Bong-Joon Kwon

Electrically Driven Quantum Dot/Wire/Well Hybrid Light‐Emitting Diodes

Electrically driven quantum dot, wire, and well hybrid light-emitting diodes are demonstrated by using nanometer-sized pyramid structures of GaN. InGaN quantum dots, wires, and wells are formed at the tops, edges, and sidewalls of pyramids, respectively. The hybrid light-emitting diodes containing low-dimensional quantum structures are good candidates for broad-band highly efficient visible lighting sources.

Optical investigation of p-type ZnO epilayers doped with different phosphorus concentrations by radio-frequency magnetron sputtering

Optical properties of p-type ZnO epilayers doped with different amounts of phosphorus by radio-frequency magnetron sputtering are investigated by x-ray diffraction, temperature dependent photoluminescence (PL), and time-resolved PL techniques. Bound exciton, free electrons-to-acceptors, donor-to-acceptor pair, and deep-level emissions are observed at about 3.356, 3.32, 3.24, and 2.4eV at 10K for p-type ZnO, respectively. The crystal quality and luminescence efficiency are improved with increasing phosphorus doping concentration. These results show that phosphorus doping plays an important role both in reducing native defects and in generating shallow acceptors in ZnO, leading to a good p-type conductivity in ZnO.

Depth-resolved optical studies of excitonic and phonon-assisted transitions in ZnO epilayers

The authors report depth-resolved optical properties of excitonic and phonon-assisted transitions in ZnO epilayers by photoluminescence (PL) and cathodoluminescence. A weaker free exciton (FX) emission than its first longitudinal optical phonon replica (FX-1LO) is observed at elevated temperatures (T>150K) for interior area, while a stronger FX than FX-1LO is seen at all temperatures for top surface area of the sample. The authors exclude out a possible self-absorption process by PL excited at back surface of the sample. Therefore, the authors conclude that the different intensity ratios of FX and FX-1LO depending on the sample depth are strongly associated with extrinsic features of ZnO.

Optical characteristics of hexagonal GaN self-assembled quantum dots: Strong influence of built-in electric field and carrier localization

Optical characteristics of hexagonal GaN self-assembled quantum dots (QDs) were systematically studied by photoluminescence (PL), PL excitation (PLE), time-resolved PL, and cathodoluminescence (CL). We observed a Stokes-like shift between PLE absorption edge and PL emission from the GaN QDs as well as from the Al(Ga)N base layer. With decreasing emission energy, the measured lifetime of the hexagonal GaN QDs emission increased, while that of the cubic GaN QDs kept almost constant. The optical emission from the GaN QDs was measured as a function of temperature from 10 to 300 K, and their properties were compared with GaN quantum-well structures. With increasing temperature, the PL intensity of Al(Ga)N base layer or GaN quantum wells was dramatically decreased, while that of GaN QDs was not changed much. We observed CL images showing strong carrier localization in GaN QDs. Therefore, we conclude that the GaN QD emissions are strongly influenced by built-in electric field as well as by carrier localization i...

Electroluminescence emission from light-emitting diode of p-ZnO/(InGaN/GaN) multiquantum well/n-GaN

We report on the fabrication and characteristics of light-emitting diodes (LEDs) which consist of antimony (Sb) doped p-ZnO, (InGaN/GaN) multiquantum well (MQW), and n-GaN. An electroluminescence (EL) emission at a wavelength of 468 nm is observed from the hybrid LEDs after thermal annealing at 750 °C, showing that Sb-doped p-ZnO can be used as a hole supplying layer in hybrid LEDs. Furthermore, the EL peaks are redshifted as the injection current is increased, indicating that the compressive strain in MQW layers is relaxed due to Sb-doped p-ZnO layer.

Optical transition dynamics in ZnO/ZnMgO multiple quantum well structures with different well widths grown on ZnO substrates

We report the optical properties of ZnO/ZnMgO multiple quantum well (MQW) structures with different well widths grown on ZnO substrates by molecular beam epitaxy. Photoluminescence (PL) spectra show MQW emissions at 3.387 and 3.369 eV for the ZnO/ZnMgO MQW samples with well widths of 2 and 5 nm, respectively, due to the quantum confinement effect. Time-resolved PL results show an efficient photogenerated carrier transfer from the barrier to the MQWs, which leads to an increased intensity ratio of the well to barrier emissions for the ZnO/ZnMgO MQW sample with the wider well width.

A highly efficient warm white light-emitting Eu2+-activated silicate host: another striking application of mesoporous silica

This study outlines a novel approach to the synthesis of an efficient yellow-emitting Sr0.975Ca0.975Eu0.05SiO4 phosphor with the adequate red spectral components. This was achieved by the use of a well-known silicate source, mesoporous silica. The findings showed that the synthesized phosphor gave a broad emission from blue to red, along with a stable emission color till 450 K and an emission decay time of 1 μs. When the phosphor was fabricated using UV (400 nm) and blue LED (450 nm) chips, it promisingly generated warm white light at a color temperature 80.

Quenching dynamics in CdSe/ZnS core/shell quantum dots-gold nanoparticle conjugates in aqueous solution

We investigated carrier dynamics of luminescence quenching in CdSe/ZnS core/shell quantum dots (QDs) in the absence and presence of gold nanoparticles (Au NPs) in aqueous solution. We emphasized that the observed quenching phenomenon in the QDs-Au NP composites can be a result of the interplay of the Forster resonance energy transfer and surface plasmon enhancement process of Au NPs on QDs. Different QDs with emission wavelengths varying from 545 to 619 nm were used to investigate the quenching dynamics. The recovery dynamics of the quenching was also confirmed by the addition of sodium chloride in the QDs-Au NP composites.

Effect of VI/II Gas Ratio on the Epitaxial Growth of ZnO Films by Metalorganic Chemical Vapor Deposition

We report the effect of the VI/II ratio on the metalorganic chemical vapor deposition (MOCVD) growth of ZnO film. The surface of ZnO film becomes very smooth as the VI/II ratio increases. Atomic force microscopy measurement shows that ZnO films grown at a VI/II ratio of 25,000 have atomically flat terraces with a root-mean-square roughness of 0.2 nm. Low-temperature photoluminescence spectra also reveal a very sharp excitonic emission comprised of a neutral donor bound exciton emission and a free exciton emission with first and second longitudinal optical (LO) phonon replicas, indicating that the ZnO epilayer is of a high optical quality.

Carrier transfer and recombination dynamics of a long-lived and visible range emission from multi-stacked GaN/AlGaN quantum dots

We have investigated the optical properties of multi-stacked GaN/AlGaN self-assembled quantum dots (QDs) grown by molecular beam epitaxy. The QDs that emit visible light have a broad spectral range without incorporation of indium alloy because of the quantum-confined Stark effect. We found differences in the structural and optical properties between the layers of multi-stacked QDs. The carriers are more effectively transferred from the AlGaN barrier to the low energy side of the GaN QD emission than to the high energy side. We also observed long-lived carrier recombination dynamics for the visible range emission from QDs.