RELIABILITY TESTING & BAYESIAN MODELING OF HIGH POWER LEDS FOR USE IN A MEDICAL DIAGNOSTIC APPLICATION

While use of LEDs in fiber optics and lighting applications is common, their use in medical diagnostic applications is rare. Since the precise value of light intensity is used to interpret patient results, understanding failure modes is very important. The contributions of this thesis is that it represents the first measurements of reliability of AlGaInP LEDs for the medical environment of short pulse bursts and hence the uncovering of unique failure mechanisms. Through accelerated life tests (ALT), the reliability degradation model has been developed and other LED failure modes have been compared through a failure modes and effects criticality analysis (FMECA). Appropriate ALTs and accelerated degradation tests (ADT) were designed and carried out for commercially available AlGaInP LEDs. The bias conditions were current pulse magnitude and duration, current density and temperature. The data was fitted to both an Inverse Power Law model with current density J as the accelerating agent and also to an Arrhenius model with T as the accelerating agent. The optical degradation during ALT/ADT was found to be logarithmic with time at each test temperature. Further, the LED bandgap temporarily shifts towards the longer wavelength at high current and high junction temperature. Empirical coefficients for Varshini's equation were determined, and are now available for future reliability tests of LEDs for medical applications. In order to incorporate prior knowledge, the Bayesian analysis was carried out for LEDs. This consisted of identifying pertinent prior data and combining the experimental ALT results into a Weibull probability model for time to failure determination. The Weibull based Bayesian likelihood function was derived. For the 1st Bayesian updating, a uniform distribution function was used as the Prior for Weibull á-â parameters. Prior published data was used as evidence to get the 1st posterior joint á-â distribution. For the 2nd Bayesian updating, ALT data was used as evidence to obtain the 2nd posterior joint á-â distribution. The predictive posterior failure distribution was estimated by averaging over the range of á-â values. This research provides a unique contribution in reliability degradation model development based on physics of failure by modeling the LED output characterization (logarithmic degradation, TTF â<1), temperature dependence and a degree of Relevance parameter `R' in the Bayesian analysis

Optical and Electrical Characterisation of AlGaInP Photodiodes

The quaternary alloy of group III-V, the (AlxGa1-x)0.52In0.48P is the widest bandgap material that can be grown lattice-matched to GaAs. Seven different aluminium compositions of x=0, 0.31, 0.47, 0.61, 0.64, 0.78 and 1.0 were grown with the same nominal i-region thicknesses of 1µm. These p-i-n photodiodes were fabricated with the standard fabrication and etching technique. The aim of this project is to characterise the optical and electrical properties of GaInP to AlInP, across the composition range for application such as top junction in multi-junction solar cell. This work comprises of obtaining the dynamic range of absorption coefficient, α through the spectral response characterisation. The α was extracted down to the bandgap absorption region from 106 to as low as 100 cm-1. The photocurrent measurement was first carried out accurately and converted to the quantum efficiency. The model of quantum efficiency derived from the current continuity equation is used to iteratively fit the experimental data. The sensitivity of the model was taken into account through the variation of the minority carrier diffusion length, the surface recombination velocity and the cladding thicknesses. Initially, the α of the direct bandgap material is rapidly blue-shifted. However, the rate of change reduces as the composition becomes indirect. From here, the bandgap is extracted through the determination of the direct and indirect bandgap between the gamma-valley and x-valley. The material started to become indirect at the aluminium content of x≥0.48, which is similar to AlGaAs. The investigation of current-voltage (I-V) measurement across the composition range was also carried out from 300K to 600K. The heater stage system was used in the characterisation of the dark current and photocurrent. Preliminary studies of photocurrent were undertaken as a function of temperature. From the forward I-V, the activation energy through the Arrhenius plot was extracted as a function of biased voltage for GaInP and AlInP. With the ideality factor of ≈1.7, the activation energy for both ends of the composition are 1.16eV and 1.29eV respectively. The ability of the devices to function at high temperature with slight degradation and to endure multiple heating cycles, proves growth and fabrication work well for this characterisation. Subject to this temperature range, the AlGaInP is considered suitable for high temperature solar cell and space exploration applications

Design and analysis of solar cells by coupled electrical - optical simulation

Careful electrical design and optical design are both crucial for achieving high-efficiency solar cells. It is common to link these two aspects serially; the optical design is first done to minimize reflection and maximize light trapping, and then the resulting optical generation rate is input to the electrical simulation. For very high efficiency solar cells that approach the Shockley-Queisser limit, however, electrical and optical transports are tightly coupled in both directions. Photons generated by radiative recombination can be reabsorbed to create additional electron-hole pairs (so-called photon recycling), which decreases losses. A variety of novel photon management schemes are currently being explored. To achieve the promise of these new approaches, a self-consistent simulation framework that rigorously treats both photons and electrons is needed. In this work, the thin-film GaAs solar cell, the single nanowire solar cell, and the GaInP/GaAs tandem solar cell are investigated. For solar cell characterization, this work examines the validity of the reciprocity theorem and quantitative lifetime parameter extraction using Time-Resolved Photoluminescence (TRPL) and Photoluminescence Excitation Spectroscopy (PLE). Overall, this thesis work has created a new simulation tool for advanced photovoltaic devices based on the self-consistent coupling of wave optics with electronic transport, which lead to accurate predictions of the characteristics and performance. Optimization of photon recycling facilitates improved design strategies to approach the Shockley-Queisser limit, which will eventually pave the way for extension to advanced designs, capable of approaching or even exceeding the Shockley-Queisser limit in the future

Narrow Bandgap (0.7–0.9 eV) Dilute Nitride Materials for Advanced Multijunction Solar Cells

Aurinkosähköllä on merkittävä rooli maailmanlaajuisessa siirtymässä kohti kestävää energiantuotantoa, sillä aurinkopaneelit tuottavat vihreää sähköä suoraan auringonvalosta. Yksi aurinkosähkön avainteknologioista on III–V puolijohteisiin perustuvat moniliitosaurinkokennot, joiden avulla on saavutettu korkeimmat hyötysuhteet sekä maanpäällisessä energiantuotannossa että avaruussovelluksissa. Moniliitosaurinkokennoilla onkin saavutettu jopa 47,6 %:n hyötysuhde käyttäen keskitettyä valoa, mutta ponnisteluista huolimatta 50 %:n rajaa ei ole vielä saavutettu. Näin korkeiden hyötysuhteiden saavuttaminen edellyttää auringon spektrin erittäin tehokasta hyödyntämistä, mikä käytännössä vaatii viiden tai useamman liitoksen käyttämistä rakenteissa, mikä puolestaan edellyttää uusien alikennojen ja materiaalien kehitystyötä. Etenkin hilasovitettuja moniliitoskennoja ajatellen uusien materiaalien kehittäminen on tärkeää hilasovitettujen materiaalien määrän rajallisuuden vuoksi. Tämä väitöskirjatyö keskittyy hilasovitettujen kapean energia-aukon omaavien laimeiden typpiyhdisteiden ja niihin pohjautuvien moniliitosaurinkokennojen kehitykseen, viimekädessä tähdäten 50 %:n hyötysuhteen saavuttamiseen. Ensimmäisenä askeleena kohti tätä tavoitetta kehitettiin neliliitosaurinkokennoja, jotka sisältävät kaksi laimeisiin typpiyhdisteisiin perustuvaa alikennoa. Näissä rakenteissa pohjaliitoksen energia-aukkoa siirrettiin kohti 0,9 eV:n energiaa. Kokeellisilla neliliitoskennoilla saavutettiin 39 %:n hyötysuhde keskitetyn valon alla. Lisäkehitystyöllä kyseisillä rakenteilla olisi mahdollista saavuttaa yli 46 % hyötysuhde. Merkittävä osa tämän väitöskirjan kokeellisesta työstä liittyi 6–8 % typpeä sisältävien kapean energia-aukon GaInNAsSb-alikennojen valmistukseen, joiden avulla voidaan paremmin kattaa energiakaista germaniumin ja vakiintuneiden hilasovitettujen materiaalien välillä. Tässä työssä esitellään kehitystyötä ensimmäisistä kapean energia-aukon GaInNAsSb-liitoksista kohti korkean suorituskyvyn alikennoja rakenteellisten ja valmistusteknisten kehitysaskelten avulla. Kapean energia-aukon (0,8 eV) GaInNAsSb-kennojen toiminnassa saatiin aikaan merkittäviä parannuksia takapeilin avulla sekä molekyylisuihkuepitaksia-prosessin optimoinnilla. Parhailla työssä esitetyllä kapean energia-aukon alikennolla onkin mahdollista saavuttaa virtasovitus seuraavan sukupolven moniliitoskennoissa, joiden avulla yli 50 %:n hyötysuhde voitaisiin saavuttaa.A prominent role in the worldwide transition towards sustainable energy production is played by photovoltaics that is used to convert sunlight directly into green electricity. One of the key photovoltaic technologies is multijunction solar cell architecture based on III–V compound semiconductors, which provides the highest conversion efficiencies to date in terrestrial and space applications of solar cells. Currently, up to 47.6% conversion efficiency has been achieved under concentrated illumination with this approach. Still, despite major efforts, the milestone efficiency of 50% has not been realized. Reaching this efficiency level practically requires implementation of five or more junctions into multijunction solar cell devices, which allows more efficient utilization of the solar spectrum. In turn, this requires the development of new sub-cells and related materials. This is especially true for lattice-matched multijunction architecture, where the library of materials is more strictly limited. To this end, the thesis focuses on the development of narrow bandgap dilute nitrides and related multijunction solar cells lattice-matched to GaAs, ultimately targeting at 50% conversion efficiencies. As the initial steps towards realization of this, four-junction solar cells employing two dilute nitride subcells were demonstrated. To this end, the bandgap of the bottom junction was shifted towards 0.9 eV. The experimental four-junction devices yielded efficiencies of up to 39% under concentration, yet with fine-tuning and higher concentration factors over 46% could be attainable. A major part of the experimental work in this thesis involved fabrication of narrow bandgap GaInNAsSb subcells with 6–8% nitrogen concentrations for bridging the gap to Ge with lattice-matched materials. The thesis covers the progress from the first proof-of-concept narrow-gap GaInNAsSb junctions towards high performance subcells enabled by structural and epitaxial developments. Significant improvements for the performance of 0.8 eV GaInNAsSb solar cells were obtained by employing a back reflector behind the dilute nitride junction, and by optimizing the molecular beam epitaxy growth of the narrow-gap materials. The best narrow bandgap subcells presented in this work would already enable current-matching in next-generation multijunction devices with projected efficiencies exceeding 50%

Catastrophic Optical Damage in High-Power AlGaInP Diode Lasers

High-power red-emitting lasers with high reliability are strongly desired by applications like photodynamic therapy. Semiconductor lasers based on AlGaInP have emerged as the best candidates in this spectral range. However, compared to infrared emitters, high-power performance is still limited by major degradation effects, especially by catastrophic optical damage (COD). An innovative combination of concepts, namely microphotoluminescence (µPL) mapping, focused ion beam (FIB) microscopy, micro-Raman spectroscopy, and high-speed thermal imaging has been employed to reveal the physics behind COD, its related temperature dynamics, as well as associated defect and near-field patterns. µPL showed that COD-related defects are composed of highly nonradiative complex dislocations, which start from the output facet and propagate deep inside the cavity. Moreover, FIB analysis confirmed that those dark line defects are confined to the active region, including the quantum wells and partially the waveguide. In addition, the COD dependence on temperature and power was analyzed in detail by micro-Raman spectroscopy and thermal imaging. For AlGaInP lasers in the whole spectral range of 635 to 650 nm, it was revealed that absorption of stimulated photons at the laser output facet is the major source of facet heating, and that a critical facet temperature must be reached in order for COD to occur. A linear relationship between facet temperature and near-field intensity has also been established. This understanding of the semiconductor physics behind COD is a key element for further improvement in output power of AlGaInP diode lasers

Prognostics and health management of light emitting diodes

Prognostics is an engineering process of diagnosing, predicting the remaining useful life and estimating the reliability of systems and products. Prognostics and Health Management (PHM) has emerged in the last decade as one of the most efficient approaches in failure prevention, reliability estimation and remaining useful life predictions of various engineering systems and products. Light Emitting Diodes (LEDs) are optoelectronic micro-devices that are now replacing traditional incandescent and fluorescent lighting, as they have many advantages including higher reliability, greater energy efficiency, long life time and faster switching speed. Even though LEDs have high reliability and long life time, manufacturers and lighting systems designers still need to assess the reliability of LED lighting systems and the failures in the LED. This research provides both experimental and theoretical results that demonstrate the use of prognostics and health monitoring techniques for high power LEDs subjected to harsh operating conditions. Data driven, model driven and fusion prognostics approaches are developed to monitor and identify LED failures, based on the requirement for the light output power. The approaches adopted in this work are validated and can be used to assess the life of an LED lighting system after their deployment based on the power of the light output emitted. The data driven techniques are only based on monitoring selected operational and performance indicators using sensors whereas the model driven technique is based on sensor data as well as on a developed empirical model. Fusion approach is also developed using the data driven and the model driven approaches to the LED. Real-time implementation of developed approaches are also investigated and discussed

Status report on emerging photovoltaics

\ua9 2023 Society of Photo-Optical Instrumentation Engineers (SPIE).This report provides a snapshot of emerging photovoltaic (PV) technologies. It consists of concise contributions from experts in a wide range of fields including silicon, thin film, III-V, perovskite, organic, and dye-sensitized PVs. Strategies for exceeding the detailed balance limit and for light managing are presented, followed by a section detailing key applications and commercialization pathways. A section on sustainability then discusses the need for minimization of the environmental footprint in PV manufacturing and recycling. The report concludes with a perspective based on broad survey questions presented to the contributing authors regarding the needs and future evolution of PV

Luminescence study of III-nitride semiconductor nanostructures and LEDs

In this work, cathodoluminescence (CL) hyperspectral imaging, photoluminescence (PL) and electroluminescence are used to study the optical properties of III-nitride semiconductor materials. III-nitride semiconductors have successfully opened up the solid-state lighting market. Light-emitting diodes (LEDs) fabricated using III-nitrides, however, still suffer from numerous deficiencies such as high defect densities, efficiency droop and the 'green gap'. In order to investigate the type and properties of the defects, CL and electron channelling contrast imaging (ECCI) were performed on the same micron-scale area of a GaN thin film. A one-to-one correlation between isolated dark spots in CL and threading dislocations (TDs) in ECCI showed that TDs of pure edge character and TDs with a screw component act as non-radiative recombination centres. Secondary electron imaging of planar InGaN/GaN multiple quantum well (MQW) structures identified trench defects of varying width. CL imaging revealed a strong redshift (90 meV) and intensity increase for trench defects with wide trenches compared with the defect-free surrounding area. Narrower trench defects showed a small redshift (10 meV) and a slight reduction in intensity. The optical properties of nanorods fabricated from planar InGaN/GaN MQW structures were investigated using PL and CL. PL spectroscopy identified reduced strain within the MQW stack in the nanorods compared with the planar structure. CL imaging of single nanorods revealed a redshift of 18 meV of the MQW emission along the nanorod axis and provided an estimate of 55 nm for the carrier diffusion length. Colour conversion using novel organic compounds as energy down-converters was studied. The first molecules absorbed in the ultra-violet and emitted in the yellow spectral region. Further modification of the organic compound shifted the absorption into the blue and white light generation was investigated by coating blue-emitting nanorods and blue LEDs. Determination of the colour rendering index and colour temperature showed "warm white" light emission with values of 70 and 3220 K, respectively.In this work, cathodoluminescence (CL) hyperspectral imaging, photoluminescence (PL) and electroluminescence are used to study the optical properties of III-nitride semiconductor materials. III-nitride semiconductors have successfully opened up the solid-state lighting market. Light-emitting diodes (LEDs) fabricated using III-nitrides, however, still suffer from numerous deficiencies such as high defect densities, efficiency droop and the 'green gap'. In order to investigate the type and properties of the defects, CL and electron channelling contrast imaging (ECCI) were performed on the same micron-scale area of a GaN thin film. A one-to-one correlation between isolated dark spots in CL and threading dislocations (TDs) in ECCI showed that TDs of pure edge character and TDs with a screw component act as non-radiative recombination centres. Secondary electron imaging of planar InGaN/GaN multiple quantum well (MQW) structures identified trench defects of varying width. CL imaging revealed a strong redshift (90 meV) and intensity increase for trench defects with wide trenches compared with the defect-free surrounding area. Narrower trench defects showed a small redshift (10 meV) and a slight reduction in intensity. The optical properties of nanorods fabricated from planar InGaN/GaN MQW structures were investigated using PL and CL. PL spectroscopy identified reduced strain within the MQW stack in the nanorods compared with the planar structure. CL imaging of single nanorods revealed a redshift of 18 meV of the MQW emission along the nanorod axis and provided an estimate of 55 nm for the carrier diffusion length. Colour conversion using novel organic compounds as energy down-converters was studied. The first molecules absorbed in the ultra-violet and emitted in the yellow spectral region. Further modification of the organic compound shifted the absorption into the blue and white light generation was investigated by coating blue-emitting nanorods and blue LEDs. Determination of the colour rendering index and colour temperature showed "warm white" light emission with values of 70 and 3220 K, respectively

Theory and optimisation of metamorphic photonic devices

Metamorphic growth of semiconductor materials – in which a “virtual” substrate with a desired lattice constant is obtained by growing a lattice-mismatched metamorphic buffer layer (MBL) on a conventional substrate such as InP or GaAs – is beginning to attract increasing interest due to its potential to facilitate the development of improved optoelectronic technologies. For example, by growing a relaxed InxGa1−xAs MBL on a GaAs substrate heterostructures can then be grown with a lattice constant intermediate between that of GaAs and InP, thereby providing enhanced scope for band structure engineering and semiconductor device design and optimization starting from a GaAs substrate. However, despite significant progress in material growth and device engineering, there has been very little theoretical analysis of metamorphic devices. We are particularly interested in the development of metamorphic AlInGaAs-based lasers operating at the technologically important 1.3 µm wavelength, as well as efficient AlInGaP-based 610 nm Light-Emitting Diodes (LEDs) for maximised white light efficiency. In this thesis we investigate the electronic and optical properties of these emitters and compare their performance with existing photonic devices. Using the continuum based multiband k·p model within the planewave expansion method we quantify the potential of lattice mismatched MBLs and identify the trends in device performance. We show that by employing an InGaAs MBL we can extend the ranges of strain and composition accessible for a direct band gap AlInGaAs or AlInGaP alloy, which allow the suppression of the amount of defects and CuPt atomic ordering created during the epitaxial growth. Using the model solid theory we demonstrate that the electron confinement strongly benefits from the use of an InGaAs MBL, bringing a reduced current leakage from the active region. After performing a detailed analysis over a series of metamorphic lasers and LEDs, which include such nanostructures in the active region as quantum wells, dots and wires, we identify the trends in electronic and optical properties which compare very favourably with existing devices, and we provide guidelines for the design of optimised devices. Using the experimental data available in the literature for metamorphic lasers we are able to estimate the defect-related current losses in such devices, and find that there remains opportunity to further improve laser performance. In addition, the micro-photoluminescence measurements performed on a prototype 610 nm metamorphic LED confirm our prediction of enhanced internal quantum efficiency compared to GaAsbased structures, suggesting that this novel type of LEDs is an excellent candidate for efficient white light emission