First-principles Calculations Based On ?-N Two-dimensional Materials

More and more two-dimensional materials with fascinating properties have been gradually applied to the new generation of electronic devices,the theoretical and experimental research on two-dimensional materials will set off a further upsurge.The successful application of traditional ?-N materials in the field of optoelectronics and power electronics has fully confirmed broad application prospect of ?-N in future.Thus,the exploration of various potential physical and chemical properties of ?-N material family under two-dimensional?2d?phase has become a research hotspot in recent years,attracting increasing attention from research groups at home and abroad.However,there are few systematic researches on ?-? two-dimensional material system,so it is of significance to calculate the physical properties of 25 two-dimensional structures of ?-? family as well as summarize the corresponding inherent regularity.Preliminary studies have found that BN,AlN and GaN have ideal flat graphene-like structures and have been successfully prepared experimentally.Therefore,based on the summary of the relevant physical properties of 2d materials in the ?-? group,this paper focuses on in-depth research on the above three 2d ?-N materials.It mainly explores the effect of two-dimensional heterojunction,strain,and various point defects on physical properties.The specific research work includes the following four aspects:1. Inorder to explore the internal regularity of some basic physical properties of group ?-? 2d materials and to figure out the uniqueness of group ?-N 2d materials,this paper firstly makes systematic calculations on 25 kinds of binary monolayer materials of group ?-?,mainly including in-depth studies on structural properties,mechanical properties and electrical properties.The results show that,among the 25 structures,BN,AlN,GaN,InN,TlN,BP,BAs,BSb and AlP are flat or very close to flat,while the other structures have different degrees of folding.Secondly,this paper summarizes the electrical quantities such as band edge electrons,hole effective mass and band gap of 25 kinds of ?-? 2d compounds,as well as the mechanical quantities such as Young's modulus and Poisson's ratio.Inaddition,based on the atomic number,a relative radius is defined in this paper,and 25 of compounds are grouped according to the relative radius.Finally,the variation of energy band with uniaxial strain,biaxial strain and shear strain is studied systematically.2. Regarding graphene-like AlN?g-AlN?2d material,based on the graphene/g-AlN?Gr/g-AlN?heterojunction system,this paper mainly focuses on the Alvacancy and N vacancy defects,as well as how the biaxial strain regulates the physical and chemical properties of the system.The results showed that the introduction of N-vacancy?Gr/g-AlN-V_N?was more stable than that of Alvacancy(Gr/g-AlN-V_Al),and the Gr/g-AlN-V_N showed no magnetism,while the total magnetic moment was1?_B for g-AlN-V_N.This means that graphene in contact with g-AlN can tune its magnetism.Inaddition,it was found that graphene acts as an electron acceptor in the Gr/g-AlN-V_N heterojunction system,and an electron donor in the Gr/g-AlN and Gr/g-AlN-V_Al systems.Traditional functional?hybrid functional?to calculate the n type of Schottky barrier height?_B,n of Gr/g-AlN,Gr/g-AlN-V_Aland Gr/g-AlN-V_N are 2.35?3.69?,2.77?3.23?,and 1.10?0.98?eV,suggest that vacancy defects can effectively tune the?_B,n.The p-type Schottky contact type of Gr/g-AlN will change to n-type after applying 10%biaxial compression strain,while it will change to Ohmic contact under large tensile strain.Similarly,the contact type of Gr/g-AlN-V_N will change from n-type to p-type when7.5%tensile strain is applied.3. For the hexagonal BN?h-BN?system,this paper systematically studies the structural,magnetic,electronic and defect properties of four intrinsic defects:N vacancy?V_N?,B vacancy?V_B?,N anti-site?N_B?,B anti-site?B_N?.Inthe research process,this paper proposes a simple method to correct the charge transition level?CTL?of the traditional functional?Perdew John P.,Burke Kieron,Ernzerhof Generalized gradient approximation,GGA-PBE?to the hybrid functional?Heyd-Scuseria-Ernzerhof,HSE06?accuracy,saving a lot of time for defect calculation.Based on this method,this paper systematically studied defect formation energies in h-BN,and found a method to quickly obtain special vacuum size?SVS?,and took N vacancy as an example to verify the reliability of this method by using hybrid functionals.Based on the results of CTLs,it is found that all intrinsic defects act as holes or electron trap centers,thus inhibiting the mobility of p-type or n-type carriers for h-BN-based electrical devices.4. For the hexagonal GaN?h-GaN?system.Based on the first-principles method and 2d charged defect correction theory,the CTLs under GGA-PBE level is effectively converted to HSE06precision.The structural properties,magnetic properties and defect properties of 12 n-type and p-type charged doping systems of h-GaN are systematically studied in this paper.The n-type system includes C_Ga,Si_Ga,Ge_Ga,O_N,S_N,Se_N,while the p-type system includes Be_Ga,Mg_Ga,Ca_Ga,C_N,Si_N,Ge_N.The results show that Ge_Ga and Be_Ga are the most stable defects in n-type and p-type,respectively.Inaddition,the most stable charge states of the n-type system are 0 and 1+,which is manifested as the characteristics of shallow donor energy level,which will also capture holes in p-type h-GaN and affect hole conductivity.The most stable charge state of the p-type system is 1-,0?except for Ge_N?,and 1+,and the main ionization energy is distributed in the energy range of higher than the valence band maximum?VBM??1.25 to 2.85 eV,which is characterized by deep acceptor energy level and capture electrons?holes?in n?p?h-GaN and affect the conductivity of n?p?carriers.Therefore,it is difficult for two-dimensional h-GaN system to achieve n or p-type doping through single defect,and it is necessary to consider the complex defects to achieve bipolar doping experimentally.Inconclusion,based on ?-? 2d material systems,this thesis firstly calculated and summarized the structure properties,mechanical properties and electrical properties of 25 kinds of ?-? 2d materials.And then,?-N family under constructed heterojunction,applied strain and introduced defects condition were studied and various physical and chemical properties were concluded.Finally,two relatively efficient computational solutions of a two-dimensional charged defects are proposed.On the one hand,the research results are of great significance for a deep understanding of the physical properties of ?-N two-dimensional material system,for a clearer understanding of the potential application of ?-N system,and for providing necessary theoretical guidance for experimental research.On the other hand,it lays a data foundation for further in-depth and systematic research on ?-N two-dimensional system.