Tuning The Electronic Structure Of The Topological Insulator Bi₂Te₃ Thin Films With Dopants

In recent years, Topological Insulators that are a new class of materials different from an ordinary insulator and metal are discovered. The topological surface state possess unique physical properties that may realize a number of applications. Widespread attention has been focused on Tls and there are a lot of studies on Tls.Recently,many materials have been predicted and confirmed to be topological Insulators. Because of crystal intrinsic defects in Tls prepared in experiments, the transport properties of the suface state are easy to be masked and difficult to be observed in experiments. We focused our attention on this problem, and used the first-principles calculations based on DFT for designing and modifying t he electronic structure of Bi2 Te3. In the process of design and modification, we used the surface control method based on non- magnetic atoms doping. Some meaningful results were obtained. Our results could guide the preparation of high quality Bi2 Te3 in experiments and the studies on topological transport properties.1 First of all,we studied the electronic structure of the 5QL-(Bi1-x Sbx)2Te3 film with x=0, 20%,80%,respectively. By adjusting the concentration ratio x, we could tune the position of Fermi level and DP, without changing the topological properties of the films. With increasing x, the position of DP is relative to BVB on the rise and a crossover from n-type to p-type materials. When the Sb concentration ratio is increased to x=80%, both DP and Fermi level located within the gap. Now the film is an ideal Tl with a truly insulating bulk and the DP is very close to the Fermi level. Our results were consistent with the observed results in experiments.2 We used the surface control method based on non- magnetic atoms doping for tuning the electronic structure of the 5QL- Bi2Te3 film. By changing the chemical potential of Bi or Te, we could detach the DP from the bulk valence bands. Firstly, we replaced the top and lower end layers of Te in the 5QL- Bi2Te3 film with Se, S, O, respectively. Because of O atom with the largest electronegativity and the smallest radius, we found that only O atom can lead to an isolated Dirac cone in the gap with the Fermi level crossing the DP. Now the film is an ideal Tl. Secondly, we replaced the top and lower end layers of Bi in the 5QL- Bi2Te3 film with Tl, In, Ga, Al, respectively. We found that all of them can separate the Dirac point from the bulk valence bands without changing the topological properties of the films. We also compared the charge distributions of these systems and found that these systems which can separate the Dirac point from the bulk valence the charge is mainly distribute at the Te1 atoms facing the van der Waals layer with almost no charge distribution in the middle of the QLs, making a true insulator region with the QLs. Accordingly, the surface Dirac cone state is confined with the van der Waals layer and the Dirac cone is separate from the bulk states and appears inside the bulk band gap region.