Physics
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Item In Situ Growth and Doping Studies of Topological Insulator Bismuth Selenide(2015) Hellerstedt, John Thery; Fuhrer, Michael; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The past decade has borne witness to the rapid development of a new field of theoretical and experimental condensed matter physics commonly referred to as "topological insulators". In a (experimentalist's) single sentence a topological insulator can be thought of as material that behaves like an empty metal box: the i-dimensional material (i = 2,3) is insulating, but conducting states exist at the (i-1)-dimensional boundaries. These edge or surface states possess non-trivial properties that have generated interest and excitement for their potential utility in solving practical problems in spin electronics as well as the creation of condensed matter systems for realizing and testing new physics. The most extensively studied materials systems with these properties suffer a major drawback in that the interior of the metal box is not "empty" (insulating) but instead filled with metal. The goal of this work has been to understand why the box is full instead of empty, and explore a particular pathway to making it empty. To address these outstanding questions in the literature pertaining to the persistent n-doping of topological insulator Bi2Se3, a custom apparatus was developed for combined epitaxial thin film growth with simultaneous, in situ measurement of transport characteristics (resistivity, Hall carrier density and mobility). Bi2Se3 films are found to be n-doped before exposure to ambient conditions and this doping appears to be interfacial in origin. This work and methodology was extended to study the efficacy of an amorphous MoO3 capping layer. MoO3 acts as an electron acceptor, p-doping the Bi2Se3 up to a |∆n| = 1x10^13 cm−2 change in carrier density. Complimentary X-ray photoemission spectroscopy (XPS) on bulk crystals showed that this was enough to put the Fermi level into the topological regime. Thin films were too highly n-doped initially to reach the topological regime, but the same magnitude change in doping was observed via the Hall effect. Finally, a Bi2Se3 film with a 100 nm capping layer of MoO3 was vented to ambient, and found to retain a stable doping for days of ambient exposure, demonstrating the effectiveness of MoO3 for passivation.Item Characterizing Unusual Behavior in Pristine Bi2Se3(2015) Syers, Paul James Howard; Paglione, Johnpierre; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Bismuth Selenide is a material of great interest to physicists as it is one of the first materials proven to be a strong topological insulator (TI). Despite its promise as an ideal TI, defects in the material that have proven difficult to elminate, keep the material's conductive states at its surface hidden by metallic behavior in the bulk. The work discussed in this thesis focuses primarily on techniques aimed at improving the material quality of pure Bi2Se3 and better understanding the effects of air exposure on both the surface and bulk of the material. The goal is to reliably produce samples in which signatures of the surface states can be seen in simple, bulk measurements. Changes in sample quality were achieved through the manipulation parameters such as Se flux and environmental pressure during the growth process. Through these techniques, the intrinsic carrier concentration of Bi2Se3 samples was lowered to the lowest levels ever reported. Samples showing nonmetallic behavior were also produced and investigated. Furthermore, it was discovered that samples of Bi2Se3 with low carrier concentrations showed strong linear magnetoresistance. The nature of this linear magnetoresistance, its angular dependence, and its evolution over time were also investigated. This thesis will discuss the results of these experiments as examples of how growth techniques influence sample quality, which in turn affects the properties of Bi2Se3.