During the investigation of current instabilities in semi-insulating single CdS crystals, caused by applying a high voltage pulse and using various illumination distributions, some completely new phenomena have been observed. The major phenomena are discussed and explanations have been offered for the observed effects with various types of illumination. Phenomena observed have been compared with the results of other workers. One type of instability was initiated by applying high voltage pulses to CdS specimens in complete darkness. It is observed from the field measurements that the period of oscillation is equal to the transit time of the acoustic wave from the position of high field region to the anode end of the specimen and not related to the wave transit time between electrodes. High frequency non-sinusodial electrical oscillations were observed in uniformly illuminated specimens. These were attributed to the effect of longitudinal potential gradients which were found when the electric field was measured under oscillatory conditions. Continuous acousto-electric current oscillations were observed in a specimen by shading a length near to the anode end. Extremely high fields were detected in the high resistivity region so formed when amplification occurred. Obscuring a narrow strip perpendicular to the length of the sample and applying a high voltage pulse resulted in current oscillations and two step saturation of the current. A number of oscillatory modes were often found in a specimen, depending on the position of the shielded strip on the specimen. Extremely high electricfields were observed near the position of the obscured strip of the specimen. In two-step current saturation phenomena the changes in current pulses were recorded. Low frequency current oscillations were observed after the current had fallen to a final steady value. The conductivity of the material at different times after the application of a high voltage pulse was recorded and it remained constant below threshold voltages and thereafter differed from the ohmic low voltage value. The electric field reached maximum values 0.5mm from the cathode and 1mm from the anode ends of the specimen. High frequency non-sinusodial current oscillations were recorded by localised illumination of the specimens with an He-Ne laser beam. With low voltage pulses, the I-V characteristics of these specimens fell below the linear ohmic relation, but at high voltages the current became proportional to the square of the applied voltage. Qualitative explanations of these oscillations have been given and it has been shown that they are consistent with existing theory.

The design parameters and the construction of a pulsed solid state laser are described. Q-switching techniques are reviewed and a novel form of the exploding film Q-switch is decribed. This Q-switch was capable of generating high power giant pulses which were comparable with those obtained using conventional dye-stuff switches. Techniques for generating ultrashort optical pulses are reviewed and in particular the author's procedures for generating mode locked pulses in ruby are described. The successful use of a random stack mirror circumvented the damage problem which besets thin film dielectric mirrors in the presence of mode locked pulses. The construction and use of random stack mirrors with lasers operating in various modes were also investigated. Spatially modulated structures defined to 0.2 were thermally etched on thin metallic films and such structures were found to he duplicated on the substrate. Assessments of such structures for applications to the manufacture of optical diffraction gratings and grid polarizers have also been made. The hologram of the laser cavity, recorded in gold film, was found to he capable of instant 'read out'.Spatially modulated thin films have been deposited onglass substrates by a process in which the condensation of the film material was inhibited by the intense electromagnetic fields within the cavity of a giant pulse ruby laser. Such an inhibitive process constitutes a new phenomenon, discovered by the author and roported very recently in Nature.

Positron annihilation has been studied in a number of solids, and at some condensed monolayer surfaces. The bulk specimens, which included cadmium, tin, selenium, and graphite, yielded information on lattice defects and phase transitions. Two dimensional layers of condensed gases, including argon, nitrogen, oxygen and helium result in positronium formation suggesting the existence of positron surface traps. The Doppler broadening method has been applied to these studies. The 511-Kev Gamma-rays resulting from the annihilation of positrons with electrons was detected by a Germanium detector with high resolution. The cadmium and tin specimens were plastically deformed at 77K and measurements with increasing temperature provided information on phase transitions, annealing processes and on the nature of the defects concerned. The application of the positron trapping model provides values of the vacancy formation energy (enthalpy) and of the concentration of monovacancies and divacancies. The energy spectra were analysed, with Gaussian and Parabolic components convoluted with instrumental resolution function, to indicate the proportion of annihilation of positrons with core and conduction electrons. An important result of the work on tin was the observation of the phase transition of white-tin to gray-tin at 240K. Positron trapping at graphite surfaces, and the formation of positronium has been observed by introducing a parameter R, related to the positronium fraction in this work. The growth of monolayers of gases condensed on graphite has been observed in the changing of the total area of the annihilation line shape spectrum, and also in the changing positronium fraction parameter. Estimated values of adsorption energies are discussed.

Diamonds from mines in South Africa, and the Argyle Mine in Western Australia, have been compared using synchrotron transmission Laue photography (Laue topography), and the Argyle stones were found to be more variable in quality. By measuring the asterism of the Laue spots, quantitative estimates of lattice distortion have been made. The various types of crystal distortion, and their effects on Laue patterns, have been considered, and it has been determined that distortion in the Argyle diamonds primarily takes the form of mosaic structure. A modification of the technique of x-ray spike topography, where parts of the specimen were obscured by a tantalum screen, has been used to estimate platelet dimensions at different positions in type 1a diamonds. Direct measurements of the platelets in some of the samples have been made from transmission electron micrographs, and show that the method is reasonably reliable for platelets below about 400 A in diameter. In the range 1361.3 to 1371.3 wavenumbers, a correlation has been confirmed between 'platelet peak' position in the infrared spectrum and platelet size, in the sense of smaller wavenumbers being associated with larger platelets. Topographs of gallium arsenide wafers, used for device fabrication, have been taken. A dislocation lineage has been studied in a (100) wafer with field effect transistor arrays fabricated onto it. It has been found to cause a drop of approximately 40 mV in the pinch-off voltage of transistors it touches. The feature has been shown to have an associated Burgers vector of the 211-type, and to be accompanied by a lattice tilt in the wafer of nearly 30". Previous work in each area of investigation is reviewed, and brief introductions on diamond and gallium arsenide given, together with introductions to the techniques of topography, using both characteristic x-rays and synchrotron radiation.

Positron annihilation has now become a much-used technique for the study of defects in solids, particularly in metals and alloys. The work in this thesis describes measurements made on a Doppler-broadening spectroscopy system designed to study the annihilation radiation emerging from metal samples as a consequence of electron-positron interaction. Defects can be produced in metals by thermal, mechanical or irradiation treatments. For the metals Cu, Al, Ni, Fe and Ti, an assessment has been made of the nature and number of defect types encountered in the mechanically deformed state by analysis of the annihilation lineshapes. Defects in molybdenum produced by neutron-irradiation have also been studied. The defect environment is seen to be dependent on the irradiation temperature. Running integrals of the difference between the annihilation lineshapes from defective and reference (annealed) samples have provided defect-specific parameters characterising the shape of the centre-of-mass momentum distributions resulting from annihilations at defect traps. In order to assess the defect species in the mechanically deformed state, these parameters have been used to monitor positron annihilation behaviour during isochronal annealing. Under favourable circumstances, individual defect-types have been characterised. The nature of the electron environment and of the positron behaviour at a specific defect site has been assessed by fitting a model to the observed lineshapes to account for the electron and positron momentum distributions. Such analysis has yielded estimates of the positron zero-point energy and local Fermi electron energies associated with individual defect-types. In the case of copper and aluminium, the calculated zero-point energy of ~ 6 eV associated with positron trapping at vacancies is found to be identical to that derived for dislocation trapping. It is concluded that positrons undergo point-like trapping at dislocations, either by trapping at jogs or at other irregularities along the dislocation line. Trapping model fits to data from mechanically deformed samples reveal the nature of the stress-strain, work-hardening relationship associated with the mechanical deformation process.

Detailed models of crystal growth have, for the most part, been limited to simple sublimation systems. These systems have little relevance to the complex chemical vapour transport systems used to produce modern semiconductors such as the III-V compounds, and the development of these systems has frequently been by trial and error. The work described in this thesis has attempted to bridge the gap between the crystal growth theories and technologically important growth systems. Crystal growth is a sequential process in which reactants are first transported to the surface of the growing crystal and then undergo a series of surface reactions. A major difficulty in investigating heterogeneous reaction kinetics is separating the two processes. In chapter 2 a formalism is developed which enables transport and surface kinetics to be uncoupled. The practical application of this is demonstrated in chapters 3 and 4 in which the sublimation of red phosphorus and the GaAs/HBr reaction are investigated using the modified entrainment method (MEM). The choice of experimental systems, and the depth to which they could be studied, was dictated largely by the interests of British Telecom. Consequently, the experimental work could only demonstrate the potential of MEM as a kinetic probe. More detailed examination of the dependence of the GaAs/HCl and GaAs/HBr reactions on reactant pressures has still to be carried out. The later chapters are concerned with the growth of the mixed semiconductor systems GalnAs and GalnAsP. These materials are required for the new generation of optical communications systems currently under development. Not only were the surface kinetics for these systems unknown, but even the transport equations for these systems had not been developed. Chapters 5-7 describe the development of a vapour phase system for the growth of these ternary and quaternary compounds. The basic principles are discussed for InP growth in chapter 5. Chapter 6 then develops thermodynamic and transport models for the mixed compounds and chapter 7 discusses the results obtained with practical growth system. Although many features of growth can be explained by the models developed in chapter 6, it is clear that a complete understanding of growth requires close study of the various processes occurring on the surface.

Over the temperature range 770 to 1310 K, however, two bromides compete for prominence, dependent upon temperature. In both instances, it is shown that vapour transport becomes rate limited at low temperature. Further to the chemical vapour transport of indium phosphide, the dissociative sublimation of the compound has also been investigated. Raman spectroscopy has been used to identify high temperature molecular species involved in vapour transport of III-V semiconductor materials. Supplementary work has been performed on the thermochemistry of indium monobromide. The heat of formation of indium bromide crystals has been determined using a solution calormetric technique. Differential scanning calorimetry was used to measure the heat capacity and heat of fusion, of the salt. An entrainment study of the evaporation of liquid indium monobromide was undertaken to yield a value for its heat of vaporisation. Using a statistical thermodynamic approach, the heat capacity of the vapour was calculated. Collating the information, a value for the heat of formation of indium monobromide gas at 1000 K has been calculated for use in other thermodynamic calculations.

Wannier functions provide us with a means for the development of rigorous theorems which are analogous to the crude theorems we might derive with a tight binding approximation. However, a good and accurate Wannier function would be extremely difficult to calculate and its structure would be quite complicated. Excluding calculations obtained using the O.A.O. approximation few works exist on the mathematical properties of Wannier functions and only very few have been explicitly determined. The present study attempts to provide a partial solution for this situation and seeks to extend the use and knowledge of these functions. First, a theoretical investigation is made of the mathematical properties of Wannier functions and then some calculations are carried out to obtain some of these functions for various forms of potential fields. The original results obtained concern; I) Free electron Wannier functions for many of the familiar crystal types. II) A method for obtaining Wannier functions from relatively simple functions (unperturbed Wannier functions). The method has been appli ed to several particular cases (potential field with inversional symmetry etc.) and has been modified for other applications. III) Nearly free electron Wannier functions calculated both using analytical methods and the method outlined in section II. IV) The extension of Kohn's one dimensional treatment to three dimensions using a new approach.

Positron annihilation in copper, silver and indium has been studied by means of the Doppler broadening method. High resolution germanium detectors have been used to observe the changes in the annihilation line profiles due to the trapping of positrons by metallic defects. Positron trapping by thermally generated vacancies have been investigated. The smaller temperature effects which occur in addition to the dominant vacancy trapping effect have been discussed and taken into account in deducing mono-vacancy formation energies. The plausible hypothesis of metastable seIf-trapping of positrons has also been discussed. The best values of the mono-vacancy formation energies are estimated to be 1.26 eV for copper and 0.95 eV for silver. For indium a precise value could not be deduced from the data. From the analysis of the line shapes, the probabilities of positron annihilation with core and conduction electrons at different temperatures have been calculated. The results of the measurements on plastically deformed copper indicate that the specific trapping rate of positrons by dislocations is constant in the range 77-293 K. On extending the range of measurements down to 4.2 K, a significant decline in the specific trapping rate has been observed. Alsco included is a chapter on the annihilation of positrons in liquid helium-3 and helium-4. Analysis of the line shapes as well as the deconvolution of the spectra indicate no significant difference in the amount of Positronium formation in liquid helium-3 and helium-4.

A selection of free-standing fine-wire grids with different wire spacings have been wound for use as spectroscopic components at millimetre and submillimetre wavelengths. The performance of the grids has been investigated extensively and found to be in good overall agreement, both qualitatively and quantitatively, with calculations made using a least-squares method developed by Beunen. Where discrepancies occur they are attributed to the effect of uneven spacing of the wires. A microcomputing system based on the Z80 8-bit microprocessor has been built to provide data acquisition and Fourier transformation for a Fourier spectrometer. Measurements have been made of the far infrared optical constants of KC1 and KBr crystals by dispersive Fourier transform spectroscopy at temperatures in the range 7-500K using instruments equipped with both mylar and wire grid beam dividers. The results have been used to calculate the dielectric functions and the an harmonic self energy functions of the q = 0 transverse optic modes in these crystals. The frequencies of the transversive optic (TO) and longitudinal optic (LO) modes determined from the dielectric functions are in good agreement with the accepted values, and in the case of KBr, the frequency-dependence of the an harmonic self-energy functions determined at room temperature is in good agreement with calculations made by Bruce.