The optically active nature of direct bandgap semiconductors makes them well suited for applications in quantum optics. Semiconductor quantum dots (QDs) are particularly promising, due to their discrete atom-like energy levels. In this thesis, transitions between these energy levels are used to investigate the effects of electric and magnetic fields on the energy structure of single QDs, with a view to developing applications in the field of quantum computing. In the work presented here a novel method of creating entangled photon pair emitters is presented, in which an electric field is used to tune the energy structure of single QDs to allow the fidelity of the emitted entangled state to be increased. In addition, a technique for the creation of energy-tunable entangled photon pairs is proposed and shown to be feasible with current technology. Furthermore, the potential of QDs to act as an interface between photonic and spin qubits is explored. Application of a time varying electric field is used to dynamically tune the QD energy levels, allowing the evolution of excitons confined within single QDs to be manipulated. Using this system a controlled phase rotation of the exciton spin state is implemented. Finally, indistinguishable single photons, emitted by the radiative decay of the exciton state, are used to generate the input state for an integrated photonic two-qubit quantum logic gate. This is the first demonstration of a two-qubit gate using on-demand single photons. It is also the first demonstration of such a gate with all components realised using semiconductor materials.

Due to their high charge carrier mobility, optical transparency and mechanical flexibility, thin-film transistors (TFTs) based on metal oxide semiconductors represent an emerging technology that offers the potential to revolutionise the next-generations of large-area electronics. This thesis focuses on the development of high-performance TFTs based on low-temperature, solution-processed metal oxide semiconductors that are compatible with inexpensive flexible plastic substrates. The first part of the dissertation describes an ultraviolet light assisted processing method suitable for room-temperature activation of ZnO nanoparticles and their application as semiconducting channels in TFTs. The impact of the semiconductor/dielectric interface on electrical performance is studied using different device configurations and dielectric surface-passivation methods. Furthermore, a nanocomposite concept is proposed in order to assist electron transport between different crystalline domains. Using this approach, TFTs with electron mobilities of ~3 cm2/Vs are demonstrated. The second part of this work explores a carbon-free, aqueous-based Zn-ammine complex route for the synthesis of polycrystalline ZnO thin-films at low temperature and their subsequent use in TFTs. Concurrently, the development of a complementary high-κ oxide dielectric system enables the demonstration of high-performance ZnO TFTs with electron mobilities > 10 cm2/Vs and operation voltage down to ~ 1.2 V. This low-temperature aqueous chemistry is further explored using a facile n-type doping approach. Enhancement in electrical performance is attributed to the different crystallographic properties of the Al-doped ZnO layers. The final part of the thesis introduces a novel TFT concept that exploits the enhanced electron transport properties of low-dimensional polycrystalline quasi-superlattices (QSLs), consisting of sequentially spin-cast layers of In2O3, Ga2O3 and ZnO deposited at temperatures < 200 °C. Optimised oxide QSL transistors exhibit electron mobility values of > 40 cm2/Vs - an order of magnitude higher than devices based on single binary oxide layers. Based on temperature dependent electron transport and capacitance-voltage measurements, it is reasoned that the enhanced electrical performance arises from the presence of quasi two-dimensional electron gas-like systems formed at the carefully engineered oxide heterointerfaces buried within the QSLs.

No description available.

This thesis investigates the development of a low cost fabrication process for flexible electronics and interconnects. By using a ‘direct writing’ process, the use of vacuum-­‐based metal evaporation and photoresist steps is not necessary and so less complex equipment is needed. The process forms silver embedded on top of a polyimide substrate and was first tested using a UV laser to perform writing before switching to a blue laser due to excessive substrate degradation observed from UV exposures. The blue light was combined with a biologically friendly photo reducing agent, which was found to be much more efficient at the creation of silver. The methods of silver formation by various means are the main focus of investigation in this thesis but process expansion and improvement were the main goals. To this end, a chemical, rather than light-­‐based, process for silver creation was found to produce more consistent silver coatings, however the patterning by this method was found to be more challenging. The process was also extended to a different substrate in polyetherimide.

A thorough investigation has been carried out in order to determine the suitability of diamond and silicon carbide for active interrogation applications. This included electrical and radiological characterisation of single crystal diamond (D-SC) and polycrystalline diamond (D-PC) detectors; epitaxial silicon carbide (SiC-EP) and semi-insulating silicon carbide (SiC-SI); all compared against the performance of a commercial silicon PIN photodiode (Si-PIN) from Hamamatsu. This work aided in determining whether the detectors were suitable for radiation detection purposes, as well as obtaining the operational criteria for use. Characterisation work was also conducted on semi-insulating silicon carbide detectors from three different suppliers, as well as on detectors fabricated via different techniques. This work demonstrated the robustness of the material, as the charge collection properties were unaffected by contact fabrication technique. Changes in current-voltage characteristics were observed for different contact fabrication methods, but were generally still low (≈nA) over the ranges tested (±500V). Following this work the performance of selected detectors was measured against criteria for the AWE active interrogation project. Radiation dose dependent performance deterioration was observed in the SiC-SI and D-PC detectors, with decreased charge collection efficiency (-45±4%) and intrinsic efficiency (-40% at -400V) observed respectively. It is not clear as to whether these effects are a result of bulk material damage or contact/surface/mount damage, but an increase in the current-voltage relationship was also observed on these detectors, as well as the Si-PIN (SiC-SI≈+25% and D-PC≈+20% at -400V; Si- PIN≈+300% at -25V). Instability of the peak position and/or counting rate with irradiation time was observed in D-SC, D-PC and all the semi-insulating SiC (polarisation effect). For D-SC this was primarily with alpha particles and stability would be maintained after a period of time, with that period decreasing as the incident flux increases. For D-PC and the semi-insulating SiC, this effect was observed on most radiation types tested (alpha, beta, X-ray, gamma, neutron and protons) with polarisation rate increasing as the the number of charge carriers created per incident particle increased. However, it has been shown that combinations of ambient light and 0V bias could depolarise a semi-insulating SiC detector and even decrease its polarisation rate for future irradiations. D-SC, SiC-EP and semi-insulating SiC material were also shown to operate from -60◦C to +100◦C. For D-SC and SiC-EP the charge collection efficiency was similar (±10%) over the entire range, apart from at +100◦C for D-SC where it was ≈50% down. For SiC-SI, the charge collection efficiency peaked at room temperature, but became more stable at +100◦C (lower polarisation rate). All the detectors demonstrated the ability to detect and discriminate between both different energy neutrons and ionising photon (gamma) energies using simple energy threshold discrimination. Comparison of the endpoint energy for AmBe (< 4.1MeV >) and Cf-252 (< 2.1MeV >) or mono-energetic 1MeV and 5MeV neutrons, give ratios (Emax(High Energy)/Emax(Low Energy)) of ≈3.5, 2.5, 5.0, 4.9 and 2.0 for D-SC, D-PC-, SiC-EP, SiC-SI and Si-PIN respectively. Similarly comparison of the endpoint energy for Co-60 gammas (1.2MeV and 1.3MeV) and AmBe neutron (Emax(AmBe)/Emax(Co − 60)) give ratios of 8.1, 16.0, 6.4, 6.9 and 9.1 respectively. It was also shown that the neutron-gamma detection ability can be improved through simple design optimisation techniques, including: the use of high atomic number filtration to reduce gamma detection; hydrogenous proton conversion layers to improve neutron detection; and large area detection arrays to improve counting statistics.

Semiconductor materials have a vast range of applications varying from basic electronic products to astronomy and their semiconducting properties can be altered through the growth of binary or ternary compound materials. CdZnTe and CdTe are prominent materials in radiation detector and photovoltaic solar cell applications. For radiation detectors, in the fabrication process, surface preparation (chemical polishing and passivation) and contact deposition are key to the detector performance. This thesis investigates the effect of these two processing steps on CdZnTe detectors through varying the passivation procedures and gold contact configurations. The surface composition, layer thickness and non-uniformity resulting from the passivation treatments have been investigated using X-ray Photoelectron Spectroscopy (XPS), Scanning Transmission Electron Microscopy (STEM), Energy Dispersive X-ray spectroscopy (EDX) and other materials characterisation techniques. The device electrical and spectroscopic responses were measured using the I-V characteristics and alpha spectroscopy respectively. Passivation using 30 % H2O2 and 5% NaClO treatments develops a very thin oxide layer of up to ~2 nm, while NH4F/H2O2 and KOH+NH4F/H2O2 treatments yield oxide layers of varying thickness (30 – 142 nm) and metal oxides comprising of Te2/Te3, CdO and ZnO. Devices were fabricated in metal-semiconductor (MS) and metal-insulator-semiconductor (MIS) configurations. The MIS configuration improves the mobility-lifetime product, partial charge collection and leakage current of a CdZnTe device. The MIS device barrier heights were calculated to be 0.83 ± 0.02 eV and 0.86 ± 0.02 eV for very thin and thick oxide layers respectively. For ultra-thin (0.5 µm CdTe layer in) CdTe/CdZnS solar cells, XPS and X-ray diffraction (XRD) were employed to study the effect of varying the CdCl2 processing step. Increasing the degree of CdCl2 activation and annealing treatment was found to increase sulphur diffusion into the CdTe layer (up to a concentration of ~ 2 at.%). Cell performance measurements showed that the increase in S concentration is directly related to the open-circuit voltage (Voc), and increasing the degree of CdCl2 treatment gives higher Voc values.

This thesis is concerned with investigations of the features of two types of electronic thin film materials: chemical vapour deposition (CVD) diamond and copper oxide based materials. CVD Diamond possesses excellent electrochemical properties. This thesis was concerned with investigating the fabrication and electrochemical properties of certain such diamond electrodes. The fabrication of diamond Ultramicroelectrodes (UMEs) was explored by coating tungsten needles with CVD diamond film under optimised. conditions, followed by selective insulation with different media. It was found that small grain diamond made the best electrode; large grain diamond coatings suffered from electrolyte leakage whereas nanodiamond had poor electrochemical properties. A range of tip insulation methods were examined, with most defined tips being produced by insulation with electrophoretic paint, followed by milling using Focused Ion Beam (FIB) methods. The utility of the tips prepared in this way in the SECM was demonstrated by imaging in biological media. The use of electrical conductive diamond as optically transparent electrode (OTE) opens novel applications for spectroelectrochemical studies due to the superior properties of diamond. The HFCVD diamond growth on fused silica quartz, ITO and AZO substrates was explored. The diamond membrane/ ITO structure was proposed and fabricated, exhibiting the best combination of optical transparency and electrical conductivity. Finally the changes in electrode properties as the diamond varied from macrocrystalline to nanocrystalline morphologies were studied. The second material investigated is copper oxide, specifically, cuprite (CU20) and SrCu202, a ternary Cu(I) oxide with a direct bandgap that arouses widespread interest as a p-type TCO. Their electronic structure and the nature of the hole charge carriers are topics of major current interest. The valence band and conduction band of both materials were studied by XPS in Daresbury, and XAS and XES measurements in ALS. The spectra are in good agreement with the PDOS from B3LYP calculations, showing strong hybridisation between Cu 3d and 0 2p states. Resonant Inelastic X-ray Scattering (RIXS) due to interband excitation close to Cu L3 edge threshold was first observed, conforming selection rule .6.L=O. This is the first observation of RIXS in close shell compound (dIO ). The UPS spectra of SrCu202 were measured with synchrotron radiation, and the changes in intensities of spectral features with varying photon energy were used to distinguish the contribution of 0 2p and Cu 3d states. Spectra showed that states at top of valance band are of dominant Cu 3d character and there is strong hybridisation between 0 2p and Cu 3d states which accounts for the hole mobility.

No description available.

No description available.

No description available.