• 한국분말재료학회지
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• 제26권1호
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• pp.34-39
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• 2019

Current synthesis processes for titanium dioxide ($TiO_2$) nanoparticles require expensive precursors or templates as well as complex steps and long reaction times. In addition, these processes produce highly agglomerated nanoparticles. In this study, we demonstrate a simple and continuous approach to synthesize $TiO_2$ nanoparticles by a salt-assisted ultrasonic spray pyrolysis method. We also investigate the effect of salt content in a precursor solution on the morphology and size of synthesized products. The synthesized $TiO_2$ nanoparticles are systematically characterized by X-ray diffraction, transmission electron micrograph, and UV-Vis spectroscopy. These nanoparticles appear to have a single anatase phase and a uniform particle-size distribution with an average particle size of approximately 10 nm. By extrapolating the plots of the transformed Kubelka-Munk function versus the absorbed light energy, we determine that the energy band gap of the synthesized $TiO_2$ nanoparticles is 3.25 eV.

• 전기전자학회논문지
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• 제22권4호
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• pp.916-921
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• 2018

본 논문에서는 고전압, 고전류 동작을 위한 전력 MOSFET 소자에 대한 전기적 특성을 시뮬레이션을 통해 분석하였다. 소자의 정적 특성을 향상시키기 위해 기존의 Si대신 4H-SiC를 이용했다. 4H-SiC는 넓은 에너지 밴드 갭에 의한 높은 한계전계를 갖기 때문에 고전압, 고전류 동작에서 Si보다 유리한 특성을 갖는다. 4H-SiC를 사용한 기존 VDMOSFET 구조는 p-base 영역 모서리에 전계가 집중되는 현상으로 인해 항복 전압이 제한된다. 따라서 본 논문에서는 p-base 영역의 모서리에 곡률을 주어 전계의 집중을 완화시켜 항복 전압을 높이고, 정적 특성을 개선한 곡률 VDMOSFET 구조를 제안하였다. TCAD 시뮬레이션을 통해 기존 VDMOSFET과 곡률 VDMOSFET의 정적 특성을 비교, 분석 하였다. 곡률 VDMOSFET은 기존 구조에 비해 온저항의 증가 없이 68.6% 향상 된 항복 전압을 갖는다.

• 한국전기전자재료학회논문지
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• 제32권5호
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• pp.376-381
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• 2019

In this work, we evaluated the structural, electrical and optical properties of $Ge_8Sb_2Te_{11}$ and Cu-doped $Ge_8Sb_2Te_{11}$ thin films prepared by rf-magnetron reactive sputtering. The 200-nm-thick deposited films were annealed in a range of $100{\sim}400^{\circ}C$ using a furnace in an $N_2$ atmosphere. The amorphous-to-crystalline phase changes of the thin films were investigated by X-ray diffraction (XRD), UV-Vis-IR spectrophotometry, a 4-point probe, and a source meter. A one-step phase transformation from amorphous to face-centered-cubic (fcc) and an increase of the crystallization temperature ($T_c$) was observed in the Cu-doped film, which indicates an enhanced thermal stability in the amorphous state. The difference in the optical energy band gap ($E_{op}$) between the amorphous and crystalline phases was relatively large, approximately 0.38~0.41 eV, which is beneficial for reducing the noise in the memory devices. The sheet resistance($R_s$) of the amorphous phase in the Cu-doped film was about 1.5 orders larger than that in undoped film. A large $R_s$ in the amorphous phase will reduce the programming current in the memory device. An increase of threshold voltage ($V_{th}$) was seen in the Cu-doped film, which implied a high thermal efficiency. This suggests that the Cu-doped $Ge_8Sb_2Te_{11}$ thin film is a good candidate for PRAM.

• 한국재료학회지
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• 제29권9호
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• pp.553-558
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• 2019

$Cu_2ZnSn(S,Se)_4(CZTSSe)$ thin film solar cells areone of the most promising candidates for photovoltaic devices due to their earth-abundant composition, high absorption coefficient and appropriate band gap. The sputtering process is the main challenge to achieving high efficiency of CZTSSe solar cells for industrialization. In this study, we fabricated CZTSSe absorbers on Mo coated soda lime glass using different pressures during the annealing process. As an environmental strategy, the annealing process is performed with S and Se powder, without any toxic $H_2Se$ and/or $H_2S$ gases. Because CZTSSe thin films have a very narrow stable phase region, it is important to control the condition of the annealing process to achieve high efficiency of the solar cell. To identify the effect of process pressure during the sulfo-selenization, we experiment with varying initial pressure from 600 Torr to 800 Torr. We fabricate a CZTSSe thin film solar cell with 8.24 % efficiency, with 435 mV for open circuit voltage($V_{OC}$) and $36.98mA/cm^2$ for short circuit current density($J_{SC}$), under a highest process pressure of 800 Torr.

• Current Photovoltaic Research
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• 제7권1호
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• pp.9-14
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• 2019

Carrier selective solar cell structure has allured curiosity of photovoltaic researchers due to the use of wide band gap transition metal oxide (TMO). Distinctive p/n-type character, broad range of work functions (2 to 7 eV) and risk free fabrication of TMO has evolved new concept of heterojunction intrinsic thin layer (HIT) solar cell employing carrier selective layers such as $MoO_x$, $WO_x$, $V_2O_5$ and $TiO_2$ replacing the doped a-Si layers on either front side or back side. The p/n-doped hydrogenated amorphous silicon (a-Si:H) layers are deposited by Plasma-Enhanced Chemical Vapor Deposition (PECVD), which includes the flammable and toxic boron/phosphorous gas precursors. Due to this, carrier selective TMO is gaining popularity as analternative risk-free material in place of conventional a-Si:H. In this work hole selective materials such as $MoO_x$, $WO_x$ and $V_2O_5$has been investigated. Recently $MoO_x$, $WO_x$ & $V_2O_5$ hetero-structures showed conversion efficiency of 22.5%, 12.6% & 15.7% respectively at temperature below $200^{\circ}C$. In this work a concise review on few important aspects of the hole selective material solar cell such as historical developments, device structure, fabrication, factors effecting cell performance and dependency on temperature has been reported.

• 한국세라믹학회지
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• 제56권1호
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• pp.98-103
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• 2019

Optical filters to control light wavelength of displays or cameras are fabricated by multi-layer stacking process of low and high index thin films. The process of multi-layer stacking of thin films has received much attention as an optimal process for effective manufacturing in the optical filter industry. However, multi-layer processing has disadvantages of complicated thin film process, and difficulty of precise control of film morphology and material selection, all of which are critical for transmittance and coloring effect on filters. In this study, the composite $TiO_2$, which can be used to control of UV absorption, coated on nano hollow silica sol, was synthesized as a coating material for optical filters. Furthermore, systematic analysis of the process parameters during the chemical reaction, and of the structural properties of the coating solutions was performed using SEM, TEM, XRD and photo spectrometry. From the structural analysis, we found that the 85 nm nano hollow silica with 2.5 nm $TiO_2$ shell formation was successfully synthesized at proper pH control and titanium butoxide content. Photo luminescence characteristics, excited by UV irradiation, show that stable absorption of 350 nm-light, correlated with a 3.54 eV band gap, existed for the $TiO_2$ shell-nano hollow silica reacted with 8.8 mole titanium butoxide solution. Transmittance observed on substrate of the $TiO_2$ shell-nano hollow silica showed effective absorption of 200-300 nm UV light without deterioration of visible light transparency.

• 한국재료학회지
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• 제29권1호
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• pp.1-6
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• 2019

Recent industrialization has led to a high demand for the use of fossil fuels. Therefore, the need for producing hydrogen and its utilization is essential for a sustainable society. For an eco-friendly future technology, photoelectrochemical water splitting using solar energy has proven promising amongst many other candidates. With this technique, semiconductors can be used as photocatalysts to generate electrons by light absorption, resulting in the reduction of hydrogen ions. The photocatalysts must be chemically stable, economically inexpensive and be able to utilize a wide range of light. From this perspective, cuprous oxide($Cu_2O$) is a promising p-type semiconductor because of its appropriate band gap. However, a major hindrance to the use of $Cu_2O$ is its instability at the potential in which hydrogen ion is reduced. In this study, gold is used as a bottom electrode during electrodeposition to obtain a preferential growth along the (111) plane of $Cu_2O$ while imperfections of the $Cu_2O$ thin films are removed. This study investigates the photoelectrochemical properties of $Cu_2O$. However, severe photo-induced corrosion impedes the use of $Cu_2O$ as a photoelectrode. Two candidates, $TiO_2$ and $SnO_2$, are selected for the passivation layer on $Cu_2O$ by by considering the Pourbaix-diagram. $TiO_2$ and $SnO_2$ passivation layers are deposited by atomic layer deposition(ALD) and a sputtering process, respectively. The investigation of the photoelectrochemical properties confirmed that $SnO_2$ is a good passivation layer for $Cu_2O$.

• 한국재료학회지
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• 제29권5호
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• pp.322-327
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• 2019

Hole carrier selective MoOx film is obtained by atomic layer deposition(ALD) using molybdenum hexacarbonyl[$Mo(CO)_6$] as precursor and ozone($O_3$) oxidant. The growth rate is about 0.036 nm/cycle at 200 g/Nm of ozone concentration and the thickness of interfacial oxide is about 2 nm. The measured band gap and work function of the MoOx film grown by ALD are 3.25 eV and 8 eV, respectively. X-ray photoelectron spectroscopy(XPS) result shows that the $Mo^{6+}$ state is dominant in the MoOx thin film. In the case of ALD-MoOx grown on Si wafer, the ozone concentration does not affect the passivation performance in the as-deposited state. But, the implied open-circuit voltage increases from $576^{\circ}C$ to $620^{\circ}C$ at 250 g/Nm after post-deposition annealing at $350^{\circ}C$ in a forming gas ambient. Instead of using a p-type amorphous silicon layer, high work function MoOx films as hole selective contact are applied for heterojunction silicon solar cells and the best efficiency yet recorded (21 %) is obtained.

• 한국전자통신학회논문지
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• 제16권2호
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• pp.233-240
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• 2021

다양한 함량으로 네오디뮴이 도핑된 황화아연 박막제작은 RF 마그네트론 스퍼터링 장비를 이용하여 황화아연과 네오디뮴을 동시 증착하여 박막을 제작하였고, 후처리 공정으로 급속열처리를 400℃ 에서 30분간 실시하였다. 다양한 네오디뮴의 도핑 함량(0.35at.%, 1.31at.%, 1.82at.% 및 1.90at.%)을 갖는 ZnS 박막의 구조, 형태, 광학적 특성을 연구하였다. X-선 회절 패턴은 모든 박막에서 (111)방향의 큐빅 구조로 성장하였다. SEM 이미지와 AFM 이미지를 통해 네오디뮴 도핑 함량에 의한 박막의 표면 및 구조적 형태에 대하여 설명하였다. EDAX를 통해 다른 불순물이 포함되지 않은 Zn, S 및 Nd의 원소만을 확인하였다. UV-vis 스펙트럼을 이용하여 제작된 박막의 투과율과 밴드갭을 확인하였다.

• 전력전자학회논문지
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• 제26권3호
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• pp.214-221
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• 2021

SiC MOSFETs require a faster and more reliable short-circuit protection circuit than conventional methods due to narrow short-circuit withstand times. Therefore, this research proposes a short-circuit protection circuit using a current-sensing circuit based on Rogowski coil. The method of designing the current-sensing circuit, which is a component of the proposed circuit, is presented first. The integrator and input/output filter that compose the current-sensing circuit are designed to have a wide bandwidth for accurately measuring short-circuit currents with high di/dt. The precision of the designed sensing circuit is verified on a double pulse test (DPT). In addition, the sensing accuracy according to the bandwidth of the filters and the number of turns of the Rogowski coil is analyzed. Next, the entire short-circuit protection circuit with the current-sensing circuit is designed in consideration of the fast short-circuit shutdown time. To verify the performance of this circuit, a short-circuit test is conducted for two cases of short-circuit conditions that can occur in the half-bridge structure. Finally, the short-circuit shutdown time is measured to confirm the suitability of the proposed protection circuit for the SiC MOSFET short-circuit protection.