High Pixel Density Research Articles

Ultrathin, Wavelength‐Multiplexed and Integrated Holograms and Optical Neural Networks Based on 2D Perovskite Nanofilms

AbstractHolography, as a technique for coherent wavefront reconstruction, is extensively used in numerous optical applications such as optical imaging, 3D display, photolithography, and optical artificial intelligence. In order to achieve highly compact and functional integration with optoelectronic devices, the hologram needs to possess ultrathin thickness as well as multicolor functionality. However, its thickness is typically limited to optical wavelength ranges due to the requirement for pronounced amplitude or phase modulation, and it generally operates at a single wavelength band without wavelength‐multiplexed channels. Here, the hologram is decreased thickness to sub‐ten nanometers by exploiting the large refractive index and strong exciton absorption of 2D perovskites. Ultrathin perovskite holograms and holographic neural networks with high stability are successfully developed by using femtosecond laser direct writing, and their operation wavelength can be rationally tuned from 400–515 nm through halide anion engineering. Consequently, the wavelength can be multiplexed with low cross‐talks by stacked perovskite nanolayers for applications of holographic display and neural networks without the usages of complex optical structures or filters. This work provides a feasible and promising technical route for integrating ultrathin, high pixel density, and multiplexed holographic structures with flat optoelectronic devices for next‐generation integrated optical systems.

Ultra-high brightness Micro-LEDs with wafer-scale uniform GaN-on-silicon epilayers

Owing to high pixel density and brightness, gallium nitride (GaN) based micro-light-emitting diodes (Micro-LEDs) are considered revolutionary display technology and have important application prospects in the fields of micro-display and virtual display. However, Micro-LEDs with pixel sizes smaller than 10 μm still encounter technical challenges such as sidewall damage and limited light extraction efficiency, resulting in reduced luminous efficiency and severe brightness non-uniformity. Here, we reported high-brightness green Micro-displays with a 5 μm pixel utilizing high-quality GaN-on-Si epilayers. Four-inch wafer-scale uniform green GaN epilayer is first grown on silicon substrate, which possesses a low dislocation density of 5.25 × 108 cm−2, small wafer bowing of 16.7 μm, and high wavelength uniformity (standard deviation STDEV < 1 nm), scalable to 6-inch sizes. Based on the high-quality GaN epilayers, green Micro-LEDs with 5 μm pixel sizes are designed with vertical non-alignment bonding technology. An atomic sidewall passivation method combined with wet treatment successfully addressed the Micro-LED sidewall damages and steadily produced nano-scale surface textures on the pixel top, which unlocked the internal quantum efficiency of the high-quality green GaN-on-Si epi-wafer. Ultra-high brightness exceeding 107 cd/m2 (nits) is thus achieved in the green Micro-LEDs, marking the highest reported results. Furthermore, integration of Micro-LEDs with Si-based CMOS circuits enables the realization of green Micro-LED displays with resolution up to 1080 × 780, realizing high-definition playback of movies and images. This work lays the foundation for the mass production of high-brightness Micro-LED displays on large-size GaN-on-Si epi-wafers.

Size and temperature effects on optoelectronic properties of Micro-LED arrays for display applications

In recent years, Micro-light-emitting diode (Micro-LED) has attracted much attention in the display field, because it provides the possibility of high pixel density, etc. The implementation of high pixel density is accompanied by the reduction of size, during which the performance degradation caused by sidewall defects gradually increases. At the same time, the increase in power density and difficulty in heat dissipation caused by high pixel density can also lead to poor thermal management, resulting in an increase in junction temperature, thus affecting the display effect. In this paper, we prepared seven different sizes of Micro-LED and studied the influence of size and temperature effects on the optoelectronic properties. Smaller-sized Micro-LED could withstand higher current densities because it was not affected by the current crowding effect. However, it tends to have a larger series resistance (RS) and a lower external quantum efficiency (EQE) due to sidewall defects created during etching. The luminance of the 100 μm and 7 μm Micro-LED was 340000 cd/m2 (at 83 A/cm2) and 32000 cd/m2 (at 773 A/cm2), respectively. The peak EQE of 100 μm Micro-LED was 16.87 %. On this basis, the temperature-dependence of the optoelectronic properties of 10 μm and 50 μm Micro-LED were studied. The larger the size of Micro-LED, the better the operational stability it exhibited. From the room temperature (RT) rose to 100 ℃, the EQE of the 10 μm and 50 μm Micro-LED decreased by 16 % and 13.1 %, respectively. These experimental results provide important data for the design and preparation of Micro-LED of different sizes and for exploring the operational stability at high temperatures.

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.

Monolithic full-color micro-LED displays featuring three-dimensional chip bonding and quantum dot-based color conversion layer.

What we believe to be a novel fabrication process for monolithic full-color (RGB) micro-LED (µLED) display technology, featuring three-dimensional (3D) and quantum dot (QD)-based color conversion layer, has been proposed. This method offers advantages such as a wide color gamut, high pixel density, high yield, and low cost. A 16 × 16 passive matrix (PM) RGB µLED array, with a pitch size of 80 µm and a pixel density of 328 pixels per inch (PPI), has been successfully realized using flip-chip bonding technology. When measuring the electroluminescence (EL) spectra of the green and red pixels with the addition of color filters, the color gamut can achieve a maximum of 124% of the National Television System Committee (NTSC) standard. Additionally, this process significantly reduces the risk of damage to the QD film during photolithography compared to using two different colored QDs for RGB µLED arrays. The proposed manufacturing process shows considerable promise for commercialization.

Size-dependent optoelectronic characteristics of InGaN/GaN red micro-LEDs on 4-inch Si substrates: high pixel density arrays demonstration.

In this study, we explored the size-dependent optoelectronic characteristics of InGaN/GaN red micro-LEDs grown on Si substrates. We successfully demonstrated the fabrication of 4-inch wafer-scale InGaN/GaN micro-LEDs, showcasing the feasibility of large-scale production. Additionally, we presented the binary pixel display with 6 µm pitch, achieving a resolution of 4232 PPI. We also introduced a method of driving the display using PWM to linearly control and counteract the blue shift in peak wavelength caused by the QCSE and band-filling effect. Our research represents a significant milestone in the development of InGaN/GaN red micro-LEDs on Si substrates, establishing them as a key component for full-color micro-LED displays.

70‐4: Late‐News Paper: Integral 3D Display Using 2D Image Time‐Division Multiplexing and Eye‐Tracking Technologies

We propose an integral three‐dimensional (3D) display method using time‐division multiplexing and eye‐tracking technologies to enhance the maximum pixel density and viewing zone angle of 3D images. We prototyped a display system and confirmed that 3D images with high pixel density and wide viewing zone angle can be displayed.

20‐1: Invited Paper: Development of Photolithgraphic Patterning of Quantum Dots for Electroluminescent Applications

In this paper the photolithgraphic patterning approaches of quantum dots in electroluminescent devices and demoes were summerized. Photolithograpy process meets the requirements for QLED mass production, including high pixel density, large‐area process and low cost. We summerized three approaches of quantum dot photolithographic procrsses, including direct photolithography, photolithography with lift‐off process and sacrificial layer assisted patterning (SLAP).

Pushing Patterning Limits of Drop‐On‐Demand Inkjet Printing with Cspbbr3/PDMS Nanoparticles

AbstractPatterning is crucial for advancing perovskite materials into color conversion micro‐display applications, but achieving inkjet‐printed patterns with high performance and precision remains challenging. Here, novel CsPbBr3/PDMS nanoparticles as a promising candidate is proposed for achieving precise and perfect inkjet printing patterns. The as‐prepared nanoparticles ensure exceeding brightness, exceptional stability, uniform fluorescence emission, and high resolution reaching the highest performance of drop‐on‐demand inkjet printing. Specifically, the performance and stability of CsPbBr3/PDMS nanoparticles are enhanced through a two‐step optimization involving dodecyl benzene sulfonic acid (DBSA) ligand modification and polydimethylsiloxane (PDMS) in situ coating, resulting in the highest photoluminescence quantum yield (PLQY) of 92% among CsPbBr3/polymer core‐shell composites so far. The branched structure of DBSA and the PDMS shell provide steric hindrance and effectively prevent agglomeration during storage or patterning. The viscous PDMS coating inhibits the coffee ring effect, not only leading to excellent near‐unity uniform emission but also promoting the formation of smaller and more elaborate droplets, increasing the printing resolution by up to surprisingly 300%. Importantly, the impeccably displayed, high‐resolution patterns formed by versatile CsPbBr3/PDMS nanoparticles have demonstrated significant potential for high pixel density Micro‐LED displays, enabling efficient and stable color conversion.

Ga2O3 Photon‐Controlled Diode for Sensitive DUV/X‐Ray Detection and High‐Resolution Array Imaging Application

AbstractSensitive high‐energy photon detection from UV to X‐ray and high‐resolution array imaging are critical for medical diagnosis, space exploration, and scientific research. The key challenges for high‐performance photodetector and imaging arrays are the effective material and device design strategies for the miniaturization and integration of the device. Here, photon‐controlled diodes (i.e., the detector has rectifying characteristics only under light irradiation) are proposed for high‐resolution and anti‐crosstalk array imaging applications without integrating the switching element. Based on ultra‐wide bandgap semiconductor Ga2O3, the sensitive DUV/X‐ray photon‐controlled diodes are realized by the design of high‐resistance Ga2O3 film and high‐barrier contact. The device exhibits remarkable detection performance, including high photo‐responsivity (168 A W−1) and specific detectivity (1.45 × 1015 Jones) under DUV illumination, as well as a high sensitivity (1.23 × 105 µC Gyair−1 cm−2) under X‐ray light. Moreover, the low dark current and excellent rectification characteristics are obtained. Furthermore, its potential for high‐density and anti‐crosstalk array imaging applications is verified. These results not only bring forth new insights in the implementation of high‐performance DUV/X‐ray photodetector, but also pave a feasible way to realize high pixel density detector array through the simplified fabrication process for high‐resolution imaging applications.

High‐luminance, large‐size 4K OLED microdisplays for VR/MR applications

AbstractThis work introduces LG Display's (LGD's) 1.3″ 4K full‐color organic light‐emitting diode (OLED) microdisplay panels with a peak brightness of more than 10,000 nits, which have been newly developed for high‐quality virtual reality/mixed reality (VR/MR) head‐mounted displays. To achieve these advanced high‐luminance OLED microdisplay panels, we have developed a well‐controlled optical microcavity and outcoupling method, along with optimized multi‐stack white OLED devices and CF‐on‐encapsulation technologies tailored to the high pixel density of the panels.

Color arrestor pixels for high-fidelity, high-sensitivity imaging sensors.

Silicon is the dominant material in complementary metal-oxide-semiconductor (CMOS) imaging devices because of its outstanding electrical and optical properties, well-established fabrication methods, and abundance in nature. However, with the ongoing trend toward electronic miniaturization, which demands smaller pixel sizes in CMOS image sensors, issues, such as crosstalk and reduced optical efficiency, have become critical. These problems stem from the intrinsic properties of Si, particularly its low absorption in the long wavelength range of the visible spectrum, which makes it difficult to devise effective solutions unless the material itself is changed. Recent advances in optical metasurfaces have offered new possibilities for solving these problems. In this study, we propose color arrestor pixels (CAPs) as a new class of color image sensors whose composite spectral responses directly mimic those of the human eye. The key idea is to employ linearly independent combinations of standardized color matching functions. These new basis functions allow our device to reproduce colors more accurately than the currently available image sensors with red-green-blue filters or other metasurface-based sensors, demonstrating an average CIEDE2000 color difference value of only 1.79 when evaluating 24 colors from the Gretag-Macbeth chart under standard illuminant D65. Owing to their high fidelity to the human eye response, CAPs consistently exhibit exceptional color reproduction accuracy under various spectral illumination compositions. With a small footprint of 860 nm height and 221 nm full-color pixel pitch, the CAPs demonstrated high absorption efficiencies of 79 %, 81 %, and 63 % at wavelengths of 452 nm, 544 nm, and 603 nm, respectively, and good angular tolerance. With such a high density of pixels efficiently capturing accurate colors, CAPs present a new direction for optical image sensor research and their applications.

High-resolution display screen as programmable illumination for Fourier ptychography

We introduce a new programmable illumination strategy for Fourier ptychographic microscopy (FPM), utilizing a high-resolution organic light-emitting diode (OLED) display screen at the vicinity of the sample to achieve super-resolution and quantitative phase imaging of biological samples. High-density pixels in the OLED screen allow for the angle-scanning illumination required for FP by placing the sample slide directly on the screen, where multiple images of the sample are captured while scanning the pixels on the screen. We discuss the design of the OLED-FP illumination and the corresponding image reconstruction strategies and experimentally validate the super-resolution and phase imaging capabilities of FP using a smartphone screen as illumination. Further, we extend our method to patterned illumination strategies, which multiplexes multiple illumination pixels to achieve full-field FP imaging with a reduced number of measurements, where we identify an optimal illumination pattern and density to maximize the imaging speed without sacrificing the image quality. Our approach offers a simple and compact programmable illuminator that can effectively transform any microscope into a compact FPM with resolution improvement and phase imaging capabilities.

A Data-Driven Machine Learning Approach for Turbulent Flow Field Prediction Based on Direct Computational Fluid Dynamics Database

A novel approach is presented for predicting compressible turbulent flow fields using a neural network-based data-driven method. Accurate prediction in turbulent regions heavily relies on the resolution of available data. Traditional methods, employing image-based techniques by mapping scattered computational fluid dynamics (CFD) data onto Cartesian grids, encounter data scarcity in critical areas such as the boundary layer and wake. Recently, convolutional neural networks (CNN) have gained prominence as the most widely referenced technique in fluid dynamics, utilizing flow field images as datasets for flow field prediction. However, CNN requires datasets with a high pixel density to enhance training accuracy in crucial regions, thereby increasing the input data volume and machine training time. To address this challenge, our proposed method deviates from using flow field images and instead generates datasets directly from the flow field properties of CFD grid points. By employing this approach, several advantages are realized. Firstly, the network benefits from the favorable characteristics of unstructured grids, such as varying point spacing near the object surface and in the far field, which effectively reduces the amount of input data and consequently the machine training cost. Secondly, the construction of the training dataset eliminates the need for interpolation or extrapolation, thereby preserving the accuracy of CFD data. In this case, a simple multilayer perceptron can be trained using the proposed dataset. Various flow field properties, including static pressure, turbulent kinetic energy, and velocity components, can be predicted with high accuracy within a few seconds.

Electrodeposited Nanowires for High Density Interconnects of GaN-Based MicroLEDs

For next-generation applications, fully controllable gallium nitride (GaN)-based micro light emitting diode (microLED) arrays will play a crucial role. These feature high pixel densities, high intensities, and high efficiencies. Well-established top-down etching processing schemes of pristine LED wafers allow for an easily accessible and scalable fabrication approach for microLED arrays down to the single digit µm dimensions with dies featuring several 100,000 on a square centimeter. At this scale, classical bonding approaches like wire bonding fail to harvest the true potential of such an array due to inherit technological restrictions. To allow for the individual controllability of every pixel a highly scalable integration approach to a CMOS backplane is necessary. Established processes for the realization of such an integration often feature major drawbacks. Indium bumps deposited via thermal evaporation generally fulfill the scale requirements but fail to adapt to the unique challenges of such hybrid integration. High non-planarity of the substrate, high wafer bow, and mismatching thermal expansion coefficients can lead to diminishing bonding yields and hence functioning microLEDs during the high-temperature bonding step necessary for indium bumps. Additionally, Indium is an inferior electrical and thermal conductor to other metals and limits the active performance of the device while also restricting its usage area due to its low melting point. In the currently ongoing EU Project SMILE, we are developing GaN-based microLED arrays up to 512 x 512 pixels and thoroughly examine established bonding technologies as well as an in-house developed metal nanowire approach. The necessary perquisites for electrochemical deposition, namely the conductive seed and the removal thereof can easily be integrated into the device’s processing scheme. Nanowires are electrochemically deposited out of a liquid solution at room temperature onto the microLED array and the CMOS backplane. The direction, length, and diameter of the nanowires can easily be controlled to accustom different proportions and bonding geometries. Inline monitoring via the recorded amperogram allows for the differentiation of the individual stages of nanowire growth during the electrochemical deposition. Mechanically stable and electrically sufficient interconnects can then be realized by pressing the nanowires of both substrates into each other. The high aspect ratio of the wires results in a high surface area and allows for nanowires of both substrates to intertwine forming stable connections with large contact surfaces at low pressures without any additional heating steps necessary. Due to the variability of the wire length and ongoing intertwining during the bonding, non-planarities and wafer bow can be accommodated. This flexible bonding behavior is a unique advantage of bonds based on metal nanowires. The junctions approach the conductivity of bulk material and are crucial for the thermal management of the microLED array. The final LED device generates a substantial amount of heat, which cannot be cooled meaningfully via convection to the surrounding air or by its radiation. Consequently, a capable thermal conduction towards the silicon chip is necessary for heat management. Here the metal nanowires play a crucial role in providing this thermal connection via the bonding interface, which potentially allows for improved device performance over other bonding technologies.

Monolithic full-color active-matrix micro-LED micro-display using InGaN/AlGaInP heterogeneous integration

A prototype of full-color active-matrix micro-light-emitting diode (micro-LED) micro-display with a pixel density of 391 pixel per inch (ppi) using InGaN/AlGaInP heterogeneous integration is demonstrated. InGaN blue/green dual-color micro-LED arrays realized on a single metal organic chemical vapor deposition (MOCVD)-grown GaN-on-Si epiwafer and AlGaInP red micro-LED arrays are both monolithically fabricated, followed by the integration with a common complementary metal oxide semiconductor (CMOS) backplane via flip-chip bonding technology to form a double-layer thin-film display structure. Full-color images with decent color gamut and brightness are successfully displayed through the fine adjustment of driving current densities of RGB subpixels. This full-color display combines the advantages of high quantum efficiency of InGaN material on blue/green light and AlGaInP material on red light through heterogeneous integration and high pixel density through monolithic fabrication approach, demonstrating the feasibility and prospects of high brightness, good color performance, and high-resolution micro-LED micro-displays in future metaverse applications.

Solution Processed Active Materials for Pixel Sensor Element and Integrated Circuits

We describe an integrated planar circuit for an active pixel sensor consisting of Metal-semiconductor-Metal (MSM) photodetector with hybrid perovskite as the semiconductor and organic field effect transistors (OFETs) as a readout circuitry. The adopted fabrication approach reduces the complexity of implementing these circuits over a large area, for a variety of substrates from flexible to rigid ones. The features of this circuit include low threshold voltage and a reasonable switching speed of ≈ 100 μs. The achievable channel length of ≈ 2.5 μm for both the photodetector element and OFETs, implies possibility of high pixel density for imaging applications.

High‐performance MEMS shutter display with metal‐oxide thin‐film transistors and optimized MEMS element

AbstractActive matrix prestressed microelectromechanical shutter displays enable outstanding optical properties as well as robust operating performance. The microelectromechanical systems (MEMS) shutter elements have been optimized for higher light outcoupling efficiency with lower operation voltage and higher pixel density. The MEMS elements have been co‐fabricated with self‐aligned metal‐oxide thin‐film transistors (TFTs). Several optimizations were required to integrate MEMS process without hampering the performance of both elements. The optimized display process requires only seven photolithographic masks with ensuring proper compatibility between MEMS shutter and metal‐oxide TFT process.

74‐1: Invited Paper: Highly Stretchable and Shrinkable AMOLED for Free Deformation

Flexible display technology has been successfully commercialized with foldable and rollable displays. Because of the limitation of the flexible technology, stretchable displays have attracted great attention which capable of realizing freeform design as a post‐flexible display. However, no technology has yet provided sufficient stretchability and resolution to realize genuine free‐form displays. We report on a highly deformable AMOLED display which capable of elongation over 20%. Moreover, our display is capable of more than 15% shrinkage deformation. We also developed low‐temperature thermal shrink process to make the most of the potential of the display. The fabricated full‐color stretchable AMOLED shows excellent deformability with high pixel density.

High-resolution single pulse LA-ICP-MS mapping via 2D sub-pixel oversampling on orthogonal and hexagonal ablation grids – A computational assessment

Laser beam profiles in analytical laser ablation – inductively coupled plasma – mass spectrometry (LA-ICP-MS) instruments are in general homogenized to produce a flat-top beam profile. However, in practice, they are mostly super-Gaussian in nature, and for small laser beam sizes (<5 μm) they even approach a Gaussian profile. This implies that the amount of surface material sampled by the laser (=ablation volume) directly depends on the beam profile and ablation grid. By contraction of the ablation grid (=sub-pixel mapping) not only more accurate surface sampling is realized, but also a higher pixel density, an improved spatial resolution, and a better signal-to-noise ratio. Although LA sampling is predominantly performed on an orthogonal grid, hexagonal or staggered/interleaved sampling may further improve the image quality as regular hexagons are more compact than squares (=lower perimeter/area) and suffer less from orientation bias (=lower anisotropy). Due to the current limitations of LA stages in executing precise hexagonal sampling with small beam sizes, computational protocols were employed to simulate LA-ICP-MS mapping. Simulation was performed by discrete convolution using the crater profile as the kernel, followed by the application/addition of Poisson/Flicker noise related to the local concentration and instrumental sensitivity/noise. A freely accessible online app was developed (https://laicpms-apps.ki.si/webapps/home/) to study the effect of sampling grid contraction (orthogonal and hexagonal) on the image map quality (spatial resolution and signal-to-noise ratio) by virtual ablation of phantoms. Comparison of experimental LA-ICP-MS maps obtained through orthogonal and hexagonal sampling methods could only be performed using a beam size of 150 μm and a macroscale inkjet-printed resolution target. This was due to the unavailability of precise hexagonal sampling stages and microscale resolution targets, which prevented the use of smaller beam sizes.