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Structure and chemical composition of InAs/GaAs quantum dots and GaInNAs/GaAs quantum wells were investigated using analytical transmission electron microscopy. These material systems are of importance for fabricating optoelectronic devices capable of operating in the 1.3 – 1.55 μm wavelength range. The samples were grown by molecular beam epitaxy. The optical properties of the quantum dots and quantum wells are largely determined by their structure and composition. Properties such as the shape, size and composition of the InAs/GaAs quantum dots and the morphology and composition of the GaInNAs/GaAs quantum wells were studied using high angle annular dark field imaging and electron energy-loss spectroscopy. In the case of the InAs/GaAs quantum dots, in particular, the effects of the GaAs spacer layer thickness in bilayer quantum dots and overgrowing the quantum dots with GaInAs instead of GaAs on the properties of the dots were investigated. The results indicated that the GaAs spacer layer can have a significant influence on the shape, size and nucleation position of the quantum dots in the second layer. In addition to this, it was demonstrated that overgrowing the quantum dots with GaInAs instead of GaAs introduces smaller changes to the structure and composition of the dots. In case of the quantum wells, the influence of the growth temperature and In composition on their morphology and composition were examined. Higher growth temperatures and In contents were shown to lead to the degradation of the 2D morphology and formation of inhomogeneities in the chemical composition of the quantum wells. When correlated with the optical performance of these structures, these results can provide important information about their structure – property relationships. Finally, the possibility of utilising electron energy-loss spectroscopy in monochromated transmission electron microscopy instruments to measure and map the band gap of semiconductor structures was studied and the results discussed.

In recent years, silicon nanowires (SiNW) have generated great interest for the fabrication of nanometre-scale transistors, thermoelectric devices, solar cells, and biological/chemical sensors. SiNWs, with minimum diameter ~10 nm, and lengths up to ~100 μm, may be prepared by a variety of growth, etching and high-resolution lithographic techniques. In particular, metal-assisted chemical etching (MACE) provides a low-cost method of producing large arrays of high aspect ratio SiNWs. This thesis investigates field-effect transistors (FETs) using SiNWs prepared by MACE. Source/drain contacts to the FET are defined by titanium silicide. FETs using large-area back-gates are found to be dominated by Schottky barriers (SB) at the source and drain. The ISD-VSD and ISD-VBG characteristics are determined by thermionic emission across the source SB, which may be lowered by the image-force potential, and by the local electric field generated by the source/drain and gate potentials. These results demonstrate that complete FET operation may be obtained by considering only the effect of SB lowering. An inverted-channel SiNW FET is also presented, where the characteristics are determined by both the contact SBs and the inversion layer in the NW. After subtracting the effect of the SBs from the data, a long-channel MOSFET model is used to find the field-effect electron mobility μFE ~100 cm2/Vs. FETs using parallel arrays of SiNWs are also investigated. These devices show similar source/drain relationship to single SiNW devices, but a weakened gate dependence, attributed to the aggregate response of multiple SiNWs in parallel. Low-temperature measurements of these multi-wire devices from 300K to 20K are used to extract the effective SB heights.

Over the last three decades, organic semiconductors, both polymeric and small-molecule compounds, have raised significant interest in academia and industry in view of the attractive combination of their versatile optoelectronic properties, lightness, flexibility and potential for low-cost and straight-forward manufacturing that makes them a valid alternative to conventional inorganic semiconductors. Thereby, 6,13-bis(triisopropylsilylethynyl) (TIPS) pentacene and other pentacene and anthradithiophene derivatives are interesting candidate materials for electronic applications such as organic field-effect transistors (OFETs) as they feature highly promising device performance and offer the possibility of processing them from solution, originating from their good solubility in common solvents. However, the small-molecule nature of these compounds often renders the control of the solid-state morphology of architectures deposited from solution challenging, thus, resulting in low reproducibility of their transistor characteristics. This thesis explores possible pathways to control the thin-film microstructure of such small molecules. By doing so, we aim to provide model systems that permit the elucidation of relevant electronic processes in these materials and to provide architectures for future technological exploitation. A thorough analysis is presented including the influence of the selection of solvent, casting temperature, coating techniques and the presence of small-molecular additives on the morphology of such semiconducting small-molecule thin films. Various strategies for chemical modification of TIPS pentacene are also discussed with focus of the effect of sidechain substitution on the electronic properties of the resulting architectures. Furthermore, investigations into the supramolecular arrangements that can be realised with some of those low-molecular-weight materials are presented and how this affects their optoelectronic features.

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Fabricating electronic devices using solution-based processing methods opens up a broad range of potential applications that are inaccessible to conventional semiconductor fabrication technologies. The chemically diverse family of carbon-based materials are suitable for this purpose with almost limitless possibilities for molecular tailoring. The present work is a study of some of the materials for and device physics of field-effect transistors based on solution processable layers. Each aspect of this work is chosen to address a current difficulty in the development solution-processable carbon-based electronics. For portable and battery-powered applications, low-power circuits are required. This can be achieved by using a complementary logic circuit architecture (that requires both electron and hole transporting semiconductors) where the discrete devices operate at low voltages. Practically, this requires a high capacitance gate dielectric which is compatible with solution processing of a range of semiconductor materials. One family of molecules suitable for this purpose are self-assembling phosphonic acids that can form molecular monolayers. In the present study, molecular tailoring of this family of molecules is investigated as a route towards improving the compatibility of these dielectrics with solution processed semiconductors. One of the difficulties with utilising a complementary logic circuit architecture is the requirement of a suitable electron transporting semiconductor. This semiconductor must be solution-processable, exhibit a high electron mobility and be stable against degradation upon atmospheric exposure. Although many p-channel semiconductors fulfil these requirements, equivalent performance in many families of n-channel semiconductors remains challenging. In the present study, the use of fullerenes, a widely used family of semiconductors, is explored for implementation as an n-channel material in field-effect transistors. Their electronic structure is controlled by chemical tailoring of each molecule and the impact of this parameter variation on the air-stability of these fullerenes is assessed. Graphene, potentially one of the most important materials for future electronics, is currently impractical to prepare over large areas. Chemical derivation routes are sought which allow processing of graphene from solution. One of the most important routes is solution phase exfoliation of graphene oxide followed by thermal or chemical reduction. Unfortunately this introduces a high density of defects within the final graphene layer which ultimately limits the charge-carrier mobility. Here, a milder oxidation with surfactant-assisted solution phase exfoliation is investigated as a route to improving the quality of graphene films following reduction. The electronic properties of thin- films of these chemically-derived graphene layers are explored as the active layer in field-effect transistors.

This thesis is a study of material blends involving organic semiconductors and their applicationto opto-electronic devices, particularly photovoltaic diodes. Its principal aim is to examinethe microstructure of the blend, where microstructure is defined as molecular ordering andspatial arrangement on the nanometer to micrometer scale. In general, each chapter of thethesis presents a novel means by which to influence the microstructure of organic semiconductorblends. These techniques are used as a means to understand how the photophysics of optoelectronicdevices is influenced by microstructure. We establish some general principles of howmicrostructure relates to device performance and also find high performance in some entirelynovel device structures and architectures. It is hoped that understanding developed here willlead to improvements in the performance of organic photovoltaic devices.

Quantum dots (QDs) are a class of semiconductor structure widely studied for their unique electronic and opto-electronic properties. Its extreme narrow line width property has been long predicted before first successful fabrication, and currently it is one of the main nano-structures used in light emitting devices. The QDs Idescribed here are engineered to form in a self-assembled manner where large quantities ofthem are produced at once. In the last decade, huge research efforts have been pumped ip.to controlling the property of the QD ensembles, such as areal density, wavelength, homogeneity and defect density. Towards the other extreme of the research interest, single QD fabrication is desirable for single electron and single photon devices, which are crucial tools both for quantum phenomenon observation and for the development ofquantum cryptography and quantum information processing. Two approaches are used to fabricate single QDs. In the micro-mesa approach areas on the dielectric mask are 'lectively opened so that the crystal structure is allowed to grow on the opened area. Usually the structures grown have a pyramidal shape. Choosing the base orientation correctly can result in pyramids with reducing top area and non-QD-growing sidewalls. Only a single QD would form at the apex of the pyramid if the pyramid size and maturity are well-controlled. The other approach described here is the micro-dimple approach. Since the formation of self-assembled QDs is a strain driven process, pin-point disturbance on a plain growth surface such as a dimple promotes QD nucleation on that point. Thus, a deposition of QD material with less than critical thickness may result in QD nucleation only on the dimples. For the micro-mesa approach, successful single QD growth has been performed in metal organic vapour phase epitaxy (MOVPE) and chemical beam epitaxy (CBE), and is demonstrated here in solid source molecular beam epitaxy (MBE). Performing lnAs/GaAs selective area QD growth in MBE appears to be problematic. The low GaAs growth selectivity between the dielectric mask and the GaAs epilayer in MBE destroys the shape of the pyramid grown. A combination of high growth temperature and low growth rate by periodic supply epitaxy (PSE) is used to reduce the polycrystalline fonnation on the mask. It is discovered that the polycrystals fonned on the dielectric mask reduce the neighbouring InAs QD areal density, whereas in MOVPE the dielectric mask increases the InAs QD areal density and/or height. Growing single QDs with the micro-dimple approach by MBE without atomic . hydrogen cleaning (ARC) is also investigated. During oxygen desorption in MBE, the native oxide on the processed sample fonns pits with a density as high as the selfassembled QDs and size comparable to the purposely made dimples. In order to realise a plain surface with only the dimples made during the fabrication processing, atomic hydrogen cleaning is usually employed to stop the pit fonnation during the oxygen desorption step. The alternative Ga beam supplement to reduce the pits is investigated and used in single QD fabrication by the micro-dimple approach in MBE.

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