The physics of carrier recombinations in III-nitride light emitters are reviewed, with an emphasis on experimental investigations. After a discussion of various methods of measuring recombination dynamics, important results on recombination physics are examined. The radiative rate displays a complex behavior, influenced by Coulomb interaction and carrier screening. Non-radiative recombinations at low and high current are shown to scale with the overlap of electron-hole wavefunctions, similarly to the radiative rate, leading to a compensation effect which explains the high efficiency of III-nitride emitters. Finally, the droop current is decomposed into two contributions: the well-known Auger scattering, and a defect-assisted droop process, which is shown to play an important role in the green gap.

The properties of quantum well carrier escape were studied by varying barrier thicknesses in InGaN/ GaN multi-quantum well solar cell devices. The dependence of the photocurrent on applied bias and temperature exhibited properties indicative of the quantum well carrier escape mechanisms of thermionic emission and tunneling, with tunneling dominating for thin barriers and high fields. Simulations using a self-consistent drift-diffusion and Schr€ odinger solver with analytical formulas extracted carrier escape lifetimes. By employing sufficiently thin barriers, it was found that escape lifetimes can be made small compared to recombination lifetimes, leading to high internal quantum efficiency. V

Carrier lifetime measurements reveal that, contrary to common expectations, the high-current non-radiative recombination (droop) in III-Nitride light emitters is comprised of two contributions which scale with the cube of the carrier density: an intrinsic recombination -most likely standard Auger scattering-and an extrinsic recombination which is proportional to the density of point defects. This second droop mechanism, which hasn't previously been observed, may be caused by an impurity-assisted Auger process. Further, it is shown that longer-wavelength emitters suffer from higher point defect recombinations, in turn causing an increase in the extrinsic droop process. It is proposed that this effect leads to the green gap, and that point defect reduction is a strategy to both vanquish the green gap and more generally improve quantum efficiency at high current.

The mechanism responsible for efficiency droop in InGaN light-emitting diodes (LEDs) has long been elusive due to indirect measurement techniques used for its identification. Auger recombination is unique among proposed efficiency droop mechanisms, in that it is the only mechanism capable of generating hot carriers. In a previous study [J. Iveland et al., Phys. Rev. Lett. 110, 177406 (2013)], we performed electron energy analysis of electrons emitted into vacuum from a forward biased InGaN LED that had been brought into negative electron affinity by cesiation. Three peaks were observed in the energy spectrum of vacuum emitted electrons. In this Letter, we unambiguously identify the origin of the peaks. The two higher energy peaks correspond to accumulation of electrons transported to the surface in the bulk Γ and side L conduction band valleys. The L-valley peak is a direct signature of a hot Auger electron population. The lower energy peak results from surface photoemission induced by the internal LED light emitted from the InGaN quantum wells. Two control experiments were performed. In the first, a simple GaN pn junction generated only a single Γ peak in electroemission. In the second, selective detection of the photoemission from an LED under modulated light excitation and DC electrical injection confirms that only the low energy peak is photogenerated and that LED light is incapable of generating Γ or L-valley peaks, the latter only occurring due to the Auger effect in the LED active region.

InGaN/GaN multiple quantum well concentrator solar cells Appl. Phys. Lett. 97, 073115 (2010); 10.1063/1.3481424 Effect of indium fluctuation on the photovoltaic characteristics of InGaN/GaN multiple quantum well solar cells Appl. Phys. Lett. 96, 081103 (2010); 10.1063/1.3327331 Effects of In composition on ultraviolet emission efficiency in quaternary InAlGaN light-emitting diodes on freestanding GaN substrates and sapphire substrates