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Han N.,Chonbuk National University | Cuong T.V.,Vietnam National University, Ho Chi Minh City | Han M.,Chonbuk National University | Ryu B.D.,Chonbuk National University | And 12 more authors.
Nature Communications | Year: 2013

The future of solid-state lighting relies on how the performance parameters will be improved further for developing high-brightness light-emitting diodes. Eventually, heat removal is becoming a crucial issue because the requirement of high brightness necessitates high-operating current densities that would trigger more joule heating. Here we demonstrate that the embedded graphene oxide in a gallium nitride light-emitting diode alleviates the self-heating issues by virtue of its heat-spreading ability and reducing the thermal boundary resistance. The fabrication process involves the generation of scalable graphene oxide microscale patterns on a sapphire substrate, followed by its thermal reduction and epitaxial lateral overgrowth of gallium nitride in a metal-organic chemical vapour deposition system under one-step process. The device with embedded graphene oxide outperforms its conventional counterpart by emitting bright light with relatively low-junction temperature and thermal resistance. This facile strategy may enable integration of large-scale graphene into practical devices for effective heat removal. © 2013 Macmillan Publishers Limited. Source

Katharria Y.S.,Chonbuk National University | Park Y.J.,Chonbuk National University | Ryu J.H.,Korea Institute of Lighting Technology | Ko K.B.,Chonbuk National University | And 3 more authors.
Current Applied Physics | Year: 2013

Selective growth by metal-organic chemical vapor deposition (MOCVD), and electrochemical etching of a heavily Si-doped GaN (n+-GaN) interlayer were employed to obtain air-gaps embedded in a u-GaN layer. As confirmed by Raman spectroscopy, the introduction of an n+-GaN, which was later etched to obtain air-gaps, also enhanced the strain-compliance of GaN epilayer on sapphire substrate. An enhanced electroluminescence emission was observed from the light-emitting diodes (LEDs) fabricated on the air-gap embedding template. Using theoretical LED simulation, it was discerned that the increase in optical emission from the LED was caused predominantly by the redirection of photons at GaN/air-gap interface. Finite-difference time domain (FDTD) simulation method was employed to understand the mechanism of optical emission enhancement and its spatial variation over the LED surface. © 2013 Elsevier B.V. Source

Kong H.J.,Korea Institute of Lighting Technology | Kim J.T.,Kyung Hee University
Energy and Buildings | Year: 2015

It is crucial to acknowledge the quantitative data on daylight illuminance and solar irradiance from a vertical plane or an incline plane for architects and architectural engineers in order to design energy efficient buildings and to create luminous environments. Also, the information of diffuse skies and the direct sun light from the vertical plane are essential in creating a comfortable working environment and a healthy indoor environment. This paper presents an evaluation of six inclined illuminance and irradiance models for Korea using two year of measured daylight weather data. In addition, a global vertical illuminance model for Korea was proposed. The evaluation was carried out for both global vertical illuminance and irradiance. The performance of all models was assessed with two common statistical parameters: the relative root mean square error (RMSE) and the relative mean bias error (MBE). As a result, all six models show high MBE and RMSE values and percentages for north-facing surfaces. On the other hand, predictions for south-facing surfaces produce good agreement with measured data. Kong's model shows the best prediction for global vertical illuminance as compared to those by Perez, Klucher, Hay, Temps-Coulson and Muneer. It is expected that the proposed model can be used as the basic data to maximize energy saving and to promote a pleasant building environment in Korea. © 2015 Published by Elsevier B.V. Source

Ryu J.-H.,Korea Institute of Lighting Technology | Lee J.H.,Korea Institute of Lighting Technology | Sun W.Y.,Korea Institute of Lighting Technology | Cho M.R.,Korea Institute of Lighting Technology
Journal of Information Display | Year: 2013

This paper analyses the degradation of the optical, electrical, and thermal characteristics of InGaN/GaN-based light-emitting diodes (LEDs) with different chip sizes. Their optical, electrical, and thermal characteristics were periodicity-monitored during the test. The small chip showed an increase in ideality factor and high series resistance. This led to the degradation of the electrical characteristic, and hence to high thermal resistance, finally resulting in low light output power. In the large chip, the high operating current made the junction temperature high. This, together with the thermal degradation on the LED surface and metal pad, led to a decrease in light output power. In the middle chip, the optical and electrical degradation ratio was lower than that in the other samples and, therefore, the light output power decreased to a lesser degree. © 2013 The Korean Information Display Society. Source

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