Applied Sciences

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7 pages, 1146 KiB

Open AccessArticle

Toward Ultra-Low Efficiency Droop in C-Plane Polar InGaN Light-Emitting Diodes by Reducing Carrier Density with a Wide InGaN Last Quantum Well

by Yongbing Zhao and Panpan Li

Appl. Sci. 2019, 9(15), 3004; https://doi.org/10.3390/app9153004 - 26 Jul 2019

Cited by 5 | Viewed by 2764

Abstract

We demonstrate an ultra-low efficiency droop in c-plane polar InGaN blue light-emitting diodes (LEDs) by reducing the carrier density using a wide InGaN last quantum well (LQW). It is found that the LEDs with a 5.2 nm thick LQW show a negligible efficiency [...] Read more.

We demonstrate an ultra-low efficiency droop in c-plane polar InGaN blue light-emitting diodes (LEDs) by reducing the carrier density using a wide InGaN last quantum well (LQW). It is found that the LEDs with a 5.2 nm thick LQW show a negligible efficiency droop, with an external quantum efficiency (EQE) reducing from a peak value of 38.8% to 36.4% at 100 A/cm² and the onset-droop current density is raised from 3 A/cm² to 40 A/cm² as the LQW thickness increases from 3.0 nm to 5.2 nm. The analysis based on the ABC model indicates that small efficiency droop is caused by the reduced carrier density using a wide LQW. The peak efficiency is reduced with a wide LQW, which is caused by the reduction of the electron-hole wavefunction overlap and the deterioration of the crystal quality of the InGaN layer. This study suggests that the application of the InGaN LEDs with a wide LQW can be a promising and simple remedy for achieving high efficiency at a high current density. Full article

(This article belongs to the Special Issue Internal Quantum Efficiency of III-Nitride Light-Emitting Diodes)

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12 pages, 2041 KiB

Open AccessFeature PaperArticle

Enhanced Radiative Recombination Rate by Local Potential Fluctuation in InGaN/AlGaN Near-Ultraviolet Light-Emitting Diodes

by Abu Bashar Mohammad Hamidul Islam, Dong-Soo Shim and Jong-In Shim

Appl. Sci. 2019, 9(5), 871; https://doi.org/10.3390/app9050871 - 28 Feb 2019

Cited by 16 | Viewed by 3396

Abstract

We investigate the differences in optoelectronic performances of InGaN/AlGaN multiple-quantum-well (MQW) near-ultraviolet light-emitting diodes by using samples with different indium compositions. Various macroscopic characterizations have been performed to show that the strain-induced piezoelectric field (F_PZ), the crystal quality, and the [...] Read more.

We investigate the differences in optoelectronic performances of InGaN/AlGaN multiple-quantum-well (MQW) near-ultraviolet light-emitting diodes by using samples with different indium compositions. Various macroscopic characterizations have been performed to show that the strain-induced piezoelectric field (F_PZ), the crystal quality, and the internal quantum efficiency increase with the sample’s indium composition. This improved performance is owing to the carrier recombination at relatively defect-free indium-rich localized sites, caused by the local in-plane potential-energy fluctuation in MQWs. The potential-energy fluctuation in MQWs are considered to be originating from the combined effects of the inhomogeneous distribution of point defects, F_PZ, and indium compositions. Full article

(This article belongs to the Special Issue Internal Quantum Efficiency of III-Nitride Light-Emitting Diodes)

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9 pages, 1696 KiB

Open AccessArticle

Enhanced Device Performance of GaInN-Based Green Light-Emitting Diode with Sputtered AlN Buffer Layer

by Seiji Ishimoto, Dong-Pyo Han, Kengo Yamamoto, Ryoya Mano, Satoshi Kamiyama, Tetsuya Takeuchi, Motoaki Iwaya and Isamu Akasaki

Appl. Sci. 2019, 9(4), 788; https://doi.org/10.3390/app9040788 - 23 Feb 2019

Cited by 11 | Viewed by 3607

Abstract

In this study, we compared the device performance of GaInN-based green LEDs grown on c-plane sapphire substrates with a conventional low temperature GaN buffer layer to those with a sputtered-AlN buffer layer. The light output power and leakage current characteristics were significantly [...] Read more.

In this study, we compared the device performance of GaInN-based green LEDs grown on c-plane sapphire substrates with a conventional low temperature GaN buffer layer to those with a sputtered-AlN buffer layer. The light output power and leakage current characteristics were significantly improved by just replacing the buffer layer with a sputtered-AlN layer. To understand the origin of the improvement in performance, the electrical and optical properties were compared by means of electro-reflectance spectroscopy, I–V curves, electroluminescence spectra, L–I curves, and internal quantum efficiencies. From the analysis of the results, we concluded that the improvement is mainly due to the mitigation of strain and reduction of the piezoelectric field in the multiple quantum wells active region. Full article

(This article belongs to the Special Issue Internal Quantum Efficiency of III-Nitride Light-Emitting Diodes)

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12 pages, 6097 KiB

Open AccessArticle

Internal Quantum Efficiency of UV μLED Chips

by Yoshihiko Muramoto, Masahiro Kimura and Akihiro Kondo

Appl. Sci. 2019, 9(3), 450; https://doi.org/10.3390/app9030450 - 28 Jan 2019

Cited by 2 | Viewed by 4211

Abstract

Micro light emitting diode (μLED) displays have been in development since 2017, aimed for application in 2020. However, when using three-color, i.e., red, blue, and green LEDs, or blue LEDs that excite red and green phosphors, many challenges arise in mass production, cost, [...] Read more.

Micro light emitting diode (μLED) displays have been in development since 2017, aimed for application in 2020. However, when using three-color, i.e., red, blue, and green LEDs, or blue LEDs that excite red and green phosphors, many challenges arise in mass production, cost, and quality. Our group has devised an ultraviolet (UV)-excited red, green, and blue (RGB) display that excites red, green, and blue phosphors using UV-LEDs. This paper studies how the composition and crystal defects of a light-emitting layer affect the luminous efficiency of a UV μLED chip from the perspective of internal quantum efficiency (IQE). It was confirmed that the luminous efficiency improves by making the LED chips in the near ultraviolet range μ-size. The UV μLED chip emitting at 385 nm exhibited a more linear output than a 400-nm purple μLED chip. Full article

(This article belongs to the Special Issue Internal Quantum Efficiency of III-Nitride Light-Emitting Diodes)

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8 pages, 18584 KiB

Open AccessArticle

Study on Optical Properties and Internal Quantum Efficiency Measurement of GaN-based Green LEDs

by Boyang Lu, Lai Wang, Zhibiao Hao, Yi Luo, Changzheng Sun, Yanjun Han, Bing Xiong, Jian Wang, Hongtao Li, Kaixuan Chen, Xiangjing Zhuo, Jinchai Li and Junyong Kang

Appl. Sci. 2019, 9(3), 383; https://doi.org/10.3390/app9030383 - 23 Jan 2019

Cited by 12 | Viewed by 4459

Abstract

In this paper, the optical properties of GaN-based green light emitting diode (LED) are investigated and the internal quantum efficiency (IQE) values are measured by temperature dependent photoluminescence (TDPL) and power dependent photoluminescence (PDPL) methods. The "S-shaped” shift of peak wavelength measured at [...] Read more.

In this paper, the optical properties of GaN-based green light emitting diode (LED) are investigated and the internal quantum efficiency (IQE) values are measured by temperature dependent photoluminescence (TDPL) and power dependent photoluminescence (PDPL) methods. The "S-shaped” shift of peak wavelength measured at different temperature disappears gradually and the spectra broadening can be observed with increasing excitation power. The IQE calculation results of TDPL, which use the integrated intensity measured at low temperature as unity, can be modified by PDPL in order to acquire more accurate IQE values. Full article

(This article belongs to the Special Issue Internal Quantum Efficiency of III-Nitride Light-Emitting Diodes)

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8 pages, 3607 KiB

Open AccessArticle

Tuning the Resonant Frequency of a Surface Plasmon by Double-Metallic Ag/Au Nanoparticles for High-Efficiency Green Light-Emitting Diodes

by Ryoya Mano, Dong-Pyo Han, Kengo Yamamoto, Seiji Ishimoto, Satoshi Kamiyama, Tetsuya Takeuchi, Motoaki Iwaya and Isamu Akasaki

Appl. Sci. 2019, 9(2), 305; https://doi.org/10.3390/app9020305 - 16 Jan 2019

Cited by 7 | Viewed by 3417

Abstract

Currently, the internal quantum efficiency (IQE) of GaInN-based green light-emitting diodes (LEDs) is still low. To overcome this problem, surface plasmon (SP)-enhanced LEDs have been intensively studied for the last 15 years. For an SP effect in green LEDs, Au and Ag are [...] Read more.

Currently, the internal quantum efficiency (IQE) of GaInN-based green light-emitting diodes (LEDs) is still low. To overcome this problem, surface plasmon (SP)-enhanced LEDs have been intensively studied for the last 15 years. For an SP effect in green LEDs, Au and Ag are typically employed as the plasmonic materials. However, the resonance wavelength is determined by their material constants, which are theoretically fixed at ~537 nm for Au and ~437 nm for Ag. In this study, we aimed to tune the SP resonant wavelength using double-metallic nanoparticles (NPs) composed of Au and Ag to match the SP resonance wavelength to the LED emission wavelength to consequently improve the IQE of green LEDs. To form double-metallic NPs, Au/Ag multilayers were deposited on a GaN layer and then thermally annealed. We changed the thicknesses of the multilayers to control the Ag/Au ratio in the NPs. We show that the SP resonant wavelength could be tuned using our approach. We also demonstrate that the enhancement of the IQE in SP-enhanced LEDs was strongly dependent on the SP resonant wavelength. Finally, the highest IQE was achieved by matching the SP resonant wavelength to the LED emission wavelength. Full article

(This article belongs to the Special Issue Internal Quantum Efficiency of III-Nitride Light-Emitting Diodes)

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9 pages, 1862 KiB

Open AccessArticle

Influence of Quantum-Well Number and an AlN Electron Blocking Layer on the Electroluminescence Properties of AlGaN Deep Ultraviolet Light-Emitting Diodes

by Shuxin Tan, Jicai Zhang, Takashi Egawa and Gang Chen

Appl. Sci. 2018, 8(12), 2402; https://doi.org/10.3390/app8122402 - 26 Nov 2018

Cited by 5 | Viewed by 3050

Abstract

The influence of quantum-well (QW) number on electroluminescence properties was investigated and compared with that of AlN electron blocking layer (EBL) for deep ultraviolet light-emitting diodes (DUV-LEDs). By increasing the QW number, the band emission around 265 nm increased and the parasitic peak [...] Read more.

The influence of quantum-well (QW) number on electroluminescence properties was investigated and compared with that of AlN electron blocking layer (EBL) for deep ultraviolet light-emitting diodes (DUV-LEDs). By increasing the QW number, the band emission around 265 nm increased and the parasitic peak around 304 nm was suppressed. From the theoretical calculation, the electron current overflowing to the p-type layer was decreased as the QW number increased under the same injection. Correspondingly, the light output power also increased. The increment of output power from 5 QWs to 10 QWs was less than that from 10 QWs to 40 QWs, which was very different from what has been reported for blue and near-UV LEDs. The parasitic peak was still observed even when the QW number increased to 40. However, it can be suppressed efficiently by 1 nm AlN EBL for LEDs with 5 QWs. The simulation showed that the insertion of a thin EBL increased the barrier height for electron overflow and the electron current in p-type layers decreased significantly. The results contributed to the understanding of behavior of electron overflow in DUV-LEDs. Full article

(This article belongs to the Special Issue Internal Quantum Efficiency of III-Nitride Light-Emitting Diodes)

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8 pages, 1898 KiB

Open AccessArticle

Effects of a Reduced Effective Active Region Volume on Wavelength-Dependent Efficiency Droop of InGaN-Based Light-Emitting Diodes

by Panpan Li, Yongbing Zhao, Xiaoyan Yi and Hongjian Li

Appl. Sci. 2018, 8(11), 2138; https://doi.org/10.3390/app8112138 - 2 Nov 2018

Cited by 7 | Viewed by 2916

Abstract

In this study, wavelength-dependent efficiency droop phenomena in InGaN-based light-emitting diodes (LEDs) by a reduced effective active region volume were investigated. Different effective active region volumes can be extracted from theoretical fitting to the efficiency-versus-current curves of standard high efficiency InGaN near-ultraviolet, blue, [...] Read more.

In this study, wavelength-dependent efficiency droop phenomena in InGaN-based light-emitting diodes (LEDs) by a reduced effective active region volume were investigated. Different effective active region volumes can be extracted from theoretical fitting to the efficiency-versus-current curves of standard high efficiency InGaN near-ultraviolet, blue, and green LEDs. It has been found that the effective volume of the active region reduces more significantly with increasing emission wavelength, resulting in a lower onset-droop current density, as well as a more severe droop. Increasing the quantum well (QW) thickness to reduce carrier density is proposed as an effective way to alleviate the efficiency droop. Full article

(This article belongs to the Special Issue Internal Quantum Efficiency of III-Nitride Light-Emitting Diodes)

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10 pages, 2051 KiB

Open AccessArticle

Performance of InGaN/GaN Light Emitting Diodes with n-GaN Layer Embedded with SiO₂ Nano-Particles

by Hong-Seo Yom, Jin-Kyu Yang, Alexander Y. Polyakov and In-Hwan Lee

Appl. Sci. 2018, 8(9), 1574; https://doi.org/10.3390/app8091574 - 6 Sep 2018

Cited by 3 | Viewed by 5295

Abstract

We demonstrate high-performance InGaN/GaN blue light emitting diodes (LEDs) embedded with an air-void layer produced by a dry-etch of nano-pillars in an n-GaN layer grown on patterned sapphire substrate (PSS), filling the space between nano-pillars with SiO₂ nano-particles (NPs) and subsequent epitaxial [...] Read more.

We demonstrate high-performance InGaN/GaN blue light emitting diodes (LEDs) embedded with an air-void layer produced by a dry-etch of nano-pillars in an n-GaN layer grown on patterned sapphire substrate (PSS), filling the space between nano-pillars with SiO₂ nano-particles (NPs) and subsequent epitaxial overgrowth. The structure exhibits enhanced output power compared to similarly grown reference conventional LED without the air-void layer. This change in growth procedure contributes to the increase of internal quantum efficiency (IQE) and light extraction efficiency (LEE) resulting in a 13.5% increase of light output. LEE is 2 times more affected than IQE in the modified structure. Simulation demonstrates that the main effect causing the LEE changes is due to the emitted light being confined within the upper space above the air-void layer and thus enhancing the light scattering by the SiO₂ NPs and preferential light via front surface. Full article

(This article belongs to the Special Issue Internal Quantum Efficiency of III-Nitride Light-Emitting Diodes)

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