Zhiyuan Liu, Haicheng Cao, Xiao Tang, Tingang Liu, Yi Lu, Zixian Jiang, Na Xiao & Xiaohang Li
Light: Science & Applications volume 14, Article number: 64 (2025)
Published: 26 January 2025
Introduction
InGaN-based micro-LEDs have attracted significant attention and development in both academia and industry due to their outstanding characteristics, such as high contrast, high brightness, wide color gamut, and long lifespan, which make them strong contenders for next-generation display technology. In recent years, numerous advanced techniques have been proposed and demonstrated to mitigate the sidewall effects in micro-LEDs and improve the light emission efficiency of the devices. To comprehensively address the physical origins of the micro-LED sidewall effect and evaluate the advanced methods and technologies designed to mitigate it, Professor Xiaohang Li’s research team at King Abdullah University of Science and Technology (KAUST) has published a literature review titled “Advanced technologies in InGaN micro-LED fabrication to mitigate the sidewall effect” in the internationally renowned journal Light: Science & Applications.
Abstract
The size of InGaN micro-LEDs is continuously decreasing to meet the demands of various emerging applications, especially in tiny micro-displays such as AR/VR. However, the conventional pixel definition based on plasma etching significantly damages the mesa sidewalls, leading to a severe reduction in efficiency as the micro-LED size decreases. This seriously impedes the development and application of micro-LEDs. In this work, we comprehensively explain the origin of micro-LED sidewall effects and corresponding physical models. Subsequently, we systematically review recent progress in micro-LED fabrication aiming at suppressing sidewall effects. Furthermore, we discuss advancements in micro-LED fabrication with “damage-free” techniques, which hold the potential to fundamentally address the issue of plasma damage in the micro-LED process. We believe this review will deepen the understanding of micro-LED sidewall effects and provide a better insight into the latest associated fabrication technologies for high-efficient InGaN micro-LEDs.
Background
Recently, InGaN micro-LEDs have gained widespread attention due to their remarkable features, such as high contrast, high brightness, wide color gamut, and long lifespan, which make them powerful candidates for next-generation display technology.
Micro-LED technology involves shrinking large-sized LEDs, typically larger than a hundred micrometers, to sizes as small as tens of micrometers or even a few micrometers. The reduction in LED size allows for higher resolution in display applications and brings many positive impacts on device performance. For instance, the reduction in mesa size results in a more uniform current distribution across the device, effectively mitigating current crowding and reducing device resistance. At the same time, this reduced current crowding leads to more uniform heat generation, which enhances the overall heat dissipation capability of the device. The decrease in series resistance and self-heating effects enables micro-LEDs to maintain extremely high current densities, which helps to increase modulation bandwidth in visible light communication applications. Additionally, the smaller mesa size enhances sidewall light reflection, improving the light extraction efficiency (LEE) of the device. The partial removal of the active region also helps to relax the strain at the mesa edges, alleviating the quantum-confined Stark effect and improving the radiative recombination rate.
However, the downsizing of micro-LEDs introduces more severe challenges. These challenges are referred to as the “micro-LED sidewall effects,” which are closely related to the fabrication process of micro-LEDs. The traditional micro-LED manufacturing process relies on plasma etching techniques to define the mesa or pixel area by etching away the excess active region. During this process, the mesa sidewalls are subjected to substantial plasma bombardment and UV photon irradiation, which causes severe etching damage and leads to the creation of defects such as lattice distortion and impurity contamination. These defects act as leakage paths for current and non-radiative recombination centers for Shockley-Read-Hall (SRH) recombination, which severely degrade the light emission efficiency of the device. Additionally, surface states at the sidewalls induce band bending, which significantly affects the carrier injection and distribution within the device. In recent years, numerous advanced techniques have been proposed to mitigate the sidewall effects and improve the light emission efficiency of micro-LED devices.
Solutions
In order to comprehensively explore the physical origins of micro-LED sidewall effects and evaluate and summarize the advanced techniques and ideas aimed at mitigating these effects, Professor Xiaohang Li’s team at King Abdullah University of Science and Technology (KAUST) has recently published a review article titled “Advanced technologies in InGaN micro-LED fabrication to mitigate the sidewall effect”.
The review highlights that the sidewall effect can be mitigated through two main approaches: (1) applying various post-etching treatments to suppress the sidewall effects, and (2) utilizing non-traditional etching methods to fabricate micro-LED pixels, referred to as “etching-free” or “damage-free” technologies. Post-etching treatments are generally classified into three categories. The first involves using chemical etching methods to remove the sidewall damage region. Typical methods include alkaline solutions such as KOH and TMAH; acidic solutions such as HF and HCl; and (NH4)2S sulfurization solutions. These solutions can remove defects caused by lattice distortion, surface unstable oxides, and other surface contaminants, thereby mitigating the presence of surface defects and improving the micro-LED sidewall effects, which ultimately enhances the device’s external quantum efficiency. It is important to note that after treatment, a large number of dangling bonds still remain on the sidewall surface, and these surface states can continue to influence device performance. Therefore, surface passivation becomes crucial as a second post-etching treatment process. Effective passivation not only reduces surface dangling bonds but also prevents sidewall surfaces from being contaminated by external environments. Common materials used for passivating micro-LED sidewalls include silicon dioxide, aluminum oxide, and aluminum nitride, deposited via PECVD, ALD, and sol-gel techniques. Among these, ALD (atomic layer deposition) has gained significant attention in recent studies due to its ability to achieve high-quality, uniform thin films with precise atomic-level thickness control and excellent step coverage. Additionally, techniques such as N-ion treatment and rapid thermal annealing can also be used to fill surface nitrogen vacancies, achieving a good passivation effect. In fact, due to the termination of the surface lattice structure or the transformation of the interface lattice structure, defects and defect states will inevitably exist, and even with solution etching and sidewall passivation, they cannot be completely eliminated. When the micro-LED size decreases to a sufficiently small scale, these residual defects may still negatively impact device performance. As a result, a third category of post-etching treatment, which involves regulating carrier transport pathways, can also contribute to mitigating the sidewall effect. By optimizing the micro-LED structure, more carriers can be injected into the bulk of the device, away from the damaged sidewall regions, thereby reducing non-radiative recombination at the sidewalls.
The advent of etching-free fabrication technologies provides a novel approach for mitigating micro-LED sidewall effects. Recently reported methods include selective epitaxy techniques, ion implantation, neutral beam etching (a low-damage etching technique distinct from traditional plasma etching), metal-assisted chemical etching, CHF3 and H2 plasma treatment, and the selective thermal oxidation technology proposed by Professor Li’s team at KAUST. By avoiding the use of traditional plasma etching, these techniques enable micro-LED devices to exhibit unique physical characteristics and performance, highlighting the promising future of these innovative technologies. However, it is worth noting that many of these methods are still in the early stages of verification and development and require further in-depth research to optimize their mechanisms and conditions. Even with the introduction of “etching-free” concepts and technologies, further advancements in traditional plasma etching-based “sidewall treatment engineering” are still necessary. Parallel development of both micro-LED fabrication routes will enable comparative studies, competition, and long-term progress.
Conclusion and Outlook
With the advancement of micro-LED technology, there has been a deeper understanding of the origins of sidewall effects and various solutions have been proposed. Through the optimization of post-etching treatment techniques and the development of etching-free fabrication processes, researchers continue to improve the light emission efficiency and reliability of micro-LEDs, laying a solid foundation for their widespread application in high-end displays, AR/VR, wearable devices, and optical communications.
Paper Information
Liu, Z., Cao, H., Tang, X. et al. Advanced technologies in InGaN micro-LED fabrication to mitigate the sidewall effect. Light Sci Appl 14, 64 (2025).
https://doi.org/10.1038/s41377-025-01751-y
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