Dual-Ligand Red Perovskite Ink for Electrohydrodynamic Printing Color Conversion Arrays over 2540 dpi in Near-Eye Micro-LED Display.

The lack of stability of red perovskite nanocrystals (PeNCs) remains the main problem that restricts their patterning application. In this work, the dual-ligand passivation strategy was introduced to stabilize PeNCs and inhibit their halogen ion migration during high-voltage electrohydrodynamic (EHD) inkjet printing. The as-printed red arrays exhibit the highest emisson intensity and least blue shift compared with samples with other passivation strategies under a high electric field during EHD inkjet printing. Combining with blue and green PeNC inks, single-color and tricolor color conversion layer arrays were successfully printed, with minimum pixel size of 5 µm and the highest spatial resolution of 2540 dpi. The color coordinate of CsPbBrI2 NCs arrays are located close to the red point, with a color gumat of 97.28% of Rec. 2020 standard. All of these show great potential in the application of color conversion layers in a near-eye micro-LED display.

Spectrally Pure, High Operational Dynamic Range, Deep Red Micro-LEDs.

III-nitride-based micro-light-emitting diodes (micro-LEDs) are currently under rapid development for next-generation high-resolution and high-brightness displays and augmented/virtual reality (AR/VR) technologies. However, it remains elusive to achieve red-emitting III-nitride micro-LEDs with a microscale size, high efficiency, and high spectral stability, posing significant impediments to the development of full-color micro-LEDs. In this work, through detailed strain engineering and control of charge carrier transport, we achieved pure red emission (≥620 nm) micro-LEDs over 2 orders of magnitude of injection current variation. We show both theoretically and experimentally that the combination of a short-period InGaN/GaN superlattice and a thick n-type GaN interlayer can not only relieve the quantum-confined Stark effect in the active region but also suppress parasitic emission from the superlattice. The optimized deep red micro-LEDs with a device lateral dimension of ∼1 µm feature a maximal external quantum efficiency of over 3% emitting at ∼660 nm.

An Ultrahigh Efficiency Excitonic Micro-LED.

High efficiency micro-LEDs, with lateral dimensions as small as one micrometer, are desired for next-generation displays, virtual/augmented reality, and ultrahigh-speed optical interconnects. The efficiency of quantum well LEDs, however, is reduced to negligibly small values when scaled to such small dimensions. Here, we show such a fundamental challenge can be overcome by developing nanowire excitonic LEDs. Harnessing the large exciton oscillator strength of quantum-confined nanostructures, we demonstrate a submicron scale green-emitting LED having an external quantum efficiency and wall-plug efficiency of 25.2% and 20.7%, respectively, the highest values reported for any LEDs of this size to our knowledge. We established critical factors for achieving excitonic micro-LEDs, including the epitaxy of nanostructures to achieve strain relaxation, the utilization of semipolar planes to minimize polarization effects, and the formation of nanoscale quantum-confinement to enhance electron-hole wave function overlap. This work provides a viable path to break the efficiency bottleneck of nanoscale optoelectronics.

InGaN/GaN superlattice underlayer for fabricating of red nanocolumnµ-LEDs with (10-11) plane InGaN/AlGaN MQWs.

In this study, the growth behavior of Indium gallium nitride (InGaN)-based nanocolumn arrays was investigated, and red emission nanocolumn micro-light emitting diodes (µ-LEDs) were fabricated. The internal structure of the InGaN/GaN superlattice (SL) layer under the multiple-quantum-well (MQW) active layers was evaluated using scanning transmission electron microscopy (STEM) analysis. It was revealed that the InGaN crystal plane at the top of the nanocolumn changed from the c-plane, (1-102) plane, to the (10-11) plane as the number of SL pairs increased. A semipolar (10-11) plane was completely formed on top of the nanocolumn by growing InGaN/GaN SLs over 15-20 pairs, where the InGaN/GaN SL layers were uniformly piled up, maintaining the (10-11) plane. Therefore, when InGaN/AlGaN MQWs were grown on the (10-11) plane InGaN/GaN SL layer, the growth of the (10-11) plane semipolar InGaN active layers was observed in the high-angle annular dark field (HAADF)-STEM image. Moreover, the acute nanocolumn top of the (10-11) plane of the InGaN/GaN SL underlayer did not contribute to the formation of the c-plane InGaN core region. Red nanocolumnµ-LEDs with anφ12µm emission window were fabricated using the (10-11) plane MQWs to obtain the external quantum efficiency of 1.01% at 51 A cm-2. The process of nanocolumnµ-LEDs suitable for the smaller emission windows was provided, where the flat p-GaN contact layer contributed to forming a fine emission window ofφ5µm.

Rectangular Amplitude Mask-Based Auto-Focus Method with a Large Range and High Precision for a Micro-LED Wafer Defects Detection System.

Auto-focus technology plays an important role in the Micro-LED wafer defects detection system. How to accurately measure the defocus amount and the defocus direction of the Micro-LED wafer sample in a large linear range is one of the keys to realizing wafer defects detection. In this paper, a large range and high-precision auto-focus method based on a rectangular amplitude mask is proposed. A rectangular amplitude mask without a long edge is used to modulate the shape of the incident laser beams so that the spot shape distribution of the reflected laser beam on the sensor changes with the defocus amount of the wafer sample. By calculating the shape of the light spots, the defocus amount and the defocus direction can be obtained at the same time. The experimental results show that under the 20× microscopy objective, the linear range of the auto-focus system is 480 µm and the accuracy can reach 1 µm. It can be seen that the automatic focusing method proposed in this paper has the advantages of large linear range, high accuracy, and compact structure, which can meet the requirements of the Micro-LED wafer defects detection equipment.

Efficiency improvement of GaN-based micro-light-emitting diodes embedded with Ag NPs into a periodic arrangement of nano-hole channel structure by ultra close range localized surface plasmon coupling.

NH-µLED, namely a micro light-emitting diode structure with nano-holes dug all the way through the active region, is designed to make silver nanoparticles in extremely close contact with the quantum wells for improving the coupling between the localized surface plasmon and the quantum wells (LSP-QWs coupling) and thus enhancing the optical properties of theµLED. The experimental results show that, thanks to this deep nanohole structure, the LSP-QWs coupling can be realized effectively, which ultimately increases the optical performance of theµLED. The internal quantum efficiency of the NH-µLED filled with silver nanoparticles is increased by 12%, and the final optical output power is also enhanced. We have further carried out a comparison study which measures the transient lifetime of two different types ofµLEDs, and the results provide convincing evidence for the existence of the ultra close range LSP-QWs coupling effect.

Photolithographic Patterning of Perovskite Thin Films for Multicolor Display Applications.

Metal halide perovskites are emerging as attractive materials for light-emitting diode (LED) applications. The external quantum efficiency (EQE) has experienced a rapid progress and reached over 21%, comparable to the state of the art organic and quantum dot LEDs. For metal halide perovskites, their simple solution-processing preparation, facile band gap tunability, and narrow emission line width provide another attractive route to harness their superior optoelectronic properties for multicolor display applications. In this work, we demonstrate a high-resolution, large-scale photolithographic method to pattern multicolor perovskite films. This approach is based on a dry lift-off process which involves the use of parylene as an intermediary and the easy mechanical peeling-off of parylene films on various substrates. Using this approach, we successfully fabricated multicolor patterns with red and green perovskite pixels on a single substrate, which could be further applied in liquid crystal displays (LCDs) with blue backlight. Besides, a prototype green perovskite micro-LED display under current driving has been demonstrated.

Monolithic Integration of GaN-Based Transistors and Micro-LED.

Micro-LED is considered an emerging display technology with significant potential for high resolution, brightness, and energy efficiency in display applications. However, its decreasing pixel size and complex manufacturing process create challenges for its integration with driving units. Recently, researchers have proposed various methods to achieve highly integrated micro-structures with driving unit. Researchers take advantage of the high performance of the transistors to achieve low power consumption, high current gain, and fast response frequency. This paper gives a review of recent studies on the new integration methods of micro-LEDs with different types of transistors, including the integration with BJT, HEMT, TFT, and MOSFET.

Advancements in Micro-LED Performance through Nanomaterials and Nanostructures: A Review.

Micro-light-emitting diodes (µLEDs), with their advantages of high response speed, long lifespan, high brightness, and reliability, are widely regarded as the core of next-generation display technology. However, due to issues such as high manufacturing costs and low external quantum efficiency (EQE), µLEDs have not yet been truly commercialized. Additionally, the color conversion efficiency (CCE) of quantum dot (QD)-µLEDs is also a major obstacle to its practical application in the display industry. In this review, we systematically summarize the recent applications of nanomaterials and nanostructures in µLEDs and discuss the practical effects of these methods on enhancing the luminous efficiency of µLEDs and the color conversion efficiency of QD-µLEDs. Finally, the challenges and future prospects for the commercialization of µLEDs are proposed.

Efficiency improvement by using metal-insulator-semiconductor structure in InGaN/GaN micro-light-emitting diodes.

InGaN/GaN micro-light-emitting diodes (micro-LEDs) with a metal-insulator-semiconductor (MIS) structure on the sidewall are proposed to improve efficiency. In this MIS structure, a sidewall electrode is deposited on the insulating layer-coated sidewall of the device mesa between a cathode on the bottom and an anode on the top. Electroluminescence (EL) measurements of fabricated devices with a mesa diameter of 10 µm show that the application of negative biases on the sidewall electrode can increase the device external quantum efficiency (EQE). In contrast, the application of positive biases can decrease the EQE. The band structure analysis reveals that the EQE is impacted because the application of sidewall electric fields manipulates the local surface electron density along the mesa sidewall and thus controls surface Shockley-Read-Hall (SRH) recombination. Two suggested strategies, reducing insulator layer thickness and exploring alternative materials, can be implemented to further improve the EQE of MIS micro-LEDs in future fabrication.

Study on different isolation technology on the performance of blue micro-LEDs array applications.

In this study, a 3 × 3 blue micro-LED array with a pixel size of 10 × 10 µm2 and a pitch of 15 µm was fabricated on an epilayer grown on a sapphire substrate using metalorganic chemical vapor deposition technology. The fabrication process involved photolithography, wet and dry etching, E-beam evaporation, and ion implantation technology. Arsenic multi-energy implantation was utilized to replace the mesa etching for electrical isolation, where the implantation depth increased with the average energy. Different ion depth profiles had varying effects on electrical properties, such as forward current and leakage currents, potentially causing damage to the n-GaN layer and increasing the series resistance of the LEDs. As the implantation depth increased, the light output power and peak external quantum efficiency of the LEDs also increased, improving from 5.33 to 9.82%. However, the efficiency droop also increased from 46.3 to 48.6%.

Series-Biased Micro-LED Array for Lighting, Detection, and Optical Communication.

Micro-LED arrays exhibit high brightness, a long lifespan, low power consumption, and a fast response speed. In this paper, we have proposed a series-biased micro-LED array by using a nitride layer with multi-quantum wells epitaxial on sapphire substrate. The III-nitride multiple quantum wells serving as the micro-LED active material enable both luminescence and detection functionalities. The micro-LED array combines lighting, detection, and communication capabilities. We have conducted a thorough analysis of the micro-LED array's optoelectronic features in both lighting and detection modes. We also explore visible light communication performance across different arrangements of single micro-LED devices within the series-biased array. Our research achieves 720p video transmission via visible light communication using the micro-LED array, supporting a communication rate of up to 10 Mbps. Our contributions encompass the successful integration of lighting and detection functions and a comprehensive assessment of optoelectronic and communication performance. This study highlights the multifunctional micro-LED array's potential as a transceiver terminal in visible light communication systems, expanding its applications from smart lighting to visible light communication and photonic integrated chips. These innovations enhance our understanding of micro-LED technology and its versatile applications.

Double-Layer Metasurface Integrated with Micro-LED for Naked-Eye 3D Display.

Naked-eye 3D micro-LED display combines the characteristics of 3D display with the advantages of micro-LED. However, the 3D micro-LED display is still at the conceptual stage, limited by its intrinsic emission properties of large divergence angle and non-coherence, as well as difficulties in achieving large viewing angles with high luminous efficiency. In this work, we propose a double-layer metasurface film integrating functions of collimation with multiple deflections, constituting a micro-LED naked-eye 3D display system. The system is characterized through numerical simulations using the 3D finite-difference time-domain method. The simulation results show that the double-layer metasurface film restricts 90% of the emitted light of the micro-LED to the vicinity of the 0° angle, improving its spatial coherence. Subsequently, a large-angle, low-crosstalk outgoing from -45° to 45° is achieved, while providing a deflection efficiency of over 80% and a pixel density of up to 605. We believe this design provides a feasible approach for realizing naked-eye 3D micro-LED displays with a large field of view, low crosstalk, and high resolution.

Real-Time Tunable Gas Sensing Platform Based on SnO2 Nanoparticles Activated by Blue Micro-Light-Emitting Diodes.

Micro-light-emitting diodes (µLEDs) have gained significant interest as an activation source for gas sensors owing to their advantages, including room temperature operation and low power consumption. However, despite these benefits, challenges still exist such as a limited range of detectable gases and slow response. In this study, we present a blue µLED-integrated light-activated gas sensor array based on SnO2 nanoparticles (NPs) that exhibit excellent sensitivity, tunable selectivity, and rapid detection with micro-watt level power consumption. The optimal power for µLED is observed at the highest gas response, supported by finite-difference time-domain simulation. Additionally, we first report the visible light-activated selective detection of reducing gases using noble metal-decorated SnO2 NPs. The noble metals induce catalytic interaction with reducing gases, clearly distinguishing NH3, H2, and C2H5OH. Real-time gas monitoring based on a fully hardware-implemented light-activated sensing array was demonstrated, opening up new avenues for advancements in light-activated electronic nose technologies.

Electrohydrodynamic Inkjet Printing of Three-Dimensional Perovskite Nanocrystal Arrays for Full-Color Micro-LED Displays.

Perovskite nanocrystal (PeNC) arrays are showing a promising future in the next generation of micro-light-emitting-diode (micro-LED) displays due to the narrow emission linewidth and adjustable peak wavelength. Electrohydrodynamic (EHD) inkjet printing, with merits of high resolution, uniformity, versatility, and cost-effectiveness, is among the competent candidates for constructing PeNC arrays. However, the fabrication of red light-emitting CsPbBrxI(3-x) nanocrystal arrays for micro-LED displays still faces challenges, such as low brightness and poor stability. This work proposes a design for a red PeNC colloidal ink that is specialized for the EHD inkjet printing of three-dimensional PeNC arrays with enhanced luminescence and stability as well as being adaptable to both rigid and flexible substrates. Made of a mixture of PeNCs, polymer polystyrene (PS), and a nonpolar xylene solvent, the PeNC colloidal ink enables precise control of array sizes and shapes, which facilitates on-demand micropillar construction. Additionally, the inclusion of PS significantly increases the brightness and environmental stability. By adopting this ink, the EHD printer successfully fabricated full-color 3D PeNC arrays with a spatial resolution over 2500 ppi. It shows the potential of the EHD inkjet printing strategy for high-resolution and robust PeNC color conversion layers for micro-LED displays.

Investigation and comparison of the influence of modified DBR and yellow color filters for quantum dot color conversion-based micro LED applications.

This study compares how a modified distributed Bragg reflector (DBR) and yellow color filter (Y-CF) increase the color purity, viewing angle, and brightness of the quantum dot color conversion layer (QDCC) for micro-LED displays. We designed and built a 53-layer high-performance modified DBR with almost total blue leakage filtering (T %: 0.16 %) and very high G/R band transmittance (T %: 96.97 %) for comparison. We also use a Y-CF that filters blue light (T %: 0.84 %) and has good G/R band transmittance (T %: 94.83 %). Due to DBR's angle dependency effect, the modified DBR/QDCC structure offers a remarkable color gamut (117.41 % NTSC) at the forward viewing angle, but this rapidly diminishes beyond 30°. The Y-CF/QDCC structure retains 116 % NTSC color at all viewing angles. Because of its consistent color performance at all viewing angles, sufficient brightness, and outstanding color gamut, the Y-CF/QDCC structure is the best option for contemporary QDCC-based micro-LED displays.

Performance comparison of flip-chip blue-light microLEDs with various passivation.

In this study, arrays of µLEDs in four different sizes (5 × 5 µm2, 10 × 10 µm2, 25 × 25 µm2, 50 × 50 µm2) were fabricated using a flip-chip bonding process. Two passivation processes were investigated with one involving a single layer of SiO2 deposited using plasma-enhanced chemical vapor deposition (PECVD) and the other incorporating Al2O3 deposited by atomic layer deposition (ALD) beneath the SiO2 layer. Owing to superior coverage and protection, the double-layers passivation process resulted in a three-order lower leakage current of µLEDs in the 5 µm chip-sized µLED arrays. Furthermore, higher light output power of µLEDs was observed in each chip-sized µLED array with double layers passivation. Particularly, the highest EQE value 21.9% of µLEDs array with 5 µm × 5 µm chip size was achieved with the double-layers passivation. The EQE value of µLEDs array was improved by 4.4 times by introducing the double-layers passivation as compared with that of µLEDs array with single layer passivation. Finally, more uniform light emission patterns were observed in the µLEDs with 5 µm × 5 µm chip size fabricated by double-layer passivation process using ImageJ software.

Monolithic Integration of Full-Color Microdisplay Screen with Sub-5 µm Quantum-Dot Pixels.

Monolithic integration of color-conversion materials onto blue-backlight micro-light-emitting-diodes (micro-LEDs) has emerged as a promising strategy for achieving full-color microdisplay devices. However, this approach still encounters challenges such as the blue-backlight leakage and the poor fabrication yield rate due to unsatisfied quantum dot (QD) material and fabrication process. Here, the monolithic integration of 0.39-inch micro-display screens displaying colorful pictures and videos are demonstrated, which are enabled by creating interfacial chemical bonds for wafer-scale adhesion of sub-5 µm QD-pixels on blue-backlight micro-LED wafer. The ligand molecule with chlorosulfonyl and silane groups is selected as the synthesis ligand and surface treatment material, facilitating the preparation of high-efficiency QD photoresist and the formation of robust chemical bonds for pixel integration. This is a leading record in micro-display devices achieving the highest brightness larger than 400 thousand nits, the ultrahigh resolution of 3300 PPI, the wide color gamut of 130.4% NTSC, and the ultimate performance of service life exceeding 1000 h. These results extend the mature integrated circuit technique into the manufacture of micro-display device, which also lead the road of industrialization process of full-color micro-LEDs.

Clinical Validation of Face-Fit Surface-Lighting Micro Light-Emitting Diode Mask for Skin Anti-Aging Treatment.

Photobiomodulation therapy based on micro light-emitting diodes (µLEDs) holds remarkable potential for the beauty industry. Here, a cosmetically effective face-fit surface-lighting µLED mask for skin anti-aging is introduced. The face-conformable mask enables deep tissue treatment through proximal light irradiation, with a 3D origami structure capable of adapting to complex facial contours with closed adherence. A blister-assisted laser transfer achieves rapid and accurate µLEDs transfer at a high throughput of 50 chips per second, facilitating a mass-producible and large-area process. Finally, clinical trials demonstrate significant improvements in elasticity, sagging, and wrinkles across six facial areas, with a maximum enhancement of 340% in deep skin elasticity of the perioral area compared to the conventional LED mask group.

Power-Dependent Optical Characterization of the InGaN/GaN-Based Micro-Light-Emitting-Diode (LED) in High Spatial Resolution.

Spatially resolved photoluminescence at the sub-micro scale was used to study the optical non-uniformity of the micro-LED under varied power density excitation levels. The trend of the efficiency along injection levels were found to be highly dependent on the location of the chip mesa. The sidewall was 80% lower than the center under low-power density excitation, but was 50% higher under high-power density excitation. The external quantum efficiency droop at the center and the sidewall was 86% and 52%, respectively. A 2 µm band area near the sidewall was characterized where the efficiency and its trends changed rapidly. Beyond such band, the full width at half maximum and peak wavelength variation across the chip varied less than 1 nm, indicating high uniformity of the material composition. The sudden change = in the band, especially under high level excitation indicates the indium composition change formed by ion residues on the sidewall affect the distribution of charge carriers. These findings contribute to the understanding of cause of efficiency disadvantage and non-uniformity problems in small-size micro-LEDs.