Won-Kyung Kim

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.

Stable high conductive amorphous InGaZnO driven by hydrogenation using hot isostatic pressing

We carried out hydrogen injection in amorphous InGaZnO (a-IGZO) by hot isostatic pressing (HIP) treatment under different pressures ranging from 10 to 1000 bar and investigated the stable site of hydrogen in a-IGZO. The HIP process efficiently injected hydrogen in the whole sample without the formation of indium clusters. Despite oxygen annealing, hydrogenated a-IGZO maintained a high electrical conductivity (n∼1019 cm−3 and μ∼16 cm2/V s) without any noticeable physical degradation. In this paper, we discuss the dependence of the preferential position of hydrogen in a-IGZO on the magnitude of pressure and its contribution on the electrical characteristics.

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.

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.

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.

Ferromagnetic spin ordering in amorphous Co-doped InGaZnO based on the Co–H–Co complex

We report on ferromagnetic spin ordering in amorphous Co-doped InGaZnO (Co-IGZO) based on hydrogen mediation. The amorphous structure was maintained after hydrogenation using hot isostatic pressing. Changes in the electrical and optical characteristics were attributed to interactions between hydrogen and each element in Co-IGZO. The ferromagnetism of hydrogenated Co-IGZO, induced in the amorphous phase without long-range ordering, was manifested by spin-spin interactions of the Co–H–Co complex acting as an individual magnetic unit, similar to a single molecular magnet. It is suggested that the electron carriers mediate the correlation between Co–H–Co units.

Magnetic domains in H-mediated Zn0.9Co0.1O microdisk arrays

In this study, we have fabricated and studied magnetic domains in the periodic H-mediated Zn0.9Co0.1O (H-ZCO) microdisk structures at room temperature with the MFM technique. The results of MFM show that the z-component of the remanent magnetic moment is uniform even though the value is much smaller than that of the saturation magnetic moment. In the investigation of H-ZCO microdisk arrays in different volumes, the magnetic domains observed on different sizes of the H-ZCO microdisks exhibited the same magnetic domain characteristics, perpendicular magnetization, regardless of the volume. Also, we confirmed that the ferromagnetism in the H-ZnCoO system is mediated by hydrogens with the MFM results of ZnCoO hydrogen injected and dehydrogenated.

Formation of ferromagnetic Co–H–Co complex and spin-polarized conduction band in Co-doped ZnO

Magnetic oxide semiconductors with wide band gaps have promising spintronic applications, especially in the case of magneto-optic devices. Co-doped ZnO (ZnCoO) has been considered for these applications, but the origin of its ferromagnetism has been controversial for several decades and no substantial progress for a practical application has been made to date. In this paper, we present direct evidence of hydrogen-mediated ferromagnetism and spin polarization in the conduction band of ZnCoO. Electron density mapping reveals the formation of Co–H–Co, in agreement with theoretical predictions. Electron spin resonance measurement elucidates the ferromagnetic nature of ZnCoO by the formation of Co–H–Co. We provide evidence from magnetic circular dichroism measurements supporting the hypothesis that Co–H–Co contributes to the spin polarization of the conduction band of hydrogen-doped ZnCoO.