Identification of nnp and npp Auger recombination as significant contributor to the efficiency droop in (GaIn)N quantum wells by visualization of hot carriers in photoluminescence

Abstract

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]

Determination of recombination coefficients in InGaN quantum-well light-emitting diodes by small-signal time-resolved photoluminescence

“…However, their determination becomes possible, if the measured DLT of non-equilibrium carriers is additionally involved. Starting from the rate equation for the unsteady carrier density 4) and considering δn(t) to be a small perturbation of the steady-state carrier density n, one can show that the perturbation decays mono-exponentially with the DLT τ = (A + 2Bn + 3Cn 2 ) −1 . In terms of the normalized optical power p the DLT can be expressed by…”

“…the reduction of efficiency with increasing drive current, 1,2) which has been discussed widely in literature and has been attributed to Auger recombination [3][4][5] experimentally. In addition, an extension of the emission wavelength into the "Green Gap" 1,6) has proven to be challenging.…”

On the Monte Carlo Description of Hot Carrier Effects and Device Characteristics of III‐N LEDs

“…[5] For example, recent measurements have shown that III-N LEDs exhibit notable hot carrier distributions already close to the peak efficiency, [6][7][8][9] suggesting that hot carriers might have a more profound effect on the device characteristics than previously anticipated.…”

“…This suggests that the details of the band structure do not lead to crucial qualitative differences in describing the hot electron signature of Auger electron emission. This may also imply that the hydrodynamic (HD) model, also based on evaluating the moments of the BTE [8] like DD, could be able to provide and alternative and relatively fast approach that would, unlike DD, be able to account for the overflow currents at high biases while being computationally lighter than MC. However, as we have not yet made any practical comparisons of the HD model with our present results, it is too early to speculate if it can indeed provide any practical improvements over DD.…”

“…[5] For example, recent measurements have shown that III-N LEDs exhibit notable hot carrier distributions already close to the peak efficiency, [6][7][8][9] suggesting that hot carriers might have a more profound effect on the device characteristics than previously anticipated.To date, conventional LED simulations have mainly relied on the drift-diffusion (DD) model (see, e.g., refs. [10-13]), which cannot genuinely account for hot carriers and generally also produces larger turnon voltages for MQW LEDs than observed experimentally.…”