Playing Ping Pong with Light: Directional Emission of White Light

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

Soft and Hard Failures of InGaN-Based LEDs Submitted to Electrostatic Discharge Testing

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, \u201csoft\u201d and \u201chard\u201d degradations, can be induced by ESD pulses. The \u201csoft\u201d 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. \u201cHard\u201d 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

Analysis of efficiency-droop mechanisms in GaN-based light-emitting diodes, related technological solutions and discriminating experiments

none7GaN LEDs suffer from the efficiency droop, limiting their use in applications like general lighting and projection displays. Different mechanisms have been held responsible for the droop. Different technological solutions have correspondingly been devised, and discriminating experiments have been proposed. We will report on a systematic analysis of above aspects (mechanisms, solutions, and experiments) carrier out by means of numerical device simulations and their comparison with experimental results. Aims are (i) to improve the insight into the droop mechanism(s), (ii) to provide guidelines for LED optimizationnoneD. Saguatti; G. Verzellesi; M. Meneghini; G. MENEGHESSO; E. Zanoni; R. Butendeich; B. HahnD., Saguatti; G., Verzellesi; Meneghini, Matteo; Meneghesso, Gaudenzio; Zanoni, Enrico; R., Butendeich; B., Hah

Reliability of InGaN-based LEDs submitted to reverse-bias stress

This paper describes a study of the electro-optical characteristics and of the degradation of green LEDs submitted to reverse-bias stress. The results of this work indicate that: (i) under reverse-bias, the dominant conduction mechanism is tunnelling, and LEDs can show a weak luminescence signal (reverse-bias luminescence, RBL); (ii) reverse-current flows through localized preferential paths, that can be identified by means of emission microscopy; (iii) reverse-bias stress can induce a significant increase in the reverse-current of the LEDs, corresponding to a decrease in the breakdown voltage, and an increase in reverse-bias luminescence; (iv) the degradation rate has a linear dependence on the stress current level, suggesting that degradation is determined by the flow of accelerated carriers through pre-existing defects. -500 \u3bcA 100 \u3bcA Electroluminescence micrograph of one of the analyzed green LEDs under reverse bias (left) and forward bias (right). At the center of the LED there is no significant light emission, due to the presence of the bondpa

Analysis of efficiency-droop mechanisms in single-quantum-well InGaN/GaN light-emitting diodes

In this work, we investigate different mechanisms that have been proposed to be at the origin of the efficiency droop, by comparing numerical device simulations with measurements from single-quantum-well (SQW) InGaN/GaN LEDs. The suitability of each mechanism to explain the droop as observed in the adopted devices and the impact on the droop effect of possible technological modifications are investigated

Investigation of Efficiency-Droop Mechanisms in Multi-Quantum-Well InGaN/GaN Blue Light-Emitting Diodes

Efficiency-droop mechanisms and related technological remedies 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 remedy in case the phenomenon is induced by Auger recombination. Possible technology-dependent variation of droop-controlling parameters and/or multiple droop mechanisms can, however, make discrimination of droop origin on the basis of the effects of applied technological remedies very difficult

A study on the reverse-bias and ESD instabilities of InGaN-based green LEDs

Therefore the aim of this paper is to describe a detailed investigation on the reverse-bias degradation of GaN-based LEDs. The results described in this paper indicate that: (i) under reverse bias, LEDs can show a weak luminescence signal, due to the recombination of carriers injected in the quantum-wells; (ii) reverse-bias stress can induce the degradation of the electrical characteristics of the LEDs (increase in reverse-current, decrease in breakdown voltage), due to the generation of point defects in proximity of pre-existing defective regions. (iii) Furthermore, our tests indicate that the defective regions responsible for reverse-current conduction can constitute weak points with respect to ESD events: ESD failures are determined by the shortening of the junction in proximity of one of the defective sites responsible for reverse-current conduction

Delta-doping superlattices in multiple quantum wells

Abstract The quantum conÿned Stark e ect has been extensively used for amplitude modulation. One way of improving the performance of multiple quantum well structures to be used in light modulation at high bit rates is by increasing the Stark shift for a given externally applied voltage. GaAs=AlGaAs multiple quantum well structures containing an nipi delta-doping superlattice, where the n-type doping is inserted in the quantum wells and the p-type in the barriers, are expected to double the Stark shift, according to Batty and Alsopp (Electron. Lett. 29 (1993Lett. 29 ( ) 2066. Such structures have been studied in detail to evaluate their potential for use in the fabrication of optical modulators. It has been observed that the required balance between n-and p-type doping levels is not trivial to achieve due to the presence of interface hole traps whose population depends on the quantum well doping concentration. It is estimated that for undoped quantum wells around 15% of the holes provided by the p-doping are trapped at the interfaces. Photoluminescence measurements, supported by calculations, point out that even though an indirect transition between electrons in the quantum wells and holes in the barriers is present at low temperatures at energies below the quantum well fundamental transition energy, at room temperature such a transition is absent and the observed optical emission occurs at essentially the same energy as that of an equivalent undoped structure

A study of Failure of GaN-based LEDs submitted to reverse-bias stress and ESD events

This paper describes an extensive analysis of the degradation of InGaN-based LEDs submitted to reverse-bias stress and Electrostatic Discharge events. Results described within the paper indicate that: (i) reverse-bias current flows through localized leakage paths, related to the presence of structural defects; (ii) the position of these paths can be identified by means of emission microscopy; (iii) reverse-bias stress can induce a degradation of the electrical characteristics of the devices (decrease in breakdown voltage); (iv) degradation is due to the injection of highly accelerated carriers through the active region of the LEDs, with the subsequent generation/propagation of point defects; (v) the localized leakage paths responsible for reverse-current conduction can constitute weak regions with respect to reverse-bias ESD events