Vibrational modes in ZnGeN2:: Raman study and theory

Polarized Raman spectra were measured for single crystals and polycrystalline ZnGeN2 grown by a vapor-liquid-solid method. A group-theoretical analysis of the selection rules governing the predicted dependence of the spectra on incoming and outgoing wave polarizations and propagation direction is presented. First-order Raman spectra corresponding to the zone center phonons are calculated from first principles using density functional perturbation theory. The Brillouin zone integrated density of phonon states is also calculated. Comparison of theory and experiment allows us to identify the a(1t) symmetry modes. However, vibrational density of states features deviating from the pure a(1t) spectrum are also visible in the experimental spectra and indicate some relaxation of momentum conservation rules. Differences in the experimental spectra under different polarization conditions are compared to the calculations. These differences allow us to identify different a(1t) Raman tensor components as well as to obtain at least partial information on the b(2t) modes. The much weaker polarization dependence in experiment than in theory, however, can at least in part be explained by using wurtzite-type selection rules. The observation of features explainable with wurtzite together with orthorhombic selection rules suggest that there is only partial ordering of the cations on their sublattice.