26 August 2021
More evidence for Auger causing GaN LED efficiency droop
University of California Santa Barbara (UCSB) in the USA and Ecole Polytechnique in France have analyzed electron emission spectroscopy (EES) results on indium gallium nitride (InGaN) light-emitting diodes (LEDs), concluding that Auger recombination is the dominant cause of efficiency droop [Wan Ying Ho et al, Appl. Phys. Lett., v119, p051105, 2021].
Efficiency droop is a particular problem in III-nitride LEDs. External quantum efficiency (EQE) tends to peak in the injection current density range of 1-10A/cm2.
The researchers used LED material grown by Seoul VioSys through metal-organic chemical vapor deposition (MOCVD) on patterned sapphire. The structure featured an 8-period quantum well. The p-contact metal consisted of palladium/gold (Pd/Au) with 2257 hexagonal apertures in a honeycomb arrangement. The hexagons had a 3.5μm apothem (perpendicular distance from a side to the center). The metal strips between the apertures were 3μm wide.
EES was enabled by depositing a sub-monolayer of cesium to create a negative electron affinity (NEA). The cesium layer was optimized by monitoring photo-excited electrons from the p-GaN contact layer.
The energy of the emitted electrons was measured using a spherical sector electrostatic analyzer operated in a constant-pass energy mode. The light output power (LOP) was monitored using a photodetector in continuous-wave mode at room temperature. The peak external quantum efficiency was achieved at an injection current density of order 10A/cm2. The EES experiments were made at current densities up to 45A/cm2 in pulse mode to avoid self-heating effects.
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Tags: Auger GaN LEDs GaN LEDs Efficiency droop MOCVD
The author Mike Cooke is a freelance technology journalist who has worked in the semiconductor and advanced technology sectors since 1997.