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Zn(_x)Co(_2-x)GeO(_4) materials were prepared and the variation in structure with composition was investigated using XED, SEM and EDX analysis. Limited series of solid solution were identified at both ends of the compositional range. D C electrical measurements were carried out to characterize the variation in the resistivity of the materials with humidity. Resistivities of the order of 10(^8) Ω m were observed in dry conditions, decreasing by 4 to 5 orders of magnitude with increasing humidity. Resistivity was not found to vary greatly with composition. Resistivity was temperature dependent, increasing by 1 to 2 orders of magnitude for a 70 C decrease in temperature. A C impedance measurements were performed to gain an understanding of the mechanism of the humidity dependent conductivity. At low frequencies impedance was found to be independent of frequency and humidity dependent. At high frequencies impedance was found to be inversely proportional to frequency and independent of humidity. The break point frequency was also humidity dependent and an increase in the impedance indicated inductive-type behaviour. Complex plane representation of the impedance gave a distorted semicircle at high frequencies and a low frequency tail. At high humidities the tail appears as a straight line, inclined at approximately 45 . At medium levels of humidity a distinctive loop is apparent at the intersection between the semicircle and the tail, corresponding to the inductive behaviour indicated at the break point frequency. The impedance response was modelled by an equivalent circuit consisting of various ideal and constant phase (dispersive) elements. The proposed mechanism of humidity-dependent conductivity is due to chemisorption and physisorption of water vapour from the atmosphere at the surface of the material, It is suggested that conduction occurs by hopping of protons between cheraisorbed hydroxyl groups at low humidities, by diffusion of H(_3)O(^+) ions between the hydroxyl groups at intermediate humidities and by hopping of protons between physisorbed H(_3)O(^+) ions (Grotthus Chain reaction) at high humidities.

This thesis addresses the issue of a potential future microelectronics technology, namely the possibility of utilising the optical properties of nanocrystalline silicon for optoelectronic circuits. The subject is subdivided into three chapters. Chapter 1 is an introduction. It formulates the oncoming problem for microelectronic development, explains the basics of Integrated Optoelectronics, introduces porous silicon as a new light-emitting material and gives a brief review of other competing light-emitting material systems currently under investigation. Examples of existing porous silicon devices are given. Chapter 2 reviews the basic physics relevant to the subject of this thesis and informs on the present situation in this field of research, including both experimental and theoretical knowledge gained up-to-date. The chapter provides the necessary background for correct interpretation of the results reported in Chapter 3 and for a realistic decision on the direction for future work. Chapter 3 describes my own experimental and computational results within the framework of the subject, obtained at De Montfort University. These include: onestep preparation of laterally structured porous silicon with photoluminescence and microscopy characterisation, Raman spectroscopy of porous silicon, a polarisation study of the photoluminescence from porous silicon, computer simulations of the conductivity of two-component media and of laser focused atomic deposition for nanostructure fabrication. Thus, this thesis makes a dual contribution to the chosen field: it summarises the present knowledge on the possibility of utilising optical properties of nanocrystalline silicon in silicon-based electronics, and it reports new results within the framework of the subject. The main conclusion is that due to its promising optoelectronic properties nanocrystalline silicon remains a prospective competitor for the cheapest and fastest microelectronics of the next century.

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