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An automated ellipsometer was used to study three topics associated with the anodic oxide films of tantalum and niobium. The electro-optic effect was measured on tantalum and niobium oxides and was found to be quadratic. The change in refractive index upon application of a field occurred in two phases: an instantaneous change followed by a slower change. The effects on Nb₂O₅ were greater than on Ta₂O₅. The effect of ultra violet light on the two oxides was found to cause a change in the refractive index before appreciable photo-induced growth occurred. The results indicated that it was unlikely that the u.v.-induced change in refractive index occurred uniformly throughout the film. The effects of annealing and temperature are discussed in relation to the constant field current transient. The change in the refractive index during the transient was monitored with the ellipsometer. / Applied Science, Faculty of / Electrical and Computer Engineering, Department of / Graduate

The present thesis is concerned mainly with space charge and high field effects on the electrical properties of thin amorphous films. A theory of space charge contribution to the polarization current in thin dielectric films is proposed. The transient current on short-circuiting a thin dielectric film is believed to consist of two components, one due to the dielectric polarization and the other due to trapped space charge. The space charge contribution is investigated using a model for a film containing distributed traps. Computed results seem to be consistent with experimental results on Ta/Ta₂O₅/Au diodes, so that space charge effects are more important at low preapplied fields. The applicability of step response techniques to determine low frequency dielectric losses is discussed and the effect of space charge on the dielectric losses is analysed. The theory of thermoluminescence and thermally stimulated currents is extended to the case of traps with distributed binding energies to investigate the possibility of distinguishing between distributed and discrete trap levels. It seems possible to distinguish experimentally between distributed and discrete traps by using different doses of optical radiation to obtain initially different amounts of trapped charges, and by varying the frequency of optical excitation over a suitable frequency range to allow only certain energy levels to be occupied by excited electrons. High field electronic conduction through very thin films sandwiched between two metal electrodes is analysed. In view of the fast tunneling time of electrons through very thin films, MIM structures can be used for microwave detection. It is shown that the maximum responsivity-bandwidth product of such detectors is obtained when they are biased at a voltage equal to the anode work function (in volts), and that the presence of invariant positive space charge increases the magnitude of this maximum. In considering high field switching in thin films of semiconducting glasses, it is suggested that Joule heating, which could account for the delay times observed experimentally, serves only to initiate an electronic switching mechanism. A model for current-controlled negative resistance due to space charge formation is proposed and its dc characteristics are computed. Carrier injection from the electrodes is taken to occur either by Schottky thermionic emission or a Fowler-Nordheim tunneling mechanism. The injected carriers develop space charge regions near the electrodes by impact ionization. The position dependent generation-recombination rate is discussed. The small ac signal equivalent circuit of the model is given. The formation of current filaments is analysed. Memory devices are discussed in terms of filament formation and phase change mechanisms due to excessive heating. Filamentary breakdown has been observed in anodic films grown on Ta, Al, Nb and Ti. A detailed experimental study of film growth and the effects of growth conditions, film thickness, counterelectrodes and temperature on breakdown strength has been carried out. A possible mode of breakdown, in which breakdown can result from thermal effects following a non-destructive electron avalanche, is proposed and its limitations are pointed out. It is concluded that breakdown in thin anodic films would occur due to disruption of the chemical bonds as the applied field approaches the formation field. The product of the molecular dissociation and the presence of energetic electrons could start an accumulative process which might end with the formation of a highly conducting channel. The injected electrons, field distortion and thermal runaway could assist in the channel development. Once the channel is developed, the sample's stored energy starts to dissipate through the channel. The voltage collapse has been found experimentally to occur in a time of less than 200 nanoseconds. / Applied Science, Faculty of / Electrical and Computer Engineering, Department of / Graduate

An organic Schottky barrier cell, consisting of a thin layer of the organic semiconductor, GaPc-Cl, in contact with gold on one side and an electrolyte containing a redox couple on the other, was developed as a solar energy conversion device. Schottky barriers were formed at both interfaces. Film morphology, as determined by the sublimation rate, was the major determinant of the photoelectrochemical behavior. An optimum film consisted of a single layer of crystallites about 1.0 micron in thickness, tightly packed together to give a non-porous film. Thinner films did not develop the full theoretical photopotential, and pores acted as recombination sites, decreasing the efficiency. Both negative and positive photopotentials could be developed, depending on the redox couple used. The photopotential, was found to be proportional to the differences between the Fermi level of the Au and the formal potential of the redox couple. Hydrogen evolution was possible with up to 0.1% solar efficiency on a platinized version of the optimum electrode. Results from photocurrent action spectra and pulsed laser photocoulostatics, showed the potential drop across the film was not linear, but formed a potential well about 0.1 eV deep, which captured charge carriers and decreased the efficiency. From scanning electron microscope studies, phthalocyanines, such as AlPc-Cl, GaPc-Cl, and InPc-Cl, with bulky anions were found to form block-like crystal structures favorable for use in Schottky barrier cells. Phthalocyanines with transition metals in the +2 oxidation state, such as FePc and MgPc, were found to form long needles, which were not favorable for use in Schottky barrier cells.

by Chan Hing Nam = 壓力對La₀.₆₇Ca₀.₃₃MnO₃薄膜的導電能的效應 / 陳慶楠. / Thesis (M.Phil.)--Chinese University of Hong Kong, 2001. / Includes bibliographical references. / Text in English; abstracts in English and Chinese. / by Chan Hing Nam = Ya li dui La₀.₆₇Ca₀.₃₃MnO₃ bo mo de dao dian neng de xiao ying / Chen Qingnan. / Acknowledgements --- p.i / Abstract --- p.ii / 論文摘要 --- p.iii / Table of contents --- p.iv / Chapter Chapter 1 --- Introduction / Chapter 1.1 --- Introduction to colossal magnetoresistance --- p.1-1 / Chapter 1.2 --- Effects of chemical pressure and strain on LCMO thin films --- p.1-7 / Chapter 1.3 --- Review of pressure effects on bulk LCMO --- p.1-11 / Chapter Chapter 2 --- Instrumentation and Characterization / Chapter 2.1 --- Preparation of LCMO thin films --- p.2-1 / Chapter 2.2 --- X-ray diffraction (XRD) --- p.2-3 / Chapter 2.3 --- Self-clamping pressure cell --- p.2-5 / Chapter 2.3.1 --- Electrical feedthroughs --- p.2-5 / Chapter 2.3.2 --- Teflon cell --- p.2-8 / Chapter 2.3.3 --- Pressure variations in the pressure-transmitting medium --- p.2-9 / Chapter Chapter 3 --- Pressure effect on LCMO thin films grown on different substrates / Chapter 3.1 --- Annealing effect --- p.3-1 / Chapter 3.2 --- Thickness effect --- p.3-4 / Chapter 3.3 --- Lattice effect and pressure effect --- p.3-5 / Chapter 3.4 --- Crystallinity effect --- p.3-13 / Chapter Chapter 4 --- Activation energy of small polaron in LCMO thin films / Chapter 4.1 --- Motivation --- p.4-1 / Chapter 4.2 --- Basic theory --- p.4-2 / Chapter 4.3 --- Activation energy --- p.4-4 / Chapter Chapter 5 --- Conclusion --- p.5-1

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