Figure - available from: Journal of Physics D: Applied Physics
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(a) EQE measurement with a current density range from 1 to 100 A cm⁻² of 6, 10, 30, 60, 100 μm; Inset is the EQE value variation under ultra high current density injection with a log-scale demonstration; (b) luminance characterization of 6, 10, 30, 60, 100 μm devices and the inset is the optical power density values of 6 μm.
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In this paper, the GaN-based green Micro-LEDs with various sizes (from 3 to 100 μm) were fabricated and electro-optically characterized. Atom layer deposition (ALD) passivation and potassium hydroxide (KOH) treatment were applied to eliminate the sidewall damage. The size dependence of Micro-LED was systematically analyzed with current-versus-volta...
Citations
... In Figure 2b, a blue shift in the peak wavelength can be observed and the difference (∆λ) of emission peaks can be as large as 5.5 nm between 1 mA and 10 mA. This phenomenon can be explained by the screening effect and the reduction of the inherent quantum-confined Stark effect (QCSE) or enhanced band-filling effects [21][22][23][24][25][26]. shown in Figure 2b. ...
... In Figure 2b, a blue shift in the peak wavelength can be observed and the difference (∆λ) of emission peaks can be as large as 5.5 nm between 1 mA and 10 mA. This phenomenon can be explained by the screening effect and the reduction of the inherent quantum-confined Stark effect (QCSE) or enhanced band-filling effects [21][22][23][24][25][26]. ...
A special micro LED whose light emitting area is laid out in a U-like shape is fabricated and integrated with colloidal quantum dots (CQDs). An inkjet-type machine directly dispenses the CQD layer to the central courtyard-like area of this U-shape micro LED. The blue photons emitted by the U-shape mesa with InGaN/GaN quantum wells can excite the CQDs at the central courtyard area and be converted into green or red ones. The U-shape micro LEDs are coated with Al2O3 by an atomic layer deposition system and exhibit moderate external quantum efficiency (6.51% max.) and high surface recombination because of their long peripheries. Low-temperature measurement also confirms the recovery of the external quantum efficiency due to lower non-radiative recombination from the exposed surfaces. The color conversion efficiency brought by the CQD layer can be as high as 33.90%. A further continuous CQD aging test, which was evaluated by the strength of the CQD emission, under current densities of 100 A/cm² and 200 A/cm² injected into the micro LED, showed a lifetime extension of the unprotected CQD emission up to 1321 min in the U-shape device compared to a 39 min lifetime in the traditional case, where the same CQD layer was placed on the top surface of a squared LED.
... The negative impact from plasma damage is not limited to the sidewall surface but, depending on etching conditions, penetrates to a certain depth, known as the "dead zone." Due to a higher area ratio between the sidewall and mesa top surface, sidewall damage has a more pronounced impact on the efficiency of smaller devices, known as the efficiency size effect or sidewall effect in micro-LEDs 12,13 . Several mainstream methods have been reported to mitigate this issue: ...
The traditional plasma etching process for defining micro-LED pixels could lead to significant sidewall damage. Defects near sidewall regions act as non-radiative recombination centers and paths for current leakage, significantly deteriorating device performance. In this study, we demonstrated a novel selective thermal oxidation (STO) method that allowed pixel definition without undergoing plasma damage and subsequent dielectric passivation. Thermal annealing in ambient air oxidized and reshaped the LED structure, such as p -layers and InGaN/GaN multiple quantum wells. Simultaneously, the pixel areas beneath the pre-deposited SiO 2 layer were selectively and effectively protected. It was demonstrated that prolonged thermal annealing time enhanced the insulating properties of the oxide, significantly reducing LED leakage current. Furthermore, applying a thicker SiO 2 protective layer minimized device resistance and boosted device efficiency effectively. Utilizing the STO method, InGaN green micro-LED arrays with 50-, 30-, and 10-µm pixel sizes were manufactured and characterized. The results indicated that after 4 h of air annealing and with a 3.5-μm SiO 2 protective layer, the 10-µm pixel array exhibited leakage currents density 1.2 × 10 ⁻⁶ A/cm ² at −10 V voltage and a peak on-wafer external quantum efficiency of ~6.48%. This work suggests that the STO method could become an effective approach for future micro-LED manufacturing to mitigate adverse LED efficiency size effects due to the plasma etching and improve device efficiency. Micro-LEDs fabricated through the STO method can be applied to micro-displays, visible light communication, and optical interconnect-based memories. Almost planar pixel geometry will provide more possibilities for the monolithic integration of driving circuits with micro-LEDs. Moreover, the STO method is not limited to micro-LED fabrication and can be extended to design other III-nitride devices, such as photodetectors, laser diodes, high-electron-mobility transistors, and Schottky barrier diodes.
... 74 Additionally, Al 2 O 3 deposited using ALD is considered an effective dielectric material for passivation. [83][84][85][86][87][88][89][90][91][92] Figure 6(a) presents the EQEs of 20 Â 20 lm 2 blue lLEDs fabricated using different passivation methods. 83 For LED-4 (ALD passivation and HF etching), the maximum EQE was 32% higher than LED-1 (without passivation). ...
... 86 For GaN-based green lLEDs, ALD-Al 2 O 3 passivation and KOH treatment produced ideality factors lower than 1.5 for all samples (varying from 3 to 100lm). 88 Treated lLEDs (6 Â 6 lm 2 ) produced a peak EQE of 16.59% at 20 A⸱cm À2 and over 600 k cd cm À2 at 1 A cm À2 . On the other hand, compared to ALD Al 2 O 3 , ALD AlN passivation has been found to offer a stronger ability to remove sidewall defects in lLEDs as a result of the uniform passivation interface. ...
... Figure 9(b) presents the EQE peak from recently published papers for InGaN-based blue, green, and red lLEDs and AlGaInP-based red lLEDs as a function of the ratio of the peripheral length of the device to the area. 77,80,85,88 For InGaN lLEDs, the slope of EQE peak increases as the wavelength decreases from red to blue. This is in good agreement with experimental results demonstrating that v varies with the indium content. ...
Display technology has developed rapidly in recent years, with III-V system-based micro-light-emitting diodes (uLEDs) attracting attention as a means to overcome the physical limitations of current display systems related to their lifetime, brightness, contrast ratio, response time, and pixel size. However, for uLED displays to be successfully commercialized, their technical shortcomings need to be addressed. This review comprehensively discusses important issues associated with uLEDs, including the use of the ABC model for interpreting their behavior, size-dependent degradation mechanisms, methods for improving their efficiency, novel epitaxial structures, the development of red uLEDs, advanced transfer techniques for production, and the detection and repair of defects. Finally, industrial efforts to commercialize uLED displays are summarized. This review thus provides important insights into the potential realization of next-generation display systems based on uLEDs.
... Also, for application to near-eye displays, the resolution should exceed 1500 PPI. A high-resolution display provides enhanced immersion, a diminished screen-door effect, a sharply projected enlarged image, and heightened visual comfort during use [11]. ...
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.
... Also, for application to near-eye displays, the resolution should exceed 1500 PPI. A high-resolution display provides enhanced immersion, a diminished screen door effect, a sharply projected enlarged image, and heightened visual comfort during use [11]. ...
Wafer-scale blue micro-light-emitting diode (micro-LED) arrays are 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 exhibit a densely packed morphology and lower resistivity compared to those of the e-beam-deposited ITO (E-ITO) films, the forward voltage (VF) values of a micro-LED with the S-ITO films are higher than those with the E-ITO films. The VF values for a single pixel and for four pixels with E-ITO films are 2.82 V and 2.83 V, respectively, while the corresponding values for S-ITO films are 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 are only 0.09 V and 0.10 V, respectively. This extremely uniform VF variation is suitable for realizing 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.
... Meanwhile, the structure of the quaternary material AlGaInP μLED is more fragile than that of the ternary material nitride μLED; thus, improving the efficiency of AlGaInP μLED is an urgent issue to expand its applications. Many researchers have optimized sidewall etching (ICP parameter optimization, laser direct writing lithography) [1][2][3], sidewall processing (hydrochloric acid, potassium hydroxide, tetramethylammonium hydroxide treatment) [4][5][6], and sidewall passivation (ammonium sulfide, octadecene, atomic layer deposition, distributed Bragg reflector) [7][8][9][10][11][12][13][14][15] to improve the characteristics of micro-LEDs. Luming Yu mentioned that fine and smooth sidewalls can be obtained by laser direct writing lithography, thereby reducing sidewall damages [3]. ...
The efficiency of AlGaInP micro light-emitting diodes (micro-LEDs) was far weaker than that of GaN-based micro-LEDs in structure and performance. Consequently, there was an urgent demand to enhance their efficiency. In this study, a citric acid treatment strategy is proposed to improve the efficiency of red micro-LEDs, and the etching uniformity of different concentrations was first confirmed. We optimized the concentration of citric acid to 1:1 and modulated the wet etching time at 0, 30, 60, 90, and 120 s to treat the sidewalls of devices. Under an injection current density of 68 nA/cm
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, the forward voltage (Vf) of micro-LEDs after soaking in citric acid ranged from 1.40 to 1.45 V. Compared with the sample operated at the forward voltage without citric acid sidewall treatment, AlGaInP micro-LEDs displayed significantly enhanced forward voltage. This indicates that citric acid effectively removed N-GaAs without damaging the electrical properties of the devices. Among all citric acid-treated micro-LEDs, the sample with a 60 s wet etching process showed the best improvement, with the light output power and external quantum efficiency (EQE) increased by 31.08% and 5.4%, respectively. Our proposed method to treat AlGaInP micro-LEDs presents promising opportunities for the future development of high-performance optoelectronics.
... Importantly, upon scaling micro-LEDs down to nanoscale dimensions, surface defects become more pronounced and detrimental to their performances. Thus, ALD clearly offers a precise and controlled method to passivate the surfaces of micro-LEDs, leading to higher luminous efficiencies and improved overall device reliabilities [43]. The future development of ALD is therefore expected to focus on enhancing luminous efficiencies, stabilities, and temperature resistances of luminescent materials based on new materials, such as rare-earth elements, perovskites, and metal-organic frameworks. ...
Driven by the growing demand for next-generation displays, the evolution of advanced luminescent materials with exceptional photoelectric properties, such as quantum dots and phosphors are accelerating rapidly. Nevertheless, the primary challenge confronting the practical applications of these luminescent materials lie in meeting high durability requirements. This perspective delves into atomic layer deposition (ALD) developed for stabilizing luminescent materials, which is employed in the fabrication of flexible display devices through material modification, surface and interface engineering, encapsulation, cross-scale manufacturing, and simulations. To satisfy low-cost, high-efficiency, and high-reliability manufacturing requirements, equipments such as spatial ALD and fluidized ALD have been developed. The strategic approach establishes the groundwork for the development of ultra-stable luminescent materials, highly efficient LEDs, and thin-film packaging. This significantly enhances their potential applicability in LED illumination and backlight displays, marking a notable advancement in the display industry.
... To achieve these goals, research and development of liquid crystals, 8) organic electroluminescence, 9) quantum dots, 10) and μLEDs (light-emitting diodes with device sizes ranging from a few micrometers to several tens of meters square) [11][12][13] are underway; besides high resolution and high definition, μLEDs have excellent characteristics such as high efficiency, high brightness and high colour saturation, which is an essential requirement for the metaverse. GaInN-based LEDs are considered extremely useful because their efficiency rarely decreases, [14][15][16][17] even when the luminescence diameter is miniaturised to a few micrometres. It was extremely difficult to obtain red LEDs using nitride semiconductors in the past. ...
We report a 330 ppi monolithic RGB μLED (ultra-compact light-emitting diode) array of blue, green and red GaInN-based LEDs stacked on the same wafer. Considering it is challenging to form ohmic electrodes on the plasma-etched p-type GaN surface, GaInN-based tunnel junctions were used to connect each LED, and anode electrodes for the blue and green LEDs were formed on n-type GaN. The fabricated stacked monolithic μLED arrays were tested at room temperature (approximately 26°C) and DC. Each μLED emitted blue, green and red with peak wavelengths of 486, 514 and 604 nm at a current density of 50 A/cm ² .
... Light output power (LOP) and the external quantum efficiency (EQE) values of Micro-LED arrays before and after LLO were calculated using the following equation [31]: where E represents the averaged photon energy across the emission peak, while e, P, and I correspond to the electron charge, radiometric power, and injection current, respectively. The LOP of different Micro-LEDs was integrated using EL spectra, which were measured under the same conditions for comparative analysis. ...
... Light output power (LOP) and the external quantum efficiency (EQE) values of Micro-LED arrays before and after LLO were calculated using the following equation [31]: ...
This work explores the pivotal role of laser lift-off (LLO) as a vital production process in facilitating the integration of Micro-LEDs into display modules. We specifically investigate the LLO process applied to high-performance gallium nitride (GaN)-based green Micro-LED arrays, featuring a pixel size of 20 × 38 μm on a patterned sapphire substrate (PSS). Scanning electron microscopy (SEM) observations demonstrate the preservation of the GaN film and sapphire substrate, with no discernible damage. We conduct a comprehensive analysis of the optoelectrical properties of the Micro-LEDs both before and after the LLO process, revealing significant enhancements in light output power (LOP) and external quantum efficiency (EQE). These improvements are attributed to more effective light extraction from the remaining patterns on the GaN backside surface. Furthermore, we examine the electroluminescence spectra of the Micro-LEDs under varying current conditions, revealing a slight change in peak wavelength and an approximate 10% decrease in the full width at half maximum (FWHM), indicating improved color purity. The current–voltage (I–V) curves obtained demonstrate the unchanged forward voltage at 2.17 V after the LLO process. Our findings emphasize the efficacy of LLO in optimizing the performance and color quality of Micro-LEDs, showcasing their potential for seamless integration into advanced display technologies.
... Figure 1(b) depicts the top-view layouts of the 10 × 10 µm 2 blue and green micro-LEDs. The fabrication process in detail can refer to our prior research [21]. (I-V) and semi-log current density-voltage (J-V) characteristics of three different sizes: 10 µm (10 × 10 µm 2 ), 30 µm (30 × 30 µm 2 ) and 80 µm (80 × 80 µm 2 ) blue and green micro-LEDs. ...
... In Fig. 2 (b), the semi-log curve of the IV data demonstrates the reverse leakage current at reverse bias. The low leakage values, ranging from 10 × 10 −13 to 10 × 10 −12 , can be attributed to the effective sidewall treatment and repair processes employed during fabrication [21]. Both blue and green exhibit a consistent J-V performance within Shockley model region (from 0.001 to 10 A/cm 2 ). ...
... To our surprise, IQE values for both blue and green are approximately 90%, indicating our micro-LEDs are improved with well-designed ITO current spreading layer, hydroxide sidewall treatment and subsequently ALD Al 2 O 3 passivation. The detailed fabrication can refer to [21]. The theoretical IQE curves are fitted by assuming the CIE is 100% in the range of ultra-low current density level (smaller than the current density at peak EQE). ...
In this paper, we investigate the efficiency droop phenomenon in green and blue GaN-based micro-LEDs of various sizes. We discuss the distinct carrier overflow performance in green and blue devices by examining the doping profile extracted from capacitance-voltage characterization. By combining the size-dependent external quantum efficiency with the ABC model, we demonstrate the injection current efficiency droop. Furthermore, we observe that the efficiency droop is induced by injection current efficiency droop, with green micro-LEDs exhibiting a more pronounced droop due to more severe carrier overflow compared to blue micro-LEDs.
