More Efficiency for Green LEDs

Cross sectional SEM and EBSD images of cubic GaN grown on U-grooved Si(100) (Source: UIUC)

Cross sectional SEM and EBSD images of cubic GaN grown on U-grooved Si(100) (Source: UIUC)

Researchers at the Univer­sity of Illinois at Urbana Champaign have developed a new method for making brighter and more efficient green light-emitting diodes. Using an industry-standard semi­conductor growth technique, they have created gallium nitride cubic crystals grown on a silicon substrate that are capable of producing powerful green light for advanced solid-state lighting.

“This work is very revolutionary as it paves the way for novel green wave­length emitters that can target advanced solid-state lighting on a scalable CMOS-silicon platform by exploiting the new material, cubic gallium nitride,” said Can Bayram, assistant professor of electrical and computer engi­neering at Illinois. “The union of solid-state lighting with sensing and net­working  to enable smart visible lighting, is further poised to revo­lutionize how we utilize light. And CMOS-compatible LEDs can faci­litate fast, efficient, low-power, and multi-functional techno­logy solutions with less of a footprint and at an ever more affor­dable device price point for these appli­cations.”

Typically, GaN forms in one of two crystal structures: hexagonal or cubic. Hexagonal GaN is thermo­dynamically stable and is by far the more conven­tional form of the semi­conductor. However, hexagonal GaN is prone to a phenomenon known as polari­zation, where an internal electric field separates the negatively charged electrons and posi­tively charged holes, preventing them from combining, which, in turn, diminishes the light output efficiency. Until now, the only way researchers were able to make cubic GaN was to use molecular beam epitaxy, a very expensive and slow crystal growth method when compared to the widely used metal-organic chemical vapor depo­sition (MOCVD) method that Bayram used.

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,” 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 the 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.

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. (Source: UIUC)

Reference: R. Liu & C. Bayram: Maximizing cubic phase gallium nitride surface coverage on nano-patterned silicon (100), Appl. Phys. Lett. 109, 042103 (2016); DOI: 10.1063/1.4960005

Link: Micro and Nanotechnology Lab., University of Illinois at Urbana-Champaign, Urbana, Illinois, USA

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