Simulation Study On Optical Properties And Mechanical Bebaviors Of Nanosized Silicon Nitride

Silicon nitride,as an advanced structural and functional ceramics,has captured a comprehensive application in a majority of fields.With the scope of technology progress to smaller and smaller length scales,nanosized Si₃N₄ ceramics,well-known for the special mechanical,electrical and optical properties that arising from nanostructure,will show great potential at the nanodevices.However,the measure and evalution of Si₃N₄ properties is difficult to realize by experiments owing to a variety of reasons.Therefore,in this paper,we conduct the electronic structure and optical properties of doped and absorbed silicon nitride systems using first principles and mechanical properties of nanowires and nano-thin layers of silicon nitride employing molecular dynamics,which improves the theoretical basis for further applications.?1?Electronic structure and optical properties of silicon nitride systems doped by Al,Ga,As and P,respectively,are calculated by first principles.The results show that the band gaps are 4.21 and 4.12 e V after Al and Ga doping and decrease after As and P doping.It indicates that the latter two systems have an important influence on the band gap.The bonding energies increase after doping according to the sequence of Al,Ga,P and As,which is accordance with the trend of formation energy and reveals that the stability of Al doping system is more than that of other systems.Differential charge density map results indicate that blue area of P doped system increases a little,i.e.electronic loss increases,revealing that the covalent of N-P bond increases.However,the blue areas of system doped by As,Ga and Al,respectively,decrease,i.e.electronic loss drops,indicating that the covalent decreases.The degree of conversion of the bond with respect to Al doping is highest due to the electron cloud is near N atom and the electronic loss disappears.The covalent order of bonding strength is P>As>Ga>Al,which is consistent with the calculated population value.The static dielectric constants of doped systems decrease firstly and increase with covalent radius increasing,in which the dielectric loss of As doped system in the low energy region is lowest and it is conductive to the applications in photoelectric materials.?2?Electronic structure and optical properties of rare earth?Yb,Gd,Sc,Sm,La and Lu?adsorbed silicon nitride systems.The data show that electronic loss decrease in terms of differential charge density map,indicating that the transition of covalent-ionic occurs.The electronic enrichment can be observed in Yb,Ga and Sc atoms around,displaying that the uneven distribution of electron cloud increases.The covalent order of bonding strength is Yb>Gd>Sc>Sm>?La,Lu?,which is in agreement with population values.The static dielectric constant and loss increase firstly and then decrease,and at last remain constant.The value of Sc adsorbed system is lowest.The contribution of [010] polarization direction is larger than that of other two directions,indicating that the adsorbed systems exhibit a certain degree of anisotropy.In the near infrared region with the wavelength of 780-2500 nm,the reflectivities of the systems adsorbed by La,Yb and Ga,respectively,are lowest,and the values are lower than 6%.While the values of Lu,Sm adsorbed systems are up to 20%,the reflection degree is relatively high and correspondingly the refractive index is lower.It indicates that light spread more easily in the first four adsorption systems.In the visible region with wavelength of 390-780 nm,the six kinds of adsorption systems have lower absorption coefficient and reflectivities,indicating that the systems have “clear-type” properties.In the UV region with wavelength of 80-390 nm,the adsorption systems can absorb large amount of light and display “Barrier-type” characteristics.?3?The models of silicon nitride nanowires in the [001] direction are built using the combination of first principles and molecular dynamics.The compression and tensile behaviors of nanowires are investigated by molecular dynamics.The results show that elastic limit is independent of aspect ratio and most occurs at ?=0.05 in tension.The fracture stress decreases with the increasing of aspect ratio,and at the same time a lot of defects that Si-Si bonds and N atoms clusters can be observed.In the compression behavior,new quadruple Si atom defect can be seen in the middle of low surface of nanowires and two symmetrical positions on the surface when the stress comes to maximum value,leading to the decrease of the stress value.?4?The three-point bending strength testing process is simulated by molecular dynamics.The results show that the stress-displacement curve consists of quasi-elastic phase and nonlinear phase.The fracture stress decreases with the aspect ratio?3:1,5:1 and 7:1?increasing.The bending stress increases whit the increasing of displacement after the indenter contacts nanowires.The stress drops dramatically when the initial fracture is present.It can be observed that the initial Si-Si bond and N atom defects with a coordination number of 2,subsequently evolving to 0 and 1,with Si evolving from 5 to 6 and 7,in the middle of top surface of the nanowire.?5?The nano-thin layers model of silicon nitride are built with the atom number of 5600,7406 and 9464,respectively.The stable configuration is obtained by the steepest descent method.The tensile behaviors of nano-thin layers are simulated.The results show that the non-linear stress-strain relationship is displayed at ?<0.06 in the course of deformation,and then linear stress-strain relationship is observed at 0.06<?<0.09,and finally non-linear stress-strain relationship is taken at ?>0.09.The fracture stress and strain increase with thin film side.The Young's modulus remains constant.The fracture stress of y direction is larger than that of x direction.The deterioration in mechanical properties derive from the N6h-Si bonds where the fracture is initiated.The location and strain point of N6h-Si bond breaking defects are the same with the strain rate increasing,which is independence of strain rate.The strain of separation decreases to 0.113 when the strain rate increases to 5.3×109s-1,which is attributed to the presence of N2c-Si bond breaking defect in the expansion process of N6h-Si bond breaking,promoting the crack spread.The maximum tensile stress first increases and then decreases as the tensile temperature is high,and corresponding strain decreases.