The strain introduced by lattice mismatch is a built-in characteristic in Si/SiGe heterostructures, which has significant influences on various material properties. Proper design and precise control of strain within Si/SiGe heterostructures, i.e. the so-called “strain engineering”, have become a very important way not only for substantial performance enhancement of conventional microelectronic devices, but also to allow novel device concepts to be integrated with Si chips for new functions, e.g. Si-based optoelectronics. This thesis thus describes studies on two subjects of such strain-engineered Si/SiGe heterostructures grown by molecular beam epitaxy (MBE). The first one focuses on the growth and characterizations of delicately strain-symmetrized Si/SiGe multi-quantum-well/superlattice structures on fully relaxed SiGe virtual substrates for light emission in the THz frequency range. The second one investigates the strain relaxation mechanism of thin SiGe layers during MBE growth and post-growth processes in non-conventional conditions. Two types of THz emitters, based on different quantum cascade (QC) intersubband transition schemes, were studied. The QC emitters using the diagonal transition between two adjacent wells were grown with Si/Si0.7Ge0.3 superlattices up to 100 periods. It was shown that nearly perfect strain symmetry in the superlattice with a high material quality was obtained. The layer parameters were precisely controlled with deviations of ≤ 2 Å in layer thickness and ≤ 1.5 at. % in Ge composition from the designed values. The fabricated emitter devices exhibited a dominating emission peak at ~13 meV (~3 THz), which was consistent with the design. An attempt to produce the first QC THz emitter based on the bound-to-continuum transition was made. The structures with a complicated design of 20 periods of active units were extremely challenging for the growth. Each unit contained 16 Si/Si0.724Ge0.276 superlattice layers, in which the thinnest one was only 8 Å. The growth parameters were carefully studied, and several samples with different boron δ-doping concentrations were grown at optimized conditions. Extensive material characterizations revealed a high crystalline quality of the grown structures with an excellent growth control, while the heavy δ-doping may introduce layer undulations as a result of the non-uniformity in the strain field. Moreover, carrier lifetime dynamics, which is crucial for the THz QC structure design, was also investigated. Strain-symmetrized Si/SiGe multi-quantum-well structures, designed for probing the carrier lifetime of intersubband transitions inside a well between heavy hole 1 (HH1) and light hole 1 (LH1) states with transition energies below the optical phonon energy, were grown on SiGe virtual substrates. The lifetime of the LH1 excited state was determined directly with pump-probe spectroscopy. The measurements indicated an increase of lifetime by a factor of ~2 due to the increasingly unconfined LH1 state, which agreed very well with the theory. It also showed a very long lifetime of several hundred picoseconds for the holes excited out of the well to transit back to the well through a diagonal process. Strained SiGe grown on Si (110) substrates has promising potentials for high-speed microelectronics devices due to the enhanced carrier mobility. Strain relaxation of SiGe/Si(110) subjected to different annealing treatments was studied by X-ray reciprocal space mapping. The in-plane lattice mismatch was found to be asymmetric with the major strain relaxation observed in the lateral [001] direction. It was concluded that this was associated to the formation and propagation of conventional a/2<110> dislocations oriented along [110]. This was different from the relaxation observed during growth, which was mainly along in-plane [110]. A novel MBE growth process to fabricate thin strain-relaxed Si0.6Ge0.4 virtual substrates involving low-temperature (LT) buffer layers was investigated. At a certain LT-buffer growth temperature, a dramatic increase in the strain relaxation accompanied with a decrease of surface roughness was observed in the top SiGe, together with a cross-hatch/cross-hatch-free transition in the surface morphology. It was explained by the association with a certain onset stage of the ordered/disordered transition during the growth of the LT-SiGe buffer. / Kisel(Si)-baserad mikroelektronik har utvecklats under en femtioårsperiod till att bli basen för vår nuvarande informationsteknologi. Förutom att integrera fler och mindre komponenter på varje kisel-chip så utvecklas metoder att modifiera och förbättra materialegenskaperna för att förbättra prestanda ytterligare. Ett sätt att göra detta är att kombinera kisel med germanium (Ge) bl.a. för att skapa kvantstrukturer av nanometer-storlek. Eftersom Ge-atomerna är större än Si-atomerna kan man skapa en töjning i materialet vilket kan förbättra egenskaperna, ex.vis hur snabbt laddningarna (elektronerna) rör sig i materialet. Genom att variera Gekoncentrationen i tunna skikt kan man skapa skikt som är antingen komprimerade eller expanderade och därmed ger möjlighet att göra strukturer för tillverkning av nya typer av komponenter för mikroelektronik eller optoelektronik. I detta avhandlingsarbete har Si/SiGe nanostrukturer tillverkats med molekylstråle-epitaxi-teknik (molecular beam epitaxy, MBE). Med denna teknik byggs materialet upp på ett substrat, atomlager för atomlager, med mycket god kontroll på sammansättningen av varje skikt. Samtidigt kan töjningen av materialet designas så att inga defekter skapas alternativt många defekter genereras på ett kontrollerat sätt. I denna avhandling beskrivs detaljerade studier av hur töjda i/SiGe-strukturer kan tillverkas och ge nya potentiella tillämpningar ex.vis som källa för infraröd strålning. Studierna av de olika töjda skikten har framför allt gjorts med avancerade röntgendiffraktionsmätningar och transmissionselektronmikroskopi.

Au cours des dix dernières années, les performances des lasers à cascade quantique dans le moyen infrarouge ont connu une progression rapide: Les rendements ont atteint des valeurs supérieures à 20% avec une puissance d'émission de 5W en régime continu, à température ambiante. Ces valeurs ont été atteintes notamment grâce à la diminution de la sensibilité des lasers à l'échauffement, avec des températures caractéristiques T0 s'approchant de 300K. Les performances sont donc actuellement limitées par la puissance injectée, qui est proportionnelle à la taille de la zone de gain. Les travaux de cette thèse présentent une solution innovante, consistant à combiner un réseau d'émetteurs de petites tailles de façon monolithique. Nous démontrons expérimentalement pour la première fois, des dispositifs jusqu'à 32 émetteurs de 2µm de larges, émettant en phase par couplage évanescent. De plus, nous mettons en évidences des résistances thermiques record. Ces résultats mettent en évidence la possibilité de fabriquer des sources de hautes puissances (supérieures à 10W) dans le moyen-infrarouge avec une très bonne qualité de faisceau.

The purpose of the current thesis was to investigate the role of gaze direction, when making preference decisions. Previous research has reported a progressive gaze bias towards the preferred stimuli as participants near a decision, termed the gaze cascade effect (Shimojo, Simion, Shimojo & Scheir, 2003). The gaze cascade effect is strongest during the final 1500 msec prior to decision (Shimojo et al.). Previous eye-tracking research has displayed natural viewing biases towards the upper visual field. However, previous investigations have not investigated the impact of image placement on the gaze cascade effect. Study 1 investigated the impact of presenting stimuli vertically on the gaze cascade effect. Results indicated that natural scanning biases towards the upper visual field impacted the gaze cascade effect. The gaze cascade effect was reliably seen only when the preferred image was presented in the upper visual field. Using vertically paired stimuli study 2 investigated the impact of choice difficulty on the gaze cascade effect. Similar to study 1 the gaze cascade effect was only reliably seen when the preferred image was presented in the upper visual field. Additionally choice difficulty impacted the gaze cascade effect where easy decisions displayed a larger gaze cascade effect than hard decisions. Study 3 investigated if the gaze cascade effect is unique to preference decisions or present during all visual decisions. Judgments of concavity using perceptually ambiguous spheres were used and no gaze cascade effect was observed. Study 3 indicated that the gaze cascade effect is unique to preference decisions. Results of the current experiments indicate the gaze cascade effect is qualified by the spatial layout of the stimuli and choice difficulty. Results of the current experiments are consistent with previous eye-tracking research demonstrating biases towards the upper visual field and offering support for Prevics theory on how we interact in visual space.

A two-dimensional thin-layer Navier-Stokes cascade flow solver for turbomachinery is developed. A second-order finite-difference scheme and a second and fourth-difference dissipation scheme are used. Periodic and non-reflecting inlet and outlet boundary conditions are implemented into the approximate-factorization numerical method. Turbulence is modeled through the one-equation Spalart-Allmaras model. A two-dimensional turbomachinery cascade structured grid generator is developed to produce six-block H-type grids. The validity of this work is tested in various ways. A grid convergence study is performed showing the effect of grid density. The non-reflecting inlet and outlet boundary conditions are tested for boundary placement influence. Comparisons of the flow solver numerical results are performed against experimental results. A Mach number sweep and angle of attack sweep are performed on two similar transonic turbine cascades.

A two-dimensional thin-layer Navier-Stokes cascade flow solver for turbomachinery is developed. A second-order finite-difference scheme and a second and fourth-difference dissipation scheme are used. Periodic and non-reflecting inlet and outlet boundary conditions are implemented into the approximate-factorization numerical method. Turbulence is modeled through the one-equation Spalart-Allmaras model. A two-dimensional turbomachinery cascade structured grid generator is developed to produce six-block H-type grids. The validity of this work is tested in various ways. A grid convergence study is performed showing the effect of grid density. The non-reflecting inlet and outlet boundary conditions are tested for boundary placement influence. Comparisons of the flow solver numerical results are performed against experimental results. A Mach number sweep and angle of attack sweep are performed on two similar transonic turbine cascades.

No description available.

Robotic manipulators have been introduced in industry as a form of increasing productivity. Today, there exist an interest to enlarge the application of these manipulators to outdoor environments. Forestry cranes used in the forestry industry are a clear example. A long term goal in this industry is the development of autonomous systems to increase the logging efficiency. In this thesis, we consider how to control the rotator of these cranes, which is an electro-hydraulically actuated motor, and is used to control the end ffector tool. Control system design for the rotator is a challenging task since the sensing is not available to full extent. The main reason is the harsh environment that these machines are exposed to and sensors such as encoders are very fragile and cannot be used. In this thesis we use alternative sensing devices, such as a magnetic sensor and a stereo camera. In the case of the camera we face a problem with big delay. A prediction method has been used to compute desired values. Due to various reasons certain measuring devices can not be used in the industry. We consider four cases for control system design where dfferent combinations of available sensors have been used. Initially angular position of the rotator is controlled using only the magnetic sensor. A cascade control setup is used where pressure and position are measurable, first using the magnetic sensor and later using the camera. When only pressure measurements are available identified models have replaced sensors for position feedback. All tests and experiments have been done using a scaled version of a real forestry crane. The available crane has similar configuration and dynamics as the real one and is therefor useful for experimental purposes.

We consider the distributed access enforcement problem for Role-Based Access Control (RBAC) systems. Such enforcement has become important with RBAC’s increasing adoption, and the proliferation of data that needs to be protected. Our particular interest is in the evaluation of a new data structure that has recently been proposed for enforcement: the Cascade Bloom Filter. The Cascade Bloom Filter is an extension of the Bloom filter, and provides for time- and space-efficient encodings of sets. We compare the Cascade Bloom Filter to the Bloom Filter, and another approach called Authorization Recycling that has been proposed for distributed access enforcement in RBAC. One of the challenges we address is the lack of a benchmark: we propose and justify a benchmark for the assessment. Also, we adopt a statistically rigorous approach for empirical assessment from recent work. We present our results for time- and space-efficiency based on our benchmark. We demonstrate that, of the three data structures that we consider, the Cascade Bloom Filter scales the best with the number of RBAC sessions from the standpoints of time- and space-efficiency.

The purpose of the current thesis was to investigate the role of gaze direction, when making preference decisions. Previous research has reported a progressive gaze bias towards the preferred stimuli as participants near a decision, termed the gaze cascade effect (Shimojo, Simion, Shimojo & Scheir, 2003). The gaze cascade effect is strongest during the final 1500 msec prior to decision (Shimojo et al.). Previous eye-tracking research has displayed natural viewing biases towards the upper visual field. However, previous investigations have not investigated the impact of image placement on the gaze cascade effect. Study 1 investigated the impact of presenting stimuli vertically on the gaze cascade effect. Results indicated that natural scanning biases towards the upper visual field impacted the gaze cascade effect. The gaze cascade effect was reliably seen only when the preferred image was presented in the upper visual field. Using vertically paired stimuli study 2 investigated the impact of choice difficulty on the gaze cascade effect. Similar to study 1 the gaze cascade effect was only reliably seen when the preferred image was presented in the upper visual field. Additionally choice difficulty impacted the gaze cascade effect where easy decisions displayed a larger gaze cascade effect than hard decisions. Study 3 investigated if the gaze cascade effect is unique to preference decisions or present during all visual decisions. Judgments of concavity using perceptually ambiguous spheres were used and no gaze cascade effect was observed. Study 3 indicated that the gaze cascade effect is unique to preference decisions. Results of the current experiments indicate the gaze cascade effect is qualified by the spatial layout of the stimuli and choice difficulty. Results of the current experiments are consistent with previous eye-tracking research demonstrating biases towards the upper visual field and offering support for Prevics theory on how we interact in visual space.

The principal focus of this thesis is the exploration of sparse structures in a variety of statistical modelling problems. While more comprehensive models can be useful to solve a larger number of problems, its calculation may be ill-posed in most practical instances because of the sparsity of informative features in the data. If this sparse structure can be exploited, the models can often be solved very efficiently. The thesis is composed of four projects. Firstly, feature sparsity is incorporated to improve the performance of support vector machines when there are a lot of noise features present. The second project is about an empirical study on how to construct an optimal cascade structure. The third project involves the design of a progressive, rate-distortionoptimized shape coder by combining zero-tree algorithm with beamlet structure. Finally, the longest run statistics is applied for the detection of a filamentary structure in twodimensional rectangular region. The fundamental ideas of the above projects are common — extract an efficient summary from a large amount of data. The main contributions of this work are to develop and implement novel techniques for the efficient solutions of several dicult problems that arise in statistical signal/image processing.