This thesis reports an investigation of deep level defects in narrow bandgap semiconductors, namely GaAs and GaAsN, and wide-gap GaN materials and devices that have potential applications in photovoltaics and betavoltaic microbatteries. Indeed, for such applications it is of paramount importance to determine the characteristics of the defects present in the materials, which will help understand their effects on the quality of the materials and the performance of devices. In particular, the investigation is done on: (i) a set of GaAs (311)A solar cell structures gown by molecular beam epitaxy (MBE); (ii) dilute GaAsN epitaxial layers containing different nitrogen concentrations grown by MBE; and (iii) betavoltaic microbattery based on a GaN p–i–n homojunction structures grown by metal-organic vapour phase epitaxy (MOVPE) technique using current-voltage (I-V), capacitance-voltage (C-V), deep level transient spectroscopy (DLTS), and Laplace DLTS measurements. The results of this study show that the defects affected significantly the electrical properties of different advanced semiconductor structures and devices. In particular, InGaAs Quantum Wires (QWr) Intermediate Band Solar Cells based nanostructures grown by MBE were studied. The DLTS and Laplace DLTS results showed that the efficiency measurements and external quantum efficiency (EQE) at different temperatures correlated with the appearance of defect peaks in QWr devices in the same temperature ranges. Additionally, this thesis reports the effect of a high dose of gamma (γ-) irradiation on MBE grown dilute GaAsN epilayers with nitrogen concentrations ranging from 0.2 to 1.2% with post-irradiation stability. The DLTS measurements revealed that after irradiation the number of traps either decreased, remained constant, or new traps are created depending on the concentration of nitrogen. Moreover, this thesis reports the effect of beta particle irradiation on the electrical properties of a betavoltaic microbattery based on a GaN p–i–n homojunction with 200 nm and 600 nm thicknesses of undoped layer (i-GaN). The experimental studies demonstrate that, only the sample with thinner i-GaN layer shows the creation of new shallow donor traps upon irradiation on the p-side of the p-i-n junction. While the sample with thicker i-GaN is more resistant to irradiation.

Colloidal quantum dots (QDs) allow for the tuning of dopant concentration as well as flexibility in the engineering of the surrounding medium. This thesis explores the use of magnetic resonance techniques and the development of hardware in order to characterize paramagnetically doped materials, in particular Mn-doped PbS colloidal QDs, and assess their potential for applications in quantum technologies such as quantum information processing (QIP). Colloidal PbS:Mn QDs capped with thioglycerol/dithiolglycerol ligands were synthesised in aqueous solution. Methods of tailoring the Mn-Mn and Mn-1H interactions, with the aim of maximizing phase memory times, were investigated. The distance between spins was optimized by initially, overgrowing the QDs with an undoped shell and secondly, by dispersing the QDs in solution. The use of a deuterated solution was found to further reduce the dephasing effects of Mn-1H interactions. This resulted in unprecedentedly long phase memory (TM ~ 8 μs) and spin–lattice relaxation (T1 ~ 10 ms) time constants for Mn2+ ions at T= 4.5 K, and in the observation of electron spin coherence (TM ~ 1 μs) near room temperature. Further improvements to relaxation times, as well as enhanced optical properties useful for the initialization and readout of spin qubits, were also studied by embedding the QDs in photonic crystals. Magnetic resonance techniques combined with paramagnetic Mn-impurities in PbS QDs are used for sensitive probing of the QD surface and environment. We report inequivalent proton spin relaxations of the capping ligands and solvent molecules. We determine the strengths and anisotropies of the Mn-1H spin interactions, and establish Mn-1H distances with ~1 Å sensitivity. These findings demonstrate the potential of magnetically doped QDs as sensitive magnetic nano-probes and the use of electron spins for surface sensing. We explore a means of characterizing mechanisms responsible for the functionality of paramagnetically doped materials. The development of instrumentation to identify and quantify interactions between paramagnetic and ordered magnetic phases is described. A probe was designed and built with a fast response time and with the aim of facilitating fast field jump experiments to identifying interactions between the different magnetic phases by correlating the response of a sample to mw irradiation with its response to a field jump.

In this thesis, the growth of metal-free naphthalocyanine (Nc) and copper naphthalocyanine (CuNc) on both bare Si/SiO2 and octadecyltrichlorosilane (OTS) modified Si/SiO2 surface were studied. The effects of the substrate temperature on morphology and structure of Nc and CuNc thin film growth were presented. For these purposes thin films of Nc and CuNc prepared by thermal vacuum evaporation were studied using atomic force microscopy (AFM) and X-ray diffraction (XRD). We observed that the increase of substrate temperature during growth affects the morphology, preferential molecular orientation and degree of crystallinity of both Nc and CuNc thin film, which were used as active layers in organic field effect transistor (OFET) devices. Organic thin film transistors (OFETs) were fabricated using these molecules as the active layers and their electrical characteristics were measured under both vacuum and atmospheric conditions and they were found to exhibit p-type transistor action. A series of samples of the Nc and CuNc thin films were grown on Si/SiO2 and OTS-modified oxide surface at different substrate temperature but fixed equivalent deposited thickness. The growth conditions, particularly the substrate temperature strongly affect nucleation size and shape of the organic thin film. In general, the thin film morphology shows a near circular grain and elongated grain shape at low substrate temperature, while the thin Nc film shows small needle-like structure and extended needle-like crystalline structures with large gaps at high substrate temperature. The optimum substrate temperature during the growth of Nc on both surfaces is achieved at 200 °C, and this occurs for growth of CuNc at 180 °C and 160 °C on Si/SiO2 and OTS surfaces, respectively, for which the naphthalocyanine thin film shows the best morphological and electrical properties. We used Nc and CuNc thin films prepared at different substrate temperatures as active layers to fabricate bottom and top-contact organic field effect transistors. Their electrical characteristics were measured at room temperature in vacuum and air in the dark. We plotted the output characteristic and transfer characteristic of all OFET devices so that the effects of grain size and crystal structure on the performance characteristic of Nc OFET device could be investigated. Then we studied the effects of hysteresis and charge traps on device performance when exposed to air. We found that the changes generated by exposure of the device to atmosphere may be reversed by annealing the thin film to ∼100 °C in vacuum. We reported the highest mobility of (5.16 ± 0.23) × 10-2 cm2 /Vs for top-contact Nc device prepared at 200ºC on SiO2 after annealing in vacuum, and also we reported the highest mobility of (3.56 ± 0.14) × 10-2 cm2 /Vs for top-contact CuNc device prepared at 180ºC on SiO2 after annealing in vacuum. We found that the top-contact device always performs better than the bottom-contact device. We attributed this to the change of morphology of active layer in the interface between contact metal and SiO2. Solvent induced self-assembly, self-trapping, and self-organizing of c-P30 cyclic porphyrin polymers on the Au surface that are deposited from two solutions and various concentrations in ambient condition was also studied. This results in the arrangement of cyclic polymers in different configurations such as stacking columnar, supramolecular nesting and uniform height hexagonal close packed structure. These conformations are observed using scanning tunnelling microscopy. Highly covered surface stacking columnar like porous array is also observed. We show that toluene:methanol mixture can play a crucial role in self-assembly of supramolecular structure in two dimensions, π-π stacking conformation perpendicular over surface in three dimensions and single in double nested nanoring conformation. Cyclic porphyrin polymers deposited from toluene shows nested nanorings structure, such as single nanoring self-trapped inside a near-circular shape single ring on surface. Diluted solutions using a large volume of methanol relative to the toluene can suppress the adsorption of nanorings to the surface. Interestingly, adsorption of the cyclic polymer from toluene:methanol 3:5 can result in the formation of uniformly height hexagonal close packing on surface, where nanorings aggregate as columnar stacks in two layers, dependent on concentration. Our results show that the self-assembly of artificial cyclic polymers is dependent on solvent and concentration provides a significant step towards control of the three-dimensional arrangement of supramolecular conformation on surfaces using non-covalent interactions.

Over the last decade, phase measurements derived from gradient echo MRI have increasingly been used as a source of quantitative information, allowing tissue composition and microstructure to be probed in vivo and opening up many new avenues of research. However, the non-local nature of phase contrast and the complexity of the underlying sources of phase variation mean that care must be taken in the interpretation and exploitation of phase information. The work described in this thesis explores the application of phase-based quantitative susceptibility measurements in vivo, and uses theory, experiment, and simulation to investigate the contribution of local structural effects to measurements of MRI signal phase. In initial work, the use of phase imaging and quantitative susceptibility mapping (QSM) is compared in the analysis of white matter lesions in multiple sclerosis, demonstrating in vivo the dipolar distortions inherent in phase images, and the correction of such artefacts through the application of QSM, based on a thresholded k-space division method . Visual analysis of the lesions with a focus on the presence of the peripheral rings that occur in some white matter lesions allows comparison of our data with previous studies. A theoretical description of effects of magnetic susceptibility anisotropy using a susceptibility tensor model is then presented, and its predictions tested using macroscopic phantoms composed of pyrolytic graphite sheet, a highly anisotropic and diamagnetic material. The results of these experiments confirm that the full tensor model must be used to predict the effects of structures composed of such materials on the magnetic field. Finally, Monte Carlo simulation is used to demonstrate the effects of perturber shape and diffusion on the MRI signal phase measured from a volume containing oriented, NMR-invisible, spheroidal perturbers with constant bulk magnetic susceptibility. The rate of phase accumulation over time is shown to be highly dependent on perturber shape and diffusion, and the possible implication of these results on real MRI measurements are discussed.

The focus of this thesis is to produce highly polarised Nuclear Magnetic Resonance (NMR) samples for use in vivo applications. This work focuses on using the brute force method to polarise relevant molecules, for example, 13C labelled pyruvic acid and 13C labelled sodium acetate. The brute force method uses the Boltzmann distribution to polarise a sample by exposing it to large magnetic fields, 15 T, and ultra-low temperatures, ~20 mK. The disadvantage of using this method is the long polarisation time. To counteract the long relaxation times, two sets of relaxation agents were assessed: paramagnetic lanthanides and nanoparticles. Chelated gadolinium is routinely used as a spin-lattice, T1, contrast agent in clinical Magnetic Resonance Imaging (MRI). It is known that when the electron spin flip time is similar to the Larmor frequency, the T1¬ time of the nuclei is reduced. Each lanthanide has a different electron spin flip time, therefore, one lanthanide may be effective at low temperatures. Unfortunately the lanthanides do not prove to be efficient in the millikelvin regime, where the brute force method is at its most effective, so the lanthanides are of limited use. Metals are known to have short T1 times in the millikelvin regime due to the Korringa effect. The conduction electrons of the metal can contribute or absorb energy from nuclei, resulting in a reduction of the T1 of relevant molecules. By having a strong interaction between conduction electrons and the nuclei of interest, it could be possible to reduce the T1¬ of any nuclei of interest. To maximise the contact between the metals and the nuclei, metal nanoparticles were used. Copper and platinum nanoparticle samples are shown to enhance the relaxation rate of nearby protons, however, aluminium and silver nanoparticle samples, which are also expected to be effective, are not. This contradicts the idea that the Korringa effect is the only relaxation mechanism which relaxes the nuclei. The magnetic properties of nanoparticles can be different from their bulk counterpart, therefore, could be contributing to the relaxation of nearby nuclei. It would therefore be advantageous to study the nanoparticle’s magnetisation in a Superconducting Quantum Interference Device (SQUID). Unfortunately, the interpretation of the magnetisation becomes very complicated, as the nanoparticles can react with the solvents. These reactions can result in a 1000-fold increase in the magnetisation of the sample. With the limited magnetic data collected in this work, it is difficult to correlate the nanoparticles magnetic properties with their effectiveness as a T1 relaxation agent.

This thesis describes the growth and characterisation of gallium nitride, indium nitride and indium gallium nitride semiconductors primarily carried out using a novel growth technique called Anion Modulation Epitaxy (AME) and also plasma-assisted MBE (PA-MBE). Characterisation was typically performed by x-ray diffraction, scanning electron microscopy and optical reflectance studies. All of the work in this thesis was carried out in the hope to improve layer structure and quality which in turn would create higher efficiency solar cells. Nanorods were grown using PA-MBE as these are known to form entirely defect-free material and this would be an attractive quality when trying to increase the efficiency. InN rods were grown at temperatures between 350 C and 450 C on SiC substrates of both Si- and C-polar faces at various indium fluxes to establish optimal growth conditions. It was found that a BEP flux of approximately 2x10-7 Torr and a growth temperature approximately 400 C provided a large array of rods. Samples produced tall, thin nanorods as well as short, fat ones. CBED analysis revealed that the tall nanorods were growing In-polar which mimics the behaviour seen for GaN. Photoluminscence (PL) data for the rods agrees with the bulk PL measurement of InN in the literature confirming that reasonable quality films have been produced. Coalescence of the rods was achieved by increasing the flux to 2x10-6 Torr. Also, p-n junctions were grown on both faces of SiC and preliminary tests have shown a response to light. A new growth method was developed from conventional PA-MBE known as Anion Modulation Epitaxy (AME) and gives rise to improved growth compared with equivalent samples by PA-MBE as the growth temperature is decreased. It also allows p-doping for GaN to be carried out at lower temperatures and more consistently. Direct comparison of GaN samples grown at equivalent temperatures by PA-MBE and AME show improved structural, electrical and optical properties for the samples grown using AME. It has also proven to be a useful tool for studying temperature changes at the substrate surface when using any pulsed growth technique. Substrate temperature was shown to vary by approximately 15 C each time the flow was interrupted. Slower, long-term trends were also monitored depending on the average nitrogen to metal ratio. An increase in overall temperature is derived from increasing metal rich growth, whereas the opposite effect is true for increased nitrogen rich growth. AME was also used for the growth of intermediate band solar cells (IBSC). The entire growth is easily monitored and altered using AME without altering the growth parameters drastically. Pulsing the nitrogen allows for variations in the metal cell fluxes to be kept under control at the surface. The discovery of `hidden' metal in the layer would have taken a lot longer to discover, and would have ruined the sample without utilising AME.

This thesis presents a study of the electron-phonon interaction in an n-doped weakly coupled semiconductor superlattice (SL). Two experiments were performed which studied different aspects of this interaction. Firstly, a coherent phonon optics chip was designed. This was used in an experiment where a phonon beam was passed through the SL while an electrical bias was applied to it. The experiment provided a sensitive measurement of the effects caused by bias in the SL on the phonon beam. Secondly, a train of strain pulses was passed through the SL and the charge transferred in the device due to the strain was investigated. A coherent phonon optics chip was formed using a semiconductor superlattice as a transducer structure and a p-i-n photodiode as a coherent phonon detector on the opposite side of the substrate. The doped weakly coupled superlattice structure, which is the main subject of investigation in this thesis was grown between the transducer and detector structures. Optical access mesas were processed on both sides of the substrate to allow the application of bias to both the doped superlattice and the p-i-n structures. A photocurrent pump-probe experiment was then performed using a femtosecond laser to excite the transducer structure and activate the detection mechanism. The application of bias to the weakly coupled SL was found to cause a small attenuation to the 378 GHz phonon beam passing through it. An investigation of the possible causes of this attenuation ruled out several trivial explanations, suggesting that it was caused by the interaction between electrons and phonons in the structure. The active control of phonon amplitude by electrical means has not previously been demonstrated and may offer exciting new prospectives for phonon devices and experiments. The coherent phonon optics technique was shown to be very sensitive and it will be a useful technique to increase our understanding of future acousto-electric devices. The electrical signal that acoustic excitation caused in the SL device was investigated using a pulse shaping technique in combination with an amplified femtosecond laser. A Fabry-Perot cavity was used in the laser path to create a train of equally spaced laser pulses with an adjustable pulse spacing. Focusing these pulses on an aluminium film transducer creates a train of equally spaced acoustic pulses simulating a monochromatic acoustic wave packet. The SL was processed and electrically contacted so that the charge transferred through it due to the acoustic pulse train could be monitored using a 12.5 GHz-bandwidth digital oscilloscope. The variation in charge transfer seen as a function of the DC bias applied to the device and as a function of the total energy of the acoustic pulse train was investigated. The behavior was compared to a theoretical model developed in the style of previous theories of electrical conversion in SLs excited by electromagnetic waves. The dependencies of the charge transfer on the bias and energy of the pulse train were well reproduced in the theory. The theory predicted that magnitude of the signal in the superlattice was independent of the frequency of the acoustic pulse train. This was verified by measuring the frequency dependence of the signal seen for a variety of transducer films. The frequency dependencies seen were well explained through simulations presuming the device response was independent of train frequency. This confirms the predictions of the theory. Both the experiments detailed in this thesis have helped increase our understanding of the nature of electron-phonon interactions in superlattices. It is hoped that a fuller understanding of these interactions may be instrumental in the creation of exciting new acousto-electrical devices.

This thesis discusses experiments that have been performed to investigate ultrafast acoustoelectric effects in semiconductor devices. Current commonly employed techniques to generate ultrafast acoustic pulses and detect them with spectral resolution require a powerful pulsed laser system that is bulky, expensive and complicated. If the acoustic pulses could instead be generated and detected by electrical methods, picosecond acoustic techniques could become more readily available as a tool for other users. This thesis focusses on the electrical detection of acoustic pulses with spectral resolution. In many of the key experiments described in this thesis a picosecond strain pulse was generated optically on the opposite face of the sample to the semiconductor device of interest. The strain was generated either in a thin Al film thermally deposited on the sample surface, or directly in the GaAs substrate. Acoustic phonons generated by this method propagated across the substrate to the device. Transient voltages across the semiconductor device caused by the incident phonons were detected using a high frequency real-time oscilloscope. The first evidence of heterodyne mixing of coherent acoustic phonons with microwaves was obtained, for frequencies up to about 100 GHz. First, it was confirmed that Schottky diodes can produce a fast transient voltage in response to an incident acoustic wavepacket. The detection process occurs at the semiconductor-metal interface, and is due to the deformation potential. Bow-tie antenna fabricated directly onto the GaAs substrate proved to be ineffective at coupling microwaves from free space to the Schottky diode. A waveguide-coupled beam-lead Schottky diode provided by e2v had a sufficient response to the incident microwaves to proceed with the mixing experiments. The microwave local oscillator signal was mixed with a tunable narrow frequency band acoustic signal that was produced using a Fabry-Perot etalon external to the laser cavity. The intermediate frequency components were in the range of 1-12 GHz, which could be detected on the oscilloscope. Mixing was performed using both the fundamental frequency acoustic wave and the second harmonic generated in the sample. Semiconductor superlattices were also investigated as electrical detectors for ultrafast acoustic pulses. In this case, the transient voltage measured across the device contained an unexpected contribution in the form of a peak with a width of approximately 2 ns. This signal is too slow to be caused by a strain pulse and too fast for a heat pulse. It is proposed that this peak is caused by long-lived phonon modes from the centre of the mini-Brillouin zone being confined in the superlattice due to Bragg reflections. The peak caused by confined phonons and the two peaks caused by heat pulses also present in the detected signal were investigated for a range of experimental conditions. This allowed comparisons to be made to previous works. A similar superlattice structure had a very different response to the incident acoustic wavepacket. The polarity of the transient voltage detected was inverted and there was no evidence of an electronic response to the confined phonon modes, which would have been present in both samples. It is proposed that the barriers of the NU1727 superlattice sample are thicker than expected, and this strongly affects the electron transport through the structure. This thesis shows that semiconductor devices can be suitable for the electrical detection of ultrafast strain pulses. For this technique to reach its full potential, it is also necessary to be able to generate these strain pulses electrically. A step recovery diode has been considered for this purpose as part of the suggested future work.

The aim of this project is the conception, implementation, and application of a simulation tool for the accurate modeling of electromagnetic fields within inhomogeneous materials with complex shapes and the propagation of the resulting fields in the surrounding environment. There are many methods that can be used to model the scattering of an electromagnetic field, however one of the most promising for hybridisation is the Boundary Element Method (BEM), which is a surface technique, and the Unstructured Transmission Line Modeling (UTLM) method, which is a volume technique. The former allows accurate description of the scatterer's boundary and the field's radiation characteristics, but cannot model scattering by materials characterized by a non-uniform refraction index. The latter, on the contrary, can model a very broad range of materials, but is less accurate, since it has to rely on approximate absorbing boundary conditions. A method resulting in the hybridisation of BEM and UTLM can be used to construct a tool that takes into account both the interaction with non-uniform tissue and propagation in its environment. The project aims to describe in detail the implementation of the novel method, and deploy it in a heterogeneous distributed computing environment.

The properties of cubic nitrides grown by molecular beam epitaxy (MBE) on GaAs (001) have been studied using optical and electrical techniques. The aim of these studies was the improvement of the growth techniques in order to improve the quality of grown nitrides intended for bulk substrate and optoelectronic device applications. We have also characterised hexagonal nanocolumn structures incorporating indium. Firstly, bulk films of cubic AlxGa1-xN with aluminium fractions (x) spanning the entire composition range were tested using time-integrated and time-resolved photoluminescence (PL) plus reflectivity measurements. Strong PL emission was recorded from the samples, with improved intensity for higher aluminium concentrations. Temperature dependent and time-resolved PL showed the increasing role of carrier localisation at larger AlN fractions. The reflectivity results showed a near-steady increase in the bandgap energy with increasing AlN content. Alternative interpretations that did and did not involve a transition from direct-gap to indirect-gap behaviour in cubic AlxGa1-xN were considered. We next looked at cubic AlxGa1-xN/GaN/AlxGa1-xN single quantum well (QW) structures with varying AlN content in the barrier regions. The PL studies indicated that carrier escape from the QWs and non-radiative recombination at layer interfaces were limiting factors for strong well emission. Higher AlN concentration in the barriers appeared to exacerbate these problems. The doping of cubic GaN with silicon (n-type) and magnesium (p-type) was also studied. For Mg-doped GaN, a strong blue band emission was noted in the PL spectrum, which became more intense at higher doping levels. The Mg-doped GaN layers had low conductivity and their mobility could not be measured due to strong compensation effects. The cubic film had similar time-resolved PL properties for the blue band emission compared to hexagonal Mg:GaN. These results suggested that the blue band was the result of recombination between a shallow Mg acceptor and deep donor, believed to be a complex including a nitrogen vacancy and an Mg atom. This complex was also associated with the compensation effect seen in the electrical measurements. With the Si-doped cubic GaN, we observed PL spectra that were consistent with other sources. Thicker layers of GaN:Si did not have measurable mobility. This was likely caused by the rough surface structure that was imaged using a scanning electron microscope. The thin layer had a very smooth surface in comparison. The mobility of sub-micron thickness layers with a carrier concentrations of n = 2.0×1018cm-3 and n = 9.0×1017cm-3 were μ = 3.9cm2/Vs and μ = 9.5cm2/Vs respectively. The mobility values and structural issues indicated that growth improvements were needed to reduce scattering defects. In addition to cubic structures, we have considered nanocolumn growth of InGaN and InN. InxGa1-xN nanocolumns were grown on Si (111) by MBE with a nominal indium concentration of x = 0.5. PL emission was obtained from samples grown at higher temperature, but overall intensity was low. A second set of samples, where nanocolumn growth was followed by growth of a continuous coalesced film exhibited much stronger PL emission, which was attributed to the elimination of a phase separated core-shell structure in the nanocolumns. Next, a coalesced InxGa1-xN structure with vertically varying indium fraction was characterised. PL readings showed evidence of successful concentration grading. Finally, the PL spectra of coalesced InN layers were recorded, for which a specialised infrared PL system needed to be used. The results highlighted how increased growth temperature and indium flux can improve PL properties. For the binary alloy however, coalescence growth can decrease PL intensity compared to the nanocolumns stage.