Correlating electroluminescence characterization and physics-based models of InGaN/GaN LEDs: Pitfalls and open issues

Abstract

Electroluminescence (EL) characterization of InGaN/GaN light-emitting diodes (LEDs), coupled with numerical device models of different sophistication, is routinely adopted not only to establish correlations between device efficiency and structural features, but also to make inferences about the loss mechanisms responsible for LED efficiency droop at high driving currents. The limits of this investigative approach are discussed here in a case study based on a comprehensive set of currentand temperature-dependent EL data from blue LEDs with low and high densities of threading dislocations (TDs). First, the effects limiting the applicability of simpler (closed-form and/or one-dimensional) classes of models are addressed, like lateral current crowding, vertical carrier distribution nonuniformity, and interband transition broadening. Then, the major sources of uncertainty affecting state-of-the-art numerical device simulation are reviewed and discussed, including (i) the approximations in the transport description through the multi-quantum-well active region, (ii) the alternative valence band parametrizations proposed to calculate the spontaneous emission rate, (iii) the difficulties in defining the Auger coefficients due to inadequacies in the microscopic quantum well description and the possible presence of extra, non-Auger high-current-density recombination mechanisms and/or Auger-induced leakage. In the case of the present LED structures, the application of three-dimensional numerical-simulation-based analysis to the EL data leads to an explanation of efficiency droop in terms of TD-related and Auger-like nonradiative losses, with a C coefficient in the 10−30 cm6/s range at room temperature, close to the larger theoretical calculations reported so far. However, a study of the combined effects of structural and model uncertainties suggests that the C values thus determined could be overestimated by about an order of magnitude. This preliminary attempt at uncertainty quantification confirms, beyond the present case, the need for an improved description of carrier transport and microscopic radiative and nonradiative recombination mechanisms in device-level LED numerical models

The influence of random indium alloy fluctuations in indium gallium nitride quantum wells on the device behavior

“…In a real device, current crowding 40 issues can make local current density quite higher than expected, making the droop effect worse, as discussed in earlier studies. 41,42 Figure 4(d) shows the IQE curve with different Auger recombination coefficient, C. With larger values of C, IQE curves show earlier droop onset, lower IQE peak, and more severe droop effects.…”

The Effect of Imbalanced Carrier Transport on the Efficiency Droop in GaInN-Based Blue and Green Light-Emitting Diodes

“…Auger recombination (i.e., an electron drops non-radiatively from the conduction band to the valence band while transferring its energy to another electron or hole) has been regarded as a major cause of efficiency droop. However, the Auger recombination coefficient, even if defect-assisted, has very small theoretical values ranging between~10 −34 and~10 −30 cm 6 /s in nitride-based LEDs and weakly increases with temperature, making it difficult to explain various aspects of the efficiency droop [7][8][9][10][11]. Recently, a simple analytical theoretical model was formulated in support of electron leakage the cause of efficiency droop [7].…”

Model for carrier capture time through phonon emission in InGaN/GaN quantum wells

“…Nitride‐based light‐emitting diodes (LEDs) have emerged as an important technology for lighting applications, but their performance is limited by a reduction (droop) of their internal quantum efficiency (IQE) as the driving current density is increased beyond A cm, whose physical origin is still under intense debate . IQE depends critically on carrier transport across the active region, and in particular is determined by a number of time scales, including all the radiative/nonradiative recombination lifetimes and the carrier capture time from the three‐dimensional (3D) unconfined states at or above the barrier energy to the confined two‐dimensional (2D) quantum well (QW) states.…”