No description available.

High brightness light emitting diodes based on the InAs/GaSb superlattice material system have been developed for use in mid-wave and long-wave infrared optoelectronic systems. By employing a multiple active region device configuration, high optical output has been demonstrated from devices in the 3-5μm and 7-12μm spectral bands. Mid-wave infrared optical output in excess of 0.95mW/sr has been observed from 120×120μm2 devices with peak emission at 3.8μm, and nearly 160μW/sr has been measured from devices of the same size operating at 8μm. Larger devices (1×1mm^2) with output as high as 8.5mW/sr and 1.6mW/sr have been demonstrated with mid-wave and long-wave devices, respectively, under quasi-DC bias conditions. The high switching speed inherent to small area light emitting diodes as well as potentially high optical output make these devices appealing candidates to improve upon the current state-of-the-art in infrared projection technology. Simulation of thermal scenes with wide dynamic range and high frame rates is desirable for calibration of infrared detection systems. Suitable projectors eliminate the need for observation of a live scene for detector calibration, thereby reducing costs and increasing safety. Current technology supports apparent temperature generation of up to approximately 800 Kelvin with frame rates of hundreds of frames per second; strong desire exists to break these barriers. Meeting the requirements of the aforementioned application requires development of the InAs/GaSb superlattice material system on multiple levels. Suppressing parasitic recombination channels via band structure engineering, improving carrier transport between active regions and confinement within active regions, reduction of defect-assisted recombination by optimizing device growth, and improving device fabrication and packaging are all routes requiring exploration. This work focuses on the latter two components of the optimization process, with emphasis on molecular beam epitaxial growth of high quality devices. Particular attention was paid to tailoring devices for thermal imaging applications and the design tradeoffs and limitations which impact that technology. Device performance and optimization success were gauged by electronic, optical, morphological, and structural characterization.

Since decades, nucleation, or the ability of certain organic or inorganic substances to trigger the crystal growth, has been empirically used in the plastics industry. Talc, for instance, is a well-known nucleating agent of poly(ethylene terephthalate) (PET) and other polymers, that allows one to enhance the crystallisation rate of the polymer material and to control its spherulites size. The exact mechanism involved in this nucleation had however remained unknown at the onset of this thesis. Through electron diffraction, performed on thin PET films nucleated by macroscopic talc particles as model samples, this work demonstrates an epitaxial relationship between polymer and substrate and thus confirms the seemingly ubiquitous role of epitaxy in the nucleation of polymers. However, in order to compare the talc-nucleated morphology of PET with the virgin one, new methods of sample preparation for transmission electron microscopy (TEM) have also been developed. Coupled with theoretically justified image analysis techniques, they allow the direct observation of PET crystalline lamellae, both in the bulk and in thin films. Analyses of the semicrystalline structure in the reciprocal and direct spaces were performed from small-angle X-ray scattering (SAXS) data and from observations by TEM on melt-crystallised samples. These independent results were shown to be in good agreement and bring strong evidence in favour of a semicrystalline space-filling model, where the average crystalline thickness is slightly smaller than the average width of the amorphous regions. Discrepancies between characteristic distances derived by several methods from the same experimental results were attributed to the broad distribution of thicknesses, in contrast with the ideal linear stack model commonly used to analyse the data.

This thesis studies p-i-n GaAs solar cells with self-assembled InAs quantum dots (QDs) inserted. The values of this work lie in three aspects. First, by comparing the cell performance with QDs in the i-region and the n-region, the photocurrent (PC) production from QDs by thermal activation and/or intermediate band (IB) absorption is proved to be much lower in efficiency than tunneling. Second, the efficiency of PC production from QDs, characterized by PC spectrum, is helpful to design QD-based photodetectors. Third, closely spaced InAs QD layers allow a strong inter-layer tunneling, leading to an effective PC production from QD deep states, potential for solar cell application. Fourth, from the temperature-dependent PC spectra the minority photohole thermal escape is found to be dominant on PC production from QDs in the n-region. The thermal activation energy reflects the potential variations formed by electron filling in QDs. Apart from InAs QDs, this thesis also explores the blinking correlation between two colloidal CdSe QDs. For QD distance of 1 µm or less, there is a bunched correlation at delay τ = 0, meaning that the two QDs blink synchronously. Such correlation disappears gradually as QD distance increases. The correlation is possibly caused by the stimulated emission between the two nearby QDs. / QC 20120507

Thermal annealing 4H-SiC(0001) substrates to produce epitaxial graphene on Si-terminated SiC was performed using five different procedures, i.e. direct and indirect current heating at different based pressures and a temperature of about 1300 . The aim is to study the effects of graphene growth under different conditions and also to produce large homogeneous graphene. To investigate the prepared samples, two surface analytical techniques, i.e. low energy electron microscopy (LEEM) and photoelectron spectroscopy (PES) have been used. LEEM was first used to observe the surface morphologies of the prepared samples. In combination with LEEM instrument, low energy electron diffraction (LEED) was used to verify the existence of graphene on SiC substrate. The number of graphene layer was determined by collecting electron reflectivity at different electron energies. The number of dips observed in the electron reflectivity curve corresponds to the number of graphene layer. The experimental results obtained from LEEM and LEED have demonstrated that a film consisting of fairly large domains of 1 and 2 monolayer (ML) graphene was obtained by direct current heating of SiC under high vacuum (HV) condition with the based pressure of 10-6 Torr. A domain size in the range of up to about 5 to 10 μm have been observed. Meanwhile another graphene film prepared by the same method and the same temperature but under ultra high vacuum (UHV) condition with the based pressure of 10-10 Torr has much smaller domain size of 1 ML graphene compared to that grown under HV condition. We therefore suggested that the based pressure during the graphene growth has a strong influence on the morphology of graphene. This is because the Si evaporation rate is suppressed when heated in a high pressure environment, which normally leads to the improvement of the surface quality. The suppression of the Si evaporation rate has also been verified by a result obtained from the other sample directly heated under much higher based pressure, i.e. in an argon (Ar) environment of 1 atm. In addition to LEEM and LEED, the existence of graphene on SiC substrate has also been verified by the PES measurement. The C1s spectrum of graphene sample grown on SiC(0001) substrate showed three components, i.e. bulk SiC, graphene (G) and the buffer layer (B) located at 283.7 eV, 284.5 eV and 285.1 eV, respectively. The intensity ratios of the three components in the C1s spectrum were also used to estimate the number of graphene layer. The estimated number of graphene layer corresponds to the result obtained from LEEM.

The Early Permian (Sakmarian to Kungarian) Raanes and Great Bear Cape formations of the Sverdrup Basin were deposited at a time of ocean cooling, and are interpreted to reflect a subtropical setting. Pelmatozoans, bryozoans, and brachiopods are the predominant fossils throughout the extent of these two units, with local occurrences of large fusulinids and colonial corals. This mixed photozoan-heterozoan assemblage is similar to the sediments of modern-day subtropical settings. Although the Raanes and Great Bear Cape have warm-water rocks below, and cool-water rocks above, the fossil assemblages in these formations were dependent upon changes in oceanography and sea-level. Three distinct phases, as determined by water depth and temperature, occur. First, the rocks of the Raanes and lower Great Bear Cape are deep water and heterozoan in nature. Second, the middle Great Bear Cape limestones record a time of shallow, subtropical waters. Finally, the upper Great Bear Cape is shallow-water, but cooling had progressed to a point that precluded the occurrence of any photozoan components, regardless of depth. Due to evolutionary changes in other subtropical biota, the most reliable fossil indicator of subtropical deposition in the rock record is large benthic foraminifera (including fusulinids) in an otherwise heterozoan assemblage. The identification of limestones representative of these conditions should, therefore, be identifiable at times in the Earth’s history when large benthic foraminifera lived in shallow marine environments. The Great Bear Cape Formation subtropical facies underwent post-depositional changes that are manifest as calcite cements, iron-oxides, glauconite, and silica. Isopachous calcite cements precipitated in intraskeletal pore spaces as well as around the outside of grains. Glauconite, which is an authigenic marine mineral, has been oxidized to iron oxide, and both minerals post-date, or are included within, the isopachous cements. The isopachous cements must, therefore, have also formed in the marine environment. Where they are precipitated around pelmatozoan fragments, these originally high magnesium calcite cements have been neomorphosed to single-crystal epitaxial cements at the same time as mineral stabilization of the biofragments. These cements then seeded the growth of further epitaxial cement in the meteoric environment. / Thesis (Master, Geological Sciences & Geological Engineering) -- Queen's University, 2007-08-21 10:58:18.958

No description available.

The use of SiGe/Si heterostructures in the fabrication of electronic devices results in an improvement of the device performances with respect to bulk silicon. Ion implantation has been proposed as one of the possible technologies to produce these structures and, thus, the aim of this work is to develop an ion beam technology to fabricate strained SiGe heterostructures. The formation of extended defects in SiGe alloy layers formed by high dose Ge+ ion implantation followed by Solid Phase Epitaxial Growth (SPEG) has been investigated by transmission electron microscopy. Rutherford backscattering spectroscopy has also been used to determine the chemical composition and the crystalline quality of the synthesised structures. In addition, X-ray diffraction has been used to evaluate the strain level in selected samples. Two different structures have been studied in this project. The first consisted of "all-implanted" layers, where the Ge+ implants were followed in some cases by additional implants of Si+ and/or C+ ions, prior to SPEG, to investigate methods to inhibit defect formation. The second was achieved by capping the ion beam synthesised SiGe alloy layer by the deposition of a thin film of silicon, in order to realise structures compatible with device dimensions. Single crystal device worthy SiGe alloy layers have been achieved by implantation of Ge+ ions at energies ranging from 70 keV to 400 keV, where the only extended defects observed are EOR defects at a depth correspondent to the a/c interface formed during the Ge+ implant. In some cases, "hairpin" dislocations have also been observed in the vicinity of the EOR defects and extending up to the surface. Both types of defects are annihilated after post-amorphisation with 500 keV Si+ and replaced with dislocation loops at a depth of about 1 fj,m. For each Ge+ implantation energy a critical value of the peak germanium concentration exists above which the structures relax through the formation of stacking faults or "hairpin" dislocations nucleated in the vicinity of the peak of the germanium concentration depth profile and extending up to the surface. A critical value of the elastic energy stored in the structures (~300 mJ/m2) has been determined above which ion beam synthesised SiGe alloys relax, independently of the implantation energy. This empirical approach has been found to successfully account for the results obtained in this work as well as in many other studies reported in the literature. "Hairpin" dislocations formed under different experimental conditions have been investigated by plan view TEM and have been found to have the same crystallographic orientation () and Burgers vector (b= a ). Their formation has been explained within a "strain relaxation model". For a regrowth temperature of 700° C, all samples investigated by XRD have been found to be almost fully strained, including samples containing relaxation-induced defects, indicating that, under these conditions, the energy transferred to the defects is very low. C+ co-implantation has been successfully used to reduce both relaxation-induced defects and EOR dislocation loops. It is noted that a mixed technology entailing both layer deposition and ion implantation to produce the Si/SiGe/Si device structures requires extra process steps to control surface contaminations, pre cleaning and/or native oxide formation, resulting in increased fabrication costs. In this work an " all-implanted" route to the synthesis of Si/SiGe/Si device structures is therefore described, which exploits all of the advantages given by ion implantation.

No description available.

Gallium antimonide (GaSb) is a promising low-bandgap binary substrate for the fabrication of various infrared-based optoelectronic devices, particularly thermophotovoltaics (TPV). In order to make GaSb-based technologies like TPV more widely available, non-epitaxial dop- ing methods for GaSb must be pursued. Ion implantation is relatively unexplored for GaSb, and can offer advantages over the more common method of zinc diffusion, including higher flexibility with regards to substrate type and control over the resulting doping profile. Pre- vious work has shown beryllium (Be+) implantation to be a suitable method for fabricating a diode in an n-type GaSb substrate, opening the possibility for other ions to be considered for implanting into both n-type and p-type substrates. This work identifies sulfur (S+) as another species to investigate for this purpose. To do so, material and electrical characterization was done on S+ and beryllium implanted GaSb films grown onto a semi-insulating gallium arsenide (GaAs) substrate. X-ray Diffraction spectroscopy (XRD) and Atomic Force Microscopy (AFM) indicate that the post-implant anneal of 600 for 10 s repaired the implant damage in the bulk material, but left behind a damaged surface layer composed of coalesced vacancies. While the beryllium implant resulted in moderate doping concentrations corresponding to an activation percentage near 15 %, Hall Effect data showed that implanting S+ ions induced a strongly p-type behavior, with hole concentrations above 1 × 19 cm^3 and sheet hole densities 3.5 times higher than the total implanted dose. This strong p-type behavior is attributed to the remaining lattice damage caused by the implant, which induces a large density of acceptor-like defect states near the valence band edge. This technique was used on an unintentionally-doped p-type GaSb substrate to create a + /p junction. The implant process succeeded in producing a potential barrier similar to that of a hole-majority camel diode with a thin delta-doped region suitable for collecting diffused carriers from the p-type substrate. A post-fabrication etching process had the effect of strongly increasing the short circuit current density to as high as 41.8 mA/cm^2 and the open circuit voltage as high as 0.21 V by simultaneously removing a high carrier recombination surface layer. This etching process resulted in a broadband spectral response, giving internal quantum efficiencies greater than 90 %. / Doctor of Philosophy / Thermophotovoltaics (TPV) is a technology that converts light and other forms of electromagnetic energy into electrical power, much like a typical solar panel. However, instead of sunlight, the energy source used in a TPV system is a terrestrial heat source at a temperature range of 1250–1750 ◦C, whose radiation is primarily infrared (IR). The IR-absorbing qualities and commercial availability of the compound semiconductor gallium antimonide (GaSb) have made it a key component in the development of absorber devices for TPV-related systems. GaSb-based devices have most often been fabricated using epitaxy, a method in which layer(s) of material are ‘grown’ in a layer-by-layer fashion atop a substrate GaSb wafer to induce an interface between negatively-charged (n-type) and positively-charged (p-type) regions. In order to improve upon the scalability of TPV production, device fabrication methods for GaSb that avoid the use of epitaxy are sought after as a lower-cost alternative. In this work, sulfur ion implantation is examined as one of these methods, in which elemental sulfur ions are injected at a high energy into a p-type GaSb substrate. The implanted ions then alter the charge characteristics at the surface of the material, producing an electric field from which a photovoltaic (PV) device can be fabricated. The results of this study showed that by implanting sulfur ions, an extremely p-type (p++) layer was formed at the surface of the GaSb substrate, which was attributed to residual damage induced by the implant process. The resulting interface between the p++ surface and the moderately p-type GaSb substrate was found to induce an electric field suitable for a PV device. Removing the excess surface damage away from the device’s metal contacts resulted in an improvement in the output electrical currents, with measured values being significantly higher than that of other devices made using more common non-epitaxial fabrication methods. The success of this work demonstrates the advantages of using a p-type GaSb substrate in place of an n-type substrate, and could help diversify the types of TPV-related devices that can be produced.