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The atomic structure of the CaF2-Si(111) interface and the repeated unit cell of MnF2-CaF2-Si(111) superlattices have been determined using surface x-ray diffraction. Specular reflectivity and non-specular rod scans were measured for a number of superlattice samples. All samples had a type-B interface. Ultra-thin layers of MnF2, below a critical thickness of 5 monolayers, adopted the fluorite structure of CaF2. These layers are likely to have different magnetic properties to bulk anti-ferromagnetic MnF2, possibly leading to novel magnetic applications. We have developed software to interface with the structure factor calculation software ROD to calculate the semi-kinematical reflectivity and non-specular rods of a superlattice. Ion bombardment and annealing were used to produce atomically clean, well ordered InSb(001)-c(4x4) and GaSb(001)-c(2x6) surface reconstructions from antimony capped and uncapped samples. Auger electron spectroscopy showed that a 1000A antimony capping layer was sufficient to prevent atmospheric oxidation of the substrate. A substantial re-ordering of the cap was observed at 180°C. Desorption of the cap was achieved by annealing the sample at 300°C under an Sb overpressure. X-ray reflectivity measurements were used to determine the out-of-plane structure of the GaSbc(2x6) reconstruction. Theoretical fits indicate that both the top and the second layer of antimony dimers lie close to their bulk positions. In-plane x-ray measurements were used to verify the symmetry of the reconstruction. The InSb anti-phase x-ray position was monitored during the growth of Tl on the InSb-c(4x4) reconstruction. Fits to the data showed that the growth mode is Stranski-Krastanow at room temperature, and revealed details about the formation of the initial wetting layer. A specular reflectivity ridge scan after deposition showed reflections produced by Tl and Tl 2O. In-plane x-ray diffraction peaks can be attributed to either Tl, TlSb, or Tl7Sb2 and indicate a preferential alignment of Tl.

A new type of pulse compressor based on sweep frequency pulse compression has been developed using a helically corrugated waveguide as a dispersive medium. This structure provides selective coupling between a TE₁₁ travelling wave and a near cut-off TE₂₁ wave, creating an eigenwave where the dispersion characteristics of one mode gradually converts into that of other. As the compressor works far from cut-off the reflections associated with operation close to cut-off are reduced and allows the compressor to be used at the output of a powerful amplifier. An experimental and theoretical study of swept-frequency based pulse compression as well as a theoretical and experimental investigation of the dispersive properties of a circular waveguide with a helical corrugation on its inner surface was carried out. Measurements of the helically corrugated compressor obtained a maximum optimum power compression ratio of 10.9 for a helically corrugated waveguide of length 208.08cm. A 1 kilowatt input pulse with a frequency sweep from 9.60GHz to 9.35GHz over 70ns was compressed by the helically corrugated waveguide. The compressed pulse had a duration of 3ns and gave a compression efficiency of 44%. The helical compressor experiments conducted were the first of their kind to be carried out and were found to be in good agreement with theory. A numerical study of the potential of the helically corrugated waveguide to produce multi-GW output pulses using frequency modulation at the falling edge of a microwave pulse produced from a high-power BWO was investigated.

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The electronic transport properties of certain organii c semIi conductors are expected to exhibit a quasi one-dimensional nature. Pulsed laser techniques have been used to study transient photoconductivity in a number of such linear molecular systems. This thesis explores carrier motion of zeolite encapsulated conjugated polymers such as polyacetylene and polypropyne, columnar discotic liquid crystals and single walled carbon nanotubes. At the time of writing, this thesis presents the first observations of transient photoconductivity for carbon nanotubes. In the systems studied: electric field, temperature and spectral dependencies are explored and the results are used to calculate a number of parameters, such as: carrier mobilities, carrier range and quantum efficiencies. Also, the effect of sample preparation has been investigated. A variation on the Auston switch technique has enabled picosecond time resolved photocurrents to be measured on carbon nanotubes, with a rise time of the order of 100ps. A similar technique was utilised to study the encapsulated polymers, but no measurable effect was observed. The Kepler-LeBlanc Time of Flight technique has been employed to find the carrier mobility in a number of columnar discotic liquid crystals along with the quantum efficiency for carrier generation in those systems. The results presented in this thesis have led to a greater understanding of charge transport on carbon nanotubes from which a ID bimolecular recombination model has been proposed. We have demonstrated a novel polymeric DLC where electrons are the majority carrier and demonstrated a photogeneration mechanism controlled by Poole- Frenkel barrier lowering. We have also been able to refute the proposal that the 3D Onsager model is applicable for describing the photogeneration mechanism in most DLC's.

The III-V ferromagnetic semiconductor Gallium Manganese Arsenide ((Ga,Mn)As) is one of the most interesting and well studied materials in spintronics research area. The first chapter is a brief introduction to spintronics, the properties of (Ga,Mn)As and the growth technique molecular beam epitaxy (MBE). Then the thesis presents a detailed study of the effect on the Curie temperature (TC) of varying the growth conditions and post-growth annealing procedures for epitaxially grown (Ga,Mn)As materials. The results indicate that it is necessary to optimize the growth parameters and post-growth annealing procedure to obtain the highest TC. From detailed magnetotransport studies, the carrier densities of high TC (Ga,Mn)As and H-doped (Ga,Mn)As have been achieved. It is found that the anomalous Hall resistance is the dominant contribution even at room temperature for these samples, which means it is incorrect to obtain carrier densities directly from Hall slope at high temperature. The results also show that the as-grown and lightly annealed H-doped (Ga,Mn)As samples have relatively high Curie temperatures down to low carrier density which make them good candidates for showing strong gate control of ferromagnetism. Besides (Ga,Mn)As, this thesis also discusses the studies of III-V ferromagnetic semiconductors (Ga,Mn)(As,P), (Al,Ga,Mn)As and some heterostructures based on these materials. The experimental investigation shows that a (Ga,Mn)(As,P) single layer grown on GaAs substrate has perpendicular anisotropy easy axis after annealing. It also demonstrates a method to suppress the diffusion of interstitial Mn ions during low temperature annealing from specific layers in (Al,Ga,Mn)As based heterostructures. The magnetometry study shows that the individual layers in the heterostructure have tailored magnetic properties, which makes this material useful for the further investigation of tunnelling magnetoresistance and spin transfer torque phenomena.

GaMnN layers grown by plasma assisted molecular beam epitaxy (PAMBE) as a function of Ga/N ratio, Mn flux and growth temperature are assessed using a variety of structural characterisation techniques. At 680 C, the Ga/N ratio is found to have a dominant impact on the zinc-blende GaMnN epilayer growth rate and the resultant composition, morphology and microstructure. A maximum growth rate and an optimised microstructure are associated with growth under slightly Ga-rich conditions. A reduced growth rate and enhanced Mn incorporation are associated with growth under slightly N-rich conditions. Increasing Mn flux under N-rich conditions is considered to lead to a build up of a Mn surfactant layer during the early stages of growth and to a transition from zinc-blende single phase growth to zinc-blende/wurtzite mixed phase growth. Further, under Ga-rich conditions at low temperature, GaMnN films adopt a tilted growth mode, with close packed planes for both hexagonal and cubic phases being tilted roughly parallel to the growth surface, and this way of modified growth is also accompanied by improved Mn incorporation which is not commonly found for samples grown under Ga-rich conditions at elevated temperature. In addition, alpha-MnAs inclusions and voids extending into the GaAs buffer layer were identified in all samples, but are considered not to have a detrimental effect on layer electrical and magnetic properties.

Polycrystalline Zn1-xCrxO (0.01 ≤ x ≤ 0.09) samples synthesized by solid state reaction technique were sintered at different temperatures following slow step sintering schedule. Structural, micro-structural, optical, magnetic properties and homogeneity were investigated using suitable characterisation techniques. Cr2O3 and CrO2 phases have been detected in the XRD patterns and Raman spectra of Zn1-xCrxO samples with x ≥ 0.05. Photoluminescence study has indicated improved optical property of the samples compared to undoped ZnO. While low percentage Cr doped samples showed diamagnetic behaviour, higher percentage doped samples (≥ 5%) exhibited ferromagnetic, paramagnetic and anti-ferromagnetic behaviours depending upon the sintering temperatures. The magnetic properties have been analysed through Electron Spin Resonance study. A g-value of 1.97 indicates Cr in +3 valence state in doped ZnO system. Presence of Cr3+ and Cr4+ in ZnO is understood to facilitate super exchange interactions to promote ferromagnetism at room temperature. ESR study shows improved magnetic homogeneity achieved by slow step sintering process. / Physics / M. Sc. (Physics)