Gallium nitride (GaN)-based light-emitting diodes (LEDs) have revolutionized the lighting industry, offering superior energy efficiency, extended operating life, and enhanced environmental sustainability over conventional lighting technologies. Recently, the push toward miniaturizing LEDs has gained momentum, driven by advancements in display devices, augmented reality, virtual reality, and other emerging technologies. However, the lack of cost-effective native substrates and high threading dislocation density in heteroepitaxial films grown on sapphire substrates remain significant obstacles to improving device performance. Additionally, Fresnel reflections at the epitaxy-substrate interface, caused by abrupt changes in refractive indices, further reduce light energy utilization.

Inspired by the compound eyes of moths, which exhibit excellent anti-reflective properties and strong light-absorption capabilities, researchers have sought to improve light utilization in LEDs. The challenge, however, lies in the rapid and precise processing of microstructures on the curved surfaces of optoelectronic devices. “Common projection lithography methods are highly sensitive to substrate shape, leading to reduced accuracy in microstructure definition on substrates with large warps or irregular shapes,” explains Professor Shengjun Zhou. “We propose a flexible nanoimprint lithography technique that enables high-throughput and high-quality processing of bionic microstructures on curved surfaces.”

The research team first designed a bionic microstructure array and flexible nanoimprinting molds. The microstructures were fabricated using thermo-compression molding, imprinting, UV exposure, and a specially designed two-step etching process to create a compound-eye-like silica microstructure (NCSM) template. Flexible nanoimprinting proved adaptable to substrate warpage, allowing for precise microstructure definition and durability. Compared to projection lithography, this method increased productivity by a factor of 6.4 and reduced economic costs by 25%.

The researchers studied the growth behavior of GaN crystals on the NCSM template through epitaxial growth interruption experiments. They discovered that adjusting the morphology of the nucleation layer with the NCSM template helps control the growth orientation of GaN, resulting in epitaxial films with low dislocation density. Moreover, the NCSM template significantly enhances the light extraction efficiency of devices by modulating photons with the bionic microstructures.

Professor Sheng Liu highlights the impact of these advancements: “Thanks to these improvements, the light output capability of mini-LEDs based on the NCSM template has been greatly enhanced.” He concludes, “This study is promising for the efficient fabrication of microstructures and performance enhancement of optoelectronic devices. In the future, we will continue to explore flexible nanoimprinting techniques and expect broader applications for this technology.”

By Impact Lab