Main objective of the present work is to develop wide-band gap InGaN solar cells in the 2.4 - 2.9 eV range that can be an integral component of photovoltaic devices to achieve efficiencies greater than 50%. In the present work, various challenges in the novel III-nitride technology are identified and overcome individually to build basic design blocks, and later, optimized comprehensively to develop high-performance InGaN solar cells. Due to the unavailability of a suitable modeling program for InGaN solar cells, PC1D is modified up to a source-code level to incorporate spontaneous and piezoelectric polarization in order to accurately model III-nitride solar cells. On the technological front, InGaN with indium compositions up to 30% (2.5 eV band gap) are developed for photovoltaic applications by controlling defects and phase separation using metal-organic chemical vapor deposition. InGaN with band gap of 2.5 eV is also successfully doped to achieve acceptor carrier concentration of 1e18 cm-3. A robust fabrication scheme for III-nitride solar cells is established to increase reliability and yield; various schemes including interdigitated grid contact and current spreading contacts are developed to yield low-resistance Ohmic contacts for InGaN solar cells. Preliminary solar cells are developed using a standard design to optimize the InGaN material, where the band gap of InGaN is progressively lowered. Subsequent generations of solar cell designs involve an evolutionary approach to enhance the open-circuit voltage and internal quantum efficiency of the solar cell. The suitability of p-type InGaN with band gaps as low as 2.5 eV is established by incorporating in a solar cell and measuring an open-circuit voltage of 2.1 V. Second generation InGaN solar cell design involving a 2.9 eV InGaN p-n junction sandwiched between p- and n-GaN layers yields internal quantum efficiencies as high as 50%; while sixth generation devices utilizing the novel n-GaN strained window-layer enhance the open circuit voltage of a 2.9 eV InGaN solar cell to 2 V. Finally, key aspects to further InGaN solar cell research, including integration of various designs, are recommended to improve the efficiency of InGaN solar cells. These results establish the potential of III-nitrides in ultra-high efficiency photovoltaics.

Since Shuji Nakamura, Hiroshi Amano, and Isamu Akasaki won the 2014 Nobel prize in Physics owing to theircontributions on the invention of efficient blue GaN light emitting diodes, GaN became an even more appealingmaterial system in the research field of optoelectronics. On the other hand, quantum structures or low-dimensionalstructures with properties derived from quantum physics demonstrate superior and unique electrical and opticalproperties, providing a significant potential on novel optoelectronic applications based on the employment of quantumconfinement.   In 2012, our research team at Linköping University utilized pyramid templates, which is an established approach toform quantum structures, to successfully grow GaN pyramids with InGaN hybrid quantum structures, includingquantum wells, quantum wires, and quantum dots. This growth enabled site-controlled pyramids based on selectivearea growth (SAG). After numerous studies on the photoluminescence properties, the mature and controlled growthtechnique was proposed to be adapted for fabrication of AlGaN pyramids on which GaN hybrid quantum structurescan be hosted.   This thesis is dedicated to the subsequent problems of the growth of AlGaN pyramids. It was found that there wasan undesired deposition of a considerable thickness on top the desired AlGaN pyramid with GaN multiple quantumwells. In this thesis, two different directions are explored to find the key solution with a potential of furtheroptimization. On one hand, the growth parameters such as precursors cut-off, carrier gas during cooling, temperatureholding, cooling pressure, III/V ratio, and the possible effect of GaN surfaces are investigated. However, due to theactual inherent properties of the metal-organic chemical vapor deposition reactor used, no promising parameter tuningcan been identified. On the other hand, from post-growth point of view, a KOH aqueous etching solution exhibits apositive result toward removing the undesired deposition. This etching process is suggested to be further optimized toachieve the final goal of eliminating the undesired deposition.

8-band k.p theory was applied to bulk GaN and InN. The optical transitionintensity was computed and results show > 80-90% degree of polarization inthe direction of compression. Polarization switching is observed when strainwas reversed from compressive to tensile. 6 band k.p theory was used tostudy InGaN quantum dot/GaN elliptical pyramid structures. The opticaltransition intensity was calculated for different elongations of the pyramid.Elongation of the pyramid gives rise to a small polarization in the directionof the pyramid elongation. The optical transition intensity was calculatedfor elongated quantum dots and was strongly in uencing the polarization inthe direction of the quantum dot elongation, with a degree of polarization of >90%.

Afin de répondre à l’exigence croissante de densité de puissance aux hautes fréquences, les chercheurs se sont intéressés aux matériaux à large bande interdite tels que le nitrure de gallium GaN. Les HEMTs (transistors à haute mobilité électronique) AlGaN/GaN ne sont pas stabilisées et donc l’analyse de défaillance de ces composants est difficile (défauts multiples).Les mécanismes de défaillance des HEMTs GaAs sont difficilement transposables sur les HEMTs GaN et nécessitent donc une étude approfondie. De plus que les données actuelles sur les effets de pièges ne permettent pas d’expliquer facilement des effet parasites comme l’effet de coude. Ce qui nécessité de développer de nouvelles procédures d’analyse de défaillance adaptées aux composant GaN. Les dégradations induites par les électrons chauds sont difficilement détectables par la technique d ‘émission de lumière standard ce qui a nécessité le développement de la microscopie à émission de lumière dans le domaine de l’UV. L’objectif principal de ce travail est la mise au point d'une méthodologie d'analyse de défaillance pour les filières GaN et l’optimisation des techniques electro-optiques non destructives de localisation de défauts. En vue de l’amélioration des procédés technologiques, et de la fiabilité des HEMT GaN. / There are several economic and technological stakes, which require the development of suitable techniques for failure analysis on microwave devices, the HEMT (High Electron Mobility Transistor) AlGaN/GaN play a key role for power and RF low noise applications.The technologies are not completely stabilized and the failure analysis is difficult. Which need the development of a non destructive investigation techniques such as electroluminescence technics. To improve the GaN HEMT performance and reliability, understanding the failure mechanisms is critical. The standard emission light is not sufficient for hot-elctron detection in GaN material. And the development of UV light emission become necessary in the AlGaN/GaN HEMT.

In this thesis, I will present and discuss research performed on InGaN/GaN multiple quantum well (QW) structures. The results of which were taken using photoluminescence (PL) spectroscopy and PL time decay spectroscopy. In the first two experimental chapters, I report on the effects of QW growth methodology on the optical properties of c-plane InGaN/GaN QWs. I compare structures grown using the single temperature (1T), quasi-two temperature (Q2T), temperature bounced (T-bounced) and two temperature (2T) QW growth methodologies. The T-bounced and 2T structures are observed to have gross well width fluctuations (GWWF), where the QW width varies from 0 to 100 % created when the QWs are exposed to a temperature ramp. Whereas, the 1T and Q2T structures have continuous QWs with only one or two monolayer well width fluctuations. The structures with GWWFs are observed to have a larger room temperature internal quantum efficiency (RT-IQE) at low excitation conditions i.e. below efficiency droop compared to those without. The larger RT-IQE is ascribed to several factors which include an increased radiative recombination rate, increased thermal activation energy of non-radiative recombination and reduced defect density of the QWs. The effect of barrier growth temperature is also investigated. No clear trend is observed between barrier growth temperature and RT-IQE.In the last experimental chapter I report on studies of carrier localisation in InGaN/GaN QWs using resonant PL spectroscopy. The effect of carrier localisation on the independently localised electrons and holes are investigated and the resonant PL spectrum is studied in detail. The InGaN/GaN QW structure is observed to exhibit an effective mobility edge at 12 K where delocalised carriers are created above a particular excitation energy. The emission from the resonantly excited localised states which are accompanied by the emission of a longitudinal optical phonon (resonant LO feature) is investigated as a function of temperature and excitation energy. The integrated PL intensity of the resonant LO feature is observed to quench rapidly with temperature up to around 45 K, independent of excitation energy. The integrated PL intensity of the resonant LO feature is fitted to an Arrhenius model and a thermal activation energy of ∼ 1(±1) meV is extracted. This activation energy is speculated to be consistent with the localisation energy of electrons.

Gallium Nitride (GaN) high electron mobility transistors (HEMT) have superior performance characteristics compared to Silicon (Si) and Gallium Arsenide (GaAs) based transistors. GaN is a wide bandgap semiconductor which allows it to operate at higher breakdown voltages and power. Unlike traditional semiconductor devices, the GaN HEMT channel region is undoped and relies on the piezoelectric effect created at the GaN and Aluminum Gallium Nitride (AlGaN) heterojunction to create a conduction channel in the form of a quantum well known as the two dimensional electron gas (2DEG). Because the GaN HEMTs are undoped, these devices have higher electron mobility crucial for high frequency operation. However, over time and use these devices degrade in a manner that is not well understood. This research utilizes low frequency noise (LFN) as a method for analyzing changes and degradation mechanisms in GaN-on-Si devices due to gate stress. LFN is a useful tool for probing different regions of the device that cannot be measured through direct means. LFN generation in GaN HEMTs is based on the carrier fluctuation theory of 1/f noise generation which states fluctuations in the number of charge carriers results in conductance fluctuations that produce a Lorentzian noise spectrum. The summing Lorentzian noise spectra from multiple traps leads to 1/f and random telegraph signal (RTS) noise. The primary cause of carrier fluctuations are electron traps near the 2DEG and in the AlGaN bulk. These traps occur naturally due to dislocations and impurities in the manufacturing process, but new traps can be generated by the inverse-piezoelectric effect during gate stress. This thesis introduces noise and presents a circuit to bias the devices and measure gate and drain LFN simultaneously. Three measurements are performed before and after gate DC stress at three different temperatures: DC characterization, capacitance-voltage (C-V) measurements, and LFN measurements. The DC characteristics show an increase in gate leakage after stress caused by an increase in traps after degradation consistent with trap assisted tunneling. However, the leakage current on the drain and source side differ before and after stress leading to the conclusion that the source side of the gate is more sensitive to gate stress. Gate leakage current on the drain side is also sensitive to temperature due to thermionic trap assisted tunneling. Hooge parameter calculations agree with previous research. The LFN results show an increase in gate and drain noise power, SIg(f) and SId(f), in accordance with increased gate leakage current under cutoff bias. RTS noise is also observed to increase in frequency with increased temperature. Activation energies for RTS noise are extracted and qualitatively linked to trap depth based on the McWhorter trap model.

Hlboké neurónové siete sa bežne používajú v oblasti odstraňovania šumu. Trénovací proces neurónovej siete je možné rožšíriť využitím druhej neurónovej siete, ktorej cieľom je vložiť šum do čistej rečovej nahrávky. Tieto dve siete sa môžu spolu využiť k rekonštrukcii pôvodných čistých a zašumených nahrávok. Táto práca skúma efektivitu tejto techniky, zvanej cyklická konzistencia. Cyklická konzistencia zlepšuje robustnosť neurónovej siete bez toho, aby sa daná sieť akokoľvek modifikovala, nakoľko vystavuje sieť na odstraňovanie šumu rôznorodejšiemu množstvu zašumených dát. Avšak, táto technika vyžaduje trénovacie dáta skladajúce sa z párov vstupných a referenčných nahrávok. Tieto dáta niesu vždy dostupné. Na trénovanie modelov s nepárovanými dátami využívame generatívne neurónové siete s cyklickou konzistenciou. V tejto práci sme vykonali veľké množstvo experimentov s modelmi trénovanými na párovaných a nepárovaných dátach. Naše výsledky ukazujú, že využitie cyklickej konzistencie výrazne zlepšuje výkonnosť modelov.

The main goal of this thesis was to increase reconstruction quality of sparse sampled microscopic images by using neural networks. The thesis will cover various approaches for image reconstruction and will also include descriptions of implementations, which were used. Implementations will be evaluated based on quality of reconstruction, but also based on segmentation, which could be their main possible application.

This thesis investigates the use of Generative Adversarial Networks (GANs) for detecting images containing non-natural objects in natural environments and if the introduction of stereo data can improve the performance. The state-of-the-art GAN-based anomaly detection method presented by A. Berget al. in [5] (BergGAN) was the base of this thesis. By modifiying BergGAN to not only accept three channel input, but also four and six channel input, it was possible to investigate the effect of introducing stereo data in the method. The input to the four channel network was an RGB image and its corresponding disparity map, and the input to the six channel network was a stereo pair consistingof two RGB images. The three datasets used in the thesis were constructed froma dataset of aerial video sequences provided by SAAB Dynamics, where the scene was mostly wooded areas. The datasets were divided into training and validation data, where the latter was used for the performance evaluation of the respective network. The evaluation method suggested in [5] was used in the thesis, where each sample was scored on the likelihood of it containing anomalies, Receiver Operating Characteristics (ROC) analysis was then applied and the area under the ROC-curve was calculated. The results showed that BergGAN was successfully able to detect images containing non-natural objects in natural environments using the dataset provided by SAAB Dynamics. The adaption of BergGAN to also accept four and six input channels increased the performance of the method, showing that there is information in stereo data that is relevant for GAN-based anomaly detection. There was however no substantial performance difference between the network trained with two RGB images versus the one trained with an RGB image and its corresponding disparity map.

ermal desorption spectroscopy (TDS) is a common method for surface analysis of adsorbed molecules. In chapter 1 the work deals with the theoretical background of this method and shows the principles of a desorption process influenced by subsurface diffusion. Chapter 2 first shows application of TDS for detection of surface molecules and determination of binding energy.Experiments were mainly focused on ditermination of surface adsorbents and impurities on Si wafers. The second part of chapter 2 describes desorption of atoms of a Ga layer on Si surface and their subsurface diffusion. A Ga diffusion process was also observed by with secondary ion mass spectrometry (SIMS) and numerically simulated.