The decrease of the light emission efficiency at high currents in the nitride based light emitting diodes (LEDs) – nicknamed as “droop” attracts a lot of attention because of its importance for the production of very bright white LEDs which could in perspective completely replace the incandescent bulbs. The gradual decrease of the diode efficiency at high currents density was reported by many groups and interpreted in various ways. The dominant interpretations is the contribution of Auger effect which indeed should manifest at high currents. However, a conventional Auger effect is hardly expected for wide band-gap semiconductor like GaN or InGaN with low indium content. The other, non direct Auger mechanisms were proposed like defect-assisted Auger, or phonon-assisted Auger.
We have measured the electroluminescence from LEDs and laser diodes fabricated on top of various types of substrates like sapphire, HVPE-GaN, high pressure grown GaN. These structures were characterized by the density of dislocations ranging from 109cm-2down to 105cm-2. In all cases the similar efficiency droop was observed. We also studied the structures with the different emission wavelength (between 410 and 440 nm). All these structures showed similar “droop”. Finally, we investigated the droop effect in nitride laser diodes. We observed a strong competition between the stimulated emission and the nonradiative droop-mechanism. In some cases, the shortening of the recombination time in laser diodes below the lasing threshold caused by the stimulated recombination may somehow suppress the Auger-like nonradiative recombination.