Epitaxial growth of topological materials

Recently, in 2007, a new electronic phase was discovered – termed as Topological Insulator (TI) – a bulk insulator that has conducting states bound to its surface. An apt analogy would be a ceramic block coated with a nanometer thick layer of metallic paint, so that electrical conduction occurs only on the surface, except in this case, the material is the same throughout. The understanding of topological insulators is based on the previously thought to be complete band theory, but taking into account the topological effects. The term topological implies the presence of certain bulk invariants that help differentiate between different systems having the same symmetry. The work presented here concerns the understanding of the transport properties of these electronic states that emerge only on the surfaces of this very special class of materials. The spin of electrons in the heavy metals of the TIs is linked to their intrinsic angular momentum; this spin-orbit coupling (SOC) leads to twisting of the electronic states in certain regions of momentum space, establishing a topological order. The term topology comes from mathematics, and deals with quantities remaining invariant under continuous modifications – it is the topology of the electronic band structures originating from SOC that protects these metallic surface states against disorders. The SOC also coerces the motion of spin-up electrons in one direction and spin-down electrons in the other – a distinguishing feature that forbids complete backscattering and localization i.e. the electrons can move freely with little or no resistance in their preferred direction. This ‘spin-momentum locking’ makes these materials interesting for future spintronic devices that require generation, control and detection of spins as information carriers. The dissertation begins by reviewing the developing field of TIs which inspired this work, followed by an introduction to the many aspects involved in the growth of atomically precise thin TI films, mainly Bismuth Selenide (Bi2Se3). The details of electrical transport are mentioned next; the experimental techniques thus introduced are used to examine the interplay of bulk and surface contributions to the transport in thin grown Bi2Se3 films. Growth of thin films using molecular beam epitaxy (MBE) with atomic precision requires precise control of each flux, thus, we first discuss the flux stability in harsh oxidation conditions and derive the optimal configuration that helps grow stoichiometric thin-films. Following this, we discuss the growth of Bi2Se3 films on various substrates and study how this affects the electronic transport. The final work in the dissertation involves the transfer of these grown thin films to other substrates, including plastic, so as to provide a platform for future device applications.Ph.D.Includes bibliographical referencesby Namrata Bansa