| Description |
The discovery of three-dimensional (3D) topological insulators (TIs) has offered the unprecedented candidates for the study of topological phases in quantum states of matter. Over the past decade, the research on 3D TI devices has been focused on the isolation of surface transport from the bulk and the elimination of bulk carriers for the convenient of probing their surface states. This thesis research continues the trend of development in 3D TIs and tackles their shortcomings from two aspects, namely the crystal growth and device structure, to achieve high-quality TI based devices. With optimization in growth conditions, Bi2-xSbxTe3-ySey (BSTS) 3D TI single crystals with significant improvement in surface mobility were grown. These together foster the development of surface integer quantum Hall effect (QHE) at low magnetic field. Next, to realize more advanced TI devices, we introduce van der Waals (vdW) heterostructures in the form of TI/insulator/graphite (Gr) to effectively control chemical potential of the topological surface states (TSS). The hBN/Gr gating in the QH regime shows improved quantization of TSS by suppression of magnetoconductivity of massless Dirac fermions. The TI vdW heterostructures also provide the local gates necessarily for quantum capacitance measurements, which allow a quantitative evaluation of the surface states' LL energies in 3D TI. With quantum capacitance studies, the top and bottom TSS can be separated and individually probed by applying excitation voltages to the gates coupled capacitively to different surfaces. Furthermore, in the studies of variable thickness 3D TI devices towards iv 2D thin limit, we establish a tunable capacitive coupling between the top and bottom TSS and study the effect of this coupling on QH plateaus and LL fan diagram via dual-gate control. We observe a splitting of N= 0 LL in magnetic field for the thin devices indicates intersurface hybridization possibly beyond single-particle effects. The studies are extended to the intersurface hybridization regime where a thermally-activated transport gap clearly resolved at the Dirac point for film thickness less than 10 nm. The layer-dependent hybridization gaps scale up exponentially with decreasing layer thickness. The perpendicular electric field and magnetic field responses to the hybridization gap are further analyzed and discussed. These works show the promise of the vdW platform in creating advanced high-quality and tunability 3D TI-based devices. |
