Deep-Ultraviolet Micro-LEDs Exhibiting High Output Power and High Modulation Bandwidth Simultaneously

Atomic-scale visualization of defect-induced localized vibrations in GaN

Phonon engineering is crucial for thermal management in GaN-based power devices, where phonon-defect interactions limit performance. However, detecting nanoscale phonon transport constrained by III-nitride defects is challenging due to limited spatial resolution. Here, we used advanced scanning transmission electron microscopy and electron energy loss spectroscopy to examine vibrational modes in a prismatic stacking fault in GaN. By comparing experimental results with ab initio calculations, we identified three types of defect-derived modes: localized defect modes, a confined bulk mode, and a fully extended mode. Additionally, the PSF exhibits a smaller phonon energy gap and lower acoustic sound speeds than defect-free GaN, suggesting reduced thermal conductivity. Our study elucidates the vibrational behavior of a GaN defect via advanced characterization methods and highlights properties that may affect thermal behavior.

A Comprehensive Review of Group-III Nitride Light-Emitting Diodes: From Millimeter to Micro-Nanometer Scales

This review describes the development history of group-III nitride light-emitting diodes (LEDs) for over 30 years, which has achieved brilliant achievements and changed people's lifestyles. The development process of group-III nitride LEDs is the sum of challenges and solutions constantly encountered with shrinking size. Therefore, this paper uses these challenges and solutions as clues for review. It begins with reviewing the development of group-III nitride materials and substrates. On this basis, some key technological breakthroughs in the development of group-III nitride LEDs are reviewed, mainly including substrate pretreatment and p-type doping in material growth, the proposal of new device structures such as nano-LED and quantum dot (QD) LED, and the improvement in luminous efficiency, from the initial challenge of high-efficiency blue luminescence to current challenge of high-efficiency ultraviolet (UV) and red luminescence. Then, the development of micro-LEDs based on group-III nitride LEDs is reviewed in detail. As a new type of display device, micro-LED has drawn a great deal of attention and has become a research hotspot in the current international display area. Finally, based on micro-LEDs, the development trend of nano-LEDs is proposed, which is greener and energy-saving and is expected to become a new star in the future display field.

Highly efficient AlGaN-based deep-ultraviolet light-emitting diodes: from bandgap engineering to device craft

AlGaN-based light-emitting diodes (LEDs) operating in the deep-ultraviolet (DUV) spectral range (210-280 nm) have demonstrated potential applications in physical sterilization. However, the poor external quantum efficiency (EQE) hinders further advances in the emission performance of AlGaN-based DUV LEDs. Here, we demonstrate the performance of 270-nm AlGaN-based DUV LEDs beyond the state-of-the-art by exploiting the innovative combination of bandgap engineering and device craft. By adopting tailored multiple quantum wells (MQWs), a reflective Al reflector, a low-optical-loss tunneling junction (TJ) and a dielectric SiO2 insertion structure (IS-SiO2), outstanding light output powers (LOPs) of 140.1 mW are achieved in our DUV LEDs at 850 mA. The EQEs of our DUV LEDs are 4.5 times greater than those of their conventional counterparts. This comprehensive approach overcomes the major difficulties commonly faced in the pursuit of high-performance AlGaN-based DUV LEDs, such as strong quantum-confined Stark effect (QCSE), severe optical absorption in the p-electrode/ohmic contact layer and poor transverse magnetic (TM)-polarized light extraction. Furthermore, the on-wafer electroluminescence characterization validated the scalability of our DUV LEDs to larger production scales. Our work is promising for the development of highly efficient AlGaN-based DUV LEDs.

20 µm Micro-LEDs Mass Transfer via Laser-Induced In Situ Nanoparticles Resonance Enhancement

Ultrafast laser is expected as a promising strategy for micro-LEDs (µ-LEDs) transfer due to its inherent property of suppressing thermal effects. However, its ultrahigh peak power and the unclear transfer mechanism make its transfer quality and efficiency unsatisfactory. Here, the study reports the high-precision mass transfer of 20 µm fine-pitch µ-LEDs via in situ nanoparticles (NPs) resonance enhancement in burst mode ultraviolet picosecond laser irradiation. This technique suppresses the thermal melting effect and rapid cooling behavior of plasma by temporal modulation of the burst mode, generating NPs-induced resonance enhancement that accurately and controllable drives a single unit up to tens of thousands of µ-LEDs. The transfer of large µ-LED arrays with more than 180 000 chips is also demonstrated, showing a transfer yield close to 99.9%, a transfer speed of 700 pcs s-1, and a transfer error of <±1.2 µm. The transferred µ-LEDs perform excellent optoelectronic properties and enable reliable device operation regardless of complex strain environments, providing a reliable strategy for preparing broader classes of 3D integrated photonics devices.

Biocompatible Electronic Skins for Cardiovascular Health Monitoring

Cardiovascular diseases represent a significant threat to the overall well-being of the global population. Continuous monitoring of vital signs related to cardiovascular health is essential for improving daily health management. Currently, there has been remarkable proliferation of technology focused on collecting data related to cardiovascular diseases through daily electronic skin monitoring. However, concerns have arisen regarding potential skin irritation and inflammation due to the necessity for prolonged wear of wearable devices. To ensure comfortable and uninterrupted cardiovascular health monitoring, the concept of biocompatible electronic skin has gained substantial attention. In this review, biocompatible electronic skins for cardiovascular health monitoring are comprehensively summarized and discussed. The recent achievements of biocompatible electronic skin in cardiovascular health monitoring are introduced. Their working principles, fabrication processes, and performances in sensing technologies, materials, and integration systems are highlighted, and comparisons are made with other electronic skins used for cardiovascular monitoring. In addition, the significance of integrating sensing systems and the updating wireless communication for the development of the smart medical field is explored. Finally, the opportunities and challenges for wearable electronic skin are also examined.

Ring geometric effect on the performance of AlGaN-based deep-ultraviolet light-emitting diodes

In this study, we fabricated and characterized various parallel flip-chip AlGaN-based deep-ultraviolet (DUV) micro-ring LEDs, including changes in ring dimensions as well as the p-GaN-removed region widths at the outer micro-ring, respectively (PRM LEDs). It is revealed that the LED chips with smaller mesa withstand higher current density and deliver considerably higher light output power density (LOPD), under the same proportion of the hole to the entire mesa column (before it is etched into ring) within the limits of dimensions. However, as the ring-shaped mesa decreases, the LOPD begins to decline because of etching damage. Subsequently, at the same external diameter, the optical performance of micro-ring LEDs with varied internal diameters is studied. Meanwhile, the influence of different structures on light extraction efficiency (LEE) is studied by employing a two-dimensional (2D)-finite-difference time-domain (FDTD) method. In addition, the expand of the p-GaN-removed region at the outer micro-ring as well as the corresponding effective light emission region have some influence to LOPD. The PRM-23 LED (with an external diameter of 90 µm, an internal diameter of 22 µm, and a p-GaN-removed region width of 8 µm) has an LOPD of 53.36 W/cm2 with a current density of 650 A/cm2, and an external quantum efficiency (EQE) of 6.17% at 5 A/cm2. These experimental observations provide a comprehensive understanding of the optical and electrical performance of DUV micro-LEDs for future applications.

Plasmon-enhanced deep ultraviolet Micro-LED arrays for solar-blind communications

Localized surface plasmon resonance (LSPR)-enhanced deep ultraviolet (DUV) Micro-light emitting diodes (Micro-LEDs) using Al nanotriangle arrays (NTAs) are reported for improving the -3 dB modulation bandwidth. Through self-assembled nanospheres, the high-density Al NTAs arrays are transferred into the designated p-AlGaN region of the Micro-LEDs, realizing the effect of LSPR coupling. A 2.5-fold enhancement in photoluminescence (PL) intensity is demonstrated. Combined with the PL intensity ratio at 300 K and 10 K, internal quantum efficiency (IQE) may be increased about 15-20% by the plasmonic effect and the carrier lifetime decreases from 1.15 ns to 0.82 ns, suggesting that LSPR accelerates the spontaneous emission rate. Resulting from the improvement of the IQE, the electroluminescence intensity of Micro-LED arrays with LSPR is obviously increased. Meanwhile, the -3 dB bandwidth of 6 × 6 Micro-LED arrays is increased from 180 MHz to 300 MHz at a current density of 200 A/cm². A potential way is proposed to further increase both the IQE and the modulation bandwidth of DUV Micro-LEDs.

Lead-Free Cesium Thulium Halide Perovskite Microcrystals for Near Ultraviolet Luminescence

Lead halide perovskites attract tremendous research attention due to excellent optoelectronic properties. However, realizing efficient near ultraviolet (NUV) luminescence with these materials is still a big challenge. Herein, a novel rare-earth perovskite cesium thulium chloride (CsTmCl₃ ) with high crystallinity has been synthesized via a simple hot-injection method. The obtained CsTmCl₃ microcrystals have a size distribution of around 1-5 µm, and demonstrate a highly efficient NUV emission at 337 nm with a full width at half maximum (FWHM) of 68 nm. The determined band gap of CsTmCl₃ microcrystals is ≈3.92 eV, which is supported by theoretical calculations. Moreover, a high photoluminescence quantum yield (PLQY) of up to 12% in NUV region has been achieved in such a lead-free perovskite. The findings suggest that CsTmCl₃ perovskite microcrystal is a promising low-toxic material for applications in NUV optoelectronic devices.

Structurally Regulated Carbazole-Pyridine Derivatives Based on Space-Crowded Theory for Efficient Narrowband Ultraviolet Nondoped Organic Light-Emitting Diodes from the High-Lying Reverse Intersystem Crossing Process

Achieving ultraviolet and narrowband emission simultaneously in nondoped organic light-emitting diodes (OLEDs) remains a tremendous challenge. Here, a "space-crowded donor-acceptor-donor" molecular design strategy is proposed for developing ultraviolet pure organic fluorophores by the nearby substituted positions at the phenyl linker between carbazole and pyridine units. Benefitting from the large steric hindrance effect, multiple intramolecular interactions, and low-frequency vibronic coupling dominated excited state property, all the emitters exhibit excellent fluorescence efficiencies at the solid state as well as the narrow full width at half maximums (FWHMs). Moreover, the effect of different substitution positions of pyridine on the structure-property relationship is also revealed. Consequently, the nondoped OLEDs exhibit an electroluminescence emission peak of 397 nm with FWHMs of 17 and 22 nm. Due to the high-lying reverse intersystem crossing process, external quantum and exciton utilization efficiencies of 3.6 and 54.55%, respectively, have been achieved based on the emitter with para-linkage. These findings may pave an avenue for the development of high-performance narrowband ultraviolet materials and OLEDs.

Thermal behavior of AlGaN-based deep-UV LEDs

This study utilized thin p-GaN, indium tin oxide (ITO), and a reflective passivation layer (RPL) to improve the performance of deep ultra-violet light-emitting diodes (DUV-LEDs). RPL reflectors, which comprise HfO₂/SiO₂ stacks of different thickness to maintain high reflectance, were deposited on the DUV-LEDs with 40 nm-thick p-GaN and 12 nm-thick ITO thin films. Although the thin p-GaN and ITO films affect the operation voltage of DUV-LEDs, the highly reflective RPL structure improved the WPE and light extraction efficiency (LEE) of the DUV-LEDs, yielding the best WPE and LEE of 2.59% and 7.57%, respectively. The junction temperature of DUV-LEDs with thick p-GaN increased linearly with the injection current, while that of DUV-LEDs with thin p-GaN, thin ITO, and RPL was lower than that of the Ref-LED under high injection currents (> 500 mA). This influenced the temperature sensitive coefficients (dV/dT, dLOP/dT, and dWLP/dT). The thermal behavior of DUV-LEDs with p-GaN and ITO layers of different thicknesses with/without the RPL was discussed in detail.