On the electronic nature of silicon and germanium based oxynitrides and their related mechanical, optical and vibrational properties as obtained from DFT and DFPT

Electronic structure, bonding and optical properties of the orthorhombic oxynitrides Si2N2O and Ge2N2O are studied using the density function theory as implemented in pseudo-potential plane wave and full-potential (linearized) augmented plane wave plus local orbitals methods. Generalized gradient approximation is employed in order to determine the band gap energy. Indeed, the Si2N2O exhibits a large direct gap whereas Ge2N2O have an indirect one. Bonding is analyzed via the charge densities and Mulliken population, where the role of oxygen is investigated. The analysis of the elastic constants show the mechanical stability of both oxynitrides. Their bulk and shear modulus are slightly smaller than those reported on nitrides semiconductors due to the oxygen presence. The optical properties, namely the dielectric function, optical reflectivity, refractive index and electron energy loss, are reported for radiation up to 30 eV. The phonon dispersion relation, zone-center optical mode frequency, density of phonon states are calculated using the density functional perturbed theory. Thermodynamic properties of Si2N2O and Ge2N2O, such as heat capacity and Debye temperature, are given for reference. Our study suggests that Si2N2O and Ge2N2O could be a promising potential materials for applications in the microelectronics and optoelectronics areas of research.