The efficiency droop in GaInN∕GaN multiple-quantum well (MQW) light-emitting diodes is investigated. Measurements show that the efficiency droop, occurring under high injection conditions, is unrelated to junction temperature. Furthermore, the photoluminescence output as a function of excitation power shows no droop, indicating that the droop is not related to MQW efficiency but rather to the recombination of carriers outside the MQW region. Simulations show that polarization fields in the MQW and electron blocking layer enable the escape of electrons from the MQW region and thus are the physical origin of the droop. It is shown that through the use of proper quaternary AlGaInN compositions, polarization effects are reduced, thereby minimizing droop and improving efficiency.

Room-temperature photoluminescence (PL) measurements are performed on GaInN/GaN multiple-quantum-well heterostructures grown on GaN-on-sapphire templates with different threading-dislocation densities. The selective optical excitation of quantum wells and the dependence of integrated PL intensity on excitation power allow us to determine the internal quantum efficiency (IQE) as a function of carrier concentration. The measured IQE of the sample with the lowest dislocation density (5.3×108 cm−2) is as high as 64%. The measured nonradiative coefficient A varies from 6×107 to 2×108 s−1 as the dislocation density increases from 5.3×108 to 5.7×109 cm−2, respectively.

We model the carrier recombination mechanisms in GaInN/GaN light-emitting diodes as R=An+Bn2+Cn3+f(n), where f(n) represents carrier leakage out of the active region. The term f(n) is expanded into a power series and shown to have higher-than-third-order contributions to the recombination. The total third-order nonradiative coefficient (which may include an f(n) leakage contribution and an Auger contribution) is found to be 8×10−29 cm6 s−1. Comparison of the theoretical ABC+f(n) model with experimental data shows that a good fit requires the inclusion of the f(n) term.

The reverse leakage current of a GaInN light-emitting diode (LED) is analyzed by temperature dependent current–voltage measurements. At low temperature, the leakage current is attributed to variable-range-hopping conduction. At high temperature, the leakage current is explained by a thermally assisted multi-step tunneling model. The thermal activation energies (95–162 meV), extracted from the Arrhenius plot in the high-temperature range, indicate a thermally activated tunneling process. Additional room temperature capacitance–voltage measurements are performed to obtain information on the depletion width and doping concentration of the LED.

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Reduction in the light-output power in GaN-based light-emitting diodes (LEDs) with increasing temperature is a well-known phenomenon. In this work, temperature dependent external-quantum-efficiency versus current curves are measured, and the mechanisms of recombination are discussed. Shockley-Read-Hall recombination increases with temperature and is found to greatly reduce the light output at low current densities. However, this fails to explain the drop in light-output power at high current densities. At typical current density (35 A/cm2), as temperature increases, our results are consistent with increased Shockley-Read-Hall recombination and increased electron leakage from the active region. Both of these effects contribute to the reduction in light-output power in GaInN/GaN LEDs at high temperatures.

Recently, photoluminescence studies using resonant optical excitation in GaInN layers have been used to investigate the physical origin of efficiency droop in GaInN/GaN light-emitting diodes. In these studies, it has been assumed that in the case of resonant excitation, where electron-hole pairs are generated in the GaInN layers only, carrier transport effects play no role. We report that in contrast to this assumption, carrier escape from quantum wells does take place and shows strong dependence upon the duration of excitation and bias conditions. We also discuss the time scales required to reach steady-state conditions under pulsed optical excitation.

Optimization of solid-state lighting spectra is performed to achieve beneficial and tunable circadian effects. First, the minimum spectral circadian action factor (CAF) of 2700 K white light-emitting diodes (LEDs) is studied for applications where biologically active illumination is undesirable. It is found that white-LEDs based on (i) RGB chips, (ii) blue & red chips plus green phosphor, and (iii) blue chip plus green & red phosphors are the corresponding minimum-CAF solutions at color-rendering index (CRI) requirements of 80, 90, and 95, respectively. Second, maximum CAF tunability of LED clusters is studied for dynamic daylighting applications. A dichromatic phosphor-converted blue-centered LED, a dichromatic phosphor-converted green-centered LED, and a monochromatic red LED are grouped to obtain white spectra between 2700 K and 6500 K. A maximum CAF tunability of 3.25 times is achieved with CRI above 90 and luminous efficacy of radiation of 313 - 373 lm/W. We show that our approaches have advantages over previously reported solutions on system simplicity, minimum achievable CAF value, CAF tunability range, and light source efficacy.