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Transferable single-crystal GaN thin films grown on chemical vapor-deposited hexagonal BN sheets
Chung, Kunook,Oh, Hongseok,Jo, Janghyun,Lee, Keundong,Kim, Miyoung,Yi, Gyu-Chul Nature Publishing Group 2017 NPG Asia Materials Vol.9 No.7
<P>Single-crystal gallium nitride (GaN) layers were directly grown on centimeter-scale hexagonal boron nitride (h-BN). Using chemical vapor deposition (CVD), centimeter-scale h-BN films were synthesized on a single-crystal Ni(111) and readily transferred onto amorphous fused silica supporting substrates that had no epitaxial relationship with GaN. For growing fully coalescent GaN layers on h-BN, the achievement of high-density crystal growths was a critical growth step because the sp(2)-bonded h-BN layers are known to be free of dangling bonds. Unlike GaN layers grown on a typical heterogeneous sapphire substrate, the morphological and microstructural results strongly suggest a high-density growth feature that is driven by the atomic cliffs inherent in the CVD-grown h-BN layers. More importantly, the GaN layers grown on CVD-grown h-BN exhibited a flat and continuous surface morphology with well-aligned crystal orientations both along the c-axis and in-plane, indicating the characteristics of GaN heteroepitaxy on h-BN.</P>
Transferable GaN Layers Grown on ZnO-Coated Graphene Layers for Optoelectronic Devices
Chung, Kunook,Lee, Chul-Ho,Yi, Gyu-Chul American Association for the Advancement of Scienc 2010 Science Vol.330 No.6004
<P>We fabricated transferable gallium nitride (GaN) thin films and light-emitting diodes (LEDs) using graphene-layered sheets. Heteroepitaxial nitride thin films were grown on graphene layers by using high-density, vertically aligned zinc oxide nanowalls as an intermediate layer. The nitride thin films on graphene layers show excellent optical characteristics at room temperature, such as stimulated emission. As one of the examples for device applications, LEDs that emit strong electroluminescence emission under room illumination were fabricated. Furthermore, the layered structure of a graphene substrate made it possible to easily transfer GaN thin films and GaN-based LEDs onto foreign substrates such as glass, metal, or plastic.</P>
Microstructures of GaN Thin Films Grown on Graphene Layers
Yoo, Hyobin,Chung, Kunook,Choi, Yong Seok,Kang, Chan Soon,Oh, Kyu Hwan,Kim, Miyoung,Yi, Gyu‐,Chul WILEY‐VCH Verlag 2012 ADVANCED MATERIALS Vol.24 No.4
<P><B>Plan‐view and cross‐sectional transmission electron microscopy</B> images show the microstructural properties of GaN thin films grown on graphene layers, including dislocation types and density, crystalline orientation and grain boundaries. The roles of ZnO nanowalls and GaN intermediate layers in the heteroepitaxial growth of GaN on graphene, revealed by cross‐sectional transmission electron microscopy, are also discussed.</P>
Epitaxial GaN Microdisk Lasers Grown on Graphene Microdots
Baek, Hyeonjun,Lee, Chul-Ho,Chung, Kunook,Yi, Gyu-Chul American Chemical Society 2013 Nano letters Vol.13 No.6
<P>Direct epitaxial growth of inorganic compound semiconductors on lattice-matched single-crystal substrates has provided an important way to fabricate light sources for various applications including lighting, displays and optical communications. Nevertheless, unconventional substrates such as silicon, amorphous glass, plastics, and metals must be used for emerging optoelectronic applications, such as high-speed photonic circuitry and flexible displays. However, high-quality film growth requires good matching of lattice constants and thermal expansion coefficients between the film and the supporting substrate. This restricts monolithic fabrication of optoelectronic devices on unconventional substrates. Here, we describe methods to grow high-quality gallium nitride (GaN) microdisks on amorphous silicon oxide layers formed on silicon using micropatterned graphene films as a nucleation layer. Highly crystalline GaN microdisks having hexagonal facets were grown on graphene dots with intermediate ZnO nanowalls via epitaxial lateral overgrowth. Furthermore, whispering-gallery-mode lasing from the GaN microdisk with a <I>Q</I>-factor of 1200 was observed at room temperature.</P><P><B>Graphic Abstract</B> <IMG SRC='http://pubs.acs.org/appl/literatum/publisher/achs/journals/content/nalefd/2013/nalefd.2013.13.issue-6/nl401011x/production/images/medium/nl-2013-01011x_0006.gif'></P><P><A href='http://pubs.acs.org/doi/suppl/10.1021/nl401011x'>ACS Electronic Supporting Info</A></P>
Lee, Keundong,Park, Jong-woo,Tchoe, Youngbin,Yoon, Jiyoung,Chung, Kunook,Yoon, Hosang,Lee, Sangik,Yoon, Chansoo,Ho Park, Bae,Yi, Gyu-Chul IOP 2017 Nanotechnology Vol.28 No.20
<P>We report flexible resistive random access memory (ReRAM) arrays fabricated by using NiO<SUB> <I>x</I> </SUB>/GaN microdisk arrays on graphene films. The ReRAM device was created from discrete GaN microdisk arrays grown on graphene films produced by chemical vapor deposition, followed by deposition of NiO<SUB> <I>x</I> </SUB> thin layers and Au metal contacts. The microdisk ReRAM arrays were transferred to flexible plastic substrates by a simple lift-off technique. The electrical and memory characteristics of the ReRAM devices were investigated under bending conditions. Resistive switching characteristics, including cumulative probability, endurance, and retention, were measured. After 1000 bending repetitions, no significant change in the device characteristics was observed. The flexible ReRAM devices, constructed by using only inorganic materials, operated reliably at temperatures as high as 180 °C.</P>
Microtube Light-Emitting Diode Arrays with Metal Cores
Tchoe, Youngbin,Lee, Chul-Ho,Park, Jun Beom,Baek, Hyeonjun,Chung, Kunook,Jo, Janghyun,Kim, Miyoung,Yi, Gyu-Chul American Chemical Society 2016 ACS NANO Vol.10 No.3
<P>We report the fabrication and characteristics of vertical microtube light-emitting diode (LED) arrays with a metal core inside the devices. To make the LEDs, gallium nitride (GaN)/indium gallium nitride (InxGa1-xN)/zinc oxide (ZnO) coaxial microtube LED arrays were grown on an n-GaN/c-aluminum oxide (Al2O3) substrate. The micro tube LED arrays were then lifted-off the substrate by wet chemical etching of the sacrificial ZnO microtubes and the silicon dioxide (SiO2) layer. The chemically lifted-off LED layer was then transferred upside-down on other supporting substrates. To create the metal cores, titanium/gold and indium tin oxide were deposited on the inner shells of the microtubes, forming n-type electrodes inside the metal-cored LEDs. The characteristics of the resulting devices were determined by measuring electroluminescence and current voltage characteristic curves. To gain insights into the current spreading characteristics of the devices and understand how to make them more efficient, we modeled them computationally.</P>