MBE Growth and Plasmonic Study of 2–3D V2-VI3 Topological Insulators

• Main Author: Nasir, Saadia
• Format: Dissertation
• Language: English
• Published: ProQuest Dissertations & Theses 01-01-2024
• Subjects: Physics; Plasma physics; Quantum physics
• ISBN: 9798381722246

The discovery of topological insulators has revolutionized many fields of condensed matter physics. The bulk band gap of these non-trivial insulators is crossed by the linearly dispersive surface state. The Dirac electrons in the surface states are two-dimensional and massless, exhibit strong spin-orbit coupling, and obey the time-reversal symmetry. The conductive surface states arising due to the strong spin-orbit coupling of electrons are robust against any non-magnetic disorder in the system. It makes the topological insulators a promising candidate in spintronics and high-speed signal processing devices. The V2-VI3 3D topological insulators have gained massive popularity in recent decades due to the simple electronic structure of their surface states, consisting of a single Dirac cone. When the thickness of these topological insulators is reduced below the critical thickness of around 6 nm, the surface states become gapped at the Dirac point. The resulting 2D system is predicted to act as the quantum spin Hall insulator (QSHI) state with insulating surfaces but conductive edge states.The experimentally grown binary thin films of the V2-VI3 topological insulator get doped with the inevitable n-type (Bi2Se3, Bi2Te3) or p-type (Sb2Te3) intrinsic bulk charge carriers, depending on the type of defect. This, in turn, pushes the Fermi level away from the Dirac point into the conduction or valence band. It makes it challenging to investigate and exploit the physical properties of the exotic surface states. To tune the Fermi level into the bulk band gap, I have explored the growth and transport properties of ternary and quaternary alloys of these binary topological insulator materials.Furthermore, the resonance frequencies of the plasmons excited on the surface states of these topological insulators fall in the terahertz (THz) gap of the light spectrum. Although the resonance frequencies of many molecules lie in the THz frequency range, the THz industry didn’t flourish much in the past because of the difficulty in its generation and detection using conventional semiconductor electronic devices. The investigation and better understanding of the THz plasmons and how they interact with themselves or other excitations in the system will help in designing devices that can work in the THz frequency regime. There has been sufficient insight into the vertical coupling of Dirac plasmons excited on both interfaces of 3D-TI thin film. I have explored the coupling of localized plasmons excited on the stripe arrays of 3D-Bi2Se3 in the same plane. The results showed that the localized Dirac plasmon polariton displays the dipole-dipole type in-plane coupling. I also wanted to explore the effect of TI film thickness on the resonance frequency of excited plasmons when the thin films transition from the 3D topological insulators to the 2D system, which is predicted to be the non-trivial QSHI above 3 nm. For this, I have developed a recipe to grow well-coalesced wafer scale 4nm ultra-thin films and demonstrated an abrupt shift in the Dirac plasmon resonance frequency when the thickness is reduced to the critical point.Overall, this dissertation improved our knowledge of the growth of V2-VI3 compounds and the understanding of THz Dirac plasmons that can be used for designing TI-based metasurfaces.