Browsing by Subject "Germanium"
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Item A dark matter search using the final CDMS-II data and 100 mm SuperCDMS germanium detector ionization test(2014-07) Zhang, JianjieAstrophysical observations indicate that approximately 85% of the matter in the universe is nonluminous, nonbaryonic, and nonrelativistic (cold) dark matter. Weakly Interacting Massive Particles (WIMPs) are a particularly well motivated dark matter particle candidate. They would be thermally produced in the early universe and their relics account for the current dark matter abundance. WIMP candidate particles are naturally provided by extensions to the Standard Model of particle physics, such as supersymmetry. The Cryogenic Dark Matter Search (CDMS) experiment operates cryogenic germanium and silicon particle detectors in the low-background environment of the Soudan Underground Laboratory in northern Minnesota to search for WIMP-nucleus scatters while rejecting electron-recoil background. The detectors simultaneously measure the ionization and phonon energies of each scattering event. The difference in ionization yield (ratio of ionization energy to recoil energy) discriminates nuclear recoils from the electron-recoil background efficiently.More sensitive detectors are required to probe the WIMP parameter space with lower WIMP-nucleon scattering cross sections. To support the R&D effort especially the detector R&D and characterization of the SuperCDMS experiment, a new CDMS test facility has been developed on the University of Minnesota campus. This thesis documents the test facility and the work involved in its development. In the test facility, we performed the first ionization collection efficiency measurements of the ionization test devices. The test devices are fabricated with detector-grade germanium crystals that are 100 mm in diameter, which is the largest available, and 33 mm in thickness. The measured efficiencies are consistent with the earlier measurements conducted with smaller Ge crystals, demonstrating that these 100 mm crystals can be used for development of the next generation dark matter detectors.Improvements of data analysis methods can also potentially improve the sensitivity of an experiment. The data taken during the last four runs of CDMS II with total raw exposure 612 kg-day were reprocessed with improved ionization pulse reconstruction algorithm. We present the classic timing analysis with the reprocessed data in this thesis. For the four runs combined, this analysis resulted in a new WIMP-nucleon cross section 4.4×10-44cm2 for a WIMP mass of 70 GeV, which is a factor of 1.6 improvement compared to the original c58 classic timing analysis.Item Electronic transport in nanocrystalline germanium/hydrogenated amorphous silicon composite thin films(2015-02) Bodurtha, Kent EdwardRecent interest in composite materials based on hydrogenated amorphous silicon (a-Si:H) stems in part from its potential for technical applications in thin film transistors and solar cells. Previous reports have shown promising results for films of a-Si:H with embedded silicon nanocrystals, with the goal of combining the low cost, large area benefits of hydrogenated amorphous silicon with the superior electronic characteristics of crystalline material. These materials are fabricated in a dual-chamber plasma-enhanced chemical vapor deposition system in which the nanocrystals are produced separately from the amorphous film, providing the flexibility to independently tune the growth parameters of each phase; however, electronic transport through these and other similar materials is not well understood. This thesis reports the synthesis and characterization of thin films composed of germanium nanocrystals embedded in a-Si:H. The results presented here describe detailed measurements of the conductivity, photoconductivity and thermopower which reveal a transition from conduction through the a-Si:H for samples with few germanium nanocrystals, to conduction through the nanocrystal phase as the germanium crystal fraction XGe is increased. These films display reduced photosensitivity as XGe is increased, but an unexpected increase in the dark conductivity is found in samples with XGe > 5% after long light exposures. Detailed studies of the conductivity temperature dependence in these samples exposes a subtle but consistent deviation from the standard Arrhenius expression; the same departure is found in samples of pure a-Si:H; a theoretical model is presented which accurately describes the actual conductivity temperature dependence.Item Non-equilibrium Quantum Transport in Two Dimensional Electron Gases in Modulation Doped Heterostruvtures(2017-08) Ebner, QuentinThis thesis focuses on experimental investigations of transport in two dimensional electron gases (2DEG) driven out of equilibrium by microwaves and dc current. Part I presents the background information in four sections that describe the basics of magneto transport, Shubnikov-de Haas oscillations (SdHO), microwave-induced resistance oscillations (MIRO), and Hall field-induced resistance oscillations (HIRO). The second part of this thesis is broken into three sections that each discuss experimental findings of MIRO and HIRO in various 2DEGs. The fifth section discusses the observation of MIRO and HIRO in p-type Germanium quantum wells. This is significant because previous studies into a variety of different materials never showed any MIRO response. P-type Germanium is the second material for which MIRO and HIRO have been observed, making MIRO and HIRO not unique to GaAs quantum wells. The sixth section looks into the role of introducing alloy disorder into the quantum well on MIRO and HIRO. The quantum scattering rate was found to increase with the amount of alloy disorder and the MIRO prefactor that is decided by scattering times was not found to have a measurable change with increasing alloy disorder concentration. It was found the amplitude of HIRO is controlled by sharp disorder scattering contribution in mobility. The seventh section provides a systematic study into the effects of density on MIRO, which is not well understood. The analysis focuses on when one energy sub-band is populated, but also gives some observations of when two sub-bands are populated. When one sub-band is populated, increasing density was found to suppress the effective mass extracted from MIRO. The change in MIRO amplitude was found to be described by a combination of the zero-field resistance and the increase of quantum lifetime as density increased. When the second sub-band was populated, MIRO was found to be highly suppressed and this suppression cannot be explained by the MIRO mechanisms, quantum lifetime, or MIRO’s power factor.Item Surface Engineering of Colloidal Group IV Nanocrystals for Optoelectronics(2014-06) Wheeler, LanceColloidal nanocrystals (NCs), often synonymous with"quantum dots," represent a burgeoning class of next-generation optoelectronic materials. The promise of NCs is twofold: (i) Their optical properties are tunable and offer unique opportunities for enhanced energy conversion due to quantum confinement effects. (ii) The NCs can be processed into thin films using cost-efficient roll-to-roll printing techniques for large-scale integration into devices. Taken together, these two attributes enable a new platform for optoelectronic technology where energy-efficient devices can be produced at low costs. There is an array of research efforts to produce NC-based optoelectronic devices such as photovoltaic cells, light emitting devices, and photodetectors. Much of the recent progress in this direction hinges on the ability to manipulate the NC surface. Conventional solution synthesis yields NCs with ligands bound to metal surface atoms through a labile acid-base complex. The electrically-insulating native ligands are thus routinely exchanged to produce conductive NC arrays for devices integration. Just as surface manipulation has launched metal-based NCs to the forefront of optoelectronic technology, it is the inability to do so with the covalent surface of group IV (germanium and silicon) NCs that has greatly hindered progress. The motivation of this research is to bridge the gap between group IV and metal-based NCs in order to establish an abundant, non-toxic alternative to NCs that contain toxic lead or cadmium. The bridge is built by developing new Si NC surface chemistries, understanding how they interact with molecules, and applying chemical and physical models to uncover the mechanism of NC colloidal stability. The research begins by developing nonthermal plasma synthesis of Si NCs from a new precursor, silicon tetrachloride. This work builds on previous studies on chlorine-terminated germanium NCs synthesized from germanium tetrachloride, which were observed to form stable colloids without covalent ligand attachment. Synthesis from silicon tetrachloride offers the same flexibility for tuning size and crystallinity as typical silane synthesis but yields a new chlorinated surface chemistry. Si-Cl surface groups of the NCs are shown to be crucial for achieving the same colloidal stability observed in Ge NCs. It was determined spectroscopically the polarized Si-Cl surface bond renders the surface Si atoms Lewis acidic and capable of hypervalent interactions with Lewis basic molecules. The NCs were thus dispersible in select Lewis basic solvents. Interestingly, these interactions are also shown to be responsible for a reversible "surface doping" effect, which was also explored spectroscopically and by electrical characterization of a thin film device. The notion of a Lewis acidic surface gave rise to the development of a more robust Si NC surface chemistry. In this work, plasma synthesis that includes diborane is applied. The resulting Si NC surface is then terminated by a classic Lewis acid, boron, which is demonstrated to be an even more versatile chemistry than the Si-Cl surface. These NCs are also used as a model system for uncovering the mechanism of colloidal stability due to these surface interactions with solvent molecules. It is found that conventional theory cannot account for the stability observed, and a simple alternative model is developed. In light of this model, we are able to demonstrate stable Si NC colloids in media that spans hexane to water. The thesis concludes with a peripheral effort on Ge NCs, a material lacking in maturity even to Si NCs. In this work, the NC surface is modified to enhance the optical properties of the material as opposed to the ability to process the NCs into films from solution. Size-tunable band gap emission is demonstrated for the first time in gas-phase synthesized Ge NCs by applying Grignard chemistry to the Ge-Cl surface groups. The emission is narrower than any previous report, and emission near the bulk band gap of Ge is attained for the first time.Item Understanding and Improving Plasma Synthesized Silicon Germanium Films for Thermoelectric Applications(2017-10) Mork, Kelsey CLaser sintering and Phenyl Acetylene surface functionalization of plasma synthesized silicon germanium nanoparticle films and powders were studied to improve electrical conductivities of these materials. Laser sintering greatly improved the electrical conductivity in thin films with peak values of 70.42 S/cm. Phenyl Acetylene functionalization was successful in both doped and undoped silicon germanium nanoparticles. Further studies should be performed to quantify the electrical conductivity values in bulk, compacted pellets of the functionalized nanoparticles. Finally, research into activation energies of compacted silicon germanium films showed drastic differences between energies obtained when using Seebeck coefficient and energies obtained when using electrical conductivity. This is attributed to traps on the surface of the nanoparticles and the potential barrier between nanoparticles.