First-principles study of phonons and related ground-state properties and spectra in Zn-IV-N2 compounds

The Zn-IV-N-2 compounds, with the group-IV element Si, Ge, and Sn, which have a common crystal structure closely related to the wurtzite- structure form a series analogous to the group- III nitrides GaN, AlN, and InN, respectively. Calculations of the phonons and related quantities in these materials are reported here using the density-functional perturbation-theory linear-response approach in the local-density approximation and using a plane-wave pseudopotential method. We focus on spectra, such as the imaginary part of the dielectric function and the energy-loss function as measurable by infrared absorption or reflectivity, and the Raman spectra. We also present phonon densities of states, band dispersions, and related integrated thermodynamic quantities such as the specific heat, and Helmholtz free energy and entropy as functions of temperature. Structural and elastic properties such as the lattice constants and bulk moduli are also reported. Finally, high-frequency and static dielectric tensors are presented. The trends in the series and the relation to the corresponding III-N nitrides are discussed. It is found that the bimodal bond-length distribution with IV-N bonds shorter than the Zn-N bonds (even for Sn) strongly modifies the spectra from those in III-N nitrides. While in ZnGeN2 and ZnSnN2 folded acoustic-like modes are clearly separated from the optic type modes, this is not the case in ZnSiN2. The calculated Born effective charges indicate that ZnGeN2 has the lowest ionicity of the three materials.