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  • About
  • The Global ETD Search service is a free service for researchers to find electronic theses and dissertations. This service is provided by the Networked Digital Library of Theses and Dissertations.
    Our metadata is collected from universities around the world. If you manage a university/consortium/country archive and want to be added, details can be found on the NDLTD website.
21

Magnetic characterisation of thin film NiFe/FeMn multilayers

O'Reilly, S. January 2014 (has links)
Exchange biased thin films form the basis of many high speed electronic devices such as hard disc drives, field sensors and microwave communication devices. It is important to have a detailed understanding of the electromagnetic properties of such thin films in order to determine their suitability for specific applications. As the operating speeds of electronic devices increase the necessary characterisation range expands into' the microwave region i.e. the gigahertz frequency range. One of the most important properties in investigating thin films for potential applications is permeability. The aim of this thesis was to characterise the magnetic properties and permeabilities of exchange biased thin films. Multilayers of the structure [Ni80Fe20 (x nm)/ Fe50Mn50 (y nm)]z were deposited using a sequential sputtering system where x is the thickness of the ferromagnetic layer, y is the thickness of the antiferromagnetic layer and z is the number of times the bilayer is repeated. The hysteresis loops of the multilayers were measured using a vibrating sample magnetometer to investigate their multiple reversal stages. A two port coplanar waveguide permeameter was built to measure the permeability spectrum of the multilayers at various applied magnetic fields. The multilayer hysteresis loops exhibited a stepped structure corresponding to multiple reversals. Each reversal stage was assigned to a particular NiFe layer based on the magnetisation ratios, exchange bias and NiFe layer thickness. NiFe layers of the same thickness with the same relative position within a stack appeared to have the same value of exchange bias. The resonant permeabilities of the multilayers shifted towards higher frequencies as the applied field was increased, in agreement with ferromagnetic resonance theory. This indicated that the resonances originated from the multilayers and were not due to systematic errors or artefacts. Trends within the sample range were observed but further work is needed to confirm these observations.
22

Reduction of variable speed drive IGBT switching loss, utilising the IGBT gate drive, without increasing radio frequency radiated emissions

Gibson, Richard Samuel January 2013 (has links)
Radio frequency radiated emission from a variable speed drive must be limited below defined limits to avoid interference with electronic equipment. It is possible to comply with the international standards however, the current state of the art methods such as large gate drive resistance and output filters significantly reduce the efficiency or increase the cost of the products. It is known that the main source of emissions emanate from the switching transients associated with the output power semiconductors, however the exact mechanisms and specific sources are unknown. This thesis examines the interaction of power devices during the switching transient identifying features which can be controlled by a sophisticated gate drive design. Analysis of the frequency content of the signals is presented together with methods to minimise power losses while maintaining compliance with radiated emission standards. A research program has been undertaken to identify the sources responsible for radiated emissions and predict a figure of merit as an indication of compliance. Measuring radio frequency content on high voltage and current signals is difficult and several techniques to accurately achieve this are presented. Simple passive gate drive solutions which can be easily implemented are examined along with a discussion on more complicated optimised solutions.
23

Semiconducting polymer nanospheres : organic alternatives to inorganic quantum dots?

Hashim, Zeina January 2013 (has links)
Semiconducting polymer nanospheres are organic conjugated polymer nanoparticles which are synthesized from benign materials and exhibit excellent fluorescence properties. The nanoparticles are generally larger than inorganic quantum dots with a relatively broad size distribution. Quantum dots, on the other hand, which have extensively been developed and synthesized with precise and narrow distributions of a few nanometers in dimensions, are now being widely investigated as bio-imaging agents, despite the rising concerns about their toxic compositions. Therefore, advances in the synthesis of the organic nanoparticles and investigations into their suitability as alternatives to quantum dots need to be explored. The ‘size problem’ of semiconducting polymer nanospheres – polymer particles are significantly larger than quantum dots – was first tackled in this work. With modifications to the miniemulsion-evaporation synthesis method, narrowly distributed quantum dot-sized nanoparticles with diameters as small as 2 nm were synthesized. These organic nanoparticles which were capped/entwined with poly(ethylene) glycol (PEG), a Food and Drug Administration (FDA) approved surfactant, were found to conserve most of the optical properties of their constituent polymers, and are therefore expected to be useful in bio-imaging applications similar to their larger counterparts. A second nanoparticle system with a dual-modality was then prepared; semiconducting polymer nanospheres capped/entwined with three amphiphilic lipids one of which was gadolinium – diethylene triamine pentacetate, a Magnetic Resonance Imaging (MRI) active ligand. These bimodal nanoparticles also maintained their optical properties, were readily taken up by two cell lines, were distinguishable from the auto-fluorescence of animal tissue, and were found to be MRI-active as revealed by their MRI relaxivity measurements. Finally, the optimized organic nanoparticles and similarly coated quantum dots were investigated for their potential to interact with human blood components, a physiological system which may be very relevant for semiconducting polymer nanospheres used as medical diagnostic agents. The preliminary ex-vivo studies performed revealed that similarly coated organic nanoparticles and quantum dots did not induce platelet aggregation or alter aggregation behaviour in response to a physiological agonist. Further, no evidence of platelet activation, neutrophil activation or increases in platelet-monocyte adhesion was observed. This implied that introduction of the nanoparticles to the blood stream at the concentrations tested may not elicit acute pro-inflammatory effects or alter normal coagulation pathways, although further rigorous evaluation in this area is still required. Fluorescence imaging showed that the organic nanoparticles were taken up by different blood cells and also showed some evidence of adhesion to their surfaces, a property which might find an application in the future. Ultimately, more short-term and long-term safety studies (in-vitro, ex-vitro, and in-vivo) must be conducted before deriving any further conclusions.
24

Thin films of CdS and the CdS-Cu₂S heterojunction

Wilson, J. I. B. January 1971 (has links)
The thin film CdS-Cu(_2)S solar cell is potentially a cheap and efficient device for converting sunlight into electrical energy, but there are various difficulties which present its ready fabrication. These centre on the CdS base layer and concern the uncertainties in the evaporation process. Moreover, the properties of the heterojunction itself are not well understood despite the profusion of theoretical models. This thesis describes an investigation into the physical and electrical properties of vacuum evaporated CdS layers on a variety of substrates, as a function of the preparative conditions, i.e. evaporation rate, substrate temperature, film thickness, and source purity. The Hall mobility, resistivity and photosensitivity have been measured in both epitaxial and polycrystalline layers. X-ray studies revealed that a fibre-axis orientation existed in the thicker polycrystalline films, and transmission electron microscopy enabled several defects in the epitaxial layers to be identified. It was essential to use a 'hot-wall' technique to deposit reproducible films. Since source purity affected the growth and subsequent electrical properties of the films, it was necessary to use re-crystallised CdS as the evaporant, and it was preferable to employ electron beam evaporation. Dopants such as indium were readily incorporated into the films. Much of the CdS film behaviour can be explained in terms of the evaporation and condensation kinetics. An Appendix describes similar work on the properties of CdSe layers. Investigations into the electrical properties of junctions prepared by chemi-plating Cu(_2)S on boule CdS suggested that the best cells were made with copper-doped CdS. Open circuit voltage was highest if the base resistivity was a few hundred ohm cm. The existence of a photoconductive i-CdS region in heat-treated cells was demonstrated by the presence of long time constants and quenching effects. The importance of the CdS surface treatment was also shown.
25

Low temperature (<150°C) hydrogenated amorphous silicongrown by PECVD with source gas heating

Cross, Richard Barrie Michael January 2005 (has links)
Low temperature « 150°C) hydrogenated amorphous silicon grown by PECVD with source gas heating Richard Barrie Michael Cross Hydrogenated amorphous silicon (a-Si:H) is a semiconductor that is widely used in a variety of applications. A particularly important development has been the incorporation of this material into thin film transistor (TFT) arrays for the active matrix addressing of liquid crystal displays. Plasma Enhanced Chemical Vapour Deposition (PECVD) is one of the most successful techniques currently in use for the deposition of device quality a-Si:H. However, there is an increasing desire to improve process compatibility with low cost, plastic substrates. This entails trying to reduce the deposition temperature from approximately 250 - 300°C to below 150 °C, whilst maintaining material quality. This thesis describes the design of a novel, low temperature PECVD system incorporating the facility to pre-heat the deposition source gases. The physical and electronic properties of a-Si:H deposited at <150°C are investigated, and the performance of TFT structures incorporating optimised material as the active layer is described. It is shown that the physical properties of a-Si:H produced at a substrate temperature of 125°C with the source gas line heated to 400 °C are commensurate with films deposited at 250-300 °C. The hydrogen content of the optimised film was found to be 10.5 %, with a Tauc bandgap of 1.66 eV. Pre-heating of the source gases also leads to an increase in the proportion ofhydrogen bonded in the monohydride configuration. It is suggested that the diffusion of the film-forming gaseous species is enhanced by this technique, resulting in a reduction in the degree of disorder within the film and hydrogen elimination. Consequently, the concentration of hydrogen and the Tauc bandgap also decrease, leading to an increase in photoconductivity of one order of magnitude. TFTs exhibit a switching ratio of 105 , which is approximately an order of magnitude smaller than high temperature a-Si:H TFTs, but a comparable OFF current of approximately 10.12A. However, the field effect mobility of these devices is very poor (10.3 cm2V·1s·I). This is thought to be due to a high interface state density at the boundary between the low temperature, gas-heated a-Si:H layer and the high temperature silicon nitride gate insulator.
26

Failure Modes of Organic Devices

Sessford, Joanna Ruth January 2008 (has links)
Research into Organic Electronics has increased dramatically over the past 20 years. Organic Electronics cannot compete with single crystal silicon for speed and circuit complexity, but it can be regarded as a competitor because of its potential to produce very large area circuits at low process temperatures and low cost. Potential applications for conjugated polymer devices include photovoltaic cells (low cost solar panel), optical amplifiers, memory, displays (in conjunction with e-paper and large area LCD's) and radio frequency (RFID). Many of these devices will use Thin Film Transistors (TFTs). TFT characteristics are therefore discussed with 'an emphasis on the hysteresis observed; the results observed indicate that one ofthe causes ofhysteresis is ion movement. A number of different current-voltage equations have been derived for the drain current channel. It is important to develop these equations to determine the effect of treating the real variation in the concentration of carriers, normal to the interface. In addition, how this large variation in the density of carriers gives rise to an enhancement of carrier mobility at the interface is approached, has been examined. It has been shown that for devices fabricated using PTAA; the most appropriate equation is that which assumes the exponential distribution of states depends on electron concentration. MOS capacitors are important test structures; they are useful because of the light they shed on electrical instability in organic devices, MOS capacitors characteristics have therefore been discussed before the hysteresis they display has been examined. The following mechanisms of hysteresis in MOS capacitors have been examined: carrier hopping, carrier percolation, gate leakage current, ion movement in the semiconductor and instability in the gate dielectric. The main causes of instability appear to be oxygen ions in the semiconductor and in the gate dielectric. The hysteresis observed in Schottky diodes has also been examined; contributing factors appear to include ion movement and diffusion. Values for Meyer Neldels energy for ; . Schottky diodes fabricated using Lisicon (undoped) have been calculated to be 5.0.10-21 ; this value increases when dopant is added; this is expected as doping introduces more states and at high levels of doping leads to Fermi level pinning. As Organic Electronics is growing rapidly into an industry, it is vital to have aroadmap; to allow equipment manufacturers and semiconductor technologists to plan targets for the future, this piece of work provides a roadmap for Organic Electronics. One of the major potential uses of Organic Electronics is believed to be in the area of auto hI/rf tags, for example, the replacement of the barcode, the roadmap therefore concentrates on this area; it has been predicted that by the year 2030 full barcode replacement will be available at an economic cost.
27

Quantum transport in carbon nanotubes and single molecules

Grace, Iain January 2004 (has links)
No description available.
28

New technologies for lead-free flip chip assembly

Gao, Shubo January 2005 (has links)
No description available.
29

Pulse compression using mini-stop bands in photonic crystal waveguides

Cao, Tun January 2007 (has links)
No description available.
30

Energy-efficient SOC design technology and methodology

Flynn, David Walter January 2007 (has links)
No description available.

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