A flash evaporation system is constructed in order to deposit thin films of Gallium Antimonide. The system includes a substrate heater-holder, a film thickness monitoring device and a powdered evaporant feeder. Thin films of GaSb, 2 to 4 microns thick are deposited on glass, sapphire and silicon substrates maintained at temperatures up to 365°C. The deposited films are observed to have many structural defects. The films exhibit semiconductor-like properties. Optical studies of the fundamental absorption edge at liquid-nitrogen temperature show that the films deposited on heated sapphire substrates have a polycrystalline structure and a band gap of 0.78 eV. The nature of the films is dependent on the type and temperature of the substrate. / Science, Faculty of / Physics and Astronomy, Department of / Graduate

This research is related to the molecular beam epitaxy (MBE ) to grow GaSb . The fabrication of GaSb/InGaSb strained quantum well and superlattice structures are used for photodetection . They are carefully investigated to obtain high quality of GaSb films. The growth mechanisms related to the major factors of (1) Subtrate temperature (2) Beam flux ratio(V/III). The properties of GaSb epilayers are characterized by different methods such as the X-ray diffraction , I-V curve and Raman spectra . The optimum growth conditions 500¢J of substrate temperature and the V/III flux ratio about 2~3 have been obtained. On the basis of structure, the best growth conditions is identified by the peak intensity and FWHM related to the quality of the GaSb films by the X-ray diffraction. On the basis of electrical property, the best growth conditions is identified by the lowest leakage current for the p-n junction related to the quality of the GaSb films by the I-V curve. On the basis of optical property, the best growth conditions is identified by the LO mode phonon intensity related to the quality of the GaSb films by the Raman spectra. Based on the GaSb growth studied here, the study will be focused in the quantum well and quantum dot laser devices furtherly by us.

The production and fabrication of semi-conductor core optical fibers was shown to be feasible and controllable. This was accomplished through the step sequence of fabrication and characterization of 4 fiber types, an experiment on controlling the grain length in the core and a simple model of the heat transfer during fabrication. Fibers were first made with a silicon core, followed by a phosphorous doped n-type silicon core, then a boron doped p-type silicon core, and a tellurium doped n-type gallium antimonide core. Characterization of the fibers was accomplished with energy dispersive spectroscopy (EDS) for compositional analysis, electron backscatter diffraction (EBSD) for crystal orientation and grain size, optical and electron microscopy for physical fiber quality and optical transmission for core optical quality. A model was developed to relate the heat transfer with the grain structure of the fiber core. All of the fibers fabricated had a polycrystalline core with either no detectable oxygen in the case of the silicon fibers or low amounts of oxygen diffusion into the core as in the case of the GaSb fibers. Fiber lengths ranged from 7 cm for the initial silicon fibers to 60 cm and outside diameters down to 100 µm for n and p type silicon fibers. Core diameters for all fiber types ranged from 10 – 200 µm depending on the fabrication parameters. Lengths of major grains in the core are dependent on the core diameter and the pulling speed. The grain lengths of the major grains in the core generally increase in length with an increase in core diameter. Grain lengths in all fibers are thought to be suitable for use in fabrication of electronic structures in the core region with even the smallest average grain length of around 300 µm. This grain structure satisfies the grain boundary requirements for fabrication of boundary free p-n junctions and other more complicated electronic structures. Small core diameter fibers had better physical quality with fewer cracks and longer continuous length than the larger core fibers. / Ph. D.

No description available.

A TPV-system consists of a hot surface emitting heat radiation on a solar cell with a narrow bandgap.  A unit consisting of a boiler and a TPV-system has been constructed for testing of the performance of TPV cells. The emitter is heated by a fuel consisting of RME-oil. The radiation is collected and concentrated through two reflecting cones formed like a Faberge-egg, with an edge-type optical filter between the cones. The Faberge-egg is treated with electro-polishing in order to obtain a high reflectance of radiation. The edge filter transmits radiation of short wavelengths towards the solar cells and reflects long wavelengths back to the emitter. This increase the temperature of the emitter to prevent the TPV-cells to be overheated. The construction made was working as expected and can be used for further experiments. The performance of the TPV-cells were however very poor because of a low emitter temperature. The main problem was to obtain a good heat transport from the flame to the emitter. It is required that the emitter temperature is considerably increased for justifying a continued work on TPV-systems in combination with boilers.

The quantum Spin Hall Insulator (QSHI) is a two-dimensional variant of a novel class of materials characterized by topological order, whose unique properties have recently triggered much interest and excitement in the condensed matter community. Most notably, the topological properties of these systems hold great promise in mitigating the difficult problem of decoherence in implementations of quantum computers. Although QSHI has been theoretically predicted in a few different materials, prior to the work presented in this thesis, only the HgTe/CdTe semiconductor system has shown direct evidence for the existence of this phase. Ideally insulating in the bulk, QSHI is characterized by one-dimensional channels at the sample perimeter, which have a helical property, with carrier spin tied to the carrier direction of motion, and protected from elastic back-scattering by time-reversal symmetry. In this thesis we present low temperature transport measurements, showing strong evidence for the existence of proposed helical edge channels in InAs/CaSb quantum wells, which thus emerge as an important alternate to HgTe/CdTe quantum wells in studies of two-dimensional topological insulators and superconductors. Surprisingly, edge modes persist in spite of comparable bulk conduction of non-trivial origin and show only weak dependence on magnetic field in mesoscopic devices. We elucidate that the seeming independence of edge on bulk transport comes due to the disparity in Fermi wave-vectors between the bulk and the edge, leading to a total internal reflection of the edge modes. Furthermore, low Schottky barrier of this material system and good interface to superconductors allows us to probe topological properties of helical channels in Andreev reflection measurements, opening a promising route towards the realization of topologically superconducting phases hosting exotic Majorana modes.

Understanding and control of the flow structures in metallic fluids is important for the development of optimal crystal growth processes. One of the techniques used to control flow structures is the application of a rotating magnetic field (RMF) in the plane perpendicular to the growth direction, which induces two magnetic body force components; one in the radial direction and the other one in the circumferential direction. These two body force components alter the fluid flow in the growth system, leading to enhanced mixing, flatter growth interface, and more homogeneous crystal composition. The application of RFM was therefore considered in three separate projects: 1) the zone refining of cadmium and tellurium, 2) the synthesis of cadmium telluride (CdTe) by the travelling heater method (THM), and 3) the THM growth of gallium antimonide (GaSb). In the zone refining of tellurium, the objective was to enhance the transport of selenium in the melt since the selenium segregation coefficient is close to unity. A magnetic field with intensity of 0.6 mT and frequency of 100 Hz was selected based on the results of earlier numerical simulations. Due to the very low electrical conductivity of tellurium, the numerical simulations predicted a very small effect of RMF on selenium transport. The designed zone refining experiments for the tellurium system have verified this numerical simulation result. On the other hand, cadmium is an electric conductor, and thus the numerical simulations predicted a notable effect of RMF. However, experiments on the cadmium system could not be carried out because of the instability of molten zones caused by cadmium’s very high thermal conductivity. The commercial synthesis of CdTe is presently done by THM, which produces materials with much better stoichiometry than other techniques, but very slow process speeds make THM very costly. An application of RMF was considered in order to improve the speed of the process. A 1.3 mT, 0.5 Hz field was applied during the THM synthesis of CdTe. Under the experimental conditions employed, the examination of samples has shown that the application of RMF did not increase the maximum synthesis speed. The use of higher intensity RMF was not possible with the present system, but it is thought that higher fields might worsen the mixing of Cd and Te to produce non-stoichiometry. The objective of the third project was to carry out preliminary THM growth experiments for GaSb under RMF in order to prepare a basis for future THM growth experiments aimed at reducing the cost of THM by using higher growth rates and smaller seeds with tapered ampoules. The substantially redesigned THM furnace permits rotation of the growth ampoule, better control of the experimental environment, and a stronger temperature gradient at the growth interface. Two crystals have been grown at 25 mm diameter with and without the application of a magnetic field of 0.6-mT intensity and 100-Hz frequency. These preliminary experiments have shown that the system can be used for the planned THM experiments; however, further experiments are required to attribute any effect to RMF.

Le développement de sources moyen infrarouge compactes et accordables dans les gammes de transmission de l’atmosphère présente un intérêt majeur dans les secteurs de la défense et de la sécurité. Les sources paramétriques à quasi-accord de phase en configuration guidée sont prometteuses pour gagner en compacité puisque l’on réduit la puissance de pompe nécessaire par rapport aux sources « massives ». Le premier axe de la thèse consiste à étudier des guides d’onde en arséniure de gallium périodiquement orientés (OP-GaAs) adaptés à un pompage par laser fibré et à des puissances relativement élevées. Le second vise à étudier de façon novatrice la possibilité d’intégrer dans un composant monolithique une diode laser en matériaux antimoniures avec un convertisseur de fréquence en antimoniure de gallium (GaSb). L’enjeu dans les deux cas est de réduire au maximum les pertes à la propagation dans ces guides d’onde pour exploiter pleinement leurs propriétés non-linéaires.Ce travail de thèse a permis de modéliser des structures de guides d’onde ambitieuses pour réduire les pertes, de développer les briques technologiques nécessaires à la fabrication de guides d’onde OP-semi-conducteur faibles pertes et de faire de premières caractérisations de ces composants dans le moyen-infrarouge. Les performances de guides d’onde GaAs ruban enterrés ou non ont pu être comparées, donnant une réduction des pertes d’un facteur trois avec des rubans enterrés. Plusieurs générations de guides d’onde GaSb ont vu le jour, et montrent des performances à l’état de l’art des structures en GaAs. En conséquence, diverses solutions ont été explorées pour intégrer une diode laser en matériaux antimoniures avec le guide d’onde convertisseur de fréquence. / The development of compact and tunable mid-infrared laser sources in the atmospheric transmission windows presents a major interest for several security and defense applications. Quasi-phase-matched parametric sources in guided wave configuration are promising solutions to enhance compactness, because of the reduction in pump power requirements with respect to bulk devices.The first axis of this thesis consists in studying orientation-patterned gallium arsenide (OP-GaAs) waveguides, adapted to fiber laser pumping and to relatively high pump power. The second axis is devoted to the original idea of integrating an antimonide based laser diode with a gallium antimonide (GaSb) frequency converter in a monolithic component. The goal in both cases is to minimize propagation losses in those waveguides to exploit the whole potential of their non-linear properties.This work led to model ambitious low-loss waveguides structures, to develop the technological fabrication steps necessary for OP-semiconductor waveguides manufacturing, and to characterize these components in the mid-infrared. The first buried ridge GaAs waveguide structure has been compared to the ridge one, giving a reduction of a factor three in the propagation losses. Several generations of GaSb waveguides have come forward, with constant losses improvement and reach GaAs state-of-the-art performances. Lastly, multiple solutions have been explored in order to integrate an antimonide-based laser diode with the frequency converter waveguide.

The semiconducting compounds CdTe and CdZnTe have important applications in high-energy radiation detectors and as substrates for infrared devices. The materials offer large band gaps, high resistivity, and excellent charge transport properties; however all of these properties rely on very precise control of the material composition. Growing bulk crystals by the travelling heater method (THM) offers excellent compositional control and fewer defects compared to gradient freezing, but it is also much slower and more expensive. A particular challenge is the current need to grow new crystals onto existing seeds of similar size and quality. Simulations and experiments are used in this work to investigate the feasibility of growing these materials by THM without the use of large seed crystals. A new fixed-grid, multiphase finite element model was developed based on the level set method and used to calculate the mass transport regime and interface shapes inside the growth ampoule. The diffusivity of CdTe in liquid tellurium was measured through dissolution experiments, which also served to validate the model. Simulations of tapered THM growth find conditions similar to untapered growth with interface shapes that are sensitive to strong thermosolutal convection. Favourable growth conditions are achievable only if convection can be controlled. In preliminary experiments, tapered GaSb crystals were successfully grown by THM and large CdTe grains were produced by gradient freezing. Beginning with this seed material, 25 mm diameter CdTe and CdZnTe crystals were grown on 10 mm diameter seeds, and 65 mm diameter CdTe on 25 mm seeds. Unseeded THM growth was also investigated, as well as ampoule rotation and a range of thermal conditions and ampoule surface coatings. Outward growth beyond one or two centimeters was achieved only at small diameters and included secondary grains and twin defects; however, limited outward growth of larger seeds and agreement between experimental and numerical results suggest that tapered growth may be achievable in the future. This would require active temperature control at the base of the crystal and reduction of convection through thermal design or by rotation of the ampoule or applied magnetic fields. / Graduate / 0346 / 0794 / 0548 / jordan.roszmann@gmail.com