Researchers at the University of Illinois at Urbana-Champaign have developed a new method for making brighter and more efficient green light-emitting diodes (LEDs). Using an industry-standard semiconductor growth technique, they have created gallium nitride (GaN) cubic crystals grown on a silicon substrate that are capable of producing powerful green light for advanced solid-state lighting.
Bayram and his graduate student Richard Liu made the cubic GaN by using lithography and isotropic etching to create a U-shaped groove on Si(100). This non-conducting layer essentially served as a boundary that shapes the hexagonal material into cubic form.
“Our cubic GaN does not have an internal electric field that separates the charge carriers—the holes and electrons,” explained Liu. “So, they can overlap and when that happens, the electrons and holes combine faster to produce light.”
Ultimately, Bayram and Liu believe their cubic GaN method may lead to LEDs free from a “droop” phenomenon that has plagued the LED industry for years. For green, blue, or ultra-violet LEDs, their light-emission efficiency declines as more current is injected, which is characterized as “droop.”
“Our work suggests polarization plays an important role in the droop, pushing the electrons and holes away from each other, particularly under low-injection current densities,” said Liu, who was the first author of the paper, “Maximizing Cubic Phase Gallium Nitride Surface Coverage on Nano-patterned Silicon (100)”, appearing in Applied Physics Letters. “Being polarization-free, cubic LEDs can have thicker active layers eliminating the reduced electron-hole overlap and current overflow. There are recent efforts by other groups to produce cubic GaN LEDs, but they are made on silicon carbide substrates, which is very expensive and have two orders of magnitude higher defectivity compared to our cubic GaN on silicon.”
Having better performing green LEDs will open up new avenues for LEDs in general solid-state lighting. For example, these LEDs will provide energy savings by generating white light through a color mixing approach. Other advanced applications include ultra-parallel LED connectivity through phosphor-free green LEDs, underwater communications, and biotechnology such as optogenetics and migraine treatment.
Enhanced green LEDs aren’t the only application for Bayram’s cubic GaN, which could someday replace silicon to make power electronic devices found in laptop power adapters and electronic substations, and it could replace mercury lamps to make ultra-violet LEDs that disinfect water.