Добірка наукової літератури з теми "GaN. InN. InGaN"

In this study, we undertake a Bayesian optimization of the Hubbard U parameters of wurtzite GaN and InN. The optimized Us are then tested within the Hubbard-corrected local density approximation (LDA+U) approach against standard density functional theory, as well as a hybrid functional (HSE06). We present the electronic band structures of wurtzite GaN, InN, and (1:1) InGaN superlattice. In addition, we demonstrate the outstanding performance of the new parametrization, when computing the internal electric-fields in a series of [InN]1–[GaN]n superlattices (n = 2–5) stacked up along the c-axis.

Reducing defects in InGaN films deposited on GaN substrates has been critical to fill the “green” gap for solid-state lighting applications. To enable researchers to use molecular dynamics vapor deposition simulations to explores ways to reduce defects in InGaN films, we have developed and characterized a Stillinger-Weber potential for InGaN. We show that this potential reproduces the experimental atomic volume, cohesive energy, and bulk modulus of the equilibrium wurtzite / zinc-blende phases of both InN and GaN. Most importantly, the potential captures the stability of the correct phase of InGaN compounds against a variety of other elemental, alloy, and compound configurations. This is validated by the potential’s ability to predict crystalline growth of stoichiometric wurtzite and zinc-blende InxGa1-xN compounds during vapor deposition simulations where adatoms are randomly injected to the growth surface.

Étant donné leurs propriétés physiques structurales et optoélectroniques très prometteuses, une attention particulière a été accordée aux semiconducteurs d’éléments III-V. L’utilisation des nitrures de géométrie 1D, les micro et nanofils, permet de répondre aux défis relatifs aux difficultés rencontrés lors de la croissance des couches 2D InGaN engendrées par le désaccord de maille entre substrat et matériau à élaborer, la ségrégation d’indium et le manque de substrat adéquats et à bas coût pour la croissance épitaxiale. Dans ce contexte, ce travail de thèse présente pour la première fois une étude de la croissance de micro et nanofils (In,Ga)N par épitaxie en phase vapeur par la méthode aux hydrures (HVPE). L’objectif de ce travail de thèse est de contrôler la composition, la morphologie et l’homogénéité des micro et nanofils InGaN élaborés par voie sélective SAG (pour selective area growth en anglais). Afin d’atteindre cet objectif, il est important d’étudier la croissance sélective des deux binaires d’InGaN qui sont le GaN et l’InN. Dans un premier temps, nous avons alors effectué une étude de la phase vapeur afin de fixer les conditions optimisées dans le cadre de ma thèse pour la croissance sélective des micro et nanofils GaN et InN. La croissance sélective des nanofils GaN est aussi montré pour la première fois par HVPE. L’effet des paramètres expérimentaux sur la croissance sélective des structures GaN et InN ont été ainsi discutés. La deuxième partie de ce travail de thèse est consacrée à la croissance sélective des micro et nanofils InGaN sur substrat de GaN/saphir par HVPE. Nous avons démontré qu’il est possible de contrôler la compositions en indium dans les nanofils InGaN en modifiant les conditions de croissance expérimentales et spécialement les pressions partielles des gaz utilisés. Des analyses EDS réalisées sur des nanofils élaborés par HVPE sous différentes conditions de phase vapeur ont montré des compositions allant de 0 à 100% en indium. Dans le but d’étudier les propriétés optiques des nanofils InGaN, des mesures en PL ont été effectuées
Given their very promising structural and optoelectronic physical properties, special attention has been given to III-V element semiconductors. The use of nitrides with 1D geometry, micro and nanowires, allows to address the challenges related to the difficulties encountered during the growth of 2D InGaN layers caused by the lattice mismatch between substrate and material to be grown, indium segregation and the lack of suitable and low cost substrates for epitaxial growth. In this context, this thesis presents for the first time a study of the growth of (In,Ga)N micro and nanowires by the hydride vapor phase epitaxy (HVPE) method. The objective of this thesis is to control the composition, morphology and homogeneity of InGaN micro and nanowires grown by selective area growth (SAG). In order to achieve this objective, it is important to study the selective growth of the two InGaN binaries which are GaN and InN. As a first step, we then performed a vapor phase study in order to set the conditions optimized in my thesis for the selective growth of GaN and InN micro and nanowires. The selective growth of GaN nanowires is also shown for the first time by HVPE. The effect of experimental parameters on the selective growth of GaN and InN structures have been discussed. The second part of this thesis is devoted to the selective growth of InGaN micro and nanowires on GaN/sapphire substrate by HVPE. We have demonstrated that it is possible to control the indium composition in InGaN nanowires by modifying the experimental growth conditions and especially the partial pressures of the gases used. EDS analyses performed on HVPE grown nanowires under different vapor phase conditions showed compositions ranging from 0 to 100% indium. In order to study the optical properties of InGaN nanowires, PL measurements were performed

The study of the electro-optical properties of semiconductors has represented one of the major topics in the research of physicists and material scientists of the last century. Extensive efforts have been especially put in the last four decades into the search for materials emitting in the visible region, with the aim of developing Light Emitting Diodes (LEDs) for general outdoor lighting as well as for new applications, where micro-size and device integration are necessary requirements. GaN/InGaN/AlGaN-based LEDs lead nowadays the solid-state lighting market, which is in continuous expansion and finds always new applications, such as display back-lighting and projection, full color displays and automotive lighting. Due to the increasing market competitiveness, further improvement of LED brightness and long term reliability, together with the reduction of production costs, are of primary concern for each LED supplier. The enhancement of device effciency and reliability requires the understanding of the physics of these devices and the optimization of the LED design. Despite of the high long-term stability reached, degradation mechanisms could still potentially occur. In this work, loss mechanism causing aging are investigated for GaN-based LEDs. Operation-induced degradation of high-brightness LEDs internal quantum e’±ciency is analyzed both experimentally and theoretically. The dynamics of the aging process are studied by means of electroluminescence relative intensity measurements in dependence of time. A model able to interpret this behavior is proposed. In particular, a thermally activated mechanism, likely correspondent to the p-type-dopant diffusion process, is believed to be responsible for long-term DC aging. In section 1, properties of Gallium Nitride material are analyzed in detail. Growing and doping methods, employed substrates and fabrication of contacts are briefly described. A description of related GaN alloys is then given. Information wrapped in this chapter will be frequently mentioned in the entire work. In section 2, an overview of the Light Emitting Diode structure is given. Topics as radiative recombination theory, non radiative recombination mechanisms and LED theory basics are examined. Finally, recently developed LED improvement techniques are shortly described. In section 3, LED degradation theory is analyzed. Principal degradation modes are first briefly traced. Then, samples used in this study are described together with measuring methods and general dynamics of aging. Acceleration concept isfinally depicted. In section 4, a new model based on dopant diffusion is developed. The model is able to interpret most of the degradation phenomena experimentally encountered and gives a first complete description of GaN/InGaN/AlGaN-based LEDs aging. A general introduction to diffusion theory is first given, together with literature results. The model is then mathematically described. An experimental application explaining temperature dependence of LED samples is presented. Finally, the model is developed from a physical point of view, and a derivation of the acceleration factor is given. In section 5, a description of an experiment based on EL and PL analysis over double and multi quantum well structures is given. The aim of the experiment is to monitor the behavior of a single QW in terms of effciency and reliability in dependence of its positioning. In section 6, current density impact over the structures is analyzed. The diffusion model is systematically applied to find experimental indications to the effects of a current over-stress on GaN-based LEDs. The current-induced phenomena are reduced to simple thermal degradation. In section 7, LED aging characteristics are monitored in dependence of device wavelength. A set of samples emitting at different wavelengths is compared in terms of aging properties. In section 8, some diffusion evidences are considered. Diffusion is proved to be actual already during sample growth. Moreover, defect density impact is taken into consideration, in particular related to the initial aging phase. Finally, a theoretical interpretation is given for a contactless case of degradation named photoaging. Results of the study are summarized in the last section.

This thesis reports the main results obtained from the Ph.D. research activity of the candidate. The activity was focused on the study of the physical mechanisms responsible for the degradation of ultraviolet light emitting diodes (UV-LEDs) and InGaN-based laser diodes (LDs), in particular, we tried to understand the role of defects in the device degradation by means of advanced techniques such Deep-Level Transient Spectroscopy (DLTS) and Photocurrent (PC) Spectroscopy. The first part of the thesis will be dedicated to the work carried out on UV LEDs, in particular on four groups of UV-A LEDs, each of them with a different emission wavelength, and another group of (In)AlGaN-based UV-B LEDs. From this study, it was possible to demonstrate that UV LEDs submitted to constant current stress show a gradual degradation and this degradation has a strong dependence on the emission wavelength. The degradation process is ascribed to the generation of point defects within the active region of the device, causing an increase in the non-radiative recombination. Moreover, stress current level and temperature have a certain impact on the degradation. Regarding the UV-B LEDs, the analysis of the degradation of the optical power highlighted that degradation is related to an increase in the Shockley-Read-Hall (SRH) recombination. DLTS analysis and PC spectroscopy measurements allowed to identify a band of defects centered around 2.5 eV below the conduction band, close to the mid-gap, that can explain the increased SRH recombination. Moreover, DLTS measurements allowed to identify the signature of Mg-related acceptor traps. These UV-B LEDs present three parasitic bands ascribed to different processes. This section on the study of the degradation of AlGaN-based UV-B LEDs will be concluded with the investigation of the involved degradation mechanisms when the devices are submitted to a stress current level higher than the nominal value. The existence of two degradation mechanisms was demonstrated, and these have a significant impact on the relative amplitude of the main peak, related to the QW emission, and of the parasitic peak at ~340 nm, related to the electron overflow towards the last quantum barrier. The second part of thesis will be focused on the work carried out on a group of InGaN-based LDs with a nominal emission wavelength around 418 nm and another group with emission wavelength between 422-426 nm. The analysis carried out on the first group of samples has the aim to study the relation between the degradation and the presence of defects in these devices when submitted to constant current stress. We were able to show that constant current stress induces an increase in the threshold current of the devices, instead, from the capacitance-temperature measurements, it was possible to identify two main defects, which physical origin, studied by means of DLTS, could be related to acceptor-like defects, associated with dislocation. To conclude the study, the results obtained from the analysis carried out on three group of InGaN-based LEDs with different dislocation density, in order to have a better comprehension of the impact of dislocation on the degradation and generation of defects, will be presented. DLTS analysis was carried out before and after the stress and allowed to identify the presence of a hole trap in each sample, whose concentration increases with the dislocation density, together with an electron trap generated after the stress, probably related to the dislocation density. Moreover, devices submitted to constant current stress showed a significant decrease in their optical power, not related exclusively to the dislocation density.
Questa tesi riporta i principali risultati ottenuti dal candidato durante la sua attività di ricerca. Il lavoro si è focalizzato sullo studio dei meccanismi fisici responsabili del degrado di diodi ad emissione di luce ultravioletta (UV-LED) e di diodi laser basati su InGaN (LDs), in particolare, si è cercato di capire il ruolo dei difetti nel degrado del dispositivo mediante tecniche avanzate come Deep Level Transient Spectroscopy (DLTS) e la spettroscopia di fotocorrente (PC). La prima parte della tesi sarà dedicata al lavoro svolto sui LED UV, in particolare su quattro gruppi di LED UV-A, ciascuno con una diversa lunghezza d'onda di emissione, e un altro gruppo di LED UV-B in (In)AlGaN. Da questo studio è stato possibile dimostrare che i LED UV sottoposti a stress a corrente costante mostrano un degrado graduale e tale degrado ha una forte dipendenza dalla lunghezza d'onda di emissione. Questo processo è dovuto alla generazione di difetti puntiformi all'interno della regione attiva del dispositivo, che causa un aumento della ricombinazione non-radiativa. Anche il livello di corrente e la temperatura di stress hanno un certo impatto sulla degradazione. Per quanto riguarda i LED UV-B, l'analisi del degrado della potenza ottica ha evidenziato che questo degrado è correlato ad un aumento della ricombinazione Shockley-Read-Hall (SRH). Le misure di analisi di DLTS e della fotocorrente hanno permesso di identificare una banda di difetti centrata intorno a 2,5 eV sotto la banda di conduzione, circa a metà band-gap, che può spiegare l'aumento della ricombinazione SRH. Inoltre, le misure di DLTS hanno permesso di identificare la presenza di trappole di tipo accettore relative al magnesio. Questi LED UV-B presentano tre bande parassite attribuite a diversi processi. Questa sezione si conclude con lo studio dei meccanismi di degrado coinvolti quando i dispositivi sono sottoposti a un livello di corrente di stress superiore al valore nominale. Si è dimostrata l'esistenza di due meccanismi di degrado, che presentano un impatto significativo sull'ampiezza relativa del picco principale, correlato all'emissione dalle buche quantiche, e del picco parassita a ~ 340 nm, correlato all’overflow di elettroni verso l'ultima barriera quantica. La seconda parte della tesi sarà incentrata sul lavoro svolto su un gruppo di diodi laser basati su InGaN con una lunghezza d'onda di emissione nominale intorno a 418 nm e un altro gruppo con lunghezza d'onda di emissione tra 422-426 nm. L'analisi effettuata sul primo gruppo di campioni ha lo scopo di studiare la relazione tra il degrado e la presenza di difetti in questi dispositivi quando sottoposti a stress a corrente costante. Si è dimostrato che lo stress induce un aumento della corrente di soglia dei dispositivi, invece, dalle misure di capacità-temperatura, è stato possibile identificare due difetti principali, la cui origine fisica, studiata per mezzo del DLTS, potrebbe essere correlata a difetti di tipo accettatore, associati a dislocazioni. Per concludere lo studio, si presenteranno i risultati ottenuti dall'analisi effettuata su tre gruppi di LED basati su InGaN con diversa densità di dislocazione, al fine di avere una migliore comprensione dell'impatto delle dislocazioni sul degrado e la generazione di difetti. L'analisi DLTS, effettuata prima e dopo lo stress, ha permesso di identificare la presenza di una trappola per lacune in ogni campione, la cui concentrazione aumenta con la densità di dislocazioni, insieme ad una trappola per elettroni generata dopo lo stress, probabilmente sempre correlata alla densità di dislocazioni. Inoltre, i dispositivi sottoposti a stress a corrente costante hanno mostrato una significativa riduzione della loro potenza ottica, non correlata esclusivamente alla densità di dislocazioni.

Presented in this thesis are extensive theoretical investigations into the causes and effects of carrier localization in InGaN/GaN quantum wells. The results of the calculations agree well with experimental data, where it is available, and provide additional insights into the mechanisms that lead to some of the experimentally observed effects of localization. Firstly, the wave functions of the electrons and holes in InGaN/GaN quantum wells have been calculated by numerical solution of the effective-mass Schrödinger equation. In our calculations we have assumed a random distribution of indium atoms, as suggested by the results of atom probe tomography: this allows us to find the contributions to the carriers' potential energy that arise from band gap fluctuations, the deformation potential and the spontaneous and piezoelectric fields. We show that the fluctuations in alloy composition can be sufficient to localize the carriers; our results are in good agreement with the results of experiment and more detailed ab-initio calculations, but we also obtain information about the distribution of localized states which those methods cannot yet provide. We find that the holes are localized on a short scale in randomly-occurring regions of high indium content, whereas the electrons are localized on a longer length scale. We consider the effect of well width fluctuations and find that these contribute to electron localization, but not to hole localization. We also simulate the low-temperature photoluminescence spectrum and find good agreement with experiment for the energy, width and shape of the photoluminescence peak. Secondly, we have used first-order time-dependent perturbation theory to study the diffusion of the carriers between their localized states at non-zero temperatures. The rates for scattering via the interaction with acoustic phonons are calculated using the carrier wave functions, and the resulting master equation for the distribution of the carriers is solved by a Monte Carlo method. We find that, even towards room temperature, the carriers are localized to a small number of states, and that their diffusion lengths are proportional to a combination of the density of localized states and the localization length. The experimentally-observed `S-shape' of the photoluminescence peak energy as a function of temperature is reproduced in our results and is explained by the thermal redistribution of holes among the localized states. A reduction of the depth of this S-shape is found as the excitation power is increased, as has been observed experimentally, and which we attribute to the saturation of the localized states.

This thesis reports the main results obtained from the Ph.D. research activity of the candidate. The activity was focused on the study of defects and physical mechanisms that limit the efficiency of Light Emitting Diodes (LEDs) based on gallium nitride (GaN). In the first part of the thesis, the fundamental properties of gallium nitride will be briefly discussed, including its alloys, crystalline structure and internal polarization fields. After that, we will present an updated review based on the literature on the most important point defects and dislocations in GaN. Particular attention will be given to the role of Ga and N vacancies, since these are native GaN defects that are very commonly introduced during the crystal growth, and to the role of threading dislocations. The discussion will be supported by both theoretical studies and experimental evidence. The second part of the work will be dedicated to the quantum efficiency of LEDs. Initially we will give a description of its fundamental parameters, after which we will focus on the physical processes that limit the efficiency. In particular, Shockley-Read-Hall non-radiative recombination is known for lowering the efficiency for low-moderate current densities. On the other hand, at high injection levels the efficiency of GaN-based LEDs is also lowered by the so-called efficiency droop. Both these processes will be analyzed, providing theoretical as well as experimental evidence and explanations. In the third chapter some advanced techniques useful for studying the deep levels in the semiconductors as well as for estimating the recombination coefficients will be discussed, including capacitive deep level transient spectroscopy (DLTS), differential carrier lifetime and deep level optical spectroscopy (DLOS). In particular, for each technique will be provided a theoretical description, together with its advantages and main limitations both under a theoretical and experimental point of view. After that, in the fourth chapter, we will give an extensive review of the state-of-the-art knowledge of the degradation mechanisms of the active region of modern LEDs, focusing on the degradation kinetics of the optical power and of the electrical characteristics of the devices. The discussion will aim at identifying the most common degradation mechanisms usually reported in the literature for real devices. The fifth chapter will be dedicated to the presentation of the most relevant results obtained during this research activity and will consist on three sections. In the first one, an extensive analysis on the origin of SRH recombination in InGaN-based LEDs will be reported. The aim of the study was to investigate the relationship between the SRH recombination A of the ABC model and the lattice defects detected by DLTS and DLOS measurements. In fact, although SRH recombination on a device is obviously strongly correlated with its defectiveness, several defects may contribute in different ways to the overall non-radiative recombination, depending on their energy position in the band diagram and on other parameters. The analysis revealed that one deep level, labeled “e2” and with activation energy E_{C}-E_{T}\sim0.7 eV, was present in all the measured samples and its concentration was well correlated with the estimated SRH coefficient, obtained by means of differential carrier lifetime measurements. That trap was therefore ascribed as the main responsible for non-radiative recombination in the analyzed devices. Moreover, further investigations of the trapping kinetics suggested it behaved like an extended defect. On the other hand, a research in the literature revealed that other papers previously reported on similar traps, ascribing them to native GaN defects, possibly vacancies and their complexes with other impurities. For this reason, trap “e2” was ascribed to native GaN defects aligned along threading dislocations. The second section of the fifth chapter will present two experimental works that investigated the degradation mechanisms in InGaN-based LEDs. In the first work the devices, that belonged to the same set studied before, were submitted to accelerated aging by means of constant dc current and temperature stress, which induced a significant decrease in the emitted optical power. By investigating the variations in the external quantum efficiency, in the photoluminescence emission and in the photocurrent signal, we shown that the degradation was due to the increase in the SRH recombination rather than a reduced injection efficiency. Deep level transient spectroscopy analysis then revealed that stress caused a remarkable increase in the concentration of the deep level “e2” previously observed, confirming the key role played by this trap in determining the SRH recombination rate of these devices. The second work was more focused on the degradation kinetics observed during the stress of another set of samples. In particular, it was revealed that stress resulted in the accumulation of charge in the active area of the devices and that the amount of accumulated charge was linearly correlated to the estimated SRH coefficient A. Both the accumulation of charge and the increase in the SRH coefficient followed an almost square-root dependence on stress time. This result suggested that degradation was induced by a diffusion process, possibly impurities diffusing toward the active region. Remarkably, the stress also induced the decrease in the concentration of a shallow level (E_{C}-E_{T}\sim0.37 eV) and the increase in the concentration of a deeper level (E_{C}-E_{T}\sim0.9 eV). These changes in the defects concentration were compatible with the observed variations in the capacitance and in the SRH recombination. In particular the increase in the concentration of the deeper level, a more efficient non-radiative center, may have caused the increase in SRH recombination, while the decrease in the shallow defect could result in the observed increase in the junction capacitance. Based on these experimental results and on other literature reports, we ascribed the degradation to the gradual diffusion of impurities, likely magnesium or hydrogen, toward the active region of the devices. The third and last section of the fifth chapter will be dedicated to an extensive analysis carried out on the thermal droop in InGaN-based LEDs. In particular, a correlation between the drop of the optical power with increasing temperature and the concentration of defects was observed. To explain this finding, several mechanisms that are believed to be related to the efficiency of the device were investigated, including SRH recombination, thermionic escape of carriers from the quantum well, phonon-assisted tunneling and thermionic trap-assisted tunneling. Each of these processes has some specific properties that, for the samples under investigation, made them unable to fully explain the experimental data. Therefore, a new model was developed consisting of a two-step zero bias extended phonon-assisted tunneling from the quantum well to the trap, which acts as an intermediate reservoir of electrons, and from the trap to the border of the conduction band. Such new model will be discussed and will be shown how, by using parameters from simulations of the energetic structure of the quantum wells and from DLTS characterization of the deep levels, the obtained closed-form equation was able to accurately fit the experimental data on thermal droop of all the investigated samples. Useful information on the research activity can be also found in the papers co-authored by the candidate and listed in the next section.
La seguente tesi riassume i risultati principali ottenuti durante lo svolgimento dell'attività di ricerca del Dottorando candidato. L'attività si è concentrata principalmente nello studio dei difetti e dei meccanismi fisici che limitano l'efficienza dei diodi emettitori di luce (LED) basati su nitruro di gallio (GaN). Nella prima parte della tesi, verranno brevemente discusse le proprietà fondamentali del nitruro di gallio, comprese le sue leghe principali, la struttura cristallina ed i campi di polarizzazione interni. Successivamente, verrà presentata una situazione aggiornata basata sulla letteratura dei difetti di punto e delle dislocazioni tipici del nitruro di gallio. Particolare attenzione verrà riservata al ruolo svolto delle dislocazioni e dalle vacanze di gallio e azoto, poiché essi sono difetti nativi del materiale che vengono comunemente introdotti durante l'accrescimento del cristallo. Tale discussione verrà supportata da risultati sia teorici sia sperimentali. La seconda parte della tesi sarà dedicata allo studio dell'efficienza quantica dei LED. Inizialmente verrà data una descrizione dei suoi parametri principali, dopodiché ci si concentrerà sui processi fisici che ne determinano eventuali limiti. In particolare, la ricombinazione non radiativa Shockley-Read-Hall (SRH) è nota per causare una riduzione dell'efficienza a basse densità di corrente. Contemporaneamente, ad alti livelli di iniezione l'efficienza dei LED basati su GaN risulta compromessa da un effetto noto come efficiency droop. Entrambi questi processi verranno analizzati, fornendo spiegazioni supportate da dati sia teorici che sperimentali. Nella terza parte del lavoro saranno introdotte alcune tecniche avanzate, utili per lo studio dei livelli profondi nei semiconduttori nonché per stimare i coefficienti di ricombinazione. Le tecniche analizzate sono capacitive deep level transient spectroscopy (DLTS), il tempo di vita differenziale dei portatori e il deep level optical spectroscopy (DLOS), e per ognuna di esse verrà fornita una descrizione teorica e verranno analizzati i principali vantaggi e svantaggi sia sotto un punto di vista teorico che sperimentale. Successivamente, nel quarto capitolo, sarà effettuata una trattazione approfondita e aggiornata allo stato dell'arte sui meccanismi di degrado della regione attiva dei LED moderni, focalizzandosi sulle cinetiche di degrado della potenza ottica e delle caratteristiche elettriche dei dispositivi. La discussione avrà come scopo l'identificazione dei meccanismi di degrado più comunemente osservati nei dispositivi reali e riportati in letteratura. Il quinto capitolo sarà dedicato alla presentazione dei risultati più rilevanti ottenuti durante lo svolgimento dell'attività di ricerca e sarà composto da tre sezioni. Nella prima sezione verrà descritta un'analisi dettagliata effettuata su dispositivi LED basati su InGaN. Lo studio aveva lo scopo di investigare la relazione tra il coefficiente di ricombinazione SRH A del modello ABC e la concentrazione dei difetti osservati tramite misure DLTS e DLOS. Infatti, sebbene la ricombinazione SRH sia ovviamente correlata con la difettosità del materiale, i vari difetti possono impattare in maniera diversa sul tasso di ricombinazione non radiativa complessivo, a seconda della loro posizione energetica nel diagramma a bande e di altri parametri. L'analisi ha rivelato la presenza di un livello profondo comune a tutti i campioni, denominato “e2” e avente energia di attivazione E_{C}-E_{T}\sim0.7 eV, la cui concentrazione risulta ben correlata con il coefficiente A stimato con misure di tempo di vita differenziale dei portatori. L'origine della ricombinazione SRH nei dispositivi analizzati è stata dunque ricondotta alla presenza di tale difetto. Successive analisi delle cinetiche di intrappolamento hanno poi rivelato che la trappola in questione si comportava come un difetto di tipo esteso. Contemporaneamente, da una approfondita ricerca in letteratura si è visto che livelli profondi simili a quello riportato erano stati già descritti in passato e associati a difetti nativi del GaN, probabilmente vacanze o complessi che esse formano con altre impurità. Per tali ragioni, l'origine del livello “e2” è stato associato a difetti nativi del GaN allineati lungo le dislocazioni del materiale. La seconda sezione del quinto capitolo presenterà due lavori svolti per studiare i meccanismi di degrado dei LED basati su InGaN. Nel primo di essi i dispositivi, appartenenti allo stesso set analizzati precedentemente, sono stati sottoposti a stress ad elevate temperature e corrente costante, che ha provocato un sensibile calo della potenza ottica emessa. Dall'analisi delle variazioni dell'efficienza quantica esterna, della fotoluminescenza e della fotocorrente, è stato dimostrato che in questi dispositivi il degrado è stato causato dall'incremento della ricombinazione SRH e non da un calo dell'efficienza di iniezione. Misure DLTS hanno poi dimostrato che lo stress ha indotto un aumento notevole della concentrazione dello stesso livello profondo “e2” individuato precedentemente, confermando il ruolo chiave di tale trappola nel determinare il tasso di ricombinazione non radiativa in questi LED. Il secondo lavoro si è più focalizzato nello studio delle cinetiche di degrado osservate durante lo stress di un altro set di campioni. In particolare, si è visto che lo stress ha provocato l'accumulo di carica in prossimità della regione attiva dei dispositivi e che l'ammontare di tale carica è risultato correlato linearmente con il coefficiente SRH A . Sia l'accumulazione della carica che l'incremento del coefficiente SRH hanno mostrato una dipendenza dalla radice quadrata del tempo di stress. Tale risultato ha suggerito che il degrado fosse causato da processi di tipo diffusivo, probabilmente diffusione di impurità verso la regione attiva. Inoltre, lo stress ha anche indotto la riduzione della concentrazione di un livello superficiale ( E_{C}-E_{T}\sim0.37 eV) e l'aumento della concentrazione di un livello più profondo ( E_{C}-E_{T}\sim0.9 eV). Queste variazioni nella difettosità del materiale si sono rivelate compatibili con le variazioni osservate nella capacità e nel coefficiente di ricombinazione SRH. In particolare, la maggiore concentrazione del difetto più profondo, che è più efficiente in termini di ricombinazione non radiativa, ha probabilmente causato l'aumento del coefficiente A , mentre la riduzione della concentrazione del livello più superficiale può aver portato all'aumento di capacità osservato. Sulla base di questi risultati sperimentali nonché su altri lavori pubblicati in letteratura, il degrado è stato imputato alla graduale diffusione di impurità, probabilmente magnesio e idrogeno, verso la regione attiva dei dispositivi. La terza ed ultima sezione del quinto capitolo verrà dedicata ad un'analisi dettagliata effettuata sul fenomeno noto come thermal droop. In particolare, è stata osservata una correlazione tra il calo di potenza ottica a seguito dell'aumento di temperatura e la concentrazione dei difetti. Per giustificare queste osservazioni, sono stati analizzati diversi meccanismi che generalmente vengono collegati all'efficienza dei dispositivi: la ricombinazione SRH, l'emissione termoionica dalla buca quantica, il tunneling assistito da fononi e l'emissione termoionica combinata al tunneling attraverso uno stato trappola. Nessuno di questi meccanismi si è rivelato in grado di riprodurre accuratamente i dati sperimentali, nel caso dei dispositivi in esame. Di conseguenza, è stato sviluppato un nuovo modello basato su una estensione del tunneling assistito da fononi a doppio step, dalla buca quantica allo stato trappola, che funge da riserva di elettroni, e da quest'ultimo al bordo della banda di conduzione. Tale modello verrà discusso e verrà in particolar modo mostrato come usando parametri estratti dalle simulazioni e dalle misure DLTS effettuate, sia stato possibile ottenere un'equazione in grado di riprodurre accuratamente i dati sperimentali su tutti i campioni analizzati. Utili informazioni sull’attività di ricerca possono essere trovate negli articoli in cui ha collaborato il candidato ed elencati nella successiva sezione.

Im Rahmen dieser Arbeit wurden selbstorganisierte, verspannte InP-Quantenpunkte mittels Gasquellen-Molekularstrahlepitaxie hergestellt und deren strukturelle und optische Eigenschaften untersucht. Die Quantenpunkte wurden sowohl in InGaP-Matrix gitterangepasst auf GaAs-Substrat als auch in GaP-Matrix auf GaP-Substrat realisiert. Die starke Gitterfehlanpassung von 3,8% im InP/InGaP- bzw. 7,7% im InP/GaP-Materialsystem ermöglicht Inselbildung mittels des Stranski-Krastanow-Wachstumsmodus: Ab einer kritischen InP-Schichtdicke findet kein zweidimensionales, sondern ein dreidimensionales Wachstum statt. Die kritische Schichtdicke wurde mit etwa 3 Monolagen für das InP/InGaP- und mit etwa 1,8 Monolagen für das InP/GaP-System bestimmt. Die strukturellen Untersuchungen zeigen, dass InP Quantenpunkte in GaP im Vergleich zu solchen in InGaP größer sind und stärker zum Abbau von Verspannung tendieren. Die in InGaP-Matrix eingebettete InP-Quantenpunkte zeigen sehr ausgeprägte optische Emissionen, die, in Abhängigkeit von den Wachstumsparametern, im Bereich von 1,6 bis 1,75eV liegen. Die Emissionslinie wird der strahlenden Rekombination von in den Quantenpunkten lokalisierten Elektronen und Löchern zugeordnet. Dies wird auch durch das Bänderschema bestätigt, das mit Hilfe der Model-Solid-Theorie modelliert wurde. Darüber hinaus weist die Lebensdauer der Ladungsträger von einigen hundert Pikosekunden darauf hin, dass die InP/InGaP Quantenpunkte vom Typ I sind. Zusätzlich zu den optischen Eigenschaften wurde die Anordnung von dicht gepackten InP-Quantenpunkten in und auf InGaP mittels zweidimensionaler Fourier-Transformation der Daten aus der Atomkraftmikroskopie, Transmissionelektronmikroskopie und diverser Röntgen-Streuexperimente untersucht sowie die planaren und vertikale Ordnungseffekte der Quantenpunkte studiert. Die Untersuchungen zeigen, dass die Ordnung der Quantenpunkte sowohl hinsichtlich ihrer Packungsdichte als auch ihrer Orientierung mit wachsender InP-Bedeckung zunimmt. Darüber hinaus wurde die Verspannungsverteilung in den InP/InGaP-Quantenpunkten mit Hilfe von diffuser Röntgen-Streuung in Verbindung mit kinematischen Simulationen studiert und eine asymmetrische Form der Quantenpunkte festgestellt, die auch Ursache für die gemessene Polarisationsanisotropie der Photolumineszenz sein kann. Die in GaP-Matrix eingebetteten InP-Quantenpunkte wurden im Rahmen dieser Arbeit erstmals erfolgreich auf ihre aktiven optischen Eigenschaften hin untersucht. Sie zeigen eine optische Emission zwischen 1,9 und 2 eV im sichtbaren Bereich. Diese strahlende Rekombination wird ebenfalls dem direkten Übergang zwischen Elektronen- und Löcherzuständen zugeordnet, die in den InP Quantenpunkten lokalisiert sind. Auch Photolumineszenzmessungen unter mechanischem Druck weisen darauf hin, dass es sich in diesem System hauptsächlich um einen direkten räumlichen Übergang handelt. Dieses Ergebnis wird dadurch untermauert, dass die Lebensdauer der Ladungsträger im Bereich von etwa 2 ns liegt, was nicht untypisch für Typ-I-Systeme ist. Die Ergebnisse für zweidimensionale, in GaP eingebettete InP-Schichten zeigen im Gegensatz zu den Quantenpunkten, dass die strahlende Rekombination in InP/GaP Quantentöpfen aufgrund eines indirekten Übergangs (sowohl in Orts- als auch in Impulsraum) zwischen Elektronen- und Löcherzuständen erfolgt. Die optischen Emissionslinien liegen für Quantentöpfe im Bereich von 2,15 bis 2,30eV. Die nachgewiesene sehr lange Lebensdauer der Ladungsträger von etwa 20ns weist weiter darauf hin, dass die Quantentöpfe ein Typ-II-System sind. Nach Modellierung des Bänderschemas für das verspannte InP/GaP-System und Berechnung der Energieniveaus von Löchern und Elektronen darin mit Hilfe der Effektive-Masse-Näherung in Abhängigkeit von der InP-Schichtdicke zeigt sich ferner, dass für InP-Quantentöpfe mit einer Breite kleiner als 3nm die Quantisierungsenergie der Elektronen so groß ist, dass der X-Punkt in GaP energetisch tiefer liegt als der Gamma-Punkt in InP. Dieser Potentialverlauf führt dazu , dass die Elektronen im X-Minimum des GaP lokalisieren, während die Löcher in der InP-Schicht bleiben. Optische Untersuchungen nach thermischer Behandlung der Quantenpunkte führen sowohl im InP/InGaP- als auch im InP/GaP-System zur Verstärkung der Lumineszenz, die bis zu 15 mal internsiver als bei unbehandelten Proben sein kann. Insgesamt zeigt diese Arbeit, dass InP-Quantenpunkte durch ihre optischen Eigenschaften sehr interessant für optoelektronische Anwendungen sind. Die Verwendung von durchsichtigem GaP (mit einer größeren Bandlücke und kleineren Gitterkonstante im Vergleich zu GaAs und InGaP) als Matrix und Substrat hat nicht nur den Vorteil, dass die InP-Quantenpunkte hierbei im sichtbaren Bereich Licht emittieren, sondern man kann in der Praxis auch von einer hochentwickelten GaP-basierten LED-Technologie profitieren. Hauptergebnis dieser Arbeit ist, dass die in indirektes GaP eingebetteten InP-Quantenpunkte aktive optische Eigenschaften zeigen. Sie können daher als aktive Medien zur Realisierung neuartiger effizienter Laser und Leuchtdioden verwendet werden.
The growth and structural properties of self-assembled InP quantum dots are presented and discussed, together with their optical properties and associated carrier dynamics. The QDs are grown using gas-source molecular-beam epitaxy in and on the two materials InGaP (lattice matched to GaAs) and GaP. Under the proper growth conditions, formation of InP dots via the Stranski-Krastanow mechanism is observed. The critical InP coverage for 2D-3D transition is found to be 3ML for the InP/ InGaP system and 1.8ML for the InP/GaP system. The structural characterization indicates that the InP/GaP QDs are larger and, consequently, less dense compared to the InP/ InGaP QDs; hence, InP dots on GaP tend to be strain-relaxed. The InP/ InGaP QDs tend to form ordered arrays when InP coverage is increased. Intense photoluminescence from InP quantum dots in both material systems is observed. The PL from InP/GaP QDs peaks between 1.9 and 2 eV and is by about 200 meV higher in energy than the PL line from InP/ InGaP QDs. The optical emission from dots is attributed to direct transitions between the electrons and heavy-holes confined in the InP dots, whereas the photoluminescence from a two-dimensional InP layer embedded in GaP is explained as resulting from the spatially indirect recombination of electrons from the GaP X valleys with holes in InP and their phonon replicas. The type-II band alignment of InP/GaP two-dimensional structures is further confirmed by the carrier lifetime above 19 ns, which is much higher than in type-I systems. The observed carrier lifetimes of 100-500 ps for InP/ InGaPQDs and 2 ns for InP/GaP QDs support our band alignment modeling. Pressure-dependent photoluminescence measurements provide further evidence for a type-I band alignment for InP/GaP QDs at normal pressure, but indicate that they become type-II under hydrostatic pressures of about 1.2 GPa and are consistent with an energy difference between the lowest InP and GaP states of about 31 meV. Exploiting the visible direct-bandgap transition in the GaP system could lead to an increased efficiency of light emission in GaP-based light emitters.

碩士
中原大學
化學工程研究所
103
A zero-dimension reactor model and the chemical mechanisms of GaN / InN / InGaN metal organic chemical vapor deposition (MOCVD) have been investigated in this research. The influence of the process parameters is discussed by Rate-Of-Production analysis. It is found that GaN reaction mechanism from literature can be successfully reduced and the model predictions agree with the experimental data. The model shows that the optimal temperature range for GaN MOCVD is from 900K to 1300K at diffusion-limit regime. The results of InN model show that the growth rate increases as temperature increases at kinetic-limit regime. However, the profile of growth rate at diffusion-limit regime is different from GaN model because the decomposition of InN occurs. The results indicate that the resident time is responsible for the effect of pressure on the growth rate in addition to the change of the surface species coverage. Lastly, we combine both GaN and InN models to establish the InGaN model and investigate the effects of the inlet gas ratio, temperature and pressure to the InGaN growth rate and indium composition of the film. This research has successfully established the model to deal with the complex chemical mechanism in InGaN MOCVD. All models agree with the experimental data. Thus, it enables to reduce the complexity of 2-D and 3-D modeling calculation.

The present research work focuses on the growth and characterization of group III-Nitride (InGaN) epitaxial layers as well as nanostructures on Si(111) substrates. The growth system used in this study was a plasma-assisted molecular beam epitaxy (PAMBE) system equipped with a radio frequency (RF) plasma source. Device quality GaN epilayers were obtained and InGaN/Si(111) heterojunctions were studied. In- GaN based multi-quantum well LED structure has been realized for green emission. Further catalyst free ultra fine GaN nanorods were grown using a two step method and further InGaN nanostructures were embedded in the as-grown nanorods. InGaN quantum dots were grown using droplet epitaxy and were characterized by Scanning Tunnelling Microscopy and Spectroscopy. It gives a brief introduction about III-nitride materials, growth, substrate selection, significance of III-Nitrides and Si integration and role of dimensionality. It deals with experimental techniques including the details of PAMBE system used in this work, substrate preparation, and detailed characterization of III-nitride epitaxial layers as well as nanostructures. It deals with the optimization of GaN epilayers on AlN/Si (111) templates. AlN underlayer was chosen to minimize the concentration of defects and also acts as an insulating layer which is crucial when it comes to integration of many other devices. The growth temperature was optimized under nitrogen rich growth regime and with the use of a thinner and better quality AlN underlayer and Si doping we could achieve device quality epilayers ( 1500 arc sec) for a thickness of 150 nm. The electron concentration and mobility were found to be -1.374 _1019cm􀀀3 (indicating n-type) and 72 cm2/V.s. Current-voltage measurements were carried out in temperature range of 77K-400K and the current conduction mechanisms at room temperature were identified. An in-depth analysis of temperature dependent current-voltage measurements reveal that the barrier height at the interface is not uniform and is found to have a double Gaussian distribution of barrier heights. It deals with the growth of InGaN epilayers on Si (111) with various substrate treatments. Actual indium composition was determined considering the bi-axial strain present in the epilayers. The effect of substrate treatment on epilayers evolution and quality are discussed. We could observe room temperature photoluminescence from the as-grown epilayers indicating that the epilayers are of good optical quality. InGaN/Si heterojunctions were studied for UV-detection applications. It was found that the heterojunction behaved as a self-powered device, i.e., the device showed a sharp rise in the photocurrent under UV illumination at zero bias. The rise and decay times were found to be 20ms and 33 ms respectively. The bandgap of grown InGaN epilayers were tuned for emission in Green wavelength range. (500nm-550nm) It discusses the sequential process involved in the unition of individual layers to successfully achieve a multi- quantum well structure. In the previous chapter, InGaN epilayers with emission in the green (500nm) region were obtained and having identified the growth conditions for green emission, InGaN epilayers were further grown on GaN/Si (111) and we could tune the bandgap to obtain the emission in blue region. The effect of InGaN growth on thickness was studied which finally led us to develop a growth sequence for successfully obtaining a multi quantum well structure. It deals the growth, structural and optical characterization of InGaN nanostructures embedded in GaN nanorods. The first part deals with the spontaneous growth of very fine (20nm diameter) GaN nanorods on Si (111). Low temperature photoluminescence spectroscopy (LTPL) was used to determine the optical properties of the GaN nanorods. The second part discusses the growth conditions for embedding InGaN in the earlier formed GaN nanorods. The effect of substrate temperature on the evolution of InGaN structures is assessed. Scanning Transmission Electron Microscopy along with Energy Dispersive Spectroscopy (STEM/EDS) is used to determine the elemental distributions in the as-grown nanostructures. LTPL was carried out to determine the emission characteristics of the InGaN/GaN nanostructures. We could successfully obtain room temperature emission in blue region from the core-shell nanorods which happens to be rare achievement. It deals with the growth of high indium content InGaN QDs by droplet epitaxy has been attempted for the first time. The experimental conditions behind InGa droplet formation have been discussed. The influence of droplet formation temperature on the transition from nanoscale structures to quantum dots has been discussed. Room temperature scanning tunnelling microscopy and spectroscopy measurements were carried out. It was found that the QDs exhibited compositional variations even at nanoscale from STM/STS studies. It gives the summary and conclusions of the present study and also discusses about future research directions in this area.

We report non-planar regrowth of InGaN quantum wells on triangular InGaN buffer layers grown on sub-200nm-wide GaN ridges. Photo-pumped internal quantum efficiencies above 20% at yellow wavelengths hold promise for semiconductor laser gain regions.

The PRCI Technical Committees have carried out many research projects related to the subject of natural gas fueled engine emissions. This report, therefore, presents a summary of past PRCI funded studies in the area of Gas Engine Emissions. Since studies sponsored by other industry groups (e.g., INGAA, EPRG, APIA, CEPA, etc.) or by individual pipeline companies are not included, this report does not represent a review of the topic in the broader industry-wide sense. Instead, it is designed to provide a guide to past work so that current TC members can identify and locate project reports that might be of use in addressing their current day-to-day Gas Engine Emissions issues.

This work was funded in part, under the Department of Transportation, Pipeline and Hazardous Materials Safety Administration. The views and conclusions contained in this document are those of the authors and should not be interpreted as representing the official policies, either expressed or implied, of the Pipeline and Hazardous Materials Safety Administration, the Department of Transportation, or the U.S. Government. This project builds on mechanical damage (MD) assessment and management tools, developed on behalf of Pipeline Research Council International (PRCI), Interstate Natural Gas Association of America (INGAA), Canadian Energy Pipeline Association (CEPA), American Petroleum Institute (API), other research organizations and individual pipeline operators and were included in API RP 1183. These include dent shape, restraint condition and interacting feature characterization; operational maximum and cyclic internal pressure characterization, screening tools defining non-injurious dent shapes based on pipe size and operating condition, failure pressure and fatigue assessment tools for dents with/without interacting features (e.g., corrosion, welds, gouges) in the restrained and unrestrained condition, and direction on available remedial action and repair techniques. In completing this development, areas for improvement were identified. The current project enhances previously developed tools being adopted in an industry recommended practice (API RP 1183) for pipeline MD integrity assessment and management considering: - Enhancement of indentation crack formation strain estimation, - Understanding the role of ILI measurement accuracy on dent integrity assessment, and - Quantification of assessment method conservatism to support safety factor definition. Safety factors (Modeling bias) defined in the present study and evaluated for different fatigue life estimation approaches in the present work refer to the conservatism inherent in different in different fatigue life models and is represented as the ratio of experimental lives to predicted lives.

This work develops a comprehensive guidance for the pipeline industry on the use of hydrostatic testing as an integrity management tool with carbon steel mainline piping as the focus of this work. Selected hydrostatic testing principles considered herein may also be applicable to other forms of transmission and facility piping. At the time of this guideline development, several parallel pipeline industry initiatives with focus on hydrostatic testing have been recently completed, or are currently on-going. Such initiatives include work performed by the Pipeline and Hazardous Materials Safety Administration (PHMSA), the American Petroleum Institute (API), the Association of Oil Pipe Lines (AOPL), and the Interstate Natural Gas Association of America (INGAA). While this completed or on-going work is of considerable value to the pipeline industry, lacking within these industry initiatives is a focus on the specific application of hydrostatic testing as a pipeline assessment tool to support integrity management plan (IMP) requirements and objectives. The scope of this work is the development of a document which will provide the necessary guid-ance to pipeline operators on the appropriate use of hydrostatic testing within the integrity and risk management process. Consideration is given to the essentials of known or assumed pipe material properties, threat type and defect populations, operating and failure history, potential risks, unintended or detrimental effects and the importance of a clear understanding of objectives, as key elements within a hydrostatic test program design process. This document does not attempt to provide practical guidance for the on-site implementation of a hydrostatic testing program. The focus of this study has instead been on hydrostatic test program decision processes and development; the selection of hydrostatic testing as an appropriate pipe-line assessment tool, the subsequent technical design of a hydrostatic test program and the necessary considerations in order to achieve IMP requirements and objectives. This report has a related webinar.

8+fjordar-området är topografiskt väldigt varierande. Vattenutbytet är långsamt och tillförseln av näringsämnen stor. Största delen av näringsämnen stannar kvar i fjordarna. Mest i Havstens- och Hakefjordar där 40-50 % av den lokala näringstillförseln stannar kvar. Utsläppen från jordbruk och skogsbruk är stora i 8+fjordar-området. T.ex. står dessa för 62 % av kvävetillförseln och 69 % av fosfortillförseln till Byfjorden. Men utsläpp från punktkällor direkt till havet är också betydande (21 % av kvävetillförseln och 19 % av fosfortillförseln till Byfjorden). Man har genom en mängd olika tilltag de senaste decennierna försökt sänka utsläppen av kväve och fosfor och även om koncentrationerna av kväve och fosfor fortsatt är höga har halterna av främst kväve sjunkit i 8+fjordar-området. De stora utsläppen av näringsämnen ökar växtplanktonproduktionen och när denna sjunker till botten och bryts ner av bakterier orsakar det syrebrist och höga koncentrationer av giftigt svavelväte i de djupare delarna av fjordarna. I de grundare delarna av fjordsystemet finns grunda vikar med ålgräsängar, blåmusselbankar och klippkuster med tångskogar. Dessa biotoper har förändrats under de senaste decennierna med en ökande påväxt av fintrådiga alger. Den pelagiska miljön (de fria vattenmassorna) är främst påverkad av avrinning av sötvatten från älvar och åar och av inflöde av salthaltigt vatten från Skagerrak genom Marstrandsfjorden och norrut i fjordsystemet. Närsaltkoncentrationen är hög i hela vattenpelaren vilket ses speciellt för nitrat. Detta ger förhöjda klorofyllkoncentrationer även om dessa alltså har minskat betydligt under de senaste decennierna. 8+fjordar-området har länge haft höga koncentrationer av giftiga dinoflagellater, men de senaste decennierna har giftalgsblomningarna minskat betydligt. De flesta områden uppnår nu minst god ekologisk status avseende vinterkoncentrationer av totalt kväve med undantag för By-, Havstens-, Askerö- och Älgöfjordar. Status för växtplankton klassas som hög i nästan hela området. År 2006 kom den amerikanska kammaneten Mnemiopsis leidyi till svenska västkusten. Den är en effektiv predator på djurplankton och kan vissa år minska djurplanktonbiomassan avsevärt under hösten. Detta kan ha allvarliga konsekvenser för överlevnad av fisk och fisklarver som livnär sig på djurplankton. De allra flesta fiskarter livnär sig på djurplankton i larvstadiet. Den bentiska miljön innefattar livsmiljön från de djupa mjukbottnarna till tångskogarnas och ålgräsängarnas topp. 8+fjordar-områdets bentiska miljö kännetecknas av klippstränder med hårdbottnar beväxta med bland annat snärjtång, blåstång, sågtång, sockertång och fintrådiga alger, där blåmusslor och andra evertebrater lever och de djupare delarna täcks av fastsittande bentiska evertebrater som till exempel havsanemoner och havsnejlikor. De djupare sedimentbottnarna hyser grävande evertebrater som livnär sig på den biomassa som sjunker ner från pelagialen. Här dominerar havsborstmasken Scalibregma inflatum, slätbukig Sammanfattning trådormstjärna och pepparmussla. Vatten från Skagerrak strömmar in i 8+fjordarområdet söderifrån och de södra bassängerna inklusive Hake- och Askeröfjordar är normalt syresatta under hela året även i sina djupaste delar, medan de djupa delarna av de nordligare Kalvö-, Borgile-, Koljö- och Byfjordar samt även Havstensfjorden ofta präglas av långvarig syrebrist. I dessa områden försvinner bottenfaunan ofta helt eller befinner sig i någon fas av återkolonisering efter vattenutbyten. Den bentiska miljön hade bättre ekologisk status under 1980- och 90-talet än under det tidiga 2000-talet. De flesta åren därefter visar måttlig status i Hake-, Halse- och Havstensfjordar. I de grunda vikarna har mängden fintrådiga alger ökat betydligt sedan slutet på 1990-talet. Dessa alger bildar påväxt på ålgräs och tång och täta mattor av ruttnande alger på botten vilket försvagar ålgräs och tång och orsaker lokal syrebrist i ängarna. Dessa alger har ökat i biomassa delvis på grund av avsaknaden av betande evertebrater (t.ex. märlkräftor) som i sin tur har minskat i antal eftersom mesopredatorer som t.ex. strandkrabba och läppfiskar har ökat när deras predatorer, som t.ex. torsk, har fiskats bort. Utbredningen av ålgräs i 8+fjordar-området har minskat ganska dramatiskt de senaste decennier, speciellt kring Kungälv och Uddevalla där 80–85% av arean har försvunnit. Status för ålgräs och tång är otillfredsställande eller måttlig i nästan hela 8+fjordar-området. Fiskbestånden i 8+fjordar-området har varit överfiskade under lång tid och liksom i Västerhavets övriga kuststområden är förekomsten av större bottenlevande fisk, främst torskfiskar, mycket reducerad i 8+fjordar-området. Det fanns ett betydande fiske av torsk, näbbgädda, rödspotta, sill och skarpsill i området på 1960- talet men 2004–2008 hade de flesta fisken kollapsat med undantag av fisket på sill och skarpsill som fortfarande fiskas även on mängden landad sill har minskat betydligt från 2014 till 2021. Av alla fångster av sill och skarpsill fiskades i genomsnitt 14 % av sillen och 87 % av skarpsillen med lysfiske. SLU:s undersökningar visar dessvärre inte på någon återhämtning av fiskbestånden under de senaste 20 åren, trots att det riktade fisket efter flera av dessa arter stoppats och det införts ett fiskefritt område i Havstensfjorden. Genetiska studier visar dock att det fortfarande finns ett lokalt lekande bestånd av torsk i området och det har observerats bättre rekryteringar 2016 och 2019. Sportfisket efter havsöring är betydande, men störst är fisket efter makrill både med spö och med dörj. Det finns ingen officiell statistik på landningar men i Fiskeriverkets rapport från 1999 angavs att fritidsfiskets sammanlagda fångster översteg yrkesfiskets för lax och havsöring och vissa arter av plattfisk. Det finns en del vattenbruk i 8+fjordar-området. Företaget Scanfjord är störst, med blåmusselodlingar i yttre Stigfjorden, Havstensfjorden och Koljöfjorden. En rad andra aktörer har odlingar främst i norra delen av 8+fjordar-området och företaget Marine Taste odlar sjöpungar nära Stenungsund. Sjöfåglar är viktiga länkar mellan näringsvävar i havet och på land och de kan vara bra indikatorer på förändringar i de marina ekosystemen. Efter mitten av 1990- talet har ejderpopulationen minskat till hälften och arten är numera (2020) rödlistad som ”starkt hotad” (EN) i både Sverige och Europa. Strandskatan har minskat med omkring 40 % i Sverige de senaste 30 åren och är sedan 2020 rödlistad som "nära hotad" (NT). Troliga faktorer för dessa två arters minskning är minskad mängd och/eller kvalitet, inklusive vitaminbrist, på bytesdjur, ökad utbredning av syrefria bottnar, klimatförändringar, predation och sjukdom. Stora förändringar har också ägt rum hos många måsfågelbestånd de senaste 20–30 åren. Fisktärna och silltrut (på västkusten) har klarat sig bra, medan övriga måsfåglar minskat så kraftigt att de blivit rödlistade. Bestånden av grågås, kanadagås och vitkindad gås har ökat kraftigt de senaste 30 åren. Tillgången på höst- och vintergröna grödor har ökat genom ändrat jordbruk och varmare och snöfattigare vintrar vilket har gynnat gässen. Storskarven var tidigare utdöd i Sverige men under 1980- och 1990-talet spred sig skarven längs den svenska kusten och i 8+fjordar-området etablerades de första kolonier i början av 2000-talet. Det finns ingen regelbunden inventering av storskarv i Sverige men år 2021 räknades 1300 bon i 7 kolonier från Nordre Älvs mynning i söder till Havstensfjord i norr, absoluta majoriteten väster om OrustTjörn. I 8+fjordar-området är storskarvens vanligaste byten smörbult och plattfisk, men även andelen torskfisk och sötvattensfisk är betydande. Knubbsäl, gråsäl och tumlare förekommer i 8+fjordar-området. Knubbsäl och tumlare är de överlägset vanligaste marina däggdjuren medan antalet gråsälar är betydligt färre. Efter att jakten förbjöds och sälarna skyddades, samtidigt som mängden miljögifter minskade, började sälpopulationen längs västkusten återhämta sig under 1980-talet. Knubbsälpopulationen har ökat sedan dess även om virussjukdom (Phocine Distemper Virus, PDV) reducerade antalet betydligt år 1988 och 2002. Under 2010-talets senare hälft uppskattades antalet knubbsälar i Västerhavet till över 20 000 djur. I 8+fjordar-området vistas de flesta sälar på utsidan Orust och Tjörn med mycket färre sälar i själva 8+fjordar-området. Prover insamlade 2015-2016 visade att knubbsälens födoval är helt dominerat av plattfisk (viktandel >70 %) följt av sill, rötsimpa och vitling. Miljögifter finns i stora koncentrationer i vissa delar av 8+fjordar-området, främst i Byfjorden där föroreningar från hamnen länge har varit stora och kring Stenungsund där utsläpp från den kemiska industrin dominerar. Men inom 8+fjordar-området finns även ett stort antal mindre båtvarv och marinor som genom åren har bidragit till giftiga utsläpp främst från båtbottenfärg. I Byfjorden har bottensedimenten höga eller mycket höga koncentrationer av olja, polycykliska aromatiska kolväten (PAH) och PCB. Där är koncentrationerna av TBT, som tidigare användes i båtbottenfärg, och dess två nerbrytningsprodukter DBT och MBT mycket höga. Flera metaller så som zink, kadmium, koppar och nickel finns också i höga koncentrationer. Kvicksilver- och PBDE-halten i blåmusslor överskrider Vattendirektivets gränsvärde för fisk. Dessa föroreningar finns också i Havstensfjorden och Halsefjorden men i lägre koncentrationer. Kring Stenungsund är bottensedimentens halter av hexaklorbensen (HCB) höga eller mycket höga men det avspeglas dock inte i förhöjda HCB-halter i blåmusslor från samma område. Halterna av DBT och MBT är också höga här, medan halterna av TBT inte är förhöjda. Kopparhalten i sediment från Stenungsundsområdet är förhöjda och på en lokal är de långt över Vattendirektivets gränsvärde. Även koncentrationen av PBDE i blåmussla överstiger vattendirektivets gränsvärde. Blåstång från en lokal vid nordvästra Stenungsön har höga eller mycket höga koncentrationer av kadmium, arsenik och koppar. I syrefria bottnar förekommer ingen omblandning av sedimentet av infauna såsom havsborstmaskar vilket gör att lagrade ämnen inte i så hög utsträckning frigörs till vattnet och miljögifter koncentreras som mest i dessa områden. Nedbrytningshastigheten av organiska miljögifter är också betydligt långsammare i en syrefattig miljö än i en syrerik och det kan ta många decennier att bryta ned gifterna. Om syrefria bottnar innehållande miljögifter åter syresätts och omblandningen sätts igång, är det därför risk för kontaminering av kringliggande vatten. Mikroplast som härstammar från den lokala plastindustrin finns i större mängder i bottensedimenten kring Stenungsund. En studie visade att minst 3 miljoner och i värsta fall 36 miljoner polyetylenpellets större än 2 mm, motsvarande 73–730 kg, släpps ut via Stenunge å årligen. När mindre fraktioner ner till 300 µm inkluderades i mätningarna var det totala partikelantalet hundrafaldigt högre. Dessa partiklar har direkt effekt på djur och växter i fjorden. Elfiskeundersökningar visade att 62 % av öring fångade i Stenunge å hade plastpartiklar i magen. Flera invasiva arter har etablerat sig i 8+fjordar-området de senaste decennierna. Den amerikanska kammaneten Mnemiopsis leidyi finns i stora mängder sensommar och höst under de flesta år. Under år med dessa maneter kan de äta upp största delen av djurplanktonbiomassan vilket skapar problem för fisklarver som livnär sig på detta plankton. Stillahavsostronet eller det japanska jätteostronet har observerats på stränder i hela 8+fjordar-området. Dessa ostron kan tränga undan blåmussla när det bildas stora ostronbankar och de europeiska ostronen kan smittas av nya typer av parasiter. Ostronens skal är också vassa och kan orsaka skador på människor som går på bottnen i grunda områden. Mellan 20 och 40 % av 8+fjordar-områdets kustlinje är bebyggd inom 100 meters avstånd till vattenlinjen. I Kungälv och på Tjörn och Orust har bebyggd kustlinje fördubblats på bara 10 år trots förbud mot uppförande av nya byggnader närmre än 100 meter från strandlinjen enligt strandskyddslagstiftningen. 20–25 % av kusten i dessa kommuner är nu bebyggd. Friluftslivet till sjöss har också ökat kraftigt under senare år. Intervjuundersökningar visar att det under 2004 uppskattningsvis fanns totalt 26 600 båtar på svenska västkusten men att antalet har fyrdubblats fram till 2010. Denna ökning av mänsklig närvaro stör på många olika sätt. Pirar eller bryggor kan störa djurs naturliga migrationsrutter längs kusten, speciellt i topografiskt komplexa områden som 8+fjordar-området och större strukturer som t.ex. brofästen eller bortsprängningar och utgrävningar kan ändra vattengenomströmningen. I 8+fjordar-området kan det ha extra stor effekt eftersom vattenutbytet är naturligt långsamt. Fåglar störs kraftigt av snabbgående båtar. Många fåglar undviker ofta platser med mycket trafik under ruggningen och på rastoch övervintringslokaler kan störningar från båtar leda till att de oftare tar till flykt med energiförluster som följd. Marint skräp är ett särskilt stort problem i Bohuslän där stora mängder makroskräp driver i land på grund av havsströmmarna (Jutska strömmen). Detta är faktiskt ett av Europas mest nedskräpade marina områden. 96 % av det marina skräpet längs stränderna i Bohuslän utgörs av plastartiklar, och det vanligaste är snören och linor som till allra största del kommer från fisket. Effekter av skräp på det marina djurlivet är väldokumenterade, t ex insnärjning av marina djur och intag av skräpföremål av fåglar, fiskar och evertebrater. Förlorade tinor, garn och ryssjor utgör också en betydande del av skräpet. Efter intervjuundersökningar uppskattades antalet förlorade hummertinor till 3900 per år på västkusten bara från fritidsfisket. Förlorade fiskredskap fiskar vidare och studier har visat att så mycket som 163 800 humrar och krabbtaskor fångas per år på västkusten i detta spökfiske. Klimatförändringarna leder till att haven runt Sverige blir allt varmare, att kustnära vatten utsötas när nederbörden ändras och att haven försuras när ökade mängder koldioxid tas upp i havet. Varmare vatten tar upp mer plats så havsnivån stiger med stigande temperatur och detta förvärras när polernas fastlandsisar smälter. I 8+fjordar-området ökar ytvattentemperaturen fyra gångar så snabbt som den globala medeluppvärmningen och enligt SMHI har temperaturen ökat med 3,5 °C sedan 1960. Salthalt och skiktning förändras i kustnära miljöer när nederbörden varierar. I svenska kustnära marina miljöer har saliniteten minskat under perioden från 1992, då mätningarna började, fram till ca 2010, men under de senare åren har den ökat så mycket att den nu är tillbaka på samma nivå som 1990. En tredjedel av den koldioxid som släpps ut absorberas av världens hav där den bildar kolsyra. Under industrialiseringen har det globala medel-pH minskat från cirka 8,11 till 8,06, en minskning som motsvarar en ökning i surhet med 30 %. Det finns ingen marin övervakning av pH i 8+fjordar-området men data från danska fjordar visar en försurning som är dubbel så snabb som globala medelvärdet. Av alla belastningar relaterade till klimatförändringarna är det temperaturökningen som har störst effekt på djur och växter i havet. Först och främst förflyttas utbredningsområden för djur och växter mot norr. Torsken i Skagerrak/Kattegatt föredrar temperaturer som är låga jämfört med de medeltemperaturer de upplever i området i dag och den temperaturökningen vi ser i 8+fjordar-området minskar därför torskens lekmöjlighet i området. Ålgräs påverkas också av ökande temperaturer och studier visar att en 5 °C ökning minskar ålgräsets skottäthet. Havsförsurning påverkar främst bottnens kalcifierande arter som kräftdjur, blötdjur och tagghudingar. Till exempel har det visat sig att sjöborrelarvers utveckling försämras av även mycket små minskningar i pH och bottensamhällen påverkas så att både artrikedom och antal individer minskar under försurning.