발간논문

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In this study, an effective method is demonstrated for fabricating titanium sputtering targets, which are used to fabricate thin films in the semiconductor industry. The method is an alternative to the existing electron beam melting (EBM) process under high vacuum. Titanium sputtering targets used in the production of semiconductors must have very low concentrations of gaseous impurities, especially oxygen, as well as metal impurities. Currently, the oxygen concentration in titanium sputtering targets used for industrial purposes is less than 400 ppm. To develop an effective alternative method, powder metallurgy and melting processes were performed to prepare a low-oxygen titanium ingot with less than 400 ppm oxygen. First, titanium powder was deoxidized using calcium vapor, and then the powder was subjected to vacuum arc melting (VAM). The oxygen in the titanium powder was reduced with calcium vapor from an initial concentration of 2200 ppm to 800 ppm, and the resulting powder was melted using VAM, resulting in titanium ingots with low oxygen content, 400 ppm or less. It was also confirmed that all lattice constants, i.e., d, a, c, and c/a, decreased as oxygen concentration decreased in both the titanium powder and the ingots. (Received December 28, 2020; Accepted February 1, 2021)

keyword : titanium, sputtering target, oxygen, deoxidation, powder, melting

In this study, we report a methodology that simultaneously improved the corrosion resistance and visible transparency of ZnO/Cu/ZnO transparent heaters with the addition of reactively sputtered Al₂O₃ layers. To assess corrosion resistance, ZnO and Al₂O₃ layers with thicknesses in the range of 0-20 nm were deposited onto 20 nm-thick Cu layer, and the corrosion behavior of the underlying Cu layers was investigated by evaluating the sheet resistance change in an 85 °C/85% humidity test. While the 20 nm-thick ZnO layer was not an effective moisture barrier, showing a sheet resistance increase of more than 50% after 10,000 m, the sheet resistance of the Cu layer below the 5 nm-thick Al₂O₃ layer did not show an observable increase for equal duration. Optical transmittance was also investigated by constructing glass/ZnO/Cu/ZnO/Al₂O₃ structures with varying thicknesses for the top ZnO and Al2O3 layers, in the range of 10-30 and 10-60 nm, respectively The thicknesses of the bottom ZnO and Cu layers were fixed at 30 nm and 10 nm. The results revealed that the average visible light transmittance of the ZnO/Cu/ZnO/Al₂O₃ structure increased by 2.5% over the optimized ZnO/Cu/ZnO structure. (Received January 6, 2021; Accepted January 20, 2021)

keyword : Al_{2O3, reactive sputtering, transparent heaters, sheet resistance, optical transmittance, corrosion}

Solution-processed zinc tin oxide (ZTO) thin-film transistors (TFTs) have great potential uses in next-generation wearable and flexible electronic products. Zinc and tin precursor materials are naturally abundant and have low fabrication costs. To integrate a single ZTO TFT into logic circuits including inverters, NAND, and NOR gates will require the development of a facile patterning process to replace conventional and complicated photolithography techniques which are usually time-consuming and toxic. In this study, selfpatterned ZTO thin films were prepared using a photo-patternable precursor solution including a photoacid generator, (4-methylthiophenyl)methyl phenyl sulfonium triflate. Solution-processed ZTO precursor films fabricated with the photoacid generator were successfully micropatterned by UV exposure, and transitioned to a semiconducting ZTO thin film by heat treatment. The UV-irradiated precursor films became insoluble in developing solvent as the generated proton from the photoacid generator affected the metal-containing ligand and changed the solubility of the metal oxide precursors. The resulting ZTO thin films were utilized as the active layers of n-type TFTs, which exhibited a typical n-type transfer, and output characteristics with appropriate threshold voltage, on/off current ratio, and field-effect mobility. We believe that our work provides a convenient solution-based route to the fabrication of metal-oxide semiconductor patterns. (Received January 21, 2021; Accepted February 3, 2021)

keyword : oxide thin-film transistors, zinc tin oxide, self-patterning, photoacid generator, solution process

Nanocrystalline Fe-Si-B-Nb-Cu Finemet alloys show low saturation magnetic flux density compared to amorphous Fe-Si-B alloys. In the Fe-Si-B-Cu base amorphous alloys, Cu atoms form clusters which act as heterogeneous nucleation sites for α-Fe crystals. The addition of Ca element atoms, distributed along grain boundaries, helps inhibit grain growth and increase resistivity. These alloys can be crystallized into fine nanograins through proper heat treatment, with increased saturated flux density and decreased core loss. According to previous studies, the addition of Zr element can also reduce nanograin size and suppress grain growth by its distribution mainly along the grain boundaries. In this experiment, the effects of added Ca and Zr on the microstructural changes and magnetic properties of Fe-Si-B-Cu were evaluated in detail. Fe-Si-B-Cu alloys containing Ca, and Zr elements were melt-spun to make rapidly solidified ribbons ~20 μm in thickness. The ribbons were then wound into toroidal shaped ribbon cores and heat treated to obtain the nanocrystalline soft magnetic ribbon cores. The microstructure was observed using TEM, and the magnetic characteristics were evaluated using an B-H meter and impedance analyzer. Based on the results, the Fe- Si-B-Cu ribbon core containing 0.037 wt.% Ca and 1.68 wt.% Zr was determined to have the lowest core loss among the alloys, when annealed at 440 oC for 30 min. It was also confirmed that the added Ca and Zr elements were distributed along the grain boundary, and suppress the growth of crystals. In conclusion, the addition of minor elements Ca and Zr to the nanocrystalline ribbon core was very effective at reducing core loss, and the saturated flux density of the core also increased pronouncedly compared to the Fe-Si-B-Nb-Cu Finemet alloys. (Received October 14, 2020; Accepted January 26, 2021)

keyword : soft magnetic properties, Fe-Si-B-Cu alloys, Ca and Zr addition, core loss, microstructure

In this study, we deposited a WO₃ thin-film photoelectrode on a fluorine-doped tin oxide (FTO) substrate using a spin-coating method, and we investigated the photocurrent density and dark current density of the WO₃ photoelectrode with various amounts of H₂O₂ additive. The morphological, structural, optical, electrical and photoelectrochemical properties of the WO₃ photoelectrode with various amounts of H₂O₂ additive were analyzed using FE-SEM, XRD, UV-vis spectroscopy, EIS and a three-electrode potentiostat/ galvanostat system, respectively. The amount of H₂O₂ additive has a large influence on the thickness of the WO₃ photoelectrode, XRD (100) peak intensity, light absorption, optical energy bandgap, flat-band potential, donor density value, etc., and thus has a large influence on photoelectrochemical properties. Specifically, the H₂O₂ additive had a large influence on the growth of the WO₃ photoelectrode, and the photocurrent density and dark current density characteristics of the WO₃ photoelectrode grown to a uniform and thick thickness were largely improved. As a result, the WO₃ photoelectrode fabricated with 0.2 mL of added H₂O₂ exhibited a high photocurrent density value of 1.17 mA/㎠, which was about 23 times higher than that of the WO₃ photoelectrode fabricated without H₂O₂ additive, and had a dark current density value of a low 0.04 mA/㎠, which was a reduction of about 87%. (Received October 19, 2020; Accepted February 2, 2021)

keyword : WO_{3, H2O2 addition amount, photoelectrochemical, PEC, photocurrent density, dark current density}

We grew ZnO nanorods (NRs) using a hydrothermal method while rotating the precursor solution, and report the effect of rotation on surface modification, and the optical and ultraviolet (UV) photoresponse properties of the resulting ZnO nanorods. The ZnO NRs grown in the rotating precursor solution at 100 rpm had the longest length and diameter, which decreased with increasing rotation rate above 100 rpm. Also, the intensity of the diffraction peaks from ZnO (002) and (100) was strongest and weakest for the ZnO NRs grown in a rotating solution at 150 rpm, respectively, indicating improvement in the c-axis orientation of the ZnO NRs. In the PL spectra, near-band-edge (NBE) and deep-level (DL) emissions were observed from all of the ZnO NRs. The intensity of the NBE emission gradually increased with increasing rotation rate, due to the increase in surface area. Also, the intensity of the DL emission gradually increased with increasing rotation rate because of the increasing number of interstitial oxygens. Analysis of the UV photoresponse found the photocurrent of the ZnO NRs grown in the rotating precursor solution was higher than that of ZnO NRs grown in a non-rotated precursor solution. In particular, ZnO NRs grown in a rotating precursor solution at 150 rpm exhibited the highest value of photosensitivity, with high reproducibility. (Received November 23, 2020; Accepted January 20, 2021)

keyword : ZnO nanorods, hydrothermal, rotating precursor solution, photoluminescence, ultraviolet photodetector, photosensitivity

Zinc oxide is transparent inorganic UV absorber and widely used as a sunscreen. But sunscreen on the skin is easily erased by sebum, so zinc oxide with a sebum curing function was studied. In this study, a zinc oxide powder was prepared using zinc chloride and sodium hydroxide via a hydrothermal synthesis method. The process conditions including reaction temperature and sodium hydroxide ratio were determined. Particle size was determined using XRD and SEM, and process tendencies were checked using the gelling test method. When the sodium hydroxide ratio was lower than 58.7 wt%, the gelling time was reduced, and when the temperature was lower than 60 o, the gelling time decreased. ZnO and Simonkolleite confirmed that the gelling time was reduced when the mixture comes out. (Received November 2, 2020; Accepted January 11, 2021)

keyword : ZnO powder, sebum harden, hydrothermal synthesis, oleic acid reaction, ultraviolet blocking

Cemented carbide is a kind of composite material in which fine particles of carbide are embedded into the matrix of a binder metal. It has a long service life because of its superior mechanical properties. In this study, the overall tensile behavior of a cemented carbide, WC-Co, was investigated by considering its characteristic microstructure parameters. Tensile strength and strain to fracture were evaluated by measuring the stress-strain curves of a standard tensile specimen. Scanning Electron Microscopy (SEM) was used to analyze both the average size and contiguity of WC carbide particles, as well as the mean free path of the Co (cobalt) binder. Specific correlations between mechanical and microstructural features were examined and elucidated for various volume fractions of the binder metal. The Co content and the mean free path of the Co binder were in a proportional relationship, and tensile strength showed an opposite tendency to Co content. Regarding Young’s modulus and strain, it was confirmed that a large difference appears depending on the crystal structure of the Co phase. Furthermore, by probing topology of the fractured surface of the tensile specimen it was determined that the existence of irregular voids could contribute to the statistical variance in the measured values. (Received December 28, 2020; Accepted January 28, 2021)

keyword : WC-Co cemented carbide, tensile strength, elongation, microstructure, correlation analysis

Zinc oxide (ZnO) simultaneously exhibits semiconducting and piezoelectric properties. ZnO in the form of nanorods has been studied intensively for application in self-powering devices. The power generation in piezoelectric nanogenerators based on ZnO nanorods can be improved via several approaches, including an oxygen plasma treatment. When ZnO nanorods are exposed to oxygen plasma, the charge carrier concentration decreases and the piezoelectric output voltage consequently increases. However, the effects of oxygen plasma on the mechanical properties of ZnO nanorods has not been systematically studied using a precise measurement technique. Given the size of ZnO nanorods, atomic force microscopy (AFM) is a suitable method for manipulating individual ZnO nanorods and measuring their elastic properties. In the present work, we observed the effects of oxygen plasma on the elemental composition and microstructure of ZnO nanorods. First of all, the surface roughness of the ZnO nanorods was analyzed using AFM, revealing that it increased due to the etching effect of the oxygen plasma. From X-ray photoelectron spectroscopy (XPS) measurements, three distinct peaks corresponding to lattice oxygen, oxygen vacancies, and absorbed oxygen on the surface were identified. The XPS analysis results showed that oxygen vacancy defects on the ZnO nanorods were decreased by oxygen plasma treatment. Next, the effects of oxygen plasma on the elastic properties of ZnO nanorods were studied using lateral force microscopy. It was confirmed that the elastic modulus of ZnO nanorods increased due to the reduced number of defects originating from oxygen vacancies. (Received December 21, 2020; Accepted January 19, 2021)

keyword : zinc oxide, nanorod, oxygen plasma, lateral force microscopy, nanogenerator, piezoelectric