Study on the efficiency droop in high-quality GaN material under high photoexcitation intensity

III–V nitride semiconductors, represented by GaN, have attracted significant research attention. Driven by the growing interest in smart micro-displays, there is a strong desire to achieve enhanced light output from even smaller light-emitting diode (LED) chips. However, the most perplexing phenomenon and the most significant challenge in the study of emission properties under high-injection conditions in GaN has always been efficiency droop for decades, where LEDs exhibit a substantial loss in efficiency at high driving currents. In this paper, we present our investigation into the intrinsic emission properties of high-quality GaN material, based on the density of states and the principles of momentum conservation. Our theoretical calculations demonstrate a growing mismatch in momentum distribution between the non-equilibrium excess electrons and holes, which becomes more pronounced as the carrier concentration increases. Our excitation-dependent photoluminescence measurements at 6 K revealed a significant droop in all exciton recombinations, while phonon-assisted recombination exhibited no droop. This behavior can be attributed to phonons compensating for the momentum mismatch. These findings indicate that the momentum distribution mismatch between the non-equilibrium excess electrons and holes is one of the intrinsic causes of the efficiency droop, which originates from the intrinsic band properties of GaN. Improving the design of the active region to reduce this mismatch will be crucial for the development of highly efficient lighting devices in the future, as indicated by the discussion of typical experimental results from the past decade.