• Journal of the Korean Crystal Growth and Crystal Technology
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• v.28 no.2
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• pp.63-68
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• 2018

Natural convection of a two dimensional laminar steady-state incompressible fluid flow in a rectangular enclosure has been investigated numerically for low aspect ratios with the physical vapor transport crystal growth. Results show that for aspect ratio (Ar = L/H) range of $0.1{\leq}Ar{\leq}1.5$, with the increase in Grashof number by one order of magnitude, the total mass flux is much augmented, and is exponentially decayed with the aspect ratio. Velocity and temperature profiles are presented at the mid-width of the rectangular enclosure. It is found that the effect of Grashof number on mass transfer is less significant when the enclosure is shallow (Ar = 0.1) and the influence of aspect ratio is stranger when the enclosure is tall and the Grashof number is high. Therefore, the convective phenomena are greatly affected by the variation of aspect ratios.

• Journal of the Korean Institute of Electrical and Electronic Material Engineers
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• v.23 no.5
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• pp.418-423
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• 2010

Carbon nanotubes (CNTs) were synthesized by Fe-catalytic chemical vapor deposition (CVD) method about $800^{\circ}C$. The influence of process parameters such as pretreatment conditions, gas flow ratio, processing time, etc on the growth of CNTs was investigated by field emission scanning electron microscopy, transmission electron microscopy, and Raman spectroscopy. Ammonia was added to acetylene source gas before and during the CNT growth. Different types of CNTs formed depending upon the processing condition. It was found that ammonia prevented amorphous carbons from adsorbing to the outer wall of CNT, resulting in purification of CNTs during CNT growth.

• Proceedings of the Korean Society of Precision Engineering Conference
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• 2006.05a
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• pp.473-474
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• 2006

Aligned carbon nanotubes(CNTs) array were synthesized using direct current plasma-enhanced chemical vapor deposition. The nickel microgrids catalyzed the growth of carbon nanotubes which take on the area of the nickel microgrids. Selective growth of areas of nanotubes was achieved by patterning the nickel film. CNTs were grown on the pretreated substrates at 30% $C_2H_2:NH_3$ flow ratios for 10min. Carbon nanotubes with diameters about 20 nanometers and lengths approximately 720 nanometers were obtained. Morphologies of carbon nanotubes were observed by FE-SEM and TEM.

• Transactions on Electrical and Electronic Materials
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• v.16 no.4
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• pp.198-200
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• 2015

The microwave plasma enhanced chemical vapor deposition (PECVD) system was used to grow a carbon nanowall (CNW) on a silicon (Si) substrate with hydrogen (H₂) and methane (CH₄) gases. To find the growth mechanism of CNW, we increased the growth time of CNW from 5 to 30 min. The vertical and surficial conditions of the grown CNWs according to growth time were characterized by field emission scanning electron microscopy (FE-SEM). Energy dispersive spectroscopy (EDS) measurements showed that the CNWs consisted solely of carbon.

• Journal of Ceramic Processing Research
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• v.13 no.spc1
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• pp.42-46
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• 2012

The effect of the parameters for few-layered graphene growth by thermal CVD on nickel substrate was investigated. Graphene can be synthesized by using different strategies. Chemical vapor deposition (CVD) has known as one of the most attractive methods to produce graphene due to its good film uniformity, compatibility and large scale production. The control of parameters such as temperature, growth time and pressure in CVD process has been widely recognized as the most important process in graphene growth. Different carbon precursors, methane and acetylene, were introduced in the quartz tube with a variety of growth conditions. Raman spectroscopy was used to confirm the presence of a few- or multi-layered graphene.

• Journal of Ceramic Processing Research
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• v.13 no.spc1
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• pp.154-157
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• 2012

We present the simultaneous growth of single-walled carbon nanotubes and graphene with the optimal conditions of the synthesizing parameters. The dense and vertically aligned SWNTs having the length of over 100 ㎛ was grown by 2 nm-thick Fe catalytic layer. From 650 ℃, the vertically well-grown SWNTs were obtained by increasing the temperature. The severallayered graphene was synthesized with the gas mixing ratio of 15 : 1(H2 : C2H2) at 650 ℃ and higher temperatures. With these optimal conditions, the vertically well-grown SWNTs and the several-layered graphene were synthesized simultaneously. The presence of SWNTs and the layer of graphene were verified by field emission scanning electron microscopy and high resolution transmission electron microscopy. From the result of this simultaneous synthesizing approach, the possibility of one step growth process of CNTs and grapheme could be verified.

• Proceedings of the Korean Vacuum Society Conference
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• 2016.02a
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• pp.125.2-125.2
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• 2016

Semiconductor nanowires are essential building blocks for various nanotechnologies including energy conversion, optoelectronics, and thermoelectric devices. Bottom-up synthetic approach utilizing metal catalyst and vapor phase precursor molecules (i.e., vapor - liquid - solid (VLS) method) is widely employed to grow semiconductor nanowires. Al has received attention as growth catalyst since it is free from contamination issue of Si nanowire leading to the deterioration of electrical properties. Al-catalyzed Si nanowire growth, however, unlike Au-Si system, has relatively narrow window for stable growth, showing highly tapered sidewall structure at high temperature condition. Although surface chemistry is generally known for its role on the crystal growth, it is still unclear how surface adsorbates such as hydrogen atoms and the nanowire sidewall morphology interrelate in VLS growth. Here, we use real-time in situ infrared spectroscopy to confirm the presence of surface hydrogen atoms chemisorbed on Si nanowire sidewalls grown from Al catalyst and demonstrate they are necessary to prevent unwanted tapering of nanowire. We analyze the surface coverage of hydrogen atoms quantitatively via comparison of Si-H vibration modes measured during growth with those obtained from postgrowth measurement. Our findings suggest that the surface adsorbed hydrogen plays a critical role in preventing nanowire sidewall tapering and provide new insights for the role of surface chemistry in VLS growth.

• Korean Journal of Materials Research
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• v.15 no.4
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• pp.246-251
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• 2005

Plasma enhanced chemical vapor deposition(PE-CVD) method has an advantage in synthesizing carbon nanotubes(CNTs) at lower temperature compared with thermal enhanced chemical vapor deposition(TE-CVD) method. In this study, CNTs was prepared by using PE-CVD method. The growth rate of CNT was faster more than 100 times on using Invar alloy than iron as catalyst. It was found that chrome silicide was formed at the interface between chrome layer and silicon substrate which should be considered in designing process. Nanoparticles of Invar catalyst were found oxidized on their surfaces with a depth of 10 m. Microstructure was analyzed by scanning electron microscopy, transmission electron microscopy, scanning transmission electron microscopy, and energy dispersive x-ray spectrometry. Based on the result of analysis, growth mechanism at an initial stage was suggested.

• Journal of Mechanical Science and Technology
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• v.14 no.10
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• pp.1143-1150
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• 2000

The effects of ambient conditions on vaporizing sprays from a high-pressure swirl injector were investigated by an exciplex fluorescence method. Dopants used were 2% fluorobenzene and 9% DEMA (diethyl-methyl-amine) in 89% solution of hexane by volume. In order to examine the behavior of liquid and vapor phases inside of vaporizing sprays, ambient temperatures and pressures similar to engine atmospheres were set. It was found that the ambient pressure had a significant effect on the axial growth of spray, while ambient temperature had a great influence on the radial growth. The spatial distribution of vapor phase at temperatures above 473K became wider than that of liquid phase after half of injection duration. From the analysis of the area ratio for each phase, the middle part (region II) in the divided region was the region which liquid and vapor phases intersect. For liquid phase, fluorescence-intensity ratio was greatly changed at lms after the start of injection. However, the ratio of vapor phase was nearly uniform in each divided region throughout the injection.

• Korean Journal of Materials Research
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• v.16 no.8
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• pp.479-484
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• 2006

Highly c-axis oriented poly-crystalline GaN with a dimension of $1{\sim}3\;{\mu}m$ was deposited on $c-Al_2O_3$ substrate by vapor transport epitaxy (VTE) method at the temperature range of $900{\sim}1150^{\circ}C$. XRD intensities from (00'2) plane of grown GaNs were increased with reaction conditions which indicate the improvement of the crystal quality. In the PL spectra measured at 10 K, the spectrum composed with the neutral-donor bound exciton-related emission at 3.47 eV, crystal defect-related emission band at 3.42 eV and with its phonon replicas. The fact that intensity of $I_2$ were increased and FWHM were decreased with growth conditions means that the quality of GaN crystals were improved. With this simple VTE technology, we confirm that the GaNs were simply deposited on sapphire substrate and crystal quality related to optical properties of GaN grown by VTE were relatively good. PL emission without deep level emission in spite of polycrystalline structure can be applicable to the fabrication of large area and low cost optical devices using poly-GaN grown by VTE.