Structure and lattice dynamics of the wide band gap semiconductors MgSiN$_{2}$ and MgGeN$_{2}$

Abstract: We have determined the structural and lattice dynamical properties of the orthorhombic, wide band gap semiconductors MgSiN${2}$ and MgGeN${2}$ using density functional theory. In addition, we present structural properties and Raman spectra of a MgSiN${2}$ powder. The structural properties and lattice dynamics of the orthorhombic systems are compared to wurtzite AlN. We find clear differences in the lattice dynamics between MgSiN${2}$, MgGeN${2}$ and AlN, for example we find that the highest phonon frequency in MgSiN${2}$ is about 100~cm$^{-1}$ higher than the highest frequency in AlN and that MgGeN${2}$ is much softer. We also provide the Born effective charge tensors and dielectric tensors of MgSiN${2}$, MgGeN$_{2}$ and AlN. Phonon related thermodynamic properties, such as the heat capacity and entropy, are in very good agreement with available experimental results.