We report the direct observation of hot carriers generated by Auger recombination via photoluminescence spectroscopy on tailored (AlGaIn)N multiple quantum well (QW) structures containing alternating green and ultra-violet (UV) emitting (GaIn)N QWs. Optically pumping solely the green QWs using a blue emitting high power laser diode, carrier densities similar to electrical light-emitting diode (LED) operation were achieved, circumventing possible leakage and injection effects. This way, luminescence from the UV QWs could be observed for excitation where the emission from the green QWs showed significant droop, giving direct evidence for Auger generated hot electrons and holes being injected into the UV QWs. An examination of the quantitative relation between the intensity of the UV luminescence and the amount of charge carriers lost due to drooping of the QWs supports the conclusion that Auger processes contribute significantly to the droop phenomenon in (AlGaIn)N based light-emitting diodes. Due to their high lifetimes and efficiencies along with rapidly declining prices, light-emitting diodes (LEDs) based on (AlGaIn)N multiple quantum well (MQW) structures are on their way to replace incandescent as well as fluorescent lighting. Despite great progress in recent years, resulting in peak power conversion efficiencies of up to 81%, 1 one obstacle still to overcome is the decrease in efficiency towards high operating current densities, a phenomenon commonly known as droop. 2,3 The current dependency of the internal quantum efficiency (IQE) can be modeled in good quantitative agreement with experimental data using an ABC rate equation model [4][5][6]

We investigate theoretically the influence of type and density of background carriers in the active region on the quantum efficiency of InGaN-based light emitters using an extension of the ABC rate model. A method to determine experimentally whether a certain type of Auger recombination is relevant in InGaN quantum wells is derived from these considerations. Using this approach, we show that the physical process which is the dominant cause for the efficiency droop is superlinear in the electron density and can thus be assigned to nnp-Auger recombination.

We investigated the influence of a μ-pixelated chip process on the photoluminescence (PL) and electroluminescence (EL) of a monolithic InGaN/GaN based blue light emitting diode with a continuous n-GaN layer. Particularly, we observed the impact of the metallic p-contact on the PL emission wavelength. A PL wavelength shift in the order of 10 nm between contacted and isolated areas was assigned to screening of internal piezoelectric fields due to charge carrier accumulation. μPL and μEL mappings revealed correlated emission wavelength and intensity inhomogeneities, caused by the epitaxial growth process. The edges of single pixels were investigated in detail via resonant confocal bias-dependent μPL. No influence on the intensity was observed beyond 300 nm away from the edge, which indicated a good working edge passivation. Due to the low lateral p-GaN conductivity, the μPL intensity was enhanced at isolated areas.

Recent experimental investigations on the reduction of internal quantum efficiency with increasing current density in (AlInGa)N quantum well structures show that Auger recombination is a significant contributor to the so-called "droop" phenomenon. Using photoluminescence (PL) test structures, we find Auger processes are responsible for at least 15 % of the measured efficiency droop. Furthermore, we confirm that electron-electronhole (nnp) is stronger than electron-hole-hole (npp) Auger recombination in standard LEDs. The ratio of respective Auger coefficients is determined to be in the range 1 < C nnp /C npp ≤ 12. This asymmetry is shown to limit the detection efficiency of Auger processes in our PL-based approach.