Light emitting diodes (LEDs) have gained widespread adoption in high-resolution display technology due to their superior characteristics, including high contrast ratio, high brightness, and high energy efficiency. While GaN-based LEDs are commonly fabricated on sapphire substrates, these conventional configurations face significant challenges regarding crystal quality and luminous efficiency. Patterned sapphire substrate (PSS) has emerged as an effective solution to enhance both GaN crystal quality and light extraction efficiency (LEE) of LEDs. However, when applied to small-size LED applications, conventional microscale PSS demonstrates inherent constraints, particularly in achieving optimal LEE and uniform light distribution. Recent investigations have demonstrated that LEE can be significantly improved through enhancing diffraction effects after reducing pattern dimensions to the nanoscale. Additionally, after the introduction of patterned Al₂O₃ - SiO₂ substrate, which is termed multi-material substrate (MMS), the LED performance can be optimized by increasing axial luminescence intensity and reducing the luminous angle. We fill the research gap in the optical design and optimization of nanoscale patterned substrate. Through optical simulations and calculations, the underlying mechanisms by which the nanoscale MMS enhances the optical performance of flip-chip LEDs have been revealed. Moreover, we aim to design the optimal parameters of nanoscale MMS patterns for flip-chip LEDs. The designed nanoscale MMS has been successfully fabricated by employing semiconductor manufacturing techniques.

We employ two simulation methods. First, the wave optics method in COMSOL software is used to simulate the transmittance of MMS with different pattern periods. By calculating the transmittance at different incident angles and wavelengths of plane waves, the influence of pattern periods on the transmittance is analyzed. Second, the Monte Carlo ray-tracing method in TracePro software is applied to simulate the impact of patterns with different materials and parameters on LEE and luminous angle. Different materials are selected for comparison, and the effects of pattern diameter and height on LED performance are studied while keeping the pattern period constant. The designed nanoscale MMS is fabricated using plasma enhanced chemical vapor deposition (PECVD), flexible nanoimprint lithography (NIL) and inductively coupled plasma (ICP) etching technology (Fig.3). NIL is selected due to its low cost and high throughput. Its flexible template fits various substrates, regardless of their shape, surface profile, or warpage. This enables the production of highly uniform patterned mask, thereby offering an innovative approach to nanoscale patterning. The etching gas used in ICP etching is BCl₃. The SiO₂ thin film deposited by PECVD is etched by ICP using the patterned mask formed by NIL, so as to form conical patterns on the SiO₂.

Wave optics simulations reveal that reducing the pattern period of MMS to 1.0 μm significantly enhances average transmittance. Notably, at an incident wavelength of 460 nm, MMS with a 1.0 μm pattern period exhibits a transmittance of 23.00%, compared to 15.63% for a 3.0 μm pattern period. This enhancement translates to a 47.15% increase in axial light emission when comparing flip-chip LEDs on nanoscale versus microscale MMS (Fig.1). Monte Carlo ray-tracing simulations demonstrate that SiO₂, owing to its low refractive index, proves optimal for patterned composite substrates. The axial LEE of flip-chip LEDs on MMS shows improvements of 9.85% and 112.53% compared to PSS of equivalent dimensions and flat sapphire substrate (FSS), respectively, while achieving a 7.46% reduction in luminous angle relative to PSS (Fig.2). The underlying mechanism for enhanced axial light output and reduced emission angle in LEDs on MMS can be attributed to the SiO₂/sapphire interface. This interface reduces the refracting angle of transmitted light. As a result, the light that was previously outside the critical angle of the sapphire/air interface can now fall within it. This enables more light to be emitted from the sapphire into the air. Following systematic optimization of the pattern parameters, the optimal pattern parameters for the period of 1.0 μm are determined to be 800?900 nm in diameter and 550?650 nm in height. Scanning electron microscope (SEM) and atomic force microscope (AFM) are employed for the morphological characterization of the fabricated nanoscale MMS. The results show that microstructures are well defined and precisely conform to the designed specifications (Fig.4).

We successfully develop an optimal nanoscale MMS for flip-chip LEDs, specifically designed for high-resolution self-emitting displays. Through wave optics simulations, we reveal the fundamental mechanism by which nanoscale patterns enhance diffraction effects, leading to significantly improved MMS transmittance in flip-chip LEDs. The optimization of material and pattern parameters is achieved through comprehensive Monte Carlo ray-tracing simulations. Theoretical calculations further demonstrate how MMS substantially increases the axial LEE while narrowing the luminous angle of flip-chip LEDs. A key finding demonstrates that SiO₂, with its low refractive index, plays a crucial role in light regulation, particularly at the SiO₂/sapphire interface, where it effectively directs light towards the axial direction. This interface thereby enables enhanced light transmission from the sapphire into the air, thereby improving axial LEE. However, our research has certain limitations. The current optimization under idealized conditions primarily focuses on enhancing LEE through pattern design, whereas practical LED optical performance requires comprehensive co-optimization across multiple interdependent aspects, including epitaxial process compatibility, crystal quality, electrical properties, and chip architecture. We plan to systematically investigate this integrated framework in future work to address existing research gaps. Furthermore, the introduction of MMS into LEDs increases thermal resistance due to the low thermal conductivity of SiO₂ (about 1.5 W/m·K, compared to sapphire’s about 30 W/m·K), which may adversely affect heat dissipation in LED devices. Therefore, subsequent efforts will elucidate the thermal resistance mechanisms of MMS and develop heat management strategies to minimize thermal compromises while preserving optical enhancements, thereby advancing LED display technology with balanced performance.