| Nano-silicon nitride has been widely used in electric equipments and basic devices owing to better mechanical,electric and optical properties.However,a large of studies focus on preparation of nano-wires,rods and bands,and the description of optical and mechanical properties is lack.Meanwhile,it is different to investigate and access the performances.Based on these reasons,in this present contribution,electronic structure and optical properties of silicon nitride are investigated by first-principles calculation.The tensile behavior of nano wires is studied by molecular dynamics simulation.The results can provide the data for design and controlling in devices.(1)Single crystal structure model is built by means of lattice constants and space parameters.The stable structure is obtained by relaxing and the data is compared with references.The super cell model is obtained and then silicon nitride doped by In is taken.The best stable model is built.(2)The band gaps are 4.5 and 2.1 for systems after and before doping,indicating that the effect of doping on the property is present.There are 81 energy bands for In-doped system and the density is improved.The low valence band is located at the range of-18 to-12 eV,and the top valence band is in the range of-10 to 0 eV.The conduction band is at 3-8 eV.The low valence band derives from the contributions of 5s orbital electrons in In and 2s orbital electrons in N atoms.The top valence band comes from the contributions of 2p orbital electrons in N and 2p orbital electrons in Si atoms.The conduction band is from 2p orbital electrons in Si and 5s orbital electrons in In atoms.(3)Differential charge density map results show that the blue area of In doped system decreases a little,i.e.electronic loss decreases,revealing that the covalent of N-In bond decreases.The calculated population values decrease from 0.65 to 0.35,indicating that the covalent decreases,which is in agreement with the results of differential charge density.The imaginary part of system increases in low energy region,demonstrating that dielectric loss increases and life span decreases.(4)In the UV region with wavelength of 80-400 nm,the absorption is strong and the reflection degree is high,showing the feature of “Barrier-type”.In the visible region with wavelength of 400-800 nm,the systems before and after doping have lower absorption coefficient and reflectivities,indicating that the systems have the “clear-type” properties.In addition,the reflectance spectrum of In-doped system becomes strong,also revealing that part light is reflected.(5)The tensile loading simulations of silicon nitride nanowires with the aspects fo 4:1,6:1 and 8:1,respectively,are conducted by molecular dynamics.The results show that the stress increases firstly and becomes smooth with the increasing of strain.The stress-strain curves consist of quasi-linear phase and nonlinear stage and the elastic limit is about 0.02.The fracture stress decreases with the increasing of aspect ratio.The peak values are 32.61,31.8 and 31 GPa,respectively.The effect of aspect ratio on the fracture behavior is obvious.(6)The snapshots of nanowires with different aspect ratios are captured in the course of loading by visual software.It indicates that the fracture occurs at the center of nanowire for the aspect ratios of 4:1 and 8:1.While the fracture is present at the right side of nanowire.(7)In the initial stage of loading,part Si-Si bond defects and single N atoms defects are observed in the interior of nanowires.The number of Si-Si bond and single N atom defects increases greatly at strain of 0.048 when fracture occurs.The new emerging defects of N atom with the coordination numbers of 0,1 and 2 and Si atoms with the coordination numbers of 5,6 and 7 are present,which strengthens the stress concentration enormously. |
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