Lee-Woon Jang

Localized surface plasmon enhanced quantum efficiency of InGaN/GaN quantum wells by Ag/SiO 2 nanoparticles

Optical properties of InGaN/GaN multi-quantum-well (MQW) structures with a nanolayer of Ag/SiO2 nanoparticle (NP) on top were studied. Modeling and optical absorption (OA) measurements prove that the NPs form localized surface plasmons (LSP) structure with a broad OA band peaked near 440-460 nm and the fringe electric field extending down to about 10 nm into the GaN layer. The presence of this NP LSP electrical field increases the photoluminescence (PL) intensity of the MQW structure by about 70% and markedly decreases the time-resolved PL (TRPL) relaxation time due to the strong coupling of MQW emission to the LSP mode.

Nanopillar InGaN/GaN light emitting diodes integrated with homogeneous multilayer graphene electrodes

InGaN/GaN nanopillar light-emitting diodes (LEDs) were fabricated using a highly homogeneous multilayer graphene (h-MLG) electrode. Four layers of h-MLG were prepared homogeneously using chemical vapor deposition and layer-by-layer transfer methods. The h-MLG exhibited excellent optical, structural and electrical properties for use as an electrode in the LEDs. The h-MLG was applied as a transparent top electrode by suspending only on the tip of nanopillar LEDs. The current-driven InGaN/GaN nanopillar LED with the h-MLG electrode was successfully operated at a high current injection and exhibited bright electroluminescence.

Enhanced light output of InGaN/GaN blue light emitting diodes with Ag nano-particles embedded in nano-needle layer

2.7 times increase in room temperature photoluminescence (PL) intensity and 3.2 times increase in electroluminescence (EL) intensity were observed in blue multi-quantum-well (MQW) GaN/InGaN light emitting diodes (LEDs) as a result of introduction of nano-needle structure embedded with Ag nanoparticles (NPs) into n-GaN film underlying the active MQW region and thick p-GaN contact layer of LEDs. The nano-needle structure was produced by photoelectrochemical etching. Simultaneously a measurable decrease in room temperature decay time from 2.2 ns in control samples to 1.6 ns in PL was observed. The results are explained by strong coupling of recombination in GaN/InGaN MQWs with Ag NPs related localized surface plasmons.

Quantum efficiency control of InGaN/GaN multi-quantum-well structures using Ag/SiO2 core-shell nanoparticles

Photoluminescence (PL) efficiency increase up to 2.8 times was observed for GaN/InGaN multi-quantum-well (MQW) structures as a result of deposition of a thin layer of about 40-nm-diameter Ag nanoparticles (NPs) surrounded by SiO2 shell. These Ag/SiO2 NPs were prepared by sol-gel method. The amount of PL intensity enhancement decreased with increasing the SiO2 shell thickness. PL intensity increase was accompanied by corresponding decrease of PL decay time and is ascribed to a strong coupling of MQW region to localized surface plasmons (LSPs) associated with Ag/SiO2 NPs.

Metal-organic chemical vapor deposition growth of InGaN/GaN high power green light emitting diode: Effects of InGaN well protection and electron reservoir layer

We investigated the effects of the well protection layer (WPL) and electron reservoir layer (ERL) on the emission properties of InGaN/GaN green multiple quantum wells (MQWs). In order to increase their emission wavelength by preventing the volatile InGaN well, a thin GaN WPL was coated subsequently on each well layer at the same temperature before ramping-up the temperature to grow the GaN barrier. It was found that the WPL directly influenced the indium content and optical properties of the MQW. The indium content was in fact increased, as was evident from the x-ray diffraction and photoluminescence experiments. Then, to explore the possibility of enhancing the quantum efficiency by increasing the electron capture rate, a superlattice ERL composed of ten pairs of InGaN/GaN was embedded between the MQW and n-GaN. The electroluminescence intensity of the green light emitting diode with the ERL was up to three times higher than that of the diode without the ERL. These results imply that the carrier capture ...

A well protection layer as a novel pathway to increase indium composition : a route towards green emission from a blue InGaN/GaN multiple quantum well

We have investigated the effects of a well protection layer (WPL) on the optical and crystal properties of an InGaN/GaN multiple quantum well (MQW). The five-pair MQW, consisting of an InGaN well grown at 750 °C and a GaN barrier grown at 850 °C, was simply embedded between GaN cladding layers on a sapphire (0001) substrate. While this dual-temperature MQW growth scheme seemed better suited to the GaN barrier quality, it exposed the volatile InGaN well to a higher temperature ambient during the ramping-up process to grow the barrier. In order to prevent damage to the fragile well, a thin GaN WPL was subsequently coated on each well layer at the same temperature before ramping-up the temperature. Consequently, it was found that the WPL directly influenced the indium composition and optical properties of the MQW. The indium composition was in fact increased, as was evident from x-ray diffraction experiments. In addition, photoluminescence measurements showed that the emission peak wavelength was increased from 464 to 520 nm. These results provide evidence that the WPL effectively suppresses indium re-evaporation during the ramping-up time. The present study proposes that the WPL leads to a new way to increase the wavelength of InGaN/GaN MQWs.

Energy coupling processes in InGaN/GaN nanopillar light emitting diodes embedded with Ag and Ag/SiO2 nanoparticles

We synthesized Ag and Ag/SiO2 nanoparticles (NPs) and investigated the energy coupling processes between the localized surface plasmons of NPs and the active quantum well regions of nanopillar light-emitting diodes (LEDs). Nanopillar LEDs embedded with Ag NPs exhibited a decreased photoluminescence (PL) intensity, while the PL was markedly enhanced for Ag/SiO2 NP embedded nanopillar LEDs. Though the PL decay times decreased in both cases compared to the sample without NPs, the difference in observed optical behavior suggests that different types of energy coupling (EC) are involved.

Free-Standing GaN Layer by Combination of Electrochemical and Photo-Electrochemical Etching

A free-standing GaN layer was produced by combining electrochemical (EC) etching from the front surface, photo-electrochemical (PEC) etching from the back surface, and subsequent regrowth of GaN on the porous template thus produced. The EC etching resulted in the formation of etch channels on the surface portion of the starting film, whereas the back-side PEC etching gave rise to a columnar structure supporting the entire film. When the n-GaN layer was regrown on such template, the underlying columnar structure provided weak places for easy separation and transfer of the film by mechanical bonding.

Enhanced optical characteristics of light emitting diodes by surface plasmon of Ag nanostructures

We investigated the surface plasmon coupling behavior in InGaN/GaN multiple quantum wells at 460 nm by employing Ag nanostructures on the top of a roughened p-type GaN. After the growth of a blue light emitting diode structure, the p-GaN layer was roughened by inductive coupled plasma etching and the Ag nanostructures were formed on it. This structure showed a drastic enhancement in photoluminescence and electroluminescence intensity and the degree of enhancement was found to depend on the morphology of Ag nanostructures. From the time-resolved photoluminescence measurement a faster decay rate for the Ag-coated structure was observed. The calculated Purcell enhancement factor indicated that the improved luminescence intensity was attributed to the energy transfer from electron-hole pair recombination in the quantum well to electron vibrations of surface plasmon at the Ag-coated surface of the roughened p-GaN.

Characteristics of a-GaN films and a-AlGaN/GaN heterojunctions prepared on r-sapphire by two-stage growth process

The electrical properties, presence of deep electron and hole traps and photoluminescence spectra were measured for undoped a-GaN films grown by metal-organic chemical vapor deposition (MOCVD) in a two-stage process using a high V/III ratio at the first stage and low V/III ratio at the second stage. Growth was performed on r-sapphire substrates with a high temperature GaN nucleation layer. The films showed a full width at half maximum of 450-470 arcseconds for the (11-20) x-ray rocking curve with little anisotropy with respect to the sample rotation around the growth direction. The stacking fault (SF) density determined by selective etching was ∼5 × 104 cm−1. The residual donor concentration was 1014–1015 cm−3, with a very low density (2.5 × 1013 cm−3) of electron traps located at Ec − 0.6 eV, which are believed to be one of the major non-radiative recombination centers in nonpolar GaN. Consequently, the films showed a high intensity of bandedge luminescence with negligible contribution from defect bands ...