The expansion of the market for wireless communications and sensors has led to the recent increase in demand for highly integrated MMICs for millimetre-wave wireless applications. These applications require MMICs that offer low cost, high integration, high functionality and high performance as well as simpler, more rapid development. An effective way of meeting these requirements and realising highly integrated MMICs is by employing multilayer three-dimensional (3-D) MMIC technology. The research work described in this thesis presents the modelling and characterisation of newly developed passive components such as coplanar waveguides (CPWs), thin-film microstrips (TFMSs) and transition transmission line structures using 3-D multilayer technology. These structures have been developed with low losses in mind, along with variable characteristic impedances and miniaturised size. With the knowledge obtained from the design and optimisation of CPW and TFMS transmission lines, new and improved compact CPW-to-TFMS transitions have been successfully achieved. Accurate electromagnetic modelling was carried out using the 2.5-dimensional ADS Momentum simulator. Newly improved fabrication techniques were employed to produce reported compact microwave components and circuits, in order to lower cost and simplify the process. Compact MMIC components were fabricated using a seven-layer fabrication procedure on semi-insulating GaAs substrate where pseudomorphic high electron mobility transistors (pHEMTs) pre-fabricated by the manufacturer. High frequency on-wafer RF measurements were carried out using Agilent 8510 series vector network analysers (VNAs). In-depth analysis and comparisons between the simulated and measured results are provided. Analysis of active MMIC components was achieved by developing small-signal equivalent circuits of the GaAs pHEMTs, and knowledge extracted from this analysis was employed in the development of large signal models of the pHEMT devices. Furthermore, the design and characterisation of a few MMIC circuits, such as limiters and amplifiers, demonstrates the integration of multilayer CPW passive components with prefabricated pHEMTs. These components are compatible with RF systems-on-chip sub-systems providing low cost, low loss performance with their ease of fabrication.

L’objectif de cette thèse était d’étudier la possibilité de compromissions électromagnétiques par le rayonnement des lignes d’énergie électrique. Nous nous sommes concentrés sur deux cas de figure. Le premier, à l’intérieur de bâtiments, concerne la technologie CPL « courant porteur en ligne ». Le second, en extérieur, sur un site industriel, porte sur la problématique de la propagation et du rayonnement de signaux caractéristiques d’un matériel industriel.Dans le premier cas, nous détaillons les mécanismes mis en jeu dans le rayonnement des câbles « basse tension » (BT) par la mesure et la modélisation. On distingue deux canaux de propagation. Le premier « filaire » correspond au canal usuel d’une liaison CPL entre deux modems. Le second est le canal « sans fil », qui correspond à la réception, aux bornes d’une antenne magnétique, d’une injection en mode différentiel sur 2 fils du câble. Ces deux types de canaux de propagation sont caractérisés expérimentalement, ce qui permet de déduire grâce à un logiciel les possibilités de décodage de l’information portée par le rayonnement compromettant. Dans le second cas, qui prend en compte les lignes « haute tension » (HT), nous considérons les fonctions de transfert successives du courant, associées à la propagation sur les lignes BT, au passage du transformateur BT/HT, puis à la propagation sur les lignes HT. Des modélisations ont été réalisées à l’aide du logiciel commercial EMTP, dont les résultats ont été comparés aux mesures expérimentales obtenues au cours de ce travail. Le rayonnement des lignes HT est abordé tant d’un point de vue théorique qu’expérimental. / The aim of this thesis was to study the possibility for power lines to emit compromising electromagnetic radiations. We focused on two cases. The first one was inside buildings, on PLC “Power Line Communication”. The second one was outside, on an industrial site, and about propagation and radiation of the characteristic signals produced by an industrial material.In the first case, we describe, measuring and modeling, the mechanisms involved in radiation cable Low Voltage (LV). There are two channels of propagation. The first is "wired" and the usual channel of a PLC link between two modems. The second channel is "wireless", which corresponds to the reception, across a magnetic antenna, of a differential injection of 2 lines of the cable. Both types of propagation channels are characterized experimentally, which enable us to deduced, using software, the decoding possibility of the information carried out by the compromising emanations.In the second case, considering "High Voltage" (HV) lines, we studies the successive transfer functions of the current associated with the spread on LV lines, crossing the transformer LV / HV, and then spreading on HV lines. Modelings were performed using commercial software EMTP. The results were compared with experimental measurements obtained during our study. HV lines radiations are discussed both from a theoretical and an experimental perspective.

This thesis focuses on the imaging of nanomagnetic systems using Lorentz transmission electron microscopy (LTEM), through which their complex structure, magnetisation processes and interactions may be understood. Magnetic domain walls (DWs) in cylindrical and square nanowires, that possess a complex three dimensional structure, were studied initially. Micromagnetic simulations were used to characterise the structure and stability of cylindrical nanowires and nanotubes across a range of dimensions thus calculating a phase diagram. The phase diagram showed that the less well known asymmetric transverse DW is a metastable state. Square nanowires were fabricated using electron beam lithography and imaged using LTEM. Three different DW types were identified, which were shown to be metastable states. The effect of anti-symmetric exchange induced at the interface of a magnetic thin film, the Dzyaloshinkii-Moriya interaction (DMI), was simulated for magnetic vortices in permalloy (Py) discs. The results were used to assess methods by which its effect could be measured. The DMI was found to cause the magnetisation around the vortex core to become divergent, depending on its strength. In addition, the DMI modified the magnetisation processes of the disc, causing the vortex to be more easily expelled with increasing DMI strength. By depositing Py thin films capped with Pt and a control sample with Cu, the vortex expulsion field was determined for both samples using LTEM in-situ magnetising experiments. This allowed the strength of DMI to be estimated. Finally, the magnetostatic interactions of an array of nanomagnets, a variation of artificial spin ice, was investigated. By using LTEM to image the entire array, a reversal sequence was obtained. This showed that the array behaved in a similar manner to a ferromagnetic thin film with strong anisotropy, since the net moment of the array preferred to be directed along the same direction. Micromagnetic simulations revealed that, contrary to artificial spin ice systems, the system did not display frustration. Further simulations suggested that the behaviour of the array could be changed from ferromagnetic to anti-ferromagnetic by reducing the space between elements.

Progress towards large format, background limited detector arrays in and around the terahertz or sub-millimetre region of the electromagnetic spectrum has – until recently – been hampered by the complexities in fabrication and cryogenic electronic readout associated with increasing pixel counts. Kinetic inductance detectors or KIDs are a superconducting pair breaking detector technology designed to overcome these complexities. Traditionally, KID arrays have been developed for imaging in astronomy. However, the high sensitivities, broadband responses, fast time constants and high detector counts that are achievable – along with the simplicity of fabrication and readout compared to other contemporary technologies – make them suitable (and in fact desirable) for a variety of applications. This thesis documents the development of a concept instrument to demonstrate KID technology for general purpose imaging applications. Specifically, I present the design, construction and performance of a near background limited, quasi-video rate, passive imaging system based on arrays of Aluminium lumped-element KIDs. The camera operates in two atmospheric windows at 150 GHz (2 mm) and 350GHz (850 μm) with 60 and 152 pixels, respectively. Array fabrication was achieved with a single photolithographic cycle of thin film deposition, patterning and etching. Full array readout is with a single cryogenic amplifier and room temperature FPGA based frequency domain multiplexing electronics. The camera is the first of its kind in applying KID arrays to imaging systems outside of pure astrophysics research and is the result of efforts from the staff and students of the Astronomy Instrumentation Group (AIG) in the School of Physics and Astronomy with support from QMC Instruments Ltd. The system exemplifies the AIG’s world-leading expertise in the development of far-infrared/sub-mm instrumentation as well as QMCI’s vision to provide the highest quality terahertz optical components and detector systems to the global marketplace.

A low frequency, waveguide array transducer, for operation in hostile environments, is studied and optimised for operation in fluids. The design consists of multiple stainless steel, rectangular cross-section strips which are used to support Lamb-like guided waves, which with appropriate delays allows the steering of the emitted beam. Wave propagation within the waveguide strips is discussed and the effect of the strip geometry on the supported wave modes is studied using comprehensive finite element modelling that is validated experimentally. Deviations from Lamb wave behaviour is observed due to coupling that occurs across the finite width of the strip, leading to dispersive behaviour that is slightly different to that of Lamb waves in a plate of the same thickness. As a result of this study, suggestions are made for modifications to the waveguide geometry that may favourably change this dispersive behaviour, over a desired frequency range. The effect of thermal gradients on the propagation of ultrasonic waves within the waveguide strips is also studied. Using Lamb waves as a basis for the analysis, general trends in the wave behaviour were identified before a series of experiments were conducted to demonstrate similar effects in the waveguide strips. Computational fl uid dynamics models were also used to study the heat distribution within the waveguide strips of the transducer to allow the in uence of these effects in a practical application to be assessed. Finally, the phased array capabilities of the strip waveguide array transducer were demonstrated. Initially, finite element modelling was conducted to allow the optimisation of the array geometry before the construction of a prototype. Using this prototype and a custom low frequency phased array controller, experimental steering of the beam emitted from the transducer was demonstrated up to angles of 45°.

Resonant tunnelling diode (RTD) is known to be the fastest electronics device that can be fabricated in compact form and operate at room temperature with potential oscillation frequency up to 2.5 THz. The RTD device consists of a narrow band gap quantum well layer sandwiched between two thin wide band gap barriers layers. It exhibits negative differential resistance (NDR) region in its current-voltage (I-V) characteristics which is utilised in making oscillators. Up to date, the main challenge is producing high output power at high frequencies in particular. Although oscillation frequencies of ~ 2 THz have been already reported, the output power is in the range of micro-Watts. This thesis describes the systematic work on the design, fabrication, and characterisation of RTD-based oscillators in microwave/millimetre-wave monolithic integrated circuits (MMIC) form that can produce high output power and high oscillation frequency at the same time. Different MMIC RTD oscillator topologies were designed, fabricated, and characterised in this project which include: single RTD oscillator which employs one RTD device, double RTDs oscillator which employs two RTD devices connected in parallel, and coupled RTD oscillators which combine the powers of two oscillators over a single load, based on mutual coupling and which can employ up to four RTD devices. All oscillators employed relatively large size RTD devices for high power operation. The main challenge was to realise high oscillation frequency (~ 300 GHz) in MMIC form with the employed large sized RTD devices. To achieve this aim, proper designs of passive structures that can provide small values of resonating inductances were essential. These resonating inductance structures included shorted coplanar wave guide (CPW) and shorted microstrip transmission lines of low characteristics impedances Zo. Shorted transmission line of lower Zo has lower inductance per unit length. Thus, the geometrical dimensions would be relatively large and facilitate fabrication by low cost photolithography. A series of oscillators with oscillation frequencies in the J-band (220 – 325 GHz) range and output powers from 0.2 – 1.1 mW have been achieved in this project, and all were fabricated using photolithography. Theoretical estimation showed that higher oscillation frequencies ( > 1 THz) can be achieved with the proposed MMIC RTD oscillators design in this project using photolithography with expected high power operation. Besides MMIC RTD oscillators, reported planar antennas for RTD-based oscillators were critically reviewed and the main challenges in designing high performance integrated antennas on large dielectric constant substrates are discussed in this thesis. A novel antenna was designed, simulated, fabricated, and characterised in this project. It was a bow-tie antenna with a tuning stub that has very wide bandwidth across the J-band. The antenna was diced and mounted on a reflector ground plane to alleviate the effect of the large dielectric constant substrate (InP) and radiates upwards to the air-side direction. The antenna was also investigated for integration with the all types of oscillators realised in this project. One port and two port antennas were designed, simulated, fabricated, and characterised and showed the suitability of integration with the single/double oscillator layout and the coupled oscillator layout, respectively.

Small scale fabrication processes have led to the advent of very thin flexible devices such as RFID tags, flexible PCBs and smart clothing. In a geometrical sense, these present themselves as curved two dimensional surfaces embedded in a three dimensional domain. When simulating electromagnetic behaviour on these surfaces at low frequencies, a full 3D field model is not always necessary. Using 3D algorithms to solve these problems can result in a large portion of the computer memory and runtime being used to mesh and simulate areas of the domain that present little electromagnetic activity. The theme of this thesis is concerned with the improvement of the runtime and memory consumption of electromagnetic simulations of these surfaces. The main contributions of this work are presented as an investigation into the feasibility of applying a 2D Unstructured Transmission Line Modelling method (UTLM) simulation to open, curved surfaces embedded in 3D space, by providing a one-to-one mapping of the geometry to a 2D flat plane. First, an investigation into the various methods of how a computer represents unstructured meshes in its memory is presented, and how this affects the runtime of the simulation. The underlying mesh data structures used to represent the geometrical problem space can have a huge impact on the efficiency and memory consumption of the simulation. This investigation served to demonstrate that it is not just simply the optimisation of the simulation algorithms that facilitate improvements to the runtime and memory consumption of a simulation. How a computer understands the connectivity of the mesh can have far greater impacts to the computational resources available. The concepts of surface parameterisation are then introduced; a process of mapping curved surfaces embedded in a three dimensional domain to a flat two dimensional plane. By providing a one-to-one mapping of the geometry from the 3D domain to the 2D flat plane, a low frequency 2D unstructured TLM simulation can be applied, negating the need for 3D algorithms. Because this mapping is one-to-one, the results of the simulation can then be mapped back to 3D space for visualisation. Parameterisations will almost always introduce distortion to angle and area, and minimising this distortion is paramount to maintaining an accurate simulation. Test cases were used to measure the extent of this distortion, and the investigation concluded that Angle Based Flattening (ABF) and Least Squares Conformal Mapping (LSCM) methods resulted in the best quality parameterisations. Simulations were then conducted on these test cases as a demonstration of how UTLM can be performed on 2D surfaces, embedded in a 3D domain.

Les matériaux semi-conducteurs à grand gap tels que le carbure de silicium (SiC) et le nitrure de gallium (GaN) sont utilisés pour fabriquer des composants semi-conducteurs de puissance, qui vont jouer un rôle très important dans le développement des futurs systèmes de conversion d'énergie. L'objectif est de réaliser des convertisseurs avec de meilleurs rendements énergétiques et fonctionnant à haute température. Pour atteindre cet objectif, il est donc nécessaire de bien connaître les caractéristiques de ces nouveaux composants afin de développer des modèles qui seront utilisés lors de la conception des convertisseurs. Cette thèse est donc dédiée à la caractérisation et la modélisation des composants à grand gap, mais également l'étude des dispositifs de mesure des courants des commutations des composants rapides. Afin de déterminer les caractéristiques statiques des composants semi-conducteurs, une méthode de mesure en mode pulsé est présentée. Dans le cadre de cette étude, une diode SiC et un JFET SiC "normally-off" sont caractérisés à l'aide de cette méthode. Afin de mesurer les capacités inter-électrodes de ces composants, une nouvelle méthode basée sur l'utilisation des pinces de courant est proposée. Des modèles comportementaux d'une diode Si et d'un JFET SiC sont proposés en utilisant les résultats de caractérisation. Le modèle de la diode obtenu est validé par des mesures des courants au blocage (recouvrement inverse) dans différentes conditions de commutation. Pour valider le modèle du JFET SiC, une méthode de mesure utilisant une pince de courant de surface est proposée. / Compared to traditional silicon (Si) semiconductor material, wide bandgap (WBG) materials like silicon carbide (SiC) and gallium nitride are gradually applied to fabricate power semiconductor devices, which are used in power converters to achieve high power efficiency, high operation temperature and high switching frequency. As those power devices are relatively new, their characterization and modeling are important to better understand their characteristics for better use. This dissertation is mainly focused on those WBG power semiconductor devices characterization, modeling and fast switching currents measurement. In order to measure their static characteristics, a single-pulse method is presented. A SiC diode and a "normally-off" SiC JFET is characterized by this method from ambient temperature to their maximal junction temperature with the maximal power dissipation around kilowatt. Afterwards, in order to determine power device inter-electrode capacitances, a measurement method based on the use of multiple current probes is proposed and validated by measuring inter-electrode capacitances of power devices of different technologies. Behavioral models of a Si diode and the SiC JFET are built by using the results of the above characterization methods, by which the evolution of the inter-electrode capacitances for different operating conditions are included in the models. Power diode models are validated with the measurements, in which the current is measured by a proposed current surface probe.

Les nanocristaux semi-conducteurs possèdent des tailles qui se situent entre celles des molécules et des matériaux cristallins. Leurs propriétés physiques sont donc dominées par des effets de confinement quantique et par des états électroniques discrets. Une étude approfondie de leur structure électronique et en particulier de la localisation des porteurs de charge s’avère nécessaire pour pouvoir à plus long terme faire de l’ingénierie de structure de bande des hétérostructures semi-conductrices. La microscopie à effet tunnel est l’outil idéal pour imager et sonder les propriétés électroniques de nanocristaux. Le système peut être comparé à une jonction tunnel à doublé barrière tunnel (chapitre 1). Pour caractériser les effets de Coulomb dans des objets quantiques par spectroscopie tunnel (technique détaillée au chapitre 2), mes travaux de recherche ont tout d’abord porté sur un système modèle : une liaison pendante silicium, dont l’état de charge a pu être modifié de manière contrôlée (chapitre 3). Des nanocristaux cœur-coquille (PbSe/CdSe) à symétrie sphérique ont ensuite été étudiés (chapitre 4). Contrairement aux nanocristaux sans coquille, les expériences révèlent que le transport est dominé par le même type de porteurs de charge à polarisation positive et négative de la jonction. Ces mesures donnent également accès à l’énergie de charge des nanocristaux. Un régime de transport similaire est obtenu pour des nanobâtonnets constitués d’un cœur sphérique CdSe enfermé dans un bâtonnet de CdS (chapitre 5), démontrant la reproductibilité des phénomènes observés par l’hétérostructures cœur-coquille. / Semiconductor colloidal nanocrystals are quite attractive, because of their physical properties, such as discrete energy levels. However, devices prepared from semiconductor nanocrystals are still facing limitations due to a high environmental sensitivity of their organic shell. In order to increase their optical properties, core-shell nanocrystals have thus been synthesized. Scanning tunneling microscopy is the appropriate tool to image and probe the electronic properties of individual nanostructures and. This system can be compared to a double barrier tunnel junction, where the transport properties are governed by the transmission probability across both potential barriers (chapter 1). In order to investigate the Coulomb effect in those quantum objects by tunneling spectroscopy (this technique being described in chapter 2), the thesis has first focussed on a prototypical model: an isolated silicon dangling bond, where its charge state has been changed in a controlled manner (chapter 3). Then, PbSe/CdSe core-shell nanocrystals have been studied and a general method is described to correctly identify the electrical nature of the charge carriers in the tunneling spectra (chapter 4). In contrast to the core nanocrystals the transport through core-shell structures reveals, for a majority of nanocrystals, that the same type of charge carrier tunnel on both sides of the apparent gap. Charging peaks are also observed and allow the measurements of the charging energy in these systems. A similar transport regime is obtained for CdSe/CdS dot in rod nanocrystals (chapter 5), demonstrating the reproducibility of the characterized transport phenomena of nanoheterostructure.

In this thesis, the design, fabrication and characterisation of graphene electromechanical resonators have been presented. Graphene features ultrahigh Young’s modulus and large surface to volume ratio that make it ideal for radio frequency (RF) components, sensors and other micro/nano-electromechanical systems (MEMS/NEMS). A novel batch fabrication process for graphene electromechanical resonators has been developed by using poly-Si film as sacrificial layer. Previously reported fabrication processes of graphene resonators use SiO2 as sacrificial layer only because graphene is visible on 300nm SiO2/Si substrate. However, the wet etching of SiO2 involves HF, which is not compatible with metal connections or SiO2 serving as dielectric or passivation layer in graphene NEMS devices. Moreover, the liquid surface tension during drying after wet etching could damage graphene bridges even critical point drying is used. Therefore, in this work, poly-Si is adopted as the sacrificial material. To facilitate the fabrication of graphene resonators, a poly-Si/SiO2/Si substrate has been designed and optimised to make graphene visible under optical microscope for the first time to the author’s knowledge. Chemical vapour deposition (CVD)-grown monolayer graphene sheet has been transferred onto the optimised poly-Si/SiO2/Si substrate and patterned into strips. Metal electrodes have been deposited by lift-off process to make electrical connections, which is prerequisite for integrating graphene resonator into practical devices. The graphene bridges have been released by etching the poly-Si layer with XeF2 in vapour phase, which completely avoids the capillary force induced damage to the graphene bridges. De-fluorination process has been performed by hydrazine reduction to recover graphene’s conductivity. This fabrication process is scalable for massive production of graphene electromechanical resonators, thus furthering their practical application. One-source current mixing characterisation setup has been constructed to test the graphene resonators. Besides the fundamental peak, the activation and enhancement of the second mode of doubly clamped resonator by electrostatic actuation have been observed for the first time. The second mode amplitude reaches 95% of the fundamental mode, whereas only odd higher modes of small intensity have been reported before in other MEMS/NEMS resonators actuated by electrostatic force or magnetomotive force. The findings in this thesis could lead to substantial increase of the sensitivity of sensors based on the graphene resonators. Modal analysis based on Euler-Bernoulli equation has been performed to understand the mechanism behind the activation and enhancement of the second mode. Finite element analysis agrees very well with experimental results and complies with the theoretical model. Finally, a set of novel alignment marks has been designed, which can be incorporated to process mechanically exfoliated 2D material flakes of micron size and irregular shape with conventional photolithography tools, as have been demonstrated by the successful fabrication of a graphene transistor. This optical alignment technique provides an alternative for prototype device development besides electron beam lithography to prevent electron-induced damage to fragile 2D materials.