In the dynamic arena of display technology, augmented reality (AR) displays represent a pivotal advancement, seamlessly bridging the digital and physical worlds. This dissertation delves into the realm of AR display technologies, spotlighting the challenges and limitations of current systems, including transparent and near-eye displays, and proposes innovative solutions to enhance user experience and display performance. With a focus on overcoming issues such as diffraction-induced image blur, the trade-off between resolution and field of view (FoV) in near-eye displays, and FoV constraints in waveguide-based displays, this research introduces new evaluation methods, optimization techniques, and system designs. First, the dissertation presents a quantitative evaluation of diffraction effects on background objects, leading to the development of a pixel structure optimization method aimed at reducing diffraction in transparent displays with small aperture ratios. This advancement promises to enhance image clarity and visibility, addressing one of the key challenges in the deployment of AR technology for transparent displays. Next, we introduce a novel Maxwellian-type foveated AR system that leverages a single light engine. This system employs a temporal polarization-multiplexing method to encode both high-resolution foveal and low-resolution peripheral images through the same light engine. With the aid of polarization-selective lenses, this system effectively separates the two views, delivering a wide FoV and high angular resolution in the foveal region, effectively minimizing the resolution-FoV compromise in near-eye displays. Furthermore, the dissertation conducts a detailed analysis of FoV limitations in single-layer waveguides, proposing a strategic combination of a gradient-pitch polarization volume grating (PVG) with a butterfly exit-pupil expansion (EPE) scheme. This approach aims to extend the FoV in single-layer waveguides towards the theoretical full-color limit. This research addresses pivotal challenges in waveguide-based AR technology, marking a significant step towards realizing more immersive and user-friendly AR systems.

Yes / A dual-port transparent multiple-input multiple-output (MIMO) antenna resonating at sub-6 GHz 5G band is proposed by using patch/ground material as transparent conductive oxide (AgHT-8) and a transparent Plexiglas substrate. Two identical circular-shaped radiating elements fed by using a microstrip feedline are designed using the finite element method (FEM) based highfrequency structure simulator (HFSS) software. The effect of the isolation mechanism is discussed using two cases. In case 1, the two horizontally positioned elements are oriented in a similar direction with a separate ground plane, whereas in case 2, the elements are vertically placed facing opposite to each other with an allied ground. In both cases, the transparent antennas span over a −10 dB band of 4.65 to 4.97 GHz (300 MHz) with isolation greater than 15 dB among two elements. The diversity parameters are also analyzed for both the cases covering the correlation coefficient (ECC), mean effective gain (MEG), diversity gain (DG), and channel capacity loss (CCL). The average gain and efficiency above 1 dBi and 45%, respectively with satisfactory MIMO diversity performance, makes the transparent MIMO antenna an appropriate choice for smart IoT devices working in the sub-6 GHz 5G band by mitigating the co-site location and visual clutter issues. / This work is supported by the Moore4Medical project, funded within ECSEL JU in collaboration with the EU H2020 Framework Programme (H2020/2014-2020) under grant agreement H2020-ECSEL-2019-IA-876190, and Fundação para a Ciência e Tecnologia (ECSEL/0006/2019).

Numerous bats species have evolved biosonar to obtain information from their habitats with dense vegetation. Different from man-made sensors, such as stereo cameras and LiDAR, bats' biosonar has much lower spatial resolution and sampling rates. Their biosonar is capable of reliably finding narrow gaps in foliage to serve as a passageway to fly through. To investigate the sensory information under such capability, we have used a biomimetic sonar robot to collect the narrow gap echoes from an artificial hedge in a laboratory setup and from the natural foliage in outdoor environments respectively. The work in this dissertation presents the performance of a conventional energy approach and a deep-learning approach in the classification of echoes from foliage and gap. The deep-learning approach has better foliage versus passageway classification accuracy than the energy approach in both experiments, and it also shows good robustness than the latter one when dealing with data with great varieties in the outdoor experiments. A class activation mapping approach indicates that the initial rising flank inside the echo spectrogram contains critical information. This result corresponds to the neuromorphic spiking model which could be simplified as times where the echo amplitude crosses a certain threshold in a certain frequency range. With these findings, it could be demonstrated that the sensory information in clutter echoes plays an important role in detecting passageways in foliage regardless of the wider beamwith than the passageway geometry. / Doctor of Philosophy / Many bats species are able to navigate and hunt in habitats with dense vegetation based on trains of biosonar echoes as their primary sources for sensory information on the environment. Drones equipped with man-made sensory systems such as optical, thermal, or LiDAR sensors, still face challenges when navigating in dense foliage. Bats are not only able to achieve higher reliability in detecting narrow gaps but accomplish this with much lower spatial resolutions and data rates than those of man-made sensors. To study which sensory information is accessible to bat biosonar for detecting passageways in foliage, a robot consisting of a biomimetic sonar and a camera system has been used to collect a large number of echoes and corresponding images (∼130k samples) from an artificial hedge constructed in the laboratory and various natural foliage targets found outdoors. We have applied a conventional energy approach which is widely used in engineered sonar but is limited by the biosonar's wide beamwidth and only achieves a foliage-versus-passageway classification accuracy of ∼70%. To deal with this situation, a deep-learning approach has been used to improve performance. Besides that, a transparent AI approach has been applied to overcome the black-box property and highlight the region of interest of the deep-learning classifier. The results achieved in detecting passageways were closely matched between the artificial hedge in the laboratory setup and the field data. With the best classification accuracy of 97.13% (artificial hedge) and 96.64% (field data) by the deep-learning approach, this work indicates the potential of exploring sensory information based on clutter echoes from complex environments for detecting passageways in foliage.

GaInN has the potential to revolutionize the solar cell industry, enabling higher efficiency solar cells with its wide bandgap range spanning the entire solar spectrum. However, material quality issues stemming from the large lattice mismatch between its binary endpoints and questionable range of p-type doping has thus far prevented realization of high efficiency solar cells. Nonetheless, amorphous and multi-crystalline forms of GaInN have been theorized to exhibit a defect-free bandgap, enabling GaInN alloys at any indium composition to be realized. But the range of possible p-type doping has not yet been determined and no device quality material has been demonstrated thus far. Nonetheless, a Schottky barrier design (to bypass the p-type doping issue) on single-crystal GaInN can be used to provide some insight into the future of amorphous and micro-crystalline GaInN Schottky barrier solar cells. Through demonstration of a functional single crystalline GaInN Schottky barrier solar cell and comparison of the results to the best published reports for more conventional p-i-n GaInN solar cells, this work aims to establish the feasibility of amorphous and multi-crystalline GaInN solar cells. / Ph. D.

Realization of low-resistance electrical contacts on both sides of a solar cell is essential for obtaining the best possible performance. A key component of a solar cell is a metal contact on the illuminated side of the cell which should efficiently collect carriers. These contacts can be formed using an opaque metal grid/finger pattern. The metal electrode may be used alone or in combination with a broad-area transparent conductive film. This work aims at investigating the impact of the electrical contacting scheme employed in InGaN/GaN Schottky barrier solar cells on their performance. InGaN is a III-V compound semiconductor and has a tunable direct band-gap (0.7 eV to 3.4 eV) which spans most of the solar spectrum; this fact, along with other beneficial material properties, motivates the study of InGaN photovoltaic devices. A number of groups have recently investigated InGaN-based homo-junction and hetero-junction p-i-n solar cells. However, very few groups have worked on InGaN Schottky solar cells. Compared to p-n junctions, Schottky barrier solar cells are cheaper to grow and fabricate; they are also expected to improve the spectral response because of near surface depletion regions in the shorter wavelength regions. In this particular work on InGaN based solar cells, a Schottky diode structure was used to avoid the issue of highly resistive p-type InGaN. In this study, platinum (Pt) is used to form a Schottky barrier with an InGaN/GaN absorber region. Electrical and optical properties of platinum films are investigated as a function of their thickness. InGaN/GaN Schottky solar cells with platinum as the transparent conductive film are reported and their performance is evaluated as a function of the metal thickness. / Master of Science

In dieser Arbeit wurden einerseits die elektronischen Eigenschaften von Graphenen und andererseits die Verwendung von Graphenen und Kohlenstoff-basierten Hybridmaterialien als transparente Elektroden untersucht. Entsprechend ist der erste, umfangreichere Teil der Arbeit Grundlagen-orientiert und fokussiert auf die elektrostatische Wechselwirkung zwischen Graphen und dem Substrat Glimmer. Der zweite, kleinere Teil befasst sich mit der Entwicklung leitfähiger Tinten auf der Basis von Graphenen und anderen Kohlenstoff-basierten Hybridmaterialien für Anwendungen in der druckbaren Elektronik, insbesondere für die Herstellung transparenter Elektroden. Graphen auf Glimmer ist ein sehr wohldefiniertes System, in dem das Graphen über mehrere Quadratmikrometer atomar flach ist. Schichtdickenabhängige Variationen des Oberflächenpotentials von einzel- und mehrlagigen Graphenen auf Glimmer wurden mittels Kelvin Probe Rasterkraftmikroskopie untersucht. Damit konnte die elektrostatische Abschirmlänge von Graphen auf Glimmer bestimmt werden. Lokale Variationen des Oberflächenpotentials innerhalb einer Graphenlage, verursacht durch eingeschlossene Wasserschichten zwischen Graphen und Glimmer, wurden mit Rasterkraftmikroskopie, elektrostatischer Rasterkraftmikroskopie und der Raman-Spektroskopie untersucht. Dies ermöglichte es, die Dotierung von Graphen durch eingeschlossene Wasserschichten zu quantifizieren. Außerdem wurde gezeigt, dass Graphen auf molekular modifiziertem Glimmer lokal auf der Nano-Skala dehnbar ist. Dabei wurde der Glimmer durch das Aufbringen von dendronisierten Polymeren verschiedener Generationen auf Nanometer-Skala modifiziert. Dies eröffnet neue Möglichkeiten, die lokalen elektronischen Eigenschaften von Graphen durch Dehnung zu kontrollieren.Schließlich wurden Kohlenstoff-basierte leitfähige Tinten hergestellt, daraus transparente Elektroden hergestellt, und die Formulierungen der Tinten für das Drucken auf Plastiksubstrate optimiert. / This work focusses on the electronic properties of graphene on the one hand, and on the application of graphenes and other carbon-based hybrid materials for transparent electrodes on the other hand. Accordingly, the first part of the work, which is the larger one, is of fundamental nature and focusses on the electronic interaction between graphene and mica as a substrate. The second, smaller part deals with the design of novel conductive inks based on graphene and other carbon-based hybrid materials for applications in printed electronics, in particular for the production of transparent electrodes. Graphene on mica is a very well defined system, which provides atomically flat graphene extending over several square micrometers. Layer-dependent surface potential variations of single and few layered graphenes on mica were probed with Kelvin Probe Force Microscopy. This allowed to estimate the screening length of graphene on mica. Local variations of the surface electrostatic potential above single layer graphene, originating from confined fluid interfacial monolayers of water between the mica and the graphene, were monitored with Scanning Force Microscopy, Electrostatic Scanning Force Microscopy and Raman spectroscopy. This allowed to quantify the doping of graphene by the confined water layers. Exfoliation of graphene onto adsorbed nanostructures on mica allowed to control the strain of graphene at the nano-scale. Nanostructuring was achieved by first coating mica with submonolayers of dendronized polymers of different generations and subsequently depositing graphene. This approach provides new opportunities for the control of the electronic properties of graphene by strain.Finally, novel conducting carbon-based inks were designed and transparent electrodes were fabricated therefrom. The formulations of the inks were optimized for printing on plastic substrates.

Les travaux de recherche effectués lors de cette thèse s'articulent autour de matériaux graphène. Une méthode est développée pour produire graphène en masse avec solution de graphenure. Les études effectuées les solutions de graphenure sont basées sur les composés d'intercalation du grpahite (GICs) synthétisé avec du potassium et l'exfoliation de GIC dans un solvant organique. Différentes techniques d'analyse ont été employées pour caractériser les graphène produits. Afin de tirer parti des propriétés électriques du graphène, les solutions de graphenure ont ensuite été utilisées pour produire des films transparents conducteurs. Des traitements de recuit à sous atmosphère d'argon ont été effectués pour améliorer les propriétés électriques du film. Les résultats de caractérisation montrent que l'élimination des groupes fonctionnels contenant des atomes d'oxygène et l'amélioration structurale peuvent largement améliorer les propriétés électriques des films de graphène avec ce traitement de recuit. / The graphene is promising materials in future industrial applications due to its excellent properties. In recent years, different production methods have been developed in order to pave the way for applications. One topic of this thesis focuses on graphenidesolutions, which provide an efficient route to produce graphene. Using this method, graphite intercalation compounds(GICs)can be exfoliated into negativelz charged grapheme organic solvent under inert atmosphere. Withits high conductivity and bendable feature, one of the promising applications of graphene is flexible transparent conductive films. The second main topic of this thesis consists in applying produced graphene to produce transparent conductive films.With mild thermal treatments, the electrical properties of graphene film can be largely improved.

Transparent oxide semiconductors (TOSs) are materials that exhibit electrical conduction and optical transparency. The traditional applications of these materials are transparent conducting oxides in flat-panel displays, light-emitting diodes, solar cells, and imaging sensors. Recently, significant research has been driven to extend state-of-the-art applications such as thin-film transistors (TFTs). A new and rapidly developing field is emerging, called transparent electronics. This thesis advances transparent electronics through developing a new technique to fabricate TOSs and demonstrating their applications to active semiconductor devices such as diodes and TFTs. Ion beam assisted evaporation (IBAE) is used to deposit two common TOSs: zinc oxide (ZnO) and indium oxide (In2O3). The detailed material study is carried out through various characterization of their electrical properties, chemical composition, optical properties, crystal structure, intrinsic stress, topology, and morphology, as well as an investigation of thin-film property as a function of the deposition parameters: ion flux and energy, and deposition rate. The study proves that IBAE technique provides the capability for fabricating TOSs with controllable properties. By utilizing the newly developed semiconducting ZnO, p-NiO/i-ZnO/n-ITO and n-ITO/i-ZnO/p-NiO heterostructure photodiodes with a low leakage are proposed and assessed. Analysis of their current-voltage characteristics and current transient behaviour reveals that the dominant source of leakage current stems from the deep defect states in the intrinsic zinc oxide layer, where its dynamic response at low signal levels is limited by the charge trapping. The exploration of the photoconduction mechanism and spectral response confirms that such photodiodes are potentially applicable for ultraviolet (UV) sensors. The comparative study of both device structures provides further insights into the leakage current mechanisms, p-i interface properties, and quantum efficiency. Secondly, with the novel semiconducting In2O3, TFTs are fabricated and evaluated. The device performance is optimized by addressing the source/drain contact issue, lowering the intrinsic channel resistance, and improving the dielectric/channel interface. The best n-channel TFT has a high field-effect mobility of ~30 cm^2/Vs, a high current ON/OFF ratio of ~10^8, and a sub-threshold slope of 2.0 V/decade. More important, high-performance indium oxide TFTs here are integrated with the silicon dioxide and silicon nitride gate dielectrics by conventional plasma-enhanced chemical vapour deposition, which makes indium oxide TFT a competitive alternative for next generation TFTs to meet the technical requirements for flat-panel displays, large area imager arrays, and radio frequency identification tags. The stability study shows that indium oxide TFTs are highly stable with a very small threshold voltage shift under both a long-term constant voltage and long-term current stress. The dynamic behaviour indicates factors that affect the operation speed of such TFTs. A descriptive model is proposed to link the material properties and the processing issues with the device performance to facilitate further research and development of TOS TFTs. The research described in this thesis is one of the first investigations of the fabrication of TOSs by the IBAE and their applications to a variety of thin-film devices, particularly UV sensors and TFTs.

Transparent oxide semiconductors (TOSs) are materials that exhibit electrical conduction and optical transparency. The traditional applications of these materials are transparent conducting oxides in flat-panel displays, light-emitting diodes, solar cells, and imaging sensors. Recently, significant research has been driven to extend state-of-the-art applications such as thin-film transistors (TFTs). A new and rapidly developing field is emerging, called transparent electronics. This thesis advances transparent electronics through developing a new technique to fabricate TOSs and demonstrating their applications to active semiconductor devices such as diodes and TFTs. Ion beam assisted evaporation (IBAE) is used to deposit two common TOSs: zinc oxide (ZnO) and indium oxide (In2O3). The detailed material study is carried out through various characterization of their electrical properties, chemical composition, optical properties, crystal structure, intrinsic stress, topology, and morphology, as well as an investigation of thin-film property as a function of the deposition parameters: ion flux and energy, and deposition rate. The study proves that IBAE technique provides the capability for fabricating TOSs with controllable properties. By utilizing the newly developed semiconducting ZnO, p-NiO/i-ZnO/n-ITO and n-ITO/i-ZnO/p-NiO heterostructure photodiodes with a low leakage are proposed and assessed. Analysis of their current-voltage characteristics and current transient behaviour reveals that the dominant source of leakage current stems from the deep defect states in the intrinsic zinc oxide layer, where its dynamic response at low signal levels is limited by the charge trapping. The exploration of the photoconduction mechanism and spectral response confirms that such photodiodes are potentially applicable for ultraviolet (UV) sensors. The comparative study of both device structures provides further insights into the leakage current mechanisms, p-i interface properties, and quantum efficiency. Secondly, with the novel semiconducting In2O3, TFTs are fabricated and evaluated. The device performance is optimized by addressing the source/drain contact issue, lowering the intrinsic channel resistance, and improving the dielectric/channel interface. The best n-channel TFT has a high field-effect mobility of ~30 cm^2/Vs, a high current ON/OFF ratio of ~10^8, and a sub-threshold slope of 2.0 V/decade. More important, high-performance indium oxide TFTs here are integrated with the silicon dioxide and silicon nitride gate dielectrics by conventional plasma-enhanced chemical vapour deposition, which makes indium oxide TFT a competitive alternative for next generation TFTs to meet the technical requirements for flat-panel displays, large area imager arrays, and radio frequency identification tags. The stability study shows that indium oxide TFTs are highly stable with a very small threshold voltage shift under both a long-term constant voltage and long-term current stress. The dynamic behaviour indicates factors that affect the operation speed of such TFTs. A descriptive model is proposed to link the material properties and the processing issues with the device performance to facilitate further research and development of TOS TFTs. The research described in this thesis is one of the first investigations of the fabrication of TOSs by the IBAE and their applications to a variety of thin-film devices, particularly UV sensors and TFTs.

Ce travail porte sur la réalisation d'un micro-écran OLED haute luminance sur silicium. L'efficacité limitée des structures WOLED associées à des filtres colorés est un frein au développement de cette technologie pour des applications dans des dispositifs de type « see-through ». Nous proposons une approche tirant parti de l'effet de microcavité optique présent dans les écrans OLED à émission vers le haut pour générer des couleurs sans filtres. Les modulations de cavité à l'échelle du sous-pixel étant assurées par l'insertion d'oxyde transparent conducteur entre l'anode et l'OLED.L'étude offre selon un raisonnement cohérent de suivre les différentes phases de la réalisation d'un démonstrateur de ce type. Seront abordées dans la première partie les étapes technologiques de structuration de l'oxyde à l'échelle d'un sous-pixel de 16µm². Nous traiterons ensuite du développement d'un empilement OLED tandem utilisant des émetteurs fluorescent et phosphorescents. Une approche par simulation optique sera utilisée pour l'optimisation de cette architecture à un fonctionnement sur microcavité. Puis la discussion autour de la mise en commun des blocs technologiques précédents permettra d'aborder des écueils spécifiques au micro-écran OLED et de proposer des pistes de résolution. / This study focuses on the realization of a high brightness OLED micro-écran on silicon. The limited efficiency of White-OLED combined with color filters prevents the use of this technology in “see-through” applications. We propose a novel approach getting benefits from the optical micro-cavity effect in Top-Emitting OLED to generate colors without using color filters. Cavity modulations at a sub-pixel scale are realized by using a Transparent Conducting Oxide between the anode and the OLED.Following a step-by-step reasoning the work offers to follow all the phases of the realization of a prototype using this principle. In the first part, the technological steps of the processing of oxide cavities with a surface of 16µm² will be discussed. Then we will work on the development of a tandem OLED structure using both fluorescent and phosphorescent emitters optimized for micro-cavities. To this end optical simulation will be used. The two technological blocs will finally be put together to enlighten some issues specific for micro-écran technology and to give some clues to solve them.