Microscopic Observation of Low Efficiency in Green Light-Emitting Diodes

The low efficiency of green light-emitting diodes (LEDs), a phenomenon known as the green gap, is a key obstacle hindering the application of LEDs as next-generation light sources to pioneer a plethora of new applications in the optical, medical, and communication sectors. Based on a microscopic photoluminescence analysis of green GaN-based multiple quantum wells, we find that In-enriched emission clusters on the submicrometer scale, previously thought to be efficient luminescent centers, do not emit light effectively. Such emission clusters can localize an excessively large amount of carriers, leading to the subsequent occurrence of vigorous nonradiative recombination processes. We also observe that the effective volume of the LED active region is significantly reduced, possibly because the generation of these In-enriched clusters via metastable phase separation significantly degrades the surrounding crystal quality. The microscopic analysis of luminescent clusters gives insight into the low efficiency of green LEDs, which may guide future directions for the development of LEDs.