 4 0H solution, which was used as a single source for the reactant and nitrogen doping for ALD ZnO:N. The carrier concentrations in ALD ZnO:N decreased down to 6.13 × 10 13 /cm 3 using 29% NH 4 OH solution. Thin film transistors (TFTs) were fabricated using ALD ZnO:N thin films with different N contents as active channel layers. The device properties were significantly changed by the amount of nitrogen incorporation due to the change in the electrical properties of ZnO:N films. Especially, threshold voltages were changed from 20.0 to 3.1 V by adjusting nitrogen doping. Additionally, dc bias stability was enhanced by the increment in nitrogen concentration, producing a robust TFT at high nitrogen incorporation. Finally, a high performance flexible TFT was fabricated using ALD ZnO:N as an active layer on poly(ethylene naphthalate) substrate.

Saturation magnetizations and Curie temperatures of CoZn Y-type ferrites

Abstract This work studies the σ s and T C of CoZn mixed Y-type ferrite (BaCo 1− x Zn x Fe 6 O 11 , x =0–1) powders, prepared by the co-precipitation method. The σ s, O K and T c change linearly with zinc content. CoY shows the highest T c (595 K) and the lowest σ s, 0 K (32.1 emu/g), while ZnY has the lowest T c (373 K) and the highest σ s, O K (73.6 emu/g). At room temperature, however, the maximum σ s appears at around x = 0.75 (34.9 emu/g). The presence of zinc weakens the superexchange interaction, without changing the spin directions of other cations, over the whole composition range. It simply increases the net magnetic moment.

Electronic, magnetic, and structural properties of L10FePtxPd1−x alloys

We present theoretical and experimental results on the electronic, magnetic, and structural properties of L10 FePd, FePt, and FePtxPd1−x alloys. These alloys have large magnetizations and magnetocrystalline anisotropies and as such are potentially technologically important for applications as permanent magnets or high-density storage media. Results of first principles electronic structure calculations show that the system is a strong ferromagnet with an almost full majority Fe band, and that magnetization and magnetocrystalline anisotropy remain large over the range of composition. Total energy calculations predict phase separation on the disordered Pd/Pt layer at low temperatures. Experimental studies using energy dispersion x-ray line scans confirm regions that are Pt rich and Pd rich forming at low temperatures.

Two-dimensional planar size effects in epitaxial PbTiO3 thin films

In order to test a critical lateral dimension in two-dimensional (2D) planar ferroelectrics, epitaxial PbTiO3 thin films are patterned into discrete islands lithographically with different lateral sizes. As the pattern size decreases, the substrate clamping effect is significantly reduced and thus the misfit strain in the films could be relaxed further. Evolution of 90° domain structures as a function of lateral dimensions was characterized extensively by reciprocal space mapping using synchrotron x ray. As the lateral 2D planar size decreases in the PbTiO3 patterns on MgO(001), some of the a domains turned into c domains due to the relaxed tensile strain. In the PbTiO3 patterns on Pt(001)∕MgO(001), on the other hand, the formation of 90° domains is enhanced by the reduction in compressive misfit strain. Equilibrium domain structures in the PbTiO3 thin film islands are also analyzed by the finite element simulation and found to be consistent with the experimental observation.

On the correlation between the hyperfine field and the particle size of fine goethite synthesized in chloride solution

Pure goethites of various particle sizes were synthesized in chloride solution by controlling the reaction rate. The particle size was measured by BET and TEM analyses. The samples were also characterized by transmission Mossbauer spectroscopy. This study provides the correlation between the hyperfine field at 25°C and the particle size of goethite which enables one to estimate the particle size from the Mossbauer spectroscopic analysis.

Hardness improvement of TiC-reinforced ferrous surface composites fabricated by high-energy electron beam irradiation

Metal matrix composites in general have excellent performance compared with metallic materials because of the ductile fracture behavior in addition to high strength and elastic modulus. Particularly, TiC-reinforced ferrous composites have highly stabilized TiC particles inside the matrix, and thus can be applied to structures requiring resistance to abrasion and corrosion and high-temperature properties. They are typically fabricated by powder metallurgy route, casting, liquid phase sintering, self-sustaining high-temperature synthesis, plasma spraying, and surface alloying using laser, but these methods usually demand complex processes to achieve homogeneous distribution of TiC particles. Recently, an attempt has been made in direct irradiation with high-energy electron beam in order to achieve surface hardening or surface alloying. In this study, a simple process was presented to fabricate a TiC-reinforced ferrous surface composite with enhanced surface hardness by depositing TiC powders on the surface of a plain carbon steel substrate and then irradiating high-energy electron beam.