Beo Deul Ryu

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.

Impact of Interlayer Processing Conditions on the Performance of GaN Light-Emitting Diode with Specific NiOx/Graphene Electrode

This paper reports on the evaluation of the impact of introducing interlayers and postmetallization 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 NiO(x) interlayers were studied using transmission line model with circular contact geometry. Direct graphene/p-GaN interface was identified to be highly rectifying and postmetallization 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 postmetallization annealing. Temperature-dependent I-V measurements revealed that the current transport was modified from thermionic field emission for the direct graphene/p-GaN contact to tunneling for the graphene/metal/p-GaN contacts. The tunneling mechanism results from the interfacial reactions that occur between the metal and p-GaN during the postmetallization annealing. InGaN/GaN light-emitting diodes with NiO(x)/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 NiO(x) during the postmetallization annealing. The observed low light output is therefore correlated to the phase change induced transmittance loss in the NiO(x)/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.

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 ...

Enhanced light output power of GaN-based light-emitting diodes by nano-rough indium tin oxide film using ZnO nanoparticles

We demonstrate the performance improvement of GaN-based light-emitting diodes (LEDs) using zinc oxide (ZnO) nanoparticles inserted between the p-GaN and the indium tin oxide (ITO) layers. Upon deposition of an ITO film over the dispersed ZnO nanoparticles, the ITO surface tends to attain a nano-rough morphology due to the presence of ZnO nanoparticles. The light output power of the fabricated LEDs with ZnO nanoparticles is 39% higher than that of conventional LEDs at an injection current of 20 mA. This is attributed to the improved light extraction favored by the light scattering tendency of ZnO nanoparticles and the nano-roughened ITO film. In addition, the intermediate refractive index (n ∼2) of ZnO materials between those of the p-GaN (n ∼2.5) and the ITO (n ∼1.9) results in a broader critical angle and a reduction of total internal reflection.

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.

Solution-processed multidimensional ZnO/CuO heterojunction as ultraviolet sensing

We report on a solution-based method to fabricate a multidimensional heterojunction composed of one-dimensional zinc oxide nanorods (ZnO NRs) decorated with zero-dimensional cupric oxide nanoparticles (CuO NPs). The ZnO NRs were vertically grown on a reduced graphene oxide (rGO) thin film, and the sensing properties of the synthesized heterojunction were investigated under ultraviolet (UV) irradiation at room temperature to assess their practical application. The CuO NPs decorated on the ZnO NRs play an essential role in creating numerous p-n heterojunctions at the interface and remediating oxygen-related defects in the ZnO NRs. Relative to the ZnO NR structures without CuO NPs, the CuO/ZnO multidimensional heterostructures show higher sensitivity and faster response, demonstrating their potential use as UV sensors.

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.

Insights into annealing-induced ohmic contact formation at graphene/p-GaN interface with a NiOx contact layer

The effect of rapid thermal annealing on the graphene/p-GaN interface with a thin NiOx contact layer in a GaN light-emitting diode (LED) was investigated. Raman and ultraviolet photoemission measurements revealed charge transfer in graphene on a NiOx contact layer. Rapid thermal annealing at temperatures above 350 °C induced the formation of metallic Ni with a simultaneous increase of Ni3+ ions within the NiOx layer covered by the graphene. As a result, the graphene electrode on a NiOx/p-GaN surface of a blue GaN LED formed a low-resistance ohmic contact with a specific contact resistance of 5.3 × 10–4 ohm cm2. The ensuing LED chip offered a low forward voltage, comparable to that of the Ni/Au counterpart. This is attributed to a combination of distinct phenomena arising due to a work function modulation in graphene, increase in the carrier concentrations at the near surface region incited during the formation of NiOx, and thermal reduction induced modifications within the NiOx contact layer. At elevated annealing temperature, however, oxidation of graphene led to poor current spreading and thus a low optical output, but both these constraints were eliminated by using a few layer graphene.