High-Uniform and High-Efficient Color Conversion Nanoporous GaN-Based Micro-LED Display with Embedded Quantum Dots

InGaN blue resonant cavity micro-LED with RGY quantum dot layer for broad gamut, efficient displays

The technology of RGBY micro resonant cavity light emitting diodes (micro-RCLEDs) based on quantum dots (QDs) is considered one of the most promising approaches for full-color displays. In this work, we propose a novel structure combining a high color conversion efficiency (CCE) QD photoresist (QDPR) color conversion layer (CCL) with blue light micro RCLEDs, incorporating an ultra-thin yellow color filter. The additional TiO2 particles inside the QDPR CCL can scatter light and disperse QDs, thus reducing the self-aggregation phenomenon and enhancing the eventual illumination uniformity. Considering the blue light leakage, the influences of adding different color filters are investigated by illumination design software. Finally, the introduction of low-temperature atomic layer deposition (ALD) passivation protection technology at the top of the CCL can enhance the device's reliability. The introduction of RGBY four-color subpixels provides a viable path for developing low-energy consumption, high uniformity, and efficient color conversion displays.

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.

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.

Current Confinement Effect on the Performance of Blue Light Micro-LEDs with 10 μm Dimension

The current confinement effect on the micro-LED (μLED) with a 10 μm dimension was simulated using SpeCLED software. In this study, three p-contact sizes were considered: 2 μm × 2 μm, 5 μm × 5 μm, and 8 μm × 8 μm dimensions for μLEDs with a 10 μm dimension. According to the simulation data, the highest external quantum efficiency (EQE) of 13.24% was obtained with a 5 μm × 5 μm contact size. The simulation data also showed that the μLEDs with narrow contact sizes experienced higher operating temperatures due to the current crowding effect. The experimental data revealed a red-shift effect in narrow contact sizes, indicating higher heat generation in those devices. As the contact sizes increased from 2 to 8 μm, the turn-on voltage decreased due to lower equivalent resistance. Additionally, the leakage current increased from 44 pA to 1.6 nA at a reverse voltage of -5 V. The study found that the best performance was achieved with a contact ratio of 0.5, which resulted in the highest EQE at 9.95%. This superior performance can be attributed to the better current confinement of the μLED compared to the μLED with a contact ratio of 0.8, resulting in lower leakage current and improved current spreading when compared to the μLED with a contact ratio of 0.2.

Nanostructure Control of GaN by Electrochemical Etching for Enhanced Perovskite Quantum Dot LED Backlighting

Upgraded technology has realized miniaturization and promoted transformation in each field. Miniaturized light-emitting diode (LED) chips enable higher resolution and create a full sense of immersion in displays. Porous GaN is a structure that can reduce excitation light leakage and enhance the light conversion efficiency. Perovskite quantum dots with the highest optical density as candidate materials for loading in pores can significantly decrease the aggregation phenomenon and increase the path of light absorption. Here, the porous tunability is explored by electrochemical etching under a range of voltages, concentrations, and etching times with acid and base electrolytes, such as oxalic acid and potassium hydroxide. Based on scanning electron microscopy images, the distribution of the pores and the morphology of pore channels can be distinguished under acid and base etching. Larger pore sizes and distorted channels (∼680 nm) are presented on the oxalic acid-etched GaN chip. In contrast, smaller pore sizes and straight-deeper channels (∼5650 nm) are demonstrated on the GaN by potassium hydroxide etching. Therefore, the hybrid nanostructure is etched by oxalic acid and potassium hydroxide, separately. The green and red light conversion efficiencies of perovskite quantum dots pumped by a blue LED can be improved by 3 and 10 times, respectively, resulting in a color gamut of approximately 124%.

Ameliorating Uniformity and Color Conversion Efficiency in Quantum Dot-Based Micro-LED Displays through Blue-UV Hybrid Structures

Quantum dot (QD)-based RGB micro light-emitting diode (μ-LED) technology shows immense potential for achieving full-color displays. In this study, we propose a novel structural design that combines blue and quantum well (QW)-intermixing ultraviolet (UV)-hybrid μ-LEDs to achieve high color-conversion efficiency (CCE). For the first time, the impact of various combinations of QD and TiO₂ concentrations, as well as thickness variations on photoluminescence efficiency (PLQY), has been systematically examined through simulation. High-efficiency color-conversion layer (CCL) have been successfully fabricated as a result of these simulations, leading to significant savings in time and material costs. By incorporating scattering particles of TiO₂ in the CCL, we successfully scatter light and disperse QDs, effectively reducing self-aggregation and greatly improving illumination uniformity. Additionally, this design significantly enhances light absorption within the QD films. To enhance device reliability, we introduce a passivation protection layer using low-temperature atomic layer deposition (ALD) technology on the CCL surface. Moreover, we achieve impressive CCE values of 96.25% and 92.91% for the red and green CCLs, respectively, by integrating a modified distributed Bragg reflector (DBR) to suppress light leakage. Our hybrid structure design, in combination with an optical simulation system, not only facilitates rapid acquisition of optimal parameters for highly uniform and efficient color conversion in μ-LED displays but also expands the color gamut to achieve 128.2% in the National Television Standards Committee (NTSC) space and 95.8% in the Rec. 2020 standard. In essence, this research outlines a promising avenue towards the development of bespoke, high-performance μ-LED displays.

Structural and optical analyses for InGaN-based red micro-LED

This study presents a comprehensive analysis of the structural and optical properties of an InGaN-based red micro-LED with a high density of V-shaped pits, offering insights for enhancing emission efficiency. The presence of V-shaped pits is considered advantageous in reducing non-radiative recombination. Furthermore, to systematically investigate the properties of localized states, we conducted temperature-dependent photoluminescence (PL). The results of PL measurements indicate that deep localization in the red double quantum wells can limit carrier escape and improve radiation efficiency. Through a detailed analysis of these results, we extensively investigated the direct impact of epitaxial growth on the efficiency of InGaN red micro-LEDs, thereby laying the foundation for improving efficiency in InGaN-based red micro-LEDs.

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.

Self-array of one-dimensional GaN nanorods using the electric field on dielectrophoresis for the photonic emitters of display pixel

Recently, high-efficiency III-nitride photonic emitters (PEs) for next-generation displays have been studied. Although micro-light-emitting diodes (μ-LEDs), one of the III-nitride PEs, have attracted considerable attention because of their high efficiency and size flexibility, they have encountered technical limitations such as high defect rate, high processing cost, and low yield. To overcome these drawbacks of μ-LEDs, a lot of research on PEs using one-dimensional (1D) gallium nitride-related nanorods (GNRs) capable of horizontally self-positioning on the electrodes has been carried out. The degree of array of GNRs on the interdigitated electrodes (IDEs) is an important factor in the efficiency of the PEs using GNRs to obtain excellent single-pixel characteristics. Therefore, in this study, we demonstrate that the improved performance of self-arrayed GNRs was realized using the dielectrophoresis technique by changing the thickness of IDEs. In addition, the shape and size of vertically aligned GNRs were controlled by the wet process, and GNR-integrated PEs (GIPEs) were driven by perfectly horizontally self-arrayed GNRs on IDEs. The electroluminescence (EL) intensity of the GIPEs was measured at 4-20 V and showed a maximum intensity value at 15 V. Over the injection voltage at 20 V, the EL intensity decreased due to the high current density of GIPEs. The external quantum efficiency (EQE) property of the GIPEs showed a similar efficiency droop as that of conventional III-nitride PEs.

Modified Distributed Bragg Reflectors for Color Stability in InGaN Red Micro-LEDs

The monolithic integration of InGaN-based micro-LEDs is being of interest toward developing full-color micro-displays. However, the color stability in InGaN red micro-LED is an issue that needs to be addressed. In this study, the modified distributed Bragg reflectors (DBRs) were designed to reduce the transmission of undesired spectra. The calculated optical properties of the InGaN red micro-LEDs with conventional and modified DBRs have been analyzed, respectively. The CIE 1931 color space and the encoded 8-bit RGB values are exhibited for the quantitative assessment of color stability. The results suggest the modified DBRs can effectively reduce the color shift, paving the way for developing full-color InGaN-based micro-LED displays.

Research on Simulation Design of MOS Driver for Micro-LED

Micro-LED is a new technology applied in the display field, which has the advantages of self-illumination, low power consumption, high brightness, long life and ultra-high resolution, and has broad application prospects. Using MOS devices to drive micro-LED can enable each unit to have its own drive, thus improving the yield and reducing the subsequent repair processes. In this paper, Sentaurus TCAD simulation software is used to design and simulate NMOS/PMOS devices and their driving circuits. For the first time, CMOS inverters are used to directly drive Micro-LED. Three kinds of driving circuits are compared and analyzed according to their simulation results in output characteristics and transient characteristics. In terms of switching characteristics caused by output characteristics, a CMOS inverter driving a micro-LED circuit has no problems of incomplete turn-off and has greater advantages. In the switching characteristics aspect caused by transient characteristics, PMOS driving a micro-LED circuit has the shortest turn-on time and greater advantages. When compared with a micro-LED driven by an access current-limiting resistor, a micro-LED driven by a direct drive has a smaller on-time value and greater advantages.

Study of High Polarized Nanostructure Light-Emitting Diode

In this study, we investigated the characteristic difference between the two different configurations of the three-dimensional shell–core nanorod LED. We achieve a degree of polarization of 0.545 for tip-free core–shell nanorod LED and 0.188 for tip core–shell nanorod LED by combining the three-dimensional (3D) structure LED with photonic crystal. The ability of low symmetric modes generated by photonic crystals to enhance degree of polarization has been demonstrated through simulations of photonic crystals. In addition, light confinement in GaN-based nanorod structures is induced by total internal reflection at the GaN/air interface. The combination of 3D core–shell nanorod LED and photonic crystals cannot only produce a light source with a high degree of polarization, but also a narrow divergence angle up to 56°. These 3D LEDs may pave the way for future novel optoelectronic components.

Correction: Huang et al. High-Uniform and High-Efficient Color Conversion Nanoporous GaN-Based Micro-LED Display with Embedded Quantum Dots. Nanomaterials 2021, 11, 2696

The authors wish to make following corrections in this paper [...].

Optical Properties of GaN-Based Green Light-Emitting Diodes Influenced by Low-Temperature p-GaN Layer

GaN-based green light-emitting diodes (LEDs) with different thicknesses of the low-temperature (LT) p-GaN layer between the last GaN barriers and p-AlGaN electron blocking layer were characterized by photoluminescence (PL) and electroluminescence (EL) spectroscopic methods in the temperature range of 6–300 K and injection current range of 0.01–350 mA. Based on the results, we suggest that a 20 nm-thick LT p-GaN layer can effectively prevent indium (In) re-evaporation, improve the quantum-confined Stark effect in the last quantum well (QW) of the active region, and finally reduce the efficiency droop by about 7%.