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The identification and characterization of molecular biomarkers using mass spectrometry on an orbiting or fly-by spacecraft is one of the preferred analytical techniques in the search for life beyond the Earth. However, analysis is complicated by unwanted molecular dissociation occurring when sampled native molecules impact the instrument at high velocity. The mechanisms of chemical changes produced in high velocity impacts have been studied experimentally in some cases; however, there are significant experimental limitations to these techniques. Here I present the design, construction, and testing of a new experimental technique to produce high velocity molecular and microparticle collisions under a controlled lab setting using a high-speed spinning rotor. Chapter 1 of this manuscript gives a scientific review of the astrobiological importance of this project for future and current space missions as well as describing previous techniques used to produced hypervelocity impacts and their limitations. Chapter 2 presents the design, construction, calibration, and preliminary experiments of the new technique involving the high-speed rotor. Chapter 3 describes the fabrication of a molecular beam system from the ground up to be coupled with the high-speed rotor. Chapter 4, describes future project directions and presents future experiments using the rotor as a stand-alone instrument. Lastly, the appendix contains the standard operation procedures and design notes regarding the operation of these two instruments.

Wide band gap semiconductors such as MgxZn1-xO represent an excellent choice for making optical photodetectors and emitters operating in the UV spectral region. High crystal and optical quality MgxZn1-xO thin films were grown epitaxially on c-plane sapphire substrates by plasma-assisted Molecular Beam Epitaxy. ZnO thin films with high crystalline quality, low defect and dislocation densities, and sub-nanometer surface roughness were achieved by applying a low temperature nucleation layer. The critical growth conditions were discussed to obtain a high quality film: the sequence of Zn and O sources for initial growth of nucleation layer, growth temperatures for both ZnO nucleation and growth layers, and Zn/O ratio. By tuning Mg/Zn flux ratio, wurtzite MgxZn1-xO thin films with Mg composition as high as x=0.46 were obtained without phase segregation. The steep optical absorption edges were shown with a cut-off wavelength as short as 278nm, indicating of suitability of such material for solar blind photo detectors. Consequently, Metal-Semiconductor-Metal photoconductive and Schottky barrier devices with interdigital electrode geometry and active surface area of 1 mm2 were fabricated and characterized. Photoconductor based on showed ~100 A/W peak responsivity at wavelength of ~260nm. ZnO homoepitaxial growth was also demonstrated which has the potential to achieve very low dislocation densities and high efficiency LEDs. Two types of Zn-polar ZnO substrates were chosen in this study: one with 0.5° miscut angle toward the [1-100] direction and the other iv without any miscut angle. We have demonstrated high quality films on both substrates with a low growth temperature (610°C) compared to most of other reported work on homoepitaxial growth. An atomically flat surface with one or two monolayer step height along the [0001] direction was achieved. By detail discussions about several impact factors for the epitaxial films, ZnO films with high crystallinity verified by XRD in different crystal orientations, high PL lifetime (~0.35 ns), and not obvious threading dislocations were achieved. Due to the difficulty of conventional p-type doping with p dopant, we have explored the possibility of p-type doping with the assistance of other novel method, i.e. polarization induced effect. The idea is the sheet layer of two dimensional hole gases (2DHG) caused by the wurtzite structure’s intrinsic polarization effect can be expanded to three dimension hole distribution by growing a MgZnO layer with a Mg concentration gradient. By simulation of LED structure with gradient MgZnO structure, the polarization effect was found not intense as that for III-nitrides because the difference of spontaneous polarization between ZnO and MgO is smaller than that of GaN and AlN, and the piezoelectric polarization effect may even cancel the spontaneous polarization induced effect. We have grown the linear gradient MgZnO structure with Mg composition grading from 0% to 43%, confirmed by SIMS. Hall measurement did not show any p-type conductivity, which further indicates MgZnO’s weak polarization doping effect. However, the gradient MgZnO layer could act as an electron blocking layer without blocking holes injected from p layer, which is useful for high efficiency light emitters.

In this work, a body of knowledge is presented which pertains to the growth, characterization and exploitation of high quality, novel II-IV oxide epitaxial films and structures grown by plasma-assisted molecular beam epitaxy. The two compounds of primary interest within this research are the ternary films NixMg1-xO and ZnxMg1-xO and the investigation focuses predominantly on the realization, assessment and implementation of these two oxides as optoelectronic materials. The functioning hypothesis for this largely experimental effort has been that these cubic ternary oxides can be exploited - and possibly even juxtaposed - to realize novel wide band gap optoelectronic technologies. The results of the research conducted presented herein overwhelmingly support this hypothesis in that they confirm the possibility to grow these films with sufficient quality by this technique, as conjectured. NixMg1-xO films with varying Nickel concentrations ranging from x = 0 to x = 1 have been grown on lattice matched MgO substrates (lattice mismatch ε < 0.01) and characterized structurally, morphologically, optically and electrically. Similarly, cubic ZnxMg1-xO films with Zinc concentrations ranging from x = 0 to x ≈ 0.53, as limited by phase segregation, have also been grown and characterized. Photoconductive devices have been designed and fabricated from these films and characterized. Successfully engineered films in both categories exhibit the desired deep ultraviolet photoresponse and therefore verify the hypothesis. While the culminating work of interest here focuses on the two compounds discussed above, the investigation has also involved the characterization or exploitation of related films including hexagonal phase ZnxMg1-xO, ZnO, CdxZn1-xO and hybrid structures based on these compounds used in conjunction with GaN. These works were critical precursors to the growth of cubic oxides, however, and are closely relevant. Viewed in its entirety, this document can therefore be considered a multifaceted interrogation of several novel oxide compounds and structures, both cubic and wurtzite in structure. The conclusions of the research can be stated succinctly as a quantifiably successful effort to validate the use of these compounds and structures for wide bandgap optoelectronic technologies.

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Thesis advisor: Ilija Zeljkovic / The splendid world of quantum materials is being unveiled in modern condensed matter physics, thanks to the advanced material synthesis methods, refined experimental probing techniques and deeper theoretical understanding. Unconventional superconductivity and topological phenomena are two of the main themes in this realm. Many outstanding problems are waiting to be solved and there is also a great potential in future technological applications. Among many routes of studying the quantum materials, creating thin film structures provides a special opportunity to learn the physical properties in low dimensions, to explore the effect of substrate and strain and to make novel electronic devices.In this thesis, I will present successful molecular beam epitaxy thin film synthesis of: (1) unconventional superconductor FeSe, (2) topological insulator Bi2Se3 doped with magnetic Fe atoms and (3) kagome structure magnets FeSn and Fe3Sn2. For (1), I will describe the finding of a dislocation network, its impact on the spatially-modulated strain field and its interesting interplay with the spontaneous symmetry-broken nematic phase. This is a new finding in the FeSe/SrTiO3 heterostructure and also provides fresh insights in the understandings of nematicity. For (2), I will show how we cross-check the doping ratio using different characterization techniques. Our observation indicates the possible formation of Fe clusters or impurity phases and sets the foundation for future synthesis of similar structures. For (3), I will demonstrate the novel selective synthesis of FexSny thin films. A plethora of spectral features were found in Fe3Sn2, implying a link with the Weyl physics. The FexSny thin films can potentially be a platform for the exploration of correlated, topological quantum phases in low dimensions. / Thesis (PhD) — Boston College, 2022. / Submitted to: Boston College. Graduate School of Arts and Sciences. / Discipline: Physics.

A molecular beam apparatus has been designed and constructed for the purpose of studying gas phase interactions between ions and molecules. A neutralization-reionization technique has been developed which permits the lifetimes of some short lived molecules to be estimated. In this way the existence of He₂, HeH and HeD molecules has been demonstrated but similar experiments have produced no evidence for the existence of H₃, D₃ or D₂H molecules with the same order of lifetimes. A collisional dissociation technique has been devised which permits analysis of molecular ions in beams and demonstrated the existence of He₃⁺ ions in low temperature helium discharges. / Thesis / Doctor of Philosophy (PhD)

The aggressive reduction of feature size in silicon (Si)-based complimentary metal-oxide-semiconductor (CMOS) technology has resulted in an exponential increase in computing power. Stemming from increases in device density and substantial progress in materials science and transistor design, the integrated circuit has seen continual performance improvements and simultaneous reductions in operating power (VDD). Nevertheless, existing Si-based metal-oxide-semiconductor field-effect transistors (MOSFETs) are rapidly approaching the physical limits of their scaling potential. New material innovations, such as binary group IV or ternary III-V compound semiconductors, and novel device architectures, such as the tunnel field-effect transistor (TFET), are projected to continue transistor miniaturization beyond the Si CMOS era. Unlike conventional MOSFET technology, TFETs operate on the band-to-band tunneling injection of carriers from source to channel, thereby resulting in steep switching characteristics. Furthermore, narrow bandgap semiconductors, such as germanium (Ge) and InxGa1-xAs, enhance the ON-state current and improve the switching behavior of TFET devices, thus making these materials attractive candidates for further study. Moreover, epitaxial growth of Ge on InxGa1-xAs results in tensile stress (ε) within the Ge thin-film, thereby giving device engineers the ability to tune its material properties (e.g., mobility, bandgap) via strain engineering and in so doing enhance device performance. For these reasons, this research systematically investigates the material, optical, electronic transport, and heterointerfacial properties of ε-Ge/InxGa1-xAs heterostructures grown on GaAs and Si substrates. Additionally, the influence of strain on MOS interfaces with Ge is examined, with specific application toward low-defect density ε-Ge MOS device design. Finally, vertical ε-Ge/InxGa1-xAs tunneling junctions are fabricated and characterized for the first time, demonstrating their viability for the continued development of next-generation low-power nanoelectronic devices utilizing the Ge/InxGa1-xAs material system. / Doctor of Philosophy / The aggressive scaling of transistor size in silicon-based complimentary metal-oxide-semiconductor technology has resulted in an exponential increase in integrated circuit (IC) computing power. Simultaneously, advances in materials science, transistor design, IC architecture, and microelectronics fabrication technologies have resulted in reduced IC operating power requirements. As a consequence, state-of-the-art microelectronic devices have computational capabilities exceeding those of the earliest super computers at a fraction of the demand in energy. Moreover, the low-cost, high-volume manufacturing of these microelectronic devices has resulted in their nigh-ubiquitous proliferation throughout all aspects of modern life. From social engagement to supply chain logistics, a vast web of interconnected microelectronic devices (i.e., the "Internet of Things") forms the information technology bedrock upon which 21st century society has been built. Hence, as progress in microelectronics and related fields continues to evolve, so too does their impact on an increasingly dependent world. Moore's Law, or the doubling of IC transistor density every two years, is the colloquialism used to describe the rapid advancement of the microelectronics industry over the past five decades. As mentioned earlier, parallel improvements in semiconductor technologies have spearheaded great technological change. Nevertheless, Moore's Law is rapidly approaching the physical limits of transistor scaling. Consequently, in order to continue improving IC (and therefore microelectronic device) performance, new innovations in materials and fabrication science, and transistor and IC designs are required. To that end, this research systematically investigates the material, optical, and electrical properties of novel semiconductor material systems combining elemental (e.g., Germanium) and compound (e.g., Gallium Arsenide) semiconductors. Additionally, alternative transistor design concepts are explored that leverage the unique properties of the aforementioned materials, with specific application to low-power microelectronics. Therefore, through a holistic approach towards semiconductor materials, devices, and circuit co-design, this work demonstrates, for the first time, novel transistor architectures suitable for the continued development of next-generation low-power, high-performance microelectronic devices.

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