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

Commercialization of gallium nitride nanorod arrays on silicon for solid-state lighting

Thesis (M. Eng.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 2008.Includes bibliographical references (p. 37-40).One important component in energy usage is lighting, which is currently dominated by incandescent and fluorescent lamps. However, due to potentially higher efficiencies and thus higher energy savings, solid-state lighting (SSL) is seriously being considered as a replacement. Currently, state-of-the-art white LEDs are made up of thin films of GaN and InGaN grown on sapphire substrates. A new LED structure design is proposed, in which GaN nanorod arrays are grown on silicon substrates. This new structure could be fabricated using anodized aluminum oxide's (AAO) ordered arrangement of pores as a template for growth of the nanorod array. AAO is selected for its high porosity and simple controllability of pore size and separation, which can in turn produce high density monocrystalline nanorod arrays with adjustable rod size and separation. Several advantages are enjoyed by LEDs based on rod arrays: lower cost, better yield and reliability and higher efficiencies. Two more LED designs, other than the current state-of-the-art GaN LED and the proposed LED structure, are included for comparisons. It is found that the proposed LED structure design is the best after considering costs and efficiency. For commercialization of this new LED design, the market penetration plan is to have a partnership with one of the major players in the current white LED industry. This has the advantage of having minimal capital investment and the product could be sold under an established brand. A simplified projection of earnings is calculated to illustrate sustainability of this business plan.by Qixun Wee.M.Eng

Optical Studies of Indium Gallium Nitride Nanostructures

Indium gallium nitride (InGaN) is a semiconductor material that is in widespread use in blue light emitting diodes (LEDs) and blue laser diodes and is being used in solid-state lighting, displays, and scientific applications. The scientific understanding of the physical mechanisms responsible for the performance of these devices is still developing; this includes the description of localization of carriers in this material – a fundamental issue which is believed to be responsible for the origin of luminesce in blue LEDs – as well as that of performance limitations of existing devices, including the so-called “green gap” and “efficiency droop,” which are related in part to the exciton-phonon interaction. This thesis studies top-down etched InGaN quantum disks (QDs) embedded in GaN nanopillars, focusing on the effect of localization and of the exciton-phonon interaction. The exciton-phonon interaction is studied with two experiments which examine the effect of quantum dot size and shape on the strength of the optical phonon replica in the PL. First, we study the effect of asymmetrical strain on the exciton-phonon coupling by examining the optical phonon replica in the PL of nineteen individual elliptical QDs with dimensions of 22nm x 36nm. We show that the effect of strain on the phonon coupling strength should be observable by a reduction in the degree of polarization (DOP) of the optical phonon replica. Measurements confirm that there is a reduction in the DOP of the optical phonon replica, with reasonable agreement with theory for the high DOP dots. Lower DOP dots, which arise to due irregularities in the shape and size of the fabricated nanopillars, also show a reduction in DOP of the phonon replica but are more sensitive to the effect of asymmetrical phonon coupling and warrant further study. Second, we examine the effect of nanopillar diameter on exciton-phonon coupling strength in InGaN quantum disks. We observe an enhancement of the phonon replica as the nanopillar diameter is reduced from 1000nm to 60nm. This effect is explained by a reduction in the lateral Bohr radius of the exciton which accompanies the decrease in vertical electron-hole separation in smaller nanopillar diameters. To quantify this effect, a simple model is used to infer that, based on the measured phonon coupling strengths, the Bohr radius reduces from approximately 2.5nm to 2nm as the diameter is reduced over the observed range. In order to study the effect of localization, we measure the Stokes shift, which is the energy difference between emission and absorption. By measuring this quantity as a function of nanopillar diameter, we demonstrate the ability to separately determine the contributions of the strain-induced quantum confined Stark effect and of localization to the observed Stokes shift. In our case, we find that the two effects have approximately equal contributions for the range of nanopillar diameters studied here. Furthermore, the site control of InGaN/GaN quantum disks using this top-down fabrication method assists the integration of advanced device structures with individual nanopillars. We demonstrate the enhancement of light collimation by a factor of 1.8x from single nanopillar LEDs using an integrated nanolens. Additionally, we report measurements of enhanced QD brightness and radiative emission rate using an open-top plasmonic cavity; this demonstration is tailored for applications in quantum technologies such as quantum cryptography.PHDPhysicsUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttps://deepblue.lib.umich.edu/bitstream/2027.42/147624/1/akatcher_1.pd

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

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

Device Engineering for Internal Quantum Efficiency Enhancement and Efficiency Droop Issue in III-Nitride Light-Emitting Diodes

Over the past few decades, III-Nitride semiconductors have found the tremendous impacts in solid state lighting, power electronics, photovoltaics and thermoelectrics. In particular, III-nitride based light-emitting diodes (LEDs) with long lifetime and eco-friendliness are fundamentally redefining the concepts of light generation due to the superior material properties of direct bandgap, efficient light emission and robustness. The industry of LED based solid state lighting is fulfilling the potential of reducing the 20% of the total US energy consumed by lighting to half of this usage. However, several major obstacles are still hindering the further development of LEDs for general illuminations. They include efficiency droop phenomenon at high operating current, low efficiency in green spectrum, and low extraction efficiency due to the large difference in refractive index. The report will present both experimental and theoretical works on III-nitride semiconductor materials and devices for solid state lighting, including 1) novel barrier design for efficiency-droop suppression, 2) novel active region design for radiative efficiency enhancement, and 3) fabrication of ultrahigh density and highly uniform III-nitride based quantum dots (QDs) for high efficiency optoelectronics and photovoltaic cells. In addition to the three main topics, a new topic on the p-type III-nitrides doping sensitivity will be investigated in the latter part of this report.Firstly, the use of large bandgap thin barrier layers surrounding the InGaN QWs in LEDs will be proposed for efficiency droop suppression. The efficiency of LED devices suffers from reduction at high current injection, which is referred as efficiency droop phenomenon. Although the origin is still inconclusive up till now, the carrier leakage issue is widely considered as one of the major reasons. The increased effective barrier heights from the use of a thin (d \u3c 2 nm) lattice-matched AlGaInN barriers are shown to improve current injection efficiency and internal quantum efficiency. The optimization of epitaxial conditions of lattice-matched AlInN material has been carried out by metal-organic chemical vapor deposition (MOCVD) for the fabrication of InGaN QW LEDs with the insertion of AlInN thin barrier. The device characterizations of cathodoluminescence and electroluminescence show the great potential of the InGaN-AlInN design in addressing the efficiency droop issue at high current density. Secondly, the staggered InGaN QW and InGaN-delta-InN QW are investigated for the high efficiency LEDs emitting at green or longer emission spectrum region to provide solutions for greengap challenge. The introduction of energy local minima in QW region by the novel structures of staggered InGaN QWs enables the spatial shift of electron and hole wavefunction towards the center of active region. Therefore, the approach leads to the enhancement of electron-hole wavefunction overlap and thus the radiative recombination rate and optical gain. The analysis of InGaN-delta-InN QW LED with the potential of effectively extending the emission wavelength without sacrificing the radiative recombination rates will also be presented. Thirdly, the sensitivity study of the doping levels of p-type layers in InGaN/GaN MQW LEDs will be discussed for industrial application. Due to the difficulty in activating the acceptor magnesium in III-nitrides, thermal annealing process is employed to increase the hole concentration in p-type semiconductors. The uniform temperature distributions in the annealing chambers will lead to non-uniformity in p-type doping levels. The effect of doping levels on LED device performance will be examined, and the doping sensitivity of light output power and internal quantum efficiency will be investigated in this report. The results will provide guidance for the parameter optimization of the fabrication process for commercial product line to increase the yield.Fourthly, the growths of ultra-high density and highly uniform InGaN QDs on GaN/ sapphire template as an important alternative active region for high-efficiency optoelectronic devices will be discussed. The growths of ultra-high density and highly uniform InGaN QDs by employing selective area epitaxy were realized on nanopatterned GaN template fabricated by diblock copolymer lithography. It results in well-defined QD density in the range of 8x1010 cm-2, which represents the highest QD density reported for nitride-based QDs. In comparison, the InGaN QD density by the prevailing Stranski-Krastanow (S-K) growth mode is around mid 109 cm-2 with non-uniformity in dot sizes and distributions. The availability of highly-uniform and ultra-high density InGaN QDs formed by this approach has significant and transformational impacts on developing high-efficiency light-emitting diodes for solid state lighting, ultra-low threshold current density visible diode lasers, and intermediate-band nitride-based solar cells

New generation light emitting diodes:fundamentals and applications

Light emitting diodes (LEDs) have made tremendous progress in last 15 years andhave reached to a point where they are reinventing and redefining artificial lighting.The efficiency and better control over light quality parameters have been the keyattributes of LEDs that makes them better than the existing lighting solutions.Nevertheless, in their own realm they suffer from decrease in efficiency at highercurrents, i.e. the “efficiency droop” phenomenon. Thus, a better understandingof the mechanisms leading to droop is of utmost importance. Moreover, the fullpotential in terms of light quality, i.e. colour rendering index (CRI) and correlatedcolour temperature (CCT) that can be offered by these devices can be furtherimproved with existing or alternative schemes and device configurations.In this thesis, a novel phosphor covered approach is investigated towards improvingthe CRI for indoor lighting applications. A monolithic di-chromatic LEDemitting at blue and cyan wavelengths is used to pump a green-red phosphor mixtureand a warm (CCT ∼ 3400 K) white light with a superior CRI of 98.6 is achieved.An alternate phosphor free solution to achieve warm white light emission is alsostudied. These monolithic di-chromatic QW devices emitting at blue and greenwavelengths under electrical pumping demonstrated tuneable emission from cool(CCT ∼ 22000 k) to warm (CCT ∼ 5500 K) white light. A maximum CRI of 67,which is the highest value demonstrated for such devices till date to the best of myknowledge, is also achieved.On the subject of efficiency of LEDs, temperature dependence of LEE andIQE of commercial InGaN/GaN based blue LED is studied in light of a step-wiseprocessing procedure based on the ABC-model to determine these quantities. Adecrease in both IQE and LEE with temperature is noted. On the other hand,efficiency decrease in the investigated AlGaInP based red LEDs under pulsed currentshows a shift in the onset of efficiency decrease towards higher current values withdecreasing pulse width with < 1% duty cycle. For sub-nanosecond pulses a linearrelation between applied peak current and peak output power is obtained. Theseobservations indicate device self-heatin

Study of the III-nitride materials grown by mixed-source HVPE for white LED applications emitting multi spectrum range

The purpose of this study is to explore the possibility of phosphor-free white-emitting LED？s based in the gallium nitride material system. The structures are to be grown using mixed source hydride vapor phase epitaxy (MS-HVPE). It is unique crystal growth technology different from conventional HVPE and MOCVD system using mixed metal source of aluminum, indium and gallium. The first step in this project is the optimization of MS-HVPE growth process. This was achieved successfully, as binary, ternary and quaternary films are demonstrated. Successful n and p-type doping are also demonstrated introducing Te and Mg. The second step in this project is fabricating broadband spectrum emitting device of phosphor-free white LED by MS-HVPE. The device structure consisted of conventional double-hetero (DH) structure, which was the undoped InAlGaN active layer and n, p-AlGaN cladding layers. We observed that the device of AlInGaN quarternary active grown by MS-HVPE emitted multi spectrum from UV to red area. We also found that its spectrum was variable as indium mole fraction and controllable. It was nano phase epitaxy phenomenon being only observed in HS-HVPE process. An extensive growth study of GaN based material was also carried out. The effects of several growth parameters on emission characteristics were presented. PL emission wavelengths for each structure were demonstrated. And EL emission wavelengths were also demonstrated after wafer fabrication process. Additionally, x-ray diffraction and x-ray photoelectron spectroscopy (XPS) showed to verify crystal quality of MS-HVPE. The dissertation presented herein demonstrates achieving phosphor-free solid-state white lighting. But it still has unknown physical characteristics. Continuation of this study will lead to future industry. And hopefully it will be commercialized and applied to residential illumination due to this technology.Chapter 1. Introduction 1 1.1. Overview of LED 1 1.2. Wide bandgap compound semiconductor 6 1.3. Overview of white LED 10 1.4. Purpose and outline of this project 15 Chapter 2. Fundamentals of Gallium Nitride 21 2.1. Introduction 21 2.1.1. Current Issues in GaN-based LED 23 2.2. Crystallography of Gallium Nitride 26 2.3. Characteristics of Gallium Nitride 32 2.3.1. Doping of Gallium Nitride 33 2.3.2. Optical Properties of Gallium Nitride 35 2.3.3. Polarity in Gallium Nitride 38 2.4. Substrates for GaN Epitxial Growth 40 2.4.1. Substrate issues 40 2.4.2. Sapphire 41 2.4.3. SiC 45 Chapter 3. Overview of Epitaxial Growth Experimental 57 3.1. Hydride vapor phase epitaxy 57 3.1.1. Introduction to HVPE 57 3.1.2. Mixed source HVPE system 59 3.1.3. Some parameters for optimized GaN growth 62 3.2. Wafer fabrication process 63 3.2.1. Selective area growth 63 3.2.2.Metallization of GaN 64 3.3. Measurements 66 3.3.1. Photoluminescence 66 3.3.2. DXRD 67 3.3.3. SEM/CL 70 3.3.4. E-CV 75 3.3.5. Hall measurement 77 Chapter 4. Mixed Source HVPE Growth Experiment for Bulk Characteristics 82 4.1. GaN growth 82 4.1.1 Buffer growth for GaN layer 83 4.1.2. Mg-doped GaN layer 87 4.2. AlGaN growth 90 4.3. InGaN growth 97 Chapter 5. Fabrication of AlInGaN-Based LED for White Emission 115 5.1. AlInGaN SAG-DH structure growth 115 5.2. Characterization of AlInGaN SAG-DH epitaxial structure 123 5.3. Device fabrication 127 Chapter 6. Experimental Results for Active layer’s Condition 135 6.1. Performance of AlInGaN white LED 136 6.2. EL characteristics of AlGaN and AlInGaN active 139 6.2.1 GaN active layer 139 6.2.2 Al(0.1g)GaN active layer 141 6.2.3 Al(0.3g)GaN active layer 144 6.2.4 Al(0.4g)GaN active layer 144 6.2.5 Al(0.5g)GaN active layer 146 6.2.6 Al(0.6g)GaN active layer 149 6.2.7 In(0.1g) Al(0.6g) GaN active layer 151 6.2.8 In(0.2g)Al(0.6g)GaN active layer 153 6.2.9 In(0.3g)Al(0.6g)GaN active layer 155 6.2.10 In(0.4g)Al(0.6g)GaN active layer 158 6.2.11 In(0.5g)Al(0.6g)GaN active layer 160 6.3. XRD characteristics 163 Chapter 7. Phosphor &#8211Free White LED Lamp 180 7.1. Manufacturing of white LED lamp 180 7.2. Analysis of White LED Spectra and Color Rendering 181 7.3. Measurement of Phospohor free white LED 189 7.4. Future research 197 Chapter 8. Conclusions 200 Publications 202 Conference 204 Biography 209 Acknowledgements 21