Research

7 pages, 2715 KiB

Open AccessArticle

Enhancement of Light Extraction Efficiency of UVC-LED by SiO₂ Antireflective Film

by Yu Wang, Zhenxing Lv, Shengli Qi, Yazhu Liu and Hao Long

Crystals 2022, 12(7), 928; https://0-doi-org.brum.beds.ac.uk/10.3390/cryst12070928 - 30 Jun 2022

Cited by 3 | Viewed by 1850

In order to achieve high quantum efficiency of AlGaN-based deep ultraviolet light-emitting diodes (UVC-LED), it is important to improve the light extraction efficiency (LEE). In this paper, theoretical simulation and experiment of SiO₂ anti-reflective film deposited on UVC-LED were investigated. The effect [...] Read more.

In order to achieve high quantum efficiency of AlGaN-based deep ultraviolet light-emitting diodes (UVC-LED), it is important to improve the light extraction efficiency (LEE). In this paper, theoretical simulation and experiment of SiO₂ anti-reflective film deposited on UVC-LED were investigated. The effect of different SiO₂ thickness on the light extraction efficiency of 275 nm UVC-LED was studied, showing that 140 nm SiO₂ anti-reflective film can effectively improve the light output power of UVC-LED by more than 5.5%, which were also confirmed by the TFCALC simulation. The enhancement of UVC-LED light extraction efficiency by this antireflective film is mainly due to the

\frac{3 λ}{2}

light coherent effect at the SiO₂/Al₂O₃ interface. Our work proved the promising application of antireflective coating on UVC-LED. Full article

(This article belongs to the Special Issue Advances in GaN-Based Optoelectronic Materials and Devices)

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9 pages, 1498 KiB

Open AccessArticle

Reduction in the Photoluminescence Intensity Caused by Ultrathin GaN Quantum Barriers in InGaN/GaN Multiple Quantum Wells

by Wei Liu, Feng Liang, Degang Zhao, Jing Yang, Ping Chen and Zongshun Liu

Crystals 2022, 12(3), 339; https://0-doi-org.brum.beds.ac.uk/10.3390/cryst12030339 - 28 Feb 2022

Cited by 3 | Viewed by 2094

Abstract

The optical properties of InGaN/GaN violet light-emitting multiple quantum wells with different thicknesses of GaN quantum barriers are investigated experimentally. When the barrier thickness decreases from 20 to 10 nm, the photoluminescence intensity at room temperature increases, which can be attributed to the [...] Read more.

The optical properties of InGaN/GaN violet light-emitting multiple quantum wells with different thicknesses of GaN quantum barriers are investigated experimentally. When the barrier thickness decreases from 20 to 10 nm, the photoluminescence intensity at room temperature increases, which can be attributed to the reduced polarization field in the thin-barrier sample. However, with a further reduction in the thickness to 5 nm, the sample’s luminescence intensity decreases significantly. It is found that the strong nonradiative loss process induced by the deteriorated crystal quality and the quantum-tunneling-assisted leakage of carriers may jointly contribute to the enhanced nonradiative loss of photogenerated electrons and holes, leading to a significant reduction in photoluminescence intensity of the sample with nanoscale ultrathin GaN quantum barriers. Full article

(This article belongs to the Special Issue Advances in GaN-Based Optoelectronic Materials and Devices)

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11 pages, 2375 KiB

Open AccessArticle

Enhanced Light Extraction Efficiency and Modulation Bandwidth of Deep-Ultraviolet Light-Emitting Diodes with Al Nanospheres

by Xiaolong Hu, Xu Liang, Lingyun Tang and Wenjie Liu

Crystals 2022, 12(2), 289; https://0-doi-org.brum.beds.ac.uk/10.3390/cryst12020289 - 18 Feb 2022

Cited by 4 | Viewed by 1606

Abstract

Planar, nanopillar and Al nanosphere structure AlGaN-based deep-ultraviolet light-emitting diodes (DUV-LEDs) were numerically investigated via a three-dimensional finite difference time domain (3D FDTD) method. The three types of DUV-LEDs were compared and analyzed in terms of light extraction efficiency (LEE), Purcell factor ( [...] Read more.

Planar, nanopillar and Al nanosphere structure AlGaN-based deep-ultraviolet light-emitting diodes (DUV-LEDs) were numerically investigated via a three-dimensional finite difference time domain (3D FDTD) method. The three types of DUV-LEDs were compared and analyzed in terms of light extraction efficiency (LEE), Purcell factor (F_P) and modulation bandwidth. The results showed that nanopillar structure DUV-LEDs with optimal nanopillar height, width and spacing can enhance transverse electric (TE)-polarized LEE to 39.7% and transverse magnetic (TM)-polarized LEE to 4.4%. The remarkable improvement was mainly due to the increased scattering effect, decreased absorption of the p-GaN layer and total internal reflection (TIR) effect. After adopting the Al nanospheres, the TE-polarized modulation bandwidth was increased by 71 MHz and the TM-polarized LEE was enhanced approximately 4.3-fold as compared to the nanopillar LED structure, while the Al nanosphere diameter was 120 nm. The reasons for promotion are mainly attributed to the coupling behavior of diploe and localized surface plasmon induced by Al nanospheres. The designed structures provide a meaningful solution for realization of high-efficiency DUV-LEDs. Full article

(This article belongs to the Special Issue Advances in GaN-Based Optoelectronic Materials and Devices)

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8 pages, 3530 KiB

Open AccessArticle

High-Speed GaN-Based Superluminescent Diode for 4.57 Gbps Visible Light Communication

by Dong Li, Chicheng Ma, Junfei Wang, Fangchen Hu, Yuqi Hou, Shanshan Wang, Junhui Hu, Shulan Yi, Yingnan Ma, Jianyang Shi, Junwen Zhang, Ziwei Li, Nan Chi, Liang Xia and Chao Shen

Crystals 2022, 12(2), 191; https://0-doi-org.brum.beds.ac.uk/10.3390/cryst12020191 - 27 Jan 2022

Cited by 12 | Viewed by 2657

Abstract

Visible light communication (VLC) is a promising technology for next-generation high-speed optical wireless data links. Among various transmitters, GaN-based superluminescent diodes (SLDs) show interesting characteristics, including a large modulation bandwidth, droop free and low speckle noise, which makes them attractive for VLC applications. [...] Read more.

Visible light communication (VLC) is a promising technology for next-generation high-speed optical wireless data links. Among various transmitters, GaN-based superluminescent diodes (SLDs) show interesting characteristics, including a large modulation bandwidth, droop free and low speckle noise, which makes them attractive for VLC applications. In this work, we design and fabricate a blue-emitting SLD utilizing tilted facet configuration. Using SLD as the light source, a VLC system is experimentally demonstrated. A record data rate of 4.57 gigabit per second (Gbps) is achieved with adaptive bit-loading discrete multiple tone (DMT) modulation, while the highest modulation format reaches 256 quadrature amplitude modulation (QAM). The corresponding bit error rate (BER) is ~3.5 × 10⁻³, which is below the forward error correction (FEC) threshold of 3.8 × 10⁻³. Full article

(This article belongs to the Special Issue Advances in GaN-Based Optoelectronic Materials and Devices)

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12 pages, 2031 KiB

Open AccessArticle

The Investigation of Carrier Leakage Mechanism Based on ABC-Models in InGaN/GaN MQW and Its Effect on Internal Quantum Efficiency under Optical Excitation

by Yuhao Ben, Feng Liang, Degang Zhao, Jing Yang, Ping Chen and Zongshun Liu

Crystals 2022, 12(2), 171; https://0-doi-org.brum.beds.ac.uk/10.3390/cryst12020171 - 25 Jan 2022

Cited by 4 | Viewed by 2354

Abstract

In this work, a GaN-based multiple quantum well (MQW) sample has a much higher IQE although it has a stronger non-radiative recombination. Through experimental verification, the higher IQE is attributed to the suppressed carrier leakage mechanism, which is normally neglected under optical excitation. [...] Read more.

In this work, a GaN-based multiple quantum well (MQW) sample has a much higher IQE although it has a stronger non-radiative recombination. Through experimental verification, the higher IQE is attributed to the suppressed carrier leakage mechanism, which is normally neglected under optical excitation. To achieve a more reasonable IQE expression in a GaN MQW structure, leakage factor m is introduced into the ABC-models. Meanwhile, by analyzing the Arrhenius fitting of the plot of IQE-temperature and leakage factor m, the key temperature and excitation power turning on the carrier leakage mechanism was roughly determined to be below 220 K and 10 mW, respectively. Such a low turn-on temperature and excitation power indicates a much easier carrier leakage mechanism in GaN-based MQW, which may be caused by the small effective electron mass of InGaN (0.11–0.22 m*) and the narrow thickness of quantum well via the model calculation of energy band structure via simulation software LASTIP. Moreover, higher IQE can be achieved by suppressing the carrier leakage mechanism via structural optimization (such as electron block layer) in GaN-based MQW. Full article

(This article belongs to the Special Issue Advances in GaN-Based Optoelectronic Materials and Devices)

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9 pages, 3713 KiB

Open AccessArticle

Optical Properties of InGaN/GaN QW with the Same Well-Plus-Barrier Thickness

by Huan Xu, Xin Hou, Lan Chen, Yang Mei and Baoping Zhang

Crystals 2022, 12(1), 114; https://0-doi-org.brum.beds.ac.uk/10.3390/cryst12010114 - 15 Jan 2022

Cited by 4 | Viewed by 1866

Abstract

Optical properties of wurtzite violet InGaN/GaN quantum well (QW) structures, with the same well-plus-barrier thickness, grown by metal-organic chemical vapor deposition (MOCVD) on c-plane sapphire substrates, were investigated using temperature-dependent photoluminescence (TDPL) and excitation-power-dependent photoluminescence (PDPL). Two samples were compared: one had a [...] Read more.

Optical properties of wurtzite violet InGaN/GaN quantum well (QW) structures, with the same well-plus-barrier thickness, grown by metal-organic chemical vapor deposition (MOCVD) on c-plane sapphire substrates, were investigated using temperature-dependent photoluminescence (TDPL) and excitation-power-dependent photoluminescence (PDPL). Two samples were compared: one had a thicker well (InGaN/GaN 3/5 nm); the other had a thicker barrier (InGaN/GaN 2/6 nm). It was found that the GaN barrier thickness in the InGaN/GaN MQWs plays an important role in determining the optical characteristics of the MQWs. The peak energy of the two samples varied with temperature in an S-shape. The thicker-barrier sample had a higher turning point from blueshift to redshift, indicating a stronger localization effect. From the Arrhenius plot of the normalized integrated PL intensity, it was found that the activation energy of the nonradiative process also increased with a thicker barrier thickness. The radiation recombination process was dominated in the sample of the thicker barrier, while the non-radiation process cannot be negligible in the sample of the thicker well. Full article

(This article belongs to the Special Issue Advances in GaN-Based Optoelectronic Materials and Devices)

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9 pages, 1953 KiB

Open AccessEditor’s ChoiceArticle

AlGaN-Based 1.55 µm Phototransistor as a Crucial Building Block for Optical Computers

by Daniel Hofstetter, Cynthia Aku-Leh, Hans Beck and David P. Bour

Crystals 2021, 11(11), 1431; https://0-doi-org.brum.beds.ac.uk/10.3390/cryst11111431 - 22 Nov 2021

Cited by 2 | Viewed by 1994

Abstract

An optically activated, enhancement mode heterostructure field effect transistor is proposed and analytically studied. A particular feature of this device is its gate region, which is made of a photovoltaic GaN/AlN-based superlattice detector for a wavelength of 1.55 µm. Since the inter-subband transition [...] Read more.

An optically activated, enhancement mode heterostructure field effect transistor is proposed and analytically studied. A particular feature of this device is its gate region, which is made of a photovoltaic GaN/AlN-based superlattice detector for a wavelength of 1.55 µm. Since the inter-subband transition in this superlattice does normally not interact with TE-polarized (or vertically incoming) radiation, a metallic second-order diffraction grating on the transistor gate results in a re-orientation of the light into the horizontal direction—thus providing the desired TM-polarization. Upon illumination of this gate, efficient inter-subband absorption lifts electrons from the ground to the first excited quantized state. Due to partial screening of the strong internal polarization fields between GaN quantum wells and AlN barriers, this slightly diagonal transition generates an optical rectification voltage. Added to a constant electrical bias, this optically produced gate voltage leads to a noticeable increase of the transistor’s source-drain current. The magnitude of the bias voltage is chosen to result in maximal transconductance. Since such a phototransistor based on high-bandgap material is a device involving only fast majority carriers, very low dark and leakage currents are expected. The most important advantage of such a device, however, is the expected switching speed and, hence, its predicted use as an optical logic gate for photonic computing. In the absence of a p-n-junction and thus of both a carrier-induced space charge region, and the parasitic capacitances resulting thereof, operation frequencies of appropriately designed, sufficiently small phototransistors reaching 100 GHz are envisaged. Full article

(This article belongs to the Special Issue Advances in GaN-Based Optoelectronic Materials and Devices)

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9 pages, 1985 KiB

Open AccessArticle

Understanding the Luminescence Characteristics of Ultraviolet InGaN/AlGaN Multiple Quantum Wells with Different In Gradients

by Jie Zhang, Wei Liu and Shuyuan Zhang

Crystals 2021, 11(11), 1390; https://0-doi-org.brum.beds.ac.uk/10.3390/cryst11111390 - 15 Nov 2021

Cited by 5 | Viewed by 1789

Abstract

The electroluminescence (EL) properties of InGaN/AlGaN ultraviolet light-emitting multiple quantum wells (MQWs) with identical average In content but different In gradients (In content increases linearly, along the growth direction) are investigated numerically. It is found that the luminescence efficiency is improved, and the [...] Read more.

The electroluminescence (EL) properties of InGaN/AlGaN ultraviolet light-emitting multiple quantum wells (MQWs) with identical average In content but different In gradients (In content increases linearly, along the growth direction) are investigated numerically. It is found that the luminescence efficiency is improved, and the EL spectral peak wavelength becomes longer for the MQW sample with a larger In gradient. Since the influence of In gradient is different for the conduction and valence bands in InGaN layers, the distribution of electrons and holes in QWs may be changed, leading to a redshift of EL spectra. In particular, when the In gradient increases, the overlap integral of electron-hole wavefunction in InGaN QWs increases, resulting in a higher radiative recombination rate and an enhanced EL intensity. Full article

(This article belongs to the Special Issue Advances in GaN-Based Optoelectronic Materials and Devices)

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12 pages, 2294 KiB

Open AccessArticle

Investigation of the Optimum Mg Doping Concentration in p-Type-Doped Layers of InGaN Blue Laser Diode Structures

by Chibuzo Onwukaeme and Han-Youl Ryu

Crystals 2021, 11(11), 1335; https://0-doi-org.brum.beds.ac.uk/10.3390/cryst11111335 - 01 Nov 2021

Cited by 4 | Viewed by 2460

Abstract

In GaN-based laser diode (LD) structures, Mg doping in p-type-doped layers has a significant influence on the device performance. As the doping concentration increases, the operation voltage decreases, whereas the output power decreases as a result of increased optical absorption, implying that optimization [...] Read more.

In GaN-based laser diode (LD) structures, Mg doping in p-type-doped layers has a significant influence on the device performance. As the doping concentration increases, the operation voltage decreases, whereas the output power decreases as a result of increased optical absorption, implying that optimization of the Mg doping concentration is required. In this study, we systematically investigated the effect of the Mg doping concentration in the AlGaN electron-blocking layer (EBL) and the AlGaN p-cladding layer on the output power, forward voltage, and wall-plug efficiency (WPE) of InGaN blue LD structures using numerical simulations. In the optimization of the EBL, an Al composition of 20% and an Mg doping concentration of 3 × 10¹⁹ cm⁻³ exhibited the best performance, with negligible electron leakage and a high WPE. The optimum Mg concentration of the p-AlGaN cladding layer was found to be ~1.5 × 10¹⁹ cm⁻³, where the maximum WPE of 38.6% was obtained for a blue LD with a threshold current density of 1 kA/cm² and a slope efficiency of 2.1 W/A. Full article

(This article belongs to the Special Issue Advances in GaN-Based Optoelectronic Materials and Devices)

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10 pages, 4304 KiB

Open AccessArticle

Investigation of Micromorphology and Carrier Recombination Dynamics for InGaN/GaN Multi-Quantum Dots Grown by Molecular Beam Epitaxy

by Xue Zhang, Wenxian Yang, Zhiwei Xing, Haibing Qiu, Ying Gu, Lifeng Bian, Shulong Lu, Hua Qin, Yong Cai, Yuta Suzuki, Sakuya Kaneko, Yuki Matsuda, Shinji Izumi, Yuichi Nakamura and Atsushi Tackeuchi

Crystals 2021, 11(11), 1312; https://0-doi-org.brum.beds.ac.uk/10.3390/cryst11111312 - 27 Oct 2021

Cited by 8 | Viewed by 1740

Abstract

InGaN quantum dots (QDs) are promising candidates for GaN-based all-visible optoelectronic devices such as micro light-emitting diode and laser. In this study, self-assembled InGaN/GaN multi-quantum dots (MQDs) have been grown by plasma-assisted molecular beam epitaxy on c-plane GaN-on-sapphire template. A high density of [...] Read more.

InGaN quantum dots (QDs) are promising candidates for GaN-based all-visible optoelectronic devices such as micro light-emitting diode and laser. In this study, self-assembled InGaN/GaN multi-quantum dots (MQDs) have been grown by plasma-assisted molecular beam epitaxy on c-plane GaN-on-sapphire template. A high density of over 3.8 × 10¹⁰ cm⁻² is achieved and InGaN QDs exhibit a relatively uniform size distribution and good dispersity. Strong localization effect in as-grown InGaN QDs has been evidenced by temperature-dependent photoluminescence (PL). The variation of peak energy is as small as 35 meV with increasing temperature from 10 K to 300 K, implying excellent temperature stability of emission wavelength for InGaN MQDs. Moreover, the radiative and nonradiative recombination times were calculated by time-resolved PL (TRPL) measurements, and the temperature dependence of PL decay times reveal that radiative recombination dominates the recombination process due to the low dislocation density of QDs structure. Full article

(This article belongs to the Special Issue Advances in GaN-Based Optoelectronic Materials and Devices)

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