The first experiment presents a novel method for continuously tracking the energies of the 1D subbands as a function of carrier density. I show a peculiar dc conductance feature in the region where the so-called 0.85(2<i>e</i>²/<i>h</i>) plateau in differential conductance is observed, directly demonstrating the pinning effect of the energy level of the minority spin-up electrons. A model concerning providing for non-linear population of the 1D subband with dc bias is proposed to explain the unusual differential conductance value of the 0.85(2<i>e</i>²/<i>h</i>) plateau. The second experiment shows that a fully spin-polarised current, consisting of a single spin-type only, can be created without external magnetic fields. When a source-drain bias lifts the momentum degeneracy, the dc measurements show that it is possible to achieve a unidirectional transport with a ferromagnetic order and this ordered spin array is destroyed once transport in both directions commences. The degree of spin polarisation of currents, between full spin polarisation, partial spin polarisation, and spin degeneracy, is thus simply controlled by source-drain bias and split-gate voltage, something of considerable value for spintronics. I then present four odd-even spin phenomena in the third experiment, showing clear evidence that a quasi-one-dimensional system tends to spontaneous spin polarisation with the energy band of minority spin-up electrons being reluctant to populate, thus widening the energy gap between two spin types. Variation of <i>g</i>-factor within a single subband is measured using a dc conductance technique, showing the <i>g</i>-factor oscillates as the 1D subbands are filled one by one with increasing carrier density. The last experiment introduces new experimental data of the zero-bias anomaly (ZBA), showing clear evidence that the ZBA observed in quantum wires in fact has a different origin from the Kondo effect seen in quantum dots. I propose a phenomenological model wherein the zero-bias anomaly in 1D quantum wires is in fact attributed to a upward shift of the 1D subband energy with source-drain bias.

A simple model is developed to extract the intrinsic decay rates of excitonic states in chiral supramolecular assemblies of OPV derivatives and those rates are found to decrease significantly upon aggregation. In stacks of HBC derivatives, the photoluminescent anisotropy reveals the existence of two excited states with complex characteristics illustrating the subtle interplay between molecular structure, packing and optoelectronic properties. Ultrafast spectroscopy is used to probe the dynamics of excitonic states in a chiral supramolecular assembly of an OPV derivative. Fast exciton diffusion leads to diffusion to trap states, exciton depolarisation, dynamic photoluminescence red-shift and exciton-exciton annihilation. A Monte-Carlo model based on resonance energy transfer steps is presented and yields microscopic properties such as the diffusion coefficient. These excitonic properties are very similar to those of polymeric semiconductors and justify the term “supramolecular polymers”. The study of mixed assemblies of OPV derivatives gives evidence of efficient energy transfer from the short oligomers to longer ones. The results are analysed with a model based on one-step Förster transfers. The supramolecular assembly is shown to speed up dramatically the energy transfers. The transfer rate and dimensionality are controlled by the molecular configuration and external parameters such as the temperature. Thus, I show that the supramolecular assemblies possess polymeric properties while offering more control than organic polymers.

The principal method of probing the structure of these films was the non-linear optical technique of Sum Frequency Generation (SFG). The experimental techniques used in this work are first introduced together with a theoretical description of SFG. There then follows an overview which describes semiconductor nanoparticle monolayers and the application of SFG to them. The work was subsequently extended to the investigation of multilayers of semiconductor nanoparticles incorporated within fatty acid films. Whilst SFG of monolayers is well understood SFG of multilayer films is not and most of this dissertation has been devoted to providing a systematic description of SFG of multilayer films adsorbed onto both metal and dielectric substrates. The observed multilayer SFG spectrum is shown to be a superposition of SFG signals coming from the surface layer, in contact with air, and the lowermost layer, in contact with the substrate. The SFG spectrum is a simple numerical addition of the two signals when gold is used as a substrate and as a subtle convolution of the two signals when a dielectric substrate is used. Through the incorporation of a single perprotonated fatty acid layer within an otherwise fully per-deuterated film it has been possible to study both the surface layer and buried interfaces. The resonant line shapes of SFG spectra obtained on gold substrates were found to depend on layer thickness, something which has been quantified experimentally and explained theoretically in terms of a nanoscale thin film interference effect. In the final part of the dissertation the capacity of SFG for studying buried interfaces has been exploited to investigate nanoparticle-induced structural changes within fatty acid multilayer films, demonstrating the versatility and potential of this novel application of SFG. It was found that, following nanoparticle formation within the fatty acid film, the surface layer remained highly ordered whilst the buried layers became disordered.

The aim is to link the unit <i>ampere </i>to the elementary charge <i>e</i>, a true invariant of nature. The high level of control over single electrons needed to realise such a quantum current standard has led to much interest in devices employing Coulomb blockade of tunnelling[2]. The time taken for an electron to tunnel through the barriers defining the dot, limits the output current of such devices to several pA[3,4]. A substantial increase in the current would allow these devices to be of a real practical use as a standard. This thesis explores another pumping mechanism for single charges which does not rely on Coulomb blockade of tunnelling. Single electrons are transported through a periodically formed decoupled quantum dot (QD). The electrons transported can be regarded as particles “surfing” on the time dependent potential of the well generated by the three barriers, rather than tunnelling through the barriers as waves. Two phase-shifted sinusoidal signals applied directly to the metallic finger gates on an etched GaAs/AlGaAs quantum wire, pump the electrons at frequencies of up to <i>f</i> = 3.4 GHz, corresponding to a current level of 0.54 nA. This approach represents an alternative path not only in the realization of a high current high accuracy quantum standard for electrical current but also in single photon production and electron spin based quantum computing.

This thesis describes the studies of the spin polarized current transport across Schottky barriers. Partially spin polarized electrons (~50%) are optically excited using circularly polarized light in GaAs. Under forward bias, these electrons are detected by a ferromagnetic (FM) layer at the FM/Semiconductor (Sc) interface using the spin split density of states at the Fermi-level in the FM. On average, a 3% change in the helicity dependent photo-current is observed. This confirms that about 6% of the spin polarized electrons, excited in the GaAs layer, transport into the FM across the FM/Sc interface without loosing their spin coherence. It is observed that the efficiency of the spin polarized electron transport across the Schottky barrier increases with increasing forward bias. Photo-current measurements are also performed for different excitation energies. An unusual resonant peak in the photo-current is observed at an excitation energy 20 meV below the band gap. We associate this peak with the modulation of the Schottky barrier. Barrier modulation is due to the neutralization of the ionized donor states when electrons are photo-excited to the empty donor states in the depletion region. Our results indicate that the efficiency of the spin polarized current transport increases at this resonant peak. We explain this increase by the decrease in the spin flip scattering due to ionized impurities. One of the possible routes to study room temperature spin polarized current transport across the semiconductor is by investigating the magneto-resistive properties of the devices, where a very thin semiconductor layer is sandwiched between ferromagnetic metallic layers. In this thesis two processing techniques which can be used to sandwich a thin GaAs layer between metallic electrodes are presented. Using these techniques a GaAs layer less than 100 nm thin is sandwiched between permalloy films. These techniques can potentially be used to sandwich even thinner GaAs layers between metals.

Research in High Voltage Thin Film Transistors (HVTFTs) has been driven by the need for devices with reduced on-state resistance and high blocking capability to improve the performance of large area electronic applications. Conventional HVTFTs give unsatisfactory performance because of the high on-state resistance, low breakdown voltage (Offset Drain HVTFT), high possibility of oxide failure and requirement for extra external bias line (Metal Field Plate HVTFT) etc. This thesis presents a novel high voltage thin film transistor structure - Semi-Insulating Field Plate HVTFT (SIFP HVTFT) which utilises a semi-insulating layer as the field plate to modify the conductivity in the offset region. The new structure has demonstrated and enhanced on-state performance relative to conventional offset drain device and a much improved blocking capability compared with all existing high voltage thin film transistor structures. Unlike conventional offset drain TFTs, during the "on-state", the channel formed under the gate can be extended into the offset region by the potential on the semi-insulating field plate which is controlled by the bias on the gate and the drain. Equivalent circuit models for the SIFP HVTFT have been developed for the device analysis. New concepts such as "extended channel" have been proposed for the first time to illustrate the device physics underlying the improvement in performance of the new device. The superior blocking capability is attributed to a very small leakage current flowing through the semi-insulating field plate which increases the radius of potential curvature near the drain. This leakage current, however, does not significantly contribute to the main current flowing through the device. It is concluded that the SIFP HVTFT presents a new concept to the high voltage thin film transistor family and has clearly shown many advantages over conventional HVTFTs.

There has been a shift in focus towards systems that integrate “nanomagnetic” components with existing device technologies, particularly for the further miniaturisation of magnetic storage devices. This thesis presents results, analysis and conclusions of studies on novel hybrid magnetic/metallic (Au/Co/Au) compositions in conjunction with low dimensional structures AlGaAs/GaAs quantum point contacts (QPC) and quantum dots (QD). The Au/Co/Au gates are used as surface confinement gates and also to apply a local inhomogeneous magnetic strayfield. Magneto-transport measurements were performed in low temperature cryostats with a minimum temperature of 350mK. It was concluded that the gating properties of these magnetic/metallic structures was significantly poor to allow for accurate characterisation of the devices. In addition, the QPC and QD themselves showed inhomogeneous effects that were dependent on the fabrication processes and material impurities. Novel broadband microwave spectroscopy measurements were used to study the transport mechanisms of a QPC. A continuous frequency range of 1-20GHz was studied where there have been no previous investigations on such devices. The experimental setup utilizes a non-invasive microwave source via a coaxial transmission line, based on the Corbino approach. The response of the device was seen to be very sensitive to these microwaves and showed ‘aperiodic’ fluctuations in the source-drain current as a function of the frequency and the onset of negative current features. These fluctuations are determined to be a result of semi-classical chaotic trapped electrons states that can be modified by the electric field of the microwave at these frequencies.

This dissertation introduces the concept of a dual-gate high electron mobility transistor (HEMT) using a multiple-split-gate structure. The device is a field effect transistor in which the current is under the control of two independent gates; one of these is a multiple-split-gate, the other is a continuous gate in close proximity to the split-gate openings. The ability of a split-gate to laterally deplete a channel, and hence modulate its width, has been applied to dual-gate HEMTs to provide a device with a variable effective channel width. A second independent electrode adjacent to the split-gate gaps supplies added functionality and a gating action not observed in conventional dual-gate HEMTs. Three fundamental limitations of a split-gate structure have previously excluded it from consideration for practical applications: the small current handling ability, the small transconductance and the fixed impedance levels. These arise from the small active width the device (<1μm). To overcome these limitations, a multiple-split-gate structure has been devised and developed; this allows the simultaneous operation of many split-gate elements in parallel. These split-gates are integrated into a single compact structure placed within a single source drain gap. Furthermore, this structure has been combined with a second gate electrode in close proximity. This gate incorporates projections that approach or actually penetrate the multiple-split-gate openings. In this way a three-dimensional structure can be achieved with the multiple-split-gate electrode bridging the protrusions of the second gate. This dual-gate structure, implemented on GaAs based heterostructures, has been demonstrated and characterised at dc and at low frequencies up to 100 MHz. Dual-gate structures with up to 100 split-gate elements acting in parallel are reported. The current handling ability and the transconductance are considerably improved over that of a single-split-gate device. These measurements are complemented by extensive two-dimensional simulations. The predictions of these simulations agree well with the measured characteristics and furnish an understanding of the interaction of the two independent gates.

Silicon on Insulator membranes are increasingly finding applications in many semiconductor devices and circuits. This thesis studies thermal and mechanical behaviour of two very different membrane based devices: Smart Gas Sensors and Membrane Power Devices. Both the devices are CMOS compatible and can easily be integrated with circuitry in a smart IC. The same fabrication process has been used for both these devices – a standard SOI CMOS process, followed by Deep Reactive Ion Etching (DRIE). Gas sensors need to micro-hotplate to heat the sensing material for better sensitivity and faster response time. The use of a membrane greatly reduces the power consumption of the heater. In lateral SOI power devices the membrane can greatly enhance the breakdown voltage and switching time. However, this comes at the expense of higher temperatures within the device – which can significantly reduce the lifetime. While in gas sensors it is desirable to have a high temperature for a given amount of power, in power devices the aim is to have a low operating temperature. Novel tungsten based SOI micro-hotplates are presented. A thorough thermal analysis of the power consumption (via conduction, convection and radiation), transient time and temperature uniformity of the micro-hotplate is presented by extensive simulation and analytical analysis. Following the study, micro-hotplate devices were fabricated at a commercial foundry. The measured results were analysed and matched with the simulations. The devices have very low power consumption (14 mW at 600°C), fast response time (2 ms for 600°C), good mechanical stability and excellent uniformity within a wafer and from wafer to wafer.

The achievement of luminescent nanocrystalline solid films, with good optical quality, will be crucial to the development of opto-electronic devices based on such materials. Although (CdSe)ZnS "core-shell" nanocrystals are typically found to have solution photoluminescence (PL) efficiencies in excess of 60%, the values associated with solid films are found to be an order of magnitude lower. Care of surface chemistry and control of nanocrystal/matrix interactions are of paramount importance. Furthermore, the PL efficiency exhibits a dependence on nanocrystal concentration consistent with a semi-quantitative model describing the effects of Förster energy transfer between nanocrystals and the associated trapping at surface sites. In addition to the ability to control optical properties by variation of the nanocrystal dimensions, it is also possible to alter the optical environment in which the nanocrystals are situated. By placing films of nanocrystals into high-Q, planar microcavities, it is possible to produce significant alteration of photoluminescence into very narrow resonant modes of the cavity. This is an important technical step towards the realisation of a nanocrystal laser. The combination of robust semiconductor emitters with the convenience of solution processing also offers considerable advantages over conventional molecular beam epitaxy (MBE) techniques. Finally, the PL emission from close-packed core-shell nanocrystalline thin films under intense picosecond UV excitation is studied. Strong, stable line-narrowing features are observed as the excitation intensity is increased, both at 77K and at room temperature; these are attributed to waveguiding and amplified spontaneous emission (ASE) in the films. Such behaviour would usually be considered as the signature of optical gain. Lasing from microcavities based on these films has yet to be observed, however, and a semi-empirical model of line-narrowing threshold intensities and cavity-photon lifetimes suggests that higher gain, lower losses or greater cavity finesse may be required for this.