Going Cubic Halves InGaAlN LED Efficiency Droop
University of Illinois team shows that switching from polar h- LEDs to c-LEDs quenches efficiency droop from 45 percent to 22 percent
Today, it is widely accepted that the large Auger coefficient is the main cause for the large (~50 percent) efficiency droop in traditional hexagonal-phase InGaAlN LEDs. Yet, this explanation is inadequate to account for the low efficiency droop in GaAs- and GaP-based LEDs, as those have similar Auger coefficients.
In IEEE Trans. Electron Devices, Can Bayram, Jean-Pierre Leburton and Yi-Chia Tsai at the University of Illinois show that the coexistence of strong internal polarisation and large carrier effective mass accounts for ~51 percent of the efficiency droop under high current densities in hexagonal-phase green InGaAlN LEDs (h-LEDs) compared to cubic-phase InGaAlN green LEDs (c- LEDs).
Previously, the efficiency droop reduction in non-polar h-LEDs was attributed to the decrease of carrier leakage from active region, overlooking the interplay between internal polarisation and Auger recombination. Indeed, recent experiments suggest that the efficiency droop reduction in non-polar h-LEDs is in fact due to carrier delocalisation, (a situation different than in polar h-LEDs) that results in stronger electron-hole wave function overlap, lower quantum well carrier densities, and lower Auger recombination rates. The University of Illinois team found out that large carrier effective mass promotes carrier localisation and degrades band-to-band optical transition matrix element.
The graph above shows normalised internal quantum efficiency (IQE) (left y-axis) and efficiency droop (right y-axis) as a function of current density. Red hexagons and green rhombuses refer to the hexagonal-phase (h-) and cubic-phase (c-) InGaAlN LEDs, whereas blue pentagons refer to the non- polar h-LED grown on m-plane GaN substrates.
According to this new interpretation, the researchers show that switching from polar h- LEDs to c-LEDs quenches the efficiency droop from 45 percent to 22 percent (i.e. a 51 percent reduction) thanks to polarisation elimination and effective mass reduction. It is further found that the quantum efficiency of c-LEDs is much immune to the Auger electron-hole asymmetry, the increase of Auger coefficient, and thus efficiency degradation mechanisms. Hence, cubic-phase InGaAlN green LEDs offer an appropriate solution to quench the efficiency droop.
Y.-C. Tsai, J.P. Leburton, C. Bayram, 'Quenching of the Efficiency Droop in Cubic Phase InGaAlN Light-Emitting Diodes,' IEEE Trans. Electron Devices (2022)