Transfer-printed, tandem microscale light-emitting diodes for full-color displays.

Inorganic semiconductor-based microscale light-emitting diodes (micro-LEDs) have been widely considered the key solution to next-generation, ubiquitous lighting and display systems, with their efficiency, brightness, contrast, stability, and dynamic response superior to liquid crystal or organic-based counterparts. However, the reduction of micro-LED sizes leads to the deteriorated device performance and increased difficulties in manufacturing. Here, we report a tandem device scheme based on stacked red, green, and blue (RGB) micro-LEDs, for the realization of full-color lighting and displays. Thin-film micro-LEDs (size ∼100 µm, thickness ∼5 µm) based on III-V compound semiconductors are vertically assembled via epitaxial liftoff and transfer printing. A thin-film dielectric-based optical filter serves as a wavelength-selective interface for performance enhancement. Furthermore, we prototype arrays of tandem RGB micro-LEDs and demonstrate display capabilities. These materials and device strategies provide a viable path to advanced lighting and display systems.

Single-crystalline GaN microdisk arrays grown on graphene for flexible micro-LED application.

We report the growth of single-crystalline GaN microdisk arrays on graphene and their application in flexible light-emitting diodes (LEDs). Graphene layers were directly grown onc-sapphire substrates using chemical vapor deposition and employed as substrates for GaN growth. Position-controlled GaN microdisks were laterally overgrown on the graphene layers with a micro-patterned SiO2mask using metal-organic vapor-phase epitaxy. The as-grown GaN microdisks exhibited excellent single crystallinity with a uniform in-plane orientation. Furthermore, we fabricated flexible micro-LEDs by achieving heteroepitaxial growth ofn-GaN, InxGa1-xN/GaN multiple quantum wells, andp-GaN layers on graphene-coated sapphire substrates. The GaN micro-LED arrays were successfully transferred onto bendable substrates and displayed strong blue light emission under room illumination, demonstrating their potential for integration into flexible optoelectronic devices.

Enhanced contact performance of high-brightness micro-LEDs via ITO/Al anode stack and annealing process.

Micro-light-emitting diodes (Micro-LEDs) are a new type of display device based on the third-generation semiconductor gallium nitride (GaN) material which stands out for its high luminous efficiency, elevated brightness, short response times, and high reliability. The contact between anode layers and P-GaN is one of the keys to improving the performance of the devices. This study investigates the impact of electrode structure design and optimized annealing conditions on the anode contact performance of devices. The Micro-LED device with the size of 9.1 µm whose electrode structure is ITO/Ti/Al/Ni/Cr/Pt/Au (100/50/350/100/500/500/5000 Å) exhibits a significant improvement in contact performance after annealing under the Ar gas atmosphere at 500 °C for 5 min. The optimized device exhibited a current of 10.9 mA and a brightness of 298,628 cd/m2 under 5 V. The EQE peak value of Device A is 10.06% at 400 mA.

Advanced Mechanical Transfer of Micro-LEDs Enabled by Structurally Modified Wide Sapphire Nanomembranes through Thermal Reflow of Photoresist.

Micro light-emitting diodes (micro-LEDs) are pivotal in next-generation display technologies, driven by the need for high pixel density. This study introduces a novel methodology utilizing wide sapphire nanomembranes (W-SNM) as a dual-purpose template for high-quality epitaxial growth and the mechanical lift-off of individual micro-LEDs. Micro-LEDs grow individually on W-SNM, obviating the chip singulation process. By employing mechanical fracturing of the thin W-SNM, our method facilitates the transfer of micro-LEDs without the conventional laser lift-off (LLO) process. Previously introduced sapphire nanomembranes (SNM) have shown promise in enhancing epitaxial layer quality; however, they encountered challenges in managing micro-LED size variation and achieving efficient mechanical transfer. Here, we apply simple yet effective adjustments to the SNM structure, specifically, its elevation and widening. This strategic modification allows micro-LEDs to endure applied forces without incurring cracks or defects, ensuring that only the targeted W-SNM are selectively fractured. The mechanically transferred vertical 15 × 15 µm2 micro-LED device operates at an optimal turn-on voltage of 3.3 V. Finite element simulations validate the mechanical strain distribution between the W-SNM and GaN when pressure is applied, confirming the efficacy of our design approach. This pioneering methodology offers a streamlined, efficient pathway for the production and mechanical transfer of micro-LEDs, presenting new avenues for their integration into next-generation, high-performance displays.

A Colloidal-Quantum-Dot Integrated U-Shape Micro-Light-Emitting-Diode and Its Photonic Characteristics.

A special micro LED whose light emitting area is laid out in a U-like shape is fabricated and integrated with colloidal quantum dots (CQDs). An inkjet-type machine directly dispenses the CQD layer to the central courtyard-like area of this U-shape micro LED. The blue photons emitted by the U-shape mesa with InGaN/GaN quantum wells can excite the CQDs at the central courtyard area and be converted into green or red ones. The U-shape micro LEDs are coated with Al2O3 by an atomic layer deposition system and exhibit moderate external quantum efficiency (6.51% max.) and high surface recombination because of their long peripheries. Low-temperature measurement also confirms the recovery of the external quantum efficiency due to lower non-radiative recombination from the exposed surfaces. The color conversion efficiency brought by the CQD layer can be as high as 33.90%. A further continuous CQD aging test, which was evaluated by the strength of the CQD emission, under current densities of 100 A/cm2 and 200 A/cm2 injected into the micro LED, showed a lifetime extension of the unprotected CQD emission up to 1321 min in the U-shape device compared to a 39 min lifetime in the traditional case, where the same CQD layer was placed on the top surface of a squared LED.

Recent Advances in Micro-LEDs Having Yellow-Green to Red Emission Wavelengths for Visible Light Communications.

Visible light communication (VLC), which will primarily support high-speed internet connectivity in the contemporary world, has progressively come to be recognized as a significant alternative and reinforcement in the wireless communication area. VLC has become more popular recently because of its many advantages over conventional radio frequencies, including a higher transmission rate, high bandwidth, low power consumption, fewer health risks, and reduced interference. Due to its high-bandwidth characteristics and potential to be used for both illumination and communications, micro-light-emitting diodes (micro-LEDs) have drawn a lot of attention for their use in VLC applications. In this review, a detailed overview of micro-LEDs that have long emission wavelengths for VLC is presented, along with their related challenges and future prospects. The VLC performance of micro-LEDs is influenced by a number of factors, including the quantum-confined Stark effect (QCSE), size-dependent effect, and droop effect, which are discussed in the following sections. When these elements are combined, it has a major impact on the performance of micro-LEDs in terms of their modulation bandwidth, wavelength shift, full-width at half maximum (FWHM), light output power, and efficiency. The possible challenges faced in the use of micro-LEDs were analyzed through a simulation conducted using Crosslight Apsys software and the results were compared with the previous reported results. We also provide a brief overview of the phenomena, underlying theories, and potential possible solutions to these issues. Furthermore, we provide a brief discussion regarding micro-LEDs that have emission wavelengths ranging from yellow-green to red colors. We highlight the notable bandwidth enhancement for this paradigm and anticipate some exciting new research directions. Overall, this review paper provides a brief overview of the performance of VLC-based systems based on micro-LEDs and some of their possible applications.

Comparative Analysis of Optoelectrical Performance in Laser Lift-Off Process for GaN-Based Green Micro-LED Arrays.

This work explores the pivotal role of laser lift-off (LLO) as a vital production process in facilitating the integration of Micro-LEDs into display modules. We specifically investigate the LLO process applied to high-performance gallium nitride (GaN)-based green Micro-LED arrays, featuring a pixel size of 20 × 38 µm on a patterned sapphire substrate (PSS). Scanning electron microscopy (SEM) observations demonstrate the preservation of the GaN film and sapphire substrate, with no discernible damage. We conduct a comprehensive analysis of the optoelectrical properties of the Micro-LEDs both before and after the LLO process, revealing significant enhancements in light output power (LOP) and external quantum efficiency (EQE). These improvements are attributed to more effective light extraction from the remaining patterns on the GaN backside surface. Furthermore, we examine the electroluminescence spectra of the Micro-LEDs under varying current conditions, revealing a slight change in peak wavelength and an approximate 10% decrease in the full width at half maximum (FWHM), indicating improved color purity. The current-voltage (I-V) curves obtained demonstrate the unchanged forward voltage at 2.17 V after the LLO process. Our findings emphasize the efficacy of LLO in optimizing the performance and color quality of Micro-LEDs, showcasing their potential for seamless integration into advanced display technologies.

Investigation on the Optical Properties of Micro-LEDs Based on InGaN Quantum Dots Grown by Molecular Beam Epitaxy.

InGaN quantum dots (QDs) have attracted significant attention as a promising material for high-efficiency micro-LEDs. In this study, plasma-assisted molecular beam epitaxy (PA-MBE) was used to grow self-assembled InGaN QDs for the fabrication of green micro-LEDs. The InGaN QDs exhibited a high density of over 3.0 × 1010 cm-2, along with good dispersion and uniform size distribution. Micro-LEDs based on QDs with side lengths of the square mesa of 4, 8, 10, and 20 µm were prepared. Attributed to the shielding effect of QDs on the polarized field, luminescence tests indicated that InGaN QDs micro-LEDs exhibited excellent wavelength stability with increasing injection current density. The micro-LEDs with a side length of 8 µm showed a shift of 16.9 nm in the peak of emission wavelength as the injection current increased from 1 A/cm2 to 1000 A/cm2. Furthermore, InGaN QDs micro-LEDs maintained good performance stability with decreasing platform size at low current density. The EQE peak of the 8 µm micro-LEDs is 0.42%, which is 91% of the EQE peak of the 20 µm devices. This phenomenon can be attributed to the confinement effect of QDs on carriers, which is significant for the development of full-color micro-LED displays.

Technological Breakthroughs in Chip Fabrication, Transfer, and Color Conversion for High-Performance Micro-LED Displays.

The implementation of high-efficiency and high-resolution displays has been the focus of considerable research interest. Recently, micro light-emitting diodes (micro-LEDs), which are inorganic light-emitting diodes of size <100 µm2 , have emerged as a promising display technology owing to their superior features and advantages over other displays like liquid crystal displays and organic light-emitting diodes. Although many companies have introduced micro-LED displays since 2012, obstacles to mass production still exist. Three major challenges, i.e., low quantum efficiency, time-consuming transfer, and complex color conversion, have been overcome with technological breakthroughs to realize cost-effective micro-LED displays. In the review, methods for improving the degraded quantum efficiency of GaN-based micro-LEDs induced by the size effect are examined, including wet chemical treatment, passivation layer adoption, LED structure design, and growing LEDs in self-passivated structures. Novel transfer technologies, including pick-up transfer and self-assembly methods, for developing large-area micro-LED displays with high yield and reliability are discussed in depth. Quantum dots as color conversion materials for high color purity, and deposition methods such as electrohydrodynamic jet printing or contact printing on micro-LEDs are also addressed. This review presents current status and critical challenges of micro-LED technology and promising technical breakthroughs for commercialization of high-performance displays.

Optimization of Ternary InxGa1-xN Quantum Wells on GaN Microdisks for Full-Color GaN Micro-LEDs.

Red, green, and blue light InxGa1-xN multiple quantum wells have been grown on GaN/γ-LiAlO2 microdisk substrates by plasma-assisted molecular beam epitaxy. We established a mechanism to optimize the self-assembly growth with ball-stick model for InxGa1-xN multiple quantum well microdisks by bottom-up nanotechnology. We showed that three different red, green, and blue lighting micro-LEDs can be made of one single material (InxGa1-xN) solely by tuning the indium content. We also demonstrated that one can fabricate a beautiful InxGa1-xN-QW microdisk by choosing an appropriate buffer layer for optoelectronic applications.

Investigations of Sidewall Passivation Using the Sol-Gel Method on the Optoelectronic Performance for Blue InGaN Micro-LEDs.

The optoelectronic effects of sidewall passivation on micro-light-emitting diodes (Micro-LEDs) were investigated using sol-gel chemical synthesis. Blue InGaN/GaN multi-quantum well (MQW) Micro-LEDs, ranging in size from 20 × 20 µm to 100 × 100 µm and with high EQE, were fabricated and distinguished by the passivation method used, including no passivation, sol-gel SiO2, and plasma-enhanced chemical vapor deposition (PECVD) SiO2. Impressively, the sol-gel method is advantageous in improving the optoelectronic performance of Micro-LEDs. The fabricated 20 × 20 µm Micro-LEDs showed an EQE of 27.7% with sol-gel passivation, which was a 14% improvement compared to devices without sidewall passivation. Sol-gel sidewall passivation allows Micro-LEDs to effectively achieve sharper edge emission, superior surface luminous uniformity, and intensity, providing the possibility for the fabrication of low-cost and high-efficiency Micro-LEDs.

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

Deep-ultraviolet (DUV) solar-blind communication (SBC) shows distinct advantages of non-line-of-sight propagation and background noise negligibility over conventional visible-light communication. AlGaN-based DUV micro-light-emitting diodes (µ-LEDs) are an excellent candidate for a DUV-SBC light source due to their small size, low power consumption, and high modulation bandwidth. A long-haul DUV-SBC system requires the light source exhibiting high output power, high modulation bandwidth, and high rate, simultaneously. Such a device is rarely reported. A parallel-arrayed planar (PAP) approach is here proposed to satisfy those requirements. By reducing the dimensions of the active emission mesa to micrometer scale, DUV µ-LEDs with ultrahigh power density are created due to their homogeneous injection current and enhanced planar isotropic light emission. Interconnected PAP µ-LEDs with a diameter of 25 µm are produced. This device has an output power of 83.5 mW with a density of 405 W cm-2 at 230 mA, a wall-plug efficiency (WPE) of 4.7% at 155 mA, and a high -3 dB modulation bandwidth of 380 MHz. The remarkable high output power and efficiency make those devices a reliable platform to develop high-modulation-bandwidth wireless communication and to meet the requirements for bio-elimination.

Use of Recycling-Reflection Color-Purity Enhancement Film to Improve Color Purity of Full-Color Micro-LEDs.

A common full-color method involves combining micro-light-emitting diodes (LEDs) chips with color conversion materials such as quantum dots (QDs) to achieve full color. However, during color conversion between micro-LEDs and QDs, QDs cannot completely absorb incident wavelengths cause the emission wavelengths that including incident wavelengths and converted wavelength through QDs, which compromises color purity. The present paper proposes the use of a recycling-reflection color-purity-enhancement film (RCPEF) to reflect the incident wavelength multiple times and, consequently, prevent wavelength mixing after QDs conversion. This RCPEF only allows the light of a specific wavelength to pass through it, exciting blue light is reflected back to the red and green QDs layer. The prototype experiment indicated that with an excitation light source wavelength of 445.5 nm, the use of green QDs and RCPEFs increased color purity from 77.2% to 97.49% and light conversion efficiency by 1.97 times and the use of red QDs and RCPEFs increased color purity to 94.68% and light conversion efficiency by 1.46 times. Thus, high efficiency and color purity were achieved for micro-LEDs displays.

Morphology Control and Crystalline Quality of p-Type GaN Shells Grown on Coaxial GaInN/GaN Multiple Quantum Shell Nanowires.

The morphology and crystalline quality of p-GaN shells on coaxial GaInN/GaN multiple quantum shell (MQS) nanowires (NWs) were investigated using metal-organic chemical vapor deposition. By varying the trimethylgallium (TMG) flow rate, Mg doping, and growth temperature, it was verified that the TMG supply and growth temperature were the dominant parameters in the control of the p-GaN shape on NWs. Specifically, a sufficiently high TMG supply enabled the formation of a pyramid-shaped NW structure with a uniform p-GaN shell. The ratio of the growth rate between the c- and m-planes on the NWs was calculated to be approximately 0.4545. High-angle annular dark-field scanning transmission electron microscopy characterization confirmed that no clear extended defects were present in the n-GaN core and MQS/p-GaN shells on the sidewall. Regarding the p-GaN shell above the c-plane MQS region, only a few screw dislocations and Frank-type partial dislocations appeared at the interface between the serpentine c-plane MQS and the p-GaN shell near the tips. This suggested that the crystalline quality of the MQS structure can trigger the formation of screw dislocations and Frank-type partial dislocations during the p-GaN growth. The growth mechanism of the p-GaN shell on NWs was also discussed. To inspect the electronic properties, a prototype of a micro light-emitting diode (LED) with a chip size of 50 × 50 µm2 was demonstrated in the NWs with optimal growth. By correlating the light output curve with the electroluminescence spectra, three different emission peaks (450, 470, and 510 nm) were assignable to the emission from the m-, r-, and c-planes, respectively.

Highly Reflective Thin-Film Optimization for Full-Angle Micro-LEDs.

Displays composed of micro-light-emitting diodes (micro-LEDs) are regarded as promising next-generation self-luminous screens and have advantages such as high contrast, high brightness, and high color purity. The luminescence of such a display is similar to that of a Lambertian light source. However, owing to reduction in the light source area, traditional secondary optical lenses are not suitable for adjusting the light field types of micro-LEDs and cause problems that limit the application areas. This study presents the primary optical designs of dielectric and metal films to form highly reflective thin-film coatings with low absorption on the light-emitting surfaces of micro-LEDs to optimize light distribution and achieve full-angle utilization. Based on experimental results with the prototype, that have kept low voltage variation rates, low optical losses characteristics, and obtain the full width at half maximum (FWHM) of the light distribution is enhanced to 165° and while the center intensity is reduced to 63% of the original value. Hence, a full-angle micro-LEDs with a highly reflective thin-film coating are realized in this work. Full-angle micro-LEDs offer advantages when applied to commercial advertising displays or plane light source modules that require wide viewing angles.

Investigation of Electrical Properties and Reliability of GaN-Based Micro-LEDs.

In this paper, we report high-performance Micro-LEDs on sapphire substrates, with pixel size scaling to 20 µm and an ultra-high current density of 9902 A/cm2. The forward voltages (VF) of the devices ranged from 2.32 V to 2.39 V under an injection current density of 10 A/cm2. The size and structure-dependent effects were subsequently investigated to optimize the device design. The reliability of Micro-LED devices was evaluated under long-aging, high-temperature, and high-humidity conditions. It was found that Micro-LED devices can maintain comparable performance with an emission wavelength of about 445 nm and a full width at half maximum (FWHM) of 22 nm under extreme environments. Following this, specific analysis with four detailed factors of forward voltage, forward current, slope, and leakage current was carried out in order to show the influence of the different environments on different aspects of the devices.

Nanoscale Characterization of Surface Plasmon-Coupled Photoluminescence Enhancement in Pseudo Micro Blue LEDs Using Near-Field Scanning Optical Microscopy.

The microcave array with extreme large aspect ratio was fabricated on the p-GaN capping layer followed by Ag nanoparticles preparation. The coupling distance between the dual-wavelength InGaN/GaN multiple quantum wells and the localized surface plasmon resonance was carefully characterized in nanometer scale by scanning near-field optical microscopy. The effects of coupling distance and excitation power on the enhancement of photoluminescence were investigated. The penetration depth was measured in the range of 39-55 nm depending on the excitation density. At low excitation power density, the maximum enhancement of 103 was achieved at the optimum coupling distance of 25 nm. Time-resolved photoluminescence shows that the recombination life time was shortened from 5.86 to 1.47 ns by the introduction of Ag nanoparticle plasmon resonance.

Realization of Ultrahigh Quality InGaN Platelets to be Used as Relaxed Templates for Red Micro-LEDs.

In this work, arrays of predominantly relaxed InGaN platelets with indium contents of up to 18%, free from dislocations and offering a smooth top c-plane, are presented. The InGaN platelets are grown by metal-organic vapor phase epitaxy on a dome-like InGaN surface formed by chemical mechanical polishing of InGaN pyramids defined by 6 equivalent {101̅1} planes. The dome-like surface is flattened during growth, through the formation of bunched steps, which are terminated when reaching the inclined {101̅1} planes. The continued growth takes place on the flattened top c-plane with single bilayer surface steps initiated at the six corners between the c-plane and the inclined {101̅1} planes, leading to the formation of high-quality InGaN layers. The top c-plane of the as-formed InGaN platelets can be used as a high-quality template for red micro light-emitting diodes.

Flexible and stretchable opto-electric neural interface for low-noise electrocorticogram recordings and neuromodulation in vivo.

Optogenetic-based neuromodulation tools is evolving for the basic neuroscience research in animals combining optical manipulation and electrophysiological recordings. However, current opto-electric integrated devices attaching on cerebral cortex for electrocorticogram (ECoG) still exist potential damage risks for both brain tissue and electrode, due to the mechanical mismatch and brain deformation. Here, we propose a stretchable opto-electric integrated neural interface by integrating serpentine-shaped electrodes and multisite micro-LEDs onto a hyperelastic substrate, as well as a serpentine-shaped metal shielding embedded in recording electrode for low-noise signal acquisition. The delicate structure design, ultrasoft encapsulation and independent fabrication followed by assembly are beneficial to the conformality, reliability and yield. In vitro accelerated deterioration and reciprocating tensile have demonstrated good performance and high stability. In vivo optogenetic activation of focal cortical areas of awaked mouse expressing Channelrhodopsin-2 is realized with simultaneous high-quality recording. We highlight the potential use of this multifunctional neural interface for neural applications.

A CMOS Time-Resolved Fluorescence Lifetime Analysis Micro-System.

We describe a CMOS-based micro-system for time-resolved fluorescence lifetime analysis. It comprises a 16 × 4 array of single-photon avalanche diodes (SPADs) fabricated in 0.35 µm high-voltage CMOS technology with in-pixel time-gated photon counting circuitry and a second device incorporating an 8 × 8 AlInGaN blue micro-pixellated light-emitting diode (micro-LED) array bump-bonded to an equivalent array of LED drivers realized in a standard low-voltage 0.35 µm CMOS technology, capable of producing excitation pulses with a width of 777 ps (FWHM). This system replaces instrumentation based on lasers, photomultiplier tubes, bulk optics and discrete electronics with a PC-based micro-system. Demonstrator lifetime measurements of colloidal quantum dot and Rhodamine samples are presented.