Kang Bok Ko

Improved heat dissipation in gallium nitride light-emitting diodes with embedded graphene oxide pattern

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.

Work-function-tuned multilayer graphene as current spreading electrode in blue light-emitting diodes

This letter reports on the implementation of multilayer graphene (MLG) as a current spreading electrode in GaN-based blue light-emitting diodes. We demonstrate two facile strategies to maneuver the electrical coupling between p-GaN layer and MLG. Using a work-function-tuned MLG and a thin gold (Au) metal interlayer, the current spreading and thus the device forward voltage are considerably improved. We attribute these improvements to the diminution in work function difference between p-GaN and MLG, the decrease of specific contact resistance, and the enhancement in the conductivity of MLG film as a result of doping. In addition, rapid thermal annealing at elevated temperature is found to provide additional pathway for enhanced carrier injection.

Performance evaluation of GaN light-emitting diodes using transferred graphene as current spreading layer

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 ...

Stress-relaxed growth of n-GaN epilayers

A significant stress-relaxation was observed in GaN epilayers by integrating a heavily Si-doped GaN (n+-GaN) sacrificial layer in the undoped GaN templates grown on sapphire substrates by metal-organic chemical vapor deposition. Selective GaN growth and electrochemical etching were exploited to achieve embedded air-gaps. Stress-relaxation and its local variations were probed by Raman mapping of high-frequency transverse-optical E2 (high) phonon mode of GaN. Enhanced In incorporation and improved light emission were observed in InGaN/GaN multi-quantum well visible light emitting diode structures fabricated on stress-relaxed GaN-epilayers with embedded air-gaps. Relevant sources for stress reduction and improved optical emission have been discussed.

Fabrication and Characteristics of GaN-Based Light-Emitting Diodes with a Reduced Graphene Oxide Current-Spreading Layer

A reduced graphene oxide (GO) layer was produced on undoped and n-type GaN, and its effect on the current- and heat-spreading properties of GaN-based light-emitting diodes (LEDs) was studied. The reduced GO inserted between metal electrode and GaN semiconductor acted as a conducting layer and enhanced lateral current flow in the device. Especially, introduction of the reduced GO layer on the n-type GaN improved the electrical performance of the device, relative to that of conventional LEDs, due to a decrease in the series resistance of the device. The enhanced current-spreading was further of benefit, giving the device a higher light output power and a lower junction temperature at high injection currents. These results therefore indicate that reduced GO can be a suitable current and heat-spreading layer for GaN-based LEDs.

Nanostructural Effect of ZnO on Light Extraction Efficiency of Near-Ultraviolet Light-Emitting Diodes

The effect of ZnO nanostructures on the light output power of 375 nm near-ultraviolet light-emitting diodes NUV-LEDs was investigated by comparing one-dimensional 1D nanorods NR-ZnO with two-dimensional 2D nanosheets NS-ZnO. ZnO nanostructures were grown on a planar indium tin oxide ITO by solution based method at low temperature of 90°C without degradation of the forward voltage. At an injection current of 100 mA, the light output efficiency of NUV-LED with NR-ZnO was enhanced by around 30% compared to the conventional NUV-LEDs without ZnO nanostructures. This improvement is due to the formation of a surface texturing, resulting in a larger escape cone and a multiple scattering for the photons in the NUV-LED, whereas the light output efficiency of NUV-LED with NS-ZnO was lower than that of the conventional NUV-LEDs due to the internal reflection and light absorption in the defective sites of NS-ZnO.

Reduced junction temperature and enhanced performance of high power light-emitting diodes using reduced graphene oxide pattern

Thermal management has become a crucial area for further development of high-power light-emitting didoes (LEDs) due to the high operating current densities that are required and result in additional joule heating. This increased joule heating negatively affects the performance of the LEDs since it greatly decreases both the optical performance and the lifetime. To circumvent this problem, a reduced graphene oxide (rGO) layer can be inserted to act as a heat spreader. In this study, current–voltage and light-output-current measurements are systematically performed at different temperatures from 30 to 190 °C to investigate the effect that the embedded rGO pattern has on the device performance. At a high temperature and high operating current, the junction temperature (Tj) is 23% lower and the external quantum efficiency (EQE) is 24% higher for the rGO embedded LEDs relative to those of conventional LEDs. In addition, the thermal activation energy of the rGO embedded LEDs exhibits a 30% enhancement as a function of the temperature at a bias of −5 V. This indicates that the rGO pattern plays an essential role in decreasing the junction temperature and results in a favorable performance in terms of the temperature of the high power GaN-based LED junction.

High performance of InGaN light-emitting diodes by air-gap/GaN distributed Bragg reflectors.

The effect of air-gap/GaN DBR structure, fabricated by selective lateral wet-etching, on InGaN light-emitting diodes (LEDs) is investigated. The air-gap/GaN DBR structures in LED acts as a light reflector, and thereby improve the light output power due to the redirection of light into escape cones on both front and back sides of the LED. At an injection current of 20 mA, the enhancement in the radiometric power as high as 1.91 times as compared to a conventional LED having no DBR structure and a far-field angle as low as 128.2° are realized with air-gap/GaN DBR structures.

Investigations of the Air Gap Embedded Green InGaN/GaN Light-Emitting Diodes

We investigated the effect of double air gaps embedded between sapphire and undoped GaN on the strain reduction in the InGaN/GaN-based green LED structure. Selective GaN growth and electrochemical etching were exploited to achieve embedded air gaps. Raman spectroscopy and photoluminescence were employed to demonstrate the relation between strain relaxation and indium incorporation. The double air gaps caused strain relaxation and led to a higher In incorporation in InGaN layers, which in turn caused a redshift of the PL spectra. As a result, three different peak wavelengths according to the existence of air gaps were observed.

Effect of curved graphene oxide in a GaN light-emitting-diode for improving heat dissipation with a patterned sapphire substrate

This article reports on the development of a reduced graphene oxide (rGO)-embedded light-emitting diode (LED) on a patterned sapphire substrate (PSS), for improving heat dissipation and reducing threading dislocations. The prototype device fabrication involves the generation of scalable curved graphene oxide microscale patterns on a PSS, followed by thermal reduction and epitaxial lateral overgrowth of GaN in a metal–organic chemical vapor deposition system with a one-step process. Using the forward voltage method, the junction temperature T j of the rGO-embedded LED was found to be reduced by about 17 °C from the ~62 °C of the LED grown without the rGO buffer layer. Temperature-dependent light output power and chip surface temperature measurements were also performed and the results are discussed.