First-principles Study Of Doped ?-Si₃N₄ And Surfaces Of ?-Si₃N₄ With Adsorptions

As a kind of significant ceramic material in the engineering applications,silicon nitride?Si₃N₄?has been widely applied due to its excellent properties,such as high temperature resistance,wear resistance and antioxidation,besides,it owns a wide band gap of about 4.5eV,which indicates its great application prospects in the field of microelectronic and photoelectronic devices with narrower band gap and improved optical properties by certain means.Up to now,the researches about Si₃N₄ mainly focus on the preparations of thin films by various chemical vapor deposition methods,and many works about silicon nitride can not be accomplished as a result of the restrictions from diverse experimental conditions.Based on it,in this paper,we carry out the first-principles calculations about electronic structures and optical properties of doped?-Si₃N₄ and surfaces of?-Si₃N₄ with adsorptions,the calculated results are shown as follows:1.Here,the optical properties and electronic structures of pristine and doped?-Si₃N₄with elements from?A,C,Ge,Sn and Pb were demonstrated.Firstly,it should be induced that Ge-doped system possessed a more stable structure with lower binding energy compared with other doped cases.The band gap of C-,Ge-,Sn-and Pb-doped system was 2.901eV,4.190eV,3.188eV,1.599e V,respectively,in which C-and Pb-doped cases owned relatively narrow band gap as a result of the impurity levels that were created by C-p,Si-p and Pb-p,N-p orbital electrons,respectively.The Sn-doped system had the highest dielectric loss,which would shorten the service life period in practical engineering.While as for C-and Ge-doped systems,a lower dielectric loss could be present at low energy region,which predicted it much more beneficial to the applications in dielectric materials.2.The calculated formation energy of?-Si₃N₄ single-and co-doped with elements from?and?A revealed that the structural stability order of Al-,Ga-,P-,and As-doped systems was as shown in following:Ga>As>Al>P.The band gap of As-and P-doped system decreased to 0.950e V and 1.922eV,respectively,indicating that the conduction process of systems was motivated mostly by the contribution of“free electrons”carriers.Viewing from the difference charge density maps of systems,the covalent property order of bonds was Ga-N>Al-N and Si-P>Si-As,which was in good agreement with the results of calculated Mulliken population values of bonds.When it came to the co-doped systems,the Ga-P co-doped system owned the most structural stability,moreover,the binding energy of co-doped systems created by P atom was smaller than that by As atom,which implied that the contribution of P atom to the stability of system was greater than that of As atom.The Al-,Ga-and As-doped systems exhibited increasing dielectric loss and static dielectric constant in the low energy range,which might decrease the service life of materials when applied in dielectric materials,in contrast,the P-doped system behaved differently compared with the cases above,which provided greater possibilities to obtain materials with longer service and lower energy loss.Furthermore,the co-doping caused rapid increase for static dielectric constant of system,which would be of great value to expand its scope of applications in the field of dielectric materials.As for Al-P,Al-As,Ga-P and Ga-As co-doped systems,the absorption,reflectivity spectrum and energy loss peaks are lower than those of single-doped systems,indicating that the co-doped systems had greater potential in the applications of antireflection coatings and wave-transparent-materials than single-doped ones.3.The electronic and optical properties of?-Si₃N₄?200?surface with 3d transition metal adsorption behaviors were studied based on DFT+U method here,the results were shown in the following:First,the surfaces with Ti,Ni,Co adsorptions were more stable than those with Mn and Fe adsorptions;after the adsorptions of 3d transition metal elements,the degree of electron-depletion on the surface became lower gradually,resulting in the enhanced ionic property for bonds between transition metal and N atom,the intensity order of covalent bond was Ti>Mn>Fe>Co>Ni>Cu,which was consistent with the evolution law of Mulliken population values for related bonds.Moreover,the dielectric loss and static dielectric constant of the systems with adsorptions followed a similarly overall trend that increased first and then decreased with the increase of the ionic radius of adsorbed atoms.The dielectric constant in low energy range of the systems was mainly derived from the contribution of[010]polarization direction,from which it could be obtained that the properties of the surface structures were of anisotropy to some extent.The surfaces with adsorptions had a relatively obvious"Fortress type"characteristic to the light wave in the ultraviolet region.In the visible range of 390⁷80nm?photon energy range of 1.59³.18eV?,the absorption spectra of the surfaces was quite low,which showed that the systems exhibited strong"Permission type"characteristic.In the near-infrared region?wavelength longer than 760nm,photon energy range of 0¹.632eV?,a much lower absorption peak was present for Mn-adsorbed surface,implying that this system owned the capacity of long wave absorption,which indicated great significance in promoting its applications in the field of long-wave-absorption materials.