Precisely fabricated deep-UV LED reaches wavelength of 232 nm
The thicknesses of the GaN/AlN layers are controlled to monolayer precision.
|Members of the Cornell group stand in front of a molecular beam epitaxy system. (Image: Cornell University)|
UV-C (200 to 280 nm) light sources are useful for sterilization, as these particular wavelengths of UV light penetrate the membranes of viruses, bacteria, mold, and dust mites, attacking their DNA and killing them.
Now, researchers from Cornell University (Ithaca, NY) and the University of Notre Dame (Notre Dame, IN), using atomically controlled thin monolayers of gallium nitride (GaN) and aluminum nitride (AlN) as active regions, have created LEDs that produce deep-UV emission at 232, 246, and 270 nm wavelengths.1 The 232 nm emission represents the shortest recorded wavelength using GaN as the light-emitting material.
One of the major challenges with UV LEDs is efficiency, which is measured in three areas: injection efficiency (the proportion of electrons passing through the device that are injected into the active region); internal quantum efficiency (IQE; the proportion of all electrons in the active region that produce photons); and light-extraction efficiency (the proportion of photons generated in the active region that can be extracted from the device and are actually useful).
"If you have 50% efficiency in all three components, multiply all of these and you get one-eighth," notes Moudud Islam, one of the Cornell researchers. "You’re already down to 12% efficiency."
Using GaN instead of AlGaN
In the deep-UV range, all three efficiency factors suffer, but the researchers found that by using GaN instead of conventional AlGaN, both IQE and light-extraction efficiency are enhanced.
Injection efficiency is improved through the use of a polarization-induced doping scheme for both the negative (electron) and positive (hole) carrier regions, a technique the group explored in previous work.
Now that the group has proven its concept of enhanced deep-UV LED efficiency, its next task is packaging it in a device that could one day go on the market. Deep-UV LEDs are used in food preservation and counterfeit currency detection, among other things.
Further study will include packaging both the new technology and existing technologies in otherwise similar devices, for the purpose of comparison.
"In terms of quantifying the efficiency, we do want to package it within the next few months and test it as if it was a product, and try to benchmark it against a product with one of the available technologies," says Debdeep Jena, another Cornell researcher.
1. S. M. Islam et al., Applied Physics Letters (2017); doi: http://dx.doi.org/10.1063/1.4975068