UMD Theses and Dissertations
Permanent URI for this collectionhttp://hdl.handle.net/1903/3
New submissions to the thesis/dissertation collections are added automatically as they are received from the Graduate School. Currently, the Graduate School deposits all theses and dissertations from a given semester after the official graduation date. This means that there may be up to a 4 month delay in the appearance of a given thesis/dissertation in DRUM.
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Item In Situ Enrichment and Epitaxial Growth of 28Si Films via Ion Beam Deposition(2017) Dwyer, Kevin Joseph; Cumings, John; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Isotopically enriched 28Si is an ideal material for solid state quantum computing because it interacts weakly with the spin states of embedded qubits (quantum bits) resulting in long coherence times. This is the result of eliminating the roughly 4.7 % 29Si isotopes present in natural abundance Si, which possesses nuclear spin I = 1/2 that is disruptive to qubit operation. However, high-quality 28Si is scarce and the degree to which it improves the performance of a qubit is not well understood. This leads to an important question in the Si-based quantum information field, which can be stated as "how good is good enough?" regarding the perfection of 28Si as a host medium for qubits. The focus of this thesis is to engineer a material that can address this question, specifically in terms of the enrichment. Secondary requirements for ideal 28Si films that are also pursued are crystalline perfection and high chemical purity. I report on the production and characterization of 28Si thin films that are the most highly enriched of any known 28Si material ever produced with a maximum 28Si enrichment of 99.9999819(35) % and a residual 29Si isotopic concentration of 1.27(29) x 10^-7. A hyperthermal energy ion beamline is used to produce this extreme level of enrichment starting from a natural abundance silane gas (SiH4) source. The Si is enriched in situ by mass separating the ions in a magnetic field just before deposition onto Si(100) substrates. Initial proof of principle experiments enriching 22Ne and 12C were also conducted. In the course of achieving this 28Si enrichment, I also pursue the epitaxial deposition of 28Si thin films. Characterizations of the film morphology and crystallinity are presented showing that smooth, epitaxial 28Si films are achieved using deposition temperatures between 349 C and 460 C. Crystalline defects present in these films include {111} stacking faults. When using higher deposition temperatures, I find that trace impurity compounds such as SiC cause step pinning and faceting of the growth surface leading to severely rough films. Assessments of the chemical purity of 28Si films are also presented, which show major impurities N, C, and O are present in the purest film at an atomic concentration of approximately 1 x 10^19 cm^-3, resulting in a Si purity of 99.96(2) %. Additionally, I introduce a model that describes the residual 29Si and 30Si in 28Si films, i.e. the enrichment, as the result of adsorption of diffusive natural abundance SiH4 gas from the ion source into the 28Si films during deposition. This model correlates the measured enrichments of 28Si films with the SiH4 partial pressures during deposition. An incorporation fraction for SiH4 adsorption at room temperature of s = 6:8(3) x 10^-4 is extracted. Finally, the temperature dependence of the sample enrichment is analyzed using a thermally activated incorporation model that gives an activation energy of Ec = 1.1(1) eV for the reactive sticking coefficient of SiH4.Item EVALUATION OF MASS FILTERED, TIME DILATED, TIME-OF-FLIGHT MASS SPECTROMETRY(2010) Demoranville, Leonard Thomas; Mignerey, Alice C; Chemistry; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The Naval Research Laboratory's Trace Element Accelerator Mass Spectrometer (NRL-TEAMS) system offers a unique opportunity to develop a new type of time-of-flight (TOF) SIMS. This opportunity derives from use of a Pretzel magnet as a recombinator and mass filter in the injector to the accelerator. Mass filtering prior to time-of-flight analysis removes extraneous species, shortening the analysis time for a single beam pulse, thereby improving the duty cycle. Using this approach, it is possible to obtain an expanded portion of a narrow segment of the entire time-of-flight spectrum created by a single beam pulse. A longer flight path for greater momenta in the Pretzel magnet introduces time dilation. Potential benefits derived from time dilation and mass filtering include improved duty cycle, shorter analysis time, increased precision, and better resolution. While the NRL-TEAMS system is not designed for TOF work, it has been used as a test bed to prove the theoretical benefit of such a design. Theoretical treatments of the spectrometer have shown improved resolution is possible under certain conditions, when compared to a traditional TOF spectrometer. SIMION 8.0 computer simulations were used to model the system and provide insight to the theoretical capabilities of the Pretzel magnet. As expected, models have shown that as field decreases, and therefore path length increases, mass resolution improves. Generally, the model matched well to experimental results provided by the NRL TEAMS system. These experimental results have predicted fundamental parameters of the system accurately and consistently, and confirmed the validity of the model. This research improved the current system's performance through improved electronics and pulsing and further uses the model to predict the theoretical benefits of a system designed for use with a Pretzel magnet.