발간논문

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High-performance thin-film transistors (TFTs) produced at low temperatures are required for ultrahigh- resolution and flexible display applications. The scientific community has been studying unconventional techniques to investigate low voltage flexible devices and low power flexible circuits for the past decade. In particular, metal oxide semiconductors, such as indium gallium zinc oxide (IGZO), have made significant progress as amorphous silicon replacements for electronics and commercial displays. On the other hand, developing metal oxide transistors with low processing temperatures remains a difficulty. The hightemperature annealing process causes very instability in the plastic substrate. Here, we introduce IGZO TFTs that shows enhanced electrical properties environmental stability by laser post-annealing. After annealing process, the laser post-annealing process was given at 80 MHz, pulse width 140 fs, for 50 seconds. The improved electrical characteristics of this laser post-annealed IGZO TFTs were: 9.03 cm²/Vs; 2.2×10⁷ on/off current ratio. The IGZO TFT to which laser post-annealing was applied had a flat surface, and it was confirmed that the combination of internal metal and oxygen was urged, and the leakage current problem was improved by suppressing excessive generation of oxygen vacancy. Furthermore, the voltage transfer curve was measured after fabricating the N-MOS logic inverter circuit, this inverter showed a value of 80.8% the total noise margin. (Received 23 March, 2022; Accepted 23 May, 2022)

keyword : metal oxide transistor, IGZO, laser annealing, post-annealing, logic inverter

Transparent heaters are widely used for defrosting to improve visibility, insulation or heating of buildings, and thermal treatment. Indium tin oxide (ITO), which has excellent transmittance and electrical conductivity, is one of the representative materials used for these transparent heaters. However, it has several drawbacks including high material price, limited processability in large-area, and brittleness. Here, we fabricated a mesh-patterned transparent heater utilizing an inexpensive and solution-processable material, reduced graphene oxide, as an alternative to ITO. In this study, electrical conductivity was improved by the synthesis of large-sized reduced graphene oxide (LrGO). Over 80% of transmittance was obtained by adapting mesh patterns. Moreover, to alleviate the decrease in transmittance due to diffraction by the mesh pattern, transparent heaters were prepared with six different mesh patterns by changing the space between the repeating pattern units. Comparing the transmittance values from calculations and measurements, the HH (honeycomb-honeycomb) mesh pattern was determined to have the minimum degradation of transmittance due to diffraction. In addition, the electrical/optical properties and heating performances of LrGO meshpatterned transparent heaters were compared to confirm the optimal mesh pattern. As a result, frost was completely removed within 30 seconds under a low power of 0.07 W using a H-H 85% mesh patterned heater. (Received 15 May, 2022; Accepted 2 June, 2022)

keyword : graphene, large-sized reduced graphene oxide, mesh pattern, transparent heater

Transparent heaters are promising devices because of their versatile applications in vehicles, smart windows, and sensors, etc. Indium tin oxide is widely used for transparent heater materials due to its high electronic conductivity and visible light transmittance. However, the cost of indium is too high, and its fabrication needs sophisticated processes, so that many studies have focused on alternative materials which are inexpensive and easy-to-synthesize. Graphene is a two-dimensional material in which carbon atoms bond to form a hexagonal structure, and it can be an alternative material due to its superior electronic/ thermal conductivity and cost-effectiveness. Here, we chemically treated graphite to synthesize large-sized graphene oxide (LGO), and coated it on a glass substrate, followed by reduction using hydrogen iodide for large-sized reduced graphene oxide (LrGO) on glass. From surface characterizations, we confirmed that the lateral size of the LGO was over 50 μm and the LGO sheets were uniformly coated on the glass, which minimized intersheet contact resistance. Structural characterizations demonstrated that the LGO sheets were reduced to LrGO and the LrGO sheets coated on the glass showed a transmittance of 76.2 % at 550 nm with a sheet resistance of 0.98 kΩ. Finally, the temperature of the substrate increased up to 30 oC when 30 V of voltage was applied for 5 min, and the frost on the glass surface vanished within 1 min. (Received 19 May, 2022; Accepted 30 May, 2022)

keyword : large-sized reduced graphene oxide, solution process, large-area transparent heater, defrosting

We fabricated a Li doped CuO photoelectrode by doping CuO with Li to improve the photoelectrochemical properties of the CuO photoelectrode. The fabricated Li doped CuO photoelectrode was optimized by experimentally investigating Li doping concentration, annealing temperature, and spin coating deposition cycle. It was confirmed that Li doped CuO had increased light absorption, decreased energy band gap, and improved crystallinity. The Li-doped CuO photoelectrode had a porous surface, unlike the bare CuO photoelectrode, and had a low charge transfer resistance as well as a high flat band potential. The Li doping concentration experiment demonstrated that the 2 at% Li doped CuO photoelectrode had a superior photocurrent density value compared with a bare CuO photoelectrode. In the annealing temperature optimization experiment with a 2 at% Li doped CuO photoelectrode, it was found to have the best photocurrent density value at 500℃. In experiments with various spin coating deposition cycles of the Li-doped CuO photoelectrode, the light absorption, energy bandgap, crystallinity, and electrical properties were affected by changes in the film thickness of the photoelectrode. In particular, we confirmed that a sample deposited with 4 spin coating cycles had the lowest interfacial resistance between the photoelectrode and the electrolyte, and the highest flat-band potential value. Consequently, we were able to obtain an improved photocurrent density of -1.68 mA/cm² compared to the bare CuO photoelectrode using the Li-doped CuO photoelectrode under the optimized conditions of Li 2 at%, an annealing temperature of 500℃, and 4 cycles of spin coating depositions. (Received 28 February, 2022; Accepted 19 May, 2022)

keyword : CuO, Li-doped CuO, photoelectrode, photoelectrochemical (PEC), annealing temperature, spin coating deposition cycle

In this study, the effect of Sb incorporation on the thermoelectric transport properties of tetradymite-type Pb(Bi_2-xSb_x)Te₄ (0≤x≤1) compounds is presented. PbBi₂Te₄ (x = 0) possesses a high electron concentration of ~1.6×10²⁰/cm³ at room temperature and exhibits n-type degenerate semiconductor behavior, in which electrical conductivity decreases and negative Seebeck coefficient increases almost linearly with increasing temperature. For x = 0.5, the electron concentration decreased to ~5.5×10¹⁹/cm³ and n-type semiconductor characteristics was observed. When the Sb content increased to x = 1, the majority charge carrier was found to be hole (p ~ 3.3×10¹⁹/cm³), thus a change in polarity from n-type to p-type semiconductor was observed. It was suggested that the change in polarity with Sb incorporation observed in this study is due to the formation of a p-type antisite defect, which is formed because the electronegativity of Sb is closer to that of Te than that of Bi. Due to the decrease in charge concentration with the increase in Sb content, it was possible to increase the power factor near room temperature for x = 0.5. However, the overall output factor decreased with increasing x. These results are expected to be helpful in efforts to improve the thermoelectric properties of tetradymite-type homologous compounds through antisite defect control. (Received 25 March, 2022; Accepted 23 May, 2022)

keyword : thermoelectric, tetradymite, antisite defect, spark plasma sintering

In this study, mechanical alloying was performed to synthesize permingeatite Cu₃Sb_1-x-ySn_xBi_ySe₄ (0.02 ≤ x ≤ 0.06 and 0.02 ≤ y ≤ 0.04) doped with Sn and Bi. Hot pressing was subsequently conducted to achieve dense sintered bodies. When the Bi content was constant, the carrier concentration increased with the Sn content, but the mobility decreased owing to the increased carrier concentration. In contrast, when the Sn content was constant, the carrier concentration and mobility were not significantly affected by the Bi content. Higher electrical conductivity was observed in specimens with a higher Sn content or lower Bi content; consequently, Cu₃Sb_0.92Sn_0.06Bi_0.02Se₄ exhibited the highest electrical conductivity. The Seebeck coefficient increased with temperature, and it is inferred that the permingeatite doped with Sn/Bi does not undergo an intrinsic transition until 623 K. In contrast to the electrical conductivity, a higher Seebeck coefficient was obtained in the specimens with a lower Sn content or higher Bi content; consequently, Cu₃Sb_0.94Sn_0.02Bi_0.04Se₄ exhibited the highest Seebeck coefficient. Cu₃Sb_0.92Sn_0.06Bi_0.02Se₄ exhibited the maximum power factor, depending on the electrical conductivity and Seebeck coefficient. The electronic thermal conductivity was not significantly affected by temperature, but the lattice thermal conductivity decreased as the temperature increased. However, the thermal conductivity decreased with increasing temperature. Sn doping effectively reduced the lattice thermal conductivity, whereas Bi doping effectively reduced the electronic thermal conductivity; consequently, Cu₃Sb_0.94Sn_0.02Bi_0.04Se₄ exhibited the lowest thermal conductivity. Finally, the highest dimensionless figure-of-merit of 0.75 was achieved at 623 K by Cu₃Sb_0.92Sn_0.06Bi_0.02Se₄. (Received 6 April, 2022; Accepted 19 May, 2022)

keyword : thermoelectric, permingeatite, mechanical alloying, hot pressing, double doping

Hexagonal-shaped ZnO nanodisks were synthesized at temperatures above 1000℃ via thermal evaporation of a mixture of ZnO, SnO, and graphite powders as the source materials. Notably, the ZnO nanodisks were easily formed in ambient air at atmospheric pressure. The ZnO nanodisks could not be obtained without SnO in the source material, which indicated that the SnO powder played a crucial role in the growth of the hexagonal disk shaped ZnO nanocrystals. The ZnO powder in the source material was reduced to Zn vapor due to the graphite and SnO powders. In particular, the SnO is considered to play a role in promoting the reduction of ZnO. As a result, the concentration of Zn vapor increased quickly, leading to the fast nucleation of Zn crystals with a hexagonal close packed crystal structure. In the growth habit of the Zn crystal, the growth rate of <1010> is much faster than that of [0001] because (0001) has the lowest surface energy, resulting in the hexagonal disk shape. Then the hexagonal-shaped ZnO nanodisks were obtained by oxidation of the Zn nanodisks under air atmosphere. The XRD and EDS results showed that the ZnO nanodisks had a hexagonal wurtzite crystal structure and high purity. The ZnO nanodisks formed at 1000℃ exhibited a strong ultraviolet emission centered at 380 nm, which was indicative of high crystalline quality. (Received 1 April, 2022; Accepted 16 May, 2022)

keyword : zinc oxide, nanodisks, hexagonal shape, thermal evaporation, air, atmospheric pressure

The ΔEVPSC numerical code based on the elasto-visco-plastic HEM (Homogeneous Effective Medium) provides a multiscale constitutive modeling framework that is suitable for describing a wide range of mechanical behaviors of polycrystalline metals. In this study, an AA6061-T6 aluminum sample was chosen to validate the predictive capability of the ΔEVPSC stand-alone (ΔEVPSC-SA) code on stress triaxiality and its evolution until fracture. The model parameters were calibrated by fitting the uniaxial flow stress-strain curve, and the initial crystallographic orientation distribution (COD) was obtained using X-ray diffraction and electron backscattered diffraction (EBSD) methods. The statistical representativeness of the COD was further examined by comparing the experimental R-values with model predictions based on a set of CODs obtained via the two mentioned diffraction methods. The results suggest that the X-ray scan does not represent the texture very well, and instead, an entire cross-sectional EBSD scan is required, even though the texture gradient along the through-thickness direction is not very significant. The model-calculated triaxiality based on the ΔEVPSC-SA code was verified by comparison with the experimental results from the uniaxial tension, the notched tension, and the plane strain tests. The results were in good agreement with the ΔEVPSC finite element (FE) simulation results and other similar experimental results reported in the literature. (Received 16 February, 2022; Accepted 19 May, 2022)

keyword : elasto-visco-plasticity, crystal plasticity, triaxiality, finite element simulation, texture

In this study, the mechanical properties of materials were predicted using machine learning to search for superhard materials. Based on an AFOW database consisting of DFT quantum calculation values, the mechanical properties of materials were predicted using various machine learning models. For supervised learning, the entire data was divided into training data and test data at a ratio of 8:2. Since the discovery of superhard materials can be predicted based on the bulk modulus and shear modulus, the bulk modulus was primarily predicted using only the chemical compositional ratio (chemical formula), and then the shear modulus was obtained using the predicted bulk moduli. To obtain good prediction performance, cross-validation and hyper-parameter tuning were carried out. Each characteristic was predicted using XGBoost, one of the ensemble algorithms, and its performance was compared to the treebased machine learning of RandomForest and Support Vector Machine regression using the coefficient of determination (R²) and root-mean-square-error (RMSE) as metrics. For the recently introduced four superhard materials (Mo_0.9W_1.1BC, ReWC_0.8, MoWC₂, and ReWB), the results of this study were similar to those of previous studies including the experimental values or the DFT quantum calculations. The shear modulus was underpredicted, which can be understood since structural properties were not considering as a feature in our machine learning models. (Received 14 March, 2022; Accepted 9 May, 2022)

keyword : machine learning, superhard materials, mechanical properties