| Summary: | 博士 === 國立清華大學 === 物理學系 === 106 === Topological insulators (TI), a new state of quantum matter, have received unparalleled attentions in recent years in the scientific community due to exotic physical properties such as a time reversal symmetry protected surface state and potential applications in dissipationless electronics and quantum computing.
In this dissertation, the study was focused on properties of topological insulators grown by molecular beam epitaxy. High quality Bi2Se3 thin films were obtained via van der Waals epitaxy in various substrates. Sharp surface state and Dirac cone were observed in our ARPES spectra of Bi2Se3 on various substrates. However, the Fermi level of Bi2Se3 is always located in bulk conduction band due to intrinsic defects. This indicates the bulk bands also contribute to the transport.
Besides, we report a crystalline Se capping layer obtained by a new growth method. Upon extended exposure to UHV or humid air, we show by x-ray photoemission spectroscopy (XPS) that the stability and resistance to oxidation of crystalline Se capping layers are superior to that of amorphous Se capping layer, which has been commonly used by current communities. Furthermore, time-dependent Hall measurements showed crystalline Se capping layers had a much stronger ability to sustain the intrinsic transport properties of Bi2Se3.
In order to let the transport dominated by the surface state, we look for an elemental material which is also a TI. Thin film Sn provides several advantages: With its elemental nature, Sn is free from the stoichiometry issue and the related defect problems in compound TI families. It also offers potentially rich structures with different band diagram spanning from 3-D TI to 2-D TI. We have successfully grown monolayer Sn film with stanene like structure on the Bi2Te3 substrate. The electronic structures of epitaxial stanene films were determined by ARPES. The hole bands of stanene were observed at the Garmma point with additional 2D states which come from the reaction between stanene and Bi2Te3. Furthermore, we investigate the bonding configuration during the growth of Stanene by XPS and pointed out a possible bonding configuration and the reason why the Dirac like the state is not observed at the K point in the ARPES spectrum.
High quality Alpha-Sn films were attained on InSb(001) by MBE. The XRD scans show only Alpha phase Sn was grown on InSb(001) with clear Pendellösung fringes. Clear bulk bands (Γ_8^+, Γ_7^+, Γ_7^-) and two TSSs were observed in our ARPES spectrum of 30BL Alpha-Sn on InSb(001). Besides, the Fermi level of Alpha-Sn can be tuned by varying the growth rate of Alpha-Sn. The Fermi level of low growth rate sample is located in the top of Dirac cone without overlapping with bulk conduction band. This indicates the band gap is larger than 0.1eV. These results suggest Alpha-Sn is an ideal topological insulator, whose transport properties dominated by the topological surface state.
|
|---|
