Search

Fluctuations in the light we receive from the Sun happen constantly. Much of this variation can be thought of as random, however within solar timeseries there are many true periodic signals to be found, if one knows where to look. Identifying the source of these oscillations can give us a wealth of information about the underlying physical environment of the solar atmosphere. One area which has been particularly exploited is the search for oscillations due to solar flares which can be short-lived, bursty, and variable. Periodic signals detected during solar flares can help us understand the physics of flares, as well as how they interact with the lower layers of the atmosphere. The lower layers of the atmosphere have been known to present periodic fluctuations on timescales of a few minutes in quiet Sun conditions, and there have been some recent reports of energetic events such as flares affecting the quiescent oscillations, even exciting them. This thesis aims to investigate the effect of flares on the lower atmosphere by identifying and categorising oscillatory signals in a variety of solar data. In Chapter 1 an overview of necessary background physics is given. The layers of the Sun's atmosphere and the standard model of solar flares are described. An introduction to different periodic signals which are present in solar data is given, including quiet Sun periodicities and those observed during transient phenomena. The methods used throughout the thesis are presented in Chapter 2, starting with descriptions of the instrumentation which obtained the analysed datasets. A discussion is presented on best practices when searching for oscillations in data, and the spectral fitting method is outlined. Techniques used to obtain physical solar parameters from observations are also introduced. In Chapter 3, a study of chromospheric intensity oscillations in a flaring active region is presented. Maps were produced showing the oscillatory signals which were present before and after the occurrence of the flare. When comparing the results after the flare to those before, oscillations were found to have changed their locations and typical periods in the vicinity of a sunspot located near the flare ribbons. These changes were interpreted as the result of a changing magnetic environment connected to the flare. The interpretations of these results were investigated using magnetohydrodynamic simulations in Chapter 4. Three different sets of simulations were carried out, to identify the most likely effect to have been responsible for the results of Chapter 3. The sets of simulations were concerned with the magnetic field inclination, the chromospheric temperature profile, and the length of the chromospheric cavity. The inclination angle was found to be the most likely of these three effects to change the periodic signals. Chapter 5 presents analysis of a second dataset, from an active region which featured much less powerful flare activity than the event from Chapter 3. A similar analysis was performed, and extended to also feature Doppler velocity data. Velocity oscillations were identified in pixels which exhibited some of the strongest flare heating. The possible cause of the induced velocity oscillations was explored using measurements of the magnetic field from spectropolarimetric observations, and inversions using the STiC code. In Chapter 6 concluding remarks are made, including a summary of the main results of the thesis and descriptions of possible future avenues for investigation.

As silicon (Si) complementary metal oxide semiconductor (CMOS) technology continues to scale into the 10 nm node, chip power consumption is approaching 200 W/cm2 and any further increase is unsustainable. Incorporating III-V compound semiconductor n-type devices into future CMOS generations could allow for the the reduction in supply voltage, and therefore, power consumption, while simultaneously improving on-state performance. The advanced state of Si CMOS places stringent demands on III-V devices, however: the current 14 nm Si tri-gate devices employ high aspect ratio, densely spaced fins which serve to significantly increase current per chip surface area. III-V devices need to significantly out perform state of the art Si devices in order to merit their disruptive incorporation into the well established CMOS process. This necessitates that they too exploit the vertical dimension. To this end, this thesis reports on the fabrication, measurement and analysis of high aspect ratio junctionless InGaAs FinFETs. The junctionless architecture was first demonstrated in 2010 and was shown to circumvent pro- hibitive fabrication challenges for devices with ultra short gate lengths. This work investigated the impact of fin width on both the on and off-state performance of 200 nm gate length devices, with nominal fin widths of 10, 15 and 20 nm. Excellent subthreshold performance was demonstrated, with the narrowest fin width exhibiting a minimum subthreshold swing (SS) of 73 mV/Dec., and an average SS of 80 mV/Dec. over two decades of current. A maximum on-current, Ion, of 80.51 μA/cm2 was measured at a gate overdrive of 0.5 V from an off-state current, Ioff, of 100 nA/cm2 and a drain voltage, Vd, of 0.5 V, with current normalised by gated perimeter. This is competitive with other III-V junctionless devices at similar gate lengths. With current normalised to base fin width, however, Ion increases to 371.8 μA/cm2, which is a record value among equivalently normalised non-planar III-V junctionless devices at any gate length. This technology, therefore, clearly demonstrates the feasibility of incorporating scaled, etched InGaAs fins into future logic generations. Perhaps the greatest bottleneck to the incorporation of III-V compounds into future CMOS technology nodes, however, is the lack of a suitable III-V PMOS candidate: co-integrating different material systems onto a common substate incurs great fabrication complexity, and therefore, cost. III-V antimonides, however, have recently emerged as promising candidates for III-V PMOS and exhibit the highest bulk electron mobility of all III-Vs in addition to a hole mobility second only to germanium. InGaSb ternary compounds have been shown to offer the best combined performance for electrons and holes in the same material, and as such, have the potential to the enable the most simplistic incarnation of III-V CMOS; provided, of course, that is possible to form a gate stack to both device polarities with sufficient electrical properties. To date, however, there has been no investigation into the high-k dielectric interface to InGaSb. To this end, this thesis presents results of the first investigation into the impact of in-situ H2 plasma exposure on the electrical properties of the p/n-In0.3Ga0.7Sb-Al2O3 interface. The parameter space was explored systematically in terms of H2 plasma power and exposure time, and further, the impact of impact of in-situ trimethylaluminium (TMA) pre-cleaning and annealing in forming gas was assessed. Metal oxide semiconductor capacitors (MOSCAPs) were fabricated subsequent to H2 plasma processing and Al2O3 deposition, and the correspond- ing capacitance-voltage and conductance-voltage measurements were analysed both qualita- tively and quantitatively via the simulation of an equivalent circuit model. X-Ray photoelectron spectroscopy (XPS) analysis of samples processed as part of the plasma power series revealed a combination of ex-situ HCl cleaning and in-situ H2 plasma exposure to completely remove In and Sb sub oxides, with the Ga-O content reduced to Ga-O:InGaSb < 0.1. The optimal process, which included ex-situ HCl surface cleaning, in-situ H2 plasma and TMA pre-cleaning, and a post gate metal forming gas anneal, was unequivocally demonstrated to yield a fully unpinnned MOS interface with both n and p-type MOSCAPs explicitly demonstrating a genuine minority carrier response. Interface state and border trap densities were extracted, with a minimum Dit of 1.73x1012 cm-2 eV-1 located at ~110 meV below the conduction band edge and peak border trap densities approximately aligned with the valence and conduction band edges of 3x1019 cm-3 eV-1 and 6.5x1019 cm-3 eV-1 respectively. These results indicate that the optimal gate stack process is indeed applicable to both p and n- type InGaSb MOSFETs, and therefore, represent a critical advancement towards achieving high performance III-V CMOS.

The subject of acoustic fatigue first came to prominence with the advent of using jet engines on commercial aircraft in the 1950’s. Despite the period of time which has elapsed since then and the work carried out to help develop our understanding of the response of structures subject to high intensity noise, acoustic fatigue problems still occur. The novel contributions which this thesis makes to knowledge in the area have been in the application of the dynamic stiffness method which has been used to predict stress and strain response due to acoustic loading. The dynamic stiffness method can under certain circumstances provide very accurate results for natural frequency and displacement. Indeed for certain configurations it can provide exact solutions. It is largely independent of the number of degrees of freedom necessary to give an accurate result unlike the finite element method. The thesis documents how the dynamic stiffness method can offer a very favourable alternative to available analysis techniques. An alternative method of formulating the dynamic stiffness method is presented and is further extended to the analysis of orthotropic plates. The response of actual structures is discussed and previously unpublished data is also presented.

In order to effectively model the behaviour of energetic materials under operational conditions it is essential to obtain detailed structural information for these compounds at elevated temperature and/or pressures. The structural characterisation of the high explosives RDX and CL-20 and a series of inorganic azides [Mn+(N3)n] at extreme conditions is described herein. In addition to the characterisation of a highly metastable β -form of RDX (1,3,5- trinitrohexahydro-1,3,5-triazine) at atmospheric pressure, the structure solution of a high-pressure/ high-temperature polymorph is described. This form, obtained above 4.3 GPa and 450 K, has been shown to be distinct from the β -form and has therefore been denoted - RDX. Furthermore, ε -RDX is sufficiently metastable to allow its recovery to ambient pressure at 150 K; it only transforms to the α -form upon warming to 230 K. Finally, the ambient-temperature compression of RDX has been investigated to a maximum pressure of 23.0 GPa, using methanol:ethanol (4:1) as the pressure-transmitting medium; no phase transition was observed under these conditions, other than the α → γ transition at 3.9 GPa. The structure of a high-pressure polymorph of CL-20 (2,4,6,8,10,12- hexanitrohexaazaisowurtzitane) has also been determined by a combination of powder and single-crystal X-ray diffraction. Compression of γ -CL-20 to above 0.7 GPa using Fluorinert (FC-77) as the pressure-transmitting medium results in a phase transition to the ζ -form, which has been found to display structural similarities with both theγ γ - and ε -forms. The high-pressure behaviour of CL-20, however, depends markedly on the starting polymorph and the pressure-transmitting medium selected. Compression of γ -CL-20 in MeOH:EtOH (4:1) results in the formation of a 2:1 CL-20:MeOH solvate at 0.5 GPa. This solvate is stable upon compression to P > 5.0 GPa. It may also be recovered to ambient pressure at 293 K. Meanwhile, no phase transition is observed during the compression of ε -CL-20 to a maximum pressure of 7.2 GPa. Finally, a series of inorganic azides [NaN3, CsN3, TlN3, NH4N3, AgN3 and Pb(N3)2] has been characterised under a range of pressure and temperature conditions. Of the six compounds studied, all displayed at least one polymorphic transition – 5 new forms have been structurally characterised in this work and evidence of another 5 is presented. The combined effect of pressure and temperature results in sodium azide adopting a tetragonal structure common to larger alkali metal azides. Caesium azide has been shown to undergo three phase transitions during compression to 6.0 GPa – the structure of the first high-pressure form is reported. A variable temperature X-ray powder diffraction study of TlN3 has allowed the structural characterisation of the low-temperature TlN3-IV (at 230 K) as well as providing evidence for a phase transition to a high-temperature form above 550 K. The high-pressure form III (obtained above 0.76 GPa) has also been determined by neutron powder diffraction. Silver, ammonium and lead(II) azides have all been shown to undergo a phase transition at high pressures. Compression of silver azide (P > 0.80 GPa) removes an orthorhombic distortion observed at atmospheric pressure, resulting in the tetragonal structure adopted by CsN3 and TlN3 under ambient conditions. Moreover, NH4N3 and Pb(N3)2 have been found to undergo phase transitions at 2.6 GPa, although their high-pressure structures have still to be determined.

Lymphocytes express antigen receptors which are formed by re-arrangement of gene segments. Mutations acquired during this process, predominantly in the complementarity determining regions (CDRs), result in generation of non-germline sequences. Through analysing the CDR3 sequence, this study attempts to determine whether editing of the lymphocyte repertoire is present in an HLA-dependant manner. Data presented demonstrates a decrease frequency of CDR3-derived HLA-A2 binding peptides in HLA-A2+ donors (0.03% (SYFPEITHI) and 0.35% (BIMAS)) compared with HLA-A2- donors (0.24% (SYFPEITHI, p=0.01) and 0.54% (BIMAS, p=0.19)). Trends similar to those seen in HLA-A2 were observed in other HLA alleles as well suggesting that there may be a process by which potentially dangerous B cell populations are edited from the B cell repertoire. Similar analysis of the TCR CDR3 did not reveal any such process in all of the HLA alleles tested suggesting that there is no immunoediting of the T cell repertoire. Simultaneously, this study attempts to determine the processing and presentation of CDR3-derived peptides at the cell surface using lymphocyte antigen receptor models containing CDR3-encoded viral epitopes. The apparent presence of these peptides on the cell surface leads to the hypothesis that antibodies enter the antigen processing pathway and potentially deliver an immunogenic peptide to a target cell. Using antibodies specific for B cells, this study has shown that cells labelled with an antibody-peptide complex are targeted and lysed by cytotoxic CD4+ T cells in a peptide-specific manner. The use of such technology in antibody immunotherapy may be of considerable therapeutic benefit.