Simulation Study Of Intrinsic Point Defects And Heterostructures On β-Si₃N₄（0001） Surface

As a structural and functional ceramic material,silicon nitride has attracted much attention in engineering practice applications.It is expected to become the substrate material for next-generation power devices because of its good stability,high strength,high thermal conductivity and other characteristics.However,the intrinsic point defects have become one of the important factors influencing the performance of optoelectronic devices,at the same time,exploring the microstructure and properties of the Si₃N₄-based heterogeneous phase interface has become particularly important when serving in various environments.Based on this,we conduct the electronic structure and optical properties of vacancies(V_Nt1,V_Nt2 and V_Si)and interstitial(I_N and I_Si)with different Si-N atomic layers in the H-passivatedβ-Si₃N₄（0001）surface from the atomic scale by the first-principles calculation method.The graphene/β-Si₃N₄（0001）heterostructure is constructed and the electronic and optical properties is calculated,and it provides theory and guidance for its further practical application.（1）For the two-dimensional single-atom vacancy defect system with different layers ofβ-Si₃N₄,it is found after structural optimization that the closer the vacancy is to the inner layer,the smaller the distortion around the atoms,especially the change of the atom around N_t1 is closer to the bulk V_Nt1 system.The formation energy indicates that under the Si-rich condition,the lower layer is likely to form N_t2 vacancy defects.Si vacancies are more difficult to generate in the inner layer.The existence of N vacancies causes the conduction band minimum to move downward,the valence band maximum to move away from the Fermi level,and a new additional energy level is present in the middle band,which has a great influence on carrier migration.When the N vacancy is near the surface layer,the dielectric constant gets smaller.However,the Si vacancies makes the system have the characteristics of p-type semiconductors.The closer the N vacancy is to the inner layer,the narrower the additional energy band range is,the more localized the electron is,and the peak becomes sharper of density of states.The closer the vacancy is to the surface layer,the smaller the photon energy value corresponding to the maximum absorption coefficient.In the low-energy region,the Si vacancy system has a higher reflectivity for light than that of the N vacancy system.（2）After relaxation of the interstitial defect structure,it was found that the interstitial Si atoms only moved in the z-axis direction,and the interstitial N atoms formed new Si-N bonds and N-N bonds with Si atoms and N_t2 atoms on the six-membered ring.The formation energy results indicate that the N atom interstitial defects are easily generated in the surface layer,while Si atom interstitial defects are prone to appear in the inner layer.Compared with vacancy system,N atom interstitial defect is more difficult to generate.The N interstitial system has a new additional energy band at the Fermi level.The closer the interstitial N atoms are to the inner layer,the narrower the energy range of the additional energy band is.However,the Si interstitial system has new additional energy band at the Fermi level and the middle band near the conduction band minimum.The dielectric constant of the I_Si-3 system is close to 0 when it is greater than 2.5 eV,and it has strong absorption capacity for visible light when the photon energy is less than 2 eV.（3）The calculated results of the graphene/β-Si₃N₄（0001）heterostructure show that the N_t1 atoms on the surface ofβ-Si₃N₄（0001）tended to be close to the graphene layer,but the wrinkled state cannot be observed in graphene laye.The binding energy shows that the structure of N_t1 correspronding to the center of the C ring is more stable,and the interlayer distance D₀ is 2.914?.The band structure is similar to the overlap of two band energy,and retains the shape of the Dirac cone,with an energy gap value of 67 meV.The difference charge density map shows that the graphene layer and theβ-Si₃N₄（0001）layer have charge transfer phenomenon.The change of the interlayer distance makes the position and band gap of Dirac cone change.The absorption capacity of the graphene/β-Si₃N₄（0001）heterostructure becomes stronger in the visible region,and it has a stronger reflection ability in the ultraviolet region.