Currently the transistors required for active matrix liquid crystal displays (AMLCDs) are fabricated using hydrogenated amorphous silicon (a-Si:H) owing to its large area capability and compatibility with a wide range of low cost substrates. Future displays will however require a material with a higher field effect mobility than a-Si:H and while polycrystalline silicon (poly-Si) can meet these requirements it does so currently at the expense of large area or low temperature substrate compatibility. The thesis investigates the suitability of hydrogenated microcrystalline silicon (μc-Si:H) for channel layers in thin film transistors (TFTs). μc-Si:H is a biphasic material consisting of crystalline regions in an amorphous matrix, and potentially offers a large area, low temperature deposition process similar to that of a-Si:H while providing an enhanced field effect mobility. Using the hydrogen dilution method in a conventional plasma enhanced chemical vapour deposition (PECVD) system μc-Si:H films were deposited and characterised. Films deposited by this method exhibited only moderate crystallinity but a wide range of conductivities, suggesting that impurity incorporation may have a more significant effect on microcrystalline films than their amorphous counterparts. TFTs fabricated using μc-Si:H channel layers exhibited clear transistor action, but field effect mobilities were uniformly lower than for equivalent structure a-Si:H channel devices. Significantly, attempts to improve the crystallinity of the channel layer resulted in degraded TFT performance consistent with an increase in defect rich material. The temperature dependence of mobility of μc-Si:H channel devices suggests that the reduced performance is a consequence of an increased density of conduction band tail states in μc-Si:H compared to a-Si:H. This increased density is in turn proposed to be due to the introduction of crystallites into the amorphous matrix and the subsequent increase in the density of weak Si-Si bonds.

The ease of processing conjugated polymers in conjunction with their novel optical and electrical properties has attracted great interest from both theoretical and applied perspectives. There has been particular interest in the development of organic transistors for mass produced low speed electronics. In this dissertation, metal-insulator-semiconductor field-effect transistors (MISFETs) are used to study the physical mechanisms of charge transport in solution-processable conjugated polymers and oligomers. The effect of dopants upon the electrical characteristics of MISFET devices is investigated using a soluble polythiophene derivative. By carefully controlling the doping level, the relationship between the conductivity and field-effect mobility is explored as a function of the dopant level. Temperature dependent measurements of the conductivity for various doping levels are presented, which suggest variable range hopping as a viable mechanism for charge transport. Combining these observations with magnetic susceptibility measurements allows the relationship between the conductivity and field-effect mobility to be modelled within the framework of variable range hopping. The applicability of this model to other systems is discussed in terms of impurities and molecular order. Recently solution-processable polyacetylene has been produced with mesogenic end-groups attached to the end of the chains. These materials are designed in an attempt to improve the molecular order in the films. Charge transport is investigated using MISFET devices by comparing results from materials with and without mesogenic end-groups. The molecular order is explored using electron microscopy and absorption spectroscopy of field-induced charges. Charge transport in precursor-route conjugated materials is studied and shown to result in improvements in device characteristics. In particular, the recent development of a precursor-route to pentacene is utilised and the device characteristics are studied as a function of the conversion conditions. The temperature dependence of the charge mobility is studied for this system and for other conjugated materials. The measurements are compared with those of inorganic systems and discussed in terms of the physical differences between inorganic materials and conjugated polymers.

This thesis describes how a scanning tunnelling microscope (STM) can be utilised for <I>in situ </I>analysis of molecular beam epitaxially grown III-V semiconductors. The initial research required the integration of an STM into an <I>in situ </I>III-V processing facility which included two growth chambers and a focused ion beam (FIB) lithography chamber. By minimising vibrations from the rest of the system, post growth STM observations of GaAs(100) with a vicinal surface showed that the STM produced reliable atomic scale images. This was extended to show how, as the substrate temperature increased from 630°C to 645°C, growth changed from an island growth regime to a step flow regime, thus preserving the vicinal structure. If there is a lattice mismatch in heterostructure growth then misfit dislocations form at the interface. The growth of GaSb on GaAs(100) was studied to determine the effect on surface morphology of a 7% lattice mismatch. It was found that the misfit dislocations propagate with continued growth and terminate at the surface as a screw dislocation, thereby inducing spiral growth. The rms surface roughness resulting from this spiral growth increased with decreasing substrate temperature, however the dislocation density decreased. Photoluminescence (PL) experiments were undertaken to determine whether the surface roughness translates to interface roughness in a single quantum well. A similar lattice mismatch was observed to produce sub-micron coherent islands of InAs on GaAs(100) substrates. The evolution of these islands for given parameters was studied, noting the formation of a wetting layer and the critical thickness (˜1.6 monolayers) at which island nucleation occurred. These islands were very uniform in width (+/- 10% size variation) yet had two height regimes. PL experiments detected emission at 1.2 eV suggesting that there is structural coherency of the islands which is confining the band energies. The interaction of a 30 keV Ga<SUP>+</SUP> FIB with GaAs(100) was studied for line doses of 1x10<SUP>8</SUP> ions cm<SUP>-2</SUP>. Topographic analysis showed there was little surface damage, however, electrically active defect sites were spatially resolved using the spectroscopic mode of the STM. These sites were found to lie below the conduction band as determined from I-V measurements.

This report describes experimental work performed on highly phosphorous doped silicon single electron transistors (SET) both with and without a nearby isolated double-dot structure (IDD). Measurements are performed at milli-Kelvin temperatures and at 4K for magnetic fields from 0T to 15T. It is found for an SET that in some cases different single particle levels are being used depending on the direction of the voltage bias across the quantum dot. This is confirmed by the differing behaviour of the corresponding Coulomb peaks with magnetic field. Peak splitting with field is observed which is shown not to be an excited-state effect. A method is suggested involving charge-reorganisation on the dot in combination with a large variation of tunnel couplings of differing levels to the drain. A four-way splitting of a Coulomb peak is also observed. A spin-filling scheme is created using the behaviour of Coulomb peaks with magnetic field. Higher order spin transitions of ΔS=3/2 are observed. High polarisation is observed with as many as four electrons of the same spin being added consecutively. Strong hysterysis is noted between forwards and reverse-swept gate voltages using an IDD device. This hysterysis is absent with a bare SET device. Oscillations of current with field, identical over many Coulomb peaks are noted for an IDD device operating in the multi-electron transport regime. Much more variable oscillations with field oscillations with magnetic field are noted for an SET operating in the single-electron transport regime.

A study of interactions between molecules of the same material was performed on a model polymer system. Regio-regular poly(3-hexylthiophene) (P3HT) self-organises into two-dimensional π-stacked lamellar structures. Although the chains pack at a distance of only 3.8Å, the resonant coulomb interaction between nearest-neighbour molecules is small, owing to large average conjugation lengths within the lamellae. When the resonant coulomb (or excitonic) interaction is smaller than the vibrational relaxation energy, the excitation is localised on a single molecule and the coupling is termed <i>weak. </i>Here we demonstrate that the polymer absorption and emission spectra can be comprehensively explained using a weakly interacting H-aggregate model. Using an analytic form of the model we can estimate the excitonic interaction energy from the ratio of the 0-0 and 0-1 absorbance peaks. It was found to vary between 5-30meV. We use this powerfully simple tool to determine how film micromorphology depends on processing. We find that films spun from low boiling point solvents show a decrease in crystalline quality, an increase in excitonic coupling and more amorphous regions, than films spun from high boiling point solvents. We correlate these findings with field-effect transistor characteristics to produce a model of film micro-morphology. We also demonstrate preliminary results that show that due to the aggregate nature of the primary neutral excitation, photo-induced charge generation in P3HT is due to exciton-exciton annihilation. Finally, we study the intermolecular interactions at an interface between two different molecules (a hole transporting polymer, TFB (Poly(9,9-dioctylfluorene –<i>co</i>-N-(4-butylphenyl)diphenylamine)) and an electron-transporting disoctic, HATNA-SC12 (Hexa-azatrinaphthylene)). Depending on the energetics of the interface between two materials, the inter-molecular state can be stabilised by either coulomb interactions (as in the case of single material interfaces) or charge-transfer interactions. In this case, we find that the charge-transfer interactions dominate and an excited-state complex, or exciplex, is formed. The exciplex is generated efficiently in light-emitting diodes, producing a pure red emission. In polymer/polymer blends efficient bulk exciton emission can occur due to endothermic transfer from the exciplex to the bulk exciton. In this system, however the exciplex is stable at the interface, acting as an energy bottleneck with inefficient transfer to bulk exciton states. Furthermore, the yield of charge separation is low.

This thesis examines the properties of hydrogenated tetrahedrally bonded amorphous carbon (ta-C:H) grown using a plasma beam source(PBS) and assesses its potential as a semiconductor material for electronic applications. Production of high quality ta-C:H requires optimisation of the deposition conditions and it is therefore of importance to analyse the behaviour of the PBS. It is found that the operation of the PBS agrees with its theoretical description as a capacitively coupled system and the plasma has the required properties to produce ta-C:H. Characterisation of the structural properties confirms the tetrahedral nature of the deposited films. Characterisation of the electronic and optical properties show that ta-C:H is a wide band gap semiconductor which conducts by a thermally activated localised hopping mechanism with a defined activation energy. Measurements of photoconductivity show that ta-C:H grown using methane gas has a photosensitivity greater than in any other form of amorphous carbon. A high density of mid-gap defects will degrade the electronic properties of an amorphous material, however defects can often be passivated by hydrogen. Hydrogen only has a weak passivation effect in ta-C:H and the defect density remains high. Improved passivation is shown to be achieved by post-deposition annealing of ta-C:H, although experimental results suggest that the presence of hydrogen increases localisation of the conduction states. The ability to dope ta-C:H is also important for electronic applications. Strong evidence is presented which suggests that ta-C:H is slightly p-type and can be doped n-type by the addition of nitrogen. This n-type behaviour is further verified by the manufacture of n-type ta-C:H:N thin film transistors (TFTs). The TFTs show a small on/off ratio and require a large turn-on voltage. This is attributed to the relatively high defect density in the mid-gap region, despite its reduction by annealing, illustrating the main problem at present with the use of ta-C:H in device applications.

A reliable procedure has been developed which facilitates the fabrication of low-leakage front and back gated lateral transport devices in which electrons and holes reside in adjacent layers. This gating permits the alteration of the electron and hole densities in such devices almost independently of one another. A simple processing technique has also been developed for resonant inter-band and hybrid inter/intra-bond tunnelling devices. In the absence of a barrier between the layers in which the charge resides, it is shown that the wavefunctions of electrons and holes hybridise, causing an energy gap to form in the dispersion relation of the hybrid particles. This manifests itself in lateral transport measurements as a resistance peak when the Fermi energy in the system is made to lie in this "gap" by the action of the gates. The resistance resonance is seen to disappear as a function of temperature and in-plane magnetic field. The results of a simple numerical model are shown to predict the broad features in the experimental data and allow the extraction of the magnitude of the energy gap in the system. Anomalous low-field positive magnetoresistance in an in-plane magnetic field is discussed in the context of wavefunction movement in the growth plane and the antilocalisation effects of asymmetry. In a perpendicular magnetic field, Landau levels are formed in the system and the transport shows Shubnikov-de Haas oscillations and quantum Hall plateaux originating from either carrier type. So-called "compensated Hall plateaux" are observed and the absence of a completely compensated "v=0" plateau is discussed. Magnetic field induced charge transfer between the electron and hole layers which depends on the inter-layer tunnelling is observed. Tilted magnetic field measurements are performed to extract the effective g-factor of the electrons in the system and the effect of electron-hole coupling on its value is discussed.

This thesis reports on the work done in studying coherent dynamics of excitons in quantum wells and exciton spins in quantum dots. Four-wave-mixing experiments were performed in a magnetic field on GaAs/AlGaAs single quantum wells known to contain spatially large monolayer islands. The properties of the dynamics of the signals are discussed, and compared to previous values. Beating was observed between the monolayer islands, and the variation of the phase of these beats was examined in an attempt to determine the mechanism for coupling between them. The majority of the data suggests the monolayer islands are not coherently coupled, however, the behaviour under some conditions brought this into question and required further modelling. Models for a three-level system and two on-interacting two-level systems including the effects of a local field are presented. Neither model was able to reproduce the experimental data on its own, however, a combination of the two was successful. On the basis of the modelling, it was shown that the contribution to the signal from coherently coupled transitions decreases as a function of magnetic field. This gives some insight into the possible coupling mechanisms, and these are discussed. Pump-probe experiments are performed on InGaAs/GaAs quantum dots in an attempt to obtain details of the coherent exciton-spin dynamics in such systems. Decoherence of the exciton spin is observed in sufficiently high magnetic fields, and the dependence of the dephasing time on field strength and temperature are studied. To my knowledge, these are the first reported experimental results showing decoherence of exciton spin. Comparisons of the observed behaviour with existing predictions are made in an attempt to determine the mechanism for loss of exciton spin coherence in quantum dots. None of the predicted mechanisms were able to fit my experimental data adequately, suggesting the presence of some other more efficient mechanism.

In the first structure presented a single two-dimensional electron gas is positioned in a region of graded (0 ≤ <i>x</i> ≤ 0.1) InGaAs composition. Through a series of MBE grown wafers the technique of successfully growing InGaAs as an InAs/GaAs superlattice was demonstrated. Varying the period of the superlattice was used to achieve the graded InGaAs region in the final device design. The exchange-enhanced <i>g</i>-factor was measured via thermal excitation seen to increase with the application of positive back-gate voltages is. One-dimensional conductance is observed in a graded alloy system for the first time, a stepping-stone to the implementation of single electron devices. Through analysis of the low-field resistivity, the appearance of a second subband in the two-dimensional electron gas, attributed to the zero-field spin-splitting from the Rashba interaction, was seen, and a dependence on back-gate voltage observed. Gate-voltage control of the spin-orbit interaction has only previously been observed in much higher indium concentration samples. A second structure investigated consists of two two-dimensional electron gases and forms a new kind of double quantum well device. Two-dimensional electron gases are located in separate GaAs and InGaAs quantum wells, separated by an AlGaAs barrier. Devices presented in this thesis allow two different gating schemes to be investigated. Firstly large-area front- and back-gates allow the isolation of a two dimensional electron gas in either well. This means two-dimensional conduction can be limited to either the GaAs or InGaAs layer. Secondly through use of a split-gate midline device it is possible to select the conduction pathway through the device with quasi-one-dimensional channels. This technique uses surface gates only, and again it gives the ability to select the material composition in which the electron wavefunction is situated. Such a double quantum well system gives the possibility of investigating the effect of the local <i>g-</i>factor and spin-orbit coupling on various low-dimensional spin-related phenomena.

In this thesis, we demonstrated that divinyltetramethyldisiloxane-benzocyclobutene (BCB), which has previously been used as an isolation dielectric in III-IV semiconductor devices, in fact makes an excellent gate dielectric material in OFETs after suitable purification. Robust ultra-thin films with high glass transition temperature and high dielectric breakdown strength can be obtained by simple spin-coating followed by rapid-thermal-anneal to above 250°C. With this material, we were able to demonstrate remarkable performance in polymer OFETs and explore several aspects of their physics. In Chapter 2, we introduce the use of BCB as a good candidate for solution-processable organic gate dielectric. Pinhole-free ultra-thin gate dielectric film as thin as 50nm can be made from this material. With this gate dielectric, robust continual cyclic operation of poly[(9,9-dioctylfluorene-2,7-diyl)-<i>alt-</i>(phenylene-(N-(<i>p</i>-2-butylphenyl-imino-phenylene)) (TFB) FETs at 120°C was achieved. Previously, the thinnest practical solution-processable gate dielectric thickness was >300 nm-thick. In Chapter 3, we demonstrated self-organised polymer semiconductor/dielectric FETs fabricated using a spontaneous and an unusual vertical phase separation of the TFB polymer semiconductor and the BCB dielectric materials during film spinning. This method enables the formation of semiconductor and dielectric layers at the same time without exposing their interface to air. Using these devices, we established that a critical root-mean-square interface roughness of 0.7 nm (measured on the 100 nm length scale) could be tolerated without loss of mobility of the devices, probably related to the hopping of the carries at the interface. In Chapter 4, we demonstrated using this non-trapping BCB dielectric the generality of n-type field-effect conduction across a wide range of polymer organic semiconductors. We showed that this was previously suppressed by interface trapping of the accumulated electrons by the –OH group in the gate dielectrics that have often been used. We found electron mobilities very similar to, if not larger than, hole mobilities across a range of organic semiconductors. Therefore, many (though not all) <i>π</i>-conjugated materials are by their nature ambipolar and can support both electron and hole conduction nearly equally well. Their previous classification into “n-type” and “p-type” materials is thus somewhat arbitrary. Finally, in Chapter 5, we used BCB as the top gate dielectric and fabricated fully functional double-gate OFETs over a bottom gate dielectric. We showed that such devices exhibit electrostatic coupling of the two gates occurs to produce an “<i>AND”</i> logic gate.