In situ growth and doping studies of topological insulator bismuth selenide

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 MoO3, 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). MoO3 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 MoO3 up to a |Delta n| = 1x1013 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 MoO3 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 MoO 3 for passivation.