First-principles Study Of Defect Evaluation Method And Its Physical Properties In Two-dimensional Semiconductors

Two-dimensional semiconductors are substances with atomic-level thickness in which electrons are allowed to move freely in two-dimensional plane but restricted in the third direction.Dimensionality reduction brings the materials quantum confinement and weak dielectric screening which in turn endow them unique properties.They show great potential in advanced technological applications,especially in electronics.The inherent advantages of two-dimensional semiconductors make them promising candidates of silicon and expected to dominate the future high-performance electronics(high-level device integration and lower power consumption).The realization of controllable and stable N-type and P-type conductivity of these important materials through defect engineering is crucial for the industrialization of two-dimensional devices.However the current studies of defect physics in two-dimensional semiconductors such as doping properties and carrier transport supplied by defect,are still in infancy.More seriously,conventional calculation method of charged defects in three-dimensional semiconductors causes energy divergence and thus fails in two-dimensional systems,which no doubt obstacles the accurate and quantitative evaluation of electronic properties of defects in two-dimensional semiconductors.In this thesis,an accurate and efficient approach to evaluate the energies of charged defects in two-dimensional semiconductors is proposed,which overcomes the drawback of energy divergence accompanied with the use of conventional calculation method.With the approach,the defect behaviors and regularity of defect property are explored.The main results of the thesis are as following:1.Evaluation approach of charged defects in two-dimensional semiconductors.The conventional calculation scheme of defects which uses Jellium approximation under the periodic boundary condition,works well for three-dimensional semiconductors but encounters an energy divergence with vacuum dimensions for charged defects in two-dimensional semiconductors.This means that different vacuum size give widely scattered results which is physically forbidden.The divergence originates from the Coulomb interaction between charged defects and oppositely charged Jellium.An analytic expression for the asymptotic behavior of the Coulomb divergence is derived,showing converged ionization energy as the zero Jellium density limit.This formulation is combined with first-principles calculations to determine the energies of charged defects in all monolayer materials.The approach lays the foundation for first-principles determination of charged defects in monolayer two-dimensional semiconductors using the standard Jellium background approximation.2.Generalization of the evaluation approach of charged defects in two-dimensional semiconductors.While the evaluation approach of charged defects in monolayer semiconductors are proposed in previous section,it becomes a new problem that how to calculate the cases which are mostly used in the practical electronic devices,i.e.few/multi-layer two-dimensional semiconductors instead of monolayer,and a layer of two-dimensional semiconductor on a substrate.One possibility to tackle the physics of defects in multilayer could be the use of a supercell with extremely large vacuum size to mimic that the few layer is still thin enough compared to this vacuum size and the application of the existing monolayer formalism.However,such calculations are usually prohibitive due to the associated computational cost.Based on this,we generalize the method to evaluate charged defect energies in two-dimensional and quasi-two-dimensional materials with arbitrary thickness and geometry including few-layer-thick,two-dimensional systems on a substrate,as well as surfaces and interfaces.Application to few-layer black phosphorus in particular,to substitutional Te(donor)and phosphorus vacancy(acceptor)reveals that that enhanced screening with increasing thickness shoal the defect levels with decreasing ionization energies.The generalized approach lays the theoretical foundation for design of materials with “customized” properties for two-dimensional electronics.3.Defect exciton in two-dimensional semiconductors and its effect on carrier transport.Application the evaluation method of charged defects in two-dimensional semiconductors to the native and substitutional defects in monolayer MoS2 reveal that they are all deep defects with large ionization energies.Even for the shallowest donor(Re substitution Mo,ReMo)and acceptor(Nb substitution Mo,NbMo)among them,the ionization energies are still up to 0.45 eV and 0.55 eV.As a result,theory suggests that defect ionization is exceedingly difficult and such materials cannot be doped to yield reasonable carrier concentrations.This theoretical understanding is seemingly contradicted by a number of experimental studies which have directly shown the stable N-type or P-type conduction in Re-doped and Nb-doped MoS2.Motivated by the contradiction,we explore the defect exciton and its effect on carrier transport.When ionized to band edges,carriers are still strongly bound by the charged defects due to the weak screening in two-dimensional materials.The strong Coulomb interaction between the charged defects and carriers bind them into a “carrier-charged defect” exciton.The exciton binding energies need to be overcome to totally free carriers.This means that ionization energies of defects in two-dimensional semiconductors include considerable exciton binding energies.The exciton binding energies of ReMo and NbMo are 0.45 eV and 0.39 eV,respectively.Although large ionization energies and exciton binding energies,the wavefunctions of carriers at bound band edges overlaps in real space with facilitation of transport.The overlap can be enhanced readily by external dielectric environments.The present investigations shed light on the key understanding of carrier supply and its transport through defect engineering for the emerging two-dimensional semiconductor devices.4.N-type and P-type conductivity of graphene oxide by nitrogen and boron doping.Graphene oxide with natural band gap and low-cost prepration has potential to be electronic materials in future electronics industry.We systematically investigated the behavior and electronic properties of dopants(nitrogen and boron)in graphene oxide in order to explore its possible N-type and P-type conductivity.The well-accepted hydroxyl chains and epoxy chains constitute the sp3 region of the graphene oxide.The oxygen coverage is 50%,which results in a band gap of 1.80 eV.Boron has tendency to replace sp3 carbon as acceptor while nitrogen prefers to substitute carbon atom at the boundary between sp2-region and sp3-region as donor.Their ionization energies are 0.24 ~ 0.42 eV for boron and 0.32 ~ 0.67 eV for nitrogen.However,a special case of nitrogen doped at the boundary can change to be an acceptor with assistance of its neighboring(epoxy)oxygen “Lift-off”,leading to the shallowest ionization energy of 0.12 eV in the GO model and the best candidate for p-type conductivity.The present study offers a microscopic picture of doping behaviors in graphene oxide for future electronic devices applications.In summary,we firstly propose an evaluation approach for charged defects in monolayer semiconductors which solves the problem of energy divergence when traditional calculation method is used.On this basis,we derive a more general formalism of the approach for not only two-dimensional semiconductors with arbitrary thickness but also quasi-two-dimensional systems such as substrates.Application to defects in typical two-dimensional semiconductors reveals that the ionization energies are all large,which originates from the weak screening.Meanwhile,the weak screening of two-dimensional semiconductors induces defect exciton and defect bound excitonic states,which further lead to a different physical picture of carrier transport.This thesis advances the physics of defect of two-dimensional semiconductors and provides the theoretical support for the industrialization of two-dimensional electronics.