Role of the Oxidizing Agent in the Etching of 4H-SiC Substrates with Molten KOH

Moonkyong Na,In-Ho Kang,Jeong Hyun Moon,Wook Bahng 한국물리학회 2016 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.69 No.11

A novel etching solution using molten potassium hydroxide (KOH) for the identification of dislocation types in a silicon-carbide (SiC) epilayer is identified. Threading screw dislocations (TSDs) and threading edge dislocations (TEDs) are rarely useful for size-based differentiation of etch pits in highly nitrogen (N)-doped SiC through conventional KOH etching. In this study, we report the role of sodium peroxide (Na2O2) and potassium dioxide (KO2) as oxidizing agent additives to the etchant for identifying the dislocation types in highly N-doped 4H-SiC. A Na2O2-KOH phase diagram was calculated to predict the chemical composition of the etchant. Solid-phase Na2O2 remained in the system when added to the etchant at concentrations greater than 13-wt% Na2O2, and it provided excess oxygen to the etchant. We experimentally confirmed that etch pit shapes became more hexagonal and that the etch pit sizes of TSDs and TEDs differed more greatly when more than 20-wt% Na2O2 was added to the etchant. We also found that the size distribution of TEDs was much smaller than that of TSDs after etching using Na2O2-KOH. Dissolved oxygen played an essential role in enhancing the anisotropic etching of highly N-doped SiC and allowed the dislocation types to be identified.

Dielectric Properties of Polymer Thin Films for the Organic Gate Dielectric Layer

Na, Moonkyong,Park, Hoyyul,Ahn, Myeongsang TaylorFrancis 2009 MOLECULAR CRYSTALS AND LIQUID CRYSTALS - Vol.510 No.1

<P> Polymethylmethacrylate (PMMA)'s thermal and mechanical stability, high resistivity, and suitable dielectric constant make it an ideal candidate for the polymer thin film for the dielectric layer. PMMA thin films were fabricated on a glass substrate, using the spin coating process, at room temperature. The thermal-degradation temperature of PMMA was about 280°C, and the glass transition temperature (Tg) was about 110°C. To determine the effect of annealing, the coating films were annealed at 70-200°C for 60 m under argon atmosphere. The surfaces of the coating films were compact and uniform at all the annealing temperatures. The surface energies of the coating films were obtained by measuring the contact angles with deionized water and di-iodomethane. The coating films were found to have low surface energies. Up to below Tg, the dielectric constants of the coating films slightly increased owing to an increase of the total polarization arising from dipoles and trapped charge carriers. Above Tg, the coating films began to degrade; as such, their dielectric constants decreased. To ensure the reliability of the thermal endurance of the dielectric properties of the coating films, annealing was repeated three times, at 100°C. The coating films then showed no degraded dielectric properties.</P>

Synthesis of organic-inorganic hybrid sols with nano silica particles and organoalkoxysilanes for transparent and high-thermal-resistance coating films using sol-gel reaction.

Na, Moonkyong,Park, Hoyyul,Ahn, Myeongsang,Lee, Hyeonhwa,Chung, Ildoo American Scientific Publishers 2010 Journal of nanoscience and nanotechnology Vol.10 No.10

<P>Organic-inorganic hybrid sols were synthesized from nano silica particles dispersed in water and from organoalkoxysilanes, using the sol-gel reaction. This work focuses on the effects of the three multifunctional organoalkoxysilanes dimethyldimethoxysilane (DMDMS), methyltrimethoxysilane (MTMS), and tetramethoxysilane (TMOS) to form a transparent and high-thermal-resistance coating film. The stability of the hybrid sol was evaluated as a function of the reaction time for 10 d through the variation of the viscosity. The viscosity of the silica/DMDMS and silica/MTMS sol was slightly increased for 10 d. The multifunctional organoalkoxysilanes formed dense silica networks through hydrolysis and condensation reaction, which enhanced the thermal resistance of the coating films. No thermal degradation of the silica/DMDMS sample occurred up to 600 degrees C, and none of the silica/MTMS and silica/TMOS samples occurred either up to 700 degrees C. The organic-inorganic hybrid sols were coated on the glass substrate using a spin-coating procedure. The organic-inorganic hybrid sols formed flat coating films without cracks. The transmittance of the hybrid sol coating films using MTMS and DMDMS was shown to be over 90%. The transmittance of the silica/TMOS sol coating film reacted for 10 d abruptly decreased due to faster gelation. The silica/DMDMS and silica/MTMS hybrid sols formed smooth coating films while the surface roughness of the silica/TMOS coating film markedly increased when the hybrid sol reacted for 10 d. The increase of the surface roughness of the silica/TMOS coating film can be attributed to the degradation of the stability of the hybrid sol and to the loss of transmittance of the coating film. It was confirmed in this study that the use of organic-inorganic hybrid sol can yield transparent and high-thermal-resistance coating films.</P>

Properties of nano-hybrid sol-gel coating films synthesized with colloidal silica and organoalkoxy silanes

Moonkyong Na,Hoyyul Park,Myeongsang Ahn 한양대학교 세라믹연구소 2008 Journal of Ceramic Processing Research Vol.9 No.5

Colloidal silica (CS)-silane sols were synthesized as a dispersion medium of dispersoid, ceramic powders. Functionalizing silanes for nano-hybrid sols were methyltrimethoxysilane (MTMS) and γ-glycidoxypropyltrimethoxysilane (GTMS). GTMS was added to sols in different amounts. The stability of the sol was evaluated as a function of reaction time up to 10 days through the variation of the viscosity and the transmittance. The viscosity of the sols increased only slightly with reaction time before 10 days. When the sols were reacted for 10 days, the viscosity increased and the transmittance decreased markedly, as well. The coating films were formed on a glass substrate using a spin coating procedure. The coating films were flat and transparent without cracks. Also, the properties of the coating films such as contact angle, roughness, and thermal property were characterized. The CS/MTMS coating films showed a large contact angle. When a large amount of GTMS was added, the contact angle decreased due to the epoxy group. The roughness of the coating films was found to be very low, less than 20 nm. However the roughness of the coating films reacted for 10 days increased due to degradation of the stability of the sol. No thermal degradation of the CS/MTMS coating film occurred up to 600 oC. The CS/MTMS/GTMS coating films degraded at 400 oC due to decomposition of the epoxy group which has a long organic chain easily degraded by the thermal shock. Colloidal silica (CS)-silane sols were synthesized as a dispersion medium of dispersoid, ceramic powders. Functionalizing silanes for nano-hybrid sols were methyltrimethoxysilane (MTMS) and γ-glycidoxypropyltrimethoxysilane (GTMS). GTMS was added to sols in different amounts. The stability of the sol was evaluated as a function of reaction time up to 10 days through the variation of the viscosity and the transmittance. The viscosity of the sols increased only slightly with reaction time before 10 days. When the sols were reacted for 10 days, the viscosity increased and the transmittance decreased markedly, as well. The coating films were formed on a glass substrate using a spin coating procedure. The coating films were flat and transparent without cracks. Also, the properties of the coating films such as contact angle, roughness, and thermal property were characterized. The CS/MTMS coating films showed a large contact angle. When a large amount of GTMS was added, the contact angle decreased due to the epoxy group. The roughness of the coating films was found to be very low, less than 20 nm. However the roughness of the coating films reacted for 10 days increased due to degradation of the stability of the sol. No thermal degradation of the CS/MTMS coating film occurred up to 600 oC. The CS/MTMS/GTMS coating films degraded at 400 oC due to decomposition of the epoxy group which has a long organic chain easily degraded by the thermal shock.

The Effect of Threading Dislocation on Current-Voltage Characteristics of 3.3 kV 4H-SiC Schottky Barrier Diode

Na, Moonkyong,Keum, Juyeon,Moon, Jeong Hyun,Kang, In Ho,Bahng, Wook The Electrochemical Society 2018 ECS Transactions Vol.85 No.7

<P>We investigated the relationship between threading dislocation and current-voltage (I-V) characteristics of 3.3 kV 4H-SiC Schottky barrier diodes (SBDs). The crystal defects were observed using non-destructive methods. X-ray topography (XRT) and electron beam induced current (EBIC) were used to compare the current-voltage characteristics of fabricated SBDs with defects. The surface and process defects were found to be fatal to devices performance such as ideality factor and leakage current increased and the breakdown deterioration. However, the threading dislocations rarely affected forward I-V characteristics and blocking voltage of fabricated 3.3 kV SBDs. The leakage current of the SBDs with threading dislocations in active region was increased by 10<SUP>3</SUP> compared with those of the SBDs without defect.</P>

Characteristics of Inorganic-Organic Hybrid Coating Films Synthesized from Nano Boehmite and Methyltrimethoxysilane

Park, Hoyyul,Na, Moonkyong,Ahn, Myeongsang,Kang, Dongjun,Lee, Hyeonhwa,Chung, Ildoo TaylorFrancis 2009 Molecular Crystals and Liquid Crystals Vol.510 No.1

<P> We studied the properties of inorganic-organic hybrid coating films prepared by boehmite sol. Sols were synthesized by sol-gel process using nano boehmite and methyltrimethoxysilane in variation with the amount of methyltrimethoxysilane at different reaction time. In order to understand physical and chemical properties of sols prepared from boehmite and methyltrimethoxysilane, coating films were fabricated on glass substrates by dip-coating process. The crystalline pattern and morphology of sol powder of boehmite were observed. Contact angle, surface roughness, transmittance, chemical bond and thermal stability of coating films were investigated.</P>

The inclination of threading dislocation in chemical vapor deposition-grown single-crystal diamond analyzed by synchrotron white beam X-ray topography

Jang Hyemin,Na Moonkyong,Bahng Wook,Lee Jung Woo 한국물리학회 2022 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.80 No.2

Diamond is a promising material for next-generation power electronic devices used in high voltage and high temperature, but intrinsic defects introduced during the growth cause the degradation of electrical properties. There is a need to improve the crystal defect analysis method of analyzing large areas as non-destructive. Herein, the inclination of threading dislocations in an on-axis single-crystal free-standing diamond grown by microwave plasma-enhanced chemical vapor deposition (MPCVD) was analyzed using synchrotron white beam X-ray topography (SWBXRT). The majority of dislocations in CVD-grown diamond were known as [001] threading dislocation; however, SWBXRT indicated that the dislocations were not exact [001], but inclined from [001] direction. Dislocation inclination vector was calculated by g = <444> diffraction images, which determined direction and angle from [001] direction. The exact [001] dislocation accounted for 20% of investigated dislocations, and the remaining dislocations inclined randomly from the [001] direction with 7°–12°. The threading dislocations in the off-axis sample inclined in a particular direction due to step-flow dominant growth. The sample analyzed in this paper has no dependence on a particular direction due to localized step-flow caused by on-axis growth. Threading dislocations in an on-axis diamond inclined with a similar degree of well-controlled off-axis sample.

Characteristics of coating films synthesized by a sol-gel reaction using nanosized boehmite

Hoyyul Park,Moonkyong Na,Myeongsang Ahn,Dongjun Kang,Dongsik Bae 한양대학교 세라믹연구소 2008 Journal of Ceramic Processing Research Vol.9 No.5

We investigated the properties of organic-inorganic hybrid coating films prepared by a sol-gel reaction using a nanosized boehmite sols. The sol was synthesized as a protective coating, functional coating, binder, etc. The sol was prepared from spherically shaped boehmite, and a mixture of spherical and fibrous shaped boehmite. Sol solutions were used to deposit onto substrates by way of a dip-coating process. In order to investigate the surface hydrophobicity, roughness and transmittance of coating films, a contact angle meter, a surface profiler and a UV-Vis spectrophotometer were used. In addition, surface modification of boehmite was investigated by a FT-IR spectrometer. Contact angle and surface roughness of coating films gradually increased with increasing reaction time due to the gelation of the sol by successive condensation reactions. The transmission rate of coating films within the visible-wavelength region was greater than 90% when the reaction time was less than 48 h. FT-IR measurement showed the spectra of condensation of MTMS and boehmite. We investigated the properties of organic-inorganic hybrid coating films prepared by a sol-gel reaction using a nanosized boehmite sols. The sol was synthesized as a protective coating, functional coating, binder, etc. The sol was prepared from spherically shaped boehmite, and a mixture of spherical and fibrous shaped boehmite. Sol solutions were used to deposit onto substrates by way of a dip-coating process. In order to investigate the surface hydrophobicity, roughness and transmittance of coating films, a contact angle meter, a surface profiler and a UV-Vis spectrophotometer were used. In addition, surface modification of boehmite was investigated by a FT-IR spectrometer. Contact angle and surface roughness of coating films gradually increased with increasing reaction time due to the gelation of the sol by successive condensation reactions. The transmission rate of coating films within the visible-wavelength region was greater than 90% when the reaction time was less than 48 h. FT-IR measurement showed the spectra of condensation of MTMS and boehmite.

Fabrication of a 1.7-kV Schottky Barrier Diode with Improved Forward Current-voltage Characteristics

In-Ho Kang,Moonkyong Na,석오균,Jeong Hyun Moon,Wook Bahng,Him-Chan Park,Chang Heon Yang 한국물리학회 2016 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.68 No.6

This paper presents the effects of thermal annealing performed at different temperatures on the forward current-voltage (I-V) characteristics to fabricate a 1.7-kV 4H-SiC Schottky barrier diode (SBD) with improved forward performances. To optimize the thermal annealing temperature, the SBDs were characterized by using precise low-current and high-current I-V measurements, a twodiode model analysis and grazing-incidence X-ray diffraction measurements. The results showed that a degradation in the ideality factor and a dramatic decrease in the turn-on resistance began at temperature above 550 C. In particular, the turn-on resistance for the SBD annealed at 550 C was reduced by 32% compared to that for the SBD without thermal annealing without any degradation in the Schottky barrier height. This was attributed to an expansion of Al crystallites caused by the thermal annealing.

Roles of AgSbTe₂ nanostructures in PbTe: controlling thermal properties of chalcogenides

Dow, Hwan Soo,Na, Moonkyong,Kim, Sang Jun,Lee, Jung Woo The Royal Society of Chemistry 2019 Journal of Materials Chemistry C Vol.7 No.13

<P>In this study, we report on the formation behavior of homogeneous nanostructures and the thermal properties of quaternary system AgPbmSbTem+2 (silver- and antimony-doped PbTe, <I>m</I> = 18) that were synthesized <I>via</I> nanostructure crystallization engineering by controlling thermal processes. It is well known that with the addition of excess amounts of Ag and Sb into a PbTe system, nano-grained structures can be dispersed in the matrix of grains or segregated as clusters at the grain boundary of PbTe. Here, we observed the nanostructures with grain size ranging from 1 nm to 15 nm, which were synthesized under consecutive thermal procedures. They were examined by elaborated transmission electron microscopy. For thermal conductivity, the reduction rate was 20-30%, which mainly originated from decrease of the lattice thermal conductivity due to crystal phonon scattering (lattice vibrations) from formation of AgSbTe2 nanostructures in the PbTe grains. The lattice thermal conductivities of the entire system, with controlled AgSbTe2 nanostructures in PbTe matrix, are substantially lower than in other cases. Finally, the best thermal conductivity reduction rate is shown in AgPb18SbTe20 compound heat-treated at 923 K. As a result, these nano-grained PbTe materials could be expected to increase <I>ZT</I> from thermal conductivity reduction.</P>