Dae-Woo Jeon

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.

Efficient nonradiative energy transfer from InGaN/GaN nanopillars to CdSe/ZnS core/shell nanocrystals

In this study, we propose and demonstrate efficient electron-hole pair injection from InGaN/GaN multiple quantum well nanopillars (MQW-NPs) to CdSe/ZnS core/shell nanocrystal quantum dots (NQDs) via Forster-type nonradiative energy transfer. For that we hybridize blue-emitting MQW-NPs with red-emitting NQDs and the resultant exciton transfer reaches a maximum rate of (0.192 ns)−1 and a maximum efficiency of 83.0%. By varying the effective bandgap of core/shell NQDs, we conveniently control and tune the excitonic energy transfer rate for these NQD integrated hybrids, and our measured and computed exciton transfer rates are found to be in good agreement for all hybrid cases.

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.

Electrical properties of undoped GaN films grown by maskless epitaxial lateral overgrowth

Electrical properties, deep traps spectra, microcathodoluminescence (MCL) spectra measurements, MCL imaging, and electron beam induced current (EBIC) imaging were performed for undoped GaN films grown by metalorganic chemical vapor deposition using maskless epitaxial lateral overgrowth on basal plane sapphire. The films showed a low dislocation density of ∼108 cm−2 in the laterally overgrown wings and an order of magnitude higher dislocation density in vertical growth seed regions, as determined by MCL and EBIC imaging. The polarity of EBIC signal measurements and the room temperature capacitance-voltage characteristics suggested that the high-dislocation-density seed regions were high-resistivity p-type, with the Fermi level pinned near Ev + 0.4 eV, as determined by admittance spectroscopy. The wing regions were n-type, with low residual donor concentration of some 1014 cm−3 near the surface. The donor concentration further decreased upon movement towards the sapphire substrate. Some possible explanation...

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.

Electrical and luminescent properties and deep traps spectra in GaN nanopillar layers prepared by dry etching

Electrical properties, microcathodoluminescence spectra, and spectra of deep traps were studied for nanopillar structures prepared by dry etching of undoped GaN films using natural masks formed by Ni nanoparticles. It is shown that as-prepared nanopillar structures have low bandedge intensity, very high leakage current of Schottky diodes, their electrical properties are determined by 0.2 eV electron traps or, after etching in aqua regia, 0.14 eV electron traps that are commonly associated with radiation defects. Deep levels transient spectroscopy spectra measured after aqua regia etching are dominated by 1 eV electron traps, other common radiation defects. Annealing at 600 °C is instrumental in eliminating the 0.2 eV and 0.14 eV electron traps, but not the 1 eV traps. A higher temperature annealing at 900 °C is required for strongly suppressing the latter and increasing the bandedge luminescence peak magnitude by 2 times compared to control sample. The best results in terms of luminescence efficiency incr...

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.