Haifeng Wu, Xiao Lin, Qin Shuai, Youliang Zhu, Yi Fu, Xiaoqin Liao, Yazhou Wang, Yizhe Wang, Chaowei Cheng, Yong Liu, Lei Sun, Xinyi Luo, Xiaoli Zhu, Liancheng Wang, Ziwei Li, Xiao Wang, Dong Li & Anlian Pan
October 2024 Light Science & Applications 13(1)
DOI: 10.1038/s41377-024-01639-3
Abstract
Owing to high pixel density and brightness, gallium nitride (GaN) based micro-light-emitting diodes (Micro-LEDs) are considered revolutionary display technology and have important application prospects in the fields of micro-display and virtual display. However, Micro-LEDs with pixel sizes smaller than 10 μm still encounter technical challenges such as sidewall damage and limited light extraction efficiency, resulting in reduced luminous efficiency and severe brightness non-uniformity. Here, we reported high-brightness green Micro-displays with a 5 μm pixel utilizing high-quality GaN-on-Si epilayers. Four-inch wafer-scale uniform green GaN epilayer is first grown on silicon substrate, which possesses a low dislocation density of 5.25 × 108 cm−2, small wafer bowing of 16.7 μm, and high wavelength uniformity (standard deviation STDEV < 1 nm), scalable to 6-inch sizes. Based on the high-quality GaN epilayers, green Micro-LEDs with 5 μm pixel sizes are designed with vertical non-alignment bonding technology. An atomic sidewall passivation method combined with wet treatment successfully addressed the Micro-LED sidewall damages and steadily produced nano-scale surface textures on the pixel top, which unlocked the internal quantum efficiency of the high-quality green GaN-on-Si epi-wafer. Ultra-high brightness exceeding 107 cd/m2 (nits) is thus achieved in the green Micro-LEDs, marking the highest reported results. Furthermore, integration of Micro-LEDs with Si-based CMOS circuits enables the realization of green Micro-LED displays with resolution up to 1080 × 780, realizing high-definition playback of movies and images. This work lays the foundation for the mass production of high-brightness Micro-LED displays on large-size GaN-on-Si epi-wafers.
With the rapid advancement of display technology and the rise of micro-display applications such as Virtual Reality (VR) and Augmented Reality (AR), miniaturization and high integration have become new trends in the development of display technology. Against this backdrop, Micro-Light Emitting Diode (Micro-LED) display technology has emerged, addressing the shortcomings of traditional micro-display technologies in key metrics like resolution and brightness, and is recognized as an important direction for the next generation of micro-display technology.
As the most critical direct bandgap optoelectronic semiconductor, Gallium Nitride (GaN) possesses excellent properties such as high thermal conductivity, high breakdown field strength, and high saturation electron drift velocity, making it a key material platform for the development of Micro-LED micro-display technology. Currently, GaN-based Micro-LED micro-display technology has become a hotspot for global academic research and industrial development. However, due to limitations in material crystal quality and significant performance degradation caused by sidewall damage after chip miniaturization, existing Micro-LED micro-displays struggle to meet practical application requirements in terms of brightness and uniformity. Furthermore, the large-scale application of Micro-LED micro-displays necessitates strict control over processing yield, device reliability, and production costs, making large-diameter silicon substrate GaN wafer fabrication technology the optimal industrialization route for Micro-LEDs. The challenges in GaN-based Micro-LED wafer processing are significant, facing a series of technical and process challenges, with few successful cases reported to date.
In response to these challenges, a team led by Prof. Anlian Pan and Prof. Dong Li from Hunan University, in collaboration with Norsight Technology, Crystal Clear Optoelectronics, and others, successfully developed IC-level GaN-based Micro-LED wafer fabrication technology, which includes large-size high-quality silicon-based Micro-LED epitaxial layer preparation processes, non-aligned bonding integration technology, and atomic-level sidewall passivation technology. This technology achieved the highest brightness green Micro-LED micro-display module reported to date on silicon substrate GaN epitaxial layers.
The related results were published under the title “Ultra-high brightness Micro-LEDs with wafer-scale uniform GaN-on-silicon epilayers” in Light: Science & Applications.
High-Quality Silicon-Based Green Micro-LED Epitaxial Material
Silicon substrate GaN-based Micro-LED epitaxial layers offer core advantages such as large size, easy substrate peeling, and compatibility with CMOS processes, making them the optimal industrialization route for Micro-LED micro-display technology. Figure 1a shows a physical photo and structural schematic of a 4-inch silicon substrate GaN-based Micro-LED epitaxial layer. The research team developed a “Ga atomic surfactant-assisted growth” method to grow a high-quality AlN buffer layer under moderate temperature conditions at 1100℃, resulting in green Micro-LED epitaxial structures with low dislocation density (~5.25×10^8 cm^-2), low warping (16.7 μm), and excellent wavelength uniformity (standard deviation 0.939 nm) (as shown in Figures 1b-1d). This technology has been successfully extended to 6, 8, and 12-inch silicon substrate GaN wafers.
Ultra-High Brightness Micro-LED Devices
Based on the high-quality silicon substrate GaN-based green epitaxial material, the research team developed a wet etching and roughening technique for Micro-LED sidewalls. This technique takes advantage of the anisotropic etching rates of alkaline solutions on the polar and non-polar surfaces of GaN, successfully removing etching damage from the Micro-LED sidewalls. Coupled with atomic-level sidewall passivation, this effectively reduces non-radiative recombination rates at the sidewalls. This technology also achieves sub-micron level roughening of the light-emitting surface, significantly enhancing the Micro-LED’s light extraction efficiency. As shown in Figure 2, this wet etching and roughening technique led to a substantial increase in Micro-LED brightness, with pixel sizes of 5 μm in the Micro-LED micro-display array reaching a maximum luminous intensity of 10 million nits.
High-Resolution, High-Uniformity Micro-LED Micro-Displays
The research team further developed a vertically non-aligned bonding integration technology, successfully constructing independently addressable Micro-LED display chips closely integrated with silicon-based CMOS. The 0.39-inch display exhibits excellent brightness uniformity, with a standard deviation of only 720 Cd/m² (2.2%), achieving a pixel density of up to 3400 PPI for high-definition image and video display.
Conclusion and Outlook
This work developed IC-level Micro-LED wafer processing technology, including wet processing technology, atomic-level passivation technology, and vertical non-aligned bonding technology, based on high-quality silicon substrate GaN-based Micro-LED epitaxial materials. It successfully fabricated ultra-high brightness Micro-LED micro-display arrays and Micro-LED micro-displays integrated with silicon-based CMOS, providing crucial support for the large-scale manufacturing and application of high-brightness GaN-based Micro-LED micro-displays.
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