1 August 2016

UIUC uses MOCVD growth of cubic GaN on silicon to boost efficiency and brightness of green LEDs

Researchers at the University of Illinois at Urbana Champaign (UIUC) say that they have developed a new method for making brighter and more efficient green LEDs (R. Liu and C. Bayram, 'Maximizing cubic phase gallium nitride surface coverage on nano-patterned silicon (100)', Appl. Phys. Lett. 109, 042103 (2016)). Using the industry-standard metal-organic chemical vapor deposition (MOCVD) 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 solid-state lighting.

The work paves the way for novel green-wavelength emitters that can target advanced solid-state lighting on a scalable CMOS-silicon platform by exploiting cubic gallium nitride, says Can Bayram, assistant professor of electrical and computer engineering (who first began investigating this material several years ago while at IBM T.J. Watson Research Center).

"The union of solid-state lighting with sensing (e.g. detection) and networking (e.g. communication) to enable smart (i.e. responsive and adaptive) visible lighting, is further poised to revolutionize how we utilize light," says Bayram. "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. However, it is prone to polarization, where an internal electric field separates the negatively charged electrons and positively charged holes, preventing them from combining (diminishing the light output efficiency).

Until now, the only way researchers were able to make cubic GaN was to use molecular beam epitaxy (MBE), which is an expensive and slow crystal growth technique compared with the more widely used MOCVD.

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," says Liu. "So, they can overlap and, when that happens, the electrons and holes combine faster to produce light."

Picture: A new method of cubic phase synthesis: hexagonal-to-cubic phase transformation. The scale bars represent 100nm in all images. (a) Cross-sectional and (b) top-view SEM images of cubic GaN grown on U-grooved Si(100). (c) Cross-sectional and (d) top-view EBSD images of cubic GaN grown on U-grooved Si(100), showing cubic GaN in blue, and hexagonal GaN in red.

Ultimately, Bayram and Liu believe that their cubic GaN method may lead to LEDs free from the phenomenon of droop (whereby the light-emission efficiency of green, blue or ultra-violet LEDs declines as more current is injected) that has plagued the LED industry.

"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," says Liu.

Having better performing green LEDs could open up new avenues for LEDs in general solid-state lighting, it is reckoned, e.g. providing energy savings by generating white light through a color mixing approach. Other applications include ultra-parallel LED connectivity through phosphor-free green LEDs, underwater communications, and biotechnology such as optogenetics and migraine treatment.

In addition, cubic GaN could eventually be used to replace silicon in power electronic devices, and could replace mercury lamps to make ultra-violet LEDs that disinfect water, it is reckoned.

Tags: Green LEDs GaN MOCVD

Visit: http://scitation.aip.org/content/aip/journal/apl/109/4/10.1063/1.4960005

Visit: http://mntl.illinois.edu