Ju Ho Lee

Dankook University, Eidō, North Chungcheong, South Korea

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Publications (35)113.38 Total impact

  • [Show abstract] [Hide abstract]
    ABSTRACT: We report on the structural and optical properties of non polar a-plane MgxZn1−xO (0 ⩽ x ⩽ 0.57) films on r-plane sapphire substrates grown by plasma-assisted molecular beam epitaxy. Reflection high energy electron diffraction (RHEED) revealed a formation of cubic MgO phase when an Mg concentration increases. Room temperature (RT) photoluminescence (PL) and transmission electron microscopy consistently revealed the formation of cubic MgO phase from the Mg0.21Zn0.79O film. The Mg0.11Zn0.89O film showed a band edge emission at ∼360 nm, which is a shorter wavelength than the ZnO (∼373 nm), from the RT PL measurements. Photoluminescence excitation (PLE) measurements at RT showed that band-gap energies of MgxZn1−xO films could be tuned up to ∼4.65 eV (∼270 nm) although cubic MgO phase were mixed for high Mg concentration. For the single phase wurtzite MgZnO film, band-gap energy of 3.48 eV was obtained from the Mg0.11Zn0.89O film.
    Journal of Alloys and Compounds 02/2015; 623. · 2.73 Impact Factor
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    ABSTRACT: ZnO films were spin-coated on silicon substrates by using the sol-gel method and were characterized as a function of the sintering temperature (600 ̃ 1000 °C). The phase transition from the ZnO phase to the Zn2SiO4 phase was investigated by using X-ray diffraction (XRD), field emission scanning electron microscope (FE-SEM), energy-dispersive X-ray spectroscopy (EDX), and cathodoluminescence (CL). The XRD pattern of the ZnO film sintered at 600 °C showed a typical polycrystalline hexagonal wurtzite structure. However, at a sintering temperature of 1000 °C, only the 21.72° and the 32.64° peaks were found, which could be attributed to the (300) and the (013) planes of Zn2SiO4, respectively. In FE-SEM, the grain size of the ZnO film sintered at 900 °C increases abruptly, and the samples sintered at 1000 °C showed dense, grains with enlarged grains. In EDX, as the sintering temperatures were increased, the relative amounts of silicon atom were decreased, and the relative amounts of oxygen were increased. In CL, the UV emission (375 ̃ 379 nm) was well measured in the ZnO films sintered at 600 ̃ 800 °C. However, the intensity of the UV emission at 379 nm in the ZnO film decreased abruptly when the film was sintered at 900 °C, and the very intensive peaks were measured at 284 nm and 589 nm. At a sintering temperature of 1000 °C, the UV emission at about 375 nm in the ZnO film disappeared. From this study, we could confirm that a phase transition toward the Zn2SiO4 phase had completely occurred at a sintering temperature at 1000 °C.
    02/2014; 64(6).
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    ABSTRACT: The fast and accurate identification of unknown liquids is important for the safety and security of human beings. Recently, sensors based on the localized surface plasmon resonance (LSPR) effect demonstrated an outstanding sensitivity in detecting chemical and biological species. In the present study, we suggest that a dual-responsive nanocomposite composed of two polymer brushes and two noble metal nanoparticles provides a significantly improved selectivity (improvement of a factor of 30 in figure-of-merit) for distinguishing diverse liquids compared to a single-responsive LSPR sensor. The dual-responsive LSPR sensor platform was realized by the synergic combinations of two hydrophobic and hydrophilic polymer brushes, which respond differently depending on the degree of interaction between the polymer brushes and the surrounding liquids. Moreover, the mixing ratio of two solvents can also be estimated with high accuracy using the dual-nanocomposite LSPR sensor, suggesting that this approach would be highly practical for in situ process monitoring systems that can instantly detect the change of solvent composition.
    Nanoscale 07/2013; · 6.73 Impact Factor
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    ABSTRACT: Cu2O thin films were synthesized on Si (100) substrate with thermally grown 200-nm SiO2 by sol-gel spin coating method and post-annealing under different oxygen partial pressure (0.04, 0.2, and 0.9 Torr). The morphology of Cu2O thin films was improved through N2 post-annealing before O2 annealing. Under relatively high oxygen partial pressure of 0.9 Torr, the roughness of synthesized films was increased with the formation of CuO phase. Bottom-gated copper oxide (CuxO) thin film transistors (TFTs) were fabricated via conventional photolithography, and the electrical properties of the fabricated TFTs were measured. The resulting Cu2O TFTs exhibited p-channel operation, and field effect mobility of 0.16 cm2/V-s and on-to-off drain current ratio of ~102 were observed in the TFT device annealed at PO2 of 0.04 Torr. This study presented the potential of the solution-based process of the Cu2O TFT with p-channel characteristics for the first time.
    ACS Applied Materials & Interfaces 03/2013; · 5.90 Impact Factor
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    ABSTRACT: This study reports the microstructural characterization and formation mechanism of the 21° top facets of ZnO-based nanowall structures. The ZnO-based nanowall structures reported previously by many other research groups have {112¯0} planes as major planes and top facets with a specific angle in common, irrespective of the growth techniques and growth conditions. These nanowalls were found to exist between two adjacent nanowires with a c-axis preferred orientation, and the atoms at the junction of the nanowalls and nanowires perfectly coincided with each other at an atomic level, without any defects. The top facets of the nanowalls showed periodically stepped surfaces and were identified as {011¯5} planes, which were perpendicular to the {112¯0} major planes. On the basis of the microstructural characterization of the synthesized ZnO-based nanowall structures, the formation mechanism and atomic structure model of the 21° top facets of the nanowall structures are proposed.
    Physica B Condensed Matter 03/2013; 412:12–16. · 1.28 Impact Factor
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    ABSTRACT: We report the usage of ZnO material as an alternative for n-GaN for realizing III-nitride based solar cell. The fabricated solar cell shows large turn-on voltage of around 8 volts and a rapid decrease of photocurrent at low bias voltage under darkness and 1-sun illumination conditions, respectively. This phenomenon can be attributed to the formation of high-resistive ultra-thin layers at the ZnO/ p-GaN junction interface during high temperature deposition. Transmission electron microscopy (TEM) studies carried out on the grown samples reveals that the ultra-thin layer consists of ZnGa2O4. It is found that the presence of insulating ZnGa2O4 film is detrimental in the performance of proposed heterostructure for solar cells.
    Journal of Nanoscience and Nanotechnology 01/2013; 13(1):448-51. · 1.15 Impact Factor
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    ABSTRACT: Hybrid ZnO/Al2O3 layers grown by atomic layer deposition (ALD) at extremely low temperature (60 °C) have been investigated as thin film encapsulations (crystalline ZnO and amorphous Al2O3 films) on polymer substrates. All single and laminated film thicknesses are approximately 60 nm. As the thickness of ZnO layer decreased from 60 nm to 0 nm, the physical properties of laminated structures were systematically manipulated such as film crystallinity, surface roughness, density, transmittance and stress. The multi-laminated structure with 10 nm thick ZnO and 10 nm thick Al2O3 layers exhibited very lower crystallinity, smoother surface (root mean square ∼ 0.2 nm), higher transmittance (over 90% at 550 nm wavelength) and similar film stress, to compare with these of a single ZnO film. As a transparent gas barrier layer, the multi-laminated structure with a thinner ZnO and Al2O3 had better barrier property than that of single ZnO and Al2O3 layers, showing that the water vapor transmission ratio of multi-laminated ZnO/Al2O3 layer was 10 times lower than that of the single layer.
    Current Applied Physics 09/2012; 12:S19–S23. · 2.03 Impact Factor
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    ABSTRACT: Al-doped ZnO (AZO) thin films with various Al concentrations were synthesized on Si(001) substrates with native oxide layers by atomic layer deposition process. The effects of the Al concentration on the microstructural characteristics of the AZO thin films grown at 250 degrees C and the correlation between their microstructural characteristics and electrical properties of the AZO thin films were investigated by AFM, XRD, HRTEM and Hall measurements. The XRD and HRTEM results revealed that the crystallinity and electrical properties of the undoped ZnO thin films were enhanced by 2.48 at% Al doping. However, 12.62 at% Al doping induced the deterioration of their crystallinity and electrical properties due to the formation of nano-sized metallic Al clusters and randomly oriented ZnO-based nano-crystals. To enhance the electrical properties of the AZO thin films while maintaining their crystallinity and electrical properties, a moderate Al concentration has to be chosen under the solubility limit of Al in ZnO.
    Journal of Nanoscience and Nanotechnology 07/2012; 12(7):5598-603. · 1.15 Impact Factor
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    ABSTRACT: This paper reports the effects of substrate temperature on the microstructural characteristics and electrical properties of p-type semiconducting NiO thin films grown on (0001) Al2O3 substrates. NiO thin films were biepitaxially grown on (0001) Al2O3 substrates by radiofrequency magnetron sputtering, and they showed specific crystallographic orientation relationships: [1̅1̅0]NiO[011̅0]Al2O3, [1̅12̅]NiO[21̅1̅0]Al2O3 (in-plane), and [1̅11]NiO[0001̅]Al2O3 (out-of-plane). Thus, a low lattice mismatch of 7.52% was obtained between the NiO thin films and the (0001) Al2O3 substrates. The film grown at 600 °C consisted of cubic and rhombohedral NiO grains, while the NiO thin films grown at substrate temperature below 400 °C only consisted of cubic NiO grains. Atoms at the grain boundaries between the cubic and the rhombohedral NiO grains perfectly coincided with each other because of the same atomic stacking sequences along [11̅1̅]c-NiO and [0003]r-NiO and with equal interatomic distances. Further, the paper discussed the observations of the perfectly coinciding nickel and oxygen atoms at the grain boundaries between the cubic and the rhombohedral NiO grains using high-resolution transmission electron microscopy (HRTEM) along with atomic modeling on the atomic scale. In addition, the dependence of the electrical properties of the NiO thin films on the substrate temperature and crystallinity is presented in this paper.
    Crystal Growth & Design 04/2012; 12(5):2495–2500. · 4.69 Impact Factor
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    ABSTRACT: Electrical properties of Nb-doped titanium oxide films were evaluated with respect to annealing temperatures. Although an amorphous phase is preserved up to 450 °C, x-ray absorption spectroscopy analyses indicate that crystal field splitting in the conduction band begins to take place at this temperature. Such molecular orbital ordering effects induce a semiconducting behavior, which is manifested by working thin film transistor devices with field effect mobility values as high as 0.64 cm2/Vs. X-ray photoelectron spectroscopy studies disclose a drastic increase in Nb+5 states upon heat treatment, and these may be attributed to oxygen deficient states that generate free electrons.
    Applied Physics Letters 04/2012; 100(14). · 3.52 Impact Factor
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    ABSTRACT: A semi-empirical molecular clustering based lattice fluid model is presented to improve the classical lattice model for volumetric properties in the critical region. This model is based on the two assumptions: (1) The Helmholtz energy is individually divided into classical and long-range density fluctuation contribution (2) All molecules form cluster near the critical region due to long-range density fluctuation. To formulate such molecular clustering, we extended the Veytsman statistics originally developed for the cluster due to hydrogen bonding. The probability function in the statistics is modified to represent the characteristics of long-range density fluctuation vanishing far from critical region. The proposed fluctuation contribution was incorporated into the Sanchez-Lacombe model and the combined model with 6 adjustable parameters has been tested against experimental VLE data for polar and non-polar components. The combined model is found to good agreements with experimental vapor pressure, saturated density and supercritical PVT data.
    Journal of Industrial and Engineering Chemistry - J IND ENG CHEM. 03/2012;
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    ABSTRACT: The development of p-channel tin oxide thin-film-transistors spurred the research into microstructural analysis of tin oxide phases and control of conduction type, as it is widely known that tin oxide thin films exhibit both n- and p-type conduction depending on growth conditions. This study reports the relationship between the microstructural properties and the ambiguity of the electrical conduction type observed in nonstoichiometric tin oxides. Nonstoichiometric tin oxide thin films have been produced by RF magnetron sputtering with a dependence on the growth gas atmosphere. The crystal phase of the tin oxide deposited under low ambient oxygen content was mainly SnO1+x with relatively stable p-type conduction. On the other hand, for deposition under high ambient oxygen content, phase separation with structural modulation in the tin oxide film occurred in SnO-like and SnO2-like regions. These phases with different conduction types caused electrically unstable dual conduction types in the tin oxide films, despite their low electrical resistivity.
    Journal of the American Ceramic Society 12/2011; 95(1):324 - 327. · 2.43 Impact Factor
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    ABSTRACT: Pure ultraviolet (UV) light emitting diodes (LEDs) using n-ZnO nanowires as an active layer were fabricated with an insulating MgO dielectric layer as a carrier control layer, where all depositions were continuously performed by metalorganic chemical vapor deposition. The current-voltage curve of the LEDs showed obvious rectifying characteristics, with a threshold voltage of about 7 V in the sample with 4 nm i-MgO. Under the forward bias of the samples with proper MgO thickness, a sharp UV electroluminescence, located at around 380 nm, was emitted from the active ZnO nanowires, while weak visible emission of around 450–700 nm were observed. The pure UV emission from the ZnO nanowires in the n-ZnO/i-MgO/p<sup>+</sup>-Si heterostructures was attributed to the electron accumulation in the ZnO by asymmetric band offset and preemptive hole tunneling from Si to ZnO by i-MgO.
    Journal of The Electrochemical Society 12/2011; 159(2):H102-H106. · 2.59 Impact Factor
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    ABSTRACT: We investigated transparent conducting aluminium-doped zinc oxide thin films fabricated by atomic layer deposition (ALD). For the thermal ALD, diethylzinc and trimethylaluminium were used as the Zn and Al precursors, respectively. The electrical, structural and optical properties were systematically investigated as functions of the Al doping contents and deposition temperature. The best resistivity and transmittance (4.2 mΩ cm and ~85%) were observed at an Al doping concentration of about 2.5 at% at 250 °C. An increase in the carrier concentration was observed with increasing deposition temperature and doping concentration. This can be explained by the effective field model of layered structures. Also, the enhancement of the mobility with increasing doping concentration was studied by the grain-boundary scattering and percolative conduction mechanism. By correlating the electrical and structural properties, it was found that varying the carrier concentration was more effective in changing the mobility than the grain-boundary scattering, due to the hopping conduction.
    Journal of Physics D Applied Physics 10/2011; 44(44):445305. · 2.53 Impact Factor
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    ABSTRACT: We develop the sculpting method to synthesize well-defined nanocrater catalysts with the hollow core/metal shell structure within an array via chemical and plasma processes. Also, we successfully demonstrate that unique morphologies of nanocrater catalysts as effective templates allow the exclusive control of the number of walls and outer diameters in carbon nanotubes.
    Journal of Materials Chemistry 09/2011; 21(39):15175-15178. · 6.63 Impact Factor
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    ABSTRACT: Transparent indium tin oxide (ITO) was directly deposited by magnetron sputtering to form two-dimensional top electrodes on vertically arrayed one-dimensional ZnO nanowires. Deposition at 5 mTorr resulted in almost complete coverage and interconnection of the nanowires, which maintained high transmittance. The use of lower pressure reduced ITO coverage over large areas of the nanowires due to the retardation of lateral coalescence. The sputtered ITO layers did not cover the entire lengths of the ZnO nanowires, although thin ITO coating was observed along the sidewalls of the nanowires for some distance from the surface. The ITO layers on the nanowire arrays had high conductivity, comparable to conventional In metal, because of the use of a plasma atmosphere and high temperature during deposition. This resulted in improved Ohmic contact and photodiode properties, which allowed direct electrical connection of the active nanowires without filling the spaces between them with insulating materials.
    Electrochemical and Solid-State Letters 09/2011; 14(11):H446-H449. · 2.01 Impact Factor
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    ABSTRACT: This research reports on morphological changes depending on the growth temperature in MgZnO nanostructures grown on GaAs substrates by metalorganic chemical vapor deposition as well as the investigation of their optical properties. As the growth temperature increased, the morphology of the MgZnO nanostructure changed from one-dimensional nanowires (480 degrees C) to pseudo-two-dimensional nanowalls (500 degrees C) to pyramid-shaped structures (520 degrees C). Among these structures, the nanowalls exhibited the best optical properties due to the large active surface area and high crystalline quality.
    Journal of Nanoscience and Nanotechnology 08/2011; 11(8):7327-30. · 1.15 Impact Factor
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    ABSTRACT: This study reports that the visible-blind ultraviolet (UV) photodetecting properties of ZnO nanowire based photodetectors were remarkably improved by introducing ultrathin insulating MgO layers between the ZnO nanowires and Si substrates. All layers were grown without pause by metal organic chemical vapor deposition and the density and vertical arrangement of the ZnO nanowires were strongly dependent on the thickness of the MgO layers. The sample in which an MgO layer with a thickness of 8 nm was inserted had high density nanowires with a vertical alignment and showed dramatically improved UV photosensing performance (photo-to-dark current ratio = 1344.5 and recovery time = 350 ms). The photoresponse spectrum revealed good visible-blind UV detectivity with a sharp cut off at 378 nm and a high UV/visible rejection ratio. A detailed discussion regarding the developed UV photosensing mechanism from the introduction of the i-MgO layers and highly dense nanowires in the n-ZnO nanowires/i-MgO/n-Si substrates structure is presented in this work.
    Nanotechnology 07/2011; 22(26):265506. · 3.84 Impact Factor
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    ABSTRACT: We present the epitaxial growth of Mg-containing Ag film on p-type GaN using domain matching epitaxy. No epitaxial growth was found in the as-deposited Ag film, but it changed to epitaxial growth as Mg atoms were added into Ag film. This is due to the fact that Mg atoms were preferentially bonded to O ones, which played a role in shrinking the surrounded Ag lattice from 4.0871 to 4.0858. In addition to that, the volume expansion induced by Mg–O chemical bonding moves the Ag atoms to the site where they are energetically most stable at the Ag/GaN interface, resulting in strong adhesion between Ag films and GaN substrate. As a result, nine domains of the (111)-oriented Ag layer (2.558 nm) are matched with eight domains of the (0001) GaN layer (2.551 nm). This arrangement has the lowest domain mismatch of 8.9%, resulting in the epitaxial growth of the (111) Ag film with the lowest surface energy. Consequently, the Mg-containing Ag film shows better thermal stability, resulting in the suppression of Ag agglomeration.
    Crystal Growth & Design 05/2011; 11(6). · 4.69 Impact Factor
  • Article: Comments on
    Ju Ho Lee, Ki-Pung Yoo
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    ABSTRACT: Among the thermodynamic models applicable to solid-liquid-vapor phases, Yokozeki's model is considered as the first repulsion-based analytic equation of state (EOS) in which a discontinuity is introduced in the isotherm. However, it was found that the model violates some physical constraints due to the empirically introduced discontinuity. This work focuses on the evaluation of the empirical basis of the model through scaled-particle theory (SPT) and a modification of the model to satisfy the physical constraint.
    International Journal of Thermophysics 03/2011; 32(3):553-558. · 0.57 Impact Factor

Publication Stats

37 Citations
113.38 Total Impact Points

Institutions

  • 2014
    • Dankook University
      • Department of Physics
      Eidō, North Chungcheong, South Korea
  • 2013
    • Korea Electronics Technology Institute
      Sŏngnam, Gyeonggi Province, South Korea
  • 2012
    • Sogang University
      • Department of Chemical and Biomolecular Engineering
      Sŏul, Seoul, South Korea
  • 2009–2011
    • Korea Advanced Institute of Science and Technology
      • Department of Materials Science and Engineering
      Seoul, Seoul, South Korea
  • 2010
    • Sungkyunkwan University
      • School of Advanced Materials Science and Engineering (AMSE)
      Seoul, Seoul, South Korea
  • 2008
    • Seoul National University
      • Department of Chemical and Biological Engineering
      Sŏul, Seoul, South Korea