Researchers at the University of Illinois at Urbana-Champaign have developed a new method to make green LEDs brighter and more efficient.

They grew cubic gallium nitride (GaN) crystals on a silicon substrate, which produce powerful green light for advanced solid-state lighting. This breakthrough opens up possibilities for scalable CMOS-silicon platforms and novel green wavelength emitters, fundamentally changing the way light is utilized. "The integration of solid-state lighting with sensing, such as detection, and networking, such as communication, can enable intelligent visible light illumination, further changing the way we utilize light," said Can Bayram, an assistant professor of electrical and computer engineering at the University of Illinois. "LEDs compatible with CMOS can offer fast, efficient, low-power, and multifunctional technological solutions that occupy less space and are more affordable."

Traditionally, GaN exists in hexagonal or cubic forms. Hexagonal GaN is thermodynamically stable and more commonly found in semiconductors. However, hexagonal GaN is prone to polarization, where an internal electric field separates negatively charged electrons and positively charged holes, preventing them from recombining and reducing the efficiency of light output.

Previously, the only way to fabricate cubic GaN was through molecular beam epitaxy, an expensive and slow crystal growth method compared to the widely used metal-organic chemical vapor deposition (MOCVD) method.

Bayram and his graduate student Richard Liu created a U-shaped trench on Si (100) using lithography and isotropic etching, forming a non-conductive layer that transforms hexagonal material into cubic. "Our cubic GaN lacks an internal electric field that separates charge carriers (holes and electrons), so they can overlap, and when they overlap, electrons and holes combine faster, resulting in light emission," explained Liu.

Finally, Bayram and Liu believe their cubic GaN approach may address the long-standing issue of "efficiency droop" that has plagued the LED industry for years. For light-emitting colors such as green, blue, or ultraviolet LEDs, their light emission efficiency decreases with increasing injected current, known as "efficiency droop." "Our research suggests that polarization effects play a significant role in efficiency droop, particularly in pushing electrons and holes apart at low injected current densities," said Liu.

The development of brighter and more efficient green LEDs holds significant implications for solid-state lighting. These LEDs can save energy by generating white light through color mixing. Other advanced applications include superparallel LED connections using phosphor-free green LEDs, underwater communication, and biotechnological applications such as optogenetics and migraine treatment.