First Principles Study Of The Effects Of Defects And Impurities In Topological Instulators

In recent years, with the research development of topological insulators (TIs), the traditional materials such as Bi2Se3class and Hg1-xCdxTe class have attracted great attention again. In experiments, defects and impurities exist and inevitably affect the physical, chemical, mechanical and electrical properities of the materials. For instance, the cation vacancy defects in Hg1-xCdxTe make the materials p-type. And Bi2Se3always behaves as an n-type semiconductor due to its intrinsic defects such as antisites or vacancies introduced in growth process. It is interesting to note that the p-to-n or n-to-p type conversion should be happen by introducing extrinsic doping. In addition, monolayer or few layers of transition metal dichalcogenides MS2(M=Mo, W) were successfully synthesized. Finding proper modulation ways to tune their electronic, mechanical, magnetic properties is catching the attention of the scientific community.In this dissertation, by using the first-principles method within the framework of density functional theory, we study the effects of defects and impurities in Bi2Se3, Hg0.75Cd0.25Te and MS2(M=Mo, W). The research contents are as follows:1. The intrinsic defects VSe1，VBi，SeBi，BiSe and extrinsic impurities CdBi，CaBi in Bi2Se3are investigated. The results indicate that, under Se-rich and Bi-rich growth conditions, SeBi and VSe1are the most stable native defects respectively. They are the main source of n-type doping. Under Se-rich growth conditions, CdBi and CaBi can effectively compensate the n-type native defects, VSe1and SeBi. And such compensation effects become weaker gradually with the growth conditions changing from Se-rich to Bi-rich. Therefore, a p-to-n or n-to-p type conversion for Bi2Se3can be achieved by changing the growth conditions. Simultaneously, the Fermi level can be effectively tuned.2. The multi-hydrogen–mercury vacancy complex impurities (nH–VHg, n=1,2,3,4) and the multi–hydrogen-telluride antisite complex impurities (nH-TeHg, n=1,2) in Hg0.75Cd0.25Te are investigated. The results indicate that, the1H–VHg can effectively compensate the deep acceptor level introduced by VHg. As the hydrogen concentration increases,2H–VHg will form and the p-type dopant nature of VHg is neutralized completely. The formation of3H–VHg and4H–VHg makes p-type VHg change to n-type. For n-type telluride antisite (TeHg), the formation of1H–TeHg complex is favorable and it can partially compensate the donor effects introduced by TeHg. The formation of2H–TeHg complex is relatively difficult. Moreover, it cannot neutralize all the donor effects introduced by Te antisite in Hg0.75Cd0.25Te because it is also a double donor. Our results and conclusion can well explain the experimental p-to-n type conversion phenomenon.3. The MS2(M=Mo，W) monolayer with double-S-vacancy-doped (DSVD) is studied under different strains. The results indicate that a reversible phase transition of defect-reconstruction from a nonmagnetic semiconducting triangle to a magnetic half-metallic circle in DSVD-MS2(M=Mo，W) can be induced by strain. The needed critical strain for the transition in DSVD-MoS2(WS2) is about12%(11%) which is experimentally touchable. Under proper compressive stress, the circle defect-reconstruction will change back to the triangular one. The calculations of electronic band structures under different strains show that the circle defect-reconstruction of Cir-3Mo-12S (Cir-3W-12S) possesses strain-tunable electronic and magnetic properties, including the half-metallic states with spin polarization of100%, which may be very useful for applications in spin-electronics.