Microstructured Air Cavities as High-Index Contrast Substrates with Strong Diffraction for Light-Emitting Diodes

Area of Science:

• Optoelectronics
• Materials Science
• Nanotechnology

Background:

• Two-dimensional dielectric gratings enhance light outcoupling in LEDs by overcoming total internal reflection.
• Limited index contrast in conventional patterned media restricts outcoupling efficiency.

Purpose of the Study:

• To develop and investigate cavity engineered substrates (CESs) with high-index-contrast for enhanced light extraction in LEDs.
• To demonstrate the effectiveness of CESs in improving optical power and light extraction efficiency.

Main Methods:

• Fabrication of hexagonally arranged hemispherical air cavities with crystallized alumina shells on GaN substrates.
• Wavelength-resolved diffraction measurements and Fourier analysis to study diffraction modes.
• Fabrication and characterization of InGaN/GaN LEDs on CESs and patterned sapphire substrates.
• Full-vectorial simulations to analyze light extraction efficiency and optical power.

Main Results:

• CESs exhibit strong diffraction and dramatic excitation of low-order diffraction modes due to high-index-contrast air/alumina core/shell patterns.
• LEDs fabricated on CESs showed a ~39% enhancement in optical power compared to state-of-the-art patterned sapphire substrates.
• Simulations confirmed enhanced optical power and showed increasing extraction efficiency with expanded air cavity volume.
• CES LEDs demonstrated a significant beaming effect and improved light extraction via the top escape route.

Conclusions:

• Cavity engineered substrates offer a promising approach to significantly enhance light extraction efficiency in LEDs.
• The high-index-contrast and specific morphology of CESs enable efficient outcoupling of guided modes.
• This technology improves LED performance without negatively impacting essential electrical characteristics.