Efficiency-droop mechanisms and related technologicalremedies are critically analyzed in multi-quantum-well (QW) InGaN/GaN blue light-emitting diodes by means of numerical device simulations and their comparison with experimental data. Auger recombination, electron leakage, and incomplete QW carrier capture can separately produce droop effects in quantitative agreement with experimental data, but “extreme” values, at the limit of or outside their generally accepted range, must be imposed for related droop-controlling parameters. Less stringent conditions are needed if combinations of the aforementioned mechanisms are assumed to act jointly. Applying technological/structural modifications like QW thickness or number increase and barrier p-type doping leads to distinctive effects on droop characteristics depending on the assumed droop mechanism. Increasing the QW number appears, in particular, to be the most effective droop remedyin case the phenomenon is induced by Auger recombination. Possible technology-dependent variation of droop-controlling parameters and/or multiple droop mechanisms can, however, makediscrimination of droop origin on the basis of the effects of applied technological remedies very difficult
This letter reports an extensive analysis of the degradation
mechanisms of InGaN-based light-emitting diodes (LEDs)
submitted to reverse-bias electrostatic discharge (ESD). The results
of this analysis indicate that two different failure modes,
namely, “soft” and “hard” degradations, can be induced by ESD
pulses. The “soft” failure mode takes place as a consequence of
ESD events with moderate voltage/current levels and consists in a
decrease in the reverse-bias leakage current of LEDs. This effect is
due to the annihilation of some of the defective paths responsible
for leakage-current conduction, possibly triggered by the injection
of relatively high reverse-bias current densities. “Hard” failure
takes place when high-voltage/current ESD pulses are applied to
an LED. After hard failure, LEDs behave as short circuits. This
process is due to the high voltage levels reached by the junction
during an ESD event (with subsequent dielectric rupture) or to
the injection of extremely high current densities through one of
the localized paths responsible for reverse-current conduction
Over the last decades, light-emitting diodes (LED) have replaced common light bulbs in almost every application, from flashlights in smartphones to automotive headlights. Illuminating nightly streets requires LEDs to emit a light spectrum that is perceived as pure white by the human eye. The power associated with such a white light spectrum is not only distributed over the contributing wavelengths but also over the angles of vision. For many applications, the usable light rays are required to exit the LED in forward direction, namely under small angles to the perpendicular. In this work, we demonstrate that a specifically designed multi-layer thin film on top of a white LED increases the power of pure white light emitted in forward direction. Therefore, the deduced multi-objective optimization problem is reformulated via a real-valued physics-guided objective function that represents the hierarchical structure of our engineering problem. Variants of Bayesian optimization are employed to maximize this non-deterministic objective function based on ray tracing simulations. Eventually, the investigation of optical properties of suitable multi-layer thin films allowed to identify the mechanism behind the increased directionality of white light: angle and wavelength selective filtering causes the multi-layer thin film to play ping pong with rays of light.
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