First-principles Study On Defect Physics In Ⅰ-Ⅶ Two-dimensional Semiconductor

Two-dimensional（2D）materials are a class of materials with a planar structure,usually only one or a few atomic layers thick.The electron distribution in this type of materials is confined in an extremely thin plane,which brings unique physical properties to materials that are different from traditional bulk materials.2D materials have great potential in various integrated devices,which can bring more novel and excellent performance to devices.In the future,devices developed based on 2D materials will shoulder the responsibility of continuing to write"Moore’s Law"in the development of semiconductor devices.After rapid development in the past two decades,systems such as group IV,group V,group III-V,group II-VI,group III-VI,and transition metal chalcogenides have been reported and even demonstrated application potential.The study of 2DⅠ-Ⅶsemiconductors have been blank for a long time.Until2018,it was theoretically predicted for the first time that the 2DⅠ-Ⅶsemiconductors with a double-layer honeycomb（DLHC）structure are stable novel 2D materials,but related property research and experimental verification are still in their infancy.As a kind of practical semiconductor,introducing suitable defects into 2DⅠ-Ⅶsemiconductors is the main way to realize n-type and p-type conductivity of them,and is an important basis for designing this new type of semiconductor device.The formation energy and ionization energy analysis of defects can be used to evaluate the difficulty of defect formation and carrier ionization,so it is very important to evaluate the defect physical behavior of 2DⅠ-Ⅶsemiconductors.When evaluating the defect ionization properties of traditional three-dimensional semiconductor materials,the jellium background charge model can reasonably predict the ionization energy of defects,and then propose n/p type defect doping schemes.However,in the two-dimensional system,the background charge（simulated ionization charge）in the jellium model diffuses into the entire vacuum layer,which is significantly different from the distribution of the ionization charge limited in the two-dimensional plane in the real situation,causing the ionization energy and formation energy of the defect to diverge with the size of the vacuum region.Therefore,in the research work of 2DⅠ-Ⅶsemiconductor defect evaluation in this thesis,we will use the WLZ extrapolation method to correct the energy divergence problem in the defect ionization calculation,and provide a reliable method for the reasonable evaluation of the defect properties of2D materials.This thesis focuses on the basic properties of two 2DⅠ-Ⅶsemiconductor materials represented by Ag I and Cu I.Based on their stability,it is the first time in the world to discuss the defect behavior of 2DⅠ-Ⅶsemiconductors using Ag I as an example.The research results are as follows:1.The energy band,electronic density of states,and phonon spectrum calculations of 2D Ag I and 2D Cu I with DLHC structure illustrate that the stability of the two materials at 0 K and the excellent electronic structure characteristics of the direct band gap are maintained.Furthermore,by analyzing the results of their molecular dynamics simulations at 300 K,and counting the magnitude and range of changes in bond lengths,atomic positions,and energy,it is found that 2D Ag I with DLHC structure is more stable than 2D Cu I at room temperature.And the 2D Ag I can still remain stable when the temperature is raised to 400 K.So in the following defect physics research section,our analysis is based on 2D Ag I.2.In the defect study of 2D Ag I with DLHC structure,we considered four types of thirteen defects that are expected to be used to control the conductivity type,including（1）intrinsic defects,including iodine vacancies（V_I）and silver vacancies(V_Ag);（2）groupⅥA substitution on I site defects,including sulfur substitution on I site（S_I）,selenium substitution on I site（Se _I）,tellurium substitution on I site（Te _I）;（3）groupⅡB substitution on Ag site defects,including zinc substitution on Ag site(Zn _Ag),cadmium substitution on Ag site(Cd _Ag),mercury substitution on Ag site(Hg _Ag);（4）groupⅡA substitution on Ag site defect,including beryllium substitution on Ag site(Be _Ag),magnesium substitution on Ag site(Mg _Ag),calcium substitution on Ag site(Ca _Ag),strontium substitution on Ag site(Sr _Ag),barium substitution on Ag site(Ba _Ag).Considering the conditions of easy formation and ionization of defects,the following conclusions are obtained:V_Ag is the most suitable acceptor defect with the ionization energy of 0.432 e V;Be _Ag is the most suitable donor defect with the ionization energy of 0.260 e V.Although their ionization energy is larger than the thermodynamic energy at room temperature,there are no obvious defect energy levels inside the band gap of their energy band diagrams,indicating that the defects have the characteristics of self-ionization to the band edge,and have a large exciton-like binding energy.3.In 2D Ag I with DLHC structure,V_I,Zn _Ag,and Cd _Ag exhibit the nature of negative U-defects.From the formation energy,it can be concluded that these three defect systems will directly transit between the+1 and the-1 valence states,making the defect transition energy level becomes shallower.By comparing the effect on the ionization energy of the ionized structure before and after optimization,it is determined that the negative U behavior is caused by the significant local structure changes of the defects during the ionization process.In summary,our work is the first systematic study on defects of 2DⅠ-Ⅶsemiconductors in the current international arena,and we have found out the defects most likely to form n-type and p-type conductivity from the thirteen defects of 2D Ag I,providing a conductivity regulation scheme for subsequent applications of this new two-dimensional material at the device level.In fact,at the end of our research,the successful synthesis of 2DⅠ-Ⅶsemiconductors has just been reported experimentally,making the study of defects in 2DⅠ-Ⅶmaterials imperative.This study on the defect properties of this material will lay the foundation for the future design and implementation of electronic and optoelectronic devices based on this brand-new 2DⅠ-Ⅶmaterial.