First Principles Study On The Intrinsic Conductivity Of Novel Two-dimensional Semiconductor Materials

In this paper,the fundamental properties of intrinsic defects in black phosphorene,black arsenene and single-layer Ge S are systematically studied by the"transfer to real state"model（TRSM）based on the first principles calculation of density functional theory.Our calculation reveals that（i）the physical origin of the intrinsic p-type conductivity of black phosphorene,black arsenene and monolayer Ge S;（ii）the physical origin of the shallow acceptor transition energy levels of the intrinsic vacancy defects in black phosphorene,black arsenene and monolayer Ge S.Some valuable research results have been obtained,and the core conclusions are as follows:（1）In this paper,the formation energies and defect transition energy levels of vacancy and interstitial defects in black phosphorene and black arsenene are calculated through the"transfer real state"model,resulting that the natural defects in black phosphorene and black arsenene are acceptor defects,and the vacancy defects in the two materials have the characteristics of low formation energies and shallow defect transition energy levels.These properties of defects make black phosphorene and black arsenene both natural p-type semiconductors,and it is very easy to obtain considerable p-type conductivity samples by doping.Due to the strong compensation effect of intrinsic p-type defects,however,it is difficult to fabricate n-type samples by doping.（2）The symmetry analysis indicating that both host black phosphorene and black arsenene have D_2hsymmetry,while the symmetry of the local structure containing vacancy defects becomes C_1h after fully relaxation,and around the vacancy site formed a four-coordinated pyramid-like local structure.The formation of the pyramid-like local structure makes the symmetry of the two defect states locate in the bandgap change from B_2u（p_y-state）and B_3u（p_x-state）to the same A^′symmetry.Therefore,large energy level splitting occurs between two defect states with the same A^′symmetry due to strong coupling,resulting in shallow acceptor transition energy levels.These results figure out the origin of intrinsic p-type conductivity of black phosphorene and black arsenene,and reconcile the contradiction between the experimental discovery that black phosphorene and black arsenene are natural p-type semiconductors and the theoretical study that all the intrinsic defects in the two materials are deep acceptor.（3）The concentration of defects and carriers in black phosphorene and black arsenene under 300K rapidly quenching from different temperatures（300～1300K）at thermodynamic equilibrium growth condition were studied by"freezing-in approximation".The results suggest that a lot of vacancy defects can be formed and almost completely ionized at 1300K,which is the main source of intrinsic p-type conductivity of both materials.The hole carrier concentration reaches 10¹⁰cm^-2 under1300K growth condition.The formation energy of vacancy in black arsenene is lower than that of interstitial.Therefore,under any growth temperature,vacancies are the major source of intrinsic p-type conductivity of black arsenene.The hole carrier concentration of black arsenene can reach 10¹¹cm^-2 when it is rapidly annealed to300K from 1300k.The results show that black phosphorene and black arsenene with good p-type conductivity can be easily manufactured under high temperature growth conditions,which makes black phosphorene and black arsenene have wide application prospects in novel 2D CMOS.（4）We predict that the intrinsic p-type conductivity of black phosphorene and black arsenene is extremely related to their unique puckered structure.Since both materials belong to nonpolar covalent semiconductors,we forecast that if the nonpolar atoms of the puckered structure are replaced by cations and anions,shallow vacancy defects and good conductivity will also be obtained.（5）The calculated formation energies of intrinsic defects in monolayer Ge S reveal that Ge-vacancy have shallow acceptor transition energy and lower formation energy.Therefore,Ge-vacancy is the dominant intrinsic acceptor defect and the main source of intrinsic p-type conductivity of monolayer Ge S,which is consistent with the experimental observation of a lot of Ge vacancies generated on the surface of monolayer Ge S.The shallow acceptor transition energy level of Ge vacancy originates from the large local structural relaxation caused by the large Coulomb repulsive interaction between the anions around the vacancy site.Therefore,under equilibrium growth condition,abundant Ge vacancies can be formed,and partial ionization contributes to the intrinsic p-type conductivity of monolayer Ge S.In addition,depending on the chemical potential,there will be fewer intrinsic donor defects to compensate for Ge vacancies in the S-rich growth condition,and obtain monolayer Ge S samples with good p-type conductivity easily.