Gun Hee Lee

Direct growth of GaN layer on carbon nanotube-graphene hybrid structure and its application for light emitting diodes.

We report the growth of high-quality GaN layer on single-walled carbon nanotubes (SWCNTs) and graphene hybrid structure (CGH) as intermediate layer between GaN and sapphire substrate by metal-organic chemical vapor deposition (MOCVD) and fabrication of light emitting diodes (LEDs) using them. The SWCNTs on graphene act as nucleation seeds, resulting in the formation of kink bonds along SWCNTs with the basal plane of the substrate. In the x-ray diffraction, Raman and photoluminescence spectra, high crystalline quality of GaN layer grown on CGH/sapphire was observed due to the reduced threading dislocation and efficient relaxation of residual compressive strain caused by lateral overgrowth process. When applied to the LED structure, the current-voltage characteristics and electroluminescence (EL) performance exhibit that blue LEDs fabricated on CGH/sapphire well-operate at high injection currents and uniformly emit over the whole emission area. We expect that CGH can be applied for the epitaxial growth of GaN on various substrates such as Si and MgO, which can be a great advantage in electrical and thermal properties of optical devices fabricated on them.

Improving the graphene electrode performance in ultra-violet light emitting diode using silver nanowire networks

This paper reports a systematic study on the characteristics of silver nanowires (AgNWs) coated graphene and its application as a transparent current spreading electrode in ultra-violet light emitting diodes (UV-LEDs). The optimized values of optical transmittance and sheet resistance of AgNWs covered graphene were 87.7% at 375 nm and 50 ± 5 Ω/sq, respectively. Upon applying the AgNWs on graphene electrode, the UV-LED exhibited uniform bright light emission with a reduction in the forward voltage and about four-fold increase in the electroluminescence intensity. We attribute the observed performance improvements to a reduction in the sheet and contact resistances.

The role of graphene formed on silver nanowire transparent conductive electrode in ultra-violet light emitting diodes

This paper reports a highly reliable transparent conductive electrode (TCE) that integrates silver nanowires (AgNWs) and high-quality graphene as a protecting layer. Graphene with minimized defects and large graphene domains has been successfully obtained through a facile two-step growth approach. Ultraviolet light emitting diodes (UV-LEDs) were fabricated with AgNWs or hybrid electrodes where AgNWs were combined with two-step grown graphene (A-2GE) or conventional one-step grown graphene (A-1GE). The device performance and reliability of the UV-LEDs with three different electrodes were compared. The A-2GE offered high figure of merit owing to the excellent UV transmittance and reduced sheet resistance. As a consequence, the UV-LEDs made with A-2GE demonstrated reduced forward voltage, enhanced electroluminescence (EL) intensity, and alleviated efficiency droop. The effects of joule heating and UV light illumination on the electrode stability were also studied. The present findings prove superior performance of the A-2GE under high current injection and continuous operation of UV LED, compared to other electrodes. From our observation, the A-2GE would be a reliable TCE for high power UV-LEDs.

Correlation between reflectance and photoluminescent properties of al-rich ZnO nano-structures

Al rich zinc oxide nano-structured films were synthesized using spin coating sol-gel technique. The films were annealed in oxygen ambient in the temperature range of 200-700 °C. The structural, optical, and photoluminescence (PL) properties of the films were studied at various annealing temperatures using X-ray diffraction spectroscopy, field emission scanning electron microscopy, photoluminescence emission spectra measurement, and Raman and UV-Vis spectroscopy. The optical band gap was found to decrease with the increase of the annealing temperature following the Gauss Amp function due to the confinement of the exciton. The PL peak intensity in the near band region (INBE) was found to increase with the increase of the annealing temperature up to 600 °C, then to decrease fast to a lower value for the annealing temperature of 700 °C due to crystalline quality. The Raman peak of E2 (low) was red shifted from 118 cm-1 to 126 cm-1 with the increase of the annealing temperature. The intensity of the second order phonon (TA+LO) at 674 cm-1 was found to decrease with the increase of the annealing temperature. The normalized values of the reflectance and the PL intensity in the NBE region were highest for the annealing temperature of 600 °C. A special correlation was found between the reflectance at λ = 1000 nm and the normalized PL intensity in the green region due to scattering due to presence of grains.

Efficiency enhancement of nanorod green light emitting diodes employing silver nanowire-decorated graphene electrode as current spreading layer

We report nanorod green light emitting diodes (NGR-LEDs) with a silver nanowire-decorated graphene electrode (S-DGE) as a current spreading electrode to connect each individual nanorod and to give improved emission efficiency. The current–voltage and electroluminescence characteristics of NGR-LEDs with an S-DGE are enhanced remarkably compared to NGR-LEDs with a graphene electrode. The effective current spreading in the whole emission area after thermal annealing of the S-DGE was attributed to reduced sheet and contact resistances caused by the p-type doping of graphene and enhanced adhesion at the graphene–GaN and graphene–silver nanowire interfaces.

Hybrid electrode based on carbon nanotube and graphene for ultraviolet light-emitting diodes

We report on a transparent current-spreading electrode that combines carbon nanotubes (CNTs) and graphene for UV-LEDs. We compared the device performance and long-term stability of this CNT–graphene hybrid electrode (CGE) with those of the silver nanowire–graphene electrode (SGE) reported previously. Both types of electrode offered excellent UV transmittance and reduced sheet resistance through the bridging effect for grain boundaries and defects in graphene by nanotubes or nanowires. UV-LEDs using such electrodes offered reduced forward voltage and enhanced electroluminescence intensity, but the device with the CGE showed excellent stability whereas the SGE degraded with time.

Carbon-nanotube-assisted nanoepitaxy of Si-doped GaN for improved performance of InGaN/GaN light-emitting diodes

Using single-walled carbon nanotubes (SWCNTs) as nanomasks on an undoped GaN template, a significant biaxial stress relaxation was achieved in the subsequently-grown Si-doped n-GaN layer. Enhanced near band edge (NBE) emission intensity, similar free carrier concentrations, and the reduced peak width of the asymmetric (102) crystallographic plane all confirmed the suppression of threading dislocations due to the nanoepitaxial growth process. Temperature-dependent photoluminescence (PL) revealed improved internal quantum efficiency (IQE) of InGaN/GaN multi-quantum wells (MQWs) grown on this n-GaN layer. Furthermore, enhanced light output power and a remarkable reduction in efficiency droop were observed for the blue light-emitting diodes (LEDs), especially at higher injection currents. Our results emphasize the strong potential for SWCNTs as nanomasks in the heteroepitaxy of GaN-based devices without the exploitation of complicated lithography or etching processes.

Enhanced optical output power by the silver localized surface plasmon coupling through side facets of micro-hole patterned InGaN/GaN light-emitting diodes

Light extraction efficiency of GaN-based light emitting diodes were significantly enhanced using silver nanostructures incorporated in periodic micro-hole patterned multi quantum wells (MQWs). Our results show an enhancement of 60% in the wall-plug efficiency at an injection current of 100 mA when Ag nano-particles were deposited on side facet of MQWs passivated with SiO2. This improvement can be attributed to an increase in the spontaneous emission rate through resonance coupling between localized surface plasmons in Ag nano-particles and the excitons in MQWs.