• Proceedings of the Korean Vacuum Society Conference
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• 2000.02a
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• pp.166-166
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• 2000

We studied electronic structure of magnetic semiconductors EuO, EuS, and EuTe. The photoemission spectra show localized Eu 4f states and broad anion p bands. As the size of anion increases from oxygen to tellurium, anion p band width increases and eventually overlaps Eu 4f states. Hence in EuO and EuS, Eu 4f states are the highest occupied stated lying above anion p band, while Te 5p band spreads widely over Eu 4f states to become valence band maximum in EuTe. It was also observed that Eu 4f states have width of 0.7eV and dispersion of 0.2eV in EuS by angle resolved photoemission spectroscopy. The width of the 4f spectra mainly originates from atomic multiplets, but the much larger dispersion than that of Eu metal is due to p-f mixing.

• Journal of the Korean Crystal Growth and Crystal Technology
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• v.25 no.4
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• pp.135-139
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• 2015

$Bi_2Te_3-Bi_2Se_3$ alloy which is typical n-type thermoelectric material were grown by traveling heater method (THM) technique. We investigate the effect of the composition of $100-x(Bi_2Te_3)-x(Bi_2Se_3)$ and doping of n-type dopants such as $SbI_3$ and $CdCl_2$. Maximum figure of merit of $Bi_2Te_3-Bi_2Se_3$ alloy was observed with $CdCl_2$ 0.1 wt% (Z: $2.73{\times}10^{-3}/K$) and $SbI_3$ 0.05 wt% (Z: $2.29{\times}10^{-3}/K$). Deviation along the length of $Bi_2Te_3-Bi_2Se_3$ ingot grown by THM method is low, which indicates that the ingot is very homogenized. Also we observed the close relationship of between anisotropy ratio and dopant in the $90(Bi_2Te_3)-10(Bi_2Se_3)$ alloys. And we confirmed the fact that anisotropy ratio exerts thermoelectric performance in $Bi_2Te_3$ based n-type thermoelectric material.

• Journal of the Korean Vacuum Society
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• v.8 no.3B
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• pp.297-301
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• 1999

High quality CdTe single crystal for the solar cell fabrication was grown by vertical Bridgman method. The etch pits patterns of {111} surfaces of CdTe etched by Nakagawa solution was observed the {11} A composed of Cd atoms with typical triangle etch pits of pyramid mode. From the photoluminescence measurement of {111} A, we observed free exciton $(E_x)$ existing only high quality crystal and neutral acceptor bound exciton ($A^{\circ}$, X) having very strong peak intensity. Then, the full width at half maximum and binding energy of neutral acceptor bound exciton were 7 meV and 5.9 meV, respectively. By Haynes rule, and activation energy of impurity was 59meV. Therefore, the origins on impurity level acting as a neutral acceptor were associated Ag or Cu elements.

• Korean Journal of Materials Research
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• v.16 no.5
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• pp.292-296
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• 2006

Effects of rapid thermal annealing of bismuth telluride thin films on their thermoelectric properties were investigated. Films with four different compositions were elaborated by co-sputtering of Bi and Te targets. Rapid thermal treatments in range of $300{\sim}400^{\circ}C$ were carried out during 10 minutes under the reducing atmosphere (Ar with 10% $H_2$). As the temperature of thermal treatment increased, carrier concentrations of films decreased while their mobilities increased. These changes were clearly observed for the films close to the stoichiometric composition. Rapid thermal treatment was found to be effective in improving the thermoelectric properties of $Bi_2Te_3$ films. Recrystallization of $Bi_2Te_3$ phase has caused the enhancement of thermoelectric properties, along with the decrease of the carrier concentration. Maximum values of Seebeck coefficient and power factor were obtained for the films treated at $400^{\circ}C$ (about $-128{\mu}V/K$ and $9{\times}10^{-4}\;W/K^2m$, respectively). With further higher temperature ($500^{\circ}C$), thermoelectric properties deteriorated due to the evaporation of Te element and subsequent disruption of film's structure.

• The Journal of Korean Institute of Electromagnetic Engineering and Science
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• v.10 no.4
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• pp.530-538
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• 1999

In this paper, design of rectangular iris waveguide filter for the microwave broadcasting transponder using WR137 is applied to the modified ${TE^x}_{mn}$ mode theory in order to exactly analyze the characteristic of the waveguide discontinuity. We designed and fabricated the rectangular iris waveguide filter with the bandwidth 47 MHz for the microwave broadcasting transponder applied the conjugate gradient method to the modified ${TE^x}_{mn}$ mode theory. The results show a good characteristics that the $S_{11}$ and $S_{21}$ are -23 ㏈, -0.56 ㏈ and the maximum VSWR is below 1.02 in the passband.

• Proceedings of the Korean Vacuum Society Conference
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• 2015.08a
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• pp.197.1-197.1
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• 2015

현재 화합물 반도체 나노구조는 적외선 검출기, 레이저, 발광 다이오드, 단전자 트랜지스터, 태양전지 등과 같은 고효율 광전자 소자에서의 응용을 위해 활발한 연구가 진행 되고 있다. 특히 양자점은 3차원으로 구속되어 있는 상태 밀도를 갖고 있어 레이저 응용 시 낮은 문턱 전류 밀도, 높은 이득, 높은 열적 안정성을 기대되고 있다. 하지만 양자점의 크기가 불규칙적이고 운반자 수집의 한계로 인하여 기대 이하의 온도 안정성을 갖고 있어 이를 극복하기 위해 양자점의 크기와 운반자 수집을 제어하기 위해 다양한 방법이 연구되고 있다. 본 연구에서는 분자 선속 에피 성장법(Molecular Beam Epitaxy; MBE)과 원자 층 교대 성장법(Atomic Layer Epitaxy; ALE)으로 크기가 다른 CdTe/ZnTe 이중 양자점을 ZnTe 장벽층의 두께에 변화하면서 성장 후 광학적 특성을 연구하였다. 저온 광루미네센스 측정(Photoluminescence; PL) 측정 결과 장벽층 두께가 작아질수록 작은 양자점의 광 루미네센스의 세기가 감소하면서 큰 양자점의 세기가 증가하는 것을 관찰할 수 있었는데, 이는 장벽층 두께가 작아질수록 작은 양자점의 운반자들이 큰 양자점으로 이동되는 양이 많아지기 때문이다. 또한 장벽층 두께가 작아질수록 큰 양자점의 반치폭(Full Width at Half Maximum; FWHM)이 단층 양자점의 반치폭 보다 감소하는 것을 관찰 할 수 있었는데 이는 작은 양자점과 결합된 큰 양자점이 작은 양자점의 strain을 받아 크기의 균일함이 증가했기 때문이다. 이와 같은 결과 두 양자점이 결합된 이중 양자점 구조가 단층 양자점의 한계인 운반자 수집과 크기의 균일함을 향상할 수 있는 좋은 구조임을 제시하고 있다.

• Journal of the Korean Institute of Electrical and Electronic Material Engineers
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• v.21 no.12
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• pp.1135-1140
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• 2008

Bismuth-telluride based thin film materials are grown by Metal Organic Chemical Vapor Deposition(MOCVD). A planar type thermoelectric device has been fabricated using p-type $Bi_{0.4}Sb_{1.6}Te_3$ and n-type $Bi_2Te_3$ thin films. Firstly, the p-type thermoelectric element was patterned after growth of $4{\mu}m$ thickness of $Bi_{0.4}Sb_{1.6}Te_3$ layer. Again n-type $Bi_2Te_3$ film was grown onto the patterned p-type thermoelectric film and n-type strips are formed by using selective chemical etchant for $Bi_2Te_3$. The top electrical connector was formed by thermally deposited metal film. The generator consists of 20 pairs of p- and n-type legs. We demonstrate complex structures of different conduction types of thermoelectric element on same substrate by two separate runs of MOCVD with etch-stop layer and selective etchant for n-type thermoelectric material. Device performance was evaluated on a number of thermoelectric devices. To demonstrate power generation, one side of the sample was heated by heating block and the voltage output measured. As expected for a thermoelectric generator, the voltage decreases linearly, while the power output rises to a maximum. The highest estimated power of $1.3{\mu}W$ is obtained for the temperature difference of 45 K. we provide a promising procedure for fabricating thin film thermoelectric generators by using MOCVD grown thermoelectric materials which may have nanostructure with high thermoelectric properties.

• Journal of the Korean Vacuum Society
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• v.9 no.4
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• pp.406-410
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• 2000

Using the hot-wall epitaxy method, we grew a $Hg_{1-x}Cd_xTe$ (MCT) thin film in-situ after growing (111) CdTe of 9 $mu \textrm{m}$ as a buffer layer. The value of FWHM of double crystal x-ray diffraction rocking curve was 125 arcsec and the surface morphology was clean with a small roughness of 10 nm. From measuring the photocurrent of the grown MCT thin film, the maximum peak wavelength and the cut-off wavelength were 1.1050 $\mu\textrm{m}$ (1.1220 eV) and 1.2632 $\mu\textrm{m}$ (0.9815 eV), respectively. This peak wavelength corresponds to the peak of the band gap due to the intrinsic transition of the photoconductor. Therefore, the MCT thin film could be used as the photoconducting detector sensing a near-IR wavelength band from 1.0 to 1.6 $\mu\textrm{m}$.

• Bulletin of the Korean Chemical Society
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• v.35 no.10
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• pp.2895-2900
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• 2014

We fabricated quantum dot-sensitized solar cells (QDSSCs) using cadmium sulfide (CdS) and cadmium telluride (CdTe) quantum dots (QDs) as sensitizers. A spin coated $TiO_2$ nanoparticle (NP) film on tin-doped indium oxide glass and sputtered Au on fluorine-doped tin oxide glass were used as photo-anode and counter electrode, respectively. CdS QDs were deposited onto the mesoporous $TiO_2$ layer by a successive ionic layer adsorption and reaction method. Pre-synthesized CdTe QDs were deposited onto a layer of CdS QDs using a direct adsorption technique. CdS/CdTe QDSSCs had high light harvesting ability compared with CdS or CdTe QDSSCs. QDSSCs incorporating single-walled carbon nanotubes (SWNTs), sprayed onto the substrate before deposition of the next layer or mixed with $TiO_2$ NPs, mostly exhibited enhanced photo cell efficiency compared with the pristine cell. In particular, a maximum rate increase of 24% was obtained with the solar cell containing a $TiO_2$ layer mixed with SWNTs.

• Journal of the Korean Institute of Electrical and Electronic Material Engineers
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• v.24 no.4
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• pp.340-344
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• 2011

Thermoelectric bismuth telluride ($Bi_2Te_3$) films were deposited on $4^{\circ}$ off oriented (001) GaAs substrates using a modified metal organic chemical vapor deposition (MOCVD) system. The effects of substrate temperature on surface morphologies, crystallinity, electrical properties and thermoelctric properties were investigated. Two dimensional growth mode (2D) was observed at substrate temperature lower than $400^{\circ}C$. However, three dimensional growth mode (3D) was observed at substrate temperature higher than $400^{\circ}C$. Change of growth mechanism from 2D to 3D was confirmed with environmental scanning electron microscope (E-SEM) and X-ray diffraction analysis. Seebeck coefficients of all samples have negative values. This result indicates that $Bi_2Te_3$ films grown by modified MOCVD are n-type. The maximum value of Seebeck coefficient was -225 ${\mu}V/K$ and the power factor was $1.86{\times}10^{-3}\;W/mK^2$ at the substrate temperature of $400^{\circ}C$. $Bi_2Te_3$ films deposited using modified MOCVD can be used to fabricate high-performance thermoelectric devices.