Sungyeon Kim

Yonsei University, Seoul, Seoul, South Korea

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Publications (12)12.83 Total impact

  • Source
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    ABSTRACT: a b s t r a c t Ga-doped zinc oxide (ZnO:Ga) thin films were prepared on glass substrate by magnetron sputtering at room temperature (RT) and thermally annealed in hydrogen atmosphere for 1 h. The effects of film thick-ness and annealing temperature on sheet resistance, transmittance and figure of merit of ZnO:Ga thin films were analyzed and modeled using the artificial neural networks (NNets). The NNet models presented the good prediction on sheet resistance, transmittance and figure of merit of ZnO:Ga thin films and it was found that the electrical and optical properties of ZnO:Ga thin films were enhanced by thermal annealing. After NNet models were verified, genetic algorithm (GA) was used to search the optimized recipe for the desired figure of merit of ZnO:Ga thin films. The methodology allows us to estimate the optimal process condition with a small number of experiments.
    Expert Syst. Appl. 01/2011; 38.
  • physica status solidi (a) 01/2010; 207(7):1572-1576. · 1.21 Impact Factor
  • Sungyeon Kim, Jang-Won Lee
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    ABSTRACT: In most cases in wireless networks, a user has a two-way communication that consists of two sessions: uplink and downlink sessions, and its overall satisfaction to the communication depends on the quality of service of both sessions. However, in most previous approaches in wireless resource allocation, the satisfactions of a user for its uplink and downlink sessions are modeled separately and treated independently, which fails to accurately model user's overall satisfaction to its communication. Hence, in this paper we model user's overall satisfaction to its communication considering both its uplink and downlink sessions. To this end, we propose a novel concept for a utility function to model user's overall satisfaction to its communication, which is called a user-level utility function, considering user's satisfaction to uplink and downlink sessions jointly. To show the appropriateness of our approach, we apply our user-level utility functions to scheduling problems in TDMA wireless networks and show the performance improvement of our approach over the traditional approach that does not treat uplink and downlink sessions of a user jointly.
    IEICE Transactions. 01/2010; 93-B:1037-1040.
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    ABSTRACT: Ti1-xSixO2 dielectric thin films were prepared by co-sputtering deposition at room temperature. Electrical properties of high-k Ti1-xSixO2 dielectric thin film were characterized and the leakage current mechanism was analyzed. As the TiO2 power increases, the dielectric constant is increased from 14 to 43 and the dominant leakage current mechanism is changed from Schottky emission to Poole-Frenkel emission.
    01/2010;
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    ABSTRACT: The Ga-doped ZnO thin films were deposited on glass substrate by sputtering and annealed at 350°C in hydrogen atmosphere for 1h. The optical bandgap of thin films showed the lower blueshift than the theoretical value of the Burstein–Moss (BM) effect. The shift of bandgap was dependent on the carrier concentration and acquired by combining the nonparabolic BM effect and bandgap narrowing (BGN). The modified BM effect equation was proposed to substitute the nonparabolic BM effect and BGN. The exponent in the modified BM equation was affected by carrier concentration and it was decreased with carrier concentration.
    Thin Solid Films 01/2010; 518(22):6304-6307. · 1.60 Impact Factor
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    ABSTRACT: TiO2 thin films were deposited by reactive radio frequency magnetron sputtering of Ti target at low working pressure (1 mTorr) and various O2 and Ar flow ratios (RO2 = O2/(O2 + Ar)). Properties of TiO2 films were measured by field emission scanning electron microscopy, atomic force microscopy, X-ray diffractometry, UV–VIS transmission spectroscopy, X-ray photoelectron spectroscopy and evaluated by contact angle analysis. Well defined columnar structure with rough surface was obtained at relatively lower RO2. As RO2 increased, the oxygen vacancies in the films decreased and the stoichiometric ratio of O/Ti changed from 1.79 to 1.98. Transition from anatase to rutile phase was observed with increase in RO2. The optical bandgap showed a red shift with increase in RO2. In addition, the film with co-existing of rutile and anatase showed a high Urbach tail. Lower RO2 resulted in higher photo-hydrophilicizing efficiency due to the change in morphology, structure and composition. However, very low RO2 (less than 10%), which led to transition mode, resulted in poor photo-hydrophilicizing efficiency.
    Surface and Coatings Technology 01/2010; 204:3436-3442. · 1.94 Impact Factor
  • Source
    Sungyeon Kim, Jang-Won Lee
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    ABSTRACT: In most of researches in resource allocation for wireless networks, uplink and downlink problems are considered separately, especially when resources for uplink and downlink are statically partitioned, as in FDD and static TDD systems. However, even in those systems, joint resource allocation for uplink and downlink can improve system efficiency and we study this issue in this paper with the concept of the user-level utility function. In most cases, a user has a two-way communication that consists of two sessions: uplink and downlink sessions and its overall satisfaction to its communication depends on its satisfaction to each of its sessions. To model user's overall satisfaction to its communication, we define a user-level utility function, which is defined as a function of its session-level utility functions. We then formulate and solve the optimization problem with user-level utility functions for cell-level resource scheduling that jointly considers uplink and downlink resource allocation. Simulation results show that our cell-level scheduling in which resource allocation in both uplink and downlink is done jointly outperforms link-level scheduling, in which resource allocation in each of uplink and downlink is done separately in most cases, especially when the asymmetry between uplink and downlink is large.
    Vehicular Technology Conference, 2009. VTC Spring 2009. IEEE 69th; 05/2009
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    ABSTRACT: Gallium doped zinc oxide (ZnO:Ga) thin films were grown on glass substrates using RF magnetron co-sputtering, followed by H2 ambient annealing at 623 K to explore a possibility of steady and low-cost process for fabricating transparent electrodes. While it was observed that the ZnO:Ga thin films were densely packed c-axis oriented self-textured structures, in the as-deposited state, the films contained Ga2O3 and ZnGa2O4 which had adverse effect on the electrical properties. On the other hand, post-annealing in H2 ambient improved the electrical properties significantly via reduction of Ga2O3 and ZnGa2O4 to release elemental Ga which subsequently acted as substitutional dopant increasing the carrier concentration by two orders of magnitude. Transmittance of the ZnO:Ga thin films were all over 90% that of glass while the optical band gap varied in accordance with the carrier concentrations due to changes in Fermi level. Experimental observation in this study suggests that transparent conductive oxide (TC
    Applied Surface Science 02/2009; 255(8):4616-4622. · 2.11 Impact Factor
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    ABSTRACT: High-k Ti1−xSixO2 gate dielectric layers were prepared at room temperature by RF magnetron sputtering using SiO2 and TiO2 targets to investigate their applicability to transparent thin-film transistors as well as metal–oxide–semiconductor field-effect transistors. Based on XRD and XPS analyses, it was found that, regardless of the deposition time, the Ti1−xSixO2 gate dielectric layers had more stable Si-based phases with stronger Si–O bonds with increasing SiO2 RF power. As SiO2 RF power increased, the capacitance of the dielectric layers decreased due to the higher fraction of the Si-based phases, and the leakage current decreased, dominantly because of the decrease in oxygen vacancies due to the formation of stoichiometric SiO2. The Ti1−xSixO2 gate dielectric layers exhibited high transparency above 80% and moderate bandgap of 4.1–4.2 eV, which can be applied to transparent thin-film transistors.
    Applied Surface Science 04/2008; 254(13):3943–3948. · 2.11 Impact Factor
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    ABSTRACT: We report on high-k TixSi1−xO2 thin films prepared by RF magnetron co-sputtering using TiO2 and SiO2 targets at room temperature. The TixSi1−xO2 thin films exhibited an amorphous structure with nanocrystalline grains of 3–30 nm having no interfacial layers. The XPS analyses indicate that stoichiometric TiO2 phases in the TixSi1−xO2 films increased due to stronger Ti–O bond with increasing TiO2 RF powers. In addition, the electrical properties of the TixSi1−xO2 films became better with increasing TiO2 RF powers, from which the maximum value of the dielectric constant was estimated to be ∼30 for the samples with TiO2 RF powers of 200 and 250 W. The transmittance of the TixSi1−xO2 films was above 95% with optical bandgap energies of 4.1–4.2 eV. These results demonstrate a potential that the TixSi1−xO2 thin films were applied to a high-k gate dielectric in transparent thin film transistors as well as metal-oxide-semiconductor field-effect transistors.
    Microelectronic Engineering 01/2008; 85(1):100–103. · 1.22 Impact Factor
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    ABSTRACT: We studied the effects of liquid crystal (LC) alignment characteristics on a SiOx thin films of various thicknesses deposited at an oblique angle of 45° by RF magnetic sputtering. A uniform LC alignment characteristic was achieved on the SiOx thin film, and the pretilt angle was about 90°. The thermal stability of the SiOx thin film was sustained until 200 °C. There was no difference in LC aligning capability owing to the thickness of SiOx thin film obliquely deposited at 45°. The vertical alignment liquid crystal display (VA-LCD) on the SiOx thin film deposited at the oblique angle of 45° by sputtering showed good electrooptical characteristics. These results suggest that a uniform homeotropic LC alignment can be achieved and a large VA-LCD can be produced by sputtering.
    Japanese Journal of Applied Physics 01/2006; 45. · 1.07 Impact Factor
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    ABSTRACT: Fabrication of Zn/ZnO nanocables by thermal oxidation of Zn nanowires grown by RF magnetron sputtering is reported. Single crystalline Zn nanowires could be grown by controlling supersaturation of source material through the adjustment of temperature and Zn RF power. X-ray diffraction and high-resolution transmission electron microscopy showed that surfaces of these Zn nanowires, grown along the [0 1 0] direction, gradually oxidized inward the Zn core to form coaxial Zn/ZnO nanocables in the subsequent oxidation at 200 °C. In the Zn/ZnO nanocable, epitaxial relations of [1 0 0]Zn//[1 0 0]ZnO, and (0 0 1)Zn//(0 0 1)ZnO existed at the interface between the Zn core and ZnO shell. A number of dislocations were also observed in the interface region of the Zn/ZnO nanocable, which are attributed to large differences in the lattice constants of Zn and ZnO. With further increasing the oxidation temperature over 400 °C, Zn nanowires were completely oxidized to form polycrystalline ZnO nanowires. The results in this study suggest that coaxial Zn/ZnO nanocable can be fabricated through controlled thermal oxidation of Zn nanowires, yielding various cross-sectional areal fractions of Zn core and ZnO shell.
    Journal of Crystal Growth 01/2006; · 1.55 Impact Factor

Publication Stats

23 Citations
12.83 Total Impact Points

Institutions

  • 2008–2010
    • Yonsei University
      • • Department of Materials Science and Engineering
      • • Department of Electrical and Electronic Engineering
      Seoul, Seoul, South Korea