The demand for near-eye displays in augmented and mixed reality applications is driving the need for displays with ultrafast response times, high resolution, high luminance, and a wide dynamic range suitable for outdoor use. Current technologies, like liquid crystal displays (LCDs) and organic light-emitting diodes (OLEDs), have limitations in meeting all these requirements. LCDs are transmissive, leading to lower energy efficiency and contrast, while OLEDs, despite their superior efficiency and dynamic range, face challenges in achieving ultra-high pixel densities due to the solvent sensitivity of organic materials. High-resolution manufacturing methods are limited by these material challenges. The inherent susceptibility of organic polymers to solvent dissolution poses a major obstacle to achieving the high pixel density needed for near-eye displays through direct lithography, thus prompting the use of shadow masking techniques. Micro-LEDs based on inorganic semiconductors like GaN offer a promising alternative due to their high energy efficiency, long lifetime, and high luminosity. Recent advances in flip-chip bonding of GaN epitaxial layers onto silicon wafers have enabled CMOS integration. However, miniaturizing individual pixels to a few micrometers presents significant challenges related to surface warping, pixel resolution, and large-scale array formation. This study addresses these challenges by developing a novel approach combining advanced microfabrication techniques with QD-based color conversion to create a full-color, emissive display exceeding the retinal resolution limit.

Existing display technologies such as LCDs and OLEDs struggle to meet the demanding specifications of near-eye displays, particularly in terms of resolution and response speed. While OLEDs offer improved efficiency and dynamic range over LCDs, their reliance on organic materials and the challenges associated with solvent-based patterning limit their pixel density. Recent advancements, such as the use of nanopatterned metasurface mirrors and advanced patterning techniques, have pushed OLED resolution to impressive levels (e.g., 10,000 PPI). However, these approaches often involve trade-offs in terms of manufacturing complexity and cost. Inorganic micro-LEDs based on GaN are considered promising alternatives due to their inherent advantages in terms of efficiency, lifetime, and color purity. However, the integration of GaN micro-LEDs with high resolution presents challenges, including substrate warping, alignment issues during wafer bonding, and difficulties in patterning high-quality color conversion layers. This study builds on previous research in GaN-on-Si technology and QD-based color conversion to address these challenges and achieve a breakthrough in display resolution.

This study employed a multi-pronged approach to create a high-resolution, full-color micro-LED display. First, a wafer-scale epilayer transfer technique was used to overcome the challenges associated with fabricating GaN LEDs on a foreign silicon substrate. This involved a 'bond-before-pattern' strategy, where a thin eutectic bonding alloy (Ti, Sn, Ni) was used to bond the GaN epilayer to a silicon substrate before patterning. This approach, unlike conventional methods, significantly improved alignment accuracy. A low-temperature, low-stress bonding process and a reliable substrate removal process (using HNA solution and RIE) were crucial for minimizing wafer warping and maintaining alignment. The use of a hybrid transparent conductor, combining ITO and a metallic mesh structure, improved the electrical properties and light extraction efficiency of the 20,000-pixel LED array. Second, to address the challenge of patterning quantum dots (QDs) with high resolution, a novel solvent-free patterning technique was developed. This involved careful surface energy control using self-assembled monolayers (SAMs, specifically perfluorooctyl trichlorosilane (FOTS)) and a photopatternable elastomer-based mask. This approach allowed for the creation of sub-10 µm QD pixels without exposing the QDs to solvents, thus preserving their optical properties. Red and green QDs (CdSe core/CdZnSe shell with TiO2 ligands) were patterned sequentially using double-layer lithography and O2 plasma etching, with a SiO2 passivation layer used between the green and red QD layers. Finally, a full-color display was created by integrating the patterned QD layers onto the GaN micro-LED array. The optical properties of the QD layers, including quantum efficiency (EQE), were characterized as a function of film thickness using a quantum efficiency measurement system (QE-1100, Otsuka Electronics). The effect of a distributed Bragg reflector (DBR) structure on color purity was also investigated using optical simulations and experimental fabrication of a DBR on a separate glass substrate.

The researchers successfully fabricated a wafer-scale full-color micro-LED display with 5 µm pixels, achieving a resolution of 1270 PPI—significantly exceeding the retinal limit. The bond-before-pattern technique, using a thin eutectic bonding layer, enabled lithography-level alignment precision during wafer bonding, effectively minimizing misalignment issues inherent in conventional flip-chip methods. The solvent-free QD patterning process, relying on surface energy manipulation and elastomeric topographical masks, successfully generated high-resolution (sub-10 µm) QD patterns for color conversion without compromising QD quality. The study demonstrated that the light extraction efficiency was significantly improved by utilizing a hybrid transparent conductor incorporating both ITO and a metallic mesh structure, especially for larger arrays. Analysis of the QD color conversion process revealed that the external quantum efficiency (EQE) exhibited a saturation behavior with increasing QD film thickness. The optimal thickness was determined experimentally, balancing the increased light absorption with reduced transmission and self-absorption effects. The implementation of a DBR structure was shown to enhance color purity by reducing optical crosstalk, resulting in RGB spectra with full-width-half-maximum (FWHM) values comparable to those achieved using black and grey photoresist matrices. The high-resolution, full-color display was characterized through microscopic imaging, spectral analysis, and quantum efficiency measurements, confirming the effectiveness of the developed techniques.

This study's success in creating a high-resolution (1270 PPI) micro-LED display with integrated QD color conversion demonstrates a significant advancement in near-eye display technology. The combination of wafer-scale GaN epilayer transfer, precise lithography-level alignment during bonding, and solvent-free QD patterning addresses key limitations of existing display technologies. The findings directly contribute to the development of high-performance head-mounted displays (HMDs) for augmented and virtual reality applications, as well as other high-resolution display systems. The achievement of a resolution beyond the retinal limit opens new possibilities for immersive visual experiences. The solvent-free QD patterning method offers a significant advantage over conventional techniques, ensuring the preservation of QD optical properties and simplifying the manufacturing process. The study's success in achieving high EQE values, while also demonstrating the impact of the DBR on color purity, indicates that further optimization of these aspects will significantly enhance display performance.

This research successfully demonstrated a novel approach for creating high-resolution, full-color micro-LED displays by integrating several advanced technologies. The use of a bond-before-pattern technique along with a solvent-free QD patterning method resulted in a 1270 PPI display, surpassing the retinal limit. Future research should focus on improving the fill factor, reducing light leakage, and exploring the integration of additional functionalities, such as advanced driver circuits, to enhance display performance further. This combined technology platform using QDs, GaN, and silicon electronics paves the way for superior near-eye displays.

While the study achieved a significant breakthrough in display resolution, some limitations should be noted. The current design may have limitations in terms of fill factor and light leakage, although these were partially addressed by implementing a DBR structure. Further research and optimization of the DBR and other peripheral technologies are needed to enhance the overall performance of the display. The quantum efficiency measurements were conducted on non-patterned QD films, and the actual EQE of the patterned QD layers in the integrated device might be slightly lower. Scaling up the production process to a larger scale while maintaining yield and uniformity remains a challenge.