The past three decades have witnessed substantial progress in the performance of GaN-based light-emitting diodes. GaN light-emitting diodes, combined with wavelength-converting materials, enable highly efficient white-light sources that have caused a revolution in lighting applications, reaching more than 50% market adoption in the USA. This Review article discusses the underlying physics of GaN-based light-emitting diodes, focusing on how the quantum efficiency of the active region is affected by three key physical effects: intrinsic polarization fields, carrier localization by random alloy disorder, and crystal defects. Early theories are critically reviewed in view of current understanding and checked against the behaviour of real-world devices. We also offer perspectives on the prospects of extending III-nitride light emitters towards extreme wavelengths, at the ultraviolet and red ends of the spectrum. Because gallium nitride light-emitting diodes (LEDs) have made most of the visible spectrum available to LEDs, they have become the backbone of modern lighting applications. This Review examines three effects governing the physics of GaN-based LEDs: polarization fields, carrier localization and non-radiative defects.

A model including random alloy disorder is used to account for the outstanding optical properties of InGaN quantum wells (QW). The model provides excellent agreement to experimental observations on various structures. This study clarifies the prevalent role played by disorder in optical features such as the luminescence lineshape, the Stokes shift, and the radiative rate. Finally, the relationship between disorder and the peculiar properties of long-wavelength InGaN emitters is investigated.

This article describes a method for calculating and specifying light source chromaticity using the International Commission on Illumination (CIE) 2015 10° color matching functions (CMFs), which, according to analysis of existing psychophysical experiment data, can reduce visual mismatch compared to specifications based on the traditional CIE 1931 2° CMFs in architectural lighting applications. Specifically, this work evaluates, documents, and recommends for adoption by lighting standards organizations a supporting system of measures to be used with the CIE 2015 10° CMFs: a new uniform chromaticity scale (UCS) diagram with coordinates (s, t), a measure of correlated color temperature (CCTst), and a measure of distance from the Planckian locus (Dst). It also presents options for updating nominal classification quadrangles. A complete method of this nature has not yet been standardized, which may be contributing to the slow uptake of the CIE 2015 CMFs. The proposed tools are analogous to u, v, CCT, Duv, and the American National Standards Institute (ANSI) C78.377 chromaticity specifications that are all currently defined in the CIE 1960 UCS diagram using the CIE 1931 2° CMFs. While conceptually equivalent, the differences between the current standard method and the proposed st system are important for reducing unintended visual mismatch in the chromaticity of light. The implications of changing chromaticity specification methods are identified by a comparison over a diverse set of real light source spectral power distributions.

Excitons in InGaN quantum wells are investigated numerically, considering random alloy disorder and Coulomb interaction on equal footing in the Schrodinger equation. Their statistical properties are systemically explored as a function of the quantum well thickness and composition, revealing a complex competition between disorder-induced carrier localization, Coulomb attraction, and field-induced wave function separation. This results in a class of semiconductor quasiparticle with hybrid properties in between hydrogenoid excitons and disorder-localized free particles. Exciton screening by free carriers is investigated and shows distinct behavior from the screening of bulk excitons. Finally, a highly accurate approximate solution of the excitonic Schrodinger equation, with reduced numerical complexity, is introduced.

We study the high-temperature electroluminescence properties of 600 nm InGaN red 40 × 40 μm2 micro-light-emitting diodes (μLEDs) with a peak external quantum efficiency (EQE) of 3.2%. Temperature-dependent peak wavelength measurements show a low redshift of 0.05 nm/K. The injection efficiency improves with increasing temperature. The hot/cold (HC) factor is used to quantify the thermal droop: at 400 K, the EQE and wall-plug efficiency HC factors at 50 A/cm2 reach high values of 0.72 and 0.85, respectively. This demonstrates the robustness of InGaN red μLEDs up to high temperature, with a much-improved stability over conventional AlInGaP red μLEDs.

Photoluminescence measurements on high-quality InGaN quantum wells reveal that carriers diffuse laterally to long distances at room temperature, up to tens of microns. This behavior, which shows a pronounced dependence on the excitation density, contrasts with the common expectation of a short diffusion length. The data is well explained by a diffusion model taking into account the full carrier recombination dynamics, obtained from time-resolved measurements. These observations have important implications for understanding the high efficiency of III-nitride emitters, but also to properly interpret photoluminescence experiments and to design efficient small-scale devices.

This article describes a way to distinguish between two distinct components of light source–induced color shifts, base color shift and metameric color shift, to provide a more complete understanding of color rendition. Working within the existing framework of IES TM-30-18 and CIE 224:2017, it shows that base color shift varies smoothly with location in color space in a pattern that is determined by the spectral power distribution (SPD) of the light source. Patterns of smooth variation can often be approximated well with a low-order polynomial function. Here, a vector field model is presented, based on a second-order polynomial function. The polynomial coefficients are adjusted, for a given light source, to provide a least squares fit to the calculated color shifts of a standard set of color samples. The adequacy of this model was verified by comparing it to another approach for characterizing base color shift that is based on discretization of color space and a much larger set of color samples. Once the vector field model of base color shifts for a given light source is determined, the metameric color shifts can be calculated from the residuals and the distribution of those shifts can be statistically summarized. Based on this information, a metameric uncertainty index (Rt) is proposed to provide new information about a light source. In particular, it can be used to estimate the likelihood of noticeable metameric mismatches induced by a given light source, which could lead to improved predictions of the perceived color quality of light.

Thermal droop is investigated in high-quality InGaN light-emitting diodes (LEDs). To determine whether it is caused by intrinsic variations in recombination or by transport effects, photoluminescence and electroluminescence measurements are compared. The former does not show signs of pronounced thermal droop, with a near-constant internal quantum efficiency and recombination lifetime, regardless of temperature. In contrast, strong thermal droop is observed in the latter, pointing to transport effects as a leading contributor. Finally, high-efficiency LEDs with near-ideal thermal droop are demonstrated.

The physics of carrier recombinations in III-nitride light emitters are reviewed, with an emphasis on experimental investigations. After a discussion of various methods of measuring recombination dynamics, important results on recombination physics are examined. The radiative rate displays a complex behavior, influenced by Coulomb interaction and carrier screening. Non-radiative recombinations at low and high current are shown to scale with the overlap of electron-hole wavefunctions, similarly to the radiative rate, leading to a compensation effect which explains the high efficiency of III-nitride emitters. Finally, the droop current is decomposed into two contributions: the well-known Auger scattering, and a defect-assisted droop process, which is shown to play an important role in the green gap.

Although light sources are designed assuming the same color sensitivity for all viewers, inter-user variability can in fact cause significant discrepancies in individual perception. Here, perception variability related to short-wavelength effects is investigated. An experimental study is reported on LED sources with reduced blue content, which cause reduced circadian stimulation. Perceived chromaticity is strongly dependent on the viewer's age and spectral shape, in excellent agreement with a model based on modern colorimetry. Broader implications for LED sources in lighting and displays are discussed, and significant effects are found. These results confirm the inadequacy of conventional colorimetry and support the use of modern color science in the design and engineering of lighting products.

The magnitude of radiative and Auger recombinations in polar InGaN quantum wells is studied. Lifetime measurements show that these two processes are related by a power law as the electron-hole wavefunction overlap varies, leading to a near-compensation of their relative contributions. Theoretical investigation reveals that, in systems with wavefunction separation, recombination rates are controlled by the spatial tails of decaying wavefunctions. Such recombinations observe a general power law whose exponent is determined only by the ratio of the carriers' effective masses. These findings explain why III-nitride emitters remain efficient despite significant wavefunction separation.

The joint impact of Anderson localization and many-body interaction is observed in the optical properties of strongly-disordered III-nitride quantum wells, a system where the Coulomb interaction and the fluctuating potential are pronounced effects with similar magnitude. A numerical method is introduced to solve the 6-dimensional coupled Schrodinger equation in the presence of disorder and Coulomb interaction, a challenging numerical task. It accurately reproduces the measured absorption and luminescence dynamics of InGaN quantum wells at room-temperature: absorption spectra reveal the existence of a broadened excitonic peak, and carrier lifetime measurements show that luminescence departs from a conventional bimolecular behavior. These results reveal that luminescence is governed by the interplay between localization and Coulomb interaction, and provide practical insight in the physics of modern light-emitting diodes.

Carrier lifetime measurements reveal that, contrary to common expectations, the high-current non-radiative recombination (droop) in III-Nitride light emitters is comprised of two contributions which scale with the cube of the carrier density: an intrinsic recombination --most likely standard Auger scattering-- and an extrinsic recombination which is proportional to the density of point defects. This second droop mechanism, which hasn't previously been observed, is likely caused by an impurity-assisted Auger process. Further, it is shown that longer-wavelength emitters suffer from higher point defect recombinations, in turn causing an increase in the extrinsic droop process. It is proposed that this effect leads to the green gap, and that point defect reduction is a strategy to both vanquish the green gap and more generally improve quantum efficiency at high current.

Human vision provides useful information about the shape and color of the objects around us. It works well in many, but not all, lighting conditions. Since the advent of human-made light sources, it has been important to understand how illumination affects vision quality, but this has been surprisingly difficult. The widespread introduction of solid-state light emitters has increased the urgency of this problem. Experts still debate how lighting can best enable high-quality vision—a key issue since about one-fifth of global electrical power production is used to make light. Photometry, the measurement of the visual quantity of light, is well established, yet significant uncertainties remain. Colorimetry, the measurement of color, has achieved good reproducibility, but researchers still struggle to understand how illumination can best enable high-quality color vision. Fortunately, in recent years, considerable progress has been made. Here, we summarize the current understanding and discuss key areas for future study.

This article discusses the use of light-emitting diodes to generate white light - a research forefront in Physics and Ergonomics. We first present various technological approaches to white-light generation. After a general introduction to the human vision system, we discuss two key aspects of the quality of white light: the color of the light itself, and the color rendering of illuminated objects. We present the tools underlying modern color science, and review key color rendering metrics, from the well-known color rendering index to the latest improvements in the field. (C) 2018 Academie des sciences. Published by Elsevier Masson SAS. All rights reserved.

An all-optical measurement of differential carrier lifetimes is performed in a specially designed single-quantum-well structure. The measurement reveals the complex carrier-dependence of radiative and non-radiative recombinations, which directly manifest wavefunction-overlap and field-screening effects. This analysis clarifies the range of applicability of the common ABC model and its limitations.

The physical process driving low-current non-radiative recombinations in high-quality III-Nitride quantum wells is investigated. Lifetime measurements reveal that these recombinations scale with the overlap of the electron and hole wavefunctions and show weak temperature dependence, in contrast with common empirical expectations for Shockley-Read-Hall recombinations. A model of field-assisted multiphonon point defect recombination in quantum wells is introduced, and shown to quantitatively explain the data. This study provides insight on the high efficiency of III-Nitride light emitters.

Though sometimes referred to as a two-measure system for evaluating color rendition, IES TM-3015 includes key components that go beyond the two high-level average values. Fidelity Index (IES R-f) and Gamut index (IES R-g). This article focuses on the Color Vector Graphic and Local Chroma Shift (IES R-cs,R-hj), discussing the calculation methods for these evaluation tools and providing context for the interpretation of the values. We illustrate why and how the Color Vector Graphic and Local Chroma Shift values capture information about color rendition that is impossible to describe with average measures (such as CIE R-a, IES R-f, or IES R-g) but that is pertinent to more completely quantifying color rendition and to understanding human evaluations of color quality in the built environment. We also present alternatives for quantifying the Color Vector Graphic and Local Chroma Shift values, which can inform the development of future measures.