Designs of InGaN Micro-LED Structure for Improving Quantum Efficiency at Low Current Density

Dual polarization for efficient III-nitride-based deep ultraviolet micro-LEDs

The deep ultraviolet (DUV) micro-light emitting diode (μLED) has serious electron leakage and low hole injection efficiency. Meanwhile, with the decrease in the size of the LED chip, the plasma-assisted dry etching process will cause damage to the side wall of the mesa, which will form a carrier leakage channel and produce non-radiative recombination. All of these will reduce the photoelectric performance of μLED. To this end, this study introduces polarized bulk charges into the hole supply layer (p-HSL) and the electron supply layer (n-ESL) respectively (dual-polarized structure) of the DUV μLED at an emission wavelength of 279 nm to enhance the binding of carriers and increase the injection efficiency of carriers. This is because the polarization-induced bulk charge can shield the polarized sheet charge on the interface and reduce the polarization electric field. The reduced polarization electric field in p-HSL can increase the effective barrier height of the conduction band in the p-type region and reduce the effective barrier height of the valence band. The decrease in the polarized electric field of n-HSL can reduce the thermal velocity of electrons, thereby enhancing the electron injection efficiency, reducing the Shockley-Read-Hall (SRH) recombination, and increasing the effective barrier height of the valence band. The study results indicate that the electron concentration and hole concentration of a μLED with dual polarization were increased by 77.93% and 93.6%, respectively. The optical power and maximum external quantum efficiency of μLED reached 31.04 W/cm2 and 2.91% respectively, and the efficiency droop is only 2.06% at 120 A/cm2. These results provide a new approach to solving the problem of insufficient carrier injection and SRH recombination in high-performance DUV μLEDs.

Wafer-Scale Characterization of 1692-Pixel-Per-Inch Blue Micro-LED Arrays with an Optimized ITO Layer

Wafer-scale blue micro-light-emitting diode (micro-LED) arrays were fabricated with a pixel size of 12 μm, a pixel pitch of 15 μm, and a pixel density of 1692 pixels per inch, achieved by optimizing the properties of e-beam-deposited and sputter-deposited indium tin oxide (ITO). Although the sputter-deposited ITO (S-ITO) films exhibited a densely packed morphology and lower resistivity compared to the e-beam-deposited ITO (E-ITO) films, the forward voltage (VF) values of a micro-LED with the S-ITO films were higher than those with the E-ITO films. The VF values for a single pixel and for four pixels with E-ITO films were 2.82 V and 2.83 V, respectively, while the corresponding values for S-ITO films were 3.50 V and 3.52 V. This was attributed to ion bombardment damage and nitrogen vacancies in the p-GaN layer. Surprisingly, the VF variations of a single pixel and of four pixels with the optimized E-ITO spreading layer from five different regions were only 0.09 V and 0.10 V, respectively. This extremely uniform VF variation is suitable for creating micro-LED displays to be used in AR and VR applications, circumventing the bottleneck in the development of long-lifespan and high-brightness organic LED devices for industrial mass production.

Enhanced light extraction efficiency of GaN-based green micro-LED modulating by a thickness-tunable SiO2 passivation structure

Green micro-light emitting diodes (micro-LEDs) is one of the three primary color light sources as full-color display, which serves as a key research object in the field of micro-LED display. As the micro-LED size decreases, the surface-area-to-volume ratio of the device increases, leading to more serious damage on the sidewall by inductively coupled plasma (ICP) etching. The passivation process of SiO2 provides an effective method to reduce sidewall damage caused by ICP etching. In this work, green rectangular micro-LEDs with passivation layer thickness of 0∼600 nm was designed using the finite-difference time-domain (FDTD) simulation. In order to verify the simulation results, the micro-LED array was fabricated by parallel laser micro-lens array (MLA) lithography in high speed and large area. The effect of the SiO2 passivation layer thickness on the performance of the green micro-LED was analyzed, which shows that the passivation layer thickness-light extraction efficiency curve fluctuates periodically. For the sample with 90 nm thickness of SiO2 passivation layer, there exists a small leakage current and higher operating current density, and the maximum external quantum efficiency (EQE) is 2.8 times higher than micro-LED without SiO2 passivation layer.

Efficiency enhancement mechanism of piezoelectric effect in long wavelength InGaN-based LED

Improving the luminescence efficiency of InGaN-based long wavelength LEDs for use in micro-LED full-colour displays remains a huge challenge. The strain-induced piezoelectric effect is an effective measure for modulating the carrier redistribution at the InGaN/GaN heterointerfaces. Our theoretical results reveal that the hole injection is significantly improved by the diminution of the valence band offset (VBO) of the InGaN/GaN heterointerfaces along the [0001] direction, and inversely, the VBO increases along the [0001] direction. The energy band structures showed that the tensile strain of the GaN film grown on a silicon (Si) substrate could weaken the internal electric field of the InGaN well layer leading to a flattening of the energy band, which increases the overlap of electron and hole wave functions. In addition, the strain-induced piezoelectric polarisation of the InGaN layer on the Si substrate generates opposite sheet-bound charges at the heterointerfaces, which causes a reduction in the depletion region of the InGaN/GaN quantum wells (QWs). A systematic analysis illustrates that the control of the piezoelectric polarisation of the InGaN QW layer is available improve the internal quantum efficiency of the InGaN-based LEDs.

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.

Preparation and usage of nanomaterials in biomedicine

Nanotechnology is one of the most promising and decisive technologies in the world. Nanomaterials, as the primary research aspect of nanotechnology, are quite different from macroscopic materials because of their unique optical, electrical, magnetic, thermal properties, and more robust mechanical properties, which make them play an essential role in the field of materials science, biomedical field, aerospace field, and environmental energy. Different preparation methods for nanomaterials have various physical and chemical properties and are widely used in different areas. In this review, we focused on the preparation methods, including chemical, physical, and biological methods due to the properties of nanomaterials. We mainly clarified the characteristics, advantages, and disadvantages of different preparation methods. Then, we focused on the applications of nanomaterials in biomedicine, including biological detection, tumor diagnosis, and disease treatment, which provide a development trend and promising prospects for nanomaterials.

Size-dependent sidewall defect effect of GaN blue micro-LEDs by photoluminescence and fluorescence lifetime imaging

Sidewall defects play a key role in determining the efficiency of GaN-based micro-light emitting diodes (LEDs) for next generation display applications, but there still lacks direct observation of defects-related recombination at the affected area. In this Letter, we proposed a direct technique to investigate the recombination mechanism and size effect of sidewall defects for GaN blue micro-LEDs. The results show that mesa etching will produce stress release near the sidewall, which can reduce the quantum confinement Stark effect (QCSE) to improve the radiative recombination. Meanwhile, the defect-related non-radiative recombination generated by the sidewall defects plays a leading role under low-power injection. In addition, the effective area of the mesas affected by the sidewall defects can be directly observed according to the fluorescence lifetime imaging microscope (FLIM) characterization. For example, the effective area of the mesa with 80 µm is affected by 23% while the entire area of the mesa with 10 µm is almost all affected. This study provides guidance for the analysis and repair of sidewall defects to improve the quantum efficiency of micro-LEDs display at low current density.

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.

Ultra-low-current driven InGaN blue micro light-emitting diodes for electrically efficient and self-heating relaxed microdisplay

InGaN-based micro-light-emitting diodes have a strong potential as a crucial building block for next-generation displays. However, small-size pixels suffer from efficiency degradations, which increase the power consumption of the display. We demonstrate strategies for epitaxial structure engineering carefully considering the quantum barrier layer and electron blocking layer to alleviate efficiency degradations in low current injection regime by reducing the lateral diffusion of injected carriers via reducing the tunneling rate of electrons through the barrier layer and balanced carrier injection. As a result, the fabricated micro-light-emitting diodes show a high external quantum efficiency of 3.00% at 0.1 A/cm² for the pixel size of 10 × 10 μm² and a negligible J_{max EQE} shift during size reduction, which is challenging due to the non-radiative recombination at the sidewall. Furthermore, we verify that our epitaxy strategies can result in the relaxation of self-heating of the micro-light-emitting diodes, where the average pixel temperature was effectively reduced.

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.

Comprehensive Investigation of Electrical and Optical Characteristics of InGaN-Based Flip-Chip Micro-Light-Emitting Diodes

Micro-light-emitting diodes (micro-LEDs) have been regarded as the important next-generation display technology, and a comprehensive and reliable modeling method for the design and optimization of characteristics of the micro-LED is of great use. In this work, by integrating the electrical simulation with the optical simulation, we conduct comprehensive simulation studies on electrical and optical/emission properties of real InGaN-based flip-chip micro-LED devices. The integrated simulation adopting the output of the electrical simulation (e.g., the non-uniform spontaneous emission distribution) as the input of the optical simulation (e.g., the emission source distribution) can provide more comprehensive and detailed characteristics and mechanisms of the micro-LED operation than the simulation by simply assuming a simple uniform emission source distribution. The simulated electrical and emission properties of the micro-LED were well corroborated by the measured properties, validating the effectiveness of the simulation. The reliable and practical modeling/simulation methodology reported here shall be useful to thoroughly investigate the physical mechanisms and operation of micro-LED devices.

Optical-electrical characteristic of green based on GaN micro-LED arrays

A detailed theoretical derivation and calculation method of the difference coefficient between a light distribution pattern of a 30×20µm² green micro-LED array and Lambert source is proposed first in this paper, to the best of our knowledge, which establishes an accurate relationship between external quantum efficiency and current efficiency (cd/A). The variation of capacitance with voltage and wavelength blueshift is illustrated by a carrier recombination mechanism. The current efficiency reaches 132.5 cd/A for the 60×50µm² and 121.7 cd/A for the 25×15µm² arrays, and the mechanism caused by size dependence is analyzed in detail combined with the classical ABC model.

Study of the Factors Limiting the Efficiency of Vertical-Type Nitride- and AlInGaP-Based Quantum-Well Micro-LEDs

The efficiency of micro-light-emitting diodes (μ-LEDs) depends enormously on the chip size, and this is connected to sidewall-trap-assisted nonradiative recombination. It is known that the internal quantum efficiency (IQE) of aluminum gallium indium phosphide (AlGaInP)-based red μ-LEDs is much lower than that of nitride-based μ-LEDs. To establish the major reasons giving rise to this huge IQE discrepancy, we examined the limiting factors in the two structures. For the nitride-based InGaN quantum wells, the influences of random alloy fluctuations were examined. A two-dimensional Poisson and drift-diffusion solver was applied to analyze these issues.

Tailoring the light distribution of micro-LED displays with a compact compound parabolic concentrator and an engineered diffusor

We propose a novel optical design to tailor the angular distribution of a micro-LED (µLED) display system and use vehicle display as an example to illustrate the design principles. The display system consists of a µLED array with a tailored LED structure, a small formfactor compound parabolic concentrator (CPC) system, and a functional engineered diffusor. It provides high efficiency, high peak brightness, and small formfactor. In the design process, a mix-level optical simulation model, including the angular distribution of polarized emission dipole (dipole emission characteristics), Fabry-Perot cavity effect (wave optics), and light propagation process (ray optics), is established to analyze the angular distribution of µLEDs. Such an optical design process from dipole emission to display radiation pattern can be extended to other µLED display systems for different applications.