• Proceedings of the Korean Vacuum Society Conference
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• 2012.02a
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• pp.565-565
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• 2012

그래핀(Graphene) 기반의 전계효과 트랜지스터(Field effect transistor) 응용에 있어, 가장 핵심적인 도전과제중 하나는 에너지 밴드갭(Energy bandgap)을 갖는 그래핀 채널의 제작이다. 그래핀은 에너지 밴드갭이 존재하지 않는 반금속(semi metal)의 특성을 지니고 있어, 그 본래의 물리적 특성을 지니고서는 소자구현에 어려움이 있다. 그러나 폭이 수~수십 나노미터인 그래핀 나노리본(Graphene nanoribbon)의 경우 양자구속효과(Quantum confinement effect)에 의하여 에너지 밴드갭이 형성되며, 갭의 크기는 리본의 폭에 반비례한다는 연구결과가 보고된 바 있다. 이러한 이유에서, 효과적이며 실현가능한 그래핀 나노리본의 제작은 필수적이다. 본 연구에서는 은 나노 와이어(Ag nanowire)를 기반으로 한 그래핀 나노리본의 합성을 연구하였다. 은 나노와이어를 열화학 기상증착법(Thermal chemical vapor deposition)을 이용, 아세틸렌(Acetylene, C2H2) 가스를 탄소공급원으로 하여 그래핀을 나노와이어 표면에 합성하였다. 합성과정에서 구조에 영향을 미치는 요인인 합성온도와 가스의 비율, 압력 등을 조절하여 최적화된 합성조건을 확립하였다. 합성된 나노리본의 특성을 라만분광법(Raman spectroscopy)과 주사전자 현미경(Scanning electron microscopy), 투과전자현미경(Transmission electron microscopy), 원자힘 현미경(Atomic force microscopy)를 통하여 분석하였다.

• Proceedings of the Korean Vacuum Society Conference
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• 2012.02a
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• pp.424-424
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• 2012

나노 구조의 반도성 산화물은 독특한 구조적 특성으로 전기적, 광학적 특성을 향상 시킬 수 있다. 현재 연구되고 있는 나노 구조의 반도성 산화물 중 Zinc oxide (ZnO)는 3.37 eV의 bandgap를 갖는 wurtzite 구조체로서 상온에서 60 meV의 exciton binding energy 등 우수한 특성으로 인하여 최근 많이 연구되고 있다. 특히 단파장 light emitting diode 재료로써 기대를 모으고 있는데, 이를 실현하기 위한 가장 큰 문제점이 바로 안정적인 p-type ZnO 박막의 제조이다. 지금까지 알려진 바에 따르면 P를 doping한 후 급속 열처리한 경우 p-type의 전기전도도를 갖는 ZnO 박막을 제조할 수 있다고 보고되어 있으나 vacancy 농도에 따른 불안정적인 요소가 해결해야 할 문제로 남아 있다. 최근 Ag를 doping 시킨 ZnO 박막의 p-type 반도체로서 가능성에 대한 보고가 제기되고 있다. 합성 방법과 조건에 따라서 수 nm에서 수십 또는 수백 nm 크기의 구형 입자나, 리본, 와이어, 로드 그리고 꽃모영 등 다양한 형상을 갖는 나노 구조체를 합성 할 수 있다. 본 연구에서는 ZnO:Ag 박막을 radio-frequency sputtering 방법으로 증착하여 그 물성을 분석하였다. 보통의 sputtering 증착법에서 사용되는 sintering된 타겟과 달리 본 실험은 분말 타겟을 이용하여 박막을 증착하였다. 타겟은 95 wt% ZnO와 5 wt% Ag를 서로 혼합하여 제조하였다. 본 발표에서는 박막의 증착압력 및 증착 온도의 변화에 따른 ZnO:Ag 박막의 구조적, 광학적 특성에 대하여 논의 할 것이다.

• Proceedings of the Korean Vacuum Society Conference
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• 2011.02a
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• pp.438-438
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• 2011

White light-emitting diodes (LEDs), the so-called next-generation solid-state lighting, offer benefits in terms of reliability, energy-saving, maintenance, safety, lead-free, and eco-friendly. Recently, rare-earth-doped oxynitride or nitride compounds have attracted a great deal of interest as a photoluminescent material because of their unique luminescent property, especially for white LEDs applications. Ce doped ${\beta}$-SiAlON has been studied as a wavelength conversion phosphor in white LEDs thanks to its high absorption rates, high quantum efficiency, and excellent thermal stability. Previously researches were not enough to understand the detail mechanism and characteristics of ${\beta}$-SiALON. The bandgap structures and electronic structures were not exact due to limitation of calculation methods. In this study, to elucidate the Ce doping effect on the SiAlON system, accurate band structures and electronic structure of the Ce doped ${\beta}$-SiAlON was intensively investigated using density functional theory calculations. In order to get a better description of the band gaps, MBJLDA method were used. We have found a single Ce atom site in ${\beta}$-SiAlON super cell. Furthermore, the density of state, band structure and lattice constant were intensively investigated.

• Electrical & Electronic Materials
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• v.10 no.10
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• pp.1005-1010
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• 1997

ZnS:Mn thin film was made by coevaporation with Electron Beam Evaparation(EBE) method. And structural and optical characteristics of ZnS:Mn thin films were investigated by substrate temperature annealing temperature and dopant Mn. When ZnS:Mn thin film was well deposited with cubic crystalline at substrate temperature of 30$0^{\circ}C$ its surface index was [111] and its lattice constant of a was 5.41$\AA$. Also When ZnA:Mn thin film was well made with hexagonal crystalline at substrate temperature of 30$0^{\circ}C$annealing temperature of 50$0^{\circ}C$and annealing time of 60min its miller indices were (0002) (1011), (1012) and (1120). And its lattice constant of a and c was 3.88$\AA$and 12.41$\AA$ respectively. Finally hexagonal ZnS:Mn thin film with dopant Mn of 0.5wt% had fundamental absorption wavelength of 342nm. And so its energy bandgap was about 3.62eV.

• Proceedings of the Korean Magnestics Society Conference
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• 2013.05a
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• pp.19-20
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• 2013

Thermoelectic properties of the typical thermoelectric host materials, the tellurides and selenides, are known to be noticeably changed by their volume change due to the strain [1]. In the bismuth telluride ($Bi_2Te_3$) crystal, a substitution of rare-earth element by replacing one of the Bi atoms may cause the change of the lattice parameters while remaining the rhombohedral structure of the host material. Using the first-principles approach by the precise full potential linearized augmented plane wave (FLAPW) method [2], we investigated the Ce substitution effect on the thermoelectric transport coefficients for the bismuth telluride, employing Boltzmann's equation in a constant relaxation-time approach fed with the FLAPW wave-functions within the rigid band approximation. Depending on the real process of re-arrangement of atoms in the cell to reach the equilibrium state, $CeBiTe_3$ was found to manifest a metal or a narrow bandgap semiconductor. This feature along with the strong correlation effect originated by the 4f states of Ce affect significantly on the thermoelectric properties. We showed that the position of the strongly localized f-states in energy scale (Fig. 1, f-states are shaded) was found to alter critically the transport properties in this material suggesting an opportunity to improve the thermoelectric efficiency by tuning the external strain which may changing the location of the f-sates.

• The Transactions of The Korean Institute of Electrical Engineers
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• v.57 no.12
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• pp.2276-2280
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• 2008

In this paper, we present thermal dependancy of LED package element by changing temperature of MCPCB for design high efficiency LED lamp, and confirmed influence of LED chip against temperature with analysis of thermal resistance and thermal capacitance. As increasing temperature, WPOs were decreased from 25 to 22.5 [%] and optical power were also decreased. that is decreased reason of optical power that forward voltage was declined by decrease of energy bandgap. Therefore optical power by temperature of MCPCB should consider to design lamp for street light and security light. Moreover, compensation from declined optical efficiency is demanded when LED package is composed. Also, thermal resistances from chip to metal PCB were decreased from 12.18 to 10.8[$^{\circ}C/W$] by changing temperature. Among the thermal resistances, the thermal resistance form chip to die attachment was decreased from 2.87 to 2.5[$^{\circ}C/W$] and was decreased 0.72[$^{\circ}C/W$] in Heat Slug by chaning temperature. Therefore, because of thermal resistance gap in chip and heat slug, reliability and endurance of high power LED affect by increasing non-radiative recombination in chip from heat.

• Journal of the Korean Institute of Telematics and Electronics A
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• v.29A no.9
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• pp.56-64
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• 1992

Because of a small bandgap energy, a high doping density, and a low operating temperature, the dark current in HgCdTe photodiode is almost composed of a tunneling current. The tunneling current is devided into an indirect tunneling current via traps and a band-to-band direct tunneling current. The indirect tunneling current dominates the dark current for a relatively high temperature and a low reverse bias and forward bias. For a low temperature and a high reverse bias the direct tunneling current dominates. In this paper, to verify the tunneling currents in HgCdTe photodiode, the new tunneling-recombination equation via trap is introduced and tunneling-recombination current is calculated. The new tunneling-recombination equation via trap have the same form as SRH (Shockley-Read-Hall) generation-recombination equation and the tunneling effect is included in recombination times in this equation. Chakrabory and Biswas's equation being introduced, band to band direct tunneling current are calculated. By using these equations, HgCdTe (mole fraction, 0.29 and 0.222) photodiodes are analyzed. Then the temperature dependence of the tunneling-recombination current via trap and band to band direct tunneling current are shown and it can be known what is dominant current according to the applied bias at athe special temperature.

• Proceedings of the Materials Research Society of Korea Conference
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• 2009.05a
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• pp.23.1-23.1
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• 2009

Transparent thin film transistor(TTFT)는 기존의 디스플레이가 가지고 있는 공간적, 시각적 제약을 해소하는 것이 가능하며, 이는 디스플레이 산업 및 기술이 지향하는 대면적, 저가격, 공정의 단순함을 해결해 줄 수 있기 때문에 최근 TTFT에 관한 연구가 급증하고 있다. 산화물 기반의 TFT는 유리, 금속, 플라스틱 등등 그 기판 종류에 상관없이 균일한 제작이 가능하며, 상온 및 저온에서 대면적으로 제작 가능하고, 저렴한 비용으로 제작 가능하다는 장점 때문에 최근 산화물을 기반으로 하는 TFT 연구가 많이 이루어지고 있다. 현재 TTFT 물질로 많이 연구되고 있는 산화물은 ZnO(3.4 eV)나 $InO_x$(3.6 eV), $GaO_x$(4.9 eV), $SnO_x$(3.7 eV)등의 물질과 각각의 조합으로 구성된 재료들이 주로 사용되고 있다. 가장 많은 연구가 이루어진 ZnO 기반의 TFT는 mobility와 switching 속도에서 우수한 특성을 보이나, amorphous ZnO 기반의 TFT의 경우 소자의 안정성이 떨어지는 것으로 보고되고 있다. 따라서 본 연구에서는 ZnO 보다 넓은 bandgap energy를 가질 수 있으며, n-type 특성을 보이고, amorphous 구조로 제작 가능한 IGZO 물질을 사용하여 RF magnetron sputtering 방법으로 박막 증착 온도의 변화를 주어 증착하였고, 증착된 IGZO 박막의 열처리를 통해 이에 따른 특성 변화를 분석하였다. Field emission scanning electron microscope(FESEM)와 surface profiler를 이용하여 IGZO 박막의 표면의 형상과 두께를 확인하였으며, x-ray diffraction(XRD) 분석을 통해 박막의 결정학적 특성을 관찰하였다. TTFT 물질로서 IGZO 박막의 적합성 여부를 확인하기 위하여 TFT를 만든 후 I-V를 측정하였으며, UV-vis를 이용하여 IGZO 박막의 투과율을 분석하여 TTFT로의 응용 가능성을 확인하였다.

• Korean Journal of Optics and Photonics
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• v.8 no.3
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• pp.213-217
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• 1997

We have measured photoluminescence (PL) and time-resolved PL in doped-GaAs. As increasing doping concentration, the PL spectra of n-type GaAs shift to higher energies while the PL spectra of p-type GaAs shift to lower energies than the bandgap of the undoped GaAs. The contribution of the Burstein-Moss effect overrules the band-gap narrowing in n-type GaAs, contrary to p-type GaAs. The PL rise time and decay time become shorter as increasing doping concentration. The PL rise and decay time in doped-GaAs depend on the type of majority carriers and their concentrations, which imply that the carrier-carrier interaction plays an important role in the energy relaxation processes.

• Journal of the Korean Vacuum Society
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• v.12 no.S1
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• pp.113-115
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• 2003

We investigate transmission of light in nanoscale structures. We present spatial and temporal domain measurements of the dephasing of surface plasmon excitations in metal films with periodic nano-hole arrays. By probing coherent spatial SP propagation lengths of a few f1. $\mu$m and an ultrafast decay of the SP polarization on a 10 fs timescale, we demonstrate that the SP transmission peaks are homogeneously broadened by the SP radiative lifetime. The pronounced wavelength and hole size dependence of the dephasing rate shows that the microscopic origin of the conversion of SP into light is a Rayleigh-like scattering by the periodic hole array. We have experimentally studied the dephasing of surface plasmon excitations in metallic nano-hole arrays. By relating nanoscopic SP propagation, ultrafast light transmission and optical spectra, we demonstrate that the transmission spectra of these plasmonic bandgap structures are homogeneously broadened. The spectral line shape and dephasing time are dominated by Rayleigh scattering of SP into light and can varied over a wide range by controlling the resonance energy and/or hole radius. This opens the way towards designing SP nano-optic devices and spatially and spectrally tailoring light -matter interactions on nanometer length scales.