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Moon, Yoon-Jong,Moon, Daeyoung,Jang, Jeonghwan,Na, Jin-Young,Song, Jung-Hwan,Seo, Min-Kyo,Kim, Sunghee,Bae, Dukkyu,Park, Eun Hyun,Park, Yongjo,Kim, Sun-Kyung,Yoon, Euijoon American Chemical Society 2016 Nano letters Vol.16 No.5
<P>Two-dimensional high-index-contrast dielectric gratings exhibit unconventional transmission and reflection due to their morphologies. For light-emitting devices, these characteristics help guided modes defeat total internal reflections, thereby enhancing the outcoupling efficiency into an ambient medium. However, the outcoupling ability is typically impeded by the limited index contrast given by pattern media. Here, we report strong-diffraction, high-index contrast cavity engineered substrates (CESs) in which hexagonally arranged hemispherical air cavities are covered with a 80 nm thick crystallized alumina shell. Wavelength-resolved diffraction measurements and Fourier analysis on GaN-grown CESs reveal that the high-index-contrast air/alumina core/shell patterns lead to dramatic excitation of the low-order diffraction modes. Large-area (1075 x 750 mu m(2)) blue-emitting InGaN/GaN light-emitting diodes (LEDs) fabricated on a 3 mu m pitch CES exhibit similar to 39% enhancement in the optical power compared to state-of-the-art, patterned-sapphire-substrate LEDs, while preserving all of the electrical metrics that are relevant to LED devices. Full-vectorial simulations quantitatively demonstrate the enhanced optical power of CES LEDs and show a progressive increase in the extraction efficiency as the air cavity volume is expanded. This trend in light extraction is observed for both lateral- and flip-chip-geometry LEDs. Measurements of far-field profiles indicate a substantial beaming effect for CES LEDs, despite their few-micron-pitch pattern. Near-to-far-field transformation simulations and polarization analysis demonstrate that the improved extraction efficiency of CES LEDs is ascribed to the increase in emissions via the top escape route and to the extraction of transverse-magnetic polarized light.</P>
Jaehee Cho,Euijoon Yoon,Hyunsoo Kim,Yongjo Park,Joon Seop Kwak IEEE 2008 Photonics Technology Letters Vol.20 No.13
<P>Improved emission efficiency in InGaN near-ultraviolet light-emitting diodes (LEDs) was demonstrated using reverse bias in pulsed voltage operation. Pulsed voltage operation of the LEDs from 3 to 3.2 V with a duty cycle of 50% at Hz produced a radiant flux of 4.0 mW, while pulsed operation from 0 to 3.2 V showed a radiant flux of 3.2 mW. The radiant flux further increased as the reverse voltage increased, at the same forward voltage. The improved radiant flux was attributed to uniform carrier redistribution at the multiquantum wells, due to a periodically applied voltage from reverse voltage to forward voltage with a frequency ranged from to Hz, resulting in improved emission efficiency in InGaN LEDs.</P>
Detailed formation processes of stable dislocations in graphene.
Lee, Gun-Do,Yoon, Euijoon,He, Kuang,Robertson, Alex W,Warner, Jamie H RSC Pub 2014 Nanoscale Vol.6 No.24
<P>We use time-dependent HRTEM to reveal that stable dislocation pairs in graphene are formed from an initial complex multi-vacancy cluster that undergoes multiple bond rotations and adatom incorporation. In the process, it is found that the transformation from the formed complex multi-vacancy cluster can proceed without the increase of vacancy because many atoms and dimers are not only evaporated but also actively adsorbed. In tight-binding molecular dynamics simulations, it is confirmed that adatoms play an important role in the reconstruction of non-hexagonal rings into hexagonal rings. From density functional theory calculations, it is also found from simulations that there is a favorable distance between two dislocations pointing away from each other (i.e. formed from atom loss). For dislocation pairs pointing away from each other, the hillock-basin structure is more stable than the hillock-hillock structure for dislocation pairs pointing away from each other (i.e. formed from atom loss).</P>