First-principles study of lattice dynamics in Mg-IV-N₂ semiconductors

Mg-IV-N₂ compounds with IV=Si, Ge, Sn are ultra-wide band gap semiconductors with various potential electronic and optoelectronic applications. Here we performed Density Functional Perturbation Theory (DFPT) calculations to study the vibrational modes of these materials in the Pna2₁ space group crystal structure. We focus on the vibrational modes at the zone center which are measurable by infrared and Raman spectroscopy. We provide a group theoretical analysis of the modes and calculated Raman and infrared spectra for the various relevant light polarizations and Raman scattering geometries. Related quantities, such as Born effective charges are reported. Furthermore, it was recently reported that MgSiN₂ may exhibit ferroelectricity. Relatedly, we here calculate spontaneous polarization, piezoelectric and elastic constants.