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

An all-optical measurement of differential carrier lifetimes is performed in a specially designed single-quantum-well structure. The measurement reveals the complex carrier-dependence of radiative and non-radiative recombinations, which directly manifest wavefunction-overlap and field-screening effects. This analysis clarifies the range of applicability of the common ABC model and its limitations.

We demonstrate InGaN/GaN multiple quantum well solar cells grown by metalorganic chemical vapor deposition on a bulk (0001) substrate with high-performance broadband optical coatings to improve light absorption. A front-side anti-reflective coating and a back-side dichroic mirror were designed to minimize front surface reflections across a broad spectral range and maximize rear surface reflections only in the spectral range absorbed by the InGaN, making the cells suitable for multijunction solar cell integration. Application of optical coatings increased the peak external quantum efficiency by 56% (relative) and conversion efficiency by 37.5% (relative) under 1 sun AM0 equivalent illumination.

The effect of employing an AlGaN cap layer in the active region of green c-plane light-emitting diodes (LEDs) was studied. Each quantum well (QW) and barrier in the active region consisted of an InGaN QW and a thin AlGaN cap layer grown at a relatively low temperature and a GaN barrier grown at a higher temperature. A series of experiments and simulations were carried out to explore the effects of varying the AlGaN cap layer thickness and GaN barrier growth temperature on LED efficiency and electrical performance. We determined that the AlGaN cap layer should be around 2 nm and the growth temperature of the GaN barrier should be approximately 75° C higher than the growth temperature of the InGaN QW to maximize the LED efficiency, minimize the forward voltage, and maintain good morphology. Optimized AlGaN cap growth conditions within the active region resulted in high efficiency green LEDs with a peak external quantum efficiency (EQE) of 40.7% at 3 A/cm. At a normal operating condition of 20 A/cm, output power, EQE, forward voltage, and emission wavelength were 13.8 mW, 29.5%, 3.5 V, and 529.3 nm, respectively.