Peak radiative efficiency and electron-drift-induced efficiency droop in iii-v nitride-based light-emitting diodes

III-V nitride-based light-emitting diodes (LEDs) have become promising lighting sources because of their high luminous efficiency and long lifetime compared to traditional lighting sources. Generally, the efficiency of nitride-based LEDs rises to a peak efficiency occurring at a relatively low current density (~10 A/cm2) and then gradually decreases ("droops") with increasing current density. This well-known phenomenon, the "efficiency droop", prevents these LEDs from maintaining their high efficiency in the high current density range. In this dissertation, a novel method to determine the peak radiative efficiency of GaN-based quantum wells (QWs) is introduced. The method is based on a power-dependent photoluminescence (PL) measurement performed at room temperature. The accuracy of this effective method is verified by a conventional temperature-dependent PL measurement. Based on this new method, a high peak radiative efficiency (~90%) is found in blue-emitting GaInN/GaN QWs. The efficiency droop is found for LEDs operated under electroluminescence conditions (EL droop) and for LEDs operated under photoluminescence conditions (PL droop). The EL droop starts to occur at a much smaller excitation density than the PL droop. Furthermore, the EL droop is much more severe than the PL droop. Based on an ABC-model analysis, the EL droop has a stronger cubic dependence on carrier concentration in the QW than the PL droop. In a structure with carriers well-confined to the QW, the PL droop can only be related to non-radiative recombination inside the QW; therefore, given that the EL droop is much stronger, we link it to carrier leakage out of the QW. According to the ABC-model analysis for EL and PL experiments, we develop a drift-leakage model to both quantitatively and qualitatively account for the cause of the EL efficiency droop. The strong carrier asymmetry (i.e. mobility and concentration) in GaN-based LEDs easily causes high-level injection. The onset of high-level injection can be identified experimentally from the sudden slope decrease in the logarithmic-current-versus-voltage curve of the LED. In the high-level injection regime, a strong electrical field develops in the resistive p-type region so that high-mobility electrons are dragged out of the active region. This theoretical drift-leakage model fits well the experimental efficiency curves of a variety of LEDs. The AlGaInP/AlInP LEDs are widely used for the long-wavelength visible spectrum (red to yellow-green). These LEDs do not exhibit the efficiency droop at room temperature. Based on the drift-leakage model, the carrier asymmetry is the key reason for the EL efficiency droop. By decreasing the ambient temperature, the carrier asymmetry can be intentionally enhanced. Therefore, we are able to demonstrate the existence of the efficiency droop for AlGaInP/AlInP LEDs at cryogenic temperature. Moreover, temperature-dependent measurements show the following four trends at cryogenic temperature: (i) higher peak efficiency (ii) smaller droop-onset current (iii) larger droop-onset voltage and (iv) stronger efficiency droop. All of these characteristics can be explained by the drift-leakage model. Based on the drift-leakage model, novel and effective concepts to reduce the efficiency droop can be developed. Two successful strategies are demonstrated in this dissertation. First, an LED with p-type GaInN spacer is carefully designed and is proved experimentally to have 12% light output enhancement at the current density of 111 A/cm2 when compared to the reference LED. The analysis of the drift-leakage model shows that the loss mechanism at high current density is largely suppressed by the enhanced hole concentration in the p-type spacer. Second, a p-type ZnO-capped green GaInN LED is demonstrated to have an average 18% light output enhancement when compared to the reference green LED. Based on the analysis of the drift-leakage model, the enhancement is caused by better carrier injection when the polarization charge density at the EBL/p-type GaN interface is modified by ZnO-induced strain.