Preparation And Properties Of Topological Insulators And Topological Crystalline Insulators

Topological insulators（TIs） and topological crystalline insulators（TCIs） are a new state of quantum matter with a band gap in bulk and conductiong spin-polarized surface states. The surface states insensitive to the structure of the surface are determined by the topology of the bulk states. The exotic properties of TIs and TCIs lead to promising application prospect in spintronics and topological quantum computation. Growing high-quality TI and TCI materials is very crucial for the fabrication of the electronic devices and the research on the nature of the new quantum matter.In this thesis, we have investigated the growth dynamics and electronic band structure of high-quality topological crystalline insulator films of NaCl-type SnSe by UHV molecular beam epitaxy（MBE）-scanning tunneling microscopy（STM）-angle resolved photoemission spectroscopy（ARPES） combo system. And the topological phase transition and the thermoelectric properties in TI materials also have been investigated. The main results are summarized as follows:（1） High-quality NaCl-type SnSe films have been epitaxially grown on Bi₂Se₃ films. The growth dynamics conditions have been identified by the real time reflection high energy electron diffraction（RHEED） and STM. By conbining the ARPES results and the first-principles calculations, we, for the first time, have identified NaCl-type SnSe as a new topological crystalline insultor.（2） By adopting MBE, we have grown high-quality In doped Bi₂Se₃ films（（Bi1-xInx）2Se3, 0 ≤ x ≤ 1） on graphitized single-layer Graphene on SiC substrate. ARPES measurements have been carried out on variable thickness（Bi1-xInx）2Se3（0≤x≤1） films. We have found that there is a topological phase transition when In concentration is between 5% and 9%.（Bi1-xInx）2Se3 film is a topological non-trivial insulator with x<5% and a topological trivial insulator with x>9%. By analyzing the ARPES results, we, for the first time, have verified that the bulk gap decreases and the penetration depth of the Dirac surface states increases with increasing In concentration, which will result in a new gap opening in the Dirac cone before the topological phase transition happens. And we have found a strong modulated band structure in the van der Waals heterostructure between α-In2Se3（thickness<3QL） and single-layer Graphene. We speculate that this modulated band structure is caused by the superlattice Dirac points. Theoretical simulations are needed to prove our speculation.（3） High-quality Bi₂Se₃ films have been grown on high resistive STO（111） surface by using MBE. Thickness dependent thermoelectric transport measurements have been carried out. We have found that the value of the thermopower and power factor of the Bi₂Se₃ films increases with increasing the thickness of the Bi₂Se₃ films. By combining the theoretical simulations, we hope our results can pave the way to enhance the thermoelectric properties of the TIs, such as Bi₂Se₃ and Sb2Te3.