First-principle Study Of The Electronic Structures Of The Bi₂Te₃-type Topological Insulators

Extensive attention has recently been focused on a newly discovered quantum state ofmatter, the so-called topological insulator （TI）. TIs are narrow-band gap semiconductors withband inversion driven by strong spin–orbit coupling （SOC） and have a nontrivial Z2topologyof the bulk band structure, but simultaneously possess topologically protected edge or surfacestates consisting of odd spin-filtered Dirac modes near time-reversal-invariant momenta andbeing robust against weak perturbations preserving time-reversal symmetry. Suchextraordinary states feature spin-momentum-locked helical Dirac fermions, making TIs tohold great promise in spintronics and fault-tolerant quantum computing.Recently, the studies on TIs in experiments encounter a major challenge for revealing theelectron transport properties of the topological surface state （TSS） by electron transportmeasurements due to the large concentration of bulk residual carriers originating from crystalintrinsic defects or environmental doping, and these mask the contribution of surface carriersin these materials. In this thesis, we focused our insight on this problem, and performed thefirst-principles calculations of two kinds of Bi₂Te₃-type TI materials to design and modifydevice performance. We obtained some meaningful results and expected that our results couldguide device frabication to improve the device performance to measure the topologicaltransport properties in related experiment. Our works include:1. Using the first-principles pseudopotential plane-wave methods based on densityfunctional theory，we studied the electronic properties and finite-size effect of thin films of arecently theoretically predicted three-dimensional TI Bi₂Te₂Se. Our results show that SOCplays an important role in determining the electronic properties of Bi₂Te₂Se and predict thatthe topological thickness limit of Bi₂Te₂Se is3QLs （Quintuple Layers）. It is worth noting thatthe newly discovered ternary tetradymite compound Bi₂Te₂Se could be fabricated with thinfilms with QL structures as a potential system without doping for studying spin helical Diractransport due to its structural perfection and naturally large bulk resistivity.2. The effect of S atom adsorption on the TSS of the TI Bi₂Te₃was studied usingpseudopotential first principles methods. Firstly, we investigated the topological phasetransition mechanism of the bulk material Bi₂Te₃and then demonstrated that the TSS derivedfrom topological insulator Bi₂Te₃（111） surface was immune to the nonmagnetic perturbations that preserve the time reversal symmetry. Our results show that Dirac-cone-like surface stateis still stable after sulfur adsorption on Bi₂Te₃（111） surface and the Fermi level relatived tothe Dirac point gradually moves up with the increasing of sulfur coverage. Our theoreticalresults put a way for tuning the Fermi level of the p-type doped TI Bi₂Te₃to the regime of thespin helical Dirac transport.