High resolution, wide field of view (FoV), lightweight and compact form factor, and low power consumption are required for augmented reality (AR) and virtual reality (VR) displays. Compared to liquid crystal display (LCD) and organic light-emitting diode (OLED) displays, microLEDs have attracted a lot of attention due to their high peak brightness, excellent dark state, high resolution density, small size, and long lifespan. On the other hand, as the size decreases, the efficiency of microLEDs decreases due to sidewall defects. Therefore, the trade-off between high resolution density and external quantum efficiency (EQE) is a major challenge in using microLEDs as AR/VR light engines.
Nanowire LEDs have great potential to achieve high resolution density and high EQE at the same time. Since each pixel is formed from an array of submicron nanowires, the efficiency of nanowire LEDs is independent of pixel size. In 2018, Aledia reported a nanowire LED whose EQE was independent of the pitch size when the pixel pitch size was reduced from 1000 μm to 5 μm. Among the different nanowire structures, InGaN/GaN dot-in-wire LEDs are particularly attractive due to their diameter-dependent emission wavelengths and excellent electrical properties. It is important to note that the emission wavelength of InGaN/GaN dot-in-wire LEDs depends on their diameter, which greatly reduces the difficulty of fabrication. However, the far-field angle distribution of the RGB primary colors does not match in such nanowire LEDs, which results in significant angular color shifts. In addition, in AR/VR imaging systems, the lens acceptance cone for the optical engine is usually in the range of ±20°. As a result, highly directional optical engines can greatly improve light utilization and reduce power consumption. Therefore, the geometry of this nanowire LED needs to be optimized to achieve the angular distribution matching of the three primary colors at the same time, improve the light extraction efficiency (LEE), and narrow the angular distribution.
University of Central Florida, USAProf. Shin-Tson WuThe research team used the commercial wave optics simulation software Finite-Difference Time-Domain (FDTD, Ansys inc.) Optimization of InGaN/GaN nanowire LED geometries with a 3D dipole cloud. Based on the experimental results of Ra, they proposed a multicolor hexagonal prism InGaN/GaN nanowire LED model[1]。 Figure 1(a) shows that each nanowire consists of a 300 nm n-GaN layer, six vertically aligned InGaN/GaN quantum dot light-emitting layers at 60 nm, a p-GaN layer at 150 nm, a GaN hexagonal pyramid overlay at 150 nm, and a transparent electrode ITO layer at 100 nm. Blue, green, and red nanowires are 630 nm, 420 nm, and 220 nm in diameter, respectively. They set up a 3D large box receiver and a small box receiver to calculate the transmit power of the nanowire LED and the dipole power in the light emitting layer, respectively, and define LEE by their ratios. In addition, the far-field map is captured by a two-dimensional power receiver placed above the structure. As shown in Figure 1(b), they simulate two sets of dipoles defined by inscribed circles and circumscribed circles, respectively, due to hexagonal symmetry. The emission wavelength of the dipole source follows the unfiltered measured emission spectrum (solid line in Figure 1(c)). Here, sidelobe excitation is present in all trichromatic nanowire LEDs, because the atomic concentration of In depends on its atomic diffusion, and this process is difficult to control perfectly. As shown by the dotted line in Figure 1(c), this sidelobe excitation can be suppressed by applying a color filter.
Figure 1 (a) Schematic representation of the FDTD simulation model in the x-z plane; (b) Top view of a blue hexagonal nanowire LED; (c) Measured EL spectra of a single nanowire LED of different diameters[1]
Figures 2(a-c) depict the normalized 2D angular distributions of the blue, green, and red nanowire LEDs they calculated, respectively. Among them, the blue nanowire LED (Fig. 2(a)) shows the widest angular distribution because the blue nanowire LED has the largest diameter and the shortest wavelength of light. As a result, the higher-order waveguide mode is excited, resulting in a larger emission angle. In addition, the intensity peak of the angular distribution of the green nanowire LED (Figure 2(b)) is not in the center, which is referred to as the batwing profile. On the other hand, the red nanowire LED (Figure 2(c)) can effectively concentrate the light in the vertical direction. The solid line in Figure 2(d) depicts the angular distribution of the blue, green, and red nanowire LEDs at azimuth φ = 0°.[蓝、绿、红] The full width of the nanowire LED at half height (FWHM) is: [48°、47°、35°], the angular mismatch between the RGB nanowire LEDs results in a color shift at different viewing angles (θ) (blue line in Figure 2(e)). At the ±20° receiver cone, the average color shift is 0.23, which is more than the level perceptible to the human eye.
By optimizing the height of the p-GaN overlay and the vertical position of the light-emitting layer, they found that the optimal conditions for blue, green, and red nanowire LEDs were: 1) completely remove the p-GaN overlay, and 2) set the thickness of the underlying n-GaN to 120 nm, 240 nm, and 250 nm, respectively. As shown by the dotted line in Figure 2(d),[蓝色、绿色、红色] The angular distribution of the nanowire LEDs FWHM is from[48°、47°、35°] Decrease to [37°、33°、24°]。 Due to the more matched angular distribution, the mean color shift decreases from 0.023 to 0.013 at θ = 20° until θ> 25° (Fig. 2(e)).
Figure 2 (a-c) Normalized 2D angular distribution of (a) blue, (b) green, and (c) red LEDs before optimization; (d) Comparison of normalized one-dimensional angular distributions between unoptimized (solid line) and optimized (dashed line) nanowire LEDs; (e) Comparison of simulated average color shift from 0° to 30° viewing angles before and after optimization
Considering that the AR imaging system receiver cone is typically ±20°, they define the effective LEE as the light extraction efficiency within ±20°. After optimization,[蓝、绿、红]The effective LEE of nanowire LEDs are from [9.3%、18.8%、30.6%]Increase to: [10.0%、25.6%、33.0%]。 Blue and green InGaN/GaN nanowire LEDs can achieve 58.5% internal quantum efficiency (IQE) [2]The red nanowire LED can achieve an IQE of 32.2%. [3], then its effective EQE will be blue = 5.9%, green = 15.0%, and red = 10.6%. with blue[4-5]and green InGaN μLEDs[4, 6-9]In comparison, their blue nanowire LEDs outperform those of 10 μm and smaller (Figure 3(a)). In addition, Figure 3(b) shows that the effective LEE of a green nanowire LED is even higher than that of an 80 μm μLED. with AlGaInP μLEDs in red[10]In comparison, their red nanowire LEDs are more efficient than 20 μm μLEDs (Figure 3(c)). At the same time, their blue nanowire LEDs can provide close brightness compared to 10 μm μLEDs, while the green and red nanowire LEDs are 1.6 and 1.4 times more efficient, respectively. As a result, nanowire LEDs are significantly more efficient than μLEDs at small pixel sizes.
Figure 3 Comparison between the calculated effective EQE (horizontal dashed line) of a Nanowire LED and the EQE measurements of different sizes of μLEDs (a) Blue InGaN μLEDs[4-5]and (b) green InGaN μLEDs[4, 6-9]and (c) red AlGaInP μLEDs[10]
With the help of a 3D dipole cloud model, Prof. Shin-Tson Wu’s team optimized the structure of nanowire LEDs, which increased the effective light extraction efficiency and reduced the angular color shift caused by the angular distribution mismatch. At the same time, they compared the effective light extraction efficiency of nanowire LEDs with μLEDs, and concluded that the efficiency of nanowire LEDs is significantly higher than that of μLEDs at small pixel sizes. This work lays a theoretical foundation for the future application of nanowire LED technology in AR/VR. This work was supported by AUO (Project No. 6501-8A64) and was published as a cover article in Opto-Electronic Science No. 12, 2022 with the title “Directional high-efficiency nanowire LEDs with reduced angular color shift for AR and VR displays”. (Source: Web of Science)
References
1. Ra Y-H, Wang R, Woo SY, Djavid M, Sadaf SM, et al. Full-color single nanowire pixels for projection displays. Nano Letters 16, 4608-4615 (2016).
2. Jain B, Velpula RT, Bui HQT, Nguyen H-D, Lenka TR, et al. High performance electron blocking layer-free InGaN/GaN nanowire white-light-emitting diodes. Optics Express 28, 665-675 (2020).
3. Nguyen HPT, Zhang S, Cui K, Korinek A, Botton GA, et al. High-efficiency InGaN/GaN dot-in-a-wire red light-emitting diodes. IEEE Photonics Technology Letters 24, 321-323 (2011).
4. Smith JM, Ley R, Wong MS, Baek YH, Kang JH, et al. Comparison of size-dependent characteristics of blue and green InGaN microLEDs down to 1 μm in diameter. Applied Physics Letters 116, 071102 (2020).
5. Olivier F, Tirano S, Dupré L, Aventurier B, Largeron C, et al. Influence of size-reduction on the performances of GaN-based micro-LEDs for display application. Journal of luminescence 191, 112-116 (2017).
6. Templier F. GaN‐based emissive microdisplays: a very promising technology for compact, ultra‐high brightness display systems. Journal of the Society for Information Display 24, 669-675 (2016).
7. Zhanghu M, Hyun B-R, Jiang F, Liu Z. Ultra-bright green InGaN micro-LEDs with brightness over 10M nits. Optics Express 30, 10119-10125 (2022).
8. Wang L, Wang L, Chen CJ, Chen KC, Hao Z, et al. Green InGaN Quantum Dots Breaking through Efficiency and Bandwidth Bottlenecks of Micro‐LEDs. Laser & Photonics Reviews 15, 2000406 (2021).
9. Hashimoto R, Hwang J, Saito S, Nunoue S. High‐efficiency green‐yellow light‐emitting diodes grown on sapphire (0001) substrates. Physica Status Solidi (c)10, 1529-1532 (2013).
10. Fan K, Tao J, Zhao Y, Li P, Sun W, et al. Size effects of AlGaInP red vertical micro-LEDs on silicon substrate. Results in Physics 36, 105449 (2022).
Brief introduction of the research team
Professor Wu is a CREOL Chair Professor in the School of Optics and Photonics at the University of Central Florida. He received his B.S. in Physics from National Taiwan University and his Ph.D. in Physics from the University of Southern California. He was inducted into the first inaugural Fellows of the American Academy of Inventors (2012) and was one of the first six inductees to the Florida Inventors Hall of Fame (2014). In addition, he has received the Optica Edwin H. Land Medal (2022), SPIE Maria Goeppert-Mayer Award (2022), OSA Esther Hoffman Beller Medal (2014), SID Slottow-Owaki Prize (2011), OSA Joseph Fraunhofer Award (2010), SPIE G. G. Stokes Award (2008), and SID Jan Rajchman Award (2008). His research group focuses on augmented reality (AR) and virtual reality (VR), including optical engines (LCOS, mini-LED, micro-LED, and OLED), optical systems (light guides, diffractive optics, and projection optics), and display materials (liquid crystals, quantum dots, and perovskites).
At present, there are nine doctoral students, one master’s student, one undergraduate student and two visiting scholars in Professor Wu’s group. Professor Wu’s students have received many awards and scholarships. For example, in 2020, Zhan Tao (now at Apple) won the ILCS-FRL Platinum Award. In 2021, Xiong Jianghao (currently teaching at Beijing Institute of Technology) and Yin Kun (currently working at Amazon) won the ILCS-FRL Diamond Award and Platinum Award, respectively. In 2022, Yan Nanqi Li won the ILCS-FRL Gold Award, and Enlin Xiang won the SPIE Optics and Photonics Education Scholarship. In 2023, Fenglin Peng (now at Meta Reality Labs) received the SPIE Early Career Achievement Award. Group Homepage: https://lcd.creol.ucf.edu/
Read the original article
Qian YZ, Yang ZY, Huang YH, Lin KH, Wu ST. Directional high-efficiency nanowire LEDs with reduced angular color shift for AR and VR displays . Opto-Electron Sci 1, 220021 (2022).
DOI: 10.29026/oes.2022.220021
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