Chae-Ryong Cho

Hydrogen-induced ferromagnetism in ZnCoO

We have investigated experimentally the effects of n-type impurities such as Al and H on the magnetic properties of ZnCoO. The carrier density changes slightly upon the introduction of hydrogen but increases significantly in the case of Al. A measurement of magnetic circular dichroism indicates that, of the two impurities, only H induces ferromagnetism in ZnCoO. Our experimental data suggest that, unlike Al, H plays an important role in the enhancement of ferromagnetic spin-spin interactions that goes much beyond a carrier-mediated effect.

Structural reconstruction of hexagonal to cubic ZnO films on Pt/Ti/SiO2/Si substrate by annealing

It is well known that ZnO generally has the wurtzite structure. We report the growth of both hexagonal and cubic ZnO on Pt(111)/Ti/SiO2/Si substrate by a solution deposition. The wurtzite structure of ZnO film is enhanced up to the annealing temperature of 600 °C, and disappeared for annealing temperatures above 700 °C. The Pt(111)/Ti/SiO2/Si substrate is reoriented to hexagonal-Pt3Ti(004)/Ti/SiO2/Si when annealed at 700 °C and above due to the Ti out-diffusion and the ZnO thin film grown on the substrate has a cubic structure. The diffusion of Ti was evidenced by Auger electron spectroscopy measurements. From the photoluminescence measurement, the band gap of the wurtzite structure of ZnO film grown by the annealing below 600 °C was 3.37 eV, as is already known, but the cubic ZnO had a band gap of 3.28 eV, which suggests a zinc blende structure. The stability of the zinc blende structure on Zn2TiO4 layer was checked by the calculation of the lattice mismatch using the extended atomic distance mismatch mo...

Effects of Co-doping level on the microstructural and ferromagnetic properties of liquid-delivery metalorganic-chemical-vapor-deposited Ti1−xCoxO2 thin films

Polycrystalline Ti1−xCoxO2 thin films on SiO2 (200 nm)/Si (100) substrates were prepared using liquid-delivery metalorganic chemical vapor deposition, and the microstructure and ferromagnetic properties were investigated as a function of doped Co concentration. Ferromagnetic behaviors of polycrystalline films were observed at room temperature, and the magnetic and structural properties strongly depended on the Co distribution, which varied widely with doped Co concentration. The annealed Ti1−xCoxO2 thin films with x⩽0.05 showed a homogeneous structure without any clusters, and pure ferromagnetic properties of thin films are only attributed to the Ti1−xCoxO2 (TCO) phases. On the other hand, in case of thin films above x=0.05, Co1−xTix clusters having a soft magnetic (SM) property formed in a homogeneous Ti1−xCoxO2 phase, and the overall ferromagnetic (FM) properties depended on both FMTCO and SMCo–Ti. Co1−xTix clusters with about 150 nm size decreased the value of Hc (coercive field) and increased the satu...

Dielectric characterization of transparent epitaxial Ga2O3 thin film on n-GaN∕Al2O3 prepared by pulsed laser deposition

(2¯01)-oriented β-Ga2O3∕GaN thin films were epitaxially grown by pulsed laser deposition. These films have the specific in-plane orientation, which was confirmed by φ scans of Ga2O3 (111) and (3¯11) reflections. When oxygen flow rate was increased, the surface morphologies and roughness of β-Ga2O3 drastically changed. The β-Ga2O3∕GaN structure showed a stable and sharp interface and uniform elemental distribution in depth. The dielectric constant and memory window of β-Ga2O3∕GaN were about 13.9 and 0.50V for oxygen flow rate of 5SCCM (SCCM denotes cubic centimeter per minute at STP).

ZnO nanobarbed fibers: Fabrication, sensing NO2 gas, and their sensing mechanism

ZnO nanobarbed fibers (NBFs) are synthesized by the epitaxial growth of 1–D ZnO nanorods (NRs) on ZnO nanofibers (NFs) via electrospinning and chemical bath deposition. The change in resistance of the NBFs in the presence or absence of a gas was explained using the surface depletion model and modified grain boundary model, in which the junction points of the NFs and NRs are considered. The NBF structure showed extraordinary NO2 sensing performance for gas concentrations up to 30 ppb. This indicates that NBF structures have great potential for use as gas sensing materials.

THE FORMATION MECHANISM OF BARIUM TITANATE THIN FILM UNDER HYDROTHERMAL CONDITIONS

Barium titanate thin films were produced under the hydrothermal conditions, and their formation mechanism was investigated. The phase structure and surface morphology of the resultant films were directly dependent on the reaction conditions. The films that have a reaction time shorter than 8 h below the processing temperature of 150 °C in the 0.8 N Ba(OH)2 solution were composed of compounds with the component ratio m(Ba)/ m(Ti) < 1, and a flower-like surface morphology. With extended reaction time under the higher temperature, the films showed a multiphase structure and a mosaic or island-like surface morphology. In the case of 1.0 N, 1.5 N Ba(OH)2 solution, well-crystallized, monoperovskite BaTiO3 thin films having uniform, mirror-like, and visible-defectless surfaces were produced at a processing temperature of 180 °C after 24 h. The experimental results were explained by hypothesizing that the formation mechanism consists of a “dissolutions-crystallization process”.

Abnormal drop in electrical resistivity with impurity doping of single-crystal Ag

Resistivity is an intrinsic feature that specifies the electrical properties of a material and depends on electron-phonon scattering near room temperature. Reducing the resistivity of a metal to its potentially lowest value requires eliminating grain boundaries and impurities, but to date few studies have focused on reducing the intrinsic resistivity of a pure metal itself. We could reduce the intrinsic resistivity of single-crystal Ag, which has an almost perfect structure, by impurity doping it with Cu. This paper presents our results: resistivity was reduced to 1.35 μΩ·cm at room temperature after 3 mol% Cu-doping of single-crystal Ag. Various mechanisms were examined in an attempt to explain the abnormal behavior.

Uniform superhydrophobic surfaces using micro/nano complex structures formed spontaneously by a simple and cost-effective nonlithographic process based on anodic aluminum oxide technology

This paper presents a uniform micro/nano double-roughened superhydrophobic surface with a high static contact angle (CA) and low contact angle hysteresis (CAH). The proposed micro/nano complex structured surfaces were self-fabricated simply and efficiently using a very simple and low-cost nonlithographic sequential process, which consists of aluminum (Al) sputtering, anodization of the Al layer and pore widening, without specific equipment and additional subsequent processes. The wetting properties of the fabricated surfaces were characterized by measuring the static CAs and the CAHs after plasma polymerized fluorocarbon coating with a low surface energy. The measured static CA and CAH were 154 ± 2.3° and 5.7 ± 0.8°, respectively, showing that the fabricated double-roughened surfaces exhibit superhydrophobic behaviors clearly. In addition, the proposed double-scaled surfaces at a wafer-level exhibited uniform superhydrophobic behaviors across the wafer with an apparent CA and CAH of 153.9 ± 0.8° and 4.9 ± 1.3°, respectively.

Enhanced photocatalytic activity of TiO2 nanobarbed fibers treated with atmospheric pressure plasma using O2 gas

TiO2 nanobarbed fibers (NBFs) were prepared by growing rutile TiO2 nanorods on anatase TiO2 nanofibers via electrospinning and hydrothermal reaction processes. There was a large increase in the photocatalytic activity of O2-plasma-treated (OP)-TiO2 NBFs relative to that of the TiO2 NBFs; this is due to the hydroxylation of the surface of the TiO2 NBFs by OP treatment. The repeatability of the photocatalytic activity of the OP-TiO2 NBFs was found to be high and the decolorization rate after ten cycles was 88.3% of the initial value. These results indicate that OP-TiO2 NBFs have great potential for use as a photocatalyst.

Ferromagnetism of heteroepitaxial Zn1-xCuxO films grown on n-GaN substrates

Ferromagnetic Zn1-xCuxO (ZnO:Cu) films were epitaxially grown on n-GaN(0001)/Al2O3(0001) by radio-frequency magnetron sputtering at a substrate temperature as low as 700°C. X-ray diffraction θ-2θ scans, -scans, and surface morphology studies revealed the degree of crystallinity, epitaxiality, and grain size. The full-width at half-maximum of the peak for the ZnO:Cu films deposited on GaN was below 0.18° according to ω-scans. The chemical bonding states and depth profiles of the films were investigated by X-ray photoelectron spectroscopy (XPS) and with a glow discharge spectrometer (GDS). A magnetic property measurement revealed that Cu-doped ZnO films exhibit ferromagnetic behavior with a strong exchange interaction between sp-band carriers and localized d electrons at room temperature.