Xuefeng Lu

Mechanical properties of β-Si3N4 thin layers in basal plane under tension: A molecular dynamics study

The mechanical properties and failure mechanisms of the β-Si3N4 thin layers in basal plane under uniaxial tension are investigated by using molecular dynamics simulations. It is found that the thin layers display a nonlinear stress-strain relationship first at e < 0.06, and then a linear response at 0.06 < e < 0.09, and finally the stresses increase nonlinearly with the strains until fracture occurs. The fracture stresses and strains increase with increasing the side lengths of the thin layers, and the trend is same for Youngs moduli accompanying little anisotropy. The deterioration in mechanical properties derives from the N6h-Si bonds where the fracture is initiated.

First principles calculations of electronic structures and optical properties of Al- and Ca-doped β-Si3N4

Abstract The crystal structures, formation energies, electronic structures and optical properties of Al- and Ca-doped β-Si3N4 were studied using first principles calculations based on density functional theory within the generalised gradient approximation. Results show that the band gaps Eg of Al- and Ca-doped β-Si3N4 decrease distinctly in comparison to that of β-Si3N4. The band structures of Ca-doped supercell behave like semiconductors. The binding energy Eb and formation energy Ef of β-Si3N4 doped with Al are lower than those of Ca-doped β-Si3N4, indicating that the former has a more stable crystal structure than the latter. The static dielectric constant ϵ(0) increases significantly to ∼35 after Al doping and almost does not change after Ca doping. The strong absorption band located at 5·11–20·81 eV becomes much sharper and shows a red shift after Al and Ca doping. The offset of the Al-doped system is larger than that of the Ca-doped system. The absorption coefficient can be remarkably modulated by Al and Ca doping, indicating the potential applications of Al- and Ca-doped β-Si3N4 in optical system.

Effects of rare earth adsorption on electronic structure and optical properties in β-Si3N4 ceramics from first principles

Electronic structure and optical properties of silicon nitride adsorbed by rare earths are explored by density functional theory. The fully relaxed structural parameters are found to be in good agreement with experimental data. Covalent character of polar covalent bonds by rare earth adsorption is in the order of Gd, Yb, Sc, Sm, La and Lu, which is verified by population values. Gd, Yb, Sc and La adsorption systems may exhibit longer life in applications as dielectric materials in the low energy regions because of the low static dielectric constant and loss, as well as transparent type characteristic exhibited in visible regions.

Effects of element doping on electronic structures and optical properties in cubic boron nitride from first-principles

Attractive potential applications of cubic boron nitride (c-BN) derive from the properties of semiconductors, widely used in optoelectronic and microelectronic devices. In this paper, the effects of element doping on the electronic structures and optical properties in cubic boron nitride are investigated. The Al- and Ga-doped systems have the lower bonding energies of −11.544 eV and −5.302 eV, respectively, indicating better stability. Difference charge density maps demonstrate that the electron loss increases after P doping and decreases after Al, Ga and As dopings, indicating that the covalent character of polar covalent bonds decreases by doping in the range of P, Al, Ga and As, which is in accordance with the calculated atom population values. The Al- and Ga-doped systems show lower dielectric loss, absorption and reflectivity in the lower energy region, displaying the “transparent-type” characteristic and their potential applications in electron devices.