• Journal of the Korean Magnetics Society
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• v.5 no.1
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• pp.42-47
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• 1995

FeN thin films for inductive recording heads were sputter deposited using RF diode sputtering mehtod from a pure iron target onto 7059 glass substrates, and their magnetic properties were measured. The magnetic properties were greatly affected by film thickness, gas pressure, sputter power and flow ratio of $N_{2}$ to Ar. Single layer FeN films with their thickness varied from $1,000\;{\AA}$ to $6,000\;{\AA}$ were doposited. 800 W sputter power, 3 mT gas pressure, $N_{2}$ to Ar flow ratio of 6.6 : 100 were the sputtering conditions. Up to 7 layers of FeN films having total thickness of $6,000\;{\AA}$ were deposited using $SiO_{2}$ of $30\;{\AA}$ thickness as intermediate layers and their coercivity and saturation magnetization were measured. The sputtering conditions were the same as those in the single layer films. Easy axis coercivity of the single layer FeN films gradually decreased as their thickness was increased, but for the films with their thicknesses above $3,000\;{\AA}$, the coercivity changed very little. As the number of the FeN layers were increased, the coercivity decreased We estimated the grain size of FeN films from the FWHM (Full Width at Half Maximum) of X-ray diffraction peaks. The grain size steadily decreased from about $200\;{\AA}$ to $120\;{\AA}$ as the number of layers were increased. Minimum hard axis coercivity of 0.4 Oe was obtained when the number of layers was four. Maximum relative permeability was 2,900 when the number of layers was three. The cut off frequeocy of the multilayer films were above 100 MHz.

• Journal of the Korea Institute of Information and Communication Engineering
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• v.14 no.3
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• pp.691-698
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• 2010

In this study Al-doped ZnO (AZO) thin films have been fabricated on Eagle 2000 glass substrates at various substrate temperature ($100{\sim}500^{\circ}C$) and working pressure (10 ~ 40 mTorr) by RF magnetron sputtering in order to investigate the structural, electrical, and optical properties of the AZO thin films. The obtained films were polycrystalline with a hexagonal wurtzite structure and preferentially oriented in the (002) crystallographic direction. The AZO thin films, which were deposited at $T=300^{\circ}C$ for 10 mTorr, shows the highest (002) orientation, and the full width at half maximum (FWHM) of the (002) diffraction peak is $0.42^{\circ}$. The lowest resistivity ($2.64{\times}10^{-3}\;{\Omega}cm$) with the highest cartier concentration ($5.29{\times}10^{20}\;cm^{-3}$) and a Hall mobility of ($6.23\;cm^2/Vs$) are obtained in the AZO thin films deposited at $T=300^{\circ}C$ for 10 mTorr. The optical transmittance in the visible region is approximately 80%, regardless of process conditions. The optical band-gap depends on the Al doping level as the substrate temperature increases and the working pressure decrease. The optical band-gap widening is proportional to cartier concentration due to the Burstein-Moss effect.

• Journal of the Institute of Electronics Engineers of Korea SC
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• v.40 no.3
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• pp.163-171
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• 2003

To evaluate the performance of the digital radiography(DR) system developed in our group, the modulation transfer function(MTF) was measured and compared with that of an analog X- ray detector, film/screen system. The DR system has an amorphous selenium(a-Se) layer vacuum-evaporated on a TFT flat panel detector. The speed class 400 film/screen (Fuji) system has been being used in the clinical field as analog X-ray detectors. Both the square wave and slit method were used to evaluate their MTF. The square method was applied to both film/screen and the DR system. The slit method, however, was applied to only DR system. The full-width half maximum resolution of film/screen was 357${\mu}{\textrm}{m}$(1.4 lp/mm at 50% spatial frequency), and the resolution of DR was limited to 200${\mu}{\textrm}{m}$(2.5 lp/mm at 30%). These results indicate the measured resolution limitations approximate to the pixel pitch, 139 ${\mu}{\textrm}{m}$ of TFT. The MTF of DR is higher than that of film/screen by the factor of 1.785. It is proved that our a-Se based DR system has potential usefulness in the clinical field.

• Journal of the Korean Vacuum Society
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• v.22 no.6
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• pp.321-326
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• 2013

The effect of rapid thermal annealing (RTA) on the optical properties of digital-alloy InGaAlAs multiple quantum well (MQW) structures have been investigated by using photoluminescence (PL) and time-resolved PL measurements as a function of RTA temperature. The MQW samples were annealed from $700^{\circ}C$ to $850^{\circ}C$ for 30 s in a nitrogen atmosphere. The MQW sample annealed at $750^{\circ}C$ exhibited the strongest PL intensity and the narrowest FWHM (Full width at half maximum), indicating the reduced nonradiative recombination centers and the improved interfaces between the wells and barriers. The MQW samples annealed at $800^{\circ}C$ and $850^{\circ}C$ showed the decreased PL intensities and blueshifted PL peaks compared to $750^{\circ}C$-annealed sample. The blueshift of PL peak with increasing RTA temperatures are ascribed to the increase of aluminum due to intermixing of gallium (Ga) and aluminum (Al) in the interfaces of InGaAs/InAlAs short-period superlattices. The decrease of PL intensity after annealing at $800^{\circ}C$ and $850^{\circ}C$ are attributed to the interface roughening and lateral composition modulation caused by the interdiffusion of Ga and Al and indium segregation, respectively. With increasing RTA temperature the PL decay becomes slower, indicating the decrease of nonradiative defect centers. The optical properties of digital-alloy InGaAlAs MQW structures can be improved significantly with optimum RTA conditions.

• Journal of Astronomy and Space Sciences
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• v.27 no.2
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• pp.153-160
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• 2010

We modified the optical system of 500 mm wide-field telescope of which point spread function showed an irregularity. The telescope has been operated for Near Earth Space Survey (NESS) located at Siding Spring Observatory (SSO) in Australia, and the optical system was brought back to Korea in January 2008. After performing a numerical simulation with the tested value of surface figure error of the primary mirror using optical design program, we found that the surface figure error of the mirror should be fabricated less than root mean square (RMS) $\lambda$/10 in order to obtain a stellar full width at half maximum (FWHM) below $28\;{\mu}m$. However, we started to figure the mirror for the target value of RMS $\lambda$/20, because system surface figure error would be increased by the error induced by the optical axis adjustment, mirror cell installation, and others. The radius of curvature of the primary mirror was 1,946 mm after the correction. Its measured surface figure error was less than RMS $\lambda$/20 on the table of polishing machine, and RMS $\lambda$/15 after installation in the primary mirror cell. A test observation performed at Daeduk Observatory at Korea Astronomy and Space Science Institute by utilizing the exiting mount, and resulted in $39.8\;{\mu}m$ of stellar FWHM. It was larger than the value from numerical simulation, and showed wing-shaped stellar image. It turned out that the measured-curvature of the secondary mirror, 1,820 mm, was not the same as the designed one, 1,795.977 mm. We fabricated the secondary mirror to the designed value, and finally obtained a stellar FWHM of $27\;{\mu}m$ after re-installation of the optical system into SSO NESS Observatory in Australia.

• Journal of the Institute of Electronics Engineers of Korea SD
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• v.47 no.1
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• pp.6-13
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• 2010

In this study, Ga-doped ZnO (GZO) thin films have been fabricated on Eagle 2000 glass substrates at various substrate temperatures $100{\sim}400^{\circ}C$ and thin film thickness by RF magnetron sputtering in order to investigate the structural, electrical, and optical properties of the GZO thin films. It is observed that all the thin films exhibit c-axis orientation and a (002) diffraction peak only. The GZO thin films, which were deposited at $T=300^{\circ}C$ and 400 nm, shows the highest (002) orientation, and the full width at half maximum (FWHM) of the (002) diffraction peak is $0.4^{\circ}$. AFM analysis shows that the formation of relatively smooth thin films are obtained. The lowest resistivity ($8.01{\times}10^{-4}\;{\Omega}cm$) and the highest carrier concentration ($3.59{\times}10^{20}\;cm^{-3}$) are obtained in the GZO thin films deposited at $T=300^{\circ}C$ and 400 nm. The optical transmittance in the visible region is approximately 80 %, regardless of process conditions. The optical band-gap shows the slight blue-shift with increase in doping which can be explained by the Burstein-Moss effect.

• Applied Chemistry for Engineering
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• v.33 no.1
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• pp.58-63
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• 2022

Graphite has been used as an anode material for lithium-ion batteries for the past 30 years due to its low de-/lithiation voltage, high theoretical capacity of 372 mAh/g, low price, and long life properties. Recently, all-solid-state lithium-ion batteries (ASSLB), which are composed of inorganic solid materials with high stability, have received great attention as electric vehicles and next-generation energy storage devices, but research works on graphite that works well for ASSLB systems are insufficient. Therefore, we induced the performance improvement of ASSLB anode electrode graphite material by removing the amorphous carbon present in the carbon material surface, acting as a resistive layer from the graphite. As a result of X-ray diffraction (XRD) analysis using heat treated graphite in air at 400, 500, and 600 ℃, the full width at half maximum (FWHM) at (002) peak was reduced compared to that of bare graphite, indicating that the crystallinity of graphite was improved after heat treatment. In addition, the discharge capacity, initial coulombic efficiency (ICE) and cycle stability increased as the crystallinity of graphite increased after heat treatment. In the case of graphite annealed in air at 500 ℃, the high capacity retention rate of 331.1 mAh/g and ICE of 86.2% and capacity retention of 92.7% after 10-cycle measurement were shown.

• Journal of the Korean Society of Radiology
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• v.11 no.1
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• pp.9-17
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• 2017

A high degree of precision and accuracy in Gamma Knife Radiosurgery(GKRS) is a fundamental requirement for therapeutical success. Elaborate radiation delivery and dose gradients with the steep fall-off of radiation are clinically applied thus necessitating a dedicated Quality Assurance(QA) program in order to guarantee dosimetric and geometric accuracy and reduce all the risk factors that can occur in GKRS. In this study, as a part of QA we verified the accuracy of single-shot dose profiles used in the algorithm of Gamma Knife Perfexion(PFX) treatment planning system employing Variable Ellipsoid Modeling Technique(VEMT). We evaluated the dose distributions of single-shots in a spherical ABC phantom with diameter 160 mm on Gamma Knife PFX. The single-shots were directed to the center of ABC phantom. Collimating configurations of 4, 8, and 16 mm sizes along x, y, and z axes were studied. Gamma Knife PFX treatment planning system being used in GKRS is called Leksell GammaPlan(LGP) ver 10.1.1. From the verification like this, the accuracy of GKRS will be doubled. Then the clinical application must be finally performed based on precision and accuracy of GKRS. Specifically the width at the 50% isodose level, that is, Full-Width-of-Half-Maximum(FWHM) was verified under such conditions that a patient's head is simulated as a sphere with diameter 160mm. All the data about dose profiles along x, y, and z axes predicted through VEMT were excellently consistent with dose profiles from LGP within specifications(${\leq}1mm$ at 50% isodose level) except for a little difference of FWHM and PENUMBRA(isodose level: 20%~80%) along z axis for 4 mm and 8mm collimating configurations. The maximum discrepancy of FWHM was less than 2.3% at all collimating configurations. The maximum discrepancy of PENUMBRA was given for the 8 mm collimator along z axis. The difference of FWHM and PENUMBRA in the dose distributions obtained with VEMT and LGP is too small to give the clinical significance in GKRS. The results of this study are considered as a reference for medical physicists involved in GKRS in the whole world. Therefore we can work to confirm the validity of dose distributions for all collimating configurations determined through the regular preventative maintenance program using the independent verification method VEMT for the results of LGP and clinically assure the perfect treatment for patients of GKRS. Thus the use of VEMT is expected that it will be a part of QA that can verify and operate the system safely.

• Proceedings of the Korean Vacuum Society Conference
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• 2012.08a
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• pp.189-189
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• 2012

Lead sulfide (PbS) nanocrystal quantum dots (NQDs) are promising materials for various optoelectronic devices, especially solar cells, because of their tunability of the optical band-gap controlled by adjusting the diameter of NQDs. PbS is a IV-VI semiconductor enabling infrared-absorption and it can be synthesized using solution process methods. A wide choice of the diameter of PbS NQDs is also a benefit to achieve the quantum confinement regime due to its large Bohr exciton radius (20 nm). To exploit these desirable properties, many research groups have intensively studied to apply for the photovoltaic devices. There are several essential requirements to fabricate the efficient NQDs-based solar cell. First of all, highly confined PbS QDs should be synthesized resulting in a narrow peak with a small full width-half maximum value at the first exciton transition observed in UV-Vis absorbance and photoluminescence spectra. In other words, the size-uniformity of NQDs ought to secure under 5%. Second, PbS NQDs should be assembled carefully in order to enhance the electronic coupling between adjacent NQDs by controlling the inter-QDs distance. Finally, appropriate structure for the photovoltaic device is the key issue to extract the photo-generated carriers from light-absorbing layer in solar cell. In this step, workfunction and Fermi energy difference could be precisely considered for Schottky and hetero junction device, respectively. In this presentation, we introduce the strategy to obtain high performance solar cell fabricated using PbS NQDs below the size of the Bohr radius. The PbS NQDs with various diameters were synthesized using methods established by Hines with a few modifications. PbS NQDs solids were assembled using layer-by-layer spin-coating method. Subsequent ligand-exchange was carried out using 1,2-ethanedithiol (EDT) to reduce inter-NQDs distance. Finally, Schottky junction solar cells were fabricated on ITO-coated glass and 150 nm-thick Al was deposited on the top of PbS NQDs solids as a top electrode using thermal evaporation technique. To evaluate the solar cell performance, current-voltage (I-V) measurement were performed under AM 1.5G solar spectrum at 1 sun intensity. As a result, we could achieve the power conversion efficiency of 3.33% at Schottky junction solar cell. This result indicates that high performance solar cell is successfully fabricated by optimizing the all steps as mentioned above in this work.

• Journal of the Korean Crystal Growth and Crystal Technology
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• v.19 no.1
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• pp.6-10
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• 2009

In this paper, a selective area growth (SAG) of a GaN/AlGaN double heterostructure (DH) has been performed on r-plane sapphire substrate by using the mixed-source hydride vapor phase epitaxy (HVPE) with multi-sliding boat system. The SAG-GaN/AlGaN DH consists of GaN buffer layer, Te-doped AlGaN n-cladding layer, GaN active layer, Mg-doped AlGaN p-cladding layer, and Mg-doped GaN p-capping layer. The electroluminescence (EL) characteristics show an emission peak of wavelength, 439 nm with a full width at half maximum (FWHM) of approximately 0.64 eV at 20 mA. The I-V measurements show that the turn-on voltage of the SAG-GaN/AlGaN DH is 3.4 V at room temperature. We found that the mixed-source HVPE method with a multi-sliding boat system was one of promising growth methods for III-Nitride LEDs.