Showing papers by "Tao Tao published in 2023"
TL;DR: In this paper , the unidirectional lasing emission of room-temperature current-injected GaN-based laser was realized by coating metal Ag on the microring sidewall and integrating a direct coupled waveguide.
Abstract: Abstract GaN-based microring lasers grown on Si are promising candidates for compact and efficient light sources in Si-based optoelectronic integration and optical interconnect due to their small footprints, low mode volume, low power consumption, and high modulation rate. However, the high symmetry of circular microcavity leads to isotropic emission, which not only reduces the light collection efficiency, but also affects other adjacent devices during data transmission. In this study, the unidirectional lasing emission of room-temperature current-injected GaN-based microring laser was realized by coating metal Ag on the microring sidewall and integrating a direct coupled waveguide. The light was efficaciously confined in the cavity and only emitted from the waveguide, which avoided optical signal crosstalk with other adjacent devices. Furthermore, we integrated a microdisk at the other end of the waveguide as a photodetector, which could effectively detect the output power of the microring laser from the direct coupled waveguide. Therefore, a preliminary on-chip integration of GaN-based microring laser, waveguide and photodetector on Si substrate was successfully demonstrated for the first time, opening up a new way for on-chip integration and optical interconnect on a GaN-on-Si platform.
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TL;DR: In this paper , semi-polar green micro-LEDs have achieved high bandwidth at low current injection due to the reduced quantum-confined Stark effect (QCSE), which was significantly greater than that of typical c-plane at the same current injection.
Abstract: Micro-light-emitting diodes (micro-LEDs) with high modulation rates and low power consumption could attract growing attention as visible light communication (VLC) technology advances. The designed and fabricated semi-polar micro-LEDs have achieved high bandwidth at low current injection due to the reduced quantum-confined Stark effect (QCSE), which was significantly greater than that of typical c-plane at the same current injection. Semi-polar green micro-LEDs got a −3 dB bandwidth that surpasses 500 MHz and 1 GHz at low current densities of 43.8 A/cm2 and 120.6 A/cm2, while blue micro-LEDs exceed 500 MHz at low current densities of 76.6 A/cm2, respectively. Additionally, the free space VLC system has shown semi-polar blue and green micro-LED transmission data rates of 3.495 Gbps (433 A/cm2) and 3.483 Gbps (402 A/cm2) respectively. Semi-polar micro-LEDs, which can achieve low power consumption and high bandwidth, are anticipated to play a significant role in the development of energy-efficient VLC in the future.
1 citations
TL;DR: In this article , the authors reported the synthesis and properties of a bisazapentalene dication (BAP2+) obtained from in situ two-electron oxidation of neutral species.
TL;DR: In this paper , a detailed mechanism of ISC in thermally activated delayed-fluorescence (TADF) molecules that possess a small singlet−triplet energy gap is discussed with a focus on its deeper understanding and the impact of molecular design.
Abstract: Triplet-triplet annihilation photon up-conversion (TTA-PUC) has gained immense attention among the scientific community in the last decade due to its application in the fields of energy, biology, and photocatalytic organic synthesis. One of the main aims to improve the efficiency of these low-to-high photon-energy conversion is to reduce energy losses during the intersystem crossing (ISC). Since 2015, many strategies have been reported to address this challenge and a significant update has been noticed in this field. This review is aimed to critically analyze these updates and provide an outlook for the future. A detailed mechanism of ISC in thermally activated delayed-fluorescence (TADF) molecules that possess a small singlet−triplet energy gap, is discussed with a focus on its deeper understanding and the impact of molecular design. In this context, a range of selected organic and inorganic TADF molecules are thoroughly evaluated. Osmium(II) complexes that exhibit a spin-forbidden metal-to-ligand charge-transfer (3MLCT) transition in their Vis-NIR-IR absorption spectra and can be excited directly into their triplet state, thereby bypassing the energy loss during ISC, are also debated in sufficient detail for their advantages as well as shortcomings in being used in TTA-PUC. This work aims at reviewing the latest progress in this field, understanding the fundamental ISC mechanism of these photosensitizers, and critically addressing the challenges that are faced in this field. This review is anticipated to serve as a helpful script for identifying future directions and designing molecular sensitizers for TTA-PUC, which can sensitize the triplet state with minimum energy loss during ISC and can be helpful for increasing the anti-Stokes shift in TTA-PUC.
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TL;DR: In this paper , a GaN-based transparent single-color micro-LED display chip based on double-side polished sapphire substrate was designed with a single pixel size of $20,\,\mu \text{m}\times 20\,α,α-γ,γ-γ-m$ , and a resolution of 254 pixel per inch (PPI).
Abstract: GaN-based Micro-LED has become a research hotspot as a novel display technology due to its numerous unique advantages. Especially in augmented reality and smart glasses applications, Micro-LED display chip with high transparency possesses unparalleled opportunities. In this work, GaN-based transparent single-color Micro-LED display chip based on double-side polished sapphire substrate was designed with a single pixel size of $20\,\,\mu \text{m}\times 20\,\,\mu \text{m}$ , and a resolution of 254 pixel per inch (PPI). The transparency reaches 80% among the whole display area ~ 0.18 inch, and the luminance of display chip can reach up to 25000 nits. It demonstrates a broad application prospects in augmented reality, smart glasses, and etc., providing promising vista for the development of “Metaverse” in the future.
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TL;DR: In this paper , an AC-driven single contact LED (SC-LED) was fabricated and a new equivalent circuit model was established and simulated to understand the dominant mechanism of SC-LEDs.
Abstract: A new type of Micro-LED driving technique has attracted wide attention, for its advantages and potential applications in the field where direct current-drive (DC-drive) LED is restricted. However, the understanding of the dominant mechanism in AC-driven LED is still vague. In this work, AC-driven single contact LED (SC-LED) devices were fabricated. Electrical and optical characteristics are investigated. A new equivalent circuit model is established and simulated. The injection efficiency (INJ) is discussed, which understands the dominant mechanism of AC-driven SC-LEDs. These results could promote the applications of AC-driven Micro-LED devices in the future.
TL;DR: In this paper , the modulation bandwidth and optical performance of micro-LEDs were investigated in series and parallel in order to acquire high speed and high brightness for VLC application, and the electrical characteristics, optical characteristics and communication characteristics of single pixel, 2-pixel, 4-pixel series, and parallel green microLEDs with different pixel sizes were systematically tested.
Abstract: GaN-based micro light-emitting diode (LED) has gradually become one of the promising light sources for visible light communication (VLC), of which high modulation bandwidth has been demonstrated. In order to acquire high speed and high brightness for VLC application, the modulation bandwidth and optical performance of micro-LEDs were investigated in series and parallel in this work. The electrical characteristics, optical characteristics and communication characteristics of single-pixel, 2-pixel, 4-pixel series and parallel green micro-LEDs with different pixel sizes were systematically tested. High modulation bandwidth as much as 1.4 GHz was acquired by using 10 μm micro-LEDs with 4-pixel in parallel. It is found that parallel micro-LED array can achieve higher modulation bandwidth, higher brightness and higher stability as well, which can be used as the preferred mode for the development of micro-LED based VLC.
TL;DR: In this paper , the effect of Ag-Bi co-modified nanostructured TiO2 on photodegradation of gaseous formaldehyde (HCHO) was investigated.
Abstract: This study investigated the effect of Ag-Bi co-modified nanostructured TiO2 on photodegradation of gaseous formaldehyde (HCHO). The Ag/Bi-TiO2 materials were synthesized using the hydrothermal method with various concentrations of Ag (0-2%). The synthesized materials were characterized using XRD, SEM, TEM, EDS, PL, UV-Vis, XPS, and ESR. Their performance under visible-light irradiation was found to be significantly influenced by the amount of Ag doping, the calcination temperature and the air contact area. The results demonstrated higher Ag doping greatly enhanced the degradation of HCHO. The photocatalytic degradation efficiency of HCHO over 2%Ag/Bi-TiO2 reached 87.5%, which was significantly higher compared to Bi-TiO2 (74.0%) and TiO2 (65.6%), and the concentration of HCHO decreased from 1.069 to 0.134 mg/m3 within 48 h. The heterostructure of Ag2O/Ag and Bi-TiO2 facilitated effective electron transfer and suppressed the recombination of electron-hole pairs. The enhanced adsorption of visible-light and the improved photocatalytic performance were attributed to the synergistic effect of surface hydroxyl and adsorbed oxygen.
TL;DR: TPEP-m-2DCV and TPE-m 2DCV were designed and synthesized, in which the strategy of incorporating two dicyanovinyl (DCV) units into meso-positions of one benzene ring of the TPEP and TPEP skeleton was realized for the first time as discussed by the authors .
Abstract: The development of versatile luminescent materials, especially those exhibiting high-contrast mechano-responsive luminescence (MRL) characteristics, is urgently desirable but a daunting challenge. Herein, two novel MRL materials named TPEP-m-2DCV and TPE-m-2DCV were designed and synthesized, in which the strategy of incorporating two dicyanovinyl (DCV) units into meso-positions of one benzene ring of the TPEP and TPE skeleton was realized for the first time. Both TPEP-m-2DCV and TPE-m-2DCV exhibited solvatochromism, aggregation-induced emission (AIE), and reversible MRL properties. More interestingly, two different polymorphs of TPEP-m-2DCV (TPEP-m-2DCV(B) and TPEP-m-2DCV(O)) with distinguishing emission (468 and 580 nm) could be separated by adjusting the solvent polarity in the process of growing single crystals. Furthermore, TPEP-m-2DCV(B) with blue fluorescence showed ultrahigh red-shifted emission up to 141 nm after grinding, while TPEP-m-2DCV(O) with orange emission did not exhibit MRL property. Single-crystal structural analysis of two polymorphs and corresponding theoretical calculations were used to explain their significant differences in photophysical properties. TPEP-m-2DCV(O) possessed a higher coplanarity and a more tightly packed dimer. According to the interaction region indicator (IRI) and density functional theory (DFT) analysis, the TPEP-m-2DCV(O) dimer showed stronger noncovalent interactions (NCI) and a significantly reduced energy gap. TPEP-m-2DCV(B) underwent a morphology transformation from crystalline to amorphous state upon grinding, with a tremendous red shift in emission. However, TPEP-m-2DCV(O) was MRL inactive because it could remain in the original crystalline state with the aid of fused hydrogen-bonded rings in the crystal packing when suffering an external force. It is believed that this work reveals the relationship between intermolecular interactions/molecular conformations and MRL performance and provides useful insights for designing high-contrast MRL materials.
TL;DR: In this paper , an optimized InGaN/GaN Micro-LED structure, incorporating a 2-nmthick quantum well (QW) and a 6-nm-thick Quantum barrier (QB), demonstrated superior photonic performance at current densities ranging from 0.1 to 10 A/cm.
Abstract: As the field of state-of-the-art displays continues to evolve, micro light-emitting diodes (Micro-LEDs) with their diminutive pixel size and exceptional efficiency are emerging as a pivotal player. It is essential for displays to achieve the highest possible efficiency under low current injection. Through the investigation of Micro-LEDs incorporating various quantum structures, we have exhibited remarkable enhancement of their optoelectronic properties at low current injection. The optimized InGaN/GaN Micro-LED structure, incorporating a 2-nm-thick quantum well (QW) and a 6-nm-thick quantum barrier (QB), demonstrated superior photonic performance at current densities ranging from 0.1 to 10 A/cm $^{\text{2}}$ . It is discovered that the electron–hole wave function overlap can be enhanced by thinning the InGaN QW, thereby increasing radiation efficiency. Meanwhile, uniform carrier distribution and reduced quantum confinement stark effect (QCSE) in QWs can be achieved by a thinner GaN barrier, resulting in more uniform luminescence at each QW in multiple QWs (MQWs). The optimized Micro-LEDs devices exhibit uniform emission and remarkably high brightness, making them a suitable emission source to be combined with quantum dots (QDs) for the realization of full-color displays. Convincing results proved that optimized Micro-LEDs array can serve as the solution for high-resolution and brightness displays.
TL;DR: In this article, the effects of Mg-doping temperature on the structural and electrical properties of nonpolar p-type GaN film are investigated in detail, and it is found that all the surface morphology, crystalline quality, strains, electrical properties are interconnected, and are closely related to the Mg doping temperature.
Abstract:
Nonpolar (11-20) a-plane p-type GaN films were successfully grown on r-plane sapphire substrate with metal organic chemical vapor deposition (MOCVD) system. The effects of Mg-doping temperature on the structural and electrical properties of nonpolar p-type GaN film are investigated in detail. It is found that all the surface morphology, crystalline quality, strains, and electrical properties of nonpolar a-plane p-type GaN films are interconnected, and are closely related to the Mg-doping temperature. This means that a proper performance of nonpolar p-type GaN can be expected by optimizing the Mg-doping temperature. In fact, a hole concentration of 1.3×1018 cm-3, a high Mg activation efficiency of 6.5 %, an activation energy of 114 meV for Mg acceptor, and a low anisotropy of 8.3 % in crystalline quality were achieved with a growth temperature of 990 ℃. This approach to optimize the Mg-doping temperature of the nonpolar a-plane p-type GaN film provides an effective way to fabricate high-efficiency optoelectronic devices in the future.
TL;DR: In this paper , a 2D hole gas (2DHG) induced by polarization charges at the GaN/AlGaN hetero-interface was investigated, and the 2DHG sheet density reached 3.6××1013 and 2.1× 1013 cm−2 at room temperature and 77 K, respectively.
Abstract: A two-dimensional hole gas (2DHG) induced by polarization charges at the GaN/AlGaN hetero-interface is attracting much attention because of its potential to develop p-channel transistors required for GaN complementary logic integrated circuits. This platform is compatible with commercial AlGaN/GaN n-channel electronics, but the performance of GaN p-channel transistors has been far behind. In this work, 2DHGs in GaN/AlGaN/GaN heterostructures grown by plasma-assisted molecular beam epitaxy have been investigated. The Al composition of the AlGaN barrier has been pushed as high as possible without obvious strain relaxation, and the record high 2DHG sheet density and conductivity on the GaN/AlGaN/GaN platform have been obtained. By adopting a parallel conduction model, a dependent relationship of the 2DHG density on temperature has been extracted. The temperature dependent Hall-effect results have demonstrated that the 2DHG density boosts by 75 times and 46 times at room temperature and 77 K, respectively, when the Al composition is pushed from 0.18 to 0.45 for the AlGaN barriers. The 2DHG sheet density reaches 3.6 × 1013 and 2.1 × 1013 cm−2 at room temperature and 77 K, respectively, and the lowest sheet resistance is 8.9 kΩ/□ at 77 K. Such a 2DHG is beneficial for fabrication of p-channel GaN transistors with lower on-resistance on the already-industrialized platform.
TL;DR: In this paper , the authors demonstrated the monolithic integration of GaN-based driving metal-oxide-semiconductor field effect transistor (MOSFET) on micro-light-emitting diodes by regrowing a hybrid tunnel junction (TJ) on top of a commercial green LED wafer.
Abstract: We demonstrated the monolithic integration of GaN-based driving metal-oxide-semiconductor field effect transistor (MOSFET) on micro-light-emitting diodes ( $\mu $ LEDs) by regrowing a hybrid tunnel junction (TJ) on top of a commercial green LED wafer. The hybrid TJ served not only as the current spreading layer for $\mu $ LEDs, but the n/p/n structure of the LED + TJ stack could also be utilized for fabricating a quasi-vertical driving MOSFET. The $\mu $ LED was connected to the MOSFET via the conductive n-GaN layer. By modulating the gate supply voltage, the MOSFET effectively controlled the injection current of the $\mu $ LED, allowing for precise modulation of its output performance. The integrated 60- $\mu \text{m}~\mu $ LED exhibited a high output power of 0.12 mW (~4.2 W/cm2) at a current of 0.3 mA (around 10 A/cm2) when the MOSFET was modulated with $\text{V}_{\text {DD}}$ = 5 V and $\text{V}_{\text {G}}$ = 16 V, demonstrating good $\mu $ LED performance and comparable driving capability to previous GaN-based MOSFETs and oxide-based thin film transistors. This work provides a new method for GaN FET/LED monolithic integration, which paves the way for potential applications in visible light communication and flexible $\mu $ LED displays.
TL;DR: In this article , a simple method for achieving green and red colors by patterning quantum dots (QDs) on GaN-based blue micro-light-emitting diodes (μLEDs) using standard photolithography and dry etching process was demonstrated.
Abstract: We demonstrated a simple method for achieving green and red colors by patterning quantum dots (QDs) on GaN-based blue micro-light-emitting diodes (μLEDs) using standard photolithography and dry etching process. The QDs could be patterned as small as 2 μm in size on a 4-inch wafer, revealing the ultra-high resolution and throughput capability of this method. The individual color-converted green and red μLEDs in a size of 10 μm × 10 μm exhibited a peak external quantum efficiency (EQE) of 9.6% and 14.7%, respectively. We also achieved the monolithic integration of red, green, and blue (RGB) μLEDs as a single pixel by sequentially patterning red and green QDs. The color gamut of the RGB μLEDs covered 83.4% of the Rec. 2020 color space in the CIE 1931 diagram. This method is very effective for patterning color-converted QDs with down to 2μm-size-resolution and achieving high performance for red/green μLEDs, while enabling scalable fabrication of full-color μLED displays.