Adaptive and smart systems are growing in popularity as we shift toward personalization as a culture. With progressive demands on energy efficiency, it is increasingly important to focus on the utilization of energy in a novel way. This thesis investigates a microelectromechanical system (MEMS) mirror with the express intent to provide flexibility in solid state lighting (SSL). By coupling the micromirror to an optical source, the reflected light may be reshaped and directed so as to optimize the overall illumination profile. In addition, the light may be redirected in order to provide improved signal strength in visible light communications (VLC) with negligible impact on energy demands. With flexibility and full analog control in mind, the design of a fully integrated tip-tilt-piston micromirror with an additional variable focus degree of freedom is outlined. Electrothermal actuators are used to both steer the light and tune the focal length. A detailed discussion of the underlying physics behind composite beams and thermal actuators is addressed. This leads directly into an overview of the two main mirror components, namely the segmented mirror and the deflection actuators. An in-depth characterization of the dynamics of the mirror is discussed including the linearity of the thermal response. Frequency domain analysis of such a system provides insight into tunable mechanical properties such as the resonant frequency and quality factor. The degenerate resonant modes can be separated significantly. It is shown that the frequency response may be tuned by straining specific actuators and that it follows a predictable pattern. As a result, the system can be scanned at increasingly large angles. In other words, coupled mechanical modes allow variable damping and amplification. A means to determine the level of coupling is examined and the mode shape variations are tracked as a function of the tuning parameters. Finally, the applications of such a device are explored and tested. Such applications include reliable signal-to-noise ratio (SNR) enhancements in VLC of 30 dB and color tunable steerable lights using laser diodes. A brief discussion of the implications of dynamic illumination and tunable systems is juxtaposed with an explanation behind the integration of an electrothermal micromirror and an all digital driver.

An Investigation into Red Emitting Phosphors for Display and Lightings Two very different classes of red phosphors were evaluated in this thesis; sulphide based phosphors (CaS:Eu2+) and phosphors based on the formulae:- LiEu1- xMx(MoO4)2-y(WO4)y, [M=Al3+, Y3+ and Gd3+]. CaS:Eu2+ was coated with Al2O3 using atomic layer deposition technique and placed into a humidity chamber at high temperature and high humidity for a period of time. The emission spectra were measured and evaluated against uncoated CaS:Eu2+. The results from the humidity tests revealed that Al2O3 coated CaS:Eu2+ increased the life span of the phosphor and therefore can be the potential use for the purpose of application at moderate humidity and temperature. For the phosphors based on LiEu1-xMx(MoO4)2-y(WO4), all the findings reported herein showed that, depending upon the types of cations introduced to the host lattices, incorporation of M at some point offered very much the same or better luminous efficacy when compared to the parent compound with 100 mol% of Eu3+. This work also attempted to examine the correlative relationship between the crystal structures and the luminous efficacies although some attempts were unsuccessful. The findings discovered within this research are beneficial for solid state lighting industries where the cost of using rare earth metals has become significant and recycling is difficult.

The past decade witnessed the rapid development of III-nitride light-emitting diodes (LEDs) and laser diodes (LDs), for smart lighting, visible-light communication (VLC), optical storage, and internet-of-things. Recent studies suggested that the GaN-based LDs, which is free from efficiency droop, outperform LEDs as a viable high-power light source. Conventionally, the InGaN-based LDs are grown on polar, c-plane GaN substrates. However, a relatively low differential gain limited the device performance due to a significant polarization field in the active region. Therefore, the LDs grown on nonpolar m-plane and semipolar (2021)-plane GaN substrates are posed to deliver high-efficiency owing to the entirely or partially eliminated polarization field. To date, the smart lighting and VLC functionalities have been demonstrated based on discrete devices, such as LDs, transverse-transmission modulators, and waveguide photodetectors. The integration of III-nitride photonic components, including the light emitter, modulator, absorber, amplifier, and photodetector, towards the realization of III-nitride photonic integrated circuit (PIC) offers the advantages of small-footprint, high-speed, and low power consumption, which has yet to be investigated. This dissertation presents the design, fabrication, and characterization of the multi-section InGaN laser diodes with integrated functionalities on semipolar (2021)-plane GaN substrates for enabling such photonic integration. The blue-emitting integrated waveguide modulator-laser diode (IWM-LD) exhibits a high modulation efficiency of 2.68 dB/V. A large extinction ratio of 11.3 dB is measured in the violet-emitting IWM-LD. Utilizing an integrated absorber, a high optical power (250mW), droop-free, speckle-free, and large modulation bandwidth (560MHz) blue-emitting superluminescent diode is reported. An integrated short-wavelength semiconductor optical amplifier with the laser diode at ~404 nm is demonstrated with a large gain of 5.32 dB at 6 V. A high-performance waveguide photodetector integrated LD at 405 nm sharing the single active region is presented, showing a significant large modulation bandwidth of 230 MHz. Thus these seamlessly integrated elements enable photonic IC at the visible wavelength for many important applications, such as smart lighting and display, optical communication, switching, clocking, and interconnect. The findings are therefore significant in developing an energy-saving platform technology that powers up human activities in a safe, health- and environmental-friendly manner.

With experimental realization, energy harvesting capabilities of chaotic microstructures were explored. Incident photons falling into chaotic trajectories resulted in energy buildup for certain frequencies. As a consequence, many fold enhancement in light trapping was observed. These ellipsoid like chaotic microstructures demonstrated 25% enhancement in light trapping at 450nm excitation and 15% enhancement at 550nm excitation. Optimization of these structures can drive novel chaos-assisted energy harvesting systems. In subsequent sections of the thesis, prospect of broadband light extraction from white light emitting diodes were investigated, which is an unchallenged but quintessential problem in solid-state lighting. Size dependent scattering allows microstructures to interact strongly with narrow-band light. If disorder is introduced in spread and sizes of microstructures, broadband light extraction is possible. A novel scheme with Voronoi tessellation to quantify disorder in physical systems was also introduced, and a link between voronoi disorder and state disorder of statistical mechanics was established. Overall, in this thesis some nascent concepts regarding disorder and chaos were investigated to efficiently manage electromagnetic waves in optoelectronic devices.

Ces travaux de thèse portent sur l'évaluation des propriétés de nanofils InGaN/GaN en vue de la réalisation de diodes électroluminescentes (LEDs). Deux types d'architecture, obtenus par des techniques de croissance différentes, ont été étudiés. La technique MBE a conduit à la réalisation de LEDs en structure axiale émettant du domaine spectral bleu au rouge. Les émetteurs uniques présentent dans ce cas des diamètres typiquement inférieurs à 100 nm. La technique MOCVD a conduit quant à elle la fabrication de LEDs émettant des longueurs d'onde plus courtes à partir d'hétérostructures InGaN/GaN en Coeur/Coquille présentant des dimensions micrométriques. Dans les deux cas, la croissance est réalisée de manière spontanée sur un substrat Silicium (111) de conductivité élevée permettant l'injection verticale du courant dans les dispositifs intégrés à l'échelle macroscopique. L'ensemble des briques technologiques nécessaires à la fabrication de LEDs a été évalué par un panel important de techniques expérimentales adaptées aux structures à fort rapport de forme. Ainsi, l'effet de l'incorporation d'espèces dopantes de type n (Silicium) et de type p (Magnésium) a été caractérisé par des expériences de spectroscopie optique couplées à des mesures électriques sur fils uniques. De plus, la cathodoluminescence basse température a été largement utilisée afin d'étudier les propriétés optiques de la zone active à base d'InGaN dans les deux architectures considérées. Après intégration technologique, des caractérisations électro-optiques résolues à l'échelle du fil unique ont montré que les performances des LEDs à nanofils restent principalement limitées par la fluctuation des propriétés électriques et optiques entre émetteurs uniques. / This thesis aims at studying the intrinsic properties of InGaN/GaN nanowires (NWs) in order to fabricate efficient light emitting diodes (LEDs). Two active region designs, obtained through different growth techniques, have been extensively investigated. Axial NW-based LEDs emitting from the blue to the red spectral range have been grown by MBE. In this case, single emitters present diameters typically smaller than 100 nm. MOCVD allowed the fabrication of LEDs emitting shorter wavelengths from Core/Shell heterostructures with typical dimensions in the micrometre range. In both cases, the spontaneous growth has been conducted on Silicon (111) highly conductive substrates in order to inject the current vertically into macroscopically contacted devices. Technological building blocks needed to fabricate LEDs have been investigated using a wide range of characterization techniques adapted for high aspect ratio structures. Thus, n-type (Silicon) and p-type (Magnesium) dopings have been assessed thanks to optical spectroscopy techniques, and these results have been confirmed by electrical measurements carried out on single wires. Furthermore, low temperature cathodoluminescence has been widely used to study the optical properties of InGaN-based active regions. After technological integration, electro-optical characterizations with spatial resolution down to the single wire level have revealed that device performances are mainly limited by the fluctuation of electrical and optical properties between single emitters.

abstract: White organic light emitting diodes (WOLEDs) are currently being developed as the next generation of solid state lighting sources. Although, there has been considerable improvements in device efficiency from the early days up until now, there are still major drawbacks for the implementation of WOLEDs to commercial markets. These drawbacks include short lifetimes associated with highly efficient and easier to fabricate device structures. Platinum (II) complexes are been explored as emitters for single emissive layer WOLEDs, due to their higher efficiencies and stability in device configurations. These properties have been attributed to their square planar nature. Tetradentate platinum (II) complexes in particular have been shown to be more rigid and thus more stable than their other multidentate counterparts. This thesis aims to explore the different pathways via molecular design of tetradentate platinum II complexes and in particular the percipient engineering of a highly efficient and stable device structure. Previous works have been able to obtain either highly efficient devices or stable devices in different device configurations. In this work, we demonstrate a device structure employing Pt2O2 as the emitter using mCBP as a host with EQE of above 20% and lifetime values (LT80) exceeding 6000hours at practical luminance of 100cd/m2. These results open up the pathway towards the commercialization of white organic light emitting diodes as a solid state lighting source. / Dissertation/Thesis / Masters Thesis Materials Science and Engineering 2016

Ce mémoire s'inscrit dans la construction d'une méthodologie d'analyse de défaillance pour l'évaluation de la fiabilité de diodes électroluminescentes, par une approche basée sur l'analyse physique de dégradation et l'extraction de signatures de défaillance électriques et optiques pour localiser les zones dégradées. L'ajout d'analyses physico-chimiques réduit le nombre de composants et peut confirmer les mécanismes de dégradation induits par les vieillissements en stockage actif. Un projet, en collaboration avec le CNES, a permis la mise en évidence des zones sensibles de DELs à MPQ InGaN/GaN à faible puissance (30mW) soumises à un vieillissement en conditions opérationnelles (1500h/85°C/Inominal). L'analyse de défaillance de ces DELs a permis d'expliquer une perte de 65% de puissance optique par la polymérisation de l'huile silicone activée photothermiquement induisant une perte de fluorescence de 69% et une très forte diminution de l'absorption de la lumière de la DEL (90%). Nous avons également démontré (projet CEA-LETI  éclairage public) que le même mécanisme est présent dans le mélange gel silicone/phosphore YAG:Ce de DELs blanches à MPQ InGaN/GaN soumises à un vieillissement similaire (85°C/550mA/500h). A 450nm, le rendement de fluorescence a augmenté de 1,2% malgré des pertes en absorption (> 94%) et en réémission de fluorescence (> 85%). La polymérisation de l'huile silicone a induit une perte de puissance optique des DELs de 45% et une dérive de la couleur blanche vers le jaune (≈ 3,6%). Cette dérive est due à un décalage spectral de la fluorescence de l'UV (5nm) vers le bleu entraînant un décalage vers le rouge (2nm) de la lumière de la DEL. / GaN-based LEDs are currently used in a wide range of applications as solid-state lighting, backlighting or full-color displays. Up to date, polymer-based packaging degradation mechanisms are not fully understood. The purpose of this thesis is to work out a methodology of failure analysis contributing towards reliability estimation of GaN-based LEDs under active storage ageing tests. The methodology consists in extracting electro-optical failure signatures to locate degraded zones. A second step is based on physico-chemical analyses used to both confirm failure mechanisms and reduce the number of components to study. Environmental ageing tests (1500h/85°C/Inominal) have been performed on low power InGaN/GaN MQW LEDs (30mW) through a project in collaboration with the French Space Agency (CNES). A 65% loss of optical power has been reported after ageing. Through the methodology, we have found out that optical loss is due to the silicone oil (i.e. chip coating) polymerization activated by photothermal mechanism thereby involving both a 69% fluorescence emission loss and a strong decrease of LED light absorption (90%). A similar failure mechanism has been reported on YAG:Ce/silicone oil mixture located in phosphor converted high power white InGaN/GaN MQW LEDs (CEA-LETI collaboration - Solid-State Lighting project). Fluorescence efficiency has increased (1,2% at 450 nm) despite both strong absorption (94%) and fluorescence emission (85%) losses. Actually, silicone oil polymerization has induced a 45% loss of optical power and a 3,6% yellow shift of white light. Such drift has been linked to both a 5nm blue shift of UV fluorescence involving a 2nm red shift of LED light.

A novel solid state illumination source has been developed. A two terminal dual LED has been created with the ability to control the relative intensities of the two emission peaks by varying drive current. Doping profiles have been used to extend the dynamic range of the dual LED over other reported devices. Operation of the dual LEDs is explained as a function of drive current. In addition, novel use of phosphor mixtures allows the creation of a broadband spectral power distribution that can be varied using a dual LED as an excitation source. Combinations of phosphors that have varied excitation spectra provide the ability to selectively excite different phosphors with the different LED emission peaks. First and second generations of the two terminal dual LED and the phosphor combination are discussed. The final source has the ability to mimic the light of a blackbody radiator over a range of 3200 K - 5300 K. The development of a three terminal dual LED as a pump source was prohibited by the need for a III-nitride tunnel junction, that proved unattainable in the scope of this work. However, several novel doping schemes were investigated toward this end. Finally, a circadian light source has also been developed that can affect physiological changes in humans, and a light box for entrainment of circadian rhythms in rats has been built.

This dissertation includes two parallel topics: optical modeling of wavelength converters based on semiconductor optical amplifiers (SOA) and optical modeling for LEDs and solid state lighting. A steady-state numerical model of wavelength converters based on cross-gain SOAs is developed. In this model, a new model of the gain coefficient is applied. Each physical variable, such as the carrier density, gain coefficient, differential gain, and internal loss, spatially varies across the SOA cavity and is numerically calculated throughout the device. Increased accuracy over previous studies is achieved by including such spatial variations. This model predicts wavelength-dependent characteristics of a wavelength converter of the SOA in both large and small signal regimes. Some key performance factors of SOA wavelength converters. A hybrid method incorporating both guided wave optics and optical ray tracing is also developed to model LEDs and solid state lighting. This method can model either single wavelength or dual-wavelength LED structures with different die shapes and packages. The waveguide and diffraction optics are mainly used to model the near-field optics inside LED chips and its vicinity and to identify guided modes and leakage modes. Geometrical ray tracing is applied to model the far-field pattern and light interactions at different material interfaces, such as LED chip structures, LED package materials, and light scattering at those rough surfaces and textures. To improve LED light extraction efficiency, different LED die shapes and device structures can also be optimized using this method. New technologies for future research on SOAs and LEDs are also proposed.

The objective of the research was to develop high quality GaN epitaxial growth on alternative substrates that could result in higher external quantum efficiency devices. Typical GaN growth on sapphire results in high defect materials, typically 10⁸⁻¹⁰cm⁻², due to a large difference in lattice mismatch and thermal expansion coefficient. Therefore, it is useful to study epitaxial growth on alternative substrates to sapphire such as ZnO which offers the possibility of lattice matched growth. High-quality metalorganic chemical vapor deposition (MOCVD) of GaN on ZnO substrate is hard to grow due to the thermal stability of ZnO, out-diffusion of Zn, and H₂back etching into the sample. Preliminary growths of GaN on bare ZnO substrates showed multiple cracks and peeling of the surface. A multi-buffer layer of LT-AlN/GaN was found to solve the cracking and peeling-off issues and demonstrated the first successful GaN growth on ZnO substrates. Good quality InGaN films were also grown showing indium compositions of 17-27% with no indium droplets or phase separation. ZnO was found to to sustain a higher strain state than sapphire, and thereby incorporating higher indium concentrations, as high as 43%, without phase separation, compared to the same growth on sapphire with only 32%. Si doping of InGaN layers, a known inducer for phase separation, did induce phase separation on sapphire growths, but not for growths on ZnO. This higher strain state for ZnO substrates was correlated to its perfect lattice match with InGaN at 18% indium concentration. Transmission electron microscopy results revealed reduction of threading dislocation and perfectly matched crystals at the GaN buffer/ZnO interface showing coherent growth of GaN on ZnO. However, Zn diffusion into the epilayer was an issue. Therefore, an atomic layer deposition of Al₂O₃was grown as a transition layer prior to GaN and InGaN growth by MOCVD. X-ray and PL showed distinct GaN peaks on Al₂O₃/ZnO layers demonstrating the first GaN films grown on Al₂O₃/ZnO. X-ray photoelectron spectroscopy showed a decrese in Zn diffusion into the epilayer, demonstrating that an ALD Al₂O₃layer was a promising transition layer for GaN growth on ZnO substrates by MOCVD.