Salehi, Maryam. Unveiling the topological quantum effects in defect-engineered topological insulator films. Retrieved from https://doi.org/doi:10.7282/t3-6w75-sk39
DescriptionTopological insulators (TIs) are a class of quantum materials where the bulk is insulating, while the surfaces host in-gap metallic states. The topological surface states (TSSs) are described by the Dirac Hamiltonian whose spin is locked to momentum, and as a result their metallic behavior is protected against crystal imperfections. These features of TSS make TIs a unique test bed for exploration of various new physics and applications. However, almost all the first-generation TIs suffered from a high level of defect densities which pushed the Fermi level away from the bulk energy gap (where the Dirac TSS lies) into the conduction band, thereby obscuring the electronic signature of the TSS.
This dissertation seeks to shed light on the physical origins of defects in pnictogen-chalcogenide TIs, as well as the various defect and interface engineering schemes that have been exploited to effectively suppress these defects. Here, we explain how growing TI films on virtually-grown structurally/chemically-compatible substrates led to films with 10 to 100 times lower defect densities compared to the first-generation films, and with the Fermi level being only few tens of meV above the Dirac point. This is followed by a discussion about the important role of an effective capping layer in stabilizing the properties of low-carrier-density TI films. We further discuss how employing such optimally-designed buffer and capping layers along with proper compensation-doping resolved the previously challenging task of carrier-type tunability in TI films. We then expound on how the ultralow-carrier-density Bi2Se3 and Sb2Te3 films enabled the observation of a series of previously unseen topological quantum effects. Additionally, we present how the ultralow-carrier-density Sb2Te3 films allowed us to reach an extreme quantum limit and explore the previously inaccessible zeroth Landau level. Finally, we conclude by providing a summary of results as well as an outlook towards the future of TI research.