micro-LED Display Research Articles

Preview the Products on Display at Display Week 2022

Preview the Products on Display at Display Week 2022

Wafer-scale monolithic integration of full-colour micro-LED display using MoS2 transistor.

Large-scale growth of transition metal dichalcogenides and their subsequent integration with compound semiconductors is one of the major obstacles for two-dimensional materials implementation in optoelectronics applications such as active matrix displays or optical sensors. Here we present a novel transition metal dichalcogenide-on-compound-semiconductor fabrication method that is compatible with a batch microfabrication process. We show how a thin film of molybdenum disulfide (MoS2) can be directly synthesized on a gallium-nitride-based epitaxial wafer to form a thin film transistor array. Subsequently, the MoS2 thin film transistor was monolithically integrated with micro-light-emitting-diode (micro-LED) devices to produce an active matrix micro-LED display. In addition, we demonstrate a simple approach to obtain red and green colours through the printing of quantum dots on a blue micro-LED, which allows for the scalable fabrication of full-colour micro-LED displays. This strategy represents a promising route to attain heterogeneous integration, which is essential for high-performance optoelectronic systems that can incorporate the established semiconductor technology and emerging two-dimensional materials.

CES 2022: Less is More

AbstractScaled‐down show still delivers top‐notch exhibits and innovation aplenty.

Novel Micro-LED Display Featuring Fingerprint Recognition Without Additional Sensors

This paper proposes a new architecture for a micro light emitting diode (micro-LED) display featuring fingerprint detection without any additional light emitting and sensing devices in mobile applications. This paper proposes that conventional LEDs can be used as photodetectors in active-matrix pixel circuits. In the area where a finger touched, some LEDs are used as photodetectors and others are used as light emitting sources. The photocurrents of reverse-biased LEDs under various illumination conditions were investigated. The active-matrix circuit was fabricated using a conventional p-type low-temperature polycrystalline silicon (LTPS) process. The fabricated pixel circuit could distinguish the intensity of the incident light to the sensing LED. The voltage across the sensing LED (V_LED) was investigated when the pixel density was 400 pixels per inch (ppi) and the cover glass thickness was 30 μm. The experimental results showed that the V_LED changed by 0.5 V for 16.7 ms when red, green, or blue LEDs emitted 150 cd/m², 400 cd/m², or 100 cd/m², respectively. 1×9 LED array with 1.5 mm pitch (p) and 2 mm thick cover glass was fabricated to emulate the LED display with 400 ppi and 30 μm thick cover glass. It is found that an emitting LED affected the sensing LEDs within the distance of 3p by comparing V_LEDs. V_LEDs were measured when the material on the cover glass was air and water with white pigments (water/W). ΔV_LEDs were 0.29 V and 1.13 V for glass-air and glass-water/W cases, respectively. It is found that the lights incident on the sensing LED comprised not only reflected lights by Fresnel reflection at the glass interface but also scattered lights in the water/W. A 75× magnified fake fingerprint was fabricated. Its material was poly-lactic acid with refractive index of 1.465 which is very close to that of a fingertip. Measured ΔV_LEDs at ridges and valleys were 0.48 V and 0.22 V, respectively. The ratio of 0.48 / 0.22 was 2.18 that is very close to 1.91 that is the ratio of light intensity at the ridge to that at the valley obtained by the calculation. This verifies that the proposed micro-LED display distinguishes the ridges and valleys of fingerprint without any help from any additional devices.

Gateway towards recent developments in quantum dot-based light-emitting diodes.

Quantum dots (QDs), with their excellent photoluminescence, narrow emission linewidth, and wide color coverage, provide unrivaled advantages for advanced display technologies, enabling full-color micro-LED displays. It is indeed critical to have a fundamental understanding of how QD properties affect micro-LED display performance in order to develop the most energy-efficient display device in the near future. However, to take a more detailed look at the stability issues and passivation ways of QDs is essential for accelerating the commercialization of QD-based LED technologies. Knowing about the most recent breakthroughs in QD-based LEDs can give a good indication of how they might be used in shaping the future of displays. In this review, we discuss the characteristics of QD-based LEDs for the applications of display and lighting technologies. Various approaches for synthesis and the stability improvement of QDs are addressed in detail, along with recent advancements towards QD-based LED breakthroughs. Moreover, we summarize our latest research findings in QD-based LEDs, providing valuable information about the potential of QD-based LEDs for future display technologies.

Research progress of Micro-LED display and its driving technology

Research progress of Micro-LED display and its driving technology

Synthesis and Conversion Efficiency Optimization of Quantum Dots Layer for Full-color Micro-LED Display

Synthesis and Conversion Efficiency Optimization of Quantum Dots Layer for Full-color Micro-LED Display

PixelEngineTM All-in-One: a Printable Pixel-Driver MicroIC with Three-Dimensionally Integrated Red, Green, and Blue MicroLEDs

Displays that utilize millions of tiny microLEDs promise a future generation of phones, computers and televisions that are bright enough for sunlit environments yet consume less power. The microLEDs are made using wafer-level semiconductor processes, and key to manufacturing displays is the capability to assemble billions of microLEDs onto panels. Elastomer stamp transfer printing is a mass transfer technology with the yield, accuracy, speed, and scalability required to meet the demands of display manufacturing. The versatility of elastomer stamp transfer leads to further innovations for displays, these include the ability to integrate miniature integrated circuits, called microICs, for advanced backplanes and strategies where microLEDs are combined into PixelEngine packages on intermediate substrates before final assembly to the display panel. This paper reviews how transfer printing has been applied to microLED display development to date and describes a pixel-driver microIC with vertically integrated microLEDs, called PixelEngineAll-in-One, that performs all pixel functions and is designed to enable displays realized through assembly onto passive wiring panels.

Narrow-Band Phosphors for Next Generation Mini-LED and Micro-LED Displays

Narrow-Band Phosphors for Next Generation Mini-LED and Micro-LED Displays

Proposal of a Combined PAM and PWM Driving Scheme for<br />Micro-LED Displays

Proposal of a Combined PAM and PWM Driving Scheme for<br />Micro-LED Displays

Eu-Doped GaN-Based Red LEDs as a Key Technology for Micro-LED Displays with Ultrahigh Resolution

Eu-Doped GaN-Based Red LEDs as a Key Technology for Micro-LED Displays with Ultrahigh Resolution

Elastomer Stamp Mass Transfer of PixelEngine Devices for High-Performance Micro-LED Displays

Elastomer Stamp Mass Transfer of PixelEngine Devices for High-Performance Micro-LED Displays

Active-Matrix Micro-LED Display Driven by Metal Oxide TFTs Using Digital PWM Method

This article presents a new micro-light-emitting diode (LED) pixel circuit driven by the digital pulsewidth modulation (PWM) method, in which the threshold voltage compensation of the driving thin-film transistor (TFT) is included. A scan driver integrated by metal oxide (MO) TFTs is also designed to provide the scan signals for pixel circuits array. A micro-LEDs array with <inline-formula> <tex-math notation="LaTeX">$76\,\, \times $ </tex-math></inline-formula> BBB <inline-formula> <tex-math notation="LaTeX">$\times \,\,78$ </tex-math></inline-formula> and a scan driver with 80 stages are simultaneously fabricated by MO TFTs backplane with back-channel etch (BCE) structure. A respective micro-LED display is consequently realized by a peripheral driving system based on a field-programmable gate array (FPGA), which is used as a core control unit to synchronize the scan signals with the data signals. It is shown that the micro-LED in the pixel circuit can operate at a constant current even at the 5-<inline-formula> <tex-math notation="LaTeX">$\mu \text{s}$ </tex-math></inline-formula> pulsewidth of the emission (EM) signal. The micro-LED display can achieve 256 linear grayscales with an approximately 60 Hz flash rate under the minimum pulsewidth of 40 <inline-formula> <tex-math notation="LaTeX">$\mu \text{s}$ </tex-math></inline-formula> of EM signal.

(Invited) The Application of ALD and ALE Passivated MicroLED Display

Micro LED has been engaged to the next generation display technology and evolved for various applications from several electronic manufacturers and institutions. The less than 10×10 μm2 area of micro-LEDs is desired for the high pixel per inch (PPI) of micro display but the sidewall effect from etching process will drop the quantum efficiency. Therefore, a good passivation process is essential to be introduced into micro-LED process to overcome the efficiency droop issue. The passivation technique of atomic layer deposition (ALD) enables to improve the sidewall damages from dry etching and hence the quantum efficiency of micro-LEDs can be enhanced [1, 2]. Furthermore, the improvement can be enhanced within smaller size of micro LEDs. In particular, due to the excellent uniformity and step coverage passivation by ALD Al2O3 thin films, the interest in efficient ALD equipment has gradually increased. The atomic layer etching (ALE) is an advance etching process that promises an excellent depth control in shallow device structures. This paper provides an overview of ALD and ALE process for improving the micro-LED performance.The epitaxial structure of the micro-LED structures was grown on a c-plane sapphire substrate using metal-organic chemical vapor deposition (MOCVD). Following the development of MOCVD, a transparent and ohmic p-contact of indium tin oxide (ITO) with a thickness of 100 nm was deposited by electron-beam evaporation as a transparent and ohmic p-contact. The ITO layer can result in efficient electrical current spreading and provide a transparent emission window for topside emitting devices. N-type GaN, p-type GaN, and the multiple quantum wells (MQWs) active region, which consists of twelve pairs of InGaN wells and a GaN barrier with an emission wavelength of around 445 nm, are the major epitaxial layers. The growth phases of micro-LED processing included mesa etching, ohmic contact metallization, sidewall passivation deposition using Al2O3 of thickness 21.3 nm and SiO2 of thickness 300 nm, and interconnects. The inductively coupled plasma (ICP) dry etching process is used for the mesa etching method, and SiO2 and Al2O3 passivation layers are deposited using PECVD and ALD techniques. Since Al2O3 and SiO2 are well-known dielectric materials for photonic applications, they were selected. Because of the field effect passivation produced by negative fixed charges, Al2O3 has emerged as a unique passivation material for commercial solar cells. The passivation layers tend to prevent surface recombination and reabsorption of emitted light near the etched surface during forward-biased activity. Following the deposition of the dielectric passivation layers, a hole was created using the ICP etching technique for the deposition of metal contacts. The electrode metal consisted of 50/300 nm of Ti/Au for p- and n- metal contacts deposited using electron-beam evaporation followed by rapid thermal annealing. The process flow of ALD passivated micro-LED is shown in Fig. 1(a).Fig. 1(b) showed the effect of the ALD-Al2O3 passivation technique on the EQE enhancement of µ-LEDs. According to the literature, the peak EQE of a small LED is considerably small due to the surface and nonradiative recombination of the mesa layer caused by plasma-assisted dry etching. ALD sidewall passivation is an important method for reducing plasma damage in LEDs. We can observe the 21.3 nm Al2O3 passivation on micro-LED in Fig.1(a). Therefore, Fig. 1 (b) presents the EQEs of 10 × 10-µm2 µ-LEDs with and without an Al2O3 passivation layer deposited using the ALD technology for reducing plasma damage. Compared with the device not subjected to ALD-Al2O3 passivation, the 10 × 10-µm2 array subjected to ALD-Al2O3 passivation exhibited an EQE enhancement of 37.5%. This result proves that ALD sidewall passivation can effectively reduce the sidewall damage caused by SRH nonradiative recombination and thus increase the EQE of small LEDs.

Full-color micro-LED display based on a single chip with two types of InGaN/GaN MQWs.

In this Letter, we have successfully realized the full-color micro-LED display on a single-chip utilizing multi-wavelength multi-quantum wells (MQWs). The epitaxial wafer used for micro-LED array chips is designed with two types of MQWs including In0.1Ga0.9N/GaN and In0.55Ga0.45N/GaN grown by metal-organic chemical vapor deposition (MOCVD). A single-chip broad-spectrum multi-wavelength emission from 620 to 450 nm can be realized by changing the injection current to realize the regulation of carrier injection in the MQWs with different emission wavelengths. And the full-color micro-LED display with uniform brightness can be achieved by adopting the pulse width modulation (PWM) to adjust the duty cycle of micro-LED pixels at different pulse voltages. We expect this study will provide a promising research direction for full-color micro-LED displays, thus effectively avoiding the problems caused during the massive transfer and color conversion.

11.1: Invited Paper: Narrow‐Band Phosphors for Next Generation MiniLED and MicroLED Displays

Both phosphors and quantum dots are currently used in liquid crystal displays (LCDs), with GE's KSF narrow red phosphor demonstrating the best possible red color point, reliability, and efficiency. Next generation miniLED and microLED based displays require excellent color quality (gamut) and high brightness which will require color conversion materials with high EQE and narrow emission peaks. Here we describe recent advancements in narrow‐band phosphors which are being implemented in next‐generation miniLED displays, and being considered for enabling full‐color microLED displays of thefuture due to their excellent optical performance and robust nature.

Dual Role of Quantum Dots as Color Conversion Layer and Suppression of Input Light for Full-Color Micro-LED Displays.

In micro-light-emitting diode (micro-LED) displays with color-conversion layers, a facile and efficient technology getting rid of the use of the color filters leads to a big technical leap in cost-effective fabrication. In this study, it is demonstrated that quantum dot (QD) color conversion layers can significantly suppress residual blue excitation light because of the high extinction coefficients of QDs, ∼0.1% transmittance of blue light for green and red core/shell CdSe/ZnS QD film with thickness of less than 17 μm, and produce green and red colors. Incorporation of TiO2 nanoparticles into QD solutions enhances more than 10% of the luminous intensity by the scattering effect. It is found that the suppression of QD reabsorption is essential to achieve a high color-conversion efficiency. Our results provide a clear path to a cost-effective fabrication of QD conversion layer micro-LED displays over the full range of their applications.

Pixel crosstalk in naked-eye micro-LED 3D display.

Micro-LED display is considered to be the most promising display technology, and naked-eye 3D display represents the most advanced frontier of display. Naked-eye 3D micro-LED display has both the advantages of micro-LED display and the characteristics of naked-eye 3D display. Only conceptual solutions for 3D micro-LED display have been reported so far. The effect of luminescence distribution and the direction of micro-LED devices on crosstalk in the naked-eye 3D display is not clear. In this paper, we first investigated the chip-shaping effect on the emission characteristics, mainly the luminescence distribution and direction of the micro-LED, and then we investigated the influence of luminescence distribution and direction of micro-LED on the pixel crosstalk in naked-eye micro-LEDs 3D display. Our work should advance the micro-LED display, especially the micro-LED 3D display.

Journal of the Society for Information Display

Jiun-Haw Lee Editor in Chief Distinguished Papers, Best of Display Week 2021 Fifteen papers won the title of Distinguished Papers of Display Week 2021 and are published in the May 2021 issue of the Journal of the Society for Information Display (JSID). Congratulations!! The program committee of SID Display Week nominated these papers, which cover different research topics, including active-matrix devices, liquid crystal technology, flexible displays, display electronics, display systems, display measurements, micro light-emitting diode (microLED), and augmented reality (AR) displays. After strict and expedited reviews, JSID has published these high-quality papers before Display Week 2021 (May 17–21). Note that these papers will remain open access until the end of 2021. They are available at https://onlinelibrary.wiley.com/doi/toc/10.1002/(ISSN)1938-3657.DW21. The June 2021 issue of JSID contains two invited papers. The first paper is about microLED technologies by Sony Corporation, which is the pioneer in this field that demonstrated the prototype of microLED in 2012 (called crystalLED at that time). Technology details were disclosed in this manuscript, which provided clear illustrations of LED devices, pixel design, driving innovations, and assembly of this emerging display technology. The second invited paper is a review that compares three major display technologies: liquid crystal display (LCD) lit by miniLED backlight, organic light-emitting diode (OLED), and microLED. Note that the same group (Shin-Tson Wu's research group at the University of Central Florida) had published a similar review paper entitled, “Prospects and challenges of mini-LED and micro-LED displays,” in the July 2019 issue of JSID, which has attracted 41 citations thus far. The 2021 paper provides updates on the progress during the past two years, broadens the topic focus, provides an overview comparison from different display performances, and provides insight on future perspectives. 9.4-inch 228-ppi flexible micro-LED display | Seok-Lyul Lee, et al. | DOI: 10.1002/jsid.1022 This flexible microLED display was fabricated on a low-temperature polysilicon thin-film transistor (LTPS-TFT) backplane, which achieved a contrast ratio over 1,000,000:1 because of the self-emission characteristics of microLED. Additionally, the microLED display's device structure and process flow are disclosed. No degradation was observed after bending tests were conducted (1,000 times with a bending radius of 40 mm). This display also exhibited higher brightness and less color shift than OLED displays when observed from oblique angles. In other words, this microLED display had better viewing-angle performances compared to OLED display. Prospects and challenges of mini-LED, OLED, and microLED displays | En-Lin Hsiang, et al. | DOI: 10.1002/jsid.1058 LCD with mini-LED backlight, OLED, or microLED: who wins? It is an interesting question. This invited review paper attempts to provide an apples-to-apples comparison based on recent products and technologies from nine display performances, including ambient contrast ratio (ACR), motion picture response time (MPRT), viewing angle, color gamut, resolution, power consumption, cost, lifetime, and flexibility. Each display has its own advantages and disadvantages. For the current stage, flexibility, lifetime, and cost are the main issues for miniLED/LCD, OLED, and microLED, respectively. JSID is published monthly by Wiley. Subscription fees apply, but SID members and student members have free online access via sid.org/Publications.aspx. Many universities also have an institutional subscription. JSID is indexed in the Web of Science. Submit your original manuscript online via mc.manuscriptcentral.com/sid. Author guidelines can be found on the Journal's homepage: tinyurl.com/jsidhome. Editorial board: tinyurl.com/jsideb. Please direct any questions about JSID to its editor in chief at editor@sid.org. EarlyView: accepted papers about to be published can be accessed online via tinyurl.com/jsidev. The Journal is soliciting review papers on any display-related topic. If you have a great idea for a review paper, please contact the editor at editor@sid.org. Page charges for invited review papers will be waived.

Development of Metal Bonding for Passive Matrix Micro-LED Display Applications

Recently, there are many breakthroughs of display in the high resolution and vivid colors for applications in virtual/augmented reality. In this study, we have successfully developed a $64 \times 32$ array inline structure of a micro-LED ( $\mu $ LED) display by metal bonding an LED array with a glass circuit for connecting a passive matrix IC driver. The dimension of each $\mu $ LED was $100\,\,\mu \text{m}\,\,\times 100\,\,\mu \text{m}$ . After bonding, the optoelectronic characteristics were measured to evaluate the best process. By the circuit fabricated on glass and by using a bonding design, we were able to increase the usage ratio of the epitaxy layer and reduce the cost of the $\mu $ LED display. Au–In metal bonding demonstrated an excellent bonding result that can withstand a shear force up to 12.58 kg after being reinforced with UV glue. The output power of the display was 61 mW under 120 mA (@1.9 V) current injection to the whole display and the wall plug efficiency is 26.75%.