In radio astronomy wider observing bandwidths are constantly desired for the reasons of improved sensitivity and velocity coverage. As observing frequencies move steadily higher these needs become even more pressing. In order to process wider bandwidths, components that can perform at higher frequencies are required. The chief limiting component in the area of digital spectrometers and correlators is the digitiser. This is the component that samples and quantises the bandwidth of interest for further digital processing, and must function at a sample rate of at least twice the operating bandwidth. In this work a range of high speed digitiser integrated circuits (IC) are designed using an advanced InP HBT semiconductor process and their performance limits analysed. These digitiser ICs are shown to operate at up to 10 giga-samples/s, significantly faster than existing digitisers, and a complete digitiser system incorporating one of these is designed and tested that operates at up to 4 giga-samples/s, giving 2 GHz bandwidth coverage. The digitisers presented include a novel photonic I/O digitiser which contains an integrated photonic interface and is the first digitiser device reported with integrated photonic connectivity. In the complementary area of analogue correlators the limiting component is the device which performs the multiplication operation inherent in the correlation process. A 15 GHz analogue multiplier suitable for such systems is designed and tested and a full noise analysis of multipliers in analogue correlators presented. A further multiplier design in SiGe HBT technology is also presented which offers benefits in the area of low frequency noise. In the effort to process even wider bandwidths, applications of photonics to digitisers and multipliers are investigated. A new architecture for a wide bandwidth photonic multiplier is presented and its noise properties analysed, and the use of photonics to increase the sample rate of digitisers examined.

No description available.

The thesis describes measurements of the velocity-field characteristic of indium phosphide, below threshold by a direct method and above threshold by domain measurements. Comparisons, with good agreement, are made with microwave measurements, below threshold on material from the same slices, and above threshold on material with very similar properties. A historical description of the Gunn effect and domain theory precedes a description of the structure of indium phosphide and a survey of published velocity-field calculations and measurements, showing the difference between 2- and 3-level conduction band models. A value for the T valley deformation potential has previously been deduced from the temperature variation of low-field mobility by adding reciprocal mobilities due to different scattering processes. This method is examined and experimental results of other workers are shown consistent with a lower deformation potential than supposed. Sub-threshold results agree closely with other, published, measurements, using various techniques. Extended to 77 K, the subthreshold method gave results agreeing reasonably with predictions for this temperature, and very well with extrapolations from other calculated and measured data. Published high-field domain measurements made with a pointcontact probe differ greatly from others. Experiments to produce a point-contact probe and associated differentiator with a known performance, and analysis of a simple circuit model indicate that the specimen resistivity controls the probe response, necessitating different probe resistances for different material resistivities. The probe was still found fundamentally difficult to use and is suspected of causing specimen damage due to localised heating under the point. Domain shapes measured agreed with published measurements from capacitive probes, but domain velocity was higher, giving a higher valley velocity (0.76 - 0.99 x 1O ⁷ cm/s). Comparison of the deduced velocity-field curve with published calculations strongly supports a 2-level transfer mechanism.

In radio astronomy wider observing bandwidths are constantly desired for the reasons of improved sensitivity and velocity coverage. As observing frequencies move steadily higher these needs become even more pressing. In order to process wider bandwidths, components that can perform at higher frequencies are required. The chief limiting component in the area of digital spectrometers and correlators is the digitiser. This is the component that samples and quantises the bandwidth of interest for further digital processing, and must function at a sample rate of at least twice the operating bandwidth. In this work a range of high speed digitiser integrated circuits (IC) are designed using an advanced InP HBT semiconductor process and their performance limits analysed. These digitiser ICs are shown to operate at up to 10 giga-samples/s, significantly faster than existing digitisers, and a complete digitiser system incorporating one of these is designed and tested that operates at up to 4 giga-samples/s, giving 2 GHz bandwidth coverage. The digitisers presented include a novel photonic I/O digitiser which contains an integrated photonic interface and is the first digitiser device reported with integrated photonic connectivity. In the complementary area of analogue correlators the limiting component is the device which performs the multiplication operation inherent in the correlation process. A 15 GHz analogue multiplier suitable for such systems is designed and tested and a full noise analysis of multipliers in analogue correlators presented. A further multiplier design in SiGe HBT technology is also presented which offers benefits in the area of low frequency noise. In the effort to process even wider bandwidths, applications of photonics to digitisers and multipliers are investigated. A new architecture for a wide bandwidth photonic multiplier is presented and its noise properties analysed, and the use of photonics to increase the sample rate of digitisers examined.

Thesis (Ph. D.)--University of Hong Kong, 1990.

Thesis (M.S.) in Electrical Engineering--University of Maine, 2006. / Includes vita. Includes bibliographical references (leaves 48-50).

No description available.

Master of Science / Department of Chemistry / Emily McLaurin / Quantum dot (QD) or semiconductor nanocrystal research has propagated extensively over the past few years due to increasing interest in long lasting, renewable, and safe applications such as solar cells and LEDs. Quantum dots are utilized for their size dependent optical properties that are based on the quantum confinement effect. Cadmium-based materials dominated early quantum dot research, which led to honing of syntheses and expansion of our understanding of various mechanisms. Recently, however, current applications, such as solar cells, LEDs, and displays, for everyday consumers require less toxic materials. Indium phosphide (InP) is a possible substitute for cadmium-based materials as it is not intrinsically toxic and emits in the visible region from 450-700 nm. Despite the potential benefits to using indium phosphide, reproducible synthetic methods for obtaining stable QDs with narrow size distribution and high quantum yield still need to be refined. Using single-source precursors such as magic-sized clusters is a good starting place for addressing some of these challenges. InP magic-sized clusters are stable intermediates that are homogenously sized and readily isolable for later growth into InP nanocrystals. Our goal with the InP clusters was to determine their long-term stability and reproducibility as an InP precursor. The InP clusters are can be reproduced, have longer stability when stored as a solid, and we can produce luminescent nanocrystals. Producing highly luminescent InP nanocrystals without the use of HF or shell growth is a challenge. We used the 1-methyl-3-butylimidazolium tetrafluoroborate as our ionic liquid to determine the effect of various ratios of ionic liquid to an InP separate-source precursor on quantum yield. The 1:10 ratio of precursor to ionic liquid provided the highest quantum yield of 21%. These reactions were difficult to reproduce, because there were many factors that affected the synthesis, such as how soon the precursor is used, when the reactions are conducted in the microwave, and how the ionic liquid interacts with the microwave. When using 1-methyl-3-butylimidazolium hexafluorophosphate as our ionic liquid and the magic-sized cluster precursor, there was a spike in pressure in the microwave, and the reaction could not proceed due to the production of a gas. This ionic liquid is still capable of producing nanocrystals with an absorption feature. Understanding the mechanism of how these ionic liquids improve luminescence can lead to safer and more efficient syntheses. Ligand stripping and exchange is also a valuable tool for uncovering information about the surface chemistry. The Lewis acid, BF3, formed adducts with native surface ligands and produces polar, stable nanocrystals. Refining the precursor synthesis so that it's reproducible and producing luminescent nanocrystals were both time consuming processes. This work serves an entry into understanding the process of surface passivation and surface composition of the luminescent InP nanocrystals produced with magic-sized clusters and ionic liquids.

The objective of this project is to investigate hydrodeoxygenation (HDO), a crucial step in the treatment of bio-oil, on transition metal phosphide catalysts. The study focuses on reactions of simple oxygenated compounds present in bio-oil -- ethanol and 2-methyltetrahydrofuran (2-MTHF). The findings from this project provide fundamental knowledge towards the hydrodeoxygenation of more complex bio-oil compounds. Ultimately, the knowledge contributes to the design of optimum catalysts for upgrading bio-oil. A series of transition metal phosphides was prepared and tested; however, the focus was on Ni2P/SiO2. Characterization techniques such as X-ray diffraction (XRD), temperature-programmed reduction and desorption (TPR and TPD), X-ray photoelectron spectroscopy (XPS), and chemisorption were used. In situ Fourier transform infrared (FTIR) spectroscopy was employed to monitor the surface of Ni2P during various experiments such as: CO and pyridine adsorption and transient state of ethanol and 2-MTHF reactions. The use of these techniques allowed for a better understanding of the role of the catalyst during deoxygenation. / Ph. D.

published_or_final_version / Physics / Doctoral / Doctor of Philosophy