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Growth, transfer printing and colour conversion techniques towards full-colour micro-LED display
Abstract
Micro light-emitting diode (micro-LED) display, mainly based on inorganic GaN-based LED, is an emerging technique with high contrast, low power consumption, long lifetime and fast response time compared to liquid crystal display (LCD) and organic light-emitting diode (OLED) display. Therefore, many research institutes and companies have conducted in-depth research on micro-LED in the full-colour display, gradually realizing the commercialization of micro-LED. And the current research results of micro-LED indicate that it can be widely used in display, visible light communication (VLC), biomedicine and other fields. Although micro-LED has broad commercial prospects, it still faces great challenges, such as the effect of size reduction on performance, the realization of high-density integration on a single wafer for independent addressing of full-colour micro-LED display, the improvement of repair technique and yield, et al. This paper reviews the key solutions to the technical difficulties of the full-colour micro-LED display. Specifically, this review analyzes and discusses a variety of advanced full-colour micro-LED display techniques with a focus on three aspects: growth technique, transfer printing technique and colour conversion technique. This review demonstrates the opportunities, progress and challenges of these techniques, aiming to guide the development of full-colour micro-LED display.
... Micro LEDs are a promising candidate for next-generation display technology, offering better contrast and saturation than a liquid crystal display panel [1]. Compared to organic light-emitting diodes (OLEDs), III-nitride-based micro LEDs exhibit higher resolution and reliability [2][3][4]. ...
... The convolution step size is set as the micro LED data period. After the convolutional calculation, the Image[x, y] can be simplified as a matrix Conv [i, j] in Equation (2). ...
... The calculated light power distribution data from a 3 × 3 micro LED array ( Figure 8) are processed using Equation (2), and the Conv[i, j] is derived. Then, the F C_T are used to find the cross-correlation between the central LED [1,1] and the other eight surrounding LEDs. ...
Monolithic micro LED display arrays show potential for application in small-area display modules, such as augmented reality (AR) displays. Due to the short distance between micro LEDs and the monolithic transparent substrate, a light crosstalk phenomenon exists between adjacent micro LED pixels, decreasing the array’s display definition. In this paper, a 64 × 64 GaN micro LED monolithic display array was fabricated on a silicon-based drive circuit. The micro LED size was 20 μm × 20 μm, and the pitch between micro LEDs was 28 μm. To suppress the optical crosstalk between adjacent micro LEDs in the array, we etched a photonic crystal structure using a focused ion beam (FIB) on the micro LED sapphire substrate. Measurements of the micro LED nearfield electroluminescence (EL) and finite element method (FEM) calculations demonstrated that the light expansion was confined in the photonic crystal micro LED with a thinner substrate. The presented work provides references regarding the fabrication of monolithic micro LED arrays and the control of crosstalk in displays.
... [1][2][3][4][5] It has various advantages over traditional technologies such as Liquid Crystal Display (LCD) and Organic Light-Emitting Diode (OLED). [6][7][8] The rationale behind the miniaturization of LEDs lies in the need for better performance, efficiency, and versatility in modern electronic devices and displays. By reducing the size of LEDs to Micro-LEDs, manufacturers can achieve significantly higher resolution and pixel density, which are crucial for applications such as ultra-high-definition TVs, medical imaging, virtual reality (VR), and augmented reality (AR). ...
... Numerous review articles have been published on Micro-LEDs, primarily focusing on recent advancements in fabrication and integration. [4][5][6][11][12][13] Nevertheless, a gap exists in the scientific literature concerning a comprehensive review that summarizes the recent strategies employed to increase optoelectronic performance in this field. In this paper, our objective is to address this gap by providing the reader with advanced solutions to enhance performance. ...
III‐V semiconductors, known for their optoelectronic properties and versatile engineering capabilities, play a crucial role in the fabrication of Micro light‐emitting diodes (Micro‐LEDs). Recent advances in research underscore that the optoelectronic performance of Micro‐LEDs can be significantly enhanced using various strategies, such as passivation and distributed Bragg reflectors (DBRs), the incorporation of metamaterials and plasmonics, and the integration of 2D materials. By implementing these diverse integration strategies, Micro‐LEDs based on III‐V semiconductors have demonstrated remarkably high External Quantum Efficiency (EQE) spanning orders of magnitude across the spectrum, from deep‐ultraviolet (DUV) to the long‐wavelength infrared (LWIR) regions. In this review, the main III‐V semiconductors used in Micro‐LEDs are discussed. Additionally, an overview of the fabrication processes and integration techniques relevant to Micro‐LED‐based technologies is provided. Furthermore, the factors that influence the figure of merit in a wide range of Micro‐LEDs based on III‐V semiconductors, taking into account quantum efficiency, emission wavelength, and electrical injection, are examined. Finally, the discussion highlights several applications of Micro‐LEDs, provides a summary, and outlines future directions for the development of Micro‐LEDs based on III‐V semiconductors.
... Recognized for these exceptional performance characteristics, μLEDs are considered at the forefront of next-generation display technology. They find applications across various domains, from wearable devices like wristbands and watches to large-scale commercial billboards, public displays, and immersive technologies such as virtual reality (VR) or augmented reality (AR) devices [5][6][7]. However, as device dimensions shrink to the microscale, the influence of sidewall effects and non-radiative recombination becomes increasingly pronounced [8,9]. ...
... As carriers approach the sidewall surfaces, they experience surface recombination and scattering, leading to non-uniform carrier distribution and reduced injection efficiency. Moreover, sidewall roughness and defects can trap carriers, increasing the likelihood of non-radiative recombination and reducing light output [7,8,13]. Experimental studies have revealed the detrimental impact of sidewall effects on device performance, highlighting the need for strategies to minimize sidewall-related losses. ...
This study fabricated 10 μm chip size μLEDs of blue-light GaN based epilayers structure with different mesa processes using dry etching and ion implantation technology. Two ion sources, As and Ar, were applied to implant into the LED structure to achieve material isolation and avoid defects on the mesa sidewall caused by the plasma process. Excellent turn-on behavior was obtained in both ion-implanted samples, which also exhibited lower leakage current compared to the sample fabricated by the dry etching process. Additionally, lower dynamic resistance (Rd) and series resistance (Rs) were obtained with Ar implantation, leading to a better wall-plug efficiency of 10.66% in this sample. Consequently, outstanding external quantum efficiency (EQE) values were also present in both implant samples, particularly in the sample implanted with Ar ions. This study proves that reducing defects on the mesa sidewall can further enhance device properties by suppressing non-radiative recombination behavior in small chip size devices. Overall, if implantation is used to replace the traditional dry etching process for mesa fabrication, the ideality factor can decrease from 11.89 to 2.2, and EQE can improve from 8.67 to 11.03%.
... The evolution of display technology has undergone transitions from cathode ray tubes (CRT) to liquid crystal displays (LCD) and eventually to organic light-emitting diodes (OLED), Mini LEDs, and Micro LEDs [1][2][3][4][5][6][7][8][9][10] . The need for improved display quality, energy efficiency, and innovation in visual technologies has driven this progression. ...
... The inherent trade-off between transfer speed and precision becomes increasingly pronounced, high-speed transfer often results in reduced accuracy, whereas high-precision transfer tends to comprise transfer efficiency. This issue has led to challenges in terms of precision, yield rates, and overall manufacturing costs, although ongoing research and development efforts aim to address these challenges brought forth by the relentless miniaturization of LEDs and unlock the potential of smaller LED technology, especially [1,[11][12][13][14][15][16][17][18][19][20][21] . ...
... 7 So far, Micro-LEDs have shown extensive application potential in areas such as augmented reality (AR)/virtual reality (VR) display, 8 high-definition televisions, 9 projectors, 10 smart devices, 11 etc. 12 Micro-LED commercialization faces technological obstacles and high production expenses despite its potential for various applications. [11][12][13][14] The mass transfer process requires transferring a large number of Micro-LED chips from the native substrate to the target substrate, making it difficult to achieve high yield, high precision, and high efficiency. Mass transfer is a crucial step in Micro-LED manufacturing and one of the biggest obstacles limiting their further development. ...
... The mass transfer includes accurately and efficiently releasing millions or tens of millions of Micro-LED chips from the native substrate and parallel transfer to the target substrate. 14 This process needs complex transfer and bonding methods. To address the challenges of mass transfer in the fabrication of Micro-LEDs, diverse methods have been proposed, 11 such as electrostatic transfer, 15 stamp transfer, 16,17 fluid self-assembly transfer, 18 roll-to-roll transfer, 19 and laser transfer. ...
Micro light-emitting diode (Micro-LED) is a highly promising technology in the field of new displays, with the mass transfer process involved in its manufacturing process widely regarded as a major barrier to their further development. This study adopts laser transfer technology as the primary solution, using ablation-type transfer release materials that improve chip utilization rates over blister-type release materials. In addition, measurement of the laser transfer parameters, inspection, and laser repair technology are combined to achieve a transfer yield of about 100% to the carrier substrate and a cumulative transfer displacement of less than 1 µm in the Micro-LED inverted chip array. Furthermore, cleaning agents were used to remove adhesive residue from the receiving substrate after transfer, improving the bonding yield between the chip and the thin film transistor driver circuit board. This study provides a feasible solution for Micro-LED mass transfer, which could boost its further development toward commercial application.
... Naturally, smaller pixel sizes correspond to higher resolutions [4]. Micro light-emitting diodes (Micro-LEDs) have gained significant attention due to their exceptional features, including high brightness, high resolution, high contrast, long life time, and low power consumption, and they hold a significant edge in the competition among display technologies [5][6][7]. ...
... In modern society, individuals no longer find contentment in the quest for a solitary color display but rather exhibit a greater inclination towards full color displays [5,6]. Red Micro-LED has gained significant attention in recent years as an essential component of full color displays [7][8][9]. ...
In this paper, an AlGaInP-based red Micro-LED display measured 17.78 mm (0.7 in), with a resolution of 1920 × 1080, a light-emitting mesa size of 6 μm, a pixel pitch of 8 μm and a pixel density of 3175 PPI was designed and fabricated with a CMOS driver backplane. The metal bump preparation technology of the complementary metal-oxide semiconductor driver backplane was optimized to enhance the bonding yield and create an optimal display effect. Improper sizing of the etched window in the SiO2 insulation and passivation layer can have a detrimental impact on the metal bump preparation and subsequent bonding process. By optimizing the settings of lithography and dry etching, the appropriate size of the etched aperture in the passivation layer was achieved. The high density, small size, and large aspect ratio of the photoresist openings for the bump fabrication made it challenging to remove the photoresist following the metal evaporation. To successfully remove the photoresist, it is important to carefully choose suitable experimental conditions for the removal. Afterwards, an 8 μm AlGaInP-based red Micro-LED display was effectively integrated with complementary metal-oxide semiconductor using flip-chip bonding technology. This work may be of reference value to those who work on ultrahigh density red Micro-LEDs that is challenging but crucial for future full color micro displays.
... MLO involves physically detaching the LED layers, potentially offering faster processing but necessitating careful control to avoid damaging the delicate structures. The transfer stage integrates the lifted micro-LEDs onto flexible substrates using methods like pick-and-place, transfer printing, roll-to-roll, or self-assembly [41][42][43][44]. Pick-and-place offers high precision but can be e-skins (ix), and epidermal micro-LED arrays (x), highlighting the adaptability of this technology. ...
Flexible micro light-emitting diodes (micro-LEDs) have garnered significant attention due to their exceptional properties, including high luminance, energy efficiency, and mechanical robustness, positioning them as a promising technology for next-generation displays and electronic devices. As the Internet of Things (IoT) paradigm advances, the demand for portable and adaptable devices has led to an acceleration in flexible micro-LED research. This review comprehensively examines advanced fabrication techniques for flexible micro-LEDs, encompassing epitaxial growth, various lift-off processes, and mass transfer strategies. These methods are systematically integrated to optimize device performance and scalability. Furthermore, it explores diverse applications of flexible micro-LEDs, ranging from flexible displays and biomedical sensors to IoT and smart devices. These applications harness the unique properties of flexible micro-LEDs, enabling their integration into various form factors and opening up new possibilities for user interfaces and information displays. This work emphasizes the transformative role of flexible micro-LEDs in driving innovations across multiple fields, paving the way for the next generation of flexible and intelligent technologies.
Graphical Abstract
... The fabrication of μLEDs typically requires high-temperature epitaxial growth on rigid substrates, necessitating a subsequent transfer process to flexible and stretchable substrates for integration into soft optoelectronic systems. [73,74] However, this transfer process is constrained by a critical trade-off between yield and precision, significantly limiting the scalability and reliability of μLED-based applications, such as phototherapy. ...
Phototherapy based on micro light‐emitting diodes (µLEDs) has gained enormous attention in the medical field as a patient‐friendly therapeutic method due to its advantages of minimal invasiveness, fewer side effects, and versatile device form factors with high stability in biological environment. Effective cosmetic and medical phototherapy depends on deep light penetration, precise irradiation, and simultaneous multi‐site stimulation, facilitated by three‐dimensional (3D) optoelectronics specifically designed for complex human matters, defined here as 3D µLEDs. This perspective article aims to present the functionalities and strategies of 3D µLEDs for human‐centric phototherapy. This study investigates the effectiveness of phototherapy enabled by three key functionalities such as shape morphing, self‐adaptation, and multilayered spatiotemporal mapping of 3D µLEDs. Finally, this article provides future insights of 3D µLEDs for human‐centric phototherapy applications.
... Recently, blue light leakage was effectively suppressed by embedding QDs into nanoporous GaN, [15][16][17] and adding a color filter (CF) or a distributed Bragg reflector (DBR) to the device to absorb or reflect the residual blue light that cannot be absorbed by the QDs. [14,[18][19][20][21][22] However, these approaches inevitably result in increased power consumption, a reduced viewing angle, and increased surface temperature of micro-LED displays. Furthermore, for conventional QD displays, the QD CCL is sandwiched between two water-oxygen barrier layers with thicknesses of up to 260 μm to improve the reliability of the displays. ...
Quantum dot (QD)‐converted micrometer‐scale light‐emitting diodes (micro‐LEDs) are regarded as an effective solution for achieving high‐performance full‐color micro‐LED displays because of their narrow‐band emission, simplified mass transfer, facile drive circuits, and low cost. However, these micro‐LEDs suffer from significant blue light leakage and unsatisfactory electroluminescence properties due to the poor light conversion efficiency and stability of the QDs. Herein, the construction of green and red QD luminescence microspheres with the simultaneously high conversion efficiency of blue light and strong photoluminescence stability are proposed. These luminescence microspheres exhibit high external photoluminescence quantum yields exceeding 46% under 450 nm excitation, along with excellent reliability against blue light, heat, and water‐oxygen degradation, owing to the waveguide and spatial confinement effects of the microspheres. The microsphere‐based green and red micro‐LEDs achieve world‐record external quantum efficiencies of 40.8% and 22.1%, respectively, and high brightness values of 1.7 × 10⁸ and 7.6 × 10⁷ cd m⁻², respectively. Finally, 0.6 inch red, green, and blue monochrome micro‐LED displays are demonstrated by integrating microsphere‐converted micro‐LED arrays with thin‐film transistor backplanes, which show a pixel resolution as high as 1700 PPI and brightness exceeding 10 000 cd m⁻².
... Micro-displays that use μLEDs are considered the nextgeneration display [1][2][3][4][5][6] for mobile, virtual, and augment reality devices, and large-size displays with >4 K resolution characterized by high image quality, energy efficiency, and brightness. [7][8][9][10] K. Hwang A key technology to manufacture μLED display is the mass transfer of μLED chips at large scale with high accuracy, alignment speed, and minimal need for midproduction repair. ...
Recent advances in mass transfer technology are expected to bring next‐generation micro light‐emitting diodes (µLED) displays into reality, although reliable integration of the active‐matrix backplane with the transferred µLEDs remains as a challenge. Here, the µLED display technology is innovated by demonstrating pixel circuit‐integrated micro‐LEDs (PIMLEDs) and integrating them onto a transparent glass substrate. The PIMLED comprises of low‐temperature poly‐silicon transistors and GaN µLED. The square‐shaped PIMLED is designed to secure a larger process margin but to reduce misalignment in the subsequent bonding process. Its unique four‐fold rotational symmetric design together with concentric circular pixel electrodes of the substrate makes their massive transfer compatible with intrinsic randomness of fluidic‐based transfer and free from angular misalignment during wafer bonding. The vertically integrated pixels show similar optical and electrical properties regardless of four possible arrangements. It is demonstrated that a 96 × 96 PIMLED display on a transparent glass substrate, which is expected to open a door to novel form‐factor free and transparent µLED displays.
... Owing to the fact that GaN material is a wide-bandgap semiconductor with good chemical stability and high electron mobility, its emission wavelength can be continuously adjusted from the ultraviolet to the visible light range, these make it widely used in fields such as full-color display, and optical communication 1-3 . GaN-based Micro-LED, as an extension of traditional GaN-based LED technology, possesses the inherent advantages of GaN-based LEDs and are solid-state light sources with high contrast, long lifespan, and high integration density [4][5][6][7][8][9][10][11][12] . With the rapid development of GaN-based Micro-LEDs and their increasing use high-resolution display, AR/VR, and medical fields, there is a growing demand for Micro-LED devices with high brightness and narrower linewidth [13][14][15][16][17][18][19][20] . ...
High-efficiency micro-light-emitting diodes (Micro-LEDs) are key devices for next-generation display technology. However, when the mesa size is reduced to around tens of micrometers or less, the luminous efficiency is constrained by the "efficiency-on-size effect". This work details the fabrication of gallium nitride (GaN) based Micro-LEDs with various mesa shapes and a single porous layer under the active region. A modified green LED epitaxial structure with different doped n-GaN layers combined with electrochemical etching created the porous layer. The strong light confinement achieved by the porous layer and the polygonal mesa greatly enhances spontaneous emission. The luminous intensity of the Micro-LEDs with the porous layer is approximately 22 times greater than those Micro-LEDs without the porous layer. A significant reduction in minimum full width at half maximum (FWHM) was observed in polygonal devices, suggesting a change in the luminescence mechanism. The influence of varying device geometry on emission performance was investigated. Experimental results reveal that, unlike circular porous Micro-LEDs, square and hexagonal porous Micro-LEDs exhibit more pronounced resonant emission, which provides a new technological approach for the further development of high-performance Micro-LEDs and lasers.
... Microdisplays are expected to be initially implemented in virtual reality, augmented reality, and smartwatches. However, significant challenges remain in the fabrication of micro-LED microdisplays [4,5]. For example, sidewall damage can increase non-radiative carrier recombination, resulting in a reduction in the EQE. ...
In this study, we designed and fabricated parallel-connected green micro-LEDs with three different P-electrode configurations: rounded (Sample A), cross-shaped (Sample B), and circular (Sample C). We then systematically evaluated the impact of these electrode shapes on the devices’ optoelectronic performance. The results show that the shape of the P-electrode significantly influences the optoelectronic performance of micro-LEDs. With a round mesa, Sample C exhibits the lowest operating voltage and the smallest dynamic resistance and achieves a peak external quantum efficiency (EQE) of 19.57%, which is 25.53% and 11.13% higher than that of Sample A (15.59%) and Sample B (17.61%), respectively. The analysis suggests that this improvement is mainly due to enhanced uniformity in current spreading and shorter current injection paths. COMSOL simulations, along with thermal resistance and surface temperature measurements, confirm that different P-electrode shapes affect the uniformity of current distribution in micro-LEDs, which in turn impacts the device’s thermal performance. TracePro simulation results further demonstrated that circular P-electrodes optimize the light output of the device. We believe that this study provides a valuable reference for the design and fabrication of micro-LED chips.
... During the past years, researchers have dedicated tremendous effort to the hybrid integration and mass transfer technologies, as can also be seen by the high number of publications dedicated to this topic. 24,[44][45][46][47][48][49][50] While the industrial race for cost-effective commercial production of microLED displays is still ongoing, it is worth to take a closer look at some of the most promising transfer technologies described in the literature. ...
MicroLEDs, particularly when integrated with CMOS microelectronics, represent a significant advancement in nitride technology. While large-area, high-power LEDs for solid-state lighting have seen extensive optimization, microLEDs present unique fabrication and characterization challenges. Utilizing standard CMOS design and foundry services for silicon driver electronics, a new hybrid interconnect technology must be developed for chip–chip or wafer–wafer integration, necessitating much higher lateral resolution than current bonding technologies. Beyond display technology, microLED integration opens avenues for groundbreaking applications such as highly efficient nanosensors, miniaturized optical neuromorphic networks, and robust chip-based microscopy. This paper explores recent advancements in nitride/CMOS hybrid modules, providing an overview of current technologies and future possibilities in this dynamic field.
... Micro light-emitting diode (micro-LED) display technology has attracted widespread attention in emerging display applications such as augmented reality, virtual reality, flexible displays, and ultra-large TVs [1]. However, micro-LEDs still face the challenge of size-dependent external quantum efficiency (EQE) reduction. ...
Red–green–blue (RGB) micro light-emitting diodes (micro-LEDs) without distributed Bragg reflector (DBR), with air-separating DBR, and with integrated DBR, were demonstrated. The effect of the DBRs as reflectors on the external quantum efficiency (EQE) and electroluminescence spectra enhancement of RGB micro-LEDs was systematically investigated for realizing higher-performance micro-LEDs for display applications. At 5 A cm⁻², the EQEs of the RGB micro-LEDs with integrated DBR were improved by 38%, 33%, and 32%, respectively, with comparison to the RGB DBR free micro-LEDs. Further, the full width at half maximum (FWHM) of the red micro-LEDs was reduced by 4.3 nm at 50 A cm⁻² with the integrated DBR due to the higher enhancement of the central wavelength spectrum. The green and blue micro-LEDs with integrated DBR had higher EQE and the red micro-LEDs with integrated DBR had narrower FWHM compared to those with air-separating DBR. However, the peak wavelength of the RGB micro-LEDs with integrated DBR shifted, resulting in a lower color gamut in CIE 1931. The above work provides guidance for future full-color micro-display applications based on RGB InGaN micro-LED technology.
... The widespread availability of blue LEDs has significantly expanded the potential applications of µLEDs in the field of optogenetics. µLEDs possess inherent characteristics such as compact size and low power consumption [67]. These features enable their integration with flexible materials and direct placement in proximity to neurons requiring stimulation, thereby greatly enhancing stimulation-targeting precision. ...
The brain–computer interface (BCI) is one of the most powerful tools in neuroscience and generally includes a recording system, a processor system, and a stimulation system. Optogenetics has the advantages of bidirectional regulation, high spatiotemporal resolution, and cell-specific regulation, which expands the application scenarios of BCIs. In recent years, optogenetic BCIs have become widely used in the lab with the development of materials and software. The systems were designed to be more integrated, lightweight, biocompatible, and power efficient, as were the wireless transmission and chip-level embedded BCIs. The software is also constantly improving, with better real-time performance and accuracy and lower power consumption. On the other hand, as a cutting-edge technology spanning multidisciplinary fields including molecular biology, neuroscience, material engineering, and information processing, optogenetic BCIs have great application potential in neural decoding, enhancing brain function, and treating neural diseases. Here, we review the development and application of optogenetic BCIs. In the future, combined with other functional imaging techniques such as near-infrared spectroscopy (fNIRS) and functional magnetic resonance imaging (fMRI), optogenetic BCIs can modulate the function of specific circuits, facilitate neurological rehabilitation, assist perception, establish a brain-to-brain interface, and be applied in wider application scenarios.
... In recent years, with the rapid development of wireless communication and big data technology, the demand for data transfer rates and communication capacity has been increasing [1][2][3]. GaNbased Micro-LEDs, due to their inherent advantages such as high efficiency, high modulation bandwidth, brightness, and stability, have gained widespread attention in various fields, including high-resolution displays [4], visible light communication (VLC) [5], and biomedical imaging [6]. Modulation bandwidth is a critical performance parameter determining the device's response speed and information transfer quality [7]. ...
GaN-based micro-LEDs are applied to visible light communication due to their high modulation bandwidth with reduced chip size. It requires a deep understanding of recombination processes and their impact on the bandwidth, which is mainly determined by the carrier lifetime. We employed confocal time-resolved photoluminescence (TRPL) to characterize the variation of carrier lifetime with optical excitation power density on micro-LEDs. We observed an initial increase followed by a sudden decrease within the power density range of 96.7 kW/cm² to 546 kW/cm² on a blue micro-LED with a chip size of 80 µm. We attribute this phenomenon to increased optical excitation power, gradually saturating the defect-dominated non-radiative recombination centers, with radiative recombination processes gradually taking over. We compared the power density at the inflection point for different regions on the sample and the samples with different sizes and sidewall structures. The power density for the lifetime inflection point at the center of the sample is smaller than that at the edge. We also find that the value is smaller for the sample with a chip size of 40 µm which prompts fewer total defects. The power density for sudden lifetime drop on samples with inclined sidewall structures is also smaller than those with vertical sidewall structures. Furthermore, we find that the excitation power density corresponding to the highest luminous efficiency is higher than that corresponding to the sudden drop at the beginning of the lifetime. This opens up possibilities for simultaneously achieving high modulation bandwidth and high efficiency between the two inflection points.
... It has enabled light-emitting diodes (LEDs) with high emission efficiency and long operational lifetime. [1][2][3][4] With the development in the porosification of epitaxial GaN via dopingselective electrochemical etching, [5,6] porous GaN offers an opportunity to produce composite materials through the infiltration of pores and thus broaden its application in numerous electronic and optoelectronic devices, [7][8][9][10][11][12] including color-DOI: 10.1002/adom.202400221 converting micro-light emitting diodes (Micro-LEDs). ...
Blue gallium nitride (GaN) light‐emitting diodes (LEDs), combined with red/green fluorescent converters, have broad potential for display applications. Metal halide perovskites now show excellent luminescence properties and may be suitable as light converters. Here a simple solution‐processed method is reported to prepare a methylammonium lead bromide (MAPbBr3) nanoporous GaN composite. Fast (within 2 ps) energy transfer is demonstrated from photoexcited nanoporous GaN to encapsulate MAPbBr3 nanocrystals, as observed by transient absorption spectroscopy. The spatial confinement of the perovskite within the nanoporous GaN is shown to increase the perovskite radiative recombination rate. These results offer guidelines for developing high‐performance perovskite/nanoporous GaN optoelectronics.
... Not only the high quality epilayer, the complex fabrication process for high-resolution displays, including thin film deposition, etching, mass-transfer, and circuit bonding processes, also exists the challenges [17,18]. Moreover, downsizing pixel sizes reduces the efficiency of micro-LEDs due to damage to the mesa sidewalls induced by non-radiative recombination centers caused by the dry etch process, particularly using inductively coupled plasma reactive-ion etching (ICP-RIE) [19][20][21]. ...
In this study, a 3 × 3 blue micro-LED array with a pixel size of 10 × 10 μm ² 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%.
... The devices are bonded either directly via van der Waals forces or with a thin adhesive layer to the new substrate by slowly peeling the PDMS stamp away from the printed area releasing the device. µTP has been successfully demonstrated for devices such as III-V lasers and semiconductor optical amplifiers (SOA) [18,19], µLEDs [20,21], and photovoltaic cells [22] onto Si platforms. Recently, QD lasers have been integrated with Si waveguides achieving 7 mW of coupled power at 250 mA under pulsed operation [23]. ...
We integrate edge-emitting etched-facet InAs/GaAs quantum dot (QD) lasers to an AlGaN/GaN-on-sapphire waveguide platform via micro-transfer printing. The lasers are placed into a trench etched into the sapphire substrate so as to transversely align the waveguides. The AlGaN/GaN waveguide structure is designed to allow for tolerant alignment of the active lasing mode to the passive waveguide mode. 4 μm wide waveguides show a TE propagation loss of 4.5 dB/cm. For a QD laser with a 1.2 mm long cavity and 3.5 μm wide ridge waveguide, 2.3 mW was measured at 80 mA under continuous wave conditions from the end of a 0.64 cm long 4 μm wide waveguide with an estimated coupling efficiency of >20 %. The measured coupled output power is 1.5 mW at 70°C and 100 mA. To our knowledge, this is the first demonstration of the heterogeneous integration of a GaAs lasing device to a GaN-based PIC by any means. The results show real promise for GaN to be a suitable platform for integrated photonics applications requiring O-band operation.
... Lately, micro-LEDs (light-emitting diodes) have found widespread applications in modular large-screen TVs [1], automotive displays [2], and augmented reality (AR) glasses [3] due to their small mesa size, ultra-high brightness, excellent thermal stability, perfect dark state, and long lifetime [4][5][6][7][8]. Still, the requirement to produce full-color ultra-small µLED chips with high efficiency is demanding [9,10]. For transparent head-up displays in vehicles, µLEDs with a mesa size smaller than 20 µm and luminance over 3000 nits are required to maintain a high ambient contrast ratio under direct sunlight [11]. ...
Micro-LEDs have found widespread applications in modular large-screen TVs, automotive displays, and high-resolution-density augmented reality glasses. However, these micron-sized LEDs experience a significant efficiency reduction due to the defects originating from the dry etching process. By controlling the current distribution via engineering the electrode size, electrons will be less concentrated in the defect region. In this work, we propose a blue InGaN/GaN compound parabolic concentrator micro-LED with a metallic sidewall to boost efficiency by combining both an optical dipole cloud model and electrical TCAD (Technology Computer-Aided Design) model. By merely modifying the p-GaN contact size, the external quantum efficiency (EQE) can be improved by 15.6%. By further optimizing the passivation layer thickness, the EQE can be boosted by 52.1%, which helps enhance the display brightness or lower power consumption.
... However, the maximum output value achievable by µLED display technology is far beyond this, and there are many factors constraining its rapid and large-scale application in the market. Factors such as epitaxial transfer [5][6][7], defect management [8,9], and bonding technology [10,11] leading to high manufacturing costs are the main reasons hindering its commercialization. Among them, the low external quantum efficiency (EQE) of µLEDs [12][13][14] and achieving full-color display [15][16][17][18] are among the main factors affecting its commercialization. ...
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.
Full-colour tuning in rare-earth doped monolithic tellurite glass is realised via excitation pulse modulation, enabling a novel platform for laser-based transparent displays. This advancement demonstrates the potential of upconversion emission for future display technologies.
The laser-assisted micro-transfer printing (Laser-assisted
TP) technology is considered a highly promising solution for the mass transfer of micro-LEDs, which offers a series of benefits, including high transfer accuracy, fast transfer speed, and selective transfer capability, but it also presents a risk of micro-LED damage due to laser exposure. To solve this issue, this article proposes an improvement to the Laser-assisted
TP by introducing a novel carbon particle-doped polydimethylsiloxane (PDMS) stamp (CPD stamp). The carbon particles doped in the stamp enhance the laser absorption capability of the stamp, thereby reducing chip damage. At the same time, the pristine adhesion strength of CPD stamp declines by 98.35% attributed to the design of surface microstructure, accelerating chip ejection and improving the transfer speed. After optimization, the optimal CPD stamp demonstrates the potential for high-speed transfer, as it is demonstrated that the detachment of the chips from the optimal CPD stamp occurs within 117 ms. Meanwhile, the micro-LEDs are successfully transferred by optimal CPD stamp without degradation. In conclusion, the CPD stamp proposed in this article provides an effective solution to the drawbacks of Laser-assisted
TP, potentially advancing the commercialization process of this technology
Achieving full‐color emission with just two emitters presents a significant challenge. Two N‐heterocyclic carbene metallacycles (NHC‐M; M ═ Ag, Au) featuring a tetraphenylethene core, combining covalent and coordination bonds are synthesized to restrict rotation within the NHC‐M‐BC (BC = bicyclic) metallacycles and greatly enhanced their quantum yields. The enhancement is accomplished by adjusting the CH3CN/H2O solvent mixture, allowing emission tuning from blue to green for NHC‐Ag‐BC. Further diversification of the emission spectrum, including access to high‐quality white light (CIE coordinates 0.33, 0.34), is facilitated through the addition of sulforhodamine B via fluorescence resonance energy transfer (FRET). The compatibility of NHC‐Ag‐BC with agarose gel extends its applicability to UV‐LEDs chromic coatings, as well as information encryption and anti‐counterfeiting materials. The results underscore the viability of dual‐fluorophore systems for achieving full‐color emission and highlight the potential for developing versatile, multi‐colored functional materials.
Active wavelength‐scale optoelectronic components are widely used in photonic integrated circuitry, however coherent sources of light – namely optical lasers – remain the most challenging component to integrate. Semiconductor nanowire lasers (NWLs) represent a flexible class of light source where each nanowire (NW) is both gain material and cavity; however, strong coupling between these properties and the performance leads to inhomogeneity across the population. While this has been studied and optimized for individual material systems, no architecture‐wide insight is available. Here, nine NWL material systems are studied and compared using 55,516 NWLs to provide statistically robust insight into performance. These results demonstrate that, while it may be important to optimize internal quantum efficiency for certain materials, cavity effects are always critical. The study provides a roadmap to optimize the performance of NWLs made from any material: this can be achieved by ensuring a narrow spread of lengths and end‐facet reflectivities.
In this research BaAl2O4: Eu2+/Li+ luminescent materials were synthesized through a facile solid-state approach at 1100 °C. To study the effect of Li+ ions, different amounts of LiCl were used within the synthesis of BaAl2O4-based compounds. The crystal structural, microstructure, surface characteristics, and photoluminescence properties were scrutinized by X-ray diffraction (XRD), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), photoluminescence (PL), Fourier transform infrared (FTIR) analyses. It was shown that the addition of LiCl flux material results in very slight changes in the crystallographic properties of BaAl2O4 crystal structure while the use of LiCl has severely reduced the average particle size from about 225 nm to 120 nm. Upon the excitation procedure at the wavelength of 343 nm, BaAl2O4: Eu2+ shows a strong and wide emission in the range of 435–610 nm that is attributed to the 4f65d1-4f7 transition of Eu2+. The highest emission occurs when 2.5 wt% LiCl was utilized as flux material. Moreover, through the employment of XPS characterization, it was proved that the addition of Li+ ions causes higher binding energies of elements and improvement of crystallization.
The micro light-emitting diodes (
LEDs) offer advantages that make it an attractive option for various applications, such as displays, lighting, AR/VR, and consumer electronics. The light output power (LOP) and external quantum efficiency (EQE) of
LEDs are the crucial parameters that impact the performance and suitability of these devices. Ongoing studies are focused on addressing the issue of
LEDs. In this study, we propose several treatment strategies to modify the sidewall of
LEDs and improve the performance of devices. The results show that the
LEDs treated with citric acid (CA) and ammonium sulfide (NH
Sx for 1 h have the best improvement in LOP by 93.3%, and EQE increases by 91.3%. In addition, the reliability of
LEDs with different sidewall treatments was studied under long-term aging and high-temperature and high-humidity conditions. The treatment method for
LEDs has made significant contributions to the performance of devices, bringing about advancements in various key areas.
The alternating current (ac)-driven GaN-based single contact light-emitting diode (SC-LED) has garnered significant attention due to its unique driving technique and potential applications, especially in areas where direct current-driven (dc-driven) LEDs face limitations. Our previous research emphasizes the importance of reducing the operating voltage of SC-LED. In this study, we have developed and manufactured a novel SC-LED featuring a tunnel junction (TJ) structure, which exhibits a lower breakdown voltage (
V
) compared to conventional LEDs with an ITO contact layer. The simulation and experimental data illustrate a significant performance gap between TJ SC-LED and ITO SC-LED. The working voltage of TJ SC-LED is 34 V, which is 29% lower than that of ITO SC-LED. Specifically, under ac power at 80 V, TJ SC-LED exhibits a current of 0.94 mA and a WPE of 3.22%, both higher than the 0.77 mA and 3.02% values recorded for the ITO SC-LED. This comparison underscores the superior performance of TJ SC-LED over ITO SC-LED. These findings enhance our understanding of SC-LEDs and pave the way for the advancement of new driving techniques in nano-sized displays.
This study uses advanced 3D packaging planarization and a copper process to manufacture a co-planarized common cathode Micro-LED display. The experimental results show that SiO
2
and copper electrodes are at the same height and lie in a horizontal line after a chemical mechanical polishing (CMP) process. Compared with a conventional Micro-LED display structure, the copper electrode has a lower starting voltage of 2.72V. In addition, when the electrodes were operated for 19 mA, the temperature of the copper electrode structured Micro-LED also is 2.2 °C cooler. Therefore, the self-heating effect that is inherent in light-emitting diodes (LEDs) is mitigated by the superior thermal conductivity of copper.
Anisotropic conductive film (ACF) is an essential layer used in Micro-LEDs and is applied through laser processing during the repair process. However, research related to the repair process using step-by-step morphology processing of ACF has hardly been conducted so far. Therefore, in this study, we proposed and thoroughly investigated a processing morphology suitable for the repair process through step-by-step processing using femtosecond lasers with wavelengths of 1026 (NIR), 513 (Green), and 257 (DUV) nm. By appropriately processing the ACF according to the intended purpose, it is expected that the desired repair process can be applied and can also be applied to all fields where ACF is utilized.
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.
A 1.37” smart‐watch size active‐matrix microLED display prototype was fabricated by Cd‐free R/G/B quantum dots pumped by UV microLEDs in an innovative 4‐sub‐pixel architecture. The temperature‐dependent emission properties from such quantum dot display have been studied based on the competing processes between LTPS TFT output current and UV microLED radiance. Our studies show that with proper thermal treatment, the photo brightening of QDs can be stabilized within 5%. This presents a promising route to develop microLED displays with brightness insensitive to operation temperature up to 90ºC.
Micro light‐emitting diodes (µLEDs) have been considered as next‐generation display technologies, but they suffer from low productivity due to poor heat dissipation, high electrical resistance, and repair difficulty of conventional µLED bonding methods. Simultaneous transfer and bonding (SITRAB) technology enables electrically/thermally conductive soldering for high‐resolution µLED displays. In this study, the transfer, bonding, and repair of InGaN/GaN µLEDs via the SITRAB method are reported. µLEDs grown on sapphire substrates are assembled on a backplane via the SITRAB process. The SITRAB‐based repair of defective pixels is demonstrated by transferring redundant LEDs to the display substrates.
Micro‐LEDs have been hailed as the next‐generation display technology, with mass transfer being the key to achieving mass production of Micro‐LED displays. During the mass transfer process, it is crucial to identify and remove defective dies in order to ensure high yield and optimal display performance. In this paper, a novel in‐situ observation, imaging, and laser processing system is proposed, enabling online, in‐situ, and zero‐misalignment removal of faulty dies during the mass transfer process. This system holds great promise for significantly improving the efficiency of mass transfer and further driving the realization of mass production for Micro‐LED displays.
In this work, based on transfer printing technology, planar integration of RGB InGaN‐based micro‐LEDs on flexible transparent material was realized. By measuring the optoelectronic parameters, we demonstrated the feasibility of this integrated device in full‐color display applications. In addition, based on the on‐off keying modulation and wavelength division multiplexing, visible light communication in parallel channels can be achieved. Relevant studies have demonstrated the broad prospects of InGaN‐based micro‐LED devices in multifunctional flexible integrated optoelectronics applications.
As the utilization of roadways increases, the probability of accidents happening also increases. So to overcome this, Uturn paths were created in place of signals. So, waiting on the signal was totally avoided, eliminating pollution in traffic signal areas. The design and development of manual and voice-assisted U-turn indicators for automobiles are discussed in this paper. The design of this system requires the development of a new circuit to accommodate the voice control system for the indicators using Google Nest Mini and the button operated, which is controlled by a MQTT app in a tab with Wi-Fi connectivity. These devices are connected to the power supply unit to charge them through USB connectors, and the NodeMCU is used for the controlling of the whole system and responsible for the displaying of the indicator signs on the 16x16 LED wall placed on the vehicle. There are conventional indicators also present in the vehicle, along with the newly developed voice-controlled and button operated MQTT app, which will control the left and right turn indicators and the left and right U-turn indicators when a specific voice command is given to the microcontroller through Google Nest Mini. By using this technology, people can reduce the lack of communication during the U-turn and control the indicator, which is a hands-free operation. Placing this technology on the vehicle will increase the safety of the automobile and reduce the risk of accidents. This system allows people to control the voice-assisted control system and an integrated dashboard control system to control the U-turn indicators and the normal indicators with ease.
Compared with conventional display technologies, liquid crystal display (LCD), and organic light emitting diode (OLED), micro-LED displays possess potential advantages such as high contrast, fast response, and relatively wide color gamut, low power consumption, and long lifetime. Therefore, micro-LED displays are deemed as a promising technology that could replace LCD and OLED at least in some applications. While the prospects are bright, there are still some technological challenges that have not yet been fully resolved in order to realize the high volume commercialization, which include efficient and reliable assembly of individual LED dies into addressable arrays, full-color schemes, defect and yield management, repair technology and cost control. In this article, we review the recent technological developments of micro-LEDs from various aspects.
The direct printing of microscale quantum dot light‐emitting diodes (QLEDs) is a cost‐effective alternative to the placement of pre‐formed LEDs. The quality of printed QLEDs currently is limited by nonuniformities in droplet formation, wetting, and drying during inkjet printing. Here, optimal ink formulation which can suppress nonuniformities at the pixel and array levels is demonstrated. A solvent mixture is used to tune the ejected droplet size, ensure wetting, and provoke Marangoni flows that prevent coffee stain rings. Arrays of green QLED devices are printed at a resolution of 500 pixels in.⁻¹ with a maximum luminance of ≈3000 cd m⁻² and a peak current efficiency of 2.8 cd A⁻¹. The resulting array quality is sufficient to print displays at state‐of‐the‐art resolutions.
In this work, the size-dependent effect for InGaN/GaN-based blue micro-light emitting diodes (µLEDs) is numerically investigated. Our results show that the external quantum efficiency (EQE) and the optical power density drop drastically as the device size decreases when sidewall defects are induced. The observations are owing to the higher surface-to-volume ratio for small µLEDs, which makes the Shockley-Read-Hall (SRH) non-radiative recombination at the sidewall defects not negligible. The sidewall defects also severely affect the injection capability for electrons and holes, such that the electrons and holes are captured by sidewall defects for the SRH recombination. Thus, the poor carrier injection shall be deemed as a challenge for achieving high-brightness µLEDs. Our studies also indicate that the sidewall defects form current leakage channels, and this is reflected by the current density-voltage characteristics. However, the improved current spreading effect can be obtained when the chip size decreases. The better current spreading effect takes account for the reduced forward voltage.
Inorganic‐based micro light‐emitting diodes (µLEDs) have witnessed significant improvements in terms of display and biomedical applications, which can shift the paradigm of future optoelectronic systems. In particular, µLED displays are on the verge of becoming the next big interface platform for visual communications, expanding to various internet of things and wearable/bioapplications. Novel µLED concepts need to be upgraded to be able to satisfy their potential optoelectric applications, such as virtual reality, smart watches, and medical sensors for individual computing in this hyperconnected society. Here, representative progresses in the field of flexible µLEDs are reviewed with regard to device structures, massive µLED transfers, methods for performance enhancement, and applications.
Color-converted micro-LED displays consist of a mono-color micro-LED array and color conversion materials to achieve full color, while relieving the burden of epitaxial growth of three-color micro-LEDs. However, it usually suffers from low efficiency and color crosstalk due to the limited optical density of color conversion materials. With funnel-tube array, the optical efficiency of the color-converted micro-LED display can be improved by ~3X, while the crosstalk is eliminated. After optimization of the tapper angle, the ambient contrast ratio is also improved due to higher light intensity.
Visible light communications (VLC) is an emerging technology that uses LEDs, such as found in lighting fixtures and displays, to transmit data wirelessly. Research has so far focused on LED transmitters and on photoreceivers as separate, discrete components. Combining both types of devices into a single transceiver format will enable bi-directional VLC and offer flexibility for the development of future advanced VLC systems. Here, a proof of concept for an integrated optical transceiver is demonstrated by transfer printing a microsize LED, the transmitter, directly onto a fluorescent optical concentrator edge-coupled to a photodiode, the receiver. This integrated device can simultaneously receive (downlink) and transmit (uplink) data at rates of 416 Mbps and 165 Mbps, respectively. Its capability to operate in optical relay mode at 337 Mbps is experimentally demonstrated.
Displays based on inorganic light-emitting diodes (LED) are considered as the most promising one among the display technologies for the next-generation. The chip for LED display bears similar features to those currently in use for general lighting, but it size is shrunk to below 200 microns. Thus, the advantages of high efficiency and long life span of conventional LED chips are inherited by miniaturized ones. As the size gets smaller, the resolution enhances, but at the expense of elevating the complexity of fabrication. In this review, we introduce two sorts of inorganic LED displays, namely relatively large and small varieties. The mini-LEDs with chip sizes ranging from 100 to 200 μm have already been commercialized for backlight sources in consumer electronics applications. The realized local diming can greatly improve the contrast ratio at relatively low energy consumptions. The micro-LEDs with chip size less than 100 μm, still remain in the laboratory. The full-color solution, one of the key technologies along with its three main components, red, green, and blue chips, as well color conversion, and optical lens synthesis, are introduced in detail. Moreover, this review provides an account for contemporary technologies as well as a clear view of inorganic and miniaturized LED displays for the display community.
A red-light micro LED display made of an AlGaInP epilayer with a resolution of 64 × 32 pixels, a pitch of 175 μm and a luminous area of 1 cm × 0.5 cm was fabricated and characterized in this study. The AlGaInP epilayer was bonded to double polished sapphire substrate by wafer-bonding technique and then removing the absorbing GaAs substrate. In this design, the ITO was applied as one of the conducting electrodes of the emitting surface, which can be beneficial since the emitting light is not shielded by metal electrodes. The other key process for LED panel fabrication is planarization. Polymer material was used to fill the gap between each pixel, which was used to prevent a short or open circuit using the planarization process. The driving mode of this display is passive multi-electrode addressable controlling. The luminance of this micro-LED panel is more than 450 nits with an operating voltage of 3 V which is three times higher than that of the OLED operating in the same driving mode.
Optoelectronic effects of sidewall passivation on micro-sized light-emitting diodes (µLEDs) using atomic-layer deposition (ALD) were investigated. Moreover, significant enhancements of the optical and electrical effects by using ALD were compared with conventional sidewall passivation method, namely plasma-enhanced chemical vapor deposition (PECVD). ALD yielded uniform light emission and the lowest amount of leakage current for all µLED sizes. The importance of sidewall passivation was also demonstrated by comparing leakage current and external quantum efficiency (EQE). The peak EQEs of 20 × 20 µm² µLEDs with ALD sidewall passivation and without sidewall passivation were 33% and 24%, respectively. The results from ALD sidewall passivation revealed that the size-dependent influences on peak EQE can be minimized by proper sidewall treatment.
A transfer printing (TP) method is presented for the micro-assembly of integrated photonic devices from suspended membrane components. Ultra thin membranes with thickness of 150nm are directly printed without the use of mechanical support and adhesion layers. By using a correlation alignment scheme vertical integration of single-mode silicon waveguides is achieved with an average placement accuracy of 100±70nm. Silicon (Si) μ-ring resonators are also fabricated and show controllable optical coupling by varying the lateral absolute position to an underlying Si bus waveguide.
We investigated the optical and electrical properties of red AlGaInP light-emitting diodes (LEDs) as functions of chip size, p-cladding layer thickness, and the number of multi-quantum wells (MQWs). External quantum efficiency (EQE) decreased with decreasing chip size. The ideality factor gradually increased from 1.47 to 1.95 as the chip size decreased from 350 μm to 15 μm. This indicates that the smaller LEDs experienced larger carrier loss due to Shockley-Read-Hall nonradiative recombination at sidewall defects. S parameter, defined as ∂lnL/∂lnI, increased with decreasing chip size. Simulations and experimental results showed that smaller LEDs with 5 pairs of MQWs had over 30% higher IQE at 5 A/cm² than the LED with 20 pairs of MQWs. These results show that the optimization of the number of QWs is needed to obtain maximum EQE of micro-LEDs.
In this investigation, we experimentally demonstrated a high-speed Gbps long-distance real-time visible light communication (VLC) system based on non-return-to-zero on-off keying (NRZ-OOK) modulation by using a high-bandwidth Gallium nitride (GaN)-based micro-LED with a modulation bandwidth of ~230 MHz and a size of 40 μm × 40 μm. The maximum real-time data rate obtained is up to 1.3 Gbps at a 3 m free-space transmission distance with a bit-error rate (BER) of 3.4 × and 1 Gbps at a 10 m distance with a BER of 3.2 × , both of which are underneath the forward error correction (FEC) threshold of 3.8 × required for free-error operation. In addition, when the transmission distance is increased to 16 m in free space through reflecting the emission light beam by blue reflectors mounted on the wall, a data rate of 0.87 Gbps with a BER of is achieved successfully.
To enable high-speed long-distance underwater optical wireless communication (UOWC) supplementing traditional underwater wireless communication, a low-power 520 nm green laser diode (LD) based UOWC system was proposed and experimentally demonstrated to implement maximal communication capacity of up to 2.70 Gbps data rate over a 34.5 m underwater transmission distance by using non-return-to-zero on-off keying (NRZ-OOK) modulation scheme. Moreover, maximum data rates of up to 4.60 Gbps, 4.20 Gbps, 3.93 Gbps, 3.88 Gbps, and 3.48 Gbps at underwater distances of 2.3 m, 6.9 m, 11.5 m, 16.1 m and 20.7 m were achieved, respectively. The light attenuation coefficient of ~0.44 dB/m was obtained and the beam divergence angle is 0.35°, so the aallowable underwater transmission distance can be estimated to be ~90.7 m at a data rate of 0.15 Gbps with a corresponding received light-output power of −33.01 dBm and a bit-error rate (BER) of 2.0 × 10⁻⁶. In addition, when the data rate is up to 1 Gbps, the UOWC distance is predicted to be ~62.7 m for our proposed UOWC system. The achievements we make are suitable for applications requiring high-speed long-distance real-time UOWC.
Red-, orange-, and green-emitting integrated optoelectronic sources are demonstrated by transfer printing blue InGaN µLEDs onto ultra-thin glass platforms functionally enhanced with II-VI colloidal quantum dots (CQDs). The forward optical power conversion efficiency of these heterogeneously integrated devices is, respectively, 9%, 15%, and 14% for a blue light absorption over 95%. The sources are demonstrated in an orthogonal frequency division multiplexed (OFDM) visible light communication link reaching respective data transmission rates of 46 Mbps, 44 Mbps and 61 Mbps.
This paper reports the utilization of colloidal semiconductor quantum dots as color converters for Gb/s visible light communications. We briefly review the design and properties of colloidal quantum dots and discuss them in the context of fast color conversion of InGaN light sources, in particular in view of the effects of self-absorption. This is followed by a description of a CQD/polymer composite format of color converters. We show samples of such color-converting composites emitting at green, yellow/orange and red wavelengths and combine these with a blue-emitting microsize LED to form hybrid sources for wireless visible light communication links. In this way data rates up to 1 Gb/s over distances of a few tens of centimeters have been demonstrated. Finally, we broaden the discussion by considering the possibility for wavelength division multiplexing as well as the use of alternative colloidal semiconductor nanocrystals.
In this research, nano-ring light-emitting diodes (NRLEDs) with different wall width (120 nm, 80 nm and 40 nm) were fabricated by specialized nano-sphere lithography technology. Through the thinned wall, the effective bandgaps of nano-ring LEDs can be precisely tuned by reducing the strain inside the active region. Photoluminescence (PL) and time-resolved PL measurements indicated the lattice-mismatch induced strain inside the active region was relaxed when the wall width is reduced. Through the simulation, we can understand the strain distribution of active region inside NRLEDs. The simulation results not only revealed the exact distribution of strain but also predicted the trend of wavelength-shifted behavior of NRLEDs. Finally, the NRLEDs devices with four-color emission on the same wafer were demonstrated.
The essential functionality of photonic and electronic devices is contained in thin surface layers leaving the substrate often to play primarily a mechanical role. Layer transfer of optimised devices or materials and their heterogeneous integration is thus a very attractive strategy to realise high performance, low-cost circuits for a wide variety of new applications. Additionally, new device configurations can be achieved that could not otherwise be realised. A range of layer transfer methods have been developed over the years including epitaxial lift-off and wafer bonding with substrate removal. Recently, a new technique called transfer printing has been introduced which allows manipulation of small and thin materials along with devices on a massively parallel scale with micron scale placement accuracies to a wide choice of substrates such as silicon, glass, ceramic, metal and polymer. Thus, the co-integration of electronics with photonic devices made from compound semiconductors, silicon, polymer and new 2D materials is now achievable in a practical and scalable method. This is leading to exciting possibilities in microassembly. We review some of the recent developments in layer transfer and particularly the use of the transfer print technology for enabling active photonic devices on rigid and flexible foreign substrates.
GaN-based light emitting diodes (LEDs) have been fabricated on sapphire substrates with different thicknesses of GaN buffer layer grown by a combination of hydride vapor phase epitaxy and metalorganic chemical vapor deposition. We analyzed the LED efficiency and modulation characteristics with buffer thicknesses of 12 μm and 30 μm. With the buffer thickness increase, cathodoluminescence hyperspectral imaging shows that the dislocation density in the buffer layer decreases from ~1.3 × 10⁸ cm⁻² to ~1.0 × 10⁸ cm⁻², and Raman spectra suggest that the compressive stress in the quantum wells is partly relaxed, which leads to a large blue shift in the peak emission wavelength of the photoluminescence and electroluminescent spectra. The combined effects of the low dislocation density and stress relaxation lead to improvements in the efficiency of LEDs with the 30 μm GaN buffer, but the electrical-to-optical modulation bandwidth is higher for the LEDs with the 12 μm GaN buffer. A rate equation analysis suggests that defect-related nonradiative recombination can help increase the modulation bandwidth but reduce the LED efficiency at low currents, suggesting that a compromise should be made in the choice of defect density.
High-speed underwater optical wireless communication (UOWC) was achieved using an 80 μm blue-emitting GaN-based micro-LED. The micro-LED has a peak emission wavelength of ~440 nm and an underwater power attenuation of 1 dB/m in tap water. The −3 dB electrical-to-optical modulation bandwidth of the packaged micro-LED increases with increasing current and saturates at ~160 MHz. At an underwater distance of 0.6 m, 800 Mb/s data rate was achieved with a bit error rate (BER) of 1.3 × 10⁻³, below the forward error correction (FEC) criteria. And we obtained 100 Mb/s data communication speed with a received light output power of −40 dBm and a BER of 1.9 × 10⁻³, suggesting that UOWC with extended distance can be achieved. Through reflecting the light emission beam by mirrors within a water tank, we experimentally demonstrated a 200 Mb/s data rate with a BER of 3.0 × 10⁻⁶ at an underwater distance of 5.4 m.
We review recent advances in quantum dot (QD)- enhanced liquid crystal displays (LCDs), including material formulation, device configuration, and system integration. For the LCD system, we first compare the color gamut difference between the commonly used Gaussian fitting method and that using real emission spectra. Next, we investigate the Helmholtz–Kohlrausch effect. Our simulation results indicate that QD-enhanced LCD appears 1.26X more efficient than OLED due to its wider color gamut. Finally, two new trends for QD-LCDs are discussed: 1) replacing conventional color filters with a QD array, and 2) emerging quantum rod (QR)-enhanced backlight. Their inherent advantages, technical challenges, and potential solutions are presented. We believe the prime time for QD-enhanced LCDs is around the corner.
Blue LEDs and HEMTs based on III-Nitride have been flourishing commercially across the globe, thanks largely to breakthroughs in the material quality of the wide-bandgap compound semiconductor back in the 1990s. The realizations of white-light LEDs, blu-ray systems, and lately efficient compact chargers have drastically changed the way we live and have contributed tremendously to global energy saving efforts. The maturity and diversity of modern discrete GaN-based devices open up opportunities for an integrated GaN platform with extended functionalities and applications. In this review paper, we present an overview of the monolithic and heterogeneous integration of GaN devices and components. Various methods for the integration of electronic, optoelectronic, and optical components based on GaN are discussed.
This work reports the use of the chip-based GaN-based micro-LED (μLED) arrays for multifunctional applications as micro-display, data transmitters, photodetectors and solar cells. The functions of display and transmitter have been reported, and particularly we experimentally demonstrated that μLED arrays could be used as self-powered, high-performance and wavelength-selective photodetectors (PDs) enabling high-speed multiple-input multiple-output (MIMO) visible light communications (VLC) under on-off keying (OOK) modulation scheme using 405 nm violet laser diodes (LDs) as transmitters. The optoelectronic and communication characteristics of the μLED-based PDs with diameters of 40-μm, 60-μm and 100-μm were systematically studied. The optoelectronic analysis shows superior performances of μLED-based PDs at 405 nm wavelength compared with other previously reported GaN-based PDs. Under a bias voltage of -5 V, the comparable peak responsivities of 0.27, 0.31 and 0.24 A/W, specific detectivities of 1.1 × 10¹¹, 2.3 × 10¹² and 2.1 × 10¹² cm ∙ H1/2 ∙ W⁻¹, and linear dynamic ranges (LDRs) of 152, 162 and 164 dB were achieved for 40-μm, 60-μm and 100-μm μLEDs, respectively. Even at zero-bias, i.e. self-powered mode, we have achieved high peak responsivities of 0.24, 0.29 and 0.21 A/W, high specific detectivities of 7.5 × 10¹², 1.5 × 10¹³ and 1.3 × 10¹³ cm ∙ H1/2 ∙ W⁻¹ and high LDR up to 186, 196 and 197 dB for 40-μm, 60-μm and 100-μm μLEDs, respectively. The μLEDs could also be used to harvest the optical energy of the system, working as solar cells. The μLED-based PD arrays were tested as receivers in VLC system to implement high-speed parallel communication, which yields maximum data rates of 180 Mbps, 175 Mbps and 185 Mbps for a single 40-μm, 60-μm and 100-μm μLED-based PDs at a distance of 1 m with BERs of 3.5 × 10⁻³, 3.7 × 10⁻³ and 3.5 × 10⁻³, respectively. Furthermore, 2 × 2 MIMO parallel VLC system was achieved to increase the VLC data rate, which suggests the potential of using a large μLED-based PD arrays for multiple Gbps and even Tbps VLC applications.
The excitement about microLEDs has grown exponentially since Apple acquired technology startup Luxvue in 2014. All major display makers have now invested in the technology. But there are still overlooked technical challenges, and a new miniLED keyword has entered the field. We try and explain all of this here.
Emissive displays based on light‐emitting diodes (LEDs), with high pixel density, luminance, efficiency, and large color gamut, are of great interest for applications such as watches, phones, and virtual displays. The high pixel density requirements of some emissive displays require a particular class of LEDs that are sub‐20‐micrometers in length, called micro‐LEDs. While state‐of‐the‐art emissive displays incorporate organic LEDs, an alternative is inorganic III‐nitride LEDs with potential reliability and efficiency benefits. Here we explore the performance, challenges, and prospective outcomes for III‐nitride micro‐LEDs to produce efficient emissive displays and provide insight to advance this technology. Calculations are performed to determine the operating points for the micro‐LEDs and the efficiency of the overall emissive display. It is shown that III‐nitride micro‐LEDs suffer from some of the same problems as their larger‐sized solid‐state lighting LED cousins; however, the operating conditions of micro‐LEDs can result in different challenges and research efforts. These challenges include improving efficiency at low current densities; improving the efficiency of longer wavelength (green and red) LEDs; and creating device designs that can overcome low coupling efficiency, high surface recombination, and display assembly difficulties. III‐Nitride micro light‐emitting diodes (micro‐LEDs) are investigated to create high pixel density, luminance, efficiency, and large color gamut emissive displays. Challenges for implementation include improving efficiency at low current densities; improving the efficiency of green and red LEDs; and creating device designs that can overcome low coupling efficiency, high surface recombination, and display assembly difficulties.
In this paper, some of the technological bottlenecks of the micro‐light‐emitting diode (µLED) for a micro‐display were reviewed. Although there remain other issues, the µLED technology has a potential for some micro‐display applications which especially demand high brightness and reliability such as augmented reality (AR) display, automotive head‐up display (HUD), and so on. The efficiency of µLED needs to be improved, and an integration technology for active matrix driving and technologies for full‐color realization as well as defective pixel control must be developed. An optimum specification of a µLED display was proposed to maximize reality in micro‐display products.
Full-color displays based on micro light-emitting diodes (μLEDs) can be fabricated on monolithic epitaxial wafers. Nanoring (NR) structures were fabricated on a green LED epitaxial wafer; the color of NR-μLEDs was tuned from green to blue through strain relaxation. An Al2O3 layer was deposited on the sidewall of NR-μLEDs, which improved the photoluminescence intensity by 143.7%. Coupling with the exposed multiple quantum wells through nonradiative resonant energy transfer, red quantum dots were printed to NR-μLEDs for a full-color display. To further improve the color purity of the red light, a distributed Bragg reflector is developed to reuse the excitation light.
Low-dimensional nanowires have received increased interest as building blocks for electronic and photonic devices. In this article, we review the recent progress made on III -nitridenanowire photonic devices by molecular beam epitaxy (MBE), with a focus on micro-LED s, deep-ultraviolet (UV)-light emitters, and solar fuel and artificial photosynthesis devices. The challenges and prospects of III -nitride nanowires for these applications are also discussed.
Gallium nitride (GaN)-based light-emitting diodes (LEDs) are being investigated for the next generation display technology. The persistent issue, however, has been the lack of ability to integrate transistors with LEDs for control. Here, a novel vertical integration scheme is utilized to fabricate nanowire LEDs with nanowire field effect transistors (FETs) for the first time. This approach utilizes the unintentionally doped GaN template layer which is common to LED growth for the fabrication of nanowire FETs. The demonstrated voltage-controlled light-emitting unit provides area savings, scaling, and easier fabrication due to the vertical integration. For these initial nanowire devices, light modulation is demonstrated with LED turn OFF at -10 V. Due to the nanowire approach, these devices show over two times improvement in the ION to I
OFF
ratio compared with the alternative integration schemes.
Alopecia is considered as an aesthetic, psychological, and social issue among modern people. Although laser-induced skin stimulation is utilized for depilation treatment, such treatment has significant drawbacks of high energy consumption, huge equipment size, and limited usage in daily life. Here, we present a wearable photostimulator for hair-growth applications using high-performance flexible red vertical light-emitting diodes (f-VLEDs). Flexible microscale LEDs were effectively fabricated by simple monolithic fabrication process, resulting in high light output (~ 30 mW mm-2), low forward voltage (~2.8 V), and excellent flexibility for wearable biostimulations. Finally, trichogenic stimulation of a hairless mouse was achieved using high-performance red f-VLEDs with high thermal stability, device uniformity and mechanical durability.
Solid-state highly photoluminescent quantum dots (QDs)-based phosphors attract great scientific interest as color converters due to an increasing demand for white light-emitting devices. Herein, a microwave-assisted heating method is presented to fabricate multi-color QDs-based phosphors within 30 seconds through microwave-assisted heating the mixture of QDs and sodium silicate aqueous solution. In the composites, the formed cross-linked networks not only play as a matrix to prevent QDs aggregation in solid state, but also cause the variation of the refractive index around QDs and the QDs surface optimization, which contributes to good stabilities and twice enhancement in photoluminescence quantum yields (69%) compared with the initial QDs aqueous solution (33%). Using the QDs-based phosphors as color conversion layer, white light-emitting diodes were realized with controllable colour temperature, high colour purity, and high colour rendering index (90.3), which show great potential in display and illumination. Furthermore, the luminescence lifetime of the QDs-based phosphors is less than 25 ns. The potential application of the QDs-based phosphors in visible light communication was also demonstrated, with the modulation bandwidth achieving 42 MHz.
MicroLED is a new, self‐emissive display technology. It offers unique features that could disrupt the display market as well as trigger significant changes in the supply chain. The authors have thoroughly analyzed the microLED industry landscape, including microLED's technological status and its strengths and weaknesses for all major display applications.
Micro‐LED displays offer potential advantages such as high brightness and low energy consumption; however mass adoption requires that manufacturing yield and cost targets are met. In this presentation we explore key manufacturing requirements and present solutions for MOVCD epitaxy and mass transfer to enable Micro‐LED display adoption for consumer applications.
We developed a complete wafer‐to‐panel technology for extremely high rate assembly of µLEDs. The process involves transferring the µLEDs directly from the source epi‐wafer to a quartz carrier from where they are selectively transferred to the panel using our Laser‐Enabled Massively Parallel Transfer method with >100M units/hr.
Hybridization of silicon integrated circuits (ICs) with compound semiconductor device arrays are crucial for making functional hybrid chips, which are found to have enormous applications in many areas. Although widely used in manufacturing hybrid chips, the flip‐chip technology suffers from several limitations that are difficult to overcome, especially when the demand is raised to make functional hybrid chips with higher device array density without sacrificing the chip footprint. To address those issues, Beida Jade Bird Display Limited has developed its unique wafer‐level monolithic hybrid integration technology and demonstrated its advantages in making large‐scale hybrid integration of functional device arrays on Si IC wafers. Active matrix micro‐light‐emitting diode micro‐displays with a resolution of 5000+ pixel per inch were successfully fabricated using Beida Jade Bird Display Limited's monolithic hybrid integration technology. The general fabrication method is described, and the result is presented in this paper. The fabricated monochromatic micro‐light‐emitting diode micro‐displays exhibit improved device performance than do other micro‐display technologies and have great potentials in applications such as portable projectors and near‐to‐eye projection for augmented reality. More importantly, the wafer‐scale monolithic hybrid integration technology offers a clear path for low‐cost mass production of hybrid optoelectronic IC chips. Wafer‐scale epi‐transfer process has been developed for hybrid monolithic integration of Si‐based IC and micro‐LED arrays. Active‐matrix micro‐LED displays with a resolution of 5000+ ppi and a brightness of >500,000 nits is demonstrated for the first time on the basis of wafer‐scale epi‐transfer technique.
This work proposes a high-bandwidth white-light system consisting of a blue gallium nitride (GaN) micro-LED (μLED) exciting yellow-emitting CsPbBr1.8I1.2 perovskite quantum dots (YQDs) for high-speed real-time visible light communication (VLC). The packaged 80 μm × 80 μm blue-emitting μLED has a modulation bandwidth of ~160 MHz and a peak emission wavelength of ~445 nm. The achievable bandwidth of the white-light system is up to 85 MHz in the absence of filters and equalization technology. Meanwhile, the bandwidth of the YQDs as a color-converter is as high as 73 MHz with the blue GaN μLED as the pump source. A maximum data rate of 300 Mbps can be achieved by taking advantage of the high bandwidth of the white-light system using non-return-to-zero on-off keying (NRZ-OOK) modulation scheme. The resultant bit-error rate (BER) is 2.0 × 10⁻³, well beneath the forward error correction (FEC) criterion of 3.8 × 10⁻³ required for error-free data transmission. In addition, the YQDs which we proposed as a color-converter possess high stability for VLC. After half a year, the achievable bandwidths of the white-light system and the YQDs are still up to 83 MHz and 70 MHz, respectively. This study provides the direction of developing high-bandwidth white-light system for both high-efficiency solid state lighting (SSL) and high-speed VLC.
In this work, monolithic red, green, and blue (RGB) micro-light-emitting diodes (LEDs) were fabricated using gallium nitride (GaN)-based blue micro-LEDs and quantum dots (QDs). Red and green QDs were sprayed onto individual region surrounded by patterned black matrix photoresist on the blue micro-LEDs to form color conversion layers. Owing to its light-blocking capability, the patterned black matrix photoresist improved the contrast ratio of the micro-LEDs from 11 to 22. To enhance the color conversion efficiency and the light output intensity, a hybrid Bragg reflector (HBR) was deposited on the bottom side of the monolithic RGB micro-LEDs, thus reflecting the RGB light emitted to the substrate. To further improve the color purity of the red and green light, a distributed Bragg reflector (DBR) with high reflection for the blue light was deposited on the top side of the QDs/micro-LEDs. The red and green light output intensities of the micro-LEDs with HBR and DBR were enhanced by about 27%.
Large quantities of microscopic red, green, and blue light-emitting diodes (LEDs) made of crystalline inorganic semiconductor materials micro-transfer printed in large quantities onto rigid or flexible substrates form monochrome and color displays having a wide range of sizes and interesting properties. Transfer-printed micro-LED displays promise excellent environmental robustness, brightness, spatial resolution, and efficiency. Passive-matrix and active-matrix inorganic LED displays were constructed, operated, and their attributes measured. Tests demonstrate that inorganic micro-LED displays have outstanding color, viewing angle, and transparency. Yield improvement techniques include redundancy, physical repair, and electronic correction. Micro-transfer printing enables revolutionary manufacturing strategies in which microscale LEDs are first assembled into miniaturized micro-system “light engines,” and then micro-transfer printed and interconnected directly to metallized large-format panels. This paper reviews micro-transfer printing technology for micro-LED displays.
In this study, a full-color emission red-green-blue (RGB) quantum-dot (QD)-based micro-light-emitting-diode (micro-LED) array with the reduced optical cross-talk effect by a photoresist mold has been demonstrated. The UV micro-LED array is used as an efficient excitation source for the QDs. The aerosol jet technique provides a narrow linewidth on the micrometer scale for a precise jet ofQDson the micro-LEDs. To reduce the optical cross-talk effect, a simple lithography method and photoresist are used to fabricate the mold, which consists of a window for QD jetting and a blocking wall for cross-talk reduction. The cross-talk effect of the well-confined QDs in the window is confirmed by a fluorescence microscope, which shows clear separation between QD pixels. A distributed Bragg reflector is covered on the micro-LED array and the QDs’ jetted mold to further increase the reuse of UV light. The enhanced light emission of the QDs is 5%, 32%, and 23% for blue, green, and red QDs, respectively.
High pixel per inch and high-resolution micro-LED displays are attracting more and more attentions. The increasing pixel number requires a large amount of bonding pads and brings huge difficulties to micro-LED system design and lowers power efficiency as well. It is urgent to integrate row and column driving circuits onto the micro-LED panel. Here, we report a fully integrated active matrix programmable micro-LED system on panel (SoP) with ultraviolet and blue emission wavelengths. The micro-LED SoP has a resolution of 60 × 60 and pixel pitch of 70 μm. The micro-LED SoP was achieved by integrating micro-LED arrays with silicon-based p-channel metal-oxide semiconductor driving panel using fine-toned flip-chip bonding technology. With fully integrated scan and data circuits, the number of bonding pads was greatly reduced from 136 to 28, and large amount of metal interconnection lines were saved. The micro-LED SoP panel was mounted on a periphery driving board, and representative characters were displayed successfully.
An active matrix-type stretchable display is realized by overlay-aligned transfer of inorganic light-emitting diode (LED) and single-crystal Si thin film transistor (TFT) with roll processes. The roll-based transfer enables integration of heterogeneous thin film devices on a rubber substrate while preserving excellent electrical and optical properties of these devices, comparable to their bulk properties. The electron mobility of the integrated Si-TFT is over 700 cm2 V−1 s−1, and this is attributed to the good interface between the Si channel and the thermally grown SiO2 insulator. The light emission properties of the LED are of wafer quality. The resulting display stably operates under tensile strains up to 40%, over 200 cycles, demonstrating the potential of stretchable displays based on inorganic materials.
Inorganic light emitting diodes (LEDs) serve as bright pixel-level emitters in displays, from indoor/outdoor video walls with pixel sizes ranging from one to thirty millimeters to micro displays with more than one thousand pixels per inch. Pixel sizes that fall between those ranges, roughly 50 to 500 microns, are some of the most commercially significant ones, including flat panel displays used in smart phones, tablets, and televisions. Flat panel displays that use inorganic LEDs as pixel level emitters (μILED displays) can offer levels of brightness, transparency, and functionality that are difficult to achieve with other flat panel technologies. Cost-effective production of μILED displays requires techniques for precisely arranging sparse arrays of extremely miniaturized devices on a panel substrate, such as transfer printing with an elastomer stamp. Here we present lab-scale demonstrations of transfer printed μILED displays and the processes used to make them. Demonstrations include passive matrix μILED displays that use conventional off-the shelf drive ASICs and active matrix μILED displays that use miniaturized pixel-level control circuits from CMOS wafers. We present a discussion of key considerations in the design and fabrication of highly miniaturized emitters for μILED displays.
Displays using direct light emission from micron-scale inorganic light emitting diodes (µILEDs) have the potential to be very bright and also very power efficient. High-throughput technologies that accurately and cost-effectively assemble microscale devices on display substrates with high yield are key enablers for µILED displays. Elastomer stamp transfer printing is such a candidate assembly technology. A variety of µILED displays have been designed and fabricated by transfer printing, including passive-matrix and active-matrix displays on glass and plastic substrates.
Development of unconventional technologies for wireless collection and analysis of quantitative, clinically relevant information on physiological status is of growing interest. Soft, biocompatible systems are widely regarded as important because they facilitate mounting on external (e.g., skin) and internal (e.g., heart and brain) surfaces of the body. Ultraminiaturized, lightweight, and battery-free devices have the potential to establish complementary options in biointegration, where chronic interfaces (i.e., months) are possible on hard surfaces such as the fingernails and the teeth, with negligible risk for irritation or discomfort. Here, the authors report materials and device concepts for flexible platforms that incorporate advanced optoelectronic functionality for applications in wireless capture and transmission of photoplethysmograms, including quantitative information on blood oxygenation, heart rate, and heart rate variability. Specifically, reflectance pulse oximetry in conjunction with near-field communication capabilities enables operation in thin, miniaturized flexible devices. Studies of the material aspects associated with the body interface, together with investigations of the radio frequency characteristics, the optoelectronic data acquisition approaches, and the analysis methods capture all of the relevant engineering considerations. Demonstrations of operation on various locations of the body and quantitative comparisons to clinical gold standards establish the versatility and the measurement accuracy of these systems, respectively.
Micro-displays based on an array of micro-sized light emitting diodes (µLEDs) are a promising technology for a wide range of applications. In these 2-dimensional arrays, each µLED works as a single pixel of a whole image. In this paper we investigate the effect of size reduction on light emission and efficiency on InGaN/GaN LED devices ranging from 10*10 to 500*500 µm².
Electroluminescence characterizations together with current–voltage–luminance (IVL) measurements are conducted to study the homogeneity of light emission and correlate with efficiency measurements. The results show a strong size-dependent efficiency. Smaller LEDs exhibit lower maximum efficiency. The light emission homogeneity is shown to be dependent on current densities. At low current density, light emission is homogeneous across the surface of the LED, while inhomogeneity appears at higher current levels. We believe that fabrication process is partly responsible of this phenomenon through a degradation of the electrical injection.
Our results are of high importance to understand the properties of µLED arrays. Indeed, such devices would allow the fabrication of very-high brightness emissive micro-displays.