HYBRID WHITE LIGHT-EMITTING DIODES: STUDY AND FABRICATION OF THIN-FILM AND NANOWIRE-BASED DEVICES

I diodi ad emissione luminosa sono dei dispositivi a semiconduttore che posseggono straordinarie proprietà fisiche di generazione della luce. Il primo LED bianco si è mosso silenziosamente dalla metà degli anni 90 fino a un punto in cui, oggi, il mercato dell’illuminazione è stato totalmente rivoluzionato. A fronte dell’industria dell’illuminazione a stato solido che richiede LED sempre più luminosi ed economici, la ricerca scientifica risponde con strutture innovative, nuovi materiali più efficienti, anche combinati con l’uso delle nanotecnologie applicate. I diodi bianchi ibridi ad emissione luminosa, ad esempio, fanno anch’essi uso di un LED blu, come in un tradizionale diodo bianco, ma in essi il materiale inorganico convenzionale per la conversione del colore è sostituito da un colorante organico cha una maggiore resa quantica. La combinazione di una sorgente blu nanostrutturata con tali coloranti costituisce un’interessante struttura non ancora esaustivamente studiata in letteratura. In questo lavoro, due strutture per diodi bianchi ibridi innovativi sono state progettate, fabbricate e caratterizzate. La prima consiste in un dispositivo a eterogiunzione ZnO/GaN, basato su nanofili di ossido di zinco, ottenuto con una tecnica di crescita in soluzione acquosa a basso costo; la conversione del colore, ottenuta tramite un colorante organico giallo a base di perilene, ha portato alla realizzazione di un LED ibrido bianco a luce fredda. Il secondo dispositivo ha sfruttato al massimo le proprietà dello strato organico di conversione del colore, facendo uso di una sorgente blu di pompa basata su una struttura GaN/InGaN ad alta luminosità. Inoltre, combinando opportunamente coloranti organici gialli e rossi, è possibile ottenere il controllo fine sul colore di emissione della struttura ibrida. Infine, viene presentato uno studio dettagliato sulla stabilità dello strato di conversione, insieme ad alcune soluzioni che possono essere adottate per ottenere le migliori prestazioni dal diodo ibrido bianco.Light emitting diodes are very compact and durable semiconductor devices that possess extraordinary physical properties for the generation of light. The white LED quietly moved from the middle of the 1990s to a point where, today, the lighting market is completely revolutionized. With the industry of solid state lighting demanding for always brighter and economic LEDs, the scientific research responds with innovative layouts and more efficient materials also mixed with applied nanotechnologies. Hybrid white light emitting diodes, for example, also use blue LEDs but replace the conventional inorganic material for color conversion with an organic dye that has a higher quantum conversion yield. The combination of a nanostructured blue light source with organic dyes can lead to another interesting structure that it is worth investigating on. In this work, two layouts for novel hybrid white light emitting diodes were designed, fabricated and characterized. The first one consisted in a heterojunction ZnO/GaN nanowire-based hybrid device that showed an easy processing with a low-cost acqueous solution growth technique; the color conversion achieved with an organic perylene-based yellow dye coating led to the realization of a cold white hybrid LED. The second device exploited the properties of the organic conversion layer at the maximum of its capabilities with the use of an high-brightness GaN/InGaN blue pump source. Moreover, mixing yellow and red organic dyes resulted in the color tuning of the hybrid structure’s emission. Finally, a detailed study of the conversion layer stability was also reported together with the solutions that can be adopted to obtain the best performances out of the hybrid white light emitting diode

Characterization and reliability of blue and white GaN-based LEDs submitted to current and thermal stress

The aim of this thesis work is to analyze reliability of blue and white GaN-based commercial LEDs. Reliability is indeed one of the key factors for devices success in the market of lighting solutions. Temperature and driving current are the main causes of degradation of LEDs. In order to understand the degradation mechanisms of these devices two types of stresses have been carried out, current and thermal stress and pure thermal stress, and electrical, optical and thermal measurements have been performed. The results obtained at the end of this work show several types of degradation mechanisms which influence LEDs both electrical and optical properties / Caratterizzazione e affidabilità di LED blu e bianchi basati su nitruro di gallio sottoposti a stress in corrente ed in temperatur

Optimising Light Source Spectrum to Reduce the Energy Absorbed by Objects

Light is used to illuminate objects in the built environment. Humans can only observe light reflected from an object. Light absorbed by an object turns into heat and does not contribute to visibility. Since the spectral output of the new lighting technologies can be tuned, it is possible to imagine a lighting system that detects the colours of objects and emits customised light to minimise the absorbed energy. Previous optimisation studies investigated the use of narrowband LEDs to maximise the efficiency and colour quality of a light source. While these studies aimed to tune a white light source for general use, the lighting system proposed here minimises the energy consumed by lighting by detecting colours of objects and emitting customised light onto each coloured part of the object. This thesis investigates the feasibility of absorption-minimising light source spectra and their impact on the colour appearance of objects and energy consumption. Two computational studies were undertaken to form the theoretical basis of the absorption-minimising light source spectra. Computational simulations show that the theoretical single-peak spectra can lower the energy consumption up to around 38 % to 62 %, and double-peak test spectra can result in energy savings up to 71 %, without causing colour shifts. In these studies, standard reference illuminants, theoretical test spectra and coloured test samples were used. These studies are followed by the empirical evidence collected from two psychophysical experiments. Data from the experiments show that observers find the colour appearance of objects equally natural and attractive under spectrally optimised spectra and reference white light sources. An increased colour difference, to a certain extent, is found acceptable, which allows even higher energy savings. However, the translucent nature of some objects may negatively affect the results

Study of the reliability of power LEDs for color mixing applications

The aim of this thesis is the reliability analysis of high-power RGB and white LEDs\nprovided by six different manufacturers. In order to evaluate the characteristics of these devices in terms of lifetime and optical power maintenance combined thermal and current stress tests have been carried out, verifying the changes of the optical and electrical characteristics of the devices during\nthe stress. The results of this work have revealed several types of degradation of various\ndevices

Enhanced optical characteristics of light emitting diodes by surface plasmon of Ag nanostructures

We investigated the surface plasmon coupling behavior in InGaN/GaN multiple quantum wells at 460 nm by employing Ag nanostructures on the top of a roughened p-type GaN. After the growth of a blue light emitting diode structure, the p-GaN layer was roughened by inductive coupled plasma etching and the Ag nanostructures were formed on it. This structure showed a drastic enhancement in photoluminescence and electroluminescence intensity and the degree of enhancement was found to depend on the morphology of Ag nanostructures. From the time-resolved photoluminescence measurement a faster decay rate for the Ag-coated structure was observed. The calculated Purcell enhancement factor indicated that the improved luminescence intensity was attributed to the energy transfer from electron-hole pair recombination in the quantum well to electron vibrations of surface plasmon at the Ag-coated surface of the roughened p-GaN. © 2011 SPIE

Dynamic solid state lighting

Thesis (S.M.)--Massachusetts Institute of Technology, School of Architecture and Planning, Program in Media Arts and Sciences, 2010.Cataloged from PDF version of thesis.Includes bibliographical references (p. 142-148).Energy conservation concerns will mandate near-future environments to regulate themselves to accommodate occupants' objectives and best tend to their comfort while minimizing energy consumption. Accordingly, smart energy management will be a needed and motivating application area of evolving Cyber-Physical Systems, as user state, behavior and context are measured, inferred, and leveraged across a variety of domains, environments, sensors, and actuators to dynamically mitigate energy usage while attaining implicit and explicit user goals. In this work, the focus in on the efficient control of a LED-based lighting network. This thesis presents a first-of-its-kind pentachromatic LED-based lighting network that is capable of adjusting its spectral output in response to ambient conditions and the user's preferences. The control of the intensity is formulated as a nonlinear optimization problem and the mathematics governing sensed illuminance, color, and corresponding control (feedback and adjustment) are formally defined. The prototype adjustable light source is capable of maintaining an average color rendering index greater than 92 (nearly the quality of daylight) across a broad adjustable range (2800 K - 10,000 K) and offers two modes of control, one of which is an energy efficient mode that reduces the total power consumption by 20%. The lighting network is capable of measuring the illuminance and color temperature at a surface and adjusting its output with an overall update rate of 11 Hz (limited by the MATLAB kernel). The sensor node features an optical suite of sensors with a dynamic range of 10000 : 1 lx (rms error: 2 lx). The sensor node measures the color temperature of daylight within ±500 K (kelvin). Device testing and validation were performed in a series of experiments in which the radiant power was collected using a radiometrically calibrated spectrometer with an expanded uncertainty (k = 2) of 14% and validated against a model derived by measuring the individual spectra of the system using custom MATLAB tools. A digital multimeter measured the current in the experiments. The work concludes by estimating the energy savings based on the measured optical and electrical data. In environments with moderate ambient lighting, the networked control reduces power consumption by 44% with an additional 5-10% possible with spectral optimization.by Matthew Aldrich.S.M

III-Nitride Self-assembled Nanowire Light Emitting Diodes and Lasers on (001) Silicon.

Substantial research is being devoted to the development of III-nitride light emitting diodes (LEDs) and lasers, which have numerous applications in solid state lighting. In particular, white LEDs play an increasingly important role in our daily lives. Current commercially available white LEDs are nearly all phosphor-converted, but these have some serious disadvantages. Planar quantum well (QW) devices on foreign substrates exhibit large threading dislocation densities, strong strain induced polarization field, and In-rich nanoclusters resulting in poor electron-hole wavefunction overlap, large emission peak shift with injection, and large efficiency drop at high injection currents in LEDs and large threshold current densities in lasers. The objective of this doctoral research is to investigate the prospects of self-assembled InGaN/GaN disks-in-nanowire (DNW) LEDs and lasers for solid state lighting. The research described here embodies a detailed study of the optical and structural characteristics of the nanowire heterostructures by varying the growth conditions and by surface passivation, and using the disks as the active region in high performance nanowire LEDs and gain medium in nanowire lasers on (001) silicon. Self-assembled InGaN/GaN DNWs are grown in a plasma-assisted molecular beam epitaxy (PA-MBE) system. Due to their large surface to volume ratio, the growth optimized and surface passivated DNWs on (001) silicon are relatively free of extended defects and have smaller polarization field resulting in higher radiative efficiencies. Blue-, green- and red-emitting DNW LEDs, with optimized nanowire densities, are demonstrated with reduced efficiency droop and smaller peak shift with injection. Phosphor-free white nanowire LEDs are realized by incorporating InGaN/GaN disks with different color emissions in the active region. The first ever monolithic edge-emitting electrically pumped green and red nanowire lasers on (001) silicon are demonstrated using DNWs as the gain media and are characterized by low threshold current densities of 1.76-2.88 kA/cm2, small peak shifts of 11-14.8 nm, large T0 of 234 K and large differential gain of 3x10-17 cm-2. Dynamic measurements performed on these lasers yield a maximum small signal modulation bandwidth of 5.8 GHz, extremely low value of chirp (0.8 Å) and a near-zero linewidth enhancement factor at the peak emission wavelength.PhDElectrical EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/111490/1/shafat_1.pd