• Proceedings of the Korean Vacuum Society Conference
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• 2012.08a
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• pp.440-440
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• 2012

Nanomaterials have emerged as new building blocks to construct light energy harvesting assemblies. Size dependent properties provide the basis for developing new and effective systems with semiconductor nanoparticles, quantized charging effects in metal nanoparticle or their combinations in 2 and 3 dimensions for expanding the possibility of developing new strategies for photovoltaic system. As top-down approach, we developed a simple and effective method for the large scale formation of self-assembled Cu(In,Ga)$Se_2$ (CIGS) nanostructures by ion beam irradiation. The compositional changes and morphological evolution were observed as a function of the irradiation time. As the ion irradiation time increased, the nano-dots were transformed into a nano-ridge structure due to the difference in the sputtering yields and diffusion rates of each element and the competition between sputtering and diffusion processes during irradiation. As bottom-up approach, we developed the growth of CIGS nanowires using thermal-chemical vapor deposition (CVD) method. Vapor-phase synthesis is probably the most extensively explored approach to the formation of 1D nanostructures such as whiskers, nanorods, and nanowires. However, unlike binary or ternary chalcogenides, the synthesis of quaternary CIGS nanostructures is challenging because of the difficulty in controlling the stoichiometry and phase structure. We introduced a method for synthesis of the single crystalline CIGS nanowires in the form of chalcopyrite using thermal-CVD without catalyst. It was confirmed that the CIGS nanowires are epitaxially grown on a sapphire substrate, having a length ranged from 3 to 100 micrometers and a diameter from 30 to 500 nm.

• Proceedings of the Korean Vacuum Society Conference
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• 2012.08a
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• pp.161-161
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• 2012

본 연구에서는 InGaAs을 이용한 테라헤르쯔(THz) 발생과 검출 특성을 GaAs에 의한 특성과 비교, 조사하였다. 고온성장(HTG, $530^{\circ}C$) InGaAs를 이용하여 photo-Dember (pD) 효과(표면방출)에 의한 THz 발생 특성을 조사하였으며, THz 검출 특성에는 저온성장(LTG, $530^{\circ}C$) InGaAs: Be을 이용하였다. HTG-InGaAs 기판 위에 패턴한 금속전극 (Ti/Au, ${\sim}500{\times}500{\mu}m$)의 가장자리에 Ti: Sapphire fs 펄스 레이저(30 ps/90 MHz)를 조사하여 LTG-GaAs 수신기(Rx)로 THz를 검출, 전류신호(a)와 Fourier transform (FT) 주파수 스펙트럼(b)을 얻었다. HTG-InGaAs에서 얻은 파형은 SI-GaAs에서와 거의 비슷한 모양이었으나, 주파수 범위(0.5~2 THz)는 SI-GaAs의 1~3 THz 보다 좁고 FT 스펙트럼의 세기는 약 1/8 정도로 낮았다. LTG-InGaAs 수신기 (Rx)의 안테나는 쌍극자 ($5/20{\mu}m$) 형태를 가지고 있으며, SI-GaAs Tx로 발생시킨 광원을 사용하여 THz 영역의 검출 특성을 조사하였다. HTG-InGaAs Tx 및 LTG-InGaAs Rx의 이득은 각각 약 $5{\times}10^{-8}$ A/W과 $2.5{\times}10^{-8}$ A/W인 것으로 분석되었다.

• Proceedings of the Korean Institute of Electrical and Electronic Material Engineers Conference
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• 2009.11a
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• pp.172-172
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• 2009

In this study, we presented comparative discrimination methods to identify various line and planar defects observed in nonpolar a-GaN epilayers on r-sapphire substrates. Unlike the case of conventional c-GaN, which is dominated by perfect threading dislocations, systematic identification of undistinguishable defects using transmission electron microscopy (TEM) is necessary to suppress the propagation of defects in nonpolar GaN epilayers. Cross-sectional TEM images near the [0001] zone axis revealed that perfect mixed and pure screw type dislocations are visible, while pure edge, partial dislocations, and basal stacking faults (BSFs) are not discernible. In tilted cross-sectional TEM images along the [$1\bar{2}10$] zone axis, the dominant defects were BSFs and partial dislocations for the $g=10\bar{1}0$ and 0002 two-beam images, respectively. From plan view TEM images taken along the [$11\bar{2}0$] axis, it was found that the dominantpartial and perfect dislocations were Frank-Shockley with b=${\pm}1/6$<$20\bar{2}3$> and mixed type without an 1 component including b=${\pm}1/3$<$1\bar{2}10$> and ${\pm}1/3$<$\bar{2}110$>, respectively. Prismatic stacking faults were observed as inclined line contrast near the [0001] zone axis and were visible as band contrast in the two-beam images along the [$1\bar{2}10$] and [$11\bar{2}0$] zone axes.

• Journal of the Korean Crystal Growth and Crystal Technology
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• v.9 no.6
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• pp.574-579
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• 1999

A counter-flow type horizontal reactor of metal organic chemical vapor deposition was designed with the Reynolds and the Rayleigh numbers of Re = 4.5 and Ra = 215.8, respectively. The GaN thin films were grown and characterized by Hall measurement, double crystal X-ray diffraction analysis and photoluminescence measurement. The Si and Mg were also used for doping of GaN films. The dislocation density of $2.6{\times}10^8/\textrm {cm}^2$ was included in GaN films representing the geometrical lattice mismatch between sapphire substrates and GaN films. The Si doped n-GaN films provide the electron carrier density and mobility in the regions of $10^{17}~10^{18}/\textrm{cm}^3$ and 200~400 $\textrm{cm}^2$/V .sec, respectively. Mg doped p-GaN films were post-annealed and activated with the hole carrier density of $8{\times}10^{17}/{\textrm}{cm}^3$.

• JSTS:Journal of Semiconductor Technology and Science
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• v.14 no.5
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• pp.557-565
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• 2014

We report the effect of basal-plane stacking faults (BSFs) on X-ray diffraction (XRD) of non-polar (11$\underline{2}$0) a-plane GaN films with different $SiN_x$ interlayers. Complete $SiN_x$ coverage and increased three-dimensional (3D) to two-dimensional (2D) transition stages substantially reduce BSF density. It was revealed that the Si-doping profile in the Si-doped GaN layer was unaffected by the introduction of a $SiN_x$ interlayer. The smallest in-plane anisotropy of the (11$\underline{2}$0) XRD ${\omega}$-scan widths was found in the sample with multiple $SiN_x$ layers, and this finding can be attributed to the relatively isotropic GaN mosaic resulting from the increase in the 3D-2D growth step. Williamson-Hall (WH) analysis of the (h0$\underline{h}$0) series of diffractions was employed to determine the c-axis lateral coherence length (LCL) and to estimate the mosaic tilt. The c-axis LCLs obtained from WH analyses of the present study's representative a-plane GaN samples were well correlated with the BSF-related results from both the off-axis XRD ${\omega}$-scan and transmission electron microscopy (TEM). Based on WH and TEM analyses, the trends in BSF densities were very similar, even though the BSF densities extracted from LCLs indicated that the values were reduced by a factor of about twenty.

• Journal of the Korean Society for Aeronautical & Space Sciences
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• v.44 no.12
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• pp.1027-1034
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• 2016

In this research, experimental study about size effect of solid particles on erosion resistance is presented. A high-density polyethylene particle with a mm-sized diameter is accelerated using a two-stage light gas gun up to Mach number of approximately 3.0. An accelerated particle impacts aluminum alloy such as Al1050 and Al6061 T6, and infrared windows such as ZnS and sapphire specimens. For the aluminum alloy, craters that form on the surface of the specimens are measured to characterize the erosion resistance of the material. For the infrared windows, repetitive tests are conducted until a linear or circumferential crack is found to create damage threshold curves that define a material's erosive resistance. From the comparison of test data for various sizes of high-density polyethylene particles, it is found that erosion resistance of material is linearly dependent on the size of particles.

• Proceedings of the Korean Vacuum Society Conference
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• 2016.02a
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• pp.280.1-280.1
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• 2016

Generally InGaN/GaN green light emitting diode (LED) exhibits the low quantum efficiency (QE) due to the large lattice mismatch between InGaN and GaN. The QE of InGaN-based multiple quantum wells (MQWs) is drastically decreased when an emission wavelength shifts from blue to green wavelength, so called "green gap". The "green gap" has been explained by quantum confined Stark effect (QCSE) caused by a large lattice mismatch. In order to improve the QE of green LED, undoped graded short-period InGaN/GaN superlattice (GSL) and Si-doped GSL (SiGSL) structures below the 5-period InGaN/GaN MQWs were grown on the patterned sapphire substrates. The luminescence properties of InGaN/GaN green LEDs have been investigated by using photoluminescence (PL) and time-resolved PL (TRPL) measurements. The PL intensity of SiGSL sample measured at 10 K shows stronger about 1.3 times compared to that of undoped GSL sample, and the PL peak wavelength at 10 K appears at 532 and 525 nm for SiGSL and undoped GSL, respectively. Furthermore, the PL decay of SiGSL measured at 10 K becomes faster than that of undoped GSL. The faster decay for SiGSL is attributed to the increased wavefunction overlap between electron and hole due to the screening of piezoelectric field by doped carriers. These PL and TRPL results indicate that the QE of InGaN/GaN green LED with GSL structure can be improved by Si-doping.

• Proceedings of the Materials Research Society of Korea Conference
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• 2009.11a
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• pp.23.1-23.1
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• 2009

ZnO is of great interest for various technological applications ranging from optoelectronics to chemical sensors because of its superior emission, electronic, and chemical properties. In addition, vertically well-aligned ZnO nanorods on large areas with good optical and structural properties are of special interest for the fabrication of electronic and optical nanodevices. To date, several approaches have been proposed for the growth of one-dimensional (1D) ZnO nanostructunres. Several groups have been reported the MOCVD growth of ZnO nanorods with no metal catalysts at $400^{\circ}C$, and fabricated a well-aligned ZnO nanorod array on a PLD prepared ZnO film by using a catalyst-free method. It has been suggested that the synthesis of ZnO nanowires using a template-less/surfactant-free aqueous method. However, despite being a well-established and cost-effective method of thin film deposition, the use of magnetrons puttering to grow ZnO nanorods has not been reported yet. Additionally,magnetron sputtering has the dvantage of producing highly oriented ZnO film sat a relatively low process temperature. Currently, more effort has been concentrated on the synthesis of 1D ZnO nanostructures doped with various metal elements (Al, In, Ga, etc.) to obtain nanostructures with high quality,improved emission properties, and high conductance in functional oxide semiconductors. Among these dopants, Ga-doped ZnO has demonstrated substantial advantages over Al-doped ZnO, including greater resistant to oxidation. Since the covalent bond length of Ga-O ($1.92\;{\AA}$) is nearly equal to that of Zn-O ($1.97\;{\AA}$), high electron mobility and low electrical resistivity are also expected in the Ga-doped ZnO. In this article, we report the successful growth of Ga-doped ZnO nanorods on c-Sapphire substrate without metal catalysts by magnetrons puttering and our investigations of their structural, optical, and field emission properties.

• Journal of the Korean Ceramic Society
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• v.39 no.3
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• pp.272-277
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• 2002

Aluminum nitride (AlN) powders and whiskers were synthesized by a modified carbothermal reduction and nitridation where a ($NH_4)[Al(ethylenediaminetetraacetate)]{\cdot}2H_2O$ complex is used as precursor. The AlN powders were obtained by calcining the complex without mixing any carbon source under a flow of nitrogen in the temperature range 1200∼1500$^{\circ}$C and then burning out the residual carbon. The nitridation process was investigated by $^{27}Al$ magic-angle spinning (MAS) unclear magnetic resonance, infrared spectroscopy and X-ray diffraction. The complex is pyrolyzed, converted to ${\rho}$- and ${\gamma}$- alumina and then nitridated to AlN without ${\gamma}-{\alpha}$ alumina transition. The morphology of ${\gamma}$-alumina, when it was converted to AlN, was retained, strongly indicating that ${\gamma}$-alumina is converted to AlN through solid-state $AlO_xN_y$, not through gaseous intermediates such as aluminum and aluminaum suboxides. AlN whiskers were obtained, when a (0001) sapphire was used as a catalyst.

• The Journal of Korean Institute of Electromagnetic Engineering and Science
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• v.15 no.8
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• pp.752-758
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• 2004

This paper reports on the DC and RF characteristics of AlGaN/InGaN/GaN high electron-mobility transistors (HEMTs) grown by molecular beau epitaxy(MBE) on sapphire substrates. The devices with a 0.5 ${\mu}$m gate-length exhibited relatively flat transconductance(g$\_$m/), which results from the enhanced carrier confinement of the InGaN channel. The maximum drain current was 880 mA/mm with a peak g$\_$m/ of 156 mS/mm, an f$\_$T/ of 17.3 GHz, and an f$\_$MAX/ or 28.7 GHz. In addition to promising DC and RF results, pulsed I-V and current-switching measurements showed little dispersion in the unpassivated AlGaN/InGaN HEMTs. These results suggest that the addition of In to the GaN channel improves the electron transport characteristics as well as suppressing current collapse that is related to the surface trap states.