• Korean Journal of Optics and Photonics
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• v.26 no.2
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• pp.98-102
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• 2015

The radiative quantum efficiency of a solid-state laser was measured by photoacoustic spectroscopy with a PZT as the detector. The radiative quantum efficiency was about 58.3 % for a laser-diode pumped Nd:S-VAP laser under lasing conditions. The measurement of radiative quantum efficiency was presented as an effective method for the optimization of a laser resonator.

• Proceedings of the Korean Vacuum Society Conference
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• 2012.08a
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• pp.184-184
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• 2012

Recently, InGaN/GaN multi-quantum well grown on GaN pyramid structures have attracted much attention due to their hybrid characteristics of quantum well, quantum wire, and quantum dot. This gives us broad band emission which will be useful for phosphor-free white light emitting diode. On the other hand, by using quantum dot emission on top of the pyramid, site selective single photon source could be realized. However, these structures still have several limitations for the single photon source. For instance, the quantum efficiency of quantum dot emission should be improved further. As detection systems have limited numerical aperture, collection efficiency is also important issue. It has been known that micro-cavities can be utilized to modify the radiative decay rate and to control the radiation pattern of quantum dot. Researchers have also been interested in nano-cavities using localized surface plasmon. Although the plasmonic cavities have small quality factor due to high loss of metal, it could have small mode volume because plasmonic wavelength is much smaller than the wavelength in the dielectric cavities. In this work, we used localized surface plasmon to improve efficiency of InGaN qunatum dot as a single photon emitter. We could easily get the localized surface plasmon mode after deposit the metal thin film because lnGaN/GaN multi quantum well has the pyramidal geometry. With numerical simulation (i.e., Finite Difference Time Domain method), we observed highly enhanced decay rate and modified radiation pattern. To confirm these localized surface plasmon effect experimentally, we deposited metal thin films on InGaN/GaN pyramid structures using e-beam deposition. Then, photoluminescence and time-resolved photoluminescence were carried out to measure the improvement of radiative decay rate (Purcell factor). By carrying out cathodoluminescence (CL) experiments, spatial-resolved CL images could also be obtained. As we mentioned before, collection efficiency is also important issue to make an efficient single photon emitter. To confirm the radiation pattern of quantum dot, Fourier optics system was used to capture the angular property of emission. We believe that highly focused localized surface plasmon around site-selective InGaN quantum dot could be a feasible single photon emitter.

• Current Optics and Photonics
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• v.2 no.5
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• pp.468-473
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• 2018

We investigate the temperature dependence of efficiency droop in InGaN/GaN multiple-quantum-well (MQW) blue light-emitting diodes (LEDs) in the temperature range from 20 to $80^{\circ}C$. When the external quantum efficiency (EQE) and the wall-plug efficiency (WPE) of the LED sample were measured as injection current and temperature varied, the droop of EQE and WPE was found to be reduced with increasing temperature. As the temperature increased from 20 to $80^{\circ}C$, the droop ratio of EQE was decreased from 16% to 14%. This reduction in efficiency droop with temperature can be interpreted by a temperature-dependent carrier distribution in the MQWs. When the carrier distribution and radiative recombination rate in MQWs were simulated and compared for different temperatures, the carrier distribution was found to become increasingly homogeneous as the temperature increased, which is believed to partly contribute to the reduction in efficiency droop with increasing temperature.

• Proceedings of the Korean Vacuum Society Conference
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• 2012.08a
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• pp.172-173
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• 2012

Currently GaN based LED is known to show high internal or external efficiency at low current range. However, this LED operation occurs at high current range and in this range, a significant performance degradation known as 'efficiency droop' occurs. Auger process, carrier leakage process, field effect due to lattice mismatch and thermal effects have been discussed as the causes of loss of efficiency, and these phenomena are major hindrance in LED performance. In order to investigate the main effects of efficiency loss and overcome such effects, it is essential to obtain relative proportion of measurements of internal quantum efficiency (IQE) and various radiative and nonradiative recombination processes. Also, it is very important to obtain radiative and non-radiative recombination times in LEDs. In this research, we measured the IQE of InGaN/GaN multiple quantum wells (MQWs) LEDs with PSS and Planar substrate using modified ABC equation, and investigated the physical mechanism behind by analyzing the emission energy, full-width half maximum (FWHM) of the emission spectra, and carrier recombination dynamic by time-resolved electroluminescence (TREL) measurement using pulse current generator. The LED layer structures were grown on a c-plane sapphire substrate and the active region consists of five 30 ${\AA}$ thick In0.15Ga0.85N QWs. The dimension of the fabricated LED chip was $800um{\times}300um$. Fig. 1. is shown external quantum efficiency (EQE) of both samples. Peak efficiency of LED with PSS is 92% and peak efficiency of LED with planar substrate is 82%. We also confirm that droop of PSS sample is slightly larger than planar substrate sample. Fig. 2 is shown that analysis of relation between IQE and decay time with increasing current using TREL method.

• Journal of Sensor Science and Technology
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• v.33 no.4
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• pp.224-229
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• 2024

Quantum dot light-emitting diodes (QD-LEDs) are emerging as next-generation displays owing to their high color purity, wide color gamut, and solution processability. Enhancing the efficiency of QD-LEDs involves preventing non-radiative recombination mechanisms, such as Auger and interfacial recombination. Generally, ZnO serves as the electron transport layer, which is known for its higher mobility compared to that of organic semiconductors and can lead to excessive electron injection. Some of the injected electrons pass through the quantum dot emissive layer and undergo non-radiative recombination near or within the organic hole transport layer (HTL), resulting in HTL degradation. Therefore, the implementation of electron blocking layers (EBLs) is essential; however, studies on all-solution-processed inverted InP QD-LEDs are limited. In this study, poly(9-vinylcarbazole) (PVK) is introduced as an EBL to mitigate HTL degradation and enhance the emission efficiency of inverted InP QD-LEDs. Using a single-carrier device, PVK was confirmed to effectively inhibit electron overflow into the HTL, even at extremely low thicknesses. The optimization of the PVK thickness also ensured minimal disruption of the hole-injection properties. Consequently, a 1.5-fold increase in the maximum luminance was achieved in the all-solution-processed inverted InP QD-LEDs with the EBL.

• Journal of the Korean Crystal Growth and Crystal Technology
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• v.1 no.1
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• pp.82-91
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• 1991

Molecular beam epitaxial growth of GaAs on Si substrate and the results on its analysis are reported. Epitaxy was performed on two different types of the substrate under various grwth conditions, and was analyzed by scanning and transmission electron microscopes, X-ray diffractometer, photoluminescence and Hall measurements. GaAs epitaxial layer has better crystalline quality when it was grown on a tilt-cut substrate. The stress seems to be releaxed more easily when multi-quantum well was introduced in the buffer layer. The epilayer was doped unintentionally with Si during growth due to the diffusion of the substrate. Also observed is that the quantum efficiency of excitonic radiative recombination of the heteroepitaxy is not as good as that of the homoepitaxy in the same doping level.

• Current Optics and Photonics
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• v.4 no.4
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• pp.380-390
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• 2020

In this study, the traditional electron-blocking layer (EBL) in (In,Ga)N/GaN light-emitting diodes is replaced by a circular EBL that is the same size as the n-pad. The three-dimensional (3D) nonlinear Poisson, drift-diffusion, and continuity equations are adopted to simulate current transport in the LED and its characteristics. The results indicate that the local carrier-density distribution obtained for the circular EBL design is more uniform than that for the traditional EBL design. This improves the uniformity of local radiative recombination and local internal quantum efficiency (IQE) at high injection levels, which leads to a higher lumped IQE and lower efficiency droop. With the circular EBL, the lumped IQE is higher in the outer active region and lower in the active region under the n-pad. Since most emissions from the active region under the n-pad are absorbed by the n-pad, obviously, an LED with a circular EBL will have a higher external quantum efficiency (EQE). The results also show that this LED works at lower applied voltages.

• Proceedings of the Korean Vacuum Society Conference
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• 2011.02a
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• pp.47-47
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• 2011

There have been numerous efforts to enhance the efficiency of light-emitting diodes (LEDs) by using low dimensional structures such as quantum dots (QDs), wire (QWRs), and wells (QWs). We demonstrate QD/QWR/QW hybrid structured LEDs by using nano-scaled pyramid structures of GaN with ~260 nm height. Photoluminescence (PL) showed three multi-peak spectra centered at around 535 nm, 600 nm, 665 nm for QWs, QWRs, and QDs, respectively. The QD emission survived at room temperature due to carrier localization, whereas the QW emission diminished from 10 K to 300 K. We confirmed that hybrid LEDs had zero-, one-, and two-dimensional behavior from a temperature-dependent time-resolved PL study. The radiative lifetime of the QDs was nearly constant over the temperature, while that of the QWs increased with increasing temperature, due to low dimensional behavior. Cathodoluminescence revealed spatial distributions of InGaN QDs, QWRs, and QWs on the vertices, edges, and sidewalls, respectively. We investigated the blue-shifted electroluminescence with increasing current due to the band-filling effect. The hybrid LEDs provided broad-band spectra with high internal quantum efficiency, and color-tunability for visible light-emitting sources.

• Journal of Adhesion and Interface
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• v.23 no.2
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• pp.17-24
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• 2022

Colloidal quantum dots (QDs) have gained attention for applications in quantum dot light emitting diodes (QLEDs) due to their high photoluminescence quantum yield, narrow emission spectra, and tunable bandgap. Nevertheless, non-radiative recombination induced by electron and hole imbalance deteriorates the device efficiency and stability. To overcome the problem, researchers have been trying to enhance hole transport properties of hole transporting layers (HTL) and/or slow down the electron injection in electron transport layer (ETL). Here, we summarize two approaches: i) development of interfacial materials between QD and ETL (or HTL); ii) engineering of HTL by blending or multi-layer approaches.

• Journal of Sensor Science and Technology
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• v.32 no.6
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• pp.441-446
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• 2023

Electrical and optical characteristics of the GaN-based light-emitting diode (LED) with the improved multi-quantum well (MQW) structure have been studied for light source in bio-sensing systems. Novel GaN/In_0.1GaN/In_0.2GaN/In_0.1GaN/GaN and Al_0.1GaN/GaN/In_0.2GaN/GaN/Al_0.1GaN (MQW) structures were suggested, and their radiative recombination rate, light output power, electroluminescence, and external quantum efficiency were compared with those of the conventional GaN/In_0.2GaN/GaN MQW structure using device simulation. The LED with the GaN/In_0.1GaN/In_0.2GaN/In_0.1GaN/GaN MQW structure showed an excellent recombination rate of 5.57 × 10²⁸ cm^-3·s^-1 that was more than one order improvement over that of the conventional LED. In addition, the efficiency droop was relieved by the suggested stepped MQW structure.