Nanomaterial integration in micro LED technology: Enhancing efficiency and applications

The micro-light emitting diode (µLED) technology is poised to revolutionise display applications through the introduction of nanomaterials and Group III-nitride nanostructures. This review charts state-of-the-art in this important area of micro-LEDs by highlighting their key roles, progress and concerns. The review encompasses details from various types of nanomaterials to the complexity of gallium nitride (GaN) and III nitride nanostructures. The necessity to integrate nanomaterials with III-nitride structures to create effective displays that could disrupt industries was emphasised in this review. Commercialisation challenges and the economic enhancement of micro-LED integration into display applications using monolithic integrated devices have also been discussed. Furthermore, different approaches in micro-LED development are discussed from top-down and bottom-up approaches. The last part of the review focuses on nanomaterials employed in the production of micro-LED displays. It also highlights the combination of III-V LEDs with silicon LCDs and perovskite-based micro-LED displays. There is evidence that efficiency and performance have improved significantly since the inception of the use of nanomaterials in manufacturing these

Beyond solid-state lighting: Miniaturization, hybrid integration, and applications og GaN nano- and micro-LEDs

Gallium Nitride (GaN) light-emitting-diode (LED) technology has been the revolution in modern lighting. In the last decade, a huge global market of efficient, long-lasting and ubiquitous white light sources has developed around the inception of the Nobel-price-winning blue GaN LEDs. Today GaN optoelectronics is developing beyond lighting, leading to new and innovative devices, e.g. for micro-displays, being the core technology for future augmented reality and visualization, as well as point light sources for optical excitation in communications, imaging, and sensing. This explosion of applications is driven by two main directions: the ability to produce very small GaN LEDs (microLEDs and nanoLEDs) with high efficiency and across large areas, in combination with the possibility to merge optoelectronic-grade GaN microLEDs with silicon microelectronics in a fully hybrid approach. GaN LED technology today is even spreading into the realm of display technology, which has been occupied by organic LED (OLED) and liquid crystal display (LCD) for decades. In this review, the technological transition towards GaN micro- and nanodevices beyond lighting is discussed including an up-to-date overview on the state of the art

The Role Of Photonics In Energy

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)In celebration of the 2015 International Year of Light, we highlight major breakthroughs in photonics for energy conversion and conservation. The section on energy conversion discusses the role of light in solar light harvesting for electrical and thermal power generation; chemical energy conversion and fuel generation; as well as photonic sensors for energy applications. The section on energy conservation focuses on solid-state lighting, flat-panel displays, and optical communications and interconnects. (C) The Authors. Published by SPIE under a Creative Commons Attribution 3.0 Unported License. Distribution or reproduction of this work in whole or in part requires full attribution of the original publication, including its DOI.5Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)U.S. National Science Foundation [DMR-1309459, ECCS 1408051, DMR 1505122]U.S. Office of Naval ResearchEngineering and Physical Sciences Research Council of the UK [EP/K00042X, EP/L012294]European Research Council of the European Union [321305]Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP

III-nitride nanowire light-emitting diodes: design and characterization

III-nitride semiconductors have been intensively studied for optoelectronic devices, due to the superb advantages offered by this materials system. The direct energy bandgap III-nitride semiconductors can absorb or emit light efficiently over a broad spectrum, ranging from 0.65 eV (InN) to 6.4 eV (AlN), which encompasses from deep ultraviolet to near infrared spectrum. However, due to the lack of native substrates, conventional III-nitride planar heterostructures generally exhibit very high dislocation densities that severely limit the device performance and reliability. On the other hand, nanowire heterostructures can be grown on lattice mismatched substrates with drastically reduced dislocation densities, due to highly effective lateral stress relaxation. Nanowire light-emitting diodes (LEDs) with emission in the ultraviolet to visible wavelength range have recently been studied for applications in solid-state lighting, flat-panel displays, and solar-blind detectors. In this thesis, investigation of the systematic process flow of design and epitaxial growth of group III-nitride nanoscale heterostructures was done. Moreover, demonstration of phosphor-free nanowire white LEDs using InGaN/AlGaN nanowire heterostructures grown directly on Si(111) substrates by molecular beam epitaxy was made. Full-color emission across nearly the entire visible wavelength range was realized by controlling the In composition in the InGaN active region. Strong white-light emission was recorded for the unpackaged nanowire LEDs with an unprecedentedly high color rendering index of 98. Moreover, LEDs with the operating wavelengths in the ultraviolet (UV) spectra, with emission wavelength in the range of 280-320 nm (UV-B) or shorter wavelength hold tremendous promise for applications in phototherapy, skin treatments, high speed dissociation and high density optical recording. Current planar AlGaN based UV-B LEDs have relatively low quantum efficiency due to their high dislocation density resulted from the large lattice mismatch between the AlGaN and suitable substrates. In this study, associated with the achievement of visible LEDs, the development of high brightness AlGaN/GaN nanowire UV-LEDs via careful design and device fabrication was shown. Strong photoluminescence spectra were recorded from these UV-B LEDs. The emission peak can be tunable from 290 nm to 320 nm by varying the Al content in AlGaN active region which can be done by optimizing the growth condition including Al/Ga flux ratio and also the growth temperature. Such visible to UV-B nanowire LEDs are ideally suited for future smart lighting, full-color display, phototherapy and skin treatments applications

Critical assessment 23: Gallium nitride-based visible light-emitting diodes

Solid-state lighting based on light-emitting diodes (LEDs) is a technology with the potential to drastically reduce energy usage, made possible by the development of gallium nitride and its alloys. However, the nitride materials family exhibits high defect densities and, in the equilibrium wurtzite crystal phase, large piezo-electric and polarisation fields arising at polar interfaces. These unusual physical properties, coupled with a high degree of carrier localisation in devices emitting visible light, result in ongoing challenges in device development, such as efficiency ‘droop’ (the reduction in efficiency of nitride LEDs with increasing drive current density), the ‘green gap’ (the relatively low efficiency of green emitters in comparison to blue) and the challenge of driving down the cost of LED epitaxy.This is the author accepted manuscript. The final version is available from Taylor & Francis via http://dx.doi.org/10.1080/02670836.2015.111622

Epitaxial growth of iii-nitride nanostructures and their optoelectronic applications

Light-emitting diodes (LEDs) using III-nitride nanowire heterostructures have been intensively studied as promising candidates for future phosphor-free solid-state lighting and full-color displays. Compared to conventional GaN-based planar LEDs, III-nitride nanowire LEDs exhibit numerous advantages including greatly reduced dislocation densities, polarization fields, and quantum-confined Stark effect due to the effective lateral stress relaxation, promising high efficiency full-color LEDs. Beside these advantages, however, several factors have been identified as the limiting factors for further enhancing the nanowire LED quantum efficiency and light output power. Some of the most probable causes have been identified as due to the lack of carrier confinement in the active region, non-uniform carrier distribution, and electron overflow. Moreover, the presence of large surface states and defects contribute significantly to the carrier loss in nanowire LEDs. In this dissertation, a unique core-shell nanowire heterostructure is reported, that could overcome some of the aforementioned-problems of nanowire LEDs. The device performance of such core-shell nanowire LEDs is significantly enhanced by employing several effective approaches. For instance, electron overflow and surface states/defects issues can be significantly improved by the usage of electron blocking layer and by passivating the nanowire surface with either dielectric material / large bandgap energy semiconductors, respectively. Such core-shell nanowire structures exhibit significantly increased carrier lifetime and massively enhanced photoluminescence intensity compared to conventional InGaN/GaN nanowire LEDs. Furthermore, AlGaN based ultraviolet LEDs are studied and demonstrated in this dissertation. The simulation studies using Finite-Difference Time-Domain method (FDTD) substantiate the design modifications such as flip-chip nanowire LED introduced in this work. High performance nanowire LEDs on metal substrates (copper) were fabricated via substrate-transfer process. These LEDs display higher output power in comparison to typical nanowire LEDs grown on Si substrates. By engineering the device active region, high brightness phosphor-free LEDs on Cu with highly stable white light emission and high color rendering index of \u3e 95 are realized. High performance nickel?zinc oxide (Ni-ZnO) and zinc oxide-graphene (ZnO-G) particles have been fabricated through a modified polyol route at 250?C. Such materials exhibit great potential for dye-sensitized solar cell (DSSC) applications on account of the enhanced short-circuit current density values and improved efficiency that stems from the enhanced absorption and large surface area of the composite. The enhanced absorption of Ni-ZnO composites can be explained by the reduction in grain boundaries of the composite structure as well as to scattering at the grain boundaries. The impregnation of graphene into ZnO structures results in a significant increase in photocurrent consequently due to graphene\u27s unique attributes including high surface area and ultra-high electron mobility. Future research directions will involve the development of such wide-bandgap devices such as solar cells, full color LEDs, phosphor free white-LEDs, UV LEDs and laser diodes for several applications including general lighting, wearable flexible electronics, water purification, as well as high speed LEDs for visible light communications

Atomic-Scale Insights into Light Emitting Diode

In solid-state lightning, GaN-based vertical LED technology has attracted tremendous attention because its luminous efficacy has surpassed the traditional lightning technologies, even the 2014 Nobel Prize in Physics was awarded for the invention of efficient blue LEDs, which enabled eco-friendly and energy-saving white lighting sources. Despite today’s GaN-based blue VLEDs can produce IQE of 90% and EQE of 70-80%, still there exist a major challenge of efficiency droop. Nonetheless, state-of-the-art material characterization and failure analysis tools are inevitable to address that issue. In this context, although LEDs have been characterized by different microscopy techniques, they are still limited to either its semiconductor or active layer, which mainly contributes towards the IQE. This is also one of the reason that today’s LEDs IQE exceeded above 80% but EQE of 70-80% remains. Therefore, to scrutinize the efficiency droop issue, this work focused on developing a novel strategy to investigate key layers of the LED structure, which play the critical role in enhancing the EQE = IQE x LEE factors. Based on that strategy, wafer bonding, reflection, GaN-Ag interface, MQWs and top-textured layers have been systematically investigated under the powerful advanced microscopy techniques of SEM-based TKD/EDX/EBSD, AC-STEM, AFM, Raman spectroscopy, XRD, and PL. Further, based on these correlative microscopy results, optimization suggestions are given for performance enhancement in the LEDs. The objective of this doctoral research is to perform atomic-scale characterization on the VLED layers/interfaces to scrutinize their surface topography, grain morphology, chemical composition, interfacial diffusion, atomic structure and carrier localization mechanism in quest of efficiency droop and reliability issues. The outcome of this research advances in understanding LED device physics, which will facilitate standardization in its design for better smart optoelectronics products

The 2020 UV emitter roadmap

Solid state UV emitters have many advantages over conventional UV sources. The (Al,In,Ga)N material system is best suited to produce LEDs and laser diodes from 400 nm down to 210 nm—due to its large and tuneable direct band gap, n- and p-doping capability up to the largest bandgap material AlN and a growth and fabrication technology compatible with the current visible InGaN-based LED production. However AlGaN based UV-emitters still suffer from numerous challenges compared to their visible counterparts that become most obvious by consideration of their light output power, operation voltage and long term stability. Most of these challenges are related to the large bandgap of the materials. However, the development since the first realization of UV electroluminescence in the 1970s shows that an improvement in understanding and technology allows the performance of UV emitters to be pushed far beyond the current state. One example is the very recent realization of edge emitting laser diodes emitting in the UVC at 271.8 nm and in the UVB spectral range at 298 nm. This roadmap summarizes the current state of the art for the most important aspects of UV emitters, their challenges and provides an outlook for future developments

Novel nanocrystal-integrated LEDs utilizing radiative and nonradiative energy transfer for high-quality efficient light generation

Ankara : The Department of Electrical and Electronics Engineering and the Institute of Engineering and Sciences of Bilkent University, 2011.Thesis (Ph. D.) -- Bilkent University, 2011.Includes bibliographical references leaves 200-216.To combat environmental issues escalating with the increasing carbon footprint, combined with the energy problem of limited resources, innovating fundamentally new ways of raising energy efficiency and level of energy utilization is essential to our energy future. Today, to this end, achieving lighting efficiency is an important key because artificial lighting consumes about 19% of total energy generation around the globe. There is a large room for improving lighting efficacy for potential carbon emission cut. However, the scientific challenge is to reach simultaneously high-quality photometric performance. To address these problems, we proposed, developed and demonstrated a new class of color-conversion light emitting diodes (LEDs) integrated with nanophosphors of colloidal quantum dots. The favorable properties of these semiconductor nanocrystal quantum dots, including size-tuneable and narrow-band emission with high photostability, have provided us with the ability of achieving highquality, efficient lighting. Via using custom-design combinations of such nanocrystal emitters, we have shown that targeted white luminescence spectra can be generated with desired high photometric performance, which is important for obtaining application-specific white LEDs, e.g., for indoors lighting, street lighting, and LED-TV backlighting. Furthermore, dipole-dipole coupling capability of these semiconductor nanocrystals has allowed us to realize novel device designs based on Förster-type nonradiative energy transfer. By mastering exciton-exciton interactions in color-conversion LEDs, we have demonstrated enhanced color conversion via recycling of trapped excitons and white light generation based on nonradiative pumping of nanocrystal quantum dots for color conversion. This research work has led to successful demonstrators of semiconductor nanocrystal quantum dots that photometrically outperform conventional rareearth phosphor powders in terms of color rendering, luminous efficacy of optical radiation, color temperature and scotopic/photopic ratio for the first time.Nizamoğlu, SedatPh.D

Nanoscale Spectroscopic Characterization of InGaN/GaN Multiple Quantum Wells on GaN Nanorods

This thesis investigates the photophysics of InGaN/GaN multiple quantum wells (MQW) on top of GaN nanorods. InGaN/GaN MQW is a promising candidate material for high-performance light-emitting diodes and solar cells. In this thesis, InGaN/GaN MQW nanorods were fabricated by nanosphere lithography, a top-down method using reactive ion etching (RIE) of c-plane GaN with InGaN quantum wells. The nanorod arrays presents a hexagonal perodicity with uniform morphology. InGaN/GaN MQW nanorods demonstrate significantly improved optical and electronic properties compared to their planar counterparts. However, the exact nature of the processes whereby nanorod structures impact the carrier dynamics in InGaN quantum wells is not well understood. Using a confocal microscopy, associated with time-resolved spectroscopy, combining selective one- and two- photon excitations steady and time-resolved photoluminescence characterization provides detailed carrier dynamic analysis. The depth- and spatial-resolution at the nanoscale is helpful for understanding the optical properties of InGaN/GaN MQW nanorods. While nanostructured surfaces enhance luminescence performances of InGaN/GaN MQW, the increased surface defects impair the device performance, which is dealt with by surface treatments in this thesis. By studying the intensity-dependent PL of InGaN/GaN MQW, this thesis proves that photoexcited electrons and holes are strongly bound by Coulomb interactions (i.e., excitons) in planar MQWs due to the large exciton binding energy in InGaN quantum wells. In contrast, free electron-hole recombination becomes the dominant mechanism in nanorods, which is ascribed to efficient exciton dissociation. The nanorod sidewall provides an efficient pathway for exciton dissociation that significantly improves the optical performance of InGaN/GaN MQW. This thesis provides new insights into excitonic and charge carrier dynamics of quantum confined materials as well as the influence of surface states. The optical characterization techniques provide depth-resolved and time-resolved carrier dynamics with nanoscale spatially-resolved mapping, which is crucial for a comprehensive and thorough understanding of nanostructured materials