Lattice mismatched heterostructures grown by Interfacial Misfit (IMF) technique, which allows the strain energy to be relieved both laterally and perpendicularly from the interfaces, are investigated. However, electrically active defects are created at the interface and away from the interface with energy levels deep in the bandgap of the host materials. These defects dramatically affect the optical and electrical properties of the devices. In this thesis, an investigation of deep level defects is carried out on GaSb/GaAs uncompensated and Te compensated heterostructures grown by the IMF method using DLTS, Laplace DLTS, I-V, C-V, C-F and C-G-F measurements. Furthermore, the effect of thermal annealing treatments on the defect states is also studied on both types of samples. It was found that the well-known EL2 electron trap is commonly observed near to the interface of both uncompensated and Te compensated GaSb/GaAs IMF samples. However, several additional electron defects are detected in Te compensated samples. Rapid thermal annealing performed on uncompensated samples resulted in the annihilation of the main electron trap EL2 at a temperature of 600 oC. On the other hand rapid thermal annealing and conventional furnace annealing were carried out on Te compensated samples, and it was observed that rapid thermal annealing process is more effective in terms of defects reduction. The density of interface states is determined from C-G-F and forward bias DLTS measurements. Te compensated samples exhibit the highest density of interface states and have additional hole traps as compared to uncompensated samples. The electrical properties of p-i-n GaInAsSb photodiodes grown on uncompensated and Te compensated GaSb/GaAs templates on GaAs substrates using special growth mode are investigated. The non-radiative defects which could have detrimental effects on the performance of these photo diodes are studied here for the first time. Both electron and hole defects are detected, and their capture cross-section measurements reveal that some of defects originate from threading dislocations. The double pulse DLTS measurements are performed and the concentration distributions of the detected defects are determined.

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.

The phenomenon of superconductivity has so for defied all attempts of explanation since it was first discovered in 1911. Although two phenomenological theories have been put forward and proved very successful, yet no atomic theories based on quantum mechanics have proved adequate. In the present thesis a new theory is proposed which will be shown to account for most of the important properties of a superconductor. In this part we briefly review the experimental facts and the earlier theories respectively ~1 and ~2.

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.

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.

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.

In this work, I detail the reconstruction and upgrades performed on the axial cavity ion trap in the ITCM group at the university of Sussex, and the measurement of the coupling of multiple ions to the cavity mode. This enables the optimal coupling between the ions and the cavity by adjusting the ions position in the radial and axial positions. This covers new ground in extending the optimal coupling beyond two ions which is of great importance for experiments with several ions in an optical cavity. The thesis outlines the background theory of light-matter interaction and cavity QED, before describing the physical ion trap hardware and its assembly. A description of the laser and cavity systems is provided, including techniques for locking both to stable references. A number of novel measurement techniques for measuring and maximising the stability of the ions and cavities are presented, including micromotion minimisation, spectroscopy, magnetic field compensation using the ground state Hanle effect, and Raman spectroscopy. These techniques enable the measurement of crucial parameters of the atomic transitions and the cavity. The work culminates in a description of the optimisation of the coupling between ion strings and the cavity first by adjusting the radial trap position by means of variable capacitors attached to RF electrodes, and then axially by means of adjusting the endcap potentials and therefore the spacing between ions to obtain the greatest localisation while still positioning the ions close to the antinodes of the cavity field.

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.

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.

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.