First-Principle Study Of B,C And N Doping Topological Insulator Bi₂Se₃

The discovered Topological insulator?TI?with a bulk energy gap and gapless metallic surface state is different from metal and insulator.The band structures are spin split and protected by the time-reversal symmetry?resistant to defects and nonmagnetic impurities?.Bi₂Se₃ is the ideal three dimensional?3D?topological insulator materials in bulk,with controllable stoichiometric ratio,easy synthesize and low toxicity.Its Fermi energy is located in the energy gap and there is one single Dirac cone at Dirac point in Brillouin zone.Particularly,it is ideal device for potential spintronic and technological application because its bulk band gap is much larger than the energy scale of room temperature.The time-reversal symmetry can be destroyed by the complex interaction between the topological order and the ferromagnetic order.The energy gap appears at the Dirac point with the magnetism introduced.TI is expected to realize the quantized anomalous Hall Effect?QAHE?,magneto-optical effect,magnetic monopole and Majorana fermion,and promote the development of new devices.Thus,in this paper,the geometrical structure,electronic structure and magnetic properties of the non-magnetic elements X?B?C?N?doping at Bi?Se1?Se2 and gap sites of Bi₂Se₃ are calculated using first-principles pseudo-potential plane wave method based on density functional theory?DFT?.The calculated results are as follows:?1?The van der Waals?vdw?density-function D2 correction is quite necessary for structure optimizing.All structures and atoms are relaxed.The optimized results show that the lattice parameters of the structures and the distance between atoms with X doping become small,the bond length becomes short,and the interaction is enhanced.The formation energy corresponds to the bond length.The structure is stable with small formation energy.The structures of B and C doping at gap site is the most stable and likely to be formed with the minimum formation energy.The formation energy of N atom doping at Se1 site is lowest.The formation energy of X atoms doping at Bi site is the largest,and the possibility of replacing Bi atoms is the least.?2?Due to the spin-orbit coupling?SOC?,the electronic structure of Bi₂Se₃ can be accurately described,especially the energy band near the Fermi level.The energy band of X-doping is consistent with Bi₂Se₃.The valence band is mainly from Se-4p states and the conduction band is mainly from Bi-6p states.?3?The electronic structures of B atom doping at Bi site and Se2 site,C atom doping at all sites,N atom doping at Se2 site and gap site are changed due to the coupling of impurity state and time-reversal symmetry state.The impurity energy band appears near the Fermi level,and the band gap is introduced at the Dirac point.?4?It is similar to the transition metals,non-magnetic atom X doping Bi₂Se₃ can induce spin-polarized ferromagnetic states and destroy the time inversion symmetry.Further research reveals that the magnetic moments are related to the covalent bond length between X and surrounding atoms.C atoms doping at all sites and all X atoms doping at Se2 site can induce magnetic moments.In order to have a stable magnetic state,the 2p state of the X atom must be sufficiently localized,which form impurity energy bands and open band gaps at the Dirac point.This provides a feasible method for X-atom doping to manipulate the topological properties of Bi₂Se₃ and promote the application of topological insulators in spintronic devices.