As described in the Nanotechnology journal, in a paper titled “UV LEDs Based on p-i-n Core-Shell AlGaN/GaN Nanowire Heterostructures Grown by N-polar Selective Area Epitaxy”, the brighter LEDs are fabricated from nanowires with a so-called “p-i-n” structure, a tri-layer design that injects electrons and holes into the nanowire. The addition of aluminium to the shell helps confine electrons to the nanowire core, boosting the electroluminescence fivefold.
The authors attribute the improved characteristics to the localization of spontaneous recombination to the NW core, reducing carrier overflow losses through the NW shell, and eliminating current shunting.
“The role of the aluminium is to introduce an asymmetry in the electrical current that prevents electrons from flowing into the shell layer, which would reduce efficiency, and instead confines electrons and holes to the nanowire core,” first author Matt Brubaker explains on the NIST website.
The nanowire test structures were about 440nm long with a shell thickness of about 40nm. The final LEDs, including the shells, were almost 10 times larger. Researchers found that the amount of aluminium incorporated into fabricated structures is dependent on nanowire diameter.