Blue micro-LED Research Articles

Quantitative Analysis and Characterization of Sidewall Defects in InGaN-Based Blue Micro-LEDs

Quantitative Analysis and Characterization of Sidewall Defects in InGaN-Based Blue Micro-LEDs

Quantitative Evaluation of Carrier Dynamics in Blue Micro-LEDs: The Impact of Size and Current Density on Recombination and Injection Efficiency

Quantitative Evaluation of Carrier Dynamics in Blue Micro-LEDs: The Impact of Size and Current Density on Recombination and Injection Efficiency

InGaN quantum dots for micro-LEDs

Micro-scale light-emitting diodes (micro-LEDs) have received widespread attention in recent years for applications in display and optical communication. Compared with conventional quantum well active regions, quantum dots (QDs) can increase the carrier concentration at the same current density, which is beneficial for improving the efficiency and bandwidth of LEDs at low current densities. This is exactly what micro-LEDs need for display and communication applications. In this Perspective, we give a general introduction to InGaN QDs and provide an overview of the growth of InGaN QDs by metal-organic chemical vapor deposition. We then discuss the advances in green and red micro-LEDs based on InGaN QDs for display applications. This is followed by recent progress on high-speed blue micro-LEDs, which have great potential for use in chip-to-chip optical interconnections. Finally, we address the remaining challenges for a further improvement in InGaN QD-based micro-LEDs.

Performance improvement of red, green and blue InGaN micro-LEDs with distributed Bragg reflector

Abstract 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 (EL) spectra enhancement of RGB micro-LEDs was systematically investigated for realizing higher-performance micro-LEDs for display applications. At 5 A/cm2, 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/cm2 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.

External quantum efficiency enhancement of GaN-based blue LEDs by treating their full-M-sided hexagonal mesa with TMAH solution.

In recent years, III-Nitride-based micro light-emitting diodes (micro-LEDs) have emerged in many fields and gained more attention. However, fabricating high-efficiency micro-LEDs still remains a challenge due to the presence of sidewall damage. In this study, a GaN-based single blue micro-LED with a full-M-sided hexagonal mesa was prepared. The mesa has a circumradius of 10 µm and was treated with a tetramethylammonium hydroxide (TMAH) solution. Experimental results show that the sidewall defects introduced by dry etching damage act as non-radiative recombination centers and greatly impair the performance of the device. By constructing a full-M-sided hexagonal structure and soaking in a TMAH solution, the etching damage on the sidewall can be eliminated to the greatest extent, thereby reducing sidewall defects. In consequence, the peak EQE of the devices treated with the TMAH solution exceeded 10% at low current density, an increase of 9% compared with the untreated samples. This work provides, to our knowledge, a new approach to improving the efficiency of GaN-based micro-LEDs.

Research on Characteristics Matching of Micro-LED Devices

This paper presents the design of a 40 × 40 micro-light-emitting (micro-LED) test array based on a 20 mm × 20 mm substrate. A study of the relationship between luminous brightness, driving current, and driving voltage revealed that the data voltages of the red, green, and blue micro-LED array from 0 to 255 grey levels are 0.31 V, 0.29 V, and 0.30 V, respectively, under the condition that the target brightness of the white field is 1000 nits and the color temperature is 9300 K. The brightness range of the red micro-LED array is 64.8–101.2%, the brightness range of the green micro-LED array is 66.5–102.8%, and the brightness range of the blue micro-LED array is 53.5–129.2%. In order to overcome the luminance nonuniformity, a grey level depth of 12 -bit is required. A 10T3C pixel driver circuit based on low-temperature polysilicon (LTPS) with a depth of 12 bits greyscale is designed and fabricated into the micro-LED display. A brightness uniformity of 84.1–97.1% can be achieved by brightness correction combined with a 12-bit greyscale depth system for micro-LED display. This provides a valuable reference point for subsequent improvements in the quality of micro-LED displays.

Variation of carrier lifetime with optical excitation power density in micro-LEDs

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.

Impacts and effectiveness of sidewall treatment on the spatially resolved optical properties and efficiency enhancement for GaN-based blue and green micro-LEDs

InGaN-based micro-light-emitting diodes (micro-LEDs) are crucial for efficient full-color micro-displays, but suffer from severe degradation of external quantum efficiency (EQE) with size reduction. In this study, the impacts of chemical treatment on the sidewall condition and the EQE of the 10 μm InGaN-based blue and green micro-LEDs were demonstrated. The sidewall treatment can effectively alleviate the residual damage caused by dry etching and suppress the nonradiative recombination, facilitating the achievement of high internal quantum efficiency (IQE). Nevertheless, the sidewall light extraction of the micro-LEDs with smoother sidewall morphology after tetramethylammonium hydroxide (TMAH) treatment is lower than those with rough and prismatic sidewall surface. By utilizing microscopic hyperspectral imaging, the difference in chromatic properties between the inner region and the whole micro-LED caused by dry etching can hinder the stability of chromatic properties of InGaN-based blue micro-LEDs, which can be alleviated by sidewall treatment. The optimal duration for TMAH treatment is obtained as 10 min for both blue and green micro-LEDs, which is the consequence of the compromise between sidewall light extraction efficiency (LEE) and IQE, whereas the improvement of IQE by sidewall treatment for green micro-LEDs is less significant compared to that for blue ones, which can be attributed to the higher dislocation density and recombination via localized states of green InGaN micro-LEDs. This work demonstrates the evolution of IQE and LEE for blue and green micro-LEDs with different sidewall treatment strategies to achieving the ultimate EQE.

Limited impact of the sidewall effect in dependence of temperature for InGaN-based blue micro-LEDs grown on a silicon substrate.

The electroluminescence (EL) properties of InGaN-based micro-LEDs grown on a silicon substrate are investigated in this Letter to reveal the dominant mechanism in dependence on different temperatures and dimensions. The invalidation of sidewall nonradiative recombination and the impact of localization-induced carrier tunneling on the external quantum efficiency (EQE) are analyzed systematically to realize high performance silicon-based micro-LEDs. Microscopic EL mapping exhibits that the localized carriers in the silicon-grown micro-LED mainly recombine in the central region of mesa. The defects in the multiple quantum wells (MQWs) grown on the silicon substrate can lead to carrier tunneling and EQE reduction at cryogenic temperatures below 200 K, which is more conspicuous for the 30 μm device with a larger inner area ratio. The low-temperature EQE evolution can be attributed to the trade-off between localization-induced tunneling and Shockley-Read-Hall (SRH) recombination.

Enhancing the efficiency of InGaN-based micro-LEDs using indium tin oxide p-electrodes

Here, we demonstrate replacing opaque Cr/Pt/Au metal p-electrodes with transparent indium tin oxide (ITO) p-electrodes to increase the light output of InGaN-based micro-light-emitting diodes (micro-LEDs). ITO p-electrodes exhibit high transmittance of ∼ 80% across the visible spectrum and low resistivity, while metal p-electrodes exhibit negligible transmittance and significant absorption. The 20 × 20 µm2 and 50 × 50 µm2 green micro-LED arrays with ITO p-electrodes yield 1.25 and 1.20 times improvement in light output power compared to conventional metal p-electrodes. The on-wafer external quantum efficiency (EQE) of ITO p-electrode devices reach 7.36% and 7.35% at a current density of ≤ 1.6 A/cm2 for 20 × 20 µm2 and 50 × 50 µm2 arrays, while the on-wafer EQE of metal-based ones remain at 5.98% and 6.16%, respectively. This work opens a straightforward yet universal strategy for enhancing micro-LEDs’ performance, as ITO p-electrodes can be seamlessly integrated into red, green, and blue micro-LED configurations.

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

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

Optical analysis of III-nitride micro-light-emitting diodes with different sidewall treatments at low current density operation

In this work, the optical efficiency of III-nitride blue micro-LEDs (μLEDs) ranged from 5 × 5 to 60 × 60 μm2 with different sidewall treatments at low current density range was investigated. The results showed dielectric sidewall passivation using atomic layer deposition (ALD) has superior optical enhancement compared to conventional RF sputtering, where most of the enhancement occurred at low current density range. Additionally, the use of ALD sidewall passivation and chemical treatment offered significant efficiency improvement for different sizes of μLEDs at operating less than 1 A cm−2 and the devices without sidewall treatments did not emit light. The effect of sidewall treatments to the effective Shockley–Read–Hall (SRH) nonradiative recombination coefficient, or the effective A coefficient from the ABC model, was estimated. The effective SRH nonradiative recombination coefficient was suppressed by two orders of magnitude for devices with sidewall treatments compared to devices without sidewall passivation.

Optimizing Peak Light-Emitting-Diode (LED) Wavelengths for Enhanced Color Performance and Visual Health in Blue Micro-LED Displays with Color-Conversion Layers

Optimizing Peak Light-Emitting-Diode (LED) Wavelengths for Enhanced Color Performance and Visual Health in Blue Micro-LED Displays with Color-Conversion Layers

Broadband beam collimation metasurface for full-color micro-LED displays.

Near-eye displays are widely recognized as a groundbreaking technological advancement with the potential to significantly impact daily life. Within the realm of near-eye displays, micro-LEDs have emerged as a highly promising technology owing to their exceptional optical performance, compact form factor, and low power consumption. However, a notable challenge in integrating micro-LEDs into near-eye displays is the efficient light collimation across a wide spectrum range. In this paper, we propose what we believe to be a novel design of a broadband beam collimation metasurface for full-color micro-LEDs by harnessing wavefront phase modulation based on Huygens' principle. Our results demonstrate a substantial reduction in the full width at half maximum (FWHM) angles, achieving a reduction to 1/10, 1/10, and 1/20 for red, green, and blue micro-LEDs compared to those without the metasurface, which is the best collimation result as far as we know. The central light intensity increases by 24.60, 36.49, and 42.15 times. Furthermore, the significant enhancement in the light energy within ±10° is achieved, with the respective multiplication factors of 14.16, 15.60, and 13.00. This metasurface has the potential to revolutionize the field by enabling high-performance, compact, and lightweight micro-LED displays, with applications in near-eye displays, micro-projectors, and beyond.

Chromatic properties of InGaN-based red, green, and blue micro-LEDs grown on silicon substrate

The optical and chromatic characteristics of InGaN-based micro-light-emitting diodes (micro-LEDs) grown on silicon substrates with blue, green, and red emissions have been investigated. The red micro-LEDs exhibited a large full-width at half maximum and a strong blueshift upon increasing current density. The 20 μm blue, green, and red micro-LEDs achieve a maximum external quantum efficiency of 8.2% at 2.65 A/cm2, 5.1% at 5 A/cm2, and 0.62% at 0.1 A/cm2, respectively, which is superior to their 8 μm counterparts. The dependences of a color gamut on the temperature and current density of InGaN-based tri-color micro-LEDs can provide guidance for improving the performance of full-color micro-LED displays.

Blue micro-LED with a red/green blended polymer film for 3.5-Gbps visible light communication employing adaptive SNR-Flattening Algorithm

By the semipolar blue single GaN μ-LED and blended Ir(piq)2(acac) + CC-MP5 polymer thin film color converter, a high-speed white-light μ-LED-based system is built up for the applications of short-distance VLC. The characteristics and properties of both devices are analyzed to understand the requirements for transmission and illumination. By selecting the growing orient, the influence of QCSE is reduced in this GaN μ-LED. Meanwhile, possessing the low reflection characteristic, it is beneficial for signal modulation. For the polymer thin film color converter with a lifetime of 7.8 ns, low surface reflection and high conversion efficiency are thought of good properties. Color-converted cool white light has a CCT of around 7000 K and high color accuracy with a CRI of about 90. The APD-combined frequency responses of the GaN μ-LED and GaN μ-LED + polymer are measured as 750 MHz and 600 MHz, respectively. After the optimization by utilizing the adaptive SNR-flattening pre-emphasis algorithm, the transmission performance of the white-light μ-LED is significantly promoted. For NRZ-OOK encoding, a 1.4 Gbps 0.15-m free-space transmission is achieved with a rising time of 656.33 ps, a falling time of 493.32 ps, and a Q-factor of 4.75. Besides, in more advanced data formats, the performance of this white-light μ-LED can be better highlighted. For the same 0.15-m free-space VLC, a high-speed 3 Gbps broadband 8-QAM-OFDM transmission is fulfilled with an EVM of 23.9%, an average SNR of 12.5, and a BER below 3.8 × 10-3; while to the best of our knowledge, a record 3.5 Gbps BL-DMT transmission is implemented as well. This white-light μ-LED can also be integrated into large-scale arrays for multi-functional VLC applications.

The improvement of bonding metal layers for high resolution micro-LED display application

In this article, a resolution of 15 × 30 blue flip-chip micro-LED array with a pixel size of 20 × 35 μm2 was fabricated. The micro-LED array was hybridized with the silicon backplane via flip-chip bonding technology and liftoff process. The Au/In/Au multilayers could provide lower electrical resistivity and reduce cracks and voids at the film interface, leading to a micro-LED display that had uniform brightness and high reliability. The blue micro-LED display exhibits excellent optical and electrical characteristics, including a low turn-on voltage of 2.8 V and a brightness reach of 1.78 × 107 cd/m2 (nits) at 5 V. Additionally, there appears in the emission spectrum at a voltage of 3 V a blue shift of a peak wavelength from 455.51 to 453.96 nm. Further, the average shear strength of the micro-LED bonded by Au/In/Au multilayers increased to 2.24 g. The presented method has tremendous potential in high resolution micro-LED displays for augmented and virtual reality devices.

Inhomogeneous Light Emission From the Vertical Structured Blue Micro-LEDs

Inhomogeneous Light Emission From the Vertical Structured Blue Micro-LEDs

Ultra-thick inkjet-printed quantum dots layer for full-color micro-LED displays.

Micro-LEDs have promising development potential in display applications because of their outstanding performance. Achieving a full-color display based on micro-LEDs is one of the most important issues in commercial applications. In this paper, an effective method based on quantum dots and blue micro-LEDs was developed. Using an etching method, a thick black matrix was fabricated to reduce crosstalk and form a thick bank for quantum dots. Quantum dots were deposited in a thick black matrix using inkjet printing technology. With blue micro-LEDs, inkjet-printed quantum dot films can realize effective color conversion. The integrated blue micro-LEDs and red/green quantum dot films can achieve full-color displays without color filters, because the blue light leakage in the color conversion film can be reduced by the quantum dots themselves. The results suggest that inkjet-printed quantum dots are a promising way to achieve full-color micro-LED displays.

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

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