MoS_(2)中S原子空位形成的非绝热动力学研究

Non-adiabatic dynamic study of S vacancy formation in MoS_(2)

缺陷是半导体领域中最核心的问题.采用含时密度泛函方法,模拟了S原子脱离MoS_(2)晶格形成空位缺陷过程中的电子动力学行为,发现该过程中存在显著的非绝热效应.非绝热效应导致S原子需要消耗更多能量以脱离晶格形成空位缺陷.随着S原子的初始动能增大,其脱离晶格形成空位的能量势垒也持续增大,并且在初始动能达到22 eV附近时发生了阶跃式的增长.这是由朗道-齐纳电子跃迁和能级间库仑作用共同导致的.非绝热效应还改变了脱离晶格的S原子上电荷的轨道分布,以及晶格中缺陷附近的电荷分布.此外,还发现该过程中自旋轨道耦合十分重要,必须被考虑.本文阐明了MoS_(2)中S原子空位的形成机制,尤其是电子非绝热动力学的重要作用,为进一步研究缺陷对材料物理性质的调控提供了理论基础.

Defect is one of the central issues in semiconductors.MoS_(2) is sensitive to irradiation and can be damaged by electron beams,accompanied with the generation of sulfur vacancies.However,the dynamics for the defect generation process is still unclear.In this work,we employ the time-dependent density functional theory to simulate the process of a sulfur atom sputtering from the MoS_(2),producing a sulfur vacancy defect in the lattice.We find that there exists a strong non-adiabatic effect in the process.During the formation of the sulfur vacancy,there exist electron transitions which can be described by the Landau-Zener model.As the sulfur atom leaves away from the lattice,two energy levels from the valence bands rise up and one energy level from the conduction band falls down.When the spin-orbit coupling(SOC)is not considered,those levels do not couple with each other.However,when the SOC is taken into account,electrons can transit between those levels.The transition probability is related to the kinetic energy of the sputtered sulfur atom.As the kinetic energy of the sulfur atom increases,the non-adiabatic electron transitions are enhanced.The evolution of the energy levels is also strongly dependent on the kinetic energy of the sputtered sulfur atom,which is induced by the non-adiabatic electron transition.It is worth noting that the SOC plays a key role in sputtering sulfur atoms,although the system produces no magnetic moments in the whole process.The non-adiabatic effect enhances the energy barrier of the sulfur sputtering.As the initial kinetic energy of the sputtered sulfur atom increases,the energy barrier increases,and exhibits a jump around the initial kinetic energy of~22 eV,which can be explained by the non-adiabatic electron occupation and the Coulomb repulsion.Beside the energy barrier,the non-adiabatic effect also modifies the charge distribution.When the kinetic energy of the sputtered sulfur atom is relatively low,more electrons occupy the pz level;when the kinetic energy is relatively high,more electrons occupy the pxy level instead.The sputtered sulfur atom always carries a bit more electrons,leaving holes around the vacancy defect.Our work reveals the dynamics of the sulfur sputtering and vacancy formation in MoS_(2),particularly the non-adiabatic effect in the process.It builds the theoretical foundation for defect engineering.