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Dilute magnetic semiconductors (DMSs) are ideal candidates for spintronic devices as they exhibit both semiconducting and magnetic properties. The defining feature of a DMS material is the exchange interactions between the magnetic ions and the band electrons and holes, which leads to many of the spin behaviours observed. A fundamental property of DMSs is that a relatively small external magnetic field can cause enormous Zeeman splittings of the electronic energy levels, which allows separating of states with different spins. The giant Zeeman effect present in the DMS systems also leads to the possibility of trapping quasiparticles in an inhomogeneous magnetic field. In this thesis the effect of inhomogeneous magnetic fields on excitons in a DMS quantum well is investigated. We look at the possibility of trapping excitons in hybrid structures composed of a DMS quantum well placed a few nanometres below a nanoscale and microscale ferromagnetic disk and a ferromagnetic strip. Quasiparticles in a DMS quantum well are shown to undergo a splitting between band states for different spin components due to the giant Zeeman interaction. Due to the inhomogeneous magnetic field created by a nanoscale ferromagnetic disk in the vortex state the quasiparticles are found to be confined in a small region on the quantum well. The behaviours of excitons in the presence of both a homogeneous and inhomogeneous magnetic field is then discussed. The binding energy of a heavy hole exciton in a finite DMS quantum well in the presence of a homogeneous is calculated. The study is extended to look at excitons in the presence of an inhomogeneous magnetic field. The behaviour of excitons in the presence of a inhomogeneous magnetic field, is found to depend on the type of magnetic field, and is shown to be different for a magnetic field created by a microscale and nanoscale ferromagnetic disk and a ferromagnetic strip.

In this thesis I present and discuss the results of optical spectroscopy performed on InGaN/GaN single and multiple quantum well (QW) structures. I report on the optical properties of InGaN/GaN single and multiple QW structures, measured at high excitation power densities. I show a correlation exists between the reduction in PL efficiency at high excitation power densities, the phenomenon so-called ``efficiency-droop'', and a broadening of the PL spectra. I also show a distinct change in recombination dynamics, measured by time-resolved photoluminescence (PL), which occurs at the excitation power densities for which efficiency droop is measured. The broadening of the PL spectra at high excitation power densities is shown to occur due to a rapidly redshifting, short-lived high energy emission band. The high energy emission band is proposed to be due to the recombination of weakly localised/delocalised carriers occurring as a consequence of the progressive saturation of the local potential fluctuations responsible for carrier localisation, at high excitation power densities. I report on the effects of varying threading dislocation (TD) density on the optical properties of InGaN/GaN multiple QW structures. No systematic relationship exists between the room temperature internal quantum efficiency (IQE) and the TD density, in a series of nominally identical InGaN/GaN multiple QWs deposited on GaN templates of varying TD density. I also show the excitation power density dependence of the PL efficiency, at room temperatures, is unaffected for variation in the TD density between 2 x107 and 5 x109 cm-2. The independence of the optical properties to TD density is proposed to be a consequence of the strong carrier localisation, and hence short carrier diffusion lengths. I report on the effects of including an InGaN underlayer on the optical and microstructural properties of InGaN/GaN multiple QW structures. I show an increase in the room temperature IQE occurs for the structure containing the InGaN underlayer, compared to the reference. I show using PL excitation spectroscopy that an additional carrier transfer and recombination process occurs on the high energy side of the PL spectrum associated with the InGaN underlayer. Using PL decay time measurements I show the additional recombination process for carriers excited in the underlayer occurs on a faster timescale than the recombination at the peak of the PL spectrum. The additional contribution to the spectrum from the faster recombination process is proposed as responsible for the increase in room temperature IQE.

In this thesis ultrafast exciton dynamics of several colloidal quantum dots have been studied using visible transient absorption spectroscopy. The resultant transient decays and differential transmission spectra were analysed to determine the ultrafast relaxation channels, multiple exciton generation (MEG) efficiency and multi-exciton interactions in the observed materials. All QDs were preliminarily optically characterized using steady state absorption and photoluminescence spectroscopies. In addition, a high repetition infrared femtosecond pump probe experiment was designed and built to detect the picosecond intraband carrier relaxations in quantum dots. Picosecond carrier dynamics of type-II ZnTe/ZnSe and of CuInSe2 and CuInS2 type-I quantum dots were investigated. The common feature of these materials is that they are eco-friendly materials, being alternatives to the toxic Cd- and Pb- based materials. It was found that surface trapping occurred in both cases for electrons in the hot states, and in the minimum of the conduction band for ZnTe/ZnSe core/shell materials. Trion formation was observed in ZnTe/ZnSe core/shell dots at high power and unstirred conditions. The hot and cold electron trapping processes in type-II dots and CuInS2 and CuInSe2 dots shifted, distorted and moderately cancelled the bleach features. In addition, intra-gap hole trapping was observed in CuInS2 and CuInSe2 dots which results in a long decay feature in the recorded transients. MEG competes with Auger cooling, surface mediated relaxation and phonon emission. To enhance the MEG quantum yield, the rival mechanisms were suppressed in well-engineered CdSe/CdTe/CdS and CdTe/CdSe/CdS core/shell/shell and CdTe/CdS core/shell type-II quantum dots. The MEG slope efficiency and threshold for a range of different core size and shell thickness were found to be (142±9)%/Eg and (2.59±0.16)Eg, respectively. The observed threshold was consistent with the literature, whereas, the obtained slope efficiency was about three times higher than the previously reported values. The biexciton interaction energy of the dots stated in the previous paragraph was also studied. To date, time-resolved photoluminescence (TRPL) has been employed to study exciton interactions in type-II quantum dots and large repulsive biexciton interaction energy values between 50-100 meV have been reported. However, unlike the TRPL method, the TA experiment ensures that only two excitons remain in the band edge of the dot. Using this method, large attractive biexciton interaction energies up to ~-60 meV was observed. These results have promising implications regarding enhancing the MEG quantum yield.

This work studies the use of Ga0.12-0.i6In0.88-0.84Sb quantum wells grown by molec­ular beam epitaxy (MBE) on a highly mismatched substrate for use in light emit­ting diodes (LEDs) and lasers emitting in the 3-4 /μm spectral range. Quantum well samples were grown at Lancaster which had abrupt interfaces and showed room temperature photoluminescence (PL) emission between 3.6 and 4.0 /μm. Trans­mission electron microscope (TEM) imaging revealed a very high defect density of more than 10[10]10 cm-2 in the buffer layer and due to this, Shockley-Read-Hall (SRH) recombination was found to dominate the temperature quenching of the PL emission. Despite the structural problems with the material, the PL quenching performance compared well with other materials designed for this spectral range. Modelling of the quantum wells found that a conduction band offset ratio of 80% gave the best agreement with the experimentally determined transition energies. LEDs fabricated at Lancaster emitted in pulsed mode up to room temperature at 3.6 μm and with an efficiency of 34%. At room temperature SRH recombination was found to dominate the total recombination up to 350 mA drive current and this was also reflected in the temperature quenching of the LED output. From the fitting of the temperature dependence of the LED efficiency the SRH recombination centres were calculated to be 30-50 meV from the centre of the band gap. Fitting the room temperature LED emission spectra revealed that the emission comprised of transitions involving the first two heavy hole states as well as holes in the valence band of the barrier. The emission spectra from the edge of the LED mesa contained amplified spontaneous emission modes which were attributed to radial modes formed due to current crowding under the LED top contact. Lasers fabricated by QinetiQ were examined and from the gain spectra the internal loss was found to be -94 cm- 1 . This was attributed to the high defect density in the structure. The devices emitted at 3.2 /μm at 130 K and from po­larisation measurements it was found that the emission was completely polarised in the TE mode corresponding to emission from the heavy hole band. The lasers tested failed to reach operating temperatures above 130 K due to a sharp increase in the threshold current. An analysis of the temperature dependence of the thresh­old current provided evidence for hole current leakage as the cause of the increase in threshold current between 80 and 130 K.

In this thesis the transport of light through disordered, densely packed semiconductor nanowire mats is studied. It is found that the extremely high photonic strength of these samples leads to corrections to the traditional diffusion picture of light transport due to mesoscopic interference. Such effects are characterized by large intensity fluctuations and correlations, and it is found the transport is dominated by only a few independent transmission channels, close to the Anderson localisation regime. In addition to the strongly scattering nanowire samples, comparatively weakly scattering samples of ZnO are investigated, demonstrating mesoscopic effects in a less exotic, isotropic multiple scattering material. Control is obtained over the transmission by a combination of shaping the incident wavefront and harnessing the intrinsic nonlinearity of the semiconductor with ultrafast optical excitation. Through these techniques, a bright focus at an arbitrary point through the nanowires is created which can be modulated by up to 60% in a demonstration of a reconfigurable photonic switch.

Quasi-one-dimensional quantum models are ideal for theoretically exploring the physical phenomena associated with strong correlations. In this thesis we study three examples where strong correlations play an important role in the static or dynamic properties of the system. Firstly, we examine the behaviour of a doped fermionic two-leg ladder in which umklapp interactions are present. Such interactions arise at special band fillings and can be induced by the formation of charge density wave order in an array of two-leg ladders with long-range (three-dimensional) interactions. For the umklapp which arises from the half-filling of one of the bands, we show that the low-energy theory has a number of phases, including a strong coupling regime in which the dominant fluctuations are superconducting in nature. These superconducting fluctuations carry a finite wave vector – they are the one-dimensional analogue of Fulde-Ferrell-Larkin-Ovchinnikov superconductivity. In a second example, we consider a quantum spin model which captures the essential one-dimensional physics of CoNb2O6, a quasi-one-dimensional Ising ferromagnet. Motivated by high-resolution inelastic neutron scattering experiments, we calculate the dynamical structure in the paramagnetic phase and show that a small misalignment of the transverse field can lead to quasi-particle breakdown – a surprising broadening in the single particle mode observed in experiment. Finally, we study the out-of-equilibrium dynamics of a model with tuneable integrability breaking. When integrability is broken by the presence of weak interactions, we show that the system relaxes to a non-thermal state on intermediate time scales, the so-called “prethermalization plateau”. We describe the approximately stationary behaviour in this regime by constructing a generalised Gibbs ensemble with charges deformed to leading order in perturbation theory. Expectation values of these charges are time-independent, but interestingly the charges do not commute with the Hamiltonian to leading order in perturbation theory. Increasing the strength of the integrability breaking interactions leads to behaviour compatible with thermalisation. In each case we use a combination of perturbative analytical calculations and non-perturbative numerical computations to study the problem at hand.

This thesis presents the results obtained from de Haas-van Alphen experiments in ironbased superconductors. Measurements of the quasi particle mass in the quantum critical system BaFe2(Asl- xPxh are presented, which show strong enhancement towards the critical composition Xc = 0.3. This is found to be in good agreement with the prediction of a diverging behaviour of the effective mass close to a quantum critical point. Further results obtained on the lower and upper superconducting critical field of this system will be presented, which are found to contradict the expectations from Ginzburg-Landau theory based on results of the quasi particle mass and London penetration depth. However we can reconcile the different experimental findings on superconducting and normal state properties, in this quantum critical system, by considering a significant contribution from Abrikosov vortex core states. The importance of understanding the normal state electronic structure and interactions is shown to be essential for an understanding of the superconducting ground state of a quantum critical system. Further we will show a detailed de Haas-van Alphen study of the il1-type iron-based superconductor LiFeP and its isovalent partner LiFeAs. To understand the formation of nodes on the superconducting gap structure in systems with little change in their Fermi surface topology, is essential for the formulation of a microscopic theory of the pairing mechanism in pnictide-superconductors. 'While we find both systems to be close to the geometric nesting condition, we are able to point to different possible scenarios of the origin of nodes based on quasi particle mass enhancement. Extending the study of quasi particle mass and its relation to the superconducting properties to the stoichiometric high-temperature cuprate superconductor YBa2 CU4 Os, we aim to study the Fermi surface evolution under hydrostatic pressure. As the system has a very stable oxygen stoichiometry which does not allow it to be doped by oxygen ordering, we use hydrostatic pressure to tune the system unexpectedly leading to an increase in the superconducting critical temperature with almost no change to the quasi particle mass .

Specific heat measurements are presented on the unconventional superconductors BaFe2(Asl-xP xb YBa2Cu408 and Lio.gM060 17 · The development of a novel highprecision AC micro-calorimeter is also presented. An inexpensive and simple to fabricate SiN membrane-based AC micro-calorimeter was developed and successfully calibrated to an accuracy of 1 % between 6 K and 200 K. An appropriate thermal model was developed and the device shown to be capable of resolving transitions in heat capacity with an absolute accuracy of 5%, with good resolution of transitions down to 10 pJ /K. The total heat capacity of a sample at any given temperature can be determined to within 10%. Specific heat measurements of the supel'conducting transition in BaFe2(Asl_xP xh show that the quantum critical fluctuations renormalising the electron effective mass are robust to both temperature and magnetic field and that inter-band scattering is dominant giving equal renormalisation across the Fermi surface. The upper critical field has been shown to contradict theoretical predictions through a lack of enhancement around the quantum critical point, possibly due to mixing of antiferromagnetism in the vortex cores with the superconducting state. With reference to lower critical field measurements, the condensation energy is used to infer a strongly renormalised vortex core energy that indicates a direct enhancement of the superconducting state due to quantum criticality. The heat capacity of YBa2Cu408 was studied with magnetic fields applied along each of the orthogonal crystallographic directions. An enhancement in the density of states along the chain direction of 0.52 mJ mol- 1 K-2 was found in relatively low fields «1 T) consistent with the quenching of the superfluid density in the unhybridised region of the chain Fermi surface. Two values of the electron effective mass are deduced, mi = 3.9 ± OAme and m2 = 3 ± 0.3me, in excellent agreement with quantum oscillations. A large increase in the electron effective mass is seen on approach to the pseudogap end point up to a value of m* ~ 9.9me by analysing published data in a similar fashion. The condensation energy of Lio.gM06017 was determined and with reference to magnetoresistance data shows a breaching of any reasonable Pauli-limiting field thereby invoking an unconventional superconducting state. The superconducting critical temperature, the normal state specific heat, and the size of the superconducting transition are found to loosely correlate between different samples. Hall effect measurements on the same samples reveal that the strong upturn seen in the Hall effect cannot be a result of carrier loss thus refuting a density wave gap. A long-localisation scenario is proposed as an alternative explanation. This study also suggests that the sample dependence in Lio.gM06017 is a doping effect at all temperatures, likely due to the removal of electrons by excess oxygen. The Hall coefficient is observed to change sign at low temperatures and moderate fields.

Existing as much more than just a gemstone or refractory material, boron-doped diamond is a semiconductor with enormous potential as an electronic material. Diamond will form the basis of future high-power electronic devices, radiation-hard electronics and radiation detectors. It is also a highly effective electron emitter and a unique, biocompatible material for biomedical devices. Still, many questions remain surrounding the versatile, easy to grow polycrystalline form of boron-doped diamond. What is the surface atomic structure of these films after growth? How uniform is the boron-induced conductivity? How and why does the work function vary across the films? All of these properties affect how diamond electronic devices can be designed, fabricated and used. In this thesis we investigate nanoscale variation in properties across the surfaces of a number of differently grown boron-doped diamond films under ultra-high vacuum and evaluate the potential impact of the changes in these properties to surface electronic applications of diamond. Kelvin probe force microscopy results demonstrate significant variation in work function across the diamond surface, with a step change in work function of 0.8 eV measured between hydrogen and oxygen surface terminations, and variations across a single diamond surface are calculated to correspond to a 310% change in dopant concentration . The effect of this dopant variation is demonstrated by conductive atomic force microscopy studies in which entire crystallites exhibit insulating behaviour, with significant variation in conduction also observed across the surface. Finally, scanning tunneling microscopy studies of the diamond surface demonstrate that nanoscale roughness on large microcrystals is caused by the existence of a layer of nanocrystalline diamond at the surface. These nanocrystals persist throughout the growth process and exhibit many different surface reconstructions. The implications of all of these discoveries are discussed, with possibilities and suggestions for further work given also.

The nature of ultrafast acoustic strain generation and effects in III-V semiconductor-based nanostructures is explored in this thesis via experimental observations that are supported by theoretical analysis. Specifically, coherent phonon generation processes in bulk gallium arsenide (GaAs) are investigated through remote hypersonic detection using a double quantum well-embedded p-i-n diode, after which strain-induced effects in a double barrier quantum well resonant tunnelling diode are examined. Finally, preliminary studies on acoustic modulation of a double barrier quantum dot resonant tunnelling diode are also considered, with recommendations for future experimentation. It was experimentally observed that the transduction of strain in bulk GaAs produces an initial acoustic wavepacket that is strongly asymmetric with a heavily damped leading edge. This was determined to be due to photogeneration of a supersonically expanding electron-hole plasma near the irradiated GaAs surface. Coupled with its propagation from the free surface, the plasma generates stress and therefore strain in the system that is caused by a combination of the deformation potential and thermoelasticity; the former and latter are shown to be dominant for low and high optical excitation densities, respectively. These acoustic waves cannot escape the plasma until it has decelerated to subsonic velocities, which is achieved in a finite time, thus resulting in the observed asymmetry and damped leading edge. This finite acoustic escape time was reduced at high optical excitation densities due to plasma expansion limitation by increased non-radiative Auger recombination of electron-hole pairs. This conclusion is substantiated by analytical expressions derived from the inhomogeneous wave equation, and analysis of the spatially- and temporally-expanding plasma density based on the deformation potential mechanism only. Numerical simulations based on these expressions are fitted to the experimental data, and the thermoelasticity contribution at high excitation densities is deduced from a non-linear deviation of the electron-hole recombination rate and a change in the duration of the leading edge. This contribution expressed a square-law behaviour in the former parameter, which is attributed to non-radiative Auger processes. Strain-induced effects on a double barrier quantum well resonant tunnelling diode resulted in the detection of current modulation on a picosecond timescale only when the device was biased within its resonance region, with the largest modulations at the resonance threshold and peak biases. Through analysis of the device structure and stationary current-voltage characteristics, it is demonstrated that the observed current changes are due to variations of the resonant tunnelling rate caused by acoustic modulation of the confined ground state energies in the diode itself. Numerical analysis of the tunnelling rates provided excellent agreement with the experimental data, particularly when comparing charge transfer rates, where the limited temporal response of the experimental device could be ignored. Furthermore, the charge transferred at the resonance threshold and peak has a set polarity regardless of optical excitation density, and therefore the device possesses “rectifying” behaviour. As such, it has been demonstrated that, by exploiting this acoustoelectronic pumping effect, control of picosecond charge transfer in a resonant tunnelling diode or its application as a hypersonic detector are possible. In closing, the mechanisms for strain generation in bulk GaAs and the utilisation of the acoustoelectronic pumping effect in a double barrier quantum well resonant tunnelling diode are both exhibited in this work, and provide promising evidence and novel hypersonic detection methods for future research into ultrafast acoustic effects in semiconductor nanostructures.

This thesis is concerned with the development of condition monitoring for future design of high temperature superconducting (HTS) power apparatus. In particular, the use of UHF sensing for detecting PD activity within HTS has been investigated. Obtained results indicate that fast current pulses during PD in LN2 radiate electromagnetic waves which can be captured by the UHF sensor. PD during a negative streamer in LN2 appears in the form of a series of pulses less than 1 μs apart. This sequence cannot be observed using conventional detection method due to its bandwidth limitation. Instead, a slowly damped pulse is recorded which shows the total amount of charge transferred during this period. A study into PD streamer development within LN2 has been undertaken that reveals the characteristics of pre-breakdown phenomena in LN2. For negative streamers, when the electric field exceeds a threshold value, field emission from the electrode becomes effective which leads to the formation of initial cavities. Breakdown occurs within these gaseous bubbles and results in the development of negative streamers. For positive streamers, the process is much less well-understood due to the lack of initial electrons. However, from the recorded current pulses and shadow graphs, the physical mechanism behind positive streamer development is likely to be a more direct process, such as field ionisation, compared with the step-wise expansion in the case of negative streamers. The mechanisms that cause damage to solid dielectrics immersed in LN2 have been investigated. Obtained results indicate that pre-breakdown streamers can cause significant damage to the solid insulation barrier. Damage is the result of charge bombardment and mechanical forces rather than thermal effects. Inhomogeneous materials, such as glass fibre reinforced plastic (GRP), tend to introduce surface defects which can create local trapping sites. The trapped charges when combined with those from streamers can create much larger PD events. Consequently, damage observed on GRP barriers is much more severe than that on PTFE barriers under similar experimental conditions. Thus, design of future HTS power apparatus must consider this degradation phenomenon in order to improve the reliability of the insulation system.

This thesis presents a study on several series of unconventional Fe-based superconductors; namely, derivatives of NaFeAs and LiFeAs, as well as molecular-intercalated FeSe. Primarily using muon spin rotation (SR), but also x-ray diffraction (XRD) and magnetic susceptibility measurements, the nature of both magnetic and superconducting phases within these systems is studied. Particular attention is focussed on how these states compete or coexist with one another. The aforementioned experimental techniques are first used to explore the phase diagram of NaFe1xCoxAs. This phase diagram includes regions of long-range antiferromagnetism and short-range order, that both coexist with superconductivity. Magnetism is gradually destroyed, primarily through a diminishment of the size of the ordered moment, as superconductivity is enhanced by Co substitution. This interplay is explored in detail. By contrast, superconductivity in LiFeAs cannot be enhanced by transition metal substitution, suggesting that it is intrinsically optimally-doped. I investigate this conclusion by studying the evolution of the penetration depth in superconducting compositions of LiFe1xCoxAs and LiFe1xNixAs, and comparing these data to those from other electron-doped systems. I also study an unusual and emergent magnetic phase in Li1yFe1+yAs. This work suggests that LiFeAs supports a superconducting phase that resembles those in other Fe-pnictides, but is uniquely close to an additional magnetic instability. I then move on to the study of a recently discovered series, based on FeSe intercalated with ammonia and various metals. I study both the penetration depth and the intrinsic magnetic phases in these systems using SR and compare them with other compounds based on FeSe. I find that these intercalated systems support spacially separated regions of dynamic magnetism and superconductivity and I discuss how much these phases depend on the precise chemical details of the intercalated layer. Finally, I return to the experimental study of NaFe1xCoxAs, extending the range of techniques employed by using high-field magnetometry and high-pressure SR. These studies reveal new features of this system that were not accessible using low-field and ambient-pressure methods. In particular, I show that the magnetic moment size in NaFeAs unexpectedly increases with pressure, suggesting that the electronic structure of this compound may be unique amongst known Fe-based superconductors.

This thesis describes inelastic neutron scattering (INS) experiments on several iron-based materials. The experiments were primarily designed to investigate the link between magnetic dynamics and superconductivity. The work contributes to evidence that magnetic fluctuations influence or are influenced by superconductivity. It is demonstrated that the INS response of a material, in conjunction with theoretical models, can provide valuable information about both superconductivity and magnetism. I measured the magnetically ordered parent-compound SrFe2As2 to investigate the nature of magnetism in iron-based systems. Comparison of the data to models based on both itinerant and localised magnetism showed that an itinerant model offers the best description of the data. LiFeAs is a superconductor that shows no magnetic order, however I was able to distinguish a magnetic signal in its INS spectrum. The signal is consistent with the magnetic resonance observed in several other iron-based superconductors. This indicates that LiFeAs likely hosts an s± gap symmetry. I investigated two iron-phosphide systems, LaFePO and Sr2ScO3FeP, and in this case I was unable to identify any magnetic scattering. Comparison to LiFeAs showed that any signal in LaFePO is at least 7 times weaker. These results suggest that magnetic fluctuations are not as influential to the electronic properties of iron-phosphide systems as they are in other iron-based superconductors. In CsxFe2−ySe2 I found two independent signals that appear to be related to phase-separated magnetic and superconducting regions of the sample. I showed that fluctuations associated with the magnetically ordered phase are consistent with localised magnetism, and do not respond to superconductivity. The second signal, however, increases in intensity below the superconducting transition temperature Tc = 27K, consistent with a magnetic resonance. This could be indicative of a pairing symmetry in CsxFe2−ySe2 that is distinct from most other iron-based superconductors. Finally, the molecular intercalated FeSe compound Li0.6(ND2)0.2(ND3)0.8Fe2Se2 revealed strong magnetic fluctuations. Again the signal was consistent with a magnetic resonance responding to Tc = 43 K. The results suggest that Lix(ND2)y(ND3)1−yFe2Se2 is similar to the superconducting phase of CsxFe2−ySe2, placing constraints on theoretical models to describe the molecular intercalated FeSe compounds.

Charge transport is the one the most fundamental concepts in organic semiconductors. The key quantity that characterises this transport behaviour is carrier mobility. The ability to transport carriers in a fast and unimpeded nature in organic devices such as Organic Photovoltaics (OPV) or Organic Light Emitting Diodes (OLED) is a key parameter for building more efficient devices. Significant steps have so far been taken to understand and model this phenomenon, however there are still many questions that need to be answered. One such fundamental question is the role of excited states on the charge transport properties of organic materials which historically has been ignored. This thesis aims to investigate the transport properties of two of the most widely used organic materials, N,N′-bis-[(1-naphthyl)-N,N′-diphenyl]-1,1′-biphenyl)-4,4′-diamine (NPB) and are N,N’-diphenyl-N,N’-bis 3-methylphenyl-1,1’-biphenyl-4,4’-diamine (TPD). We demonstrate how excitons are generated in a single organic layer OLED devices and how traditionally hole transport materials are capable of fast long range electron transport. We provide a comprehensive analysis of the charge transport properties of both materials with respect to the Gaussian Disorder Model (GDM) and demonstrate how both types of carriers can easily be transported in these materials. We then investigate the effects of exciton generation on the transport properties of the materials and propose some numerical modeling to analyse the effects of such excited states and the distribution of energetic traps in our system. We show that the swing of carrier mobility in either direction depends on the interplay and dominance of each mechanism (triplet/carrier interaction and trap filling). We also investigate the effects of 5 removing excited states from our device by deliberately introducing impurities via doping of a phosphorescent molecule to alter their concentration. Finally we propose some future direction that one can take to model charge transport behaviour in disordered organics based on the experimental work discussed in thesis.

Measurement of the I(V ) characteristics of single molecules is the first step towards the realisation of molecular electronic devices. In this thesis, the electronic transport properties of alkanedithiol (ADT) and alkylthiol-terminated oligothiophene molecules are investigated under ultra high vacuum (UHV) using a scanning tunnelling microscope (STM). Two techniques are employed that rely upon stochastic molecular bridge formation between gold STM tip and substrate; a novel I(V; s) method is proven to be a powerful alternative to the well-known I(s) method. For ADTs, three temperature-independent (180 - 390 K) conduction groups are identified, which arise from different contact-substrate coordination geometries. The anomalous reduction of conductance at small chain lengths reported by other groups for non-UHV conditions is far less pronounced here; all groups closely follow the anticipated exponential decay with chain length, β = (0.80 ± 0.01) Å ¹, until a small deviation occurs for the shortest molecule. Thus, the likely explanation for the anomalous effect is hydration of thiol groups. The I(V ) curves were fitted using a rectangular tunnel barrier model, with parameters in agreement with literature values; m = (0.32 ± 0.02) m, φ = 2 eV. For the oligothiophene molecules, one common temperature-independent (295-390 K) conduction group was identified; the conductance decays exponentially with molecular length, with different factors of β = (0.78 ± 0.15) Å ¹ and β = (0.16 ± 0:04) Å ¹ for length changes to the alkylthiol chains and thiophene backbone, respectively. An indented tunnel barrier model, anticipated from the physical and electronic structure of the molecules, was applied to fit the measured I(V ) curves; φ1 = φ3 = 2 eV, φ2 = 1.3 to 1.6 eV, m = 0.17 to 0.24 m. These UHV measurements provide an important baseline from which to better understand recent reports indicating hydration-dependent, and hydration-induced temperature-dependent, transport properties.

The negatively charged nitrogen vacancy centre is a promising candidate for use as a single photon source for linear optical quantum information, and as a solid state spin for solid state quantum information and room temperature magnetometry. However low photon collection efficiency is a problem for each of these applications. We demonstrate how photon losses due to refraction can be eliminated through the use of Solid Immersion Lenses (SILs) nano-fabricated on the surface of diamond. Coherent electron spin manipulation and readout is demonstrated on NV- centres under SILs. We show initial investigations into the effects of FIB fabrication on the NV- centre's coherence time, and demonstrate unitary quantum process discrimination on between two non orthogonal processes. In order to improve collection efficiency further it is necessary to couple NV- centres to optical micro cavities. This requires a higher degree of precision in the measurement of the NV- centres position than is possible using conventional confocal microscopy. We investigate spectral self interferometric microscopy as a method for precision measurement of the depth of an NV- centre. Finally we show coherent manipulation of photons emitted from a near infra-red colour centre in diamond using a single integrated waveguide chip. This is used to verify wave particle duality of the photons.

As a multipotent tool for scientific exploration, semiconductor nanoparticles, or quantum dots (QDs), have gained enormous interest in nanoscience in the past two decades. The research presented here focused on cadmium telluride (CdTe) QDs: novel synthetic methodologies were used to prepare previously inaccessible nanomaterials based on CdTe QDs. -- CdTe/CdSe/ZnSe core/shell/shell QDs were prepared by a one-pot synthesis. The resulting QDs exhibited near infrared emission, were readily dispersed in aqueous media and applied to deep tissue imaging where emission through the skin indicated the gradual transition of the QDs via the lymphatic tract. -- Using a different synthetic approach, CdTe QDs, which were dispersed in organic media, were exposed to mercury cations in a toluene/methanol solution, resulting in CdHgTe nanoalloy formation. The optical characteristics of the resulting materials were substantially red-shifted from those of the original CdTe QDs. Structural changes were also examined and the influence of the addition of metal cations to other colloidal QDs. -- The organometallic compound Cd(TeC6H5)2 was synthesised and used as a single-source precursor for CdTe QDs. Products isolated after thermal decomposition of the single-source precursors showed strong emission of various wavelengths depending on the reaction time. The underlying chemistry on QDs formation was investigated, and CdTe/ZnS QDs were prepared using only single-source precursors. -- To make the QDs useful in biology, the surface of organically synthesised CdTe/ZnS QDs was modified with an amphiphilic protein (hydrophobin) to phase transfer the particles into aqueous solution. The QDs exhibited bright emission after phase transfer, and were applied to cell imaging in order to examine the validity as a fluorophore and the influence on a cell.

In this work the Path Integral quantum Monte Carlo (PI-QMC) method has been used to study exciton complexes in semiconductor nanostructures. This powerful technique allows for coulomb correlations in these complexes to be correctly treated, and at the same time allowing for nite temperature simulations in an arbitrary external potential without the need for complicated trial function or basis set information. Quantum dots and rings were modelled using both analytic potentials, and by potentials derived from atomistic models of these structures, including strain and piezoelectric e ects. The e ect of strain and the piezoelectric potential on quantum rings is explored, and rings are shown to have a unique strain and piezoelectric pro le which directly impacts observables. This unique piezoelectric potential in quantum rings is exploited by use of vertical electric elds, to induce a novel lateral switching of the exciton and biexciton probability distributions when the direction of the applied eld is switched. Calculations of in-plane polarizability suggest the switching would be observable experimentally. The diamagnetic susceptibility of quantum rings and dots are investigated, and accurate reproduction of experimental results are shown { which require the proper treatment of coulomb correlations. Finally, the transition between a bound and anti-bound biexciton in a core/shell Type-II colloidal quantum dot, with increasing shell thickness is for the rst time theoretically shown. Excellent agreement with experimental results are seen, and these results are contrasted with previous perturbative results which miss this transition from the literature.

In this thesis the development of new analysis methods that study the carrier distributions in quantum dots (QDs) directly from experimental measurement of spontaneous emission and gain spectra are described. These were applied to three InAs QD structures that are nominally identical except for the doping type in the active region, one p-doped, one n-doped and one left un-doped for comparison. The effect that carrier localisation within individual dots had on this temperature dependence of the carrier distribution under injection was studied and related to key aspects associated with laser device performance. The nature of QD occupation in the three samples was determined through measurement of the carrier temperature (TC) of the electrons populating the QD states. It was found that the un-doped samples QDs were in thermal equilibrium with the bulk lattice down to 200 K. Below this temperature the sample’s QD states become decoupled from the lattice and at 60 K QD occupation was shown to be random. The p-doped sample was shown to be non-thermal between 300 K and 200 K where at 150 K the occupation of QDs became random. The TC was observed to decrease for this sample below 200 K and this was attributed to fewer dopants ionising as the temperature decreased. The n-doped sample was also shown to be non-thermal between 300 K and 200 K with the QD occupation becoming random at 100 K. In all three samples, above 300 K, the measured TC was lower than that predicted by a Fermi-Dirac distribution and this was attributed to the these QDs having a large number of closely spaced hole states leading to a size dependence of the number of these states. This means an individual ΔEf exists for a given set of dot sizes. So emission from an ensemble of dots is “smoothed” across different ΔEf levels leading to a reduction in the apparent TC. These results have a significant effect on the threshold current densities of these samples and suggest that the differences observed due to doping will not be reproduced by calculations assuming a quasi-thermal equilibrium across the QD structure. The temperature dependence of the shift in gain peak energy was determined for the un and p-doped samples. This showed that the blue-shift of the gain peak due to state-filling in un-doped QD structures is independent of temperature, at a given value of peak gain, over the temperature range studied (200 K to 350 K). In the pdoped sample however, the state filling is temperature dependent at any fixed gain with a shift of 8meV observed between 200 K and 350 K. This was attributed to the wide electron state distribution and the lowering of the electronic quasi-Fermi level by the p-doping. This renders p-doped materials unsuitable for any technology application where gain peak wavelength temperature stability is required for efficient operation.

This thesis comprises a theoretical study of the dynamics of indirect excitons in coupled quantum wells at low lattice temperatures. The results of numerical simulations of the exciton photoluminescence pattern are presented and compared to available experimental data. The in-plane transport of quantum well excitons created by laser excitation is modeled using a non-linear drift-diffusion equation. Combined with a model of exciton relaxation thermodynamics, a complete description of the evolution of the exciton density and temperature is built. The optical decay of indirect excitons is included in the modeling. This is used to make predictions of the spatial photoluminescence patterns which have been observed experimentally. The transport of dipole orientated excitons via externally applied electrostatic potentials is also studied. The drift-diffusion equation is adapted to include the inplane electric field. This is done for some specific forms of the potential landscapes such as a linear potential energy gradient and a propagating lattice. These correspond to some recent experiments for which results are available. The combined theoretical and experimental studies reveal a deeper insight into the transport properties of indirect excitons. Finally, the external ring structure in the indirect exciton emission pattern is studied. Its formation is modeled using a set of coupled transport equations for electrons, holes and indirect excitons. The Coulomb interactions between all three species are incorporated in the model. It is shown that these interactions lead to an instability in the external ring and are responsible for its fragmentation into a periodic array of islands which has been observed experimentally.

Neutron scattering from the free surface of liquid He has been used to investigate the distribution of 3He atoms in extremely dilute 3He-4He solutions at temperatures down to 0.08 K In particular evidence has been sought and found for the existence of Andreev trapping states for 3He just below the surface. The experiments were conducted in ISIS at the Rutherford Appleton Laboratory using the CRISP instrument (a general purpose reflectometer). The neutron scattering from the surface of 4 He (industrial and high purity) also was investigated and compared with dilute 3He-4He solutions. The 3He surface layer was observed for the first time using a small-angle neutron scattering (SANS) technique. The thickness of the layer is strongly dependent on temperature and 3He concentration. Even in the case of industrial 4He and even at 2.3 K (higher than superfiuid transition) a substantial amount of 3He atoms was found close to the surface. In the case of extremely pure 4He the 3He atoms on the surface were not observed. At low temperature (0.4 K) a very thin layer of 3He atoms was observed and the calculated thickness (based on our experimental results) is approximately ~10 A of pure 3He on the top of the mixture. It is followed by a diffusive interface area (also observed experimentally) of 3He-4He mixture of thick layer approximately ~200 A, just below that almost pure 3He thin layer on the top. With increase of the concentration the thickness of the top layer increases until it reaches a limit above which the 3He starts dissolving in the bulk superfiuid liquid 4He. Neutron critical opalescence from 3He gas/vapour was observed for the first time and the information for 3He density was obtained in a broad range of temperatures 1.4 - 50 K. The data was compared with the results gathered by using different techniques. Far from the critical temperature all methods yield similar results. However, near the critical temperature of Tc = 3.32 K, the densities obtained from neutron transmission results are lower than other published results.

The major challenges in trapped-ion quantum computation are to scale up few-ion experiments to many qubits and to improve control techniques so that quantum logic gates can be carried out with higher fidelities. This thesis re- ports experimental progress in both of these areas. In the early part of the the- sis we describe the fabrication of a surface-electrode ion trap, the development of the apparatus and techniques required to operate it and the successful trap- ping of 40Ca+ ions. Notably we developed methods to control the orientation of the principal axes and to minimise ion micromotion. We propose a repumping scheme that simplifies heating rate measurements for ions with low-lying D levels, and use it to characterise the electric field noise in the trap. Surface-electrode traps are important because they offer a route to dense integration of electronic and optical control elements using existing microfabrication technology. We explore this scaling route by testing a series of three traps that were microfabricated at Sandia National Laboratories. Investigations of micromotion and charging of the surface by laser beams were carried out and improvements to future traps are suggested. Using one of these traps we also investigated anomalous electrical noise from the electrode surfaces and discovered that it can be reduced by cleaning with a pulsed laser. A factor of two de- crease was observed; this represents the first in situ removal of this noise source, an important step towards higher gate fidelities. In the second half of the thesis we describe the design and construction of an experiment for the purpose of replacing laser-driven multi-qubit quantum logic gates with microwave-driven ones. We investigate magnetic-field-independent hyperfine qubits in 40Ca+ as suitable qubits for this scheme. We make a design study of how best to integrate an ion trap with the microwave conductors required to implement the gate and propose a novel integrated resonant structure. The trap was fabricated and ions were successfully loaded. Single-qubit experiments show that the microwave fields above the trap are in excellent agreement with software simulations. There are good prospects for demonstrating a multi-qubit gate in the near future. We conclude by discussing the possibilities for larger-scale quantum computation by combining microfabricated traps and microwave control.

The main focus of the dissertation is description, modeling and understanding of the mechanisms underpinning electroluminescence from quantum wells. The dissertation contains original contribution of methodological and phenomenological character. We have described in detail the eight band model within the envelope function approximation(EFA) using the Löwd in perturbation method used for band structure calculations. Although not novel, a detailed derivation of this is rarely done in the literature. We have derived a theoretical expression for electroluminescence spectral emittance based entirely on quantum mechanical model, unlike the more usual semi classical models used in semiconductor physics. The final expression for the spectral emittance has a different dependence compared to the semi classical expression, namely the prefactor in the newly derived expression is proportional to 2 . We use the combination of 8 band EFA method and the newly derived expression for spectral emittance to interpret experimental measurements on unpolarized spectral emittance from several InSb/AlxIn1-xSbquantum wells. We do that using slightly novel procedure and identify several transitions unreported in InSb/AlxIn1-xSb material system up to now. In simplified models these are regarded as forbidden. We show that in 8 band EFA model there aren’t any forbidden transitions. Instead all transitions are allowed and we discuss the product of momentum matrix elements and 2D density of states, to which we refer as "generalized selection rule", as the quantity which determines the strength of the individual transitions in different energy ranges. Furthermore we discuss three groups of mechanisms which determine various properties of the electroluminescence spectrum. These groups are entirely general to electroluminescence from all sorts of quantum wells. They are: (i) band structure embodied in the "generalized selection rules" ; (2) broadening effects and (3) statistical effects. Very important are the effects of structure inversion asymmetry (SIA) on the "generalized selection rules" and the spectral emittance, which we describe and explain. Finally we discuss aspects of two other major themes related to the two characteristic properties of InSb:(i) the broken space inversion invariance and (ii) the relativistic correction of spin-orbit coupling.

The focus of this work is to investigate the effects of applying an external magnetic field to a solidifying liquid metal melt. The principle is that thermoelectric currents that are naturally inherent to solidification processes will interact with this magnetic field, resulting in a Lorentz force. This force will exist in a microscopic region in the vicinity of the solidification front, generating microscopic fluid flow in the liquid region which can significantly effect the mechanism of dendritic growth. The work contained in this thesis provides an initial insight into the complex behaviour of this process, through the use of numerical models. To model the soldification dynamics, an enthalpy based model for dendritic growth in a supercooled melt is used in 2-dimensions and extended into 3-dimensions. The dendrite is defined as being equiaxed in nature and, for purely diffusion driven growth, numerical calculations show a good agreement with other methods under similar growth parameters. To investigate the effects of fluid dynamics, dendritic growth is tested under forced convection conditions and significant morphological changes occur. The incident tip velocity is increased and the downstream tip velocity is decreased; in agreement with many other authors investigating similar situations. In the presence of a magnetic field the Lorentz force will form in planes perpendicular to the direction of the magnetic field. Due to the morphology and anisotropy of the surface temperature, the nature of the flow is dependent on the relative orientation of the magnetic field and the crystallographic orientation of the lattice. Using a low magnetic field strength approximation, thus removing the non-linear and resistive terms in Navier-Stokes equation, the resulting fluid velocity is arbitrarily small so that convective transport is negated. At some time, when the morphological features of a dendrite are apparent, steady state simulations show the flow fields that exist with different orientations of the magnetic field. The results are compared to an analytic solution for the Lorentz force, which is described by reducing the morphology of a dendrite to a sphere and assuming that the surface temperature is equivalent to the anisotropy in the surface energy. When the thermoelectric currents are large and the magnetic field strength is substantial the convective transport, non-linear and resistive terms become significant. The problem is purely 3-dimensional and it is shown that classical 2-dimensional boundary conditions lead to stagnant conditions. A 2-dimensional quasi 3-dimensional approximation is proposed and, with the magnetic field orientated in the (001) direction, the effect of heat and solute redistribution through convection on the crystal morphology is modelled. Two significant morphological changes occur; the first is a deflection of the dendrite tip and the second is the initiation of secondary branching into the incident flow. The deflection is caused by circulations at the tips of the dendrite; the circulations continuously provide a region of higher free energy on the incident side while lowering it on the other. The net effect is a bias of growth in the direction of incident flow. The increase in secondary branching, in a similar fashion to the deflection, is caused by both a circulation at the tip and also a global circulation around the entire dendrite, destabilising the incident interface and initiating secondary growth. To qualify the quasi 3-dimensional approximation, a moving mesh technique is developed that tracks a single tip of 3-dimensional growth and the similar morphological features are observed in comparison to the quasi 3-dimensional case. Finally a discussion into possible extensions of this work is proposed and preliminary results for grain growth in the presence of a magnetic field are given.

Dilute magnetic semiconductors are an important family of materials that have many potential applications in spintronics; (Ga,Mn)As, (In,Ga,Mn)As and (Ga,Mn)N are of major interest. This thesis investigates dierent aspects of these, using the synchrotron radiation techniques of x-ray magnetic circular dichroism (XMCD) and x-ray magnetic linear dichroism (XMLD), supported by superconducting quantum interference device (SQUID) magnetometry and magnetotransport measurements. A large anisotropic XMLD signal is observed for the Mn L-edge in (Ga,Mn)As. In unannealed (Ga,Mn)As, an apparently reduced Mn magnetic moment is commonly observed. It is thought to be related to compensation of both carriers and magnetic moment, caused by interstitial Mn. This issue is investigated using combined data from XMCD, XMLD and SQUID magnetometry. The findings suggest that substitutional and interstitial Mn form `non-magnetic' pairs which do not have a preferred spin orientation. (Ga,Mn)N is studied by x-ray absorption and field-dependent XMCD at the Mn L-edge. Two distinct Mn congurations are identified: Mn2+ is prevalent towards the surface with nearly paramagnetic behaviour, while a weakly ferromagnetic Mn2+/Mn3+ mixed valence exists within the bulk. The weak ferromagnetism, often observed in (Ga,Mn)N, is attributed to coupling between the impurities by the double exchange mechanism. Finally, XMCD is used to measure the orbital polarization of As 4p states of (III,Mn)As materials. These states correspond to those of the holes involved in the itinerant exchange interaction in ferromagnetic semiconductors. The coupling between the localized d states of the magnetic impurities and the valence band p states of the host is demonstrated by an anisotropy in the orbital moment of these states. This is experimental confirmation of the origin of the magnetocrystalline anisotropy in dilute magnetic semiconductors.

This thesis tells the story of the Balkanization of the theory community in High Temperature Superconductivity (HTS) and of the many roles experimental evidence has been playing in the battles there. In the twenty-five years that followed the discovery of HTS, the Condensed Matter Physics (CMP) community has experienced extreme difficulty in trying to reach a consensus on a 'final' theory. I will explore some of the reasons for such dissent, starting from testimonies that I collected through personal interviews with HTS physicists. I will focus on the way experiments actively contribute to the formulation of theories. I claim that there is a tension between the different methods and aims of two scientific traditions as they implement the contribution from experiments. This tension will be illustrated through the discussion of several episodes from the history of Superconductivity and CMP research. In particular the paradigmatic quarrels between two of the major players in the history of superconductivity, physicists PW Anderson and B Matthias, will be presented to explore the meeting of theoretical and experimental driving forces and their impact on the evaluation of theories and research programmes. I will also argue that the ambiguity in the theories of evidence employed by the warring camps in HTS allows each of them to claim empirical adequacy for itself and deny it to the opponents, and I shall raise questions about whether the standards of evidence employed there are consistently applied and grounded.

The dilute nitrides on GaAs (e.g. Ga(In)NxAs1-x where typically x < 5%) have provided a large amount of interesting physics and led to the development of improved telecommunication lasers. In the last 5 years, novel dilute nitride materials (such as GaN(As)P) have grown in popularity due to applications such as monolithic growth on silicon and improved device characteristics in red-amber-yellow solid state illumination. The Band Anti-Crossing model, using a single weighted nitrogen level (found through photocurrent spectroscopy), is found to successfully predict the band gap energy in GaNP/GaP and GaNAsP/GaP and bulk GaInNAs/GaAs with nitrogen composition, temperature and high hydrostatic pressure. An investigation over a range of host materials found that the coupling parameter can be modelled in terms of the energy separation between the host conduction band minimum and the nitrogen level. GaNAsP/GaP single quantum well lasers are a realistic possibility for an efficient laser source grown on silicon for monolithic optoelectronic integrated circuits. Large threshold current densities of over 0.75kA/cm2 at 80K and 890nm and large temperature variations in the threshold current are shown to be due to non-radiative loss processes. Low characteristic temperatures of the threshold current and differential efficiency (T0=60K and T1=30K at 130K) are found. The sub-linear dependence of the spontaneous emission vs current curve and increasing threshold current with high pressure reveal this loss process is carrier leakage. It is shown that the leakage is not into the X-minima of the GaP barriers, but associated with nitrogen levels in the GaP barriers. If this leakage path can be eliminated, a low room temperature threshold current density should be achieved. (In)GaNP/GaP LEDs are studied for use in red-amber-yellow illumination applications. Through high pressure measurements, it is found that 90 +/-10% of the current flowing through a GaN0.006P0.994 bulk LED is lost to carrier leakage at room temperature and pressure, at a current density of 40A/cm2. When using an In0.14Ga0.86N0.006P0.994 quantum well (of thickness 100 A) active region, the electron potential barrier is increased and carrier leakage accounts for 50+/-15% of the device current density (40A/cm2) at room temperature and pressure. At 100K it was found the leakage reduces to below 10% at current density of 40A/cm2 in all devices. If the leakage path can be reduced, these devices may offer better device characteristics than current red AlInGaP LEDs. Initial investigations on bulk GaInNAs avalanche photodiodes show that with an increase of nitrogen concentration and reduction in band gap, the breakdown voltage increases, consistent with the nitrogen suppressing electron impact ionization as predicted by theory. The pressure coefficient of the breakdown voltage in GaInNAs was found to be larger in magnitude and opposite in sign (dVbd/dP=+5.5+/-0.5 x10-3kbar-1) than the pressure coefficient of the breakdown voltage in GaAs, showing that nitrogen induces a profound change in the carrier scattering mechanisms, which may provide low-noise, high sensitivity detectors at telecommunication wavelengths.

This dissertation consists of a theoretical investigation into the transport and coherence properties of indirect excitons in coupled quantum wells (QWs) at helium temperatures. The motion of excitons along the quantum well plane is described through a quantum diffusion equation and the possibility of excitonic cloud formation is studied both due to the natural potential fluctuations and externally applied confining potentials. The photoluminescence (PL) of decaying excitons is used as a probe for their properties such as concentration, effective temperature and optical lifetime. The exciton thermalisation from an initial high energy to the lattice temperature is achieved within their lifetime due to a very effective coupling between the exciton states and a continuum of phonon states, a direct consequence of the relaxation of momentum conservation along the growth direction of a QW. Moreover, the natural spatial separation between electrons and holes prevents their recombination, resulting in long lifetimes. The dynamics of the system of excitons in optically-induced traps is also studied and the numerical solution of the quantum diffusion equation provides an insight into the extremely fast loading times of the trap with a highly degenerate exciton gas. The hierarchy of timescales in such a trap allows for the creation of a cold and dense gas confined within the trap, opening a new route towards the long sought Bose-Einstein Condensation (BEC) in solid state. Finally the issue of exciton spatial coherence is studied and an analytic expression for the coherence function, i.e., the measure of the coherence in a system, is derived. A direct comparison with large coherence lengths recently observed in systems of quantum well excitons and microcavity polaritons is attempted and interesting conclusions are drawn regarding the build up of spontaneous coherence in these systems.

In-situ surface x-ray scattering measurements of the electrode/electrolyte interface have been made along with complementary electrochemical and ultra-high vacuum measurements. Using these techniques, two main systems 'have been studied; bimetallic surfaces which have shown enhanced activity for the oxygen reduction reaction (ORR) and single-crystal surfaces to probe the effect of temperature on the interfacial structure and surface reactions. The Pt3Ni(111) electrode surface has been found to be the most highly catalytically active surface ever detected for the ORR, 1/202 + 2H+ + 2e- = H20. , In-situ surface x-ray scattering measurements of this surface revealed that the Pt concentration profile oscillates over three atomic layers, whereby the oute'rmost atomic layer was found to be pure Pt, followed by a Ni-rich second layer and a slightly Pt-rich third layer, before reaching the bulk occupation value. It is the concentration profile which causes the surface' electronic structure of Pt3Ni(111) to be significantly different from the atomically identical surface of pure Pt(lll). The surface electronic structure of Pt3Ni(111) acts to optimise conditions for the adsorption of reactant species involved in the ORR. Density-functional theory calculations have indicated that 670-atom octahedral Pt3Ni nanoparticles, where each surface plane is [111] orientated, would be thermodynamically stable and have approximately the same Pt concentration profile as the Pt3Ni(111) single crystal, which, if synthesised, would prove to be an improved catalyst for practical application in fuel cells. The solution/substrate temperature plays an important role in determining the molecular adsorbate structure of CO (COad ) formed on the Pt(111) surface. At electrode potentials approaching hydrogen evolution, 0.05 V (versus the reversible hydrogen electrode), the (2x2) and vII9 CO-structures coexist at high temperature (rv45°C). This is due to a significant negative shift in the onset of OH adsorption and subsequent partial oxidation of COad through the LangmuirHinshelwood reaction (COad +OHad --+ CO2 +H+ +e-). The second investigation into the role of temperature in electrochemical systems has been made on the surface reconstruction of the AU(100) and Au(111) electrodes. Surface reconstruction involves the rearrangement of surface atoms and can be directly monitored by in-situ surface x-ray scattering measurements. The AU(100) reconstruction is greatly enhanced at high temperature, rv45°C, however, the Au(lll) reconstruction is unaffected by temperature changes. The apparent discrepancy is due to the initial step by which the respective reconstructions form. As observed in the CO/Pt(l11) system, the onset of OH adsorption is shifted negatively with increased temperature. The negative shift increases the nucleation sites of hex-strings, the initial step in forming the Au(100) reconstruction, which causes an enhancement of the reconstruction.

Torque magnetometry techniques have been employed to study the quantum Hall effect in several AIGaAs/GaAs-heterostructure two-dimensional electron gas samples at filling factors between 1 and 4. Two magnetometers were employed to acquire measurements for three samples an existing instrument was used to acquire data for two samples and a novel instrument has been developed in which the output signal sensitivity is increased by 700% during experiments on a third sample. The third sample was also illuminated in situ. The samples exhibit breakdown-like behaviour in two forms. The first is the saturation of the magnetic moment peak size with respect to increasing sweep rate. A simple charge-up model, which treats the charge density of the sample in terms of a capacitance around the edge of the sample, was used to analyse experimental data. Temperature dependence of the longitudinal conductivity is analysed with respect to a published model of Polyakov and Shklovskii. The charge-up time constant, rc (10--105) seconds, decreased with increasing temperature and was found to follow a straight line when plotted on a logarithmic scale against temperature. Characteristic temperatures extracted from the data lie in the range T0 * (0.2--2.0) Kelvin. Decay time measurements were performed to acquire the decay time constant rd. Two regimes of decay were observed, the first exhibiting a time constant of the order of several seconds followed by a second phase with a much larger time constant many minutes or hours. The second form of breakdown was demonstrated as a type of noisy breakdown clearly observed at filling factor 2 in two samples. This noise, of the form of sudden jumps followed by more gradual growth, was interpreted using an edge charge-up model and is thought to be consistent with the sand-pile model of the theory of self-organised criticality. As a result the frequency of occurrence of noisy jumps as a function of their particular size is seen to follow a power law. Time constants of individual noise jumps were found to lie mainly in the range (1--10) seconds.

The problem of design optimisation of thin film direct current Superconducting QUantum Interference Device (SQUID) magnetometers made of YBCO (YBa2Cu3O7-x) was considered. The inductances and effective areas were calculated using the software package 3D-MLSI. Resolution and reliability issues were first tested on simple superconducting systems, showing good agreement with analytical formulae and experimental results, and demonstrating that a remarkable precision can be obtained though at the expense of CPU time and memory. The software was then used to simulate a SQUID magnetometer fabricated in the Device Materials Group of the Department of Materials Science and Metallurgy, proving that 3D-MLSI can be used to predict the parameters of real systems with acceptable accuracy.

Studies have been undertaken trying to produce new high Tc, superconducting cuprates. As a mechanism for superconductivity in these materials is sought, it is important to produce new materials with new structural features. Studies were carried out on three systems: bismuthates, quadruple perovskites and ruthenates. Bismuthate work was aimed at trying to intergrow Sillen type phases with layered cuprate phases. This work yielded four new Sillen related structures: CaBiO2Cl [P21/m : a = 7.7311(1)Å, b = 4.1234(1)Å, c = 6.3979(2)Å and ß = 105.21(1)0] has a puckered halide layer due to the lower co-ordination preference of the small Ca2+ ion. SrBiO2C1 [Cmcm : a = 5.7109(2)Å, b = 12.4081(5)Å, c = 5.5888(2)Å] is a Sillen X1 type material with an ordering of Sr and Bi in the double fluorite layers as previously observed in BaBiO2C1. Bi2LaO4C1 [I4/mmm: a = 3.9547(1)Å and c = 9.1275(3)Å] has triple fluorite layers with ordering such that La occupies the central layer only. These layers are separated by planar Cl layers. Isostructural Bi2NdO4C1 and Bi2YO4CI have also been prepared. Bi6Ca5O11Cl6[Cmcm: a = 3.865(6)Å, b = 12.57(2)Å and c = 43.05(7)Å] has an X1 related, Arppe type structure, with undulating (M2O2) layers, determined from microcrystal diffraction data. Microcrystals formed on reaction of the stoichiometry Bi1.7Ca2.7O4-yCl4 were shown to possess a Sillen X1X3 type structure, confirming a previous report. Work on perovskites involved doping studies of the Quadruple Perovskite series Ln2Ba2Cu2Ti2O11 (Ln = Nd, Gd and Tb). Investigation of Sr doping for Ba in this series is reported, as is Cu substitution for Ti in the Nd system. Work on trying to extend the observation of both superconductivity and ferromagnetism in the ruthenate RuSr2GdCu2O8 involved investigation of RuSr2TbCu2O8 1-2-1-2 and quadruple perovskite Ln2Ba2Cu2Ru2O11 (Ln = La, Nd, Gd and Tb) and NdBa3Cu2TiNb1-xRuxO11 systems.

The techniques for the computational simulation of solids have, in recent years, been developed to the level that they now provide a predictive tool. The increased understanding of interatomic forces in solids coupled with improvement in numerical and computational methods allows the study of solid state properties, including structural, thermodynamic and transport properties. This thesis presents both development and application studies on two topical classes of materials: first organic superconductors, and secondly metal oxides and their surfaces. A detailed computational study of the organic superconducting charge transfer BEDT-TTF (ET) salts has been undertaken in Part I of this thesis. Molecular mechanics calculations have been used to investigate the extended lattice, of a perfect crystal, for various ET salts. The chosen forcefield, ESFF, reproduces known crystal structures very well, although the complexity of the materials of interest limits the ability to predict new structure. As the electronic properties of ET salts depend strongly on the packing of the donor- radical cations, electronic structure calculations have been performed for various salts at the molecular and periodic level using Density Functional Theory (DFT). These calculations have been used to investigate the magnetic ordering effect of the BEDT-TTFFeBr4 salt. By fixing the overall spin-state of the system we compared the relative energies of the various spin states. The intermolecular spin ordering was found to be primarily mediated via the covalent sulphur-sulphur interaction on neighbouring BEDT-TTF cations. We have performed optimisation calculations with periodic boundary conditions for some of the smaller examples of ET salts. Part II of the thesis details the parameterisation of the COMPASS forcefield to include electronic polarisation by implementation of the shell model for metal oxide systems to allow the study of surface adsorption by interatomic potential methods. The capabilities of the interatomic potentials are demonstrated by calculations of the adsorption of various small molecules on surface models. Finally, an investigation of the nucleation of silicate fragments around sodium cations, by calculations using DFT, is reported in Appendix I.

With the aid of Lang-Firsov transformation, a single particle Green's function describing the propagation of small Fröhlich polaron has been derived. One- and two-dimensional spectral functions are studied by expanding Green's function perturbatively. SIS tunnelling cuprates can also be described by some closely-related type of this Green's function. The theory allows us to determine the characteristic phonon frequencies, gap parameter, inverse-lifetime parameter and the electron-phonon coupling from the tunnelling data.

The work described in this thesis involves magnetotransport measurements performed on n-channel Si0.7Ge0.3/Si modulation doped quantum well structures. These systems are grown by gas source molecular beam epitaxy, and are found to be quasi two-dimensional. Most of the work is carried out at low temperatures between 0.05K and 4.2K using magnetoresistivity measurements on Hall bar devices, in order to understand the physics of the two-dimensional electron gas (2DEG). Shubnikov-de Haas measurements between 0.05 and 1.6K have been performed on three modulation-doped n-type Si/Si0.7Ge0.3 heterostructures, and the results analysed to extract the quantum relaxation time. Use of the conventional Dingle formula resulted in deviations from the expected theoretical behaviour above 0.3K. The corresponding quantum relaxation time appeared to increase with temperature in the same range. The data are then re-analysed using a modified expression, in which the thermal damping term is neglected. This gave plots with the correct characteristics, and as expected for ionised impurity scattering in a degenerate electron gas, the quantum relaxation time is found to be approximately constant with temperature. SdH oscillations corresponding to the spin-Landau levels of a 2DEG at 100mK are also studied. The method used to obtain the effective Lande g factor can be described as the method of coincidences, where the tilt angle that causes adjacent SdH minima to be equal, is used to determine the g* factor. The effective g factor for v=6 – 8 is 3.34 ± 0.05. The results demonstrated that the effective g factor oscillates as a function of the Landau level. The temperature dependence of the σxx in their ρxx minima in SdH oscillation at integer filling factor is analysed. Above 1K, the resistivity is thermally activated. However, the activation energy obtained from the experiments is much lower than the cyclotron energy. This has been attributed to the fact that the Lande g factor should be larger than the bulk value of g=2 and the Landau levels are broadened due to impurity scattering. The temperature dependence of the resistivity deviated from the expected behaviour below 1K, as also found in measurements on disordered systems, and this is interpreted as being due to hopping conduction.

This dissertation describes low-temperature electronic transport measurements on semiconductor structures of restricted dimensionality. The experiments fall into two sets. The first concerns anisotropic magnetotransport measurements and electron focusing in a varying external magnetic field. These are performed using MBE-grown high mobility two-dimensional electron gases formed on (311)B GaAs substrates. The second is a study of magnetic field induced insulator-quantum Hall liquid transitions performed on GaAs-AIGaAs heterostructures in which a number of InAs monolayers are inserted in the centre of a GaAs quantum well. The sample structures were characterised by STM, TEM, STEM, and AFM. Interest in electron transport on high-index GaAs surfaces is increasing, especially since the advent of patterned substrate regrowth. An anisotropic mobility in orthogonal directions seems to be universal for electron gases grown on (311)B-oriented GaAs substrates. The anisotropy depends on the two-dimensional electron gas carrier density, but mobilities are always higher in the [233] direction. The interface roughness scattering is a possible cause of the mobility anisotropy. The electron focusing results demonstrate that the effective mass and Fermi surface are isotropic even through the mobility is anisotropic. An explanation is proposed based on interface roughness scattering. In the second part, a magnetically induced direct transition from an insulating state at zero magnetic field to quantum Hall effect states with Hall resistance Pxy = h/2e2 and Pxy = h/e2 and back to an insulating state at higher field is observed. The phase boundaries are plotted as a function of disorder and magnetic field using two methods, firstly the temperature independent Pxx points and secondly the maxima in CJxx. This experimental phase diagram is related to the disorder induced collapse of spin splitting in the lowest Landau level obtained from activation energy studies.

Several alternative techniques for improving the transport properties of delta-doped quantum well structures have been investigated. The most successful have used edge delta-doping and compositional grading to produce significantly enhanced electron transport characteristics in AlxGa1-xAs/GaAs, AlxGa1-xAs/InyGa1-yAs, and Al xIn1-xAs/InyGa1-yAs quantum well structures. This compositional grading has been achieved using the digital alloying growth technique—which has been confirmed as a viable method of producing good quality, delta-doped material. Improvements of up to 100% in mobility (to 2630cm2/Vs) and 67% in electron saturation drift velocity (to 1.7 × 107 cm/s) have been achieved using these techniques. The dependence of electron saturation drift velocity (vsat) on mobility (μ) has been determined as vsat α μ0.8±0.3. In addition, we present our investigations of the free-carrier loss observed in a series of highly Si delta-doped AlxGa1-xAs/GaAs quantum well structures. We interpret the results of Hall measurements and self-consistent Poisson Schrodinger modelling in terms of a model of DX centre formation which includes Coulomb interactions, a result of the charged nature of the DX state. Our interpretation implies a strongly growth dependent DX centre energy—which explains the range in published values for the GaAs DX centre energy. The data allows investigation of the DX centre distribution between 214 and 249 meV above the F minima, and confirms a considerably broadened DX centre density of states ≥35 meV).

The dynamics of exciton saturation in GaAs/AlGaAs multiple quantum wells are investigated using picosecond optical pulses. The main contributions to exciton saturation have been shown to be phase space filling. Coulomb screening and exciton lineshape broadening. The spin dependent nature of the phase space filling nonlinearity has allowed its separation from the effects due to Coulomb screening and broadening. The effect of lineshape broadening on exciton saturation has been investigated through its density dependence for a number of quantum well widths. The results show that the effects due to broadening are important in narrow wells of high quality. An investigation into electron spin relaxation at room temperature has been carried out as a function of the well width. The observed decrease in the electron spin relaxation time with decreasing well width is in good agreement with previously reported results and suggests the D'Yakonov-Perel mechanism is the dominant spin-flip mechanism for electrons in quantum wells at room temperature. The first demonstration of an electron spin grating in a quantum well semiconductor is reported by utilising the optical selection rules for quantum wells and optical polarisation gratings. Time resolved studies of the grating decay have allowed the first optical measurement of the in-plane electron diffusion coefficient in a quantum well semiconductor.

The search for the synthesis, and the characterisation of new superconducting materials such as the Y-Ba-Cu-O, Tl-Ba-Ca-Cu-O, Bi-Sr-Ca-Cu-O, Pb-Sr-Y-Ca-Cu-O systems have initiated an almost unparalleled degree of activity during the past several years because of their various possible technological applications. Different existing techniques of synthesis have been applied or modified for the fabrication of these superconducting copper oxide materials (cuprate), with the motive of obtaining single phase and better superconducting properties. The bulk synthesis and reproducibility of lead-cuprates are not straightforward because of the formation of complex co-existing phases, the volatile nature of lead and the required oxygen adjustment in the structure to obtain good superconducting properties. Usually, different superconducting phases give rise to different superconducting transition temperatures which are also modified by the cationic ratios (Pb:Sr:(Y,Ca);Cu) of the constituent elements of lead-cuprates. The synthesis of various cationic ratios of lead-based materials have been performed through a two- step ceramic technique or in a single-step with some modifications in synthesis procedure and optimisation, and characterised using EDX, XRD, resistive and diamagnetic transition temperature measurement. The different cationic ratios of Pb:Sr:(Y,Ca):Cu such as Pb-2213, (Pb,Cu)-1212, Pb-1213, Pb-1313, and Pb-3324 have been proposed, and those are shown to exhibit superconducting properties at various temperatures up to 83K. The pulsed laser deposition technique (PLD) has been used for the synthesis and fabrication of cuprate superconducting thin layers since the discovery of the Y-Ba-Cu- O system. This technique enables very precise and stoichiometric evaporation from an irradiated target due to its pulsed nature. It also allows thin layers of complex materials to be produced. This thesis also reports on the growth and synthesis of single phase Pb2Sr2(Y0.5Ca0.5)Cu3O8 layers using PLD and the highest transition temperature achieved so far in this material by any technique. The synthesis of (Pb,Cu)-1212 phase layers on MgO substrates has also been demonstrated using PLD multi-layering of nonsuperconducting Sr2(Y0.5Ca0.5)Cu3O8, and PbO targets. Ex-situ annealing of these structures in air provided superconducting (Pb,Cu)-1212 phase layers. Ex-situ and in-situ thin film growth of the Pb-2213 phase on MgO substrate by PLD are also reported, whereby the growth parameters such as annealing temperature and duration are optimised. The superconducting properties of the thin films are found to be sensitive to the annealing period at certain temperatures in a nitrogen ambient. In-situ oxygen-adjustment and minimisation of lead loss with high substrate temperature are shown to be excessively difficult task. In-situ adjustment of the oxygen in nitrogen ambient after deposition has been demonstrated with maximum onset transition temperature of 79K and zero resistance at 63K. Layers prepared by both annealing routes are found to be highly c-axis oriented. Uniformity and large area deposition are inherently serious problems with PLD. A simple approach to understanding and alleviating these difficulties using a four sided substrate holding geometry is presented. A substrate holder placed inside, mid way, and along the forward motion of plume at certain inclinations enable the main problem of droplets, non-uniformity and large-area deposition without any scanning of laser beam, target or substrate rotation to be minimised. A slight variation in superconducting transition temperature in 7X7mm samples deposited at distances from 3 to 12 cm from the target is demonstrated after ex-situ annealing. The critical current density is an important parameter for the potential use of any superconducting material. This was found to vary with oxygen content in the thin Pb2Sr2(Y0.5Ca0.5)Cu3O8 layers. Different reduction periods provided variations in the critical currents. The critical current density in Pb2Sr2(Y0.5Ca0.5)Cu3O8 was found to be as high as 0.32xl06A/cm2 in zero field and 0.5x 104A/cm2 in 1Tesla applied field at 55K. These values are much higher than for the bulk where a critical current density value of typically 5.5A/cm2 in zero field and 0.6A/cm2 in 2 Tesla field at 65K was found. The bulk (Pb, Cu)-1212 compositions with some impurity phases showed its highest onset transition temperature of 82K and TC, zero of 76K without the requirement of high pressure oxygen anneal. The critical current density was found to be as high as 130A/cm2 at 50K in zero field. The bulk composition Pb3Sr3(Y0.5Ca0.5)2Cu4Oz has shown maximum TC, onset 81.5K, TC, zero and a critical current density as high as 75A/cm2 in zero field and 1.2A/cm2 in 1 Tesla at 60K.

The optical properties of InAs/GaSb heterostructures under applied magnetic fields are studied in experimental and theoretical detail. The InAs/GaSb system is a type-II "crossed-gap" system, where the valence band edge of GaSb lies higher in energy than the conduction band edge of InAs. This leads to a region of energy above the InAs conduction band where conduction and hole states mix. Thin-layer superlattices remain semiconducting due to confinement effects, but thick-layer superlattices experience charge transfer which leads to intrinsic carrier densities approaching 1012 cm-2 per layer. Existing multi-band modeling techniques based on the k·p formalism are discussed, and a method of solving superlattice band structure (the "momentum-matrix" technique) is presented. The quantizing effects of the superlattice layers and applied magnetic fields are investigated, and the selection rules for optical transitions are derived. Standard cyclotron resonance (CR) is used to study effective masses in InAs/GaSb structures. The heavy hole mass is found to be strongly orientation-dependent, with a mass in the [111] orientation reduced 25% from the [001] mass. The electron mass is found to be roughly isotropic with respect to growth orientation, but shows variation with the InAs width due to quantum confinement effects. CR of InAs/GaSb heterojunctions display hitherto unexplained oscillations in linewidth, intensity, and effective mass. A model is proposed which explains the oscillations, based on the intrinsic nature of the InAs/GaSb system. CR is performed on an InAs/GaSb heterojunction using a free-electron laser, where due to the high intensities (on the order of MW/cm2) the absorption process saturates. This saturation allows for a determination of non-radiative relaxation lifetimes, and through the energy dependence of these lifetimes the magnetophonon effect is observed, allowing a direct measurement of LO-phonon-assisted energy relaxation rates. Coupling is introduced into the standard CR experiment, either by tilting the sample with respect to the magnetic field, or by applying a metal grating to the surface. These coupled CR experiments have striking qualitative results which allow for determination of subband separation energies and coupling matrix elements. Photoconductivity experiments are performed on thin-layer (semiconducting) superlattices, showing optical response at far-infrared wavelengths (5-20 μm). The results are compared with k·p calculations. One sample is processed for vertical transport, in which conduction occurs perpendicular to the superlattice layers. Strong optical response from this sample indicates the viability of InAs/GaSb-based far-infrared detectors. The momentum-matrix technique is used to predict optimum parameters for semiconducting superlattices with band gaps in the far-infrared. Semimetallic structures are studied via a multi-band self-consistent model, with results corroborating with and extending previous work. Intrinsic structures under applied magnetic field are modeled theoretically for the first time.

In this thesis a theoretical study is made of the behaviour of single- electronic devices and systems. It is argued that the properties of single- electronic systems can be determined if the densities of the legal soliton states are known. The dynamics of the tunnelling electrons in such systems are modelled as a traffic process. An exact analytical technique based on the conventional Traffic Theory is formulated in the thesis. This new technique is compared with the traditional Monte-Carlo method and is shown to be superior, both in accuracy and speed. An algorithm that correctly and efficiently determines the set of active soliton states and the relationship between the states is also described. Tunnelling dynamics in a double-junction system are modelled as a Birth-Death process and an exact analytical solution is obtained. The model is then used to study the effects of discrete energy spectrum at the central dot of a double-junction system. A numerical solution for the Fokker-Planck like equation of the charge density function in a single tunnel junction circuit is implemented and used to study the coherence of the single-electron tunnelling (SET) oscillations. To avoid the heavy computations involved in the numerical solution a simple technique based on the distribution of the time between successive tunnel events is presented. It is shown that the SET oscillations do exist but the predicted ensemble average oscillations damp out exponentially with time. The regimes of operation of the double-junction system, the turnstile, are investigated. It is argued that the addition of resistive components to the circuit will enhance the reliability of the device and dramatically reduce the probability of unwanted events at the cost of reduced clocking rate. The Master-Equation formalism is extended to the double-junction system. The charge fluctuations are shown to be less than the fluctuations in a single-junction case. The processes degrading the reliability of single-electronic systems are studied. These include thermal and quantum fluctuations, the charge macroscopic quantum tunnelling and single charges that are being momentarily or permanently trapped in the vicinity of the system's electrodes. The Traffic model is extended to include these processes.

This thesis describes some of the ways in which the coupling between closely spaced quantum wells may be used to enhance the performance of quantum well optical modulators. These are identified as having a useful role as electro-optic modulators for which operating voltage and contrast ratios are two crucial parameters. The background to Airy function solutions of Schrödinger's equation using a matrix matching technique is outlined and absorption spectra calculated at various applied electric fields for a range of quantum well structures. Through a consideration of coupling effects, the width of the barrier separating a pair of wells is optimised for optical modulation at room temperature and this is confirmed from photocurrent measurements. The electric field changes required are shown to be smaller than comparable multiple quantum well devices. Based upon this and an experimental determination of the absorption coefficient an asymmetric Fabry Perot reflection modulator employing coupled quantum wells is designed and fabricated. This exhibits the expected zero field reflectivity and similar devices should operate at low voltages giving good contrast ratios. A p-i-n diode with the wells in the intrinsic region is used to apply bias to these well structures. The diode has a built in voltage and for any low electric field device this can degrade performance. Allowance was made for this in the design of the device above by the use of an extra layer between the p and n regions. As a possible alternative, the use of a particular asymmetric well structure is investigated. Finally, the use of barriers narrower than normal in multiple quantum wells is modelled taking coupling into account. Modelling and photocurrent results show that a reduction in operating voltage through a reduction in barrier width is possible without coupling worsening the optical modulation.

This thesis describes a wide-ranging investigation of the multiple quantum well electroabsorption modulator, from the physical effect governing its operation to the systems in which it might be used. First, the important properties of the device are discussed and the parameters describing the modulation performance defined. A physical model for the field dependent absorption spectrum of quantum well material is developed and used to assess, understand and model the modulator operation. For realistic results this calculation includes the reduction in oscillator strength and the field-enhanced broadening mechanisms. These are inhomogeneous broadening mechanisms caused by well width fluctuations and field variations within the device. The theory is used to explain results from an experimental investigation into modulators of different well widths. Using the above model the optimal design of a non-resonant modulator is determined. It is found that in a transverse modulator, for operation at a particular voltage, there is an optimum thickness of device and this is used to investigate expected device performance. The work is briefly extended to embrace resonant devices such as the asymmetric Fabry-Perot modulator. Some applications of the modulator are then considered. The design of an optical interconnect for an electronic chip is investigated, with the intent of minimising the power dissipation for a optical link and comparing that dissipation with that of the equivalent electronic link. Finally, new fibre-optic architectures are proposed that utilise the functional duality of the MQW diode as both modulator and detector.