발간논문

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CaO-added Mg alloys were cast in air, hot extruded to thin plates, and oxidized in air at high temperatures. During the casting process, the CaO was decomposed into Ca in the α-Mg matrix and formed Al2Ca precipitates along grain boundaries. A thin, nonuniform CaO-rich layer was formed on the surface during the casting and oxidation. Based on this study, the oxidation mechanism of the CaO-added Mg alloys during casting and oxidation is proposed. (Received August 25, 2015)

keyword : alloy, casting, oxidation, thermal analysis, magnesium

The aim of this study was to reinforce Al-Si/SiCp metal matrix composites (MMCs), which have homogeneous microstructures, by adding the Ti fibers and the Al-Mg powder. The Al-Si/SiCp MMC powder was fabricated by gas atomization for homogeneous distribution of the SiC particles. The Ti fibers were mixed with the Al-Si/SiCp MMC powder to structurally reinforce and enhance the interfacial bonding force between the Al-Si matrix and the Ti fibers. The Al-Mg powder was added as a binder to promote liquid phase sintering and to remove the oxide layer on the Al-Si/SiCp MMC powder. The Al-Si/SiCp system had homogeneous microstructures without any clustering of the SiC particles. The Ti fibers formed a diffusion layer at the interface of the Al-Si matrix and the Ti fibers. By adding the Ti fibers, the ultimate tensile strength and elongation increased from 194 to 234 MPa and from 6.2% to 7.6%, respectively. The Al-Mg powder liquefied during sintering and filled the pores between each particle, but the toughness of the Al-MMC decreased. (Received November 2, 2015)

keyword : metals, composites, sintering, mechanical properties, tensile test

A dense nanostuctured AlCr2-Al2O3 composite was sintered by pulsed current activated heating up to 1200 °C within 2 min from mechanically milled powders. The advantage of this process is that it allows very quick densification to near theoretical density and prohibition of grain growth in nanostructured materials. A highly dense AlCr2-Al2O3 composite with a relative density of up to 96% was produced under application of a 80 MPa pressure and a pulsed current of 2000 A. The fracture toughness and hardness of the AlCr2-Al2O3 composite in this study was better than those of previous studies due to the grain refinement. The microstructure and mechanical properties of the composite were investigated using field emission scanning electron microscopy and a Vickers hardness tester. (Received November 24, 2015)

keyword : sintering, composite materials, nanomaterials, mechanical properties, synthesis

The nickel (Ni) electroless plating process is an encouraging method for selectively depositing Ni onto silicon (Si) wafers to form front electrodes of Si solar cells. Ni has the ability to block diffusion in copper (Cu), which is used for further thickening of the Ni contacts. The contact between Ni and Si can be optimized by an additional sintering step to form nickel-silicide (NiSix). This paper presents the characterization of NiSix according to bath conditions, deposition time, and sintering process. The selective electroless deposition of Ni was achieved by plating from an alkaline nickel chloride (NiCl2) bath with ammonia as the pH control and sodium hypophosphite as the reducing agent. Thin clusters of Ni were deposited on top of Si, which ensured the formation of uniform Ni seed layers with better surface coverage on both textured and non-textured Si surfaces. The dominant phase of NiSi that offered the lowest series resistance was confirmed by X-ray diffraction (XRD) analysis at sintering temperatures of 350 ºC and 400 ºC. Selective deposition of Cu over the Ni seed layer was achieved by light induced plating arrangements, and thin finger lines with widths up to 28.4 μm were realized. (Received July 1, 2015)

keyword : plating, solar cells, textured, X-ray diffraction (XRD), light induced plating (LIP)

We studied the dependence of the optical band gap energy and structural properties of the MgxZn1-xO (MZO) thin films on Mg concentration to explore applications in flexible optoelectronics. The MZO thin films were grown on muscovite mica substrates via sol-gel spin-coating. Their surface morphology was smoother and denser than that of ZnO thin film. The peaks were observed for the ZnO thin films at 31.7° and 34.4°, corresponding to the ZnO (100) and (002) planes, but the (100) peak did not appear when Mg was added. The (002) peak of the films was found to be increased with increasing Mg concentration. In the photoluminescence spectra, there were strong UV emissions in the range 3.25.3.45 eV and broad visible emissions were observed for the MZO thin films. The optical band gap calculated from the Tauc``s plot was blue-shifted with increasing Mg concentration. (Received August 13, 2015)

keyword : Mg doped zinc oxide, optical properties, mica, sol-gel, band gap, SEM

Zn thin films were deposited by thermal evaporation on Si substrates and then annealed at temperatures of 300.600 °C to form ZnO thin films. The effects of the annealing temperature on the structural and optical properties of the resulting ZnO thin films were investigated. The highest diffraction intensity for the ZnO (002) plane was obtained in the film formed at 500 °C. In the photoluminescence spectra of the ZnO films, the intensity of the ultraviolet (UV) emission increased with an increase in the oxidation temperature until 500 °C but decreased with further increases. The intensities of the defect-related emissions decreased with an increase in the annealing temperature from 300 to 500 °C. The ratio of the intensity of the UV emission to that of the visible emission (INBE/IDLE) was the highest for the ZnO thin film formed by annealing at 500 °C. (Received September 3, 2015)

keyword : zinc oxide, metallic Zn, thermal oxidation, structural properties, X-ray diffraction, annealing

In this study, cupric oxide (CuO) nanorods were grown on the fluorine-doped tin oxide (FTO) glass substrate using a modified-chemical bath deposition (M-CBD) method. We investigated the morphology, structural, optical and photoelectrochemical properties of the cupric oxide nanorods with various growth durations by using field-emission scanning-electron microscopy (FE-SEM), X-ray diffraction (XRD), UV-visible spectroscopy (UV-vis) and three-electrode potentiostat, respectively. In this work, we found that the morphologies, thickness, growth rate, crystallinities, grain sizes and optical bandgap were controllable on the growth duration, which affected photocurrent density and photo-stability. The highest growth rate of CuO nanorods was 126 nm/min. From the XRD measurement, we also confirmed that (020) directional growth affected the growth of the CuO nanorods. A maximum photocurrent density of-1.88 mA/cm² at -0.55 V (vs. SCE) and high photo-stability value about 40 % was obtained with 10 minutes growth duration. (Received November 5, 2015)

keyword : cupric oxide, photoelectrode, photoelectrochemical, nanorod, growth duration

We added 0.0~0.5 wt% YBO3:Eu3+ nano powders to an electrolyte to improve the energy conversion efficiency (ECE) of a dye-sensitized solar cell (DSSC). TEM and XRD were used to characterize the microstructure and phases. Photoluminescence(PL) was used to determine fluorescence and the composition of the YBO3:Eu3+ Phosphor. UV-VIS-NIR spectroscopy was used to determine the optical absorbance of the DSSC with the YBO3:Eu3+ phosphor. A solar simulator and a potentiostat were used to confirm the photovoltaic properties of the DSSC with YBO3:Eu3+. From the result of microstructure analysis, we found that the YBO3:Eu3+ phosphor had a particle size of less than 100 nm. The absorbance in a visible light regime increased as the amount of YBO3:Eu3+ increased. The photovoltaic property results revealed that the maximum ECE was 5.11% with 0.5 wt% YBO3:Eu3+ compared to that of 3.88% without YBO3:Eu3+. Our results suggest that we might be able to enhance the ECE of a DSSC by employing a phosphor-added electrolyte. (Received November 24, 2015)

keyword : solar cell, chemical synthesis, electrical properties, transmission electron microscopy

In- and Ga-doped ZnO (IGZO) films were deposited on 5 nm thick Cu film buffered poly-carbonate substrates with RF magnetron sputtering and the effects of the Cu buffer layer on the optical and electrical properties of the films were investigated. The IGZO single layer films exhibited an electrical resistivity of 1.2×10-1 Ω cm while the IGZO/Cu bi-layered films exhibited a lower resistivity of 1.6×10-3 Ω cm. With respect to optical properties, the optical band gap of the IGZO films appeared to decrease as a result of an increasing carrier concentration due to the Cu buffer layer. In addition, the RMS roughness (8.2 nm) of the IGZO films also decreased to 6.8 nm by a Cu buffer layer in AFM observation. Although the optical transmittance in the range of visible wavelengths was deteriorated by the Cu buffer layer, the IGZO films with a 5 nm thick Cu buffer layer exhibited a higher figure of merit of 2.6×10-4 Ω-1 compared with the IGZO single layer films due to enhanced optoelectrical performance. (Received December 8, 2015)

keyword : thin films, sputtering, optical properties, atomic force microscope, figure of merit

Increasing the numerical aperture (NA) of an optical system is considered as a resolution enhancing technology. In extreme ultraviolet (EUV) lithography, however, it is hard to use optics with a NA value higher than 0.45 without interference between the incident and reflected light cones at the mask. To avoid this, the incident angle must be increased from 6° to 9°, but the resulting imbalance in reflected EUV light with the different incidence angle, in addition to the mask shadowing effect, can deteriorate the imaging performance of the mask. In this paper, a phase-shifting absorber stack is proposed for high-NA EUVL applications. Aerial image simulation was performed at 0.45NA, an incident angle of 9° and 6× demagnification to compare the imaging properties of a TaBN binary intensity mask with the new TaBN/Mo phase shift mask. The mask 3D effect was mitigated by adopting the phase shifting absorber stack. As a result, image contrast and normalized image log-slope were improved and horizontal-vertical critical dimension bias was significantly decreased. (Received April 5, 2016)

keyword : ceramics, deposition, optical properties, computer simulation, phase shift mask (PSM)