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A surge in the developments in nanoscience in recent years has opened the door to new possibilities in the field of semiconductors, in particular the fabrication of high quality semiconductor nanocrystals. These materials are important to the semiconductor photovoltaics and photo induced water splitting devices. The aim of this thesis is to advance understanding regarding the band edge energy level (Eedge) alignment of as-fabricated nanoscale semiconductors (such as quantum dots, QDs) through 'hot injection' methodologies, as well as investigation of the chemical reaction mechanisms behind this synthetic approach. Eedge evolution of binary and ternary cadmium chalcogenide QDs were studied electrochemically, employing an ultramicroelectrode. It was found that, with change in particle size or composition, variations in the band gap were mainly attributed to changes in the conduction Eedge; a non-linear conduction Eedge bowing phenomenon also revealed when tuning the composition of alloyed QDs. A radical reaction during the phosphine-free QDs synthetic approach was detected through NMR, FTIR. and mass spectroscopy, which further led to studies of the chemical reaction mechanism of formation of quaternary CuZnSnS(Se4) (CZTS, CZTSe) and binary iron chalcogenide nanocrystals. By employing a modified electrode approach, Eedge of as-prepared quaternary CZTS(Se) and binary ion chalcogenide nanocrystals were also determined. It was discovered that the conductivity of CZTSe is three orders of magnitude higher than the CZTS nanocrystals; the detection of metallic copper (formed in the precursor preparation step) and different oxidative power of S and Se species, are found to be the reasons behind the- difference in charge transport ability. The scientific contributions reported in this thesis add precious value to the picture of energy level distribution of semiconductors, and the chemical mechanism study provides guidance for scale up production and new material synthesis, which are crucial factors in the fabrication of electrochemical semiconductor devices.

The fabrication of SiGe layers with ingrown crystal lattice strain allows engineering of the electronic properties of the alloy. This phenomenon is of scientific and ultimately commercial interest (Meyerson 1994). In this work the information available via the microdiffraction technique developed on a VG HB501 Scanning Transmission Electron Microscope (STEM) is investigated by analysing the strain information retrievable from nanometer scale Si/Si<SUB>1-x</SUB>Ge<SUB>x</SUB> multilayers. Deficit Higher Order Laue Zone (HOLZ) line patterns are recorded from localised lattice positions within strained Si/SiGe superlattices. It is shown that by employing a simple model for the production of the HOLZ lines, direct measurement of the crystal lattice strain and lattice parameter can be achieved by measuring displacement of the HOLZ lines. The two main problems concerned with the determination of lattice strain by electron microscopy have been addressed. The first is due to the fact that strained materials which are thinned down to electron transparency may undergo varying degrees of bulk and surface relaxation which are difficult to predict. This has been observed as a splitting and blurring of deficit HOLZ lines or by significant deviation of line positions from that predicted by CBED simulations. Investigations of the relaxation of materials prepared with different specimen geometries have been investigated. The second problem is the possibility that the pattern changes observed in the two dimensional diffraction plane of the microscope could be caused by more than one possible displacement in the 3D reciprocal lattice. This uniqueness problem results from the fact that it is impossible to separate out which displacements of a single deficit line in the 2D microdiffraction plane have been caused by the three possible different displacements in the 3D lattice. A rigorous geometrical description of the movement of the HOLZ lines is developed and inter-relationships between the straining of the lattice along the different lattice axes are used to solve a unique solution to the strain tensor.

There are a number of proposals that involve single electrons in multiple quantum dot systems, using either electron spin or dot location to store and manipulate information for quantum computation. In this dissertation a technique for fabricating double quantum dot systems potentially useful for quantum computation is described. This method allows single charge electrometers to be fabricated close to the dots so that measurement of the charge states can be made, which is a requirement for quantum computation. Low temperature measurements of the charge transport through these quantum dot structures are presented for a variety of different double dot structures. A number of phenomena observed in the experiments are described. These include period doubling and peak splitting of the Coulomb oscillations, carrier energy filtering effects due to energy level misalignment, observation of the double dot stability diagram, and magnetic field dependence of the Coulomb oscillations. Other measurements in a dilution refrigerator reveal that photon assisted tunnelling from black body radiation within the refrigerator - a source of decoherence for quantum computation, can take place. The device can be used as a sensitive radiation detector to ensure that the radiation is minimized or reduced for measurements of quantum coherence. Single hole electrometers fabricated very close to the double dot structures are shown to be sensitive enough to measure the charge states in the double dot structure.

Gallium nitride (GaN) and its related alloys - aluminium gallium nitride (AlGaN) and indium gallium nitride (InGaN) are becoming increasingly important in the world of semiconductors and in modern life - being used to make high power transistors and LEDs. However GaN is much less understood than other commercial semiconductors, and this project looks at aspects of GaN growth, with emphasis on defect reduction and strain management. The core of this work is the MOVPE growth of III-nitrides, in particular GaN and AlGaN, on silicon substrates. In order to understand growth processes more fully, several techniques have been used to characterise the samples grown, mainly atomic force microscopy and high resolution X-ray diffraction. Additionally, photoluminescence has been used, along with transmission electron microscopy. The samples have been grown on 2 inch and 6 inch silicon substrates, with structures ranging from basic templates to Bragg reflectors and LEDs. The majority of the samples grown in this work have been test structures which have been designed to help understand certain aspects of the growth. This has lead to more sophisticated structures, with development of growth schemes which allow thick uncracked AlGaN/GaN Bragg reflectors to be grown for the first time on silicon. These Bragg reflectors have been high quality and have allowed the growth of blue LED structures on top of them. Work on LEDs has been limited, but it has been possible to grow blue emitting LEDs on 2 inch wafers. Work on 6 inch wafers has been mainly limited to analysis of uniformity of growth across the wafers, but is has been possible to grow blue LED strucutures on 6 inch silicon wafers, one of the first groups in the world to have done this.

We investigate the room temperature polaritons dynamics of wide bandgap semiconductor microcavities. Our efforts are focused on microcavities based on wide bandgap semiconductors, such as GaN and ZnO, which possess large exciton binding energies and consequently exhibit enormous Rabi splittings, ensuring the thermal stability of excitons even at room temperature. In this context, the strong coupling regime is indeed observed for the first time in a hybrid bulk GaN microcavity at room temperature. The polaritonic properties are studied with a range of spectroscopic techniques and reveal the occurrence of a polaritonic lasing transition, characterized by an ultralow threshold. The condensate originated emission exhibits a stochastic change of its polarization state. This result is regarded as evidence of spontaneous symmetry breaking. Our observations thus indicate the realization of the first polaritonic Bose-Einstein condensate at room temperature.

Degenerate semiconductors: Assuming that there is large number of degenerate conduction band minima provides a useful route to developing an exact analytical treatment of semiconductors. The new formalism, which was also verified computationally, gives an exact expression for the total electron energy, and provides convenient access to the electron dynamical response. Ultracold atomic gases: These are a new tool that offers investigators an exquisite level of control over a many-body systems. Firstly we show how an atomic gas could be used to unravel a long-standing mystery about textured superconductors, secondary we explore the properties of collective models, and thirdly we investigate a novel form of ferromagnetism. Critical phenomena in correlated quantum systems: As the temperature falls thermal excitations give way to quantum fluctuations. These can couple leading to unexpected phases; firstly we search for a putative textured phase that could pre-empt the first order ferromagnetic transition, and secondly predict a metaelectric phase transition in ferroelectrics.