• Progress in Superconductivity and Cryogenics
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• v.19 no.3
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• pp.40-43
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• 2017

Carbon coating approach is used to prepare carbon-doped $MgB_2$ bulk samples using low-cost naphthalene ($C_{10}H_8$) as a carbon source. The coating of carbon (C) on boron (B) powders was achieved by direct pyrolysis of naphthalene at $120^{\circ}C$ and then the C-coated B powders were mixed well with appropriate amount of Mg by solid state reaction method. X-ray diffraction analysis revealed that there is a noticeable shift in (100) and (110) Bragg reflections towards higher angles, while no shift was observed in (002) reflections for $MgB_2$ doped with carbon. As compared to un-doped $MgB_2$, a systematic enhancement in $J_c(H)$ properties with increasing carbon doping level was observed for naphthalene-derived C-doped $MgB_2$ samples. The substantial enhancement in $J_c$ is most likely due to the incorporation of C into $MgB_2$ lattice and the reduction in crystallite size, as evidenced by the increase in the FWHM values for doped samples.

• Korean Journal of Crystallography
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• v.7 no.1
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• pp.73-80
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• 1996

The single crystal growth has been carried out with the newly designed 1-T HB(single temperature zone horizontal Bridgman) system for GaAs crystals of 2 inch diameter doped with Si, Zn or undoped. With this method, incidence probability of single crystallinity was shown to be 0.73. Lattice defects evaluated from EPD(etch pit density) measurement were in the range of 5,000-20,000/cm2, dependent upon the doping condition. For the undoped GaAs crystals, carrier concentrations from the Hall measurement were ∼1×1016/cm3 at the seed part, which were less than half the concentrations of double of triple temperature zone(2-T, 3-T) HB grown crystals. By the 1-T HB method, therefore, GaAs crystals can be grown successfully with better yield and higher purity.

• Korean Journal of Materials Research
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• v.8 no.8
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• pp.667-671
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• 1998

The photoluminescence (pL) characteristics of hydride vapor phase epiyaxy (HVPE) grown GaN films on $MgAl_{2}O_{4}$ substrate were investigated with several growth conditions. The GaN films on $MgAl_{2}O_{4}$ substrate is autodoped with Mg atoms which thermally out-diffused from substrate lead to a PL characteristics of impurity doped ones. The Mg-related emission band intensity decreased with growth temperature may due to the evaporation of Mg atoms at the GaN film surfaces. and it also decreased with GaN film thicknesses. We can estimate the diffusion coefficient of Mg atoms in GaN under the consideration of diffusion phenomena between two infinite solids lead to a value of D= 2$\times$$lO^{-10}\textrm{cm}^2/sec.

• Proceedings of the Korean Institute of Electrical and Electronic Material Engineers Conference
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• 2007.06a
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• pp.11-11
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• 2007

1차원 구조체인 반도체 나노선은 앙자제한효과 (quantum confinement effect) 등을 이용하여 고밀도/고효율의 소자 개발이 기대되고 있다. GaN는 상온에서 3.4 eV의 밴드갭 에너지를 갖는 III-V 족 반도체 재료로써 박막의 경우 광전자 소자로 폭넓게 응용되고 있다. 최근 GaN 나노선의 합성에 성공하면서 발광소자, 고효율의 태양전지, HEMT 등으로의 응용을 위한 많은 연구가 활발히 이루어지고 있다. 하지만, 아직까지 GaN 나노선의 전기적 특성을 제어하는 기술은 확립되지 않고 있다. 본 연구에서는 Vapor solid (VS)법을 이용하여 GaN 나노선을 합성하였으며, GaN 분말과 함께 $Mg_2N_3$ 분말을 첨가하여 (Ga,Mg)N 나노선을 성공적으로 합성하였다. 합성시에 GaN와 Mg 소스간의 거리 변화를 통해 Mg 도핑농도를 제어하고자 하였다. 이 같은 방법으로 합 된 (Ga,Mg)N 나노선의 Mg 도핑농도에 따른 결정학적 특성을 알아보고, (Ga,Mg)N 나노선을 이용하여 소자를 제작한 후 그 전기적 특성을 살펴보고자 한다. X-ray diffraction (XRD)과 high-resolution transmission electron microscopy (HRTEM), EDX를 이용하여 합성된 나노선의 결정학적 특성과 Mg의 도핑 농도를 확인하였다. Photo lithography와 e-beam lithography법을 이용하여 (Ga,Mg)N 나노선 field-effect transistor (FET)를 제작하고, channel current-drain voltage ($I_{ds}-V_{ds}$) 와 channel current-gate voltage ($I_{ds}-V_g$) 측정을 통해 (Ga,Mg)N 나노선이 도핑 농도에 따라 n형에서 p형으로 전기적 특성이 변화함을 확인하였다.

• Proceedings of the Korean Institute of Electrical and Electronic Material Engineers Conference
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• 2007.11a
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• pp.126-127
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• 2007

Sn-filled and Fe-doped $CoSb_3$ skutterudites were synthesized by encapsulated induction melting. Single ${\delta}$-phase was successfully obtained by subsequent annealing and confirmed by X-ray diffraction analysis. Temperature dependences of Seebeck coefficient, electrical resistivity and thermal conductivity were examined from 300 K to 700 K. The positive Seebeck coefficient confirmed the p-type conduction. Electrical resistivity increased with increasing temperature, which shows that the $Sn_zCo_3FeSb_{12}$ skutterudite is highly degenerate. Thermal conductivity was reduced by Sn-filling because the filler atoms acted as phonon scattering centers in the skutterudite lattice. Thermoelectric figure of merit was enhanced by Sn filling and its optimum filling content was considered to be z=0.3 in the $Sn_zCo_3FeSb_{12}$ system.

• Bulletin of the Korean Chemical Society
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• v.15 no.9
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• pp.713-718
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• 1994

Manganese-incorporated neodymium oxide systems with a variety of Mn mol% were prepared to investigate the effect of doping on the electrical properties of neodymium oxide. XRD, XPS, SEM, DSC, and TG techniques were used to analyze the specimens. The systems containing 2, 5, 8, and 10 mol% Mn were found to be solid solutions by X-ray diffraction analysis and the lattice parameters were obtained for the single-phase hexagonal structure by the Nelson-Riley method. The lattice parameters, a and c, decreased with increasing Mn mol%. Scanning electron photomicrographs of the specimens showed that the grain size decreased with increasing Mn mol%. The curves of log conductivity plotted as a function of 1/T in the temperature range from 500 to 1000$^{\circ}C$ at $PO_2$'s of $10^{-5}$ to $10^{-1}$ atm for the specimens were divided into high-and low-temperature regions with inflection points near 820-890$^{\circ}C$. The activation energies obtained from the slopes were 0.53-0.87 eV for low-temperature region and 1.40-1.91 eV for high-temperature region. The electrical conductivities increased with increasing Mn mol% and $PO_2$, indicating that all the specimens were p-type semiconductors. At $PO_2$'s below $10^{-3}$ atm, the electrical conductivity was affected by the chemisorption of oxygen molecule in the temperature range of 660 to 850$^{\circ}C$. It is suggested that electron holes generated by oxygen incorporation into the oxide are charge carriers for the electrical conduction in the high-temperature region and the system includes ionic conduction owing to the diffusion of oxygen atoms in the low-temperature region.

• Bulletin of the Korean Chemical Society
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• v.19 no.6
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• pp.661-666
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• 1998

Active reaction sites and electrochemical O2 reduction kinetics on La_{1-x}Sr_xMnO_{3+{\delta}} (x=0.1-0.4)/YSZ (yttria-stabilized zirconia) electrodes are investigated in the temperature range of 700-900 ℃ at $Po_2=10^{-3}$-0.21 atm. Results of the steady-state polarization measurements, which are formulated into the Butler-Volmer formalism to extract transfer coefficient values, lead us to conclude that the two-electron charge transfer step to atomically adsorbed oxygen is rate-limiting. The same conclusion is drawn from the $Po_2$-dependent ac impedance measurements, where the exponent m in the relationship of $I_o$ (exchange current density) ∝ $P_{o_{2}}^m$ is analyzed. Chemical analysis is performed on the quenched Mn perovskites to estimate their oxygen stoichiometry factors (δ) at the operating temperature (700-900 ℃). Here, the observed δ turns out to become smaller as both the Sr-doping contents (x) and the measured temperature increase. A comparison between the 8 values and cathodic activity of Mn perovskites reveals that the cathodic transfer coefficients $({\alpha}_c)$ for oxygen reduction reaction are inversely proportional to δ whereas the anodic ones $({\alpha}_a)$ show the opposite trend, reflecting that the surface oxygen vacancies on Mn perovskites actively participate in the $O_2$ reduction reaction. Among the samples of x= 0.1-0.4, the manganite with x=0.4 exhibits the smallest 8 value (even negative), and consistently this electrode shows the highest ${\alpha}_c$ and the best cathodic activity for the oxygen reduction reaction.

• Nuclear Engineering and Technology
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• v.51 no.5
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• pp.1373-1380
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• 2019

Besides promising implications as fertile nuclear materials, thorium carbonitrides are of great interest owing to their peculiar physical and chemical properties, such as high density, high melting point, good thermal conductivity. This paper reports first-principles simulation results on the structural, electronic and magnetic properties of cubic thorium carbonitrides $ThC_xN_{(1-x)}$ (X = 0.03125, 0.0625, 0.09375, 0.125, 0.15625) employing formalism of density-functional-theory. For the simulation of physical properties, we incorporated full-potential linearized augmented plane-wave (FPLAPW) method while the exchange-correlation potential terms in Kohn-Sham Equation (KSE) are treated within Generalized-Gradient-Approximation (GGA) in conjunction with Perdew-Bruke-Ernzerhof (PBE) correction. The structural parameters were calculated by fitting total energy into the Murnaghan's equation of state. The lattice constants, bulk moduli, total energy, electronic band structure and spin magnetic moments of the compounds show dependence on the C/N concentration ratio. The electronic and magnetic properties have revealed non-magnetic but metallic character of the compounds. The main contribution to density of states at the Fermi level stems from the comparable spectral intensity of Th (6d+5f) and (C+N) 2p states. In comparison with spin magnetic moments of ThSb and ThBi calculated earlier with LDA+U approach, we observed an enhancement in the spin magnetic moments after carbon-doping into ThN monopnictide.

• Korean Journal of Materials Research
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• v.28 no.11
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• pp.680-684
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• 2018

We investigate the effect of light intensity and wavelength of a solar cell device using photoconductive atomic force microscopy(PC-AFM). A $POCl_3$ diffusion doping process is used to produce a p-n junction solar cell device based on a polySi wafer, and the electrical properties of prepared solar cells are measured using a solar cell simulator system. The measured open circuit voltage($V_{oc}$) is 0.59 V and the short circuit current($I_{sc}$) is 48.5 mA. Moreover, the values of the fill factors and efficiencies of the devices are 0.7 and approximately 13.6 %, respectively. In addition, PC-AFM, a recent notable method for nano-scale characterization of photovoltaic elements, is used for direct measurements of photoelectric characteristics in limited areas instead of large areas. The effects of changes in the intensity and wavelength of light shining on the element on the photoelectric characteristics are observed. Results obtained through PC-AFM are compared with the electric/optical characteristics data obtained through a solar simulator. The voltage($V_{PC-AFM}$) at which the current is 0 A in the I-V characteristic curves increases sharply up to $18W/m^2$, peaking and slowly falling as light intensity increases. Here, $V_{PC-AFM}$ at $18W/m^2$ is 0.29 V, which corresponds to 59 % of the average $V_{oc}$ value, as measured with the solar simulator. Furthermore, while the light wavelength increases from 300 nm to 1,100 nm, the external quantum efficiency(EQE) and results from PC-AFM show similar trends at the macro scale but reveal different results in several sections, indicating the need for detailed analysis and improvement in the future.

• Proceedings of the KIPE Conference
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• 2019.07a
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• pp.230-232
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• 2019

Light Triggerd Thyristor (LTT)는 HVDC 및 산업용 스위치 등에 사용되는 대전력 반도체소자이다. 일반적인 Thyristor가 전기적 신호에 의해 trigger 되는 것과는 다르게 LTT는 광신호에 의해 동작하는 소자이다. 본 논문에서는 5,000V, 2,200A 급의 4인치 LTT 소자의 제작 및 전기적인 특성평가 결과를 기술하였다. 4인치 LTT의 구조적인 특징은 전면부 중앙에 광신호가 주입되는 수광부가 위치해 있으며 입력 전류 증폭을 위한 4-단계 증폭 게이트 (gate) 구조를 가지도록 설계하였다. $400{\Omega}{\cdot}cm$ 비저항을 갖는 1mm 두께의 n-형 실리콘 웨이퍼에 boron 이온주입과 열처리 공정으로 약 $30{\mu}m$ 깊이의 p-base를 형성하였으며, 고내압 저지를 위한 edge termination은 VLD (variable lateral doping) 기술을 적용하였다. 제작된 4인치 LTT는 6,500 V의 순방향 항복전압 ($V_{DRM}$) 특성을 나타내었으며, 100V의 어노드전압 ($V_A$)과 20 mA의 게이트전류 ($I_G$)에 의하여 thyristor가 trigger 됨을 확인하였다. 제작한 LTT 소자는 disk형 press-pack 패키지를 진행한 후, LTT의 수광부에 $10{\mu}s$, 50 mW의 900 nm 광 펄스를 조사하여 전류 특성을 평가하였다. LTT 패키지 샘플에 60 Hz 주파수의 광 펄스를 조사한 경우 2,460 A의 순방향 평균전류 ($I_T$)와 $336A/{\mu}s$의 반복전류상승기울기 (repetitive di/dt)에 안정적으로 동작함을 확인하였다. 또한, 펄스 전류 시험의 경우 61.6 kA의 최대 통전 전류 (ITSM, surge current)와 $1,050A/{\mu}s$의 펄스전류 상승 기울기 (di/dt of on-state pulse current)에도 LTT의 손상 없이 동작함을 확인하였다.