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 highoperating 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 selfheating 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.

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.

Design and development of the growth-process for the production of wafer-scale spatially homogeneous thickness controlled atomically thin transition metal dichalcogenides (TMDs) is one of the key challenges to realize modern electronic devices. Here, we demonstrate rapid and scalable synthesis of MoS2 films with precise thickness control via gas-phase chemical vapor deposition approach. We show that a monolayer MoS2 can be synthesized over a 2-in. sapphire wafer in a growth time as low as 4 min. With a linear growth rate of 1-layer per 4 min, MoS2 films with thicknesses varying from 1- to 5-layers with monolayer precision are produced. We propose that, in addition to Raman spectroscopy, the energy splitting of exciton bands in optical-absorbance spectra may be another choice for layer thickness identification. With suitable precursor selection, our approach can facilitate the rapid synthesis of spatially homogeneous atomically thin TMDs on a large scale.

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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.

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The chemical vapor deposition (CVD) method to obtain tailored graphene as a transparent and flexible gas barrier has been developed. By separating nucleation step from growth, we could reduce early graphene nucleation density and thus induce better stitching between domain boundaries in the second growth step. Furthermore, two step growth in conjunction with electrochemical polishing of Cu foils achieved large graphene domains and improved graphene quality with minimized defects. The performance of resulting graphene as a gas barrier was superior to the graphene obtained by one-step growth on polished or unpolished Cu foils. The CVD graphene reported here could open up the possibility for exploring graphene-based gas barrier due to the minimized density of defect area.