Ji Hun Park

Cu Mesh for Flexible Transparent Conductive Electrodes

Copper electrodes with a micromesh/nanomesh structure were fabricated on a polyimide substrate using UV lithography and wet etching to produce flexible transparent conducting electrodes (TCEs). Well-defined mesh electrodes were realized through the use of high-quality Cu thin films. The films were fabricated using radio-frequency (RF) sputtering with a single-crystal Cu target—a simple but innovative approach that overcame the low oxidation resistance of ordinary Cu. Hybrid Cu mesh electrodes were fabricated by adding a capping layer of either ZnO or Al-doped ZnO. The sheet resistance and the transmittance of the electrode with an Al-doped ZnO capping layer were 6.197 ohm/sq and 90.657%, respectively, and the figure of merit was 60.502 × 10–3/ohm, which remained relatively unchanged after thermal annealing at 200 °C and 1,000 cycles of bending. This fabrication technique enables the mass production of large-area flexible TCEs, and the stability and high performance of Cu mesh hybrid electrodes in harsh environments suggests they have strong potential for application in smart displays and solar cells.

Fabrication of high-quality single-crystal Cu thin films using radio-frequency sputtering

Copper (Cu) thin films have been widely used as electrodes and interconnection wires in integrated electronic circuits, and more recently as substrates for the synthesis of graphene. However, the ultra-high vacuum processes required for high-quality Cu film fabrication, such as molecular beam epitaxy (MBE), restricts mass production with low cost. In this work, we demonstrated high-quality Cu thin films using a single-crystal Cu target and radio-frequency (RF) sputtering technique; the resulting film quality was comparable to that produced using MBE, even under unfavorable conditions for pure Cu film growth. The Cu thin film was epitaxially grown on an Al2O3 (sapphire) (0001) substrate, and had high crystalline orientation along the (111) direction. Despite the 10−3 Pa vacuum conditions, the resulting thin film was oxygen free due to the high chemical stability of the sputtered specimen from a single-crystal target; moreover, the deposited film had >5× higher adhesion force than that produced using a polycrystalline target. This fabrication method enabled Cu films to be obtained using a simple, manufacturing-friendly process on a large-area substrate, making our findings relevant for industrial applications.

Effects of Al doping on the magnetic properties of ZnCoO and ZnCoO:H

We investigated the effects of Al doping on ferromagnetism in Co-doped ZnO and the mechanisms that give rise to ferromagnetism in hydrogen-injected ZnCoO. The aim of this study was to determine whether the occurrence of ferromagnetism or the strength of the magnetization is related to the charge carrier mobility, charge carrier density, or the presence of defects in the crystal lattice. Al doping increased the carrier density, as well as the density of oxygen vacancies and the lattice strain; however, these physical properties were not related to the changes in magnetism. Al-doped and undoped ZnCoO showed an increase in ferromagnetism as a function of the hydrogen plasma treatment time. Al doping suppressed the hydrogen-mediated ferromagnetism in ZnCoO:H by trapping hydrogen via oxygen vacancies created by Al doping.

Hydrogen lithography for nanomagnetic domain on Co-doped ZnO using an anodic aluminum oxide template

Based on hydrogen-mediated ferromagnetism and a selective hydrogen exposure technique, i.e., hydrogen lithography, we attempted to produce magnetic domains in a paramagnetic host. Hydrogen lithography on Co-doped ZnO with an anodic aluminum oxide template was used to produce nanomagnetic domains in paramagnetic Co-doped ZnO. The domains showed in-plane magnetization with a head-to-tail configuration at room temperature, which is consistent with the object-oriented micro-magnetic framework simulations.

Analysis of oxygen vacancy in Co-doped ZnO using the electron density distribution obtained using MEM

Oxygen vacancy (VO) strongly affects the properties of oxides. In this study, we used X-ray diffraction (XRD) to study changes in the VO concentration as a function of the Co-doping level of ZnO. Rietveld refinement yielded a different result from that determined via X-ray photoelectron spectroscopy (XPS), but additional maximum entropy method (MEM) analysis led it to compensate for the difference. VO tended to gradually decrease with increased Co doping, and ferromagnetic behavior was not observed regardless of the Co-doping concentration. MEM analysis demonstrated that reliable information related to the defects in the ZnO-based system can be obtained using X-ray diffraction alone.

Hydrogen-induced anomalous Hall effect in Co-doped ZnO

The electrical transport characteristics and anomalous Hall effect (AHE) were investigated for a hydrogen-injected Co-doped ZnO thin film. Based on the measurements of resistivity and the Hall effect between 5 K and 300 K, the existence of Co-H-Co complexes was observed to introduce the AHE and enable the AHE to persist up to room temperature. The observed H-induced AHE originates from the asymmetric scattering of carrier hopping between the localized states driven by ferromagnetic Co-H-Co complexes, and a theoretical study using first-principle calculations supports the experimental results well. This large ferromagnetic response of charge carriers by the hydrogen-induced AHE on semiconducting oxides will stimulate the further investigation of room-temperature spintronic applications.

Control of magneto-transport characteristics of Co-doped ZnO by electron beam irradiation

Electron beam irradiation can be used to modify the physical properties of materials, but its effects have never been studied in Co-doped ZnO (ZnCoO), a promising magnetic semiconductor material. Here, we demonstrate that electron beam irradiation enables modification of the magneto-transport properties of ZnCoO. The electron beam irradiation cannot affect the electrical and magnetic properties of ZnCoO significantly, due to the insulating nature of pristine ZnCoO showing paramagnetic behavior. However, intentional hydrogen doping increases carrier concentration and induces ferromagnetism in ZnCoO, which allows ZnCoO to be affected by electron beam irradiation. As a consequence of s–d exchange interaction, hydrogen-doped ZnCoO shows positive magnetoresistance and anomalous Hall conductivity. Electron beam irradiation reduced the carrier concentration in hydrogen-doped ZnCoO by removing shallow donor-type hydrogen and consequently increasing s–d exchange interaction, which resulted in an increase in positive magnetoresistance and a decrease in the anomalous Hall conductivity. These findings demonstrate the novel applicability of electron beam irradiation for tailoring the physical properties of ZnO-based materials.

Study on the formation of magnetic nanoclusters and change in spin ordering in Co-doped ZnO using magnetic susceptibility

Manipulation of spin ordering in oxides without extrinsic contribution is an important issue in current spintronics. This study introduced a mechanism to ascertain the formation of magnetic clusters and the magnetism of a Co-doped ZnO system by temperature-dependent magnetic susceptibility measurements. The measurements demonstrate the possibility of detecting tiny portions of Co clusters that could be created from Co oxides that were undetectable in X-ray diffraction or transmission electron microscopy. By fitting the magnetic susceptibility results, we were also able to ascertain that pure Co-doped ZnO showed a negative Curie–Weiss temperature, implying the existence of antiferromagnetic ordering; however, the Curie–Weiss temperature was gradually increased by intentional hydrogen introduction, with results suggesting that the antiferromagnetic ordering declined proportionally to the hydrogen contents by hydrogen mediation in Co–Co spin ordering and induced ferromagnetism. These results provide a useful methodological approach as well as experimental clues for identifying the origin of magnetism.

Fabrication of ZnCoO nanowires and characterization of their magnetic properties

Hydrogen-treated ZnCoO shows magnetic behavior, which is related to the formation of Co-H-Co complexes. However, it is not well known how the complexes are connected to each other and with what directional behavior they are ordered. In this point of view, ZnCoO nanowire is an ideal system for the study of the magnetic anisotropy. ZnCoO nanowire was fabricated by trioctylamine solution method under different ambient gases. We found that the oxidation of trioctylamine plays an essential role on the synthesis of high-quality ZnCoO nanowires. The hydrogen injection to ZnCoO nanowires induced ferromagnetism with larger magnetization than ZnCoO powders, while becoming paramagnetic after vacuum heat treatment. Strong ferromagnetism of nanowires can be explained by the percolation of Co-H-Co complexes along the c-axis.

Gallium codoping for high visible and near-infrared transmission in Al-doped ZnO thin films for industrial-scale applications

Transparent conductive oxide (TCO) materials have been constantly studied and developed in laboratory research scale, but few of them were successfully connected to the industrial production. One of main reasons is a lack of reproducibility of the high TCO performance due to the completely different fabrication conditions, equipment, and scale between academic science and industry. In this work, the authors report high visible and near-infrared transmission in Zn(Al,Ga)O thin films that were fabricated under suitable industrial conditions by using a large-scale, high-throughput in-line sputtering system. Zn(Al,Ga)O exhibits sheet resistance of ∼8.05 Ω/sq. and average optical transmittance of 92.07% in the visible region, with a figure of merit that is three times higher than that of ZnAlO. Zn(Al,Ga)O also shows high near-infrared transmittance tunability of more than 10% without significant change in the visible light transmittance, where the difference is <1%. The authors reveal the significant roles of ...