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With the increasing application of active-matrix organic light-emitting diode (AMOLED) displays which are basically current-driven devices, a current-driven switch with larger current capability and higher frequency operation will prove to be beneficial. The growth of the Internet of Things (IoT) also results in increasing demand for transparent and conformable electronics, which requires materials that are highly uniform in electrical properties over large areas. All these factors have contributed to the prominent interest in metal oxides for semiconductor devices in the recent years, due to their high uniformity, high carrier mobility and low resistivity as a disordered material, when compared to the alternatives such as amorphous and polycrystalline silicon and organic semiconductors. In addition, metal oxides can be fabricated at much lower temperatures compared to high-temperature polycrystalline silicon (HTPS) and yet still achieve reasonable performance, enabling them to be easily deposited on flexible substrates like plastics and paper. Metal oxides generally with their larger band gaps are hard to be doped both n- and p-type, so heterojunctions are the feasible way to construct bipolar devices. Extensive literature review indicates that most research was focused on nickel oxide and oxides of copper for p-type materials, and variants of zinc oxide for n-type materials. This work identifies that, among these materials, cuprous oxide (Cu2O) and amorphous zinc-tin oxide (a-ZTO) are great materials with potential to form a good heterojunction, but this combination has been mostly unexplored. The simple mathematic model presented in this work shows that the charge carrier injection is possible with the proposed p-n heterojunction configuration, an important design requirement for heterojunction bipolar transistors which is the long-term goal of this work, but extreme care should be exercised in forming the junction as its quality heavily affects the injection efficiency. There is a need for a good quality thin film diode of Cu2O/a-ZTO p-n heterojunction as it is an essential component for the realization of flexible large-area electronics. However, this Cu2O/a-ZTO diode initially showed poor rectification characteristics. A systematic study of the origins of the poor performance is performed based on several experiments and measurements on the metal-semiconductor junctions and the p-n heterojunction. A good choice of metal contacts is crucial, and the contacts should be Ohmic so that diode rectification is only controlled by the p-n heterojunction. Experiments suggests that molybdenum and gold are good Ohmic contacts to a-ZTO and Cu2O, respectively. Further investigation and analysis are targeted on the properties of the p-n heterojunction. The results suggests that multiple carrier trapping and thermal release of carriers in defect states stemming from oxygen vacancies and oxygen related atomic coordination at the heterojunction interface is the primary cause of poor rectification. It is demonstrated that a plasma treatment is the simplest yet most effective way to optimize the population of oxygen vacancies at the heterojunction interface based on extensive material analyses, allowing a significant improvement in the diode performance with a much-enhanced rectification ratio from ~20 to 10 000, and a consequent facilitation of the next-generation of ubiquitous electronics. Finally, a more in-depth analysis on the p-n junction model, with a focus on capacitance-voltage (C-V) characteristics, is presented, with discussions on analytical derivations of the model and analyses of simulated junctions with various doping profiles. Some junction parameters in this thesis are obtained using the classical C-V model which was derived for more ideal situations. Hence, it is important to understand the accuracy of such a simple model when applied on real scenarios that differ significantly from the usual, basic assumptions. The analytical model of a p-n junction, even in only one dimension, is severely limited due to the complexity in mathematical representation. The results indicate that numerical simulation is the practical way to further investigate p-n junction properties. It is shown that the classical C-V analysis, which is simple, may be sufficient in extracting p-n junction parameters such as depletion depth and doping profiles, provided that the method's limitations are understood. These findings allow readers further insight to the implementation challenges such as low rectification ratio of bipolar heterojunction thin film diodes made with Cu2O and a-ZTO, and future research direction for plausible solutions to these difficulties.

Monolayer graphene has been under the spotlight of research since its discovery in 2004, due to its unusual properties such as massless Dirac fermions, exceptional tensile strength, ultrahigh thermal conductivity, superior charge carrier mobility, remarkable optical properties etc. Graphene-based devices are expected to be promising building blocks in nanotechnology for a range of applications. However, it has no intrinsic band gap which means that graphene FETs will have a very low on/off ratio. Several groups have tried to introduce a band gap in graphene artificially, such as etching graphene into nanoribbons. This thesis suggests the possibility to modify graphene's band structure from appropriately engineered periodic potential patterns. Ferroelectrics is widely applied in everyday technologies, such as non-volatile memories and medical imaging, due to the presence of spontaneous polarisation in the material and that being reversible by an externally applied electric field. A ferroelectric material PZT (PbZrxTi1−xO3), is utilised in this work to produce 1D periodic potentials. A piece of single crystal thin film PZT undergoes domain engineering to create periodic potentials. With single layer graphene transferred on top, this structure qualifies as an 1D electrostatic graphene superlattice (EGSL), whose effect is measurable as a broadening of the current valley near the CNP point and variations in conductance on the gate sweep curve. A PZT-graphene device is designed, fabricated, and electrically characterised at different temperatures. The device size is kept around 300∼400nm to ensure coherence transport of electrons in the device. A unique room temperature fabrication process is developed and put in use. Evidence of a bandgap is observed, and the measurements prove to match theoretical predictions on the 1D electrostatic graphene superlattice effect.

Solar energy remains one of the most actively researched areas in the field of renewable energy. Harnessing light energy from the sun is an attractive solution because of its high abundance, for example, it is estimated that the sunlight received by 8 % of the earth's deserts is sufficient to meet the current global energy consumption.1 A promising alternative chemical fuel is hydrogen gas. This energy carrier can be used in fuel cells to power many applications and also as a chemical feedstock for producing commodity chemicals such as ammonia. One method for producing hydrogen is photocatalytic water-splitting which involves the use of light and a semiconductor photocatalyst to produce hydrogen from water. The main challenge is the low efficiency of the process. The research discussed in this thesis focusses on producing more efficient photocatalysts to provide a pathway for hydrogen fuel production. More specifically the project investigates the use of ultrasonic sonochemistry for the formation of nanostructured photoactive semiconductors, especially semiconductor heterojunctions, for light-driven hydrogen production. It was shown previously that ultrasonication can be used to improve the photoelectrochemical activity of ZnO nanoparticles by modifying the defect concentrations within the material.2 In this work, CdS and WO3 semiconductor photocatalysts were tested before and after ultrasonic treatment to observe if their photocatalytic activities could be enhanced in the same way. Analysis using XPS and UV-Vis absorption spectrometry suggested that the treatment introduced oxygen vacancies into WO3, however, the defects did not appear to affect the material's activity. Initially, WO3 and CdS were tested separately using methyl orange dye degradation and water-splitting experiments, before and after ultrasonication, to see if the treatment affected their activities. It was found that after ultrasonication the dye-degradation activity for WO3 remained relatively unchanged whereas the oxygen evolution activity increased after prolonged treatment. For CdS, the dye-degradation activity increased after ultrasonication whilst the hydrogen evolution activity declined after the treatment. Next, the materials were combined using ultrasonication to form CdS-WO3 composite materials based on a direct z-scheme heterojunction structure that could be used for complete water-splitting. The composites showed enhanced activity for dye degradation compared to the component materials on their own. All materials were found to produce hydrogen in the presence of hole scavengers with two materials, 15 and 60US-CdS-WO3, displaying higher activities than 15US-CdS and 60US-CdS and 15US-CdS + 15US-WO3 and 60US-CdS + 60US-WO3 physical mixtures. The results showed that formation of the composite enhanced the photocatalytic activity of the materials. g-C3N4 was explored as a substitute for CdS to increase the photostability of the composite material and also to experiment with organic-inorganic systems. The g-C3N4-WO3 materials were capable of producing hydrogen and the proportion of WO3 used to prepare the composites was found to have a significant effect on their hydrogen evolution activity. The results of this research have shown that ultrasonication can be used to i) modify the photocatalytic activities of CdS and WO3 ii) produce composite materials with enhanced photocatalytic activity.

The twin fields of superconducting circuits and circuit quantum electrodynamics now form the basis for a major part of the effort towards building a quantum computer. Yet many fundamental problems remain. These may range from very practical considerations, such as how to construct a qubit with a sufficiently long coherence time, to questions of how best to understand and model the complex nonlinear dynamics arising in superconducting circuits. In this thesis we take a broad look at these fields and explore many questions within them. We begin by studying critical slowing down in a dissipative phase transition of a coupled qubit-cavity system, before examining the underlying dynamics of switching between metastable states which causes this slowdown. We then examine an unexplained phenomenon of resonance narrowing in another qubit-cavity system and suggest it may also be related to metastable states. Finally, we examine a circuit which harnesses long range interactions, and present it as a promising candidate for building a qubit with a long coherence time.

Organic light-emitting diodes (OLEDs) have attracted a lot of attentions because of their high performance in display applications. Organic emitters are still being developed to improve efficiency, colour gamut and sustainability, and thermally activated delayed fluorescence (TADF) materials are widely regarded as one of the most promising next-generation OLED emitters. To date, green TADF emitters have been developed, and the corresponding OLEDs show high light-emitting efficiency and long operation lifetimes. However, the performance of red and blue TADF OLEDs still lag their green-emitting counterparts. For this reason, this work focuses on developing new red and deep-blue TADF materials. A group of red and blue TADF emitters were designed and synthesized. Their photophysical properties and electroluminescence performance were also studied. In addition, it was found that doping some of these new emitters into common host materials, such as 2,8-bis(diphenyl-phosphoryl)dibenzo[b,d]thiophene (PPT), 2,2',2"-(1,3,5-Benzinetriyl)-tris(1-phenyl-1-H-benzimidazole) (TPBi) or poly(methyl methacrylate) (PMMA), can lead to organic long-persistent luminescence (OLPL) lasting for thousands of seconds at room temperature. As traditional room-temperature OLPL materials are based on exciplex emitters, the new OLPL systems discovered here demonstrates that exciplex formation is not required for harvesting OLPL. This enables a wide range of host materials to be used including materials as simple as PMMA. Expanding this concept further, the author developed a method for large-scale PMMA-based OLPL sample fabrication to take full advantage of its low expense. This method gives thick (> 2 mm) and clear OLPL products, and all the required equipment is easily accessed in lab condition. Combining the flexible design of TADF emitters and mature PMMA industry, this work opens the 'door' of large-scale, colour tuneable and cost-efficient room-temperature OLPL materials. At the same time, the light-emitting properties and mechanism of these new OLPL emitters were studied, which provides a guideline for further OLPL emitter improvements.

The power module is an important building block of a power electronic converter used in high power applications. It provides the placement of power semiconductor devices. One of the most important design constraints of power module is maximum junction temperature of semiconductor devices. Therefore, thermal modelling of the power module is important because it gives the maximum junction temperature of the devices. There are three different approaches to do the thermal modelling of the power module: analytical, finite element analysis and thermal network. In this work, the steady-state and transient thermal modelling of a rectangular N-layer structure with an arbitrary number of heat sources on the top surface is obtained by a Fourier series solution based on the separation of variables method. Although a typical power module does not exactly resemble an N-layer rectangular structure but it can be closely approximated as the latter. As the structure of power modules can be closely approximated as a rectangular N-layer structure. Various simplified structures are analyzed to understand the effects of structural approximation on the temperature field. The steady-state model (RNLF method) is compared with the finite-element method simulation, and an excellent matching (approximately maximum 1.00% temperature error) is found in the centres of the semiconductor dies for this case. Experimental temperature measurements taken at the surface of a commercial SiC power module are also presented demonstrating agreement in the centres of the dies to within 3.5%. The transient model is compared with a finite-element model and an excellent matching (less than 2% error) in the transient region is found in the centres of semiconductor dies. However, in the sub-transient region (this is the region where the rate of die temperature rise is much higher ), the error is higher (approximately 15% error) because of finite thermal conductivity of the semiconductor material (SiC is used here). An experimental validation is performed using a high frame-rate thermal camera. After the model outputs have been compensated to match the limited frequency response of the camera, a good agreement is found between model and experiment (less than 6% error). For the points where the rate of rise of temperature is relatively slow, this compensation is not necessary.

There is a pressing need for high-efficiency linear power amplifiers (PA) due to consumer demand for greater mobile data rates and battery lifetimes. The outphasing transmitter is one such technique that warrants research as it has yet to achieve commercial viability. It operates by decomposing a modulated signal into two constant-amplitude signals with a phase difference corresponding to the normalised envelope amplitude, amplifying them with high-efficiency PAs, and then summing them to reconstruct the envelope. Silicon is a promising semiconductor for outphasing as the signal processing circuitry and PA can reside on the same die however silicon PAs suffer from low output power due to the sub-1V supply voltages and lossy passive components. This thesis focuses on improving the design of CMOS outphasing PAs which utilise unisolated power combiners and switched-mode voltage-source PA stages. Power combiner design is advanced with the creation of two novel three-port coupled coils which allow multiple inductors to be merged into a single compact low-loss structure. Four Wilkinson power combiners were fabricated to demonstrate and evaluate the coupled coils with one combiner requiring only three components by harnessing resistive coil parasitics. The most popular voltage-source CMOS PA, the complementary Class-D, suffers from shoot-through current losses where the supply voltage is shorted to ground during switching. The short-circuit can be prevented by driving the two transistors with dead-time-separated voltages to stop them from switching simultaneously. To investigate this technique, a hard-switching CMOS PA with a novel driver that generates two dead-time-separated signals was designed, fabricated, and evaluated. A thorough analysis of the dead-time technique is presented, something hitherto absent in the literature. The third fabricated chip completes the work by pairing an unisolated coupled-coil power combiner with a switched-mode PA stage featuring an improved dead-time technique.

Zinc oxide is regarded as an attractive semiconductor alternative to the most commonly used silicon and GaAs owing to its abundancy, thermodynamic stability and the large variety of morphologies it can be grown into. Among these morphologies, nanowires (NWs) have gathered vast attention as an ideal research platform for new and enhanced functionalities of ZnO. One of these functionalities is the integration of ZnO into the renewable production of hydrogen from water splitting. In this thesis, the growth of ZnO NWs, its doping and surface functionalization are studied with the aim of developing highly efficient photoelectrochemical (PEC) water splitting photoanodes. Before delving in the functional application, a growth study of the ZnO NWs was necessary to understand the factors that control this growth. For this, a seed mediated chemical bath deposition (CBD) approached was explored in detail and adopted. Control over NW growth was obtained by tuning the seed layer deposition with two different techniques (atomic layer deposition and sol-gel processing) and by controlling the CBD parameters. This study demonstrated that NW diameter, length, growth orientation and crystallinity can be controlled by this approach. To modulate the optoelectronic properties, ZnO NWs were doped with two different transition metals, copper and cobalt. A detailed study of the optoelectronic properties of these doped-ZnO NWs revealed that the introduction of cobalt into the ZnO lattice considerably improved the optoelectronic properties of ZnO. This enhancement was induced by the introduction of traps states in the bandgap of ZnO prompted by the interaction between the sp orbitals of ZnO and the d orbitals of Co. In particular, a 1% nominal doping yielded the most promising results of this study. Further improvement of the ZnO properties towards PEC water splitting was achieved by functionalizing the surface of the NWs with iridium and a metal-organic framework the zeolitic imidazolate framework-8 (ZIF-8). The successful integration was demonstration by electron microscopic analysis that showed the control of this conformal surface functionalization. The integration of Co-doping and ZIF-8 functionalization resulted in a large enhancement of the PEC performance of the ZnO NWs, doubling the photogenerated current and the stability over time while also increasing the incident photon-to-current efficiency from 11% for ZnO NWs to 75% in the blue and ultraviolet region for ZnO:Co@ZIF-8 core-shell NWs.

This thesis presents a complete characterisation and an assessment of the technology readiness of a new Electron Multiplying Charge Coupled Devices (EMCCD) technology. Several factors of interest are studied here including, charge transfer efficiency, gain ageing and radiation effects from protons. Many light-starved and high-speed image applications (e.g. observation from space and automated visual inspection) can benefit from Time Delay Integration (\acrshort{tdi}) as it allows photoelectrons from multiple exposures to be summed in the charge domain with no added noise. Electron multiplication (EM) can be used to further increase the signal to noise ratio for extremely faint light signals. There is a growing demand for Complementary metal-oxide-semiconductor (\acrshort{cmos}) sensors with the same or greater functionality and even better performance. The research presented here analyses the functionality of a recently designed EMCCD in a CMOS process. This device (EMTC1) incorporates two novel EM pixel structures which enable high gain at relatively low voltages. The theory and architecture of Charge Coupled Devices (CCDs) and EMCCDs are discussed, providing a technical background for the results. Furthermore, the practical methods, including experimental techniques developed for this device's testing, are presented here. Ageing within the device is a primary focus within this thesis, as is the effect of proton irradiation. The effects of the radiation damage on parameters such as dark current and Charge Transfer Inefficiency (CTI) have been documented along with its effect on EM gain. These results have been corroborated with simulations of device operation in TCAD.

Direct growth of III-V infrared photodetectors on silicon substrates is a promising so- lution for realising low-cost and large-format infrared focal plane arrays. However, this heteroepitaxial growth technique will generate various defects due to the dissimilarities between III-V materials and Si. These defects can severely damage the performance of a detector. In this thesis, different III-V quantum structured infrared photodetectors directly grown on Si are investigated to understand how different structures react to the defects. The experimental chapters begin with reporting an InGaAs/GaAs quantum dot infrared pho- todetector (QDIP) on Si. By utilising a Si substrate with a high degree of offcut along with dislocation filter layers, antiphase domains have been eliminated and the threading dislocation density has been reduced by ∼4 orders of magnitude. The QDIP shows a dual-band photoresponse at 80 K. To reduce the noise, a sub-monolayer QD quantum cascade photodetector on Si was designed. This structure has led to a distinct reduction of dark current and noise, achieving a high operating temperature of 160 K. To further boost the quantum efficiency of infrared photodetectors on Si, InAs/GaSb type-II superlattice (T2SL) photodetectors were also studied in this thesis. Generally, T2SL photodiode structures are more sensitive to material defects than QDs. Moreover, the surface leakage current contributes to a high level of dark current. InAs/GaSb T2SL photodiodes and barrier detectors have been grown on GaAs and Si substrates. Transmission electron microscopy and X-ray diffraction results confirm that the strain energy has been released at the heteroepitaxy interface and the threading dislocation density has been reduced by ∼3 orders of magnitude. The bulk dark current has been reduced by implementing an nBp barrier design. As a result, a T2SL nBp detector on GaAs with surface passivation has been shown to be capable of operating at 190 K without external bias. The work described in this thesis shows that there is great potential to improve the detector performance by using novel detector designs. Future work should focus on structure optimisation, as well as material quality im- provements, in order to achieve both low dark current and high quantum efficiency. High- operating temperature detectors on Si can be attempted to further explore the potential of III-V quantum structured infrared photodetectors. Specific recommendations are made for candidate structures. In order to be compatible with the mainstream Si micro-electronics industry, fabrication on (001) Si substrates will be required. Research towards this objec- tive is therefore also proposed.

The work in this thesis is related to the strongly correlated electron interaction effects taking place within 2D and 1D electron systems in IIIV semiconductor devices. Initially the work conducted on GaAs/AlGaAs was to investigate the effects of incompressible/compressible strips on the transverse and longitudinal resistance. Effects due to these strips lead to resistance anomalies in the transverse resistance, when the incompressible strips are in the evanescent regime, as described by the screening theory. In the study described in this thesis, such anomalies are observed not just for integer states, but for fractional states as well, i.e v = 4/3, 3/2, 5/3, 8/3, 3, 10/3, 7/2 and 5, which have been predicted theoretically but not studied experimentally. Additionally, longitudinal resistance hysteresis was noticed which increases in size with lower-valued v and by increasing the constriction of the quasi-1D channel within the system. The relaxation times of the longitudinal resistance within these hysteretic areas were found to be linked to two mechanisms with τ_1 and τ_2 being in the order of a 102 s and 106 s. These hysteretic loops were discussed in terms of dynamic nuclear polarisation and Ising ferromagnetism and the screening theory. It is discussed that the latter provides a better t in the explanation of the hysteresis. Further studies were conducted using the GaAs/AlGaAs device, but instead the transverse voltage was measured while setting up the system in measuring the quasi-1D conductance. By varying the Magnetic fi eld it was found that various oscillations were measured indicating unusual features. These oscillations seem to be linked to the 's in the 2DEG regions and the change in the peaks height and position are linked to the constriction size within the quasi-1D channel. These effects seem to be related to the crossings of Landau levels. In addition when spin-polarisation is enhanced a set of peaks increase in size compared to others, which is why it is thought that they are linked to enhanced spin polarisation within the constriction. By applying source-drain bias it was found that peaks that seem to be linked to even fi lling factors tend to disappear with positive voltage bias but for negative voltage bias, peaks related to both odd and even filling factors seem to persist. This is explained by scattering being induced due to quasi-elastic inter-Landau-level scattering as well as through the spin gap model. The data from these two chapters will provide a better understanding on the physical phenomena taking place and how the Landau levels and electron-electron interactions affect the behaviour of the systems. Furthermore the oscillations observed at 3.25 T are thought that they could be linked to the Aharonov-Bohm effect. Finally an InGaAs/InAlAs device was used to study the interaction effects due to perpendicular magnetic field and spin-orbit interactions within a quasi-1D channel. While applying lateral voltage bias within the quasi-1D channel, the asymmetric voltage on the split-gates acts as a type of electric Stern-Gerlach apparatus inducing spin-splitting as well as Rashba spin-orbit-coupling (RSOC). As a consequence exotic phases occur which lead to fractional conductance states appearing within the system, which are enhanced by increasing the magnetic fi eld. Such states are the 5/2 and 12/5 fractional states which if proven to be non-Abelian can help in the creation of topological fault tolerant quantum computers. These fractional conductance states are thought to be a consequence of backscattering and umklapp scattering. Also some of the fractional conductance states noticed have been observed experimentally in other 2D systems, and the fact that fractional states like 3/2, 5/2 and 7/2 weaken and strengthen with certain perpendicular magnetic fields and asymmetric voltages within the system are explained by the RSOC inducing in-plane magnetic fi eld components in the system leading to similar behaviour observed in 2D systems in tilted B-fi eld setups. In the InGaAs/InAlAs system though more fractional states appear in higher sub-band levels, compared to the measurements conducted in other two-dimensional systems, which is thought to be due to the intrinsically stronger spin-orbit coupling InGaAs/InAlAs systems have. These fractional states would be highly valuable for spintronic devices and the construction of quantum computers by utilising lower perpendicular magnetic fi elds than what is required in conventional 2D systems.

This thesis covers research on the efficiency of green light emitting diodes (LEDs). The work presented focusses on the characterisation of conventional wurtzite (wz)-GaN based LEDs and novel zincblende (zb)-GaN based LEDs. The investigated wz-GaN based multiple quantum well structures have been found in other studies to have higher internal quantum efficiency (IQE) when the quantum wells (QWs) were grown at relatively high temperatures in MOVPE. In this thesis, more QW fluctuations were found as a result of using higher QW growth temperatures for green emitting material. QW fluctuations tended to localise the carriers away from defects, suppressing non-radiative recombination rate and this is hence a possible mechanism for improvement of the IQE of LED devices. Conventional wz-GaN based LEDs are limited in IQE by a strong in-build electric field when a large amount of indium is used to achieve green emission. Despite being a metastable phase, zb-GaN does not suffer from the same problem and exhibits good potential in green emission. A full MOVPE-grown zb-GaN LED structure was characterised in this thesis. The density of stacking faults (SFs) in the MOVPE-grown material is shown to be comparable to the state-of-art MBE-grown samples. A SF-induced alloy segregation effect was discovered in correlated structural and compositional characterisation and was further confirmed by the measurements in atom probe tomography (APT). Indium was found segregating next to the SFs whereas aluminium was found segregating to the SFs. Protruding surface features were produced by the growth of QWs and were suggested to correlate with SF bunches. Cathodoluminescence of a zb-GaN LED structure was characterised in both plan-view and in cross sections. Free exciton transition, donor-acceptor pair (DAP) emission and QW emission of 2.58 - 2.82 eV were observed in a Si doped layer, a Mg doped layer and QW layers, respectively. Low-energy QW emission peaks were also found in large wurtzite inclusions, which have been identified by in-depth correlated STEM and CL characterisation. The understanding given by this thesis on the luminescence and the defects of wz-GaN and zb-GaN based LEDs can provide useful suggestions for future growth strategies to achieve high efficiency green LEDs.

The recent research trends on the advanced display and smart lighting system are focused on the wide colour gamut, high colour purity, higher brightness, stability and flexibility with respect to the performance and form factors. Considering the above factors on the basis of the self-emitting light source candidates, a quantum dot light-emitting device (QD-LED) is strongly recommended for the next-generation display and lighting application. Herein, this thesis touches the studies on the science and technologies of QD-LEDs from the material, the process technology, the device design and architecture to the system integration and its characterization for the specified applications. Especially by scrutinizing QD material and its characteristics, unique fabrication technologies, various design of device architecture, and system level innovation, the QD-LEDs devices and systems are fabricated and intensively studied for the display and smart lighting applications. On the materials, Cd-embedded and Cd-free QD materials are used for QD-LED, and the electrical and optical analyses are studied from the materials level to the specific system level of QD-LEDs. With respect to the process level for the colour pixel patterning of QD-LED, unique printing technology called as transfer printing is used and its process technologies are intensively studied with respect to various materials, devices architecture, and system integration, in addition to the conventional process technology such as spin-coating. On the device perspective, the framework of design of device and structuring of various device architecture based upon passive-matrix (PM) based QD-LEDs and active-matrix (AM) based QD-LEDs with thin-film transistor (TFT) backplane is deeply studied for the desired full-colour display and smart lighting system. The control parameters for higher performance devices are extracted and studied by way of modelling the desired architecture and discovering unique and innovative fabrication. On the system, various QD-LED device architectures are embedded into the displays and smart lighting systems and analysed with respect to the PM and AM based systems. The transparent display system, the smart QD lightings system with mixed mode, stacked mode, and patterned mode pixel architecture, and the displays with PM and AM based mono and full-colour displays are integrated and deeply studied with electro-optical analysis. Especially, the transparent model of displays is considered to be a strong candidate to replace the top emission displays which require more difficult fabrication processing steps. The PM based smart QD lighting with the emission areas of 2 by 2-inch is studied with respect to the various pixel architectures. This smart QD lighting is also first demonstrated as textile lighting by conventional weaving technology, and this is considered to be a major engineering breakthrough in the area of systems with free form factor, free size-ability, and offering full flexibility such as rollable, bendable, and foldable applications. Finally, the various AM QD-LED display systems with Cd- or Cd-free red (R), green (G), and blue (B) pixels, are fabricated by way of spin coating and transfer printing and studied deeply. In order to enhance the functional properties of the display and lighting system, in this study, the TFT technology is implemented and optimised for large and higher pixelated display system using in-house research facilities. To achieve improved AM TFT backplane performance, In-Ga-Zn-O (IGZO) is used for thin-film transistor (TFT) channel material due to its ultra-thin channel and high electron mobility leading to fast turn-on current and low-temperature processability. Vacuum-deposited IGZO TFT demonstrates ON/OFF current ratio of higher than 109 for pixel control and a stiff subthreshold swing (SS) of 0.2 V decade-1 for fast switching behaviour. The developed TFT, low gate leakage current (<10 pA) with high mobility of 15 cm2 V-1 s-1 delivers sufficient current of hundredth of micro ampere to manipulate pixel colour brightness. Having this TFT backplane, multi-purpose AM QD-LEDs are integrated for the stable and scalable QD-LED. A unique transfer printing technology is implemented for RGB pixels with various device architecture on the TFT backplane. Finally, the 1.5-inch diagonal display composed of 120 x 90 x 3 colour pixels (120: horizon, 90: vertical, and 3 colours) with Cd and Cd-free based QDs and the IGZO TFT array backplane has been successfully integrated as full-colour displays. The world's first display with Cd-free InP (Red), InP (Green) and ZnTeSe (Blue) QDs on AM QD-LED display system is demonstrated in this thesis. Work conducted in this research thus, sets out the route from the QD materials, process, and device architecture and design to system integration for QD-LEDs as a framework for developing cutting-edge QD-based display and smart white lighting applications.

This dissertation is concerned with the charge carrier dynamics in semiconductors based on metal-halide perovskites. These materials have shown remarkable performance in optoelectronic applications like solar cells or light-emitting devices. They are solution-processable at low temperature using inexpensive earth-abundant reagents, have low Urbach energies, high carrier mobilities and long diffusion lengths, while their bandgap can be tuned across the visible and near-infrared spectrum through the chemical composition. In this thesis, two different perovskite-based systems are studied with respect to their carrier dynamics and related photoluminescence yields as a probe for their performance in devices. The first study compares a variety of perovskite thin films containing mixed cations (cesium, methylammonium, formamidinium) and mixed halides (bromide, iodide). I find that the disordered energetic landscape arising from domains that are bromide- or iodide-rich allows charge carriers to accumulate in low-bandgap regions. Recombination of charges at these sites follows quasi-first-order kinetics and the locally high carrier density allows bimolecular radiative recombination to outcompete trap-mediated loss channels. Thus, the photoluminescence yields in mixed-halide compositions remain high even at low excitation densities. This unearths a new route towards highly efficient light-emitting devices or solar cells through micro-structuring of the energy landscape in these materials. The second study investigates the consequences of manganese-doping for the carrier dynamics of cesium lead halide nanocrystals. Photoluminescence quantum yields are shown to double upon doping. This is found to be not only a consequence of reduced non-radiative losses, but of an increased intrinsic radiative excitonic recombination rate as well. The origin of this stronger emission lies in a carrier localisation effect induced by the manganese dopants which locally break the periodicity of the host crystal lattice. This leads to an increased overlap of electron and hole wave functions and thus favours radiative recombination. The mechanism provides a new strategy of transition-metal doping for highly efficient light-emitting devices.

Thin-film photovoltaics (TF-PV) offer several advantages over crystalline silicon due to reduced material usage, including both reduced weight and material costs. TF-PV panels are fabricated monolithically, with material layers deposited onto a large glass panel and divided into series-interconnected cells. This interconnection is conventionally done in a multi-step process in between each layer deposition step. M-Solv has a novel, patented One-Step-Interconnect (OSI) for TF-PV whereby interconnects are produced after all material deposition is complete, by a combination of laser processing and functional material deposition. OSI has been shown to have benefits over conventional interconnection, including reduced capital costs and increased production efficiency. Copper-Indium-Gallium-Selenide (CIGS) is considered as potentially the most efficient of the TF-PV technologies, and is also fastest growing in terms of market share. This thesis presents the first example of functional CIGS-based OSI devices, produced using novel 1550 nm laser scribing processes, and a novel viscous-paste conductor deposition method. CIGS OSI device performance approaches that of reference cells with conventional interconnects, with Fill Factors (FF) >0.69 in some cases, and areas of further development are identified. In addition, a new suite of laser scribing processes for both OSI and non-OSI CIGS scribing have been demonstrated, and a combined optical-thermal-mechanical finite element model has been developed to evaluate the laser scribing process.

Silicon Carbide (SiC) based power devices receive more and more popularity in the field of power electronics as they operate at higher voltages, higher switching frequencies and higher temperatures compared to traditional Silicon (Si) based power modules. As for SiC-based power devices, the temperature of SiC chip must be monitored in order to operate the device within its limit. However, it is not straight forward to directly measure the junction temperature (Tj) of a power device non-intrusively due to the package obstruction. Therefore, indirect Tj measurement methods like Temperature Sensitive Electrical Parameters (TSEPs) are preferred by researchers and been intensively investigated for Si devices as the dominant power devices in the past. However, those TSEPs which are effective for Si devices are mostly not applicable to SiC devices. This is due to different physical and electrical behaviour between SiC-based device and Si-based device. Thus, it is necessary to develop new method to implement indirect Tj measurement for SiC devices. This thesis presents a new on-line technique to estimate the Tj of discrete SiC MOSFET devices. In this work, small amplitude, high frequency chirp signals are injected into the gate of a discrete SiC device during its off-state operation. Then, the gate-source voltage (VGS) is measured and its frequency response (FR) characteristic is determined by using Discrete Fast Fourier Transform (DFFT) analysis. The captured VGS signal is a direct function of the gate-source loop impedance. The derived function becomes a linear function in respect Tj as it represents only the resistive elements of the gate-source loop. As the gate channel resistance of the SiC MOSFET (Rint) is the largest resistance in that loop and it is temperature dependent. As a result, the temperature of the SiC MOSFET chip can be estimated. The new method in the thesis will be explained in details and the theory will be backed up by analytical simulations. A 3D numerical model for the discrete SiC MOSFET is also established and simulated. Furthermore, a network analyser is used for initial validation of the new method and finally a boost circuit was built with signal injection circuit integrated within the gate driver circuit to demonstrate the feasibility of using this innovative method to extract junction temperature of a discrete SiC MOSFET.

Organic semiconductors have a huge potential for low cost, scalable, lightweight, flexible, and semi-transparent electronic devices; which can revolutionize the semiconductor device industry upon their technological maturity. Semiconducting properties and device performances are critically dependent upon their molecular attributes such as conformation, order, and vibronic coupling. In this respect, this thesis aims to identify the critical role of molecular scale morphology on device performances (including efficiency and stability), with the focus on molecular nanomorphology (includes molecular-level structures and processes). Molecular vibrational Raman spectroscopy, photoluminescence, and several other advanced structural/energetic probes are used extensively in conjunction with electronic device characterizations to deduce the morphology and performance relationships in organic devices such as photovoltaics (OPVs), photodetectors (OPDs) and chemiresistors (organic diodes) within the context of organic small molecules and conjugated polymers. Firstly, the impact of a model polymer:fullerene nanomorphology on long term device performances (i.e. operational stability) is investigated demonstrating that fine-tuning the nanomorphology of the photoactive layer of OPV devices is vital not only for power conversion efficiency but also for operational stability. The study proposes that a thermal annealing preconditioning of a general polymer:fullerene layer below the molecular scale phase segregation temperature (T_ps) is essential to develop a stable bulk-heterojunction (BHJ) morphology for improved operational stability whilst maintaining optimum efficiency. Secondly, molecular morphology and associated photophysics of highly intermixed all small-molecule BHJ blend system comprising of a novel dipolar donor and buckminsterfullerene yielding state-of-the-art OPD performances are investigated. It is found that such highly intermixed small molecule blends morphology is poorly optimized for OPV operation due to dominant geminate recombination and strong emissive charge-transfer states, however, it shows high photocurrent generation for OPD operation due to efficient field-driven dissociation of interfacial charge-transfer (CT) states. The molecular conformation of the dipolar donor critically determines the device energetic landscape and thus the device parameters such as dark current, CT state binding energy, and photoresponse time. The study highlights key differences in ideal BHJ morphology required for optimum OPV and OPD operations. While OPVs require a fine balance of mixed and pure phases via optimized donor/acceptor phase segregations, OPDs can be efficient even when entirely finely mixed. Finally, the electronic properties of organic diodes comprising of a model π-conjugated polymer blended with several modified ionic liquids (MIL, solid-state in room temperature) is investigated revealing an application as chemiresistors; showing sensitive and selective detection of polar and non-polar chemical analytes; operating at low power (µW) and room temperature. The study proposes that molecular-level electrochemical doping of the π-conjugated polymer by MIL is responsible for the conductivity tuning of the diode while gas-specific interaction between the polymer and MIL results in specific transduction of the dielectric environment into specific electronic signals. This thesis highlights the key role of molecular properties responsible for determining the operational mechanism of the associated organic electronic devices. As such, optimizations at the molecular scales are paramount which can include new molecular structural designs, more compatible organic blends, or conformational optimizations at molecular heterointerfaces.

The awarding in 2016 of the Nobel Prize in Chemistry to Prof. J. Sauvage, J. F. Stoddart and B. L. Feringa, "for the design and synthesis of molecular machines" proves the interest of the scientific community and high-tech industry towards stimuli-responsive multifunctional materials and devices. In this direction, the research activity described in this thesis is focused on light-responsive emissive devices, which can be controlled remotely and reversibly via irradiation with light of specific wavelengths. A range of photochromic diarylethene derivatives were judiciously selected in combination with commercially available organic semiconductors to generate the light-responsivity in our devices. At first, optically switchable green-emitting OLEDs and micro-OLEDs were investigated achieving the maximum ON/OFF ratio of ~20 and ~90 for current density and luminance, respectively. Additionally, through the studying of the performance of light-responsive single carrier devices, it was demonstrated that electrons are more affected than holes by the switching of the photo-active dopant. The device emissive area was further scaled-down working on the characterization of light-emitting transistors (OLETs) having channel length of 2.5 µm. For the first time such devices covered the whole visible spectrum and a maximum ON/OFF ratio exceed 500 was achieved for both drain current and luminance. Finally, another macro-trend of the display community was addressed, the constant search for innovative chromophores as alternative to poorly stable and highly expensive iridium-based phosphorescent materials or highly toxic cadmium-based colloidal quantum dots. In this direction, air stable and potentially non-toxic red-emitting graphene quantum dots (GQDs) were embedded in electroluminescent devices. The LEDs showed high colour purity (FWHM= 30 nm), maximum luminance of 1300 cd/m2 and EQE of 0.67 % which are among the best performance ever reported for LEDs based on red-emitting GQDs.

Organic-inorganic metal-halide perovskites and narrow-gap semiconductors have been studied extensively over the last few decades. Their peculiar optoelectronic properties make them ideal candidates for future photovoltaic applications and ultrafast electronic devices. Specifically, knowledge of the charge-carrier transport and electronic band structure is key to understanding the fundamental principles that underpin their optoelectronic behaviors, and hence to improve the performance of devices based on them. Terahertz spectroscopy provides an ideal pathway to probe the optoelectronic properties of these semiconductor materials not only because of its non-contact nature which avoids damaging the sample and removes the complications arising from metallic contacts, but also due to its ability to measure the charge-carrier dynamics on an ultrafast (sub-picosecond) timescale. The versatility of terahertz spectroscopy allows a large phase space of optoelectronic parameters to be probed, such as charge-carrier mobility, carrier diffusion length, and photoconductivity. A comparative study on the prototypical metal-halide perovskite MAPbI3 as both single crystals and polycrystalline films suggests that Frohlich scattering is the dominant mechanism limiting charge-carrier mobility, which arises from the electron-phonon coupling effect. Further investigations of the phonon modes in MAPbI3 show a phonon-hardening effect under photo-excitation, which is attributed to the increase in dielectric screening due to photo-induced electron doping. In addition, grain-boundary scattering is shown to further reduce mobility in polycrystalline films at cryogenic temperature. By taking into account the effect of photon reabsorption, the wide range of values for charge recombination parameters and charge diffusion lengths reported previously is reconciled. The benign grain boundaries observed in polycrystalline films offer great promise for future high-performance devices. Time-resolved magneto-terahertz spectroscopy is utilized to investigate the cyclotron effective mass of a narrow-gap semiconductor InSb. A significant reduction of effective mass observed in the first 50ps after photoexcitation indicates a fast hot-electron cooling process. Meanwhile, the increase in effective mass with increasing carrier density agrees with the non-parabolic conduction band predicted by the ab initio calculations of the quasiparticle band structure based on the Kane model. This thesis, which combines a thorough experimental study utilizing terahertz spectroscopy with a complementary theoretical study based on first-principles calculations, helps to uncover a complete picture of the fundamental optoelectronic properties of metal-halide perovskites and narrow-gap semiconductors.

During the past decades, the near-infrared (NIR) light emissions of Yb3+ and Nd3+ ions have been favoured for their applications in the area of biology, sensors and telecommunication etc. However, due to their low absorption cross-sections, high pump power density is required to produce bright emission intensity. Sensitisation is one of the most commonly used approaches to reduce the power density to photosensitise lanthanide ions. In this thesis, we report a composite material system in which the organic sensitiser Zn(F-BTZ)2 is doped with lanthanide complexes Ln(FTPIP) 3 (Ln= Nd or Yb) in thin films. Both Zn(F-BTZ)2 and Ln(F-TPIP)3 are perfluorinated molecules, which are designed to improve the quantum efficiency of lanthanide ions. For these Zn(F-BTZ)2 and Yb(F-TPIP)3 co-doped samples, Yb3+ demonstrated efficient photoluminescence (PL) with a prolonged lifetime up to ~ 0.3 s at 1 μm, far exceeding the intrinsic Yb3+ lifetime of ~ 1ms. The dynamic equilibrium is studied to demonstrate that this prolonged emission is caused by the energy transfer from the long-lived organic triplet excitons. Experiment and simulation results suggest that we discovered a novel route to develop bright and ultralong-lived 1 μm emitting materials by coupling Yb3+ with other existing organic persistent luminescence materials. Furthermore, different concentration Zn(F-BTZ)2 and Nd(F-TPIP)3 codoped films were fabricated. Sensitised PL spectra demonstrate bright Nd3+ NIR emission peaks that have PL intensities comparable to the emission of Zn(F-BTZ)2 chromophore. Series of experiments and simulations were conducted to quantitatively study the effect of sensitisation. The reduction of triplet lifetime with the presence of Nd3+ as an energy acceptor indicates the triplet energy transfer rate 7 at room temperature is ~ 1200 s-1. Hence, both the photoluminescence excitation (PLE) measurement and steady state rate equation simulation suggest the sensitisation via Zn(F-BTZ)2 could enhance the PL intensity of Nd3+ by maximumly 3000 times.

The limitations of the down-scaling of the silicon metal-oxide-semiconductor (MOS) field-effect transistor (FET) and the demand for energy-efficient appliances motivate the study of alternative devices. The device under consideration is the biaxial tensile-strained Germanium on InGaAs alloy tunnel FET (TFET). The type of device depends on the band alignments at the heterojunction. For example, it determines if it can work as a TFET or a MOSFET. In turn, the band alignments depend on the atomistic details of the interface structure. First-principles calculations were performed to obtain the band alignments of the simpler Ge/GaAs system. The difference of the Ge and GaAs band gaps, calculated at Many-Body Perturbation Theory (MBPT) level, is added to the potential line-up of a supercell structure containing the Ge/GaAs interface, calculated using Density Functional Theory (DFT). It was studied how the band alignment is affected by the disorder, the stoichiometry composition and the diffusion of atoms at the interface. Calculations on the relative interface formation energy were performed to determine the relative stability of the various models for the interface. Results show how the heterojunction type changes when going from As-rich to Ga-rich conditions and with the diffusion of atoms over a few layers. These results explain the interval range of the experimental measurements and highlight the challenge of obtaining a Ge/GaAs TFET as this implies the control of atomic diffusion at the interface. Results also show that As-rich interface conditions are more favourable than Ga-rich conditions and that the contribution of disorder at the interface to the potential line-up is negligible. Further, preliminary studies were performed for band gaps of strained Ge and InGaAs alloys. Besides, an alternative approach to the expensive MBPT was developed based on enforcing Koopman's theorem to DFT to obtain accurate band gaps at a low computational cost.

New electronic devices fabricated from organic molecules have been greatly improved over the past two decades. Yet, understanding the electronic transport mechanism of free carriers and excitons (bound electron-hole pairs) in organic semiconductors (OSs) is still a pertinent challenge. The soft molecular nature of these materials gives rise to an intricate interplay between electronic and nuclear motion as well as unique solid-state physical properties. Standard (analytic) treatments describing electronic transport often rely on one of two extremes: a travelling wave propagating through the material or a particle hopping from one molecular unit to the next. These are often unsuitable to fully describe the complex dynamics, which falls in between these regimes. In this regard, non-adiabatic molecular dynamics simulations permit a direct view into the transport mechanism, thus providing new important insights. In this thesis, I have further developed and improved in terms of efficiency and accuracy a fully atomistic non-adiabatic molecular dynamics algorithm, called fragment orbital-based surface hopping (FOB-SH). This allows the propagation of the coupled electron-nuclear motion in large nano-scale systems. After validating the accuracy of this methodology and discussing important physical requirements (i.e. energy conservation, detailed balance and internal consistency), I will present the application of FOB-SH to the calculation of room temperature charge mobility of a series of molecular organic crystals. I will discuss the agreement with experimental mobility values and the role of the disorder, induced by thermal fluctuations, on the delocalization of the states and the subsequent formation of a polaronic charge state. This polaronic charge propagates through the crystal by diffusive jumps over several lattice spacings at a time during which expands to more than twice its size. I will show that FOB-SH can recover the crossover from hopping to band-like transport depending on the strength of the electronic coupling and the temperature, thus successfully bridging the gap between these two extreme transport regimes. Finally, I will discuss a further extension of FOB-SH to the treatment of exciton transport in OSs. This opens up new exciting avenues for the application of FOB-SH to the study of electronic processes occurring in organic photovoltaic cells.

Transition Edge Sensors (TESs) are ultra-sensitive superconducting detectors having a wide range of applications, from quantum cryptography to x-ray spectroscopy. Models of TESs often employ a circuit theory approach, taking TES parameters such as the temperature and current dependent resistance as inputs, but there are few microscopic models which allow prediction of these parameters. In this thesis, I develop a microscopic model to describe TESs based on the Usadel equations, allowing predictions of resistance surfaces, critical currents and the small signal electrothermal parameters. To test my model, I calculate I(V) curves for device designs used by different research groups and compare them to experimental measurements. Using my model, I design a TES wafer for a systematic study of the effects of bilayer size and patterning on performance. To investigate the effects of magnetic field, I also design and characterise a new cryogenic magnetic field test facility to apply fields of variable direction and magnitude. I present the results of this study, showing that bilayers of around 10-20 square micrometres present a good balance between low field susceptibility and predictable behaviour. I then investigate the use of a backing plate as an on-chip shield, to reduce the field susceptibility of the TESs by attenuating the perpendicular field component. I present the models used in this design process to ensure a good shielding factor as well as a high reflection coefficient, and repeat key measurements from my systematic study to show the reduction in field achieved using this prototype on-chip shield. TES readout typically uses Superconducting Quantum Interference Detectors (SQUIDs), placing limitations on the readout frequency. I investigate the possibility of using a High Electron Mobility Transistor (HEMT) amplifier in parallel with an inductor for readout, increasing the bandwidth and allowing higher frequency operation, and discuss how TESs might be optimised for this new readout scheme. Through this thesis I have explored a range of microscopic physics and discussed how this understanding can be used to enhance the performance of transition edge sensors.

A key phenomenon to improve even further the efficiency of photovoltaic technology is hot carrier dynamics: the possibility of harvesting hot, energetic carriers in photovoltaics could increase the efficiency of such devices beyond the Shockley-Queisser limit. In this thesis advances in the understanding of hot carrier dynamics in metal halide perovskite semiconductors (MHP) are reported: the influence of the composition on the hot carrier cooling process was investigated by means of optical pump terahertz probe spectroscopy (OPTP) and transient absorption spectroscopy (TA). In particular the role of the metal in controlling the electron-phonon coupling and phonon-phonon coupling was studied. A new phenomenological model aimed at describing the cooling dynamics of hot carriers is introduced, and its parameter linked to the microscopical physical processes underlying energy relaxation. A fully inorganic tin-based perovskite semiconductor is studied using OPTP and the hot carrier cooling time connected to the first stage of cooling is measured, compared to the prototypical III-V semiconductor GaAs, and linked to the Fröhlich electron-LO phonon interaction. The influence of the bandstructure on the dynamics is also assessed. A series of mixed lead-tin perovskites with controlled Pb/Sn ratio is investigated by both OPTP and TA spectroscopy. The numerical outcome of the two techniques is investigated and the differences linked to the different physical processes the two techniques probe, helping clarify discrepancies that appeared in previous works on the subject. The influence of the metal fraction on the cooling dynamic is also established, and linked to the modification of the electron-phonon and phonon-phonon interaction caused by alloying. Finally the simpler, narrow-gap semiconductor InSb is studied for the first time via terahertz cyclotron spectroscopy. The effective mass of InSb was measured during the cooling of hot carriers towards the band edge, finding that the change in effective mass during this process is indeed measurable and can be described by a simple model assuming a nonparabolic band dispersion. These findings suggest possible new directions for the design and implementation of future of future semiconductor materials and devices with optimised carrier cooling profiles.

There is a growing requirement for high brightness light emitting diodes which can operate in the technologically important mid-infrared spectral range (3-5 μm), for applications such as environmental monitoring, industrial process control and spectroscopy. To date, mid-infrared (λ> 4 μm) LEDs exhibit a poor 300 K external efficiency with broadband emission spectra, resulting in low available output power at the target wavelength. The resonant cavity structure is an attractive solution to improve the performance of these LEDs by locating the active region inside a Fabry-Perot cavity which is formed between two distributed Bragg reflectors (DBRs). In this thesis, we demonstrated four novel mid-infrared resonant cavity LED (RCLED) structures operating near 4.0 μm, 4.2 μm, 4.5 μm, and 4.6 μm at room temperature. Three samples were grown on GaSb substrate and one on InAs substrate using molecular beam epitaxy (MBE). Different III-V semiconductor materials; bulk InAsSb, AlInAs/InAsSb strained multi quantum wells (MQWs), and InAs/GaAsSb superlattices (SLs) were used as active regions, positioned at the antinode of the electric field inside the micro-cavity. The 1λ-thick cavities, containing the active regions, were sandwiched between two high contrast latticematched AlAs0.08Sb0.92/GaSb DBR mirrors for GaSb-based RC structures and lattice-matched AlAs0.16Sb0.84/GaAs.08Sb0.92 DBR mirrors for the InAs-based RC structure respectively. The RC structures were first designed theoretically, where the thickness of the micro-cavity layers and the DBR layers were evaluated corresponding to the target wavelength of RC emission spectra. Then, the reflectivity of the top and bottom mirror were investigated to achieve high emission enhancement as determined by the number of layers in the DBR. The simulated results show that 13.5 pairs in the bottom DBR mirror with reflectivity (R>98%) and 5 pairs in the top DBR mirror with reflectivity (R>83%) are sufficient to achieve very high enhancement factors. The temperature dependence of the transmission spectra of the RC samples was measured over the range from 77 K to 300 K. It was found that the position of the optical cavity mode and the DBR stopband centre shift towards longer wavelength very slowly as the temperature increases at a rate of <0.4 nm/K and <0.3 nm/K, respectively. At room temperature the optical modes exhibit a narrow linewidth in the transmission spectrum (Δλ<100 nm). The room temperature quality factors were found to be in the range from ~60 to ~100, predicting that the emission of the RCLEDs should have a strong enhancement. Although all the RC samples exhibit some detuning, between the wavelength of the DBR stopband centre and the position of the optical cavity mode, the simulated results show that the spectral linewidth remains narrow and the emission enhancement factors are still high. Two entirely new GaSb-based mid-infrared RCLEDs were fabricated, (i) using bulk InAsSb operating at ~4.2 μm and (ii) with AlInAs/InAsSb MQWs operating at ~4.5 μm. The temperature dependence of the electroluminescence spectra and I-V characterization of both these RCLEDs and their reference LEDs were measured experimentally over the range from 20 K to 300 K. The RCLEDs show significantly better temperature stability, narrower emission linewidth, and high enhancement factors. The bulk InAsSb RCLED exhibits a significantly narrower (10x) spectral linewidth, (6x) superior temperature stability, (70x) higher peak intensity, (33x) higher integrated output power compared to that of the reference without a resonant cavity. For the MQWs RCLED, the peak intensity and the integrated emission were enhanced by a factor of ~85 and ~13, respectively. It also shows a superior temperature stability of ~0.35 nm/K and a narrow emission linewidth of ~70 nm (which are less than that of the reference LED by 7x and 16x, respectively). The optical and electrical properties of the RCLEDs without top DBR mirror were also investigated. The results show that the intensity and integrated enhancement are still achieved, but somewhat less than that of the full RCLED structures, due to low reflectivity (~33%) of the interface semiconductor/air top mirror. The high brightness, better spectral purity, narrow linewidth and superior temperature stability, of the RCLEDs developed in this work are rather attractive features, enabling these devices to be readily implemented in the next generation of optical gas sensor instrumentation.

Quantum computing is predicted to provide unprecedented enhancements in computational power. A quantum computer requires implementation of a well-defined and controlled quantum system of many interconnected qubits, each defined using fragile quantum states. The interest in a spin-based quantum computer in silicon stems from demonstrations of very long spin-coherence times, high-fidelity single spin control and compatibility with industrial mass-fabrication. Industrial scale fabrication of the silicon platform offers a clear route towards a large-scale quantum computer, however, some of the processes and techniques employed in qubit demonstrators are incompatible with a dense and foundry-fabricated architecture. In particular, spin-readout utilises external sensors that require nearly the same footprint as qubit devices. In this thesis, improved readout techniques for silicon quantum devices are presented and routes towards implementation of a scalable and high-sensitivity readout architecture are investigated. Firstly, readout sensitivity of compact gate-based sensors is improved using a high-quality factor resonator and Josephson parametric amplifier that are fabricated separately from quantum dots. Secondly, an integrated transistor-based control circuit is presented using which sequential readout of two quantum dot devices using the same gate-based sensor is achieved. Finally, a large-scale readout architecture based on random-access and frequency multiplexing is introduced. The impact of readout circuit footprint on readout sensitivity is determined, showing routes towards integration of conventional circuits with quantum devices in a dense architecture, and a fault-tolerant architecture based on mediated exchange is introduced, capable of relaxing the limitations on available control circuit footprint per qubit. Demonstrations are based on foundry-fabricated transistors and few-electron quantum dots, showing that industry fabrication is a viable route towards quantum computation at a scale large enough to begin addressing the most challenging computational problems.

As a part of a wide and term project, where quantum micro and nano-electromechanical (MEMS and NEMS) implemented in a superconducting quantum interference device (SQUID) made of boron doped diamond (BBD) to be explored, experimental and theoretical investigations, and solutions for related technical issues linked to these investigations, are presented in this thesis. Experimentally, current-voltage, I(V), characteristics and the differential resistance measurements have been performed for SIS tunnel junctions, and nanobridge devices made of BBD. On the basis of analyses of these measurements, temperature dependence of the critical current of a nanobridge device with bridge dimensions of L = 118 nm, and W = 109 nm, was attributed to the proximity effect described by Likharev’s theory of SNS weak-link junctions. Furthermore, temperature dependence of I(V) characteristics between 20 and 700 mK for another device with W=108 nm, and L=78 nm, shows resistive steps in the transition region of the I(V) curves around Ic. As phase slip events may arise due to vortex kinematics and the granularity of the superconducting films, the observed steps have been attributed to vortex kinematics, and granularity of BBD films from which the device have been fabricated. On the other side, for SIS junctions, fitting for I(V) characteristics measured for a typical SIS junction of a 6 nm vacuum gap, shows good agreement with the RSCJ model. An SIS junction with a wide vacuum gap of 76 nm, have been also investigated, where the measured temperature dependence of the I(V) characteristics of the junction, shows two transitions, the first transition, Ic1(T) was observed in the superconducting region, while the second one, Ic(T), occurs just before the normal state. The transition Ic(T) was attributed to Ambegaokar and Baratoff formula and BCS theory. Other measurements for I(V), and R(T) curves of superconducting strips of different strictures, have been performed. The measurements show resistive steps around transition regions of the I(V) curves, and corresponding spikes have been observed in the R(T) curves. Such behaviour is attributed to a collective effect that involves kinematic vortices, thermal fluctuations and/or quasiparticles diffusion (SBT model), and the granularity of BBD films from which the strips have been made. Technically, as superconducting devices are being influenced by a considerable amount of noise such as radio frequency (RF) noise, high performance RF filters were developed and fabricated. As results, the fabricated filters have shown that they are quite competitive to the earlier RF filters, efficient tools to attenuate the RF noise, and appropriate to be used for simultaneous measurements that take place in a cryogenic system where the temperatures down to few mK. In terms of theoretical aspect, simulations to quantitatively describe the interaction between a dc SQUID and an integrated doubly clamped cantilever were performed, where the unscaled dc SQUID equations coupled to the equations of motion of an integrated cantilever, have been numerically solved. In these simulations, an existing experimental configuration was selected to explore the motion of the integrated cantilever, and the voltage-displacement traces of a displacement detector were determined. Furthermore, the effect of the back-action between the SQUID and the doubly clamped cantilever have been analysed via the shift in the cantilever frequency, the line width, intensity, and shift in the position of the normal state. The simulations show how a sharp transition state drives the system into a nonlinear-like regime, and modulates the cantilever displacement amplitude, by tuning the bias current, and the external flux, which set the system in different regions of voltage-flux curve.

Excited states in organic semiconductors are generated, transported, and converted through π-orbital interactions between multiple molecules. Hence, efficiency depends on the relative geometries between molecules. Yet, control over spatial assembly of organic semiconductors is challenging. Large aromatic molecules will self-assemble into extended structures with no size or positional control. Deterministic interactions between two different semiconductor molecules is even rarer. In contrast, selective and high-yielding DNA base pairing interactions can control discrete, self-assembly of nanoscale structures. After introducing relevant synthetic and theoretical background to DNA assembly and interchromophore coupling, the synthesis of DNA-conjugated organic semiconductors are described. Semiconductors were inserted into the phosphodiester backbone of DNA, maintaining DNA’s self-assembling properties. The appended DNA direct the interactions between predefined numbers of the same molecule, followed by hetero-assemblies composed of different molecular backbones. DNA-directed changes are interrogated by excitonic couplings, using steady-state time-resolved optical spectroscopy to track the excited state evolution. Homo-aggregate molecules exhibit excitonic coupling. In contrast, heterostacks were capable of charge transfer between an electron-donor and electron-acceptor on DNA. Finally, singlet-fission active molecules were attached to DNA. This process generates two triplet excitons from one singlet exciton. The evolution of this process was tracked using time-resolved optical and magnetic spectroscopy. The combined observations in this thesis show multi-chromophore dependent phenomena are replicated in the aqueous DNA environment. Importantly, DNA successfully restricts and controls the extent of chromophore interaction to produce size-dependent phenomena and impart spatial control of multiple, different semiconductors.

Continuous rapid shrinking of feature size made the authorities to seek alternative patterning methods as the conventional photolithography process is reaching its intrinsic resolution limit. In this regard, some promising techniques have been proposed as the next generation lithography (NGL) that have the potentials to achieve both high volume production and very high resolution. Among them, several methods such as Extreme Ultraviolet Lithography (EUVL), Electron Beam lithography (EBL), Nanoimprint Lithography (NIL), Directed Self Assembly (DSA) and Scanning Probe Lithography (SPL) have demonstrated excellent potentials as promising candidates for future industrial nanofabrication. However, all these technologies are in their development phases and still need further work to overcome some challenges in terms of flexibility, uniformity, high throughput, high resolution, high reliability, high- efficiency, defectivity, and cost of ownership. On the other hand, nanoelectrode nanolithography (NEL) has been developed in the laboratory and demonstrated as an efficient lithographic tool. It has been strengthened in recent years as one of the most promising methods due to its high reproducibility, low cost, and ability to manufacture nano-sized structures. This method is based on the spatial confinement of the anodic oxidation between a conductive stamp and the sample surface. However, the non-uniformity issue severely limits the existing nanoelectrode lithography to be applied for large area nanopatterning. Besides, other issues such as stamp lifetime and low-cost stamp fabrication method need to be addressed to make this lithography technique viable for commercial applications. A clear and explicit understanding of the mechanism at a molecular level helps to improve this technique. Therefore, this PhD thesis firstly aims to gain an in-depth understanding of nanoscale mechanisms involved in the anodic oxidation process and the parametric influence in nanoelectrode lithography through molecular dynamics (MD) simulations. To do this, three-dimensional MD models of oxidation nanocell were developed, and a reactive force field (ReaxFF) was adopted to describe the interactions between atoms. The MD simulations were implemented in LAMMPS software and were performed by using a High-Performance Computing (HPC) service, ARCHIE-WeSt. The simulation results demonstrated two forms of adsorption of water molecules: molecular adsorption and dissociative adsorption. After breaking the adsorbed hydroxyls, the oxygen atoms insert into the substrate to form the Si−O−Si bonds so as to make the surface oxidized. A linear dependency of the electric field intensity on oxidation growth was observed. The relative humidity also showed the same linear behavior after a certain value (40%). The simulation results have been compared qualitatively with the experimental results, and they show in good agreement. MD simulation results also showed that the crystallographic orientation of the substrate has a great impact on the oxidation process. It was revealed that the thickness of the oxide film and the initial oxygen diffusion rate follow an order of (100) > (110) > (111) at lower electric field intensities. It also confirmed that surfaces with higher surface energy are more reactive at lower electric field intensity. Crossovers occurred at a higher electric field intensity (7 V/nm) under which the thickness of the oxide film yields an order of T(110) > T(100) > T(111). Atomic force microscope (AFM) oxidation experiments were performed to validate these results, which showed different orders for the (100) and (111) substrates, while (110) remained the largest for the oxide thickness. A good correlation has been found between the oxide growth and the orientation-dependent parameters where the oxide growth is proportional to the areal density of the surfaces. The oxide growth also follows the relative order of the activation energies, which could be another controlling factor for the oxide growth. However, the differences between simulation and experimental results probably relate to the empirical potential as well as different time and spatial scales of the process. Another objective of this thesis is to develop a new NEL process with a brass stamp that does not require conductive layer deposition. The brass material was chosen as it has high elastic modulus and high breaking strength, which ensures higher life expectancy. Therefore, this thesis reports the feasibility of using brass materials as the conductive stamps for NEL to shorten the process steps and reduce the production cost. The fabrication of nanostructures on the brass stamp was performed on a single point diamond turning (SPDT) machine. Some burrs were formed during the machining process, that prohibit the stamps from achieving a homogeneous contact with the substrates. Oxidation experiments were carried out with a home built NEL system. The results showed that an introduction of a thin layer of polymer (PS-OH) on the silicon substrate could improve the contact uniformity so as the oxidation. Finally, a rolling nanoelectrode lithography process was proposed, for the first time, to scale up the nanoelectrode lithography technique for large-area nanofabrication. A test-bed was developed to realize uniform pressure distribution over the whole contact area so that the local oxidation process occurs uniformly over a large area of the samples. A brass roller wrapped with a fabricated polycarbonate strip has been used as a stamp to generate nanopatterns on a silicon surface. The experimental results indicated that a significant improvement in pattern uniformity compared to the other results was obtained with the conventional NEL process. Moreover, the impact of pattern direction has been investigated, which shows no significant variation in the oxide pattern. Lastly, the rolling speed and the applied bias voltage were identified as the primary control parameters for the oxide growth.

This work includes studies into optically induced dielectric changes in organic heterostructure devices. The goal is experimentally measure the changes in the dielectric constant under illumination in organic devices. The organics used in this study is a 95:5 ratio of Poly(3-hexylthiophene) : Phenyl-C61-Butyric-acid-Methyl-ester, P3HT:PCBM blend. The permittivity change under illumination is studied at both high (GHz�THZ) frequencies as well as low (sub-MHz and DC) frequencies. To facilitate this study into permittivity, charge transport properties also need to be investigated. The charge carrier mobility is measured by transient time of ight method, which resulted in typical mobilities of 2 10�4 cm2V�1s�1. This is then used, together with I-V characteristics of the blended organic devices, to calculate charge carrier concentrations upon illumination of 1014�1015 cm�3. This is then compared to unblended devices which give a lower charge density of 1012�1013 cm�3. This increase of carrier concentration is further investigated upon by calculating the photogenerated charge carrier e ciency which we report to be 1 10�4 and 7 10�2 for the pure and blended samples respectively. This is a substantial improvement of two orders of magnitude with only the inclusion of 5% acceptor, which denotes that this donor acceptor ratio has viability in non-photovoltaic applications which rely on the polarisability of the material. Quasi-Optic Transmissometry is used to measure the phase and magnitude change under illumination for encapsulated organic devises. iv We report that these (non-contact electrode based) devices result in a change in the imaginary dielectric constant of "Im = 0:6 to 1:2 at 60 mWcm�2 illumination and GHz frequency. This corresponds with real permittivity change of "Re = �0:05 to �0:55. Low frequency changes under illumination were also investigated via impedance spectroscopy. No conclusions can be drawn about the viability of the organic devices tested for optically tunable dielectric applications at low frequencies. This is due to practical limitations in impedance spectroscopy not being able to measure geometrical capacitance, due to the system being dominated by the chemical capacitance of the organic-metallic interface. The measurement of this interfacial capacitance is further masked by a leakage current at low illumination. From interpretation of Nyquist plots we are able to report a photocapatience of 0:8 nF under 60mWcm2. The outcome of this study is that 95% P3HT:5%PCBM blends are viable as optically tuneable dielectric devices for use in radio frequency applications. This is due to the relatively large dielectric changes reported and the substantial increase in photocarrier generation e ciency of the blend compared to pure P3HT devices.

Despite a number of attempts over the past two decades, research into reliable, controlled induction of long term evoked responses, mimicking low level learning and memory in dissociated cell cultures remains challenging. In addition, a full understanding of the stimulus-response relationships that underlie synaptic plasticity has not yet been achieved, and many of the underlying principles remain largely unknown. Plasticity studies have been predominantly limited to low density Multi/Micro Electrode Arrays (MEAs). With the advent of complementary metal-oxide-semiconductor (CMOS) based High-Density (HD) MEAs, unprecedented spatial and temporal resolution is now possible. In this thesis, an attempt to bridge the gap between studies of neural plasticity and the use of CMOS based HD-MEAs with thousands of electrodes, is reported. Additionally, since such HD-MEAs generate a large volume of data and require advanced analytics to efficiently process and analyse recordings, computational tools and novel algorithms to infer connectivity during plasticity have been developed. The study showed that the responsiveness, stability and initial firing rate of neuronal cultures are the deciding factors to reliably induce evoked responses. With multi-site stimulation, sustained long term potentiation was achieved, which was validated both by evoked response plots and overall firing rates measured at five different time points - before and after repeated stimulation, and at a three day time points. In contrast, while depression responses were observed, it was found that the effects were not sustained over many days. The findings of the study suggest that appropriate selection of neuronal cultures is crucial for inducing desired evoked responses and criteria for this have been developed. Furthermore, it is concluded that the initial responses to test stimuli can be used to determine whether potentiated or depressed responses are to be expected. To analyse the recordings, pipeline of computational tools was developed. Firstly, neuronal synchrony metrics were adapted for the first time for large HD-MEA recordings and shown to correspond effectively to the firing dynamics. To analyse functional connectivity, an information theoretic approach, Transfer Entropy(TE), was utilised. The method showed accurate estimation of functional connectivity with mid 80th percentile accuracy on simulated data. A superimposition method was proposed to enhance confidence in the connectivity estimation. To statistically evaluate connectivity estimation, a new surrogate method, based on ISI distribution approach, was proposed and validated with a simulated Izhikevich network. The method achieved improved accuracy, compared to the existing ISI shuffling method. This newly developed method was later utilised to infer connectivity and refine connections during the learning process of real neuronal cultures over many days of stimulation. The connectivity inference corresponded accurately to both the spontaneous and stimulated networks during evoked responses and the proposed method permitted observation of the evolution of connections for the potentiated network.

The aim of this work is to characterize the impact ionization characteristics of AlAs0.56Sb0.44 towards its use as the avalanche medium in separate absorption and multiplication avalanche photodiodes (SAM-APDs) based on InP substrate for optical communication systems. The previous studies of the AlAs0.56Sb0.44 material were only undertaken on very thin p-i-n structures where we cannot accurately estimate the impact ionization coefficient and excess noise behavior due to “dead-space” effects. In this work, much thicker AlAs0.56Sb0.44 homojunction diodes were investigated systematically. The absorption coefficient was fitted by 1-D quantum efficiency model. Comprehensive multiplication and excess noise measurements based on AlAs0.56Sb0.44 homojunction diodes over a wide range of thickness were performed at room temperature. The bulk electron and hole ionization coefficients, α and β respectively, were found to be very disparate and ‘silicon like’ at low electric fields and α > β over the whole electric field range. The ionization coefficients were determined from 220-1250 kV/cm for α and from 360-1250 kV/cm for β. The β was found to rapidly drop at the low electrical field, but the α was similar to that of InP and InAlAs. Noise measurements carried out on the thickest p-i-n structure exhibits the best reported excess noise based on InP substrate, k = 0.005 at room temperature. The SAM-APDs using AlAsSb show potentially better performance than those using InP/InAlAs as the multiplication layer.

Organic semiconductors exhibit excellently suited material properties for a variety of applications and offer the possibility of low-cost and large-scale production. This thesis analyses photonic properties of several organic semiconductors, which are of great interest for different applications. To explore these materials, a combination of steady-state and time-resolved optical measurement techniques is employed and energy transfer dynamics within organic semiconductors are modelled. Research was focused on the following areas: The influence of the fraction of chain segments that adapt β-phase conformation on film microstructure of thin films of the blue-emitting polymer PFO is consistently investigated for the first time. An analysis of emission and absorption properties of PFO films with systematically varying β-phase content indicates that with increasing β-phase in PFO films, the chain conformation becomes more planar and includes more repeat units. Energy transfer, which occurs upon photoexcitation from glassy-phase to β-phase chain segments in PFO thin films is evaluated in terms of Förster resonant energy transfer theory. Differences in Förster radii calculated from ultrafast emission dynamics and spectral overlap between steady-state emission and absorption indicate that the energy transfer process is influenced by exciton diffusion within the glassy phase. Parasitic absorption of light in a molecular sun-facing charge extraction layer (CTL) of a metal halide perovskite (MHP)-based solar cell structure is addressed. Experimentally observed energy transfer between layers of photoexcited conjugated polymers and a MHP layer is combined with modelling the efficiency of the energy transfer process. Efficient energy transfer is found to counteract parasitic light absorption in the CTL for thin layers (≤ 10 nm) and/or high PLQE materials. Morphological changes in films of the hole-transport material P3HT deposited on MAPbI₃ are explored. Changes in aggregation are observed through careful analysis of absorption and emission spectra from P3HT films of varying thickness and spin speed during their fabrication on a MAPbI₃ layer.

Strong light-matter interaction inside planar microcavities has been the object of intense scrutiny thanks to a wide variety of technological and theoretical achievements for which future applicability relies on understanding and controlling both photonic and excitonic environments. Recently, demonstrations of polariton lasing, non-equilibrium Bose Einstein Condensation (BEC) and superfluidity at room temperature have been reported thanks to the large oscillator strengths and exciton binding energies of Frenkel excitons in organic semiconductors. Among this class of material, semiconducting conjugated polymers are promising thanks to their versatility, remarkable optical properties and tunability. In this thesis, we explore different methods to manipulate the intra- and inter-chain properties of a subgroup of semiconducting polymers, the polyfluorenes, to shape the interactions of their excitons with light. We begin our work by examining the essential properties of semiconducting conjugated polymers, which we then embed inside microcavities to observe strong light-matter interactions. We present a brief overview of exciton-polaritons in organic microcavities, highlight recent developments in their understanding and introduce the measurement and fabrication techniques that we use throughout the thesis. In our first experimental investigation, we perform conformational changes to a polymer's backbone inside metallic microcavities thereby inducing two sub-populations of excitons with a pre-determined fraction. We present the mode characteristics of these structures with ultrastrong coupling (USC) of the first disordered population of excitons and a splitting of the lower polariton branch induced via gradual introduction of the second population of excitons. We measure the photoluminescence (PL) only emanating from the lowermost polariton branch, which allows us to exert conformational control over the emission energy and its angular variation to create dispersion-free cavities with highly saturated blue-violet emission. In our second experimental realization, we report an inter-chain tunability via the fabrication and optical characterization of organic microcavities containing liquid-crystalline conjugated polymers (LCCP)s aligned on top of a thin transparent Sulfuric Dye 1 (SD1) photoalignment layer. Transition dipole moment alignment enables a systematic increase in the interaction strength, with unprecedented solid-state Rabi splitting energies ~Ω0 of up to 1.80 eV, the first to reach energies comparable to those in the visible spectrum, accompanied by the highest-to-date organic microcavity coupling ratio: 65%. We also demonstrate that the coupling strength is polarization-dependent with bright polariton PL for TE polarization parallel to the polymer chains and either no emission or weakly coupled emission from the corresponding TM polarization. We finally examine the reflectivity and transmissivity of TE-polarized waves incident on microcavities containing excitons with in-plane uniaxially oriented transition dipole moments both theoretically and experimentally. We discuss the propagation of the electric field through the cavity and compare our results with previous reports. We confirm that in all cases, the reflected and transmitted electric fields derive from photons leaking parallel and perpendicular to the transition dipole moment orientation. We conclude our work by examining the opportunities that are enabled from our two experimental techniques for the future of polaritonics.

Ultimate lighting sources for general illumination are monolithic on-chip white light emitting diodes (LEDs) containing multiple colour emissions, either red-green-blue or blue-yellow, but without involving any yellow phosphors. It is highly likely that current white LEDs fabricated by using a “blue LED+ yellow phosphors” approach will be eventually replaced by monolithic on-chip white LEDs. One of the direct routes for the fabrication of monolithic white LEDs is to utilize InGaN quantum wells (QWs) with different emission wavelengths as an active region, which will involve a number of fundamental issues, such as the design of an active region, carrier transport, etc. So far, these fundamental issues have not been understood. In this work, a systematic simulation study on these challenging issues has been carried out, achieving a full understanding of these issues and thus leading to the design of optimized white LED structures on (11-22) semi-polar substrates by taking the major advantages of semi-polar LEDs in comparison with their c-plane counterparts. Finally, the monolithic on-chip white LED epiwafer based on these designs have been successfully grown on our well-established (11-22) GaN templates with a step-change in crystal quality. Detailed device characterization has been performed on these LEDs, validating these approaches and designs. The design of dual-colour (11-22) semi-polar LEDs aiming at white LEDs and their carrier transport issues have been systemically studied by using one-dimensional drift-diffusion simulations. Due to the much heavier effective mass of holes than that of electrons and also the much larger activation energy of p-GaN than n-GaN, the distribution of injected carrier (mainly holes) is extremely uneven during LED operation. Furthermore, the residual polarization of semi-polar LEDs makes the case even more complicated. Based on a systematic study, carrier transport issues for (11-22) semi-polar white LEDs and their c-plane counterparts have been fully understood, demonstrating their major differences. In addition a novel structure utilizing an extra thin GaN spacer prior to the growth of blue InGaN quantum well, has been design to effectively improve hole transportation and a dual-colour emission LED has been achieved. A tri-chromatic emission has been subsequently designed by further optimizing two key factors, indium content in InGaN quantum wells and barrier thicknesses. In order to validate our simulation results dual-color emission LEDs have been grown on our high quality (11-22) semi-polar GaN templates. Simulations have agreed very well with experimental results demonstrating that both the growth order of the yellow and blue InGaN quantum wells and the growth of a thin GaN spacer are of vital importance. A different approach has been developed, leading to the growth and then the fabrication of monolithically integrated white light LEDs on (11-22) semi-polar GaN template. In this approach, an electrically injected semi-polar blue LED is firstly grown, followed by a yellow multiple quantum well structure as a down conversion layer. This forms a white LED. For the first time, a systematic and comprehensive study on optical polarization properties has been conducted on (11-22) semi-polar LEDs with a wide spectral region from blue to yellow as a function of indium concentration and injection current. Fundamental understanding of the polarization properties and the emission mechanisms of (11-22) semi-polar LEDs has been achieved. Detailed polarization dependent electroluminescence measurements have demonstrated that both indium content and current injection play crucial roles in the optical polarization properties of (11-22) semi-polar LEDs.

Silicon Single Photon Avalanche Diodes (SPADs) have become increasingly important due to a rise in applications requiring very sensitive, low level light detectors. This thesis focuses on the development of a simple monte carlo simulator for the modelling of Si SPADs, along with the fabrication of a Si mesa SPAD. The simulator was validated against experimental and reported Si results. Simulations are performed to compare an n-on-p to a p-on-n SPAD design. These simulations find the n-on-p design offers better timing performance for a given breakdown probability, however the p-on-n design achieves a greater breakdown probability for a given bias. A new temperature-dependent simple monte carlo parameter set is presented for InP APDs. This parameter set is extensively validated from 150-290 K, showing that the simulator is capable of temperature dependent modelling. Finally, a Si mesa SPAD is demonstrated. Follow on work from this thesis could include further development of the simulator to add the simulation of external quenching mechanisms and the validation of the InP parameter set for Geiger-mode simulation. Fabrication of a planar Si SPAD using the same active device structure would allow for the direct comparison of dark current contributions due to the etching process.

This thesis is a report of work carried out at the Jeremiah Horrocks Institute at the University of Central Lancashire, the Stephenson Institute For Renewable Energy at the University of Liverpool and the Center for Nanoscale Materials at Argonne National Lab. The focus of study are systems of interest to developing molecular electronics and systems that facilitate the synthesis of graphene patterned with ordered defects. The approach taken to developing the latter is via the deposition of precursor molecules atop symmetry conflicting substrates.

Organic semiconductors offer significant advantages as photoactive materials for solar energy conversion, such as ease of synthesis, processing and tunability of properties. However, their efficiency and stability are significantly lower than competing technologies and this hinders their widespread commercialisation. Organic semiconductors are characterised by the relatively soft nature of the materials along with a strong coupling between electronic and vibrational modes. The aim of this work is to understand how the molecular nature of the materials limits the attainable power conversion efficiency of a device. Properly understanding how the properties of the molecules affect the device performance will serve as the basis to develop new chemical compounds for efficient and stable organic solar cells. In this thesis I have focused on understanding the origin of the open-circuit voltage (V_oc) losses in organic solar cells. V_oc Corresponds to the difference between the quasi-fermi levels at the contacts in an illuminated solar cell at zero current flow and is one of the characteristics of the device that controls power conversion efficiency (PCE). I used a combination of modelling and simulation to explain the experimentally measured voltage losses in series of organic photovoltaic (OPV) devices, aiming to understand which properties of the molecules impact the open circuit voltage of the devices. The main results of this thesis are presented in two chapters: In Chapter 4, I introduce a model for the voltage losses in OPV devices based on the radiative and non-radiative decay of the lowest energy charge transfer (CT) state and discuss its use. This CT state is formed at the interface between the donor and acceptor molecules in bulk-heterojunction solar cells. Using the model, I highlight the properties of the CT state that best explain trends in the voltage losses of OPV devices. These properties are related to the chemical structure of the molecules and their spatial arrangement. Therefore, these results serve as guidelines for developing molecules that would reduce the open circuit voltage losses. In chapter 5, I explore how to reliably characterise the recombination of the free charge carriers in OPV devices using optoelectronic techniques. Using a device simulation tool, I show that a commonly used characterisation technique (transient photovoltage (TPV)) is often unreliable. From the conclusions of the simulation I developed a modified characterisation technique (transient photo-charge (TPQ)) that overcomes the limitations of TPV. In the last section of chapter 5, I show how the combined use of the model in chapter 4 and the optoelectronic techniques can help further understand the photo-generation and recombination processes in OPV devices. Overall using a combination of molecular and device simulation approaches, I explored the correlation between the molecular properties of the organic semiconductors and the losses in the OPV devices. This work helps refine our understanding of the voltage losses in OPV devices and how to improve them.

This thesis looks at growing thin films of heavy group V elements as well as using Pt to study high spin-orbit coupling for topological materials. We start with Bi/Ni bilayers that had been claimed to exhibit an usual form for superconductivity at the interface, instead we found that Bi will diffuse across the interface at room temperature over a timescale of days. The Bi would then alloy with the Ni to create the superconducting alloy NiBi3 and that elevated temperatures would accelerate this process even faster. After this we continued on to heavy half Heusler alloys, growing epitaxial thin films of YPtSb and YbPtBi on c-plane sapphire. The YPtSb thin films grew as smooth continuous layers with high quality crystallinity, but without any transport properties to point to a topological state. YbPtBi on the other hand, grew as aligned triangular islands and we observed a negative longitudinal magnetoresistance that may originate from the chiral anomaly. The chiral anomaly is an expected property of a topological Weyl semimetal. Both YPtSb and YbPtBi were matched well with the c-plane sapphire surface with induced strain and so this process could be used for other half Heusler alloys in the future.

Potential Well Barrier (PWB) and the Hybrid PWB/PDB diodes are novel forms of submillimterwave devices with potential applications in detectors and mixers. The diodes have a potential well inserted between two intrinsic regions of AlGaAs in which charge accumulates to form a triangular potential barrier. The PWB diode is a majority carrier diode with similar operation to the Planar Doped Barrier diodes (PDB) though, with further potential advantages. The PWB diode therefore offers the possibility of providing alternative to applications for which Schottky barrier and PDB diodes are used. The barrier height of the PWB diode depends on other factors such as bias, temperature and length of intrinsic regions. With a voltage responsivity of 6.4 mV/μW at 10 GHz, the diode has demonstrated a promising radio frequency (RF) signal detection potentiality. This thesis describes the experimental design of PWB diodes and analyses their complex operation with respect to the sensitivity of the current-voltage (I-V) characteristics to various design parameters. This reveals a notable dependence of barrier height on the length of the intrinsic regions, depth and size of well and bias. The variation is reflected in the non-constancy of the ideality factor of the device with bias. The study also investigates the temperature operation of the PWB device and it was found that, the barrier height increases with increasing temperature at the rate of 0.093-0.36 meV/K in the temperature range of 200-380 K. Temperature changes cause electrons to diffuse from doped regions of AlGaAs(Si) into the intrinsic regions which modifies the electric field hence, modifying the barrier height. The band offset effect on diode operation was investigated in terms of electron velocity and energy, electric field and density of charge along the intrinsic regions. The hot electron effect on these devices was investigated, with the Monte Carlo (MC) model predicting lower currents flowing through the diode due to back scattering and carrier heating compared to the drift diffusion (DD) models. The MC and DD models have been in use for semiconductor device simulation for several decades and are used extensively in this study. This thesis gives a detailed description of the implementation of both models with emphasis on GaAs based heterostructures. The MC model proves to be more reliable and gives more details than the DD model though, takes a longer time to realise one voltage point. Therefore, in this study, the MC model is used mainly to study the devices where non-local fields and non-homogeneous transport of carriers is of the utmost importance.