Will nanowire LEDs be the ultimate light engine?

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Image: Figure 1 | (a) Schematic diagram of the xz-plane FDTD simulation model. (b) Top view of a blue hexagonal nanowire LED. (c) Measured EL spectra for nanowire LEDs with different diameters
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New post from Optoelectronics; DOI 10.29026 / oes.2022.220021 It is studying whether nanowire LEDs could be the ultimate lighting driver for augmented and virtual reality displays.

High resolution density, wide field of view (FoV), lightweight and compact form factor, and low power consumption are pressing requirements for Augmented Reality (AR) and Virtual Reality (VR) displays. Compared with liquid crystal displays (LCD) and organic light-emitting diode (OLED) displays, microLED attracts more attention due to its higher peak brightness, excellent dark state, high resolution density, small form factor, and long life. On the other hand, as the wafer size decreases, the microLED efficiency decreases due to side wall defects. Therefore, the trade-off of high resolution density and external quantum efficiency (EQE) is a major challenge for the application of microLED as an AR/VR light engine, in addition to its high manufacturing cost.

Nanowire LEDs show great potential for simultaneously achieving high-resolution intensity and high EQE. Since each pixel is composed of a nanowire array, the LED nanowire efficiency is independent of the pixel size. In 2018, Aledia reported a micro-LED whose EQE is independent of pitch size when the pitch size is reduced from 1000 µm to 5 µm. Among the different nanowire structures, the InGaN/GaN six-dot LED is attractive because the emission wavelength can be controlled by wire diameter and its electrical performance is excellent. The first advantage greatly reduces manufacturing difficulties. However, these nanowires display different angular radiation patterns for red, green and blue colors in the far field, resulting in a remarkable angular color shift. In addition, a directional light drive is preferred since the acceptance cone of an AR/VR imaging system is usually within ±20°. Therefore, the nanowire geometry must be optimized to achieve identical radiation patterns for the three primary colors, high light extraction efficiency (LEE), and narrow angular luminance distribution simultaneously.

The authors of this article optimized the geometry of the InGaN/GaN nanowire LED by a 3D bipolar cloud with commercial finite-difference time-domain (FDTD, Ansys inc.) simulation software. They propose a multi-color InGaN/GaN dot-in-nanowire LED hexagonal LED model based on the experimental Ra results. They set a large hologram screen and a small box screen to calculate the emission energy and dipole energy, respectively, which determine the light extraction efficiency (LEE) by their ratio. Besides, the far-field distribution map is captured by a two-dimensional energy screen placed on top of the structure. As shown in Fig. 1(b), due to hexagonal symmetry, they simulate two sets of dipoles that are delineated by an inscribed circle and a circumscribed circle, respectively. The emission wavelength of bipolar sources follows the measured unfiltered emission spectra (solid lines in Fig. 1(c)). All three nanowires without color filters have side-lobe emission where it is difficult to control indium adatom diffusion perfectly. As shown by the dashed lines in Fig. 1(c), side-lobe emission is significantly suppressed after applying color filters.

By considering a cone-accepting AR imaging system, the authors determine the effective LEE to be LEE to be within ±20°. After optimization, the effective LEE of the blue, green and red nanowire LEDs increases from [9.3%, 18.8%, 30.6%] to [10.0%, 25.6%, 33.0%], respectively. In comparison to the blue-green size-dependent InGaN µLED and assuming that 100% of the output light can be coupled to the imaging system, the blue nanowire LED shows better performance than the µLED whose mesa size is smaller than 10 µm as shown in Fig. 3. (a). In addition, Fig. 3(b) indicates that the effective LEE of green nanowire LEDs is higher than that of 80 μm µLED. Compared to the red AlGaInP µLEDs, the red nanowire LEDs are more efficient than those with a wafer size of 20 μm (Fig. 3(c)). Remarkably, compared with a mesa size of 10 μm, the blue nanowire LEDs provide similar brightness, while the green and red nanowire LEDs can provide 1.6x and 1.4x higher efficiencies, respectively. Therefore, nanowire LEDs show clearly higher efficiency than µLED under small pixel size and high-resolution intensity.

Key words: nanowire LED / microdisplay / AR / VR light engines / angular color change

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Shin-Tson Wu is the Trustee Chair Professor in the College of Optics and Photonics at the University of Central Florida. He obtained his Ph.D. in Physics from the University of Southern California and a BA in Physics from National Taiwan University. He is an Academician of Academia Sinica, Charter Fellow of the National Academy of Inventors, Recipient of the Optica Edwin H. Land Medal (2022), SPIE Maria Goeppert-Mayer Award (2022), Optica Esther Hoffman Beller Medal (2014), SID Slottow-Owaki Award (2011), Optica Joseph Fraunhofer Award (2010), SPIE GG Stokes Award (2008), and SID Jan Rajchman Award (2008). His research group focuses on augmented and virtual reality, including light engines (LCOS, mini-LED, micro-LED, and OLED), optical systems (light guide, reflective optics, projection optics), and display materials (liquid crystals, and quantity). points and perovskites).

Currently, Professor Wu’s group holds 9 Ph.D. A student, a master’s student, one university student, and two visiting researchers. Professor Wu’s students have received many prestigious awards and scholarships. For example, in 2020 Tao Zhan (now at Apple) received the ILCS-FRL Platinum Award. In 2021, Jianghao Xiong (now at BIT, China) and Kun Yin (now at Amazon) took home Diamond and Platinum awards from ILCS-FRL, respectively. In 2022, Yannanqi Li received the ILCS-FRL Gold Award and En-Lin Hsiang received the SPIE Optics and Photonics Education Scholarship. In 2023, Fenglin Peng (now at Meta Reality Labs) received the SPIE Award for Early Career Achievement.

More information can be found at: https://lcd.creol.ucf.edu/

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Optoelectronics (OES) is an international, peer-reviewed, open-access interdisciplinary journal published by the Institute of Optics and Electronics, Chinese Academy of Sciences as the sister journal of optoelectronic advances (OEA, IF = 9.682). OES is dedicated to providing a professional platform to enhance academic exchange and accelerate innovation. OES publishes articles, reviews, and letters on fundamental breakthroughs in the basic sciences of optics and optoelectronics.

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Article reference Qian YZ, Yang ZY, Huang YH, Lin KH, Wu ST. High efficiency directional nanowire LEDs with low angular color shift for augmented reality and virtual reality displays. Optoelectronics 1220021 (2022). doi: 10.29026 / oes.2022.220021

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