Structure and lattice dynamics of the wide band gap semiconductors MgSiN₂ and MgGeN₂

We have determined the structural and lattice dynamical properties of the orthorhombic, wide band gap semiconductors MgSiN₂ and MgGeN₂ using density functional theory. In addition, we present the structural properties and Raman spectra of MgSiN₂ powder. The structural properties and lattice dynamics of the orthorhombic systems are compared to those of wurtzite AlN. We find clear differences in the lattice dynamics between MgSiN₂, MgGeN₂ and AlN, for example, we find that the highest phonon frequency in MgSiN₂ is about 100 cm⁻¹ higher than the highest frequency in AlN, and that MgGeN₂ is much softer. We also provide the Born effective charge tensors and dielectric tensors of MgSiN₂, MgGeN₂ and AlN. Phonon related thermodynamic properties, such as the heat capacity and the entropy, have also been evaluated and are found to be in very good agreement with available experimental results.