Byung Jin Cho

Korea Advanced Institute of Science and Technology , Sŏul, Seoul, South Korea

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Publications (157)352.58 Total impact

  • Onejae Sul, Jaehoon Bong, Alex Yoon, Byung Jin Cho
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    ABSTRACT: We report that oxygen plasma treatment of CVD-grown graphene can improve the integrity of an aluminum oxide layer deposited by atomic layer deposition. There is an optimum process window for treatment with O2 plasma which does not cause serious degradation in the quality of the graphene, but provides significant improvement in the gate dielectric integrity in relation to capacitance uniformity, leakage current, and dielectric breakdown voltage.
    Journal of Nanoscience and Nanotechnology 01/2015; 15(1). · 1.15 Impact Factor
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    ABSTRACT: A method of graphene transfer without metal etching is developed to minimize the contamination of graphene in the transfer process and to endow the transfer process with a greater degree of freedom. The method involves direct delamination of single-layer graphene from a growth substrate, resulting in transferred graphene with nearly zero Dirac voltage due to the absence of residues that would originate from metal etching. Several demonstrations are also presented to show the high degree of freedom and the resulting versatility of this transfer method.
    Small 08/2014; · 7.82 Impact Factor
  • Ju Hyung We, Sun Jin Kim, Byung Jin Cho
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    ABSTRACT: TEG (Thermoelectric power generator) modules are attractive energy harvesters, as they can deliver electrical output power from the temperature difference of all sorts of things. Recently, growing interests in self-powered wearable mobile electronics provoke the necessity of flexible TEG modules. However, the technology on flexible TEG modules is still at a very early stage. Here we demonstrate flexible high-performance TEG modules using a screen-printed inorganic thermoelectric thick film and organic conducting polymer hybrid composite. By infiltrating the organic conducting polymer, poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate) (PEDOT:PSS), into the micropores of the screen-printed thermoelectric thick film, the flexibility of the module is greatly enhanced without degradation of the output characteristics of the module. This work provides a promising new approach which has the potential to achieve a flexible high-performance TEG module.
    Energy 08/2014; 73:506–512. · 4.16 Impact Factor
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    ABSTRACT: Layered structures of transition metal dichalcogenides stacked by van der Waals interactions are now attracting the attentions of many researchers because they have fascinating electronic, optical, thermoelectric and catalytic properties emerging at the monolayer limit. However, the commonly used methods for preparing monolayers have limitations of low yield and poor extendibility into large-area applications. Herein, we demonstrate the synthesis of large area MoSe2 with high quality and uniformity by selenization of MoO3 via chemical vapor deposition on arbitrary substrates such as SiO2 and sapphire. The resultant monolayer was intrinsically doped, as evidenced by the formation of charged excitons under low temperature photoluminescence analysis. A van der Waals heterostructure of MoSe2 on graphene was also demonstrated. Interestingly, the MoSe2/graphene heterostructures show strong quenching of the characteristic photoluminescence from MoSe2, indicating the rapid transfer of photo-generated charge carriers between MoSe2 and graphene. The development of highly controlled heterostructures of two dimensional materials will further advances in the physics and chemistry of reduced dimensional systems and will provide novel applications in electronics and optoelectronics.
    ACS Nano 07/2014; · 12.03 Impact Factor
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    ABSTRACT: Crystallization of materials has attracted research interests for a long time, and its mechanisms in three-dimensional materials have been well studied. However, crystallization of two-dimensional (2D) materials is yet to be challenged. Clarifying the dynamics underlying growth of 2D materials will provide the insight for the potential route to synthesize large and highly crystallized 2D domains with low defects. Here we present the growth dynamics and recrystallization of 2D material graphene under a mobile hot-wire assisted chemical vapor deposition (MHW-CVD) system. Under the local but sequential heating by MHW-CVD system, the initial nucleation of nano-crystalline graphenes, which were not extended into the growth stage due to the insufficient thermal energy, took a recrystallization and converted into a grand single crystal domain. While this process, the stitching-like healing of graphene was also observed. The local but sequential endowing thermal energy to nano-crystalline graphenes enabled us to simultaneously reveal the recrystallization and healing dynamics in graphene growth, which suggests an alternative route to synthesize a highly crystalline and large domain size graphene. Also, this recrystallization and healing of 2D nano-crystalline graphenes offers an interesting insight on the growth mechanism of 2D materials.
    Nano Letters 06/2014; · 13.03 Impact Factor
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    ABSTRACT: We report a post-synthetic n-doping method for chemical-vapor-deposition (CVD) grown graphene using wet chemical processing. An ammonium fluoride solution was found effective in converting pristine hole doping into electron doping in addition to the mobility improvement of charge carriers. We verified the doping by electrical measurements, Raman spectroscopy and X-ray photoelectron spectroscopy (XPS) analyses and suggest that the mechanism of n-doping is electrostatic doping by ionic physisorption of ammonium ions on the graphene surface. This simple chemical doping method provides a facile and robust route to n-doping of large area graphene for the realization of high performance graphene-based electronic devices.
    Nanoscale 06/2014; · 6.73 Impact Factor
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    ABSTRACT: We investigated the ultrafast carrier dynamics and phonon relaxation of CVD-grown monolayer and 9-layer graphene on a quartz substrate. Excitation was performed at 400 and 800 nm. The normalized change in optical density ΔOD was probed over the range 260–640 nm (1.94–4.77 eV), reaching down into the region of graphene’s Fano resonance, previously not investigated in femtosecond broadband pump–probe experiments. Time constants of 160 fs and 4 ps were found and assigned to carrier–optical phonon scattering and slower phonon relaxation processes, respectively. The carrier distribution at early times was clearly hotter for 400 nm excitation than for 800 nm excitation. A pronounced spectral bleach feature was observed below 300 nm. It immediately formed after photoexcitation and recovered slowly, with a time constant of 35 ps for monolayer and time constants of 120 and 970 ps for 9-layer graphene. The same dynamics were found for weak transient absorption features above 300 nm, which emerged after ca. 0.5 ps. The slow dynamics were assigned to interfacial heat flow from graphene to the quartz substrate. The bleach and absorption features were well described by a simple model assuming a red-shift of the Fano resonance. This red-shift disappeared with progressive cooling of graphene. We therefore suggest that the red-shift is induced by shrinking of the band separation due to lattice heating.
    The Journal of Physical Chemistry C. 03/2014; 118(12):6454–6461.
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    ABSTRACT: This study elucidates the correlation among conductivity of graphene and interface aspects in GaN light-emitting diodes (LEDs). Using a multilayer graphene of low sheet resistance, it is demonstrated that graphene alone can make ohmic contact with p-GaN without necessitating additional interlayer. Large-area blue LED with relatively low contact resistance in the order of 10−2 ohm-cm2 and improved forward voltage of 3.2 ± 0.1 V was realized irrespective of the use of the interlayer. The results from parallel evaluation experiments performed by varying the layer numbers of graphene with ultrathin NiOx interlayer revealed that the poor lateral conductivity of monolayer or few layer graphene can be well compensated by the interlayer. A combination of three layer graphene and NiOx offered device with enhanced electro-optical performance. But the Schottky barrier associated with the inadequate adhesion of transferred graphene dominates all the benefits and becomes a major bottleneck preventing the formation of low resistance stable ohmic contact.
    Journal of Applied Physics 02/2014; 115(5):054503. · 2.21 Impact Factor
  • Seung Min Song, Jae Hoon Bong, Byung Jin Cho
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    ABSTRACT: Understanding of the contact between graphene and metal is a key issue to improve device performance. We extend the previous finding of work function pinning of monolayer graphene under various metals to multilayer graphene and discover that the work function of graphene under metal can be tuned from 4.3 eV to 5.1 eV by controlling the number of graphene layers. The work function of graphene is found to gradually shift with the number of graphene layers and four layers of graphene successfully screen this shift. These findings provide an alternative approach to control the work function of graphene electrodes.
    Applied Physics Letters 01/2014; 104(8):083512-083512-3. · 3.52 Impact Factor
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    ABSTRACT: A soft program method is proposed for charge-trap flash (CTF) memory devices. By adding a subsequent small positive gate pulse after main Fowler–Nordheim (FN) injection programming, early charge loss is greatly reduced. The multi-level cell performance as well as the initial flat-band voltage (VFB) instability can thereby be improved by removing the trapped electrons at the shallow traps in the blocking oxide layer. The proposed soft program method is a simple but very effective way to improve the fast retention property without changing the memory structure, especially for cases where the κ-value of the blocking oxide is high.
    Solid-State Electronics. 01/2014; 94:86–90.
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    ABSTRACT: We demonstrate top-gate graphene field effect transistors (FETs) on an aluminum nitrite (AlN) substrate with high surface phonon energy. Electrical transport measurements reveal significant improvement of the carrier mobility of graphene FETs on AlN compared to those on SiO2. This is attributed to the suppression of surface phonon scattering due to the high surface phonon energy of the AlN substrate. The RF cut-off frequency of the graphene FET is also greatly increased when the AlN substrate is used. AlN can easily be formed on a Si or SiO2 substrate using a standard semiconductor process and thus provides a practical way to improve the performance of graphene FETs.
    Applied Physics Letters 01/2014; 104(19):193112-193112-4. · 3.52 Impact Factor
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    ABSTRACT: A junctionless-accumulation-mode (JAM) p-channel MOSFET is successfully implemented based on a junction-isolated bulk FinFET for the first time. The JAM devices with a fin width of 16 nm show outstanding transfer characteristics: 1) subthreshold swing (SSmin) = 68 mV/dec; 2) drain-induced-barrier-lowering is 9 mV/V; and 3) ION/IOFF ratio >1 × 106. The JAM devices with smaller fin widths or longer gate lengths give superior short-channel characteristics and higher threshold voltages (Vth) due to their enhanced gate electrostatic controllability. The reverse back bias modulates Vth and SS favorably by virtue of a body-tied device, maintaining the substrate current due to junction leakage of <;1 × 10-11 A.
    IEEE Electron Device Letters 12/2013; 34(12):1479-1481. · 2.79 Impact Factor
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    ABSTRACT: We report penetration and lateral diffusion behavior of environmental molecules on synthesized polycrystalline graphene. Penetration occurs through graphene grain boundaries resulting in local oxidation. However, when the penetrated molecules diffuse laterally, the oxidation region will expand. Therefore, we measured the lateral diffusion rate along the graphene-copper interface for the first time by the environment-assisted crack growth test. It is clearly shown that the lateral diffusion is suppressed due to the high van der Waals interaction. Finally, we employed bilayer graphene for a perfect diffusion barrier facilitated by decreased defect density and increased lateral diffusion path.
    Nanoscale 11/2013; · 6.73 Impact Factor
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    ABSTRACT: Ultrathin functionalized graphene (FG) is demonstrated to work as an effective seed layer for the atomic layer deposition (ALD) of high-k dielectrics on graphene that is synthesized via chemical vapor deposition (CVD). The FG layer is prepared using a low-density oxygen plasma treatment on CVD graphene and is characterized using Raman spectroscopy and X-ray photoelectron spectroscopy (XPS). While the ALD deposition on graphene results in a patchy and rough dielectric deposition, the abundant oxygen species provided by the FG seed layer enable conformal and pinhole-free dielectric film deposition over the entire area of the graphene channel. The metal-insulator-graphene (MIG) capacitors fabricated with the FG-seeded Al2O3 exhibit superior scaling capabilities with low leakage currents when compared with the co-processed capacitors with Al seed layers.
    ACS Applied Materials & Interfaces 10/2013; · 5.90 Impact Factor
  • Jeong Hun Mun, Byung Jin Cho
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    ABSTRACT: For the chemical vapor deposition (CVD) of graphene, grain growth of the catalyst metal and thereby surface roughening are unavoidable during the high temperature annealing for the graphene synthesis. Considering that nano-scale wrinkles and poor uniformity of synthesized graphene originate from the roughened metal surface, improving surface flatness of metal thin films is one of the key factors to synthesize high quality graphene. Here, we introduce a new method for graphene synthesis for fewer wrinkle formation on a catalyst metal. The method utilizes a reduced graphene oxide (rGO) interfacial layer between the metal film and the wafer substrate. The rGO interlayer releases the residual stress of the metal thin film and thereby suppresses stress-induced metal grain growth. This technique makes it possible to use much thinner nickel films, leading to a dramatic suppression of RMS roughness (~3 nm) of the metal surface even after high temperature annealing. It also endows excellent control of the graphene thickness due to the reduced amount of total carbon in the thin nickel film. The synthesized graphene layer having negligible amount of wrinkles exhibits excellent thickness uniformity (95% coverage of monolayer) and very high carrier mobility of ~15000 cm(2)/V·s.
    Nano Letters 05/2013; · 13.03 Impact Factor
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    ABSTRACT: Group IV transition metal dichalcogenides such as WS2 and WSe2 are one of attracting material classes which have a physical two dimension of one atomic layer and atomically thin layers like graphene. These materials have interesting features such as an indirect bulk gap makes a transition to a direct band gap in monolayer. Recent research results of FETs showed that a high effective hole mobility of 250 cm^2 /V s with subthreshold swing of 60 mV/dec from an exfoliated monolayer. Indeed it is natural to think that artificial large area synthesis is needed for practical applications. Here we report the large-area tungsten diselenide layers on SiO2 substrate using vapor phase deposition method. Selenium source was evaporated from certain distances to a tungsten thin film on SiO2/Si wafer. Nitrogen gas was flowed during all processes as a carrier gas. Growth was performed at 700 ˜ 900 Celsius degree. The size of atomic tungsten diselenide layers simply depends on a wafer and quartz tube size. Good qualities of selected tungsten diselenide layers were investigated by AFM/EFM, SEM/TEM, and Raman spectroscopy. FET and PL data also will be presented.
    03/2013;
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    ABSTRACT: We report the realization of field-effect transistors (FETs) made with chemically synthesized multilayer 2D crystal semiconductor MoS2. Electrical properties such as the FET mobility, subthreshold swing, on/off ratio, and contact resistance of chemically synthesized (s-) MoS2 are indistinguishable from that of mechanically exfoliated (x-) MoS2, however flat-band voltages are different, possibly due to polar chemical residues originating in the transfer process. Electron diffraction studies and Raman spectroscopy show the structural similarity of s-MoS2 to x-MoS2. This initial report on the behavior and properties of s-MoS2 illustrates the feasibility of electronic devices using synthetic layered 2D crystal semiconductors.
    01/2013;
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    ABSTRACT: This paper reports on the evaluation of the impact of introducing interlayers and post-metallization annealing on the graphene/p-GaN ohmic contact formation and performance of associated devices. Current-voltage characteristics of the graphene/p-GaN contacts with ultrathin Au, Ni and NiOx interlayers were studied using transmission line model with circular contact geometry. Direct graphene/p-GaN interface was identified to be highly rectifying and post-metallization annealing improved the contact characteristics as a result of improved adhesion between the graphene and the p-GaN. Ohmic contact formation was realized when interlayer is introduced between the graphene and p-GaN followed by post-metallization annealing. Temperature dependent I-V measurements reveled that the current transport was modified from thermionic emission for the direct graphene/p-GaN contact to tunneling for the graphene/metal/p-GaN contacts. The tunneling mechanism results due to the interfacial reactions that occur between the metal and p-GaN during the post-metallization annealing. InGaN/GaN light-emitting diodes with NiOx/graphene current spreading electrode offered a forward voltage of 3.16 V comparable to that of its Ni/Au counterpart, but ended up with relatively low light output power. X-ray photoelectron spectroscopy provided evidence for the occurrence of phase transformation in the graphene-encased NiOx during the post-metallization annealing. The observed low light output is therefore correlated to the phase change induced transmittance loss in the NiOx/graphene electrode. These findings provide new insights into the behavior of different interlayers under processing conditions that will be useful for the future development of opto-electronic devices with graphene-based electrodes.
    ACS Applied Materials & Interfaces 01/2013; · 5.90 Impact Factor
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    Seung Min Song, Byung Jin Cho
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    ABSTRACT: The performance of graphene-based electronic devices is critically affected by the quality of the graphene-metal contact. The understanding of graphene-metal is therefore critical for the successful development of graphene-based electronic devices, especially field-effect-transistors. Here, we provide a review of the peculiar properties of graphene-metal contacts, including work function pinning, the charge transport mechanism, the impact of the process on the contract resistance, and other factors.
    Carbon letters. 01/2013; 14(3).
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    ABSTRACT: The high quality contact between graphene and the metal electrode is a crucial factor in achieving the high performance of graphene transistors. However, there is not sufficient research about contact resistance reduction methods to improve the junction of metal-graphene. In this paper, we propose a new method to decrease the contact resistance between graphene and metal using directly grown graphene over a metal surface. The study found that the grown graphene over copper, as an intermediate layer between the copper and the transferred graphene, reduces contact resistance, and that the adhesion strength between graphene and metal becomes stronger. The results confirmed the contact resistance of the metal-graphene of the proposed structure is nearly half that of the conventional contact structure.
    Carbon letters. 01/2013; 14(3).

Publication Stats

951 Citations
352.58 Total Impact Points

Institutions

  • 2009–2014
    • Korea Advanced Institute of Science and Technology
      • Department of Electrical Engineering
      Sŏul, Seoul, South Korea
  • 2013
    • Graphene Research Institute
      Sŏul, Seoul, South Korea
  • 2010
    • National NanoFab Center
      Daiden, Daejeon, South Korea
  • 1998–2010
    • National University of Singapore
      • Department of Electrical & Computer Engineering
      Singapore, Singapore
  • 2004
    • National Chiao Tung University
      • Department of Electronics Engineering
      Hsinchu, Taiwan, Taiwan