Advances in sensor technology have made possible the recoding of three and four-dimensional signals which afford a better representation of our actual three-dimensional world than the ''flat view'' one and two-dimensional approaches. Although it is straightforward to model such signals as real-valued vectors, many applications require unambiguous modeling of orientation and rotation, where the division algebra of quaternions provides crucial advantages over real-valued vector approaches. The focus of this thesis is on the use of recent advances in quaternion-valued signal processing, such as the quaternion augmented statistics, widely-linear modeling, and the HR-calculus, in order to develop practical adaptive signal processing algorithms in the quaternion domain which deal with the notion of phase and frequency in a compact and physically meaningful way. To this end, first a real-time tracker of quaternion impropriety is developed, which allows for choosing between strictly linear and widely-linear quaternion-valued signal processing algorithms in real-time, in order to reduce computational complexity where appropriate. This is followed by the strictly linear and widely-linear quaternion least mean phase algorithms that are developed for phase-only estimation in the quaternion domain, which is accompanied by both quantitative performance assessment and physical interpretation of operations. Next, the practical application of state space modeling of three-phase power signals in smart grid management and control systems is considered, and a robust complex-valued state space model for frequency estimation in three-phase systems is presented. Its advantages over other available estimators are demonstrated both in an analytical sense and through simulations. The concept is then expanded to the quaternion setting in order to make possible the simultaneous estimation of the system frequency and its voltage phasors. Furthermore, a distributed quaternion Kalman filtering algorithm is developed for frequency estimation over power distribution networks and collaborative target tracking. Finally, statistics of stable quaternion-valued random variables, that include quaternion-valued Gaussian random variables as a special case, is investigated in order to develop a framework for the modeling and processing of heavy-tailed quaternion-valued signals.

In order for a photovoltaic device to be commercially viable it must have a production cost and operational stability commensurate with its final application. Both of these properties are influenced by many factors, including the production of the active materials and the deposition techniques used to fabricate it. In this thesis, the stability and manufacturability of two emerging photovoltaic materials are examined: organic semiconducting polymers and organic-inorganic perovskites. Organic semiconducting polymers are commonly synthesised through reactions utilising metal catalysts, which can remain with the polymer after synthesis, necessitating the investigation of their influence on photovoltaic devices. This work shows that the presence of the residual catalyst palladium in PCDTBT organic photovoltaic (OPV) devices caused significant reductions in power conversion efficiency and an additional increase in efficiency loss during the first 60 hours of operation. It is also shown, however, that only minor losses occurred in PFD2TBT-8 OPV devices at high Pd concentrations, highlighting the need to examine individual material systems. Despite being a very new technology, perovskite solar cells (PSCs) have already achieved comparable performance to silicon solar cells, making it important to investigate the stability of such devices. The operational stability of PSCs in the inverted architecture was characterised, showing lifetimes of < 300 hours. Using spectroscopic and device characterisation techniques, the major loss mechanisms were revealed to be reactions with water and oxygen, resulting the in the decomposition of the perovskite. It is also examined how the addition of hydroiodic acid to the perovskite precursor solution affects the performance and stability of spin and spray coated PSCs. Finally, the effects of deposition temperature and additional annealing on the operational stability of PSCs was investigated.

This thesis describes the optical spectroscopic measurements of III-V nano-photonic circuit elements with integrated self-assembled quantum dot single-photon sources, as a step towards achieving an on-chip quantum optical circuit. An electrically controllable optical router consisting of an embedded quantum dot within a photonic crystal cavity which is selectively coupled to separate waveguides is presented. By tuning the voltage, the quantum dot emission can be directed into either waveguide. This is experimentally demonstrated with spatially resolved microphotoluminescence measurements. An electrically driven single-photon source monolithically integrated with nano-photonic circuitry is investigated. In this device, lectroluminescent emission from a single quantum dot is channelled through a suspended nanobeam waveguide. The emission is shown to be highly coherent with coherence properties which are sufficient to observe non-classical interference. Correlation and cross-correlation measurements are used to confirm the single-photon nature of the source and the propagation of the single photons. A detailed investigation of the on-chip two-photon interference of two dissimilar sources is presented. Photons emitted by a quantum dot embedded in one arm of a directional coupler are combined with photons originating from an external laser. The occurrence of Hong-Ou-Mandel interference is confirmed with cross-correlation measurements.

Using the interfacial misfit (IMF) array growth mode, GaSb p-i-n diodes were grown on Si and GaAs lattice-mismatched substrates by molecular beam epitaxy (MBE) under optimised growth conditions. For the sample grown on Si, an AlSb nucleation layer was used to reduce the occurrence of twinning defects. In addition to the samples grown on mismatched substrates, an equivalent structure was further grown on a native GaSb substrate, for comparison. X-ray diffraction (XRD) was used to demonstrate that the layers were fully relaxed, and transmission electron microscopy (TEM) imaging showed arrays of 90° misfit dislocations with measured periodicities in agreement with atomistic modelling. However, after processing, device dark current densities of 0.9 Acm^-2 and 0.18 Acm^-2 were recorded for the sample grown on Si and the sample grown on GaAs, respectively, at -1.0 V and 300 K. These were compared to the sample grown on native GaSb, which had a dark current density of 0.01 Acm^-2 under the same conditions. Furthermore, TEM analysis revealed relatively high threading dislocation densities (TDDs) of ~10^8 cm^-2. It was proposed that not all the interfacial strain could be accommodated by the IMF arrays, since the array periods (9:8 for AlSb/Si and 13:14 for GaSb/GaAs) were not in exact agreement with ratio of the lattice constants (of AlSb to Si and GaSb to GaAs), i.e. a population of 60° misfit dislocations was still formed. It was therefore decided to investigate the use of nBn detector structures as lattice mismatched photodetectors. Using a design based on an InAsSb bulk-material absorber, a comparison was again drawn between two samples, one grown on mismatched GaAs and a second grown on native GaSb. This time, device dark current densities were found to be relatively similar when comparing the two samples (1.6×10^-5 Acm^-2 vs 3×10^-6 Acm^-2 at 200 K). D^* performance figures were also found to be within one order of magnitude (1.5×10^10 cmHz^1/2 W^-1 vs 9.8×10^10 cmHz^1/2 W^-1 at 200 K). Furthermore, diffusion limited performance was exhibited at all temperatures tested, so that the effects of Shockley Read Hall (SRH) generation were established to be absent (or at least much less significant). It was also found that absorption layer doping of around ~4×10^17 cm^-3 was necessary to ensure diffusion limited performance for the sample grown on GaAs and that, with this modification, diffusion limited performance was achieved even for a sample with a highly lattice-mismatched absorption layer (with higher Sb content and longer cut-off wavelength).While nBn detector structures offer very low dark currents, it will sometimes be necessary to have a detector which is sensitive to very weak signals. In telecoms applications, avalanche photodiode (APD) structures are often used as receivers for long-haul fibre optic systems. However, relatively few avalanche photodiode designs exist for wavelengths beyond 1.55 μm. Two novel separate-absorption-and-multiplication (SAM) APD structures were therefore demonstrated based on the IMF growth mode. In particular, by transitioning the lattice from 5.65 Å to 6.09 Å, it was possible to combine GaSb absorption layers with GaAs and (for improved noise performance) Al0.8Ga0.2As multiplication layers. Multiplication profiles were established using capacitance voltage modelling (together with ionisation coefficients from the literature) and excess noise measurements were then carried out. Through the presence of 1.55 μm photocurrent, it was confirmed that absorption took place in the GaSb regions, with transport to the p-n junction (in the multiplication region) taking place by diffusion. Through measurements showing 0.2<k_eff<0.4 and 0.1<k_eff<0.2 it was confirmed that multiplication of the photocurrent took place in the GaAs and Al0.8Ga0.2As layers. Extension of the designs for sensitivity at longer wavelengths would then be possible using other absor-ption layer materials which are lattice matched to GaSb. It should be noted that these include InGaAsSb (short-wave infrared) InAsSb (mid-wave infrared) and strained layer superlattices based on InAs/GaSb or InAs/InAsSb (long-wave infrared).

This work studied the replacement of incandescent airfield lighting systems with light emitting diodes. The focus was on the replacement of the infrared component of the incandescent spectra. A series of LEDs with a variety of nanostructures in different material systems were produced and tested to determine their suitability in replacing incandescent airfield lighting systems. Utilising quantum dashes in the active region, a surface emitting LED achieved an output power of 1.2mW at 1.97 um. This device had a wall-plug efficiency of 0.7%, an efficiency greater than that obtained in comparable commercially available surface emitting devices. The output power of this device was limited by the connement of electrons within the quantum dashes at room temperature. Another device characterised in this study was an LED with sub-monolayer InSb/GaSb quantum dots in the active region. The sub-monolayer InSb quantum dots were grown at Lancaster on GaSb substrates using molecular beam epitaxy and fabricated into surface emitting LEDs. These were investigated using x-ray diffraction, transmission electron microscopy and electroluminescence. This is the first reported electroluminescence from such devices. Emission was measured at temperatures up to 250 K. Room temperature emission was from the quantum wells in which the quantum dots where grown, output power was 80 uW at a wavelength of 1.66 um. Further devices with InSb sub-monolayer insertions were fabricated into edge emitting diodes. These samples were grown on GaAs using interfacial misfit arrays, defect densities were reduced through the use of defect filtering layers. The threading dislocation density decreased by a factor of 6 from 2.5x10^9/cm^2 to 4x10^8/cm^2 between the bottom and top of the defect filtering layer. The edge emitting devices achieved lasing up to 200 K with a characteristic temperature of 150 K. These devices were limited by Shockley-Read-Hall recombination and weak confinement of carriers within the InSb regions. The inclusion of AlGaSb barriers improved room temperature operation with output power increasing from 2 uW to 42 uW. In addition, increased confinement also resulted in a decrease in peak wavelength from 2.01um to 1.81um. GaInSb quantum well samples were produced on GaAs substrates utilising an interfacial misfit array. This included the first reported instances of ternary inter-facial misfit array interfaces with threading dislocation densities of < 2 x 10^9/cm^2 for an AlGaSb/GaAs interface and 5 x 10^10/cm^2 for an InAlSb/GaAs interface. By utilising an AlGaSb interfacial misfit array it was possible to improve the confinement of carriers within the GaInSb quantum wells, resulting in a twenty fold increase in room temperature photoluminescence intensity.

Type II InSb/InAs quantum dots (QDs) emitting in the 3-4 µm range are promising candidate as the gain medium for semiconductor laser diodes. The molecular beam epitaxy (MBE) growth of the QDs on GaAs and InP substrates can largely cut down the costs for future devices and massively broaden its application possibilities using the more mature material platforms. Different metamorphic growth techniques including inter-facial misfit (IMF) arrays were experimented for the integration of the InSb QDs on GaAs substrates. The density of threading dislocations and the quality of the QDs were investigated using cross-sectional transmission electron microscopy (TEM) images, high resolution X-ray diffraction (XRD) and photo-luminescence (PL). The 4 K PL intensity and linewidth of InSb QDs grown onto a 3 μm thick InAs buffer layer directly deposited onto GaAs proved to be superior to that from QDs grown on 0.5 μm thick InAs buffer layers using either AlSb or GaSb interlayers with IMF technique. Even though the dislocation densities are still high in all the 3 samples (~109 cm-2), they all achieved comparable PL intensity as the QDs grown on InAs substrates. Electro-luminescence (EL) from the QDs on GaAs substrates were obtained up to 180 K, which was the first step towards making mid-infrared InSb QD light sources on GaAs. From the study of PL temperature quenching, thermal excitation of holes out of the QDs was identified as one of the major reasons for weaker PL/EL signals at higher temperature range. To compensate the thermal leakage problem, the QDs integrated on InP substrates were grown between InGaAs barriers, which can provide a larger valence band offset compared with InAs. The QD PL peak moved to shorter wavelength (~2.7 μm) partly due to the stronger confinement, and the PL quenching was significantly slower for T > 100 K. From microscopy images, PL characteristics and calculations, the size and composition of the QDs were estimated. The InSb QD laser structures on InAs substrates emitting at around 3.1 µm were improved by using liquid phase epitaxy (LPE) grown InAsSbP p-cladding layers and two step InAs n-cladding layers. The maximum working temperature was increased from 60 K to 120 K. The gain was determined to be 2.9 cm-1 per QD layer and the waveguide loss was around 15 cm-1 at 4 K. The emission wavelength of these lasers showed first a blue shift followed by a red shift with increasing temperature, identical with the PL characteristics. Multimodal spectra were measured using Fourier transform infrared spectroscopy (FTIR). Spontaneous emission measurements below threshold revealed a blue shift of the peak wavelength with increasing current, which was caused by the charging effect in the QDs. The characteristic temperature, T0 = 101K below 50 K, but decreased to 48K at higher temperatures. Current leakage from the active region into the cladding layers was possibly the main reason for the increase of threshold current and decay of T0 with rising temperature.

The secure identification of an object or electronic system is carried out through the provision of some unique internal or external characteristic. The most obvious examples of these include passwords and fingerprints that can identify a person or an electronic device, and holograms that can tag any given object to provide a check of its authenticity. Unfortunately, modern technology provides resources that enable the trust of these everyday techniques to be undermined. Identification schemes have been proposed to address these issues by extracting the identity of a system from its underlying physical structure, which is constructed such that the system is hard‐to‐ clone or predict. These systems are known as Unique Objects (UNOs) and Physically Unclonable Functions (PUFs). The aim of the work in this thesis is to create a novel type of UNO/PUF that utilises the atomic‐scale uniqueness of semiconductor devices by measuring a macroscopic quantum property of the system. The variations in these quantum properties are amplified by the existence of such atomic‐scale imperfections, meaning these devices would be the hardest possible system to clone, use the least resources and provide robust security. Such devices would be of great societal and political significance and would provide the biggest technological barrier between the good guys and the bad. Specifically, this work has introduced three distinct devices based on semiconducting systems that could provide atomic‐scale unique identification: • Electronically ‐ Fluctuations in the current‐voltage characteristics of Resonant Tunneling Diodes (RTDs) were found to provide a simple measurement of the underlying quantum state electronically. • Optically ‐ Macroscopic thin films of the two‐dimensional material, MoS2, were created by the Langmuir‐Blodgett technique for the first time and have laid the foundations for the formation of an optical analogue of an atomic‐PUF/UNO system. • Optoelectronically ‐ The Langmuir‐Blodgett technique’s flexibility was utilised to fabricate complex heterostructures that couple graphene to semiconducting nanoparticles. This system should provide an ideal system with efficient electronic and optical characteristics that would be useful in a range of applications, including unique identification.

In this thesis, semiconductor nanostructures are studied, both experimentally and theoretically, to help aid the development of two diverse and important technologies. Firstly, charge-carrier confinement in stacked sub-monolayer (SML) InAs in GaAs: SML deposition results in the formation of In-rich agglomerations within a lateral InGaAs quantum well (QW) with lower In content. Low-temperature photoluminescence (PL) and magneto-PL reveals strong vertical and weak lateral confinement, indicative of a two-dimensional (2D) excitons. Paradoxically, high-temperature magneto-PL allows excited-state peaks to become resolved, which can be fitted by a Fock-Darwin spectrum, characteristic of a zero-dimensional (0D) system. To solve this contradiction, we postulate that stacked SML InAs in GaAs forms a heterodimensional system, in which electrons are 2D, and see only the lateral InGaAs QW, whilst the heavier holes are 0D, and are confined in the In-rich agglomerations. This description is fully supported by single-particle effective-mass and eight-band k · p calculations, which show heterodimensional confinement is probable for a large variation in In content. SML vertical-cavity surface-emitting lasers (VCSELs) — which prove to be one of the most promising candidates for datacoms applications — have been demonstrated at >20 Gb/s, and we postulate that heterodimensionality is fundamental to this high-speed operation. Efficient carrier injection is achieved by the lack of a wetting layer, along with the 2D electrons coupling to several In-rich agglomerations, making them quickly available to states that are lasing. Furthermore, the shallow confining potential of the In-rich agglomerations means that excess holes cannot build up in states that aren't lasing. Secondly, semiconductor photoelectrolysis for the solar-powered generation of renewable hydrogen by water splitting is researched. The novel use of nanostructures at the semiconductor-electrolyte interface (SEI) in a photoelectrochemical cell (PEC) is proposed to help increase the maximum potential that can be photo-generated, thus increasing the likelihood of a given PEC being able to split water. By solving the Schrödinger, Poisson and drift-diffusion equations, we simulate the band alignment, confined carrier energy states and carrier densities for a variety of different material systems. ZnO quantum dots on InxGa1-xN show the most promising band alignment, with electron-donating and -accepting states straddling the hydrogen- and oxygen-production potentials (respectively) for small x (x < 0.3), indicating an ability to split water.

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